Reconciling Disparate Models of the Involvement of Vinyl Radicals in Cobalamin-Mediated Dechlorination Reactions

Inner-sphere (nonradical) and outer-sphere (radical-based) mechanisms have been suggested for cobalamin-mediated dechlorination of tetra chloroethylene (PCE) and trichloroethylene (TCE). In this study, the role of free vinyl radicals was investigated using deuterated radical traps: de-isopropanol and d(8)-tetrahydrofuran. For both substrates, addition of trap resulted in production of deuterated dechlorination products, and higher concentrations of trap resulted in increased amounts of deuterated products. However, only a finite proportion of the products were trappable: 8% of the PCE-derived products and 86% of the TCE-derived products result from free radicals. The data show that the reaction does not proceed solely by either an inner-sphere or an outer-sphere mechanism and led to the hypothesis that caged radical intermediates were involved in the mechanism. The untrappable fraction of products are hypothesized to result from in-cage reactions. This hypothesis was investigated using d(5)-glycerol as a radical trap and viscogen. Although increased viscosity resulted in decreased formation of free-radical-derived products, consistent with the cage hypothesis, these data were inconclusive. The role of d(8)-isopropanol in enhancing the production of radicals in this system via an acetone ketyl radical chain mechanism was also investigated, and no evidence for such an effect was found.