Racemic domains

Although in first approximation the individual layers keep their chirality during switching between AFE and FE states, extended application of electric fields can alter racemic domains to chiral or vice versa. Racemic domains can be rendered chiral by surface interactions, too. ... 

Experiments by Binet et al. [72], carried out on chiral bent core materials containing biphenyl (BP) cores with S or R hydrocarbon chains, and on achiral biphenyl core molecules with chiral dopants, reveal the effect of the molecular chirality on the polarization stmcture. In addition the polarization Pb due to the closed packing of the bent-shape molecules, another polarization, P is introduced due to the chiral and tilted molecular structure. It was found [72] that in the antiferroelectric racemic domains at low fields, of the synclinic - racemic... 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

Fig. 21 In situ epifluorescence micrographs of (a) fluid racemic and (b) crystalline enantiomeric SSME monolayers at the air-water interface at 25°C. Lighter domains are fluorescing probe l-NBD PC darker domains are SSME. Total magnification is 5000x. Reprinted with permission from Harvey et al, 1989. Copyright 1989 American Chemical Society.

Conversely, the racemic film system appears to be solubilized by the achiral fatty acid component. At compositions of 10-33% palmitic acid, the ESP of the racemic system varies linearly with film composition, indicating that the monolayer in equilibrium with the racemic crystal is a homogeneous mixture of racemic SSME and palmitic acid. At compositions of less than 33% palmitic acid, the ESP is constant, indicating that three phases consisting of palmitic acid monolayer domains, racemic SSME monolayer domains, and racemic SSME crystals exist in equilibrium at the surface. 

EO switching. Since the phase is a macroscopic racemate and achiral, the EO behavior of the SmCsPA phase can reasonably be expected to be achiral. But, what of the chiral minority domains ... 

If instead of enantiomeric heptahelicene a racemic mixture is sublimated onto the surface, the molecules self-organize in enantiomorphous mirror domains. Thus, as in the case of cysteine on Au(l 10), the surface is stereoselective. 

The complexity of the tetrahydroiso-a-acids from /3-acids extends to the hexahydroiso-a-acids (also derived from /3-acids), with the added complication that borohydride reduction of a carbonyl to a carbinol moiety results in yet another racemic, optically active center, theoretically giving rise to eight hexahydroiso-a-acids for each of the co-, -, and ad-variants. This number of compounds has, as far as the author is aware, precluded the derivation of individual response factors for each of these compounds. Indeed, identification of all of the significant bands evident in a chromatogram of the hexahydroiso-a-acids has not yet been reported in the public domain. 

Abstract It is well known that spontaneous deracemization or spontaneous chiral resolution occasionally occurs when racemic molecules are crystallized. However, it is not easy to believe such phenomenon will occur when forming liquid crystal phases. Spontaneous chiral domain formation is introduced, when molecules form particular liquid crystal phases. Such molecules possess no chiral carbon but may have axial chirality. However, the potential barrier between two chiral states is low enough to allow mutual transformation even at room temperature. Therefore the systems are essentially not racemic but nonchiral or achiral. First, enhanced chirality by doping chiral nematic liquid crystals with nonchiral molecules is described. Emphasis is made on ester molecules for their anomalous behavior. Second, spontaneous chiral resolution is discussed. Three examples with rod-, bent-, and diskshaped molecules are shown to give such phenomena. Particular attention will be paid to controlling enantiomeric excess (ee). Actually, almost 100% ee was obtained by applying some external chiral stimuli. This is very noteworthy in the sense that we can create chiral molecules (chiral field) without using any chiral species. 

A conglomerate in real liquid crystalline phases was first observed in the smectic phase of a rod-shaped mesogen with two stereogenic centers in its tail [42], We used a racemic mixture which was supposed not to electrically switch. Evidence for conglomerate formation was provided by clear electro-optic switching and texture observation under a polarizing microscope domains with stripes, which themselves display fine stripes. These stripes are tilted in two different directions with respect to the primary stripes. This is a still very rare example now that fluid soft matter is known to resolve spontaneously into a three-dimensional conglomerate. 

The formation of two-dimensional nanocrystals, by peptide amphiphiles is also influenced by the chirality of the peptide building block.125 Three types of two-dimensional crystals were observed after the assembly of the functionalized peptide amphiphiles 17-19 shown in Figure 7.11 (above) at the air-water interface, which was followed by polymerization. These two dimensional crystals include (/) a racemic compound, in which each enantiomer is packed with its mirror image in a crystalline order, (it) enantiomorphous conglomerates, in which each enantiomer is segregated into small domains, and (iii) a solid solution, in which all molecules are randomly distributed without crystalline order. Interestingly, in the case of the two-dimensional system arising from the racemic compound,... 

The use of click chemistry has also been applied to the synthesis of benzophe-none-modified y-secretase probes. The group of Yao reported the preparation of a compound library built up from Bpa-containing alkyne 77 and azide 78 (Fig. 7) [81]. The azide part contains a racemic hydroxyethylene moiety, and variations were made in its aryl sulfonamide domain. The compound library was screened for its potency against y-secretase inhibition and the most potent compounds were used to label active PS1 in a cell lysate. In addition, Fuwa and coworkers reported a divergent synthesis of y-secretase A/BPs by means of click chemistry with alkyne 79 and azide 80 [82]. Variations were made in the aryl part of the alkyne (dibenzoazepine or benzodiazepine) and in the type of spacer between the benzo-phenone moiety and biotin in the azide. PAL using these probes provided the authors with evidence that the molecular target of this type of probe is the N-terminal fragment of PS1. 

Fig. 26 STM image (27 nm x 27 nm) showing two enantiomorphous domains of R- and S-phenylglycine on Cu(l 10) after adsorption of the racemate. Courtesy of N.V. Richardson...

Although devoid of alkyl chains, [7]H on Cu(lll) forms, similar to 9,10-iodo-octadecanol and the anthracene derivative shown in Fig. 28a, a racemic lattice structure. Conglomerate formation was initially concluded from LEED, because the mirror domain pattern observed for the racemate was identical to the superimposed patterns of the pure enantiomers [92]. STM images, however, delivered different lattice structures for the mirror domains of the racemate and the pure enantiomers [93]. High-resolution STM and MMC finally showed that the enantiomorphous domains are racemic [88]. We will return to this system in more detail in Sect. 4. 

A coverage dependence of 2D conglomerate or racemate formation was first observed for NN on Au(lll). In that case, the transition from homochiral chain structures to heterochiral 2D domains is strongly influenced by the hcp-fcc substrate reconstruction sites of the substrate [83]. 

The coadsorption of chiral molecules into racemic layers is an efficient way to induce further asymmetrization towards single handedness. While in heterogeneous chiral catalysis the stationary ratio of modifier and reactant at the surface is assumed to be one, a small amount of chiral dopant can be sufficient for induction of homochirality on the entire surface SU on Cu( 110), for example, forms two enantiomorphous domains in its bisuccinate phase [27]. 

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