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.