14 Pathways for Formation of Transition Metal-Carbon
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Bonds i n Protic Media JAMES H. ESPENSON Ames Laboratory and Department of Chemistry, Iowa State University, Ames, IA 50011 I plan to review work from my group and from the literature concerning the title subject. My intention is to be rather selective, focusing upon only a limited family of compounds and reactions. This main emphasis is on the reactions themselves and the mechanisms by which they occur. R-ML Complexes. The compounds of interest are sigmabonded organometallic complexes, chiefly of cobalt and chromium. This includes a series of tetradentate macrocyclic complexes of cobalt, RCo(chel)B withB=H2O,py, etc., aquochromium complexes, [(H2O) Cr-R ], and chromium complexes containing the macrocyclic ligand [15]ane N . Representative structures are shown in Figure 1. The prototype of the organocobalt structures is that of the vitamin B derivative methylcobalamin. The most successful and abundant family of model compounds are the bis (dimethylglyoximato) complexes developed by Schrauzer (1); these are the compounds R-Co(dmgH) B. Other cobalt chelate structures are also shown in Figure 1 with semi-systematic names as suggested by Busch (2). Detailed reviews of their preparation and chemistry have been published (3,4). A large number of compounds with the general formula (H20)5Cr-R are now known (5-11). Chromium complexes analogous to those of the organocobalts, although known, are much less abundant. West (12) has prepared some perfluoroalkyls Rp-Cr(chel)B, and we have recently prepared an extensive series of the alkyl compounds [R-Cr([15]ane Ν4)Η2θ] + (x") (13). Many other classes and examples of organochromium compounds are of course known (14), but their formation will not be reviewed here. A limited number of related organoiron compounds are also known (15,16,17). Our consideration of synthetic methods for such sigma organometallics, as well as their reaction chemistry, can be systematized by means of somewhat artificial oxidation state 5
2+
5
4
12
2
2+
2
2
0-8412-0461-6/78/47-082-235$05.00/0 © 1978 American Chemical Society Brinckman and Bellama; Organometals and Organometalloids ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
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236
ORGANOMETALS
OÙ Co([14]aneN4)2+
Co(Mo4teteneN4)2+
00
00
Co(meso-Me6[14]aneN4)2+
Co(dpnH) +
AND ORGANOMETALLOIDS
Co(Me6-4,ll-d1eneN4)2+
Co(dr.igH) 2
Journal of the American Chemical Society
Figure 1. Representative structures of cobalt chelates which form organometallic compounds of the form R-Co(chel)B. Chelate abbreviations are based on the suggestions of Busch (2).
Brinckman and Bellama; Organometals and Organometalloids ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
14.
Transition Metal-Carbon Bonds
ESPENSON
237
assignments. The organic group R i s c u s t o m a r i l y regarded as a carbanion R:~. Thus the f o l l o w i n g s p e c i f i c compounds, c o n s i d e r i n g t h e i r i o n i c charges and those on the other l i g a n d s ( i f any), are a l l d e r i v a t i v e s o f M ( I I I ) : CH CH -Co(dmgH)opy, [(H2O)5Cr-CH CH CH3] +, [CH -Co([14]ane N ) H 0 ] + , and [CH3-Co(dpnH)H20]+. 3
2
2
2
2
3
3
4
2
Survey o f P r e p a r a t i v e Methods. The f o l l o w i n g r e a c t i o n scheme summarizes the modes of r e a c t i v i t y o f R(Co)B and (H 0) Cr-R : Scheme I . R e a c t i v i t y p a t t e r n s : τττ + ο, electrophiles R:~+(Co ) o r Cr [1]
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2 +
2
5
>
(
2
or R-Cr + ^
h
u
o
r
1
"
e l n
reagents^
,
11
R +(Co ) or C r +
nucleophiles
2 +
1
^ R +(Co )"
[2] [3]
+
The organic groups shown as R , R°, and R a r e , o f course, combined w i t h other reagents as shown i n these s p e c i f i c r e a c t i o n s : 2
+
R(Co)H 0 + Hg + -> RHg + ( H 0 ) ( C o 2
2
(H 0) Cr-R 2
5
2 +
+ Br2 + RBr + C r ( H 0 )
R(Co)H 0 + Cr^q 2
I I I
2
2
2
)
3 + 6
+
[4]
+ Br"
[5]
1 1
(H 0) Cr-R + + (Co ) 2
5
[6]
