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

Site-Selective Radical Aromatic C-H Functionalization of Alloxazine and Flavin through Ground-State Single Electron Transfer

Flavins and their alloxazine isomers are key chemical scaffolds for bioinspired electron transfer strategies. Their properties can be fine-tuned by functional groups, which must be introduced at an early stage of the synthesis as their aromatic ring is inert towards post-functionalization. We show that the introduction of a remote metal-binding redox site on alloxazine and flavin activates their aromatic ring towards direct C-H functionalization. Mechanistic studies are consistent with a synthetic sequence involving ground-state single electron transfer (SET) with an electrophilic source followed by radical-radical coupling. This unprecedented reactivity opens new opportunities in molecular editing of flavins by direct aromatic post-functionalization and the utility of the method is demonstrated with the site-selective C6 functionalization of alloxazine and flavin with a CF3 group, Br or Cl, that can be further elaborated into OH and aryl for chemical diversification.

Generation and Reactivity of μ-1,2-Peroxo CuIICuII and Bis-μ-oxo CuIIICuIII Species and Catalytic Hydroxylation of Benzene to Phenol with Hydrogen Peroxide

Tetradentate-N4 ligands stabilize dinuclear {CuII(μ-1,2-peroxo)CuII} and {CuIII(μ-O)2CuIII} species, and CuII complexes of these ligands were reported to catalyze the oxidation of benzene with H2O2. Here, we report {CuII(μ-1,2-peroxo)CuII} and {CuIII(μ-O)2CuIII} intermediates of dinucleating bis(tetradentate-N4) ligands depending on the absence or presence of 6-methyl substituents on the terminal pyridine donors, respectively, generated either from {CuICuI} precursors with O2 or from {CuIICuII} precursors with H2O2 and NEt3. Both intermediates are not stable even at low temperatures, but they show no electrophilic HAT reactivity with DHA. Catalytic investigations on the hydroxylation of benzene with excess H2O2 between 30 and 50 °C indicate that both radical-based and {Cu2On}-based mechanisms depend strongly on the catalytic conditions. In the presence of a radical scavenger, TONs of ∼920/∼720 have been achieved without/with the 6-methyl group of the ligand. Although {CuII(μ-OH)CuII} reacts with excess H2O2 at -40 °C to {CuII(OOH)}2 species, these are only stable for seconds at 20 °C and cannot account for catalytic oxidations over a period of 24 h at 30-50 °C.

The bridge towards a more stable and active side-on-peroxido (Cu2II(µ-η2:η2-O2)) complex as a tyrosinase model system

A novel dinucleating bis(pyrazolyl)methane ligand was developed for tyrosinase model systems. After ligand synthesis, the corresponding Cu(I) complex was synthesized and upon oxygenation, formation of a µ-η2:η2 peroxido complex could be observed and monitored using UV/Vis-spectroscopy. Due to the high stability of this species even at room temperature, a molecular structure of the complex could be characterized via single-crystal XRD. Additional to its promising stability, the peroxido complex showed catalytic tyrosinase activity which was investigated via UV/Vis-spectroscopy. Products of the catalytic conversion could be isolated and characterized and the ligand could be successfully recycled after catalysis experiments. Furthermore, the peroxido complex was reduced by reductants with different reduction potentials. The characteristics of the electron transfer reactions were investigated with the help of the Marcus relation. The combination of the high stability and catalytic activity of the peroxido complex with the new dinucleating ligand, enables the shift of oxygenation reactions for selected substrates towards green chemistry, which is furthered by the efficient ligand recycling capability.

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

The CuI salts [Cu(CH3 CN)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 CuI salts towards monooxygenation of 2,4-di-tert-butylphenol (DTBP-H) were compared to those of [Cu(CH3 CN)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 CuII -semiquinone complexes as catalysts, indicating that formation of a CuII 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.

Formal dual C(sp2)–H cross-dehydrogenative C–O bond formation to construct highly functionalized diaryl ethers with O2

A formal dual C(sp2)–H cross-dehydrogenative C–O bond formation reaction between phenols and naphthylamine derivatives to construct diaryl ethers has been developed under mild conditions.

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.

