KETO ENOL AND IMINE ENAMINE TAUTOMERISM OF 2-PHENACYLPYRIDINES, 3-PHENACYLPYRIDINES AND 4-PHENACYLPYRIDINES

Equilibrium constants for keto-enol tautomerisation and migration of h ydrogen from carbon to nitrogen to form enamine or zwitterion tautomer s have been measured for 2-, 3- and 4-phenacylpyridines (PyCH2COPh) in aqueous solution at 25-degrees-C. Relative tautomeric stabilities fal l in the order ketoimine > enamine > enol and (for the 3-isomer) enol > zwitterion. Values of pK(T), (-log K(T)) where K(T) = [enamine (or z witterion)]/[imine] or [enol]/[ketone], are 1.05, 5.87 and 2.42 for th e enamine or zwitterion tautomerism and 2.0, 4.86 and 4.4 for keto-eno l tautomerism of the 2-, 3- and 4-isomers respectively. For the enamin es K(T) was determined kinetically by quenching the enolate anion at a pH below its pK(a) and monitoring its relaxation to the ketoimine spe ctrophotometrically: combining rate constants for this process and the reverse reaction measured by halogen trapping of the enol or enamine gave K(T). Values are compared with results of earlier indirect measur ements by Katritzky. For the 3-isomer, the N-methyl-3-phenacylpyridini um 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 an d equilibrium ionisation measurements corrected for a methyl substitue nt effect. The unusually large enol content of 2-phenacylpyridine in n on-polar solvents was measured spectrophotometrically and extrapolated to water. For the 4-isomer the enol content could be obtained from a correlation of pK(a)s of phenacylpyridine enols and vinylogously relat ed phenols. Acidic and basic pK(a)s of all tautomers are reported incl uding kinetically determined values for O-protonation of the enaminone s. Proton activating factors for ionisation and tautomerisation have b een calculated and are compared with values for the vinylogous hydroxy pyridines. The influence of charge delocalisation and electrostatic in teractions on the stability of enolate and carboxylate anions is discu ssed.