BiX3 and FeX3‐Promoted Prins Cyclization of Enol Ethers in CH2Cl2

The Prins cyclization of enol ethers has been realized by employing BiX₃ (or FeX₃) as catalyst and TMSX (X=Br, Cl) as the halogen source. The presence of a tiny amount of water in the solvent dichloromethane played a key role for the reaction to proceed. The reaction is believed to be catalyzed by Lewis acid‐assisted Brønsted acids, which were generated in situ from MX₃ and water in the solvent.     

Selectivity enhancement of silica-supported sulfonic acid catalysts in water by coating of ionic liquid

Coating of silica-supported sulfonic acid catalysts with hydrophobic ionic liquid leads to a significant improvement of catalyst selectivity. Many organic reactions, including Prins cyclization, cycloaddition of epoxide to aldehyde, and dehydrative etherification of secondary benzyl alcohols, proceed well in formalin or pure water. In particular, tandem dehydration/Prins cyclization reactions of tertiary and secondary alcohols with formaldehyde were developed for the first time.     

Prins Cyclization of Styrenes or Acetophenone Catalyzed by DBSA in Water

Dodecylbenzenesulfonic acid (DBSA) was proved to be an efficient catalyst for Prins cyclization of styrenes and formaldehyde or acetaldehyde in water. A tandem dehydration/Prins cyclization reaction using a tertiary alcohol and formaldehyde as substrates proceeded very well by using DBSA as catalyst. Acetophenone, which is less reactive compared with styrene, can also react with formaldehyde when catalyzed by DBSA in water to afford 1,3-dioxan-5-ylphenylmethanone in good yield.     

MIL-101(Fe)高效催化 β-蒎烯与甲醛的Prins缩合制备诺卜醇

通过水热晶化法制备了MIL-101(Fe)金属有机骨架材料, 利用X射线衍射(XRD)、 傅里叶变换红外光谱(FTIR)、 热重分析(TG)、 扫描电子显微镜(SEM)、 透射电子显微镜(TEM)和X射线光电子能谱(XPS)对催化剂的结构和形貌进行了表征. 结果表明, 该材料用于催化β-蒎烯与甲醛的Prins缩合制备诺卜醇反应的效果优异; 催化剂合成温度、 合成时间、 催化剂用量、 反应溶剂、 反应温度和反应时间对β-蒎烯的反应结果均有一定影响. 在相似的反应条件下, 合成的MIL-101(Fe)催化β-蒎烯制备诺卜醇反应的最佳条件为使用150 ℃下反应15 h合成的催化剂MIL-101(Fe), 在90 ℃下反应8 h得到的β-蒎烯转化率高达97.3%, 诺卜醇选择性达到96.7%.     

o-Benzenedisulfonimide as a Recyclable Homogeneous Organocatalyst for an Efficient and Facile Synthesis of 4-Amidotetrahydropyran Derivatives Through Prins–Ritter Reaction

A highly diastereoselective synthesis of 4-amidotetrahydropyran derivatives has been achieved using a catalytic amount of o-benzenedisulfonimide under mild conditions involving sequential allylation and Prins–Ritter amidation. The oxo-carbenium ion formed in Prins cyclization could be successfully trapped with nitriles through Ritter amidation. The catalyst is highly efficient in promoting both allylation and Prins cyclization with a net addition of nitrile. The catalyst can be easily recovered and reused in subsequent reactions.     

缺位Dawson型K10Na2H2P2W16O60在环己烯氧化中的催化作用

合成了二缺位杂多化合物K10Na2H2P2W16O60·18H2O,用IR、UV-vis、XRD及TG表征了其结构,考察了该化合物在过氧化氢氧化环己烯反应中的催化作用.研究结果表明,溶剂种类和反应温度对二缺位杂多化合物的催化性能有显著的影响,在叔丁醇中主要生成环己烯酮,在丙酮中主要生成环己二醇.在乙腈中主要生成环氧环己烷,其选择性随着反应温度升高和催化剂用量增加而降低,环己烯酮的选择性则逐步升高.催化剂与产物可通过温控固-液相分离,可以重复使用.     

