Protonation of pyridines by phenols, thiophenol, trifluoroacetic acid and methanesulfonic acid in aprotic solvents

Abstract

At moderately low temperatures, 293–213 K, phenols and pyridines form complexes linked by hydrogen bridges as a way for proton migration: A—H⋯B ⇄ A^–⋯H—B⁺. The enthalpy and entropy variations induced by this intracomplex proton transfer have been evaluated for the 2,6-dichloro-4-nitrophenol-3,5-lutidine complex from the temperature dependences of the phenol ¹³C chemical shifts. At lower temperatures, down to 110 K, ¹H NMR results provided direct evidence for the homoconjugation equilibrium: A^–⋯H—B⁺+ B ⇄(A^–)(B₂—H⁺). Separate ¹H signals of the B—H⁺ and B₂—H⁺ species were recorded for solutions of pyridines with thiophenol, trifluoroacetic acid or methanesulfonic acid with molar excess of the base. The hydrogen-bridge protons in the homoconjugated cations were strongly deshielded, at ca. 20.5 ppm, and this observation suggests their central or near-central position in the bonds: ^δ+B⋯H⋯B^δ+. The thermodynamic quantities of the homoconjugation equilibrium were calculated for selected systems.