TEMPO-catalyzed Aerobic Oxygenation and Nitrogenation of Olefins via C═C Double-Bond Cleavage

Visible-Light Driven Synthesis of Vinyl Amines without Photocatalyst

We developed a visible-light-induced vinyl amination of activated alkenes using TMSN3 and CsF through EDA complex formation under an oxygen atmosphere. Without light, the EDA complex forms between activated alkene, CsF, and oxygen. Upon exposure to light, oxygen in the complex gets excited, initiating the HAT process. This method efficiently synthesizes vinyl-amine derivatives via a radical pathway, demonstrating good functional group tolerance and high yields in a short time. Further, the late-stage functionalization enables the synthesis of biologically active heterocycles.

Synergistic catalysis: cobalt-catalyzed cascade reaction for the oxidative cleavage of olefins to access esters

An efficient strategy for the oxidative cleavage of CC bonds in olefins to form esters with one or multiple carbon atoms less over heterogeneous cobalt/nitrogen-doped carbon catalyst with dioxygen as the oxidant was described. The protocol features a wide substrate range including the challenging inactive aliphatic and long-chain alkyl aryl olefins. The reactivity of the catalyst did not decrease after reused for seven times. Characterization and control experiments reveal that synergistic effects between the metallic Co nanoparticles and Co-Nx sites provide access to the excellent catalytic activity.

Metal-Free Cyanation of gem-Difluoroalkenes via Azide-Mediated C-C Double Bond Fragmentation

Activation and cleavage of C-C double bonds are long-standing challenges in synthetic chemistry. Herein, we report an unprecedented azide-mediated C-C double bond fragmentation of gem-difluoroalkenes under mild and metal-free conditions, enabling the efficient synthesis of structurally diverse aromatic nitriles in moderate to good yields. This protocol is also amenable to the cyanation of gem-dichloro and dibromo alkenes. This reaction features simple operation and good functional group compatibility and can be implemented at a gram scale.

Iron-Catalyzed Cyanide-Free Synthesis of Alkyl Nitriles: Oxidative Deconstruction of Cycloalkanones with Ammonium Salts and Aerobic Oxidation

A sustainable, cyanide-free synthesis of alkyl nitriles via the aerobic oxidative deconstruction of unstrained cycloalkanones with ammonium salts has been developed. Using inexpensive and stable ammonium salts as the nitrogen source, a variety of alkyl nitriles containing a distal carbonyl group were obtained in good yields under visible-light-promoted iron catalysis. This protocol is characterized by mild conditions, abundant and environmentally benign materials, and high atom and step economy with minimal waste generation. The primary mechanism study revealed that 1O2 is likely to be involved in this reaction.

Late-stage modification of bioactive compounds: Improving druggability through efficient molecular editing

Synthetic chemistry plays an indispensable role in drug discovery, contributing to hit compounds identification, lead compounds optimization, candidate drugs preparation, and so on. As Nobel Prize laureate James Black emphasized, "the most fruitful basis for the discovery of a new drug is to start with an old drug"1. Late-stage modification or functionalization of drugs, natural products and bioactive compounds have garnered significant interest due to its ability to introduce diverse elements into bioactive compounds promptly. Such modifications alter the chemical space and physiochemical properties of these compounds, ultimately influencing their potency and druggability. To enrich a toolbox of chemical modification methods for drug discovery, this review focuses on the incorporation of halogen, oxygen, and nitrogen-the ubiquitous elements in pharmacophore components of the marketed drugs-through late-stage modification in recent two decades, and discusses the state and challenges faced in these fields. We also emphasize that increasing cooperation between chemists and pharmacists may be conducive to the rapid discovery of new activities of the functionalized molecules. Ultimately, we hope this review would serve as a valuable resource, facilitating the application of late-stage modification in the construction of novel molecules and inspiring innovative concepts for designing and building new drugs.

Dioxygen compatible electron donor-acceptor catalytic system and its enabled aerobic oxygenation

The photochemical properties of Electron Donor-Acceptor (EDA) complexes present exciting opportunities for synthetic chemistry. However, these strategies often require an inert atmosphere to maintain high efficiency. Herein, we develop an EDA complex photocatalytic system through rational design, which overcomes the oxygen-sensitive limitation of traditional EDA photocatalytic systems and enables aerobic oxygenation reactions through dioxygen activation. The mild oxidation system transfers electrons from the donor to the effective catalytic acceptor upon visible light irradiation, which are subsequently captured by molecular oxygen to form the superoxide radical ion, as demonstrated by the specific fluorescent probe, dihydroethidine (DHE). Furthermore, this visible-light mediated oxidative EDA protocol is successfully applied in the aerobic oxygenation of boronic acids. We believe that this photochemical dioxygen activation strategy enabled by EDA complex not only provides a practical approach to aerobic oxygenation but also promotes the design and application of EDA photocatalysis under ambient conditions.

