What is "phosphole"

n. (context organic compound English) The unsaturated five-membered heterocycle having four carbon atoms, one phosphorus atom and two double bonds; an analogue of pyrrole

Phosphole is the organic compound with the chemical formula CHPH; it is the phosphorus analog of pyrrole. The term phosphole also refers to substituted derivatives of the parent heterocycle. These compounds are of theoretical interest but also serve as ligands for transition metals and as precursors to more complex organophosphorus compounds.

The first phosphole, pentaphenylphosphole, was reported in 1953, and the parent phosphole itself was first described in 1987. The usual route to phospholes is via the McCormack reaction, involving the addition of a 1,3- diene to a phosphonous chloride followed by dehydrohalogenation. Phenylphospholes can be prepared via zirconacyclopentadienes by reaction with PhPCl.

Unlike the related 5-membered heterocycles pyrrole, thiophene, and furan, the aromaticity of phospholes is diminished, reflecting the reluctance of phosphorus to delocalize its lone pair. For example, phospholes undergo Diels-Alder reactions with electrophilic alkynes.