Phosphonium salt

A phosphonium salt is a salt containing the phosphonium (PH₄⁺) ion such as phosphonium iodide (PH₄⁺I⁻). More commonly, phosphonium refers to a quaternary organic derivative such as tetraphenylphosphonium chloride, (C₆H₅)₄P⁺ Cl^- and tetramethylphosphonium iodide, [P(CH₃)₄]⁺I⁻.

Alkyltriphenylphosphonium salts are widely used for the preparation of Wittig reagents for the Wittig reaction. Such salts are readily made by the reaction of triphenylphosphine with an alkyl halide:

Synthesis of phosphonium halides from triphenylphosphine and an alkyl halide

Note that Ph stands for phenyl and X is a halide

The reaction works well if the alkyl group is methyl or an unhindered primary alkyl group (as shown), but it is usually poor with secondary alkyl halides. Tertiary alkyl groups cannot form the ylide. The phosphonium salt is a stable compound which can often be purified by recrystallisation from ethanol.

To form the Wittig reagent (ylide), the phosphonium salt is suspended in a solvent such as diethyl ether or THF and a strong base such as phenyllithium or n-butyllithium is added.

One study ^[1] demonstrates the use of benzyl alcohols as starting material for the synthesis of phosphonium acetates provided that the arene carries activating groups:

Phosphonium Acetate Synthesis from benzyl alcohols

Note that Ac stands for acetyl, the ester group is hydrolyzed to a phenol

The phosphonium acetate group does not have an impact on the subsequent Wittig reaction.

Phosphonium halides

Phosphines (R₃P) react with halogens (X₂) to phosphonium halides of the type R₃PX₂. The compound Ph₃PBr₂ (high melting solid) formed by reaction of triphenylphosphine and bromine is called bromotriphenylphosphonium bromide or dibromotriphenylphosphorane (now defined as a phosphorane) or simply triphenylphosphine dibromide. The dibromide and dichloride are commercially available as halogenation reagents for instance in halogenation of alcohols and phenols.^[2]

The compound triphenylphosphine dichloride, Ph₃PCl₂, is reported as being an ionic compound (PPh₃Cl)⁺Cl⁻ in polar solutions and a molecular species with trigonal bipyramidal molecular geometry in apolar solution and in the solid state.^[3]

References

^ One-pot synthesis of benzyltriphenylphosphonium acetates from the corresponding activated benzyl alcohols Paola Hernández, Alicia Merlino, Alejandra Gerpe, Williams Porcal, Oscar E. Piro, Mercedes González and Hugo Cerecetto Arkivoc 2006 (xi) 128-136 Online article
^ Studies in Organophosphorus Chemistry. I. Conversion of Alcohols and Phenols to Halides by Tertiary Phosphine Dihalides G. A. Wiley, R. L. Hershkowitz, B. M. Rein, B. C. Chung J. Am. Chem. Soc., 1964, 86 (5), pp 964–965 doi:10.1021/ja01059a073
^ Structural dependence of the reagent Ph3PCl2 on the nature of the solvent, both in the solid state and in solution; X-ray crystal structure of trigonal bipyramidal Ph₃PCl₂, the first structurally characterised five-coordinate R₃PCl₂ compound Stephen M. Godfrey, Charles A. McAuliffe, Robin G. Pritchard and Joanne M. Sheffield Chem. Commun., 1998 921 doi:10.1039/a800820e

[1] One-pot synthesis of benzyltriphenylphosphonium acetates from the corresponding activated benzyl alcohols Paola Hernández, Alicia Merlino, Alejandra Gerpe, Williams Porcal, Oscar E. Piro, Mercedes González and Hugo Cerecetto Arkivoc 2006 (xi) 128-136 Online article

[2] Studies in Organophosphorus Chemistry. I. Conversion of Alcohols and Phenols to Halides by Tertiary Phosphine Dihalides G. A. Wiley, R. L. Hershkowitz, B. M. Rein, B. C. Chung J. Am. Chem. Soc., 1964, 86 (5), pp 964–965 doi:10.1021/ja01059a073

[3] Structural dependence of the reagent Ph3PCl2 on the nature of the solvent, both in the solid state and in solution; X-ray crystal structure of trigonal bipyramidal Ph₃PCl₂, the first structurally characterised five-coordinate R₃PCl₂ compound Stephen M. Godfrey, Charles A. McAuliffe, Robin G. Pritchard and Joanne M. Sheffield Chem. Commun., 1998 921 doi:10.1039/a800820e

[1]

[2]

[3]

Phosphonium halides

See also

References