Hexahydropyridazines and Tetrahydropyridazines

2-Dimethylhexahydropyridazine and related compounds can exist in eight conformations. It was recognized109,248,249 in 1971 that the barriers 

The JV-CH2 protons will be equivalent in the H-NMR spectrum of 1,2-dimethylhexahydropyridazine only if the two equivalent sets separated by the vertical line in Fig. 11 are interconverting rapidly on the NMR timescale. The N-Me group protons will, however, absorb as a singlet if rapid equilibration is occurring within a set, even though the sets are interconverting slowly on the NMR time scale. 

The original H-NMR work in this series251 was reinterpreted109 on the basis of the 13C-NMR spectra, cis-1,2,4,5-Tetramethylhexahydropyridazine shows a barrier of 12.6 kcal due to freezing of the passing interactions. 

4-Tetramethylhexahydropyridazine (242 243) has only four confor-mers that do not possess /3-diaxial methyl groups the high-energy barrier is 

5 kcal mol-1 and the nonpassing ring inversion is frozen at lower temperatures with a barrier of 10.0 kcal mol l. 

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