I cleavage of carbon—cobalt bonds

Perhaps the best studied mode I cleavages are the base-catalyzed /8-elimination reactions of 2-substituted alkylcobalt complexes with electron-withdrawing substituents (Eqn. 34). 

Such reactions are formally the reverse of the alkylation of cobalt(I) nucleophiles by suitably activated olefins (Eqn. 8). Indeed, Schrauzer et al. [53] have presented spectroscopic as well as other evidence that for cobaloximes where X = —CN or —COOCH2CH3 these reversible reactions proceed via intermediate formation of a cobaloxime(I)-olefin w complex, i.e. the microscopic reverse of Eqns. 10 and 11. However, Barnett et al. [73] have studied the kinetics of the analogous base-catalyzed elimination of 2-cyanoethylcobalamin to produce cob(I)alamin and acrylonitrile. These authors found no general base catalysis and a rate law which was first order in organocobalamin and first order in hydroxide ion and determined a second-order rate constant of 230/M/min. As these authors pointed out, this rate constant is several orders of magnitude greater than the second-order rate constant for ionization of acetonitrile so that the mechanism must either by a concerted E2 elimination (or possibly direct elimination of hydridocobalamin) or, if stepwise, the rate of /8-proton dissociation must be substantially enhanced by the cobalt-containing substituent. 

In what appears to be a similar process, adenosylcobalamin decomposes in base to give cob(I) ilamin and /8-elimination products of the 5 -deoxyadenosyl moiety (Eqn. 35) [74]. 

However, in an interesting contrast, alkoxyethylcobaloximes do not, apparently, undergo /8 elimination in base, producing ethylene in an apparent mode III process (Eqn. 36) instead of the expected vinyl ethers [75]. 

The mechanisms of these reactions are still under study. 

---