Rh-DuPHOS-catalyzed enantioselective hydrogenation of enol esters.: Application to the synthesis of highly enantioenriched α-hydroxy esters and 1,2-diols

The asymmetric hydrogenation of alpha-(acetyloxy)-and alpha-(benzoyloxy)acrylates 4 catalyzed by cationic rhodium-DuPHOS complexes has been examined. A wide range of substrates (4) were prepared via a convenient Horner-Emmons condensation protocol, and subsequently hydrogenated under mild conditions (60 psi of H-2) at substrate-to-catalyst ratios (S/C) of 500. Overall, enol ester substrates 4 were reduced by the cationic Et-DuPHOS-Rh catalysts with very high levels of enantioselectivity (93-99% ee). Importantly, substrates 4 bearing beta-substituents could be employed as E/Z isomeric mixtures with no detrimental effect on the selectivity. Labeling studies indicated that no significant E/Z isomerization of the substrates occurs during; the course of these reactions. Details concerning optimization of the reaction, interesting solvent effects, and deprotection procedures for the synthesis of highly enantioenriched alpha-hydroxy esters and 1,2-diols also are provided.