Electrocatalytic Alcohol Oxidation with TEMPO and Bicyclic Nitroxyl Derivatives: Driving Force Trumps Steric Effects

Molecular catalyst and co-catalyst systems based on transition metal complexes for the electrochemical oxidation of alcohols

Molecular catalysts allow deeper study of underlying mechanisms relative to heterogeneous systems by offering a discrete active site to monitor. Mechanistic study with knowledge of key intermediates subsequently enables the development of design principles through an understanding of how improved reactivity or selectivity can be achieved through modification of the catalyst structure. The co-catalytic inclusion of redox mediators (RM), which are small molecules that can aid in the transfer of protons and electrons, has been shown to improve product conversion and selectivity in many molecular systems, through intercepting key intermediates to direct reaction pathways. The primary focus for the majority of molecular electrocatalysts has been on optimizing design for reductive reactions, such as the hydrogen evolution reaction (HER), the oxygen reduction reaction (ORR), and the carbon dioxide reduction reaction (CO2RR). By comparison, there has been much less focus on key oxidative reactions by molecular species, apart from the oxygen evolution reaction (OER). The focus of this review is to highlight molecular catalyst systems optimized for the electrochemical oxidation of alcohols. The electrochemical alcohol oxidation reaction (AOR) can serve a role in synthesizing value-added chemicals and can serve as the counterpart to the CO2RR by releasing electricity from energy-rich molecules. State-of-the-art molecular systems for the AOR are divided between single-site catalysts and co-catalytic systems with redox mediators. The AOR is contextualized as an energy relevant reaction, an overview of the area is provided, foundational improvements in catalyst systems are highlighted, and future development principles for incorporating redox mediators are suggested.

TEMPO-Mediated Electrochemical α-Allylation of Tetrahydroisoquinolines

A C(sp3)-H allylation of tetrahydroisoquinolines has been developed by combining Shono oxidation with a vinylogous Mannich-type reaction. TEMPO was used as the electrocatalyst to lower the electrode potential, improving functional group compatibility. This method provided a practical and efficient tandem procedure for the α-allylation of tetrahydroisoquinolines. The reaction proceeded through the formation of an iminium cation intermediate, which was generated in situ by anodic oxidation, followed by nucleophilic addition of 2-allylazaarenes.

A dual-radical process for tri/di-fluoromethylarylation of alkenes enabled by indirect electroreduction

This study presents a stable and mild electrochemical dual-radical strategy for tri/di-fluoromethylarylation of alkenes. The synergistic combination of cyanoarene and phenanthrene as dual redox mediators constructs an efficient catalytic system.

Scaling Organic Electrosynthesis: The Crucial Interplay between Mechanism and Mass Transport

Organic electrosynthesis opens new avenues of reactivity and promises more sustainable practices in the preparation of fine chemicals and pharmaceuticals. The full value of this approach will be realized by taking these processes to the production scale; however, achieving this goal will require a better understanding of the influence of mass transport on reaction behavior and the interactions between reactive species and electrodes inherent to organic electrosynthesis. The limited options for cell geometries used on small scale limit elucidation of these features. Here, we show how advanced cell geometries allow us to control the interplay between reaction mechanism and mass transport, leading to improved performance of three modern organic electrosynthetic reactions. Each reaction shows a unique relationship with mass transport, highlighting the importance of understanding this relationship further to maximize the utility of organic electrosynthesis at scale.

Anodic Potential Driven Electrochemiluminescence of Luminol/O2 System with the Immobilized TEMPO Mediator

The oxygen reduction reaction (ORR) has been recognized well as an essential step to trigger the electrochemiluminescence (ECL) of the luminol/O2 system but suffers from sluggish reaction kinetics. Herein, ECL of the luminol/O2 system without involving ORR is reported by using a 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) grafted glassy carbon electrode (4-amino-TEMPO/GCE). This electrode exhibits high catalytic activity toward the oxidation of luminol, though it is covered with an organic film. Such promoted luminol oxidation on the electrode results in a 5.0-fold ECL enhancement at an anodic potential, ensuring the ECL detection as more sensitive. The mechanism studies indicate that the catalysis of 4-amino-TEMPO/GCE can provide additional luminol anion radical (L-•) for enhancing the ECL of the luminol/O2 system. Hence, this work introduces a different approach to modulate luminol oxidation for enhancing ECL emission based on an immobilized organic oxidation catalyst.

Closed-Loop Navigation of a Kinetic Zone Diagram for Redox-Mediated Electrocatalysis Using Bayesian Optimization, a Digital Twin, and Automated Electrochemistry

Molecular electrocatalysis campaigns often require tuning multiple experimental parameters to obtain kinetically insightful electrochemical measurements, a prohibitively time-consuming task when performing comprehensive studies across multiple catalysts and substrates. In this work, we present an autonomous workflow that combines Bayesian optimization and automated electrochemistry to perform fully unsupervised cyclic voltammetry (CV) studies of molecular electrocatalysis. We developed CV descriptors that leveraged the conceptual framework of the EC' (where EC' denotes an electrochemical step followed by a catalytic chemical step) kinetic zone diagram to enable efficient Bayesian optimization. The CV descriptor's effect on optimization performance was evaluated using a digital twin of our autonomous experimental platform, quantifying the accuracy of obtained kinetic values against the known ground truth. We demonstrated our platform experimentally by performing autonomous studies of TEMPO-catalyzed ethanol and isopropanol electro-oxidation, demonstrating rapid identification of kinetically insightful conditions in 10 or less iterations through the closed-loop workflow. Overall, this work highlights the application of autonomous electrochemical platforms to accelerate mechanistic studies in molecular electrocatalysis and beyond.

