Zigzag-Type Molecular Belts: Synthesis, Structure, and Properties

ConspectusSince Heilbronner proposed cyclacenes as hypothetical molecules in 1954, zigzag hydrocarbon belts and their heteroatom-doped analogs have captivated chemists and materials scientists with their aesthetically appealing structures, intriguing properties, and potential applications. Except for the work reported in the late 1980s and early 1990s by Stoddart, who employed iterative Diels-Alder reactions to construct such belts, the field has remained dormant for decades. This stagnation is primarily due to the lack of synthetic methods. At the beginning of this millennium, we created heteracalix[n]aromatics, macrocyclic hosts that have since been developed into privileged macrocyclic hosts in supramolecular chemistry due to their ease of synthesis and versatile applications. One of the salient structural features of heteracalix[n]aromatics and related calix[n]arenes and resorcin[n]arenes is their ability to adopt distinct conformations. Notably, these conformations can be tuned by varying the ring sizes, substituents, and synthetic conditions. We envisioned that the preorganized macrocycles with aromatic rings in close proximity could provide ideal scaffolds, and chemical bonding to stitch each fjord encompassed by adjacent aromatic rings would lead to zigzag-type molecular belts. We initiated our study with the aim of synthesizing strained belt[8]arenes. Leveraging the cone-conformational resorcin[4]arenes and facile transformations of phenolic hydroxyl groups in fjords, we successfully prepared H₈-belt[8]arenes by stitching fjords through quadruple intramolecular Friedel-Crafts alkylation. Subsequent oxidative aromatization with DDQ produced a belt[8]arene-(DDQ)₄ adduct. Upon laser irradiation, the adduct underwent retro-Diels-Alder reactions, enabling for the first time the formation of belt[8]arene, a fully conjugated zigzag hydrocarbon belt. The fjord-stitching method has since evolved into a versatile and powerful strategy. A diversity of hydrocarbon belts having different sizes, shapes, and functional groups have been synthesized through multiple fjord-stitching reactions such as Friedel-Crafts alkylation and acylation and olefin metathesis. To bridge fjords with oxygen and nitrogen atoms through S_NAr reactions and transition-metal-mediated carbon-heteroatom bond coupling, we have achieved the construction of heteroatom-doped hydrocarbon belts. In addition, C_Ar-C_Ar homocoupling reactions to seal all fjords of azacalix[n](3,5-dibromopyridine)s furnished globally aromatic zigzag-type belts consisting of pyrrole and pyridine subunits. Depending on the combinations of constitutional hexagonal and nonhexagonal carbocyclic or heterocyclic subrings, zigzag-type belts adopt unique and appealing structures to give cavities that range from distinct prisms to truncated cones and nearly spherical frustums. Governed by the steric and electronic effects, zigzag-type belts exhibit intriguing photophysical and electrochemical properties and can form complexes with diverse guests with outstanding selectivity. In this Account, we summarize our endeavors to explore the chemistry of zigzag-type molecular belts with a focus on the synthesis of diverse belt molecules, their unique structural features, and their fascinating molecular recognition properties. We hope that this Account will ignite intensive research interest in this revitalized field, propelling zigzag-type molecular belts toward practical applications.