Cleavage Of Aryl Ethers Research Articles

Mesoporous SBA‐15 Supported CdS through Organosilane Ligand for Photo Driven Cleavage of Aryl Ether in Lignin and its Model Compound

AbstractMesoporous silica (SBA‐15) was synthesized hydrothermally followed by calcination and then functionalization with 3‐(triethoxysilyl)‐1‐propanethiol through surface grafting. The thiol moiety of the grafted ligand was utilized for the uniform anchoring of cadmium into the channels of the material. The synthesised material was subjected for its photo catalytic applications after characterization through standard characterization protocol. The synthesised mesoporous silica (i.e. SAB‐15), ligand modified material (i.e.SBA‐15‐S and Cd loaded ( i.e. SAB‐15‐CdS) materials were characterized through standard characterization protocol like powder XRD, FT‐IR, UV‐DRS, MAS NMR, N2 sorption, FE‐SEM, HR‐TEM and TG‐DTA studies. After successful characterization, the screening of the catalyst was carried out on a model reaction involving photo catalytic cleavage of α‐O‐4 aryl ether linkage present in benzyl phenyl ether, which is an important lignin model compound. The catalyst exhibited good efficiency along with the selectivity for desired monoaromatic platform chemicals. The synthesized catalyst was also carried out for the photo catalytic depolymerization of alkali lignin at the optimized reaction conditions which further showed the promising results for getting value added chemicals from lignin. The reaction products were characterized by using Gas chromatography‐mass spectrometry (GC‐MS) techniques.

Allyl Aryl Ether Cleavage by Blautia sp. MRG-PMF1 Cocorrinoid O-Demethylase.

ABSTRACTCoabalamin-dependent O-demethylase in Blautia sp. strain MRG-PMF1 was found to catalyze the unprecedented allyl aryl ether cleavage reaction. To expand the potential biotechnological applications, the reaction mechanism of the allyl aryl ether C-O bond cleavage, proposed to utilize the reactive Co(I) supernucleophile species, was studied further from the anaerobic whole-cell biotransformation. Various allyl naphthyl ether derivatives were reacted with Blautia sp. MRG-PMF1 O-demethylase, and stereoisomers of allyl naphthyl ethers, including prenyl and but-2-enyl naphthyl ethers, were converted to the corresponding naphthol in a stereoselective manner. The allyl aryl ether cleavage reaction was regioselective, and 2-naphthyl ethers were converted faster than the corresponding 1-naphthyl ethers. However, MRG-PMF1 cocorrinoid O-demethylase was not able to convert (2-methylallyl) naphthyl ether substrates, and the conversion of propargyl naphthyl ether was extremely slow. From the results, it was proposed that the allyl ether cleavage reaction follows the nucleophilic conjugate substitution (SN2′) mechanism. The reactivity and mechanism of the new allyl ether cleavage reaction by cobalamin-dependent O-demethylase would facilitate the application of Blautia sp. MRG-PMF1 O-demethylase in the area of green biotechnology.IMPORTANCE Biodegradation of environmental pollutants and valorization of biomaterials in a greener way is of great interest. Cobalamin-dependent O-demethylase in Blautia sp. MRG-PMF1 exclusively involves anaerobic C1 metabolism by cleaving the C-O bond of aromatic methoxy group and also produces various aryl alcohols by metabolizing allyl aryl ether compounds. Whereas methyl ether cleavage reaction is known to follow the SN2′ mechanism, the reaction pattern and mechanism of the new allyl ether cleavage reaction by cobalamin-dependent O-demethylase have never been studied. For the first time, stereoselectivity and the SN2′ mechanism of allyl aryl ether cleavage reaction by Blautia sp. MRG-PMF1 O-demethylase is reported, and the results would facilitate the application of Blautia sp. MRG-PMF1 O-demethylase in the area of green biotechnology.