Synthesis of diethyl [(2<Emphasis Type="Italic">S</Emphasis>,4<Emphasis Type="Italic">S</Emphasis>)-4-hydroxytetrahydrofuran-2-ylmethyl]phosphonate

A convenient method for the stereoselective synthesis of diethyl [(2S,4S)-4-hydroxytetrahydrofuran-2-ylmethyl]phosphonates was developed based on preparation of phophorylated glycol followed by its intramolecular cyclization and hydrogenation. It was shown that the nature of the hydrogenation products depends on the presence of water in the solvent.     

Racemization in Prins cyclization reactions

Isotopic labeling experiments were performed to elucidate a new mechanism for racemization in Prins cyclization reactions. The loss in optical activity for these reactions was shown to occur by 2-oxonia-Cope rearrangements by way of a (Z)-oxocarbenium ion intermediate. Reaction conditions such as solvent, temperature, and the nucleophile employed played a critical role in whether an erosion in enantiomeric excess was observed. Additionally, certain structural features of Prins cyclization precursors were also shown to be important for preserving optical purity in these reactions.     

HBF4·OEt2 as a versatile reagent for the Hosomi-Sakurai allylation and Prins cyclization: one-pot synthesis of symmetrical 4-fluorotetrahydropyrans

The synthesis of symmetrical 2,6-disubstituted 4-fluorotetrahydropyran derivatives has been achieved using HBF₄·OEt₂ via a tandem allylation and Prins cyclization. This is a highly efficient and diastereoselective approach for the preparation of 4-fluorotetrahydropyrans in a single step. The use of readily available and easy to handle reagent HBF₄·OEt₂ makes this method simple, convenient and practical.     

Unprecedented synergistic effects between weak Lewis and Brønsted acids in Prins cyclization

Novel synergistic effects between Lewis and Br?nsted acids in Prins cyclization are reported. Non-reactive Lewis acids and non-reactive Br?nsted acids, which failed to perform Prins cyclization when used alone, have shown remarkable synergistic effects when used in combination to perform the reaction successfully.     

Water‐Enhanced Synthesis of Higher Alcohols from CO2 Hydrogenation over a Pt/Co3O4 Catalyst under Milder Conditions

The effect of water on CO₂ hydrogenation to produce higher alcohols (C₂–C₄) was studied. Pt/Co₃O₄, which had not been used previously for this reaction, was applied as the heterogeneous catalyst. It was found that water and the catalyst had an excellent synergistic effect for promoting the reaction. High selectivity of C₂–C₄ alcohols could be achieved at 140 °C (especially with DMI (1,3‐dimethyl‐2‐imidazolidinone) as co‐solvent), which is a much lower temperature than reported previously. The catalyst could be reused at least five times without reducing the activity and selectivity. D₂O and ¹³CH₃OH labeling experiments indicated that water involved in the reaction and promoted the reaction kinetically, and ethanol was formed via CH₃OH as an intermediate.     

Click approach to the three‐component synthesis of novel β‐hydroxy‐1,2,3‐triazoles catalysed by new (Cu/Cu2O) nanostructure as a ligand‐free,green and regioselective nanocatalyst in water

Copper nanostructures were produced as an effective and regioselective catalyst for the synthesis of 1,2,3‐triazoles from a wide range of raw materials, such as sodium azide, epoxides and terminal alkynes, in water via a one‐pot three‐component click reaction. The new heterogeneous catalyst was prepared by a simple ball mill reduction of CuO with NaBH₄ using a ball‐to‐powder weight ratio of 50:1 under air atmosphere at room temperature. The catalyst was fully characterized using scanning electron microscopy, energy‐dispersive X‐ray analysis, Fourier transform infrared spectroscopy and X‐ray diffraction. The copper nanostructures catalysed both ring opening and triazole cyclization steps. Products were obtained in high yields and short reaction times. The reactions were performed at ambient temperature in water as a green solvent. The Cu/Cu₂O nanostructures revealed high reusability and high stability via a simple recycling process.     

Lewis acid-catalyzed one-pot crossed Prins cyclizations using allylchlorosilane as allylating agent

A one-pot multi-component Lewis acid-catalyzed Prins cyclization was developed with high yield and selectivity. The crossed 2,4,6-trisubstituted tetrahydropyran products were formed with high stereoselectivity. This catalytic method could also be used with α,β-unsaturated aldehydes affording moderate yields of products.     

