Sustainable synthesis of quinolines and pyrroles enabled by chromium-catalyzed acceptorless dehydrogenative coupling using a bipyridine ligand

A straightforward and atom-economical method for the synthesis of quinolines and pyrroles has been reported. Using Earth-abundant, commercially available CrCl₂ salt as a catalyst and the inexpensive, bench-stable 6,6'-dimethyl-2,2'-dipyridyl as a ligand, this chromium-catalyzed acceptorless dehydrogenative coupling provides important heterocycles, quinolines and pyrroles, in synthetically useful yields with good functional group tolerance, releasing water and hydrogen gas as by-products. Notably, the described protocol was also found to be applicable to the sustainable synthesis of highly substituted fused polycyclic quinolines and pyrroles. Moreover, the synthetic value of this operationally simple protocol was demonstrated by gram-scale synthesis of quinolines.