Indole synthesis: palladium-catalyzed C–H bond amination via reduction of nitroalkenes with carbon monoxide

Nitroalkenes have been called ‘chemical chameleons’ due to their versatility in numerous synthetic transformations. Herein, we describe the first transition metal-catalyzed transformation of conjugated nitroalkenes into indoles. Under mild reaction conditions (1 atm carbon monoxide, 110 °C), palladium catalyzes the reductive cyclization of nitroalkenes to form a putative nitrosoalkene intermediate, which then rearranges to provide 3-arylindoles in high yields. Notably, this novel C–H bond amination takes advantage of carbon monoxide as an inexpensive stoichiometric reductant and produces carbon dioxide as the major byproduct.