Organometals and Organometalloids - American Chemical Society

13 Mechanisms for Alkyl Transfers in Organometals JAY K. KOCHI

Organometals and Organometalloids Downloaded from pubs.acs.org by UNIV LAVAL on 07/10/16. For personal use only.

Department of Chemistry, Indiana University, Bloomington, IN 47401

The class of vitamin B (methylcorrinoid) derivatives is one of three major coenzymes which is available for methyl transfer in biological systems, being particularly effective with inorganic substrates. Thus methylcobalamin has been i m p l i cated in methyl transfers to a variety of metal ions including mercury, lead, tin, and thallium, as well as platinum, palladium, and gold (1). The mechanism of methyl transfer from cobalt to another metal center, that is, transmetallation, is the subject of extensive study. Two general mechanisms have been proposed for the methylation of metal ions by methylcobalamin (2). In type I reactions, the metal ion acts as an electrophile during the transfer of a methyl anion equivalent. In type II reactions, methyl radicals are transferred between metal centers. Thus type I reactions are considered to involve heterolytic cleavage of the cobalt-carbon bond of methylcobalamin, whereas the homolytic cleavage of the same bond in type II reactions is induced by a reduced member of a redox couple. In a more general sense, type I and type II reactions can be considered as two-equivalent and one-equivalent processes, respectively. The concept of electron transfer relates two-equivalent processes with their one-equivalent counterparts. For example, consider the carbonium ion as the key reactive intermediate in solvolysis reactions, which historically have served as prototypes for numerous ionic processes. Electron transfer by one— equivalent reduction produces alkyl radicals, 12

CH

+ 3

^

w

CH * 3

which are crucial to homolytic processes. The same interchange between ionic and radical species applies to electron transfer processes between carbanions and alkyl radicals. CH " 3

^F=*r CH 3

0-8412-0461-6/78/47-082-205$07.50/0 © 1978 American Chemical Society

ORGANOMETALS AND ORGANOMETALLOIDS

206

F i n a l l y , an o v e r a l l t w o - e q u i v a l e n t change i n t e r r e l a t e s c a r b o n i u m i o n s and c a r b a n i o n s . ^ 2e CH · CH


3

3

V i e w e d i n t h i s way, t h i s h y p o t h e t i c a l t r a n s f o r m a t i o n i s b e t t e r considered as a two-step p r o c e s s i n v o l v i n g s u c c e s s i v e e l e c t r o n t r a n s f e r s . T h u s , i n e l e c t r o c h e m i c a l p r o c e s s e s , o n l y one e l e c t r o n i s t r a n s f e r r e d i n a s i n g l e a c t s i n c e the s i m u l t a n e o u s t r a n s f e r of two e l e c t r o n s , l i k e a b i p h o t o n i c p r o c e s s , i s a m u c h l e s s p r o b a b l e event. A l t h o u g h the o b s e r v a t i o n of e l e c t r o n t r a n s f e r p r o c e s s e s between a l k y l r a d i c a l s and c a r b o n - c e n t e r e d i o n s a r e as y e t r e l a t i v e l y r a r e , those between m e t a l c o m p l e x e s of c o u r s e r e p r e s e n t a w e l l - e s t a b l i s h e d p a r t of i n o r g a n i c c h e m i s t r y . Outer-sphere and i n n e r - s p h e r e m e c h a n i s m s d e v e l o p e d by T a u b e s e r v e a s an e x c e l l e n t b a s i s f o r e l e c t r o n t r a n s f e r p r o c e s s e s between two m e t a l c o m p l e x e s (3.). T h e p r e s e n c e of a b i m e t a l l i c i n t e r m e d i a t e either as a p r e c u r s o r or successor (postcursor) complex can play an important r o l e i n inner-sphere electron transfer pro c e s s e s . O u t e r - and i n n e r - s p h e r e m e c h a n i s m s h a v e a l s o been a p p l i e d to the 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 a l k y l r a d i c a l s w i t h m e t a l c o m p l e x e s (4). H o w e v e r , t h e d e t a i l e d e x a m i n a t i o n of the m e c h a n i s m of o x i d a t i o n - r e d u c t i o n p r o c e s s e s w i t h a l k y l r a d i c a l s i s m a d e d i f f i c u l t b y t h e i r t r a n s i e n t n a t u r e . A s a r e s u l t , the m e c h a n i s m s have been d e r i v e d h e r e t o f o r e f r o m d e d u c t i o n s b a s e d on k i n e t i c o b s e r v a t i o n s and p r o d u c t a n a l y s e s , and a few i n t e r m e d i a t e s have only r e c e n t l y been d e t e c t e d . F o r e x a m p l e , the i n n e r - s p h e r e c o m p l e x between m e t h y l r a d i c a l s and c o p p e r ( l l ) , CH * 3

+

Cu

1

>-

CH3-CU

1

has been o b s e r v e d by flash p h o t o l y t i c t e c h n i q u e s and found to d e c a y w i t h f i r s t - o r d e r k i n e t i c s (k = 7χ 1 0 sec" ) i n aqueous s o l u t i o n s at 25° (.§.). Indeed, the a s s o c i a t i o n of a l k y l r a d i c a l s w i t h m e t a l c o m p l e x e s t h r o u g h i n n e r - s p h e r e c o m p l e x e s m a y be t h e r o u t e b y w h i c h m o s t , i f not a l l , 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 a l k y l r a d i c a l s w i t h m e t a l c o m p l e x e s o c c u r , i r r e s p e c t i v e of w h e t h e r they have been p r e v i o u s l y c l a s s i f i e d a s i n n e r - o r o u t e r s p h e r e p r o c e s s e s . In t h i s r e g a r d , the r o o t of the d i f f e r e n c e between w h o l l y i n o r g a n i c s y s t e m s and the h y b r i d a l k y l m e t a l s y s t e m s p r o b a b l y l i e s i n the g r e a t p r o p e n s i t y of the c a r b o n c e n t e r e d i o n s , both c a r b o n i u m i o n s and c a r b a n i o n s , to be h i g h l y s o l v a t e d , e i t h e r a s i o n - p a i r s or i n n e r - s p h e r e c o m p l e x e s . A n i n n e r - s p h e r e a l k y l m e t a l i n t e r m e d i a t e s u c h a s that i n the a b o v e equation m a y be d e r i v e d by an a l t e r n a t i v e r o u t e i n v o l v i n g e i t h e r o x i d a t i o n o r r e d u c t i o n of a s t a b l e a l k y l m e t a l c o m p l e x , e.g., 2

CH -Cu 3

tt

g

" >

CH -Cu

1

m

3

In t h i s i n s t a n c e , the p r e c u r s o r i t s e l f i s u n s t a b l e .

However,

13.

