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Benzodifurantrione: A Stable Phenylogous Enol

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Anthony J. Lawrence†, Michael G. Hutchings†, Alan R. Kennedy‡ and Joseph J. W. McDouall§

^† DyStarUK Ltd., School of Chemistry, University of Manchester, Manchester M13 9PL, U.K.

^‡ Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, U.K.

^§ School of Chemistry, University of Manchester, Manchester M13 9PL, U.K.

J. Org. Chem., 2010, 75 (3), pp 690–701

DOI: 10.1021/jo9022155

Publication Date (Web): January 7, 2010

anthony.lawrence@manchester.ac.uk; mike.hutchings@manchester.ac.uk

Section:

Physical Organic Chemistry

Abstract

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 (β_ΟΗ), 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.

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This article has been cited by 1 ACS Journal articles (1 most recent appear below).

Subtle Trade-off Existing between (Anti)Aromaticity, Push−Pull Interaction, Keto−Enol Tautomerism, and Steric Hindrance When Defining the Electronic Properties of Conjugated Structures
Erich Kleinpeter, Ute Bölke and Andreas Koch
The Journal of Physical Chemistry A2010 114 (28), 7616-7623
- Subtle Trade-off Existing between (Anti)Aromaticity, Push−Pull Interaction, Keto−Enol Tautomerism, and Steric Hindrance When Defining the Electronic Properties of Conjugated Structures
  Erich Kleinpeter, Ute Bölke and Andreas Koch
  The Journal of Physical Chemistry A2010 114 (28), 7616-7623
  The spatial magnetic properties (through space NMR shieldings, TSNMRS) of conjugated structures (benzenoid/quinonoid keto/enol tautomers, 1,3-dihydroxyaryl-2-aldehydes, Don-π-Acc chromophores with trade-off existing push−pull vs aromatic behavior) have ...
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