Cobalt hydrocarbonyl, reactions acylcobalt tetracarbonyls

A useful study has just been completed by Roos and Orchin (125), who have examined the effect of ligands such as benzonitrile on the stoichiometric hydroformylation of olefins. A variety of such reagents (acetonitrile, anisole) were found to act in a similar manner to carbon monoxide by suppressing the formation of branched products and the isomerization of excess olefin. The yield of aldehyde was also increased by increasing ligand concentration up to 2 moles per mole of cobalt hydrocarbonyl. Benzonitrile was not found to affect the rate of the reaction of cobalt hydrocarbonyl with acylcobalt tetracarbonyl, so the ligand must have affected an earlier step in the reaction sequence. It seems most likely that cobalt hydrocarbonyl reacts with olefin in the presence of benzonitrile to form an acylcobalt tricarbonyl-benzonitrile complex which is reduced more rapidly than the acylcobalt tetracarbonyl. 

Two general methods are available for the synthesis of acylcobalt tetracarbonyls. One method is the reaction of acyl halides with salts of cobalt hydrocarbonyl (7). This reaction is applicable to all types of acid halides,... 

This reduction is very likely the last step in the industrially important hydroformylation or oxo reaction for converting olefins into aldehydes (4). The catalytic species seems to be cobalt hydrocarbonyl, which first adds to the olefin as in Eq. (2). The alkylcobalt tetracarbonyl so formed then probably isomerizes to the acylcobalt tricarbonyl [Eq. (25)] and is reduced by hydrogen as in Eqs. (45) and (46). 

Cobalt hydrocarbonyl also readily reduces acylcobalt tetracarbonyls, producing aldehydes and cobalt octacarbonyl (4). In this reaction, as in the hydrogen reduction, carbon monoxide is an inhibitor. The reaction probably involves the addition of cobalt hydrocarbonyl to an acylcobalt tricarbonyl followed by a hydride shift from cobalt to carbon, producing aldehyde and dicobalt heptacarbonyl. 

Warming pure alkylcobalt tetracarbonyls, or their solutions, to room temperature or above causes a further reaction to take place, producing dialkyl ketones (J). These products can be accounted for by assuming that the acylcobalt tetracarbonyl adds to the alkylcobalt tricarbonyl as cobalt hydrocarbonyl probably adds to an acylcobalt tricarbonyl [Eq. (48)]. The cobalt(III) intermediate could then decompose and form ketone and a mixture of cobalt carbonyls. 

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