Use of Electrochemistry in the Synthesis of Heterocyclic Structures

Ruthenium(ii)-catalyzed electrooxidative [4+2] annulation of benzylic alcohols with internal alkynes: entry to isocoumarins

A new ruthenium(ii)-catalyzed electrooxidative [4+2] annulation of primary benzylic alcohols with internal alkynes is described, which enables benzylic alcohols as weakly directing group precursors to access isocoumarins via multiple C-H functionalization. The reaction works well with a broad substrate scope, tolerates a wide range of functional groups, and incorporates practically the isocoumarin unit into diverse bioactive molecules. Mechanistic studies indicate that activation of aryl C(sp²)-H bonds is achieved through the generation of the active benzoyloxy-Ru(ii) intermediates via oxidation of benzylic alcohols using an electrooxidative Ru(ii) catalyst.

Synergy of anodic oxidation and cathodic reduction leads to electrochemical deoxygenative C2 arylation of quinoline N-oxides

We herein report an electrochemical deoxygenative C2 arylation protocol of quinoline N-oxides. By employing both anodic oxidation and cathodic reduction, a variety of 2-arylquinolines were obtained under environmentally benign conditions.

The Mn-catalyzed paired electrochemical facile oxychlorination of styrenes via the oxygen reduction reaction

Reported herein is the electrochemical engendering of chlorine radicals by a manganese catalyst with a controllable pattern, and inexpensive MgCl₂ as the chlorine source.

Electrochemical 1,4-reduction of α,β-unsaturated ketones with methanol and ammonium chloride as hydrogen sources

This protocol presented a sustainable, chemoselective 1,4-reduction of α,β-unsaturated ketones by means of an electrochemical method with ammonium chloride (NH₄Cl) as the only additive.

Electrochemically driven P–H oxidation and functionalization: synthesis of carbamoylphosphonates from phosphoramides and alcohols

Electrochemical synthesis of carbamoylphosphonates via P–H phosphorylation and oxygenation of phosphinecarboxamides with alcohols by using n-Bu₄NI (10 mol%) as an iodine source.

Electrochemical vicinal aminotrifluoromethylation of alkenes: high regioselective acquisition of β-trifluoromethylamines

A high-regioselective electrochemical aminotrifluoromethylation of alkenes leading to β-trifluoromethylamines using CF₃SO₂Na as a trifluoromethyl precursor and acetonitrile as an N-nucleophile was achieved.

Triphenylphosphine-assisted dehydroxylative Csp3–N bond formation via electrochemical oxidation

A dehydroxylative Csp³–N coupling by electrochemical oxidation with readily available alcohols as substrates and a wide variety of azoles and amides as N-nucleophiles.

Electrochemical Minisci-type trifluoromethylation of electron-deficient heterocycles mediated by bromide ions

An electrochemical methodology for the Minisci-type trifluoromethylation of electron-deficient heterocycles mediated by cheap and easily available bromide ions has been developed.

External oxidant-free oxidation/[3+2] cycloaddition/aromatization cascade: electrochemical synthesis of polycyclic N-heterocycles

The extent of the electrochemical oxidative [3+2] cycloaddition/aromatization cascade can be regulated with the assistance of redox mediators. Isoquinolinium salts as alternative azomethine ylide precursors afforded fully aromatized N-heterocycles.

Exogenous-oxidant- and catalyst-free electrochemical deoxygenative C2 sulfonylation of quinoline N-oxides

By employing quinoline N-oxides as the starting materials, the electrochemical C–H sulfonylation of electron-deficient quinolines was indirectly achieved at room temperature.

A metal- and oxidizing-reagent-free anodic para-selective amination of anilines with phenothiazines

A highly para-selective amination of anilines with phenothiazines for producing various functionalized 10-aryl-10H-phenothiazines is reported.

Catalyst-free electrochemical decarboxylative cross-coupling of N-hydroxyphthalimide esters and N-heteroarenes towards C(sp3)–C(sp2) bond formation

We formed C(sp³)–C(sp²) bonds under electrochemical conditions by using NHP esters and N-heteroarenes without any catalysts. Our approach could be a complement to the Kolbe reaction and a promising strategy for finding more new reactions.

