Oxidation of Cyclohexene in the Presence of Transition-Metal-Substituted Phosphotungstates and Hydrogen Peroxide: Catalysis and Reaction Pathways

Graphical That′s radical ! In the homogeneous catalytic oxidation of cyclohexene by transition-metal-substituted phosphotungstates with hydrogen peroxide, allylic oxidation and epoxidation proceed through radical pathways based on transition metals and nonradical pathways based on PW 4 , respectively. Homogeneous catalytic oxidations of cyclohexene by transition-metal-substituted phosphotungstates [PW 11 M(L)O 39 ] m− (PW 11 M, M=Co II , Cu II , Fe III , Ni II , Mn II , L=H 2 O 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 PW 11 Co showed the highest activity. The product distribution and the catalyst stability were dominated by mole ratio of hydrogen peroxide to PW 11 M, whereby low or high mole ratios led to stable structure of PW 11 M and predominant formation of allylic oxidation products or decomposition of PW 11 M, respectively. Different from the activation of the 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 nonradical processes was proposed from NMR, EPR, and kinetic data, to describe the reaction pathways of cyclohexene oxidation.