Phosphine oxide

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). Such compounds are thermally stable, decomposing only above 450 °C.Phosphoryl refers to a functional group drawn with a phosphorus-oxygen double bond.

Phosphine oxides feature tetrahedral phosphorus centers. 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; This proposal, which is supported by ab initio calculations, has gained consensus in the chemistry community.

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.

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

Another route to phosphine oxides is the thermolysis of phosphonium hydroxides:

In the laboratory, phosphine oxides are usually generated by the oxidation, often accidentally, of tertiary phosphines:

Basic phosphines, such as trialkyl derivatives, are prone to this reaction.

The hydrolysis of phosphorus(V) dihalides also affords the oxide:

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:

One common way of synthesizing tertiary phosphine ligands is by deoxygenation of phosphine oxides (OPR₃) with trichlorosilane. Deoxygenation proceeds with retention or inversion of configuration, depending on conditions. Inversion is favored by a combination of trichlorosilane and triethylamine. In the absence of the base, the reaction proceeds with retention. The stereochemical course is explained mechanistically by complexation followed by intramolecular hydride transfer. On the other hand, the inversion of configuration in the presence of triethylamine is explained by complexation followed by intermolecular hydride transfer from a 1:1 triethylamine-trichlorosilane complex in an SN2 reaction. Based on these observations, it is thought that complexation of OPR₃ by trichlorosilane occurs with retention of configuration.

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