Cyclometallated Imides as Templates for the H‐Bond Directed Iridium‐Catalyzed Asymmetric Hydrogenation of N‐Methyl, N‐Alkyl and N‐Aryl Imines

Abstract

AbstractA combined computational and experimental approach allowed us to develop overall the most selective catalyst for the direct hydrogenation of N‐methyl, N‐alkyl and N‐aryl imines described to date. Iridium catalysts with a cyclometallated cyclic imide group provide selectivity of up to 99 % enantiomeric excess. Computational studies show that the selectivity results from the combined effect of H‐bonding of the imide C=O with the substrate iminium ion and a stabilizing π–π interaction with the cyclometallated ligand. The cyclometallated ligand thus exhibits a unique mode of action, serving as a template for the H‐bond directed approach of the substrate which results in enhanced selectivity. The catalyst (2) has been synthesized and isolated as a crystalline air‐stable solid. X‐ray analysis of 2 confirmed the structure of the catalyst and the correct position of the imide C=O groups to engage in an H‐bond with the substrate. 19F NMR real‐time monitoring showed the hydrogenation of N‐methyl imines catalyzed by 2 is very fast, with a TOF of approx. 3500 h−1.