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]

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 = )

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 (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

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, 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