Reactive Intermediates: Volume 1 [1st ed.] 978-0-306-40220-3;978-1-4613-2973-2

Table of contents :
Front Matter ....Pages i-xiii
The Preparation and Reactions of Atomic Carbon (Philip B. Shevlin)....Pages 1-36
Metal Atoms as Reactive Intermediates (Kenneth J. Klabunde)....Pages 37-149
Aminium Radicals (Yuan L. Chow)....Pages 151-262
The Behavior of Arylcarbenes and Arylnitrenes in the Gas Phase (Curt Wentrup)....Pages 263-319
Metal-Salt-Catalyzed Carbenoids (David S. Wulfman, B. Poling)....Pages 321-512
Back Matter ....Pages 513-522

Citation preview

Reactive Intermediates J6/ume 1

89

Metal Atoms as Reactive Intermediates

In the case of Ge atoms, oxidative addition of CCl 4 occurs, but the resulting Cl 3 CGeCI abstracts CI atoms from excess CCI 4 , resulting in a respectable yield of CI 3 CGeCI 3 • Chloroform and silicon tetrachloride reacted similarly as shown below 110:

~(....:.C..:..;H,-C...:Ia,-

Ge

r

ClaCGeCIa 20%

~

ClaSiGeCla

10%

B. Acyl Halides with Metal Atoms (Macroscale Co condensation) Acyl chlorides oxidatively add to palladium atoms and nickel atoms.27i.95 In all the examples studied, the resultant RCOMCI species liberated CO very

o 1/

RCCI

+

o II

M atom - - + RC-M-CI

M = Ni, Pd R = CHa, CFa, C.H s , C.Fs , n-CaF7

readily. By carrying out low-temperature trapping experiments with Et 3 P we were ahle to reach some conclusions about thermal stabilities and decomposition pathways. Table 10 summarizes our findings,27i.95 and Figure 20 illustrates the

TABLE /0. Acyl Chloride-Metal-Atom Reactions, Products, and Comments271.95

Reactants Pd, CF 3 COCI Pd, CF aCF 2 CF 2 COCI Pd, C 6 FsCOCl Pd, C 6 H sCOCI Pd, CH 3 COCI Ni, CF 3 COCI

Comments CFaCOPdCl eliminates CO at -80T, at which point Et 3 P addition yielded almost equal quantities of (Et 3 PhPdCI(CF 3 ), (Et 3 P),PdCI(COCF 3 ) and (Et 3 P),PdCI 2 CF 3 CF 2 CF 2 COPdCI stable above -80 v C (but -

:C:

+

stable fragments

(7)

one must choose an energetic carbene in which the fragment(s) are of sufficiently low energy to provide a thermodynamic driving force. A carbene which appears to meet the above criteria is tetracyclo[3.2.0.02.7.04.6]-heptane-3-ylidene (1), which is reported to generate atomic carbon by the sequence shown in equation (8).42 Pyrolysis of the tosylhydrazone lithium salt 2 produces benzene and a carbon atom. The carbon atom then reacts N- N-Ts

tb II

+

C

J:;

(8)

2

with benzene to produce toluene, a known reaction of atomic carbon. 43 That the toluene is not simply formed by the rearrangement of 1 is established by adding benzene- l4 C and observing radioactive toluene. Although other reactions characteristic of atomic carbon are observed in this system, the high reactivity of carbon toward benzene renders investigations of these reactions difficult. A more satisfactory carbene precursor to atomic carbon appears to be 3. Thermal decomposition of 5-tetrazoyldiazonium chloride (4) by the reaction sequence shown in equation (9) generates atomic carbon.44 Reactions of atomic

8

Philip B. Shevlin N 2 +C1-

NAN/ \

C

H

/

N=N

N/ "N

~

II

\\

-----* -HCl

N-N

4

-- C

+

2N2

(9)

3

carbon are investigated in this system by pyrolyzing 4 in the presence of a gaseous reactant. A number of reactions of these chemically generated carbon atoms have been reported and will be discussed in subsequent sections of this review. Investigations of reactions involving the thermolysis of 4 have the advantage of generating carbon atoms at convenient temperatures (~JOO°C) with little excess kinetic energy. However, it has not been definitely established that free carbon atoms are produced in these systems. It is possible that the reactions that have been reported are those of a carbon atom donor such as 3 or CNN rather than those of a free carbon atom. Clarification of this point awaits spectroscopic observation of carbon atoms in this system. It is gratifying, however, that decomposition of 4 in the presence of various reactants gives the same products as do carbon atoms generated by other methods. Another chemical reaction that has been reported to produce atomic carbon is the reaction of nitrogen atoms with the cyano radical [equation (10)].45.46 In :N:

+

·CN - - N2

+

C

tJ.H = -43 kcal/mol

(10)

this system, atomic nitrogen is produced in a microwave discharge and reacted further downstream with ·CN. Atomic carbon ep) has been observed spectroscopically and its reactions with hydrogen and nitrogen atoms studied in this manner.

5. Miscellaneous Methods of Producing Carbon Atoms A number of ingeniocs methods of providing the energy for carbon atom production have been developed. Martinotti, Welch, and Wolf 47 reported that atomic carbon is produced when carbon suboxide or carbon monoxide is passed through a microwave discharge. These workers were able to measure the amount of free carbon atoms by titrating with oxygen added at various points downstream from the discharge. In this way rate constants for the dimerization of ground state carbon and for its reaction with oxygen were measured. That carbon atoms are indeed produced in this system has been confirmed by spectroscopic studies in which all three low-lying states of carbon were identified. 48 Spangler, Lott, and 10ncich 49 have observed the production of atomic carbon in the explosion of graphite filaments. The reaction of carbon and C 2 with hydrogen has been examined in this system. Other energetic methods of producing carbon atoms include shock tube decompositions 50 and pulse radiolysis. 51 The pulse radiolysis studies are noteworthy in that they provide rate data for reactions of C(1S).

Preparation and Reactions of Atomic Carbon

9

Carbon atoms have also been produced by neutralization of a beam of carbon ions. In this technique, which has been developed by Lemmon and coworkers,52 l4C + ions are formed by electron impact on 14C02 and are focused upon the frozen substrate. In this way beams of carbon ions ranging in energies from 2 to 5000 eV have been studied. The carbon ion beam can be neutralized to atoms by charge exchange on tungsten wire grids prior to impacting on the target. Since products are very similar in the absence of the tungsten grid, it is felt that the carbon ions are neutralized before they reach energies at which chemical reactions can occur. Carbon-14-labeled ions are used to facilitate detection of products and to provide mechanistic information. A detailed study of the reaction of carbon with benzene has been reported. 53 ,54 As the foregoing survey of carbon atom syntheses indicates, there is no method that is entirely free of complicating factors. Most techniques require the indiscriminate input of high energy which perturbs the system and introduces additional complexity. In other methods, the intermediary of a free monatomic carbon has not been unequivocally established. The ideal method would utilize a highly discriminating reaction to produce a carbon atom whose concentration could be continually monitored by a convenient analytical technique. In the absence of such a method, the chemistry of atomic carbon must be studied within the framework of current technology. Reactions of atomic carbon must be discussed with an awareness of the shortcomings of the method by which the carbon atom has been produced. In the following sections we shall attempt to do this.

III. REACTIONS OF ATOMIC CARBON 1. Reaction with Hydrogen The two ways in which atomic carbon can react with hydrogen are shown in equations (11) and (12), Insertion into the C-H bond is exothermic for all three C C

+ +

(1J)

H-H ----+ H-C -H H-H ----+ :C-H

+

H'

(12)

atomic carbon states. Abstraction to yield C-H and H is exothermic for the 1 D (!1H = - 6 kcaljmol) and IS states (!1H = - 38 kcal/mol) but endothermic by 23 kcal/mol for Cep). Insertion into the C-H bond to give methylene is highly exothermic in all cases. The exothermicity of the insertion would be expected to promote dissociation to CH and H in the case of singlet carbon. However, Cep) insertion to form triplet methylene is expected to be reversible unless stabilization by collision with a third body occurs. In accordance with this prediction, a number of measurements of the rate constant for Cep) + H2 indicate that that reaction is third order. A recent value of 6.9 ± 1.2 x 10- 32 cm 6 molecule- 2 sec- 1 has been reported using helium as the third body,32

Philip B. Shevlin

10

Donovan and Husain 55 have presented correlation diagrams which indicate that C(l D) and Cep) should react rapidly with H 2 • The noncrossing rule predicts a slower reaction between hydrogen and C(lS). Rate constants that have been reported tend to support this view: C(lD), 2.6 x 10- 10 cm 3 molecule- 1 sec-I,30 and C(1S), = +[>- +>- +~ +~ +~ 35%

10%

17%

6%

27%

(18)

1%

are those in equation (18). Equation (19) shows the major products of reaction of gas phase llC with propane. 65 Finally, when the tetrazoyldiazonium chloride 4 is

l1C +~ ~ >= +/"--../ + 1.8%

2.4%

6.2%

1.6%

26.0%

14.2~~

0.5%

decomposed in an atmosphere of propane, the products shown in equation (20) are formed. 66

C+~~

>=+[>-+ 12.4%

4.1%

3.3%

4.9%

4.1%

20.7%

36.4~.

14.1%

Preparation and Reactions of Atomic Carbon

13

An examination of equations (18)-(20) reveals that the products are similar but not identical in the three reactions. It is generally true that differing energetics and reaction conditions in the various methods of carbon atom production lead to some dissimilarities in products and product ratios. However, in all systems, C4HS products of C-H insertion are produced. A consideration of the yields of I-butene and isobutene in equations (16) and (17) shows that, if 5 and 6 are intermediates in the reaction of carbon with propane, 6 must predominate. This is so as a statistical insertion of carbon into the I ° and 2° C-H bonds of propane is expected to produce a 4.6: I ratio of I-butene to isobutene. In none of the above carbon atom reactions does this ratio approach this value. These facts lead to the conclusion that, when carbon reacts by C-H insertion, it is a selective reaction preferring the weaker C-H bonds. This rather unusual selectivity has been rationalized through the use of MINDO/3 semiempirical molecular orbital calculations. 66 Calculation of the reaction coordinates for insertion into the I ° and 2° C-H bonds of propane reveals that reaction at the 2° carbon is the lower-energy pathway. Inspection of geometries generated along these reaction coordinates reveals that, in all cases, attack of carbon atoms on propane is calculated to proceed via initial strong bonding of the attacking carbon to one of the hydrogens. This stage of the reaction is followed by formation of the carbon-carbon bond. Thus attack of atomic carbon on a C-H bond is calculated to proceed via a species which resembles a complex between a methyne and a radical [equation (21)]. Since this R-H

+

.. C

~

R·

+

.. ·C-H

(21)

reaction initially involves cleavage of a C-H bond, it is selective toward weaker bonds. Molecular orbital calculations of the energetics of the reaction of methylene with methane 67 indicate that the insertion proceeds by initial transfer of hydrogen to methylene in much the same manner as is calculated for carbon atom insertion. Carbon appears to be considerably more selective in its C-H insertions than methylene, however. A consideration of the thermodynamics of the complete hydrogen transfer in the reaction of methylene and carbon with methane may provide a clue as to the reason for the observed differences in selectivities: !:.H = - 5 kcal/mol !:.H = - 15 kcal/mol

Thus, heats of reaction indicate that the initial hydrogen transfer is more favorable in methylene insertion than in insertion by atomic carbon. It is quite obvious that all of the products in equations (18)-(20) cannot be rationalized by a C-H insertion mechanism. However, many of them may be expected to result from an initial hydrogen transfer as shown in Scheme I.

Philip B. Shevlin

14 SCHEME 1

~H-C' + C+

~~

H-C· +

C.HlO products -

.
- NaCI + ·CHCl z reaction has been studied quite carefully. This reaction is accompanied by luminescence, which is due to the sodium D doublet. 11 • 59

Metal Atoms as Reactive Intermediates

67

TABLE 7. Gas Phase Sodium Atom Reactions with Simple Alkyl Halides (24Q-520°C) 11 Reactions: collisions

Reaction Na Na Na Na Na Na Na Na Na Na Na

+ + + + + + + + + + +

CH3I ---+ NaI + ·CH3 CH3Br ---+ NaBr + . CH3 CH3CI-> NaCl + ·CH3 CH3F ---+ NaF + ·CH3 CCI. ---+ NaCI + . CCIo CF3I --;. NaI + ·CF3 CF3Br ---+ NaBr + ·CF3 CF3CI---+ NaCI + ·CF3 CF 4 -> NaF + ·CF3 CNCI---+ NaCN + ·CI CNCI --;. NaCi + ·CN

Na + (CN), ---+ NaCN + ·CN Na + COCI 2 -> NaCi + . COCI Na + CF3COOH ---+ NaF + ·CF2 COOH Na + CHCI=CHCI(trans)-> NaCi + ·CH=CHCI Na + CHCI=CHCI(cis) ---+ NaCl + ·CH=CHCI Na + CH 2 =CHI -> NaI + CH 2 =CH Na + CH 2 =CHBr -. NaBr + CH 2 =CH Na + CH 2 =CHCI ---+ NaCi + CH 2 =CH Na + C 2 H5CI > NaCI + ·C 2 H5 Na + (CH 3),CHCI -> NaCI + (CH 3 )2 CH Na + (CH3hCCI> NaCi + (CH 3 hC, Na + CoH5I -, NaI + C 6 H5• Na + CoH5Br -, NaBr + CoH5' Na + COH5C1 . NaCi + COH5' Na + COH5F • NaF + C 6 H5• Na + CH 2 Cb • NaCI + ·CH 2 CI Na + C5H l l ONO • C5H l l ON + NaO

Activation Exoenergy thermicity (kcaljmol) (kcaljmol) Reference

1: 1.6 (240°C) I : 25 (240°C) 1 : 5000 (260°C) I : 100,000 (500°C) I :5.5 (240°C) - (290°C) - (290°C) - (290°C) -

(-)

I : 6.3 (250°C) (NaCNjNaCi Ij200(320-400°C)] I : 15,000 (260°C) I : 7.4 (270°C)

0.30 3.2 8.8 25 1.7 1.7 2.3 7.0 14 2.0

26.8 28.8 24.5 10

11,56 11,56,60 11,56,60 11,56,60 11,58 11,61 11,61 11,61 11,62 11,56,63

12 2.3

11.55 11,56 11,55

I :48.5 (220°C)

4.0

11,64

I: 2600 (275°C)

8.6

II, 57

I: 1800 (275°C)

8.2

II, 57

1:20 (263°C)

3.2

II, 63

I :218 (269°C)

6.8

11,63

10.1 7.3

11,57 11,56

I: 3300 (275°C)

8.9

11,57

I: 1500 (275°C) 1:2.3 (240°C) 1:36 (255°C) I : 1980 (281' C) very slow 1:760 (250'C)

8.0 0.83 3.8 8.3 6.8

11,57 11,56 11,65 11,66 11,64 11,58

I: 14

2.8

II, 67

I: 11,000 (275°C) I : 900 (270°C)

36.2 28.1 12.0

Kenneth J. Klabunde

68 6

2

CCI 4 ----+ CCI3Br ----+ CCI 2 Br 2 ----+ CCIBr3 ---+ CBr4

~

~4

50

3

CHCI 3 ----+ CHCI 2 Br ---+ CHCIBr2

" " 760

""

26

/

/

1

/

------+ CHBr3

13 /

CH 2CI 2 ---+ CH 2CIBr ---+ CH 2Br2

~

""7100/ 135 / CH 3CI ---+ CH3Br

FIGURE 16. Relative reactivities of halomethanes with Na atoms (gas phase)Y·58

Na + CH 2 C1 2 ----+ NaCl + . CH 2 C1 Na + ·CH 2 CI ---+ NaCl + :CH 2 :CH 2 + :CH 2 ---+ CH 2 =CH 2

(I) (2)

(3)

In the proposed sequence the first reaction is exothermic by about 25-30 kcal, and since the Na excitation energy needed is 47.6 kcal, it cannot be responsible for the luminescence observed. By this reasoning, the second reaction also is probably not the one responsible, unless excited states are involved. Reaction (3) is also probably not responsible since it needs a third body (heterogeneous) and the Na luminescence is homogeneous. After a number of studies concerning this process, it was finally concluded that reaction (2) must involve some type of excited state process.ll.59.67 A final interesting observation concerning the diffusion flame and lifeperiod work is that oxygen abstraction can occur. An example is the Na + C 5H ll ONO -+ C S H l1 0N + NaO reaction, which is quite efficient collisionwise with a low activation energy (2.8 kcal).11.68

B. Alkali-Metal-Halide Reactions by Microscale Cocondensation Microscale cocondensation of lithium, sodium, or potassium atoms with OF 2 and OCI 2 has been employed as a means of generating the reactive OF and OCI radicals for low-temperature spectral analyses: M atom + OX 2 ---+ MF + OX + O 2 + X 2 M = Li, Na, or K VO-F = 1028.6 cm- I X = F or CI VO-CI = 850 em - 1

These experiments, mainly due to Andrews and coworkers,35e often showed spectroscopically the presence of new unexpected species. These were attributed to materials such as Li +OF-, O 2 , F 2 , Li +, ClO-, and CIO-CIO.

Metal Atoms as Reactive Intermediates

69

Sodium atom reactions have been employed as a tool for production and subsequent low-temperature trapping of organic radicals. Mile 69 has reviewed this area of research which deals with Na atom reactions with alkyl, vinyl, and acyl halides at low temperature (usualIy 77 K). Mile describes a rotating cryostat where Na atoms and organic halide are cocondensed simultaneously on a small rotating drum, and he believes this apparatus alIows more control of the depositions than with more conventional matrix deposition methods. 69 Using this technique, Mile and coworkers 69 prepared a series of organic radicals and. in some cases, compared their esr spectra with those of the same radicals prepared by radiolysis techniques. GeneralIy, the spectra matched those obtained in the radiolysis work, for example, with I-heptyl and 4-heptyl radicals. These studies indicated that" free" radicals were formed in the RX + Na ---'?- NaX + R· reactions (NaX did not appear to be complexed to the radical). This conclusion might also be inferred from experiments with 3-chloro-l-butene and l-chloro-2butene. The resultant radicals exhibited identical esr spectra, ilIustrating total CH 2 =CHCHCICH 3 C1CH 2 CH=CHCH 3

+ Na + Na

CH 2 =CHCHCH 3 + NaCi • CH 2 CH=CHCH 3 + NaCI

~ ~

resonance equivalence. 69 Phenyl radicals produced in the rotating cryostat (from C6H51 + Na) could be trapped in matrices of water, benzene, deuterobenzene, or perfluorocyclohexane. 69 •7o In 1966, Andrews and Pimentel27c.47 published their first matrix isolation investigations of Li atom reactions with CH31 (slightly earlier than the Mile Na atom work). They were able to produce and trap the elusive methyl radical, and they observed an ir absorption presumably due to CH 3 at 730 cm -1. MilIigan and Jacox 71 found, however, this absorption to be at 611 cm -1 based on vacuum uv studies of matrix-isolated methane. Further work by Tan and Pimentel 72 showed that the 730-cm -1 absorption was probably due to Lil-complexed ·CH 3 rather than to free ·CH 3 • These results contrast with the Na + RX work

Li atoms + CH31

~

{

[Lil---CH31 or [ILi--- CH 3J

described by Mile 69 where the radicals formed were "free." The CH 3 or the I group must impart special properties to the system, however, since further studies by Andrews and coworkers indicated that essentially free halogen-substituted methyl radicals were produced in the folIowing reactions 27c:

70

Kenneth J. Klabunde Ref.

Li atoms + CCI. Li atoms + CBr. Li atoms + CHCI 3 Li atoms + CHBr3 Li atoms + CHBrF2 Li atoms + CH 212 Li atoms + CH 2X 2

~ ~ ~

~ ~

~ ~

LiCI + ·CCI3 LiBr + ·CBr3 LiCI + ·CHCI2 LiBr + ·CHBr2 LiBr + ·CHF2 LiI + ·CH21 LiX + ·CH2X

73

74 75 75

76a-76c 76a-76c 76a-76c

(X = F, CI, Br, or I)

Without direct comparisons of spectra obtained for the same radicals by different generation techniques, it is difficult to tell if LiX complexation is important. Intuitively, it would seem that interaction would be important since an isolated metal salt molecule would have some moderate reactivity in its own right. Also, the radical should certainly be extremely reactive and the LiX and R· would be in close proximity. Further work devoted to understanding these interactions, including theoretical calculations, would be in order. C. Synthetic Applications of Sodium-Potassium Organohalide Gas Phase Reactions

Returning again to gas phase studies of organic halide-alkali-metal reactions, Skell and coworkers 350 carried out macroscale reactions with Na-K vapor and dihalides, and by careful analyses of organic products were able to deduce certain information about intermediate radicals and diradicals. Tetramethylcyclobutadiene was produced by interaction of 1,2-dichlorotetramethylcyclobut-3-ene with Na-K vapor (employing the gas phase flow system described earlier). Petersen and Skell 77 believe that either singlet or triplet diradical

singlet or triplet

L--l

1r-(CH2

CH,

+

t t

CH 2

CH 2 2

1 was produced, with triplet being favored and probably lowest in energy. The presence of excess Na-K vapor apparently allowed facile intersystem crossing. Also, if triplet :CH 2 were produced, the diradical could be converted to the triene 2. It is interesting that the presence of methyl radicals instead of triplet

Metal Atoms as Reactive Intermediates

71

: CH 2 did not cause formation of triene 2. In similar work, Doerr and Ske1l 78a produced trimethylenemethane from the necessary diiodohalide. Dimerization of the triplet diradical took place. ICH 2 " /

·CH 2 C=CH 2

Na-K

~

·CH 2

ICH 2

"-C=CH 2 triplet /

1

=0= Since other similar diradicals could be produced but did not dimerize, the authors believe that monoradicals were not discrete intermediates, but instead the Na-K reagent abstracted both halogens in concert, or nearly so. In a related intriguing study, Doerr and Ske1l 78b were able to produce a series of cyc\opropanones by dihaloketone-Na-K reactions shown below:

o

o

II

Na-K

II

C1CH 2 -C-CH 2 Cl - - - - + ·CH 2 -C-CH 2

•

[~l 1

co + CH

r

o

2

=CH 2

~[~l~>+co

6 ->l61->o+co Due to the exothermicity of the Na-K reactions, Doerr and Skell proposed that the formation of cyc\opropanone would occur with 40-50 kcal excess vibrational energy. Since the decomposition of cyc\opropanone to CO and alkene is 56 kcal exothermic itself, and since the excess 40-50 kcal would provide plenty of activation energy, decomposition took place readily. Comparisons of (·CH2)2C=O and (·CH 2}zC=CH 2 are interesting since molecular orbital calculations predict that the ketone has a singlet ground state

72

Kenneth J. Klabunde

and the alkene a triplet ground state. Therefore, the singlet diradical ketone should rapidly close to cyclopropanone, and the triplet diradical alkene should favor dimerization, which was found experimentally. It appears that the Na-K method yields diradicals in their ground state, meaning that collisional deactivation and facile intersystem crossing are readily allowable in the system. It is important to note in this regard that excess Na-K vapor was always present in the reaction system. It is unlikely that collisions with Na or K atoms or particles were the cause of the facile relaxation and intersystem crossing. Skell and coworkers also used the Na-K reactor to carry out macro scale syntheses by coupling of diradicals or by insertion of carbenes or silylenes. One of these studies, the insertion of (CH3)2Si: into Si-H bonds is important to note 35c, 79: (CH3hSiX 2 ~ (CH3hSi: (CH 3hSiH + (CH 3hSi: - - - - * (CH3hSiSi(CH3hH

(See also the chapter by Y. -N. Tang in volume 2 of this series.)

D. Abstractions by Excited State Mg Atoms A number of very interesting abstraction reactions have been studied employing macroscale cocondensation techniques. Skell and Girard, in the first comparison of ground state versus excited state metal atom chemistry, demonstrated that excited triplet state magnesium atoms were made to undergo reaction by abstraction processes with alkyl halides, ammonia, and butyne. 27b ,35c.so The reaction products using thermally vaporized magnesium (yielding only ground lS Mg) versus arc-vaporized magnesium (brilliant purple arc yielding a high concentration of first excited state triplet 3p Mg) with alkyl halides and ammonia were compared. The lS Mg with R-X yielded RMgX, while the 3Mg reacted in radical-like processes. so The presence of inert diluent apparently helped relax (CH 3hCHBr

+

3Mg(arc)

~

[(CH3hCH

+

·MgBr]

1

(CH 3 )2CHBr

MgBr2

+ 2(CH 3hCH·

1

disproportion.

CH3CH2CH 3

+ CH 3 CH =CH2

excited states and perhaps cage the radicals, so that coupling to Grignard reagents occurred. Excited 3Mg reacted with ammonia in radical-like processes also, while ground state lMg only yielded a Mg-NH3 charge transfer complex.

73

Metal Atoms as Reactive Intermediates NHa

+

aMg(arc) -----+ H·

+

·MgNH2

(-1960C)~ /H. 1

NH 3 (warmup)

H2

+

Mg(NH 2),

H·

H2

In this experiment t mole of H2 was formed upon codeposition, while an additional t mole was released upon matrix ( - 196°C) warmup (25°C), implying that .MgNH2 was stable until warmup, then yielded Mg(NH2h.80

E. Cu, Ag, and Au with Halides; Formation of RM Compounds I n the earliest report of a synthetic application of the macroscale cocondensation technique (1968), Timms reported a very efficient abstraction of halogen from BCI 3 by copper atoms. 81 The resultant . BCI 2 coupled to form B2CI 4 in BCla

+

2Cu atoms -----+ 2CuCI

+

B 2C1.

high yields. This method was employed for synthesis of 10-g batches of B2C1 4 , a considerable improvement over previous synthetic methods. Timms also used the method to prepare alkyl-substituted diboron chlorides in 40-70'70 yields. 27g With PCI 3 , Cu atoms yielded smaller yields of P 2CI 4 ('" 15%).27 g Timms and coworkers have compared eu atoms with Na atoms in dehalogenations of alkyl halides (using macrococondensation techniques at - 196°C). Both metals reacted with alkyl iodides and bromides to yield a mixture of alkyl coupling products and disproportionation products. 27g Copper and silver atoms, apparently coordinated to the alkyl radical, formed so that the radical was never in a free state, as demonstrated by experiments with R( - )-sec-butyl chloride where, in each case, the coupling product S,S( - )-3.4dimethylhexane was formed in about 70'70 optical purity.27 g Sodium atoms yielded an inactive produce which supports earlier discussion that Na atoms and alkyl halides indeed do yield" free" radicals. 69


- RMX reaction could be carried out at low temperature in the absence of complicating donor ligands or solvents. We produced new, reactive, sometimes isolable RMX species, some of which had been proposed as important reaction or catalysis intermediates but never before purposely prepared and characterized. 95 - 98 Many of these new species for the transition metals are highly coordinatively unsaturated (formally), and show a very interesting chemistry.94-98 (ii) To gain some understanding of the mechanism of M atom + RX--,>RMX reaction could have implications regarding mechanistic understanding of oxidative addition to metal atoms on metal surfaces as well as the aforementioned solution oxidative addition studies (ligand complications eliminated). (iii) It would be useful to determine what bonds are most susceptible to oxidative insertion (e.g., C-C1, C-Br, C-I, C-C, C-O, Si-CI, Si-H, P-CI, etc.) and what transition metals favor oxidative insertion (addition) processes. (iv) The metal atom macroscale cocondensation method could be developed as a useful means of synthesizing new RMX species and studying their chemistry.

80

Kenneth J. Klabunde

The importance of systematic studies in this area should be emphasized. A great many R-X---M combinations are possible, and so for reasons of convenience in vaporization and ~ood product stabilities, we have chosen to study one metal (Pd) in some detail with many RX species, followed by study of other metals with the RX species showing the most promise. 54 A mechanistic study of Pd atom + RX ~ RPdX (only saturated alkyl halides) was carried out by macro scale cocondensation techniques. 99 Both unstable RPdX and stable isolable RPdX species were produced. The important experimental results, their meaning, and a mechanistic summary are enumerated as follows.99.100 (i) Pd atoms condensed with CF31 or C2F51 yielded stable isolable CF3Pdl and C 2F 5Pdl, respectively. Some PdI 2 was also formed, but no gases such as CF 4 , C 2F s, or C 4 F lO were formed from decomposition processes. Since the hydro analogs did yield decomposition gases, it may be concluded that the gases came from decomposition of RPdX, not from a prior process leading to RPdX. (ii) Detailed analyses of gaseous products in combination with HX doping experiments (HX, RX, Pd all condensed together, or HX deposited on top of RX-Pd matrix) showed that the probable decomposition pathway for C 2H 5PdX was HPdX elimination with the HPdX capable of reducing RX to RH: C.H5X + Pd atom

~

[C.HsPdXj

~

C.H, + HPdX

1

C.H.X

C.H.

+ PdX.

(iii) Neopentyl bromide [(CH 3hCCH 2 Br] with Pd yielded as major products methane and isobutylene, apparently involving a vibrationally excited radical species [(CH3hCCH~] which split out CH 3 which then picked up H· from the matrix. (iv) Doping experiments with radical scavengers did not affect product yields or distributions. Therefore, radical chain processes were not operating. (v) Use of bare tungsten crucibles rather than the normal Al 2 0 3-coated crucibles shielded with Si0 2-AI 20 3 wool, had no effect on product yield or distribution with C2H51-Pd. So photon energy released from the unshielded crucibles apparently had no effect. (vi) Tertiary halides reacted as well as primary halides, indicating that an SN2 process is probably not involved. (vii) In view of the previously discussed Ag-RX reactions (Section IV. I.E, Ag being very similar to Pd in its properties) it is likely that upon cocondensation an R-X --Pd complex forms which converts to RPdX via a close-radical pair mechanism on warming, with the R· instantaneously possessing excess vibrational energy. A proposed general mechanistic scheme for normal alkyl halidePd-atom reactions is shown in Figure 18.99.100

81

Metal Atoms as Reactive Intermediates +

R-X:

+

Pd

R-X-Pd

-196°C)

I

Sli9ht

warming

(CH 3l2C =CH 2

+

R = (CH 3),CCH 2 •

·CH3 * ( - - - - - - -

CH4~ H· from matrix

HP.dX

+

alkene (

most important

R-H

+

PdX 2

:CH 2

/

CH 2 =CH 2

pathway

+

HPdl

1

CH31

CH 3CH 2 1 Pdl 2

+ CH 4 +

/

~

R·

.X-Pd +

isolated if

RPdX: - - - - - - + ) RPdX thermally stable

·CH 3 + .Pdl

1

CH31'

• CH 3 + Pdl 2 (. CH 3 combines with Pd-Pd1 2 residues; also some CH 3CH 3 formation)

FIGURE 18. Proposed mechanistic scheme for product formations in alkyl halide-Pd-atom reactions. 99

For benzyl, aryl, or other unsaturated halides we believe that a complex is also formed at - 196°C prior to RPdX formation, but that this complex is of the 7T type 19.99.100:

@-Pd

Visually, the - 196°C matrices are very similar, whether the aryl substrate is benzyl chloride, toluene, bromobenzene, or pentafluorobromobenzene. On matrix warming drastic visual changes occur, and in this stage differences are seen if different arene substrates are employed (matrix is dark red changing to black, metallic, dark brown, yellow-green, or other colors often with metal particle formation). Similarly, in a particularly striking experiment, l-bromo-5hexene was condensed with Pd yielding a clear glassy matrix (an obvious efficient

82

Kenneth J. Klabunde

complexation at - 196°C), but on warmup the matrix darkened, yielding only Pd metal particles and I-bromo-4-hexene in 70%' yield based on starting halide (catalytic in Pd) but with no products indicating that any RPdBr had been formed. This experiment demonstrated that good 1T complexation can occur without C-Br insertion. Of course this 1T complexation is also demonstrated by toluene, mesitylene, and other arene complexes of Fe, Co, Ni, Pd that form at -l96°C but decompose on warming. Therefore, we believe that with unsaturated organic halides, 1T complexation occurs first, followed by insertion into the C-X bond on warmup of the matrix. The exact mechanism of the insertion is probably similar to the close radical pair process for normal alkyl halides.99.100 For perfluoro unsaturated halides, however, addition-rearrangement-elimination processes may be occurring, as previously discussed for Ca-atom-perfluoroalkenes 89 (Section IV.2.A.a). It should be pointed out that the efficiency of metal atom reactions with organic halides is in the order C-I > C-Br > C-Cl > C-F. Only in special activated cases do C-F and C-Cl bonds react at all. Secondly, unsaturated organic halide (benzyl, aryl, alkenyl) react more efficiently than do saturated halides, and this is probably due to the aforementioned initial 1T complexation serving to trap or preserve the metal atom (from metal repolymerization) until warmup when C-X---M reaction occurs.19 Matrix isolation spectroscopists have so far totally ignored this interesting oxidative addition research area, which is unfortunate since important mechanistic information could probably be gained from such studies. So far, only macroscale cocondensation techniques have been applied to these processes, and mainly in our laboratories. From a synthetic viewpoint, macro scale cocondensation reactions of organic halides and metal atoms has been very fruitful for us and others. In the last six years we have studied literally dozens of M-RX combinations, and this work has led to the synthesis, isolation, and characterization of a series of new organometallic reactive RMX and R2M intermediates.19.271.54.94-98 It was indeed surprising to find that many of these formally coordinatively unsaturated species were isolable. Many other RMX species were not isolated, however, and the wide-ranging stability dependence on R turned out to be one of the most interesting features of the work. These materials were formed at relatively low temperature, but decomposed on warmup before reaching room temperature. Evidence for their existence was obtained by low-temperature trapping experiments with added ligands to stabilize RMX and/or by observation of decomposition products resulting from RMX. Table 9 lists the nonisolable and isotable RMX species produced along with stability data and comments. From Table 9 it is clear that perfluoro ligands yield the most stable RMX compounds. In particular, C6F5NiBr, C 6F 5PdBr, and CF 3 PdI are strikingly stable when compared with their hydrocarbon analogs. The stabilizating effect of perfluoroalkyl and aryl groups has been noted before for Cu and Ag com-

Metal Atoms as Reactive Intermediates

83

TABLE 9. Transition Metal RMX Species Produced from Metal Atoms; Approximate Thermal Stabilities; and Comments about Their Chemistry RMX Unstable (not isolated) CF3ZnI (CF3hCFZnI CF 3CF=CCaF(CF3) CSF5CaF C sH 5PdBr (or CI) (CF3hCFPdI CH 3PdBr (or I) C 2H 5PdI CsF5NiCI (or Br) CF3NiBr (or I) CH3NiI C2H5NiI Stable, isolated C s F 5PdBr (or CI, I) CF 3 PdI

CF 3CF 2PdI CF 3 CF 2CF2PdI C sH 5CH 2 PdCl C SF5PtBr CH2=CHCH2NiCl CH 2=CHCH 2PdCl CH2=CHCH 2 PtCl

Comments

Reference

Facile :CF2 formation near -80°C Readily hydrolyzed at low temperature Eliminates CaF2 at low temperatures yielding CF3C==CCF3 Readily hydrolyzed at low temperature Decomposes to PdBr2 + CSH5CSH5 at < -100°C; can be trapped with PEt3 Could not be trapped Decomposes to PdBr2 + CH., C 2H., C 2Hs at < -100°C Decomposes to PdI, + C2H., C2HS at < - 100°C Can be trapped with PEt3 at -80°C or lower; above -80°C decomposes to CSF5CSF5 and NiCI 2 Trace can be trapped by PEt3 at - BO°C CH. formation during - 196°C codeposition CH. or H2 formation during -196°C codeposit ion Stable to 130°C; reacts with many ligands to yield C s F5PdBr(Lh Stable to 85°C; reacts with ligands to yield CF 3PdI(Lh; very reactive, weakly coordinates toluene Stable in acetone at > 25°C Stable in acetone at > 25°C 11 3 bonding; stable to 105°C; reacts with ligands to yield CsH5CH2PdCl(Lh Reacts with PEt 3 to yield cis- and transC€F 5PtBr(PEt3h, stable at 25°C 11 3 bonding; previously known 11 3 bonding; previously known 11 3 bonding; previously known

101 101 89 89 95 95 95 95 27i 27i 27i 27i

95,102

95, 103 95,99 95 96, 97 19 104 19 105

pounds,.B2 and for many transition metal systems. 106 Although there has been a considerable amount of discussion concerning the reasons for this stabilization, no clear understanding has been obtained. Most agree that backbonding of the metal to anti bonding 1T* orbitals of perfluoroarene or to a* orbitals of perfluoroalkyl groups is important. Additional ligands should have an effect on this interaction and, in light of our work, we now have available a series of RrPdX(Lh species which can be compared with the nonligand-stabilized R rPdX. 102 Ligands react with CF 3 Pdl and C 6F 5 PdBr quantitatively in toluene or acetone solution (acetone is preferable). Spectroscopic investigation of the RrPdX and RrPd(Lh compounds are being carried out.

