Ring-chain tautomerism 1,2,3-triazoles

NMR has also been used as a basis for distinguishing 1- and 2-acetyl-triazoles - - and for investigating ring-chain tautomerism in tri-azoles. - - 0-H coupling constants are a useful means of determining the site of methylation of triazolo oxides, and triazolo sulfides. -... 

In this section, the reactivity of the major types of aromatic 1,2,3-triazoles and benzotriazoles is considered and compared with that which would be expected on the basis of electronic theory and spectroscopic data presented in Section 4.11.3. Tautomeric equilibria and ring-chain tautomerisms are discussed in Section 4.11.4.1.2(i), Dimroth rearrangements in Section 4.11.4.1.2(iii) and acid-base considerations are discussed in the section immediately following. 

Besides the annular tautomerism and the Dimroth rearrangement (Section 4.02.3.5) ring-chain tautomerism of certain 1,2,3-triazoles have been discussed and, in some cases, shown by means of spectroscopic methods. Thus, cyanogen azide reacts with acetylene to form a 1 1 adduct, which was proved by means of IR, UV and temperature-dependent... 

H NMR data to exist as a ring-chain tautomeric mixture of 1-cyano-l,2,3-triazole (71a) and a-diazo-iV-cyanoethylideneimine (71b). The equilibrium is sensitive to temperature and solvent (67JA4760). 

Similarly, 1,3 -dipolar cycloadditions of benzenesulfonyl azides to N,N- diethylaminoprop-1-yne led to 1,2,3-triazoles (72a) and a-diazoamidines (72b). With the aid of IR and H NMR data these were shown to exist in a tautomeric equilibrium (70JOC3444). With IR and H NMR data a 5-aminotriazole-diazoamidine equilibrium (73a) s (73b) was established (70TL2823, 72CB2963, 72CB2975). However, 4-amino-l,2,3-triazole (74a) proved to be stable and no ring-chain tautomerism could be identified (73TL1137). 

Besides ring-chain tautomerism (Section 4.11.4.1.2(0) and the Dimroth rearrangement (Section 4.11.4.1.2(iii)) cleavage reactions of some amino-, azido and diazo-methyltriazoles have been reported. Thus, 5-azido-l,4-diphenyl-l,2,3-triazole (121) proved to be unusually labile above 50 °C. Nitrogen is evolved with the formation of a conjugated nitrile (122) (64JA2025, 70JOC2215). Similarly the diazomethyltriazole (123) decomposes to (124). Both reactions can be explained in terms of concerted processes. 

Molecules containing a fused 1,2,3-triazole ring, as in (8), (11), (12), (26), (27), (33) and (36), frequently display a diazo characteristic in their chemical reactivity that leads to homolytic and heterolytic ring fission. This is a result of an equilibrium that can exist between the closed and open forms (A) and (B) analogous to the ring-chain tautomerism exhibited by fused tetrazoles. Some studies have been undertaken (7UCS(C)2156) to evaluate the role that this equilibrium plays in the physical and chemical properties of fused 1,2,3-triazoles. 

TL2823, 72CB2963, 72CB2975). However, 4-amino-l,2,3-triazole (74a) proved to be stable and no ring-chain tautomerism could be identified (73TLH37). 

Studies along lines similar to those of this German work, carried out in the United States, led to similar results. In addition, it was shown that 1- N,N-dimethylamino)-2-phenylacetylene gave phenylacetamidines with little tendency to tautomerize to triazoles (70JOC3444). 4-Aminotriazole has been shown not to take part in ring-chain tautomerism (73TL1137). 

As already mentioned, 1,2,3-triazoles are very stable heterocycles. However, those that bear a strong electron-withdrawing group at N1 are known to undergo ring-chain tautomerization. The ring-chain isomerism of 1 -sulfonyl 1,2,3-triazoles can be exploited in synthesis. As reported recently, 1-sulfonyl triazoles could serve as... 

Within the huge field of heterocyclic rearrangements [53], the Dimroth rearrangement gives 5-mercapto-l,2,3-triazoles from 5-amino-l,2,3-thiadiazoles and allows interconversion of 5-amino-l,2,3-triazoles into a mixture of isomers under basic condition [54]. Thus, this reaction, discovered in 1909, involves a linear intermediate that can cyclize into two isomeric compounds (Scheme 15) [55]. This ring-chain tautomerization is reversible, but the equilibrium can be pushed toward an end if one of the two isomers is stabilized. 

Tautomerism was only treated briefly in CHEC-I <84CHEC-i(5)67l> under the section of thermal and photochemical reactions. Tremendous effort has been devoted to the study of the tautomerism, both prototropic and ring chain, of 1,2,3-triazoles and benzotriazoles. Therefore, tautomerism is described is considerably more detail in CHEC-II. For tautomerism of 1,2,3-triazolines, see Section 4.01.5. 

Of the various polyfunctional u-triazoles reported, an overwhelming number contain the sulfonamido group. One reason for this interest lies in the phenomenon of ring-open chain tautomerism (Eq. 20). " The position of the triazole-diazoamidine equilibrium (10.2-1 and 10.2-2) is heavily dependent on structure ... 

---