Phase Transfer Catalysts

Heterogeneous organic reactions are often encountered in organic synthesis. These reactions are generally slow, have low yields, and have incomplete reactivity. However, if a water-soluble inorganic salt is used, the organic substance is dissolved in an organic solvent having a small polarity, and a small amount (0.05 mol or less) of a quaternary ammonium salt or a quaternary phosphonium salt is added, and the reaction proceeds easily. Such a substance which promotes the reaction rate and transfers negative ions between two phases is called a phase transfer catalyst. In other words, phase transfer catalysts (PTCs) are a class of catalysts that can assist in the transfer of reactants from one phase to another that is capable of reacting, thereby accelerating the reaction rate of the heterogeneous system.

Applications

• Nucleophilic substitution reaction: The use of halogenated compounds and potassium cyanide to prepare nitriles is one of the earliest types of reactions using phase transfer catalysts. At present, these reactions can obtain good results not only with quaternary ammonium salts (or quaternary phosphonium salts) but also with crown ethers, and high yields can also be obtained with three-phase catalysts. For example, under the catalysis of a three-phase catalyst, 1-bromooctane (dissolved in benzene) is reacted with an aqueous solution of KCN to give a yield of 95%. Under normal conditions, 1-bromooctane does not react with C₆H₅SK or KSCN. However, in the solid-liquid two-phase, the reaction is easily catalyzed by a newly synthesized phase transfer catalyst, and the yield is high.
• Nucleophilic hydrocarbylation: The nucleophilic hydrocarbylation reaction is one of the most widely used types reaction using phase transfer catalysts. Under phase transfer catalysis, the hydrocarbon group can be easily introduced to the C, O, S, and N atoms. For example, multi-step reaction is required in the synthesis of O, S-diethyl, olylmethyl-methylthiomalonate. However, under the action of phase transfer catalyst (C₄H₉) ₄NHSO₄, the reaction can be completed in just two steps.
• Addition reaction: In the classical addition reaction, the hydrogen halide of the olefin and the gas must be reacted in a polar solvent in the absence of light and without a free radical initiator to obtain an addition product conforming to the Markov law. Under phase transfer conditions, olefin and hydrohalic acid aqueous solutions are easily carried out according to Markov's law.
• Eliminate the reaction: The reaction of removing two molecules of hydrogen halide to form an alkyne by a vicinal or homo-dihalogen is generally carried out in two steps. In the second step of removing the hydrogen halide, a strong base of NaNH₂ is used. However, in the presence of the phase transfer catalyst (C₈H₁₇)₄NBr, the reaction is completed in one step. For example, under the catalysis of (C₈H₁₇)₄NBr, the following o-dibromo compound is eliminated, and the yield of diphenylacetylene is up to 96%.

  Figure 1. Phase transfer catalytic reaction

Classification

The phase transfer catalyst has the advantages of accelerating the reaction rate, lowering the reaction temperature, high product yield, and simple synthesis operation. It can usually be divided into the following categories:

• Polyether: Chain polyethylene glycol H(OCH₂CH₂)_nOH and chain polyethylene glycol dialkyl ether R(OCH₂CH₂)_nOR.
• Cyclic crown ethers: 18 crown 6, 15 crowns 5, cyclodextrin and so on.
• Quaternary ammonium salts: Commonly used quaternary ammonium salt phase transfer catalysts are benzyltriethylammonium chloride (TEBA), tetrabutylammonium bromide (TBAB), tetrabutylammonium chloride, tetrabutylammonium hydrogen sulfate, trioctylmethylammonium chloride, dodecyltrimethylammonium chloride, tetradecyltrimethylammonium chloride and so on.
• Tertiary amine: R₄NX, pyridine, tributylamine and the like.
• Quaternary ammonium base: Its alkalinity is similar to that of sodium hydroxide, and it is easily soluble in water and has strong hygroscopicity.

  Figure 2. Phase transfer quaternary ammonium catalyst

References

1. Alberto, R.; Ortner, K.; Wheatley, N.( 2001). ”Synthesis and properties of boron carbonate”. J. Am. Chem. Soc. 121: 3135–3136.
2. Herriott, A.W. Picker, D.( 1975). ”Phase transfer catalysis. Evaluation of catalysis”. J. Am. Chem. Soc. 97 (9): 2345–2349.
3. Mieczyslaw Makosza.( 2000). “Phase-transfer catalysis: A general green methodology in organic synthesis.” Pure Appl. Chem. 72 (7): 1399–1403.