Achiral systems crystals

The culmination of the studies on asymmetric photodimerization reactions in the solid state was the successful elaboration of chemical systems that are achiral but crystallize in chiral structures, and that yield, on irradiation, dimers, trimers, and higher oligomers in quantitative enantiomeric yield (175,258). 

As discussed in detail in Ref. 36, for use in optoelectronics only systems crystallizing in non-centrosymmetric crystal lattices are of interest if the use of expensive enantiomers of chiral molecules is to be avoided. This considerably limits the available crystal lattices since most organic achiral molecules crystallize into centrosymmetric space groups. An interesting example of enantioselective inclusion complexation was reported by Gdaniec and coworkers [37]. 

It is known since 1975 [10] that crystal chemical reactions can be used in absolute asymmetric synthesis When achiral molecules crystallize in chiral space groups and the reaction of the crystals leads to chiral products, one speaks justly and correctly of absolute asymmetric synthesis. For this, the use of chiral agents and thus also the crystal-selecting human hand must be dispensed with. However, if autoseeding does indeed occur in a system, manipulations by seeding with crystals of a desired chirality must not detract from that term still. Most absolute asymmetric syntheses have been performed photochemi-cally. Some short reviews have appeared. [11]... 

A chirality classification of crystal structures that distinguishes between homochiral (type A), heterochiral (type B), and achiral (type C) lattice types has been provided by Zorkii, Razumaeva, and Belsky [11] and expounded by Mason [12], In the type A structure, the molecules occupy a homochiral system, or a system of equivalent lattice positions. Secondary symmetry elements (e.g., inversion centers, mirror or glide planes, or higher-order inversion axes) are precluded in type A lattices. In the racemic type B lattice, the molecules occupy heterochiral systems of equivalent positions, and opposite enantiomers are related by secondary lattice symmetry operations. In type C structures, the molecules occupy achiral systems of equivalent positions, and each molecule is located on an inversion center, on a mirror plane, or on a special position of a higher-order inversion axis. If there are two or more independent sets of equivalent positions in a crystal lattice, the type D lattice becomes feasible. This structure consists of one set of type B and another of type C, but it is rare. Of the 5,000 crystal structures studied, 28.4% belong to type A, 55.6% are of type B, 15.7% belong to type C, and only 0.3% are considered as type D. 

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... 

In our approach we searched for a monomer which is achiral and crystallizes in a chiral structure, or of which a racemic mixture crystallizes with disorder in such a structure. Further, we focused our efforts on (2+2)-photocyclopolymerizations, for reasons we have given above. A detailed analysis was performed of the chemical and crystallographic requirements for conversion of monomer molecules in the crystal into chiral products, including polymers, and a number of structural systems appeared to be suitable [9]. 

Bearing in mind the odd-even effect, I tried to synthesize new polymer types in order to express the ferroelectric phase in an achiral system. The opportunity to study chiral ferroelectric liquid crystals was ripe at that time, as I already mentioned in the opening sentence. Their discoverer Mayer [118] argued that by introducing a chiral molecule into a Sc phase, the twofold axis of the Sc phase becomes the polar axis because of the extinction of a vertical mirror symmetry (Fig. 9.20). The reduction of the symmetry by the introduction of chirality into the system is in a sense conventional, simple, and straightforward. 

Liquid crystal molecules usually tilt in the same direction over the smectic layers (synclinic [212]) in the smectic C (SmC) phase. However, in one of the smectic A (SmA) phases, called de-Vries phase [213,214], molecules tilt but the tile direction is random so that the overall molecular tilt cannot be recognized optically. Frustration can be produced between aligning and random orders [215]. There is another style of tilt, in which the tilting direction is aligning in one direction in each smectic layer however, tilting direction alternates between the adjacent layers (anticlinic [212]). It has been well known that the introduction of chirality into the synclinic and anticlinic stmctures produces the ferroelectric and antiferroelectric properties, respectively. Frustration between the ferroelectric and antiferroelectric properties produces the ferroelectric structure in which the spontaneous polarization is partially canceled by the different magnitude between plus and minus polarization directions [216, 217]. The anticlinic order, NOT the antiferroelectric order, has been reported to be created by achiral systems [218, 219], indicating that the frustration between synclinic and anticlinic structures occurs, without any polar effects. The clinicity is determined by the style of the molecular order between the adjacent smectic layers, and therefore, the molecular structures at the peripheral... 

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. 

So far we have considered the formation of tubules in systems of fixed molecular chirality. It is also possible that tubules might form out of membranes that undergo a chiral symmetry-breaking transition, in which they spontaneously break reflection symmetry and select a handedness, even if they are composed of achiral molecules. This symmetry breaking has been seen in bent-core liquid crystals which spontaneously form a liquid conglomerate composed of macroscopic chiral domains of either handedness.194 This topic is extensively discussed in Walba s chapter elsewhere in this volume. Some indications of this effect have also been seen in experiments on self-assembled aggregates.195,196... 

