Designing catalysts for functionalization of unactivated C-H bonds based on the CH activation reaction

Transition Metal Mimetic π-Activation by Cationic Bismuth(III) Catalysts for Allylic C-H Functionalization of Olefins Using C═O and C═N Electrophiles

The discovery and utilization of main-group element catalysts that behave similarly to transition metal (TM) complexes have become increasingly active areas of investigation in recent years. Here, we report a series of Lewis acidic bismuth(III) complexes that allow for the catalytic allylic C(sp3)-H functionalization of olefins via an organometallic complexation-assisted deprotonation mechanism to generate products containing new C-C bonds. This heretofore unexplored mode of main-group reactivity was applied to the regioselective functionalization of 1,4-dienes and allylbenzene substrates. Experimental and computational mechanistic studies support the key steps of the proposed catalytic cycle, including the intermediacy of elusive Bi-olefin complexes and allylbismuth species.

Potential of Molecular Catalysts with Electron-Rich Transition Metal Centers for Addressing Long-Standing Chemistry Enigmas

Molecular complexes with electron-rich metal centers are highlighted as potential catalysts for the following five important chemical transformations: selective conversion of methane to methanol, capture and utilization of carbon dioxide, fixation of molecular nitrogen, water splitting, and recycling of perfluorochemicals. Our initial focus lies on negatively charged metal centers and ligands that can stabilize anionic metal atoms. Catalysts with electron-rich metal atoms (CERMAs) can sustain catalytic cycles with a "ping-pong" mechanism, where one or more electrons are transferred from the metal center to the substrate and back. The donated electrons can activate the chemical bonds of the substrate by populating its antibonding orbitals. At the last step of the catalytic cycle, the electrons return to the metal and the product interacts only weakly with the formed anion, which enables the solvent molecules to remove the product fast from the catalytic cycle and prevent subsequent unfavorable reactions. This process resembles electrocatalysis, but the metal serves as both an anode and a cathode (molecular electrocatalysis). We also analyze the usage of CERMAs as the base of Frustrated Lewis pairs proposing a new type of bimetallic catalysts. This Featured Article aspires to initiate systematic experimental and theoretical studies on CERMAs and their reactivity, the potential of which has probably been underestimated in the literature.

On-Surface Synthesis of Non-Benzenoid Nanographenes Embedding Azulene and Stone-Wales Topologies

The incorporation of non-benzenoid motifs in graphene nanostructures significantly impacts their properties, making them attractive for applications in carbon-based electronics. However, understanding how specific non-benzenoid structures influence their properties remains limited, and further investigations are needed to fully comprehend their implications. Here, we report an on-surface synthetic strategy toward fabricating non-benzenoid nanographenes containing different combinations of pentagonal and heptagonal rings. Their structure and electronic properties were investigated via scanning tunneling microscopy and spectroscopy, complemented by computational investigations. After thermal activation of the precursor P on the Au(111) surface, we detected two major nanographene products. Nanographene Aa-a embeds two azulene units formed through oxidative ring-closure of methyl substituents, while Aa-s contains one azulene unit and one Stone-Wales defect, formed by the combination of oxidative ring-closure and skeletal ring-rearrangement reactions. Aa-a exhibits an antiferromagnetic ground state with the highest magnetic exchange coupling reported up to date for a non-benzenoid containing nanographene, coexisting with side-products with closed shell configurations resulted from the combination of ring-closure and ring-rearragement reactions (Ba-a , Ba-s , Bs-a and Bs-s ). Our results provide insights into the single gold atom assisted synthesis of novel NGs containing non-benzenoid motifs and their tailored electronic/magnetic properties.

