Phosphite ester

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The general structure of a phosphite ester showing the lone pairs on the P Phosphite.svg
The general structure of a phosphite ester showing the lone pairs on the P

In organic chemistry, a phosphite ester or organophosphite usually refers to an organophosphorous compound with the formula P(OR)3. They can be considered as esters of an unobserved tautomer phosphorous acid, 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.

Contents

Synthesis

From PCl3

Phosphite esters are typically prepared by treating phosphorus trichloride with an alcohol. For alkyl alcohols the displaced chloride ion can attack the phosphite, causing dealkylation to give a dialkylphosphite and an organochlorine compound. [1] [2] 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: [3]

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. [4]

By transesterification

Phosphite esters can also be prepared by transesterification, as they undergo alcohol exchange upon heating with other alcohols. [5] 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

Tris(2,4-di-tert-butylphenyl)phosphite, a widely used stabilizer in polymers BigPhosphite31570-04-4.png
Tris(2,4-di-tert-butylphenyl)phosphite, a widely used stabilizer in polymers

Phosphites are oxidized to phosphate esters:

P(OR)3 + [O] → OP(OR)3

This reaction underpins the commercial use of some phosphite esters as stabilizers in polymers. [6]

Alkyl phosphite esters are used in the Perkow reaction for the formation of vinyl phosphonates, and in the Michaelis–Arbuzov reaction to form phosphonates. Aryl phosphite esters may not undergo these reactions and hence are commonly used as stabilizers in halogen-bearing polymers such as PVC.

Autoxidation of a keto steroid with oxygen to the hydroperoxide (not depicted) followed by reduction with triethylphosphite to the alcohol Triethylphosphitereduction.png
Autoxidation of a keto steroid with oxygen to the hydroperoxide (not depicted) followed by reduction with triethylphosphite to the alcohol

Phosphite esters may be used as reducing agents in more specialised cases. For example, triethylphosphite is known to reduce certain hydroperoxides to alcohols formed by autoxidation [7] (scheme). In this process the phosphite is converted to a phosphate ester. This reaction type is also utilized in the Wender Taxol total synthesis.

Homogeneous catalysis

Phosphite esters are Lewis bases and hence can form coordination complexes 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 angles than the structurally related phosphine ligand family. Phosphite ligands are components of industrial catalysts for hydroformylation and hydrocyanation. [8]

BiPhePhos is representative diphosphite ligand used in homogeneous catalysis. BiPhePhos.svg
BiPhePhos is representative diphosphite ligand used in homogeneous catalysis.

Chemistry of HP(O)(OR)2

Structure of a diethylphosphite. (EtO)2POH.png
Structure of a diethylphosphite.

Diorganophosphites are derivatives of phosphorus(V) and can be viewed as the di-esters of phosphorous acid ((HO)2P(O)H). They exhibit tautomerism, however, the equilibrium overwhelmingly favours the right-hand (phosphonate-like) form: [11] [12]

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

See also

Related Research Articles

<span class="mw-page-title-main">Ester</span> Compound derived from an acid

In chemistry, an ester is a compound derived from an acid in which the hydrogen atom (H) of at least one acidic hydroxyl group of that acid is replaced by an organyl group. Analogues derived from oxygen replaced by other chalcogens belong to the ester category as well. According to some authors, organyl derivatives of acidic hydrogen of other acids are esters as well, but not according to the IUPAC.

<span class="mw-page-title-main">Phosphite anion</span> Ion

A phosphite anion or phosphite in inorganic chemistry usually refers to [HPO3]2− but includes [H2PO3]− ([HPO2(OH)]−). These anions are the conjugate bases of phosphorous acid (H3PO3). The corresponding salts, e.g. sodium phosphite (Na2HPO3) are reducing in character.

<span class="mw-page-title-main">Phosphorus trichloride</span> Chemical compound

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 and reacts readily with water to release hydrogen chloride.

<span class="mw-page-title-main">Phosphorus tribromide</span> Chemical compound

Phosphorus tribromide is a colourless liquid with the formula PBr3. The liquid fumes in moist air due to hydrolysis and has a penetrating odour. It is used in the laboratory for the conversion of alcohols to alkyl bromides.

<span class="mw-page-title-main">Triphenylphosphine</span> Chemical compound

Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to PPh3 or Ph3P. It is versatile compound that is widely used as a reagent in organic synthesis and as a ligand for transition metal complexes, including ones that serve as catalysts in organometallic chemistry. PPh3 exists as relatively air stable, colorless crystals at room temperature. It dissolves in non-polar organic solvents such as benzene and diethyl ether.

<span class="mw-page-title-main">Michaelis–Arbuzov reaction</span>

The Michaelis–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 common types of substrates undergoing the Arbuzov reaction; phosphite esters (1) react to form phosphonates (2), phosphonites (3) react to form phosphinates (4) and phosphinites (5) react to form phosphine oxides (6).

