Bis(oxazoline)-copper complexes, supported by electrostatic interactions, as heterogeneous catalysts for enantioselective cyclopropanation reactions:Influence of the anionic support

Several bis(oxazoline)-copper complexes have been supported by cation excha nge in nafion and nafion-silica nanocomposite. The catalytic performance of these solids and that of the materials prepared with laponite clay have be en compared in the benchmark cyclopropanation reaction of styrene with ethy l diazoacetate at room temperature in 1,2-dichloroethane. The results depen d on the nature of both the chiral auxiliary and the support. Nafion-type s upports are better than laponite for the complex between Cu(II) and 2,2'-is opropylidene- [(4R)-4-phenyl-2-oxazoline] because of their weaker electrost atic interaction with the complex, and the nafion-silica nanocomposite is t he best support due to its high surface area. In this case the enantioselec tivities (58% ee in the trans-cyclopropanes and 47% ee in the cis-cycloprop anes) are similar to those obtained in the homogeneous phase and the cataly st is recoverable and retains almost the same catalytic performance. Howeve r, 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-bu tyl-2-oxazoline] leads to good results (69% ee in the trans-cyclopropanes a nd 64% ee in the cis-cyclopropanes) when supported on laponite, but it is t he worst catalyst when exchanged in sulfonic supports, in spite of being th e best catalyst in the homogeneous phase. This effect might only be explain ed by an interaction between a support and ligand which must have a steric nature. (C) 1999 Academic Press.