Transition-metal-free coupling reactions

Nucleophilic Amination of Aryl Halides with an Azanide Surrogate

We report the development of an azanide (-NH2) surrogate which enables the facile conversion of electron-deficient (hetero)aryl halides into primary N-aryl amines under transition-metal-free conditions. The designed amidine reagent is easy to prepare, bench stable, and undergoes facile N-arylation under basic conditions at 40 °C. Intermediate N-aryl amidines are readily cleaved to form N-aryl amines in situ through hydrolysis or base-promoted elimination. The developed surrogate is a safer and more selective alternative to existing anionic N-nucleophiles, such as alkali metal amides or azide salts.

Computational study on catalyst-free BCl3-promoted chloroboration of carbonyl compounds

DFT calculations were performed to investigate the possible reaction mechanisms underlying catalyst-free chloroboration reactions of carbonyl compounds with BCl3. The interaction between BCl3 and the C[double bond, length as m-dash]O moiety of carbonyl compounds is a two-step reaction. In the first step, B of BCl3 forms a bond with the O of the C[double bond, length as m-dash]O moiety, followed by the 1,3-Cl migration process from BCl3 to the C of the carbonyl group. To indicate the versatility of our synthetic methodology, a catalyst-free chloroboration of a variety of aldehydes and ketones with a broad range of electron-donating and electron-withdrawing groups with BCl3 was checked. According to DFT results, BCl3-induced chloroboration of aldehydes and ketones progressed under a kinetically favorable condition with <20 kcal mol-1 of activation free energy.

N-heterocyclic nitrenium-catalyzed photosynthesis of 3,3-disubstituted oxindoles from α-chloroanilides

The radical cascade reaction of α-halogenated anilides represents an efficient approach for synthesizing 3,3-oxindoles. However, most methods have focused on α-bromoanilides, with limited utilization of the more stable and readily available α-chloroanilides. In addition, the transition-metal-free preparation of 3,3-oxindoles has been far less explored. Here, we present a photoreductive N-heterocyclic nitrenium-catalyzed radical cyclization reaction of α-chloroanilides for the construction of 3,3-oxindoles under transition-metal-free conditions.

Carboxylations of (Hetero)Aromatic C-H Bonds Using an Alkyl Silyl Carbonate Reagent

In this paper, we report that the use of an alkyl silyl carbonate reagent combined with CsF and 18-crown-6 facilitates efficient direct carboxylations of (hetero)aromatic C-H bonds. This system also enables benzylic carboxylation of a toluene derivative and double carboxylation of methyl heteroarene. The carbonate reagent is characterized by its ease of handling and storage. Moreover, we demonstrate the application of this system in 13C-labeling experiments.

TFA-promoted (hetero)arylation/hydroxylation of quinoxaline-2-one derivatives with electron-rich aromatic compounds

A metal- and solvent-free, one-pot and TFA-promoted method for the construction of hetero/aryl-substituted quinoxalin-2-ones, pyrazin-2(1H)-ones, and pyrimidin-4(3H)-ones is reported. This method involves the reaction of chloro-derivatives of nitrogen heterocycles with electron-rich arenes/heteroarenes, followed by hydroxylation. This protocol is easy to use, providing access to (hetero)aryl-substituted N-heterocycles in good yields.

Conversion of Phenols and Anilines to trans-Olefins via Oxidation, Wittig Olefination, and Stereoselective Olefin Isomerization

Development of metal-free conversion of naturally abundant phenols and anilines to the corresponding olefins remains a formidable challenge. The current state of the art relies on the TM-catalyzed Heck coupling of activated phenols (triflates, tosylates, and more) with the olefins. While these advancements are promising, the reaction suffers from branch vs linear selectivity and requires an expensive TM-ligand combination, hazardous organotin reagents, and very high reaction temperature. Herein, we reported a one-pot conversion of phenol and anilines to the corresponding E-olefins, which goes via oxidation, Wittig olefination, and stereoselective olefin isomerization.

Micelle-Assisted C(sp2)-H Functionalization for C-Se and C-X Bond Formation in the Aqueous Medium

An environmentally sustainable, versatile, and cost-effective approach for C-Se and C-X (X = I, Br, and Cl) bond formation through C-H functionalization assisted by micellar catalysis in water is developed. The reaction utilizes a minimum amount of diorganyl diselenides and potassium halides for the respective functionalizations. The present protocol was suitable for scale-up synthesis, which directly provided the desired selenylated products without the need for chromatographic purification, in sufficient purity. The aqueous micellar catalysis system was reusable for up to 5 reaction cycles without compromising the reaction yield.

Nucleophilic Ring Opening of Donor-Acceptor Cyclopropanes through Umpolung Reactivity of Organochlorophosphines: Phosphine Oxide-Functionalized Boron-Pendanted Compounds

We present a novel set of frustrated Lewis pair (FLP) systems that exhibit a remarkable ability to promote the ring opening of donor-acceptor cyclopropanes (DACs). This FLP-promoted protocol offers umpolung reactivity of R1R2PCl/CN (R1, R2 = aryl, alkyl) toward DACs via nucleophilic ring-opening reactions to provide phosphinated boron-pendanted diester compounds. This novel approach exhibits the dual role of BF3·OEt2 as an activator and a reactant. The resulting compounds were found in both the keto and enol forms, with the majority being in the keto form, according to NMR analysis. The enol form was identified by single-crystal XRD analysis, and DFT calculations indicated that the keto form is more stable than the corresponding enol form.

