Mechanism of pyrazoline formation from the reactions of substituted hydrazines and Mannich bases

Abstract

Aminoguanidinium nitrate reacts with Mannich bases, ArCO·CH₂·CH₂·NMe₂ to form, depending upon the pH, di-β-keto-hydrazines, 1-amidino-Δ²-pyrazolines, or amidinohydrazone dinitrates. These substituted hydrazones do not display hydrazine anchimerism to form pyrazolines, probably because they exist in the wrong geometric form. The suggested mechanism for pyrazoline formation involves elimination from the Mannich base to form a vinyl ketone, which then forms a hydrazone (of the right configuration); subsequently this cyclises to the pyrazoline.