Controlling product selectivity during the catalytic aerobic oxidation of phenols remains a significant challenge that hinders reaction development. This work provides a mechanistic picture of a Cu-catalyzed, aerobic functionalization of phenols that is selective for phenoxy-coupled ortho-quinones. We show that the immediate product of the reaction is a Cu(II)-semiquinone radical complex and reveal that ortho-oxygenation precedes oxidative coupling. This complex is the resting state of the Cu catalyst during turnover at room temperature. A mechanistic study of the formation of this complex at low temperatures demonstrates that the oxygenation pathway mimics the dinuclear Cu enzyme tyrosinase by involving a dinuclear side-on peroxodicopper(II) oxidant. Unlike the enzyme, however, the rate-limiting step of the ortho-oxygenation reaction is the self-assembly of the oxidant from Cu(I) and O2. We provide details for all steps in the cycle and demonstrate that turnover is contingent upon proton-transfer events that are mediated by a slight excess of ligand. Finally, our knowledge of the reaction mechanism can be leveraged to diversify the reaction outcome. Thus, uncoupled ortho-quinones are favored in polar, coordinating media, highlighting unusually high levels of chemoselectivity for a catalytic aerobic oxidation of a phenol.
Charge-assisted hydrogen bond-directed self-assembly of a zwitterionic quinonemonoimine was investigated at the liquid/solid interface using scanning tunnelling microscopy. Factors governing morphology, chirality and multilayer formation are discussed, presenting an important foundation for understanding the properties of a large family of related molecules with interesting potential in supramolecular design.
Bismuth
metallic nanoparticles have evoked considerable interest
in catalysis owing to their small size, high surface area-to-volume
ratio, and low toxicity. However, the need for toxic reductants and
organic solvents in their synthesis often limits their desirability
for application development. Here, we describe a green strategy to
synthesize bismuth nanodots via the redox reactions between bismuth
nitrate and d-glucose, in the presence of poly(vinylpyrrolidone)
in the basic aqueous phase. Both reagents play a crucial role in the
formation of monodisperse bismuth nanodots acting as mild reducing
and capping agents, respectively. We further demonstrate that the
catalytic activity of these dots via the successful reduction of the
environmental contaminant 4-nitrophenol to its useful 4-aminophenol
analogue requiring only 36 μg/mL nanocatalyst for 20 mM of the
substrate. Moreover, they can be recovered and recycled in multiple
reactions before the onset of an appreciable loss of catalytic activity.
The proposed facile synthetic route and inexpensive matrix materials
lead the way to access bismuth nanodots for both the fundamental study
of reactions and their industrial catalysis applications.
Substitution on the aromatic bridge of a nickel(II) salophen complex with electron-donating dimethylamino substituents creates a ligand with three stable, easily and reversibly accessible oxidation states. The one-electron-oxidized product is characterized as a nickel(II) radical complex with the radical bore by the central substituted aromatic ring, in contrast to other nickel(II) salen or salophen complexes that oxidize on the phenolate moieties. The doubly oxidized product, a singlet species, is best described as having an iminobenzoquinone bridge with a vinylogous distribution of bond lengths between the dimethylamino substituents. Protonation of the dimethylamino substituents inhibits these redox processes on the time scale of cyclovoltammetry, but electrolysis and chemical oxidation are consistent with deprotonation occurring concomitantly with electron transfer to yield the mono- and dioxidized species described above.
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