Chiral Primary Amine-Polyoxometalate Acid Hybrids as Asymmetric Recoverable Iminium-Based Catalysts

A new strategy for the immobilization of iminium organocatalysts through the acid-base assembly of multidentate chiral primary amines and solid polyacids is presented. A suitable structurally distinctive C-2-symmetric chiral primary amine (CPA) was identified in this study and the optimal CPA-POM hybrid obtained catalyzed the Diels-Alder cycloaddition of a-substituted acroleins in high yields and fair-to-high selectivity under aqueous conditions. The primary amine in the metal-organic-framework (MOF)-like catalyst acted as the catalytic center as well as multidentate basic centers, whereas phosphotungstic acid played dual roles as both catalyst anchors and modulators of the activity and stereoselectivity. Furthermore, the MOF-like catalyst showed both high reactivity and physical stability and thus could be recycled and reused six times with only a small loss of activity and selectivity. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)