Hydride Relay Exchange Mechanism for the Heterocyclic C–H Arylation of Benzofuran and Benzothiophene Catalyzed by Pd Complexes

The mechanism and regioselectivity of the heterocyclic C–H arylation of benzofuran and benzothiophene catalyzed by Pd(OAc)2 complexes were investigated using the density functional theory (DFT) method. The Pd(0)L2(PhI) complex (L = HOAc) is proposed to be the catalytic species. Compared to the traditional Heck-type mechanism, concerted metalation-deprotonation (CMD) mechanism, and electrophilic aromatic substitution (SEAr) mechanism for the C–H arylation, a new hydride relay exchange mechanism was proposed for the benzoheterocyclic C–H arylation catalyzed by Pd complexes, which consists of two redox processes between Pd(II) and Pd(0) species to complete the regioselective C–H activation. The calculated results indicate that the reaction along the hydride relay exchange mechanism is more favorable than those along other mechanisms, including the traditional Heck-type mechanism and the base-assisted anti-H elimination mechanism. This agrees well with the experimental results. Meanwhile, the origin for the regioselective C–H arylation was unveiled in which the α-C-H arylation products are major for the heterocyclic C–H arylation of benzofuran, but the β-C-H arylation products are major for that of benzothiophene. This study might provide a deep mechanistic understanding on the regioselective C–H activation and arylation of benzoheterocycle compounds catalyzed by transition-metal complexes.