Catalytic Aerobic Oxidation of C(sp3)−H Bonds

Oxidation reactions are a key technology to transform hydrocarbons from petroleum feedstock into chemicals of a higher oxidation state, allowing further chemical transformations. These bulk‐scale oxidation processes usually employ molecular oxygen as the terminal oxidant as at this scale it is typically the only economically viable oxidant. The produced commodity chemicals possess limited functionality and usually show a high degree of symmetry thereby avoiding selectivity issues. In sharp contrast, in the production of fine chemicals preference is still given to classical oxidants. Considering the strive for greener production processes, the use of O₂, the most abundant and greenest oxidant, is a logical choice. Given the rich functionality and complexity of fine chemicals, achieving regio/chemoselectivity is a major challenge. This review presents an overview of the most important catalytic systems recently described for aerobic oxidation, and the current insight in their reaction mechanism.     

Kinetics and mechanisms of decomposition reaction of hydrogen peroxide in presence of metal complexes

Hydrogen peroxide was discovered in 1818 and has been used in bleaching for over a century [ 1 ]. H₂O₂ on its own is a relatively weak oxidant under mild conditions: It can achieve some oxidations unaided, but for the majority of applications it requires activation in one way or another. Some activation methods, e.g., Fenton's reagent, are almost as old [ 2 ]. However, by far the bulk of useful chemistry has been discovered in the last 50 years, and many catalytic methods are much more recent. Although the decomposition of hydrogen peroxide is often employed as a standard reaction to determine the catalytic activity of metal complexes and metal oxides [ 3 , 4 ], it has recently been extensively used in intrinsically clean processes and in end‐of‐pipe treatment of effluent of chemical industries [ 5 , 6 ]. Furthermore, the adoption of H₂O₂ as an alternative of current industrial oxidation processes offer environmental advantages, some of which are (1) replacement of stoichiometric metal oxidants, (2) replacement of halogens, (3) replacement or reduction of solvent usage, and (4) avoidance of salt by‐products. On the other hand, wasteful decomposition of hydrogen peroxide due to trace transition metals in wash water in the fabric bleach industry, was also recognized [ 7 ]. The low intrinsic reactivity of H₂O₂ is actually an advantage, in that a method can be chosen which selectively activates it to perform a given oxidation. There are three main active oxidants derived from hydrogen peroxide, depending on the nature of the activator; they are (1) inorganic oxidant systems, (2) active oxygen species, and (3) per oxygen intermediates. Two general types of mechanisms have been postulated for the decomposition of hydrogen peroxide in the presence of transition metal complexes. The first is the radical mechanism (outer sphere), which was proposed by Haber and Weiss for the Fe(III)‐H₂O₂ system [ 8 ]. The key features of this mechanism were the discrete formation of hydroxyl and hydroperoxy radicals, which can form a redox cycle with the Fe(II)/Fe(III) couple. The second is the peroxide complex mechanism, which was proposed by Kremer and Stein [ 9 ]. The significant difference in the peroxide complex mechanism is the two‐electron oxidation of Fe(III) to Fe(V) with the resulting breaking of the peroxide oxygen‐oxygen bond. It is our intention in this article to briefly summarize the kinetics as well as the mechanisms of the decomposition of hydrogen peroxide, homogeneously and heterogeneously, in the presence of transition metal complexes. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 643–666, 2000     

Catalytic Enantioselective Flow Processes with Solid‐Supported Chiral Catalysts

Sustainability concerns are the wind in the sails for the development of novel, more selective catalytic processes. Hence, chiral catalysts play a crucial role in the green production of enantioenriched compounds. To further increase the green profile of this approach, the use of solid‐supported catalytic species is appealing due to the reduced generation of waste, as well as the possibility of reusing the precious catalyst. Even more attractive is the implementation of flow processes based on these immobilized catalysts, a flexible strategy that allows to generate from milli‐ to multi‐gram amounts of chiral product with a reduced footprint set‐up. Herein, we will present the efforts devoted in our laboratory towards the immobilization of chiral catalysts and their use in single‐pass, highly enantioselective, flow processes. Proline, diarylprolinols, other aminocatalysts, squaramides, thioureas, phosphoric acids and even chiral ligands and metal‐based catalysts constitute our current toolkit of supported species for enantioselective catalysis.     

Design and Catalytic Application of Functional Porous Organic Polymers: Opportunities and Challenges

This review article encompasses the progress and conventional overview of current research activities of porous organic polymers (POPs), especially in catalysis, as they have garnered colossal interest in the scientific fraternity due to their intriguing characteristic features. Various synthetic strategies with possible modification of functionality of POPs have been used to improve the catalytic efficiency towards value‐added chemicals production. Accordingly, this review article is mainly focused on the design, development of various functionalized POPs by employing Friedel‐Crafts alkylation, FeCl₃ assisted oxidative polymerisation and polymerisation in nonaqueous medium, and a comprehensive understanding in potential catalytic applications namely, acetalization, hydrodeoxygenation (HDO), hydrogenation, coupling, photocatalytic hydrogen evolution and biomass conversion towards the production of value‐added chemicals in biodiesel and chemical industries.     

