Hydrophobic Vitamin B12. V. Electrochemical Carbon-Skeleton Rearrangement as Catalyzed by Hydrophobic Vitamin B12: Reaction Mechanisms and Migratory Aptitude of Functional Groups

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Abstract
The carbon-skeleton rearrangements as catalyzed by heptamethyl cobyrinate perchlorate, [Cob(II)7C1ester]ClO4, were investigated under electrochemical conditions. The controlled-potential electrolysis of 2,2-bis(ethoxycarbonyl)-1-bromopropane, which is considered to be a model substrate for methylmalonyl–CoA mutase, was catalyzed by [Cob(II)7C1ester]ClO4 in N,N-dimethylformamide to give the rearrangement product, 1,2-bis(ethoxycarbonyl)propane, as a major one at −1.5V vs. SCE in the presence of acetic acid and at potentials more cathodic than −1.8V vs. SCE without acetic acid in the dark. The electrochemical carbon-skeleton rearrangement was postulated to proceed via formation of anionic intermediates. The electrolyses of 1-bromo-2-cyano-2-ethoxycarbonylpropane, 2-acetyl-1-bromo-2-ethoxycarbonylpropane, and 1-bromo-2-[(ethylthio)carbonyl]propane with [Cob(II)7C1ester]ClO4 also afforded the corresponding carbon-skeleton rearrangement products. The results indicated that substrates with two electron-withdrawing groups placed on the β-carbon atom with combinations of one carboxylic ester and one of carboxylic ester, acetyl, and cyano moieties readily gave the corresponding rearrangement products which were derived from individual migration of the substituent groups. Substrates with only one of the electron-withdrawing groups, carboxylic ester, acetyl, and cyano, did not give any rearrangement product but a substrate with one thioester group afforded the corresponding rearrangement product. The migratory aptitude of electron-withdrawing groups was found to decrease in the order: COSR>COR>COOR>CN. Both electronic character and steric bulkiness of the migrating groups are apparently reflected on this tendency, even though relative contributions of these effects are much dependent on the nature of β-substituents.

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