Hydrogen-bonded cyclic dimer formation in temperature-induced reversal of tautomerism of salicylideneanilines.

Tautomeric equilibrium of salicylideneanilines is in favor of the enol form in most organic solvents at room temperature. We have previously reported that the equilibrium in saturated hydrocarbon solvents is reversed at low temperature, being totally in favor of the cis-keto form, which is unfavorable at room temperature. Although this stabilization of the cis-keto form has been explained in terms of aggregation of molecules at low temperature, the structure of the aggregate has remained uncharacterized. In this study, we measured UV-vis absorption spectra of salicylideneanilines including those which have tert-butyl groups at various positions of the benzene rings. The cis-keto form did not appear at low temperature, when salicylideneanilines have a tert-butyl group at a specific position or is restrained to a planar conformation. From these results we propose here that the temperature-induced reversal of the tautomeric equilibrium proceeds through the formation of intermolecular hydrogen-bonded cyclic dimers and the subsequent formation of the higher-order aggregates.