Controlling the Catalytic Aerobic Oxidation of Phenols

Solar-Light-Driven Photocatalytic Oxidative Coupling of Phenol Derivatives over Bismuth-Based Porous Metal Halide Perovskites

The selective oxidative coupling of phenol derivatives, involving carbon-carbon (C-C) and carbon-oxygen (C-O) bond formation, has emerged as a critical approach in the synthesis of natural products. However, achieving precise control over the selectivity in coupling reactions of unsubstituted phenols utilizing solar light as the driving force remains a big challenge. In this study, we report a series of porous Cs3Bi2X9 (X=Cl, Br, I) photocatalysts with tailored band gaps and compositions engineered for efficient solar-light-driven oxidative phenol coupling. Notably, p-Cs3Bi2Br9 exhibited about 73 % selectivity for C-C coupling, displaying a high formation rate of 47.3 μmol gcat -1 h-1 under solar radiation. Furthermore, this approach enables control of the site-selectivity for phenol derivatives on Cs3Bi2X9, enhancing C-C coupling. The distinctive porous structure and appropriate band-edge positions of Cs3Bi2Br9 facilitated efficient charge separation, and surface interaction/activation of phenolic hydroxyl groups, resulting in the kinetically preferred formation of C-C over C-O bond. Mechanistic insights into the reaction pathway, supported by comprehensive control experiments, unveiled the crucial role of interfacial charge transfers and Lewis acid Bi sites in stabilizing phenolic intermediates, thereby directing the regioselectivity of diradical couplings and resulting in the formation of unsymmetrical biphenols.

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.

Oxygenation of Phenols with Water as the Oxygen Source and Oxoammonium Salt as the Oxidant

Aromatic C-H oxygenation is important in both industrial production and organic synthesis. Here we report a metal-free approach for phenol oxygenation with water as the oxygen source using oxoammonium salts as the renewable oxidant. Employing this protocol, various alkyl-substituted phenols were converted into benzoquinones in yields of 59-98%. On the basis of 18O-labeling and kinetic studies, the hydroxy-oxoammonium adduct was proposed to attack the aromatic ring similarly to electrophilic aromatic substitution. We suppose that the findings described here not only provide an efficient and highly selective protocol for aromatic C-H oxygenation but also may encourage further developments of possible transition-metal-free catalytic methods.

Ligand Hydrogenation during Hydroformylation Catalysis Detected by In Situ High-Pressure Infra-Red Spectroscopic Analysis of a Rhodium/Phospholene-Phosphite Catalyst

Phospholane-phosphites are known to show highly unusual selectivity towards branched aldehydes in the hydroformylation of terminal alkenes. This paper describes the synthesis of hitherto unknown unsaturated phospholene borane precursors and their conversion to the corresponding phospholene-phosphites. The relative stereochemistry of one of these ligands and its Pd complex was assigned with the aid of X-ray crystal structure determinations. These ligands were able to approach the level of selectivity observed for phospholane-phosphites in the rhodium-catalysed hydroformylation of propene. High-pressure infra-red (HPIR) spectroscopic monitoring of the catalyst formation revealed that whilst the catalysts showed good thermal stability with respect to fragmentation, the C=C bond in the phospholene moiety was slowly hydrogenated in the presence of rhodium and syngas. The ability of this spectroscopic tool to detect even subtle changes in structure, remotely from the carbonyl ligands, underlines the usefulness of HPIR spectroscopy in hydroformylation catalyst development.

O2 Activation by a Coordinated -NH- Function: Hydrogen Atom Transfer and Aromatic Ring Oxidation

