Bis(oxazoline)–copper complexes supported by electrostatic interactions: scope and limitations

Bis(oxazoline)–copper complexes have been supported by cation exchange in laponite and nafion-like solids. The results obtained with these catalysts in cyclopropanation reactions depend on the nature of both the chiral ligand and the support. On using the complex of 2,2′-isopropylidenebis[(4S)-4-tert-butyl-2-oxazoline] with Cu(II), direct exchange in methanol leads to low enantioselectivities in nafion-type solids. Some improvement in enantioselectivity can be obtained by using the less coordinating nitroethane as the exchange solvent or by exchanging the support prior to use with a more soluble tetraalkylammonium cation. Another strategy consists of the exchange of copper(II) and subsequent treatment with the chiral bis(oxazoline) ligand. This system leads to slower reactions and similar enantioselectivities to those obtained by direct exchange. The best enantioselectivity obtained in the reaction between styrene and ethyl diazoacetate is around 70% ee. The recovery of the catalysts leads to a partial loss of enantioselectivity. The stability of the bis(oxazoline)–copper complex is the crucial factor for this behavior. The selectivity can be improved with the bulkier (1R,2S,5R)-menthyl diazoacetate, up to 83% ee, although the reactivity in this case is much lower.