Conformational enantiomerism

Molecules that are chiral as a result of barriers to conformational interconversion can be racemized if the enantiomeric conformers are interconverted. The rate of racemization will depend upon the conformational barrier. For example, -cyclooctene is chiral. E-Cycloalkenes can be racemized by a conformational process involving reorienting of the... 

The latter exists as only one achiral polycentric configuration. For systems like 10 there are not only achiral rigid models 10b and 10d but also chiral ones, 10a and 10a, as well as 10c and 10c where the observable achirality must be interpreted by looking at the whole ensemble of conformations. Such systems are achiral, because the enantiomeric conformations are mutually interconvertible via achiral conformations and occur at equal equilibrium concentrations. 

Fig. 12.27 Enantiomeric conformations of gauche ethylenediamine (1.2-diaminoethane). Note that S represents a right-handed, helix and A a left-handed helix.

The enantiomeric conformers 121a and 121b were resolved using low-temperature HPLC on a chiral stationary phase, and their differential absorption (Ai —Ar) was reported as a signal from an on-line CD detector. 

Amidic groups were also incorporated into the structure of receptor 37 [67]. This receptor was used for the complexation of substrates capable of quick interchange between their conformational enantiomers in solution. These guests can be fixed in the cavity of 37 predominantly in one of the possible enantiomeric conformations. Using CD spectroscopy, an increase in the amount of (S)-helical bilirubin (BR) conformer upon addition of (+)-37 was observed. The (R)-helical conformer was preferred on adding (-)-37. The complexation constants were determined from CD spectroscopy data 6.4X105 and 6.9X105 M 1... 

Conformational enantiomerism. trans-Cyclooctene is strained, unable to achieve a symmetric planar conformation. It is locked into one of these two enantiomeric conformations. Either pure enantiomer is optically active, with [a] = 430°. 

Meso configurations can have enantiomeric conformers in crystals. Ribitol and xylitol have meso configurations and are optically inactive in solution. In the crystalline state, they have the bent-chain enantiomorphic conformations. Xylitol crystallizes in space group P212121, so each crystal contains either only the optically active left-handed or the optically active right-handed conformers. There are, of course, an equal number of left- and right-handed crystals in any one batch, which on dissolution give no optical activity.1... 

The N-CH2-CH2-N group forms a five-membered puckered ring with a twofold axis of symmetry as shown in (73) and (74). When undistorted, the C-C bond makes a dihedral angle of 48.8° with respect to the N-M-N plane. The two enantiomeric conformations are labelled 5 (73) (left-handed helicity) and )((74) (right handed helicity) with an inversion energy barrier of about 20kJmoL that increases with N-alkylation. 

L-forms. In myo-inositol, the plane of symmetry is C-2/C-5 unsymmetrically substituted derivatives on C-1, C-3, C-4, and C-6 are chiral. Substitution at C-2 and/or C-5 gives a meio-product. fl/to-Inositol appears to have a plane of symmetry, but at room temperature it is actually a racemate formed of two enantiomeric conformers in rapid equilibrium. 

The twist-boat-chair is chiral and hence there are actually two mirror-image or enantiomeric conformations. Correspondingly, there are two mirror-image pseudorotation paths to the twist-boat-chairs from the boat-chair, which is achiral. These paths are conveniently labelled by the mirror-image letters b and d. If we arbitrarily assign the letter h to the process shown in Fig. 3, and if we use the proton in the 1-equatorial position as a label, we can write down the effect of pseudorotation as shown in Table 6. 

Replacement of a CH2 group in cyclooctane by a CHX or CX2 group, where X is achiral, or by a heteroatom such as O or S, leads to a chemically achiral molecule. If the conformational preference of the group or heteroatom is for the 1 or 5 positions in the boat-chair, the molecule is also con-formationally achiral. However, if the preference is for any other positions, two enantiomeric conformers will result. In the latter case, dynamic nmr can be used to study the sequence of pseudorotations which leads to conformational racemization. 

The TCNQ and TCNQF4 CT complexes of octaethylferrocene ([Fe(fy -C5Et4H)2] have been described by Sitzmann and co-workers [54]. Whereas the former material adopts a ID-DADA structure in the solid state, the second is found to comprise DDAA stacks. These are constituted by a side-by-side arrangement of the cations and by parallel anion dimers. Interestingly, the cation pairs correspond to pairs of enantiomeric conformations of the octaethylferrocene, related by a crystallographic inversion center. No magnetic properties of these two materials have been reported. 

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