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 O2 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 × 109 L mol-1 s-1), I2 •- (k = 5.1 × 108 L mol-1 s-1), CCl3O2 • (k = 5 × 108 L mol-1 s-1), and N3 • 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 (λmax ∼ 270 nm, ε ∼ 1300 L mol-1 cm-1) and weaker absorptions in the visible range. This species undergoes rapid dimerization (2k ∼ 5 × 108 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 O2 by TMP in organic solvents, including electron transfer and σ-bonding via peroxyl radical addition.
The oxidation products in acetonitrile of hexapyrrolylbenzene (HPyB) and hexakis(3-octylpyrrolyl)benzene (HOPyB), two star-shaped compounds consisting of 6-fold N-pyrrolyl-substituted benzenes, were characterized by MALDI-TOF. These products result from intramolecular coupling of pyrrolyl residues without dimer or polymer products. Transient electrochemical and flash photolysis experiments on HPyB indicate that this intramolecular coupling occurs via a radical cation/substrate mechanism and not by a radical cation/radical cation mechanism, as usually observed in intermolecular coupling that occurs during the polymerization of pyrrole derivatives.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.