Phase Transfer Catalysis in Green Synthesis

The mechanism of the PTC reaction is well known and is described in relevant literature.  

Phase transfer catalysts used are the quaternary onium salts, such as ammonium, phosphonium, antimonium and tertiary sulphonium salts. In practice, however, only a limited number of ammonium and phosphonium salts are widely used. Some of the PTCs normally used are  

Besides the above, another catalyst, crown ether is also widely used as PTC. 

It is now well known that PTC reactions have considerable advantage over the conventional procedures. PTC reactions  

A number of phase transfer catalysts are commercially available. However, these can be conveniently synthesised. Some of the advantages of phase-transfer catalysts which are relevant to green synthesis are as follows  

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A recent literature report described a green procedure for the condensation of arylacetonitriles with cyclic ketones using phase-transfer catalysis. This process was applied to the synthesis of venlafaxine, which was realized in overall 30% yield in two steps from commercially available 14. The condensation step was run in aqueous sodium hydroxide in the presence of tetrabutylammonium sulfate, to provide quantitative yield of intermediate 15. Hydrogenation in a formalin-methanol mixture provided the final product venlafaxine (1) in 30% overall yield. This protocol did not necessitate intermediate purification steps, making it attractive from the commercial standpoint. 

Makosza, M. Phase-transfer catalysis. A general green methodology in organic synthesis. Pure Appl. Chem. 2000, 72. 1399-1403. 

M. Makosza, Pure Appl. Chem., 72,1399 (2000). Phase Transfer Catalysis. A General Green Methodology in Organic Synthesis. 

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