Carbon-oxygen Bond Cleavage Research Articles

Electrocatalytic Reduction of CO2 to CO by Molecular Cobalt-Polypyridine Diamine Complexes.

Cobalt complexes have previously been reported to exhibit high faradaic efficiency in reducing CO2 to CO. Herein, we synthesized capsule-like cobalt-polypyridine diamine complexes [Co(L1)](BF4)2 (1) and [Co(L2) (CH3CN)](BF4)2 (2) as catalysts for the electrocatalytic reduction of CO2. Under catalytic conditions, complexes 1 and 2 demonstrated the electrocatalytic reduction of CO2 to CO in the presence or absence of CH3OH as a proton source. Experimental and computational studies revealed that complexes 1 and 2 undergo two consecutive reversible one-electron reductions on the cobalt core, followed by the addition of CO2 to form a metallocarboxylate intermediate [CoII(L)-CO22-]0. This crucial reaction intermediate, which governs the catalytic cycle, was successfully detected using high resolution mass spectrometry (HRMS). In situ Fourier-transform infrared spectrometer (FTIR) analysis showed that methanol can enhance the rate of carbon-oxygen bond cleavage of the metallocarboxylate intermediate. DFT studies on [CoII(L)-CO22-]0 have suggested that the doubly reduced species attacks CO2 on the C atom through the dz2 orbital, while the interaction with CO2 is further stabilized by the π interaction between the metal dxz or dxz orbital with p orbitals on the O atoms. Further reductions generate a metal carbonyl intermediate [CoI(L)-CO]+, which ultimately releases CO.

Controlled degradation of chemically stable poly(aryl ethers) via directing group-assisted catalysis.

To establish a sustainable society, the development of polymer materials capable of reverting into monomers on demand is crucial. Traditional methods rely on breaking labile bonds such as esters in the main chain, which limits applicability to polymers that consist of robust covalent bonds. We found that the integration of directing groups allowed the engineering of resilient polymers with built-in recyclability. Our study showcases phenylene ether-based polymers fortified with directing groups, which can be selectively disassembled under nickel catalysts via selective cleavage of carbon-oxygen bonds. Notably, these polymers exhibit exceptional chemical stability towards acids, bases, and oxidizing agents, while being degradable to well-defined, repolymerizable molecules in the presence of a catalyst. Our findings allow for the development of next-generation polymer materials that are chemically recyclable by design.

H2O˙+ and OH+ reactivity versus furan: experimental low energy absolute cross sections for modeling radiation damage.

Radiotherapy is one of the most widespread and efficient strategies to fight malignant tumors. Despite its broad application, the mechanisms of radiation-DNA interaction are still under investigation. Theoretical models to predict the effects of a particular delivered dose are still in their infancy due to the difficulty of simulating a real cell environment, as well as the inclusion of a large variety of secondary processes. This work reports the first experimental study of the ion-molecule reactions of the H2O˙+ and OH+ ions, produced by photoionization with synchrotron radiation, with a furan (c-C4H4O) molecule, a template for deoxyribose sugar in DNA. The present experiments, performed as a function of the collision energy of the ions and the tunable photoionization energy, provide key parameters for the theoretical modelling of the effect of radiation dose, like the absolute cross sections for producing protonated furan (furanH+) and a radical cation (furan˙+), the most abundant products, which can amount up to 200 Å2 at very low collision energies (<1.0 eV). The experimental results show that furanH+ is more fragile, indicating how the protonation of the sugar component of the DNA may favor its dissociation with possible major radiosensitizing effects. Moreover, the ring opening of furanH+ isomers and the potential energy surface of the most important fragmentation channels have been explored by molecular dynamics simulations and quantum chemistry calculations. The results show that, in the most stable isomer of furanH+, the ring opening occurs via a low energy pathway with carbon-oxygen bond cleavage, followed by the loss of neutral carbon monoxide and the formation of the allyl cation CH2CHCH2+, which instead is not observed in the fragmentation of furan˙+. At higher energies the ring opening through the carbon-carbon bond is accompanied by the loss of formaldehyde, producing HCCCH2+, the most intense fragment ion detected in the experiments. This work highlights the importance of the secondary processes, like the ion-molecule reactions at low energies in the radiation damage due to their very large cross sections, and it aims to provide benchmark data for the development of suitable models to approach this low collision energy range.

Mechanistic Differences between Electrochemical Hydrogenation and Hydrogenolysis of 5-Hydroxymethylfurfural and Their pH Dependence.

