Investigations of the Richter reaction in a series of vicinal alkynylpyrazolediazonium salts

All positional isomers of via-alkynylaminopyrazoles, when treated with sodi um nitrite in hydrochloric or hydrobromic acid, are transformed into pyrazo lopyridazines via cyclisation of the corresponding alkynylpyrazolyldiazoniu m salts. The ease of cyclisation was found to depend markedly on the positi on of the alkynyl and diazonium groups around the pyrazole nucleus: 4-alkyn ylpyrazole-5-diazonium salts derived from the corresponding 4-alkynyl-5-ami nopyrazoles 1a-c cyclized at 0-20 degrees C to 4-chloro- and 4-bromo-1H-pyr azolo[3,4-c]pyridazines 2a-c and 3a; the heterocyclization of 5-alkynylpyra zole-4-diazonium salts 5a,b required heating to 100-105 degrees C and gave 7-chloro-1H-pyrazolo[4,3-c]pyridazines 6a,b in good yield. The cyclisation of 1-methyl-3-alkynylpyrazole-4-diazonium salts 8a,b was accompanied by met hyl group migration towards the neighbouring nitrogen atom to give the same 7-chloro-1H-pyrazolo[4,3-c]pyridazines 6a,b arising from cyclisation of th e 5-alkynylpyrazole-4-diazonium salts 5a,b. When treated with sodium hydrog en carbonate, the 1,5-dimethylpyrazol-4-diazonium salts 8a-c gave the 5-(py razol-4-ylazomethyl)pyrazole-4-diazonium chloride derivatives 9a-c which cy clized in the presence of base to give the corresponding 6-(pyrazol-4-ylazo )-4H-pyrazolo[4,3-c]pyrazoles 10a-c. In contrast, the heterocyclization of 4-alkynylpyrazole-3-diazonium salts d erived from 4-alkynyl-3-aminopyrazoles 11a,b at 50-60 degrees C gave 4-hydr oxy-2H-pyrazolo[3,4-c]pyridazines 12a,b as the major components together wi th the corresponding 6-halogeno-pyridazines 13a,b and 14a as minor componen ts.