Aromatic transition states in nonpericyclic reactions: Anionic 5-endo cyclizations are aborted sigmatropic shifts

  • Gilmore K
  • Manoharan M
  • Wu J
 et al. 
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Abstract

The transition states (TSs) of 5-endo-dig and 5-endo-trig anionic ring closures are the first unambiguous examples of nonpericyclic reactions with TSs stabilized by aromaticity. Their five-center, six-electron in-plane aromaticity is revealed by the diatropic dissected nucleus-independent chemical shifts, -24.1 and -13.7 ppm, respectively, resulting from the delocalization of the lone pair at the nucleophilic center, a sigma CC bond, and an in-plane alkyne (or alkene) pi bond. Other seemingly analogous exo and endo cyclization TSs do not have these features. A symmetry-enhanced combination of through-space and through-bond interactions explains the anomalous geometric, energetic, and electronic features of the 5-endo ring closure transition state. Anionic 5-endo cyclizations can be considered to be "aborted" [2,3]-sigmatropic shifts. The connection between anionic cyclizations and sigmatropic shifts offers new possibilities for the design and electronic control of anionic isomerizations.

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Authors

  • Kerry Gilmore

  • Mariappan Manoharan

  • Judy I.Chia Wu

  • Paul V.R. Schleyer

  • Igor V. Alabugin

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