The McCormack reaction is a method for the synthesis of organophosphorus compounds. In this reaction, a 1,3-diene and a source of R<sub>2</sub>P<sup>+</sup> are combined to give phospholenium cation. The reaction is named after W. B. McCormack, a research chemist at duPont.
An illustrative reaction involves phenyldichlorophosphine and isoprene:
The reaction proceeds via a pericyclic [2+4]-process. The resulting derivatives can be hydrolyzed to give the phosphine oxide. Dehydrohalogenation gives the phosphole.