Enantioselective epoxidation reaction of non-functionalised prochiral alkenes using optically resolved [brucine](R)-[Ru(PDTA-H)Cl] and [brucine](S)-[Ru(PDTA-H)Cl] complexes

The racemic metal complex K[Ru(PDTA-H)CI](1) has been resolved into its opt ical isomers using brucine as the resolving agent counter ion, [brucine](S) -[Ru(PDTA-H)CI] (1) and [brucine](R)-[Ru(PDTA-H)CI] (2) and their structure s are determined by single crystal X-ray methods. Longer Ru-Cl bonds in bot h the complexes (2.3974(13)A in 1 and 2.415(6) in 2 along with one relative ly weaker and strained chelation ring could be responsible for their cataly tic activity. The CD pattern of the complex 1 shows the presence of the two isomers lambda and delta with more contribution of lambda form while the c omplex 2 acquire only lambda conformation. Catalytic activity of 1 and 2 fo r enantioselective epoxidation of non-functionalised alkenes viz. styrene, 4-chloro-, 4-methyl-, 4-nitrostyrene, 1,2-dihydronaphthalene and indene was accomplished by using molecular oxygen and iodosyl benzene as terminal oxi dant. Excellent conversions (85-89%) were obtained in case of 1,2-dihydrona phthalene with both the catalysts while catalyst 2 gave good conversion wit h styrene and 4-methylstyrene. The enantiomeric excess of the epoxide was d etermined by H-1 NMR using chiral shift reagent Eu(hfc)(3)/ by chiral capil lary column. The extent of enantioselectivity with respect to the substitue nts on substrate is shown on Hammet plot. A possible mechanism at the oxo t ransfer stage is also envisaged. (C) 1999 Elsevier Science B.V. All rights reserved.