Phase Transfer Catalysis

Phase transfer catalysis (PTC) is a powerful tool to carry out organic reactions in a practical fashion, both in laboratory and industrial scale. Significant cost savings and major process improvements can be achieved in reactions performed in water-organic solvent mixtures under PTC conditions. Accordingly, the market of phase transfer catalysts has continuously grown in the past few years reaching 1,000 USD Million size. Particularly advantageous is the possibility to replace expensive solvents and dangerous bases such as sodium hydride, requiring strictly anhydrous conditions, with aqueous bases and apolar, environmentally friendly and easily recyclable solvents. During the last decade notable results have been obtained in stereoselective reactions performed in the presence of chiral, non-racemic quaternary ammonium salts. In addition, new avenues have been opened by the use of bulky chiral phosphate anions to generate lipophilic ion pairs enabling the stereoselective construction of important building blocks. Large scale industrial applications of asymmetric PTC have also been described. The dissemination of PTC concepts has been facilitated by the possibility to visualize the actual transfer of the ion pair to the organic phase as well as its consumption.