Electroorganic Reactions Mediated by Vitamin B₁₂ Model Complexes

Vitamin B₁₂-dependent enzymes, involving the cobalt species as a catalytic center, mediate various isomerization reactions accompanied by carbon-skeleton rerragements. In order to simulate the catalytic functions of vitamin B₁₂ as excerted in the hydrophobic active sites of enzymes concerned, we have been dealing with hydrophobic vitamin B₁₂ derivatives which have ester groups in place of the peripheral amide moieties of the naturally occurring vitamin B₁₂. In this work, the carbon-skeleton rearrangements as mediated by hydrophobic vitamin B₁₂ derivatives were investigated under electrochemical conditions. The controlled-potential electrolyses of alkyl halides with various electron-withdrawing groups were carried out, and the electrochemical carbon-skeleton rearrangements proceeded effectively via formation of anionic intermediates. These reactions can be also applied to the ring-expansin reactins. We have carried out the electrolysis of an alkyl halide having two ester groups and one phenyl group on the β-carbon atom in the presence of a catalytic amount of the hydrophobic vitamin B₁₂. The migration of the pheny group was observed under the conditions forming a radical intermediate. The ester-migrated product was detected under the conditions forming an anionic intermediate. This is the first successful example in selecting a migrating group by electroylsis potential. We have also prepared a strapped hydrophobic vitamin B₁₂ in order to change the enantioselectivity. The controlled-potential electrolysis of a racemic alkyl halide having phenyl, methoxy, and carboxylic ester groups on the same carbon atom was carried out. The simple hydrophobic vitamin B₁₂ tends to bind S-enantiomers more favorably. On the other hand, strapped hydrophobic vitamin B₁₂ acts to bind R-enantiomers more favorably. These results suggested that the stability of alkylated complexes was dominated the enantioselectivity of reduction products.