Chirality surface-induced

Charra F, Cousty J (1998) Surface-induced chirality in a self-assembled monolayer of discotic liquid crystal. Phys Rev Lett 80 1682-1685... 

A mechanistic consideration is as follows the initial reaction of aldehyde 11 and z-Pr2Zn proceeded on the chiral surface of the crystal so that a small enantiomeric excess was induced. Then, subsequent asymmetric autocatalysis with an amplification of chirality produced alkanol 12 in high enantiomeric excess and with the corresponding absolute configuration. Further mechanistic details are now under investigation. 

For example, Wakayama and coworkers discovered the formation of two oppositely orientated domains of 5,10,15,20- lclrakis(3,5-di-ferf-bulylphcnyl)-porphine deposited onto a Cu surface using low-temperature scanning tunneling microscopy [124]. It was demonstrated that the twin domains were produced by a pair of chiral conformations induced by a combination of symmetrical tilting and twisting rotation of the phenyl rings with respect to... 

Fig. 9 Top-view on enantiomers and diastereoisomers of adsorbed glycine and alanine. The amino nitrogen and the two oxygen atoms form bonds to the surface and create a chiral footprint configuration. The surface-induced absolute configuration, as indexed with a superscripted i, is specified using CIP-rules and by giving an atom or group closer to the surface a higher priority...

A molecule may also lose mirror symmetry if the presence of the surface induces non-equivalence of otherwise identical groups. This is shown for the theoretical example of methane in tilted adsorption geometry (Fig. 8). Another example is the amino acid glycine, hi tilted geometry, it will become chiral on the surface [22], This surface-induced handedness, in turn, gives rise to four diastereomeric configurations for alanine on a surface (Fig. 9). 

Concentration of the organic reactants on surfaces or in the pores of clay materials prior to reaction has been suggested by Bernal [219] and Cairns-Smith [220]. Pores of different sizes might have operated as prebiotic reactors for asymmetric synthesis, since within their confined environment one may find chiral catalytic sites as well as chiral surfaces. One could envisage that such pores might have provided a plausible environment for the formation of diastereoisomeric self-assemblies of the types described in this review and as required for the stochastic mirror symmetry breaking scenarios. In addition, within such pores the chiral material once formed would be protected from racemization that could have been induced by impact with heavy bodies or by intense cosmic radiation. 

Abstract Electron transmission through chiral molecules induced by circularly polarized light can be very different for mirror image structures. This behavior is described in terms of current transfer, the transfer of both charge and momentum. We review recent theoretical developments on the theory of current transfer and discuss related experimental studies of electron transmission through chiral molecular structures adsorbed on surfaces. 

Enantiopure thiahelicenes can transfer their molecular chirahty to the whole phase of an achiral liquid crystal phase thus acting as dopant systems. Coupling the analysis of CD spectra with the study of cholesteric meso-phases induced in nematic liquid crystals (LC), a model has been proposed for the transfer of chirality from thiahehcene (Al)-114 (Figure 13) to the whole liquid crystal phase (cholesteric induction) (1996JO2013). The dopant thiahehcene presents a twisted chiral surface, which is homochiral with the induced cholesterics as a consequence of the interaction of its... 

For electric fields which are not extreme, however, intermediate structures must exist between the spherical and planar solutions. The transition induced by the field has been observed to be continuous. In first approximation the helical structure of the cholesteric is considered to be unaltered, so new chiral surfaces have to be found that would then give the intermediate structure in a weak electric field. Flattened ellipsoids qualify as a natural generalization of the spheres (Figure 12.9(a)), but unfortunately such chiral surfaces are not equidistant, resulting in pitch variations. In first approxi-... 

In the presence of lecithin and using AIBN, copolymers of indene and MA are produced with optical activity. The rate of copolymerization and intrinsic viscosities of the copolymers decreased in the presence of the chiral, surface-active (amphiphilic) lecithin. The optical activity observed is presumed to be due to asymmetry induced in the backbone of the copolymer by the chirality of the lecithin. Asymmetry is thought to be induced at the site of the MA moiety. Copolymerization is influenced by the electrostatic interaction between MA and the polar hard groups of the lecithin to give 82, where the indicates possible asymmetric centers. 

As witli tlie nematic phase, a chiral version of tlie smectic C phase has been observed and is denoted SniC. In tliis phase, tlie director rotates around tlie cone generated by tlie tilt angle [9,32]. This phase is helielectric, i.e. tlie spontaneous polarization induced by dipolar ordering (transverse to tlie molecular long axis) rotates around a helix. However, if tlie helix is unwound by external forces such as surface interactions, or electric fields or by compensating tlie pitch in a mixture, so tliat it becomes infinite, tlie phase becomes ferroelectric. This is tlie basis of ferroelectric liquid crystal displays (section C2.2.4.4). If tliere is an alternation in polarization direction between layers tlie phase can be ferrielectric or antiferroelectric. A smectic A phase foniied by chiral molecules is sometimes denoted SiiiA, altliough, due to the untilted symmetry of tlie phase, it is not itself chiral. This notation is strictly incorrect because tlie asterisk should be used to indicate the chirality of tlie phase and not tliat of tlie constituent molecules. 

Figure 1.14 (a) STM image (lOnmx lOnm, tip bias +0.52V, tunneling current 0.5 nA) of a PVBA-induced sawtooth blade in a restructured Ag(l 1 0) surface terrace, (b) Structural model of the chiral kink arrangements induced by lateral interaction of molecular carboxylate end groups with Ag l 0 0 microfacets. (Reprinted with permission from Ref. [48]. Copyright 2004, American Institute of Physics.)... 

In Section 1.2.1, we discussed the phenomenon of adsorbate-induced chirality whereby the adsorption of achiral species (e.g., glycine) results in the formation of two mirror equivalent domains on the surface. It has recently been shown that the presence of relatively small mole fractions of chiral dopants can result in the exclusive... 

There exists the possibility that one can imprint a chiral arrangement on a catalytic surface. Although chiral imprinting is now occurring on silica surfaces14 and in polymers,15 it has not been used to induce chirality in metal surfaces. This is difficult to explore. 

This sort of analysis could be extended to any metal-catalyzed chemistry in which a large runaway chiral excess is induced in the product by way of a small chiral excess of the molecules that serve as ligands to the metal. It is only necessary that the D,L-metal center be kinetically slower and thermodynamically more stable than the d,d- or L,L-complexes in order that any small e.e. of a chiral ligand be translated into chiral dominance of the reaction product. That the initial e.e. resulting in chiral takeover within a reacting system can be induced by asymmetric mineral surfaces indicates that a general chemical route to the asymmetry of life may exist. 

The continuous availability of trillions of independent microreactors greatly multiplied the initial mixture of extraterrestrial organics and hydrothermal vent-produced chemicals into a rich variety of adsorbed and transformed materials, including lipids, amphiphiles, chiral metal complexes, amino add polymers, and nudeo-tide bases. Production and chiral amplification of polypeptides and other polymeric molecules would be induced by exposure of absorbed amino adds and organics to dehydration/rehydration cydes promoted by heat-flows beneath a sea-level hydro-thermal field or by sporadic subaerial exposure of near-shore vents and surfaces. In this environment the e.e. of chiral amino adds could have provided the ligands required for any metal centers capable of catalyzing enantiomeric dominance. The auto-amplification of a small e.e. of i-amino adds, whether extraterrestrially delivered or fluctuationally induced, thus becomes conceptually reasonable. 

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