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

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.

Biomimetic Total Synthesis of Bimagnolignan: A Natural Anti-Breast Cancer Agent

A short scalable biomimetic route to bioactive natural product bimagnolignan (1) was accomplished. Compound 1 was successfully prepared through a three-step metal-free synthesis from honokiol (2). Alternatively, 1 was also synthesized by biomimetic transformations that mimic tyrosinase in four steps. The key reactions feature a regioselective acetylation, a highly efficient C(sp2)-H oxidation, a cascade aerobic oxidative cyclization/coupling, and a Cu-catalyzed direct oxidative coupling. In addition, cell-based assays validate that 1 is a promising natural lead for HER2-positive breast cancer treatment.

Recent advances in oxidative phenol coupling for the total synthesis of natural products

Covering: 2008 to 2023This review will describe oxidative phenol coupling as applied in the total synthesis of natural products. This review covers catalytic and electrochemical methods with a brief comparison to stoichiometric and enzymatic systems assessing their practicality, atom economy, and other measures. Natural products forged by C-C and C-O oxidative phenol couplings as well as from alkenyl phenol couplings will be addressed. Additionally, exploration into catalytic oxidative coupling of phenols and other related species (carbazoles, indoles, aryl ethers, etc.) will be surveyed. Future directions of this particular area of research will also be assessed.

Indole-5,6-quinones display hallmark properties of eumelanin

Melanins are ubiquitous biopolymers produced from phenols and catechols by oxidation. They provide photoprotection, pigmentation and redox activity to most life forms, and inspire synthetic materials with desirable optical, electronic and mechanical properties. The chemical structures of melanins remain elusive, however, creating uncertainty about their roles, and preventing the design of synthetic mimics with tailored properties. Indole-5,6-quinone (IQ) has been implicated as a biosynthetic intermediate and structural subunit of mammalian eumelanin pigments, but its instability has prevented its isolation and unambiguous characterization. Here we use steric shielding to stabilize IQ and show that 'blocked' derivatives exhibit eumelanin's characteristic ultrafast nonradiative decay and its ability to absorb light from the ultraviolet to the near-infrared. These new compounds are also redox-active and a source of paramagnetism, emulating eumelanin's unique electronic properties, which include persistent radicals. Blocked IQs are atomistically precise and tailorable molecules that can offer a bottom-up understanding of emergent properties in eumelanin and have the potential to advance the rational design of melanin-inspired materials.

A Bioinspired Synthesis of 1,4-Benzothiazines by Selective Addition of Sulfur Nucleophiles to ortho-Quinones

Herein, we report a bioinspired approach to the synthesis of 1,4-benzothiazines by drawing inspiration from the biosynthesis of pheomelanin pigments (pheomelanogenesis). In this context, general conditions for the regioselective coupling reaction between ortho-quinones and thiols were developed. The mild conditions proved amenable to a wide scope of both thiol and ortho-quinone coupling partners while simultaneously suppressing redox-exchange. The utility of this methodology was demonstrated by a synthesis of 1,4-benzothiazines, following a biomimetic, oxidative cyclization.

Catalytic Oxidative Coupling of Phenols and Related Compounds

Phenols and their derivatives are the elementary building blocks for several classes of complex molecules that play essential roles in biological systems. Nature has devised methods to selectively couple phenolic compounds, and many efforts have been undertaken by chemists to mimic such coupling processes. A range of mechanisms can be involved and with well-studied catalysts, reaction outcomes in phenol-phenol oxidative coupling reactions can be predicted with a good level of fidelity. However, reactions with catalysts that have not been studied or that do not behave similarly to known catalysts can be hard to predict and control. This Perspective provides an overview of catalytic methods for the oxidative coupling of phenols, focusing on the last 10 years, and summarizes current challenges.

Recent advances in catalytic oxidative reactions of phenols and naphthalenols

This critical review aims to provide an overview of oxidative phenol and naphthalenol transformations in nature and synthetic chemistry.

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.

Iron-catalyzed arene C-H hydroxylation

[Figure: see text].

