Perspectives of Hydrosilylation Reactions [72] 9783031459597

Table of contents :
Cover
Half Title
Topics in OrganometallicChemistry Series: Volume 72
Perspectives of Hydrosilylation Reactions
Copyright
Preface
Contents
Silicometallics vs. Organometallics and Catalysis: General Guidelines
Abstract
1. Contents
2. Conclusions
References
Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts
Contents
Abstract
Abbreviations
1. Hydrosilylation of Alkenes
1.1 Platinum Catalysts
1.1.1 Homogeneous Platinum Catalysts
1.1.2 Heterogeneous Platinum Catalysts
1.1.3 Activity Control of Platinum Catalysts
1.2 Palladium Catalysts
1.3 Nickel Catalysts
2. Hydrosilylation of Alkynes
2.1 Platinum Catalysts
2.1.1 Homogeneous Platinum Catalysts
2.1.2 Heterogeneous Platinum Catalysts
2.2 Palladium and Nickel Catalysts
3. Hydrosilylation of Dienes, Allenes, Enynes, and Diynes
4. Hydrosilylation of Carbon-Heteroatom Multiple Bonds and Epoxides
4.1 Hydrosilylation of Aldehydes, Ketones, Esters, Amides, Imines, and Epoxides
4.2 Hydrosilylation of CO2
5. Conclusions
References
State of the Art in Rhodium- and Iridium-Catalyzed Hydrosilylation Reactions
Contents
Abstract
1. Introduction
2. Hydrosilylation of C-C Multiple Bonds
2.1 Selectivity Issues on the Hydrosilylation of Terminal and Internal Alkynes
2.2 Hydrosilylation of Alkynes
2.3 Selectivity Issues on the Hydrosilylation of Alkenes
2.4 Hydrosilylation of Alkenes
2.5 Mechanistic Considerations on the Hydrosilylation of C-C Multiple Bonds
3. Hydrosilylation of C=O Bonds
3.1 Hydrosilylation of Ketones
3.2 Hydrosilylation of CO2
3.3 Hydrosilylation of Amides
4. Hydrosilylation of C-N Multiple Bonds
5. Conclusions
References
Recent Development of Cobalt-Catalyzed Hydrosilylation Reactions: New Catalysts, Mechanistic Understandings, and Future Trends
Contents
Abstract
1. Introduction
2. Cobalt-Catalyzed Hydrosilylation of Alkenes
2.1 Anti-Markovnikov Hydrosilylation of Alkenes
2.1.1 Anti-Markovnikov Hydrosilylation of Alkenes Using Cobalt-Isocyanide Catalysts
2.1.2 Anti-Markovnikov Hydrosilylation of Alkenes Using Cobalt-NHC Catalysts
2.1.3 Anti-Markovnikov Hydrosilylation of Alkenes Using Cobalt-Nitrogen Ligand-Based Catalysts
2.1.4 Anti-Markovnikov Hydrosilylation of Alkenes Using Cobalt-Phosphine Catalysts
2.2 Markovnikov Hydrosilylation of Alkenes
2.3 Hydrosilylation of Allenes
3. Cobalt-Catalyzed Hydrosilylation of Alkynes
3.1 Markovnikov Hydrosilylation of Terminal Alkynes
3.2 Anti-Markovnikov Hydrosilylation of Terminal Alkynes
3.3 Hydrosilylation of Internal Alkynes
3.4 Hydrosilylation of Enynes and Diynes
3.5 Double Hydrosilylation of Alkynes
4. Conclusions
References
Iron and Manganese Catalyzed Hydrosilylation Reactions
Contents
Abstract
1. Introduction
1.1 Iron Catalyzed Hydrosilylation Reactions
1.1.1 Hydrosilylation of Carbonyl Compounds
1.1.2 Hydrosilylation of Alkenes
1.1.3 Hydrosilylation of Alkynes
1.1.4 Hydrosilylation of Imines
1.2 Manganese Catalyzed Hydrosilylation
1.2.1 Hydrosilylation of Carbonyl Compounds
1.2.2 Hydrosilylation of Alkenes
1.2.3 Hydrosilylation of Alkynes
1.2.4 Hydrosilylation of Carbon Dioxide
2. Conclusions
References
Catalysis of Hydrosilylation Processes with the Participation of Ionic Liquids
Contents
Abstract
1. Introduction
2. Ionic Liquids as Solvents and Immobilizing Agents
3. Heterogeneous Catalysts with Ionic Liquids
4. Ionic Liquids as Complexes or Ligands in Metal Complexes
5. Application of Catalytic Systems Containing Ionic Liquids in Continuous Processes
6. Conclusions
References
Hydrosilylation Catalysis for One-Pot Synthesis
Contents
Abstract
1. Introduction
2. One-Pot Synthesis with Si as a Leaving Group
3. One-Pot Synthesis with Si as an Intersection
4. One-Pot Hydrosilylation-Functionalization
5. Conclusions
References
Hydrosilylation of Carbon-Carbon Multiple Bonds in Organic Synthesis
Contents
Abstract
1. Introduction
2. Classical Applications of Intra- and Intermolecular Hydrosilylation in Organic Synthesis
3. Transition Metal-Catalyzed Sequential Double Hydrofunctionalization of Alkynes
4. Asymmetric Hydrosilylation of Alkenes
5. Conclusions
References

Citation preview

Topics in Organometallic Chemistry 72

Bogdan Marciniec Hieronim Maciejewski Editors

Perspectives of Hydrosilylation Reactions

Perspectives of Hydrosilylation Reactions

72

Topics in Organometallic Chemistry Series Editors Matthias Beller, Leibniz-Institut für Katalyse e.V., Rostock, Germany Pierre H. Dixneuf, Université de Rennes 1, Rennes CX, France Jairton Dupont, UFRGS, Porto Alegre, Brazil Alois Fürstner, Max-Planck-Institut fur Kohlenforschung, Mülheim, Germany Frank Glorius, WWU Münster, Münster, Germany Lukas J. Gooßen, Ruhr-Universität Bochum, Bochum, Germany Steven P. Nolan, Ghent University, Ghent, Belgium Jun Okuda, RWTH Aachen University, Aachen, Germany Luis A. Oro, University of Zaragoza-CSIC, Zaragoza, Spain Michael Willis, University of Oxford, Oxford, UK Qi-Lin Zhou, Nankai University, Tianjin, China

Aims and Scope The series Topics in Organometallic Chemistry presents critical overviews of research results in organometallic chemistry. As our understanding of organometallic structure, properties and mechanisms increases, new ways are opened for the design of organometallic compounds and reactions tailored to the needs of such diverse areas as organic synthesis, medical research, biology and materials science. Thus the scope of coverage includes a broad range of topics of pure and applied organometallic chemistry, where new breakthroughs are being achieved that are of significance to a larger scientific audience. The individual volumes of Topics in Organometallic Chemistry are thematic. Review articles are generally invited by the volume editors. All chapters from Topics in Organometallic Chemistry are published Online First with an individual DOI. In references, Topics in Organometallic Chemistry is abbreviated as Top Organomet Chem and cited as a journal.

Bogdan Marciniec • Hieronim Maciejewski Editors

Perspectives of Hydrosilylation Reactions With contributions by T. Aneeja
G. Anilkumar
I. Dąbek
L. Deng
F. J. Fernández-Alvarez
M. Iglesias
M. Jankowska-Wajda
H. Maciejewski
B. Marciniec
K. Motokura
L. A. Oro
P. Pawluć
S. Shimada
P. X. T. Rinu
D. Wang
M. Zaranek

Editors Bogdan Marciniec Faculty of Chemistry and Center for Advanced Technologies Adam Mickiewicz University Poznań, Poland

Hieronim Maciejewski Faculty of Chemistry Adam Mickiewicz University Poznań, Poland

ISSN 1436-6002 ISSN 1616-8534 (electronic) Topics in Organometallic Chemistry ISBN 978-3-031-45959-7 ISBN 978-3-031-45960-3 (eBook) https://doi.org/10.1007/978-3-031-45960-3 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Paper in this product is recyclable.

Preface

Despite the fact that almost 8 decades have passed since the discovery by Sommer and co-workers of the Si-H addition reaction to octene, the interest in hydrosilylation processes is not waning. Hydrosilylation, due to the wide range of substrates used, creates virtually unlimited possibilities for the production of any material containing organosilicon components. Therefore, it is the most popular and widely used method for the synthesis of organosilicon compounds from small-scale laboratory syntheses to large-scale industrial processes. This method is used not only for the production of molecular compounds (organofunctional silanes or silsesquioxanes) and polymeric compounds (organofunctional silicones, cross-linking of silicones), but also for the production of hybrid and preceramic materials, for the modification of organic polymers and surface functionalization. Hydrosilylation also plays an important role in organic synthesis, among others as a selective method of reducing various functional groups (carbonyls, nitriles, imines). All these directions of application and the possibility of obtaining transformations with high selectivity and efficiency are possible thanks to the variety of catalysts. Noble metals such as platinum, rhodium, ruthenium, palladium, and iridium have been catalytic workhorses for decades and most catalytic systems rely on them. Although excellent activity and selectivity can be obtained, the high price of these metals, necessary of purification, and the unacceptable presence of metal residue in the final products are problems in the silicone industry. Therefore, more effective (allowing to reduce their concentration) and sustainable catalysts that can be recycled and used many times are sought. More than 30 years have passed since our team’s first “Comprehensive Handbook on Hydrosilylation” and during this period the scope of the hydrosilylation reaction had been extended to include a wide range of unsaturated substrates as well as new catalytic systems. In recent years, great strides have been made in hydrosilylation catalyzed by earth-abundant transition metals. However, noble metals have also been the subject of research and a number of new, more effective complexes (including pincer complexes) as well as many heterogeneous catalysts have been developed with their participation, which significantly improve the economics of the process. In v

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this book, we have attempted to review, analyze, and summarize the state of the art in the field of catalysis of hydrosilylation processes, focusing on the most recent literature. As editors, we are very grateful to all authors whose excellent contributions will hopefully make this book a useful reference and guide to modern hydrosilylation processes. We also wish to thank Springer, our publishing company and our publishing editor for expert help, understanding, and superb realization of the final text during the production process. Poznań, Poland Poznań, Poland August 2023

Bogdan Marciniec Hieronim Maciejewski

Contents

Silicometallics vs. Organometallics and Catalysis: General Guidelines ................................................................................................. Bogdan Marciniec

1

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts .......................................................................................................... Shigeru Shimada

13

State of the Art in Rhodium- and Iridium-Catalyzed Hydrosilylation Reactions .......................................................................................................... Manuel Iglesias, Francisco J. Fernández-Alvarez, and Luis A. Oro

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Recent Development of Cobalt-Catalyzed Hydrosilylation Reactions: New Catalysts, Mechanistic Understandings, and Future Trends . . . . . . 141 Dongyang Wang and Liang Deng Iron and Manganese Catalyzed Hydrosilylation Reactions . . . . . . . . . . . 225 Thaipparambil Aneeja, Pulluparambil Xavier Thresia Rinu, and Gopinathan Anilkumar Catalysis of Hydrosilylation Processes with the Participation of Ionic Liquids ............................................................................................................. 253 Hieronim Maciejewski, Magdalena Jankowska-Wajda, and Izabela Dąbek Hydrosilylation Catalysis for One-Pot Synthesis . . . . . . . . . . . . . . . . . . . 285 Ken Motokura Hydrosilylation of Carbon–Carbon Multiple Bonds in Organic Synthesis .......................................................................................................... 305 Maciej Zaranek and Piotr Pawluć

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Top Organomet Chem (2023) 72: 1–12 https://doi.org/10.1007/3418_2023_98 # The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 Published online: 12 October 2023

Silicometallics vs. Organometallics and Catalysis: General Guidelines Bogdan Marciniec

Contents 1 Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Abstract Great progress in the research on hydrosilylation processes, mainly those based on catalysis by TM complexes, achieved over the last 70 years, has inspired the writing of this book. It consists of 8 chapters and is concentrated not only on the most attractive publications and excellent reviews from the last decade, but above all on the presentation of perspectives of research in this area in the next few decades. The aim of Chapter “Silicometallics vs. Organometallics and Catalysis: General Guidelines” is to formulate a vision for such perspective development of this field, including new mechanistic aspects of TM-catalyzed hydrosilylation involving intermediates containing transition metal–silicon (TM–Si), i.e., silicometallics and TM– H bonds leading to selective synthesis of silicon-containing products—fine chemicals and precursors of materials. However, the guideline idea is to highlight a crucial role of TM–Si intermediates, using homogeneous and heterogeneous systems, in application of hydrosilylation reactions. The most industrially important processes for the synthesis of organosilicon molecular and polymeric compounds will lead to development of advanced technologies and, finally, to their transfer to innovative firms for production of new and hybrid materials of expected and unexpected properties. Keywords Alkenes · Alkynes · Catalysis · Hydrosilylation · Materials · Silicometallics · Transition metals

B. Marciniec (✉) Faculty of Chemistry and Center for Advanced Technologies, Adam Mickiewicz University, Poznań, Poland e-mail: [email protected]

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1 Contents The importance of organometallics in catalysis is a consequence of the reactivity of TM–C bonds, occurring in compounds which are active intermediates in crucial steps of important processes in organic synthesis (e.g., olefin oxidation, hydroformylation, carbonylation, hydrogenation, olefin metathesis) and the preparation of polymers (Ziegler–Natta polymerization, oligomerization of olefins). On the other hand, if p-block elements such as boron, silicon, germanium, tin, arsenic, antimony, and tellurium, known in the literature as metalloids (E), form at least one chemical bond with a metal atom or atoms (from a main group element, a TM, or a lanthanide or actinide) then the resulting species are referred to as inorganometallics. Such compounds are the subjects of a new field of study, called inorganometallic chemistry [1, 2]. The compounds containing TM–E bonds, although distinctly different from organometallics, also act as active intermediates, particularly in transformations of p-block compounds as those with silicon, boron, aluminum, and others. Advances in organometallic chemistry, including the results of intensive research on organometallic chemistry of boron, silicon, and other metalloid compounds, have clearly shown that the chemical properties of compounds containing a metal– metalloid bond, particularly when the metal is a TM, are significantly different from those having a metal–non-metal (O, N, P) bond. Therefore, it is reasonable to narrow the concept of reactivity of the metalloid derivatives as shown below (Scheme 1a) [2, 3]. From the catalytic point of view, all transformations involving the TM–E bond are important, but when it comes to the production of materials, the silicon derivatives are of the greatest importance, as all other derivatives are in most cases raw materials for the production of new organometalloid compounds (Scheme 1b) [3, 4]. The formation of the TM–Si bond is a key step in the catalytic processes of hydrosilylation, which is a well-documented area of silicon chemistry, and several excellent reviews and books give details also on the catalyst selection [5–14]. Our contribution to this field is the Comprehensive Handbook on Hydrosilylation edited in 1992, considered by many as the “Bible on hydrosilylation” [6] and “Hydrosilylation. A Comprehensive Review of Recent Advances” [7] edited in

Scheme 1 Concept of inorganometallic (1a) and silicometallic (1b) vs. organometallic chemistry

Silicometallics vs. Organometallics and Catalysis: General Guidelines

Si

C

Si

CH C

C Si

N

3

C

CH

C C

N Si

CH

C

C

O Si

H

O

CH

N O

C N N N Si

N

CH

Si Si

N

O

NH

NH

Scheme 2 Hydrosilylation of multiple bonds of carbon–carbon, carbon–heteroatom (oxygen, nitrogen), and heteroatom–heteroatom

2008/2009 which acquired its reputation as the “New Testament.” In this chapter, only the novelties on mechanistic aspects published in the last 12 years for catalysis of hydrosilylation by TM are discussed. Hydrosilylation (or hydrosilation) is a term describing an addition reaction of organic and inorganic silicon hydrides to multiple bonds mainly of carbon–carbon, carbon–oxygen, carbon–nitrogen and nitrogen–nitrogen, according to Scheme 2. Although the reaction can occur by a mechanism involving free radicals generated in the reaction mixture, the catalysts based on the transition metal (TM) complexes enable the process through a heterolytic mechanism. A general mechanism presented in Scheme 3 for hydrosilylation to alkenes catalyzed by late TM complexes was disclosed in 1965 by Chalk and Harrod for hydrosilylation catalyzed by Pt complex [5]. Intermediates with metal–silicon bond (i.e., silicometallics) play a decisive role in mechanistic implications of all catalytic reactions of silicon-containing substrates [3, 4]. The mechanism includes a conventional oxidative addition of Si–H to a metal alkene complex configuration (usually d8-d10), followed by insertion of alkene into the TM–H bond (then the resulting metal–silyl alkyl complex undergoes reductive elimination by Si–C bond formation), and regeneration of the TM catalyst. However, also a modified version of the mechanism has been proposed, which involves the alkene insertion into the metal–inorganometallic bond followed by reductive elimination. It is known that hydrosilylation process is commonly used in the production of silane coupling agents, in the cross-linking of silicones, and in the production of hybrid and pre-ceramic materials. It is also used in the synthesis of nanoparticles

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Scheme 3 General mechanism of the hydrosilylation of alkenes by TM complexes

3 Scheme 4 Hydrosilane binding mode as a function of electron density of the metal site

with a strictly defined spatial structure (silsesquioxanes and dendrimers) and in the production of nanocomposites containing nanoparticles [15]. The methods for synthesizing polysilanes (crucial raw materials for the production of sensors, LEDs, photovoltaic devices, and nonlinear optics) using new catalytic systems have been also continuously developing. The low electronegativity and relatively large size allow silicon to develop a variety of bonds exhibiting secondary interactions, for example, with hydrides. The polarization of the Si–H unit in hydrosilanes combined with the steric effect of H makes the Si–H unit a good electron donor for TM complexes. A variety of structures can formally be dictated by the electron richness of the metal. For an electron-poor acceptor, the interaction with hydrosilane occurs via η1-coordination (see Scheme 4) [16]. Most TM compounds with hydrosilanes involve some M–Si interactions (π-back bonding into σ*SiH) whose ultimate aim is to bring about Si–H oxidative addition. However, for an oxidative addition process to occur, a coordinatively unsaturated metal precursor is required, which can be obtained via a reductive elimination such as H2 and HX. When, especially for early TMs, oxidative addition is not favored,

Silicometallics vs. Organometallics and Catalysis: General Guidelines

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Scheme 5 Simplified comparison of an oxidative addition reaction pathway and σ-bond metathesis pathway

Scheme 6 General mechanism for the dehydrogenative silylation of styrene catalyzed by Fe and Co triad

another reaction pathway can be followed: σ-bond metathesis may provide a low-energy step to a TM–silyl complex (Scheme 5) [17]. In contrast to the well-established platinum-catalyzed hydrosilylation, the iron and cobalt triad complexes particularly cationic ones and also nickel complexes appeared to be attractive catalysts for the dehydrogenative silylation. A general scheme for dehydrogenative silylation of styrene catalyzed by Fe and Co triad is given in Scheme 6, but an analogous mechanism for nickel equivalent of Karstedt catalyst via insertion of olefin into Ni–Si bond has been proposed by us earlier [1]. Great progress in the research on hydrosilylation processes accomplished over the last 70 years has been summarized by excellent specialized reviews, while this book

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consists of 8 chapters concentrated on the most attractive publications of the last decade, concerning catalysis of hydrosilylation by TM complexes. However, a vision of perspective development of this area, including fundamental research but leading simultaneously to advanced technologies for production of fine chemicals and a new generation of materials, is formulated in this chapter. All aspects of homogeneous as well as heterogeneous catalysis of hydrosilylation by TM complexes have been described in the next 7 chapters of this review monograph, starting from group 10 transition metal complexes, mainly Pt but also Pd and Ni (Chapter “Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts”). Although homogeneous platinum complexes, mainly Karstedt’s and Speier’s catalysts, have been still predominantly used in hydrosilylation reactions in industry as well as academia, a wide variety of new polysiloxanes and (poly)carbosilanes hybrid and nano-materials based on Pt-catalyzed hydrosilylation of carbon–carbon multiple bonds have been proposed since 2015 and are discussed in Chapter “Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts”. Mechanistic studies of well-known Pt complexes have permitted discovery of many side reactions and have been aimed at stabilization of Pt complexes (also immobilized) as well as reduction of Pt consumption during the reaction. However, simultaneously, in the last decade a significant advancement has been made in the research of non-precious hydrosilylation catalysts of 10th group (mainly nickel). However, we will observe the actual practical application of Ni-catalyzed processes from the perspective of the next few decades. Novel and more efficient rhodium- and iridium-catalyzed hydrosilylation processes are presented in Chapter “State of the Art in Rhodium and Iridium Catalyzed Hydrosilylation”, focused on review of results published since 2009 on the use of Rh and Ir complexes in hydrosilylation of carbon–carbon as well as carbon–heteroatom (O, N) and heteroatom–heteroatom bonds. A survey of novel Rh and Ir complexes applied as hydrosilylation catalysts includes a discussion of mechanistic problems. The authors concentrate on strategies for the design of ligand systems, taking into account the role of given TM complexes in selectivities (especially enantioselectivities), heterogenization, and metal–ligands cooperation. The conclusion is that there are real perspectives for successful development of more efficient catalytic systems involving Rh and Ir to be used in chemical industry, especially in production of fine chemicals, biochemicals, and precursors of new materials. On the other hand, cobalt complexes as new non-precious metal catalysts of hydrosilylation of alkenes and alkynes have been comprehensively surveyed in view of the synthesis of organosilicon products (Chapter “Recent Development of CobaltCatalyzed Hydrosilylation Reactions: New Catalysts, Mechanistic Understandings and Future Trends”). Well-defined cobalt complexes bearing structurally diverse nitrogen-based ligands, phosphorous, and isocyanate have appeared as new catalytic systems in hydrosilylation of alkenes and alkynes with different chemo-regio- and stereo-selectivity as well as enantioselectivity. The diversified ligands used in cobaltcatalyzed hydrosilylation may lead to different reactivities as well as different reactions involving different mechanisms. Thus, straightforward comprehensive mechanistic studies, including identification of intermediates and with precise

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Scheme 7 TM complexes immobilized into phosphinated polymer (a) or siloxyl ligand (b)

control of products selectivity, may bring application of cobalt-catalyzed hydrosilylation of alkenes and alkynes in synthesis of fine chemicals and precursors of new nano-hybrid materials of expected or unexpected properties. Recent advances in the chemistry of less expensive transition metals, like Fe and manganese as hydrosilylation catalysts, are summarized in Chapter “Iron and Manganese Catalyzed Hydrosilylation Reactions”. Iron as the most abundant TM with high catalytic activity and low toxicity as well as excellent tolerance of functional group forms complexes which have been effective catalysts for hydrosilylation (also asymmetric) of ketones and aldehydes but have also been explored in the reactions of alkenes, alkynes, and other carbonyl compounds. Similarly to iron complexes, recently increasing interest in manganese complexcatalyzed hydrosilylation permitted obtaining aldehydes and ketones, although new examples of manganese complex-catalyzed alkenes and alkynes have also been published. A summary of the attempts at replacement of expensive platinum by catalytically active and cheaper derivatives of Fe, Co, Ni, and others as well as novel applications of potent hydrosilylation and dehydrogenative silylation products in the synthesis of fine chemicals and precursors of new materials has been presented in a separate review [1]. It is a collection of papers published over the last decade by the research groups of Chirik [14, 18], Naganawa [19], Huang [9], Wiesbrock [12], Beller [8], Lu [13], Li [20], Findlater [10, 21], Nagashima [22], Gade [23], and others. However, the most promising perspective direction for industrial application of hydrosilylation processes is related to development of a new generation of heterogenized TM complex catalysts which combine the advantage of heterogeneous catalysis (easy catalyst recovery) with high activity and selectivity of homogeneous complexes. Transition metal complexes can be immobilized either on organic polymers (mostly through cross-linking) such as polystyrene and polyvinyl chloride or on an inorganic support such as silica, glass, and molecular sieves. The metal complexes can be attached to the support via functional groups which act as ligands (e.g., phosphines (Scheme 7), amines, acac) followed by anchoring the precursor onto each heterogenized ligand [2]. Most recent reviews, also mentioned in our chapters (except Chapter “Recent Development of Cobalt-Catalyzed Hydrosilylation Reactions: New Catalysts, Mechanistic Understandings and Future Trends”), emphasize the search for recyclable heterogeneous catalysts, mostly employing immobilization of TM complexes on the appropriate polymers or inorganic supports. The mechanisms of catalysis by supported metal complexes are still a subject of intensive study [1]. For instance, a

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Scheme 8 Mechanism of heterogeneous catalysis of alkene hydrosilylation by surface rhodium (diene)siloxide complex

well-defined rhodium–siloxane complex immobilized by direct reaction of molecular siloxide precursor with Aerosil [24] has been characterized by the solid-state NMR study and identified as a product of oxidative addition of PhSiMe2 leading to the surface siloxide complex 1 → 2 (Scheme 8). The lack of the disiloxane elimination (recorded by GC-MS) is also evidence for such a key intermediate in the heterogeneous system. The coordination of alkene to surface-siloxide (2 → 3) Rh complex is followed by its insertion into Rh–H bond (3 → 4) with a final elimination of the product and regeneration of stable surface complex 1. The interaction of the silanol group in 1 is responsible for high stability of such a single-site rhodium catalyst, which can be recycled at least 10–20 times with no decline in the high yield and selectivity. Also metals supported on inorganic materials or carbon can be used as effective hydrosilylation catalysts. A new concept of such heterogeneous catalysts has been

Silicometallics vs. Organometallics and Catalysis: General Guidelines

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Scheme 9 One-pot synthesis with hydrosilylation catalysis: (a) double hydrofunctionalization of alkynes (b) synthesis of organic molecules through sequential hydrosilylation/cross-coupling with silyl as a leaving group, (c) synthesis of organosilicon compounds with silicon as an intersection, and (d) sequential hydrosilylation/oxidation of alkenes

introduced by the Beller group [25]. They are simple atom catalysts (SACs) which contain isolated metal centers stabilized by the neighboring salts [25]. One of the main directions of the search for heterogeneous catalysts of high catalytic activity and, simultaneously, easy product isolation seems to be the employment of ionic ligands as agents for immobilization of metal complexes. The studies of mechanistic aspects of this new attractive synthetic method, including recent progress in the employment of ionic ligands as agents for the immobilization of metal complexes (Chapter “Catalysis of Hydrosilylation Processes with the Participation of Ionic Liquids”), have provided convincing evidence for integration of homo- and heterogeneous catalysis, realized in several variants, leading to more cost-effective hydrosilylation processes. Catalytic hydrosilylation processes, which have been intensively developed in recent years, have revolutionized modern synthetic organic chemistry, ensuring straightforward access to a wide variety of organosilicon compounds. Two chapters are devoted to the progress in this field. Chapter “Hydrosilylation Catalysis for Onepot Synthesis” describes recent advances in one-pot syntheses triggered by hydrosilylation. Motokura has succinctly described the traditional sequences, including cross-coupling and oxidation of silyl intermediates, focusing more attention on the use of CO2 hydrosilylation in the synthesis of formyl esters and aldehydes. The chapter is completed by one-pot synthesis with silicon as an intersection, describing reactions involving dihydrosilanes in the synthesis of organosilicon compounds (e.g., oligosiloxanes and siloles). The simple functionalizations (cycloaddition and dipolar cycloaddition) of the reaction intermediates obtained by hydrosilylation are also briefly presented (Scheme 9). Zaranek and Pawluc, in Chapter “Hydrosilylation of Carbon: Carbon Multiple Bonds in Organic Synthesis” have provided a concise overview of the sequential reactions involving hydrosilylation of functional alkenes and alkynes as the initial step, followed by desilylation (oxidation, cross-coupling), double

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Scheme 10 General scheme for the synthesis and advanced technologies of production of fine chemicals and precursors of materials

hydrofunctionalization of alkynes, and asymmetric hydrosilylation of prochiral alkenes. The applications of new catalysts based on the first-row transition metal complexes in consecutive (also tandem) hydrosilylation/desilylation reactions have been highlighted. Special attention has been paid to enantioselective hydrosilylation of olefins, which has been experiencing great progress in recent years as an attractive method for the synthesis of chiral silanes, alcohols, and their derivatives. All literature data from the last few decades show that the processes of silicon substrates catalyzed by transition metal complexes occur via a mechanism involving intermediates containing a metal–silicon bond (i.e., silicometallics) and a metal– hydrogen bond, occasionally accompanied by intermediates containing a metal– carbon bond [2]. However, since in catalytic conversions of silicon compounds only hydrosilylation reactions are well known as processes of industrial importance [4], the perspective of catalytic hydrosilylation in the next few decades should be based on the synthesis of molecular and polymeric fine chemicals via hydrosilylation, as well as precise mechanistic studies which have confirmed a crucial role of TM–Si intermediates using predominantly heterogeneous systems based on immobilized complexes. On the other hand, detailed recognition of applications of hydrosilylation reactions leading to sustainable products and materials precursors will substantiate the general idea of the role of catalysis via silicometallics (Scheme 10).

Silicometallics vs. Organometallics and Catalysis: General Guidelines

11

2 Conclusions This model study “from invention to innovation” is a very good example of knowledge-based economy, first formulated in 1987 as the Brundtland Report containing “sustainable development”. The role of chemistry as “central science” introduced in 2010 can be greatly illustrated just by developing original catalytic methods for syntheses of chemicals (e.g., silicon-containing fine chemicals) as well as a new generation of nanomaterials and their precursors, designed in cooperation between chemists and physicochemists. This research activity should be followed by the development of advanced technologies for the production of fine chemicals and precursors of materials to be used in optoelectronics, ceramics, medicine, and other fields of high-tech industry.

References 1. Marciniec B, Pietraszuk C, Pawluć P, Maciejewski H (2022) Inorganometallics (transition metal-metalloid complexes) and catalysis. Chem Rev 122:3996–4090 2. Marciniec B, Pawluć P, Pietraszuk C (2007) Inorganometallic chemistry. In: Bertini I (ed) Encyclopedia of life support systems (EOLSS). Eolss Publ., Co. Ltd. www.eolss.net 3. Marciniec B, Pietraszuk C, Kownacki I, Zaidlewicz M (2005) Vinyl- and arylsilicon, germanium, and boron compounds. In: Katritzky AR, Taylor RJK (eds) Comprehensive organic functional group transformations II, vol 1. Elsevier, Amsterdam, pp 941–1024 4. Marciniec B (2000) Silicometallics and catalysis. Appl Organomet Chem 14:527–538 5. Chalk AJ, Harrod JF (1965) Homogeneous catalysis. II. The mechanism of the hydrosilation of olefins catalyzed by group VIII metal complexes. J Am Chem Soc 87:16–21 6. Marciniec B, Gulinski J, Urbaniak W, Kornetka ZW (1992) Marciniec B (ed) Comprehensive handbook on hydrosilylation. Pergamon Press, Oxford 7. Marciniec B, Maciejewski H, Pietraszuk C, Pawluć P (2009) Marciniec B (ed) Hydrosilylation. A comprehensive review on recent advances. Springer, Berlin 8. de Almeida LD, Wang H, Junge K, Cui X, Beller M (2021) Recent advances in catalytic hydrosilylations: developments beyond traditional platin catalysts. Angew Chem Int Ed 60: 550–565 9. Du X, Huang Z (2017) Advances in base-metal-catalyzed alkene hydrosilylation. ACS Catal 7: 1227–1243 10. Tamang SR, Findlater M (2019) Emergence and applications of base metals (Fe, Co, and Ni) in hydroboration and hydrosilylation. Molecules 24:3194 11. Sun J, Deng L (2016) Cobalt complex-catalyzed hydrosilylation of alkenes and alkynes. ACS Catal 6:290–300 12. Hofmann RJ, Vlatkovi’c M, Wiesbrock F (2017) Fifty years of hydrosilylation in polymer science: a review of current trends of low-cost transition-metal and metal-free catalysts, non-thermally triggered hydrosilylation reactions, and industrial applications. Polymers 9:534 13. Chen J, Guo J, Lu Z (2018) Recent advances in hydrometallation of alkenes and alkynes via the first row transition metal catalysis. Chin J Chem 36:1075–1109 14. Obligacion JV, Chirik PJ (2018) Earth-abundant transition metal catalysts for alkene hydrosilylation and hydroboration. Nat Rev Chem 2:15–34 15. Lee VY (ed) (2017) Organosilicon compounds, vol 2. Academic Press, London 16. Whited MT, Taylor BLH (2020) Metal/organosilicon complexes: structure, reactivity, and considerations for catalysis. Comment Inorg Chem 40:217–276

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17. Corey JY (2016) Reactions of hydrosilanes with transition metal complexes. Chem Rev 116: 11291–11435 18. Pappas I, Treacy S, Chirik PJ (2016) Alkene hydrosilylation using tertiary silanes with α-diimine nickel catalysts. Redox-active ligands promote a distinct mechanistic pathway from platinum catalysts. ACS Catal 6:4105–4109 19. Naganawa Y, Inomata K, Sato K, Nakajima Y (2020) Hydro-silylation reactions of functionalized alkenes. Tetrahedron Lett 61:151513 20. Zhou H, Sun H, Zhang S, Li X (2015) Synthesis and reactivity of alkydrido CNC pincer cobalt III complex and its application in hydrosilylation of aldehydes and ketones. Organometallics 34: 1479–1486 21. Smith AD, Saini A, Singer LM, Phadke N, Findlater M (2016) Synthesis, characterization and reactivity of iron- and cobalt-pincer complexes. Polyhedron 114:286–291 22. Sunada Y, Tsutsumi H, Shigeta K, Yoshida R, Hashimotoa T, Nagashima H (2013) Catalyst design for iron-promoted reductions: an iron disilyl-dicarbonyl complex bearing weakly coordinating η2-(H–Si) moieties. Dalton Trans 42:16687–16692 23. Sauer DC, Wadepohl H, Gade LH (2012) Cobalt alkyl complexes of a new family of chiral 1,3-Bis(2-pyridylimino)-isoindolates and their application in asymmetric hydrosilylation. Inorg Chem 51:12948–12958 24. Marciniec B, Szubert K, Potrzebowski MJ, Kownacki I, Łęszczak K (2008) Synthesis, characterization and catalytic activity of the well-defined rhodium siloxide complex immobilized on silica. Angew Chem Int Ed 47:541–544 25. Cui X, Junge KK, Dai X, Kreyenschulte C, Pohl MM, Wohlrabe S, Shi F, Brucner A, Beller M (2017) Synthesis of single atom based heterogeneous platinum catalysts: high selectivity and activity for hydrosilylation reactions. ACS Cent Sci 3:580–585

Top Organomet Chem (2023) 72: 13–94 https://doi.org/10.1007/3418_2023_99 # Copy the VE in proof email 2023 Published online: 31 October 2023

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts Shigeru Shimada

Contents 1 Hydrosilylation of Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Platinum Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Palladium Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Nickel Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Hydrosilylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Platinum Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Palladium and Nickel Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Hydrosilylation of Dienes, Allenes, Enynes, and Diynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Hydrosilylation of Carbon-Heteroatom Multiple Bonds and Epoxides . . . . . . . . . . . . . . . . . . . . . 4.1 Hydrosilylation of Aldehydes, Ketones, Esters, Amides, Imines, and Epoxides . . . . . 4.2 Hydrosilylation of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

16 16 31 33 41 42 49 53 62 63 68 70 72

Abstract This chapter summarizes recent advances of group 10 transition metal hydrosilylation catalysts from 2015 onward. Since the discovery of Speier’s catalyst in 1957, homogeneous Pt catalysts have been mainly used for the hydrosilylation reaction of C-C multiple bonds. Although research on homogeneous Pt hydrosilylation catalysts has still been intensive, recent trends of research that meet the demands for sustainable development significantly increased the research on heterogeneous and non-precious metal hydrosilylation catalysts including Ni catalysts. Although Pd catalysts are very popular in organic transformation, its usefulness in hydrosilylation reaction is relatively limited, except for several special cases such as asymmetric hydrosilylation.

S. Shimada (✉) National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan e-mail: [email protected]

14

S. Shimada

Keywords Catalysis · Heterogeneous catalysts · Homogeneous catalysts · Hydrosilylation · Nickel · Palladium · Platinum · Silicone

Abbreviations acac AGE Bn CNT cod COF dba DCM DME DMS dppe dvtms EDTA HMTS IMes MeCp MOF MTBE NHC NMP NP PDMS PEG PMDS PMHS SIPr TMDS TOF TON

Acetylacetonato Allyl glycidyl ether Benzyl Carbon nanotube 1,4-Cyclooctadiene Covalent organic framework Dibenzylideneacetone Dichloromethane 1,2-Dimethoxyethane Dimethyl sulfide 1,2-Bis(diphenylphosphino)ethane 1,3-Divinyl-1,1,3,3-tetramethyldisiloxane Ethylenediaminetetraacetic acid 1,1,1,3,5,5,5-Heptamethyltrisiloxane 1,3-Bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene Methylcyclopentadienyl Metal organic framework Methyl tert-butyl ether N-heterocyclic carbene N-methyl-2-pyrrolidinone Nanoparticle Polydimethylsiloxane Poly(ethylene glycol) 1,1,1,3,3-Pentamethyldisiloxane Polymethylhydrosiloxane 1,3-Bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene 1,1,3,3-Tetramethyldisiloxane Turn over frequency Turn over number

Homogeneous Pt catalysts have been used as the standards in the hydrosilylation reaction since the discovery of Speier’s catalyst (H2PtCl6•6H2O in iPrOH) [1] in 1957 and the subsequent development of Karstedt’s catalyst (Pt2(dvtms)3) [2] in 1973. Karstedt’s catalyst is currently most widely used in industry as well as in academia. In industry, Pt catalysts are used for the production of organosilicon compounds including organofunctional silanes (such as γ-functionalized propylsilanes) and functionalized siloxanes, silicone polymers, and copolymers of

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

15

polysiloxanes and organic polymers. These homogeneous Pt catalysts are also used in the curing of silicone compositions containing multi-vinyl and multi-Si-H polysiloxanes for the production of silicone elastomers and resins. The Pt catalysts can be recovered in the production of small molecule organosilicon compounds, while they permanently remain in the cured silicone products, resulting in the consumption of large amounts of Pt in silicone industry (5.6 t in 2007, which is about 3% of annual worldwide Pt production) [3]. Particularly, the production of release coating materials requires high speed curing and uses high concentration of Pt catalysts (up to 100–200 ppm), which composes the main Pt consumption in silicone industry. In addition to the large Pt consumption, another important drawback in the current Pt catalysts is the selectivity of the reaction. Hydrosilylation of alkenes and alkynes will form regio- and/or stereoisomers. Karstedt’s and Speier’s catalysts often produce a mixture of isomeric products depending on the substrates. In addition, side reactions, such as alkene isomerization, oligomerization, and hydrogenation, dehydrogenative silylation, and dehydrocoupling and redistribution of hydrosilanes, often happen. It is known that Karstedt’s catalyst decomposes during catalysis and forms Pt colloids, which catalyze the side reactions. Therefore, one of the directions of the research to solve the selectivity problem is to develop Pt catalysts with higher stability. Indeed, Markó and co-workers reported that NHC-Pt(dvtms) catalysts, which are stabilized by highly electron-donating NHC ligands and do not form Pt colloids during the catalysis, showed higher selectivity than Karstedt’s catalyst [4]. To reduce the Pt consumption, the development of Pt catalysts with higher activity and stability or non-Pt catalysts that can replace Pt catalysts is necessary. For the usage that requires high reaction rate such as release coating production, catalysts with higher activity (higher TOF) are required to reduce the catalyst loading. On the other hand, in the processes that do not require very high curing speed (e.g., silicone rubber curing), catalysts with higher stability (higher TON) with appropriate activity are required. Improvement of catalytic activity and stability of Karstedt’s catalyst has been continuously investigated to date. During the last decade, significant development was made in the field of non-precious metal hydrosilylation catalysts including nickel, though Pt catalysts (mainly Karstedt’s type catalyst) are still used in most practical applications. From the viewpoint of practical application in silicone curing, another important aspect is the switching of the catalysis. Under the storage conditions (room temperature or below under dark) of silicone curing compositions, the catalysis should be stopped to achieve long storage periods, while the catalysis should be switched on when it is necessary by a trigger such as heat or photo-irradiation. Since review articles including group 10 transition metal hydrosilylation catalysts have been continuously published [5–22], this chapter mainly summarizes the developments published in 2015 or later.

16

S. Shimada

1 Hydrosilylation of Alkenes 1.1

Platinum Catalysts

1.1.1

Homogeneous Platinum Catalysts

Karstedt’s catalyst is the most widely used hydrosilylation catalysts. Although huge numbers of new catalysts/catalyst systems have been developed, practical applications of hydrosilylation reactions still largely depend on this catalyst. Recent detailed mechanistic study on the hydrosilylation reaction with Karstedt’s catalyst further supported the Chalk–Harrod mechanism [23] and added some new insights [24]. Figure 1 shows the proposed mechanism including by-product formation (isomerization of alkenes) steps, based on the detailed experimental study including 2H-labeling experiments, 195Pt NMR, detection of Pt-H species B, and in-depth kinetic study. Oxidative addition of R3Si-H to alkene-coordinated Pt species A forms (alkene)Pt (H)SiR3 species B (this step is reversible). Migratory insertion of the alkene into the Pt-H bond forms alkyl(silyl)Pt species C, and subsequent reductive elimination and alkene coordination provide the hydrosilylation product and regenerate A. Although the reductive elimination step III was believed to be the rate-determining step, the new study concluded that the migratory insertion step II is rate-determining and not reversible. Side reaction, alkene isomerization, takes place through the intermediates D and E. The above mechanistic study also clarified that higher concentration of Pt (≥250 ppm relative to the alkene) causes a rapid Pt colloid formation and decreases catalytic performance. A comprehensive study of the influence of experimental parameters on side reactions in the hydrosilylation of allyl-terminated polyethers

’R

R’ R’

’R

VI H

SiR3

[Pt]

SiR3

[Pt] A R’

III

I

V R’

C

[Pt] II

E

R’

R’ [Pt] SiR3

SiR3 H

SiR3 H B

R’ IV [Pt] SiR3 D

rds

Fig. 1 Proposed reaction mechanism of the Pt-catalyzed hydrosilylation reaction [24]

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts Fig. 2 Structures of bulky NHC ligands IPr* and IPr*OMe

Ph

17

Ph N

R

Ph N

R

Ph Ph Ph

Ph

Ph

IPr*: R = Me IPr*OMe: R = OMe

with (EtO)3SiH by Karstedt’s catalyst was undertaken by applying a fractional factorial design of experiments [25]. Side reactions can be reduced considerably by using (MeOCH2CH2O)3SiH instead of (EtO)3SiH in the hydrosilylation reaction of allyl-terminated PEG with Karstedt’s catalyst [26]. In situ monitoring method by IR and Raman spectroscopies was described for the hydrosilylation of allylterminated PEG with PMHS by Karstedt’s catalyst [27]. Hydrosilylation of a wide variety of functionalized alkenes with (EtO)3SiH was studied using Karstedt’s catalyst [28]. Usefulness of a microreactor system in combination with real-time IR monitoring was demonstrated in the hydrosilylation reaction of 1-octene, 3-allyloxy-1,2-propanediol, and AGE with HMTS catalyzed by Karstedt’s catalyst [29]. Karstedt’s catalyst was used for the hydrosilylation of allylgermanes with tertiary hydrosilanes [30]. Markó and co-workers reported NHC-Pt0-dvtms complexes as excellent hydrosilylation catalysts [4, 31]. As compared with Karstedt’s catalyst, NHC-Pt complexes are slightly less active, but show higher regio- and chemoselectivities and significantly improved stability avoiding Pt colloids formation, which causes side reactions. Steric bulkiness of NHC ligands significantly affects the hydrosilylation reaction. NHC-Pt0-dvtms complexes bearing very bulky aryl ligands (IPr* and IPr*OMe, Fig. 2) exclusively afforded β-adducts in excellent yields in the hydrosilylation of terminal alkenes with tertiary hydrosilanes and showed very high stability (up to 107 TON) [32]. The same complex was also used for the hydrosilylation of unconjugated dienes, conjugated and unconjugated enynes, and unconjugated diynes, showing high regio- and chemoselectivity [33]. A series of NHC-PtII(L)Cl2 (L = dimethyl sulfide (DMS), tetrahydrothiophene (THT), and pyridine) were synthesized and tested for the hydrosilylation of 1-octene with HMTS [34]. Among many NHC-PtII(L)Cl2 complexes, IPr*-PtII(L)Cl2 (L = DMS, THT, but not pyridine) showed exceptionally high catalytic activity (up to 970,000 TON, 4 × 104 h-1 TOF) and exclusive β-selectivity. The catalyst was also active for the hydrosilylation of various combinations of terminal alkenes with tertiary hydrosilanes. User-friendly green synthetic procedures of NHC-Pt complexes, including those with very bulky NHC ligands, using a weak base (K2CO3) under open-air conditions were reported [35, 36]. Recently, continuous flow reaction systems attract considerable interest for fine chemicals synthesis [37–39]. IPr*-Pt(DMS)Cl2 was used in a continuous flow

18

S. Shimada R Si

N Pt

Me3Si

O

N

N

N

(CH2CH2O)45CH3

Si

Fig. 3 Structures of new NHC-Pt complexes (left) and a PEG-substituted NHC ligand (right) Ar N

Si Si:

Pt

O

tBu

N P Me2Si N

Me3Si

SiMe3 Si:

N

Ph

Si

Ph

tBu Me3Si

Si

Si:

O

Pt

SiMe3

Si

Ad

Si

Pt

O Si

Me3Si

SiMe3

Fig. 4 Structures of silylene-Pt complexes

process for hydrosilylation of 1-octene with HMTS under neat conditions [40]. The homogeneous IPr*-Pt catalyst was efficiently separated from the product using a commercially available membrane (bulky IPr*-Pt molecules did not go through the membrane, while the hydrosilylation product passed it) and reused in the flow process. NHC-Pt complexes bearing pyridine-fused NHC (imidazo[1,5-a]pyridine-3ylidenes) (Fig. 3, left; R = 4-CNC6H4 or 4-CF3C6H4) showed more than 10 times higher TOF as compared with Markó’s original catalyst in the hydrosilylation of 1-octene with HMTS under air, although these catalysts are photo-sensitive and decompose in solution by light exposure [41]. Pt(II) complex bearing PEG-substituted NHC ligands (Fig. 3, right) showed good catalytic activity and βselectivity for the hydrosilylation of terminal alkenes with tertiary hydrosilanes under air [42]. The catalyst can be reused 27 times by centrifugation without loss of catalytic performance. The success of Markó’s NHC-Pt catalysts in the hydrosilylation reaction prompted us to investigate Pt complexes bearing heavier congeners of carbenes, silylenes, and germylenes. Kato’s [43] and Iwamoto’s [44, 45] groups reported cyclic silylene-Pt(dvtms) complexes as catalysts for the alkene hydrosilylation (Fig. 4). Although the NHC-Pt(dvtms) complexes generally show lower catalytic activity with higher selectivity in comparison with Karstedt’s catalyst, these silylenePt(dvtms) complexes exhibit high catalytic activity similar to Karstedt’s catalyst

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts Fig. 5 Structures of germylene ligands

19

R1 N Ge: N

R1, R2 = tBu R1= tBu, R2 = 2-pyridyl R1, R2 = 2-pyridyl

R2

Table 1 Pt1@[15]crown-5-catalyzed hydrosilylation of terminal alkenes

Alkene 1a

Silane 2a

1b

2b 2c 2d 2a 2c 2c 2c

1c 1d a

Cat (mol%) 5 × 10-5 5 × 10-6 5 × 10–7 a 5 × 10-3 5 × 10-4 5 × 10-3 5 × 10-5 5 × 10-3 5 × 10-3 5 × 10-3

Yield of β-isomer (%) 89 69 93 83 >99 89 88 85 85 80

β-Selectivity (%) 99 95 98 92 >99 93 >99 85 85 80

TOF (h-1) 1.1 × 108 8.3 × 108 3.7 × 108 1.0 × 106 1.2 × 107 1.0 × 106 1.0 × 108 1.0 × 106 1.0 × 106 1.0 × 106

Reaction time, 30 min

with higher stability and selectivity in the hydrosilylation of terminal alkenes with HMTS. In the patent by Wacker Chemie, effects of germylene ligands were described. The hydrosilylation reaction of 1-octene with HMTS by Karstedt’s catalyst with germylene ligands (Fig. 5) showed much higher selectivity (up to 97% vs 79% without germylene ligands at 100°C). Addition of NHC ligands significantly slowed down the reaction, while the reaction with the germylene ligands was rapid with high selectivity [46]. Pt1@PDMS-PEG (mononuclear Pt species stabilized by PDMS-PEG, prepared by the reduction of H2PtCl6 in PDMS-PEG EtOH solution) was reported to show remarkably high TOF (~ 108 h-1, 2 orders higher than that of Karstedt’s catalyst) and TON (5 × 108) in the hydrosilylation of 1-octene with HMTS. The catalyst can be reused at least 3 times without loss of activity and selectivity [47]. The same authors prepared Pt1@[15]crown-5 by the similar procedure using [15]crown-5 instead of PDMS-PEG [48]. Pt1@[15]crown-5 also showed remarkably high TOF (up to 8.3 × 108 h-1) in the hydrosilylation of 1-octene with PhMe2SiH with 95% β-selectivity and TON (1 × 109) in the hydrosilylation of 1-octene with HMTS (Table 1). The catalyst was tested for the various combinations of alkenes and

20 Fig. 6 Pt complex bearing 2,2-dimethyl-4pyridylpentane-3,5-dione ligands

S. Shimada 2+

tBu

– N Pt (NO3 )2

O HO

4

Fig. 7 Pt complex bearing an amphiphilic N,C,Npincer ligand

(CH2)11CH3

CH3(OCH2CH2)3

O

N Pt Cl

CH3(OCH2CH2)3

O

N

(CH2)11CH3

tertiary hydrosilanes and showed excellent performance. For example, styrene reacted with PhMe2SiH exclusively giving β-adduct with extra-high TOF (1 × 108 h-1). A simple and convenient recyclable Pt(0) catalyst system can be prepared by simply mixing K2PtCl4 in ethylene glycol. In the reaction of 1-octene with HMTS, the catalyst solution forms a separate phase, and the product can be separated by decantation. The catalyst system was reused 36 times with ≥95% product yields [49]. Cationic Pt complexes bearing 4-pyridyl or 3-pyridyl-pentanedione ligands (Fig. 6) showed high catalytic activity comparable to Karstedt’s catalyst and exclusive β-selectivity for terminal alkenes including styrenes with tertiary hydrosilanes [50]. It was proposed that the reaction follows Chalk–Harrod mechanism with Pt(II)/ Pt(IV) intermediates. Amphiphilic N,C,N-pincer platinum complex having hydrophilic tri(ethylene glycol) and hydrophobic dodecyl chains (Fig. 7) efficiently catalyzed the hydrosilylation reaction of various terminal alkenes with PhMe2SiH in water with 1 mol ppm Pt [51]. Cyclopropenium-derived triplatinum complex (Fig. 8, left) showed high catalytic activity comparable to Karstedt’s catalyst for the hydrosilylation of terminal alkenes. The complex was also active for the reaction of internal cyclic alkenes with Me (MeO)2SiH [52]. LPt(cod) bearing a bidentate ligand (L = pinacolato, catecholato, salicylato, and related bidentate O,N, N,N, and O,S ligands) were reported to show better catalytic performance than Karstedt’s catalyst [53]. Very recently, cyclobutadiene Pt

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts Fig. 8 Cyclopropeniumderived triplatinum complex (left) and cyclobutadiene Pt (II) complex (right)

21

NCMe

MeCN

Pt Et

Ph

Et

Ph Ph Pt MeCN

Ph

Et Cl

Pt

Pt

Et Cl

Ph

MeCN Ph

R

N3 + N3

R3SiH

Karstedt’s cat (0.25 mol%) toluene 70 ºC

N3 R

SiR3

R

Scheme 1 Selective hydrosilylation of allylic azides

(II) complex (Fig. 8, right) was reported and used for the hydrosilylation of alkenes and alkynes [54]. Bidentate bis-phosphonite ligated LPtCl2 [55] and Rh, Pt-bimetallic complex bearing a tetradentate phosphine ligand [56] were used for the hydrosilylation of styrene with PhMe2SiH. Hydrosilylation of various allylamines and other functionalized terminal alkenes with (EtO)3SiH was studied using Cp2PtCl2 (0.033 mol%), affording terminally silylated products as main products (77–98% selectivity) [57]. Allylic azides are known to isomerize easily and form an equilibrating isomeric mixture, whose transformation often provides an isomeric mixture of products. However, hydrosilylation of allylic azides with tertiary hydrosilanes catalyzed by Pt complexes such as Karstedt’s catalyst, Pt(cod)Cl2, Pt(dba)3, K2PtCl4, and NHC-Pt (dvtms) afforded terminally silylated products highly selectively (Scheme 1) [58]. Hydrosilylation of allylamines and vinylpyridines with TMDS [59] and limonene with HMTS and 1,1,3,3,5,5-hexamethyltrisiloxane [60] was studied by using Karstedt’s catalyst. Asymmetric hydrosilylation of borylalkenes with tertiary hydrosilanes was efficiently catalyzed by Pt(dba)3/pyrrolidine-derived phosphoramidite ligand system (Scheme 2) [61]. The resulting geminally boryl(silyl)-substituted compounds are useful synthetic building blocks. Pt nanoparticles (ca. 2 nm diameter) prepared from Karstedt’s catalyst or Pt(dba)2 showed high catalytic activity comparable to Karstedt’s catalyst for the hydrosilylation of PMHS with 1-octene [62]. Cyclometallated Pt complexes (Fig. 9) [63, 64], cis- and trans-PtCl2(RCN)2 [65], and siloxane-substituted-pyridine ligated Pt complex [66] were used for silicone curing.

