Mechanistic studies of the epoxidation of olefins catalyzed by molecular molybdenum complexes

The epoxidation of olefins is a convenient method to obtain functionalized organic materials from crude oil fractions, with molybdenum complexes being among the best catalysts in homogeneous phase. More than three decades ago, Mimoun and Sharpless proposed mechanisms for the Mo-catalyzed olefin epoxidation, however, the true mechanism is still unclear. Therefore, our research focus is set to shed some light on the mechanism by an experimental approach. Our investigations are based on the synthesis and characterization of eight compounds of the type [(η⁵-C₅R₅)Mo(O)₂X] and [(η⁵-C₅R₅)Mo(O₂)OX] (R = H, CH₃; X = CH₃, Cl) by means of IR and NMR spectroscopy. In order to obtain an insight into the nature of the active species, the dioxo complexes were used as catalysts for the epoxidation of cyclooctene with tert-butylhydroperoxide as oxidant, whereas the oxo-peroxo complexes were reacted with a) equimolar amounts (without additional oxidant) and b) an excess of the olefin.