PdCl2 and N-hydroxyphthalimide co-catalyzed C(sp2)-H hydroxylation by dioxygen activation

A Porphyrin-Faced Zn8L6 Cage for Selective Oxidation of C(sp3)-H Bonds and Sulfides

Catalytic oxidation of benzyl C-H bonds and sulfides from fuel oils stands as an attractive proposition in the quest for clean energy, yet their simultaneous oxidation with a singular, economically friendly catalyst is not well established. In this work, the combination of a cobalt(II) porphyrin ligand with 2-pyridinecarboxaldehyde and ZnII yielded a Zn8L6 cage (Co cube). The three-dimensional conjugated structure effectively enhances energy transfer efficiency, enabling the Co cube to show a good ability to activate oxygen under light conditions for photooxidation. Moreover, this catalytic system demonstrates high selectivity for the photocatalytic oxidation of C(sp3)-H bonds and sulfides, employing the Co cube as a single component catalyst, molecular oxygen as the oxidant, and activating oxygen into 1O2 under mild reaction conditions. This provides significant insights for organic synthesis and future design of photocatalysts with complex molecular components.

Chemoselective C-H Hydroxylation and Borylation of N-Phenylbenzamides using BBr3

A novel metal-free chemoselective C-H hydroxylation and borylation of N-phenylbenzamides using BBr3 is described. The protocol generates the corresponding phenols and arylboronic esters in moderate to excellent yields under mild conditions with brilliant chemoselectivity. Additionally, this strategy can be realized in one pot, and several potential bioactive derivatives can be synthesized efficiently. Density functional theory calculations certify that the preferred pathway for this metal-free C-H hydroxylation process is the formation of a five-membered boracycle.

Hydroxylation of Substituted Anilides with Metallaphotocatalysis

We report the combination of organo-photocatalysis with transition metal (TM) catalysis for directed ortho-hydroxylation of substituted anilides for the synthesis of α-aminophenol derivatives under mild conditions. The developed metallaphotocatalysis utilizes N-pivaloyl as a directing group and phenyl iodine(III) bis(trifluoroacetate) (PIFA) in the combination of the 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) photocatalyst and [RuCl2(p-cymene)]2 TM catalyst under visible-light irradiation at room temperature. The hydroxylation reaction works well for a wide range of substrates containing electron-withdrawing substituents and could be applied to late-stage functionalization and ortho-hydroxyl metabolite generation for drug compounds-containing anilides with electron-withdrawing substituents in a single mild reaction.

Synthesis of 2-(Pyridin-2-yl)phenols and 2-(Pyridin-2-yl)anilines

Herein, we report a new synthetic strategy for 2-(pyridin-2-yl)phenols and 2-(pyridin-2-yl)anilines catalyzed by a Pd/C-ethylene system. The starting materials, 2-(pyridin-2-yl)cyclohexan-1-ones, can be easily prepared by the reaction of substituted pyridine N-oxide and cyclohexanones. The most useful feature of this method is that both 2-(pyridin-2-yl)phenols and 2-(pyridin-2-yl)anilines are easily synthesized independently using the same compound as a starting material, simply by adding or not adding a nitrogen source.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Optimizing the Synthetic Potential of O2: Implications of Overpotential in Homogeneous Aerobic Oxidation Catalysis

Molecular oxygen is the quintessential oxidant for organic chemical synthesis, but many challenges continue to limit its utility and breadth of applications. Extensive historical research has focused on overcoming kinetic challenges presented by the ground-state triplet electronic structure of O2 and the various reactivity and selectivity challenges associated with reactive oxygen species derived from O2 reduction. This Perspective will analyze thermodynamic principles underlying catalytic aerobic oxidation reactions, borrowing concepts from the study of the oxygen reduction reaction (ORR) in fuel cells. This analysis is especially important for "oxidase"-type liquid-phase catalytic aerobic oxidation reactions, which proceed by a mechanism that couples two sequential redox half-reactions: (1) substrate oxidation and (2) oxygen reduction, typically affording H2O2 or H2O. The catalysts for these reactions feature redox potentials that lie between the potentials associated with the substrate oxidation and oxygen reduction reactions, and changes in the catalyst potential lead to variations in effective overpotentials for the two half reactions. Catalysts that operate at low ORR overpotential retain a more thermodynamic driving force for the substrate oxidation step, enabling O2 to be used in more challenging oxidations. While catalysts that operate at high ORR overpotential have less driving force available for substrate oxidation, they often exhibit different or improved chemoselectivity relative to the high-potential catalysts. The concepts are elaborated in a series of case studies to highlight their implications for chemical synthesis. Examples include comparisons of (a) NOx/oxoammonium and Cu/nitroxyl catalysts, (b) high-potential quinones and amine oxidase biomimetic quinones, and (c) Pd aerobic oxidation catalysts with or without NOx cocatalysts. In addition, we show how the reductive activation of O2 provides a means to access potentials not accessible with conventional oxidase-type mechanisms. Overall, this analysis highlights the central role of catalyst overpotential in guiding the development of aerobic oxidation reactions.

