Diaxial-diequatorial

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. 

One interesting example is (+)-trans -2-ehloro-5-methyleyelohexanone (32). The sign of the Cotton effeet in its ORD spectrum is reversed on transferring it from water to -heptane (molar optical rotation at X — 330 nm and 25 °C [< ] = +382 and — 1486, respectively) [189], This can be ascribed to a diaxial/diequatorial conformational equilibrium (32a) (32b), which lies more to the left-hand side in n-heptane than in water. 

In the case of diaxial/diequatorial equilibria of trans -l,2-dihalocyclohexanes (with and without a 4-ter+butyl group), AG° [aa ee) also shows a pronounced dependence on the medium, varying from about 4 kJ/mol in apolar solvents to about —2 kJ/mol in polar solvents [90-94, 192-194]. This is mainly due to the very different dipole moments of the two conformers. In general, the more dipolar diequatorial isomer is favoured in polar solvents. 

To establish the configuration at C-5, the free telrol (m.p. 193°) was examined in deuterium oxide at 100 me. (see Fig. 5-A). The four methylene protons produced the pair of doublets at 2.22 p.p.m. When all four 0-C-H protons were irradiated, the methylene pattern collapsed to a singlet (see Fig. 5-B). Spectroscopic equivalence of the two protons within each methylene results from the trans configuration of the neighboring gi oups at C-1 and C-5 (and C-3 and C-5), and also from time-averaging of two diaxial-diequatorial conformations (see the formulas in Fig. 5). The all-cfs configuration (67) was thus excluded. 

The observed ratio of diaxial/diequatorial isomers is very little influenced by the presence of ammonium chloride in the reaction mixture in contrast to the marked effect of the A -substituent. The diaxial isomers predominate in most reactions, although the amounts of diequatorial isomers are sometimes significant. In the... 

Without additional reagents Diaxial-diequatorial rearrangement with positional interchange of arylthio and sulfonyl groups... 

Dictatorial s. Diaxial-diequatorial rearrangement Dietbylalnminam chloride as reagent 27,142 28,658... 

Menger and Boyer [34] have recently presented optical rotary dispersion data which indicate appreciable penetration of water into the micellar core. A large change in the sign of the Cotton effect is induced on transferrance of (-h )trans-2-chloro-5-methylcyclohexanone from heptane to water. This is ascribed to a diaxial-diequatorial equilibrium which responds to the nature of its environment, lying further to the right in water than in heptane. 

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 )],... 

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