Add time:08/05/2019         Source:sciencedirect.com

Rate constants in the pH range 3–9 for formation of the enethiolate product upon hydrolysis of 3,4-dimethylthiazolium ion, a model for the coenzyme thiamin, have been determined by irreversible iodination of the enethiolate at 25°C and ionic strength 1.0 m in aqueous solution. It is concluded that the rate-limiting step for hydrolysis of 3,4-dimethylthiazolium ion in the pH range 3–11 is breakdown of the neutral tetrahedral addition intermediate (T0) to product: general acid catalysis for enethiolate formation is observed and is inconsistent with rate-limiting formation of T0; buffer catalysis results provide no evidence for a change in rate-limiting step. Brønsted values are α = 0.53 for general acid and β = 0.31 for general base catalysis of the formation of the hydrolysis product by oxygen-containing buffers and primary amines: nucleophilic attack of the buffer bases has been ruled out. Catalysis by buffer acids is formulated as concerted general acid catalysis of the departure of the enethiol from T0. The buffer base- and water-catalyzed reactions are formulated as concerted general base catalysis of the expulsion of the enethiolate from T0. It is suggested that these mechanisms are general for hydrolysis reactions of 3-substituted-4-methylthiazolium ions where the substituent on the nitrogen atom of the thiazolium ring is not an intramolecular nucleophilic catalyst.

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