Mediated Electrochemical Oxidation of Nucleosides to Their 5'-Carboxylic Acid Derivatives

Nucleoside 5'-carboxylic acids are important synthetic targets, but most existing methods to access these compounds use (super)stoichiometric chemical oxidants to oxidize the primary alcohol. The present study introduces an aminoxyl-mediated electrochemical method for oxidation of nucleosides and ribosugars to afford their 5'-carboxylic acid derivatives. The optimized conditions employ 4-acetamido-2,2,6,6-tetramethylpiperidine 1-oxyl (ACT) as the aminoxyl mediator in a water/acetonitrile solvent mixture with tetraethylammonium bicarbonate or monohydrogen phosphate as a base within an undivided electrolysis cell. The reaction tolerates diverse functionality at the nucleobase and sugar positions and is showcased in the oxidation of 10 different substrates, including a 10 g scale oxidation of 2'-O-methylcytidine.

Redox-Detecting Deep Learning for Mechanism Discernment in Cyclic Voltammograms of Multiple Redox Events

In electrochemical analysis, mechanism assignment is fundamental to understanding the chemistry of a system. The detection and classification of electrochemical mechanisms in cyclic voltammetry set the foundation for subsequent quantitative evaluation and practical application, but are often based on relatively subjective visual analyses. Deep-learning (DL) techniques provide an alternative, automated means that can support experimentalists in mechanism assignment. Herein, we present a custom DL architecture dubbed as EchemNet, capable of assigning both voltage windows and mechanism classes to electrochemical events within cyclic voltammograms of multiple redox events. The developed technique detects over 96% of all electrochemical events in simulated test data and shows a classification accuracy of up to 97.2% on redox events with 8 known mechanisms. This newly developed DL model, the first of its kind, proves the feasibility of redox-event detection and electrochemical mechanism classification with minimal a priori knowledge. The DL model will augment human researchers' productivity and constitute a critical component in a general-purpose autonomous electrochemistry laboratory.

Synergistic enhancement of electrochemical alcohol oxidation by combining NiV-layered double hydroxide with an aminoxyl radical

Electrochemical alcohol oxidation (EAO) represents an effective method for the production of high-value carbonyl products. However, its industrial viability is hindered by suboptimal efficiency stemming from low reaction rates. Here, we present a synergistic electrocatalysis approach that integrates an active electrode and aminoxyl radical to enhance the performance of EAO. The optimal aminoxyl radical (4-acetamido-2,2,6,6-tetramethylpiperidine 1-oxyl) and Ni0.67V0.33-layered double hydroxide (LDH) are screen as cooperative electrocatalysts by integrating theoretical predictions and experiments. The Ni0.67V0.33-LDH facilitates the adsorption and activation of N-(1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl)acetamide (ACTH) via interactions with ketonic oxygen, thereby improving selectivity and yield at high current densities. The electrolysis process is scaled up to produce 200 g of the steroid carbonyl product 8b (19-Aldoandrostenedione), achieving a yield of 91% and a productivity of 243 g h-1. These results represent a promising method for accelerating electron transfer to enhance alcohol oxidation, highlighting its potential for practical electrosynthesis applications.

Rotaxane-catalyzed aerobic oxidation of primary alcohols

Nitroxide radicals are widely utilized as catalysts for the oxidation of primary alcohols. Here, the aerobic catalytic oxidation cycle of nitroxide radicals has been implemented within a mechanically interlocked rotaxane architecture consisting of a paramagnetic crown ether, which is confined by a molecular axle containing a dialkylammonium station and a 1,2,3-triazole unit. The rotaxane is engineered to exploit the oxidation of a primary alcohol: the primary catalyst is the wheel, a nitroxide radical capable of altering its oxidation state during the catalytic cycle, while the co-oxidant is the Cerium(IV)/O2 couple. The synthesis of the proposed rotaxane, along with its characterization using EPR, HRMS, voltammetry and NMR data, is reported in the paper. The aerobic catalytic oxidation cycle was further investigated using EPR, NMR and GC-MS analyses. This study can aid in the design of autonomously driven molecular machines that exploit the aerobic catalytic oxidation of nitroxide radicals.

Oxoammonium-Catalyzed Oxidation of N-Substituted Amines

We report the development of oxoammonium-catalyzed oxidation of N-substituted amines via a hydride transfer mechanism. Steric and electronic tuning of catalyst led to complementary sets of conditions that can oxidize a broad scope of carbamates, sulfonamides, ureas, and amides into the corresponding imides. The reaction was further demonstrated on a 100-g scale using a continuous flow setup.

NHPI-Catalyzed Electro-Oxidation of Alcohols to Aldehydes and Ketones

A practical and recyclable electro-oxidation of alcohols to aldehydes and ketones by using N-hydroxyphthalimide (NHPI) as the catalyst is presented. Through an undivided pool, under constant current conditions, various alcohols can be oxidized to the corresponding aldehydes or ketones in a high yield. Compared with previous methods, this system has the following characteristics: (1) the catalyst, electrode, electrolyte, and solvent (mainly water) are recyclable; (2) it has many advantages such as mild reaction conditions, easy operation, and good tolerance of functional groups; and (3) it can be smoothly scaled up to kilogram-scale production.

