Transition-metal-free coupling reactions

Generation of aryl radicals by redox processes. Recent progress in the arylation methodology

Arylation methods based on the generation and use of aryl radicals have been a rapidly growing field of research in recent years and currently represent a powerful strategy for carbon – carbon and carbon – heteroatom bond formation. The progress in this field is related to advances in the methods for generation of aryl radicals. The currently used aryl radical precursors include aryl halides, aryldiazonium and diaryliodonium salts, arylcarboxylic acids and their derivatives, arylboronic acids, arylhydrazines, organosulfur(II, VI) compounds and some other compounds. Aryl radicals are generated under mild conditions by single electron reduction or oxidation of precursors induced by conventional reagents, visible light or electric current. A crucial role in the development of the radical arylation methodology belongs to photoredox processes either catalyzed by transition metal complexes or organic dyes or proceeding without catalysts. Unlike the conventional transition metal-catalyzed arylation methods, radical arylation reactions proceed very often at room temperature and have high functional group tolerance. Without claiming to be exhaustive, this review covers the most important advances of the current decade in the generation and synthetic applications of (het)aryl radicals. Examples of reactions are given and mechanistic insights are highlighted.

The bibliography includes 341 references.

Transition-metal-free decarboxylative ipso amination of aryl carboxylic acids

An unprecedented DMAP-catalysed decarboxylative amination of carboxylic acids has been achieved under metal free conditions, enabling the convenient synthesis of structurally diverse aryl and alkyl amines.

Metal-free synthesis of biarenes via photoextrusion in di(tri)aryl phosphates

A metal-free route for the synthesis of biarenes has been developed. The approach is based on the photoextrusion of a phosphate moiety occurring upon irradiation of biaryl- and triaryl phosphates. The reaction involves an exciplex as the intermediate and it is especially suitable for the preparation of electron-rich biarenes.

Catalyst-Free Four-Component Polymerization of Propiolic Acids, Benzylamines, Organoboronic Acids, and Formaldehyde toward Functional Poly(propargylamine)s

Multicomponent polymerizations (MCPs) are a group of fascinating polymer synthesis approaches that are developed rapidly in the recent decade. As a popular alkyne-based MCP, the A³ -polycouplings of alkynes, aldehydes, and amines are developed for the synthesis of poly(propargylamine)s under the catalysis of metal catalysts. In this work, through the design of carboxylic acid group-activated alkyne monomers, a catalyst-free, four-component polymerization of propiolic acids, benzylamines, organoboronic acids, and formaldehyde is reported under mild condition at 45 °C in dichloroethane. This four-component polymerization is applicable to different monomer structures, which can afford seven poly(propargylamine)s with up to 94% yields and molecular weights of up to 13 900 g mol^-1 . Moreover, the poly(propargylamine)s demonstrate good solubility and processibility, high thermal stability and light refractivity, unique photophysical property, and so on. The simple monomers, mild condition, low cost, high efficiency, and procedure simplicity of this catalyst-free four-component polymerization demonstrates an elegant example of functional polymer synthesis.

Transition Metal-Free Synthesis of Carbamates Using CO2 as the Carbon Source

Utilization of carbon dioxide as a C1 synthon is highly attractive for the synthesis of valuable chemicals. However, activation of CO₂ is highly challenging, owing to its thermodynamic stability and kinetic inertness. With this in mind, several strategies have been developed for the generation of carbon-heteroatom bonds. Among these, formation of C-N bonds is highly attractive, especially, when carbamates can be synthesized directly from CO₂ . This Minireview focuses on transition metal-free approaches for the fixation of CO₂ to generate carbamates for the production of fine chemicals and pharmaceuticals. Within the past decade, transition metal-free approaches have gained increasing attention, but traditional reviews have rarely focused on these approaches. Direct comparisons between such methods have been even more scarce. This Minireview seeks to address this discrepancy.

Synthesis of Tetrahydroindolizino[8,7-b]indole Derivatives in the Presence of Fe(OTf)3 or CF3SO3H through Intramolecular Dearomatization of Indole

We have developed a convenient synthesis of tetrahydroindolizino[8,7-b]indole derivatives via intramolecular dearomatization of indole. Highly functionalized tetrahydroindolizinoindoles can be prepared in the presence of trifluoromethanesulfonic acid in good to excellent yields (up to >99% yield) with novel designed pyrrole-tethered indoles. The same products can also be synthesized through a mild Fe(OTf)₃-catalyzed process in acceptable to good yields (up to 75% yield).