Effect of Chemical Charging/Discharging on Plasmonic Behavior of Silver Metal Nanoparticles Prepared using Citrate‐Stabilized Cadmium Selenide Quantum Dots

The thermodynamics and kinetics of the chemical and electrochemical charging of a catalyst surface are very important to understand its applicability as a catalyst material, particularly in redox catalysis. Through the present study, we hereby communicate the results obtained from our detailed investigations related to the effect of chemical charging on the plasmonic behavior of silver metal nanoparticles (Ag MNPs) as redox catalysts. Two different batches of Ag MNPs were prepared through thermally assisted chemical reduction of silver ions. The difference in these batches was the use or not of citrate‐capped cadmium selenide quantum dots (Q‐CdSe) for the reduction of solution‐phase silver ions to their colloidal plasmonic phase. The charge on the surfaces of the Ag MNPs was varied by the chemical electron injection method by using BH₄^? ions from a NaBH₄ solution. The processes of charging and discharging were monitored by using UV/Vis absorption spectroscopy. The impact of the concentration of the reductant on the charging and discharging processes was also investigated. The Ag MNPs were also tested for their voltammetric response, wherein it was observed that it was more difficult to oxidize the Ag MNPs prepared with Q‐CdSe seeds than to oxidize Ag MNPs prepared without Q‐CdSe particles. Our results demonstrate that Q‐CdSe seeds not only enhance the redox catalytic activity of Ag MNPs but also provide stability towards polarization of their plasmonic behavior.     

Polyhalides as Efficient and Mild Oxidants for Oxidative Carbene Organocatalysis by Radical Processes

Simple and inexpensive polyhalides (CCl₄ and C₂Cl₆) have been found to be effective and versatile oxidants in removing electrons from Breslow intermediates under N‐heterocyclic carbene (NHC) catalysis. This oxidative reaction involves multiple single‐electron‐transfer (SET) processes and several radical intermediates. The α, β, and γ‐carbon atoms of aldehydes and enals could be readily functionalized. Given the low cost of the oxidants and the broad applicability of the reactions, this study is expected to greatly enhance the feasibility of oxidative NHC catalysis for large‐scale applications. Also this new SET radical process with polyhalides as single‐electron oxidants will open a new avenue in the development of NHC‐catalyzed radical reactions.     

Precisely Designed Isopeptide Bridge‐Crosslinking Endows Artificial Hydrolases with High Stability and Catalytic Activity under Extreme Denaturing Conditions

Enzymes normally lose their activities under extreme conditions due to the dissociation of their active tertiary structure. If an enzyme could maintain its catalytic activity under non‐physiological or denaturing conditions, it might be used in more applications in the pharmaceutical and chemical industries. Recently, we reported a coiled‐coil six‐helical bundle (6HB) structure as a scaffold for designing artificial hydrolytic enzymes. Here, intermolecular isopeptide bonds were incorporated to enhance the stability and activity of such biomolecules under denaturing conditions. These isopeptide bridge‐tethered 6HB enzymes showed exceptional stability against unfolding and retained or even had increased catalytic activity for a model hydrolysis reaction under thermal and chemical denaturing conditions. Thus, isopeptide bond‐tethering represents an efficient route to construct ultrastable artificial hydrolases, with promising potential to maintain biocatalysis under extreme conditions.     

Transition‐Metal‐Catalyzed Redox‐Neutral and Redox‐Green C–H Bond Functionalization

Transition‐metal‐catalyzed C–H bond functionalization has become one of the most promising strategies to prepare complex molecules from simple precursors. However, the utilization of environmentally unfriendly oxidants in the oxidative C–H bond functionalization reactions reduces their potential applications in organic synthesis. This account describes our recent efforts in the development of a redox‐neutral C–H bond functionalization strategy for direct addition of inert C–H bonds to unsaturated double bonds and a redox‐green C–H bond functionalization strategy for realization of oxidative C–H functionalization with O₂ as the sole oxidant, aiming to circumvent the problems posed by utilizing environmentally unfriendly oxidants. In principle, these redox‐neutral and redox‐green strategies pave the way for establishing new environmentally benign transition‐metal‐catalyzed C–H bond functionalization strategies.     

