Aromaticity is a phenomenon that is linked to the improved stability of organic compounds. Its occurrence is predicted by Hückel’s rule, which states that an aromatic compound must be flat, fully conjugated and have (4n + 2) π electrons. The most encountered aromatic structure is the benzenoid ring, which is observed in a wide variety of pharmaceuticals and organic materials. Expanded monocyclic structures are also of substantial interest as they would allow one to experimentally investigate the relationships between aromaticity, stability, reactivity and ring size. However, larger rings such as annulene are often non-planar and non-aromatic due to angle strain present in their planar conformation. Inspired by earlier computational work, we show here that the presence of both a fused cyclopropane and an internal alkyne in a annulene framework provides a planar and highly aromatic structure. The resulting dehydroannulene is bench stable and can be stored for extended periods of time.
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General information, experimental procedures, compound characterization data, X-ray crystallographic data, 1H NMR/13C NMR spectra, HPLC data, computational details and xyz coordinates of computed structures are all available in the Supplementary Information. Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition number CCDC 2092414 (cocrystal of trans-22 and 23). Copies of the crystallographic data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/.
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We thank the Natural Sciences and Engineering Research Council of Canada (grant no. RGPIN-2017–06230 to M.G.), the Canada Foundation for Innovation and the Province of Saskatchewan (Saskatchewan Innovation and Opportunity Scholarship to K.P.) for financial support. We thank the Saskatchewan Structural Sciences Centre and J. Zhu. We thank D. Ward and S. Langdon for helpful discussions and suggestions.
The authors declare no competing interests.
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General information, experimental procedures, compound characterization data, X-ray crystallographic data, 1H NMR/13C NMR spectra, HPLC data, computational details and xyz coordinates of computed structures.
X-ray data for trans-22/23 cocrystal, CCDC 2092414.
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Parmar, K., Blaquiere, C.S., Lukan, B.E. et al. Synthesis of a highly aromatic and planar dehydro annulene derivative. Nat. Synth 1, 696–700 (2022). https://doi.org/10.1038/s44160-022-00135-z