Brønsted acidic ionic liquid [HMIm]BF4 promoted simple and efficient one-pot green synthesis of isoxazolyl-1,3-benzoxazines at ambient temperature

Abstract

Brønsted acidic ionic liquid [HMIm]BF₄ has been used as a nonvolatile, ecofriendly solvent, and catalytic medium for the one-pot green synthesis of isoxazolyl-1,3-benzoxazines. The reaction afforded excellent yield in short reaction time, and avoids multistep synthesis. The easy of recyclability of the IL makes the reaction economically and potentially viable for commercial applications.     

酸性离子液体催化油酸酯化合成生物柴油简

酸性离子液体具有催化活性好、选择性高及易于回收等优点,是一种应用前景非常好的环境友好的酸性催化剂,在生物柴油合成反应中具有重大的理论意义和应用价值. 本文以油酸和甲醇为原料,探讨了7种不同酸性离子液体在生物柴油合成反应中的催化效应. 研究表明,离子液体酸性越强,催化酯化活性越高;引入磺酸基团可大大增强离子液体Brönsted酸性,使其在酯化反应中发挥溶剂/催化剂的双重作用,促进酯化反应向产物方向进行,达到高产率,因而1-丁基磺酸-3-甲基咪唑硫酸氢盐（[BHSO₃MIM]HSO₄）催化效果最好. 此外,系统研究了[BHSO₃MIM]HSO₄催化油酸与甲醇酯化反应,并采用响应面法优化了反应条件. 结果发现,该反应的最适醇酸摩尔比、催化剂用量、反应温度及反应时间分别为4：1,10%（基于油酸的质量）,130 ℃和4 h;在此条件下,生物柴油产率为97.7%. [BHSO₃MIM]HSO₄连续使用10批次后,仍能保持初始催化活性的95.6%,表现出极好的操作稳定性. 另外,利用该离子液体催化游离脂肪酸含量为72%的废油脂生产生物柴油,反应6 h可获得产率94.9%. 可见,[BHSO₃MIM]HSO₄在酯化生产生物柴油方面具有巨大的应用潜力.     

Stable poly (ionic liquids) with unique cross‐linked mesoporous‐macroporous structure as efficient catalyst for alkylation of o‐xylene and styrene

In this work, using divinylbenzene (D), 1‐vinylimidazole (V) and 1‐vinyl‐3‐butylimidazolium bromide ([VBIM][Br]) as monomers, the binary‐monomer poly (ionic liquids) (PILs) and ternary‐monomer PILs were successfully synthesized, via hydrothermal polymerization and anion exchange, sequentially. Compared with each other, the ternary polymeric acidic IL catalyst has a clear spongy porous structure, while having a more stable macroporous structure, a larger specific surface area, more acidic groups and more active sites. Catalytic performance of catalyst was investigated through the alkylation of o‐xylene and styrene. The effect of the amount of IL added and the length of the cation chain on the ternary polymerization of acidic IL was systematically investigated. Under optimal reaction conditions (molar ratio of monomers was D:V:[VBIM][Br] = 2:1:1, the most suitable cation chain length was C₄), the synthesized MPD‐[C₄V]‐[VBIM][SO₃CF₃] has a larger specific surface area (89.47 m²/g), large pore volume (0.29 cm³/g), and abundant mesopores and macropores, which help to improve the contact between the active site and reactants. Moreover, the catalyst could maintain a relatively high conversion of styrene (99.0%), 1,2‐diphenylethane yield (98.7%) and high thermostability under reaction and be easily be divided from the solution, which is critical for heterogeneous solid catalysts.     

