Cobalt-catalyzed carbonylations proposed mechanism

Scheme 5. Proposed mechanism for the unpromoted cobalt-catalyzed carbonylation of methanol.

Scheme 6. Proposed mechanism of the iodide-promoted cobalt-catalyzed carbonylation of alcohols encompassing the effects discussed in Section IV,B.

In addition, Coates et al. have developed a cobalt-catalyzed carbonylation of epoxides for the synthesis of substituted 3-hydroxy-8-lactones [113] (Scheme 66). After screening for several catalysts, HCo(CO)4 was identified as the best catalyst to effect this transformation. The proposed mechanism of the carbonylation involves protonation and ring opening of the epoxide 302 by the catalyst to form cobalt alkyl complex 303, followed by insertion of CO and subsequent cyclization to generate the 3-hydroxy-8-lactone framework 305. 

A typical example of this is the dicobalt octacarbonyl catalyzed hydroformylation of olefins to yield aldehydes. According to the classical mechanism proposed by Heck and Breslow /29/ (Equations 28-31), the cobalt carbonyl reacts with hydrogen to form hydrido cobalt tetracarbonyl, which is in equilibrium with the coordinatively unsaturated HCo(C0)2. The tricarbonyl coordinates the olefin, and rearranges to form the alkyl cobalt carbonyl. 

Duckett and Perutz have shown the stoichiometric reaction of the CpRh(C2H4)(SiR3)H (R = Et, i-Pr) complexes (Scheme 33)201. These complexes have been found to act as precursors to the catalytically active species for the hydrosilylation of ethene with Et3SiH but are not within the catalytic cycle. The mechanism proposed in Scheme 34 for the hydrosilylation of ethene was found to be equivalent to the Seitz-Wrighton hydrosilylation mechanism catalyzed by cobalt carbonyls complexes202. 

Industrially the straight chain isomer is generally the most desired product and hence the normal/iso product ratio obtained for a given catalyst is of importance. Further, the hydrogenation activities of catalysts vary considerably such that alcohols can in some cases be obtained in a single step (222). The first catalysts developed for this reaction were based on cobalt carbonyl and later cobalt carbonyl phosphine complexes. However, more recently attention has been focused on the intrinsically much more active rhodium catalysts (222, 223). A simplified mechanism for (223) cobalt- and rhodium-catalyzed hydroformylation has been proposed which involves the following steps ... 

The amidocarbonylation reaction was discovered by Wakamatsu [1], who demonstrated the synthesis of a range of A -acyl amino acids through the cobalt carbonyl-catalyzed reactions of various combinations of aldehyde plus amide, with carbon monoxide (eq. (1)). Some aspects of the mechanism of aliphatic aldehyde amidocarbonylation have been examined by both Pino and co-workers [2] and by Getman [3], Magnus and Slater [4] subsequently investigated the scope of this synthesis for variety of A -substituted amide co-reactants and C-sub-stituted aldehydes. Further mechanistic revisions were proposed involving acyl-iminium species. 

The accepted mechanism for the carbonylation of epoxides is shown in Scheme 17.26, and the basic steps of this cycle are also thought to occur during the carbonylation of aziridines. Alper first proposed a catalytic cycle for the expansion carbonylation of aziridines by [Co(CO)J, and Coates has proposed a similar cycle for epoxide carbonylation catalyzed by complexes containing both Lewis acids and cobalt-carbonyl anions (Scheme 17.26). This mechanism consists of four steps (1) the activation of substrate by coordination to a Lewis acid (2) the S 2 attack on the substrate by [Co(CO)J (3) the insertion of CO into the new cobalt-carbon bond, and the subsequent uptake of CO and (4) ring closing with extrusion of product and regeneration of the catalytic species. 

Heck has formulated a mechanism which accounts for hydroformylation of olefins catalyzed by cobalt carbonyl (68). A modification of this mechanism is presented in Fig. 5. Cobalt octacarbonyl reacts with hydrogen to form the tetracarbonyl hydride. It is proposed that this coordinatively saturated complex loses a CO group to form the four-coordinate hydride (LX). Coordination of an olefin yields the olefin complex (LXI). Migration of hydride yields an unsaturated alkyl complex (LXII). Further insertion of a CO group (undoubtedly by a migration mechanism) affords the four-coordinate acyl cobalt(I) complex (LXIII). Oxidative addition of hydrogen affords the hypothetical dihydride (LXIV), which eliminates the product aldehyde and regenerates the cobalt(I) hydride catalyst (LX). This latter... 

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