Synthetic Organic Electrochemistry: An Enabling and Innately Sustainable Method

Electrochemical Hydroxylation of Arenes Catalyzed by a Keggin Polyoxometalate with a Cobalt(IV) Heteroatom

The sustainable, selective direct hydroxylation of arenes, such as benzene to phenol, is an important research challenge. An electrocatalytic transformation using formic acid to oxidize benzene and its halogenated derivatives to selectively yield aryl formates, which are easily hydrolyzed by water to yield the corresponding phenols, is presented. The formylation reaction occurs on a Pt anode in the presence of [Co^III W₁₂ O₄₀ ]^5- as a catalyst and lithium formate as an electrolyte via formation of a formyloxyl radical as the reactive species, which was trapped by a BMPO spin trap and identified by EPR. Hydrogen was formed at the Pt cathode. The sum transformation is ArH+H₂ O→ArOH+H₂ . Non-optimized reaction conditions showed a Faradaic efficiency of 75 % and selective formation of the mono-oxidized product in a 35 % yield. Decomposition of formic acid into CO₂ and H₂ is a side-reaction.

Synthetic Organic Electrochemistry: Calling All Engineers

Unmet potential: Electrochemistry is the most simple and basic way of altering the redox-states of organic molecules. Despite extensive studies and its demonstrated promise, it has yet to take off in mainstream synthesis. The reason is due to engineering challenges in instrument design.

Electrochemical Corey-Winter reaction. Reduction of thiocarbonates in aqueous methanol media and application to the synthesis of a naturally occurring α-pyrone

An electrochemical version of the Corey-Winter reaction was developed giving excellent results in aqueous methanol media (MeOH/H₂O (80:20) with AcOH/AcONa buffer 0.5 M as supporting electrolyte), using a reticulated vitreous carbon as cathode in a divided cell. The electrochemical version is much more environmentally friendly than the classical reaction, where a large excess of trialkyl phosphite as reducing agent and high temperatures are required. Thus, cathodic reduction at room temperature of two cyclic thiocarbonates (-1.2 to -1.4 V vs Ag/AgCl) afforded the corresponding alkenes, trans-6-(pent-1-enyl)-α-pyrone and trans-6-(pent-1,4-dienyl)-α-pyrone, which are naturally occurring metabolites isolated from Trichoderma viride and Penicillium, in high chemical yield and with excellent stereo selectivity.

Stepwise radical cation Diels-Alder reaction via multiple pathways

Herein we disclose the radical cation Diels–Alder reaction of aryl vinyl ethers by electrocatalysis, which is triggered by an oxidative SET process. The reaction clearly proceeds in a stepwise fashion, which is a rare mechanism in this class. We also found that two distinctive pathways, including “direct” and “indirect”, are possible to construct the Diels–Alder adduct.

Electrochemical Oxidative C-H Amination of Phenols: Access to Triarylamine Derivatives

Dehydrogenative C-H/N-H cross-coupling serves as one of the most straightforward and atom-economical approaches for C-N bond formation. In this work, an electrochemical reaction protocol has been developed for the oxidative C-H amination of unprotected phenols under undivided electrolytic conditions. Neither metal catalysts nor chemical oxidants are needed to facilitate the dehydrogenation process. A series of triarylamine derivatives could be obtained with good functional-group tolerance. The electrolysis is scalable and can be performed at ambient conditions.

Investigating radical cation chain processes in the electrocatalytic Diels-Alder reaction

Single electron transfer (SET)-triggered radical ion-based reactions have proven to be powerful options in synthetic organic chemistry. Although unique chain processes have been proposed in various photo- and electrochemical radical ion-based transformations, the turnover number, also referred to as catalytic efficiency, remains unclear in most cases. Herein, we disclose our investigations of radical cation chain processes in the electrocatalytic Diels-Alder reaction, leading to a scalable synthesis. A gram-scale synthesis was achieved with high current efficiency of up to 8000%. The reaction monitoring profiles showed sigmoidal curves with induction periods, suggesting the involvement of intermediate(s) in the rate determining step.

Functionalization of N-arylglycine esters: electrocatalytic access to C-C bonds mediated by n-Bu4NI

An efficient electrocatalytic functionalization of N-arylglycine esters is reported. The protocol proceeds in an undivided cell under constant current conditions employing the simple, cheap and readily available n-Bu₄NI as the mediator. In addition, it is demonstrated that the mediated process is superior to the direct electrochemical functionalization.