<span class="mw-page-title-main">Phosphorous acid</span> Chemical compound (H3PO3)

Phosphorous acid is the compound described by the formula H3PO3. This acid is diprotic, not triprotic as might be suggested by this formula. Phosphorous acid is an intermediate in the preparation of other phosphorus compounds. Organic derivatives of phosphorous acid, compounds with the formula RPO3H2, are called phosphonic acids.

In chemistry, phosphorus oxoacid is a generic name for any acid whose molecule consists of atoms of phosphorus, oxygen, and hydrogen. There is a potentially infinite number of such compounds. Some of them are unstable and have not been isolated, but the derived anions and organic groups are present in stable salts and esters. The most important ones—in biology, geology, industry, and chemical research—are the phosphoric acids, whose esters and salts are the phosphates.

<span class="mw-page-title-main">Phosphonate</span> Organic compound containing C–PO(OR)2 groups

In organic chemistry, phosphonates or phosphonic acids are organophosphorus compounds containing C−PO(OR)2 groups. Phosphonic acids, typically handled as salts, are generally nonvolatile solids that are poorly soluble in organic solvents, but soluble in water and common alcohols.

Organophosphorus chemistry is the scientific study of the synthesis and properties of organophosphorus compounds, which are organic compounds containing phosphorus. They are used primarily in pest control as an alternative to chlorinated hydrocarbons that persist in the environment. Some organophosphorus compounds are highly effective insecticides, although some are extremely toxic to humans, including sarin and VX nerve agents.

Organophosphines are organophosphorus compounds with the formula PRnH3−n, where R is an organic substituent. These compounds can be classified according to the value of n: primary phosphines (n = 1), secondary phosphines (n = 2), tertiary phosphines (n = 3). All adopt pyramidal structures. Organophosphines are generally colorless, lipophilic liquids or solids. The parent of the organophosphines is phosphine (PH3).

<span class="mw-page-title-main">Trimethylolpropane phosphite</span> Chemical compound

Trimethylolpropane phosphite, C2H5C(CH2O)3P, is a phosphite ester used as a ligand in organometallic chemistry. Trimethylolpropane phosphite is sometimes abbreviated to EtCage. It is a white solid that is soluble in organic solvents. It is also highly toxic.

<span class="mw-page-title-main">Triethyl phosphite</span> Chemical compound

Triethyl phosphite is an organophosphorus compound, specifically a phosphite ester, with the formula P(OCH2CH3)3, often abbreviated P(OEt)3. It is a colorless, malodorous liquid. It is used as a ligand in organometallic chemistry and as a reagent in organic synthesis.

In organophosphorus chemistry, the Kabachnik–Fields reaction is a three-component organic reaction forming α-aminomethylphosphonates from an amine, a carbonyl compound, and a dialkyl phosphonate, (RO)2P(O)H (that are also called dialkylphosphites). Aminophosphonates are synthetic targets of some importance as phosphorus analogues of α-amino acids (a bioisostere). This multicomponent reaction was independently discovered by Martin Kabachnik and Ellis K. Fields in 1952. The reaction is very similar to the two-component Pudovik reaction, which involves condensation of the phosphite and a preformed imine.

<span class="mw-page-title-main">Diphenylphosphine oxide</span> Chemical compound

Diphenylphosphine oxide is an organophosphorus compound with the formula (C6H5)2P(O)H. It is a white solid that soluble in polar organic solvents.

Aminophosphonates are organophosphorus compounds with the formula (RO)2P(O)CR'2NR"2. These compounds are structural analogues of amino acids in which a carboxylic moiety is replaced by phosphonic acid or related groups. Acting as antagonists of amino acids, they inhibit enzymes involved in amino acid metabolism and thus affect the physiological activity of the cell. These effects may be exerted as antibacterial, plant growth regulatory or neuromodulatory. They can act as ligands, and heavy metal complexes with aminophosphonates have had medical applications investigated.

<span class="mw-page-title-main">Diethylphosphite</span> Chemical compound

Diethyl phosphite is the organophosphorus compound with the formula (C2H5O)2P(O)H. It is a popular reagent for generating other organophosphorus compounds, exploiting the high reactivity of the P-H bond. Diethyl phosphite is a colorless liquid. The molecule is tetrahedral.

<span class="mw-page-title-main">Dimethylphosphite</span> Chemical compound

Dimethylphosphite is an organophosphorus compound with the formula (CH3O)2P(O)H, known as dimethyl hydrogen phosphite (DMHP). Dimethylphosphite, is a minor tautomer of the phosphorus(V) derivative. It is a reagent for generating other organophosphorus compounds, exploiting the high reactivity of the P-H bond. The molecule is tetrahedral. It is a colorless liquid. The compounds can be prepared by methanolysis of phosphorus trichloride or by heating diethylphosphite in methanol.

