Showing papers on "Ether published in 2017"

Recent progress in the design and applications of fluorescence probes containing crown ethers

Abstract: Crown ethers, discovered by the winner of the Nobel Prize Charles Pedersen, are cyclic chemical compounds that consist of a ring or multiple rings containing several ether groups that are capable of binding alkali ions. A smart fluorescent probe containing a crown ether moiety could be developed as a sensor for metal ions, anions and other bio-molecules and be further applied to monitor the relevant biological process in vivo. This review highlights recent advances which can be divided into seven parts: (i) fluorescent probes containing a simple crown ether or an aza-crown ether structure; (ii) fluorescent probes containing an azathia crown ether; (iii) fluorescent probes containing a cryptand; (iv) fluorescent probes containing two or more binding sites; (v) crown ether derivatives-metal complex assisted chemosensing of bioactive species; (vi) crown ether-based chemosensors for bioactive molecular detection; and (vii) efforts to improve biological relevance.

Highly conductive and durable poly(arylene ether sulfone) anion exchange membrane with end-group cross-linking

Abstract: Here, we demonstrate the improved electrochemical performance and stability of end-group cross-linked anion exchange membranes (AEM) for the first time via the introduction of imidazolium groups in poly(arylene ether sulfone) (Imd-PAES). A novel feature of the cross-linking reaction is that basic additives are not required to prevent gelation with the cationic functional groups. In this work, the sodium salt of 3-hydroxyphenylacetylene acted directly as the end-group cross-linker, and it was cross-linked by thermal treatment at 180 °C. The gel fraction and hydroxide conductivity of the cross-linked membranes (XE-Imds) depended on the cross-linking temperature and time. The prepared XE-Imd70 (70 refers to the degree of functionalization) membranes with an ion exchange capacity (IEC) of 2.2 meq g−1 achieved a high hydroxide conductivity (107 mS cm−1). This material also showed good single cell performance (XE-Imd70: 202 mA cm−2 at 0.6 V and a maximum power density of 196.1 mW cm−2) at 80 °C, 100% relative humidity (RH), and improved durability and alkaline stability. The excellent hydroxide conductivity and electrochemical performance of XE-Imd70 was due to the fact that the ion cluster size of XE-Imd membranes was larger (12.1–14 nm) than that of E-Imd (5.5–8.14 nm), indicating that XE-Imd membranes have a closely associated ion-clustered morphology, which was confirmed by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) measurements.

Promoting Lignin Depolymerization and Restraining the Condensation via an Oxidation-Hydrogenation Strategy

Abstract: For lignin valorization, simultaneously achieving the efficient cleavage of ether bonds and restraining the condensation of the formed fragments represents a challenge thus far. Herein, we report a two-step oxidation–hydrogenation strategy to achieve this goal. In the oxidation step, the O2/NaNO2/DDQ/NHPI system selectively oxidizes CαH–OH to Cα═O within the β-O-4 structure. In the subsequent hydrogenation step, the α-O-4 and the preoxidized β-O-4 structures are further hydrogenated over a NiMo sulfide catalyst, leading to the cleavage of Cβ–OPh and Cα–OPh bonds. Besides the transformation of lignin model compounds, the yield of phenolic monomers from birch wood is up to 32% by using this two-step strategy. The preoxidation of CαH–OH to Cα═O not only weakens the Cβ–OPh ether bond but also avoids the condensation reactions caused by the presence of Cα+ from dehydroxylation of CαH–OH. Furthermore, the NiMo sulfide prefers to catalyze the hydrogenative cleavage of the Cβ–OPh bond connecting with a Cα═O rathe...

Photoinduced Nickel-Catalyzed Chemo- and Regioselective Hydroalkylation of Internal Alkynes with Ether and Amide α-Hetero C(sp3)–H Bonds

Abstract: A direct hydroalkylation of disubstituted alkynes with unfunctionalized ethers and amides was achieved in an atom-efficient and additive-free manner through the synergistic combination of photoredox and nickel catalysis. The protocol was effective with a wide range of internal alkynes, providing products in a highly selective fashion. Notably, the observed regioselectivity is complementary to conventional radical addition processes. Mechanistic investigations suggest that the photoexcited iridium catalyst facilitated the nickel activation via single-electron transfer.

Multicomponent Oxyalkylation of Styrenes Enabled by Hydrogen-Bond-Assisted Photoinduced Electron Transfer.