Palladium-catalyzed C-H activation/C-C cross-coupling reactions via electrochemistry

Palladium-catalyzed C-H activation/C-C cross-coupling reactions typically require stoichiometric chemical oxidants and exogenous ligands. However, there are significant disadvantages associated with the use of traditional stoichiometric oxidants. To overcome these issues, we have developed an electrochemical strategy to achieve methylation and acylation.

Oxidant-Free C(sp2)-H Functionalization/C-O Bond Formation: A Kolbe Oxidative Cyclization Process

An anodic oxidation/cyclization of 2-arylbenzoic acids for the synthesis of dibenzopyranones has been developed. The reaction proceeds at room temperature with no oxidant or electrolyte required and exhibits a high atom economy with H₂ being the only byproduct. A series of dibenzopyranones was obtained in good to excellent yields. Urolithins A, B, and C are formally synthesized by adopting this method as a key step to demonstrate its synthetic utility.

Ni-Catalyzed Electrochemical Decarboxylative C-C Couplings in Batch and Continuous Flow

An electrochemically driven, nickel-catalyzed reductive coupling of N-hydroxyphthalimide esters with aryl halides is reported. The reaction proceeds under mild conditions in a divided electrochemical cell and employs a tertiary amine as the reductant. This decarboxylative C(sp³)-C(sp²) bond-forming transformation exhibits excellent substrate generality and functional group compatibility. An operationally simple continuous-flow version of this transformation using a commercial electrochemical flow reactor represents a robust and scalable synthesis of value added coupling process.

Electrochemical Difluoromethylarylation of Alkynes

An unprecedented radical difluoromethylarylation reaction of alkynes has been developed by discovering a new difluoromethylation reagent, CF₂HSO₂NHNHBoc. This air-stable and solid reagent can be prepared in one step from commercially available reagents CF₂HSO₂Cl and NH₂NHBoc. The CF₂H radical, generated through ferrocene-mediated electrochemical oxidation, participates in an unexplored alkyne addition reaction followed by a challenging 7-membered ring-forming homolytic aromatic substitution step to afford fluorinated dibenzazepines.

Electrochemical C-H/N-H Activation by Water-Tolerant Cobalt Catalysis at Room Temperature

Electrochemistry enabled C-H/N-H functionalizations at room temperature by external oxidant-free cobalt catalysis. Thus, the sustainable cobalt electrocatalysis manifold proceeds with excellent levels of chemoselectivity and positional selectivity, and with ample scope, thus allowing electrochemical C-H activation under exceedingly mild reaction conditions at room temperature in water.

Selective electrochemical generation of benzylic radicals enabled by ferrocene-based electron-transfer mediators

The generation and intermolecular functionalisation of carbon-centred radicals has broad potential synthetic utility. Herein, we show that benzylic radicals may be generated electrochemically from benzylboronate derivatives at low electrode potentials (ca. -0.3 V vs. Cp2Fe0/+) via single electron oxidation. Use of a catalytic quantity of a ferrocene-based electron-transfer mediator is crucial to achieve successful radical functionalisation and avoid undesirable side reactions arising from direct electrochemical oxidation or from the use of stoichiometric ferrocenium-based oxidants.

Metal-Free Electrocatalytic Aerobic Hydroxylation of Arylboronic Acids

Hydroxylation of arylboronic acids to aryl alcohols was realized by a scalable electrocatalytic method. The present electrochemical hydroxylation employs low-cost methyl viologen as an organic cathodic electrocatalyst and involves O₂ as a green and sustainable reactant. The electrochemical kinetic studies shown here can be a powerful tool to gain rich mechanistic and kinetic information and thus an in-depth understanding of the electrocatalytic mechanism.

Palladium-Catalyzed C-H Bond Acetoxylation via Electrochemical Oxidation

Here we describe the development of a method for the Pd-catalyzed electrochemical acetoxylation of C-H bonds. The oxidation step of the catalytic cycle is probed through cyclic voltammetry and bulk electrolysis studies of a preformed palladacycle of 8-methylquinoline. A catalytic system for C-H acetoxylation is then developed and optimized with respect to the cell configuration, rate of oxidation, and chemistry at the counter electrode. This transformation is then applied to substrates containing various directing groups and to the acetoxylation of both C(sp2)-H and C(sp3)-H bonds.

