Cyclohexane, stable axial conformers

In opposition to a central principle of alicyclic conformational analysis, Kang and Yin reported that a complex O-cyclohexyl nitronate and the corresponding O-cyclohexyloxime constitute the first stable axial conformer of monosubstituted cyclohexanes at ambient temperature (97JA8562). Snyder and co-workers (99JA11864) reevaluated the corresponding... 

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. 

More evidence comes from the reactions of substituted cyclohexanes. You saw in Chapter 18 that substituents on cyclohexanes can be parallel with one another only if they are both axial. An equatorial C-X bond is anti-periplanar only to C-C bonds and cannot take part in an elimination. For unsubstituted cyclohexyl halides treated with base, this is not a problem because, although the axial conformer is less stable, there is still a significant amount present (see the table on p. 462), and elimination can take place from this conformer. 

In monosubstituted cyclohexanes the chair conformation is again more stable, and the substituent can be either equatorial or axial. 

Miscellaneous Photochemical Reactions.—14a-Steroids are converted into an equilibrium mixture of 14oc- and 14j8-isomers ca. 1 19) on irradiation at 254 nm, in cyclohexane containing mercuric chloride or bromide/ The reaction comprises abstraction of 14-H and recombination, probably involving bromine atoms. Other isomerisations at tertiary hydrogen permit equilibrations, e.g. of cis- and trans-decalins, but the 5)3 — 5a-steroid conversion is very slow compared with reaction at C-14. Distortion from ideal bond-angles at C-14 probably loosens the hydrogen atom here. Axial secondary methyl substituents can also be epimerised into the more stable equatorial conformation. 

Cyclohexane rapidly interconverts between two stable chair conformations because of the ease of rotation about its carbon-carbon bonds. This interconversion is known as ring flip (Figure 2.8). When the two chair conformers interconvert, bonds that are equatorial in one chair conformer become axial in the other chair conformer and vice versa. 

Section 23.9 Pyranose forms of carbohydrates resemble cyclohexane in their conformational preference for chair forms. The anomenc effect causes an electronegative substituent at the anomeric (C-1) carbon to be more stable when it is axial than when it is equatorial. The effect is believed to result from the delocalization of an electron pair of the ring oxygen into an antibonding orbital of the anomeric substituent. 

On a cyclohexane ring, an axial carboxyl group has a conformational energy of 5.9 kj (1.4 kcal)/mol relative to an equatorial carboxyl group. Consider the equilibrium for the alternative chair conformations of trans-l,4-cyclohexanedicarboxylic acid. Draw the less stable chair conformation on the left of the equilibrium arrows and the more stable chair on the right. Calculate AG for the equilibrium as written and calculate the ratio of the more stable chair to the less stable chair at 25"C. 

PROBLEM 5.35 Draw the most stable chair conformation of fri-butylcyclohexane. Put a circle around each of the axial hydrogens on the cyclohexane. Put a square around each of the equatorial hydrogens attached to the ring. 

For the sake of clarity in the above scheme, the absolute configuration for one of the two possible enantiomeric syn- and anti-products is presented. Note that on the route to an anri-isomer both larger groups R and R are found in a pseudo-equatorial position in the cyclic transition state, present in the more stable chair conformation. In the transition state leading to the yn-isomer group R is found in the less preferred, crowded pseudo-axial position. Here we follow the suggestion to use the prefixes axial/equatorial only for substiments on the cyclohexane ring in the... 

Solution to 4Sa If the two substituents of a 1,2-disubstituted cyclohexane are to he on the same side of the ring, one must be in an equatorial position and the otho" must he in an axial position. The more stable chair conformer is the one in which the largo" of the two suhstituents (the ethyl group) is in the equatorial position. 

Cycloalkanes C H2 hydrocarbons n = 3, 4 Cyclopropane, cyclobutane (strained bond angles) n = 6 Cyclohexane (most stable, chair conformation) axial (less stable) equatorial (more stable) substituent positions (4-2, 3)... 

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