Iridium-catalyzed enantioselective hydrogenation of terminal alkenes

Abstract

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-arylalkenes. Among six iridium complexes that were tested, the most selective catalyst was a complex with a phosphinite-oxazoline ligand derived from threonine (IrThrePHOX). In contrast to the hydrogenation of trisubstituted alkenes, a strong pressure effect was observed for this class of substrates. Lowering the hydrogen pressure from 50 to 1 bar resulted in a strong increase of the ee values. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.