Palladium-Catalyzed Dehydrogenative Cross-Couplings of Benzazoleswith Azoles

摘要

Biaryl compounds play an important role in nature and many functional materials. Classical transition-metal-catalyzed methods for the synthesis of biaryls, such as Kumada, Negishi, Stille, Suzuki, and Hiyama-Denmark reactions, require functionalized arenes for the selective linkage of two arenes through a C-C bond. Recently, catalytic direct arylations have emerged which avoid the introduction of functional groups in at least one of the two coupling partners prior to cross-coupling by OH bond activation. The development of direct selective intermolecular heteroarylations of hetero-arenes appears particularly beneficial because prefunctional-izations of heteroarenes are often difficult. From the viewpoint of atom economy, twofold C-H bond activation is the ideal strategy for interconnecting two heteroarenes, and the groups of Fagnou and DeBoef showed independently that palladium(II) catalysis can be used for oxidative C-H/C-H cross-couplings of heteroarenes with carbocyclic arenes. The Pd(OAc)2-catalyzed oxidative cross-coupling of electron-deficient polyfluoroarenes with thiophenes, furans, and imidazoles was achieved by Zhang and co-workers by using Ag2CO3 in the presence of 1 equivalent of acetic acid. Hu, You, and co-workers reported on Pd(OAc)2-catalyzed copper-salt-activated C-H/C-H cross-couplings of xanthines, azoles, and electron-poor pyridine N-oxides with thiophenes and furans. However, to date, efficient C-H/C-H cross-couplings between very similar partners, such as different azoles, remains a challenge because of their tendency to undergo homocoupling. Hence, decarboxylative C-H arylations were employed by Zhang and Greaney to link differently substituted azoles in moderate to good yield, but homocoupling was not fully suppressed and remained a limiting factor.