Quaternary Onium Salts Immobilized on Inorganic Matrices

Onium salts immobilized on silica gel and alumina show unusual properties because of the presence of the polar inorganic support. Under liquid—liquid phase-transfer catalysis conditions these systems can also catalyze reactions carried out without stirring, and unlike the corresponding onium salts supported on polystyrenic matrix, they do not need the use of polar solvent. Moreover, different matrices with different geometries in the micropores are differently functionalized and may induce selectivity towards reagents with different molecular size. The polar and hydrophilic matrix strongly affects the affinities of the immobilized onium salts in the anion exchange process, producing a higher affinity towards hydrophilic inorganic and lipophilic organic anions, with respect to that observed for the homogeneous and polystyrene supported onium salts. In aqueous solution the onium salts supported on inorganic matrices are able to adsorb the reactants and to vary the reaction microenvironment as the micellar catalysts do.