Electrocatalytic cleavage of aryl ether C-O linkages in lignin model dimers

Electrocatalytic hydrodeoxygenation (ECH) is a promising strategy for upgrading lignin-derived compounds to hydrodeoxygenation products under mild conditions. Here, we investigated the C − O bonds cleavage and oxygen-containing intermediates hydrogenation of lignin-derived aryl ether dimers, including Diphenyl ether (4-O-5 linkage), Benzyl phenyl ether (α-O-4 linkage), and Phenethoxybenzene (β-O-4 linkage), in a Phosphotungstic acid-Pt/C based fluidized ECH system. The ECH of three kinds of aryl ether dimers all afforded >90% of the corresponding hydrodeoxygenation products under the optimized conditions (80 °C and a current density of 25 mA cm−2 within 90 min). It showed some difference in the reactivity of α-O-4, 4-O-5, and β-O-4. The reaction rate of the CO bond decreased in the order of α-O-4 > 4-O-5 > β-O-4. The CO bond of the α-O-4 and β-O-4 linkages was mainly cleaved by hydrogenolysis and reductive hydrolysis, respectively. In comparison, the CO bonds of 4-O-5 linkage were cleaved by parallel hydrogenolysis and reductive hydrolysis. This difference was mainly attributed to the reaction rate of the further hydrogenolysis of the Caliphatic-OH fragment (produced by the hydrolysis of the α-O-4 and β-O-4 linkages). While phenol (produced by hydrolysis of the 4-O-5 linkage) mainly generated cyclohexanol by hydrogenation pathway.

Electrocatalytic Cleavage of Aryl Ether C−O Linkages in Lignin Model Dimers

Electrocatalytic Cleavage of Aryl Ether C−O Linkages in Lignin Model Dimers

Electron-Donating C-NH2 Link Backbone for Highly Alkaline and Mechanical Stable Anion Exchange Membranes.

Aryl-ether cleavage and benzylic quaternary ammonium (QA) group degradation are promoted by C═O groups in most commercial anion exchange membrane materials. Herein, a novel strategy of converting C═O groups to the electron-donating C-NH2 linkages in conventional poly(arylene ether ketone)s is proposed by reductive amination via Leuckart reaction. Density functional theory (DFT) calculations indicate that the model compound containing C-NH2 linkage exhibits much higher barrier heights for aryl-ether cleavage and QA group degradation by enhancing the electronic cloud density on both the ether-connected carbon and the benzylic carbon. The C-NH2 linkages also induce hydrogen bond networks in the membranes, which enhance intermolecular interaction and provide additional hydroxide transport sites. As a result, the C-NH2 linkage membranes exhibit excellent hydroxide conductivity (108.2 mS cm-1 at 80 °C) and tensile strength (48.4 MPa) with high elongation at break (50.8%). The C-NH2 linkage membranes also show outstanding alkaline stability with no detectable backbone degradation even in 4 M KOH at 80 °C for 400 h, which is at the top-level among the state-of-the-art main chain architecture AEMs. This study proposes a new strategy for the synthesis of highly stable AEMs based on electron-donating C-NH2 link backbone.

Stable and Antisintering Tungsten Carbides with Controllable Active Phase for Selective Cleavage of Aryl Ether C–O Bonds

Transition-metal carbides are important materials in heterogeneous catalysis. It remains challenging yet attractive in nanoscience to construct the active phase of carbide catalysts in a controllable manner and keep a sintering-resistant property in redox reactions, especially hydroprocessing. In this work, an integrated strategy was presented to synthesize stable and well-defined tungsten carbide nanoparticles (NPs) by assembling the metal precursor onto carbon nanotubes (CNTs), wrapping a thin polymeric layer, and following a controlled carburization. The polymer served as a soft carbon source to modulate the metal/carbon ratio in the carbides and introduced amorphous carbons around the carbides to prevent the NPs from sintering. The as-built p-WxC/CNT displayed high stability in the hydrogenolysis of aryl ether C-O bond in guaiacol for more than 150 h. Its activity was more than two and six times higher than those prepared via typical temperature-programmed reduction with gaseous carbon (WxC/CNT-TPR) and carbothermal reduction with intrinsic carbon support (WxC/CNT-CTR), respectively. Our p-WxC/CNT catalyst also achieved high efficiency for selective cleavage of the aryl ether C-O bonds in lignin-derived aromatic ethers, including anisole, dimethoxylphenol, and diphenyl ether, with a robust lifespan.

