Title

A New Asymmetric Synthesis of Cyanohydrins and Oxygen-Functionalized Derivatives: Stereoselective Addition of Cyanide to Chiral Rhenium Aldehyde and Ketone Complexes of the Formula [(η5-C5H8)Re(NO)(PPh3)(O=CRR′)]+BF4

Abstract

Reactions of racemic and optically active η 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 °C) give cyanohydrin alkoxide complexes (η5-C5H5)Re(NO)(PPh3)(OCH(CN)R) (3a-e, 78-95%) in 89-53% 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 (96-85%; 90-56% de). Analogous reactions of racemic σ-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, 60-88%) in 71-99% de. A similar reaction of 2e and (CH3)3SiC≡CLi gives (η5-C5H5)Re(NO)(PPh3)(OCCH3(C≡CSi(CH3)3)C6H5) (88%) in 94% de. Mechanisms of diastereoselection are briefly discussed. © 1991, American Chemical Society. All rights reserved.

Publication Title

Journal of Organic Chemistry

Share

COinS