Surface confinement effects in enantioselective catalysis:: Design of new heterogeneous chiral catalysts based on C1-symmetric bisoxazolines and their application in cyclopropanation reactions

Surface confinement effects are extremely important in determining the stereochemical course of cyclopropanation reactions catalyzed by C-2-symmetric bisoxazoline-copper complexes immobilized on laponite. In the case of the PhBox ligand. complete reversal of trans/cis-diastereoselectivity is observed, as well as a reversal in the absolute configuration of the major cis-cyclopropane. Furthermore, the enantioselectivity obtained with the immobilized catalyst is better than that obtained in the homogeneous phase, indicating that support effects can be beneficial to the stereoselectivity of the reaction. The design of C-1-symmetric bisoxazoline ligands specifically for heterogeneous catalysis allows improvement of the cis-selectivity of the benchmark cyclopropanation reaction up to 91% cis-cyclopropanes. This finding is in agreement with a closer complex surface approach. Experiments carried out with reagents with a greater steric demand highlight the existence of reaction channels in which the ester group points toward the support surface, a factor that was disregarded in the former stereoinduction model. (C) 2008 Elsevier Inc. All rights reserved.