Phosphine oxides

General formula of organophosphine oxides

Phosphine oxides are phosphorus compounds with the formula OPX3. When X = alkyl or aryl, these are organophosphine oxides. Triphenylphosphine oxide is an example. An inorganic phosphine oxide is phosphoryl chloride (POCl3).[1] The parent phosphine oxide (H3PO) remains rare and obscure.


Tertiary phosphine oxides

Principal resonance structures for phosphine oxides

Tertiary phosphine oxides are the commonly encountered phosphine oxides. With the formula R3PO, they are tetrahedral compounds.

They are usually prepared by oxidation of tertiary phosphines. The P-O bond is short and polar. According to molecular orbital theory, the short P–O bond is attributed to the donation of the lone pair electrons from oxygen p-orbitals to the antibonding phosphorus-carbon bonds.[2] The nature of the P–O bond was once hotly debated. Some discussions invoked a role for phosphorus-centered d-orbitals in bonding, but this analysis is not supported by computational analyses. In terms of simple Lewis structure, the bond is more accurately represented as a dative bond, as is currently used to depict an amine oxide.[3][4]

Preparation and occurrence

Phosphine oxide are typically produced by oxidation of organophosphines. The oxygen in air is often sufficiently oxidizing to fully convert trialkylphosphines to their oxides at room temperature:

R3P + 1/2 O2 → R3PO

Oxidation of less basic phosphines, such as methyldiphenylphosphine can be achieved using hydrogen peroxide:[5]

PMePh2 + H2O2 → OPMePh2 + H2O (Me = Category:Pages using Chem2 with parameter issues#*)

Phosphine oxides are by-product of the Wittig reaction:

R3PCR'2 + R"2CO → R3PO + R'2C=CR"2

Another route to phosphine oxides is the thermolysis of phosphonium hydroxides:

[PPh4]Cl + NaOH → Ph3PO + NaCl + PhH

The hydrolysis of phosphorus(V) dihalides also affords the oxide:[6]

R3PCl2 + H2O → R3PO + 2 HCl

Secondary phosphine oxides

Secondary phosphine oxides (SPOs), formally derived from secondary phosphines (R2PH), are again tetrahedral at phosphorus.[7] One commercially available example of a secondary phosphine oxide is diphenylphosphine oxide. SPOs are used in the formulation of catalysts for cross coupling reactions.[8]

Unlike tertiary phosphine oxides, SPOs often undergo further oxidation:

R2P(O)H + H2O2 → R2P(O)OH + H2O

These reactions are preceded by tautomerization to the phosphinous acid (R2POH):

R2P(O)H → R2POH
R2POH + H2O2 → R2PO2H + H2O

Syntheses

A nonoxidative route is applicable secondary phosphine oxides, which arise by the hydrolysis of the chlorophosphine. An example is the hydrolysis of chlorodiphenylphosphine to give diphenylphosphine oxide:

Ph2PCl + H2O → Ph2P(O)H + HCl

P-chiral phosphine oxides[9][10] are valuable intermediates in the synthesis of P-chiral phosphines[11] and phosphates, important as ligands in catalysis[12] and in the synthesis of oligonucleotide drugs.[13]

Primary phosphine oxides

Primary phosphine oxides, formally oxidized derivatives of primary phosphines, are again tetrahedral at phosphorus. With four different substituents (O, OH, H, R), they are chiral. The primary phosphine oxides subject to tautomerization, which leads to racemization. Like SPO's they are susceptible to further oxidation. Primary phosphine oxides disproportionate to the phosphinic acid and the primary phosphine:[14]

2 RP(O)H2 → RP(O)(H)OH + RPH2
2 RP(O)H2 → RP(O)(H)OH + 2 RPH2

Reactions

Transition metal complexes of phosphine oxides are numerous.

Some phosphine oxides are well-known photoinitiators in photopolymer chemistry. UV/LED exposure induces a type I Norrish fission to free radicals, which then polymerize in a radical chain. An example is 2,4,6‑trimethylbenzoyl­diphenyl­phosphine oxide, which absorbs around 380-410nm (near UV).[15]

Deoxygenation

Phosphine oxide deoxygenation has been extensively developed because some useful reactions convert stoichiometric tertiary phosphines to the corresponding oxides. Regenerating the tertiary phosphines requires strongly oxophilic reagents,[16] and can retain or invert chirality at P, depending on the reductant.[17]

Industrial deoxygenation usually begins with treatment with phosgene or equivalents. The resulting chlorotriphenylphosphonium chloride is then reduced.[18]

In the laboratory, phosphine oxides are usually reduced with silicon derivatives,[16] typically inexpensive trichlorosilane. Trichlorosilane and triethylamine reduce phosphine oxides with inversion, whereas the reaction proceeds with retention absent the base:[17]

HSiCl3 + Et3N ⇋ SiCl3− + Et3NH+
R3PO + Et3NH+ ⇋ R3POH+ + Et3N
SiCl3− + R3POH+ → PR3 + HOSiCl3

Other perchloropolysilanes, e.g. hexachlorodisilane (Si2Cl6) or Si3Cl8, can reduce phosphine oxides and generally give higher yields:

R3PO + Si2Cl6 → R3P + Si2OCl6
2 R3PO + Si3Cl8 → 2 R3P + Si3O2Cl8

Boranes and alanes also deoxygenate phosphine oxides.[16] Phosphoric acid diesters ((RO)2PO2H) catalyze deoxygenation with hydrosilanes.[19]

Use

Phosphine oxides are ligands in various types of homogeneous catalysis.

