Kinetics and Mechanism for Acid-Catalyzed Disproportionation of 2,2,6,6-Tetramethylpiperidine-1-oxyl

Disproportionation of cyclic nitroxyl radicals (NRs) in acid solutions is of key importance for the chemistry of these compounds. Meanwhile, the data reported on the mechanism of this reaction in dilute acids are inconsistent with those on the stability of NRs in concentrated acids. Here we have examined the kinetics and stoichiometry for the disproportionation of 2,2,6,6-tetramethylpiperidine-1-oxyl (1) in aqueous H(2)SO(4) (1.0-99.3 wt%) and found that (1) the disproportionation of 1 proceeds by the same mechanism over the entire range of acid concentrations, (2) the effective rate constant of the process exhibits a bell-shaped dependence on the excess acidity function X peaked at X= -pK(1H+) = 5.8 +/- 0.3, (3) a key step of the process involves the oxidation of 1 with its protonated counterpart 1H(+) yielding oxopiperidinium cation 2 and hydroxypiperidine 3 at a rate constant of (1.4 +/- 0.8) x 10(5) M(-1). s(-1), and (4) the reaction is reversible and, upon neutralization of acid, disproportionation products 2 and 3H(+) comproportionate to starting 1. In highly acidic media, the protonated form 1H(+) is relatively stable due to a low disproportionation rate. Based on the known and newly obtained values of equilibrium constants, both the standard redox potential for the 1H(+)/3 pair (955 +/- 15 mV) and the pH-dependences have been calculated for the reduction potentials of 1 and 2 to hydroxylamine 3 that is in equilibrium with its protonated 3H(+) and deprotonated 3(-) forms. The data obtained provide a deeper insight into the mechanism of nitroxyl-involving reactions in chemical and biological systems. Copyright (c) 2008 John Wiley & Sons, Ltd.