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

ABSTRACT

A 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.