Catalytic Asymmetric Ionic Hydrogenation of α-Alkyl Styrenes
Wencke Leinung1, Nobuya Tsuji2, Michael Merher1
1Max-Planck-Institut für Kohlenforschung, 45470 Mülheim an der Ruhr, Germany.
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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.
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