Tuning light-driven oxidation of styrene inside water-soluble nanocages

Selective functionalization of innate sp2 C-H bonds under ambient conditions is a grand synthetic challenge in organic chemistry. Here we combine host-guest charge transfer-based photoredox chemistry with supramolecular nano-confinement to achieve selective carbonylation of styrene by tuning the dioxygen concentration. We observe exclusive photocatalytic formation of benzaldehyde under excess O2 (>1 atm) while Markovnikov addition of water produced acetophenone in deoxygenated condition upon photoexcitation of confined styrene molecules inside a water-soluble cationic nanocage. Further by careful tuning of the nanocage size, electronics, and guest preorganization, we demonstrate rate enhancement of benzaldehyde formation and a complete switchover to the anti-Markovnikov product, 2-phenylethan-1-ol, in the absence of O2. Raman spectroscopy, 2D 1H-1H NMR correlation experiments, and transient absorption spectroscopy establish that the site-selective control on the confined photoredox chemistry originates from an optimal preorganization of styrene molecules inside the cavity. We envision that the demonstrated host-guest charge transfer photoredox paradigm in combination with green atom-transfer reagents will enable a broad range of sp2 carbon-site functionalization.

Combining Donor Strength and Oxidative Stability in Scorpionates: A Strongly Donating Fluorinated Mesoionic Tris(imidazol-5-ylidene)borate Ligand

Strongly donating scorpionate ligands support the study of high-valent transition metal chemistry; however, their use is frequently limited by oxidative degradation. To address this concern, we report the synthesis of a tris(imidazol-5-ylidene)borate ligand featuring trifluoromethyl groups surrounding its coordination pocket. This ligand represents the first example of a chelating poly(imidazol-5-ylidene) mesoionic carbene ligand, a scaffold that is expected to be extremely donating. The {NiNO}10 complex of this ligand, as well as that of a previously reported strongly donating tris(imidazol-2-ylidene)borate, has been synthesized and characterized. This new ligand's strong donor properties, as measured by the υNO of its {NiNO}10 complex and natural bonding orbital second-order perturbative energy analysis, are at par with those of the well-studied alkyl-substituted tris(imidazol-2-ylidene)borates, which are known to effectively stabilize high-valent intermediates. The good donor properties of this ligand, despite the electron-withdrawing trifluoromethyl substituents, arise from the strongly donating imidazol-5-ylidene mesoionic carbene arms. These donor properties, when combined with the robustness of trifluoromethyl groups toward oxidative decomposition, suggest this ligand scaffold will be a useful platform in the study of oxidizing high-valent transition-metal species.

Visible light-mediated C (sp3)-H bond functionalization inside an all-organic nanocavity

Ultrafast C-H bond activation and functionalization in confinement using visible light will enable engineering chemical reactions with extraordinary speed and selectivity. To provide a transition metal-free route, here we demonstrate C-H bond activation reactions on poly-aromatic hydrocarbons (PAH) in all-organic cationic nanocage ExBox4+ for the first time. Visible light excitation in the host-guest charge transfer (CT) state allows the formation of oxidized photoproducts with high selectivity. Mechanistic understanding of this CT-mediated photoreaction using femtosecond broadband transient absorption revealed a few ∼100 ps timescale for C-H bond breaking on the attached -CH3 group via sequential electron transfer and proton transfer steps. We envision that our photosensitizer-free method will open up new avenues to pursue organic reactions using cavities that could serve both as photoredox catalysts and hosts for reactive reaction intermediates.

Improving a Methane C-H Activation Complex by Metal and Ligand Alterations from Computational Results

We present results for a series of complexes derived from a titanium complex capable of activating C-H bonds under mild conditions (PNP)Ti═CHtBu(CH2tBu), where PNP = N[2-PiPr2-4-methylphenyl]2-. In addition to the initial activation of methane, a tautomerization reaction to a terminal methylidene is also explored due to methylidene's potential use as a synthetic starting point. Analogous complexes with other low-cost 3d transition metals were studied, such as scandium, titanium, vanadium, and chromium as both isoelectronic and isocharged complexes. Our results predict that V(IV) and V(V) complexes are promising for methane C-H bond activation. The V(V) complex has a low rate-determining barrier for methane activation, specifically 16.6 kcal/mol, which is approximately 12 kcal/mol less than that for the Ti complex, as well as having a moderate tautomerization barrier of 29.8 kcal/mol, while the V(IV) complex has a methane activation barrier of 19.0 kcal/mol and a tautomerization barrier of 31.1 kcal/mol. Scandium and chromium complexes are much poorer for C-H bond activation; scandium has very high barriers, while chromium strongly overstabilizes the alkylidene intermediate, potentially stopping the further reaction. In addition to the original PNP ligand, some of the most promising ligands from a previous work were tested, although (as shown previously) modification of the ligand does not typically have large effects on the activity of the system. Our best ligand modification improves the performance of the V(V) complex via the substitution of the nitrogen in PNP by phosphorus, which reduces the tautomerization barrier by 5 to 24.4 kcal/mol.

