Intrinsic preference

Takeuchi and coworkers prepared a series of methyl and phenyl substituted germacy-clohexanes to evaluate conformational preferences of these substituents (Scheme 17)35,36. Based on analysis of 13C NMR spectra and molecular mechanics calculations, they concluded that a C-methyl prefers the equatorial orientation by ca 1.4 kcal mol 1 but the Ge-methyl substituent actually shows a slight intrinsic preference for the axial orientation (Figure 9). A similar conclusion was reached for the Ge-phenyl substituent. 

The remarkable success of LiCoOz as an electrode material is likely related to the strong intrinsic preference of Co for octahedral sites over the +3 to +4 valences as indicated by the high calculated value of AF oct-tet in Tables 2 and 3. This strong preference of Co for octahedral sites clearly suggests that Co will not easily migrate through an close-packed oxygen framework at these oxidation states. 

This result can be rationalized, if, as mentioned earlier, the gain in covalency is taken to be roughly 30 kcal/mol. By approximating the intrinsic preference of both chlorocarbene subunits in 22 for a local configuration, as twice the A/Tst of phenyl chlorocarbene (about —7 kcal/mol. Table 1), the energetic price to be paid for the promotion of each divalent carbon center to a an/aTi configuration is about 14 kcal/mol, significandy less than the benefit due to the extra -ir bond that is formed in the (a ir/a-ir)- A state. 

In the case of carbenonitrenes, the nitrene subunit has an intrinsic preference for a triplet configuration, and, thus, for a A" ground state, a carbene with a very strong preference for a closed-shell singlet ground state is required. Such a case has not been realized yet. On the other hand, if both subunits are carbenes, then. 

Figure 6.13 Radical polymerization of a growing polymer chain in the presence of two distinct monomers (i.e., copolymerization conditions) can at every step incorporate one monomer or the other. How might one quantitatively go about estimating the intrinsic preference for one monomer over the other What other molecular properties expected to correlate with this discrimination might be subject to computation ...

This model calculation illustrates a recurring theme in this book the notion of an intrinsically preferred curvature implies profound consequences for structure. The analysis of mesostructure of these microemulsions is helped by the fact that DDAB resides exclusively at the interface. In many microemulsions this is not the case, so that more detailed calculations are... 

The key component of the cell wall lipopolysaccharide of Gram-negative bacteria, KDO (3-deoxy-D-manno-2-octulosonic acid), was synthesized by S.D. Burke and co-workers. One of the key transformations in the synthetic sequence was a doubie SAD of a 6-vinyldihydropyran-2-carboxylate template. This 1,4-diene was cleanly converted to a mixture of two C7 epimeric tetraols in a 20 1 ratio. The endocyclic olefin had an intrinsic preference for dihydroxyiation from the 3-face and not from the desired a-face. This stereofacial bias was impossible to override with any ligand normally used in the SAD, so later in the synthesis these two stereocenters had to be inverted in order to give the required stereochemistry at C4 and C5. 

Figure 25. Crystal structure of the /3-tripeptide t Boc/ 3-HVal-/33 IIAla fp IILeuOMe (3) reported by Seebach223 showing an intrinsic preference for a turned conformation. Two of the /3-peptide residues are rendered as thick cylinders. The Newman protections about the Ca—Cp bonds for these residues are shown, indicating the apparent preference of the /33 substituted residue bearing the bulky substituents to adopt a sc conformation while the residue with the smaller methyl substituents takes on an ap conformation.

In all cases where the structures have been unambiguously established, tetrahalocyclopropenes add exclusively exo to 1,3-dienes and furans. Earlier reports of endo addition need to be considered with care. Addition of l-bromo-2-chlorocyclopropenes to furans occurs preferentially in an exo fashion, but endo addition predominates with open-chain dienes. The parent cyclopropene has an intrinsic preference for endo addition to cyclopentadiene and open-chain dienes, but addition to diphenylisobenzofuran is exclusively exo. The presence of a substituent at C3 may overrule the endo preference even with acyclic dienes, and 1- and 2-halogens on the cyclopropene have themselves a strong endo preference. 

Such simple calculations on pairs of chain segments are mainly useful as bridges between the single-chain and the many-chain assembly levels of calculation. They provide information on the preferred interactions of two chain segments in the absence of other chain segments. Comparison of their results with the results for isolated chains and multi-chain assemblies can facilitate the identification of which effects are primarily caused by (i) intrachain factors, (ii) the intrinsically preferred patterns of interaction between pairs of chains, and (iii) constraints and/or superpositions of effects induced by large-scale packing. [34]... 

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