Development of heterogeneous catalyst systems for the continuous synthesis of chiral amines via asymmetric hydrogenation

Continuous-flow synthesis of fine chemicals has several advantages over batch synthesis in terms of environmental compatibility, efficiency and safety. Nevertheless, most preparative methods still rely on conventional batch systems. For instance, chiral amines are ubiquitous functionalities in pharmaceutical compounds, but methods for their continuous synthesis with broad substrate generality remain very challenging. Here we show the development of heterogeneous iridium complexes combined with chiral phosphoric acids for the asymmetric hydrogenation of imines towards the continuous synthesis of chiral amines. Direct asymmetric reductive amination of ketones under a hydrogen atmosphere also proceeded smoothly using the same catalyst systems. Various chiral aromatic and aliphatic amines including pharmaceutical intermediates could be prepared in high yields with high enantioselectivities. It was found that continuous-flow reactions that use columns packed with the heterogeneous iridium complexes afforded chiral amines continuously for more than two days even at pressures lower than those in the corresponding batch reactions. The synthesis of chiral amines is of crucial importance for the pharmaceutical industry, but it remains a challenging task and is often inefficient. Now, a heterogeneous iridium complex is developed for the asymmetric hydrogenation of imines and the asymmetric reductive amination of carbonyl compounds in continuous flow with high yields and enantioselectivities.