Two ways of formation of pyrazoles in aroylation of 2-Phenacyl-1H-benzimidazole hydrazone

Reaction of 2-phenacyl-1H-benzimidazole hydrazone with aroyl chlorides can yield pyrazolyl-benzimidazoles of two types depending on the experimental c onditions, With benzoyl chloride in pyridine at 95-100 degrees C, heterocyc lization proceeds involving the active methylene group to give a 2-(4-pyraz olyl)benzimidazole derivative. In boiling N,N-dimethyaniline the formation of the pyrazole ring involves cleavage of the benzimidazole ring followed b y the formation of a new benzimidazole fragment to produce 1-[3(5)pyrazolyl ]benzimidazole derivatives. Isomeric diphenyl-substituted 2-(4-pyrazolyl)- and 1-[3(5)-pyrazolyl] benzimidazoles are studied by single crystal X-ray d iffraction.