Phosphorine

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. Phosphorine is a planar aromatic compound with 88% of the aromaticity of that of benzene. The P-C bond length is 173 pm and the C-C bond lengths center around 140 pm and show little variation.

Phosphorine is generally stable against air and moisture and can be handled without special inert atmosphere equipment, so it is different from silabenzene, which is usually not only air- and moisture-sensitive but also thermally unstable without extensive steric protection. This stability of phosphorine comes from the close electronegativities of phosphorus (2.1) and carbon (2.5). The physical and chemical properties of metal complexes bearing phosphorine as a ligand as well as phosphorine itself have been studied extensively.

History

The first phosphorine is 2,4,6-triphenylphoshorine, which was synthesized by Gottfried Märkl in 1966 from the corresponding pyrylium salt and phosphorus sources, such as phosphine, P(CH₂OH)₃, and P(SiMe₃)₃.^[2]

Unsubstituted phosphorine, which was reported by Arthur J. Ashe III in 1971,^[3]^[4] is a distillable liquid that is somewhat air-sensitive but stable against hydrolysis. In 1990s, François Mathey developed a methodology for the synthesis of functionalized phosphorines using transition metal mediated reactions including palladium- or nickel-catalyzed coupling reactions.

More recently, Mathey and Grundy developed a new ring-opening approach, synthesising a six-membered ring phosphabenzene in a one-pot method from a five-membered phosphole ring.^[5]

Properties and reactions

Although phosphorine is a heavier element analog of pyridine, the electronic structure is very different from that of pyridine. The lone pair of pyridine is its HOMO, so pyridine has good σ-donating ability. The HOMO and LUMO of phosphorine, on the other hand, are its π and π^* orbitals, respectively, and the lone pair is located at the lower energy level. Thus phosphinines are much better π-acceptor ligands, but less good σ-donors compared to pyridines. Phosphorine with a 0.55 positive charge on phosphorus and pyridine with a -0.67 negative charge on nitrogen also show different reactivities against nucleophiles. Pyridine reacts with nucleophiles at the C-2 position due to the higher electronegativity of the nitrogen atom. Nucleophiles, however, attack the phosphorus atom of phosphorine, producing λ⁴-phosphorine anion, which reacts with electrophiles to give λ⁵-phosphorine. Lately it was reported that anionic λ⁴-phosphorine coordinates to transition metals as a π-donor ligand.

Phosphorine undergoes electrophilic substitution reactions like ordinary aromatic compounds: bromination, acylation, and so on.

References

↑ "Phosphate-Binding Proteolipid - Compound Summary". The PubChem Project. USA: National Center of Biotechnology Information.
↑ G. Märkl, 2,4,6-Triphenylphosphabenzol in Angewandte Chemie 78, 907–908 (1966)
↑ Ashe, A. J. (1971). "Phosphabenzene and Arsabenzene". Journal of the American Chemical Society 93 (13): 3293–3295. doi:10.1021/ja00742a038.
↑ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 544. ISBN 0-08-037941-9.
↑ Grundy, J.; Mathey, F. (2005). "One-Pot Conversion of Phospholide Ions into β-Functional Phosphinines". Angewandte Chemie International Edition 44 (7): 1082–1084. doi:10.1002/anie.200462020.

Quin, L. D. (2000). A Guide to Organophosphorus Chemistry. Wiley-Interscience. ISBN 0-471-31824-8.

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Phosphorine

History

Properties and reactions

See also

References