Substituent effect 8-gauche

Some other mechanisms will be discussed in later sections The Grant-Cheney formulation (88) of steric compression effects will appear in the context of y-gauche substituent effects (Section III-C), and hyperconjugative orbital interactions will be cited in interpretations of y-anti substituent (Section III-D) and intramolecular interaction effects (Section IV). 

Many authors have tried to identify other contributions to y-gauche substituent effects instead of or in addition to sterically induced bond polarizations. For electronegative substitutents, for example F, OR, NR2, and Cl, the most obvious interpretation is to assume a through-space electric-field transmission of yg-SCS... 

The 13C NMR spectra of monohydroxylated steroids demonstrate that the y-gauche substituent effects (the hydroxy group and the y-carbon are gauche to each other) are different for secondary (average value — 6.5 ppm) and tertiary (average value — 7.8 ppm) y-carbons. For the y-trans shifts much smaller values in the range of + 1.3 to — 3.0 ppm are found [575],... 

It was demonstrated in the preceding chapter that the chemical shifts observed for dissolved polymers depend sensitively on their local microstructures, and that the connection [1] between and microstructure is the local conformation about the observed carbon nucleus. This connection is provided by the conformationally sensitive y-gauche substituent effect [18] on chemical shifts. 

There are two different relative orientations of X and a y-carbon atom in staggered n-propyl derivatives. Accordingly, we have to make a distinction between y-gauche (yg-SCS) and y-antiperiplanar (ya-SCS) substituent effects. 

The substituent effect data indicates that the charge polarity in the vinyl ether transition states is less than in the corresponding ester elimination transition states. The activation energies (corrected) naturally parallel the rate coefficient variations and show a systematic decrease of 2.3 to 2.4 kcal.mole for successive methyl substitutions for hydrogen at the a-carbon. Corrected for gauche destabilizations of the ether ground states, this indicates a transition state charge stabilization by methyl of about 1.6 kcal per CH3. Ester stabilizations have previously been estimated to be about 3.3 kcal per CH3. 

Substituent effects in organoselenium compounds (e.g., in Ph-Se-R with various R) are qualitatively similar as compared to homologous carbon analogues (Ph-CH2-R) but they are 3-5 times larger on the ppm-scale. The diagnostically valuable /-gauche effect in olefins has attracted some attention. 

PA s are larger than in (17). TheNa,Na-dimethyl derivative is the most basic compound. The high basicity of the Nc, atom in gauche configuration is also due to a combination of the substituent effect and the internal interaction. 

The gauche-trans equilibrium in the substituted diazadiphosphetidines (35) has been investigated by means of a detailed and P n.m.r. study. Among the parameters evaluated was J(P—P), which, particularly for the t-butyl derivative, showed a large isomer shift. Substituent effects in the mixed chloride fluorides (36) have been similarly investigated. ... 

Analogous effects can be observed in confonnationally well-defined saturated compounds. The substituent effect on the y-carbon of cyclohexane ring depends on the conformation. Because an axial substituent is gauche to the y-carbon of the ring, it causes a significant upfield shift of 4-7 ppm. On the other hand, an equatorial substituent is trans to the y-carbon and causes amuch smaller upfield shift of 0-3 ppm (Figure 12.32). 

The preference for eclipsed conformations around the sp -sp bond of alkenes is general, but the reasons for the preference are not as yet understood. The rotational barriers are relatively low, that for propene being about 2 kcal/mol. Simple substituent effects appear straightforward. Methyl substitution at C(2), as in 2-methyl-1-butene, introduces a methyl-methyl gauche interaction in the conformation analogous to B, with the result that in 2-methyl-l-butene, the two eclipsed conformations are of approximately equal energy. ... 

Reasonable predictions for the shift values for trans-decalin can be based on the simple substituent effects outlined in Chapter 1 and the stereochemical considerations mentioned above. Thus, the bridgehead carbons would be predicted to absorb at 45.3 (27.3, + 24.0 for three additional P substituents, and -6.0 for a 3 carbon with four p substituents, 43.7 obs.). Similarly, the prediction for C-2 would be 35.3 (27.3 + 8.0), 34.3 observed, and for C-3 would be 27.3 (26.9 obs.) since there are no additional a or P substituents relative to cyclohexane and the y carbons are anti, not gauche. There is a small upfield shift, typically 0.5 for an anti, y carbon, but in most cases this effect is too small to be of significant use in st eochemical predictions. The situation with cis-decalin is more complicated because of the superposition of two equal energy conformations. Nonetheless, the shifts observed are understandable once this factor is combined with stereochemical considerations to be detailed in Chapter 3. 

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