22

S. Shimada

Ar Pt(dba)3 (3 mol%) Ligand (6 mol%)

Bpin

R +

Bpin

R

MTBE, RT

SiMe2Ph

Ph2MeSiH

Up to 92% ee

Ar

O

O

O

O

P N Ar

Ar

Ar = 3,5-(iPrO)2C6H3 Ligand

Scheme 2 Asymmetric hydrosilylation of borylalkenes + R2N L

CNCy

Pt N

Pt

Cl N

L

X–

O

N N

NXyl Pt

Cl

CNXyl

L = CNCy, PPh3; X = BF4, OTf

Fig. 9 Cyclometallated Pt complexes

1.1.2

Heterogeneous Platinum Catalysts

Preceding the discovery of Speier’s catalyst, heterogeneous Pt catalysts were discovered by Wagner and Strother and commercialized for the production of trichloro (vinyl)silane in the 1950s [67–71]. After the discovery of Speier’s [1] catalyst, homogeneous Pt catalysts became the standard of hydrosilylation catalysts. However, recent academic studies of Pt hydrosilylation catalysts focused more on the development of heterogeneous catalysts. Various types of silica- and modified silica-supported Pt catalysts were reported [72–88]. Pt NP embedded into the walls of a mesostructured silica framework is a highly active catalyst for the hydrosilylation of PMHS with 1-octene (TON ~105) with no Pt leaching [75]. Pt NP@(cross-linked polysiloxane-SiO2) was prepared by the hydrosilylation of Pt NP-@PMHS with vinyl-modified SiO2 [88]. The catalyst showed good catalytic activity for the hydrosilylation of terminal alkenes with HMTS and hydrogen-terminated polydimethylsiloxane. In the first run of the reaction, Pt leaching (0.18 ppm) was observed, while almost no Pt leaching was observed in the reactions with reused catalyst (6 recycles). Pt NP@(poly-carboxylic acids or boronic acid-modified SiO2) showed high β-selectivity in the hydrosilylation of terminal alkenes with MeCl2SiH [77–79]. Schiff base ligated Pt complex immobilized on mesoporous silica SBA-15 (Fig. 10, left) catalyzed the hydrosilylation of terminal alkenes with (EtO)3SiH at 60°C showing high β-selectivity, though the catalyst showed significant deactivation after 4 cycles [82]. N,N,N-pincer-Pt complex immobilized on SiO2 (Fig. 10, right)

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

O O Si O SBA-15

(cod) Pt N O

O Si O O SiO2 O O O Si

23

N Pt

N

N

Fig. 10 Pt complexes immobilized on silica

showed good catalytic activity and high β-selectivity for the hydrosilylation of terminal alkenes with (EtO)3SiH at 90°C [86]. “Single atom catalyst (SAC)” is a new frontier in heterogeneous catalysis and significant advances have been achieved in recent years. SACs are catalysts with atomically dispersed metals on the supports and often show very high catalytic activity [89–94]. Single atom Pt on Al2O3 nanorod (Pt/NR-Al2O3) was prepared from H2PtCl6 and showed high catalytic activity (up to 3 × 105 TON and 1.5 × 105 h-1 TOF) and selectivity in the hydrosilylation of various functionalized terminal alkenes with tertiary hydrosilanes at 100–120°C [95]. Preparation methods significantly affected the catalyst performance; catalysts prepared by impregnation precipitation showed much better activity and recyclability than that prepared by reductive precipitation. Partially charged single atom Pt on TiO2 nanotube catalyzed the selective hydrosilylation of alkenes with (EtO)3SiH with moderate TOF (780 h1 ) [96]. The catalyst can be reused 5 times with 99

β-Selectivity (%) >99 99 99 >99 81 85 82 >99

TOF (h-1) 3.0 × 107 1.2 × 106 3.1 × 104 1.1 × 106 6.4 × 104 1.0 × 105 9.7 × 104 1.2 × 105

Reaction temperature, 50°C Reaction time, 10 min

cage structure comprising eight BL1 and six Cu2(ArCO2)4. Then the cage structure was fixed by the intramolecular olefin metathesis reaction of the vinyl groups before removing Cu atoms. Finally, the OCC-Pt was obtained by the reaction of the cage with Na2PtCl4 and following reduction with PhMe2SiH. 1H NMR analysis suggested the coordination of Pt with olefinic groups in the OCC. OCC-Pt showed high catalytic activity (more than 10 times higher TOF than that of Karstedt’s catalyst for the reaction of 1-octene with (EtO)3SiH). By the steric effect of the cage, OCCPt exhibits unique size/site selectivity. The reaction of PhMe2SiH with common

26

S. Shimada

(CH2)6 (CH2)6

Si

Si

CO2H

X

X

HO2C

Si (CH2)6

Si

Si (CH2)6

Si

(CH2)6

(CH2)6

CO2H

X

BL1

X = CHO or CO2H

BL2

Fig. 13 Structure of BL1 and BL2 CHO S NH2 +

H2N

BTT-BPh-COF S

BPh OHC

S BTT

CHO

Scheme 3 Synthesis of BTT-BPh-COF

terminal alkenes efficiently proceeds, while the reaction of PhMe2SiH with bulky alkenes or that of Ph3SiH with common terminal alkenes does not proceed at all. This selectivity was successfully applied to the selective hydrosilylation of olefinic compounds containing two or more C-C double bonds. The catalyst seems to dissolve in organic solvents but can be reused 5 times with less than 10% activity loss using dialysis tube (regenerated cellulose, molecular weight cutoff of 2 k). The same authors also reported efficient COF-Pt catalysts using similar building blocks BL2 (Fig. 13) [119]. Pt NPs (1–4 nm) supported on a 2-aminoethanethiol-functionalized MIL-101 (MIL-101 is a MOF made from Cr3+ and terephthalic acid) was used for the hydrosilylation of terminal alkenes with HMTS showing up to 1.1 × 104 h-1 TOF [120]. The catalyst was reused 5 times, though a slight decrease of catalytic activity was observed. Heterogeneous Pt NP catalyst with (3-phenylaminopropyl) trimethoxysilane-modified MIL-88(Fe) (MIL-88(Fe) is a MOF made from Fe3+ and terephthalic acid) was also examined for the hydrosilylation of terminal alkenes with HMTS [121]. A mesoporous COF made from benzidine (BPh) and fusedthiophenecarbaldehyde BTT was used as a support of Pt single atoms (Scheme 3).

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

27

Pt@BTT-BPh-COF catalyst showed high β-selectivity for the hydrosilylation of 1-octene and allyl glycidyl ether with tertiary hydrosilanes, while regioselectivity for styrenes was low [122]. The catalyst can be reused at least 9 times without any loss of catalytic performance. Heterogeneous Pt catalysts separable by magnetic field were prepared using Fe3O4 as a support core material. Pt NPs (2 nm) were supported on SiO2/Fe3O4 NPs (ca. 15 nm Fe3O4 core surrounded by SiO2 shell, total NP size 40–50 nm) [123]. The Pt NPs@SiO2@Fe3O4 catalyst (30 ppm) efficiently catalyzed the hydrosilylation of allyl polyether with Si-H containing silicone oil at 100°C. The catalyst can be separated from the viscous oil product by a magnet and reused for the same hydrosilylation, though Pt content decreased 30% after 7 recycles. The same authors also reported Pt@vinyl-modified-SiO2@Fe3O4 catalyst by modifying SiO2 surface with triethoxy(vinyl)silane [124]. In this catalyst, coordination of the vinyl groups to Pt was suggested. This vinyl-modified catalyst showed higher activity than the original Pt NPs@SiO2@Fe3O4 catalyst for the hydrosilylation of allyl polyether with Si-H containing silicone oil. Pt@[(3-aminopropyl) triethoxysilane]-modified-SiO2@Fe3O4 catalyst was effective for the hydrosilylation of terminal alkenes including styrenes with (EtO)3SiH giving β-isomers exclusively [125]. After magnetic separation of the catalyst, no further reaction took place in the solution, showing that the catalysis takes place heterogeneously. The catalyst was reused 5 times with slight loss of activity. Pt@EDTA or DTPA-modifiedSiO2@Fe3O4 catalysts (DTPA = diethylenetriaminepentaacetic acid) were tested for the hydrosilylation of hexenes with MeCl2SiH. The catalysts afforded terminally silylated product with high selectivity not only from 1-hexene but also from internal hexenes [126]. Pt@boronic acid-modified-SiO2@Fe3O4 catalyst showed superior activity and selectivity than that of Speier’s catalyst for the hydrosilylation of 1-hexene with MeCl2SiH, while the reaction of styrene with (EtO)3SiH gave a mixture of α- and β-isomers in 4:6 [127]. The catalyst can be reused by magnetic separation at least 9 times with similar catalytic performance. PtCl2@bis-imidazolemodified SiO2@Fe3O4 catalyst showed high β-selectivity for the hydrosilylation of terminal alkenes with (EtO)3SiH [128]. The catalyst can be reused at least 5 times without any loss of activity. Pt@[(3-aminopropyl)triethoxysilane]-modifiedmesoporous-SiO2@Fe3O4 catalyst was tested for the hydrosilylation of 1-octene with HMTS [129]. The catalyst showed a good activity and recyclability. A unique synergetic effect of Fe2O3 and Pt NPs was reported. DMF-protected Fe2O3 and Pt NPs were inactive for the hydrosilylation 1-dodecene with (EtO)3SiH when each NPs were used separately. On the other hand, if the Fe2O3 and Pt NPs were used together (0.05 mol% each), β-silylated product was obtained quantitatively. The catalyst system was effective for various combinations of terminal alkenes and hydrosilanes and can be reused by separation with hexane/DMF [130]. Several reports described Pt-catalyzed hydrosilylation of alkenes and alkynes using ionic liquids, which are detailed in chapter “Catalysis of Hydrosilylation Processes with the Participation of Ionic Liquids” [87, 131–138].

28

1.1.3

S. Shimada

Activity Control of Platinum Catalysts

For the practical application of hydrosilylation reaction in silicone curing, it is highly important to develop switchable catalysts/catalyst systems that show no catalytic activity under ambient conditions, while exhibiting high efficiency (high TOF and/or high TON depending on the applications) by a trigger such as heat or photoirradiation. Such catalysts/catalyst systems allow a long pot life for practically useful one-component silicone curing compositions and reduce the platinum consumption in silicone industry. Various compounds that coordinate to Pt have been used to inhibit the catalytic activity of Pt at lower temperature and increase the pot life of silicone curing compositions [139]. Representative examples are alkyne compounds such as alkynols and acetylene dicarboxylates, fumarates and maleates, and phosphorous compounds such as phosphines and phosphites. Recent patent described TFAA (1,1,1-trifluoroacetylacetone) as an easily removable inhibitor for silicone curing [140]. Addition of TFAA (≥15 wt%) completely inhibits the hydrosilylation curing even at 80°C, while it can be removed from the mixture at 60°C under open air condition. Phosphites, such as P(OiPr)3 [141], P(OSiR3)3 [142], and alkynol-derived phosphites [143], Buchwald-type bulky phosphines [144], tri- or tetra-acrylates, such as EtC(CH2OCOCH=CH2)3 and O[CH2(Et)C(CH2OCOCH=CH2)2]2 [145], and nitrogen-containing compounds, such as bipyridines, phenanthroline, and poly (vinylpyridine) [146], are useful inhibitor for the hydrosilylation-curable silicon rubber compositions with a long pot life. Pt(II) complexes bearing a nitrile and an imidate-type ligand (Fig. 14) are active upon heating while inactive at room temperature for the hydrosilylation of polysiloxanes, and would be suitable for the hydrosilylation-curable silicone compositions with a long pot life [147]. 1-Ethynylcyclohexane-1-ol (EC) is one of the most used inhibitors for Pt catalysts for hydrosilylation. The mechanism of its action was investigated by using 13 C-labeled 1-ethynylcyclohexane-1-ol (13C-EC) [148]. A mixture of vinyl-PDMS ([vinyl] 0.27 M), Si-H containing PDMS ([Si-H] 0.55 M), 13C-EC (0.014 M), and Karstedt’s catalyst ([Pt] 0.44 mM) was monitored by NMR and showed that 13C-EC was slowly and selectively (over vinyl-PDMS) hydrosilylated during 8 days at room temperature. After complete consumption of 13C-EC, hydrosilylation of vinyl groups started and the mixture turned to a soft gel quickly. At 70°C, most of 13CEC was consumed in 25 min in the same mixture, and a gel was formed in 32 min. The study clearly showed that the coordination of EC to Pt and slow consumption of EC at room temperature inhibit the curing of silicones. Fig. 14 Pt(II) complexes bearing imidate-type ligands

Cl

Cl

O

Pt N

N R

H

R

N

N

O

NH2 R = Et, tBu, Ph

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

O

Si

Si

O

Pt

R O

Si O

S

Si O Si Pt

Si O

29

Me n

O

Si

O

Pt

P(OAr)3

Si O Ar = 2,4-tBuC6H3

R = H or Me Si

Fig. 15 Modifications of Karstedt’s catalyst

Polyethylene-encapsulated microparticle Pt catalysts were prepared from low-molecular weight polyethylene waxes and H2PtCl6 solution [149]. The catalysts are useful for one-component hydrosilylation-curable silicone compositions, which can be stored for 6 months at 10°C while they can be cured at 100°C. Similar polymer-encapsulated Pt catalysts were also prepared using polystyrene and poly (methyl methacrylate) [150]. Hydrosilylation-curable silicone compositions containing the polymer-encapsulated Pt catalysts were stable for 6 months at room temperature while they can be cured at elevated temperature. The curing temperature can be adjusted by changing the molecular weights of the polymers. Various methods to improve the catalytic properties of Speier’s and Karstedt’s catalysts are described in patents. Addition of cod to Speier’s and Karstedt’s catalysts improved product yields and selectivity in the alkene hydrosilylation [151]. Addition of sulfides such as nBu2S and MeSCH2CH2OH to Karstedt’s catalyst can improve the catalytic activity and stability for the silicone curing [152]. Modified Karstedt’s catalyst bearing diallyl sulfide ligand (Fig. 15, left) has higher stability while keeping the catalytic activity for silicone curing [153]. Catalyst system containing Karstedt’s catalyst and acrylate-modified linear polysiloxane (Fig. 15, center) has high stability under hermetically closed conditions, while showing high catalytic activity for the curing of silicon release coating material even with low catalyst loading (~50 ppm) [154]. Catalyst system whose main Pt species is probably modified-Karstedt’s catalyst bearing cyclic vinylsiloxane and bulky triaryl phosphite (Fig. 15, right) is useful for one-component curable silicone composition that shows high stability at room temperature for a long period of time [155]. Bis(vinyl- or allyl(dimethyl)silylmethyl)PtII(cod) (Fig. 16) was used as “slowrelease” precatalysts for the hydrosilylation of terminal alkenes with Et3SiH and HMTS [156]. With the Pt complex (5 × 10-5 mol%), the reaction of AGE with HMTS was very slow at room temperature (no reaction after 30 min), while it was

30

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Fig. 16 “Slow-release” Pt catalysts

Me2Si

n

Pt Me2Si n = 0, 1

O

MeO MeO Si

O

O SiO2

O

n

O Si

O

O

MeO NB1

NB1-SiO2

Fig. 17 Norbornene derivatives for controlling of the catalytic activity of Karstedt’s catalyst

iPr O O

Pt

Me

O

Me2 Si

Me R

O Me PtMe3 + isomers

PtMe3 R = H, SiMe2allyl, (CH2)3SiMe(OMe)2

Fig. 18 Photo-activatable Pt catalysts

completed at 50°C after 4 h. Similar thermal behavior was observed for the hydrosilylation of various terminal alkenes. Latency of catalytic activity was controlled by norbornene-modified SiO2 (NB1SiO2, Fig. 17) [157]. Hydrosilylation reaction of 1-octene with HMTS with Karstedt’s catalyst at room temperature was significantly inhibited with NB1SiO2, while untethered NB1 (1,000 equiv. per Pt, Fig. 17) had no inhibition effect. The inhibition effects depended on the density of norbornene on SiO2 as well as the structure of the bridge between norbornene and the Si atom. As a trigger to switch on the activity of Pt hydrosilylation catalysts, photoirradiation is also used [21]. Representative examples of such photo-activatable catalysts are Pt(acac)2 [158] and (MeCp)PtMe3 [159–161]. Pt(II) complex bearing two salicylaldehyde (Fig. 18, left) ligands showed higher curing activity of silicones as compared with Pt(acac)2 [162]. New complexes bearing dihydroguaiazulenyl ligands (Fig. 18, center) [163] and allyldimethylsilyl-Cp ligands (Fig. 18, right) [164] similar to (MeCp)PtMe3 are described in patents. The (allylMe2Si)CpPtMe3

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts Fig. 19 Pt complexes bearing 2-pyridylphenyl and salen ligands

31

R1

R1 N

Pt O

N

NC

CN

NC

CN

S

S

R2

UV Ph

S S Open-form

Ph Visible Ph

Ph

Closed-form

Scheme 4 Activity control of Pt catalyst using photochromism of dicyanoalkene compound

catalysts have higher stability under dark than (MeCp)PtMe3 in the hydrosilylationcurable silicone compositions. Pt(II) complexes bearing 2-pyridylphenyl and salen ligands (Fig. 19) are described in a patent as hydrosilylation catalysts whose activity can be accelerated by UV irradiation [165]. By the combination of (MeCp)PtMe3 and a sensitizer such as 2-chlorothioxanthen-9-one, visible light instead of UV can be used to activate the Pt catalyst [166]. Activity control of Pt hydrosilylation catalyst by a photochromic inhibitor was demonstrated by photochromic dicyanoalkene compound [167]. Dicyanodithienylethene (Scheme 4, Open-form) is a photochromic molecule and is converted to the closed form by UV irradiation, while the reverse reaction takes place upon visible light irradiation (Scheme 4). The open-form molecule inhibits the hydrosilylation reaction of styrene with Et3SiH catalyzed by Karstedt’s catalyst to some extent, while the closed form has no inhibition effect. It was confirmed that visible light irradiation of the reaction mixture containing the closed-form molecule (2.5 equiv. to Pt) slowed down the reaction.

1.2

Palladium Catalysts

Since palladium catalysts are generally less efficient than the platinum catalysts for the hydrosilylation reaction, the number of publications on Pd-based hydrosilylation catalysts is limited. Cationic C,N,C-pincer type bis(NHC) Pd complexes (Fig. 20) [168] and Pd/Rhand Pd/Ir-bimetallic complexes bearing a tetradentate phosphine ligand [56] were

32

S. Shimada

Fig. 20 Cationic C,N,Cpincer type bis(NHC) Pd complexes

N N

N

PF6

Pd N R

Cl

N R

Scheme 5 Pd-catalyzed hydrosilylation of electron-deficient alkenes with tertiary hydrosilanes

synthesized and used for the hydrosilylation of styrene, though catalytic activity and selectivity were not high. Hydrosilylation of cis- and trans-CF3CH=CHCF3 with Me2ClSiH, MeCl2SiH, and Cl3SiH was examined using H2PtCl6, (PPh3)3RhCl, and (PPh3)2PdCl2. The Pd catalyst afforded hydrosilylation products selectively, while the Pt and Rh catalysts were not active or gave hydrogenation products [169]. Hydrosilylation of norbornadiene with PMDS and HMTS was examined using Rh, Pt, and Pd catalysts. Rh(acac)(CO)2 and Karstedt’s catalysts were more active, but much less selective than [(allyl)PdCl]/R-MOP (R-MOP: (R)-(+)-2-diphenylphosphino-2′-methoxy-1,1′-binaphthyl). The reaction with the Rh and Pt catalysts proceeded at room temp giving a mixture of exo- and endo-silylated products, while the reaction with [(allyl)PdCl]/R-MOP took place at 75°C giving exo-silylated products with high selectivity [170]. Pd(diallylether)(PMe3) catalyzes the hydrosilylation of electrondeficient terminal and internal alkenes with tertiary hydrosilanes to give α-silylated products selectively (Scheme 5) [171]. Although Pd catalysts are not commonly used in hydrosilylation reaction, asymmetric versions of alkene hydrosilylation are often performed by Pd catalysts since the discovery of highly selective asymmetric hydrosilylation of terminal alkenes with Cl3SiH by Pd/MOP system (MOP: chiral binaphthyl-based monophosphine ligands) by Hayashi in 1991 [172, 173]. Hydrosilylation of substituted styrenes with Cl3SiH with Pd/MOP-phosphinite was studied, giving higher selectivity for 4-methoxystyrene than with original H-MOP ligand [174]. The same reaction was studied with Pd/chiral phosphoramidite ligands giving chiral benzyl alcohols in high yields with high enantioselectivity [175]. Hydrosilylation of β-silylstyrenes with Cl3SiH catalyzed by Pd/spirocyclic phosphoramidite system gave bis(silyl) compounds with good to high enantioselectivity [176]. Asymmetric hydrosilylation of cyclopropene derivatives with Ph2SiH2 was selectively catalyzed by Pd/chiral

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

O

R

Pd2(dba)3 (3 mol%) Ligand (6 mol%)

+

Toluene, 50 ºC

N

O

Ph2MeSiH

SiMePh2

O

N

33

Ar

Ar

O

O

O

O

P N

O

R Up to 99% ee

Ar

Ar Ligand

Scheme 6 Desymmetric hydrosilylation of maleimides

Scheme 7 NHC-Ni-catalyzed α-selective hydrosilylation of vinylarenes

phosphoramidite ligand [177]. Desymmetric hydrosilylation of a C=C double bond of maleimides with Ar2MeSiH was efficiently catalyzed by Pd2(dba)3/2 L (L = chiral TADDOL-derived phosphoramidite ligand) with high selectivity (Scheme 6) [178].

1.3

Nickel Catalysts

The number of publications on nickel-based hydrosilylation catalysts significantly increased during the last decade due to the growing interest in non-precious metal hydrosilylation catalysts. Highly α-selective hydrosilylation of vinylarenes with secondary and tertiary hydrosilanes was attained by Ni(0) NHC complex catalysts (Scheme 7) [179]. IMes-Ni(styrene)2 catalyzed the hydrosilylation of variously substituted styrenes with secondary and tertiary hydrosilanes to give α-silylated products highly selectively in moderate to high yields. On the other hand, the catalyst showed normal β-selectivity for terminal aliphatic alkenes. Steric size of NHC ligands, coordinating olefins, and reaction temperature significantly affected the selectivity and/or catalytic activity. IMes-Ni(dvtms)2 complex was less efficient than IMes-Ni(styrene)2. Smaller NHC ligands showed higher activity and α-selectivity. In situ-generated ITMe-Ni(0) species from Ni(cod)2/ITMe was proved to be a better catalyst than

34

S. Shimada

Fig. 21 N,N,N-pincer-Ni (OMe) complex

NMe2 N

Ni OMe NMe2

+ +

Ni cat. 0.5 mol% HSi(OEt)3 (1.2 equiv)

Si(OEt)3

neat, 60 ºC, 2h +

81%

Scheme 8 Ni-catalyzed isomerization-hydrosilylation reaction

IMes-Ni(styrene)2 for the hydrosilylation of styrene with (4-MeOC6H4)3SiH, Ph2MeSiH, PhMe2SiH, (EtO)3SiH, and HMTS. N,N,N-pincer-Ni(OMe) complex (1 mol%, Fig. 21) efficiently catalyzed the hydrosilylation of various functionalized terminal alkenes with H2SiPh2 [180]. Even alkenes bearing ketone or aldehyde functionalities were selectively hydrosilylated at C=C double bonds. Internal linear alkenes reacted to give terminally silylated products through chain-walking. Later, the authors proved that the active species in this catalysis is Ni NPs by detailed mechanistic investigations [181]. Although the above Ni NPs are not active for tertiary hydrosilanes, Ni NPs in situ generated from Ni(OtBu)2•xKCl (1 mol%) are effective for tertiary hydrosilanes such as (MeO)3SiH, (EtO)3SiH, Me(EtO)2SiH, and Me2(MeO)SiH [182]. Various functionalized terminal alkenes were selectively hydrosilylated with (MeO)3SiH giving β-silylated products in good to high yields. Internal linear alkenes reacted to give terminally silylated products through chain-walking. By using this isomerization-hydrosilylation, a mixture of terminal and internal octenes was converted to terminally silylated octyl(triethoxy)silane in 81% yield (Scheme 8). A dimeric cationic N,P,N-pincer Ni complex bearing bis(quinolyl)phosphine ligand (0.1–3 mol% Ni) efficiently catalyzed the hydrosilylation of terminal alkenes as well as cyclopentene with tertiary, secondary, and primary hydrosilanes at room temperature (Fig. 22) [183]. The mechanism of this reaction was studied by DFT calculations (Fig. 23). A cationic Ni–H species INT-1 is initially formed from the cationic Ni-Cl precursor by the reaction with the hydrosilane. Then INT-1 is converted to a Ni-Si species INT-2 by the reaction with the hydrosilane or cyclopentene and the hydrosilane. Coordination of cyclopentene to INT-2 forms INT-3, in which insertion of cyclopentene in the Ni-Si bond takes place to form an alkyl-Ni species INT-4, where C-Si bond coordination to Ni stabilizes the intermediate. Formation of the hydrosilylation product and regeneration of the Ni-Si species

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

35

OAr P

2+

Cl N

(PNP)Ni

N

Ni(PNP)

2 [B(C6F5)4–]

Cl PNP Ligand Ar = 3,5-tBuC6H3

Ni catalyst precursor

Fig. 22 Structures of the PNP ligand and cationic (PNP)Ni dimer complex [Si] [Si]

P

H

[Si]

Ni

[Si]

H

N N TS-1 N P

Ni N

N

[Si]-H, –H2 H

P H2, – [Si]-H

INT-1

Ni

[Si]

P

N

Ni N

[Si]

N INT-4

INT-2

+ [Si]-H

P

Ni

[Si]

N N INT-3

Fig. 23 Proposed mechanism of (PNP)Ni-catalyzed hydrosilylation reaction Ar

Ar

N

N

Ni X

Ni(cod)

N

O PiPr2 O PiPr2 Ar = Ph, 2,6-iPr2C6H3

N

N Ni Br2

Fig. 24 N,C,P-pincer Ni complexes (left and center) and terpyridine-Ni complex (right)

INT-2 take place by σ-bond metathesis through the transition state TS-1. Through this catalytic cycle, oxidation state of the Ni does not change. The calculations suggested that the catalytic cycle based on the Ni-H species INT-1 is energetically unfavorable and INT-1 is easily converted to the Ni-Si species INT-2. N,C,P-pincer-Ni(II) complexes and their Ni(0) analogue (Fig. 24, left and center) [184] as well as a terpyridine-Ni(II) complex [185] (Fig. 24, right) were used for the hydrosilylation of terminal alkenes, aldehydes, and ketones with PhSiH3 or Ph2SiH2.

36

S. Shimada

N Ar

N

N

Ni

Ni

N

Ar

Ar

N

N

H Ni

Ar

H

N

Ni N

Ar Ar = 2,6-iPr2C6H3

N

Ni N

PPh2 PPh2

Ar

Ar = 2,6-iPr2C6H3

Fig. 25 Dinuclear Ni complexes (left & center) and a phosphine-substituted α-diimine Ni complex (right) Ar N

Ar

N

N

H Ni

N

Ar

Ar R

Ni H

Ar N

N

Ar

Ar

A

Ni

N [Si]

N

Ar

R

N Ar

H

R

olefin isomers

E

H R

B

Ar

Ar

N Ni

Ni

N

R N

[Si] Ar

D

[Si]

Ni

R

C

H

Fig. 26 Proposed mechanism for the Ni-H-catalyzed hydrosilylation reaction

A dinuclear Ni(I)-Ni(I) complex supported by a doubly reduced naphthyridinediimine ligand (5 mol%, Fig. 25, left) was used as a catalyst for the hydrosilylation of alkenes, alkynes, 1,3-dienes, and carbonyl compounds with Ph2SiH2 [186]. A redoxactive α-diimine ligand in combination with air-stable Ni(2-ethylhexanoate)2 generated a highly active Ni catalyst for the hydrosilylation of 1-octene with tertiary hydrosilanes such as (EtO)3SiH, Me(EtO)2SiH, and Me2(EtO)SiH, HMTS, and PMDS [187]. The catalyst (100 ppm) was also effective for curing of silicone polymers. Ni-H dimer (Fig. 25, center) was separately synthesized from Ni (2-ethylhexanoate)2, α-diimine ligand, and (EtO)3SiH. DFT calculations suggested that the formally Ni(I)-H dimer was best described as two Ni(II) with one-electronreduced ligands (A, Fig. 26). Hydrosilylation starts with the dissociation of the Ni-H

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

THF O O Ni O O THF

37 Bn

O

N

O

N

N

O N

Bn

O catechol-alloxazine ligand

Fig. 27 Structures of phenalenyl Ni complex (left) and catechol-alloxazine ligand (right)

dimer to form monomer B (Fig. 26). Insertion of an alkene into the Ni-H bond forms alkyl-Ni species C. Both oxidative addition/reductive elimination through intermediate D and σ-bond metathesis are viable pathways for the formation of the hydrosilylation product by the reaction of C and a hydrosilane. Isomerization of alkenes was observed during catalysis, while only the terminally silylated products were observed. Internal alkenes were also hydrosilylated to give the terminally silylated products, which can be explained by the rapid reversible alkene isomerization through β-hydride elimination/alkene insertion via intermediate E (Fig. 26). A nickel complex bearing a phosphine-substituted α-diimine ligand (1 mol%, Fig. 25, right) efficiently catalyzed the hydrosilylation of mono-substituted terminal alkenes with Ph2SiH2 at room temperature [188]. The same complex was also active for the hydrosilylation of geminally disubstituted terminal alkenes with Ph2SiH2, though higher reaction temperature (70°C) and longer reaction time (7 days) are required. A Ni(II) complex ligated by two phenalenyl ligands (0.25 mol%, Fig. 27, left) in combination with K (Ni:K = 1:3) as a reductant is an efficient catalyst for the hydrosilylation of terminal alkenes with primary and secondary hydrosilanes [189]. The catalyst system is also active for tertiary hydrosilanes with decreased activity. In this catalytic system, two-electron-reduced biradical species was suggested as a key catalytic species. A hexanuclear Ni6L6 complex was generated by the reaction of catechol-alloxazine ligand (Fig. 27, right) and Ni(NO3)2. The Ni6L6 complex (0.25 mol%) in combination with reductants such as Na or NaEt3BH (7.5 mol%) catalyzed the hydrosilylation of terminal alkenes and cyclopentenes with Ph2SiH2 [190]. Aliphatic internal alkenes were also hydrosilylated to give terminally silylated products. (Salicylaldiminato)Ni(II) complex (0.5 mol%, Fig. 28, left) is active for the hydrosilylation of terminal alkenes with secondary hydrosilanes [191, 192]. Cationic π-allylnickel(II) complexes bearing an arene ligand (0.1–0.5 mol%, Fig. 28, center) were used for the hydrosilylation of 1-octene with secondary and primary hydrosilanes [193]. Mechanistic study by DFT calculations suggested that the πallyl ligand plays an important role in the catalytic cycle as a non-innocent ligand (Fig. 29). First, the arene ligand was replaced by the alkene to form alkene complex INT-1. Then R2SiH2 reacts with INT-1 to form σ-silane complex INT-2. Subsequent hydronickelation of the alkene takes place with the assistance of the allyl ligand by forming a Si-C bond, giving allylsilane-coordinated alkyl-nickel intermediate INT-3. Then, silylation of the alkyl ligand takes place through the transition

38

S. Shimada

Rn iPr

iPr N

Ni

BAr4

CH3

Cy2P

Ni

NXyl

OTf

Ni O

N

Ar = 3,5-(CF3)2C6H3

Fig. 28 Structures of (salicylaldiminato)Ni(II) complex (left), cationic (arene)π-allylnickel (II) complexes (center), and cationic (iminophosphine)π-allylnickel(II) complex (right)

Rn

R2SiH2

R Ni

Ni

R2HSi H Ni

INT-1

R

INT-2

arene R

SiHR2

R2HSi

SiHR2 H R

Ni H TS-1

Ni R INT-3

Fig. 29 Proposed mechanism for the cationic (arene)π-allylnickel(II) complex-catalyzed hydrosilylation reaction

state TS-1, giving the hydrosilylation product and regenerating the starting complex by the arene coordination. Cationic π-allylnickel(II) complexes bearing an iminophosphine ligand (Fig. 28, right) are active for the hydrosilylation of geminally disubstituted alkenes with Ph2SiH2, giving terminally silylated products selectively [194]. The catalyst (5 mol%) is also useful for the hydrosilylation of internal alkenes such as trans-β-methylstyrene, indene, cyclohexene, and 2,3-dihydrofuran with Ph2SiH2. The proposed reaction mechanism includes Ni(II)-H species as an active catalytic species, which reversibly adds to an alkene and subsequently reacts with a hydrosilane via σ-bond metathesis giving a hydrosilylation product and regenerating the Ni(II)-H species. A simple catalytic system, a combination of NiCl2•6 H2O and KOtBu, efficiently catalyzed the hydrosilylation of various functionallized terminal alkenes with primary hydrosilanes to give β-adducts selectively [195]. Various functional groups, such as nitrile, ester, amide, epoxy, and sulfide, are tolerant and up to 5,500 TON was achieved. Proposed reaction mechanism is shown in Fig. 30. KOtBu is expected to activate hydrosilanes by the formation of a pentacoodinate silicate, which replaces

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

39

H tBuO

R Sol SiH2Ph

R

H

Ni0

Ph

Si

PhSiH3 + tBuO

H

R

+

A

tBuO

I

III ‡ Ph

OtBu H Si H H

R Ni0 Ph R

C

II

OtBu H Si H H

R Ni0 R

B

Fig. 30 Proposed mechanism for the hydrosilylation reaction catalyzed by NiCl2•6 H2O/KOtBu catalyst system

the solvent molecule of (alkene)Ni0 species A to form a silicate-coordinated Ni (0) species B. Hydrosilylation of alkene takes place through a concerted transition state C to form the hydrosilylation product and regenerate A. The oxidation state of the Ni does not change throughout the catalytic cycle. Ni(CNtBu)4 is described in a patent as a hydrosilylation catalyst for alkenes [196], though its catalytic performance is much less efficient than the corresponding Co and Fe complexes [197]. Ni(acac)2 and related Ni complexes are active catalysts for silicone curing [198]. Formal hydrosilylation reaction of alkenes by a silylborane reagent, PhMe2Si-B(pinacolato), and H2O with Ni(cod)2/PCy3 catalyst system was reported [199]. Gaseous hydrosilanes such as SiH4, MeSiH3, Me2SiH2, and Me3SiH are highly flammable (some of which are explosive) and are difficult to handle particularly in academic laboratories. 3-silylated-hexa-1,4-dienes work as surrogates of these gaseous hydrosilanes for the hydrosilylation in the presence of B(C6F5)3 catalyst [200– 202]. Alkoxysilanes such as (MeO)3SiH, Me(EtO)2SiH, and Me2(MeO)SiH, respectively, work as surrogates of SiH4, MeSiH3, and Me2SiH2 in the presence of N,N,Npincer-Ni complex and NaOtBu (Scheme 9) [203]. Redistribution of the alkoxysilanes takes place generating SiH4, MeSiH3, or Me2SiH2 in situ. TMDS also works as a surrogate of Me2SiH2 in the presence of P,N,O-pincer–-Ni complex and NaOMe (Scheme 10) [204]. Styrenes and other aromatic alkenes afforded αadducts with high selectivity, while aliphatic alkenes gave β-adducts exclusively. Heterogeneous Ni(0)@SiO2 (mainly consists of 1–2 nm Ni NP) was prepared and tested for the hydrosilylation of triethoxy(vinyl)silane with (EtO)3SiH, though the

40

S. Shimada

Scheme 9 Alkoxyhydrosilanes as surrogates of gaseous hydrosilanes in the presence of N,N,Npincer-Ni complex

Scheme 10 TMDS as a surrogate of Me2SiH2 in the presence of P,N,O-pincer-Ni complex

catalytic activity and selectivity were not high [205]. Heterogeneous nickel complex catalyst ligated by silica-supported terpyridine ligands was used for the hydrosilylation of 1-octene with Ph2SiH2, giving β-isomer exclusively [206]. Ni-MOF prepared from NiCl2•6 H2O and 4,4′-biphenyldicarboxylic acid showed high catalytic acitivity for the hydrosilylation of terminal alkenes, cyclopentene, and norbornene with Ph2SiH2 [207]. The catalyst showed up to 9,500 TON and was reused 10 times with a slight loss of activity. Asymmetric hydrosilylation of difluoroalkenes RCH=CF2 with Ar2SiH2 was achieved by Ni(cod)2/chiral oxazoline-substituted ferrocenylphosphine ligand/B (C6F5)3 catalyst system (Scheme 11) [208]. Variously substituted difluoro alkenes were hydrosilylated in good to excellent yields with up to 96% ee.

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

F

R F + Ar2SiH2

Ni(cod)2 (5 mol%) Ligand (5 mol%) B(C6F5)3 (5 mol%) p-anisidine (10 mol%) 4Å MS toluene, 30 ºC

41

PPh2 O

SiHAr2 Fe R

CF2H

N iPr Ligand

Up to 96% ee

Scheme 11 Asymmetric hydrosilylation of difluoroalkenes

Scheme 12 Hydrosilylation of alkynes Table 3 Selectivity in the hydrosilylation of terminal alkynes with tertiary hydrosilanes catalyzed by Karstedt’s catalyst (0.004 mol %) [210]

Alkyne

Hydrosilane

PhC  CH

Et3SiH Ph3SiH (EtO)3SiH Et3SiH Ph3SiH (EtO)3SiH Ph3SiH

nPrCCH

ˆtBuCCH

Conditions Temp, time 60°C, 1 h a , 2–3 min rt, 1 h rt, 1 h rta, 1 h rt, 1 h a , immediate

Selectivity β-(E)/β-(Z )/α 81/1/17 78/15/7 55.5/6/38.5 89/0/11 77/0/23 55.5/6/38.5 100/0/0

a

Ph3SiH (or Ph3SiH/alkyne mixture) was heated to melt Ph3SiH before the addition of Pt catalyst

2 Hydrosilylation of Alkynes Selective hydrosilylation of alkynes is generally more challenging than that of alkenes because the former can form not only regioisomers but also stereoisomers [209]. For example, hydrosilylation of terminal alkynes can produce α and β regioisomers as well as E and Z stereoisomers of β-isomer (Scheme 12). In addition, by-products such as alkyne-hydrogenation, dehydrogenative silylation, and doublehydrosilylation products can be produced. In the case of unsymmetrical internal alkynes, four regio- and stereoisomers can be formed. Table 3 shows typical examples of selectivity in the hydrosilylation of terminal alkynes with tertiary hydrosilanes catalyzed by Karstedt’s catalyst [210]. Traditional Pt catalysts generally provide β-(E) isomers as the main products, though the selectivity depends on various factors such as catalyst systems, steric and electronic effects of substrates, and reaction conditions (temperature, solvents, etc.) [211].

42

2.1 2.1.1

S. Shimada

Platinum Catalysts Homogeneous Platinum Catalysts

Bis(silyl) NHC-Pt complexes (Fig. 31, left), which can be in situ generated from NHC-Pt(dvtms) complex with hydrosilanes, showed higher activity and selectivity than the original NHC-Pt(dvtms) for the hydrosilylation of alkynes [212]. Terminal alkynes were selectively converted to β-(E) hydrosilylation products (>20:1 selectivity in most cases) with 0.1 mol% catalyst. Internal silyl alkynes, RCCSiMe3 (R = nHex, Ph), were hydrosilylated to give (E)-R(R3Si)C=C(H)SiMe3 selectively (≥13:1). NHC-Pt(dvtms) with very bulky NHC (IPr*, Fig. 2) shows very high selectivity in the alkyne hydrosilylation [32]. Terminal alkynes, ArCCH and R3SiCCH, were hydrosilylated with various tertiary hydrosilanes (0.05–0.1 mol % catalyst) to give β-(E) hydrosilylation products (>95% selectivity, exclusively in many cases). Hydrosilylation of internal silyl alkynes, ArCCSiMe3, with tertiary hydrosilanes by this catalyst exclusively afforded (E)-Ar(R3Si)C=C(H)SiMe3. Dinuclear Pt2 complex bearing a bulky bis(NHC) ligand (Fig. 31, right) in situ generated from the ligand precursor and Karstedt’s catalyst with a base showed higher catalytic activity than Karstedt’s catalyst or Pt catalysts with mono-NHC ligands for the hydrosilylation of internal alkynes [213]. Silylalkynes, ArCCSiMe3, exclusively afforded (E)-Ar(R3Si)C=C(H)SiMe3. Pt(0) complexes bearing NHC ligands with NaSO3 substituents (Fig. 32, left) were synthesized as catalysts for the hydrosilylation of alkynes in water

Ar 2+

SiR3

N Pt N

Ar

SiR3

Ar Ar = 2,6-iPr2C6H3

N

N

N

N

Cl– Ar

Ar = 2,6-(Ph2CH)2-4-MeO-C6H2

Fig. 31 Structures of bis(silyl) NHC-Pt complexes (left) and bis(NHC) ligand (right) NaO3S N

N

N Mes

(CH2CH2O)3CH3

N

Br Pt

X

Br

N N NaO3S

N

SO3Na

N

(CH2CH2O)3CH3

X = CH2, C6H2Br2

Fig. 32 Structures of NaSO3-substituted NHC ligands (left) and Pt complexes bearing PEG-substituted bis(NHC) ligand (right)

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

43

NXyl H N

H N N

Pt

C

+ Xyl

Cl Cl–

C N Xyl

Cl

NXyl

Pt N

N N

Pt

Xyl C N

Xyl

Cl

Fig. 33 Pyridyl-diaminocarbene-Pt complexes Fig. 34 Structure of XPhos-SO3Na

iPr SO3Na Cy2P

iPr

XPhos-SO3Na

[214, 215]. The catalysts (0.1 mol%) afforded hydrosilylation products with moderate β-selectivity. The water-soluble catalysts can be reused by separating the products with organic solvents. Pt(II) complexes bearing bis(NHC) ligands with PEG substituents (Fig. 32, right) were used for the hydrosilylation of alkynes in water/MeOH mixture [216]. Hydrosilylation of terminal alkynes exclusively afforded β-isomers, but as a mixture of E/Z stereoisomers. Pyridyl-diaminocarbene-Pt complexes catalyzed the hydrosilylation of alkynes under photolysis (Fig. 33). Blue LED irradiation significantly improved the catalytic performance [217]. Terminal alkynes afforded a mixture of β-(E) and α products. Bulkier hydrosilanes such as nPr3SiH and iPr3SiH gave more α products (63–81% selectivity) with PhCCH and 4-tBuC6H4CCH. Several other diaminocarbene-Pt and Fisher-carbene-Pt complexes active for alkyne hydrosilylation were reported [218–220]. Effects of seven Buchwald ligands (2-dialkylphosphinobiphenyls) in combination with Karstedt’s catalyst were examined for the hydrosilylation of PhCCH with Ph2SiH2 [221]. In all cases, the addition of the ligands improved the selectivity (up to 96.4% β-(E) selectivity with 0.05–0.1% Pt). Exclusive β-(E) selectivity in the terminal alkyne hydrosilylation was attained by a catalyst system K2PtCl4/XPhosSO3Na in PEG-400/H2O (2/1) at 60°C (1 mol% Pt loading) (Fig. 34) [222]. Various functionalized terminal alkynes were hydrosilylated with Et3SiH and PhMe2SiH giving β-(E) products. Although the catalyst loading is rather high, the catalyst system can be easily separated from the products by cyclohexane extraction and reused at least 8 times without any loss of activity. Ester-substituted internal alkynes (RCCCO2Et, R = Ph and pentyl) were also hydrosilylated with PhMe2SiH to give (E)-RCH=C(SiMe2Ph)CO2Et in 93–95% selectivity.

44

S. Shimada

Fig. 35 Pt(II) complexes bearing fluoroalkylsubstituted bipyridine ligands

Rf

Rf O

O N

N Pt

Cl

Cl

Rf = C4F8Cl, C4F8H, C11F23

Me2Si O

+

iPr3SiH

1) Karstedt’s cat. (0.2 mol%) 80 ºC 2) TBAF, rt

R R = H, Me, Cy, tBu

Si3iPr HO

R

84-92% yield

Scheme 13 Highly regioselective hydrosilylation of propargyl alcohol derivatives using dimethyl (vinyl)silyl group as a directing group

Pt(II) complexes bearing fluoroalkyl-substituted bipyridine ligands were used as separable catalysts under thermomorphic conditions for the alkyne hydrosilylation (Fig. 35) [223, 224]. The reaction was performed in Bu2O as a solvent at 120°C under homogeneous conditions. Phase separation took place by cooling to room or ice temperature and the catalyst can be separated by centrifugation. The catalysts were reused 8 times without loss of catalytic performance, although regioselectivity in the hydrosilylation of terminal alkynes was not high. Pt(ethylenediamine)Cl2 (1 mol%) was used for the hydrosilylation of terminal and internal alkynes with tertiary and secondary hydrosilanes [225]. The authors suggested that the catalyst works as heterogeneous catalyst under neat conditions and reused 5 times without any loss of activity. K2PtCl4 or Karstedt’s catalyst (0.5 mol%) in combination with two ligands, 1,1′-binaphthalene-based diol and amide-substituted triarylphosphine, increased β-(E) selectivity in the hydrosilylation of ethynylarenes with Et3SiH [226]. Hydrosilylation of various terminal and internal alkynes with tertiary hydrosilanes by Karstedt’s catalyst in supercritical CO2 was studied, and the advantages and the limitations of supercritical CO2 as a reaction and extraction medium were clarified [227]. As described above, Pt-catalyzed hydrosilylation of terminal alkynes generally shows β-(E) selectivity. High α-selectivity was reported for other transition metal catalysts such as Ru [228–230], Co [231–233], Rh [234], Ir [235], and Cu [236]. Exclusive α-selectivity was attained in Pt-catalyzed hydrosilylation of propargyl alcohol derivative with iPr3SiH by using dimethyl(vinyl)silyl group as a directing group (Scheme 13) [237]. Not only a terminal alkyne but also internal alkynes were hydrosilylated with excellent regioselectivity. A recent report disclosed that Pt3 cluster in situ generated from Karstedt’s catalyst at 110°C shows α-selectivity in the hydrosilylation of terminal alkynes [238]. Regioselectivity of the reaction of PhC  CH with Et3SiH with Karstedt’s catalyst was found to be significantly temperature dependent (Table 4). Up to 60°C,

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

45

Table 4 Temperature dependence of regioselectivity in the alkyne hydrosilylation

Cat Karstedt’sa Karstedt’s Karstedt’s Karstedt’s Karstedt’s Karstedt’s H2PtCl6 PtO2b Pt/SiO2c

Silane Et3SiH Et3SiH Et3SiH Et3SiH Me2PhSiH Me2BnSiH Et3SiH Et3SiH Et3SiH

Temp (°C) 25 60 80 110 110 110 110 110 110

Time (h) 1 17 7 0.5 1 5 6 0.7 1

Yield (%) β 76 43 17 26 24 26 25 25 25

α/β α 15 5 47 72 66 73 74 74 73

0.20 0.12 2.76 2.77 2.75 2.81 2.96 2.96 2.92

a

0.5 mol% Pt 0.05 mol% Pt c Pt/SiO2 1 wt% b

β-(E) isomer was formed in more than 80% selectivity. On the other hand, at 80 or 110°C, α-isomer was predominant (>73% selectivity). Other hydrosilanes, such as Me2PhSiH and Me2BnSiH, showed similar α-selectivity. This α-selectivity was independent on Pt precursors; H2PtCl6, PtO2, and Pt/SiO2 showed similar α-selectivity. It was proved that Pt3 clusters, generated at higher temperature, were active species that show α-selectivity. Pt cluster complexes [NEt4]2[Pt3(CO)6]3, Na2[Pt3(CO)6]5, and Na2[Pt3(CO)6]10, which have layered structures with terminal Pt3 units, were also examined for the hydrosilylation of PhCCH with Et3SiH and showed α-selectivity even at room temperature. Karstedt’s catalyst (0.005 mol%) was applied to the hydrosilylation of various functionalized terminal alkynes with tertiary hydrosilanes in toluene at 110°C giving α-regioisomers as main products. The α-selectivity was explained by the modified Chalk–Harrod mechanism, where insertion of the alkyne into the Pt-Si bond preferentially takes place to give αsilylated intermediate. Later, detailed experimental and theoretical study supported this mechanism [84]. A silane and alkyne-coordinated-Pt3 species is suggested. Insertion of the coordinated alkyne into the Pt-Si bond takes place through the transition state TS-1 avoiding the steric repulsion between alkyne R’ group and bulky Pt3 moiety to form α-silylated intermediate INT-1 (Scheme 14). On the other hand, in the case of Pt1 catalyst, five-membered Pt(Si-H)2 cyclic transition state TS-2 was suggested based on kinetic studies and supported by DFT calculations (Scheme 15). In this case, the reaction follows Chalk–Harrod mechanism giving intermediate INT-2, from which reductive elimination produces β-(E) isomer. PtCl2/XPhos (0.5/1.0 mol%) catalyst system showed high β-(E) selectivity for the hydrosilylation of propargylic alcohols with tertiary, secondary, and primary

46

S. Shimada R’ R’

R’ Pt

R’ R’

Pt

R’

Pt

Pt

H

SiR3 R’

Pt R’

Pt R’ SiR3

H ’R INT-1

TS-1

Scheme 14 Proposed transition state TS-1 for Pt3-catalyzed hydrosilylation of alkynes R’ Pt

R’

R3 Si

Pt

H R’

H

SiR3

Si R3

’R

TS-2

INT-2

Scheme 15 Proposed transition state TS-2 for Pt1-catalyzed hydrosilylation of alkynes OH R2 R1 + BnMe2SiH

PtCl2 (0.5 mol%) XPhos (1 mol%)

OH

+ I

R2

THF, 50 ºC

SiMe2Bn

R1

R Pd cat TBAF

OH R2

R

R1

Scheme 16 1,3-Diene synthesis by PtCl2/XPhos-catalyzed hydrosilylation of propargylic alcohols and Hiyama cross-coupling reaction

R1

R2

OH

PtCl2 (1 mol%) XPhos (2 mol%)

R1

R2

R2

OH

NIS

R1

O

I

THF, 50 ºC + BnMe2SiH

SiMe2Bn

SiMe2Bn

Scheme 17 Oxetane synthesis by PtCl2/XPhos-catalyzed hydrosilylation of propargylic alcohols and iodocyclization reaction

hydrosilanes [239–241]. The resulting vinylsilane products were used for Hiyama cross-coupling reaction to provide a variety of 1,3-dienes (Scheme 16) [241]. PtCl2/ XPhos system was also used for the highly β-(E)-selective hydrosilylation of homopropargylic alcohols with BnMe2SiH (Scheme 17) [242]. The resulting vinylsilanes are useful intermediates and can be converted to oxetanes by iodocyclization reaction. Hydrosilylation of internal alkynes with dihydrosilanes

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

47

Scheme 18 Hydrosilylation of borylalkynes

R1

R1 X OH

+ R2SiH2

Pt2(dba)3 (1 mol%) RuPhos (2 mol%)

X

O PCy2

SiR2 iPrO

OiPr

80 - 100 ºC R2

X = CH2 or SiR32

R2

RuPhos

Scheme 19 Tandem dehydrogenative silylation of alcohols/intramolecular hydrosilylation

(R1R2SiH2) in the presence of 1 equiv. of alcohols (R3OH) was catalyzed by Pt2(dba)3/tBuMe2SiO-MOP system to selectively afford R1R2(R3O)Si-substituted alkenes [243]. Hydrosilylation of borylalkynes with tertiary hydrosilanes was studied using Pt (PtO2/XPhos, Karstedt’s, Pt(PPh3)4) and Ru ([CpRu(MeCN)3][PF6], Ru(CO)Cl(H) (PCy)2) catalysts [244]. Pt catalysts generally showed higher activity than the Ru catalysts, while Pt catalysts and [CpRu(MeCN)3][PF6] often showed opposite selectivity. One example is shown in Scheme 18. PtO2/XPhos system was also used for the synthesis of bifunctional disiloxanes by the selective hydrosilylation of monofunctionalized hydrodisiloxanes with alkynes [245]. Hydrosilylation of CF3CCCF3 with tertiary hydrosilanes such as Et3SiH, Me2ClSiH, MeCl2SiH, and Cl3SiH was quantitatively catalyzed by Cp2Pt and/or Speier’s catalyst [246]. Reaction of hydroxy- and silanol-substituted diarylalkynes with secondary hydrosilanes afforded cyclic hydrosilylation products in moderate to high yields (Scheme 19) [247]. The reaction was catalyzed by Pt2(dba)3/RuPhos system and proceeded through the formation of Ar-X-O-SiR2H intermediates by dehydrogenative silylation of the OH group and subsequent intramolecular hydrosilylation.

2.1.2

Heterogeneous Platinum Catalysts

Super-microporous silica-supported Pt catalyst was used for the hydrosilylation of ethyne, 1-octyne, and phenylacetylene with Cl2MeSiH, giving β-products with >97% selectivity [74]. Pt NPs on mesoporous silica SBA-15 (Pt@SBA-15) was

48

S. Shimada

Fig. 36 Pt complexes immobilized on bipyridineperiodic organosilicas

O OX Si O Si OX O OX Si O

N

N Pt Me2

N

N

XO O Si O XO Si XO O Si O

X = H or SiMe3

Table 5 Hydrosilylation of alkynes with Pt1@NaY catalyst

R1 Ph Ph Ph 2-MeC6H4 2-ClC6H4 3-MeO C6H4 Cy Cy Ph Ac HOCH2CH2

R2 H H H H H H H H Et H H

Silane Et3SiH PhMe2SiH PhSiH3 Et3SiH Et3SiH BnMe2SiH Et3SiH PhMe2SiH Et3SiH Et3SiH PhMe2SiH

Yield (%) 82 68 51 86 91 52 14 55 77 56 35

α: β-(E) 3.5: 1 3: 1 1: 1 9: 1 7: 1 4: 1 1: 6 1.5: 1 5: 1 1.5: 1 1.4: 1

tested for the hydrosilylation of PhCCH with Et3SiH [248]. Catalytic performance depended on the Pt particle sizes (1.6, 5.0 and 7.0 nm); catalytic activity (TOF) was in the order 7.0 nm > 5.0 nm > 1.6 nm with β-(E) isomer as a main product, though the selectivity was not high. Hydrosilylation of ethyne with (EtO)3SiH was studied with Pt/ZrO2 catalysts. Pt contents significantly affected the catalytic performance, and 2 wt% Pt/ZrO2 showed slightly better performance than Speier’s catalyst [249]. Pt/ZrO2 catalyst can be reused, though significant loss of activity was observed even after 2 cycles. Pt complexes immobilized on bipyridine-periodic organosilicas (Fig. 36) were used for the hydrosilylation of PhCCH with (MeO)3SiH, showing usual β-(E)/α selectivity (ca. 7/3) with 5 mol% Pt [250]. Heterogeneous Pt1@NaY showed α-selectivity in the hydrosilylation of terminal alkynes when the reaction was performed in toluene at 110°C (Table 5) [84]. An

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts Fig. 37 Pt complex bearing a pyrene-tagged NHC ligand

49 Mes N N

Cl Br Pt N

Br

internal alkyne, but-1-yn-1-ylbenzene, was also hydrosilylated with good regioselectivity. It is supposed that Pt3 is formed under the reaction conditions and well trapped in the zeolite cavity. Fresh catalyst showed some induction period, while used catalyst can be recycled up to five times without induction period. Pt NPs@CNT modified by two layers of organic polymers, which are useful for the alkene hydrosilylation, was also applied to the alkyne hydrosilylation [103]. The reactions of PhCCH or Me(CH2)3CCH with ClMe2SiH and PhCCPh or 1-ethynylcyclohexan-1-ol with PhMe2SiH were efficiently catalyzed by the catalyst (0.04 mol%) at room temperature under air. In the case of terminal alkynes, β-(E) isomers were exclusively obtained. Immobilization of NHC-Pt complex on the surface of reduced graphene oxide was easily accomplished by using a pyrene-tagged NHC ligand (Fig. 37) [251]. The immobilized complex showed similar or slightly better catalytic activity than the unimmobilized complex for the hydrosilylation of alkynes. Terminal alkynes gave mixtures of β-(E) and α isomers with low selectivity. The catalyst also effective for internal alkynes. Insoluble cross-linked polysiloxane-stabilized Pt NPs catalyst was prepared by the reaction of Me2Pt(cod) and polyhydrosiloxane and subsequent exposure to air [252]. The catalyst (0.1 mol%) is active for the hydrosilylation of terminal alkynes with Et3SiH, Me2ClSiH, or Cl3SiH at room temperature or 70°C, giving vinylsilanes in excellent yields with good β-(E) selectivity (≥84% in most cases). Internal alkynes were also hydrosilylated in excellent yields. RCCSiMe3 gave (E)-R (R3Si)C=C(H)SiMe3 isomer with high selectivity. Pt NPs stabilized by tris (imidazolium) compounds were used for the hydrosilylation of terminal and internal alkynes [253].

