Chiral inorganic crystals

Asymmetric Autocatalysis Triggered by Chiral Inorganic Crystals... 

Quartz is a naturally occurring chiral inorganic crystal. It exhibits either a dextrorotatory (d) or levorotatory (f) enantiomorph. Quartz has been considered as one of... 

As described, a chiral organic compound with high ee is formed using chiral inorganic crystals in conjunction with asymmetric autocatalysis. 

TABLE 3. Asymmetric autocatalysis in reaction of 87 initiated by chiral inorganic crystals, inorganic-organic hybrid materials and chiral cocrystals of achiral compounds... 

The achiral inorganic ionic sodium chlorate (NaClOs) and sodium bro-mate (NaBrOs) crystallize in enantiomeric forms belonging to the P2i3 space group for which the same crystal structures exhibit opposite optical rotation [89]. The levo-(Z) and dextrorotatory (d) crystals can be obtained in equal proportions [90]. The chiral ionic crystals of NaClOs and NaBrC>3 were subjected to asymmetric autocatalysis as the initial seed of chirality to study the correlation between the organic compound with high ee and the chiral inorganic crystal composed of achiral ionic components. 

Soai et al. discovered and developed asymmetric autocatalysis (Figure 9), in which the structures of the chiral catalyst (5)-54 and the chiral product (5)-54 are the same after the addition of diisopropylzinc to aldehyde 53. Consecutive asymmetric autocatalysis starting with (S)-54 of 0.6% ee amplifies its ee, and yields itself as the product with >99.5% ee. Even chiral inorganic crystals, such as quartz or sodium chlorate, act as chiral inducers in this reaction. Soai et alls asymmetric autocatalysis gives us an insight to speculate on the early asymmetric reactions on this planet Earth. However, it can be argued whether such strictly anhydrous organometallic reactions are possible under the nonartificial conditions or not. 

Thus, the Soai reaction is a template-directed self-replicating system that successfully maintains exponential growth kinetics and high autocatalytic efficiency over many turnovers. The results support the view that multiple and diverse ways exist to obtain chiral biomolecules via CPL or chiral inorganic crystals such as quartz combined with asymmetric autoctalysis. It is, however, important to remember that the Soai reaction must be carried out in nonaqueous solvents under prebiotically unrealistic conditions. 

By far the most reliable method to determine the absolute configuration of chiral inorganic and organic compounds is by single-crystal X-ray crystallographic analysis.4 By 1974 Saito listed 59 chiral metal complexes whose absolute stmctures were determined by X-ray diffraction.181 Advances in the speed of computer processing and other aspects of X-ray instrumentation have resulted in the ability to determine absolute structures from much smaller crystals. As a consequence, the number of crystal structure determinations of inorganic compounds has increased enormously in recent years. 

Asymmetric Autocatalysis Utilizing Enantiomorphous Inorganic Crystals as an Initial Source of Chirality... 

Scheme 14 Asymmetric autocatalysis utilizing inorganic crystals as an initial source of chirality...

A remarkable stereospecificity between the chiral templates and the chiral inorganic structural motifs is illustrated in open-framework zinc phosphate [Con(en)3] [Zn4(H2 P04)3(HP04)2(P04)(H20)2] (JLU-9).[90] It crystallizes in the orthorhombic space group Pbcn with a = 10.4787(8) A, b = 20.0091(14) A, and c= 14.9594(10) A. The asymmetric unit, as shown in Figure 7.52, contains two unique Zn atoms and four unique P atoms. Both Zn(l) and Zn(2) are tetrahedrally coordinated, making four Zn—O—P... 

This result supports the view that diverse ways exist to obtain chiral biomolecules via CPL or chiral inorganic or organic crystals combined with asymmetric autoctalysis. Kenso Soai and his team studied the effect of the structure of the substituents at position 2 of the pyrimidyl alkanol (Shibata et al. 1996). They found that using 2-alkynyl-pyrimidyl alkanol after three rounds of asymmetric autocatalysis, an astonishing amplification factor of 630,0000 was reached. In the reaction, either (+) or (—) crystals of Cytosine serve as initiators that were formed spontaneously by stirring. In the Soai reaction of chiral amplification, it is crucial that dimers of the O-Zinc diisopropyl intermediate are the active catalysts Racemic pyrimidine alcohols subjected to photolysis with either right- or left-handed CPL produced an ee of one isomer as shown in Fig. 3.4. 

One other aspect of inorganic morphology concerns the handedness of crystals. Indeed, Werner employed this in work on the chirality of crystals of K3[Rh(C204)3] ... 

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