Functionalization of the 3-position of thiophene and benzo[b]thiophene moieties by palladium-catalyzed C-C bond forming reactions using diazonium salts

The palladium-catalyzed Matsuda-Heck and Suzuki-Miyaura cross-couplings of 2-methoxycarbonylthiophene-3-diazonium tetrafluoroborate 1 were performed to synthesize a series of thiophene derivatives functionalized in the β-position. Good to excellent yields of the cross-coupling products were obtained using palladium acetate [Pd(OAc)₂], a ligandless palladium catalyst, without a base, at room temperature, under aerobic conditions and with short reaction times. A diazotization and cross-coupling sequence can also be performed in a one-pot process avoiding the isolation of the thiophenediazonium salt derivative. In addition, 2-methoxycarbonylbenzo[b]thiophene-3-diazonium tetrafluoroborate 8 was also efficiently arylated by applying the same Suzuki-Miyaura optimized reaction conditions.