Glucaric Acid Production from Miscanthus sacchariflorus via TEMPO-Mediated Oxidation with an Efficient Separation System

In this study, production and isolation of glucaric acid from lignocellulosic biomass were performed via potassium cation-based TEMPO-mediated oxidation for the ease of glucaric acid isolation. To optimize the oxidation conditions, response surface methodology (RSM) was adopted using standard glucose as the raw material. Among the oxidation conditions, the dosage of oxidant and pH of reaction affected the glucaric acid production, and the optimum conditions were suggested by RSM analysis: 5 °C of reaction temperature, 4.23 equiv dosage of KClO per mole of glucose, and pH of 12. Furthermore, glucaric acid was produced from lignocellulosic biomass-derived enzymatic hydrolysate from Miscanthus under optimum conditions. The impurities such as xylose and lignin in enzymatic hydrolysate inhibited the efficiency of glucose oxidation. As a result, more oxidant was required to produce sufficient glucaric acid from the enzymatic hydrolysate compared to standard glucose. The produced glucaric acid was simply isolated by controlling the pH in the form of glucaric acid monopotassium salt, which showed lower solubility in water, and the purity of isolated glucaric acid was over 99%. The overall mass balance of feedstock to glucaric acid was analyzed, suggesting that 86.38% (w/w) glucaric acid could be produced from initial glucan in feedstock.

o-Alkynylaryl 2-Cyanoacrylates: A Synthon for the Construction of Diverse Isoquinolones/Napthyridinones

o-Alkynylaryl 2-cyanoacrylates have been disclosed as a new synthon for the regioselective synthesis of isoquinolones and naphthyridinones under mild reaction conditions. The attractive feature of this investigation includes carbon-carbon double bond cleavage under metal-free conditions by an intramolecular SN2 reaction. Incorporating two distinct C-I bonds in the resulting products provides facile opportunity for structural elaboration. The viability of the present protocol was unveiled by postfunctionalization with drug analogues and gram-scale synthesis.

Olefin skeletal rearrangement enabling access to multiarylated N-sulfonyl amidines

A base-promoted olefin skeletal rearrangement strategy from para-quinone methides (p-QMs) and N-fluoroarenesulfonamides is reported, enabling direct nitrogen insertion of olefins to produce a series of multiarylated (Z)-N-sulfonyl amidines with complete stereoselectivity and generally good yields. Using p-QMs without o-hydroxy substituents gave triarylated N-sulfonyl amidines, whereas tetraarylated N,N'-disulfonyl amidines were synthesized with the existence of o-hydroxy groups.

TEMPO-Mediated Interrupted 6π-Photocyclization of ortho-Biaryl-Appended 1,3-Dicarbonyl Compounds toward 10-Phenanthrenols

In this paper, we present an example of a photoinduced catalyst, halogen-, and base-free TEMPO-mediated interrupted 6π-photocyclization/dehydrogenative aromatization of ortho-biaryl-appended 1,3-dicarbonyl compounds for the preparation of 10-phenanthrenols. The reaction involves rapid photocycloaddition via a 1,2-biradical of 1,3-dicarbonyl compounds, followed by subsequent dehydrogenative aromatization of 1,4-biradical intermediates using TEMPO as the commercially available oxidant rather than trapped by TEMPO to form an alkoxyamine product.

