Search

A small library of Ir–MaxPHOX catalysts has been applied to the asymmetric hydrogenation of N-aryl imines. A structure–activity analysis of the three-chiral-center MaxPHOX ligand has been performed. Using complex 1b, the hydrogenation of N-aryl imines took place with up to 96% enantiomeric excess at atmospheric pressure of hydrogen and low temperature. The impact of the stereochemical information at the phosphorus center is small with respect to the selectivity, but large with respect the catalyst activity. Non-P-stereogenic analogs of MaxPHOX were also synthesized and tested, but they provided lower selectivity. The selectivity observed could be explained by taking into account that the actual catalysts were cyclometalated imine complexes formed in situ. [IrHCl(MaxPHOX)(imine)] complexes 9and 10were synthesized and characterized by X-ray crystallography. These complexes, via chloride abstraction, provided the active catalytic species with the same levels of selectivity. Finally, the influence of the counterion on the catalyst performance was also studied.