Alkyl Transfers

KOCHi

207

t h e r e a r e a v a r i e t y of other s t a b l e a l k y l m e t a l c o m p l e x e s extant f r o m which electron transfer is possible. Reversible d i s s o c i a tion of s u c h i n t e r m e d i a t e s then r e l a t e s a l k y l r a d i c a l s to o r g a n o metals derived by conventional two-equivalent processes, i . e . ,


R

+

M

+

=c=^

^

R-M

^-=^=

R-MÎ"

R-

+

M

+

T h i s d i c h o t o m y i s i n h e r e n t i n a l l of t h e s e p r o c e s s e s , and t h e r e i s a s e v e r e p r o b l e m of r i g o r o u s l y d e m o n s t r a t i n g how e a c h m a y participate i n a particular organometal reaction. S e v e r a l m a j o r q u e s t i o n s a r i s e i n the t r e a t m e n t of a l k y l m e t a l s as i n t e r m e d i a t e s i n t r a n s m e t a l l a t i o n s : (a) the m e c h a n i s t i c d i s t i n c t i o n between e l e c t r o p h i l i c and e l e c t r o n t r a n s f e r m e c h a n i s m s i n the c l e a v a g e of a n a l k y l - m e t a l bond, (b) the s e p a r a t i o n of c o n c e r t e d , t w o - e q u i v a l e n t p r o c e s s e s f r o m s u c c e s s i v e , o n e - e q u i v a l e n t p r o c e s s e s , and (c) the l a b i l i z a t i o n of a l k y l m e t a l s e s p e c i a l l y with r e g a r d to e l e c t r o n t r a n s f e r p r o c e s s e s . B i n a r y a l k y l m e t a l s of m e r c u r y , l e a d and tin a r e u s e f u l m o d e l s for the study of t h e s e q u e s t i o n s s i n c e they a r e s u b s t i t u t i o n - s t a b l e c o m p o u n d s and g e n e r a l l y w e l l b e h a v e d i n s o l u t i o n for k i n e t i c s t u d i e s . In t h i s r e p o r t , we s h a l l d e s c r i b e how v a r i o u s a l k y l d e r i v a t i v e s c a n be e x a m i n e d s y s t e m a t i c a l l y to a l l o w d i r e c t photoe l e c t r o n s p e c t r o s c o p i c study of the b o n d i n g o r b i t a l s i n t h e s e organometals. T w o s e r i e s of w e l l d e l i n e a t e d c l e a v a g e s of a l k y l m e t a l s w i l l then be d i s c u s s e d . T h e s e i n c l u d e : (a) the e l e c t r o p h i l i c p r o t o n o l y s i s of d i a l k y l m e r c u r y compounds, R'HgR and

+

HOAc

RH +

R'HgOAc

(b) the e l e c t r o n t r a n s f e r o x i d a t i o n of the s a m e o r g a n o m e r c u r i a l s with h e x a c h l o r o i r i d a t e ( l V ) . R'HgR

+

IrCl " 6

Rt

H g

2

RÎ

f

a

s

t

^

R'HgR*

>.

R.

+

+

R Hg l

IrCl " 6

+

,

3

etc.

U s i n g t h e s e s y s t e m s , we w i l l c o m p a r e and c o n t r a s t e l e c t r o p h i l i c and e l e c t r o n t r a n s f e r m e c h a n i s m s i n the c l e a v a g e s of a l k y l - m e t a l bonds i n both m a i n g r o u p and t r a n s i t i o n m e t a l c o m p l e x e s . I.

O r g a n o m e t a l s as E l e c t r o n D o n o r s — I o n i z a t i o n P o t e n t i a l s

A. D i a l k y l m e r c u r y C o m p o u n d s . T h e He(l) photoelectron s p e c t r a of d i a l k y l m e r c u r y c o m p o u n d s show two p r i n c i p a l bands of i n t e r e s t ( £ . ) . T h e f i r s t i o n i z a t i o n p o t e n t i a l , Ij> o c c u r r i n g i n a r a n g e between 7.5 7 eV ( d i - t - b u t y l m e r c u r y ) and 9.33 eV ( d i methylmercury) is included i n a fairly broad, u n s y m m e t r i c a l b a n d . A s e c o n d , w e a k e r band o c c u r r i n g between 14.4 and 15.0 eV i s due to i o n i z a t i o n f r o m the m e r c u r y 5 d shell. T h e ionization e n e r g i e s for t h e s e two bands a r e tabulated i n T a b l e I. Repre1 0

208

ORGANOMETALS

AND ORGANOMETALLOIDS

T a b l e I. T h e F i r s t and 5 d V e r t i c a l I o n i z a t i o n P o t e n t i a l s (eV) of D i a l k y l m e r c u r y C o m p o u n d s . 1 0

R—Hg—R CH CH C H CH CH CH C H n-C H i-C H C H i-C H n-C H i-C H t—C H i-C H neo-C H 3

2

5

3


3

5

3

7

3

7

2

7

4

9

4

9

4

9

3

7

3

7

7

9

4

9

4

9

5

9

4

4

9

4

7

4

3

5

3

5

3

n

9

4

9

4

9

5

n

5

n

D

5d

1 0

I

14.93 14.85 14.71 14.86 14.74

9.33 8.84 8.45 8.47 8.75 8.32 8.18 8.29 8.03 8.06 7.73 8.35 8.30 7.57 8.33 8.30

5

2

3

2

CH C H C H i-C H i-C H t-C H i-C H n-C H i-C H t—C H t—C H n-C H i-C H t-C H neo-C H neo-C H 3

3

2

First I

1

14.61 14.63 14.46

14.47 14.49 14.41

s e n t a t i v e s p e c t r a of only the f i r s t band a r e r e p r o d u c e d i n F i g u r e 1 f o r one s e r i e s of a l k y l m e t h y l m e r c u r y c o m p o u n d s , i . e . , R-HgCH . T h e effect of a l k y l s u b s t i t u t i o n on the f i r s t i o n i z a t i o n p o t e n t i a l of a s e r i e s of a l k y l d e r i v a t i v e s i s a t t r i b u t e d p r i m a r i l y to p o l a r i z a t i o n effects i n the m o l e c u l a r i o n f i n a l state. It has been r e c o g n i z e d that s u c h e l e c t r o n i c effects a r e a d d i t i v e a l o n g the series: M e , E t , i - P r , and t - B u . T h u s , the e n e r g y effect of r e p l a c i n g M e b y E t i s e x p e c t e d to e q u a l that of r e p l a c i n g E t by i-Pr, or of r e p l a c i n g i - P r by t - B u . In e a c h c a s e , α - h y d r o g e n s in C H a r e being sequentially replaced by methyl groups. Addi t i v e e n e r g y effects have been u s e d by T a f t (7) as a c r i t e r i o n for i d e n t i f y i n g p o l a r effects as denoted by the e m p i r i c a l substituent constant σ * : 3

3

(σ*)

M e : Et : i - P r : t - B u

=

0 : 0.10

: 0.19 :