Universal high-efficiency electrocatalytic olefin epoxidation via a surface-confined radical promotion

Production of epoxides via selective oxidation of olefins affords a fundamental source of key intermediates for the industrial manufacture of diverse chemical stocks and materials. Current oxidation strategy generally works under harsh conditions including high temperature, high pressure, and/or request for potentially hazardous oxidants, leading to substantial challenges in sustainability and energy efficiency. To this end, direct electrocatalytic epoxidation poses as a promising solution to these issues, yet their industrial applications are limited by the low selectivity, low yield, and poor stability of the electrocatalysts. Here we report a universal electrochemical epoxidation approach via a kinetically confined surface radical pathway. High epoxidation efficiency can be achieved under mild working conditions (e.g., >99% selectivity, >80% yield and >80% Faraday efficiency for cyclohexene-to-cyclohexene oxide conversion), which can be extended to broad scope of olefin substrates. The catalytic performance originated from a surface bimolecular (L-H) reaction mechanism involving formation and surface confinement of bromine radicals due to kinetic restriction, which effectively activates inert C=C bonds while avoiding the homogenous radical side reactions. With the use of renewable energy and water as green oxygen source, successful implementation of this approach will pave the way for more sustainable chemical production and manufacturing.

Bright luminol electrochemiluminescence mediated by a simple TEMPO radical for visualized multiplex detection

In this work, a series of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) radicals bearing different functional groups were exploited as a simple catalyst to promote electrochemiluminescence (ECL) generation in luminol/H2O2 system. These TEMPO radicals were found to facilitate the electrochemical oxidation of H2O2 and luminol through different catalytic mechanisms, as well as the subsequent ECL generation of luminol/H2O2 system. The electrochemical oxidation and luminol ECL generation could be tuned by the functional group on the para-position of TEMPO, for which the structure/activity relationship was revealed. Finally, with the combination of enzymatic system, luminol ECL enhancement up to 9.6-fold was obtained through the catalysis of 4-hydroxyl-TEMPO. The enhanced luminol ECL allows acquiring brighter ECL images in a single-electrochemical system (SEES) for multiplex detection of cholesterol, H2O2 and glucose.

Strategies in photochemical alcohol oxidation on noble-metal free nanomaterials as heterogeneous catalysts

Using light absorbing semiconductors to harvest solar energy has long been considered as a promising approach for driving chemical reactions. Photochemical oxidation of alcohols catalyzed by noble-metal free nanomaterials is particularly attractive due to the bio-derivable nature of the substrate and the wide availability of such catalysts. Here, a number of strategies that could facilitate the progress are discussed, where it is concluded that the photochemical alcohol oxidation process can benefit from both mechanistic optimization of the electron-transfer steps and structural engineering of the light-absorbing nanomaterials.

Covalently linked molecular catalysts in conjugated polymer dots boost photocatalytic alcohol oxidation in neutral condition

As a new class of organic photocatalysts, polymer dots show a potential application in photocatalytic hydrogen peroxide production coupled with chemical oxidation such as methanol oxidation. However, the poor methanol oxidation ability by polymer dots still inhibits the overall photocatalytic reaction occurring in the neutral condition. In this work, an organic molecular catalyst 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl radical is covalently linked to a fluorene unit in a polymer skeleton, eventually enabling photocatalytic hydrogen peroxide production coupled with methanol oxidation in the neutral condition. By conducting various spectroscopic measurements, charge transfer between components in this molecular catalyst-immobilized polymer dots system is studied and found to be very efficient for hydrogen peroxide production coupled with alcohol oxidation. This work proves a strategy for designing polymer dots photocatalysts with molecular catalysts, facilitating their future development and potential applications in other fields such as water splitting, CO2 reduction, photoredox catalysis and photodynamic therapy.

Electroreductive deuteroarylation of alkenes enabled by an organo-mediator

Electroreduction mediated by organo-mediators has emerged as a concise and effective strategy, holding significant potential in the site-specific introduction of deuterium. In this study, we present an environmentally friendly electroreduction approach for anti-Markovnikov selective deuteroarylation of alkenes and aryl iodides with D2O as the deuterium source. The key to the protocol lies in the employment of a catalytic amount of 2,2'-bipyiridine as an efficient organo-mediator, which facilitates the generation of aryl radicals by assisting in the cleavage of the C-X (X = I or Br) bonds in aryl halides. Because its reduction potential matches that of aryl iodides, the organo-mediator can control the chemoselectivity of the reaction and avoid the side reactions of competitive substrate deuteration. These phenomena are theoretically supported by CV experiments and DFT calculations. Our protocol provides a series of mono-deuterated alkylarenes with excellent deuterium incorporation through two single-electron reductions (SER), without requiring metal catalysts, external reductants, and sacrificial anodes.

Fluorescence Quenching Effect of a Highly Active Nitroxyl Radical on 7-amino-4-methylcoumarin and Glutathione Sensing

Nitroxyl radical compounds, such as 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), are stable radical compounds with a variety of unique characteristics, including fluorescence quenching. In this study, we investigated the fluorescence quenching effect of nortropine N-oxyl (NNO), which is a highly active nitroxyl radical that is more active than TEMPO in oxidation catalysis. The fluorescence intensity of 7-amino-4-methylcoumarin (AMC) was quenched by NNO and TEMPO to 5% and 95% of the initial fluorescence intensity, respectively, indicating highly efficient quenching by NNO. In addition, we used this reaction to measure glutathione concentration. The quenching effect of NNO was abrogated by the chemical reaction with glutathione, resulting in restoration of AMC fluorescence. This response was observed at glutathione concentrations from 10 µM to 1 mM, and good calibration curves were obtained from 10 to 250 µM.

