Phosphite ester

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

In 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.

Synthesis

From PCl3

Phosphite esters are typically prepared by treating phosphorus trichloride with an alcohol. Depending on the synthetic details, this alcoholysis can give the diorganophosphites:[1]

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:[2]

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, as they undergo alcohol exchange upon heating with other alcohols.[3] 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 trialkoxyphosphites

Reactions

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.[4]

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

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[5] (scheme). In this process the phosphite is converted to a phosphate ester. This reaction type is also utilized in the Wender Taxol total synthesis.

Coordination chemistry

Phosphite esters are lewis basic 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). In contrast to phosphine ligands, phosphites exhibit a smaller ligand cone angles, making them appealing as ligands. They remain somewhat less important that the structurally related phosphine ligand family.[6]

Chemistry of HP(O)(OR)2

Structure of a diorganophosphite.

Diorganophosphites are derivatives of phosphorus(V) and can be viewed as the di-esters of phosphorous acid. They exhibit tautomerism, however the equilibrium overwhelmingly favours the right-hand (phosphonate-like) form:[7]

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

See also

References

  1. ↑ Malowan, John E. (1953). "Diethyl phosphite". Inorganic Syntheses. 4: 58–60. doi:10.1002/9780470132357.ch19.
  2. ↑ A. H. Ford-Moore; B. J. Perry (1963). "Triethyl Phosphite". Organic Syntheses. ; Collective Volume, 4, p. 955
  3. ↑ 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.
  4. ↑ Rainer Wolf; Bansi Lal Kaul (2000). "Plastics, Additives". Ullmann's Encyclopedia Of Industrial Chemistry. doi:10.1002/14356007.a20_459.
  5. ↑ 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.
  6. ↑ 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. Phosphorus(III) Ligands in Homogeneous Catalysis: Design and Synthesis. John Wiley & Sons. doi:10.1002/9781118299715.ch3.
  7. ↑ Doak, G. O.; Freedman, Leon D. (1 February 1961). "The Structure and Properties of the Dialkyl Phosphonates". Chemical Reviews. 61 (1): 31–44. doi:10.1021/cr60209a002.
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