Catalytic Asymmetric Ionic Hydrogenation of α-Alkyl Styrenes.

Over the past two decades, chemists have made significant advances in the field of catalytic asymmetric transfer hydrogenation of various unsaturated compounds with biomimetic hydrogen donors. The reduction of carbon-carbon double bonds, however, has been limited to activated substrate classes, such as enals, enones, nitroolefins, or α-(2-hydroxyaryl) styrenes. Here we report a highly enantioselective Brønsted acid-catalyzed ionic hydrogenation of α-alkyl styrenes using a hydrosilane in combination with a protic additive. Mechanistic and computational investigations support a pathway proceeding through a carbocation intermediate and a transient silylated catalyst species, with catalyst turnover dependent on the presence of a protic additive. Moreover, the exemplary stereoconvergent reduction of a trisubstituted alkene highlights the potential of organocatalytic approaches over traditional metal-catalyzed hydrogenations.