Growing Utilization of Radical Chemistry in the Synthesis of Pharmaceuticals

Our current unhealthy lifestyle and the exponential surge in the population getting affected by a variety of diseases have made pharmaceuticals or drugs an imperative part of life, making the development of innovative strategies for drug discovery or the introduction of refined, cost-effective and modern technologies for the synthesis of clinically used drugs, a need of the hour. Ever since their discovery, free radicals and radical cations or anions as reactive intermediates have captivated the chemists, resulting in an exceptional utilization of these moieties throughout the field of chemical synthesis, owing to their unprecedented and widespread reactivity. Sticking with the idea of not judging the book by its cover, despite the conventional thought process of radicals being unstable and difficult to control entities, scientists and academicians around the globe have done an appreciable amount of work utilizing both persistent as well as transient radicals for a variety of organic transformations, exemplifying them with the synthesis of significant biologically active pharmaceutical ingredients. This review truly accounts for the organic radical transformations including radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling with metal-complexes and radical cations coupling with nucleophiles, that offers fascinating and unconventional approaches towards the construction of intricate structural frameworks of marketed APIs with high atom- and step-economy; complementing the otherwise employed traditional methods. This tutorial review presents a comprehensive package of diverse methods utilized for radical generation, featuring their reactivity to form critical bonds in pharmaceutical total synthesis or in building key starting materials or intermediates of their synthetic journey, acknowledging their excellence, downsides and underlying mechanisms, which are otherwise poorly highlighted in the literature. Despite great achievements over the past few decades in this area, many challenges and obstacles are yet to be unraveled to shorten the distance between the academics and the industry, which are all discussed in summary and outlook.

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.

Fe-Catalyzed Direct Synthesis of Nitriles from Carboxylic Acids with Electron-Deficient N-Cyano-N-aryl-arylsulfonamide

Established carboxylic acids to nitriles conversion methods suffer from expensive catalysts, tedious steps, high temperatures (>200 °C), high pressure, or a narrow substrate range. Herein, we demonstrate a concise and efficient access to diverse nitrile compounds from ubiquitous carboxylic acids with electron-deficient N-cyano-N-aryl-arylsulfonamide (NCAS) in moderate to excellent yields. This strategy is promoted by an inexpensive iron catalyst and is generally compatible with primary, secondary, tertiary, and aryl carboxylic acids, as well as a variety of functional groups.

Cobalt-Catalyzed Nitrogen Atom Insertion in Arylcycloalkenes

Developing strategies enabling the modification of underlying molecular frameworks facilitates access to underexplored chemical spaces. Skeletal editing is an emerging technology for late-stage diversification of bioactive molecules. However, the current state of this knowledge remains undeveloped. This work describes a simple protocol that "inserts" a nitrogen atom into arylcycloalkenes to form the corresponding N-heterocycles. The use of an inexpensive cobalt catalyst under aqueous and open-air conditions makes this protocol very practical. Examples of late-stage modification of compounds of pharmaceutical interest and complex fused ring compounds further demonstrated the potentially broad applicability of this methodology.

Selective desaturation of amides: a direct approach to enamides

C(sp3)-H bond desaturation has been an attractive strategy in organic synthesis. Enamides are important structural fragments in pharmaceuticals and versatile synthons in organic synthesis. However, the dehydrogenation of amides usually occurs on the acyl side benefitting from enolate chemistry like the desaturation of ketones and esters. Herein, we demonstrate an Fe-assisted regioselective oxidative desaturation of amides, which provides an efficient approach to enamides and β-halogenated enamides.

Electrochemical Aerobic Oxygenation and Nitrogenation of Cyclic Alkenes via C═C Bond Cleavage or Oxygenation and Azidation of Open-Chain Alkenes

An efficient strategy involving electrochemical C═C double-bond cleavage and functionalization of cyclic alkenes for the synthesis of ketonitriles is described. This transformation features environmentally friendly conditions and utilizes relatively safe TMSN₃ as the nitrogenation reagent and molecular oxygen as the oxidant. For the open-chain alkenes, the reaction gave 1,2-difunctionalized products. A wide range of cyclic alkenes and open-chain alkenes were found to be compatible, providing the corresponding ketonitriles and α-azido aromatic ketones in moderate to good yields.

1,2-Amino oxygenation of alkenes with hydrogen evolution reaction

1,2-Amino oxygenation of alkenes has emerged as one of the most straightforward synthetic methods to produce β-amino alcohols, which are important organic building blocks. Thus, a practical synthetic strategy for 1,2-amino oxygenation is highly desirable. Here, we reported an electro-oxidative intermolecular 1,2-amino oxygenation of alkenes with hydrogen evolution, removing the requirement of extra-oxidant. Using commercial oxygen and nitrogen sources as starting materials, this method provides a cheap, scalable, and efficient route to a set of valuable β-amino alcohol derivatives. Moreover, the merit of this protocol has been exhibited by its broad substrate scope and good application in continuous-flow reactors. Furthermore, this method can be extended to other amino-functionalization of alkenes, thereby showing the potential to inspire advances in applications of electro-induced N-centered radicals (NCRs).

