Chiral anion recognition

The application of anion receptors in sensing has been mentioned earlier. Dioxatetraazamacrocycles 381 were synthesized for the application in chiral anion recognition [43]. Mesoporous films by 382 were developed to be used as sensors for volatile organic compounds [44]. Receptors for anions were also studied to mimic their transport through membranes [45]. 

Intramolecular asymmetric induction has also been used in electrochemistry as in the reduction of optically active alcohol esters or amides of a-keto [469,470] and unsaturated [471] acids and oximes [472] and in the oxidation of olefins [473]. A maximum asymmetric yield of 81% was obtained in the reduction of (5 )-4-isopropyl-2-oxazolidinone phenyl-glyoxylate [470]. Nonaka and coworkers [474,475] found that amino acid A-carboxy anhydrides were polymerized with various electrogenerated bases as catalyst to give the poly(amino acids) with high chirality in high yields. Conductive chiral poly(thiophenes) prepared by electropolymerization can be used for chiral anion recognition [476]. 

Nonetheless, much excellent use has been made of this system. Receptor 5, for example, extracts p-nitrobenzoate quantitatively from water into chloroform (43), and the chirality of the receptor allows the possibility of chiral anion recognition (44). Guanidinium has also been incorporated into devices, such as a hydrogen sulfite selective electrode (45). Recently, Mendoza and co-workers (46) reported a chiral double helical array of polyguanidinium strands assembled around sulfate templating anions, the first anion centered helical structure. 

Type II sorbents are based on an inclusion mechanism. Chiral recognition by optically active polymers is based solely on the helicity of that polymer. Optically active polymers can be prepared by the asymmetric polymerization of triphenylmethyl methacrylate using a chiral anionic initiator [264]. Helical polymers are unique from the previously discussed chromatographic approaches because polar functional groups are not required for resolution [265]. These commercially available sorbents have been used to resolve enantiomers of a-tocopherol [266]. The distinction between this group (lib) and the sorbents containing cavities is vague (Ila). 

Guanidines have been implemented early as recognition elements, guided by the apparent function of arginine in protein structures. The C2-symmetric, chiral anion receptor 52 was introduced by Lehn, Schmidtchen and de Mendoza consecutively and studied in various modifications (Scheme 13) [23c]. For example, an elaborate system based on 52 provided reasonable enantioselective recognition of amino acids [23c, 28]. Furthermore, bis(guanidinium) compounds catalyze RNA hydrolysis in the presence of external base via phosphodiester complexation [29]. The,se functional elements were joined in receptor 53 to yield a functional transesterification catalyst [30]. 

Chiral Recognition Using the Chiral Anion Strategy... 

Recently, Lee and co-workers have shown that abinol-strapped calix[4]pyr-role (49) can be used in the enantioselective recognition of carboxylate anions [66]. Both the R- and S-enantiomers of the strapped calixpyrrole were isolated and characterized. Detailed studies of the enantioselectivity of the S enantiomer were carried out by isothermal titration calorimetry experiments in acetonitrile with the chiral anions (R)-2-phenylbutyrate and (S)-2-phenylbutyrate. Stability constants were determined and revealed... 

Amino Acids Applications, p. 42 Anion-Directed Assembly, p. 51 Chiral Guest Recognition, p. 236... 

Amide- and Urea-Based Anion Receptors, p. 51 Amino Acids Applications, p. 42 Chiral Guest Recognition, p. 236 Deoxycholic, Cholic, and Apocholic Acids, p. 441 Fluorescence Sensing of Anions, p. 566 Guanidinium-Based Anion Receptors, p. 615 Hydrogen Bonding, p. 658 lonophores, p. 760... 

Later in this chapter we will show the state of the art in recognition of chiral anions. But first we will start with an overview of reeognition of achiral anions by sugar containing anion receptors. 

Table 3 Recognition ability of chiral anions (anions used as tetrabutylammonium salts) by receptor 23.

The synthesis of novel CILs with a spiro skeleton has been reported by Sasai and coworkers [97]. The influence of Al-substituents and the counteranions on their chiral discrimination abilities was investigated. In addition, the diastereomeric interaction between the novel spiro imidazolium-based CIL 17 and (5)-Mosher s potassium salt was examined. The H NMR spectrum of racemic spiro imidazolium salt 17 exhibited two pairs of counteranion from bromide to a chiral anion. The counteranion was changed in situ by treating racemic 17 with the potassium salt of Mosher s acid in the presence of 18-crown-6. The NMR spectrum exhibited excellent splitting in each pair of doublets. These results demonstrate spiro imidazolium salts are potentially useful in chiral molecular recognition (Fig. 13). 

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