Single Electron Research Articles

Unlocking the potential of Allium dictyoprasum C.A. Meyer ex Kunth: quantum chemical insights into radical scavenging, chemical composition, phenolic content, and antimicrobial activity

ABSTRACT Allium dictyoprasum C.A. Meyer ex Kunth (A. dictyoprasum) underwent comprehensive analysis, encompassing quantum chemical computations to assess its radical scavenging potential, chemical and elemental composition, total phenolic content, and antimicrobial activity. Experimental and theoretical investigations focused on elucidating the radical scavenging properties of polyhydroxy phenolic compounds present in the plant. Quantum chemical calculations were employed to evaluate the antioxidants employed to evaluate selected polyhydroxy phenolic molecules including flavonoids, hydrocinnamic acid derivatives, and hydroxybenzoic acid derivatives from natural sources. Thermochemical parameters of these compounds were calculated by the B3LYP/6–311 G++(d,p) level in both gas and solvent phases to elucidate the radical scavenging mechanism including hydrogen atom transfer (HAT), single electron transfer followed by proton transfer (SET-PT) and sequential proton loss electron transfer (SPLET). Analysis of A. dictyoprasum extracts obtained via various extraction methods revealed the presence of several major compounds, including dimethyl trisulfide, 3,5-Dihydroxy-6-methyl-2,3-dihydro-4 H-pyran-4-one, 2-Methoxy-4-vinylphenol, Dimethyl phthalate, Methyl palmitate, Methyl oleate, Methyl stearate, (9Z)-9-Octadecenamide. Notably, Malic acid and Quinic acid were identified as major compounds, with concentrations of 43.31 and 17.47 mg kg−1 extract, respectively, based on LC-MS/MS analysis. The total phenolic content of the extract was measured as 17.83 mg gallic acid/mL, while its free radical scavenging activity was 80.89% per mg/mL. Elemental analysis revealed significant levels of Mg, K, Na, Fe, and P, with minor concentrations of elements such as Ti, Tl, B, and Be. Furthermore, A. dictyoprasum exhibited notable antibacterial activity against various bacteria strains, surpassing the efficacy of some commercial antibiotics.

Photoinduced Intermolecular Hydroamination and Hydroetherification of Electron-rich Alkenes with Low Catalyst Loadings.

A photochemical method based on visible-light irradiation (blue LEDs/sunlight) has been developed for the intermolecular hydroamination and hydroetherification of electron-rich alkenes. This photochemical method is compatible with a wide range of azoles and electron-rich alkenes, such as vinyl ethers, vinyl sulfides and enamides, and is performed with low concentrations of photocatalyst (1000 ppm). Comprehensive mechanistic studies indicate that this process is initiated by the formation of an active radical cation intermediate through single electron oxidation of azole, which is mediated by excited Acr-Mes+ BF4-.

Visible Light-Induced Electron-Donor-Acceptor-Mediated C-3 Coupling of Quinoxalin-2(1H)-ones with Unactivated Aryl Iodides.

Visible light-induced C-3 arylation of quinoxalin-2(1H)-ones with abundantly available aryl iodides with good yields via an electron-donor-acceptor (EDA)-complex formation have been accomplished. The radical scavenging, Electron paramagnetic resonance (EPR), UV-visible experiments, density functional theory (DFT), and quantum yield studies revealed that the reaction went through a radical pathway via a single electron transfer (SET) process. Furthermore, the protocol could also be applied to the synthesis of biologically active molecules, illustrating the practicality of the present protocol.

Modifying Proton Relay into Bioinspired Dye-Based Coordination Polymer for Photocatalytic Proton-Coupled Electron Transfer.

Proton-coupled electron transfer (PCET) imparts an energetic advantage over single electron transfer in activating inert substances. Natural PCET enzyme catalysis generally requires tripartite preorganization of proton relay, substrate-bound active center, and redox mediator, making the processes efficient and precluding side reactions. Inspired by this, a heterogeneous photocatalytic PCET system was established to achieve higher PCET driving forces by modifying proton relays into anthraquinone-based anionic coordination polymers. The proximally separated proton relays and photoredox-mediating anthraquinone moiety allowed pre-assembly of inert substrate between them, merging proton and electron into unsaturated bonds by photoreductive PCET, which enhanced reaction kinetics compared with the counter catalyst without proton relay. This photocatalytic PCET method was applied to a broad-scoped reduction of aryl ketones, unsaturated carbonyls, and aromatic compounds. The distinctive regioselectivities for the reduction of isoquinoline derivatives were found to occur on the carbon-ring sides. PCET-generated radical intermediate of quinoline could be trapped by alkene for proton relay-assisted Minisci addition, forming the pharmaceutical aza-acenaphthene scaffold within one step. When using heteroatom(X)-H/C-H compounds as proton-electron donors, this protocol could activate these inert bonds through photooxidative PCET to afford radicals and trap them by electron-deficient unsaturated compounds, furnishing the direct X-H/C-H functionalization.