Herein, we disclose a unique method of oxidation of a 1,4-naphthoquinone ring in air. We report that (1,4-naphthoquinone)-NH-N=C(OH)Ph (H3L) coordinated to octahedral ruthenium(II) and osmium(II) ions activates an 3O2 molecule spontaneously. Hydrogen atom transfer (HAT) from the -NH- function of H3L to 3O2 and subsequent (2e + 2H+) oxidation forming (1,3,4-trioxonaphthalen)=N-N=C(OH)Ph (HLOX) have been established. The H3L → HLOX transformation occurs via (3-hydroperoxy-1,4-naphthoquinone)=N-N=C(O-)Ph (HLOOH-) as an intermediate. The primary step is HAT generating H2L•- and hydroperoxide (OOH•) radicals. H2L•- is delocalized over the aromatic ring and incites coupling reactions via ortho carbon and produces coordinated HLOOH-. In solution, the homolytic cleavage of the peroxo bond leads to aromatic ring oxidation, affording LOX-. Ruthenium(II) and osmium(II) complexes of the types [MII(H2L-)(PPh3)2X], [MII(HLOOH-)(PPh3)2X], and trans-[MII(LOX-)(PPh3)2X] were successfully isolated in good yields. Notably, the cyclic voltammograms of all of the complexes exhibit reversible anodic waves due to MIII/MII redox couples. The rate constants of the [MII(H2L-)(PPh3)2X] → [MII(HLOOH-)(PPh3)2X] conversions determined by time-driven UV-vis spectroscopy in dry CH2Cl2, wet CH2Cl2, and D2O wet CH2Cl2 in air at 298 K follow the order k CH 2 Cl 2 -H 2 O > k CH 2 Cl 2 -D 2 O > k CH 2 Cl 2 . It is established that the rate constants are dependent on the 3O2 content of the solution but not on the concentration of the complex.

Building Porous Ni(Salen)-Based Catalysts from Waste Styrofoam via Autocatalytic Coupling Chemistry for Heterogeneous Oxidation with Molecular Oxygen

The development of robust and industrially viable catalysts from plastic waste is of great significance, and the facile construction of high performance heterogeneous catalyst systems for phenol-quinone conversions remains a grand challenge. Herein, a feasible strategy is demonstrated to reclaim Styrofoam into hierarchically porous nickel-salen-loaded hypercrosslinked polystyrene (PS@Ni-salen) catalysts with high activities through an unusual autocatalytic coupling route. The salen is immobilized onto PS chain by Friedel-Crafts alkylation of benzyl chloride derivatives, and the generated hydrogen chloride coordinately promotes the simultaneous crosslinking and bridge formation between aromatic rings via a Scholl coupling route, leading to hierarchically porous networks. After the metallization with Ni, the resultant networks exhibit high catalytic activity for the oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-1,4-benzoquinone under mild conditions (303 K, 1 bar of O2 ). This catalyst also demonstrates attractive recycling performance without an obvious loss of catalytic efficiency over five consecutive cycles. This methodology might provide a potential sustainable alternative to construct environmentally benign and cost-effective catalysts for specific organic transformation.

Thiol-Aldehyde Polycondensation for Bio-based Adaptable and Degradable Phenolic Polymers

Designing sustainable materials with tunable mechanical properties, intrinsic degradability, and recyclability from renewable biomass through a mild process has become vital in polymer science. Traditional phenolic resins are generally considered to be not degradable or recyclable. Here we report the design and synthesis of linear and network structured phenolic polymers using facile polycondensation between natural aldehyde-bearing phenolic compounds and polymercaptans. Linear phenolic products are amorphous with Tg between -9 °C and 12 °C. Cross-linked networks from vanillin and its di-aldehyde derivative exhibited excellent mechanical strength between 6-64 MPa. The connecting dithioacetals are associatively adaptable strong bonds and susceptible to degradation in oxidative conditions to regenerate vanillin. These results highlight the potential of biobased sustainable phenolic polymers with recyclability and selective degradation, as a complement to the traditional phenol-formaldehyde resins.

Direct conversion of lignin to functionalized diaryl ethers via oxidative cross-coupling

Efficient valorization of lignin, a sustainable source of functionalized aromatic products, would reduce dependence on fossil-derived feedstocks. Oxidative depolymerization is frequently applied to lignin to generate phenolic monomers. However, due to the instability of phenolic intermediates, repolymerization and dearylation reactions lead to low selectivity and product yields. Here, a highly efficient strategy to extract the aromatic monomers from lignin affording functionalized diaryl ethers using oxidative cross-coupling reactions is described, which overcomes the limitations of oxidative methods and affords high-value specialty chemicals. Reaction of phenylboronic acids with lignin converts the reactive phenolic intermediates into stable diaryl ether products in near-theoretical maximum yields (92% for beech lignin and 95% for poplar lignin based on the content of β-O-4 linkages). This strategy suppresses side reactions typically encountered in oxidative depolymerization reactions of lignin and provides a new approach for the direct transformation of lignin into valuable functionalized diaryl ethers, including key intermediates in pharmaceutical and natural product synthesis.

