Catalytic aerobic oxidation of phenols to ortho-quinones with air-stable copper precatalysts

Copper‐Catalyzed Monooxygenation of Phenols: Evidence for a Mononuclear Reaction Mechanism

The Cu^I salts [Cu(CH₃CN)₄]PF and [Cu(oDFB)₂]PF with the very weakly coordinating anion Al(OC(CF₃)₃)₄⁻ (PF) as well as [Cu(NEt₃)₂]PF comprising the unique, linear bis‐triethylamine complex [Cu(NEt₃)₂]⁺ were synthesized and examined as catalysts for the conversion of monophenols to o‐quinones. The activities of these Cu^I salts towards monooxygenation of 2,4‐di‐tert‐butylphenol (DTBP‐H) were compared to those of [Cu(CH₃CN)₄]X salts with “classic” anions (BF₄⁻, OTf⁻, PF₆⁻), revealing an anion effect on the activity of the catalyst and a ligand effect on the reaction rate. The reaction is drastically accelerated by employing Cu^II‐semiquinone complexes as catalysts, indicating that formation of a Cu^II complex precedes the actual catalytic cycle. This result and other experimental observations show that with these systems the oxygenation of monophenols does not follow a dinuclear, but a mononuclear pathway analogous to that of topaquinone cofactor biosynthesis in amine oxidase.

Influence of the amine donor on hybrid guanidine-stabilized Bis(μ-oxido) dicopper(III) complexes and their tyrosinase-like oxygenation activity towards polycyclic aromatic alcohols

The tyrosinase-like activity of hybrid guanidine-stabilized bis(μ-oxido) dicopper(III) complexes [Cu₂(μ-O)₂(L)₂](X)₂ (L = 2-{2-((Diethylamino)methyl)phenyl}-1,1,3,3-tetramethylguanidine (TMGbenzNEt₂, L2) and 2-{2-((Di-isopropylamino)methyl)phenyl}-1,1,3,3-tetramethylguanidine (TMGbenzNiPr₂, L3); X = PF₆^-, BF₄^-, CF₃SO₃^-) is described. New aromatic hybrid guanidine amine ligands were developed with varying amine donor function. Their copper(I) complexes were analyzed towards their ability to activate dioxygen in the presence of different weakly coordinating anions. The resulting bis(μ-oxido) species were characterized at low temperatures by UV/Vis and resonance Raman spectroscopy, cryo-ESI mass spectrometry and density functional theory calculations. Small structural changes in the ligand sphere were found to influence the characteristic ligand-to-metal charge transfer (LMCT) features of the bis(μ-oxido) species, correlating a redshift in the UV/Vis spectrum with weaker N-donor function of the ligand. DFT calculations elucidated the influence of the steric and electronic properties of the bis(μ-oxido) species leading to a higher twist of the Cu₂O₂ plane against the CuN₂ plane and a stretching of the Cu₂O₂ core. Despite their moderate stability at -100 °C, the bis(μ-oxido) complexes exhibited a remarkable activity in catalytic oxygenation reactions of polycyclic aromatic alcohols. Further the selectivity of the catalyst in the hydroxylation reactions of challenging phenolic substrates is not changed despite an increasing shield of the reactive bis(μ-oxido) core. The generated quinones were found to form exclusively bent phenazines, providing a promising strategy to access tailored phenazine derivatives.

Room temperature stable multitalent: highly reactive and versatile copper guanidine complexes in oxygenation reactions

Inspired by the efficiency of natural enzymes in organic transformation reactions, the development of synthetic catalysts for oxygenation and oxidation reactions under mild conditions still remains challenging. Tyrosinases serve as archetype when it comes to hydroxylation reactions involving molecular oxygen. We herein present new copper(I) guanidine halide complexes, capable of the activation of molecular oxygen at room temperature. The formation of the reactive bis(µ-oxido) dicopper(III) species and the influence of the anion are investigated by UV/Vis spectroscopy, mass spectrometry, and density functional theory. We highlight the catalytic hydroxylation activity towards diverse polycyclic aromatic alcohols under mild reaction conditions. The selective formation of reactive quinones provides a promising tool to design phenazine derivatives for medical applications.

Recent Advances in the Selective Oxidative Dearomatization of Phenols to o-Quinones and o-Quinols with Hypervalent Iodine Reagents

ortho-Quinones are valuable molecular frameworks with diverse applications across biology, materials, organic synthesis, catalysis, and coordination chemistry. Despite their broad utility, their synthesis remains challenging, in particular via the direct oxidation of readily accessible phenols, due to the need to affect regioselective ortho oxidation coupled with the sensitivity of the resulting o-quinone products. The perspective looks at the emergence of I(V) hypervalent iodine reagents as an effective class of oxidants for regioselective o-quinone synthesis. The application of these reagents in regioselective phenol oxidation to both o-quinones and o-quinols will be discussed, including a recent report from our laboratory on the first method for the oxidation of electron-deficient phenols using a novel nitrogen-ligated I(V) reagent. Also included are select examples of total syntheses utilizing this methodology as well as recent advancements in chiral I(V) reagent design for asymmetric phenol dearomatization.

