Mechanisms of oxidation of 1,2,5-trimethylpyrrole: Kinetic, spectroscopic, and electrochemical studies

The oxidation of 1,2,5-trimethylpyrrole (TMP) in aqueous and organic solvents is studied by various techniques. Heating oxygenated chlorobenzene solutions of TMP results in autoxidation that is initiated via reaction of TMP with O-2 and partly propagated via oxidation of TMP by a TMP-derived peroxyl radical. In radiolytic experiments, TMP is oxidized rapidly by Br2(.-) (k = 2.3 x 10(9) L mol(-1) s(-1)), I-2(.-) (k = 5.1 x 10(8) L mol(-1) s(-1)), CCl3O2. (k = 5 x 10(8) L mol(-1) s(-1)), and N-3(.) radicals in aqueous solutions and by peroxyl radicals in organic solvents. One-electron oxidation forms the radical cation, which exhibits significant absorption in the UV (lambda(max) similar to 270 nm, epsilon similar to 1300 L mol(-1) cm(-1)) and weaker absorptions in the visible range. This species undergoes rapid dimerization (2k similar to 5 x 10(8) L mol(-1) s(-1)), and the dimer is very easily oxidized to a stable product absorbing around 460 nm. NMR analysis of the product formed in irradiated CH2Cl2 solutions is in accord with a dication of dimeric TMP. Other products are also formed under different conditions, probably resulting from addition of peroxyl radicals to the pyrrole ring. In cyclic voltammetry experiments at low scan rates, an irreversible peak at a potential of 0.72 V vs SCE is found for oxidation of TMP in acetonitrile solutions, and a stable product absorbing at 460 nm is formed. The formation of this product involves the transfer of more than one electron per TMP monomer. At very high scan rates, a reversible oxidation step is observed, from which a redox potential of 0.87 V vs SCE is derived for the couple TMP/TMP.+. Several mechanisms are suggested for the consumption of O-2 by TMP in organic solvents, including electron transfer and sigma-bonding via peroxyl radical addition.