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 σ CC bond, and an in-plane alkyne (or alkene) π 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. © 2012 American Chemical Society.
CITATION STYLE
Gilmore, K., Manoharan, M., Wu, J. I. C., Schleyer, P. V. R., & Alabugin, I. V. (2012). Aromatic transition states in nonpericyclic reactions: Anionic 5-endo cyclizations are aborted sigmatropic shifts. Journal of the American Chemical Society, 134(25), 10584–10594. https://doi.org/10.1021/ja303341b
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