Five Nitrogen Oxidation States from Nitro to Amine: Stabilization and Reactivity of a Metastable Arylhydroxylamine Complex

Redox noninnocent ligands enhance the reactivity of the metal they complex, a strategy used by metalloenzymes and in catalysis. Herein, we report a series of copper complexes with the same ligand framework, but with a pendant nitrogen group that spans five different redox states between nitro and amine. Of particular interest is the synthesis of a unprecedented copper(I)-arylhydroxylamine complex. While hydroxylamines typically disproportionate or decompose in the presence of transition metal ions, the reactivity of this metastable species is arrested by the presence of an intramolecular hydrogen bond. Two-electron oxidation yields a copper(II)-(arylnitrosyl radical) complex that can dissociate to a copper(I) species with uncoordinated arylnitroso. This combination of ligand redox noninnocence and hemilability provides opportunities in catalysis for two-electron chemistry via a one-electron copper(I/II) shuttle, as exemplified with an aerobic alcohol oxidation.

Total Synthesis of Pyrolaside B: Phenol Trimerization through Sequenced Oxidative C-C and C-O Coupling

A facile method to oxidatively trimerize phenols using a catalytic aerobic copper system is described. The mechanism of this transformation was probed, yielding insight that enabled cross-coupling trimerizations. With this method, the natural product pyrolaside B was synthesized for the first time. The key strategy used for this novel synthesis is the facile one-step construction of a spiroketal trimer intermediate, which can be selectively reduced to give the natural product framework without recourse to stepwise Ullmann- and Suzuki-type couplings. As a result, pyrolaside B can be obtained expeditiously in five steps and 16 % overall yield. Three other analogues were synthesized, thus highlighting the utility of the method, which provides new accessibility to this area of chemical space. A novel xanthene was also synthesized through controlled Lewis acid promoted rearrangement of a spiroketal trimer.

Copper catalysis with redox-active ligands

Copper catalysis finds applications in various synthetic fields by utilizing the ability of copper to sustain mono- and bielectronic elementary steps. Further to the development of well-defined copper complexes with classical ligands such as phosphines and N-heterocyclic carbenes, a new and fast-expanding area of research is exploring the possibility of a complementing metal-centered reactivity with electronic participation by the coordination sphere. To achieve this electronic flexibility, redox-active ligands can be used to engage in a fruitful “electronic dialogue” with the metal center, and provide additional venues for electron transfer. This review aims to present the latest results in the area of copper-based cooperative catalysis with redox-active ligands.

Modular bismacycles for the selective C-H arylation of phenols and naphthols

Given the important role played by 2-hydroxybiaryls in organic, medicinal and materials chemistry, concise methods for the synthesis of this common motif are extremely valuable. In seeking to extend the lexicon of synthetic chemists in this regard, we have developed an expedient and general strategy for the ortho-arylation of phenols and naphthols using readily available boronic acids. Our methodology relies on in situ generation of a uniquely reactive Bi(V) arylating agent from a bench-stable Bi(III) precursor via telescoped B-to-Bi transmetallation and oxidation. By exploiting reactivity that is orthogonal to conventional metal-catalysed manifolds, diverse aryl and heteroaryl partners can be rapidly coupled to phenols and naphthols under mild conditions. Following arylation, high-yielding recovery of the Bi(III) precursor allows for its efficient re-use in subsequent reactions. Mechanistic interrogation of each key step of the methodology informs its practical application and provides fundamental insight into the underexploited reactivity of organobismuth compounds.

Total Synthesis of (S,S)-Tetramethylmagnolamine via Aerobic Desymmetrization

We describe a concise synthesis of the pseudodimeric tetrahydroisoqunoline alkaloid (S,S)-tetramethylmagnolamine by a catalytic aerobic desymmetrization of phenols. Desymmetrization reactions increase molecular complexity with high levels of efficiency, but those that do so by aerobic oxidation are uncommon. Our conditions employ molecular oxygen as an oxygen atom transfer agent and a formal acceptor of hydrogen, enabling two mechanistically distinct aromatic C-H oxygenation reactions with high degrees of selectivity.

