Chichibabin‐Type Phosphonylation of 2‐(Hetero)aryl Pyridines: Selective Synthesis of 4‐Phosphinoyl Pyridines via an Activated N‐Benzylpyridinium Salt

Abstract A direct C−H phosphonylation of pyridine by N ‐benzylating activation is reported, which afforded 4‐pyridylphosphine oxides with high regioselectivity, without the employment of metal catalyst or expensive activator. By increasing the electrophilicity of pyridinium with electron‐withdrawing substituents on the N ‐benzyl group, the phosphonylation process could be realized at room temperature. Control experiments and NMR investigation confirmed the interaction between DBU and diphenylphosphine oxide which generated the true phosphorus nucleophile. Moreover, DFT calculations offered enough insight into an intermolecular cooperative dehydrogenation‐debenzylation mechanism, which helped to elucidate the origin of C4‐regioselectivity in this metal‐free Chichibabin‐type phosphonylation. magnified image