Phosphonium salt is a phase transfer catalyst newly developed in recent years, characterized by high catalytic efficiency, low toxicity, high thermal stability, and recyclability. The development of a phase transfer catalyst of the phosphonium salt type started later than the ammonium salt. With the vigorous development of asymmetric chemistry, quaternary phosphonium salt phase transfer catalysts have rapidly developed. Representative quaternary phosphonium salt catalysts are mainly divided into biphenyl naphthalene skeleton, spiro ring skeleton and amino acid skeleton derivatization catalyst. The development of these novel catalysts has significantly expanded the types and applications of quaternary phosphonium salt phase transfer catalysts.
Figure 1. Quaternary phosphonium salts phase transfer catalyst
- Catalytic alkylation reaction: Some researchers have reported that the chiral quaternary phosphonium salt as a phase transfer catalyst catalyzed benzylation of β-carbonyl ester, achieving moderate yield and moderate enantioselectivity, which is the first time that a quaternary phosphonium salt phase transfer catalyst has been used in the catalytic reaction.
- Catalytic asymmetric amination reaction: Some researchers have reported that chiral C2-symmetric bi-naphthalene-derived quaternary phosphonium salt phase transfer catalysts catalyze the asymmetric amination of β-ketoesters. The catalyst can efficiently catalyze the reaction, and the product is obtained with good yield and ee value.
Figure 2. Quaternary phosphonium salt phase transfer catalyst for asymmetric amination reaction
- Catalytic cycloaddition reaction: In the reaction catalyzed by the phosphonium salt phase transfer catalyst, there is little catalytic cycloaddition reaction. However, some researchers have reported a fluorinated quaternary phosphonium salt phase transfer catalyst which can be used to catalyze the cycloaddition reaction of an acetal to an activated imine. Even though some quaternary ammonium salts of fluoride do not give satisfactory results, this phosphorus salt phase transfer catalyst gives a disubstituted lactam product in high yield and diastereoselectivity.
Figure 3. Quaternary phosphonium salt phase transfer catalyst for cycloaddition reaction.
The phosphonium salt phase transfer catalysts are mainly classified into the following types: the alkyl or aryl-substituted quaternary phosphonium salt, the quaternary phosphonium salt supported on a polymer, the N-alkyl substituted quaternary phosphonium salt, and the conjugated ion-type quaternary phosphonium salt.
- The alkyl or aryl-substituted quaternary phosphonium salt: For example, common tetraphenylphosphonium bromide or tetraphenylphosphonium chloride. These catalysts have good thermal stability and the quaternary phosphonium ion PR4+ has strong binding ability to F-, which increases the reaction rate and yield. It has been applied to decarboxylation reaction and denitrofluorination reaction.
- Quaternary phosphonium salt supported on a polymer: It has a good catalytic effect and can be separated by simple filtration and can be recycled. Therefore, it is possible to reduce costs and reduce pollution.
- N-alkyl substituted quaternary phosphonium salt: These catalysts have good catalytic properties, high-temperature resistance, and low toxicity.
- Conjugated ion-type quaternary phosphonium salt: This type of catalyst is mainly used in the Halex reaction and is a highly efficient catalyst. It can increase the reaction rate and avoid the generation of reaction by-products and decomposition of products at high temperatures for a long time. Therefore, energy can be saved and the yield can be greatly improved.
- Manabe K. (1998). "Asymmetric phase-transfer alkylation catalyzed by a chiral quaternary phosphonium salt with a multiple hydrogen-bonding site".Tetrahedron Letters. 39(32): 5807-5810.
- Enders D, Nguyen T V. (2012). "Chiral quaternary phosphonium salts: A new class of Organocatalysts". Organic&Biomolecular Chemistry. 10(28):5327-5331.
- Liu S, Kumatabara Y, Shirakawa S.(2016). "Chiral quaternary phosphonium salts as phase-transfer catalysts for environmentally benign asymmetric Transformations". Green Chemistry. 18(2): 331-341.