Oxidation of cyclohexene in the presence of transition metal substituted phosphotungstates and hydrogen peroxide: catalysis and reaction pathways

Homogeneous catalytic oxidations of cyclohexene by transition metal substituted phosphotungstates [PW₁₁M(L)O₃₉]m- (PW₁₁M, M = Co(II), Cu(II), Fe(III), Ni(II), Mn(II), L = H₂O or absence) with hydrogen peroxide in acetonitrile were experimentally studied. The catalytic activities of allylic oxidation were found to strongly depend on the transition metals, and PW₁₁Co showed the highest activity. The product distribution and the catalyst stability were dominated by mole ratio of hydrogen peroxide to PW₁₁M, whose low or high led to stable structure of PW₁₁M and predominant formation of allylic oxidation products or decomposition of PW₁₁M. Different from the activation of allylic C-H bond by radicals, the oxidation of C=C double bond was based on tungsten-peroxo species. A reaction mechanism composed of radical and non-radical processes was proposed from nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR) and kinetic data, to describe the reaction pathways of cyclohexene oxidation.

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