In coordination chemistry, they are known to have labilizing effects to CO ligands cis to it in organometallic reactions. The cis effect describes this process.

Phosphine oxides are excellent hydrogen-bond acceptors.[20][21][22][23]

The 31P NMR shift of triethylphosphine oxide when bound to a Lewis acid, is commonly used to determine the effective Lewis acidity.[24][25]

References

  1. ↑ D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam 1995. ISBN 0-444-89307-5.
  2. ↑ D. B. Chesnut (1999). "The Electron Localization Function (ELF) Description of the PO Bond in Phosphine Oxide". Journal of the American Chemical Society. 121 (10): 2335–2336. Bibcode:1999JAChS.121.2335C. doi:10.1021/ja984314m.
  3. ↑ Gilheany, Declan G. (1994). "No d Orbitals but Walsh Diagrams and Maybe Banana Bonds: Chemical Bonding in Phosphines, Phosphine Oxides, and Phosphonium Ylides". Chemical Reviews. 94 (5): 1339–1374. doi:10.1021/cr00029a008. PMID 27704785.
  4. ↑ In fact, the N-O bonds in amine oxides are more likely to be closer to double bonds than are those of the P-O bonds in phosphine oxides; see e.g. https://pubs.rsc.org/en/content/articlelanding/2015/sc/c5sc02076j#:~:text=Quantitative%20analysis%20of%20known%20species%20of%20general%20formulae,high%20degree%20of%20covalent%20rather%20than%20ionic%20bonding.
  5. ↑ Denniston, Michael L.; Martin, Donald R. (1977). "Methyldiphenylphosphine Oxide and Dimethylphenylphosphine Oxide". Inorganic Syntheses. Vol. 17. pp. 183–185. doi:10.1002/9780470132487.ch50. ISBN 9780470132487.
  6. ↑ W. B. McCormack (1973). "3-Methyl-1-Phenylphospholene oxide". Organic Syntheses; Collected Volumes, vol. 5, p. 787.
  7. ↑ Gallen, Albert; Riera, Antoni; Verdaguer, Xavier; Grabulosa, Arnald (2019). "Coordination Chemistry and Catalysis with Secondary Phosphine oxides". Catalysis Science & Technology. 9 (20): 5504–5561. doi:10.1039/C9CY01501A. hdl:2445/164459. S2CID 202885438.
  8. ↑ Ackermann, Lutz (2007). "Catalytic Arylations with Challenging Substrates: From Air-Stable HASPO Preligands to Indole Syntheses and C-H-Bond Functionalizations". Synlett. 2007 (4): 0507–0526. doi:10.1055/s-2007-970744.
  9. ↑ Xu, Ronghua; Gao, Zhenhua; Yu, Yiteng; Tang, Yehua; Tian, Duanshuai; Chen, Tian; Chen, Yibing; Xu, Guangqing; Shi, Enxue; Tang, Wenjun (2021). "A facile and practical preparation of P -chiral phosphine oxides". Chemical Communications. 57 (27): 3335–3338. doi:10.1039/D1CC00646K. ISSN 1359-7345.
  10. ↑ Mondal, Anirban; Thiel, Niklas O.; Dorel, Ruth; Feringa, Ben L. (2021-12-23). "P-chirogenic phosphorus compounds by stereoselective Pd-catalysed arylation of phosphoramidites". Nature Catalysis. 5 (1): 10–19. doi:10.1038/s41929-021-00697-9. hdl:11370/2dea4dbb-72e1-4c34-bbee-a10cd49463a3. ISSN 2520-1158.
  11. ↑ Imamoto, Tsuneo (2024-07-24). "P-Stereogenic Phosphorus Ligands in Asymmetric Catalysis". Chemical Reviews. 124 (14): 8657–8739. doi:10.1021/acs.chemrev.3c00875. ISSN 0009-2665.
  12. ↑ Meng, Yinggao; Wang, Qian; Yao, Xinyu; Wei, Donghui; Liu, Ying-Guo; Li, Er-Qing; Duan, Zheng (2022-12-23). "Rigid P-Chiral Phosphorus Ligands for Highly Selective Palladium-Catalyzed (4+2) and (4+4) Annulations". Organic Letters. 24 (50): 9205–9209. doi:10.1021/acs.orglett.2c03706. ISSN 1523-7060.
  13. ↑ Xu, Dongmin; Rivas-Bascón, Nazaret; Padial, Natalia M.; Knouse, Kyle W.; Zheng, Bin; Vantourout, Julien C.; Schmidt, Michael A.; Eastgate, Martin D.; Baran, Phil S. (2020-03-25). "Enantiodivergent Formation of C–P Bonds: Synthesis of P-Chiral Phosphines and Methylphosphonate Oligonucleotides". Journal of the American Chemical Society. 142 (12): 5785–5792. doi:10.1021/jacs.9b13898. ISSN 0002-7863.