Recent Advances in the Synthesis of Borinic Acid Derivatives

Borinic acids [R2B(OH)] and their chelate derivatives are a subclass of organoborane compounds used in cross-coupling reactions, catalysis, medicinal chemistry, polymer or optoelectronics materials. In this paper, we review the recent advances in the synthesis of diarylborinic acids and their four-coordinated analogs. The main strategies to build up borinic acids rely either on the addition of organometallic reagents to boranes (B(OR)3, BX3, aminoborane, arylboronic esters) or the reaction of triarylboranes with a ligand (diol, amino alcohol, etc.). After general practical considerations of borinic acids, an overview of the main synthetic methods, their scope and limitations is provided. We also discuss some mechanistic aspects.

Rhodium(III)-Catalyzed C-H/O2 Dual Activation and Macrocyclization: Synthesis and Evaluation of Pyrido[2,1-a]isoindole Grafted Macrocyclic Inhibitors for Influenza H1N1

The development of environment-friendly, step economic couplings to generate structurally diverse macrocyclic compounds is highly desirable but poses a marked challenge. Inspired by the C-H oxidation mechanism of cytochromes P450, an unprecedented and practical Rh^III -catalyzed acylmethylation macrocyclization via C-H/O₂ dual activation has been developed by us. The process of macrocyclization is facilitated by a synergic coordination from pyridine and ester group. Interestingly, the reaction mode derives from a three-component coupling which differs from established olefination and alkylation paths. Density functional theory (DFT) calculations and control experiments revealed the mechanism of this unique C-H/O₂ dual activation. The newly achieved acylmethylation macrocyclic products and their derivatives showed a potent anti-H1N1 bioactivity, which may provide an opportunity for the discovery of novel anti-H1N1 macrocyclic leading compounds.

Development of lucigenin-N-hydroxyphthalimide chemiluminescence system and its application to sensitive detection of Co2

N-hydroxyphthalimide (NHPI) is an efficient organic catalyst and an important chemical raw material which can be used as an intermediate in organic synthesis of drugs and pesticides. In this study, NHPI has been used as a coreactant of lucigenin chemiluminescence (CL) for the first time. The CL of the developed system is significantly enhanced in the presence of Co²⁺. Therefore, we developed a novel lucigenin-NHPI CL method coupled with flow injection analysis for the sensitive, precise, and selective determination of Co²⁺. The linear range of this method is 1-1000 nM, and the detection limit is 67 pM (S/N = 3). In addition, this method has a good selectivity for Co²⁺. It has been applied to the detection of Co²⁺ in lake water, and the standard recovery rate is 95.9-103.2%, indicating that the method is feasible.

Ligand-Enabled C-H Hydroxylation with Aqueous H2O2 at Room Temperature

With the large number of Pd(II)-catalyzed C-H activation reactions of native substrates developed in the past decade, the development of catalysts to enable the use of green oxidants under safe and practical conditions has become an increasingly important challenge. Notably, the compatibility of Pd(II) catalysts with sustainable aqueous H2O2 has been a long-standing challenge in catalysis including Wacker-type oxidations. We report herein a bifunctional bidentate carboxyl-pyridone (CarboxPyridone) ligand that enables room-temperature Pd-catalyzed C-H hydroxylation of a broad range of benzoic and phenylacetic acids with an industry-compatible oxidant, aqueous hydrogen peroxide (35% H2O2). The scalability of this methodology is demonstrated by a 1000 mmol scale reaction of ibuprofen (206 g) using only a 1 mol % Pd catalyst loading. The utility of this protocol is further illustrated through derivatization of the products and synthesis of polyfluorinated natural product coumestan and pterocarpene from phenol intermediates prepared using this methodology.

