Measurement of Keto-Enol Tautomeric Equilibrium of Oxalacetates by ¹H-NMR

Abstract

Keto-enol tautomeric equilibriums of diethyl oxalacetate 1, dimethyl oxalacetate 2, diethyl α-chlorooxalacetate 3 and dimethyl α-chlorooxalacetate 4 were determined by ¹H-NMR spectroscopy. Enol tautomers, which were stabilized by intramolecular H-bonding to give six-membered ring, existed in almost E coofigiiration. In 1 and 2 enolic forms existed richly i n chloroform, and the tautomeric equilibrium of 1 was not altered with changing the temperature or using benzene as solvent. While keto tautomers were superior to enol in α-chlorooxalacetates, by reason that they had only one proton which 'can be enolized and that the enolic forms were unstabilized by cross conjugation with +M effect of the chlorine atom.