Organic electrosynthesis: electrochemical alkyne functionalization

We outline examples of electrochemical alkyne functionalization reactions in connection with green and sustainable chemistry that proceed with excellent atom economy.

Electrochemically induced oxidative S–O coupling: synthesis of sulfonates from sulfonyl hydrazides and N-hydroxyimides or N-hydroxybenzotriazoles

A variety of sulfonates were synthesized from sulfonyl hydrazides and N-hydroxy compounds via electrochemically induced oxidative S–O bond formation.

Electrochemically Enabled Carbohydroxylation of Alkenes with H2O and Organotrifluoroborates

Unprecedented hydroxy-alkynylation and -alkenylation reactions of arylalkenes have been developed through electrochemically enabled addition of an organotrifluoroborate reagent and H₂O across the double bond of the alkene. The use of electrochemistry to promote these oxidative alkene 1,2-difunctionalization reactions not only obviates the need for transition-metal catalysts and oxidizing reagents but also ensures high regio- and chemoselectivity to afford homopropargylic or homoallylic alcohols. The possibility of extending the electrochemical alkene difunctionalization strategy to other alkene carbo-heterofunctionalization reactions has been demonstrated.

A general electrochemical strategy for the Sandmeyer reaction

Herein we report a general electrochemical strategy for the Sandmeyer reaction.

Herein we report a general electrochemical strategy for the Sandmeyer reaction. Using electricity as the driving force, this protocol employs a simple and inexpensive halogen source, such as NBS, CBrCl₃, CH₂I₂, CCl₄, LiCl and NaBr for the halogenation of aryl diazonium salts. In addition, we found that these electrochemical reactions could be performed using anilines as the starting material in a one-pot fashion. Furthermore, the practicality of this process was demonstrated in the multigram scale synthesis of aryl halides using highly inexpensive graphite as the electrode. A series of detailed mechanism studies have been performed, including radical clock and radical scavenger study, cyclic voltammetry analysis and in situ electron paramagnetic resonance (EPR) analysis.

A Multicomponent Electrosynthesis of 1,5-Disubstituted and 1-Aryl 1,2,4-Triazoles

A novel electrochemical route has been developed for the synthesis of 1,5-disubstituted and 1-aryl 1,2,4-triazoles from aryl hydrazines, paraformaldehyde, NH₄OAc, and alcohols. In this multicomponent reaction system, alcohols act as solvents as well as reactants and NH₄OAc is used as the nitrogen source. With the assistance of reactive iodide radical or I₂ and NH₃ electrogenerated in situ, this process could effectively avoid the use of strong oxidants and transition-metal catalysts and be smoothly carried out at room temperature to give a wide array of 1,2,4-triazole derivatives in good to high yields. Preliminary studies reveal that the reaction mechanism involves a radical process.

Dehydrogenative reagent-free annulation of alkenes with diols for the synthesis of saturated O-heterocycles

Dehydrogenative annulation reactions are among the most straightforward and efficient approach for the preparation of cyclic structures. However, the applications of this strategy for the synthesis of saturated heterocycles have been rare. In addition, reported dehydrogenative bond-forming reactions commonly employ stoichiometric chemical oxidants, the use of which reduces the sustainability of the synthesis and brings safety and environmental issues. Herein, we report an organocatalyzed electrochemical dehydrogenative annulation reaction of alkenes with 1,2- and 1,3-diols for the synthesis of 1,4-dioxane and 1,4-dioxepane derivatives. The combination of electrochemistry and redox catalysis using an organic catalyst allows the electrosynthesis to proceed under transition metal- and oxidizing reagent-free conditions. In addition, the electrolytic method has a broad substrate scope and is compatible with many common functional groups, providing an efficient and straightforward access to functionalized 1,4-dioxane and 1,4-dioxepane products with diverse substitution patterns.

