Tautomerism of dihydropyridazines

Dihydropyridazines without substituents on the nitrogens exist as 1,4-and/or 4,5-dihydro tautomers, often in equilibrium with one another. 

Niklas, R. Aumuller, and B. Olsen, Justus Liebigs Ann. Chem. 585, 81 (1954). 

Crossland and Avellen110 observed that addition of trifluoroacetic acid to a chloroform solution of 28a and 28b shifts the equilibrium entirely toward the 4,5-dihydro tautomer (28a). Neutralizing the solution, however, restores the original equilibrium. Equilibrium positions were also determined for two other 3,6-diaryl compounds, and in both cases the equilibrium favored the 1,4-dihydro tautomer. The ratio of 4,4-dimethyl-3,6-diphenyl-1,4-dihydropyri-dazine (81) to the 4,5-dihydro tautomer is 3.4 1, the 3,5,6-triphenyl compound 35 is found solely in the 1,4-dihydro form, probably because of conjugation stabilization. 

As expected, the position of equilibrium varies significantly with the substituents at the diene termini. 3,6-Di-tei-r-butyldihydropyridazine (82) shows 3.4 1 for the ratio between the 4,5-dihydro tautomer to the 1,4-dihydro species and, as was noted earlier, the 3,6-dimethoxy- and 3,6-bis(dimethylamino)dihydropyridazines exist completely in the 4,5-dihydro form. 

The difference in free energy, AG°, between the two tautomers can be calculated from the ratio of the two forms in solution using Eq. (23), in which KT = [4,5-dihydropyridazine]/[l,4-dihydropyridazine]. 

---