Modifying Proton Relay into Bioinspired Dye‐Based Coordination Polymer for Photocatalytic Proton‐Coupled Electron Transfer

AbstractProton‐coupled electron transfer (PCET) imparts an energetic advantage over single electron transfer in activating inert substances. Natural PCET enzyme catalysis generally requires tripartite preorganization of proton relay, substrate‐bound active center, and redox mediator, making the processes efficient and precluding side reactions. Inspired by this, a heterogeneous photocatalytic PCET system was established to achieve higher PCET driving forces by modifying proton relays into anthraquinone‐based anionic coordination polymers. The proximally separated proton relays and photoredox‐mediating anthraquinone moiety allowed pre‐assembly of inert substrate between them, merging proton and electron into unsaturated bonds by photoreductive PCET, which enhanced reaction kinetics compared with the counter catalyst without proton relay. This photocatalytic PCET method was applied to a broad‐scoped reduction of aryl ketones, unsaturated carbonyls, and aromatic compounds. The distinctive regioselectivities for the reduction of isoquinoline derivatives were found to occur on the carbon‐ring sides. PCET‐generated radical intermediate of quinoline could be trapped by alkene for proton relay‐assisted Minisci addition, forming the pharmaceutical aza‐acenaphthene scaffold within one step. When using heteroatom(X)−H/C−H compounds as proton‐electron donors, this protocol could activate these inert bonds through photooxidative PCET to afford radicals and trap them by electron‐deficient unsaturated compounds, furnishing the direct X−H/C−H functionalization.

Synthesis of 1‐azabicyclo[2.1.1]hexanes via formal single electron reduction of azabicyclo[1.1.0]butanes under photochemical conditions

C(sp3)‐rich heterocycles are privileged building blocks for pharmaceuticals and agrochemicals. Therefore, synthetic methods that provide access to novel saturated nitrogen‐containing heterocycles are in high demand. Herein, we report a general synthesis of 1‐azabicyclo[2.1.1]hexanes (1‐aza‐BCH) via a formal cycloaddition of azabicyclo[1.1.0]butanes (ABB) with styrenes under photochemical conditions. To overcome the challenging direct single electron reduction of ABBs, we designed a polar‐radical‐polar relay strategy that leverages a fast acid‐mediated ring‐opening of ABBs to form bromoazetidines, which undergo efficient debrominative radical formation to initiate the cycloaddition reaction. The reaction is applicable to a broad range of ABB‐ketones and we demonstrate the 1‐aza‐BCH products can be further functionalised to access larger saturated, conformationally rigid heterocycles.

Synthesis of 1-azabicyclo[2.1.1]hexanes via formal single electron reduction of azabicyclo[1.1.0]butanes under photochemical conditions.

C(sp3)-rich heterocycles are privileged building blocks for pharmaceuticals and agrochemicals. Therefore, synthetic methods that provide access to novel saturated nitrogen-containing heterocycles are in high demand. Herein, we report a general synthesis of 1-azabicyclo[2.1.1]hexanes (1-aza-BCH) via a formal cycloaddition of azabicyclo[1.1.0]butanes (ABB) with styrenes under photochemical conditions. To overcome the challenging direct single electron reduction of ABBs, we designed a polar-radical-polar relay strategy that leverages a fast acid-mediated ring-opening of ABBs to form bromoazetidines, which undergo efficient debrominative radical formation to initiate the cycloaddition reaction. The reaction is applicable to a broad range of ABB-ketones and we demonstrate the 1-aza-BCH products can be further functionalised to access larger saturated, conformationally rigid heterocycles.

Visible Light-Induced, Nickel-Catalyzed Late-Stage 4-Alkylation of Hantzsch Esters with Alkyl Bromide.

Herein, visible light-induced, nickel-catalyzed direct functionalization of the Hantzsch esters (HEs) with readily accessible alkyl bromides has been successfully achieved by taking advantage of HE as the reductant and substrate through an aromatization-dearomatization process. In this strategy, the single electron reduction of alkyl bromides by reactive Ni(I) species is essential for the success of this late-stage transformation. A wide range of 4-alkyl-1,4-dihydropyridines were rapidly assembled in moderate to good yields under mild conditions, rendering this photoinduced approach attractive for synthetic and medicinal chemistry.

