Reaction Mechanism and Factors Influencing Phase-Transfer Catalytic Activity of Crown Ethers Bonded to a Polystyrene Matrix

Article abstract of DOI:10.1021/ja00316a006

Polymer-supported crown ethers 1 and 2 were prepared by reaction of 1percent cross-linked chloromethylated polystyrene, with (hydroxymethyl)- and (ο-hydroxynonyl)-18-crown-6, respectively.Their phase-transfer catalytic activity was tested in anion-promoted nucleophilic aliphatic substitutions and compared with that of structurally similar soluble crown ethers and of polymer-supported and soluble phosphonium salts.Catalytic efficiency of crown ethers 1 and 2 depends on a combination of three parameters: the nature of the nucleophile, the percent ring substitution, and the presence of a spacer chain.Complexation of potassium salts largely depends on the anion, with a high degree for soft nucleophiles like I^- and SCN^- and a lower degree for smaller and less polarizable nucleophiles like Br^- and CN^-.This corresponds to high and low catalytic efficiency, respectively.Spaced catalysts 2 are on the average 2-4 times more reactive than directly bonded catalysts 1.The extent of ring substitution noticeably influences catalytic activity, the variation depending on the nature of the nucleophile.With soft nucleophiles the observed rates progressively diminish as loading increases, following a linear correlation on a semilogarithmic scale, whereas with harder nucleophiles the observed rates reach a maximum at 30percent ring substitution.All reactions follow a pseudo-first-order kinetics, and rates are linearly dependent on molar equivalents of polymer-supported crown ethers.Hydrophilicity of catalysts and the extent of complexation increase with the extent of loading.Phenol is exclusively O-alkylated, even in the presence of the most hydrophilic catalysts.The data, as a whole, lead to the conclusion that the reactions occur in the organic shell surrounding a complexed crown ether, following a mechanism analogous to that demonstrated for immobili zed quaternary salts.