Chiral anion

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

Polymerization of triphenylmethyl methacrylate in the presence of a chiral anion catalyst results in a polymer with a helical structure that can be coated onto macroporous silica [742,804). Enantioselectivity in this case results from insertion and fitting of the analyte into the helical cavity. Aromatic compounds and molecules with a rigid nonplanar structure are often well resolved on this phase. The triphenylmethyl methacrylate polymers are normally used with eluents containing methanol or mixtures of hexane and 2-propanol. The polymers are soluble in aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran which, therefore, are not suitable eluents. 

Lammerhofer, M., Maier, N. M., and Lindner, W., Chiral anion exchange-type stationary phases based on cinchonan alkaloids, Am. Lab., 71, April 1998. 

Similarly, use of the chiral anionic initiator (+) or (—) potassium menthoxide with the same masked disilene afforded a polysilane with a PSS helical conformation. Optically active polysilanes are discussed further in Section 3.11.6. 

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. 

Buffered mobile phases are inherently used to adjust and control the adsorption-desorption process. These CSPs are especially useful for the separation of very polar charged analytes, such as sulphonic acids. Chiral anion-exchangers are the most successful CSPs and among them the cinchona alkaloids, quinine and quinidine (Figure... 

Cobalt(lll).—Complexes. Ammine complexes. Optical activity can be induced in the complexes [Co(NH3) ] and [Cofenlj] by means of outer-sphere association with chiral anions, e.g. (- - )-tartrate. Circular dichroism is observed in the d-d bands of the cations and it is suggested that this is due to (a) direct interaction between the chiral anion and the metal f/-orbitals and (b) the preferred conformation adopted by the inner-sphere ligands in the presence of a helical outer-sphere ligand. 

Chiral Anions Using Secondary and Primary Amines as Catalysts. 330... 

Subsequently, List reported that although the method described above was not applicable to the reduction of a,P-unsaturated ketones, use of a chiral amine in conjunction with a chiral anion provided an efficient and effective procedure for the reduction of these challenging substrates [210]. Transfer hydrogenation of a series of cyclic and acyclic a,P-unsaturated ketones with Hantzsch ester 119 could be achieved in the presence of 5 mol% of valine tert-butyl ester phosphonate salt 155 with outstanding levels of enantiomeric control (Scheme 64). A simple mechanistic model explains the sense of asymmetric induction within these transformations aUowing for reliable prediction of the reaction outcome. It should also be noted that matched chirality in the anion and amine is necessary to achieve high levels of asymmetric induction. 

In conjunction with the chiral anion TRIP (156) (10 mol%), diamine 157 (10 mol%) can be used in the catalytic asymmetric epoxidation of a,p-unsaturated ketones (>90% ee) [196], while the secondary amine 158 (10 mol%) can be used for the epoxidation of both di- and trisubstituted a,P-unsaturated aldehydes (92-98% ee) (Fig. 15) [211], The facile nature of these reactions, using commercially available peroxides as the stoichiometric oxidant, together with the synthetic utility of the epoxide products suggests application in target oriented synthesis. 

A further example of the use of a chiral anion in conjunction with a chiral amine was recently reported by Melchiorre and co-workers who described the asymmetric alkylation of indoles with a,P-unsaturated ketones (Scheme 65) [212]. The quinine derived amine salt of phenyl glycine (159) (10-20 mol%) provided the best platform with which to perform these reactions. Addition of a series of indole derivatives to a range of a,P-unsaturated ketones provided access to the adducts with excellent efficiency (56-99% yield 70-96% ee). The substrates adopted within these reactions is particularly noteworthy. For example, use of aryl ketones (R = Ph), significantly widens the scope of substrates accessible to iminium ion activation. Expansion of the scope of nucleophiles to thiols [213] and oximes [214] with similar high levels of selectivity suggests further discoveries will be made. 

Scheme 65 Alkylation of indoles using a chiral anion/chrral amine combination...

Recently the group of Leitner was able to achieve high enantioselectivities in the aza-Baylis-Hillman reaction by the application of enantiopure ionic liquid with a chiral anion (Scheme 82) [208]. 

Scheme 82 Enantioselective aza-Baylis-Hillman reaction in a chiral anion based ionic liquid...

Figure 20. A Structure of immobilized dihydroquinidine carbamate serving as chiral anion exchanger type CSP. B Chromatogram showing a direct resolution of the herbicide 2-(2,4-dichlorophenoxy)propanoic acid (Dichlorprop). Reprinted with permission from ref 140.

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]. 

Recently, we found that the polymerization of TrMA with chiral anionic catalysts gave an optically active polymer the chirality of which is caused by helicity (12). This is the first example of optically active vinyl polymer the activity of which arises only from the helicity. This article describes the detailed results of the polymerization of TrMA by chiral anionic catalysts in addition to a brief review on our earlier studies described above. 

Preparation of addition polymers having the oxolene (dihydrofuran) functionality can be envisioned to occur in two possible ways (Scheme 13). Both, in fact, have been observed (77MI11102). Whereas furan (53) or its derivatives do not homopolymerize under free radical conditions, 1 1 alternating copolymers possessing the 1,4-structure are produced with maleic anhydride (50). Intermediate formation of a CT complex between monomers (50) and (53) is believed to be necessary before polymerization can occur. On the other hand, cationic polymerization is quite facile. The outcome is straightforward with benzo[f>]furan derivatives, producing 1,2-polymers. Optically active poly(benzofurans) are formed when the cationic polymerizations are conducted in the presence of a chiral anion. 

In 1988, Hosomi et al. established that hydride transfer from hydrosilanes can be rendered enantioselective by using chiral anionic activators such as the dilithium salts of the diol 15 or of phenylalaninol 16 (Scheme 11.6) [22], In the presence of stoichiometric amounts of the dilithium salt of 15, isobutyrophenone was reduced by trialkoxysilanes with 69% ee, whereas 40 mol% of the corresponding salt of 16 was sufficient to effect reduction of acetophenone with 49% ee [23],... 

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