Highly Active Cp*Ir Catalyst at Low Temperatures Bearing an N‑Heterocyclic Carbene Ligand and a Chelated Primary Benzylamine in Transfer Hydrogenation

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.