Direct synthesis of 3-aryl-1,2,4,5-tetrazine N-1-oxides by the oxidation with methyl(trifluoromethyl)dioxirane

1,2,4,5-Tetrazines are oxidized by methyl(trifluoromethyl)dioxirane (TFD) to their hitherto unknown(1a) N-oxides in excellent yields. A detailed NMR study (H-1, C-13, N-15) shows that the N-1 atom of 3-aryl-1,2,4,5-tetrazines 1 is oxidized regioselectively. A Hammett plot (r(2) = 0.970) affords a rho value of -1.53. The correlation of the logarithm of the reaction rates versus the ionization potentials, which were calculated by AM1, is much worse for ionization out of the aromatic system (IPR; r(2) = 0.804) than for ionization out of the higher-energy nitrogen lone pairs (IPN:; r(2) = 0.940). This implies that an electron transfer mechanism is unlikely and an S(N)2 attack by the nitrogen lone pair of the tetrazine on the dioxirane peroxide bond appears to operate.