Triphenylphosphine oxide explained

Triphenylphosphine oxide (often abbreviated TPPO) is the organophosphorus compound with the formula OP(C₆H₅)₃, also written as Ph₃PO or PPh₃O (Ph = C₆H₅). It is one of the more common phosphine oxides. This colourless crystalline compound is a common but potentially useful waste product in reactions involving triphenylphosphine. It is a popular reagent to induce the crystallizing of chemical compounds.

Structure and properties

Ph₃PO is structurally related to POCl₃.^[1] As established by X-ray crystallography, the geometry around P is tetrahedral, and the P-O distance is 1.48 Å.^[2] Other modifications of Ph₃PO have been found: For example, a monoclinic form crystalizes in the space group P2₁/c with Z = 4 and a = 15.066(1) Å, b = 9.037(2) Å, c = 11.296(3) Å, and β = 98.47(1)°.The orthorhombic modification crystallizes in the space group Pbca with Z = 4 and 29.089(3) Å, b = 9.1347(9), c = 11.261(1) Å.^[3]

The oxygen center is relatively basic. The rigidity of the backbone and the basicity of the oxygen center make this species a popular agent to crystallize otherwise difficult to crystallize molecules. This trick is applicable to molecules that have acidic hydrogen atoms, e.g. phenols.^[4]

As a byproduct of organic synthesis

Ph₃PO is a byproduct of many useful reactions in organic synthesis including the Wittig, Staudinger, and Mitsunobu reactions. It is also formed when PPh₃Cl₂ is employed to convert alcohols into alkyl chlorides:

Ph₃PCl₂ + ROH → Ph₃PO + HCl + RCl

Triphenylphosphine can be regenerated from the oxide by treatment with a variety of deoxygenation agents, such as phosgene or trichlorosilane/triethylamine:^[5]

Ph₃PO + SiHCl₃ → PPh₃ + 1/n (OSiCl₂)_n + HCl

Triphenylphosphine oxide can be difficult to remove from reaction mixtures by means of chromatography. It is poorly soluble in hexane and cold diethyl ether. Trituration or chromatography of crude products with these solvents often leads to a good separation of triphenylphosphine oxide. Its removal is facilitated by conversion to its Mg(II) complex, which is poorly soluble in toluene or dichloromethane and can be filtered off.^[6] An alternative filtration method where ZnCl₂(TPPO)₂ is formed upon addition of ZnCl₂ may be used with more polar solvents such as ethanol, ethyl acetate and tetrahydrofuran.^[7]

Coordination chemistry

Ph₃PO forms a variety of complexes. A representative complex is the tetrahedral species NiCl₂(OPPh₃)₂.^[8]

Ph₃PO is a common impurity in PPh₃. The oxidation of PPh₃ by oxygen, including air, is catalyzed by many metal ions:

2 PPh₃ + O₂ → 2 Ph₃PO

Notes and References

1. D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. .
2. 10.1107/S0108270187092345. Structure of a Second Monoclinic Polymorph of Triphenylphosphine Oxide. 1987. Spek. Anthony L.. Acta Crystallographica. C43. 6. 1233–1235. 1987AcCrC..43.1233S .
3. 10.1007/BF01160986. Crystal structure of triphenylphosphine oxide. 1992. Al-Farhan. Khalid A.. Journal of Crystallographic and Spectroscopic Research. 22. 6. 687–689. 98335827.
4. M. C. Etter and P. W. Baures . Triphenylphosphine oxide as a crystallization aid . 1988 . . 110 . 2 . 639–640 . 10.1021/ja00210a076.
5. van Kalkeren, H. A.; van Delft, F. L.; Rutjes, F. P. J. T.. Organophosphorus Catalysis to Bypass Phosphine Oxide Waste. ChemSusChem. 2013. 6. 9. 1615–24. 10.1002/cssc.201300368. 24039197. 2013ChSCh...6.1615V . 2066/117145. free.
6. Patent WO 1998007724. "Process for the preparation of 7-alkoxyalkyl-1,2,4-triazolo[1,5-a] pyrimidine derivatives"
7. Batesky. Donald C.. Goldfogel. Matthew J.. Weix. Daniel J.. Removal of Triphenylphosphine Oxide by Precipitation with Zinc Chloride in Polar Solvents. The Journal of Organic Chemistry. en. 2017. 82. 19. 9931–9936. 10.1021/acs.joc.7b00459. 28956444. 5634519. free.
8. D. M. L. Goodgame and M. Goodgame . Near-Infrared Spectra of Some Pseudotetrahedral Complexes of Cobalt (II) and Nickel(II) . 1965 . . 4 . 2 . 139–143 . 10.1021/ic50024a002.