Substituent and temperature controlled tautomerism: multinuclear magnetic resonance, X-ray, and theoretical studies on 2-phenacylquinolines

Proton-transfer equilibria in chloroform solution of twelve 2-phenacylquino lines were studied by H-1, C-13 and N-15 NMR spectroscopies. The (Z)-enamin one form stabilized by an intramolecular hydrogen bond was found to prevail in all cases. Electron-donating substituents in the phenacyl part of the m olecule lead to an increase of the ketimine form (to 33% for p-NMe2). Varia ble temperature H-1 NMR measurements show that higher temperatures have the same effect. The negative logarithm values of the equilibrium constant, pK (T), were found to be linearly dependent on Hammett sigma substituent const ants. The pK(T) vs. temperature correlation also has a linear character. In general, strong electron-withdrawing substituents cause transformation of the ketimine to the enaminone form to become more exothermic but values of the heat of reaction for 2-phenacylquinolines studied are not linearly depe ndent on sigma. X-Ray data show that the strength of the internal hydrogen bond in the enaminone form increases for strong electron-withdrawing substi tuents. Rough estimation shows this bond to be stronger in chloroform solut ion than in the crystalline state. pi-Electron delocalization in the six-me mbered quasi-ring involving the H ... O bond is very strong. This effect is responsible for the predominance of the tautomeric enaminone form in 2-phe nacylquinolines. On the other hand, semiempirical AM1 and PM3 calculations show that in the gas phase the ketimine tautomer is energetically favored i n most cases.