Double-bond tautomers in nonconjugated systems

NMR studies have shown that the 1,4-dihydropyridazine is the dominant tautomer in 3,6-di- and 

6-triphenyl-, and 4,4-dimethyl-3,6-diphenyldihydropyridazines the 4,5-dihydro form is a minor tautomer 83JHC855 . 3,6-Diaryl-4,5-dimethyldihydropyridazines exist in the 4,5-dihydro tautomeric form as indicated by the symmetrical H NMR spectrum, while 1-substituted analogues have the 

4-dihydro conformation. In contrast, it is claimed that 3,6-dimethyl-1-(4-sulfamoylphenyl)-dihydropyridazines exist in the 1,2-dihydro form 87JHC1745 . 

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