Investigation on the mechanism of formation and consumption of vinylcobalamins involved in vitamin B(12)-catalyzed reductive dechlorination of perchloroethylene

Vitamin B₁₂ reductively dechlorinates perchloroethylene (PCE), a common environmental pollutant and human carcinogen, into less toxic molecules, such as ethene, making B₁₂ a possible catalyst for the bioremediation of PCE from contaminated sites. Application of B₁₂ as a catalyst requires a detailed understanding of the mechanism by which B₁₂ dechlorinates PCE. While vitamin B₁₂ dechlorinates PCE to form trichloroethylene (TCE) via an electron transfer mechanism, further dechlorination of TCE to form ethene involves chlorovinylcobalamins as intermediates. Despite their detection, the structure and reactivity of these cobalamin intermediates is poorly understood.; Cobaloximes, B₁₂ model complexes, were used to investigate the cobalamin intermediates. It was found that cob(I)aloxime catalyzed the dechlorination of PCE to TCE, and TCE reacted with cob(I)aloxime to form dichlorinated vinylcobaloxime 4. Cob(I)aloxime further catalyzed the dechlorination of 4 to monochlorinated vinylcobaloximes (5 and 6) and could fully dechlorinate 5 and 6 to vinylcobaloxime 7. Electrochemical reduction of the chlorinated vinylcobaloximes indicated that the reduction of the cobaloximes became more facile as the number of chlorides on the vinyl ligand increased.; Similar experiments were undertaken using cobalamins to further investigate the vitamin B₁₂-catalyzed dechlorination of PCE. Chlorovinylcobalamin 16, previously detected in the PCE dechlorination reaction, was synthesized by treating cob(I)alamin with chloroacetylene. The structure of 16 was confirmed by X-ray crystallography, and is the first reported structure of an organocobalamin containing a bond to a sp2 hybridized carbon in its axial ligand. Although treating cobalamin 16 with either titanium(III)citrate or cob(I)alamin led to the formation of vinylcobalamin 17, cob(I)alamin was found to catalyze this transformation at a much faster rate. Vinylcobalamin 17 was resistant toward reduction by titanium(III)citrate or cob(I)alamin, which prevents efficient dechlorination of PCE by sequestering the active catalyst. Electrochemical experiments on 16 and 17 indicate that the chlorovinylcobalamins become more susceptible to reduction as the number of chlorides on the vinyl ligand increases. It would be expected that the presence of three chlorines on the vinyl ligand would further shift the peak potential toward the CoII /CoI couple of B₁₂ making reduction of a polychlorinated vinylcobalamins by cob(I)alamin even more facile.