Phosphaalkenes (IUPAC name:alkylidenephosphanes) areorganophosphorus compounds withdouble bonds between carbon and phosphorus(III) with theformula R₂C=PR. In the compoundphosphorine one carbon atom in benzene is replaced by phosphorus. The reactivity of phosphaalkenes is often compared to that ofalkenes and not to that ofimines because theHOMO of phosphaalkenes is not the phosphoruslone pair (as in imines the amine lone pair) but the double bond. Therefore like alkenes, phosphaalkenes engage inWittig reactions,Peterson reactions,Cope rearrangements andDiels-Alder reactions.

The firstphosphaalkene discovered was aphosphabenzene, by Mërkl in 1969. The first localized phosphaalkene was reported in 1976 byGerd Becker^[1] as aketo-enol tautomerism akin aBrook rearrangement:

In the same yearHarold Kroto established spectroscopically thatthermolysis of Me₂PH generates CH₂=PMe. A general method for the synthesis of phosphaalkenes is by1,2-elimination of suitable precursors, initiated thermally or by base such asDBU,DABCO ortriethylamine:^[2]

The Becker method is used in the synthesis of the phosphorus pendant ofPoly(p-phenylene vinylene):^[3]

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The reduction or oxidation of phosphaalkenes can produceradical phosphorus ions.

• Organophosphanes
• Functional groups
• Unsaturated compounds