2.2

Palladium and Nickel Catalysts

Alkynoates were hydrosilylated with secondary and tertiary hydrosilanes by a cationic palladium complex ligated by an iminophosphine ligand (Scheme 20) [254]. The reaction proceeded with complete regio- and stereoselectivity to give single isomer as a product. The same catalyst also selectively hydrosilylated enynes with Ph2SiH2, though regioselectivity reversed depending on the substrates (Scheme 21). The Pd catalyst was not active for the hydrosilylation of inactivated internal

50

S. Shimada

R1

CO2Me

+

R22SiH2

R1 = H, nPr, Ph

Pd-cat (2 mol%)

SiR22H Ph2P

CDCl3 rt or 0 ºC

R1

Pd

NtBu OTf

CO2Me 64 - 93%

R22SiH2 = PhSiH3, Ph2SiH2, iPr2SiH2, PhMeSiH2

Pd-cat

Scheme 20 Hydrosilylation of alkynoates

H HPh2Si H 92%

R1 = H Pd-cat (5 mol%) CDCl3 rt

R1 = Et Pd-cat (5 mol%)

R1 + Ph2SiH2

CDCl3 rt

Et H

SiPh2H 81%

Scheme 21 Regio- and stereoselective hydrosilylation of enynes

alkynes such as nPrCCnPr, PhCCPh, and PhCCMe. On the other hand, Ni complex bearing the same ligands was active for the hydrosilylation of inactivated internal alkynes. Polymer-phosphine ligands containing dppe structures (POL-dppe) were synthesized by radical polymerization of dppe having vinyl groups on the aromatic substituents. Heterogeneous Pd catalysts prepared from POL-dppe and PdCl2(PPh3)2 showed good catalytic activity and high stereoselectivity for the hydrosilylation of internal alkynes with Ph2SiH2, PhMe2SiH, Ph2MeSiH, and Ph3SiH [255]. Heterogeneous Pd NPs catalyst supported on N,O-doped hierarchical porous carbon, which was prepared from bamboo shoots, is active for the hydrosilylation of alkynes in the presence of tetrabutylammonium iodide (TBAI) [256]. In the absence of TBAI, hydrogenation took place instead of hydrosilylation. In the case of terminal alkynes, high β-(E) selectivity (>90% in most cases) was observed. Heterogeneous bimetallic NPs catalysts, Pd1Cu2@SiO2, showed high β-(E) selectivity for the hydrosilylation of terminal alkynes with Et3SiH with 0.4 mol% Pd [257]. Other combination of metals, such as Pd1Nix (x = 1–6), Pd1Co2, and Pd1Fe2, showed similar catalytic activity with lower selectivity. Pd1Cu2@SiO2 catalyst can be recycled at least 5 times without loss of catalytic performance. Bimetallic NPs catalyst, Pd1Au5@Nb2O5, was also reported to be active for the hydrosilylation of internal alkynes with Et3SiH and (EtO)3SiH [258]. Mechanism of the Pd-Au system was studied by DFT calculations, showing that the reaction follows Chalk–Harrod mechanism and Pd is the active center of the catalysis [259]. Xantphos-polymer ligand (POL-Xantphos, Fig. 38) was prepared by the radical polymerization of vinyl-substituted Xantphos. Ni(acac)2/POL-Xantphos system showed higher catalytic performance (better yield and selectivity) than Ni(acac)2/

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

51

Fig. 38 Structure of POL-Xantphos O P

P

POL-Xantphos

Table 6 Effects of phosphine ligands in the nickel-catalyzed hydrosilylation of 1-decyne with Ph2SiH2

β:α >20:1 >20:1 >20:1 5:1 22:1

Ligand PPh3 dppe dppb Xantphos POL-Xantphos

nPr

Ph

E:Z 1:2 1:3 1:2 >20:1 >20:1

Ph

HO

SiHPh2 96%

95%

Yield (%) 8 99:1

1,2:1,4c >99:1 – >99:1

Karstedt’s OCC-Pt

99:1

5:95 >99:1

Karstedt’s

Complex mixture

–

–

a

Yield of 1,2-addition product b Ratio of mono- and di-addition products c Ratio of 1,2- and 1,4-addition products

anionic bis(silyl)Pt(II) A is formed from the catalyst precursor via the reaction with MeMgCl and (EtO)3SiH. Subsequent oxidative addition of (EtO)3SiH generates Pt (IV) species B, which is converted to η2-butadiene-coordinated Pt(IV) intermediate C via phosphine dissociation. Regioselective hydroplatination forms intermediate D, from which reductive elimination takes place to form 3-butenylsilane and A. Since coordinatively saturated intermediate C does not allow η4-coordination of butadiene, π-allylplatinum species, which leads to 1,4-addition, cannot be formed. Selective 1,2-hydrosilylation of 1,3-dienes was also achieved by organic cage compound-single atom Pt catalyst (OCC-Pt, Fig. 13) [118]. Steric effect of the organic cage allowed selective 1,2-hydrosilylation at the least sterically congested C=C double bond. Table 7 shows the results with OCC-Pt and Karstedt’s catalyst, showing significant difference between OCC-Pt and Karstedt’s catalyst. Hydrosilylation of various 1,3-dienes with Ar3SiH and Ph2SiH2 was studied using Pd(PR3)(diallyl ether) as catalysts, showing unique regiodivergence depending on the phosphine ligand and hydrosilanes. Representative examples are shown in Scheme 25 [268]. Ph3SiH with PhMe3-Pd catalyst afforded (E)-4silylpent-2-enoate exclusively, while (Z )-2-silylpent-3-enoate was selectively obtained from (4-CF3C6H4)3SiH with PPh3-Pd catalyst. Ni(acac)2/NaEt3BH (0.5 mol%) system was used for the hydrosilylation of 1,3-dienes with tertiary and secondary hydrosilanes, giving 1,4-addition products with good regioselectivity [269]. Unique hydrosilylation/cyclization reaction of 1,n-dienes with tertiary hydrosilanes was catalyzed by cationic 1,10-phenanthroline Pd complexes (Scheme 26) [270]. A proposed mechanism is shown in Fig. 41. A cationic Pd-Me species

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

CO2Me + Ar3Si-H

Pd cat-Me or Pd cat-Ph

55

SiAr3

SiAr3 + CO2Me

C6D6, 30 ºC

CO2Me

Ph3SiH Pd-cat-Me (5 mol%)

99% yield

(100

:

0)

O

Ph3SiH Pd-cat-Ph (10 mol%)

60% yield

(28

:

72 )

Pd cat-Me: R = Me Pd cat-Ph: R = Ph

(4-CF3C6H4)3SiH Pd-cat-Ph (10 mol%)

97% yield

(0

:

100)

R3P

Pd

Scheme 25 Pd-catalyzed hydrosilylation of methyl (E)-penta-2,4-dienoate

Pd-cat (2.5 mol%) R1 R1

n + R2

3SiH

NaBArf4 (3 mol%) DCM 40 ºC

N

R1

Cl Pd

R1 n

SiR23

N

Me

n = 1-3 Pd-cat

Scheme 26 Chain-walking hydrosilylation/cyclization reaction of 1,n-dienes

generated from (ligand)Pd(Cl)Me and NaBArf4 reacts with R3SiH to form [Pd]-SiR3 species, which reacts with a diene substrate to form alkene-coordinated species A. Migratory insertion of the C=C double bond to the Pd-Si bond gives intermediate B. Chain-walking takes place between intermediates B and C through repeated βhydrogen elimination/migratory insertion. Migratory insertion of the C=C double into the Pd-C bond in C forms bicyclic intermediate D. Deuterium-labeling experiments suggested that further chain-walking takes place between intermediates D and E, from which the final product is formed by the reaction with R3SiH. Regiodivergent hydrosilylation/cyclization reaction of 1,6-enynes was attained by Ni complex catalysts (Scheme 27) [271]. Ni(cod)2 alone catalyzed the reaction to give alkene-silylated cyclization products P1, while Ni(cod)2 in combination with a NHC ligand afforded alkyne-silylated cyclization products P2 selectively. Regioselective hydrosilylation/cyclization of 1,7-enyne amides with Ph3SiH took place in the presence of Ni/bipyridine catalyst, giving quinolinone products in good to high yields (Scheme 28) [272]. Regio- and stereoselective mono- and bis-hydrosilylation of symmetrical 1,3-diynes with tertiary hydrosilanes was reported by using Pt catalysts [273]. Representative examples are shown in Scheme 29. Under the suitable reaction

56

S. Shimada

R1 R1 [Pd] SiR3

[Pd] SiR3

R1

Product

R1

A

R3Si-H [Pd]

R1

[Pd]

R1 SiR23

R1

R1

E

SiR3 B chain walking

chain walking

[Pd]

R1

[Pd] R1

SiR23

R1

R1

D

SiR3 C

Fig. 41 Proposed mechanism for the chain-walking hydrosilylation/cyclization reaction of 1, n-dienes R Ni(cod)2 R

DME, 80 ºC

X

[Si] P1

H

Ni(cod)2/L1 CsOAc

N

N Ar Cl

R

+ [Si]

Ar

[Si]

L1 Ar = 2,6-iPr2C6H3

MeCN, 80 ºC [Si]-H = Ph2SiH2, PhMe2SiH, PhMeSiH, etc.

P2

Scheme 27 Ni-catalyzed regiodivergent hydrosilylation/cyclization reaction of 1,6-enynes

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

R3

R4

R1

+ Ph3SiH

O R2

R3

NiBr2 (10 mol%) Ligand (10 mol%) Zn (2 equiv)

N

57

N

O

R1

R4

THF, 40 ºC

SiPh3

Ligand = 6,6’-dimethyl-2,2’-bipyridine

R2

Scheme 28 Regioselective hydrosilylation/cyclization of 1,7-enyne amides with Ph3SiH R

R +

toluene or xylene

Et3SiH

R

SiEt3

Pt cat

SiEt3

+ R

SiEt3 R

R

R = Me, Pt(PPh3)4 (1 mol%), diyne:SiH = 1:1, RT:

:

0)

R = Me, Karstedt (0.04 mol%), diyne:SiH = 1:2, 100 ºC: 99% conv. (0

99% conv. ( 99

:

99)

R = Ph, PtO2 (2 mol%), diyne:SiH = 1:1, 40 ºC:

:

0)

:

93)

99% conv. ( 99

R = Ph, Karstedt (0.04 mol%), diyne:SiH = 1:2, 140 ºC: 99% conv. (0

Scheme 29 Regio- and stereoselective mono- and bis-hydrosilylation of symmetrical 1,3-diynes

R1

R2

+

R3R4SiH2

SiHR3R4

Ni(acac)2 (2 mol%) POL-Xantphos THF, RT or 70 ºC

R1

R2

SiH2Ph SiH2Ph

SiHPh2

SiHPh2 SiMe3

Ph

SiMe3 85%

Ph

nHex

tBu 75%

S

78%

SiMe3 80%

Scheme 30 Regio- and stereoselective hydrosilylation of unsymmetrical 1,3-diynes

conditions, Pt(PPh3)4 and PtO2 provided mono-hydrosilylation products exclusively, while more active Karstedt’s catalyst provided bis-hydrosilylation products with high selectivity. Ni(acac)2 (2 mol%) in combination with POL-Xantphos ligand (Fig. 38) achieved highly regio- and stereoselective hydrosilylation of unsymmetrical 1,3-diynes with primary and secondary hydrosilanes [274]. Representative examples are shown in Scheme 30. POL-Xantphos ligand can be reused by centrifugation at least 5 times without any loss of performance, though Ni(acac)2 (2 mol%) is needed to add in each time. Hydrosilylation of allenes can provide various allylsilane and vinylsilane products. For example, 6 products can be formed from unsymmetrically substituted terminal allenes; addition at C1-C2 position can form E/Z-isomers of allylsilanes

58

S. Shimada R1 R1

3

2

•

1

+ [Si]-H

cat

R2

R1 [Si]

R2

[Si]

R2

[Si]

R1

[Si]

R1

R2

R2

linearallylsilane

R1 [Si] R2

R1 R2 [Si]

internalvinylsilane

terminalvinylsilane

branchedallylsilane

Scheme 31 Hydrosilylation of allenes Table 8 Hydrosilylation of cyclohexylallene with Et3SiH catalyzed by Pd or Ni/NHC catalyst systems cat •

Cy

+

SiEt3

SiEt3

Et3SiH

+

Cy

THF, rt

Cy A B cat: Ni(cod)2 (10 mol%) or Pd2(dba)3 (5 mol%), L1–L4 (10 mol%), KOtBu (10 mol%) Ph

Ph –

Ar1

+ Cl N Ar1

N L1

–

Ar2

+ Cl N Ar2

N

Ar2

– + BF4 N Ar2

N

L2

Ar3

N Ar3

N L4

L3

Ar1 = Mes, Ar2 = 2,6-iPr2C6H3, Ar3 = 2,6-(Ph2CH)2-4-MeOC6H2

Pd or Ni Pd Ni Pd Ni Ni Pda a

Ligand L1 L1 L2 L2 L3 L4

Yield (%) 80 22 75 58 84 90

A:B 2:>98 33:67 12:88 85:15 >98:2 >98:2

The reaction was conducted in the absence of KOtBu

(linear-allylsilanes) and vinylsilanes (internal-vinylsilanes), while addition at C2-C3 position can form one vinylsilane (terminal-vinylsilane) and one allylsilane (branched-allylsilane) (Scheme 31). High level of regio- and stereo-control is required to make the reaction useful. Although study on the hydrosilylation of allenes was very rare [275, 276], significant advances have been attained during the last decade. Hydrosilylation of monosubstituted terminal allenes catalyzed by NHC-Pd or NHC-Ni complex proceeds at internal C=C double bond selectively. Regioselectivity of the reaction highly depends on the metal as well as ligand bulkiness [277, 278]. As shown in Table 8, Pd catalyst with less bulky NHC ligand L1 afforded branched allylsilane B highly selectively. On the other hand, Ni

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts [Si]

59

R

R

[Si]

ML

A

H

[Si]

B

[Si]

H R

R INT-2 H

H

M

L

[Si]

[Si]

ML

M

INT-3 L

allene R

R •

[Si] H

[Si]

R

M TS1

L

H • ML

•

H

[Si]

M L

TS2

INT-1

Fig. 42 Proposed mechanism for the regioselective hydrosilylation of monosubstituted terminal allenes

catalysts tend to produce vinylsilane A; with bulkier NHC ligand L3, vinylsilane A was obtained with >98% selectivity. Even with Pd catalyst, very bulky ligand L4 almost exclusively afforded vinylsilane A. These catalyst systems were successfully applied to the hydrosilylation of several monosubstituted allenes with tertiary hydrosilanes. The mechanism of the selectivity difference was explained by Chalk–Harrod and modified Chalk–Harrod mechanism, which was later supported by DFT calculations, although oxidative addition of Si-H to the metal and subsequent insertion of allenes proceed concertedly (Fig. 42) [279]. Coordination of a hydrosilane and an allene forms INT-1. In the case of nickel catalyst with a bulkier NHC ligand, silylated-π-allylmetal intermediate INT-2 is formed by concerted oxidative addition-silylmetallation through TS1, and subsequent reductive elimination gives vinylsilane product A. In the case of palladium catalyst with a less bulky NHC ligand, concerted oxidative addition-hydrometallation through TS2 preferably takes place to form intermediate INT-3, from which branched-allylsilane product B is formed. The same Pd and Ni NHC catalyst systems were also applied to the hydrosilylation of 1,3-disubstituted allenes, showing regioselectivity trend similar to that observed for monosubstituted allenes [280]. As shown in Table 9, Pd/L1 or L2 system afforded branched-allylsilane product B, while Ni/L4 system exclusively gave vinylsilane A. In this case, not only regioisomers but also stereoisomers can be

60

S. Shimada

Table 9 Hydrosilylation of pentadeca-7,8-diene with BnMe2SiH catalyzed by Pd or Ni/NHC catalyst systems R

•

+

R

R

THF, rt

Pd or Nia Pd Pd Ni Ni a

Liganda L1 L2 L2 L4

+

R

[Si] = BnMe2Si

R = nHex

[Si]

[Si]

cat

[Si]-H

R

A

Yield (%) 80 75 84 89

R B

E:Z for A – – 2:>98 2:>98

A:B 2:>98 2:>98 60:40 >98:2

E:Z for B 75:25 >98:2 75:25 –

See Table 8 for catalyst precursors and ligands

Cy

•

nHex

+ BnMe2SiH

Ni(cod)2 (10 mol%) L4 (10 mol%) THF, rt

BnMe2Si Cy

nHex

BnMe2Si +

Cy

nHex

main product Z:E = >98:2, >98/2 regioselectivity

Scheme 32 Ni-catalyzed hydrosilylation of unsymmetrically disubstituted allene

formed. Pd/L2 and Ni/L4 systems respectively showed very high E and Z selectivities. Unsymmetrically disubstituted allene was highly regio- and stereoselectively hydrosilylated with BnMe2SiH by Ni/L4 system (Scheme 32, see Table 8 for the structure of L4) [280]. The origins of these high regio- and stereoselectivities were theoretically investigated by DFT calculations [281]. Cationic Pd complex bearing iminophosphine ligand (1 mol%; see Scheme 20 for the structure of the Pd complex) efficiently catalyzed hydrosilylation of cyclohexylallene and 1,1-dimethylallene [282]. Primary hydrosilane, PhSiH3, and secondary hydrosilanes such as Ph2SiH2, PhMeSiH2, and iPr2SiH2 selectively afforded branched-allylsilane products, while tertiary hydrosilanes such as Et3SiH, nPr3SiH, PhMe2SiH, Ph2MeSiH, and 1,4-(HMe2Si)2C6H4 exclusively gave terminal-vinylsilane products. Heterogeneous Pd catalyst made from microporous polymer bipyridine ligand (Fig. 43, left), which contains alkyne backbones, and Pd(PPh3)2Cl2 (CMP-Pd) efficiently catalyzed the hydrosilylation of various monosubstituted allenes with PhSiH3 and Ph2SiH2 to give branched allylsilane products highly selectively [283]. The catalyst is also effective for the hydrosilylation of internal disubstituted and trisubstituted allenes. CMP-Pd (0.02 mol% Pd) can be recycled at least 5 times without any loss of catalytic performance. The same authors reported that Pd (PPh3)2Cl2/phenylethynyl-substituted bithiophene ligand (Fig. 43, right; 0.05 mol %/10 mol%) system selectively catalyzed the hydrosilylation of monosubstituted allenes and 1-methyl-1-phenylallene with PhSiH3 and Ph2SiH2 to give branched allylsilane products [284].

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

61

N N N

N

S

S

Ph

Ph

N N

Fig. 43 Structures of microporous polymer bipyridine ligand (left) and bithiophene ligand (right) Fig. 44 Structure of porous triarylphosphine polymer ligands

X

X

P

X POL-Ar3

A similar branched-allylsilane selective hydrosilylation of monosubstituted allenes with PhSiH3 was reported by Pd(acac)2/Xantphos system (2 mol% Pd) [285] and Ni(acac)2/Xantphos system (1 mol% Ni) [286]. Highly active heterogeneous Pd catalyst for allene hydrosilylation was prepared by using porous triarylphosphine polymer ligands (POL-Ar3, Fig. 44) [287]. Among Pd catalysts prepared by the reaction of POL-Ar3 with PdCl2(PPh3)2, m-F-substituted one showed the highest performance. The catalyst afforded branched-allylsilane products selectively from monosubstituted allenes with Ph2SiH2 in the presence of Cs2CO3 (0.1 equiv.). Even with 1 ppm Pd, the catalyst was effective, and 770,000 TON was attained. The catalyst can be reused at least 10 times without any loss of catalytic performance. Heterogeneous Pd-Au alloy catalyst Pd1Au5@Al2O3 was examined for the hydrosilylation of monosubstituted and disubstituted allenes with Et3SiH, giving terminal- and internal-vinylsilane products as a mixture of regio- and stereoisomers [288]. Ni-catalyzed enantioselective hydrosilylation of 1,1-disubstituted allenes with PhSiH3 was reported very recently [289]. Various functionalized-aryl methyl allenes were converted to chiral allylsilanes with high regioselectivity and up to 98% ee by NiBr2•DME (5 mol%)/chiral spirophosphite ligand (6 mol%) catalyst system

62

S. Shimada SiH2Ph

Ni/Ligand Ar

+

•

PhSiH3 DCE, 30 ºC

O

Ar

O P O

Si

O

O P O

Ligand

Scheme 33 Asymmetric hydrosilylation of 1,1-disubstituted allenes OCC-Pt (0.06 mol%) •

+

PhMe2SiH THF, RT, 24 h

SiMe2Ph 94% yield >99% selectivity

Scheme 34 Hydrosilylation of 1,1-dimethylallene catalyzed by OCC-Pt

(Scheme 33). Not only PhSiH3 but also various primary hydrosilanes can be used, while secondary and tertiary hydrosilanes are inactive. Steric effect of organic cage compound-single atom Pt catalyst (OCC-Pt, Fig. 13) allowed selective hydrosilylation of 1,1-dimethylallene with PhMe2SiH at less hindered position, giving a linear allylsilane product exclusively (Scheme 34) [118].

4 Hydrosilylation of Carbon-Heteroatom Multiple Bonds and Epoxides Although platinum catalysts have been widely used for the hydrosilylation of alkenes and alkynes, the first report on carbonyl hydrosilylation was made using rhodium catalyst, Rh(PPh3)3Cl, by Ojima [290, 291]. Studies of platinum catalysts for the hydrosilylation are mostly for C-C multiple bonds because of its practical importance. Research on the hydrosilylation of carbon-heteroatom multiple bonds with platinum catalysts is relatively limited. On the other hand, recent interest in non-precious metal catalysts for the hydrosilylation reaction increased the usage of nickel catalysts for the hydrosilylation of carbon-heteroatom multiple bonds. Hydrosilylation of epoxides is also included in this section.

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts

4.1

63

Hydrosilylation of Aldehydes, Ketones, Esters, Amides, Imines, and Epoxides

P(O)C(O)P-, P(N)C(N)P-, and P,B,P-pincer Pt complexes were used for the carbonyl hydrosilylation reaction. Aldehydes and ketones were efficiently hydrosilylated with PhSiH3 by the P(O)C(O)P-pincer-Pt-H complexes (Fig. 45, left; 0.05–0.4 mol%) to give corresponding alcohols in good to high yields after hydrolysis [292]. P(N)C(N)P-Pt-Cl complex (Fig. 45, center) was reported to be more efficient than P(O)C(O)P-pincer-Pt-Cl for the hydrosilylation of aldehydes and ketones with PhSiH3 [293]. P,B,P-pincer-Pt-SR complexes (Fig. 45, right) were also reported for aldehydes and ketones hydrosilylation with PhSiH3 or PMHS, being more efficient than P(O)C(O)P-pincer-Pt-SR complexes [294]. Hydrosilylation with PMHS required much longer reaction time than that with PhSiH3, though product yields were comparable. Pt(PPh3)3 and silylene-bridged trinuclear Pt complex [Pt (PMe3)(μ-SiPh2)]3 were also active as catalysts for the hydrosilylation of aromatic aldehydes and acetophenone with Ph2SiH2 [295]. Unique Pd-catalyzed hydrosilylation/aminomethylation of isatin derivatives was reported [296]. Isatin derivatives were reacted with Et3SiH in DMF with Pd(OAc)2 as a catalyst to give products whose C=O was converted to (Me2NCH2)C-O(SiEt3) moiety (Scheme 35). Not only DMF but also N-formylmorpholine can be used. Plausible reaction mechanism is shown in Fig. 46. DMF was hydrosilylated to give iminium intermediate A, while isatin was hydrosilylated to give intermediate B. Nucleophilic addition reaction of B to A forms the observed product. Various NHC-Ni complexes were studied for the hydrosilylation of carbonyl compounds. A cationic Ni complex bearing a pyridyltriazolylidene ligand was used HN

O PR2

PtBu2

PtBu2

N Pt Cl

Pt X

B Pt SR N

HN

O PR2 P(O)C(O)P-pincer-Pt R = iPr, tBu X = H, Cl, SH, SPh

PtBu2

PtBu2

P,B,P-pincer-Pt R = H, Ph

P(N)C(N)P-pincer-Pt

Fig. 45 Structures of pincer-Pt complexes

O R2

O N R1

+

Et3SiH

NMe2

Et3SiO

DMF Pd(OAc)2 (5 mol%) R2 70 ºC

O N R1

Scheme 35 Pd-catalyzed hydrosilylation/aminomethylation reaction of isatin derivatives

64

S. Shimada O H

OSiEt3 H

NMe2

NMe2

H

[Et3SiO]–[CH2=NMe2]+

A Et3SiH

Et3SiO

Et3SiH

Pd(OAc)2

H

Pd0

SiEt3 PdII

NMe2

O N R

H OSiEt 3

O O

OSiEt3

O

N R

OH

N R

N R

B

Fig. 46 Proposed mechanism for the Pd-catalyzed hydrosilylation/aminomethylation reaction of isatin derivatives

R1 Ni

Ni

nBu N

N

N

R2S

OTf

PF6

N

N N Me

R1 = Mes or Bn, R2 = tBu or Ph

Fig. 47 Cationic NHC Ni complexes

O

O

N

N Mes

Ni N N

N N

N RR N N Me Me R = Ph, Bu

N

Ni

N

X Ph2P

PPh2

Ni PhS

SPh

X = Br, PF6

Fig. 48 Structures of Ni complexes used for hydrosilylation of carbonyl compounds

for the hydrosilylation of aromatic aldehydes (Fig. 47, left) [297]. PhSiH3 reacted efficiently to give alcohols after hydrolysis, while Ph2SiH2 and Et3SiH were much less efficient. Up to 23,000 h-1 TOF (at 50% conversion) was attained. Cationic Ni complexes bearing sulfide-substituted NHC ligands (Fig. 47, right) [298] and Ni complexes bearing phenoxy-triazolylidene ligands (Fig. 48, left) [299] were also active catalyst for the hydrosilylation of aromatic aldehydes with PhSiH3. Various

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts HN PtBu2 N N Ni H

O PtBu2

PtBu2

Ni SR

B Ni H N

N PtBu2

65

PtBu2

O PtBu2

Fig. 49 Structures of pincer-Ni complexes used for hydrosilylation of carbonyl compounds

N

PtBu2

N

PtBu2 INT-1

H Ph Ph Si H O PR2 Ph

B Ni

B Ni O N

PR2

Ph

N

TS-1

Fig. 50 Structures of the intermediate INT-1 and the transition state TS-1 for the hydrosilylation of benzaldehyde catalyzed by the P,B,P-pincer Ni-H complex

aromatic and aliphatic aldehydes and ketones were hydrosilylated with PhSiH3 in the presence of a cationic Ni complex bearing a pyridyl-substituted NHC ligand (Fig. 48, center) [300] and Ni benzenethiolato complex bearing a bis(phosphine)NHC ligand (Fig. 48, right) [301]. A nickel hydride complex bearing a P,N,P-pincer ligand (Fig. 49, light) was used for the hydrosilylation of aldehydes [302]. Various aldehydes were converted to the corresponding alcohols after hydrolysis with 0.2 mol% Ni catalyst and 0.4 equiv. PhSiH3. P,B,P-pincer ligated Ni-H complex (Fig. 49, center) is active for the hydrosilylation of aldehydes and ketones with Ph2SiH2 [303]. Based on the experimental and theoretical studies, the hydrosilylation of benzaldehyde was supposed to proceed through 1) insertion of the C=O double bond into the Ni-H bond to form PhCH2O-Ni intermediate INT-1 and 2) subsequent σ-bond metathesis with Ph2SiH2 via the transition state TS-1, giving the product, PhCH2OSiHPh2, and regenerating the Ni-H species (Fig. 50). Ni-SR (R = H, Bn) complexes bearing a P,C,P-pincer ligand (Fig. 49, right) are also active for the hydrosilylation of various aldehydes with PhSiH3 with 0.2 mol% catalyst loading [304]. It was confirmed that the stoichiometric reaction of the Ni-SH complex with PhSiH3 forms the corresponding Ni-H complex, which was reported to catalyze the hydrosilylation reaction of aldehydes [305]. A nickel complex bearing a phosphine-substituted α-diimine ligand (Fig. 25, right) is active for the hydrosilylation of aldehydes (0.1 mol% catalyst) and ketones (1 mol% catalyst) with PhSiH3 [306, 307]. A cationic π-allylnickel(II) complex bearing a iminophosphine ligand (2 mol%, Fig. 51, left) catalyzed the hydrosilylation of various ketones and benzaldehyde with Ph2SiH2 at room temperature [254]. Imine-bridged dinuclear nickel complex (Fig. 51, right) is an efficient catalyst for the hydrosilylation of various aldehydes with Ph2SiH2 at 45°C with 0.6 mol% loading [308]. With the same catalyst, selective 1,2-addition took place for

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Fig. 51 Cationic πallylnickel(II) complex bearing a iminophosphine ligand (left) and iminebridged dinuclear nickel complex (right)

F

Ph2P

Ni

NtBu

OTf

Me

F

N

Me3P

Ni

Ni N

F

Me PMe3

F Ar1

Ar2

Ar1

PhCHO N

N

NiII

Me3P

Me

Si Ph

Ph

NiIV

Me3P

H

H

Ar2

PhCH2OSiHPh2

INT-1

Ph2SiH2

Ph2HSi

Me

OCH2Ph

INT-2

Scheme 36 Proposed Ni(II)/Ni(IV) catalytic cycle

conjugated enals giving the corresponding allyl alcohols after hydrolysis. A Ni(II)/ Ni(IV) catalytic cycle was proposed based on experimental observations (Scheme 36). A silane-coordinated mononuclear Ni(II) intermediate INT-1, which was observed by NMR, reacts with PhCHO to form Ni(IV) intermediate INT-2. Reductive elimination of the product from INT-2 and subsequent coordination of Ph2SiH2 regenerate INT-1. During catalysis, the Ni-Me bond is preserved, which was confirmed by NMR. Heterogeneous Ni NPs@graphene catalyst (0.06 mol% Ni) is active for the hydrosilylation of aldehydes with PhMe2SiH, Et3SiH, and Ph2SiH2, giving the corresponding silyl ethers with good to high selectivity at relatively high temperature, 120°C [309]. Co-Ni bimetallic NPs encapsulated in N-doped carbon (CoNi@NC) were prepared by using MOF as a template [310]. The CoNi@NC efficiently catalyzed the hydrosilylation of ketones with tertiary hydrosilanes such as PhMe2SiH, Et3SiH, EtMe2SiH, and Ph2MeSiH, but not Ph3SiH, giving the corresponding silyl ethers at 90°C. The catalyst was reused 6 times with some loss of catalytic activity. Ni3Ga NPs@SiO2 catalyst (4 mol% Ni) is active for the hydrosilylation of aldehydes and ketones with Et3SiH at room temperature [311]. 1,4-Addition took place for the hydrosilylation of cyclohexenone with the same catalyst. Heterogeneous Pt(II) catalyst supported on PMHS-modified graphene (Pt@PMHS-graphene) was prepared by performing the reduction of amides to amines with PMHS-modified graphene as a hydrosilane reductant with chloroplatinic acid (1–5 mol%) as a catalyst [312]. Various amides were reduced

Recent Advances of Group 10 Transition Metal Hydrosilylation Catalysts O

Pt@PMHS-graphene or PtCl2(PPh3)2 (5 mol%)

X

R2

R1

67

O

SiR3 X

THF, 60 ºC, 3 h

+ X = CH or N

R3SiH

R1

+ E-isomer

R2

Scheme 37 Hydrosilylation of chalcones NiCl2 (10 mol%) tBuNC (0.5 equiv)

O + R

n n=1–4

OSiPh3

Ph3SiH xylene, 120 ºC, 12 h

R

n

Scheme 38 Hydrosilylation of unconjugated enones

in high yields by this method. The resulting Pt-containing material (Pt@PMHSgraphene) was not active for the reduction of amides by hydrosilanes. On the other hand, it was found that Pt@PMHS-graphene was active catalyst for the hydrosilylation of chalcones with tertiary hydrosilanes (Scheme 37). Karstedt’s catalyst was not active for this reaction. Various substituted chalcones were hydrosilylated with tertiary hydrosilanes by Pt@PMHS-graphene (5 mol% Pt) to give the corresponding silyl enol ethers (1,4-addition products) in 71–94% yields with 75–99% Z-selectivity. It was also found that PtCl2(PPh3)2 (5 mol%) is more efficient than Pt@PMHS-graphene, giving the products in 81–94% yields with 92 – >99% Z-selectivity (Scheme 37). Stereoselective 1,4-hydrosilylation of enals and enones was also reported using Pd/C [313] and bimetallic NPs catalyst, Pd1Au5@Nb2O5 [258]. Unconjugated enones were selectively converted to the silyl enol ethers by the hydrosilylation with Ph3SiH catalyzed by NiCl2/tBuNC system (Scheme 38) [314]. The reaction was supposed to proceed through 1) in situ generation of Ni0(tBuNC)n, 2) coordination of the enone and Ph3SiH to the Ni0 forming INT-1, 3) migratory insertion of the C=C double bond into the Ni-H to form INT-2, 4) chain-walking by repeated β-hydrogen elimination/migratory insertion and isomerization to generate INT-3, and 5) product formation and regeneration of the Ni0 by the reductive elimination from INT-3 (Scheme 39). Hydrosilylation of esters with (EtO)3SiH to the corresponding alkyl silyl ethers proceeded quantitatively in the presence of 1 mol% of NHC-Pt complex, SIPr-Pt (dvtms), at 100°C [315]. Hydrosilylation of imines was reported using pincer-type Pt complexes and a cationic Pd complex. The P(N)C(N)P-pincer-Pt complex (0.2 mol%, Fig. 45, center) catalyzed the hydrosilylation of N-phenylaldimines and -ketimines with PhSiH3, giving the corresponding secondary N-phenylamines in good to high yields after hydrolysis [293]. The P(O)C(O)P-pincer-Pt complexes (Fig. 45, left, R = tBu,

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R

SiPh3

Ph3Si NiL2 O

H SiPh3 O

NiL2

H

R

n

O

Ni(L2)

n

R

INT-2

INT-1

n INT-3

Scheme 39 Proposed reaction mechanism for the hydrosilylation of unconjugated enones Fig. 52 Structures of Ni complexes (left) and diamide ligands (right) used for the hydrosilylation of epoxides

MeO2C

NHR O

N O

N

Ni

N

O

O MeO2C

F R

H

MeO2C

R

R = iPr, Ph

NHR

R = Ph, nBu diamide ligands

X = SH or R = iPr, X = H, 0.1–2 mol%) are active for the hydrosilylation of Narylaldimines with PhSiH3 at 70°C to give secondary N-arylamines after hydrolysis [316]. The catalytic efficiency highly depended on the substrates and the catalysts, and up to 137 h-1 TOF was observed. A cationic palladium complex ligated by an iminophosphine ligand (5 mol%, Scheme 20) was used for the hydrosilylation of Nallylaldimines and -ketimines with PhSiH3 in CDCl3 at 40°C [317]. Various secondary N-allylamines were obtained in good to high yields after hydrolysis. Hydrosilylation of pyridine and quinoline with PhSiH3 by (bipyridine)NiI-H catalyst was theoretically studied with DFT calculations [318]. Regioselective hydrosilylation of substituted styrene oxides and other arylepoxides with PhSiH3 was catalyzed by N,N,N-pincer nickel fluoride complexes (Fig. 52, left) [319]. The reaction smoothly proceeds at room temperature with 5 mol % Ni catalysts, giving primary alcohol products after hydrolysis. Hydrosilylation of aliphatic epoxides with Ph2SiH2 was catalyzed by Ni(OAc)2/diamide ligand (Fig. 52, right) /NaOtBu/BF3•OEt catalyst system [320]. Terminal epoxides were converted to primary alcohols (after hydrolysis) with good to high regioselectivity. Internal epoxides were also efficiently hydrosilylated.

4.2

Hydrosilylation of CO2

Recent global interests in CO2 increased the number of publications on CO2 hydrosilylation. Pd complexes ligated by unique P,M,P-metallopincer ligands (M = Al, Ga, In) (Fig. 53, left) were synthesized and examined for the hydrosilylation of CO2 with PhMe2SiH, giving a silylformate (Scheme 40) [321]. Among the three complexes, P,Al,P-ligated Pd complex showed high

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Fig. 53 Pd complexes bearing metalloligands

CO2

+

PhMe2SiH

Pd cat Cs pivalate DMF

(1 atm)

O H

OSiMe2Ph

Scheme 40 Hydrosilylation of CO2 with PhMe2SiH

N

N Pt

tBu

N

tBu BAr4

N

tBu

N

PtBu2

O B(C6F5)3

B Ni O N

PtBu2

H

Ar = 3,5-(CF3)2C6H4

Fig. 54 Pt and Ni complexes used for CO2 hydrosilylation

catalytic activity at room temperature with high TOF, 19,300 h-1. Another type of Pd complexes bearing P,M,P-metalloligands containing Zn, Li, and Cu (Fig. 53, right) was also used as catalysts for the same CO2 hydrosilylation. Zn-containing Pd complex showed good catalytic activity [322]. Cationic Pt complex bearing NHC ligands (Fig. 54, left) catalyzed the hydrosilylation of CO2 with nBuSiH3 and Ph2SiH2, giving silylformates HCO2SiH2Bu and HCO2SiHPh2, respectively, though catalytic activity was not very high [323]. Bis(silyl)acetals (R3SiO)2CH2 were selectively obtained by the hydrosilylation of CO2 with Et3SiH, Ph2MeSiH, and PhMe2SiH catalyzed by a nickel complex bearing a P,B,P-pincer ligand (Fig. 54, right) with up to 1,200 TON and 56 h-1 TOF [324]. The mechanism of this reaction was investigated in detail by DFT calculations [325, 326]. Nickel hydride complex ligated by P,N,P-pincer ligand (Scheme 41) efficiently catalyzed the CO2 hydrosilylation with Ph2SiH2 giving MeOH after hydrolysis with up to 4,900 TON [327]. The same catalyst selectively converted secondary and primary amines to the corresponding formamides by the reaction with CO2 and

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S. Shimada

CO2

+ Ph2SiH2

(8.2 atm) R1R2NH +

CO2 (2.7 atm)

+ Ph2SiH2

O

Ni cat MeCN, RT Ni cat

H

HN NR1R2

R1R2NMe

MeCN, 120ºC

PtBu2

N Ni H N

PtBu2

Ni cat

Scheme 41 Amide and amine formation via hydrosilylation of CO2

Ph2SiH2 in MeCN at room temperature (Scheme 41). On the other hand, the same reaction at 120°C afforded methylamines selectively (Scheme 41). Theoretical study on the CO2 hydrosilylation by (bpy)NiI-H was conducted showing that (bpy)NiI-H would be an efficient catalyst and the reaction proceeds via bond metathesis steps with keeping Ni(I) oxidation state [328].

5 Conclusions Intensive research has continuously been conducted on the development of group 10 transition metal hydrosilylation catalysts as summarized in this chapter. Research on heterogeneous hydrosilylation catalysts and Ni catalysts rapidly increased in recent years, and significant progress has been made. On the other hand, Karstedt’s and Speier’s catalysts have still been used in most applications in industry as well as in academia. Recent examples of applications of hydrosilylation reactions are listed below, in which Karstedt’s and Speier’s catalysts were used in most cases. A wide variety of polysiloxane-based materials were prepared by Pt-catalyzed hydrosilylation reaction [329–337], including self-healing poly(dimethylsiloxane) materials [338], asymmetric Janus adhesive tape [339], silicone-aerogels prepared in supercritical-CO2 [340, 341], polysiloxane-matrix-stabilized quantum dots [342– 344], thermally conductive silicone materials [345, 346], Cu-MOF-embedded polysiloxanes [347, 348], silicone materials for dental use [349–352], silicone compositions for wound treatment [353, 354], silicone materials for drug-delivery systems [355, 356], a capsaicin-containing silicone patch [357], silicone rubbers for airbags [358, 359], silicone rubber foams for secondary battery packs [360, 361], polyimide-siloxane composites [362], and polysiloxanes containing biomassderived compounds such as bio-phenol [363], eugenol [364], and cellulose [365]. Modification of Si-H containing polysiloxanes with various functionalized alkenes was studied using Karstedt’s catalyst, Speier’s catalyst, Pt/C, and Wilkinson’s catalyst [366–369]. Karstedt’s catalyst was used for the iterative synthesis of discrete dimethylsiloxane oligomers [370]. Various polycarbosilanes were prepared by the hydrosilylation using Karstedt’s catalyst and used for the preparation of SiC [371–373], SiOC [373, 374], and SiONC [375] ceramics. Thiophene-containing polycarbosilanes [376],

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polycarbosilane-dendrimers [377], ferrocene-rich carbosilane- and siloxane-based dendrons, dendrimers, and dendronized polymers [378], and recyclable polycarbosilane from a biomass-derived bifuran-based monomers [379] were prepared by the hydrosilylation reaction using Karstedt’s catalyst. Speier’s catalyst was used for the preparation of σ–π-conjugated aluminum-containing polyphenylcarbosilanes [380] and 5-(dimethylsilyl)pentylalkylferrocene-grafted hydroxy-terminated polybutadiene [381]. Various POSS modifications were reported by the hydrosilylation reaction of Si-H containing POSS with alkenes [382–384], alkynes [385–387], dienes [388], and 1,3-diynes [389] and vinyl-substituted POSS [390–392] or ethynyl-substituted POSS [393] with hydrosilanes. Hydrosilylation polymerization of divinylsubstituted POSS with HMe2SiOSiMe2OSiMe2H by Karstedt’s catalyst was also reported [394]. Photo-activatable catalysts such as Pt(acac)2 and (MeCp)PtMe3 were used for UV curing of silicone materials [395]. Such UV curing are useful for 3D printing, and various ink compositions are reported [396–399]. Thermal curing is also used for 3D printing [400–403]. Organosilica membranes [404, 405], POSS-polysiloxane membranes [406], and SiOC membranes from organosilicas [407, 408] for gas separation were prepared using hydrosilylation reaction catalyzed by Speier’s catalyst. Copolymers with disiloxane-polyimide-alternating structure were prepared by the hydrosilylation reaction catalyzed by Pt(cod)Cl2 and tested as a membrane for O2 separation [409]. PDMS pervaporation membranes were prepared by UV-curing system [410]. A series of liquid crystalline disiloxane-substituted 4-cyanobiphenyls were prepared by the flow reactor system using Karstedt-type heterogeneous Pt@vinylmodified SiO2 catalyst [411]. Karstedt’s catalyst was used for the synthesis of photo-responsive liquid crystalline siloxane polymers [412]. Composite materials of liquid crystal-silicone elastomer/Au or amino-functionalized graphene oxide NPs were synthesized by the hydrosilylation reaction catalyzed by Pt(cod)Cl2 [413]. Karstedt’s catalyst was used for the synthesis of densely grafted bottlebrush polymers [414], terminally silylated polypropylene-Si(OEt)3, which was used for the surface modification of SiO2 NPs [415], and highly aromatic bisphenol-A-based elastomers [416]. Hydrosilylation of polybutadiene was studied using Karstedt’s, Speier’s, and some Rh catalysts [417]. Tridentate Si Lewis acids [418], trisiloxane-chain-substituted biphenylcarbazole liquid materials [419], spiropyran-functionalized polystyrenes [420], silole amino acids with aggregation-induced emission property [421], and α,ω-bis(benzoazine) compounds (from alkenyl-modified benzoazines and TMDS or 1,4-bis(HMe2Si) benzene) [422] were synthesized by the hydrosilylation reaction catalyzed by Karstedt’s or Karstedt-type catalyst. Site and regioselective hydrosilylation of botryococcene, an algal biomass-triterpene having six C-C double bonds, with tertiary hydrosilanes took place with Karstedt’s catalyst [423]. Azide-substituted organotrialkoxysilanes were synthesized by the hydrosilylation with Karstedt’s catalyst, immobilized on SiO2, and used for copper-catalyzed alkyne-azide cycloaddition reaction [424].

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Permethylated β-cyclodextrin-chiral stationary phase for HPLC was prepared by the hydrosilylation of allylcarbamido-modified β-cyclodextrin with (EtO)3SiH catalyzed by Pt(PPh3)4 and subsequent immobilization on SiO2 [425]. Cyclodextrincontaining silicone gels useful for cosmetics and personal care products were also synthesized by the Pt-catalyzed hydrosilylation reaction [426]. As summarized in this chapter, significant advances were made in recent years for group 10 transition metal hydrosilylation catalysts. Although practical application of these new catalysts is still very limited, not only the new homogeneous Pt catalysts but also heterogeneous and Ni hydrosilylation catalysts would play important roles in practical applications in the near future.

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Top Organomet Chem (2023) 72: 95–140 https://doi.org/10.1007/3418_2023_100 # The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 Published online: 4 October 2023

State of the Art in Rhodium- and Iridium-Catalyzed Hydrosilylation Reactions Manuel Iglesias, Francisco J. Fernández-Alvarez, and Luis A. Oro

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Hydrosilylation of C–C Multiple Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Selectivity Issues on the Hydrosilylation of Terminal and Internal Alkynes . . . . . . . . 2.2 Hydrosilylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Selectivity Issues on the Hydrosilylation of Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Hydrosilylation of Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Mechanistic Considerations on the Hydrosilylation of C–C Multiple Bonds . . . . . . . 3 Hydrosilylation of C=O Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Hydrosilylation of Ketones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Hydrosilylation of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Hydrosilylation of Amides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Hydrosilylation of C–N Multiple Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract This chapter reviews the state of the art of rhodium- and iridium-catalyzed hydrosilylation reactions, demonstrating the utility of rhodium and iridium catalysts for organic synthesis. Nowadays, this field has solid and proven mechanistic foundations that allow the design, development, and optimization of new catalytic systems. As this chemistry will undoubtedly continue to make relevant progress, it deserves constant attention. Keywords Homogenous catalysis · Hydrosilylation · Iridium · Rhodium

M. Iglesias (✉), F. J. Fernández-Alvarez (✉), and L. A. Oro (✉) Departamento de Química Inorgánica, Facultad de Ciencias, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Universidad de Zaragoza – CSIC, Zaragoza, Spain e-mail: [email protected]; [email protected]; [email protected]

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1 Introduction Hydrosilylation reactions encompass a diverse range of catalytic transformations wherein a Si–H bond is added to an unsaturated functional group. Due to the broad range of applicable substrates, these reactions find utility in various chemical processes, from small-scale laboratory synthesis to large-scale industrial processes [1–5]. Homogeneous catalysts based on rhodium and iridium complexes have played a fundamental role, together with Pt derivatives, in the development of catalytic hydrosilylation processes for more than four decades. However, despite the large number of works published on this topic, as we will see in this chapter, rhodium and iridium still possess significant untapped potential within this area of chemistry. The purpose of this chapter is to provide the readership with an updated overview of the utilization of rhodium and iridium complexes as homogeneous catalysts in selected processes such as hydrosilylation of alkynes, alkenes, ketones, CO2, amides, imines, and nitriles. This chapter focuses on the results published subsequent to 2009, since previous research on this topic has been comprehensively reviewed [5].

2 Hydrosilylation of C–C Multiple Bonds 2.1

Selectivity Issues on the Hydrosilylation of Terminal and Internal Alkynes

The hydrosilylation of alkynes may lead to a mixture of vinylsilanes, the syn- and anti-addition products, with Markovnikov or anti-Markovnikov regioselectivity. Scheme 1 summarizes the possible isomers that result from the hydrosilylation of

Scheme 1 Possible isomers obtained from the hydrosilylation of alkynes

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a generic alkyne. It must be mentioned that, in contrast with terminal alkynes (R1 = H), the Markovnikov or anti-Markovnikov regioselectivity of internal alkynes is arbitrarily chosen in this example. Note that both Markovnikov isomers are identical for terminal alkynes. In the case of terminal alkynes, three possible isomers may be found. The most commonly obtained is the anti-addition product with an anti-Markovnikov regioselectivity, the β-(E)-isomer. The syn-addition leads to the formation of the β-(Z )-isomer, for which selective catalysts are somewhat less frequent. The Markovnikov product (α-isomer) is the most difficult to obtain selectively, although several interesting catalytic systems have been described (vide infra). In the hydrosilylation of internal alkynes, the regioselectivity depends strongly on the nature of the substituents, which makes it difficult to control the selectivity of the reaction. Therefore, examples of selective systems for disubstituted alkynes are scarce.

2.2

Hydrosilylation of Alkynes

The most recent developments on the hydrosilylation of alkynes have primarily focused on the design of novel catalysts, where ligand development has played a key role in the search for more active and selective catalysts. Among these ligands, Nheterocyclic carbenes (NHCs) have garnered significant attention. Complexes of general formula [Rh(Cl)(NBD)(NHC)] (NBD = 2,5norbornadiene), 1a-c (Fig. 1), show that the steric hindrance of the NHC, modified by functionalization with increasingly encumbered wingtip groups, drastically affects the activity of the catalyst—the most active catalyst being the one that features the least hindered metal center (1a). No significant improvement of the selectivity is observed upon modification of the NHC. However, the mixture of the β-(E)-, β-(Z )-, and α-vinylsilanes usually obtained with these catalysts seems to depend on the steric hindrance of the NHC, with the E/Z ratio being higher for the most encumbered metal centers (1c) [6]. A more recent study by Salazar, Suarez et al. has explored the impact of the steric encumbrance of the ligand on the E/α ratio. Complexes 2a-c feature an NHC ligand

Fig. 1 Depiction of catalysts 1a-c

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Fig. 2 Depiction of catalysts 2a-c

Fig. 3 Depiction of catalysts 3a-c and 4a-c

equipped with a hemilabile picolyl group functionalized at the 6-position (Fig. 2). The use of more sterically crowded R groups prevents the formation of the α-vinylsilanes, leading to almost the exclusive formation of the E-isomers for a variety of substrates in the case of 2c [7]. NHC and mesoionic carbene (triazolylidene) Rh complexes that present functionalized sidearms, 3 [8] and 4 [9], respectively, have shown activity in the hydrosilylation of alkynes, also yielding the E-vinylsilane as major product (Fig. 3). However, selectivities somewhat lower than those obtained with 2c have been reported. Catalyst 3b also proved efficient for the hydrosilylation of a broad variety of internal alkynes, showing a good group tolerance. The hydrosilylation of symmetric disubstituted alkynes affords preferentially the E-isomer, with higher temperatures favoring E-selectivity. In contrast, Ir complexes with coumarin-functionalized NHC ligands 5 and 6 (Fig. 4) promote the preferential formation of β-(Z)-vinylsilanes with selectivities up to 88%, and moderate conversions for several substrates [10]. Rh(III) and Ir(III) complexes containing bis-NHC ligands have shown interesting activities and selectivities for a wide range of terminal alkynes. Rh(III) complex

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Fig. 4 Depiction of catalysts 5a-e and 6a-c

Fig. 5 Depiction of catalysts 7–10

7 and Ir(III) complex 8 afford β-(Z)-vinylsilanes in good yields and selectivities [11, 12]. Remarkably, the substitution of the iodido ligands in 7 by one or two trifluoroacetate groups to give rise to complexes 9 and 10 results in a drastic change of the selectivity. The use of Rh complex 9 as catalyst brings about the preferential formation of α-vinylsilanes, with yields up to 76% for this isomer. The use of the bis-trifluoroacetate complex 10 further improves the a-selectivity of these catalysts, giving rise to yields up to 87% (Fig. 5) [13]. The rhodium and iridium bis-NHC complexes 11a-b and 12a-b (Fig. 6), reported by Jiménez, Pérez-Torrente et al., were tested as catalysts for the hydrosilylation of terminal alkynes. Remarkably, 11a leads to excellent selectivities toward the β-(Z)vinylsilane for an assortment of aliphatic and aromatic alkynes, with quantitative formation of this isomer in many cases. In sharp contrast with 11a, the use of 11b and 12a-b as catalysts under analogous conditions results in mixtures of the β-(Z)-, β-(E)-, and α-vinylsilanes [14].

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Fig. 6 Depiction of catalysts 11a-b and 12a-b

Fig. 7 Depiction of catalysts 13 and 14

Bis-NHC ligands have also been used to prepare bimetallic catalysts. Several examples of bimetallic Rh complexes have shown markedly higher activities than their monometallic counterparts. Messerle et al. described a significant improvement of the reaction rate when binuclear complex 13 was used as catalyst, instead of mononuclear 14 (Fig. 7), in the hydrosilylation of diphenylacetylene. In fact, 13 leads to a 68% conversion after 1.5 h compared to a 0% for 14 [15]. Similarly, bimetallic Rh complex 15 allows a 96% conversion of diphenylacetylene into the corresponding E-vinylsilane in 6 min, while its monometallic counterpart, 16, requires 60 min to achieve the same conversion (Fig. 8) [16]. Related bimetallic Rh systems based on bis-NHC scaffolds also proved active for the hydrosilylation of terminal alkynes, giving rise to selectivities that very much depend on the halide ligand [17]. The design of ligands different from NHCs has also resulted in interesting catalytic systems for alkyne hydrosilylation. Young et al. described that the use of Rh complexes featuring phosphino allyl ligands 17a-e (Fig. 9) as catalysts gives rise to good yields for the hydrosilylation of a variety of terminal and internal alkynes with HSiMe(OEt)2. Very high E-selectivities were reported for most substrates [18].