Electrocatalytic water-to-oxygenates conversion: redox-mediated versus direct oxygen transfer

Electrocatalytic oxygenation of hydrocarbons with high selectivity has attracted much attention for its advantages in the sustainable and controllable production of oxygenated compounds with reduced greenhouse gas emissions. Especially when utilizing water as an oxygen source, by constructing a water-to-oxygenates conversion system at the anode, the environment and/or energy costs of producing oxygenated compounds and hydrogen energy can be significantly reduced. There is a broad consensus that the generation and transformation of oxygen species are among the decisive factors determining the overall efficiency of oxygenation reactions. Thus, it is necessary to elucidate the oxygen transfer process to suggest more efficient strategies for electrocatalytic oxygenation. Herein, we introduce oxygen transfer routes through redox-mediated pathways or direct oxygen transfer methods. Especially for the scarcely investigated direct oxygen transfer at the anode, we aim to detail the strategies of catalyst design targeting the efficient oxygen transfer process including activation of organic substrate, generation/adsorption of oxygen species, and transformation of oxygen species for oxygenated compounds. Based on these examples, the significance of balancing the generation and transformation of oxygen species, tuning the states of organic substrates and intermediates, and accelerating electron transfer for organic activation for direct oxygen transfer has been elucidated. Moreover, greener organic synthesis routes through heteroatom transfer and molecular fragment transfer are anticipated beyond oxygen transfer.

Improved Electrosynthesis of Biomass Derived Furanic Compounds via Nitroxyl Radical Redox Mediation

Biomass is an abundantly available, underutilized feedstock for the production of bulk and fine chemicals, polymers, and sustainable and biodegradable plastics that are traditionally sourced from petrochemicals. Among potential feedstocks, 2,5-furan dicarboxylic acid (FDCA) stands out for its potential to be converted to higher-value polymeric materials such as polyethylene furandicarboxylate (PEF), a bio-based plastic alternative. In this study, the sustainable, electrocatalytic oxidation of stable furan molecule 2,5-bis(hydroxymethyl)furan (BHMF) to FDCA is investigated using a variety of TEMPO derivative electrocatalysts in a mediated electrosynthetic reaction. Three TEMPO catalysts (acetamido-TEMPO, methoxy-TEMPO, and TEMPO) facilitate full conversion to FDCA in basic conditions with >90% yield and >100% Faradaic efficiency. The remaining three TEMPO catalysts (hydroxy-TEMPO, oxo-TEMPO, and amino-TEMPO) all perform intermediate oxidation of BHMF in basic conditions but do not facilitate full conversion to FDCA. On the basis of pH studies completed on all TEMPO derivatives to assess their electrochemical reversibility and response to substrate, pH and reversibility play significant roles in the catalytic ability of each catalyst, which directly influences catalyst turnover and product formation. More broadly, this study also highlights the importance of an effective and rapid electroanalytical workflow in mediated electrosynthetic reactions, demonstrating how voltammetric catalyst screening can serve as a useful tool for predicting the reactivity and efficacy of a catalyst-substrate electrochemical system.

Operando film-electrochemical EPR spectroscopy tracks radical intermediates in surface-immobilized catalysts

The development of surface-immobilized molecular redox catalysts is an emerging research field with promising applications in sustainable chemistry. In electrocatalysis, paramagnetic species are often key intermediates in the mechanistic cycle but are inherently difficult to detect and follow by conventional in situ techniques. We report a new method, operando film-electrochemical electron paramagnetic resonance spectroscopy (FE-EPR), which enables mechanistic studies of surface-immobilized electrocatalysts. This technique enables radicals formed during redox reactions to be followed in real time under flow conditions, at room temperature and in aqueous solution. Detailed insight into surface-immobilized catalysts, as exemplified here through alcohol oxidation catalysis by a surface-immobilized nitroxide, is possible by detecting active-site paramagnetic species sensitively and quantitatively operando, thereby enabling resolution of the reaction kinetics. Our finding that the surface electron-transfer rate, which is of the same order of magnitude as the rate of catalysis (accessible from operando FE-EPR), limits catalytic efficiency has implications for the future design of better surface-immobilized catalysts.

Axial Ligation Impedes Proton-Coupled Electron-Transfer Reactivity of a Synthetic Compound-I Analogue

The nature of the axial ligand in high-valent iron-oxo heme enzyme intermediates and related synthetic catalysts is a critical structural element for controlling proton-coupled electron-transfer (PCET) reactivity of these species. Herein, we describe the generation and characterization of three new 6-coordinate, iron(IV)-oxo porphyrinoid-π-cation-radical complexes and report their PCET reactivity together with a previously published 5-coordinate analogue, FeIV(O)(TBP8Cz+•) (TBP8Cz = octakis(p-tert-butylphenyl)corrolazinato3-) (2) (Cho, K. A high-valent iron-oxo corrolazine activates C-H bonds via hydrogen-atom transfer. J. Am. Chem. Soc. 2012, 134, 7392-7399). The new complexes FeIV(O)(TBP8Cz+•)(L) (L = 1-methyl imidazole (1-MeIm) (4a), 4-dimethylaminopyridine (DMAP) (4b), cyanide (CN-)(4c)) can be generated from either oxidation of the ferric precursors or by addition of L to the Compound-I (Cpd-I) analogue at low temperatures. These complexes were characterized by UV-vis, electron paramagnetic resonance (EPR), and Mössbauer spectroscopies, and cryospray ionization mass spectrometry (CSI-MS). Kinetic studies using 4-OMe-TEMPOH as a test substrate indicate that coordination of a sixth axial ligand dramatically lowers the PCET reactivity of the Cpd-I analogue (rates up to 7000 times slower). Extensive density functional theory (DFT) calculations together with the experimental data show that the trend in reactivity with the axial ligands does not correlate with the thermodynamic driving force for these reactions or the calculated strengths of the O-H bonds being formed in the FeIV(O-H) products, pointing to non-Bell-Evans-Polanyi behavior. However, the PCET reactivity does follow a trend with the bracketed reduction potential of Cpd-I analogues and calculated electron affinities. The combined data suggest a concerted mechanism (a concerted proton electron transfer (CPET)) and an asynchronous movement of the electron/proton pair in the transition state.

