Acylcobalt tetracarbonyls reaction with triphenylphosphine

In support of the existence of an acylcobalt tricarbonyl, Heck and Breslow cited the appearance of an infrared band at 5.8 p, similar to that occurring in acylcobalt tetracarbonyls when alkylcobalt tetracarbonyls are examined in solution. They postulated the equilibrium, Eq. (20). There is now some doubt of the value of this evidence since the 5.8 /x band is due in part at least to the acylcobalt tetracarbonyl formed by some kind of disproportionation, or decomposition during the preparation (53). However, evidence for Eq. (23) has since been found in a study of the reaction of acylcobalt tetracarbonyls with triphenylphosphine, where a first-order dissociation was indicated (52). 

This reaction was studied by Heck (52), who found that in the reaction of acylcobalt tetracarbonyls with triphenylphosphine, which proceeds via the acylcobalt tricarbonyl, the rate increases nearly two orders of magnitude as R goes from CH3 to (CH3)3C. This steric acceleration of dissociation would be expected to affect ks to an even greater extent, thus explaining the increase of k5/k4 with increasing branching of R. 

Kinetic measurements have shown that the reaction of acylcobalt tetracarbonyls with triphenylphosphine is first order in the cobalt compound and zero order in the triphenylphosphine down to at least 0.02 M (12). A first-order dissociation of the acylcobalt tetracarbonyl into an acylcobalt tricarbonyl and carbon monoxide has been proposed as the rate-determining step to explain the kinetics. The fast second step of the mechanism is then the reaction of the acylcobalt tricarbonyl with the triphenylphosphine, forming the acyl(triphenylphosphine)cobalt tricarbonyl. 

The mechanism of this reaction was investigated to find out if carbon monoxide dissociation is the rate-determining step (16). The rate of the reaction of acetylcobalt tetracarbonyl with iodine is too fast to measure under conditions which allow dissociation rate to be measured easily. Thus, dissociation is not rate-determining, and the acylcobalt tetracarbonyl and the iodine must be reacting directly. Further studies with the less reactive acyl(triphenylphosphine)cobalt tricarbonyls showed that the first step in the reaction with iodine is a rapid cleavage of the cobalt-carbon bond to form acyl iodide and iodo(triphenylphosphine)cobalt tricarbonyl. 

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