Bis(oxazoline)–Copper Complexes, Supported by Electrostatic Interactions, as Heterogeneous Catalysts for Enantioselective Cyclopropanation Reactions: Influence of the Anionic Support

Abstract

Several bis(oxazoline)–copper complexes have been supported by cation exchange in nafion and nafion–silica nanocomposite. The catalytic performance of these solids and that of the materials prepared with laponite clay have been compared in the benchmark cyclopropanation reaction of styrene with ethyl diazoacetate at room temperature in 1,2-dichloroethane. The results depend on the nature of both the chiral auxiliary and the support. Nafion-type supports are better than laponite for the complex between Cu(II) and 2,2′-isopropylidene-[(4R)-4-phenyl-2-oxazoline] because of their weaker electrostatic interaction with the complex, and the nafion–silica nanocomposite is the best support due to its high surface area. In this case the enantioselectivities (58% ee in the trans-cyclopropanes and 47% ee in the cis-cyclopropanes) are similar to those obtained in the homogeneous phase and the catalyst is recoverable and retains almost the same catalytic performance. However, this is not the case for the other two Cu–bis(oxazoline) complexes. For example, the complex between Cu(II) and 2,2′-isopropylidene-[(4S)-4-tert-butyl-2-oxazoline] leads to good results (69% ee in the trans-cyclopropanes and 64% ee in the cis-cyclopropanes) when supported on laponite, but it is the worst catalyst when exchanged in sulfonic supports, in spite of being the best catalyst in the homogeneous phase. This effect might only be explained by an interaction between a support and ligand which must have a steric nature.