Harnessing the Reactivity of Nitroarene Radical Anions to Create Quinoline N-Oxides by Electrochemical Reductive Cyclization

Electrochemical reduction of 2-allyl-substituted nitroarenes using a simple, undivided electrochemical cell with non-precious electrodes to generate nitroarene radical anions was developed. The nitroarene radical anion intermediates participate in 1,5-hydrogen atom transfer reactions to construct quinoline N-oxides bearing aryl-, heteroaryl-, alkenyl-, benzyl-, sulfonyl-, or carboxyl groups.

Simple and versatile electrochemical synthesis of highly substituted 2,1-benzisoxazoles

A sustainable, general and scalable electrochemical protocol for direct access to 3-(acylamidoalkyl)-2,1-benzisoxazoles by cathodic reduction of widely accessible nitro arenes is established. The method is characterised by a simple undivided set-up under constant current conditions, inexpensive and reusable carbon-based electrodes, and environmentally benign reaction conditions. The versatility of the developed protocol is demonstrated on 39 highly diverse examples with up to 81% yield. A 50-fold scale-up electrolysis highlights its relevance for preparative applications.

In-Situ Electrolyte for Electrosynthesis: Scalable Anodically-Enabled One-Pot Sequence from Aldehyde to Isoxazol(in)es

Electrochemical transformations are considered a green alternative to classical redox chemistry as it eliminates the necessity for toxic and waste producing redox reagents. Typical electrochemical reactions require the addition of a supporting electrolyte - an ionic compound to facilitate reaction medium conductivity. However, this is often accompanied by an increase in the amount of produced waste. Here, we report an "in-situ electrolyte" concept for facile, transition-metal-free, additive-free one-pot electrochemical preparation of isoxazol(in)es, important scaffolds for biologically active natural and synthetic molecules, from the respective aldehydes. The protocol utilizes no halogenated solvents and no external oxidants, while salt side-products provide the ionic conductivity necessary for the electrosynthesis. The electrolysis is performed in an undivided cell, using the state-of-the-art electrodes for the chlor-alkali industry dimensionally stable and scalable mixed metal oxide anode and platinized titanium cathode of high durability. The cascade transformation comprises the condensation of aldehyde to oxime followed by its anodic oxidation and subsequent intra- and/or intermolecular [3+2] cycloadditions with an appropriate dipolarophile. Chemical yields up to 97 %, and good Faradaic efficiency, scalability, and stability are observed for most substrates in a broad scope.

Highly Selective Electrosynthesis of 1H-1-Hydroxyquinol-4-ones-Synthetic Access to Versatile Natural Antibiotics

1H-1-Hydroxyquinolin-4-ones represent a broad class of biologically active heterocycles having an exocyclic N,O motif. Electrosynthesis offers direct, highly selective, and sustainable access to 1-hydroxyquinol-4-ones by nitro reduction. A versatile synthetic route starting from easily accessible 2-nitrobenzoic acids was established. The broad applicability of this protocol was demonstrated on 26 examples with up to 93% yield, highlighted by the naturally occurring antibiotics Aurachin C and HQNO. The practicability and technical relevance were underlined by multigram scale electrolysis.

Exploiting Nitroreductases for the Tailored Photoenzymatic Synthesis of Structurally Diverse Heterocyclic Compounds

N-heterocyclic compounds have a broad range of applications and their selective synthesis is very appealing for the pharmaceutical and agrochemical industries. Herein we report the usage of the flavin-dependent nitroreductase BaNTR1 for the photoenzymatic synthesis of various anthranils and quinolines from retro-synthetically designed o-nitrophenyl-substituted carbonyl substrates, achieving high conversions (up to >99 %) and good product yields (up to 96 %). Whereas the effective production of anthranils required the inclusion of H2O2 in the reaction mixtures to accumulate the needed hydroxylamine intermediates, the formation of quinolines required the use of anaerobic or reducing conditions to efficiently generate the essential amine intermediates. Critical to our success was the high chemoselectivity of BaNTR1, performing selective reduction of the nitro group without reduction of the carbonyl moiety or the activated carbon-carbon double bond. The results highlight the usefulness of an innocuous chlorophyll- and nitroreductase-based photoenzymatic system for the tailored synthesis of diverse N-heterocycles from simple nitro compounds.

