Polymer-bound thiamine models. V. On the stability of a macroporous polystyrene-bound thiazolium salt in the (semi)-continuous production of benzoin

Abstract

The stability of a macroporous polystyrene-bound thiazolium salt ( 1) in the benzoin condensation reaction was examined in batch-wise re-utilization experiments and in a continuous flow stirred-tank reactor. Both experimental techniques show that the immobilized thiazolium salts are steadily deactivated under the reaction conditions, leading to a 50 % decrease in catalytic activity of 1 after seven batch-wise re-utilization experiments in methanol at 50°C. It was observed that the stability of 1 in continuous flow experiments was not essentially affected by the co-catalyst concentration in the feed stream. On the basis of these results and analysis of data on re-use or continuous use of 1, the origin of the catalyst degradation is discussed. It was found that, when the hydroxyethyl group of the thiazole moieties was protected as thiazolium salt 4, the stability of the polymeric catalyst was improved significantly.