0.30

It has been shown that the T a f t r e l a t i o n s h i p holds for the i o n i z a tion p o t e n t i a l s of a l c o h o l s and other a l k y l c o m p o u n d s . Figure 2 i l l u s t r a t e s the l i n e a r r e l a t i o n s h i p between σ * v a l u e s and the i o n i z a t i o n p o t e n t i a l s of a s e r i e s of a l c o h o l s , a l k y l b r o m i d e s , a l k y l h y d r a z i n e s , and a l d e h y d e s . T h e c o r r e l a t i o n of the i o n i z a t i o n potentials of a s e r i e s of a l k y l m e r c u r i a l s R - H g C H a l s o plotted i n F i g u r e 2 i s d i s t i n c t l y n o n l i n e a r . T h e i n c r e m e n t a l changes i n energies become p r o g r e s s i v e l y s m a l l e r or ' ' s a t u r a t e d " as one p r o c e e d s f r o m m e t h y l to t e r t - b u t y l . M o r e o v e r , the s a m e p a t t e r n of s a t u r a t i o n obtains f o r the a n a l o g o u s s e r i e s of G r i g n a r d r e a g e n t s R - M g X and t r i 3


13. KOCHi

Alkyl Transfers 209

210

ORGANOMETALS AND

ORGANOMETALLOIDS

Or*


0

OJ

0.2

03

No. OF α-METHYL GROUPS Inorganic Chemistry

Figure 2. Correlation of the first ionization po tential of various alkyl-substituted compounds vs. Taft's σ* constant (top scale) and number of α-methyl groups (bottom scale) (6).

13.

KOCHi

Alkyl Transfers

211

m e t h y l t i n c o m p o u n d s R S n M e m e a s u r e d i n d e p e n d e n t l y (8,2,). T h e d i f f e r e n c e between e n e r g y effects w h i c h a r e s a t u r a t e d and those that a r e a d d i t i v e m a y be r e l a t e d to the n a t u r e of the highest o c c u p i e d m o l e c u l a r o r b i t a l ( H O M O ) . F o r those s y s t e m s c o n t a i n i n g nonbonding e l e c t r o n s , the i o n i z a t i o n p r o c e e d s f r o m a H O M O w h i c h i s l a r g e l y o r t h o g o n a l to the σ b o n d i n g o r b i t a l s , p a r t i c u l a r l y those a s s o c i a t e d with the bonding of the h e t e r o a t o m to carbon. T h u s , although i n t e r a c t i o n s between the a l k y l g r o u p and the h e t e r o a t o m c a n be o b s e r v e d , i o n i z a t i o n f r o m the H O M O i s only w e a k l y c o u p l e d to the b o n d i n g s y s t e m and a d d i t i v i t y i s o b s e r v e d . In c o n t r a s t , the i o n i z a t i o n p r o c e s s i n o r g a n o m e t a l s such as M e H g p r o c e e d s f r o m a m o l e c u l a r o r b i t a l that has s u b s t a n t i a l m e t a l - c a r b o n bonding c h a r a c t e r . T h i s conclusion is portrayed with a s i m p l e L C B O d i a g r a m as:


3

2

Η 6ρ·0~ PbR

Et

2

+ 2IrCl

6

2

Me-Cl

+ EtPbR

2

+

,

etc.

,

etc.

" —(

[10] ν—> Et-Cl

+ MePbR

2

A f t e r n o r m a l i z a t i o n for e a c h type of a l k y l g r o u p i n the r e a c t a n t , the r e l a t i v e y i e l d s of ethyl c h l o r i d e and m e t h y l c h l o r i d e a r e r a t h e r constant at about 25 i n a c e t o n i t r i l e , but v a r y s o m e w h a t i n acetic a c i d . T e t r a a l k y l l e a d c o m p o u n d s r e a c t with h e x a c h l o r o i r i d a t e ( l V ) at d i f f e r i n g r a t e s , w h i c h w e r e f o l l o w e d s p e c t r o p h o t o m e t r i c a l l y by the d i s a p p e a r a n c e of the a b s o r p t i o n b a n d s at 490 and 585 n m . T h e k i n e t i c s showed a f i r s t - o r d e r d e p e n d e n c e on t e t r a a l k y l l e a d

220

ORGANOMETALS A N D ORGANOMETALLOIDS

and h e x a c h l o r o i r i d a t e ( l V ) i n both a c e t o n i t r i l e and a c e t i c a c i d solution. T h e s e c o n d - o r d e r rate constants determined i n aceto-d[IrCl "]/dt 6

=

2

2 k[R Pb] [IrCl ~] 4

6

[11]

2

n i t r i l e solutions i n c r e a s e p r o g r e s s i v e l y f r o m M e P b , M e P b E t , M e P b E t , M e P b E t , to E t ^ P b as l i s t e d i n T a b l e VIII. 4


2

2

3

3

T a b l e VIII. T h e C o r r e l a t i o n of S e l e c t i v i t i e s and Rates of O x i d a t i v e C l e a v a g e of T e t r a a l k y l l e a d s b y H e x a c h l o r o i r i d a t e ( l V ) with the E n e r g e t i c s of E l e c t r o n D e t a c h m e n t P r o c e s s e s . PbMe Et . n

4

k

ID

( M " sec )

MeCl

(eV)

(V)

(cm- )

26 11 3.3 0.57 0.02

—

24 25 24

8.13 8.26 8.45 8.65 8.90

1.67 1.75 1.80 2.01 2.13

20,400 22,600 23,200 24,300

1

PbEt PbEt Me PbEt Me PbEtMe PbMe 4

3

2

2

3

4

Ε

EtCl

n

l

-

1

—

T w o i m p o r t a n t c r i t e r i a c a n be u s e d to d i s t i n g u i s h the r e a c tion of t e t r a a l k y l l e a d with h e x a c h l o r o i r i d a t e ( l V ) f r o m the m o r e c o n v e n t i o n a l e l e c t r o p h i l i c p r o c e s s e s , e . g . , those i n v o l v i n g B r o n s t e d a c i d s , s i l v e r ( l ) , c o p p e r ( l ) or c o p p e r ( l l ) c o m p l e x e s , etc. (15,19*24,25). F i r s t , the r a t e of r e a c t i o n of E t » P b M e w i t h h e x a c h l o r o i r i d a t e ( l V ) i n c r e a s e s s u c c e s s i v e l y as m e t h y l i s r e p l a c e d b y ethyl g r o u p s [ s e e η = 4 to 0 i n T a b l e VIII, c o l u m n 2]. S e c o n d , a g i v e n e t h y l g r o u p i s c l e a v e d a p p r o x i m a t e l y 25 t i m e s f a s t e r than a m e t h y l g r o u p [ c o l u m n 3]. Both of t h e s e r e a c t i v i t y t r e n d s a r e d i a m e t r i c a l l y opposed to a n e l e c t r o p h i l i c c l e a v a g e w h i c h o c c u r s d i r e c t l y at the l e s s h i n d e r e d m e t h y l site f a s t e r than at a n ethyl site u n d e r equivalent c o n d i t i o n s . T h e s e r e s u l t s suggest that the r a t e - l i m i t i n g step with h e x a c h l o r o i r i d a t e ( l V ) o c c u r s p r i o r to a l k y l t r a n s f e r . T h e m e c h a n i s m g i v e n i n S c h e m e I i n v o l v e s e l e c t r o n t r a n s f e r i n eq 12 a s the r a t e limiting process. 4

n

n

S c h e m e I; R Pb 4

+ Ir^Cl " 6

2

k

R Pbt

f

a

s

R« + I r ^ C l / "

f

a

s

4

t

t

>

R Pbt

>

R- + R P b

>

RC1 + I i F c i / ' , etc.

