Highly Regioselective\nModular Assembly of 3‑Phosphonyl\nPolysubstituted Pyridines through Radical Cascade Cyclization of 1,5-Enynes\nwith Phosphine Oxide by Photoinitiation

A series of 3-phosphonyl polysubstituted pyridine were\nfirst synthesized\nby photocatalysis, combining a phosphonyl radical cascade reaction,\nBoc deprotection, and aromatization. This strategy can avoid the difficulties\nof activating the C3–H bond on pyridine to synthesize 3-phosphonylpyridine\nunder mild conditions. Furthermore, by constructing different enynes,\nwe can achieve the metal-free modular synthesis of 3-phosphonyl polysubstituted\npyridine, which will be transferred into a new type of phosphine ligand.\nThis is of significance for organometallic catalysis. The regioselective\ncontrol and detailed reaction mechanism of the cascade reaction are\nexplained by DFT calculations.