Immobilisation of bis(oxazoline)-copper complexes on clays and nanocomposites. Influence of different parameters on activity and selectivity

Abstract

Bis(oxazoline)–copper complexes, immobilised by electrostatic interactions on Laponite and Nafion/silica nanocomposites, have been tested as catalysts in the cyclopropanation of styrene with ethyl diazoacetate. Several factors play a decisive role in the outcome of the reaction. Firstly, the nature of the solid counterion is important, with perfluorosulfonic solids being the best in this regard. Secondly, the nature of the solvent used has a marked influence as some solvents cause the solid support to modify the stereochemical course of the reaction. Finally, the nature of the chiral ligand is also very important, in particular the strength of its copper complex, in order to avoid the formation of non-chiral catalytic copper centres. This particular effect has been highlighted by using a ligand that is able to give strong complexation, namely an iminobis(oxazoline) ligand, and this leads to the best enantioselectivities being obtained for reactions using electrostatically immobilised catalysts.