Tetrahydropyridazines and Hydropyridazines with Fused Rings

Alkylation of 1,2-dimethyl-1,2,3,6-tetrahydropyridazine, as in 1,1,2-trimethyl-1,2,3,6-tetrahydropyridazinium tetrafluoroborate, reduces the percentage of 2-methyl(ax) conformer to 3.5—5.5% at — 80°C.254 

The similarity in AG for the ae aa nitrogen inversion process in 1,2-dimethylhexahydropyridazine and for the ae - aa process in trans-1,2,3,6-tetramethylhexahydropyridazine (245) (AG 7.85 kcal mol-1 at 25°C), in which the inversion involves NMe-CMe passing, supports256 the view243 (see above) that the difference between AG for the fast ea aa and slow ee aa nitrogen inversion is due in part to the differing electronic destabilizations in the transition states. 

In the bicyclic systems AG values are greater than in the monocyclic analogs (e.g., AG 14.43 kcal mol 1) for the ae ee process in trans-2,5-dimethyl-l,6-diazabicyclo[4.4.0]decane, (depicted in Table XIX) inasmuch as nitrogen inversion and ring inversion are now coupled processes. 

Comparison of Conformational Equilibria in 1,6-Diazabicyclo[4.4.0]dec-3-enes AND IN 1,6-DlAZABICYCLO[4.4.0]DEC A NES2 5 8 

X-Ray studies252 on the tetrahydropyridazine 249 (ae in the crystal) show appreciable flattening of the nitrogen atoms and a decrease in the N—N bond length compared to 244. 

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