An Electrochemical Approach to Directed Fluorination

Electrosynthesis has made a revival in the field of organic chemistry and, in particular, radical-mediated reactions. Herein, we report a simple directed, electrochemical C-H fluorination method. Employing a dabconium mediator, commercially available Selectfluor, and RVC electrodes, we provide a range of steroid-based substrates with competent regioselective directing groups, including enones, ketones, and hydroxy groups, as well as never reported before lactams, imides, lactones, and esters.

Transition metal oxide complexes as molecular catalysts for selective methane to methanol transformation: any prospects or time to retire?

Transition metal oxides have been extensively used in the literature for the conversion of methane to methanol. Despite the progress made over the past decades, no method with satisfactory performance or economic viability has been detected. The main bottleneck is that the produced methanol oxidizes further due to its weaker C-H bond than that of methane. Every improvement in the efficiency of a catalyst to activate methane leads to reduction of the selectivity towards methanol. Is it therefore prudent to keep studying (both theoretically and experimentally) metal oxides as catalysts for the quantitative conversion of methane to methanol? This perspective focuses on molecular metal oxide complexes and suggests strategies to bypass the current bottlenecks with higher weight on the computational chemistry side. We first discuss the electronic structure of metal oxides, followed by assessing the role of the ligands in the reactivity of the catalysts. For better selectivity, we propose that metal oxide anionic complexes should be explored further, while hydrophylic cavities in the vicinity of the metal oxide can perturb the transition-state structure for methanol increasing appreciably the activation barrier for methanol. We also emphasize that computational studies should target the activation reaction of methanol (and not only methane), the study of complete catalytic cycles (including the recombination and oxidation steps), and the use of molecular oxygen as an oxidant. The titled chemical conversion is an excellent challenge for theory and we believe that computational studies should lead the field in the future. It is finally shown that bottom-up approaches offer a systematic way for exploration of the chemical space and should still be applied in parallel with the recently popular machine learning techniques. To answer the question of the title, we believe that metal oxides should still be considered provided that we change our focus and perform more systematic investigations on the activation of methanol.

Trinuclear and Cyclometallated Organometallic Dinuclear Pt-Pyrazolato Complexes: A Combined Experimental and Theoretical Study

Two differently substituted pyrazole ligands have been investigated with regard to the topology of their Pt complexes: upon deprotonation, two mononuclear 1:2 PtII-pyrazole complexes—one of the sterically unhindered 4-Me-pzH and one of the bulky 3,5-tBu-pzH (pzH = pyrazole)—yield the corresponding 1:2 PtII-pyrazolato species; the former a triangular, trinuclear metallacycle (1), and the latter a dinuclear, half-lantern species (2) formed via the unprecedented cyclometallation of a butyl group. Stoichiometric oxidation of the colorless PtII2 complex produces the deep-blue, metal–metal bonded PtIII2 analog (3) with a rarely encountered unsymmetrical coordination across the Pt-Pt bond. All three complexes have been characterized by single crystal X-ray structure determination, 1H-NMR, IR, and UV-vis-NIR spectroscopic methods. The XPS spectra of the PtII2 and PtIII2 species are also reported. Density functional theory calculations were carried out to investigate the electronic structure, spectroscopic properties, and chemical bonding of the new complexes. The calculated natural population analysis charges and Wiberg bonding indices indicate a weak σ-interaction in the case of 2 and a formal Pt-Pt single bond in 3.

Electrochemical organic reactions: A tutorial review

Although the core of electrochemistry involves simple oxidation and reduction reactions, it can be complicated in real electrochemical organic reactions. The principles used in electrochemical reactions have been derived using physical organic chemistry, which drives other organic/inorganic reactions. This review mainly comprises two themes: the first discusses the factors that help optimize an electrochemical reaction, including electrodes, supporting electrolytes, and electrochemical cell design, and the second outlines studies conducted in the field over a period of 10 years. Electrochemical reactions can be used as a versatile tool for synthetically important reactions by modifying the constant electrolysis current.

