Conformational studies by dynamic nuclear magnetic resonance spectroscopy. Part 27. Kinetics and mechanism of annular tautomerism in isomeric triazoles

Abstract

The rates for the prototropic shift of the NH hydrogen in the two isomeric triazoles [(1)= 1,2,4- and (2)= 1,2,3-triazole] have been measured at low temperature by ¹H n.m.r. (100 and 300 MHz). Only the less symmetric (c_s) of the two possible annular tautomers was found to be populated in the case of (1), whereas both (c_s and c_2v) were detected in (2). The relative proportions in the latter compound are dramatically dependent on temperature, concentration, and solvent. In either triazoles the kinetic process appears to be intramolecular and the mechanism corresponds to a 1,2 prototropic shift. The parameters of activation yielded similar ΔH^‡(5.8 and 6.5 kcal mol^–1) and quite negative ΔS^‡(–29 and –19 cal mol^–1 K^–1) values, thus indicating a relatively low probability of attaining the ordered (three-membered) cyclic transition state.