Kinetic studies of fast equilibrium by means of high-performance liquid chromatography. Part 11. Keto-enol tautomerism of some β-dicarbonyl compounds

Abstract

The keto-enol tautomerism of some β-dicarbonyl compounds, including ethyl acetoacetate (1), acetylacetone (2), benzoylacetone (3), dibenzoylmethane (4), and 1,1,1-trifluoro-3-(2-thenoyl)acetone (5), has been investigated in various solvents at 25°C, by using low-temperature (-20 to -50°C) high-performance liquid chromatography (h.p.l.c.) for analytical measurements. Base-line separation of keto and enol tautomers of these compounds has been achieved on silica gel packings with a mobile phase composed of hexane and propan-1-ol containing a small amount of acetic acid. The ratios of keto and enol tautomers in different solvents have been determined; the percentages of enol tautomers have been found to be higher in polar solvents, in accord with n.m.r. data previously reported. In all solvents the proportions of enol tautomers of acetylacetone derivatives have been found to increase in the following order; (2) < (3) < (4); this has been attributed to the fact that enol tautomers of phenyl-substituted acetylacetones have been stabilized, owing to the formation of extended conjugated systems. For the ester (1), the presence of a very small amount of a third tautomer (0.1%) has been found, probably an unconjugated enol tautomer.