Triphenylphosphine

Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C₆H₅)₃ and often abbreviated to P Ph₃ or Ph₃P. It is widely used in the synthesis of organic and organometallic compounds. PPh₃ exists as relatively air stable, colorless crystals at room temperature. It dissolves in non-polar organic solvents such as benzene and diethyl ether.

Preparation and structure

Triphenylphosphine can be prepared in the laboratory by treatment of phosphorus trichloride with phenylmagnesium bromide or phenyllithium. The industrial synthesis involves the reaction between phosphorus trichloride, chlorobenzene, and sodium:

:PCl₃ + 3 PhCl + 6 Na → PPh₃ + 6 NaCl

Triphenylphosphine crystallizes in triclinic and monoclinic modification. In both cases, the molecule adopts a pyramidal structure with propeller-like arrangement of the three phenyl groups.

Principal reactions with chalcogens, halogens, and acids

Oxidation

Triphenylphosphine undergoes slow oxidation by air to give triphenylphosphine oxide, Ph₃PO:
:2 PPh₃ + O₂ → 2 OPPh₃
This impurity can be removed by recrystallisation of PPh₃ from either hot ethanol or isopropanol. This method capitalizes on the fact that OPPh₃ is more polar and hence more soluble in polar solvents than PPh₃.

Triphenylphosphine abstracts sulfur from polysulfide compounds, episulfides, and elemental sulfur. Simple organosulfur compounds such as thiols and thioethers are unreactive, however. The phosphorus-containing product is triphenylphosphine sulfide, Ph₃PS. This reaction can be employed to assay the "labile" S⁰ content of a sample, say vulcanized rubber. Triphenylphosphine selenide, Ph₃PSe, may be easily prepared via treatment of PPh₃ with red (alpha-monoclinic) Se. Salts of selenocyanate, SeCN⁻, are used as the Se⁰ source. PPh₃ can also form an adduct with Te, although this adduct primarily exists as (Ph₃P)₂Te rather than PPh₃Te.

Aryl azides react with PPh₃ to give phosphanimines, analogues of OPPh₃, via the Staudinger reaction. Illustrative is the preparation of triphenylphosphine phenylimide:
:PPh₃ + PhN₃ → PhNPPh₃ + N₂
The phosphanimine can be hydrolyzed to the amine. Typically the intermediate phosphanimine is not isolated.
:PPh₃ + RN₃ + H₂O → OPPh₃ + N₂ + RNH₂

Chlorination

Cl₂ adds to PPh₃ to give triphenylphosphine dichloride ( Ph₃Cll), which exists as the moisture-sensitive phosphonium halide. This reagent is used to convert alcohols to alkyl chlorides in organic synthesis. Bis(triphenylphosphine)iminium chloride (PPN⁺Cl⁻, formula C₆H₅)₃P)₂Nl is prepared from triphenylphosphine dichloride:
:2 Ph₃PCl₂ + NH₂OH·HCl + Ph₃P → Cl + 4HCl + Ph₃PO

Protonation

PPh₃ is a weak base. It forms isolable triphenylphosphonium salts with strong acids such as HBr:Hercouet, A.; LeCorre, M. (1988) Triphenylphosphonium bromide: A convenient and quantitative source of gaseous hydrogen bromide. Synthesis, 157–158
:P(C₆H₅)₃ + HBr → P(C₆H₅)₃sup>+Br⁻

Organic reactions

PPh₃ is widely used in organic synthesis. The properties that guide its usage are its nucleophilicity and its reducing character. The nucleophilicity of PPh₃ is indicated by its reactivity toward electrophilic alkenes, such as Michael-acceptors, and alkyl halides. It is also used in the synthesis of biaryl compounds, such as the Suzuki reaction.

Quaternization

PPh₃ combines with alkyl halides to give phosphonium salts. This quaternization reaction is particularly fast for benzylic and allylic halides:
:PPh₃ + CH₃I → H₃PPh₃sup>+I⁻
These salts, which can often be isolated as crystalline solids, react with strong bases to form ylides, which are reagents in the Wittig reactions.

