Reversible hydrolysis of the 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium cation in aqueous solution

Abstract

The hydrolysis of 3-benzyl-5-(2-hydroxethyl)-4-methylthiazolium bromide has been studied using a stopped-flow technique with spectrophotometric recording. This reaction consists of three steps: the binding of a hydroxide ion to the thiazolium cation to form the pseudobase, the ring-opening of the pseudobase, and the loss of a proton from the ring-opened form (Breslow mechanism). The pseudobase and ring-opened form are intermediates and cannot be observed at equilibrium. However, in the stopped-flow experiment the three steps can be observed separately, and all equilibrium and rate constants have been obtained. The pH dependence of the reaction rates indicates the existence of a further intermediate, which is formed by N-protonation of the pseudobase. The pK value of the catalytically active ylide is larger than 15, therefore its concentration is too small to be detected.