Water co-catalysis in aerobic olefin epoxidation mediated by ruthenium oxo complexes.

We report the development of a versatile Ru-porphyrin catalyst system which performs the aerobic epoxidation of aromatic and aliphatic (internal) alkenes under mild conditions, with product yields of up to 95% and turnover numbers (TON) up to 300. Water is shown to play a crucial role in the reaction, significantly increasing catalyst efficiency and substrate scope. Detailed mechanistic investigations employing both computational studies and a range of experimental techniques revealed that water activates the Ru VI di-oxo complex for alkene epoxidation via hydrogen bonding, stabilises the Ru IV mono-oxo intermediate, and is involved in the regeneration of the Ru VI di-oxo complex leading to oxygen atom exchange. Distinct kinetics are obtained in the presence of water, and side reactions involved in catalyst deactivation have been identified.