Photobiocatalysis: Activating Redox Enzymes by Direct or Indirect Transfer of Photoinduced Electrons

Biocatalytic transformation has received increasing attention in the green synthesis of chemicals because of the diversity of enzymes, their high catalytic activities and specificities, and mild reaction conditions. The idea of solar energy utilization in chemical synthesis through the combination of photocatalysis and biocatalysis provides an opportunity to make the “green” process greener. Oxidoreductases catalyze redox transformation of substrates by exchanging electrons at the enzyme′s active site, often with the aid of electron mediator(s) as a counterpart. Recent progress indicates that photoinduced electron transfer using organic (or inorganic) photosensitizers can activate a wide spectrum of redox enzymes to catalyze fuel‐forming reactions (e.g., H₂ evolution, CO₂ reduction) and synthetically useful reductions (e.g., asymmetric reduction, oxygenation, hydroxylation, epoxidation, Baeyer–Villiger oxidation). This Review provides an overview of recent advances in light‐driven activation of redox enzymes through direct or indirect transfer of photoinduced electrons.     

Chemistry in Confinement: Copper and Palladium Catalyzed Ecofriendly Organic Transformations within Porous Frameworks

A concise account on the use of transition metals copper (Cu) and palladium (Pd), as their cations as well as nanoparticles exchanged/immobilized onto porous frameworks such as zeolites, metal organic frameworks (MOFs), covalent organic polymers (COPs) and hollow nanostructures, functioning as catalysts in organic synthesis is presented. This biomimetic account, “focusing on catalytic systems in confinement” within zero‐dimensional microenvironments and second sphere coordination covers primarily results from our group on N‐sulfonylketenimine mediated cycloaddition, hydrogenation and C−C bond forming reactions, thus providing an interesting insight into the versatility and utility of these Cu and Pd catalysts. Other significant advantages and green credentials of confinement such as stability, selectivity, reusability, promotion of multicomponent reactions, use of green solvents, atom economy, and use of ambient conditions are highlighted at appropriate places. In the final section, our views on the current achievements and the future prospects in this area are summarized.     

Pickering‐Emulsion Inversion Strategy for Separating and Recycling Nanoparticle Catalysts

With the recent advances in nanoscience and nanotechnology, more and more nanoparticle catalysts featuring high accessibility of active sites and high surface area have been explored for their use in various chemical transformations, and their rise in popularity among the catalysis community has been unprecedented. The industrial applications of these newly discovered catalysts, however, are hampered because the existing methods for separation and recycling, such as filtration and centrifugation, are generally unsuccessful. These limitations have prompted development of new methods that facilitate separation and recycling of nanoparticle catalysts, so as to meet the burgeoning demands of green and sustainable chemistry. Recently, we have found that Pickering‐emulsion inversion is an appealing strategy with which to realize in situ separation and recycling of nanoparticle catalysts and thereby to establish sustainable catalytic processes. We feel that at such an early stage, this strategy, as an alternative to conventional methods, is conceptually new for readers but that it has potential to become a popular method for green catalysis. This Concept article aims to provide a timely link between previous efforts and both current and future research on nanoparticle catalysts, and is expected to facilitate further investigation into this strategy.     

Electrochemical Cross-Dehydrogenative Coupling between Phenols and β-Dicarbonyl Compounds: Facile Construction of Benzofurans

Preparative electrochemical synthesis is an ideal method for establishing green, sustainable processes. The major benefits of an electro-organic strategy over that of conventional chemical synthesis are the avoidance of reagent waste and mild reaction conditions. Here, an intermolecular cross-dehydrogenative coupling between phenols and β-dicarbonyl compounds has been developed to build various benzofurans under undivided electrolytic conditions. Neither transition metals nor external chemical oxidants are required to facilitate the dehydrogenation and dehydration processes. The key factor in success was the use of nBu₄NBF₄ as the electrolyte and hexafluoroisopropanol as the solvent, which play key roles in the cyclocondensation step. This electrolysis is scalable and can be used as a key step in drug synthesis. On the basis of several experimental results, the mechanism, particularly of the remarkable anodic oxidation and cyclization process, was illustrated.     

Nanocomposite Incorporating V2O5 Nanowires and Gold Nanoparticles for Mimicking an Enzyme Cascade Reaction and Its Application in the Detection of Biomolecules

Artificial enzyme mimics are a current research interest, and many nanomaterials have been found to display enzyme‐mimicking activity. However, to the best of our knowledge, there have not hitherto been any reports on the use of pure nanomaterials to construct a system capable of mimicking an enzyme cascade reaction. Herein, we describe the construction of a novel nanocomposite consisting of V₂O₅ nanowires and gold nanoparticles (AuNPs) through a simple and facile chemical method, in which V₂O₅ and AuNPs possess intrinsic peroxidase and glucose oxidase (GOx)‐like activity, respectively. Results suggest that this material can mimic the enzyme cascade reaction of horseradish peroxidase (HRP) and GOx. Based on this mechanism, a direct and selective colorimetric method for the detection of glucose has been successfully designed. Because single‐strand and double‐strand DNA (ssDNA and dsDNA) have different deactivating effects on the GOx‐like activity of AuNPs, the sensing of target complementary DNA can also be realized and disease‐associated single‐nucleotide polymorphism of DNA can be easily distinguished. Our study opens a new avenue for the use of nanomaterials in enzyme mimetics, and holds promise for the further exploration of nanomaterials in creating alternative catalytic systems to natural enzymes.     

