Bromide-Mediated Silane Oxidation: A Practical Counter-Electrode Process for Nonaqueous Deep Reductive Electrosynthesis

The counter-electrode process of an organic electrochemical reaction is integral for the success and sustainability of the process. Unlike for oxidation reactions, counter-electrode processes for reduction reactions remain limited, especially for deep reductions that apply very negative potentials. Herein, we report the development of a bromide-mediated silane oxidation counter-electrode process for nonaqueous electrochemical reduction reactions in undivided cells. The system is found to be suitable for replacing either sacrificial anodes or a divided cell in several reported reactions. The conditions are metal-free, use inexpensive reagents and a graphite anode, are scalable, and the byproducts are reductively stable and readily removed. We showcase the translation of a previously reported divided cell reaction to a >100 g scale in continuous flow.

A guide to troubleshooting metal sacrificial anodes for organic electrosynthesis

The development of reductive electrosynthetic reactions is often enabled by the oxidation of a sacrificial metal anode, which charge-balances the reductive reaction of interest occurring at the cathode. The metal oxidation is frequently assumed to be straightforward and innocent relative to the chemistry of interest, but several processes can interfere with ideal sacrificial anode behavior, thereby limiting the success of reductive electrosynthetic reactions. These issues are compounded by a lack of reported observations and characterization of the anodes themselves, even when a failure at the anode is observed. Here, we weave lessons from electrochemistry, interfacial characterization, and organic synthesis to share strategies for overcoming issues related to sacrificial anodes in electrosynthesis. We highlight common but underexplored challenges with sacrificial anodes that cause reactions to fail, including detrimental side reactions between the anode or its cations and the components of the organic reaction, passivation of the anode surface by an insulating native surface film, accumulation of insulating byproducts at the anode surface during the reaction, and competitive reduction of sacrificial metal cations at the cathode. For each case, we propose experiments to diagnose and characterize the anode and explore troubleshooting strategies to overcome the challenge. We conclude by highlighting open questions in the field of sacrificial-anode-driven electrosynthesis and by indicating alternatives to traditional sacrificial anodes that could streamline reaction optimization.

Parallel Paired Photoelectrochemical Bromination of Alkylarenes with Electrochemical Pinacol Coupling

A paired electrochemical method for paralleling benzylic bromination of alkylarenes under irradiation with reductive pinacol coupling in a divided cell has been developed. A variety of benzyl bromides at the anode and pinacols at the cathode were obtained simultaneously in moderate-to-high faradaic efficiency. This parallel paired electrochemical protocol showed a broad substrate scope and high chemoselectivity as well as high synthetic and faradaic efficiencies.

Rare Earths-The Answer to Everything

Rare earths, scandium, yttrium, and the fifteen lanthanoids from lanthanum to lutetium, are classified as critical metals because of their ubiquity in daily life. They are present in magnets in cars, especially electric cars; green electricity generating systems and computers; in steel manufacturing; in glass and light emission materials especially for safety lighting and lasers; in exhaust emission catalysts and supports; catalysts in artificial rubber production; in agriculture and animal husbandry; in health and especially cancer diagnosis and treatment; and in a variety of materials and electronic products essential to modern living. They have the potential to replace toxic chromates for corrosion inhibition, in magnetic refrigeration, a variety of new materials, and their role in agriculture may expand. This review examines their role in sustainability, the environment, recycling, corrosion inhibition, crop production, animal feedstocks, catalysis, health, and materials, as well as considering future uses.

Electroreductive Desulfurative Transformations with Thioethers as Alkyl Radical Precursors

Thioethers are highly prevalent functional groups in organic compounds of natural and synthetic origin but remain remarkably underexplored as starting materials in desulfurative transformations. As such, new synthetic methods are highly desirable to unlock the potential of the compound class. In this vein, electrochemistry is an ideal tool to enable new reactivity and selectivity under mild conditions. Herein, we demonstrate the efficient use of aryl alkyl thioethers as alkyl radical precursors in electroreductive transformations, along with mechanistic details. The transformations proceed with complete selectivity for C(sp3 )-S bond cleavage, orthogonal to that of established transition metal-catalyzed two-electron routes. We showcase a hydrodesulfurization protocol with broad functional group tolerance, the first example of desulfurative C(sp3 )-C(sp3 ) bond formation in Giese-type cross-coupling and the first protocol for electrocarboxylation of synthetic relevance with thioethers as starting materials. Finally, the compound class is shown to outcompete their well-established sulfone analogues as alkyl radical precursors, demonstrating their synthetic potential for future desulfurative transformations in a one-electron manifold.

