2023 Volume 81 Issue 12 Pages 1168-1177
Non-alternant hydrocarbons, such as azulene, pentalene, and heptalene, are intriguing compounds. The π-conjugations within these consecutive odd-membered carbocycles provide unique physical properties not shared by alternant-benzenoid hydrocarbons. Incorporating these non-alternant frameworks into polycyclic aromatic hydrocarbons (PAHs) is a curious strategy to develop unusual optical/magnetic properties, such as near-IR absorption/emission, (anti)aromaticity, and spin polarization. The design of a relatively simple and efficient methodology to construct polycyclic systems with odd-membered rings is in high demand. We have been demonstrating the efficient synthesis of π-extended molecules with non-alternant frameworks using the interactions of Lewis acidic metals with sp-hybridized carbon-carbon bonds to promote tandem annulations. Among the rich chemistry of non-alternant hydrocarbons, pursuing the non-alternant isomers of pyrene, which has seven possible non-alternant isomers, has been the most significant project. Through enormous dedication by many pioneering works, six of the seven possible non-alternant isomers of pyrene have been isolated and characterized as stable aromatic molecules. However, the only unsynthesized isomer, bis-periazulene (cyclohepta[def]fluorene), had remained an uncharacterized hydrocarbon until 2022, despite many synthetic and theoretical investigations since it was first reported in 1955. This article describes the synthesis and characterization of bis-periazulene derivatives. The historical perspectives, synthetic approaches employing Lewis acid-mediated cyclization, fundamental properties, and comparisons to related systems are documented.
