Design of iridium n-heterocyclic carbene amino acid catalysts for asymmetric transfer hydrogenation of aryl ketones

Abstract

A series of chiral complexes of the form Ir(NHC)2 (aa)(H)(X) (NHC = N-heterocyclic carbene, aa = chelated amino acid, X = halide) was synthesized by oxidative addition of α-amino acids to irid-ium(I) bis-NHC compounds and screened for asymmetric transfer hydrogenation of ketones. Follow-ing optimization of the reaction conditions, NHC, and amino acid ligands, high enantioselectivity was achieved when employing the Ir(IMe)2 (L-Pro)(H)(I) catalyst (IMe = 1,3-dimethylimidazol-2-ylidene), which asymmetrically reduces a range of acetophenone derivatives in up to 95% enantiomeric excess.