Mechanism of the formation of reactive N-amidinohydrazonyl bromides and their conversion to N-amidinobenzohydrazides and 3-amino-5-aryl-1,2,4-triazoles

Abstract

A new group of reactive N-amidinohydrazonyl halides is described. These materials may be isolated as stable hydrobromide salts but undergo rapid halide displacement on dissolution in ionizing solvents. Their solvolytic behaviour was investigated in detail in aqueous dioxan; because of the rapidity of the reaction, measurements were possible (at 25°) only in solutions of high acidity and low aqueous content. The initial product formed at all pH values was the corresponding hydrazide (formed by water attack on an intermediate azocarbonium ion) rather than the expected 3-amino-5-aryl-1,2,4-triazoles. The latter is obtained at higher temperatures from the hydrazide in basic solution (t_½ca. 1 h for cyclization at pH 9 at 90°). The mechanism of formation of the N-amidinohydrazonyl bromides by direct bromination of the corresponding hydrazones was also investigated and the rate-determining step for this process was shown to be a slow geometric isomerisation (E–Z) about the azomethine bond.