Bifunctional Gyroidal MOFs: Highly Efficient Lewis Base and Lewis Acid Catalysts

A family of gyroidal metal–organic frameworks (STUs) composited with transition metal ions and bi‐imidazolate ligands (BIm) were prepared and applied as both Lewis base and acid catalysts. Benefiting from the intrinsic basicity of the ligands and the Lewis acidic sites of the open metal centres, the STUs materials show excellent catalytic activities as Lewis base for the Knoevenagel condensation reaction between various aldehydes and malononitrile, and as Lewis acid for cyanosilylation reactions. Among these STUs, STU‐4 (Ni(BIm)) shows the best catalytic efficiency (conversions >99 %) in both Knoevenagel condensation and cyanosilylation reactions under mild conditions, providing thus an advanced material for both Lewis base and Lewis acid catalysis.     

碱催化剂g-C3N4在液相反应中溶胀特性的研究

Graphitic carbon nitrides (g-C₃N₄) with different surface areas were prepared by pyrolysis using different precursors including melamine, dicyandiamide, thiourea and urea, and subsequently characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA) and N2 adsorption. Their basicities were measured by temperature-programmed desorption of CO₂ (CO₂-TPD) and acid-base titration. The catalytic properties for the Knoevenagel condensation of benzaldehyde and malononitrile were investigated in various solvents. In non-polar toluene solution, the benzaldehyde conversions of the g-C₃N₄ catalysts were low and changed according to their respective surface areas and basicities. However, in polar ethanol solution, the benzaldehyde conversions of all catalysts were similar, and much higher than those in toluene. This could not be explained by the results obtained from either of the two conventional basicity measurements. Further experimental results proved that g-C₃N₄ catalysts swelled in polar solutions, and more basic sites were exposed on the surface of the swollen catalysts, leading to the imminent increase in catalytic activity. This was proved by the catalyst poisoning data, which showed that the g-C₃N₄ catalyst lost its activity completely in toluene by adding 40.9 mmol·g^-1 benzoic acid, while the same catalyst was still active in ethanol until the added amount exceeded 143.3 m·g^-1. Additionally, the reaction tests in various solutions showed that the swelling effect was enhanced according to the polarity of the solvent used. A similar conclusion could be reached for the Knoevenagel condensation of furfural and malononitrile in various solvents. The reusability of g-C₃N₄ catalyst in Knoevenagel condensation was also studied, which showed that g-C₃N₄ was stable in liquid-phase reactions, whose activity dropped from 74.2% to 63.8% after three regeneration processes.     

A New Multicomponent Synthetic Route to Hexahydropyrido[2,3‐d]pyrimidine Derivatives

A new one‐pot multicomponent reaction of equimolar amounts of malononitrile and benzaldehyde in ethanol for the synthesis of hexahydropyrido[2,3‐d]pyrimidine derivatives through a sequence of Knoevenagel condensation, Michael addition, and Addition of Nucleophile, Ring Opening, and Ring Closure process is described.     

Tetra‐n‐butylammonium Hydroxide (TBAH)–Catalyzed Knoevenagel Condensation: A Facile Synthesis of α‐Cyanoacrylates, α‐Cyanoacrylonitriles,and α‐Cyanoacrylamides

Tetra‐n‐butylammonium hydroxide (TBAH) has been utilized as a novel and efficient catalyst for the Knoevenagel condensation of aldehydes with acidic methylene compounds such as methyl‐ and ethylcyanoacetate, malononitrile, and cyanoacetamide to afford substituted olefins.     

Preparation of substituted cinnamic acids labelled with deuterium and tritium at the ring positions

A series of substituted cinnamic acids labelled at the ring positions with deuterium or tritium has been easily obtained via the Doebner modification of Knoevenagel condensation reaction between labelled benzaldehyde derivatives and malonic acid in pyridine. The substituted benzaldehyde precursors were synthesized by isotope exchange method in deuterated or tritiated water at 80 °C in acidic medium. Ring-tritiated substituted cinnamic acids with specific activity ranging from 207 GBq /5.6 Ci/ mol^–1 to 4366 GBq /118 Ci/ mol^–1 was obtained using ca. 37 GBq /1 Ci/ of HTO.     