Formate and CO2 Enable Reductive Carboxylation of Imines: Synthesis of Unnatural α-Amino Acids.

Herein, a photocatalytic umpolung strategy for reductive carboxylation of imines for the synthesis of α-amino acids was disclosed. Carbon dioxide radical anion (CO2•-) generated from formate is the key single electron reductant in the reactions. An unprecedentedly broad substrate scope of imines with excellent reaction yields was obtained with carbon dioxide (CO2) and formate salt as carbon sources.

Engineering an enzyme-like catalytic sensor for on-site dual-mode evaluation of total antioxidant capacity.

Anovel Fe-MoOx nanozyme, engineered with enhanced peroxidase (POD)-like activity through strategic doping and the creation of oxygen vacancies, is introducedto catalyze the oxidation of TMB with high efficiency. Furthermore, Fe-MoOx is responsive to single electron transfer (SET) and hydrogen atom transfer (HAT) mechanisms related to antioxidants and can serve as a desirable nanozyme for total antioxidant capacity (TAC) determination. The TAC colorimetric platform can reach a low LOD of 0.512μM in solution and 24.316μM in the smartphone-mediated RGB hydrogel (AA as the standard). As proof of concept, the practical application in real samples was explored. The work paves a promising avenue to design diverse nanozymes for visual on-site inspection of food quality.

Stereoelectrochemistry of Tetranitrocalix[4]arenes

The present article reveals the differences in voltammetric reduction curves of isomeric nitrocalix[4]arenes (a kind of their “fingerprints”). This observation represents a contribution to a newly introduced utilization of electrochemical approach called “stereoelectro­chemistry” and extends the knowledge on these polycyclic molecules. In stereoelectrochemistry, the electrochemical data (potentials and their shifts, number of individual redox steps, proportions of their currents, reversibility etc.) in correlation with a series of derivatives can offer important information concerning the shape, geometry, conformation and static/dynamic behaviour of studied molecules in solution. Four possible atropoisomers of para-tetranitrocalix[4]arene were investigated by DC polarography and cyclic voltammetry in aprotic media (anhydrous N,N-dimethylformamide). Under such conditions, a typical reduction mechanism of a nitro compound involves a reversible single electron reduction step yielding an anion radical, followed by three-electron irreversible process assisted by protons (from moisture traces). While the already published most symmetric cone-conformer accepts first four electrons in two separated steps (which is in accordance with the “pinched” C2v shape of the molecule), the same behaviour was observed also in the case of 1,3-alt-derivative, the equilibrium geometry of which was predicted to be highly symmetric (S4), with no distinction among the nitro substituents. Hence, the 1,3-alt-molecule must oscillate around the equilibrium geometry (slower than the time scale of electron transfer) to produce a pinched limiting geometry with two different pairs of nitro groups (distal and proximal). The paco-derivative (and also its dinitro analogue) is reduced in three steps, during which the “tilted” aromatic unit is reduced first, followed by the opposite one (distal), and eventually another pair of proximal nitro groups (equivalent) is reduced simultaneously. The least symmetric 1,2-alt-compound (C1) is reduced in four single-electron, difficultly resolved steps due to small differences in the individual nitro groups energy.

Photoinduced Electron Donor Acceptor Complex-Enabled α-C(sp3)-H Alkenylation of Amines.

Allylic amines are prevalent and vital structural components present in many bioactive compounds and natural products. Additionally, they serve as valuable intermediates and building blocks, with wide-ranging applications in organic synthesis. However, direct α-C(sp3)-H alkenylation of feedstock amines, particularly for the preparation of α-alkenylated cyclic amines, has posed a longstanding challenge. Herein, we present a general, mild, operationally simple, and transition-metal-free α-alkenylation of various readily available amines with alkenylborate esters in excellent E/Z - and diastereoselectivities. This method features good compatibility with water and oxygen, broad substrate scope, and excellent functional group tolerance, thereby enabling the late-stage modification of various complex molecules. Mechanistic studies suggest that the formation of a photoactive electron donor-acceptor complex between 2-iodobenzamide and the tetraalkoxyborate anion, which subsequently undergoes photoinduced single electron transfer and intramolecular 1,5-hydrogen atom transfer to generate the crucial α-amino radicals, is the key to success of this chemistry.

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.