Effects of Cu Species on Liquid-Phase Partial Oxidation of Methane with H2O2 over Cu-Fe/ZSM-5 Catalysts

In this study, a Cu-promoted Fe/ZSM-5 catalyst was examined to reveal the effects of Cu species in selective oxidation of methane into methane oxygenates using H2O2 in water. Cu/ZSM-5, Cu-Fe/ZSM-5, and Fe/ZSM-5 catalysts were prepared using wet impregnation, solid-state ion exchange, and ion-exchange methods. Various techniques, including nitrogen physisorption, temperature-programmed reduction with H2, UV-vis spectroscopy, and FT-IR spectroscopy after NO adsorption, were utilized to characterize the catalysts. The promotional effect of Cu on the Cu-Fe/ZSM-5 catalyst in terms of methanol selectivity was confirmed. The preparation method has a considerable influence on the catalyst performance, and the ion-exchange method is the most effective. However, leaching of the Cu species was observed during this reaction, which can affect the quantification of formic acid by 1H-NMR. The homogeneous Cu species increase hydrogen peroxide decomposition and CO2 selectivity, which is undesirable for this reaction.

Experimental Demonstration and Density Functional Theory Mechanistic Analysis of Arene C-H Bond Oxidation and Product Protection by Osmium Tetroxide in a Strongly Basic/Nucleophilic Solvent

Reactions that result in the oxy-functionalization of sp² C-H bonds to give phenols are relatively rare. Here we report experiments and density functional theory (DFT) calculations that demonstrate selective C-H bond hydroxylation of nitroarenes to their corresponding mono-phenoxide as the exclusive product using OsO₄ in a highly basic solvent mixture of water, hydroxide, and pyridine. DFT calculations using a mixed explicit/continuum solvent approach indicate that there is likely a mixture of OsO₄-hydroxide/pyridine ground-state structures that have competitive reactivity and that the mechanism involves the nucleophilic addition of an anionic metal-oxo species to the arene followed by a hydride transfer process that is different from the standard [3 + 2] mechanism often invoked for the OsO₄ oxidation of σ and π bonds. This work demonstrates the utility of using a strongly basic solvent for C-H bond oxidation reactions as this effectively converts any reactive phenolic product into the corresponding phenoxide, which is protected and essentially inert to further oxidation by the nucleophilic metal-oxo species.

Being negative can be positive: metal oxide anions promise more selective methane to methanol conversion

Computational studies are performed to show that metal oxide anionic complexes promote the CH₄ + N₂O → CH₃OH + N₂ reaction with low activation barriers for the C-H activation and the formation of the CH₃-OH bond. The energy for the release of the produced methanol is minimal, reducing the residence time of methanol around the catalytic center and preventing its overoxidation.

Gas-Phase Selective Oxidation of Methane into Methane Oxygenates

Methane is an abundant resource and its direct conversion into value-added chemicals has been an attractive subject for its efficient utilization. This method can be more efficient than the present energy-intensive indirect conversion of methane via syngas, a mixture of CO and H2. Among the various approaches for direct methane conversion, the selective oxidation of methane into methane oxygenates (e.g., methanol and formaldehyde) is particularly promising because it can proceed at low temperatures. Nevertheless, due to low product yields this method is challenging. Compared with the liquid-phase partial oxidation of methane, which frequently demands for strong oxidizing agents in protic solvents, gas-phase selective methane oxidation has some merits, such as the possibility of using oxygen as an oxidant and the ease of scale-up owing to the use of heterogeneous catalysts. Herein, we summarize recent advances in the gas-phase partial oxidation of methane into methane oxygenates, focusing mainly on its conversion into formaldehyde and methanol.

Recent progress in rare-earth metal-catalyzed sp2 and sp3 C–H functionalization to construct C–C and C–heteroelement bonds

The review presents rare-earth metal-catalyzed C(sp2/sp3)–H functionalization accessing C–C/C–heteroatom bonds and olefin (co)polymerization, highlighting substrate scope, mechanistic realization, and origin of site-, enantio-/diastereo-selectivity.