Transition metal-free and regioselective vinylation of phosphine oxides and H-phosphinates with VBX reagents

A series of phosphine oxides and H-phosphinates were vinylated in the presence of the iodine(iii) reagents vinylbenziodoxolones (VBX), providing the corresponding alk-1-enyl phosphine oxides and alk-1-enyl phosphinates in good yields with complete chemo- and regioselectivity. The vinylation proceeds in open flask under mild and transition metal-free conditions.

Arylation of aryllithiums with S-arylphenothiazinium ions for biaryl synthesis

Aryllithiums are one of the most common and important aryl nucleophiles; nevertheless, methods for arylation of aryllithums to produce biaryls have been limited. Herein, we report arylation of aryllithiums with S-arylphenothiazinium ions through selective ligand coupling of intermediary sulfuranes. Various unsymmetrical biaryls could be obtained without transition-metal catalysis.

Potassium tert-butoxide mediated C-C, C-N, C-O and C-S bond forming reactions

Potassium tertiary butoxide (KO^tBu) mediated constructions of C-C, C-O, C-N, and C-S bonds are reviewed with special emphasis on their synthetic applications. KO^tBu can be used to perform reactions already known to be carried out using transition metals, but it has advantages in terms of environmental congruence and economic cost. KO^tBu is widely employed in organic synthesis to mediate the construction of C-C, C-O, C-N, C-S and miscellaneous bonds in good to excellent yields. Synthetic uses of KO^tBu in coupling, alkylation, arylation, α-phenylation, cyclization, Heck-type, annulation, photo-arylation, aromatic-substitution, amidation, and silylation reactions are summarized and discussed. The mechanisms through which KO^tBu carries out a specific reaction are also discussed. One of the goals of this review is to attract the attention of chemists as to the benefits of using KO^tBu as an environmentally benign alternative to transition metals and its applications in the construction of chemical bonds with predominant importance in organic synthesis. This review completely covers the synthetic protocols that have been performed using KO^tBu in the last two decades.

Hypervalent iodine reagent-mediated reactions involving rearrangement processes

Hypervalent iodine reagents have been extensively employed in various types of oxidative organic reactions including oxidative coupling/cyclization, bifunctionalization of olefins and cyclopropane, C-H functionalization, and oxidative rearrangement reactions. In this review, the developments of the exclusive hypervalent iodine-mediated reactions involving oxidative rearrangement processes, including [1,2]-migration, Hofmann rearrangement, Beckmann rearrangement, ring contraction, ring expansion, [3,3]-sigmatropic/iodonium-Claisen rearrangement and some miscellaneous rearrangements, have been summarized.

Metal-Free Intramolecular Aminophosphination of Allenes

A metal-free intramolecular aminophosphination of sulfonamidoallenes with diarylphosphine oxides and Tf₂O was developed. This method offers a general and practical procedure to construct valuable alkenylphosphine-substituted N-heterocycles via the bifunctionalization reaction of allenes in good yields under mild conditions.

Transition-Metal-Free Cross-Coupling Reaction of Iodocarboranes with Terminal Alkynes Enabled by UV Light: Synthesis of 1-Alkynyl-o-Carboranes and Carborane-Fused Cyclics

A transition-metal-free coupling protocol between iodocarboranes and terminal alkynes enabled by light at room temperature has been developed, leading to the synthesis of a variety of 1-alkynyl-o-carboranes. Moreover, following this strategy, the introduction of 1-I-3-aryl-o-carboranes or 1-I-2-aryl-o-carboranes results in the formation of o-carborane-fused cyclics. Interestingly, when 1-I-3-(p-R-C₆H₄)-o-carboranes are chosen as coupling partners, unexpected R-group migration products are also isolated. On the basis of the results of control experiments and isolation of the key intermediates, a possible reaction mechanism is then proposed, involving the formation of spiro radical species.

Indole Carbonized Polymer Dots Boost Full-Color Emission by Regulating Surface State

Carbonized polymer dots (CPDs) are impressive imaging probes with great potential for enriching the library of metal-free fluorescent materials, yet current strategies have struggled to achieve products that emit full-color light in a single reaction system. Establishing an efficient and robust synthesis approach that unlocks the color barrier to the luminescence centers of specific CPDs remains a challenge. Herein, the surface-state engineering of pyridine and amide in the indole system to create a palette of resolvable full-color light-emissive CPDs is reported. Detailed structural analysis revealed that cationic polymerization and oxidation reactions potentially contribute to the formation of the main frameworks and emission centers of the final CPDs, with emissive oxygen- and nitrogen-based centers fixed by cross-linked polymer structures. This study provides valuable insight into the energy absorbance and photoluminescence mechanism of CPDs and introduces additional reactants (benzo heterocycle) into CPD research.

