Choice of the Right Supporting Electrolyte in Electrochemical Reductions: A Principal Component Analysis.

We present an analysis of a set of molecular, electrical, and electronic properties for a large number of the cations of quaternary ammonium salts usually employed as supporting electrolytes in cathodic reduction reactions. The goal of the present study is to define a measure for the quality of a supporting electrolyte in terms of the yield of the reaction considered. We performed a principal component analysis using the normalized values of the properties in order to lower the number of relevant reaction coordinates and find that the integral variance of 13 properties can well be represented by three principal components. The yield of the electrochemical hydrodimerization of acrylonitrile employing different quaternary ammonium salts as supporting electrolytes was determined in a series of experiments. We found only a very weak correlation between the yield and the values of the properties but a strong correlation between the yield and the values of the most important principal component. Very similar results are obtained for two further existing systematic experimental studies of the impact of the supporting electrolyte on the yield of cathodic reductions. For all three example reactions, a supervised regression using the two most important principal components as variables yields excellent values for the coefficients of determination. For comparison, we also applied our methodology to sets of purely structure-based features that are usually employed in cheminformatics and obtained results of almost similar quality. We therefore conjecture that our methodology in combination with a small number of experiments can be used to predict the yield of a given cathodic reduction on the basis of the properties of the supporting electrolyte.

Nickel-Electrocatalyzed Synthesis of Bifuran-Based Monomers.

Bifuran motifs can be accessed with nickel-bipyridine electrocatalyzed homocouplings of bromine-substituted methyl furancarboxylates, which, in turn, can be prepared from hemicellulose-derived furfural. The described protocol uses sustainable carbon-based graphite electrodes in the simplest setup - an undivided cell with constant current electrolysis. The reported method avoids using a sacrificial anode by employing triethanolamine as an electron donor.

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.

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.

Divergent Carbocatalytic Routes in Oxidative Coupling of Benzofused Heteroaryl Dimers: A Mechanistic Update.

Mildly thermal air or HNO3 oxidized activated carbons catalyse oxidative dehydrogenative couplings of benzo[b]fused heteroaryl 2,2'-dimers, e.g., 2-(benzofuran-2-yl)-1H-indole, to chiral 3,3'-coupled cyclooctatetraenes or carbazole-type migrative products under O2 atmosphere. DFT calculations show that the radical cation and the Scholl-type arenium cation mechanisms lead to different products with 2-(benzofuran-2-yl)-1H-indole, being in accord with experimental product distributions.

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

Invited for the cover of this issue is the group of Siegfried R. Waldvogel at Johannes Gutenberg-Universität Mainz. The image depicts the idea that synthetic electrochemistry is green chemistry in its essence. Read the full text of the article at 10.1002/chem.201905874.

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.

Carbocatalytic Oxidative Dehydrogenative Couplings of (Hetero)Aryls by Oxidized Multi-Walled Carbon Nanotubes in Liquid Phase.

HNO3 -oxidized carbon nanotubes catalyze oxidative dehydrogenative (ODH) carbon-carbon bond formation between electron-rich (hetero)aryls with O2 as a terminal oxidant. The recyclable carbocatalytic method provides a convenient and an operationally easy synthetic protocol for accessing various benzofused homodimers, biaryls, triphenylenes, and related benzofused heteroaryls that are highly useful frameworks for material chemistry applications. Carbonyls/quinones are the catalytically active site of the carbocatalyst as indicated by model compounds and titration experiments. Further investigations of the reaction mechanism with a combination of experimental and DFT methods support the competing nature of acid-catalyzed and radical cationic ODHs, and indicate that both mechanisms operate with the current material.

Photoreductive Removal of O-Benzyl Groups from Oxyarene N-Heterocycles Assisted by O-Pyridine-pyridone Tautomerism.

Facile photoreductive protocols have been developed to remove benzyl O-protective groups from oxyarene N-heterocycles at positions capable for 2-/4-O-pyridine-2-/4-pyridone tautomerism. Blue light irradiation, a [Ru] or [Ir] photocatalyst, and ascorbic acid in a water-acetonitrile solution debenzylates a variety of aryl N-heterocycles cleanly and selectively. Ascorbic acid has two functions in the reaction. On the one hand, it protonates the N-heterocycles that reduces their reduction potentials notably and on the other hand it acts as a sacrificial reductant. Reduction potentials and free energy barriers calculated at the CPCM-B3LYP/6-31+G** level can predict the reactivities of the studied substrates.

Gold(III)-catalyzed enynamine-cyclopentadiene cycloisomerization with chirality transfer: an experimental and theoretical study indicating involvement of dual Au(III) push-pull assisted cis-trans isomerism.

A synthetic approach for asymmetric ring-fused cyclopentadienes (Cps) with a chiral carbon at the ring junction has been established from chiral enynamines by achiral Au(III) catalysis. On the basis of experimental and theoretical data, the proposed mechanistic pathway from enynamines to Cps occurs via a Au(III) ene cis-trans isomerization step. Computational studies at DFT and NEVPT2 levels advocate that the cis-trans isomerization step proceeds via a dual Au(III) push-pull assisted intermediate with a low computed rotation barrier. The chirality transfer occurs through a helical-shaped transition state with allenic character. The scope of the catalysis encompasses sterically bulky enynamines including terpene natural products.