酸碱双功能MgO/HMCM-22催化剂上Knoevenagel缩合反应

采用浸渍法制备了一系列MgO改性催化剂MgO/HMCM-22, 利用X射线衍射、N₂物理吸附-脱附、扫描电镜、傅里叶变换红外光谱、NH₃及CO₂程序升温脱附等技术对所制催化剂进行了表征. 结果表明, MgO改性后MCM-22分子筛仍保持原有的结构; 随着MgO负载量的增加, 催化剂的碱强度和碱含量显著增加, 而强酸含量明显减少, 弱酸酸位有所增加. 以Knoevenagel缩合为探针反应, 考察了所制催化剂的性能. 在优化的反应条件下, MgO/HMCM-22上苯甲醛转化率高达92.6%. 催化剂 MgO/HMCM-22和MgO/NaMCM-22的催化性能明显优于HMCM-22和MgO. 酸中心和碱中心均对该缩合反应起着重要的促进作用. MgO/HMCM-22对Knoevenagel缩合反应显示出较高的催化活性, 体现出明显的酸碱协同催化作用.     

Schiff Base Structured Acid–Base Cooperative Dual Sites in an Ionic Solid Catalyst Lead to Efficient Heterogeneous Knoevenagel Condensations

An acid–base bifunctional ionic solid catalyst [PySaIm]₃PW was synthesized by the anion exchange of the ionic‐liquid (IL) precursor 1‐(2‐salicylaldimine)pyridinium bromide ([PySaIm]Br) with the Keggin‐structured sodium phosphotungstate (Na₃PW). The catalyst was characterized by FTIR, UV/Vis, XRD, SEM, Brunauer–Emmett–Teller (BET) theory, thermogravimetric analysis, ¹H NMR spectroscopy, ESI‐MS, elemental analysis, and melting points. Together with various counterparts, [PySaIm]₃PW was evaluated in Knoevenagel condensation under solvent and solvent‐free conditions. The Schiff base structure attached to the IL cation of [PySaIm]₃PW involves acidic salicyl hydroxyl and basic imine, and provides a controlled nearby position for the acid–base dual sites. The high melting and insoluble properties of [PySaIm]₃PW are relative to the large volume and high valence of PW anions, as well as the intermolecular hydrogen‐bonding networks among inorganic anions and IL cations. The ionic solid catalyst [PySaIm]₃PW leads to heterogeneous Knoevenagel condensations. In solvent‐free condensation of benzaldehyde with ethyl cyanoacetate, it exhibits a conversion of 95.8 % and a selectivity of 100 %; the conversion is even much higher than that (78.2 %) with ethanol as a solvent. The solid catalyst has a convenient recoverability with only a slight decrease in conversion following subsequent recyclings. Furthermore, the new catalyst is highly applicable to many substrates of aromatic aldehydes with activated methylene compounds. On the basis of the characterization and reaction results, a unique acid–base cooperative mechanism within a Schiff base structure is proposed and discussed, which thoroughly explains not only the highly efficient catalytic performance of [PySaIm]₃PW, but also the lower activities of various control catalysts.     

A recyclable bifunctional acid-base organocatalyst with ionic liquid character. The role of site separation and spatial configuration on different condensation reactions

A series of bifunctional organic catalysts containing acid and basic sites with ionic liquid characteristics have been prepared and their catalytic activity and reaction coordinate for aldol and Knoevenagel condensations have been compared. While the only factor controlling catalyst activity for the Knoevenagel condensation was the distance between the acid and base sites, the spatial orientation of the organocatalyst is also key to achieve high activity and selectivity in the Claisen-Schmidt condensation. Mechanistic studies based on theoretical DFT calculations show that the acid-base bifunctional organocatalyst follows a mechanism inspired in natural aldolases for the synthesis of trans-chalcones, being able to produce a large variety of these compounds of industrial interest. The combination of the acid-base pairs within the proper geometry and the ionic liquid nature makes this catalyst active, selective and recyclable.     

Knoevenagel Condensation Catalyzed by 1,1,3,3‐Tetramethylguanidium Lactate

Knoevenagel condensation between malononitrile or ethyl cyanoacetate and carbonyl compounds could be effectively promoted by 1,1,3,3‐tetramethylguanidium lactate, a basic ionic liquid, under mild conditions. Good to high isolated yields could be obtained. The ionic liquid can enhance the reaction rates significantly even though it was used in catalytic amount. The ionic liquid could be prepared simply and reused efficiently.     