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Achieving a palette of full-color light-emissive CPDs in a single reaction system

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Providing the surface-state engineering rules for CPDs' emission centers

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Enriching the guidance library for studying the fluorescence mechanism of CPDs

De Novo Protocol for the Construction of Benzo[a]fluorenes via Nitrile/Alkene Activation

Unprecedented chemo- and regioselective synthesis of benzo[a]fluorenes and naphthamide-substituted benzo[a]fluorenes were constructed from the reaction of (E)-2-aroyl-3-(2-(arylalkynes/alkenes)aryl)acrylonitrile scaffolds under metal-free conditions via the activation of nitriles and alkenes, respectively. A tentative reaction mechanism was proposed for this homofunctionalization of nitriles. Control experiments showed that the reaction proceeds via selective nitrile or alkene protonation, depending upon the substrates. Additionally, we demonstrated an alternative expeditious route for the synthesis of disubstituted benzo[a]fluorenes in the presence of TfOH alone.

Benzylic Methylene Functionalizations of Diarylmethanes

Diarylmethanes are cardinal scaffolds by virtue of their unique structural feature including the presence of a benzylic CH2 group that can be easily functionalized to generate a variety of fascinating molecules holding immense importance in pharmaceutical, agrochemical, and material sciences. While the originally developed protocols for benzylic C-H functionalization in diarylmethanes employing base-mediated and metal-catalyzed strategies are still actively used, they are joined by a new array of metal-free conditions, offering milder and benign conditions. With the recent surge of interest towards the synthesis of functionalized diarylmethanes, numerous choices are now available for a synthetic organic chemist to transform the benzylic C-H bond to C-C or C-X bond offering the synthesis of any molecule of choice. This review highlights benzylic methylene (CH2 ) functionalizations of diaryl/heteroarylmethanes utilizing various base-mediated, transition-metal-catalyzed, and transition-metal free approaches for the synthesis of structurally diverse important organic molecules, often with a high chemo-, regio- and enantio-selectivity. This review also attempts to provide analysis of the scope and limitations, mechanistic understanding, and sustainability of the transformations.

Construction of Binuclear Benzimidazole-Fused Quinazolinones and Pyrimidinones Using Aryl Isocyanates as Building Blocks by Transition-Metal-Free C(sp2)-N Coupling

A class of binuclear N-fused hybrid scaffolds was constructed by the reaction of 2-(2-bromoaryl)- and 2-(2-bromovinyl)benzimidazoles with aryl isocyanates as building blocks in the presence of a base under microwave irradiation. A nucleophilic addition followed by an unprecedented transition-metal-free C(sp²)-N coupling is proposed as a reaction pathway of this green process.

Metal-Free Indole-Phenacyl Bromide Cyclization: A Regioselective Synthesis of 3,5-Diarylcarbazoles

A metal-free, simultaneous triple C-C coupling cyclization reaction between phenacyl bromides and indoles is discovered in a highly regioselective fashion to furnish 3,5-diarylcarbazoles. DMAP is utilized as the only reagent for the unusual and rapid cyclization reaction to furnish all new carbazole compounds through installation of a great diversity of substituents. A plausible radical mechanism for the new reaction is predicted by conducting various control experiments, competitive reactions, furoindole formation, and ESI-MS analyses of the ongoing cyclization reaction.

Metal-Free, Redox-Neutral, Site-Selective Access to Heteroarylamine via Direct Radical-Radical Cross-Coupling Powered by Visible Light Photocatalysis

Transition-metal-catalyzed C-N bond-forming reactions have emerged as fundamental and powerful tools to construct arylamines, a common structure found in drug agents, natural products, and fine chemicals. Reported herein is an alternative access to heteroarylamine via radical-radical cross-coupling pathway, powered by visible light catalysis without any aid of external oxidant and reductant. Only by visible light irradiation of a photocatalyst, such as a metal-free photocatalyst, does the cascade single-electron transfer event for amines and heteroaryl nitriles occur, demonstrated by steady-state and transient spectroscopic studies, resulting in an amine radical cation and aryl radical anion in situ for C-N bond formation. The metal-free and redox economic nature, high efficiency, and site-selectivity of C-N cross-coupling of a range of available amines, hydroxylamines, and hydrazines with heteroaryl nitriles make this protocol promising in both academic and industrial settings.

