Dependence of Mechanism on pH for Deuterium–Hydrogen Exchange in 1-Methyltetrazole-5-d. Transition Metal Ion Catalysis of a Deprotonation Process

The pH–rate profile of protium for deuterium exchange of 1-methyltetrazole-5-d in aqueous solution at 67.6° has been determined and used as a control experiment in a demonstration that σ complexation of a heteroaromatic substrate with a transition metal cation (Cu²⁺ and Zn²⁺ used) can have a major rate-enhancing effect on processes in which substrate ring substituents are eliminated to produce formal sp² carbanion intermediates. The control experiment is itself of interest, since in addition to a flattened exchange rate minimum at pH 4, a rate maximum is found at –H₀ 1–1.5, well below the experimental H₀ (ca. –2.4) for half-protonation of 1-methyltetrazole.