Phosphorine (IUPAC name: phosphinine) is a heavier element analog of pyridine, containing a phosphorus atom instead of an aza- moiety. It is also called phosphabenzene and belongs to the phosphaalkene class. It is a colorless liquid that is mainly of interest in research.
Phosphorine is an air-sensitive oil In contrast, silabenzene, a related heavy-element analogue of benzene, is not only air- and moisture-sensitive but also thermally unstable without extensive steric protection.
History
The first phosphorine to be isolated is 2,4,6-triphenylphosphorine. It was synthesized by Gottfried Märkl in 1966 by condensation of the corresponding pyrylium salt and phosphine or its equivalent ( P(CH<sub>2</sub>OH)<sub>3</sub> and P(SiMe<sub>3</sub>)<sub>3</sub>).
500px|Synthesis of Triphenylphosphabenzene
The (unsubstituted) parent phosphorine was reported by Arthur J. Ashe III in 1971 by the reaction of 1,4-dihydro-1,1 dibutylstannabenzene and phosphorus tribromide. Ring-opening approaches have been developed from phospholes.
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|center|thumb|620px|Bond lengths and angles of benzene, [[pyridine, phosphorine, arsabenzene, stibabenzene and bismabenzene]]
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Although phosphorine and pyridine are structurally similar, phosphorines are far less basic. The pK<sub>a</sub> of C<sub>5</sub>H<sub>5</sub>PH<sup>+</sup> and C<sub>5</sub>H<sub>5</sub>NH<sup>+</sup> are respectively −16.1 and +5.2. The P-oxides are extremely unstable, rapidly adding nucleophiles to a species tetracoordinate at phosphorus. Strongly backbonding Lewis acids (e.g. tungsten pentacarbonyl) can stabilize a dative bond from phosphorus.
Both electrophiles and strong, hard nucleophiles preferentially attack at phosphorus, but the ring aromaticity is sufficiently weak that the result is an addition reaction, and not aromatic substitution. Halophosphorines do undergo noble-metal- or zirconocene-catalyzed substitution, and λ<sup>5</sup>-phosphorines exhibit a much more traditional substitution chemistry.