Asymmetric epoxidation of alkenes using a mixed-ligand complex of ruthenium(III) containing a sugar-based ligand

A mixed-ligand ruthenium(III) catalyst complex, [RuIII(TDL*)(bipy)(H2O)]Cl (1) (TDL* = N-3,5-di-(t-butyl)salicylidine-d-glucosamine; bipy = 2,2′-bipyridine) exhibited catalytic activity toward enantioselective alkene epoxidation using tert-butyl hydroperoxide as terminal oxidant. Styrene, 4-chlorostyrene, 4-methylstyrene, 4-methoxystyrene, 1-methylcyclohexene and 1,2-dihydronaphthalene were effectively converted to their organic epoxides with moderate enantioselectivity (37–47% ee) at ambient temperature. A mechanism involving the formation of a high-valent Ru(V)-oxo species, and the subsequent oxo-transfer to the alkene through a metallaoxetane intermediate is proposed.