Synthesis of Unsymmetrical 4,4′-Bipyridines through Base-Mediated Cross-Coupling between 4-Pyridyl Phosphonium Salts

Syntheses of unsymmetrical 4,4'-bipyridines were achieved through base-mediated cross-couplings of diverse 4-pyridyl phosphonium salts. This transition-metal- and oxidant-free protocol operates under mild conditions and tolerates a broad range of functional groups. Subsequent derivatization of the bipyridine products enabled the selective preparation of viologens and other valuable aromatic scaffolds. Mechanistic studies indicate the formation of an anionic pyridyl intermediate, which undergoes C-C bond formation through phosphorus ligand coupling or an SNAr mechanism.