Synergy of carbon defect and transition metal on tungsten carbides for boosting the selective cleavage of aryl ether C[sbnd]O bond

Transition-metal carbides are attractive heterogeneous catalysts for the transformation of lignin-derived compounds. However, the effects of the surface carbon defect (C-defect) remain unclear due to coke deposition and exposed C terminations. Herein, we transform inert tungsten carbides (WC) into active catalysts via surficial C-etching using transition metals in the presence of H2. An optimized 1% Pt–WC catalyst with synergy of Pt and C-defects can perform a 78.6% conversion in the selective hydrogenolysis of guaiacol to phenol with 89.0% selectivity at 300 °C, while the intact WC only gives a 7.1% guaiacol conversion with 51.0% phenol selectivity under the same conditions. As a result, the 1% Pt–WC catalyst exhibits 19-fold higher reaction rate and 12.6-fold higher turnover frequency than those of WC. Moreover, no substantial loss of catalytic performance has been observed with 1% Pt–WC for 50 h on stream. A rational reaction pathway is accordingly proposed through in-depth characterizations.

Photocatalytic Reductive C-O Bond Cleavage of Alkyl Aryl Ethers by Using Carbazole Catalysts with Cesium Carbonate.

Methods to activate the relatively stable ether C-O bonds and convert them to other functional groups are desirable. One-electron reduction of ethers is a potentially promising route to cleave the C-O bond. However, owing to the highly negative redox potential of alkyl aryl ethers (Ered < -2.6 V vs SCE), this mode of ether C-O bond activation is challenging. Herein, we report the visible-light-induced photocatalytic cleavage of the alkyl aryl ether C-O bond using a carbazole-based organic photocatalyst (PC). Both benzylic and non-benzylic aryl ethers underwent C-O bond cleavage to form the corresponding phenol products. Addition of Cs2CO3 was beneficial, especially in reactions using a N-H carbazole PC. The reaction was proposed to occur via single-electron transfer (SET) from the excited-state carbazole to the substrate ether. Interaction of the N-H carbazole PC with Cs2CO3 via hydrogen bonding exists, which enables a deprotonation-assisted electron-transfer mechanism to operate. In addition, the Lewis acidic Cs cation interacts with the substrate alkyl aryl ether to activate it as an electron acceptor. The high reducing ability of the carbazole combined with the beneficial effects of Cs2CO3 made this otherwise formidable SET event possible.

Highly stable electron-withdrawing C[dbnd]O link-free backbone with branched cationic side chain as anion exchange membrane

To break the alkaline stability limitation of aryl-ether cleavage in anion exchange membranes (AEMs), novel electron-withdrawing CO link-free polymer backbone with spacer-tunable branched ionic side-chain is synthesized via the Leuckart-Menshutkin route. Density functional theory studies show that replacing CO links in conventional poly(aryl ether ketone)s (PAEKs) with electron-donating C–NH2 groups followed by grafting long flexible spacer cationic side chain can not only elevate the barrier height of aryl-ether cleavage by increasing electron cloud density on the ether-connected carbon atom, but also result in higher barrier height of quaternary ammonium (QA) group degradation by providing stronger steric hindrance effect and higher lowest unoccupied molecular orbital (LUMO) energy. Long pendant QAs also facilitate microphase separation and ion conduction. The PEAM-2C6 AEM with flexible hexyl-spacer branched cationic side chain achieves high ion conductivity (128.2 mS cm−1 at 80 °C), as well as excellent alkaline stability (no backbone degradation and 81% retaining of original conductivity) after immersion in 4 M KOH at 80 °C for 400 h. H2/O2 fuel cell assembled with PEAM-2C6 exhibits a maximum peak power density of 499 mW cm−2 at 80 °C. Electron-withdrawing link-free strategy provides an effective way to fabricate alkaline stabilized polyelectrolytes along with high ion conductivity.

Electrocatalytic Cleavage of Aryl Ether C−O Linkages in Lignin Model Dimers

Electrocatalytic Cleavage of Aryl Ether C−O Linkages in Lignin Model Dimers

Gut Metabolism of Furanocoumarins: Proposed Function of Co O-Methyltransferase.