The Formation of a Unique 2D Isonicotinate Polymer Driven by Cu(II) Aerobic Oxidation

The isolation and structural characterization of a unique Cu(II) isonicotinate (ina) material with 4-acetylpyridine (4-acpy) is provided. The formation of [Cu(ina)₂(4-acpy)]_n (1) is triggered by the Cu(II) aerobic oxidation of 4-acpy using O₂. This gradual formation of ina led to its restrained incorporation and hindered the full displacement of 4-acpy. As a result, 1 is the first example of a 2D layer assembled by an ina ligand capped by a monodentate pyridine ligand. The Cu(II)-mediated aerobic oxidation with O₂ was previously demonstrated for aryl methyl ketones, but we extend the applicability of this methodology to heteroaromatic rings, which has not been tested so far. The formation of ina has been identified by ¹H NMR, thus demonstrating the feasible but strained formation of ina from 4-acpy in the mild conditions from which 1 was obtained.

Cu4Ti4-oxo clusters functionalized by in situ - generated 2,2'-biphenolate ligands from the oxidative coupling of phenols

Herein, we report a twisted cubic Cu(I)₄Ti(IV)₄-oxo cluster stabilized by in situ - formed 2,2'-biphenolate ligands from the oxidative coupling of phenols. The 2,2'-biphenolate-functionalized Cu(I)₄Ti(IV)₄O₄ cluster shows short Cu⋯C contacts and exhibits smaller HOMO-LUMO gaps than those of reported Ti(IV)₄O₄.

Atmosphere- and Solvent-Controlled Coupling and Acetylation of Phenols Induced by Visible Light

A tunable coupling or acetylation of phenol derivatives with diacetyl was enabled through the switch of the atmosphere and solvent induced by visible light under metal-free conditions. Symmetric and asymmetric diphenols or binaphthols were obtained under oxygen in water or 1,1,1,3,3,3-hexafluoroisopropanol, whereas phenol acetates were formed under argon in the presence of diacetyl and acetic acid. The possibility to control the chemo- and regioselectivities enriches the synthetic versatility of photoreactions.

Electron Donor-Acceptor (EDA) Complex between a Triarylamine and B(C6F5)3 for the Photocatalytic Dehydrogenative Cross-Coupling of Phenols

In this article, an electron donor-acceptor (EDA) complex between a triarylamine and B(C6F5)3 that catalyzes the dehydrogenative cross-coupling of phenols is described. We demonstrate, for the first time, that the use of both components of the radical ion pairs generated by the photoexcitation of the EDA complex as co-catalysts, and the triarylaminium radical cation (+·NAr3) successfully promotes dehydrogenative cross-coupling between electron-rich phenols and 2-naphthols to provide electron-rich biphenol motifs using molecular oxygen as a terminal oxidant.

Metal-Free Oxidative Annulation of Phenols and Amines: A General Synthesis of Benzoxazoles

The ubiquity of benzoxazoles in natural products, drugs, and functional materials has stimulated numerous efforts toward their synthesis; however, the developed methods rely on prefunctionalized substrates and lack generality. Under metal-free conditions, a highly general synthesis of benzoxazoles direct from abundant and easily available phenols and amines is developed via a modular phenol functionalization controlled by TEMPO. In the reaction, various phenols and primary amines with a broad range of functional groups are compatible, producing structurally and functionally diverse benzoxazoles (64 examples) without or with trace observation of the byproducts of phenol transformation with amines. The practical synthesis, especially for drug tafamidis, demonstrates decisive advantages in generality, selectivity, efficiency, and atom- and step-economies over traditional methods, even in the cases of low yields. Mechanistically, the radical adducts of TEMPO with ortho-cyclohexa-2,4-dien-1-one radicals rather than the well-recognized cyclohexa-3,5-diene-1,2-diones may serve as intermediates.

Tris(Imidazolyl) Dicopper(I) Complex and its Reactivity to Exert Catalytic Oxidation of Sterically Hindered Phenol Substrates via a [Cu2O]2+ Core

The Cu ion ligated with histidine residues is a common active site motif of various Cu-containing metalloenzymes exerting versatile catalytic oxidation reactions. Due to the scarce of structurally characterized biomimetic...

