Mechanistic studies on the epoxidation of alkenes with molecular oxygen and aldehydes catalysed by transition metal-β-diketonate complexes

The scope, mechanism and kinetics of the aerobic epoxidation of alkenes with an aldehyde and substituted beta-diketonate-transition metal complexes as catalysts were studied. beta-Diketonate complexes of nickel(rr) proved to be among the best catalysts for this reaction. The epoxidation is not dependent on substrate concentration and is first order in aldehyde, catalyst concentration and oxygen partial pressure. It was shown by reactivity studies and EPR experiments that the reaction is radical in nature. Additional evidence for this was obtained from stereochemical investigations. The metal catalyst is not only an efficient initiator of the reaction, but is also believed to enhance the reactivity of intermediate species in the oxidation process by allowing these to co-ordinate to the metal center. A mechanism is proposed for the catalytic reaction.