CONFORMATIONAL ANALYSIS .8. VALIDITY OF NUCLEAR MAGNETIC RESONANCE METHOD OF ESTABLISHING CONFORMATIONAL EQUILIBRIA

The basis for the nmr method of determining conformational equilibria in substituted cyclohexanes is examined. Comparison of conformationally analogous 3- and 4-t-butyl-substituted cyclohexyl derivatives indicates that the signals for the C1 proton therein are not the same; conformational equilibria derived from the signal position in the 4-t-butyl compounds and the corresponding monosubstituted compound (Figure 1) are consistent with similar equilibria obtained by other methods, but those derived from 3-t-butyl compounds are out of line. However, in proton resonance of 2-t-butyl- and 5-t-butyl-substituted 1,3-dioxanes, the t-butyl group does affect the signals of certain ring and methyl substituent hydrogens in positions corresponding to the 4 position in cyclohexane. In the case of 19F resonance it is found that in both 3-t-butyl- and 4-t-butyl-1,1-difluorocyclohexanes the position of the fluorine signals is affected by the alkyl substituent. Comparison of cis- and trans-4-t-butylcyclohexyl chlorides with cyclohexyl chloride at –81 to –91° as a means of ascertaining the equatorial and axial CHC signals suggests that the 4-t-butyl compounds are better suited for this purpose inasmuch as the signal position changes with temperature and an extrapolation is required to utilize the measured low-tem-perature shifts. However, 4-t-butyl compounds are apparently not good models for 19F shifts or for proton snifts in heterocyclic systems. © 1968, American Chemical Society. All rights reserved.