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 tetrachloroethylene (PCE) and trichloroethylene (TCE). In this study, the role of free vinyl radicals was investigated using deuterated radical traps: d(8)-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.
Kliegman
Sarah
S
Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455-0431, USA.
McNeill
Kristopher
K
D005609:Free Radicals / Q000737:chemistry*; D054879:Halogenation; D007700:Kinetics; D008956:Models, Chemical; D013237:Stereoisomerism; D013750:Tetrachloroethylene / Q000737:chemistry; D014241:Trichloroethylene / Q000737:chemistry; D014753:Vinyl Compounds / Q000737:chemistry*; D014783:Viscosity; D014805:Vitamin B 12 / Q000737:chemistry*
D005609:Free Radicals; D014753:Vinyl Compounds; D014241:Trichloroethylene; D014805:Vitamin B 12; D013750:Tetrachloroethylene