Other Asymmetric Phase-Transfer Reactions

In addition to the above-mentioned asymmetric transformations, some novel phase-transfer catalytic reactions, such as dihydroxylation [143], Neber-rearrangement [144], ring-opening reaction [145], and hydrolysis [146], have emerged in recent years. Under mild PTC conditions, these asymmetric transformations involve 

Applications, and Industrial Perspectives, Chapman Hall, New York. 

(a) Dehmlow, E.V. and Dehmlow, S.S. (1993) Phase Tranter Catalysis, 3rd edn, Wiley-VCH Verlag GmbH, Weinheim. (b) Sasson, Y. and Neumann, R. (eds) (1997) Handbook of Phase Transfer Catalysis, Blackie Academic Professional, London, (c) Weber, W.P. and Gokel, G.W. (2012) Phase Transfer Catalysis in Organic Synthesis, Springer, New York. 

(a) Mamoka, K. (ed.) (2008) Asymmetric Phase Transfer Catalysis, Wiley- 

VCH Verlag GmbH, Weinheim. (b) Shirakawa, S. and Maruoka, K. (2010) in Catalytic Asymmetric Synthesis, 3rd edn (ed. 1. Ojima), John Wiley Sons, Inc, Hoboken, NJ, pp. 95-117. 

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Aqueous alkali hydroxides can be used to replace flammable bases of sodium metal, sodium hydride, sodamide, and other alkoxides. The reaction temperature is lowered while the reaction rate improves because the increased reactivity of anions in the nonpolar solvent (Goldberg, 1989 Dehmlow and Dehm-low, 1993 Starks et al., 1994) as have asymmetric phase-transfer reactions (O Donnell, 1993). 

As is the case in all other quinine-catalyzed reactions, the quininium-salt-catalyzed phase-transfer reactions are subject to strong solvent effects (Table 8) (81). The fact that, in the presence of water, polar solvents lower the e.e., whereas apolar solvents raise the e.e., indicates that these are true phase-transfer reactions in which the ion pairs within the organic layer are responsible for the asymmetric induction. 

Application Asymmetric Phase Transfer Catalysis to Other Important Reactions 1129... 

Some other very important events in the historic development of asymmetric organocatalysis appeared between 1980 and the late 1990s, such as the development of the enantioselective alkylation of enolates using cinchona-alkaloid-based quaternary ammonium salts under phase-transfer conditions or the use of chiral Bronsted acids by Inoue or Jacobsen for the asymmetric hydro-cyanation of aldehydes and imines respectively. These initial reports acted as the launching point for a very rich chemistry that was extensively developed in the following years, such as the enantioselective catalysis by H-bonding activation or the asymmetric phase-transfer catalysis. The same would apply to the development of enantioselective versions of the Morita-Baylis-Hillman reaction,to the use of polyamino acids for the epoxidation of enones, also known as the Julia epoxidation or to the chemistry by Denmark in the phosphor-amide-catalyzed aldol reaction. ... 

The asymmetric aziridination of a, P-unsaturated carboxylic acid derivatives is a direct route to optically active aza-cyclic a-amino acids, and this class of chiral aziridines can also be used as chiral building blocks for the preparation of other amino acids, P-lactams, and alkaloids. Prabhakar and coworkers carried out an asymmetric aziridination reaction of tert-butyl acrylate with O-pivaloyl-N-arylhydroxylamine 25 in the presence of cinchonine-derived chiral ammonium salt 2e under phase-transfer conditions, which furnished the corresponding chiral N-arylaziridine 26 with moderate enantioselectivity (Scheme 2.24) [46],... 

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). 

Aldol reactions using a quaternary chinchona alkaloid-based ammonium salt as orga-nocatalyst Several quaternary ammonium salts derived from cinchona alkaloids have proven to be excellent organocatalysts for asymmetric nucleophilic substitutions, Michael reactions and other syntheses. As described in more detail in, e.g., Chapters 3 and 4, those salts act as chiral phase-transfer catalysts. It is, therefore, not surprising that catalysts of type 31 have been also applied in the asymmetric aldol reaction [65, 66], The aldol reactions were performed with the aromatic enolate 30a and benzaldehyde in the presence of ammonium fluoride salts derived from cinchonidine and cinchonine, respectively, as a phase-transfer catalyst (10 mol%). For example, in the presence of the cinchonine-derived catalyst 31 the desired product (S)-32a was formed in 65% yield (Scheme 6.16). The enantioselectivity, however, was low (39% ee) [65],... 

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. 

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