Effects of Imidazole-Type Ligands in Cu^I/TEMPO-Mediated Aerobic Alcohol Oxidation

Abstract

Selective aerobic oxidation of benzyl alcohol to benzaldehyde by a (bpy)Cu(IM)/TEMPO catalyst (IM represents differently substituted imidazoles) has been studied by simultaneous operando electron paramagnetic resonance/UV-vis/attentuated total reflectance infrared spectroscopy in combination with cyclic voltammetry to explore the particular role of imidazole in terms of ligand and/or base as well as of its substitution pattern on the catalytic performance. For molar ratios of IM/Cu ≥ 2, a (bpy)Cu(IM)(IM) complex is formed, in which the Cu-N distances and/or angles for the two IM ligands a and b are different. The coordination of a second IM molecule boosts the oxidation of Cu to Cu and, thus, helps to activate O by electron transfer from Cu to O. The rates of Cu oxidation and Cu reduction and, thus, the rates of benzaldehyde formation depend on R of the R-N moiety in the IM ligand. Oxidation is fastest for R = H and alkyl, while reduction is slowest for R = H. The Cu/Cu interplay leads to decreasing total benzaldehyde formation rates in the order R (I+ effect) > R (conjugated system) > R = H.

Biomimetic Cu/Nitroxyl Catalyst Systems for Selective Alcohol Oxidation

“…NMI has a bifunctional role in the aerobic alcohol oxidation. Firstly, NMI is used to coordinate and stabilise the (bpy)Cu I -intermediate to afford the (bpy)(NMI)Cu I intermediate, XII, which is oxidised to (bpy)(NMI)Cu II O2 ·ˉ, XIII, through a SET to O2 [9,46,86]. The magnetic interaction between the paramagnetic Cu II and O2 ·ˉ has an influence on the EPR studies, and therefore the (bpy)(NMI)Cu II O2 ·ˉ intermediate might not be seen [9].…”

Multivariate Analysis of Coupled Operando EPR/XANES/EXAFS/UV–Vis/ATR‐IR Spectroscopy: A New Dimension for Mechanistic Studies of Catalytic Gas‐Liquid Phase Reactions

“…Very recently, such a kind of complex with direct bonding between the oxygen atom of TEMPO and the Cu II site has been postulated, yet without coordination of NMI to the Cu center . In light of the EXAFS results, we suggest that the fifth coordination site at Cu II is occupied by NMI, which is known to enhance the Cu I redox potential . This is also in line with our previous UV‐Vis and EPR results showing clearly that NMI coordinates to Cu II when it is present in solution .…”

“…We suppose that this non‐reducible Cu II may arise from partial oxidation of BzOH to BzOOH which reacts with the active [(bpy)(NMI)Cu II O 2 ] intermediate to a (bpy)(NMI)Cu II ‐OOBz site that cannot activate gaseous O 2 anymore, since transfer of an electron like in step 1 of Scheme is no longer possible (Scheme ). This assumption has been confirmed by an experiment in which 1 equiv of BzOOH was added to the reaction mixture containing the active Cu complex, which led to no conversion of BzOH to BA . The formation of BzOOH was detected by Gas Chromatography analysis of the product after the catalytic test but not by operando ATR‐IR, probably due to its low concentration.…”

Ultraschnelle Synthese multivalenter radikalischer Nanopartikel durch ringöffnende Metathesepolymerisations‐induzierte Selbstorganisation

“…Stabile Radikalverbindungen mit ungepaarten Elektronen, wie Nitroxide, werden seit langem in so unterschiedlichen Bereichen wie der organischen Synthese, der kontrollierten radikalischen Polymerisation, magnetischen Materialien, organischen Batterien und der Biomedizin eingesetzt . Der Einbau von Nitroxiden in polymere Materialien verbessert deren Anwendbarkeit durch die Kombination der chemischen und physikalischen Vielseitigkeit von Polymeren mit den einzigartigen Eigenschaften radikalischer Verbindungen .…”