Synthesis of a Mobius aromatic hydrocarbon

The defining feature of aromatic hydrocarbon compounds is a cyclic molecular structure stabilized by the delocalization of pi electrons that, according to the Huckel rule, need to total 4n + 2 (n = 1,2,.,,); cyclic compounds with 4n pi electrons are antiaromatic and unstable. But in 1964, Heilbronner predicted(1) on purely theoretical grounds that cyclic molecules with the topology of a Mobius 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 1 2, p electrons. The prediction stimulated attempts to synthesize Mobius 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 Huckel topology(2,3). Although transition-state species(4), an unstable intermediate(5) and a non-conjugated cyclic molecule(6), all with a Mobius topology, have been documented, a stable aromatic Mobius 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 Mobius compound stabilized by its extended pi system.