Iridium-catalyzed enantioselective hydrogenation of terminal alkenes

Iridium complexes derived from chiral P,N ligands are efficient catalysts for the enantioselective hydrogenation of 2-aryl-substituted terminal alkenes. Using 0.1 - 1 mol % of catalyst at room temperature and ambient hydrogen pressure, high enantioselectivities (88-94% ee), full conversions after short reaction times and essentially quantitative yields were obtained for a range of differently substituted 2-arylal-kenes. Among six iridium complexes that were tested, the most selective catalyst was a complex with a phosphinite-oxazoline ligand derived from threonine (Ir-ThrePHOX). In contrast to the hydrogenation of tri-substituted alkenes, a strong pressure effect was observed for this class of substrates. Lowering the hydrogen pressure from 50 to I bar resulted in a strong increase of the ee values.