Efficient enhancement of copper-pyridineoxazoline catalysts through immobilization and process design

Abstract

Copper-pyridineoxazoline (Cu-pyox) complexes are poor homogeneous catalysts for asymmetric cyclopropanation reactions. Pyox ligands have been immobilized by polymerization of monomers possessing a vinyl group directly attached to position 6 with styrene and divinylbenzene. The corresponding heterogeneous catalysts show a significant enhancement in enantioselectivity, up to 7-fold that of the analogous homogeneous Cu-pyox catalysts. This effect is due to a synergic effect between the proximity of the polymeric backbone and the presence of a bulky substituent in the chiral oxazoline ring around copper. The obtained values of enantioselectivity are similar to those found with supported C2-symmetric bis(oxazolines), but with only half the chiral information given the presence of only one oxazoline ring in pyox. Besides, the co-polymerization in the presence of the right porogen inside a column allows the preparation of monolithic mini-flow reactors. Continuous flow processes contribute to further improve the catalytic efficiency in both classical solvents (dichloromethane) and neoteric greener ones, such as supercritical CO2. The use of scCO2 as solvent yields the same selectivities obtained in batch processes in combination with higher productivity avoiding the use of VOC.