Exceptional Substrate Diversity in Oxygenation Reactions Catalyzed by a Bis(μ-oxo) Copper Complex

The enzyme tyrosinase contains a reactive side-on peroxo dicopper(II) center as catalytically active species in C-H oxygenation reactions. The tyrosinase activity of the isomeric bis(μ-oxo) dicopper(III) form has been discussed controversially. The synthesis of bis(μ-oxo) dicopper(III) species [Cu2 (μ-O)2 (L1)2 ](X)2 ([O1](X)2 , X=PF6 - , BF4 - , OTf- , ClO4 - ), stabilized by the new hybrid guanidine ligand 2-{2-((dimethylamino)methyl)phenyl}-1,1,3,3-tetramethylguanidine (L1), and its characterization by UV/Vis, Raman, and XAS spectroscopy, as well as cryo-UHR-ESI mass spectrometry, is described. We highlight selective oxygenation of a plethora of phenolic substrates mediated by [O1](PF6 )2 , which results in mono- and bicyclic quinones and provides an attractive strategy for designing new phenazines. The selectivity is predicted by using the Fukui function, which is hereby introduced into tyrosinase model chemistry. Our bioinspired catalysis harnesses molecular dioxygen for organic transformations and achieves a substrate diversity reaching far beyond the scope of the enzyme.

Selectivity in the Aerobic Dearomatization of Phenols: Total Synthesis of Dehydronornuciferine by Chemo- and Regioselective Oxidation

We describe a selective aerobic oxidation of meta-biaryl phenols that enables rapid access to functionalized phenanthrenes. Aerobic oxidations attract interest due to their efficiency, but remain underutilized in complex molecule settings due to challenges of selectivity. We discuss these issues in the context of Cu catalysis, and highlight the advantages of confining oxygen activation and substrate oxidation to the catalyst's inner-coordination sphere. This gives rise to predictable selectivity that we use for a concise synthesis of the aporphine dehydronornuciferine.

Ligand and electronic effects on copper–arylnitroso self-assembly

The topology and degree of electron transfer in self-assembled redox reactions between copper(i) species and nitrosoarenes are controlled by ligand properties.

Decoding the Mechanism of Intramolecular Cu-Directed Hydroxylation of sp3 C-H Bonds

The use of copper in directed C-H oxidation has been relatively underexplored. In a seminal example, Schönecker showed that copper and O2 promoted the hydroxylation of steroid-containing ligands. Recently, Baran (J. Am. Chem. Soc. 2015, 137, 13776) improved the reaction conditions to oxidize similar substrates with excellent yields. In both reports, the involvement of Cu2O2 intermediates was suggested. In this collaborative article, we studied the hydroxylation mechanism in great detail, resulting in the overhaul of the previously accepted mechanism and the development of improved reaction conditions. Extensive experimental evidence (spectroscopic characterization, kinetic analysis, intermolecular reactivity, and radical trap experiments) is provided to support each of the elementary steps proposed and the hypothesis that a key mononuclear LCuII(OOR) intermediate undergoes homolytic O-O cleavage to generate reactive RO• species, which are responsible for key C-H hydroxylation within the solvent cage. These key findings allowed the oxidation protocol to be reformulated, leading to improvements of the reaction cost, practicability, and isolated yield.

Redox-promoted associative assembly of metal-organic materials

We develop an associative synthesis of metal-organic materials that combines solid-state metal oxidation and coordination-driven self-assembly into a one-step, waste-free transformation. The methodology hinges on the unique reactivity of ortho-quinones, which we introduce as versatile oxidants for mechanochemical synthesis. Our strategy opens a previously unexplored route to paramagnetic metal-organic materials from elementary metals.

A bio-inspired synthesis of oxindoles by catalytic aerobic dual C-H functionalization of phenols

We report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of oxygen.

Nitrogen-containing heterocycles are fundamentally important to the function of pharmaceuticals, agrochemicals and materials. Herein, we report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of molecular oxygen (O₂). Our method is inspired by the biosynthesis of melanin pigments (melanogenesis), but diverges from the biosynthetic polymerization. Mechanistic analysis reveals the involvement of Cu^II-semiquinone radical intermediates, which enable dehydrogenative carbon–heteroatom bond formation that avoids a catechol/quinone redox couple. This mitagates the deleterious polarity reversal that results from phenolic dearomatization, and enables a high-yielding phenolic C–H functionalization under catalytic aerobic conditions. Our work highlights the broad synthetic utility and efficiency of forming C–N bonds via a catalytic aerobic dearomatization of phenols, which is currently an underdeveloped transformation.

A Biomimetic Mechanism for the Copper-Catalyzed Aerobic Oxygenation of 4-tert-Butylphenol

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.