Abstract: Herein, we disclose a strategy for the activation of N-(acyloxy)phthalimides towards photoinduced electron transfer through hydrogen bonding. This activation mode enables efficient access to C(sp3)-centered radicals upon decarboxylation from bench-stable and readily available substrates. Moreover, we demonstrate that the formed alkyl radicals can be successfully employed in a novel redox-neutral method for constructing sp3−sp3 bonds across styrene moieties that gives straightforward access to complex alcohol and ether scaffolds.

Cation Radical Accelerated Nucleophilic Aromatic Substitution via Organic Photoredox Catalysis

Abstract: Nucleophilic aromatic substitution (SNAr) is a direct method for arene functionalization; however, it can be hampered by low reactivity of arene substrates and their availability. Herein we describe a cation radical-accelerated nucleophilic aromatic substitution using methoxy- and benzyloxy-groups as nucleofuges. In particular, lignin-derived aromatics containing guaiacol and veratrole motifs were competent substrates for functionalization. We also demonstrate an example of site-selective substitutive oxygenation with trifluoroethanol to afford the desired trifluoromethylaryl ether.

Recent Advances of Oxidative Radical Cross-Coupling Reactions: Direct α-C(sp3)–H Bond Functionalization of Ethers and Alcohols

Abstract: Oxygen-containing heterocycles are an important class of compounds with diverse biological activities. In recent years, direct α-C–H functionalization of inexpensive and abundant readily available ethers and alcohols by oxidative radical cross-coupling with different nucleophilic partners (C–H, N–H, O–H and S–S), leading to the construction of C–C and C–X (X=C, N, O, S) bonds, has emerged as one of the vital strategies among oxidative cross-coupling reactions. Owing to the features of being atom-economic, environmentally benign, having a simple operation and biologically properties, a series of ether α-C(sp3)–H bond activation reactions have been developed by metal or metal-free systems via the radical oxidative coupling pathway, since the radical oxidative coupling reactions have wide range of applications. This review aims to highlight the recent advances in direct α-C(sp3)–H bond functionalization of ethers and alcohols via radical oxidative process.

Direct Cα-heteroarylation of structurally diverse ethers via a mild N-hydroxysuccinimide mediated cross-dehydrogenative coupling reaction.

Abstract: An important challenge in the Cα-heteroarylation of ethers is the requirement of a large excess amount of ethers (that are used as solvents in many cases) to achieve effective transformations. This drawback has significantly restricted the Cα-heteroarylation of ethers to the use of simple and easily accessible ether substrates. To overcome this limitation, a new, efficient, N-hydroxysuccinimide (NHS) mediated, mild and metal-free CDC strategy for the direct Cα-heteroarylation of diverse ethers has been developed. Different to our previous benzaldehyde mediated photoredox Cα-heteroarylation, we have identified NHS as a new and efficient mediator without using light. A distinct non-photoredox engaged hydrogen-atom-transfer (HAT) mechanism that used a nitrogen-centered radical cation produced from NHS is initially revealed. Notably, only 5–10 equivalents of ethers as coupling partners are used, which allows for structurally diverse and complex ethers to engage in this process, to create highly medicinally relevant Cα-heteroarylated ethers. Furthermore, more structurally diverse heterocyclics can serve as reactants for this process.

Radical Alkylation of Imines with 4-Alkyl-1,4-dihydropyridines Enabled by Photoredox/Brønsted Acid Cocatalysis.

Abstract: Radical alkylation of imines with 4-alkyl-1,4-dihydropyridines cocatalyzed by iridium/ruthenium complex and Bronsted acid under visible light irradiation has been achieved. Both aldimines and ketimines can undergo this transformation. Common functional groups, such as hydroxyl groups, ester, amide, ether, cyanide, and heterocycles, can be tolerated in this reaction. A variety of structurally diverse amines (57 examples) have been produced with up to 98% isolated yields using this method.

Thermoresponsive Pickering Emulsions Stabilized by Silica Nanoparticles in Combination with Alkyl Polyoxyethylene Ether Nonionic Surfactant.

