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Reversible Hydrolysis of the 3-Benzyl-5-(2-hydroxyethyl)-4-methylthiazolium Cation in Aqueous Solution
Heiber-Langer, Irmgard,Winter, Ingrid,Knoche, Wilhelm
, p. 1551 - 1557 (2007/10/02)
The hydrolysis of 3-benzyl-5-(2-hydroxyethyl)-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.
