Palladium(II) ion promoted hydroamination of di(phenylethynyl)phenylphosphine and aniline: a facile synthesis of a six-membered P–N heterocycle

The six-membered aromatic heterocycle 1,4-2H-1,2,4,6-tetraphenyl-1,4-azaphosphabenzene was generated efficiently via a [PdCl2(NCMe)2] promoted hydroamination between PhP(CCPh)2 and aniline. The heterocyclization reaction adopts a stepwise mechanism during which the unstable intermediate imino complex cis-dichloro{1,2-diphenyl-3-phenyl(phenylethynyl)phosphino-1-aza-1-propene}palladium(II) is formed. The end product of the cyclization reaction is a palladium complex in which the six-membered aromatic P–N heterocyclic ring is coordinated to the metal center as a typical monodentate ligand via phosphorus. The azaphospha heterocycle can subsequently be liberated from the palladium reaction activator as an air-stable yellow solid by treating the palladium complex with KCN. In contrast to the intermediate imino-complex, where the CN is readily hydrolyzed by HCl, the aromatic heterocycle is stable in strongly acidic environments.