Abstract: We put forward a simple protocol to prepare thermoresponsive Pickering emulsions. Using hydrophilic silica nanoparticles in combination with a low concentration of alkyl polyoxyethylene monododecyl ether (C12En) nonionic surfactant as emulsifier, oil-in-water (o/w) emulsions can be obtained, which are stable at room temperature but demulsified at elevated temperature. The stabilization can be restored once the separated mixture is cooled and rehomogenized, and this stabilization–destabilization behavior can be cycled many times. It is found that the adsorption of nonionic surfactant at the silica nanoparticle–water interface via hydrogen bonding between the oxygen atoms in the polyoxyethylene headgroup and the SiOH groups on particle surfaces at low temperature is responsible for the in situ hydrophobization of the particles rendering them surface-active. Dehydrophobization can be achieved at elevated temperature due to weakening or loss of this hydrogen bonding. The time required for demulsification decr...

Identification of Novel Hydrogen-Substituted Polyfluoroalkyl Ether Sulfonates in Environmental Matrices near Metal-Plating Facilities

Abstract: Environmental occurrence and behaviors of 6:2 chlorinated polyfluoroalkyl ether sulfonate (Cl-6:2 PFESA, with trade name F-53B) have been receiving increased attention recently. Nevertheless, its potential fates under diversified conditions remain concealed. In this study, susceptibility of Cl-6:2 PFESA to reductive dehalogenation was tested in an anaerobic super-reduced cyanocobalamin assay. A rapid transformation of dosed Cl-6:2 PFESA was observed, with a hydrogen-substituted polyfluoroalkyl ether sulfonate (1H-6:2 PFESA) identified as the predominant product by a nontarget screening workflow. With the aid of laboratory-purified standards, hydrogen-substituted PFESA analogues (i.e., 1H-6:2 and 1H-8:2 PFESA) were further found in river water and sediment samples collected from two separate regions near metal-plating facilities. Geometric mean concentrations of 560 pg/L (river water) and 11.1 pg/g (sediment) for 1H-6:2 PFESA and 11.0 pg/L (river water) and 7.69 pg/g (sediment) for 1H-8:2 PFESA were measured, and both analytes consisted average compositions of 1% and 0.1% among the 18 monitored per- and polyfluoroalkyl sulfonate and carboxylate pollutants, respectively. To our knowledge, this is the first to report existence of polyfluoroalkyl sulfonates with both hydrogen and ether functional group in the environment.

Mechanism study on the pyrolysis of a synthetic β-O-4 dimer as lignin model compound

Abstract: In this study, a β-O-4 dimer with abundant oxygen substituents was successfully synthesized, and was used as the model compound for lignin to study the pyrolysis mechanism. Py-GC/MS (micro pyrolyzer coupled with gas chromatography/mass spectrometry) was employed to identify the distribution of pyrolytic products under temperatures of 150–850 °C. It was found that the yields of methoxylated monoaromatics underwent tendencies of first increase and then decrease at high temperature. Polyaromatics and benzofuran were only detected at high temperature. Based on experimental analysis, a detailed pyrolysis kinetic model was developed by combining the density functional theory (DFT) and the transition state theory (TST). The homolysis of C β -O was the most favorable route for the initial depolymerization of dimer rather than the concerted retro-ene fragmentation. For the evolution of intermediate products, the breakage of ether bond and the keto-enol tautomerization of enols were the most favorable routes, while intramolecular cyclization and group dissociation exhibited high energy barriers and low reaction rates. The methoxyl in guaiacol preferred to undergo demethylation leading to the formation of catechol.

Supramolecular Recognition Allows Remote, Site-Selective C−H Oxidation of Methylenic Sites in Linear Amines

Abstract: Site-selective C-H functionalization of aliphatic alkyl chains is a longstanding challenge in oxidation catalysis, given the comparable relative reactivity of the different methylenes. A supramolecular, bioinspired approach is described to address this challenge. A Mn complex able to catalyze C(sp3 )-H hydroxylation with H2 O2 is equipped with 18-benzocrown-6 ether receptors that bind ammonium substrates via hydrogen bonding. Reversible pre-association of protonated primary aliphatic amines with the crown ether selectively exposes remote positions (C8 and C9) to the oxidizing unit, resulting in a site-selective oxidation. Remarkably, such control of selectivity retains its efficiency for a whole series of linear amines, overriding the intrinsic reactivity of C-H bonds, no matter the chain length.