Gut metabolism of natural products is of great interest due to the altered biological activity of the metabolites. To study the gut metabolism of the dietary furanocoumarins, the biotransformation of Angelica dahurica was studied with human gut microbiota. The major components of Avenula dahurica, including xanthotoxin (1), bergapten (2), imperatorin (3), isoimperatorin (4), oxypeucedanin (5), and byakangelicol (6), were all metabolized by the human fecal sample, and each furanocoumarin was also biotransformed by Blautia sp. MRG-PMF1 responsible for intestinal O-demethylation. Oxypeucedanin (5) and byakangelicol (6) were converted to oxypeucedanin hydrate (9) and desmethylbyakangelicin (12), respectively. The gut microbial conversion of xanthotoxin (1) and bergapten (2) with the MRG-PMF1 strain resulted in the production of xanthotoxol (7) and bergaptol (8), respectively, due to the methyl aryl ether cleavage by O-methyltransferase. Unexpectedly, the biotransformation of prenylated furanocoumarins, imperatorin (3), and isoimperatorin (4) resulted in the corresponding deprenylated furanocoumarins of xanthotoxol (7) and bergaptol (8), respectively. The cleavage of the prenyl aryl ether group by gut microbiota was unprecedented metabolism. Our data presented the first deprenylation of prenylated natural products, presumably by the anaerobic prenyl aryl ether cleavage reaction catalyzed by Co-corrinoid enzyme.

Acid-Catalyzed α-O-4 Aryl-Ether Cleavage Mechanisms in (Aqueous) γ-Valerolactone: Catalytic Depolymerization Reactions of Lignin Model Compound During Organosolv Pretreatment.

In this study, acidolysis of benzyl phenyl ether (BPE), being a representative lignin model compound with the α-O-4 linkage, was examined in γ-valerolactone (GVL) and a GVL/water mixture, each time acidified with sulfuric acid. The product distribution was strongly affected by water used as a cosolvent, which was found to be advantageous by inhibiting the formation of larger structures and introducing reactive OH groups instead. The experimental results indicate the GVL/water ratio as an important parameter to attain an optimal hydrolytic α-ether bond cleavage. Differences between the organosolv lignins (molecular weight distribution, OH group content, and structural features with reaction time), isolated under moderate reaction conditions, supported the findings obtained using BPE. A beneficial effect of the added water is reflected in the higher aliphatic OH group content and less intact structure. Analysis of the reaction mechanism represents an initial step toward kinetics and structure–activity correlation of biorefining industrial resources.

High-Performance Anion Exchange Membranes with Para-Type Cations on Electron-Withdrawing C═O Links Free Backbone

As critical triggers, both the electron-withdrawing links and neighboring cationic groups induce and accelerate the aryl ether cleavage in the widely used aryl ether containing anion exchange membr...

Fractionation of herbaceous biomass using a recyclable hydrotropic p–toluenesulfonic acid (p–TsOH)/choline chloride (ChCl) solvent system at low temperatures

Lignin valorization is significantly influenced by the degrees of lignin condensation measured by the amount of cleaved βO4 ether linkages and interunit CC linkages formed during biomass fractionation. To minimize the cleavage of aryl ether and formation of CC linkages, a p–toluenesulfonic acid (p–TsOH)/choline chloride (ChCl) solvent system with hydrotropic properties was evaluated. The ChCl was chosen to control the H+ concentration of the p–TsOH/ChCl solvent system. Compared with p–TsOH/H2O, the degradation of βO4 was kept in a similar rate during p–TsOH/ChCl fractionation; however, the formation of CC linkage was slower. Under 75/25 p–TsOH/ChCl mass ratio at 80 °C for 20 min, 76.6 % of miscanthus lignin, with βO4 content of 30.9 %, and 88.9 % of wheat straw lignin, with βO4 content of 33.6 %, can be dissolved. In addition, the low polar and layered structure of hydrotropic p–TsOH/ChCl system has a strong ability to dissolve lignin, which resulted in lignin with larger particle size and lower surface charge. The results also indicated that lignin from p–TsOH/ChCl solvent system has great potential for lignin depolymerization to aromatics for valorization.