Theory and Experiment Demonstrate that Sb(V)-Promoted Methane C-H Activation and Functionalization Outcompete Superacid Protonolysis in Sulfuric Acid

Sb(V) in strong Brønsted acid solvents is traditionally assumed to react with light alkanes through superacid protonolysis, which results in carbocation intermediates, H₂, and carbon oligomerization. In contrast to this general assumption, our density functional theory (DFT) calculations revealed an accessible barrier for C-H activation between methane and Sb(V) in sulfuric acid that could potentially outcompete superacid protonolysis. This prompted us to experimentally examine this reaction in sulfuric acid with oleum, which has never been reported because of presumed superacid reactivity. Reaction of methane at 180 °C for 3 h resulted in very high yields of methyl bisulfate without significant overoxidation. Our DFT calculations show that a C-H activation and Sb-Me bond functionalization mechanism to give methyl bisulfate outcompetes methane protonolysis and many other possible reaction mechanisms, such as electron transfer, proton-coupled electron transfer, and hydride abstraction. Our DFT calculations also explain experimental hydrogen-deuterium exchange studies and the absence of methane carbo-functionalization/oligomerization products. Overall, this work demonstrates that in very strong Brønsted acid solvent, Sb(V) can induce innersphere reaction mechanisms akin to transition metals and outcompete superacid reactivity.

Metal Nanoparticles as Sustainable Tools for C-N Bond Formation via C-H Activation

The design of highly active metal nanoparticles to be employed as efficient heterogeneous catalysts is a key tool for the construction of complex organic molecules and the minimization of their environmental costs. The formation of novel C-N bonds via C-H activation is an effective atom-economical strategy to access high value materials in pharmaceuticals, polymers, and natural product production. In this contribution, the literature of the last ten years on the use of metal nanoparticles in the processes involving direct C-N bond formation will be discussed. Where possible, a discussion on the role and influence of the support used for the immobilization and/or the metal chosen is reported. Particular attention was given to the description of the experiments performed to elucidate the active mechanism.

Electrochemical activation of C-H by electron-deficient W2C nanocrystals for simultaneous alkoxylation and hydrogen evolution

The activation of C-H bonds is a central challenge in organic chemistry and usually a key step for the retro-synthesis of functional natural products due to the high chemical stability of C-H bonds. Electrochemical methods are a powerful alternative for C-H activation, but this approach usually requires high overpotential and homogeneous mediators. Here, we design electron-deficient W2C nanocrystal-based electrodes to boost the heterogeneous activation of C-H bonds under mild conditions via an additive-free, purely heterogeneous electrocatalytic strategy. The electron density of W2C nanocrystals is tuned by constructing Schottky heterojunctions with nitrogen-doped carbon support to facilitate the preadsorption and activation of benzylic C-H bonds of ethylbenzene on the W2C surface, enabling a high turnover frequency (18.8 h-1) at a comparably low work potential (2 V versus SCE). The pronounced electron deficiency of the W2C nanocatalysts substantially facilitates the direct deprotonation process to ensure electrode durability without self-oxidation. The efficient oxidation process also boosts the balancing hydrogen production from as-formed protons on the cathode by a factor of 10 compared to an inert reference electrode. The whole process meets the requirements of atomic economy and electric energy utilization in terms of sustainable chemical synthesis.

Fe-Catalysed Coupling Reactions Between Alkynes and Alcohols

In this review, we discussed about the synthetic developments in the field of Fe-catalysis for the formation of - bonds through the coupling of alkynes and alcohols, for the period of 13 years (2008-2020). These strategies fulfil important Green Chemistry principles, as they are highly atom economic (up to 100 %), no toxic by-products (only water), employs highly abundant and low toxic alcohols (no need for any pre-functionalization hence step economy) and cheaper Fe-catalysts. Having these advantages, one can predict that in the coming years a large number of fascinating new iron-catalyzed reactions will be developed for the organic synthesis. We hope that this review article will be highly useful for the synthetic community to design and develop new Fe-catalyzed coupling reactions and keeps the content in the right prospect.

