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 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 indicate that water activates the key oxo species by hydrogen bonding and likely also facilitates catalyst regeneration by disproportionation. Distinct kinetics are obtained in the presence of water, and side reactions involved in catalyst deactivation have been identified.