Simple electrochemical synthesis of cyclic hydroxamic acids by reduction of nitroarenes

The electrochemical reduction of nitroarenes allows direct access to manifold nitrogen containing heterocycles. This work reports the simple and direct electro-organic synthesis of 18 different examples of 2H,4H-4-hydroxy-1,4-benzoxazin-3-ones in up to 81% yield. The scalability of the method was demonstrated on a gram-scale.

Roughness-Dependent Electro-Reductive Coupling of Nitrobenzenes and Aldehydes on Copper Electrodes

The electro-reductive coupling of nitro and carbonyl compounds enables a facile, environmentally friendly and energy benign transformation toward value-added nitrones or imines, but the selectivity is still challenging. Here, the surface roughness of Cu electrodes is introduced for the first time as the determinant to switch products from nitrones to imines owing to the controllable reduction of nitroarenes to hydroxylamines or amines on tailored Cu^I /Cu⁰ interfaces. The roughness-dependent selectivity, that is the decrease of nitrones and the increase of imines with enhanced roughness, is visible in the electro-reductive coupling of nitrobenzene and furfural. Thus, the high selectivity of nitrone (98 %) and imine (80 %) can be achieved on a surface smooth Cu foil and the one electrochemically roughened in the presence of I^- , respectively. Such roughness-dependence of nitrone/imine selectivity on Cu electrodes is further verified in a wide substrate scope, highlighting the promise of surface/interfacial engineering for electrochemical synthesis.

Simple and scalable electrosynthesis of 1H-1-hydroxy-quinazolin-4-ones

Cathodic synthesis provides sustainable access to 1-hydroxy- and 1-oxy-quinazolin-4-ones from easily accessible nitro starting materials. Mild reaction conditions, inexpensive and reusable carbon-based electrode materials, an undivided electrochemical setup, and constant current conditions characterise this method. Sulphuric acid is used as a simple supporting electrolyte as well as a catalyst for cyclisation. The broad applicability of this protocol is demonstrated in 27 differently substituted derivatives in high yields of up to 92%. Moreover, mechanistic studies based on cyclic voltammetry measurements highlight a selective reduction of the nitro substrate to hydroxylamine as a key step. The relevance for preparative applications is demonstrated by a 100-fold scale-up for gram-scale electrolysis.

Electrochemical Synthesis of Pyrazolines and Pyrazoles via [3+2] Dipolar Cycloaddition

Pyrazolines and pyrazoles are common and important motifs of pharmaceutical agents and agrochemicals. Herein, the first electrochemical approach for their direct synthesis from easily accessible hydrazones and dipolarophiles up to decagram scale is presented. The application of a biphasic system (aqueous/organic) even allows for the conversion of highly sensitive alkenes, wherein inexpensive sodium iodide is employed in a dual role as supporting electrolyte and mediator. In addition, mechanistic insight into the reaction is given by the isolation of key step intermediates. The relevance of the presented reaction is underlined by the synthesis of commercial herbicide safener mefenpyr-diethyl in good yields.

Counter Electrode Reactions-Important Stumbling Blocks on the Way to a Working Electro-organic Synthesis

Over the past two decades, electro-organic synthesis has gained significant interest, both in technical and academic research as well as in terms of applications. The omission of stoichiometric oxidizers or reducing agents enables a more sustainable route for redox reactions in organic chemistry. Even if it is well-known that every electrochemical oxidation is only viable with an associated reduction reaction and vice versa, the relevance of the counter reaction is often less addressed. In this Review, the importance of the corresponding counter reaction in electro-organic synthesis is highlighted and how it can affect the performance and selectivity of the electrolytic conversion. A selection of common strategies and unique concepts to tackle this issue are surveyed to provide a guide to select appropriate counter reactions for electro-organic synthesis.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

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.

