Metal- and Reagent-Free Electrochemical Synthesis of Alkyl Arylsulfonates in a Multi-Component Reaction

Exploring new 5-Nitroimidazole Derivatives as Potent Acetylcholinesterase and Butyrylcholinesterase Enzyme Inhibitors

Discovering new compounds capable of inhibiting physiologically and metabolically significant drug targets or enzymes is of paramount importance in biological chemistry. With this aim, new 5-nitroimidazole derivatives (1-4) were designed and synthesized, and their inhibitory activities against acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) were discovered using acetyl (butyryl) thiocholine and Ellman's reagents for spectrophotometric assay. The inhibitory profiles of the synthesized compounds were assessed by comparing their IC50 and Ki values. Results demonstrate significant inhibitory activity of all synthesized compounds against both AChE and BuChE compared to the reference compound, donepezil. Notably, compound 4 exhibited dual inhibition of these enzymes, showing the highest activity against Electrophorus electricus AChE (EeAChE) with a Ki value of 0.024±0.009 nM and against equine BuChE (eqBuChE) with a Ki value of 0.087±0.017 nM. Furthermore, molecular modeling was conducted to study the interaction modes of the most potent compound (4) and donepezil in the active site of their related enzymes' crystal structures (PDB ID: 4EY7 and 4BDS, respectively). Additionally, drug-likeness, ADME, and toxicity profiles of the compounds and metronidazole were predicted. The above results indicated that the dual inhibition of these enzymes is considered as a promising strategy for the treatment of neurological disorder especially Alzheimer's disease.

Electrochemical Synthesis of S-Aryl Dibenzothiophenium Triflates as Precursors for Selective Nucleophilic Aromatic (Radio)fluorination

A novel electrosynthetic approach to aryl dibenzothiophenium salts, including the direct intramolecular formation of a C-S bond in a metal-free, electrochemical key step under ambient conditions, is reported. The broad applicability of this method is demonstrated with 14 examples, including nitrogen-containing heterocycles in isolated yields up to 72%. The resulting sulfonium salts can be used as precursors for fluorine labeling to give [18F]fluoroarenes as found in PET tracer ligands.

Electrochemical Multicomponent Synthesis of Alkyl Alkenesulfonates using Styrenes, SO2 and Alcohols

A novel electrochemical approach to access alkyl alkenesulfonates via a multicomponent reaction was developed. The metal-free method features easy-to-use SO2 stock solution forming monoalkylsulfites from alcohols with an auxiliary base in-situ. These intermediates serve a dual role as starting materials and as supporting electrolyte enabling conductivity. Anodic oxidation of the substrate styrene, radical addition of these monoalkylsulfites and consecutive second oxidation and deprotonation preserve the double bond and form alkyl β-styrenesulfonates in a highly regio- and stereoselective fashion. The feasibility of this electrosynthetic method is demonstrated in 44 examples with yields up to 81 %, employing various styrenes and related substrates as well as a diverse set of alcohols. A gram-scale experiment underlines the applicability of this process, which uses inexpensive and readily available electrode materials.

An efficient and mild oxidative approach from thiols to sulfonyl derivatives with DMSO/HBr

A mild and practical method for synthesizing sulfonyl derivatives, which have a wide range of applications in pharmaceuticals, materials, and organic synthesis, was described through the oxidative functionalization of thiols with DMSO/HBr. The simple conditions, low cost and ready availability of DMSO/HBr, as well as the versatility of the transformations, make this strategy very powerful in synthesizing a variety of sulfonyl derivatives including sulfonamides, sulfonyl fluorides, sulfonyl azides, and sulfonates. Mechanistic studies revealed that DMSO served as the terminal oxidant, and HBr acted as both a nucleophile and a redox mediator to transfer the oxygen atom.

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.