Hydrogenation and hydrogenolysis are two important reactions for electrochemical reductive valorization of biomass‐derived oxygenates such as 5‐hydroxymethylfurfural (HMF). In general, hydrogenolysis (which combines hydrogenation and deoxygenation) is more challenging than hydrogenation (which does not involve the cleavage of carbon–oxygen bonds). Thus, identifying factors and conditions that can promote hydrogenolysis is of great interest for reductive valorization of biomass‐derived oxygenates. For the electrochemical reduction of HMF and its derivatives, it is known that aldehyde hydrogenation is not a part of aldehyde hydrogenolysis but rather a competing reaction; however, no atomic‐level understanding is currently available to explain their electrochemical mechanistic differences. In this study, combined experimental and computational investigations were performed using Cu electrodes to elucidate the key mechanistic differences between electrochemical hydrogenation and hydrogenolysis of HMF. The results revealed that hydrogenation and hydrogenolysis of HMF involve the formation of different surface‐adsorbed intermediates via different reduction mechanisms and that lowering the pH promoted the formation of the intermediates required for aldehyde and alcohol hydrogenolysis. This study for the first time explains the origins of the experimentally observed pH‐dependent selectivities for hydrogenation and hydrogenolysis and offers a new mechanistic foundation upon which rational strategies to control electrochemical hydrogenation and hydrogenolysis can be developed.

GAS PHASE DECOMPOSITION OF T-BUTYL METHYL ETHER CATALYZED BY DIFFERENT HYDROGEN HALIDES: A DFT STUDY

The gas phase decomposition of t-butyl methyl ether catalyzed by hydrogen halides is studied. Four different hydrogen halides (fluorine, chlorine, bromine, and iodine) were evaluated to determine the electronic influence of the halogen in the reaction mechanism. To describe the mechanism, the ωB97XD/LANL2DZ level of theory was used. The reactivity order found was F&lt;Cl&lt;Br&lt;I. Interestingly, the activation free energy estimated for the HCl model was 133.9 kJ/mol, which is in good agreement with the experimental one (134.3 kJ/mol). Furthermore, a correlation above 0.804 was noticed when the electronegativity, the hydrogen-halide distance, and the pKa of the hydrogen halides were compared to the thermodynamic parameters (activation free energy, enthalpy, and entropy). Analyzing the mechanism in depth through the intrinsic reaction coordinate, reaction force, and reaction electronic flux plots, it was observed that though the reaction occurs in a one-step concerted way. The mechanism could be divided into two events; the first one composed by a proton transfer from the halide to the oxygen and the carbon-oxygen bond cleavage, and the second one being the rate-limiting event which includes the proton transfer from the t-butyl to the halide and the double bond formation.&#x0D;

Generation of microplastic particles during degradation of polycarbonate films in various aqueous media and their characterization

A 250-day batch experiment was conducted to examine the generation of microplastic particles from degradation of polycarbonate films in 3 aqueous media of environmental relevance. The microplastic particles generated from the experiment were characterized by SEM/EDS and micro-FTIR analysis. Hydrolysis was responsible for the cleavage of carbon-oxygen bonds in the carbonate group of polycarbonate backbone and detachment of micro-sized plastic particles from the PC film surfaces. The deionized water treatment had the highest concentration of total organic carbon and the greatest number of microplastic particles among the three treatments. Either elevated acidity or the presence of hydroxyl radical did not enhance the hydrolytic degradation of the PC films and generation of microplastic particles though hydroxyl radical caused oxidative degradation of polycarbonate by attacking the organic group but not the carbonate group. Bisphenol A was not detected in any of the treatments. The microplastic particles generated from the current experiment were highly irregular, which may have different physicochemical and toxic behaviours from the spheric synthetic ones that were frequently used in toxicity experiments.

Efficient nickel(II) immobilized on EDTA‐modified Fe3O4@SiO2 nanospheres as a novel nanocatalyst for amination of heteroaryl carbamates and sulfamates through the cleavage of C-O bond

In this report, there was immobilized Nickel(II) on EDTA‐modified Fe3O4@SiO2 nanospheres and catalytic activity of which was described in the arylation reaction of nitrogen nucleophiles via carbon-oxygen bond cleavage of (hetero)aryl carbamates and sulfamates. This protocol was applied to various nitrogen nucleophiles such as amines, anilines and N-heterocyclic compounds (pyrroles, indoles and imidazoles) in good to excellent yields. This reaction was promoted without the use of any external ligands under simple and mild conditions. The synthesized catalyst was well characterized by FT‐IR, XRD, TEM, FE-SEM, TGA, DLS,XPS, VSM, EDX, ICP and nitrogen adsorption-desorption isotherm analysis. The recycling studies revealed that the catalyst could be easily recovered by used the external magnetic field and directly reused for at least 7 times without the significant decrease in its catalytic activity.