C-H bond activation in light alkanes: a theoretical perspective

Alkanes are the major constituents of natural gas and crude oil, the feedstocks for the chemical industry. The efficient and selective activation of C-H bonds can convert abundant and low-cost hydrocarbon feedstocks into value-added products. Due to the increasing global demand for light alkenes and their corresponding polymers as well as synthesis gas and hydrogen production, C-H bond activation of light alkanes has attracted widespread attention. A theoretical understanding of C-H bond activation in light hydrocarbons via density functional theory (DFT) and microkinetic modeling provides a feasible approach to gain insight into the process and guidelines for designing more efficient catalysts to promote light alkane transformation. This review describes the recent progress in computational catalysis that has addressed the C-H bond activation of light alkanes. We start with direct and oxidative C-H bond activation of methane, with emphasis placed on kinetic and mechanistic insights obtained from DFT assisted microkinetic analysis into steam and dry reforming, and the partial oxidation dependence on metal/oxide surfaces and nanoparticle size. Direct and oxidative activation of the C-H bond of ethane and propane on various metal and oxide surfaces are subsequently reviewed, including the elucidation of active sites, intriguing mechanisms, microkinetic modeling, and electronic features of the ethane and propane conversion processes with a focus on suppressing the side reaction and coke formation. The main target of this review is to give fundamental insight into C-H bond activation of light alkanes, which can provide useful guidance for the optimization of catalysts in future research.

Single-Atom Catalysts: A Sustainable Pathway for the Advanced Catalytic Applications

A heterogeneous catalyst is a backbone of modern sustainable green industries; and understanding the relationship between its structure and properties is the key for its advancement. Recently, many upscaling synthesis strategies for the development of a variety of respectable control atomically precise heterogeneous catalysts are reported and explored for various important applications in catalysis for energy and environmental remediation. Precise atomic-scale control of catalysts has allowed to significantly increase activity, selectivity, and in some cases stability. This approach has proved to be relevant in various energy and environmental related technologies such as fuel cell, chemical reactors for organic synthesis, and environmental remediation. Therefore, this review aims to critically analyze the recent progress on single-atom catalysts (SACs) application in oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and chemical and/or electrochemical organic transformations. Finally, opportunities that may open up in the future are summarized, along with suggesting new applications for possible exploitation of SACs.

Methane to Methanol Conversion Facilitated by Anionic Transition Metal Centers: The Case of Fe, Ni, Pd, and Pt

Density functional theory and high-level ab initio electronic structure calculations are performed to study the mechanism of the partial oxidation of methane to methanol facilitated by the titled anionic transition metal atoms. The energy landscape for the overall reaction M^- + N₂O + CH₄ → M^- + N₂ + CH₃OH (M = Fe, Ni, Pd, Pt) is constructed for different reaction pathways for all four metals. The comparison with earlier experimental and theoretical results for cationic centers demonstrates the better performance of the metal anions. The main advantage is that anionic centers interact weakly with the produced methanol. This fact facilitates the fast removal of methanol from the catalytic center and prevents the overoxidation of methane. Moreover, a moderate or high energy barrier for the M^- + CH₄ → HMCH₃^- reaction step is observed, which protects the metal center from deactivation. Future work should focus on the identification of proper ligands, which stabilize the negative charge on the metal (electronic factors) and prevent the formation of the global CH₃MOH^- minimum (steric factors). Finally, a composite electronic structure method (combining size extensive coupled clusters approaches and accurate multireference configuration interaction) is proposed for computationally demanding systems and is applied to Fe^-.

Reductive C-C Coupling from Molecular Au(I) Hydrocarbyl Complexes: A Mechanistic Study