Kinetics and mechanism of the defluorination of 8-fluoropurine nucleosides in basic and acidic media

For investigating the stability of C(8)-fluorine bond in 8-fluoropurine nucleosides some protected 8-fluoroguanosine, 8-fluoroinosine and 8-fluoroadenosine derivatives were prepared by direct fluorination of acetyl-protected purine nucleosides with elemental fluorine in solvents such as chloroform, acetonitrile and nitromethane. Fluorination reactions conducted in chloroform medium gave better yields of 8-fluoropurines. The fluorination yields were slightly lower when acetonitrile or nitromethane was used as solvent, but the product purification was found to be much easier. When the synthesized, protected fluoronucleosides were subjected to standard basic (NH₃ in methanol or 2-propanol) and acidic (HCl in methanol) deprotection conditions relevant to nucleoside chemistry, an efficient defluorination reaction took place. The kinetics of these defluorination reactions were conveniently followed, under pseudo-first-order reaction conditions, using ¹⁹F NMR spectroscopy. ¹H NMR, LC-MS and mass spectroscopy identified the products of the kinetic reaction mixtures. The defluorination reaction rate constants (k_obs) in basic media depended upon the electron density at C(8) while the k_obs data in acidic medium were determined by the pK_a of N⁷. An addition-elimination based mechanism (S_NAr) has been proposed for the defluorination reactions of these 8-fluoropurine nucleosides.     

Cationic polymerization of α-methylstyrene in dichloromethane initiated with system 2,5-dichloro-2,5-dimethylhexane-BCl3. I. Molecular weight distribution and stereoregularity of poly (α-methylstyrene)

In the polymerization of α-methylstyrene (α-MeSt) in dichloromethane in the temperature interval between ?60 and ?20°C the polymer yield decreased with increasing temperature depending on the initiating system used (I-IV) in the series II > I > IV > III, where I was a freshly prepared solution of 2,5-dichloro-2,5-dimethylhexane (DDH) with BCl₃ in dichloromethane, was the same solution as in the preceding case, but stored at room temperature one month and then used, III was a freshly prepared BCl₃ solution in dichloromethane, and IV was the initiation system “H₂O”/BCl₃. The polymer samples synthesized at ≤ ?30°C had a bimodal molecular weight distribution (MWD), which was attributed to the different participation of ionic pairs and free ions in the propagation reaction. The stereoregularity of the polymer observed (ca. 85% syndiotactic and ca. 15% heterotactic triads) determined from the ₁H-NMR spectra was not affected by the difference in the initiation system. MWD of the polymer samples was investigated by the GPC method     

Green and diasteroselective oxidative cyclization of bisnaphthols to spirans

Hydrogen peroxide/MoO₃, as an efficient and clean oxidizing system was used to afford diasteroselective oxidative cyclization of bisnaphthols to spirans in ethanol at 60 °C with high yields. Bisnaphthols were prepared by the reaction of a series of aldehydes and 2-naphthol in the presence of a catalytic amount of H₃[P(Mo₃O₁₀)₄].nH₂O (HPa) in refluxing dichloromethane.     

A facile and highly efficient protocol for synthesis of poly(ether sulfone)s in ionic liquid/zwitterion

A benign approach for efficient preparation of poly(ether sulfone)(PES)(η_(inh) = 0.10-0.31 dL×g~(-1)) has been presented by using ionic liquid(IL)/zwitterion(ZI) as reaction medium. It has been found that the interaction between 4,4′-dihydroxydiphenylsulfone(Bisphenol-S) and ZI at elevated temperatures results in efficient reduction of dehydration time, which is confirmed by TGA curves, melting point of the mixture and FTIR spectra. Furthermore, no agglomeration is observed at dehydration stage, which is attributed to the high solubility of Bisphenol-S dipotassium salt in IL/ZI. This also makes the polymerization temperature(150 °C) much lower than a conventional method(220-300 °C). In order to demonstrate the efficiency of IL/ZI as reaction medium, the polymerization was also performed in sulfolane under the same reaction condition as that in IL/ZI.     

