Single Electron Transfer Process Research Articles

Photocatalyzed chemodivergent aerobic oxidation of naturally occurring Viridicatin and derivatives

A chemodivergent reaction is an appealing way to construct molecules with enriched structure diversity in a controlled manner. A plethora of methodologies were developed based on this concept, and transition metal-catalysis plays a central role among others catalysis systems own to its longstanding history. While a chemodivergent reaction based on organo-catalysis, especially photocatalysis, is rather limited as these concepts were only prevalent in the last two decades. With the ever-increasing importance of photocatalysis, a chemodivergent reaction based on such an activation pathway would be a meaningful direction. Herein, an efficient chemodivergent strategy for visible light photocatalysis is developed. By employing commercially available Rose Bengal as a photocatalyst, naturally occurring Viridicatin and its derivatives can be transformed into three different types of products through switchable single electron transfer (SET) or energy transfer (EnT) processes. Mechanistic studies have revealed that the oxygen as a reactive center, rather than carbon, is favored, which accounts for the first example of a C–O homo-dimerization product.

Activation of peroxymonosulfate and peroxydisulfate by nitrogen-doped carbon nanotubes for effective degradation of neonicotinoid insecticides

Nitrogen-doped carbon nanotubes (N-CNTs) have been considered as an excellent metal-free activator for persulfate-based advanced oxidation processes. However, the critical role of N-doping in the activation of peroxymonosulfate (PMS) and peroxydisulfate (PDS) remains a significant difference even opposed in scientific understanding. Herein, the CNTs and N-CNTs were used to active PMS and PDS for degradation of neonicotinoid insecticides (NNIs), one of the emerging refractory organic pollutants. The N-CNTs achieve 92.6% of nitenpyram (Nit) degradation within only 10 min under PMS system, much higher than that of CNTs/PMS system (∼10%). Moreover, the N-CNTs can active PMS to conduct degradation through a combined 1O2 and electron transfer process with k of 4.94 × 10−2 L·mg−1·min−1, which was quite different from N-CNTs/PDS system (single electron transfer process, k = 1.84 × 10−3 L·mg−1·min−1). A positive correlation between degradation rates of five NNIs and their electron-donating ability is established, further verifying the importance of electron transfer in NNIs degradation. This study proves the superiority of N-CNTs/PMS over N-CNTs/PDS in Nit degradation and the underlying mechanisms, and reveals the importance of electron-donating ability of NNIs in their degradation by N-CNTs/PMS. These findings are of great significance for their removal from the environment.

Role of low-molecular-weight organic compounds on photochemical formation of Mn(III)-ligands in aqueous systems: Implications for BPA removal

Aqueous trivalent manganese [Mn(III)], an important reactive intermediate, is ubiquitous in natural surface water containing humic acid (HA). However, the effect of low-molecular-weight organic acids (LMWOAs) on the formation, stability and reactivity of Mn(III) intermediate is still unknown. In this study, six LMWOAs, including oxalic acid (Oxa), salicylic acid (Sal), catechol (Cat), caffeic acid (Caf), gallic acid (Gal) and ethylene diamine tetraacetic acid (EDTA), were selected to investigate the effects of LMWOAs on the degradation of BPA induced by in situ formed Mn(III)-L in the HA/Mn(II) system under light irradiation. The chromophoric constituents of HA could absorb light radiation and generate superoxide radical to promote the oxidation of Mn(II) to form Mn(III), which was further involved in transformation of BPA. Our results implied that different LMWOAs did significantly impact on Mn(III) production and its degradation of BPA due to their different functional group. EDTA, Oxa and Sal extensively increased the Mn(III) concentration from 50 to 100 μM compared to the system without LMWOAs, following the order of EDTA > Oxa > Sal, and also enhanced the degradation of BPA with the similar patterns. In contrast, Cat, Caf and Gal had an inhibitory effect on the formation of Mn(III), which is likely because they consumed the superoxide radicals generated from irradiated HA, resulting in the inhibition of Mn(II) oxidation and further BPA removal. The product identification and theoretical calculation indicated that a single electron transfer process occurred between Mn(III)-L and BPA, forming BPA radicals and subsequent self-coupling products. Our results demonstrated that the LMWOAs with different structures could alter the cycling process of Mn via complexation and redox reactions, which would provide new implications for the removal of organic pollutants in surface water.

Copper-Catalyzed 1,2-Sulfonyletherification of 1,3-Dienes.