Alkoxysulfonyl radical species: acquisition and transformation towards sulfonate esters through electrochemistry

Sulfonyl radical mediated processes have been considered as a powerful strategy for the construction of sulfonyl compounds. However, an efficient and high atom-economical radical approach to the synthesis of sulfonate esters is still rare, owing to the limited tactics to achieve alkoxysulfonyl radicals. Herein, an electrochemical anodic oxidation of inorganic sulfites with alcohols is developed to afford alkoxysulfonyl radical species, which are utilized in subsequent alkene difunctionalization to provide various sulfonate esters. This transformation features excellent chemoselectivity and broad functional group tolerance. This new discovery presents the potential prospect for the construction of sulfonate esters, and enriches the electrochemical reaction type.

Scalable, Chemoselective Nickel Electrocatalytic Sulfinylation of Aryl Halides with SO2

Simple access to aryl sulfinates from aryl iodides and bromides is reported using an inexpensive Ni-electrocatalytic protocol. The reaction exhibits a broad scope, uses stock solution of simple SO2 as sulfur source, and can be scaled up in batch and recycle flow settings. The limitations of this reaction are clearly shown and put into context by benchmarking with state-of-the-art Pd-based methods.

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.

Homolytic Aromatic Sulfonation with K2S2O5 Promoted by a Combination of Mn(OAc)3·2H2O and HFIP

Herein, we reported a so far unprecedented Mn(OAc)₃·2H₂O-promoted homolytic aromatic sulfonation. The reaction was performed under mild conditions with K₂S₂O₅ employed as a green sulfonating reagent. Various arenes were successfully converted into desired sulfonic acids or sulfonates in high efficiency. Preliminary mechanistic studies demonstrated that the present reaction proceeds via a homolytic aromatic substitution-type mechanism involving an SO₃^- radical. The combination of Mn(OAc)₃·2H₂O and HFIP plays a crucial role.

Multicomponent reactions and photo/electrochemistry join forces: atom economy meets energy efficiency

Visible-light photoredox catalysis has been regarded as an extremely powerful tool in organic chemistry, bringing the spotlight back to radical processes. The versatility of photocatalyzed reactions has already been demonstrated to be effective in providing alternative routes for cross-coupling as well as multicomponent reactions. The photocatalyst allows the generation of high-energy intermediates through light irradiation rather than using highly reactive reagents or harsh reaction conditions. In a similar vein, organic electrochemistry has experienced a fruitful renaissance as a tool for generating reactive intermediates without the need for any catalyst. Such milder approaches pose the basis toward higher selectivity and broader applicability. In photocatalyzed and electrochemical multicomponent reactions, the generation of the radical species acts as a starter of the cascade of events. This allows for diverse reactivity and the use of reagents is usually not covered by classical methods. Owing to the availability of cheaper and more standardized photo- and electrochemical reactors, as well as easily scalable flow-setups, it is not surprising that these two fields have become areas of increased research interest. Keeping these in view, this review is aimed at providing an overview of the synthetic approaches in the design of MCRs involving photoredox catalysis and/or electrochemical activation as a crucial step with particular focus on the choice of the difunctionalized reagent.

Electrochemical Nitration with Nitrite

Aromatic nitration has tremendous importance in organic chemistry as nitroaromatic compounds serve as versatile building blocks. This study represents the electrochemical aromatic nitration with NBu4 NO2 , which serves a dual role as supporting electrolyte and as a safe, readily available, and easy-to-handle nitro source. Stoichiometric amounts of 1,1,1-3,3,3-hexafluoroisopropan-2-ol (HFIP) in MeCN significantly increase the yield by solvent control. The reaction mechanism is based on electrochemical oxidation of nitrite to NO2 , which initiates the nitration reaction in a divided electrolysis cell with inexpensive graphite electrodes. Overall, the reaction is demonstrated for 20 examples with yields of up to 88 %. Scalability is demonstrated by a 13-fold scale-up.

