Cyclohexanes axial

The steric parameters for the estimation of reactant state effects were chosen to be the conformational free energy differences for cyclohexane axial-equatorial equilibria (A-values) (8). In order to establish the methyl group as the standard size group, modified A-values (A ) for the various groups were used, by simply subtracting the A value for the methyl group (1.70) from the A values of the various substituents ... 

Gawronski et al.21 have used NMR spectroscopy, especially NOE experiments in order to assign conformation of the chiral calixsalen-type macrocycles, products from the [3 + 3] cyclocondensation of the trans-1,2-diaminocyclohexane with hydroxydialdehydes. It was shown that the macrocycles had C3-symmetrical structure and s-syn conformation of imine C—H/cyclohexane axial C—H bond systems and s-trans conformation of the bis-imine unit. 

For the cases where all carbon atoms are fully substituted, competitive migration occurs to C and N, giving 129 (or its degradation products) and 131. Normally, both products are observed, R3 having been aryl,18,59,64,67,68,70,80,142 143 and in one example methyl.144 In some instances only the product from migration to carbon was observed,11,15,16,104,141 whereas in others, only the N-substituted product.15,16,47,92 Some examples are shown (Scheme 48)47,59,104 the unexpected isolation of 153 rather than 151 from rearrangement of 152 was explained in terms of steric inhibition of phenyl migration by the cyclohexane axial protons.104... 

In the static TBP shown in Fig. 13 there are two types of methine protons, two types of cyclohexane-equatorial and two types of cyclohexane-axial methylene protons, and two types of fluorines. Yet, the n.m.r. spectra of this compound in solution at + 30°, which is summarized in Table 13, exhibits only one signal (tCh) for the methine protons, one signal for the equatorial and one signal for the axial methylene protons of the cyclohexane (rCHl), and one signal for the fluorines. These spectra... 

Figure 5.6. Cyclohexane axial and radial profiles in a CMR. Adapted from Tayakout et al [5.19].

Reverse Perlin effect In contrast to cyclohexane, axial protons at the p-carbons of heterosubstituted cyclohexanes may have coupling constants larger than those of the equatorial protons (i.e. the reverse Perlin effect). In 1,3-dioxanes and 1,3-dithianes, equatorial C-H bonds at the five-position have been shown to display this behavior (Figure 12.42). 

Substituted Cyclohexanes Axial and Equatorial Hydrogen Groups 171... 

SUBSTITUTED CYCLOHEXANES AXIAL AND EQUATORIAL HYDROGEN GROUPS... 

FIGURE 4.17 The chair conformation of cyclohexane. Axial hydrogen atoms are shown in red, equatorial hydrogens are shown in black. 

The classic example of this phenomenon involves the spectrum of cyclohexane. As you know, there are two kinds of hydrogen in cyclohexane, axial and equatorial, and these are interconverted by a ring flip, with an activation energy of about 11 kcal/mol (p. 198). Despite the existence of two kinds of hydrogen, the room-temperature spectrum of cyclohexane shows only a single line at 81.4 ppm (Fig. 15.58). 

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