Organometallic gold complexes are used in a range of catalytic reactions, and they often serve as catalyst precursors that mediate C-C bond formation. In this study, we investigate C-C coupling to form ethane from various phosphine-ligated gem-digold(I) methyl complexes including [Au2(μ-CH3)(PMe2Ar')2][NTf2], [Au2(μ-CH3)(XPhos)2][NTf2], and [Au2(μ-CH3)(tBuXPhos)2][NTf2] {Ar' = C6H3-2,6-(C6H3-2,6-Me)2, C6H3-2,6-(C6H2-2,4,6-Me)2, C6H3-2,6-(C6H3-2,6-iPr)2, or C6H3-2,6-(C6H2-2,4,6-iPr)2; XPhos = 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl; tBuXPhos = 2-di-tert-butylphosphino-2',4',6'-triisopropylbiphenyl; NTf2 = bis(trifluoromethyl sulfonylimide)}. The gem-digold methyl complexes are synthesized through reaction between Au(CH3)L and Au(L)(NTf2) {L = phosphines listed above}. For [Au2(μ-CH3)(XPhos)2][NTf2] and [Au2(μ-CH3)(tBuXPhos)2][NTf2], solid-state X-ray structures have been elucidated. The rate of ethane formation from [Au2(μ-CH3)(PMe2Ar')2][NTf2] increases as the steric bulk of the phosphine substituent Ar' decreases. Monitoring the rate of ethane elimination reactions by multinuclear NMR spectroscopy provides evidence for a second-order dependence on the gem-digold methyl complexes. Using experimental and computational evidence, it is proposed that the mechanism of C-C coupling likely involves (1) cleavage of [Au2(μ-CH3)(PMe2Ar')2][NTf2] to form Au(PR2Ar')(NTf2) and Au(CH3)(PMe2Ar'), (2) phosphine migration from a second equivalent of [Au2(μ-CH3)(PMe2Ar')2][NTf2] aided by binding of the Lewis acidic [Au(PMe2Ar')]+, formed in step 1, to produce [Au2(CH3)(PMe2Ar')][NTf2] and [Au2(PMe2Ar')]+, and (3) recombination of [Au2(CH3)(PMe2Ar')][NTf2] and Au(CH3)(PMe2Ar') to eliminate ethane.

Synthesis of N-alkoxyphthalimide derivatives via PIDA-promoted cross dehydrogenative coupling reaction

A PIDA-promoted cross-dehydrogenative coupling reaction between N-hydroxyphthalimide (NHPI) and aryl ketones for efficient synthesis of N-alkoxyphthalimide products in moderate to good yields has been described. This methodology is distinguished by catalyst-free conditions, readily available starting materials, wide substrate scope and operational simplicity. In addition, a gram-scale reaction and synthetic transformation of the product into synthetically useful intermediates has been demonstrated.

A cross dehydrogenative coupling reaction of aryl ketones with N-hydroxyphthalimide was realized. The reactions afforded a clean and facile access to diverse N-alkoxyphthalimide derivatives in high yields (up to 99%).

Deep eutectic solvents: alternative reaction media for organic oxidation reactions

Deep eutectic solvents (DESs) are a valid alternative to common organic solvents and ionic liquids (ILs) as solvent media for organic oxidation reactions.

Catalytic Oxidation of Methane to Oxygenated Products: Recent Advancements and Prospects for Electrocatalytic and Photocatalytic Conversion at Low Temperatures

Methane is an important fossil fuel and widely available on the earth's crust. It is a greenhouse gas that has more severe warming effect than CO2. Unfortunately, the emission of methane into the atmosphere has long been ignored and considered as a trivial matter. Therefore, emphatic effort must be put into decreasing the concentration of methane in the atmosphere of the earth. At the same time, the conversion of less valuable methane into value-added chemicals is of significant importance in the chemical and pharmaceutical industries. Although, the transformation of methane to valuable chemicals and fuels is considered the "holy grail," the low intrinsic reactivity of its C-H bonds is still a major challenge. This review discusses the advancements in the electrocatalytic and photocatalytic oxidation of methane at low temperatures with products containing oxygen atom(s). Additionally, the future research direction is noted that may be adopted for methane oxidation via electrocatalysis and photocatalysis at low temperatures.

Progress and prospects in copper-catalyzed C-H functionalization

Copper-catalyzed C-H functionalization is becoming a significant area in organic chemistry. Copper is now widely used as a catalyst in organic synthesis as it is inexpensive and not very toxic. Functionalization of C-H bonds to construct wide varieties of organic compounds has received much attention in recent times. This review focuses on the recent advances in Cu-catalyzed C-H functionalization and covers literature from 2018-2020.

