Homochiral recognition

Octahedral hexa-coordinated ruthenium(ll) complexes bearing 2,2 -bipyridine ligands (bpy) have received considerable attention because they have numerous applications for instance, as chiral building blocks for supramolecular assemblies, or as chiral probes for biological molecules (polynucleotides, Hence efforts have been devoted to 

Schematic drawing of A-TRISPHAT (4.7) and some octahedral ruthenium complexes that show (A, A) homochiral recognition. 

The strategy of using A-TRISPHAT to resolve or differentiate other organometallic or coordination compounds has been investigated by several groups with success. These studies are presented in Section 4.2. 

Cyciodextrins (CDs) are cyclic oligosaccharides composed usually of six to eight D-glucopyranoside units, linked by 1,4-glycosidic bond. The three most important 

Schematic drawing of per-C02 -/(-CD (4.12) and the hucket-t)fpe model of this heptaanion cyclodextrin. 

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AAA. This system represented a rare example of selective homochiral recognition processes. 

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). 

The carboxylic acid functionality is not involved in any significant interaction with the gold substrate but rather dominates the pairing interaction between enantiomers. This three-point bonding of each molecule, Au—S, Au—N, and O—H—O, drives the self-recognition preference for homochiral pairs [58]. 

A more subtle example of homochiral preference, which draws attention to the conformational changes in the molecules needed to achieve self-recognition, is that... 

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... 

Shi, X. Fettinger, J. C. Cai, M. Davis, J. T. Enantiomeric self-recognition Cation-templated formation of homochiral isoguanosine pentamers. Angew. Chem. Int. Ed. 2000, 39, 3124-3127. 

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... 

Enantiomer recognition is a general principle in chemistry. Molecular recognition is achieved by numerous electronic and steric factors including chirality. This is also the case among molecules with the same atomic composition and connectivity. As illustrated in Scheme 27, chiral (/ ,/ )- or (5,5)-tartaric acid may be seen as a homochiral dimer of the / - or 5-pyramidal radicals, respectively meso tartaric acid is a result... 

Scheme 41 shows a striking stereospecificity in coupling of a metal-complexed carbene species (67). Crossover experiments using the enantiomerically configured Re-carbene complexes revealed that the reaction proceeds through strict self-recognition of the enantiomers to form only homochiral coupling products. 

Self-assembly may occur with self-recognition, mixtures of components yielding defined superstructures without interference or crossover (see Section 9.5). A special case is self-resolution which results in the formation of homochiral supramolecular entities from racemic components (see Sections 9.4.2, 9.4.4 and 9.7). 

Only two articles considered the effect of solvation on the chiral recognition. The first one, already discussed in the previous section, checks the effect of water as solvent in the relative stability of a dimer of a-aminoalcohols [32]. While in the gas phase, one of the heterochiral dimers is always the more stable one, the inclusion of the water solvation effect provides, as the most stable configuration, a homochiral one in three of the four systems studied. 

From the abundance ratio, the enantiodifferentiation energy of the clusters is obtained, having found seven cases in which the heterochiral complexes are more stable than the corresponding homochiral, while only four cases of the homochiral complex are more stable. A chiral recognition of between —1.8 kJ mol-1 and +2.3 kJ mol 1 has been found depending on the metallic center. 

A theoretical study of the chiral recognition of the bis(amino(phenyl)methanol) chromium(O) complexes and chiral discrimination of the dimers of amino(phenyl) methanol linked by HB has been carried out by means of a DFT method, the B3LYP/6-31+G. The results (Fig. 3.24) show that in the chromium complexes, the homochiral one is preferred over the heterochiral one, while the opposite... 

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)... 

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. 

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