In Situ Generation of Ruthenium Carbonyl Phosphine Complexes as a Versatile Method for the Development of Enantioselective C-O Bond Arylation.

We report here a method for in situ generation of various ruthenium carbonyl phosphine catalysts for arylation via cleavage of inert aromatic carbon-oxygen bonds. The use of catalyst systems consisting of [RuCl2 (CO)(p-cymene)], CsF, styrene, and phosphines enabled facile screening of phosphine ligands. Asymmetric C-O arylation was also achieved for atropo-enantioselective biaryl synthesis using a chiral monodentate phosphine ligand.

Molecular Nitrogen as an Extraordinary Accelerator for Hydrodeoxygenation

Molecular nitrogen (N2) has been long believed to be an inert gas. Recently in Nature Catalysis, Duan et al. reported that N2 could dramatically promote the catalytic performance of hydrodeoxygenation over ruthenium-based catalysts and that as-formed hydrogenated nitrogen species contributed to the reduction of the activation energy required for carbon-oxygen bond cleavage inside p-cresol.

Regioselective oxidative carbon-oxygen bond cleavage catalysed by copper(II) complexes: A relevant model study for lytic polysaccharides monooxygenases activity

Lytic polysaccharide monooxygenases (LPMOs) are copper-containing monooxygenase and catalyzing the oxidative cleavage of recalcitrant polysaccharides using dioxygen. The copper(II) complexes [Cu(L1)(H2O)ClO4]ClO41, [L1 = 4-methyl-1-[(pyridin-2-yl-methyl)]-1,4-diazepane]; [Cu(L2)(H2O)ClO4]ClO42, [L2 = 4-methyl-1-[(2-(pyridine-2-yl)ethyl]-1,4-diazepane] and [Cu(L3)(H2O)ClO4]ClO43, [L3 = 1-(4-methoxy-3,5-dimethyl-pyridin-2-yl)methyl)-4-methyl-1,4-diazepane] have been synthesized and characterized as the novel models for LPMOs. The molecular structures exhibit distorted square pyramidal geometry (τ, 0.183–0.388) as similar to LPMOs. The CuN (1.99–2.02 Å) bond distances of the model complexes are almost identical to those of native LPMOs enzyme (1.9–2.2 Å). The 1, 4-diazepane backbone and pyridine unit of complexes provide reasonable structural resemblances to ‘histidine brace’ and histidine residues of LPMOs respectively. The spectral, redox and kinetic studies were performed in water to mimic accurate enzymatic reaction conditions. The well-defined Cu(II)/Cu(I) reduction couples were observed around 8–112 mV versus NHE, which is lower than that of LPMOs. The electronic spectra of the complexes showed the d-d transitions around 600–635 nm and axial EPR parameter (g||, 2.28–2.29; A||, 160–168 × 10−4 cm−1), which are almost identical to that of LPMOs. The model complexes were catalyzed oxidative cleavage of model substrate p-nitrophenyl-β-D-glucopyranoside into p-nitrophenol and D-allose with a maximum yield up to 78.4% and TON, 300. The kinetics of reaction monitored by following the formation of an absorption band around at 400 nm corresponds to p-nitrophenol, showed the rate of 3.19–5.26 × 10−3 s−1. The oxidative cleavage reaction may occur via CuII-OOH intermediate, whose formation accompanied by an electronic spectral signature around 375 nm with the rate of 1.61–9.06 × 10−3 s−1. The CuII-OOH intermediate was characterized by spectral methods and its geometry was optimized by DFT calculations.

Fundamentals of C–O bond activation on metal oxide catalysts

Fundamental knowledge of the active site requirements for the selective activation of C–O bonds over heterogeneous catalysts is required to design multistep processes for the synthesis of complex chemicals. Here we employ reaction kinetics measurements, extensive catalyst characterization, first principles calculations and microkinetic modelling to reveal metal oxides as a general class of catalysts capable of selectively cleaving C–O bonds with unsaturation at the α position, at a moderate temperature and H2 pressure. Strikingly, metal oxides are considerably more active catalysts than commonly employed VIIIB and IB transition metal catalysts. We identify the normalized Gibbs free energy of oxide formation as both a reactivity and a catalyst stability descriptor and demonstrate the generality of the radical-mediated, reverse Mars–van Krevelen C–O bond activation mechanism on oxygen vacancies, previously established only for RuO2. Importantly, we provide evidence that the substrate plays an equally key role to the catalyst in C–O bond activation. Metal oxides have been identified as a promising class of catalysts for carbon–oxygen bond cleavage in the context of biomass valorization, although the systematic understanding of their reactivity remains elusive. Now, a combination of catalytic screening and first principles calculations provide important insights into this family of catalysts.