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Fig. 8 Depiction of catalysts 15 and 16

Fig. 9 Depiction of catalysts 17a-e Fig. 10 Depiction of catalyst 18

More recently, related bimetallic rhodium complexes based on phosphinine ligands also proved drastically more active than their related monometallic counterparts in the hydrosilylation of terminal alkynes, with excellent selectivities toward the β-(E)-vinylsilanes [19]. Campos et al. reported that germyl-rhodium complex 19 (Fig. 10) catalyzes the hydrosilylation of phenylacetylene, giving rise to modest yields and a mixture of β-(E)- and α-vinylsilanes. Remarkably, significant amounts of the α-isomer were

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Fig. 11 Depiction of catalysts 19 and 20

Fig. 12 Depiction of catalysts 21a-e

observed in some cases, while formation of the β-(Z )-vinylsilane was not detected [20]. Ir(III) complex 19, reported by Pérez-Torrente, Modrego et al., features the tridentate ligand κ3-hqa, which derives from 8-hydroxyquinoline-2-carboxylic acid (H2hqa). 19 can be converted into 20 by coordination of 1-hexyne followed by migratory insertion reaction into the Ir–H bond (Fig. 11). Both catalysts show good activities for the hydrosilylation of a variety of terminal alkynes, with β-(Z )vinylsilane usually being the major isomer. Nevertheless, substantial quantities of the β-(E)-isomer were obtained in several instances. It is noteworthy that the observed selectivity of the reaction is influenced by β-(Z) to β-(E) isomerization, catalyzed by 19 after consumption of the alkyne [21]. IrCp* (Cp* = pentamethylcyclopentadienyl) scaffolds stabilized by pyridylidene amide (PYA), described by Albrecht et al., result in efficient catalysts for the hydrosilylation of terminal alkynes with excellent selectivities toward the β-(Z)vinylsilane, especially in the case of 21d. This selectivity has been attributed to the presence of the PYA ligand, which prevents the β-(Z ) to β-(E) isomerization [22] (Fig. 12). The functionalization of ligands has facilitated the immobilization of homogeneous catalysts onto various materials, thereby leading to several examples where hybrid systems exhibit compelling attributes such as catalyst recycling and complexsupport synergies.

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Fig. 13 Depiction of catalysts 22 and 23

The NHC complexes 22 and 23 reported by Peris et al. were evaluated as catalysts for the hydrosilylation of terminal alkynes. Their immobilization onto reduced graphene (RGO) oxide generated heterogenized catalysts 22-RGO and 23-RGO. The β-(Z)-vinylsilane was obtained as the major reaction product, but significant amounts of β-(E)- and marginal amounts of α-vinylsilane were also obtained in the hydrosilylation of 1-hexyne and phenylacetylene. Regarding the recyclability of the catalyst, 23-RGO showed a significantly better performance than 22-RGO due to the presence of two anchoring sites (pyrene tags), which prevent the leaching of the catalyst. It is noteworthy that in the case of the hydrosilylation of 1-hexyne using 23-RGO, a rise in selectivity for the β-(Z )-vinylsilane was observed, indicating the existence of a collaborative effect between 23 and the RGO support [23] (Fig. 13). Rh(III)Cp* complex 24, reported by Alvarez, Jiménez et al., which contains cyclometallated triazolylidene MICs (mesoionic carbenes), has shown excellent activities at room temperature, with yields up to 99% and exclusive formation of the β-(Z )-vinylsilane for a range of aliphatic and aromatic terminal alkynes (Fig. 14). A related hybrid material (24-TRGO), based on a triazolylidene-Rh(III)Cp* complex covalently bound to thermally reduced graphene oxide (TRGO), was prepared and tested as catalyst. The hydrosilylation reaction with the heterogeneous catalyst 24-TRGO requires a higher temperature (60 °C) than its homogeneous analogue,

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Fig. 14 Depiction of catalysts 24 and 24-TRGO

Fig. 15 Depiction of catalysts 25 and 25-TRGO

but quantitative yields and complete β-(Z )-selectivity were observed, even after multiple recycling experiments [24]. Further work by the same authors also showed excellent activities with catalyst 25 (Fig. 15). However, although an excellent β-(Z )-selectivity was observed for 1-hexyne, the formation of significant amounts of β-(E)- and α-vinylsilanes was described for tert-butylacetylene and phenylacetylene. Complex 25 proved efficient in the hydrosilylation of internal alkynes, giving rise to excellent Z-selectivities [25]. The related graphene-oxide-supported catalyst 25-TRGO displays similar selectivities to those described for its homogeneous counterpart and can be reused without a loss of activity even under an air atmosphere. In sharp contrast, a related

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Fig. 16 Depiction of catalysts 26–33

hybrid material with the Rh complex anchored via Rh–O bonds to the graphene oxide shows poor recyclability, plausibly due to the labile complex-TRGO linkage. Neutral and cationic Rh(I) and Rh(III) complexes containing NHC ligands decorated with pyridine and thiofuran moieties (Fig. 16) proved active catalysts for the hydrosilylation of terminal alkynes. The best selectivities were obtained for aromatic alkynes with catalysts 30–33, especially in the case of 31, the latter leading to nearly quantitative yields of the β-(Z )-vinylsilane. Remarkably, Buchmaiser et al. reported that the immobilization of catalysts 26–33 on the mesoporous silica material SBA-15 via the trimethoxysilyl moiety resulted in improved β-(Z )-selectivities for all the catalysts, which the authors ascribed to confinement effects. Note that long reaction times lead to isomerization to the β-(E)-vinylsilane [26]. Messerle et al. described that 34 and 34-CB (CB = carbon black) are very active catalysts for the hydrosilylation of diphenylacetylene, providing exclusively the Zisomer. 34-G (G = graphene) is slightly less efficient than 34 and 34-CB. 34-GC

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Fig. 17 Depiction of catalysts 34, 34-GC, 34-CB, and 34G

Fig. 18 Examples of heterobimetallic Rh/Ir catalysts. Depiction of catalysts 35 and 36

(GC = glassy carbon electrode) is the least active of these catalysts, likely due to the low surface area of the support; however, a very high TON was obtained in this case. The hybrid catalysts can be recycled 10 times without significant leaching of the rhodium complex [27] (Fig. 17). Heterobimetallic catalysts that comprise organometallic Rh and Ir complexes supported on carbon black through tethers of different lengths have been described (representative examples are showed in Fig. 18). These catalysts were successfully tested in the hydrosilylation of terminal and internal alkynes. In the case of the former, the Rh-enriched hybrid catalysts promote α-selectivity, while the Ir-enriched analogues favor β-(E)-selectivity. For the latter, only the Rh(III) head-group is active, producing selectively the E-isomer [28]. Commercially available Rh and Ir complexes, with or without the use of a ligand to generate in situ the active species, have also allowed the development of interesting alkyne hydrosilylation reactions. For instance, Song et al. proved that [{Rh (μ-Cl)(CO)2}2] is a proficient catalyst for the hydrosilylation of ynamines, giving rise to excellent β-regioselectivities and anti-stereoselectivities, thus allowing for the selective preparation of β-silyl (Z )-enamides. This selectivity has been ascribed to the O-coordination of the ynamine to the rhodium center [29] (Scheme 2).

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Scheme 2 Hydrosilylation of ynamines catalyzed by [Rh(CO)2Cl]2

Scheme 3 Hydrosilylation of internal alkynes functionalized with directing groups

Scheme 4 Markovnikov hydrosilylation of terminal alkynes featuring a functional group (X) Scheme 5 α,synHydrosilylation of internal thioalkynes catalyzed by [{Ir(μ-Cl)(COD)}2]

Ding et al. showed that the aforementioned directing effect also plays a crucial role in the selectivity of hydrosilylation reactions involving functionalized alkynes. The inclusion of directing groups in unsymmetrical internal alkynes serves to enhance the selectivity of these hydrosilylation reactions, which is typically challenging to control (Scheme 3). Efficient directing groups are alcohol, acetate, benzoate, tetrahydropyran, or oxazolidinone. The use of HSi(TMSO)3 (TMS = trimethylsilyl) has also proved pivotal to achieve a selective process [30]. The same authors also reported that [Ir(μ-Cl)(COD)]2 (COD = 1,5cyclooctadiene) catalyzes the Markovnikov hydrosilylation of functionalized terminal alkynes employing HSi(TMSO)3. The unusual α-selectivity has been explained invoking the coordination ability of the functional groups at the alkyne’s substituent (Scheme 4) [31]. Internal thioalkynes can be hydrosilylated selectively with [{Ir(μ-Cl)(COD)}2] as catalyst using HSi(EtO)3 as silane source, providing excellent α-regioselectivities and syn-stereoselectivities (Scheme 5). The migratory insertion of the alkyne into the

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Scheme 6 Dynamic kinetic asymmetric intramolecular hydrosilylation of alkynes

Ir–H bond has been proposed to control the selectivity of the reaction, with the sulfur atom directing this step [32]. The intramolecular hydrosilylation of alkynes has the potential to yield siliconstereogenic silanes. Therefore, there is a significant interest in the development of enantioselective catalysts that enable the production of organosilanes in enantiopure form. Processes exemplifying this approach have been developed using chiral catalysts synthesized in situ by the addition of suitable chiral ligands and metal precursors. Xu et al. showed that the in situ addition of ligand SiMOS-Phos (Scheme 6) and [Rh(μ-Cl)(COD)]2 catalyzes the dynamic kinetic asymmetric intramolecular hydrosilylation of alkynes, which allows for the preparation of chiral benzosiloles from racemic hydrosilanes in good yields and enantioselectivities [33]. The same authors reported a related reaction that employs bisalkynes as starting materials and the system Ar-BINMOL-Phos/[{Rh(μ-Cl)(COD)}2] as catalyst (Scheme 7), thus obtaining the corresponding benzosiloles in moderate yields and high enantioselectivities [34].

2.3

Selectivity Issues on the Hydrosilylation of Alkenes

The hydrosilylation of alkenes can occur according to a Markovnikov or antiMarkovnikov regioselectivity, which may potentially generate two stereocenters (in the case of internal alkenes). In the case of terminal alkenes (R1/R4 = H),

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Scheme 7 Obtaining of benzosiloles by asymmetric intramolecular hydrosilylation of alkynes

Scheme 8 Products of Markovnikov and anti-Markovnikov addition in the hydrosilylation of alkenes

which is by far the most widely explored version of this reaction, the antiMarkovnikov addition affords a linear silylalkane, while the Markovnikov selectivity yields a branched silylalkane (Scheme 8). The latter always results in the creation of a chiral center. In contrast, the former only produces a chiral center if R2 and R3 are dissimilar non-hydrogen substituents. Moreover, the use of prochiral hydrosilanes results in the formation of a silicon-stereogenic center. Therefore, the development of enantioselective alkene hydrosilylation catalysts is a topic of considerable interest.

2.4

Hydrosilylation of Alkenes

The development of asymmetric processes based on Rh or Ir catalysts for the preparation of enantiopure organosilanes has mainly focused on the use of chiral

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Scheme 9 Enantioselective synthesis of chiral spirosilabiindanes

catalysts generated in situ by addition of chiral phosphines to suitable metal precursors. The regioselectivity of Si–H bond addition may be influenced by the steric constraints imposed by intramolecular reactions. In such cases, the catalytic system is only required to control the enantioselectivity of the process. The asymmetric synthesis of chiral spirosilabiindanes has been accomplished by Wang, Li et al. via intramolecular hydrosilylation of bis(alkenyl)dihydrosilanes, employing [{Rh(μ-Cl) (1,5-hexadiene)}2] as metal precursor. Several chiral bisphosphine ligands were tested, with the best yields and selectivities being obtained with (R,R)-Et-DuPhos. The use of (R)-QuinoxP as chiral ligand allowed for the selective preparation of the corresponding enantiomers (Scheme 9) [35]. The catalytic system [{Rh(μ-Cl)(1,5-hexadiene)}2]/(R)-QuinoxP was also employed for the intramolecular hydrosilylation of dihydrosilanes to obtain silicon-stereogenic monohydrosilanes in excellent enantioselectivities and yields. It is noteworthy that this methodology affords six-membered tetrahydrobenzosiline monohydrosilanes by endo-regioselective intramolecular hydrosilylation with excellent enantioselectivities and yields (Scheme 10) [36]. Chiral tetrasubstituted silicon centers have been synthesized by Naganawa, Nishiyama et al. through the enantioselective desymmetrization of monohydrosilanes using intramolecular hydrosilylation. This process involves the utilization of [{Rh(μ-OMe)(COD)}2] as the metal precursor and a binol-derived phenanthroline as chiral ligand (Scheme 11) [37]. The intramolecular hydrosilylation of alkenes permits the synthesis of 5- or 6-membered oxasilacycles depending on the nature of the ligand. The use of [{Rh (μ-Cl)(rac-BINAP)}2] (BINAP = 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) in

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Scheme 10 Enantioselective synthesis of chiral monohydrosilanes

Scheme 11 Synthesis of chiral silicon centers via desymmetrization of monohydrosilanes

the presence of substoichiometric amounts of norbornene results in the formation of the trans/exo isomer (1,3-trans-oxasilacyclopentanes) in good regio- and diastereoselectivities. Notably, [{Rh(μ-Cl)(dpph)}2] (dpph = 1,2-bis (diphenylphosphino)hexane) catalyzes the 6-endo-trig cyclization reaction, showing excellent regioselectivities toward the 1,4-oxasilacyclohexanes (Scheme 12) [38]. The effect of the presence of substoichiometric amounts of a diene donor ligand instead of nbe was explored by Jeon et al., displaying again good selectivities

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Scheme 12 Synthesis of 5- or 6-membered oxasilacycles

Scheme 13 Generation of Si-stereogenic centers from prochiral secondary silanes

toward 1,3-trans-oxasilacyclopentanes. The authors proposed that the role of nbe or the diene is to act as a hydride shuttle in a modified Chalk-Harrod mechanism via alkene-assisted syn-hydrorhodation, followed by silylrhodation and β-hydride elimination [39]. The intermolecular hydrosilylation of terminal alkenes with prochiral dihydrosilanes leads to the formation of a silicon-stereogenic center. Zang, He et al. reported that the use of [{Rh(μ-Cl)(COD)}2] as metal precursor and (R)MeO-DM-NBiphep as chiral ligand gives rise to good yields (up to 88%) and enantioselectivities up to 80% ee for a variety of alkenes functionalized with ethers and amines (Scheme 13) [40]. The intermolecular hydrosilylation of internal alkenes poses a great challenge due to the difficult control of the regio- and enantioselectivity of the reaction. The inclusion of a directing group on one of the substituents of the olefin is an attractive strategy to enhance the regio- and stereoselectivity of the reaction. Amides are interesting directing groups owing to the ubiquitous presence of chiral amides in APIs (active pharmaceutical ingredients) and their role as building blocks in organic synthesis. Li et al. reported that the hydrosilylation of β,β-substituted enamides with [{Rh(μ-Cl)(COD)}2] in the presence of binaphthyl-based phosphite ligands leads to good yields, complete diastereoselectivity, and ees up to 98% for a variety of

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Scheme 14 Enantioselective hydrosilylation of β,β-substituted enamides

Scheme 15 Enantioselective hydrosilylation of internal alkenes

monohydrosilanes (Scheme 14) [41]. O-coordination of the Rh center to the carbonyl group drives the reaction toward the selective formation of α-aminosilanes. β,γ-Unsaturated amides are more prone to undergo alkene isomerization than their related enamines; consequently, regio- and enantioselectivity are more difficult to control in the case of the former. [Rh(COD)2]BF4 with binaphthyl-based phosphite ligands are able to selectively catalyze this reaction, with yields up to 96% and ees up to 88%. In this case, O-coordination affords enantioenriched β-silylated amides as shown by Li et al. (Scheme 15) [42]. The enantioselective hydrosilylation of 1,1′-disubstituted cyclopropenes has been accomplished employing secondary silanes in the presence of catalytic amounts of [Rh(COD)2]BF4, NaBPh4, and (R)-DTBM-SEGPHOS as chiral ligand (Scheme 16). The silylcyclopropanes were obtained in good yields, diastereoselectivities up to >99:1 d.r. and enantioselectivities up to >99% [43]. It is noteworthy that a diastereodivergent version of this reaction has been reported employing palladium catalysts [44]. Excellent linear selectivities in terminal alkene hydrosilylations have been achieved by Ding et al. and Nakajima, Shimada et al. in the presence of substituents

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Scheme 16 Enantioselective hydrosilylation of cyclopropenes

[45, 46, 47]. Scheme 17 Hydrosilylation of sulfur and oxygen containing alkenes

functionalized with ethers and thioethers. The O- or S-coordination of the substrate to the metal center has been deemed responsible for the low amounts of branched product observed in these reactions, since the reductive elimination step to form the Si–C bond of a branched silane is disfavored in these systems. [{Ir(μ-Cl)(COD)}2] has proved to be an excellent catalyst for these transformations, allowing for the selective formation of a wide variety of linear silanes, being even compatible with late-stage functionalization (Scheme 17) [45–47]. The hydrosilylation of terminal alkenes with hydrosilylalkynes can be achieved employing [{Ir(μ-Cl)(COD)}2] as catalyst in the presence of excess COD, giving rise to linear silylalkanes chemo- and regioselectively in good yields under solventless conditions as showed by Rahaim et al. (Scheme 18) [48].

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Scheme 18 Hydrosilylation of alkenes with hydrosilylalkynes

Scheme 19 Enantioselective Markovnikov hydrosilylation of terminal alkynes

Scheme 20 Transformation of terminal and internal alkenes into linear alkylsilanes under solventless conditions

Other systems based on Rh have also been reported to yield the linear silylalkynes in moderate to good regioselectivities [49–51]. The synthesis of well-defined metal complexes has also received much attention for the design of catalysts for the hydrosilylation of alkenes, although somewhat less than that in the case of alkynes. Nishiyama et al. proved that the use of the bis (oxazolinyl)phenyl as ligand for the synthesis of Rh(III)-acetate catalysts allows for the hydrosilylation of internal and terminal alkenes with good Markovnikov selectivity and excellent enantioselectivities (Scheme 19) [52]. The Rh(III)-hydrido-silyl complex 38 reported by Huertos et al. is able to selectively catalyze the synthesis of linear silanes in excellent yields and selectivities under mild, solventless conditions (Scheme 20). Terminal alkenes have been converted with complete anti-Markovnikov selectivity with HSiEt3. Notably, also

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Scheme 21 Rh(I)-catalyzed cross-linking of polyhydrosiloxane (EHDMS) and vinyl-terminated polydimethylsiloxane (PDMS-1) or polydimethylsiloxane-co-polystyrene (PDMS-2) Fig. 19 Depiction of catalyst 39

internal alkenes have afforded linear silanes as a result of an isomerizationhydrosilylation tandem reaction (Scheme 20) [53]. Rh(I)-biscarbonyl complexes with acetylacetonato or 4-arylimino-2-pentanoato ligands are efficient catalysts for the cross-linking of polysiloxanes functionalized with terminal alkynes by hydrosilylation at room temperature (Scheme 21). Remarkably, these catalysts reported by Roodt, Boyarskiy, Islamova et el. show better activities than Wilkinson’s or Karstedt’s catalysts [54]. Binuclear Rh(I) complex 39 (Fig. 19) reported by Reilly et al. catalyzes the hydrosilylation of a series of terminal alkynes with good selectivities toward the linear silane employing HSiMe2Ph as silane source at 100 °C with a 0.5 mol% catalyst loading [55]. [Rh(OH)(COD)(NHC)] complexes 40a-d (Fig. 20) reported by Nolan et al. showed good activities for the hydrosilylation of terminal alkenes with complete anti-Markovnikov selectivity, with the activity increasing according to the trend 40a < 40b < 40c < 40d. It is noteworthy that competition with dehydrogenative

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Fig. 20 Depiction of catalysts 40a-d

Fig. 21 Depiction of catalysts 41a-d

silylation was reported for these catalysts. Optimization of the reaction conditions led to preferential formation of the β-(E)-vinylsilane with good selectivities [56]. Complexes 41a-d (Fig. 21) reported by Camp et al. are able to catalyze the hydrosilylation of terminal alkenes with excellent regio- and chemoselectivity to afford the corresponding linear silanes. The authors describe that the stronger the Rh–O bond the better the catalytic activity; thus catalyst 41b shows the best performance. Moreover, 41b is also a proficient catalyst for the tandem isomerization-hydrosilylation of internal olefins, affording the corresponding linear silanes with excellent selectivities [57].

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The use of ionic liquids and supercritical CO2 as solvents in the presence of Rh-NHC catalyst precursors has been explored in several research works, resulting in interesting selectivities and activities, with preferential formation of the linear silane [58, 59]. Other reports on the hydrosilylation of alkenes in ionic liquids have been published in recent years [60–64]. The hydrosilylation of allyl compounds has been studied with [{Ir(μ-Cl) (COD)}2] as metal precursor, allowing for the synthesis of the related γ-substituted propylsilanes in good selectivities, thus avoiding undesired hydrosilane-mediated reduction of the functional group [65, 66].

2.5

Mechanistic Considerations on the Hydrosilylation of C– C Multiple Bonds

Most of the catalytic cycles so far proposed for the hydrosilylation of alkenes and alkynes catalyzed by Rh and Ir complexes have been based on Chalk-Harrod-type mechanisms. The Chalk-Harrod mechanism has been postulated to provide an explanation for the formation of β-(E)-vinylsilanes in the hydrosilylation of alkynes. The initial step involves the oxidative addition of the Si–H bond to the metal center. Subsequently, alkyne coordination followed by 1,2-migratory insertion into the M– H bond affords the (E)-alkenyl intermediate. The β-(E)-vinylsilane is then generated through reductive elimination, along with regeneration of the active species (Scheme 22a; right-hand side). The formation of β-(Z )-vinylsilanes cannot be explained invoking the Chalk-Harrod mechanism [67], which led to the postulation of the modified Chalk-Harrod mechanism (Scheme 22a; left-hand side) [68]. In contrast with the Chalk-Harrod mechanism, the migratory insertion step in the modified Chalk-Harrod mechanism entails a 2,1-migratory insertion of the alkyne into the M–Si bond instead of the M–H bond. Then, isomerization of the (E)-alkenyl to a

Scheme 22 Chalk-Harrod and modified Chalk-Harrod mechanisms for alkynes (a) and alkenes (b)

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(Z )-alkenyl takes place, which leads to the formation of the β-(Z)-vinylsilane upon reductive elimination. The isomerization of the alkenyl intermediate is driven by steric interactions between the ancillary ligands and the SiR3. This process has been proposed to occur via formation of a metallacyclopropene [69] or a zwitterionic carbene [70]. Although scarce, ionic outer-sphere mechanisms have also been proposed for the hydrosilylation of alkynes. In these cases, the Ir(III) center is able to abstract a hydride from the hydrosilane, thus generating an Ir(III)-H species and a R3Si+ cation. Subsequently, the R3Si+ reacts with the alkyne to give a silyl carbocation, which accepts the hydride from the Ir(III)-H complex to furnish the corresponding vinylsilane [12, 14]. Regarding alkene hydrosilylation by Rh or Ir catalysts, Chalk-Harrod (Scheme 22b; right-hand side) and modified Chalk-Harrod (Scheme 22b; left-hand side) mechanisms have been postulated exclusively, the difference between them being the migratory insertion of the alkene, which may occur into the M–H bond for the former or the M–Si for the latter. However, it must be mentioned that these mechanisms present subtle variations depending on the substrate and metal–ligand system that account for the regio- and/or enantio-selectivity of the process.

3 Hydrosilylation of C=O Bonds 3.1

Hydrosilylation of Ketones

One of the most important applications of the catalytic hydrosilylation of unsaturated molecules is the asymmetric hydrosilylation of ketones to afford enantiomerically pure silyl ethers, which after acid hydrolysis are transformed easily into the corresponding enantiomerically pure alcohol [71–73] (Scheme 23). Homogeneous rhodium-based catalysts have been at the center of this area of chemistry since the 1970s; however, lately, the number of studies on rhodiumcatalyzed asymmetric hydrosilylation of ketones published per year has been significantly reduced [73, 74]. Examples of iridium-based catalysts effective for the asymmetric hydrosilylation of ketones are considerably fewer in number than those of rhodium [73, 75]. Below are summarized some of the recent results on homogenously rhodium- and/or iridium-catalyzed hydrosilylation of ketones.

Scheme 23 Catalytic hydrosilylation of asymmetric ketones

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42

43

44

45

Fig. 22 Depiction of Rh-NHC-based enantioselective ketones hydrosilylation catalysts

A number of rhodium(I) complexes of the type [Rh(Cl)(NHC)(COD)] (NHC = N-heterocyclic carbene) have been used as catalyst precursors for the hydrosilylation of aldehydes and ketone. The most relevant outcomes in this field until 2008 were reviewed by Nolan et al. [76]. Moreover, [Rh(Cl)(NHC)(COD)] species with Si(OiPr)3-functionalized N-substituents at the NHC ligand have been successfully supported on MCM-41, and the supported catalysts have shown to be active for the hydrosilylation of acetophenone and for the synthesis of high molecular weight polysilylethers by catalytic hydrosilylation of terephthaldehyde with hexamethyltrisiloxane [77–79]. One of the most important applications of the hydrosilylation of ketones is the preparation of enantiomerically enriched alcohols via asymmetric hydrosilylation. The first examples of homogeneous Rh-NHC-catalyzed asymmetric hydrosilylation of ketones were reported by Lappert et al. in 1984 by using NHC ligands with chiral N-substituents [80]. This strategy was developed in the following decades by several groups Herrmann et al. [81, 82], Enders et al. [83], Faller et al. [84], Fernández, Lassaletta et al. [85], and Galan et al. [86]. However, the use of rhodium mono-NHC complexes with monodentate NHC ligands bearing chiral N-substituents as catalyst precursors is limited by their moderate enantiomeric excesses (Fig. 22).

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Fig. 23 Depiction of Rh complexes with NHC-based bidentate neutral ligands effective for the enantioselective ketones hydrosilylation catalysts

In 2003, Shi et al. communicated that using the axially chiral complex [Rh(I)2(κ2O2CMe)(bis-NHC)] (46), with a bis-NHC ligand derived from 1,1′-binaphthalenyl2,2′-diamine (BINAM), as catalyst precursor, it was possible to achieve highly enantioselective (92–98 ee %) hydrosilylation of aryl methyl ketones with H2SiPh2; however, a lack of enantioselectivity was observed for dialkyl ketones (Fig. 23) [87]. One year later, Gade et al. reported that Rh-oxazolinylcarbene complexes (47) are excellent catalysts precursors for the enantioselective hydrosilylation of dialkyl ketones (77–95 ee %) and aryl methyl ketones (88–91 ee %) with H2SiPh2 (Fig. 23) [88]. In 2009, Gade, Hofmann et al. published a mechanism for the Rh-oxazolinylcarbene-catalyzed hydrosilylation of ketones with H2SiMe2 based on theoretical calculations, which involve the participation of Rh-silylene species as key intermediates [89]. Some years later, Shi et al. reported on the synthesis of the chiral Rh-phosphine-NHC species (48), with N-phosphinefunctionalized NHC ligands based on a 1,1′-binaphthyl backbone and different substituents (Me, Et, i-Pr) at the other nitrogen atom of the NHC. These species have found to be active for the enantioselective hydrosilylation of acetophenone with H2SiPh2; however, a moderate enantioselectivity was observed (49–72 ee %) (Fig. 23) [90]. Recently, de Ruiter et al. have shown that using rhodium (I) complexes with bidentate chiral imidazo[1,5-a]pyridine-3-ylidene-derived NHC ligands (49) in the presence of AgBF4. it is possible to achieve the enantioselective hydrosilylation of aryl methyl ketones (88–93 ee %) with H2SiPh2 in CH2Cl2 after 24 h at 40°C. However, under the same conditions, low to moderate enantioselectivities were obtained with dialkyl ketones (40–81 ee %) (Fig. 23)

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[91]. Therefore, the enantioselectivity of Rh complexes with NHC-based bidentate neutral ligands as catalysts for the hydrosilylation of ketones is higher than that so far found for rhodium complexes with monodentate NHC ligands (Fig. 23). In this regard, it should be mentioned that NHC-free rhodium(I)-complexes with chiral tridentate neutral ligands of the type bis(oxazolinyl)pyridine (PyBOX) have also been found to be active for the enantioselective hydrosilylation of aryl methyl ketones [92, 93]. In this context, it should be mentioned that the electronic properties of remote substituents of the PyBOX ligand play a role not only in the activity but also in the enantioselectivity of the resulting catalysts. The best performances (94% conversion, 98% ee) were obtained when ligands with 4-ethyl-phenyl substituents at the 4-position of the pyridine ring were used. Other examples of chiral N-donor ligands based on oxazolines, such as bis(oxazolinyl)(3,4-dihydroxy-furane) (FuBOX), have shown to be highly effective for the enantioselective rhodiumcatalyzed hydrosilylation of aryl methyl ketones [94]. It is worth mentioning that silyl enol ethers have often been observed as side products in the homogenous rhodium-catalyzed hydrosilylation of aryl methyl ketones [95–98]. The formation of silyl enol ethers in such reactions has been explained by dehydrogenative silylation of the ketone. It has been demonstrated that the in situ generated silyl enol ether reacts with the hydrogen to afford the same silyl ether that should be expected from the corresponding catalytic hydrosilylation reaction. Therefore, in these reactions, the formation of silyl ethers by two competitive processes, ketone hydrosilylation and/or silyl enol ether hydrogenation, is plausible. In this regard, it has been found that by tuning the pressure of H2 it is possible to control the selectivity of these reactions toward the formation of the corresponding silyl ether (high H2 pressure) or silyl enol ether (low H2 pressure) [97, 98]. Moreover, the nature of the silane can also play a key role [98]. The catalytic systems effective for the asymmetric hydrosilylation of ketones based on iridium complexes are scarce in comparison to those based on rhodium, and the enantioselectivity of most of them is low or moderate. However, there are some examples of iridium-based catalytic systems that have shown to be highly enantioselective. To the best of our knowledge the first one was reported by Uemura et al. in 1996. They showed that using iridium(I) complexes with chiral oxazolylferrocene-phosphine ligands it was possible to achieve the asymmetric hydrosilylation of aryl methyl ketones (88–96 ee %) with H2SiPh2 at 273 K [99]. It should be mentioned that Sakaguchi’s research group (2012–2022) has developed intense work in the field of asymmetric hydrosilylation of ketones catalyzed by iridium complexes. Using the system [Ir(cod)2][BF4]/chiral azolium salt to generate in situ the corresponding active species, [Ir(NHC)(cod)][BF4], they have managed to reach enantiomeric excesses of up to 94% [100–104].

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Hydrosilylation of CO2

The catalytic reaction of CO2 with hydrosilanes is a straightforward and thermodynamically favored methodology for its reduction to the corresponding silylformate (Scheme 24). The selectivity of this type of reaction represents a challenge because, under the reaction conditions, the formation of mixtures of silylformate, bis(silyl)acetal, methoxysilane, and methane is frequently observed [105–107] (Fig. 24). In addition, methylsilylcarbonates have also been identified as intermediates in catalytic CO2 hydrosilylation processes (Fig. 24) [108]. Therefore, not only activity but also selectivity is a key parameter when designing CO2 hydrosilylation catalysts. The first examples of homogeneous catalytic systems effective for the reduction of CO2 with hydrosilanes were based on ruthenium [109, 110] and iridium [111] complexes. Since then, several examples of transition-metal-based catalysts as well as of metal-free catalytic systems effective for the selective reduction of CO2 with hydrosilanes have been reported [105–107]. Among them, iridium catalysts stand out. Conversely, a few examples of catalytic systems based on rhodium, effective for the hydrosilylation of CO2, have been reported [105–107]. To the best of our knowledge, the first example of a rhodium-catalyzed homogeneous hydrosilylation of CO2 was reported by Mizuno and coworkers. They showed that the catalytic system [Rh2(μ-O2CCH3)4]/K2CO3 promotes the reaction of CO2 (1 atm) in acetonitrile at 353 K with hydrosilanes to afford the corresponding silylformate. This system was also effective for the one-pot synthesis of formamides by reaction of secondary amines with CO2 and hydrosilanes [112]. The catalytic

Scheme 24 Catalytic hydrosilylation of CO2

Fig. 24 Reported products from the catalytic reduction of CO2 with silicon-hydrides

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Fig. 25 Inner- versus outersphere transition state (TS) found for Ir-triflate and Ir-trifluoroacetate species, respectively

hydrosilylation of CO2 has also recently been proposed as a key step in rhodiumcatalyzed amine formylation processes [113, 114]. The iridium complex [Ir(CF3SO3)(SiR3)(NSiN)(NCMe3)] (NSiN = fac-bis-(pyridine-2-yloxy)methylsilyl; SiR3 = SiMe(OSiMe3)2) (50) was the first example of an iridium catalyst efficient for the selective hydrosilylation of CO2 to give the corresponding silylformate [115]. Studies on the influence of the ancillary ligands on the activity of Ir-NSiN species as CO2 hydrosilylation catalysts showed that the best catalytic performance was obtained using the iridium(III) complex [Ir(H) (CF3CO2)(NSiNMe)(coe)] (NSiNMe = fac-bis-(4-methyl-pyridine-2-yloxy) methylsilyl) (51), which contains a trifluoroacetate instead of a triflate ligand and a NSiNMe ligand with 4-methylated pyridinic rings as catalyst precursor [116, 117]. The nature of the ancillary ligand, triflate or trifluoroacetate, plays a key role in the CO2 activation mechanism. Thus, while Ir-trifluoroacetate species follow an inner-sphere mechanism, an outer-sphere mechanism is favored for Ir-triflate derivatives (Fig. 25) [117]. The steric hindrance around the active center, which hampers further reduction of silylformates, is hypothesized to be the reason for the selectivity of Ir-NSiN CO2hydrosilylation catalysts. Interestingly, the related catalyst precursor [Ir(CF3CO2) (κ2-NSiMe)2] (NSiMe = 4-methylpyridine-2-yloxydimethylsilyl) (52), containing two Ir–Si bonds trans-located to the catalyst active positions, allows the selective formation of silylformate [118], methoxysilane [118], or bis(silyl)acetal [119] by tuning the reaction conditions (Scheme 25). The iridium(III) cationic species [Ir(H)(η1-HSiR3)(POCOP)][B(C6F5)4] (POCOP = 2,6-bis((di-tert-butylphosphanyl)oxy)benzen-1-yl) (53) reported by Brookhart et al. in 2012 [120] has proved to be effective for the reduction of CO2 (1 bar, 296 K) to methane with different hydrosilanes (HSiEt3, HSiPh3, HSiMe2Et, HSiMe2Ph, and HSiEt2Me) using C6H5Cl as solvent. This catalytic system works reasonably well with HSiMe2Ph at 296 K (TOF = 115 h-1); moreover, increasing the temperature to 333 K produces a positive effect on the catalytic activity (TOF = 661 h-1) (Scheme 26) [120]. The zwitterionic iridium(III) half-sandwich species [IrClCp*{(MeIm)2CHCOO}] ((MeIm = 3-methylimidazol-2-yliden-1-yl; Cp* = pentamethylcyclopentadienyl)

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Scheme 25 Complex [Ir(CF3CO2)(κ2-NSiMe)2] (52)-catalyzed reaction of CO2 with hydrosilanes

Scheme 26 Ir-(POCOP)-catalyzed CO2 reduction to methane with HSiMe2Ph

(54) has proven to be active for the hydrosilylation of CO2 with HSiMe2Ph to give the corresponding silylformate (Scheme 27) [121].

3.3

Hydrosilylation of Amides

The deoxygenative hydrosilylation of amides is a straightforward methodology for the synthesis of amines. These reactions are two steps processes; firstly the amide is hydrosilylated to give the corresponding silylhemiaminal, which in some cases has been isolated, and secondly, the silylhemiaminals reacts with one equivalent of silane to afford the desired amine and the corresponding siloxane O(SiR3)2 [122– 124] (Scheme 28).

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Scheme 27 CO2 hydrosilylation {(MeIm)2CHCO2)}]+ (54)

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catalyzed

by

the

zwitterionic

species

[Cp*IrCl

Scheme 28 Catalytic reduction of amides to amines with hydrosilanes

Examples of homogeneous rhodium-catalyzed hydrosilylation of amides to amines are scarce. To the best of our knowledge, the first example of a rhodiumbased catalytic system effective for the reduction of amides to the corresponding tertiary amine was reported in 1998 by Ito et al. [125]. They described that the rhodium (I) hydride species [Rh(H)(CO)(PPh3)2] and [Rh(H)(PPh3)3] (0.1 mol%) are highly effective catalyst precursors for the reduction of tertiary amides to amines with H2SiPh2 (2.5 equiv.) in THF at room temperature. Conversely, a drastic decrease of activity was observed on using [Rh(Cl)(PPh3)3] or [Rh(COD)2][BF4]/2 PPh3 [125]. Some years later, in 2015, Beller et al. showed that the reduction of β-lactams to azetidines was possible using the cationic species [Rh(COD)2][BF4] in the presence of two equivalents of bis(diphenylphosphino)propane (dppp) [126], as well as the selective reduction of the tertiary amides in amino acid esters and peptides [127] using HSiPh3 as reducing agent in THF under mild reaction conditions. Beller et al. also found that the phosphine-free catalytic system based on [Rh(acac)(cod)] and H3SiPh is effective for the selective reduction of secondary amides and N-acyl amino esters in THF at 50°C [128]. Driess et al. did a comparative study of the activity of [Rh(μ-Cl)(cod)]2, [Rh(Cl) (cod)(NHSi)], and [Ir(Cl)(cod)(NHSi)] (NHSi = LSi; L = [HC(CMeN-Ar)(C(CH2) N-Ar]; Ar = 2,6-iPr2C6H3) as catalyst precursors for the hydrosilylation of N-acetyldibenzoazepine. The results of this study show that the rhodium complexes are less

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Scheme 29 Modified Chalk-Harrod catalytic cycle proposed for the Rh(I)-catalyzed hydrosilylation of tertiary amides

active than the iridium species. However, while the iridium catalyst affords a mixture of N-ethyl-dibenzoazepine (78%) and dibenzoazepine (9%) the rhodium species selectively promotes the formation of N-ethyl-dibenzoazepine. It should be mentioned that no significative difference was observed in the reaction performance between [{Rh(μ-Cl)(cod)}2] and [Rh(Cl)(cod)(NHSi)] [129]. There is a lack of mechanistic studies on these types of reactions. Ito et al. proposed in 1998 a classical modified Chalk-Harrod mechanism [130] based on three steps: (i) oxidative addition of the silane, (ii) insertion of the carbonyl group in the Rh–Si bond, and (iii) finally reductive elimination of the resulting silylhemiaminal [125] (Scheme 29). A similar mechanism was proposed by Beller et al. in 2015 for the hydrosilylation of tertiary amides [127]. One of the first examples of iridium-catalyzed reduction of amides with silanes was reported in 2009 by Nagashima et al. They found that the complex [Ir(Cl)(CO) (PPh3)2] (0.05 mol%) is a highly active catalyst precursor for the synthesis of enamines from the reaction of amides with 1,1,3,3-tetramethyldisiloxane (TMDS) in toluene at 25°C [131]. Some years later, this group published an interesting work on the effect of the nature of the PR3 ligands on the activity of [Ir(Cl)(CO)(PR3)2] species as catalyst precursors for the reduction of amides to π-conjugated enamines with TMDS. They found the best reaction performance when using the iridium complexes with P(OC6F5)3 or P{OCH(CF3)2}3 ligands [132]. They reported a classical modified Chalk-Harrod mechanism for the [Ir(Cl)(CO)(PPh3)2]-catalyzed hydrosilylation of amines to the corresponding silylhemiaminal, similar to that

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Scheme 30 Reaction mechanism proposed by Nagashima et al. for the formation of enamines from O-slylated hemiaminals [131, 132]

shown in Scheme 8 for rhodium(I) catalysts [128]. Under excess of hydrosilane mixtures of the corresponding enamine and amines were obtained. The formation of the enamine was explained via an iminium intermediate species, which evolves by elimination of one of the α-protons to generate the corresponding enamine (Scheme 30) [132, 133]. In this regard, Dixon et al. have recently reported that silylhemiaminals prepared according to Nagashima’s methodology can be further transformed into α-functionalized tertiary amines by reaction with an electrophilic dehydroalanine acceptor in the presence of a proper iridium(III)-based photocatalyst [134]. Brookhart and Cheng found in 2012 that the iridium (I) species [{Ir(μ-Cl) (coe)2}2] (0.5 mol%) is an effective catalyst precursor for the reduction of secondary amines to silyl amines with 4 equiv. of H2SiEt2. The resulting silyl amines can be easily converted into the corresponding secondary amine by acidic workup. The chemoselectivity of the process is strongly dependent on the catalyst and silane loading; thus using [{Ir(μ-Cl)(coe)2}2] (0.1 mol%), instead of 0.5 mol%, the reactions of secondary amines with 2.0 equiv. of H2SiEt2 selectively afford the corresponding imine [135]. The same group, also in 2012, reported that the iridium (III) species [Ir(H)2(κ3mer-P,O,P´-POCOP)] (POCOP = 2,6-bis(di-tert-butylphosphinito)phenyl) (55) is an effective catalyst precursor for the reduction of tertiary amides to the corresponding tertiary amine with H2SiEt2 in the presence of the ammonium borate salt Et3NH-B(C6F5)3 and under H2 (1 atm). The active silylating species is the iridium(V) intermediate [Ir(H)3(SiEt2H)(κ3-mer-P,O,P´-POCOP)] (56), which was observed in solution (Scheme 31). Intermediate 56 is generated in the reaction medium by oxidative addition of H2SiEt3 to 55 and/or by reaction of 53 with H2SiEt2 and H2 in the presence of NEt3. The best catalytic performance was achieved at 60°C with 0.5 mol% of catalyst loading and 3.0 equiv. of H2SiEt2 [136]. Recently, Michon, Agbossou-Niedercorn et al. have reported on the Ir-Cp* (metallacycle)-catalyzed selective reduction of secondary and tertiary amides to the corresponding amine with TMDS. The active species consist in a cationic unsaturated Ir(III) complex stabilized with the anion BArF20¯. The authors propose a two-cycles mechanism; the first cycle involves the iridium-promoted heterolytic cleavage of the Si–H bond and subsequent transfer of the silylium cation to the amide to generate an Ir–H neutral species and a silyloxy carbonium. Finally, the silyloxy carbonium species reacts with the iridium–hydride to afford the

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Scheme 31 Reaction mechanism proposed by Brookhart et al. for the [Ir(H)2(κ3-mer-P,O,P´POCOP)] (55)-catalyzed hydrosilylation of amides [136]

corresponding silyl hemiacetal and initial the active species. The second cycle consists in the reaction of the silylium cation with the silyl hemiacetal to give the corresponding siloxane, an iminium salt, and an Ir–H neutral species (Scheme 32). The reaction of the iminium salt with the Ir–H species produces the desired amine and regenerates the active catalyst [137, 138]. Therefore, silylhemiaminals, also denominated O-silylated hemiaminals, have been proposed as intermediates of the catalytic reduction of amides to amines or enamines. In this context, it should be mentioned that some of us recently reported that the reaction of formamides with one equivalent of HSiMe2Ph and in the presence of the iridium(III) complex [Ir(H)(Cl)(κ2-NSitBu2)(coe)] (NSitBu2 = 4methyl-pyridine-2-iloxy)ditertbutylsilane) (57) (0.5 mol%) in C6D6 at r.t. affords the corresponding silylhemiaminal. The temperature is an important factor to control the selectivity of the process. When the reactions were performed at 353 K, the formation of the corresponding methylamine is unavoidable. On the other hand, using the iridium (III)-triflate derivative [Ir(H)(Of)(κ2-NSitBu2)(coe)] (58) as catalyst precursor no evidence for the formation of silylhemiaminals was observed. Indeed, complex 58 has proven to be a highly active catalyst for the reduction of formamides to methylamines with two equivalents of HSiMe2Ph in C6D6 at 353 K [139].

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R3Si

O

Ph R3Si Ph

O

H

N(Bn)2

C

Cp* HSiR3

Ir [BArF20]

N

N(Bn)2 C

[BArF20] C

Cp* Ir

N

Cp* Ir

N

H

H

SiR3 [BArF20]

O Ph R3Si Ph

O

N(Bn)2

[BArF20] N(Bn)2

Scheme 32 Reaction mechanism proposed by Michon, Agbossou-Niedercorn et al. for the Ir–Cp* (metallacycle)-catalyzed hydrosilylation of amides

4 Hydrosilylation of C–N Multiple Bonds To the best of our knowledge, the first examples of rhodium-catalyzed hydrosilylation of enamines to give amines were reported during the 1970s decade by Kagan, Dang, and Langlois [140, 141]. Some years later, in 1999, Hidai, Uemura et al. reported on the asymmetric hydrosilylation of imines to chiral amines with H2SiPh2 in the presence of [M(Cl)(cod)]2 (M = Rh, Ir; 0.5 mol%) and chiral s (0.1 mol%) as chiral ligands. The best reaction performance (>95% yield and 88% ee) was obtained with the iridium precursor in the presence of the oxazolinylferrocenylphosphine with a phenyl substituent on the oxazolyne ring [142]. In 2005, Tsuji et al. published a study on the activity of the system [Rh(Cl) (CH2 = CH2)2]2/n PR3 as catalyst in the hydrosilylation of ketones and imines. They compared bowl-shaped phosphines (BSPs) with conventional phosphines. The best catalytic performance was obtained when using tris(2,2″,6,6″-tetramethyl-mterphenyl-5′-yl)phosphine (cone angle of 205°). Other BSP ligands with higher cone angles 210° and 223° bring about a slight reduction of the activity. On the other hand, conventional phosphine ligands such as PPh3, PEt3, PCy3, or P(t-Bu)3 led to poorly active hydrosilylation catalysts. The reason, as confirmed by 31P{1H} NMR spectroscopy, is that, while using BSP ligands the formation of unsaturated

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Scheme 33 Reaction mechanism proposed by Freixa et al. for the iridacycle-catalyzed hydrosilylation of imines [146]

Rh-mono(phosphine) is favored, the use of conventional PR3 ligands results in less active Rh-bis(phosphine) species [143]. Cationic iridacycles of the type [IrCp*(κ2-C,N-L)][BArF24] (L = benzoquinoline derivatives, 2-phenylpyridine derivatives; BArF24 = tetrakis[(3,5-trifluoromethyl) phenyl]borate) have also been reported as catalysts for the hydrosilylation of imines and subsequent protodesilylation to give the corresponding secondary amine [144– 146]. Cations of the type [IrCp*(κ2-C,N-L)H → SiR3]+ have been proposed as key intermediate for the hydrosilylation process. The silylium, [SiR3]+, is transferred to the imine to generate the corresponding silyliminium cation and the neutral Ir–H species [IrCp*(H)(κ2-C,N-L)]. Hydride transfer from [IrCp*(H)(κ2-C,N-L)] to the silyliminium species affords the corresponding silylamine, which by protodesilylation leads to the secondary amine (Scheme 33) [146]. It should be mentioned that the catalytic systems based on [IrCp*(κ2-C,N-L)] [BArF24] species are also effective for the catalytic conversion of nitriles into amines by their reaction with hydrosilanes [147]. In this regard, Takaya, Iwasawa et al. have recently reported that rhodium complexes with the pincer-type 6,6″-bis(phosphino)terpyridine chlorogallylene ligand are effective catalysts for the hydrosilylation of nitriles to the imine level [148].

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5 Conclusions Homogeneous rhodium and iridium catalysts have been pivotal for the development of the hydrosilylation reaction. However, despite the vast number of Rh and Ir catalysts so far described for this reaction, and their outstanding performances, there is still room for improvement, mainly regarding their selectivity and recyclability. The nature of the metal center significantly influences the catalytic performance of rhodium and iridium hydrosilylation catalysts. In this regard, it should be mentioned that it is difficult to establish a general behavior pattern. For example, while rhodium(I) complexes are much more active than their iridium analogues as catalysts for the hydrosilylation of ketones, examples of rhodium-based catalytic systems effective for the hydrosilylation of CO2, amides, and/or enamines are scarce. Regarding the hydrosilylation of C–C multiple bonds, rhodium and iridium catalysts have been reported, but the former generally lead to better performances. Especially remarkable is the enantioselective hydrosilylation of alkenes, for which the role of rhodium catalysts has been preponderant. In conclusion, the current trends in the development of novel and more efficient Rh and Ir catalysts for hydrosilylation reactions primarily rely on various strategies for the design of the ligand system. Among these strategies, we would like to highlight (i) metal–ligand cooperation, (ii) the use of tethered ligands for catalyst heterogenization, and (iii) the design of new chiral ligands to enhance enantioselectivities. It is also noteworthy that the use of binuclear complexes has shown promise for the design of new architectures with improved activities. Further investigation into these research topics may hold the key to successfully developing more efficient catalytic systems that can be applied in the chemical industry.

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Top Organomet Chem (2023) 72: 141–224 https://doi.org/10.1007/3418_2023_101 # The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 Published online: 31 October 2023

Recent Development of Cobalt-Catalyzed Hydrosilylation Reactions: New Catalysts, Mechanistic Understandings, and Future Trends Dongyang Wang and Liang Deng

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Cobalt-Catalyzed Hydrosilylation of Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Anti-Markovnikov Hydrosilylation of Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Markovnikov Hydrosilylation of Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Hydrosilylation of Allenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Cobalt-Catalyzed Hydrosilylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Markovnikov Hydrosilylation of Terminal Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Anti-Markovnikov Hydrosilylation of Terminal Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Hydrosilylation of Internal Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Hydrosilylation of Enynes and Diynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Double Hydrosilylation of Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

142 143 143 171 181 184 185 194 196 205 213 217 218

Abstract This chapter presents a comprehensive summary of cobalt-catalyzed hydrosilylation of alkenes and alkynes from 2016 to 2023 with the objective of providing readers with the status of this field. Various well-defined cobalt complexes bearing structurally diverse nitrogen-based ligands, phosphines, NHCs, and isocyanides are developed. Their catalytic application in the hydrosilylation D. Wang State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, China L. Deng (✉) State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, China School of Chemistry and Materials Science, Hangzhou Institute for Advanced Study, University of Chinese Academy of Sciences, Hangzhou, China e-mail: [email protected]

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reactions of alkenes and alkynes led to the development of new catalytic systems with different chemo-, regio-, and stereoselectivity. Different substrate activation and chemical bond construction modes are proposed based on mechanistic study, which contributes to a better understanding of the chemistry of cobalt-catalyzed hydrosilylation reaction. In addition, the problems and perspectives in this field are also presented. Keyword Alkene · Alkyne · Cobalt · Hydrosilylation · Silane

1 Introduction The searching of non-precious metal alternatives for noble metal catalysts in hydrosilylation reactions has intrigued explorations on late 3d metal-catalyzed hydrosilylation reactions in recent years, among which cobalt-catalyzed hydrosilylation reactions have evidenced the fastest development. The increase of cobalt-catalyzed hydrosilylation reactions seems to have its inevitability when considering the proximity of cobalt to rhodium, palladium, iridium, and platinum in the periodical table. The latter ones have been popularly used as catalysts in hydrosilylation reactions. In addition to the phenomenal similarity, one should not ignore the uniqueness of cobalt as a first-row late transition metal relative to those 4d and 5d congeners, which includes small electronegativity, small atomic radii, and being relatively “hard” according to the soft-hard acid-base theory [1, 2]. These characters lead to the coordination chemistry of cobalt species being much different from the noble metals. Therefore, cobalt catalysts effecting hydrosilylation reactions have to embrace different ligand systems and/or reaction conditions as compared to those applied for the noble metal-catalyzed reactions. The unique chemical properties of cobalt compounds, such as relatively low bond dissociation energies and high polarity of cobalt-ligand bonds versus the rhodium- and iridium-ligand bonds, also pose challenges in achieving cobalt-catalyzed hydrosilylation reactions with high catalytic activity and high selectivity. Targeting the practical use of cobalt-catalyzed hydrosilylation reactions for the synthesis of organosilicon compounds and polymers, great recent efforts on these aspects were thus exercised. In this chapter, we present a comprehensive summary of the studies on cobaltcatalyzed hydrosilylation of alkenes and alkynes from 2016 to 2023. Previous advances in the area can be found in our last review published in 2016 [2]. There are also excellent reviews on non-precious metal-catalyzed hydrosilylation reactions and hydro-functionalization reactions [3–11], which also introduce cobalt-catalyzed hydrosilylation reactions. Readers who are interested in those broad topics are encouraged to read them. This chapter is organized by the type of unsaturated organic substrates. Inasmuch as advances in cobalt-catalyzed hydrosilylation reactions of alkenes were mainly achieved in the reactions of terminal alkenes, the section of hydrosilylation of alkenes is further categorized by the structures of the resultant alkylsilanes, namely, the formation of linear and branched alkylsilanes as

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well as chiral alkylsilanes. The cobalt-catalyzed hydrosilylation of alkynes is classified according to alkyne substrates, including the hydrosilylation reactions of terminal alkynes, internal alkynes, enynes, and diynes. Owing to the limitation of space, cobalt-catalyzed hydrosilylation reactions of the heterogeneous catalysis type are not included in this chapter [12–17].

2 Cobalt-Catalyzed Hydrosilylation of Alkenes Since the discovery of Co2(CO)8-catalyzed hydrosilylation of terminal alkenes in the 1960s [18, 19], lots of cobalt-based catalytic systems are developed for the regioand enantioselective hydrosilylation of alkenes, wherein ligands play a significant role in selectivity. Isocyanides, NHCs, nitrogen-based ligands, and phosphines are applied to these systems.

2.1

Anti-Markovnikov Hydrosilylation of Alkenes

Transition-metal-catalyzed hydrosilylation of terminal alkene have been extensively studied and the anti-Markovnikov hydrosilylation products, linear alkylsilanes, are formed most often as the major products.