Transition-Metal-Free Oxidative Cross-Coupling of Tetraarylborates to Biaryls Using Organic Oxidants

Readily prepared tetraarylborates undergo selective (cross)-coupling through oxidation with Bobbitt's salt to give symmetric and unsymmetric biaryls. The organic oxoammonium salt can be used either as a stoichiometric oxidant or as a catalyst in combination with in situ generated NO2 and molecular oxygen as the terminal oxidant. For selected cases, oxidative coupling is also possible with NO2 /O2 without any additional nitroxide-based cocatalyst. Transition-metal-free catalytic oxidative ligand cross-coupling of tetraarylborates is unprecedented and the introduced method provides access to various biaryl and heterobiaryl systems.

Slicing and Splicing of Bromodifluoro-N-arylacetamides: Dearomatization and Difunctionalization of Pyridines

Copper-catalyzed dearomatization and difunctionalization of pyridines have been disclosed, in which bromodifluoro-N-arylacetamide was sliced into five fragments and three or four of them were transferred to pyridine partners. Through this reaction, novel N-difluoromethyl-2-imine dihydropyridine derivatives can be conveniently accessed from commercially available 4-amino substituted pyridines. This strategy demonstrates a novel fluorination method featuring high atom economy, environmental friendliness, an easily available catalyst, and simple operation.

Main-Group Metal Complexes in Selective Bond Formations Through Radical Pathways

Recent years have witnessed remarkable advances in radical reactions involving main-group metal complexes. This includes the isolation and detailed characterization of main-group metal radical compounds, but also the generation of highly reactive persistent or transient radical species. A rich arsenal of methods has been established that allows control over and exploitation of their unusual reactivity patterns. Thus, main-group metal compounds have entered the field of selective bond formations in controlled radical reactions. Transformations that used to be the domain of late transition-metal compounds have been realized, and unusual selectivities, high activities, as well as remarkable functional-group tolerances have been reported. Recent findings demonstrate the potential of main-group metal compounds to become standard tools of synthetic chemistry, catalysis, and materials science, when operating through radical pathways.

The C3-H Bond Functionalization of Quinoxalin-2(1H)-Ones With Hypervalent Iodine(III) Reagents

The modification of quinoxalin-2(1H)-ones via direct C-H bond functionalization has begun to receive widespread attention, due to quinoxalin-2(1H)-one derivatives' various biological activities and pharmaceutical properties. This mini review concentrates on the accomplishments of arylation, trifluoromethylation, alkylation, and alkoxylation of quinoxalin-2(1H)-ones with hypervalent iodine(III) reagents as reaction partners or oxidants. The reaction conditions and mechanisms are compared and discussed in detail.

Catalytic conversion of ketones to esters via C(O)-C bond cleavage under transition-metal free conditions

The catalytic conversion of ketones to esters via C(O)-C bond cleavage under transition-metal free conditions is reported. This catalytic process proceeds under solvent-free conditions and offers an easy operational procedure, broad substrate scope with excellent selectivity, and reaction scalability.

Transition-Metal-Free Site-Selective γ-C(sp2)-H Monoiodination of Arenes Directed by an Aliphatic Keto Group

A general γ-C(sp²)-H iodination method directed by an aliphatic keto group has been developed under transition-metal-free conditions for the first time, generating iodoarenes in good to excellent yields with excellent site selectivity. This protocol features a wide range of aryl-substituted ketones, short reaction times, mild reaction conditions, and scalable synthetic procedures. A possible reaction mechanism was also proposed based on several control experiments.

Transition-Metal-Free C(sp2 )-C(sp2 ) Cross-Coupling of Diazo Quinones with Catechol Boronic Esters

A transition-metal-free C(sp² )-C(sp² ) bond formation reaction by the cross-coupling of diazo quinones with catechol boronic esters was developed. With this protocol, a variety of biaryls and alkenyl phenols were obtained in good to high yields under mild conditions. The reaction tolerates various functionalities and is applicable to the derivatization of pharmaceuticals and natural products. The synthetic utility of the method was demonstrated by the short synthesis of multi-substituted triphenylenes and three bioactive natural products, honokiol, moracin M, and stemofuran A. Mechanistic studies and density functional theory (DFT) calculations revealed that the reaction involves attack of the boronic ester by a singlet quinone carbene followed by a 1,2-rearrangement through a stepwise mechanism.

Highly Reactive Cyclic Monoaryl Iodoniums Tuned as Carbene Generators Couple with Nucleophiles under Metal-Free Conditions

Cross-coupling reactions between aryl iodide and nucleophiles have been well developed. Iodoniums equipped with a reactive C-I(III) bond accelerate cross-coupling reactions of aryl iodide. Among them, cyclic diaryliodoniums are more atom economical; however; they are often in the trap of metal reliance and encounter regioselectivity issues. Now, we have developed a series of highly reactive cyclic monoaryl-vinyl iodoniums that can be tuned to construct C-N, C-O, and C-C bonds without metal catalysis. Under promotion of triethylamine, coupling reactions with aniline, phenol, aromatic acid, and indole proceed rapidly and regioselectively at room temperature. The carbene species is conceptualized as a key intermediate in our mechanism model. Furthermore, the coupling products enable diversity-oriented synthesis strategy to further build up a chemical library of diverse heterocyclic fragments that are in demand in the drug discovery field. Our current work provides a deep insight into the synthetic application of these highly reactive cyclic iodoniums.