Selective hydrogenolysis of lignin-derived aryl ethers over Co/C@N catalysts

Low aromaticity and a large number of oxygen-bridged bonds among the aromatic structural units of lignin make it possible to obtain chemicals directly. A novel Co/C@N catalyst with high activity towards hydrogenolysis of lignin-derived aryl ethers was synthesized according to the pyrolysis process of a predesigned ZIF-67. According to the results of characterization by powder X-ray diffraction (XRD), N2 adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), Co was reduced by the carbonized organic linker of ZIF-67, and N was doped into the carbon skeleton of the catalyst. The effects of holding time, temperature, and initial H2 pressure on the catalytic performance of Co/C@N were evaluated in the selective hydrogenolysis of benzyl phenyl ether (BPE). BPE was completely converted and the selectivity of monomer reached to 98.2% under optimized reaction conditions. The Calk-O bond in BPE was first dissociated to form toluene and phenol, and then phenol was rapidly hydrogenated to form cyclohexanol. Furthermore, the Co/C@N catalyst shows high activity for hydroprocessing and selective cleavage of other lignin-derived aryl ethers, including phenylethyl phenyl, diphenyl ether, dibenzyl ether, dinaphthalene ether, guaiacol, anisole and veratrole.

Ni-Catalyzed Cycloisomerization between 3-Phenoxy Acrylic Acid Derivatives and Alkynes via Intramolecular Cleavage and Formation of the C–O Bond To Give 2,3-Disubstituted Benzofurans

Reactions based on transition-metal-catalyzed C-O bond cleavage have attracted much attention as a new synthetic method. Until now, several intermolecular reactions via C-O bond cleavage of aryl ethers, alkenyl ethers, esters, and others have been reported. Here we report an unprecedented C-O bond cleavage of 3-phenoxy acrylic acid derivatives, followed by intramolecular C-O bond formation with alkynes. This reaction gave 2,3-disubstituted benzofurans having useful functional groups-silyl substituents and acrylic acid derivatives-at the 2- and 3-positions, respectively. This report also described theoretical (DFT) insights into the mechanism.

Photocatalytic Cleavage of Aryl Ether in Modified Lignin to Non-phenolic Aromatics

Depolymerization of lignin meets the difficulty in cleaving the robust aryl ether bond. Herein, through installing an internal nucleophile in the β-O-4′ linkage, the selective cleavage of aryl ethe...

Palladium-Catalyzed Reductive Insertion of Alcohols into Aryl Ether Bonds.

Palladium on carbon catalyzes C-O bond cleavage of aryl ethers (diphenyl ether and cyclohexyl phenyl ether) by alcohols (R-OH) in H2 . The aromatic C-O bond is cleaved by reductive solvolysis, which is initiated by Pd-catalyzed partial hydrogenation of one phenyl ring to form an enol ether. The enol ether reacts rapidly with alcohols to form a ketal, which generates 1-cyclohexenyl-O-R by eliminating phenol or an alkanol. Subsequent hydrogenation leads to cyclohexyl-O-R.

Palladium‐Catalyzed Reductive Insertion of Alcohols into Aryl Ether Bonds

AbstractPalladium on carbon catalyzes C−O bond cleavage of aryl ethers (diphenyl ether and cyclohexyl phenyl ether) by alcohols (R−OH) in H2. The aromatic C−O bond is cleaved by reductive solvolysis, which is initiated by Pd‐catalyzed partial hydrogenation of one phenyl ring to form an enol ether. The enol ether reacts rapidly with alcohols to form a ketal, which generates 1‐cyclohexenyl−O−R by eliminating phenol or an alkanol. Subsequent hydrogenation leads to cyclohexyl−O−R.

Efficient cleavage of aryl ether C-O linkages by Rh-Ni and Ru-Ni nanoscale catalysts operating in water.

Bimetallic Ru-Ni and Rh-Ni nanocatalysts coated with a phase transfer agent efficiently cleave aryl ether C-O linkages in water in the presence of hydrogen. For dimeric substrates with weaker C-O linkages, i.e. α-O-4 and β-O-4 bonds, low loadings of the precious metal (Rh or Ru) in the nanocatalysts quantitatively afford monomers, whereas for the stronger 4-O-5 linkage higher amounts of the precious metal are required to achieve complete conversion. Under the optimized, relatively mild operating conditions, the C-O bonds in a range of substituted ether compounds are efficiently cleaved, and mechanistic insights into the reaction pathways are provided. This work paves the way to sustainable approaches for the hydrogenolysis of C-O bonds.