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-amin0-5-aryl-l.2.4-triazoles.The latter is obtained a t higher temperatures from the hydrazide in basic solution (tt ca. 1 h for cyclization at pH 9 at 900). 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.

Synthesis of 5-aryl-3-C-glycosyl- and unsymmetrical 3,5-diaryl-1,2,4-triazoles from alkylidene-amidrazones

“…9 Hydrazonoyl bromides XIII may be prone to loss of bromide ion to form carbocation XIV as it was made likely in kinetic studies of several hydrazonoyl halides. [38][39][40] Cation XIV may form the triazole V directly or via XII depending on the reaction conditions. The presence of this cation in the reaction mixtures may also account for the formation of the observed/isolated by-products.…”

“…General procedure IX for the synthesis of unsymmetrical 3,5disubstituted-1,2,4-triazoles (39,40) An arylidene amidazone (35 or 36, 0.331 mmol) was dissolved in CH 2 Cl 2 (10 mL), and NBS (0.059 g, 0.331 mmol) was added. The reaction mixture was stirred at room temperature.…”

“…General procedure X for the synthesis of unsymmetrical 3,5disubstituted-1,2,4-triazoles (39,40) An arylidene amidazone (35 or 36, 0.83 mmol) and ammonium acetate (0.13 g, 0.1.66 mmol) was dissolved in glacial AcOH (16 mL), then NBS (0.148 g, 0.83 mmol) was added. The mixture was stirred and heated at 110 C overnight.…”

“…arylidene-arenecarboxamidrazones(35,36) and 3,5-diaryl-1,2,4-triazoles(39)(40)(41) method constitutes the shortest synthetic route to 3-(b-Dglycopyranosyl)-5-substituted-1,2,4-triazoles, among which the glucose derivatives belong to the most efficient inhibitors of glycogen phosphorylase, and are thereby potential antidiabetic agents.…”