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

A para- to meta-isomerization of phenols

Phenols and their derivatives are ubiquitous in nature and critically important industrial chemicals. Their properties are intimately linked to the relative substitution pattern of the aromatic ring, reflecting well-known electronic effects of the OH group. Because of these ortho-, para-directing effects, meta-substituted phenols have historically been more difficult to synthesize. Here we describe a procedure to transpose phenols that hinges on a regioselective diazotization of the corresponding ortho-quinone. The procedure affords the meta-substituted phenol directly from its more common and accessible para-substituted isomer, and demonstrates good chemoselectivity that enables its application in late-stage settings. By changing the electronic effect of the OH group and its trajectory of hydrogen bonding, our transposition can be used to diversify natural products and existing chemical libraries, and potentially shorten the length and cost of producing underrepresented arene isomers.

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.

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.

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.

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.

Influence of the Increasing Number of Organic Radicals on the Structural, Magnetic, and Electrochemical Properties of the Copper(II)-Dioxothiadiazole Family of Complexes

The preparation, structures, and electrochemical and magnetic properties supported by density functional theory (DFT) calculations of three new copper(II) compounds with [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide (td) and its radical anion (td·-) are reported: {[CuIICl(td)](μ-Cl)2[CuIICl(td)]} (1), which incorporates only neutral td ligands; [CuIICl(td·-)(td)]·2MeCN (2), which comprises one neutral td and one radical td·-; and PPN[CuIICl(td·-)2]·2DMA (3), where CuII ions are coordinated by two radical anions td·- (DMA, dimethylacetamide; PPN+, the bis(triphenylphosphine)iminium cation). All three compounds show interesting paramagnetic behavior with low-temperature features indicating significant antiferromagnetic coupling. The magnetic properties of 1 are dominated by CuII···CuII interactions (JCuCu) mediated through the Cl- bridges, while the magnetic properties of 2 and 3 are governed mainly by the td·-···td·- (Jtdtd) and CuII-td·- (JCutd) exchange interactions. The structure of 2 features only two major magnetic coupling pathways enabling the fitting of experimental data with Jtdtd = -36.0(5) cm-1 and JCutd = -12.6(2) cm-1 only. Compound 3 exhibits a complex network of magnetic contacts. Attempt to approximate its magnetic behavior using only a local magnetic contacts model resulted in Jtdtd = -5.6(1) cm-1 and two JCutd constants, -12.4(2) and -22.6(4) cm-1. The experimental fitting is critically compared with the results of broken symmetry density functional theory (BS DFT) calculations for inter- and intramolecular contacts. More consistent results were obtained with the M06 functional as opposed to popular B3LYP, which encountered problems reproducing some of the experimental intermolecular exchange interactions. Electrochemical measurements of 2 and 3 in MeCN showed three reversible nearly overlapping redox peaks appearing in a narrow potential range of -600 to -100 mV vs Fc/Fc+. Small differences between the redox events suggest that such compounds may be good candidates for new switchable materials, where the electron transfer between the metal and the ligand center is triggered by temperature, pressure, or light (valence tautomerism).

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.

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.

Cu(i)-O2 oxidation reactions in a fluorinated all-O-donor ligand environment

Investigation of Cu-O2 oxidation reactivity is important in biological and anthropogenic chemistry. Zeolites are one of the most promising Cu/O based oxidation catalysts for development of industrial-scale CH4 to CH3OH conversion. Their oxidation mechanisms are not well understood, however, highlighting the importance of the investigation of molecular Cu(i)-O2 reactivity with O-donor complexes. Herein, we give an overview of the synthesis, structural properties, and O2 reactivity of three different series of O-donor fluorinated Cu(i) alkoxides: K[Cu(OR)2], [(Ph3P)Cu(μ-OR)2Cu(PPh3)], and K[(R3P)Cu(pinF)], in which OR = fluorinated monodentate alkoxide ligands and pinF = perfluoropinacolate. This breadth allowed for the exploration of the influence of the denticity of the ligand, coordination number, the presence of phosphine, and KF/O interactions on their O2 reactivity. KF/O interactions were required to activate O2 in the monodentate-ligand-only family, whereas these connections did not affect O2 activation in the bidentate complexes, potentially due to the presence of phosphine. Both families formed trisanionic, trinuclear cores of the form {Cu3(μ3-O)2}3-. Intramolecular and intermolecular substrate oxidation were also explored and found to be influenced by the fluorinated ligand. Namely, {Cu3(μ3-O)2}3- from K[Cu(OR)2] could perform both monooxygenase reactivity and oxidase catalysis, whereas those from K[(R3P)Cu(pinF)] could only perform oxidase catalysis.

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.

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.

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.

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 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.