Even starting from achiral molecules it is in some systems possible to achieve crystallization in a chiral structure. Perhaps one of the most striking achievements in organic solid-state chemistry has been the trapping of the chirality of such a crystal as the chirality of the stable product of chemical reactions in the crystal. Such asymmetric synthesis has been reviewed (255), and a recent book (256) also provides a thorough discussion of chirality in crystals. The related and fascinating topic of the chemical consequences of the presence of a polar axis in some organic crystals has also been reviewed (257). 

In these systems, after the crystal chirality induced the chirality of asymmetric carbon in external organic compound, the subsequent asymmetric autocatalysis gives the greater amount of enantiomerically amplified product. These results clearly demonstrate that the crystal chirality of achiral organic compound is responsible for the enantioselective addition of /-Pr2Zn to pyrimidine-5-carbalde-hyde Ic. 

Katsuki has extended his earlier work on asymmetric induction using achiral catalysts such as 13. In these systems, the stereochemical bias is imbued by a chiral non-racemic axial ligand, such as (+)-3,3 -dimethyl-2,2 -bipyridine A2,A -dioxide (14), which was purified by crystallization with (5)-binaphthol. Epoxidation using these conditions resulted in good ee s and fair yields, as exemplified by the preparation of chromene epoxide 16 <99SL783>. 

Systematic studies of topochemical reactions of organic solids have led to the possibility of asymmetric synthesis via reactions in chiral crystals. (A chiral crystal is one whose symmetry elements do not interrelate enantiomers.) (Green et al, 1979 Addadi et al, 1980). This essentially involves two steps (i) synthesis of achiral molecules that crystallize in chiral structures with suitable packing and orientation of reactive groups and (ii) performing a topochemical reaction such that chirality of crystals is transferred to products. The first step is essentially a part of the more general problem of crystal engineering. An example of such a system where almost quantitative asymmetric induction is achieved is the family of unsymmetrically substituted dienes ... 

In six-membered chelate rings such as those formed by 1,3-propanediamine (tn), at least two conformational energy minima, which correspond to an achiral chair structure (23a) and the chiral skew- or twist-boat forms (25b) and (25c),166-168 occur and both have been observed in crystal structures.169 176 The possible conformational isomerism in tn complexes is therefore even more involved than in en complexes.177 In (OC-6-12)-[CoCl2(tn)2]+ (tratns-[CoCl2(tn)2]+), for example, there are two achiral bis(chair) forms (26a and 26b), two enantiomeric (chair, skew-boat) forms (26c and 26d), one achiral form (26e) and two enantiomeric bis (skew-boat) forms (26f and 26g). Several of these have been isolated in conformationally locked systems.178 180... 

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. 

Actually, it is very hard to determine whether the crystals (or solid) are doped in chiral or achiral. The space group can be determined by only X-ray crystallographic analysis. Now, we can conveniently survey the crystal from the measurement of CD spectra (KBr or nujor method)[8] or activity of SHG (Second Harmonic Generation). [9] However, sense of high accuracy is required for the use of both methods. When we could observe a Cotton effect from the CD spectra, the crystal system should be chiral. On the other hand, there are many examples of chiral crystals that exhibit quite a little Cotton effect. As a matter of course, it is also difficult to recognize chiral crystals when the enantiomeric purity is poor. 

A great number of chiral TTFs have been prepared [31,32], and some of their charge transfer salts have been crystallised. The 7r-donor 1 can be elec-trocrystallised to give a salt 12 PF6, which has a conductivity of 5 2 1 cm 1 at room temperature and metallic behaviour when cooled down [33]. While the structure of the crystals is chiral, the structure of this salt and other related ones [34] has an essentially achiral stack of donor molecules, which are pseudo-centrosymmetric [35]. The methyl groups at the periphery of the molecule are apparently not sufficient to cause a truly chiral stack of donors, which prefer to stack in parallel arrangements with partial overlap of the tr-systems in the solid state, a situation which is true for the majority of the efforts to prepare salts of this type (even if the salts are metallic). 

There are many host systems that are inherently chiral or attain chirality upon crystallization. When achiral molecules are included in these chiral hosts induced chirality may result. If this occurs the induced chirality of the guest reactant can be preserved when it is converted to a product [175-178]. This has been shown to be possible with several systems. Once again, the reactants are nonchiral and give racemic products in isotropic solution. Structures of a few chiral molecules that serve as hosts are listed in Sch. 35. The following examples in which l,6-Z /s(o-chlorophenyl)-l,6-diphenyl-2,4-dyine-l,6-diol and cyclodextrin serves as the host illustrate the point. 

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