Classical Keggin Intercalated into Layered Double Hydroxides: Facile Preparation and Catalytic Efficiency in Knoevenagel Condensation Reactions

The family of polyoxometalate (POM) intercalated layered double hydroxide (LDH) composite materials has shown great promise for the design of functional materials with numerous applications. It is known that intercalation of the classical Keggin polyoxometalate (POM) of [PW₁₂O₄₀]^3? (PW₁₂) into layered double hydroxides (LDHs) is very unlikely to take place by conventional ion exchange methods due to spatial and geometrical restrictions. In this paper, such an intercalated compound of Mg_0.73Al_0.22(OH)₂ [PW₁₂O₄₀]_0.04?0.98 H₂O (Mg₃Al‐PW₁₂) has been successfully obtained by applying a spontaneous flocculation method. The Mg₃Al‐PW₁₂ has been fully characterized by using a wide range of methods (XRD, SEM, TEM, XPS, EDX, XPS, FT‐IR, NMR, BET). XRD patterns of Mg₃Al‐PW₁₂ exhibit no impurity phase usually observed next to the (003) diffraction peak. Subsequent application of the Mg₃Al‐PW₁₂ as catalyst in Knoevenagel condensation reactions of various aldehydes and ketones with Z‐CH₂‐Z′ type substrates (ethyl cyanoacetate and malononitrile) at 60 °C in mixed solvents (V_2‐propanol:V_water=2:1) demonstrated highly efficient catalytic activity. The synergistic effect between the acidic and basic sites of the Mg₃Al‐PW₁₂ composite proved to be crucial for the efficiency of the condensation reactions. Additionally, the Mg₃Al‐PW₁₂‐catalyzed Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate demonstrated the highest turnover number (TON) of 47 980 reported so far for this reaction.     

富氮多孔有机聚合物催化制备α⁃氰基肉桂酸乙酯

α-氰基肉桂酸乙酯作为含多种官能团的缺电子烯烃, 是一种极具应用价值的有机合成反应底物, 主要通过催化Knoevenagel缩合反应获得. 本文以多聚甲醛和三聚氰胺为前驱体, 采用溶剂热法制备富氮多孔有机聚合物mPMF, 经K₂CO₃处理得到K₂CO₃-mPMF-X(X=1, 10, 50). 考察了mPMF在苯甲醛和氰乙酸乙酯Knoevenagel缩合反应中的催化性能, 通过mPMF与K₂CO₃-mPMF-X催化活性的比较, 探讨了碱性强弱对Knoevenagel缩合反应的影响, 并对催化反应机理进行了探索. 结果表明, 催化剂中丰富的氮物种为反应提供了碱性环境和大量的碱性活性位点, 催化剂碱性强弱的控制是催化合成α-氰基肉桂酸乙酯的关键因素. mPMF在甲醇溶剂中于60 ℃反应3 h后, 苯甲醛转化率为97%, 目标产物选择性在99.9%以上.     

On Water: An Efficient Knoevenagel Condensation using 12‐Tungstophosphoric Acid as a Reusable Green Catalyst

12‐Tungstphosphoric acid is found to be an efficient, environmentally attractive, and reusable catalyst for Knoevenagel condensation of malononitrile and ethylcyanoacetate with various aldehydes.     

ATR-IR spectroscopy of pendant NH2 groups on silica involved in the Knoevenagel condensation

The liquid-phase Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate catalyzed by aminopropyl-modified silica has been investigated using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. The aim of the work was to demonstrate the different levels of information on the reaction mechanism that can be achieved by operating the spectroscopic cell in the absence and in the presence of a solvent, in flow-through and stop-flow modes and in combination with concentration modulation spectroscopy. The reaction mechanism involves the formation of an imine intermediate whose existence has been verified in situ by combining in one experiment continuous and stop-flow operations. Identical information has been gained more elegantly using concentration modulation spectroscopy, which additionally provided information on the possible origin of the solvent effect observed in the Knoevenagel reaction. Faster production and consumption of the imine intermediate was observed in cyclohexane solvent than in toluene. Identification of other species evolving on the catalyst surface and monitoring of the effluents of the spectroscopic cell provided some insight in possible catalyst deactivation.     

An Efficient and Environment Friendly Procedure for the Synthesis of Arylmethylenemalononitrile Catalyzed by Strong Base Anion‐Exchange Resin in Water

Abstract

Knoevenagel condensation of malononitrile with aromatic aldehydes catalyzed by strong base anion‐exchange resin in water results arylmethylenemalononitrile in 74–98% yield at room temperature.     

Multifunctional hybrid organic-inorganic catalytic materials with a hierarchical system of well-defined micro- and mesopores

Novel layered zeolitic organic-inorganic materials (MWW-BTEB) have been synthesized by intercalation and stabilization of arylic silsesquioxane molecules between inorganic zeolitic MWW layers. The organic linkers are conformed by two condensed silyl-arylic groups from disilane molecules, such as 1,4-bis(triethoxysilyl)benzene (BTEB), which react with the external silanol groups of the zeolitic layers. The hybrids contain micropores within the inorganic layers and a well-defined mesoporous system in between the organic linkers. An amination post-treatment introduces basic groups in the organic linkers close to the acid sites present in the structural inorganic counterpart. Through this methodology it has been possible to prepare bifunctional acid-base catalysts where the acid sites are of zeolitic nature located in the inorganic building blocks and the basic sites are part of the organic structure. The resultant materials can act as bifunctional catalysts for performing a two-step cascade reaction that involves the catalytic conversion of benzaldehyde dimethylacetal into benzylidene malononitrile.     

