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

The keto-enol (K reversible arrow E) and enol-enol (E reversible arrow E) equilibria of a variety of trifluoromethyl-beta-diketones were investigated using H-1, C-13, F-19 NMR spectroscopy, infrared spectroscopy and ultraviolet-visible spectrophotometry in nonpolar solvents. In general, NMR, IR and UV spectral evidence indicates that trifluoromethyl-beta-diketones exist as mixtures of two chelated cis-enol forms in nonpolar media. Infrared spectroscopy and ultraviolet spectrophotometry show the E reversible arrow E equilibrium lies in the direction of the enol form which maximizes conjugation in most cases. Exceptions are noted and discussed. (c) 2006 Elsevier B.V. All rights reserved.