Direct functionalization of C-H bonds by electrophilic anions

Alkanes and [B₁₂X₁₂]^2- (X = Cl, Br) are both stable compounds which are difficult to functionalize. Here we demonstrate the formation of a boron-carbon bond between these substances in a two-step process. Fragmentation of [B₁₂X₁₂]^2- in the gas phase generates highly reactive [B₁₂X₁₁]^- ions which spontaneously react with alkanes. The reaction mechanism was investigated using tandem mass spectrometry and gas-phase vibrational spectroscopy combined with electronic structure calculations. [B₁₂X₁₁]^- reacts by an electrophilic substitution of a proton in an alkane resulting in a B-C bond formation. The product is a dianionic [B₁₂X₁₁C_nH_2n+1]^2- species, to which H⁺ is electrostatically bound. High-flux ion soft landing was performed to codeposit [B₁₂X₁₁]^- and complex organic molecules (phthalates) in thin layers on surfaces. Molecular structure analysis of the product films revealed that C-H functionalization by [B₁₂X₁₁]^- occurred in the presence of other more reactive functional groups. This observation demonstrates the utility of highly reactive fragment ions for selective bond formation processes and may pave the way for the use of gas-phase ion chemistry for the generation of complex molecular structures in the condensed phase.

Balancing the decomposable behavior and wet tensile mechanical property of cellulose-based wet wipe substrates by the aqueous adhesive

With the current global outbreak of novel coronaviruses, the fabrication of decomposable wet wipe with sufficient wet strength to meet daily use is promising but still challenging, especially when renewable cellulose was employed. In this work, a decomposable cellulose-based wet wipe substrate is demonstrated by introducing a synthetic N-vinyl pyrrolidone-glycidyl methacrylate (NVP-GMA) adhesive on the cellulose surface. Experimental results reveal that the NVP-GMA adhesive not only significantly facilitates the chemical bonding between cellulose fibers in the wet state, but also increase the surface wettability and water retention. The as-fabricated cellulose-based wet wipe substrate displays a superb water retention capacity of 1.9 times, an excellent water absorption capacity (completely wetted with 0° water contact angle), and a perfect wet tensile index of 3.32 N.m.g⁻¹. It is far better than state-of-the-art wet toilet wipe on the market (non-woven). The prepared renewable and degradable cellulose-based substrate with excellent mechanical strength has potential application prospects in diverse commercially available products such as sanitary and medical wet wipes.

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A decomposable wet wipe substrate was prepared from the bio-based materials.

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Synthetic adhesive enhanced the wet strength of the cellulose sheet.

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Enhancement of cellulose-based material was achieved under aqueous conditions.

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As-prepared cellulose substrate balanced the dispersibility and wet strength.

Photocatalytic deaminative benzylation and alkylation of tetrahydroisoquinolines with N-alkylpyrydinium salts

A ruthenium-catalyzed photoredox coupling of substituted N-aryltetrahydroisoquinolines (THIQs) and different bench-stable pyridinium salts was successfully developed to give fast access to 1-benzyl-THIQs. Furthermore, secondary alkyl and allyl groups were also successfully introduced via the same method. Additionally, the typically applied N-phenyl group in the THIQ substrate could be replaced by the cleavable p-methoxyphenyl (PMP) group and successful N-deprotection was demonstrated.

Recent advances in the synthesis of bridgehead (or ring-junction) nitrogen heterocycles via transition metal-catalyzed C–H bond activation and functionalization

An update on recent advances in the synthesis of bridgehead nitrogen fused heterocycles via transition metal-catalyzed C–H activation and functionalization is reported.

The synthesis of biphenyl through C–H bond activation in benzene over a Pd catalyst supported on graphene oxide

This is the first report on carbon–hydrogen (C–H) bond activation in benzene over a palladium catalyst supported on graphene oxide (GO) leading to the sole formation of biphenyl with a yield of 78%.

Rhodium(iii)-catalyzed synthesis of spirocyclic isoindole N-oxides and isobenzofuranones via C–H activation and spiroannulation

Rhodium(iii)-catalyzed C–H activation of oximes and benzoic acids has been realized in oxidative annulation with quinone diazides for synthesis of spirocycles.