Commonly used phase transfer catalysts are chiral onium salt and chiral crown ethers. The chiral onium salts phase transfer catalyst may be a chiral quaternary ammonium salt or a chiral quaternary phosphonium salt. Among them, the chiral quaternary ammonium salt phase transfer catalyst is one of the most studied catalysts in the asymmetric catalytic reaction, and many bonding reactions are completed under mild phase transfer catalysis. The chiral quaternary ammonium salt type phase transfer catalyst is currently the most mature type of phase transfer catalysts. Up to now, according to the main skeleton of the catalyst, it can be classified into a cinchona base skeleton, a binaphthyl skeleton, a chiral guanidine, and an amino acid skeleton. The phase transfer catalysts of the cinchona base skeleton and the binaphthyl skeleton have achieved great success in asymmetric transformation. The phase transfer catalyst of the amino acid skeleton has been developed in recent years, and the application in asymmetry needs further research.
Figure 1. Chiral quaternary ammonium salt catalyst
- Catalytic asymmetric alkylation reaction: Phase transfer catalysts have been widely used in asymmetric alkylation reactions, particularly asymmetric alkylation of glycine Schiff bases, which is an efficient method for the synthesis of optically active α-amino acids. Some researchers have found that the binaphthyl quaternary ammonium salt phase transfer catalyst can achieve selective monoalkylation of glycine Schiff base in a mild liquid-liquid reaction system, and further acidolysis can obtain α-amino acid. The yield is 99% and the enantioselectivity is 98%.
- Catalytic cyclization reaction: The epoxidation of α,β-unsaturated ketones under ammonium salts phase transfer catalysis has been extensively studied. Some scholars have reported the asymmetric epoxidation of chalcone in NaOCl aqueous solution under the action of the quaternary ammonium salt phase transfer catalysts of cinchona. The product has enantioselectivity of 86% and a yield of up to 98%. Subsequently, the phase transfer catalyst derived from N-2,3,4-trifluorophenylrholine base further enhances the enantioselectivity of the reaction.
- Catalytic mannich reaction: Some researchers have reported the aza-Henry reaction under the action of cinchona quaternary ammonium phase transfer catalyst. The corresponding N-carbamine imine is formed in situ by using a-amino sulfone, and the target product is obtained by reacting with nitromethane. The reaction is equally applicable to aliphatic amines and aromatic amines. Other types of cinchona base-derived quaternary ammonium salt phase transfer catalysts have also been used in this system with enantioselectivity of up to 99%.
- Catalytic aldol reaction: The direct Aldol reaction of glycine donor and aldehyde under ammonium salts phase transfer catalyst is an important method for constructing β-hydroxy-α-amino acid, which is of great significance in pharmaceutical synthesis. Some researchers have reported the asymmetric Aldol reaction of N-diphenylmethylene glycine t-butyl ester with phenylpropanoid. A trans-β-hydroxy-α-amino acid derivative is obtained using a symmetric naphthyl phase transfer catalyst.
- Catalytic conjugate addition: Some researchers have used the third generation of cinchona base phase transfer catalyst to carry out a conjugate addition reaction of N-diphenylmethylene glycine tert-butyl ester and vinyl ketone, followed by imine exchange under acid catalysis. Further Catalytic hydrogenation gives the cis 5-substituted valine ester.
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- Maruoka K, (2012). "Designer chiral phase-transfer catalysts for green sustainable chemistry". Pure and Applied Chemistry, 84(7): 1575-1585.