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

The reductive amination of α‐dialkylphosphine acetaldehydes with enantiopure β‐aminophosphines is a new, versatile route to unsymmetrical tridentate (pincer) ligands P‐NH‐P′. Four new ligands PR 2 CH 2 CH 2 NHCHR′CHR′′PPh 2 (R= i Pr, Cy, R′=Ph, CH(CH 3 ) 2 , R′′=Ph, H) prepared in this way are used to make the iron(II) complexes mer ‐FeCl 2 (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 % KO t Bu, 5–10 bar, 50 °C) to the ( S )‐alcohols, usually in enantiomeric excess ( ee ) greater than 90 %. DFT calculations provided transition‐state structures for the enantiodetermining hydride‐transfer step.