Keto–Enol and imine–enamine tautomerism of 2-, 3- and 4-phenacylpyridines

Abstract

Equilibrium constants for keto-enol tautomerisation and migration of hydrogen from carbon to nitrogen to form enamine or zwitterion tautomers have been measured for 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-, 3-and 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 * This pK, was earlier estimated4 as 8.4 instead of 8.1 and the derived pK, for the keto-enol tautomerisation as 4.2 instead of 4.4. The present values are preferred.

On the roles of close shell interactions in the structure of acyl-substituted hydrazones: An experimental and theoretical approach

“…at δ 3.74 and 3.83 ppm for methyl protons of esters -CO 2 CH 3 and -CH 2 CO 2 CH 3 , respectively, besides the aromatic protons in the range δ= 6.97-7.34 ppm. In the 13 can be formally related with the migration of hydrogen from carbon to nitrogen to form the enamine tautomer [34]. The later is computed to be higher in energy by ca.…”

Alkylation of Carbonyl Compounds in Water: Formation of CC and CO Bonds in the Presence of Surfactants

“…The investigated alkyl halides are: benzyl chloride, benzyl bromide, methyl iodide, cyclohexyl bromide and cyclohexyl iodide. These starting materials have been chosen for the following reasons: benzyl phenyl ketone has a molecular structure and a p K a value7 (≈15 in water) which are very similar to those of 2‐phenylacetylthiophene, whose keto–enol interconversion has been kinetically investigated5 in the presence of micelles of CTAB and the binding constant of its enolate with the micelle has been accurately measured. Cyclohexanone and γ ‐phenylcyclohexanone have quite different hydrophobic properties, but presumably very similar acidities.…”

Nitropyridines: X. Palladium-catalyzed cross-coupling of 2-bromo-5-nitropyridine with terminal acetylenes

“…Tautomerism of 2-, 3-, and 4-phenacylpyridines having no nitro group in the pyridine ring was studied previously. It was found that 2-phenacylpyridine can exist as three tautomers, the third tautomer being enamino ketone; its concentration depended on the solvent polarity [24][25][26][27][28].…”