Transition Metal-Free Difunctionalization of Unactivated Alkenes: Arylation/Azidation, Arylation/Chlorination, and Arylation/Cyanation

Arylethylamines represent a privileged scaffold in pharmaceutical compounds and form the backbone of many medical drugs, including those used for treating neurological diseases and pain. Their biomedical significance has inspired new synthetic methods that rely on transition metal-catalyzed aminoarylation reaction to an alkene, often in conjunction with a photoredox catalyst or a photosensitizer, and guided by a directing or stabilizing group. Here, we introduce a simple and effective method for azidoarylation of unactivated alkenes under transition metal-free conditions. Visible or near-UV light irradiation of readily available triarylbismuth dichlorides generates an aryl radical that selectively adds to the alkene, and the resulting homobenzyl radical is intercepted by an amine equivalent. This method offers a broad substrate scope and also enables aryl chlorination and arylcyanation of unactivated alkenes.

Truxinates and truxillates: building blocks for architecturally complex polymers and advanced materials

Significant advancements in the syntheses of cyclobutane containing small molecules and polymers are described in the last 15 years. Small molecule cyclobutanes are under investigation for their diverse pharmacological activities, while polymers with cyclobutane backbones are emerging as novel mechanophores, stress-responsive materials, and sustainable plastics. Within these chemistries, [2 + 2] photocycloadditions to yield truxinates and truxillates are highly efficient offering a versatile strategy to access complex scaffolds. This article provides a comprehensive review on the synthetic methodologies, properties, and applications of polymer truxinates and truxillates, providing the background necessary to understand current developments and envision future applications. Additionally, we highlight the links between the development, discoveries, and synthetic methodologies of small molecules and cyclobutane polymers. We emphasize structure property relationships and discuss methods to control composition and structure for desired applications. We begin with a discussion of synthetic techniques for small molecule and polymer cyclobutanes followed by their greater applications, including pharmacological and material properties with examples including sustainable plastics and stimuli-responsive systems.

Base-Mediated Visible-Light-Driven C-H Arylation of Quinoxalin-2(1H)-Ones in Ethanol

Prior methods for visible-light-driven C-H arylation of quinoxalin-2(1H)-ones relied on external photocatalysts. Herein, we report a photocatalyst-free approach for this arylation. In this approach, β-dicarbonyl iodonium ylides, combined with t-BuOK in ethanol, act as aryl precursors, forming electron donor-acceptor (EDA) complexes. These complexes enhance light absorption, facilitating efficient single electron transfer and aryl radical formation. Consequently, various quinoxalin-2(1H)-ones undergo precise and efficient arylation without external photocatalysts. This protocol exhibits excellent tolerance toward diverse functional groups, with mild reaction conditions and eco-friendly solvents, revealing a high Ecoscale value.

Photoinduced Single Electron Reduction of the 4-O-5 Linkage in Lignin Models for C-P Coupling Catalyzed by Bifunctional N-Heterocyclic Carbenes

Catalytic activation of Caryl-O bonds is considered as a powerful strategy for the production of aromatics from lignin. However, due to the high reduction potentials of diaryl ether 4-O-5 linkage models, their single electron reduction remains a daunting challenge. This study presents the blue light-induced bifunctional N-heterocyclic carbene (NHC)-catalyzed one-electron reduction of diaryl ether 4-O-5 linkage models for the synthesis of trivalent phosphines. The H-bond between the newly devised bifunctional NHC and diaryl ethers is responsible for the success of the single electron transfer. Furthermore, this approach demonstrates selective one-electron reduction of unsymmetric diaryl ethers, oligomeric phenylene oxide, and lignin model.

Metal-Free Synthesis of Pharmaceutically Relevant Sulfonylureas via Direct Reaction of Sulfonamides with Amides

A metal-free process has been developed for the sustainable synthesis of medicinally important sulfonylureas in one pot via the direct reaction of sulfonamides with amides in green solvent (DMC). The reaction proceeded efficiently at room temperature, and the products were obtained in good to excellent yields. The use of readily accessible, inexpensive, and environmentally benign starting materials and reagents, metal-free mild reaction conditions, wide substrate scope, tolerance to air and moisture, operational simplicity, and good atom economy are the salient features of this reaction protocol. Gram-scale synthesis of antidiabetic drugs tolbutamide and chlorpropamide in excellent yields further revealed the practical utility of this procedure. Additionally, the synthetic value of this straightforward method is showcased by the late-stage modification of drug molecules, including drug-drug conjugation with good yields. Preliminary mechanistic studies indicated the in situ generation of an isocyanate intermediate, which further reacts with sulfonamide to form sulfonylurea. As compared to other related methods reported for sulfonylurea synthesis, the current method obviates the requirement of traditional multistep protocols involving isolation of hazardous isocyanates and avoids the use of toxic phosgene.

