Comparative product analysis of carton tetrachloride dehalogenation catalyzed by cobalt corrins in the presence of thiol or titanium(III) reducing agents

Abstract

Pathways of dehalogenation catalyzed by reduced Co corrins in the presence of various Co reductants are not well understood. Dehalogenation of CCl4 was determined in reaction systems containing a corrin (vitamin B12, cobinamide dicyanide, or aquocobalamin), reductant [Ti(III), dithiothreitol, or S2-/cysteine], buffer (pH 8.2), and CCl4. Transformation products in both the headspace and soluble reaction mixture were determined using GC/MS, HPLC, NMR, or TLC/ autoradiography. Products in the presence of titanium-(III) citrate, which yields the Co(I) form of the corrin, were mostly hydrogenolytic and included predominantly CH3Cl and CH4. More highly halogenated products were formed in the presence of thiol reducing agents, which yield the Co(II) form of the corrin. These products included CH2Cl2, CO, and formate in the case of dithiothreitol. Sulfide/cysteine reductants were least effective in promoting reductive dehalogenation with the major products identified as CHCl3, CS2, 2-oxothiazolidine carboxylic acid, and 2-thioxo-4-thiazolidinecarboxylic acid. Unidentified water-soluble products were produced in all assays with highest concentrations found in the presence of cobinamide dicyanide and thiol reductants. A proposed pathway includes the formation of a trichloromethyl radical as the first intermediate. This radical forms an adduct with the reductant and, in the case of S2-/cysteine, ultimately produces CS2 or thiazolidines by way of thiophosgene. Alternatively, the radical can be further reduced to form CHCl3 and CH2Cl2 or CO and formate byway of a dichlorocarbene intermediate. Conjugation of thiol reductants with reactive intermediates is potentially important in remediation and in predicting abiotic dehalogenation pathways in reduced environments.