Bis(oxazoline)-metal complexes immobilised by electrostatic interactions as heterogeneous catalysts for enantioselective Diels-Alder reactions

The immobilisation by cationic exchange of complexes of a bis(oxazoline) wi th Cu(II), Mg(II) and Zn(II) has been studied with two types of anionic sol id, a laponite clay and a nafion-silica nanocomposite. The results obtained show that the cationic exchange depends on both the geometry of the comple x and the nature of the cation. Whereas the immobilisation of square-planar complexes takes place easily, the situation is not the same for tetrahedra l complexes. The solids prepared have been used as catalysts in a benchmark Diels-Alder reaction. These solids are efficient catalysts but the enantio selectivities are low, a fact that is related to the presence of non-chiral catalytic sites and to the nature of both the solid counter-ion and the bi s(oxazoline) structure. (C) 2001 Elsevier Science B.V. All rights reserved.