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. The molecule is tetrahedral.

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

Under similar conditions but in the presence of base, triethyl phosphite results:
 * PCl3 + 3 EtOH + 3 R3N → P(OEt)3 + 3 R3NH + 3 Cl−

Many analogues of diethyl phosphite can be prepared. 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.


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

Hydrolysis and alcoholysis
Diethyl phosphite hydrolyzes to give phosphorous acid. Hydrogen chloride accelerates this conversion.:

Diethyl phosphite undergoes transesterification upon treating with an alcohol. For alcohols of high boiling points, the conversion can be driven by removal of ethanol:
 * (C2H5O)2P(O)H + 2 ROH → (RO)2P(O)H + 2 C2H5OH

Similarly amines can displace ethoxide:
 * (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):
 * (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. 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. 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, in both cases forming aminophosphonates