Cyclometallated Imides as Templates for the H‐Bond Directed Iridium‐Catalyzed Asymmetric Hydrogenation of N‐Methyl, N‐Alkyl and N‐Aryl Imines
摘要:
A combined computational and experimental approach allowed us to develop the most selective catalysts for the direct hydrogenation of N‐methyl and N‐alkyl 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.
A novel cationic [IrH(THF)(P,N)(imine)] [BArF] catalyst containing a P-stereogenic MaxPHOX ligand is described for the direct asymmetrichydrogenation of N-methyl and N-alkyl imines. This is the first catalytic system to attain high enantioselectivity (up to 94% ee) in this type of transformation. The labile tetrahydrofuran ligand allows for effective activation and reactivity, even at low temperatures