Enantioselective hydrogenation of imines with chiral (phosphanodihydrooxazole)iridium catalysts

Cationic iridium(I) complexes of chiral phosphanodihydrooxazoles were used as catalysts for the enantioselective hydrogenation of prochiral N-alkyl and N-aryl imines. The complexes are air-stable crystalline solids that can be readily prepared and are easy to handle. The structures of two complexes were determined by X-ray analysis. For N-alkyl imines of acetophenone, enantiomeric excesses of up to 79 % were obtained. Di-alkyl ketimines and cyclic imines showed lower reactivity and selectivity. A remarkable dilution effect was observed for the hydrogenation of the N-phenyl imine of acetophenone: decreasing the substrate and catalyst concentration led to a significant improvement of the enantioselectivity. Thus, up to 89 % ee could be achieved using 0.1 mol% of catalyst. The highest enantioselectivities were obtained in weakly coordinating solvents such as CH2Cl2. Additives such as halides, imides, or amines were found to poison the catalyst. Hydrogen pressures of 100 bar were usually employed, but in some cases identical results were achieved with only 1 bar H-2.