4

+ Ir Cl M

3

6

3

"

+

[12] [13]

[14]

Indeed, t h e r e i s a good l i n e a r c o r r e l a t i o n of the r a t e s (log k) with the o n e - e l e c t r o n o x i d a t i o n or i o n i z a t i o n potentials of t e t r a a l k y l l e a d c o m p o u n d s p r e s e n t e d i n T a b l e V . S e l e c t i v i t y i n the c l e a v a g e of a l k y l g r o u p s f r o m o r g a n o l e a d a c c o r d i n g to S c h e m e I o c c u r s d u r i n g f r a g m e n t a t i o n of the c a t i o n - r a d i c a l i n a fast subsequent

13.

Alkyl Transfers

KOCHi

221

s t e p 13, w h i c h i s c o n s i s t e n t w i t h the m a s s s p e c t r a l study. Thus, a q u a n t i t a t i v e d e t e r m i n a t i o n of the c r a c k i n g p a t t e r n s of the s e r i e s of P b M e E t . showed that s c i s s i o n of the E t - P b bond i s f a v o r e d o v e r the M e - P b bond i n the p a r e n t m o l e c u l a r i o n s , l a r g e l y due to bond e n e r g y d i f f e r e n c e s . n

4

n

Me

Me* + E t P b R >bR

2

Et


2

(

f

[15]

+

. ν—•

Et-

+ MePbR

[16]

2

E x a m i n a t i o n of the e l e c t r o n s p i n r e s o n a n c e s p e c t r u m d u r i n g the r e a c t i o n w i t h h e x a c h l o r o i r i d a t e ( l V ) d i d not r e v e a l the p r e s e n c e of the c a t i o n - r a d i c a l P b E t t , w h i c h m u s t be h i g h l y u n s t a b l e e v e n at t e m p e r a t u r e s a s l o w as - 2 0 ° C . N o n e t h e l e s s , the f o r m a t i o n of ethyl r a d i c a l s i n high y i e l d s was evident f r o m s p i n - t r a p p i n g e x p e r i m e n t s w i t h n i t r o s o i s o b u t a n e and p h e n y l - t - b u t y l n i t r o n e , i n w h i c h the w e l l - r e s o l v e d s p e c t r u m of the e t h y l adduct c o u l d be obtained. 4

T h e u s e of h e x a c h l o r o i r i d a t e ( l V ) as an e f f i c i e n t s c a v e n g e r f o r a l k y l r a d i c a l s i s i m p l i e d i n S c h e m e I b y the i s o l a t i o n of a l k y l c h l o r i d e s i n h i g h y i e l d s . In s u p p o r t , s e p a r a t e e x p e r i m e n t s do i n d e e d show that e t h y l r a d i c a l s g e n e r a t e d u n a m b i g u o u s l y f r o m the t h e r m o l y s i s of p r o p i o n y l p e r o x i d e a r e q u a n t i t a t i v e l y c o n v e r t e d by h e x a c h l o r o i r i d a t e ( l V ) to e t h y l c h l o r i d e i n eq 14. T h e r e i s an a l t e r n a t i v e p o s s i b i l i t y that a l k y l h a l i d e i s f o r m e d d i r e c t l y f r o m the c a t i o n - r a d i c a l without the i n t e r m e d i a c y of a n a l k y l r a d i c a l . R Pbt + Ir^Cl/' 4

R Pb 3

+

+ R - C l + Ir^Clg " , 2

etc.

T h e d i f f e r e n c e between t h i s f o r m u l a t i o n and that p r e s e n t e d i n eq 13 of S c h e m e I r e s t s on the d e g r e e of m e t a s t a b i l i t y of the c a t i o n r a d i c a l toward fragmentation. T h e f a i l u r e to o b s e r v e the e s r s p e c t r a of R Pb+ and the i r r e v e r s i b i l i t y of the o x i d a t i o n w a v e i n c h r o n o p o t e n t i o m e t r y s u g g e s t s that i t s l i f e t i m e i s s h o r t . 4

B. D i a l k y l m e r c u r y Compounds. Hexachloroiridate(lV) a l s o r e a d i l y c l e a v e s d i a l k y l m e r c u r y c o m p o u n d s by s e c o n d - o r d e r k i n e t i c s s i m i l a r to eq 15 f o r t e t r a a l k y l l e a d (26). M o r e o v e r , the p r o d u c t s , both o r g a n i c and i r i d i u m ( l l l ) , as w e l l as the s t o i c h i o m e t r y of the r e a c t i o n a r e a l s o e q u i v a l e n t to that g i v e n i n eq 9> viz., Me Hg + 2 I r C l " 2

2

MeCl + MeHg

+

+ IrCl " + IrCl (S) " 6

3

5

2

N m r s t u d i e s i n d i c a t e that M e H g i s bound to I r C l ( S ) " i n s o l u t i o n . T h e s a m e s t o i c h i o m e t r y a p p l i e s to the h i g h e r h o m o l o g s ; the only d i f f e r e n c e lies i n the c o m p l e x i o n of the p r o d u c t s , i n c r e a s i n g a m o u n t s of a l k e n e s and a l k y l a c e t a t e s b e i n g f o r m e d at the e x p e n s e of a l k y l c h l o r i d e s on going f r o m e t h y l , i s o p r o p y l to t - b u t y l . T h e c l e a v a g e of d i a l k y l m e r c u r y by h e x a c h l o r o i r i d a t e ( l V ) i s h i g h l y dependent on the s t r u c t u r e of the a l k y l g r o u p s . T h u s , i n +

5

2

222


the h o m o l o g o u s s e r i e s of R H g C H , the r e l a t i v e r a t e s of c l e a v a g e i n c r e a s e f r o m R = m e t h y l : ethyl : i s o p r o p y l : t e r t - b u t y l , roughly i n the o r d e r of 1 0 ° : 10 · 10 · 1 0 . T h e s e r e s u l t s r u n c o u n t e r to the p a t t e r n o b s e r v e d i n the e l e c t r o p h i l i c c l e a v a g e of the s a m e m e r c u r i a l s d e s c r i b e d i n the f o r e g o i n g s e c t i o n , or to expectations b a s e d on i n c r e a s i n g s t e r i c h i n d r a n c e . Instead, it s u g g e s t s that the r a t e - l i m i t i n g step o c c u r s p r i o r to a l k y l t r a n s f e r . 3

3

5

6

S c h e m e II; R Hg Organometals and Organometalloids Downloaded from pubs.acs.org by UNIV LAVAL on 07/10/16. For personal use only.