Selective Catalytic Synthesis Using the Combination of Carbon Dioxide and Hydrogen: Catalytic Chess at the Interface of Energy and Chemistry

The present Review highlights the challenges and opportunities when using the combination CO₂/H₂ as a C₁ synthon in catalytic reactions and processes. The transformations are classified according to the reduction level and the bond‐forming processes, covering the value chain from high volume basic chemicals to complex molecules, including biologically active substances. Whereas some of these concepts can facilitate the transition of the energy system by harvesting renewable energy into chemical products, others provide options to reduce the environmental impact of chemical production already in today's petrochemical‐based industry. Interdisciplinary fundamental research from chemists and chemical engineers can make important contributions to sustainable development at the interface of the energetic and chemical value chain. The present Review invites the reader to enjoy this exciting area of “catalytic chess” and maybe even to start playing some games in her or his laboratory.     

Rhodium‐Catalyzed Electrooxidative C−H Olefination of Benzamides

Metal‐catalyzed chelation‐assisted C?H olefinations have emerged as powerful tools for the construction of functionalized alkenes. Herein, we describe the rhoda‐electrocatalyzed C?H activation/alkenylation of arenes. The olefinations of challenging electron‐poor benzamides were thus accomplished in a fully dehydrogenative fashion under electrochemical conditions, avoiding stoichiometric chemical oxidants, and with H₂ as the only byproduct. This versatile alkenylation reaction also features broad substrate scope and used electricity as a green oxidant.     

过渡金属催化醇氧化反应的研究进展

氧化醇为相应的羰基化合物在有机化学基础研究及其工业应用中占有非常重要的地位.在众多关于醇氧化反应的研究中,如何使用对环境友好的氧化剂替代传统的氧化剂,选择具有高性能、廉价、清洁、可回收的催化剂,一直是化学工作者研究的一个热点.综述了近年来该研究领域中的一些重要过渡金属催化体系,并简要介绍了各类催化氧化体系的催化效果.     

Asymmetric Electrochemical Transformations

The field of electrochemical synthesis has developed rapidly over the last decade and has provided alternative synthetic methods with the absence of stoichiometric amounts of chemical oxidants or reductants. Although sustainable electrosynthetic procedures have been developed, relatively few examples of highly enantioselective catalytic electrosynthesis have been reported to date. The development of general strategies for electrochemical enantiocontrol has thus proven to be a considerable challenge. This Minireview highlights the current knowledge and recent advances in the synthetic utility of electrochemical transformations for asymmetric synthesis. Specifically, three major types of catalytic enantioselective strategy in electrosynthesis are outlined, including electrochemical activation of chiral catalyst‐bound substrates, asymmetric cascade electrochemical processes, and chemically modified chiral electrodes.     

Microfluidic Studies of Carbon Dioxide

Carbon dioxide (CO₂) sequestration, storage and recycling will greatly benefit from comprehensive studies of physical and chemical gas–liquid processes involving CO₂. Over the past five years, microfluidics emerged as a valuable tool in CO₂‐related research, due to superior mass and heat transfer, reduced axial dispersion, well‐defined gas–liquid interfacial areas and the ability to vary reagent concentrations in a high‐throughput manner. This Minireview highlights recent progress in microfluidic studies of CO₂‐related processes, including dissolution of CO₂ in physical solvents, CO₂ reactions, the utilization of CO₂ in materials science, and the use of supercritical CO₂ as a “green” solvent.     

Non-thermal plasma technology for the conversion of CO2

The conversion of carbon dioxide is vital if we are to avoid the catastrophic consequences that will result from further global temperature rise as a result of burning fossil fuels. Current techniques, such as catalytic conversion and biochemical processes, are each associated with their own drawbacks such as catalyst deactivation and high energy input. Plasma processes are gaining increasing interest as they have the potential to reduce a greater amount of atmospheric environmental pollutants at any one time due to an increased throughput, whilst using a smaller reactor with improved energy efficiency and near-zero emissions. Non-thermal plasma can dissociate stable molecules, such as CO₂, at temperatures as low as room temperature. It is this key feature which makes plasma conversion such a promising technology in the conversion and utilisation of CO₂. Furthermore, possible products from plasma processes include fuels and chemicals, such as methanol and syngas, which have a high market value; hence potentially making the process feasible on an industrial scale. This paper discusses recent advances in the use of plasma processes for carbon dioxide conversion, along with the future outlook of this technology and the impact these techniques could have on the chemical and energy industries.