Letters to Nature

Nature 426, 819-821 (18 December 2003) | doi:10.1038/nature02224; Received 18 September 2003; Accepted 21 November 2003

Synthesis of a Möbius aromatic hydrocarbon

D. Ajami1, O. Oeckler2, A. Simon2 & R. Herges1

  1. Institut für Organische Chemie, Universität Kiel, Otto-Hahn-Platz 4, 24098 Kiel, Germany
  2. Max-Planck-Institut für Festkörperforschung, Heisenbergstras zlige 1, D-70569 Stuttgart, Germany

Correspondence to: R. Herges1 Email: rherges@oc.uni-kiel.de

The defining feature of aromatic hydrocarbon compounds is a cyclic molecular structure stabilized by the delocalization of pi electrons that, according to the Hückel rule, need to total 4n + 2 (n = 1,2,...); cyclic compounds with 4n pi electrons are antiaromatic and unstable. But in 1964, Heilbronner predicted1 on purely theoretical grounds that cyclic molecules with the topology of a Möbius band—a ring constructed by joining the ends of a rectangular strip after having given one end half a twist—should be aromatic if they contain 4n, rather than 4n + 2, pi electrons. The prediction stimulated attempts to synthesize Möbius aromatic hydrocarbons, but twisted cyclic molecules are destabilized by large ring strains, with the twist also suppressing overlap of the p orbitals involved in electron delocalization and stabilization. In larger cyclic molecules, ring strain is less pronounced but the structures are very flexible and flip back to the less-strained Hückel topology2, 3. Although transition-state species4, an unstable intermediate5 and a non-conjugated cyclic molecule6, all with a Möbius topology, have been documented, a stable aromatic Möbius system has not yet been realized. Here we report that combining a 'normal' aromatic structure (with p orbitals orthogonal to the ring plane) and a 'belt-like' aromatic structure (with p orbitals within the ring plane) yields a Möbius compound stabilized by its extended pi system.