Clay-supported bis(oxazoline)-copper complexes as heterogeneous catalysts of enantioselective cyclopropanation reactions

Several chiral bis(oxazoline)-copper complexes are supported by cation exchange in laponite, bentonite and K10 montmorillonite, and their catalytic performances are compared in the benchmark cyclopropanation reaction of styrene with ethyl diazoacetate. The results depend on the nature of the support, the chiral auxiliary, and the precursor and solvent used to carry out the cation exchange. However, the use of Cu-I or Cu-II is not so decisive. The best results are obtained when laponite is used as the support. The interaction with the support leads to a modification of the stereochemical course of the reaction, giving rise to an increase of the cis-cyclopropanes with regard to that observed in the homogeneous phase. Some of the catalysts can be recovered and reused, keeping the same catalytic activity and leading to the same enantioselectivity. (C) 1998 Elsevier Science Ltd. All rights reserved.