Small-Scale Phenomena in Reactive Bubbly Flows: Experiments, Numerical Modeling, and Applications

Improving the yield and selectivity of chemical reactions is one of the challenging tasks in paving the way for a more sustainable and climate-friendly economy. For the industrially highly relevant gas-liquid reactions, this can be achieved by tailoring the timescales of mixing to the requirements of the reaction. Although this has long been known for idealized reactors and time- and space-averaged processes, considerable progress has been made recently on the influence of local mixing processes. This progress has become possible through joint research between chemists, mathematicians, and engineers. We present the reaction systems with adjustable kinetics that have been developed, which are easy to handle and analyze. We show examples of how the selectivity of competitive-consecutive reactions can be controlled via local bubble wake structures. This is demonstrated for Taylor bubbles and bubbly flows under technical conditions. Highly resolvednumerical simulations confirm the importance of the bubble wake structure for the performance of a particular chemical reaction and indicate tremendous potential for future process improvements.

Aerobic Copper-Catalyzed Intramolecular Cascade Oxidative Isomerization/[4+4] Cyclization of 2,2'-Disubstituted Stilbenes

An aerobic copper-catalyzed cascade oxidative isomerization/[4+4] cyclization of 2,2'-disubstituted stilbenes is described. Under the mild CuCl/DBED/air catalytic system, various 5,10-heteroatom-containing tetrahydroindeno[2,1-a]indenes were efficiently prepared through the difunctionalizations of alkenes in a highly atom economic manner. Mechanistic investigations suggested the bicyclic product was likely formed through a sequence of rapid single-electron oxidation/[4+4] cyclization from 2,2'-disubstituted stilbene. The antarafacial manner of the thermally allowed [4+4] cyclization was further proven by series of control experiments and density functional theory calculations. Our findings provide an important addition to the aerobic copper-catalyzed oxidative cyclization.

Modification of N-terminal α-amine of proteins via biomimetic ortho-quinone-mediated oxidation

Naturally abundant quinones are important molecules, which play essential roles in various biological processes due to their reduction potential. In contrast to their universality, the investigation of reactions between quinones and proteins remains sparse. Herein, we report the development of a convenient strategy to protein modification via a biomimetic quinone-mediated oxidation at the N-terminus. By exploiting unique reactivity of an ortho-quinone reagent, the α-amine of protein N-terminus is oxidized to generate aldo or keto handle for orthogonal conjugation. The applications have been demonstrated using a range of proteins, including myoglobin, ubiquitin and small ubiquitin-related modifier 2 (SUMO2). The effect of this method is further highlighted via the preparation of a series of 17 macrophage inflammatory protein 1β (MIP-1β) analogs, followed by preliminary anti-HIV activity and cell viability assays, respectively. This method offers an efficient and complementary approach to existing strategies for N-terminal modification of proteins.

Methods for selective modification of the N-terminus of proteins are of high interest, but mostly require specific amino acid residues. Here, the authors report a selective and fast method for N-terminal modification of proteins based on quinone-mediated oxidation of the alpha-amine to aldehyde or ketone, and apply it to diverse proteins.

Total Synthesis of (S)-Cularine via Nucleophilic Substitution on a Catechol

Catechols are part of many essential chemicals and are valuable, typically nucleophilic intermediates used in synthesis. Here we describe an unexpected transformation in which they play the role of the electrophile in a formal nucleophilic aromatic substitution. We made this discovery while studying a seven-membered dioxepin ring formation during a synthesis of the benzyltetrahydroisoquinoline (S)-cularine. We suggest a chain mechanism for this new transformation that is triggered by molecular oxygen and that propagates an electrophilic ortho-quinone.

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.

Regioselective Oxidative Arylation of Fluorophenols

A metal free and highly regioselective oxidative arylation reaction of fluorophenols is described. The relative position of the fluoride leaving group (i.e., ortho or para) controls the 1,2 or 1,4 nature of the arylated quinone product, lending versatility and generality to this oxidative, defluorinative, arylation concept.

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.

Dearomatization of Electron-Deficient Phenols to ortho-Quinones: Bidentate Nitrogen-Ligated Iodine(V) Reagents

Despite their broad utility, the synthesis of ortho-quinones remains a significant challenge, in particular, access to electron-deficient derivatives remains an unsolved problem. Reported here is the first general method for the synthesis of electron-deficient ortho-quinones by direct oxidation of phenols. The reaction is enabled by a novel bidentate nitrogen-ligated iodine(V) reagent, a previously unexplored class of compounds which we have termed Bi(N)-HVIs. The reaction is extremely general and proceeds with excellent regioselectivity for the ortho over para isomer. Functionalization of the ortho-quinone products was examined, resulting in a facile one-pot synthesis of catechols, as well as the incorporation of a variety of heteroatom nucleophiles. This method represents the first synthetic application of Bi(N)-HVIs and demonstrates their potential as a platform for the further development of highly reactive, but also highly tunable, I(V) reagents.

