Tautomerism in heterocycles

For a monograph on tautomerism in heterocyclic compounds, see Elguero Marzin Katrizky Linda The Tautomerism of Heterocycles Academic Press New York, 1976. For reviews, see Katritzky Karelson Harris Heterocycles 1991, 32, 329-369 Beak Acc. Chem. Res. 1977,10, 186-192 Katritzky Chimia 1970, 24, 134-146. 

Tautomerism is a complicated and often contentious subject. Tautomerism in heterocycles in general and for thiophene in particular is no exception. The complexity arises with thiophenes chiefly because the aromaticity is. not so great that it cannot be partially or completely overcome within a tautomeric structure with the double bonds external to the thiophene ring. 

A treatise on tautomerism in heterocycles (76AHC(Sl)l) reviews the work in this area on pyrans and fused pyrans. 

Fig. 4.13. illustrates how to apply carbon-13 NMR for analysis of tautomerism in heterocyclic chemistry 3-Methyl-5-oxo-l-phenyl-4,5-dihydropyrazole (the Knorr-pyrazolone ) is shown to exist as the CH tautomer B with a CH2 carbon at 43.1 ppm in chloroform solution (Fig. 4.13(a)), while the OH tautomer A predominates (90%) in hexadeuteriodimethyl sulfoxide (Fig. 4.13(b) [73 i]. 

Tautomerism in heterocyclic five-membered rings was thoroughly studied by Katritzky and co-workers beginning in 1961193 and reviewed in 1970194 and in the first two volumes of this series.195 The confusing situation described by Boyer196 has been resolved by a combination of pK, measurements,60,194 the most useful single method of study, and IR, UV, and NMR spectroscopy.60, 150-153... 

Tautomerism in heterocyclic five-membered rings was recently reviewed by Elguero et al. (76AHC(S 1)266). 

AHOMO and ALUMO are not very different from each other, the same procedure can be applied to such compounds in order to gain information about the protonation site, for example, in the various aza analogues of indolizine (10), or about the tautomerism in heterocycles as exemplified by the lactam-lactim isomerism in 2-pyridone (11) and about positional isomerism such as in the tautomeric 3,5- and 3,6-diazaindoles (12, 13). This is possible since the protonation of a nitrogen or the replacement of a carbon by a heteroatom can be viewed as a special case of substitution. 

Molecular design of tautomeric systems 81ACR210 77MI3. Metallotropic tautomerism in heterocyclic complexes 81 UK 1304. Ring-chain tautomerism of heteroeycies 85MI13 78MI7. 

The equilibrium position depends on the type of substituent and on the solvent used. In the gas phase and in apolar solvents, the 2,4-dihydro-3//-pyrazol-3-one dominates. Substituent tautomerism in heterocycles is observed in oxo compounds as well as in the corresponding thiones and imines. 

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