Keto-enol tautomerism is one of the fundamental concepts of organic chemistry and is a subject of detailed discussions in most of organic chemistry textbooks. The enol forms of simple ketones are thermodynamically less stable than the keto forms and only small amounts of the enols can usually be detected in solution. Enols can be stabilized when the steric hindrance in the keto form is larger and in cases when intramolecular hydrogen bonds can form. For instance, the content of the enol form in solution for open chain 1,3-diketones such as acetylacetone can reach 80-90%, which is explained by intramolecular hydrogen bonding:

The NMR spectra of enols of the symmetrically substituted diketones show symmetrical patterns, such that the chemical shifts of the carbonyl and enolic carbon atoms and those of the substituents R are identical. This is explained by rapid proton migration from one oxygen atom to the other so that only the average signals are measured by the NMR technique, known to be sensitive to dynamic phenomena.However, this approach meets considerable difficulties when applied to the cyclic 1,3-diketones, which cannot form intramolecular hydrogen bonds but their NMR spectra are still symmetrical. The results of our reinvestigation on keto-enol tautomerism of dimedone (1, R=Me) and phenindione (2), which show that the species assumed to be the enols are, in fact, anionic complexes, will be presented. These are both commercially available, and have been intensively studied in the past. The existence of the enol form of 2 has been postulated by A. Hantzsch as long as 94 years ago.

The spectroscopic (NMR, UV-VIS) behavior indicates that their free enol forms do not exist in solution. The behavior of both derivatives is better described by the two-stage equilibria. The anionic species formed at the first ionization stage strongly interact with the neutral species at the second complexation stage, shifting the equilibrium on the first stage to the right. Existence of the two-stage equilibria involving the anionic species accounts for the observed anomalies in the spectroscopic behavior of the cyclic 1,3-diketones and is well supported by the quantum mechanical calculations. Fast exchange between the anionic and enol form of 1 and the anionic and diketo form of 2 in the dimeric complexes gives rise to the observed symmetrical patterns in the NMR spectra.

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