{{confuse|diphosphenes}} [[File:Mesityldiphenylmethylenephosphine-from-xtal-1986-3D-balls.png|thumb|right|220px|Ball-and-stick model of mesityldiphenylmethylenephosphine, a phosphaalkene]]
'''Phosphaalkenes''' (IUPAC name: '''alkylidenephosphanes''') are organophosphorus compounds with double bonds between carbon and phosphorus(III) with the formula R<sub>2</sub>C=PR. In the compound phosphorine one carbon atom in benzene is replaced by phosphorus. The reactivity of phosphaalkenes is often compared to that of alkenes and not to that of imines because the HOMO of phosphaalkenes is not the phosphorus lone pair (as in imines the amine lone pair) but the double bond. Therefore like alkenes, phosphaalkenes engage in Diels-Alder reactions and Wittig and Cope rearrangements.{{cn|date=July 2025}}
Phosphaalkenes appear in academic research as polymerization monomers and as π-accepting ligands (similar to a strained alkene).<ref name=PWH>{{cite journal|pmc=3738942|journal=Angewandte Chemie|edition=International|date=May 7, 2013|volume=52|issue=25|pp=6484–6487|doi=10.1002/anie.201301469|title=Mechanism of the phospha-Wittig–Horner reaction|first1=Anna I.|last1=Arkhypchuk|first2=Yurii V.|last2=Svyaschenko|first3=Andreas|last3=Orthaber|first4=Sascha|last4=Ott}}</ref>
A special case of the phosphaalkenes are the phosphaallenes, in which the carbon atom is attached to a second double bond, to another carbon atom.
The first phosphaalkene discovered was a phosphabenzene, by Mërkl in 1969. The first localized phosphaalkene was reported in 1976 by Gerd Becker<ref>''Bildung und Eigenschaften von Acylphosphinen. I. Monosubstitutionsreaktionen an substituierten Disilylphosphinen mit Pivaloylchlorid'' Gerd Becker, Zeitschrift für anorganische und allgemeine Chemie '''1976''', 423, pp 242 - 254</ref> as a keto-enol tautomerism akin a Brook rearrangement: ::[[Image:Becker Reaction.svg|300px|Becker reaction to phosphaalkenes (R = methyl or phenyl)]] Becker's method continues to see use in, e.g., a phosphorus analogue to poly(''para''-phenylene vinylene):<ref>''Phosphorus Copies of PPV: π-Conjugated Polymers and Molecules Composed of Alternating Phenylene and Phosphaalkene Moieties'' Vincent A. Wright, Brian O. Patrick, Celine Schneider, and Derek P. Gates J. Am. Chem. Soc.; '''2006'''; 128(27) pp 8836 - 8844; (Article) {{doi|10.1021/ja060816l}}</ref> ::800px|Poly(p-phenylenephosphaalkene).
In 1976, Harold Kroto also established spectroscopically that thermolysis of Me<sub>2</sub>PH generates CH<sub>2</sub>=PMe. A general method for the synthesis of phosphaalkenes is by 1,2-elimination of suitable precursors, initiated thermally or by base such as DBU, DABCO or triethylamine:<ref>''Synthesis of mesityldiphenylmethylenephosphine: a stable compound with a localized phosphorus:carbon bond'' T. C. Klebach, R. Lourens, F. Bickelhaupt J. Am. Chem. Soc., '''1978''', 100 (15), pp 4886–4888 {{doi|10.1021/ja00483a041}}</ref> ::400px|Phosphaalkene general method
A more modern synthesis is the ''phospha''-Wittig-Horner reaction, which combines a carbonyl compound with a phosphoryl organophosphide (diphosphorus ylide).<ref name=PWH/>
The reduction or oxidation of phosphaalkenes can produce radical phosphorus ions.
==References== <references/>
Category:Organophosphanes Category:Functional groups Category:Unsaturated compounds