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

Selective aerobic oxidation of benzyl alcohol to benzaldehyde by a (bpy)Cu^I(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^I/II(IM)_a(IM)_b 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^I to Cu^II and, thus, helps to activate O₂ by electron transfer from Cu^I to O₂. The rates of Cu^I oxidation and Cu^II 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^I/Cu^II interplay leads to decreasing total benzaldehyde formation rates in the order R (I+ effect) > R (conjugated system) > R = H.