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
CAS Number	762-04-9 Y
3D model (JSmol)	Interactive image
Beilstein Reference	4-01-00-01329
ChemSpider	12437
ECHA InfoCard	100.010.992
PubChem CID	12977
UNII	U9X9YBA22W Y
CompTox Dashboard (EPA)	DTXSID9041861
InChI InChI=1S/C4H11O3P/c1-3-6-8(5)7-4-2/h5H,3-4H2,1-2H3 Key: SULWMEGSVQCTSK-UHFFFAOYSA-N
SMILES CCOP(OCC)=O
Properties
Chemical formula	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)₂P^III(OH) ⇌ (C₂H₅O)₂P^V(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 (Michaelis–Becker reaction):^[10]

(C₂H₅O)₂P(O)H + KO^tBu → (C₂H₅O)₂P(O)K + HO^tBu

(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

• Dimethylphosphite
• Diisopropylphosphite
• Diphenylphosphite