Iridium-catalysed asymmetric hydrogenation of allylic alcohols via dynamic kinetic resolution

Dynamic kinetic resolutions (DKRs) allow for the conversion of both enantiomers of starting material into a single enantiomer of product, hence avoiding the 50% yield limit observed in traditional kinetic resolutions. Transition-metal-catalysed variants have become an important and useful method in asymmetric synthesis. Here we report an asymmetric hydrogenation of allylic alcohols using an Ir–N,P-ligand complex via DKR. In contrast to the many DKRs involving carbonyl reduction, this methodology allows for DKR during alkene reduction. Mechanistic studies support the hypothesis that racemization of the substrate is achieved by cleavage and reforming of the oxygen–carbon bond. Under the cooperative dynamic kinetic asymmetric hydrogenation, a broad range of chiral alcohols containing two stereogenic centres were produced with excellent diastereoselectivities (up to 95:5) and enantioselectivities (up to 99%). Dynamic kinetic resolution (DKR) allows the conversion of both enantiomers of a racemic mixture into a single enantiomer of product, and requires both a stereoselective reaction and a means of rapidly racemizing the starting materials. Here, a highly stereoselective iridium-catalysed DKR of secondary allylic alcohols is reported, with mechanistic studies implying that substrate racemization is achieved through carbon–oxygen bond cleavage.

Metal-Organic Frameworks with Open Metal Sites for Sensing, Catalysis, and Energy Storage

Metal-Organic Frameworks (MOFs) are a class of coordination polymer in which metal ions and organic electron donors (“linkers”) assemble to form crystalline structures with stable nanoporosity. As a result of well-understood principles governing inorganic coordination chemistry and the rigidity and topology of the linkers, rational design principles have been established that allow MOFs to be designed and synthesized with properties selected for specific applications. Among the thousands of known MOFs, those with incomplete coordination shells around the metal ion (also known as open metal sites, or OMS) exhibit unique properties, including increased uptake of weakly interacting gases such as H2 and CH4, catalytic behavior resulting from substrate coordination to the OMS, and guest-dependent electrical conductivity that provides an avenue to electronic devices and sensors. This presentation will provide an introduction to MOFs and MOFs with OMS, then illustrate their possibilities using examples from our recent research: 1) catalytic cleavage of carbon-oxygen bonds found in lignin using activated H2; 2) colorimetric sensing; and 3) creation of electrically conducting MOFs using redox-active guest molecules such as TCNQ (7,7,8,8-tetracyanoquinodimethane). In each of these examples, guest molecules in the MOF pores interact directly with the OMS, resulting in bond polarization, modification of the electronic structure of the metal ion, and/or charge transfer between the metal ion and the guest. In the case of C-O bond cleavage, chemisorption of substrate ether molecules to Mg(II) ions in the IRMOF-74 series weakens the C-O bond. In addition, the MOF activates H2 molecules in solution with the substrate, as shown by isotope exchange experiments. Colorimetric sensing is enabled by guest binding to Cu(II) ions in the MOF known as HKUST-1, resulting in new charge-transfer transitions in the visible and near-UV. These perturbations of the electronic structure are relatively minor compared with the binding of TCNQ, a pi acid that bridges between two dimeric copper units in HKUST-1, creating a pathway for charge transport via a hopping mechanism. These results demonstrate that the novel features of MOFs with OMS provide the means to achieve completely new classes of catalysts and materials for chemical sensing and other electronic devices.

Alkyl Carbon–Oxygen Bond Cleavage of Aryl Alkyl Ethers by Iridium–Porphyrin and Rhodium–Porphyrin Complexes in Alkaline Media

Alkyl C–O bond cleavage in aryl alkyl ethers was achieved with Rh(ttp)Cl (1a; ttp = 5,10,15,20-tetrakis(p-tolyl)porphyrinato dianion) together with competitive alkyl C–H bond activation in alkaline media. In contrast, selective alkyl C–O bond cleavage occurred with the iridium–porphyrin Ir(ttp)(CO)Cl (1b)/KOH. Mechanistic investigations indicate the coexistence of MI(ttp)− and M2II(ttp)2 (M = Rh, Ir) under basic conditions. With a weaker Rh(ttp)–Rh(ttp) bond, RhII(ttp)· metalloradical exists in an appreciable amount to cleave the alkyl C–H bond, competing with the alkyl C–O bond cleavage via RhI(ttp)−. In contrast, the more nucleophilic IrI(ttp)− cleaves the alkyl C–O bond exclusively.