A Stereoselective Tyrosinase Model Compound Derived from an m-Xylyl-l-histidine Ligand

The aim of mimicking enzyme activity represents an important motivation for the development of new catalysts. A challenging objective is the development of chiral complexes for bioinspired enantioselective oxidation reactions. Herein, we report a new chiral dinuclear copper(II) complex based on a m-xylyl-bis(histidine) ligand (mXHI) as a biomimetic catalyst for tyrosinase and catechol oxidase. The new ligand improves a previous system also containing two tridentate N3 units derived from l-histidine that were connected by a short, rigid ethanediamine bridge. In mXHI the bridge is provided by the more extended m-xylyl moiety. The dicopper(II) complex [Cu₂(mXHI)]⁴⁺ was studied as a catalyst for stereoselective oxidations of enantiomeric couples of chiral catechols of biological interest (L/D-dopa, L/D-dopa methyl ester, and ( R/ S)-norepinephrine), showing excellent discrimination capability, particularly for the methyl esters of dopa enantiomers. The catechol oxidation was studied in acetate buffer as slightly acidic medium, and a role of acetate as bridging ligand between the two coppers, preorganizing the dinuclear center in a more catalytic efficient structure, could be established. The oxidation of β-naphthol and 3,5-ditertbutylphenol was studied as a model monophenolase reaction. The oxidation proceeds stoichiometrically, and the partial incorporation of ¹⁸O into β-naphthol when the reaction was performed using ¹⁸O₂ suggests the existence of two competitive reaction pathways, a genuine monooxygenase mechanism and a radical pathway. However, the more challenging reaction on derivatives of l-/d-tyrosine did not lead to the desired monooxygenase product but only to products of radical oxidation. Complex [Cu₂(mXHI)]⁴⁺ was also used for the catalytic sulfoxidation of thioanisole in the presence of hydroxylamine as cosubstrate, in a preliminary attempt to model the reaction of external monooxygenases. The reaction proceeds with 25 turnovers, but the enantiomeric excess of sulfoxide was modest.

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

Copper is one of the most abundant and less toxic transition metals. Nature takes advantage of the bioavailability and rich redox chemistry of Cu to carry out oxygenase and oxidase organic transformations using O₂ (or H₂O₂) as oxidant. Inspired by the reactivity of these Cu-dependent metalloenzymes, chemists have developed synthetic protocols to functionalize organic molecules under enviormentally benign conditions. Copper also promotes other transformations usually catalyzed by 4d and 5d transition metals (Pd, Pt, Rh, etc.) such as nitrene insertions or C-C and C-heteroatom coupling reactions. In this review, we summarized the most relevant research in which copper promotes or catalyzes the functionalization of organic molecules, including biological catalysis, bioinspired model systems, and organometallic reactivity. The reaction mechanisms by which these processes take place are discussed in detail.

Dual oxidase/oxygenase reactivity and resonance Raman spectra of {Cu3O2} moiety with perfluoro-t-butoxide ligands

A mechanism for the formation of O-donor trinuclear {Cu₃O₂} moiety is reported.

Selective Oxidation of Exogenous Substrates by a Bis-Cu(III) Bis-Oxide Complex: Mechanism and Scope

Cu(III)₂(μ-O)₂ bis-oxides (O) form spontaneously by direct oxygenation of nitrogen-chelated Cu(I) species and constitute a diverse class of versatile 2e^-/2H⁺ oxidants, but while these species have attracted attention as biomimetic models for dinuclear Cu enzymes, reactivity is typically limited to intramolecular ligand oxidation, and systems exhibiting synthetically useful reactivity with exogenous substrates are limited. O _tmpd (TMPD = N ¹ , N ¹ , N ³ , N ³ -tetramethylpropane-1,3-diamine) presents an exception, readily oxidizing a diverse array of exogenous substrates, including primary alcohols and amines selectively over their secondary counterparts in good yields. Mechanistic and DFT analyses suggest substrate oxidation proceeds through initial axial coordination, followed by rate limiting rotation to position the substrate in the Cu(III) equatorial plane, whereupon rapid deprotonation and oxidation by net hydride transfer occurs. Together, the results suggest the selectivity and broad substrate scope unique to O _tmpd are best attributed to the combination of ligand flexibility, limited steric demands, and ligand oxidative stability. In keeping with the absence of rate limiting C-H scission, O _tmpd exhibits a marked insensitivity to the strength of the substrate C^α-H bond, readily oxidizing benzyl alcohol and 1 octanol at near identical rates.

Fluorescent Bis(guanidine) Copper Complexes as Precursors for Hydroxylation Catalysis

Bis(guanidine) copper complexes are known for their ability to activate dioxygen. Unfortunately, until now, no bis(guanidine) copper-dioxygen adduct has been able to transfer oxygen to substrates. Using an aromatic backbone, fluorescence properties can be added to the copper(I) complex which renders them useful for later reaction monitoring. The novel bis(guanidine) ligand DMEG2tol stabilizes copper(I) and copper(II) complexes (characterized by single crystal X-ray diffraction, IR spectroscopy, and mass spectrometry) and, after oxygen activation, bis(µ-oxido) dicopper(III) complexes which have been characterized by low-temperature UV/Vis and Raman spectroscopy. These bis(guanidine) stabilized bis(µ-oxido) complexes are able to mediate tyrosinase-like hydroxylation activity as first examples of bis(guanidine) stabilized complexes. The experimental study is accompanied by density functional theory calculations which highlight the special role of the different guanidine donors.

