Super-delocalized valence isomer of coronene

J Org Chem. 2006 Sep 1;71(18):6840-5. doi: 10.1021/jo060898w.

Abstract

Coronene (1) has been proposed to be "superaromatic", but energetic, geometric, and magnetic criteria of global and local aromaticity fail to support this proposal, and indeed, the calculated current-density map shows opposition of currents: diatropic on the 18-carbon rim and paratropic on the 6-carbon hub. However, [7,5,7,5,7,5]-isocoronene (2) ([7,5,7,5,7,5:6]-circulene, or isocoronene, for short), which is a valence isomer in which alternate pentagons and heptagons replace the hexagons surrounding the central ring, is predicted to have a single, unopposed, intense diatropic perimeter current arising from its four pi HOMO electrons, such as in the ipsocentric description of classically aromatic [4n + 2]-annulenes, hence, qualifying 2 as superaromatic on the magnetic criterion. This conclusion is in excellent agreement with anisotropy of magnetic susceptibility (359 cgs-ppm for isocoronene vs 247 cgs-ppm for coronene) and exaltation of magnetic susceptibility (isocoronene exceeds coronene by 51.4 cgs-ppm). Central and perimeter bond lengths suggest an increased aromaticity of isocoronene. In contrast, the energetic criterion shows that isocoronene is destabilized with respect to coronene by ca. 105 kcal/mol of which only ca. 30 kcal/mol can be attributed to differential strain.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anisotropy
  • Isomerism
  • Models, Chemical
  • Polycyclic Compounds / chemistry*

Substances

  • Polycyclic Compounds
  • coronene