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

-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.