Selective hydrogenation of benzylideneacetone catalyzed by iridium diphosphine complexes

Hydrogenation of benzylideneacetone (PhCH=CHCOCH3) is catalysed by iridium diphosphine complexes of the type [Ir(P-P)2]+ (P-P = dpe, dpp, dpb, (S, S)-chiraphos, (R)-prophos,, (+)-diop and (S)-prolophos) and by systems formed in situ from [Ir(cod)Cl]2 or [Ir(cod)(OCH3)]2 (cod = 1,5-cyclooctadiene) and diphosphines. 1-Phenyl-1-buten-3-ol [PhCH=CHCH(OH)CH3] is selectively produced in high yields using [Ir(P-P)2]+ (P-P=diop, prolophos) as catalyst precursor or high P/Ir ratios in the systems prepared in situ. Optical yields up to 30% in the S(-)-unsaturated alcohol are obtained with chiral diphosphines as ligands. Catalytic results are given together with spectroscopie data and some equilibria are proposed to account for the different activities and selectivities observed in the hydrogenation. The length of the carbon atom chain in the diphosphine seems to play a crucial role in determining the distribution of products.