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 s olvents is studied by various techniques. Heating oxygenated chloroben zene solutions of TMP results in autoxidation that is initiated via re action of TMP with O-2 and partly propagated via oxidation of TMP by a TMP-derived peroxyl radical. In radiolytic experiments, TMP is oxidiz ed 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 o rganic solvents. One-electron oxidation forms the radical cation, whic h exhibits significant absorption in the UV (lambda(max) similar to 27 0 nm, epsilon similar to 1300 L mol(-1) cm(-1)) and weaker absorptions in the visible range. This species undergoes rapid dimerization (2k s imilar to 5 x 10(8) L mol(-1) s(-1)), and the dimer is very easily oxi dized to a stable product absorbing around 460 nm. NMR analysis of the product formed in irradiated CH2Cl2 solutions is in accord with a dic ation of dimeric TMP. Other products are also formed under different c onditions, 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 oxidat ion of TMP in acetonitrile solutions, and a stable product absorbing a t 460 nm is formed. The formation of this product involves the transfe r 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 mechanis ms are suggested for the consumption of O-2 by TMP in organic solvents , including electron transfer and sigma-bonding via peroxyl radical ad dition.