Y-gauche shielding

In 2,2-difluoronorbornanes, exo methyl groups in the 3, 5, and 6 position cause a shielding of C-7 while an endo methylation in the 3 and 5 position shields C-5 and C-3, respectively, due to y gauche interaction via the y CH bond (Table 4.20(a) [229]). In endt)-6-methyl-2,2-difluoronorbornane, however, the fluorines at the 2 position attenuate the y gauche shielding as expected for C-2. 

While the steric polarization model may be called into question by the considerations presented above, it would be hard to reject the obvious parallels of steric interaction and the y-gauche shielding effect. A case in point is the highly strained molecule [14] (compound 480). The effects... 

If we know how much gauche character (Bg = fractional population of 0 = 120° conformations (See J. D. Honeycutt in this volume who describes the methodology used to calculate the bond conformational populations), then we can estimate the y-gauche shielding (yc-c) produced at the methyl carbons in butane, for example. When the observed shielding ASCHs = SCHslbutane) —5CH3 (propane) =... 

Figure 5.10 Newman projections of a n-alkane chain in the (a) trans ((/> = 0°) and (b) gauche ((j) = 120°) conformations, (c) Derivation of the y-gauche shielding produced by the y-substituents...

Figure Z4 Derivation of the y-gauche shielding produced by the y-substituents C, OH, and Cl (see text)...

PPO carbon chemical shifts to stereochemistry is very small. The total spread of is only 0.12, 0.20, and 0.25 ppm for the methyl, methine, and methylene carbons, respectively. This can be contrasted to atactic PP [11], where the range of chemical shifts due to stereosequences is 2.0, 0.5, and 2.0 ppm for the same carbon types. The reduced sensitivity in PPO reflects the presence of three bonds between chiral centers in contrast to the two bonds in vinyl polymers. The limited chemical shift sensitivity is predicted by the RIS model for PPO [50]. On the basis of y-gauche shielding interactions, a spread of H-T chemical shifts of 0.5 ppm is predicted for each of the three carbon types. 

In addition to eliminating the need for triple resonance observation [52] of C NMR spectra, F NMR spectra of fluoropolymers [54] are much more sensitive to their microstructures, brcause the conformationally sensitive y-gauche shield-... 

Poly(l-butene) exists in three different helical conformations in the solid state [34] (Fig. 9.11). The CP-MAS spectra of the three crystalline polymorphs and the amorphous polymer of poly(l-butene) (Fig. 9.12) have been reported, and the spectra were interpreted by using a y-gauche shielding parameter that is dependent on the dihedral angle. The y-gauche shielding parameter is reduced to half its standard value when the angle is increased to 82 1°. 

Finally, it should be noted that y-gauche effects are not confined to carbon atoms. Analogous shifts have been reported for l9F, 31P (191), and 15N nuclei (206). Distinct shieldings of I70 nuclei caused by y-gauchc-oriented methyl or methylene groups were found, too (207). 

The observation of deshielding 8M-SCSs was a crushing blow to the Grant-Cheney concept (33,88) correlating shielding yg-SCSs with steric compression (cf. Section III-C), since in the 8M-orientation even more severe spatial interference occurs. An interpretation (252) in which Ss,-SCS in split into loss of y-gauche and introduction of 8-interactions, though quite artificial, may be useful. 

Similiarly, an exo-amino group at C-2 in norbornane shields C-7 while endo-2-NH, particularly shields C-6 [229] due to y-gauche type interacton with CH bonds. 

Similar, shielding, y-gauche effects are evidenced at carbons 9 and 14 when a 7ct-hydroxy (compound 414) or a 7cr-hydroperoxy (compound 416) group is substituted into cholesterol (compound 211). The effects of bromine, hydroxyl, and hydroperoxy are —6 to —7 ppm for groups without hydrogen or with hydrogen and certainly of differing physical size. 

The presence of 14 -axial hydroxyl compared to those with no hydroxyl produces a gauche shielding effect of 5-8 ppm on C-7, C-9 and C-12 in rings B and C. A similar Y-interaction might have also been expected on carbons-16 and 17 in the pentacyclic D ring but only C-17 has been found shielded. This has been explained that in a five-membered ring no more than one carbon experiences a Y-effect. The 17 B-side chain forces the 17 -H into an axial position to receive the y -effect. However, if a 17 -hydroxyl is present no change is observed in the shifts of either C-16 or C-17 since C-17 carbon is non-protonated. 

The P-configuration assigned to this tertiary hydroxyl group in teupestalin A (108) was supported by the very deshielded resonance of one of the C-11 methylene protons [H-1 la (pro-S), 8 3.47 dd, J= 1.5 and 3.7 Hz] which is close to the lOP-hydroxyl group, and by comparison of the NMR spectral data of 113 and 114 (Table 13). Since the observed differences in the chemical shifts of the C-8 [A8 = 8(1)- 8(3), -6.2 ppm] and C-11 (A8-7.7) y-carbons were only compatible with a y-gauche arrangement [66], whereas for a 10a-hydroxyl substituents, smaller shielding effects are expected on these carbon atoms, at least on C-11 (y-trans carbon) [67]. 

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