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 [Mo^VO(S₂CNEt₂)₂] formed by a one-electron-transfer reaction from [Mo^IVO(S₂CNEt₂)₂] to dioxygen.