1,2-Aminoxygenation of alkenes without extra oxidant is a practical yet challenging way to prepare β-amino alcohols. Here, the authors report an electro-oxidative route achieving such a goal with H2 evolution, exhibiting broad scope and application potential.

Copper-Catalyzed Oxidative C-C Bond Cleavage of Alkyl-(Hetero)arenes Enabling Direct Access to Nitriles

The cleavage and functionalization of C-C bonds has emerged as a powerful tool for discovery of new transformations. Herein, we report a protocol that enables direct synthesis of nitriles via copper-catalyzed oxidative cleavage and cyanation of C-C bonds in a wide variety of multicarbon alkyl-substituted (hetero)arenes. Detailed mechanistic studies reveal that a tandem oxidative process is involved in this transformation.

Photochemical Organocatalytic Aerobic Cleavage of C═C Bonds Enabled by Charge-Transfer Complex Formation

A novel visible-light-driven organocatalytic protocol to access aerobic oxidative cleavage of olefins, promoted by sodium benzene sulfinate, is described herein. An array of alkenes smoothly delivered the corresponding aldehydes and ketones under transition-metal-free conditions. Notably, α-halo-substituted styrenes proceeded with photoinduced oxidation to finally afford α-halo-acetophenones with halogen migration. Crucial to this oxidation was the formation of charge-transfer complexes between sodium benzene sulfinate with molecular O₂ to ultimately deliver the carbonyl products.

Iodine-Mediated C═C Double Bond Cleavage toward Pyrido[2,1-b]quinazolinones

A transition-metal-free C═C double bond cleavage reaction employing molecular iodine is described. In the presence of K₂CO₃ as the base, I₂-mediated C═C bond cleavage followed by intramolecular annulation of N-(2-vinylaryl)pyridin-2-amine substrates produces pyrido[2,1-b]quinazolinones and related heterocyclic compounds. This reaction can be completed on a gram scale and has been successfully applied to the synthesis of compounds with important biological properties, including efflux pump inhibitory and antiallergic activities.

Chemical versatility of azide radical: journey from a transient species to synthetic accessibility in organic transformations

The generation of azide radical (N₃˙) occurs from its precursors primarily via a single electron transfer (SET) process or homolytic cleavage by chemical methods or advanced photoredox/electrochemical methods. This in situ generated transient open-shell species has unique characteristic features that set its reactivity. In the past, the azide radical was widely used for various studies in radiation chemistry as a 1e^- oxidant of biologically important molecules, but now it is being exploited for synthetic applications based on its addition and intermolecular hydrogen atom transfer (HAT) abilities. Due to the significant role of nitrogen-containing molecules in synthesis, drug discovery, biological, and material sciences, the direct addition onto unsaturated bonds for the simultaneous construction of C-N bond with other (C-X) bonds are indeed worth highlighting. Moreover, the ability to generate O- or C-centered radicals by N₃˙ via electron transfer (ET) and intermolecular HAT processes is also well documented. The purpose of controlling the reactivity of this short-lived intermediate in organic transformations drives us to survey: (i) the history of azide radical and its structural properties (thermodynamic, spectroscopic, etc.), (ii) chemical reactivities and kinetics, (iii) methods to produce N₃˙ from various precursors, (iv) several significant azide radical-mediated transformations in the field of functionalization with unsaturated bonds, C-H functionalization via HAT, tandem, and multicomponent reaction with a critical analysis of underlying mechanistic approaches and outcomes, (v) concept of taming the reactivity of azide radicals for potential opportunities, in this review.

PhIO-Mediated Oxidative C═C Bond Cleavage and Reassembly toward Highly Functionalized Oxazolones

An efficient PhIO-mediated oxidative C═C bond cleavage and reassembly of enaminone toward oxazolone with high regioselectivity has been reported. DFT calculations revealed that the reaction proceeded through an oxygen atom transfer, C═C bond cleavage, alkylthio migration, and reassembly cascade. This strategy is highlighted by high atom and step economy with formation of five bonds in one pot and generation of a high-valued oxazolone skeleton under mild conditions.

