Iron Complexes with Chiral N/P Macrocycles as Catalysts for Asymmetric Transfer Hydrogenation

The present account describes the development of chiral, C₂-symmetric N/P macrocyclic ligands that attune to the size and electronic properties of the iron(II) ion to give robust complexes under catalysis conditions. This is not trivial, as the complexes of base metals are substantially less stable than those of precious metals. Also, as these N₂P₂ macrocycles feature stereogenic P atoms, the control of the stereochemistry at phosphorus is a key synthetic issue. Still, as the macrocyclic effect was insufficient to give robust catalysts under hydrogen transfer conditions, we had to dig deeper into the toolbox of coordination chemistry and use strong-field ancillary ligands other than CO, specifically isonitriles, which additionally offer a further handle to tune the catalyst. The reward was the discovery of the first iron(II) catalyst for the asymmetric transfer hydrogenation of polar double bonds that is highly enantioselective for a broad scope of substrates.