Catalytic air and amine N-oxide oxidation of p-substituted benzoin by molybdenum(VI) complexes. Identification of the deactivation process by dioxygen

Abstract

The catalytic oxidation of benzoin and two p-substituted benzoins, e.g. 4,4′-dimethoxy- and 4,4′-dichloro derivatives, by dioxygen or pyridine N-oxide, in the presence of [MoO2(cysS-OMe)2](cysS-OMe = S-deprotonated cysteinate methyl ester) and [MoO2(S2CNEt2)2] was studied kinetically. The catalytic oxidation rates indicate a trend, MeO > H > Cl, which is related to the ease in apparent hydride release from the C–H group of benzoin derivatives. The stoicheiometric oxidation indicates a similar oxo-transfer reactivity of the above complexes towards the three p-substituted benzoins. Only one 18O atom from [18O] dioxygen was found in one of the benzil carbonyl groups. The catalytic dioxygen oxidation by [MoO2(S2CNEt2)2] suffers from deactivation through the formation of [MoVO(S2CNEt2)2] formed by a one-electron-transfer reaction from [MoIVO(S2CNEt2)2] to dioxygen.