Chiral Onium Salts Phase-Transfer Reactions

Chiral Onium Salts (Phase-Transfer Reactions) 

Comprehensive Enantioselective Organocatalysis Catalysts, Reactions, and Applications, First Edition. 

In contrast, the development of asymmetric phase-transfer catalysis based on the use of structurally well-defined chiral, non-racemic catalysts has progressed rather slowly, despite its potential to create a new area of asymmetric catalysis by taking full advantage of structurally and stereochemically modifiable tetraalkylonium ions (Q ). However, recent efforts in this direction have resulted in notable achievements, thus making it feasible to perform various bond-formation reactions under the mild conditions used in phase-transfer catalysis [4], 

Based on the design of chiral quaternary onium bromides and chlorides as chiral phase-transfer catalysts, chiral quaternary onium fluorides [5] and phenoxides [6] have been developed as chiral base catalysts for homogeneous reactions. 

This chapter aims to assist the reader to generate an intuitive understanding of the catalyst types, reaction classes, and mechanism of chiral onium salts catalyzed asymmetric reactions. 

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I 74 Chiral Onium Salts (Phase-Transfer Reactions)... 

Abstract Phase transfer catalysts including onium salts or crown ethers transfer between heterogeneous different phases and catalytically mediate desired reactions. Chiral non-racemic phase transfer catalysts are useful for reactions producing new stereogenic centers, giving chiral non-racemic products. Recent developments in this rapid expanding area will be presented. 

Considerable efforts made for the synthesis of biologically relevant molecules by means of asymmetric phase-transfer catalysis are summarized in this chapter. Because the phase-transfer reaction is usually insensitive to the contamination of air, moisture, and even acidic or inorganic-salt impurities, and it is set up with simple and user-friendly protocols. It is recognized as one of the easiest methods for large-scale, stereoselective production of functional molecules as exemplified by the studies reported from pharmaceutical companies. In addition, ready accessibility of chiral onium salts as a catalyst facilitates an initial trial in... 

Asymmetric phase-transfer catalysis using chiral nonra-cemic onium salts or crown ethers has now grown into a practical method whereby a large number of reactions can be performed and some optically pure compounds can be produced effectively on a large scale. 

Over the past decades, quaternary ammonium- and phosphonium salts have been widely employed as effective phase-transfer catalysts in reactions between snb-stances located in different immiscible phases. Recently, efforts have been made to unlock the full potential of chiral non-racemic onium salts as versatile catalysts for asymmetric carbon-carbon bond formation. These reactions can be condncted nnder mild biphasic conditions and the phase-transfer catalysts can often be derived from readily available naturally occurring alkaloids. The reaction proceeds since the catalyst forms a well-defined chiral ion pair with the electrophile. As a result one enantiotopic face is shielded and enantioselective carbon-carbon bond formation can be realized. 

In the present paper we wish to report on the most significant examples of chiral polymer-supported onium salts and polymeric amines which have been used in heterophase reactions. In the first part attention will be paid to describe the Michael-type reactions that have been rather extensively studied by several research groups. Even though they constitute an example of application placed right at the borderline of base catalyzed reactions performed under conventional and phase transfer conditions, they appear worthy of comment by virtue of the achieved valuable and reproducible optical yields in the chemical transformation of several prochiral substrates. 

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