Tetramethylpiperidine N-Oxyl (TEMPO), Phthalimide N-Oxyl (PINO), and Related N-Oxyl Species: Electrochemical Properties and Their Use in Electrocatalytic Reactions

Recent advances in electrochemical 1,2-difunctionalization of alkenes: mechanisms and perspectives

In recent years, significant achievements have been made in the field of electroorganic chemistry regarding the difunctionalization of alkenes. Researchers have developed innovative strategies utilizing the unique reactivity of electrochemical processes to synthesize complex molecules with high regioselectivity and stereoselectivity. This technology is widely applied in the total synthesis of natural products and in the pharmaceutical industry. This article reviews the research progress in the electrochemical difunctionalization of alkenes through three different radical-mediated pathways over the past five years. It includes discussions on 1,2-stereoselective and non-diastereoselective difunctionalization reactions, rearrangements, intramolecular migrations, and cyclization processes. The summary emphasizes innovative electrode designs, reaction mechanisms, and the integration with other emerging technologies, highlighting the potential of this method in modern organic chemistry. Additionally, it aims to address current challenges and propose possible solutions, providing a promising direction for electrochemically mediated difunctionalization reactions of alkenes.

Metalloporphyrins in bio-inspired photocatalytic conversions

Numerous natural systems contain porphyrin derivatives that facilitate important catalytic processes; thus, developing biomimetic photocatalytic systems based on synthetic metalloporphyrins constitutes a rapidly advancing and fascinating research field. Additionally, porphyrins are widely investigated in a plethora of applications due to their highly versatile structure, presenting advantageous photoredox, photophysical and photochemical properties. Consequently, such metallated tetrapyrrolic macrocycles play a prominent role as photosensitizers and catalysts in developing artificial photosynthetic systems that can store and distribute energy through fuel forming reactions. This review highlights the advances in the field of metalloporphyrin-based biomimetic photocatalysis, particularly targeting water splitting, including both hydrogen and oxygen evolution reactions, carbon dioxide reduction and alcohol oxidation. For each photocatalytic system different approaches are discussed, concerning either structural modifications of the porphyrin derivatives or the phase in which the process takes place, i.e. homogenous or heterogenous. The most important findings for each porphyrin-based photocatalytic reaction are presented and accompanied by the analysis of mechanistic aspects when possible. Finally, the perspectives and limitations are discussed, providing future guidelines for the development of highly efficient metalloporphyrin-based biomimetic systems towards energy and environmental applications.

5-Hydroxymethyl Furfural Oxidation by Perylene Diimide-Sensitized Electrodes Boosted by Photoinduced Doping

We explored the electrochemical behavior of antimony-doped tin oxide (ATO) and perylene diimide (PDI)-sensitized ATO (ATO-PDI) for the (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) mediated conversion of 5-hydroxymethyl furfural (HMF) to 2,5-furandicarboxylic acid (FDCA), a value-added substrate for alternative polymer synthesis. We first showed that ATO displayed good electrocatalytic properties towards TEMPO, affording a quasi-reversible response with a heterogeneous rate constant on the order of 2×10-4 cm s-1. We then evaluated the performance of ATO under exhaustive electrolysis of HMF in basic aqueous electrolyte, reaching 80 % Faradaic Efficiency (FE) for FDCA production. Interestingly, a significantly enhanced current (up to 2.5 mA cm-2) was recorded over time when ATO-PDI was exposed to prolonged visible illumination in a Dye-Sensitized Photoelectrochemical Cell (DSPEC) configuration, which we ascribed to the photoinduced doping of ATO resulting from the oxidative quenching of PDI excited states. The proposed system enabled the production of FDCA with ca. 75 % FE in <2 h reaction time, and an almost quantitative HMF conversion when both the mono- and di-acid products were considered. To the best of our knowledge, this is the first example of a molecular dye-sensitized interface used for the TEMPO-mediated oxidation of HMF.

OH-Detected Aromatic Microsolvation of an Organic NO Radical: Halogenation Controls the Solvation Side

The persistent organic radical 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) protects its NO radical center by four methyl groups. Two of them are arranged tightly (t) on one side of the six-membered puckered heterocycle, and the other two more openly (o) on the other side. It is shown by OH stretching infrared spectroscopy in heated supersonic jet expansions that the hydrogen bond and aromatic ring of a first solvating benzyl alcohol have almost no preference for either side. An increased preference for the t side develops in para-halogenated benzyl alcohols, and it is inverted for ortho-halogenated benzyl alcohols. The experimental dependence on the actual halogen (Cl, Br, and I) is weak, whereas different quantum chemical approaches predict more or less pronounced trends along the halogen series. Some of the benzyl alcohol in the pre-expansion reservoir reduces the TEMPO radical to the closed-shell heterocyclic hydroxylamine TEMPO-H (1-hydroxy-2,2,6,6-tetramethylpiperidine), to the extent that the TEMPO-H···TEMPO complex is observed as an impurity.

