Regiochemistry of the microwave-assisted reaction between aromatic amines and alpha-bromoketones to yield substituted 1H-indoles

The scope and regioselectivity of the Bischler (or Bischler - Mohlau) reaction between aromatic amines and alpha-bromoketones has been studied by computational and experimental techniques. It has been found that in many cases the reaction yields are improved under microwave irradiation and working in the absence of solvent. When di- and trisubstituted amines are used as substrates the regioselectivity of the reaction is different to that obtained with the corresponding primary anilines. The reaction between benzene-1,2-diamine and alpha-bromoacetophenones under the same conditions yields 2-substituted quinoxalines instead of indoles. Finally, when pyridin-2-amines and pyrimidine-2-amines are allowed to react with the corresponding alpha-bromoacetophenones, the corresponding imidazo[1,2-a]pyridines and imidazo[1,2-a]pyrimidines are obtained, respectively.