# Triphenylphosphine

Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C$_6$H$_5$)$_3$ and often abbreviated to PPh$_3$ or Ph$_3$P. It is versatile compound that is widely used as a reagent in organic synthesis and as a ligand for transition metal complexes, including ones that serve as catalysts in organometallic chemistry. PPh$_3$ 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:
    PCl$_3$ + 3 PhCl + 6 Na → PPh$_3$ + 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, Ph$_3$PO:
    2 PPh$_3$ + O$_2$ → 2 OPPh$_3$
This impurity can be removed by recrystallisation of PPh$_3$ from either hot ethanol or isopropanol. This method capitalizes on the fact that OPPh$_3$ is more polar and hence more soluble in polar solvents than PPh$_3$.
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, Ph$_3$PS. This reaction can be employed to assay the "labile" S$^0$ content of a sample, say vulcanized rubber. Triphenylphosphine selenide, Ph$_3$PSe, may be easily prepared via treatment of PPh$_3$ with red (alpha-monoclinic) Se. Salts of selenocyanate, SeCN$^−$, are used as the Se$^0$ source. PPh$_3$ can also form an adduct with Te, although this adduct primarily exists as (Ph$_3$P)$_2$Te rather than PPh$_3$Te.
Aryl azides react with PPh$_3$ to give phosphanimines, analogues of OPPh$_3$, via the Staudinger reaction. Illustrative is the preparation of triphenylphosphine phenylimide:
    PPh$_3$ + PhN$_3$ → PhNPPh$_3$ + N$_2$
The phosphanimine can be hydrolyzed to the amine. Typically the intermediate phosphanimine is not isolated.
    PPh$_3$ + RN$_3$ + H$_2$O → OPPh$_3$ + N$_2$ + RNH$_2$

### Chlorination

Cl$_2$ adds to PPh$_3$ to give triphenylphosphine dichloride ([PPh$_3$Cl]Cl), 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 [(C$_6$H$_5$)$_3$P)$_2$N]Cl is prepared from triphenylphosphine dichloride:
    2 Ph$_3$PCl$_2$ + NH$_2$OH·HCl + Ph$_3$P → {[Ph$_3$P]$_2$N}Cl + 4HCl + Ph$_3$PO

### Protonation

PPh$_3$ is a weak base (aqueous pK$_{aH}$ = 2.73, determined electrochemically), although it is a considerably stronger base than NPh$_3$ (estimated aqueous pK$_{aH}$ < –3). It forms isolable triphenylphosphonium salts with strong acids such as HBr:
    P(C$_6$H$_5$)$_3$ + HBr → [HP(C$_6$H$_5$)$_3$]$^+$Br$^−$

## Organic reactions

PPh$_3$ is widely used in organic synthesis. The properties that guide its usage are its nucleophilicity and its reducing character. The nucleophilicity of PPh$_3$ 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

PPh$_3$ combines with alkyl halides to give phosphonium salts. This quaternization reaction is particularly fast for benzylic and allylic halides:
    PPh$_3$ + CH$_3$I → [CH$_3$PPh$_3$]$^+$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 PPh$_3$ to give tetraphenylphosphonium salts:
    PPh$_3$ + PhBr → [PPh$_4$]Br
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 PPh$_3$ is oxidized to OPPh$_3$.

### Appel reaction

In the Appel reaction, a mixture of PPh$_3$ and CX$_4$ (X = Cl, Br) is used to convert alcohols to alkyl halides. Triphenylphosphine oxide (OPPh$_3$) is a byproduct.
    PPh$_3$ + CBr$_4$ + RCH$_2$OH → OPPh$_3$ + RCH$_2$Br + HCBr$_3$
This reaction commences with nucleophilic attack of PPh$_3$ on CBr$_4$, an extension of the quaternization reaction listed above.

### Deoxygenation

The easy oxygenation of PPh$_3$ is exploited in its use to deoxygenate organic peroxides, which generally occurs with retention of configuration:
    PPh$_3$ + RO$_2$H → OPPh$_3$ + 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 PPh$_3$ gives tris(3-sulfophenyl)phosphine, P(C$_6$H$_4$-3-SO$_3$$^−$)$_3$ (TPPTS), usually isolated as the trisodium salt. In contrast to PPh$_3$, 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 PPh$_3$ to give Ph$_2$PM (M = Li, Na, K). These salts are versatile precursors to tertiary phosphines. For example, 1,2-dibromoethane and Ph$_2$PM react to give Ph$_2$PCH$_2$CH$_2$PPh$_2$. Weak acids such ammonium chloride, convert Ph$_2$PM (M = Li, Na, K) into diphenylphosphine:
    (C$_6$H$_5$)$_2$PM + H$_2$O → (C$_6$H$_5$)$_2$PH + 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, PPh$_3$ has a Tolman cone angle of 145°, which is intermediate between those of P(C$_6$H$_{11}$)$_3$ (170°) and P(CH$_3$)$_3$ (115°). In an early application in homogeneous catalysis, NiBr$_2$(PPh$_3$)$_2$ was used by Walter Reppe for the synthesis of acrylate esters from alkynes, carbon monoxide, and alcohols. The use of PPh$_3$ was popularized by its use in the hydroformylation catalyst RhH(PPh$_3$)$_3$(CO).

## Polymer-anchored PPh$_3$ derivatives

Polymeric analogues of PPh$_3$ are known whereby polystyrene is modified with PPh$_2$ groups at the para position. Such polymers can be employed in many of the applications used for PPh$_3$ 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 (PPh$_2$Cl).