Ortho-selective C–H arylation of phenols with N-carboxyindoles under Brønsted acid- or Cu(i)-catalysis

Control over chemo- and regioselectivity is a critical issue in the heterobiaryl synthesis via C–H oxidative coupling. To address this challenge, a strategy to invert the normal polarity of indoles in the heterobiaryl coupling was developed. With N-carboxyindoles as umpoled indoles, an exclusively ortho-selective coupling with phenols has been realized, employing a Brønsted acid- or Cu(i)-catalyst (as low as 0.01 mol%). A range of phenols and N-carboxyindoles coupled with exceptional efficiency and selectivity at ambient temperature and the substrates bearing redox-active aryl halides (–Br and –I) smoothly coupled in an orthogonal manner. Notably, preliminary examples of atropselective heterobiaryl coupling have been demonstrated, based on a chiral disulfonimide or a Cu(i)/chiral bisphosphine catalytic system. The reaction was proposed to occur through S_N2′ substitution or a Cu(i)–Cu(iii) cycle, with Brønsted acid or Cu(i) catalysts, respectively.

Control over chemo- and regioselectivity is a critical issue in the heterobiaryl synthesis via C–H oxidative coupling. To address this challenge, a strategy to invert the normal polarity of indoles was developed.

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.

Hypoiodite-catalysed oxidative homocoupling of arenols and tandem oxidation/cross-coupling of hydroquinones with arenes

We report the hypoiodite-catalyzed oxidative C-C homocoupling of arenols to biarenols or biquinones using aqueous hydrogen peroxide as an oxidant. In addition, by combining hypoiodite catalysis and lipophilic Lewis acid-assisted Brønsted acid catalysis under aqueous conditions, we achieved a tandem oxidation/cross-coupling reaction of hydroquinones with electron-rich arenes. These results highlight the substantial scope of hypoiodite/acid co-catalysis for use in oxidative coupling reactions.

Organophotocatalytic Aerobic Oxygenation of Phenols in a Visible-Light Continuous-Flow Photoreactor

A mild photocatalytic phenol oxygenation enabled by a continuous-flow photoreactor using visible light and pressurized air is described herein. Products for wide-ranging applications, including the synthesis of vitamins, were obtained in high yields by precisely controlling principal process parameters. The reactor design permits low organophotocatalyst loadings to generate singlet oxygen. It is anticipated that the efficient aerobic phenol oxygenation to benzoquinones and p-quinols contributes to sustainable synthesis.

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.

Computational exploration of copper catalyzed vinylogous aerobic oxidation of unsaturated compounds

Selective oxidation is one of the most important and challenging transformations in both academic research and chemical industry. Recently, a highly selective and efficient way to synthesize biologically active γ-hydroxy-α,β-unsaturated molecules from Cu-catalyzed vinylogous aerobic oxidation of α,β- and β,γ-unsaturated compounds has been developed. However, the detailed reaction mechanism remains elusive. Herein, we report a density functional theory study on this Cu-catalyzed vinylogous aerobic oxidation of γ,γ-disubstituted α,β- and β,γ-unsaturated isomers. Our computational study unveils detailed mechanism for each elementary step, i.e. deprotonation, O2 activation, and reduction. Besides, the origin of regioselectivity, divergent reactivities of substrates as well as reducing agents, and the byproduct generation have also been investigated. Notably, the copper catalyst retains the + 2 oxidation state through the whole catalytic cycle and plays essential roles in multiple steps. These findings would provide hints on mechanistic studies and future development of transition metal-catalyzed aerobic oxidation reactions.

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.

Oxidative Photocatalytic Homo- and Cross-Coupling of Phenols: Nonenzymatic, Catalytic Method for Coupling Tyrosine

The oxidative photocatalytic method for phenol-phenol homo-coupling and cross-coupling is described and isolated yields of 16-97% are obtained. Measured oxidation potentials and computed nucleophilicity parameters support a mechanism of nucleophilic attack of one partner onto the oxidized neutral radical form of the other partner. Understanding of this model permitted development of cross-coupling reactions between nucleophilic phenols/arenes and easily oxidized phenols with high selectivity and efficiency. A highlight of this method is that one equivalent of each coupling partner is utilized. Building on these findings, a non-enzymatic, catalytic method for coupling tyrosine was also developed.

