Spin trapping of sterically unhindered phenoxyl radicals with nitrosobenzene and nitrosodurene

Abstract

Unhindered pheaols were oxidized witb PbO, in the presence of nitrosobeazew and nitrosodmete. The p k o x y l radicals generated are trapped in their resonance form as orrkarboaeentred radicals. Depending on tbe substitution of the spin trap, two types of amimxyl radicals are formed. Stable diarylamiaoxyls are the t i d products of the spin-trapping reaction witb nitrdurene. The adduets of the ortho-carbobceatred pbenoxyl radicals witb nitrosobenzene are rapidly rearranged to the phenoxazise-lO-oxyl radicals, the only ndial prod~~~ts obsened. Some exceptiom to this rule are discussed. Radical products were characterized by their EPR parameters.

Spin Trapping of Radical Intermediates Generated by the Oxidation of Substituted 4‐Methylphenols

“…Instead of them, the EPR signal of hydroxyphenyl adduct 18 was detected. The EPR parameters of the latter (a N (NO) ¼ 0.980 mT, a H (1 H) ¼ 0.060 mT, a H (o,p) ¼ 0.261 mT, and a H (m) ¼ 0.085 mT) were similar to those of the species obtained by the spin trapping of the 3,5-dimethylphenoxy radical with nitrosobenzene [4]. Nevertheless, the PbO 2 oxidation of 2e in CHCl 3 in the presence of 3 revealed the simultaneous formation of small amounts of benzyl radical adduct 17e as deduced from the detailed analysis of the EPR side lines present besides the dominating EPR signal of radical adduct 18.…”

“…Although the generation of phenoxy radicals resulting from the abstraction of the phenolic H-atom is the dominating radical process during the oxidation, the tendency towards the abstraction of an H-atom from a Me group in 4-methyl-substituted phenols was also observed. The benzyl radicals formed were detected by a spin-trapping technique by using nitroso spin traps [4] [5]. The recombination products of benzyl radicals observed in the reaction mixture of the oxidized phenols also support this mechanism, although other theories interpreting their formation have been proposed [6].…”

“…Similarly to the above discussed phenols 1a -1c, the simultaneous generation of corresponding phenoxy and benzyl radicals is also expected in the course of the oxidation with PbO 2 of sterically unhindered phenols 2a -2e. Due to their instability caused by structural factors (unsubstituted or partially substituted ortho position), the phenoxy radical cannot be directly detected by the EPR method, and only indirect evidence by means of the spin-trapping technique is suitable [4]. Due to the tendency towards the abstraction of an H-atom from the 4-methyl substituent, the benzyl radicals also reacted with nitroso spin traps and the generated adducts mostly dominated in the EPR spectrum.…”

“…Consequently, the tendency towards its homolytic scission, leading in the case of phenols 1a and 1b to the formation of benzyl radicals, is partially reduced in phenol 1c. The EPR signal of nitroxide radicals can possibly be ascribed to adducts of generated phenoxy radicals with nitrosobenzene [4].…”

“…For this reason, 2,6-di(tert-butyl)-4R-phenols are frequently utilized as very efficient antioxidants [2]. Monohydric phenols with 2,6-dialkyl substituents containing an a-CH bond, as well as phenols with unsubstituted or partially substituted ortho-position (sterically unhindered phenols) provide on oxidation unstable phenoxy radicals, which can be detected only by using special EPR techniques, e.g., flow or spin-trapping methods [3] [4]. Although the generation of phenoxy radicals resulting from the abstraction of the phenolic H-atom is the dominating radical process during the oxidation, the tendency towards the abstraction of an H-atom from a Me group in 4-methyl-substituted phenols was also observed.…”