Polymer-bound thiamine models: Part II. Polymer effects on the catalytic activity of a macroreticular polystyrene-bound thiazolium salt in the benzoin condensation reaction

Abstract

A macroporous polymer-bound thiazolium salt (4) was prepared from 4-methyl-5-hydroxyethylthiazole and chloromethylated XAD-2 according to Scheme 1. The catalytic activity of this polymer was tested in the benzoin condensation reaction (Scheme 2) and compared with that of the low molar mass analogues 1 – 3. The catalytic activity of 4 was about two times lower, which can be explained by two effects: (i) steric hindrance of the polymer support on the pendant catalytic sites; (ii) site isolation of the catalytic moieties on the polymeric support (prevention of thiazolium ylid dimer formation). Furthermore, it is shown that the polymeric thiazolium salt-catalyzed benzoin condensation can be performed in the absence of cocatalyst, provided that, prior to use, 4 is ‘washed’ with a cocatalyst solution. Apart from the fact that this result has important implications for the use of polymer-bound thiazolium salts in organic synthesis, it is also direct support for the hypothesis that the one and only catalytic function of the basic cocatalyst is conversion of the inactive thiazolium salt into its active ylid form.