Synthesis of Secondary Boronates via Deaminative Cross-Coupling of Alkyl Nitroso Carbamates and Boronic Acids

A strategy for transition metal-free cross-coupling of alkyl nitroso-carbamates and boronic acids is reported. The N-nitroso carbamates are easily prepared from the corresponding amine in two simple steps. This method allows for the synthesis of a wide variety of secondary boronates, benzylic boronates and formal Csp3-Csp2 cross-coupling products under operationally simple conditions. Functional group tolerance is also demonstrated and applied in the modification of lysine to make non-canonical amino acids.

Substituent-Controlled Copper-Catalyzed Trifluoromethylation of 1,7-Dienes: Synthesis of Mono- and Bis-trifluoromethylated Benzoxepines

A copper-catalyzed trifluoromethylation of benzene-linked 1,7-dienes with 1-trifluoromethyl-1,2-benziodoxole via a radical cascade cyclization process for the synthesis of mono- and bis-trifluoromethylated benzoxepines is developed. The selectivity depends on substituents on the double bond of the allyl group in 1,7-dienes. The large-scale operation and late-stage functionalization of bioactive molecules reveal the promising utility of this protocol.

Magnetically Recyclable Borane Lewis Acid Catalyst for Hydrosilylation of Imines and Reductive Amination of Carbonyls

Fluorinated arylborane-based Lewis acid catalysts have shown remarkable activity and serve as ideal examples of transition metal-free catalysts for diverse organic transformations. However, their homogeneous nature poses challenges in terms of recyclability and separation from reaction mixtures. This work presents an efficient technique for the heterogenization of boron Lewis acid catalysts by anchoring Piers' borane to allyl-functionalized iron oxide. This catalyst demonstrates excellent activity in the hydrosilylation of imines and the reductive amination of carbonyls using various silanes as reducing agents under mild reaction conditions. The catalyst exhibits broad tolerance towards a wide range of functional substrates. Furthermore, it exhibits good recyclability and can be easily separated from the products using an external magnetic field. This work represents a significant advance in the development of sustainable heterogenous metal-free catalysts for organic transformations.

Arylation of Diethyl Acetamidomalonate with Diaryliodonium Salts En Route to α-Arylglycines

Diethyl acetamidomalonate (DEAM) has been widely used for the synthesis of α-amino acids via C-alkylation under basic conditions followed by hydrolysis/decarboxylation. In contrast, the C-arylation of this reagent remains undeveloped. Herein, we report a novel strategy for the synthesis of racemic α-arylglycines based on the selective arylation of DEAM with diaryliodonium salts under mild, transition metal-free conditions. The reaction features good functional group tolerance and easy scalability and is applicable to the chemoselective C-H-modification of arenes including approved drugs, thus enabling a straightforward approach to complex α-arylglycines that would be challenging to make otherwise.

The Versatile and Strategic O-Carbamate Directed Metalation Group in the Synthesis of Aromatic Molecules: An Update

The aryl O-carbamate (ArOAm) group is among the strongest of the directed metalation groups (DMGs) in directed ortho metalation (DoM) chemistry, especially in the form Ar-OCONEt2. Since the last comprehensive review of metalation chemistry involving ArOAms (published more than 30 years ago), the field has expanded significantly. For example, it now encompasses new substrates, solvent systems, and metalating agents, while conditions have been developed enabling metalation of ArOAm to be conducted in a green and sustainable manner. The ArOAm group has also proven to be effective in the anionic ortho-Fries (AoF) rearrangement, Directed remote metalation (DreM), iterative DoM sequences, and DoM-halogen dance (HalD) synthetic strategies and has been transformed into a diverse range of functionalities and coupled with various groups through a range of cross-coupling (CC) strategies. Of ultimate value, the ArOAm group has demonstrated utility in the synthesis of a diverse range of bioactive and polycyclic aromatic compounds for various applications.

On the mechanism of sp2 C-H borylation using ortho-N-substituted pyridinium cations

ortho-N-Substituted pyridinium cations with the weakly coordinating anion [B(C6F5)4]- have been studied and crucial structural features in the sp2 C-H borylation catalysis of 3-methylthiophene have been identified. The electron-deficiency of the aromatic core of the cation is essential for activity together with accessible protons. The spectroscopic yield of the borylation of 3-methylthiophene with catecholborane (CatBH) was optimized up to 86% and the method was further applied to other substrates such as N-alkylbenzenes. A mechanistic DFT study revealed the rate-limiting step in the catalysis to be the liberation of molecular H2 (ΔG‡ = 27.5 kcal mol-1), whereas the overall reaction was found to be exergonic by 5.1 kcal mol-1.

Redox Relay-Induced C-S Radical Cross-Coupling Strategy: Application in Nontraditional Site-Selective Thiocyanation of Quinoxalinones

Oxidative cross-coupling is a powerful strategy to form C-heteroatom bonds. However, oxidative cross-coupling for constructing C-S bond is still a challenge due to sulfur overoxidation and poisoning transition-metal catalysts. Now, electrochemical redox relay using sulfur radicals formed in situ from inorganic sulfur source offers a solution to this problem. Herein, electrochemical redox relay-induced C-S radical cross-coupling of quinoxalinones and ammonium thiocyanate with bromine anion as mediator is presented. The electrochemical redox relay comprised initially the formation of sulfur radical via indirect electrochemical oxidation, simultaneous electrochemical reduction of the imine bond, electro-oxidation-triggered radical coupling involving dearomatization-rearomatization, and the reformation of the imine bond through anodic oxidation. Applying this strategy, various quinoxalinones bearing multifarious electron-deficient/-rich substituents at different positions were well compatible with moderate to excellent yields and good steric hindrance compatibility under constant current conditions in an undivided cell without transition-metal catalysts and additional redox reagents. Synthetic applications of this methodology were demonstrated through gram-scale preparation and follow-up transformation. Notably, such a unique strategy may offer new opportunities for the development of new quinoxalinone-core leads.

