Fulvalene- and [3]Cumulene-Bridged Conjugated Organoboranes: Pentagon Annulation and Olefination Enabled by One-Step Alkyne Cyclization

Abstract Fulvalenes and cumulene analogues are hydrocarbons bearing two pentagonal rings linked by one or more consecutive double bonds. Their unique topological structures and physical properties have drawn great research interest. However, the synthesis of such hydrocarbons remains very challenging, owing to their inherent instability and the difficulty in the simultaneous construction of five-membered rings and double bonds. Herein, we disclose an ICl-mediated one-step alkyne-cyclization reaction that achieves concurrent pentagon annulation and olefination. Relying on an unexpected yet critical 5-endo cyclization pathway, we successfully synthesized two types of organoboranes featuring two boron-doped polycyclic π-skeletons tethered by either a fulvalene bridge or a pentagon-capped [3]cumulene motif. In comparison to the all-hexagon helicene generated via 6-exo cyclization, these two molecules exhibit distinct physical properties, such as broad light absorption, low-lying LUMO energy levels, high electron affinity, and short-lifetime excited states. Studies have demonstrated that the fulvalene substructure or [3]cumulene analogue, together with the boron centers, dominates their optoelectronic characteristics. Moreover, the chemical reduction of the fulvalene-bridged organoborane affords a radical anion in which the unpaired electron is fully delocalized over the fulvalene unit. This observation verifies that the fulvalene moiety possesses a superior electron-accepting ability compared with boron atoms. The established synthetic protocol and elucidated structure–property relationships lay a solid foundation for interdisciplinary studies spanning organoboron, fulvalene, and cumulene chemistry.