Aryl halides will quaternize PPh₃ to give tetraphenylphosphonium salts:
:PPh₃ + PhBr → Ph₄r
The reaction however requires elevated temperatures and metal catalysts.

Mitsunobu reaction

In the Mitsunobu reaction, a mixture of triphenylphosphine and diisopropyl azodicarboxylate ("DIAD", or its diethyl analogue, DEAD) converts an alcohol and a carboxylic acid to an ester. DIAD is reduced as it serves as the hydrogen acceptor, and the PPh₃ is oxidized to OPPh₃.

Appel reaction

In the Appel reaction, a mixture of PPh₃ and CX₄ (X = Cl, Br) is used to convert alcohols to alkyl halides. Triphenylphosphine oxide (OPPh₃) is a byproduct.
:PPh₃ + CBr₄ + RCH₂OH → OPPh₃ + RCH₂Br + HCBr₃
This reaction commences with nucleophilic attack of PPh₃ on CBr₄, an extension of the quaternization reaction listed above.

Deoxygenation

The easy oxygenation of PPh₃ is exploited in its use to deoxygenate organic peroxides, which generally occurs with retention of configuration:
:PPh₃ + RO₂H → OPPh₃ + ROH (R = alkyl)

It is also used for the decomposition of organic ozonides to ketones and aldehydes, although dimethyl sulfide is more popular for the reaction as the side product, dimethyl sulfoxide is more readily separated from the reaction mixture than triphenylphosphine oxide. Aromatic ''N''-oxides are reduced to the corresponding amine in high yield at room temperature with irradiation:

:

Sulfonation

Sulfonation of PPh₃ gives tris(3-sulfophenyl)phosphine, P(C₆H₄-3-SO₃⁻)₃ ( TPPTS), usually isolated as the trisodium salt. In contrast to PPh₃, TPPTS is water-soluble, as are its metal derivatives. Rhodium complexes of TPPTS are used in certain industrial hydroformylation reactions.

Reduction to diphenylphosphide

Lithium in THF as well as Na or K react with PPh₃ to give Ph₂PM (M = Li, Na, K). These salts are versatile precursors to tertiary phosphines. For example, 1,2-dibromoethane and Ph₂PM react to give Ph₂PCH₂CH₂PPh₂. Weak acids such ammonium chloride, convert Ph₂PM (M = Li, Na, K) into diphenylphosphine:
:(C₆H₅)₂PM + H₂O → (C₆H₅)₂PH + MOH

Transition metal complexes

Triphenylphosphine binds well to most transition metals, especially those in the middle and late transition metals of groups 7–10. In terms of steric bulk, PPh₃ has a Tolman cone angle of 145°, which is intermediate between those of P(C₆H₁₁)₃ (170°) and P(CH₃)₃ (115°). In an early application in homogeneous catalysis, NiBr₂(PPh₃)₂ was used by Walter Reppe for the synthesis of acrylate esters from alkynes, carbon monoxide, and alcohols.*{{cite journal , title = Cyclisierende Polymerisation von Acetylen. III Benzol, Benzolderivate und hydroaromatische Verbindungen , author1=Reppe, W. , author2=Schweckendiek, W. J. , journal = Justus Liebigs Annalen der Chemie , volume = 560 , issue = 1 , pages = 104–116 , year = 1948 , doi = 10.1002/jlac.19485600104 The use of PPh₃ was popularized by its use in the hydroformylation catalyst RhH(PPh₃)₃(CO).

Polymer-anchored PPh₃ derivatives

Polymeric analogues of PPh₃ are known whereby polystyrene is modified with PPh₂ groups at the para position. Such polymers can be employed in many of the applications used for PPh₃ with the advantage that the polymer, being insoluble, can be separated from products by simple filtration of reaction slurries. Such polymers are prepared via treatment of 4-lithiophenyl-substituted polystyrene with chlorodiphenylphosphine (PPh₂Cl).

See also

* Tris(o-tolyl)phosphine
* Decyl(triphenyl)phosphonium

References

External links

International Chemical Safety Card 0700

Tertiary phosphines
Phenyl compounds