Synthesis, Kinetics, and Mechanism of Catalytically Active Aminic Radical-Trapping Antioxidants & Development of the Fluorometric and Spectrophotometric Tools Used in Their Analysis

Abstract

Amine and nitroxide based radical-trapping antioxidants (RTAs) have long been known to display remarkable efficacy as inhibitors of hydrocarbon autoxidation. Their unique ability to catalytically trap the chain-carrying peroxyl radicals responsible for oxidative degradation of organic materials has led to their widespread use in petroleum-derived materials. While a great deal of research has been done to understand and expand upon this reactivity, little improvement in the chemistry behind diarylamine and nitroxide RTAs has emerged. In recent years our group has established that heterocyclic analogues of phenolic and diarylaminic RTAs are more stable to one-electron oxidation than the equivalent phenyl derivatives. This has allowed substitution of these RTAs with strong electron donating groups without compromising their stability to oxidation, and has led to the development of some of the most effective RTAs ever reported – compounds which often have reactivities ca. 200-fold greater than the current industrial standards. Herein, we describe the development of novel fluorometric and spectrophotometric methods to measure the reactivities of these RTAs, which replace more traditional approaches that are often laborious and require highly specialised equipment. Co-autoxidations with the highly absorbent probes PBD-BODIPY and STY-BODIPY allow for rapid and convenient measurement of RTA activity under a wide variety of conditions by UV/Vis spectrophotometry. Similarly, the high temperature activity of these RTAs can be measured in heavy hydrocarbon autoxidations, where hydroperoxide formation is monitored through the use of a pro-fluorescent phosphine. The key step in Korcek’s proposed diarylamine catalytic cycle has been studied and found to proceed through different mechanisms depending on the structure of the intermediate N,N-diarylalkoxyamine. While unactivated alkoxyamines widely react through N-O homolysis/disproportionation to regenerate the diarylamine RTA, activation of either the aryl or alkyl fragments allows regeneration through a more efficient, pericyclic retro-carbonyl-ene (RCE) reaction. Additionally, the mechanism behind the high temperature RTA activity of dialkylnitroxides – key intermediates in the activity of hindered amine light stabilizers (HALS) – has been evaluated and found to be dependent on in situ formation of carboxylic acids. Upon protonation by these acids, dialkylnitroxides become potent RTAs capable of trapping oxygen-centered radicals. The oxoammonium ions arising from this reaction then oxidize alkyl radicals competitively with O2 addition to regenerate the nitroxide. Lastly, we have extended the strategy used for heterocyclic phenols and diarylamines to the development of highly reactive azaphenoxazine and azaphenothiazine RTAs. While synthesis of these compounds is complicated by the presence of a favorable smiles rearrangement, synthesis of the ‘correct’ isomers yields extremely potent RTAs, capable of trapping peroxyl radicals under diffusion control. Applying these compounds in both ambient and high temperature autoxidations reveals that they may be some of the most effective RTAs ever reported, outperforming even the most reactive of the heterocyclic diarylamines previously studied.