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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 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, 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 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 π system.

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Figure 1: Strategy to stabilize the Möbius structure of annulenes.
Figure 2
Figure 3: X-ray structures and photographs of the crystals of the C2 Möbius and the Cs Hückel isomer.

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We thank C. Näther for measuring the X-ray structures of the Hückel Cs and the Möbius C1 compounds, and C. Wolff for the interpretation of the NMR spectra. We also thank E. Heilbronner for providing additional information on ref. 1, and T. Bally for hints on how to improve the synthesis of syn-tricyclooctadiene. D.A. was supported by a scholarship from the federal state of Niedersachsen. Authors' contributions. R.H. together with D.A. conceived the experiment; D.A. carried it out; R.H. wrote the Letter; and A.S. and O.O. determined the X-ray structure of the Möbius C2 compound.

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Correspondence to R. Herges.

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Supplementary Information: This file contains details about experimental procedure, UV spectra and X-ray data of Moebius c2 (DOC 650 kb)

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Ajami, D., Oeckler, O., Simon, A. et al. Synthesis of a Möbius aromatic hydrocarbon. Nature 426, 819–821 (2003).

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