Electronic and steric effects of bis(oxazolinyl)pyridine ligands on asymmetric Diels–Alder reactions

A series of bis(oxazolinyl)pyridine (Pybox) ligands with different electronic and steric properties were synthesized and evaluated in the Sc(III)-catalyzed asymmetric Diels–Alder reaction of alkenoyl-1,3-oxazolidin-2-ones with cyclopentadiene. The results show that electron-withdrawing groups increase the enantioselectivity, which is more significantly influenced by steric effects arising near the metal center. Up to 96% ee was obtained under mild reaction conditions when using a ligand containing the sterically bulky tBu substituent and electron-withdrawing chloride.

Graphical abstractHigh enantioselectivity was obtained in the asymmetric Diels–Alder reaction when the Pybox ligand contains an electron-withdrawing substituent X at the 4 position and a sterically bulky moiety R at the 4′ position near the Lewis acid center.Figure optionsDownload full-size imageDownload as PowerPoint slide