Colorimetric quantification of vancomycin by highly active nitroxyl radical compounds

Nitroxyl radicals, represented by 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO), are highly stable organic free radicals with unique properties and are used as functional molecules in various fields. However, TEMPO had low reactivity and sometimes did not provide enough response. Therefore, highly active nitroxyl radical compounds have been developed in which bicyclo and tricyclo structures stabilize the radicals. In this study, we found that nortropine N-oxyl (NNO), a type of highly active nitroxyl radical, can oxidize the 2,2'-dihydroxybiphenyl structure under physiological conditions, and succeeded in the colorimetric quantification of vancomycin containing 2,2'-dihydroxybiphenyl moieties in the molecular structure. The reaction took only a few minutes to complete and could be confirmed with the naked eye, with a quantitative range of 10-100 μM. High-performance analytical probes are expected to be developed that use highly active nitroxyl radical derivatives to replace TEMPO derivatives.

One-Pot, Telescoped Aryl Nitrile Synthesis from Benzylic Silyl Ethers

A one-pot, telescoped transformation of silyl ethers into cyanides that proceeds via silyl-ether oxidation mediated by nitroxyl-radical catalyst 1 and [bis(trifluoroacetoxy)iodo]benzene followed by an imine formation-oxidation sequence using iodine and aqueous ammonia is reported. This transformation is effective for the site-selective transformation of benzylic and allylic silyl ethers in the presence of other silyl ethers. Using an O-protected oxime and a catalytic amount of triflic acid instead of iodine/aqueous ammonia is also effective for cyanation.

Metal-Free Electrochemical Allylic C-H Aerobic Oxidation

A scalable and sustainable electrochemical protocol for allylic C-H aerobic oxidation has been developed, enabling the formation of enones without the use of stoichiometric toxic oxidants or metal catalysts and offering an environmentally benign alternative to traditional chemical oxidation techniques. The process has been successfully applied to selectively oxidize a series of natural products and drug molecules, underscoring its potential for widespread adoption in both academic and industrial contexts.

Enantioselective Electrocatalysis for Cross-Dehydrogenative Heteroarylation with Indoles, Pyrroles, and Furans

Oxidative cross-dehydrogenative C-H/C-H functionalizations represent an exemplary approach for synthesizing carbonyl compounds via α-heteroarylation. Here we present the development of a direct anodic oxidative coupling process between 2-acylimidazoles and divergent heterocyclic systems including indole, pyrrole, and furan, facilitated by ferrocene-assisted asymmetric nickel electrocatalysis with high levels of enantioselectivity. Mechanistic investigations indicate that the reaction initially involves the formation of a chiral Ni-bound α-carbonyl radical, which is then captured by the heteroarene radical cation via intermolecular stereoselective radical/radical cation coupling. The mild, scalable, and robust reaction conditions allow for a broad substrate scope and excellent functional group tolerance, enabling access to a wide range of chiral hetero-compounds. The consequential α-heteroaromatic carbonyl products can potentially be transformed into a plethora of synthetically valuable frameworks, as exemplified by their application in the asymmetric total synthesis of (-)-COX-2 inhibitor, (+)-acremoauxin A, and (+)-pemedolac.

Copper-Catalyzed Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran Assisted by TEMPOL in Liquid Sunlight Methanol

2,5-diformylfuran (DFF) is a significant biomass-derived compound with diverse applications in novel furan-based materials, fragrances, fuel additives, and drug synthesis. A pivotal challenge in DFF synthesis was developing a method to produce DFF under mild conditions using sustainable feedstocks. In this study, an affordable 4-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOL)- assisted Cu(OAc)2 catalytic system for aerobic oxidation reaction of HMF to DFF in liquid sunlight methanol solvent was developed. The effects of parameters such as metal species, catalyst amount, solvent species, base structure, and reaction temperature were systematically investigated. The evolution of product distribution in the reaction solution at various times was monitored and analyzed using 1H-NMR spectroscopy. FT-IR and ESI-MS characterizations were employed to integrate experimental findings and elucidate the reaction mechanism. The highest DFF yield of 96 % and complete conversion of HMF were obtained. Furthermore, a total DFF yield of 68.6 % was achieved from fructose using a two-steps method, demonstrating the potential for scalable production. The establishment of this catalytic system presents a novel approach for the selective preparation of DFF from sustainable feedstock.

Elucidating the Mechanism of Electrooxidative Allene Dioxygenation: Dual Role of Tetramethylpiperidine N-Oxyl (TEMPO)

The cumulated π system of a nonsymmetric allene contains three distinct unsaturated carbons that imbue it with unique reactivity toward radicals as compared to its alkene and alkyne counterparts. Despite the synthetic potential of these versatile building blocks, electrochemical transformations of allenes have been historically underexplored. Myriad strategies for easy access to allenes, coupled with the resurgence of interest in sustainable oxidative transformations of hydrocarbons, prompted our efforts to conduct an in-depth investigation of a rare example of an electrochemical TEMPO-mediated allene dioxygenation. The resultant vinyl-TEMPO motif is readily postfunctionalized to install a heteroatom at each allene carbon. Mechanistic investigations, including cyclic voltammetry (CV) studies, computations, and monitoring by operando NMR (ReactNMR) were performed to lay the groundwork for future electrochemical allene functionalizations that deliver unique synthetic building blocks.

