Ferrosalen and Ferrosalen-Type Ligands: Structural Modulation and Applications in Asymmetric Catalysis

Several ferrosalen and ferrosalen-type ligands, 1−10, were prepared and fully characterized. The structure of one of these ligands ligated to Cu(II) was determined by single-crystal X-ray diffraction analysis. In comparison with the parent ligand 1, the two six-membered rings of ligand 2 are not aromatized and the synthesis was more facile. Diastereoisomers 3 and 4, containing substituents on the ethylene chain, were synthesized. The aromatized versions of 3 and 4, namely 5 and 6, were also prepared. Further modulation of the ethylene backbone produced ligands 7 and 8. Replacing the Cp ligands with Cp* and CpPh5 gave ligands 9 and 10, respectively. All these ligands were tested for catalytic asymmetric reactions, including the Co(III)-catalyzed carbonyl-ene reaction, Al(III)-catalyzed Strecker reaction, and Al(III)-catalyzed silylcyanation of aldehyde.