Catalytic activity of copper-bis(oxazoline) grafted on mesoporous silica in enantioselective cyclopropanation

Chiral phenyl substituted bis(oxazoline) (PhBox) was covalently immobilized through carbamate linkers onto mesoporous silica materials. These supports were previously prepared by the sol-gel method and they exhibit different textural properties. The presence and the integrity of the bis(oxazoline) ligand was checked by C-13-CP-MAS-NMR. These chiral mesoporous materials were complexed with copper(II) triflate. In spite of the different textural properties of these supports, the copper loading, determined by ICP-AES, was nearly the same (0.041-0.044 mmol Cu/g of solid). The supported Cu(II) complexes were tested as catalysts in the enantioselective cyclopropanation of styrene with ethyl diazoacetate. Enantioselectivities are consistently lower than those obtained in homogeneous phase. Different analyses point to a difficulty in the formation of the expected chelate, due to the presence of a coordinating functional group in the linker, as responsible for the loss in enantioselectivity. The textural properties of the materials do significantly affect the behavior upon recovery.