Inherently dissymmetric

The presence of an inherently dissymmetric chromophore in a molecule clearly provides a key to obtaining stereochemical information from both electronic CD and, as demonstrated by the CH2CH2C H fragment, from VCD spectra. As with any spectroscopic structural probe, the characteristic spectral features of the chromophore must not significantly mix with nor be overlapped by other vibrations in the molecule. 

Many chromophores are suitable for use in the exciton chirality method. One of the features required for such a chromophore is its planarity or near-planarity. Nonplanar (inherently dissymmetric) chromophores would contribute to the CD spectra by other mechanisms. The other limiting factor is the position of the transition in the spectral region studied. For example, the 1B transition in the alkyl-substituted benzene chromophore appears near the short-wavelength recording limit around 200 nm, making its use in the exciton chirality method less attractive. Furthermore, the direction of polarization of the lB transition in alkyl-substituted benzene derivatives is not readily determined. In such cases calculation of the rotatory strength is more reliable than qualitative analysis. 

Previous, unsuccessful attempts to demonstrate the principle of a molecular switch based on CPL irradiation include inherently dissymmetric fluorene derivative l,1161 atropisomeric bridged binaphthyls 2,1171 and l,l -binaphthylpyran 3J18 Inefficient photoracemization, low g-values, and insufficient sensitivity for detection were some of the problems encountered. 

Figure 2 shows the structures of four classes of so-called overcrowded alkenes (see Section 5.3.1), designed as molecular components for a chiroptical switch based on CPL irradiation.1191 Thanks to unfavorable steric interactions around the central ole-finic bond, the molecules are forced to adopt a helical shape. The chirality in these inherently dissymmetric alkenes - denoted M and P for left-handed and right-handed helices, respectively - therefore originates from distortion of the molecular... 

Fig. 2 Inherently dissymmetric over-TyP 3 Type 4 crowded alkenes.

Tab. 1 Anisotropy factors of different types of inherently dissymmetric alkenes.

The sterically overcrowded alkenes shown in Scheme 6 have been exploited in our group since, from the perspective of molecular switches design, they combine a number of attractive structural features. Steric interactions between the groups attached to the central olefmic bond force these molecules to adapt a non-planar helical shape. The chirality of these so-called inherently dissymmetric alkenes 3, is therefore the result of distortion of the entire molecular structure. Beside the heli-cene-like geometry, both a cis- and a trans-stilbene chromophore are present in the same molecule. 

The biphenyl group in both lactonic and metacyclophane alkaloids poses an interesting question about chirality. Ferris et al. (39) reported that the biphenyl moiety in lactonic alkaloids was inherently dissymmetric. Its chirality was determined by comparison of the circular dichroism (CD) curves with those of di-hydrothebaines with known chiralities. The biphenyl group of metacyclophane alkaloids with a piperidine ring, however, should exist as an equilibrium between two rotamers with (R) chirality (126a) and (5) chirality (126b) in solution as... 

Typical for aromatic hydrocarbons and their derivatives are the Lb, Lj, Bb, and Bj, bands. Chirally twisted aromatic compounds, for example, hexa-helicene (2), are inherently dissymmetric n chromophores. The rotatory strength of the various transitions can be calculated by means of the common r-electron methods. 

All chemical bonds, except for symmetrically substituted ones, are inherently dissymmetric in two respects charge dissymmetry and hard-soft dissymmetry. The reversal of charge dissymmetry by modifying the structural unit (umpolung) has been well documented and has become one of the important principles in synthetic organic chemistry (16). On the other hand,... 

The fourth, the so-called inherent dissymmetric chromophore, does not possess local symmetry. Therefore, the transitions belonging to these chromophores are magnetically and electrically allowed. Inherent dissymmetric chromophores are often found with so-called form chiral molecules for which atropisomers like binaphthols are typical representatives. Further examples are chromophores that come into being by exciton coupHng. In both cases the dissymmetry factor g is in the order of 10 to 10 and the CD is easily measurable in spite of the fact that the absorption coefficients of these compounds are often very high (e between 10" and 10 ). 

The assignment by semiempirical and empirical rules known as sector and helicity rules. Sector rules can be applied to inherent symmetric chrom-ophores. Helicity rules have been deduced for inherent dissymmetric chromophores. 

Electronic circular dichroism for compounds without a chromophore To determine the absolute configuration of compounds without a chromophore in the accessible spectral region one or more proper groups can be derivatized to obtain an appropriate chromophoric system. The substitution of di-hydroxysteroids with two p-methoxybenzoates can be mentioned as an example. The substitution leads to an inherent dissymmetric chromophore for which the exciton chirality method can be applied. For chiral diols and amino alcohols another interesting possibility does exist. Complexing of these diols and amino alcohols with a transition metal... 

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