Visible-Light-Induced Bond Homolysis in Titanacyclopentadienes for the Catalytic Cyclodimerization of Internal Alkynes.

Ligand-to-metal charge transfer (LMCT) processes offer significant potential in photochemical synthesis but remain comparatively underdeveloped, relative to metal-to-ligand charge transfer (MLCT) pathways. Titanium organyls are particularly promising in this context, enabling the direct generation of carbon-centered radicals. However, reports on their photochemistry have remained scarce. Herein, we present the visible-light-induced homolytic cleavage of a Ti-C bond in a titanacyclopentadiene complex supported by a bulky pyridine-diamido ligand. The resulting biradicaloid undergoes a selective skeletal rearrangement via a H atom shift to form a titanacyclopentene. This stoichiometric transformation was translated into a novel catalytic cyclodimerization of internal alkynes. With 2-butyne, the unusual dimer 1,2,3-trimethyl-4-methylenecyclobutene, a strained and readily functionalizable small ring, was obtained quantitatively. Mechanistic investigations of this previously unreported catalytic transformation including quantum chemical calculations, single-turnover experiments, quantum yield determination by actinometry, and reaction kinetics reveal key features of the underlying reactivity and indicate that visible-light-induced Ti-C bond homolysis is the rate-determining step. Overall, our findings demonstrate that the combination of a sterically demanding ligand, which suppresses alkyne cyclotrimerization, with visible-light-induced bond homolysis offers a new entry point into catalytic alkyne oligomerization chemistry.