Facile Cleavage of Activated Ketones: An Access to Thioethers via In Situ Generation of Anhydrides by Pummerer-Type Rearrangement

Herein, we report an oxygen insertion in activated ketones from simple inorganic carbonates for the synthesis of symmetric aromatic anhydrides. For the first time, Li2CO3 acts as an oxygen source and the in situ generated symmetric aromatic anhydrides undergo Pummerer-type rearrangement to access α-benzoyloxy-thioethers. Attractively, this protocol occurs under metal-, ligand-, and oxidant-free conditions and is compatible with a wide range of substrates. Control experiments reveal the reaction pathway.

Photoredox Synthesis of Silicon-Containing Isoindolin-1-ones and Deuterated Analogues Through Hydrosilylation and Deuterium-silylation

An efficient, practical, and metal-free protocol for the synthesis of silicon-containing isoindolin-1-ones and deuterated analogues via the synergistic combination of an organic photoredox and hydrogen atom transfer process is described. This strategy features mild reaction conditions, high atom economy, and excellent functional group compatibility, delivering a myriad of structurally diverse and valuable products with good to excellent yields.

Regioselective C(sp2)-H halogenation of pyrazolo[1,5-a]pyrimidines facilitated by hypervalent iodine(iii) under aqueous and ambient conditions

An efficient and mild approach has been developed for the regio-selective direct C3 halogenation of pyrazolo[1,5-a]pyrimidines employing readily available potassium halide salts and a hypervalent iodine(iii) reagent at ambient temperature. The protocol is both practical and environmentally friendly, utilizing water as a green solvent, potassium halides as an inexpensive and bench stable halogen source and PIDA as a non-toxic reagent, enabling clean and efficient halogenation at room temperature. The procedure yields a range of C3 halogenated pyrazolo[1,5-a]pyrimidines in good to excellent yields. Mechanistic studies suggest the involvement of electrophilic substitution mechanism in the halogenation process.

Pushing Boundaries: What's Next in Metal-Free C-H Functionalization for Sulfenylation?

The synthesis of thioether derivatives has been explored widely due to the potential application of these derivatives in medicinal chemistry, pharmaceutical industry and material chemistry. Within this context, there has been an increasing demand for the environmentally benign construction of C-S bonds via C-H functionalization under metal-free conditions. In the present article, we highlight recent developments in metal-free sulfenylation that have occurred in the past three years. The synthesis of organosulfur compounds via a metal-free approach using a variety of sulfur sources, including thiophenols, disulfides, sulfonyl hydrazides, sulfonyl chlorides, elemental sulfur and sulfinates, is discussed. Non-conventional strategies, which refer to the development of thioether derivatives under visible light and electrochemically mediated conditions, are also discussed. The key advantages of the reviewed methodologies include broad substrate scope and high reaction yields under environmentally benign conditions. This comprehensive review will provide chemists with a synthetic tool that will facilitate further development in this field.

Cross-Coupling Between Aryl Halides and Aryl Alkynes Catalyzed by an Odd Alternant Hydrocarbon

Catalytic cross-coupling between aryl halides and alkynes is considered an extremely important organic transformation (popularly known as the Sonogashira coupling) and it requires a transition metal-based catalyst. Accomplishing such transformation without any transition metal-based catalyst in the absence of any external stimuli such as heat, photoexcitation or cathodic current is highly challenging. This work reports transition-metal-free cross-coupling between aryl halides and alkynes synthesizing a rich library of internal alkynes without any external stimuli. A chemically double-reduced phenalenyl (PLY)-based molecule with the super-reducing property was employed for single electron transfer to activate aryl halides generating reactive aryl radicals, which subsequently react with alkyne. This protocol covers not only various types of aryl, heteroaryl and polyaryl halides but also applies to a large variety of aromatic alkynes at room temperature. With a versatile substrate scope successfully tested on more than 75 entries, this radical-mediated pathway has been explained by several control experiments. All the key reactive intermediates have been characterized with spectroscopic evidence. Detailed DFT calculations have been instrumental in portraying the mechanistic pathway. Furthermore, we have successfully extended this transition-metal-free catalytic strategy for the first time towards solvent-free cross-coupling between solid aryl halide and alkyne substrates.

Copper-Catalyzed Chemoselective O-Arylation of Oxindoles: Access to Cyclic Aryl Carboxyimidates

We have developed a highly efficient base- and additive-free chemoselective CuO-catalyzed strategy for the O-arylation of 2-oxindoles to synthesize 2-phenoxy-3H-indole and 2-phenoxy-1H-indole derivatives in the presence of diaryl iodonium salts. This method offers a variety of O-arylated oxindoles in good to excellent yields under relatively milder reaction conditions. Furthermore, this methodology was extended for the O-arylation of 2-pyridinone and isoindoline-1-one derivatives as well.