A selective three-component 1,2-sulfonyl etherification of aryl 1,3-dienes enabled by copper catalysis to afford biologically interesting alkenyl 1,2-sulfone ether derivatives through C-S and C-O bond formation is described. The protocol proceeds with the sulfonyl chloride and alcohols under simple, mild, and base-free conditions, providing a straightforward route to sulfonylated allyl ether compounds with broad functional group tolerance and excellent chemo- and regioselectivity. Mechanistic studies indicate that the selective alkene difunctionalization includes a key copper-mediated single-electron transfer process.

Syntheses, Structures, and Electrochemical Properties of Metallacyclic Oxidovanadium(V) Complexes with Asymmetric Multidentate Linking Ligands.

Trinuclear metallacyclic oxidovanadium(V) complexes, [{VO(L3+2R)}3] (1-3) with asymmetric multidentate linking ligands (H3L3+2R: R = H, Me, Br), were synthesized. The molecular structure of 1 is characterized as a tripod structure, with each V(V) ion coordinated by ONO-atoms from a tridentate Schiff base site and ON-atoms from a bidentate benzoxazole site of two respective H3L3+2H ligands. The intramolecular V⋯V distances range from 8.0683 to 8.1791 Å. Complex 4 is a mononuclear dioxidovanadium(V) complex, (Et3NH)[VO2(HL3+2H)]. Cyclic voltammograms of 1-3 in DMF revealed redox couples attributed to three single-electron transfer processes.

Photoactive Ni-Complexes in Metallaphotoredox Catalysis: A Successful Match in C–C Cross-Coupling Reactions

AbstractNickel catalysis is a well-established and powerful tool for C–C cross-coupling reactions, and its versatility has expanded significantly over past decades by its combination with visible-light photocatalysis in metallaphotoredox chemistry. Photocatalysis enables the activation of traditionally inert substrates and turnover of the Ni catalyst through a single-electron transfer processes. In recent years, dual catalysis has been further empowered by photoactive Ni intermediates, which exhibit distinct reactivity profiles from their ground states and complement existing protocols. This short review focuses on the emergent subclass of metallaphotoredox catalysis in which the synergy of a photoactive Ni catalyst and a typical photocatalyst (e.g., a polypyridyl Ir complex) provide solutions to challenging C–C bond formation.1 Introduction2 Photoactive Nickel Complexes3 HAT-Mediated C–C Cross-Coupling4 Halofunctionalization of π-Systems5 Photoelimination of an Aryl Radical6 Conclusion

Asymmetric photoredox catalytic formal de Mayo reaction enabled by sensitization-initiated electron transfer.

Visible-light-driven photoredox catalysis is known to be a powerful tool for organic synthesis. Its occurrence critically depends on the twice exothermic single-electron transfer processes of photosensitizers, which are governed by the redox properties of the species involved. Hence, the inherently narrow range of redox potentials of photosensitizers inevitably constrains their further availability. Sensitization-initiated electron transfer has recently been found to effectively overcome this substantial challenge. However, feasible and practical strategies for designing such complicated catalytic systems are rather scarce. Herein we report an elaborate dual-catalyst platform, with dicyanopyrazine as a visible light photosensitizer and a pyrenyl-incorporated chiral phosphoric acid as a co-sensitizer, and we demonstrate the applicability of this sensitization-initiated electron transfer strategy in an asymmetric formal de Mayo-type reaction. The catalysis platform enables otherwise thermodynamically unfavourable electron transfer processes to close the redox cycle and allows for precise access to valuable enantioenriched 1,5-diketones with a wide substrate range.

Mechanistic Investigation into Single-Electron Oxidative Addition of Single-Atom Cu(I)-N4 Site: Revealing the Cu(I)-Cu(II)-Cu(I) Catalytic Cycle in Photochemical Hydrophosphinylation.

Understanding the valency and structural variations of metal centers during reactions is important for mechanistic studies of single-atom catalysis, which could be beneficial for optimizing reactions and designing new protocols. Herein, we precisely developed a single-atom Cu(I)-N4 site catalyst via a photoinduced ligand exchange (PILE) strategy. The low-valent and electron-rich copper species could catalyze hydrophosphinylation via a novel single-electron oxidative addition (OA) pathway under light irradiation, which could considerably decrease the energy barrier compared with the well-known hydrogen atom transfer (HAT) and single electron transfer (SET) processes. The Cu(I)-Cu(II)-Cu(I) catalytic cycle, via single-electron oxidative addition and photoreduction, has been proven by multiple in situ or operando techniques. This catalytic system demonstrates high efficiency and requires room temperature conditions and no additives, which improves the turnover frequency (TOF) to 1507 h-1. In particular, this unique mechanism has broken through the substrate limitation and shows a broad scope for different electronic effects of alkenes and alkynes.

