Catalytic Oxidative Cracking of Benzene Rings in Water

Abstract

Efficient degradation of harmful benzene rings in water is indispensable for achieving a clean water environment. We report herein unprecedented catalytic oxidative benzene cracking (OBC) in water using a ruthenium­(II)–aqua complex having an N-heterocyclic carbene ligand as a catalyst and a cerium­(IV) salt as a sacrificial oxidant under mild conditions. The OBC reactions produced carboxylic acids such as formic acid, which can be converted to dihydrogen directly from the OBC solution using a rhodium­(III) catalyst with adjustment of the solution pH to 3.3. The OBC reactions can be applied to monosubstituted benzene derivatives such as ethylbenzene, chlorobenzene, and benzoic acid. Initial rates of the OBC reactions showed a linear relationship in the Hammett plot with a negative slope, indicating the electrophilicity of a Ru­(III)–oxyl complex as the reactive species in the catalytic OBC reaction. Also, we discuss a plausible mechanism of the catalytic OBC reactions based on the kinetic analysis and the product stoichiometry for the OBC reaction of nonvolatile sodium m-xylene sulfonate. The addition of an electrophilic radical to the aromatic ring to form arene oxide/oxepin is proposed as the initial step of the OBC reaction.

Photocatalytic Oxygenation Reactions Using Water and Dioxygen

“…[47] Av ariety of high-valent metal-oxo complexes were produced by ET oxidation of metal complexes by one-electron oxidants, such as [Ru(bpy) 3 ] 3 + and cerium(IV) ammonium nitrate (CAN), with H 2 O, catalyzing oxygenation of substrates by using H 2 Oa sa no xygen source. [35][36][37][38][39][40][41][42][43][44][45][46][47] [Ru(bpy) 3 ] 3 + was used as ao neelectron oxidant to produce high-valent metal-oxo complexes, but it can be replaced by much milder oxidants such as [Co III (NH 3 ) 5 Cl] 2 + under photoirradiation. [48][49][50][51][52][53][54][55][56][57][58] Scheme2 shows the catalytic cycle of the photocatalytic oxygenation of watersoluble substrates (S) with [Co III (NH 3 ) 5 Cl] 2 + as an oxidanta nd [Ru(bpy) 3 ] 2 + as ap hotocatalyst and am anganese(III)-hydroxo porphyrin ([(Por)Mn(OH)]) as an oxygenation catalyst.…”

“…[24][25][26][27][28][29][30][31][32][33][34] In such cases, oxygen in the high-valent metal-oxo species comes from O 2 ,w hich is the oxygen source in oxygenated products. [35][36][37][38][39][40][41][42][43][44][45][46] If metal-oxo speciesa re produced by photochemical oxidation of metal complexes with H 2 Ob y O 2 ,p hotocatalytic oxygenation of substrates with metal-oxo speciesw ould occur using H 2 Oa sa no xygen source andO 2 as an oxidant, both of which are the greenest availables pecies for such purposes. [35][36][37][38][39][40][41][42][43][44][45][46] If metal-oxo speciesa re produced by photochemical oxidation of metal complexes with H 2 Ob y O 2 ,p hotocatalytic oxygenation of substrates with metal-oxo speciesw ould occur using H 2 Oa sa no xygen source andO 2 as an oxidant, both of which are the greenest availables pecies for such purposes.…”

“…[47] Such preference of cyclohexene over styrene in the catalytic oxygenation has also been observedu nder photocatalytic oxygenation of other alkenes by porphyrins. [35][36][37][38][39][40][41][42][43][44][45][46][47] [Ru(bpy) 3 ] 3 + was used as ao neelectron oxidant to produce high-valent metal-oxo complexes, but it can be replaced by much milder oxidants such as [Co III (NH 3 ) 5 Cl] 2 + under photoirradiation. [47] Ethylbenzene was also hydroxylated to yield 1-phenylethanol using [47] The oxygen source in the oxygenatedp roducts was confirmed by using 18 Oi sotopic labeling to be H 2 O.…”

“…[24][25][26][27][28][29][30][31][32][33][34] High-valent metal-oxo speciesc an also be generated by the oxidation of metal complexesw ith H 2 O, when the oxygen atom in highvalent metal-oxo speciesc omes from H 2 O, as does that in the oxygenated products. [35][36][37][38][39][40][41][42][43][44][45][46] If metal-oxo speciesa re produced by photochemical oxidation of metal complexes with H 2 Ob y O 2 ,p hotocatalytic oxygenation of substrates with metal-oxo speciesw ould occur using H 2 Oa sa no xygen source andO 2 as an oxidant, both of which are the greenest availables pecies for such purposes.…”

Photoinduced Generation of Superoxidants for the Oxidation of Substrates with High C−H Bond Dissociation Energies

“…However, neither the direct conversion of methane with O 2 to generate methanol nor the direct conversion of benzene with O 2 to produce phenol under mild conditions has been well established. Metal‐oxo species and other strong oxidants are often involved as reactive intermediates responsible for the catalytic oxidation of substrates including methane and benzene …”