Chiral diphosphine ddppm-iridium complexes: Effective asymmetric imine hydrogenations at ambient pressures

Complexes of the type [Ir(ddppm)(COD)]X were prepared and tested in the asymmetric hydrogenation of a range of imine substrates. Contrary to known iridium catalysts, the ddppm complexes formed efficient catalysts under an atmospheric hydrogen pressure, whereas at higher pressures the catalytic activity of the system was drastically reduced. Depending upon the reaction conditions, N-aryl-imines, Ar'N=CMeAr, were hydrogenated to the corresponding secondary amines in high yields and enantioselectivities (80-94% ee). In contrast to the [BF4](-) and [PF6](-) complexes, coordinating anions such as chloride did not form active Ir-ddppm hydrogenation catalysts. The cationic Ir-ddppm hydrogenation system performed well in chlorinated solvents, whereas coordinating solvents deactivated the system. Dimeric and trimeric Ir(III) polyhydride complexes were formed from the reaction of [Ir(ddppm)(COD)]PF6 with molecular hydrogen at atmospheric pressure and were found to inhibit catalytic activity.