Ferf-Butylcyclohexane

Kessler and co-workers (136) reported that the barrier to rotation in cis-di-ferf-butylcyclohexane (92) was 16.3 kcal/mol at 298 K. This relatively low... 

To take this general principle to its extreme, we noted above that ferf-butyl groups are sufficiently large that they never occupy an axial position. It is possible to make di-ferf-butylcyclohexanes where conformational mobility would predict that one of these groups would have to be axial, namely cis-1,2-, trans-1,3- or d -1,4-derivatives. As a result, in these cases, we do not see an axial ferf-butyl, but... 

The trans and cis isomers of l-bromo-4-ferf-butylcyclohexane can be taken as models to estimate the chemical shift of the proton of the CHBr group when it is axial and equatorial, respectively, in the two chair conformations of bromocyclohexane. An axial proton is more shielded (8 3.81 ppm for t ra nx -1 -bromo -4 -1 e r/ - b u t y Icyclolicxanc) than an equatorial one (8 4.62 ppm for cis-1 -bromo-4- < rt-buty Icy clohexane). 

G. Gill, D. M. Pawar and E. A. Noe, Conformational study of c/s-l,4-di-ferf-butylcyclohexane by dynamic NMR spectroscopy and computational methods. Observation of chair and twist-boat conformations. J. Org. Chem., 2005, 26, 10726-10731. 

PROBLEM 2.15 For ferf-butylcyclohexane, only one conformation, with the tert-butyl group equatorial, is detected experimentally. Explain why this conformational preference is greater than that for methylcyclohexane (see Figure 2.7). 

Which will be more stable, cis- or trans-1,4-di- ferf-butylcyclohexane Explain your answer by drawing conformational structures for each compound. 

It is known that 1,2-di-ferf-butylcyclohexane exists in a boat conformation to a large extent. Draw a chair conformation for this molecule and suggest a reason why this should be so. 

TraKS-l,4-di-ferf-butylcyclohexane exists in a normal chair conformation. Cis-l,4-di-tert-butylcyclohexane, however, adopts a twist-boat conformation. Draw both isomers and explain why the cis isomer is more stable in a twist-boat conformation. 

Cyclohexane rings that bear fm-butyl substituents are examples of conformationally biased molecules. A fert-butyl group has such a pronounced preference for the equatorial orientation that it will strongly bias the equilibrium to favor such conformations. This does not mean that ring inversion does not occur, however. Ring inversion does occur, but at any instant only a tiny fraction of the molecules exist in conformations having axial ieri-butyl groups. It is not strictly correct to say that ferf-butylcyclohexane and its derivatives are... 

Which would you expect to undergo E2 elimination faster, frans-l-bromo-4-ferf-butylcyclohexane or cis-l-bromo-4-ferf-butylcyclohexane Draw each molecule in its more stable chair conformation, and explain your answer. 

In c -l-bromo-4-ferf-butylcyclohexane, the proton on carbon-4 is found to give resonance at 4.33 ppm. In the tram isomer, the resonance of the C4 hydrogen is at 3.63 ppm. Explain why these compounds should have different chemical shift values for the C4 hydrogen. Can you explain the fact that this difference is not seen in the 4-bromomethylcyclohexanes except at very low temperatnre ... 

An ethereal 50%-soln. of 4-ferf-butylcyclohexanone added at 5-10° to a suspension of triphenylmethylpotassium in ether until the color disappears, transferred under Ng to a flask containing Dry Ice, and allowed to stand for 20 min. 5-ferf-butylcyclohexan-2-onecarboxylic acid. Y 89.5%. J. Sicher, F. Sipo , and M. Tichy, Coll. 26, 847 (1961) method s. L. Levin, E. Baumgarten, and G. R. Hauser, Am. Soc. 66, 1230 (1944). 

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