BF3·OEt2 catalyzed chemoselective CC bond cleavage of α,β-enones: an unexpected synthesis of 3-alkylated oxindoles and spiro-indolooxiranes

A BF₃·OEt₂ catalyzed highly chemoselective formal CC double bond cleavage reaction of α,β-enones with diazoamides for the synthesis of 3-alkylated oxindoles is developed. Boron trifluoride etherate is found to be an effective catalyst for the chemoselective C_α-C_β cleavage of enones to obtain 3-alkylated oxindoles. The product formation indicates a selective β-carbon elimination pathway of α,β-enones using the inexpensive BF₃·OEt₂ as a catalyst, transition metal-free conditions, an open-air environment, good functional tolerance and broad substrate scope. The synthetic utility of this protocol is highlighted by synthesizing spiro-indolooxiranes.

A computational study of the reaction mechanism of 2,2-azobis(isobutyronitrile)-initiated oxidative cleavage of geminal alkenes

A computational study of 2,2-azobis(isobutyronitrile) (AIBN)-initiated aerobic oxidative cleavage of alkenes is carried out employing density functional theory (DFT) and high-level coupled-cluster methods, such as coupled-cluster singles and doubles with perturbative triples [CCSD(T)]. Our computations show that the barriers for the formation of dioxetane derivatives suggested by Xu and co-workers (J. Org. Chem., 2014, 79, 7220-7225) for the reaction mechanism of aerobic oxidative cleavage of alkenes are computed to be higher than 65 kcal mol^-1. This barrier is relatively high under the reaction conditions. Our results for the Xu mechanism indicate that the reaction does not proceed via the formation of a dioxetane ring under the reaction conditions. Our results demonstrate that the reaction of aerobic oxidative cleavage of geminal alkenes in the presence of AIBN is initiated by the peroxyl radical 9, contrary to the isobutyronitrile radical 2. Our results show that the 2-(2-hydroxyl-1,1-diarylethoxy)-2-methylpropanenitrile radical (15) does not appear throughout the reaction scheme and the reaction progresses over the 2-(2-hydroxyl-2,2-diarylethoxy)-2-methylpropanenitrile radical (13) rather than the 2-(2-hydroxyl-1,1-diarylethoxy)-2-methylpropanenitrile radical (15). Our results are in agreement with the experimental results for the aerobic oxidative cleavage of the geminal disubstituted alkenes. Our results also demonstrate that the epoxide derivatives can be formed as an intermediate under the reaction conditions. This reaction is not applicable for pyridine derivatives due to the conversion of vinylpyridine derivatives to N-oxide derivatives.

1,2-Dibutoxyethane-Promoted Oxidative Cleavage of Olefins into Carboxylic Acids Using O2 Under Clean Conditions

Herein, we report the first example of an effective and green approach for the oxidative cleavage of olefins to carboxylic acids using a 1,2-dibutoxyethane/O₂ system under clean conditions. This novel oxidation system also has excellent functional-group tolerance and is applicable for large-scale synthesis. The target products were prepared in good to excellent yields by a one-pot sequential transformation without an external initiator, catalyst, and additive.

Palladium-Catalyzed Cascade Carbonylation to α,β-Unsaturated Piperidones via Selective Cleavage of Carbon-Carbon Triple Bonds

A direct and selective synthesis of α,β-unsaturated piperidones by a new palladium-catalyzed cascade carbonylation is described. In the presented protocol, easily available propargylic alcohols react with aliphatic amines to provide a broad variety of interesting heterocycles. Key to the success of this transformation is a remarkable catalytic cleavage of the present carbon-carbon triple bond by using a specific catalyst with 2-diphenylphosphinopyridine as ligand and appropriate reaction conditions. Mechanistic studies and control experiments revealed branched unsaturated acid 11 as crucial intermediate.

Radical Annulation of 2-Cyanoaryl Acrylamides via C═C Double Bond Cleavage: Access to Amino-Substituted 2-Quinolones

A novel annulation of 2-cyanoaryl acrylamides via C═C double bond cleavage has been developed for facile and efficient access to a broad spectrum of functionalized 4-amino-2-quinolones, which are important N-heterocycles. In this transformation, the solvent THF is demonstrated to play a crucial role, and the addition of alkyl radicals to nitrile, 1,5-hydride shift, and cleavage of the C-C bond are involved in the mechanism.

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.

