Conformational and tautomeric eccentricities of 2-acetyl-1,8-dihydroxynaphthalenes

Poul Erik Hansen, Fadhil S. Kamounah, Bjarke Knud Vilster Hansen, Jens Spanget-Larsen

Research output: Contribution to journalJournal article

Abstract

Tautomerism in aromatic systems with oxygen substiuents 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 13C caused by deuteration at the CH3C=O 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 neighbouring 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..O-1 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 13C 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.

Original languageEnglish
JournalMagnetic Resonance in Chemistry
Volume45
Issue number2
Pages (from-to)106-117
Number of pages12
ISSN0749-1581
Publication statusPublished - 2007

Keywords

  • 2-acetyl-1,8-dihydroxynaphthalenes
  • tautomerism
  • intramolecular hydrogen bonding
  • isotope effects
  • chemical shifts
  • OH,OH spin-spin coupling
  • solid-state NMR

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