Electroreduction of Benzaldehyde with a Metal–Ligand Bifunctional Hydroxycyclopentadienyl Molybdenum(II) Hydride

Cobaltocene-Mediated Catalytic Hydride Transfer: Strategies for Electrocatalytic Hydrogenation

The selective electrocatalytic hydrogenation of organics with transition metal hydrides is a promising strategy for electrosynthesis and energy storage. We report the electrocatalytic hydrogenation of acetone with a cyclopentadienone-iridium complex in a tandem electrocatalytic cycle with a cobaltocene mediator. The reductive protonation of cobaltocenium with mild acids generates (C5H5)CoI(C5H6) (CpCoI(CpH)), which functions as an electrocatalytic hydride mediator to deliver a hydride to cationic Ir(III) without generating hydrogen. Electrocatalytic hydride transfer by CpCoI(CpH) to a cationic Ir species leads to the efficient (Faradaic efficiency > 90%) electrohydrogenation of acetone, a valuable hydrogenation target as a liquid organic hydrogen carrier (LOHC). Hydride-transfer mediation presents a powerful strategy to generate metal hydrides that are inaccessible by stepwise electron/proton transfer.

Electrohydrogenation of Nitriles with Amines by Cobalt Catalysis

Catalytic hydrogenation of nitriles represents an efficient and sustainable one-step synthesis of valuable bulk and fine chemicals. We report herein a molecular cobalt electrocatalyst for selective hydrogenative coupling of nitriles with amines using protons as the hydrogen source. The key to success for this reductive reaction is the use of an electrocatalytic approach for efficient cobalt-hydride generation through a sequence of cathodic reduction and protonation. As only electrons (e- ) and protons (H+ ) as the redox equivalent and hydrogen source, this general electrohydrogenation protocol is showcased by highly selective and straightforward synthesis of various functionalized and structurally diverse amines, as well as deuterium isotope labeling applications. Mechanistic studies reveal that the electrogenerated cobalt-hydride transfer to nitrile process is the rate-determining step.

Developing electrochemical hydrogenation towards industrial application

Electrochemical hydrogenation reactions gained significant attention as a sustainable and efficient alternative to conventional thermocatalytic hydrogenations. This tutorial review provides a comprehensive overview of the basic principles, the practical application, and recent advances of electrochemical hydrogenation reactions, with a particular emphasis on the translation of these reactions from lab-scale to industrial applications. Giving an overview on the vast amount of conceivable organic substrates and tested catalysts, we highlight the challenges associated with upscaling electrochemical hydrogenations, such as mass transfer limitations and reactor design. Strategies and techniques for addressing these challenges are discussed, including the development of novel catalysts and the implementation of scalable and innovative cell concepts. We furthermore present an outlook on current challenges, future prospects, and research directions for achieving widespread industrial implementation of electrochemical hydrogenation reactions. This work aims to provide beginners as well as experienced electrochemists with a starting point into the potential future transformation of electrochemical hydrogenations from a laboratory curiosity to a viable technology for sustainable chemical synthesis on an industrial scale.

Water oxidation couples to electrocatalytic hydrogenation of carbonyl compounds and unsaturated carbon–carbon bonds by nickel

Artificial photosynthesis, an umbrella term, is a chemical process that biomimetics natural photosynthesis. In natural photosynthesis, electrons from the water-oxidation reaction are used for carbon dioxide reduction. Herein, we report the reducion of aldehydes and ketones to corresponding alcohols in a simple undivided cell. This reaction utilized inexpensive nickel foam electrodes (1 cm2) and LiClO4 (0.05 M) as a commercially accessible electrolyte in an aqueous medium. Under electrochemical conditions, a series of alcohols (21 examples) produces high selectivity in good yields (up to 100%). Usage the current method, 10 mmol (1060 mg) of benzaldehyde is also successfully reduced to benzyl alcohol (757 mg, 70% isolated yield) without any by‑products. This route to alcohols matched several green chemistry principles: (a) atom economy owing to the use of H2O as the solvent and the source of hydrogen, (b) elimination of the homogeneous metal catalyst, (c) use of smooth reaction conditions, (d) waste inhibition due to low volumetric of by-products, and (e) application of safe EtOH co-solvent. Moreover, the ability of the system to operate with alkyne and alkene compounds enhanced the practical efficiency of this process.

Cobalt-electrocatalytic HAT for functionalization of unsaturated C-C bonds

The study and application of transition metal hydrides (TMHs) has been an active area of chemical research since the early 1960s1, for energy storage, through the reduction of protons to generate hydrogen2,3, and for organic synthesis, for the functionalization of unsaturated C-C, C-O and C-N bonds4,5. In the former instance, electrochemical means for driving such reactivity has been common place since the 1950s6 but the use of stoichiometric exogenous organic- and metal-based reductants to harness the power of TMHs in synthetic chemistry remains the norm. In particular, cobalt-based TMHs have found widespread use for the derivatization of olefins and alkynes in complex molecule construction, often by a net hydrogen atom transfer (HAT)7. Here we show how an electrocatalytic approach inspired by decades of energy storage research can be made use of in the context of modern organic synthesis. This strategy not only offers benefits in terms of sustainability and efficiency but also enables enhanced chemoselectivity and distinct, tunable reactivity. Ten different reaction manifolds across dozens of substrates are exemplified, along with detailed mechanistic insights into this scalable electrochemical entry into Co-H generation that takes place through a low-valent intermediate.

Transition Metal Complex Catalyzed Photo- and Electrochemical (De)hydrogenations Involving C=O and C=N Bonds

Herein, we summarize the photo- and electrochemical protocols for dehydrogenation and hydrogenations involving carbonyl and imine functions. The three basic principles that have been explored to interconvert such moieties with transition metal complexes are discussed in detail and the substrate scope is evaluated. Furthermore, we describe some general thermodynamic and kinetic aspects of such electro- and photochemically driven reactions.

1 Introduction

2 Dehydrogenation Reactions

2.1 Electrochemical Dehydrogenations Using High-Valent Metal Species

2.2 Electrochemical Dehydrogenations Involving Metal Hydride species

2.3 Photochemically Driven Dehydrogenation

3 Hydrogenation Reactions

3.1 Electrochemical Protocols

3.2 Photochemical Protocols

4 Conclusion

5 Abbreviations

Taming Electron Transfers: From Breaking Bonds to Creating Molecules

The electron is the ultimate redox reagent to build and reshape molecular structures. Understanding and controlling the parameters underlying dissociative electron transfer (DET) reactivity and its coupling with proton transfer is crucial for combining selectivity, kinetics and energy efficiency in molecular chemistry. Reactivity understanding and mechanistic elements in DET processes are traced back and key examples of current research efforts are presented, demonstrating a large variety of applications. The involvement of DET pathways indeed encompasses a broad range of processes such as photoredox catalysis, CO₂ reduction and alcohol oxidation. Interplay between these experimental examples and fundamental mechanistic study provides a powerful path to the understanding of driving force-rate relationships, which is crucial for the development of future generations of energy efficient catalytic schemes in redox organic chemistry.