Regioselectivity in palladium-catalysed direct arylation of 5-membered ring heteroaromatics

Abstract

In recent years, palladium-catalysed arylation of heteroaromatics via C–H bond activation has become a popular method for generating carbon–carbon bonds. For this reaction, a wide variety of heteroaromatics such as (benzo)furans, (benzo)thiophenes, pyrroles, indoles, thiazoles, oxazoles, imidazoles, pyrazoles or triazoles can be employed. In most of these heterocycles, several reactive C–H bonds are present. If specific C–H bonds of such heteroarenes can be coupled with arenes, this becomes one of the most simple methods to access bi(hetero)arenes. In the past few years, several results using modified and improved catalysts and new reaction conditions have been reported permitting better control of the regioselectivity of such arylations. For example, initially only C2- or C5-arylated thiophenes were accessible via palladium-catalysed direct arylation, whereas now regioselective C3- or C4-arylations are possible when appropriate reaction conditions are used. In this review, the influence of the reactants, catalysts and reaction conditions on the regioselectivity of palladium-catalysed arylation of heteroaromatics is reported. The recent progress in the regioselectivity control now allows the synthesis of a wide variety of complex molecules using only a few steps, and will certainly provide simpler access to new heteroaryl derivatives in the next years.