Induced dissymmetry

The design of viable, highly enantioselective epoxidation catalysts based on porphyrin ligands is confronted by the inherent difficulty associated with inducing dissymmetry from remote parts of an sp -hybridized coordination sphere, and the relative difficulties in constructing the chiral porphyrin rings. As research in this field has been the subject of an insightful review [90], only more recent developments will be covered here. 

X-ray studies also indicate that the same type of structure may be assigned to the basic propionate, triacetate tripropionate, isobutyrate, butyrate, and pivalate. It may therefore be assumed as general for all members of the series. In the basic formate and acetate, complete crystal symmetry is predicted by structures of this type. Replacement of the hydrogen or methyl with radicals such as ethyl and propyl should induce dissymmetry. That this is true is shown by the crystal habits of these compounds. The basic acetate crystallizes in the cubic system, but the basic propionate, triacetate tripropionate, and pivalate are monoclinic in form, and the basic isobutyrate crystallizes in the triclinic system. The low melting point of the basic butyrate made a complete x-ray investigation difficult, but the crystals were shown to be of very low symmetry. ... 

A more serious limit to this implementation is due to the volume of the recycling pump and associated equipment such as flowmeters and pressure sensors. As the pump moves with respect to the zones, its volume leads to a dead volume dissymmetry, which can lead to a decrease extract and raffinate purities. This decrease can be significant for SMB with short columns and/or compounds with low retention. However, it can be easily overcome by using a shorter column or asynchronous shift of the inlets/outlets [54, 55]. This last solution is extremely efficient and does not induce extra costs because it is a purely software solution. 

The fringes contrasts are subject to degradation resulting from dissymmetry in the interferometer. The optical fields to be mixed are characterized by a broadband spectrum so that differential dispersion may induce a variation of the differential phase over the spectrum. Detectors are sensitive to the superposition of the different spectral contributions. If differential dispersion shifts the fringes patterns for the different frequency, the global interferogramme is blurred and the contrast decreases. Fig. 5 shows corresponding experimental results. 

In another example, the cumulative effect of equatorial attack in prochiral cyclohex-anoneimines with diastereoselectivity induced by a chiral nitrogen substituent allowed the synthesis of spirocyclic oxaziridines with a high induction of axial dissymmetry. The major oxaziridine isomer results from both the favored equatorial attack and oxidation anti to the chiral nitrogen substituent (equation 45)204... 

The enantiomeric atropoisomers of 1,1 -binaphthyl-2,2 -diol (BINOL) and bis-diphenylphosphonate derivatives (BINAP) are completely synthetic molecules that have been developed to exploit the axial dissymmetry induced by the restricted rotation about the biaryl bond (Scheme 1.8) [64]. During the past 15 years, these compounds have become the most widely used ligands for both stoichiometric and catalytic asymmetric reactions, with many analogues and derivatives having been developed recently. 

We will show in this chapter that a designed combination of thermodynamic and/or kinetic control of the self-assembly processes has allowed us to develop various applications all based on the induced chirality in the porphyrin Soret band absorption region. Chiral information transfer has been followed using circular dichroism, a spectroscopic technique particularly useful to characterize (supra) molecular dissymmetry or chirality [3]. 

Multiple Cotton effects in the visible region were observed for the algal chromoprotein phycocyanin (265, 266). Interpretations included inherent dissymmetry of the tetrapyrrole chromophore (267), possibly induced by binding to the protein, interactions between chromophores of different monomeric units or between these chromophores and protein transitions (265, 266). The chemistry and mode of attachment of the conjugated molecules to the protein will have to be elucidated in more detail in order to enable a better evaluation of the origin of the optical activity observed. 

Correlation of solvent and cation inclusion induced changes in the CD spectra with the computer generated molecular models supports the cyclization mechanism 51,58) pjjg rotation of the C25 carboxylate accompanying cyclization destroys the plane and center of symmetry of the aromatic chromophore rendering it dissymmetric. Induction of dissymmetry into the aromatic chromophore establishes a diastereomeric relationship with the existing stereochemistry of the ionophore backbone. Since the... 

When we reinvestigated the base-induced reaction of 1-naphthol with formaldehyde we isolated and identified three isomeric tetrameric compounds [2] which we named calix[4]naphthalenes by analogy with the calix[4]arenes and cal-ix[4]resorcinarenes. However, unlike the latter which are derived fromp-substituted phenols and resorcinol respectively, several different isomers can theoretically exist for the calix[4]naphthalenes. Additionally, the conformations that are possible for some of these isomers are further complicated due to the dissymmetry that is introduced by the naphthalene rings. In this contribution we will describe some properties of this new class of compounds. 

Spreading and Translation. Examples of motions of drops on liquid surfaces are known in literature like the pentanol droplet of Nagai et al. (2005) or the aniline droplet of Chen et al. (2009), both on aqueous solutions. The aniline drop is similar to the present system in the sense that dissolution, evaporation and density effects are also involved. In this case evaporation and solubilization are relatively slow and the drop appears as a macroscopic drop coexisting with a surrounding precursor film. Two different kinds of motion are observed bee-like motion and circular motion that could last for hours. Motility was ascribed to a surface tension imbalance at the front and rear of the drop. This difference, revealed by a difference in contact angle, was attributed to the precursor film. The film is driven by a Marangoni flow and may lose its initial symmetry (by fluctuations or induced manually). The initiated translation is then sustained by the dissymmetry of the surface active film pointing to the rear of the drop. 

Similarly, in optically active esters containing an olefinic group, the induced C C group yields one center (or two) of classical dissymmetry, either by reduction (76,136,143) or by addition of two hydroxyl groups (95) ... 

Bromination of I yielded diastereoisomerides II and IV, which were separated by fractionation and then oxidized individually. The antipodal substances, III and V, respectively, were isolated, in which the induced asymmetry of the original olefinic group has been fixed by bromination, but the original dissymmetric secondary alcoholic grouping has been oxidized to a symmetrical carbonyl group. It remains a part of the final molecule, but with its classical dissymmetry destroyed, though it probably still contributes to the total optical activity of the product through asymmetric induction. 

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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