CATALYTIC CARBON-HALOGEN BOND-CLEAVAGE CHEMISTRY BY REDOX-ACTIVE POLYOXOMETALATES

The use of redox-active polyoxotungstate complexes to effect the cleavage of carbon-balogen bonds (C-X, X = Cl or Br) by three distinct modes is demonstrated for the first time. The first mode involves direct thermal reaction of halocarbon substrates with H2W10O324- or alpha-HPW12O403-. The rate law for CCl4 dehalogenation is V = k(H2W10O324-)(CCl4) and k(H2W10O324-)/k(alpha-HPW12O403-) = 2.8. Several lines of evidence collectively establish that carbon-halogen bond cleavage likely involves dissociation electron transfer for the mode 1 reactions, although halogen atom abstraction (atom transfer) cannot be ruled out. The evidence includes comparisons of kinetic profiles for dehalogenation rate versus halocarbon substrate structure, relative reactivities of substrates (polyhalogenated more reactive (>) than monohalogenated compounds; tertiary > secondary > primary halides; bromides > chlorides), and other product distribution data including one ''radical clock'' reaction, in addition to the rate law. Interestingly one carbocation derived product, N-tert-butylacetamide, is generated in the debromination of tert-butyl bromide in acetonitrile. The second and third modes of dehalogenation involve extensions of previously reported polyoxometalate photoredox processes, and both modes are catalytic extensions of existing effective stoichiometric dehalogenation processes. The second mode proceeds by a complex rate law and involves photocatalytic transformation of organic halide (halocarbons) into inorganic halide (HX) coupled with the oxidation of sacrificial organic reductants (secondary alcohols or tertiary amides). The second mode essentially defines a method to catalytically generate reducing radicals under mild conditions; the radicals are the principal dehalogenating species. The third mode of dehalogenation is similar to the second mode but run in the presence of O2. Here the reduced polyoxotungstates reduce O2 to superoxide which then dehalogenates substrate. The third mode effects catalytic dehalogenation of a wide range of halocarbons.