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, [(RuCl)-Cl-III(TDL1*)(PPh3)(2)] {(H2TDL1* = N-3, 5-di-(tertiarybutyl)salicylidine-D-glucosamine)},(1) [(RuCl)-Cl-III(TDL2*)(PPh3)(2)] H2TDL2* = {N-3, 5-di-(tertiarybutyl)salicylidine-L-alanine} (2) and [Ru-III(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 I 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. (c) 2006 Elsevier B.V. All rights reserved.