84

Kenneth J. Klabunde L

CF3 PdI

+L

I

~

CF3 PdI

I

L

L = PEt 3 , (CH 3 ).S, (CH 3 ).NH, CsHsN L

CeFsPdBr

+L

~

I

CeF5-Pd-Br

I

L

Piper and Timms demonstrated that the bis(1)3-alkyl nickel chloride) dimer could be synthesized very efficiently employing Ni atoms. 104 Skell and Havel 105 showed that Pt atoms behaved similarly, as we did for Pd 19 (all are known com-

M = Ni, Pd, Pt

pounds previously prepared by classical solution methods). Somewhat related to the allyl dimer syntheses is our synthesis of the hitherto unknown benzyl palladium chloride derivative. 96 • 97 It is the only new nonfluorinated RPdX that we found to be isolable and readily characterized. This compound exhibits 1)3 bonding to the benzyl group and is dimeric. SpectroscoPIc studies of 5 showed it to be very fluxional at room temperature and even at

s - 90°C (facile movement so that bonding between two allylic positions (2-1-7 and 6-1-7 carbons) are equivalently populated. 97 However, a 3,4-dimethylsubstituted analog of 5 did show a temperature-dependent NMR spectrum between + 40 and - 61°C. A preferred conformation was exhibited at about - 60°C, where the 6-1-7 allylic position was favored. Careful analysis of the spectrum indicated that the mechanism for the fluxionality {equilibrium between

85

Metal Atoms as Reactive Intermediates

2-1-7 and 6-1-7 bonding positions) was due to a rapid 7T -)- a -)- 7T rearrangement. 97

In terms of metal atom chemistry, it is very interesting that this type of 7]3benzyl bonding occurs. The RPdX species formed initially is coordinatively unsaturated and, in this case, in order to partially satisfy open coordination sites. drastic distortion of the aromatic systems occurs. This allows greatly enhanced stability over similar RPdX species such as C 6H 5PdBr and CH 3 PdI (cf. Table 9). In some M atom + RX reactions, the RMX species formed disproportionates to R2M and MX 2. This behavior was found for [C6F5CoBr] (which we have not been able to trap as such) 98 and C 6F 5NiBr (which we have trapped).271 The [CsFsCoBr] ---+ (CsFs).Co 6

+

CoBr2

[CsFsNiBr] ---+ (CsFs).Ni 7

+

NiBr2

chemistry of these R2M compounds is most intriguing, again mainly because of their" desire" for further coordination. Both 6 and 7 are capable of binding arenes in hexahapto fashion to yield the first examples of R 2M-arene complexes.98.ID7 X-ray structural determination of both the (C 6F 5hCo and Ni-

arene

~

8 M = Co, NI arene = toluene, mesitylene, and others

toluene complexes have been completed in collaboration with Professor Lewis Radonovich.54.98.1D70 The two structures are very similar, and the cobalt analog is depicted in Figure 19. The molecular structure consists of a cobalt atom bonded to two F-phenyl ligands and 7T-bonded to one toluene ligand. The Co-C 1 bond distance is 1.931(5) A and the C1CoC/ bond angle is 88.3(3r. The Co atom is 1.627(2) A from the plane of the toluene ligand and the average Co-C 7T-bond distance of

86

Kenneth J. Klabunde F F

F F

F

F

F

H--=~a~--~l-"--__~__-H

H--l.!

,~

J

H

H

FIGURE 19. The structure of [1Ja-CaHsCH3(CaFshCo]. The crystallographic mirror plane contains Co, C 7 , CiO, C ll , and H io .

2.141 (7) A is similar to the results found in (C6HS)CC03(CO)6 o7T-C 6H 3(CH 3)losa of 2.15(3) A. The toluene ligand makes an angle of 86.3° with the plane defined by cobalt and the two a-bonded carbons. The molecule has m(C.) symmetry, and the carbon framework of both F-phenylligands remains planar. This Co complex is paramagnetic (BM = 1.7), exhibits a broad featureless epr spectrum at room temperature, but shows Co 5/2 fine structure and gx, y, z components at low temperature, thus implying a low-spin complex. losb The analogous Ni compound is an I8-electron system, is diamagnetic, and its formation occurs in one of the highest yields we have ever observed (~60% based on Ni(2). By comparing the X-ray structure of 7J6_C6HsCH3(C6FshCo with that of the Ni compound, we have been able to get direct comparisons of C-a-M and C-7T-M bonding for Co vs. Ni for two molecules of the same symmetry and general structural features. The data indicate that Ni 7T-bonds less strongly (to toluene) but a-bonds more strongly (to C 6F s) than does Co. Our detailed bonding analysis in these systems will be reported later.lo7c The arene ligands in both the cobalt and nickel complexes are quite labile, and are exchangeable at room temperature (toluene complex ~ benzene complex). Both the complexes serve as hydrogenation catalysts for benzene or toluene at room temperature and 1500 psi, although the turnover rate is low « 20) before decomposition of the complex occurs. Competition hydrogenations show that benzene is hydrogenated slightly more rapidly than is toluene. In a planned toluene-cyclohexene competition, addition of cyclohexene immediately caused decomposition of the Ni complex.

87

Metal Atoms as Reactive Intermediates

The perfluorophenyl (C6FS) group is the only group we have found to date that stabilizes these R 2 Co-arene and R 2 Ni-arene complexes. Attempts with CF 3 , C 6H s, and 2,4,6-C sH 2 (CH 3 h have failed. We have carried out lowtemperature trapping experiments employing Et 3 P in order to determine the thermal stability of reactive intermediates involved. The initial reaction of Ni atoms with C 6H sCl or C 6F sBr apparently is insertion to yield C 6F sNiX, which can be trapped by Et 3 P at - 80°C or lower. Warming in the absence of toluene PEt, Ni

+

CaF5X ---+ [CaF5NiX]

1

Et,P

---'---+)

-80"C

I

CaF5NiX

I

PEt,

or Et 3 P yields NiX 2 , Ni, and perfluorobiphenyl. However, warming in the presence of toluene yields the toluene-Ni(C 6F s}2 adduct. The most rational sequence is that C6FsNiX is first formed, which can disproportionate above - 80°C to (C 6F shNi and NiX 2 • In the absence of toluene, which can stabilize the (C6FshNi species, this compound decomposed to C6FS-C6FS and Ni metal. The

CaF5 NiX

(CaF5h

+

Ni

/

""" toluene

~

--@ CH,

(e,F,hN;

analogous cobalt system (C6FShCo did not decompose to C6FsC6FS' even upon warming to room temperature. Subsequent addition of toluene yielded the (C6Fs)2Co-toluene adduct, and very little C6FsC6FS was formed. When Ti, V, or Cr were condensed with C6FSBr, very unstable organometallics resulted. We have not been successful at isolating or trapping the species formed. In the case of a C 6F s Br-Ti-atom cocondensation, however, the product [CaF 5 Ti Br] served as a tremendously active catalyst for butadiene polymerization at _78 c oo (cf. Section IV.4.C for more information). In similar work we found that benzyl chloride-V, -Cr, -Mn, or -Fe depositions yielded varying quantities of catalytic self-alkylation products. This observation indicates that the benzyl metal halides or metal halides formed served as catalysts for Friedel-Crafts-type self-alkylation by benzyl chloride. 100

e

Kenneth J. Klabunde

88

@-CH2CI

+ M

~

@-CH2MCI

+ MCI.

1

C.HsCH.Cl

c. Si and Ge Silicon is best vaporized by the electron beam method because of the corrosiveness of molten silicon toward resistive heating crucibles. Silicon atoms produced in this way are relatively unreactive. Cocondensations with alkyl halides or aryl halides yielded no volatile products. However, more reactive substrates such as C1 2 , HCl, HBr, CH 30H, and (CH3)3SiH yielded doubleinsertion products as shown below lo9 : H

I

I I

(CH 3 hSiSiSi(CH 3 h

I

H

no volatile products

~x

~

(CH3h:~ ~~~ / '

0

Si CH 3 0H/ /

(CH 3 0hSiH 2 19%

~HCl

HBr

~ H 2 SiCI 2 20%

It is believed that organohalides initially reacted to yield silylene intermediates, which then polymerized. lo9 Often in these studies purple matrices were obtained probably due to trapped stray electrons from electron beam scatter. These results with Si emphasize the fact that, in metal atom chemistry, rarely do multiple-bond insertions occur. Usually the simplest process takes place, which is rarely more than one oxidative addition step. In the case of Si, the RSiX species is not reactive enough to form R 2 SiX 2 and so polymerization of RSiX is favored.

Metal Atoms as Reactive Intermediates

89

In the case of Ge atoms, oxidative addition of CCl 4 occurs, but the resulting Cl 3 CGeCi abstracts Cl atoms from excess CCI 4 , resulting in a respectable yield of CI 3 CGeCI 3 • Chloroform and silicon tetrachloride reacted similarly as shown below 110:

CI 3 SIGeCl 3 10,%

B. Acyl Halides with Metal Atoms (Macroscale Cocondensation) Acyl chlorides oXidatively add to palladium atoms and lllckel atoms.271.95 In all the examples studied, the resultant RCOMCI species hberated CO very

o II

RCCI

+

M atom -----*

M = Ni, Pd R = CH 3 , CF 3 , CsHs, C 6 Fs, n-C3F7

readtly. By carrymg out low-temperature trappmg expenments with Et 3 P we were able to reach some conclusions about thermal stabtlltles and decompositIOn pathways. Table 10 summanzes our findmgs,271.95 and Figure 20 Illustrates the

TABLE 10. Acyl Chloride-Metal-Atom Reactions, Products, and Comments271.95 Reactants Pd, CF 3 COCI

Pd, CF 3 CF2CF2COCI

Pd, CH 3 COCI NI, Cf:,COCI

Comments CF 3 COPdCI eliminates CO at -80 C, at which pOint Et 3P addition Yielded almost equal quantities of (Et 3P),PdCI(CF 3 ), (Et 3PhPdCI(COCF 3) and (Et 3 PhPdCl z CF ,CF2CF 2COPdCI stable above - 80 C (but < 0 C), where It was effiCiently trapped to Yield (Et3P)2PdCI(CO-n-C3F7) CsFsCOPdCliost CO at - -50°C to Yield CsFsPdCI effiCiently, which could be trapped With PEt 3 at 40 C C6H5CSHS, C 6 H 5 CI, and (Et,P)zPdCl z found With Et 3 P trapping at 40'C CO loss at < -IOO'C, only (Et3P)2PdCI2 found after trapping dt o or -78 C Et 3 P trapping at -80 C Yielded small amount of(Et 3P)zNICI(CF J ) and mostly (Et 3 P),NICb, but no (Et J PhNICI(COCF 3 ) CO loss at < - 100 C, and Et 3 P trapping at - 80 C Yielded only (EtJPhNICIz

90

Kenneth J. Klabunde RPdX R=

R'CH=CH 2

~Ar

R'CH.~ +

Ar-Ar

HPdX

1

+

ArX

+

Pd

+

PdX 2

R'CH 2 CH 2 X

R'CH 2 CH 3

+

PdX 2

[ RPdX stability order: R = C.F 5 > CF3 '" C.F. '" n-C3F7 > .C.H5 > CH 3 '" ] '" n-C3H7 " CF 2 Br '" (CF3)2 CF

RCPdC,

II

o

i-co RPdCI

Decomposition as above or, if stable, can be isolated or trapped [

RCOPdCI stability order: n-C3F7 > CF 3 > C.F5 > ]

>C.H. > CH 3

FIGURE 20. Scheme for decomposition of RCOPdCI and RPdX species.'71.54.95

decomposition pathway for RCOPdCI species. It appears that decarbonylation to give RPdCI occurs first and. if this species possesses some thermal stability (cf. Table 10), it can be trapped, but ifnot it decomposes as most RPdX species do by fi-H elimination or by R-R coupling. Figure 20 also summarizes decomposition pathways for RPdX species.271.95 C. Acid Anhydrides with Metal Atoms (Macroscale Cocondensation) Hexafluoroacetic anhydride codeposited with Pd atoms yielded a complex that slowly deposited a Pd metal mirror while standing at room temperature in acetone. lll We believe that this complex is zero valent in Pd, but its structure has not yet been determined. Addition of Et 3 P to a fresh acetone solution of the complex yielded cis-bis(triethylphosphine)perfluorodiacetatopalladium(I1) [(Et 3 PhPd(OCOCF 3 )2l. Thus. two molecules of the anhydride must be bonded in the complex. The preference for the cis compound is quite interesting. Formally, this is an example of C-O oxidative addition to a palladium atom. 19 .lll

91

Metal Atoms as Reactive Intermediates

o

II

o

0

I

CF3C-O-CCF3

+

II

0

I

Pd (atoms) ------+ (CF3C-O-CCF3).Pd acetone solution

25T

Pd mirror

o

PEt 3

I

I

CF3C -O-PdPEt3

I

OCCF3

II

o cis

Efner and Fox have extended these studies and have found a number of stable perfluoroanhydride-metal complexes, the structures of which are not yet elucidated. 112 Acetic anhydride codeposited with Pd atoms followed by Et 3 P addition yielded only PdCI 2 (PEt 3 )2' No RCOPdCI was trappable even at - 780C. 113 Similar results were obtained with Ni. Benzoyl chloride with Pd atoms did not yield a trappable C 6H 5 COPdCI species. The decomposition products C 6H 5 CI and C6HsC6H5 were found, however, which indicates that such a species was probably an intermediate 113:

o II

CaHsCPdCI

D. Future Work with Oxidative Addition to Metal Atoms

a. Many other bonds should be investigated (Si-X, P-X, C-c. C-O, C-X, etc.) with transition metals as well as with main group and rare earth atoms. b. Matrix isolation techniques should be utilized in order to obtain more detailed mechanistic information about the oxidative addition reaction.

3. Electron Transfer from Metal Atoms (Reaction Type 3, Figure /5) A. Alkali Metal and Alkaline Earth Metal Atoms (Microscale Cocondensation)

Employing matrix isolation spectroscopy, wide-ranging studies of alkali metal and alkaline earth metal atoms cocondensed with small molecules such

92

Kenneth J. Klabunde

as O 2 , 0 3 , N 2 , NO, N0 2 , CO 2 , CS 2 , and organic carbonyl compounds have been carried out. Table 11 summarizes the results of these studies. In each case electron transfer from the metal atoms to the substrate occurred. Some of the earliest studies in this area were published by Andrews and Pimentel 27c ,114 concerning the reaction of Li and NO. With all of the alkali metals MON (rather than MNO) was formed with a good deal of charge transfer to NO. A good approximation to the structure is M +ON-, However, ONpossesses an antibonding electron, and it was found that larger alkali metals help

TABLE 11. Summary of Alkali Metal and Alkaline Earth Metal Atom Electron Transfer to O 2 , 0 3 , N 2 , NO, N0 2 , C1 2 , F 2 , CO 2 , CS 2 , and R 2 CO, H 2 O, ROH, H 2 S, B2 H s , and Cr(CO)s (Microscale Cocondensation) Reactant

M

Products

Li N2

Li +N 2-

Ca N2

Ca+N 2-

Spectral features and comments 1800 cm-" end-on bonding (C oov ) VN-N 1800 cm-" end-on bonding (C v) VN-N 1800 cm- 1 , end-on bonding (C v) VN-N 1800 cm-" end-on bonding (Co"v) Vo-o 1097.4cm- 1 side-on bonding (Czv ) VN-N

References 35e, 121 35e, 118ab

00

Sr N2

Sr+N 2-

35e, 118ab

00

Ba N2

Ba+N 2-

Li O2

Li+0 2-

Na O2

Li 22+0 22Na+02-

K

O2

Rb O2 Cs

O2

Ba Sr

O2 O2 NO

Li

Na NO K

NO

K+0 2K+O.Rb+0 2Rb+O.Cs+0 2Cs+O. Ba +0 2Sr+02 Li+ONNa+ONK+ON-

Rb NO

Rb+ON-

Cs NO

Cs+ON-

Ca Sr

NO NO

Ca+ONSr+ON-

1094 cm- 1 side-on bonding (C2V) Vo-o 1108 cm- 1 (C2v ) 993.4 em - I, two side-on bonded O2 (D 2d ) Vo-o 1111.3 cm- 1 (C 2v ) 991.7cm- 1 (D 2d ) Vo-o 1115.6cm- 1 (C2V ) 1002.5 cm -1 (D 2d ) Vo-o 1120 cm-" C 2V Vo-o 1115.6cm-'. C 2V VL,-O 651 em- 1 , VN-O 1352 em- 1 , triangular VNa-O 361 em-" VN-O 1358 em-I, linear or bent VK-O 280 em - " VN-O 1372 em - I, linear or bent VRb-O 235 em-I, VN-O 1373 em-I, linear or bent VCs-O 219 em-I, VN-O 1374 em-I, linear or bent VN-O 1357 cm- 1 VN-O 1361 em- 1 Vo-o

35e, 118ab 35e, 119ab 119a 35e,I1ge 35e, 119f 35e, 119f 35e, 119g 35e, 118a 35e, 118a 35e, 116 35e, 116 35e, 116 35e, 116 35e, 116 35e 35e

93

Metal Atoms as Reactive Intermediates TABLE ll.-cont. M

Reactant

Ba Na Cs Ca

NO 03 03 03

Ba

03

Sr

03

Li Na K Cs Rb Li Na K Rb Cs Li

Cl2 Cl 2 Cl, Cl 2 Cl 2 F2 F2 F2 F2 F2 N02

Na

N0 2

K

N0 2

Spectral features and comments

Products Ba+ONNa+03Cs+0 3Ca+03-, CaO, Ca+02Ba +0 3-, BaO, Ba+02Sr+03-, SrO, Sr+02 Li +CI 2Na +CI 2K +Ci 2 Cs+CI 2Rb+CI 2 LiF, Li+F 2NaF, Na+F 2KF, K+F 2RbF, Rb+F2 CsF, Cs+F2 Li+, N0 2-, Li2 +N0 2 Na+N0 2-, Na2+N02K+N0 2-, K 2 +N0 2-

References

1364 cm- 1 Symmetrical (C2V ) Symmetrical (C2V ) 0 3 adduct (C 2V )

35e 35e, 118d 35e,118d 35e, 118ab

0 3 adduct (C2V )

35e, 118ab

0 3 adduct (C2V )

35e, 118ab

VN-O

Cs+, N0 2-, Cs 2 +N0 2 Na+C0 2 Na CO 2 Na +CS 2Na CS 2 Na (CH 3 )zC=0 Na +[(CH 3 )zCO] ~ (CH 3 hC=0 K +[(CH 3 )zC=0] ~ K

35e, 123ab 35e, 123ab 35e, 123ab 35e,123ab 35e, 123ab 123c 123c 123c 123c 123c 35e,115, 117 35e, 115, 117 35e, 115, 117 35e, 115, 117 Small epr coupling to Na 69 Small epr coupling to Na 69 Considerable spin density on oxygen 69 Same epr as Na species 69

Na

Considerable spin density on oxygen 69

Cs

N0 2

y N.'[C(]

246 cm - 1 225 cm - 1 VCI-C! 264 cm - 1 VC!-C! 260 cm - 1 VCI-C! 259 cm - 1 VF-F 452 cm -1 (C2v ) VF-F 474.9 cm- 1 (C2v ) VF-F 464.1 cm - 1 (C2V ) VF-F 462.4 cm- 1 (C2v ) VF-F 458.8 cm- 1 (C2v ) Antisymmetric stretch N0 21244 cm- 1 VCI-C!

Ve!-c!

0

N a O = o Na + [ 0 = 0] Na

H,O

Na +(H20)n~

Na

ROH

Na +(ROH)n ~

Na

H 2S

Na+H2S~

Na B2 H6 Na Cr(CO)6

Na +B2H6 ~ Na+[Cr(CO),]~

~ Considerable spin density on oxygen Solvated electron, octahedral protons around electron Solvated electron, warming gives a-hydroxyl alkyl radical Low-lying d orbitals accommodate electron Low-energy light required Low-energy light required

69 69 69 69 124 27k

94

Kenneth J. Klabunde

disperse this antibonding charge density and thus cause a slight increase in l'N-O (cf. Table 11).35e.115.116 Metal atom reactions with N0 2 were first studied by Milligan and coworkers,1l5.117 The N0 2- anion is very stable, and could be formed by vacuum photolysis, electron bombardment, or metal atom depositions. A 1244 cm- 1 band was assigned to matrix-isolated N0 2- . Andrews believes that this band is due to M +N0 2-, and other ir features in 1200-1230 cm -1 may be due to structural isomers of M +N0 2- and/or M2 + N0 2-. 35e Andrews stresses an obvious but important point concerning these studies as well as other matrix electron transfer processes, and that is that Coulombic energetics tell us that for any matrix metal atom-substrate electron transfer, the resultant cation and anion must remain paired no matter what dilutions or matrices are used. The metal and substrate must make a close approach to one another in the first place in order for the transfer to take place, and then they never separate. 35e In 03-alkali-metal experiments, M +0 3- was the primary product.35e With alkaline earth metals three products were formed: M+0 3-, MO, and M+0 2-. It is believed that the M +0 3- is the precursor to MO and O 2, and the decomposition of M +0 3- is aided by the presence of N 2 as a matrix diluent rather than Ar.lls Metal atom-0 2 codepositions have been extensively investigated for alkali metals, alkaline earth metals, and transition metals. 35 The transition metals yield complexes without complete electron transfer, and these will be discussed further below under the heading of Simple Orbital Mixing (Section IV.4). With alkali metals, electron transfer does take place, yielding M+0 2-, M+0 22-M+, and M +0 4 - speciesY9.12o Generally, the side-on bonded M +0 2- is formed at low temperature, and the final products formed on warmup are MO and 0 3, For example, Ca plus O 2 eventually yields 0 3 and CaO, which behaves as a .. free" metal oxide diatomic molecule absorbing at 800 cm - 1 (ve _ 0) while trapped in the matrix. 120 Codepositions of Li with N2 yielded Li +N2 - and Li 22+ (N 2)2 2-, which are believed to exist with" end-on" bonded N2 to Li.35e.llSab.121.122 (See Table II for further information and references.) Complete electron transfer from metal atoms to organic substrates has been observed in several instances by using microscale cocondensation techniques. Deposition of Na or K atoms onto solid CO 2 or solid CS 2 at 77 K resulted in the formation of Na +CO 2~ and Na + CS 2 ~. 69 Epr studies of these species revealed a small hyperfine coupling to the alkali metal nucleus, which indicated that the unpaired electron was not completely transferred to the organic moiety. Mile 69 compared these epr results with the same radical anions formed by y irradiation techniques and indeed marked differences were found, indicating that the alkali metal ion does have a real effect on the magnetic environment of the CO 2 or CS 2 radical anions. In the same manner, Mile 69 also has made a series of matrix-isolated ketyl radical anions. No differences in epr spectra were observed

95

Metal Atoms as Reactive Intermediates

o II Na or K atoms

+

0

o

R-~-R = CH)CH3,

R

/C"

17K

R ---+

U 0=0, o'

and biacetyl

on changing from Na to K, showing that the electron had been completely transferred in these cases. Comparison of R 2CO:- with R 2CH:- by epr showed that a good deal of unpaired spin density resided on oxygen as well as on carbon. R

I

R

. C- 0 -

+--------+

I

I

-c-o· I

R

R

Mile 69 has also prepared solvated electrons in ice and solid alcohols by depositing alkali metal atoms with water or alcohols at 77 K. Intensely colored matrices resulted, with epr spectra showing just a single narrow line. At higher temperatures interaction with neighboring protons became evident from epr splittings (six equivalent protons from three water molecules yielding a sevenline spectrum probably caused by octahedral placement of the protons about the electron). Warming of the alcohol-solvated electron matrices caused formation of j3-hydroxy radicals [C-C-OHJ. 69 Deposition of Na or K with H 2S did not yield a solvated electron, but instead an H 2S - species. Mile rationalizes the differences between H 20 and H 2S as H 2S not being as highly associated and with less dipole moment, and the sulfur as low-lying 3d orbitals which can accommodate the extra electron, whereas the oxygen does not. Kasai and McLeod 124 were able to carry out the reaction of Na atoms with B2H6 with argon dilution to produce Na + B2H6 -. However, low-energy light was required, indicating some type of concerted condensation-light-induced process.

The light energy used was of much lower energy than that needed to ionize Na. 27k In the same way, Cr(CO)6 can form an ion by Na atom electron transfer with visible light, again apparently a concerted or synergistic process of some type 271: Na/Ar

+

Cr(CO)6

h. (vis.»

Cr(CO), -

+ Na + + CO

96

Kenneth J. Klabunde

B. Synthetic Applications of Electron Transfer (Macroscale Cocondensation) We have been interested in applying macroscale cocondensation techniques to the production of films of ionic materials as well as "borderline" complexes [partial electron transfer: borderline between complete electron transfer and simple orbital mixing to form a covalent M-substrate bond (Section IV.4)]. As an extreme case of complete electron transfer we have chosen to study metalatom-TCNQ .cocondensations. The molecule tetracyanoquinodemethane (TCNQ) is of great interest in the "organic metals" field 125 (organic and organometallic semiconductors and hoped-for superconductors) because of its excellent electron acceptor properties.

M atom

+

NC

"

=0=

C-

NC/

-C /

-TCNQ

CN

--+

M+TCNQ~

"CN

Previously it had been shown on microscale that cesium atoms condensed on TCNQ yielded Cs+TCNQ- very efficiently.126a Aluminum atoms behave similarly.126b We have extended this work to synthesize on a macroscale transition-metal-TCNQ films in the absence of water or other solvents. 54 For example, Ni atoms cocondensed with TCNQ vapor yielded an emerald green film of Ni ++(TCNQ7)2 (using a double-deposition apparatus for subliming TCNQ simultaneously with Ni).54 The reaction with Ni was very efficient and clean. With proper control of metal and TCNQ vaporization rates, uniform films of M +TCNQ - - TCNQ should be attainable on almost any surface desired. Due to the relative nonvolatility ofTCNQ, the cocondensation can be carried out with a wall temperature as high as 25°C. C. Further Studies and their Importance Electron transfer is a particularly important reaction type for several reasons. First, many organometallic reaction mechanisms involve electron transfer as an intermediate step.127 Cocondensation methods should be useful in trapping some of these intermediate species for detailed study. Second, systematic studies with different metal atoms and ligands of varying electron demand should lead to better understanding of electron transfer versus simple orbital mixing. The study of borderline cases where 7T complexation or electron transfer can occur should have some importance. Metalfilms, vanadium in particular, have changed transition temperatures for superconductivity when exposed to arene ligands such as anthracene, 1,4,5,8-tetrachloroanthraquinone, and other similar substances. 128 Electronegativity of the ligand versus the metal appears to be quite important for these surface electron transfer processes. It is certain that a better

Metal Atoms as Reactive Intermediates

97

understanding of the interactions of the metal film surface with both electrondemanding and electron-releasing ligands is needed. Some helpful information might be gained by matrix isolation work as well as the synthesis of single metal atom-ligand complexes foIlowed by studies of their properties, including X-ray structures, to determine substituent effects on bonding parameters. In support of this area of study, we previously reported on the differences in stability of a series of transition-metal-arene complexes where arene = benzene or hexafluorobenzene. 129 For the metals with higher d-electron populations (later transition metals such as Ni), we found that hexafluorobenzene formed more stable complexes, and we attributed this to better" 17-acceptor" properties of hexafluorobenzene versus benzene. This example approaches complete electron transfer [i.e., Ni + HFB:-]. We are also involved in a study of substituent effects on the bonding and structures of bis(arene)V and Cr complexes (in collaboration with Professor L. Radonovich), which wiIl be discussed in the next section (Section IV.4.E.b).

4. Simple Orbital Mixing of Ligands with Metal Atoms (Reaction Type 4, Figure 15) (Macroscale and Microscale) "Simple orbital mixing" is intended to mean the mixing of luetal atom orbitals with ligand orbitals to form a new molecular orbital and a bond mainly covalent in character. Complete electron transfer does not occur although there may be strong polarization in the final product. In this process no a bonds are M atom

+

nL ----+ M(L)n

broken. Since metal atoms have readily available orbitals and essentially no steric restrictions, simple complexation with 17 and a donors is a very efficient process. The greatest majority of metal atom synthetic work has been carried out in this area. Professor Skell and Professor Timms and their coworkers have contributed a great deal. In the microscale matrix isolation area, Professors Ozin, Turner, and Moskovitz have made substantial contributions. A. Olefins with Metal Atoms

A series of macroscale cocondensation experiments were carried out where propene was cocondensed at - 196°C with metals Co, Ni, Pd, Pt, AI, Oy, Er, and Zr. 130 These were initial experiments intended to demonstrate differences and similarities between the metals. Organometallic products were not isolated and characterized, but rather were decomposed with 0 2 0 to mark C-M bonds. It was believed that r.-type M-propene would be destroyed by 0 2 0 to simply release unlabeled propene, while a-type M-propene bonds would yield deutereopropane.

98

Kenneth J. Klabunde

M / CH 3CH=CH 2

+

M

~

CH 2 D D,O -----+

/CH 2

I

M/CHCH3

I

CHDCH3

CH 2

M~

D,O -----+ release CH 2 =CHCH3

CHCH3

Aluminum atoms with propene followed by warming and 0 20 hydrolysis yielded deuterated propane, 2,3-dimethylbutane, 2-methylpentane, and traces of n-hexane, and the scheme shown in Figure 21 was proposed.27b.131.132 The aluminacydopropane species was believed to open to a diradical with subsequent addition of another propane molecule. Bond rotation could occur in the diradical stage, as shown by cis- and trans-2-butene experiments. However, the addition of donor-complexing ligands such as ethers resulted in better stereochemical retention, probably by slowing down the ring-opening process.

~o

FIGURE 21. Proposed reaction scheme for aluminum atoms with propene followed by D 20 hydrolysis.27b.131,132

99

Metal Atoms as Reactive Intermediates

retained stereochemistry indicated

Skell and Wolf132 carried out matrix isolation spectroscopy work as well on this system, and believed the results supported their proposal of dialuminoalkane formation, and that it was formed at -196°C (not on warming of the matrix or from active aluminum clusters or particles).132 Thus, Skell and Wolf1 32 believe AI-olefin matrices form M-C bonded species. However, somewhat conflicting evidence was found when Kasai and McLeod 133 carried out epr studies of matrix-isolated AI-ethylene-neon mixtures (roughly I: 10: 1000). The green matrix exhibited a unique sextet attributed to hyperfine interactive with the 27 Al nucleus (100'70 abundance, I = j). Of the three possible bonding structures I, II, and III, structure I can be eliminated because of the observed strong anisotropic coupling tensor to the Al nucleus. H2C-CH2

\

AI

H 2C-CH2

\/ AI

H2Cr;

~.I - RPdX(PR 3)2]'

Metal Atoms as Reactive Intermediates

119

In the cases where a phosphine is the main ligand and of main interest, the reactions are fairly straightforward, and these are listed in the earlier portion of Table 17. Only a few comments should be made about these examples. First, many M-PF 3 complexes are available by conventional high-pressure methods, but the metal atom method does serve as an attractive alternative procedure. 175 Care must be taken to use pure PF 3 in these codepositions, however, or poor results are obtained. When the restrictions are carried out properly, the products can generally be pumped out of the reactor and purified by low-temperature distillation or simply sublimation. The reason for the great interest in PF 3 in metal atom chemistry is that it behaves much like CO does in its bonding and stabilizing characteristics, and yet is condensable at - 196°C, whereas CO is not. The method serves well for the preparation of mixed complexes using mixtures of weak ligands and strong ligands such as with PF 3-PH 3 .175 It also serves well as a method of preparing complexes of thermally unstable ligands, as nicely shown by the work of Parry and Staplin. 176 Here PF 2H was prepared and kept cool during the reaction procedure, and the final Ni(PF 2H)4 complex prepared was stable, thus serving to stabilize the PF 2H itself.176

TABLE 17. Metal-Phosphine Complexes and Mixed-Ligand-Phosphine-Metal Complexes Prepared by Macroscale Cocondensation Ligands PF 3 PF 3 PF 3 PF 3 PF 3 PF 2Cl PF 3 , PH 3 PF2H P(CH 3 h P(CH 3 h P(CH 3 h P(C6H6h PN PF 3 , C6H6 PF" CeF6 PF 3 , C 6H sCH(CH 3 h PF" C O H3(CH 3 h PF3, C O H6 PF 3, NO PF 3 • NO PF 3 , NO

Metal Cr Co Ni Pd Fe Ni Ni Ni Ni Co Fe

Pertinent compounds formed

Cr(PF 3 )6 Co2(PF 3 ). Ni(PF 3 ). Pd(PF 3 ). Fe(PF 3 h + (PF 3 hFe(PF2hFe(PF 3 h Ni(PF2CI). Ni(PF 3 },(PH 3 h + Ni(PF 3 h(PH 3 ) Ni(PF 2H). Ni[P(CH 3 hl. Co[P(CH 3 hl. Fe[P(CH 3 hls or [(CH 3 hPhFe(H)CH2(PCH 3 h Ni Ni[P(C 6 Hshl. a Ag AgPN + Ag(PNh" Cr C 6H 6Cr(PF 3 h Cr C 6 F 6 Cr(PF 3 h Cr C6HsCH(CH 3 hCr(PF 3 h Cr C 6 H 3 (CH 3hCr(PF 3 h Fe C6H6Fe(PF3h Mn Mn(NOhPF 3c Fe Fe(NO)2(PF 3 )2 C Co Co(NO)(PF 3 h C

References 175 175 175 175 175 175 175 176 27g 27g 27g, 35b 27g 27g 162 162 162 162 152 162 162 162 (continued)

120

Kenneth J. Klabunde TABLE J7-cont. Ligands

Metal

Pertinent compounds formed

References

CsHsCH3' then PF J CH2=CHCH=CH2, then PF3 CH2=CHCH=CH 2, then PF3 CsHs, then PF3 C3H 7 , then PF 3 CH3CH2CH=CH2, then PF3 CF3CF=CFCF3, PEt 3 CsFsBr, PEt3 CsFsBr, PEt 3 CF 3Br, PEt 3 CF 3I, PEt3 CF 3Br, PEt 3 CsHsBr, PEt3 CsFsCI, PEt3 CsFsCI, PEt3 CsFsI, PEt3 n-C3F7I, PEt3 C2FsI, PEt3 CF 3COCl, PEt 3 n-CJ F 7COCl, PEt3 CF"COCl, PEt3 (CF 3CO)zO, PEt 3 C s H sCH 2CI, then PEt3 1,4-cyclooctadiene, PF3. 1,3-cyclohexadiene, PF3 (CsHs)zPCH2CHzP(CsHs)z, N2 (cyclooctadiene)

Fe Fe Cr Co Ni Co Pd Ni Pd Ni Pd Pd Pd Pd Ni Pd Pd Pd Pd Pd Ni Pd Pd Cr Cr

CsHsCH3Fe(PF3h (CH 2 =CHCH=CH2hFe(PF3) (CH 2=CHCH=CH 2)Cr(PF 3)4 CsH GCO(PF3h C3H 7Ni(PF3h (CH3CHCH=CH2)Co(PF3h (CF3CF=CFCF3)Pd(PEt 3h CsFsNiBr(PEt3h CsF sPdBr(PEt3h CF3NiBr(PEt3h CF3PdI(PEt 3h CF3PdBr(PEt3h CsHsPdBr(PEt3h C s F sPdCI(PEt 3 h CsFsNiCl(PEt3h CsF sPdI(PEt3h n-C 3F 7PdI(PEt 3h C2FsPdI(PEt3h CF3COPdCI(PEt3h. CF3PdI(PEt3h n-C 3F 7 COPdCI(PEt3)z CF3NiCI(PEt3h cis(CF 3C02)zPd(PEt3h C sH s CH 2PdCI(PEt3h ']s-CBHll Cr(H)(PF 3)a, C.H ,zCr(PF 3 ) . (']4-C S H.)zCr(PF 3)z

152 152, 153 131 35d, 177 35d 35d 111 27i 27i,95 27i,95 27i,95 27i,95 27i,95 27i,95 27i,95 27i,95 27i,95 27i,95 27i,95 27i, 95, 111 27i,95

Fe

(diphos)zFeN 2

178

a b

c

III

96, 97 27h

168

Prepared in methyJcyclohexane solution. Microscale at 10K; PN generated from P3N5 at 850°C. BF 3-NO adduct used as source of NO (to avoid excessive vapor pressure of pure NO at operating temperature).

The metal atom method also allows the preparation of exceedingly reactive complexes of (CH 3hP, which are strongly nucleophilic. 27g Although careful studies have not appeared, Et 3P does not behave as well as does (CH 3hP in reactions with metal atoms, and usually Et radicals, H2 gas, and C 2H 4 are formed along with a complex mixture of organometallic products. 146 .147 Another interesting area is the study of PN. This material can be formed by pyrolysis of P3NS at 850°C, and serves as an example of reaction of metal atoms with other reactive species. 27g So far, only microscale methods have been used with PN, and spectroscopic methods indicate M-P bonding rather than M-N bonding in Ag(PNh.27g The additional compounds listed in Table 17 have been discussed under sections dealing with the ligands involved, other than phosphines (e.g., NO complexes, arene complexes, diene complexes, oxidative addition to RX and RCOCI).

Metal Atoms as Reactive Intermediates

121

H. Isonitriles with Metal Atoms

Gladkowski and Scholar 179 have reported the synthesis of a series of (RNC)nM complexes by the macro scale cocondensation method. In particular, NiL4' FeL 5 , and CrLs where L = t-butylisocyanide, methylisocyanide, cycIohexylisocyanide, and vinylisocyanide were prepared. The complexes did undergo exchange reactions with phosphines and phosphites. Codepositions of isocyanides and PF 3 yielded mixed complexes.

I. Metal Atoms with Nitric Oxide (NO) Nitric oxide (NO), a three-electron ligand, possesses some vapor pressure at - 196°C, and so is difficult to cocondense with metals in macroscale preparations. The BF3 adduct of NO can be used as a source of NO in such cocondensations, however. 1s2 By itself, NO (or F3B-NO) has sometimes yielded explosive M-NO complexes so that care must be taken in its use. An attempted preparation of Co(NOh failed, as did that of Co(CsHs)NO and of Cr(CsHs)(NOh.ls2 A few mixed-ligand preparations were successful, however, and these are listed in Table 17. Microscale matrix isolation experiments have yielded a rich electron transfer chemistry (Section IV.3) with alkali metals and related systems. However, as yet little has been done with transition metals to yield M-NO complexes on microscale. J. Carbon Monoxide with Metal Atoms

Carbon monoxide is, of course, a "super" ligand, having excellent 7T-acid characteristics. It is an obvious choice for study in metal atom chemistry, except for one problem. It possesses too high a vapor pressure at - 196°C (liquid nitrogen) to be used in macro scale preparations where good vacuum must be maintained. However, for microscale studies where lower temperatures can be employed economically, much work with metal-atom-CO reactions has been carried out. Matrix ir, Raman, uv-visible, esr, and Mossbauer spectroscopy have been employed in these studies. Pure CO or CO diluted with Ar or Kr can be deposited, usually at about 10 K. Employing these dilution studies M(CO)n' where n = I, 2, 3, 4, 5, and/or 6, can often be observed spectroscopically, whereas pure CO yields the" saturated" CO analog, often a thermally stable species. M

+ co

----+ M-CO

----+

M(CO)2

M(CO).

