Green advancements towards the electrochemical synthesis of heterocycles

Heterocyclic chemistry is a large field with diverse applications in the areas of biological research and pharmaceutical advancement. Numerous initiatives have been proposed to further enhance the reaction conditions to reach these compounds without using harmful compounds. This paper focuses on the recent advances in the eco-friendly and green synthetic procedures to synthesize N-, S-, and O-heterocycles. This approach demonstrates considerable potential in accessing such compounds while circumventing the need for stoichiometric quantities of oxidizing/reducing agents or catalysts containing precious metals. Merely employing catalytic quantities of these substances proves sufficient, thereby offering an optimal means of contributing to resource efficiency. Renewable electricity plays a crucial role in generating environmentally friendly electrons (oxidant/reductant) that serve as catalysts for a series of reactions. These reactions involve the production of reactive intermediates, which in turn allow the synthesis of new chemical bonds, enabling beneficial transformations to occur. Furthermore, the utilization of metals as active catalysts in electrochemical activation has been recognized as an effective approach for achieving selective functionalization. The aim of this review was to summarize the electrochemical synthetic procedures so that the undesirable side reactions can be considerably reduced and the practical potential range of the chemical reactions can be expanded significantly.

Visible-Light-Enabled Catalytic Approach to N,O-Spirocycles through Amidyl Radical Addition/Cyclization

A rational combination of photoredox catalyst anthraquinone and hydrogen atom transfer (HAT) catalyst methyl thioglycolate allows for the rapid and straightforward conversion of a range of 2-amidated acetylenic alcohols to multifunctional N,O-spirocycles under visible light irradiation. With oxygen as the sole terminal oxidant, these reactions can be carried out efficiently at room temperature without the involvement of transition metals or strong oxidants. The successful application of this mild catalytic strategy in the late-stage functionalization of bioactive skeletons further highlights its practical value.

Electrochemical annulation of 1,2,3-benzotriazinones with alkynes to access isoquinolin-1(2H)-ones

An eco-friendly approach for electrochemical radical cascade annulation of 1,2,3-benzotriazinones with alkynes is described. Under catalyst-free and external reductant-free electrolysis conditions, a range of isoquinolin-1(2H)-ones were obtained in moderate to good yields. Cyclic voltammetry and control studies suggest that the reaction proceeds via a radical pathway. Furthermore, this approach could be easily scaled up.

Heterocyclic Electrochemistry: Renewable Electricity in the Construction of Heterocycles

Numerous applications in the realm of biological exploration and drug synthesis can be found in heterocyclic chemistry, which is a vast subject. Many efforts have been developed to further improve the reaction conditions to access this interesting family to prevent employing hazardous ingredients. In this instance, it has been stated that green and environmentally friendly manufacturing methodologies have been introduced to create N-, S-, and O-heterocycles. It appears to be one of the most promising methods to access these types of compounds avoiding use of stoichiometric amounts of oxidizing/reducing species or precious metal catalysts, in which only catalytic amounts are sufficient, and it represent an ideal way of contributing toward the resource economy. Thus, renewable electricity provides clean electrons (oxidant/reductant) that initiate a reaction cascade via producing reactive intermediates that facilitate in building new bonds for valuable chemical transformations. Moreover, electrochemical activation using metals as catalytic mediators has been identified as a more efficient strategy toward selective functionalization. Thus, indirect electrolysis makes the potential range more practical, and less side reactions can occur. The latest developments in using an electrolytic strategy to create N-, S-, and O-heterocycles are the main topic of this mini review, which was documented over the last five years.

Recent advances in electrooxidative radical transformations of alkynes

During the past few years, electrochemical oxidative reactions through radical intermediates have emerged as an environmentally-benign, powerful platform for the facile formation of C–E (E = C, N, S, Se, O and Hal) bonds through single-electron-transfer (SET) processes at the electrodes. Functionalized unsaturated molecules and unusual structural motifs can, for instance, be directly constructed under exceedingly mild reaction conditions through initial radical attack onto alkynes. This minireview highlights the recent advances in electrooxidation in radical reactions until June 2022, with a particular focus on radical additions onto alkynes.

