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 materials, i.e., matter in its various forms that contain carbon atoms.Clay ...

, a phosphite ester or organophosphite usually refers to an

organophosphorous 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 ...

compound with the formula P(OR)₃. They can be considered as

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides ...

s of an unobserved tautomer

phosphorous acid Phosphorous acid (or phosphonic acid (singular)) is the compound described by the formula H3PO3. This acid is diprotic (readily ionizes two protons), not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in the ...

, H₃PO₃, with the simplest example being

trimethylphosphite Trimethyl phosphite is an organophosphorus compound with the formula P(OCH3)3, often abbreviated P(OMe)3. It is a colorless liquid with a highly pungent odor. It is the simplest phosphite ester and finds used as a ligand in organometallic che ...

, 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 a ...

with an

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

. Depending on the synthetic details, this alcoholysis can give the diorganophosphites: :PCl₃ + 3 C₂H₅OH → (C₂H₅O)₂P(O)H + 2 HCl + C₂H₅Cl Alternatively, when the alcoholysis is conducted in the presence of proton acceptors, one obtains the C₃-symmetric trialkoxy derivatives: :PCl₃ + 3 C₂H₅OH + 3 R₃N → (C₂H₅O)₃P + 3 R₃NHCl Numerous derivatives have been prepared for both types of phosphites. ;By transesterification Phosphite esters can also be prepared by

transesterification In organic chemistry, transesterification is the process of exchanging the organic 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. The reaction ca ...

, 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) 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 for the formation of vinyl phosphonates, and in the Michaelis–Arbuzov reaction to form

phosphonate In organic chemistry, phosphonates or phosphonic acids are organophosphorus compounds containing groups (where R = alkyl, aryl, or just hydrogen). Phosphonic acids, typically handled as salts, are generally nonvolatile solids that are poorl ...

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

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 commonly reducing agents include the Earth m ...

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

(''scheme''). 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 Wender Taxol total synthesis in organic chemistry describes a Taxol total synthesis (one of six to date) by the group of Paul Wender at Stanford University published in 1997.''The Pinene Path to Taxanes. 6. A Concise Stereocontrolled Synthesis ...

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 consists 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 complexing agents. Many ...

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 (a common example being the Tolman cone angle or ''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 me ...

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

hydroformylation 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 (CHO) and a hydrogen atom to a carbon-carbon d ...

and

hydrocyanation In organic chemistry, hydrocyanation is a process for conversion of alkenes to nitriles. The reaction involves the addition of hydrogen cyanide and requires a catalyst. This conversion is conducted on an industrial scale for the production of pr ...

Chemistry of HP(O)(OR)₂

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

. They exhibit tautomerism, 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 pho ...

), 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