Alkylcobalt tetracarbonyls isomerization

Originally, Piacenti et al. explained the formation of isomeric products in terms of an equilibrium of alkylcobalt carbonyls with olefin-hydrocarbonyl complexes as in the Oxo reaction. More recently, however, they have noted that the conditions under which n-propyl orthoformate gave no isomeric products (below 150° C, carbon monoxide pressure 10 atm) are conditions under which isomerization occurs readily in the hydroformylation of olefins (115). Since alkylcobalt carbonyls were formed in both reactions they dismissed the possibility that this isomerization was due to alkyl- or acylcobalt carbonyls. The fact that Takegami et al. have found that branched-chain acylcobalt tetracarbonyls isomerize more readily than straight-chain acylcobalt tetracarbonyls would seem to fit in quite well with the results of Piacenti et al., however, and suggests that the two findings may not be so irreconcilable as might at first appear (see Section II, B,2). 

A second method for preparing alkylcobalt tetracarbonyls involves the addition of cobalt hydrocarbonyl to olefins (5). With unsymmetrical olefins, two isomeric alkylcobalt tetracarbonyls are possible, depending upon the direction of addition of the cobalt hydrocarbonyl. Both possible isomers are often obtained. 1-Pentene and cobalt hydrocarbonyl at 0°C give a 50 50 mixture of the two possible pentylcobalt tetracarbonyls. Methyl acrylate... 

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). 

The alkylcobalt tetracarbonyls decompose fairly rapidly at 0° C even in dilute solution. One of the decomposition products seems to be the corresponding acylcobalt tetracarbonyl 22). This product was first thought to be the isomeric form of the alkylcobalt tetracarbonyl, the acylcobalt tricarbonyl 4, 10, 22). The formation of acylcobalt tetracarbonyls from alkylcobalt tetracarbonyls probably involves a disproportionation reaction forming, at least as a transitory intermediate, an alkylcobalt tricarbonyl. 

The conditions under which cobalt hydrocarbonyl was reacted with olefin were also found to affect the distribution of products and the extent of isomerization of excess olefin (62, 73, 147). At low temperatures (0° C) under carbon monoxide (1 atm) very little isomerization of excess 1-pentene occurred and the main product was the terminal aldehyde. Under nitrogen or under carbon monoxide at 25° C, extensive olefin isomerization occurred and the branched aldehyde was mainly produced. The olefin isomerization is most satisfactorily accounted for by an equilibrium between alkylcobalt and olefin-hydride cobalt complexes [Eqs. (9) and (10)]. The carbon monoxide inhibition is most easily explained if the isomerization proceeds via the tricarbonyls rather than tetracarbonyls. This also explains why ethylcobalt tetracarbonyl is not in equilibrium with hydrocarbonyl and ethylene under conditions where the isomerization is rapid (62, 73). 

For a more detailed consideration of the isomerization of alkyl- and acylcobalt carbonyls, Section V,B should be consulted. It is sufficient to say here that this isomerization is normally slow at room temperature, especially for the linear acylcobalt tetracarbonyls. The reaction also appears to be quite sensitive to solvent for reasons which have not yet been adequately explained. Alkylcobalt carbonyls are rapidly converted to the acylcobalt carbonyls and do not appear to give rise to any significantly faster isomerization. 

Piacenti s results for 1- and 2-pentene at high pressure would then fit in quite well with the fact that Takegami et al. (147-149) found that linear acylcobalt tetracarbonyls were much more difficult to isomerize than their branched-chain isomers. However, Piacenti et al. reject the possibility of an isomerization of alkylcobalt carbonyls in view of their work on the hydro-formylation of orthoformic esters (Section II, D,2). 

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