2

+ Ir^Cl*" R Hgt 2

R. + I r ^ C l " 2

k

f

a

s

f a s t

t

>

R Hgt + Ir Cl

>

RHg

>»

m

2

R

+

6

3

"

[17]

+ R.

[18]

+ Ir ci X " m

o x

5

[19]

n

w h e r e X = C I or s o l v e n t T h e k i n e t i c s , p r o d u c t s and s e l e c t i v i t y as w e l l as s p i n t r a p p i n g w i t h t - B u N O and 0 a l l a c c o r d w i t h the m e c h a n i s m i n S c h e m e II. T h e o b s e r v a t i o n of p a r a m a g n e t i c i n t e r m e d i a t e s by s p i n t r a p p i n g i n d i c a t e s that a l k y l r a d i c a l s a r e f o r m e d d u r i n g the c l e a v a g e of R H g b y I r C l * " . In fact, the q u a n t i t a t i v e a c c o u n t i n g of the a l k y l f r a g m e n t s as a l k y l p e r o x y p r o d u c t s , when the r e a c t i o n i s c a r r i e d out i n the p r e s e n c e of oxygen, shows that a l l of the a l k y l g r o u p s m u s t d e p a r t f r o m m e r c u r y a s f r e e r a d i c a l s a c c o r d i n g to eq 18. T h e l a t t e r i s s t r o n g l y s u p p o r t e d by the o b s e r v a t i o n that I r C l " d i s a p p e a r s u n d e r t h e s e c o n d i t i o n s at just o n e - h a l f the r a t e o b s e r v e d i n an i n e r t a t m o s p h e r e , as p r e d i c t e d b y S c h e m e II. T h e f a i l u r e to o b s e r v e d i r e c t l y the e l e c t r o n s p i n r e s o n a n c e s p e c t r u m of R H g t s u g g e s t s that i t s l i f e t i m e i s v e r y s h o r t . It i s p r e s e n t as one of the p r i n c i p a l s p e c i e s d u r i n g e l e c t r o n i m p a c t of R H g i n the gas p h a s e , and a m e r c u r y ( l l l ) s p e c i e s has been o b s e r v e d as t r a n s i e n t i n the e l e c t r o c h e m i c a l o x i d a t i o n of H g ( c y c l a m ) +. S e l e c t i v i t y i n the c l e a v a g e of a l k y l g r o u p s f r o m u n s y m m e t r i c a l d i a l k y l m e r c u r y b y I r C l " a c c o r d i n g to S c h e m e II o c c u r s d u r i n g f r a g m e n t a t i o n of R H g t r a d i c a l - c a t i o n subsequent to the r a t e - l i m i t i n g step. T h e u n i m o l e c u l a r d e c o m p o s i t i o n of ( C H ) H g i n the gas phase has been e x a m i n e d b y p h o t o e l e c t r o n photoion c o i n c i d e n c e s p e c t r o s c o p y (27). 2

2

2

6

2

2

2

2

2

2

3

2

+

CH Hg 3

CH HgCH t 3

( \->

3

+

CH Hg3

+ CH 3

[20]

+ CH

3

[21]

T h e t h r e s h o l d e n e r g y f o r f r a g m e n t a t i o n i n eq 20 i s found to be n e a r l y 2.5 v o l t s l o w e r than that f o r eq 21. T h e e x c l u s i v e cleavage of R = t - B u and i - P r and p r e f e r e n t i a l c l e a v a g e of R = E t i n the h o m o l o g o u s s e r i e s of R H g C H i s i n a c c o r d w i t h a w e a k e r a l k y l m e r c u r y c o m p a r e d to a m e t h y l - m e r c u r y b o n d . T h e p r e d o m i n a n t f a c t o r w h i c h d e t e r m i n e s a l k y l v s . m e t h y l c l e a v a g e a r e the s t r e n g t h s of the r e l e v a n t C - H g b o n d s . T h e s e v a l u e s c a n be e v a l uated f r o m the a v e r a g e bond e n e r g i e s f o r M e H g , E t H g , and 3

2

2

13.

Alkyl Transfers

KOCHi

223 1

i - P r H g w h i c h a r e 58, 48, and 42 k c a l m o l " , r e s p e c t i v e l y (28.29)* A c c o r d i n g to S c h e m e II, the i s o l a t i o n of a l k y l c h l o r i d e s i n h i g h y i e l d s i m p l i e s that h e x a c h l o r o i r i d a t e ( l V ) i s an e f f i c i e n t s c a v enger of a l k y l r a d i c a l s i n eq 19 (Rox = RC1, X = S). H o w e v e r , i n a d d i t i o n to the r e d o x t r a n s f e r of c h l o r i n e f r o m h e x a c h l o r o i r i d a t e (IV) i n eq 14, a n a d d i t i o n a l r e d o x step i s r e q u i r e d , e s p e c i a l l y f o r R = i s o p r o p y l and t - b u t y l . T h e o b s e r v a t i o n of i s o b u t y l e n e and t e r t - b u t y l a c e t a t e f r o m t e r t - b u t y l r a d i c a l s and h e x a c h l o r o i r i d a t e (IV) i s a n a l o g o u s to e l e c t r o n t r a n s f e r o x i d a t i o n of a l k y l r a d i c a l s (30). ( C H ) 0 + Ir C l " >- I r C l " + ( C H ) C , etc. 2