Selective glucose electro-oxidation catalyzed by TEMPO on graphite felt

Long-term electrolyses of glucose in a potassium carbonate (K2CO3) aqueous electrolyte have been performed on graphite felt electrodes with TEMPO as a homogeneous catalyst. The influences of the operating conditions (initial concentrations of glucose, TEMPO, and K2CO3 along with applied anode potential) on the conversion, selectivity toward gluconate/glucarate, and faradaic efficiency were assessed first. Then, optimizations of the conversion, selectivity, and faradaic efficiency were performed using design of experiments based on the L9 (34) Taguchi table, which resulted in 84% selectivity toward gluconate with 71% faradaic efficiency for up to 79% glucose conversion. Side products such as glucaric acid were also obtained when the applied potential exceeded 1.5 V vs. reversible hydrogen electrode.

Probing Local pH Change during Electrode Oxidation of TEMPO Derivative: Implication of Redox-Induced Acidity Alternation by Imidazolium-Linker Functional Groups

The chemical degradation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-based aqueous energy storage and catalytic systems is pH sensitive. Herein, we voltammetrically monitor the local pH (pHlocal) at a Pt ultramicroelectrode (UME) upon electro-oxidation of imidazolium-linker functionalized TEMPO and show that its decrease is associated with the greater acidity of the cationic (oxidized) rather than radical (reduced) form of TEMPO. The protons that drive the decrease in pH arise from hydrolysis of the conjugated imidazolium-linker functional group of 4-[2-(N-methylimidazolium)acetoxy]-2,2,6,6-tetramethylpiperidine-1-oxyl chloride (MIMAcO-T), which was studied in comparison with 4-hydroxyl-TEMPO (4-OH-T). Voltammetric hysteresis is observed during the electrode oxidation of 4-OH-T and MIMAcO-T at a Pt UME in an unbuffered aqueous solution. The hysteresis arises from the pH-dependent formation and dissolution of Pt oxides, which interact with pHlocal in the vicinity of the UME. We find that electrogenerated MIMAcO-T+ significantly influences pHlocal, whereas 4-OH-T+ does not. Finite element analysis reveals that the thermodynamic and kinetic acid-base properties of MIMAcO-T+ are much more favorable than those of its reduced counterpart. Imidazolium-linker functionalized TEMPO molecules comprising different linking groups were also investigated. Reduced TEMPO molecules with carbonyl linkers behave as weak acids, whereas those with alkyl ether linkers do not. However, oxidized TEMPO+ molecules with alkyl ether linkers exhibit more facile acid-base kinetics than those with carbonyl ones. Density functional theory calculations confirm that OH- adduct formation on the imidazolium-linker functional group of TEMPO is responsible for the difference in the acid-base properties of the reduced and oxidized forms.

Cyclic voltammetry and chronoamperometry: mechanistic tools for organic electrosynthesis

Electrochemical methods offer unique advantages for chemical synthesis, as the reaction selectivity may be controlled by tuning the applied potential or current. Similarly, measuring the current or potential during the reaction can provide valuable mechanistic insights into these reactions. The aim of this tutorial review is to explain the use of cyclic voltammetry and chronoamperometry to interrogate reaction mechanisms, optimize electrochemical reactions, or design new reactions. Fundamental principles of cyclic voltammetry and chronoamperometry experiments are presented together with the application of these techniques to probe (electro)chemical reactions. Several diagnostic criteria are noted for the use of cyclic voltammetry and chronoamperometry to analyze coupled electrochemical-chemical (EC) reactions, and a series of individual mechanistic studies are presented. Steady state voltammetric and amperometric measurements, using microelectrodes (ME) or rotating disk electrodes (RDE) provide a means to analyze concentrations of redox active species in bulk solution and offer a versatile strategy to conduct kinetic analysis or determine the species present during (electro)synthetic chemical reactions.

Comprehensive Structural and Electronic Properties of 2-Azaadamantane N-Oxyl Derivatives Correlated with Their Catalytic Ability

Herein, a comprehensive kinetic study is performed to compare the catalytic efficiency of 2-azaadamantane N-oxyl (AZADO) derivatives with that of 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) used as radical catalysts in the aerobic oxidation of l-menthol. Furthermore, the correlation between the catalytic activity and structural/electronic parameters of AZADOs and TEMPO is elucidated. The reaction rate constants achieved with several AZADO derivatives exhibit moderate relationships with spectroscopic parameters, such as the hyperfine coupling constant of the N atom (AN) and NO stretching vibration frequency (νNO) observed in electron spin resonance and infrared spectra, respectively. The planarity C-(NO)-C angle (φ) at the N atom, determined by density functional theory (DFT) calculations, also strongly correlates with the AN and νNO. Moreover, the bond order of NO, which strongly depends on the structural and electronic properties of NO radicals, correlates with radical activity; thus, the radical activity can be predicted by DFT calculations, thereby accelerating the synthesis of new AZADO derivatives without requiring alcohol oxidation experiments.