Recent advances in iodine mediated electrochemical oxidative cross-coupling

This review article gives an overview of the recent development of iodine mediated electrochemical oxidative coupling reactions.

Electrochemical methoxymethylation of alcohols – a new, green and safe approach for the preparation of MOM ethers and other acetals

A novel electrochemical methodology was developed for preparing MOM ethers under safe, inexpensive and scalable conditions.

Electrochemical synthesis of some 2-aminobenzofuran-3-carbonitrile and 2-aminobenzofuran-3-carboxylate derivatives: product diversity by changing the applied current density

Green syntheses of two different series of new benzofuran derivatives were carried out by anodic oxidation of 4,4′-biphenol (4BP) in the presence of some CH-acid compounds (malononitrile, methyl cyanoacetate and ethyl cyanoacetate) as nucleophiles by controlling the potential during electrolysis.

Electrochemical synthesis of 7-membered carbocycles through cascade 5-exo-trig/7-endo-trig radical cyclization

An electrochemical synthesis of functionalized 7-membered carbocycles through a 5-exo-trig/7-endo-trig radical cyclization cascade has been developed.

Palladium-catalyzed reductive electrocarboxylation of allyl esters with carbon dioxide

Palladium-catalyzed regioselective electrocarboxylation of homostyrenyl acetates with CO₂ has been successfully developed, providing α-aryl carboxylic acids with good selectivity and yield.

N-Hydroxyphthalimide-Mediated Electrochemical Iodination of Methylarenes and Comparison to Electron-Transfer-Initiated C-H Functionalization

An electrochemical method has been developed for selective benzylic iodination of methylarenes. The reactions feature the first use of N-hydroxyphthalimide as an electrochemical mediator for C-H oxidation to nonoxygenated products. The method provides the basis for direct (in situ) or sequential benzylation of diverse nucleophiles using methylarenes as the alkylating agent. The hydrogen-atom transfer mechanism for C-H iodination allows C-H oxidation to proceed with minimal dependence on the substrate electronic properties and at electrode potentials 0.5-1.2 V lower than that of direct electrochemical C-H oxidation.

Coenzyme-Catalyzed Electro-RAFT Polymerization

Here we report an electrochemically switchable reversible addition-fragmentation chain transfer polymerization (eRAFT). A new family of biochemical coenzymes are discovered that can be used as highly efficient electroredox catalysts to mediate this polymerization. The oxidation of coenzyme, nicotinamide adenine dinucleotide (NADH), can promote the reduction of a chain transfer agent, triggering generation and propagation of polymer radicals. External potential can activate the reduction of the NAD⁺ oxidized state and pause the propagation. Tuning the applied potential to reversibly switch the catalyst between its reduced and oxidized states can toggle the polymerization between ON and OFF states. This new strategy is universal to a broad scope of monomers, and ppm-level coenzymes result in the desirable polymer structures with targeted molecular weight, dispersity, and excellent chain-end fidelity. We envisage that the bioorganic-based catalysts would open new directions of organocatalyzed electro-controlled polymerization and be of value in electrocatalysis for well-structured polymers.

Redox-Tag Processes: Intramolecular Electron Transfer and Its Broad Relationship to Redox Reactions in General

Explosive growth in the use of open shell reactivity, including neutral radicals and radical ions, in the field of synthetic organic chemistry has been observed in the past decade, particularly since the advent of ruthenium complexes in 2008. These complexes generally induce single-electron transfer (SET) processes via visible-light absorption. Additionally, recent significant advancements in organic electrochemistry involving SET processes to provide open shell reactivity offer a complementary method to traditional polarity-driven reactions described by two-electron transfer processes. In this Review, we highlight the importance of intramolecular SET processes in the field of synthetic organic chemistry, which seem to be more elusive than the intermolecular versions, since they are net redox-neutral and thus cannot simply be regarded as oxidations or reductions. Such intramolecular SET processes can rationally be understood in combination with concomitant bond formations and/or cleavages, and are regulated by a structural motif that we call a "redox tag." In order to describe modern radical-driven reactions involving SET processes, we focus on a classical formalism in which electrons are treated as particles rather than waves, which offers a practical yet powerful approach to explain and/or predict synthetic outcomes.