Theoretical Investigations into the Intermediacy of Chlorinated Vinylcobalamins in the Reductive Dehalogenation of Chlorinated Ethylenes

The reductive dehalogenation of perchloroethylene and trichloroethylene by vitamin B₁₂ produces 95% (Z)-dichloroethylene (DCE) and small amounts of (E)-DCE and 1,1-DCE, which are further reduced to ethylene and ethane. Chloroacetylene and acetylene have been detected as intermediates, but not dichloroacetylene. Organocobalamins (RCbls) have been proposed to be intermediates in this process. Density functional theory based approaches were employed to investigate the properties of chlorinated vinylcobalamins and chlorinated vinyl radicals. They reveal that all vinyl radicals studied have reduction potentials more positive (E° ≥ −0.49) than that of the Co^II/Co^I couple of B₁₂ (E° = −0.61 V), indicating that any (chlorinated) vinyl radicals formed in the reductive dehalogenation process should be reduced to the corresponding anions by cob(I)alamin in competition with their combination with Co(II) to yield the corresponding vinylcobalamins. The computed Co−C homolytic bond dissociation enthalpies (BDEs) of the latter complexes range from 33.4 to 45.8 kcal/mol. The substituent effects on the BDEs are affected by the stabilities of the vinyl radicals as well as steric interactions between (Z)-chloro substituents and the corrin ring. The calculated E° values of the cobalamin models were within 200 mV of one another since electron attachment is to a corrin ring π*-orbital, whose energy is relatively unaffected by chloride substitution of the vinyl ligand, and all were >500 mV more negative than that of the Co^II/Co^I couple of B₁₂. Reduction of the base-off forms of vinyl- and chlorovinylcobalamin models also involves the corrin π* orbital, but reduction of the base-off dichlorovinyl- and trichlorovinylcobalamin models occurs with electron attachment to the σ_Co-C* orbital, yielding calculated E° values more positive than that of the calculated Co^II/Co^I couple of B₁₂. Thus, cob(I)alamin is expected to reduce these base-off vinyl-Cbls. Heterolytic cleavage of the Co−C bonds is much more favorable than homolysis (>21 kcal/mol) and is significantly more exergonic when coupled to chloride elimination.