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.

Asymmetric Synthesis of Chiral Calix[4]arenes with Both Inherent and Axial Chirality via Cobalt-Catalyzed Enantioselective Intermolecular C-H Annulation

Inherently chiral calixarenes have garnered significant attention due to their distinctive properties, yet the development of efficient catalytic asymmetric synthesis methods remains a critical challenge. Herein, we report the asymmetric synthesis of calix[4]arenes featuring inherent or both inherent and axial chirality via a cobalt-catalyzed C-H activation/annulation strategy in high yield with excellent enantio- and diastereoselectivity (up to >99 % ee and >20 : 1 dr). Electrooxidation was also suitable for this transformation to obviate the sacrificial metal oxidants, underscoring the environmentally friendly potential of this approach. A key octahedral cobaltacycle intermediate was synthesized and characterized, providing valuable insights into the mode of enantio- and diastereocontrol of this protocol. Noteworthy photoluminescence quantum yields of up to 0.94 were measured, underscoring the potential of these compounds in the domain of organic fluorescent materials.

Regioselective Electrochemical Construction of Csp2-Csp2 Linkage at C5-C5' Position of 2-Oxindoles via an Intermolecular Anodic Dehydrogenative Coupling

Applying electricity as a reagent in synthetic organic chemistry has attracted particular attention from synthetic chemists worldwide as an environmentally benign and cost-effective technique. Herein, we report the construction of the Csp2-Csp2 linkage at the C5-C5' position of 2-oxindole utilizing electricity as the traceless oxidant in an anodic dehydrogenative homo-coupling process. A variety of 3,3-disubstituted-2-oxindoles were subjected to dimerization, achieving yields of up to 70 % through controlled potential electrolysis at an applied potential of 1.5 V versus Ag/Ag+ nonaqueous reference electrode. This electro-synthetic approach facilitates the specific assembly of C5-C5' (para-para coupled) dimer of 3,3-disubstituted-2-oxindole without the necessity of any external oxidants or additives and DFT (Density Functional Theory) calculations provided confirmation of this pronounced regioselectivity. Furthermore, validation through control experiments and voltammetric analyses substantiated the manifestation of radical-radical coupling (or biradical pathway) for the dimerization process.

Enantioselective Nickel-Electrocatalyzed Cross-Dehydrogenative α- and γ-Nitroalkylation

Asymmetric catalytic versions of electricity-driven processes hold immense potential for the sustainable preparation of chiral compounds. However, the involvement of anodic oxidative cross-dehydrogenative coupling events between two distinct nucleophiles makes it challenging for a chiral catalyst to regulate the stereochemistry of the products. Our current electrocatalytic strategy for enantioconvergent cross-dehydrogenative α- and γ-nitroalkylation via radical-based pathways produces an array of enantioenriched nitroesters without supplementary stoichiometric oxidants. Mechanistic investigations reveal that the nickel catalyst plays a key role in both the electrochemical activation of the substrates and the stereoselectivity-defining events, affording the electrochemically generated Lewis acid-bound α-carbonyl radicals to interact with in situ-generated nitronate anions in a stereoselective manner. This electrocatalytic approach enables transformations that are highly challenging under thermal conditions, such as umpolung reactivity with readily available substrates, all-carbon quaternary stereocenter creation, and the control of remote stereochemistry.

Electrochemically Determined and Structurally Justified Thermochemistry of H atom Transfer on Ti-Oxo Nodes of the Colloidal Metal-Organic Framework Ti-MIL-125

Titanium dioxide (TiO2) has long been employed as a (photo)electrode for reactions relevant to energy storage and renewable energy synthesis. Proton-coupled electron transfer (PCET) reactions with equimolar amounts of protons and electrons at the TiO2 surface or within the bulk structure lie at the center of these reactions. Because a proton and an electron are thermochemically equivalent to an H atom, these reactions are essentially H atom transfer reactions. Thermodynamics of H atom transfer has a complex dependence on the synthetic protocol and chemical history of the electrode, the reaction medium, and many others; together, these complications preclude the understanding of the H atom transfer thermochemistry with atomic-level structural knowledge. Herein, we report our success in employing open-circuit potential (EOCP) measurements to quantitatively determine the H atom transfer thermochemistry at structurally well-defined Ti-oxo clusters within a colloidally stabilized metal-organic framework (MOF), Ti-MIL-125. The free energy to transfer H atom, Ti3+O-H bond dissociation free energy (BDFE), was measured to be 68(2) kcal mol-1. To the best of our understanding, this is the first report on using EOCP measurements to quantify thermochemistry on any MOFs. The proton topology, the structural change upon the redox reaction, and BDFE values were further quantitatively corroborated using computational simulations. Furthermore, comparisons of the EOCP-derived BDFEs of Ti-MIL-125 to similar parameters in the literature suggest that EOCP should be the preferred method for quantitatively accurate BDFE calculations. The reported success in employing EOCP for nanosized Ti-MIL-125 should lay the ground for thermochemical measurements of other colloidal systems, which are otherwise challenging. Implications of these measurements on Ti-MIL-125 as an H atom acceptor in chemical reactions and comparisons with other MOFs/metal oxides are discussed.

