Light-Induced Enantioselective Hydrogenation Using Chiral Derivatives of Casey's Iron-Cyclopentadienone Catalyst

We herein report the first example of an asymmetric ketone hydrogenation using chirally modified derivatives of the homogeneous iron (II)-cyclopentadienone-tricarbonyl system, known as Casey's catalyst. For the synthesis of the chirally modified catalysts, one of three carbonyl ligands was exchanged for a chiral phosphoramidite. To this end, either oxidative decarbonylation using trimethylamine-N-oxide or photolysis was applied. Photolysis was also used to convert the tricarbonyl iron precatalyst (and, analogously, the dicarbonyl phosphoramidite complexes) to the coordinatively unsaturated dicarbonyl (monocarbonyl, respectively) complexes, which are intermediates in the catalytic cycle of ketone hydrogenation. Hydrogen uptake by the latter species affords the loaded hydride, as evidenced by H-1 NMR spectroscopy. Thus, the preparation of sensitive iron hydrides by the typically low-yielding Hieber reaction could be avoided. Instead, the catalytic cycle is accessed from air-stable carbonyl precursors. In the hydridic form of the phosphoramidite carbonyl catalysts, the iron atom itself becomes a stereocenter. NMR spectroscopy confirmed the generation of two hydride diastereomers. With the MonoPhos iron dicarbonyl complex, moderate enantioselectivity (up to 31% ee) was achieved in the hydrogenation of acetophenone.