Iridium-catalyzed asymmetric hydrogenation of unfunctionalized, trialkyl-substituted olefins

Chiral iridium complexes with bicyclic pyridine-based N,P ligands have emerged as efficient catalysts for the enantioselective hydrogenation of unfunctionalized trialkyl-substituted olefins. Optimization of the reaction conditions by variation of the solvent, pressure, and temperature led to enantiomeric excesses of up to 99%. Three pure alkenes, (E)-2-cyclohexyl-2-butene and (E)- and (Z)-3,4-dimethyl-2-pentene were converted into the corresponding chiral alkanes with 97%, 94%, and 93% ee, respectively. Hydrogenation of the three C=C bonds of both α- and γ-tocotrienyl acetate led to α- and γ-tocopheryl acetate with very high diastereoselectivity. The same catalysts were successfully applied in the hydrogenation of trisubstituted alkenes with a carboxylic ester or a keto group in the γ position. This reaction was used as a key step in a highly enantioselective synthesis of the pheromone of the caddisfly Hesperophylax occidentalis. The hydrogenation of a structurally analogous allylic alcohol also gave high enantioselectivities.