Asymmetric Diels-Alder reactions with hydrogen bonding heterogeneous catalysts and mechanistic studies on the reversal of enantioselectivity

Chiral bis(oxazoline) complexes of Cu(II), Zn(II) and Mg(II) have been immobilized on silica support via hydrogen-bonding interactions. Up to 93% ee is obtained in the Diels-Alder reaction between 3-((E)-2-butenoyl)-1,3-oxazolin-2-one and cyclopentadiene at room temperature with the heterogeneous bis(oxazoline) complexes, and the catalysts can be recycled without losing enantioselectivity. Experimental and theoretical studies show that the reversal of the absolute product configuration upon immobilization of the PhBOX-Cu(II) catalyst is triggered by the anion dissociation from Cu(II) onto the surface of the support. (c) 2005 Elsevier Ltd. All rights reserved.