Capturing Dichotomic Solvent Behavior in Solute-Solvent Reactions with Neural Network Potentials

Simulations of chemical reactivity in condensed phase systems represent an ongoing challenge in computational chemistry, where traditional quantum chemical approaches typically struggle with both the size of the system and the potential complexity of the reaction. Here, we introduce a workflow aimed at efficiently training neural network potentials (NNPs) to explore energy barriers in solution at the hybrid density functional theory level. The computational burden associated with training at the PBE0-D3(BJ) level is bypassed through the use of active and transfer learning techniques, whereas extensive sampling of the transition state region is accelerated by well-tempered metadynamics simulations using multiple time step integration. These NNPs serve to explore a puzzling solute-solvent reactivity route involving the ring opening of N-enoxyphthalimide experimentally observed in methanol but not in 2,2,2-trifluoroethanol (TFE). This reaction represents a challenging example characterized by intricate hydrogen bonding networks and structurally ambiguous solvent-sensitive transition states. The methodology successfully delivers detailed free energy surfaces and relative energy barriers in agreement with experiment. These barriers are associated with an ensemble of transition states involving the direct participation of up to five solvent molecules. While this picture contrasts with the single transition state structure assumed by current static models, no drastic qualitative difference is observed between the formed hydrogen bonding networks and the number of participating solvent molecules in methanol or TFE. The dichotomy between the two solvents thus essentially arises from an electronic effect (i.e., distinct nucleophilicity) and from the larger conformational entropy contributions in methanol. This example underscores the critical role that dynamic simulations at the ab initio levels play in capturing the full complexity of solute-solvent interactions.

Modified Working Electrodes for Organic Electrosynthesis

Organic electrosynthesis has gained much attention over the last few decades as a promising alternative to traditional synthesis methods. Electrochemical approaches offer numerous advantages over traditional organic synthesis procedures. One of the most interesting aspects of electroorganic synthesis is the ability to tune many parameters to affect the outcome of the reaction of interest. One such parameter is the composition of the working electrode. By changing the electrode material, one can influence the selectivity, product distribution, and rate of organic reactions. In this Review, we describe several electrode materials and modifications with applications in organic electrosynthetic transformations. Included in this discussion are modifications of electrodes with nanoparticles, composite materials, polymers, organic frameworks, and surface-bound mediators. We first discuss the important physicochemical and electrochemical properties of each material. Then, we briefly summarize several relevant examples of each class of electrodes, with the goal of providing readers with a catalog of electrode materials for a wide variety of organic syntheses.

A TEMPO-N3 Complex Enables the Electrochemical C-H Azidation of N-Heterocycles through the Cleavage of Alkoxyamines

A TEMPO-N3 charge-transfer complex enables the electrochemical C-H azidation of various N-heterocycles. The TEMPO+ ion, generated from TEMPO, assists in producing N3 ⋅ by forming a TEMPO-N3 complex with N3 -. The formation of this complex is supported by UV-vis absorption spectra, cyclic voltammetry studies, and ESI-HRMS studies. The reaction likely proceeds by forming a highly labile azidooxygenation adduct, which undergoes oxidative alkoxyamine mesolytic cleavage. Subsequent deprotonation of the resulting carbocation exclusively produces the azidation product. It is important to note that substituted olefins generally yield azidooxygenation or diazidation as the final product. Our study demonstrates that N-heterocycles deliver a selective monoazidation product, possibly due to steric reasons. ESI-HRMS studies provide evidence for forming azidooxygenation and alkoxyamine radical cation adducts. The regio- and chemoselectivity of this azidation reaction using the TEMPO-N3 complex have been discussed.

Oxoammonium-Catalyzed Ether Oxidation via Hydride Abstraction: Methodology Development and Mechanistic Investigation Using Paramagnetic Relaxation Enhancement NMR

Hydride abstraction represents a promising yet underexplored approach in the functionalization of C-H bonds. In this work, we report the oxidation of α-C-H bonds of ethers via oxoammonium catalysis using 3-chloroperbenzoic acid (mCPBA) as the terminal chemical oxidant or by means of electrochemistry. Mechanistic studies revealed intricate equilibria and interconversion events between various catalytic intermediates in the presence of mCPBA, which alone however was incompetent to drive catalytic turnover. The addition of a small amount of strong acid HNTf2 or weakly coordinating salt NaSbF6 turned on catalytic turnover and promoted ether oxidation with excellent efficiency. NMR experiments leveraging paramagnetic relaxation enhancement effect allowed for quantification of open-shell catalytic intermediates in real time during the reaction course, which aided the identification of catalyst resting states and elucidation of reaction mechanisms.

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.

