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

Abstract

Mixed-ligand complexes of ruthenium(III) containing tridentate chiral Schiff-base ligands (H 2TDL *s) derived from condensation of either d-glucose amine or l-alanine with 3,5-di-tertiarybutylsalicyldehyde, and triphenylphosphine (PPh 3) or 2,2′-bipyridine (bipy) have been synthesized. The ruthenium(III)-complexes, [ R u III Cl ( TD L 1 * ) ( PP h 3 ) 2 ] {( H 2 TD L 1 * = N - 3 , 5 -di- ( tertiarybutyl ) salicylidine- d -glucosamine )},( 1) [ R u III Cl ( TD L 2 * ) ( PP h 3 ) 2 ] H 2 TD L 2 * = { N - 3 , 5 -di- ( tertiarybutyl ) salicylidine- l -alanine } ( 2) and [ R u III ( TD L 2 * ) ( bipy ) H 2 O ] 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.