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 so lids that can be readily prepared and are easy to handle. The structur es of two complexes were determined by X-ray analysis. For N-alkyl imi nes of acetophenone, enantiomeric excesses of up to 79 % were obtained . Di-alkyl ketimines and cyclic imines showed lower reactivity and sel ectivity. A remarkable dilution effect was observed for the hydrogenat ion of the N-phenyl imine of acetophenone: decreasing the substrate an d catalyst concentration led to a significant improvement of the enant ioselectivity. Thus, up to 89 % ee could be achieved using 0.1 mol% of catalyst. The highest enantioselectivities were obtained in weakly co ordinating solvents such as CH2Cl2. Additives such as halides, imides, or amines were found to poison the catalyst. Hydrogen pressures of 10 0 bar were usually employed, but in some cases identical results were achieved with only 1 bar H-2.