The Catalytic
Mechanism of Diarylamine Radical-Trapping
Antioxidants
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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. <i>N</i>,<i>N</i>-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 <i>N</i>-phenyl-β-naphthylamine, decompose
by an unprecedented concerted retro-carbonyl-ene reaction. The alkoxyamines
that decompose by the concerted process inhibit hexadecane autoxidations
at 160 °C to the same extent as the corresponding diarylamine,
whereas those alkoxyamines that decompose by the N–O homolysis/disproportionation
pathway are much less effective. This suggests that the competing
cage escape of the alkoxyl radicals following N–O homolysis
leads to significantly less effective regeneration of diarylamines
and implies that the catalytic efficiency of diarylamine antioxidants
is substrate-dependent. 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