The hydride isonitrile complex [FeH(CNCEt )(1 a)]BF (2) containing a chiral P (NH) macrocycle (1 a), in the presence of 2-propanol as hydrogen donor, catalyzes the base-free asymmetric transfer hydrogenation (ATH) of prostereogenic ketones to alcohols and the hemihydrogenation of benzils to benzoins, which contain a base-labile stereocenter. Benzoins are formed in up to 83 % isolated yield with enantioselectivity reaching 95 % ee. Ketones give the same enantioselectivity observed with the parent catalytic system [Fe(CNCEt ) (1 a)] (3 a) that operates with added NaO Bu.

Hydride cis-β-[FeH(CNCEt 3 )(1)]BF 4 (5) (1 is a chiral N 2 P 2 macrocycle) is the catalytically active species in the asymmetric transfer hydrogenation of ketones formed upon reaction of [Fe(CNCEt 3 ) 2 ( 1)](BF 4 ) 2 (3) with base. Stoichiometric reactions show that hydride 5 is formed by H-elimination from the 2-propoxo complex [Fe(O i Pr)(CNCEt 3 )(1)]BF 4 (8a) and inserts the CO bond of acetophenone to give the diastereoisomeric alcoholato complexes [Fe(OCH(Me)Ph)-(CNCEt 3 )(1)]BF 4 (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.

The chiral iron(II) hydride complex [FeH(CNCEt3)(1a)](BF4) (3, 1a is a chiral macrocycle with an (NH)2P2 donor set) catalyzes the base-free transfer hydrogenation (ATH) of prochiral ketones and the hemireduction of benzils to the corresponding benzoins using iPrOH as hydrogen donor. Ketones give the same excellent enantio-selectivity (up to 99% ee) as the parent catalyst [Fe(CNCEt3)2(1a)](BF4)2 (2), which is only active upon treatment with NaOtBu. Benzoins, whose labile stereocenter is known to undergo racemization under basic conditions, are formed in up to 83% isolated yield with enantioselectivity as high as 95%.

The catalytic strategies developed so far for the synthesis of enantiomerically pure benzoins are reviewed. Particular attention is given to their substrate scope and limitations. Finally, this short review highlights the advantages of more atom-economic methods that have been reported recently.1 Introduction2 Benzoin Condensation2.1 Nucleophilic Carbenes as Catalysts2.2 Homocondensation of Aromatic Aldehydes2.3 Cross-Benzoin Condensation2.4 Acyloin Condensation2.5 Biocatalytic Methods3 Organocatalytic Friedel–Crafts Reaction4 Oxidative Methods4.1 α-Hydroxylation of Ketones4.2 Ketohydroxylation of Alkenes4.3 Enantiospecific Oxidation of meso-Hydrobenzoins4.4 Kinetic Resolution of Racemic Hydrobenzoins4.5 Biocatalytic Dynamic Kinetic Resolution5 Asymmetric Hemireduction of Benzils5.1 Biocatalytic Reductions – A Brief Summary5.2 Piers Hydrosilylation of Benzils5.3 Photoreduction of Benzils to Benzoins5.4 Metal-Catalyzed Hemihydrogenation of Benzils6 Conclusion and Future Challenges