Merging electrocatalytic alcohol oxidation with C-N bond formation by electrifying metal-ligand cooperative catalysts

Electrification of thermal chemical processes could play an important role in creating a more energy efficient chemical sector. Here we demonstrate that a range of MLC catalysts can be successfully electrified and used for imine formation from alcohol precursors, thus demonstrating the first example of molecular electrocatalytic C-N bond formation.This novel concept allowed energy efficiency to be increased by an order of magnitude compared to thermal catalysis. Molecular EAO and the electrification of homogeneous catalysts can thus contribute to current efforts for the electrocatalytic generation of C-N bonds from simple building blocks.

Recent advancements in the use of Bobbitt's salt and 4-acetamidoTEMPO

Recent advances in synthetic methodologies for selective, oxidative transformations using Bobbitt's salt (4-acetamido-2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate, 1) and its stable organic nitroxide counterpart ACT (4-acetamidoTEMPO, 4-acetamido-2,2,6,6-tetramethylpiperidine-1-oxyl, 2) have led to increased applications across a broad array of disciplines. Current applications and mechanistic understanding of these metal-free, environmentally benign, and easily accessible organic oxidants now span well-beyond the seminal use of 1 and 2 in selective alcohol oxidations. New synthetic methodologies for the oxidation of alcohols, ethers, amines, thiols, C-H bonds and other functional groups with 1 and 2 along with the field's current mechanistic understandings of these processes are presented alongside our contributions in this area. Exciting new areas harnessing the unique properties of these oxidants include: applications to drug discovery and natural product total synthesis, the development of new electrocatalytic methods for depolymerization of lignin and modification of other biopolymers, in vitro and in vivo nucleoside modifications, applications in supramolecular catalysis, the synthesis of new polymers and materials, enhancements in the design of organic redox flow batteries, uses in organic fuel cells, applications and advancements in energy storage, the development of electrochemical sensors, and the production of renewable fuels.

Organo-Mediator Enabled Electrochemical Deuteration of Styrenes

Despite widespread use of the deuterium isotope effect, selective deuterium labeling of chemical molecules remains a major challenge. Herein, a facile and general electrochemically driven, organic mediator enabled deuteration of styrenes with deuterium oxide (D2 O) as the economical deuterium source was reported. Importantly, this transformation could be suitable for various electron rich styrenes mediated by triphenylphosphine (TPP). The reaction proceeded under mild conditions without transition-metal catalysts, affording the desired products in good yields with excellent D-incorporation (D-inc, up to >99 %). Mechanistic investigations by means of isotope labeling experiments and cyclic voltammetry tests provided sufficient support for this transformation. Notably, this method proved to be a powerful tool for late-stage deuteration of biorelevant compounds.

Electrosynthesis and Microanalysis in Thin Layer: An Electrochemical Pipette for Rapid Electrolysis and Mechanistic Study of Electrochemical Reactions

Electrochemistry represents unique approaches for the promotion and mechanistic study of chemical reactions and has garnered increasing attention in different areas of chemistry. This expansion necessitates the enhancement of the traditional electrochemical cells that are intrinsically constrained by mass transport limitations. Herein, we present an approach for designing an electrochemical cell by limiting the reaction chamber to a thin layer of solution, comparable to the thickness of the diffusion layer. This thin layer electrode (TLE) provides a modular platform to bypass the constraints of traditional electrolysis cells and perform electrolysis reactions in the timescale of electroanalytical techniques. The utility of the TLE for electrosynthetic applications benchmarked using NHPI-mediated electrochemical C-H functionalization. The application of microscale electrolysis for the study of drug metabolites was showcased by elucidating the oxidation pathways of the paracetamol drug. Moreover, hosting a microelectrode in the TLE, was shown to enable real-time probing of the profiles of redox-active components of these rapid electrosynthesis reactions.

Redox: Organic Robust Radicals and Their Polymers for Energy Conversion/Storage Devices

Persistent radicals can hold their unpaired electrons even under conditions where they accumulate, leading to the unique characteristics of radical ensembles with open-shell structures and their molecular properties, such as magneticity, radical trapping, catalysis, charge storage, and electrical conductivity. The molecules also display fast, reversible redox reactions, which have attracted particular attention for energy conversion and storage devices. This paper reviews the electrochemical aspects of persistent radicals and the corresponding macromolecules, radical polymers. Radical structures and their redox reactions are introduced, focusing on redox potentials, bistability, and kinetic constants for electrode reactions and electron self-exchange reactions. Unique charge transport and storage properties are also observed with the accumulated form of redox sites in radical polymers. The radical molecules have potential electrochemical applications, including in rechargeable batteries, redox flow cells, photovoltaics, diodes, and transistors, and in catalysts, which are reviewed in the last part of this paper.

