Axial-equatorial coupling

Proton c can be defined by the fact that it is not equatorial and it is highly coupled. The multiplet at 3.82 ppm satisfies these requirements. It is in the right ball park for chemical shift and is highly complex in that this proton is already the X part of an ABX system coupled to both protons alpha to the chlorine (the AB part). It is then further coupled with a 10 Hz, axial-axial coupling (reciprocated in the dd at 2.07 ppm) and with a 2 Hz axial-equatorial coupling which is reciprocated in the ddd at 2.90 ppm. Note that c and d are not fully resolved from each other. Such overlap inevitably complicates the issue. 

The /3-configuration of the hydroxyl group was inferred from the small coupling constant (J 2.5 cps) corresponding to axial-equatorial coupling. 

In the first molecule, proton H has two neighbours, one axial (H ) and one equatorial (H ) so it will appear as a double doublet with characteristic large axial/axial and small axial/equatorial couplings. By contrast the two... 

Spectral data of 4 were very close to 3, and indicated it was a stereoisomer of 5. The stereochemistry at C-6 was determined indirectly as in the case of the compound 3. Assuming axial orientation of H-4, J= 9.1 Hz could be assigned to axial-axial coupling between H-4 and H-Sax. Starting from this, J = 4.2 Hz then must be due to axial-equatorial coupling H-Sax and H-6. Therefore, the configuration of the hydroxyl group at C-6 was concluded to be S. 

The case of cyclohexyl iodide provides an example of the use of NMR spectroscopy to determine the conformational equilibrium constant and the value of -AG°. At -80 C, the NMR spectrum shows two distinct peaks in the area of the CHI signal, as shown in Fig. 3.6. The multiplet at higher field is a triplet of triplets with coupling constants of 3.5 and 12 Hz. This pattern is characteristic of a hydrogen in an axial position with two axial-axial couplings and two axial-equatorial couplings. The broader peak at lower field is characteristic of a proton at an equatorial... 

In the trans isomer, the H is axial and therefore will experience two large axial-axial couplings and one smaller axial-equatorial coupling (Section 7.2C). This proton appears at 4.70 ppm (ddd, J = 10.2, 10.2, 4.4 Hz). In the cis isomer, the H is equatorial and therefore experiences three small couplings 2 axial-equatorial and 1 equatorial-equatorial. This proton appears at 5.25 ppm and is technically a ddd, but the small J values result in significant overlap of the lines as just discussed. Therefore, the cis diastereomer is the major component and the trans diastereomer is the minor component. 

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