Axial equatorial

CIS 1 2 Dimethylcyclohexane trans 1 2 Dimethylcyclohexane Axial-equatorial Diequatorial (1248 3) (1246 8) (1 5) trans... 

CIS 1 4 Dimethylcyclohexane trans 1 4 Dimethylcyclohexane Axial-equatorial Diequatorial (1247 4) (1245 7) (1 7) trans... 

The spectroscopic properties of the /V-nitrosamines, especially the nmr and mass spectra, vary widely depending on the substituents on the amine nitrogen (44—47). The nmr spectra are affected by the E—Z isomerism around the N—N partial double bond and by the axial—equatorial geometry resulting from conformational isomerism in the heterocycles (44,45). Some general spectral characteristics for typical dialkylnitrosamines and simple heterocycHc nitrosamines are given in Table 1. 

Neighbouring diaxial protons of cyclohexane can be clearly identified by their large coupling constants 11-13 Hz, Table 2.10) which contrast with those of protons in diequatorial or axial-equatorial configurations ( Jee 2-4 Hz). Similar relationships hold for pyranosides as oxy-... 

The multiplets and coupling constants of the axial) protons at = 3.15, 3.50 and 4.08 moreover confirm the equatorial positions of all three OH groups, as can be seen in formula B. Here the couplings from 10.0 to 11.5 Hz, respectively, identify vicinal protons in diaxial configurations, whilst values of 4.5 and 5.0 Hz, respectively, are typical for axial-equatorial relationships. As the multiplets show, the protons at 5 = 3.50 and 4.08 couple with two axial and one equatorial proton (triplet of doublets) respectively, whereas the proton at = 3.15 couples with one axial and one equatorial proton (doublet of doublets). 

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. 

When methylene protons at 1.82 p.p.m. are irradiated, each signal at 3.15 and 2.92 p.p.m. splits into a doublet with a coupling constant, 1.6 c.p.s., and a doublet with 9.6 c.p.s., respectively. Therefore, the assignment of the signals at 3.15 and 2.92 p.p.m. to C-2 and C-4 protons, is clearly established (C in Figure 4). The coupling constants again confirmed the axial- equatorial or equatorial-equatorial relation between C-l and C-2 protons and axial-axial relation between C-4 and C-5 protons. 

Cis/lrans substitution pattern Axial/equatorial relationships... 

Examination of the NMR spectrum of thiane 3,3,5,5-d4 oxide enabled the estimation of the axial/equatorial equilibrium constant317. The value was found to be 1.62 (at — 90 °C), corresponding to a free-energy difference of0.175 kcal mol - , which is in good agreement with field force calculations315. 

For definitions of axial, equatorial, and related terms for rings of any size, see Anet, F. A.L. Tetrahedron Lett., 1990, 31, 2125. 

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 ... 

You can do this for every carbon (and you should try on the drawing above), and you will see that each carbon has two groups (up and down). It is important to realize that there is no correlation between up/down and axial/equatorial. Look at the drawing above. For one of the carbon atoms, the up position is axial. For the other carbon atom showing its groups, the up position is equatorial. Take a close look at the two equatorial positions shown above. One of them is up and the other is down. 

In both chair conformations, Cl is down and Br is up. The difference between these drawings is the axial/equatorial positions. In the conformation on the left, both groups are equatorial. In the conformation on the right, both groups are axial. 

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. 

Size and conformational effects such as those described above have been exploited in a wide range of separation procedures. In a patented method for the separation of cis- and tranj-l-methyl-4-uohexylcyclohexane carboxylic acids only the tram isomer forms a thiourea inclusion compound 831. In solution the cis isomer always has an axial, equatorial arrangement in the chair conformation, while the trans isomer can exist as the diequatorial or diaxial chair conformers. It is not reported which of these is favoured or how the guest packs in the canals. 

Since the steric requirement of the /erf-butyl group was known to be much larger than that of the isopropyl group, as exemplified by the free energy difference involved in the axial-equatorial equilibrium of substituted cyclohexanes56), 2,7-di-terf-butylthiepin derivatives seemed to be more promising candidates for the synthesis. 

A popular measure of steric requirements is A, the free energy difference involved in the axial-equatorial equilibrium of substituted cyclohexane. Recommended d-values for i-C3H9 and tert-C4H9 are 2.1 and >4.4, respectively. Eliel, E. L. Chem. Educ. 37,126(1960) Angew. Chem. Int. Ed. Engl. 4, 761 (1965)... 

---