Metal-free Oxidation Research Articles

Metal-Free Oxidation of Acceptor-Donor Acylhydrazones into Diazo Compounds Using Phenyl Iododiacetate.

Aryl-ester acylhydrazones readily react with phenyl iododiacetate (PIDA) in methanol to produce the corresponding α-diazoesters with good to excellent yields (30 examples). The conditions have also been proven to be efficient in the synthesis of triazolopyridines. The crude mixture containing the diazo compound and acetic acid was also irradiated with low-energy blue LED light for a subsequent one-pot insertion of the in situ-generated carbene with AcOH to afford the respective acetates in high yields.

Selective Oxidation of Benzo[d]isothiazol-3(2H)-Ones Enabled by Selectfluor.

A metal-free and Selectfluor-mediated selective oxidation reaction of benzo[d]isothiazol-3(2H)-ones in aqueous media is presented. This novel strategy provides a facile, green, and efficient approach to access important benzo[d]isothiazol-3(2H)-one-1-oxides with excellent yields and high tolerance to various functional groups. Furthermore, the purification of benzoisothiazol-3-one-1-oxides does not rely on column chromatography. Moreover, the preparation of saccharine derivatives has been achieved through sequential, double oxidation reactions in a one-pot aqueous media.

Nitrogen-Rich Covalent Organic Polymer for Metal-Free Tandem Catalysis and Postmetalation-Actuated High-Performance Water Oxidation.

Development of high-performing catalytic materials for selective and mild chemical transformations through adhering to the principles of sustainability remains a central focus in modern chemistry. Herein, we report the template-free assembly of a thermochemically robust covalent organic polymer (COP: 1) from 2,2'-bipyridine-5,5'-dicarbonyl dichloride and 2,4,6-tris(4-aminophenyl)triazine as [2 + 3] structural motifs. The two-dimensional (2D) layered architecture contains carboxamide functionality, delocalized π-cloud, and free pyridyl-N site-decked pores. Such trifunctionalization benefits this polymeric network exhibiting tandem alcohol oxidation-Knoevenagel condensation. In contrast to common metal-based catalysts, 1 represents a one of a kind metal-free alcohol oxidation reaction via extended π-cloud delocalization-mediated free radical pathway, as comprehensively supported from diverse control experiments. In addition to reasonable recyclability and broad substrate scope, the mild reaction condition underscores its applicability in benign synthesis of valuable product benzylidene malononitrile. Integration of 2,2'-bipyridyl units in this 2D COP favors anchoring non-noble metal ions to devise 1-M (M: Ni2+/ Co2+) that demonstrate outstanding electrochemical oxygen evolution reaction in alkaline media with high chronoamperometric stability. Electrochemical parameters of both 1-Co and 1-Ni outperform some benchmark, commercial, as well as a majority of contemporary OER catalysts. Specifically, the overpotential and Tafel slope (280 mV, 58 mV/dec) for 1-Ni is better than 1-Co (360 mV, 78 mV/dec) because of increased charge accumulation as well as a higher number of active sites compared to the former. In addition, the turnover frequency of 1-Ni is found to be 6 times higher than that of 1-Co and ranks among top-tier water oxidation catalysts. The results provide valuable insights in the field of metal-free tandem catalysis as well as promising electrochemical water splitting at the interface of task-specific functionality fuelling in polymeric organic networks.

The degradation of tetrabromobisphenol A by metal-free graphene oxide in the dark

Brominated flame retardants (BFRs) such as tetrabromobisphenol A (TBBPA) were widely detected in the environment. The ease of synthesis of graphene-based materials (GBMs) makes it an excellent metal-free carbon-based catalyst for the reductive debromination of TBBPA. Here we explore the complete debromination of TBBPA from the aqueous environment in the dark using metal-free graphene oxide (GO) and reduced GO (rGO). More importantly, radical scavenger experiments reveal the formation of superoxide radicals and singlet oxygen from the surface of GBMs leading to the debromination of TBBPA. GO shows an enhanced catalytic activity due to enriched O-functional groups on the surface than rGO, as observed by cyclic voltammetry and electrochemical impedance spectroscopy. In addition, the TBBPA degradation pathway was proposed with BPA as the end product and by-products were identified by mass spectroscopy. The complete TBBPA degradation was observed within 240 mins of reaction time (kobs = 8.6×10−3 min−1), which was significantly more efficient than the catalytic activity of other metal-based catalysts. Our study thus provides a new insight into the debromination mechanisms of TBBPA which would exhibit potential for future efficient catalyst development.

