Noncovalent immobilization of chiral cyclopropanation catalysts on mesoporous TUD-1: Comparison of liquid-phase and gas-phase ion-exchange Original Research Article

TUD-1 has been used as Brønsted acidic mesoporous support for the immobilization of a chiral cyclopropanation catalyst. Brønsted acid sites were introduced by supporting phosphotungstic acid on all alumina TUD-1 matrix (PW-TUD-Al2O3) or by substitution of Si by Al in all silica TUD-1 matrix (Al-TUD-1). The chiral cyclopropanation catalyst Cu(I)- 2,2′-methylenebis [(4R,5S)-4,5-diphenyl-4,5-dihydro-1,3-oxazole] was immobilized by different noncovalent immobilization techniques; ion-exchange in solution and solid–gas ion-exchange. The chiral cyclopropanation catalyst was adsorbed on PW-TUD-Al2O3 during ion-exchange in solution. This catalyst system displayed high leaching of copper reaching values of 21%. While using Al-TUD-1 as support copper was introduced as CuCl at temperatures from 550 to 850 °C subsequently followed by introduction of the ligand. Noncovalent immobilization by solid–gas ion-exchange of CuCl on Al-TUD-1 resulted in heterogenous catalyst with only 1% leaching of copper but also moderate results in the cyclopropanation of styrene in terms of yield (33%) and enantioselectivities.