Chirality orientation

Stereoselectivity Mechanism of Chiral Orientation of Growing Chain... 

A large part of the stereospecific behavior of polymerization catalysts presented in this review can be rationalized in the framework of a stereoselectivity mechanism involving a chiral orientation of the growing chain. The discovery... 

Hence, this analysis indicates that the stereoselectivity of these models is due, not to direct interactions of the tt-ligands with the monomer, but to interactions of the n-ligands with the growing chain, determining its chiral orientation (0i —60° preferred to 0i +60°), which in turn discriminates between the two prochiral faces of the propene monomer.15,37... 

Molecular modeling studies relative to both preinsertion intermediates and insertion states indicate that for all the metallocenes from 1 to 39 of Scheme 1.2 (independent of their structure and symmetry), when a substantial stereoselectivity is calculated for primary monomer insertion, this is mainly due to nonbonded energy interactions of the methyl group of the chirally coordinated monomer with the chirally oriented growing chain. 

It is worth noting that the lower syndiospecificity of catalytic systems based on 31, with respect to those based on 30,9 is accounted for by these calculations. This is easily rationalized in the framework of the enantioselective mechanism which imposes to the growing chain (both in the preinsertion intermediate and in the approximated transition state) a chiral orientation toward... 

Corradini et al. examined in some detail by molecular mechanics15 and density functional studies100 the polymerization mechanism proposed by Cossee and the catalytic sites on TiC surfaces, including those proposed by Arl-man and Cossee13 and by Allegra.14 According to the calculations, for all these octahedral active sites a similar general mechanism of stereoselectivity occurs which is very similar to the one established several years later for stereospecific metallocenes (see previous section). The chirality of the site would determine a chiral orientation of the first C-C bond of the chain (for a A site,... 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

P-chirally oriented aggregates /W-chirally oriented aggregates... 

Figure 4.23 Chiral orientation of the preferred helicity of single molecules of the polysilane for the origin of bisignate Cotton effect.

The currently used stereochemical nomenclature systems for configurations with four or more ligands are chirality oriented, refering to rigid configurations, or their monocentric subunits. The preceding discussion demonstrates, however, that in many cases it is preferable to use a polycentric representation. 

For (-)-cyclohexanone-2,6-chair conformations are equivalent, with the axial and equatorial CD bonds chirally oriented. The CD stretching VCD, positive at —2200 cm and negative between 2130-2170 cm , is consistent with that predicted for coupled motion of the two CD bonds. In this case, the splitting between the modes is due to different CD bond force constants for the axial and equatorial positions. 

As noted eailier, the dynamic sensitivity of VCD carries it beyond the Bom-Oppenheimer approximation (17, 18). Even though non-Bom-Oppenheimer calculations have recently been initiated (116-119), the use of molecular orbital methods to describe these vibrationally induced currents is lagging behind the measurement and interpretation of VCD based on such currents. Nevertheless, the empirical basis that is now emerging for understanding VCD intensities based on chirally oriented oscillators and induced currents makes possible the immediate application of this methodology to stereochemical problems of widespread interest. 

There is one chiral center in this set of molecules that has been set to be the R configuration. A search of the literature has been unfruitful in an attempt to determine the correct chirality of this carbon, yet none of the QSAR studies (125-127,130) or experimental papers mentions the chiral orientation. This method of arbitrarily setting the chirality is not correct. The only reason the... 

Optical activity arises from the coupling of given electric-allowed transitions with a chiral orientation (coupled oscillator mechanism or two-electron mechanism) or from the electric or magnetic moments of a transition being pertubed by a chiral static field (asymmetrically perturbed field mechanism or one-electron mechanism) in the given one molecule. A similar mechanism of the optical activity can be expected for molecular assemblies which are composed of chiral and achiral ones. This type of optical activity is called induced optical activity and depends on types of inter-molecular interaction modes. 

If two (or more adjacent) distinct chromophores exist in a chiral arrangement with respect to one another in a dendrimer molecule, the CD spectrum shows two intense Cotton effects of opposite signs which merge with each other [80]. This phenomenon, known as an exciton couplet, arises from the interaction of two electronic transition moments in a mutually chiral orientation, such as oc-... 

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