A sustainable and metal-free advanced oxidation process for efficient water purification by electrified reduced graphene oxide membrane

A sustainable and metal-free advanced oxidation process for efficient water purification by electrified reduced graphene oxide membrane

N/Se co-doped porous carbon catalyst derived from a deep eutectic solvent and chitosan as green precursors: Investigation of catalytic activity for metal-free oxidation of alcohols

Heteroatom-doped porous carbon-based materials with high surface area compared to their metal-based homologs are considered environmentally friendly and ideal catalysts for organic reactions. In this paper, a new method for the convenient fabrication, cost-effective, and high efficiency of nitrogen/selenium co-doped porous carbon-based catalysis (marked as N/SePC-T) was designed. The N/SePC-T catalysts were created from the direct pyrolysis of a eutectic solvent containing choline chloride/urea as the nitrogen-rich carbon source, selenium dioxide as a source of heteroatom and chitosan as a secondary carbon source in different temperatures (T). The efficacy of the carbonization temperature on the pore structure, morphology, and catalytic activity of the N/SePC-T materials was investigated and displayed, the N/SePC-900 (having a surface area of 562.01 m2/g and total pore volume of 0.2351 cm3 g−1) has the best performance. The morphology, structure, and physicochemical properties of N/SePC-900 were characterized using various analyses including XRD, TEM, TGA, FE-SEM, EDX, FT-IR, XPS, and Raman. The optimized N/SePC-900 catalyst indicated excellent catalytic performance in the oxidation of benzylalcohols to corresponding aldehydes in very mild conditions.

Metal-free Oxidations with m-CPBA: An Octennial Update

Abstract: meta-Chloroperbenzoic acid (mCPBA) is a readily available peroxide that is a powerful oxidizing agent utilized in a range of oxidative reactions. Metal-free oxidations mediated by mCPBA involving selective oxidation of aliphatic amines to oximes, selective oxidation of organosulphides/ aldehydes, synthesis of heterocyclic N-oxides, domino C[sp2] hydroxylation/ annulation of enaminones (heterocyclic ring formation), acid to phenol conversion, oxidation of exocyclic C=C bond, oxidative ring contraction, etc. have been comprehensively and critically examined in this mini-review from 2015 to date.

Dual Role of AgNO3 as an Oxidizer and Chloride Remover toward Enhanced Combustion Synthesis of Low-Voltage and Low-Temperature Amorphous Rare Metal-Free Oxide Thin-Film Transistors

Dual Role of AgNO₃ as an Oxidizer and Chloride Remover toward Enhanced Combustion Synthesis of Low-Voltage and Low-Temperature Amorphous Rare Metal-Free Oxide Thin-Film Transistors

Selective deuteration of pyridine using barium oxide and D2 gas

We report a transition metal-free barium oxide catalyst for selective deuteration of pyridine using D2 as a D source. The heterolytic dissociation of D2 on BaO plays a key role in the D/H exchange reaction of pyridine.

Heterogeneous solvent-metal-free aerobic oxidation of alcohol under ambient conditions catalyzed by TEMPO-functionalized porous poly(ionic liquid)s.

Heterogeneous solvent-metal-free aerobic oxidation of alcohols under ambient conditions is interesting but remains a significant challenge. Herein, a series of porous TEMPO-functionalized poly(ionic liquid)s (TEMPO-PILs) featuring a pure polycationic framework were successfully developed through the free radical polymerization of the ionic liquid 3-(2-chloroacetic acid-2,2,6,6-tetramethyl-1-oxo-4-piperidyl)-1-vinylimidazolium chloride and bis-vinylimidazolium bromide salt. Characterizations revealed that the obtained TEMPO-PILs possessed a high TEMPO density, abundant bromide ions, and a tunable porous structure, which enabled them to serve as solvent-free heterogeneous organocatalysts for the metal-free aerobic oxidation of benzyl alcohol under ambient conditions, exhibiting high catalytic activity and stable recyclability. A high yield of 99% coupled with a turnover frequency (TOF) of 13.3 h-1 was obtainable, which is higher than most of the reported TEMPO-based heterogeneous catalysts, even superior to homogeneous TEMPO-functionalized ionic liquids. Furthermore, a broad range of alcohols were effectively converted into their corresponding ketones and aldehydes. A possible reaction mechanism is proposed for understanding the catalytic oxidation behavior, indicative of the synergistic effect of TEMPO moieties and bromide ions.

Metal-Free Oxidative Formation of Aryl Esters by Catalytic Coupling of Acyl and Sulfonyl Chlorides with Arylboronic Acids.

A practical and efficient synthesis of aryl esters was developed through metal-free oxidation. This reaction employs stable and readily available acyl or sulfonyl chlorides and arylboronic acids as the starting materials and proceeds under mild reaction conditions without additional precious metal catalysts. This new strategy exhibits broad substrate tolerance and operational simplicity and gives diverse aryl esters in moderate to high yields.

