Investigation on the Mechanism of Formation and Consumption of Vinylcobalamins Involved in Vitamin B(12)-Catalyzed Reductive Dechlorination of Perchloroethylene

Abstract

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.