0(-

M(CO),

1

0(-

122

Kenneth J. Klabunde

Many thermally stable metal carbonyls exist. However, the important aspects of the matrix M-CO work are as follows: (1) the fact that the unsaturated M(CO)n (where n is small) can be observed; (2) totally CO-saturated M(CO)n can be observed, even for metals where this configuration is not stable at room temperature; (3) the observed Vc=o in the complexes can be compared with calculated values, and thus expected or theoretical bonding interactions verified or refuted; and (4) with proper concentration manipulations, M 2(CO)n species can be observed. 27i Three tools have been used for determining the M-CO stoichiometry: (1) ligand concentration studies, (2) mixed-ligand isotopic substitution (13CO + 12CO), and (3) other mixed-ligand studies. Of course, employing these tools depends a great deal on previous knowledge of the coordination chemistry of each metal in a zero valent oxidation state. In addition, with some expertise, ligand coordination" side-on" or" end-on" can be distinguished from one another. 35a ,g For a detailed consideration of these studies, the reader is referred to several reviews that have recently appeared. 27c ,35a,g,39-42 In brief, Table 18 summarizes the important M-(CO) complexes prepared by microscale, metal atoms methods, along with Vc=o observed and comments. All of these complexes possess M-CO covalent like bonds without complete electron transfer. The geometries assumed by the molecules are quite interesting (these geometries can be determined by spectroscopic methods) and are described in Table 18 by point group symmetry notation. Although much interesting information about M-C and C=O bonding in the many complexes listed in Table 18 can be gleaned, only a few points will be made here since Moskovits and Ozin have already presented a detailed discussion. 35g ,35h The tetracarbonyls of Ni, Pd, and Pt are listed in Table 18. The Ni analog is the only stable species of the three. Comparison of the spectra (vc=o and VM-C) indicates that the effectiveness of CO as a a donor and 7T acceptor toward M is in the order Ni > Pt > Pd. It is interesting that this is also the stability trend order for most 7T complexes with these metals. However, the order is generally Pt > Pd > Ni for a complexes. In the carbonyl series, Ni appears to be a better a acceptor and 7T donor than are Pd and Pt. The charge on the metal does not vary much throughout the series, however. Some very important information about molecular geometry of thermally unstable molecules can be obtained from these studies. For example, Pt(CO)4(l2C 16 0) at 10K showed strong absorption at 2047.8 cm -1 and weak absorption at 2055.0 cm -1 in the matrix ir. The Raman spectrum showed lines at 2049 and 2119 cm -1, and these data along with matrix-Raman depolarization experiments were consistent with Pt(CO)4 being tetrahedral. Assuming this structure, calculated ir frequencies agreed very well with those found. 35g Similar clever application of these techniques, along with isotope labeling and mixed-ligand studies, showed that Re(COh is probably square pyramidal in the matrix, and that CO(CO)4 has C3V symmetry.35 g In all of these instances the M(CO) species are

123

Metal Atoms as Reactive Intermediates TABLE 18. M(CO)n, Mz(CO)n and M(CO)m(Nz)n Complexes Prepared by Microscale Metal Atom Techniques

Complexes V(CO) V(COh V(CO)a V(CO). V(CO)s V(CO)a Cr(COh Cr(CO). Cr(CO)s

Characteristic strongest Vc_o (cm -1) modes 1904 (argon) 1974/1882; 1880; 1723/1719 1920 1893 1952/1943 1971 1867 1940/1934/1896 2093/1966/1936

Cr(CO)6 Mn(CO)s Fe(CO) Fe(COh Fe(CO). Fe(CO)s CoCCO) CO(CO)2 CoCCO), Co (CO). Ni(CO) Ni(CO), Ni(CO)a Ni(CO). Cu(CO) Cu(CO), Cu(CO)a Ta(CO) Ta(CO), Ta(COh Ta(CO). Ta(CO>, Ta(CO)6 Mo(CO), Mo(CO), Mo(CO). Mo(CO>,

1990 (argon)

Mo(CO)6

W(CO)3 W(CO). W(CO)s

1993 1865 1939/1894 2097/1963/1932

W(CO)6

1987

1898 2042/1936 (argon) 1999/1994/1974 1952/1944 1914 1977 2021/2011 1996 1967 2017 2052 2010 1892 1985/1976 1819/1831 1891/1897 1916 1943 1953 1967 1915/1911 1869 1951/1895 2098/1973tI 933

Comments, probable geometry

References

Jahn-Teller instability, nonlinear 35g, 190 C ah , C 2h , D ooh, three forms D3h D'h or Td Dah Oh, stable Cav

Low-spin C 2V Interacts with rare gases, CH., SF6, C. v or Dah Stable,Oh C. v

35g, 190 35g, 190 35g, 190 35g, 190 35g, 190 180, 192 180,192

180, 187-189, 192 180, 192 189 190a Low-spin C 3V 185 Interacts with rare gases, CH., C 2v 181 Stable, D3h C oov 183,190a DOOh 183 C 3V 183 C 3V 181,182,183 184, 190a Coov 184 DOOh 184 D3h 184 Stable, Td C oov 190a, 186, 199 186, 199 DOOh 186, 199 D3h 35g, 184 35g, 184 35g, 184 35g, 184 35g, 184 35g, 184 180, 187 Dooh Cav 180, 187 C 2V 180, 187 Interacts with rare gases, CH., SF G, C. v 180,187,192 Stable, Oh 180, 187 C3u 180, 187 C 2V 180, 187 Interacts with rare gases, CH 4 , 180, 187, 192 SF6, C. v Stable, 0, 180, 187 (contmued)

124

Kenneth J. Klabunde TABLE lB.-cant.

Complexes Re(CO), Rh(CO) Rh(CO), Rh(COh Rh(CO), Ir(CO) Ir(CO), Ir(COh Ir(CO)4 Pd(CO) Pd(CO), Pd(COh Pd(CO)4 Pt(CO) Pt(CO), Pt(COh Pt(CO)4 Ag(CO) Ag(CO), Ag(COh Au(CO) Au(CO), AlxCCO), Gax(CO), Ge(CO) Gex(CO)y SnCO SnACO)y Pr(CO) Pr(CO), Pr(COh Pr(CO)4 Pr(CO), Pr(CO)6 Nd(CO) Nd(CO)2 Nd(COh Nd(CO)4 Nd(CO), Nd(CO)e Gd(CO) Gd(CO), Gd(COh

Characteristic strongest Vc~o (em-I) modes 1995/1977

2050 2044 2057 2071 2052 2058 2049 2053

Comments, probable geometry D4h

Dooh D3h

Stable to 80 K, Td C oov Dooh D3h

Td

1968/1939 D3

1936 1890, 1939 2006, 1912 1908 1873, 1878 1921 1700-2100 1835 1858 1885 1940 1965 1989 1840 1861 1891 1940 1965 1990 1841 1864 1901

Linear

References 193 35g 35g 35g 35g 35g 35g 35g 35g 194, 195 194,195 194, 195 194, 195 196 196 196 196 197,199 197,199 197,199 198 35g, 198 209 35f, 210 211 211 211 211 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200 271, 35g, 200

Metal Atoms as Reactive Intermediates

125

TABLE lB.-cant.

Complexes Gd(CO). Gd(CO)s Gd(CO)6 Ho(CO) Ho(CO)2 Ho(COh Ho(CO). Ho(CO)s Ho(CO)6 Eu(CO) Eu(CO), Eu(COh Eu(CO). Eu(CO)s EU(CO)6 Yb(CO)x U(CO) U(CO), U(COh U(CO). U(CO)s U(CO)6 N i(N 2)",( CO).-m V2(CO)12

Characteristic strongest vc=o (cm -1) modes 1945 1967 1986 1830 1859 1902 1929 1961 1982

1968 1974 2000 2008, 1995, 1985, 1976, 1966, 1958 1832(1817 1855(1846 1893 1919 1938 1961

Ir2(CO)s Re2(COho

2039(2003

35g, 200 35g, 200 35g, 200 35g, 200 35g, 200 35g, 200 35g, 200 35g, 200 35g, 200 35g, 200 35g, 200 35g, 200 35g, 200

200c 200 200 200 200 200 200 201 Both CO bridged and V-V bonded forms exist M- M weakly bonded

Fe2(CO)8 Fe,(CO)g Co 2(CO)s

References 211 211 271, 271, 271, 271, 271, 271, 271, 271, 271, 271, 271, 271, 271,

1873

Cr2(CO),O Mn 2CO Mn2(CO)2 Mn2(CO)lO

Ag 2(CO)6 CU2(CO)6 Rh 2(CO)s

Comments, probable geometry

Normal form M- M bonded, this bridged CO bridged CO bridged, D3h Both CO bridged and Co--Co bonded forms M -M bonded M-M bonded Del CO bridged; forms Rh.(CO), at -48°C CO bridged; forms Ir2(CO)!2 at -58°C M-M bonded

190b,191 27j 27j, 189 27j, 189 27j, 189 202 202 183 197 186 203 203 27j

126

Kenneth J. Klabunde

unstable, and were best prepared and studied by the metal-atom-matrix method. Table 18 indicates the experimentally predicted geometries for similar first-row transition metal carbonyl fragments. Unusual electronic effects apparently do not allow each species to adopt the highest symmetry configuration. 35g Mixed CO-N2-M depositions have been carried out in order to study (spectroscopically) the binding of CO and N2 to the same metal atom, something not possible before the advent of metal-atom-matrix-isolation work. A series of Ni(N 2)m(CO)4_m complexes were prepared employing isotopic labeling where needed.35g.201 Employing spectroscopic methods and the assumption that stretching force constants were linearly related to quantity of charge donated by N2 or CO to the metal (a) (Dewar-Chatt bonding model) and accepted (17) by the ligand, Moskovits and Ozin 35g were able to determine 17CO > 17N2 and a Co ;::: a N 2. Also, it was found that force constants for bonded CO involved a (a - 17) term but those of bonded N2 a (a + 17) term, which explains why N2 shows a greater decrease in force constant on complexation than does CO. The Dewar-Chatt bonding model deals with ligand a donation to unfilled d metal orbitals, and backdonation from filled metal orbitals to 17* orbitals of the ligand. Are p orbitals capable of this type of donation? Based on matrix-metalatom techniques, the preparation of AI, Ga, Ge and Sn carbonyls were carried out. These metals all have filled valence d shells. Moskovits and Ozin 35g proposed that P17-17* bonding (with minimal d17-17* contributions) furnished enough backbonding stabilization to allow these main group carbonyls to exist. Previous discussion of Kaisai's work 133 with AI atoms and ethylene gave further support to this idea, where by esr it appeared that the Al odd 3px electron was delocalized onto the ethylene ligand by P7T-7T* overlap (cf. Section IV.4.A). For the rare earth carbonyls the normal Dewar-Chatt model appears adequate, and the participation of 4/ orbitals does not appear necessary. Silver-vapor-CO codepositions were studied in some detail in order to demonstrate that metal-atom-matrix-isolation techniques can be employed for carrying out kinetic studies. 35g As mentioned previously, the low temperatures employed allow the study of reactive species in reactions of very low activation energies. In the Ag-CO case, it was shown that Ag(CO)3 was converted to dimer Ag 2(CO)6' The dimerization process was studied kinetically by carrying out experiments-where first metal flux was varied while CO deposition rate was held constant. In the second set of experiments the metal-CO ratio was held constant but temperatures 0[30,33,35, and 37 K were studied. Rate constants and diffusion coefficients were obtainable, the latter ranging from D (cm2jsec) = 1.8 x 10- 18 to 1.2 X 1O-15.35g Diffusion kinetics of Pd(CO)1_4 have also been studied. 35g Table 18 also lists a number of M 2 (CO)n matrix prepared compounds. Kundig, Moskovits, and Ozin27J have found that M2 species can be formed readily, especially for Mn and Sn, by dimerization in matrices. Detailed studies of these processes and metal atom migration in general have been examined by a number of workers. It has been found that very light metal atoms, such as Li and Be, diffuse readily in rare gas matrices, but that heavier atoms such as Ca, Pb, Ag, Au, and so on have much less tendency to do so. Concerning the effect of

127

Metal Atoms as Reactive Intermediates

matrix material, it was found that diffusion of metal atoms was best in Ne > Ar > Kr > Xe. 27l,35h,204,205,206 For CO studies with M and M2 species interesting comparisons can be made with (M)n surface studies. Many studies of chemisorbed CO have been made and Vc=o can be compared for (M)n-CO [chemisorbed vs. M-CO vs. M 2-CO (terminal or bridged)]. For example, the (Cr)-(CO) species shown below (with Vc=o values given in cm -1) have been characterized spectroscopically27J:

o II C

Cr-C==:O

Cr-Cr-C==:O

1840

1975

Cr/ ""cr

~/ C

II o 1821

o II C

/\

Cr-----Cr 1733

O==:C-Cr-Cr-C==:O 1952

In another example, Moskovits and Ozin believed that by the study of M-CO matrix spectra, a controversy concerning chemisorbed CO was solved. 35g That is, for CO chemisorbed on certain metal surfaces, two major bands have been found. Eischens, Francis, and Pliskin 207 assigned these to terminal and bridged CO, while Blyholder and Allen have proposed CO adsorbed on flat or central metal sites versus edge sites. 20B In the former case (terminal and bridged) a higher frequency Vco should be observed for the terminal adsorbed CO. In the latter case (edge versus flat surface adsorption) a higher frequency should be observed for the central or flat sites than for edge sites because the higher n is for (M)n-CO, the less backbonding to CO would be necessary. In a single metal atom M-CO system, the strongest 17* backbonding overlap would be expected, thus making Vco lower than for terminal CO on any metal surface sites. Since, in almost every case, Vco for M-atom-CO was found to be in between the two chemisorbed bands, it was proposed that Eischens' original idea was correct. That is, only bridged CO could fall at a lower Vco than the Vco for M-atom-CO. 35 g It appears from the data in Table 18 that, by using microscale cocondensation methods, metal carbonyls of essentially all the metals can be prepared. In addition to the transition metals, the rare-earth-CO work should be noted, as well as the work on AI,209 Ga,2Bf,210 Sn,211 and Ge.35h.211 Thus, it is apparent that pretransition and post-transition metals form carbonyls, rare earths form carbonyls, and transition metals form carbonyls. More work is needed to determine if some of these new M-carbonyls are stable enough to be isolated, or if they can be stabilized by addition of other simple ligands.

128

Kenneth J. Klabunde

K. Dinitrogen (N2 ) with Metal Atoms

Table 19 lists transition metal M-(N2) complexes that have been prepared in low-temperature matrix-isolation experiments [alkali metal and alkaline earth metal atoms yield M + N2 - species, and are discussed in the section on electron transfer (Section IV.3)]. In this series both" end-on" and" side-on" bonding can occur, although end-on is usually favored. Also, the matrix environment can sometimes affect the geometry of the complex. For example, Ni(N 2)4 appeared by ir to be tetrahedral with end-on linear N2 ligands when frozen in argon, but assumed a distorted tetrahedral shape in a pure N2 matrix.35g.212 Also with Ni, previous discussion indicated that N2-CO-Ni depositions allowed formation of mixed complexes where 7T bonding was favored for CO over N 2, as would be expected.35g.201 Comparing Ni, Pd, and Pt the mono-N 2 complexes (M-N2) showed bonding force constants in the order Ni-N > Pt-N > Pd_N,35g.213 which is the same as for M-CO complexes. L. Dioxygen (0 2 ) with Metal Atoms As with M-N2 matrices, some M-02 matrices yield charged species by electron transfer, and these processes have been discussed in Section IV.3. Other relevant M-02 complexes have been brought together here in Table 20, which is not meant to be comprehensive. For the transition metals generally side-on bonding is favored for O 2, whereas for N2 end-on bonding is usually favored. However, for nontransition metals, such as Ge and Sn, insertion to yield linear O-M-O species readily takes place. Thus, tin and O 2 yield Sn02 and Sn 20 2. It is believed that first Sn and O 2 bond side-on and then insertion occurs to yield a DOOh O-Sn-O species which can react with more Sn atoms to yield a I

I

four-membered Sn-O-Sn-O species. 35f.217 A similar insertion mechanism was TABLE 19. M-(N 2 ) Complexes Prepared by Microscale Metal Atom Techniques VN=N

Complex

(em -1)

Co(N 2) Ni(N 2) Ni(N 2h Ni(N2h Ni(N 2)4 Pd(N 2) Pd(N2h Pd(N'.)3 Pt(N 2) Pt(N2h Pt(N 2h

2101 2090 2106 2134 2175 2213 2234 2242 2170 2198 2212

Comments Side-on bonding 466 em - " linear

VM-N

T d , end-on bonded, but distorted in pure N. matrix 378 em - " linear

VM-N

Doon D 3n , C 2 in solid a-N 2

394 em - " linear End-on and side-on bonding D 3h , end-on bonding

VM-N

References

214 213 213 213 213 213 213 213 213,215,216 213,215,216 213,215,216

129

Metal Atoms as Reactive Intermediates TABLE 20. M-(02) Complexes Prepared by Microscale Metal Atom Techniques Complex

Characteristic spectral features and comments

Ge02 Sn02

D"'h,O-Ge-O D h O-Sn-O "" 0 D2h, sn-sn Kr > Ar > Ne. Also, SF 6 has been found to work well. It is believed that the higher polarizability of Xe allows better van der Waals interaction and therefore better isolation efficiency. It is necessary to carry out slow matrix depositions to prevent warmup due to latent heat of matrix fusion. Temperatures for deposition should be lower than temperature of matrix meltdownj2, as "a rule of thumb." 27J Unfortunately, in metal atom cocondensation experiments, the dense liquidlike layer continually formed just prior to freezing is a medium where unusually rapid metal atoms diffusion could occur, which complicates the studies. Heavy metals are much more mobile under these conditions. Kundig, Moskovits, and Ozin 27J believe that it is in this dense gas liquid that the diffusion of metal atoms occurs. For some metal atoms the diffusion is very efficient upon cocondensation,

Kenneth J. Klabunde

134

and very high concentrations of M2 species have been observed. For Mn-CO depositions MnlCO)IO was found to be the main product even at M/CO ratios of about I I 1000! 27j Evidently Mn has some very unusual properties in terms of mobility in the quasiliquid layer. However, the other first-row transition metal atoms also showed dimerization tendencies, although not as definite as those of Mn. The heavier transition metal atoms such as Re, Rh, and Ir showed much less tendency to diffuse and dimerize, as would be expected considering their increased mass and polarizability. However, Rh 2(CO)s and Ir2(CO)s could be produced in spite of the high masses of Ir and Rh. When Rh(CO)s and Ir 2(CO)s were warmed on the matrix window, transformations took place at - 48°C (Rh) and - 58°C (lr), and Rh 4 (CO)I2 and Ir 4 (CO)I2 were formed. At room temperature further disproportionation to form Rh 6(CO)I6 took place. Kinetic analyses of the dimerization process have been discussed.27i.35h Both a statistical frozen matrix approach (calculable probability that M and Mare neighbors and react to give M 2), and a high mobile metal atom approach (diffusion is rapid in quasiliquid layer) have been used. 27j It was found, not unexpectedly, that the diffusion mechanism approach appears to be corroborated best by experimental results, and that the eventual M2 concentration is proportional to the square of the M/substrate ratio. Concentrations of higher metal aggregates vary as some higher power of the M/substrate ratio (in these analyses "reactive matrices" are assumed). Analysis of the nonreactive matrices kinetically is somewhat more involved (e.g. M + argon + CO), and any further conclusions are not warranted at this time. Further statistical mathematical analyses concerning higher metal clusters have been carried out. These treatments have aided Moskovits and Hulse 226 in their attempts to establish the nitrogen size in matrix experiments where (Ni)nCO species were produced. For Ni 2CO two ir Vco absorptions were observed (1973 and 1938 cm- I), which could be due to linear Ni 2-CO and bridged Ni 2-CO:

o II

/C"

Ni-Ni-CO

Ni-Ni

Ni-CO

NiaCO

NiaCO

1973 cm- 1

1938 cm- 1

1999 cm- 1

1970 cm- 1

1963 cm- 1

(terminal)

However, it is disconcerting to compare the Ni-CO Vco band at 1999 cm- I (highest for series) with Vco for CO adsorbed on silica supported Ni particles, which show several bands above 2000 cm -1. Moskovits, Ozin, and Hulse35h.226 question whether silica-supported (Ni)n is zero valent or not or whether just one CO/Ni is adsorbed under such conditions. It appears that a great deal of work is still needed to unravel these puzzling questions.

/35

Metal Atoms as Reactive Intermediates C. Small Discrete Organometallic Clusters (Macroscale)

Although the potential for production of small cluster compounds employing macro scale metal atom techniques is very great indeed, only a small amount of work has been done. This potential lies in three experimental areas: (a) First the dispersion of M into a weakly stabilizing L matrix, such that on warming M(L)m decomposes slowly to yield small metal particles (M)n (where n is large). Then, trapping of (MML')m (where n is small) could be carried out by the addition of stabilizing L' at the correct time and temperature. We have warm

M+L ------+ M(L)m

)

(M).(L)m (n;:S 10)

trap (M)n(L')m

(

1 1

add L'

(n;:S 10)

(M)n

metal particles

+ - - (M).(L)m

(n»

10)

demonstrated the feasibility of this process by addition of CO to a Co-toluene complex at various temperatures to yield CO 2(CO)8 and/or C0 4(CO)12, C0 6(CO)16, and higher clusters. (b) The formation of stable, isolable clusters by dispersion of metal atoms into reactive matrices to form unstable M-(l)m complexes that spontaneously form small cluster compounds on warming and workup (l' can also be present). This spontaneity would be due to favorable kinetic or thermodynamic factors which would depend on M and/or L(L').

~ L'

warming . spontaneous

warmmg

(M).(LL')m stable cluster

(M).(L)m stable cluster

Our work in this area has involved Co, Ni, and Pd with acetylenes and CO.227 For example, Ni atoms cocondensed with excess hexafluoro-2-butyne (HFB) with subsequent addition of CO and warming yielded mainly the known 228 cluster Ni4(COMHFBh54.229 A small amount of Ni(CO)2(HFB) was also formed which was probably the precursor to the cluster. This method serves as a convenient preparation of this cluster without the need for Ni(CO)4 or highpressure equipment [normal procedure Ni(CO)4 + HFB ~ ].228

136

Kenneth J. Klabunde

With this background work, we attempted preparations of analogous (MMCOMHFBh clusters with metals where stable metal carbonyls were not available as starting materials. With Pd the preparation

M

+ CFaC===CCFa

----+ M

M = Ni,Pd

worked very well to yield the new cluster (PdMCOMHFB)3.54.229 With Pt, however, a complex formed which did not contain CO. This experiment led us to attempt preparations of Co, Ni, Pd, Pt, and Au complexes with only HFB and, in each case, we discovered stable (MMHFB)m clusters or mononuclear complexes. However, these materials were extremely labile, readily decomposing CF 3 M

+

HFB ----+ ~ (M).(HFB)m

0

CF3=$CF3

ready decomposition ;;:25°C

- - - - - - - - - + ) (M).

+

CF 3

eF 3

CF3

to yield the benzene trimer of HFB. Purposely decomposing the Ni-HFB and Pd-HFB clusters with HCI followed by quantitative analyses of MCI 2, CF 3 CH=CHCF 3, and dimer C a F12H2 yielded Ni:HFB and Pd:HFB ratios of HC)

(M).(HFB)m ----+ (MCI 2 ).

+

(CFaCH=CHCFa)m

1: I and I: I, respectively.229 Ir analyses of the Ni-HFB and Pd-HFB clusters showed that the Vc==c stretching frequencies varied considerably for the M 4(COMHFBh clusters. These analyses indicate a stronger backbonding ability for N i over Pd in these materials: (NiMCF3C-CCF 3)n (PdMCF 3C--CCF3)n Ni 4(CO)4(CF 3C-CCF 3h Pd 4(COMCF3C CCF 3)3

vc==c: Vc==c: Vc,=c: Vc=c:

1695 cm- 1 1675 cm- 1 1570,1552 cm -1 1600cm- 1

vco: Vco:

2120,2110 cm- 1 2138,2122cm- 1

The molecular weights of the M-HFB complexes indicated the existence of labile Ni(HFB)(solvent) and PdiHFBMsolvent) [solvent = acetophenone] compounds.

Metal Atoms as Reactive Intermediates

137

(c) The third possible area for macro scale discrete cluster work would deal with the reaction of metal atoms with other metal-containing compounds, in particular, other clusters. For example, metal atom oxidative insertions into other M-L bonds may yield new "ligand-deficient" clusters. So far this metal atom research area has not been investigated. M

+

(M').(L)m ~ [(M')n-(M)(L)mI

D. Large Clusters (Small Metal Particles or Crystallites, Metal Slurries) (Macroscale) a. Active Metals by Metal Atom Methods As mentioned previously, metal particles or crystallites can be formed from dispersed metal atoms in weakly coordinating solvents. If the dispersing" solvent" is really inert to the metal atoms, this process is actually a metal crystallization process from different solvents. It holds the promise that differing crystal faces and/or reactive sites, surface areas, and ligand surface protection will be obtainable for the same metal with different solvents. Indeed, we have observed differing chemical reactivity for the same metal "crystallized" from different solvents,271.38.54.222.225 for metals Mg, Fe, Ni, Zn, Cd, AI, In, Sn, Pb, and Te. This clustering process has discrete stages,38 as shown in Figure 26. Stage I involves the formation of a weak a or 7r complex when excess (> 30: I) solvent is

M atoms

+

solvent vapor

-196°C

--------+) M-solvent complex cocondense (colored) (I)

lmelt (Mn)-solvent (black, slurry) (III) vaporize

excess

further

~(---­

warming

M-solution ( II)

j

solvent

25 c C pyrolysIs

-------+) (M)n-solvent adduct (black, small crystallites)

(Mn)

+

organics (V)

(IV)

FIGURE 26. Scheme for metal atom clustering (crystallization) from weakly complexing solvents. 38

Kenneth J. Klabunde

138

codeposited with the metal vapor. Stage II results with some solvents upon lowtemperature meltdown of the matrix. If well diluted, a homogeneous metal atom (or metal telomer) solution forms. On further warming small metal particles form, and the final size and shape of these particles is dependent on the solvent employed. Evaporation of excess solvent yields stage IV with little apparent sintering on going from III to IV. Each metal will be briefly discussed. Nickel. For Ni, examination of the stage IV particles has been carried out in some detail employing scanning electron microscopy, X-ray powder methods, and chemical methods. 38 Table 21 summarizes some of the data collected for Ni from hexane, from toluene, and from tetrahydrofuran (THF), compared with a commercial sample of Raney Ni. Nickel from alkanes has been studied most. Since the ability of Ni atoms to migrate and cluster in a low-temperature matrix should be proportional to the solvent's permeability (glassiness or viscosity) and ability to complex, we undertook a study of a series of alkanes with Ni. With alkanes, very weak complexation should occur, and so any differences in particles obtained should simply be dependent on matrix permeability. We did find differences in the Ni particles obtained from different alkanes. It now appears, however, that the amount of alkane employed is more important than the structure of the particular alkane. The Ni particles (stage TV, Figure 26) from pentane were very small and nonferromagnetic if very large pentane: Ni ratios were employed. If less pentane was employed, larger ferromagnetic particles were obtained.54.230 Either of these samples were extremely active hydrogenation catalysts, as well as reactive with alkyl halides. These particles were extremely pyrophoric, and potent scavengers TABLE 21. Physical Data on Ni Particles from Hexane, Toluene, and THF38 M (solvent) Ni(hexane) Ni(toluene)

Ni(THF)

Raney Ni d a b

c d

e

Solvent: metal ratio·

Surface area (m 2 /g)b

Particle size" (I'm)

I :25

45 100 400 80-100

5 x 20 rough pieces c 0.5-0.8 spheres 0.5-1.5 spheres 90% 2-40

~

~1:40

~1:3

DetermIned by desorptIOn of organiCS upon pyrolYSIS BET methods used except With NI-THF, where solvent and gas desorption was employed to calculate surface area. Very rough edges and an average Size. Grace #28. Crystallite sizes could not be determIned quantitatively due to the large lIne broadenmgs observed These Wide lines Indicate crystallite sizes < 100 A, a regIOn where the Scherrner equatIOn does not apply Relative crystallite sizes were determIned by companng the broad lIne COInCident With the III reflectIOn (d spaCIng 201 A) for commerCial NI powder utiliZIng CU K. radiatIOn Raney NI (20 broadenIng of 2"), NI-hexane (30 broadening), NI-toluene (4° broadening), NI-THF (8 0 broadenIng), commercial NI powder (0 5° broadenIng). More detaJied analYSIS of the NI-toluene showed only the one broad lIne COInCident With the 111 reflectIOn, whereas NI-THF showed two very broad lines, one correspondIng to the 111 reflectIOn and the other at lower angle With d spacIng of about

4A.

Metal Atoms as Reactive Intermediates

139

of traces of oxygen. They are quite stable thermally with gross sintering occurring at about 250°C. The particle surfaces are covered with organic fragments from the original alkane, and heating with H2 released small organic molecules such as CH 4, C 2 H a , C 3H 8, and others.230 Currently, detailed studies of reactions with D2 and the resultant products are being studied. Nickel particles from aromatic solvents have been obtained. Nickel particles from toluene precipitate out as tiny spheres, which are reactive with alkyl halides, but fairly unreactive as a hydrogenation catalyst. 38 The particles are ferromagnetic or non ferromagnetic, depending on the toluene: Ni ratios employed. The Ni clustering in toluene occurs above - 96°C, and so Ni-atomtoluene solutions are obtainable, and serve as a useful source of active Ni atoms (solvated Ni atoms).38 When Ni and THF were cocondensed a yellow matrix was formed consisting of a low-temperature stable Ni-THF etherate. Warming of the Ni-THF complex resulted in black streams of Ni-THF flowing to the bottom of the reactor. The resulting Ni-THF slurry was very finely divided and totally syringeable. The dry Ni-THF powder served as a poor hydrogenation catalyst but a very good catalyst for cyclohexene disproportionation to cyclohexane and benzene. A great deal of residual THF remained strongly bound to the Ni particles. The particles are surprisingly uniform as tiny spheres. 38 The Ni-THF slurry was allowed to react with a series of alkyl halides forming NiX 2 and reduction products of RX, the necessary hydrogen coming apparently from the THF. Table 22 summarizes some of these results.54.231 In the past several years we have extended these studies to many main group metals, preparing slurries of metals in THF, diglyme, dioxane, trioxane, toluene, hexane, and pentane. Generally, we have found that the ether solvents yield the finest, smallest particle slurries, which are usually totally syringeable. However, with toluene, hexane, and pentane, particle sizes are larger but are usually very TABLE 22. Products of Reactions of Ni-THF Slurries with Organohalides 231 Organohalide Allyl bromide Allyl iodide Benzyl chloride Todobenzene Bromobenzene Chlorobenzene \-Bromobu tane 2-Bromobutane 2-lodopropane 1-Bromopropane

Products" Propene (19%), I ,5-hexadiene (23 '70)' \ ,4-hexadiene (21 '70) Propene (47'70)' I ,5-hexadiene (8%), \ ,4-hexadiene (5'70) Bibenzyl (33'70)' toluene (2'70) Biphenyl (6%) Biphenyl (0'70) Biphenyl (0'70) Butane (21 '70), I-butene (7'70)' cis-2-butene (2'70)' trans-2-butene (I '70) Butane (4'70)' I-butene (0.6%), cis-2-butene (0.6%), trans-2-butene (0.5%) Propane (17'70)' propene (9'70) Propane (15'70)' propene (7'70)

" After heating under reflux at 55-60°C for several hours, yields based on organohalide converted.

140

Kenneth J. Klabunde

active in alkyl halide reactions, and often prove to be of the most interest because they allow preparation of non solvated organometallics. Magnesium. The very first work carried out of these laboratories on the preparation of active metal slurries was inspired by the fascinating work of Rieke 232 on the generation of active Mg metal by reduction of MgCl 2 with potassium (cf. Section IV.5.D.b). MgCl 2

+ 2K

- - * Mg

+ 2KCI

(Rieke method 232 )

We anticipated being able to prepare active Mg particles in the absence of KCI salt and in any type of solvent desired (the Rieke method requires solvents in which MgCI2 is partially soluble). Indeed, we were able to produce active Mg slurries in THF and hexane. 222 The Mg-THF slurries reacted with CSH5Br at -78°C, with CsH5Cl at 25°C, and CsF5Brand CF 3Brat - 30°C. 222 The Mg-THF CoHsBr

+ Mg-THF

-78'C

----+)

C 6 HsMgBr

and Mg-hexane slurries did show very high reactivity, but not as high as Rieke's Mg. For example, the Rieke Mg reacted with CSH5F to yield CSH5MgF, whereas the Mg-THF and Mg-hexane slurries made by the metal atom method did not. Zinc. Active slurries of Zn in THF, diglyme, dioxane, toluene, and hexane have been prepared. 225 Alkyl bromides reacted with the Zn in any of these solvents to give good yields of R2Zn. This is the first report that active Zn in both polar and nonpolar solvents is available. In the noncoordinating solvents the R 2Zn compounds were readily isolated simply by vacuum distillation from the reaction mixtures. 231 Cadmium. We have described the first examples of direct reaction of Cd metal with alkyl halides in both polar and nonpolar solvents. 225 Cadmium slurries in diglyme, dioxane, THF, toluene, and hexane were easily prepared in 9-g batches and allowed to react with RI to yield RCdl in yields of 55-8370' with the diglyme slurry found to be superior. Aluminum. Al-diglyme and AI-dioxane slurries reacted readily with CSH5X (X = CI, Br, I) to form (CsH5hAIX.231 Indium. In-diglyme and In-dioxane slurries reacted readily with C2H51 to form (C2H5)2InI and Inl. 231 Tin. Sn-THF slurries reacted with CH31 to yield a mixture of CH 3SnI 3, (CH3)2SnI2, and (CH 3 hSnl. Of more interest is the reaction of CF3I with SnTHF yielding (CF3)2SnI2.231 With CF3Br, a mixture of (CF 3hSnBr, CF3SnBr3, and (CF3)2SnBr2 was obtained, while CSF5Br yielded (CSF5hSnBr. Reaction of CsH5I with Sn-THF yielded only Sn1 4 , however. Lead. Lead slurries in THF or diglyme reacted with CH3I to give small yields of (CH 3hPbl, as the first example of a direct Pb-RX reaction. 231

141

Metal Atoms as Reactive Intermediates

Tellurium. This element vaporizes extremely readily in the form of Te telomers. Addition of CH3I to a boiling Te-pentane slurry gave (CH 3)2 TeI 2 in 80% yield. Similarly, a Te-pentane-C 2H 5I reaction yielded (C2H5)2TeI2 in 30/0 yield. 233 b. Active Metals by Metal Salt Reductions (A Comparison) Rieke and coworkers have also extended their active metals by reduction work in the past several years. They have prepared and studied Mg,232 Zn,234 In,235 AI,236 and transition metals Cr,237 Ni,238 and Pd. 238 The Rieke reduction technique and our metal atom technique for production of active metal slurries have both found considerable use, and have turned out to MCI, M

+

+K

solvent

------+)

M* slurry

+

KCl

(Rieke and co-workers)

-196P C

solvent ------+) M-solvent complex ----+ ----+ M* slurry

(Klabunde and co-workers)

be complementary to each other. The Rieke method enjoys the advantages of: a) simplicity of required laboratory equipment, and (b) in some instances higher metal reactivity. It suffers from the disadvantages of the need for a choice of solvent that dissolves some of the metal salt employed, and the fact that some metal salts are not cleanly reduced, in particular the salts of the transition metals. The metal atom method for slurry preparations enjoys the advantages of (a) absence of potassium or potassium halide, and (b) use of both polar and nonpolar solvents. The method suffers from the disadvantage of the need for more complex equipment (metal atom reactor). ACKNOWLEDGMENTS

We gratefully acknowledge the generous and steady support of the National Science Foundation. Also, the author truly appreciates the outstanding work of students (chronologically) James Y. F. Low, M. S. Key, Curt White, Howard F. Efner, John S. Roberts, Bruce B. Anderson, Thomas O. Murdock, W. J. Kennelly, Michael Bader, Michael Brezinski, R. Roppel, K. Neuenschwander, Stephen C. Davis, Daniel Ralston, and Thomas Groshens. Also, collaborative work with Professor Lewis Radonovich is greatly valued.

V. REFERENCES I. (a) G. C. Bond, Catalysis by Metals, Academic Press, New York (1962); (b) G. C.

Bond, Heterogeneous Catalysis: Principles and Applications, Clarendon Press, Oxford (1974). 2. J. H. Sinfelt, Ace. Chen!. Res. 10, 15 (1977), and references therein.