Direct deoxygenation of lignin model compounds into aromatic hydrocarbons through hydrogen transfer reaction

Abstract: A new route is reported for the direct deoxygenation of lignin model compounds to aromatic hydrocarbons via the catalytic hydrogen transfer reactions over Ru/Nb 2 O 5 -SiO 2 catalyst with 2-PrOH as a hydrogen donor. It is found that Ru/Nb 2 O 5 -SiO 2 catalyst is active for the hydrodeoxygenation of p -cresol with 98.5% conversion and 84.0% yield of toluene at 230 °C, owing to the significant promotion effect of NbO x species on C O bond cleavage and the proper transfer hydrogenation activity of Ru. Furthermore, Ru/Nb 2 O 5 -SiO 2 catalyst also shows excellent performances in the transfer hydrodeoxygenation of various lignin model compounds such as 2-methoxy-4-methyl phenol, benzyl phenyl ether (α-O-4 model compound), 2-(2-methoxyphenyl)oxy-1-phenethanol (β-O-4 model compound) and even the real lignin extracted from birch, into aromatic hydrocarbons with 2-PrOH as the hydrogen donor. The reaction pathway studies of these model compounds demonstrate that the direct deoxygenation (DDO) is the main reaction route over Ru/Nb 2 O 5 -SiO 2 catalyst in the presence of 2-PrOH. Here, using 2-PrOH as a hydrogen source is found to be more selective to aromatic hydrocarbons than using molecular hydrogen. This work provides a new way for the high selective production of aromatic hydrocarbons from renewable lignin via the catalytic hydrogen transfer reactions.

High-Performance Semicrystalline Poly(ether ketone)-Based Proton Exchange Membrane

Abstract: A novel semicrystalline poly(ether ketone) (PEK)-based proton exchange membrane (semi-SPEK-x) has been developed. Through a one-step sulfonation and hydrolysis, a poly(ether ketimine) precursor transforms into PEK and ion-conducting groups are introduced. With an ion-exchange capacity ranging from 1.49 to 2.00 mequiv g–1, the semi-SPEK-x polymers exhibit a semicrystalline feature in both dry and hydrated states. Owing to the semicrystalline domains inside the polymer, the obtained membrane exhibits low water uptake and low volume swelling ratio. More importantly, the semicrystalline structure lowers methanol permeability and, consequently, improves the overall performances of direct methanol fuel cells.

Activation of Remote meta‐C−H Bonds in Arenes with Tethered Alcohols: A Salicylonitrile Template

Abstract: Palladium-catalyzed activation of remote meta-C-H bonds in arenes containing tethered alcohols was achieved with high regioselectivity by using a nitrile template. Computational studies on the macrocyclic transition state of the regioselectivity-determining C-H activation steps revealed that both the C-N-Ag angles and gauche comformations of phenyl ether play an extremely important role in the meta selectivity.

Thermocatalytic conversion of lignin in an ethanol/formic acid medium with NiMo catalysts: Role of the metal and acid sites

Abstract: NiMo catalysts supported on different sulfated and non-sulfated aluminas and zirconias were studied for the catalytic conversion of lignin in a formic acid/ethanol medium. All the pre-reduced NiMo-support combinations resulted in high conversion of lignin into bio-oil, with over 60% yield (mass%). The NiMo-sulfated alumina catalyst exhibited the highest activity among all the catalysts studied. The overall reaction mechanism of the catalytic lignin conversion was found to be especially complex. The oil yield and its properties are affected by a combination of successive catalytic reactions that are part of the lignin conversion process. Lignin is first de-polymerized into smaller fragments through the cleavage of the aliphatic ether bonds. This reaction can be either catalyzed by Ni0 species and strong Lewis acid sites within the alumina supports. In the presence of both active species, the Ni0 catalyzed ether bond cleavage is the prevailing reaction mechanism. In a second step, the smaller lignin fragments can be stabilized by catalytic hydrodeoxygenation (HDO) and alkylation reactions that hinder their re-polymerization into char. Mo was found to be especially active for HDO reactions while all the catalysts studied exhibited significant alkylation activity. The final bio-oil yield is strongly dependent on the aliphatic ether bond cleavage rate; the contribution of those monomer stabilization reactions (i.e. HDO and alkylation) being secondary.

Anion Exchange Membranes: Enhancement by Addition of Unfunctionalized Triptycene Poly(Ether Sulfone)s.

Abstract: Anion exchange membrane fuel cells are a clean and efficient promising future energy source. However, the development of stable high-performance membranes remains a major challenge. Herein we demonstrate that the addition of unfunctionalized triptycene poly(ether sulfones) into 1-methylimidazolium poly(ether sulfone) enhances membrane’s conductivity (up to 0.082 S/cm at 80 °C), minimizes dimensional changes over temperatures from 20 to 80 °C, and enhances stability with 30% of the initial conductivity maintained after 450 h. These enhancements appear to be the result of nanophase separation and internal free volume. Small angle X-ray scattering and transmission electron microscopy reveal that the internal domain size increases (up to 7.44 nm) with increasing triptycene fraction.