Three-Component 1,2-Dioxygenation of 1,3-Dienes Using Carboxylic Acids and TEMPO

A mild, metal-free 1,2-dioxygenation of 1,3-dienes using TEMPO and carboxylic acids is reported. This method includes examples for a variety of 1,3-dienes as well as aliphatic and aromatic carboxylic acids. This reaction also demonstrates complete site- and regioselectivity in the oxygen addition. Furthermore, extensive derivatization of the differential oxygen groups in the product has been demonstrated, including reduction of the remaining alkene to access alkyl, vicinally dioxygenated scaffolds. Finally, this reaction is shown both experimentally and computationally to occur through carboxylic acid-driven disproportionation of TEMPO.

Electrochemical and Non-Electrochemical Pathways in the Electrocatalytic Oxidation of Monosaccharides and Related Sugar Alcohols into Valuable Products

In this contribution, we review the electrochemical upgrading of saccharides (e.g., glucose) and sugar alcohols (e.g., glycerol) on metal and metal-oxide electrodes by drawing conclusions on common trends and differences between these two important classes of biobased compounds. For this purpose, we critically review the literature on the electrocatalytic oxidation of saccharides and sugar alcohols, seeking trends in the effect of reaction conditions and electrocatalyst design on the selectivity for the oxidation of specific functional groups toward value-added compounds. Importantly, we highlight and discuss the competition between electrochemical and non-electrochemical pathways. This is a crucial and yet often neglected aspect that should be taken into account and optimized for achieving the efficient electrocatalytic conversion of monosaccharides and related sugar alcohols into valuable products, which is a target of growing interest in the context of the electrification of the chemical industry combined with the utilization of renewable feedstock.

A Highly Efficient Electrosynthesis of Formaldehyde Using a TEMPO-Based Polymer Electrocatalyst

In the chemical industry, formaldehyde is an important bulk chemical. The traditional synthesis of formaldehyde involves an energy intensive oxidation of methanol over a metal oxide catalyst. The selective electrochemical oxidation of methanol is challenging. Herein, we report a catalytic system with an immobilized TEMPO electrode that selectively oxidizes methanol to formaldehyde with high turnover numbers. Upon the addition of various organic and inorganic bases, the activity of the catalyst could be tuned. The highest Faradaic efficiency that was achieved was 97.5 %, the highest turnover number was 17100. Additionally, we found that the rate determining step changed from the step in which the carbonyl specie is created from the methanol-TEMPO adduct to the oxidative regeneration of the TEMPO+ species. Finally, we showed that the system could be applied to the oxidation of other aliphatic alcohols.

The Aqueous and Acetonitrile Bond Dissociation Free Energies of N-Hydroxyphthalimide

Widely cited values of 89 and 90.9 kcal/mol for the bond-dissociation free energy of N-hydroxyphthalimide (NHPI) in water and acetonitrile, respectively, are in error. The sources of the errors leading to these values have been explored and corrected. The corrected values are confirmed through new experiments in aqueous and acetonitrile media and are found to be 84.4 ± 0.1 and 80.04 ± 0.06 kcal/mol, respectively.

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.

Dearomative Cyclization/Spirocyclization via Electrochemical Reductive Hydroarylation of Nonactivated Arenes

An electrochemical cyclization/spirocyclization hydroarylation via reductive dearomatization of a series of nonactivated arenes including N-substituted indoles, indole-3-carboxamide derivatives, and iodo-substituted benzamides is described. This protocol boasts high atom efficiency, broad substrate applicability, and excellent selectivity. Utilizing a simple undivided cell, various nonactivated arenes undergo cyclization/spirocyclization through the intramolecular addition of aryl radicals to an aromatic ring, yielding 50 indolines, spirocyclizative hydroarylation products, and phenanthridinones.

Electrochemical Postpolymerization Modification and Deconstruction of Macromolecules

Electrolysis is an emerging approach to polymer postpolymerization modification, deconstruction, and depolymerization. Electrochemical reactions are particularly appealing for macromolecular transformations because of their high selectivity, ability to be externally monitored, and intrinsic scalability. Despite these desirable features and the recent resurgent use of small-molecule electrochemical reactions, the development of macromolecular electrolysis has been limited. Herein, we highlight recent examples of polymer transformations driven by heterogeneous redox chemistry. Throughout our exploration of macromolecular electrolysis, we provide our perspective on opportunities for continued investigation in this nascent field. Specifically, we highlight how targeted reaction development through deeper mechanistic insight will expand the scope of materials that can be (de)constructed with electrochemical methods. As this insight is developed, we expect macromolecular electrolysis to emerge as a high-functioning and complementary tool for macromolecular functionalization and deconstruction.

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.

Interfacial Science for Electrosynthesis

Interfacial science and electroorganic syntheses are inextricably linked because all electrochemical reactions occur at the interface between the electrode and the solution. Thus, the surface chemistry of the electrode material impacts the organic reaction selectivity. In this short review, we highlight emergent examples of electrode surface chemistries that enable selective electroorganic synthesis in three reaction classes: (1) hydrogenation, (2) oxidation, and (3) reductive C‒C bond formation between two electrophiles. We showcase the breadth of techniques, including materials and in-situ characterization, requisite to establish mechanistic schemes consistent with the observed reactivity patterns. Leveraging an electrode's unique surface chemistry will provide complementary approaches to tune the selectivity of electroorganic syntheses and unlock an electrode's catalytic properties.

