7- hydrazones ring chain

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 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). 

Ring-chain equilibria involving cyclic tautomer formation by intramolecular reversible amidrazone NH-group addition to the C=0 bond were observed (85KGS849 86ZOR500) in solutions of the products formed in the reactions of pentane-2,4-dione and its 3-methyl derivatives with benzamidrazinium iodides. The ring-chain equilibrium was detected only in solutions of iodides or picrates. The free bases exist as the open-chain tautomers, like the hydrazones or enhydrazines. In contrast with the acyl-... 

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. 

Solid-State Structure and Ring-Chain Equilibrium Constants of Hydrazones of 3-(A -Acetylhydrazino)propanals 75A 75B"... 

A. A. Potekhin. The Chemistry of the Functionally Substituted Hydrazines and Hydrazones. The Influence of Structural Factors on Ring-Chain Equilibria (in Russian). Thesis Doct. Chem. Sci. Diss., Leningrad State University, 1989. 

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. ... 

Pyridazine aldehydes and ketones with the carbonyl group at the ring or in a side chain react in the usual manner. They form hydrazones, semicarbazides, oximes, etc. Side-chain aldehydes can be easily oxidized to pyridazinecarboxylic acids with silver nitrate and side-chain ketones are oxidized to carboxylic acids by treatment with potassium permanganate or hydrogen peroxide. 

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