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145. (a) E. M. Van Dam, W. N. Brent, M. P. Silvon, and P. S. Skell, J. Arn. Chern. Soc. 97, 465 (1975); (b) T. S. Tan, J. L. Fletcher, and M. J. McGlinchey, J. Chern. Soc. Chern. Cornrnun., 771 (1975). 146. P. L. Timms, private communications. 147. W. J. Kennelly, unpublished results from this laboratory. 148. (a) B. Bogdanovic, M. Kroner, and G. Wilke, Ann. Chern. 699, 1 (1966); (b) G. Wilke, Angew. Chern. Internat. Ed. Engl. 2, 105 (1963). 149. M. Green, J. A. K. Howard, J. L. Spencer, and F. G. A. Stone, J. Chern. Soc. Chern. Cornrnun., 449 (1975). 150. P. S. Skell, E. M. Van Dam, and M. P. Silvon, J. Arn. Chern. Soc. 96, 626 (1974). 151. M. Yevitz and P. S. Skell, unpublished results. 152. D. L. Williams-Smith, L. R. Wolf, and P. S. Skell, J. Arn. Chern. Soc. 94, 4042 (1972). 153. (a) E. Koerner von Gustorf, O. Jaenicke, and O. E. Polansky, Angew. Chern. 84, 547 (1972); Angew. Chern. Internat. Ed. Engl. 11, 532 (1972); (b) J. R. Blackborow, R. H. Grubbs, A. Miyashita, A. Scrivanti, and E. A. Koerner von Gustorf, J. Organornet. Chern. 122, C6 (1977). 154. H. F. Efner, R. R. Smardzewski, D. E. Tevault, and W. B. Fox, private communications. 155. M. Brezinski, W. Kennelly, and T. Groshens, unpublished results from this laboratory. 156. K. J. Klabunde, T. Groshens, M. Brezinski, and W. Kennelly,J. Arn. Chern. Soc. 100, 4437 (1978). 157. T. Groshens, unpublished work from this laboratory. 158. E. O. Fischer and W. Hafner, Z. Naturforsch. B 10, 665 (1955). 159. M. P. Silvon, E. M. Van Dam, and P. S. Skell, J. Arn. Chern. Soc. 96, 1945 (1974). 160. K. J. Klabunde and H. F. Efner, Inorg. Chern. 14, 789 (1975). 161. P. L. Timms, J. Chern. Ed. 49,782 (1972). 162. R. Middleton, J. R. Hull, S. R. Simpson, C. H. Tomlinson, and P. L. Timms,J. Chern. Soc. Dalton, 120 (1973). 163. M. J. McGlinchey and T. S. Tan, Can. J. Chern. 52, 2439 (1974). 164. M. J. McGlinchey and T. S. Tan, J. Arn. Chern. Soc. 98, 2271 (1976). 165. V. Graves and J. J. Lagowski, lnorg. Chern. 15,577 (1976). 166. A. N. Nesmeyanov, N. N. Zaitseva, G. A. Dormrachev, V. D. Zinovev, L. P. Yureva, and I. I. Tverdokhlebova, J. Organornef. Chern. 121, C52 (1977). 167. P. S. Skell, private communications. 168. H. F. Efner, unpublished results, from this laboratory. 169. G. Essenmacher (with P. Treichel), Ph.D. Thesis, University of Wisconsin, Madison, Wisconsin (1976); collaborative work with H. Efner and K. J. Klabunde. 170. L. Radonovich, C. Zuerner, H. F. Efner, and K. J. Klabunde, Inorg. Chern. 15,2976 ( 1976). 171. L. Radonovich and C. Zuerner, unpublished work. 172. J. Boyd, J. Lavoie, and D. M. Gruen, J. Chern. Phys. 60,4088 (1974). 173. L. H. Simons, P. E. Riley, R. E. Davis, and J. J. Lagowski, J. Arn. Chern. Soc. 98,1044 ( 1976). 174. P. E. Riley and R. E. Davis, lnorg. Chem. IS, 2735 (1976). 175. P. L. Timms, J. Chern. Soc. A, 2526 (1970). 176. D. Staplin and R. W. Parry, Symposium on "Atomic Species as Synthetic Reagents," 170th National ACS Meeting, Chicago, Illinois, August 1975, paper INOR 117. 177. J. J. Havel, Ph.D. Thesis, The Pennsylvania State University, University Park, Pennsylvania (1972). 178. R. Cable, M. Green, R. E. Mackenzie, P. L. Timms, and T. W. Turney, J. Chern. Soc. Chern. Cornrnun., 270 (1976). 179. D. Gladkowski and F. R. Scholar, 171 st (Centennial) National ACS Meeting, New York, April 1976, paper INOR 133. 180. R. N. Perutz and J. J. Turner, J. Arn. Chern. Soc. 97, 4791, 4800 (1975).

148

Kenneth J. Klabunde

181. M. Poliakoff and J. J. Turner, J. Chern. Soc. Dalton, 1351 (1973); 2276 (1974). 182. O. Crichton, M. Poliakoff, A. J. Rest, and J. J. Turner, J. Chern. Soc. Dalton, 1321 (1973). 183. L. Hanlan, H. Huber, E. P. Kiindig, B. McGarvey, and G. A. Ozin,J. Arn. Chern. Soc. 97, 7054 (1975). 184. R. L. DeKock, Inorg. Chern. 10, 1205 (1971). 185. M. Poliakoff, J. Chern. Soc. Dalton, 210 (1974). 186. H. Huber, E. P. Kiindig, M. Moskovits, and G. A. Ozin, J. Arn. Chern. Soc. 97, 2097 (1975). 187. M. A. Graham, M. Poliakoff, and J. J. Turner, J. Chern. Soc. A, 2939 (1971). 188. E. P. Kiindig and G. A. Ozin, J. Arn. Chern. Soc, 96, 3820 (1974). 189. H. Huber, E. P. Kiindig, G. A. Ozin, and A. J. Poe, J. Arn. Chern. Soc. 97, 308 (1975). 190. (a) L. Hanlan, H. Huber, and G. A. Ozin, Inorg. Chern. 15,2592 (1976); (b) T. A. Ford, H. Huber, W. Klotzbiicher, E. P. Kiindig, M. Moskovits, and G. A. Ozin, Inorg. Chern. 15, 1666 (1976). 191. H. Huber, M. Moskovits, and G. A. Ozin, unpublished results. 192. M. A. Graham, M. Poliakoff, and J. J. Turner, J. Chern. Soc. A, 2939 (1971). 193. E. P. Kiindig and G. A. Ozin, J. Arn. Chern. Soc. 96,5585 (1974). 194. E. P. Kiindig, M. Moskovits, and G. A. Ozin, Can. J. Chern. 50, 3587 (1972). 195. J. H. Darling and J. S. Ogden,J. Chern. Soc. Dalton, 1079 (1973); Inorg. Chern. 11,666 (1972). 196. E. P. Kiindig, D. McIntosh, M. Moskovits, and G. A. Ozin, J. Arn. Chern. Soc. 95, 7234 (1973). 197. D. McIntosh and G. A. Ozin, J. Arn. Chern. Soc., 98,3167 (1976). 198. J. H. Darling, M. B. Garton-Sprenger, and J. S. Ogden, J. Chern. Soc. Faraday Trans. 2, Syrnp., 75 (1973). 199. J. S. Ogden, Chern. Cornrnun., 978 (1971). 200. (a) J. L. Slater, R. K. Sheline, K. C. Lin and W. WeItner, J. Chern. Phys. 55, 5129 (1971); (b) J. L. Slater, T. C. DeVore, and V. Calder, Inorg. Chern, 13,1808 (1974); 12, 1918 (1973). 201. E. P. Kiindig, M. Moskovits, and G. A. Ozin, Can. J. Chern. 51, 2737 (1973). 202. M. Poliakoff and J. J. Turner, J. Chern. Soc. A, 2403 (1971). 203. L. Hanlan and G. A. Ozin, J. Arn. Chern. Soc. 96, 6324 (1974). 204. L. Brewer and C. Chang, J. Chern. Phys. 56, 1728 (1972). 205. J. M. Brom, W. D. Hewett, and W. Weltner, J. Chern. Phys. 62, 3122 (1975). 206. D. W. Green and D. M. Gruen,J. Chern. Phys. 60,1797 (1974); 57, 4462 (1972). 207. R. P. Eischens, S. A. Francis, and W. A. Pliskin, J. Phys. Chern. 60,194 (1956). 208. G. Blyholder and M. C. Allen, J. Arn. Chern. Soc. 91, 3158 (1969). 209. A. J. Hinchcliffe, D. D. Oswald, and J. S. Ogden, J. Chern. Soc. Chern. Cornrnun., 338 (1972). 210. A. J. Hinchcliffe and D. D. Oswald, unpublished results. 211. A. Bos, Chern. Cornrnun., 26 (1972). 212. H. Huber, E. P. Kiindig, M. Moskovits, and G. A. Ozin, J. Arn. Chern. Soc. 95, 332 (1973). 213. (a) W. Klotzbiicher and G. A. Ozin, J. Arn. Chern. Soc. 97, 2672 (1975); (b) H. Huber, E. P. Kiindig, M. Moskovits, and G. A. Ozin, J. Arn. Chern. Soc. 95, 332 (1973). 214. G. A. Ozin and A. Vander Voet, Can. J. Chern. 51, 637 (1973). 215. E. P. Kiindig, M. Moskovits, and G. A. Ozin, Can. J. Chern. 51, 2710 (1973). 216. D. W. Green, J. Thomas, and D. M. Gruen, J. Chern. Phys. 58, 5453 (1973). 217. A. Bos and J. S. Ogden, J. Phys. Chern. 77, 1513 (1973). 218. A. Bos, J. S. Ogden, and L. Orgee, J. Phys. Chern. 78, 1763 (1974).

Metal Atoms as Reactive Intermediates

149

219. (a) H. Huber and G. A. Ozin, Can. J. Chern. 50, 3746 (1972); (b) H. Huber, W. KlotzbUcher, G. A. Ozin, and A. Vander Voet, Can. J. Chern. 51, 2722 (1973); (c) D. McIntosh and G. A. Ozin, Inorg. Chern. 15,2869 (1976). 220. Z. Zsigmandy and P. A. Thiessen, Das Kolloide Gold, Akademische Verlagsgesellschaft, Leipzig (1925); N. Uyeda, M. Nishiro, and E. Suito, J. Colloid Interface Sci. 43, 264 (1973). 221. M. Faraday, Philos. Trans. R. Soc. London 147, 145 (1875). 222. K. J. Klabunde, H. F. Efner, L. Satek, and W. Donley, J. Organornet. Chern. 71, 309 (1974). 223. (a) J. Venables and G. L. Price in J. W. Matthews, Epitaxy, Academic Press, New York (1975), Chapter 4; (b) R. Niedermayer, in Advances in Epitaxy and Endotaxy, H. G. Schneider and V. Ruth, Eds., Deutscher Verlag fUr Grundstoffindustrie, Leipzig, (1974); (c) V. Halpern, J. Appl. Phys. 40, 4627 (1969); (d) K. J. Routledge and H. J. Stowell, Thin Solid Filrns 6, 407 (1970). 224. (a) K. J. Klabunde, M. Brezinski, W. Kennelly, T. Groshens, unpublished results; (b) W. Worthy, Chern. Eng. News, 23 (January 24, 1977). 225. T. O. Murdock and K. J. Klabunde, J. Org. Chern. 41, 1076 (1976). 226. M. Moskovits and J. Hulse, Surf Sci. 57, 125 (1976). 227. M. Brezinski, unpublished work from this laboratory. 228. (a) J. L. Davidson, M. Green, F. G. A. Stone, and A. J. Welch, J. Arn. Chern. Soc. 97, 7490 (1975); (b) R. B. King, M. I. Bruce, J. R. Phillips, and F. G. A. Stone, Inorg. Chern. 5,684 (1966). 229. K. J. Klabunde, T. Groshens, M. Brezinski, and W. Kennelly, J. Arn. Chern. Soc. 100, 4437 (I978). 230. (a) S. Davis and K. J. Klabunde, J. Arn. Chern. Soc., 100, 5973 (1978); (b) K. J. Klabunde, S. Davis, and C. White, unpublished. 231. K. J. Klabunde and T. O. Murdock, unpublished results. 232. R. D. Rieke and S. E. Bales, J. Arn. Chern. Soc. 96, 1775 (1974); R. D. Rieke and P. Hudnall,J. Arn. Chern. Soc. 94, 7178 (1972); J. Chern. Soc. Chern. Cornrnun., 879 (1973). 233. C. King, K. Irgolic, and K. J. Klabunde, unpublished work. 234. R. D. Rieke, S. J. Uhm, and P. M. Hudnall,J . Chern. Soc. Chern. Cornrnun., 269 (1973). 235. L. Chung Chao and R. D. Rieke, Syn. [norg. MetalOrg. Chern. 4, 373 (1974); L. Chung Chao and R. D. Rieke, J. Organornet. Chern. 67, C64 (1974). 236. R. D. Rieke and L. Chung Chao, Syn. React. Inorg. MetalOrg. Chern. 4, 101 (1974). 237. R. D. Rieke, M. Ofele, and E. O. Fischer, J. Organornet. Chern. 76, CI9 (1974). 238. R. D. Rieke, W. J. Wolf, N. Kujundzic, and A. Kavaliunas, J. Arn. Chern. Soc. 99, 4159 (I 977).

3 Aminium Radicals Yuan L. Chow

l. INTRODUCTION The commonly known isoelectronic radicals of carbon, amino, and alkoxy radicals and chlorine or bromine atoms are well-investigated species. A glance at these radicals reveals one significant aspect, namely, that radicals other than carbon may enter into acid-base equilibria with suitable proton (or carbenium ion) donors to form cation radicals. Chemical intuition also leads us to suspect that alkoxy (or ether) and hydrochloride (or alkyl chloride) cation radicals are high-energy species and form only under highly acidic conditions or on highenergy electron bombardment (mass spectroscopic conditions). Indeed, H 2 0t radical is proposed to have pKa less than zero. l Likewise, the less electronegative nitrogen atom may render amino radicals more readily protonated under ordinary

I

-C-

-N-

-0.

:Cl·

+H+j[-H+

j[

j[

-NH!

-OH!

H-Cl·

I

•• +

acidic conditions in an aqueous system. It is no wonder, therefore, that aminium radicals, i.e., cation radicals derived from amines, were proposed as reactive intermediates in 1950,2 and that their chemistry has attracted considerable attention in recent years, as shown by the number of reviews published. 3 - 15 This chapter will be devoted to a summary of all types of reactions that may be rationally interpreted to involve aryl- and alkylaminium radicals as the reactive intermediates as proposed by the respective authors or as extrapolated from these proposals. At present, besides those isolable and well-characterized R2NH~

RNH 2 ;

R = alkyl or aryl

Yuan L. Chow • Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A IS6

151

152

Yuan L. Chow

triarylaminium radicals and Wurster's salts, e.g., N,N,N',N'-tetramethylphenylenediamine cation radical,14 relatively few reactions have been established to involve aminium radicals in their pathways. In some amine oxidations the proposal of aminium intermediacy is not necessarily always accepted by others in the same field; this could cause some confusion in the reader's mind. Some examples included in this chapter come from extrapolations by this reviewer of the established results rather than from individual author's proposals and may meet with controversial comments. If this occurs, the reviewer must face the possible sin of prejudicial judgement. But it also points to the need for further work in the area. Recognition of aminium radicals as reactive intermediates in the HofmannLofHer reaction came fairly early,2.3 but that in various amine oxidations by oneelectron transfer reagents such as chlorine dioxide and potassium ferricyanide is rather new. 16 The former reagent is used in the disinfection of water and in the delignification of wood, 17 and the latter in model oxidations patterned after biogenetic pathways.18 Photosensitized oxidations of amines in the presence 19 .20 t or absence of oxygen 13 have been recognized as a possible part of biological reaction systems. 21 Evidence abounds that aminium radicals are the vital link in these sensitized oxidations of amines. 19 Indeed, various cation radicals (for example, 1-4) of nitrogen heterocycles which are building blocks for flavins and vitamins have been generated by flash photolysis or pulse radiolysis, and their R

o N l§:C*~fO \!)

(

'NH

H

0-

H

H

H

C:~IO N NH

H

I R-N~

(E)

HNQ l \ ......+ -_1\

S

N H

0 2

3

4

spectroscopic properties, acid-base equilibria, and kinetic properties have been investigated. 22- 24 Whether or not these biologically active heterocyclic derivatives mediate various life processes through cation radical intermediates is yet unclear; recently, one-electron transfer has gained more attention. 22 An inspection of the literature shows that there is pervasive implication of aminium radicals as reactive intermediates in various reactions involving amines. In recent years, aminium radicals have been included in the individual reviews of those relevant reactions. 3- 15 Persistent aromatic aminium radicals 14 and nitrogen-centered radicals in general 10.15 have also been reviewed. In an attempt to provide comparisons of the behavior of aminium radicals generated under different conditions, this discussion will endeavor to cover all those reactions that use aminium radicals as the reactive intermediates. It is hoped that readers will be able to find the links or missing links among those reaction patterns. t In Reference 20 see also earlier papers cited therein.

153

Aminium Radicals

II. GENERAL METHODS OF AMINIUM RADICAL FORMATION Because of fast acid-base equilibria of primary and secondary aminium radicals, the acidity of the solution used in their generation is vitally related to the true identity of neutral or cation radicals formed. 1.22 Tertiary aminium radicals, when formed, can be treated as bona fide cation radicals since they do not dissociate readily. Formation of aminium radicals can be summarized in the three types of general equations below, the last of which is a simple acid-base equilibrium and requires the generation of amino radical as the preliminary step in acid solution. The first two equations can be formally considered as oneelectron transfers in which equation (I) is a direct, and equation (2) an indirect, one-electron oxidation process. R3N:

+ X+

---+ R 3N;

+ X·

R3N:

+ X+

---+ R3NX + ---+ R3Nt

R2N·

+ H+

---+ R 2NH;

(I)

+ X·

(2)

(3)

Photolysis of nitramines in dilute acid conditions 25 is a typical way of generating aminium radicals according to equation (3), and electrolytic oxidation 11.12 and sensitized photooxidation of amines 13 proceed according to equation (I). Protonation of nitroxides in concentrated sulfuric acid [equation (4)] also forms the stable hydroxyaminium radicals,26 the stereochemistry of which is similar to that of the parent nitroxides. Good evidence has accumulated that (t-Bu),N---O

H+ ~

pK. -

-5.5

+-

(t-BU)2N-OH

±

(4)

I

chlorine dioxide and alkaline ferricyanide (and probably other metal ions of one-electron oxidants) oxidations 16 of tertiary amines also involve direct oneelectron transfer as in equation (I). A large number of reactions may fall into the category of equation (2). Hydrogen abstraction by hydroxyl radical from an ammonium ion, e.g., NH 30H +, is an example of the aminium radical formation by H atom abstraction [equation (5)].I On the other hand, solvated electrons (e aq - ) generate aminium radicals by reductive cleavage [equation (6)], in a similar manner to ferrous-ion-catalyzed decomposition of amine N-oxides in acid solution [equation (7)].27 Both reactions occur in radiolysis, during which OH radical +NH 30H

+ ·OH

-----+ ; NH 20H

(5) (6)

+

-

R3N-O

+ Fe 2+ + H+

-----+

R3N~

+ Fe(OH)2+

(7)

154

Yuan L. Chow

and eaq - are generated. 22 Indeed, Cole generated the parent aminium radical NH3 t trapped in a solid matrix by the radiolysis of NH 4C10 4 and was the first to identify it by its esr spectrum. 28 Most tertiary amine oxidations by chlorine 6 and bromine 29 .3o and their derivatives, such as N-bromosuccinimide,31.:32 hypochlorite,33 and acyl peroxides 33.34 occur via homolysis of the intermediate ammonium ion, as in equation (2). Decomposition of N-chloroamines in highly acidic solution (such as under Hofmann-Lamer reaction conditions 3.6) and photolysis of N-nitrosamines in dilute acid solutions 9 follow the same reaction scheme [equation (2)] as applied to secondary and/or primary amines; in the former the intermediate ammonium salts have been isolated as the conjugated bases for the subsequent homolytic reaction. In addition, triphenylamines and p-yhenylenediamines form electron transfer (charge transfer) complexes with various acceptors (A) in the solid and in various organic solvents. The corresponding aminium radical in the radical pairs has been observed by esr and optical spectroscopy.35 (C 6 H 5 hN: A = 12•

(CN),C=C(CN)"

+A

~

[(C 6 H 5 hN! A ~ I

1,3,5-(N02hC 6 H3, tetrachloro-p-benzoquinone

(8)

Persistent arylaminium radicals are generally prepared by chemical or electrolytic oxidation and are isolated as the salts of strong acids such as perchlorate, phosphorus and antimony hexahalides, or tetrafluoroborate. Their preparations and reactions, as for example those of p-substituted triaryl- and diarylaminium radicals and Wurster's salts were the subject of a detailed review published in 1968.14 In conformance with the title of this monograph, the chemistry of these persistent aminium radicals will not be dealt with in length except where they are relevant.

III. PHYSICOCHEMICAL PROPERTIES OF AMINIUM RADICALS As the lifetimes of aminium radicals range from infinity to less than microseconds, conditions of recording spectra are very much limited by the techniques available. For isolable persistent aminium radicals, the limitation is solvents required to dissolve these radical salts for various types of spectroscopy; esr, uv and visible, and ir spectra and magnetic susceptibility have been determined. 14 For less stable aminium radicals not amenable to isolation, amines may be chemically36.37 or electrochemically oxidized in situ in static 39 or flow systems 37 depending on the reactivity of the aminium radicals and conditions. Visible and ultraviolet spectra of transient aminium radicals are generally determined by applying techniques of flash photolysis 40 and pulsed radiolysis 1.22 under optimum conditions. Highly acidic media, such as H 2 S0 4 or CF 3S0 3H, and low temperature appear to moderate the decomposition of the intermediates and, at times, facilitate recording of spectraY·42

Aminium Radicals

1.

155

~sr ~pectra

The application of electron spin resonance (esr) spectroscopyt has been instrumental in the direct identification of radicals and in answering many questions of structure, stereochemistry, and spin distribution in many radicals; among the parameters, hyperfine splitting constants (hfsc, or a values) are related to the spin density at the radical center and to the extent of s character in the spin-bearing orbital. Esr results for nitrogen-containing radicals remain somewhat ambiguous in detail because of the difficulty in estimating spin density from experimental aN values. Nevertheless, correlations of the splitting constants of aminium radicals with those of carbon radicals as well as approaches from theoretical computations with various refinements28.44-48 have lead to general agreement on a planar geometry about nitrogen for alkylaminium radicalsY Some representative esr parameters of aryl- and alkylaminium radicals are collected in Tables I and 2, respectively. In comparison with their isoelectronic carbon radicals, owing to their formal positive charge aminium radicals should have a somewhat more rigid planar configuration around the nitrogen atom by analogy with that of carbenium ions, and, further, should bear a larger portion of the unpaired electron density at the esr active nitrogen atom. In stereochemical terms, this means that they have a 7T-radical configuration in which the unpaired electron resides primarily in the p orbital of an Sp2 hybridization; the alternative configuration, the a-radical configuration in which the unpaired electron resides in an Sp2 orbital, is less likely. Comparisons of arylaminium radical aN values in Table I show that electrondonating substituents enhance the delocalization of the unpaired electron, resulting in lower aN values, while 0,0' -dimethyl groups hinder the delocalization, causing higher aN and lower spin density in the ring. The aN values for the aminium radical carrying the same substituent (H, p-CH 3 , or p-OCH 3 ) exhibit the decreasing order of tertiary > secondary> primary, while the a H values for the ortho and para hydrogens are in the reverse order. These trends indicate that the extent of coplanarity of aryl rings with the nitrogen radical center increases in the order of tertiary < secondary < primary; indeed, ir studies of several tri-p-tolylaminium radical salts 58 indicate propellerlike configurations for the radicals. The aN/aNH H ratio should be sensitive to deviation from 7T-radical configuration. For dialkyl-,41 diaryl-,36 and monoarylaminium 37 radicals, the values are fairly constant and show average values of 0.84 ± 0.02, 0.84, and 0.80 ± 0.04, respectively. These values are slightly larger than the aN/a NH H for NH3 t and might indicate a small deviation from coplanarity. Forced deviation from coplanarity is revealed in the aN values of the bridgehead aminium radicals 42 in which ring strain and, therefore, the s character of the n orbital at nitrogen increase in the same direction as the aN values, i.e., 7 t < 8 t < 9 t. The aN

t

There are many references available, e.g., Reference 43.

156

Yuan L. Chow

values appear to vary from '" 19 G for the planar configuration 28 of NH3 t to 30.2 G for a near pyramidal configuration in 9 t . It is interesting to note a large long-range hyperfine coupling to 0 hydrogen in 8 t (14.3 G) and the axial 0 hydrogen in 9 t (10.8 G) due to spin interaction transmitted through a bonds. Such a positive spin interaction is largely cancelled in 7t by a negative spin interaction transmitted through the shorter C-7 methylene bridge where a4 H is only 1.8 G. The relatively stable aminium radicals derived from 1,4-diazabicyclo[2.2.2]octane (DABCO, 10) and I ,3,6,8-tetraazatricyclo[4.4.1.1 3.8]dodecane (TATCD, 11) can be readily obtained by anodic oxidation 56 .57 ; the aN values of 16.96 G coupled to two equivalent nitrogens and of 7.09 G coupled to four equivalent nitrogens suggest an extensive spin delocalization through the a bonds and through space. 59 .60 When their aN values are compared with those of 7t and 8 t , almost pyramidal nitrogen configuration of lOt and 11 t may be assumed. The hfs (hyperfine splitting) interaction of f3 hydrogens with the radical center in a 7T radical arises principally by hyperconjugation, which is apparently more effective in a cation radical. This conclusion is suggested by the afiH values of 34.27, 27.36, and 24.68 G for (CH3hNHt, (CH 3hN., and (CH3)2CH., respectively.41.61.62 Theoretical calculations of NH3 t and simple aminium radicals have been carried out by many groups using both semi empirical and ab initio methods with various modifications.28.44-48 The main aim was to find a rational correlation between the experimental and the calculated hfs constants from which the stereochemistry of the radical might be elucidated. Results of various groups appear to agree that simple aminium radicals possess planar 7T configuration with the unpaired electron located primarily in the nitrogen p orbital.

2. Electronic Absorption Spectra Some representative absorption maxima of aminium radicals are collected in Table 3. Recordings of electronic spectra of aminium radicals are subject to similar or ever more severe limitations as those applying to esr determinations of short radical lifetimes and, therefore, to the difficulties involved in achieving enough concentration. Since esr parameters can be analyzed and interpreted with the related theories to yield more information on structure and stereochemical aspects of the radicals, far more work has been carried out using esr rather than uv spectroscopy. As a means of characterization and as a monitor of the concentration changes, uv and visible spectra are useful properties. Generally, aminium radicals show absorption spectra as red shifted compared to those of the corresponding parent amine; e.g., the maxima fortri-p-anisylamine and the tri-p-anisylaminium radical perchlorate are at 300 and 714 nm, respectively.74

5.12 6.99 3.6 4.88

10.16 8.97 9.45 9.77 10.65 10.00 11.06 9.03 8.70 8.06 7.68 7.19 7.39

aN

a Hfsc for the N-CH3 hydrogens. • TFA = Trifluoroacetic acid.

p-NH2C 6 H.NH 2 P-(Me2N ),C 6 H. (p-NH 2 C 6 H.). (P-Me2NC.H.),

(C 6 HshN (p-CH3OC 6 H.hN (p-CH3C 6 H.hN (p-CH3OC 6 H.).NCH 3 (p-CH3C 6 H.).NCH3 p-CH3OC 6 H.N(CH 3), p-CH3C 6 H.N(CH 3), (C 6 H s).NH (p-CH3C 6 H.)2NH (p-CH3OC 6 H.).NH C.HsNH2 p-CH3C 6 H.NH 2 2,4,6-(CH3)aC6 H2NH2

ArNR2

+ •

H

5.67 6.76" 3.97 4.70"

10.27" 11.83 " 10.40" 12.20" 10.98 10.50 9.56 9.58 9.03 8.02

aNR

2.10 1.97 1.08 1.65

2.28 1.22 2.06 2.42 2.90 4.25 5.30 3.46 3.28 2.72 5.82 5.31

a oH

2.10 1.97 1.62 0.734

1.22 0.61 1.03 0.48 1.24 1.82 1.35 1.31 1.06 0.40 1.52 0.87 0.88

a mH

9.58

4.86

3.32

apH

11.74(p-CH3)

4.62(o-CH 3)

12.40(CH3)

6.IO(CH3) 1.06(OCH3)

0.61(OCH3) 3.89(CH3) 0.97(OCH3) 5.31 (CH3) 1.85(OCH3) 10.00(CH3)

a~tber

electrolysis at pH 4.8 (buffer) CIO.-salt oxidation by I, in CH3CN at - 40° C oxidation by AgCIO. in acetone

S02-BF3 or BF3-CH 2C1 2 CH3CN, electrolysis CH 3CN, electrolysis CH 3CN, electrolysis CH 3CN, electrolysis CH3CN, electrolysis CH 3CN, electrolysis Pb(OAc). in TFA,· flow system Pb(OAc). in TFA, flow system Pb(OAc). in TFA, flow system Pb(OAc). in CH 2Cl-TFA, flow system Pb(OAc). in CH2CI-TFA, flow system Pb(OAc). in CH 2CI-TFA, flow system

Conditions

TABLE 1. Esr Hyperfine Splitting Constants (in Gauss) of Arylaminium Radicals ArNR2

50 51 52 52

49 38 38 38 38 38 38 36 36 36 37 37 37

Ref.

~

~

..... v,

'I

.,.~

~

~

~

i:;.

:i.

+.

+.

+.

+.

llt

[(CHahCH], N +. (CHahCNHCHa St 6t 7t St 9t lOt

(CHaCH2CH~2NH

(CHaCH 2hNH

CHa)2 N H

+.

(CHa),N

NHa

+.

RR'R"N

7.09 c

19.5 19.3 18.0 20.7 20.55 19.23 19.13 19.28 18.65 18.58 18.7 19.2 18.57 21.57 21.6 25.1 30.2 16.96 a 17.02 a

aN

+.

H

21.96 22.61 22.73 22.24 21.81 22.5 22.2 21.35

25.9 25.8

aNH

7.34' 7.27' 7.68, 4.14

26.7 28.5 28.56 33.61 34.16 34.27 37.19 32.41 21.7 34.5

apH

2.0044 2.0037 2.00357 2.00354 2.00412 2.0036 2.0036 2.0036 2.0036 2.0036 2.00382 2.0046

2.0035

g factor

X-irrad. of solid NH.ClO. y-irrad. of solid NH.Cl04 at 77 K y-irrad. of solid (CHa)4NCI CI.-(CHa)aN, hv in CFaSOaH, -50-160° C Radiolysis of (CHahN in aqueous HClO. Radiolysis of (CHahNH in aqueous HClO4 (CHahNCl, hv in CFaC02H (CHahNCI, hv in 90% H 2S04 at 31°C Et 2NCI, hv in 90% H 2SO. at 31°C n-Pr 2NCI, hv in 90% H 2S04 at 31°C iso-Pr2NCl, hv in 90% H 2 S0 4 at 31°C t-BuMeNCl, hv in 90% H 2S04 at 31°C N-chloramine, hv in CFaC02H The nitroxide in H 2SO4 Cl, + amine, hv in CFaSOaH, - 50°C CI 2 + amine, hv in CFaSOaH, O°C CI 2 + amine, hv in CFaSOaH, -40°C Pt anode oxidation in CHaOH, flow system Amine + (p-BrC 6 H.hNt in CaH 7 CN, -100°C

Conditions

TABLE 2. Esr Spectra of Alkylaminium Radicals RR'R"N

28 53 54 42 55 55 26 41 41 41 41 41 26 26 42 42 42 56 57 57

Ref.

......

v,

~

~

c

Q

!:'"'

;::

I:)

00

a

C

b

295

9;

,,-vi.

~N

Split by two equivalent nuclei. Split by 12 equivalent nuclei. Split by four equivalent puclei.

15.1

/'

"t;·1.80

2.95

lOt

P'I Cr::-}

6;

5:

2.95

CH 3 CH 3

N~·OH

~~3

G

CH

3

CH 3 CH 3

NH;

~~3

280 nm).99 The reaction of aminium radicals with simple alcohols involves H abstraction from the a positions and its rate increases in the order MeOH > EtOH > isoPrOH under identical conditions; the same order of reactivity has been found for hydroxyl,100 methyl,lOl and phenyl 102 radicals. It is surprising that t-BuOH is as reactive as MeOH. These rate constants place the reactivity of the piperidinium radical toward H abstraction very close to that of the hydroxyl radical 100 but much higher than that of the methyl radical. 101 Piperidinium radicals (and most alkylaminium radicals) preferentially attack the carbon-carbon double bond rather than abstract allylic hydrogens in reaction with cyclohexene or other simple alkenes. 9 For example, the reactivity ratio of the piperidinium radical toward cyclohexene and methanol is ca. 5000. As shown in Table 8 the tbutoxy and the acetamido radicals react with olefinic and nonolefinic substrates without large differences in the rate constants, indicating that H abstraction is the common pathway for these radicals toward the substrates. Although this has not been systematically studied, acidity appears to exert a marked influence on the reactivity of aminium radicals. It is clear that while alkylaminium radicals undergo a unimolecular decay process in < I N H 2S0 4 ,96 the decay pattern changes to bimolecular26 at - Ho of 3-8. Within its own range, the decay rate constants vary only slightly with changes in acidity, e.g., the bimolecular rate constant of dimethylaminium changes by a factor of lOin going from - Ho = 3 to 8. In between these acidity ranges, alkylaminium radical decays show complex patterns obviously due to a mixing of first- and second-order reactions. 95 Further, aminium radicals decay faster at the lower end of this intermediate acidity range, e.g., the strong esr signal of the 2,2,6,6tetramethylpiperidinium radical in 1.3 M H 2SO c AcOH solution is no longer observable in 0.1 M H 2SO c AcOH solution. By analogy with chemical and electrochemical oxidations, the first-order decay of alkylaminium radicals in neutral or dilute acid ranges is most likely a deprotonation from an a position which is believed. to be catalyzed by general bases such as water, methanol, and others, as in equation (19). This base-catalyzed deprotonation is retarded in the high-acidity range in acetic acid solution, and consequently the bimolecular reaction becomes the major pathway. This proposal H

1+ I

If\ I 1 :B - - -N-C·

·N-C--H

I

I

.,

I +

BH+

(19)

Yuan L. Chow

172

is supported by various scattered observations. First, aminium radicals are often more stable as their tluoroborate, hexatluorophosphate, perchlorate, etc. saIts than as the acetate or halide,14 and are stable enough to afford their esr spectra when generated in 90% H 2S0 4 or tritluoromethanesulfonic acidY·42 Second, in electrochemical oxidation, alkylarylaminium radicals are more stable in 0.1 M TEAPjacetonitrile solution than in a 50% aqueous acetone buffer solution. 38 Third, deprotonation patterns, as suggested by the products, are different in oxidation of dimethylbenzylamines by chlorine dioxide66.108 and electrolysis 109 as the acidity changes. Finally, the tri-n-butylaminium radical, generated by Fe 2+ -catalyzed decomposition of the corresponding N-oxide [equation (20)], undergoes primarily the normal Hofmann-L6ffier reaction pattern 27 (intramolecular H abstraction) in 50'70 H 2S0 4 but only deprotonation (followed by demethylation) in 0.5 N H 2S0 4 [equation (21)]. In dilute acid or in neutral media, deprotonation is obviously more rapid than intramolecular hydrogen abstraction. (20)

(21) 39'70

Although aminium radicals carry a formal positive charge at the nitrogen radical center, they can only enter H abstraction and addition to multiple bonds by radical mechanisms; such radical reactions must have strong electrophilic characteristics, as has been suggested by the uses of acetic acid and acetonitrile as solvents without complication from H abstraction from these compounds. Photochemically generated alkylaminium radicals from the N-nitrosamines in dilute acid solution fail to add to alkenes bearing electron-withdrawing substituents 9 (such as phenylvinylsulfone, acrylonitrile, ethylacrylate, and even butylvinyl ether11 0-11 2), although those from N-chloroamines under highly acidic conditions obviously do add to halogen- or tritluoromethyl-substituted alkenesY3.114 This difference, again, underscores the faster deprotonation of the aminium radicals catalyzed by general bases under the former conditions (water or methanol), the reaction of which supersedes the slower addition to electron-poor alkenes [equation (22)]. On prolonging the lifetimes of aminium radicals, attack on these electron-poor alkenes may have some chance to occur. 113 .114 +.

B:

+.

RCH 2 NH 2 CHR +--- (RCH 2 hNH

CH.==W

+.

) (RCH 2 hNHCH 2 CH.W

(22)

Aminium Radicals

173

Quantitatively, the electrophilic nature of aminium radical attack is demonstrated by Hammett-type linear free-energy correlations in H abstraction 1l4 and addition reactions. 1l2 Benzylic chlorination of ring-substituted toluenes in 2 M H 2S0 4-AcOH solution by photolysis of N-chloropiperidine is assumed to involve benzylic H abstraction by the aminium radical as the rate-determining step [equations (23) and (24)]. The rates of H abstraction are nicely correlated with a+ -substituent constants to give a p value of 1.21. X @ - CH 3

+ (CH 2hNH!"

-

X@-CH2'

+

(CH 2 hNH2+

(23)

A Hammett-type correlation is also applied to the photoaddition of Nnitrosopiperidine to ring-substituted styrenes in which the rate-determining step is assumed to be aminium radical attack of the carbon-carbon double bond [equations (25) and (26)] and the final products are the a-piperidinoacetophenone oximes.1l2 The reactivities are better correlated with a-substituent constants (CH 2 hNNO

X@-CH=CH2

+

+

H+ -

hv

(CH 2lsNH;

(CH 2lsNH; -

+

(25)

·NO

X@-CHCH2NH(CH2ls

(26)

giving a p value of 1.34. This p is the biggest one found so far in radical reactions although it is not significantly big in comparison to those found in ionic reactions. The presence of a positive charge on the piperidinium radicals obviously did not induce unusually enhanced polar effects in the transition states in both reactions (though correlation with a+ gave a better fit than with a, p values are generally low). This may suggest that the polar effects in the radical reactions operate primarily in the ground state electrostatic attraction rather than resonance stabilization in the transition state~. The conclusion may be reached that transmission of the positive charge to the interacting sites of the substrates, as in 12 and 13 of equations (27) and (28), does not contribute significantly to the transition states.

@-

X

+

•••

CH 2--- H--- NHR2

12

(27)

80

15

1 1

0

0

0

Me2NC)

iso-Pr 2 NC)

Me 2NC)

39

23

16

13

6

88

6

2

92

0

0

0

CH3-CH2-CH2-CH2-CH2-CH2-0Me 1.0 4 83 12 1

8

CH3-CH2-CH2-CH2-CH2-CH20Me

6

CH3-CH2-CH2-CH2-CH2-0Ac

1

CH3-CH2-CH2-CH2-CH2-CH2-CH2-(C'H2).-C02H

5

Me2NC)

14

CH3-CH2-CH2-CH:z-CH2-CH2-CH2-CONH2 13 2 0 8 59 11 8

80

iso-Pr 2 N C)

)

CH3-CH2-CH2-CH2-CH2-CH2-CH2-C02H

13

iso-Pr 2 NC )

82

CH3-CH2-CH2-CH2-CH2-CH2-C02Me 6 83 10 0.8

4

CH3-CH2-CH2-CH2-CH2-CH2-C02Me

4

CH3-CH2-CH2-CH2-CH2-CH2-C02Me

Product distribution ('70)

BU2N C)

Me2NBr

Me 2NC)

Reagent

FeSO.

