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

Dimeric Ir(III) complexes [Ir(P-P)HI2](2) (P-P = enantiopure bisphosphine) 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 mu-I-2 heterodimer {[Ir(R,R)-diopHI(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 ratio of [(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.