Dehydrogenative annulation is a valuable approach to heterocycles, however, stoichiometric oxidants are often required. Here, the authors describe the electrochemical dehydrogenative annulation of diols and alkenes to generate dioxanes and dioxepanes under metal- and oxidant-free conditions.

Copper-Catalyzed Electrochemical C-H Amination of Arenes with Secondary Amines

Electrochemical oxidation represents an environmentally friendly solution to conventional methods that require caustic stoichiometric chemical oxidants. However, C-H functionalizations merging transition-metal catalysis and electrochemical techniques are, to date, largely confined to the use of precious metals and divided cells. Herein, we report the first examples of copper-catalyzed electrochemical C-H aminations of arenes at room temperature using undivided electrochemical cells, thereby providing a practical solution for the construction of arylamines. The use of n-Bu₄NI as a redox mediator is crucial for this transformation. On the basis of mechanistic studies including kinetic profiles, isotope effects, cyclic voltammetric analyses, and radical inhibition experiments, the reaction appears to proceed via a single-electron-transfer (SET) process, and a high valent Cu(III) species is likely involved. These findings provide a new avenue for transition-metal-catalyzed electrochemical C-H functionalization reactions using redox mediators.

Electrochemical strategies for C-H functionalization and C-N bond formation

Conventional methods for carrying out carbon-hydrogen functionalization and carbon-nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon-carbon and carbon-heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon-hydrogen functionalization and carbon-nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.

Facile Electrochemical Intramolecular Amination of Urea-Tethered Terminal Alkenes for the Synthesis of Cyclic Ureas

Facile intramolecular amination of unactivated alkenes has been achieved by using electricity as a catalyst that helps to generate an intermediate and accelerates formation of cyclic ureas in high yields. Using this method, no metal catalysts were used. During electrolysis, a nitrogen radical was formed at the urea substrate that cyclised with the alkene and generated a terminal carbon radical which further formed a bond with the 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO). This method of electrolysis not only gives cyclic ureas but also functionalises terminal unactivated alkenes. This method can be considered to be environmentally friendly given that it avoids the issues of toxicity or complicated metal ligands and could therefore be potentially employed in green chemistry.

Electrochemical oxidative oxysulfenylation and aminosulfenylation of alkenes with hydrogen evolution

Difunctionalization of alkenes is a valuable and versatile chemical transformation that could quickly build complex molecules. Extensive efforts have been made, and great achievement, such as Sharpless aminohydroxylation and dihydroxylation, has been reached. However, in marked contrast to the extensive research of aminohydroxylation and dihydroxylation, directly using thiophenols/thiols and O/N-nucleophiles to perform the difunctionalization of alkenes that form the C-S and C-O/N bonds together is still underexplored. The main issue is that thiophenols/thiols are often easily overoxidized to sulfoxides or sulphones under such essential oxidation conditions. We demonstrate an electrochemical oxidative oxysulfenylation and aminosulfenylation of alkenes. A critical feature of this transformation is that neither external chemical oxidants nor metal catalysts are required. This electrochemical oxidative synthetic strategy could also be applied for the hydroxysulfenylation and acyloxysulfenylation of alkenes.

Electrochemical Arylation Reaction

Arylated products are found in various fields of chemistry and represent essential entities for many applications. Therefore, the formation of this structural feature represents a central issue of contemporary organic synthesis. By the action of electricity the necessity of leaving groups, metal catalysts, stoichiometric oxidizers, or reducing agents can be omitted in part or even completely. The replacement of conventional reagents by sustainable electricity not only will be environmentally benign but also allows significant short cuts in electrochemical synthesis. In addition, this methodology can be considered as inherently safe. The current survey is organized in cathodic and anodic conversions as well as by the number of leaving groups being involved. In some electroconversions the reagents used are regenerated at the electrode, whereas in other electrotransformations free radical sequences are exploited to afford a highly sustainable process. The electrochemical formation of the aryl-substrate bond is discussed for aromatic substrates, heterocycles, other multiple bond systems, and even at saturated carbon substrates. This survey covers most of the seminal work and the advances of the past two decades in this area.