Reversal dipolar repulsion

Theoretical conformational analyses of 2-hydroxypiperidine and 2-hydroxy-hexahydropyrimidine indicated that the anomeric effect is due to charge back donation from lone pairs rather than to dipolar repulsion. A molecular orbital study of the reverse anomeric effect in AT-pyranosylimidazoles suggested that the effect is not a general phenomenon even for glycosylated quartemary ammonium compounds. The proportions of axial anomers of various glucosylamines and their conjugate acids have been determined by H-NMR spectroscopy. The changes upon iV-protonation were small and were accounted for by steric reasons rather than reverse anomeric effect. ... 

Causes of Reversal in Tautomeric Form Lone-Pair and Dipolar Repulsion... 

The corresponding scheme for maleic hydrazide 34 appears in Figure 2.8. Here the only course open for deprotonation from the dication 34d is by loss of a proton from NH" or OH+ to give 34b or 34c, respectively. This is where the problems enter. If we consider the reverse step, while 34e and 34f necessarily protonate on N and O respectively, protonation of 34a to give 34b or 34c is uncontrollable and there is no possible model compound that can overcome this deficiency. The same problem afflicts 36a, for example, where either or both of 36b and 36c can in principle be formed. While Barlin [20a] was able to establish the predominance of 36c, probably the result of NH/NR dipolar repulsion in 36b, which can be a very powerful force, it was impossible to quantify hence remains out... 

For the five enantiomers smdied in the reversed-phase mode, two terms dominated the AeE and the AvV terms, the first being positive and the second being negative (Table 5, bottom). They almost cancel each other. The AeE term encodes interactions through polarizable n and jt electrons. The e coefficient has a minor importance in overall solute retention [47]. It has a major effect on enantioselectivity. The negative enantioselective contribution of the AvV term is likely an indication of steric repulsion. Since these two terms almost cancel each other for our five test solutes, it means that the dipolar, AsS, and especially H-bonding, AbB, terms will be mainly responsible for the experimentally observed enantioselectivity factor. 

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