Amidine type tautomerism

Many aminopyrimidines do not condense directly with sugars. The lack of reactivity may be due to tautomerism of the amidine type ... 

Only for Z = O do the two columns produce similar K values. Af-type tautomerism gives similar values for Z = O and Z = S and C-type tautomerism for Z = S and Z = NR, but otherwise there is little point of contact. Amidines ( Af-type , Z=NR) are unique in that, since the same element is involved on both sides of the equilibrium, if R = R then K =1 necessarily. The values given are roughly correct throughout for R = R = R = Me and in many cases will extend to any of these as a straight-chain alkyl group, but this is not always correct and cannot be guaranteed. 

These authors conclude that the problem of internal solvation is still an experimental and theoretical challenge GB measurements for this type of molecules of low volatility are not always in good agreement194. Molecular orbital calculations may help to solve the difficult experimental problems, but they have to take into account conformational isomerisms and the prototropic tautomerisms of the amidine and guanidine moieties. In light of the above discussion, the proton affinities deduced from the experimental GB values should be based on accurate estimations of the entropy of cyclization 86. 

N-Unsubstituted 1//-azepines are rare since, like the parent system, they tautomerize readily to the 3H-isomers in whose preparation they are often considered as transient intermediates (see Section 5.16.4.1.2(h)). This rearrangement is particularly apparent with 2-amino- and 2-alkoxy derivatives since stabilization of the 37/-azepine is then possible by amidine and imidate type resonance. For the CH2-containing tautomers the order of stability appears to be 3H > AH > 2H, a fact attested to by the facile thermal and base-catalyzed rearrangements of AH- azepines to the 3H-tautomers (72CB982) and the rarity and inherent instability of 2H-azepines. The latter are well established as intermediates in the formation of 3H- azepines (74JOC3070) but have been characterized only as their benzologues. 

The most investigated type of tautomerism of dihydropyrimidines is the amidine equilibrium (Scheme 3.137). The energies of substituted dihydropyrimidines with these structures are usually similar and the existence of mixtures of tautomeric forms is typical [248]. 

The prototropic tautomerism of compounds containing an amidine moiety has been studied extensively.41-43 However, the data obtained on this type of equilibrium are rather qualitative and have not been explored systematically. The difficulties encountered in this work stem primarily from the common experience that proton transfer between electronegative atoms, such as nitrogen, is very fast.43,44... 

Geometrical isomerism of amidines of type R —C(=NR )NH2 has not been observed previously because of the occurrence of tautomeric changes (Scheme 2). In order to demonstrate the existence of geometrical isomerism in this series, a pair of tautomers, AA or BB, must be suitably stabilized. A study of i.r. spectra has indicated that monoarylamidines tend to exist... 

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