Iron(II)-Catalyzed Hydrogenation of Acetophenone with a Chiral, Pyridine-Based PNP Pincer Ligand: Support for an Outer-Sphere Mechanism

We report here the
tridentate, P-stereogenic, C2-symmetric
PNP pincer ligand (SP,SP)-2,6-bis­((cyclohexyl­(methyl)­phosphanyl)­methyl)­pyridine
(1a) and its iron­(II) complexes [FeBr2(CO)­(1a)] (2a), [FeHBr­(CO)­(1a)] (3a), and [FeH2(CO)­(1a)] (4a). In the presence of base, bromocarbonylhydride 3a catalyzes
the hydrogenation of acetophenone to (S)-1-phenylethanol
with 48% ee. The transition states of the enantiodetermining transfer
of hydride from 3a to the carbonyl group of acetophenone
were studied by density functional theory (DFT) with a full conformational
analysis of the PNP ligand for the three different mechanistic models
recently proposed for a related achiral catalyst. The DFT calculations
show that the outer-sphere monohydride mechanism originally proposed
by Milstein reproduces the experimentally observed sense of induction
(S) and enantioselectivity, whereas the dihydride
and inner-sphere pathways predict the formation of the R enantiomer.