To further gain insight into the reaction mechanism, several control experiments were carried out (
Scheme 4). The 2-methyl-2-phenyloxirane
3a was transformed into the acetophenone
2a in 94% GC–MS yield (
Scheme 4a), suggesting that the phenyloxirane might be a key intermediate during the present oxidation reaction. Only a trace amount of oxidation product
2a was detected by GC–MS analysis in the presence of a free-radical inhibitor (BHT or TEMPO) under the optimal reaction conditions (
Scheme 4c). Both the results of radical trapping experiment and radical-clock experiment [(1-cyclopropylvinyl)benzene,
1v] indicated that the present reaction might proceed through a free-radical pathway. To clarify that a free-radical species is involved in the oxidation process, the electron paramagnetic resonance (EPR) experiments were performed. As shown in
Fig. 2a, when 2-diethoxyethane was exposed under oxygen atmosphere at 90 ℃ for 5 min, by using a trapping reagent of 5, 5-dimethyl-1-pyrroline
N-oxide (DMPO), we successfully observed a weak signal with four characteristic hyperfines of the DMPO adduct of the peroxyl radical (DMPO-OOH) (g = 2.002, A N = 1.46 mT, AH = 1.37 mT). Prolonged the heating time to 20 min, strong signals were observed, which thereby confirmed the formation of peroxyl radicals in the reaction (
Fig. 2b) [
13]. The investigation on the oxidants under the nitrogen atmosphere revealed that air was pivotal in the reaction system to ensure an efficient proceeding of the metal-free oxidation reaction (
Scheme 4c). A moderate yield of
2a was observed in the presence of 0.1 equiv. of 1, 2-diethoxyethane, whereas no oxidation product was detected by GC–MS analysis in the absence of 1, 2-diethoxyethane. Taken together, these experimental results manifested that the 1, 2-diethoxyethane not only served as a reaction medium but also acted as a catalyst in the developed oxidation (
Scheme 4d). Considering that this type of oxidation reactions with ether as the solvent may involve the
in situ generated ether peroxide, we attempted to detect this compound by GC–MS, but all efforts failed. It is probably due to the unstable of ether peroxide in high temperature. In addition, approximately 5% of 1, 2-diethoxyethylane was lost during the reaction (detected by GC–MS), which might be caused by system error.