Organoboron Cycloarenes: Boron-Carbon Annulated Macrocyclic π-Systems

Organic macrocycles, especially cycloarenes, are important for aromaticity theory, synthetic chemistry, and material science. However, organoboron macrocycles are highly challenging, due to low stability and inefficient cyclization reactions caused by the boron atom. In this study, we disclose boron-carbon-based annulated macrocyclic π-systems. Our synthetic strategy is to implement macrocyclization reactions on a boron-doped building block. Two organoboron cycloarenes, featuring the C₆₄B₄ and C₉₆B₆ π-skeletons, respectively, were synthesized from chlorinated diboron-bridged diphenylnaphthalene via the Suzuki-Miyaura coupling and Bi(OTf)₃-catalyzed cyclization reactions. The unique p-π* conjugation of the boron atoms endows them with broad absorption, red fluorescence, and electron deficiency, which are rarely observed in expanded cycloarenes. Moreover, the dianionic species of the C₆₄B₄ molecule displays a largely open-shell singlet diradical nature with minimal spin-spin correlation, demonstrating that long-range reduction reactions are favorable for such organoboron cycloarene systems. Thus, this study will open a new direction of boron-containing macrocycle chemistry and materials.