Non-Tertiary Alkyl Substituents Enhance High-Temperature Radical Trapping by Phenothiazine and Phenoxazine Antioxidants

Radical-trapping antioxidants (RTAs) are an indispensable class of additive used to preserve hydrocarbon materials from oxidative degradation. Materials that are regularly subjected to elevated temperatures where autoxidation is self-initiated (i.e., >120 °C) require high concentrations of RTA for protection. Not only is this costly, but it can negatively impact material performance. Herein we show that inhibition of the autoxidation of a model hydrocarbon (n-hexadecane) by phenothiazine (PTZ) at ≥160 °C can be greatly enhanced by the incorporation of either 1° or 2° alkyl substituents in the 3- and/or 7-positions of the scaffold. Structure–reactivity studies, product analyses and computations suggest that this results from hydrogen atom transfer (HAT) from the benzylic carbon of these alkyl substituents in the PTZ-derived aminyl radical intermediate. The resultant iminoquinone methide can then undergo further radical-trapping reactions, depending on the nature of the alkyl substituent. Similar structure–reactivity relationships are observed for the phenoxazine (PNX) scaffold. These results not only have significant implications for the design and development of new high-temperature RTA technology, but also for understanding aminic RTA activity at elevated temperatures. Specifically, they suggest that a stoichiometric model better accounts for the RTA activity of aromatic amines in saturated hydrocarbons than the widely accepted catalytic model.