Selective activation of one methyl ketone enantioface via .sigma.-binding to a chiral transition-metal template: synthesis, structure, and reactivity of rhenium ketone complexes [(.eta.5-C5H5)Re(NO)(PPh3)(.eta.1-O:C(CH3)R)]+X-

Abstract

The chiral Lewis acid [(η 5 -C 5 H 5 )Re(NO)(PPh 3 )] + serves as an effective template for the stereoselective binding and reduction of methyl ketones. The mechanism of asymmetric induction entails (1) formation of a σ complex with the rhenium cis to the smaller methyl substituent and (2) hydride attack upon a rhenium-oxygen conformer in which the bulky PPh 3 ligand blocks one C=O face of the ketone ligand. Stereoselectivities match or exceed those obtained with other chiral hydride reductants. The alkoxide ligands in the reduction products can be converted to a variety of chiral, optically active organic products. Many of these reactions give optically active rhenium-containing products. Hence, the chiral auxiliary can be recycled without additional optical resolution