Solvent and counterion effects in the asymmetric cyclopropanation catalysed by bis(oxazoline)-copper complexes

The reaction of styrene with ethyl diazoacetate, promoted by the complexes of two bis(oxazolines) with Cu(I) and Cu(II) salts (chloride and triflate), was studied in CH2Cl2 and nitroethane. The use of nitroethane reduces the selectivity with regard to the diazoacetate, and also slightly reduces the trans/cis selectivity. In the reactions carried out with triflates the use of nitroethane gives rise to a slight decrease in the enantioselectivity, whereas the reverse is true for copper chlorides. The influence of the solvent and the counterion on the enantioselectivity is explained on the basis of their influence in the disproportionation of Cu(I) to Cu(II) and Cu(0), the latter acting as a non-chiral catalyst. The rate of this side reaction also depends on the structure of the bis(oxazoline) Ligand. (C) 1999 Elsevier Science B.V. All rights reserved.