Transition Metal and Photocatalyst Free Arylation via Photoexcitable Electron Donor Acceptor Complexes:Mediation and Catalysis

AbstractVisible‐light‐activated organic reactions unlock novel avenues for complex molecular transformations, impossible under standard “thermal” conditions, which makes them powerful tools in the arsenal of synthetic chemistry. However, transition metal‐based or organic photoredox catalysts are often used to ensure productive absorption of visible light, which might be not desirable to medicinal chemistry and industry due to toxicity, low sustainability, and high cost of most photocatalysts. A more environmentally and economically benign approach is based on the formation of transient electron donor‐acceptor (EDA) complexes between two reagents or a reagent and an additive, that readily absorb visible light, acting as internal photosensitizers. Within the EDA complex‐based arylation strategies, chemical transformations are mediated by noncovalent interaction between two molecules, namely between electron‐poor aryl halides or their synthetic equivalents and electron‐rich nucleophilic reagents or additives. Moreover, besides stoichiometric EDA complexes between two molecules, EDA complex based organocatalysis can be achieved in certain cases through regeneration of the donor molecules in the course of the reaction. Photoexcitation of the EDA complexes induces a single electron transfer (SET) process to generate aryl radical species for the arylation step. This Review will focus on the state‐of‐the‐art EDA complex‐based arylation strategies utilizing aryl halides, aryldiazonium, diaryliodonium, arylsulfonium and arylphosphonium salts as reactants, published mainly in the last five years.

Review and Theoretical Analysis of Fluorinated Radicals in Direct CAr-H Functionalization of (Hetero)arenes.

We highlight key contributions in the field of direct radical CAr- H (hetero)aromatic functionalization involving fluorinated radicals. A compilation of Functional Group Transfer Reagents and their diverse activation mechanisms leading to the release of radicals are discussed. The substrate scope for each radical is analyzed and classified into three categories according to the electronic properties of the substrates. Density functional theory computational analysis provides insights into the chemical reactivity of several fluorinated radicals through their electrophilicity and nucleophilicity parameters. Theoretical analysis of their reduction potentials also highlights the remarkable correlation between electrophilicity and oxidizing ability. It is also established that highly fluorinated radicals (e.g. ⋅OCF3) are capable of engaging in single-electron transfer (SET) processes rather than radical addition, which is in good agreement with experimental literature data. A reactivity scale, based on activation barrier of addition of these radicals to benzene is also elaborated using the high accuracy DLPNO-(U)CCSD(T) method.

The Pivotal Radical Intermediate [Au21(SR)15]+ in the Ligand-Exchange-Induced Size-Reduction of [Au23(SR)16]- to Au16(SR)12.

The atomic precision of sub-nanometer-sized metal nanoclusters makes it possible to elucidate the kinetics of metal nanomaterials from the molecular level. Herein, the size reduction of an atomically precise [Au23(CHT)16]- (HCHT = cyclohexanethiol) cluster upon ligand exchange with HSAdm (1-adamantanethiol) has been reported. During the 16 h conversion of [Au23(CHT)16]- to Au16(SR)12, the neutral 6e Au21(SR)15, and its 1e-reduction state, i.e. the 5e, cationic radical, [Au21(SR)15]+, are active intermediates to account for the formation of thermodynamically stable Au16 products. The combination of spectroscopic monitoring (with UV-vis and ESI-MS) and DFT calculations indicates the preferential size-reduction on the corner Au atoms on the core surface and the terminal Au atoms on longer AunSn+1 staples. This study provides a reassessment on the electronic state of the Au21 structure and highlights the single electron transfer processes in cluster systems and thus the importance of the EPR analysis on the mechanistic issues.

Hydroxytrifluoroethylation and Trifluoroacetylation Reactions via SET Processes.

Hydroxytrifluoroethyl and trifluoroacetyl groups are of utmost importance in biologically active compounds, but methods to tether these motifs to organic architectures have been limited. Typically, the preparation of these compounds relied on the use of strong bases or multistep routes. The renaissance of radical chemistry in photocatalytic, transition metal mediated, and hydrogen atom transfer (HAT) processes have allowed the installation of these medicinally relevant fluorinated motifs. This review provides an overview of the methods available for the direct synthesis of hydroxytrifluoroethyl- and trifluoroacetyl-derived compounds governed by single-electron transfer processes.

