Keto–enol and enol–enol tautomerism in trifluoromethyl-β-diketones

The keto–enol (K⇌E) and enol–enol (E⇌E) equilibria of a variety of trifluoromethyl-β-diketones were investigated using 1H, 13C, 19F NMR spectroscopy, infrared spectroscopy and ultraviolet-visible spectrophotometry in nonpolar solvents. In general, NMR, IR and UV spectral evidence indicates that trifluoromethyl-β-diketones exist as mixtures of two chelated cis-enol forms in nonpolar media. Infrared spectroscopy and ultraviolet spectrophotometry show the E⇌E equilibrium lies in the direction of the enol form which maximizes conjugation in most cases. Exceptions are noted and discussed.

Graphical abstractA variety of trifluoromethyl-β-diketones were investigated using 1H, 13C, 19F NMR, IR and UV–vis spectrophotometry in nonpolar solvents. In general, spectral evidence indicates the β-diketones exist as mixtures of two chelated cis-enol forms. IR and UV–vis spectrophotometry show the enol–enol equilibrium lies in the direction of enol form a, which maximizes conjugation in most cases.Figure optionsDownload full-size imageDownload as PowerPoint slide