Pyridazinium salts

IV-Methylated pyridazinones can be obtained from 3,6-dialkoxypyridazines by treatment with alkyl halides or dialkyl sulfates. Methyl iodide and dimethyl sulfate are most frequently used. According to the proposed mechanism, an intermediate quaternary pyridazinium salt is formed, followed by elimination of a group R from the alkoxy group. At higher temperature, l,2-dimethylpyridazine-3,6(l//,2//)-dione is formed with dimethyl sulfate. 

Alkylidene-2,3-dihydropyridazines (124) are synthesized by coupling of 3-thiomethyl-pyridazinium salts with active methylene compounds in the presence of potassium carbonate in DMF (Scheme 39) (79TL4837). 

Pyridazinium salts, 1-methoxy-reaction with pyridazines, 3, 23 Pyridazino[4,5-6]azepines synthesis, 7, 522 Pyridazino[4,5-d]azepines synthesis, 7, 522 Pyridazinofuroxans synthesis, 6, 425 Pyridazinoheteronins synthesis, 7, 729 Pyridazino[2,3-a]indole synthesis, 4, 297... 

Pyrrolo[3,4-d]pyridazinium salts, 2,3-dimethyl-synthesis, 4, 291 Pyrrolo[ 1,2-6]pyridazinones oxidation, 4, 298 Pyrrolo[2,3-6]pyridine, 1-acetyl-cycloaddition reactions, 4, 509 Pyrrolo[2,3-6]pyridine, 3-bromo-nitration, 4, 505... 

The reaction of l-amino-2-methylpyridinium tosylate and its 5-methyl derivative with benzil in the presence of NEt3 in boiling MeCN, and the treatment of the reaction mixture with 60% HCIO4 and 40% HBF4 gave 2,3-diphenylpyrido[l,2-fe]pyridazinium salt (58) (96JOC4423) and its 7-methyl derivative (01CPH77), respectively. 

In the Webb and Levy test (60) for 2-deoxy pentoses, the same hydroxyaldehyde intermediate (4) (30), formed by treating the sugars with trichloracetic acid, reacts with p-nitrophenylhydrazine to yield the pyridazinium salt (5) from which a quinonoid dye (6) absorbing at 560 m/i, is formed in alkaline medium. 

The same authors also investigated the NMR spectroscopic properties of 1- and 2-alkylated tetrazolo[l,5- ]-pyridazinium salts <2002MRC507> and concluded that the site of alkylation can be accurately determined from the 1SN NMR chemical shifts of these compounds. The obtained chemical shifts with these salts are summarized in Table 2. 

Table 2 Compilation of 13C and 15N NMR shifts of some 1 - and 2-alkylated tetrazolo[1,5-b]pyridazinium salts...

The thiazolo[3,2-h]pyridazinium salts 192 react with active methylene compounds to give ylides 1%. This reaction is initiated by nucleophilic attack at C-8 and subsequent deprotonation to 194, which is oxidized by atmospheric oxygen via radical intermediates 195 (82CPB35) (Scheme 34). 

Ab initio calculations of the ionic Diels-Alder reactions of tiiazoloisoquinolinium and tetramethoxycarbonylquinohzinium ions with electron-rich dienophiles have been reported.215 The 2++ 4-cycloadditions of arenediazonium ions with (T)-penta-l,3-diene, 2,3-dimethylbutadicnc, and ( )-2-methylpenta-l,3-diene produce dihydropyr-idazines and pyridazinium salts.216 The similarity approach has been applied to predict successfiilly the preferred regiochemistry of various types of pericyclic reaction including polar and semi-polar Diels-Alder and 2 + 2-cycloadditions.217... 

Azine approach. Pyridazinium salts (216) can be prepared by acid-catalyzed cyclodehydration of 2-/3-oxoalkyl-3(2//)-pyridazinones (77YZ422). The same cyclization of 3,4-dihydro-... 

When 3,6-dialkoxypyridazines (4) are heated with a variety of catalysts (e.g., mineral acids, p-toluene sulfonic acid, Lewis acids), rearrangement occurs to give the 3-alkoxy-l-alkylpyridazin-6(l//)-ones (5). Thus N-methylated pyridazinones can be prepared by treatment of 3,6-dialkoxypyridazines with methyl iodide or dimethyl sulfate (84MI2), the reaction mechanism presumably involving an intermediate quaternary pyridazinium salt. 

I Pyridazines, pyridazinium salts, dihydropyridazines, tetrahydropyridazines, cinnolines and phthalazines... 

Pyridazinium salts or 1,6-dihydropyridazines have been obtained in cycloaddition reactions between aromatic and heterocyclic diazo compounds and dienes [Eq. (10)]. Diazoheterocycles usually cycloadd as 1,3- poles. 

There are, however, a few exceptions since they can react also as 1,2-dipoles. For example, 3-diazopyrazolo-[3,4-b] pyridine reacts with 2,3-dimethyl-butadiene to give the pyridazinium salt 43, and in a similar manner diazocyanoimidazole and butadiene gave a dihydropyridazine derivative. ... 

Some modifications of amino and hydrazino groups were also performed. An amino group can be replaced in the reaction with amines, although in low yield. To the few known pyridazinonimines some new compounds have been added. The imines (203) are prepared from the corresponding pyridazinium salts and sodium hydride. Hydrazinopyridazines upon nitrosation are transformed into azido- or tetrazolopyrida es. The reaction can be complicated by aza-transfer reactions with a heterocyclic diazo compound. For example, 3-chloro-6-hydrazinopyridazine, when treated with... 

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