Pyridazine-3-thiones, tautomerism

It must be also emphasized that UV-vis irradiation favors the formation of the thiol conformer. A typical example is provided by 3-pyridazine, which, as we have mentioned above, is found exclusively in the thione form. However, one hour of UV-vis irradiation (A, > 330 nm) of matrix-isolated 3-thiopyrazine caused a nearly complete disappearance of the initial IR spectrum and the new observed spectrum fits well with the predicted spectrum of the thiol form, which was taken123 as evidence that the photoproduct is the thiol tautomer. A similar conversion of the thione tautomeric form into the thiol tautomer upon UV-vis irradiation was also observed for 2-thiopyridine63 and for 4-thiopyrimidine123. 

Studies on the tautomerism of pyridazines with potential tautomeric groups have continued. On the basis of recorded ionization constants and UV spectra of 3,4,5-trimercaptopyridazine and its derivatives, it is concluded that the compound exists as 3,4-dimercaptopyridazine-5(2f/)-thione. Tautomerism of hydroxy- and mercaptopyridazines was investigated with the aid of the corresponding anhydro bases as reference compounds. They exist predominantly in the -one or -thione forms. Similarly, the 3-mercapto-pyridazine-6(l/f)-thione structure in aqueous solution has been redeter-... 

S-N rearrangement, 3, 36 ionization constants, 3, 4 oxidation, 3, 37 quatemization, 3, 17 Pyridazine-3 (2 H) - thiones analysis, 3, 2 tautomerism, 3, 5 Pyridazinium betaine, 3-oxido-photolysis, 3, 11 Pyridazinium betaine, 5-oxido-photolysis, 3, 11 Pyridazinium dicyanomethylide photolysis, 3, 12... 

Spectroscopic and crystallographic techniques were used to good effect in the study of pyridazines. Spectrophotometry and H NMR spectroscopy were used to investigate the ligand substitution reactions of pyridazine in Pt(II) coordination complexes <07M1>. The electron densities and tautomeric equilibria of 6-(2-pyrrolyl)pyridazin-3-one 11 and 6-(2-pyrrolyl)pyridazin-3-thione 12 <07ARK114>. Optical, dielectric and x-ray diffraction studies of pyridazine perchlorate showed distinct structural differences between phases <07MI086219>. 

For thiones, the thione form is generally present (86JPR522), although it was claimed without proof that 4-arylmethyl-6-methyl-3-mercaptopy-ridazine exists in the thiol form (80S410). For pyridazine-3,4,6-trithiol, no firm conclusion could be reached about its preponderant tautomeric form, but it was concluded that it does not exist in the trithiol form nor as the 6(1//)- or 3(2//)-thione (78AJC389). 

Tautomerism, particularly keto-enol, thione-thiol and amino-imino, was discussed in CHEC-I <84CHEC-l(3B)l>. In general, hydroxy substituents at all positions of the heterocyclic ring of pyridazines, phthalazines, and cinnolines exist in the keto form, with the exception (noted in CHEC-I) of the 3- and 5-hydroxy derivatives of pyridazine 1-oxide. In cases where two hydroxy or thiol substituents are present, the normal situation is for only one to be in the keto or thione form. In contrast, amino derivatives usually exist as such. 

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