Chiral adsorbants

Figure Bl.5.8 Random distribution of (a) non-chiral adsorbates that gives rise to a surfaee having effeetive oo m-synnnetry (b) ehiral moleeules that gives rise to effeetive oo-synnnetry. (e) SH intensity versus the angle of an analyser for a raeemie (squares) and a non-raeemie (open eireles) monolayer of ehiral moleeules. The pump beam was p-polarized the SH polarization angles of 0° and 90° eorrespond to s- and p-polarization, respeetively. (From [70].)...

Synthetic chiral adsorbents are usually prepared by tethering a chiral molecule to a silica surface. The attachment to the silica is through alkylsiloxy bonds. A study which demonstrates the technique reports the resolution of a number of aromatic compoimds on a 1- to 8-g scale. The adsorbent is a silica that has been derivatized with a chiral reagent. Specifically, hydroxyl groups on the silica surface are covalently boimd to a derivative of f -phenylglycine. A medium-pressure chromatography apparatus is used. The racemic mixture is passed through the column, and, when resolution is successful, the separated enantiomers are isolated as completely resolved fiactions. Scheme 2.5 shows some other examples of chiral stationary phases. 

Partially successful attempts towards chiral electrochemical synthesis have involved chiral supporting electrolytes chiral solvents and chiral adsorbates, mostly alkaloids With the latter method enantiometric excess values >40% have... 

The first four facets are rotationally equivalent to each other as are the final four. The two sets are related by reflectional symmetry to each other. When a chiral adsorbate, for example, S-lysine, is used, the reflectional symmetry is no longer valid and only rotationally equivalent facets should be formed. This was demonstrated elegantly by Zhao with STM [53], The driving force for facet formation is proposed to be a three-point interaction involving the carboxylate group, the a-amino group, and the amino-terminated side chain. The simultaneous optimization of adsorbate-adsorbate and adsorbate-substrate interactions determines the stereochemistry of the facet. 

When optically inactive polystyrene was used as adsorbent, no difference in the relative peak intensity at m/z 288 to 286 was detected. Moreover, in the resolution of (RS)-1,1 -bi-2-naphthol and (if5)-l,l,-bi-2-naphthol-rf2 on the CSP, no isotope effect was observed. These findings indicate that the difference in EI-MS spectra is due to the difference in desorption between the enantiomers from the chiral adsorbent tris(5-f uoro-2-methylphenylcarbamate). This method can be used to discriminate the chirality of other enantiomers of small molecules if they show peaks in their EI-MS spectra in the presence of chiral polymers. Similar chiral recognition was detected by negative ion fast-atom bombardment mass spectrometry [34],... 

Scheme 1 a The [2 + 2] cycloaddition product of prochiral trans 2-butene with Si dimers of the Si(100) surface leads to chiral adsorbate complexes, b Hydrogenation of prochiral a-keto esters over platinum is a heterogeneously catalyzed reaction leading to chiral alcohols. Using cinchonidin as chiral modifier makes this surface reaction enantioselective. In a similar fashion, TA-modified nickel is a highly enantioselective catalyst for /3-keto ester hydrogenation... 

The pinwheel structure is not only observed for chiral adsorbates. An early example was reported for small molecules at low temperatures on graphite in UHV (Fig. 19). Neutron and electron diffraction experiments as well as... 

Moller, P, Sanchez, D., Allenmark, S. and Andersson, S. Chiral Adsorbents and Preparation thereof as well as... 

Selenoxides derived from unsymmetrical selenides are chiral and stable toward pyramidal inversion at room or even higher temperatures. They are produced enantioselectively by the use of chiral oxidants such as the Sharpless reagent or camphor-derived oxaziridines or diastereoselectively with achiral oxidants when one of the selenide substituents is itself chiral (see Section 9). Racemic selenoxides have been resolved by chromatography over chiral adsorbents. Chiral selenoxides racemize readily in water, particularly under acid-catalyzed conditions, presumably via the intermediacy of achiral selenoxide hydrates (equation 2). 

In this crystal lattice system, all surfaces with Miller indices, (hkl), satisfying the conditions h x k x 1 and h k l h are chiral [11]. Although such high Miller index surfaces have been studied for decades, it was not until recently that McFadden et al. specifically pointed out and demonstrated that their low synunetry structures render them chiral and, therefore, that they might have enantiospecific interactions with chiral adsorbates [12]. There has been a growing interest in the enantiospecific properties of naturally chiral metal surfaces and in the possibility of using such surfaces for enantioselective chemical processes. 

The thermal roughening of naturally chiral metal surfaces must have some impact on their enantiospecific interactions with chiral adsorbates. Sholl et al. have also studied the effects of step roughening on the adsorption of small chiral molecules [11, 17]. Molecular simulation of small chiral alkanes adsorbed on ideal and roughened Pt(643) surfaces revealed that enantioselectivity is preserved during... 

