Their Benzologs 1 Structure

The synthesis of betaines of several aromatic azines and their benzologs by Katritzky et al. stimulated also investigations of the corresponding pyrida-zines. They were prepared by methylation of the corresponding pyridazinones with methyl toluene-p-sulfonate to 317 and subsequent conversion into betains (319). Contrary to other related betaines, compounds of the type 319 are unreactive toward a variety of dipolarophiles. The hydroxy form of the cation (317) was shown to (ncdominate over the oxo form (318). UV, IR, and NMR data have been used for structure examination of betaines (95) and some pyridazinones. NMR spectra established... 

All these special cases involve benzologs with an ortho quinonoid structure. But even this pattern is not universal, a deviation being 2-methylindazole (40), which quaternizes 2.2 times more slowly than its parent compound 1-methylpyrazole (37).122 Indeed, except for the molecules just considered and 1,2-benzisothiazole,122 benzo-fusion is rate retarding. Thus, 1,2-benzisoxazole (indoxazene, 39), reacts 3.2 times, and 1-methylindazole (39) 7.1 times, more slowly than their parent compounds.122 Fusing a benzene ring onto an azole where the heteroatoms are situated 1,3 leads to decreases in rate constants by factors of 5.0, 6.3, and 6.8, respectively, when X of 38 is NMe, S, and O.122 These factors are not much smaller than that obtained from a comparison of pyridine and quinoline reactivities.61,78,79... 

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