Mechanistic aspects of electro-oxidative generated triazolinediones in the presence of amines-EC' versus ECE mechanism

Abstract

• Oxidation of urazoles in presence of amines proceeds through both ECE and EC' mechanisms. • A robust relation is observed between tendency for EC' and Hammett constant. • EC' and ECE mechanisms are sensitive to reaction conditions. Electrochemical oxidation of urazoles is studied in the presence of diverse amines to demonstrate competition of oxidative ring cleavage of urazoles, electrocatalytic oxidation of amines (EC' mechanism) and coupling reaction of amines and urazoles to produce products during ECE mechanism. The oxidized form of urazole is unstable due to the presence of carbonyl and azo moieties in the molecule. Instability is more critical with the presence of electron-withdrawing groups in the structure. The instability results in oxidative ring cleavage. However, the results reveal that triazolinediones (oxidized form of urazoles) promote a wide range of amine oxidations despite instability (EC' mechanism). Results of the current work indicate that depending on amine structure and also experimental conditions electrochemical oxidation of urazoles in the presence of amines can proceed via Michael addition reaction coupled with electrode process (ECE mechanism). The pathways are strongly influenced by reaction conditions. In this study, effect of different parameters such as pH, concentration ratio of amine to triazolinedione, substituent effects of triazolinedione and amine is studied to provide insight into effective parameters to proceed the reactions toward ECE or EC' mechanisms. Moreover, the effects of electronic and structure nature of amines are evaluated for benzylic, aromatic and aliphatic amines.