A New and Simple Electrocatalyst for Formaldehyde Oxidation; Nickel/poly(o‐Anisidine)/Film Modified Ionic Liquid Carbon Paste Electrode

In present work, the ionic liquid, 1‐butyl‐3‐methylimidazolium bis (trifluoromethylsulfonyl) imide was incorporated in the carbon paste electrode as the binder (IL‐CPE). O‐anisidine (OA) monomer is electropolymerized in the presence of an aqueous acidic solution onto IL‐CPE (POA/IL‐CPE). The as‐prepared substrate is used as a porous matrix for dispersion of Ni(II) ions by immersing the modified electrode in a nickel(II) nitrite solution. The modified electrodes are characterized by scanning electron microscopy (SEM) and electrochemical methods. The POA/IL‐CPE was applied successfully to highly efficient (current density of 18.2 mA cm^?2) electrocatalytic oxidation of formaldehyde in alkaline medium. Finally, the rate constant for chemical reaction between formaldehyde and redox sites of the electrode was calculated.     

Isonitrosoacetanilide as an analytical reagent

In a continuation of earlier work on the reactivity of various isonitrosoacetarylides, the simplest of the series, isonitrosoacetanilide was examined. In acidic medium, this reagent gives a specific reaction with palladium (dilution limit 1:400,000), and in ammoniacal medium a very sensitive reaction with cobalt (dilution limit 1:1,000,000). The composition of the complexes involved was established as R₂Pd and R₃Co₂. The effect of substitution on the reactivity is discussed.     

Synthesis of polyferromethylsiloxane sorbents using a sol–gel method

Trivalent iron complexes which could be easily converted into materials are formed by the reaction of FeCl₃ with K₃[O₃SiMe] in a highly concentrated aqueous alkaline solution. The presence of the liquid glass as an additive and decreasing the pH results in solutions that give rise to homogeneous gels. Polycondensation proceeds very rapidly in the higher pH range (viz. pH 9–10) and substantially slower in an acidic medium (pH 2–3). Xerogels were obtained having microporous structure after treatment of polyferromethylsiloxane gels obtained from acidic medium or mesoporous structure when obtained from alkaline medium.     

Theoretical study of DABCO-based ionic liquid: synthesis and reaction mechanism

Quaternary ammonium salt obtained from the Menshutkin reaction between DABCO and benzyl chloride has been used in the synthesis of a novel Bronsted acidic ionic liquid (IL), namely 1-benzyl-4-(sulfobutyl)-diaza-bicyclo-octane hydrogen sulfate. The reaction of DABCO with benzyl chloride is a crucial step in the synthesis of this IL. Density functional theory calculations at B3LYP/6-31G(d,p) level have been employed to investigate the mechanism of Menshutkin reaction by calculating the energy barriers through possible transition states i.e., five-membered ring transition state and S_N2 transition state in gas phase and in diethyl ether as a solvent. It was found that while DABCO reacts with benzyl chloride through the well-known S_N2 transition state mechanism, the corresponding reaction with chlorodiphenylmethane can proceed through both S_N2 and five-membered ring transition state mechanism. However, S_N2 transition state mechanism is still the strongly preferred one out of the two possible mechanisms. The electronic structure analysis shows that solvent effects and enhanced resonance stabilization may play a decisive role in guiding the reaction pathway.     

Synthesis of P123-Templated and DVB-Cross-linked Meso-macroporous Poly (ionic liquids) with High-Performance Alkylation

Meso-macroporous poly (ionic liquids) (MPILs) were successfully synthesized through a novel strategy with P123 (PEO₂₀PPO₇₀PEO₂₀) as the pore-forming agent and divinylbenzene as crosslinker. Then effects of reaction temperature and the molar ratio of ionic liquid to divinylbenzene (IL to DVB) were systematically investigated together with the morphology and structure of MPILs. It was found the MPILs owned a meso-macroporous channel-like structure, large surface area (up to 732 m²/g), large pore volume (1.42 cm³/g) and high thermal stability. Meanwhile, the Brunauer–Emmett–Teller surface of MPILs could be fabricated by adjusting the reaction temperature and especially the IL to DVB molar ratio. In the probe reaction of alkylation of styrene and o-xylene, a 100% conversion of styrene and 96.4% PXE yield was acquired, and this is more expert than common commercial liquid acid catalyst. This work might develop a novel way for synthesizing porous heterogeneous polymer catalysts in the future and owning a promising future applied in other acidic catalytic reactions.     