2

3

3

3

6

+

6

3

3

The t e r t - b u t y l cation f o r m e d under such c i r c u m s t a n c e s w i l l u n d e r g o s o l v a t i o n , f o r e x a m p l e to t e r t - b u t y l acetate, or l o s s of a β-proton to i s o b u t y l e n e . T h e r e s u l t a n t i r i d i u m ( l l l ) p r o d u c t m u s t then m a i n t a i n i t s c o o r d i n a t i o n s p h e r e i n t a c t as I r C l " . Indeed, the d i s t r i b u t i o n of I r C l ^ " and I r C l ( C H C N ) " a m o n g r e d u c e d i r i d i u m ( l l l ) products formed f r o m various a l k y l m e r c u r i a l s i s p r e c i s e l y i n a c c o r d w i t h t h i s f o r m u l a t i o n . T h u s , the r e s u l t s c l e a r l y i n d i c a t e that m e t h y l and e t h y l r a d i c a l s r e a c t w i t h I r C l ~ i n a c e t o n i t r i l e , e x c l u s i v e l y by c h l o r i n e t r a n s f e r . F o r i s o p r o p y l and t e r t - b u t y l r a d i c a l s , a p p r o x i m a t e l y 85 and 50$, r e s p e c t i v e l y , of the r e a c t i o n p r o c e e d s by c h l o r i n e t r a n s f e r and the r e m a i n d e r by e l e c t r o n t r a n s f e r . T h e l a t t e r b e c o m e s m o r e i m p o r t a n t i n a c e t i c a c i d s o l u t i o n s . T h e d e c r e a s i n g t r e n d of a l k y l r a d i c a l s to r e a c t w i t h I r C l " by e l e c t r o n t r a n s f e r i n the o r d e r : t - B u > i - P r » E t > M e f o l l o w s the e a s e of i o n i z a t i o n of the r a d i c a l i n the o r d e r : t - B u < i - P r < E t < M e as l i s t e d i n T a b l e II. Further m o r e , the opposed t r e n d i n the y i e l d s of a l k y l c h l o r i d e s i s c o n s i s t e n t w i t h the g e n e r a l l y d e c r e a s i n g a l k y l - c h l o r i n e bond energies f r o m M e C l t h r o u g h t - B u C l . W h e t h e r c h l o r i n e t r a n s f e r and c a r b o n i u m i o n f o r m a t i o n r e p r e s e n t i n n e r - and o u t e r - s p h e r e r e d o x p r o c e s s e s , r e s p e c t i v e l y , f o r m s an i n t e r e s t i n g s p e c u l a t i o n . I n n e r - and o u t e r - s p h e r e m e c h a n i s m s m e r i t c o n s i d e r a t i o n f o r the p r o c e s s by w h i c h e l e c t r o n t r a n s f e r o c c u r s f r o m R H g to I r C l " i n the r a t e - l i m i t i n g step i n eq 1 7. A l i n e a r f r e e e n e r g y r e l a t i o n s h i p b e t w e e n l o g k of r e a c t i o n and Ij) of R H g i s e x p e c t e d f o r t h i s s y s t e m i f e l e c t r o n t r a n s f e r o c c u r s by an o u t e r - s p h e r e p r o c e s s . H o w e v e r , the n e g a t i v e d e v i a t i o n of d i - t e r t - b u t y l - , d i i s o p r o p y l - , and d i e t h y l m e r c u r y f r o m the l i n e a r plot s u g g e s t s that s t e r i c f a c t o r s can be i m p o r t a n t i n the e l e c t r o n t r a n s f e r to IrCl ". 3

6

2

5

3

2

6

2

2

2

2

2

6

C. D i a l k y l ( b i s - p h o s p h i n e ) p l a t i n u m ( l I ) C o m p l e x e s . The c l e a v a g e of o r g a n o p l a t i n u m ( I I ) c o m p l e x e s w i t h o u t e r - s p h e r e o x i dants was c a r r i e d out as a c o m p a r i s o n f o r the a l k y l s of the m a i n g r o u p e l e m e n t s , l e a d and m e r c u r y , d e s c r i b e d above. Indeed, cis-dialkyl(bis-phosphine)platinum(ll) complexes are readily oxi d i z e d by h e x a c h l o r o i r i d a t e ( l V ) to a f f o r d two p r i n c i p a l types of p r o d u c t s d e p e n d i n g on the s t r u c t u r e of the a l k y l g r o u p and the

224

O R G A N O M E T A L S AND

ORGANOMETALLOIDS

c o o r d i n a t e d p h o s p h i n e (31)· T h u s , the d i e t h y l analog, c i s - E t P t ( L X ) ( P M e P h ) , a f f o r d s E t C l and e t h y l p l a t i n u m ( l l ) s p e c i e s by o x i d a t i v e c l e a v a g e of the E t - P t bond, 2

2

2

2

Et P#L + 2IrCl " 2

2

+

•

6

3

EtP^L S+

EtCl + IrCl "+IrCl

2

6

where L = P M e P h , P P h ; S = 2

2

5

S"

solvent

3

w h e r e a s M e P t ( P M e P h ) u n d e r g o e s o x i d a t i o n to d i m e t h y l platinum(lV) species. 2

2

2


2

Me PR. ( \ — R *


1

+

R Hg f

+

+ RHg

A s expected, the r a t e of t h i s c l e a v a g e (log k g ^ ) i s linearlyr e l a t e d to the i o n i z a t i o n p o t e n t i a l of the m e r c u r i a l as shown i n F i g u r e 3. A s i m i l a r c o r r e l a t i o n i s shown by o r g a n o m e t a l s u n d e r g o i n g s u b s t i t u t i o n [ G r i g n a r d r e a g e n t and p e r o x i d e (35)] or i n s e r t i o n [ t e t r a a l k y l l e a d and T C N E (36)] v i a e l e c t r o n t r a n s f e r , and they a r e a l s o i n c l u d e d i n F i g u r e 1 f o r c o m p a r i s o n . T h e r a t e constant k g f o r the e l e c t r o p h i l i c p r o t o n o l y s i s of an a l k y l - m e r c u r y bond, R-HgR

1

+

H

+

k

E

>

R-H +

R»Hg

+

can be d i s s e c t e d i n t o two p a r a m e t e r s : C, w h i c h i s f o c u s e d on the c l e a v e d g r o u p R , and I·, w h i c h depends only on the l e a v i n g g r o u p , R H g , as d e s c r i b e d by eq 8. F i g u r e 4 shows that I· r e s p o n d s l i n e a r l y to the i o n i z a t i o n p o t e n t i a l of the m e r c u r i a l . F u r t h e r m o r e , I· a l s o s t r o n g l y c o r r e l a t e s w i t h the T a f t p o l a r s u b stituent constant, σ*, f o r the a l k y l g r o u p s l i s t e d i n T a b l e X . T h e !

T a b l e X . C o r r e l a t i o n of I· and C P a r a m e t e r s w i t h T a f t P o l a r ( σ * ) and S t e r i c ( E ) C o n s t a n t s . s

σ*

R

0 0.10 0.20 0.30

CH CH3CH2 (CH ) CH (CH ) C 3

3

2

3

3

Leaving Group Effects RHg

+

CH Hg+ CH CH Hg (CH ) CHHg+ (CH )3CHg 3

3

2

3

+

2

+

3

Cleaved Group Effects R CH C H C H2 (CH ) CH (CH ) C 3

3

3

2

3

3

JL( expt.) 0 0.76 1.28 1.44

C (expt.) 0 0.55 0.29 — 1

0 -0.07 -0.47 -1.54 8.1 σ * + 0.65 (cale.)

E

s

0 0.75 1.30 1.45 8.1 σ * + 2.8 (cale.)

E

s

0 0.61 0.30 -1.9

c u r v a t u r e i n F i g u r e 4 f o r the c l e a v e d g r o u p constant, C, on p r o c e e d i n g f r o m m e t h y l to t - b u t y l can be a t t r i b u t e d to an i n c r e a s i n g s t e r i c effect as a r e s u l t of a d d i t i o n a l e n c u m b r a n c e by s u c c è s -

ORGANOMETALS

228

AND

ORGANOMETALLOIDS

3 * Log k


8.5

9.0

Log

Figure 3. Εlectron-transfer processes in electrophilic substitutions. Saturation ef fects followed by alkyl substituents in the cleavage of organometals during the treat ment with various electrophiles: scale left and bottom for ((B) tetraalkyllead with tetracyanoethylene and (Q) dialkylmercury with hexachloroiridate(IV). Scale right (Tafel potential) and top for Grignard reagents and di-tert-butyl peroxide (·).

