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

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.

Self-Standing Metal Foam Catalysts for Cathodic Electro-Organic Synthesis

Although electro-organic synthesis is currently receiving renewed interest because of its potential to enable sustainability in chemical processes to value-added products, challenges in process development persist: For reductive transformations performed in protic media, an inherent issue is the limited choice of metallic cathode materials that can effectively suppress the parasitic hydrogen evolution reaction (HER) while maintaining a high activity toward the targeted electro-organic reaction. Current development trends are aimed at avoiding the previously used HER-suppressing elements (Cd, Hg, and Pb) because of their toxicity. Here, this work reports the rational design of highly porous foam-type binary and ternary electrocatalysts with reduced Pb content. Optimized cathodes are tested in electro-organic reductions using an oxime to nitrile transformation as a model reaction relevant for the synthesis of fine chemicals. Their electrocatalytic performance is compared with that of the model CuSn7Pb15 bronze alloy that has recently been endorsed as the best cathode replacement for bare Pb electrodes. All developed metal foam catalysts outperform both bare Pb and the CuSn7Pb15 benchmark in terms of chemical yield and energetic efficiency. Moreover, post-electrolysis analysis of the crude electrolyte mixture and the cathode's surfaces through inductively coupled plasma mass spectrometry (ICP-MS) and scanning electron microscopy (SEM), respectively, reveal the foam catalysts' elevated resistance to cathodic corrosion.

Molybdenum-catalyzed deoxygenative heterocyclization of 2-nitroazobenzenes: a novel strategy for catalytic synthesis of 2-aryl-2H-benzo[d][1,2,3]triazoles

A novel strategy for the catalytic synthesis of 2-aryl-2H-benzo[d][1,2,3]triazoles bearing a wide range of functional groups in good to excellent yields by non-noble molybdenum-catalyzed deoxygenative heterocyclization of 2-nitroazobenzenes is described. The salient features of the transformation include the use of readily available substrates, valuable products and ease of scale-up. The mechanistic study indicates that the reaction occurred via double deoxygenation by the Mo(VI)/Mo(IV) catalytic cycle from 2-nitroazobenzene, through the formation of 2-aryl-2H-benzo[d][1,2,3]triazole-N1-oxide or nitrene intermediates.

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.

Direct Electrochemical Synthesis of 2,3-Disubstituted Quinoline N-oxides by Cathodic Reduction of Nitro Arenes

The use of electric current in synthetic organic chemistry offers a sustainable tool for the selective reductive synthesis of quinoline N-oxides starting from easily accessible nitro compounds. The reported method employs mild and reagent-free conditions, a simple undivided cell, and constant current electrolysis set-up which provides conversion with a high atom economy. The synthesis of 30 differently substituted quinoline N-oxides was successfully performed in up to 90 % yield. Using CV studies, the mechanism of the selective formation of the quinoline N-oxides was elucidated. The technical relevance of the described reaction could be shown in a 50-fold scale-up reaction.

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.

Electrochemical Synthesis of High-Nitrogen Materials and Energetic Materials

Electrochemical synthesis is a valuable method for the preparation of molecules. It is innately eco-friendly, as potentially hazardous oxidation and reduction agents are replaced with electrochemical potentials. Electrochemistry is commonly applied globally in the synthesis of numerous chemicals, but the energetic materials field lags in this regard. In this review, we endeavor to cover the entire history of synthetic electrochemistry for the preparation of energetic materials and detail the electrochemical transformations of high-nitrogen materials that are relevant for the preparation of new energetic molecules. We hope this review serves as a starting point to inform those involved in synthetic energetic materials chemistry, and those interested in other applications of high-nitrogen molecules, about the environmentally friendly electrochemical methods available for such compounds.

Regioselective Synthesis of N2-Aryl 1,2,3-Triazoles via Electro-oxidative Coupling of Enamines and Aryldiazonium Salts

An efficient synthetic route for the construction of N²-aryl 1,2,3-triazoles is reported via sequential C-N bond formation and electro-oxidative N-N coupling under metal-free conditions. Readily accessible 2-aminoacrylates and aryldiazonium salts were used as starting materials, and the developed protocol displays excellent functional group tolerance, allowing an extensive range of substrate scope up to 91% isolated yield. Various mechanistic studies, along with the isolation of an intermediate adduct, refer to successive ionic and radical reaction sequences.

Cathodic Corrosion of Metal Electrodes-How to Prevent It in Electroorganic Synthesis

The critical aspects of the corrosion of metal electrodes in cathodic reductions are covered. We discuss the involved mechanisms including alloying with alkali metals, cathodic etching in aqueous and aprotic media, and formation of metal hydrides and organometallics. Successful approaches that have been implemented to suppress cathodic corrosion are reviewed. We present several examples from electroorganic synthesis where the clever use of alloys instead of soft neat heavy metals and the application of protective cationic additives have allowed to successfully exploit these materials as cathodes. Because of the high overpotential for the hydrogen evolution reaction, such cathodes can contribute toward more sustainable green synthetic processes. The reported strategies expand the applications of organic electrosynthesis because a more negative regime is accessible within protic media and common metal poisons, e.g., sulfur-containing substrates, are compatible with these cathodes. The strongly diminished hydrogen evolution side reaction paves the way for more efficient reductive electroorganic conversions.

Electrochemical Heteroatom-Heteroatom Bond Construction

Heteroatom-heteroatom linkage, with S-S bond as a presentative motif, served a crucial role in biochemicals, pharmaceuticals, pesticides, and material sciences. Thus, preparation of the privileged scaffold has always been attracting tremendous attention from the synthetic community. However, classic protocols suffered from several drawbacks, such as toxic and unstable agents, poor functional group tolerance, multiple steps, and explosive oxidizing regents as well as the transitional metal catalysts. Electrochemical organic synthesis exhibited a promising alternative to the traditional chemical reaction due to the sustainable electricity can be employed as the traceless redox agents. Hence, toxic and explosive oxidants and/or transitional metals could be discarded under mild reaction with high efficiency. In this context, a series of electrochemical approaches for the construction of heteroatom-heteroatom bond were reviewed. Notably, most of the cases illustrated the dehydrogenative feature with the clean energy molecules hydrogen as the sole by-product.

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.