ENANTIOSELECTIVE CATALYTIC EPOXIDATION OF STYRENES BY IODOSYLBENZENE USING CHIRAL RUTHENIUM(II) SCHIFF-BASE COMPLEXES

Chiral ruthenium(II) Schiff base complexes 1-3 derived from L-histidine with salicylaldehyde, 5-chloro and 5-methoxy salicylaldehyde were prepared and used for catalytic enantioselective epoxidation of non-functionalised olefins, viz., styrene, 3-methyl-, 3-methoxy-, 3-chloro- and 3-nitrostyrene, with iodosylbenzene as an oxidant, giving the highest ee (80%) for nitrostyrene with catalyst 3. Each catalyst/substrate combination was examined under epoxidation conditions and the results for catalysts 1-3 are presented as Hammet plots. The existence of a possible intermediate and the mechanism of chiral induction are discussed. The stacking/charge transfer interaction between the substrate and triphenyl phosphine of chiral Ru(II) catalyst at ore-transfer stage seem to function favouring S(-) styrene oxide as dominant enantiomer.