Triphenylphosphine

Triphenylphosphine (in Europe: triphenylphosphane) is a common organophosphorus compound with the formula P(C₆H₅)₃ - often abbreviated to PPh₃ 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.

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Preparation, structure, handling

Although it is inexpensive, 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.^[1] PPh₃ is pyramidal with a chiral propeller-like arrangement of the three phenyl rings. The rigidity of PPh₃ contributes to the ease with which its derivatives crystallize.

Principal reactions with chalcogens, halogens, and acids

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 hot isopropanol.^[2] This method capitalizes on the fact that OPPh₃ is more polar and hence more soluble in hydroxylic solvents than PPh₃.

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)

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 Ph₃PS. This reaction can be employed to assay the "labile" S⁰ content of a sample, say vulcanized rubber. Triphenylphosphine selenide, Ph₃PSe, does not readily form upon treatment of PPh₃ with either allotrope of Se. Salts of selenocyanate, SeCN^-, are used as the Se⁰ source. Ph₃PTe is unknown and apparently unstable.

Aryl azides react with PPh₃ to give imido analogue of OPPh₃ via the Staudinger reaction:

PPh₃ + PhN₃ → PhNPPh₃ + N₂

The product imides can be hydrolyzed to the amine. Typically the intermediate imidophosphorane is not isolated.

PPh₃ + RN₃ + H₂O → OPPh₃ + N₂ + RNH₂

Cl₂ adds to PPh₃ to give [PPh₃Cl]Cl, which exists as the moisture-sensitive phosphonium salt, This reagent is used to convert alcohols to alkyl chlorides in organic synthesis.

PPh₃ is a weak base, but does form stable salts with strong acids such as HBr. The product contains the phosphonium cation [HPPh₃]⁺.

Principal organic reactions

PPh₃ is widely used in organic synthesis. The properties that guide its usage are its nucleophilicity and its reducing character.^[3] 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 most alkyl halides to give phosphonium salts. The facility of the reaction follows the usual pattern whereby alkyl iodides and benzylic and allylic halides are particularly efficient reactants:

PPh₃ + CH₃I → [CH₃PPh₃]⁺I^-

These salts, which are readily isolated as crystalline solids, react with strong bases to form ylides:

CH₃PPh₃⁺ + base → [CH₂PPh₃] + baseH⁺

Such ylides are key reagents in the Wittig reactions, used to convert aldehydes and ketones into alkenes. Nickel salts are required to react PPh₃ with PhBr to give [PPh₄]Br. The tetraphenylphosphonium cation is widely used to prepare crystallizable lipophilic salts.

Mitsunobu reaction

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

Appel reaction

PPh₃ is oxidized again to OPPh₃ in this application, which covert alcohols to alkyl halides using CX₄ (X = Cl, Br):

PPh₃ + CBr₄ + RCH₂OH → OPPh₃ + RCH₂Br + HBr + HCBr₃

This reaction commences with nucleophilic attack of PPh₃ on CBr₄, an extension of the quaternization reaction listed above.

Principal transition metal derivatives

Triphenylphosphine binds well to most transition metals, especially those in the middle and late transition metals of groups 7-10.^[4] In terms of steric bulk, PPh₃ has a 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.^[5] Wilkinson's further popularized the use of PPh₃, including the then revolutionary hydroformylation catalyst RhH(PPh₃)₂(CO)₂.

It is telling that the corresponding triphenylamine shows little tendency to bind to metals. The difference between the coordinating power of PPh₃ and NPh₃ reflects the greater steric crowding around the nitrogen atom, which is smaller and favors a more tetrahedral geometry. Far more similar to PPh₃ in terms of its coordinating properties is triphenylarsine, AsPh₃.

An important technique for the characterization of metal-PPh₃ compounds is³¹P NMR spectroscopy. Substantial shifts occur upon complexation and³¹P-³¹P spin-spin coupling can provide insight into the structure of complexes containing multiple phosphine ligands.

Illustrative PPh₃ complexes:

• tetrakis(triphenylphosphine)palladium(0) is widely used to catalyse C-C coupling reactions in organic synthesis, see Heck reaction.
• Wilkinson's catalyst, RhCl(PPh₃)₃ is a square planar Rh(I) complex of historical significance used to catalyze the hydrogenation of alkenes.
• Vaska's complex, trans-IrCl(CO)(PPh₃)₂, is also historically significant; it was used to establish the scope of oxidative addition reactions. This early work provided the insights that led to the flowering of the area of homogeneous catalysis.
• NiCl₂(PPh₃)₂ is a four coordinate Ni(II) complex that exists as an equilibrium mixture of paramagnetic tetrahedral and diamagnetic square planar geometries. In contrast PdCl₂(PPh₃)₂ is square planar.

Organophosphorus chemistry

Conversion to PPh₂ derivatives

Triphenylphosphine is commonly employed as a precursor to other organophosphines. Lithium in THF and Na or K in NH₃ react with PPh₃ to give Ph₂PM (M = Li, Na, K). These reactions generate equal amounts of phenyllithium (or sodium, potassium analogs thereof) C₆H₅M, which can be selectively converted to benzene by selective protonation. Treatment of the resulting alkali metal diphenylphosphide with an alkylating agent RX gives PRPh₂. This method can be used to prepare such ligands as PMePh₂, methyldiphenylphosphine. The corresponding reaction with dihaloalkanes gives bis(diphenylphosphino)alkanes. For example, 1,2-dibromoethane and Ph₂PM react to give Ph₂PCH₂CH₂PPh₂, known as 1,2-bis(diphenylphosphino)ethane or dppe. Weak acids such ammonium chloride, converts Ph₂PM (M = Li, Na, K) into Ph₂PH, known as diphenylphosphine.

Sulfonation - access to water-soluble phosphine ligands

Sulfonation of PPh₃ gives tris(3-sulfophenyl)phosphine, P(C₆H₄-3-SO₃^-)₃. This anionic phosphine is usually isolated as the trisodium salt and is known as TPPTS. In contrast to PPh₃, TPPTS is a water soluble as are its metal derivatives. Rhodium complexes of TPPTS are used in certain industrial hydroformylation reactions because the water-soluble catalyst is readily separated from the organic products.^[6]

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 PPh₂Cl.

Safety

PPh₃ should be handled in a well ventillated area, preferably a fume hood.

References

1. ^ D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.
2. ^ D. D. Perrin, W. L. F. Armarego, D. R. Perrin, Purification of Laboratory Chemicals, 2nd ed.; Pergamon: New York, 1980; p 455.
3. ^ Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289
4. ^ C. Elschenbroich, A. Salzer ”Organometallics : A Concise Introduction” (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN 3-527-28165-7
5. ^ *Reppe, W.; Schweckendiek, W. J. (1948). "Cyclisierende Polymerisation von Acetylen. III Benzol, Benzolderivate und hydroaromatische Verbindungen". Comp. Rendus 560 (2): 104-116. doi:10.1002/jlac.19485600105.
6. ^ Herrmann, W. A.; Kohlpaintner, C. W. "Synthesis of Water-Soluble Phosphines and Their Transition Metal Complexes" Inorganic Syntheses, 1998, volume 32, pages 8-25. ISBN 0-471-24921-1