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, +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:

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 demonstrates the use of benzyl alcohols as starting material for the synthesis of phosphonium acetates provided that the arene carries activating groups:

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.