B12 Organometallic Reactivity

Coenzyme B₁₂is recognised as an organometallic derivative of vitamin B₁₂. In people, methionine synthase and methylmalonyl-CoA mutase use coenzyme B₁₂ and methylcobalamin, correspondingly, B₁₂ cofactors. Organometallic compounds offer nucleophilic carbon atoms which react with electrophilic carbon, making a new carbon-carbon bond. This finds application in the creation of complex molecules from simple initial materials. To explain the general reactivity of organometallics it is expedient to view them as ionic, so R-M = R-M. The data for alkyl iodides are fitted by a third-order rate law which is consistent with an associative mechanism, e-Adenosylcobalamin (synthesis and characterisation of 8-methoxy5'-deoxyadenosylcobalamin, a coenzyme B₁₂ analogue which, following Co-C bond homolysis, avoids cyclisation of the 8-methoxy5'-deoxyadenosyl radical) has been further studied. The Co-C bond dissociation energy of alkylcobalamins including coenzyme B₁₂ has been estimated to be in the range of 20-30 kcal mo1^-1 Sterically hindered secondary alkylcobalamins with a 3-hydrogen decompose spontaneously, under both anaerobic and aerobic conditions, by syn-elimination.