PROTONATION OF PYRIDINES BY PHENOLS, THIOPHENOL, TRIFLUOROACETIC-ACID AND METHANESULFONIC-ACID IN APROTIC-SOLVENTS

At moderately law temperatures, 293-213 K, phenols and pyridines form complexes linked by hydrogen bridges as a way for proton migration: A-H...B reversible arrow 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 C-13 chemical shifts. At lower temperatures, down to 110 K,H-1 NMR results provided direct evidence for the homoconjugation equilibrium: A(-)...H-B+ + B reversible arrow (A(-))(B-2-H+). Separate H-1 signals of the B-H+ and B-2-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: B-delta+...H...B-delta+. The thermodynamic quantities of the homoconjugation equilibrium were calculated for selected systems.