Synthesis, Thermodynamic and Kinetic Studies of Novel Diarylamine Antioxidants & Development of a Fluorescent Probe for Quantifying Hydroperoxides and Measuring H-Atom Transfer Kinetics With Peroxyl Radicals

Abstract

Diarylamines (Ar2NH) and phenols (ArOH) comprise the bulk of radical-trapping antioxidant (RTA) additives to petroleum-derived products, owing to their ability to slow hydrocarbon autoxidation through rate-controlling inhibition reactions. While much work has been done to optimize and understand structure-activity relationships of phenolic antioxidants, optimization of highly-reactive diarylamine antioxidants has been comparatively difficult due to their propensity to undergo one-electron oxidation with molecular oxygen. Recently, it was demonstrated that incorporation of nitrogen atoms into the aromatic ring of phenolic antioxidants greatly improves their stability to one-electron oxidation without compromising their antioxidant efficacy. Given these results, it was our supposition that this approach could be extended to the development of highly reactive diarylamine RTAs. Herein we describe the synthesis of a small library of pyridine- and pyrimidine-containing diarylamines and characterize their thermodynamic properties (standard potentials (E°) and N-H bond dissociation enthalpies) as well as their kinetic properties (rate constants for reactions with peroxyl and alkyl radicals) – demonstrating that the approach of N-atom incorporation is also very effective at increasing diarylamines’ stability to one-electron oxidation without compromising their antioxidant efficacy. In fact, the diarylamines described herein are among the best peroxyl-radical trapping antioxidants known, having inhibition rate constants ca. 200-fold greater than the current industry standard. Through a series of mechanistic studies (measuring Arrhenius parameters, kinetic solvent effects, deuterium kinetic isotope effects and transition state calculations) we have provided strong evidence that reactions between electron-rich diarylamines and peroxyl radicals occur by a proton-coupled electron transfer (PCET) mechanism. The reaction of diarylamines with peroxyl radicals at elevated temperatures (>160 °C) is highly relevant industrially, as this reflects the operating environment of lubricants in engine applications. At these temperatures, diarylamines are known to react catalytically as peroxyl trapping RTAs, although the mechanism has yet to be fully elucidated for this important chemistry. Current methods of studying high temperature oxidations suffer from time-consuming and/or air-sensitive analytical methods (e.g. iodometry, GC analysis). To enable rapid, accurate mechanistic studies at high temperature, we have improved the analytical component by developing a fluorescent dye that can be applied in an assay to determine hydroperoxide concentrations in solution in real-time.