2.1.1

Anti-Markovnikov Hydrosilylation of Alkenes Using Cobalt-Isocyanide Catalysts

Chalk and Harrod have showed that Co2(CO)8 can efficiently catalyze the antiMarkovnikov hydrosilylation of terminal alkenes [18, 19]. Considering the analogy of isocyanides CNR with CO, it can be expected that cobalt complex with isocyanide ligands might be able to catalyze anti-Markovnikov hydrosilylation of alkene. In line with the expectation, Nagashima et al. showed that Co(OPiv)2 (Piv = pivaloyl) in combination with three equiv. of CNAd (3 mol%) can catalyze the hydrosilylation reactions of α-methylstyrene with various tertiary silanes, such as HSiMe(OTMS)2 (TMS = trimethylsilyl), HSiMe2Ph, and HSi(EtO)2Me [20]. The neat reactions in 80°C can yield the anti-Markovnikov addition products in high yields (Table 1). Hydrosilylation of 2-octene proceeded with a rate similar to that of the reaction with 1-octene, giving the same 1-silylated octane in high yield, suggesting that alkene isomerization is involved in the catalytic system. In addition, 2-norbornene, allyl glycidyl ether, and allyl benzyl ether are hydrosilylated successfully to form the corresponding product in medium to high yields (53–91%). No Markovnikov products are formed in all cases. Moreover, this catalytic system is also suitable for the production of modified silicone fluids and cross-linking of silicone polymers containing vinyl groups with polymethylhydrosiloxanes. In these reactions, hydrosiloxanes behave as both an activator of cobalt carboxylates and the reactant for hydrosilylation.

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Table 1 The Co(OPiv)2- and CNAd-catalyzed hydrosilylation of alkenes

Later, Nagashima’s group found Co2(CNR)8 can be directly used as hydrosilylation reaction catalysts. The reduction of CoI2 by KC8 or Na silica gel in the presence of CNR was found to form binuclear cobalt isocyanide complexes Co2(CNR)8 (R = Ad, tBu, Mes) [21]. The reactions of three alkenes, styrene, α-methylstyrene, and 1-octene with HSiMe2(OTMS) catalyzed by Co2(CNAd)8 and Co(OPiv)/CNAd/HSi(OEt)3, were conducted, and the two catalytic systems exhibited similar anti-Markovnikov addition selectivity. The yields of the hydrosilylation products in Co2(CNR)8-catalyzed reactions are slightly higher than those of the reactions with Co(OPiv)/CNAd/HSi(OEt)3 as catalyst (Table 2). Co2(CNAd)8 can also be used as catalyst for chemical modification of silicone fluids containing Si-H groups and for two-component silicone curing (Scheme 1). For the Co2(CNR)8- and Co(OPiv)/CNAd/HSi(OEt)3-catalyzed hydrosilylation reactions, the authors proposed that a classical Chalk–Harrod mechanism might operate (Scheme 2) [21]. Important evidence for the involvement of cobalt hydride species HCo(CNR)3 is the attainment of the same product from the hydrosilylation reaction of 2-octene and 1-octene. On the other hand, the catalytic performance of cobalt silyl complex (EtO)3SiCo(CNAd)4, formed from the reaction of Co2(CNAd)8 with HSi(OEt)3 or Co(OPiv)2, CNAd with HSi(OEt)3, in the catalytic hydrosilylation reaction proved significantly inferior to the Co2(CNAd)8. The absence of dehydrogenative silylation products in those catalytic systems does not support a modified Chalk–Harrod mechanism either.

2.1.2

Anti-Markovnikov Hydrosilylation of Alkenes Using Cobalt-NHC Catalysts

Since the report of the hydrosilylation of 1-octene with H3SiPh using silyl-anchored NHC-Co(II) complexes as catalysts [22], a handful of cobalt-NHC complexes

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Table 2 The Co2(CNtBu)8-catalyzed hydrosilylation of alkenes

Scheme 1 The Co2(CNAd)8- and Co(OPiv)2/CNAd/HSi(OEt)3-catalyzed modification of silicone fluids and silicone curing

proved effective in catalyzing the anti-Markovnikov hydrosilylation of terminal alkenes (Fig. 1). Fout and co-workers found that pincer cobalt(I) complex [(DIPPCCC)CoN2] DIPP CCC = bis(diisopropylphenyl-imidazol-2-ylidene)phenyl) can catalyze the ( anti-Markovnikov hydrosilylation of terminal alkenes with tertiary silanes [23]. In the presence of 5 mol% [(DIPPCCC)CoN2], the reactions of terminal alkenes with HSiMe2Ph or HSiMe(OTMS)2 in benzene at room temperature can afford antiMarkovnikov hydrosilylation products in 62–99% yields, and the functional groups including hydroxyl, amino, ester, epoxide, ketone, formyl, and nitrile are all tolerated in the catalytic reaction (Table 3). This catalytic system is incapable of catalyzing the hydrosilylation of 1,1-disubstituted or 1,2-disubstituted alkenes either.

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Scheme 2 The proposed mechanism for Co2(CNR)8and Co(OPiv)2/CNR/H [Si]-catalyzed hydrosilylation

Fig. 1 Cobalt catalysts used in the anti-Markovnikov hydrosilylation of alkenes with NHC ligands

Mechanism study showed that the stoichiometric reaction of [(DIPPCCC)CoN2] with H2SiPh2 can form a cobalt(III) hydride silyl complex (DIPPCCC)Co(SiHPh2)(H) (N2), which is catalytically relevant as its reaction with 1-octene furnishes the hydrosilylation product and [(DIPPCCC)CoN2] [23]. Further attempts to isolate the desired cobalt monosilyl complex led to the attainment of a cobalt disilyl complex [(DIPPCCC)Co(SiHPh2)2(N2)] (Scheme 3). The formation of cobalt(III) disilyl complex is likely the result of a σ-bond metathesis of (DIPPCCC)Co(H)(SiHPh2)(N2) with hydrosilane. The detailed mechanism is unclear. Deng’s group applied the cobalt(II) amides complexes with NHC ligation [(NHC)Co(N(TMS)2)2] as catalysts for the hydrosilylation of alkenes with tertiary silanes [24]. Catalysts screening indicated that the steric nature of NHC ligands plays a key role in selectivity and [(IMesMe)Co(N(TMS)2)2] with a smaller steric bulk NHC ligand IMesMe can achieve the highly anti-Markovnikov hydrosilylation of 1-octene with HSi(OEt)3 to give the desired product in 99% GC yield. Further substrate scope investigation showed that in the presence of [(IMesMe)Co(N (TMS)2)2] (1 mol%), various mono-substituted alkyl alkenes can react with HSi (OEt)3 selectively, giving anti-Markovnikov addition products in moderate to excellent yields (42–98%) (Table 4). The functional groups ester, epoxide, and secondary amine are tolerated. The catalytic system is incapable in promoting the hydrosilylation of styrene probably due to the coordination effect of arene toward the in situ generated low coordinate cobalt species. When other tertiary silanes, like HSiPh3, HSiEt3, and HSi(OTMS)2Me, were tested for the reaction with 1-octene, alkene isomerization products are observed.

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Table 3 The [(DIPPCCC)CoN2]-catalyzed anti-Markovnikov hydrosilylation of terminal alkenes

Scheme 3 The stoichiometric reaction of [(DIPPCCC)CoN2] with H2SiPh2

Mechanistic study showed that the stoichiometric reaction of [(IPr)Co(N (TMS)2)2] with HSi(OEt)3 can form a cobalt(I) amide complex [(IPr)Co(N (TMS)2)], which can further react with HSi(OEt)3 in benzene to give a cobalt (I) hydride complex [(IPr)Co(η6-C6H6)(H)] (Scheme 4) [24]. The cobalt(I) amide complex [(IPr)Co(N(TMS)2)] shows comparable activity and selectivity in the reaction of 1-octene with HSi(OEt)3 as those using the pre-catalyst [(IPr)Co(N (TMS)2)2]. But the cobalt(I) hydride complex [(IPr)Co(η6-C6H6)(H)] merely promotes the isomerization of alkenes, rather than hydrosilylation. These results suggest the genuine catalytic species for the hydrosilylation reactions may be produced from the reaction of cobalt(I) amide species with HSi(OEt)3, likely a low-coordinate cobalt(I) hydride or a cobalt(I) silyl species. The observation of the dehydrogenative silylation products in some of the catalytic reactions let the authors to propose that cobalt silyl species is more likely the active intermediate. Later, the same group found that cobalt(I) chloride complexes bearing NHC ligands can affect the hydrosilylation of terminal alkenes with H2SiPh2 [25]. In the presence of [(IAd)(PPh3)CoCl] (2 mol%) in toluene at 70°C, various terminal aryl-

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Table 4 The [(IMesMe)Co(N(TMS)2)2]-catalyzed anti-Markovnikov hydrosilylation of alkenes

Scheme 4 The stoichiometric reaction of [(IPr)Co(N(TMS)2)2] with HSi(OEt)3

and alkyl-substituted alkenes can react with H2SiPh2 to give anti-Markovnikov hydrosilylation products in high isolated yields (85–95%) with good to high regioselectivities (9/1- > 99/1) (Table 5). Aryl-substituted alkenes bearing either electron-withdrawing or electron-donating groups at para- or meta-position can undergo hydrosilylation to produce corresponding alkylsilanes. The catalytic system is not effective for hydrosilylation of ortho-substituted styrene and disubstituted alkenes. The [(IAd)(PPh3)CoCl]-catalyzed reaction was proposed to involve cobalt(I) silyl species, which can be generated by the interaction of cobalt(I) chloride species with two equiv. of H2SiPh2, as the active intermediate. This proposal was referred from the observation of dehydrogenative silylation products in these catalytic reactions and also the capability of the cobalt(I) silyl complex [(IAd)(PPh3)Co(SiHPh2)] in catalyzing the hydrosilylation of styrene with H2SiPh2 [25]. The steric nature of IAd is believed to the cause for the 1,2-addition to form linear alkylsilanes (Scheme 5).

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Table 5 The [(IAd)(PPh3)CoCl]-catalyzed anti-Markovnikov hydrosilylation of alkenes

Scheme 5 The proposed mechanism for [(IAd)(PPh3) CoCl]-catalyzed antiMarkovnikov hydrosilylation

2.1.3

Anti-Markovnikov Hydrosilylation of Alkenes Using Cobalt-Nitrogen Ligand-Based Catalysts

2,6-Bis(imino)pyridines and β-diketiminate ligands have proved to be effective ligands for cobalt-catalyzed hydrosilylation of alkenes [26, 27]. The high-spin cobalt(I) β-diketiminate catalyst developed by Holland and co-workers can promote rapid and regioselective hydrosilylation of alkenes [28]. Though the catalyst has a paramagnetic cobalt center, mechanistic study indicated that this catalytic system also follows the classical Chalk–Harrod or modified Chalk–Harrod mechanism. Thus, high-spin electronic configurations and weak ligand fields are not a hindrance to well-controlled organometallic catalysis [28]. Up to now, some other nitrogen ligands, including terpyridine, phosphine-iminopyridine, pyridine/quinolineoxazoline, 2-iminopyridyl-phosphine, (aminomethyl)pyridine, and iminobipyridine

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Fig. 2 Cobalt catalysts bearing nitrogen ligands used in the anti-Markovnikov hydrosilylation of alkenes

ligands were developed and tested for the cobalt-catalyzed hydrosilylation of alkenes (Fig. 2). Chirik’s group had found the aryl-substituted pyridine diamine cobalt(I) alkyl complex (MesPDI)CoCH3 can catalyze the dehydrogenative silylation of terminal alkenes [29]. Further investigation suggested that the use of sterically unencumbered ligand, N-methyl imine-substituted pyridine diimine cobalt(I) alkyl complex (MePDI)Co(CH2SiMe3), can suppress the dehydrogenative silylation and promote exclusive hydrosilylation [30]. In addition, the combination of Co(OAc)2 and phosphine ligand can affect the hydroboration of alkenes without reductants [31]. The active catalyst is likely generated from the reaction of cobalt carboxylate with pinacolborane. Considering this, Chirik and co-workers synthesized some pyridine diimine cobalt(II) acetate complexes with smaller substituents and tested the catalytic hydrosilylation performance [30]. Under 1 mol% (TFPDI)Co(2-EH)2 (EH = ethylhexanoate) at 23°C for 1 h, 1-octene can react with alkoxysilanes and siloxanes to give anti-Markovnikov hydrosilylation products with high selectivities (32/1- > 98/2) (Table 6). Using (TFPDI)Co(2-EH)2 as catalyst, a series of terminal alkenes with different functional groups were tested to react with HSi(OEt)3, from which the anti-Markovnikov hydrosilylation products are obtained as the dominated

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Table 6 The (TFAPDI)Co(2-EH)2-catalyzed hydrosilylation of alkenes

hydrosilylation products in high yields (63–97%). The study also mentioned that (TFPDI)Co(2-EH)2 can catalyze silicon cross-linking reactions. Oña-Burgos and co-workers combined cobalt(II) carboxylates with terpyridine ligand to generate catalysts in situ and achieved the hydrosilylation of alkenes in aerobic conditions and non-dried solvents [32]. Among the cobalt(II) carboxylates compounds, cobalt(II) acetylacetonate hydrate, cobalt(II) naphthenate, and cobalt (II) octanoate were found to be the better cobalt source. The catalytic system can promote the hydrosilylation reactions of a range of terminal alkenes bearing different functional groups with H3SiPh, yielding anti-Markovnikov products in good to high NMR yields (Table 7). Besides, H2SiPh2, HSi(OEt)3, and HSiMe2(OTMS) were also suitable for the catalytic system, and the corresponding anti-Markovnikov hydrosilylation products were formed in high yields. In addition, Peng, Bai, and co-workers showed that similar catalytic system containing Co(iso-octoate)2 and terpyridine derivatives can also achieve the hydrosilylation of terminal alkenes [33]. Stoichiometric reaction studies indicated that the interaction of these cobalt (II) acetates with terpyridine can yield ionic cobalt(II) complexes [(tpy)Co (THF)2(OR)][OR] along with a small amount of the homoleptic complexes

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Table 7 The anti-Markovnikov hydrosilylation of alkene catalyzed by cobalt(II) carboxylates with terpyridine ligand

[(tpy)2Co][OR]2 [32]. In situ Raman spectroscopy indicated that the reactions of these cobalt(II) acetate complexes with hydrosilanes and alkenes can yield cobalt hydride species, whose exact identity is unknown. Consequently, a classical Chalk– Harrod mechanism has been proposed for this cobalt-catalyzed hydrosilylation reaction (Scheme 6). Nakazawa, Kobayashi and co-workers reported a closely related catalytic system using [(tpy)CoBr2]/K2CO3 as catalyst [34]. The catalytic hydrosilylation reactions using 0.1 mol% of [(tpy)CoBr2] and 2.0 mol% of K2CO3 operate under neat conditions and can effectively promote the hydrosilylation of terminal alkenes with a series of hydrosilanes including tertiary silanes, yielding anti-Markovnikov addition products in good to high yields (41–99%) (Table 8). The study noted that catalytic system with NaHBEt3 as catalyst activation reagent resulted in decreased yields of the hydrosilylation products. The catalyst was immobilized on a SiO2 stationary phase and the immobilized catalyst (tpy)CoBr2@SiO2/K2CO3 was found to be applicable for catalytic hydrosilylation reactions in a continuous flow reactor [35]. Pawluć et al. synthesized a new cobalt chloride complex bearing tridentate pyridine-imine-imidazole ligand and proved its efficacy in catalyzing the hydrosilylation of alkenes [36]. The cobalt complex was found to promote both hydrosilylation and dehydrogenative silylation of terminal alkenes, and the selectivity was found to depend upon the hydrosilane (Table 9). In the presence of [(NNN)CoCl2] (3 mol%) and NaHBEt3 (12 mol%), styrene reacted with the hydrosilanes H3SiPh, H2SiPh2, and HSiMe2Ph at 80°C to give anti-Markovnikov hydrosilylation

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Scheme 6 The proposed mechanism for cobalt (II) carboxylates with terpyridine ligand-catalyzed hydrosilylation

Table 8 The hydrosilylation of alkene catalyzed by [(tpy)CoBr2] and K2CO3

as the dominated products. When the hydrosiloxane HSiMe(OTMS)2 was used, the dehydrogenative silylation product was detected as the major product with 34% conversion. Triethylsilane appears to be unreactive under the given conditions. Besides, the substituted styrenes bearing functional groups such as Me, OMe, Cl, and Br can undergo hydrosilylation reaction to form mixtures of linear and branch alkylsilanes. On the other hand, the catalytic reactions of 1-hexene and 1-octene led only to alkene isomerization. Later, Pawluć et al. synthesized a series of bench-stable cobalt(II) chloride pre-catalysts coordinated to different pyridine-imine-imidazole ligands and examined their catalytic performance in the reaction of styrene with H3SiPh [37]. Catalyst screening revealed that the cobalt(II) complex featuring benzimidazole/2Himidazole is the most efficient one in terms of hydrosilylation selectivity and catalyst loading. Under optimized conditions, substituted styrenes react with H3SiPh and H2SiPh2 in the presence of [(NNN)CoCl2] (0.25 mol%) and LiHBEt3 (0.75 mol%) at room temperature, yielding anti-Markovnikov hydrosilylation products as the sole or dominated products (Table 10). Low hydrosilane conversion (21%), however,

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Table 9 The hydrosilylation of alkenes catalyzed by cobalt (II) chloride complex with tridentate nitrogen ligand and NaHBEt3

was observed in the reaction of styrene with diethylsilane. The catalytic system is ineffective in facilitating the reactions with the tertiary silanes HSiPhMe2, HSiEt3, or HSiMe2(OTMS). Rauchfuss et al. developed a family of (PNpy)CoCl2 complexes for alkene hydrosilylation, where the ligand PNpy (2-iminopyridyl-phosphine) derives from aminoalkyl and aminoaryl phosphines and 2-keto- and 2-formylpyridines [38]. Investigating their catalytic performance in the hydrosilylation of 1-octene with H2SiPh2 showed that the reaction using monomeric complex (iPr2PC3NHpy) CoCl2 as catalyst exhibits highest rate and selectivity for anti-Markovnikov addition (97% NMR yield) (Scheme 7). Besides, dehydrogenative silylation products, vinylsilanes, were also detected as minor products in these reactions, suggesting that cobalt silyl species might be an in-cycle active intermediate. Stoichiometric reaction studies also support the effectiveness of cobalt silyl species in mediating alkene hydrosilylation [38]. The reaction of (Ph2PC6H4NPhpy) CoCl2 with LiMe in the presence of PPh3 gave the cobalt(I) methyl complex (Ph2PC6H4NPhpy)CoMe(PPh3) (Scheme 7). The interaction of the cobalt methyl complex with H2SiPh2 forms the cobalt(I) silyl complex (Ph2PC6H4NPhpy)Co(PPh3) (SiHPh2) that can further react with ethylene to yield a β-silylethyl cobalt (I) complex. On the other hand, the reaction of (Ph2PC6H4NPhpy)Co(PPh3)(SiHPh2) with styrene affords a cobalt alkene complex (Ph2PC6H4NPhpy)Co(η2-CH2CHC6H5) (SiHPh2) that can further react with H2SiPh2 to give the hydrosilylation product C6H5C2H4SiHPh2. These results strongly support that the cobalt silyl species might be the active intermediate for (PNpy)CoCl2-catalyzed hydrosilylation of alkenes. Later, Rauchfuss et al. developed phosphine-quinoline-pyridine ligands (RPQpy) (R = iPr, C6F5) and found that the corresponding cobalt halide complexes (RPQpy) CoX2 (X = Cl, Br) can also effectively catalyze the hydrosilylation and

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Table 10 The hydrosilylation of alkenes catalyzed by cobalt(II) chloride complex with the benzimidazole/2H-imidazole ligand

Scheme 7 The cobalt-catalyzed hydrosilylation of 1-octene with 2-iminopyridyl-phosphine ligands and its reactions with alkenes and hydrosilanes

dehydrogenative silylation of 1-octene, vinylsiloxanes, ethylene with H2SiPh2, HSi (OEt)3, and HSiPh3 [39]. There have been continuing research interests on the use of (bis(imino)pyridine) cobalt complexes for catalytic hydrosilylation reaction. RajanBabu and co-workers found that the the active catalyst generated from the reaction of (MesPDI)CoCl2 with

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Table 11 The hydrosilylation of alkene catalyzed by (DippPDI)CoCl2 with NaHBEt3

NaHBEt3 at -78°C can catalyze the reactions of terminal alkenes with H3SiPh, H2SiPh2, and H2SiPhMe to yield selectively anti-Markovnikov addition products (Table 11) [40], wherein dehydrogenative silylation reactions [29] can be suppressed. RajanBabu’s study also showed that the active species generated at low temperature can also promote selective hydrosilylation of dienes (Table 12) [40]. Under the catalytic reaction conditions, the reactions of alkyl-substituted 1,3-diene with H3SiPh, H2SiPh2, and H2SiMePh give anti-Markovnikov 1,2-hydrosilylation products in good to excellent yields. In these reactions, the configuration of the internal double bonds of the dienes remains unchanged. Reactions of aryl-substituted dienes yield mixtures of linear and branched products, and those of isoprene and β-myrcene were found to yield 1,4-hydrosilylation product as the dominated products. The fine performance of RajanBabu’s catalytic system was thought to originate from the selective formation of cobalt hydride species at low temperature, and a classical Chalk–Harrod mechanism was proposed (Scheme 8) [40]. The authors believed that the formation of a cobalt silyl species from the further reaction of cobalt hydride species with hydrosilane at -78°C might be kinetically less favored than the reaction with alkenes, which led to the suppression of the modified Chalk– Harrod catalytic cycle. Interestingly, Thomas et al. found that the use of tBuONa as a catalyst activator can generate reactive active species at room temperature, which can promote the hydrosilylation reactions of alkenes with high anti-Markovnikov addition selectivity (Table 13) [41]. Reaction condition screening showed that the catalytic system employing (iPrPDI)CoCl2/tBuONa as catalyst produced anti-Markovnikov

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Table 12 The hydrosilylation of dienes catalyzed by (DippPDI)CoCl2 with NaHBEt3

Scheme 8 The proposed mechanism for (PDI)CoCl2and NaHBEt3-catalyzed hydrosilylation

hydrosilylation product as the main product, whereas changing the pre-catalyst activator to a more conventionally used organometallic reagent (EtMgBr, NaHBEt3 and MeLi) is detrimental to both the yield and regioselectivity of the hydrosilylation reaction. Substituents scope study indicated that the use of (iPrPDI)CoCl2/tBuONa as

158 Table 13 The hydrosilylation Dipp PDICoCl2 with tBuONa

D. Wang and L. Deng of

alkene

catalyzed

by

catalyst allows anti-Markovnikov hydrosilylation reactions of aryl- and alkylsubstituted alkenes bearing a number of functional groups, like tertiary amine, tosylate, ester, and amide that undergo hydrosilylation with H3SiPh. The study noted that the reaction of (DippPDI)CoCl2 with tBuONa and H3SiPh can give diamagnetic species [(iPrPDI)Co] and neither cobalt hydride nor cobalt silyl species was detected from the mixture. The detailed mechanism of the catalytic reaction is unclear. Nakazawa, Kobayashi, and co-workers found that a cobalt-iminobipyridine complex ((2,2′-bpy)-6-C(Me) = N-Mes)CoBr2 can catalyze the hydrosilylation of styrene with H2SiPh2 to give anti-Markovnikov product as the sole hydrosilylation product in 91% yield (Scheme 9) [42]. When 1-octene was subjected to the catalytic system, a mixture of Markovnikov and anti-Markovnikov isomers were formed in 21% and 52% yields, respectively. Further investigation showed that the exchange of substituent on nitrogen atom on the catalyst to Cy, that is ((2,2′-bpy)-6-C (Me) = N-Cy)CoBr2, can form anti-Markovnikov hydrosilylation product in >99% yield [43]. Besides H2SiPh2, the catalytic system using ((2,2′-bpy)-6-C (Me) = N-Cy)CoBr2/NaHBEt3 also allows the use of H3SiPh, H2SiMePh, H2SiEt2, HSiMe2Ph, and HSiMePh2 as the hydrosilane sources (Table 14), though those with tertiary silanes gave the hydrosilylation products in decreased yields. The catalytic system is incapable of promoting the hydrosilylation reaction of pentamethyldisiloxane with 1-hexene. Findlater’s group found the use of pyridine-imine-phosphoramide/CoBr2 as catalyst and NaHBEt3 or NaOtBu as the activator can form reactive catalyst for the antiMarkovnikov hydrosilylation of terminal alkenes with H2SiPh2 (Table 15) [44]. The catalytic system is compatible with the functional groups of borate, epoxy, acetal,

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Scheme 9 The hydrosilylation of 1-octene and styrene with H2SiPh2 catalyzed by ((2,2′-bpy)-6-C (Me) = N-Cy)CoBr2 and ((2,2′-bpy)-6-C(Me) = N-Mes)CoBr2 with NaHBEt3 Table 14 The ((2,2′-bpy)-6-C(Me) hydrosilylation of alkenes

=

N-Cy)CoBr2-

and

NaHBEt3-catalyzed

and ester. Interestingly, the reactions using analogous iron complex as catalyst afford Markovnikov addition adducts. Imidazoline-iminopyridine ligands were also used for cobalt-catalyzed hydrosilylation reactions. Lu’s study showed that the cobalt(II) chloride complex bearing iPr- and NPh-substituted imidazoline-iminopyridine ligand [(IIP)CoCl2] can catalyze the dehydrogenative silylation of styrene with H2SiPh2 [45]. Under optimized conditions, various styrene derivates bearing electron-withdrawing and

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Table 15 The hydrosilylation of alkene catalyzed by CoBr2 and amineiminopyridine ligand

electron-donating groups on the ortho-, meta-, or para-position can smoothly react with H2SiPh2 to yield corresponding vinylsilanes in 47–88% isolated yields (Table 16). Tertiary silanes, like HSiMe2Ph, HSiMePh2, and HSi(OEt)3, are also suitable for this dehydrogenative silylation reaction. The [(IIP)CoCl2]-catalyzed dehydrogenative silylation reaction was proposed to involve cobalt silyl intermediate (Scheme 10) [45]. The insertion of alkene into the Co–Si bond of the cobalt silyl intermediate can generate a cobalt β-silylalkyl species that undergoes β-H elimination to give the dehydrogenative silylation product and the cobalt hydride species. The insertion of alkene into the cobalt hydride species generates a cobalt alkyl species, which further reacts with H2SiPh2 to regenerate cobalt silyl species and the hydrogenation by-product. There are also recent efforts in the use of bidentate nitrogen-based ligands for cobalt-catalyzed hydrosilylation reactions. As a representative, Lee’s group synthesized a series of (aminomethyl)pyridine ligands and their cobalt(II) complexes [46]. The interaction of the cobalt complex bearing the nitrogen ligand that has a 2-methylnaphthyl group and two phenyl groups with three equiv. of TMSCH2Li was found to generate active species that can effectively catalyze the anti-Markovnikov hydrosilylation reactions of vinylsilanes with alkoxy- or siloxy-substituted hydrosilanes (Table 17), giving the hydrosilylation products in >98% yields and >98% anti-Markovnikov selectivity. This catalytic system is also applied for the preparation of polymeric vinyl- and hydrosiloxanes. The mechanism of the reaction

Recent Development of Cobalt-Catalyzed Hydrosilylation Reactions:. . . Table 16 The imidazoline-iminopyridine dehydrogenative silylation of aryl alkenes

cobalt

complex

161

[(IIP)CoCl2]-catalyzed

Scheme 10 The proposed mechanism for [(IIP)CoCl2]-catalyzed dehydrogenative silylation

is unclear. The author proposed that under the reaction conditions the N-H moiety in the ligand might be deprotonated to form amido ligand. Kuai, Ji, Chen, and co-workers applied bidentate pyridine-imine ligands for Co (acac)3-catalyzed hydrosilylation reactions of isoprene with secondary hydrosilanes and found that the selectivity of the addition reaction can be tuned by the steric nature of ortho-substituents of the pyridyl motif [47]. The use of catalyst bearing simple bidentate pyridine imine ligand can ensure the 4,1-addition product as the dominate products (Table 18), whereas the use of the cobalt catalyst featuring an isopropyl group on the ortho-position of the pyridyl motif facilitates 2,1-addition reactions (Table 19). Secondary hydrosilanes bearing different substituents can be successfully applied to the regioselective hydrosilylation reactions. But tertiary silanes are inapplicable. The authors proposed cobalt silyl and cobalt hydride species as the in-cycle species for the 4,1-addition and 2,1-addition reactions, respectively. The different steric nature of the pyridine imine ligand was thought to govern the preference of the formation of cobalt silyl or cobalt hydride species.

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Table 17 The cobalt-catalyzed hydrosilylation of alkoxy- or siloxy (vinyl)silanes with alkoxy- or siloxyhydrosilanes

Trzaskowski, Tamm, Frank, and co-workers reported alkene hydrosilylation reactions using a cobalt(I) catalyst bearing an amino-imidazoline-2-imine ligand (AmIm)Co(η6-C6H6) [48]. Using 0.1 mol% catalyst, the terminal alkyl alkenes, 1-hexene, trimethyl(vinyl)silane, and vinylcyclohexane, can react with primary, secondary, and tertiary silanes to give the anti-Markovnikov addition products in high yields (81–97%) (Table 20). On the other hand, the catalytic hydrosilylation of the aryl-substituted alkenes, styrene and 1-methyl-2-vinylbenzene, afforded a mixture of Markovnikov and anti-Markovnikov products. Theoretical calculations were performed to probe the reaction mechanism [48]. It suggested that the cobalt-catalyzed hydrosilylation reaction might operate on a triplet energy surface (Scheme 11). The interaction of (AmIm)Co(η6-C6H6) with H3SiPh might generate a triplet cobalt(I) silane species (AmIm)Co(η2-H-SiPhH2) that can interact with olefin to form cobalt(III) silyl alkyl intermediate (AmIm)Co (SiPhH2)(CH2CH2R). Further interaction of the cobalt(III) species with H3SiPh resulted in reductive elimination reaction, yielding the hydrosilylation product and cobalt(I) silane species (AmIm)Co(η2-H-SiPhH2). Considering the steric demanding

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Table 18 Cobalt-catalyzed protocol for 4,1-hydrosilylation of isoprene

Table 19 Cobalt-catalyzed protocol for 2,1-hydrosilylation of isoprene

nature of the first coordination sphere of the cobalt center, the authors thought that a modified Chalk–Harrod mechanism involving cobalt silyl intermediate is unlikely to be involved in this catalytic hydrosilylation reaction.

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Table 20 The (AmIm)Co(η6-C6H6)-catalyzed hydrosilylation of alkenes

Scheme 11 The proposed mechanism for (AmIm)Co(η6-C6H6)-catalyzed hydrosilylation

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2.1.4

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Anti-Markovnikov Hydrosilylation of Alkenes Using Cobalt-Phosphine Catalysts

Phosphine ligands have been popularly used in cobalt-catalyzed hydrosilylation reactions. Haszeldine et al. had found that the cobalt phosphine complexes, CoH (N2)(PPh3)3 and CoH3(PPh3)3, can catalyze the hydrosilylation of 1-hexene with HSi(OEt)3 at room temperature [49, 50]. Brookhart and Grant found the pentamethylcyclopentadienyl cobalt(III) alkyl complex [Cp*Co(P(OMe)3) (CH2CH3)][BArF4] (where ArF = 3,5-ditrifluoromethylphenyl) can catalyze the hydrosilylation of 1-hexene with HSiEt3 [51]. Following these pioneering work, recent explorations have led to the development of a large number of cobalt phosphine catalysts effective for hydrosilylation reactions (Fig. 3). In 2017, Ge’s group reported anti-Markovnikov hydrosilylation of alkenes using Co(acac)2/phosphines as pre-catalyst [52]. The study shows that the combination of Co(acac)2 with Xantphos can catalyze the reactions of various terminal alkyl alkenes with H3SiPh, yielding anti-Markovnikov hydrosilylation products in good to high yields (61–93%) and high regioselectivities (98/2- > 99/1) (Table 21). This catalytic system can tolerate the functionalities, ether, siloxy, chloro, cyano, ester, epoxide, and acetal groups. On the other hand, the combination of Co(acac)2 with dppf can serve as catalyst for anti-Markovnikov hydrosilylation of terminal aryl-substituted alkenes with H2SiPh2 (Table 22) [52]. These reactions were proposed to have cobalt hydride species as the key intermediates. As supporting evidence, the reaction of Co (acac)2 with 8 equiv. of PhSiH3 and 1 equiv. of Xantphos was thought to yield a cobalt hydride complex (Xantphos)CoH that is characterized by 1H NMR and HR-MS (ESI). Moret and co-workers showed that the Co(I) complex (pToldpbp)CoCl bearing the diphosphine-ketone ligand can catalyze the anti-Markovnikov hydrosilylation of

Fig. 3 Cobalt catalysts used in the anti-Markovnikov hydrosilylation of alkenes with phosphine ligands

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Table 21 The hydrosilylation of alkenes catalyzed by Co(acac)2 and Xantphos ligand

Table 22 The anti-Markovnikov hydrosilylation of alkenes catalyzed by Co(acac)2 and dppf ligand

allylbenzene and 1-octene with H3SiPh (Scheme 12). When styrene is used as the substrate, the yield of the hydrosilylation product is low [53]. Li and Sun’s group has developed a series of cobalt phosphine catalysts for alkene hydrosilylation. They revealed that the simple cobalt(I) chloride complex (PMe3)3CoCl can catalyze the anti-Markovnikov hydrosilylation of the terminal alkyl alkenes with H2SiPh2 (Table 23) [54]. The reaction was proposed to have cobalt(III) hydride silyl species as the key intermediate (Scheme 13). Upon the reactions of (PMe3)3CoCl with Ph2PC6H4-o-CHO and the 1,3-disilylene compound 1,3-((PhC(tBuN)2Si)(Et)N)2C6H4, Li and Sun’s group prepared the cobalt(III) hydride complexes [mer-(Me3P)2Co(H)(Cl)(o-Ph2P-C6H4-

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Scheme 12 Cobalt(I) complex (p-toldpbp)CoCl-catalyzed hydrosilylation of 1-octene and allylbenzene with H3SiPh Table 23 The (PMe3)3CoCl-catalyzed anti-Markovnikov hydrosilylation of alkenes

C=O)] [55] and [(1,3-((PhC(tBuN)2Si)(Et)N)2C6H3)Co(PMe3)(H)(Cl)] (Table 24) [56]. Both cobalt(III) complexes also proved effective catalysts for the anti-Markovnikov hydrosilylation reactions of alkyl-substituted terminal alkenes with H2SiPh2. In addition, the cobalt(III) hydride complex [(CNC)Co(H)(PMe3)2] (Table 25) [57], the cobalt(I) acyl complex [(Me3P)3Co(o-Ph2P-C6H4-C=O)] [58], cobalt(I) complex bearing a phosphine-silyl-phosphine ligand [(2-Ph2PC6H4)2HSiCo(PMe3)2] (Table 26) [59], and cobalt(II) aryl complex [(F4C5N)Co(Cl)(PMe3)3] [60] all proved active in catalyzing the hydrosilylation reactions of alkenes with H2SiPh2. Although the yields of the hydrosilylation products in these catalytic systems are different, their α/β-selectivities are close to each other. As Li and Sun’s cobalt catalysts listed above mainly affect the hydrosilylation reactions of alkyl-substituted alkenes, their further studies showed that the cobalt(III) halides bearing pincer ligands [(2-Ph2PC6H4)2PhSiCo(H)(Cl)(PMe3)] [61] and [(2,6-((Ph2P)(Et)N)2C6H3)Co(PMe3)(H)(Cl)] [56] can catalyze the hydrosilylation reactions of H2SiPh2 with both alkyl- and aryl-substituted alkenes. The reactions

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Scheme 13 The proposed mechanism for (PMe3)3CoCl-catalyzed hydrosilylation of alkenes

Table 24 The [(1,3-((PhC(tBuN)2Si)(Et)N)2C6H3)Co(PMe3)(H)(Cl)]-catalyzed antiMarkovnikov hydrosilylation of alkenes

with [(2-Ph2PC6H4)2PhSiCo(H)(Cl)(PMe3)] as the catalyst yield anti-Markovnikov hydrosilylation products in yields of 49–95% and β/α-selectivities ranging from 70/30 to 99/1 (Table 27). Interestingly, in the reactions of aryl-substituted alkenes, the addition of pyridine N-oxide to the catalytic system can reverse the selectivity of the reaction from anti-Markovnikov to Markovnikov addition. Similarly, the cobalt (III) hydride complex [(2,6-((Ph2P)(Et)N)2C6H3)Co(PMe3)(H)(Cl)] also catalyzes

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Table 25 The [(CNC)Co(H)(PMe3)2]-catalyzed anti-Markovnikov hydrosilylation of alkenes

Table 26 The [(2-Ph2PC6H4)2HSiCo(PMe3)2]-catalyzed hydrosilylation of alkenes

anti-Markovnikov

the hydrosilylation of alkyl and aryl alkenes to form linear alkylsilanes as the dominant products (Table 28). Mechanism studies showed that the catalyst [(2-Ph2PC6H4)2PhSiCo(H)(Cl) (PMe3)] does not react with styrene, but its reaction with Ph2SiH2 can form the cobalt(III) dihydride complex [(2-Ph2PC6H4)2PhSiCo(H)2(PMe3)] that exhibits comparable catalytic activity as the pre-catalyst [61]. It was proposed that the cobalt(III) dihydride complex (2-Ph2PC6H4)2PhSiCo(H)2(PMe3) might eliminate H2 to form the in-cycle intermediate ((2-Ph2PC6H4)2PhSi)Co(PMe3) that can then

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Table 27 The [(2-Ph2PC6H4)2PhSiCo(H)(Cl)(PMe3)]-catalyzed antiMarkovnikov hydrosilylation of aryl and alkyl alkenes

Table 28 The [(2,6-((Ph2P)(Et)N)2C6H3)Co(PMe3)(H)(Cl)]-catalyzed anti-Markovnikov hydrosilylation of aryl and alkyl alkenes

interact with hydrosilane and alkene to form the linear product (Scheme 14). The catalyst [(2-Ph2PC6H4)2PhSiCo(H)(Cl)(PMe3)] contains merely one PMe3 ligand. Consequently, it was proposed that the Li and Sun’s catalysts that are incapable of catalyzing the hydrosilylation reactions of aryl alkynes might be due to the presence of two or more equiv. of PMe3 in the catalytic system, which might render the coordination of aryl-substituted alkene hard to occur.

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Scheme 14 The proposed mechanism for [(2-Ph2PC6H4)2PhSiCo(H) (Cl)(PMe3)]-catalyzed antiMarkovnikov hydrosilylation of alkenes

2.2

Markovnikov Hydrosilylation of Alkenes

Compared with anti-Markovnikov hydrosilylation, Markovnikov hydrosilylation of alkenes to give branched alkylsilanes is thought to be disfavored in thermodynamics and kinetics. Some cobalt catalysts can catalyze the Markovnikov hydrosilylation of aryl alkenes and the π–π interaction between the ligand and aryl alkenes is believed to play a key role for the “unusual” regioselectivity (Fig. 4). In 2016, Huang et al. reported that (iPrPCNNMe)CoCl2/NaHBEt3 can catalyze the addition reactions of alkyl-substituted terminal alkenes with H3SiPh to form Markovnikov hydrosilylation products with high selectivity (Table 29) [62]. The catalytic system is compatible with many functionalities on alkyl chains. However, when styrene is subjected to the catalytic system, a mixture of linear and branched hydrosilylation products are isolated. As the deuterated-labeling experiment with D3SiPh showed no deuterium incorporation into the α-carbon of the hydrosilylation product, which precludes the involvement of cobalt hydride intermediates, Huang’s catalytic system was proposed to operate via a modified Chalk–Harrod mechanism (Scheme 15). On the other hand, the similar activity of the Co(I) complexes (iPrPCNNMe)CoCl and (iPrPCNNMe)CoMe in the reaction as that using (iPrPCNNMe)CoCl2 points out the catalytic relevance of cobalt(I) species. Ge’s group found that the combination of Co(acac)2 and a bis(imino)pyridine ligand mesPDI can facilitate the Markovnikov hydrosilylation of terminal alkyl alkenes. The Co(acac)2/Xantphos catalyst can catalyze the Markovnikov hydrosilylation of terminal aryl alkenes [52]. In the presence of 3 mol% of Co (acac)2/mesPDI in toluene at 60°C, a wide range of terminal alkyl alkenes can react with PhSiH3 to form branched alkylsilanes in high yields with high to excellent regioselectivities (Table 30). The functional groups like ether, thioether, siloxy,

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Fig. 4 Cobalt catalysts used in the Markovnikov hydrosilylation of alkenes Table 29 The (iPrPCNNMe)CoCl2- and NaHBEt3-catalyzed Markovnikov hydrosilylation of alkyl alkenes

chloro, bromo, ester, amide, acetal, ketone, pyridine, and epoxide moieties are all tolerated. However, internal alkenes and alkenes containing aldehyde, nitro, unprotected hydroxyl, or unprotected amine group cannot undergo hydrosilylation reactions under these conditions. Alkenes containing allyl ether moiety reacts with

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Scheme 15 The proposed mechanism for (iPrPCNNMe) CoCl2 and NaHBEt3 catalyzed Markovnikov hydrosilylation

Table 30 Markovnikov hydrosilylation of terminal alkyl alkenes catalyzed by the Co(acac)2 and mes PDI.

modest regioselectivities, which likely stems from the interaction of the substrate ether groups with the cobalt center. In the presence of Co(acac)2 and Xantphos, various terminal aryl alkenes can react with H3SiPh smoothly to give Markovnikov hydrosilylation products in good to high yields (51–96%) and high regioselectivities (97/3- > 99/1) (Table 31). The electronic properties of the substituents on aryl groups do not have a significant influence on this cobalt-catalyzed hydrosilylation. The steric hindrance at the metaposition or the para-position of aryl groups also has little influence on the

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Table 31 The hydrosilylation of alkenes catalyzed by Co(acac)2 and Xantphos ligand

Table 32 The Markovnikov 1,2-hydrosilylation of conjugated dienes catalyzed by Co(acac)2 and Xantphos ligand

regioselectivity. Steric hindrance at the ortho-position, however, has significant influence on the regioselectivities. Ge’s group found that the catalytic system of Co(acac)2 and Xantphos can also catalyze the Markovnikov 1,2-hydrosilylation of conjugated dienes (Table 32) [63]. Various aryl- and alkyl-substituted as well as multiple-substituted trans-diene can react with H3SiPh to give (E)-allylsilanes in high isolated yields with excellent regioselectivity. The steric and electronic properties of these substituents have little effect on the yield and selectivity. This catalytic system is incapable of catalyzing the reactions of the dialkylsilane or tertiary silanes. Surprisingly, the conjugated dienes containing a mixture of (E/Z )-isomeric 1,3-dienes can undergo Markovnikov 1,2-hydrosilylation in a stereoconvergent manner, affording the corresponding (E)-

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Scheme 16 The proposed mechanism for Co(acac)2and Xantphos ligandcatalyzed hydrosilylation of conjugated dienes.

allylsilanes in high yield and selectivity. These results indicate the occurrence of alkene isomerization, which is further supported by the deuterium-labeling experiments. This cobalt-catalyzed Markovnikov hydrosilylation of conjugated dienes was proposed to have the hydrometallation of a cobalt hydride intermediate with alkenes as a key step (Scheme 16). The allyl cobalt species formed from the insertion of Zalkene to Co–H bond underwent σ-π-σ isomerization to generate the other allyl cobalt intermediate that can be formed from the insertion of E-alkene to a Co–H bond, accounting for the stereoconvergent of the (Z/E)-diene hydrosilylation. Thomas’s group had investigated the ligand-controlled regiodivergent alkene hydrosilylation [41]. While the combination of (DippPDI)CoCl2 with tBuONa proved effective in catalyzing anti-Markovnikov hydrosilylation of 1-alkenes with H3SiPh. (EtPDI)CoCl2/tBuONa was found to promote Markovnikov hydrosilylation of terminal alkenes with H3SiPh (Table 33). Interestingly, changing the pre-catalyst activator to the more conventionally used organometallic reagents EtMgBr, NaHBEt3, and MeLi proved detrimental to both the yield and regioselectivity of the hydrosilylation reaction. Along with the finding that [(IAd)(PPh3)CoCl] catalyzes anti-Markovnikov hydrosilylation of terminal alkyl alkenes with H2SiPh2, Deng’s group also found that the cobalt(I) NHC complex [(IMes)2CoCl] can mediate Markovnikov hydrosilylation of terminal aryl alkenes with H2SiPh2 [25] (Table 34). Using [(IMes)2CoCl] (5 mol%) as catalyst, a number of aryl alkenes, including 2-vinylthiophene and vinylnaphthalene, can undergo Markovnikov hydrosilylation to yield alkyldiphenylsilanes with the branched/linear ratio of 10:1 to 57:1. The catalytic reactions of terminal alkyl alkenes with H2SiPh2, however, produce Markovnikov hydrosilylation products in low yields (20–26%). Similar to anti-Markovnikov addition reaction catalyzed by [(IAd)(PPh3)CoCl], cobalt(I) silyl species was also proposed as the active intermediate for the Markovnikov addition reaction catalyzed by [(IMes)2CoCl]. The different steric nature of IAd versus IMes and the potential of IMes incurring π


π interaction

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Table 33 The Markovnikov hydrosilylation of alkene catalyzed by EtPDICoCl2 with tBuONa

Table 34 The [(IMes)2CoCl]-catalyzed Markovnikov hydrosilylation of alkenes

with aryl alkenes are believed to be the causes for the differentiated 1,2- and 2,1-addition selectivity in the two catalytic systems (Scheme 17). A number of other cobalt complexes have also proved effective in catalyzing the Markovnikov addition of terminal aryl alkenes with H2SiPh2. These include Co (PMe3)4 (Table 35) [54], [mer-(Me3P)2Co(H)(Cl)(o-Ph2P-C6H4-C=O)] [55], [(1,3-((PhC(tBuN)2Si)(Et)N)2C6H3)Co(PMe3)(H)(Cl)] (Table 36) [56], [(CNC)Co (H)(PMe3)2] [57], [(Me3P)3Co(o-Ph2P-C6H4-C=O)] [58], [(2-Ph2PC6H4)2HSiCo (PMe3)2] [59], and [(F4C5N)Co(Cl)(PMe3)3] [60], reported by Li and Sun’s group.

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Scheme 17 The proposed mechanism for [(IMes)2CoCl]-catalyzed Markovnikov hydrosilylation

Table 35 The Co(PMe3)4-catalyzed hydrosilylation of alkenes

Markovnikov

In general, all these catalytic systems exhibit good functional group compatibility. They facilitate the addition reaction with mono-substituted aryl alkenes with H2SiPh2, but are incapable of catalyzing the reactions of disubstituted alkenes or the reactions using tertiary silanes. Interestingly, the catalytic reactions using [mer-(Me3P)2Co(H)(Cl)(o-Ph2P-C6H4-C=O)] [55], [(1,3-((PhC(tBuN)2Si) (Et)N)2C6H3)Co(PMe3)(H)(Cl)] [56], and [(Me3P)3Co(o-Ph2P-C6H4-C=O)] [58] are found to use DMSO as a co-solvent and that of [(CNC)Co(H)(PMe3)2] [57] even uses pyridine N-oxide as an additive. The roles of DMSO and pyridine N-oxide in these reactions are unknown. Li and Sun have proposed two mechanistic scenarios for their Markovnikov addition reactions: a modified Chalk–Harrod mechanism for the reaction catalyzed by Co(PMe3)4 (Scheme 18) and a concerted hydrogen-atom transfer mechanism for the reactions catalyzed by pincer cobalt complexes (Scheme 19). In the catalytic system of [(CNC)Co(H)(PMe3)2], the authors also thought that the π–π interaction between the aryl rings and substrates is responsible for the 2,1-insertion to give

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Table 36 The [(1,3-((PhC(tBuN)2Si)(Et)N)2C6H3)Co(PMe3)(H)(Cl)]catalyzed Markovnikov hydrosilylation of aryl alkenes

Scheme 18 The proposed mechanism for Co(PMe3)4catalyzed Markovnikov hydrosilylation of alkenes

Markovnikov selectivity for aryl alkenes. Theoretical studies might provide supporting evidence for the proposed mechanism, which, however, is unknown yet. An important advance in cobalt-catalyzed hydrosilylation reactions of alkenes is the development of enantioselective hydrosilylation reactions of alkenes, which yield chiral hydrosilanes that are highly valuable building blocks in asymmetric synthesis of natural product. In 2017, Lu’s group reported an enantioselective Markovnikov hydrosilylation of both terminal aryl and alkyl alkenes with primary hydrosilanes using chiral oxazoline-pyridine-imine-supported cobalt complex (OIP)CoCl2 as the catalyst [64]. In the presence of (OIP)CoCl2 (1 mol%) and NaOtBu (3 mol%), various terminal alkenes can react with H3SiAr smoothly at room temperature to give chiral dihydrosilanes in good to high yields (53–97%) and high regioselectivities (≥96/4) and enantioselectivities (80.8–99.8% ee) (Table 37).

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Scheme 19 The proposed mechanism for pincer cobalt-catalyzed Markovnikov hydrosilylation of alkenes

Ar

Table 37 The (OIP)CoCl2- and NaOtBu-catalyzed enantioselective Markovnikov hydrosilylation

Similar to the aforementioned Markovnikov addition reactions, secondary and tertiary silanes cannot be applied for the enantioselective Markovnikov hydrosilylation reactions. The group also studied the reaction of D8-styrene with 4-methoxyphenylsilane using (OIP)CoCl2/NaOtBu as catalyst and found that no deuterium-scrambling occurred. This observation indicates that cobalt hydride species is unlikely to be involved in the catalytic cycle. Consequently, a catalytic cycle involving cobalt silyl species was proposed for this cobalt-catalyzed enantioselective Markovnikov hydrosilylation reaction (Scheme 20).

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Scheme 20 The proposed mechanism for (OIP)CoCl2and NaOtBu-catalyzed enantioselective Markovnikov hydrosilylation

Chiral oxazoline-pyridine-imine-supported cobalt complex is also found to be effective in catalyzing enantioselective Markovnikov addition reactions of cyclopropenes with primary silanes H3SiAr [65]. Marek et al. performed ligand screening on the catalytic reactions and identified that the tert-butyl-substituted chiral oxazoline-pyridine-imine ligand can affect the diastereoselective and enantioselective addition reactions (Table 38). Huang and co-workers achieved cobalt-catalyzed regio- and enantioselective 1,2-Markovnikov hydrosilylation of diene using chiral bidentate pyridine-oxazoline as ligand [66]. With the cobalt complex containing a tBu-substituted pyridineoxazoline ligand (QuinOxtBu)CoCl2 as pre-catalyst, a series of 1,3-dienes react with H3SiPh to form 1,2-Markovnikov addition products with up to >99:1 rr and 94% ee (Table 39). The catalytic system is also suitable for some other primary hydrosilanes, including H3Si(4-Cl-C6H4), H3Si(4-OMe-C6H4), and H3Si(3,5-Me2C6H3). Lower regioselectivity and enantioselectivity are found in the cases using H2SiPh2 as the silane source. Similar to Lu’s system, a modified Chalk–Harrod mechanism involving cobalt silyl intermediates was proposed in Huang’s catalytic reaction. In addition to the chiral nitrogen ligands, the application of chiral phosphines to cobalt-catalyzed hydrosilylation reactions of alkenes has also led to the development of enantioselective hydrosilylation reactions. Using Co(acac)2/(R)-difluorphos (4 mol%) and NaHBEt3 (5 mol%) as catalyst, Meng and co-workers showed that 1,3-dienes can react with H2SiPhMe to produce chiral allylsilanes in high yields (72–96%) and good selectivity (72–90% de, 92- > 99% ee) (Table 40) [67]. Different to other cobalt phosphine-catalyzed hydrosilylation reactions, a cobalt silyl species was proposed as the active intermediate in Meng’s system (Scheme 21). Control experiments, deuterium-labeling, and kinetic experiments indicated that the C- and Si-stereogenic centers in the allyl silanes are built in high selectivity from two

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Table 38 Cobalt-catalyzed diastereoselective and enantioselective hydrosilylation of cyclopropenes

individual steps. The regioselectivity of the catalytic system may be governed by steric repulsion between the large diphosphine ligand and the alkenes.

2.3

Hydrosilylation of Allenes

The hydrosilylation of allenes can form vinylsilanes and allylsilanes. Selective control is a key issue. The cobalt-catalyzed hydrosilylation of allenes had been studied with stoichiometric amounts of Co2(CO)8 and low selectivity was observed [68]. The catalytic reaction was achieved recently. Ma, Huang, and co-workers found that in the presence of (tBuPCNNiPr)CoCl2 (0.5 mol%) and NaHBEt3 (1.0 mol%), terminal allenes can react with H3SiPh to form linear Z-allylsilanes in 80–95% yield (Table 41). This catalytic system can also achieve the hydrosilylation of unsymmetric 1-methyl-1-phenylallene in a decent yield (81%). However, complicated mixtures are obtained when secondary hydrosilanes H2SiPh2 and H2SiEt2 and tertiary silanes HSiEt3 are subjected to the catalytic system. The combination of Co(acac)2 and BINAP can also catalyze the hydrosilylation of mono-substituted terminal allenes with H3SiPh to furnish (Z )-allylsilanes with

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Table 39 The (QuinOxtBu)CoCl2- and NaHBEt3-catalyzed enantioselective 1,2-Markovnikov hydrosilylation of diene

Table 40 Cobalt-catalyzed asymmetric Markovnikov 1,2-hydrosilylation of conjugated dienes

high regio- and stereoselectivity (Table 42) [69]. A range of reactive groups, such as chloro, bromo, iodo, siloxy, ester, and pinacol boronic ester, are compatible with the reaction conditions. On the other hand, the combined catalyst of Co(acac)2 with Xantphos was found effective for the hydrosilylation of disubstituted terminal allenes with H3SiPh, yielding Z-allylsilanes in high yield (74–92%) and high selectivity (up to 99/1) (Table 43). The later catalytic system also allows the use of secondary hydrosilanes H2SiPh2, H2SiMePh, and H2SiEt2, but is ineffective in

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Scheme 21 The proposed mechanism for cobalt-catalyzed asymmetric Markovnikov 1,2-hydrosilylation of conjugated dienes Table 41 Cobalt-catalyzed hydrosilylation of allenes with PNN tridentate ligand

catalyzing the reactions of the tertiary silanes HSi(OEt)3 or HSiMe(OTMS)2. Ge’s group studied the reaction of Co(acac)2 and H3SiPh in the presence of dppbz ligand, which was found to generate a cobalt(I) hydride complex (dppbz)2CoH. The cobalt hydride species exhibits comparable catalytic activity and selectivity as those of the reaction employing Co(acac)2/dppbz. Subsequently, a hydrometallation pathway involving a cobalt hydride species was proposed for the cobalt-catalyzed hydrosilylation reaction of allenes (Scheme 22).