Mechanistic Insights into the ortho-Defluorination-Hydroxylation of 2-Halophenolates Promoted by a Bis(μ-oxo)dicopper(III) Complex

C-F bonds are one of the most inert functionalities. Nevertheless, some [Cu₂O₂]²⁺ species are able to defluorinate-hydroxylate ortho-fluorophenolates in a chemoselective manner over other ortho-halophenolates. Albeit it is known that such reactivity is promoted by an electrophilic attack of a [Cu₂O₂]²⁺ core over the arene ring, the crucial details of the mechanism that explain the chemo- and regioselectivity of the reaction remain unknown, and it has not being determined either if Cu^II₂(η²:η²-O₂) or Cu^III₂(μ-O)₂ species are responsible for the initial attack on the arene. Herein, we present a combined theoretical and experimental mechanistic study to unravel the origin of the chemoselectivity of the ortho-defluorination-hydroxylation of 2-halophenolates by the [Cu₂(O)₂(DBED)₂]²⁺ complex (DBED = N,N'-di-tert-butylethylenediamine). Our results show that the equilibria between (side-on)peroxo (P) and bis(μ-oxo) (O) isomers plays a key role in the mechanism, with the latter being the reactive species. Furthermore, on the basis of quantum-mechanical calculations, we were able to rationalize the chemoselective preference of the [Cu₂(O)₂(DBED)₂]²⁺ catalyst for the C-F activation over C-Cl and C-H activations.

Controllable synthesis of 2- and 3-aryl-benzomorpholines from 2-aminophenols and 4-vinylphenols

We present herein a method for the controllable synthesis of 3-aryl-benzomorpholine and 2-aryl-benzomorpholine cycloadducts via cross-coupling/annulation between electron-rich 2-aminophenols and 4-vinylphenols. Molecular oxygen was successfully used in the reaction as the terminal oxidant and the complete inversion of chemoselectivity was achieved by the adjustment of the solvents and bases at room temperature.

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.

One-Pot Multicomponent Reaction of Catechols, Ammonium Acetate, and Aldehydes for the Synthesis of Benzoxazole Derivatives Using the Fe(III)-Salen Complex

The Fe(III)-salen complex has been applied successfully as a catalyst for the novel, simple, efficient, and one-pot multicomponent synthesis of benzoxazole derivatives from catechols, ammonium acetate as the nitrogen source, and aldehydes (nontoxic and cheap alternatives of amines) for the first time. Using this procedure, a wide range of benzoxazoles was successfully synthesized in the presence of a catalyst in EtOH under mild conditions, and all products were obtained in excellent yields. To the best of our knowledge, this method is the first example of the multicomponent synthesis of benzoxazole derivatives using these starting materials. The notable features such as the use of air that is considered as a benign oxidant and EtOH as a green solvent, ease of product separation, readily available and inexpensive aldehydes, and mild conditions make our procedure more efficient and practical for organic synthesis. Moreover, the current protocol is successfully applied to synthesize desirable products on a large scale.

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.

Vanadium-Catalyzed Oxidative Intramolecular Coupling of Tethered Phenols: Formation of Phenol-Dienone Products

A mild and efficient method for the vanadium-catalyzed intramolecular coupling of tethered free phenols is described. The corresponding phenol-dienone products are prepared directly in good yields with low catalyst loadings. Electronically diverse tethered phenol precursors are well tolerated, and the catalytic method was effectively applied as the key step in syntheses of three natural products and a synthetically useful morphinan alkaloid precursor.

Mechanistic Insights into Rhenium-Catalyzed Regioselective C-Alkenylation of Phenols with Internal Alkynes

A (μ-aryloxo)rhenium complex was isolated and confirmed as a key precatalyst for rhenium-catalyzed ortho-alkenylation (C-alkenylation) of unprotected phenols with alkynes. The reaction exclusively provided ortho-alkenylphenols; the formation of para or multiply alkenylated phenols and hydrophenoxylation (O-alkenylation) products was not observed. Several mechanistic experiments excluded a classical Friedel-Crafts-type mechanism, leading to the proposed phenolic hydroxyl group assisted electrophilic alkenylation as the most plausible reaction mechanism. For this purpose, the use of rhenium, a metal between the early and late transition metals in the periodic table, was key for the activation of both the soft carbon-carbon triple bond of the alkyne and the hard oxygen atom of the phenol, at the same time. ortho-Selective alkenylation with allenes also provided the corresponding adducts with a substitution pattern different from that obtained by the addition reaction with alkynes.

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.