Transition-Metal-Free, Reductive Csp2-Csp3 Bond Constructions via Electrochemically Induced Alkyl Radicals

Construction of the Csp2-Csp3 bond without the aid of transition metal catalysts has been achieved by coupling the electrogenerated alkyl radicals with electron deficient (hetero)arenes in an undivided cell. Simultaneous cathodic reduction of both unactivated alkyl halides and cyanobenzenes under high potential enables radical-radical cross-coupling to deliver alkylarenes in the absence of transition metals. Depending on the coupling partner, the electrogenerated alkyl radicals can also proceed the Minisci-type reaction with N-heteroarenes without redox agents.

Transition-Metal-Free Continuous-Flow Synthesis of 2,5-Diaryl Furans: Access to Medicinal Building Blocks and Optoelectronic Materials

The direct transformation of 1,3-dienes into valuable 2,5-diarylfurans using transition-metal-free conditions is presented. By employing a simple oxidation─dehydration sequence on readily accessible 1,3-dienes, important 2,5-diarylfuran building blocks frequently used in medicinal and material chemistry are prepared. The oxidation step is realized using singlet oxygen, and the intermediate endoperoxide is dehydrated under metal-free conditions and at ambient temperature using the Appel reagent. Notably, this sequence can be streamlined into continuous flow, thereby eliminating the isolation of the intermediate, often unstable endoperoxide. This leads to a significant improvement in isolated yields (ca. 27% average increase) of the 2,5-diarylfurans while also increasing safety and reducing waste. Our transition-metal-free synthetic approach to 2,5-diarylfurans delivers several important furan building blocks used commonly in medicinal chemistry and as optoelectronic materials, including short-chain linearly conjugated furan oligomers. Consequently, we also complete a short study of the optical and electrochemical properties of a selection of these novel materials.

One-pot multicomponent polymerization towards heterocyclic polymers: a mini review

Multicomponent polymerization (MCP) is an innovative field related to polymer-based chemistry that offers numerous advantages derived from multicomponent reactions (MCRs). One of the key advantages of MCP is its ability to achieve high efficiency. Additionally, MCP offers other advantages, including operational simplicity, mild reaction conditions, and atom economy. MCP is a versatile technique that is used for synthesizing a wide range of analogs from several classes of heterocyclic compounds. The ring structures of heterocyclic polymers give them different mechanical, photophysical, and electrical properties to other types of polymers. Because of their unique properties, heterocyclic polymers have been widely utilized in various significant applications. MCRs are a type of chemical reaction that can be used to synthesize a wide variety of compounds in a single pot, which allows researchers to quickly assemble libraries of compounds. The development of MCPs from MCRs has made it easier to access a library of polymers with tunable structures. However, MCPs related to alkynes or acetylene triple bonds have more potential. In this review study, we provide an overview of the synthesis of heteroatom-functional polymers and alkyne-based development or other reactions such as Cu-catalyzed, catalyst-free, MCCP, MCTPs, green monomers, A3 coupling reactions, Passerini reactions, and sequence- and controlled-multicomponent polymerization. The up-to-date progress provides a convenient and efficient kind of approach related to heteroatoms and MCP synthesis, and perspectives in terms of future directions are also discussed in the study.

Aryne-Enabled C-N Arylation of Anilines

Anilines are potentially high-value arylating agents, but are limited by the low reactivity of the strong C-N bond. We show that the reactive intermediate benzyne can be used to both activate anilines, and set-up an aryl transfer reaction in a single step. The reaction does not require any transition metal catalysts or stoichiometric organometallics, and establishes a metal-free route to valuable biaryl products by functionalizing the aniline C-N bond.

Simulating Electron Transfer Reactions in Solution: Radical-Polar Crossover

Single-electron transfer (SET) promotes a wide variety of interesting chemical transformations, but modeling of SET requires a careful treatment of electronic and solvent effects to give meaningful insight. Therefore, a combined constrained density functional theory and molecular mechanics (CDFT/MM) tool is introduced specifically for SET-initiated reactions. Mechanisms for two radical-polar crossover reactions involving the organic electron donors tetrakis(dimethylamino)ethylene (TDAE) and tetrathiafulvalene (TTF) were studied with the new tool. An unexpected tertiary radical intermediate within the TDAE system was identified, relationships between kinetics and substitution in the TTF system were explained, and the impact of the solvent environments on the TDAE and TTF reactions were examined. The results highlight the need for including solvent dynamics when quantifying SET kinetics and thermodynamics, as a free energy difference of >20 kcal/mol was observed. Overall, the new method informs mechanistic analysis of SET-initiated reactions and therefore is envisioned to be useful for studying reactions in the condensed phase.

Synthesis of 3-Azabicyclo[3.1.0]hexane Derivates

3-Azabicyclo[3.1.0]hexanes are an important class of nitrogen-containing heterocycles that have been found to be key structural features in a wide range of biologically active natural products, drugs, and agrochemicals. As a cutting-edge area, the synthesis of these derivatives has made spectacular progress in recent decades, with various transition-metal-catalyzed and transition-metal-free catalytic systems being developed. In this review, we provide an overview of recent advances in the efficient methods for the synthesis of 3-azabicyclo[3.1.0]hexane derivatives since 2010, emphasizing the scope of substrates and synthesis' applications, as well as the mechanisms of these reactions.

