Influence of spacer chains and percentage ring substitution on phase-transfer catalytic activity of phosphonium salts bonded to polystyrene matrix

Abstract

In nucleophilic aliphatic substitutions carried out under liquid–liquid phase-transfer conditions, the catalytic activity of tributylphosphonium groups directly bonded to a polystyrene matrix through a methylene bridge decreases by about one order of magnitude on passing from 10 to 60% ring substitution; log k_obs. is linearly related to the loading of the catalytic sites. When tributylphosphonium groups are bonded to the polymer matrix by a linear spacer chain of 13 atoms for 10–30% ring substitution, k_obs. increases 1.7–3.1 times with respect to directly bonded catalysts. However, for spaced catalysts with 60% ring substitution, the observed rate increase is 4.1–10-fold. This behaviour is explained by the combination of two opposing effects: (i) a polarity increase at the catalytic site by increasing percentage ring substitution leading to a decrease of nucleophilic activity of the anions; (ii) a polarity decrease at the catalytic site by insertion of spacer chains, due to the intrinsic lipophilicity of the alkyl chains as well as the more even distribution of catalytic sites within the polymer matrix. The reactivities of the catalysts, all prepared from chloromethylated polystyrene (200–400 mesh), are independent of the size of particles separated by sieves.