Diethylphosphite

Diethyl phosphite is the organophosphorus compound with the formula (C2H5O)2P(O)H. It is a popular reagent for generating other organophosphorus compounds, exploiting the high reactivity of the P-H bond. Diethyl phosphite is a colorless liquid.[1] The molecule is tetrahedral.

Diethylphosphite
Names
Preferred IUPAC name
Diethyl phosphonate
Other names
diethyl phosphonite; DEP; Phosphonic acid, diethyl ester
Identifiers
3D model (JSmol)
4-01-00-01329
ChemSpider
ECHA InfoCard 100.010.992
UNII
  • InChI=1S/C4H11O3P/c1-3-6-8(5)7-4-2/h5H,3-4H2,1-2H3
    Key: SULWMEGSVQCTSK-UHFFFAOYSA-N
  • CCOP(O)OCC
Properties
C4H11O3P
Molar mass 138.103 g·mol−1
Appearance colorless liquid
Density 1.072 g/cm3
Boiling point 50-51 °C at 2 mm Hg
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Synthesis and properties

The compound was probably prepared in the 1850s by combining phosphorus trichloride and ethanol, but intentional preparations came later. It arises as follows:[2]

PCl3 + 3 C2H5OH → (C2H5O)2P(O)H + 2 HCl + C2H5Cl

Under similar conditions but in the presence of base, triethyl phosphite results:[3]

PCl3 + 3 EtOH + 3 R3N → P(OEt)3 + 3 R3NH + 3 Cl−

Many analogues of diethyl phosphite can be prepared.[4][5] Despite being named as a phosphite the compound exists overwhelmingly in its phosphonate form, (C2H5O)2P(O)H, a property it shares with its parent acid phosphorous acid. Nonetheless many of its reactions appear to proceed via the minor phosphorus(III) tautomer.[6]

(C2H5O)2PIII(OH) ⇌ (C2H5O)2PV(O)H, K = 15 x 106 (25°C, aqueous)[7]

Reactions

Hydrolysis and alcoholysis

Diethyl phosphite hydrolyzes to give phosphorous acid. Hydrogen chloride accelerates this conversion.:[2]

Diethyl phosphite undergoes transesterification upon treating with an alcohol. For alcohols of high boiling points, the conversion can be driven by removal of ethanol:[8]

(C2H5O)2P(O)H + 2 ROH → (RO)2P(O)H + 2 C2H5OH

Similarly amines can displace ethoxide:[9]

(C2H5O)2P(O)H + RNH2 → (C2H5O)(RN(H)P(O)H + C2H5OH

P-alkylation

Diethyl phosphite undergoes deprotonation with potassium tert-butoxide. This reactivity allows alkylation at phosphorus:[10]

(C2H5O)2P(O)H + KOtBu → (C2H5O)2P(O)K + HOtBu
(C2H5O)2P(O)K + RBr → (C2H5O)2P(O)R + KBr

For converting aryl halides, palladium-catalysis can be employed.[1] The C-P coupling process is reminiscent of the Buchwald-Hartwig amination.

Reaction of diethyl phosphite with Grignard reagents results in initial deprotonation followed by displacement of the ethoxy groups.[11][12] This reactivity provides a route to secondary phosphine oxides, such as dimethylphosphine oxide as shown in the following pair of idealized equations:

(C2H5O)2P(O)H + CH3MgBr → (C2H5O)2P(O)MgBr + CH4
(C2H5O)2P(O)MgBr + 2 CH3MgBr → (CH3)2P(O)MgBr + 2 MgBr(OC2H5)
(CH3)2P(O)MgBr + H2O → (CH3)2P(O)H + MgBr(OH)

Hydrophosphonylation

Diethyl phosphite can add across unsaturated groups via a hydrophosphonylation reaction. For example, it adds to aldehydes in a manner similar to the Abramov reaction:

(C2H5O)2P(O)H + RCHO → (C2H5O)2P(O)CH(OH)R

It can also add to imines in the Pudovik reaction and Kabachnik–Fields reaction,[13] in both cases forming aminophosphonates