Electro-organic synthesis: an environmentally benign alternative for heterocycle synthesis

Heterocyclic compounds are considered to be one of the most established structural classes due to their extensive application in agrochemicals, pharmaceuticals and organic materials. Over the past few years, the development of heterocyclic compounds has gone through a considerable renaissance from conventional traditional methodologies to non-conventional electro-organic synthesis. Replacing metal catalysts, strong oxidants and multi-step methodologies with metal and strong oxidant-free single-step protocols has revolutionized the field of sustainable organic synthesis. Electro-organic synthesis has evolved as a scalable and sustainable approach in different synthetic protocols in an environment-benign manner. The current review outlines the recent developments in C-C, C-N, C-S and C-O/Se bond formation for heterocycle synthesis using electrochemical methods. Different synthetic strategies and their detailed mechanistic description are presented to enlighten the future applications of electrochemistry in heterocycle synthesis.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Electrocatalytic three-component synthesis of 4-halopyrazoles with sodium halide as the halogen source

The first example of the electrocatalytic multicomponent synthesis of 4-chloro/bromo/iodopyrazoles from hydrazines, acetylacetones and sodium halides under chemical oxidant- and external electrolyte-free conditions has been developed.

Electrochemically-initiated intramolecular 1,2-amino oxygenation of alkynes: facile access to formyl- and acyl-substituted indolizines

An environmentally benign electrooxidative approach to the intramolecular aminooxygenation of alkynes through an electrophilic cyclization reaction has been developed, providing an efficient approach toward diverse formyl- and acyl-substituted indolizines.

Electrochemical Oxo-Fluorosulfonylation of Alkynes under Air: Facile Access to β-Keto Sulfonyl Fluorides

Radical fluorosulfonylation is emerging as an appealing approach for the synthesis of sulfonyl fluorides, which have widespread applications in many fields, in particular in the context of chemical biology and drug development. Here, we report the first investigation of FSO₂ radical generation under electrochemical conditions, and the establishment of a new and facile approach for the synthesis of β-keto sulfonyl fluorides via oxo-fluorosulfonylation of alkynes with sulfuryl chlorofluoride as the radical precursor and air as the oxidant. This electrochemical protocol is amenable to access two different products (β-keto sulfonyl fluorides or α-chloro-β-keto sulfonyl fluorides) with the same reactants. The β-keto sulfonyl fluoride products can be utilized as useful building blocks in the synthesis of various derivatives and heterocycles, including the first synthesis of an oxathiazole dioxide compound. Furthermore, some β-keto sulfonyl fluorides and derivatives exhibited notably potent activities against Bursaphelenchus xylophilus and Colletotrichum gloeosporioides.

Synthesis of 1H-indazoles by an electrochemical radical Csp2-H/N-H cyclization of arylhydrazones

The development of efficient and sustainable C-N bond-forming reactions to N-heterocyclic frameworks has been a long-standing interest in organic synthesis. In this work, we develop an electrochemical radical C_sp²-H/N-H cyclization of arylhydrazones to 1H-indazoles. The electrochemical anodic oxidation approach was adopted to synthesize a variety of 1H-indazole derivatives in moderate to good yields. HFIP was not only employed as a solvent or the proton donor, but also can promote the formation of N free radicals. This synthetic methodology is operationally simple, and less expensive electrodes would be suitable for this chemistry.

Discovery of a tetraarylhydrazine catalyst for electrocatalytic synthesis of imidazo-fused N-heteroaromatic compounds

The development of electrocatalytic synthetic methods hinges on efficient molecular catalysts. Triarylamines are well-known redox catalysts because of the good stability of their corresponding amine radical cations. Herein we show that tris(4-(tert-butyl)phenyl)amine decomposes unexpectedly during electrolysis in MeOH/THF to afford a tetraarylhydrazine, 1,1,2,2-tetrakis(4-(tert-butyl)phenyl)hydrazine. In addition, we have applied this tetraarylhydrazine, which is either preprepared or formed in situ from tris(4-(tert-butyl)phenyl)amine, as an electrocatalyst for the synthesis of imidazopyridines and related N-heteroaromatic compounds through intramolecular [3 + 2] annulation. This metal-free electrocatalytic method provides straightforward access to the N-heteroaromatic compounds from readily available materials without the need for external chemical oxidants.

