Triphenylphosphine

Triphenylphosphine (in Europe: triphenylphosphane) is a common diethyl ether.

Preparation, structure, handling

Although it is inexpensive, triphenylphosphine can be prepared in the laboratory by treatment of 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 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 allotrope of Se. Salts of selenocyanate, SeCN^-, are used as the Se⁰ source. Ph₃PTe is unknown and apparently unstable.

Aryl 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 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 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 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:

• Heck reaction.
• catalyze the hydrogenation of alkenes.
• 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. ammonium chloride, converts Ph₂PM (M = Li, Na, K) into Ph₂PH, known as diphenylphosphine.

Sulfonation - access to water-soluble phosphine ligands

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