Tautomerism, ring-chain

Ring-chain equilibria of 1,2,3-triazoles and benzotriazoles with the isomeric a-diazo imines result in the well-known Dimroth rearrangement (see Section 4.01.4.1.1). 

3-triazoles with strong electron-withdrawing group, such as —CN and OSO2CF3 are also known. 

3-Triazolium-l-imides are in equilibrium with l,2-cis-bis(arylazo)ethenes, which have been investigated by variable-temperature NMR 87JCR(S)332 (see Section 4.01.2.5) and by ab initio calculations 90CC882 (see Section 4.01.1.5). 

All known examples of ring-chain tautomerism can be conveniently divided into two large groups (i) those without direct involvement of any substituent in the heterocyclic ring and (ii) isomerizations with participation of certain substituents. 

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An interesting ring-chain tautomerism between 2-azidothiazole (328) and thiazolotetrazole (328a) has been reported (597, 618, 619), the 328 structure predominating (Scheme 190). The solvent polarity and basicity influences this equilibrium constant significantly (1592). 

Thiol-thione tautomers have not been extensively studied, but UV and IR evidence show that 5-phenylisothiazole-3-thiol exists in the SH form. Ring-chain tautomerism of 2,3-dihydro derivatives of 1,2-benzisothiazole can occur (26a 26b) and the position of equilibrium depends very much on the solvent, physical state and nature of the substituents (69JOC919, 81KGS1209). 

Benzoxadiazines ring-chain tautomerism, 3, 1056 ring contraction, 3, 1066... 

Ring transformations heterocyclic compounds reviews, 1, 70 Ring-chain tautomerism polyheteroatom six-membered rings, 3, 1056 Ripariochromene A synthesis, 3, 751, 755 Robinson-Gabriel synthesis oxazoles, 6, 216... 

Thus the following problems are unsolved (1) Does the threefold tautomerism assumed by Gadamer really exist for heterocyclic pseudo bases and which factors determine the stability of the various forms (2) Does ring-chain tautomerism occur in the derivatives of these bases when this is formally possible, and if not, do these derivatives possess the cyclic or open-chain structure, and by what path have they been formed ... 

As an over-all conclusion, it can be stated that the assumption of ring-chain tautomerism in the pseudo bases derived from the heterocyclic quaternary ammonium salts is quite unnecessary as an explanation of the formation of two (cyclic and open-chain) types of deriva-... 

The hydrochloride of (3) holds water rather tenaciously, and the infrared spectrum indicates that the water is covalently bound. Mild oxidation of the cation (3) gives 4-hydroxyquinazoline in high yield and ring-chain tautomerism is excluded on the grounds that quinazo-line does not give a positive aldehyde test in acid solution, 2-Methyl-quinazoline also has an anomalous cationic spectrum and a high basic strength (see Table I), but 2,4-dimethylquinazoline is normal in both these respects, which supports the view that abnormal cation formation entails attack on an unsubstituted 4-position. ... 

The spectra of 1,3,5-, 1,3,7-, and 1,3,8-triazanaphthalerie cations revealed that they were predominantly hydrated, and mild oxidation to the corresponding 4-oxo compounds indicated that hydroxylation took place on C-4. Ring-chain tautomerism in the cation was excluded, at least during the first 45 min after mixing, because negative aldehyde tests were obtained with p-nitrophenylhydrazine. 1,3,6-Triazanaphthalene, under the same conditions, gave a yellow precipit-... 

The phenomenon of tautomerism comprises many different types of which the prototropic tautomerism that we consider here is only one. Prototropic tautomerism exists when the two tautomers differ only in the position of a proton (this is, of course, an approximation there are other differences between two tautomers, for example, in precise bond lengths). Other important types of tautomerism include the following (1) anioniotropy, where the two tautomers differ only in the position of an anion, which moves from one place to another in the molecule (2) cationiotropy, where the two tautomers differ in the position of a cation (other than a proton), which moves from one place to another in the molecule (3) ring-chain tautomerism and (4) bond-valence tautomerism. 

The ring-chain tautomerism of the imidazolidines 279 (80H1313) is of interest (Scheme 99). The isomer ratio is determined by the nature of the substituents and is hardly affected by the polarity of the solvent (CCI4, DMSO). 

The only example of ring-chain tautomerism of imidazoles is 1-hydroxy-4 -imidazoline-3-oxide (69MI86 78MI2, pp. 155,182). 

Ring-chain tautomerism of derivatives of 1,3,4-triazolidines 283 involves the equilibrium of three isomeric forms (Scheme 102) [90TL3927 96AHC(66)1, p. 33], In DMSO solution, the predominant form (about 70%)is 283c, the content of 283a and 283b varied between 13-25% and 5-17% respectively. 

For the isoxazolines 284 substituted at position 3, ring-chain tautomerism is depicted by the equilibrium 284 and 285-287 (Scheme 103). In general the cyclic tautomers 284 are strongly preferred. The ring-opened forms exist in equilibrium with 284 in rare cases [95ZOB705 96AHC(66)1, p. 21]. The equilibrium of the oxazolidinones 288 [78MI1, p. 107] is affected by the nature of the solvent. 

Ring-chain tautomerism was studied for 1,3,4-thiadiazolines 296 and 1,3,4-thiadiazolidines 297 (Scheme 109) [78MI2, p. 159 84CHEC-I(6)545, p. 557 88KGS3 95ZOB705 96AHC(66)1, pp. 44, 46, 52]. In all these cases the equilibria are shifted to the cyclic structures. Quantitative aspects of these equilibria have been discussed elsewhere [96AHC(66)1, pp. 44 6]. 

MI1 R. E. Valters and W. Flitsch, Ring-Chain Tautomerism (A. R. 

Azide-tetrazole isomerism, or valence tautomerism, was not discussed for [5.6]bicyclic systems in the previous survey (76AHCS1). During recent years, this type of ring-chain tautomerism has been extensively studied for both six- and hve-membered heterocyclic azides. Tire tautomerism of [5.5]bicyclic tetrazole systems is covered in Section II,C. We discuss the tautomerism of the six-membered heterocyclic azides in this section. 

Ring-chain tautomerism of hydrazones of 1,3-dicarbonyl compounds 99MI21. 

Ring-chain tautomerism via intramolecular reversible addition reactions to the C=0 group 95AHC(64)251. 

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