Unexpected opening of benzimidazole derivatives during 1,3-dipolar cycloaddition

Hydroxylamine hydrochloride adds via a Michael reaction on the acetonylidene moiety of (4Z)-(2-oxopropylidene)-1,2,4,5-tetrahydro-2H-1,5-benzodiazepin-2-one, leading only to 2-[(5-methyl-isoxazol-3-yl)]methyl]benzimidazoles. This report describes the difference of reactivity of C-phenyl-N-phenyl formohydrazonoyl chloride with benzimidazole. For the N-substituted benzimidazole, an unexpected opening of the azole ring occurs, which was confirmed by single crystal X-ray diffraction analysis.