Phosphine oxides

Phosphine oxides are phosphorus compounds with the formula OPX₃. When X = alkyl or aryl, these are organophosphine oxides. Triphenylphosphine oxide is an example. An inorganic phosphine oxide is phosphoryl chloride (POCl₃).^[1] The parent phosphine oxide (H₃PO) remains rare and obscure.

General formula of organophosphine oxides

Structure and bonding

Tertiary phosphine oxides

Principal resonance structures for phosphine oxides

Tertiary phosphine oxides are the most 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]

Secondary phosphine oxides

Secondary phosphine oxides (SPOs), formally derived from secondary phosphines (R₂PH), are again tetrahedral at phosphorus.^[5] One commercially available example of a secondary phosphine oxide is diphenylphosphine oxide. SPOs are used in the formulation of catalysts for cross coupling reactions.^[6]

Unlike tertiary phosphine oxides, SPOs often undergo further oxidation, which enriches their chemistry:

R₂P(O)H + H₂O₂ → R₂P(O)OH + H₂O

These reactions are preceded by tautomerization to the phosphinous acid (R₂POH):

R₂P(O)H → R₂POH

Primary phosphine oxides

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, and further oxidation, analogous to the behavior of SPOs. Additionally, primary phosphine oxides are susceptible to disproportionation to the phosphinic acid and the primary phosphine:^[7]

2 RP(O)H₂ → RP(O)(H)OH + RPH₂

2 RP(O)H₂ → RP(O)(H)OH + 2 RPH₂

Syntheses

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

This conversion is usually undesirable. In order to suppress this reaction, air-free techniques are often employed when handling say, trimethylphosphine.

Less basic phosphines, such as methyldiphenylphosphine are converted to their oxides with hydrogen peroxide:^[8]

PMePh₂ + H₂O₂ → OPMePh₂ + H₂O

Phosphine oxides are generated as a by-product of the Wittig reaction:

R₃PCR'₂ + R"₂CO → R₃PO + R'₂C=CR"₂

Another albeit unconventional 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:^[9]

R₃PCl₂ + H₂O → R₃PO + 2 HCl

A special 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

Reactions

Transition metal complexes of phosphine oxides are numerous.

Some phosphine oxides are well-known photoinitiators in photopolymer chemistry. UV/LED exposure induces a type I Norrish fission to free radicals, which then polymerize in a radical chain. An example is 2,4,6‑trimethylbenzoyl­diphenyl­phosphine oxide, which absorbs around 380-410nm (near UV).^[10]

Deoxygenation

Phosphine oxide deoxygenation has been extensively developed because many useful reactions convert stoichiometric tertiary phosphines to the corresponding oxides. Regenerating the tertiary phosphines requires cheap oxophilic reagents,^[11] and can retain or invert chirality at P, depending on the reductant.^[12]

Industrial deoxygenation usually occurs in two steps. Phosgene or equivalents first produce chlorotriphenylphosphonium chloride, which is then reduced separately.^[13]

In the laboratory, phosphine oxides are usually reduced with silicon derivatives,^[11] typically inexpensive trichlorosilane. Trichlorosilane and triethylamine reduce phosphine oxides with inversion, whereas the reaction proceeds with retention absent the base:^[12]

HSiCl₃ + Et₃N ⇋ SiCl₃⁻ + Et₃NH⁺

R₃PO + Et₃NH⁺ ⇋ R₃POH⁺ + Et₃N

SiCl₃⁻ + R₃POH⁺ → PR₃ + HOSiCl₃

Other perchloropolysilanes, e.g. hexachlorodisilane (Si₂Cl₆) or Si₃Cl₈, can reduce phosphine oxides and generally give higher yields:

R₃PO + Si₂Cl₆ → R₃P + Si₂OCl₆

2 R₃PO + Si₃Cl₈ → 2 R₃P + Si₃O₂Cl₈

Boranes and alanes also deoxygenate phosphine oxides.^[11] Phosphoric acid diesters ((RO)₂PO₂H) catalyze deoxygenation with hydrosilanes.^[14]

Use

Phosphine oxides are ligands in various applications of homogeneous catalysis. In coordination chemistry, they are known to have labilizing effects to CO ligands cis to it in organometallic reactions. The cis effect describes this process.