Hydrazone ring-chain tautomerism

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

Ring-chain tautomerism was observed in a series of l,2,3,6-tetrahydro-l,2, 4-triazine 4-oxides 5 in nonpolar solvents (e.g., CCI4) by NMR spectroscopy. Depending on the nature of substituents R and R, the ratios of the cyclic form of 1,2,4-triazine 5a to the open-chain form of hydrazone 5b were found to be up to 45 55 (77ZOR2617). 

Methyl hydrazones of a wide range of aldehydes and ketones (380) undergo addition-cyclization to give 1,2,4,5-tetrazines (382) via 381 (216-219). Substituent effects on the ring-chain tautomerism between 381 and 382 were studied by NMR spectroscopy. 

The ring-chain tautomerism of the hydrazone derivatives has been discussed in a monograph (79MI1) and more thoroughly in a review (88KGS3). 

Ring-chain tautomerism established by intramolecular reversible N — H group addition to the hydrazone C=N bond has been studied in a wide series of derivatives with different structures containing hydrazone N — H groups [for a review, see (88KGS3)]. The cyclic tautomers mostly involve five- or six-membered rings. 

Solvent effects similar to those described for the keto/enol equilibria can also be found for other tautomerisms, e.g. lactim/lactam, azo/hydrazone, ring/chain equilibria, etc. [62-64], The pecularities arising here can only be illustrated by means of a few representative examples. 

Ring-chain tautomerism of functionally substituted hydrazones 88KGS3. 

A careful analysis of n.m.r. spectra measured in various solvents has revealed the occurrence of ring-chain tautomerism in 1,3,4-thiadiazolidine-2-thiones (141). In solution, these compounds exist in equilibrium with their open-chain hydrazone -tautomers (142) the equilibrium is displaced entirely to the hydrazone salt form (143) in alkaline media. The discovery was made with 5-(pyrid-2-yl)-l,3,4-thiadiazolidine-2-thione (141 = R = H R = 2-C6H4N), the alkali and amine salts of which... 

The realization of this new type of ring-chain tautomerism suggested the existence of reverse changes, so that compounds of type (151), hitherto considered linear, might assume the tautomeric A -l,3,4-thiadiazoline structure (153) in acidic media ie.g. in F3CCO2D). N.m.r. measurements made with thiosemicarbazones (151 R = NHj or NHMe) and hydrazones of alkyl dithiocarbazates (151 R = SMe) fully confirmed these expectations. ... 

The structure of sugar phenylosazones, derivatives first prepared by Fischer in 1884, has remained undefined. Besides Fischer s open-chain structure, a tautomeric azo-hydrazone structure and ring structures have been proposed. Although work in the last two decades involving ultraviolet spectra studies - and examination of the acetylated - and... 

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