Cyclohexane 1,3-diaxial interaction

Chair cyclohexanes are conformationally mobile and can undergo a ring-flip, which interconverts axial and equatorial positions. Substituents on the ring are more stable in the equatorial position because axial substituents cause 1,3-diaxial interactions. The amount of 1,3-diaxial steric strain caused by an axial substituent depends on its hulk. 

Diaxial interaction (Section 4.8) The strain energy caused by a steric interaction between axial groups three carbon atoms apart in chair cyclohexane. 

Other examples of the formation of six-membered rings by means of an intramolecular alkylation of an ester enolate are given in Table 7. Entry 6, i.e., stereoselective transformation of the epoxy ester into the cyclohexane derivative, should be discussed briefly as a representative for the other cases. The probable reason for the unexpectedly high selectivity i.e., the nonappearance of the diastereomer 8, can be demonstrated by the two transition-state-like conformations 9 and 10. 9 displays a very severe 1,3-diaxial interaction in comparison to 10, thus, formation of the diastereomer 7 from conformation 10 is highly favored113. 

Determination of the relative stability of many other substituted cyclohexane stereoisomers can be done in a similar manner. However, examples in which there are complications due to 1,3-diaxial interactions between groups or examples in which the rings are substituted with polar groups, whose dipoles interact, are much more complicated. Recently, computer programs have been developed that enable the most stable conformation of many molecules, cyclic and noncyclic, to be determined. These molecular mechanics calculations can provide the most stable shape of even quite complex molecules. 

Diaxial interaction (Section 6.7) An interaction that destabilizes two axial groups on the same face of a cyclohexane ring because of steric crowding between them. 

The steric interference between substituents in axial positions is particularly severe when there are large groups on two carbon atoms that bear a 1,3-diaxial relationship (cis on Cl and C3, or Cl and C5), as in the two chair conformations of cis-l,3-dimethyl-cyclohexane shown here. The less stable conformation has both methyl groups in axial positions. The more stable conformation has both methyl groups in equatorial positions. Note the strongly unfavorable 1,3-diaxial interaction between the two methyl groups in the diaxial conformation. The molecule can relieve this 1,3-diaxial interference by flipping to the diequatorial conformation. Use your models to compare the diaxial and diequatorial forms of cis-1,3-dimelhylcyclohexane. 

Larger axial substituents create unfavorable 1,3-diaxial interactions, destabilizing a cyclohexane conformation. 

Diaxial interaction (Section 4.13A) A steric interaction between two axial substituents of the chair form of cyclohexane. Larger axial substituents create unfavorable 1,3-diaxial interactions, destabilizing a cyclohexane conformation. 

Conformational analysis of cyclohexane derivatives containing several different substituents follows along the same lines as that of the dimethylcyclohexanes. We need to keep in mind that, of two groups, the larger one will tend to call the tune. Because of its very large 1,3-diaxial interactions (Problem 9.3, p. 301), the bulky /er/-butyl group is particularly prone to occupy an equatorial position. If—as is usually the case—other substituents are considerably smaller than tert-butyl, the molecule is virtually locked in a single conformation the one with an... 

We saw in Problem 4.21 that c(.< decalin is less stable than trans-decalin. Assume that the 1,3-diaxial interactions in rctR.s-decalin are similar to thase in axial methyl-cyclohexane [that is, one CH2-H interaction costs 3.8 kJ/mol (0.9 kcal/raoDI, and calculate the magnitude of the energy difference between cis- and trccns-decalin. 

Due to a deformation of the cyclohexane ring, c/5 -hydrindane (the numbers in [ ] refer to carbon atoms between the bridgehead carbons) is only slightly less stable than the trans isomer ( 1 kcal/mol). 8-Methylhydrindane is of interest since it occurs as part structure of the C/D rings of steroids (as a trans fused ring system). Introduction of an angular CH3 group makes the cis isomer more stable than the trans because the cis isomer has fewer 1,3-diaxial interactions. 

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