Chiral induction chemically induced chirality

Cholesteric liquid crystals are optically active nematic phases as a result of their gradual twist in orientational alignment. Therefore, cholesteric liquid-crystalline solvents are expected to induce enantioselectivity in chemical reactions see reference [713] for a review on photoasymmetric induction by chiral mesophases. The existing results are not very promising. So far, the maximum photoasymmetric induction reported has... 

More recently, Hoppe has also compared (-)-sparteine 2 with (-)-a-iso-sparteine 28 and N,Ar,AT, Ar -tetramethyldiamine 4 in the enantioselective deprotonation of alkyl carbamates. It was found that a-isosparteine 28 does not support the deprotonation of alkyl carbamates at all, whereas 4 generally gave lower levels of ee (Scheme 13) compared with sparteine [42]. However, the slimmer hgand 4 facilitated enantioselective deprotonation of hindered carbamate 27 (R = t-Bu) in 79% ee, whereas (-)-sparteine 2 failed to effect deprotonation a delicate balance in the steric demand of the CH-acid (alkyl carbamate) and the inducing diamine therefore determines the success of the deprotonation. Quantum-chemical calculations (PM3, ab initio methods) on several models for the competing diastereomeric transition states of the deprotonation under the influence of 2 were found to reflect well the sense and the magnitude of the experimentally observed chiral induction. 

The most obvious stimulus to ehiral induction is the use of stereogenie eenters, but external perturbation, such as physical fields ean also be used to induce chirality. The term is applied to traditional asymmetric synthesis through covalent bonds as well as to supramolecular synthesis, which is our interest here. Mention the word "chirality" to any chemist, and thoughts will be conjured of the right-handed B-DNA double helix, the right-handed a-helices formed by peptides of the natural L-amino acids, of thalidomide, and other emblematic and dramatic examples of the importanee of stereochemistry. It is clear that the chirality of the eomponents of biological systems play a key role in their function, in which induction of chirality through noncovalent bonds is inherent. But beyond natural systems and related phenomena, there is a wealth of unnatural chemical systems that display remarkable and important properties. 

Fig. 11.11 Schematic model of the chiral induction of (rac)-Poly-4 into a N -LC phase. Upon the addition of the chiral dopant (S)-Dl at 10 wt % into the N-LC phase of (rac)-Poly-4 left), a N -LC phase is induced (right), a POM image of the N-LC phase of (rac)-Poly-4 in 10 wt % lyotropic LC solution in toluene showing a Schlieren texture, b POM image of the N -LC phase of (S)-Dl/(rac)-Poiy-4 in 10 wt % lyotropic LC solution in toluene showing a fingerprint texture. Inset shows the helical half pitch of 2.0 pm. Reprinted with permission from [18]. Copyright 2012, American Chemical Society...

In the case when the compound reacting with the optically active molecule also possesses one or more electronic transitions the evidence of the chemical interaction can become indiscutable. The very existence of an association submits the electronic transitions of the compound to the dissymmetry of the chiral molecule. They become optically active and, consequently, detectable by the intermediary of the corresponding induced COTTON effects. Figure 9 shows the induction of optical activity in the d-d transition of the Cuii ion complexed with the optically active ligand N-tosyl L-alanine(YI) in aqueous solution at pH 10 [18]. 

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