Visible-Light-Driven Multicomponent Diamination and Oxyamination of Alkene.

Herein, we describe an effective method for the synthesis of 2-alkoxyamides and 1,2-diamines through visible-light-mediated difunctionalization of alkenes. N-Aminopyridinium salts were employed as appropriate precursors to generate key amidyl radical intermediates via a photoinduced single-electron transfer (SET) process. The amidyl radicals would react with alkenes, followed by oxidation and nucleophilic addition. Excellent functional group tolerance and good yields demonstrate the synthetic potential of this transformation.

Unlocking flavin photoacid catalysis through electrophotochemistry.

Molecular flavins are one of the most versatile photocatalysts. They can coordinate single and multiple electron transfer processes, gift hydrogen atoms, form reversible covalent linkages that support group transfer mechanisms, and impart photonic energy to ground state molecules, priming them for downstream reactions. But one mechanism that has not featured extensively is the ability of flavins to act as photoacids. Herein, we disclose our proof-of-concept studies showing that electrophotochemistry can transform fully oxidized flavin quinones to super-oxidized flavinium photoacids that successfully guide proton-transfer and deliver acid-catalyzed products. We also show that these species can adopt a second mechanism wherein they react with water to release hydroxyl radicals that facilitate hydrogen-atom abstraction and sp3C-H functionalization protocols. Together, this unprecedented bimodal reactivity enables electro-generated flavinium salts to affect synthetic chemistries previously unknown to flavins, greatly expanding their versatility as catalysts.

C-H Fluorination Promoted by Pyridine N-Oxyl Radical

Inspired by C-H hydroxylation by cytochrome P450 enzymes (P450s), we report here a method for preparing organofluorides through single-electron transfer (SET) process, in which the pyridine N-oxyl radical greatly promotes...

Visible-light photoredox-catalyzed three-component radical alkyl-acylation of [1.1.1]propellane.

We described herein a three-component radical alkyl-acylation of [1.1.1]propellane via a visible-light photoredox single electron transfer process, demonstrating an efficient approach for accessing a diverse array of 1,3-disubstituted BCP ketone derivatives. The synthetic utility of the present radical protocol was further demonstrated by the Baeyer-Villiger oxidation of the BCP ketone for BCP ester formation.

Advances in Flavin-Inspired Photocatalytic Oxidations Involving Single Electron Transfer Process

Advances in Flavin-Inspired Photocatalytic Oxidations Involving Single Electron Transfer Process

Visible-light-induced direct C-H alkylation of polycyclic aromatic hydrocarbons with alkylsulfones.

Polycyclic aromatic hydrocarbons (PAHs) are fragments of graphene that have attracted considerable attention as a new class of carbon-based materials. The functionalization of edge positions in PAHs is important to enable the modulation of physical and chemical properties essential for various applications. However, straightforward methods that combine functional group tolerance and regioselectivity remain sought after. Here we report a photochemical approach for the direct alkylation of carbon-hydrogen bonds in PAHs that takes place in a regiospecific manner, an outcome that has never been achieved in related thermal reactions. A reaction mechanism involving a single electron transfer process from photo-excited PAHs to sulfones, and a rationale for the origin of regioselectivity are proposed on the basis of spectroscopic analyses and theoretical calculations.

Photocatalytic Carbosulfonylation/Cyclization of N-Homoallyl and N-Allyl Aldehyde Hydrazones toward Sulfonylated Tetrahydropyridazines and Dihydropyrazoles.

N'-Benzylidene-N-homoallylacetohydrazides were designed and synthesized as novel skeletons for the construction of functionalized tetrahydropyridazines. A series of aryl- and alkylsulfonylated tetrahydropyridazines were obtained in yields of up to 94% employing sulfonyl chlorides as the sulfonyl radical sources under visible-light irradiation. Besides, sulfonylated dihydropyrazoles were also produced from N-allyl-N'-benzylideneacetohydrazides. Mechanistic investigations indicated that both energy transfer and single electron transfer processes were involved in accomplishing the radical 6/5-endo-trig cyclization to the C═N bond.

Iron-Catalyzed carbonylative synthesis of amides from Alkyl-Boronic pinacol ester via a single electron transfer process

An iron-catalyzed oxidative aminocarbonylation of alkyl boronic pinacol esters via alkyl radical intermediate has been developed. Various aliphatic amides were produced in good yields from the corresponding alkyl boronic esters and anilines in the presence of oxidant under carbon monoxide pressure.