Reductive couplings of 2-halopyridines without external ligand: phosphine-free nickel-catalyzed synthesis of symmetrical and unsymmetrical 2,2'-bipyridines.

An unexpectedly facile synthetic approach for symmetrical and unsymmetrical 2,2'-bipyridines through the Ni-catalyzed reductive couplings of 2-halopyridines was developed. The couplings were efficiently catalyzed by 5 mol % of NiCl2·6H2O without the use of external ligands. A variety of 2,2'-bipyridines including caerulomycin F have been efficiently synthesized.