By-product of hydrothermal liquefaction: Characterization of biochar and its application in metal-free electrochemical advanced oxidation process of hydrogen peroxide to provide hydroxyl radicals

Hydrothermal liquefaction stands as a prominent method for the conversion of biomass into renewable energy sources. Alongside its production of high-quality biocrude oil, this process, however, inevitably generates by-products. This research was centered on biochar, a by-product of the hydrothermal liquefaction. The structural and morphological properties were explored in-depth, employing various techniques such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, among others. The analysis revealed that biochar showcased an initial inclination towards graphitization, possessing a rough layered structure. Additionally, the biochar surface contained a significant nitrogen content, predominantly in the forms of pyridinic nitrogen, pyrrolic nitrogen, and graphitic nitrogen. These nitrogen species contributed significantly to the electrocatalytic capabilities. Building upon this finding, a biochar-modified glassy carbon electrode was prepared and utilized as the cathode in a metal-free electrochemical advanced oxidation system, facilitating the production of hydroxyl radicals from H2O2. This work delved deep into the mechanism underlying the electrocatalytic reaction. It elucidated that the reduction of H2O2 to ⋅ OH was a diffusion-controlled, one-electron transfer with proton-independent process. Through comparative assessments between the electrocatalytic performance of biochar-modified and unmodified glassy carbon electrodes, the former substantiated a significant enhancement, specifically regarding the reduction of H2O2 for ⋅ OH generation.

Visible-Light-Mediated Oxidation of 1-Benzyl-3,4-dihydroisoquinolines with Dioxygen: A Switchable Synthesis of 1-Benzoylisoquinolines and 1-Benzoyl-3,4-dihydroisoquinolines

AbstractA visible-light-mediated metal-free oxidation of 1-benzyl-3,4-dihydroisoquinolines (1-BnDHIQs) for the switchable preparation of a wide range of 1-benzoylisoquinolines (BzIQs) and 1-benzoyl-3,4-dihydroisoquinolines (BzDHIQs) is realized using dioxygen as the oxidant. This protocol provides a facile route for the efficient synthesis of isoquinoline alkaloids. Mechanistic investigations suggest that a radical pathway and an ionic pathway may exist simultaneously, with both pathways proceeding via a common key peroxide intermediate to give the oxidized products.

Photochemical Hydrogen Atom Transfer Catalysis for Dehydrogenation of Alcohols To Form Carbonyls.

Controllable oxidation of alcohols to carbonyls is one of the fundamental transformations in organic chemistry. Herein, we report an unprecedented visible-light-mediated metal-free oxidation of alcohols to carbonyls with hydrogen evolution. By synergistic combination of organophotocatalyst 4CzIPN and a thiol hydrogen atom transfer catalyst, a broad range of alcohols, including primary and secondary benzylic alcohols as well as aliphatic alcohols, were readily oxidized to carbonyls in moderate to excellent yields. A site-selective oxidation has also been achieved by this protocol. Mechanistic investigation indicates that the oxidation proceeds through an oxidative radical-polar crossover process to obtain an α-oxy carbon cation.

Biomass derived S, N self-doped catalytic Janus cathode for flow-through metal-free electrochemical advanced oxidation process: Better removal efficiency and lower energy consumption under neutral conditions

Facing low treatment efficiency, narrow adaptive pH and high energy consumption in electro-Fenton (EF), we proposed a novel flow-through metal-free electrochemical advanced oxidation processes (EAOPs) using biomass derived S, N self-doped catalytic Janus cathode named SNJC. The SNJC was composed of a hydrophobic gas diffusion layer in the middle and hydrophilic catalytic membrane at both ends, while the catalytic membrane of S, N self-doped biomass carbon named SN-BC was derived from waste ginkgo leaves without additional supporting templates or activation processes. The conversion of graphite N, pyridinic N and thiophene S in SN-BC played a significant role in efficient oxygen reduction reaction (ORR) for H2O2 generation and in-situ active species generation. The efficient enrichment and rapid degradation of pollutants in catalytic membrane achieved almost complete removal of tetracycline within 120 min with a low energy consumption of 16.8 kWh/kg TOC. This flow-through system exhibited superior catalytic performance in wide pH ranges (3–11) due to the collective effect of radicals (•OH and O2•−) and non-radical (1O2). This work provides a new insight towards the design of S, N self-doped Janus electrode and activation mechanism of in-situ generation and metal-free catalysis of H2O2 in flow-through EAOPs.

