The chemistry of vitamin B12. Part 19. Labilisation of the cobalt–carbon bond in organocobalamins by steric distortions; neopentyl-cobalamin as a model for labilisation of the vitamin B12coenzymes

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Abstract
Neopentylcobalamin in neutral solution at 25 °C is stable under nitrogen but is decomposed by O2 and by imidazole to give a cob(III)alamin and by hydrogen-atom donors such as thiols and PriOH to give the cob(II) alamin and neopentane. The reaction is ascribed to reversible homolytic fission of the Co–C bond to give a low steady-state concentration of the cob(II)alamin and neopentyl radicals, which can then form neopentane by the abstraction of a hydrogen atom. Neopentylcobinamide is also decomposed by high concentrations of imidazole, while the purely five-co-ordinate neopentylcobalamin in acid and neopentylcobinamide in neutral solution are stable both in the presence and absence of O2. Labilisation of the Co–C bond is ascribed to steric distortion around the co-ordinated C atom in the six-co-ordinate neopentyl complexes. It is suggested that neopentylcobalamin provides a model for the labilisation of the Co–C bond in the vitamin B12 coenzymes, and possible mechanisms for the isomerase reactions are discussed.

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