Electrochemical reduction of metamitron

Metamitron (I) (4-amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one) is a herbicide which is reduced electrochemically in two 2-e(-) steps. Reduction in the first step involves the protonated form of the 1,6-azomethine bond, while the second step involves the protonated form of the 2,3-azomethine bond. The 1,6-bond is reduced at potentials ca. 0.5 V more positive than that of the 2,3-bond. Over the whole pH range, the potential of the first wave of metamitron is identical to that at which 2,3-dihydrometamitron is reduced. This manifests a lack of electronic interaction between the 1,6- and 2,3-double bonds, supported by electron density map results. Reduction of both azomethine bonds is accompanied by acid-base and hydration-dehydration equilibria, while reduction of 1,6-dihydrometamitron is also accompanied by a fast irreversible chemical reaction in acidic media, which is attributed to ring opening. (C) 1998 Elsevier Science S.A. All rights reserved.