Visible‐Light‐Gated Rh‐Catalyzed Intermolecular [2 + 2 + 2] Cycloaddition of Alkynes Enabled by Coordination‐State Control

Visible‐light‐gated catalysis enables control over chemical reactivity, yet single transition‐metal system for intermolecular [2 + 2 + 2] cycloadditions of alkynes remain underdeveloped. Here, we report a visible‐light‐gated Rh‐catalyzed intermolecular [2 + 2 + 2] cycloaddition of internal and terminal alkynes using spiro‐fluorene–indenoindenyl (SFI)–Rh(cod) complexes (cod = 1,5‐cyclooctadiene). Under blue‐light irradiation, a range of alkynes undergo cyclotrimerization under mild conditions to afford substituted benzenes. In contrast, under dark conditions, reactivity is suppressed or diverted to polymerization, highlighting a divergence in reaction pathways. Mechanistic studies suggest that this divergence arises from coordination‐state‐dependent reactivity of the Rh catalyst. Photoinduced ligand dissociation generates coordinatively unsaturated Rh species capable of binding multiple alkynes, thereby enabling cyclotrimerization, whereas the dark‐state reactivity is consistent with predominance of mono‐alkyne‐coordinated species. DFT calculations indicate that cyclotrimerization proceeds on the ground‐state surface, supporting a mechanism in which light modulates catalyst speciation rather than altering the reaction pathway. This coordination‐state control regulates competing reactivity through kinetic control of pathway selection.