An Ion-Responsive Pincer-Crown Ether Catalyst System for Rapid and Switchable Olefin Isomerization

Abstract: Rapid, selective, and highly controllable iridium-catalyzed allylbenzene isomerization is described, enabled by tunable hemilability based on alkali metal cation binding with a macrocyclic "pincer-crown ether" ligand. An inactive chloride-ligated complex can be activated by halide abstraction with sodium salts, with the resulting catalyst [κ5 -(15c5 NCOPiPr )Ir(H)]+ exhibiting modest activity. Addition of Li+ provides a further boost in activity, with up to 1000-fold rate enhancement. Ethers and chloride salts dampen or turn off reactivity, leading to three distinct catalyst states with activity spanning several orders of magnitude. Mechanistic studies suggest that the large rate enhancement and high degree of tunability stem from control over substrate binding.

Novel polymeric humidity sensors based on sulfonated poly (ether ether ketone)s: Influence of sulfonation degree on sensing properties

Abstract: Sulfonated poly (ether ether ketone)s (SPEEKs) with different values of sulfonation degree were successfully synthesized by a direct aromatic nucleophilic substitution polymerization of sulfonated monomers. By the direct-synthesis method, the number of sulfonic acid groups can be precisely controlled. They were then fabricated by spin coating on a ceramic substrate to be used as a novel polymeric humidity sensor. The morphology of the ionic polymers especially the microstructure of ionic clusters, water sorption and the complex impedance spectra of the sensors were investigated to explain the humidity sensing characteristics and the sensing mechanism under different relative humidity. In addition, SPEEK-6 polymer with sulfonation degree (D s = 1.18) was chosen as the optimal humidity sensing materials, which possessed high sensitivity and satisfactory linearity ranging from 11% RH to 97% RH. Furthermore, it exhibited a small hysteresis (less than 3% RH) during the sorption processes, an acceptable response/recovery time (100 s response and 105 s of recovery time) and an excellent repeatability. These results indicated SPEEKs could be promising for applications in humidity sensor.

Ionic crosslinking of imidazolium functionalized poly(aryl ether ketone) by sulfonated poly(ether ether ketone) for anion exchange membranes.

Abstract: Two N3-substituted imidazoles 1,2-dimethylimidazole and 1-butyl-2-methylimidazole were chosen to functionalize poly(aryl ether ketone), respectively. The generated imidazolium cations could electrostatically react with sulfonate ions of the sulfonated poly(ether ether ketone) forming the ionic crosslinking structure of the membranes. The changes in crosslinking degree and the alkyl chain-length on N3 site of the imidazoliums could highly affect the properties of the anion exchange membranes (AEMs). The AEMs functionalized by 1-butyl-2-methylimidazole exhibited superior properties compared to those functionalized by 1,2-dimethylimidazole according to the tolerance tests of the AEMs towards hot alkaline solutions. After exposed to 1M KOH at 80°C for 200h, the 1-butyl-2-methylimidazole modified AEMs maintained the ion exchange capacity of above 85%, the conductivity of about 70%, and the tensile stress at break of around 80%, respectively. The hydrophile-lipophile balance of the polymer membranes was calculated and proposed to better understand the correlation between structures and properties of the AEMs. The degradation of the imidazolium functional groups of the AEMs under the attack of hydroxide ions was evidenced by FT-IR analysis.

4,5‐Dimethyl‐2‐Iodoxybenzenesulfonic Acid Catalyzed Site‐Selective Oxidation of 2‐Substituted Phenols to 1,2‐Quinols

Abstract: A site-selective hydroxylative dearomatization of 2-substituted phenols to either 1,2-benzoquinols or their cyclodimers, catalyzed by 4,5-dimethyl-2-iodoxybenzenesulfonic acid with Oxone, has been developed. Natural products such as biscarvacrol and lacinilene C methyl ether could be synthesized efficiently under mild reaction conditions. Furthermore, both the reaction rate and site selectivity could be further improved by the introduction of a trialkylsilylmethyl substituent at the 2-position of phenols. The corresponding 1,2-quinols could be transformed into various useful structural motifs by [4+2] cycloaddition cascade reactions.