I t i s o u t s i d e the scope o f t h i s review t o d i s c u s s t h i s r e a c t i o n chemistry i n d e t a i l , and i t i s mentioned here t o demonstrate that the reverse o f each of the three general r e a c t i v i t y methods can be used to form o r g a n o m e t a l l i c d e r i v a t i v e s . Thus a source of a carbanion such as a Grignard reagent or a l k y l l i t h i u m w i l l r e a c t w i t h a M ( I I I ) d e r i v a t i v e such as a h a l i d e t o form a metal-carbon bond. L i k e w i s e , generation of a carbon-centered f r e e r a d i c a l from a s u i t a b l e source i n the presence of the M(II) complex gives R-M, provided the r a d i c a l capture r a t e i s s u f f i c i e n t l y r a p i d ( i . e . , t h a t M(II) i s s u f f i c i e n t l y l a b i l e to l i g a n d s u b s t i t u t i o n ) , v i a the reverse of r e a c t i o n 2. The n u c l e o p h i l i c process, when permitted by the p r o p e r t i e s o f M ( I ) , o f t e n a f f o r d s the best s y n t h e t i c method. Thus cobaloximes and r e l a t e d compounds are a v a i l a b l e from organic h a l i d e s o r t o s y l a t e s by r e a c t i o n s which are w e l l - c h a r a c t e r i z e d as t y p i c a l S 2 s u b s t i t u t i o n s (18): N
1
m
( C o ) " + RX -> R ( C o ) + X"
[7]
Methods Based on M(II) and Free R a d i c a l s . The accepted scheme by which many reduced metal complexes r e a c t w i t h organic h a l i d e s t o form o r g a n o m e t a l l i c products i s shown i n Scheme I I . The e a r l i e s t examples were reported f o r Co(CN)5^~ (19) and Cr2+ (20,21,22).
Brinckman and Bellama; Organometals and Organometalloids ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
ORGANOMETALS AND
238
ORGANOMETALLOIDS
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Scheme I I . Two-Step R a d i c a l Mechanism: RX + M
11
-> X M
R* + M
11
·> R M
Net:
RX + 2 M
m
+ R*
[8]
111
11
[9] + ΧΜ
1 1 1
+ RM
111
[10]
The requirements f o r t h i s to c o n s t i t u t e a s u c c e s s f u l r o u t e to R - M I H are (1) the reduced metal complex must be a s u f f i c i e n t l y s t r o n g and r e a c t i v e r e d u c i n g agent that the f i r s t step w i l l be f e a s i b l e , and (2) the r a t e of combination of M(II) and R* i n equation 9 must be s u f f i c i e n t l y h i g h t o compete w i t h other processes such as r a d i c a l d i m e r i z a t i o n and d i s p r o p o r t i o n a t i o n . The f i r s t requirement l i m i t s the u s e f u l n e s s f o r both c o b a l t and chromium. Although the M complexes Co(dmgH>2 and C r ^ r e a c t w i t h such a c t i v a t e d o r g a n i c h a l i d e s as b e n z y l bromide, they remain u n r e a c t i v e toward s a t u r a t e d a l k y l h a l i d e s such as i-C3H7Br and C H 3 I . For systems i n which the M complex i s a s t r o n g e r r e d u c i n g agent, e.g. C r S 0 i n aqueous dimethylformamide (20) or a " C r ^ - e n " complex i n the same s o l v e n t (23), the processes do occur as shown. Recent work by G. J . Samuels (13) has been based on t h i s i d e a . The complex of C r * * and the t e t r a d e n t a t e m a c r o c y c l i c l i g a n d [15]ane N4 forms r e a d i l y , and i t i s a s t r o n g e r r e d u c i n g agent than Cr|+ (E°=-0.58 V v s . -0.41 V f o r C r + ) . I t r e a c t s to form organochromium complexes w i t h a wide v a r i e t y of o r g a n i c h a l i d e s a c c o r d i n g to the s t o i c h i o m e t r y of equation 10. The r a t e s of these r e a c t i o n s f o l l o w t h i s e x p r e s s i o n 11
2
11
4
2
II
-d[Cr [15]ane N
2 + 4
11
] / d t = 2k [Cr [15]ane N 8
2 + 4
][RX]
[11]
Values of the second-order r a t e constant kg f o r a s e l e c t e d group of the h a l i d e s are summarized i n Table I . Table I Rate Constants f o r R e a c t i o n of 0 Γ [ 1 5 ] ane N/. w i t h A l k y l H a l i d e s (13) C o n d i t i o n s : 25°C i n 1:1 aq. t e r t - b u t a n o l ι χ
RX
1ko/dnAnol'^s"-*-
RX
kg/dflAnol-^s"!
O
C H Br n-C3H7Br n-C H Br 6-Br-l-hexene (CH3)3CCH2Br i-C3H7Br 2
5
4
Q
0.164 0.167 0.130 0.155 M).05 1.85 5.4