Oxidative Cleavage of Alkenes by O2 with a Non-Heme Manganese Catalyst

The oxidative cleavage of C=C double bonds with molecular oxygen to produce carbonyl compounds is an important transformation in chemical and pharmaceutical synthesis. In nature, enzymes containing the first-row transition metals, particularly heme and non-heme iron-dependent enzymes, readily activate O₂ and oxidatively cleave C=C bonds with exquisite precision under ambient conditions. The reaction remains challenging for synthetic chemists, however. There are only a small number of known synthetic metal catalysts that allow for the oxidative cleavage of alkenes at an atmospheric pressure of O₂, with very few known to catalyze the cleavage of nonactivated alkenes. In this work, we describe a light-driven, Mn-catalyzed protocol for the selective oxidation of alkenes to carbonyls under 1 atm of O₂. For the first time, aromatic as well as various nonactivated aliphatic alkenes could be oxidized to afford ketones and aldehydes under clean, mild conditions with a first row, biorelevant metal catalyst. Moreover, the protocol shows a very good functional group tolerance. Mechanistic investigation suggests that Mn–oxo species, including an asymmetric, mixed-valent bis(μ-oxo)-Mn(III,IV) complex, are involved in the oxidation, and the solvent methanol participates in O₂ activation that leads to the formation of the oxo species.

Poly(ethylene glycol) dimethyl ether mediated oxidative scission of aromatic olefins to carbonyl compounds by molecular oxygen

A simple, and practical oxidative scission of aromatic olefins to carbonyl compounds using O₂ as the sole oxidant with poly(ethylene glycol) dimethyl ether as a benign solvent has been developed. A wide range of monosubstituted, gem-disubstituted, 1,2-disubstituted, trisubstituted and tetrasubstituted aromatic olefins was successfully converted into the corresponding aldehydes and ketones in excellent yields even with gram–scale reaction. Some control experiments were also conducted to support a possible reaction pathway.

A simple and practical O₂ oxidized scission of monosubstituted, gem- and 1,2-disubstituted, trisubstituted and tetrasubstituted aromatic olefins to aldehydes and ketones in PEGDME has been developed.

Aminolactonization of Unactivated Alkenes Catalyzed by Aryl Iodine

A one-step protocol of the aryl iodine-catalyzed aminolactonization of unactivated alkenes under oxidation conditions was first reported to efficiently construct diverse amino lactones in a short time using HNTs₂ as the compatible nitrogen source. In addition, we investigated the influence of the reaction rate based on the structure of the iodoarene precatalyst, which revealed the selective adjustment effect on aminolactonization and oxylactonization. Finally, preliminary experiments verified the feasibility of asymmetric aminolactonization catalyzed by a chiral iodoarene precatalyst.

Tellurium(II)/Tellurium(III)-Catalyzed Cross-Dehydrogenative C-N Bond Formation

The TeII /TeIII -catalyzed dehydrogenative C-H phenothiazination of challenging phenols featuring electron-withdrawing substituents under mild aerobic conditions and with high yields is described. These unexpected TeII /TeIII radical catalytic properties were characterized by cyclic voltammetry, EPR spectroscopy, kinetic experiments, and DFT calculations.

Beyond osmium: progress in 1,2-amino oxygenation of alkenes, 1,3-dienes, alkynes, and allenes

Olefin 1,2-difunctionalization has emerged as a popular strategy within modern synthetic chemistry for the synthesis of vicinal amino alcohols and derivatives. The advantage of this approach is the single-step simplicity for rapid diversification, feedstock nature of the olefin starting materials, and the possible modularity of the components. Although there is a vast number of possible iterations of 1,2-olefin difunctionalization, 1,2-amino oxygenation is of particular interest due to the prevalence of both oxygen and nitrogen within pharmaceuticals, natural products, agrochemicals, and synthetic ligands. The Sharpless amino hydroxylation provided seminal results in this field and displayed the value in achieving methods of this nature. However, a vast number of new and novel methods have emerged in recent decades. This review provides a comprehensive review of modern advances in accomplishing 1,2-amino oxygenation of alkenes, 1,3-dienes, alkynes, and allenes that move beyond osmium to a range of other transition metals and more modern strategies such as electrochemical, photochemical, and biochemical reactivity.

