Other Chiral Phase-Transfer Catalysts for Asymmetric Synthesis

Other Chiral Phase-Transfer Catalysts for Asymmetric Synthesis 

Currently, the chiral phase-transfer catalyst category remains dominated by cinchona alkaloid-derived quaternary ammonium salts that provide impressive enantioselec-tivity for a range of asymmetric reactions (see Chapter 1 to 4). In addition, Maruoka s binaphthyl-derived spiro ammonium salt provides the best results for a variety of asymmetric reactions (see Chapters 5 and 6). Recently, some other quaternary ammonium salts, including Shibasaki s two-center catalyst, have demonstrated promising results in asymmetric syntheses (see Chapter 6), while chiral crown ethers and other organocatalysts, including TADDOL or NOBIN, have also found important places within the chiral phase-transfer catalyst list (see Chapter 8). 

Apart from these well-known catalysts, much effort has been expended in the synthesis and applications of chiral phase-transfer catalysts that include various quaternary ammonium salts, metal-salen complexes, phosphonium salts, and chiral amines. However, few of these catalysts have shown promising levels of asymmetric induction in asymmetric reactions. 

The aim of this chapter is to provide up-to-date information on the design and applications of other chiral phase-transfer catalysts. 

Asymmetric Phase Transfer Catalysis. Edited by Keiji Maruoka Copyright 2008 WILEY-VCH GmbH Co. KGaA, Weinheim ISBN 978-3-527-31842-1 

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Cinchona-Derived Chiral Phase-Transfer Catalysts for Other Asymmetric Synthesis... 

Several families of efficient chiral phase transfer catalysts are now available for use in asymmetric synthesis. To date, the highest enantiomeric excesses (>95% ee) are obtained using salts derived from cinchona alkaloids with a 9-anthracenylmethyl substituent on the bridgehead nitrogen (e.g. lb, 2b). These catalysts will be used to improve the enantiose-lectivity of existing asymmetric PTC reactions and will be exploited in other anion-mediated processes both in the laboratory and industrially. 

Asymmetric phase-transfer catalysis is a method that has for almost three decades proven its high utility. Although its typical application is for (non-natural) amino acid synthesis, over the years other types of applications have been reported. The unique capability of quaternary ammonium salts to form chiral ion pairs with anionic intermediates gives access to stereoselective transformations that are otherwise very difficult to conduct using metal catalysts or other organocatalysts. Thus, this catalytic principle has created its own very powerful niche within the field of asymmetric catalysis. As can be seen in Table 5 below, the privileged catalyst structures are mostly Cinchona alkaloid-based, whereas the highly potent Maruoka-type catalysts have so far not been applied routinely to complex natural product total synthesis. 

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