Industrial and Laboratory Alkylations

10 C o u p l i n g of

Alkyl

G r o u p s U s i n g Transition

M e t a l Catalysts

JAY K. KOCHI

Downloaded by RUTGERS UNIV on January 12, 2018 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0055.ch010

Department of Chemistry, Indiana University, Bloomington, IN 47401

The formation of carbon-carbon bonds is one of the most important operations in organic synthesis, and it can be repre sented by the coupling of organometallic reagents (including alkyllithium and Grignard reagents) with organic derivatives, such as alkyl halides among others. M

1

RMgX + R'-X t l » R-R + MgX

2

[l]

The organic moieties, R and R' in Equation 1, can either be saturated alkyl, aryl, vinyl or acetylenic groups leading to a wide variety of hydrocarbon structures. The most effective catalysts represented in Equation 1 as [M] are derived from transition metal complexes. The role of the metal catalysts is varied in these reac tions, but they are most commonly involved in the formation of organometallic intermediates RM which subsequently reductively eliminate to the coupled product. Reactions leading to the forma tion of the key intermediate R-M and the elucidation of the path ways for its decomposition are thus central to the understanding of these catalytic processes. We will first summarize briefly the processes involved in the formation and destruction of RM, which will be followed by our studies of various catalytic systems leading to the coupling of alkyl groups. Formation of Alkylmetal Complexes Organometals R-M are commonly prepared by metathesis of a transition metal complex with substitution-labile carbanio noid reagents such as Grignard and lithium derivatives. CHi

CHî

Œ

ID:

CH3-Au -PPh + C D L i 3

CH -Au -PPh3 + L i l

3

3

I

CD, 167

Albright and Goldsby; Industrial and Laboratory Alkylations ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

INDUSTRIAL AND LABORATORY ALKYLATIONS

168

The f o r m a l oxidation state of the m e t a l under these c i r c u m stances does not change. S i m i l a r l y , no change in the f o r m a l oxidation state of the metal results f r o m the insertion of an olefin into a ligand-metal bond.

L Pt*

+ CH =CH

2

N

2

2

—> L Pt^ C1

[3](2)

2

X

C1


On the other hand, when either a l k y l r a d i c a l s or a l k y l c a r b o n i u m ions (or their p r e c u r s o r s ) are used as alkylating agents, the metal center undergoes a change i n the f o r m a l oxidation state of either one or two, respectively. CH . 3

+cAn

+ 2

n

- CH -Cr en

2

3

CH CDjI + ( C H ) A u L i 3

2

P

P

h

+ 2

[4](3)

2

3

Œ

CD -Au -PPh

3 »

3

CH

3

+ Lil

[5](4)

3

A l k y l a t i o n s of metal centers under these c i r c u m s t a n c e s a r e considered as oxidative additions. Decomposition

of A l k y l m e t a l Complexes

T h e r e a r e a number of modes by which carbon-metal bonds can be cleaved. Conceptually, they can be represented by the m i c r o s c o p i c r e v e r s e of each of the p r o c e s s e s in Equations 2-5 which lead to the alkylation of the m e t a l center. Thus, the r e v e r s e of Equation 2 i s represented by the well-known e l e c t r o philic cleavage of organometals.(5) (CHa^Au^PPha+HOAc — ( C H ^ A u ^ O A c J P P l ^ + C H - H 3

[6]

S i m i l a r l y , β-elimination of hydrogen i s probably the most common route by which a l k y l m e t a l s decompose. CH CH CH CH L P< * CH CH CH CH 2

2

2

3

2

2

2

3

CH

2

— CHCH CH 2

3

+

2

[7](6)

L Pt^

etc.

2

CH CH CH CH 2

2

2

3

The homolytic cleavage of a l k y l m e t a l bonds, p a r t i c u l a r l y those of M a i n Group metals, i s known f r o m Paneth s c l a s s i c experiments to occur at high temperatures. T h e r e v e r s e of Equation 4, however, does not u s u a l l y represent the energeti!


10.

KOCHi

169

Transition Metal Catalysts

c a l l y m o s t f a v o r e d p a t h w a y i n the d e c o m p o s i t i o n of R - M . Alkyl c o u p l i n g as a r e s u l t of h o m o l y s i s to f r e e r a d i c a l s , f o l l o w e d b y dimerization R-M

[ M - + R-] — *

R-R,

etc.

