Phosphine oxide

Phosphine oxides are either inorganic phosphorus compounds such as phosphoryl trichloride (Cl3P=O) or organophosphorus compounds with the formula OPR3, where R = alkyl or aryl. Organophosphine oxides are considered to be the most stable organophosphorus compounds, triphenylphosphine oxide and trimethylphosphine oxide, decomposing only above 450 °C.[1]

Contents

Bonding

The nature of the phosphorus to oxygen "double bond" in phosphine oxides has long been debated. Pentavalent phosphorus is not compatible with the octet rule, but phosphorus is known to violate this rule anyway, e.g. phosphorus pentafluoride and phosphoric acid. In older literature the bond is represented as a dative bond, as is currently used to depict an amine oxide. The involvement of a phosphorus d-orbitals in bonding is not supported by computational analyses. Alternative theories favor an ionic description R3P+−O−, which explains the short bond length. Molecular Orbital Theory proposes that the short bond length is attributed to the donation of the lone pair electrons from oxygen to the antibonding phosphorus-carbon bonds. This proposal, which is supported by ab initio calculations, has gained consensus in the chemistry community.[2]

Syntheses

Phosphine oxides are most frequently generated as a by-product of the Wittig reaction:

R3PCR'2 + R"2CO → R3PO + R'2C=CR"2

Another common route to phosphine oxides is the thermolysis of phosphonium hydroxides. In the laboratory, phosphine oxides are usually generated by the oxidation, often accidentally, of tertiary phosphines:

R3P + 1/2 O2 → R3PO

Use

Phosphine oxides are made as by-products in the Wittig reaction. They themselves are usable in a Wittig-like reaction. So benzaldehyde is converted into β-methoxystyrene using methoxymethyl diphenylphosphine oxide in a two step procedure. In step one the phosphine oxide is deprotonated at −90 °C in THF/ether with lithium diisopropylamide, then the aldehyde is added. After aqueous workup, adducts are isolated. With potasium-t-butoxide the adducts are, at room temperature, converted to the styrenes. As the adducts exist as a separatable mixture of D/L compounds and a meso-form the final styrenes are obtainable as pure E- or Z-form .[3]

Parent compound

The parent compound phosphine oxide (H3PO) is unstable. It has been detected with mass spectrometry as a reaction product of oxygen and phosphine,[4] by means of FT-IR in a phosphine - ozone reaction [5] and in matrix isolation with a reaction of phosphine, vanadium oxytrichloride and chromyl chloride.[6]It is also been reported relatively stable in a water/ethanol solution by electrochemical oxidation of white phosphorus where is slowly disproportionates into phosphine and hypophosphorous acid.[7] Phosphine oxide is tautomeric with phosphinous acid (H2POH).

Phosphine oxide is reported as a intermediate in the room-temperature polymerization of phosphine and nitric oxide to solid PxHy [8]

References

  1. ^ D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam 1995. ISBN 0-444-89307-5.
  2. ^ G. L. Miessler and D. A. Tarr “Inorganic Chemistry” 3rd Ed, Pearson/Prentice Hall publisher, ISBN 0-13-035471-6.
  3. ^ Synthesis of 3- (1 -alkenyloxy) -1,2-propanediols, enol ethers of glycerol, by the horner-wittig reaction Recueil des Travaux Chimiques des Pays-Bas Volume 102, Issue 10, 1983, Pages: 465–466, T. A. M. van Schaik and A. van der Gen doi:10.1002/recl.19831021009
  4. ^ Kinetics and mechanism of the reactions of PH3 with O(3P) and N(4S) atoms Peter A. Hamilton and Timothy P. Murrells J. Chem. Soc., Faraday Trans. 2, 1985, 81, 1531-1541 doi:10.1039/F29858101531
  5. ^ FTIR spectra of the photolysis products of the phosphine-ozone complex in solid argon Robert. Withnall, Lester. Andrews J. Phys. Chem., 1987, 91 (4), pp 784–797 doi:10.1021/j100288a008
  6. ^ Matrix Isolation and Theoretical Study of the Photochemical Reaction of PH3 with OVCl3 and CrCl2O2 David A. Kayser and Bruce S. Ault J. Phys. Chem. A, 2003, 107 (33), pp 6500–6505 doi:10.1021/jp022692e
  7. ^ Yakhvarov, D., Caporali, M., Gonsalvi, L., Latypov, S., Mirabello, V., Rizvanov, I., Sinyashin, O., Stoppioni, P. and Peruzzini, M. (2011), Experimental Evidence of Phosphine Oxide Generation in Solution and Trapping by Ruthenium Complexes. Angewandte Chemie International Edition, 50: 5370–5373. doi:10.1002/anie.201100822
  8. ^ Phosphine Polymerization by Nitric Oxide: Experimental Characterization and Theoretical Predictions of Mechanism Yi-Lei Zhao, Jason W. Flora, William David Thweatt, Stephen L. Garrison, Carlos Gonzalez, K. N. Houk and Manuel Marquez Inorg. Chem., 2009, 48 (3), pp 1223–1231 doi:10.1021/ic801917a