"Phosphite" redirects here. For the anion, see
Phosphite anion.
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.
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
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.
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.
^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.
^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.
^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.
^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.