Chiral onium salts

For enantioselectivity to be possible multipoint interaction between the catalyst and the reactant in the transition state is necessary. The most effective chiral onium salts are derivatives of cinchona alkaloids (see Fig. 3.59). 

Chiral Induction.—As with many synthetic techniques in organic chemistry, PTC methods have been under investigation to determine whether asymmetric induction (chiral selection) can be achieved. In this case chiral onium salts are necessary, and the systems investigated to date have been quaternary derivatives (23) of A -methyl ephedrine. These salts have been used to catalyse two-phase metal boro-hydride reductions of ketones, and the chiral tetra-alkylammonium borohydride ion pair does accelerate this reaction " in organic solvents. However, either no asymmetric reduction is observed, " or low optical purity of products is achieved (less than 15% enantiomer excess). - A low chiral selectivity is also observed in... 

The recent results reported by Cram are extremely important because his catalysts are chiral crown ethers, and represent one of the first attempts to develop novel structures without the inherent disadvantages of the ammonium salts described above. Such crowns yield products from Michael additions of 3-ketoesters to methyl vinyl ketone with optical purities up to 99%. A series of novel chiral onium salts derived from methionine have also been reported by Colonna and his coworkers. Although so far these have displayed no asymmetric induction in their reactions, they are important as an imaginative new series of catalysts. 

Also of considerable encouragement were the results reported in the Darzens reaction some time ago . Carbonyl compounds when reacted with chloromethyl-p-tolyl sulphone or with chlorophenyl-acetonitrile in the presence of aqueous sodium hydroxide and chiral onium salts derived from [I] [reaction 7 and 8] yielded product epoxides with up to 2.5% enantiomeric excesses. More importantly. 

Chiral Onium Salts (Phase-Transfer 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. 

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

As described in this chapter, numerous recent efforts to elaborate chiral onium salts have greatly expanded their use as asymmetric catalysts in organic synthesis. However, the full potential of chiral onium salts is yet to be realized in terms of general applicability. Continuous efforts should be made toward the design and development of conceptually new catalysts, and to understanding the relationship between catalyst structure and stereocontrolling ability. 

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... 

Chiral onium salts, such as chiral quaternary ammonium and phosphonium salts, are the predominant choice for asymmetric PTC. The onium salt-triggered PTC is beheved to proceed via the formation of catalyst-substrate ion pairs. High enantioselectivity could be achieved if the chiral cation was designed with effective structural features. 

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