[8]

i s not u b i q u i t o u s , a l t h o u g h s u c h v i e w s w e r e w i d e l y h e l d due to the m i s g u i d e d b e l i e f that c a r b o n - m e t a l bonds, p a r t i c u l a r l y those involving transition metals, are extremely weak. If a l k y l r a d i c a l s a r e i n t e r m e d i a t e s i n the c a t a l y z e d c o u p l i n g of a l k y l g r o u p s , t h e y s h o u l d u n d e r g o d i s p r o p o r t i o n a t i o n i n a d d i t i o n to d i m e r i z a tion. CH


2

2

=C H

2

+ CH3CH3

[9a]

CH CH 3

2

CH CH CH CH 3

2

2

[9b]

3

E i t h e r b i m o l e c u l a r p r o c e s s i s not p o s s i b l e w i t h o u t the other, s i n c e the r a t i o of the r a t e c o n s t a n t s i s f i x e d b y the s t r u c t u r e of the r a d i c a l . ( 7 ) H o w e v e r , a n u m b e r of m e t a l - c a t a l y z e d c o u p l i n g s a r e k n o w n to p r o c e e d w i t h o u t the f o r m a t i o n of a n y d i s p r o p o r t i o n a t i o n p r o d u c t s , c e r t a i n l y i n the a m o u n t s d i c t a t e d by the v a l u e s of 1 ^ / F u r t h e r , the c o u p l i n g r e p r e s e n t e d i n E q u a tion 1 c a n o c c u r without the s c r a m b l i n g of R and R , as w o u l d be e x p e c t e d of f r e e r a d i c a l intermediates.(8) T h e m i c r o s c o p i c r e v e r s e of o x i d a t i v e a d d i t i o n i n E q u a t i o n 5 is r e p r e s e n t e d by r e d u c t i v e e l i m i n a t i o n w h i c h c a n be i n t e r m o l e c u l a r ( E q u a t i o n 10) o r i n t r a m o l e c u l a r ( E q u a t i o n 11). 1

dmgCo^CHa (CH ) Au 3

3

1 I

+ Br"

—>

dmgCo

PPh3

—>

CH AuPPh 3

1

+ CH Br

[10](9)

3

3

+ CH CH 3

3

[H](10.)

I n d e e d , the c o m b i n a t i o n of o x i d a t i v e a d d i t i o n i n E q u a t i o n 5 a n d r e d u c t i v e e l i m i n a t i o n i n E q u a t i o n 11 i s t h e b a s i s f o r a c a t a l y t i c m e c h a n i s m for alkyl coupling. Alkyl Transfers from Organometallic in Catalytic P r o c e s s e s

Intermediates

T h e o x i d a t i o n - r e d u c t i o n r e a c t i o n s of o r g a n o m e t a l l i c i n t e r m e d i a t e s p r e s e n t e d i n the f o r e g o i n g d e s c r i p t i o n c a n be a p p l i e d , i n c o m b i n a t i o n , to a v a r i e t y of c a t a l y t i c p r o c e s s e s , s u c h as the m e t a l - c a t a l y z e d a l k y l t r a n s f e r r e a c t i o n s of G r i g n a r d r e a g e n t s originally investigated by K h a r a s c h and c o w o r k e r s . ( l 1 ) W e have found that the c a t a l y t i c r e a c t i o n s b e t w e e n l a b i l e o r g a n o m e t a l s and a l k y l h a l i d e s can be g e n e r a l l y c l a s s i f i e d into two c a t e g o r i e s , c o u p l i n g i n E q u a t i o n 12 a n d d i s p r o p o r t i o n a t i o n i n E q u a t i o n 1 3 ,


170


depending on the catalyst. F o r example, silver(l) and a r e e f f e c t i v e c a t a l y s t s i n t h e c o u p l i n g of a l k y l g r o u p s ,

R-m

+ R-X

r—*

R-R

( V_>-

RH + R(-H) + m X

copper(l) whereas

+ m X

[12] [13]


i r o n effects only disproportionation except when a r y l and vinylic halides are employed. E a c h catalyst shows unique features w h i c h a r e best d e s c r i b e d within the f o l l o w i n g m e c h a n i s t i c context. A. N i c k e l C a t a l y s i s i n the C r o s s C o u p l i n g of A r y l H a l i d e s w i t h A l k y l m e t a l s. T h e R o l e of A r y l a l k y l n i c k e l ( I I ) S p e c i e s a s Intermediates. F o r the s t u d y of n i c k e l c a t a l y s i s i n the f o r m a t i o n of a r a l k a n e s , w e e m p l o y e d the s y s t e m c o n s i s t i n g of a r y l b r o m i d e s and m e t h y l l i t h i u m or m e t h y l m a g n e s i u m b r o m i d e . A r - B r

