The Catalytic Mechanism of Diarylamine Radical-Trapping Antioxidants

TLDR: The results presented here have significant implications in the future design of antioxidant additives: diarylamines designed to yield intermediate alkoxyamines that undergo the retro-carbonyl-ene reaction are likely to be much more effective than existing compounds and will display catalytic radical-trapping activities at lower temperatures due to lower barriers to regeneration.

Abstract: Diarylamine radical-trapping antioxidants are important industrial additives, finding widespread use in petroleum-derived products. They are uniquely effective at elevated temperatures due to their ability to trap multiple radicals per molecule of diarylamine. Herein we report the results of computational and experimental studies designed to elucidate the mechanism of this remarkable activity. We find that the key step in the proposed catalytic cycle–decomposition of the alkoxyamine derived from capture of a substrate-derived alkyl radical with a diarylamine-derived nitroxide–proceeds by different mechanisms depending on the structure of both the substrate and the diarylamine. N,N-Diarylalkoxyamines derived from saturated substrates and diphenylamines decompose by N–O homolysis followed by disproportionation. Alternatively, those derived from unsaturated substrates and diphenylamines, or saturated substrates and N-phenyl-β-naphthylamine, decompose by an unprecedented concerted retro-carbonyl-ene reaction....