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Table 42 Cobalt-catalyzed hydrosilylation of mono-substituted allenes with diphosphine ligand

Table 43 Cobalt-catalyzed hydrosilylation of disubstituted terminal allenes with diphosphine ligand

3 Cobalt-Catalyzed Hydrosilylation of Alkynes Transition-metal-catalyzed hydrosilylation of alkynes is an ideal method for the preparation of vinylsilanes that are valuable synthetic reagents in organic synthesis. The challenge here is the control of the regio- and stereoselectivity, because even the hydrosilylation of a terminal alkyne may result in three vinylsilanes, namely, the Markovnikov addition product α-vinylsilanes and anti-Markovnikov β-(Z), β-(E)

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Scheme 22 Proposed mechanism for cobaltcatalyzed hydrosilylation of allenes with diphosphine ligand

Scheme 23 The hydrosilylation of terminal alkynes

isomers (Scheme 23). Cobalt carbonyl complexes were tested for the hydrosilylation of alkynes [70–73]. Later, various cobalt catalytic systems with different ligands have been developed for hydrosilylation of terminal and internal alkynes.

3.1

Markovnikov Hydrosilylation of Terminal Alkynes

Cobalt-catalyzed hydrosilylation of terminal alkynes usually forms α-isomers of vinylsilanes, which is different to the precious metal catalytic systems, where the more stable β-(E)-vinylsilanes are isolated as the predominant product. The regioselectivity maybe originate from steric repulsion between the ligand and the alkyne substituents on the coordination sphere of cobalt that has small atomic radii. A number of bidentate and tridentate nitrogen ligands were introduced to cobaltcatalyzed hydrosilylation reactions of alkynes, and high Markovnikov hydrosilylation of terminal aryl alkynes with primary and secondary hydrosilanes has been achieved (Fig. 5). An exception is the Deng’s dicobalt carbonyl NHC-catalyzed reaction, wherein binuclear cobalt carbonyl complex with NHC ligation can catalyze the Markovnikov hydrosilylation of both terminal alkyl and aryl alkynes with tertiary silanes. Lu’s and Huang’s groups independently reported the Markovnikov hydrosilylation of terminal aryl alkynes catalyzed by (OIP)CoBr2/NaHBEt3 and

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Fig. 5 Cobalt catalysts used in the Markovnikov hydrosilylation of terminal alkynes

(tBuPyBox)CoCl2/NaHBEt3, respectively (Tables 44 and 45) [74, 75]. Ligand screening showed that the substituents on the oxazoline have a significant effect on the activity and selectivity of the catalytic reaction. Under these catalytic reaction conditions, lots of terminal aryl alkynes bearing electron-donating or electronwithdrawing substituents in the ortho, meta, and para position of the phenyl ring as well as heteroaryl alkynes can all react smoothly with H2SiPh2 to give αvinylsilane as the dominant or sole hydrosilylation products. Later, other tridentate and bidentate nitrogen ligands were also investigated and found effective in facilitating cobalt-catalyzed Markovnikov hydrosilylation of terminal aryl alkynes. Yang’s (amine-pyridine-imine)cobalt catalyst (Table 46) [76], the von Wangelin’s (bipyridine)cobalt catalyst (Table 47) [77], Chen’s (terpyridine)cobalt catalyst (Table 48) [78], and Jin’s (iminopyridine)cobalt catalyst (Table 49) [79] have all proved to be highly effective for the Markovnikov hydrosilylation of terminal aryl alkynes with H3SiPh and/or H2SiPh2. When it comes to terminal alkyl alkynes, the regioselectivity of the addition reactions commonly decreased and a mixture of Markovnikov and anti-Markovnikov vinylsilanes were obtained. For example, long-chain substituted alkyne 1-hexyne and 1-octyne can react with H3SiPh and H2SiPh2 to give vinylsilanes with high isolated yields (85–98%), but the α/β regioselectivities are merely 4/1–1/1. The Markovnikov addition reactions of terminal alkynes with secondary hydrosilanes can be used for the synthesis of Si-stereogenic vinyl silanes. Huang and co-workers found that a pyridine-bis(oxazoline)cobalt complex is well suited for this task. With a catalyst loading of 2 mol%, a series of silicon-stereogenic α-vinylsilanes have been obtained from the reactions of terminal aryl alkynes with H2SiPhAr’ with high regio- (>99/1) and enantioselectivity (82–91% ee) (Table 50). Mechanistic study showed these catalytic systems might obey modified Chalk– Harrod mechanism. Cobalt silyl species, formed from the reaction of pre-catalysts

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Table 44 The (OIP)CoBr2-catalyzed Markovnikov hydrosilylation of terminal alkynes

Table 45 The (tBuPyBox)CoCl2-catalyzed hydrosilylation of terminal alkynes

Markovnikov

with hydrosilanes and activator, are the active intermediates in the catalytic cycle (Scheme 24). von Wangelin and co-workers studied the catalytic system of Co (OAc)2•4H2O and 4Mebipy by LIFDI-MS measurements and detected species with m/z 746.00 that is equal to that of the cobalt silyl species (4Mebipy)2-Co (SiHPhSiHPhSiH2Ph) [77]. In these catalytic reactions, most likely for steric

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Table 46 The (API)CoCl2-catalyzed Markovnikov hydrosilylation of terminal alkynes

Table 47 The Co(OAc)2•4H2O/4-Mebipy-catalyzed Markovnikov hydrosilylation of terminal alkynes

reasons, alkynes undergo selective 1,2-insertion into the Co-Si bond of cobalt silyl species to form cobalt vinyl intermediate that then reacts with hydrosilane to form the Markovnikov addition product. In the case of Yang’s study, anti-addition products were observed from the deuterium-labeling experiments [76], which was proposed to be formed from a Crabtree-Ojima-type isomerization of cobalt vinyl intermediates. Notably, despite the popularity of cobalt(I)–cobalt(III) cycles for these cobalt-catalyzed hydrosilylation reactions, Ma’s DFT calculation study pointed out that the reaction pathways involving Co(0) rather than Co(I) species

Recent Development of Cobalt-Catalyzed Hydrosilylation Reactions:. . . Table 48 The (NNN)CoCl2-catalyzed hydrosilylation of terminal alkynes

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Table 49 The Co(OAc)2/iminopyridyl-catalyzed hydrosilylation of terminal alkynes

Markovnikov

are more energetically favored for the Markovnikov hydrosilylation reactions of phenylacetylene with silanes using (diiminopyridine)cobalt and (dioxazoline-pyridine)cobalt species as catalysts (Scheme 25) [80].

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Table 50 Asymmetric Markovnikov hydrosilylation of terminal alkynes catalyzed by cobalt complex with tridentate nitrogen ligands

Scheme 24 Plausible catalytic cycle for Markovnikov hydrosilylation of terminal alkynes catalyzed by cobalt complexes with nitrogen ligands

Pawluć’s (benzimidazole/2H-imidazole)cobalt complex was also found to be effective for the hydrosilylation of terminal alkynes with H3SiPh, H2SiPh2, and HSiMe2Ph, yielding moderate to good Markovnikov hydrosilylation products (Table 51) [81]. Except for HSiMe2Ph, the other tertiary silanes HSiPh3, HSiEt3, and HSi(OEt)3 are ineffective hydrosilane partners for this catalytic system. Intriguingly, Pawluć’s further study showed that the cobalt complex bearing pyrimidine/ 2H-imidazole ligand can catalyze the Markovnikov hydrosilylation of terminal alkynes with a range of tertiary silanes, e.g., HSiPh3, HSiEt3, HSi(OEt)3, HSi (OEt)2Me, HSiMePh2, HSiMe2(OTMS) with the yields of the Markovnikov addition products ranging from 32% to 98% and α/β selectivity being 1.5/1- > 99/1 (Table 52) [82]. Deng’s group achieved the Markovnikov hydrosilylation of terminal alkyl and aryl alkynes with tertiary silanes with the dinuclear cobalt carbonyl NHC catalyst

Recent Development of Cobalt-Catalyzed Hydrosilylation Reactions:. . . Scheme 25 DFT calculation-predicted mechanism for the Markovnikov hydrosilylation reactions of phenylacetylene with silanes

Table 51 Markovnikov hydrosilylation of terminal alkynes catalyzed by cobalt complexes with benzimidazole/2H-imidazole-based tridentate nitrogen ligand

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Table 52 Markovnikov hydrosilylation of terminal alkynes catalyzed by cobalt complexes with pyrimidine/2H-imidazole ligand

[(IPr)2Co2(CO)6] [83]. Catalyst development investigation showed that the steric hindrance of NHC ligand has a significant effect on the α/β selectivity and larger steric hindrance contributed to the better α/β selectivity. Compared with the catalytic performance of Co2(CO)8 and other phosphine ligands, the NHC ligand, especially the IPr, exhibited better catalytic activity and selectivity. Substrate scope studies indicated that the pre-catalyst [(IPr)2Co2(CO)6] is applicable to the hydrosilylation of both alkyl- and aryl-substituted terminal alkynes and shows good functional group compatibility (halides, ketone, ester, amides, hydroxyl, and nitriles, etc.) and also broad scope on tertiary silanes (HSiEt3, HSi(OEt)3, HSinBu3, HSiMe2Ph, HSiPh3, HSi(OTMS)2Me, HSiMe2Cl). These reactions can furnish α-vinylsilanes with good to high yields (23–90%) and α/β selectivity (3/1- > 50/1) (Table 53). The catalytic system was found to be applicable for hydrosilylation polymerization of terminal diynes, by which silicon-containing polymers featuring vinylidene branches have been prepared. Mechanistic study revealed that the pre-catalyst [(IPr)2Co2(CO)6] can react with PhCCH and HSiEt3 to form the bridging alkyne complexes [(IPr)(CO)2Co(μ-η2:η2HCCPh)Co(CO)3], [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)2(IPr)], and the mononuclear Co(I) silyl complex [(IPr)Co(CO)3(SiEt3)] (Scheme 26). The bridging alkyne complexes [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3] and [(IPr)(CO)2Co(μ-η2:η2HCCPh)Co(CO)2(IPr)] can both further react with HSiEt3 to give α-vinylsilane. The mono-(NHC)-dicobalt alkyne complex [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co (CO)3] exhibits catalytic activity of Markovnikov hydrosilylation at both 60°C and

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Table 53 Markovnikov hydrosilylation of terminal alkynes catalyzed by [(IPr)2Co2(CO)6]

room temperature, whereas the reaction using [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co (CO)2(IPr)] has to be run at 60°C. These results, in addition to the observation of deviation of ideal Cs-symmetry of the Co2C2-butterfly core in the mono(NHC)dicobalt alkyne complex [(IPr)(CO)2Co(μ-η2:η2-HCCPh)Co(CO)3] and deuteriumlabeling experiments, collectively point out that mono(IPr)-dicobalt alkyne species are likely the genuine in-cycle catalysts for the Markovnikov hydrosilylation reaction (Scheme 27). The α-selectivity induction in the catalytic system is proposed to be achieved by the joint play of the unique μ-η2:η2-HCCR′ coordination mode of alkyne ligands in dicobalt complexes and the steric demanding nature of IPr. Recently, Patrycja Żak and Małgorzata Bołt further increased the steric hindrance of NHC to IPr*Ph to combine with Co2(CO)8 to affect the Markovnikov hydrosilylation [84]. Their findings further confirmed that steric hindrance of NHC can effectively increase the activity and selectivity than those of analogous complexes bearing smaller ligands.

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Scheme 26 Stoichiometric reaction of [(IPr)2Co2(CO)6] with PhCCH and HSiEt3

Scheme 27 Plausible catalytic cycle for Markovnikov hydrosilylation of terminal alkynes catalyzed by [(IPr)2Co2(CO)6]

3.2

Anti-Markovnikov Hydrosilylation of Terminal Alkynes

Compared with the Markovnikov hydrosilylation of terminal alkynes, cobaltcatalyzed anti-Markovnikov hydrosilylation of terminal alkynes is less investigated. Deng et al. reported [(IAd)Co(PPh3)(CH2TMS)]-catalyzed hydrosilylation of terminal alkynes with H2SiPh2 [85]. Recent developments are achieved by the use of diiminopyridine, phosphine-iminopyridine, and chelating phosphine ligands (Fig. 6). Ge and co-workers reported anti-Markovnikov hydrosilylation of terminal alkynes using Co(OAc)2 and diiminopyridines as catalysts [86]. The steric properties of the pyridine-2,6-diimine ligands significantly influence the regioselectivity of the reaction, wherein reactions with sterically more congested ligands favor β-(Z )

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products and MesPDI and iPrPDI are chosen as the better ligands for substrate scope studies (Table 54). Under the catalytic reaction conditions, various terminal alkynes containing electronically and sterically varied groups can react with H3SiPh to afford the corresponding β-(Z ) vinylsilanes in high stereoselectivities (Z/E = 24/1- > 99/1). The study noted that the use of a catalytic amount of phenol is crucial for the catalytic reaction, and its role is believed to quench Co-H species and hence suppress the Z/E-isomerization of vinylsilanes. The anti-Markovnikov hydrosilylation reactions of terminal alkynes catalyzed by Huang’s (phosphine-iminopyridine)cobalt catalyst also give β-(Z ) vinylsilanes as the dominant products (Table 55) [87]. The catalytic system can utilize H2SiPh2 and H3SiPh as the silane source and is well suited for the reactions of alkyl-substituted alkynes. However, it is incapable of mediating the reactions of tertiary silanes or aryl alkynes. The study proposed a modified Chalk–Harrod pathway for the antiMarkovnikov addition reaction. Steric repulsion between the ligand and the silyl

Fig. 6 Cobalt catalysts used in the anti-Markovnikov hydrosilylation of terminal alkynes Table 54 Anti-Markovnikov hydrosilylation of terminal alkynes catalyzed by cobalt complexes with pyridine-2,6-diimine ligand

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Table 55 Anti-Markovnikov hydrosilylation of terminal alkynes catalyzed by cobalt complexes with phosphine-iminopyridine ligand

group in the vinyl intermediates was thought to be the factor inducing Z/E isomerization via a zwitterionic carbene or metallacyclopropene intermediate (Scheme 28). Ge and co-workers developed cobalt-catalyzed (E)-selective anti-Markovnikov hydrosilylation of terminal alkynes using phosphine ligands [88]. The combination of Co(acac)2 with diphosphine ligand DPEphos can smoothly catalyze the reactions of terminal aryl and alkyl alkynes with H3SiPh to yield β-(E) vinylsilanes in 65–95% isolated yield with 8/1- > 99/1 β/α selectivities (Table 56). The electronic properties of substituents on aryl alkynes have noticeable effects on the regioselectivity. Changing the diphosphine ligand to Xantphos, the catalytic system can mediate the anti-Markovnikov hydrosilylation of terminal aryl and alkyl alkynes with secondary hydrosilane H2SiPh2 (Table 57). von Wangelin group combined Co (OAc)2•4H2O and DPEphos to catalyze the (E)-selective anti-Markovnikov hydrosilylation of terminal alkyl alkynes (Table 58) [77]. The isolated yields of products and selectivities are comparable to those of the Co(acac)2 and DPEphos catalytic system. Besides DPEphos and Xantphos, Dppb is also tested as the ligand for the cobalt-catalyzed anti-Markovnikov hydrosilylation reaction (Table 59). Again, cobalt(I) species is believed to be the genuine catalytic active species in these reactions.

3.3

Hydrosilylation of Internal Alkynes

Compared with terminal alkynes, internal alkynes are less reactive toward metal center for steric reason. Their hydrosilylation reaction might form different isomers of vinylsilanes. Therefore, selectivity control in the hydrosilylation reactions of internal alkynes is hard to achieve.

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Scheme 28 Plausible catalytic cycle for cobalt-catalyzed anti-Markovnikov hydrosilylation of terminal alkynes with pyridine-2,6-diimine ligand and phosphine-iminopyridine ligand Table 56 The Co(acac)2/DPEphos-catalyzed (E)-selective antiMarkovnikov hydrosilylation of terminal alkynes

So far, fine selectivity control on the hydrosilylation reactions of internal alkynes is mainly restricted to the symmetric internal alkynes and internal alkynes bearing electronically and/or sterically differentiated substituents. One of the latter cases is the reactions of aryl silyl acetylenes ArCCSiR3. Petit’s group reported the HCo (PMe3)4-catalyzed hydrosilylation reaction of ArCCSiR3 with HSiPh3 and HSi (OEt)3 (Table 60) [89]. These reactions give β-(E) vinylsilanes with the SiPh3/Si (OEt)3 group on α-positions with high regioselectivity. Pawluć’s (pyrimidine/2Himidazole)cobalt complex can also catalyze the hydrosilylation of ArCCSiR3 with H2SiPh2 to form similar products (Table 61) [82].

198 Table 57 The Co(acac)2/Xantphos-catalyzed (E)-selective antiMarkovnikov hydrosilylation of terminal alkynes

Table 58 The Co(OAc)2•4H2O/DPEphos-catalyzed (E)-selective antiMarkovnikov hydrosilylation of terminal alkynes

Table 59 The Co(OAc)2•4H2O/Dppb-catalyzed (E)-selective antiMarkovnikov hydrosilylation of terminal alkynes

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Table 61 Hydrosilylation of TMS-protected arylacetylenes catalyzed by cobalt complexes with pyrimidine/2H-imidazole ligand

Petit’s group performed mechanistic study on the HCo(PMe3)4-catalyzed hydrosilylation reaction [89]. The stoichiometric reaction of pre-catalyst HCo (PMe3)4 with HSiPh3 can give dihydrocobalt(III) complex (PMe3)3Co(H2)(SiPh3), which was characterized by NMR spectroscopy. The dihydrocobalt(III) complex (PMe3)3Co(H2)(SiPh3) can catalyze the hydrosilylation of PhCCPh and HSiPh3 to afford the desired hydrosilylation product in good yields with similar selectivity to that of the catalytic reaction with HCo(PMe3)4. Based on these experimental results, a classical Chalk–Harrod mechanism was proposed (Scheme 29), which also gains support from theoretical studies [89]. For the hydrosilylation reactions of aryl-alkyl alkynes ArCCR, the cobalt (I) complex HCo(PMe3)4 (Table 62) [89], Lu’s (imine-pyridine-oxazoline)cobalt catalyst (Table 63) [74], von Wangelin’s Co(OAc)2
4H2O/Xantphos (Table 64) [77], and Deng’s cobalt(0) complex [(CyIDep)Co(vtms)2] (Table 65) [90] was found to be effective catalysts. The major products in these reactions are the syn-addition products that have the silyl groups located at the α-carbon positions.

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Scheme 29 Propose mechanism for HCo(PMe3)4-catalyzed hydrosilylation Table 62 The HCo(PMe3)4-catalyzed hydrosilylation of arylalkyl acetylenes

The [(CyIDep)Co(vtms)2]-catalyzed hydrosilylation reaction is among the rare examples of hydrosilylation reactions utilizing cobalt(0) compounds as catalysts [90]. As control experiments with other structurally well-defined cobalt-NHC catalysts indicated that the Co(I) silyl species and Co(I) hydride species are unlikely the active intermediates, it was proposed that [(CyIDep)Co(vtms)2]-catalyzed alkyne hydrosilylation reaction probably operated on a cobalt(0)/cobalt(II) catalytic cycle (Scheme 30), wherein the steric nature of the NHC ligand CyIDep might allow the alkyne and hydrosilane ligands to point their steric less demanding substituents toward the NHC ligand, resulting in the formation of α-(E) vinylsilanes with high selectivity. A number of cobalt complexes proved to be effective in catalyzing stereoselective hydrosilylation reactions of symmetric internal alkynes. These include (imine-pyridine-oxazoline)cobalt complexes [74], Co(OAc)2
4H2O/Xantphos [77], and [(CyIDep)Co(vtms)2] [90]. In these reactions, H3SiPh and H2SiPh2 are the commonly used hydrosilanes and E-vinylsilanes were formed as the dominant or sole hydrosilylation products in moderate to high isolated yields (Schemes 31 and 32).

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Table 63 The (OIP)CoBr2-catalyzed hydrosilylation of arylalkyl acetylenes

Table 64 The Co(OAc)2•4H2O/Xantphos-catalyzed hydrosilylation of aryl-alkyl acetylenes

Despite the fine stereochemical control in the reactions of symmetric alkynes, the aforementioned catalysts all failed in facilitating regioselective hydrosilylation reactions of unsymmetric internal diaryl alkynes or dialkyl alkynes, wherein mixtures of α-(E) and β-(E) vinylsilanes were usually produced [78, 89] (Schemes 33 and 34). An interesting case of regioselective hydrosilylation of dialkyl alkynes is the Co2(CO)8-catalyzed reactions of alk-2-ynes with tertiary hydrosilanes, wherein the β-(E)-isomers of vinyl silanes were obtained as the major or sole hydrosilylation products (Table 66) [91]. The catalytic reaction shows good functional group compatibility with chloride, hydroxyl, ester, and carboxylic acids. Mechanistic studies showed that the stoichiometric reaction of Co2(CO)8 with 2-pentyne can form bridging alkyne complex (CO)3Co(μ-η2:η2-CH3CH2CCCH3)Co (CO)3 that can further react with HSiEt3 to give β-(E)-vinylsilane, indicating that alkyne-bridged dicobalt species are likely the active intermediates [91]. Similar to the proposed mechanisms for [(IPr)2Co2(CO)6]-catalyzed Markovnikov

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Table 65 The [(CyIDep)Co(vtms)2]-catalyzed hydrosilylation of arylalkyl acetylenes

Scheme 30 Propose mechanism for [(CyIDep)Co(vtms)2]-catalyzed hydrosilylation

hydrosilylation reaction of terminal alkynes, a catalytic cycle involving alkynebridged dicobalt species as intermediate was proposed for the catalytic hydrosilylation reaction (Scheme 35). Steric repulsion between substituents in

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Scheme 31 Cobalt-catalyzed hydrosilylation of symmetric diaryl alkynes

Scheme 32 Cobalt-catalyzed hydrosilylation of symmetric dialkyl alkynes

Scheme 33 Cobalt-catalyzed hydrosilylation of unsymmetric diaryl alkynes

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Scheme 34 Cobalt-catalyzed hydrosilylation of unsymmetric dialkyl alkynes Table 66 Co2(CO)8-catalyzed hydrosilylation of alk-2-ynes

vinyl-bridged dicobalt silyl intermediates was proposed as the key factor inducing the observed regioselectivity. Another unusual reaction of unsymmetric dialkyl alkyne with hydrosilanes is the [(dcpe)CoBr2]/NaHBEt3-catalyzed reactions of 3-alkynes with hydrosilanes reported by Park and co-workers, which can form branched allylsilanes rather than vinylsilanes (Table 67) [92]. The study noted that the use of the electron-rich and steric demanding hydrosilane, dimethyl(3,5-dimethyl-4-methoxyphenyl)silane, is crucial for the selective formation of allylsilanes. Park and co-workers performed detailed mechanistic study on the reaction. It revealed that the pre-catalyst [(dcpe)CoBr2] can react with NaHBEt3 to form dinuclear cobalt(I) hydride complex [(dcpe)Co(H)]2 that can catalyze the reaction of 3-alkynes with dimethyl (3,5-dimethyl-4-methoxyphenyl)silane to form allylsilanes. Control experiments and deuterium-labeling experiments further suggested that cascade β-hydride elimination should occur in these reactions. Thus, a catalytic cycle involving cobalt hydride and cobalt allyl intermediates was proposed (Scheme 36). Later, Park and co-workers reported the reactions of 2-alkynes with dimethyl (3,5-dimethyl-4-methoxyphenyl)silane using [(dcpe)CoBr2]/NaHBEt3 as catalyst. This reaction yields α-vinylsilanes as the major products (Table 68) [93], being distinct from the formations of allyl silanes in the reactions of 3-alkynes. The study mentioned that the selection of alkyl substituents of 2-alkynes and hydrosilanes is critical for the formation of α-vinylsilanes.

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Scheme 35 Proposed mechanism for Co2(CO)8-catalyzed hydrosilylation of 2-pentyne Table 67 Cobalt-catalyzed migratory hydrosilylation of 3-alkynes

3.4

Hydrosilylation of Enynes and Diynes

Hydrosilylation reactions of enynes and diynes can yield vinylsilanes featuring conjugation with unsaturated C–C bonds or fused ring structures. The exploration of cobalt-catalyzed hydrosilylation reactions in recent years has led to the development of catalytic systems that can mediate the hydrosilylation reactions of 1,3-, 1,4-, and 1,6-enynes as well as 1,3-diynes. Chen and co-workers applied (Dppf)CoCl2/NaHBEt3 as catalyst for the hydrosilylation reactions of the aryl-substituted 1,3-enynes but-3-en-1-yn-1ylbenzene derivatives with H3SiPh and H2SiPh2 and achieved the synthesis of

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Scheme 36 Proposed mechanism for cobaltcatalyzed migratory hydrosilylation of 3-alkynes

Table 68 Cobalt-catalyzed migratory hydrosilylation of 2-alkynes

1,3-dien-1-ylsilanes in good to high yields and good selectivities (3.5/1–49/1) (Table 69) [94]. Investigation with (Xantphos)CoCl2/NaHBEt3 as catalyst then led to the establishment of the hydrosilylation of alkyl-substituted 1,3-enynes, which displays inverse regioselectivity and furnishes 1,3-dien-2-ylsilanes (Table 70). These reactions led to merely the hydrosilylation of the C–C triple bond of the enynes. Classical Chalk–Harrod mechanism was proposed for these cobalt-catalyzed hydrosilylation of 1,3-enynes.

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Table 69 Cobalt-catalyzed hydrosilylation aryl-substituted of 1,3-enynes

Table 70 Cobalt-catalyzed hydrosilylation of alkyl-substituted 1,3-enynes

Meng’s group introduced chiral diphosphine ligands for the hydrosilylation of but-3-en-1-yn-1-ylbenzene derivatives and achieved site- and stereoselective hydrosilylation of the 1,3-enynes with primary silanes, by which enantioenriched cyclic vinylsilanes with simultaneous construction of a carbon-stereogenic center and a silicon-stereogenic center were synthesized (Table 71) [95]. These reactions can be viewed as the results of the cascade reactions of intermolecular alkyne hydrosilylation followed by intramolecular alkene hydrosilylation. In contrast, the reactions of 1,4-enynes with primary silanes using chiral diphosphine cobalt catalyst only led to the formation of 1,3-dien-1-ylsilanes, wherein isomerization of the alkene

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Table 71 Cobalt-catalyzed site- and stereoselective hydrosilylation of 1,3-enynes

moiety followed by site- and stereoselective hydrosilylation was thought to took place (Table 72). Meng proposed cobalt hydride species as the in-cycle intermediates for these reactions (Scheme 37) [95]. For the reaction yielding cyclic vinylsilanes, OjimaCrabtree isomerization of vinylcobalt species was proposed for the Z/E-isomerization that could yield E-vinylcobalt species that can transfer into E-vinylsilanes for further alkene hydrosilylation. Lu’s group reported hydrosilylation/cyclization of 1,6-enynes using iminopyridine cobalt complex as catalyst (Table 73) [96]. Under catalytic reaction conditions, 1,6-enynes containing tethered nitrogen, carbon, or oxygen atom can be converted into the corresponding products in moderate to good yields. Functional groups tethered on aryl substitutes, such as amine, free aniline, ester, ether, cyano, halide, trifluoromethyl, and heterocycle, are all compatible with this catalytic system. The authors proposed that the reaction might start from the insertion of the alkyne moiety into the Co–H bond of cobalt hydride species formed from the reaction of pre-catalyst with NaHBEt3. Subsequent alkene-insertion reaction of the cobalt vinyl intermediate leads to the cyclization and the formation of the alkyl cobalt intermediate, which then reacts with hydrosilanes to form the final products (Scheme 38). The other cobalt catalyst capable of promoting the hydrosilylation/cyclization of 1,6-enynes is Co(acac)2/(R,Sp)-Josiphos developed by Ge and co-workers (Table 74) [97]. The catalytic system is applicable to a variety of O-, C-, and N-tethered 1,6-enynes as well as secondary and tertiary hydrosilanes, which are in line with Lu’s catalytic system [96]. More intriguingly, the use of the chiral phosphine ligand here resulted in the formation of enantioenriched β-stereogenic alkylsilanes with excellent enantioselectivity (94–99% ee). Thorpe-Ingold effect is thought to play a

Recent Development of Cobalt-Catalyzed Hydrosilylation Reactions:. . . Table 72 Cobalt-catalyzed hydrosilylation of 1,4-enynes

site-

and

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stereoselective

Scheme 37 The proposed mechanism for cobalt-catalyzed hydrosilylation of 1,3 and 1,4-enynes

key role in this cobalt-catalyzed cyclization of carbon-tethered 1,6-enynes as C (CO2Me)2-, C(CN)2-, and CH2-tethered 1,6-enynes show different reactivity in the catalytic reaction. It also revealed that the hydrosilylation/cyclization of alkylsubstituted 1,6-enyne affords vinylsilanes rather than alkylsilanes. The authors proposed that a cobalt hydride species is the key in-cycle species for the cyclization reaction (Scheme 39). So far, studies on cobalt-catalyzed hydrosilylation reactions of diynes are restricted to 1,3-diynes. Ge’s group found the combination of Co(acac)2 and diphosphine ligands can achieve the high regio- and stereoselective hydrosilylation of 1,3-diynes (Table 75) [98]. In the presence of Co(acac)2 (2 mol%) and dppp (2 mol%) in toluene at 50°C for 24 h, various unsymmetrical silyl-substituted

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Table 73 Cobalt-catalyzed hydrosilylation/cyclization of 1,6-enynes

Scheme 38 The proposed mechanism for cobaltcatalyzed hydrosilylation/ cyclization of 1,6-enynes

1,3-diynes and symmetrical 1,4-bis(aryl)- or 1,4-bis(alkyl)-substituted 1,3-diynes can smoothly react with H2SiPh2 to give silyl-functionalized 1,3-enynes in high yields (57–96%) with excellent regioselectivity (>99/1). However, when the 1,3-diyne bearing phenyl and n-butyl group was subjected to the catalytic system, a mixture of vinylsilanes was obtained. These results indicated that the steric bulky nature of TMS plays a key role for the high selectivity of the hydrosilylation reactions of TMS-substituted 1,3-diynes. Chen and co-workers found that the catalytic systems of (NCNN)CoBr2/NaHBEt3 and [(DPPP)CoCl2]/NaHBEt3 can catalyze the hydrosilylation of diynes with H2SiPh2 (Table 76) [99]. The catalytic system using NCNN-CoBr2/NaHBEt3 necessitates excess of H2SiPh2 (2.0 equiv.) to suppress the formation of divinylsubstituted silanes. The catalytic system using [(DPPP)CoCl2]/NaHBEt3 is more

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Table 74 Cobalt-catalyzed asymmetric hydrosilylation/cyclization of 1,6-enynes

Scheme 39 The proposed mechanism for cobalt-catalyzed asymmetric hydrosilylation/cyclization of 1,6-enynes

efficient as it merely needs a catalytic loading of 1 mol% and does not require the use of H2SiPh2 in excess [100]. In addition to Ge and Chen’s catalytic systems, Jin and co-workers found that the (amine-iminopyridine)cobalt complex in combination with NaHBEt3 also affects the hydrosilylation of 1,3-diynes with H3SiPh and H2SiPh2, giving silyl-substituted 1,3-enynes in moderate to good yields (Table 77) [101].

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Table 75 Hydrosilylation of diynes catalyzed by Co(acac)2 and diphosphine ligand dppp

Table 76 Hydrosilylation of diynes catalyzed by NCNN-CoBr2 or (DPPP)CoCl2 with NaHBEt3

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Table 77 Hydrosilylation of diynes catalyzed by (NNN)CoCl2 with NaHBEt3

3.5

Double Hydrosilylation of Alkynes

Ideally alkynes may be able to react two equiv. of hydrosilanes to form saturated hydrocarbons bearing two silyl groups. Inasmuch as the double addition reactions can produce different regio- and stereo-isomers of disilanes and the vinylsilane intermediates can be harder to undergo hydrosilylation reaction than its alkyne precursor due to steric reason, regio- and stereoselective double hydrosilylation of alkynes is hard to achieve. In spite of these challenges, several cobalt catalysts have been found effective in promoting this transformation. One of the catalysts is Lu’s cobalt complex supported by N-phenyl protected imidazoline-iminopyridine ligand [102]. The cobalt complex in combination with NaHBEt3 catalyzes the reactions of terminal alkyl alkynes with two equiv. of aryland alkyl-substituted primary silanes to form gem-bis(dihydrosilyl)alkanes in good yields (Table 78). Various functionalized groups such as cyclopropane, halide, ester, amide, ether, silyl protected alcohol, free alcohol, and dioxalate are well tolerated.

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Table 78 Cobalt-catalyzed double hydrosilylation of alkynes with imidazolineiminopyridine ligand

However, this catalytic system is not applicable to terminal aryl alkynes. When secondary hydrosilane H2SiPh2 serves as the silane source, the reaction only delivers anti-Markovnikov mono-hydrosilylation products. Another catalytic system is Pawluć’s (benzoimidazole-imine-imidazole)cobalt complex that is capable of catalyzing the Markovnikov double hydrosilylation of terminal aryl alkynes (Table 79) [103]. In the presence of (benzoimidazole-imineimidazole)cobalt dichloride (1 mol%) and NaHBEt3 (7 mol%), the reactions of various terminal aryl alkynes bearing electron-donating and electron-withdrawing substituents as well as heteroaryl alkynes can react with two equiv. of primary hydrosilanes to give the gem-bis(silanes) 1,1-disilyl-1-aryl-ethanes in 32–67% isolated yields with excellent selectivities. When an equimolar mixture of H3SiPh with H2SiPh2, H3SiPh with H3SiC6H13-n, or H2SiPh2 with HSiMe2Ph was applied, gembis(silanes) containing two different silyl groups can be formed. NMR experiments revealed that the NaHBEt3 activator may not only react with the metal center of the pre-catalyst but also modify the ligand and thus influence the catalytic activity of the metal center. Lu and co-workers developed a catalytic protocol that employs two cobalt complexes (Xantphos)CoBr2 and (OIP)CoBr2 for sequential hydrosilylation of alkynes and vinylsilanes, whose implementation with two types of hydrosilane in one-pot reaction has led to the synthesis of chiral gem-bis(silyl)alkanes (Table 80) [104]. Using this catalytic protocol, terminal alkyl alkynes can react with two types of secondary or primary hydrosilanes to afford chiral gem-bis(silyl)alkanes in moderate yields and with 98%- > 99% ee. Aryl alkynes, however, are not effective substrates. Considering the fast rate of (Xantphos)CoBr2/NaHBEt3-catalyzed hydrosilylation of terminal alkynes, a hydrometallation pathway with a Xantphossupported cobalt hydride intermediate was proposed for the primary β-(E) hydrosilylation of alkyne [104]. The secondary hydrosilylation of vinylsilanes to give the final chiral gem-bis(silyl)alkanes was thought to be mediated by (OIP)Co

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Table 79 Cobalt-catalyzed double hydrosilylation of terminal alkynes with N,N,N-tridentate hydrazine ligand

Table 80 Cobalt-catalyzed asymmetric double hydrosilylation of alkynes

species, involving very likely a cobalt silyl intermediate (Scheme 40). The proposed mechanism is further supported by DFT calculations. Recently, Lu and co-workers achieved cobalt-catalyzed double hydrosilylation of terminal aryl alkynes that can yield vicinal or geminal bis(silane)s using the sequential (OIP)CoBr2/NaHBEt3-(Xantphos)CoBr2/NaHBEt3 or (OIP)CoBr2/NaHBEt3NaHBEt3 catalytic protocol, respectively (Table 81) [105]. Previous report showed that (OIP)CoBr2/NaHBEt3 can catalyze the Markovnikov hydrosilylation of terminal aryl alkynes with high regioselectivity to give α-vinylsilanes [74]. Without isolation of these α-vinylsilanes, (Xantphos)CoBr2/NaHBEt3 and the second hydrosilane were added to facilitate further anti-Markovnikov hydrosilylation of

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Scheme 40 Proposed mechanism for cobalt-catalyzed asymmetric double hydrosilylation of alkynes Table 81 Cobalt-catalyzed double hydrosilylation of terminal aryl alkynes

α-vinylsilanes, by which vicinal bis(silane)s can be obtained. On the other hand, when NaHBEt3 and the second hydrosilane were added to the mixture containing α-vinylsilanes, Markovnikov hydrosilylation of α-vinylsilanes can operate, leading to geminal bis(silane)s. Pawluć’s group had found that NaHBEt3 can serve as a catalyst for hydrosilylation reactions of alkene [106]. Under these two optimized conditions, various terminal aryl alkynes bearing electron-donating or electronwithdrawing substituents can react with same or different secondary hydrosilanes to yield vicinal and geminal bis(silane)s in moderate to good yields and regioselectivities. However, these catalytic systems are not effective in catalyzing the reactions of terminal alkyl alkynes.

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4 Conclusions The large body of studies shown above have well demonstrated the fast development of cobalt-catalyzed hydrosilylation of alkenes and alkynes in recent years. The development can be observed from the different perspectives of substrates, ligands, in situ generation methods of catalytic active species, and selectivity control on the addition reaction. For cobalt-catalyzed hydrosilylation of alkenes, a number of catalytic systems have been discovered for selective anti-Markovnikov or Markovnikov addition reactions of simple terminal alkenes and 1,3-dienes, particularly the monosubstituted ones. Several catalytic systems are even capable of catalyzing enantioselective Markovnikov addition of terminal alkenes to produce chiral silanes with high ee values. A handful of examples of the hydrosilylation reactions of disubstituted alkenes, e.g., 1,1-disubstituted terminal alkenes [20, 21, 47, 103, 105], trans-vinylsilanes that are formed from the hydrosilylation of terminal alkynes with hydrosilanes [102, 104], and cyclopropenes [65], are known. The hydrosilylation reactions of other disubstituted alkenes, tri-, and tetra-substituted alkenes, whose double bonds are sterically more hindered, remain as a formidable challenge. For cobalt-catalyzed hydrosilylation of alkynes, plenty of cobalt catalysts are known to be effective in catalyzing cis-addition reactions of hydrosilanes with terminal alkynes with good Markovnikov selectivity. Many of these catalytic systems are also found to be effective in catalyzing the cis-addition of hydrosilanes with symmetric internal alkynes. Cobalt catalysts facilitating anti-Markovnikov addition reactions of terminal alkynes, however, are relatively scarce. On the other hand, cobalt-catalyzed hydrosilylation reactions of alkynes showing trans-addition selectivity are exceedingly rare [86, 87]. Regioselective hydrosilylation of unsymmetric internal alkynes that have the electronic and steric properties of the two substituents being close remains a big challenge. From the perspective of ligands, ligands that are commonly applied in other organometallic catalysis have been successfully introduced to cobalt-catalyzed hydrosilylation reactions. These include CO, isocyanides, phosphines, NHCs, pyridine- and imine-based nitrogen ligands, NNN-, NNP-, PCP-, PSiP-, and CCC-pincer ligands. There are also explorations with chiral phosphines and oxazolines that have led to the development of enantioselective hydrosilylation reactions of alkenes. As ligands can directly influence the reactivity of their metal centers, the continuing exploration on new ligands should provide solutions to the aforementioned challenges. The diversified ligands used in cobalt-catalyzed hydrosilylation reactions also pose mechanistic questions that deserve study. The traditional Chalk–Harrod and modified Chalk–Harrod mechanisms were built on the studies on cobalt carbonyl complexes. The distinct steric and electronic nature of phosphines, NHCs, pyridine- and imine-based nitrogen ligands, NNN-, NNP-, PCP-, PSiP-, and CCC-pincer ligands as compared with CO might confer different reactivities to their cobalt species, and different mechanisms might operate for their cobalt-catalyzed hydrosilylation reactions. However, so far, mechanistic studies on

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the reported catalytic systems are mainly restricted to control experiments and isotope-labeling experiments. Only a few systems allow the isolation of relevant reactive cobalt complexes. Theoretic studies on those reactions are also rarely known. This status urges further mechanistic studies upon reactive intermediates identification and theoretical calculations. Hydrosilylation reactions are useful for the synthesis of organosilicon reagents and polymers. Their practical application requires the catalytic reaction to embrace the features of high catalytic activity and are easy to operate. Considering these, the higher catalyst loading (≥1 mol%) of most of the cobalt-catalyzed hydrosilylation reactions and the high sensitivity of their cobalt catalysts to air and moisture might set hurdles for their practical usage. While the use of alkoxides as catalyst activator and the use of cobalt carboxylates as the cobalt source have exemplified the accessibility of air-stable cobalt catalysts for hydrosilylation reactions, the high sensitivity of cobalt hydrocarbyls, cobalt hydride, and cobalt silyl species toward dioxygen and moistures render the use of pre-dried alkenes/alkynes and hydrosilanes as a prerequisite for most cobalt-catalyzed hydrosilylation reactions. Again, solutions to these problems should be based on the development of new ligands that can endow cobalt catalysts with all the desired properties. Acknowledgments We thank financial support from the National Key Research and Development Program of the Ministry of Science and Technology of China (2021YFA1500203), and Natural Science Foundation of China (22231010 and 22201290).

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Top Organomet Chem (2023) 72: 225–252 https://doi.org/10.1007/3418_2023_102 # The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 Published online: 12 October 2023

Iron and Manganese Catalyzed Hydrosilylation Reactions Thaipparambil Aneeja, Pulluparambil Xavier Thresia Rinu, and Gopinathan Anilkumar

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Iron Catalyzed Hydrosilylation Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Manganese Catalyzed Hydrosilylation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

225 226 241 248 249

Abstract Hydrosilylation reaction has emerged as an important strategy for the preparation of silicone polymers and functionalized silanes. Most of the reports on hydrosilylation reactions rely on rare, toxic, and expensive noble metals such as Pd, Pt, Ir, Rh, etc. However, in view of the growing environmental concerns, the development of more ecofriendly protocols using less expensive transition metals like Fe, Co, Mn, Ni, etc. is highly demanding. Recently, transition metals are widely exploited as catalysts for hydrosilylation reactions. Considering the tremendous interest in hydrosilylation reactions and transition metal catalysis, in this chapter we summarize the recent advances in iron and manganese catalyzed hydrosilylation reactions. Keywords Alkyne · Carbonyl compound · Hydrosilylation · Iron · Manganese · Olefin

G. Anilkumar (✉) School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India e-mail: [email protected]

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1 Introduction Hydrosilylation reaction is an organic transformation in which a Si-H bond is added to C = X or C  X bond (where X = O, N, or C) [1]. Various materials like pressuresensitive adhesives, lubricating oils, liquid injection molding products, etc. could be prepared via hydrosilylation [2]. Silicone products are extensively employed as line coating in stamps, tapes, and labels. Organosilicon compounds also have various applications as performance enhancers in cosmetics, contact lenses, car tyres, textiles, etc. [3]. Metal-catalyzed hydrosilylation reaction is becoming an important area in synthetic organic chemistry. Earlier, noble metals such as Pd, Pt, Ru, Ir, etc. were mainly employed as catalysts for hydrosilylation reactions [4]. Most of the previous reports on hydrosilylation reactions rely on platinum catalysts such as Speier’s catalyst and Karstedt’s catalyst owing to their high anti-Markovnikov selectivity, high catalytic efficiency, and functional group tolerance [5, 6]. Though these metals are widely exploited in hydrosilylation reactions, high toxicity, limited availability, and high cost of these metals remain as a hurdle in this area [7]. In this scenario, the development of new methodologies utilizing earth-abundant first row transition metals such as Mn, Ni, Fe, Co, etc. as catalysts is highly demanding. Among these metals, iron and manganese have received significant attention in recent times. Iron is the most abundant transition metal with high catalytic efficiency, low toxicity, and excellent functional group tolerance [8]. Manganese is the third most abundant transition metal and it satisfies various principles of green chemistry [9]. In the last few years, a large number of reports have been disclosed on hydrosilylation reactions using iron or manganese as catalyst. In this chapter, we summarize the recent developments in iron and manganese catalyzed hydrosilylation reactions covering literature from 2015 to 2023.

1.1

Iron Catalyzed Hydrosilylation Reactions

Iron catalyzed hydrosilylation reaction has witnessed spectacular progress in recent times. Owing to the earth-abundant, low-toxic, and cost-effective characteristics of iron, it has been widely exploited for hydrosilylation reactions. Iron was found highly efficient toward the hydrosilylation of carbonyl compounds, alkenes, alkynes, etc.

1.1.1

Hydrosilylation of Carbonyl Compounds

Gade and co-workers demonstrated the hydrosilylation of a wide range of arylalkylketones using chiral iron alkyl and iron alkoxide complexes bearing

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Scheme 1 Enantioselective hydrosilylation of arylalkylketones using chiral iron catalyst

Scheme 2 Iron catalyzed asymmetric hydrosilylation of aryl ketones

boxmi pincers as catalyst (Scheme 1) [10]. High enantioselectivity and short reaction time are the major highlights of this method. Hydrosilylation of arylalkylketones bearing long unbranched alkyl chains was found to be efficient via this method with excellent enantioselectivity. In 2015, Huang and co-workers disclosed that iron complexes with chiral iminopyridine-oxazoline (IPO) ligands can catalyze asymmetric hydrosilylation of aryl ketones in the presence of 2 equiv. of NaBEt3H in toluene at 25°C for 10 h, achieving secondary aryl alcohols with high enantioselectivity (up to 93% ee) and excellent yield (up to 99%) (Scheme 2) [11]. This method was found to be suitable for aromatic ketones with electron-donating and electron-withdrawing substituents. Sterically hindered 1-mesitylethanone reacted smoothly to afford the desired product in good yields with high enantioselectivity. However, low enantioselectivity was observed in the case of 1,1-dialkyl ketones. Reduction of ketones and aldehydes via sequential hydrosilylation/hydrolysis method using amine-bis(phenolate) iron(III) catalyst was developed (Scheme 3) [12]. The major highlight of this strategy is the selective reduction of ketones over imines, esters, olefins, and conjugated olefins. This method utilized the easily available amine-bis(phenolate) iron(III) as catalyst, which is stable to air and moisture and this strategy exhibits wide substrate scope. Hydrido metal complexes have great significance in coordination chemistry, synthetic chemistry, and homogeneous catalysis. In 2015, Li et al. disclosed the efficiency of hydrido iron(II) complexes with [P,S]chelating ligands in the hydrosilylation of aldehydes and ketones in the presence of (EtO)3SiH as hydrogen source (Scheme 4) [13].

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Scheme 3 Hydrosilylation of carbonyl compounds using amine-bis(phenolate) iron(III) catalyst

Scheme 4 Hydrido iron(II) complexes with [P,S]chelating ligands for hydrosilylation of carbonyl compounds

Scheme 5 Hydrosilylation of carbonyl compounds catalyzed by iron hydrido complex

Sun and co-workers disclosed an efficient methodology for the hydrosilylation of carbonyl compounds using iron hydrido complex (Scheme 5) [14]. Initially they have synthesized (POCH2OP)-pincer ligand and then iron hydrido complex was prepared via the oxidative addition of Csp3–H bond of the methylene group to the iron(0) center. This catalyst allows feasible reduction of both aldehydes and ketones. Aromatic aldehydes with electron-donating groups offered lower yields. Findlater and co-workers demonstrated that bis(imino)acenaphthene iron arene complex, BIAN-Fe(C7H8) could be utilized as precatalyst in hydrosilylation of aldehydes and ketones [15]. Diphenylsilane was chosen as the reductant in this method under solvent-free conditions. α,β-Unsaturated ketones smoothly underwent hydrosilylation using Ph2SiH2 in the presence of BIAN-Fe(C7H8) (1 mol%) at 70°C for 30 min (Scheme 6). In this case, they observed exclusive formation of α,β-unsaturated alcohol without alkene reduction.

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Scheme 6 Hydrosilylation of carbonyl compounds using BIAN-Fe(C7H8) Scheme 7 [(DPB)Fe]2(N2) catalyzed hydrosilylation of carbonyl compounds

Scheme 8 Cyclopentadienyliron dicarbonyl dimer for the hydrosilylation of aldehydes and ketones

The research group of Peters has disclosed the hydrosilylation of carbonyl compounds using [(DPB)Fe]2(N2) (DPB = bis(o-diisopropylphosphinophenyl) phenylborane) as catalyst (Scheme 7) [16]. They compared the catalytic activity of this complex with (PhDPBMes)Ni. The advantages of this catalyst include wide substrate scope, mild reaction condition, and shorter reaction time, and this catalyst was found to be more efficient than the nickel complex, (PhDPBMes)Ni. Faster hydrosilylation was observed in the case of benzophenone than that of acetophenone in spite of the steric hindrance. Arguouarch et al. designed a new methodology for the hydrosilylation of aldehydes and ketones using cyclopentadienyliron dicarbonyl dimer in the presence of diethoxymethylsilane (Scheme 8) [17]. For hydrosilylation, most of the reported iron-based homogeneous catalytic systems need inert condition. This protocol overcomes this limitation, as cyclopentadienyliron dicarbonyl dimer efficiently catalyzes hydrosilylation under aerobic condition. The advantages of this method include ready-to-use iron-based precatalyst, solvent-free reaction condition, air- and moisture-stable catalyst, operational simplicity, and wide substrate scope. Li and co-workers explored an efficient method for the synthesis of (4-CF3C6F4) Fe(PMe3)4, (4-C5NF4)Fe(PMe3)4, (C6F5)Fe(PMe3)4, (η4-1,2,3,4-C10F8)Fe(PMe3)3, and (C6F5)FeH(PMe3)4, important catalysts in hydrosilylation of carbonyl compounds [18]. From their studies it was understood that among these synthesized catalysts, (C6F5)FeH(PMe3)4 (0.5 mol%) is the best one for hydrosilylation reaction (Scheme 9). Higher catalyst loading or longer reaction time is needed for ketones

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Scheme 9 Hydrosilylation of carbonyl compounds using (C6F5)FeH(PMe3)4

Scheme 10 Iron catalyzed hydrosilylation of carbonyl compounds

and aldehydes with electron-withdrawing groups. In the case of aliphatic aldehydes, they obtained the desired product in moderate yields. A new approach for the hydrosilylation of carbonyl compounds using anthraquinone amide-based iron complex as catalyst was disclosed. This method does not require any special neutral donors or activators and proceeds well in the presence of 0.25 mol% of catalyst in THF at room temperature (Scheme 10) [19]. Even though good conversions were observed in short time at room temperature, they could not achieve complete conversion for the reduction of acetophenone to 1-phenylethanol. Later in 2018, the same group developed a new anthraquinoid-based iron (II) complex with more flexible ligand having much higher catalytic performance (Scheme 11) [20]. This method exhibits broad functional group tolerance with high turnover frequencies. Reductive dehydration of amides to nitriles using hydrido thiophenolato iron (II)complex [cis-Fe(H)(SAr)(PMe3)4] as catalyst under hydrosilylation condition was demonstrated (Scheme 12) [21]. In this method they have selected (EtO)3SiH as the efficient reducing agent. Aryl amides with electron-donating and electronwithdrawing substituents performed well in this reaction and afforded the corresponding products in 71–93% yield. Moderate yield of the product was obtained in the case of alkyl amides and heteroaromatic amides. In 2017, Li and co-workers reported that hydrido thiophenolato iron (II) complexes [cis–Fe(H)(SAr)(PMe3)4] are effective in the hydrosilylation of

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Scheme 11 Hydrosilylation of carbonyl compounds using anthraquinoid-based iron(II) complex

Scheme 12 Iron catalyzed reductive dehydration of amides to nitriles

Scheme 13 Hydrosilylation of carbonyl compounds using hydrido thiophenolato iron(II) complex

aldehydes and ketones in presence of (EtO)3SiH as reducing agent [22]. Hydrosilylation of aldehydes proceeded smoothly in presence of 1 mol% of catalyst in THF at 50°C for 2 h (Scheme 13). But this optimized condition was not favorable for ketones, an increase in temperature to 60°C, catalyst loading to 2 mol %, and extending the reaction time to 24 h were essential for the hydrosilylation of ketones. Iron(II) piano stool complexes were identified as active catalysts for hydrosilylation of carbonyl compounds. In 2017, Albrecht et al. disclosed that 1,2,3-triazolylidene iron(II) piano stool complexes are efficient catalysts for the hydrosilylation of aldehydes and ketones ([Fe] precatalyst (1 mol%), PhSiH3 (1.2 equiv.), 1,2-DCE, 60°C) (Scheme 14) [23]. Aldehydes with different substituents at para position including nitro, dimethylamino, trifluoromethane, bromo and nitrile

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Scheme 14 Iron(II) piano stool complexes as catalyst for hydrosilylation of carbonyl compounds

Scheme 15 Reduction of aldehydes/ketones using [P,Si]-chelate silyl hydrido iron(II) complex

Scheme 16 Hydrosilylation of carbonyl compounds catalyzed by iron pincer complex

groups underwent hydrosilylation smoothly affording the desired product in excellent yield within 60 min. Sun and co-workers in 2018 synthesized [P,Si]-chelate silyl hydrido iron (II) complex through Si-H bond activation for the reduction of aldehydes/ketones (Scheme 15) [24]. In this method, (EtO)3SiH was selected as the reducing agent. This reaction was found to be suitable for the selective reduction of α,β-unsaturated carbonyls to α,β-unsaturated alcohols. Lee and co-workers reported the preparation of low-coordinate iron(II) complex (CztBu(PztBu)2)Fe[N(SiMe3)2] with NNN-pincer ligand [25]. They also demonstrated the efficiency of this complex as precatalyst in the hydrosilylation of organo carbonyl substrates. Li and co-workers designed a novel and efficient methodology for the synthesis of diphosphine–phosphine oxide ligand ((Ph2P-(C6H4))2P(O)H) [26]. They have prepared [PPP]-pincer Fe, Ni, and Co complexes and explored the catalytic activity of these complexes in the hydrosilylation of carbonyl compounds (Scheme 16).

