Highly active Cp∗Ir catalyst at low temperatures bearing an N-heterocyclic carbene ligand and a chelated primary benzylamine in transfer hydrogenation

Abstract

The synthesis of new Cp Ir complexes bearing an N-heterocyclic carbene ligand and a chelated primary benzylamine is described. The new complexes are chiral at metal and have a stereogenic carbon at the benzylamine ligand. The synthesis is diastereoselective, and the origin is thermodynamically controlled. The chiral complexes have been fully characterized. The catalytic results show that the complexes are very active in transfer hydrogenation: for example, acetophenone is reduced to 1-phenylethanol in 2 h at 50 °C using a catalyst loading of 1 mol %. More interestingly is that no base, apart from the required for catalyst activation, is needed in the process. The enantioselectivities obtained range from low to moderate, with a maximum of 58% ee in the case of 2'-methylacetophenone. Initial mechanistic studies by means of DFT calculations suggest that the mechanism is based on a direct hydrogen transfer via a highly ordered transition state centered at the iridium amido group. The calculations are in good agreement with the experimental data and support a concerted one-step mechanism process.