High activity in displacement reactions catalysed by quaternary onium salts immobilized on inorganic matrices

Abstract

High nucleophilic activity in displacement reactions by Br −, I −and SCN − on n-octylmethanesulfonate is observed when these anions are paired with bulky onium salts immobilized on inorganic matrices (silica gel and alumina). The reactions follow second-order kinetics; the observed rate constants depend on the matrix (alumina > silica gel), on the nature of the alkyl chains bonded to the quaternary atoms and to the support, on the anion, and on the solvent used in the reaction (n-heptane > toluene > chlorobenzene). The observed reaction rates are in some cases higher by more than an order of magnitude than the same reactions carried out in a homogeneous phase in the presence of strong anion activators (lipophilic onium salts and lipophilic [2.2.2]-cryptands). This dramatic increase is due both to anion activation and to adsorption of the substrate by the inorganic matrix. In spite of this high activity, the reaction microenvironment is polar, as shown by comparative reactivity of the anions (I − > SCN - > Br -) and by high C-alkylation of ambident anions (phenoxide and β-naphthoxide).