2

L

Log k

Figure 4. Correlation of cleaved-group constant C and leaving group constant L in acetolysis with the ionization potential of the dialkylmercury compound

C Bii*-HgCH

s

8

9

IONIZATION POTENTIAL,

tV

13.

Alkyl Transfers

KOCHi

229

s i v e l y m o r e α - m e t h y l groups. T h e latter i s a l s o supported by the s i z e a b l e c o n t r i b u t i o n of the T a f t s t e r i c p a r a m e t e r E to the c o r r e l a t i o n w i t h C, as shown i n T a b l e X . B a r r i n g s t e r i c effects, both I· and C a r e thus s t r o n g l y dependent on the i o n i z a t i o n p o t e n t i a l of the m e r c u r i a l ; that i s , e l e c t r o p h i l i c a t t a c k at a n a l k y l m e t a l bond i s r e s p o n s i v e to e l e c t r o n a v a i l a b i l i t y , n a m e l y the H O M O e n e r g y , i n m u c h the s a m e w a y that the i o n i z a t i o n p o t e n t i a l is. D e s c r i b e d i n an a l t e r n a t i v e way, the t r a n s i t i o n state f o r e l e c t r o p h i l i c c l e a v a g e i n eq 28 c a n be c o n s i d e r e d a s a p e r t u r b a t i o n of the o r g a n o m e t a l b y an e l e c t r o p h i l e , w h i c h i s a k i n to a virtual ionization. T h e s e c o m p a r i s o n s show that e l e c t r o n t r a n s f e r and e l e c t r o p h i l i c a t t a c k both depend h e a v i l y on the e l e c t r o n a v a i l a b i l i t y i n the organometal. It f o l l o w s that a n y c o r r e l a t i o n of the r e a c t i v i t y ( i . e . , r a t e c o n s t a n t s ) w i t h the i o n i z a t i o n or o x i d a t i o n p o t e n t i a l s i s not s u f f i c i e n t to d i f f e r e n t i a t e the two m e c h a n i s m s . The differ ence between the two i s l a r g e l y due to the v a r i a t i o n s i n the s t e r i c i n t e r a c t i o n s , w h i c h can be l a r g e i n a n i n n e r - s p h e r e , e l e c t r o p h i l i c p r o c e s s ( i . e . , f o r C but not I.) and l e s s i m p o r t a n t i n an o u t e r sphere, electron transfer process. T h i s c o n c l u s i o n s u p p o r t s the g e n e r a l f o r m u l a t i o n that e l e c t r o n t r a n s f e r and e l e c t r o p h i l i c p r o c e s s e s c a n s h a r e a c o m m o n t h e m e of c h a r g e t r a n s f e r i n t e r a c t i o n s . S u c h a c o n c l u s i o n a l s o b e a r s on the m u l t i p l i c i t y of a v a i l a b l e m e c h a n i s m s f o r the c l e a v a g e of o r g a n o m e t a l s . T h e ready a c c e s s i b i l i t y of s u c h c o n c e r t e d and s t e p w i s e p r o c e s s e s f o l l o w s n a t u r a l l y f r o m t h e i r b a s i c s i m i l a r i t y , and i t c a n m a k e the t a s k of differentiation in individual cases difficult. T h e s e s t u d i e s a l s o shed l i g h t on the e f f e c t s of p o l y a l k y l a tion of m e t a l s on t h e i r r e a c t i v i t y to e l e c t r o p h i l i c a n d e l e c t r o n transfer cleavages. T h u s the r a t e s of c l e a v a g e of a s i n g l e a l k y l l i g a n d f r o m G r o u p I V B o r g a n o m e t a l s a l w a y s d e c r e a s e i n the orderR M > R M C 1 > R M C 1 , and f o r the m e r c u r i a l s , R Hg » R H g C l , i n d e p e n d e n t of whether an e l e c t r o p h i l i c o r e l e c tron transfer process pertains. T h i s reactivity sequence natur a l l y f o l l o w s f r o m the a v a i l a b i l i t y of σ - b o n d i n g e l e c t r o n s i n the H O M O as l i s t e d i n T a b l e IV f o r the m e r c u r i a l s .


s

4

3

2

2

2

V.

Homolytic Displacements in A l k y l Transfers

O r g a n o c o b a l t ( l I I ) c o m p l e x e s a r e r e a d i l y c l e a v e d by c h r o m o n s i o n i n aqueous p e r c h l o r i c a c i d s o l u t i o n (37). RCo^(DMG)

2

+ Cr + aq 2

• > ZH'

R-Cr*+ + 4 a

Co^DMG^

where D M G = dimethylglyoximate T h e a l k y l g r o u p i s t r a n s f e r r e d to c h r o m i u m ( l l ) e s s e n t i a l l y q u a n t i t a t i v e l y . T h e c l e a v a g e f o r m a l l y r e p r e s e n t s a t r a n s f e r of a n a l k y l r a d i c a l R« to C r + , that i s an o v e r a l l r e d u c t i v e c l e a v a g e of an a l k y l - c o b a l t (III) b o n d . T h e r a t e of t r a n s m e t a l l a t i o n f o l l o w s 2

230


second-order kinetics, -d[Cr]/dt

+

=

+

k [ R C o ] [ C r ] + k«[RCoH ] [ C r ]

[30]

1

w h e r e k and k r e l a t e to the c l e a v a g e of the n e u t r a l and protonated alkylcobalt(lll) species, respectively. T h e v a r i a t i o n i n k and k w i t h the a l k y l g r o u p s f o l l o w s t h e s a m e o r d e r , a n d d e c r e a s e s i n the o r d e r ; 1

R 1

1


k'fM" s e c " )

=

Me

=

1

10 ·

>

Et >

4

n-Pr >

1

2

ΙΟ" ·*

ΙΟ" ·*

i - P r> i-Bu ΙΟ"

4

4

10" ·

2

A l k y l c o b a l a m i n s a r e a l s o c l e a v e d b y c h r o m o u s i o n (38), +

+

RCo(corrin) + C r

RCr

+ B

[31]

1 2 r

with s i m p l e second-order kinetics, f i r s t - o r d e r i n each reactant, and i n d e p e n d e n t of p H b e t w e e n 0-2.3. T h e c l e a v a g e s of the m e t h y l [k = 3 . 6 x l 0 M s e c " ] and e t h y l [k = 4.4 M** s e c " ] d e r i v atives proceed with different activation parameters: Δ Η * - 3.8 (Me), 11 (Et) k c a l m o l " ; Δ S* = -34(Me), - 1 8 ( E t ) eu. A l k y l t r a n s f e r s f r o m c o b a l t ( l l l ) t o c h r o m i u m ( l l ) as de s c r i b e d a b o v e a r e a n a l o g o u s t o the r e v e r s i b l e e x c h a n g e b e t w e e n a l k y l c o b a l t ( H I ) and c o b a l t ( l l ) (32,40,41), 2