+ C H

3

- m

(

E t

3P) NiBr 2

2 >

A

_

r

C

H

j

+

m

_

B

r

[

where m = L i or

M

]

M g X

A l t h o u g h the t r i e t h y l p h o s p h i n e c o m p l e x e s of n i c k e l ( l l ) m a y not n e c e s s a r i l y r e p r e s e n t o p t i m u m e x a m p l e s of c a t a l y s t s , ( 1 2 ) ( l 3 ) t h e y a l l o w e d u s a c c e s s t o t h e k e y i n t e r m e d i a t e , v i z . , tn~e a r y l m e t h y l - n i c k e l c o m p l e x e s s u c h as l a and b. A n y catalytic cycle PEt

JCH PEt

3

3

3

A"(Et P) Ni i - p r o p y l > n - p r o p y l ) as w e l l as the k i n e t i c s a r e the s a m e as the s i l v e r - c a t a l y z e d coupling described above and suggest a s i m i l a r m e c h a n i s m :


5

Scheme

6 Fe

1

R. R'MgBr

+ RBr

—>

Fe^Br

+ pe

- >

RFe

1

+ FeBr

RFe , R'Fe 1

—•

[4 7] [48]

1

—+> R ' F e 1

+ R-

+ MgBr

1

[49]

2

[RH, R H , R - H , R - H ] + 2 Fe , !

!

1

etc.

[50]

A c c o r d i n g to t h i s p o s t u l a t e , the d i f f e r e n c e b e t w e e n c o u p l i n g w i t h s i l v e r and d i s p r o p o r t i o n a t i o n w i t h i r o n r e s t s on the d e c o m p o s i t i o n of the a l k y l m e t a l i n t e r m e d i a t e i n E q u a t i o n 50. Indeed, it has b e e n s h o w n s e p a r a t e l y i n E q u a t i o n 42 t h a t t h e d e c o m p o s i t i o n of a l k y l s i l v e r ( l ) p r o c e e d s by r e d u c t i v e c o u p l i n g . U n f o r t u n a t e l y , the h i g h l y u n s t a b l e a l k y l i r o n i n t e r m e d i a t e i n S c h e m e 6 i s not yet a c c e s s i b l e to i n d e p e n d e n t s t u d y , but the s o m e w h a t a n a l o g o u s d i a l k y l m a n g a n e s e ( l l ) s p e c i e s i n E q u a t i o n 51 u n d e r g o e s s i m i l a r r e d u c t i v e d i s p r o p o r t i o n a t i o n b y a m e c h a n i s m ( £ 8 ) r e m i n i s c e n t of d i a l k y l p l a t i n u m ( l l ) c o m p l e x e s d e s c r i b e d i n E q u a t i o n 7. RjjMn

1

—• R H + R - H + M n °

[51]

S e l e c t i v e t r a p p i n g of a l k y l r a d i c a l s f r o m the a l k y l h a l i d e c o m p o n ent d u r i n g the c o u r s e of the c a t a l y t i c d i s p r o p o r t i o n a t i o n i s t h e s a m e as the p r e v i o u s o b s e r v a t i o n w i t h s i l v e r , and it i n d i c a t e s that the p r i m e s o u r c e of r a d i c a l s i n the K h a r a s c h r e a c t i o n l i e s i n the o x i d a t i v e a d d i t i o n of a l k y l h a l i d e to r e d u c e d i r o n i n E q u a t i o n 47. S e p a r a t e p a t h w a y s f o r r e a c t i o n of i - p r o p y l g r o u p s d e r i v e d f r o m the o r g a n i c h a l i d e and the G r i g n a r d r e a g e n t a r e a l s o s u p p o r t e d b y d e u t e r i u m l a b e l l i n g s t u d i e s w h i c h s h o w that they a r e not c o m p l e t e l y e q u i l i b r a t e d . ( 4 9 ) F u r t h e r m o r e , t h e o b s e r v a t i o n of C I D N P ( A E m u l t i p l e t effect) TrTthe l a b e l l e d p r o p a n e a n d p r o p e n e



