Highly Enantioselective Hydrogenation of N-Aryl Imines Derived from Acetophenones by Using Ru-Pybox Complexes under Hydrogenation or Transfer Hydrogenation Conditions in Isopropanol

The asymmetric reduction of N-aryl imines derived from acetophenones by using Ru complexes bearing both a pybox (2,6-bis(oxazoline) pyridine) and a monodentate phosphite ligand has been described. The catalysts show good activity with a diverse range of substrates, and deliver the amine products in very high levels of enantioselectivity (up to 99%) under both hydrogenation and transfer hydrogenation conditions in isopropanol. From deuteration studies, a very different labeling is observed under hydrogenation and transfer hydrogenation conditions, which demonstrates the different nature of the hydrogen source in both reactions.