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A Non-alternant Aromatic Belt: Methylene-bridged Cycloparaphenylene Synthesized from Pillararene
preprintsubmitted on 03.06.2020, 05:11 and posted on 04.06.2020, 05:14 by Yuanming Li, Yasutomo Segawa, Akiko Yagi, Kenichiro Itami
The synthesis, structure, and properties of methylene-bridged cycloparaphenylene (CPP), a non-alternant aromatic belt, are described. This belt-shaped methylene-bridged CPP, in which each phenylene unit is tethered to its neighbors by methylene bridges, was constructed through six-fold intramolecular nickel-mediated aryl-aryl coupling of triflate-functionalized pillararene in 15% isolated yield. Compared with the analogous CPP, the methylene bridges co-planarize neighboring paraphenylene units and enhance the degree of p-conjugation, resulting in a significant decrease in energy gap. Moreover, the incorporation of small molecules in the defined pocket of methylene-bridged CPP makes it an attractive supramolecular architecture. Methylene-bridged 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.