Diequatorial form

The case is interesting in 1,2 dibromocyclohexane. The moment for the diaxial isomer is assumed to be zero, while the calculated and observed values for the diequatorial form are respectively 3.09 D and 3.12 D. The trans diaxial form is more stable than the trans diequatorial, although the latter is favoured sterically. The reason for the lesser stability of the trans diequatorial form is that it is destabilized by dipole repulsion. 

Because of H bonding in the diequatorial form, AG includes the energy required to break the H bond. Thus the actual AG is greater than that calculated. 

The steric interference between substituents in axial positions is particularly severe when there are large groups on two carbon atoms that bear a 1,3-diaxial relationship (cis on Cl and C3, or Cl and C5), as in the two chair conformations of cis-l,3-dimethyl-cyclohexane shown here. The less stable conformation has both methyl groups in axial positions. The more stable conformation has both methyl groups in equatorial positions. Note the strongly unfavorable 1,3-diaxial interaction between the two methyl groups in the diaxial conformation. The molecule can relieve this 1,3-diaxial interference by flipping to the diequatorial conformation. Use your models to compare the diaxial and diequatorial forms of cis-1,3-dimelhylcyclohexane. 

Wiest and Houk" " examined the Claisen rearrangement that takes 1 into 2 in the gas phase at B3LYP/6-31G. The geometries of the diequatorial and diaxial form of 1, the TS 3, and the product 2 are shown in Figure 9.4. The diequatorial form is more stable than the diaxial form, by 12 kcal mol at B3LYP/6-31G, ... 

Alternatively, take the diequatorial form 18 and, as before, insert an imaginary C(l)---C(4) bond. One can now identify a dihedral angle H1-C(l) - C(4)-H4 of 180°, which again indicates a trans configuration. 

Even after trapping an intermediate bay-region PAHTC [156] there is little prospect to experimentally determine the conformation of reactive TC intermediates. The theoretical predictions can however, be checked indirectly by calculating the relative stabilities of the diaxial and diequatorial forms of PAHDE s, for which experimental estimates are available. NMR proton spin coupling experiments indicate equilibria between the two forms, with the diequatorial favoured for a n -DE s and... 

The alkyl group effect on the conformational equilibrium of trans-2-bromoalkoxycyclohexanes has been analyzed by Silla et alP by the use of vicinal proton-proton couplings measured in a variety of solvents. It has been found that the diaxial conformer is preponderant, or at least similarly populated to the diequatorial form, for the tcrt-butoxy derivative only, whereas the diequatorial conformer is prevalent for the remaining alkoxy derivatives, the OMe compound in CCI4 solution being an exception. 

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. 

In the diaxial isomer, the two methyl groups occupy anti positions, and therefore their interaction with one another is not destabilizing. However, for each axial methyl group we still have the two gauche interactions with the ring methylene groups (see p. 199), and this will cost the molecule 2 X 1.74 kcal/mol = 3.48 kcal/mol. We can predict that the diequatorial isomer should be (3.48 — 0.6) kcal/mol = 2.88 kcal/mol more stable than the diaxial isomer. In turn, this calculation predicts that at 25 °C there will be more than 99% of the diequatorial form present. 

PROBLEM 5.17 Show that the ring flip of the diequatorial form to the diaxial form will not racemize optically active tra i-l,2-dimethylcyclohexane. What is the relation between the equatorial—equatorial and axial—axial forms of trans-1,2- dim ethylcyclohexane ... 

In principle, both the diaxial and diequatorial forms should be separable into a pair of enantiomers, resolvable (p. 169)—they are both chiral molecules. In practice, we cannot isolate diaxial-1,2-dimethylcyclohexane it simply flips to the much more stable diequatorial form. The diequatorial form can be isolated and resolved. 

That the diaxial form of trani-l,4-dimethylcyclohexane is a trans isomer is easy to see the two methyl groups are clearly on opposite sides of the ring. The trans relationship of the methyl groups in the diequatorial form is not as obvious, however. 

Figure 4.22 The two chair conformations of trans-1,4-dimethylcyclohexane trans-diequatorial and trans-diaxial. The trans-diequatorial form is more stable by 15.2 kJ moF ...

Experimentally, the diequatorial form is preferred over the diaxial form by 3.4 kcal mol exactly twice the AG° value for monomethylcyclohexane. Indeed, this additive behavior of the data given in Table 4-3 applies to many other substituted cyclohexanes. For example, the AG° (diaxial diequatorial) for trflny-l-fluoro-4-methylcyclohexane is — 1.95 kcal mol [-(1.70 kcal moF for CH3 plus 0.25 kcal mol for F)]. Conversely, in d5 -l-fluoro-4-methylcyclohexane, the two groups compete for the equatorial positions and the corresponding AG° = 1.45 kcal moF [-(1.70 kcal moF minus 0.25 kcal moF )],... 

Therefore, the diaxial conformation of cw-13-dimethylcyclohexane is less stable than the diequatorial form by more than 3.4 kcal mol . ... 

For the cis-2,6-dihalogenocyclohexanones it is noted that the chlorofluoro-compound (29) exists completely in the diequatorial form, whereas (30) exists to the extent ofl0% in the diaxial conformation. These results follow the trend of earlier results from the same group which ascribe a completely diequatorial conformation to (31), a 5% diaxial conformer content to (32) and a 10% diaxial conformer content to (33). The intrinsic role of the carbonyl in determining the particular conformational preferences is seen in a comparison of (26), (exclusively axial chlorine at 25 C) and (34) which exists to the extent of 76 % fluorine axial at 25 °C. Additionally for the tran5-2,6-dihalogenocyclohexanones, one of the derived cyanohydrins (35) and (36) gives a 55 45 conformer ratio in favour of fluorine axial, whereas in (37) fluorine is axial to the extent of 80%. 

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