Mechanism of base-catalysed hydrogen–deuterium exchange in thiazolium ion: evidence for the involvement of a tetrahedral intermediate

Abstract

The mechanism of base-catalysed hydrogen–deuterium exchange of 2-H in the thiazolium ion has been examined by ¹H n.m.r. spectroscopy, using small and sterically hindered nucleophiles. Rapid H–D exchange is observed in [²H₆]Me₂SO in the cases of 3,4,5-trimethyl-(1), 3-methyl-4,5-diphenyl-(2), 3,5-dimethyl-4-phenyl-(3), and 3,4-dimethyl-5-phenyl-thiazolium iodide (4), when small nucleophiles (KOD–D₂O or KOCD₃–DOCD₃; 0.016M; 25°) are employed. Utilization of a hindered nucleophile [KOC(CD₃)₃–DOC(CD₃)₃; 0.016M; 25°] however results in a slow exchange of 2-H in (1), (3), and (4), concomitant with H–D exchange of the 4- and 5-methyl protons. The rate of exchange of (1) is found to be approximately twice as rapid as that of (3) or (4). No H–D exchange of 2-H is detected in the case of (2) with the hindered nucleophile. These observations are ascribed to the formation of a tetrahedral intermediate as a prerequisite of the exchange reaction.