  14. ↑ Horký, Filip; Císařová, Ivana; Štěpnička, Petr (2021). "A Stable Primary Phosphane Oxide and Its Heavier Congeners". Chemistry – A European Journal. 27 (4): 1282–1285. doi:10.1002/chem.202003702. PMID 32846012. S2CID 221346479.
  15. ↑ "Boosting the cure of phosphine oxide photoinitiators" (PDF). Retrieved 2025-03-27.
  16. 1 2 3 Podyacheva, Evgeniya; Kuchuk, Ekaterina; Chusov, Denis (2019). "Reduction of phosphine oxides to phosphines". Tetrahedron Letters. 60 (8): 575–582. doi:10.1016/j.tetlet.2018.12.070. S2CID 104364715.
  17. 1 2 Klaus Naumann; Gerald Zon; Kurt Mislow (1969). "Use of hexachlorodisilane as a reducing agent. Stereospecific deoxygenation of acyclic phosphine oxides". Journal of the American Chemical Society. 91 (25): 7012–7023. Bibcode:1969JAChS..91.7012N. doi:10.1021/ja01053a021.
  18. ↑ van Kalkeren, Henri A.; van Delft, Floris L.; Rutjes, Floris P. J. T. (2013). "Organophosphorus Catalysis to Bypass Phosphine Oxide Waste". ChemSusChem. 6 (9): 1615–1624. Bibcode:2013ChSCh...6.1615V. doi:10.1002/cssc.201300368. hdl:2066/117145. ISSN 1864-5631. PMID 24039197.
  19. ↑ Li, Yuehui; Lu, Liang-Qiu; Das, Shoubhik; Pisiewicz, Sabine; Junge, Kathrin; Beller, Matthias (2012). "Highly Chemoselective Metal-Free Reduction of Phosphine Oxides to Phosphines". Journal of the American Chemical Society. 134 (44): 18325–18329. Bibcode:2012JAChS.13418325L. doi:10.1021/ja3069165. ISSN 0002-7863. PMID 23062083.
  20. ↑ Kostin, Mikhail A.; Alkhuder, Omar; Xu, Luhang; Krutin, Danil V.; Asfin, Ruslan E.; Tolstoy, Peter M. (2024). "Complexes of phosphine oxides with substituted phenols: hydrogen bond characterization based on shifts of PO stretching bands". Physical Chemistry Chemical Physics. 26 (13): 10234–10242. doi:10.1039/D3CP05817D. ISSN 1463-9076.
  21. ↑ Cuypers, Ruud; Sudhölter, Ernst J. R.; Zuilhof, Han (2010-07-12). "Hydrogen Bonding in Phosphine Oxide/Phosphate–Phenol Complexes". ChemPhysChem. 11 (10): 2230–2240. doi:10.1002/cphc.201000084. ISSN 1439-4235.
  22. ↑ Tupikina, Elena Yu.; Bodensteiner, Michael; Tolstoy, Peter M.; Denisov, Gleb S.; Shenderovich, Ilya G. (2018-01-25). "P═O Moiety as an Ambidextrous Hydrogen Bond Acceptor". The Journal of Physical Chemistry C. 122 (3): 1711–1720. doi:10.1021/acs.jpcc.7b11299. ISSN 1932-7447.
  23. ↑ Hanna, Fergal E.; Root, Alexander J.; Schade, Markus; Hunter, Christopher A. (2024). "Negative cooperativity in the formation of H-bond networks involving primary anilines". Chemical Science. 15 (30): 12036–12041. doi:10.1039/D4SC03719G. ISSN 2041-6520. PMC 11290332. PMID 39092127.
  24. ↑ Beckett, Michael A.; Strickland, Gary C.; Holland, John R.; Sukumar Varma, K. (1996-09-01). "A convenient n.m.r. method for the measurement of Lewis acidity at boron centres: correlation of reaction rates of Lewis acid initiated epoxide polymerizations with Lewis acidity". Polymer. 37 (20): 4629–4631. doi:10.1016/0032-3861(96)00323-0.
  25. ↑ Beckett, Michael A; Brassington, David S; Coles, Simon J; Hursthouse, Michael B (2000-10-01). "Lewis acidity of tris(pentafluorophenyl)borane: crystal and molecular structure of B(C6F5)3·OPEt3". Inorganic Chemistry Communications. 3 (10): 530–533. doi:10.1016/S1387-7003(00)00129-5.
Category:Functional groups Category:Phosphine oxides#*
Category:Articles with short description Category:Functional groups Category:Pages using Chem2 with parameter issues Category:Phosphine oxides Category:Short description matches Wikidata