Ring Opening of π‑Delocalized 2,5-Diphenyltellurophene by Chemical or Self-Sensitized Aerobic Photooxidation

We report the self-sensitized aerobic photooxidation of 2,5-diphenyltellurophene (PT). Irradiation of PT induces rapid intersystem crossing to the triplet excited state, which undergoes energy transfer to oxygen to form singlet oxygen. Reaction of the heterocycle with singlet oxygen was found to occur at the carbon framework through a 1,4-cycloaddition reaction rather than at tellurium to form the Te­(IV) telluroxide, as is commonly observed for other tellurium-containing heterocycles. This leads to oxidative ring-opening of the tellurophene to form (Z)-1,4-diphenylbut-2-ene-1,4-dione ((Z)-ED) with extrusion of TeO2. In attempts to study the formation of the Te­(IV) telluroxide and Te­(VI) tellurone by chemical oxidation with m-chloroperoxybenzoic acid (mCPBA), we discovered that PT undergoes an oxidative ring-opening reaction to the same ene-dione product after reaction with 4 equiv of mCPBA. Extrusion of TeO2 was also observed in this case. This ring-opening reaction was studied in detail by NMR spectroscopy, GC-MS, and density functional theory calculations.