Alkynoates as Versatile and Powerful Chemical Tools for the Rapid Assembly of Diverse Heterocycles under Transition-Metal Catalysis: Recent Developments and Challenges

Heterocycles, heteroaromatics and spirocyclic entities are ubiquitous components of a wide plethora of synthetic drugs, biologically active natural products, marketed pharmaceuticals and agrochemical targets. Recognizing their high proportion in drugs and rich pharmacological potential, these invaluable structural motifs have garnered significant interest, thus enabling the development of efficient catalytic methodologies providing access to architecturally complex and diverse molecules with high atom-economy and low cost. These chemical processes not only allow the formation of diverse heterocycles but also utilize a range of flexible and easily accessible building units in a single operation to discover diversity-oriented synthetic approaches. Alkynoates are significantly important, diverse and powerful building blocks in organic chemistry due to their unique and inherent properties such as the electronic bias on carbon-carbon triple bonds posed by electron-withdrawing groups or the metallic coordination site provided by carbonyl groups. The present review highlights the comprehensive picture of the utility of alkynoates (2007-2019) for the synthesis of various heterocycles (> 50 types) using transition-metal catalysts (Ru, Rh, Pd, Ir, Ag, Au, Pt, Cu, Mn, Fe) in various forms. The valuable function of versatile alkynoates (bearing multifunctional groups) as simple and useful starting materials is explored, thus cyclizing with an array of coupling partners to deliver a broad range of oxygen-, nitrogen-, sulfur-containing heterocycles alongside fused-, and spiro-heterocyclic compounds. In addition, these examples will also focus the scope and reaction limitations, as well as mechanistic investigations into the synthesis of these heterocycles. The biological significance will also be discussed, citing relevant examples of drug molecules highlighting each class of heterocycles. This review summarizes the recent developments in the synthetic methods for the synthesis of various heterocycles using alkynoates as readily available starting materials under transition-metal catalysis.

Radical-mediated aerobic oxidation of substituted styrenes and stilbenes

A 2,2-azobis(isobutyronitrile)-catalyzed oxidative cleavage of alkenes with molecular oxygen as the oxidant was described.

Highly efficient oxidative cleavage of olefins with O2 under catalyst-, initiator- and additive-free conditions

Without employing any external catalyst, initiator and additives, an efficient and eco-friendly protocol has been developed for the synthesis of carbonyl compound via 1,4-dioxane- promoted oxidation of olefins with atmospheric O₂ as the sole oxidant.

Selective Dealkenylative Functionalization of Styrenes via C-C Bond Cleavage

As a readily available feedstock, styrene with about 25 million tons of global annual production serves as an important building block and organic synthon for the synthesis of fine chemicals, polystyrene plastics, and elastomers. Thus, in the past decades, many direct transformations of this costless styrene feedstock were disclosed for the preparation of high-value chemicals, which to date, generally performed on the functionalization of styrenes through the allylic C-H bond, C(sp²)-H bond, or the C=C double bond cleavage. However, the dealkenylative functionalization of styrenes via the direct C-C single bond cleavage is so far challenging and still unknown. Herein, we report the novel and efficient C-C amination and hydroxylation reactions of styrenes for the synthesis of valuable aryl amines and phenols via the site-selective C(Ar)-C(alkenyl) single bond cleavage. This chemistry unlocks the new transformation and application of the styrene feedstock and provides an efficient protocol for the late-stage modification of substituted styrenes with the site-directed dealkenylative amination and hydroxylation.

Sequential One-Pot Synthesis of 3-Arylbenzofurans from N-Tosylhydrazones and Bromophenol Derivatives

A divergent and efficient one-pot sequence allowing direct access to 3-arylbenzofuran derivatives has been developed. The process, involving N-tosylhydrazones and bromophenols, proceeds via a palladium-catalyzed Barluenga-Valdés cross-coupling, followed by an aerobic, copper-catalyzed, radical cyclization to form Csp²-Csp² and O-Csp² bonds. 3-Arylated benzofurans bearing various substituents were obtained with good to excellent yields (up to 90%). Mechanistic investigation strongly supports a radical process for the cyclization step.

Copper-Catalyzed Aerobic Cyclization of β,γ-Unsaturated Hydrazones with Concomitant C═C Bond Cleavage

A Cu-catalyzed aerobic oxidative cyclization of β,γ-unsaturated hydrazones for the preparation of pyrazole derivatives has been developed. The hydrazonyl radical promoted the cyclization, along with a concomitant C═C bond cleavage of β,γ-unsaturated hydrazones. This process has been verified via several control experiments, including a radical-trapping study, an ¹⁸O-labeling method, and the identification of the possible byproducts. The advantages of this reaction include operational simplicity, a broad reaction scope, and a mild selective reaction process.