Comparative Product Analysis of Carbon Tetrachloride Dehalogenation Catalyzed by Cobalt Corrins in the Presence of Thiol or Titanium(III) Reducing Agents

Pathways of dehalogenation catalyzed by reduced Co corrins in the presence of various Co reductants are not well understood. Dehalogenation of CCl 4 was determined in reaction systems containing a corrin (vitamin B 12, cobinamide dicyanide, or aquocobalamin), reductant [Ti(III), dithiothreitol, or S 2 - /cysteine], buffer (pH 8.2), and CCl 4 . 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 CH 3 Cl and CH 4 . 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 CH 2 Cl 2, CO, and formate in the case of dithiothreitol. Sulfide/cysteine reductants were least effective in promoting reductive dehalogenation with the major products identified as CHCl 3, CS 2, 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 S 2 - /cysteine, ultimately produces CS 2 or thiazolidines by way of thiophosgene. Alternatively, the radical can be further reduced to form CHCl 3 and CH 2 Cl 2 or CO and formate by way 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.