Mechanistic Insights into Ru-catalyzed Alkene Epoxidation with Nitrous Oxide as a Terminal Oxidant

Nitrous oxide (N2O) is a greenhouse gas produced in the manufacture of 6,6-nylon and nitric acid. While an attractive oxidant that releases only N-2 as a by-product, the kinetic stability of N2O typically requires high temperatures and pressures for activation. This work describes initial kinetics of oxygen transfer in the epoxidation of cholesteryl acetate with N2O catalysed by D-4-Ru(VI)(por)(O)(2) complexes in efforts to provide a better mechanistic understanding of this chemistry. Insights include a need for low concentrations of the alkene to avoid competitive binding to the metal, possible saturation behavior at high N2O pressures, transfer of only one oxygen of RuVI(O)(2) to substrate and a possible catalyst turnover involving disproportionation of RuIV(O) and RuIV(O)(N2O) to active RuVI(O)(2), RuIV(O) and N-2. These insights will be used in future designs of improved catalysts and reaction protocols that may operate efficiently at low pressures of N2O and ambident temperature.