Diphenylphosphine
# Diphenylphosphine

Diphenylphosphine, also known as diphenylphosphane, is an organophosphorus compound with the formula $(C_6H_5)_2PH.$ This foul-smelling, colorless liquid is easily oxidized in air. It is a precursor to organophosphorus ligands for use as catalysts.

## Synthesis

Diphenylphosphine can be prepared from triphenylphosphine by reduction to lithium diphenylphosphide, which can be protonated to give the title compound:

    $PPh_3$ + 2 Li → $LiPPh_2$ + LiPh
    $LiPPh_2$ + $H_2O$ → $Ph_2PH$ + LiOH

## Uses and reactions

In the laboratory, diphenylphosphine is a common intermediate. It can be deprotonated to give diphenylphosphide derivatives:

    $Ph_2PH$ + $^nBuLi$ → $Ph_2PLi$ + $^nBuH$

The preparation of phosphine ligands, Wittig-Horner reagents, and phosphonium salts are commonly accomplished by alkylating diphenylphosphine. The hydrogen atom connected to phosphorus undergoes Michael-like addition to activated alkenes, providing products with which to produce phosphine ligands such as 1,2-bis(diphenylphosphino)ethane $(Ph_2PC_2H_4PPh_2).$ Diphenylphosphine and especially diphenylphosphide derivatives are nucleophiles, so they add to carbon – heteroatom double bonds. For example, in the presence of concentrated hydrochloric acid at 100 °C, diphenylphosphine adds to the carbon atom in benzaldehyde to give (phenyl-(phenylmethyl)phosphoryl)benzene.

    $Ph_2PH$ + PhCHO → $Ph_2P(O)CH_2Ph$

Compared to tertiary phosphines, diphenylphosphine is weakly basic. The pKa of the protonated derivative is 0.03:

    $Ph_2PH_2^+$ ⇌ $Ph_2PH$ + $H^+$

### Handling properties

Diphenylphosphine readily oxidizes.

    $Ph_2PH$ + $O_2$ → $Ph_2P(O)OH$

An intermediate in this oxidation is diphenylphosphine oxide. The use of the diphenylphosphine–borane complex, $Ph_2PH•BH_3$ avoids the problem of phosphine oxidation by protecting the phosphine from oxidation and is available through chemical vendors.

