Diphosphine-palladium and -platinum complexes as catalysts for the Baeyer-Villiger oxidation of ketones: Effect of the diphosphine, oxidation of acyclic ketones, and mechanistic studies

A variety of Pd and Pt complexes of the type [(P-P)M(mu-OH)](2)(2+) (M = Pd, Pt; P-P = a series of tetraphenyldiphosphines) were tested in the Baeyer-Villiger oxidation of ketones with hydrogen peroxide. The effect of the diphosphine-metal ring size on the catalytic activity indicates that the larger the ring, the better the catalyst and that, in general, Pt complexes are superior. The complex modified with P-P = dppb is the most active catalyst and allows for the first time the oxidation of a series of acyclic ketones. The corresponding migratory aptitude series is in full agreement with the one known for the stoichiometric organic reaction employing peracids as oxidants. A test of the reactivity of different peroxidic oxidants.(H2O2, t-BuOOH, KHSO5, carbamide peroxide) shows that hydrogen peroxide is the most effective. A kinetic study of the oxidation of 2-methylcyclohexanone with [(dppb)Pt(mu-OH)](2)(2+) as the catalyst shows typical half-order dependence on the catalyst concentration, suggesting that the hydroxy dimer opens up to form the catalytically active species. The reaction is first order in ketone and hydrogen peroxide and is independent of the acidity of the system. The reaction is suggested to proceed via a quasi-peroxymetallacyclic intermediate and bears strong similarities to the stoichiometric organic reaction.