DFT Study on Mechanism and Regioselectivity in Pd(II)-Catalyzed Dehydrogenative Heck Olefination of Selenophenes

The present study deals with the theoretical investigation of mechanistic and regiochemical aspects of the Pd(II)-catalyzed dehydrogenative Heck olefination of selenophenes. The detailed reaction mechanism was established, and the roles of the catalyst and regioselectivity were well rationalized. Our results clearly showed that the whole reaction involves a concerted metalation-deprotonation (CMD) process between the C² site of selenophene and Pd(OAc)₂ in the first catalytic cycle. Afterward, the olefin coordinates to the palladium, followed by a regioselective 1,2-migratory insertion to form a C─C bond. Then, β-hydride elimination would give the final 2-monoolefinated product. Subsequently, the second catalytic cycle generates the symmetrical 2,5-diolefinated selenophene product. Another key finding of this study is that selenophene and its monoolefinated product act as nucleophiles, while the catalyst behaves as an electrophile. A global reactivity index (GRI) analysis revealed that the nucleophilicity (N_k) of C², C³, C⁴, and C⁵ atoms in selenophene plays an important role in controlling the reaction selectivity at these sites. However, the distortion energies play a more important role in controlling the selectivity of reactions at the olefin C sites.