Conformational analysis methylcyclohexane

Since carbocations are structurally similar to ketones, they are discussed here. Schleyer s force field incorporates carbocation parameters, and Harris is exploring their application in conformational analysis (188). The calculated angles in a series of rigid polycyclic carbocations correlated well with ketone infrared frequencies (188a). The calculated relative stabilities among various conformers of tertiary cations of methylcyclohexane, methylcycloheptane, and methyl-cyclooctane do not contradict the limited MNR observations of these species at low temperature (188b). 

Conformational relationships for the three cis-trans pairs of dimethyl-cyclohexanes were examined by Beckett et al. (34). Here the situation becomes more involved than for methylcyclohexane because some of the isomers are dl pairs, each of which can exist as a pair of chair conformations. However, the conformational analysis of this system merits attention because it will provide a nearly complete basis for the conformational analysis of the deoalins, methyldecalins, and dimethyl-decalins in the following sections. 

The application of /hc couplings in conformational analysis of six-membered heterocycles has been shortly overviewed by Juaristi et al. and by Alabugin et a/. The couplings have been also shortly addressed by Ribeiro and Rittner in their work on the role of hyperconjugation in the conformational analysis of methylcyclohexane and methylheterocyclohexanes. 

Aspects of cyclohexane conformational analysis. A. Interconverting chair forms of cyclohexane, with axial and equatorial locations labeled. In the left structure letters x are axial while letters y are equatorial. Note that the chair flip moves axial substituents to the equatorial position and vice versa. B. Newman projections down the C1-C2/ C5-C4 bonds of methylcyclohexane. In the axial form, there is a gauche butane interaction between the methyl and C3. C. Views of cyclohexane, equatorial methylcyclohexane, and axial methylcyclohexane. Note that chair cyclohexane is a relatively disk-shaped molecule, and an equatorial methyl does little to disrupt this shape. In contrast, an axial substituent puts a "kink" into the structure. Also evident is the steric interaction between one methyl hydrogen and the two axial hydrogens on C3 and C5. 

We can now apply our knowledge of conformational analysis to substituted cyclohexanes. Let us look at the simplest alkylcyclohexane, methylcyclohexane. 

Figure 6.15 NBO analysis of hyperconjugative interactions of the main hyperconjugative interactions that influence the conformational equilibrium of methylcyclohexane. ...

A map of the photoisomerization potential energy surface for tetraphenylethylene in alkane solvents was prepared using a fluorescence and picosecond optical calorimetry (Figure 3.4) [21]. Line shapes of the vertical and relaxed exdted-state emissions at 294 K in methylcyclohexane were obtained from the steady-state emission spec-tmm, the wavelength dependence of the time-resolved fluorescence decays, the temperature dependences of the vertical and relaxed state emission quantum yields, and of the time-resolved fluorescence decays. Analysis of these data in conjunction with values of the twisted exdted-state energy provided values for the energies of the vertical, conformationally relaxed, and twisted exdted states on the photoisomerization surface, as well as the barriers to their interconversion. The energy difference between the last two states is found to be 1.76 0.15 kcal/mol in methylcyclohexane. 

The A value of the benzyl group has been determined from an examination of the low temperature n.m.r. spectrum of cis-l-benzyl-4-methylcyclohexane. At — 97.6°C two doublets were observed for the benzyl methylene protons, with the more abundant isomer (57.4 + 2.7 %) corresponding to the conformation with an equatorial benzyl group, being more stable by 0.11 kcal mol . This led to an A value for the benzyl group of 1.81 kcal mol at — 97.6°C however, the room-temperature A value cannot be derived from this. The A value obtained is similar to that for other —CHjX groups, including ethyl. It was concluded from spectral analysis that cis-l-benzyl-4-methylcyclohexane is distorted with respect to benzylcyclohexane and that the conformation around the benzyl-cyclohexane bond is markedly temperature dependent. 

---