Catalytic Upgrading of Biomass‐Derived Methyl Ketones to Liquid Transportation Fuel Precursors by an Organocatalytic Approach

A highly efficient water‐tolerant, solid‐base catalyst for the self‐condensation of biomass‐derived methyl ketones to jet‐diesel fuel precursors was developed by grafting site‐isolated secondary amines on silica‐alumina supports. It is shown that apart from the nature and density of amine groups and the spatial separation of the acidic and basic sites, the acidity of the support material plays a critical role in defining the catalytic activity. It is also found that a combination of weakly acidic silanol/aluminol with secondary amine groups can mimic proline catalysts and are more effective in catalyzing the selective dimerization reaction than the combination of amines with organic acids. In situ FTIR measurements demonstrate that acidic groups activate methyl ketones through their carbonyl groups leading to a favorable C? C bond formation step involving an enamine intermediate. DFT analysis of the reaction pathway confirms that C? C bond formation is the rate‐limiting step.     

A Green Synthesis of 2‐Amino‐4‐(9H‐carbazole‐3‐yl)thiophene‐3‐carbonitriles by a Step‐wise and One‐pot Three‐component Gewald Reaction

An eco‐friendly method has been developed for the synthesis of 2‐amino‐4‐(9H‐carbazole‐3‐yl)thiophene‐3‐carbonitriles from preliminary carbazole ( 1 ) through an intermediate of 2‐(1‐(9H‐carbazole‐3‐yl)ethylidene)malononitriles using the Knoevenagel condensation followed by the Gewald reaction. On the other hand, the target compounds could also be prepared in a one‐pot three‐component manner by treating equimolar quantities of 1‐(9H‐carbazole‐3‐yl)ethanone ( 3 ), malononitrile, and elemental sulfur. The merits of this preparation are mild reaction conditions. The Gewald reaction is executed with inorganic base NaHCO₃ (H₂O) in tetrahydrofuran, easy work‐up procedure with good yields.     

功能化碳纳米管作为固体碱用于Knoevenagel缩合

采用液相沉积方法分别制备了聚乙烯亚胺功能化和聚4-乙烯基吡啶功能化的碳纳米管催化剂,利用N2吸附/脱附、透射电子显微镜(TEM)、热重分析(TG)以及酸碱滴定等方法对所得催化剂的结构和碱性进行了表征,测试了其对于苯甲醛和丙二腈的Knoevenagel缩合反应的催化性能。结果发现,聚乙烯亚胺功能化的碳纳米管具有比聚4-乙烯基吡啶功能化的催化剂更高的碱催化能力。对碳纳米管进行强酸预处理可增加其与修饰剂的相互作用,虽然会降低其碱催化活性,但却可使其稳定性大大提高。     

功能化碳纳米管作为固体碱用于Knoevenagel缩合

采用液相沉积方法分别制备了聚乙烯亚胺功能化和聚4-乙烯基吡啶功能化的碳纳米管催化剂,利用N₂吸附/脱附、透射电子显微镜（TEM）、热重分析（TG）以及酸碱滴定等方法对所得催化剂的结构和碱性进行了表征,测试了其对于苯甲醛和丙二腈的Knoevenagel缩合反应的催化性能。结果发现,聚乙烯亚胺功能化的碳纳米管具有比聚4-乙烯基吡啶功能化的催化剂更高的碱催化能力。对碳纳米管进行强酸预处理可增加其与修饰剂的相互作用,虽然会降低其碱催化活性,但却可使其稳定性大大提高。     

Ionic Liquid–Mediated Knoevenagel Condensation of Meldrum's Acid and Aldehydes

A simple, efficient, and green protocol for Knoevenagel condensation of Meldrum's acid and aldehydes in ionic liquid at room temperature without any catalyst is described. The reaction has been performed in different ionic liquids. The enhancement in the rate is observed in Brönsted acidic ionic liquid 1‐methylimidazolium trifluoroacetate [Hmim]Tfa, which furnishes quantitative yields with 4–30 min in most of the cases. Furthermore, ionic liquid is easily reused without any appreciable loss in activity.