Abstract
The demand ’from industry in the mid-1960s for an alternative to be found for the expensive traditional synthetic procedures led to the evolution of phase-transfer catalysis in which hydrophilic anions could be transferred into an organic medium. Several phase-transfer catalysts are available: quaternary ammonium, phosphonium and arsonium salts, crown ethers, cryptands and polyethylene glycols. Of these, the quaternary ammonium salts are the most versatile and, compared with the crown ethers, which have many applications, they have the advantage of being relatively cheap, stable and non-toxic. Additionally, comparisons of the efficiencies of various catalysts have shown that the ammonium salts are superior to the crown ethers and polyethylene glycols, and comparable with the cryptands, which have fewer proven applications and require higher concentrations of catalyst compared with the ammonium salts, which mitigates against their use in large-scale industrial applications. It is for these reasons that this chapter focuses on the utilization of quaternary ammonium salts in organic synthesis.
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