Acylcobalt carbonyls isomerization

The products of the reaction are determined by two factors (1) the initial direction of the addition of hydrocarbonyl (2) isomerization of the resulting alkyl- or acylcobalt carbonyls. 

This seems to be the most likely mechanism, both for the isomerization of the acylcobalt carbonyls and excess olefin. The fact that the isomerizing species contains two molecules of carbon monoxide less than the acylcobalt tetracarbonyl would suggest a very slow rate of isomerization. However, it is conceivable that the reaction may be catalyzed by the tricarbonyls, e.g.,... 

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. 

Prior to Takegami s studies, the effect of isomerization of acylcobalt carbonyls on the products of the reaction between cobalt hydrocarbonyl and olefins had received little attention. Terminal olefins had been found to give a mixture of linear and branched products at low temperatures under carbon monoxide, and this was taken as reflecting the mode of addition of cobalt hydrocarbonyl (62, 73, 147). In view of the slow rate of isomerization of acylcobalt carbonyls this seems justified. However, it is worth noting that branched products predominated in the reaction of 1-pentene with hydrocarbonyl under nitrogen even when the olefin had isomerized only to the extent of 50% (73). Both isobutylene and alkyl acrylates had been found to produce branched products. It was suggested that isobutylene, with an... 

Takegami et al. (147) reexamined the reaction of olefins with cobalt hydrocarbonyl to determine the effect of reaction variables and the possibility of isomerization on the structure of the acylcobalt carbonyls formed. Products were recovered as their ethyl esters as described previously. Additionally, the uptake of carbon monoxide was measured. This is important since the presence of an acylcobalt tricarbonyl can be inferred when the amount of ester produced exceeds the amount of carbon monoxide absorbed. 

The factors affecting the distribution of products formed in the hydroformylation reaction have already received attention in Section II, A,2. The isomerizations of both olefin and acylcobalt carbonyl can be of importance and the extent of these isomerizations will be dependent on carbon... 

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). 

The isomerization of epoxides is discussed in Section II, D,l. The isomerizations of olefins and of alkyl- and acylcobalt carbonyls have been considered as side reactions to hydroformylation but studies dealing principally with these reactions will now receive attention. 

The isomerization of alkyl- and acylcobalt carbonyls is important in considering the products of the hydroformylation reaction and has been dealt with in part in Section II, A. Equations (9) and (10) give the most likely mechanism for the isomerization. 

The products were isolated as esters by reaction of the acylcobalt carbonyls with an alcohol and iodine. In the case of the alkyl halides, carbon monoxide was normally absorbed, but under nitrogen, acylcobalt tricarbonyls must be formed. The reaction with alkyl halides was slow and some isomerization was noted using M-propyl iodide (formation of n-butyrates and isobutyrates). Absence of carbon monoxide promoted the isomerization. Isopropyl iodide gave no reaction. When ethyl a-bromopropionate was used, no isomerization was found at — 25 °C under carbon monoxide, but the isomerized product, diethyl succinate, was the major product at 25° C under carbon monoxide or nitrogen. Under the conditions of the experiments no isomerization of the alkyl halide itself was found. 

Acylcobalt carbonyls formed from acyl halides appear to isomerize less readily. No isomerization was found using isobutyryl bromide or n-butyryl chloride in nonpolar solvents such as hexane or benzene. Some isomerization was found with diethyl ether and ethyl acetate as solvents, again promoted by absence of carbon monoxide. Curiously, no isomerization... 

This is similar to postulating the various resonance forms for the benzyl anion but does not so easily permit para products. Takegami found that this isomerization was strongly promoted by cobalt hydrocarbonyl. It would be interesting to know if this is also true of the isomerization of aliphatic acylcobalt carbonyls. 

The base (B) used was dicyclohexylethylamine. At the higher temperatures the isomeric products that one would expect from an isomerization of the acylcobalt carbonyls were formed (see Section II, A). Amines were also used in place of alcohols to give amides. Thus benzyl chloride reacted with carbon monoxide and aniline in tetrahydrofuran solution at 35° C in the presence of sodium cobalt carbonylate catalyst to give a 47% yield of phenylacetanilide. 

The mechanism involves the elimination of and readdition of HCo(CO>3 as shown in Scheme 1. Starting with either acyl isomer, a 50% yield of the mixed aldehydes is obtained in addition to the mixture of isomeric acylcobalt compounds. Formation of aldehyde and olefin from the acylcobalt carbonyl is thus a disproportionation reaction that competes with the skeletal rearrangement. 

Heck and Breslow have postulated equilibria between the acylcobalt-carbonyls, the alkylcobaltcarbonyls and the olefin-hydrocarbonyl complexes [35] and Takegami et aL have demonstrated in quite a number of experiments that acylcarbonyls can be readily isomerized [58, 70]. Heck and Breslow [35] explain the significant change in the isomer distribution mentioned above by the different thermal stability of the organometallic... 

One final interesting isomerization achieved in the cobalt carbonyl system should be mentioned. Heck and Breslow (22b) found that acylcobalt tetracarbonyl compounds undergo alcoholysis with the formation of HCo(CO)4. With methanol, the reaction proceeds at 50° ... 

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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