Sector rule

Walborsky and coworkers44 proposed a sector rule, the planar diene rale, to correlate the sign of the Ag —> transition Cotton effect of these derivatives with the absolute... 

FIGURE 8. Sector rules for exocyclic s-trans dienes. Top bond-centred , bottom atom-centred rule. The ellipsoid represents the rest of the molecule and also defines the direction opposite to the observer. In both cases two planes are defined by the transition dipole and the plane containing the diene only the third plane is located differently. The bond-centred rule holds for cyclohexylidene compounds, while the atom-centred rule should apply to adamantylidene derivatives... 

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

For alkyl-substituted A-nitrosazetidines, it was shown that the CD spectra can be interpreted on the basis of conformational diastereoisomerism, taking into account the nonplanarity of the nitrosazetidine chromophore137. For a particular configuration, the CE sign of the n - tt transition in the 350-400 nm region is determined by the intrinsic chirality of the chromophore and obeys a spiral rule, the same as that for nonplanar nitrosaziridines where the absence of the local plane of symmetry in the nitrosamine chromophore does not permit the use of the planar sector rule 148. 

U.v. and c.d. data for 18 highly substituted steroid and triterpenoid olefins include some additional examples which for the most part follow the dissignate sector rule with respect to the lowest-energy transition and a consignate rule for the next transition of higher energy. ... 

The resulting shieldings or dcshieldings of nuclei can be described by double-cone sector rules (+) indicates shielding (smaller <5 values) and (—) deshielding (larger <5 values). 

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. 

By a reaction sequence similar to the one outlined above for the [8][8]cyclophane 32 also the levorotating [8][10]paracyclophane 33 was obtained starting from (—)-methyl[10]paracyclophanecarboxylic acid IS6. Opposite Cotton effects of (+)-32 and (—)-33 indicated that they had opposite chiralities, and hence it followed that both (+)-32 and (+)-33 had the same chirality, namely (S). It should be noted that for (-)-14 the chirality (S) had been established 40) (cf. also Ref.62) and sections 2.9.2. and 2.9.3.) it would be somewhat surprising that its levorotatory methyl derivative 15 had (/ ) chirality as deduced from the above sequence (—)-15 - (-)(/ )-33 44). Moreover, for levorotatory [m][n]paracyclophanes (S)chirality had been proposed by application of a sector rule 63) (see also 2.9.4.). 

As usual in stereochemical research, four main approaches have been applied to the problem of assigning chiralities to optically active cyclophanes. They are listed in order of their reliabilities i) anomalous X-ray diffraction (Bijvoet method), ii) chemical correlations with compounds of known chiralities (preferably established by the Bijvoet method), iii) kinetic resolutions and/or asymmetric syntheses, iv) interpretation of chiroptical properties (mainly circular dichroism) on the basis of (sector) rules including theoretical methods. 

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). 

This chapter is not an update of a previous chapter and will therefore try to review the reported chiroptical data on the carbon-carbon double bond, starting from 1968. It will refer only to the available literature on the C=C chromophore itself. It will analyze the available data of molecules which contain only one chromophore, the carbon-carbon double bond. It will not dwell on molecules which have the C=C bond as one of the chromophores which are responsible for its optical activity. It will cover the literature in the field of electronic excitations and will not provide information on vibrational CD (VCD) or Raman optical activity. The chiroptical properties provide information regarding the spectroscopy of the chromophore, as well as its absolute configuration. The latter is usually done with the help of sector rules around the chromophore of interest. In this review both aspects will be discussed. 

FIGURE 8. Comparison between the sector rules proposed by the three models... 

In order to use its CD to determine the AC of a chiral molecule, a theory is required which predicts the sign of the CD of a given enantiomer. The utilization of CD by organic chemists was greatly stimulated by the development of the Octant Rule, which predicts the CD of the n-tt electronic excitation of carbonyl functional groups [3], Subsequently, so-called Sector Rules were developed for many other electronic chromophores, extending the applicability of CD [4],... 

For purposes of discussion, we divide applications of CPL spectroscopy into three categories (1) efforts to develop reliable CPL "sector rules", (2) use of comparative CD and CPL studies to probe excited state geometry changes, and (3) the specific use of the selectivity and sensitivity of CPL to probe details of molecular and electronic structure, and dynamics. Since in this book we are primarily concerned with "analytical" applications of these chiroptical methods, we will emphasize here the last of these categories. 

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