Benzodifurantrione: A Stable Phenylogous Enol

The first example of a stable phenylogous enol, resulting from an extended keto−enol tautomerization across a benzene ring, is described. The enol has been isolated, and its structure was proven by X-ray crystallography. The equilibrium between the keto- and enol-tautomers has been extensively studied and quantified in solution by NMR and UV−vis spectroscopy. The position of equilibrium showed a linear correlation to the Kamlet−Taft solvatochromic scale for solvent H-bond acceptor strength (βΟΗ), and the equilibrium was proven to be fully dynamic, obeying first-order equilibrium kinetics. To attempt to explain why enolization occurs, at what surprisingly appears to be the expense of aromatic resonance stabilization, various structural features have been considered and explored further with the aid of MO calculations. Nucleus independent chemical shift (NICS) index of aromaticity calculations for each of the rings comprising both tautomers showed that while the central benzene ring loses aromaticity on enolization, the α-keto-lactone ring showed an unexpected and significant antiaromaticity in the keto-tautomer, which is by no means intuitive. The loss of stabilization energy associated with the central benzene ring is, therefore, to a certain degree compensated by removal of the antiaromatic destabilization of the α-keto-lactone ring rendering the two structures much closer in energy than would otherwise be expected.