Diaxial conformer

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. 

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. 

Approximately how much sleric strain does the 1,3-diaxial interaction between the two methyl groups introduce into the diaxial conformation of cis- 1,3-dimethvlcyclohexane (See Problem 4.43.)... 

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. 

The marked ANTI stereoselectivity observed with cyclohexyl systems (see above) reflects the ability to achieve, and the very marked preference to eliminate from, the so-called trans-diaxial conformation (34) ... 

Thus of the geometrical isomers of hexachlorocyclohexane, C6H6C16, one is found to undergo elimination of HC1 at a rate slower, by a factor of 7-24 x 103, than any of the others it is found to be the one (35) that cannot assume the above trans-diaxial conformation. 

In cyclic systems, the usual simple requirements of Saytzev or Hofmann rules may be overridden by other special requirements of the system, e.g. the preference for elimination from the truns-diaxial conformation in cyclohexane derivatives (cf. p. 255). Another such limitation is that it is not normally possible to effect an elimination so as to introduce a double bond on a bridgehead carbon atom in a fused ring system (Bredt s rule), e.g. (47) (48) ... 

In contrast to 21, the diol epoxide derivative of the 8,9-dihydrodiol of DMBA was relatively stable. Although only the anti isomer was isolated and identified from epoxidation of the 8,9-dihydrodiol with m-chloroperbenzoic acid (84), it is likely that the syn isomer may also be formed in this reaction. The 8,9-dihydrodiol exists predominantly in the diaxial conformation as a consequence of steric interaction between the 8-hydroxyl and 7-methyl groups (88). 

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 ratio of intensities of ion peaks [M — Br]+/M+ for cis-1,2-dibromocyclo-hexane is 1 to 55, while for its irans-isomer the ratio is 64 to 5. The difference is due to anchimeric assistance of the second bromine atom (Scheme 5.11) in diaxial conformation) [16]. 

Typical experimental data are shown in Table 52. As can be seen, the diaxial conformation is preferred for the cases in which adjacent substituent bonds are hermaphroditic, i.e. simultaneously very good donor and very good acceptor bonds. 

The molecules of trans-2,3- and -2,5-dichloro-l, 4-dioxanes in the crystal are in the chair conformation with both chlorine atoms occupying axial positions (29). In solution, too, there is a preference for this conformation however, the dipole moment for the former molecule in solution is higher than expected for the purely diaxial conformation (30). 

In the trans isomer, one methyl is written down (dotted bond) whilst the other is written up (wedged bond). If we transform this to a chair conformation, as shown in the left-hand structure, the down methyl will be equatorial and the up methyl will also be equatorial. With ring flip, both of these substituents then become axial as in the right-hand conformer. From what we have learned about monosubstituted cyclohexanes, it is now easily predicted that the diequatorial conformer will be very much favoured over the diaxial conformer. 

In the synthesis of cyclohexene oxide from cyclohexene shown, this does implicate the less favourable diaxial conformer in the epoxide-forming step. Cyclohexene oxide contains a c/s-fused ring system, the only arrangement possible, since the three-membered ring is necessarily planar (see Section 3.5.2). 

In trans-2,3-di-F- and rrans-2,3-di-Cl-l,4-dioxane, as in the OR analogs, the halogens adopt the diaxial conformation [79NJC145, 79JOC2274 84SPL307]. The corresponding cis isomers, c/s-rrans-2,3,5,6-tetrachloro-, trans-anti-trans-2,3,5,6-tetTachloTO-, 2,2,3,3-tetrachloro-, and 2,2-dichloro-... 

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