C_2-Symmetric Cationic Copper(II) Complexes as Chiral Lewis Acids: Counterion Effects in the Enantioselective Diels-Alder Reaction

摘要

We have recently documented the utility of chiral bis(oxazo-line)copper(II) complexes 1 (oxazoline = dihydrooxazole) in the catalysis of asymmetric group transfer reactions such as cyclo-propanation and aziridination. In addition, we have demonstratedthat the C_2-symmetric bis(oxazoline)copper(II) triflate complex I a (inflate = trifluoromethanesulfonate = OTf) is also capable of functioning as an effective chiral Lewis acid in the Diels-Alder reaction with two-point binding ;V-acylimide dienophiles. In an effort to broaden the utility of Lewis acidic chiral Cu~(II) complexes, we have prepared Cu~(II) complexes with tridentate bis(oxazolinyl)pyridine (pybox) ligands, [Cu~(II)-(pybox)X_2] (2) and probed their reactivity as chiral Lewis acids (Scheme1). In conjunction with this investigation, we have uncovered dramatic counterion effects that strongly influence the reactivity of these Lewis acids. In this communication, we document the scope of these catalysts in enantioselective Diels-Alder reactions with unsaturated aldehyde and imide-derived dienophiles.