Uncovering Factors Controlling Reactivity of Metal-TEMPO Reaction Systems in the Solid State and Solution

Nitroxides find application in various areas of chemistry, and a more in-depth understanding of factors controlling their reactivity with metal complexes is warranted to promote further developments. Here, we report on the effect of the metal centre Lewis acidity on both the distribution of the O- and N-centered spin density in 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) and turning TEMPO from the O- to N-radical mode scavenger in metal-TEMPO systems. We use Et(Cl)Zn/TEMPO model reaction system with tuneable reactivity in the solid state and solution. Among various products, a unique Lewis acid-base adduct of Cl2Zn with the N-ethylated TEMPO was isolated and structurally characterised, and the so-called solid-state 'slow chemistry' reaction led to a higher yield of the N-alkylated product. The revealed structure-activity/selectivity correlations are exceptional yet are entirely rationalised by the mechanistic underpinning supported by theoretical calculations of studied model systems. This work lays a foundation and mechanistic blueprint for future metal/nitroxide systems exploration.

Asymmetric Paired Electrolysis: Enantioselective Alkylation of Sulfonylimines via C(sp3)-H Functionalization

Enantioselective transformation of ubiquitous C(sp3)-H bonds into three-dimensional chiral scaffolds is of longstanding interest to synthetic chemists. Herein, an asymmetric paired electrolysis enables a highly efficient and sustainable approach to the enantioselective alkylation of sulfonylimines via C(sp3)-H functionalization. In this protocol, anodic oxidation for benzylic radical formation and Lewis acid-catalyzed sulfonylimine reduction on the cathode were seamlessly cross-coupled (up to 88 % yield). Enantioenriched chiral amines containing a tetrasubstituted carbon stereocenter are accessed with high enantioselectivity (up to 96 % ee). Mechanistic studies suggest that the amine generated in situ could serve as a base to deprotonate phenols and decrease the oxidation potential of the reaction, allowing phenols with lower potentials to be preferentially oxidized.

Tandem Electrooxidation - Reductive Amination of Biobased Isohexides Towards Bicyclic Diamines

Isohexide-derived diamines are considered preferred precursors for the production of biobased polyurethanes and polyamides. However, current synthesis methods from isohexides suffer from serious issues concerning selectivity and the recyclability of the process auxiliaries (e. g. homogeneous catalysts), which renders a translation to the industry highly unlikely. Here, we report on the production of such diamine building blocks, via a tandem electrooxidation - reductive amination process in which the process auxiliaries can be easily recycled. The application of (immobilized) TEMPO in combination with simple halides (e. g. NaBr) in the electrochemical step even enables the oxidation of the sterically hindered exo-OHs of the isohexides to the corresponding diketones (yield up to 99 %). In the subsequent reductive amination, the produced ketones are atom-efficiently converted to isohexide diamines utilizing NH3, H2, and Ru/C and an acid resin cocatalyst.

Overlooked interaction between redox-mediator and bisphenol-A in permanganate oxidation

Research efforts on permanganate (Mn(VII)) combined with redox-mediator (RM), have received increasing attention due to their significant performance for bisphenol-A (BPA) removal. However, the mechanisms underpinning BPA degradation remain underexplored. Here we show the overlooked interactions between RM and BPA during permanganate oxidation by introducing an RM-N-hydroxyphthalimide (NHPI). We discovered that the concurrent generation of MnO2 and phthalimide-N-oxyl (PINO) radical significantly enhances BPA oxidation within the pH range of 5.0-6.0. The detection of radical cross-coupling products between PINO radicals and BPA or its derivatives corroborates the pivotal role of radical cross-coupling in BPA oxidation. Intriguingly, we observed the formation of an NHPI-BPA complex, which undergoes preferential oxidation by Mn(VII), marked by the emergence of an electron-rich domain in NHPI. These findings unveil the underlying mechanisms in the Mn(VII)/RM system and bridge the knowledge gap concerning BPA transformation via complexation. This research paves the way for further exploration into optimizing complexation sites and RM dosage, significantly enhancing the system's efficiency in water treatment applications.

Ferrier Glycosylation Mediated by the TEMPO Oxoammonium Cation

The TEMPO oxoammonium cation has been proven to be both an efficient oxidizing reagent and an electrophilic substrate frequently found in organic reactions. Here, we report that this versatile chemical reagent can also be used as an efficient promoter for C- and N-glycosylation reactions through a Ferrier rearrangement with moderate to high yields. This unprecedented reactivity is explained in terms of a Lewis acid activation of glycal by TEMPO+ forming a type of glycal-TEMPO+ mesomeric structure, which occurs through an extended vinylogous hyperconjugation toward the π*(O═N+) orbital [LP(O1) → π*(C1═C2), π*(C1═C2) → σ*(C3-O3), and LP(O6) → π*(O═N+)]. This enables the formation of the respective Ferrier glycosyl cation, which is trapped by various nucleophiles. The extended hyperconjugation (or double hyperconjugation) toward the π*(O═N+) orbital, which confers the Lewis acid character of the TEMPO cation, was supported by natural bond orbital analysis at the M06-2X/6-311+G** level of theory.