In situ copper photocatalysts triggering halide atom transfer of unactivated alkyl halides for general C(sp3)-N couplings

Direct reduction of unactivated alkyl halides for C(sp3)-N couplings under mild conditions presents a significant challenge in organic synthesis due to their low reduction potential. Herein, we introduce an in situ formed pyridyl-carbene-ligated copper (I) catalyst that is capable of abstracting halide atom and generating alkyl radicals for general C(sp3)-N couplings under visible light. Control experiments confirmed that the mono-pyridyl-carbene-ligated copper complex is the active species responsible for catalysis. Mechanistic investigations using transient absorption spectroscopy across multiple decades of timescales revealed ultrafast intersystem crossing (260 ps) of the photoexcited copper (I) complexes into their long-lived triplet excited states (>2 μs). The non-Stern-Volmer quenching dynamics of the triplets by unactivated alkyl halides suggests an association between copper (I) complexes and alkyl halides, thereby facilitating the abstraction of halide atoms via inner-sphere single electron transfer (SET), rather than outer-sphere SET, for the formation of alkyl radicals for subsequent cross couplings.

Inferring Population HIV Viral Load From a Single HIV Clinic's Electronic Health Record: Simulation Study With a Real-World Example.

Population viral load (VL), the most comprehensive measure of the HIV transmission potential, cannot be directly measured due to lack of complete sampling of all people with HIV. A given HIV clinic's electronic health record (EHR), a biased sample of this population, may be used to attempt to impute this measure. We simulated a population of 10,000 individuals with VL calibrated to surveillance data with a geometric mean of 4449 copies/mL. We sampled 3 hypothetical EHRs from (A) the source population, (B) those diagnosed, and (C) those retained in care. Our analysis imputed population VL from each EHR using sampling weights followed by Bayesian adjustment. These methods were then tested using EHR data from an HIV clinic in Delaware. Following weighting, the estimates moved in the direction of the population value with correspondingly wider 95% intervals as follows: clinic A: 4364 (95% interval 1963-11,132) copies/mL; clinic B: 4420 (95% interval 1913-10,199) copies/mL; and clinic C: 242 (95% interval 113-563) copies/mL. Bayesian-adjusted weighting further improved the estimate. These findings suggest that methodological adjustments are ineffective for estimating population VL from a single clinic's EHR without the resource-intensive elucidation of an informative prior.

Environmental fate of phenobarbital in water: Homogeneous and non-homogeneous environment

In aqueous environments, phenobarbital (PB) undergoes degradation mediated by hydroxyl radicals (HO·) and sulfate radicals (SO4·−). In this study, a theoretical approach was used to investigate the oxidation of phenobarbital triggered by HO·/ SO4·− in different dissociated forms of PB at different pH values, as well as the adsorption mechanism of phenobarbital on the silica surface and the non-homogeneous reaction with HO·. Our study confirms three primary reaction models for PB degradation: radical addition, hydrogen abstraction, and single electron transfer reactions. Thermodynamic and kinetic calculations show that the reaction of PB with HO· is the main degradation pathway compared to the reaction of SO4−. At the C5 and C6 positions, HO·-addition and H-abstraction from H6 and H7 are competitive mechanisms. However, in non-homogeneous environments, HO·-addition at C5 becomes the dominant pathway. Our work identifies the primary products of these degradation reactions. At a temperature of 298 K, the total rate constants for the degradation reactions of HO· with PB neutral molecules and anions were determined to be 2.06 M−1 s−1 and 4.11 × 107 M−1 s−1, respectively. In a non-homogeneous reaction with HO·, the total rate constant is 4.47 × 1010 M−1 s−1. Despite degradation, the majority of these products retain their potential to impact aquatic organisms. In particular, compounds such as 5-ethyl-5-hydroxypyrimidine-2,4,6(1H,3H,5H)-trione, 5-(1-hydroxyethyl)-5-phenylpyrimidine-2,4,6(1H,3H,5H)-trione, and 5-acetyl-5-phenylpyrimidine-2,4,6(1H,3H,5H)-trione have a greater toxic effect on green algae. The findings underscore the need for future research to consider the environmental implications and ecotoxicity of PB. Such consideration is crucial for a comprehensive understanding of the long-term environmental impact of pharmaceutical residues in aquatic ecosystems.

Discovery and Manipulation of van der Waals Polarons in Sb2O3 Ultrathin Molecular Crystal.

