Ring-expansion from tellurophenes to telluropyrans: inhibition of C–Te bond cleavages in transition metal-catalyzed reactions

Abstract

Tellurium-fused aromatic hydrocarbons have attracted extensive attention due to their unique properties as well as the synthetic challenges. However, the synthesis of organotellurium molecules falls much behind those of other chalcogen-containing compounds since the synthetic methods to build C–S and C–Se bonds always fail to extend the C–Te bonds. Herein, a PtCl₂-catalyzed ring-expansion reaction is attempted on tellurophene derivatives. However, tellurium-free vinylogous pentafulvalene analogues instead of telluropyrans are produced unexpectedly. To prevent the Pt–Te interactions and avoid the C–Te bond cleavage, bulky phosphine ligands are incorporated into the PtCl₂ catalyst. A ring-expansion cyclization towards neutral telluropyran derivatives from the corresponding tellurophene compounds has been thus developed. The mechanisms of both the ring-opening and the ring-expansion reactions are proposed and the pathways are verified by density functional theory calculations. Furthermore, the tolerance of the obtained telluropyran derivatives in metal-related reagents is further investigated and the conjugation can be facilely extended via various transition metal-catalyzed coupling reactions. Therefore, our work provides not only a synthetic methodology towards telluropyran-fused polycyclic aromatic hydrocarbons, but also an effective pathway to avoid the C–Te bond cleavages in transition metal-catalyzed coupling reactions.