Inhibition of hydrocarbon autoxidation by nitroxide-catalyzed cross-dismutation of hydroperoxyl and alkylperoxyl radicals

Abstract

Nitroxides inhibit the autoxidation of unsaturated hydrocarbon substrates by utilizing small amounts of hydroperoxyl radicals formed in a tunneling-enhanced 1,4-HAT/elimination sequence from substrate-derived α-alkoxyalkylperoxyl radicals.

Kinetics of oxidation of benzyl alcohols with molecular oxygen catalyzed by N‐hydroxyphthalimide: Role of hydroperoxyl radicals

“…It reacts with an analogue of α‐tocopherol (TOH) and regenerates TOH by a fast H‐atom transfer: TO • + HOO • → TOH + O 2 . The nitroxides catalyze the cross‐dismutation of hydroperoxyl and alkylperoxyl radicals to yield a O 2 and a hydroperoxide, thereby halting the autoxidation chain reaction . Participation of HOO • in the chains termination explains the inhibition by aromatic nitroxide of the oxidation reaction .…”

“…30 The nitroxides catalyze the cross-dismutation of hydroperoxyl and alkylperoxyl radicals to yield a O 2 and a hydroperoxide, thereby halting the autoxidation chain reaction. 31 Participation of HOO • in the chains termination explains the inhibition by aromatic nitroxide of the oxidation reaction. 32 Extremely effective reaction HOO • with TEMPO • (2,2,6,6-tetramethylpiperidine-1-oxyl) breaks down the chain of oxidation, and measured rate constants for reactions HOO • radicals in the inhibitory cycle are very high 33 : It should be noted that by the value of ΔH it is possible to predict a greater reactivity of PINO • than the ТЕМРО • in the reaction of chain breaks with HOO • which can significantly decrease the oxidation rates.…”

The antioxidant activity of polysulfides: it's radical!

“…These experiments were carried out in a manner similar to those which we have previously reported 23,39. Briefly, a 3.5 mL cuvette was loaded with hexadecene (2.0 mL) and PhCl (440 μL) and placed in a preheated thermostatted sample holder of a spectrophotometer set to 37 °C or 100 °C.…”

Temperature-dependence of radical-trapping activity of phenoxazine, phenothiazine and their aza-analogues clarifies the way forward for new antioxidant design

“…kinh and n) using the PBD-BODIPY co-autoxidation approach. 1-Hexadecene, a low volatility oxidizable substrate in which kPBD-BODIPY has been determined (8283 M -1 s -1 ), 7,30 was used in the co-autoxidation (Figure 4A-B). From the initial rates, the RTA reactivity of the PNX and PTZ derivatives follow the same trends at 100°C as those observed at ambient temperature: 2-and 4-azaPNX ( inh PhCl = 2.5 × 10 7 and 2.0 × 10 7 M -1 s -1 , respectively) inhibit the autoxidation similarly to PNX (kinh = 2.5 × 10 7 M -1 s -1 ) whereas the 1-and 3-isomers were slower ( inh PhCl = 3.4 × 10 6 and 7.9 × 10 6 M -1 s -1 , respectively) (Figure 4C).…”

“…Alternatively, the diarylnitroxide may catalyse the cross-dismutation of alkylperoxyl and hydroperoxyl radicals (Figure 1C); the latter arising from addition reactions in autoxidations of unsaturated substrates. 7 The sulfur-and oxygen-bridged diarylamines phenothiazine (PTZ) and phenoxazine (PNX) are among the most reactive RTAs known, with rate constants for reactions with peroxyl radicals of 7.6  10 6 and 3.910 7 M -1 s -1 , respectively, at 37C -roughly 40-and 200-fold greater than ADPAs, respectively (∼210 5 M -1 s -1 under the same conditions). [8][9][10] However, PTZ, PNX and their derivatives are not commonly used to protect hydrocarbon products from autoxidation.…”