Intermetallic Copper-Based Electride Catalyst with High Activity for C-H Oxidation and Cycloaddition of CO2 into Epoxides

Inorganic electrides have been proved to be efficient hosts for incorporating transition metals, which can effectively act as active sites giving an outstanding catalytic performance. Here, it is demonstrated that a reusable and recyclable (for more than 7 times) copper-based intermetallic electride catalyst (LaCu0.67 Si1.33 ), in which the Cu sites activated by anionic electrons with low-work function are uniformly dispersed in the lattice framework, shows vast potential for the selective C-H oxidation of industrially important hydrocarbons and cycloaddition of CO2 with epoxide. This leads to the production of value-added cyclic carbonates under mild reaction conditions. Importantly, the LaCu0.67 Si1.33 catalyst enables much higher turnover frequencies for the C-H oxidation (up to 25 276 h-1 ) and cycloaddition of CO2 into epoxide (up to 800 000 h-1 ), thus exceeding most nonnoble as well as noble metal catalysts. Density functional theory investigations have revealed that the LaCu0.67 Si1.33 catalyst is involved in the conversion of N-hydroxyphthalimide (NHPI) into the phthalimido-N-oxyl (PINO), which then triggers selective abstraction of an H atom from ethylbenzene for the generation of a radical susceptible to further oxygenation in the presence of O2 .

Biomimetic catalytic aerobic oxidation of C-sp(3)-H bonds under mild conditions using galactose oxidase model compound CuIIL

Developing highly efficient catalytic protocols for C-sp(3)-H bond aerobic oxidation under mild conditions is a long-desired goal of chemists. Inspired by nature, a biomimetic approach for the aerobic oxidation of C-sp(3)-H by galactose oxidase model compound CuIIL and NHPI (N-hydroxyphthalimide) was developed. The CuIIL-NHPI system exhibited excellent performance in the oxidation of C-sp(3)-H bonds to ketones, especially for light alkanes. The biomimetic catalytic protocol had a broad substrate scope. Mechanistic studies revealed that the CuI-radical intermediate species generated from the intramolecular redox process of CuIILH2 was critical for O2 activation. Kinetic experiments showed that the activation of NHPI was the rate-determining step. Furthermore, activation of NHPI in the CuIIL-NHPI system was demonstrated by time-resolved EPR results. The persistent PINO (phthalimide-N-oxyl) radical mechanism for the aerobic oxidation of C-sp(3)-H bond was demonstrated.

Metal-Free Boron-Mediated ortho-C-H Hydroxylation of N-Benzyl-3,4,5-tribromopyrazoles

A novel route has been reported for C-H hydroxylation of benzyl compounds directed by a 3,4,5-tribromopyrazole auxiliary via boronation/oxidation using BBr₃ and NaBO₃·4H₂O. The strategy exhibits outstanding site selectivity and affords the corresponding phenols in moderate to excellent yields under metal-free conditions. Besides, this protocol can be achieved in one pot, which is highly promising as a practical method for use in a multistep organic synthetic process.

Deconstructive Cycloaromatization Strategy toward N,O-Bidentate Ligands from Indolizines and Cyclopropenones

The innovative construction of novel N,O-bidentate ligands represents a long-standing challenge for chemists. Here, we report an unprecedented approach for the construction of N,O-bidentate derivatives via the merging of ring deconstruction with cycloaromatization of indolizines and cyclopropenones. Without any catalysts, our method can deliver a series of polyaryl 2-(pyridin-2-yl)phenols in excellent yields. In addition, N,O-bidentate organic BF₂ complexes can also be constructed via this one-pot protocol.

Highly crystalline K-intercalated Se/C: An easily accessible mesoporous material catalyzing the epoxidation of β-ionone

Highly crystalline K-intercalated Se/C was fabricated by calcining the commercially available methylselenized glucose with KBr being followed by the subsequent water washing. The corrosion of water to KBr led to...

Directed evolution of cytochrome P450DA hydroxylase activity for stereoselective biohydroxylation

A colorimetric high throughput screening method was developed based on a dual-enzyme cascade and used for the directed evolution of cytochrome P450 hydroxylase activity.

Decarboxylative Hydroxylation of Benzoic Acids

Herein, we report the first decarboxylative hydroxylation to synthesize phenols from benzoic acids at 35 °C via photoinduced ligand-to-metal charge transfer (LMCT)-enabled radical decarboxylative carbometalation. The aromatic decarboxylative hydroxylation is synthetically promising due to its mild conditions, broad substrate scope, and late-stage applications.

