Organophosphorus compounds are chemical compounds containing carbon phosphorus bonds. Organophosphorus chemistry is the corresponding science exploring the properties and reactivity of organophosphorus compounds. Phosphorus shares group 14 in the periodic table with nitrogen and phosphorus compounds and nitrogen compounds have much in common .

Phosphorus can adopt oxidation states -3, -1, 1, 3 and 5. In chemical literature very often compounds with +3 or -3 oxidation state are grouped together as having a (III) oxidation state regardless of sign. In an official and more descriptive nomenclature phosphorus compounds are identified by their coordination number δ and their valency λ. In this system a phosphine is a δ³λ³ compound.

Phosphines

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The parent compound of the phosphines is phosphine (PH₃). Phosphines have oxidation state -3 (δ³λ³) and are the phosphorus analogues of simple amines. An often used organic phosphine is triphenylphosphine. Like amines, phosphines have a trigonal pyramidal molecular geometry although with larger angles. The C-P-C bond angle is 98.6° for trimethylphosphine increasing to 109.7 when the methyl groups are replaced by tert-butyl groups.

The barrier to inversion is also much higher than in amines for a process like nitrogen inversion to occur and therefore phosphines with three different substituents can display optical isomerism.

The basicity of phospines is less than that of corresponding amines, for instance phosphine itself has a pKa of -14 compared to 9.21 for ammonia; trimethylphosphine has a pKa of 8.65 compared to that of 9.76 of trimethylamine; and triphenylphosphine (pKa 11.2) is less basic than triphenylamine (pKa 19).

Amines and phosphines both have a lone pair of electrons but with a difference. Whereas the lone pair in an amine shares its electrons at every opportunity for delocalization for instance in pyridine, the phosphorus atom in a similar configuration will not. For this reason, the phosphorus equivalent of pyrrole called phosphole is not at all aromatic.

The reactivity of phosphines match that of amines with regard to nucleophilicity in the formation of Phosphonium salts with the general structure PR₄⁺X^-. This property is used in the Appel reaction converting alcohols to alkyl halides. A difference in reactivity with amines is the ease of oxidation of phosphines to phosphine oxides.

Synthetic produces for phosphines are:

• Nucleophilic displacement of phosphorous halides with organometallic reagents such as Grignard reagents.
• Nucleophilic displacement of metal phosphides, generated by reaction of potassium metal with phosphine as in sodium amide with alkyl halides.
• Nucleophilic addition of phosphine with alkenes in presence of a strong base (often KOH in DMSO), Markovnikov's rules apply . Phosphine can be prepared in situ from red phosphorus and potassium hydroxide. Primary (RPH₂) and secondary phospines (RRPH) do not require a base with electron-deficient alkenes such as acrylonitriles.

• Radical addition of phosphines to alkenes with AIBN or organic peroxides to give anti-Markovnikov adducts.
• Nucleophilic addition of phosphine and phosphines to alkynes in presence of base. Secondary phosphines react with electron-deficient alkynes such as phenylcyanoacetylene without base.
• Organic reduction of phosphine oxides for instance with chlorosilane.

The main reaction types of phosphines are:

• as nucleophiles for instance with alkyl halides to phosphonium salts.
• as reducing agents:

Phosphines are reducing agents in the Staudinger reduction converting azides to amines and in the Mitsunobu reaction converting alcohols into esters. In these processes the phosphine is oxidized to the phosphine oxide. Phosphines have also been found to reduce activated carbonyl groups for instance the reduction of an α-keto ester to an α-hydroxy ester in scheme 2 . In the proposed reaction mechanism the first proton is on loan from the methyl group in trimethylphosphine (triphenylphosphine does not react).

When modified with suitable substituents as in certain diazaphospholenes (scheme 3) the polarity of the P-H bond can actually be inverted (see: umpolung) and the resulting phosphine hydride can reduce a carbonyl group as in the example of benzophenone in yet another way .

• Multidentate phosphines such as BINAP are important ligands in organometallic chemistry.

Phosphine oxides

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Phosphine oxides (designation δ³λ³) have the general structure P=OR₃ with formal oxidation state -1. Phospines form hydrogen bonds and many phosphines are therefore soluble in water. The P=O bond is very polar with a dipole moment of 4.51 D for triphenylphosphine.