Recent Advances in C-H Functionalisation through Indirect Hydrogen Atom Transfer

The functionalisation of C-H bonds has been an enormous achievement in synthetic methodology, enabling new retrosynthetic disconnections and affording simple synthetic equivalents for synthons. Hydrogen atom transfer (HAT) is a key method for forming alkyl radicals from C-H substrates. Classic reactions, including the Barton nitrite ester reaction and Hofmann-Löffler-Freytag reaction, among others, provided early examples of HAT. However, recent developments in photoredox catalysis and electrochemistry have made HAT a powerful synthetic tool capable of introducing a wide range of functional groups into C-H bonds. Moreover, greater mechanistic insights into HAT have stimulated the development of increasingly site-selective protocols. Site-selectivity can be achieved through the tuning of electron density at certain C-H bonds using additives, a judicious choice of HAT reagent, and a solvent system. Herein, we describe the latest methods for functionalizing C-H/Si-H/Ge-H bonds using indirect HAT between 2018-2023, as well as a critical discussion of new HAT reagents, mechanistic aspects, substrate scopes, and background contexts of the protocols.

A metal-free carbon catalyst for oxidative dehydrogenation of aryl cyclohexenes to produce biaryl compounds

A metal-free route based on a carbon catalyst to synthesize biphenyls through oxidative dehydrogenation (ODH) of phenyl cyclohexene has been investigated. Among the samples examined, an air-oxidized active carbon exhibits the best activity with a 9.1 × 10-2 h-1 rate constant, yielding 74% biphenyl in 28 h at 140 °C under five bar O2 in anisole. The apparent activation energy is measured as 54.5 kJ⋅mol-1. The extended reaction scope, consisting of 15 differently substituted phenyl cyclohexenes, shows the wide applicability of the proposed method. The catalyst's good recyclability over six runs suggests this ODH method as a promising route to access the biaryl compounds. In addition, the reaction mechanism is investigated with a combination of X-ray photoelectron spectroscopy, functional group blocking, and model compounds of carbon catalysts and is proposed to be based on the redox cycle of the quinoidic groups on the carbon surface. Additional experiments prove that the addition of the catalytic amount of acid (methanesulfonic acid) accelerates the reaction. In addition, Hammett plot examination suggests the formation of a carbonium intermediate, and its possible structure is outlined.

Electrochemical synthesis of biaryls by reductive extrusion from N,N'-diarylureas

The synthesis of biaryl compounds by the transition-metal free coupling of arenes is an important contemporary challenge, aiming to avoid the toxicity and cost profiles associated with the metal catalysts commonly used in the synthesis of these pharmaceutically relevant motifs. In this paper, we describe an electrochemical approach to the synthesis of biaryls in which aniline derivatives are coupled through the formation and reduction of a temporary urea linkage. The conformational alignment of the arenes in the N,N'-diaryl urea intermediates promotes C-C bond formation following single-electron reduction. Our optimized conditions are suitable for the synthesis of a variety of biaryls, including sterically hindered examples carrying ortho-substituents, representing complementary reactivity to most metal catalysed methods.

Photoinduced C(sp3)-H Functionalization of Glycine Derivatives: Preparation of Unnatural α-Amino Acids and Late-Stage Modification of Peptides

ConspectusPeptides are essential components of living systems and contribute to critical biological processes, such as cell proliferation, immune defense, tumor formation, and differentiation. Therefore, peptides have attracted considerable attention as targets for the development of therapeutic products. The incorporation of unnatural amino acid residues into peptides can considerably impact peptide immunogenicity, toxicity, side effects, water solubility, action duration, and distribution and enhance the peptides' druggability. Typically, the direct modification of natural amino acids is a practical and effective approach for promptly obtaining unnatural amino acids. However, selective functionalization of multiple C(sp³)-H bonds with comparable chemical reactivities in the peptide side chains remains a formidable challenge. Furthermore, chemical modifications aimed at highly reactive (nucleophilic and aromatic) groups on peptide side chains can interfere with the biological activity of peptides.In recent years, the rapid advancement of photoinduced radical reactions has made photoredox radical-radical cross-coupling a practical approach for constructing C(sp³)-C(sp³) bonds under mild conditions. Glycine, a naturally occurring amino acid and the fundamental skeleton of all α-amino acids, provides a basis for the alkylated modification of its own α-C(sp³)-H bond under mild conditions. This Account describes our recent research endeavors for systematically investigating the photocatalytic α-C(sp³)-H alkylation of glycine derivatives via radical-radical coupling between N-aryl glycinate-derived radicals and various alkyl radicals. In 2018, we disclosed the photoinduced Cu-catalyzed decarboxylative α-C(sp³)-H alkylation of glycine derivatives. Subsequently, we developed a catalyst-free method for alkylating glycine derivatives and glycine residues in peptides via electron donor-acceptor (EDA)-complex-promoted single electron transfer. Moreover, we developed a photoinduced method for the radical alkylation of N-aryl glycinate α-C(sp³)-H bonds using unactivated alkyl chlorides (iodides) under photocatalyst-free conditions. Notably, by building on racemic alkylations of glycine derivatives and glycine-residue-containing peptides, we recently stereoselectively alkylated the N-aryl glycinate α-C(sp³)-H bond using a dual-functional Cu catalyst generated in situ for synthesizing a series of unnatural chiral α-amino and C-glycoamino acids.We have developed a series of methods for synthesizing unnatural amino acids through the α-C(sp³)-H alkylation of glycine derivatives using photoredox-promoted radical coupling as a key strategy. These methods are efficient and versatile and can be used for the late-stage modification of peptides in various contexts. Our work builds on the fundamental importance of glycine as the basic scaffold of all α-amino acids and highlights the potential of radical-based chemistry for developing chemical transformations in peptide synthesis. These findings have broad implications for chemical biology and may open doors for discovering peptide drugs and developing therapeutics.