Ruthenium Catalyzed Intramolecular C-X (X=C, N, O, S) Bond Formation via C-H Functionalization: An Overview

Ruthenium catalyzed C-H activation is well known for its high tolerance towards the functional group and broad applicability in organic synthesis and molecular sciences, with significant applications in pharmaceutical industries, material sciences, and polymer industry. In the last few decades, enormous progress has been observed with ruthenium-catalyzed C-H activation chemistry. Notably, the vast majority of the C-H functionalization known in the literature are intermolecular, although the intramolecular variant provides fascinating new structural facet starting from the simple molecular scaffolds. Intramolecular C-H functionalization is atom economical and step efficient, results in less formation of undesired products which is easy to purify. This has created a lot of interest in organic chemistry in developing new synthetic strategies for such functionalization. The focus of this review is to present the relatively unexplored intramolecular functionalization of C-H bonds into C-X (X=C, N, O, S) bonds utilizing versatile ruthenium catalysts, their scope, and brief mechanistic discussion.

Molecular oxygen-mediated oxygenation reactions involving radicals

Molecular oxygen as a green, non-toxic and inexpensive oxidant has displayed lots of advantages compared with other oxidants towards more selective, sustainable, and environmentally benign organic transformations. The oxygenation reactions which employ molecular oxygen or ambient air as both an oxidant and an oxygen source provide an efficient route to the synthesis of oxygen-containing compounds, and have been demonstrated in practical applications such as pharmaceutical synthesis and late-stage functionalization of complex molecules. This review article introduces the recent advances of radical processes in molecular oxygen-mediated oxygenation reactions. Reaction scopes, limitations and mechanisms are discussed based on reaction types and catalytic systems. Conclusions and perspectives are also given in the end.

Chemiluminescence-promoted oxidation of alkyl enol ethers by NHPI under mild conditions and in the dark

The hydroperoxidation of alkyl enol ethers using N-hydroxyphthalimide and molecular oxygen occurred in the absence of catalyst, initiator, or light. The reaction proceeds through a radical mechanism that is initiated by N-hydroxyphthalimide-promoted autoxidation of the enol ether substrate. The resulting dioxetane products decompose in a chemiluminescent reaction that allows for photochemical activation of N-hydroxyphthalimide in the absence of other light sources.

Palladium-Catalyzed Regiospecific peri- and ortho-C-H Oxygenations of Polyaromatic Rings Mediated by Tunable Directing Groups

An efficient divergent approach of Pd-catalyzed C-H oxygenation of polyaromatic rings is described. Reversible directing groups enable regiospecific peri- and ortho-oxygenation to readily access a wide array of polyaromatic phenols without pre- and postmanipulation of directing groups. The systematic mechanistic investigation, including deuterium-labeling experiments, palladacycle trapping, and DFT calculations, reveals that the tunable ligand-assisted C-H bond cleavage played a crucial role during the reaction process.

Silver(I)-Catalyzed Enyne Cyclization/Aromatization of Alkyne-Tethered Cyclohexadienones to Access Meta-Substituted Phenols

Herein we report a highly regioselective silver(I)-catalyzed intramolecular annulation of alkyne-tethered cyclohexadienones to access meta-substituted phenols with enone functionality, which are difficult to synthesize from conventional methods. The reaction proceeds via intramolecular 1,6-enyne cyclization followed by aromatization and subsequent oxetene ring rearrangement. This strategy has also been compatible with a wide range of C-tethered cyclohexadienones to afford indanes in high yields. The unique functionality of products allows further transformations to expand the diversity.

Insights into Ruthenium(II/IV)-Catalyzed Distal C-H Oxygenation by Weak Coordination

C-H hydroxylation of aryl acetamides and alkyl phenylacetyl esters was accomplished via challenging distal weak O-coordination by versatile ruthenium(II/IV) catalysis. The ruthenium(II)-catalyzed C-H oxygenation of aryl acetamides proceeded through C-H activation, ruthenium(II/IV) oxidation and reductive elimination, thus providing step-economical access to valuable phenols. The p-cymene-ruthenium(II/IV) manifold was established by detailed experimental and DFT-computational studies.

CuBr/NHPI co-catalyzed aerobic oxidative [3 + 2] cycloaddition-aromatization to access 5,6-dihydro-pyrrolo[2,1-a]isoquinolines

An efficient and enviromentally friendly CuBr/NHPI co-catalyzed aerobic oxidative [3 + 2] cycloaddition-aromatization cascade was realized with N-substituted tetrahydroisoquinolines and electron-deficient olefins. Under the mild conditions, the reaction proceeded smoothly and displayed excellent functional group tolerance, affording 5,6-dihydro-pyrrolo[2,1-a]isoquinolines in good to high yields. This protocol exhibits a broad substrate scope to both N-alkyl tetrahydroisoquinolines and dipolarophile substrates.

