On the Mechanism of the Reactivity of 1,3‐Dialkylimidazolium Salts under Basic to Acidic Conditions: A Combined Kinetic and Computational Study

Comprehensive spectroscopic kinetic studies illustrate an alternative mechanism for the traditional free‐carbene intermediated H/D exchange reaction of 1,3‐dialkylimidazolium salts under neutral (D₂O) and acidic conditions (DCl/D₂O 35 wt % solution). The deuteration of high purity [bmim]Cl in D₂O is studied at different temperatures, in absence of catalyst or impurities, to yield an activation energy. DFT transition‐state modelling, of a small water cluster and [bmim] cation, also yields an activation energy which strongly supports the proposed mechanism. The presence of basic impurities are shown to significantly enhance the exchange reaction, which brings into question the need for further analysis of technical purities of ionic liquids and the implications for a wide range of chemical reactions in such media.     

Theoretical study of the mechanism for the Markovnikov addition of imidazole to vinyl acetate catalyzed by the ionic liquid [bmIm]OH

By performing density functional theory calculations, we have studied the Markovnikov addition of imidazole to vinyl acetate catalyzed by 1-butyl-3-methylimidazolium hydroxide, [bmIm]OH, a novel basic ionic liquid. The purpose is to show the detailed reaction mechanism, to rationalize the experimental observations, and in particular to better understand the role of [bmIm]OH played in the reaction as a catalyst. Two different reaction pathways (stepwise and concerted) have been characterized and analyzed in detail. It was found that both the cation and anion of [bmIm]OH play important roles in the Markovnikov addition, which decrease the barrier and increase the selectivity of Markovnikov addition. The present results rationalize the early experimental findings well and provide a prototype for theoretically understanding the Markovnikov addition catalyzed by room-temperature ionic liquids.     

Chemoselective Esterification of Phenolic Acids in the Presence of Sodium Bicarbonate in Ionic Liquids

Chemoselective esterification of phenolic acids with dialkyl sulphates or alkyl halides in the presence of sodium bicarbonate in 1,3‐dialkylimidazolium ionic liquids is reported in excellent yields and less reaction time as compared to organic solvents.     

Ionic Liquids as Novel Reaction Media for the Synthesis of Condensation Polymers

The use of ionic liquids as novel solvents for the synthesis of condensation polymers was investigated. A series of ionic liquids including new ones was synthesized and purified. 1,3‐Dialkylimidazolium‐based ionic liquids seem to be suitable reaction and activating media for the synthesis of high‐molecular‐weight aromatic polyimides and polyamides. Inherent viscosities of the polymers obtained in 1,3‐dialkylimidazolium bromides range from 0.52 to 1.35 dL/g.     

Diels‐Alder Reaction in Air‐ and Moisture‐Stable Zinc‐Containing Ionic Liquids

Diels‐Alder reactions in a number of air‐ and moisture‐stable dialkylimidazolium halide‐ZnCl₂ ionic liquids are reported. High yields and high endo selectivities have been observed. The ionic liquids could then be recyclable without loss of reactivity.     

Novel synthesis of high‐molecular‐weight prepolymer of poly(p‐phenylene benzoxazole) in ionic liquids

Using ionic liquids (ILs) as the reaction solvent for the synthesis of prepolymer polyamide of poly(p‐phenylene benzoxazole) (PBO) was investigated. The optimum condition of prepolymer preparation was determined in ILs. A series of 1,3‐dialkylimidazolium ILs were used to be the reaction media of the polycondensation. The relationship between the molecular weight of prepolymer and the structure of ILs was analysed by changing the structure of the cation and species of anion of ILs. In order to prove the feasibility of the transformation, the prepolymer was used to prepare PBO in polyphosphoric acid media, and the conversion process was analyzed. The spinnability of the PBO solution was explored by the preparation of PBO fibers. The basic mechanical properties of PBO single fiber were tested. In a word, using 1,3‐dialkylimidazolium ILs as the reaction solvents was feasible for the synthesis of high‐molecular‐weight PBO prepolymer, which could be a promising PBO preparation method.     

Novel Cyclic Sulfonium‐Based Ionic Liquids: Synthesis,Characterization, and Physicochemical Properties

A new series of ionic liquids composed of three cyclic sulfonium cations and four anions has been synthesized and characterized. Their physicochemical properties, including their spectroscopic characteristics, ion cluster behavior, surface properties, phase transitions, thermal stability, density, viscosity, refractive index, tribological properties, ion conductivity, and electrochemical window have been comprehensively studied. Eight of these salts are liquids at room temperature, at which some salts based on [NO₃]^? and [NTf₂]^? ions exhibit organic plastic crystal behaviors, and all the saccharin‐based salts display relatively high refractive indices (1.442–1.594). In addition, some ionic liquids with the [NTf₂]^? ion exhibit peculiar spectroscopic characteristics in FTIR and UV/Vis regions, whilst those salts based on the [DCA]^? ion show lower viscosities (34.2–62.6 mPa s at 20 °C) and much higher conductivities (7.6–17.6 mS cm^?1 at 20 °C) than most traditional 1,3‐dialkylimidazolium salts.     