Determination of antibiotics by amperometry using nortropine N-oxyl, a highly active nitroxyl radical

Nitroxyl radical compounds oxidize hydroxy groups and some amino groups upon application of an electric potential. The resulting anodic current depends on the concentration of these functional groups in solution. Thus, it is possible to quantify compounds containing these functional groups by electrochemical methods. Cyclic voltammetry has been used to evaluate the catalytic activity of nitroxyl radicals, and the ability of such radicals to sense biological and other compounds. In this study, we evaluated a method for quantifying compounds using constant-potential electrolysis (amperometry) of nitroxyl radicals for application in flow injection analysis and high-performance liquid chromatography as an electrochemical detector. When amperometry was performed using 2,2,6,6-tetramethylpiperidine 1-oxyl, a common nitroxyl radical compound, little change was observed even with 100 mM glucose due to its low reactivity in neutral aqueous solutions. In contrast, 2-azaadamantane N-oxyl and nortropine N-oxyl, which are highly active nitroxyl radicals, showed a concentration-dependent response in neutral aqueous solution. Responses of 33.8 and 125.9 μA, respectively, were observed. By recognition of hydroxy and amino groups, we have succeeded in the electrochemical detection of some drugs by amperometry. Streptomycin, an aminoglycoside antibiotic, was quantifiable in the range of 30-1000 µM.

Oxidative Deprotection of Benzyl Protecting Groups for Alcohols by an Electronically Tuned Nitroxyl-Radical Catalyst

The oxidative deprotection of benzyl (Bn) groups using nitroxyl-radical catalyst 1 and co-oxidant phenyl iodonium bis(trifluoroacetate) (PIFA) is reported. This catalyst is highly active for the oxidation of benzylic ethers because of the electronic tuning on account of the electron-withdrawing ester groups next to the catalytically active center. This catalytic system promotes deprotections at ambient temperature and has a broad substrate scope, including substrates possessing hydrogenation-sensitive functional groups, while the deprotection hardly proceeds when using well-known nitroxyl-radical catalysts such as 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO). The 1/PIFA system also promotes the deprotection of several benzylic protecting groups, including 2-naphthylmethyl (NAP) and 4-methylbenzyl (MBn) groups. Catalyst 1 was also effective for the direct synthesis of ketones and aldehydes from Bn ethers via deprotected alcohols using an excess of the co-oxidant PIFA.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Acetamido-TEMPO mediated electrochemical oxidation of alcohols to aldehydes and ketones

A protocol for the oxidation of alcohols to aldehydes and ketones employing an electrochemical aminoxyl-mediated reaction is presented. The approach employs a catalytic amount of the radical and the use of a base is not required. It is performed using readily available electrodes in a commercially available electrochemistry apparatus and does not require a reference electrode. The methodology is applicable to a range of structurally and electronically diverse substrates, including the oxidation of primary alcohols to aldehydes rather than the more commonly formed carboxylic acids.

Electrified hydrocarbon-to-oxygenates coupled to hydrogen evolution for efficient greenhouse gas mitigation

Chemicals manufacture is among the top greenhouse gas contributors. More than half of the associated emissions are attributable to the sum of ammonia plus oxygenates such as methanol, ethylene glycol and terephthalic acid. Here we explore the impact of electrolyzer systems that couple electrically-powered anodic hydrocarbon-to-oxygenate conversion with cathodic H₂ evolution reaction from water. We find that, once anodic hydrocarbon-to-oxygenate conversion is developed with high selectivities, greenhouse gas emissions associated with fossil-based NH₃ and oxygenates manufacture can be reduced by up to 88%. We report that low-carbon electricity is not mandatory to enable a net reduction in greenhouse gas emissions: global chemical industry emissions can be reduced by up to 39% even with electricity having the carbon footprint per MWh available in the United States or China today. We conclude with considerations and recommendations for researchers who wish to embark on this research direction.

Efficient Voltage-Driven Oxidation of Water and Alcohols by an Organic Molecular Catalyst Directly Attached to a Carbon Electrode

The integration of heterogeneous electrocatalysis and molecular catalysis is a promising approach to designing new catalysts for the oxygen evolution reaction (OER) and other processes. We recently showed that the electrostatic potential drop across the double layer contributes to the driving force for electron transfer between a dissolved reactant and a molecular catalyst immobilized directly on the electrode surface. Here, we report high current densities and low onset potentials for water oxidation attained using a metal-free voltage-assisted molecular catalyst (TEMPO). Scanning electrochemical microscopy (SECM) was used to analyze the products and determine faradic efficiencies for the generation of H₂O₂ and O₂. The same catalyst was employed for efficient oxidations of butanol, ethanol, glycerol, and H₂O₂. DFT calculations show that the applied voltage alters the electrostatic potential drop between TEMPO and the reactant as well as chemical bonding between them, thereby increasing the reaction rate. These results suggest a new route for designing next-generation hybrid molecular/electrocatalysts for OER and alcohol oxidations.

TEMPO-Modified Polymethacrylates as Mediators in Electrosynthesis: Influence of the Molecular Weight on Redox Properties and Electrocatalytic Activity

Homogeneous catalysts ("mediators") are frequently employed in organic electrosynthesis to control selectivity. Despite their advantages, they can have a negative influence on the overall energy and mass balance if used only once or recycled inefficiently. Polymediators are soluble redox-active polymers applicable as electrocatalysts, enabling recovery by dialysis or membrane filtration. Using anodic alcohol oxidation as an example, we have demonstrated that TEMPO-modified polymethacrylates (TPMA) can act as efficient and recyclable catalysts. In the present work, the influence of the molecular size on the redox properties and the catalytic activity was carefully elaborated using a series of TPMAs with well-defined molecular weight distributions. Cyclic voltammetry studies show that the polymer chain length has a pronounced impact on the key-properties. Together with preparative-scale electrolysis experiments, an optimum size range was identified for polymediator-guided sustainable reaction control.