Organocatalyst in Direct C(sp2)-H Arylation of Unactivated Arenes: [1-(2-Hydroxyethyl)-piperazine]-Catalyzed Inter-/Intra-molecular C-H Bond Activation

This article describes the identification of 1-(2-hydroxyethyl)-piperazine as a new, cost-effective, highly efficient organocatalyst, which promotes both inter- and intra-molecular direct C(_sp²)-H arylations of unactivated arenes in the presence of potassium tert-butoxide. While the inter-molecular C-H arylation of unactivated benzenes with aryl halides (Ar-X; X = I, Br, Cl) toward biaryl syntheses underwent smoothly in the presence of only 10 mol % organocatalyst, the intra-molecular C-H arylation catalytic system composed of 40 mol % each of the catalyst and the additive (4-dimethylaminopyridine (DMAP)). The novel catalyst was also able to perform both inter- and intra-molecular direct arylations simultaneously in a single pot. The mechanistic studies confirmed the involvement of aryl radical anions and proceeded via a single-electron-transfer (SET) mechanism. The large substrate scope, high functional group tolerance, competition experiments, gram-scale synthesis, and kinetic studies further highlight the importance and versatile nature of the methodology as well as the compatibility of the new catalyst. To the best of our knowledge, this is the first report on any organocatalyst that reported detailed investigations of both inter- and intra-molecular direct C(_sp²)-H arylations of unactivated arenes in a single representation.

Sulfonate Versus Sulfonate: Nickel and Palladium Multimetallic Cross-Electrophile Coupling of Aryl Triflates with Aryl Tosylates

While phenols are frequent and convenient aryl sources in cross-coupling, typically as sulfonate esters, the direct cross-Ullmann coupling of two different sulfonate esters is unknown. We report here a general solution to this challenge catalyzed by a combination of Ni and Pd with Zn reductant and LiBr as an additive. The reaction has broad scope, as demonstrated in 33 examples (65% ± 11% average yield). Mechanistic studies show that Pd strongly prefers the aryl triflate, the Ni catalyst has a small preference for the aryl tosylate, aryl transfer between catalysts is mediated by Zn, and Pd improves yields by consuming arylzinc intermediates.

Transition-metal-free formal cross-coupling of aryl methyl sulfoxides and alcohols via nucleophilic activation of C-S bond

Employment of sulfoxides as electrophiles in cross-coupling reactions remains underexplored. Herein we report a transition-metal-free cross-coupling strategy utilizing aryl(heteroaryl) methyl sulfoxides and alcohols to afford alkyl aryl(heteroaryl) ethers. Two drug molecules were successfully prepared using this protocol as a key step, emphasizing its potential utility in medicinal chemistry. A DFT computational study suggests that the reaction proceeds via initial addition of the alkoxide to the sulfoxide. This adduct facilitates further intramolecular addition of the alkoxide to the aromatic ring wherein charge on the aromatic system is stabilized by the nearby potassium cation. Rate-determining fragmentation then delivers methyl sulfenate and the aryl or heteroaryl ether. This study establishes the feasibility of nucleophilic addition to an appended sulfoxide as a means to form a bond to aryl(heteroaryl) systems and this modality is expected to find use with many other electrophiles and nucleophiles leading to new cross-coupling processes.

Cross-coupling processes without the use of transition metals are challenging to achieve. Here, the authors show a transition-metal-free cross-coupling utilizing aryl(heteroaryl) methyl sulfoxides and alcohols to afford alkyl aryl(heteroaryl) ethers and propose a nucleophilic addition mechanism based on experiments and theory.

DMSO/t-BuONa/O2-Mediated Aerobic Dehydrogenation of Saturated N-Heterocycles

Aromatic N-heterocycles such as quinolines, isoquinolines, and indolines are synthesized via sodium tert-butoxide-promoted oxidative dehydrogenation of the saturated heterocycles in DMSO solution. This reaction proceeds under mild reaction conditions and has a good functional group tolerance. Mechanistic studies suggest a radical pathway involving hydrogen abstraction of dimsyl radicals from the N-H bond or α-C-H of the substrates and subsequent oxidation of the nitrogen or α-aminoalkyl radicals.