See also

References
  1. Green, Kenneth (2001). "Diethyl Phosphonite". Encyclopedia of Reagents for Organic Synthesis. doi:10.1002/047084289X.rd211. ISBN 0471936235.
  2. Malowan, J. E. (1953). "Diethyl Phosphite". Inorganic Syntheses. pp. 58–60. doi:10.1002/9780470132357.ch19. ISBN 9780470132357. {{cite book}}: |journal= ignored (help)
  3. Ford-Moore, A. H.; Perry, B. J. (1951). "Triethyl Phosphite". Org. Synth. 31: 111. doi:10.15227/orgsyn.031.0111.
  4. Pedrosa, Leandro (March 20, 2011). "Esterification of Phosphorus Trichloride with Alcohols; Diisopropyl phosphonate". ChemSpider Synthetic Pages. Royal Society of Chemistry. SyntheticPage 488. doi:10.1039/SP488. Retrieved July 10, 2017.
  5. Fakhraian, H.; Mirzaei, A. (2004). "Reconsideration of the Base-Free Batch-Wise Esterification of Phosphorus Trichloride with Alcohols". Org. Process Res. Dev. 8 (3): 401–404. doi:10.1021/op049958v.
  6. Doak, G. O.; Freedman, Leon D. (1961). "The Structure and Properties of the Dialkyl Phosphonates". Chem. Rev. 61 (1): 31–44. doi:10.1021/cr60209a002.
  7. Guthrie, J. Peter (1979). "Tautomerization Equilibria for Phosphorous Acid and its Ethyl Esters, Free Energies of Formation of Phosphorous and Phosphonic Acids and their Ethyl Esters, and p Ka Values for Ionization of the P—H Bond in Phosphonic Acid and Phosphonic Esters". Canadian Journal of Chemistry. 57 (2): 236–239. doi:10.1139/v79-039.
  8. Malowan, John E. (1953). "Dioctyl Phosphite". Inorganic Syntheses. pp. 61–62. doi:10.1002/9780470132357.ch20. ISBN 9780470132357. {{cite book}}: |journal= ignored (help)
  9. John M. Read, Yu-Pu Wang, Rick L. Danheiser (2015). "Synthesis of Phosphoryl Ynamides by Copper-Catalyzed Alkynylation of Phosphoramidates. Preparation of Diethyl Benzyl(oct-1-yn-1-yl)phosphoramidate". Org. Synth. 92: 156. doi:10.15227/orgsyn.092.0156.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  10. Boeckman, Robert K.; Perni, Robert B.; Macdonald, James E.; Thomas, Anthony J. (1988). "6-Diethylphosphonomethyl-2,2-dimethyl-1,3-dioxen-4-one (Phosphonic acid, [(2,2-dimethyl-4-oxo-4H-1,3-dioxin-6-yl)methyl]-, diethyl ester)". Organic Syntheses. 66: 194. doi:10.15227/orgsyn.066.0194.; Collective Volume, vol. 8, p. 192
  11. Hays, Hugh R. (1968). "Reaction of diethyl phosphonate with methyl and ethyl Grignard reagents". J. Org. Chem. 33 (10): 3690–3694. doi:10.1021/jo01274a003.
  12. Busacca, Carl A.; Lorenz, Jon C.; Sabila, Paul; Haddad, Nizar; Senanyake, Chris H. (2007). "Synthesis of Electron-Deficient Secondary Phosphine Oxides and Secondary Phosphines: Bis[3,5-bis(trifluoromethyl)phenyl]phosphine Oxide and Bis[3,5-bis(trifluoromethyl)phenyl]phosphine". Organic Syntheses. 84: 242. doi:10.15227/orgsyn.084.0242.
  13. Keglevich, György; Bálint, Erika (1 November 2012). "The Kabachnik–Fields Reaction: Mechanism and Synthetic Use". Molecules. 17 (11): 12821–12835. doi:10.3390/molecules171112821. PMC 6268146. PMID 23117425.open access
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