Heterochiral mixture

Heterochiral mixtures of JV-(a-methylbenzyl)stearamide and stearoylserine methyl ester show a collapse of the film at 20 k2/molecule to some unstable state (Fig. 33). The homochiral mixture can be compressed to a higher... 

We have seen that any object whose mirror image is nonsuperimposable with it, or in other terms does not have any element of inverse symmetry, is chiral. It exists in the form of two isomers which have the same chemical composition and the same connectivity of the constituent atoms. These two isomers are named enantiomers. Enantiomers have the same physical properties, only by their vectorial properties are inverted and in particular their ability to rotate the plane of polarization of light. Rotation to the right corresponds to the dextrorotatory (+) isomer, rotation to the left to the levorotatory (—) one. Molecules, whether in the gas, liquid or solid phase, are not unique, isolated objects. In the chemist s flask there are a great many molecules. If they comprise just one single enantiomer the product is termed enantiopure or homochiral. In the case where both enantiomers are present but with a majority of one of them, it is referred to as an enantio-enriched or heterochiral mixture (Figure 2.28). 

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 implications for films cast from mixtures of enantiomers is that diagrams similar to those obtained for phase changes (i.e., melting point, etc.) versus composition for the bulk surfactant may be obtained if a film property is plotted as a function of composition. In the case of enantiomeric mixtures, these monolayer properties should be symmetric about the racemic mixture, and may help to determine whether the associations in the racemic film are homochiral, heterochiral, or ideal. Monolayers cast from non-enantiomeric chiral surfactant mixtures normally will not exhibit this feature. In addition, a systematic study of binary films cast from a mixture of chiral and achiral surfactants may help to determine the limits for chiral discrimination in monolayers doped with an achiral diluent. However, to our knowledge, there has never been any other systematic investigation of the thermodynamic, rheological and mixing properties of chiral monolayers than those reported below from this laboratory. 

When compressed to surface pressures greater than their stability limits (see Table 10), diastereomeric mixtures of /V-(a-methylbenzyl)stearamides with both stearoylalanine and stearoylserine methyl esters provided clear evidence of chiral discrimination. Force-area isotherms at 35°C for homochiral and heterochiral pairs of N-(a-methylbenzyl)stearamide and stearoylalanine methyl ester show differences in both their lift-off and touchdown (the area per molecule where the surface pressure returns to zero on the expansion arm of the isotherm) areas per molecule (Fig. 32). In addition, monolayers of the heterochiral pair could be compressed to lower areas per molecule than monolayers of the homochiral pair. 

No discrimination in the pressure/area characteristics was seen for diastereomeric monolayer films spread from all possible mixtures of pure racemates (R- and S-) and their racemic mixtures (R-, S-) of stearoylalanine, stearoylserine, stearoyltyrosine, and stearoyltryptophan methyl esters. The one exception was heterochiral and homochiral mixtures of stearoylalanine methyl esters and stearoylserine methyl esters at 35°C. The force/area... 

The relative stability of the homochiral and the heterochiral dimers arising from self-CI of an equimolar mixture of the L and the D enantiomers of dimethyl- and di-isopropyltartrate has been evaluated by Nikolaev et al. using the FT-ICR... 

Alfonso, Gotor, and coworkers have also affected diasteroeselective amplification from racemic libraries and achiral guests [4]. Mixtures of cyclohexyl-1,2-diamine (rac) and 2,6-diformyl-pyridine led to a mixture of homo- and heterochiral dimers, trimers, and tetramers (Fig. 5.4). The addition of Ba(II) slightly amplified the homochiral over the heterochiral dimer (1.6 1). Templafing with Cd(II) instead led to a preponderance of trimers, as evidence by ESI-MS. However, the large number of signals in the H and C NMR spectra indicated a heterochiral trimer. Amplification of the C -symmetric heterochiral trimer (Fig. 5.4) was confirmed by... 

Stereochemically, even more interesting are the bis-l,3-oxathiane derivatives 98 and 99 <2005STC369>. All compounds are chiral and the chiral elements are carbons C-2/C-2 and the 1,3-oxathine moieties themselves (Equation 9). Due to the bis-structure, compounds 98 and 99 exhibit both homochiral iZR,2 R ZS,Z S) and hetero-chiral (2R,Z S) isomers (Scheme 4) and they reveal rapid equilibration in solution via open-chain intermediates, thereby preventing separation and individual analysis of the isomers in solution. In the solid state, the compounds crystallized either as unique heterochiral isomers or as a mixture of the two as a solid solution. 

