title: Aromatic transition states in nonpericyclic reactions: anionic 5-endo cyclizations are aborted sigmatropic shifts.
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 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.
authors.affiliation: Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, USA.
authors.forename: Kerry
authors.initials: K
authors.lastname: Gilmore
authors.forename: Mariappan
authors.initials: M
authors.lastname: Manoharan
authors.forename: Judy I-Chia
authors.initials: JI
authors.lastname: Wu
authors.forename: Paul v R
authors.initials: Pv
authors.lastname: Schleyer
authors.forename: Igor V
authors.initials: IV
authors.lastname: Alabugin