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Scheme 17 Fe-iminobipyridine complex as catalyst for the hydrosilylation of carbonyl compounds

Scheme 18 Hydrosilylation of amides using (P,N)Fe-hexamethyldisilazide complex

Among these three pincer metal complexes, hydrido iron(II) complex was identified as the best catalyst for hydrosilylation of aldehydes and ketones. Aromatic aldehydes with electron-withdrawing groups performed well compared to the electrondonating ones. Moreover, excellent selectivity was observed for α,β-unsaturated aldehydes. Nakazawa et al. in 2019 demonstrated an efficient methodology for the hydrosilylation of ketones using Fe-iminobipyridine complexes [27]. In this strategy, the highest TOF (4,190 min-1) was observed for the hydrosilylation of 2-octanone with PhSiH3 using (HBPIDipp,H)FeBr2 as the catalyst (Scheme 17). Acetophenone derivatives bearing electron-withdrawing groups at the para position were found less reactive in this method. 4-Chloroacetophenones underwent hydrosilylation slowly with low TOF (30 min-1). However, they were able to accelerate the hydrosilylation via the addition of various N-donor ligands. Turculet et al. described a new methodology for the synthesis of (P,N)Fehexamethyldisilazide complex (Fe-1) and four-coordinate (P,N)Fe (Fe-2) and (P, N)Co-alkyl complexes (Scheme 18) [28]. The catalytic efficiencies of these complexes for the hydrosilylation of amides were evaluated. The catalytic efficiency was found to be superior in the case of iron-based catalysts. Among the two iron

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Scheme 19 Hydrosilylation of carbonyl compounds catalyzed by [PSiP]-pincer iron hydrides

Scheme 20 Iron catalyzed hydrosilylation of carbonyl compounds

catalysts, Fe-2 outperforms Fe-1 in this reaction. Wide substrate scope was observed for iron catalyzed strategy under ambient temperature. The major significance of this work was that this was the first report on iron catalyzed hydrosilylation of amides at room temperature which avoids photochemical activation. In 2020, Li and co-workers used [PSiP]-pincer iron hydrides as the catalyst for hydrosilylation of carbonyl compounds (Scheme 19) [29]. They proved that addition of pyridine N-oxide in this catalytic process could enhance the catalytic performance and reduce the reaction temperature. In presence of pyridine N-oxide, aldehydes underwent smooth hydrosilylation at 30°C and ketones at 50°C. Synthesis of silylene supported iron hydride [Si,C]FeH (PMe3)3 via C(sp3)-H bond activation was established (Scheme 20) [30]. This complex showed excellent activity in the hydrosilylation of carbonyl compounds. The main attraction of this method is that hydrosilylation of both aldehydes and ketones could be possible under the same reaction conditions. Cui et al. demonstrated a method for the hydrosilylation of carbonyl compounds to prepare tris- and bis(alkoxy)silanes employing cyclopentadienyl dicarbonyl iron anion as catalyst (Scheme 21) [31]. They have tested the catalytic activity of K[CpFe (CO)2] and [NEt4][CpFe(CO)2] salts and found that these two catalysts were highly active for the reduction of carbonyl compounds with low-catalyst loading (0.05 mol %). This reaction was found suitable for aliphatic and heteroaromatic carbonyl compounds. Sterically hindered carbonyl compounds also participated well in this reaction.

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Scheme 21 Hydrosilylation of carbonyl compounds catalyzed by cyclopentadienyl dicarbonyl iron anion

Scheme 22 Hydrosilylation of 1,1-disubstituted aryl alkenes

Scheme 23 Iron catalyzed hydrosilylation of alkenes

1.1.2

Hydrosilylation of Alkenes

Lu and co-workers reported the iron catalyzed hydrosilylation of 1,1-disubstituted aryl alkenes in presence of iminopyridine-oxazoline ligands (Scheme 22) [32]. This is the first report on anti-Markovnikov hydrosilylation reactions of 1,1-disubstituted aryl alkenes to afford chiral organosilanes with high regio- and enantioselectivity. Styrenes with electron-rich and electron-deficient groups performed well in this reaction to give the expected products with good yields and high ee values. Nagashima et al. reported the unprecedented hydrosilylation of alkenes using Fe isocyanide as the catalyst [33]. The catalytic system Fe(COT)2 (1 mol%) and adamantyl isocyanide (2 mol%), with Me2PhSiH reduced alkene at 50°C for 23 h (Scheme 23). This reaction tolerated styrenes with different functional groups such as tBu, Cl, F, and CO2Et. They also performed this reaction using Me2PhSiH as silane source under the same reaction conditions and obtained the required products with high selectivity. This method provided smooth reduction of sterically hindered o-methylstyrene and vinylnaphthalene. However, they could not achieve selective hydrosilylation in the case of aliphatic or cyclic alkenes. Cobalt or iron carboxylates and isocyanide ligands were identified as active catalytic system for the hydrosilylation of alkenes with hydrosiloxanes [34]. Under

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Scheme 24 Iron catalyzed hydrosilylation of alkenes using hydrosiloxanes as silane source

Scheme 25 Iron catalyzed hydrosilylation of olefins

Scheme 26 Iron oxide nanoparticles as catalyst for hydrosilylation of alkenes

the optimized condition (1:2 mixture of Fe(OPv)2 (3 mol%) and CNAd at 50°C for 3 h) styrenes underwent smooth hydrosilylation with 1,1,3,3,3pentamethyldisiloxane (PMDS) to give the desired hydrosilylated products with high selectivity (>99%) and in good yield (>91%) (Scheme 24). Higher TON (9700) was observed for the hydrosilylation of styrenes with PMDS using 0.01 mol% of Fe(OPv)2/CNAd at 50°C for 24 h (yield = 86%). Later the same group discovered two new catalysts, Fe(CNR)5 and Co2(CNR)8 (where R = tert-butyl (tBu)) and studied the efficiency of this catalyst for the hydrosilylation of alkenes (Scheme 25) [35]. They found that the catalytic activity of this complex was lower to that of the previously reported Fe(OPv)2/CNAd system. However, this complex was found to be more effective for the hydrosilylation of allylic ethers. Nakazawa et al. prepared NNN-pincer iron complexes with ketimine-type iminobipyridine (BPI) ligands [36]. They have studied the catalytic activity of this complex for the hydrosilylation of 1-octene with primary, secondary, and tertiary silanes. In addition to the monoalkylated silanes, dialkylated silanes were also obtained in the case of primary or secondary silanes. Obora and co-workers reported the synthesis of highly efficient N, N-dimethylformamide (DMF)-stabilized iron oxide nanoparticles via DMF reduction method (Scheme 26) [37]. This synthesized iron nanoparticles were found efficient toward the hydrosilylation of various functionalized alkenes. Interestingly,

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Scheme 27 Markovnikov-selective hydrosilylation of terminal aliphatic alkenes

Scheme 28 Hydrosilylation of 1-octene under solvent-free condition

silylation of alkenes with carbonyl moiety occurs smoothly without reducing the C=O group. In 2018, Lu and co-workers demonstrated the hydrosilylation of terminal aliphatic alkenes using iron catalyst in a highly enantioselective and Markovnikovselective way (Scheme 27) [38]. Chiral oxazoline iminopyridine (OIP) iron complex (OIP-FeCl2) was chosen as the active catalyst for this transformation. A wide substrate scope was found using phenylsilane as silane source under optimized condition (OIP-FeCl2 (2 mol%) as catalyst, NaOtBu (6 mol%) THF, 0°C for 2 h) and obtained the desired product with high ee values (>99%) Different functional groups such as protected alcohol, silyl, halide, ether, ester, acetal, amine, and amide were found compatible in this strategy. Rauchfuss et al. synthesized FeBr2(MesNQpy), an efficient precatalyst for the hydrosilylation of olefins [39]. This catalytic system was found applicable to the smooth Markovnikov silylation of simple and complex 1-alkenes with various hydrosilanes. Peng and co-workers disclosed a novel and efficient strategy for the hydrosilylation of alkenes under solvent-free conditions [40]. They investigated the hydrosilylation of 1-octene using different silane sources such as MeCl2SiH, Me2ClSiH, and Ph2SiH2 in presence of 10 mol% iron powder as catalyst (Scheme 28). The use of MeCl2SiH, Me2ClSiH, and Ph2SiH2 as silane source for the hydrosilylation of 1-octene provided α-adduct in 72.9%, 85.7%, and 73.8% conversions, respectively. The selectivity observed for β-adduct was 74.8%, 80.5%, and 98.1%, respectively. Iron catalyzed highly regioselective and ligand-specific hydrosilylation of terpenes was described by Chen and co-workers (Scheme 29) [41]. While bidentate ligand in presence of iron catalyst provided allylic silanes through 4,1-addition, the tridentate nitrogen ligand offered 3,4-hydrosilylation product by switching the selectivity.

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Scheme 29 Iron catalyzed hydrosilylation of terpenes Scheme 30 Visible light mediated hydrosilylation of electron-deficient alkenes

Scheme 31 Iron catalyzed anti-Markovnikov selective hydrosilylation of conjugated dienes and terminal alkenes

A visible light induced, iron catalyzed methodology for the hydrosilylation of electron-deficient alkenes was developed (Scheme 30) [42]. This method was found to be successful for alkenes bearing different functional groups. Characteristics such as excellent atom economy, mild reaction condition, and earth-abundant metal salt as catalyst further enhance the significance of this method. Zhu et al. demonstrated an iron catalyzed, anti-Markovnikov-selective hydrosilylation of conjugated dienes and terminal alkenes using 2-imino-9-aryl1,10-phenanthroline as ligand (Scheme 31) [43]. This reaction was found to be feasible for aryl and alkyl ethylenes. A wide substrate scope was observed for conjugated dienes yielding the desired product with high 1,2-anti-Markovnikov selectivity.

1.1.3

Hydrosilylation of Alkynes

A highly regio- and stereoselective iron catalyzed hydrosilylation of terminal alkynes was developed by Zhan and co-workers [44]. They carried out the initial reaction using 1-decyne as model substrate and the optimized conditions were

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Scheme 32 Regio- and stereoselective hydrosilylation of terminal alkynes using Xantphos as ligand Scheme 33 Synthesis of geminal bis(silanes) via dihydrosilylation of alkynes

identified as PhSiH3 as silane source, FeCl2 (2 mol%) as catalyst, Xantphos (2 mol %) as ligand, NaBHEt3 (4 mol%) as additive in ether at 50°C under N2 atmosphere for 12 h (Scheme 32). This method was found suitable for branched and linear aliphatic alkynes and obtained the desired product with high selectivity and high yields. Aromatic alkynes also performed well in this reaction yielding the required products in 65% yields. Notable advantages of this strategy include use of commercially available catalyst and reagents, anti-Markovnikov selectivity, and excellent functional group tolerance. Geminal bis(silanes) are well-known compounds having wide application in synthetic organic chemistry due to their stability and potential to undergo various transformations. Zhu and co-workers introduced a new protocol for the preparation of geminal bis(silanes) via dihydrosilylation of alkynes utilizing iron catalyst (Scheme 33) [45]. This method tolerated different functional groups such as fluoro, siloxy, chloro, alkenyl, amino, alkoxy, etc. The major highlights of this methodology include high regiospecificity, mild reaction conditions, excellent atom economy, and easily available substrates. Later in 2019, the same group disclosed a regiodivergent and stereoselective hydrosilylation of internal and terminal alkynes in presence of iron catalyst with 2,9-diaryl-1,10-phenanthroline ligands (Scheme 34) [46]. Interestingly, complete reversal of regioselectivity was observed while varying the substitution pattern on ligands. This method was found to be highly useful for the preparation of di- and trisubstituted olefins in large scale under mild conditions. Huang and co-workers described an efficient iron catalyzed methodology for the synthesis of 1,3-dienylsilanes via hydrosilylation of 1,3-enynes in a highly stereoand regioselective way (Scheme 35) [47]. They have chosen the tridentate iminopyridine-oxazoline (IPO) as ligand for this reaction. This catalytic system was compatible with 1,4-disubstituted, 4-alkyl and 1,1,4-trisubstituted 1,3-enynes. This methodology exhibits good functional group tolerance. Tilley et al. investigated the hydrosilylation of terminal alkynes and internal alkynes with primary silanes using [Cp*(iPr2MeP)FeH2SiHR]+ as catalyst [48]. This methodology requires low catalyst loading of only 0.1 mol% and works under ambient temperature. This catalytic system selectively gave cis-hydrosilylated product in the case of internal alkynes.

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Scheme 34 Regiodivergent and stereoselective hydrosilylation of internal and terminal alkynes

Scheme 35 Iron catalyzed hydrosilylation of 1,3-enynes

1.1.4

Hydrosilylation of Imines

Fe(0)-catalyzed hydrosilylation of imines was explored by Mandal et al. in 2016 [49]. Selective hydrosilylation of aldimines and ketimines was possible using aNHC-based iron(0) catalyst at room temperature under low-catalyst loading with high TON (up to 17,000) (Scheme 36). The substrate scope was found wide in the case of aldimines under optimized condition (aNHC-based iron(0) catalyst (0.05 mol %), DMSO, RT, 12 h). However, a slight modification in the reaction condition was needed for ketimines (aNHC-based iron(0) catalyst (2 mol%), ethanol, RT, 24 h). Findlater et al. put forward the hydrosilylation of imines to amines using ([BIAN] Fe(η6-toluene) as the catalyst (Scheme 37) [50]. This reaction was found to be feasible to aryl aldimines with electron-withdrawing and electron-donating

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Scheme 36 Hydrosilylation of aldimines and ketimines

Scheme 37 Iron catalyzed hydrosilylation of imines

substituents. However, imines prepared from aliphatic amines or aliphatic aldehydes afforded the corresponding amines in lower yields. Imines with bromo- or hydroxylsubstituents failed to achieve the expected product.

1.2

Manganese Catalyzed Hydrosilylation

Manganese, a cost-effective, low-toxic, and earth-abundant element, has been widely exploited in various organic transformations. Manganese catalyzed hydrosilylation reactions have undergone astonishing developments in recent years. Manganese showed high catalytic efficiency in hydrosilylation of carbonyl compounds, alkenes, and alkynes.

1.2.1

Hydrosilylation of Carbonyl Compounds

Trovitch et al. synthesized manganese zero-valent dimer [(Ph2PEtPDI)Mn]2] (PDI = propylene-bridged bis(imino)pyridine) by reacting 1:1 ratio of (THF)2MnCl2 with Ph2PEtPDI followed by reduction using Na/Hg in 1,3,5,7-cyclooctatetraene [51]. They have studied the catalytic activity of this compound in the hydrosilylation of carbonyl compounds. Heteroaryl aldehydes participated well in this reaction. This catalyst was also found efficient for formate dihydrosilylation in the presence of 0.01 mol% of precatalyst at 25°C for 2 h (Scheme 38). The reaction tolerated several substrates such as benzyl, octyl, p-anisyl, and isoamyl formates.

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Scheme 38 Hydrosilylation and dihydrosilylation of aldehydes and formates in the presence of manganese zero-valent dimer

Scheme 39 Asymmetric hydrosilylation of aromatic ketones using manganese catalyst

Asymmetric hydrosilylation of aryl ketones using manganese catalyst was reported for the first time by Huang and co-workers [52]. Chiral IPO manganese pincer complex was chosen as the efficient precatalyst for this method. Optically active secondary alcohols were obtained under mild reaction condition with low catalyst loading (Scheme 39). At room temperature, differently substituted alkylaryl ketones underwent hydrosilylation reaction smoothly and gave the required products in excellent yield. This methodology was found to be highly effective for aryl ketones with electron-donating and electron-withdrawing substituents. Mandal et al. reported the hydrosilylative reduction of 1° amide into 1° amines using mononuclear manganese complex as the catalyst. Various heteroaryl, aliphatic, and aryl primary amides underwent hydrosilylation in presence of 5 mol% of Mn-catalyst, 15 mol% of KOtBu, and PhSiH3 at 50°C (Scheme 40) [53]. They also found that primary amides could be converted to nitriles under the same reaction condition in the presence of catalytic amount of secondary amine. Using SOMC/TMP (surface organometallic chemistry/thermolytic molecular precursors strategy) method, Cope’ret et al. synthesized silica supported manganese(II) sites (2@SiO2–400 and 1@SiO2–400) from bis-trimethylsilyl amide (Mn{N (SiMe3)2}2
THF) and bis-tris(tert-butoxy)siloxide (Mn2[OSi(OtBu)3]4 devoid of any organic ligands [54]. These manganese (II) sites showed high catalytic activity in hydrosilylation of carbonyl compounds (Scheme 41). From their studies, they

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Scheme 40 Mn-catalyzed hydrosilylative reduction of primary amides

Scheme 41 Silica supported manganese(II) catalyzed (2@SiO2–400) hydrosilylation reaction of carbonyl compounds

observed similar catalytic activity for both complexes. However, since 2@SiO2–400 is synthetically more accessible, they have chosen this one as the catalysts. Leitner et al. investigated the catalytic performance of Mn(I) complexes containing triazole ligands towards the hydrosilylation of carboxyl and carbonyl compounds [55]. Differently substituted aliphatic and aromatic ketones react smoothly with PhSiH3 under optimized condition (Mn-1 (0.25 mol%), THF, 80° C, 1 h). Interestingly, this is the first report on hydrosilylation of carboxylic acids to afford alcohols in presence of manganese complex with tridentate (PNN)iminotriazole ligand (Mn-2) (Scheme 42). Bengali and co-workers established the hydrosilylation of ketones and aldehydes using manganese α-diimine complex (Scheme 43) [56]. The Mn-catalyst with diazabutadiene ligand displayed excellent chemoselectivity and strong functional group tolerance toward the hydrosilylation of carbonyl compounds. Primary silanes were chosen as the reducing agent for this method, and this method was found to be more effective for aromatic aldehydes compared to aliphatic ones. Synthesis of novel bidentate manganese(I) complex containing 6-methylpyridine and benzimidazole fragments was developed by Kundu and co-workers. This manganese complex showed high catalytic performance in the hydrosilylation of nitriles and ketones (Scheme 44) [57]. This protocol was found to be successful for the

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Acids

Scheme 42 Hydrosilylation of carboxyl and carbonyl compounds using Mn(I) complexes

Scheme 43 Hydrosilylation of carbonyl compounds using manganese α-diimine complex

reduction of a wide range of nitriles and ketones to the corresponding amines and alcohols, respectively. Chemoselective reduction of unsaturated ketone to unsaturated alcohol was possible via this method. Aliphatic and aromatic ketones bearing electron-withdrawing and electron-donating substituents performed well in this reaction. Biphosphine ligated Mn-complex was utilized as efficient catalytic system for the selective reduction of esters to alcohols. The substrate scope studies showed that a wide variety of esters are compatible to afford the corresponding alcohols under the optimized condition (Mn-catalyst (1 mol%, 100°C, 6 h) (Scheme 45) [58]. Various aliphatic and aryl esters were tolerated well and provided reasonable yield of the desired products. Low catalyst loading and solvent-free reaction conditions are the main highlights of this protocol. Hydrosilylation of carbonyl compounds using Mn-PNP pincer complex and low-cost polymethylhydrosiloxane (PMHS) was disclosed by Kirchner et al (Scheme 46) [59]. In the case of carbonyl compounds bearing conjugated C=C bonds or reducible moieties like cyano or nitro groups, carbonyl compounds were

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Scheme 44 Hydrosilylation of nitriles and ketones using Mn(I)-catalyst containing benzimidazole fragment

Scheme 45 Biphosphine ligated Mn-complex as catalyst for reduction of esters

Scheme 46 Mn-PNP pincer complex catalyzed hydrosilylation reaction

selectively reduced without reducing CN and NO2 groups. Under the optimized reaction conditions, various aryl alkyl, heteroalkyl, aliphatic carbonyl compounds reacted well in this method. Mn(I) catalyzed hydrosilylation of carboxylic acids to give alcohols was reported. Aryl or aliphatic carboxylic acids underwent reduction in the presence of 2 mol% of [Mn(CO)5Br] and 2–2.5 mmol of PhSiH3 in 2-methyltetrahydrofuran (2-MTHF) at 80°C for 2 to 16 h, followed by hydrolysis to give the corresponding alcohols in 40–98% yields (Scheme 47) [60]. The gram scale synthesis was also found successful using low catalyst loading of 0.5 mol% and low amount of silane (1.5 equiv. of PhSiH3). Manganese(II)bis(supersilyl) complex was obtained when MnBr2 was reacted with KSi(SiMe3)3 in THF at room temperature for 1 h. Hydrosilylation of carbonyl compounds was achieved using 1,1,3,3-tetramethyldisiloxane [TMDS] in the presence of manganese(II)bis(supersilyl) complex Mn[Si(SiMe3)3]2(THF)2 [61]

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Scheme 47 Mn(I)catalyzed hydrosilylative reduction of carboxylic acids

Scheme 48 Hydrosilylation reactions of ketones and aldehydes using manganese(II) disilyl complexes

Scheme 49 Hydrosilylation of carbonyl compounds using binuclear 2-iminopyrrolyl manganese (II) pyridine chloride complex

(Scheme 48). When the reaction was carried out with relatively inert ketones such as aryl ketones with electron-withdrawing groups, the addition of adamantyl isocyanide (CNAd) was needed to enhance the catalytic efficiency. The novel Mn (II)-complex (Mn[Si(SiMe3)3]2(CNAd)2) was additionally produced in good yield through the stoichiometric reaction of adamantyl isocyanide with Mn[Si (SiMe3)3]2(THF)2. This resultant complex was discovered to function as a catalytic precursor for hydrosilylation reaction of comparatively inert ketones. Gomes and co-workers developed a novel methodology for the preparation of binuclear 2-iminopyrrolyl manganese(II) pyridine chloride complexes (Scheme 49) [62]. One of the prepared complexes was highly efficient toward the hydrosilylation of carbonyl compounds. Under mild condition, differently substituted ketones and aldehydes smoothly underwent hydrosilylation and afforded high yield of the corresponding alcohols with high turnover frequency (TOF) 95 min-1.

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Hydrosilylation of Alkenes

Hilal et al. developed an efficient tetra(4-pyridyl)porphyrinato-manganese(III) (MnTPyP+) catalyst for the hydrosilylation reaction of triethoxysilane with 1-octene [63]. They were able to increase the catalyst efficiency and selectivity to form linear hydrosilylation product via intercalating (MnTPyP+) ions inside layered clay support (Scheme 50). Easy recovery and the reusability of the catalyst up to three times without losing its activity are the major highlights of this supported catalyst. Manganese catalyzed hydrosilylation of alkenes in a regio-controlled manner was established [64]. NaOtBu was required in this method for the alkoxide activation which is essential for precatalyst activation. In this synthetic methodology, aryl and alkyl olefins underwent hydrosilylation with (EtO)3SiH in the presence of catalyst under the optimized condition (DIPPBIPMnBr2, NaOtBu at 25°C for 4 h) (Scheme 51). Trovitch and co-workers synthesized β-diketiminate Mn-hydride complex ([(2,6iPr2Ph BDI)Mn(μ-H)]2) via the reaction between [(2,6-iPr2PhBDI)Mn(μ-Cl)]2 and NaEt3BH. This complex was found to be highly efficient for hydrosilylation of alkenes in benzene-d6 at 130°C (Scheme 52) [65]. Interestingly, styrenes underwent Markovnikov’s hydrosilylation, while aliphatic alkenes provided antiMarkovnikov’s products. Moreover, this catalyst was found to be highly effective for carbonyl group reduction and was highly selective that it reduces carbonyl functionalities over alkenes at room temperature.

1.2.3

Hydrosilylation of Alkynes

Highly stereo- and regioselective manganese catalyzed hydrosilylation of alkynes under visible light radiation was developed (Scheme 53) [66]. This method offered a

Scheme 50 Hydrosilylation of olefins using tetra(4-pyridyl)porphyrinato-manganese(III) catalyst

Scheme 51 Manganese catalyzed hydrosilylation of alkenes

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Scheme 52 Hydrosilylation of olefins using β-diketiminate Mn-hydride complex

upto

Scheme 53 Visible light mediated Mn-catalyzed alkyne hydrosilylation

wide range of Z-vinyl silanes in excellent yield (up to 98%) with high anti-Markovnikov selectivity. Aromatic alkynes with different functional groups such as bromo, chloro, fluro, alkoxyl, alkyl, etc. were tolerated well in this reaction. The notable characteristics of this method include mild reaction conditions, excellent regio- and stereoselectivity and good functional group tolerance.

1.2.4

Hydrosilylation of Carbon Dioxide

Carbon dioxide hydrosilylation with triethyl silane (Et3SiH) utilizing bromopentacarbonylmanganese (Mn(CO)5Br) was devised by García et al. [67]. This was the first report on radical transition-metallic intermediate catalyzed carbon dioxide hydrosilylation. Triethylsilylformate was obtained in 86% yield when THF was used as the solvent at 50°C and 4 bar of carbon dioxide pressure for 1 h. From solvent optimization studies, they understood that by using a mixture of toluene and tetrahydrofuran (THF) they could obtain bis(triethylsilyl)acetal as the product in 68% yield

2 Conclusions In this chapter we have summarized the recent developments in iron and manganese catalyzed hydrosilylation reactions. Hydrosilylation reaction is one of the important hydrofunctionalization reactions. Though most of the reports on hydrosilylation reaction involve toxic, rare, and expensive noble metals like Pd, Pt, Ir, Rh, etc., recently earth abundant first row transition metals were proven to be highly efficient.

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Thus transition metals such as Fe, Co, Ni, Mn, etc. have emerged as promising catalysts in hydrosilylation reactions. Over the past few decades, iron and manganese were widely exploited as catalyst in various organic transformation. Iron-based complexes were identified as effective catalysts for the hydrosilylation of carbonyl compounds, alkenes, alkynes, etc. Most of the reported methods exhibit good functional group tolerance and high regio- and stereoselectivity. Some of the suggested methods provide new avenues toward the synthesis of chiral alcohols via asymmetric hydrosilylation of aldehydes and ketones. Different iron-based complexes such as iron piano stool complexes, iron hydrido complexes, iron pincer complexes, etc. have been proven to be highly efficient for hydrosilylation reaction. From our studies it is understood that most of the reported methods prefer phenyl silane as the silane source. Iron-based complexes were found to be highly efficient toward anti-Markovnikov and Markovnikov selective hydrosilylation of olefins. Though iron was widely utilized in hydrosilylation of aldehydes and ketones, substrates such as alkenes, alkynes, and other carbonyl compounds were less explored. Over the past few decades, manganese catalyzed hydrosilylation has received significant attention. Different manganese-based complexes such as carbonyl-Mn complexes, pincer-Mn complexes, manganese amide complexes, etc. have been devised for hydrosilylation reactions. Most of the reported manganese catalyzed methodologies include the hydrosilylation of aldehydes and ketones. Limited works are available on hydrosilylation of C=C or C  C bonds. Compared to the more explored iron-based catalytic system, manganese catalyzed hydrosilylation reaction is in its initial stage. The development of highly active manganese-based complexes with good functional group tolerance and excellent selectivity is highly demanding. Acknowledgments TA thanks the Council of Scientific and Industrial Research (CSIR, New Delhi) for a senior research fellowship.

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Top Organomet Chem (2023) 72: 253–284 https://doi.org/10.1007/3418_2023_103 # The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 Published online: 21 October 2023

Catalysis of Hydrosilylation Processes with the Participation of Ionic Liquids Hieronim Maciejewski, Magdalena Jankowska-Wajda, and Izabela Dąbek

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Ionic Liquids as Solvents and Immobilizing Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Heterogeneous Catalysts with Ionic Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Ionic Liquids as Complexes or Ligands in Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Application of Catalytic Systems Containing Ionic Liquids in Continuous Processes . . . . 6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Hydrosilylation processes are still most often catalyzed using homogeneous complexes of transition metals (mainly platinum and rhodium). However, due to economic (high prices of precious metals), ecological (unacceptable presence of heavy metals in products), and technological (difficulties in removing catalysts from products and trying to recycle them) other catalytic systems are still being sought. In addition to the new earth-abundant TM-based catalysts (which are still not as effective as platinum complexes), heterogenized catalysts are increasingly being used. This chapter presents examples of such catalysts, which were obtained with the use of ionic liquids as a platform for the heterogenization of highly active Rh and Pt complexes. Ionic liquids (ILs) in these catalytic systems play different roles, as (i) a solvent dissolving the catalyst and an immobilizing agent (biphasic system), H. Maciejewski (✉) Faculty of Chemistry, Adam Mickiewicz University, Poznań, Poland Poznań Science and Technology Park, Adam Mickiewicz University Foundation, Poznań, Poland e-mail: [email protected] M. Jankowska-Wajda Faculty of Chemistry, Adam Mickiewicz University, Poznań, Poland I. Dąbek Poznań Science and Technology Park, Adam Mickiewicz University Foundation, Poznań, Poland

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(ii) an immobilizing layer with a solid support (supported ionic liquid phase—SILP and solid catalysts with ILs layer SCILL), (iii) ligand in metal complexes, or (iv) complexes (halometallate ionic liquid). All of the above catalytic systems show high activity in hydrosilylation processes and also enable easy isolation and multiple use. Keywords Heterogeneous catalysis · Hydrosilylation · Ionic liquids · Rh and Pt complexes · SILPC

1 Introduction Hydrosilylation is the main and commonly applied method of the manufacture of organosilicon compounds both in the silicone industry (crosslinking of silicone rubbers, organofunctional silicones) and in the production of hybrid materials and preceramic materials, for the modification of organic polymers and functionalization of surfaces [1–3]. It also finds an application to the synthesis of nanomolecules with a strictly defined spatial structure (silsesquioxanes and dendrimers) as well as to the manufacture of nanomolecule-containing nanocomposites [4–7]. Currently known and applied catalytic systems enable Si–H bond addition to almost each olefin, and this causes hydrosilylation processes to create practically unlimited possibilities of producing any material that contains organosilicon components. The silicon industry (including the manufacture of organosilicon derivatives) is one of the most dynamically developing industries (the annual production increase of 5–7%) and the hydrosilylation process considerably contributes to this result and is one of the most prospective ones, although it has been known for almost 80 years. Hydrosilylation is an addition reaction that can be carried out in the presence of free radical precursors or various catalysts, e.g., amines, Lewis acids, supported metals, or metal complexes. Although a number of catalysts have been tested in the hydrosilylation process, most research and industrial syntheses are still carried out using platinum and rhodium complexes because they exhibit high catalytic activity. However, in most cases they are homogeneous catalysts. The high price of both metals, as well as the unacceptable presence of even trace amounts of these metals in final products, makes it necessary to look for other catalytic systems. The solution from an economic point of view is to use cheaper metals. Currently, more and more literature reports concern catalysis with the participation of earth-abundant transition metals for example Fe, Co, Ni, and Mn [8–10]. They have been presented in detail in previous chapters. The second trend in the search for other catalytic systems is the development of heterogeneous catalysts that enable easy isolation from the postreaction mixture and recycling and reuse of the same catalyst. Recently, there has been a lot of interest in recyclable heterogeneous catalysts based on metal nanoparticles (MNPs) and single-atom catalysts (SACs) [11]. Certainly, these are the catalysts of the future because they already show high activity and selectivity

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Fig. 1 The most popular cations and anions forming ionic liquids

(largely platinum catalysts), but the problem of obtaining high fineness of the metal, stability of the dispersion, and metal leaching from the catalytic system during the process still remains to be solved. An alternative solution combining homogeneous and heterogeneous catalysis is the immobilization of metal complexes (catalysts) on an organic or inorganic (molecular or polymer) platform. An example of this type of solution may be the use of ionic liquids, which in the last few decades have aroused great interest in many fields of science [12]. Research on ionic liquids in catalysis is one of the most interesting and prospective research areas [13–17], as evidenced by the number of publications in this field, which over the last 5 years has been increasing by over 400 reports per year. Currently, over 100 types of chemical reactions catalyzed by transition metal complexes are carried out in the presence of ionic liquids [16–18]. Among them, there are also hydrosilylation reactions, but the number of literature reports on this subject is still relatively small. Ionic liquids are defined as ionic compounds consisting of an organic cation and an anion (organic or inorganic) (Fig. 1), characterized by a melting point below 100° C [19–22]. Due to their unique properties, mainly high polarity, very low vapor pressure, and high thermal stability, ionic liquids enable chemical reactions to be carried out in a wide range of temperatures and pressures. Their directions of application are very diverse, ranging from solvents (which are an alternative to volatile organic solvents), through electrochemistry (as electrolytes or conductive polymers), chemical

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separation, liquid or solid carriers, surfactants, and stabilizers of nanoparticles, as well as broadly understood catalysis [18, 21–25]. Ionic liquids in catalytic processes can perform various functions, starting from the role of a solvent (which, due to its low vapor pressure and stability, is a “green” alternative to conventional solvents), through an agent immobilizing metal complexes, to a component (substituent, ligand) of an active catalyst [12]. The purpose of this chapter is to present the most important examples of hydrosilylation processes carried out with ionic liquids and to determine the role they play in them.

2 Ionic Liquids as Solvents and Immobilizing Agents One of the most important and widely developed directions of application of ionic liquids in catalysis is their use as solvents, which simultaneously immobilize homogeneous catalysts and produce two-phase systems, where one phase is the ionic liquid with the catalyst dissolved in it, and the other is the reagents. One of the first examples of the use of ionic liquids in hydrosilylation reactions was the use of ammonium liquids [26]. The hydrosilylation reaction of the terminal alkynes, catalyzed by Speier’s catalyst (H2PtCl6 in isopropanol) in the medium of methyltrioctylammonium bis(trifluoromethylsulfonyl)imide (Scheme 1), proceeded in a two-phase system at room temperature. The reaction proceeded much faster compared to the reaction carried out in the presence of conventional solvents, with very high yields (>94%). In the case of the reaction with trichlorosilane, a selective formation of the β-E-isomer was observed, while in the reaction with the remaining silanes, a small amount of the α-isomer was additionally formed. Nevertheless, the selectivities were very high. Moreover, after completion of the reaction, both phases could be easily separated and the catalytic system could be recycled for further syntheses. The reactions were repeated three times with the same batch of catalyst without loss of catalytic activity. Almost at the same time, a paper appeared describing the hydrosilylation of octene with dimethylphenylsilane catalyzed by a fluorinated equivalent of

Scheme 1 Hydrosilylation of alkynes catalyzed by Speier’s catalyst dissolved in IL

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Scheme 2 Hydrosilylation of octene catalyzed by fluorinated equivalent of Wilkinson’s catalyst

Scheme 3 Synthesis of silicone polyethers catalyzed by platinum complex dissolved in an imidazolium ionic liquid.

Wilkinson’s catalyst [RhCl{P(C6H4Si(CH3)2CH2CH2C6F13)}3] in an imidazolium ionic liquid containing a fluorinated anion (Scheme 2) [27]. The use of the same fluorinated derivative as a substituent in the phosphine (which is a ligand in the rhodium complex) and in the anion of the ionic liquid ensured very good solubility of the complex in this liquid and limited its leaching. In addition, the catalyst-containing phase could be easily separated from the postreaction mixture and reused. 15 catalytic cycles were carried out, in which the average yield was 92%. This resulted in TON = 4,000 mol per mol of catalyst. For comparison, the leaching of the catalyst from the ionic liquid was 3 times smaller compared to the leaching in a conventional fluorinated solvent. These first literature reports aroused the interest of the industry in the possibility of conducting hydrosilylation processes in two-phase systems. In particular, it concerned the functionalization of polysiloxanes, which, due to their high viscosity, made it impossible to isolate the catalyst in typical homogeneous systems. The Degussa S.A. concern has developed a method for the synthesis of silicone polyethers [28]. Various ionic liquids (pyridinium, ammonium, and imidazolium derivatives) as well as polysiloxanes with different chain lengths and allyl polyethers with different content of ethoxy and/or propoxy groups were used in the research (Scheme 3). The most effective catalytic system turned out to be the platinum

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Scheme 4 Hydrosilylation of hexadecene catalyzed by various Pt complexes immobilized in pyridinium ionic liquid

complex [{Pt(μ-Cl)(C6H14)Cl)}2], dissolved in trimethylimidazolium methyl sulfate [TriMIM][MeSO4]. It has been shown that in order to ensure good separation of the catalyst from the post-reaction mixture, the selection of the ionic liquid for a given catalyst must be in harmony with the hydrophilic-hydrophobic properties of the reaction products. Another example of the industrial application of ionic liquids concerns the hydrosilylation of hexadecene with poly(hydromethyl, dimethyl) siloxane, catalyzed by various catalytic systems based on platinum Pt(0) and Pt(II) complexes, dissolved in ionic liquids, which are pyridinium or piccolinium derivatives (Scheme 4) [29]. The best catalytic system turned out to be the complex [Pt(PPh3)4] in pyridinium liquid [C4py][BF4]. The contribution of our team in this field was the study and selection of many catalytic systems, mainly based on platinum and rhodium complexes, immobilized in various types of ionic liquids. Due to the ease and low cost of synthesis, the most popular group of ionic liquids are imidazolium derivatives, which is why this group of derivatives was the subject of our research in the first place. During the preparation of silicone waxes (alkylpolydimethylsiloxanes), we compared the performance of homogeneous platinum and rhodium catalysts, their heterogenized counterparts (complexes dissolved in imidazolium derivatives), and heterogeneous Pt/SDB catalysts (platinum on a styrene-divinylbenzene support). The highest catalytic activity and the possibility of multiple use were shown by a system composed of a siloxyl rhodium complex [{Rh(μ-OSiMe3)(cod)}2], immobilized in the liquid [TriMIM] [MeSO4] [30]. The same system also turned out to be the most effective catalyst for the synthesis of other organofunctional polysiloxanes (Scheme 5) [31]: Therefore, we extended our research to other siloxyl rhodium complexes (monomeric and dimeric, Fig. 2), and we also used another group of ionic liquids— phosphonium derivatives with the general formula shown in Fig. 3, differing in the type of anion: These systems have been used in polysiloxane functionalization reactions. All siloxyl rhodium complexes showed high activity; however, the best results and at the same time the highest stability were again demonstrated by the complex [{Rh (μ-OSiMe3)(cod)}2] [32]. From the point of view of the ionic liquid, no significant effect of the type of cation on the effectiveness of immobilization of rhodium

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Scheme 5 Synthesis of organofunctional polysiloxanes catalyzed by Rh complex in imidazolium IL Fig. 2 Various siloxyl rhodium complexes

Fig. 3 General formula of phosphonium ionic liquids

complexes was observed, while the type of anion has a major influence. By far the best catalytic systems, ensuring high catalytic activity, easy isolation of the catalyst from the post-reaction mixture, and above all multiple use without loss of activity, turned out to be those in which the anions were [MeSO4], [(CF3SO2)2N], [Ace], and [Sac]. A common feature of these anions is the presence of the SO2 group, which suggests that the presence of oxygen atoms as donors additionally stabilizes the metal complex dissolved in the liquid and thus prevents its leaching. Thus, earlier

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Scheme 6 Functionalization of silsesquioxanes catalyzed by platinum complexes dissolved in [TriMIM][MeSO4]

observations of studies using imidazolium liquids [30], where the most effective liquid was [TriMIM][MeSO4], are identical to the results obtained in studies on phosphonium liquids. Another confirmation of the effect of anion was the synthesis of monofunctional disiloxane derivatives. In the hydrosilylation of octene with 1,1,3,3tetramethyldisiloxane, the rhodium complex [{Rh(μ-Cl)(cod)}2], dissolved in tetraalkylphosphonium liquids, differing in the type of anion ([MeSO4], [(CF3SO2)2N], [(CN2)N], and [Cl]) was used [33]. Catalytic systems with liquids containing one of the first two anions made it possible to selectively obtain a monofunctional disiloxane derivative and, at the same time, to use the catalytic system multiple times. On the other hand, the other two systems did not show catalytic activity in the tested reaction. In the course of further research, we used the best imidazolium and phosphonium liquids for the immobilization of platinum complexes at various oxidation states ([Pt2{(CH2 = CHSiMe2)O}3], K2[PtCl4], K2[PtCl6], PtCl4) and the systems prepared in this way were used as catalysts for the functionalization of silsesquioxanes (Scheme 6) [34]: The best results were obtained for systems in which platinum compounds with higher oxidation states were used, i.e., PtCl4 and K2[PtCl6], dissolved in [TriMIM] [MeSO4], in particular in the hydrosilylation of non-polar olefins. In the case of more polar olefins (allyldimethoxybenzene or allyl-glycidyl ether) complexes with a low oxidation state (Karstedt’s catalyst) showed better activity, but still lower than the above-mentioned systems. Another anion that has stabilizing properties for metal complexes is the methylsulfonate anion. The ionic liquid formed from its combination with a pyrylium cation (Fig. 4) was used by us for the first time as a solvent for platinum and rhodium complexes and used in the hydrosilylation of 1-octene [35]. The Wilkinson’s complex [Rh(Cl(PPh3)3]) dissolved in this liquid showed the highest activity and the possibility of repeated recycling of the catalytic system (the reaction was repeated 10 times with the same catalyst activity).

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Fig. 4 Catalytic systems based on rhodium or platinum complexes dissolved in pyrylium IL

Fig. 5 Morpholinium ILs with various anions Table 1 Acidity constants (pKa) of acids conjugated with bases (anions of an ionic liquid)

Conjugated acid H2SO4 (1) CH3C6H4SO3H (2) CH3SO3H (3) HNO3 (4) HNO2 (5)

pKa -3.2 -2.8 -1.9 -1.4 3.3

The effect of the anion of the ionic liquid on the catalytic activity of the system formed with its participation was also the subject of research for morpholinium liquids (Fig. 5). in which rhodium complexes ([{Rh(μ-OSiMe3)(cod)}2], [{Rh(μ-Cl)(cod)}2], [RhCl(PPh3)3]) or platinum complexes ([Pt(PPh3)4], [PtCl2(PPh3)2], PtCl4) have been dissolved [36]. Studies have shown that all rhodium complexes show greater activity in the hydrosilylation of octene with heptamethyltrisiloxane, and the siloxyl rhodium complex is the most active of them. At the same time, it could be noticed that in each case the activity of individual metal complexes was influenced by the anion from the morpholinium liquid. Comparing the acid constants of the conjugate acids with individual anions (Table 1), it can be seen that they are differentiated. The lower the acid constant, the stronger the acid, and hence the weaker base (weaker nucleophile) conjugate to that acid. Since the strength of the acids presented in Table 1 decreases from top to bottom of the table, the coordination capacity of the corresponding anions (conjugated bases) increases (their nucleophilicity increases). Weakly coordinating ligands, located in the metal coordination sphere, easily detach from the metal center during the catalytic reaction, and when the reaction is completed, they are coordinated again, because they are present in excess as a

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Fig. 6 Catalytic system based on various Rh or Pt complexes dissolved in ammonium ionic liquids

Fig. 7 Imidazolium ILs functionalized with polyethylene glycol

component of the ionic liquid. In turn, stronger coordinating anions do not detach from the metal center, preventing the course of the catalytic reaction or significantly limiting it. In the case of morpholinium liquids, a decrease in the catalytic activity of the complexes dissolved in them in the series from (1) to (5) is observed. Considering that the previously used anions [MeSO4] and [(CF3SO2)2N] are also weak nucleophiles, liquids containing these anions in combination with metal complexes show higher catalytic efficiency, which was previously observed. Despite the evident influence of the type of anion from the ionic liquid on the catalytic activity of the systems, in some cases the cation also has an influence. An example of this would be ammonium ionic liquids containing unsaturated groups (allyl or vinyl) as substituents (Fig. 6). Most of these liquids have sulfonic groups in their anions, which, according to the above data, should result in high efficiency of catalytic systems. It turns out, however, that also the liquid with the anion [NO3] maintains the catalytic activity of rhodium or platinum complexes at a high level [37]. The reason is the presence of unsaturated substituents in the cation, which, by coordinating with the metal center, form π-type complexes and stabilize the system. Better results were shown by diallyl derivatives, which suggests stronger coordination to the metal, but at the same time, during the course of the reaction, the possibility of detaching one group and making free coordination sites, and the other group still stabilizes the system. It should be emphasized that ammonium liquids containing saturated substituents do not immobilize metal complexes well. Another example of stabilization of metal complexes by a cation (from an ionic liquid) is the use of the so-called solvating ionic liquids. In the works presented by Peng’s team [38–40], imidazolium derivatives functionalized with polyethylene glycol were used (Fig. 7).

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Such liquids were used to immobilize rhodium complexes ({RhCl(PPh3)3] or RhCl3), and the resulting systems were used in the hydrosilylation reactions of styrene with triethoxysilane. Thanks to this, the reactions proceeded with higher selectivity and were devoid of side processes. Moreover, such catalytic systems could be recycled up to five times with only a slight decrease in activity. Polyglycol substituents in the ionic liquid acted as podands and, coordinating through oxygen atoms to rhodium complexes, stabilized the system. All the examples of ionic liquid applications presented above ensured the course of the reaction in two-phase liquid–liquid systems, in which one phase was the catalyst dissolved in the ionic liquid, and the second was the reactants. Another variant, facilitating the separation of two-phase systems, is the use of thermoregulated ionic liquids, which, under the influence of lowering the temperature, solidify (together with the catalyst contained in them) and enable easy separation from the reactants. After reheating, the catalytic system becomes homogenized and shows activity in subsequent reaction cycles. A review article was published a few years ago that provides an excellent overview of this group of ionic liquids and their applications in catalysis [41]. There are several examples of this type of ionic liquids that have been used in the hydrosilylation of olefins. One of them are alkylpyridinium [42] or dialkylimidazolium liquids [43] in the system with Wilkinson’s catalyst [Rh(Cl)PPh3)3]. It was found that the type of alkyl substituent in both derivatives had a significant influence on the course of the reaction and ease of isolation. Two-phase systems, in particular liquid–liquid, are the most common. In all the above systems, ionic liquids were used as an immobilizing agent for the metal complex and the system thus prepared was successfully used as a catalyst. An effective ionic liquid should (i) be a good solvent for the metal complex, (ii) immobilize the metal complex well to avoid leaching, (iii) do not interact too strongly with the metal complex so that catalytic processes can take place—do not deactivate it, (iv) do not mix with reactants and reaction products—form two-phase systems with them, (v) allow easy isolation from the post-reaction mixture, and (vi) be resistant to reactive substrates and not react with products. In addition to the undoubted benefits of easier separation of the catalyst from the reaction mixture (e.g., by decantation or extraction) and the possibility of its recycling and reuse, there are also several problems: (i) due to the polarity of some reactants, the catalyst may leach out of the ionic liquid, (ii) adsorb moisture and various impurities through the ionic liquid, which may cause deactivation of the catalyst, (iii) the high viscosity of the ionic liquids is often an obstacle to obtaining the maximum yield of the product, and (iv) a relatively large amount of ionic liquids is needed, which are expensive and therefore the process is not economically advantageous. Low vapor pressure, which is a positive feature of ionic liquids used as solvents, may be a problem when it is necessary to separate them (by distillation) from the post-reaction mixture. Separation problems can also be caused by the good solubility of ionic liquids in various solvents. In addition, the high viscosity of most ionic liquids causes problems with mass transport, which can be a factor limiting the course of the catalytic reaction. In order to eliminate the

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above problems, various solutions are used. For example, too high a viscosity can be reduced by using an additional solvent. Recently, microemulsions based on ionic liquids and aqueous or non-aqueous solvents [44], which combine the functions of ILs, but with a much lower viscosity, have gained great interest. Ionic liquids were also used as a reaction media and at the same time a stabilizing factor for platinum nanoparticles produced by microwaves. One of such liquids are derivatives of 1-aryl-3-alkyl imidazolium bis(trifluoromethylsulfonyl)imide [45]. These liquids enabled the formation of small platinum nanoparticles (3–5 nm) and prevented their aggregation. The systems prepared in this way were used as catalysts in the hydrosilylation reaction of phenylacetylene with triethylsilane, enabling the obtained quantitative conversion in a very short time (5 min). Unfortunately, the recycling and reusing of the catalysts failed due to the high degree of Pt-NPs leaching by the products. However, the results obtained are promising and finding the right conditions to prevent leaching is a challenge for future research. All these activities are aimed at optimizing process conditions in which ionic liquids are used as a solvent. However, as mentioned above, ionic liquids are definitely more expensive than conventional solvents, so efforts are being made to reduce their quantity. The optimal system is to obtain a catalyst in which the ionic liquid will perform analogous functions, but will be present in the amounts necessary to dissolve the catalyst.

3 Heterogeneous Catalysts with Ionic Liquids It is commonly sought to obtain heterogenized systems that combine the features of homogeneous and heterogeneous catalysis, making the catalytic process more profitable [46]. Such integration of homo- and heterogeneous catalyst options is currently implemented in several variants: as a catalyst in the ionic liquid phase on a solid support (supported IL phase catalysts—SILPCs) [47–50], solid catalysts with ILs Layer—SCILLs [47, 51–53], as well as porous ionic liquids [54, 55]. One of the most popular systems of this type are SILPCs, which are obtained by impregnating a porous carrier (usually silica, activated carbon, or aluminum oxides) with a selected ionic liquid [53]. The liquid film created on the surface of the carrier is also an immobilizing agent for metal complexes or nanoparticles. The properties of SILPCs depend not only on the type of metal complex (nanoparticle) used but also on the type of carrier (its specific surface and porosity) and, of course, on the type of ionic liquid and its ability to immobilize [56]. Ionic liquids play a different role in SCILL systems. In this case, the classic heterogeneous catalyst is covered with a thin layer of ionic liquid. This layer of ionic liquid modifies the surface of the catalyst, facilitating access of the reagents to the active centers, which improves the selectivity of the catalyzed reaction and limits the course of side reactions and increases resistance to poisoning. In this case, the role of the ionic liquid (IL) is to modify the surface, unify it, and also to increase the

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selectivity of some transformations and to protect the catalyst against poisoning. Of course, the selection of the appropriate ionic liquid is crucial, as it will facilitate the transport of substrates to the catalyst surface [25, 47]. The main difference between SCILL and SILPC systems is the different role of the ionic liquid, because in the case of SCILLs it is a factor facilitating the dissolution of the reagents and their transport to the catalyst surface, while in SILPCs it plays the role of an agent immobilizing the metal complex [51]. Unfortunately, there are very few literature reports on the use of this type of systems in hydrosilylation processes. It is still a new field of research. Our research group produced SILP catalysts by impregnating silica with tetraalkylphosphonium liquids (with different alkyl chain lengths) with various anions [(MeSO4] or [(CF3SO2)2N]) and then immobilizing various rhodium complexes ([{Rh(μ-OSiMe3)(cod)}2], [{Rh(μ-Cl)(cod)}2], [RhCl(PPh3)3]) [57]. The catalytic activity of the obtained systems (in the reaction of octene with HMTS) was compared with the two-phase systems, obtained by dissolving the same complexes in analogous ionic liquids. SILP catalysts turned out to be much more effective than two-phase systems. The most active system ([{Rh(μ-OSiMe3)(cod)}2] immobilized in [P66614][CF3SO2)2N] deposited on silica) allowed for a 1,000-fold reduction in the amount of catalyst (compared to the two-phase system), a significant reduction in the time reaction (by half), increasing the number of catalytic cycles using the same portion of the catalyst (at least 20 cycles), and easy separation of the catalyst from the post-reaction mixture (by decantation). The same catalyst also proved to be very effective in the hydrosilylation reaction of olefins with tetramethyldisiloxane [58]. This catalytic system enabled not only the selective production of a monofunctional derivative, but also the performance of 50 catalytic cycles with 99% conversion and 98% selectivity. Various compounds can be used as a carrier in SILP materials. One example is the oxide systems, TiO2–SiO2 or TiO2–SiO2/lignin [59]. Such a support was impregnated with ionic liquids (imidazolium, pyridinium, phosphonium, or sulfonium containing methylsulfate or bis(trifluoromethylsulfonyl)imide) anions, and then a rhodium [RhCl(PPh3)3] or platinum [PtCl2(cod)] complex was immobilized on this material. The catalytic activity of the obtained Rh-SILP and Pt-SILP materials was tested in the hydrosilylation reaction of 1,1,1,3,5,5,5-heptamethyltrisiloxane, triethylsilane, and triethoxysilane with polar and non-polar olefins (1-octene, allylglycidyl ether, allyl-octafluoropentyl ether). The obtained results showed that the Pt-SILP system is more active than the rhodium complex system. The obtained results showed that Pt-SILP systems are more active than systems with rhodium complex. Among the Pt-SILP materials, the highest catalytic activity was observed for those containing phosphonium and sulfonium ionic liquids. A similar support based on mixed oxides (MgO-SiO2 or MgO-SiO2/lignin) was synthesized by the sol-gel method and used to obtain SILP systems using analogous ionic liquids and complexes as in the example described above [60]. Also in this case, higher catalytic activity was shown by Pt-SILP materials containing phosphonium and sulfonium ionic liquids with methylsulfate anion, in the presence of which 10–15 catalytic cycles were carried out (TON = 1,109,000).

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Fig. 8 Cationic rhodium complex immobilized in polymeric monoliths

Very recently, Buchmeiser et al. presented new SILP materials, in which supports were polymer monoliths, made on the basis of polyurethanes [61] or norbornene [62]. The polymeric monoliths were impregnated with ionic liquids (ammonium or imidazolium) and final SILP system was created by immobilizing the cationic rhodium complex (Fig. 8). The above SILP materials have been used in the hydrosilylation reaction of both aromatic and aliphatic 1-alkynes with dimethylphenylsilane. In all cases, the highest β(Z) selectivity was obtained, which was explained by the effect of confinement by the mesoporous structure of polymer monoliths. Undoubtedly, SILPC catalysts are one of the most effective and have great potential, and by significantly reducing the amount of ionic liquid used, they eliminate the problem of mass transport (occurring in the case of two-phase systems) and reduce the cost of manufacturing the catalytic system, and consequently also the cost of the final product.

