Aerobe Oxidation primärer Alkohole mit einem neuen einkernigen CuII-Radikal-Katalysator

Abstract

Primäre Alkohole wie Ethanol oder Benzylalkohol werden mit dem einkernigen Kupfer(II)‐Radikalkomplex 1 – einem funktionellen Modell des Metalloenzyms Galactose‐Oxidase – mit Luftsauerstoff bei 20°C spezifisch katalytisch oxidiert. Dabei entstehen die entsprechenden Aldehyde und H2O2 in ca. 60 % Ausbeute.

Ten Years of a Biomimetic Approach to the Copper(II) Radical Site of Galactose Oxidase

“…Mixing the tridentate ligand H 2 L 9 with copper(II) in the presence of triethylamine and air affords the EPR‐silent dinuclear Cu II ‐phenoxyl complex [Cu(L 9 · )] 2 , which exhibits absorption bands at 408 and 650 nm (Table 2). 44 The mononuclear Cu II ‐phenoxyl complex [Cu(L 10 · )(NEt 3 )] was obtained under similar conditions by replacing H 2 L 9 with H 3 L 10 and structurally characterized 45. The tridentate ligand is coordinated to a single copper(II) ion and the fourth coordination site is occupied by an NEt 3 molecule.…”

“…Several X‐ray crystal structures of transition metal complexes of ligands possessing iminosemiquinonate moieties have been reported 45,95. The first X‐ray crystal structure of coordinated phenoxyl radical was reported in 1996 by Wieghardt et al96 for a phenoxylchromium complex.…”

Structure, Spectroscopy, and Reactivity of a Mononuclear Copper Hydroxide Complex in Three Molecular Oxidation States

“…This differs from the reactivity of galactose oxidase model systems in which Cu complexes bound by redox-active ligands (L•Cu II ) typically perform the oxidation of alcohols (RCH 2 OH) to aldehydes (RCHO) via alkoxide coordination to the Cu center (L•Cu II OCH 2 R) followed by an intramolecular 1H + /2e − oxidation (L•Cu II OCH 2 R → LHCu I + RCHO). 25,61 Interestingly, the H• abstraction reactivity of the CuOH complexes reported is not triggered by the oxidation of the CuOH core, which maintains its physical oxidation state, but rather by oxidation and/or protonation of the ligand scaffold, a strategy that might also be utilized by some Cu-dependent monooxygenase enzymes. 5,62 Ongoing studies are focused on modifying our copper complexes by tuning the electronics of the redox-active backbone, altering the stereoelectronics of the H-bond donors, and utilizing various anions, which we believe will boost the oxidative power of these reactive M−O(H) cores and allow for monooxygenase-like 2e − functionalization of C−H substrates (H• abstraction followed by •OH rebound).…”

“…24 Upon addition of 1 equiv of Fc + , the EPR features of tBu CuOH O.S.2 disappeared to produce an EPR silent species, which could be due to the formation of an S = 0 [CuOH] 2+ core or the formation of a Cu-semiquinone-like species in which the unpaired electron of the [CuOH] + center is ferromagnetically coupled to the semiquinone ligand (S = 1) or antiferromagnetically coupled (S = 0). 25 An addition of 1 equiv of Fc + to the silent species tBu CuOH O.S.3 produced an EPR signal similar to tBu CuOH O.S.2 but distinct (g ⊥ = 2.05, g || = 2.24, A || = 508 MHz), which suggests that tBu CuOH O.S.4 is a [CuOH] + complex bound by the quinone form of the ligand. Magnetic susceptibilities were measured at room temperature using the Evans method for tBu CuOH O.S.2 (μ eff = 1.50 μ B ), tBu CuOH O.S.3 (μ eff = 0.00 μ B ), and tBu CuOH O.S.4 (μ eff = 1.81 μ B ).…”

Copper-Promoted Functionalization of Organic Molecules: from Biologically Relevant Cu/O2 Model Systems to Organometallic Transformations

“…In a related system, the same authors used the N,N-bis(2hydroxy-3,5-di-tert-butylphenol) ligand to synthesize a mononuclear Cu II -phenoxyl complex by oxidation of a mixture of the deprotonated ligand, Cu I and NEt 3 with O 2 (Figure 5B, i). 93 The resulting copper(II) complex was characterized by X-ray diffraction analysis and EPR spectroscopy (EPR silent). This mononuclear system catalyzed the selective oxidation of alcohols to the corresponding aldehyde products (no C−C coupling product) utlizing low catalyst loadings (0.02%) at room temperature and using O 2 as oxidant (Figure 5B, ii).…”