Properties and reactivity of chlorovinylcobalamin and vinylcobalamin and their implications for vitamin B12-catalyzed reductive dechlorination of chlorinated alkenes.

Vitamin B12-catalyzed reductive dechlorination of perchloroethylene (PCE) and trichloroethylene (TCE) is a potential strategy for cleanup of polluted environments. Presented are crystal structures of vinylcobalamin 2 and cis-chlorovinylcobalamin 1. They show a strong resistance toward photolysis. Reduction of 2 is difficult, but reduction of 1 occurs readily and produces 2. The mechanism of this latter reaction involves acetylene as an intermediate. These and other findings are discussed in the context of environmental studies on B12-catalyzed dechlorination of PCE and TCE and investigations of the haloalkene reductive dehalogenases that catalyze similar reactions.

Characterization of chlorovinylcobalamin, a putative intermediate in reductive degradation of chlorinated ethylenes.

The first X-ray structure of a vinylcobalamin is reported. Chlorovinylcobalamin is formed in the reaction of cob(I)alamin with chloroacetylene. Subsequently, cob(I)alamin catalyzes the reduction of chlorovinylcobalamin to vinylcobalamin in the presence of excess titanium(III)citrate. Introduction of a chlorine onto the vinyl group of vinylcobalamin greatly changes its reduction potential. These results are discussed with respect to vitamin B12-catalyzed dechlorination of perchloroethylene, a pollutant on the priority list of the EPA.

Dichloroacetylene is not the precursor to dichlorinated vinylcobaloxime and vinylcobalamin in cobalt catalyzed dechlorination of perchloro- and trichloroethylene.

Vitamin B(12) catalyzes the reductive dechlorination of perchloroethylene (PCE), a process for which vinylcobalamins have been proposed as intermediates. Previous model studies have shown that PCE and trichloroethlylene (TCE) react with cob(I)aloxime to form cis-1,2-dichlorovinylcobaloxime (1). This compound could be formed by nucleophilic vinylic substitution of cob(I)aloxime on TCE or its syn-addition to dichloroacetylene. To evaluate the latter possibility, dichloroacetylene was reacted in this study with cob(I)aloxime. The major product was not complex 1 but a novel cobalt complex, indicating that dichloroacetylene is not involved in the reductive dechorination of PCE catalyzed by cob(I)aloxime. An X-ray structure of the major product was obtained showing an unexpected tricyclic structure in which one of the carbons of dichloroacetylene is a ligand to the metal and the second carbon has formed a C-C bond to one of the oxime carbons. This arrangement connects the axial and equatorial ligands. The cathodic peak potential of this complex is significantly more negative than that of previously characterized chlorinated vinylcobaloximes. Cob(I)alamin also reacts with chloroacetylene to provide cis-chlorovinylcobalamin in analogy to cob(I)aloxime, but it does not provide dichlorinated vinylcobalamins in the reaction with dichloroacetylene. Hence, dichlorinated vinylcobalt complexes detected in the reductive dechlorination of PCE catalyzed by cobaloximes or vitamin B(12) are not derived from a dichloroacetylene intermediate.

Mechanistic investigation of a novel vitamin B(12)-catalyzed carbon [bond] carbon bond forming reaction, the reductive dimerization of arylalkenes.

In the presence of catalytic vitamin B(12) and a reducing agent such as Ti(III)citrate or Zn, arylalkenes are dimerized with unusual regioselectivity forming a carbon [bond] carbon bond between the benzylic carbons of each coupling partner. Dimerization products were obtained in good to excellent yields for mono- and 1,1-disubstituted alkenes. Dienes containing one aryl alkene underwent intramolecular cyclization in good yields. However, 1,2-disubstituted and trisubstituted alkenes were unreactive. Mechanistic investigations using radical traps suggest the involvement of benzylic radicals, and the lack of diastereoselectivity in the product distribution is consistent with dimerization of two such reactive intermediates. A strong reducing agent is required for the reaction and fulfills two roles. It returns the Co(II) form of the catalyst generated after the reaction to the active Co(I) state, and by removing Co(II) it also prevents the nonproductive recombination of alkyl radicals with cob(II)alamin. The mechanism of the formation of benzylic radicals from arylalkenes and cob(I)alamin poses an interesting problem. The results with a one-electron transfer probe indicate that radical generation is not likely to involve an electron transfer. Several alternative mechanisms are discussed.

Synthesis and characterization of chlorinated alkenylcobaloximes to probe the mechanism of vitamin B(12)-catalyzed dechlorination of priority pollutants.

Vitamin B(12) catalyzes the reductive dechlorination of several ubiquitous pollutants such as perchloroethylene (PCE) and trichloroethylene (TCE). Several mechanisms have been proposed for these transformations, some of which involve the intermediacy of chlorinated vinylcobalamins. To evaluate the currently unknown chemical and physical properties of such species, various chlorinated vinylcobaloxime complexes [cobaloxime = bis(dimethylglyoximato)(pyridine)cobalt(III)] were prepared and characterized. X-ray structures are reported for (cis-1,2-dichloroethenyl)cobaloxime (4), (cis-monochloroethenyl)cobaloxime (5), (alpha-chloroethenyl)cobaloxime (6), and vinylcobaloxime (7), and the reactivities of these isolated complexes were investigated. They were stable in the presence of ethanolic NaBH(4) unless external cobaloximes were added. The cob(I)aloxime formed under the latter conditions promoted the conversion of 4 to 5 and 6, and of 5 and 6 into 7. Mechanistic studies of these transformations are consistent with a pathway in which the conversion of 4 into 5 and 6 takes place via chloroacetylene as an intermediate, and the conversion of 6 to 7 involves vinyl chloride as an intermediate. Cyclic voltammetry on the chlorinated vinylcobaloximes resulted in irreversible reduction waves, with 4 displaying the least negative and 7 the most negative peak potential. These results are discussed in the context of the B(12)-catalyzed reductive dechlorination of PCE and TCE.