Substituent and temperature controlled tautomerism of 2-phenacylpyridine: the hydrogen bond as a configurational lock of (Z)-2-(2-hydroxy-2-phenylvinyl)pyridine

Abstract

2-Phenacylpyridines substituted in the benzene ring are in equilibrium with (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines when dissolved in chloroform. The substituent affects significantly the tautomeric equilibrium [the amount of the enolimine form stabilized by the intramolecular hydrogen bond is 1 and 92% for R = p-N(CH₂)₄ and p-NO₂, respectively]. The negative logarithm of the tautomeric equilibrium constant, K_T, is linearly dependent on the Hammett σ substituent constants. The dependence of K_Tvs. temperature is exponential in character: the more electron-withdrawing is the substituent, the more distinct is the influence of temperature. Unexpectedly, the tautomer present in the crystalline state is not the same for all compounds studied (it is the ketimine one for those carrying strong electron-donor groups). Among the different ab initio methods used to calculate the enthalpy of the proton transfer in chloroform solution, MP2/6-31G** gives the best results.