Tautomerism of Heterocycles: Five-Membered Rings with

The Tautomerism of Heterocycles Five-Membered Rings with Two or More Heteroatoms... 

Friedrichsen, W., Traulsen, T., Elguero, J., Katritzky, A. R., Tautomerism of Heterocycles Five-Membered Rings with One Heteroatom, 76, 85. 

A theoretical, comparative study of the tautomerism of 56 five-membered heterocyclic rings announced in (76AHC(Sl)l) has appeared (81MI40402). The stabilities of the three forms for 5-pyrazolones, 5-pyrazolethiones and 5-aminopyrazoles have been calculated by a simple Hiickel o) iterative method. The relative energies and the substituent and solvent effects are in agreement with the experimental results. 

Any heterocycle containing the OCH=CH moiety can in principle extrude the superfluous fragment and form oxirene, as illustrated for a five-membered ring in Scheme 105. Probably the most propitious AB fragment would be nitrogen, but the required 1,2,3-oxadiazole (123) is unknown (see Chapter 4.21), probably because of ready valence tautomerization to diazoethanal (Scheme 106) (this approach has been spectacularly successful with the sulfur analogue of (2) (8UA486)). The use of (123) as an oxirene precursor is thus closely linked to the important diazo ketone decompositions discussed in Section 5.05.6.3.4(f). 

Relatively little data are available on sulfonamido derivatives of heterocyclic systems with five-membered rings. The tautomeric equilibrium between structures 222 and 223 has been shown to favor the imine form by about 10 1 by comparison of the ultraviolet spectrum of the parent compound with those of both methylated forms." ... 

Volume 13 of this serial publication comprises six chapters of which four deal with general accounts of compound classes 1-azirines (F. W. Fowler), phenanthridines (B. R. T. Keene and P. Tissington), tri-thiapentalenes (N. Lozac h), and heterocyclic ferrocenes (F. D. Popp and E. B. Moynahan). The other two chapters are concerned with particular aspects of the chemistry of groups of heterocyeles the tautomerism of purines (B. Pullman and A. Pullman) and quantitative aspects of the electrophilic substitution reactions of five-membered rings (G. Marino). 

Our survey of heteroaromatic tautomerism will be completed in three further chapters, two of which will appear in Volume 77 of our series covering the tautomerism of (i) rings with three, four, and seven or more members and (ii) two or more fused five- or six-membered heterocyclic rings. The final chapter dealing with the tautomerism of six-membered monocyclic ring systems will appear in a subsequent volume. 

Annular tautomers are prototropic tautomers in which the migrating proton is restricted to ring atoms. For these five-membered heterocycles, annular tautomerism can only occur with pyrrole 2 and its polycyclic derivatives. There is no authenticated case of the monocyclic pyrrolenine tautomeric forms 211 and 212 predominating, presumably due to the required loss of resonance energy in these nonaromatic tautomers . 

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