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.