Benzene sector rule

FIGURE 8. Quadrant projections for application of the benzene sector rule to chiral monosubstituted benzene compounds... 

The benzene sector rule may be nsed to correlate the absolnte configuration of a contiguous chiral cento- of a monosnbstitnted benzene compound with the sign of the CE... 

When an inert solvent, e.g., tetrachloromethane or chloroform, is replaced by benzene or pyridine, characteristic signal shifts Ad can be observed, the extent and sign of which can be correlated with the structure by simple empirical sector rules. 

Based on the CD-spectra of various optically active carbophanes, a sector rule for the correlation of the sign of the 1Lb-Cotton effect of the benzene chromophore with the absolute chiralities of these phanes (as well as for chiral dihydro-9,10-ethano-anthracenes) has been proposed 63). 

Another example for an application of computed sector maps is the OR of phenylalanine, as far as contributions from the phenyl group are concerned [155]. Sector maps for benzene were computed as shown in Fig. 25. The findings of [155] were in agreement with a revised sector rule for the phenyl chromophore by Pescitelli et al. who investigated a set of chiral molecules with phenyl substituents (PhCH(Me)R, with R = Et, "Pr, Pr, and Bu) [265],... 

More numerous, however, are aromatic compounds with an effectively planar n system, the optical activity of which comes from chiral perturbations. Benzene derivatives such as phenylalanine (3) are particularly important. The Lb band of the benzene chromophore (A = 260 nm) is neither magnetically nor electrically allowed. The symmetry selection rule may be broken by vibronic interactions or due to substituents. The interpretation of the observed rotatory strength is not easy, but empirical sector rules have been proposed. (Cf. Charney, 1979 Pickard and Smith, 1990.)... 

Figure 11 The benzene chromophore as an example for the application of sector rules for the third sphere. On the right the schematic representation of the sector rules for the long wavelength /.j transitions (A) and for the short wavelength transitions (B) are given. The plane of the aromatic system is a nodal plane.

Important examples of such rules include the octant rule (sector rule) used for predicting the sign of the CE of the n—>71 transition (A 300nm) of saturated ketones and the helicity rule of the benzene chromo-phore, which correlates the helicity of the nonaromatic ring of tetralines, tetrahydoisoquinolines, isochromanes. 

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