Synthesis of a Möbius aromatic hydrocarbon

The defining feature of aromatic hydrocarbon compounds is a cyclic molecular structure stabilized by the delocalization of π electrons that, according to the Hückel rule, need to total 4n + 2 (n = 1,2,...) cyclic compounds with 4n π electrons are antiaromatic and unstable. But in 1964, Heilbronner predicted 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, π 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 topology. Although transition-state species, an unstable intermediate and a non-conjugated cyclic molecule, 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 π system.