The chemistry of vitamin B12. Part 25. Mechanism of the β-elimination of olefins from alkylcorrinoids; evidence for an initial homolytic fission of the Co–C bond

Abstract

Equilibrium constants (log₁₀K/dm³ mol^–1) have been determined for the co-ordination of imidazole by the five-co-ordinate alkylcobinamides with R (= alkyl)= Me (0.9), Et(–0.5), neopentyl (np)(–1.4), Prⁱ( < –1.9), and cyclohexyl (C₆)( < –1.9) in aqueous solution at 25 °C; this confirms that contact between dbzm (the heterocyclic base, 5,6-dimethylbenzimidazole, in the cobalamin side-chain) and the corrin ring can play only a minor part in the similar effect of varying R on the co-ordination of dbzm. The maximum (pH- and p_o2-independent) rates of decomposition of np- and C₆-cobalamin (at 36 °C) and of the corresponding cobinamides (at 80 °C) have been determined. Comparison of present and published kinetic data show that changing R (from Et to np and C₆) and changing from six- to five-co-ordination (for both R = np and C₆) produce very similar and large changes in rate, irrespective of whether the overall reaction corresponds to homolytic fission (R = np) or β-elimination (R = C₆); this provides indirect evidence that both reactions involve a common first step, viz. homolytic fission of the Co–C bond to produce a caged (Co^II+ radical) pair. The decomposition of C₆-cobinamide in air at pH 1 and 25 °C is catalysed by vanadyl ions in solution and by cobalt boride in suspension. Me- and C₆-cobinamide are both diamagnetic.