Mechanism of alcohol oxidations by anionic peroxomolybdenum complexes

Article abstract of DOI:10.1016/S0040-4020(01)90422-3

The rates of oxidation of a series of primary and secondary alcohols to the corresponding carbonyl compounds by seven anionic peroxomolybdenum complexes, i.e [MoO(O₂)₂(C₅H₄NCOO)]^- (BU₄N⁺ (1a), [MoO(O₂)₂(C₅H₄NOCOO)]^- (Bu)₄N⁺ (2a) and [MoO(O₂)₂(X-C₆H₄COO)]^- (Bu)₄N⁺ (X = H, p-NH₂, p-OMe, m-Cl, p-NO₂) (3a-e), have been measured under various experimental conditions. Different kinetic behaviors are exhibited by the various oxidants. In particular, different rate laws, activation parameters and kinetic isotope effects are found. Despite such differences a common general mechanistic scheme can be proposed, involving the association of the substrate to a free or releasable coordination site of the peroxocomplexes, leading to an intermediate present in a low, steady-state concentration. From such an alkoxo intermediate the carbonyl product is obtained through a series of steps whose nature is suggested only by indirect evidence. Two subsequent electron transfers to the oxidant from the coordinated alkoxo compound are considered the most likely mechanism.