Highly efficient asymmetric epoxidation of alkenes with a novel chiral complex of ruthenium(III) containing a sugar based ligand and triphenylphosphines

Mixed-ligand complexes of ruthenium(III) containing tridentate chiral Schiff-base ligands (H2TDL*s) derived from condensation of either d-glucose amine or l-alanine with 3,5-di-tertiarybutylsalicyldehyde, and triphenylphosphine (PPh3) or 2,2′-bipyridine (bipy) have been synthesized. The ruthenium(III)-complexes, [RuIIICl(TDL1*)(PPh3)2] {(H2TDL1*=N-3,5-di-(tertiarybutyl)salicylidine-d-glucosamine)},(1) [RuIIICl(TDL2*)(PPh3)2]H2TDL2*={N-3,5-di-(tertiarybutyl)salicylidine-l-alanine} (2) and [RuIII(TDL2*)(bipy)H2O]Cl (bipy = 2,2′-bipyridine) (3) were characterized by analytical, spectral (UV–vis and IR), molar conductivity, magnetic moment and electrochemical studies. Complex 1 exhibited remarkable enantioselcetivity toward epoxidation of unfunctionalized alkenes using tert-butylhydroperoxide (t-BuOOH) as terminal oxidant. Styrene, 4-chlorostyrene, 4-methylstyrene, 4-methoxystyrene, 1-methylcyclohexene and 1,2-dihydronaphthalene were effectively converted to their organic epoxides in the 70–95% ee at ambient temperature. A lesser enantioselectivity was observed when complexes 2 and 3 were used in the epoxidation of enlisted alkenes under identical experimental conditions. A mechanism involving intermediacy of a high-valent Ru(V)-oxo species is proposed for the catalytic epoxidation process.

Graphical abstractMixed-ligand ruthenium catalyst containing tridentate sugar-based chiral ligand and triphenylphosphines effect remarkably high asymmetric induction in epoxidation of styrenes using t-BuOOH as a terminal oxidant. Figure optionsDownload full-size imageDownload as PowerPoint slide