Pyridine-2,6-bis(oxazolines), Helpful Ligands for Asymmetric Catalysts

Pyboxes are a direct evolution of box ligands and take advantage from the C2-symmetry to reduce the number of possible configurations of the reacting complex formed from ligand, Lewis acid (a cation), and one of the reagents involved in the reaction. A comparison of complexed box and pybox showed that the binding site of pybox is suitable to host Ln cations that become the Lewis acids of election in pybox-based chiral catalysts, in contrast to the failure of Ln-box complexes. It also increased rigidity of the pybox scaffold when the ligand behaves as tridentate, rigidity that is further increased in the reacting complex with reagents behaving as bidentate.