Nonselective Disproportionation of Isomeric Hydroxycyclohexadienyl Radicals to Phenols

Abstract

Comparison of the isomer ratios and yields of cresol, methoxyphenol, and nitrophenol, which were obtained by the aromatic hydroxylation with hydroxyl radical in the presence or absence of benzoquinones in anhydrous media, indicates that disproportionation of hydroxycyclohexadienyl radicals occurs nonselectively.

Regioselectivity of Hydroxyl Radical Reactions with Arenes in Nonaqueous Solutions

“…The measured reactivities and product distributions in all cases show little change over the temperature ranges studied, and exhibit modest variations with solvent, where such variation could be measured. In some cases, there are distinctions between observed regioselectivities and those previously reported in the literature. ,− The origin of these differences is not entirely clear, but we note that in previous product studies, the yields of formed products are often low, suggesting the possibility of secondary oxidation. Others involve photochemical conditions where it is possible that primary photoproducts may undergo secondary photolysis.…”

“… a Additional data for temperature ranges 318–348 K in hydrocarbons, and 308–328 K in acetonitrile may be found in the Supporting Information. b Methods as described in text unless otherwise indicated. c Range of mole ratio of substrate in benzene (mixture of solvents), concentration is cited for anisole, as it is diluted in benzene. d Product ratios determined by GC–MS unless otherwise indicated, and do not include reactivity such as ipso -substitution or hydrogen abstraction from side chains, see text. Regioisomer ratios were determined relative to the meta -isomer. e Calculated relative to benzene: branching ratio = (% product distribution × product ratio (substrate/benzene)) × (6/ n ), where n is the number of each type of addition site available on the substrate. f Photolysis of azohydroperoxide, ref . g Photolysis of azohydroperoxide, ref . h Ref , phot = photocatalytic dissociation of water over Pt/TiO 2. i Acetonitrile/benzene cosolvents used. j Calculated by Effective Carbon Number, see text and SI. k γ-irradiation ( 60 Co), ref . l Electrochemical generation (Fe II /O 2 , ref ). …”

Selective Ortho Hydroxylation of Nitrobenzene with Molecular Oxygen Catalyzed by the H5PV2Mo10O40 Polyoxometalate

“…Even rarer, in fact, apparently undocumented, is the selective hydroxylation of a deactivated arene substituted with electron-withdrawing moieties, such as nitrobenzene. Thus, nitrobenzene has been hydroxylated to a mixture of nitrophenol regioisomers using various hydroxy radical sources, including hydrogen peroxide . There is also an old report of nitrobenzene hydroxylation to a mixture of products under basic aerobic conditions, as well as a report on hydroxylation by anodic electrolysis …”