Chiral Ru(III) metal complex-catalyzed aerobic enantioselective epoxidation of styrene derivatives with co-oxidation of aldehyde

Abstract

Ru(III) chiral Schiff base complexes 1–3 derived from dehydroacetic acid with 1 S,2 S-(+) diaminocyclohexane, 1 R,2 R-(−)1,2 diphenyl ethylenediamine and S-(+)1,2 diaminopropane have been prepared. The characterization of the complexes was done by microanalysis, magnetic moment, IR-, UV/Vis-, CD spectral studies, optical rotation, conductance measurements and cyclic voltammetry. The enantioselective epoxidation of styrene and substituted styrenes viz., 4-chloro-, 4-nitro- and 4-methyl styrene was achieved by the combined use of molecular oxygen and sacrificial reductant isobutyraldehyde catalyzed by the above synthesized Ru(III) chiral Schiff base complexes. Good yields of the desired epoxides were obtained with styrene and 4-chlorostyrene by GLC. Enantiomeric excess of the epoxide was evaluated by 1H-NMR using chiral shift reagent Eu(hfc) 3 and by chiral capillary column. The extent of enantioselectivity is shown on Hammet plots.