A New Aysmmetric Synthesis of Cyanohydrins and Oxygen-Functionalized Derivatives: Stereoselective Addition of Cyanide to Chiral Rhenium Aldehyde and Ketone Complexes of the Formula 5-C5H5)Re(NO)(PPh3)(O=CRR')>+BF4-

Article abstract of DOI:10.1021/jo00024a023

Reactions of racemic and optically active ? aldehyde complexes <(η⁵-C5H5)Re(NO)(PPh3)(η²-O=CHR)>⁺BF4^- (1; R = a, CH3; b, CH2CH3; c, CH(CH3)2; d, CH2C6H5; e, C6H5) with (CH3CH2)4N⁺CN^- (CH2Cl2, -80 deg C) to give cyanohydrin alkoxide complexes (η⁵-C5H5)Re(NO)(PPh3)(OCH(CN)R) (3a-e, 78-95percent) in 89-53percent de.Subsequent reactions of (+)-(RS)-3a-e with (-)-(R)-C6H5(CH3O)(F3C)CC(=O)Cl (MTPA-Cl) and DMAP in C6H6 give optically active esters MTPA-OCH(CN)R (85-96percent, 90-56percent de).Analogous reactions of racemic ?-methyl ketone complexes <(η⁵-C5H5)Re(NO)(PPh3)(η¹-O=C(CH3)R)>⁺BF4^- (2; R = a, CH3; b, CH2CH3; c, CH(CH3)2; d, C(CH3)3; e, C6H5) and (CH3CH2)4N⁺CN^- give alkoxide complexes (η⁵-C5H5)Re(NO)(PPh3)(OCCH3(CN)R) (5a-e, 60-88percent) in 71-99percent de.A similar reaction of 2e and (CH3)3SiCCLi gives (η⁵-C5H5)Re(NO)(PPh3)(OCCH3(CCSi(CH3)3)C6H5) (88percent) in 94percent de.Mechanisms of diastereoselection are briefly discussed.