FeSO.

FeSO.

ACHNa/hv

FeSO.

FeSO.

FeSO.

FeSO.

FeSO.

Conditions

TABLE 9. Oxidation of Aliphatic Compounds using Haloamines, Hydroxylamines, and Fe2 +

122

115

122

98

115

115

124

119

119,120

Ref.

~

Q

to
CH 2 > CH.1l7

Aminium Radicals

177

Titanous chloride-catalyzed decomposition of NH 20H in acidic solution generates the parent aminium radical, NH3 t [equation (31)] and not the amino radical as reported,125-128 since it has been shown that the intermediate adds to (31)

alkenes; the radical intermediates from the addition have been identified by esr spectroscopy.127.128 In 2 M H 3P0 4 solution, the relative rates of aminium radical H abstraction toward straight-chain primary amines and alcohols clearly indicate the expected electrostatic effects, similar to those observed above; the rate ratios are 125 CH3NH3 + : C2H5NH3 + : C3H7NH3 + : C 4H9NH3+ : C 5HllNH3+ : C 6H13NH3+ = 0:0:0.13:1:2.5:5.5 and CH30H:C2H50H:C3H70H:C4H90H:C5HllOH = 0.11 :0.50:0.67:1 :1.52 The final products were not isolated in these reactions.

IV. ANODIC OXIDATION OF AMINES 1. Redox Energetics Execution of a one-electron oxidation process as shown in equation (1) is a direct and clean method of generating aminium radicals. Indeed, recently, anodic oxidations 91,129 and certain chemical 16 and photochemical 13 oxidations of amines have been shown to proceed by this mechanism, though the subsequent aminium radical reactions are not necessarily clearly defined. It follows that any thermodynamic and kinetic quantities associated with equation (I) would offer a measure of the ease of the one-electron oxidation. Common thermodynamic properties are electrolytic oxidation potentials (£0) and ionization potentials (lP). The former are experimentally obtained by cyclic voltametry of the reaction at the solution-electrode interface88.130.132 which has seen considerable refinement since the advent of polarography. The latter measures the energy required for removal of one electron from substrates in the vapor phaset and use electron impact techniques, as in mass spectrometry or photoionization; recently, photoelectron spectroscopy (pes) has been developed for the same purpose.1 34 The ionization potential 133 corresponds, in principle, to the energy required to remove one electron from the highest occupied molecular orbital, the electrons of which are generally the most weakly bound in the molecule. Such an electron in amines is assumed to be that occupying the n orbital. In view of substantial

t

For a discussion of ionization potentials and their measurement, see Reference 133.

178

Yuan L. Chow

geometry changes on going from the approximately tetrahedral amine configuration to the planar aminium radical, there must be a great mismatch of the vibrational states between amine and aminium species. Two types of photoionization potential can be considered: one is the adiabatic ionization potential (IP a), which corresponds to the energy required to remove one electron from an amine in its zeroth vibrational level to give a vibrationally relaxed aminium radical; the other is the vertical ionization potential (lPJ, which corresponds to the energy gap between the amine and the aminium radical without a change in geometry. With the development of pes, 134 IP values of amines are being refined gradually. Inadequacy of our knowledge of solution and solid phases prevents a better understanding of ionization potentials as related to solution chemistry of aminium radical formation. Interested readers may find these IP data for amines in the literatures.60.133-137 Under experimental conditions where the electron transfer process is electrochemically reversible, cyclic voItametry88 produces an oxidation wave and a reduction wave of equal height. The oxidation potential EO for the substance is then equal to the half-wave potential E 1I2 , which is the average of the peak potential (Ep) for the oxidation and reduction waves. If the substrate radical ion is so reactive that it is destroyed before re-reduction on the reverse scan, no reduction wave can be observed and the Ep value may move to the higher or the lower side, depending on the follow-up reactions. Needless to say, it will be much more reliable to obtain EO of aminium radicals electrochemically where the radicals are stable enough to give reduction waves. This ideal situation is, unfortunately, not always attainable for aminium radicals and experimental Ep values are generally reported. Developments in electroanalytical chemistry, both in theory and in technique, have greatly assisted in confirming that the initial step in anodic oxidation of amines is a one-electron transfer. 88.129 As most aminium radicals, except some favorably substituted ones, are rather unstable, some Ep values (but less frequently EO values) of various types of amines are available in reviewsY·12 The number has grown rapidly in recent years130-132 and this will not be tabulated here. Considering all of these data, it is possible to draw these conclusions: Qualitatively the ease of aminium radical formations is in the increasing order 1° < 2° < 3° amines, and methyl < ethyl < n-propyl < isopropyl in the same type of amines as indicated by lowering of the Ep values of respective amines. Further, Ep values are expected to respond to the inductive and resonance effects of substituents located in the vicinity of, or in conjugation with, the nitrogen center. For tertiary alkyl_130.131 and arylamines,86.89.138 the values shift to the more positive side with increasingly electronegative substituents. The ease of one-electron transfer of amines to form aminium radicals is, therefore, controlled by the electron density at the nitrogen center as is basicity (pKa) of the amines. 13o Consequently, linear free-energy correlations of peak potentials and of pKa values with Hammett a, a+, or a* constants have been

a

s.c.e. = standard calomel electrode.

l-chlorobenzotriazole + (CH 3 hNCH2C6H.X Br2 + (CH3hNCH2C6H.X 3[FluoreneneJ* + m and p-XC 6H.N(CH 3h 3[FluoreneneJ* + m and p-XC 6H.NH 2 3[BiacetyIJ* + m- and p-XC 6H.NH 2

CI0 2 + (CH 3 ),NCH2C6H.X Fe(CN)6 3 - + (CH 3 ),NCH 2 C 6H.X

alkylamines Ep(V) vs. AgiO. 10 M AgN0 3 for dimethylbenzylamines Ep(V) vs. s.c.e. a for 3° alkylamines E.(V) vs. s.c.e. for dimethylbenzylamines

Ep(V) vs. Ag/O.IO M AgN0 3 for 2° and 3°

Reaction

Carbon, pH 11.9 Carbon, 10- 3 M NaOH, 30% aqueous MeOH H 2 0, pH 4.5-8.9, 27°C 30'7. aqueous MeOH, 0.25 M KOH, at 30°C Benzene, 280 K 50'7. aqueous AcOH at 25°C hv in benzene hv in benzene hv in benzene

Pt, CH 3 CN

Pt, CH 3 CN

Conditions

-0.71 -0.95 -1.96 -0.96 -0.85

+0.63 +0.67

+0.812 +0.99

-5.24 -4.24

-0.924 -0.989

Brensted a

+0.82

(Scatter)

-6.72

Taft p*

-0.94

-2.22

Hammett p

TABLE 10. Linear Free-Energy Correlations of Oxidation Peak Potentials and Rate Constants

141 30 227 245 250

108,130 140

131 132

130

130

Ref.

~

.......

~

£t::;-

I:)

e:

::0:,

i::' :!

:! ;;.

180

Yuan L. Chow

carried out as shown in Table 10. In some cases the experimental E1' and pKa values are linearly correlated.13l.l3l In Table 10, four cases of similar correlations involving bimolecular oxidation rate constants (instead of £1' values) are also included for comparison; these oxidations of amines, except the last one using bromine, have been shown to occur by one-electron transfer processes. 30 ,139-14l TABLE 11. Selected Peak Potentials E1' for Some Saturated Amines

Compound

E.

(C 2 H5 hN

0.78

(CH 2 ).NCH 3

0.68 a

(CH 2 ).NCH 3

0.80 a

(CH 2 )6 NCH 3

0.60 a

CH(CH 2 CH 2 hN

(14)

CH(CH 2 CH 2 CH 2 hN

'2(C(CH')'

(V)

1.l0(0.9W 0.38

0.74"

Compound

E.(V)

(~N~ (~N~

0.60"

(10)

0.56

CN)

1.20

~~ ~N~N~

,,/\/

/N

N"

1\

-N N~

~N

0.87

/

0.66

0.75

0.69

N--7 \ N

N)J LN"~-

(IS)

r?N/~ N,( N

(II)

1.37

0.58"

eN:::::}

aThe peak potentials measured at a glassy carbon electrode in MeOH 10- 3 M NaOH.'3 2 Others determined at a gold electrode in CH 3 CN.93 " These were EO values since reversible reduction potentials were observed.

Aminium Radicals

181

In addition, experimentally obtained bimolecular rate constants for the oxidation of triethylamine (log k) have been empirically correlated with the oxidation potential of oxidants (E~x) and photoionization potentials (IP) or ~a* of the amine 16 as in equations (32) and (33). These oxidants include chlorine dioxide, permanganate, molybdicyanide, ferricyanide, and several ferric phenanthroline complexes, all of which are implied to give reversible one-electron oxidations of the amine. 16 The successful correlations indicate that in the log k = -7.84 E~x - 5.431 IP

3.85

(32)

log k = -7.64 E~x - 4.78 ~a* - 3.47

(33)

+

reversible electron transfer reactions steric factors must be relatively unimportant and photoionization potentials are also related to corresponding solution properties in a linear fashion. The nonbonded electrons of amine groups conjugated with double bonds (enamines) are extensively delocalized, causing large decreases in their oxidation potentials, as shown in Table 7. 91 Indeed the first four of these, as uncharged species, are powerful reducing agents, as shown by their negative EO values. Increasing the number of dimethylamino and methyl substituents at the olefinic carbons-particularly the former-dramatically reduces the EO values of the amines. Some Ep values of tertiary mono- and polyaza compounds obtained by Nelsen and Hintz 93 are summarized in Table II as a demonstration of the relationship between structure and oxidation potentials. It is clear that 1,4diamine systems possess much lower Ep than I ,3-diamine systems, probably due to inductive effects through the shorter -CH 2- bridge in the latter; -CH2NR2 groups are inductively electron withdrawing as compared to -CH 2CR 3 groups. The low Ep values of 10 and 11 in comparisons with quinuclidine 14 and tetramethylenetetramine 15 may be intuitively attributed to favorable geometry of the n orbitals in 10 and 11. In relation to the stability of lOt and 11 t and the aminium radicals derived from dimethylaminoalkenes in Table 7, one may conclude that the electronic factors controlling amine susceptibility to one-electron oxidation and those controlling stability of the corresponding aminium radicals operate in the same fashion.

2. Reaction Patterns and Mechanisms One-electron oxidation of amines at anodes is a sure and clean method of generating aminium radical. Yet mechanisms of electrochemical oxidation are very complex since they simultaneously involve adsorption, heterogeneous catalysis, reactions within an electric field, concentration and pH gradients, and bulk reactions. However, both synthetic and mechanistic studies have been greatly facilitated in the last decade by the development of new techniques in

182

Yuan L. Chow

potential control, polarography, cyclic voltametry, coulometry, esr spectroscopy, and so on. Basic concepts and elucidation of mechanisms have been well described. BB.143 In contrast to the extensive literature available on anodic oxidation of aromatic amines, aliphatic amines have not been studied very much by this method. Most of those that have have been reviewedY·12.142 It is generally accepted that the first step in all of these amine oxidations is the generation of an aminium radical by one-electron transfer even though direct proof of this is lacking in some cases. For convenience, they will be divided into primary and secondary amine oxidations and tertiary amine oxidation.

A. Primary and Secondary Amine Oxidations The primary anodic oxidation processes of primary and secondary amines can be summarized in equation (34) using RNHR' as a model compound. An interplay of these primary processes is naturally dependent on reaction parameters (such as pH, concentration of substrates, concentration and nature of supporting electrolytes, anodic potential, nature and states of the electrode, etc.) RNHR'

-e

:;:::=:::

+.

R NH R'

-e

:;:::=:::

(R N H R')2 +

16 -H+

18

jr

RNR'

17

-H+

~

jr

(34)

+

RNR'

19

and the nature of the aminium radical. Tfthe n orbital is conjugated with stabilizing groups, all chemically active species 16-19 may have finite probability of being involved in the reactions. Under basic conditions, deprotonation of aminium radical 16 is generally rapid to give amino radical 17, which may be oxidized to the nitrenium ion 19 if both Rand R' groups can stabilize the charge. 7o .B4 In acidic conditions, if 16 is stabilized by delocalization of the unpaired electron and by steric hindrance further one-electron transfer to form a dication is a possibility.B4 In some cases, a combination of available techniques can clarify the pathway and identify the intermediate species. B4 In view of the moderately long lifetimes of arylaminium radicals and arylamino radicals, coupling of two radicals is generally thought to occur, the pattern of which is summarized in equation (35) using C6H5NHR t as a modeI.142.143 The acidity of the solution controls the concentration of aminium (20) and amino (21) radicals, which in turn affects the pattern of coupling products. Anodic oxidations of anilineB5.B6.13B.144 and diphenylamine7o.84.144-146 follow this pattern; in acidic solution benzidines (22), and in basic solution (pyridine bases,

Aminium Radicals

183

2@-NHR~

~

20 + H+

- H+

2 @-NR

22

/@-:-@-NHR ---+

@-NRNR-@ 24

21 -e

(35)

Jf

[@-NR

+----------+ -\

)=NR

+----------+]

---+

electrophilic attack

2S

etc.) hydrazines (24), are the major products, with varying amounts of phenylenediamines (23) formed in the intermediate acidity range. Products 22 and 23 are generally further oxidized at the anode to quinoids, cation radicals, or dications under the electrolysis conditions. 146 Tetrasubstituted hydrazines, 24, derived from secondary amines are generally oxidized to their cation radicals, and disubstituted ones (24, R = H) to azobenzenes. It should be pointed out that anodic oxidations of amino radical 21 to nitrenium ion 25 followed by attack on the parent amine also yields the same products 22-24. Indeed, for anodic oxidation of diphenylamines polysubstituted at para and orlho positions with electron-donating groups, pathway 20 -+ 21 -+ 25 is followed under basic conditions, yielding the nitrenium ion which reacts with parent amine to give dihydrophenazine [see equation (12)] or with nudeophiles 7o .84 as in equation (36). Di-(2,4,6-trimethoxyphenyl)amine is electrolyzed in acetonitrile containing 2,6-lutidine to give the nitrenium ion 26, which reacts either with water to give quinonimine 28 or with cyanide ion to yield 27. In acidic solution, the ftuoroborate of the amine is oxidized by two successive one-electron transfers to give dication 29, which is hydrolyzed to 28. The intermediates 26 and 29 are characterized by their uv absorptions. 84 Anodic oxidation of substituted (OCH 3 , OC 2 H s, Cl, CH 3 , C0 2 H, CN, and N0 2 ) ani lines gives the substituted 23 as the major product in which the substituent group has been eliminated either as an anion or a cation, depending on the nature of the substituent. 141 The hindered aminium radicals generated from 2,4,6-tri-t-butylaniline9o.147-149 and 9-amino-IO-phenylanthracene 1so [equations (37) and (38)] do not undergo facile chemical reaction and tend to follow the

184

Yuan L. Chow

R

Ar2NHt

R

~ CH30-0-NH-~OCH3 ~- ~ R

R 29

-H+

M.N

~

(i) H.O

(ii) base )

~ CH'~-N:cro /

CH,A

R

~ NR0 OCH,H'~

~~ R

R ----+ CH 3 0

CN-

~ 0

\

-

CN

N=Q

/-

R R = H or OCH 3

(36)