Manipulating single electrons at the atomic scale is vital for mastering complex surface processes governed by the transfer of individual electrons. Polarons, composed of electrons stabilized by electron-phonon coupling, offer a pivotal medium for such manipulation. Here, using scanning tunneling microscopy and spectroscopy (STM/STS) and density functional theory (DFT) calculations, we report the identification and manipulation of a new type of polaron, dubbed van der Waals (vdW) polaron, within mono- to trilayer ultrathin films composed of Sb2O3 molecules that are bonded via vdW attractions. The Sb2O3 films were grown on a graphene-covered SiC(0001) substrate via molecular beam epitaxy. Unlike prior molecular polarons, STM imaging observed polarons at the interstitial sites of the molecular film, presenting unique electronic states and localized band bending. DFT calculations revealed the lowest conduction band as an intermolecular bonding state, capable of ensnaring an extra electron through locally diminished intermolecular distances, thereby forming an intermolecular vdW polaron. We also demonstrated the ability to generate, move, and erase such vdW polarons using an STM tip. Our work uncovers a new type of polaron stabilized by coupling with intermolecular vibrations where vdW interactions dominate, paving the way for designing atomic-scale electron transfer processes and enabling precise tailoring of electron-related properties and functionalities.

The reactivity of organic radicals in the performic, peracetic, perpropionic acids-based advanced oxidation process: A case study of sulfamethoxazole

Advanced oxidation processes (AOPs) based on peracetic acid (PAA) displayed great potential in removing emerging contaminants by generating HO• and organic radicals. Performic and perpropionic acids (PFA and PPA) also act as disinfectants, but their application potential has not been investigated yet. Here, we investigated the degradation mechanism and kinetics of sulfamethoxazole (SMX) by HO•, RC(O)O• species (including HC(O)O•, CH3C(O)O• and CH3CH2C(O)O•) and RC(O)OO• species (including HC(O)OO•, CH3C(O)OO• and CH3CH2C(O)OO•). The results show that the calculated reaction rate constants of SMX follow the order of HC(O)O• > CH3C(O)O• > CH3CH2C(O)O• > HO• > HC(O)OO• > CH3C(O)OO• > CH3CH2C(O)OO•. The reactivity towards SMX is strongly correlated with the redox potentials of reactive radicals. Hence, the RCOO• species play dominant roles in the purification of SMX in PFA/PAA/PPA-based AOPs. The degradation of SMX mainly proceeds via addition at the benzene ring, the hydrogen abstraction from the -NH2 group as well as the single electron transfer reaction. This study highlights the fundamental aspects of PFA, PAA, and PPA in the purification of sulfamethoxazole and enhances the role of organic radicals in the AOPs based on organic peracetic acids.

Synthesis of Sulfilimines via Visible-Light-Mediated Triplet Energy Transfer to Sulfonyl Azides.

Sulfilimines and their derivatives have garnered considerable interest in both synthetic and medicinal chemistry. Photochemical nitrene transfer to sulfides is known as a conventional synthetic approach to sulfilimines. However, the existing methods have a limited substrate scope stemming from the incompatibility of singlet nitrene intermediates with nucleophilic functional groups. Herein, we report the synthesis of N-sulfonyl sulfilimines via visible-light-mediated energy transfer to sulfonyl azides, uncovering the previously overlooked reactivity of triplet nitrenes with sulfides through a postulated mechanism involving single electron transfer. This reaction features broad functional group tolerance, water compatibility, and amenability to the late-stage functionalization of drugs. Thus, this work represents an important example of energy transfer chemistry that overcomes challenges in traditional synthetic methods.

Coinage Metal Complexes of a Sterically Encumbered Anionic Pyridylborate.

Sterically loaded, anionic pyridine has been synthesized and utilized successfully in the stabilization of a isoleptic series of coinage metal complexes. The treatment of [4-(Ph3B)-2,6-Trip2Py]K (Trip=2,4,6-iPr3C6H2) with CuBr(PPh3), AgCl(PPh3) or AuCl(PPh3) (Py=pyridine) afforded the corresponding [4-(Ph3B)-2,6-Trip2Py]M(PPh3) (M=Au, Ag, Cu) complexes, via salt metathesis, as isolable, crystalline solids. Notably, these reactions avoid the facile single electron transfer chemistry reported with the less bulky ligand systems. The X-ray structures revealed that they are two-coordinate metal adducts. The M-N and M-P bond distances are longest in the silver and shortest in the copper adduct among the three group 11 family members. Computational analysis revealed an interesting stability dependence on steric bulk of the anionic pyridine (i. e., pyridyl borate) ligand. A comparison of structures and bonding of [4-(Ph3B)-2,6-Trip2Py]Au(PPh3) to pyridine and m-terphenyl complexes, {[2,6-Trip2Py]Au(PPh3)}[SbF6] and [2,6-Trip2Ph]Au(PPh3) are also provided. The Au(I) isocyanide complex, [4-(Ph3B)-2,6-Trip2Py]Au(CNBut) has been stabilized using the same anionic pyridylborate illustrating that it can support other gold-ligand moieties as well.