Direct access to substituted benzo[b]carbazoles through cascade annulation of 2-vinylbenzaldehydes with indoles.

A highly efficient palladium-catalyzed cascade annulation of 2-vinylbenzaldehydes with indoles has been achieved to afford 6-(3-indolyl)benzo[b]carbazoles under mild conditions in good yield and with excellent regioselectivity. Mechanistic investigations reveal that the reaction proceeds via double addition of indoles, unexpected intramolecular 1,4-aryl and 1,2-hydrogen migrations, and oxidative aromatization.