Aza‐Crown Ether Complex Cation Ionic Liquids: Preparation and Applications in Organic Reactions

Aza‐crown ether complex cation ionic liquids (aCECILs) were devised, fabricated, and characterized by using NMR spectroscopy, MS, thermogravimetric differential thermal analysis (TG‐DTA), elemental analysis and physical properties. These new and room‐temperature ILs were utilized as catalysts in various organic reactions, such as the cycloaddition reaction of CO₂ to epoxides, esterification of acetic acid and alcohols, the condensation reaction of aniline and propylene carbonate, and Friedel–Crafts alkylation of indole with aldehydes were investigated carefully. In these reactions, the ionic liquid exhibited cooperative catalytic activity between the anion and cation. In addition, the aza‐[18‐C‐6HK][HSO₄]₂ was the best acidic catalyst in the reactions of esterification and Friedel–Crafts alkylation under mild reaction conditions.     

A DFT study on the mechanisms for the cycloaddition reactions between 1‐Aza‐2‐azoniaallene cations and carbodiimides

The mechanisms of the title reactions between 1‐aza‐2‐azoniaallene cations and carbodiimides in the gas phase have been examined using the Becke‐3‐parameter‐Lee‐Yang‐Parr (B3LYP) at 6‐31++G** level. The theoretical results revealed that the reaction is a domino reaction that comprises two consecutive reactions: an ionic [3+2] cycloaddition reaction between 1‐aza‐2‐azoniallene cation and carbodiimide to yield the cycloadduct 3 and then a [1,2]‐shift to yield the thermodynamically more stable adduct 4 . Both stepwise and concerted pathways are accessible in the first cycloaddition in the model reaction. The activation barriers of them are almost equivalent. For the [1,2]‐shift reactions, both of the electron‐withdrawing chlorine substituent and the electron‐releasing methyl substituent on the 1‐aza‐2‐azoniaallene cation can facilitate the reaction but have little effects when substituted in the carbodiimide moiety. The model reaction has also been investigated at the QCISD (quadratic configuration interaction using single and double substitutions)/6‐31++G** and CCSD(T) (coupled cluster calculations with single and double excitations and a perturbative estimate of triple contributions calculations)/6‐31++G** levels as well as by the density functional theory. In addition, solvent effects with the isodensity‐surface polarized continuum model are also reported for all the reactions. In solvent dichloromethane, the cycloadducts of all the reactions, except model reaction and reaction d, were obtained from reactants directly as the result of the solvent effect. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

离子液体溶剂中5-氧代-5,6,7,8-四氢苯并吡喃衍生物的一种温和有效的合成

A one-pot synthesis of a series of 5-oxo-5,6,7,8-tetrahydro-4H-benzo-[b]-pyran derivatives via three-component coupling reactions of aldehydes, dimedone and malononitrilein room temperature ionic liquids （RTILs） without any catalyst has been reported. In the meantime, the reuse of ionic liquids and the effect of different ionic liquids as solvent on the reaction have also been investigated.     

[EPy][BF4]和[EPy][PF6]离子对的气相及液相阴阳离子相互作用研究

采用从头算HF/6-31G和密度泛函理论B3LYP/6-31+G(d,p)方法, 对乙基吡啶四氟硼酸盐([EPy][BF4])和乙基吡啶六氟磷酸盐([EPy][PF6])的离子对进行了结构优化和频率分析, 并利用自洽反应场(SCRF)的导体极化连续模型(CPCM)考察了离子对液态下的结构及相互作用, 得到了两种离子对的红外光谱及气相、液相下最稳定结构. 由两种离子对的几何参数可知, 阴阳离子通过氢键相互作用, 两种离子液体的红外光谱特征值与实验值比较吻合. 应用自然键轨道(NBO)理论分析了吡啶阳离子及离子对中的原子电荷分布和电荷转移情况, 结果表明两种离子对中阴阳离子间存在静电相互作用和氢键作用. 通过几何参数、相互作用能及NBO分析研究发现, 液相下由于周围电荷的中和作用, 离子对中阴阳离子的相互作用明显降低.     

