From closed-shell edge-extended kekulenes to open-shell carbonylated cycloarene diradicaloid

Abstract

The precise synthesis of cycloarenes remains a challenging topic in both organic chemistry and materials science due to their unique fully fused macrocyclic π-conjugated structure. Herein, a series of alkoxyl-...

Generalized kekulenes and clarenes as novel families of cycloarenes: structures, stability, and spectroscopic properties

“…Edge-extended kekulene derivatives with 14 and 16 rings have been synthesized by Lu and co-workers, 24 which were constructed by extending a pair of opposite sides of the kekulene molecule. In addition, Wu also synthesized similar molecules with alternating side lengths.…”

“…1(a) illustrates the concept of kekulenes, exemplified by one of the experimentally synthesized kekulenes, [2,2,4,2,2,4]. 24 The numbers in brackets indicate the side lengths, meaning the effective number of benzene rings along each side of the superhexagon for the given kekulene structure. Note that each of these numbers is one fewer than the actual number of benzene rings.…”

“…Note that some lowest-energy kekulenes predicted by this work have been successfully synthesized in the laboratory with the number of rings from 10 to 21. 1,15–17,23,24,80 In addition, some kekulenes with retalively high energy have also been synthesized, such as, [14], [16], and [18]kekulenes. 8,22–24…”

“…20 X-ray crystallographic analysis and NMR measurements in high-boiling-point solvents revealed a local aromatic character that π-electrons are localized on individual benzene-type rings for the kekulene, which was further confirmed by ultra-high-resolution atomic force microscopy (AFM) 15 and modern on-surface synthesis. 21 Although some new synthetic methods have been explored over the years, 4,8,22 only a few cycloarenes were reported, 5 such as edge-extended kekulenes, 23,24 septulenes, 22 and octulenes, 4,8 due to the challenging synthesis of this class of cata-condensed aromatic macrocyclic systems. 5,15 The lack of molecular diversity greatly limits the further development of cycloarenes and related analogs.…”

“…However, such a nomenclature applies only to structures with alternating side lengths, thus covering only a specific type of the extended kekulenes. In addition, other edge-extended homologues of kekulenes were designed and synthesized by Lu and co-workers 24 through a similar method in 2023. In light of the successfully synthesized extended kekulenes, we hope to further extend kekulenes to a more generalized structure, such that the lengths of the six sides can be any positive integer, and therefore fully enrich the structural diversity of these types of molecules.…”