The nature of the phosphorus to oxygen double bond is a matter of debate. Pentavalent phosphorus like nitrogen is not compatible with the octet rule. In older literature the bond is represented as a dative bond just like a amine oxide. The prevailing view is that of a full double bond with back bonding taking place between a filled oxygen electron pair and an empty phosphorus d-orbital (lacking in nitrogen). problem is: the P=O bond does not react as any C=C double bond as addition reactions are absent and involvement of a phosphorus d-orbital in bonding is not supported in silico. Alternative theories favor an ionic bond P⁺-O^- which on its own strength should explain the short bond length.

Phospines are easily oxidized to phosphine oxides as examplified by the template directed synthesis of a phospha crown, the phosphorus analogue of an aza crown where it is not possible to isolate the phosphine itself .

Phosphonates

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Phosphonates have the general structure R-P=0(OR)₂. In the Horner-Wadsworth-Emmons reaction and the Seyferth-Gilbert homologation phosphonates are used as stabilized carbanions in reactions with carbonyl compounds. Phosphonates have many technical applications and bisphosphonates are a class of drugs.

Phosphite & phosphate esters

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Phosphite esters or phosphites have the general structure P(OR)₃ with oxidation state +3. Phosphites are employed in the Perkow reaction and the Arbusov reaction. Phosphate esters with the general structure P=O(OR)₃ and oxidation state +5 are of great technological importance as flame retardant agents and plasticizers. Lacking a P-C bond, these compounds are technically not organophosphorus compounds.

Phosphoranes

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Phosphoranes have a -5 oxidation state (δ⁵λ⁵) with parent compound the non-stable phosphoran PH₅ or λ5-Phosphan (lambda5phosphan). Phosphorus ylides are unsaturated phosphoranes used in the Wittig reaction.

Phosphorus multiple bonds

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Many compounds exist with carbon phosphorus multiple bonds (P=C) as phosphaalkenes (R₂C::PR) and phosphaalkynes (RC:::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. The reason is that 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 Wittig reactions, Peterson reactions, Cope rearrangements and Diels-Alder reactions.

The first phosphaalkene was synthesis in 1974 by Becker as a Keto-enol tautomerism akin a Brook rearrangement:

with R = methyl or phenyl and tms representing trimethylsilyl.

In the same year Harold Kroto established spectroscopically that thermolysis of Me₂PH yielded CH₂=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:

The Becker method is used in the synthesis of the phosphorus pendant of Poly(p-phenylene vinylene) Phosphorus Copies of PPV: -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) :

Diphosphenes are compounds containing P=P phosphorous double bonds.

See also

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• Compounds of carbon with period 3 elements: organosilicon compounds, organophosphorus compounds, organosulfur compounds,

External links

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• organophosphorus chemistry @ http://users.ox.ac.uk Link

References

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1. Phosphorus. The Carbon Copy Keith B. Dillon, Francois Mathey, John F. Nixon 1997 John Wiley & Sons ISBN 0471973602
2. A Guide to Organophosphorus Chemistry Louis D. Quin 2000 John Wiley & Sons ISBN 0-471-31824-8
3. Nucleophilic and free-radical additions of phosphines and phosphine chalcogenides to alkenes and alkynes Svetlana N. Arbuzova, Nina K. Gusarova, Boris A. Trofimov Arkivoc AL-1672FP pp 29-37 2006 Article
4. Reduction of activated carbonyl groups by alkyl phosphines: formation of -hydroxy esters and ketones Wen Zhang and Min Shi Chemical Communications, 2006, 1218 - 1220 Abstract
5. P-Hydrogen-Substituted 1,3,2-Diazaphospholenes: Molecular Hydrides Sebastian Burck, Dietrich Gudat, Martin Nieger, and Wolf-Walther Du Mont J. Am. Chem. Soc.; 2006; 128(12) pp 3946 - 3955; Abstract
6. Template Synthesis of 1,4,7-Triphosphacyclononanes Peter G. Edwards, Robert Haigh, Dongmei Li, and Paul D. Newman J. Am. Chem. Soc.; 2006; 128(11) pp 3818 - 3830; Abstract
7. In step 1 diphosphinoethane coordinates to a ferrocene containing additional carbon monoxide ligands and a acetonitrile ligand. The next step is a hydrophosphination with trivinylphosphine followed by alkylation with ethyl bromide and hydrogenation with hydrogen over palladium on carbon. In the final step the iron template is removed by bromine but oxidation of the phosphine groups is unavoidable.

phosphorus compounds | organic compounds