An Adventure in Sustainable Cross-Coupling of Phenols and Derivatives via Carbon–Oxygen Bond Cleavage

Aryl halides are very useful electrophiles for synthesizing various substituted aromatic compounds via metal-catalyzed cross-coupling reactions. Because of the high cost associated with their synthesis and the stoichiometric halide waste produced when using aryl halide feedstocks, cheaper and more sustainable alternatives have been explored, such as phenols. However, phenols have a very reactive hydroxyl group and a C–O bond with high dissociation energy. To overcome such challenges, earlier studies focused on finding ways to reduce the energy of the C–O bond while removing the active proton by transforming phenols into phenol derivatives (e.g., sulfonates, esters, carbamates, ethers, and metal salts). A greater ambition is to directly cross-couple phenols with nucleophiles via C–O cleavage. In this Perspective, we briefly summarize efforts and accomplishments concerning the cross-coupling of phenol derivatives and phenols.

Nickel-catalyzed Ring-opening Cross-coupling of Cyclic Alkenyl Ethers with Arylboronic Esters via Carbon–Oxygen Bond Cleavage

We have developed a nickel-catalyzed cross-coupling reaction of cyclic alkenyl ethers with arylboronic esters that leads to the formation of unsaturated alcohols via ring-opening by the cleavage of a C(sp2)–O bond. The use of 1,3-dicyclohexylimidazol-2-ylidene as the ligand is essential to promote this cross-coupling process.

Mesolysis Processes with Benzylic Carbon–Oxygen Bond Cleavage in Radical Anions of Aryl Benzyl Ethers Studied by Electron Pulse Radiolysis in DMF

Abstract Based on transient absorption measurements obtained upon electron-pulse radiolysis of aromatic ethers in N,N-dimethylformamide (DMF), mesolysis processes of the radical anions were investigated. We observed transient absorption spectral change due to the formation of the radical anions in the temperature range (220–295 K). The transient absorption of the radical anions decreased with first-order kinetics along with the growth of the absorption spectrum of the corresponding benzylic radical. Arrhenius analysis of the decay rates of the radical anions provided the activation energies and the frequency factors for the mesolysis. These observations indicated that the mesolysis of the radical anions occurs via a stepwise mechanism, resulting in the formation of benzylic radical and phenolate anion due to the C–O bond cleavage.

Discovery of N-substituted 3-arylisoquinolone derivatives as antitumor agents originating from O-substituted 3-arylisoquinolines via [2,3] or [3,3] rearrangement

The present study discovers multiple N-substituted 3-arylisoquinolone derivatives as antitumor agents originating from O-substituted 3-arylisoquinolines via [2,3] or [3,3] rearrangement. The current [2,3] rearrangement of epoxy or acetal O-substituents converting to diol or alcohol N-substituents can be promoted by silica gel or by diluted hydrochloric acid, which is distinct from previously reported [2,3] rearrangements. Some of the derivatives displayed comparable or even stronger cytotoxicity than sorafenib and vemurafenib on HCT116 colon carcinoma and A375 melanoma cell lines. Therefore, the rearrangement via intramolecular carbon-oxygen bond cleavage and carbon-nitrogen bond formation should be a useful approach for developing novel anticancer drugs derived from isoquinolones.

Aqueous-phase hydrodeoxygenation of highly oxygenated aromatics on platinum

Utilization of renewable sugars from biomass by a hybrid chemical process produces highly oxygenated aromatic compounds, such as phloroglucinol, which require catalytic reduction for desirable aromatic products. Aqueous phase hydrodeoxygenation of phloroglucinol on carbon-supported platinum produces resorcinol, phenol, cyclohexanol, cyclohexanone, and 1,3-cyclohexanediol by combinations of carbon–oxygen bond cleavage and carbon–carbon double bond hydrogenation. Carbon–carbon σ-bond cleavage was not observed. Hydrodeoxygenation was the primary reaction of phloroglucinol, leading to the production of resorcinol in the overall rate-limiting reaction, with an activation energy barrier of Ea = 117 kJ mol−1. Subsequent reactions of resorcinol produced 1,3-cyclohexanediol and phenol with similar energy barriers, Ea = 46 and Ea = 54 kJ mol−1, respectively. Further hydrogenation of phenol (Ea = 42 kJ mol−1) occurs through the intermediate, cyclohexanone, which is further reduced (Ea = 14 kJ mol−1) to the dominant product, cyclohexanol.

ChemInform Abstract: Organoselenium and Organotellurium Oxidation and Reduction

AbstractReview: 543 refs.