Unsymmetrical Iron P-NH-P′ Catalysts for the Asymmetric Pressure Hydrogenation of Aryl Ketones

The reductive amination of alpha-dialkylphosphine acetaldehydes with enantiopure beta-aminophosphines is a new, versatile route to unsymmetrical tridentate ( pincer) ligands P-NH-P'. Four new ligands PR2CH2CH2NHCHR'CHR''' PPh2 (R=iPr, Cy, R'=Ph, CH(CH3)(2), R ''=Ph, H) prepared in this way are used to make the iron(II) complexes mer-FeCl2(CO)(P-NH-P') and mer-FeCl(H)(CO)(P-NH-P'). The hydride complex with the rigid ligand with R'= R ''= Ph is an efficient and highly enantioselective homogeneous asymmetric pressure hydrogenation (APH) catalyst. Prochiral aryl ketones are reduced under mild conditions (THF, 0.1 mol% catalyst, 1 mol% KOtBu, 5-10 bar, 50 degrees C) to the (S)-alcohols, usually in enantiomeric excess (ee) greater than 90%. DFT calculations provided transition-state structures for the enantio-determining hydride-transfer step.