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A Nonalternant Aromatic Belt: Methylene-Bridged [6]Cycloparaphenylene Synthesized from Pillar[6]arene

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posted on 09.07.2020, 20:30 by Yuanming Li, Yasutomo Segawa, Akiko Yagi, Kenichiro Itami
The synthesis, structure, and properties of methylene-bridged [6]­cycloparaphenylene ([6]­CPP), a nonalternant aromatic belt, are described. This belt-shaped methylene-bridged [6]­CPP, in which each phenylene unit is tethered to its neighbors by methylene bridges, was constructed through 6-fold intramolecular nickel-mediated aryl–aryl coupling of triflate-functionalized pillar[6]­arene in 18% isolated yield. As compared to the analogous [6]­CPP, the methylene bridges coplanarize neighboring paraphenylene units and enhance the degree of π-conjugation, which results in a significant decrease in energy gap. Moreover, the incorporation of small molecules in the defined pocket of methylene-bridged [6]­CPP makes it an attractive supramolecular architecture. Methylene-bridged [6]­CPP is characterized by high internal strain energy reaching 110.2 kcal mol–1, attributed to its restricted structure. This work not only exhibits an efficient strategy to construct a new family of aromatic belt, but also showcases their properties, which combine the merits of CPPs and pillararenes.