28

R

26

R

OCH 3

R 27

~~~ CHPH.(r ,.

I

¢ CH 3 CN lpyridone

OCH,

(37)

Aminium Radicals

185

pathway 20 - 21 - 25, particularly in the presence of a strong base (such as diphenylguanidine), wherein the nitrenium ion reacts with nucleophiles. The aminium radical 31 can be trapped by oxygen to give the corresponding peroxide 32. NH NH

o ©¢.© ¢ 1) 0

4>

- H+

---+

•

---+

4>

4>

31

1

t

NH

NH

0•

HO '

x)

II

Q

o+'

4>

4>

NH

*

(38)

OH

32

Electrochemical substitution of diphenylamines is a synthetically useful reaction. The anodic oxidation of diphenylamine in NaCN-MeOH at a potential of 0.3-0.6 V yields p-cyanodiphenylamine, which can be further oxidized to p,p' -dicyanodiphenylamine. 151 Although coulometric determination showed an average of two electrons was consumed per amine molecule, the exact sequence C 6 H SNHC 6 H S

anode MeOH-NaCN)

61~o

of this substitution is not clear. Since the applied potential was too low to permit oxidation of cyanide ion,152 the primary step must be a one-electron transfer to give an aminium radical. As a sodium cyanide solution is highly basic, the pathway 20 - 21 - 25 may be proposed in which the nitrenium is trapped by the cyanide ion to give the product. There are only a few anodic oxidations of primary and secondary alkyl amines reported; these are discussed in earlier reviews. l l • 12 Ammonium ions are resistant to anodic oxidation. As alkylamines are much stronger bases than the corresponding aminium radicals (see Section 111.3), deprotonation 16 _ 17 [equation (34)] would occur rapidly in the absence of pH control; anodic oxidations in aqueous alkaline solution (pH 11-12) should exhibit amino radical

186

Yuan L. Chow

reaction patterns. 130 An attempt to perform a Hofmann-LatHer-type reaction with the dibutylaminium radical generated by anodic oxidation in 90% aqueous methanol, using benzyltrimethyl tetrafluoroborate as the supporting e1ectrolyte,153 resulted in the formation of various dealkylation products, but none of the 0position functionalized one. This may be explained if the aminium radical preferentially abstracts a H atom from methanol or is deprotonated by the parent amine to form the dibutylamino radical. 153 The mechanistic interpretations previously proposed for anodic oxidations of primary and secondary amines l l should obviously be revised in light of acid-base equilibria in electrolytic solution.

B. Tertiary Amine Oxidations Extensive studies on anode oxidations of triphenylamines and dimethylanilines have been reviewedY·12 The lack of N-H bonds and relatively longer lifetimes of these aminium radicals suggest that the major pathway is that of 20 ~ 22 [equation (35)] to form benzidines which, however, are oxidized by two one-electron oxidations to the corresponding dication under the electrolysis conditions. 142 In addition, the oxidation of dimethylaniline yields a number of products derived from dealkylation, condensation, and further oxidations. 154 The dimethylanilinium radical also undergoes deprotonation from the a positions to give the C-radical 33, which process is facilitated by the presence of a base 155 as shown in equation (40) (see Section I1I.4). This is followed by a one-electron transfer to give the immonium ion 34 and sets up the stage for the hydrolytic dealkylation and/or an attack by a nucIeophile X - (CH 30 - or CN-)153.155 as in equation (41). The anodic oxidation of dimethylaniline in CH 2 , -H+

.. /

----+ CsHsN

"-CH 3

-e

(40)

----+

34

33

CsHsNHCH3

+

H 20

x-

/CH 2 X

CH 2 0 +--- 34 ----+ CsHsN

"-CH 3

(41)

35

methanol 155 with potassium hydroxide as the electrolyte yields 35 (X = OCH 3), which is further oxidized to N,N-dimethoxymethylaniline. However, in methanol solution containing ammonium nitrate, tetramethylbenzidine is obtained. Anodic oxidations of trialkylamines generally follow the pathways of equations (40) and (41) to afford dealkylation products or functionalized products at an a position. Two major factors make the anodic oxidation pattern of trialkylamines somewhat different from that of arylamines. Firstly, delocalization

Aminium Radicals

187

of the unpaired electron, as commonly occurs in arylaminium radicals, is no longer possible, which results in less stability and no coupling reaction [such as those of equation (35)] for alkylaminium radicals. Secondly, trialkylamines are fairly strong bases and participate in proton elimination if there is no stronger base in sight. Thus, oxidative dealkylation oftrialkylamines in aqueous solution at pH 12 gave a better conversion since the amines are not inactivated by scavenging protons. 156 In anodic oxidations of tertiary amines with different alkyl groups, an immediate question is the preference of the alkyl group being oxidized, which is directly related to the proton transfer step, as eventually it is the immonium ion 34 that reacts with nucJeophiles to give the final products as in equations (40) and (41). Using unsubstituted or p-substituted dimethylbenzylamines as model compounds, several anodic oxidation studiesl09.132.155-158 resulted in controversy which, however, seems to be satisfactorily resolved in a recent report. 109 It is interesting to compare the results obtained with those of one-electron oxidations by chemical reagents on the same amines.108.140.141 :B Bz attack

~

-e

:B

39

H 2 00r

x-

x-

)

~

CH 3

C H3 Me attack

HoOor

38

36

C s HsCH.N(CH3h

38

+

C 6 HsCHN(CH3h ------+ C 6 HsCH = N(CH 3h

CsH sCHXN(CH 3h 40

I

CsHsCH2~CH2 •

-e

------+

CsHsCH2N(CH3)CH2X

I

CsHsCH2N~CH2

37 or

(42)

+

39

C 6 HsCHO

+

(CH 3hNH

(42a)

or

+

CsHsCH 2NHCH 3

(42b)

CH 20

41

The dimethylbenzylaminium radical may undergo proton transfer by methyl attack or by benzyl attack to give the C-radicals 36 and 37, respectively, that are oxidized to the corresponding immonium ions 38 and 39; the latter react with water to give dealkylation products, or with a nucJeophile X- (e.g., CH 30-) to 40 and 41. While statistically distributed proton transfers should have the value of three for the ratio of methyl to benzyl attack, the ratios determined from product analyses for anodic oxidations in methanol with potassium hydroxide as the supporting electrolyte (i.e., in the presence of strong bases) range from two to three and approach the statistical ratio at a lower applied current. 109 In methanol containing tetra-n-butylammonium fluoroborate (i.e., the amine itself serves as the base), however, the ratio is 10 at the applied potential of 1.05 V vs. s.c.e. (standard calomel electrode), showing a strong preference for methyl attack. Shifts toward more benzyl attack at higher currents in the former and at

188

Yuan L. Chow

higher voltages in the latter are explained by preferential H-atom abstraction from the benzyl site by the hydroxymethyl radical generated under these conditions 109 [equation (43)]. -e

-H+

(43)

CH 3 0H ---+ CH 3 0H· ---+ ·CH 2 0H

It is reasonable that the stronger base (hydroxide ion and/or methoxide ion) attacks more randomly in a nondiscriminating fashion but that the much weaker base attacks more discriminately the hydrogen having a higher positive electron density. The authors 109 also quoted Eberson's INDO results, which predicted a higher charge density at the methyl hydrogens than at the methylene hydrogens in the aminium radical. This interpretation is further supported by the conclusions derived from esr studies of aliphatic a-amino radicals produced by Xirradiation in which 159 it is shown that (i) the radicals have a substantial stabilization from a three electron 7T bond between the trigonal carbon and nitrogen and (ii) the decreasing order of stability is . CH 2NH 2 > . CHRNH 2 > . CR 2NH 2, which is just the reverse of that found for alkyl radicals. The near-statistical oxidative dealkylation of N,N-dimethylbenzylamine in strongly basic solutions has been observed at a Pt 109,155,157,158 or a glassy carbon electrode. 131 ,156 It is surprising that the ratios of methyl attack to benzyl attack for substituted dimethylbenzylamines are in the range 2.0-3.0 and do not respond significantly and proportionally to the electronic nature of the substituents. 131 It is noteworthy that the ratios obtained by chlorine dioxide oxidation at pH 8-9 of p-chloro- and p-methoxy-N,N-dimethylaniline 108 are 4 and that for potassium ferricyanide oxidation of the parent amine 109 in 2 M potassium hydroxide solution is 2.8. Also, the ratio when p-substituted N,N-dimethylbenzylamines are oxidized by I-chlorobenzotriazole in methanol is higher than three. H1 On the other hand, two research groups152.155 have concluded that methyl attack is favored in the anodic oxidation even in strongly basic solution, and they explained their preference by invoking the stereochemical accessibility of the methyl groups to attack by base in an adsorbed aminium radical at the electrode surface. This proposal involves unexplored and unknown factors of stereochemistry of adsorbed aminium radicals which could play important roles. The accuracy of experimental product analyses in both electrochemical and chemical oxidation should be improved. This might lead to a better understanding for the selectivity of attack. Preference for dealkylation in anodic oxidation of other trialkylamines may be understood on the basis of the above discussion. Experimental results -e, CH 3 CN EI.N+CN-

/CH 2 CN

• C6 HSN "-

CH 2 CH 3

/CH 3

+ C6 HsN

"- CH(CN)CH

42

43

64'70

34'70

(44) 3

Aminium Radicals

189

show a slight decreasing order for 1° > 2° > 3° hydrogen in proton transfers from trialkylaminium radicals. 1l • l56 Electrolysis of N-ethyl-N-methylaniline in acetonitrile and tetraethylammonium cyanide afforded 42 and 43 in a 2: I ratio. 152 The anodic oxidation of tertiary amine 44 in methanol containing potassium hydroxide afforded oxazolidines 45 and 46 155 :

(45)

C.H 5 CH 2 N(CH 2 CH 2 0Hh

46

45

44 CH 2 CH 2 0H

CH 2 CH 2 0H -e

I "'" C.H 5 CH 2 N-CH 2 -CH 2 -O-H +.~

~OH-

I

----+ C 6 H 5 CH 2 N=CH 2

47

+

~

46

(46)

48

The latter is probably derived from intramolecular addition in the immonium ion 48 which must arise from oxidative cleavage of 47 as in equation (46). The related cleavage of 49 in a carbon anode oxidation 160 is shown in equation (47).

CH3CN-H.O. NaOCH3 )

-2e

HO

(47)

49

50

There is good evidence that oxidation of amines at passivated metal electrodes, such as silver, copper, nickel and cobalt, in aqueous bases actually involve the reaction of a metal peroxide rather than electron transfer and that aminium radicals are not involved in these oxidations. 16I Therefore amine oxidation at these electrodes and by solid metal peroxides will not be covered.

Yuan L. Chow

190

V. AMINIUM RADICAL INTERMEDIACY IN CHEMICAL OXIDATIONS Amine oxidations are a part of biogentic pathways of nitrogen-containing compounds in nature such as amino acids and alkaloids 162 and also occur in the metabolism of certain drugs in mammalian liver.163 Oxidations of amines have been investigated with every conceivable reagent and technique available to chemists and the literature on this subject is enormous. Many studies were directed to their mechanism, though cumulative results to date are not always as clear as those of electrochemical oxidation. Recognition of aminium radical formation in the oxidations of triphenylamines and phenylenediamines with various reagents is fairly 01d. 164 For alkylamines, while suggestions of aminium radical intermediacy exist in the older literature, such as that on nitrosative dealkylation 165 and various oxidations of tertiary alkylamines,33 general acceptance of their merit in oxidation of alkylamines is rather recent.

1. Reaction Patterns and Mechanisms In spite of a long history and many investigations on amine oxidations, reliable mechanistic studies are few and have only become available recently with the advent of modern techniques such as esr and stopped-flow techniques. Structure-reactivity correlations in amine oxidations are often misleading because of difficulties in controlling reaction conditions and in product analysis arising from complexity in product patterns. Sometimes this leads to conflicting proposals, which makes the scope of coverage in this section somewhat arbitrary. Probably it is fair to say that serious considerations of aminium radicals were stimulated by the rapid progress in electrolytic oxidation of amines, for the obvious reason that aminium radicals are invariably the primary electrolytic products. Six mechanisms of amine oxidation are defined below [equations (48)-(55)] using XY as an oxidant and a tertiary alkylamine as the model compound. Mechanism 1. Direct one-electron oxidation: e transfer

- - - " * ) RCH 2 NR;

+

XV;

(48)

SI +.

RCH 2 NR;

RCHNR~

(49)

S2

RCHNR'

-e

~

+

RCH=NR.

(50)

S3

Mechanism 2. Indirect one-electron oxidation via the ammonium ion 54 followed by equations (49) and (50):

Aminium Radicals RCH 2 NR 2

191

+ Xy

+

+.

RCH 2 NR2 y- ~ RCH 2 NR.

~

I

+ X· + y-

(51)

X

54

51

Mechanism 3. a-Hydrogen abstraction followed by equation (SO): RCH 2 NR.

+ Xy

~

...

RCHNR 2

+ HY + X·

(52)

52

Mechanism 4. Direct two-electron oxidations via proton elimination:

(X+ (53)

RCH-NR., ----+ RCH=NR'

IJ

H

Y-

53

~

Mechanism 5. Indirect two-electron oxidations via ammonium ion 54:

RCH 2 NR.

+ XY

(~

----+ RCH-NR. Y-

IJ

+

~

RCH=NR';

+ HX + Y-

(54)

+

H

53

54

Mechanism 6. Direct two-electron oxidations via hydride ion transfer:

().

RCH--NR';

~J

~

RCH=NR;

x-v

+ HX + Y-

(55)

53

L..

Besides hydrolysis of, and nucleophilic attack on, immonium ion 53 by analogy to that shown in equation (41), 53 may exist as the corresponding enamine by prototropy and can also undergo further oxidation to amide 55 via carbinolamine 56. In the majority of chemical oxidations of alkylamines, the observed products have mostly been those derived from the immonium ion 53, RCH=NR 2

~ [R~:NR2] ~

RCONR"

53

56

55

(56)

but occasionally those derived from a-amino radical 52, e.g., those from coupling or radical scavenging, are isolated. It is clear that these mechanisms, whether one- or two-electron oxidations, share as the common intermediate the immonium ion 53 and/or the a-amino radical 52. The criteria for aminium radical intermediacy must, therefore, be sought elsewhere than in product distributions.

192

Yuan L. Chow

One would be on more secure ground in judging the intermediacy of aminium radicals if their reactions, such as those coupling reactions [equation (35)] found in anodic oxidation, could be observed; indeed, some chemical oxidations of arylamines give these coupling products, sometimes in good yield. In alkylamine oxidations, however, the formation of Hofmann-Laffier-type products 3 or addition products (see Section VILl.A) are good criteria to use, as these are typical products formed from aminium radicals; such reaction patterns have not been reported in the chemical (as opposed to the anodic) oxidation of amines. Before proceeding to describe those chemical oxidations having aminium radicals as intermediate, it will be useful to briefly mention those which have been shown to follow other mechanisms. Amine oxidation with solid metal oxides shows the reactivity order of 1° > 2° > 3° amines (see Section IV.3), just the reverse of that of one-electron oxidation at anodes, and has been shown to proceed by a two-electron oxidation process. 161 From the formation of amine oxides or hydroxylamines, it would appear that amine oxidations by hydrogen peroxide,166 peroxy acids,166 and probably by ozone 167 occur by two-electron pathways. Mercuric acetate 168 and cyanogen bromide 169 oxidations of various tertiary amines are believed to follow Mechanism 5; the metallic mercury is oxidized by mercuric ion to mercurous ion. 168 Nitrosative dealkylation of tertiary amines 170 is now believed to occur by cis elimination of the nitrosammonium intermediate 54 (X = NO) as in equation (54). The oxidation of N,N-dimethylaniline by t-butylperoxide 171 appears to occur by a-hydrogen abstraction (Mechanism 3) as shown by the first-order kinetics of the reacton and the formation of coupling product of 52 [equation (57)]. C6HSN(CH3h

+

(I-BuO).

Il.

-----+ [C6HsN(CH3)CH 2 -1o

+

l-BuOH

(57)

Many oxidants have been mentioned in the oxidation of triarylamines and phenylenediamines to generate the corresponding aminium radicals.14 Yet, in applications of some of these reagents (e.g., bromine)29.3o to aliphatic amine oxidations, the experimental evidence is generally interpreted in favor of ionic mechanisms.

A. Chlorine Dioxide and Potassium Ferricyanide

The oxidation of tertiary amines by these two reagents has been established to occur by a one-electron transfer (Mechanism I) as a result of extensive studies by Rosenblattl08-13o and Lindsay Smith172-175; they are combined for discussion purposes to clarifying the criteria for establishing Mechanism 1. Chlorine dioxide oxidatively destroys phenols and amines and has been used to disinfect water and to delignify woodP It reacts with alkylamines to give oxidative dealkylation products in the pH 5-9 range, the mechanism of which

Aminium Radicals

193

has been proposed to be as in equations (48)-(50) in which XY = Cl0 2 and XY-:- = Cl0 2 - (chlorite anion).103,10S The elaborate kinetic studies elicited dual pathways for the amine oxidation l03 ,176 in which tertiary amines are oxidized nearly exclusively by Mechanism I but primary amines mainly by the a-hydrogen abstraction route of Mechanism 3. The experimental rate constants for the reversible one-electron transfer [equation (48)] range from 2 x 105 M -1 sec -1 for triethylamine to 1.12 x 10 - 2 M -1 sec -1 for benzylamine,103 a decrease by a factor of 107. Several lines of evidence indicate that the one-electron transfer occurs in tertiary amine oxidation. First, the rate constants lOS ,130 of the oxidations of psubstituted N,N-dimethylbenzylamines and of trialkylamines respond linearly to the Hammett a or a+ and the Taft a* constants, respectively (Table 10), suggesting that electron density at the nitrogen is regulating the oxidation rates. Further, the log of the rate constants also directly correlates linearly with oxidation potentials and pK a of the amines. 13o Secondly, the kinetic isotope effects for oxidation of trimethylamine-dg (kHlkn = 1.3) and a,a-d2-benzyl-t-butylamine (kHlkn = 1.8) are secondary rather than primary.l03 Thirdly, the reversibility of the process depicted in equation (48) as demonstrated by rate dependence on Cl0 2-Cl0 2- concentrations is best rationalized in terms of a one-electron transfer.l03.l0S Fourthly, the DABCO aminium radical (lOt) is generated by oxidation of 10 with chlorine dioxide and characterized by uv and esr spectra. 64.65,177 Trialkylamine oxidation by alkaline potassium ferricyanide is about 106 times slower than that by chlorine oxide 16 ,10s but, on similar lines of evidence,140,172-l75 it has been concluded that it proceeds by the same mechanism [equations (48)-(50)], where XY = Fe(CN)6 3 - and XY-:- = Fe(CN)6 4-. While the kinetic isotope effects on the bimolecular rate constants for N-d3 methyl-di-n-butylamine is nearly unity, a strong isotope effect (3.6) on the ratios of demethylation and debutylation is observed. 174 From a large number of experimental rate constants of ferricyanide oxidation of tertiary amines, e.g., mono and diaza compounds and amino alcohols of acyclic, cyclic, and bicyclic systems, it is seen that the rates are retarded by electron-demanding substituents placed close to the amine site owing to their inductive effect 175 (see Table 10). In contrast to near statistical distribution in dealkylation using chlorine dioxide oxidation, lOS dealkylation in ferricyanide oxidation of tertiary amines seems to depend on the relative acidities of the respective a hydrogens in the aminium radicals.14o.l72 An electron-withdrawing para substituent (e.g., N0 2 ) in the N,Ndimethylbenzylamines strongly favors, and an electron-donating one (e.g., CH 30) retards, the debenzylation. 140 The kinetics of triethanolamine oxidation with alkaline ferricyanide [equation (58)] showed first-order dependence on each reagent and the expected fragmentation products were obtained. 178 N(CH 2 CH 2 0Hh

+

6Fe(CN)6 3 -

+ -

60H - _ 6HCHO

+

NH3

+

3H 2 0

+

6Fe(CN)6 4 -

(58)

194

Yuan L. Chow

B. Metal Ions Iron(lII) complexed with various methyl-substituted phenanthrolines and potassiu'11 octacyanomolybdate reacts with triethylamine according to Mechanism 1 to generate the aminium radicaP6; generally, cupric,179 silver, and ceric 180 ions are unreactive toward aliphatic amines. Aromatic amines and vinylamines (enamines) are oxidized by ferric/ 81 ceric/80-183 cupric/ 79 lead(IV),36.37 manganic,182.184 silver,91.92 thallic,182 and cobaIt(III)182 ions, and aminium radicals have been invoked as the reactive intermediates in many cases. Metal ion oxidations are generally sensitive to the nature of solvent and ligands as the reactions occur in a complex of unknown nature. The observed substituent effects on kinetic behavior of the cupric ion oxidation of ani lines have led to a proposal of a one-electron transfer mechanism for this reaction [equations (59)-(61)].179 Oxidative dealkylation of N-ethyl-N-methylaniline and N-n-butylN-methylaniline with the acetates of Mn 3+ , C0 3 + , Pb 4+ and TP + in chloroform (C aH sNH2---CU)2+

(59)

and acetic anhydride has resulted in more attack on the methyl than on the larger groups, in agreement with the proposed intermediacy of an aminium radicaJ182 (see Section IV.2.B). Both lead tetracetate and ceric ammonium nitrate have been used to generate aromatic aminium radicals for esr studies 36 .37 and the former oxidatively dealkylates N,N-dimethyl- and diethylanilines in good yield. 185 Lead tetracetate and boron trifluoride (or perchloric acid)68.75 in acetic acid and ceric ammonium nitrate in dilute sulfuric acid 183 act as one-electron oxidants to oxidize N,Ndimethylaniline and triphenylamine to the corresponding benzidine [see equation (35)] by analogy with the anodic oxidation (see Section IV.2.B).

C. Potassium Permanganate Kinetic rate constants for permanganate oxidation of alkylamines show that the order of reactivity is 3° > 2° > 1° amines.176.186 While the kinetic isotope effect for the oxidation of a,a-d2-benzylamine 176 is 7.0, that of trimethylamine-d9 187 is only 1.8, indicating shifting of primary isotope effects to secondary ones. It is interpreted that permanganate oxidizes primary alkylamines by ahydrogen abstraction (Mechanism 3) 187 and tertiary alkylamines by one-electron transfers, as in Mechanism I, to form aminium radicals. 176

195

Aminium Radicals

D. Halogens and their Derivatives

Bromine78 and chlorine 165 have been used extensively to generate stable arylaminium radicals. However, kinetic and structure-reactivity correlation studies on the oxidation of alkylamines in buffered solution 29 and in 50% aqueous acetic acid 30 have been interpreted in favor of a direct two-electron transfer, as in Mechanism 4. Under the latter conditions,3o the kinetic isotope effect for dimethylbenzylamine-a,a-d2 (kHlkD = 1.37) is definitely secondary and the Hammett p constant ( -0.95) obtained in a correlation of the kinetic rate constants of the substituted-N,N-dimethylbenzylamines is very similar to those observed in one-electron transfer reactions (Table 10). N-Haloammonium ions 54 (X = CI or Br) were either proposed or identified as the initial intermediates in the reactions of tertiary amines with Nbromosuccinimide,31.32 hypochlorous acid 64.66.188.189 (or aqueous solutions of chlorine at various pH), and chlorine in carbon tetrachloride or in aqueous sulfuric acid. 6 While some early workers favored the decomposition of 54 by an ionic mechanism 188 (Mechanism 5), homolytic decomposition of 54 (Mechanism 2) has also been suggested. 31.33.189 Thermal decomposition of N-chlorotriethylammonium ion either in neat isopentane or in a 40'70 H 2S0 4 in isopentane chlorinates the latter at the 1°,2°, and 3° carbon positions in the ratio 0.3: I: I in which the aminium radical intermediacy is assumed by analogy with the Hofmann-Loffler reaction. 6 The N-chloro-DABco 57 obtained by hypochlorite oxidation has been demonstrated to undergo mainly heterolysis as well as a small proportion of homolysis [equation (62)].64.66

J

CH 2

1 \N+=CH =+N "----I

2

+---

1 \N N \----./ /'y/

I

+

--CI

1\+ +CI· \----./

N~N·

(62)

57

The kinetic rate constants of the oxidation of substituted N,N-dimethylbenzylamine by l-chlorobenzotriazole141 in benzene can be linearly correlated to yield a Hammett p value of -0.71 (Table 10), and the kinetic isotope effect of dimethylbenzylamine-a,a-d2 (1.3) is secondary. While oxidative debenzylation is favored in benzene solution, methyl groups are preferentially attacked in methanol solution. The same reagent also oxidizes DABCO (10) and N,N,N',N'tetramethylphenylenediamine to the corresponding aminium radicals. It is suggested that this oxidation occurs by the one-electron transfer as in Mechanism I, and elaborate reaction pathways have been presented. E. Diacy/ Peroxides

Interaction of diacyl peroxides with tertiary amines has been studied in connection with polymerization initiation and shown to give a complex mixture

196

Yuan L. Chow

of products. 34.190-192 Taking into account the observed electronic effects on the rates and the capacity to initiate polymerization, Horner and coworkers have proposed a reaction mechanisml90-192 (essentially Mechanism 2). The oxidation of N,N-dimethylaniline with benzoyl peroxide has been studied repeatedly under various conditions.193.194 Although recent kinetic studies 195 are interpreted CsHsN(CHah

+

CsHsN(CHah

I 0-

+

(C 6 HsCO.h

+ (CaHsCO),O

~ /

OCOCsHs

I

(63)

63

in terms of a radical mechanism, it is more likely that the intermediate 63 decomposes by the dual mechanisms of heterolysis and homolysis. The reaction of N,N-dimethylaniline N-oxide with acyl anhydrides [equation (63)] also generates the intermediate 63,196 decomposition of which also initiates polymerizations. I97 The reaction conditions are essentially those of the Polonovski reaction. 19B Decomposition of alkaloid N-oxides with trifluoroacetic anhydride (modified Polonovski reaction) is generally thought to involve two-electron shifts.I99 Dibenzoyl peroxide oxidation of N,N-dimethylaniline gives a plethora of products as in the anodic,146.154 cupric,20o and mangamc lB4 ion oxidations. Some of these products (~8) have been obtained as the major product,

o

0

CHa

CaHsCO " - - @ -N(CHah

(CHahN--@-CH2~-@

64

6S

CHa

I

([email protected]@- N(CHah

©()§J I

CHa 67

66

(CHahN-@--@--N(CHah 68

depending on oxidation conditions: for example, benzidine, 68, was obtained in toluene 195 at - 25°C, 67 (70%) in cumene 194 at O°C, 65 (64%) in benzene 194 at 5-45°C, and 64 (27%) in acetonitrile 194 at - 5°C. Treatment of the amine N-

197

Aminium Radicals

oxide with acetic anhydride in benzene or acetonitrile 194 gives 65 and 68, indicating the similarity of the two reactions as in equation (63). Compound 66 must be formed by multistep reactions and has been found among the products in many other types of oxidation of the same amine, some of which obviously involve the aminium radical intermediate.146.154

F. Molecular Oxygen As oxygen itself is not very reactive, oxidation with oxygen is generally catalyzed under a variety of conditions except when reactive substrates are involved. Pyrrolidine dienamine 69 derived from the a,,B-unsaturated ketone is oxygenated to give the enedione 72 in 80-85% yields in the presence of a catalytic amount of FeCI 3 , Cu(OAch, or CuCI 2, and the aminium radical 70 is proposed as an intermediate 201 : one of the possible follow-up pathways is the combination 02(FeCl 3 )

benzene

)

70

69

~

.£0

.~

~

(CH 2 ).N;J

C

(CH 2 ).N

o H

0-c?H

"0-)

71

~

oW 72

,,+./ N

"N/

YI 73

0 •• CuCi ~ CHCI 3

~

(64)

0

'\+/ N

YI h ~

~

0-0-

O2 ;

\

N

/

~>x);-< 0/

0

-»

o+>~ 50'70

(65)

198

Yuan L. Chow

of the radicals to give 71 without involving radical chain processes. Other dienamines derived from several steroidal enones are similarly oxidized. 201 By the same token, cuprous (or cupric)-ion-catalyzed oxidation of enamines 202 containing no vinylic hydrogen (e.g., 73) might be written as in equation (65). Photosensitized oxygenation of aliphatic amines in the presence of dyes or aromatic hydrocarbons or ketones has occasioned some controversies. Some advocate that singlet oxygen20.203-206 causes the oxidation, while others favor direct interactions of amines with excited states of sensitizers (S) as the first step,207-209 as in the photo-oxidation of amines with benzophenone (see Sections VI. I and VI.2). There are suggestions, among others, that in both cases aminium radicals are the primary intermediate.203.204.208.209 The elaborate kinetic studies on rose-bengal-sensitized oxidation of triethylamine in methanol S·

+

O2 + RCH 2NR'

~

102

+

RCH2NR~

Ib S;

+

+.

+•

O2 + RCH2NR" - - + RCH 2NR;

1 +

O2;

+

So

(66)

+

So

have led to the conclusion that both paths a and b [equation (66)] are operating concurrently.19 However, aromatic hydrocarbons such as anthracene, naphthalene, perylene, and phenanthrene photosensitize the oxidation of tertiary amines in acetonitrile (but not primary and secondary amines) .by direct interaction (path b).19 Direct proof of the electron transfer as in path b, equation (66), has been provided by flash photolysis of perylene and N,N-dimethylaniline in acetonitrile in which both the uv absorption maxima of the aminium radical C6H5N(CH3h t and the anion radical S-: are served 69 (Table 3). Flash photolysis of an oxygen-saturated solution shows the absence of the latter anion radical peak, indicating that the superoxide anion has superseded as the radical partner, although either mechanistic pathway (a or b) can be responsible for the change. On photosensitized oxidation, tertiary amines give better quantum yields than do secondary and primary amines and yield the products indicated in equation (67), as expected from operation of equations (42) and (56) in which the superoxide anion (pKa 4.85)1 or the anion radical act as the base to remove a proton from aminium radicals. 204 Primary and secondary aminium radicals are deprotonated to form amino radicals which may change the reaction patterns. Dye-sensitized auto-oxidation of mono-, di-, and tributylamines in organic .co. hv

o 2. benzene ) C 6 HsN(CH 3 )CHO + C 6 HSNHCH 3 + CsHsNH2 50'70

20'70

(67)

trace

solvent reveals complex product patterns owing to concurrent generation of peroxides and overlap of various secondary processes. 204 Benzophenone sensitizes photo-oxidation of all types of amines in benzene solution, giving reasonable yields of the products. 204

199

Aminium Radicals

There are many examples of uncatalyzed 210 auto-oxidations of amines or catalyzed by platinum or palladium 211 ; those of indole derivativest are numerous and might be related to their biogenetic transformations. While one may presume the operation of aminium radical intermediates in these reactions, the correct mechanism, whether it involves one- or two-electron transfers, remains to be determined. As indoles are expected to have fairly low oxidation potentials [by analogy with enamines (Table 7)], it is not unreasonable to expect indole derivatives to undergo one-electron transfer processes.

2. Applications Some examples of amine oxidations which may involve aminium radical intermediates will be discussed here; the reader may find more examples in the original publications. We must caution that these reactions are proposed to involve aminium radicals for various reasons but very few have been proved to do so beyond any doubt. These may provide the reader with intellectual exercises in mechanistic chemistry. Hopefully this may lead to new developments and further progress in amine oxidation.

A. Aliphatic and Aromatic Amines Regiospecific oxidative dealkylation, particularly demethylation, is a highly desirable method in natural product chemistry. Examples of demethylation which might involve the aminium radicals are shown in equations (68),213 (69),209 and (70).209 Photosensitized demethylation of codeine 214 has also been reported.

K 3 [Fe(CN)61. KOH

(68)

;CSNHCH'

N(CH 3 h

02.h,. MeOH methylene blue

t There are

many references available, e.g., Reference 212.

)

(69)

200

Yuan L. Chow

OH 0 •• h•• MeOH methylene blue

(70)

~

(CH 3 ).N CH 2 0H

Oxidations of aliphatic amines often lead to conversion of the a methylene to a carbonyl group through a secondary oxidation, as indicated in equation (56), giving amides. Depending on conditions, amides can be the major products, as shown in equations (71)215 and (72).206.209.213

CH'(J)?

KMnO.

CH3f~

)

o~Ny o

o

x = 0 CH _ I' X - 0 n , or 2

(i) n = 0

(ii) n = I. X = H(OH)

}h

v,

0

...

2. senSitIzer In

Fe(CN)6 0

C H

6 6

-

(71)

50-60%

87'70

Oxidative coupling of carbazoles yields the corresponding dimer 74 (100%), which could be identified as the cation radical 75 ('\max 390 and 800 nm for R = C2H5).75

C§t:@

Pb(OAc). HCIO,

I

R Pb(OAcl.

(73)

Fe(CN)6'-

74

7S

R = CH ... C 2 H s• CH(CH"),, C.Hs

The cupric ion oxidation illustrated in equations (74) and (75) may involve ligand-metal electron transfer complexes as illustrated in equation (59): the

Aminium Radicals

201

yields of various benzimidazoles (e.g., 76) and (2H)-benzotriazoles (e.g., 77) are excellent. 79

@C}-CH.

Cu(OAch

(74)

H

76 75%

o::

;;;o--N

CuSO. ) pyridine

NCaH.-o-NH2

""--N

~

(75)

77 657.

Copper-ion-catalyzed auto-oxidation of a-phenylenediamine alters the courses of reaction depending on the conditions [equations (76) and (77)].216.217

i (X ~ NAV~NH

N~NH2

(76)

I

H CN

( ~

(77)

CN

In view of their possible applications as antioxidants for hydrocarbons, rubber, and others, the roles played by substituted p-phenylenediamines in autooxidation processes have been investigated. The reaction of peroxy radicals with (CH3hN--@-N(CH3h

+

R0 2·

0/

----'>-

(CH3)2N -(O>-N(CH3)2 ------+ (CH3hN -@-N(CH3h


-CN

+=

-=-CN

+

="

CN

= \

+

eN

SCHEME 22

N~

~NI

-----+

N~ JLN--'l

""'N*

j-~N'

N:*

~N).

-----+

I

N=N

173

J:~

CN

174

175

* N=N

(NfN* I

N

I

CN*

-----+

N

*/CN N

N~ J(N H

-----+

I

1,

*

£;. * N

C)~· I --'l-~~ (N/ 1) N

N • =

15N

310

Curt Wentrup

(Scheme 21). Therefore, interconversion of the two nitrenes via 171 is excluded, and the mechanism of ring contraction is most likely mechanism 4, leading to N-cyanopyrazoles. Further details are given in Reference 16.

C. The Carbodiimide Mechanism Both 4-pyrimidyl- and pyrazinylnitrenes undergo ring contraction to N-cyanoimidazoles in the gas phase as well as in solution, the former with label migration, the latter without. The results are most easily accounted for in terms of mechanism 4, yielding N-nitriles (Scheme 22).16 The gas phase pyrolysis of 173 gave 175 in 95-100% yield, together with a very small amount of a nitrile which may be 174. Imidazole 174 may, in principle, be formed by means of either mechanism I, 2, or 3. Imidazole 175 can, however, only be formed by either mechanism 4 or 5. If mechanism 5 were correct, it would have to be completely specific [see equation (53)].

(53)

Analogous products are formed in the benzo-annelated cases summarized in Scheme 23. The label migration in the 4-quinazolylnitrene (177) is ~ 100% in the gas phase, and ;?: 7870 in solution. 16 In the gas phase, however, the quinoxalylnitrene (179) gives an 870 yield of indolo[2,3-b]quinoxaline (184); this compound was not detectable in the pyrolysates from 176. Furthermore, trapping of the nitrenes with octanethiol in solution thermolyses resulted in formation of 181 from 176, and 183 from 180. 84 Crossover products were not detectable. (The opposite was true for the pyridylnitrenes; see Scheme 17.) The ring contraction product 182 was isolated under the reaction conditions (and shown in the absence of octanethiol to be ;?: 78% labeled in the ring). Therefore, 182 can hardly be formed directly from 177, at least not when starting from the quinazoline derivative 176. The previously mentioned possibility16 that the carbodiimide 178 (or its mesomeric carbene) is responsible for the formation of I-cyanobenzimidazole (182) therefore seems to be the correct one. Carbodiimide 178 is unique inasmuch as the carbodiimide structure should be favored over the carbene structure due to loss of aromaticity in the latter (cf. calculations on the carbocyclic analogs 28 ). The whole reaction is unique in that it occurs in solution as well, despite the fact that a bicyclic intermediate 185 would be highly unlikely on energetic grounds. 16 Consequently, the reaction 177 ~ 178 is best described as a concerted 1,2-aryl shift16:

Gas-Phase Behavior of Arylcarbenes and Arylnitrenes

311

/ctN~JlN ~

177 ~

----+ 178

~

~

'"

185

This reinforces the statement (Section 11.1) that bicyclic intermediates are not necessarily involved in ring expansions [see also equation (18)].

SCHEME 23" *N-N

N=N

Q>' I

~

~

CX~X\;

N

N~C.08)

Salt

Catalyst

Reference 78.

b Relative yields in parentheses.

TABLE 4. Effect of Additives upon Yields and Product Distribution in the Reaction of Cyclohexene with Dimethyl Diazomalonate Using (CH 3 0hP·CuCI as Catalyst a Additive (30 mmol)

14

18

15

None CuCI (CH 3 OhP (CH 3 O),PO (CH 3 O),CH 3 PO [(CH 3 hNhP [(CH 3 ),NhPO CuCl 2 (CH 3 O),HPO

63.9(1.00)b 33.5(1.00) 46.1 (1.00) 68.1(1.00) 60.6(1.00) 49.9(1.00) 45.8(1.00) 55.0(1.00) 59.5(1.00)

13.3(0.21) 33.4(1.00) 21.2(0.46) 16.2(0.24) 19.2(0.32) 24.3(0.49) 17.3(0.38) 18.0(0.33) 25.1 (0.42)

4.7(0.07) 2.3(0.07) 3.8(0.08) 5.9(0.09) 5.1 (0.08) 4.6(0.09) 3.7(0.08) 5.3(0.10) 4.3(0.08)

a

Reference 3.

b

Relative yields in parentheses.

Metal-Salt-Catalyzed Carbenoids

335

TABLE 5. Product Distribution and Yields in the Reaction of Cyclohexene and Dimethyl Diazomalonate as a Function of Catalyst and Peroxide Content of the Olefin"·b Xin (CH 3 OhP'CuX (0.14 mmo\) Br e Ie Brd Id Br e Ie No catalyst e

Percent yield

3

4

7

Ratios

6S.7 74.3 22.0 19.9 71.6 78.S 9.45

15.1 12.9 1.9 2.0 22.9 15.0 I.S

6.7 7.4 2.1 I.S 5.7 5.9 0

1.00:0.22:0.10 1.00:0.17:0.10 1.00:0.09:0.10 1.00:0.10:0.09 1.00:0.32 :O.OS 1.00: 0.19: O.OS 1.00:0.19

Reference 78. All reactions were run as follows: A solution of 0.02 mol of dimethyl diazomalonate in 0.25 mol of cyclohexene was added at the rate of 6 drops per min to 0.25 mol of boiling olefin containing the catalyst or catalyst plus peroxide. After 24 h at reflux. the excess olefin was removed by distillation. and the residue analyzed by GLC using diglyme as an internal standard. C Commercial cyclohexene. d Commercial cyclohexene filtered through alumina. e Commercial cyclohexene filtered through alumina. then 0.07 mmol benzoyl peroxide added. a

b

TABLE 6. "Copper(O)-Catalyzed" Reactions" Percent yield Conditions b

14

15

18

Thermal Cu metal Metal-free filtrate

12.7

trace

38.0

8.1

36.0

7.2

0 1.7 2.S

a Reference 78. " All reactions were initiated under nitrogen to prevent atmospheric oxidation.

TABLE 7. The Effect of Peroxide upon the Cu(AcAc)zCatalyzed Decomposition of Dimethyl Diazomalonate in Cyclohexene a Percent yield Conditions"

14

15

18

Peroxide free Peroxide present

7S.5 7S.1

12.4 12.4

5.9 5.S

a Reference 78. " All reactions were initiated under nitrogen to prevent atmospheric oxidation.

336

David S. Wu/fman and B. Poling

The parallel between the reactivity of olefin in the catalyzed process with its stability in the copper(l) halide complex towards diazomalonate has also been interpreted as evidence for a ternary complex leading to products. 75 Steric factors in this carbene precursor, however, may have influenced the reactivity pattern of these olefins, since there is no evidence for olefin complexation with the catalyst used.

The objections to the original claims can be stated as follows: (a) Why does Cu(II) catalysis give better yields than Cu(I), when all other factors are constant? (b) How can one argue that the Cu(1) was recovered quantitatively in the form of a complex after the reaction when Cu(O) was added to clarify the solution and therefore would have reduced any Cu(II) to Cu(I)? An explanation supporting copper(I) over copper(II) as the catalyst was proposed 86 as follows: Among various copper complexes employed as catalysts, those in which the copper ion is relatively uncomplexed are the most effective in promoting cyclopropanation. For example, copper(I) triflate 71 is much more active than chloro(trialkylphosphite) copper(I)19.80 in catalyzing the reaction between ethyl diazoacetate and olefins. This difference underscores the care which must be exercised in interpreting the effect of adventitious peroxides on the catalytic rate. For example, Wulfman et aP8 have used the observation of a peroxide effect to conclude that copper(II) species are the actual catalysts in cyclopropanation promoted by chloro(trialkylphosphiate) copper(I) since it is known 81 that peroxides oxidize copper(I) to copper(II). However, it is also known that trialkyl phosphites are readily oxidized by peroxides to trialkyl phosphates 82 ,83 which are very poor ligands: (RO),P·Cu(I)X

+

peroxide -----+ (ROhPO

+

Cu(II)X +

etc.

Thus, the subsequent re-reduction of copper(I1) by diazo compounds 71 • 84 gives an altered copper(I) species in which the phosphite ligand has been stripped, e.g., Cu(II)X +

+ R2CN2

-----+ Cu(l)

+ N2 + !(R 2CXlz

The net effect of peroxides is to generate a more active copper(l) catalyst than the one added. Such an explanation also accords with the absence of an observable peroxide effect 78 when the copper acetylacetonate (which contains no readily oxidizable ligand) is used. In the absence of definitive evidence to the contrary, it still appears that copper(J) species are the catalysts in the cyclopropanation of olefins with diazo compounds. 86

