Keto - Enol and Imine - Enamine Tautomerism of 2 - , 3 - and 4 - Phenacylpyridines

Equilibrium constants for keto-enol tautomerisation and migration of hydrogen from carbon to nitrogen to form enamine or zwitterion tautomers have been measured for 2-, 3and 4phenacylpyridines (PyCH,COPh) in aqueous solution at 25 "C. Relative tautomeric stabilities fall in the order ketoimine > enamine > enol and (for the 3-isomer) enol > zwitterion. Values of pKT, (-log KT) where KT = [enamine (or zwitterion)]/[imine] or [enol]/[ketone], are 1.05, 5.87 and 2.42 for the enamine or zwitterion tautomerism and 2.0, 4.86 and 4.4 for keto-enol tautomerism of the 2-, 3and 4-isomers respectively. For the enamines KT was determined kinetically by quenching the enolate anion at a pH below its pKa and monitoring its relaxation to the ketoimine spectrophotometrically: combining rate constants for this process and the reverse reaction measured by halogen trapping of the enol or enamine gave KT. Values are compared with results of earlier indirect measurements by Katritzky. For the 3-isomer, the N-methyl-3-phenacylpyridinium ion was taken as a model for the zwitterion tautomer and a ratio of enol to zwitterion concentrations of 10 : 1 was derived from kinetic and equilibrium ionisation measurements corrected for a methyl substituent effect. The unusually large enol content of 2-phenacylpyridine in non-polar solvents was measured spectrophotometrically and extrapolated to water. For the 4-isomer the enol content could be obtained from a correlation of pKas of phenacylpyridine enols and vinylogously related phenols. Acidic and basic pKas of all tautomers are reported including kinetically determined values for 0-protonation of the enaminones. Proton activating factors for ionisation and tautomerisation have been calculated and are compared with values for the vinylogous hydroxypyridines. The influence of charge delocalisation and electrostatic interactions on the stability of enolate and carboxylate anions is discussed.