Pd-Catalyzed C-H Activation Route to Aryl Indoles and Pyrroles

Significance

Phosphine-free, palladium-catalyzed direct C-2 arylation of N-unsubstituted indoles is reported. The C-H activation method thus eliminates the need for N-protection which is usually obligatory for cross-coupling methodologies (B. S. Lane, D. Sames Org. Lett. 2004, 6, 2897-2900). The reaction may also be carried out on 3-subsituted indoles. Interestingly, the reaction of iodo­aryl substrates with 3-unsubstituted indoles gave predominately 2-arylation while coupling of the corresponding bromo derivatives gave a 2:1 mixture of 2- and 3-arylated indoles. A catalytic cycle is suggested.