Redox Mediators in Homogeneous Co-electrocatalysis

Homogeneous electrocatalysis has been well studied over the past several decades for the conversion of small molecules to useful products for green energy applications or as chemical feedstocks. However, in order for these catalyst systems to be used in industrial applications, their activity and stability must be improved. In naturally occurring enzymes, redox equivalents (electrons, often in a concerted manner with protons) are delivered to enzyme active sites by small molecules known as redox mediators (RMs). Inspired by this, co-electrocatalytic systems with homogeneous catalysts and RMs have been developed for the conversion of alcohols, nitrogen, unsaturated organic substrates, oxygen, and carbon dioxide. In these systems, the RMs have been shown to both increase the activity of the catalyst and shift selectivity to more desired products by altering catalytic cycles and/or avoiding high-energy intermediates. However, the area is currently underdeveloped and requires additional fundamental advancements in order to become a more general strategy. Here, we summarize the recent examples of homogeneous co-electrocatalysis and discuss possible future directions for the field.

Chemoselective Oxidation of Alcohols in the Presence of Amines Using an Oxoammonium Salt

The oxidation of alcohols in the presence of reactive amines employing the commercially available oxoammonium cation, "Bobbitt's salt" is described. The oxidation is accomplished under acidic conditions and subsequent treatment with a suitable base affords a convenient one-pot method to access imines in good to excellent isolated yields (74-99%).

Bioresource Upgrade for Sustainable Energy, Environment, and Biomedicine

We conceptualize bioresource upgrade for sustainable energy, environment, and biomedicine with a focus on circular economy, sustainability, and carbon neutrality using high availability and low utilization biomass (HALUB). We acme energy-efficient technologies for sustainable energy and material recovery and applications. The technologies of thermochemical conversion (TC), biochemical conversion (BC), electrochemical conversion (EC), and photochemical conversion (PTC) are summarized for HALUB. Microalgal biomass could contribute to a biofuel HHV of 35.72 MJ Kg-1 and total benefit of 749 $/ton biomass via TC. Specific surface area of biochar reached 3000 m2 g-1 via pyrolytic carbonization of waste bean dregs. Lignocellulosic biomass can be effectively converted into bio-stimulants and biofertilizers via BC with a high conversion efficiency of more than 90%. Besides, lignocellulosic biomass can contribute to a current density of 672 mA m-2 via EC. Bioresource can be 100% selectively synthesized via electrocatalysis through EC and PTC. Machine learning, techno-economic analysis, and life cycle analysis are essential to various upgrading approaches of HALUB. Sustainable biomaterials, sustainable living materials and technologies for biomedical and multifunctional applications like nano-catalysis, microfluidic and micro/nanomotors beyond are also highlighted. New techniques and systems for the complete conversion and utilization of HALUB for new energy and materials are further discussed.

Electrochemical organic reactions: A tutorial review

Although the core of electrochemistry involves simple oxidation and reduction reactions, it can be complicated in real electrochemical organic reactions. The principles used in electrochemical reactions have been derived using physical organic chemistry, which drives other organic/inorganic reactions. This review mainly comprises two themes: the first discusses the factors that help optimize an electrochemical reaction, including electrodes, supporting electrolytes, and electrochemical cell design, and the second outlines studies conducted in the field over a period of 10 years. Electrochemical reactions can be used as a versatile tool for synthetically important reactions by modifying the constant electrolysis current.

Molecular flavin catalysts for C-H functionalisation and derivatisation of dehydroamino acids

In nature, the isoalloxazine heterocycle of flavin cofactors undergoes reversible covalent bond formation with a variety of different reaction partners. These intermediates play a crucial role inter alia as the signalling states and in selective catalysis reactions. In the organic laboratory, covalent adducts with a new carbon-carbon bond have been observed with photochemically excited flavins but have, so far, only been regarded as dead-end side products. We have identified a series of molecular flavins that form adducts resulting in a new C-C bond at the C4a-position through allylic C-H activation and dehydroamino acid oxidation. Typically, these reactions are of radical nature and a stepwise pathway is assumed. We could demonstrate that these adducts are no dead-end and that the labile C-C bond can be cleaved by adding the persistent radical TEMPO leading to flavin regeneration and alkoxyamine-functionalised substrates. Our method allows for the catalytic oxidation of dehydroamino acids (16 examples) and we show that the acylimine products serve as versatile starting points for diversification. The present results are envisioned to stimulate the design of further catalytic reactions involving intermediates at the flavin C4a-position and their reactivity towards metal complexes or other persistent organic radicals. Our method for dehydrobutyrine derivatisation is orthogonal to the currently used methods (i.e., nucleophilic attack or radical addition) and offers new perspectives for peptide natural product diversification.

Mapping Out the Nonconjugated Organic Radical Conductors via Chemical or Physical Pathways

Stable, nitroxide-based organic radicals have gained tremendous attention in a wide range of research fields, ranging from solid-state electronics to energy storage devices. While the success of these organics has been their designer flexibility and the processability that can fully potentiate the open-shell chemistry, a significant knowledge gap exists on the solid-state electronics of small-molecular radicals. Herein, we examine the structure-property relationship that governs the solid-state electronics of a model nitroxide and its derivatives by seeking the connection to their well-established, electrolyte-based chemistry. Further, we propose a general strategy of enhancing their solid-state conductivity by systematic humidity control. This study demonstrates an open-shell platform of the device operation and underlying principles thereof, which can potentially be applied in a number of future radical-based electronic devices.