Recent advances in the electrochemical reactions of nitrogen-containing organic compounds

The electrochemical reaction of amines, nitriles, amides, nitroaromatics, and imines has been proven to be a valuable method for the synthesis of various nitrogen-containing organic compounds.

TfOH-Promoted Decyanative Cyclization toward the Synthesis of 2,1-Benzisoxazoles

A novel solvent-free, TfOH-promoted decyanative cyclization approach for the synthesis of 2,1-benzisoxazoles has been developed. The reactions are complete instantly at room temperature and result in the formation of the desired 2,1-benzisoxazoles in a 34-97% isolated yield.

Electrochemical Dehydrogenative C(sp2 )-H Amination

A transition-metal-free direct electrolytic C-H amination involving an electrochemically generated nitrenium ion intermediate has been developed. The electrosynthesis takes place in the absence of any organoiodine catalysts and is enabled by an in situ generated electrolyte. A novel, efficient intramolecular and intermolecular C-H amination has been demonstrated using a simple reaction setup.

Metal-free tandem reactions of 2-iodoaryl ynones with sodium azide for the synthesis of benzoisoxazole containing 1,2,3-triazoles

A metal-free approach has been developed for the synthesis of benzo[c]isoxazole (anthranils) containing 1,2,3-triazoles. The reaction proceeded efficiently through a [3 + 2] azide-alkyne cycloaddition, SNAr azidation and denitrogenative cyclization sequence. The metal-free protocol enabled effective construction of one N-O and three C-N bonds in one pot. In addition, the synthetic utility of the current methodology was further demonstrated by late stage modification of the obtained products.

Speedy one-pot electrochemical synthesis of giant octahedrons from in situ generated pyrrolidinyl PAMAM dendrimer

A novel electrochemical synthesis via a radical generation pathway is described here for the generation of a quaternary megamer structure from secondary dendrimers. The reaction is rapid and completes in <5 min. We have used lower/higher generation poly(amido)amine (PAMAM) dendrimers with carboxylic acid groups at the terminals. A precise electrocatalytic reaction at >3.5 V activates the carboxylic groups to undergo anodic oxidation (-e-) and produce radical carboxylate anions on the dendrimer surface. The reaction further goes through a decarboxylative elimination. Successive self-assembly creates billions of polydispersed and extremely stable ∼500 nm octahedron nanostructures, which we failed to destroy even by using a 20 kV electron beam. This is a new route for the speedy synthesis of important futuristic materials of well-defined shape. It has applications in building designer organic crystals for solar cells, organic electronics, rapid protein gelation, rapid protein crystallization, etc.

Metal-free electrochemical fluorodecarboxylation of aryloxyacetic acids to fluoromethyl aryl ethers

Electrochemical decarboxylation of aryloxyacetic acids followed by fluorination provides easy access to fluoromethyl aryl ethers. This electrochemical fluorodecarboxylation offers a sustainable approach with electric current as traceless oxidant. Using Et3N·5HF as fluoride source and as supporting electrolyte, this simple electrosynthesis affords various fluoromethoxyarenes in yields up to 85%.

Selective and Scalable Electrosynthesis of 2H-2-(Aryl)-benzo[d]-1,2,3-triazoles and Their N-Oxides by Using Leaded Bronze Cathodes

Electrosynthesis of 2H-2-(aryl)benzo[d]-1,2,3-triazoles and their N-oxides from 2-nitroazobenzene derivatives is reported. The electrolysis is conducted in a very simple undivided cell under constant current conditions with a leaded bronze cathode and a glassy carbon anode. The product distribution between 2H-2-(aryl)benzo[d]-1,2,3-triazoles and their N-oxides can be guided by simply controlling the current density and the amount of the charge applied. The reaction tolerates several sensitive functional groups in reductive electrochemistry. The usefulness and the applicability of the synthetic method is demonstrated by a formal synthesis of an antiviral compound.