Conformation diequatorial

It is interesting to note that in the most stable conformation (diequatorial) of the trans-isomer, the -—CH3 groups are exactly the same distance apart as they are in either conformation of the c/s-rsomer. Clearly, it is not repulsion between... 

Disubstituted cyclohexanes can exist as cis-trans isomers as well as axiaEequatorial conformers. Two isomers are predicted for 1,4-dimethylcyclohexane (see Fig. 1.9). For the trans isomer the diequatorial conformer is the energetically favorable form. Only one cis isomer is observed, since the two conformers of the cis compound are identical. Interconversion takes place between the conformational (equatoriaEaxial) isomers but not configurational (cis-trans) isomers. 

CHa-ring interactions might also be expected to control the conformational preferences of dimethylcylohexanes. It might even be anticipated that the energy differences between a diequatorial conformer and a diaxial conformer will be twice the equatorial-axial energy difference in methylcyclohexane. 

Obtain energies for diequatorial and diaxial conformers of cis-l,3-dimethylcyclohexane, trans-1,2-dimethyl-cyclohexane and trans-l,4-dimethylcyclohexane. 

Compare energies for both diaxial and diequatorial chair conformers of trans-2-fluoromethyIcyclohexane (X = Me). Which conformer is preferred Examine a space-filling model of each conformer. Which group is largest methyl, fluorine, or hydrogen Which is smallest Does the preferred conformer minimize steric repulsion Explain. 

A The diaxial conformation of c/s-l,3-dimethylcyclohexane is approximately 23 kj/mol (5.4 keal/mol) less stable than the diequatorial conformation. Draw the two possible chair conformations, and suggest a reason for the large energy difference. 

A study195 based on the NMR lanthanide-induced shifts (LIS) for a series of cis- and trans-3-substituted, and 3,3-disubstituted thietane oxides concluded that all cis-substituted oxides (5c R = CH3, t-Bu and aryl) exist exclusively in the diequatorial conformation. The trans-3-substituted isomers (185) prefer the equatorial oxygen conformation (R = CH3,86% t-Bu, 65-75% aryl, 75%), which means an axial preference for the substituents (e.g. 185d), at least when they are bound to a shift reagent (equation 75). 

In the pyranoses in the chair conformations, the vicinal hydroxyl groups can exist only in the anticlinal or gauche orientations. The gauche arrangement of a-glycols is normally encountered as diequatorial or axial-... 

Note that the 1,2-diequatorial substituted examples in Fig 7.10(c and d) are individual stereoisomers. The corresponding cis-species (Fig. 7.11b) is not another conformation, but another stereo isomer. The experimentally by calorimetry determined energy difference between the isomers is 6.5 kJ mol" . 

It is believed that equatorial substituents such as chlorine or bromine would increase the guest diameter beyond the allowed values (assuming that the guest molecules stack roughly parallel to the canal68)). Support for this comes from the study of fluorocyclohexane where the population of the axial conformer is not enhanced to any major extent70. Nitro-71) and cyano-cyclohexane, trans-l,2-dichloro-, trans-1,2-dibromo-, tram-1,4-dichloro-, trans-1,4-dibromo-, and trans-l-bromo-4-chloro-cyclohexane all pack most efficiently in the thiourea canals as the axial or diaxial conformer 68,72. Tram-2,3-dichloro-1,4-dioxane behaves similarly73. In contrast isocyanato-, tram-1,4-diiodo-, trans-1 -bromo-4-iodo-, and tram-1 -chloro-4-iodo-cyclohexane are present as mixtures of the axial/equatorial or diaxial/diequatorial conformations as appropriate 68,72). The reason for this anomalous behaviour of the iodosubstituted cyclohexanes is not clear. 

It should be noted that equatorial or diequatorial conformers of substituted cyclohexanes could be accommodated in the thiourea canals by the stacking of guests at an oblique angle or parallel to the canal axis. X-ray structural data is so far unavailable... 

Size and conformational effects such as those described above have been exploited in a wide range of separation procedures. In a patented method for the separation of cis- and tranj-l-methyl-4-uohexylcyclohexane carboxylic acids only the tram isomer forms a thiourea inclusion compound 831. In solution the cis isomer always has an axial, equatorial arrangement in the chair conformation, while the trans isomer can exist as the diequatorial or diaxial chair conformers. It is not reported which of these is favoured or how the guest packs in the canals. 

The diequatorial conformation is the more stable conformer and it represents at least 99% of the molecules at equilibrium. 

Epoxidation of 10 with m-chloroperbenzoic acid yielded the chrysene anti-1,2-diol-3,4-epoxide, whereas similar reaction of 11 gave a mixture of the corresponding anti and syn diol epoxides in a 5 3 ratio (57,59). These findings are in accord with previous observations that dihydrodiols free to adopt the diequatorial conformation undergo anti stereospecific epoxidation, whereas bay region diaxial dihydrodiols yield mixtures of anti and syn diastereomers. The syn-... 

Figure 12. The structures of two trans diols of benz[a]anthracene showing the diequatorial conformation of the unhindered 10,11-diol and the diaxial conformation of the hindered 1,2-diol. These trends persist in solution where the 10,11-diol exists as an equilibrium of 30% axial and 70% equatorial conformers (that is, the ring is flexible) on the other hand the 1,2-diol is 100% diaxial even in solution. If the 1-hydroxyl group were equatorial it would "bump" into the hydrogen atom on Cl2.

The conformations of the 1- and 3-nitro-BaP metabolites were determined through analysis of their NMR spectra (145-146). Both 1- and 3-nitro-BaP-trans-7,8-dihydrodiols existed predominantly in quasi-diequatorial conformations, which corresponds to the preferred conformation of the proximate carcinogen BaP-trans-7,8-dihy-drodiol (149). This suggests that these dihydrodiol metabolites may be converted into electrophilic diol epoxides and in support of this contention, the stereochemistries of 1- and 3-nitro-BaP-... 

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