Highly regioselective and diastereoselective synthesis of novel pyrazinoindolones via a base-mediated Ugi-N-alkylation sequence

An efficient base-mediated/metal-free approach has been developed for the synthesis of 1-oxo-1,2,3,4-tetrahydropyrazino[1,2-a]indole-3-carboxamide derivatives via intramolecular indole N-H alkylation of novel bis-amide Ugi-adducts. In this protocol the Ugi reaction of (E)-cinnamaldehyde derivatives, 2-chloroaniline, indole-2-carboxylic acid and different isocyanides was designed for the preparation of bis-amides. The main highlight of this study is the practical and highly regioselective preparation of new polycyclic functionalized pyrazino derivatives. This system is facilitated by Na₂CO₃ mediation in DMSO and 100 °C conditions.

Synthesis and in vitro study of pyrimidine-phthalimide hybrids as VEGFR2 inhibitors with antiproliferative activity

Aim: Thalidomide, a once notorious sedative, is now clinically used as an antitumor agent. We aimed to use it as a lead compound for designing pyrimidine-phthalimide hybrids. Materials & methods: Nucleophilic substitution reaction of thalidomide analog 4 with primary and/or secondary aliphatic amines afforded pyrimidine-phthalimide hybrids 5a-g, 6 and 7a-d. Results & conclusion: Compound 7c showed high antiproliferative activity against four cell lines: HepG-2 (IC₅₀: 7.86 ± 0.5 μM), MCF-7 (IC₅₀: 2.77 ± 0.1 μM), HCT-116 (IC₅₀: 5.73 ± 0.4 μM) and PC-3 (IC₅₀: 8.32 ± 0.5 μM), with selective cytotoxicity for WI-38 (IC₅₀: 43.2 ± 2.56 μM). 7c arrested MCF-7 cells at S phase of the cell cycle and increased the total apoptotic cells by 50-fold. 7c inhibited VEGFR2 in vitro (IC₅₀: 0.130 ± 0.02 μM). 7c was capable of binding at the VEGFR2 binding site, forming hydrogen bond interactions with Asp1046 and Glu885 in a similar way to sorafenib.

Recent Advances in the Transition-Metal-Free Synthesis of Quinazolines

Quinazolines are a privileged class of nitrogen-containing heterocycles, widely present in a variety of natural products and synthetic chemicals with a broad spectrum of biological and medicinal activities. Owing to their pharmaceutical applications and promising biological value, a variety of synthetic methodologies have been reported for these scaffolds. From the perspective of green and sustainable chemistry, transition-metal-free synthesis provides an alternative method for accessing several biologically active heterocycles. In this review, we summarize the recent progress achieved in the transition-metal-free synthesis of quinazolines and we cover the literature from 2015 to 2022. This aspect is present alongside the advantages, limitations, mechanistic rationalization, and future perspectives associated with the synthetic methodologies.

Different Performances of BF3, BCl3, and BBr3 in Hypervalent Iodine-Catalyzed Halogenations

Herein, hypervalent iodine-catalyzed halogenation of aryl-activated alkenes using BX₃ (X = Cl, Br) as the halogen source and activating reagents was reported. Various halogenated 1,3-oxazine/2-oxazoline derivatives were obtained in good-to-high yields. Using BF₃ resulted in different substitute sites from BBr₃ and BCl₃ of the products, indicating different reactive intermediates and reaction pathways. The reaction underwent a "ligand coupling/oxidative addition/intermolecular nucleophilic attack/1,2-aryl migration/reductive elimination/intramolecular nucleophilic attack" cascade when BF₃ was applied as the halogen source, while 1,2-aryl migration has "disappeared" when the halogen source was BBr₃ or BCl₃. Possible catalytic cycles were proposed, and DFT calculations were conducted to demonstrate the differences among BX₃ (X = F, Cl, Br) in the hypervalent iodine-catalyzed halogenations.

Visible-Light-Induced N-Heterocyclic Carbene-Catalyzed Single Electron Reduction of Mono-Fluoroarenes

Fluoroarenes are abundant and readily available feedstocks. However, due to the high reduction potentials of mono-fluoroarenes, their photoreduction remains a continuing challenge, motivating the development of efficient activation modes to address this issue. This report presents the blue light-induced N-heterocyclic carbene (NHC)-catalyzed single electron reduction of mono-fluoroarenes for biaryl cross-couplings. We discovered that under blue light irradiation, NHC/tBuOK combination could construct powerful photoactive architectures to promote single electron transfer for C_aryl -F bond reduction via forming highly reducing NHC radical anion. Notably, the strategy was also successful to reduce C_aryl -O, C_aryl -N, and C_aryl -S bonds for biaryl cross-couplings.

