Water, an Essential Element for a ZnII-Catalyzed Asymmetric Quinone Diels-Alder Reaction: Multi-Selective Construction of Highly Functionalized cis-Decalins

A Zn-II complex of a C-2-chiral bisamidine-type sp(2)N bidentate ligand (L-R) possessing two dioxolane rings at both ends catalyzes a highly efficient quinone asymmetric Diels-Alder reaction (qADA) between o-alkoxy-p-benzoquinones and 1-alkoxy-1,3-butadienes to construct highly functionalized chiral cis-decalins, proceeding in up to a >99:1 enantiomer ratio with a high generality in the presence of H2O (H2O:Zn-II=4-6:1). In the absence of water, little reaction occurs. The loading amount of the chiral ligand can be minimized to 0.02 mol % with a higher Zn/L-R ratio. This first success is ascribed to a supramolecular 3D arrangement of substrates, in which two protons of an H2O-Zn-II reactive species make a linear hydrogen bond network with a dioxolane oxygen atom and one-point-binding diene; the Zn-II atom captures the electron-accepting two-points-binding quinone fixed on the other dioxolane oxygen atom via an n-pi* attractive interaction. The mechanisms has been supported by H-1 NMR study, kinetics, X-ray crystallographic analyses of the related ZnLR complexes, and ligand and substrate structure-reactivity-selectivity relationship.