Electrooxidation Mechanism of Methanol at Pt-Ru Catalyst Modified GC Electrode in Electrolytes with Different pH Using Electrochemical and SERS Techniques

The electrochemical and in-situ surface-enhanced Raman spectroscopy （SERS） techniques were used to investigate the electrooxidation behavior of methanol in acidic, neutral and alkaline media at a Pt-Ru nanoparticle modified glassy carbon （Pt-Ru/GC） electrode. The results showed that methanol could be dissociated spontaneously at the Pt-Ru/GC electrode to produce a strongly adsorbed intermediate, CO. It was found that CO could be oxidized more easily in the alkaline medium than in the acidic and neutral media. The peak potential of methanol oxidation was shifted from 0.663 and 0.708 V in the acidic and neutral media to -0.030 V in the alkaline medium, which is due to that the adsorption strength of CO on the Pt surface in the alkaline medium is weaker than that in the acidic and neutral media. The final product of the methanol oxidation is CO2. However, in the alkaline medium, CO2 produced would form CO3^2- and HCO3^- resulting in the decrease in the alkaline concentration and then in the decrease in the performance of DMFC. Therefore, the performance of the alkaline DMFC is not Stable.     

Microwave-assisted rapid and efficient deprotection and direct esterification and silylation of MOM and EOM ethers catalyzed by [Hmim][HSO4] as a Br?nsted acidic ionic liquid

Abstract  

1-Methylimidazolium hydrogensulfate, [Hmim][HSO₄], a Br?nsted acidic room temperature ionic liquid, is used as a catalyst and reaction medium for facile and eco-friendly deprotection of methoxymethyl (MOM) and ethoxymethyl (EOM) ethers to their corresponding alcohols under thermal conditions (Δ) and microwave irradiation (MW). Furthermore, one-pot interconversion to the respective acetates and trimethylsilyl (TMS) ethers was also achieved.     

Rapid, efficient, room temperature aromatization of Hantzsch-1,4-dihydropyridines with vanadium(V) salts: superiority of classical technique versus microwave promoted reaction

The aromatization of 1,4-dihydropyridines (1,4-DHPs) employing group 4 (Zr and Hf) and 5 (V, Nb, Ta) elements of periodic system has been studied. The reaction with VOCl₃ in dichloromethane at room temperature afforded products, substituted pyridines, in high-to-excellent yield. For the first time, the formation of charge-transfer complexes (CTCs) has been evidenced in preorganization step between 1,4-DHP and oxidant before electron transfer. The CTC has been formed only in neutral solvents such as dichloromethane and is characterized by intensive coloration. The aromatization of 1,4-DHP with V₂O₅ in refluxing acetic acid has found to be superior over microwave promoted reaction in solventless media. The only reasonable explanation was found in polymeric structure of V₂O₅, which slowly transfer energy of microwaves needed for the activation of the reactants. The solvent polarity dependent oxidative dealkylation of 4-n-propyl-1,4-DHP has been discovered. Unexpectedly, the reaction in acetic acid afforded only 33% of dealkylated product compared to 91% obtained in dichloromethane under the same reaction conditions.     

The preparation of sol-gel materials doped with ionic liquids and trialkyl phosphine oxides for yttrium(III) uptake

A new material (IL923SGs) composed of ionic liquids and trialkyl phosphine oxides (Cyanex 923) for Y(III) uptake was prepared via a sol-gel method. The hydrophobic ionic liquid 1-octyl-3-methylimidazolium hexafluorophosphate (C₈mim⁺PF₆⁻) was used as solvent medium and pore templating material. The extraction of Y(III) by IL923SGs was mainly due to the complexation of metal ions with Cyanex 923 doped in the solid silica. Ionic liquid was stably doped into the silica gel matrix providing a diffusion medium for Cyanex 923, and this will result in higher removal efficiencies and excellent stability for metal ions separation. IL923SGs were also easily regenerated and reused in the subsequent removal of Y(III) in four cycles.     

