Chiral self-recognition

Fractionation resulting from partial sublimation of optically active samples has also been described27 29 and, more recently, modest chiral self-recognition was observed even in liquids during distillation30-31. In this case distillation of propyl 2-hydroxypropanoate (8.55% R) resulted in a distilled fraction which was enriched in the f -isomer (9.11 % 81.79% ee), while the residue was deficient in the / -isomer (7.83% 84.34% ee)31. 

The neutral, homochiral complex (SS,SS-9) is formed by stirring two equivalents of (4S,4, S)-7 with Zn(OAc)2 (Figure 9.5). Zn(OAc)2 serves a dual role in the readion, simultaneously delivering the metal center and the required base. When a racemic mixture of box ligands [i.e., one equivalent each of (4S,4 S)-7 and (4R,4 R)-7] is combined with Zn(OAc)2, three complexes could form, the homochiral complexes SS,SS)- and RR,RR)-9 (i.e., chiral self-recognition) and the heterochiral complex SS,RR)- (i.e., chiral self-discrimination). The tetrahedral coordination geometry strongly favors self-discrimination only the heterochiral complex SS,RR)- is... 

All the experimental data and most of the calculations would correspond to gas-phase results and examples of chiral self-recognition. Only exceptionally, condensed phase results of chiral self-recognition or gas-phase results of classical... 

One of the most complete studies of chiral self-recognition concerns serine. The experimental studies carried out on hydrogen-bonded gas phase clusters, provide useful data to be compared with theoretical results, as in general, the latter are... 

O. Picazo et al., Dimers of 1, 8a-dihydro-1, 8-naphthyridine derivatives as models of chiral self-recognition. Struct. Chem. 16, 339-345 (2005)... 

N. Borho et al., Chiral self-recognition in the gas phase The case of glycidol dimers. Phys. Chem. Chem. Phys. 3, 1945-1948 (2001)... 

Z. Su et al., Chiral self-recognition Direct spectroscopic detection of the homochiral and heterochiral dimers of propylene oxide in the gas phase. J. Am. Chem. Soc. 128, 17126-17131 (2006)... 

A particular point of interest included in these hehcal complexes concerns the chirality. The heUcates obtained from the achiral strands are a racemic mixture of left- and right-handed double heUces (Fig. 34) (202). This special mode of recognition where homochiral supramolecular entities, as a consequence of homochiral self-recognition, result from racemic components is known as optical self-resolution (203). It appears in certain cases from racemic solutions or melts (spontaneous resolution) and is often quoted as one of the possible sources of optical resolution in the biological world. On the other hand, the more commonly found process of heterochiral self-recognition gives rise to a racemic supramolecular assembly of enantio pairs (204). 

In principle, mass spectrometry is not suitable to differentiate enantiomers. However, mass spectrometry is able to distinguish between diastereomers and has been applied to stereochemical problems in different areas of chemistry. In the field of chiral cluster chemistry, mass spectrometry, sometimes in combination with chiral chromatography, has been extensively applied to studies of proton- and metal-bound clusters, self-recognition processes, cyclodextrin and crown ethers inclusion complexes, carbohydrate complexes, and others. Several excellent reviews on this topic are nowadays available. A survey of the most relevant examples will be given in this section. Most of the studies was based on ion abundance analysis, often coupled with MIKE and CID ion fragmentation on MS " and FT-ICR mass spectrometric instruments, using Cl, MALDI, FAB, and ESI, and atmospheric pressure ionization (API) methods. 

The same CIMS approach has been used for investigating the self-recognition processes in proton-bound tartrate trimers. The trimer chirality effect is consistent with the heterochiral trimers as more stable than the homochiral ones. The reverse is true when the proton in the proton-bound trimers is replaced by hydronium, ammonium ion, or primary aminium ions. " This changeover is... 

Self-assembly, self-recognition and replication may involve chiral components, as discussed on several instances above. This leads to some more general considerations about the role of molecular chirality in supramolecular species. 

When discussing chiral recognition, one must take into account two aspects the quantitative and the qualitative. The first refers to the magnitude, and the second, to the sign (in the general case, what enantiomer of B, the R or the S is preferred, and in self-recognition what of the homo- or the heterodimer is most stable, Scheme 3.1). 

As observed for the molecular clips reported above, the chiral scaffold is pivotal in promoting homo- or heterochiral self-discrimination. Amide hydrogen bonds were implemented on helicene chiral scaffolds as well, but in this case dimerization of the monomers was characterized by homochiral enantioselective self-recognition, that is self-association between molecules with the same helicity (Fig. 17B) [44], These species dimerized in solution with association constants of 207 M 1 by means of four non-covalent bonding interactions and, in combination with the peculiar helical shape of the monomers, forms only homochiral dimers. 

Figure 3.4 Example of anticancer tetracationic arene ruthenium metallacycle with chirality at the metal predetermined by self-recognition of chiral units. Structure adapted from Ref. [23].

Cu(lOO), chiral Fe(TMA)4 clusters are formed. This is associated with the deprotonation of the carboxylic acid groups of TMA upon adsorption on the Cu substrate at temperatures above 250 K. Upon annealing at 400 K, homochiral porous network structures can be built up from these Fe(TMA)4 clusters in a hierarchical manner (Figure 6). The nanocavities of these networks have been used as hosts for the accommodation of Ceo and small biomolecules. The same group also investigated such important key factors for self-assembly as self-recognition, self-selection, self-repair, and dynamic self-organization for a library of linear dicarboxylates and bipyridines codeposited with Fe atoms onto a Cu(lOO) surface. ... 

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