Benzene Ring Expansion to Borepins via Borylene Insertion Enabled by Thorium-Arene Cooperativity.

Cleaving aromatic C-C bonds of unfunctionalized arenes remains a formidable challenge in synthetic chemistry. Here, we report room-temperature benzene ring expansion to a seven-membered borepin via formal borylene insertion. Treatment of an inverse-sandwich thorium-benzene complex with pinacolborane affords a structurally authenticated inverse-sandwich thorium-borepin complex under mild conditions. The transformation accommodates substituted and fused arenes, as well as B-alkyl and B-aryl boranes. Mechanistic studies reveal that thorium-arene cooperativity is essential for activating the inert benzene ring and enabling ring expansion. Furthermore, we demonstrate a synthetic cycle yielding free borepins from benzene. This work establishes a previously inaccessible mode of arene skeletal editing through direct boron incorporation, expanding the toolbox for aromatic ring manipulation and underscoring f-element-arene cooperativity as a key enabler of unconventional arene functionalization.