Cyclohexane ring current

Some of the more remarkable effects of strain are found in NMR chemical shifts. Cyclopropane derivatives usually have upheld proton chemical shifts with regard to the corresponding cyclohexane derivatives, whereas cyclobutanes commonly have downfield shifts.The upfield shift for cyclopropane protons have sometimes been attributed to a ring current in the three-membered ring, but there is little evidence for such a phenomenon. The unusual shift for these protons has proven valuable in demonstrating the presence of a three membered ring. 

A substituent on a cyclohexane ring can assume either an equatorial or an axial orientation and the free energy preference for the equatorial conformer is termed the A value for the substituent (O Fig. 1). Tables of A values are available [8,18]. The values (at 300 K) most relevant for the current topic are those for methyl (7.31 [19] or 7.61kJmol [20]), for hydroxymethyl (7.36kJmol [21]), for hydroxyl (2.5 to 4.6kJmol depending on sol-... 

The same explanation, although we shall not show the molecular orbitals, has been advanced to account for the small difference in chemical shift between the axial and equatorial protons in cyclohexanes, detectable in cyclohexane itself by freezing out at -100 °C the otherwise rapid interconversion of the two chair conformations.46 The axial protons come into resonance upfield at 81.1 and the equatorial protons downfield at 81.6. It is possible that the three axial C—H bonds on each side overlap mare sense to create a trishomoaromatic system, with a diamagnetic ring current which places the axial protons in the reduced magnetic field 1.43, and the equatorial protons in the enhanced magnetic field 1.44. 

The conformational analysis of monosaccharides, disaccharides, and ohgosaccharides is reviewed. Conformational terms are introduced through examination of the conformations of cyclohexane and cyclopentane then applied to the pyranose, furanose, and septanose rings. Concepts such as the anomeric effect are discussed. Topics of current interest, such as hydroxymethyl group and hydroxyl group rotation and disaccharide conformations are summarized. Physical methods for studying conformation are outlined. 

Some perspective is necessary here. As indicated in Chapter 7, Section 4.1 on the Cope rearrangement, the free energy for formation of a cyclohexane-1,4-diyl is 50-53 kcal/mol and that for formation of two allyl radicals is roughly 57 kcal/mol. However, in the current system, the diyl is destabilized by roughly 20 kcal/mol due to the bicyclo[2.2.1]ring system that must be generated. Such a species is kinetically inaccessible due, in part, to a substantial negative entropy despite the fact that the activation enthalpy for its formation would appear to be 50-55 kcal/mol. 

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