Biocatalysis and Biomass Conversion in Alternative Reaction Media

In this Minireview, the state of the art in the use of ionic liquids (ILs) and deep eutectic solvents (DESs) as alternative reaction media for biocatalytic processes and biomass conversion is presented. Initial, proof‐of‐concept studies, more than a decade ago, involved first‐generation ILs based on dialkylimidazolium cations and non‐coordinating anions, such as tetrafluoroborate and hexafluorophosphate. More recently, emphasis has switched to more environmentally acceptable second‐generation ILs comprising cations, which are designed to be compatible with enzymes and, in many cases are derived from readily available, renewable resources, such as cholinium salts. Protic ionic liquids (PILs), prepared simply by mixing inexpensive amines and acids, are particularly attractive from both an environmental and economic viewpoint. DESs, prepared by mixing inexpensive salts with, preferably renewable, hydrogen‐bond donors such as glycerol and amino acids, have also proved suitable reaction media for biocatalytic conversions. A broad range of enzymes can be used in ILs, PILs and DESs, for example lipases in biodiesel production. These neoteric solvents are of particular interest, however, as reaction media for biocatalytic conversions of substrates that have limited solubility in common organic solvents, such as carbohydrates, nucleosides, steroids and polysaccharides. This has culminated in the recent focus of attention on their use as (co)solvents in the pretreatment and saccharification of lignocellulose as the initial steps in the conversion of second‐generation renewable biomass into biofuels and chemicals. They can similarly be used as reaction media in subsequent conversions of hexoses and pentoses into platform chemicals.     

Synthesis of poly(glycolic acid) in ionic liquids

The synthesis of poly(glycolic acid) (PGA) by polyesterification of glycolic acid was studied using ionic liquids, mainly 1,3‐dialkylimidazolium salts, as reaction media. The ¹H NMR spectra of PGA oligomers were assigned and end‐group signals were used to follow the reaction. Low PGA yields were obtained by the direct polyesterification of glycolic acid at 200–240 °C, because of monomer evaporation during the reaction. On the other hand, PGAs of DP _n up to 45 were obtained by the postpolycondensation of a preformed oligomer in 1‐butyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIm⁺Tf₂N^?). The precipitation of PGA in reaction medium at long reaction times limited the achievable molar mass. Rate constants were determined for catalyzed and noncatalyzed reactions, assuming a second‐order reaction mechanism. The efficiency of esterification catalysts such as Zn(OAc)₂ was low in these media, as only about twofold increases in reaction rate were observed. This was assigned to the preferential interaction of Zn²⁺ with ionic liquid anion instead of the polymer carboxylic acid end‐groups. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3025–3035, 2006     

烷基咪唑型卤盐类离子液体的合成机理研究

用密度泛函理论考察了甲基咪唑和一系列的卤代烷烃(氯乙烷,氯丁烷,溴乙烷,溴丁烷)反应合成咪唑类离子液体的反应机理. 在B3LYP/6-31++G**//B3LYP/6-31G*基组水平上找到了两条反应路径：路径A(反应物→TS1→P1)和路径B(反应物→TS2→P2). 在路径A中, 卤素离子和咪唑环C2上的氢质子形成氢键；在路径B中, 卤素离子和咪唑环C5上的氢质子形成氢键. 计算发现, 氢键的形成在反应中起到了非常重要的作用, 特别是咪唑环C2上的氢质子在和卤素离子成氢键后形成了一个五员环结构的过渡态, 该过渡态能量较低. 经过渡态TS1的反应途径其活化能要低于经过渡态TS2的反应途径, 反应路径A为主要的反应通道. 计算结果表明, 经过渡态TS1的反应途径是一放热过程, 这和实验观察现象一致.     

Identification of 1,3-dialkylimidazolium salt supramolecular aggregates in solution

The nature of the interactions between 1,3-dialkylimidazolium cations and noncoordinating anions such as tetrafluoroborate, hexafluorophosphate, and tetraphenylborate has been studied in the solid state by X-ray diffraction analysis and in solution by (1)H NMR spectroscopy, conductivity, and microcalorimetry. In the solid state, these compounds show an extended network of hydrogen-bonded cations and anions in which one cation is surrounded by at least three anions and one anion is surrounded by at least three imidazolium cations. In the pure form, imidazolium salts are better described as polymeric supramolecules of the type {[(DAI)(3)(X)](2+)[(DAI)(X)(3)](2-)}(n) (where DAI is the dialkylimidazolium cation and X is the anion) formed through hydrogen bonds of the imidazolium cation with the anion. In solution, this supramolecular structural organization is maintained to a great extent, at least in solvents of low dielectric constant, indicating that mixtures of imidazolium ionic liquids with other molecules can be considered as nanostructured materials. This model is very useful for the rationalization of the majority of the unusual behavior of the ionic liquids.     

