Palladium-Catalyzed C3 or C4 Direct Arylation of Heteroaromatic Compounds with Aryl Halides by C-H Bond Activation

In recent years, palladium-catalyzed direct C2 or C5 arylation of heteroaromatic compounds with aryl halides by C-H bond activation has become a popular method for generating carbon-carbon bonds. For this reaction, a wide variety of heteroaromatics, such as furans, thiophenes, pyrroles, thiazoles, oxazoles, imidazoles, pyrazoles, indoles, triazoles, or even pyridines, can be employed. C3 and C4 arylations of heteroaromatics by C-H bond activation have also been described. Such reactions initially attracted much less attention than the C2 or C5 arylations due to the lower reactivity of the C3 and C4 positions. However, in more recent years, several results from using modified and improved catalysts and reaction conditions have been reported, which permit C3 and C4 arylations in synthetically useful yields. Several intramolecular cyclizations of 2-substituted heterocycles have been described, with formation of a C-C bond on C3 resulting in the formation of five- to nine-membered rings incorporating pyrroles, indoles, thiophenes, furans, isoxazoles, or pyridines. Intermolecular C3 or C4 direct arylations are still quite rare for some heteroaromatics and are in several cases not highly regioselective. For such reactions, the best results have been obtained using pyrroles, thiophenes, or furans. For selected substrates, regioselective arylation at C3 or C4 of the heteroaromatic compounds took place under appropriate reaction conditions. Only a few examples of intermolecular couplings using oxazoles, thiazoles, imidazoles, isoxazoles, pyrazoles, triazoles, or pyridines have been reported. For most of these reactions, aryl iodides or bromides have been used as coupling partners, although a few examples with aryl chlorides are also known. This method allows the synthesis of complex molecules in only a few steps, and will provide access to a very wide variety of new heteroaryl derivatives in the next years.