Arylation of indoles using cyclohexanones dually-catalyzed by niobic acid and palladium-on-carbons

3-Arylindoles were easily constructed from indoles and cyclohexanone derivatives using a combination of catalytic niobic acid-on-carbon (Nb 2 O 5 /C) and palladium-on-carbon (Pd/C) under heating conditions without any oxidants. The Lewis acidic Nb 2 O 5 /C promoted the nucleophilic addition of indoles to the cyclohexanones, and the subsequent dehydration and Pd/C-catalyzed dehydrogenation produced the 3-arylindoles. The additive 2,3-dimethyl-1,3-butadiene worked as a hydrogen acceptor to facilitate the dehydrogenation step. 3-Arylindoles could be effectively constructed from indoles and cyclohexanones in the presence of the dual catalysts of N 2 O 5 /C and Pd/C.