Hyperconjugation, reverse

However, the Baker-Nathan effect has now been shown not to be caused by hyperconjugation, but by differential solvation. This was demonstrated by the finding that in certain instances where the Baker-Nathan effect was found to apply in solution, the order was completely reversed in the gas phase. ° Since the molecular structures are unchanged in going from the gas phase into solution, it is evident that the Baker-Nathan order in these cases is not caused by a structural feature (hyperconjugation) but by the solvent. That is, each alkyl group is solvated to a different extent. 

The anomeric configuration is set in the reductive lithiation step, which proceeds via a radical intermediate. Hyperconjugative stabilization favors axial disposition of the intermediate radical, which after another single electron reduction leads to a configurationally stable a-alkoxylithium intermediate. Protonation thus provides the j9-anomer. The authors were unable to determine the stereoselectivity of the alkylation step, due to difficulty with isolation. However, deuterium labeling studies pointed to the intervention of an equatorially disposed a-alkoxylithium 7 (thermodynamically favored due to the reverse anomeric effect) which undergoes alkylation with retention of configuration (Eq. 2). 

Although the results indicated some stabilization for the type B structures, they clearly indicate that hyperconjugation is not sufficient to alter the natural preference of cyclopropane radical cations for the type A structure. On the other hand, both conjugation and homoconjugation have been shown to reverse the stabilities... 

Even less obvious in this particular case is evidence that the less-substituted radical is actually more stabilised, presumably by negative hyperconjugation with the f3 C—F bonds, than the more-substituted radical is by the attached F atoms.997 Furthermore, in the other striking reversal of expectations, the tert-butoxy radical, which is certainly more electrophilic than methyl, adds to 1,1-difluoroethylene 7.45 at the substituted carbon with a selectivity of 80 20,998 whereas the methyl radical is normal in this case. Thus the coefficients in the LUMO are hardly likely to be the explanation, and one suggestion in this particular case is that there is a growing anomeric effect between the oxygen atom and the two fluorine substituents in the transition structure for the formation of the major intermediate 7.46.988... 

Several objections have been raised for this interpretation17 Unlike in the 260 m/x band, no definite trend has been found in the position of the 200 m/x bands in alkyl benzenes or p-alkyl substituted benzenes. In many cases, the observed wavelengths are actually found to be in the reverse order (i.e., in the inductive order) to that predicted by C-—H hyperconjugation. At this point it may be stated that the evidence for hyperconjugation from the absorption spectra of alkyl substituted benzene derivatives is not definitive. 

Wheland and Farr, 150), but their rates of reaction with hydroxyl ion are in the reverse order, so that the dependence of velocity upon acid strength is the exact opposite of that usually found. Moreover, the effect of alkyl substitution in increasing the acidity is the opposite of the usual inductive effect (e.g., in the carboxylic acids). It seems likely that the sequence of acid strengths results from the stabilization of the undissociated molecule by a hyperconjugative effect which depends on the number of hydrogen atoms on the carbon adjacent to the nitro group, e.g., in nitromethane we can write three structures of the type... 

Due to the much lower rate of allylic H-abstraction by the H-atom as compared to the OH radical, this process is generally of small importance (cf. Table 7). In so far as the base is methylated at carbon, the pattern of site preference exhibited by the H atom is influenced by the position of the methyl group in ways different from the OH radical (Table 1). It may be the effect of hyperconjugation that appears to enhance the relative attraction for the hydrogen atom to become attached, cf. the pronounced preference for addition at C(5) in 6-methyluracil, and the reversal of the preference in thymine, as compared to uracil. 

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