Speaking against the above interpretation are the following: (a) The difference in reactivity between copper triflate and copper(I) chloride-trialkyl phosphites is a fact completely in agreement with our papers showing a common ion effect and Bethell's paper showing a pre-equilibrium between catalyst and diazo compound. 85 Rejection of the common ion effect was based upon a failure to note that insufficient common ion was present to saturate the ligancy. One could reject Bethell's results as not being representative because of the solvent system and diazo compound involved. Indeed, Bethell himself warns against extrapolation to our work. (b) Copper(lI) fluoborate and copper(lI) perchlorate have sufficient solubility in cyclohexene, cyclooctene, benzene, and hexafluorobenzene to furnish good esr spectra. In order to saturate the ligancy of the phosphite-based catalysts,

Metal-Salt-Catalyzed Carbenoids

337

the tetramethylammonium iodide and tetramethylammonium chloride must also be sufficiently soluble to react with the catalysts (even to saturate the ligancy). Since there was a real, though somewhat modest effect ( '" 25% suppression of process) by adding tetramethylammonium fluoborate one must assume that this case of suppression does involve solubility and acts via an ionic strength effect, for it is clear that fluoborate has absolutely no ability to serve as a ligand. (c) Our interpretation that adventitious peroxides lead to copper(II) is based upon the use of added peroxides as well as the presence of adventitious peroxides and we noted and reported at the time that oxidation of the phosphite would also occur. Since the resulting phosphate is a poor ligand one might ask why externally added phosphate leads to increased yields. We argued that the phosphate may be a poor ligand but it is a gross improvement as a solvent for salts and anions. The net effect is to generate a better catalyst system whether it be Cu(l) or Cu(n). (d) The absence of a peroxide effect with copper(II) acetylacetonate is consistent with the lack of an oxidizable ligand and with either valence state being active. (e) Kochi 71 may have published the definitive evidence for copper(II) being the catalytic species. When he employed copper(II) triflate and copper(II) fluoborate as catalysts, the yield of products based upon diazo compound was higher than that obtained with the corresponding copper(I) salts. Since diazo compound will be consumed in altering the oxidation state from (I) to (II) or (II) to (I), a fall in yield will be evidenced when the needed species must be generated. Thus, higher yields from copper(II) suggest that more Cu(II) is required than copper(l). The most one can say in favor of Cu(I) is that some Cu(I) may be required in the presence of a relative excess of copper(II). Our own present and published feeling is that valence state is a function of many factors, e.g., ligands, anions, solvent, diazo compound, substrate, concentrations, and temperature. However, the range of valence may be 0 to + 3 and not + I or + 2. On the question of yield, the argument against Cu(II) being the catalytic species was as follows 86: (a) Copper(l) triflate is much more active catalytically than is copper(II) triflate; (b) the copper(I) salt is much more soluble than the copper(II) salt because of complexation by the olefin, whereas the copper(lI) triflate requires solubilization by methyl acetate; (c) the better yields (Table 8)71 obtained with copper(II) as catalyst may have resulted from a trace of copper(I) being present; (d) most of the copper(II) was not dissolved and the systems were heterogeneous; a small steady state of copper(l) generated by reduction of copper(lI) by diazo compound could account for the activity; and (e) there may be a competing reaction in which copper(I) reduces the diazo compound, which could be important at high concentrations of copper(I). The above interpretation points out once again one of the basic problems associated with many of the studies of catalytic decompositions of diazo compounds; the comparison of apparently comparable processes but at different

David S. Wul/man and B. Poling

338

TABLE 8. Competitive Reactions of Diazoacetic Ester with a Mono- and

Tetrasubstituted Olefin a

>=
Y) T

= 0°

T

= 45°

T

= 60°

T

,

r------,

ds-) =

&) r-------, I

Y

120'-

, \ \

\

I

I

I

I

I

I

,

I I

I

T

Eclipsed min. gauche

= 240'-

T

= 270 0

T

,

-------, \

\

Staggered droite

\

= 300' -------,

T

/

\

\

\

I

= 340°

------ "\

\

\

\

\

Eclipsed max. droite

I

I

I

I

I

X

Y

I

I

I

I

/

I I

T

~ Eclipsed min.

,

Y

Skew droite

180°

T=

\

,

Eclipsed max

T

= 90°

Eclipsed max. gauche

Staggered gauche

Eclipsed min. droite

Skew gauche

On occasion, chemists have expressed their concern with our formulation of the copper carbenoids as being pentacoordinant of the ML 4 X type on the basis that such a formulation with copper(II) involves 19 electrons and therefore violates the 16-18 electron rule. t This point initially was of concern to us as well. However, we feel there are several ways to accommodate the additional electron without necessitating the presence of 19 electrons on copper. It should be noted that the 16-18 electron rule only applies to molecules and intermediates with an even number of electrons.

R = CH 3 or t-C,H9

t For a treatment of the 16-18 electron rule see References 91-101.

David S. Wu/fman and B. Poling

350

SCHEME 20. Orientation of the p Orbital on C in TBP Carbenoids as/(r)68

,,

, \

\

I

I

I

I

I

I

./

I

I

If the valence state were + 3 the problem would vanish. The possibility of rapid electron transfer processes occurring to attain such a condition is totally plausible and the driving force might be the lack of stability of a 19-electron configuration. We have observed such electron transfer phenomena with complexes of Cu(I) and Cu(II) occurring rapidly on an nmr time scale. If Cu(III) is required to furnish a "stable carbenoid " then one might perhaps see beneficial effects by employing a one-electron oxidizing agent (e.g., N-O·) in a bridge at a position capable of interaction with the metal. There are obviously a large number of 7T and 7T* orbitals involved in a his. acetylacetonate complex of any metal in addition to the p and d orbitals of the metal. If an odd electron needs to be accommodated, it may wel1 enter into a low-lying 7T* orbital and give rise to only a minor perturbation of the system. An alternative possibility (which is equivalent to the above) is to invoke resonance structures in which alternatively one O-Cu bond is broken. If one places a full negative charge on the oxygen a 17-electron system would result. If homolytic cleavage occurs with a single electron retained on the oxygen and one electron on the copper, an 18-electron system would result. Thus at least three fairly plausible paths are open to permit the generation of a ML 4 X copper carbenoid which obviate the 16-18 electron rule. The potential paths for carbenoid reactions are numerous but manageable. SCH EME 21. Orientation of the p Orbital of C in TP Carbenoids as I( r)68

r

= 0°,90°, \80°, or 270°

Eclipsed

r

= 45° or 225

Exo

0

r

= 135 or 31 Y 0

Endo

351

Metal-Salt-Catalyzed Carbenoids SCHEME 22

XCY

Co,llp) , ,,

Cu

'---0· 0--- '

',

XCY +------+

('1.11") , (

,

'-- 0

I

:

+------+

\ ---0/ .0---

(oJI?) /., Cu ) ' '-- 0 0--- '

,

~

C'!II?) Cu ) / \ ' '---0 0---'

(

,

I

I

XCY

XCY

(

'

0---'

I

C'1.lp)" Cu

Cu

XCY

XCY (

/

+------+

Co,I?) ,

,

/

Cu \

1

1

'---0 (}::--'

We have generated a system for the purpose of classification of carbene transfer processes (MT). (The initial M for methylene was used in preference to C. CT is the standard abbreviation of charge transfer.) Since the reactions involved are often exceedingly complex and it is difficult to identify the ratedetermining step and because one of the "readily" identifiable features of a reaction is the products, we base our analyses on the product-determining step. Since we are concerned with the group CWZ which is capable of forming a maximum of two bonds, we divide the processes into two basic categories for carbene transfer: (a) those in which only a single bond is made or broken, and (b) those in which two bonds are made or broken. Accordingly, MT processes are either synchronous (MTsync) or nonsynchronous (MTnsync). For the sake of simplicity, of these two we assume that most processes are of the MTnsync type and we write these as MT, and only employ MTsync for the less common process. In terms of molecularities, the group CWZ always arises from species Q to which it is bonded and will become bonded to a substrate S. For convenience we designate the car bene as M and any metal as ft. If the intermediate Q contains M and ft, we write the species MIL. If M is not formally doubly bonded to ft, but rather is attached to some other group (residue: R) as well, e.g, N 2 -as in a diazoalkane metal complex, we designate the species as RMIL. We write a diazoalkane as RM.

352

David S. Wulfman and B. Poling SCHEME 23

~~"L" 36

35 CuL.

II CX.

+

X

36

2+2 -----+

35

CuL.

+ 37

t

C,UL.

~ CuL. +

X.

38

39

37

There are two basic ways MfL or RMfL can transfer M or RM to S to furnish a product RMS or MS: (a) dissociation to furnish M or RM, which then reacts with S to give the product, an MT-J process, or (b) a bimolecular process MT-2. The MT-J or MT-2 process may, however, involve a substrate activated by the catalyst fL. These processes we designate MTE for car bene transfer with substrate enhancement. A well-defined example of the MT-J process exists in the work of SCHEME 24A

U~

p
+

II

N2

O=P(OMe)2 428 8/0

427

8. Insertions into X-H Bonds (X "# C) Catalyzed by Lewis Acids The decomposition of diazoalkanes by Lewis acids which fail to strongly backbond leads to reactions one might easily ascribe to carbenium ions. Such processes tend to lead to the formation of a single bond to the diazo carbon and only rarely furnish cyclopropane derivatives. rn those cases where a new C-C bond is formed it is not unusual to find the same product available from the expected cyclopropane after treatment with a suitable Lewis acid. rn the case of insertion reactions into X-H bonds (X "# C), there exist numerous examples where a proton can serve as the catalyst. (A proton is a special case of a Lewis acid incapable of backbonding.) When the group X bears a nonbonded electron pair the most probable reaction sequence is that represented below in which we have employed 0 + as the acid: RR'CN2

+ D+ +

RR'DC-~2

~

RR'DCN 2+ +

~~ -

H-t

+ N2

----+ RR'DCX- H ----+ RR'DCXRH

RR'D~~~::H COCHN 2

~NA4> H

D. Insertions into M-H Bonds A[though metal hydrides are capable of reducing diazoa[kanes, a few cases are known in which insertion with loss of N2 results. These are summarized in Scheme 77 and are in no way meant to represent the total number of known examp[es.464-468

9. Insertions into C-H Bonds Insertions into C-H bonds by carbenoids is a relatively uncommon process. The cause is clearly the reduced reactivity of the intermediate as compared to SCHEME 77

~

CH 2 N 2 --~

Oc....--Mn-CO I "-H C 0

~

~

(Ref. 468)

Oc/Mn-CO "'CH 3 0

d

CH 2 N 2

-----*

/W-H OC I "'co CO

~

-----+

/W-N 2 CH 3 oc I"'co CO

~

(Ref. 468)

OC....--W-CH3 I "-CO CO (Ref. 467)

n-Bu 3SnH

+

RCHN 2 (R =

4>SiH 3

~ n- Bu 3SnCHR H,Ac,CN,4>CO,orCOOEt)

+ CH 2 N 2 ~ 4>Si(CH3h

(Ref. 465, 466)

(Ref. 464)

432

David S. Wul/man and B. Poling SCHEME 78

CX::N'+CQ DM~ ~o o

01 o

439b

photochemical assisted car bene generation. The process may of course be present to a small extent in many catalyzed diazoalkane reactions but overlooked because of interest in the cyclopropane or ease of isolation of other products. The first recognized example of a C-H insertion under the influence of copper or its salts is due to Lansbury (Scheme 78).469 When the reaction with copper powder was carried out in benzene, the tetralin was generated, 529 whereas carrying out the react on in acetonitrile or dimethylsulfoxide failed to furnish this product. In the absence of copper no reaction occurred, even after prolonged heating under reflux. The C-C insertion product (439b) was only found in the dimethyl sulfoxide reaction and only to the extent of '" I %. More recently Peace 74 and Gaspar and Jones have also observed the copper-catalyzed insertions of carbenoids into C-H bondsYu The reaction of diazo esters with cyclohexenes has already been discussed (Schemes 58 and 64).

10. Insertions into X- Yand X Rings

Y Bonds Not Generating Three-Membered

An important synthetic reaction is the addition of diazomethanes to the carbonyl group of ketones to homologate the ketone. This type of process, when catalyzed, invariably involves a non-backbonding Lewis acid and is not discussed here. A discussion can be found in References 2, 151, 161,472-475 and numerous references are cited therein. The homologation also proceeds to furnish some epoxides but this is also outside the scope of this chapter.

Metal-Salt-Catalyzed Carbenoids

433

The reactions with C=S are of interest because the possibility exists that copper catalysis is of some use. The ability of a species to insert into a bond X- Y is controlled to some degree by the energy of the attacking species and by the bond energy of the bond X-Y. There will be an important contribution from the entropy of activation when the energy of the attacking species is comparable to or lower than the bond dissociation energy of the bond being ruptured. The overall processes in many cases will be highly exothermic after initial generation of the attacking species; e.g., for the insertion of CH 2 into a C-C bond there is a net increase in the number of C-C bonds so the dissociation energy is recovered and a highly unstable species is stabilized as well. The data in Table 39 allow one to estimate the relative susceptibility to rupture of an X-Y bond and the problem is then one of classifying the reactions into types and determining the lower limit towards the type of reactions. We can readily surmise that if a process of insertion is going to occur, a benzylic or allylic system will often be most favored. Thus, if one hopes to see a particular type of insertion, the test substrate should be a benzylic or allylic system. To further favor the process, an intramolecular process possessing a reasonable degree of flexibility and permissible geometry should also be considered. Clearly, variations in these last parameters can reveal information regarding the mechanism of the reaction. When strong bonds are ruptured it is reasonable to suspect that the process is not a direct one when carbenoids are involved. This point makes the observation of insertion into C-H bonds with retention of configuration all the more noteworthy.471 Insertions into C-B bonds by diazoalkanes in the absence of metal catalysts probably proceed as shown in the Scheme 79. With C-X bonds involving an atom X possessing non bonding electron pairs the formation of an ylid such as is shown in Scheme 80 is highly probable. TABLE 39. Bond Dissociation Energies (kJ mol-1)a

X X~Y

bond

CHJ~X

C6H5~X

C6H5CH2~X

CH 2=CHCH 2 -X CH2~~CH-X

(C H3lJC-X (CH 3hCH-X CH3CH2~X a Reference 496. " EstImated by author.

H

F

Cl

Br

435 431 356 356 431 381 395 4\0

452 485

349

372 b 372 b 452 b 397 b

439 444

272"

293 301 213"

272 b

213 b

351 328 339 341

293 264 285 289

351 b

234 238 b

167 172 234 205 222 224

CH3

BR2

SR

368 389 301 301 385 335 351 356

268 b

285 b

444" 289"

213

289 b

213 b 205 b 213 b

351"

213"

David S. Wu/fman and B. Poling

434

SCHEME 79 R

R

I R-8 I

R

I

R-B--CXY

r~2+

R

-----11--+

I I

+

R-B--CXY R

1

R"B-CXYR R/

A. C-C Bonds Insertions into C-C bonds are highly unlikely processes and the example given above (Scheme 78) is perhaps almost unique. 678 The reason is quite clear: the C-C bond is of relatively high energy ( ~ 350 kJ mol- 1 ), other reactions are possible including dimer formation, and the orbitals are not readily accessible to bulky reagents.

B. C-B Bonds I nsertions into C-B bonds are potentially of synthetic importance and have received considerable attention. Papers by Hooz 477-481 and Pasto, 482.483 although not representing all of the work in the area, are typical. Some of the reactions are given in Schemes 81 and 82. A review on the reactions of boron compounds with diazoalkanes appeared in 1964. 484 C. C-Halogen Bonds

Many carbon-halogen bonds are relatively weak compared to C-H and C-C bonds (Table 40), and because the halogens possess nonbonded electron

M

+

MCYZ

N 2CYZ

+

RXR

SCHEME 80

--

M-CYZ

~

R2X-CYZ

~

I

-----+ M=CYZ

N; I

M-

~

R2X-CYZ

~

RX--CR YZ

435

Metal-Salt-Catalyzed Carbenoids SCHEME 81

o N2

MeO

MeO

82'70

°

0

Lt'

EI,B

-----+

-----+

Lr 98'70

pairs, N processes can be expected to occur with carbenes and carbenoids. Only C-F bonds should be relatively resistant to attack. Some results employing copper(II) sulfate as catalyst, diazoacetic ester and C-Br and C-C1 bonds are summarized in Scheme 83.485-488 Atkins and Trost 489 examined the catalytic and photolytic decompositon of l-chloro-4-diazoalkenes in the course of studies directed towards tetrahedrane. Their results and others' (Scheme 84 and Table 41) indicated that very little C-C1 insertion was occurring in the intramolecular process (2.5-970)' SCHEME 82

BU3B

+

COCHN 2 -----+

Bu 2 BO" /H C=C -1,/ " Bu 'f-' LiO C -1,/

¢= o

I

°

/

C, ,

CHJI

-----+ COCH(CH 3 )C.H g

Bu

°II

N2

-N2

)

H "

'f-'

2 BuLl

~

~ o

(Ref. 482, 483)

72%

o

~

436

David S. Wu/fman and B. Poling

TABLE 40. Product Distributions for the Reactions of Diazo Esters with Allyl Chloride (A), ,B-Methylallyl Chloride (B), y-Methylallyl Chloride (C), and Allyl Bromide (D)

Diazo ester

Substrate

N 2 C(C0 2 Me), N 2 C(C0 2 Me), N 2 C(C0 2 Me), N 2 CHC0 2 Et N 2 CHC0 2 Et N 2 C(C0 2 Me), N 2 C(C0 2 Meh N 2 C(C0 2 Meh N 2 CHC0 2 Et N 2 CHC0 2 Et N 2 C(C0 2 Meh N 2 C(C0 2 Me), N 2C(C0 2Me), N 2CHC02Et N 2 CHC02Et N 2CHC0 2Et N 2CHC02Et N 2C(C02Me), N 2C(C02Meh

A A A A A B B B B B C C C C C C C

Method of decomposition of diazo ester hv (direct) hv (Ph 2 C=O sens.)

110°C, CuCI hv (direct) hv (Ph 2 C=O sens.) hv (direct) hv (Ph 2 C=O sens)

110°C, CuCI hv (direct) hv (Ph 2 C=O sens.) hv (direct) hv (Ph 2 C=O sens.)

110°C, CuCl hv (direct)

80°C, 60 min, CuCI 80°C, 9 min, CuCI 80°C, 60 min, Cu hv (direct) hv (Ph 2C=O sens.)

D D

% Yield ofC-X insertion product

% Yield ofC=C addition product

Ref.

53 5 32 21 7 25 Trace 35 57 6 38 Trace 35 16 9 12 6 38 4

23 88 3 18 69 22 86 4 30 85 15 49 Trace 8 2 3 3 6 30

122 122 119 119 119 122 122 122 119 119 122 122 122 119 119 119 119 119 119

SCHEME 83

r/> . . . . . CHBr / r/>

+

N2CHC02Et - * r/>CHBrr/>CHC02Et

(Ref. 485)

440

440

+

r/>CH 2Br

440

+

CH 2=CHCH 2Cl

440

+

CICH=CHCH 2CI

440

+ CH 3CH=CHCH 2Cl

- * CH 3CH=CHCH 2CHCIC02Et

440

+

~2=CHCH(CH3)CHCIC02Et

--'>-

r/>CH 2CHBrC02Et

(Ref. 485)

~ ~ CICH~CHCH2CHCIC02Et

CH2=CHCHCICH 3

(Ref. 485)

Metal-Salt-Catalyzed Carbenoids

437

SCHEME 84

~CHN'

Hgl. CHaOCHa -18'C (N. atmos.)

44S

+

.JY CHCI

CHCI

CHCI H

+

N

CH'~

(

H

+

Jy+Jy

+

CHCI

CHCI

CHO

I I

I I

-CH 3

442

441 CHCI

CH 3

§---

+

CH 2 OCH 3

~

(Ref. 489)

444

CI

H

443

Andot has examined a number of N-type processes in which the hetero atom is allylic. The resulting ylid intermediates are therefore ideally located for a symmetry-allowed rearrangement process. Such behavior was clearly established by employing suitably substituted substrates (Schemes 85 and 86). From the data in Table 40 it is evident that" singlet car bene " and the carbenoid lead to insertion products, whereas "triplet carbene" furnishes the cyclopropane. The intermediacy of ylids is inferred on the basis that, with diazomalonates and sulfides or sulfoxides, "stable" ylids can be isolated.

D. C-O Bonds Carbon-oxygen bonds should be readily susceptible to attack by carbenes and carbenoids since the formation of oxygen carbon ylids would appear to be TABLE 41. Product Distribution from Decomposition of Diazoalkene (445)

441 Catalyst

HgI 2 CuCN CuBF. LiBr ZnCI, hv

t

442

Trans

Cis

Cis-cis

Trans-cis, trans-trans

12 25 35 25 20 17

37 47 63 50 60 31

3 4

20 6

3

10 15

References 119, 121, 122,490-511.

443

444

Ref.

52

489a 489b 489 489c 489d 489

3 9 2.5 5 5

438

David S. Wulfman and B. Poling SCHEME 85 R

"X

N.CYZ

/

CH. CH=CH 2

)

"

SCHEME 86

SCHEME 87

EtOR = HorCH,

SCHEME 88 R' N 2 C(C0 2 CH3h 446

+

RSR'

'-..+

R/

C0 2 CH 3

-/

S-C

"

CO.CH3

Metal-Salt-Catalyzed Carbenoids

439

o ~s

SCHEME 89

N 2 CHC02 Et) Cu(AcAc)2

~

-----+

s+

)

jC0 2 Et

an allowed and favored process. Such ylids in turn should be highly reactive and undergo rearrangement to furnish insertion products. Gutsche observed such behavior with two dioxolanes which reacted with diazoacetic ester at '" 140°C to furnish dioxanes (Scheme 87)117 and postulated the oxonium ylid mechanism. More recently Nozaki has inserted "carboethoxycarbene" into phenyloxetane by the copper-catalyzed decomposition of diazoacetic ester. U8 Ylid formation is probably responsible for the low yields obtained when dimethyl diazomalonatejcopper catalysts are used in ether solutions of olefins to form cyclopropanes. 3

E. C-S Bonds Ando has recently presented a very detailed summary of the formation and reaction of sulfur-derived ylids prepared from diazoalkanes (Schemes 88-94 and Table 42). This work is of considerable importance and the review is recommended for a more complete treatment. 119 SCHEME 90

o

446

---+ Cuso,

OyCO,CH, C0 2 CH 3

68'70

o s

C~,)

Q S

51

C0 2 CH 3 C0 2 CH 3

'70

440

David S. Wu/fman and B. Poling SCHEME 91

- ;QJ RNH

SCH(C0 2 CHah

446

C uSO.

o

C"",,,.

~

C0 2 CH a

RR'ON~N;\

)

)-~~

o SCHEME 92

CH 2 SCH 2 CH 2 COCHN 2

-

CuSO.

C0 2 Me

I

EtSCC(MehCH=CH 2

I

44~

EtSCH 2 CH=CMe 2

CO 2 Me

~SO.

95%

~~ EtSCH 2

C0 2 Me

C0 2 Me

SCHEME 93. 51

RCH 2 SCH 2 C0 2 Et

N 2 CHC0 2 Et)

CuC)

+

RCH 2 SCH 2 C0 2 Et

I

-

CH 2 C0 2 Et

+/

RCHS"

CH 2 C0 2 Et

-CHC0 2 Et

1

RCH 2 SCHC0 2 Et

I

CH 2 C0 2 Et

cuc'l N 2 CHC0 2 Et

RCH=CHC0 2 Et

/

CH 2 C0 2 Et

+ S" CH 2 C0 2 Et

441

Metal-Salt-Catalyzed Carbenoids SCHEME 94

11;CC;xr H

S

~N N 2 CHC0 2 Et» Cuso.

'"

0

o//

N

C0 2 Et

" ,,// h

Me02C'"

The reactions of diazoalkanes with the C=S group are of some interest in that this is a route for converting a carbonyl group into an olefin. Schonberg512-522 published extensively in this area, and more recently French workers have been very active.523-530 The work of Scho'nberg sometimes involved the use of a copper bronze catalyst [or copper(lI) sulfide] and olefins were usually obtained. In other cases, no catalyst was used and olefins and thietanes were generated. The removal of the sulfur was then accomplished with copper bronze. It is, therefore, not clear whether the reactions proceeded via copper carbenoids or whether the copper was used to facilitate ejection of sulfur. Some examples of the types of C=S groups and diazoalkanes used are given in Schemes 95-99.

F. Reactions of M-X Bonds The reactions of metal salts with diazoalkanes on a stoichiometric basis has received considerable attention. One of the earliest studies was that of Wittig and Schwarzenbach 69.531 in which diazomethane was treated with a variety of metal salts. The products isolated included halomethyl metal compounds, TABLE 42. Product Distributions in the Reactions of Ethyl Diazoacetate with

Alkyl Allyl Sulfides a

Substrate

N 2 CHC0 2 Et (mmol)

MeCH=CHCH 2 SEt MeCH=CHCH 2 SEt MeCH=CHCH 2 SEt MeCH=CHCH 2 SEt MeCH=CHCH 2 SEt

1.8 1.9 1.8 1.8 2.0

H 2 C=CHCH 2 S-n-Bu H 2 C=CHCH 2 S-t-Bu H 2 C=C(Me)CH 2 S-n-Bu

4.4 4.4 4.4

a

Reference 119.

Mode of decomposition of diazo ester hv (direct) hv (direct) 90°C,2 min, CuCI 90°C,2 hr, Cu 80°C,24 hr, no catalyst hv (direct) hv (direct) hv (direct)

'70 Yield of C=C addition '70 Yield of product C-S insertion (cis, trans) product 6

5

23 19 95 85 25

10 13 15

15 16 23

7

David S. Wul/man and B. Poling

442 SCHEME 95

450a,b

+ 447

448

% Yield a Rl

R2

Me Me

iso-Pr

Et

Me

'"

R6

447

448

70

25 35 5E 5Z 25

H H H

H H

iso-Pr

H H H

Me

H H H

t-Bu t-Bu

Me Me

H H

H H

H H

t-Bu

H H

Me

60 85 75

70

20

H

Me Me

''""

75 85 55 40

H H

Me

'"

20

45

95

Me Me

'" '"

H H

45

Me

75 H

70

H

75

'"

Me Me H

'"

Et0 2 C

'"

a

'"

H

85

90

90

H

Et0 2 C

85 45

60

H H

451

15 E 15Z

H

Et0 2 C

449

20 10

90 90 50

70 25

35

The yields for the last three thioketones are the maximum values reported. Yields are a function of temperature and order of addition. The presence of 4SOa and 4SOb was observed by NMR but these were not isolated.

443

Metal-Salt-Catalyzed Carbenoids SCHEME 96 RCSCH 3

R'RHCN,

~

II

S

R'

7"1

CH3 S R

+ CH3S-C=CR'R" + R/

R"

452a

452b R

R

\~-r"SC H

+ CH 3S \

+ CH3S

3

R"

453 CH 3:

cs. -OM.

,R

"'"SCH3

\\\\\\

S XS R'

1-(

R

S XS R' R"

454 R'

1

I

R"

-----i>-

S~S

R"I

(

R'

SyS

II

S

S

455

456

'70 Yield R

R'

R"

452a

452b

Me Et

H

H H H H

40--60 40--60 40--60

Not isolated

H H H H

'"

H

Me

Me

H

Et

Me

H

i.\o-Pr

Me

H

t-Bu

Me

H

'"

Me Me Me

H Me Me

iso-Pr t-Bu

Me t-Bu U

454

455

21 20

-20

Not isolated 40 E 40Z 30 E 40Z 50 E 40Z 3E 60Z

453

31

456

~ }"o,otw yields

15

10E

10Z 20E 10Z 4E 6Z 32 E 5Z

Relative yields absolute yields

40--60'70 60

Yield, of 452a and 452b not given. Sugge'tlon that GC converted 4508 and 450b to 452a and 452b.

444

David S. Wul/man and B. Poling SCHEME 97

S

S

I

II

R,COR.

+

R,CR3R.COR.

+

457

+

459

,

R,O

+

R.O\ /R3(.) C=C / \ R, R'(3)

,R3(.)

R,""f-\"'I/'R • '(3)

R,R3R.C

"/C=C\

+

S

R3

/

+

R,CR 3R.CR3R.COR.

+

I

R,R.R 3CCOR.

462

S'N:;::;N

S'N:;::;N

S

II

463

461

460

% Yields R,

R.

H CH3 CH 3 Et Me H Me iso-Pr Me Me

Et CH 3 Et Et Et Et Et Me Me Me Me

Et

Et

Me

a b C

R3

R.

H

H H H H H H H Me Me H H

H

H H H H H Me Me Me Me

457

458

459

460

461

462

463

50

10

30

40 60 50 30

30 30 30

a

20 50E 50Z 45 E 55Z 20E 30Z

80

20E 30Z

Observed, no yield given. Observed, not isolated. Observed in the presence of CH.,OH.

TABLE 43. Relation Between Normal Potentials of Catalyst and Product Formed from Diazomethane Normal potential (volts) Metal Poly methylene Ethylene

M(CH.X).

-2.4 -1.7 -1.5 -0.5 -0.76 -0.4 -0.34 +0.72 +0.86 Mg Be AI Ga Zn Cd Hg 1n TI

Metal-Salt-Catalyzed Carbenoids

445

SCHEME 98

o

N.

II

II

2 4>C-C4> 464 464

+

CS.

+ S=CCI 2

~

465

o

o 465

+

I

X4>CCHN 2

+

S

II

I

X4>C-CHCICCI

ethylene and polymethylene. The authors observed a correlation between products formed and the normal potential for the metal (Table 43). However, the use of diazoalkanes to prepare organometallic and organometalloid compounds was already sufficiently advanced to warrant a review by Seyferth in 1955,532 and more recently ones by Bawn and Ledwith 484 on the reactions with boron compound, and by Poland and Lappert 533 in 1970 on a-hetero diazoalkanes. More recently reviews have appeared dealing with thecomplexation of diazo compounds with metals.534.535 SCHEME 99

S

o II

4>CCHN 2

+

II

H 2 NC4>

466

~~¢ ~

~

S

II

H 2 NCNH 2

+

CH 2 N 2

O-~~C--NH

I

C C=S HC;:/ ' S /

q,C-N

II

II

II

HC

466

--*

I

c,6C-N

4>C'0/ CSMe

\\

C-R

's/

446

David S. Wu/fman and B. Poling

A summary of some of the more pertinent examples of insertion of CH 2 and substituted methylenes into M-X bonds appear in Scheme 100. More exhaustive lists are available in References 151, 532-535. Boron. The reactions of diazo compounds with boron compounds has been the subject of an extensive review. 484 There exist a number of reactions involving SCHEME 100 GROUP II Mercury

Ref. 536 536

Hg

HgCI 2 + CH 2N. ~ CIHgCH 2CI + N. CIHgCH 2CI + CH 2N 2 ~ CICH2HgCH 2C1 + N2 (C s H 5hCN 2 + HgCI2 ~ (CSH5hCCIHgCI + N2 HgCI. + C 2H 50H + CH2N 2 - - + Hg(O) p-CH,CsH.HgCI + CH.N. ~ p-CH,C 6 H.HgCH.CI + N2 2 p-CH,CsH.HgCI -----+ p-CH,CsH.HgC 6 H.CH,-p + Hg(CH 2CI).

4 N 2CHC02C2H5

+ 3HgCI 2

~ [~~O'C'H'] Hg

536 537 536

Et

+ 4 N2 + 2 CICH2C02

HgCI 2 538, 554 541 2 N 2CHC0 2C 2H 5 + HgO ~ Hg(CN2C02C2H 5) + H20 ArHg02CAr' + CH 2N 2 ~ ArHgCH 20 2CAr' + N2 2 ArHgCH 20 2CAr' ~ Ar2Hg + Hg[CH 20 2CAr'J, 536, 539, 540

Zinc ZnCI. + 2 CH 2 N 2 ~ Zn(CH 2CI). + 2 N2 Zn(CH 2 CI)2 + C 2H 50C2H 5 ~ ZnO + CICH 2CH 2CI CICH.ZnCH 2 CI

+

+ C.HlO

537

~ (CH 3 hNCH 2ZnCH2N(CH 3 h ~

2(CH,hN

2 CI-

~

~

+

(C.HshP

-

(CH 3 hNCH 21 I +

+

(CSH5hPCH2ZnCH2P(CaH5h

) HCI ~

+

(CSH5hPCH 3

537

CI-

ZnI,

+

CH 2N 2

BeCI2

~

+ 2CH 2N 2

ICH 2 ZnI

~

Beryllium ----')0 2 N2

ICH 2ZnCH 21

+ CH 2=CH 2

69,531

69

Magnesium H 20

RMgX + CH 2N. ~~ CH2=N--NHR CH 2N 2 + MgI, ~ ICH 2MgCH 21

542, 543

447

Metal-Salt-Catalyzed Carbenoids SCHEME JOO-Continued Cadmium CH2N2 + CdI. -----+ ICH2CdCH21

69

GROUP III Boron See discussion and examples presented in the text. Thallium

TICla

+ CH2N.

-----+ CICH2TlCI2

545

Aluminum

(C2Hs).AII RaAI

+

+ CH.N.

CH 2N 2 -----+ RCH 2AIR2

-----+ (C 2H s).AICH 21

+

546,547

N2 -----+ higher alkylation products

558, 559

Gallium

GaCla

CuCI

+

+2

CH.N2 -----+ CH2=CH2 + 2 N2

Inla

+

69

Indium 3CH 2 N. -----+ In(CH.lh

69

GROUP Ib Copper N.CHCO.CaHs -----+ cis, trans-C2Hs02CCH=CHCO.C2Hs

554

97'70

CuCI 2 + N2CHC02C2Hs -----+ CICH 2C0 2C2Hs

+ CI,CHC02C2Hs +

8 16;0

40 82%

+ cis- and trans-C2Hs02CCCI=CCIC02C2Hs

554

3-6'70

TRANSITION METALS Cobalt

2 CoCI2

+ 4 N2CHC02C2Hs

-----+ 2 CICH2C02C2Hs

+ co-[t' CO'C'H'] CoCI

554 ~

Iron

FeCb

+ 2 N2CHC02C2Hs (C 6 HshP],Ir(CO)C1

(CsHshPPdCI 2],

-----+ N2C=C(OC2Hs)OFeCI2

+

+ CICH 2C0 2C 2Hs

Iridium CH 2N2 -----+ (C 6 HshP12Ir(CO)CH 2CI

+ 2 NCCHN 2

Palladium -----+ (C 6 H shPPdCI(NCCHCI)],

+

622

N2

555

+ 2 N2

556

Continued

448

David S. Wu/fman and B. Poling

SCHEME lOO-Continued

[(C.HshPPtCI2].

SiCI.

+

+

Platinum 2(CFa).CN. - - - * [(C 2H shPPtCI(CF a).CCI))2

GROUP IV Silicon CH 2 N2 ---* ClaSiCH2CI + CI2Si(CH2CI)2 - 60/0

RaSiCI

+

-

+

+

CISi(CH2Clh

30/.

2 N2

557

548, 549

- 15/0

CH 2N 2 ---* no RaSiCH2C1

(R = CH 3 • C 2 H s• or C.H5)

SiBr.

+

549, 550

4 CH2N2 - - - * Si(CH2Br). Germanium C

u pwd.

)

CI 3 GeCH 2 CI

+ N2

548

- 94':"0

CHaGeCIa

+ CH2N2

----+ CH 3 GeCI 2CH 2CI

548

78'7.

(CH 3 ).GeCI2 SnCI.

a

+

+

CH2N2

CU pwd.

) NR

Tin 4 N.CHC02C2Hs - - - * [N2C=C(OC2Hs)O)2SnCI2

SnCI.

+

Pb(OAc).

+

+

2 CICH 2C0 2C 2H s 551

Et.O I CH2-SnCI2] \ CH2N2 ----+ [ CI2Sn\ ·(C2Hs)20 CH 2-SnCI,

p

Lead a CH2N2 ----+ Pb(OAc).

+

CH 2(OAc).

+

N2

550, 552

550, 553

Note however that Pb(OAc). can be used to prepare diazo compounds from hydrazones. 56 '

boron compounds in addition to the etherification of OH bonds and polymerizations. Of particular note is the ability to insert into C-B bonds (see Section IU.B). Trichloroborazole undergoes B-CI insertion reactions with diazomethane to furnish the tris-chloromethyl borazole.559.56o Gutsche 561 has examined the reactions of tris-methylmercapto borate with diazoketones. The reaction of BF3 with diazomethane in solutions leads to polymethylene, but in the vapor phase in the presence of nitrogen as an inert diluent and operating around - SO°C, insertion into a B-F bond occurs to furnish F 2BCH 2F.562 With phenylboronic acid, diazomethane in the presence of water or ammes inserts into the B-C bond to burnish benzylboronic acid. 563 G. Elimination Reactions Accompanying a number of the reactions involving insertions into C-X bonds, one finds products which result from formal f3 elimination. Marchand 195

449

Metal-Salt-Catalyzed Carbenoids

has summarized a number of examples which are promoted by heat, light, or catalysts. The examples in Table 44 are all catalyzed but the same processes may also occur under other conditions as well.

11. Formation o/Three-Membered Rings/rom C-X and X

Y

Marchand 196 has recently reviewed the addition of carbenes to multiple bonds and the reader is referred to the article. The reactions are nowhere as numerous as those with C=C and C==C. A. C

0

As already mentioned before, the additions to the carbonyl group are normally catalyzed by a Lewis acid incapable of back bonding and are thus outside the interest of this review. However, thermolyses of phenyl trihalomethyl mercury derivatives in the presence of carbonyl compounds can serve to form oxiranes (Scheme 10 I). The reaction can be viewed as a simple collapse of a +

-

carbonyl ylid (R;C=O-CR 2) and may not necessarily involve a free carbene generated by an MT- I process but rather complexation of the mercury compound to the carbonyl group. B. C

S

In Section II.J.E it was pointed out that, in the presence of copper and jts salts, diazoalkanes do not give thiiranes with thiocarbonyl groups. However, phenyl trihalomethyl mercury derivatives do react with CS 2 to furnish a thiirane formally derived from the carbene dimer. 565

c.

C

N

The addition of carbethoxycarbonylcarbene to benzalaniline has been realized employing diazoacetic ester/Cu in cyclohexane at 80°C. The expected aziridine is obtained in 15% yield accompanied by ~40'7o of a-anilino-,Bcarbethoxystyrene. 566 Some imines also add dihalocarbenes derived from arylmercury trihalomethanes. 567 SCHEME 10J

R' HgCCI2Br

+

I

RR'CO ~ R-C-O \ /

C

/ \

CI CI R = R' = CFzCI. R = R' = CF 3 • or R = CF o• R' = CI

n-BuOH

PhCH 2 0H

H 2 C=CHCH20H

d, CuCI 2

d, CuCI 2

Substrate

d, CuCI 2

Method of decomposition of diazo ester

H 2C=CHCH20CH2C0 2 Et (17%) + O(CH 2 C02Et), (9%)

PhCH20CH2C0 2 Et (49%) + O(CH 2 C02Et), (5%)

n-BuOCH 2 C02Et (15'70) + O(CH 2 C02Et), (7%)

Reactions with Alcohols

XCH2C0 2Et or XCH(C02Et), produced by diazo esters

I

(8'70)

(5'70)

C0 2 Et

+ diethyl maleate and fumarate (31 '70) + Et0 2 CCH=CCH2C0 2 Et (24'70)

C0 2 Et

y

+ \ T C H 2 0H

C0 2 Et

y

C0 2 Et \ T C H 20CH2CH=CH 2

I

C0 2 Et Product resulting from further reaction of PhCH20CH2C0 2 Et with: N 2 CHC0 2 Et (13'70) + diethyl maleate and fumarate (15%) + Et0 2 CCH=CCH 2 C02Et (11 %)

I

Products resulting from further reaction of n-BuOCH 2 C0 2Et with: N 2 CHC0 2 Et (30'70) + diethyl maleate and fumarate (23%) + Et02CCH=CCH2C0 2Et (16%)

Other products

TABLE 44. Carbalkoxycarbene-Promoted HX Eliminations with Ethyl Diazoacetate

529

529

529

Ref.

"'v."

~

~

~

~

§

I:l

~ ;:,

~

S;

~ ~

~ .

R

OC02Et

MeOOC

~I

~

~

#

~

Ht5 These same workers have now succeeded in isolating" stable" carbonyl ylids (isomunchones) from o:-diazoacetimides (Scheme 109) which are good 1,3dipoles. At ~ 200°C the cycloadducts with fumarates and maleates dissociate and the I ,3-dipoles add to acetylenes.612-621 The results with a variety of dipolarophiles are summarized in Tables 48 s24 and 49. 615

13. Dimers, Telomers, and Polymers The use of the terms" dimers," "telomers," "polymers," and related terms as applied here is loose. These products contain combinations of XYCN 2 and CXY in varying degrees of complexity, and include derivatives of systems based upon the general formula (XYCN 2MCXY)m (Scheme 110). Thus the products (C S H 5hC(OH)C(OH)(C s H 5)2 and (CSH5)2CHCH(CsH5h, both derived from (C S H 5hCN 2 , are considered. The formation of such species often goes unreported because of the fact that they are undesired byproducts, hopefully found in small quantities, and when found, often escape the eye of the abstractor. The major products of interest are olefins (" carbene dimers "), azines, pinacols and pinacol esters, pyrazolines, ethanes, and occasional cyclopropanes and cyclobutanes. Rarely, if ever, will one encounter all of these products

462

David S. Wu/finan and B. Poling SCHEME 110

2XYCN 2

X"

/X

y/

"y

C=C

~

481

+

X

Y

"C=C/ y/

"X

X

Y "C=N-N=C/ y/ "X

481

~

481

~

XYCHCHXY

481

~

XYC(OH)C(OH)XY

481

~

XYC(OCOR)C(OCOR)XY

'/"

X

481

C

+

~

&

N=N

Y

~~

Y

'\~ C / ~/

N=N

X

X"y

481

"

~

X-, $

Y

481 ----+

_ X ~

Y

+

:::::r~~Y Y

481

X

~

coming from one single diazoalkane. In many cases, they will only be encountered in catalyzed reactions and their presence can almost be taken as evidence of a catalyzed path. Kirmse 21 has summarized much of the more pertinent data to 1970 and the author has added to this through 1976. 2 Recently, extensive studies have been performed which indicate that some of the processes can be optimized in terms of yields. 56 There have been few mechanistic studies regarding the origin of these

463

Metal-Salt-Catalyzed Carbenoids SCHEME 111

2

N2~

2eON,

o

o 2 CH30 2C(CH2)8COCHN2 ~u~ CH302C(CH2)8COCH=CHCO(CH2hC02CH3 hv

products. McDaniel has discussed the formation of maleic and fumaric esters from diazoacetic esters.56.296 Although one might only expect to encounter examples involving a single diazoalkane, the use of two different diazo compounds to generate unsymmetrical olefins has been realized, as well as intramolecular reactions to furnish cyclic 0Iefins.627-630 With diazocarbonyl compounds, the resulting dimers are often good 1,3dipolarophiles and they, in turn, add a molecule of diazo compound to form a pyrazoline (Scheme III). Peace, McDaniel, and Wulfman t examined in some detail the dimerization of ethyl diazoacetate and dimethyl diazomalonate to furnish ethylenes. The reaction products are a function of catalyst concentration and solvent. High catalyst concentrations can lead to lower yields. When benzene was used it was possible to prepare tetrakis-carbomethoxyethylene in over 80% yield. The use of benzene or hexafluorobenzene as an inert solvent is preferred. With diazo compounds of the type XYCN 2 (X =1= Y) the formation of E and Z isomers is possible. With diazoacetic ester, the E/Z ratio is a function of catalyst concentration. This is only consistent with the operation of two competing paths leading to dimer. Since both the maleate and fumarate esters are good 1,3-dipolarophiles for diazoacetic esters, the use of a very active catalyst is desirable so that no appreciable concentration of diazo compound builds up during the reaction. Copper(I1) fluoborate proved to be useful for this purpose. With low concentrations of catalyst, maleate formation was favored, while at high concentrations formation of fumarate was (Tables 50-52). Other catalyst t References 2, 3, 30, 55-57, 72, 73, 76, 78, 130, 296, 301, and 304.

David S. Wu/fman and B. Poling

464

TABLE 50. Ratios of Products from Catalyzed Decompositions of Diazoacetic Ester in Cyclohexene Solutions a Catalyst Ni(C 5 H 5h Ni(CO). Cu CuBr CuSO. ZnI 2 Cr(C 5 H 5 ), a

Diethyl fumarate

Diethyl maleate

7-Carbethoxynorcarane

1.4 6.0 0.57 0.50 0.7 1.4

2.6

1.0 1.0 1.0 1.0 1.0 1.0 1.0

1.5

0.67 0.5 0.5 0.6 0.6

Reference 647.

TABLE 51. Product Distribution in the Reaction of Cyclohexene with Ethyl Diazoacetate Using [(CH 3 0hP]n· CuI as Catalysta Norcarane Catalyst (mmol) 0.140 5.00 0.140 5.00 0.140 5.00 a

Dimer

n

exo

endo

Ratio

cis

trans

Ratio

2 2 3 3

7.80 2.91 7.87 0.97 8.05 0.33

0.65 1.16 0.68 0.95 0.53 0.83

12.0 2.5 11.5 1.0 15.1 0.4

0.09 0.22 0.10 0.08 0.07 0.00

0.12 0.71 0.15 0.25 0.11 0.00

0.72 0.31 0.67 0.30 0.61

Reference 56.

TABLE 52. Effect of Cu(AcAch Concentration upon the Ratio of Diethyl Fumarate: Diethyl Maleate from the Decomposition of Diazoacetic Ester in Cyclohexene Solutions a Catalyst conc. (mgj50 ml)

o 1

4 16 32 64 256 a

Reference 56.

Diethyl fumarate Diethyl maleate 0.53 0.69 0.69 0.89 1.03 1.39 2.18

465

Metal-Salt-Catalyzed Carbenoids TABLE 53. Catalytic Decomposition of 9-Diazoftuorene by Copper(II)

Carboxylates in Aqueous Dimethyl Formamidea (Ar 2C)-2 Cu(ll) carboxylate Acetate Propionate n-Butyrate Isobutyrate Tartrate a

I

AcO

(Ar 2 C 2h

Ar 2 CO

55 47 51 36

24 17 16 36 85

15 18 23 19

(Ar 2C =N)-2

12

Reference 625.

systems have also been used to form dimers, including a combination of photolysis in the presence of copper(ll) oxide. The use of copper(l)-alkyl-sulfide complexes to decompose diazocarbonyl compounds undoubtably involves ylid intermediate. Serratosa 171-178 has used this system to generate cyclopropanes (trimers) both inter- and intramolecularly. The overall reaction yield may be poor but the second step, involving a favorable carbenoid addition to an electron-deficient olefin, does not occur with other types of catalysts. It is not clear whether or not the overall yield might be improved by operating in two stages, dimerization with a "normal catalyst" followed by cyclopropanation with an ylid catalyst. Azine formation is not a common process for catalyzed decompositions of diazoalkanes. However, Nozaki has reported such products to be formed along with glycols and glycol esters with copper catalysts as well as other transitionmetal-salt-based catalysts.623-626 His systems were all of the diaryldiazomethane type and the results are summarized in Tables 53-56; a possible mechanism for glycol and glycol esters is presented in Scheme 112. TABLE 54. Decomposition of Diphenyl Diazomethane by Copper(lI) Carboxylates in Aqueous Dimethyl Formamidea (q,2C)-2

Cu(Il) carboxylate Acetate Propionate n-Butyrate Isobutyrate Benzoate Tartrate Glycinate Salicylate a

Reference 624.

I

AcO

70 61 54 48 14 58 5 7

63 4 48 45

466

David S. Wulfman and B. Poling TABLE 55. Decomposition of Diazoalkanes by Metal Acetates"

Metal acetate

(Ar2C), Solvent

Ar2CHOAc (Ar2C)2

I

AcO

ArC(OAch

Ar2CO

42 61

55 15 56 30

27 3 21 53

30 27 40 40 32 20 50 30 50 Trace

Azine

Diazof/uorene Cr(OAc), Cu(OAch TI(OAc), Pb(OAc).

DMFaq DMFaq CH2C12 CH 2CI 2

9 24

55

Diphenyl diazomethane Cr(OAc), Cr(OAc), Cu(OAch Cu(OAch Cu(OAch AgOAc Hg(OAch Hg(OAc), TI(OAch TI(OAch a

DMF DMFaq DMF EtOH c/>H DMF DMF Et 2 0 DMF Et 2 0

35 40 52 47 52 17 5 50