182


d e r i v e d only f r o m the a l k y l halide component c a n be attributed to a b i m o l e c u l a r d i s p r o p o r t i o n a t i o n of i s o p r o p y l r a d i c a l s a r i s i n g f r o m diffusive displacements. However, the latter c a n only be a m i n o r fate of the a l k y l r a d i c a l s d e r i v e d f r o m the a l k y l halide, since the coupled d i m e r i s not f o r m e d i n amounts r e q u i r e d by the b i m o l e c u l a r r e a c t i o n of a l k y l r a d i c a l s p r e v i o u s l y d i s c u s s e d i n Equation 9·(48)(5£) C r o s s c o u p l i n g of G r i g n a r d reagents w i t h 1 - a l k e n y l halides, i n m a r k e d c o n t r a s t to a l k y l halides, o c c u r s r e a d i l y w i t h the r e d u c e d i r o n catalyst, a s d e s c r i b e d above. The ironc a t a l y z e d r e a c t i o n of G r i g n a r d reagents w i t h 1 - a l k e n y l h a l i d e s can, however, be differentiated f r o m the reaction with alkyl halides. T h u s , a m i x t u r e of p r o p e n y l b r o m i d e a n d ethyl b r o m i d e on r e a c t i o n w i t h m e t h y l m a g n e s i u m b r o m i d e a f f o r d e d b u t e n e - 2 but no c r o s s - o v e r p r o d u c t s s u c h a s p e n t e n e - 2 o r p r o p y l e n e . The latter certainly would have resulted if a propenyliron species per se w e r e i n v o l v e d i n the c a t a l y t i c p r o c e s s . C r o s s coupling under tïïese c i r c u m s t a n c e s c l e a r l y m e r i t s further study. Conclusions T h e c o m p l e x catalytic reactions leading to the coupling of organic substrates induced by metal complexes c a n be rationally d i s s e c t e d into a v a r i e t y of e l e m e n t a r y steps i n v o l v i n g o x i d a t i o n r e d u c t i o n reactions of o r g a n o m e t a l l i c i n t e r m e d i a t e s . Electron transfer interactions a r e important considerations i n differentia ting concerted f r o m stepwise processes, especially with regard to c h a i n p r o c e s s e s . C r u c i a l to the design of new synthetic p r o cedures andthe understanding of catalytic p r o c e s s e s i s the i n f o r m a t i o n to be gained f r o m the s c r u t i n y of transient a l k y l m e t a l species, w h i c h r e p r e s e n t a l a r g e p o t e n t i a l f o r a v a r i e t y of n o v e l reactions.

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10.

37. 38.


39· 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

KOCHI


185

Walborsky and M. S. Aronoff, J. Organometal. Chem. (1973), 51, 53; H. L . Goering and F. H. McCarron, J. Am. Chem. Soc. (1958), 80, 2287; R. J. Rogers, H. L . Mitchell, Y. Fujiwara and G. M. Whitesides, J. Org. Chem. (1974), 39, 857. Cf. G. M. Whitesides, C. P. Casey and J. K. Krieger, J. Am. Chem. Soc. (1971), 93, 1379. Whitesides, G. M . , E . R. Stedronsky, C. P. Casey and J. San Filippo, J r . , J. Am. Chem. Soc. (1970), 92, 1426; G. M. Whitesides, E . J. Panek and E . R. Stedronsky, Ibid. (1972), 94, 232; M. Tamura and J. K. Kochi, Ibid. (1971), 93, 1483; J. Organometal. Chem. (1972), 42, 205; Ibid. (1971), 29, 111. Tamao, Κ., Y. Kiso, K. Sumitani and M. Kumada, J. Am. Chem. Soc. (1972), 94, 9268; M. Tamura and J. K. Kochi, J. Organometal. Chem. (1972), 42, 205. Jukes, Α. Ε . , Adv. Organometal. Chem. (1974), 12, 215. Tamura, M. and J. K. Kochi, J. Am. Chem. Soc. (l 971), 93, 1485. Kochi, J. Κ., Pure App. Chem. (1971), 4, 3958. Tamura, M. and J. K. Kochi, Synthesis (1971), 303. Tamura, M. and J. K. Kochi, J. Am. Chem. Soc. (19 71), 93, 1483. Jenkins, C. L . and J. K. Kochi, J. Am. Chem. Soc. (1972), 94, 843, 856. Whitesides, G. M . , C. P. Casey and J. K. Krieger, J. Am. Chem. Soc. (1971), 93, 1379. Tamura, M. and J. K. Kochi, J. Organometal. Chem. (1971), 31, 289; Bull. Chem. Soc. Japan (1971), 44, 3063. Tamura, M. and J. K. Kochi, J. Organometal. Chem. (1971), 29, 111. Allen, R. B., R. G. Lawler and H. R. Ward, J. Am. Chem. Soc. (1973), 95, 1692. The large enhancement possible in CIDNP may not reflect its chemical importance until they are quantitatively related. A small amount of radical combination leading to CIDNP may have been overlooked in the chemical studies.


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