Carbonyl and defect of metal-free char trigger electron transfer and O2•− in persulfate activation for Aniline aerofloat degradation

Residual flotation reagents in mineral processing wastewater can trigger severe ecological threats to the local groundwater if they are discharged without treatment. Metal-free biochar-induced persulfate-advanced oxidation processes (KCBC/PS) were used in this study to elucidate the degradation of aniline aerofloat (AAF) – a typical flotation reagent. In KCBC/PS system, AAF can be removed at low doses of catalyst (KCBC, 0.05 g/L) and oxidant (PS, 0.3 mM) additions with high efficiency. The analysis revealed the dominance of O2•− among the identified reactive oxygen species (ROS), which achieved deeper mineralization for the AAF degradation in the KCBC/PS system. The role of the electron transfer mechanism was equally important; the importance was corroborated by the chemical quenching experiments, electron spin resonance (ESR) detection, probe experiments, and electrochemical analysis. It benefited from the electron transfer mechanism in the KCBC/PS system and exhibited a wide pH adaptation (3.5–11) and high resistance to inorganic anions for real mining wastewater treatment. Combined with theoretical calculations and other analyses, the carbonyl group was deemed to be the active site of the non-radical pathway of biochar, while the site of the conversion of SO4•− to superoxide radicals by biochar activation represented a defect. These findings revealed a synergistic effect of multiple active sites on PS activation in biochar-based materials. Moreover, the intermediate degradation products of AAF from mass spectrometry indicated a possible pathway through the density functional theory (DFT) method, which was effective in reducing the environmental toxicity of pollutants for the first time according to the T.E.S.T software and seed germination experiments. Overall, our study proposed a novel modification strategy for cost-effective and environmentally friendly biochar-based catalysts, while also deepening our understanding of the mechanism of activation of persulfate by metal-free carbon-based materials.

Zwitterionic hydrogen-bonded organic framework (ZHOF): a metal-free water oxidation proton relay in alkaline and neutral media

Graphite supported zwitterionic hydrogen-bonded organic framework (ZHOF) acts as a supercapacitor, and proton relay during water oxidation reactions in alkaline and natural media.

Building N-hydroxyphthalimide organocatalytic sites into a covalent organic framework for metal-free and selective oxidation of silanes.

A novel crystalline covalent organic framework COF-NHPI was built by a bottom-up strategy to guarantee highly ordered embedment of the N-hydroxyphthalimide (NHPI) units as nitroxyl radical organocatalytic sites. The COF-NHPI was demonstrated to be a metal-free, highly selective and heterogeneous catalyst for the efficient oxidation of various silanes to the corresponding silanols. Mechanistic studies revealed that the critical phthalimido N-oxyl radical was generated in situ to govern the catalysis.

Electric field-enhanced coupled with metal-free peroxymonosulfate activactor: The selective oxidation of nonradical species-dominated system

Nowadays metal-free persulfate-based advanced oxidation processes (AOPs) have been intensively investigated, however, the catalysts are often too complex to fully consider their application potential. Conventional AOPs usually suffer from severe interference in real water matrix, thus, selective oxidation is practically and scientifically challenging as it could avoid unnecessary inputs of energy and possible secondary pollutants. In this study, a remarkably synergistic effect was achieved when conventional amorphous boron/peroxymonosulfate (Boron/PMS, 0.67 × 10−2 min−1) system was combined with electrolysis (E-Boron/PMS, 1.54 × 10−2 min−1) to degrade sulfamethoxazole (SMX). Evidenced by selectively quenching tests with kinetic evaluation, electron paramagnetic resonance (EPR), solvent-exchange experiment and electrochemical analysis, the dominated reactive oxygen species in E-Boron/PMS system tended to be 1O2, instead of the •OH and SO4•−. Mechanistic study unveiled that 1O2 was generated via accelerated PMS self-decomposition, triggered by interface alkalization and hydroxyl radicals transfer at the cathode interface. 1O2 is considered to be selective to the electron-rich organic compounds, thus E-Boron/PMS system was superior to conventional radical-dominated system (Boron/PMS) for SMX removal in the co-presence of common inorganic anions, showing the great merits of selective oxidation in nonradical system. These findings provided new insights into effective and selective oxidation of SMX via E-Boron/PMS system, which shed new light on the development of nonradical system.

Unexpected Pyridinyl Group Mediated Metal-Free Wacker-Type Oxidation en Route to Pyrido[2,1-a]isoindol-5-ium Salts

AbstractA two-step approach to rapidly access a diverse array of pyrido[2,1-a]isoindol-5-ium salts from 2-pyridinylstyrenes through an unprecedented pyridinyl group mediated metal-free Wacker-type oxidation and an acid-mediated cyclization has been developed. As a part of the mechanistic investigation of this novel Wacker-type oxidation, the abnormal instability and reactivity of the pyrido[2,1-a]isoindole intermediates were studied through DFT calculations.