A new, selective and functional group tolerant method for the synthesis of 1-aryl-1H-indoles and 9-aryl-9H-carbazoles

Several methods have been described for the N-arylation of 1H-indoles and 9H-carbazoles, but some problems limit their synthetic utility. In fact, the Ullmann-type reactions suffer from a limited substrate scope, moderate yields, have been preferentially conducted with aryl halides activated by electron-withdrawing groups and involve the use of expensive phosphine ligands. On the other hand, even the Cu-catalyzed N-arylation of 1H-indoles is not entirely satisfactory since the use in this methods of a ligand entails an increase of the cost of the reaction and problems in the isolation of the required chemically pure products. Furthermore, the experimental conditions of the Ullmann-type coupling and the Pd- or Cu-catalyzed N-arylations, that involve the use of a base, cause an intolerance of base sensitive groups such as the phenolic hydroxy group. Recently, in the course of our studies on the development of new and efficient protocols for the highly regioselective arylation of azoles we found that the CuOAc-mediated N-arylation of 1H-indole derivatives and 9H-carbazole with aryl iodides in DMA at 160 °C under base-free and ligandless conditions provides with complete N-selectivity the required N-arylazoles in moderate to good yields (eqs. 1 and 2).This inexpensive new version of the classical Ullmann reaction allows an unprecedented tolerance of functional groups both in the 1H-indoles and aryl iodides and works well with electron-rich and electron-poor aryl iodides. Moreover, the experimental conditions of this protocol facilitate the work-up of the reaction mixtures and the isolation of the required chemically pure N-aryl derivatives.