Metal-Free Highly Regioselective 1,4-Sulfonyliodination of 1,3-Enynes

Herein, a novel, practical, and green synthetic method using readily available 1,3-enynes with sulfonyl hydrazides and I₂ through tert-butyl hydroperoxide (TBHP)-mediated 1,4-sulfonyliodination has been developed for synthesizing various tetrasubstituted allenyl iodides under metal-free conditions. Notably, the proposed method exhibits a broad substrate scope, operational simplicity, tolerance to air, high functional-group tolerance, satisfactory yields, and excellent regioselectivity as well as involves the use of cost-effective reagents such as green oxidants.

Organocatalytic [2 + 2] Photopolymerization under Visible Light: Accessing Sustainable Polymers from Cinnamic Acids

Herein, the successful development of a metal-free, solution [2 + 2] photopolymerization of natural cinnamic acid-derived bisolefinic monomers is reported, which is enabled by a strategy based on direct triplet state access via energy transfer catalysis. 2,2'-Methoxythioxanthone has been identified as an effective organic photocatalyst for the [2 + 2] photopolymerization in solution, which can be excited by visible light and activate the biscinnamate monomers via triplet energy transfer. This method features its metal-free conditions, visible light utilization, solution polymerization, and abundant biomass-based feedstock, as well as processable polymer products, which is different from the rigid, insoluble products obtained from solid-state photopolymerization. This solution polymerization method also shows a good compatibility to monomer structures; cinnamic acid-derived bisolefinic monomers with different linkers, including diamine, natural diol, and bisphenol, can all readily undergo [2 + 2] photopolymerization, and be transformed into colorless, sustainable polymers.

1,2-Metallate Rearrangement Using Indole Boronate Species to Access 2,3-Diarylindoles and Indolines

A transition metal-free multicomponent reaction using lithiated indole, boronic ester, pyridine, and ethyl chloroformate was developed to access C2,C3 bis-arylated indoles, which are present in several marketed drugs and bioactive compounds. One-pot access to unsymmetrical C2,C3-diaryl indole from the parent indole remains a huge synthetic challenge. Our group was able to achieve this goal through a transition metal-free 1,2-metalate rearrangement of the indole boronate complex. The reaction of indole boronate species with activated pyridine allows 1,2-migration to access pyridyl-indoleboronate species, which will convert to the corresponding indole upon oxidation and indoline after deborylation. The reaction tolerates substituted pyridines, quinolone, isoquinoline, and more. Both aryl and alkyl boronic esters were accommodated under optimized reaction conditions. Apart from mechanistic studies using ¹¹B-NMR, this methodology has been applied to the gram-scale synthesis of several bioactive compounds.

Transition-metal-free Synthesis of tetra-substituted Vinyl Iodides by Cascade Sequential Reaction of α-Keto Acids, 1-Iodoalkynes, and Alkyl Halides

The cascade sequential reaction of α-keto acids, 1-iodoalkynes, and alkyl halides are reported herein to synthesize tetra-substituted vinyl iodides. It represents an efficient protocol to access a diverse range of tetra-substituted vinyl iodides starting from simple materials in a one-pot fashion, featuring mild reaction conditions, ease of operation, and broad substrate scope.

Intramolecular cyclization of m-homoprenylphenols through oxidative nucleophilic aromatic substitution

We developed an intramolecular cyclization of m-homoprenylphenols and related m-prenylphenols to bicyclic skeletons by hypervalent iodine reagents through an oxidative nucleophilic aromatic substitution using the prenyl group as a carbon nucleophile. The reaction was applicable for the syntheses of 5/6-, 6/6-, and 7/6-fused ring systems.

Improved Metal-Free Approach for the Synthesis of Protected Thiol Containing Thymidine Nucleoside Phosphoramidite and Its Application for the Synthesis of Ligatable Oligonucleotide Conjugates

Oligonucleotide conjugates are versatile scaffolds that can be applied in DNA-based screening platforms and ligand display or as therapeutics. Several different chemical approaches are available for functionalizing oligonucleotides, which are often carried out on the 5' or 3' end. Modifying oligonucleotides in the middle of the sequence opens the possibility to ligate the conjugates and create DNA strands bearing multiple different ligands. Our goal was to establish a complete workflow that can be applied for such purposes from monomer synthesis to templated ligation. To achieve this, a monomer is required with an orthogonal functional group that can be incorporated internally into the oligonucleotide sequence. This is followed by conjugation with different molecules and ligation with the help of a complementary template. Here, we show the synthesis and the application of a thiol-modified thymidine nucleoside phosphoramidite to prepare ligatable oligonucleotide conjugates. The conjugations were performed both in solution and on solid phase, resulting in conjugates that can be assembled into multivalent oligonucleotides decorated with tissue-targeting peptides using templated ligation.

Domino Sonogashira coupling/metal carbene-involved annulation enabled by Pd/Cu relay catalysis: rapid assembly of indazole-containing biheteroaryls

An efficient and novel method has been developed for the synthesis of indazole-containing biheteroaryls via a domino Sonogashira coupling/azaenyne cycloisomerization/Barton–Kellogg reaction.