Conformations equatorial

Recall from Section 3 10 that the equatorial conformation of methylcyclohexane is 7 kJ/mol (1 7 kcal/mol) lower in energy than the confer mation with an axial methyl group... 

Because AG° will be negative when the equatorial conformation is more stable than the axial, the value of —AG° is positive for groups which favor the equatorial position. The larger the value of —AG°, the greater is the preference for the equatorial position. 

Several structural factors have been considered as possible causes of the anomeric effect. In localized valence bond terminology, it can be recognized that there will be a dipole-dipole repulsion between the polar bonds at the anomeric carbon in the equatorial conformation. This dipole-dipole interaction is reduced in the axial conformation, and this factor probably contributes to the solvent dependence of the anomeric effect. 

The axial-equatorial conformational equilibria for 2-fluoro- and 2-chlorotetrahydropyran have been investigated with several MO calculations, including calculations at the MP2/6-31G level. The MP2/6-31G calculations give values of 3.47 and 2.84kcal/mol, respectively, for the energy favoring the axial conformer. Solvent effects were also explored computationally and show the usual trend of reduced stability for the axial conformation as solvent polarity increases. 

This result shows than the initially added trichloromethyl group has little influence on the stereochemistry of the subsequent bromine atom-abstraction. The intermediate 2-(trichlor-omethyl)cyclohexyl radical presumably relaxes to the equatorial conformation faster than bromine-atom abstraction occurs. In contrast with addition to A -octahydronaphthalene, the addition is exclusively /ran -diaxial ... 

For the equilibrium between the axial and equatorial conformations of a monosubstituted cyclohexane. 

It was assumed above that the methyl group adopts an equatorial conformation. Actually, methylcyclohexanone exists as a mixture of axial and equatorial conformations. 

By the use of nuclear magnetic double (and triple) resonance, the configuration (34) of pseudo-talose was established, confirming chemical evidence, and the sidechain-equatorial conformation (38) was also... 

Even though cyclohexane rings rapidly flip between chair conformations at room temperature, the two conformations of a monosubstituted cyclohexane aren t equally stable. In methylcyclohexane, for instance, the equatorial conformation is more stable than the axial conformation by 7.6 kj/mol (1.8 kcal/mol). The same is true of other monosubstituted cyclohexanes a substituent is almost always more stable in an equatorial position than in an axial position. 

The energy difference between axial and equatorial conformations is due to steric strain caused by 1,3-diaxial interactions. The axial methyl group on Cl is too close to the axial hydrogens three carbons away on C3 and C5, resulting in 7.6 kj/mol of steric strain (Figure 4.13). 

Figure 4.13 Interconversion of axial and equatorial methylcyclohexane, as represented in several formats. The equatorial conformation is more stable than the axial conformation by 7.6 kJ/mol.

Look at Figure 4.12, and estimate the percentages of axial and equatorial conformers present at equilibrium in bromocyclohexane. 

The preference for the axial position in unhindered thiane-1-oxides has been known for some time. The spectra of the cis and trans isomers of the 2-, 3- and 4-methyl thiane-1-oxides, 169-171, were also measured. It was concluded from the 13C chemical shifts that the methyl groups preferred the equatorial positions. A comparison of the 170 chemical shifts obtained for sulfoxides 169-174 with those obtained for the cis and tram sulfoxide isomers of trans- 1-thiadecalin, 175 and 176, was consistent with this proposal. Sulfoxide 175 with the S=0 axial gave a shift about 17 ppm upfield from that of its equatorial isomer 176. For sulfoxides 169-174, the conformers proposed to have the S=0 axial gave shifts that were upfield from those of the supposed equatorial conformers. For tram-3, (rans-5-dimethylthiane-1 -oxide (177) with the oxygen axial, the 170 signal was 21 ppm upfield from the signal observed for the equatorial oxygen in cis-3, cis-5-dimethylthiane-l-oxide (178). 

The conformational preference of the monosulfoxides of 1,2-, 1,3- and 1,4-dithianes (179-181) were determined by NMR experiments which included variable-temperature studies, double irradiation, solvent effects and the influence of lanthanide shift reagents167. For 179 and 181, the axial conformers were the dominant species in CD3OD, but for 180, the equatorial conformer was in excess. 

It might well be that, compared with other thietane oxide systems, the larger pucker angle here is due to the two bulky 2,4-phenyl substituents that tend toward equatorial conformation. 

In the equilibrium mixtures of thiane oxide (2) and 1,3-, 1,4-dithiane dioxides (3) and (4), the axial conformers are present predominantly over the equatorial conformers at low temperatures (— 90 °C). For instance in the monoxide (2) a ratio of 62% axial and 48% equatorial conformers has been observed. The prevalence of the axial conformers in the sulphoxides (2) and (4) has been explained to be due to hydrogen bonds between the oxygen in the axial sulphinyl group and the hydrogen atom at the 3-position as shown in Scheme 7121 127. 

Fig. 7. Conformations of substituted 4-lithio-l,3-dioxanes calculated at B3LYP/6-31-i-G(d)// HF/3-21G. Only the lowest energy 4,6-czs and 4,6-trans conformations are listed, along with the relative energy for each pair. The (4,6-czs)-5-axial conformation is 1.81 kcal/mol higher in energy than the (4,6-ds)-5-equatorial conformation...

There were indications from other experimental work [72-74] that only the lower energy equatorial conformers are significantly populated at modest... 

N.m.r. spectroscopy has played an important part in determining the stereochemistry of the 1,3-dioxaphosphorinanes (52). The presence of the saturated six-membered ring means that there are usually conformational effects to be unravelled before configurational assignments can be made. The chair conformation is generally dominant. Phosphorus substituents which exhibit shielding effects show that in many P " phosphorinanes this substituent occupies an axial position and Sis( has been used to establish the equatorial conformation of a t-butyl substituent at C(5). Even in P" derivatives the isomer possessing the bulkiest P-substituent in an axial... 

The chloro- and bromo-cyclohexane inclusion compounds have been extensively examined by infrared (4000-30 cm-1) spectroscopy65-68 and by Raman (< 1000 cm-1) spectroscopy 69). In the canals both guests are found to exist exclusively in the chair conformation with an axial halogen substituent, while iodocyclohexane 65, 68,69) acj0pts both axial and equatorial conformations in the canal. These results should be contrasted with the familiar situation in the liquid phase where the equatorial arrangement is the lowest energy conformer and is present to the extent of about 65-70% at room temperature. 

---