- 1

1

1

1

1

m

E

R C o ( l ) + Co (2)

C o ( l ) + RCo (2)

=F=*=

M

[32]

w h e r e C o ( l ) and Co(2) r e f e r t o c o b a l t c o m p l e x e s w i t h s l i g h t l y d i f f e r e n t c h e l a t i n g l i g a n d s s u c h a s d i m e t h y l g l y o x i m a t o and c y c l o h e x a n e d i o n e d i o x i m a t o . T h e o v e r a l l p r o c e s s i n eq 32, w h i c h i s e q u i v a l e n t to e l e c t r o n t r a n s f e r , a c t u a l l y o c c u r s b y t r a n s f e r of a n a l k y l g r o u p a s a r a d i c a l a s shown b y l a b e l l i n g t h e c o b a l t a t o m s w i t h d i f f e r e n t c h e l a t i n g l i g a n d s . T h e r a t e of e x c h a n g e f o l l o w s s e c o n d - o r d e r k i n e t i c s , f i r s t - o r d e r i n cobalt(ll) and f i r s t - o r d e r i n a l k y l c o b a l t ( l l l ) . T h e second-order rate constants d e c r e a s e i n the r e l a t i v e o r d e r [ k ( E t ) = 1.1 χ 10" M" s e c " ] : 1

R k

r e l

= =

Me (2>10 ·6) 2

>

Et >

1

1

n-Pr ~ 1

(10°)

2

η-Bu 1

(ΙΟ" · )

4

(ΙΟ" · )

>

i - P r> i-Bu 2

5

(10- · )

(10-3.3)

T h e t r a n s f e r of t h e e r y t h r o - P h C H D C H D - g r o u p o c c u r s w i t h i n v e r s i o n (42). C o u p l e d w i t h the r e a c t i v i t y t r e n d of a l k y l g r o u p s , the c l e a v a g e i s b e s t c o n s i d e r e d a s a h o m o l y t i c d i s p l a c e m e n t on the c a r b o n c e n t e r (43). T h e t r a n s i t i o n state,

τη

E

V

'

η

m

i *

[Co—C—Co J i s s i m i l a r to that i n e l e c t r o p h i l i c c l e a v a g e s o c c u r r i n g w i t h i n v e r s i o n , e x c e p t t h e p r o c e s s i n v o l v e s a o n e - e q u i v a l e n t r a t h e r than a t w o - e q u i v a l e n t change. H o w e v e r , t h e l a t t e r does not a p p e a r to be

13.

Alkyl Transfers

KOCHi

231

a d e c i s i v e factor, since a l k y l t r a n s f e r f r o m alkylcobalt(lll) to cobalt(l), i.e., cJfl) + RCo (2)

m

m

R C o ( l ) + Cc?(2)

[33]

o c c u r s w i t h r a t e s and s t e r e o c h e m i s t r y m u c h l i k e that of i t s c o b a l t ( l l ) c o u n t e r p a r t i n eq 32. In p a r t i c u l a r , i n v e r s i o n of c o n f i g u r a t i o n at c a r b o n o c c u r s d u r i n g a l k y l exchange, a n d the s e c o n d - o r d e r r a t e c o n s t a n t s d e c r e a s e i n the r e l a t i v e o r d e r [ k ( E t ) = 1CT M" s e c " ] : 1


R

k

r e l

1

1

=

=

Me 2

> E t > n - P r - η-Bu > i - B u 2

fclO - )

(10°) (ίο- · )

(ίο- · )

1 4

3

(l0- -9)

1 7

A t t e m p t s to m e a s u r e the a l k y l exchange of a l k y l c o b a l t ( i l l ) w i t h c o b a l t ( i l l ) w e r e u n f o r t u n a t e l y c o m p l i c a t e d b y the s l o w r a t e s , and thus s u s c e p t i b l e to t r a c e c a t a l y s i s b y c o b a l t ( l l ) s p e c i e s . T h e l o w r e a c t i v i t y of c o b a l t ( l l l ) i s p r o b a b l y due to i t s s u b s t i t u t i o n s t a b i l i t y , w h i c h l i m i t s the a v a i l a b i l i t y of the a c t i v e 5 - c o o r d i n a t e e l e c t r o p h i l i c s p e c i e s . N o n e t h e l e s s , the t r a n s i t i o n s t a t e s f o r a l k y l e x c h a n g e s i n a l l t h r e e s y s t e m s a r e l i k e l y to b e s i m i l a r , e f f e c t i v e l y i n v o l v i n g a l i n e a r 3-atom c o n f i g u r a t i o n (see above). T h e s i m i l a r i t y i n the r a t e s of the c o b a l t ( l l ) a n d c o b a l t ( l ) r e a c t i o n s s u g g e s t s that the e x t r a 1 a n d 2 e l e c t r o n s , r e s p e c t i v e l y , a r e i n a nonbonding o r b i t a l c e n t e r e d on both c o b a l t a t o m s . T h e c l e a v a g e s of a l k y l - m e t a l bonds b y e a c h of the t h r e e c o b a l t c o m p l e x e s w i t h o x i d a t i o n s t a t e s I, II a n d III a r e r e p r e s e n t a t i v e of what i s c o m m o n l y c o n s i d e r e d t o b e e l e c t r o p h i l i c , h o m o l y t i c a n d n u c l e o p h i l i c p r o c e s s e s , r e s p e c t i v e l y , e.g., C

, °>

R-Co* + M °

LÇsL+. R - C o + M 1

R-M*

1

WïL R-Cc? + M Y e t t h e r e a d y i n t e r c o n v e r s i o n of e a c h c o b a l t s p e c i e s b y onee q u i v a l e n t changes, 1

Co

^ *

Co

^

Co

r a i s e s the i s s u e of w h e t h e r e l e c t r o n t r a n s f e r p r o c e s s e s a r e i n v o l v e d i n a l k y l t r a n s f e r a s d i s c u s s e d i n the p r e v i o u s s e c t i o n s . S u c h p r o c e s s e s a r e e s p e c i a l l y r e l e v a n t i n v i e w of the e a s e w i t h w h i c h the a l k y l c o b a l t c o m p l e x e s t h e m s e l v e s u n d e r g o o x i d a t i o n r e d u c t i o n (44). RCo

111

=

^

RCo^

F o r e x a m p l e , t h e c l e a v a g e i n eq 32 m a y i n v o l v e a t w o - s t e p p r o cess, i n which cobalt(ll) acts as a nucleophile leading to the i n i t i a l r e d u c t i o n of a l k y l c o b a l t ( l l l ) , f o l l o w e d b y e l e c t r o n t r a n s f e r .

232

ORGANOMETALS

R C o ( l ) + Cc?(2) m

Ccf(l)

+ RdF(2)

^P*=

AND ORGANOMETALLOIDS

Co(l) + R C o ^ ) C

Organometals and Organometalloids - American Chemical Society

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