A General Iron-Catalyzed Decarboxylative Oxygenation of Aliphatic Carboxylic Acids

We report an iron-catalyzed decarboxylative C(sp3)-O bond-forming reaction under mild, base-free conditions with visible light irradiation. The transformation uses readily available and structurally diverse carboxylic acids, iron photocatalyst, and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) derivatives as oxygenation reagents. The process exhibits a broad scope in acids possessing a wide range of stereoelectronic properties and functional groups. The developed reaction was applied to late-stage oxygenation of a series of bio-active molecules. The reaction leverages the ability of iron complexes to generate carbon-centered radicals directly from carboxylic acids by photoinduced carboxylate-to-iron charge transfer. Kinetic, electrochemical, EPR, UV/Vis, HRMS, and DFT studies revealed that TEMPO has a triple role in the reaction: as an oxygenation reagent, an oxidant to turn over the Fe-catalyst, and an internal base for the carboxylic acid deprotonation. The obtained TEMPO adducts represent versatile synthetic intermediates that were further engaged in C-C and C-heteroatom bond-forming reactions using commercial organo-photocatalysts and nucleophilic reagents.

Revealing the Mechanistic Features of an Electrosynthetic Catalytic Reaction and the Role of Redox Mediators through DFT Calculations and Microkinetic Modeling

Organic electrosynthesis is an emerging field that provides original selectivity while adding features of atom economy, sustainability, and selectivity. Electrosynthesis is often enhanced by redox mediators or electroauxiliaries. The mechanistic understanding of organic electrosynthesis is however often limited by the low lifetime of intermediates and its difficult detection. In this work, we report a computational analysis of the mechanism of an appealing reaction previously reported by Mei and co-workers which is catalyzed by copper and employs iodide as redox mediator. Our scheme combines DFT calculations with microkinetic modeling and covers both the reaction in solution and the electrodic steps. A detailed mechanistic scheme is obtained which reproduces well experimental data and opens perspectives for the general treatment of these processes.

Selective Transition Enhancement in a g-Engineered Diradical

A diradical with engineered g-asymmetry was synthesized by grafting a nitroxide radical onto the [Y(Pc)2]⋅ radical platform. Various spectroscopic techniques and computational studies revealed that the electronic structures of the two spin systems remained minimally affected within the diradical system. Fluid-solution Electron Paramagnetic Resonance (EPR) experiments revealed a weak exchange coupling with |J| ~ 0.014 cm-1, subsequently rationalized by CAS-SCF calculations. Frozen solution continuous-wave (CW) EPR experiments showed a complicated and power-dependent spectrum that eluded analysis using the point-dipole model. Pulse EPR manipulations with varying microwave powers, or under varying magnetic fields, demonstrated that different resonances could be selectively enhanced or suppressed, based on their different tipping angles. In particular, Field-Swept Echo-Detected (FSED) spectra revealed absorptions of MW power-dependent intensities, while Field-Swept Spin Nutation (FSSN) experiments revealed two distinct Rabi frequencies. This study introduces a methodology to synthesize and characterize g-asymmetric two-spin systems, of interest in the implementation of spin-based CNOT gates.

Unraveling the Isotropic Hyperfine Coupling Constants of Nitroxide Radicals via Molecular Electrostatic Potential Analysis

Nitroxide radicals have wide and promising applications as organic magnetic materials. Modulating the isotropic hyperfine coupling constants (HFCCs) of these radicals through proper structural design is an effective strategy for their application as spin probes and spin labels. In the present work, density functional theory calculations were carried out to develop a robust descriptor based on the molecular electrostatic potential for nitrogen HFCCs of nitroxide radicals. Forty nitroxide radicals from five distinct classes, namely, derivatives of cyclic, acyclic, imino, nitronyl, and benzimidazole nitronyl nitroxides, were selected, and the molecular electrostatic potential (MESP) at the nitrogen atom (VN) of the NO moiety was calculated. The VN values efficiently capture the electronic changes associated with the steric and electronic nature of these systems. A significant correlation between VN values and the experimental HFCCs of nitrogen nuclei demonstrates the applicability of VN as a simple and efficient descriptor for monitoring HFCCs. Furthermore, a good correlation between VN and experimental nitrogen HFCCs for each class of nitroxide radicals indicates the use of VN in the evaluation of the magnetic nature of the nitroxide radicals. The findings in this work are expected to facilitate the design of novel nitroxide radicals with desirable magnetic properties based on MESP topology analysis.

Accessing Alkoxy Radicals via Frustrated Radical Pairs: Diverse Oxidative Functionalizations of Tertiary Alcohols

Alkoxy radicals are versatile reactive intermediates in organic synthesis. Here, we leverage the principle of frustrated radical pair to provide convenient access to these highly reactive species directly from tertiary alcohols via oxoammonium-mediated oxidation of the corresponding alkoxides. This approach enabled various synthetically useful transformations including β-scission, radical cyclization, and remote C-H functionalization, giving rise to versatile alkoxyamines that can be further elaborated to various functionalities.

