Chiral anion strategy

Chiral Recognition Using the Chiral Anion Strategy... 

Monolithic columns with the chiral anion exchange-type selectors incorporated into the polymer matrix obtained through in situ copolymerization process of a chiral monomer (in situ approach) [80-83,85] or attached to the surface of a reactive monolith in a subsequent derivatization step (postmodification strategy) [84], both turned out to be viable routes to enantioselective macroporous monolithic columns devoid of the limitations of packed columns mentioned earlier. 

More recently Toste and coworkers reported the use of the chiral counter-anion strategy with a functionalized BNP (4.29), to carry out some asymmetric hydroxycyliza-tion reactions with gold (1) catalysts (see Chapter 3, Section 3.3.3) (Scheme 4.3). The authors showed that the chiral counter-anion strategy has a profound effect on the enantioselectivity and generates products in 90-99% enantiomeric excess. These results hold promise for a golden future in this field. ... 

Recently, chiral anion catalysis has been introduced as a useful strategy in metal-catalyzed asymmetric transformations. You and co-workers disclosed that chiral substituted binol phosphate could serve as a suitable counteranion in Ag-catalyzed AFC reaction of ortfto-alkynylaryl aldimines 25 with indoles, affording enantioenriched 1,2-dihydroisoquinoline 27 in moderate to good yields and ee (Scheme 6.10). ... 

Besides chiral cations, also chiral anion PTCs have recently been successfully impl euted for challenging asynunetric transformations [71, 126]. However, this strategy was much less thoroughly investigated in the past than the use of chiral cation catalysts. One fact that should be emphasized is that an exact classification of the real activation mode is not always trivial in this field. To illustrate the difficulties associated with respect to the classification of chiral anion PTCs and the exact activation mechanism, the use of asymmetric phosphoric acid catalysts should be briefly mentioned. Hereby different modes of activation are possible in principle (see also the previous chapter on chiral acids). One extreme case involves complete protonation of the substrate and formation of a chiral ion pair with the conjugated base of the catalyst. On the other hand, such... 

In 2009, Seidel et al. reported a particularly elegant dual catalytic approach to the KR of primary aryl alkyl amines [125]. Their strategy reUed on the use of a chiral anion receptor such as a chiral thiourea, which could bind in situ to an achiral acyl pyridinium salt to form a chiral ion pair (Scheme 41.49). 

Although asymmetric PTC induced by chiral cationic salts was discovered over 30years ago, an analogous charge-inverted strategy - chiral anion PTC-was just put into practice only in 2008 by Toste and coworkers [97]. The authors envisioned that a lipophilic chiral anionic salt initially could extract a cationic reagent from... 

We thus turned to alternative strategies for synthesizing aldehyde 3. Particularly attractive was the proposal that sugar-like materials could be constructed via the reaction of an allyl ether anion and an a-alkoxyaldehyde (Figure 5).10,11 For this approach to be successful, it would be necessary to control (i) the regioseiectivity of the reaction of the allyl ether anion, 0 (jj) the syn (threo) or anti (erythro) relationship generated in concert with the new C-C bond, and (iii) this new C(2)-C(3) relationship with respect to the chiral center (C(4)) already present in the aldehyde reaction partner. 

Intermolecular reactions of hydroxylamines with secondary alkyl halides and mesylates proceed slower than with alkyl triflates and may not provide sufficiently good yield and/or stereoselectivity. A nseful alternative for these reactions is application of more reactive anions of 0-alkylhydroxamic acids or 0-alkoxysulfonamides ° like 12 (equation 8) as nucleophiles. The resulting Af,0-disubstituted hydroxamic acids or their sulfamide analogs of type 13 can be readily hydrolyzed to the corresponding hydroxylamines. The same strategy is also helpful for synthesis of hydroxylamines from sterically hindered triflates and from chiral alcohols (e.g. 14) through a Mitsunobu reaction (equation 9). 

A new catalyst salt (20) that consists of an achiral ammonium ion and a chiral phosphate anion and which catalyses highly enantioselective transfer hydrogenations of ,/J-unsaturated aldehydes to the corresponding saturated derivatives has been developed. The underlying principle, namely asymmetric counteranion-directed catalysis, is claimed to be a new strategy for highly enantioselective synthesis.357... 

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