Palladium-catalyzed synthesis of indoles by reductive N-heteroannulation of 2-nitrostyrenes

A palladium-phosphine catalyzed reductive N-heteroannulation of 2-nitrostyrenes, in the presence of carbon monoxide, producing indoles has been developed. Indoles were obtained, in moderate to excellent yield, from substituted 2-nitrostyrenes having either electron-withdrawing (NO2 and CO2Me) or electron-donating (Br, OH, Me, OMe, and OTf) substituents on the aromatic ring. Best results were obtained using palladium diacetate (6 mol %) together with triphenylphosphine (24 mol %) as the catalytic system, under 4 atm of carbon monoxide in acetonitrile at 70 degrees C. Other palladium(II) and palladium(0) complexes also catalyze the reaction.