Polymer-bound thiamine models. III. Influence of site isolation on the catalytic activity of polystyrene-bound thiazolium salts

Abstract

Several polystyrene-bound thiazolium salts ( 2–4) have been prepared from 5-hydroxyethyl-4-methylthiazole and linear and macroporous chloromethylated polystyrenes. The catalytic activities and spectroscopic behavior of these polymers were tested in the benzoin condensation reaction (Scheme 4) and compared with those of the low molar mass analogues ( 1). It was found that, within the group of polymer catalysts examined, the highest relative activity (0.58), with respect to the low molar mass model, was exhibited by the spacer immobilized N-ethylthiazolium salt 4c. For the polymer-bound thiazolium salts 2, 3 and 4a–b, it was found that the relative catalytic activity decreased (from 0.42 to 0.07) with decreasing extent of site isolation (e.g., by increasing the degree of catalyst loading) on the polymeric support. This decrease in activity with increasing degree of catalyst loading has been explained in terms of accumulation of polymer-bound thiazolium ylid dimers, which have a much lower activity than the corresponding ylid “monomers”. The low catalytic activity of polymer-bound ylid dimers, in contrast to low molar mass ylid dimers (which are excellent catalysts), is explained by the entropic stabilization of a non-catalytic dimeric intermediate. This explanation was confirmed in a low molar mass model study.