25

References 624 and 625.

The formation from diazoacetic ester of tetraethyl cyclobutane-I,2,3,4tetracarboxylate is described in older literature 71.228.337 and more recently a report of the generation of the related cis-cyclopropane has also been made. These reactions bear reinvestigation. The polymerization of diazo methane occurs readily with a number of Lewis acids and normally furnishes polymethylene, though occasionally ethylene is formed (see Section IU.F).

TABLE 56. Decomposition of Ar 2 CN 2 by Copper(Il) Acetylacetonates in Benzene a

% Yield Ar C 6 Hs

C6 HS C6 HS C6 HS

p-MeOC 6 H. C6 Hs a

Reference 626.

,B-Dicarbonyl compound

ArC=CAr 2

Ar2C=N-N=CAr 2

2,4-Pentadione I-Phenyl-I,3-butadione 1,3-Dlphenyl-l ,3-propadione Acetoacetic ester Acetoacetic ester 1,1,1-Trifluoro-2,4-pentadione

60 47 41 74 48 84

30 46 50

o o

15

467

Metal-Salt-Catalyzed Carbenoids SCHEME 112 +

-

R 2 CML n _ 1

N. I

R2C-M-CR2 -----+

I

L

I

I

L

Ln -

1

L

I r.n~ / I

R 2C-Ml-CR2 -----+ R 2CLCLR2 Ln -

2

L

~

L = O-CR'

14. Rearrangements The rearrangements of interest formally involve carbenes but most probably are carbenoid processes. These reactions involve e\ectron~deficient carbon and have been the subject of several reviews.t The most common process is the Wolff rearrangement in which an a-diazo carbonyl compound is converted into a ketene or one or more of its derivatives.631.632 The Wolff rearrangement has found extensive application for ring contractions 474.475 including, in many cases, those which create highly strained systems. Aryldiazoalkanes are frequently employed in the homologation of carboxylic acids and this is normally referred to as the Arndt-Eistert synthesis.§ The Arndt-Eistert synthesis normally involves the reaction of an acid chloride with a diazoalkane (usually diazomethane), whereas the syntheses of a-diazocyc1anones are more varied. In recent years Regitz and coworkers have developed facile syntheses of a-diazoketones so that compounds for Wolff rearrangement are readily accessible. When an aryl group migrates, the rearrangements of diazo ketones are occasionally referred to as Schroter rearrangements. These rearrangements can occur under a variety of conditions: heat, light, or under the influence of a catalyst. In a number of cases the conditions and concentrations are critical and rearrangement may fail in preference to other carbene or carbenoid reactions. It is most probable that the photolyses leading to rearrangement involve the singlet state. Jones and Ando avoided Wolff rearrangements when performing cyc1opropanations by employing triplet sensitizers.635a Silver salts are most commonly employed for the catalyzed reactions, although copper(l), (II), and (0) have occasionally been used. Copper carbenoids

t References 2,151,474,475,631.632.

§ References 2. 151,158,161,634,635.

468

David S. Wu/fman and B. Poling

appear to be much more stable than the related silver complexes and consequently fail, in most instances, to bring about rearrangements, leading instead to olefinic dimers. Typical silver salt and other systems include CF3C0 2Ag,636 Ag 20/ Na2C03/Na2S203,637 and Hg02CC6H5/t-amine.638 The last has found wide usage and the Ag20/Na2C03/Na2S203 system is the classic condition for the Arndt-Eisert sequence.639.640 Some typical examples involving catalyses were given in Scheme 40. It would appear that no thorough analysis of the influence of catalysts on products has been performed in this area.

15. The Mechanisms of Metal-Salt-Catalyzed Cyclopropanations, C-H Insertions, and Dimerizations of Diazoalkanes The full story on the mechanisms of these reactions is far from complete. The studies of Moser,79.80 Peace, McDaniel, and Wulfman,3.56.57.68.76.78 Kochi and Salomon,71 Bethell,85.157.641-644 D'yakanov, Komendantov, and Vittenberg,t Hubert and Teyssie,404.405.645.646 Nozaki and Noyori,118.123.124.133 and Aratani 87.88 have revealed much but left far more in obscurity. The reasons are numerous but the major problem is the sheer complexity of the processes and the failure of some workers to recognize this fact. Chemical kinetics alone will never succeed in unraveling the situation and, unfortunately, this approach has been heavily relied upon and has resulted in obfuscation. The question of valence state is far from settled but the one "thorough" kinetic study claiming to establish the valence state of copper is equally or more consistent with copper(II), although it is given as evidence for copper(l) 71 (see Section I). The problem of mechanism was considerably complicated when Peace and Wulfman 73 observed a dependence of behavior of dimethyl diazomalonate upon the catalyst concentration (see Scheme 58 and Table 27). The presence of yield vs. catalyst concentration phenomena exhibiting two maxima (Figure 5) is consistent with either the existence of two processes going to the same products or one going to products and a second going to unidentified materials which exhibits a narrow maxima as a function of catalyst concentration as discussed before. The latter explanation cannot currently be ruled out but it seems clear that at least two processes do lead to products since the syn/anti ratio for cyclopropanation of cyclohexene by diazoacetic ester is a function of catalyst concentration. 73 Such behavior requires two reaction paths or a subsequent epimerization. (Since the epimerization is not observed independently the dual mechanistic rationale holds.) This type of analysis is clearly an important one to apply to any cyclopropanation study involving XYCN 2 (X #- Y) systems. When X = Y, yield vs. catalyst concentration studies run over a wide range of concentrations varied in small increments may reveal the presence of more than

t References 70, 77, 128, 129,244-250,252-258,263-266.

Metal-Salt-Catalyzed Carbenoids

469

one path, but this is not assured. (Peace employed a range of 210 operating with twofold variations. Since the dip in yield behavior occurs over a range of only 24, the use of small increments is obviously important.) Bethell 643 observed multipathways for copper(l)- and copper(II)-catalyzed decomposition of di pheny ldiazomethane. The results of Nozaki 118.123 on the dimerization of carbenoids to furnish pinacols and pinacol esters clearly reveal the effect of changing solvent types (see Section II. I 3), while the formation of olefins from formal carbene dimerization presents a formal tie with the process believed to be the initiation step in olefin disproportionation reactions (see Section III).

A. The Mechanisms of Cyc/opropanation and C-H Insertions Moser 79.80 proposed that the cyclopropanation of olefins by diazoacetic ester/CuX· [P(ORhln systems involved a four-center mechanism of the type shown in Scheme 23. The mechanism accounts for the formation of syn/anti isomers and assumes coordination of the carbenoid to the olefin occurring initially through the copper. This last point was assumed as a consequence of the observation of a very low level of optical induction when using a catalyst containing a chiral ligand. Unfortunately, such a low level of induction could also result from specific solvation by ligand-derived species or by complexation ofthe olefin and subsequent attack of the double bond by the diazo compound. The same objection can be raised for the optical induction studies of Nozaki. Wulfman has treated this point in detail. 68 The inductions observed by Aratani 87.88 and by Otsuka and Nakamura 237-239 are sufficiently high to fairly well preclude such an argument and probably establish the presence of a pathway in which the metal is coordinated to both the double bond and the carbene. Similar conclusions can also be reached by examining the effect of varying the catalyst with a constant olefin and diazo compounds as has been done by Peace et al.t Teyssie and his colleagues 404.450.645.646 have employed Pd- and Rh-based catalysts for cyclopropanation with diazoacetate and diazomethane. The mechanism Hubert 648 has proposed for their results is similar to that of Moser. Using diazomalonates, Wulfman initially found no compelling evidence for invoking a carbenoid; from subsequent work he did. Ultimately, a dual mechanism was required to account for the double maxima phenomenon. The key interpretation was the recognition of the fact that the two paths had to converge to a common intermediate which went to products. (The reader is referred to the original paper for the arguments. 57 ) The mechanism felt to best explain all of the existing data is that given in Scheme 25. This mechanism accounts nicely for the formation of the three

t

References 3,55-57,68,72-76,78,130,298-304.

David S. Wul/man and B. Poling

470

C-H insertion products formed with I-methylcyclohexene as well as for the single cyclopropane and the existence of synjanti isomers when the diazoalkane is of the N 2CXY type (X i= Y).

B. Dimer Formation The formation of dimers from diazoacetic ester furnishes both diethyl maleate and diethyl fumarate.t The data in Table 51 reveal that the ratio is a function of both the catalyst and its concentration. A subsequent study by McDanie156.296 showed a range of synjanti values as the catalyst concentration [copper(ll) bis-acetylacetonate] was varied. This is only consistent with the presence of two reaction paths possessing different dependencies upon catalyst concentration. The same conclusion was reached regarding the formation of tetrakis-carbomethoxyethylene from dimethyl diazomalonate. If a common intermediate were required at the same molecularity for forming cyclopropane, C-H insertion, and dimeric products, the partial rate data would remain invariant with changes in catalyst concentration. The data in Table 52 clearly show that this is not the case. Therefore the molecularities for C-H insertion and cyclopropanation differ from that required for dimer formation. Since the first two reactions involve catalyst it is assumed that some of the dimer is formed by a process involving two molecules of catalyst. When all of the possible ways of bringing diazo compound + carbenoid and carbenoid + carbenoid together t References 55, 56, 73, 78,296, 304, 647, 649, 659.

SCHEME 113

w +

I -

N 2 -C-CuXYL

I

Z

CuXY

CuXY

w~~ + z~ ~ Z C-Z C-W Z ~ dN;

~

I

W

\

/

/

\

W

W

Z

Z

I

\

/

/

\

C=C

C=C

Z

N~

Z

W

Metal-Salt-Catalyzed Carbenoids

471

SCHEME 114

---0>

z

w

w

z

\ / C=C / \

to furnish olefin are considered, it is seen that, conformationally, one would predict on an a priori basis that the cis isomer would be favored by a carbenoid + diazoalkane path and the trans isomer would be favored by a carbenoid + carbenoid mechanism (diazo compound is N 2 CXY, X =1= Y). The proposed reaction paths appear in Schemes 113 and 114. These mechanisms assume a transoid relationship in the collapse of the diazonium-copper alkyl intermediates (Scheme 113). If this step is cisoid the other isomeric olefin would result.

III. OLEFIN METATHESIS The subject of olefin metathesis has received considerable attention in the years following its discovery (1964) by Banks and Bailey. 650 The most recent and succinct review has just appeared. 651 The reaction RCH=CH 2

+

RCH=CH 2 -----+ RCH=CHR

+

CH 2 =CH 2

is of obvious industrial importance. The processes are generally believed to involve a reverse carbenoid dimerization (see Section 11.13 above): RCH=CH 2

+

2MLn ~ RCH=MLm + CH 2 =ML m

which proceeds via an MT-2 type of process to a metallocyc1obutane (Scheme 115). These processes are reversible and can occur by paths labeled b or those labeled a or c. In the b case no change occurs, but in the a and c cases the generation of a new olefin results, and the sum of a and c is similar to that of a metathesis (disproportionation or double decomposition). Since the reaction is not limited to terminal olefins, is virtually statistical in product distribution, is very

472

David S. Wu/fman and B. Poling SCHEME 115 a

RCH=CH 2

+ RCH=MLm

~

b

R-CH-:-CH2

R-CH';"-CH2

R-CH-;-ML..

L .. M-:-CHR

···I RCH 2CH= > R 2CHCH= > R 2C=.9 Functional groups normally act as catalyst poisons, particularly when located on the double bond. However, a number of examples of metatheses in the presence of remote substituents have been reported. In particular, the presence of ester functions is compatible with the catalyst systems WCI 2(CH3)4Sn and WCI 6-Al[C 2H 5].661-668 A good review on the metatheses of substituted olefins has appeared. 669 Both heterogeneous and homogeneous catalyst are employed for metathesis. The metals include Mo, W, and Re as carbonyls, Mo and W as sulfides, Mo, W, Re, V, Sn, Te, Nb, Ta, La, Ru, Os, Ir, Rh, Sr, and Ba as oxides. In general, Mo and W furnish the best catalysts. 651 The catalysts are often supported on Al 2 0 3 or Si02 • Homogeneous catalysts may only be homogeneous at the beginning of the reactions. 65 1,670,671 Typical systems involve a metal halide, e.g., WCl 6 and a metal alkyl cocatalyst such as BuLi or CH 3AlCI 2 • Originally, the cocatalyst was felt to be needed to reduce the valence state of the other component, e.g.,

Metal-Salt-Catalyzed Carbenoids

473 SCHEME 116

RXH HXR' [WI

R

H

H

R'

R'XH RX'H [WI

R

H

R

H

wvr to Wrv; however, WCl 4 is not an active catalyst. 9 •673 The system WCI 4/AlCI 3 is a very active catalyst, however. 672 The ratio of catalyst to cocatalyst is of considerable importance. The mechanism given above is now widely accepted. The carbenoid inter+

mediates apparently have the polarity M-CR 2. Gassman has isolated a cyclopropane in very low yield by employing an electron-poor olefin as the carbenoid substrate. 104-106 Metatheses have been initiated by catalytically decomposing diazoalkanes and from stable carbene complexes.674.675 Levisalles 676 has proposed an alternate sequence to account for the metathesis reaction which does not involve a carbenoid but which proceeds to a metallocyclobutane. Experimental evidence in support of the proposed mechanism has been presented which would appear to be inconsistent with a carbenoid route (Scheme 116).677 ACKNOWLEDGMENTS

It had been our original intent to review stable carbenoid reactions but time and space prevented this from being accomplished. Several reviews have appeared. It is hoped that the foregoing has demonstrated that there is a great deal that can be accomplished synthetically by employing metal-salt-catalyzed carbenoid processes and that there are many challenging problems open to the organic and inorganic chemists interested in mechanistic chemistry. Our own venture into the area was prompted by synthetic needs but quickly moved into the mechanistic arena when the doctoral studies of B. W. Peace revealed the wealth of information that could be obtained with a modest effort. Dr. Peace deserves much of the credit for the advances we have made. In addition, the writing of

474

David S. Wulfman and B. Poling

this chapter was greatly facilitated by S. Julia and G. Linstrumelle of l'Ecole Normale Superieure, who furnished the authors with copies of their reference files, and by numerous chemists who have kept D.S.W. well supplied with preprints. For the past five years we have employed Chemical Abstracts Condensates-Computer Searches to keep abreast of the area. Approximately 400 papers appear each year which bear on the subject of this article, so one can only assume the area has its applications. The authors also wish to acknowledge the support of their research by the National Science Foundation.

IV. ADDENDUM Since the foregoing was written (early 1978) much new work has appeared which bears upon the subject and is deserving of treatment. Further examination of the literature on diazoalkane chemistry (dating back to 1849) has generated some additional insights. Herrmann has published a review entitled" Organometallic Synthesis With Diazoalkanes." 680 This paper is complementary to the present treatment with a minimum amount of duplication. The previously unreported insertion of a carbene into a molecule of CO in a metal carbonyl to form a ketene is reported. The Regensburg workers have also reported the formation of bis-metalocyclopropanes from cyclopentadienyl metal carbonyls and diazomethane (Mn),681 diazoacetic ester (CO),682 and (Rh)683 (Scheme 117).

SCHEME 117

Metal-Salt-Catalyzed Carbenoids

475

Diazocyclopentadienes react with Re(CO)5X (X= CI, Br, or J) to furnish the X-cyclopentadienyl rhenium tricarbonyl complexes. 684 With the tetracarbonyl iron halides (CO)4Fe(a-C3F7)I and (CO)4FeX2 (X = Br, I), similar products are obtained along with I, I' -dibromoferrocene in one case. 685 Ibers has reported the isolation of a stable diazotetrachlorocyclopentadiene complex from the reaction of the diazo compound with IrCI(CO)[(C6H5hP]2.686 Porter has extended somewhat the work of Ando on the generation and use of thio-bis-methoxycarbonylmethylides by using Rh(II) acetate as the catalyst.687-689 Good yields (86-100/0 ) were obtained with a variety of thiophenes. Under similar conditions, ethyl diazoacetate, diazoacetophenone, and ethyl diazoacetoacetate formed the monocyclopropyl derivatives with olefins. Insertion into the a-CH bond of thiophene occurred when copper(l) chloride, iodo (triphenyl phosphite) copper(l), or silver acetate were employed as catalysts. 687 The ylid from thiophene undergoes ready rearrangement to the C-H insertion product when heated in boiling thiophene for 2 hr. Attempts to trap the free carbene using electron-rich and electron-poor substrates failed, however. Polymeric bis-carbomethylmethylene was a byproduct. 688 When 2,5-dichlorothiophenium bis-carbomethoxycarbonyl methylide was heated in cyclohexene both the cyclopropane and allylic C- H insertion products were isolated. 689 When the same reagent was used with a variety of other substrates, cyclopropanation did occur and in some cases the yields undoubtedly would be better than those obtained using catalytic decomposition of the diazomalonic ester alone. However, the best yields were obtained using the ylid and rhodium(II) acetate or preferably copper(II) bis-acetylacetonate. Unlike the catalyzed reactions employed by Peace,76 there was little necessity of carefully controlling concentrations, rate of additon, or temperature. The ylid is a stable crystalline solid at room temperature, whereas dimethyl diazomalonate is a highly stable liquid below 80°e. The use of rhodium salts and palladium salts as catalysts in the decomposition of diazoalkanes is due to the work of Hubert and his colleagues at Liege. They have found a number of fascinating processes which nicely parallel earlier postulated processes and some phenomena observed with copper-based catalysts. Variations in the anions employed with their catalysts led to variations in the exo/endo ratio in the cyclopropane generated from ethyl diazoacetate. This phenomenon was observed by Moser 79 .80 and by Wulfman 78 with copper(l)halide-trialkylphosphite complexes. In addition, variations occurred as a function of catalyst concentration, in line with the report by Wulfman et al. of this phenomena with copper systems. 56 Thus the Liege systems also involve more than a single reaction path.690-694 It was postulated that changes in ligands, anions, and substrates should lead to a range of reactive intermediates. This was clearly demonstrated with the reactions of ethyl diazoacetates with aromatic substrates in the presence of rhodium(II) carboxylates 695 (Scheme 118). The reaction is particularly useful

476

David S. Wu/fman and B. Poling SCHEME 118. Possible Path for Homologation of Arenes by Rhodium Carboalkoxy Carbenoids

-

N 2 CHC0 2 R

+ RhL n_ 1

+

LnRh-CHC0 2 R ---+ ---+ or LnRh-CHC0 2 R

with rhodium(lI) trifiuoroacetate where benzene furnished 7-carbethoxycycloheptatriene in 95% yield at room temperture. The initial impression that this reaction proceeds through an MT-I process, however, seems an unlikely one since p-xylene furnishes the 2,5-dimethylcycloheptatriene almost exclusively (20: I 2,5-dimethyl: 3,6-dimethyl), whereas the related photolysis furnishes predominantly the symmetrical 3,6-dimethyl product. Two possible explanations can readily account for this difference: (i) the carbenoid behaves as a diradical, or (2) the carbenoid behaves as an electrophile. Either process would be an MT-2 carbenoid process and clearly demonstrates that changes in metal and Iyate anions very appreciably moderate the mechanism by which metal-saltcatalyzed carbenoid reactions proceed. With some of the catalysts used at Liege, polycarbethoxymethylene was generated in appreciable yields. Some of the results are summarized in Tables 57-67 and Scheme 119. TABLE 57. Rh(02CCF 3 h-Catalyzed Addition of Ethyl Diazoacetates to Arenes

to Furnish Cyc10heptatrienes a

Substrate Benzene Toluene p-Xylene

m-Xylene a-Xylene Mesitylene Hexamethylbenzene a

Reference 695.

Overall yield (%)

Substrate

100 95 90 90 80 60 20

Indane Anisole Chlorobenzene Ethyl benzoate H exafl uoro benzene Pyridine

Overall yield (/0)

30 40 72 10

10 0

TABLE 58. Influence of the Substitution of the Oletins on the Cyclopropanation of Ethyl Diazoacetate in the Presence of Pd(OAc)za Yield (%)

Olefin

Yield (%)

Olefin

>90

o 30

42

>=

2r o a

o

o o

o

21

Traces

Reference 694.

TABLE 59. Reaction of Substituted Styrenes with Diazo Compounds in the Presence of Palladous Acetate a

,

R" R

CH

~C=CH2 +

N2CHR

Pd(OAc)2 H

/

\

) RR'C-CH2

R'

RH

(

x-( (

o

H

)-CH=CH2

COOEt

25

> 80b

)-y=CH2

COOEt

25

42

CH 3 a o

90

)-CH=CH2

Reference 694. X = OCH 3 , CH 3 , H, CI, or N02 • When X

= NR 2 , yield

=

0%.

David S. Wu/finan and B. Poling

478

SCHEME 119. Competition between C=N and C=C in the Cycloadditions of Carboalkoxy Carbenoids 694

COOR

~C

~N

N2CHC02R, CU(Tf)2

or

Pd(OAcj,

~

Oxazole +

80~o

or

30'/0

K 2PtCl,

lO~u

CH3CN+~

~,

~N

Olr)

30'/0

CU(Tf)2 ) N2CHC0 2R

+

CH,-omoi,

C

COOR 10'70

14'/0

.J.CN +

~

'f':;"'-

CU(Tf)2 N2CHC0 2 R'

.J. oxazole + r

~

~ ___ _

COOR

1:1

TABLE 60. Catalyzed Reaction of Nitriles with Diazoacetates 694 R

RC=:=N + N 2 CHCOOR'

Cu(Tf).

or

)

\

O~

Pd(OAcj,

-

N

I

OR'

'70 Yield R

CH 3 C 3 H 7C6 H 5C 6 H s-CH=CH-

Cu

Pd

80

30

15

30

30

20

60 60

25 25 25 25

50 50

R4

H

CH 3

CH 3 H

trans-2-0ctene

2-5-Dimethyl-2,4-hexadiene 1,5-Hexadiene 1,3-Cyc1ohexadiene Cyc10hexene

CH 3 H

H

R4

CH 3 C 2H 5 n-C 4 Hg CH 3 CH 3 CH 3 n-C5Hll H CH 3 n-C 4 H g H n-C 3 H 7 CH 3 C 2H5 n-C 4 Hg n-C 5 Hll CH 3 H n-C 4 Hg H C 2H 5 C.H7 H C2H 5 C.H7 C 2H 5 CH 3 C2H 5 n-C.Hg CH 3

C 2H5

R3

R5 in diazoacetic ester CH 3

80 (64)(4)

(80) (90) 84(70)

b

a

85

54(14)

60(8)

85(12) 77(20)

60(40)

80

68

t-C 3 H 7

64 25(31)

77 60 70 54

14(26)

8(26) 23

30(36) 40

15

Cu(Il) triflate as c,atalyst

Yield (based on alkene)

100 75(20) 89(12)

n-C 3 H 7

70(30)(58) 24(36) 70 (85)

70(28)' 65 90(82) 7(40)

98(90)'

56

'70 R6 in Rh(R 6C02) as catalyst

Reference 693. Reproduced with permission of Georg Thieme Verlag, Stuttgart. Values in ( ) which are italicized are yields of cyc\opropanes isolated by distillation, , Values in ( ) and in roman type are for maleate + fumarate and were determined by GLC.

1,5-Cyc1ooctadiene

H

n-C 3 H 7

CH 3 H

trans-4-0ctene

H

R2

CH 3 CH 3

Rl

2-3-Dimethyl-2-butene cis-2-0ctene

'/R' ")

C2H5

C=C

cis-3-Hexene

R2 /

C'

Alkene

10(30)

21

35 37 20

2.2(18)

11.6

< 8(26) 5(33)

15

Pd(II) acetate as catalyst

TABLE 61. Influence of Catalysts on the Reaction of Alkenes with Diazoacetic Esters to Furnish Cyclopropane Carboxylates a

\0

'-I

"'"

~

'";::c

ti-

D

I:)..

~

~

D i:)

~

I:)

....

...~

480

David S. Wu/fman and B. Poling TABLE 62. Influence of the Catalyst a on the Yield of CycIopropanation of Styrene" by Ethyl Diazoacetate C at 25°C d Catalyst

Yield (%)

None Cu(OTf), Cu(pyridineh(OTf), Rh 2 (OAc)4 Pd(OAc), a b

4')0- 4 mol. 50 m\.

C

d

o (pyrazoline, o (polymers)

1470)

65 48 98

5.10- 2 mol. Reference 694.

TABLE 63. CycIopropanation of Conjugated Diolefins by Ethyl Diazoacetatea ." Yield (%) as a function of catalyst Olefins

Cu(OTf),

Rh 2 (OAc),

~

Pd(OAc), 45"

~

80

90

60

~

85

70

30

Jy

77

85

35

yA

70

75

35

~CI

0(polym.)(50)d

60

O(polym.)

70

90

20

0

Reference 694. " [Olefin] = (10 x 10-" M. [ethyl diazoacetate] = (2.5 x 10- 3 M). and [cat.] = (2.5 x 10- 5 M). T = 25'C. No solvent was used except for butadiene. which was in benzene. C With a saturated solution of butadiene in benzene. d This olefin polymerized in the presence of Cu(OTf)2 and Pd(OAc)2 but a 50% yield was obtained with Cu(pyridine)2(OTfj,.

a

481

Metal-Salt-Catalyzed Carbenoids

TABLE 64. Cyclopropanation of Nonconjugated Diolefins with Methyl or Butyl Diazoacetate (DAM or DAB)

'70

Maleate + fumarate

6 26 10

Diolefin

Catalyst a

()

Cu(OTf), RH 2 (OAc). Pd(OAc),

48 49 40

22 18

Oc

Cu(OTf), Pd(OAc),

16 6

25 0

a

'70

'70

Dicyclopropane

Monocyclopropane

12

11

15

CondItIOns for the first three entries: [diolefin] = (3.10- 2 M); [DAM] = (3.10- 2 M); [cat.] = solvent. ConditIons for the last two entries: [dlolefin] = (3.10- 3 [cat.] = (3.10- 5 M); T = 25°C.

(3.10- 4 M); T = 25°C; no M); [OA -] = (6.10- 3 M);

The use of cobalt complexes to catalyze carbenoid reactions has been examined in great detail by employing chiral complexes based upon 1,2-dioximes of chiral 1,2-diones. Cobalt complexes, rhodium, and palladium complexes all have appreciably greater limitations in chemical synthesis than copper-based systems. The major problem lies in the apparent requirement in many instances of monosubstitution on the olefin and/or various "activating" substituents. Thus, the work of Nakamura and Otsuka 237-239.696 with cobalt furnished an excellent route to chiral cycIopropanes based on addition of diazoacetic esters to styrene, but proved to be a rather poor competitor to the Aratani synthesis of TABLE 65. Influence of the Catalyst on the Competitive Insertion-Cyclopropenation Reactions of Hydroxyacetylenes

OOH pKa of

C=CH

HC=C-CH 2 OH

Rh 2(02 CR).

RCOOH

la

A"

I/A

la

A"

t-Butyl Adamantyl Methyl Methoxy Menthoxy Trifluoromethyl

5.06

24

0.8 0.6 0.8

53 52 60

12 12

3.2

59 40

4 3

a 1= OH insertion. b

A = cyclopropenation.

4.76 3.53

10 16

29 17 12 14

0.25

16

5

II

11

I/A

4.8 4.3 5

1.1

14.7 13.3

David S. Wu/fman and B. Poling

482

TABLE 66. Competitive Intermolecular Insertion-CycJopropenation of Acetylenic Alcoholsa. b Yield ('70) Total yield (%)

Substrate Propargyl alcohol 3-Butyn-l-ol I-Pentyn-3-o1 3-Methyl-l-pentyn-3-ol I-Ethynylcyclohexan-I-ol 4-Methyl-I-pentyne-4-o1 Ethanol + l-hexyne I : I a b

72 69 75 73 22 54 98

Insertion (I) Addition (A) 60 50 54 37 10 25 74

12 19 21 36 12 29 24

Ratio JjA 5.0 2.6 2.6 1.0 0.8 0.9 3.0

Reference 692. By permission of Pergamon Press, Oxford. Catalyst = Rh,(OCOCH 3 ) ••

chrysanthemic esters. 87 • 88 The carbenoid derived from the cobalt complex is capable of adding to acrylic esters and related systems, however, whereas copper catalysts can only accomplish this type of reaction when a sulfide is present to serve as a sulfonium ylid carrier. The interesting mechanistic aspect of this is that the cobalt carbalkoxycarbenoid apparently is polarized to furnish a nucleophilic carbene carbon, whereas copper catalysts furnish a neutral or electrophilic carbalkoxycarbenoid. Thus, the two extreme cases depicted in Scheme 24 have now been demonstrated to exist, in line with the original prediction made in 1971. 72

TABLE 67. CycJopropenation of RC-CR' (10 mmol) by N 2 CHCOOCH 3 (4 mmol)a. b R n-C 4 H g n-C 4 H g n-C 4 H g t-C 4 H g Cyc10hexyl C 6 Hs CH 3 COOOCH 2 CH 3 OCH 2 C 2 H sO CH 2 =C(CH 3 ) C2 H S CH 3 OCH 2 a b C

R'

R" (catalyst)

Yield ('70)

H H H H H H H H H H C2HS CH 3 OCH 2

CH 3 C.H g CF 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3

84 70 0 86 80 0 40 46 0 17+ 16c 68 38

Reference 692. By permission of Pergamon Press, Oxford. Catalyst: (R"C0 2 )zRh, 0.05 mmo1. T = 25°C. No solvent. Two adducts were obtained but their structures were not determined.

483

Metal-Salt-Catalyzed Carbenoids SCHEME 120

l az~OH OH

""'N

,B-cqdH

a-cqdH

N

/

OH

8-cqdH

~N/OH 8-nqdH

The Nakamura and Otsuka cobalt complexes based upon the ligands given in Scheme 120 were not effective catalysts with bulky diazo compounds such as diphenyldiazomethane or 9-diazofluorene. The less bulky compound diazomalononitrile reacts in the presence of styrene to furnish the related cyclopropane in low yield and in only 4.6/0 EE (enantiometric excess). Diazomethane, diazoacetophenone, and diazoacetic ester, however, behave in a more useful fashion. A summary of these results is presented in Tables 68-75. The data on the cycIopropanation of cis-styrene-d2 is particularly revealing for the reaction occurs with considerable loss of stereochemical integrity of the deuterium atoms. This apparently occurs during the course of cycIopropanation and not before or after the event. Thus, the processes are either of the MT-2syn + fLSr or MfLr type or of the MT-2nsyn SMfL or MfLS type, and not of the MT-2syn or MT-2syn-ia type observed with copper catalysis. The authors have performed an analysis of the possible conformations assuming penta or hexa coordination not unlike that in Schemes 18-21. The complexes involved will not preclude Berry pseudo rotation, and therefore the mechanistic evidence as to how transfer of the methylene occurs to furnish larger enantiomeric excesses is not firmly established. One interesting feature of the cobalt complexes is the presence of an additional ligand such as water or pyridine which modifies the character of the carbenoid reactions observed (Table 68). In benzene solution the catalysts are dimeric and, on this basis, the structure of the catalyst Co(a-cqd)z· H 2 0 was assigned as being A and not B, for hydrogen bonding between monomers is highly likely for A but highly unlikely for B (Scheme 121). In B intramolecular hydrogen bonding is possible for both NOH

b

76 77 63

34

cis-d2

23 37

trans-d2

H

Reference 239. By permission of the American Chemical Society. The values are averages of two independent reactions and accurate to

H 2O H 2O 3-Picoline

30 0 0

a

Base

Temperature CC)

D

~O'H ~O'H

trans Isomer

± 3 '70'

37 37 38

trans-d2

H

63 63 62

cis-d2

D

W.o'H W.o,H

cis Isomer

0.73 0.72 1.32

trans/cis

TABLE 68. Results of the Cyclopropanation with cis-Styrene-d2 and Optical Yieldsa,b

65 9.0

41

trans

35 63 7.2

cis

Optical yield (%)

l:)

~

?:l ~

!:>...

;::

l:)

;::

~

S;

~

~

'"~

i::;,

~

4>-

485

Metal-Salt-Catalyzed Carbenoids

TABLE 69. Asymmetric Synthesis of Ethyl-2-phenylcyc1opropanecarboxylate with Various Co(ll) Catalysts a • b trans Isomer

cis Isomer

Catalyst Co( a-cqd).· H 2 O Co( o-cqd).· H 2 O Co(o-Prcqd).·H 2 O Co(o-nqd).·H 2 O Co(,B-cqdh· H 2 O Co(,B-nqd).· H 2 O a b C

d

e

f

Yield c ('70) trans/cis d

92 62 78 83 81 80

0.85 0.65 0.60 0.91 2.2 8.0

oye

[alD (deg)

Config.

(%)

[alD (deg)

Config.

+12 -11 +4.4 -3.7 -5.6 +0.6 f

(lS,2R) (lR,2S) (lS,2R) (I R, 2S) (lR,2S) (lS,2R)

66 63 24 21 31 4

+236 -202 -82 -237 +29 -0.6 f

(lS,2S) (lR,2R) (I R, 2R) (lR,2R) (lS,2S) (lR,2R)

OY" (%)

75 64 26 74 9 0.2

Reference 239. By permission of the American Chemical Society. Reaction was performed in neat styrene at O°C with 3 mol. % catalyst (based on ethyl diazoacetate). Based on ethyl diazoacetate. Ratio of area in the GLC peaks. OY ==optical yield; each ester was hydrolyzed to the corresponding acid and optical yield was calculated from the highest specific rotation of the acid: the cis acid, [aID 18°; the trans acid, [aID 381°. Observed rotation was 0.01 ° with a 0.5-dm cell.

SCHEME 121

B

486

David S. Wu/fman and B. Poling

TABLE 70. Asymmetric Synthesis of Ethyl-2-phenyIcyclopropanecarboxylate with Co(a-cqdh· H 2 0 as Catalyst a • b

Product Catalyst (mol %)

C

3 3 3 3 0.5

[a In

Temperature CC)

Yield c (%)

trans/cis·

trans

99 24 0 -15 0 0

95 93 92 80 87 51

0.98 0.85 0.95 0.75 0.95

1.3

+102 +236 +236 +239 +233 +186

deg cis

+ 2.9 +11 + 12 +13 +12 + 9.6

Optical yield e('7o) trans

cis

33 75 75 76 74 59

16 61 67 72

67 53

Reference 239. By permission of the American Chemical Society. Reaction was performed in neat styrene. C Based on ethyl diazoacetate. d Ratio of area in the G LC peaks. e The product was hydrolyzed to the corresponding acid and optical yield was calculated from the specific rotation of the cis acid, [aln + 30°. a b

groups, whereas in A only one can hydrogen bond intramolecularly. A pairing of two molecules of Co(a-cqdh·H 20 would permit intermolecular hydrogen bonding between the second NOH groups in A. Thus structure A appeared to be more consistent with the molecular weight determination in benzene solutions. This type of argument was necessitated by the inability to obtain a suitable single crystal for X-ray structural analysis. The influence of the valence state of the cobalt was examined and the Co(l) complex was found to be completely unreactive, while the Co(lll) complex was an order of magnitude less reactive than the Co(ll) complex. Extensive kinetic studies were also carried out and a prodigious amount of work is reported.237-239.696 TABLE 71. Asymmetric Synthesis of Ethyl-I, I-diphenyIcyc\opropanecarboxylate with Various Co(ll) Catalysts a • b

Catalyst Co(a-cqd),· H2 O Co(o-cqd),·H 2 O Co(o-Prcqd),·H 2 O Co(o-nqd),·H 2 O a b

C

d

Yield c ('70)

[aln (deg)

Config.

95

+ 156 -112 -61 -118

(IS) (lR) (lR) (lR)

77

68 87

Optical yield d ('70)

70 50 27 54

Reference 239. By permission of the American Chemical Society. Reaction was performed in neat 1, I-diphenylethylene at 5°C with 2.2-2.9 mol '70 catalyst (based on ethyl diazoacetate). Based on ethyl diazoacetate. Ester was hydrolyzed to the corresponding acid and optical yield was calculated from the highest specific rotation of the acid, [aln 230 ± 5°.

487

Metal-Salt-Catalyzed Carhenoids

TABLE 72. Cyclopropanation of Styrene with Various Alkyl Diazoacetates (NzCHCOzR) in the Presence of Co(a-cqdh· Hzoa,b [a li',3 e

R in N 2 CHC0 2 R

oyr

of trans Yield C ('70)

trans/cis"

cis

trans

acid (%)

94 92 91 94 87 72

0.69 0.85 1.15 0.92 2.34 1.46

+21 +21 +22 +21 +24 +22

+231 +286 +320 +302 +335 +298

61 75 84 80 88 78

Me

Et iso-Pr iso-Bu neo-pent Cyclohexyl

Reference 239. By permission of the American Chemical Society. Reaction was performed in neat styrene at OGC with 3 mol. /0 catalyst (based on alkyl diazoacetate). C Based on alkyl diazoacetate. " Ratio of areas of the G LC peaks. , Value of the corresponding acid obtained after alkaline hydrolysis of the ester. Measured at 23"C in CHCI 3 with a 0.5-dm cell. Concentration: cis, 5.07-5.20: trans 1.00-1.20. r Optical yield was calculated from the highest specific rotation of the trans acid, [aln + 381 c.

a

b

A closer examination of the older literature dealing with the behavior of diazoacetylnorleucine methyl ester (DAN) revealed that it has been established that the inactivation of the acid protease, pepsin, in the presence of copper(II) is reversed by hydroxylamine treatment 697 (Scheme 122). This clearly establishes that in that particular case acylation and not alkylation is the mode of inhibition. If alkylation had occurred the hydroxylamine would either fail to cleave the TABLE 73. Effect of an Axial Base on Asymmetric Synthesis U of Ethyl-2-phenylcyclopropanecarboxylate u • b [aln' (deg)

Axial base

Yield c ('70)

trans/cis"

cis

trans

H2O Pyridine 2-Picoline 3-Picoline 4-Picoline 2,6-Lutidine 3,5-Lutidine CH 3 C(CH 2 O)3 pr Triphenylphosphine

92 70 91 67 67 86 84 89 86

0.85 1.4 0.74 1.4 1.1 0.72 1.3 1.1 0.69

+ 13 +1.4 +3.1 -0.4 -1.9 +10 -1.8 +7.0 +12

+236 +60.6 +248 +36.6 +17.9 + 187 + 12.8 +232 +211

Reference 239. By permission of the American Chemical Society. Reaction was performed at O°C in neat styrene with 3 mol. /0 of the catalyst (based on ethyl diazoacetate) and a base (2.5-fold molar ratio to the catalyst). C Based on ethyl diazoacetate. " Ratio of area in the GLC peaks. e Measured in CHCI'J with a 0.5-dm cell. r 4-Methyl-2,6,7-trioxa-l-phosphabicyclo[2.2.210ctane. a

b

David S. Wu/fman and B. Poling

488

TABLE 74. Solvent Effect on Asymmetric Synthesis of Ethyl-2-phenylcyc\opropanecarboxylatea. b [a]D' (deg)

Solvent

Yield C('70)

trans/cis·

cis

None Acetone Ethyl acetate Di-n-butyl ether n-Hexane Acetophenone

92 81 92 88 67 95

0.85 1.3 1.0 0.67 0.85 0.95

+12 +5.4 +11 +12 +13 + 11

trans +236 +197 +222 +199 +223 +200

Reference 239. By permission of the American Chemical Society. Reaction was performed at O°C with 3 mol. '70 of the catalyst (based on ethyl diazoacetate) with styrene concentration 2.6-2.9 M (in twofold molar excess). C Based on ethyl diazoacetate. " Ratio of area in the OLC peaks. e Measured in CHCI 3 with a 0.5-dm cell. a

b

alkyl-X bond or would furnish a hydroxamic acid in which the hydroxamate function resided on the enzyme. However, if the diazo compound were converted into an acylating agent, the hydroxamic acid functionality would subsequently be generated on this group and not the enzyme. The carbonyl ylids or heterocycles derived therefrom (Section 1.12.8) would be potential acy lating agents. With the related compound diazo-oxonorleucine (DON) the major portion of the diazo compound is converted into the alkylating agent diazomethane by a retro-Claisen condensation (70 parts) and one part becomes attached to the protease, glutaminase A (from E. coli), in the absence of copper(II).698 This inactivation is irreversible. Patterson 699 has generated a fairly complete survey of the reported biological activities of a variety of diazo compounds and the TABLE 75. Effects of the Substituent in Styrene and of the Ester Group of Diazoacetate on the Relative Rates of the Cyclopropanation with Co(a-cqd)za Effect of X in p-XC 6 H.CH=CH 2 " in reaction with N 2 CHC0 2 Et X CH 3 0 CH 3 H CI

kObs(min - 1) 2.5 X 2.0 X 1.7 X 3.8 x

10- 2 10- 2 10- 2 10- 3

Effect of R groups in N 2 CHC0 2 Rc in reaction with PhCH=CH 2 R

Me Et iso-Pr iso-Bu neo-pentyl

kObs(min - 1) 2.5 1.7 1.4 8.4 1.2

x 10- 2 X 10- 2 x 10- 2 x 10- 3 x 10- 3

Reference 239. By permission of the American Chemical Society. Reaction conditions: O°C; [Co(a-cqdj,- H 2 0], 1.2 x 10- 2 M; [N 2 CHC0 2 Et], 0.24 M in neat olefin. e Reaction conditions: O°C; [Co(a-cqdj,- H 2 0], 1.2 X 10- 2 M, [N 2 CHC0 2 R], 0.23 M in neat styrene.

a

b

489

Metal-Salt-Catalyzed Carbenoids SCHEME 122 N 2 CHCONHCHRCOOCH 3

+

EXH

~

- - EXCH 2 CONHCHRCOOCH 3

and/or

alkylation

H2 NOH

alkylation H2 NOH

1

NOH EXCH 2 CONHCHRCONHOH

II

HOCH 2 CONHCHRCOCH 3 or

NHOH

I

HOCH 2 CONHCHRC=O

EXH

=enzyme

+ EXH

behavior of metal salt catalyzed and related systems summarized in Table 76. The National Cancer Institute has recently completed a survey of the 21 most likely heterocycles to be derived from catalyzed decomposition of diazoacetyl amino acid derivatives.t Only two have ever had representative members screened for chemotherapeutic acitivity. It has been known for some time that DDT [2,2-bis(p-chlorophenyl)-I,I,Itrichloroethane] interacts with various metallo species in living systems.725-728 Reduction to DDD (the related dichloroethane) occurs with heme proteins,730.731 vitamin B12 ,730 myoglobin,729 and cytochrome C oxidase.732.733 Recently, Mansuy 734.735 has succeeded in isolating carbene complexes derived from carbon tetrachloride and from DDT by treating the halocarbons with tetraphenylporphyrin iron(II) in the presence of excess reducing agents such as iron powder. There is claimed to be good reason to suspect that such complexes are formed during the reductive metabolism of poly halogenated compounds by hepatatic cytochrome P-450 iron(II). The formation of the tetraphenylporphyrin iron(II) complexes involves loss of three chlorine atoms from DDT and two from carbon tetrachloride, whereas DDD and chloroform result from biological reductions

t Reference 724; the senior author acknowledges the search by NCI of the 21 structures submitted for examination.

Diazoacetyl-dl-norleucine methyl ester (DAN)

Compound

Porcine pepsin Rhodotorula glutinis K-24 Cladosporium sp. No. 45-2 Aspergillus niger type A Acid prot eases from: porcine brain porcine kidney porcine skeletal muscle Nepenthes ampullaria Drosera capensis

B

Scytalidium lignicolum acid protease B Acid proteases: Scytalidium Iignicolum A-I A-2

Cathepsin D from bovine spleen Rhizopus chinensis acid protease

System

+ + +b +++

Modifier Cu 2 + Cu 2 +

+++ +++ + + + + + +

Cu 2 + Cu 2 + Cu 2 + Cu 2 + Cu 2 + Cu 2 + Cu 2 + Cu 2 +

N.D. N.D. N.D.

Inactivation of enzyme

Inactivation of various enzymes

Cu 2 + Cu 2 + Cu 2 + Cu 2 +

N.D. N.D. N.D. N.D.

+++

No inactivation of enzyme

Inactivation of enzyme with incorporation of norleucine

Inactivation of enzyme

Effect

Cu 2 +

+ +

Activity with modifiera

N.D.

N.D.

Activity of compound alone"

TABLE 76. Survey of the Catalyzed Reactions of Diazocarbonyl Compounds with Biological Systems

707

703, 705, 706

704

701-703

700

References

t:::1

~.

;;p -..

~

I:l..

::::

I:l

::::

I:l

~

S;

~

!'l

~

~

I:l

'0 C

"'"

Cathepsin E from rabbit bone marrow Thyroid acid proteinase from pigs

Pepsin

Acid proteases from Acrocylindrium sp. Aspergillus niger type B Aspergillus saitoi Mucor plirsilllls Paecilomyces varioti Trametes sangllinea Calf renin Pepsin

Mucor miehei acid protease

+

N.D.

+++ +++

Cu 2+ Ag+ Cd2+ C02+ Pb 2+ Zn 2+ Au3+ d

+ + +b ++

+++ +++ +++ +++ +++ +++ + + +b

Cu 2+ Cu 2+ Cu 2+ Cu 2+ Cu 2+ Cu 2+ Cu 2+

Cu 2+ Cu 2+

+++

Inactivation of enzyme Inactivation of enzyme but not to the extent of pepsin

Inactivation of enzyme with incorporation of norleucine Inactivation of enzyme and norleucine incorporated

Inactivation of enzyme with incorporation of norleucine

with incorporation of norleucine

+ + + b, CInactivation of enzyme

Cu 2+

Cu 2+

Continued

710, 711 712

700

709

703

708 ~

'0

""....

~

~

;::

'"