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Phase transfer catalysis, abbreviated as PT, is a new synthesis technique that has been increasingly widely used in organic synthesis since the 1970s. Heterogeneous organic reactions are often encountered in organic synthesis, which typically have slow reaction rates and low yields. But if water-soluble inorganic salts are used, organic matter is dissolved in organic solvents with low polarity, and a small amount of quaternary ammonium salts or quaternary phosphate salts are added, the reaction can easily proceed. This type of onium salt that can promote the increase of reaction rate and transfer negative ions between the two phases is called a phase transfer catalyst. Generally, reactions involving phase transfer catalysis involve two phases: aqueous solution and organic solvent. Ionic reactants are often soluble in the aqueous phase and insoluble in the organic phase, while organic substrates are soluble in the organic solvent. When there is no phase transfer catalyst, the two phases are isolated from each other, and several reactants cannot come into contact, resulting in a slow reaction. The presence of phase transfer catalysts can bind with ions in the aqueous phase and utilize their affinity for organic solvents to transfer reactants from the aqueous phase to the organic phase, promoting the reaction to occur.

Advantages of phase transfer catalysts:

(1) Not using expensive special solvents and not requiring anhydrous operation, simplifying the process;

(2) Due to the presence of phase transfer catalysts, the negative ions participating in the reaction exhibit high reactivity;

(3) It has universality and wide applications;

(4) Atomic economy.

Commonly used phase transfer catalysts listed by Ahsuperchem:

1. Polyether: chain polyethylene glycol, chain polyethylene glycol dialkyl ether

2. Cyclic crown ethers: 18-Crown-6, 15-Crown-5, cyclodextrin, etc

3. Quaternary ammonium salts: Commonly used quaternary ammonium salt phase transfer catalysts include Benzyl triethylammonium chloride, Tetrabutylammonium bromide, Tetrabutylammonium chloride, Trioctylmethylammonium chloride, Dodecyl trimethylammonium chloride, Tetradecyltrimethylammonium chloride, etc

4. Tertiary amines, pyridine, tributylamine, etc

5. Quaternary ammonium hydroxides

6. Quaternary phosphonium salts