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.

Metal-free electrocatalytic upcycling of polyethylene terephthalate plastic to C2 products

Polyethylene terephthalate (PET) is one of the most used polymers, but the non-degradable and persistent nature of PET waste in the environment is a global menace. Hence upcycling PET waste becomes indispensable. Herein, we introduce the first metal-free electrochemical-upcycling of PET into value-added chemicals and H2 fuel using an organo-electrocatalyst (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO). Electrolysis at pH 10 produces glycolate and oxalate exclusively while at pH 14, over-oxidation and subsequent C-C bond cleavage produce formate and carbonate as well. Tuning the rate and product selectivity via pH regulation with mechanistic insight displays a sustainable route to implement waste PET recycling.

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.

TEMPO/O2 Synergistically Mediated BiBrO-Photocatalyzed Decarboxylative Phosphorylation of N-Arylglycines

With both TEMPO and O2 (in air) as the homogeneous redox mediators, BiBrO as the heterogeneous semiconductor photocatalyst, the first example of semi-heterogeneous photocatalytic decarboxylative phosphorylation of N-arylglycines with diarylphosphine oxides was established. A series of α-amino phosphinoxides were efficiently synthesized.

Iron-Catalyzed, Light-Driven Decarboxylative Alkoxyamination

An iron-catalyzed visible-light driven decarboxylative alkoxyamination is disclosed. In the presence of FeBr2 and TEMPO, a large array of carboxylic acids including marketed drugs and biobased molecules is turned into the corresponding alkoxyamine derivatives. The versatility of the latter offers an entry towards molecular diversity generation from abundant starting materials and catalyst. Overall, this method proposes a unified and general approach for LMCT-based iron-catalyzed decarboxylative functionalization.

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.

Decoupling Plasmonic Hot Carrier from Thermal Catalysis via Electrode Engineering

Increased attention has been directed toward generating nonequilibrium hot carriers resulting from the decay of collective electronic oscillations on metal known as surface plasmons. Despite numerous experimental endeavors, demonstrating hot carrier-mediated photocatalysis without a heating contribution has proven challenging, particularly for single electron transfer reactions where the thermal contribution is generally detrimental. An innovative engineering solution is proposed to enable single electron transfer reactions with plasmonics. It consists of a photoelectrode designed as an energy filter and photocatalysis performed with light function modulation instead of continuously. The photoelectrode, consisting of FTO/TiO2 amorphous (10 nm)/Au nanoparticles, with TiO2 acting as a step-shape energy filter to enhance hot electron extraction and charge-separated state lifetime. The extracted hot electrons were directed toward the counter electrode, while the hot holes performed a single electron transfer oxidation reaction. Light modulation prevented local heat accumulation, effectively decoupling hot carrier catalysis from the thermal contribution.

Recent advances in oxidative degradation of plastics

Oxidative degradation is a powerful method to degrade plastics into oligomers and small oxidized products. While thermal energy has been conventionally employed as an external stimulus, recent advances in photochemistry have enabled photocatalytic oxidative degradation of polymers under mild conditions. This tutorial review presents an overview of oxidative degradation, from its earliest examples to emerging strategies. This review briefly discusses the motivation and the development of thermal oxidative degradation of polymers with a focus on underlying mechanisms. Then, we will examine modern studies primarily relevant to catalytic thermal oxidative degradation and photocatalytic oxidative degradation. Lastly, we highlight some unique studies using unconventional approaches for oxidative polymer degradation, such as electrochemistry.

Unlocking high-efficiency decontamination by building a novel heterogeneous catalytic reduction system of thiourea dioxide/biochar

Herein, we reported a new contaminant purification paradigm, which enabled highly efficient reductive denitration and dechlorination using a green, stable reducing agent thiourea dioxide (TDO) coupled with biochar (BC) over a wide pH range under anoxic conditions. Specifically, BC acted as both activators and electron shuttles for TDO decomposition to achieve complete anoxic degradation of p-nitrophenol (PNP), p-nitroaniline, 4-chlorophenol and 2,4-dichlorophenol within 2 h. During this process, multiple strongly reducing species (i.e., SO22-, SO2•- and e-/H•) were generated in BC/TDO systems, accounting for 13.3%, 9.7% and 75.5% of PNP removal, respectively. While electron transfer between TDO and H+ or contaminants mediated by BC led to H• generation and contaminant reduction. These processes depended on the electron-accepting capacity and electron-conducting domains of biochar. Significantly, the BC/TDO systems were highly efficient at a pH of 2.0-8.0, especially under acidic conditions, which performed robustly in common natural water constituents.

Simple electrochemical synthesis of cyclic hydroxamic acids by reduction of nitroarenes

The electrochemical reduction of nitroarenes allows direct access to manifold nitrogen containing heterocycles. This work reports the simple and direct electro-organic synthesis of 18 different examples of 2H,4H-4-hydroxy-1,4-benzoxazin-3-ones in up to 81% yield. The scalability of the method was demonstrated on a gram-scale.