Phosphine-free palladium-catalysed direct 5-arylation of imidazole derivatives at low catalyst loading

The regioselective 5-arylation of imidazole derivatives with aryl bromides using a low loading of a phosphine-free palladium catalyst gives a simple and economic access to the corresponding 5-arylimidazoles. The choice of the base and of the solvent was found to be crucial to form these products in high yields. Using KOAc as the base, DMAc as the solvent and only 0.5-0.01 mol % Pd(OAc)₂ as the catalyst, the target products were obtained in moderate to good yields with a wide variety of aryl bromides. Substituents such as fluoro, trifluoromethyl, formyl, acetyl, propionyl, ester or nitrile on the aryl bromide are tolerated. Sterically congested aryl bromides or heteroaryl bromides can also be employed. The nature of the substituents on the imidazole derivative has an important influence on the yields.