The enantioselectivities of reactions on chiral surfaces are of interest from a practical standpoint and are the result of enantiospecific differences in reaction energetics and reaction barriers. Another manifestation of the enantiospecific interaction between a chiral adsorbate and a chiral surface is adsorbate orientation. Enantiospecific orientations of chiral adsorbates on naturally chiral metal surfaces have been predicted by molecular simulation studies. The first studies using Monte Carlo methods to study chiral cycloalkanes adsorbed on chiral surfaces pre-... 

In this section, we focus on the Ni(llO) surface, the metal most commonly modified by tartaric acid, to yield the successful enantioselective catalytic system for the hydrogenation of P-ketoesters [6, 7], A detailed study [22, 23] of the adsorption of (/ ,/ )-tartaric acid on Ni(llO) as a function of temperature and coverage again shows a polymorphic system, (Fig. 5.8) with the local nature of the chiral adsorbate changing dynamically as conditions change, echoing the findings on Cu(llO). 

Another means of resolution is to use a chiral material in a physical separation. Currently, many resolutions are done using medium- or high-pressure chromatography with chiral column-packing materials. Resolution by chromatography depends upon differential adsorption of the enantiomers by the chiral stationary phase. Differential adsorption occurs because of the different fit of the two enantiomers to the chiral adsorbent. Figure 2.7 shows such a separation. Topic 2.1 provides additional detail on several types of chiral stationary phases. 

The resolution by the mentioned column chromatography for chiral complexes is based on the interaction between a chiral adsorbent and a chiral eluent. From such a point of view the association constants have been determined for the ion pairs. A- and A-[Co(en)3p with L(+)-tartrate dianion and with L(-i-)-tartratoanti-monate(III) dianion ((+)-tartan), on the basis of CD measurements at two wave-... 

It has long been understood that chromatography of racemates on chiral adsorbents might result in enantiomer separation. Initial efforts to use convenient chiral adsorbents (cellulose, starch, wool) usually met with scant success. With the advent of more sophisticated techniques, it has become possible to separate the enantiomers of a large number, literally tens of thousands, of compounds by liquid chromatography. Because of the close ties between life and chirality, the ability to separate enantiomers by HPLC has an eager audience of potential users. 

Ferroni and Cini tried to resolve the racemic chelate complex Be[(PhCOCH)2CO]2 using optically active crystals of NaClOs as chiral adsorbents, but results were uncertain (see Table 1.7.). 

Imprinting chiral adsorbents specifically modified silica gels... 

Unfortunately, e mlanations of the problems associated with the results of Amariglio et al. have not been incorporated into considerations of quartz as a chiral adsorbent... 

Carbohydrates are the natural compounds possessing the most asymmetric centers in molecules that make them prospective auxiliaries for the preparation of chiral adsorbents and chiral catalysts. 

Chiral adsorbents, like lactose, starch, and paper eellulose, gained applications in analytical and practical uses. Optically pure lactose was used by Henderson and Rule for the resolution of racemic / ara-phenylene-bis-iminocamphor in chromatographic columns. The (+)-enantiomer was more strongly adsorbed than the (-)-enantiomer. 

Partition chromatographic mechanisms operate on cellulose thin layers even if adsorption effects cannot be excluded (for separation of substance classes, see Table 2). Celluloses are naturally occurring chiral adsorbents and can be used for chiral separation of optically active amino acids and dipeptides. 

A recent innovation in gas chromatography is to use chiral adsorbent materials to achieve separations of stereoisomers. The interaction between a particular stereoisomer and the chiral adsorbent may be different from the interaction between the opposite stereoisomer and the same chiral adsorbent. As a result, retention times for the two stereoisomers are likely to be sufficiently different to allow for a clean separation. The interactions between a chiral substance and the chiral adsorbent will include hydrogen-bonding and dipole-dipole attraction forces, although other properties may also be involved. One enantiomer should interact more strongly with the adsorbent than its opposite form. Thus, one enantiomer should pass through the gas chromatography column more slowly than its opposite form. 

The ability of chiral adsorbents to separate stereoisomers is rapidly finding many useful applications, particularly in the synthesis of pharmaceutical agents. The biological activity of chiral substances often depends upon their stereochemistry because the living body is a highly chiral environment. A large number of pharmaceutical compounds have two enantiomeric forms that in many cases show significant... 

The structure of one important cyclodextrin-based chiral adsorbent is shown in Figure 22.7. Gas chromatography using this chiral adsorbent as a stationary phase has been used to separate a wide variety of stereoisomers. In one recent publication. 

Cyclodextrin derivative used as a chiral adsorbent in gas chromatography. 

In the pharmaceutical industry adsorbents are used for purification of anaesthetics, removal and purification of vitamins, antibiotics and others. To-day more than fifty per cents of the pharmaceuticals are enantiomers. They cannot be often obtained by means of stereoselective synthesis. Then, the only solution of the problem is to apply the so-called chiral adsorbents and to perform separation. 

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