Natural terpene derivatives as new structural task‐specific ionic liquids to enhance the enantiorecognition of acidic enantiomers on teicoplanin‐based stationary phase by high‐performance liquid chromatography

We present the specific cooperative effect of a semisynthetic glycopeptide antibiotic teicoplanin and chiral ionic liquids containing the (1R ,2S ,5R )‐(–)‐menthol moiety on the chiral recognition of enantiomers of mandelic acid, vanilmandelic acid, and phenyllactic acid. Experiments were performed chromatographically on an Astec Chirobiotic T chiral stationary phase applying the mobile phase with the addition of the chiral ionic liquids. The stereoselective binding of enantiomers to teicoplanin in presence of new chiral ionic liquids were evaluated applying thermodynamic measurements and the docking simulations. Both the experimental and theoretical methods revealed that the chiral recognition of enantiomers in the presence of new chiral ionic liquids was enthalpy driven. The changes of the teicoplanin conformation occurring upon binding of the chiral ionic liquids are responsible for the differences in the standard changes in Gibbs energy (ΔG ⁰) values obtained for complexes formed by the R and S enantiomers and teicoplanin. Docking simulations revealed the steric adjustment between the chiral ionic liquids cyclohexane ring (chair conformation) and the β‐d ‐glucosamine ring of teicoplanin and additionally hydrophobic interactions between the decanoic aliphatic chain of teicoplanin and the alkyl group of the tested salts. The obtained terpene derivatives can be considered as “structural task‐specific ionic liquids” responsible for enhancing the chiral resolution in synergistic systems with two chiral selectors.     

Tetraalkylphosphonium-based ionic liquids

Ionic liquids are salts that are liquid at or near room temperature. Their wide liquid range, good thermal stability, and very low vapor pressure make them attractive for numerous applications. The general approach to creating ionic liquids is to employ a large, unreactive, low symmetry cation with and an anion that largely controls the physical and chemical properties. The most common cations used in ionic liquids are N-alkylpyridinium and N,N′-dialkylimidazolium. Another very effective cation for the creation of ionic liquids is tetraalkylphosphonium, [PR₁R₂R₃R₄]⁺. The alkyl groups, R_n, generally are large and not all the same. The halide salts of several phosphonium cations are available as starting materials for metathesis reactions used to prepare ionic liquids. The large phosphonium cations can combine with relatively large anions to make viscous but free flowing liquids with formula mass greater than 1000 g mol⁻¹. Some other more massive salts are waxes and glasses. The synthesis and the physical, chemical, and optical properties of phosphonium-ionic liquids having anions with a wide range of masses were measured and are reported here.     

氯铝酸室温离子液体中缩醛和缩酮反应

以取代硫酸等无机酸，实现清洁合成为目的，在1－烷基吡啶和1－甲基－3－ 烷基咪唑季胺盐与无水AlCl_3构成的室温离子液体为催化剂和反应介质中，尝试了 醛和酮与甲醇的缩合反应。醛与甲醇反应，产物以缩醛为主，酮与甲醇反应则有相 当量的Aldol缩合产物。依反应底物不同，可获得中至高的转化率和选择性。同时 ，一些产物因不溶于离子液体中而分层，便于产物分离。     

A theoretical study of the solvent effect on Diels-Alder reaction in room temperature ionic liquids using a supermolecular approach

The Diels-Alder reaction between cyclopentadiene and three dienophiles (acrolein, methyl acrylate and acrylonitrile) in different room temperature ionic liquids containing imidazolium-based cations ([HBIM]⁺, [BMIM]⁺ and [BM₂IM]⁺) has been studied at the DFT level using a supermolecular approach. An analysis of the theoretical results shows that the ionic liquid cation coordination affects the equilibrium geometries and electronic structures of the reacting species throughout the reaction pathway leading to changes in reactivity and selectivity. They also indicate that the strong asynchronicity of the Diels-Alder reactions in ionic liquids is due to the polarisation effect of the cation.     

Preparation of SRN1-type coupling adducts from aliphatic gem-dinitro compounds in ionic liquids

S(RN)1-type coupling adducts are readily prepared by the reaction between a-sulfonylesters or a-cyanosulfones and gem-dinitro compounds in ionic liquids. The reactions progress smoothly and recovered ionic liquids can be used for several iterations, as long as they are washed with water to remove alkali metallic salts. The reaction rate is slower than the corresponding S(RN)1 reaction in DMSO, but no acceleration on irradiation or no inhibition in the presence of m-DNB are observed.