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 alpha-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. pi-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.