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.

Directed Hydroxylation of sp2 and sp3 C-H Bonds Using Stoichiometric Amounts of Cu and H2O2

The use of copper for C-H bond functionalization, compared to other metals, is relatively unexplored. Herein, we report a synthetic protocol for the regioselective hydroxylation of sp² and sp³ C-H bonds using a directing group, stoichiometric amounts of Cu and H₂O₂. A wide array of aromatic ketones and aldehydes are oxidized in the carbonyl γ-position with remarkable yields. We also expanded this methodology to hydroxylate the β-position of alkylic ketones. Spectroscopic characterization, kinetics, and density functional theory calculations point toward the involvement of a mononuclear LCu^II(OOH) species, which oxidizes the aromatic sp² C-H bonds via a concerted heterolytic O-O bond cleavage with concomitant electrophilic attack on the arene system.

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.

Chemoselective Borane-Catalyzed Hydroarylation of 1,3-Dienes with Phenols

A B(C₆ F₅ )₃ -catalyzed hydroarylation of a series of 1,3-dienes with various phenols has been established through a combination of theoretical and experimental investigations, affording structurally diverse ortho-allyl phenols. DFT calculations show that the reaction proceeds through a borane-promoted protonation/Friedel-Crafts pathway involving a π-complex of a carbocation-anion contact ion pair. This protocol features simple and mild reaction conditions, broad functional-group tolerance, and low catalyst loading. The obtained ortho-allyl phenols could be further converted into flavan derivatives using B(C₆ F₅ )₃ with good cis diastereoselectivity. Furthermore, this transformation was applied in the late-stage modification of pharmaceutical compounds.

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.

GO–Cu7S4 catalyzed ortho-aminomethylation of phenol derivatives with N,N-dimethylbenzylamines: site-selective oxidative CDC

An efficient ortho-selective C–H aminomethylation of phenols and naphthols with N,N-dimethylbenzylamines using a GO–Cu₇S₄ nanocatalyst is reported.

Asymmetric Oxidative Coupling of Phenols and Hydroxycarbazoles

The first examples of asymmetric oxidative coupling of simple phenols and 2-hydroxycarbazoles are outlined. Generation of a more vanadium catalyst by ligand design and by addition of an exogenous Brønsted or Lewis acid was found to be key to coupling the more oxidatively resistant phenols. The resultant vanadium complex is both more Lewis acidic and more strongly oxidizing. Good to excellent levels of enantioselectivity could be obtained, and simple trituration readily provided the products with ≥95% ee.

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.

A chlorine-free protocol for processing germanium

Replacing molecular chlorine and hydrochloric acid with less energy- and risk-intensive reagents would markedly improve the environmental impact of metal manufacturing at a time when demand for metals is rapidly increasing. We describe a recyclable quinone/catechol redox platform that provides an innovative replacement for elemental chlorine and hydrochloric acid in the conversion of either germanium metal or germanium dioxide to a germanium tetrachloride substitute. Germanium is classified as a "critical" element based on its high dispersion in the environment, growing demand, and lack of suitable substitutes. Our approach replaces the oxidizing capacity of chlorine with molecular oxygen and replaces germanium tetrachloride with an air- and moisture-stable Ge(IV)-catecholate that is kinetically competent for conversion to high-purity germanes.

Chemoselective oxidation of aryl organoboron systems enabled by boronic acid-selective phase transfer

We report the direct chemoselective Brown-type oxidation of aryl organoboron systems containing two oxidizable boron groups. Basic biphasic reaction conditions enable selective formation and phase transfer of a boronic acid trihydroxyboronate in the presence of boronic acid pinacol (BPin) esters, while avoiding speciation equilibria. Spectroscopic investigations validate a base-promoted phase-selective discrimination of organoboron species. This phenomenon is general across a broad range of organoboron compounds and can also be used to invert conventional protecting group strategies, enabling chemoselective oxidation of BMIDA species over normally more reactive BPin substrates. We also demonstrate the selective oxidation of diboronic acid systems with chemoselectivity predictable a priori. The utility of this method is exemplified through the development of a chemoselective oxidative nucleophile coupling.