Electron upconversion in reactions of 1,2,4-triazoline-3,5-dione

During the electroreduction of 4-phenyl-1,2,4-triazoline-3,5-dione under aprotic conditions, a chain reaction of its transformation to triazolo[1,2- a ]triazole-1,3,5,7-tetraone is initiated, with the mechanism including the transformation of the initially formed radical anion of the starting compound into the radical anion of the product, the difference in the formation potentials of which is 2.39 V, that corresponds to an electron upconversion value of over 50 kcal mol −1 . For complete conversion of 4-phenyl-1,2,4-triazoline-3,5-dione in both MeCN and THF, it was enough to pass 0.1 F mol −1 of electricity at the reduction potential of the starting compound. Cyclic voltammetry was used to study in detail the reaction mechanism, which includes a rapid reversible dimerization to π-dimer of the initially formed radical anions, relatively slowly undergoing conversion into the key intermediate of the entire process, a σ-dimeric dianion. Further, the σ-dimeric dianion acts as an electron donor for the molecule of the starting compound, turning itself into a separated dimeric radical anion, which eliminates nitrogen with the formation of the radical anion of the product, that also reduces the molecule of the starting substrate, thus closing the catalytic cycle.