Electrocatalytic Dehydrogenative Cyclization of 2-Vinylanilides for the Synthesis of Indoles

Indole is prevalent in bioactive compounds and natural products. The development of efficient and sustainable methods to access this privileged structural scaffold has been a long-standing interest of synthetic chemists. Herein, we report an electrocatalytic method for the synthesis of indoles through dehydrogenative cyclization of 2-vinylanilides. The reactions employ an organic redox catalyst and do not require any external chemical oxidant, providing speedy and efficient access to 3-substituted and 2,3-disubstituted indoles.

Electrochemical Generation and Use in Organic Synthesis of C-, O-, and N-Centered Radicals

During the last decade several research groups have been developing electrochemical procedures to access highly functionalized organic molecules. Among the most exciting advances, the possibility of using free radical chemistry has attracted the attention of the most important synthetic groups. Nowadays, electrochemical strategies based on these species with a synthetic purpose are published continuously in scientific journals, increasing the alternatives for the synthetic organic chemistry laboratories. Free radicals can be obtained in organic electrochemical reactions; thus, this review reassembles the last decade's (2010-2020) efforts of the electrosynthetic community to generate and take advantage of the C-, O-, and N-centered radicals' reactivity. The electrochemical reactions that occur, as well as the proposed mechanism, are discussed, trying to give clear information about the used conditions and reactivity of these reactive intermediate species.

Electrochemically Driven Radical Reactions: From Direct Electrolysis to Molecular Catalysis

Organic radicals are versatile synthetic intermediates that provide reactivities and selectivities complementary to ionic species. Despite its long history, electrochemically driven radical reactions remain limited in scope. In the past few years, there have been dramatic increase in research activity in organic electrochemistry. We have been developing electrochemical and electrophotocatalytic methods for the generation and synthetic utilization of organic radicals. In our studies, various radical species such as alkene and arene radical cations and carbon- and heteroatom-centered radicals are generated from readily available precursors through direct electrolysis, molecular electrocatalysis or molecular electrophotocatalysis. These radical species undergo various inter- and intramolecular oxidative transformations to rapidly increase molecular complexity. The simultaneous occurrence of anodic oxidation and cathodic proton reduction allows the oxidative reactions to proceed through H₂ evolution without external chemical oxidants.

Electrifying Sustainability on Transition Metal-Free Modes: An Eco-Friendly Approach for the Formation of C-N Bonds

Embracing sustainable green methodologies and techniques in chemical transformations has always been in the limelight to the synthetic community. Electrosynthesis has emerged as a powerful, sustainable synthetic tool for molecular synthesis exploiting inexpensive electricity in place of sacrificial chemical oxidizing/reducing reagents. Herein, recent advances in the incorporation of transition metal-free redox mediators in electrosynthesis for the construction of C-N bonds are outlined. Furthermore, conjugation of this strategy with flow catalysis allows easy scale up of the synthesis of molecular assembly. This comprehensive Review provides an overview of metal-free mediated electro-construction of C-N bonds, focusing on the reaction mechanisms involved and its synthetic applications.

Electrochemically facilitated oxidative C–H amination enables access to fluorescent N9-fused tricyclic xanthines

An electrochemically enabled intramolecular C−H amination route for accessing a broad range of fluorescent N9-fused tricyclic xanthines with various substitution patterns under simple, green, and mild condition is developed.

Regioselective Intramolecular Allene Amidation Enabled by an EDA Complex*

The addition of radicals to unsaturated precursors is a powerful tool for the synthesis of both carbo- and heterocyclic organic building blocks. The recent advent of mild ways to generate N-centered radicals has reignited interest in exploiting highly regio-, chemo-, and stereoselective transformations that employ these reactive intermediates. While the additions of aminyl, iminyl, and amidyl radicals to alkenes and alkynes have been well-studied, analogous additions to allenes are scarce. Allenes offer several attractive features, including potential for selective amidation at three distinct sites via judicious choice of precursor or radical source, the opportunity for axial-to-point chirality transfer, and productive trapping of vinyl or allyl radical intermediates to diversify functionality in the products. In this article, we report a regioselective addition of amidyl radicals to allenes to furnish an array of valuable N-heterocycle scaffolds.