Electrochemical Preparation of Sm(II) Reagent Facilitated by Weakly Coordinating Anions

Samarium diiodide (SmI2 ) is widely used as a strong one-electron reducing agent and is often employed to form C-C bonds in complex systems. Despite their utility, SmI2 and related salts suffer from several drawbacks that render the use of Sm reducing agents in large-scale synthesis impractical. Here, we report factors influencing the electrochemical reduction of Sm(III) to Sm(II), towards the goal of electrocatalytic Sm(III) reduction. We probe the effect of supporting electrolyte, electrode material, and Sm precursor on Sm(II)/(III) redox and on the reducing power of the Sm species. We find that the coordination strength of the counteranion of the Sm salt affects the reversibility and redox potential of the Sm(II)/(III) couple and establish that the counteranion primarily determines the reducibility of Sm(III). Electrochemically generated SmI2 performs similarly to commercial SmI2 solutions in a proof-of-concept reaction. The results will provide fundamental insight to facilitate the development of Sm-electrocatalytic reactions.

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 formal homocoupling of sec-alcohols

Electrochemical pinacol coupling of carbonyl compounds in an undivided cell with a sacrificial anode would be a promising approach toward synthetically valuable vic-1,2-diol scaffolds without using low-valent metal reductants. However, sacrificial anodes produce an equimolar amount of metal waste, which may be a major issue in terms of sustainable chemistry. Herein, we report a sacrificial anode-free electrochemical protocol for the synthesis of pinacol-type vic-1,2-diols from sec-alcohols, namely benzyl alcohol derivatives and ethyl lactate. The corresponding vic-1,2-diols are obtained in moderate to good yields, and good to high levels of stereoselectivity are observed for sec-benzyl alcohol derivatives. The present transformations smoothly proceed in a simple undivided cell under constant current conditions without the use of external chemical oxidants/reductants, and transition-metal catalysts.

Samarium(II)-Electrocatalyzed Chemoselective Reductive Alkoxylation of Phthalimides

The unprecedented samarium-eletrocatalyzed reductive alkoxylation of phthalimides in a single step is presented. Under mild conditions, using electrogenerated Sm(II) with TMSCl (trimethyl chlorosilane), N-substituted 3-alkoxyl isoindolin-1-ones are isolated in good...

Electrocatalysis as an enabling technology for organic synthesis

Electrochemistry has recently gained increased attention as a versatile strategy for achieving challenging transformations at the forefront of synthetic organic chemistry. Electrochemistry's unique ability to generate highly reactive radical and radical ion intermediates in a controlled fashion under mild conditions has inspired the development of a number of new electrochemical methodologies for the preparation of valuable chemical motifs. Particularly, recent developments in electrosynthesis have featured an increased use of redox-active electrocatalysts to further enhance control over the selective formation and downstream reactivity of these reactive intermediates. Furthermore, electrocatalytic mediators enable synthetic transformations to proceed in a manner that is mechanistically distinct from purely chemical methods, allowing for the subversion of kinetic and thermodynamic obstacles encountered in conventional organic synthesis. This review highlights key innovations within the past decade in the area of synthetic electrocatalysis, with emphasis on the mechanisms and catalyst design principles underpinning these advancements. A host of oxidative and reductive electrocatalytic methodologies are discussed and are grouped according to the classification of the synthetic transformation and the nature of the electrocatalyst.

Electrode Materials in Modern Organic Electrochemistry

The choice of electrode material is critical for achieving optimal yields and selectivity in synthetic organic electrochemistry. The material imparts significant influence on the kinetics and thermodynamics of electron transfer, and frequently defines the success or failure of a transformation. Electrode processes are complex and so the choice of a material is often empirical and the underlying mechanisms and rationale for success are unknown. In this review, we aim to highlight recent instances of electrode choice where rationale is offered, which should aid future reaction development.

Simple electrochemical reduction of nitrones to amines

The use of electricity allows the reduction of nitrones containing aromatic and heteroaromatic rings to the corresponding amines. The main advantage of this protocol relies on the fact that only electrons are needed, avoiding the use of different chemicals and harsh procedures, something that reinforces some green aspects. The conversion tolerates different moieties and a large variety of functional groups. In addition, this electrolysis can be performed on a simple undivided beaker-type cell with constant current conditions.

CO2 activation by electrogenerated divalent samarium for aryl halide carboxylation

The first combination of samarium and electrochemistry towards the effective reduction of CO₂ for the synthesis of benzoic acids from aryl halides.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

Electrochemistry represents one of the most intimate ways of interacting with molecules. This review discusses advances in synthetic organic electrochemistry since 2000. Enabling methods and synthetic applications are analyzed alongside innate advantages as well as future challenges of electroorganic chemistry.