Abstract
Substituent, temperature and solvent effects on tautomeric equilibria in several β-ketoamides have been investigated by means of nuclear magnetic resonance spectroscopy (NMR). Keto-enol equilibrium predominates over the amide-imidol one. The relative stability of the individual tautomers and the corresponding equilibrium shifts are explained considering electronic and steric effects and tautomer stabilization via internal hydrogen bonds. In solution, these compounds exist mainly as ketoamide and Z-enolamide tautomers, both presenting intramolecular hydrogen bonds.
Highlights
Keto-enol tautomerism in β-ketoesters, β-diketones and β-ketonitriles is a topic that has been extensively studied from several points of view and by means of a variety of experimental methods [1,2,3]
The reactivity of β-ketoamides is related to their structure and their tautomeric equilibria; that is why it should be useful to determine their spectral behaviour in different conditions in order to study their tautomeric distribution
Each nuclear magnetic resonance spectroscopy (NMR) spectrum is the result of the superposition of the spectra of the individual tautomers, since they are altogether in equilibrium
Summary
Keto-enol tautomerism in β-ketoesters, β-diketones and β-ketonitriles is a topic that has been extensively studied from several points of view and by means of a variety of experimental methods [1,2,3]. Structure and their tautomeric equilibria; that is why it should be useful to determine their spectral behaviour in different conditions in order to study their tautomeric distribution It is of practical and theoretical importance to investigate tautomeric equilibria in such systems. The tautomeric equilibria of some β-ketobutanamides in solution were investigated by means of 1HNMR and 13CNMR Their chemical shifts were compared with those of related β-hydroxybutanamides. Regarding β-ketoamides, internal hydrogen bonding is possible to be established in several tautomeric forms. This point has been studied for a series of 3-oxo-2-phenylbutanamides [17]. Effects of substituents, solvents and temperature on the equilibria among different tautomeric forms in eleven β-ketoamides have been studied. Differential solvation effects, electron donor and acceptor substituents and temperature variations should shift the protomeric tautomerism
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