α-Hydroxy Esters via Enantioselective Hydrogen-Mediated C−C Coupling: Regiocontrolled Reactions of Silyl-Substituted 1,3-Diynes

Catalytic hydrogenation of ethyl glyoxalate in the presence of 1,3-diynes 4a−9a using chirally modified rhodium catalysts enables formation of α-hydroxy esters 4c−9c in highly optically enriched form. Notably, for such trialkylsilyl-substituted 1,3-diynes, C−C coupling occurs exclusively at the carbon atom bearing silicon. π-Back-bonding from low valent rhodium as described by the Dewar−Chatt−Duncanson model appears to direct the regiochemistry of C−C coupling, as corroborated by calculations of the diyne LUMO coefficients.