Butenylcobalt complex

Organic groups may also provide hydrogen in forming the hydrido complex. Thus, the disproportionation of a butenylcobalt complex yielding an equimolar mixture of butenes and butadiene 21, 22) appears to involve the intermediate formation of hydrido complex (Reactions 3 and 4). 

While the parent allylcobalt complex (R = R = H) was stable in an aqueous alkaline solution, the butenylcobalt complex (R = CH3, R = H) gradually evolved an approximately equimolar mixture of butenes and butadiene, suggesting a disproportionation involving the intermediate formation of hydrido complex (Reactions 3 and 4) (20, 22). Since apparently the same butenylcobalt complex is formed by adding hydrido complex to butadiene, the reduction of allylic halides is discussed with the intimately related hydrogenation of dienes later in the text. 

Butadiene, Using butadiene as the model substrate, it has been found that the catalytic hydrogenation of dienes occurs in three steps (1) reversible addition of hydrido complex to butadiene to form a butenylcobalt complex (Reaction 3) (2) cleavage of the butenylcobalt complex by hydrido complex to form butenes and pentacyanocobaltate(II) (Reaction 4) and (3) hydrogen absorption by pentacyanocobaltate(II) to reform hydrido complex (Reaction 1) 20, 21), The butenylcobalt complex has the same properties as the complex obtained via reaction of y-methyl-allyl bromide with pentacyanocobaltate(II) (21, 22). 

With deficient hydrido complex, the chlorides gave the same product distribution as that obtained from butadiene. The formation of approximately equimolar quantities of butadiene under the latter reaction conditions indicates the intermediate formation of butenyl complexes, which decompose, forming equimolar quantities of butenes and butadiene. Thus, except for halide reduction by excess hydrido complex, it seems the reactions take place via a common butenylcobalt complex. 

Nearly identical results were obtained for all substrates under both reaction conditions. The formation of approximately equimolar quantities of butadiene and butenes again indicates the intermediate formation of butenylcobalt complexes. While the predominant product at a cyanide to cobalt ratio of 5.1 was trans-2-hutene, at a ratio of 7, the formation of 1-butene is favored. 

Since the butenylcobalt complex was too unstable, the PMR spectrum of the allylcobalt complex was studied 21, 22) to obtain information on the structure of the organocobalt intermediates in these stereoselective reductions (20). This investigation indicated that a-bonded allylic complexes are in equilibrium with 7r-allylic complexes and cyanide ion, thus providing a rationale for the stereoselectivities observed in the reduction of butadiene and butenyl chlorides. 

Presumably, in the presence of excess cyanide ion only a-bonded butenylcobalt complexes are present. These may equilibrate rapidly via intimate-ion or radical-pair transitions. The primary trans-a-complex would be expected to be the most stable species and thus would be enriched in such an equilibrium. At low cyanide concentrations, the various (T-bonded species may be in equilibrium with two 7r-allylic isomers. Of these, the syn isomer is expected to be the most stable by analogy with the corresponding cobalt tricarbonyls. 

Cobalt hydrocarbonyl reacts rapidly with conjugated dienes, initially forming 2-butenylcobalt tetracarbonyl derivatives. These compounds lose carbon monoxide at 0°C. or above, forming derivatives of the relatively stable l-methyl-ir-allyl-cobalt tricarbonyl. As with normal alkylcobalt tetracarbonyls, the 2-butenyl derivatives will absorb carbon monoxide, forming the acyl compounds but these acyl compounds also slowly lose carbon monoxide at 0°C. or above, forming 7r-allyl complexes. The acyl compounds can be isolated as the monotriphenylphosphine derivatives (47). 

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