Enantiomers, helical

Enantioseparation is typically achieved as a result of the differences in interaction energies A(AG) between each enantiomer and a selector. This difference does not need to be very large, a modest A(AG) = 0.24 kcal/mol is sufficient to achieve a separation factor a of 1.5. Another mechanism of discrimination of enantiomers involves the preferential inclusion of one into a cavity or within the helical structure of a polymer. The selectivity of a selector is most often expressed in terms of retention of both enantiomers using the separation factor a that is defined as ... 

The trisulfane molecule exists as two conformers which have been termed as cis- and trans-HzSi. While the trans-form is a helical molecule of C2 symmetry with the motif ++ (or — for the enantiomer), the cfs-form is of Q symmetry with the motif +- (identical to -+). Both forms have been detected by rotational spectroscopy [17, 45, 46]. The motif gives the order of the signs of the torsion angles at the SS bonds. The geometrical parameters [17] are presented in Table 4. The trans-isomer is by only 1 kj mol more stable than the cfs-form but the barrier to internal rotation from tmns to cis is 35 kJ mor [46]. The dipole moments were calculated by ab initio MO theory at the QCISD/TZ+P level as 0.68 D (trans) and 2.02 D (cis) [46]. For geometrical parameters of cis- and trans-trisulfane calculated at the MP2/6-311++G> > level, see [34]. 

An extension of this protocol was further developed for a diiron(II) triple helicate and afforded in separated phases the P or M enantiomers of the [Fe2L3]4+ helix 71 (Fig. 25) [135]. 

Now, consider repeating these operations, but with the use of the observer s left hand to spin the nut in an anticlockwise direction as seen by the observer. It should be apparent that, in either orientation of the rod, the nut will now travel backward (toward the observer). This must suggest that an electron would be ejected in a reversed direction following excitation by a photon of opposite helicity, again mediated through the chiral interaction of the electron with the enantiomer s potential field. 

While it is clear that a direct measurement of the angular distribution, Eq. (4), with a given hehcity of hght should be capable of yielding the h angular parameter, it is often more convenient to examine the dichroism, or difference, obtained with opposite helicities of the light (or, possibly, of the enantiomer). From Eq. (4) and the antisymmetry property Eq. (6) one obtains an expression for the PECD ... 

The oldest method of resolving enantiomers by TLC takes advantage of the natural chiral properties of cellulose and triacetylcellulose resulting from the helical structure of the polymers (98). Amino acid derivatives have been resolved on silica gel layers impregnated with chiral acids or bases, for example. 

The cyclooctapyrroles shown in Figure 55 appear predestined to form binuclear metal complexes since the loop-shaped conformation of these macrocycles exhibits two structurally identical, helical N4 cavities. Enantiomers of such complexes, which are presumably generally very stable towards racemization owing to the rigidity of the molecule imposed by the incorporation of the metal, are of interest as possible models for binuclear metalloenzymes and as potential catalysts in asymmetric synthesis. The first two ligands as well as their recently obtained palladium complexes601 were... 

In the resolution of racemates having a C2 axis and two aromatic groups using (+)-l, the more retained enantiomers have P-helicity with respect to their aromatic groups if one looks at the molecules from the direction perpendicular to a C2 axis (Figure 3.8). This could suggest that (+)-l has P-helicity that is, a right-handed helical conformation.45... 

Compound 72 was shown to display enantioselectivity in the extraction of chiral potassium salts from water into the organic phase.105 The supramolec-ular polymer possesses a homochiral helical architecture onto which one of the anionic enantiomers preferentially binds. Intriguingly, for some of the anions the octamer and polymer showed opposite selectivity, illustrating the difference in supramolecular chirality of the two systems. Furthermore, the polymer was capable of inducing a Cotton effect in the achiral compound potassium A-(2,4-dinitrophenyl)glycinate. Since the apolar side chains would... 

The material system is a Langmuir-Blodgett film of the S enantiomer of a chiral polymer deposited on a glass substrate. The polymer is a poly(isocyanide)30 functionalized with a nonlinear optical chromophore (see Figure 9.14). In this particular system the optical nonlinearity and chirality are present on two different levels of the molecular structure. The chirality of the polymer is located in the helical backbone whereas the nonlinearity is present in the attached chromophores. Hence, this opens the possibility to optimize both properties independently. 

The cylindrical helical binuclear complex [Fe2L3]4+, where L is the bis-diimine Schiff base 1, exists as two enantiomers, whose interactions with DNA differ markedly. The more strongly interacting enantiomer targets the major groove (cf. ruthenium(II) complexes, Section II.E.l below) and induces dramatic tightening of the DNA coil (118). 

Racemic complex (racemic PBG), composed of PBLG and its enantiomer poly(y-benzyl D-glutamate) (PBDG), takes an a-helical conformation in the solid state and has different physical properties from those of PBLG or... 

Another cooperativity effect, dubbed majority rule has also been noted. Where opposite enantiomer chiral groups are both present in side chains, such as (S) and (l )2-methylbutyl groups, the overall helicity is determined in a non-linear manner by the ee only, as is evident in Figure 42. 

Figure 41 Composition dependence of the enantiomer excess 2fP - 1 of the P helical state of polysilanes 109(P) O 109(S) o, 110 A and 111 (calculated solid curves experimental symbols).326 Reprinted with permission from Sato, T. Terao, K. Teramoto, A. Fujiki, M. Macromolecules 2002, 35, 5355-5357, 2002 American Chemical Society. 

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