Abstract
Tautomerism in aromatic systems with oxygen substitutents is rare. This is investigated in 2‐acetyl‐1,8‐dihydroxy‐3,6‐dimethylnaphthalene (1) and in 2,7‐diacetyl‐1,8‐dihydroxy‐3,6‐dimethylnaphthalene (2). The tautomeric nature of 2‐acetyl‐1,8‐dihydroxy‐3,6‐dimethylnaphthalene is supported by long‐range hydrogen–hydrogen coupling between the OH‐1 and the OH‐8 and by the isotope effects on ^13^C caused by deuteration at the CH~3~CO methyl group. Compound 2 participates in a degenerate equilibrium between two equivalent nonsymmetrical rotamers (2A and 2B), each having two intramolecular O···HO hydrogen bonds: one involving an acetyl oxygen and the neighboring hydroxyl group, and the other between the oxygen centers at positions 1 and 8. In addition, each rotamer is involved in a tautomeric equilibrium, with a structure having an OH‐substituted exocyclic double bond (2AT or 2BT).
DFT calculations for a large set of compounds highlight the factors controlling the unusual rotational and tautomeric behaviors. A very important factor seems to be the repulsive interaction between the O‐1 and O‐8 centers, which is modulated by formation of an OH‐1···O‐8 or OH‐8···O1 hydrogen bond. Steric interactions, mesomeric release of electrons from the oxygen at position 8, and a strong OH···O···C hydrogen bond are other factors.
Solid‐state ^13^C NMR spectra of 2,7‐diacetyl‐1,8‐dihydroxy‐3,6‐dimethylnaphthalene at different temperatures demonstrated no averaging in the solid, whereas partially deuterated 2‐acetyl‐1,8‐dihydroxy‐3,6‐dimethylnaphthalene showed an isotope effect at C‐1 of 1.5 ppm, indicating tautomerism in the solid state. Copyright © 2006 John Wiley & Sons, Ltd.