A NEW ASYMMETRIC-SYNTHESIS OF CYANOHYDRINS AND OXYGEN-FUNCTIONALIZED DERIVATIVES - STEREOSELECTIVE ADDITION OF CYANIDE TO CHIRAL RHENIUM ALDEHYDE AND KETONE COMPLEXES OF THE FORMULA [(ETA-5-C5H5)RE(NO)(PPH3)(O=CRR')]+BF4-
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Reactions of racemic and optically active aldehyde complexes [(5-C5H5)Re(NO)(PPh3)(2-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 (5-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 -methyl ketone complexes [(5-C5H5)Re(NO)(PPh3)(1-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 (5-C5H5)Re(NO))(PPh3)(OCCH3(CN)R) (5a-e, 60-88%) in 71-99% de. A similar reaction of 2e and (CH3)3SiCCLi gives (5-C5H5)Re(NO)(PPh3)(OCCH3(CCSi(CH3)3)C6H5) (88%) in 94% de. Mechanisms of diastereoselection are briefly discussed. 1991, American Chemical Society. All rights reserved.
