Hydrolyses of 2-Trimethylsilyl N1-Heterocycles. Involvement of Zwitterions in the Hydrolysis of 2-(Trimethylsilyl)-N-methylimidazole, 2-(Trimethylsilyl)pyridine, and 2-((Trimethylsilyl)methyl)-N-methylimidazole

Article abstract of DOI:10.1021/ja00332a038

The title N-containing aromatic heterocycles (1,2, and 4, respectively) with a trimethylsilyl substituent at the 2-position hydrolize relatively rapidly to form trimethylsilanol and the corresponding 2-H heterocycle.In the case of 2-((trimethylsilyl)methyl)-N-methylimidazole (4) the heterocyclic product is N,2-dimethyl imidazole. pH vs. log k_obsd profiles establish that the reaction proceeds faster at high than low pH and that above the pK_a of the heterocyclic base the reaction is independent of pH. 2-(Trimethylsilyl)-N-methylpyridinium iodide (6) and 2-(trimethylsilyl)- N,N'-dimethylimidazolium iodide (5) salts hydrolyze with a first-order dependence on -> above pH 6 with no evidence for a pH-independent region at high pH.F^- accelerates the reactions of 1 and 2 markedly at low pH but has no effect at pH 12.5.The results indicate that the dominant mechanism for hydrolysis of the title compounds over most of the pH/rate profile involves nucleophilic attack of OH^- on the trimethylsilyl unit of the N-protonated base, the leaving group being the zwitterion (ylide) of the heterocycle.The bimolecular constants for attack of OH^- on protonated 1 and 2 are comparable to those observed for attack of OH^- on 5 and 6, respectively.Compound 1 a below pH 5 attacked by H2O as is 4, but such is not observed for 2.Only compound 4 suffers nucleophilic attack of OH^- on the deprotonated base.For 1 and 2 at low pH, F^- successfully competes with the intrinsically more nucleophilic OH^- because the latter's concentration is so low, but at high pH the reaction proceeds entirely via OH^- attack.