Régiospécificité et stéréosélectivité de la cycloaddition de l'oxyde de benzonitrile sur les pyrazolines-3

Pursuing our study on the cycloaddition of benzonitrile oxide with Δ₃-pyrazolines (1,2), we can now affirm that this reaction is regiospecific and stereoselective. The behaviour in acidic medium was studied. We have then discussed the process of epimerization and degradation observed according to the C₄ substituent of the bicyclic molecule and the solvent used.     

Synthetic and natural silica‐aluminates as inorganic acidic catalysts in ring opening polymerization of cyclosiloxanes

Ring opening polymerization (ROP) of hexamethylcyclotrisiloxane (D₃) and octamethylcyclotetrasiloxane (D₄) was promoted by acid‐treated synthetic and natural silica‐aluminates. Silica‐alumina (1:3 Si/Al molar ratio) was obtained using a simple and economic route from precipitation of aluminum sulfate solutions. The material was treated in an acidic medium to improve the content of acid sites and successfully tested as inorganic acidic catalyst for ROP of D₃ or D₄ cyclosiloxanes. Natural bentonite was treated and used in a similar manner. Once the ROP reaction completed, the catalyst was easily removed and it was found that the recovered synthetic silica‐alumina was active in a second ROP reaction. The effect of the concentration and type of catalyst in respect to the molecular weight and polydispersity of polydimethylsiloxanes was analyzed: increasing the amount of silica‐alumina in ROP of D₄ from 0.05 to 0.1 g decreased the average molecular weight (M_n = 13–1.8 kDa) associated with an increase in the polydispersity (2.95 vs. 1.81). Analogous results were found with bentonite. These values suggest that an increase in the catalyst concentration led to a lower M_n, with a more homogeneous molecular chain dimension. Copyright © 2012 John Wiley & Sons, Ltd.     

Heterogeneous strong base catalysis in supercritical carbon dioxide by mesoporous alumina and sulfated mesoporous alumina

The development of solid strong base catalysts utilizable in green but acidic medium of scCO₂ is reviewed. The strong base sites on mesoporous alumina and sulfated mesoporous alumina that had been generated by severe treatment at 773 K under vacuum (10^?4 Torr) were not neutralized by the compressed Lewis acidic molecules of CO₂, promoting a representative strong base-catalyzed reaction of the Tishchenko reaction as well as a typical base-catalyzed reaction of the Knoevenagel reaction in scCO₂. Infrared spectroscopy of the adsorbed pyrrole, temperature-programmed desorption of CO₂, and the poisoning by a very weak Brönsted acid of methanol have revealed that the average strengths of the base sites on mesoporous alumina and sulfated mesoporous alumina are weaker than that on conventional γ-alumina like JRC-ALO-4, but that they have a small number of strong base sites which function even in scCO₂ medium. It was found that the addition of a slight amount of THF cosolvent into scCO₂ remarkably accelerates the Tishchenko reaction over sulfated mesoporous alumina; the reaction rate in the scCO₂–THF medium was 1.5-fold and 2-fold faster than those in ordinary organic solvents such as benzene and THF and that in pure scCO₂, respectively. The unique structures of mesoporous alumina and sulfated mesoporous alumina have been fully characterized by N₂ adsorption–desorption measurements and XRD analyses.     

Cr(VI) oxidation using cetylpicolinium dichromate: Kinetics of oxidation of benzaldehydes with a green protocol

Oxidation kinetics of benzaldehyde and some of its ortho- and para-monosubstituted derivatives have been studied using cetylpicolinium dichromates, a class of novel phase transfer oxidants, in dichloromethane medium. The rate of reaction is first order with respect to oxidant and fractional order with respect to the substrates. The Michaelis–Menten type oxidation was observed with respect to the substrates. Benzaldehydes are found to be oxidized to their corresponding acids. The mechanism of oxidation reaction has been suggested based on the solvent isotope effect, Hammett plot, and thermodynamic study. The solvent isotope effect (k_CHCl3/k_CDCl3 = 1.57) indicates the involvement of hydrogen exchange with the medium during oxidation reactions. A strong influence of specific solute–solvent interactions on the rate of the reaction is observed. Both the electron-withdrawing and electron-releasing substituents on the substrates accelerate the rate of reaction.