Radical-mediated vicinal addition of alkoxysulfonyl/fluorosulfonyl and trifluoromethyl groups to aryl alkyl alkynes

The addition of sulfonyl radicals to alkenes and alkynes is a valuable method for constructing useful highly functionalized sulfonyl compounds. The underexplored alkoxy- and fluorosulfonyl radicals are easily accessed by CF₃ radical addition to readily available allylsulfonic acid derivatives and then β-fragmentation. These substituted sulfonyl radicals add to aryl alkyl alkynes to give vinyl radicals that are trapped by trifluoromethyl transfer to provide tetra-substituted alkenes bearing the privileged alkoxy- or fluorosulfonyl group on one carbon and a trifluoromethyl group on the other. This process exhibits broad functional group compatibility and allows for the late-stage functionalization of drug molecules, demonstrating its potential in drug discovery and chemical biology.

An unprecedented method for vicinal addition of alkoxysulfonyl/fluorosulfonyl and trifluoromethyl groups to aryl alkyl alkynes has been developed to afford useful alkenylsulfonate esters and alkenylsulfonyl fluorides.

Advances in photochemical and electrochemical incorporation of sulfur dioxide for the synthesis of value-added compounds

Organic photochemistry and electrochemistry currently receive tremendous attention in organic synthesis as both techniques enable the reagent-less activation of organic molecules without using expensive and hazardous redox reagents. The incorporation of SO₂ into organic molecules is a relatively modern research topic, which likewise gains immense popularity since the discovery of the SO₂ surrogate DABSO. Sulfur-containing organic molecules are omnipresent in pharmaceuticals and agrochemicals. This review covers the recent progress in electrochemical and photochemical methodologies for the incorporation and uses of SO₂ in the synthesis of value-added compounds. Additionally, different work techniques are demonstrated for the synthetic application of SO₂.

Oxidation Potential-Guided Electrochemical Radical-Radical Cross-Coupling Approaches to 3-Sulfonylated Imidazopyridines and Indolizines

Oxidation potential-guided electrochemical radical-radical cross-coupling reactions between N-heteroarenes and sodium sulfinates have been established. Thus, simple cyclic voltammetry measurement of substrates predicts the likelihood of successful radical-radical coupling reactions, allowing the simple and direct synthetic access to 3-sulfonylated imidazopyridines and indolizines. The developed electrochemical radical-radical cross-coupling reactions to sulfonylated N-heteroarenes boast the green synthetic nature of the reactions that are oxidant- and metal-free.

Metal-Free Electrochemical Synthesis of Sulfonamides Directly from (Hetero)arenes, SO2 , and Amines

Sulfonamides are among the most important chemical motifs in pharmaceuticals and agrochemicals. However, there is no methodology to directly introduce the sulfonamide group to a non-prefunctionalized aromatic compound. Herein, we present the first dehydrogenative electrochemical sulfonamide synthesis protocol by exploiting the inherent reactivity of (hetero)arenes in a highly convergent reaction with SO2 and amines via amidosulfinate intermediate. The amidosulfinate serves a dual role as reactant and supporting electrolyte. Direct anodic oxidation of the aromatic compound triggers the reaction, followed by nucleophilic attack of the amidosulfinate. Boron-doped diamond (BDD) electrodes and a HFIP-MeCN solvent mixture enable selective formation of the sulfonamides. In total, 36 examples are demonstrated with yields up to 85 %.

Electrochemical synthesis of sulfamides

Organic electrosynthesis enables the formation of symmetrical sulfamides directly from anilines and SO₂ mediated by iodide.

Metal- and Reagent-Free Electrochemical Synthesis of Alkyl Arylsulfonates in a Multi-Component Reaction

This work presents the first electrochemical preparation of alkyl arylsulfonates by direct anodic oxidation of electron-rich arenes. The reaction mechanism features a multi-component reaction consisting of electron-rich arenes, an alcohol of choice and excess SO2 in an acetonitrile-HFIP reaction mixture. In-situ formed monoalkyl sulfites are considered as key intermediates with bifunctional purpose. Firstly, this species functions as nucleophile and secondly, excellent conductivity is provided. Several primary and secondary alcohols and electron-rich arenes are implemented in this reaction to form the alkyl arylsulfonates in yields up to 73 % with exquisite selectivity. Boron-doped diamond electrodes (BDD) are employed in divided cells, separated by a simple commercially available glass frit.