Selective Epoxidation of Olefins by Oxo(V) Alkylpreoxides. On the Mechanism of the Halcon Epoxidation Process

Article abstract of DOI:10.1021/ja00273a027

Novel vanadium(V) alkylperoxy complexes with the general formula VO(OOR)(R'-OPhsal-R'') (II) were synthesized and characterized by physicochemical methods.These complexes most probably have a pentagonal pyramidal structure, with an axial vanadyl group and, in the pentagonal plane, three positions occupied by the Schiff base planar ligand and two positions occupied by a bidendate alkylperoxy group which is presumably weakly coordinatively bonded to the metal by the alkoxy oxygen atom.These complexes are very effective reagents for the selective transformation of olefins into epoxides, with yields ranging from 40percent for 1-octene to 98percent for tetramethylethylene.The reactivity of olefins is sensitive to steric hindrance and increases with the olefin nucleophilicity.The epoxidation of olefins by complexes II is steroselective, inhibited by water, alcohols, and basic ligands or solvents, and accelerated in polar nondonor solvents.Kinetic studies showed that the olefin coordinates to the metal prior to the decomposition of the metal-olefin complex in the rate-determining step.Competitive epoxidation of several olefins vs. cyclohexene showed that the more strongly coordinated olefins exert an inhibiting effect on the epoxidation of the less strongly coordinated ones.These data, which are similar to those of the Halcon catalytic epoxidation process, are consistent with a pseudocyclic peroxy metalation mechanism.