Formation of pyridazinium salts by azo coupling of N-substituted 3-amino-1-phenylbut-2-en-1-ones and diazonium salts

Treatment of 3-(2,4-dimethoxyphenylamino)- and 3-methylamino-1-phenylbut-2-en-1-ones with some benzenediazonium tetrafluoroborates gives, besides the usual azo coupling products [i.e., 3-(substituted imino)-1-phenylbutane-1,2-di-ones 2- (4-substituted phenylhydrazones) and 2-(4-methoxy-phenyldiazenyl)-3-methylamino-1-phenylbut-2-en-1-one, respectively], the previously unreported 1,4,5,6-tetrasubstituted pyridazinium tetrafluoroborates. The pyridazinium salts have been identified by X-ray analysis and by their H-1, C-13, N-15, B-11 and F-19 NMR spectra. Their formation is most probably the result of nucleophilic attack on the carbonyl carbon by the nitrogen of the hydrazone group and subsequent dehydration. ((C) Wiley-VCH Verlag GmbH & Co, KGaA, 69451 Weinheim, Germany, 2004).