Immobilized phosphine–phosphite rhodium complexes: highly active and enantioselective catalysts for asymmetric hydrogenation under continuous flow conditions

Abstract

The asymmetric hydrogenation of benchmark substrates dimethyl itaconate and (Z)-α-acetamidocinnamic acid methyl ester with chiral pentane-2,4-diyl-based phosphine–phosphite Rh complexes immobilized on the support with heteropolyacid (phosphotungstic acid) as anchoring agent has been studied. The complexes have been supported on commercially available Al2O3 by the Augustine method. The novel heterogeneous catalysts were applied in a high-throughput flow reactor. The effect of the pressure, temperature, substrate concentration, and flow rate was thoroughly screened to optimize reaction conditions. The immobilized catalysts proved to be remarkably stable and could be used 6 h in the microfluidic-based reactor without a significant loss of activity and selectivity. Furthermore, under optimized conditions, the hydrogenation product could be obtained with high activity (TOF >2000 h−1) and enantioselectivity (up to 99% ee). As the first precedent, the potential of supported Rh(P-OP)-complexes under flow conditions has been presented.