Chiral surfaces molecules

An important distinction among surfaces and interfaces is whether or not they exliibit mirror synnnetry about a plane nonnal to the surface. This synnnetry is particularly relevant for the case of isotropic surfaces (co-synnnetry), i.e. ones that are equivalent in every azunuthal direction. Those surfaces that fail to exliibit mirror synnnetry may be tenned chiral surfaces. They would be expected, for example, at the boundary of a liquid comprised of chiral molecules. Magnetized surfaces of isotropic media may also exliibit this synnnetry. (For a review of SFIG studies of chiral interfaces, the reader is referred to [68]. ... 

Given the interest and importance of chiral molecules, there has been considerable activity in investigating die corresponding chiral surfaces [, and 70]. From the point of view of perfomiing surface and interface spectroscopy with nonlinear optics, we must first examhie the nonlinear response of tlie bulk liquid. Clearly, a chiral liquid lacks inversion synnnetry. As such, it may be expected to have a strong (dipole-allowed) second-order nonlinear response. This is indeed true in the general case of SFG [71]. For SHG, however, the pemiutation synnnetry for the last two indices of the nonlinear susceptibility tensor combined with the... 

A qualitatively new approach to the surface pretreatment of solid electrodes is their chemical modification, which means a controlled attachment of suitable redox-active molecules to the electrode surface. The anchored surface molecules act as charge mediators between the elctrode and a substance in the electrolyte. A great effort in this respect was triggered in 1975 when Miller et al. attached the optically active methylester of phenylalanine by covalent bonding to a carbon electrode via the surface oxygen functionalities (cf. Fig. 5.27). Thus prepared, so-called chiral electrode showed stereospecific reduction of 4-acetylpyridine and ethylph-enylglyoxylate (but the product actually contained only a slight excess of one enantiomer). 

Prochiral Molecules Interacting with Chiral Surfaces... 

In principle, sites a, IT, and c need not be association sites as depicted by Ogston but could be steric sites that form obstructions such that the adsorbed molecule is chirally directed. Only one active site is actually required providing the remaining two sites (protuberances or cavities) are different from each other and from the active site that catalyzes the reaction. They could be identical providing they are not symmetrically oriented with respect to the active site (not an isosceles triangle). These are the basic concepts for a chiral environment on a surface and they lead to the three basic methods for creating chiral surfaces in heterogeneous catalysis. 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

These possibilities rectify the proposed subsequent appearance and amplification of chiral autocatalytic molecules and hypercydes. [190] Any autocatalytic systems would propagate [191] throughout an extensive adjoining hydrated porous network already rich in layered amphiphiles, lipids, polymeric materials, amino acids, thiols, and so forth. In addition, amphiphiles are known to be organized into lipid membranes by interaction with the inner surfaces of porous minerals. [136] It is a small organizational jump from these membranes to frilly formed lipid vesides. 

However, in this context CPSs are defined throughout this article as very stable phy-sisorbed (physically absorbed) and/or most often covalently bound chiral selector compounds to a nonchiral (most often silica) surface. To the same category belong the CSPs, which have as their bases beads of polymeric chiral selector material. The strong irreversible adsorption of chiral selector molecules (macromolecules or small molecules onto a plain or premodified surface) depends, of course, on the nature of the mobile phase and whether or not it has some solvation strength for the adsorbed chiral selector moiety. 

Single crystal X-ray diffraction measurements have shown the importance of these attractive second order interactions in stabilisation of a relatively constant structure of TA in diastereoisomeric salts [14], DBTA in complexes [15, 16] and of DBTA derivatives in sole or solvated crystals, too. [17] In the most cases, strong H-bridges stick together the TA or DBTA molecules into long chains and chanels having chiral surface for discrimination between enantiomers. 

This contribution is restricted to supramolecular chiral phenomena on solid surfaces. Neither interaction of molecules with chiral inorganic surfaces [4], nor chiral amphiphilic molecules at the air-water interface are considered [5-8]. It is the intent of this review to present principle aspects of surface chirality. That is, examples are used to highlight typical chiral assembly mechanisms and structures. Most of these examples, however, belong to more than one aspect presented within this review. Hopefully, this set-up will be accessible to the reader who is not working with surfaces as well as informative to those who are familiar with the subject. 

Examples of planar pro-chiral molecules creating a chiral surface complex if adsorbed with their molecular plane parallel to the surface are shown in Fig. 7. 

Diastereoisomeric interactions between chiral surfaces of non-chiral crystals and chiral molecules present in solution are demonstrated by the formation of etch pits. Etch pits were only formed on the (010) face of an a-glycine crystal partially dissolved in an undersaturated solution containing D-alanine, whereas the (0-10) face does not exhibit etch pits, Fig. 7a [69]. 

Heterogeneous photochemical processes are concerned with the effect of light on interacting molecules and solid surfaces. The concept of photoinduced surface chemistry is commonly used to integrate these processes. As cited earlier, they involve surface phenomena such as adsorption, diffusion, chemical reaction and desorption [3]. Experiments and theoretical calculations make clear that the photochemical behavior of an adsorbed molecule can be very different from that of a molecule in the gas or liquid phase [4]. Photochemical reactions of this type involve molecules and systems of quite different complexity, from species composed of a few atoms in the stratosphere to large chiral organic molecules that presumably were formed in prebiotic systems. 

Finally, in a recent paper, Yeganeh et al. suggest that the large asymmetries seen in polarized electron transmission are partly due to a combination of the presence of a molecule with axial chirality, surface orientation, and cooperative effects in the monolayer [129]. They use scattering theory to show that differences in transmitted intensity arise from the preferential transmission of electrons whose polarization is oriented in the same direction as the sense of advance of the helix. 

Liu N, Haq S, Darling G, Raval R (2007) Direct visualisation of enantiospecific substitution of chiral guest molecules into heterochiral molecular assemblies at surfaces. Angew Chem Int Ed 46 1... 

One general observation is that the chiral effect of both the cinchona and vinca type alkaloids appears to change, in most cases to decrease, if the reaction is started as a racemic hydrogenation compared with the case when the chiral modifier is added at the beginning of the hydrogenation. But no clear conclusion can be made as to whether or not the modifier molecules interact less with the catalyst surface, which is covered by hydrogen and chiral product molecules, and as a result exert less asymmetric effect. 

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