title: Acid Is Key to the Radical-Trapping Antioxidant Activity of Nitroxides.
abstract: Persistent dialkylnitroxides (e.g., 2,2,6,6-tetramethylpiperidin-1-oxyl, TEMPO) play a central role in the activity of hindered amine light stabilizers (HALS)-additives that inhibit the (photo)oxidative degradation of consumer and industrial products. The accepted mechanism of HALS comprises a catalytic cycle involving the rapid combination of a nitroxide with an alkyl radical to yield an alkoxyamine that subsequently reacts with a peroxyl radical to eventually re-form the nitroxide. Herein, we offer evidence in favor of an alternative reaction mechanism involving the acid-catalyzed reaction of a nitroxide with a peroxyl radical to yield an oxoammonium ion followed by electron transfer from an alkyl radical to the oxoammonium ion to re-form the nitroxide. In preliminary work, we showed that TEMPO reacts with peroxyl radicals at diffusion-controlled rates in the presence of acids. Now, we show that TEMPO can be regenerated from its oxoammonium ion by reaction with alkyl radicals. We have determined that this reaction, which has been proposed to be a key step in TEMPO-catalyzed synthetic transformations, occurs with k ∼ 1-3 × 10(10) M(-1) s(-1), thereby enabling it to compete with O2 for alkyl radicals. The addition of weak acids facilitates this reaction, whereas the addition of strong acids slows it by enabling back electron transfer. The chemistry is shown to occur in hydrocarbon autoxidations at elevated temperatures without added acid due to the in situ formation of carboxylic acids, accounting for the long-known catalytic radical-trapping antioxidant activity of TEMPO that prompted the development of HALS.
authors.affiliation: Department of Chemistry, University of Ottawa , Ottawa, Ontario K1N 6N5, Canada.
authors.forename: Evan A
authors.initials: EA
authors.lastname: Haidasz
authors.affiliation: Department of Chemistry, University of Ottawa , Ottawa, Ontario K1N 6N5, Canada.
authors.forename: Derek
authors.initials: D
authors.lastname: Meng
authors.affiliation: Department of Chemistry "G. Ciamician", University of Bologna , Bologna I-40126, Italy.
authors.forename: Riccardo
authors.initials: R
authors.lastname: Amorati
authors.affiliation: Department of Chemistry "G. Ciamician", University of Bologna , Bologna I-40126, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Baschieri
authors.affiliation: National Research Council of Canada , Ottawa, Ontario K1A 0R6, Canada.
authors.forename: Keith U
authors.initials: KU
authors.lastname: Ingold
authors.affiliation: Department of Chemistry "G. Ciamician", University of Bologna , Bologna I-40126, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Valgimigli
authors.affiliation: Department of Chemistry, University of Ottawa , Ottawa, Ontario K1N 6N5, Canada.
authors.forename: Derek A
authors.initials: DA
authors.lastname: Pratt