Asymmetric Transfer Hydrogenation with a Bifunctional Iron(II) Hydride: Experiment Meets Computation.

Hydride cis-β-[FeH(CNCEt3)(1)]BF4 (5) (1 is a chiral N2P2 macrocycle) is the catalytically active species in the asymmetric transfer hydrogenation of ketones formed upon reaction of [Fe(CNCEt3)2(1)](BF4)2 (3) with base. Stoichiometric reactions show that hydride 5 is formed by H-elimination from the 2-propoxo complex [Fe(O iPr)(CNCEt3)(1)]BF4 (8a) and inserts the C═O bond of acetophenone to give the diastereoisomeric alcoholato complexes [Fe(OCH(Me)Ph)(CNCEt3)(1)]BF4 (10R and 10S). Complexes 5, 8a, and 10 were characterized by NMR spectroscopy, and their structures were calculated by DFT. The DFT study supports a bifunctional mechanism with the alkoxo complexes 8a and 10 as resting species. The stereochemical model reproduces the high enantioselectivity with acetophenone, which results from the combination of the rigid macrocyclic scaffold with the bulky, yet conformationally flexible isonitrile.