Enantioselective Reductions Promoted by (Cyclopentadienone)iron Complexes

(Cyclopentadienone)iron complexes have recently gained widespread interest as cheap metal-based pre-catalysts for the reduction of carbonyl compounds and imines, thanks to their air- and moisture-stability and easy synthesis/purification. In this account, several approaches are presented to achieve enantioselective C=O and C=N bond reduction using this class of iron complexes. Most of the examples, used in the asymmetric reduction of ketones, rely on chiral (cyclopentadienone)iron complexes, featuring a chiral cyclopentadienone backbone and/or a chiral monophosphoramidite ligand, introduced by replacement of a CO ligand. The enantiomeric excesses achieved so far with this strategy are at best moderate. Better ees could be obtained with an alternative approach, used in the enantioselective reduction of C=N bonds, which consists in combining an achiral (hydroxycyclopentadienyl)iron complex (i.e. the activated form of a (cyclopentadienone)iron complex) with a chiral phosphoric acid.