phosphorine

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(CH₂OH)₃ and P(SiMe₃)₃).G. Märkl, 2,4,6-Triphenylphosphabenzol in [http://onlinelibrary.wiley.com/doi/10.1002/ange.19660781817/abstract Angewandte Chemie 78, 907–908 (1966)]

file:Synthesis Triphenylphosphabenzene.svg

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.{{ cite journal | last1 = Ashe | first1 = A. J. | title = Phosphabenzene and Arsabenzene | journal = Journal of the American Chemical Society | year = 1971 | volume = 93 | issue = 13 | pages = 3293–3295 | doi = 10.1021/ja00742a038 }}{{Greenwood&Earnshaw2nd | page = 544}} Ring-opening approaches have been developed from phospholes.

Structural studies by electron diffraction reveal that phosphorine is a planar aromatic compound with 88% of aromaticity of that of benzene. Potentially relevant to its high aromaticity are the well matched electronegativities of phosphorus (2.1) and carbon (2.5). The P–C bond length is 173 pm and the C–C bond lengths center around 140 pm and show little variation.László Nyulászi "Aromaticity of Phosphorus Heterocycles" Chem. Rev., 2001, volume 101, pp 1229–1246. {{doi|10.1021/cr990321x}}

Although phosphorine and pyridine are structurally similar, phosphorines are far less basic. The pK_a of C₅H₅PH⁺ and C₅H₅NH⁺ 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.Mathey, François (2011). "Phosphorus Heterocycles" in Modern Heterocyclic Chemistry, 1st ed., edited by Álvarez-Builla, Julio; José Vaquero, Juan; and Barluenga, José. Weinheim: Wiley-VCH. §23.3. {{doi|10.1002/9783527637737.ch23}}.

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. Thus for example methyllithium adds to phosphorus in phosphorine whereas it adds to the 2-position of pyridine.Ashe III, Arthur J.; Smith, Timothy W. "The reaction of phosphabenzene, arsabenzene and stibabenzene with methyllithium." Tetrahedron Letters 1977, volume 18, pp. 407–410. {{doi|10.1016/S0040-4039(01)92651-6}} Halophosphorines do undergo noble-metal- or zirconocene-catalyzed substitution, and λ⁵-phosphorines exhibit a much more traditional substitution chemistry.

Unlike arsabenzene, phosphorine rarely participates in Diels-Alder-type cycloadditions; when it does, the coupling partner must be an extremely electron-poor alkyne. Phosphorine complexes are tolerable Diels-Alder reactants.

Coordination complexes bearing phosphorine as a ligand are known. Phosphorines can bind to metals through phosphorus center. Complexes of the diphospha analogue of 2,2′-bipyridine are known. Phosphorines also form pi-complexes, illustrated by V(η⁶-C₅H₅P)₂.

• Six-membered aromatic rings with one carbon replaced by an element from another group: borabenzene, silabenzene, germabenzene, stannabenzene, pyridine, phosphorine, arsabenzene, stibabenzene, bismabenzene, pyrylium, thiopyrylium, selenopyrylium, telluropyrylium

{{Reflist}}

• {{cite book | title = A Guide to Organophosphorus Chemistry | author = Quin, L. D. | publisher = Wiley-Interscience | year = 2000 | isbn = 978-0-471-31824-8}}

Category:Phosphorus heterocycles

Category:Six-membered rings

Category:Substances discovered in the 1970s