Chiral poisoning of rac-diop iridium complexes in the catalytic enantioselective hydrogenation of imines

Abstract

Dimeric Ir(III) complexes [Ir(P-P)HI 2] 2 (P-P = enantiopure biophospine) have previously been shown to be efficient catalysts for the enantioselective hydrogenation of imines. In the present study we have prepared the analogues using rac-diop. Among the possible dimers, there was only a slight preference for the μ-I 2 heterodimer {[Ir( R, R)-ddiopHI 2][Ir( S, S)-diopHI 2]}. Although this mixture of dimers was a moderately good catalyst for the hydrogenation of imines, it showed no enantioselectivity, as expected. Addition of the readily available aminophosphinephosphinite ligand, (+)-( S)-pronop, to this dimer mixture in a ratioof [( S)-pronop]:[Ir] tot = 1:1 produced a poor catalyst which effected only a few turnovers in 100h. However, addition of (+)-( S)-pronop to this dimer mixture in a ratio of [( S)-pronop]:[Ir] tot = 1:2 produced an effective catalyst for the enantioselective hydrogenation of imines. Significant chiral amplification was not observed in catalysis with dimers prepared from nonracemic diop.