In organophosphorus chemistry, the Pudovik reaction is a method for preparing α-aminomethylphosphonates. Under basic conditions, the phosphorus–hydrogen bond of a dialkylphosphite, (RO)2P(O)H, adds across the carbon–nitrogen double bond of an imine (a hydrophosphonylation reaction). The reaction is closely related to the three-component Kabachnik–Fields reaction, where an amine, phosphite, and an organic carbonyl compound are condensed, which was reported independently by Martin Kabachnik and Ellis Fields in 1952. In the Pudovik reaction, a generic imine, RCH=NR', would react with a phosphorous reagent like diethylphosphite as follows:

In chemistry, phosphorochloridites are a class of organophosphorus compound with the formula (RO)2PCl (R = organic substituent). They are pyramidal in shape, akin to regular phosphites (P(OR)3). They are usually colorless and sensitive toward hydrolysis and, to some extent, oxidation to the corresponding phosphorochloridates ((RO)2P(O)Cl).

References

  1. ↑ Malowan, John E. (1953). "Diethyl phosphite". Inorganic Syntheses. Inorganic Syntheses. Vol. 4. pp. 58–60. doi:10.1002/9780470132357.ch19. ISBN   9780470132357.
  2. ↑ Gerrard, W.; Whitbread, E. G. G. (1952). "165. Interaction of hydrogen halides and n-butyl phosphites". Journal of the Chemical Society (Resumed): 914. doi:10.1039/JR9520000914.
  3. ↑ A. H. Ford-Moore & B. J. Perry (1963). "Triethyl Phosphite". Organic Syntheses ; Collected Volumes, vol. 4, p. 955.
  4. ↑ Mao, Mengmei; Zhang, Le; Yao, Hanlin; Wan, Li; Xin, Zhong (28 April 2020). "Development and Scale-up of the Rapid Synthesis of Triphenyl Phosphites in Continuous Flow". ACS Omega. 5 (16): 9503–9509. doi:10.1021/acsomega.0c00716. PMC   7191834 . PMID   32363302.
  5. ↑ Hoffmann, Friedrich W.; Ess, Richard J.; Usingef, Robert P. (November 1956). "The Transesterification of Trialkyl Phosphites with Aliphatic Alcohols". Journal of the American Chemical Society. 78 (22): 5817–5821. doi:10.1021/ja01603a026.
  6. ↑ Rainer Wolf; Bansi Lal Kaul (2000). "Plastics, Additives". Ullmann's Encyclopedia Of Industrial Chemistry. doi:10.1002/14356007.a20_459. ISBN   3527306730.
  7. ↑ J. N. Gardner; F. E. Carlon & O. Gnoj (1968). "One-step procedure for the preparation of tertiary α-ketols from the corresponding ketones". J. Org. Chem. 33 (8): 3294–3297. doi:10.1021/jo01272a055. PMID   5742870.
  8. ↑ Aitor Gual; Cyril Godard; Verónica de la Fuente; Sergio Castillón (2012). "Design and Synthesis of Phosphite Ligands for Homogeneous Catalysis". In Paul C. J. Kamer; Piet W. N. M. van Leeuwen (eds.). Phosphorus(III) Ligands in Homogeneous Catalysis: Design and Synthesis. John Wiley & Sons. pp. 81–131. doi:10.1002/9781118299715.ch3. ISBN   9781118299715.
  9. ↑ Cuny, Gregory D.; Buchwald, Stephen L. (1993). "Practical, High-Yield, Regioselective, Rhodium-Catalyzed Hydroformylation of Functionalized α-olefins". Journal of the American Chemical Society. 115 (5): 2066–2068. doi:10.1021/ja00058a079.
  10. ↑ Van Rooy, Annemiek; Kamer, Paul C. J.; Van Leeuwen, Piet W. N. M.; Goubitz, Kees; Fraanje, Jan; Veldman, Nora; Spek, Anthony L. (1996). "Bulky Diphosphite-Modified Rhodium Catalysts: Hydroformylation and Characterization". Organometallics. 15 (2): 835–847. doi:10.1021/OM950549K.
  11. ↑ Guthrie, J. Peter (1979). "Tautomerization Equilibria for Phosphorous Acid and its Ethyl Esters, Free Energies of Formation of Phosphorous and Phosphonic Acids and their Ethyl Esters, and p Ka Values for Ionization of the P—H Bond in Phosphonic Acid and Phosphonic Esters". Canadian Journal of Chemistry. 57 (2): 236–239. doi: 10.1139/v79-039 .
  12. ↑ Janesko, Benjamin G.; Fisher, Henry C.; Bridle, Mark J.; Montchamp, Jean-Luc (2015-09-29). "P(═O)H to P–OH Tautomerism: A Theoretical and Experimental Study". The Journal of Organic Chemistry. 80 (20). American Chemical Society (ACS): 10025–10032. doi:10.1021/acs.joc.5b01618. ISSN   0022-3263. PMID   26372089.
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