1.3- Dimethylcyclohexane, conformational analysis

Dimethylcyclobutane, isomerization, 30 31 Dimethylcyclohexane conformational analysis of, 18 14 experimental equilibrium, 18 17 epimerization of, 25 136 vibrational spectra, 42 239 Dimethylcyclohexylbenzene, 42 432 Dimethylcyclopentanes aromatization, 30 54 isomerization, 30 34... 

One classic example is an experiment reported by Doering and Roth in 196217 (Scheme 1.XIII). Upon heating, racemic 3,4-dimethylhexa-l,5-diene (13) rearranged to a mixture of (2 ,6 )-octa-2,6-diene (90%), (2Z,6Z)-octa-2,6-diene (9%), and a trace amount of (2 , 6Z)-isomer. The experimental results are explained in terms of a six-membered transition state17 (Scheme 1.XIV). Chairlike transition state A is favored over transition state B based on the conformational analysis of 1,2-dimethylcyclohexane, in which the methyl substituents prefer to be in an equatorial position. The observation that 14 Z was formed in only trace amounts indicates that boatlike transition state C is of significantly higher energy than transition state A or B. 

Let us apply the methods of conformational analysis to the stereochemistry of cyclohexane derivatives and, since we are already somewhat familiar with interactions of the methyl group let us use the dimethylcyclohexanes as our examples. 

Conformational analysis of cyclohexane derivatives containing several different substituents follows along the same lines as that of the dimethylcyclohexanes. We need to keep in mind that, of two groups, the larger one will tend to call the tune. Because of its very large 1,3-diaxial interactions (Problem 9.3, p. 301), the bulky /er/-butyl group is particularly prone to occupy an equatorial position. If—as is usually the case—other substituents are considerably smaller than tert-butyl, the molecule is virtually locked in a single conformation the one with an... 

The same kind of conformational analysis just carried out for cis- and fra/i.s-1,2-dimethylcyclohexane can be done for any substituted cyclohexane, such as c7.s-l-ftT(-butyl-4-chlorocyclohexane (see Worked Fixample 4.3). As you might imagine, though, the situation becomes more complex as the number of... 

Use the principles of conformational analysis to predict the difference in heats of combustion of cis- and fr ns-l dimethylcyclohexane. How does your calculated value compare with the data in Table 3.3 ... 

The trans form of 1,2-dimethylcyclopropane presents a different picture. trans- -Dimethylcyclohexane can either have both methyl groups axial or both equatorial (Fig. 5.40).The ring flip converts the dismal form into the diequatorial form. Let s apply conformational analysis to predict which of these two diastereomers is more stable. 

Although analysis of the consequences of ring flip in a monosubstituted cyclohexane is pretty straightforward, the presence of two or more substituents requires careful consideration to decide which conformer, if any, is the more favoured. Let us illustrate the approach using 1,4-dimethylcyclohexane. Now, two configurational isomers of this structure can exist, namely trans and... 

Conformational mobihty, such as we get in cyclohexane rings, makes the analysis more difficult, and manipulating molecular models provides the clearest vision of the relationships. Let us look at 1,2-dimethylcyclohexane as an example. Again, we have met the cis and trans isomers when we looked at conformational aspects (see Section 3.3.2). Here, we need to consider both configuration and conformation. 

The stereochemistry of cycloalkanes is discussed extensively in the chapter by Anderson, and it is important for an understanding of the analyses offered for these compounds to distinguish between isomers and conformers. Because of the low energetic barrier for equilibration between axial and equatorial conformers, only low-temperature HNMR or CNMR as well as IR will freeze the equilibration and allow the spectral analysis of the conformers. However, disubstituted cycloalkanes will exhibit, in addition to structural isomerism, also cis, trans isomerism. Both cis and trans isomers of 1,4-dimethylcyclohexane undergo, at room temperature, a conformer equilibration. In the case of the trans isomer the conformer with two equatorial CH3 groups is more stable, while for the cis isomer both conformers are equally stable and cannot be separated. 

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