Oxygen-Centered Radicals Formed in the Reaction Mixtures Containing Chloroiron Tetraphenylporphyrin, Iodosylbenzene, and Ethanol

Abstract

Heme and nonheme high-valent FeIV═O can mediate reactions of olefin epoxidation, alkane hydroxylation, aromatic hydroxylation, S-oxidation, P-oxidation, N-dealkylation, alkylaromatic oxidation, and alcohol oxidation. Bromocycloheptane forms as a product in the reaction of high-valent FeIV═O with cycloheptane, suggesting that a cycloheptyl radical reacts with CCl3Br. However, the cycloheptyl radical has not yet been directly detected. To directly detect the radical intermediate in the reaction of the high-valent FeIV═O, we analyzed reaction mixtures containing chloroiron tetraphenylporphyrin, iodosylbenzene, ethanol, and α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) in 1,2-dichloroethane by an electron spin resonance (ESR) spin-trapping method. As a spin-trapping reagent, we used 4-POBN. Prominent ESR signals were observed in the reaction mixtures. To determine the structure of the radical, the reaction was performed using ethanol-1-13C (or ethanol-2-13C) instead of ethanol. ESR spectra with no additional hyperfine splitting were observed, indicating that the radical formed in complete reaction mixtures of the porphyrin π-cation-radical species (TPP)•+FeIV═O (TPP = 5,10,15,20-tetraphenyl-21H,23H-porphine) with ethanol has an unpaired electron at neither the α-carbon nor the β-carbon. When the reaction mixture containing ethanol-d6 instead of ethanol was analyzed using high-performance liquid chromatography-ESR-mass spectrometry, the ions m/z 240 (4-POBN/•OCH2CH3) shifted to m/z 245 (4-POBN/•OCD2CD3). Thus, the radical formed in the complete reaction mixture of (TPP)•+FeIV═O with ethanol has an unpaired electron at the oxygen atom in ethanol. We detected and identified the ethanol-derived oxygen-centered radicals in the reaction of (TPP)•+FeIV═O with ethanol for the first time in this study.