In the latter case, the applicability rests on a knowledge of the relative formation constants of homo- or heterochiral diastereomers. A special case arises when the formation of dimers occurs by self-association (being fast on the NMR timescale) under nonideal conditions leading to nonequivalence of resonance absorptions. When the enantiomeric composition differs from that of the racemic mixture, self-induced anisochrony is observed and the intensities of the signals are directly related to the enantiomeric ratio regardless of parameters such as temperature, concentration, and the ratio of homo- and heterochiral dimers116. 

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... 

Fig. 4 Production of a monomeric active catalyst from diethylzinc and a f. -aminoalcohol the resting form is an equilibrating mixture of homochiral and heterochiral dimers, which favours the latter...

The key NMR observations (i) that the proportion of homo- and heterochiral dimers is near-equal, and (ii) that their interconversion by a dissociative process is rapid compared to catalytic turnover, preclude the possibility of a monomer autocatalyst. In Kagan s classification, monomer catalysis with a positive NLE may only arise when there is an unequal concentration of homo- and heterochiral oligomers, in favour of the heterochiral form, which acts as a reservoir for the deficient enantiomer. NMR results show that the resting state for Soai s autocatalysis is an equal mixture of homo-and heterochiral species, predominantly dimeric. The lack of ground-state stereo-discrimination requires that the number of chiral entities in the resting state must be less than or equal to the number in the enantioselectivity-determining transition state, else there is no possibility of the vital non-linear effect. Even after the publication of these results in late 2004, their consequences are not always applied. For recent discussions where a monomeric catalyst for Soai s system is permitted or promoted, see [91-93]. 

When starting from chiral non-racemic mixtures of 3 7 and 4 6 L/D compositions, the short oligopeptides generated are rich in heterochiral di-astereoisomers, whereas the longer oligomers are rich with oligopeptides of... 

Dimers RR and SS are termed homochiral dimers, whereas the mixed dimer RS is termed a heterochiral dimer. A fraction of pure enantiomer is always a mixture of the monomer and the corresponding homochiral dimer, but it does not contain any heterochiral dimer. It was shown, that the separation of the excess enantiomer of a non-racemic feed mixture is the combination of two effects [31], the difference in reaction equilibrium constants of the homo- and heterochiral dimerization reaction and the difference in the adsorptivity of the homo- and heterochiral dimers. 

Thermodynamic studies favour the heterochiral complex over the homochiral complex [56]. On the other hand, crystallization from a mixture seems to favour the matched pairs. Thus, when the mismatched complex 29 48 between (R,R)-trans-1,2-diaminocyclohexane and (S, S)-1,4-cyclohexene- 1,2-diol was heated (melted) with 1 equiv. of the matched (f ,f )-diol, only crystals of the matched pair 29 35 were formed upon cooling (Scheme 21). In a similar way, when the mismatched complex (R,R/S,S)-29 49 was melted with 1 equiv. of the matched diol (f ,f )-32 and the mixture was crystallized from benzene, only crystals of the matched pair (R,RJR,R)-29 32 were obtained (Scheme 21). The corollary that matched diols and diamines compete better in the crystallization and assembly... 

The relative stability of the homochiral and the heterochiral dimers arising from self-CI of an equimolar mixture of the L and the D enantiomers of dimethyl- and di-isopropyltartrate has been evaluated by Nikolaev et al. using the FT-ICR technique.366-369 The dimer chirality effect, Khomo/= 0.33 corresponds to a AAG°98 = — RT ln(Arhomo/Arhelero) = 0.65 kcal mol-1 value at 20 °C, a value which is slightly larger than those measured in the CIMS experiments (0.25-0.50 kcal mol-1).358,359 The lack of chirality effects, observed when the used tartrates are replaced by the L and the D enantiomers of methyl lactate, alaninamide, and Af-acetyl-a-methyl-benzylamine, is attributed to their extensive racemization after protonation. 

Chiral recognition techniques have been used in photoinduced electron transfer reactions and the differential formation of homochiral against heterochiral exciplexes. In the systems studied excited states of lanthanide complexes are quenched by transition metal complexes. The complexes with trigonal dihedral (D3) symmetry which can exist as enantiomers with either left handed (a) or right-handed (A) configurational chirality are chosen. The system consists of a mixture of racemic lanthanide complex and optically resolved... 

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