Evolution of High-Valent Nickela-Electrocatalyzed C-H Activation: From Cross(-Electrophile)-Couplings to Electrooxidative C-H Transformations

C-H activation has emerged as one of the most efficient tools for the formation of carbon-carbon and carbon-heteroatom bonds, avoiding the use of prefunctionalized materials. In spite of tremendous progress in the field, stoichiometric quantities of toxic and/or costly chemical redox reagents, such as silver(I) or copper(II) salts, are largely required for oxidative C-H activations. Recently, electrosynthesis has experienced a remarkable renaissance that enables the use of storable, safe and waste-free electric current as a redox equivalent. While major recent momentum was gained in electrocatalyzed C-H activations by 4d and 5d metals, user-friendly and inexpensive nickela-electrocatalysis has until recently proven elusive for oxidative C-H activations. Herein, the early developments of nickela-electrocatalyzed reductive cross-electrophile couplings as well as net-redox-neutral cross-couplings are first introduced. The focus of this Minireview is, however, the recent emergence of nickel-catalyzed electrooxidative C-H activations until April 2020.

Strain-Promoted Oxidation of Methylenecyclopropane Derivatives using N-Hydroxyphthalimide and Molecular Oxygen in the Dark

The hydroperoxidation of alkylidenecyclopropanes and other strained alkenes using an N-hydroxylamine and molecular oxygen occurred in the absence of catalyst, initiator, or light. The oxidation reaction proceeds through a radical pathway that is initiated by autoxidation of the alkene substrate. The hydroperoxides were converted to their corresponding alcohols and ketones under mild conditions.

Boron-mediated directed aromatic C-H hydroxylation

Transition metal-catalysed C–H hydroxylation is one of the most notable advances in synthetic chemistry during the past few decades and it has been widely employed in the preparation of alcohols and phenols. The site-selective hydroxylation of aromatic C–H bonds under mild conditions, especially in the context of substituted (hetero)arenes with diverse functional groups, remains a challenge. Here, we report a general and mild chelation-assisted C–H hydroxylation of (hetero)arenes mediated by boron species without the use of any transition metals. Diverse (hetero)arenes bearing amide directing groups can be utilized for ortho C–H hydroxylation under mild reaction conditions and with broad functional group compatibility. Additionally, this transition metal-free strategy can be extended to synthesize C7 and C4-hydroxylated indoles. By utilizing the present method, the formal synthesis of several phenol intermediates to bioactive molecules is demonstrated.

Transition metal-catalysed C–H hydroxylation is one of the most notable synthetic advances to access alcohols and phenols. Here, the authors report a metal-free, mild C–H hydroxylation of (hetero)arenes via boron-mediated chelation.

Nickela-electrocatalyzed C-H Alkoxylation with Secondary Alcohols: Oxidation-Induced Reductive Elimination at Nickel(III)

Nickela-electrooxidative C-H alkoxylations with challenging secondary alcohols were accomplished in a fully dehydrogenative fashion, thereby avoiding stoichiometric chemical oxidants, with H2 as the only stoichiometric byproduct. The nickela-electrocatalyzed oxygenation proved viable with various (hetero)arenes, including naturally occurring secondary alcohols, without racemization. Detailed mechanistic investigation, including DFT calculations and cyclovoltammetric studies of a well-defined C-H activated nickel(III) intermediate, suggest an oxidation-induced reductive elimination at nickel(III).

Copper mediated C(sp2)-H amination and hydroxylation of phosphinamides

Copper mediated C(sp2)-H amination and hydroxylation of arylphosphinic acid are accomplished by adopting phosphinamide as the directing group. This method is distinguished by its wide substrate scope and excellent functional group tolerance, thus allowing for the rapid preparation of organophosphorus compounds in organic synthesis.

Direct Oxygenation of C-H Bonds through Photoredox and Palladium Catalysis

This report presents the oxygenation of C-H bonds via the merger of photocatalysis and Pd catalysis. Herein, we describe the utilization of a photocatalyst to oxidize an organopalladium(II) intermediate to high-valent Pd^III or Pd^IV intermediates, which promotes the formation of C-O bonds. The demonstrated method works efficiently with various directing groups, such as oxime ether and benzothiazole. The applicability of this direct C-O bond formation method is shown by synthesizing several metal complexes of 2-(benzo[d]thiazol-2-yl)phenol that can be used in organic light-emitting diodes and pharmaceuticals.