Cu-Catalyzed Cross-Dehydrogenative ortho-Aminomethylation of Phenols

A highly selective Cu^II‐catalyzed cross‐dehydrogenative ortho‐aminomethylation of phenols with aniline derivatives is described. The corresponding C(sp²)−C(sp³) coupling products were obtained in moderate to excellent yields under mild reaction conditions and with a broad substrate scope. A radical mechanism is proposed.

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.

Record Broken: A Copper Peroxide Complex with Enhanced Stability and Faster Hydroxylation Catalysis

Tyrosinase model systems pinpoint pathways to translating Nature's synthetic abilities for useful synthetic catalysts. Mostly, they use N-donor ligands which mimic the histidine residues coordinating the two copper centres. Copper complexes with bis(pyrazolyl)methanes with pyridinyl or imidazolyl moieties are already reported as excellent tyrosinase models. Substitution of the pyridinyl donor results in the new ligand HC(3-tBuPz)₂ (4-CO₂ MePy) which stabilises a room-temperature stable μ-η² :η² -peroxide dicopper(II) species upon oxygenation. It reveals highly efficient catalytic activity as it hydroxylates 8-hydroxyquinoline in high yields (TONs of up to 20) and much faster than all other model systems (max. conversion within 7.5 min). Stoichiometric reactions with para-substituted sodium phenolates show saturation kinetics which are nearly linear for electron-rich substrates. The resulting Hammett correlation proves the electrophilic aromatic substitution mechanism. Furthermore, density functional theory (DFT) calculations elucidate the influence of the substituent at the pyridinyl donor: the carboxymethyl group adjusts the basicity and nucleophilicity without additional steric demand. This substitution opens up new pathways in reactivity tuning.

Second-Order Biomimicry: In Situ Oxidative Self-Processing Converts Copper(I)/Diamine Precursor into a Highly Active Aerobic Oxidation Catalyst

A homogeneous Cu-based catalyst system consisting of [Cu(MeCN)₄]PF₆, N,N'-di-tert-butylethylenediamine (DBED), and p-(N,N-dimethylamino)pyridine (DMAP) mediates efficient aerobic oxidation of alcohols. Mechanistic study of this reaction shows that the catalyst undergoes an in situ oxidative self-processing step, resulting in conversion of DBED into a nitroxyl that serves as an efficient cocatalyst for aerobic alcohol oxidation. Insights into this behavior are gained from kinetic studies, which reveal an induction period at the beginning of the reaction that correlates with the oxidative self-processing step, EPR spectroscopic analysis of the catalytic reaction mixture, which shows the buildup of the organic nitroxyl species during steady state turnover, and independent synthesis of oxygenated DBED derivatives, which are shown to serve as effective cocatalysts and eliminate the induction period in the reaction. The overall mechanism bears considerable resemblance to enzymatic reactivity. Most notable is the "oxygenase"-type self-processing step that mirrors generation of catalytic cofactors in enzymes via post-translational modification of amino acid side chains. This higher-order function within a synthetic catalyst system presents new opportunities for the discovery and development of biomimetic catalysts.

A Catalyst-Controlled Aerobic Coupling of ortho-Quinones and Phenols Applied to the Synthesis of Aryl Ethers

ortho-Quinones are underutilized six-carbon-atom building blocks. We herein describe an approach for controlling their reactivity with copper that gives rise to a catalytic aerobic cross-coupling with phenols. The resulting aryl ethers are generated in high yield across a broad substrate scope under mild conditions. This method represents a unique example where the covalent modification of an ortho-quinone is catalyzed by a transition metal, creating new opportunities for their utilization in synthesis.

Copper-catalyzed divergent oxidative pathways of 2-naphthol derivatives: ortho-naphthoquinones versus 2-BINOLs

Divergent reaction pathways of 2-naphthol derivatives to either ortho-naphthoquinones or BINOLs have been achieved using two different Cu(ii) catalyst systems.

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.