"Phosphite" redirects here. For the anion, seePhosphite anion.
The general structure of a phosphite ester showing thelone pairs on the P
Inorganic chemistry, aphosphite ester ororganophosphite usually refers to anorganophosphorous compound with the formula P(OR)3. They can be considered asesters of an unobserved tautomerphosphorous acid, H3PO3, with the simplest example beingtrimethylphosphite, P(OCH3)3. Some phosphites can be considered esters of the dominant tautomer of phosphorous acid (HP(O)(OH)2). The simplest representative isdimethylphosphite with the formula HP(O)(OCH3)2. Both classes of phosphites are usually colorless liquids.
Phosphite esters are typically prepared by treatingphosphorus trichloride with analcohol. Foralkyl alcohols the displaced chloride ion can attack the phosphite, causing dealkylation to give a dialkylphosphite and anorganochlorine 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 bytransesterification, 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 astrimethyl 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
This reaction underpins the commercial use of some phosphite esters asstabilizers in polymers.[6]
Alkyl phosphite esters are used in thePerkow reaction for the formation of vinyl phosphonates, and in theMichaelis–Arbuzov reaction to formphosphonates. Aryl phosphite esters may not undergo these reactions and hence are commonly used as stabilizers in halogen-bearing polymers such asPVC.
Diorganophosphites are derivatives of phosphorus(V) and can be viewed as the di-esters ofphosphorous acid ((HO)2P(O)H). They exhibittautomerism, 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 theAtherton–Todd reaction andHirao coupling), whereas in tri-organophosphites the lone pair on phosphorus is the site of high reactivity. Diorganophosphites do however undergo transesterification.
^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.
^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.
^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.PMID5742870.
^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.
^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.ISBN9781118299715.
^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.ISSN0022-3263.PMID26372089.
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