The general structure of a phosphite ester showing the lone pairs on the P In

organic chemistry Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...

, a phosphite ester or organophosphite usually refers to an organophosphorous compound with the formula P(OR)₃. They can be considered as

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

s of an unobserved tautomer

phosphorous acid Phosphorous acid (or phosphonic acid) is the Compound (chemistry), compound described by the chemical formula, formula . It is diprotic (readily ionizes two protons), not triprotic as might be suggested by its formula. Phosphorous acid is an in ...

, H₃PO₃, with the simplest example being trimethylphosphite, P(OCH₃)₃. Some phosphites can be considered esters of the dominant tautomer of phosphorous acid (HP(O)(OH)₂). The simplest representative is dimethylphosphite with the formula HP(O)(OCH₃)₂. Both classes of phosphites are usually colorless liquids.

Synthesis

;From PCl₃ Phosphite esters are typically prepared by treating

phosphorus trichloride Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxic ...

with an

alcohol Alcohol may refer to: Common uses * Alcohol (chemistry), a class of compounds * Ethanol, one of several alcohols, commonly known as alcohol in everyday life ** Alcohol (drug), intoxicant found in alcoholic beverages ** Alcoholic beverage, an alco ...

. For

alkyl In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term ''alkyl'' is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of . A cycloalkyl group is derived from a cy ...

alcohols the displaced chloride ion can attack the phosphite, causing dealkylation to give a dialkylphosphite and an

organochlorine compound Organochlorine chemistry is concerned with the properties of organochlorine compounds, or organochlorides, organic compounds that contain one or more carbon–chlorine bonds. The chloroalkane class (alkanes with one or more hydrogens substituted ...

. The overall reaction is as follows: :PCl₃ + 3 C₂H₅OH → (C₂H₅O)₂P(O)H + 2 HCl + C₂H₅Cl Alternatively, when the alcoholysis is conducted in the presence of proton acceptors (typically an amine base), one obtains the C₃-symmetric trialkyl derivatives: :PCl₃ + 3 C₂H₅OH + 3 R₃N → (C₂H₅O)₃P + 3 R₃NHCl A base is not essential when using aromatic alcohols such as phenols, as they are not susceptible to attack by chloride, however it does catalyse the esterification reaction and is therefore often included. ;By transesterification Phosphite esters can also be prepared by

transesterification Transesterification is the process of exchanging the organic functional group R″ of an ester with the organic group R' of an alcohol. These reactions are often catalyzed by the addition of an acid or base catalyst. Strong acids catalyze the r ...

, as they undergo alcohol exchange upon heating with other alcohols. This process is reversible and can be used to produce mixed alkyl phosphites. Alternatively, if the phosphite of a volatile alcohol is used, such as trimethyl phosphite, then the by product (

methanol Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...

) can be removed by distillation, allowing the reaction to be driven to completion.

Reactions and applications of tris(organo)phosphites

Reactions

: Phosphites are oxidized to phosphate esters: :P(OR)₃ + → OP(OR)₃ This reaction underpins the commercial use of some phosphite esters as stabilizers in polymers. Alkyl phosphite esters are used in the

Perkow reaction The Perkow reaction is an organic reaction in which a trialkyl phosphite ester reacts with a haloketone to form a dialkyl vinyl group, vinyl phosphate and an alkyl halide. In the related Michaelis–Arbuzov reaction the same reactants are known ...

for the formation of vinyl phosphonates, and in the

Michaelis–Arbuzov reaction The Michaelis–Arbuzov reaction (also called the Arbuzov reaction) is the chemical reaction of a trivalent phosphorus ester with an alkyl halide to form a pentavalent phosphorus species and another alkyl halide. The picture below shows the most c ...

to form

phosphonate In organic chemistry, phosphonates or phosphonic acids are organophosphorus compounds containing Functional group, groups, where R is an organic group (alkyl, aryl). If R is hydrogen then the compound is a Phosphite_ester#Chemistry_of_HP(O)(OR ...

s. Aryl phosphite esters may not undergo these reactions and hence are commonly used as stabilizers in halogen-bearing polymers such as

PVC Polyvinyl chloride (alternatively: poly(vinyl chloride), colloquial: vinyl or polyvinyl; abbreviated: PVC) is the world's third-most widely produced synthetic polymer of plastic (after polyethylene and polypropylene). About 40 million tons o ...

. Phosphite esters may be used as

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...

s in more specialised cases. For example, triethylphosphite is known to reduce certain

hydroperoxide Hydroperoxides or peroxols are Chemical compound, compounds of the form ROOH, where R stands for any group, typically Organic compound, organic, which contain the hydroperoxy functional group (). Hydroperoxide also refers to the hydroperoxide anio ...

s to alcohols formed by

autoxidation Autoxidation (sometimes auto-oxidation) refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organi ...

In this process the phosphite is converted to a

phosphate ester In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or aromatic substituents. They can be considered a ...

. This reaction type is also utilized in the Wender Taxol total synthesis. As strong nucleophiles, phosphite esters add to

aldehyde In organic chemistry, an aldehyde () (lat. ''al''cohol ''dehyd''rogenatum, dehydrogenated alcohol) is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred ...

s in the Abramov reaction.

Homogeneous catalysis

Phosphite esters are

Lewis base A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any sp ...

s and hence can form

coordination complexes A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or ...

with various metal ions. Representative phosphite ligands include trimethylphosphite ((MeO)₃P), triethylphosphite ((EtO)₃P), trimethylolpropane phosphite, and triphenylphosphite ((PhO)₃P). Phosphites exhibit a smaller

ligand cone angle In coordination chemistry, the ligand cone angle (θ) is a measure of the steric bulk of a ligand in a transition metal coordination complex. It is defined as the solid angle formed with the metal at the vertex of a cone and the outermost edge ...

s than the structurally related phosphine ligand family. Phosphite ligands are components of industrial catalysts for

hydroformylation In organic chemistry, hydroformylation, also known as oxo synthesis or oxo process, is an industrial process for the production of aldehydes () from alkenes (). This chemical reaction entails the net addition of a formyl group () and a hydrogen ...

and hydrocyanation.

Chemistry of HP(O)(OR)₂

Diorganophosphites are derivatives of phosphorus(V) and can be viewed as the di-esters of

((HO)₂P(O)H). They exhibit

tautomerism In chemistry, tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the reloca ...

, however, the equilibrium overwhelmingly favours the right-hand (phosphonate-like) form: :(RO)₂POH ⇌ (RO)₂P(O)H The P-H bond is the site of high reactivity in these compounds (for example in the

Atherton–Todd reaction The Atherton-Todd reaction is a name reaction in organic chemistry, which goes back to the British chemists F. R. Atherton, H. T. Openshaw and A. R. Todd. These described the reaction for the first time in 1945 as a method of converting Dialkyl ph ...

and Hirao coupling), whereas in tri-organophosphites the lone pair on phosphorus is the site of high reactivity. Diorganophosphites do however undergo transesterification.

References

{{Organophosphorus Functional groups Esters Phosphorus(III) compounds