4 Ionic Liquids as Complexes or Ligands in Metal Complexes In all the above examples, catalytic systems were presented in which ionic liquids played the role of an immobilizing agent. Although the interaction of the cation or anion of the ionic liquid on the metal complex was visible, “the real catalyst” formed as a result of the metal complex–ionic liquid interactions was not isolated and thus not identified. An alternative way to obtain heterogenized catalysts and at the same time with a defined structure is the use of metal complexes in which the ligands would be the ionic liquids themselves. One of the methods of synthesis of this type of complexes is the preparation of phosphine ligands functionalized with ionic liquids, which are then attached to the metal. In particular, phosphines bearing imidazolium moieties have been synthesized and used as ligands. There are several methods for obtaining such ligands, which are summarized in Scheme 7. The phosphines obtained in this way were used for the synthesis of various rhodium or platinum complexes. Peng et al. described the hydrosilylation of olefins with triethoxysilane catalyzed by two rhodium complexes (Scheme 8) [63, 64]. Using type A complexes, it was found that all of them showed high catalytic activity, much higher in comparison to the homogeneous Wilkinson catalyst [63]. It should be noted, however, that the rhodium–phosphine complexes were dissolved in an ionic liquid analogous to that used for the phosphine modification. This means

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Scheme 7 Methods of synthesis of phosphine ligands functionalized with imidazolium IL

that the reaction was carried out in a biphasic system. In the case of the reaction with styrene, it was noticed that with the increase in the length of the alkyl chain (in the substituents at the nitrogen atoms), there is a slight decrease in the yield, but the selectivity toward formation of the β-adduct increases. In the case of reactions with aliphatic alkenes, excellent yields and selectivities were obtained. In addition, the catalytic system could be recycled and repeated four times without loss of catalytic activity. However, in the case of type B catalysts [64], it was additionally found that the catalytic activity decreases with increasing length of the alkyl chain between the imidazolium ring and the diphenylphosphine group. In addition, the influence of the anion of the ionic liquid on the efficiency of the styrene hydrosilylation reaction was found. Due to its nucleophilic nature, the BF4- anion (as opposed to the PF6- anion) caused lower solubility of the silane in the hydrophilic ionic liquid, which resulted in a lower reaction yield.

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A, where: R1 = CH3, R2 = C2H5, X = PF6 R1 = CH3, R2 = C4H9, X = PF6 R1 = CH3, R2 = C6H13, X = PF6 R1 = C2H5, R2 = C4H9, X = PF6 R1 = C4H9, R2 = C4H9, X = PF6 R1 = C4H9, R2 = C6H13, X = PF6 R1 = C4H9, R2 = C8H17, X = PF6 R1 = CH3, R2 = C4H9, X = BF4

B, where: R = CH3, n = 2, X = PF6 R = C4H9, n = 2, X = PF6 R = C6H9, n = 2, X = PF6 R = C8H17, n = 2, X = PF6 R = C8H17, n = 3, X = PF6 R = C8H17, n = 4, X = PF6 R = C8H17, n = 5, X = PF6 R = C8H17, n = 2, X = BF4

Scheme 8 Hydrosilylation of alkenes catalyzed by rhodium complexes containing functionalized phosphines as ligands

Fig. 9 Rhodium and platinum complexes containing one functionalized phosphine ligand

Our team synthesized similar complexes, which however contained only one phosphine functionalized with an imidazolium liquid (Fig. 9). Four rhodium complexes and two platinum complexes were obtained (Fig. 16) [65].

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Fig. 16 Yields of the product of hydrosilylation of octene with heptamethyltrisiloxane as determined for 10 subsequent reaction runs catalyzed by the same catalyst portion, for pyridinium derivatives

Scheme 9 Synthesis of organofunctional polysiloxanes catalyzed by rhodium and platinum complexes containing functionalized phosphine ligand

Complexes with Br- anion were solids and complexes with NTf2- anions were high viscosity oils. All complexes were thermally stable (decomposition temperatures >200°C) and insensitive to oxygen and moisture, making them easy to handle. They were used as catalysts in hydrosilylation reactions of olefins of different polarity (octene, allyl glycidyl ether) with heptamethyltrisiloxane (Scheme 9). The complexes were not soluble in the reagents; therefore, after the reaction was completed, they could be easily isolated and reused. The most active catalysts did not lose their activity even after 10 times of use. Rhodium complexes showed higher activity in the tested reactions, although the effectiveness of individual catalysts was dependent on the type of olefin and the products formed. Much more stable complexes were those containing bromide anion and in their presence it was possible to carry out 10 catalytic cycles without loss of activity. For non-polar reagents (1-octene) the activity of all complexes was similar (Fig. 10). Based on the results presented in Fig. 10, it can be easily seen that the most repeatable (in the next 10 cycles the product efficiency is the same and is over 95%) and at the same time showing the highest activity is the rhodium complex. 1. Among

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9 10

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Fig. 10 Comparison of the catalytic activity of Rh and Pt complexes, based on the yield of the octene hydrosilylation product obtained in the next 10 cycles

the platinum complexes, only the one with the bromide anion (complex 3) shows high activity and stability, while the complex with the imide anion (complex 5) shows lower activity (yield 69% after the fourth cycle) but relatively high stability. However, significant differences in the activity and stability of these catalysts can be observed in the case of the hydrosilylation reaction of allyl glycidyl ether, as shown in Fig. 11. The polar nature of allyl glycidyl ether causes complexes with the imide anion to leach more easily, which results in a significant decrease in product yield in subsequent catalytic cycles. In contrast, complexes with the bromide anion (both Rh and Pt complexes) show high stability and reproducibility, although the activity is lower than that observed in the case of the reaction with octene. Rhodium complexes with bromide anions have also been used in the hydrosilylation reaction of aliphatic and aromatic alkynes with heptamethyltrisiloxane or triethylsilane (Scheme 10) [66]. In all reactions, the above complexes showed higher activity and selectivity compared to their precursors. The reactions were much faster and in the case of aliphatic alkynes, mainly β(Z) isomers were formed, regardless of the hydrosilylating agent. However, during the hydrosilylation of phenylacetylene, different products were formed depending on the type of hydrogen silane (Table 2). In the case of the reaction with heptamethyltrisiloxane, mainly the β(Z) isomer was formed, while in the reaction with triethylsilane, mainly the β(E) isomer was formed, which can be explained by steric effects (due to the presence of a phenyl group), as well as the strongly electron-donating nature of triethylsilane and its

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Fig. 11 Comparison of the catalytic activity of Rh and Pt complexes, based on the yield of the allyl glycidyl ether hydrosilylation product obtained in the next 10 cycles

Scheme 10 Hydrosilylation of alkynes catalyzed by rhodium complexes with bromide anion (1 and 2)

interactions with the phenyl group. Since the rhodium–phosphine complexes were insoluble in the reagents, it was possible to isolate and reuse them. Therefore, for the most active catalyst 1, the possibility of using the same portion of the catalyst at least

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Table 2 Conversion of alkynes and selectivity for isomers in the hydrosilylation of RC  CH with HMTS or TES

H-Si HMTS

TES

Catalyst [RhCl(PPh3)3] 1 [{Rh(μ-Cl)(cod)}2] 2 [RhCl(PPh3)3] 1 [{Rh(μ-Cl)(cod)}2] 2

Alkyne C6H13C  CH Conv [%] β(Z)/β(E)/α 94 95/5/0 100 96/4/0 95 50/38/12 100 89/11/0 79 97/3/0 95 95/5/0 97 79/0/21 100 92/8/0

PhC  CH Conv [%] 99 70 0 87 88 89 0 96

β(Z)/β(E)/α 94/6/0 100/0/0 0 86/14/0 25/75/0 17/83/0 0 53/47/0

five times in the octyne hydrosilylation (with the same yield) significantly reduces the cost of the synthesis. In the above examples, an ionic liquid was used to functionalize the ligand, which was then attached to the metal. But an ionic liquid can also be an ionic complex if we modify it appropriately and the metal becomes a component of the ionic liquid. Examples are metallates or so-called halometallate ionic liquids, which are formed by reactions of metal halides with organic halides. The chloroaluminate ionic liquids were one of the first combination of this type [67]. Currently, derivatives containing transition metals such as Co, Ni, Ir, Au, Pd, and Pt are also known. A relatively simple method of synthesis inspired us to undertake research on this type of compounds. Platinum complexes were obtained by two methods, using potassium chloroplatinates or a cyclooctadiene complex as metal precursors, as shown in Scheme 11. In both cases, the reactions proceeded with high yields; however, in the second method, the yield is the highest due to the ease of removing the liberating cyclooctadiene. Despite the simple method of synthesis, most of these complexes were obtained for the first time, and for a few of them we were able to confirm the crystal structure (Figs. 12, 13, and 14) [68]. In the pyridinium complex as expected [PtCl4]2- ions show a square planar geometry. The 4-butyl-1-methyl-pyridinium cation is involved in seven C-H---Cl interactions between a hydrogen bond from phenyl ring and chlorine atom from [PtCl4]2-. When employing potassium hexachloroplatinate (IV) as a metal precursor in reactions with ionic liquids, hexachloroplatinates (IV) of imidazolium and pyridinium derivatives were obtained,(Fig. 13.) The coordination geometry of the Pt4+ atom of this complex can be described as a typical octahedral geometry and, similarly as in the case of the previous structure, 1-butyl-3-methyl imidazolium cation is involved in three C-H---Cl interactions between imidazolium ring hydrogen atom and chlorine from [PtCl6]2-. In addition, two C-H....Cl interactions occur between alkyl hydrogen atoms and [PtCl6]2-

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Scheme 11 Methods of synthesis of anionic platinum complexes Fig. 12 X-ray structure of [BMPy]2[PtCl4]

Fig. 13 X-ray structure of [BMIM]2[PtCl6]

Another analogous complex was synthesized, but 1,2-dimethyl-3-butylimidazole derivative was used for this purpose (Fig. 14). The structure of this complex is very similar to the previous one and fully agrees with spectroscopic analysis data. All the above compounds are basically classified into the group of halometallate ionic liquids; however, their melting points which exceed 150°C (particularly in the case of the obtained crystalline compounds) make it difficult to classify them among ionic liquids, whose melting points should be below 100°C. However, they are anionic platinum complexes. In the literature one can find information on platinum anionic complexes, i.e., chloroplatinates, but no information is available on analogous rhodium complexes.

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Fig. 14 X-ray structure of [BuMMIM]2[PtCl6]

Scheme 12 Synthesis of anionic rhodium complexes with various cations

Recently, however, we synthesized several anionic rhodium complexes (according to Scheme 12), and in the case of a complex with a piperidine cation, its crystal structure was determined. Such complexes have not hitherto been used as catalysts in hydrosilylation processes, although this process is mainly catalyzed by platinum and rhodium compounds. All anionic platinum and rhodium complexes were active in hydrosilylation processes, but their activity differed depending on the type of olefins. Fig. 15 shows product yields obtained in subsequent catalytic cycles of hydrosilylation of octene catalyzed by some anionic complexes of platinum or rhodium with imidazolium cation. The greatest stability was observed for the rhodium complexes, especially for the complex with cyclooctadiene ligand (B), in the case of which the yield was maintained at a constant level of 97% for 10 subsequent reaction runs while using the same portion of catalyst. Yields obtained in the presence of the platinum complexes, especially with hexachloroplatinate anion, (D) were lower and ranged from 71 to 58%, but the reproducibility in subsequent cycles has also been very high.

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Fig. 15 Yields of the product of hydrosilylation of 1-octene with heptamethyltrisiloxane as determined for 10 subsequent reaction runs catalyzed by the same catalyst portion, for imidazolium derivatives

The course of reactions catalyzed by anionic rhodium and platinum complexes with pyridinium cation has been similar (Fig. 16). Also in this case, the rhodium complexes showed the highest yield (96%) and catalytic stability. A lower yield in subsequent cycles was observed in the reactions catalyzed by the complex with hexachloroplatinate anion, although the yields were on the same level as those obtained in the presence of an analogous complex with imidazolium cation. On the other hand, a considerable reduction in the yield obtained in subsequent cycles was observed in the reactions catalyzed by the complex with tetrachloroplatinate anion. While comparing catalytic activity of platinum complexes, one can notice that complexes in which platinum is at lower oxidation states show a higher catalytic activity but a lower stability, which may be the result of a weaker bonding of ions in them. Even greater differences in the activity of the studied catalysts are observed in the reaction of allyl glycidyl ether hydrosilylation (Fig. 17). From among complexes with imidazolium cation, the most active and reproducible appeared to be rhodium(I) complex (A) in the presence of which the product yield in subsequent 10 runs was maintained at the same level of 95%, whereas the complex containing rhodium at the oxidation state of three (C) appeared to be less active and permitted to achieve 75% yield during the first four runs followed by a considerable activity drop during next runs. In the case of platinum complexes, one can notice that both complexes are very active in the first run; however, beginning from the second run the activity of platinum(II)-containing complex (D) drastically drops. On the other hand, the complex with hexachloroplatinate anion (B) permits us to obtain a lower yield in the first run, but during the next seven runs the yield exceeds 50%.

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Fig. 17 Yields of the product of hydrosilylation of allyl glycidyl ether with heptamethyltrisiloxane as determined for 10 subsequent reaction runs catalyzed by the same catalyst portion, for imidazolium derivatives

Fig. 18 Yields of the product of hydrosilylation of allyl glycidyl ether with heptamethyltrisiloxane as determined for 10 subsequent reaction runs catalyzed by the same catalyst portion, for pyridinium derivatives

The same tendency can be observed for reactions catalyzed by complexes with pyridinium cation (Fig. 18). But in this case the complex with hexachloroplatinate anion was the most stable and the most active since it enabled to achieve the yield above 90% during the next six cycles. Even rhodium complexes, whose catalytic performance was greater in most cases, appeared to be inferior to the complex with hexachloroplatinate anion. We extended our research with piperidinium and pyrrolidinium derivatives, with which we synthesized a number of tetra- and hexachloroplatinate complexes

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Scheme 13 Hydrosilylation of acetophenone derivatives with HMTS catalyzed by anionic rhodium complexes

[69]. Also in this case, the complexes showed high catalytic activity, but definitely greater stability and repeatability in subsequent catalytic cycles were demonstrated by piperidinium complexes, which, due to the presence of a six-membered heterocyclic ring, are more stable than pyrrolidinium complexes with a fivemembered ring. The anionic rhodium complexes also proved to be very effective catalysts for the hydrosilylation of ketones [70]. In the reaction with acetophenone (Scheme 13), they showed activity at much lower concentrations compared to the concentrations of conventional catalysts, and the reactions proceeded faster and with very good selectivity (mainly toward the product of hydrosilylation—A, and not dehydrogenative silylation—B). By examining various acetophenone derivatives, it can be concluded that anionic rhodium complexes are effective catalysts for the hydrosilylation of derivatives with electron-donating substituents (i.e., 4-methylacetophenone), while they do not show activity in the case of derivatives with electron-withdrawing substituents (i.e., 4-nitroacetophenone, 4-iodoacetophenone, and pentafluoroacetophenone). An interesting example of the use of an anionic iron complex is the use of [BMIm][FeCl4] in the reductive amination of aldehydes reaction [71]. In this reaction, hydrogen silane is a reducing agent and the final product is a secondary amine; however, during the process, a hydrosilylation product (N-silylamine) is formed (Scheme 14), which, as a result of hydrolysis with water, forms the final product. Certainly, halometallate ionic liquids (anionic metal complexes) are a good alternative to the currently used catalysts for the olefin hydrosilylation process due to the ease of their synthesis, stability in atmospheric conditions, high efficiency at low concentration, and ease of isolation from the post-reaction mixture due to their insolubility.

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Scheme 14 Proposed catalytic cycle for the [BMIM][FeCl4]-catalyzed direct reductive amination of aldehyde

5 Application of Catalytic Systems Containing Ionic Liquids in Continuous Processes One of the ways to reduce the cost of manufacturing products is the use of continuous processes that enable uninterrupted process and systematic delivery of raw materials and collection of products. This is especially important in industrialscale production. The first reports on effective two-phase catalysis with ionic liquids aroused the industry’s interest in their use in flow reactors. Wacker Chemie AG developed a method for the synthesis of 3-chloropropyltrichlorosilane (one of the most important organofunctional silanes), obtained in the process of hydrosilylation of allyl chloride with trichlorosilane using platinum complexes immobilized in imidazolium ionic liquids [72]. After the initial selection of the most effective system, a continuous method was developed using a loop reactor with integrated separation of the ionic liquid with the catalyst. Such a reactor could be operated continuously for 48 h, and the platinum leaching after this period was less than 1 ppm. In addition, an increase in the selectivity of the product was observed, as well as the possibility of using a simple platinum salt (PtCl4) instead of expensive and difficult to synthesize platinum complexes. On a laboratory scale, continuous processes can be implemented using microreactors. These types of reactors offer many advantages, including a significantly higher surface area to volume ratio, which allows for much more efficient mass transport and heat exchange, better control of the residence time of the reactants at a certain temperature, which improves efficiency, and, above all, selectivity [73–75]. Comparison of the catalytic activity of platinum and rhodium complexes immobilized in ionic liquids allowed us to select the most effective catalytic systems for use in the microreactor system. One of the first systems was the Karstedt’s catalyst, dissolved in phosphonium and imidazolium liquids [76]. The best ionic

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liquid turned out to be [P44414][NTf2]. The use of this catalytic system in the hydrosilylation of olefins with heptamethyltrisiloxane in a microreactor made it possible to intensify this process and obtain a product with higher yield and in a shorter time than in a conventional stirred reactor. Moreover, such a catalytic system could be reused at least four times without loss of activity. Comparison of other rhodium and platinum complexes showed that using the same phosphonium ionic liquid (as above), the rhodium complexes, and in particular the siloxyl rhodium complex [{Rh(μ-OSiMe3)(cod)}2], are the most effective catalysts for the hydrosilylation process in the microreactor system [77]. The conducted research showed that the use of microreactors in the hydrosilylation reaction allows us to increase the Si–H conversion in a significant way. In addition, by changing the reaction parameters, such as the length of the microreactors (which affects the residence time of the reactants) as well as the flow rate, temperature, and concentration of the catalyst, the course of the reaction can be optimized and controlled [78]. Microreactors have also been used in reactions catalyzed by platinum nanoparticles. An example is the microfluidic reactor, whose capillaries have been coated with imidazolium-based ionic polymer. This polymer layer served as a carrier for the platinum nanoparticles [79]. Microreactors prepared in this way were used in the hydrosilylation reaction of phenylacetylene with triethylsilane. An improvement in catalytic activity (2–8 times) was observed compared to batch reactions. In addition, the microreactors used allowed for a significant reduction in the amount of platinum needed to obtain a similar effect as in the case of batch reactions.

6 Conclusions The use of ionic liquids in various fields, including catalysis, continues unabated. Through the selection of new synthesis methods and the use of renewable and biodegradable raw materials, an increasing group of derivatives is recognized as safe and environmentally friendly. In the case of catalysis, ionic liquids allow for reactions with higher selectivities and yields, but above all, for easy isolation of the catalyst from the catalytic mixture. The presented examples show that also in the case of hydrosilylation reactions, the importance of ionic liquids and their application is becoming wider and wider. Currently, the most popular application is the use of ionic liquids as a solvent and immobilizing agent. The development of effective catalytic systems (metal complex–ionic liquid) and their practical application for the synthesis of organofunctional silicon compounds is justified and beneficial for the following reasons: (i) economic—the possibility of recycling the catalyst and the ease of isolation of the final product, which eliminates additional operations, (ii) ecological—removal of classic solvents from production, as well as wastegenerating operations, which significantly reduces emissions of volatile substances and waste generation, (iii) cognitive—the obtained results deepen our knowledge in the field of coordination chemistry, catalysis, and new directions of application of

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ionic liquids. Heterogeneous catalysts with the participation of ionic liquids, i.e., SILP or SCILL, as well as metal anion complexes, in which the metal is incorporated into the structure of the anion, are also gaining in importance. In this type of systems, all the problems observed in the case of catalysis in homogeneous systems are eliminated. First, there is no problem in isolating a catalyst that is heterogeneous. In the case of SILP and SCILL, the ionic liquid covers the carriers with a thin layer; therefore, a small amount is needed, which significantly reduces the cost (despite the high price of ionic liquids). Also due to this thin film, the viscosity of the liquid does not affect the process and mass transfer is not a problem. This form of catalyst can be used in flow reactors, which also improves the economics of the process. Therefore, in the future, a significant development of this type of heterogeneous catalysts and their use in hydrosilylation processes should be expected.

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Top Organomet Chem (2023) 72: 285–304 https://doi.org/10.1007/3418_2023_104 # The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 Published online: 4 October 2023

Hydrosilylation Catalysis for One-Pot Synthesis Ken Motokura

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 One-Pot Synthesis with Si as a Leaving Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 One-Pot Synthesis with Si as an Intersection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 One-Pot Hydrosilylation-Functionalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract One-pot synthesis, including the hydrosilylation of alkenes, alkynes, carbonyl compounds, CO2, and other molecules, enables the facile synthesis of final products with high efficiency. This chapter describes recent advances in three types of one-pot syntheses triggered by hydrosilylation: (i) one-pot synthesis with Si as a leaving group, (ii) one-pot synthesis with Si as an intersection, and (iii) other successive reactions, including hydrosilylation and functionalization reactions. Synthetic utilities such as the substrate scope and homogeneous/heterogeneous catalysis with mechanistic aspects are also discussed. Keywords Arylation · Carbon dioxide · Cyclization · Oligosiloxane · One-pot synthesis

K. Motokura (✉) Yokohama National University, Yokohama, Kanagawa, Japan e-mail: [email protected]

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1 Introduction One-pot syntheses promote multiple reactions in a single reactor. One-pot synthesis has several advantages over traditional multistep procedures (Fig. 1) [1–3]. The advantages of the one-pot synthesis are as follows: (1) The separation and purification of reaction intermediates is unnecessary, which means that the yield is not affected by losses during purification or the stability problems of the intermediate. (2) The reaction time, energy input, and space requirements are minimized. If the successive reactions proceed automatically in the same reactor, the total reaction time for synthesis of the final product can be minimized. (3) The economic use of organic solvents and reagents is possible, as large volumes of solvent are not required when there are no intermediate compounds to purify. However, sequences of multiple reactions are often difficult to conduct in a single reactor because of the deleterious interactions between the catalysts and reagents. In addition, the different reaction conditions required for each reaction make one-pot synthesis difficult. Therefore, the careful design of the reaction system is mandatory for the construction of one-pot synthetic reaction systems. The hydrosilylation reaction is one of the most useful reactions that can be used in one-pot reaction sequences due to its excellent reactivity and selectivity with appropriate catalysts [4– 10]. The unique reactivity of silicon atoms also confers advantages for one-pot synthesis. Scheme 1 illustrates the three types of one-pot synthetic systems, including hydrosilylation. Silyl groups are suitable leaving groups for cross-coupling reactions. The promotion of a cross-coupling reaction by incorporating a Si group into organic molecules after hydrosilylation is possible (Scheme 1a). The functionalization of reactive Si–H bonds of dihydrosilanes (R2SiH2) as useful intersections is an unique procedure for molecular design. Selective hydrosilylation with dihydrosilane, followed by the cross-coupling of the remaining Si–H bond, enables the formation of a disubstituted silane compound in a one-pot reaction (Scheme 1b). The simple functionalization of the reaction intermediate obtained

Multi-step Synthesis 多段階反応

A + B → P1 isolation ༢㞳䚸⢭〇 purification

C + P1 → P2 isolation ༢㞳䚸⢭〇 purification

D + P2 → P3 Fig. 1 Multistep synthesis and one-pot synthesis

One-Pot Synthesis ワンポット合成

A B

C D

in a single-pot A + B → P1 C + P1 → P 2 D + P2 → P 3

P3

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Scheme 1 One-pot synthesis with hydrosilylation catalysis: (a) one-pot synthesis with Si as a leaving group, (b) one-pot synthesis with Si as an intersection, and (c) one-pot synthesis through hydrosilylation and functionalization of organic molecules

by hydrosilylation is another possible one-pot synthesis, including hydrosilylation (Scheme 1c). In this chapter, recent examples of one-pot syntheses, including hydrosilylation, are classified into three groups. The reaction conditions, including the type of catalyst and proposed reaction mechanisms, are discussed. Additionally, to clarify the synthetic utility, the substrate scopes selected for each reaction are described.

2 One-Pot Synthesis with Si as a Leaving Group The most common one-pot strategy including hydrosilylation involves the use of a silyl group as the leaving group. As shown in Scheme 2, the hydrosilylation of alkenes/alkynes followed by oxidation/protodesilylation affords the corresponding alcohols, ketones, and alkenes. For desilylative oxidation reactions, H2O2 is frequently used as an oxidizing agent (Tamao–Fleming oxidation) [11–13]. For protodesilylation, fluoride reagents such as tetrabutylammonium fluoride are added to enhance desilylation. Successive hydrosilylation and oxidation/protodesilylation reactions have been discussed elsewhere [4]. The successive hydrosilylation of alkyne/Pd-catalyzed cross-coupling reactions between alkenylsilane intermediates and haloarenes is a well-known one-pot synthesis method that includes hydrosilylation. There are many literature reports on the

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Scheme 2 Hydrosilylation followed by oxidation/protodesilylation

Scheme 3 Successive hydrosilylation and Pd-catalyzed cross-coupling for total synthesis of an HMG-CoA reductase

reaction sequences, which are summarized elsewhere [4]. A brief summary of this topic is presented in this section. Pioneering work on this reaction sequence was reported by Hiyama et al. [14, 15]. As shown in Scheme 3, this is a powerful method for the total synthesis of HMG-CoA reductase. The hydrosilylation of a terminal alkyne with methyldichlorosilane using a Pt complex afforded an alkenylsilane intermediate, and the addition of an iodoarene and a Pd complex provided the corresponding cross-coupling product, which is a precursor for the preparation of NK-104, in 83% yield [15]. This hydrosilylation/Pd-catalyzed cross-coupling reaction sequence has wide applicability in the synthesis of various alkenylarenes. Denmark et al. investigated the generality of successive reactions [16]. As shown in Scheme 4, the Pt complex demonstrated high activity in the hydrosilylation of terminal alkynes to afford

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Scheme 4 Successive hydrosilylation of alkynes and Pd-catalyzed cross-coupling

Scheme 5 Regioselective allene hydroarylation through Pd-catalyzed hydrosilylation and crosscoupling reaction

alkenylsilanes, followed by cross-coupling with iodoarenes to afford the corresponding products in 67–94% yields with excellent selectivity [16]. Miller and Montgomery reported a regioselective allene hydrosilylation followed by a Pd-catalyzed cross-coupling reaction (Scheme 5) [17]. A Pd complex catalyst with an N-heterocyclic carbene (NHC) ligand promotes the hydrosilylation of an allene with a hydrosilane. Usually, a Pd catalyst with a standard NHC ligand affords allylsilane, whereas an NHC ligand with bulky substituents (Ipr*Ome) promotes alkenylsilane formation. The alkenylsilane intermediate was then converted to 1,1-disubstituted alkenes via a Pd-catalyzed cross-coupling reaction. As shown in

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Scheme 6 Proposed reaction pathway for regioselective reaction

Scheme 7 Reactions of silyl formate intermediate with amine and H2O

Scheme 5, a variety of allenes, including aliphatic and aromatic allenes, readily react with tertiary silanes such as dimethylphenylsilane and dimethylethoxysilane, followed by a reaction with iodoarenes to afford various 1,1-disubstituted alkenes with high regioselectivity (Scheme 5). The origin of the high regioselectivity of alkenylsilane intermediates is illustrated in Scheme 6. In the case of typical NHC ligands, such as Imes and Ipr, a hydride of hydrosilane connects with the center carbon atom of an allene to afford allylsilane. In contrast, the Pd catalyst with the less common bulkier NHC ligand induces steric repulsion between the silyl group and the ligand, resulting in the introduction of silicon to the central carbon atom of the allene to afford an alkenylsilane. The hydrosilylation of CO2 to silyl formate is a useful method for the reduction of CO2 to organic compounds. Various metal complexes and organocatalysts have been reported for the hydrosilylation of CO2 [18–26]. Silyl formate demonstrates good reactivity for further transformations, in which the silyl group acts as a leaving group; therefore, many reaction systems for one-pot reaction sequences triggered by the hydrosilylation of CO2 have been reported. As shown in Scheme 7, silyl formate easily reacts with polar molecules such as amines and H2O [27]. The selective production of formic acid and silanols via the hydrosilylation of CO2 has been reported using a copper complex-catalyzed hydrosilylation reaction systems [28]. Formamides can be produced via CO2 hydrosilylation in the presence of amines. Several reaction routes have been proposed, including the formation of a silyl formate as an intermediate. The successive reactions of CO2 hydrosilylation and the addition of amines to silyl formate were reported by Mizuno et al. [29]. After the Rh-catalyzed hydrosilylation of CO2, an amine was added to the reaction mixture, resulting in the formation of the corresponding formamide and formic acid in an almost 1:1 ratio (Scheme 8). The

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Scheme 8 Equimolar formation of formamide and formic acid from silyl formate intermediate

reaction of silyl formate and amine affords formamide and silanol, which can then react with other silyl formates to afford formic acid and siloxane. Both secondary and primary amines can be used as substrates to afford various formamides (Scheme 8). García et al. reported a reaction between silyl formate and alcohols affording formyl esters (Scheme 9) [30]. After the catalytic hydrosilylation of CO2 with the Ni complex, the addition of an alcohol and HBF4 led to the formation of the corresponding formyl esters in high yields. The silyl group is effectively removed by the reaction with HBF4 to afford Et3SiF. A one-pot aldehyde synthesis was reported by Ema et al. (Scheme 10) [31]. The reaction between CO2, PhSiH3, and 2-(methylamino)pyridine afforded N-methyl-N(2-pyridyl)formamide via silyl formate formation. The obtained formamide, Comins-Meyers formamide, reacts with a Grignard reagent to afford a six-membered Mg2+ chelate complex, which affords the aldehyde after hydrolysis. Both hydrosilylation and formamide formation (Scheme 11) are promoted by the Cu species. As shown in Scheme 10, two equivalents of the Grignard reagent can be used as nucleophiles to afford the corresponding aldehydes in good yields. During aldehyde formation, CO2 was replaced with an N2 balloon. Notably, the addition of three equivalents of the Grignard reagent under harsh reaction conditions affords the corresponding alcohols. The reaction between silyl formate and the Grignard reagent affords alcohols, as reported by Mizuno et al. [29] The detailed substrate scope was demonstrated by Ema et al. (Scheme 12) [31]. After the hydrosilylation of CO2 with PhSiH3 using a

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Scheme 9 One-pot synthesis of formyl esters through hydrosilylation of CO2/ester exchange reaction with alcohols

Scheme 10 One-pot aldehyde synthesis from CO2, amine, hydrosilane, and Grignard reagents Scheme 11 Proposed mechanism promoted by Cu

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Scheme 12 One-pot alcohol synthesis from CO2, hydrosilane, and Grignard reagents

Scheme 13 Hydrosilylation of imine followed by benzylation to tertiary amine

tetrabutylammonium acetate (TBAA) catalyst, the addition of three equivalents of Grignard reagent afforded the corresponding alcohols in good yields. This reaction system can be applied to the production of aromatic, heteroaromatic, and benzylic secondary alcohols. Li et al. reported the one-pot synthesis of tertiary amines via the hydrosilylation of imines (Scheme 13) [32]. The hydrosilylation of imines by [RuCl2(p-cymene)]2 followed by a reaction with benzyl bromide affords the corresponding tertiary amines. The selectivity for tertiary amines increased with the addition of PPh3 at 80°C. The high reactivity of hydrosilane for the reduction of carbonyl and carboxylic acid groups was applied in the one-pot synthesis of tetralin derivatives from 3-benzoylpropionic acids [33]. The reaction pathway for tetralin is summarized in

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Scheme 14 One-pot synthesis of tetralin derivatives from 3-benzoylpropionic acids

Scheme 14. The reaction was conducted using an excess of hydrosilane (6.5 equiv.). Initially, the silylation of the carboxylic acid and the indium-catalyzed hydrosilylation/deoxygenation of the carbonyl group afforded a silyl ester intermediate. Then, hydrosilylation/deoxygenation of the silyl ester afforded a silyl ether, followed by a reaction with the iodosilane intermediate formed in situ, resulting in tetralin formation after indium-catalyzed cyclization. This reaction did not occur without I2, which confirmed the proposed reaction pathway. Despite many reaction steps and complicated mechanisms, the corresponding tetralin derivatives were obtained in high yields (Scheme 15).

3 One-Pot Synthesis with Si as an Intersection The selective reaction of hydrosilane enables the step-by-step utilization of two Si–H groups in the same hydrosilane moiety. Kuznetsov and Gevorgyan developed a one-pot method for the synthesis of dihydrobenzosiloles from styrenes and diphenylsilane (Ph2SiH2) (Scheme 16) [34]. The selective hydrosilylation of styrene proceeds in the presence of NiBr2 with a triphenylphosphine ligand. The addition of [Ir(OMe)(cod)]2 induced intramolecular aromatic silylation to afford dihydrobenzosilole derivatives in a single reaction system. Norbornene acts as a hydrogen acceptor and is converted to norbornane. The reaction system was applicable to various substituted styrenes, which reacted with diphenylsilane, followed by cyclization to provide the corresponding products in moderate to high yields (Scheme 16).

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Scheme 15 Selected substrate scope of tetralin derivatives

Scheme 16 One-pot synthesis of dihydrobenzosiloles through hydrosilylation and cyclization

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Scheme 17 One-pot synthesis of siloxane oligomers via hydrosilylation/rearrangement/crosscoupling sequence

Scheme 18 One-pot synthesis of pentasiloxane

The selective reaction of hydrosilanes is a powerful tool for the programmed synthesis of oligosiloxanes. Matsumoto et al. demonstrated one-pot oligosiloxane synthesis through the reaction sequence of hydrosilylation/rearrangement/crosscoupling (Scheme 17) [35]. The Ir-catalyzed hydrosilylation of silyl acetate afforded a disilyl acetal, which was easily converted to disiloxane via a B(C6F5)3-catalyzed rearrangement. The ethoxy group in the disiloxane selectively reacts with another disilane to afford the corresponding trisiloxane in high yield. As shown in Scheme 18, when phenylsilane was used in the final step (cross-coupling), two disiloxanes were connected to phenylsilane, which acted as an intersection, to afford pentasiloxane in 77% yield. Matsumoto et al. modified the oligosiloxane synthesis using only B(C6F5)3 as a homogeneous catalyst because B(C6F5)3 can catalyze both dehydrocarbonative cross-coupling and hydrosilylation [36–38]. As shown in Scheme 19, successive cross-coupling/hydrosilylation reactions led to sequence-controlled oligosiloxane production.

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Scheme 19 One-pot synthesis of sequencecontrolled oligosiloxane

Scheme 20 One-pot synthesis of pentasiloxane with controlled sequence

For example, the cross-coupling of (isopropoxy)trimethylsilane and diphenylsilane affords diphenyl(trimethylsiloxy)silane, which reacts with acetone to afford (isopropoxy)disiloxane. Then, the set of cross-coupling/hydrosilylation induces a pentasiloxane with controlled sequence (Scheme 20). This procedure was applied to various oligosiloxanes with controlled elongation (Scheme 21).

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Scheme 21 Examples of sequence-specific oligosiloxanes Scheme 22 Structure of silica-supported Rh-ammonium iodide catalyst

4 One-Pot Hydrosilylation-Functionalization Heterogeneous catalysts with multifunctionality that promote multiple reactions are ideal materials for the promotion of one-pot synthesis. Various heterogeneous catalysts that exhibit high performance in one-pot reactions have been reported [39–43]. Silica-supported Rh complexes have also been reported as heterogeneous hydrosilylation catalysts [44–46]. The immobilization of both Rh complexes and organic functionalities, such as amines, enhances hydrosilylation catalysis [47, 48]. In addition, successive hydrosilylation/CO2 cycloaddition reactions were promoted by a silica-supported Rh complex-ammonium iodide catalyst [49, 50]. The surface structure of the catalyst is shown in Scheme 22. Several spectroscopic characterizations revealed that the monomeric Rh species was stabilized by an interaction with I-. The results of the one-pot hydrosilylation/ CO2 cycloaddition reaction are summarized in Scheme 23. After the Rh-catalyzed hydrosilylation of epoxyolefins, CO2 was introduced into the reactor and the CO2 cycloaddition of the epoxide to the cyclic carbonate proceeded via the catalysis of

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Scheme 23 One-pot synthesis of silyl carbonate via hydrosilylation/CO2 cycloaddition

ammonium iodide. Various silanes were introduced and the corresponding cyclic carbonates with silyl groups were obtained in high yields. Mandal et al. reported a Cu(I)-NHC catalyst for successive click reactions and hydrosilylation [51]. The Cu-complex-catalyzed click reaction between a terminal alkyne and 4-azidobenzaldehyde/4-azidoacetophenone, followed by hydrosilylation of the carbonyl group, affords the corresponding alcohols in high yields (Scheme 24). Both the click reaction and hydrosilylation are promoted by the Cu complex catalyst.

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Scheme 24 Cu(I)-catalyzed click reaction/hydrosilylation sequences

5 Conclusions Recent reports on one-pot synthesis including hydrosilylation reaction are summarized. After hydrosilylation, the silyl group in an intermediate could be used as a leaving group to introduce other functionalities, such as hydroxyl, aryl, and amino group. Selective hydrosilylation with a dihydrosilane affords an intermediate remaining Si–H bond, which reacts with other substrates through several crosscoupling reactions. The use of multifunctional catalyst is one of the efficient methods to promote one-pot synthesis including several reactions. The one-pot synthesis including hydrosilylation gives various organic compounds, such as substituted alkenes, formamides, tetralin derivatives, dihydrobenzosiloles, sequence-controlled oligosiloxanes, and silyl carbonates. The highly active and selective catalysis for hydrosilylation contributes to the rapid synthesis of complex molecules through one-pot procedures.

References 1. Hall N (1994) Chemists clean up synthesis with one-pot reactions. Science 266:32 2. Hayashi Y (2016) Pot economy and one-pot synthesis. Chem Sci 7:866–880

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Top Organomet Chem (2023) 72: 305–328 https://doi.org/10.1007/3418_2023_105 # The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 Published online: 14 October 2023

Hydrosilylation of Carbon–Carbon Multiple Bonds in Organic Synthesis Maciej Zaranek and Piotr Pawluć

Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Classical Applications of Intra- and Intermolecular Hydrosilylation in Organic Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Transition Metal-Catalyzed Sequential Double Hydrofunctionalization of Alkynes . . . . . . 4 Asymmetric Hydrosilylation of Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Hydrosilylation of alkenes and alkynes is the most fundamental and elegant method for laboratory and industrial synthesis of organosilicon compounds, which can be directly used in organic synthesis. The aim of this chapter is to present a concise overview of the most attractive results of the application of regio- and stereoselective hydrosilylation of alkenes and alkynes published mainly in the last decade. The chapter consists of sections discussing sequential reactions involving hydrosilylation of functional alkenes and alkynes as the initial step followed by desilylation (oxidation, cross-coupling), double hydrofunctionalization of alkynes including hydrosilylation, and asymmetric hydrosilylation of prochiral alkenes. In our chapter, we highlight the applications of new catalysts based on first-row transition metal complexes in consecutive (also one-pot) hydrosilylation/desilylation reactions. Special attention has been paid to enantioselective hydrosilylation of olefins, which has been experiencing a renaissance in recent years and is an extremely attractive method in the synthesis of chiral silanes, alcohols, and their derivatives.

M. Zaranek (✉) and P. Pawluć (✉) Faculty of Chemistry and Centre for Advanced Technologies, Adam Mickiewicz University, Poznań, Poland e-mail: [email protected]; [email protected]

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M. Zaranek and P. Pawluć

Keywords Asymmetric hydrosilylation · Desilylation · Hiyama–Denmark crosscoupling · Hydrofunctionalization · Prochiral alkenes · Sequential processes · Tamao–Fleming protocol

1 Introduction Organosilicon reagents play a crucial role in the realm of organic synthesis. Their growing significance can be attributed to successful protocols developed in the 1980s, such as the Tamao–Fleming procedure for stereospecific oxidation and the Hiyama–Denmark cross-coupling, applied to these valuable organometallic reagents. Moreover, the versatility of organosilicon intermediates has been further enhanced through various strategies enabling the conversion of silyl groups into different functional groups using halodesilylation, desilylative acylation, reduction (protodesilylation), and other processes [1, 2]. Hydrosilylation stands out as the most direct and adaptable method for introducing silyl groups into unsaturated molecules. The resulting products serve as highly valuable intermediates capable of participating in a wide range of synthetically significant organic reactions. This has spurred remarkable advancements in the development of stereoselective strategies for their synthesis and subsequent transformations, without the necessity of isolating intermediates (one-pot processes). Concurrently, silanes themselves have garnered attention in pharmaceutical studies due to their unique bioactivity, particularly stemming from their ability to serve as bioisosteric replacements for carbon atoms in existing drugs. Historically, most processes using hydrosilylation in applied organic synthesis were based on platinum catalysts. On the other hand, organosilicon chemistry has matured substantially over the course of the past decade, and new methods based on platinum-free catalysis have been developed for the regio- and stereoselective introduction of the silyl groups into unsaturated molecules [3–6]. Of particular importance in this context are discoveries of first-row transition metal catalysts for double hydrofunctionalization of alkynes [7] and Markovnikov hydrosilylation of alkenes [8], which, with the use of appropriate chiral ligands, can promote asymmetric variants of the reactions. As an example, the asymmetric hydrosilylation of prochiral olefins followed by Tamao–Fleming oxidation has proven to be a useful method to access a variety of enantioenriched secondary alcohols.

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2 Classical Applications of Intra- and Intermolecular Hydrosilylation in Organic Synthesis The burgeoning interest in developing sequential processes, with hydrosilylation as the initial step, arises from the capability to construct intricate molecules starting from simple substrates, namely functionalized alkenes and alkynes, through the use of organosilicon intermediates in an efficient and atom-economical manner. One well-established approach in this regard is the employment of platinum-catalyzed intramolecular hydrosilylation followed by Tamao–Fleming oxidation. This method is renowned for its ability to selectively synthesize a diverse range of alcohols, including 1,3-diols, 2-alkoxy-1,3-diols, 1,3,5-triols, 2-aminoalcohols, and carbonyl compounds such as β-hydroxyketones, γ-hydroxyketones, α,β-dihydroxyketones, and α,γ-dihydroxyketones. These compounds are derived from readily available substituted allyl- or propargyl alcohols and their analogs (Scheme 1) [9, 10]. In the context of platinum-catalyzed intramolecular hydrosilylation of protected allylamines, a distinctive regiochemical outcome is observed when compared to the corresponding allyl alcohols. Specifically, the reaction leads to the formation of 1-aza-2-silacyclobutane derivatives as single isomers through 4-exo-cyclization when using bis(dimethylsilyl)allylamines in the presence of Karstedt’s catalyst. These intermediates can subsequently be converted into 2-aminoalcohols through oxidation with hydrogen peroxide in the presence of potassium fluoride and potassium bicarbonate [11] (Scheme 2). Interestingly, a significant impact on regioselectivity is noted when the reaction is conducted in the presence of rhodium catalysts. Utilizing catalysts such as [RhCl(PPh3)3] or [{RhCl(C2H4)2}2], the intramolecular hydrosilylation of N,N-protected allylamines proceeds via 5-endohydrosilylation, yielding 1-aza-2-silacyclopentanes. Alkenylsilanes, which are prepared through the platinum or rhodium-complexcatalyzed hydrosilylation of terminal alkynes, have been employed in stereospecific

Scheme 1 Synthesis of alcohols via the hydrosilylation/Tamao– Fleming oxidation sequence

Scheme 2 Synthesis of aminoalcohols from allylamines

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Scheme 3 Synthesis of (E)-olefins and conjugated (E,E)-dienes from (E)-alkenyl triethoxysilanes

cross-coupling reactions with aryl and alkenyl halides. This synthetic strategy yields stilbenes and conjugated dienes [12, 13]. Furthermore, (E)-alkenyl-triethoxysilanes, synthesized via hydrosilylation of terminal alkynes with triethoxysilane catalyzed by the Wilkinson complex, have been effectively cross-coupled with aryl and alkenyl halides in an aqueous environment under microwave irradiation. This process results in the formation of unsymmetrical (E)-stilbenes, (E)-alkenylbenzenes, and conjugated dienes (Scheme 3) [14]. The fluoride-free cross-coupling reactions of alkenyltrialkoxysilanes with aryl iodides, bromides, and chlorides are carried out in an aqueous medium using sodium hydroxide as an activator, either under normal heating conditions or with microwave assistance at 120°C. Hydroxy-functionalized β-(E)-alkenylsilanes, obtained by platinum-catalyzed hydrosilylation of propargyl alcohols, serve as versatile platforms for the highly selective synthesis of hydroxy-substituted (E,E)-1,3-dienes [15]. Tandem hydrosilylation/Hiyama cross-coupling reactions have been accomplished using a single catalyst, [PdCl2(PTA)2] (where PTA = 1,3,5-triaza-7-phosphaadamantane) [16]. Functionalized gem-disubstituted alkenes can be obtained with good regiocontrol through a one-pot sequence involving [Pd2(dba)3]/NHC ligandcatalyzed allene hydrosilylation and cross-coupling [17]. The regioselectivity is governed by N-heterocyclic carbene ligand identity in the hydrosilylation step, favoring either allylsilane or alkenylsilane products, and is preserved in the subsequent cross-coupling reaction. For the highly selective synthesis of substituted 1,3-dienes, regioselective hydrosilylation of 1,3-enynes, catalyzed by [Pd(acac)2]/ PEt3/DIBAL-H, has been followed by palladium-catalyzed cross-coupling or TBAF-mediated protodesilylation [18]. In addition, consecutive intramolecular hydrosilylation of propargyl and homopropargyl alcohols, catalyzed by platinum (0) or ruthenium(II) complexes, in conjunction with palladium-catalyzed crosscoupling reactions, has proven to be a powerful tool for the stereoselective synthesis of various aryl-substituted (E)- and (Z)-allyl and homoallyl alcohols (Scheme 4) [19, 20]. This approach has also been applied to the synthesis of natural products [21, 22]. The revelation of the catalytic potential of a cationic ruthenium complex, [Cp*Ru (MeCN)3]PF6, by Trost and co-workers [23], ignited a wave of research focused on harnessing regioselective ruthenium-catalyzed hydrosilylation in organic synthesis. This breakthrough allowed for the development of a direct method for the

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Scheme 4 Synthesis of aryl-substituted allyl and homoallyl alcohols by sequential hydrosilylation/ Hiyama–Denmark cross-coupling

Scheme 5 Synthesis of β-hydroxy ketones

functionalization of alkynes at the α-position, thanks to the compatibility of this reaction with subsequent desilylation reactions, including protodesilylation, desilylative oxidation, or cross-coupling. The sequential process of rutheniumcatalyzed hydrosilylation followed by oxidation of propargylic alcohols and their analogs has found extensive use in generating hydroxy ketone derivatives (Scheme 5) [24, 25]. Furthermore, the ruthenium-catalyzed trans-hydrosilylation/oxidation method, leveraging the synthetic potential of well-defined organosilicon intermediates, has been employed as a pivotal step in total syntheses of natural products [25– 27]. Similarly, the [Pd2(dba)3]-catalyzed cross-coupling of α-vinylsilanes featuring benzyldimethylsilyl moieties with aryl or alkenyl iodides has been demonstrated to

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Scheme 6 Pd-catalyzed cross-coupling of α-vinylsilanes with aryl- and alkenyl iodides

Scheme 7 Synthesis of macrocyclic (E)-cycloalkenes via hydrosilylation/protodesilylation sequence

Scheme 8 Consecutive iridium-catalyzed hydrosilylation and Hiyama–Denmark cross-coupling of homopropargyl alcohols

proceed smoothly, yielding geminal-substituted alkenes in a regioselective manner (Scheme 6) [28]. The hydrosilylation/protodesilylation protocol has proven valuable for selectively reducing cycloalkynes to (E)-cycloalkenes [29, 30]. Fürstner and co-workers introduced a two-step procedure for the selective synthesis of macrocyclic (E)cycloalkenes from cycloalkynes, utilizing trans-selective hydrosilylation with triethoxysilane catalyzed by [Cp*Ru(MeCN)3]PF6 followed by AgF-mediated protodesilylation (Scheme 7) [30]. Markovnikov hydrosilylation of terminal alkynes with the iridium catalyst [Ir(μCl)(cod)]2 has been effectively employed in a one-pot sequential hydrosilylation/ desilylative cross-coupling of derivatives of various bio-relevant molecules (Scheme 8) [31]. The remaining siloxyl group, -Si(OSiMe3)3, can directly participate in organic transformations or be converted into other valuable silyl derivatives. This approach accommodates various functionalized groups, including halides, free alcohols, carboxylic acids, ketones, esters, and amides, potentially leading to further applications and late-stage derivatizations. A highly regio- and stereoselective cobalt-catalyzed Markovnikov hydrosilylation of alkynes with secondary silanes was developed by Lu and co-workers. The Hiyama–Denmark cross-coupling reactions of the resulting

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Scheme 9 Synthesis of 1,1-diarylethenes from aryl alkynes through α-vinylsilanes

Scheme 10 Co-catalyzed hydrosilylation/hydroboration of terminal aromatic alkynes

α-vinylsilanes with aryl iodides underwent smoothly to afford gem-diarylethenes (Scheme 9) [32].

3 Transition Metal-Catalyzed Sequential Double Hydrofunctionalization of Alkynes The utilization of transition metal-catalyzed asymmetric sequential double hydrofunctionalization of alkynes has emerged as a potent strategy for generating valuable chiral compounds that contain two functional groups in a highly efficient manner, using readily accessible substrates [7]. The sequential alkyne hydrosilylation/hydrogenation provides direct access to secondary chiral silanes. The research group of Lu pioneered the one-pot sequential hydrosilylation/hydrogenation of aromatic terminal alkynes. This reaction is catalyzed by cobalt(II)-chiral oxazoline iminopyridine (OIP) precatalyst in conjunction with diphenylsilane and a hydrogen balloon [33]. Mechanistic investigations revealed that the initial step involves a highly regioselective hydrosilylation of alkynes, followed by a cobalt-catalyzed asymmetric hydrogenation of the resulting α-vinylsilanes, which exhibits good enantioselectivity (78–99% ee). The same research group also disclosed a cobalt-catalyzed, highly regioselective Markovnikov hydrosilylation/hydroboration sequence, albeit with somewhat limited enantiomeric excess (~10% ee) (Scheme 10) [32]. Hayashi and co-workers reported a sequential asymmetric 1,2-dihydrosilylation of arylacetylenes followed by Tamao oxidation in the presence of platinum and

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Scheme 11 Pt/Pd-catalyzed asymmetric 1,2-dihydrosilylation of aryl alkynes

Scheme 12 Sequential hydrosilylation/hydrohydrazidation of alkynes

palladium catalysts to access chiral 1-aryl-1,2-diols with excellent selectivity (Scheme 11) [34]. More recently, there have been reports of cobalt-catalyzed regioselective tandem hydrosilylation/hydrohydrazidation of alkynes [35]. This reaction proceeds through ligand relay catalysis involving hydrosilanes and diazo compounds, yielding 1-amino-2-silylalkanes (Scheme 12). These products, bearing both N–H bonds and Si–H bonds, can be readily converted into hydrazino alcohols, diverse siliconsubstituted hydrazino silanes, and amide silanes. The asymmetric version of the reaction has also been successfully carried out, yielding chiral products with enantiomeric excess values of up to 86% ee.

4 Asymmetric Hydrosilylation of Alkenes Enantioselective hydrosilylation was a subject of research from the early 1970s [36]. Most research articles concerning enantioselective hydrosilylation focus solely on this transformation, and there are no or limited studies on desilylative functionalization of silylated products. However, as ever increasing number of silicon compounds are themselves targets of organic synthesis, we decided to cover also significant reports of enantioselective hydrosilylation without further treatment.

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Scheme 13 Asymmetric hydrosilylation of alkenes using Pd(II)/chiral phosphine catalytic system

A general remark is needed. By principle, there is no intrinsic property of chiral organosilanes which could preclude them from any of the characteristic transformations described in the previous sections of this chapter and other specialized reviews, and thus one can find examples of well-known oxidation, borylation, or crosscoupling protocols being successfully applied to those compounds. Most papers present a limited overview of such transformations (most recently a mere one example of each reaction). Although it poses a challenge to the authors of this chapter, we are fully aware that this approach is justified since these protocols are already well-established and not within the scope of the mentioned studies. The earliest somewhat successful enantioselective hydrosilylation dates back to 1972 when Kumada et al. reviewed a group of palladium(II) complexes with chiral phosphine ligands in hydrosilylation of styrene, cyclopentadiene, and 1,3-cyclohexadiene with trichlorosilane (Scheme 13). The products were methylated before analysis [37, 38]. The authors conclude that π-allylic or π-benzyl interactions between palladium and olefins are crucial in determining selectivity. The group of Kumada described also the use of dichlorobis[(R)-benzylmethylphenylphosphine]nickel(II) in asymmetric hydrosilylation of 1,1-disubstituted alkenes with methyldichlorosilane, however, with moderate success, achieving yields of at most 31% [39]. The approach of using chiral phosphine palladium(II) precatalysts was further expanded by Hayashi and Uozumi who described synthesis of chiral 2-alkanols via alkene hydrosilylation with trichlorosilane, ethanolysis of the corresponding 2-(trichlorosilyl)alkanes, and oxidation of such formed triethoxysilyl compounds (Scheme 14) [40, 41]. The catalyst they used comprised 0.1 mol% diallyldi(μ-chloro)dipalladium(II) and 0.2 mol% (S)-2-(diphenylphosphino)-2′-methoxy-1,1′-binaphthyl (MOP). The use of such catalytic system followed by ethanolysis and Tamao–Fleming oxidation allowed for efficient synthesis of chiral aliphatic alcohols with up to 75% of total yield with an enantiomeric excess of 95%. The second part of this work was hydrosilylation of aromatic alkenes to afford benzyl alcohols as final products (Scheme 15) [42]. The use of the same conditions

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Scheme 14 Hayashi’s original Pd(II)/monophosphine-catalyzed hydrosilylation and subsequent oxidation of the products

Scheme 15 Expansion of the Pd(II)/MOP catalysis over hydrosilylation of styrenes

on styrenes led to formation of benzylsilanes with usually very high yields of up to 100%. These were then again subjected to ethanolysis and oxidated to afford chiral substituted benzyl alcohols whose enantiopurity was ranging from 13 to 96% of ee. More work has been dedicated by Hayashi and co-workers in the subsequent years to enhance their flagship catalytic system [43]. One of such enhancements was a double hydrosilylation of alkynes with trichlorosilane as a method for one-pot synthesis of chiral 1-aryl-1,2-ethanediols, as has been described in Sect. 3 [34]. In the meantime, the group of Marks was working on hydrosilylation over organolanthanide catalysts. In general, enantioselectivity was hindered by differing regioselectivity; however, the authors were able to select the best catalyst backbone and prepare its asymmetric variant with a chiral (-)-menthyl substituent (Scheme 16). Its use allowed for getting 68 and 65% ee for (R) and (S) variants, respectively [44]. In 2000, Muci and Bercaw made an attempt at using chiral ansa-yttrocene in asymmetric cyclization-hydrosilylation of α,ω-dienes (Scheme 17). Enantioselectivities they reported were low to moderate, with ee ranging from