Nickel(II)-Catalyzed Asymmetric Inverse-Electron-Demand Diels-Alder Reaction of 2-Pyrones with Styrenes and Indenes

With electronically unbiased styrenes and indenes as dienophiles, a highly enantioselective all-carbon-based inverse-electron-demand Diels-Alder reaction of electron-deficient 2-pyrones is reported. Using C-1-symmetric imidazolidine-pyrroloimidazolone pyridine as the tridentate ligand and Ni(OTf)(2) as the Lewis acid, diverse bridged bicyclic lactones were obtained in high yields (88-97% yields), diastereoselectivities (>95:5 dr), and enantioselectivities (90-99% ee). Cyclic enamine and 2,3-dihydrofuran were also suitable dienophiles. DFT calculations supported a concerted [4 + 2] cycloaddition mechanism for the Ni(II) complex-catalyzed reaction with high enantioselectivity caused by steric factors.