Introduction
Quaternary phosphonium compounds (QPC), a key class of organophosphorus compounds, can be synthesized by routes involving nucleophilic phosphorus. The synthesis process of QPC shows below.
Fig 1. Synthesis of quaternary phosphonium salt
Application of Quaternary Phosphonium Compounds in Chemical Reaction
Because of its unique chemical structure, quaternary phosphates are often used as reagents in multiple chemical reactions, such as in Witting reaction, reduction reaction and others. The main chemical reactions are introduced below:
- Wittig reaction
The Wittig reaction allows the preparation of an alkene by the reaction of an aldehyde or ketone with the ylide generated from a phosphonium salt. The geometry of the resulting alkene depends on the reactivity of the ylide.
Fig 2. Application of QPC in Witting reaction
If R is the primary alkyl methyl or an unimpeded alkyl, the reaction efficiency is higher, while if R is the secondary or higher alkyl, the reaction effect is very poor, and if R is tertiary alkane the salt can't be produced.
- Reduction reaction
Quaternary phosphonium salt is stable with mild reductant, but it can be reduced by strong reducing agents like sodium, lithium aluminum hydride. The reaction equation shows below.
Fig 3. Reduction reaction of quaternary phosphonium salt
- Other reaction
A readily available quaternary phosphonium salt containing a trifluoroacetonyl group and a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF4-) counterion was demonstrated to be a highly active Bronsted acid catalyst for Friedel-Crafts-type reactions of an array of electron-rich heteroarenes and aniline derivatives with isatin-derived ketimines, even at 0.1mol% catalyst loadings.
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