杂多酸(盐)及单缺位磷钨杂多化合物在醇类氧化反应中的催化活性

在相转移条件下,研究了杂多化合物在苄醇,环己醇氧化反应中的催化活性.六种Keggin结构杂多酸的催化活性按GeMo~12(H~4GeMo~12O~40的简写,其余类推,PW~12,PMo~12,SiMo~12,GeW~12,SiW~12顺序下降,杂多酸中的质子可分别被其它阳离逐渐取代而达到酸性修饰. H~3PW~12O~40随着其质子逐步被Na^+取代,酸性下降,催化活性大大提高;杂多酸(盐)的催化活性随体系pH值的改变将发生奇妙剧烈的变化;单缺位杂多化合物显示出较饱和杂多酸(盐)更高催化活性.溶剂对催化活性有明显影响.     

Hydrolysis of cellulose by the heteropoly acid H<Subscript>3</Subscript>PW<Subscript>12</Subscript>O<Subscript>40</Subscript>

The potential of heteropoly acid H₃PW₁₂O₄₀ to catalyze the hydrolysis of cellulose to glucose under hydrothermal conditions was explored. This technology could contribute to sustainable societies in the future by using cellulose biomass. A study to optimize the reaction conditions, such as the amount of catalyst, reaction time, temperature, and the amount of cellulose used, was performed. A remarkably high yield of glucose (50.5%) and selectivity higher than 90% at 453 K for 2 h with a mass ratio of cellulose to H₃PW₁₂O₄₀ of 0.42 were achieved. This was attributed to the high hydrothermal stability and the excellent catalytic properties, such as the strong Brønsted acid sites. This homogeneous catalyst can be recycled for reuse by extraction with diethyl ether. The results illustrate that H₃PW₁₂O₄₀ is an environmentally benign acid catalyst for the hydrolysis of cellulose.     

色谱法研究杂多酸 4: 杂多酸在反相离子对色谱中的保留行为

本文研究了Keggin型杂多酸盐在四丁基铵-磷酸盐缓冲液-甲醇体系中的色谱保留行为。导出了反相离子对色谱中离子对试剂浓度, 强溶剂浓度及离子强度影响溶质保留值的三个规律式, 并用实验及文献值进行验证, 在所研究的体系中, 不同电荷数的杂多酸阴离子都可有保留, 通过调节适当的离子对试剂浓度及强溶剂浓度,可改善其选择性。     

Recent Progress on Nazarov Cyclizations: The Use of Iron Salts as Catalysts in Ionic Liquid Solvent Systems

Nazarov cyclization is an important and versatile method for the synthesis of five‐membered carbocycles, and extensive studies have been conducted to optimize the reaction. Among recent studies, several trends are recognized. One is the combination of different reactions with Nazarov cyclization in a one‐pot reaction system which enables the preparation of unique cyclization products. The second is the use of a transition‐metal catalyst, though Lewis or Brønsted acids have generally been used for the reaction. The third is the realization of the asymmetric Nazarov cyclization. The fourth is the base‐catalyzed Nazarov cyclization. Furthermore, several useful protocols for realizing Nazarov cyclization have also been developed. The recent progress on Nazarov cyclizations is summarized in Section 2. Section 3 is our chronicle in this field. We focused on the use of iron as the catalyst in Nazarov cyclizations and ionic liquids as solvents: Nazarov cyclization of thiophene derivatives using FeCl₃ as the catalyst was accomplished and we succeeded in demonstrating the first example of an iron‐catalyzed asymmetric Nazarov reaction. We next established Nazarov cyclization of pyrrole or indole derivatives using Fe(ClO₄)₃·Al₂O₃ as the catalyst with high trans selectivities in excellent yields. Since the cyclized product was reacted with a vinyl ketone in the presence of the same iron salt, the system allowed realization of the sequential type of Nazarov/Michael reaction of pyrrole derivatives. Furthermore, we demonstrated the recyclable use of the iron catalyst and obtained the desired Nazarov/Michael reaction products in good yields for five repetitions of the reactions without any addition of the catalyst using an ionic liquid, [bmim][NTf₂], as the solvent. We expect that the iron‐catalyzed Nazarov cyclization, in particular, in an ionic liquid solvent might become a useful method to synthesize functional molecules that include cycloalkene moieties.     

An expeditious synthesis of hexahydrobenzo[f]isochromenes and of hexahydrobenzo[f]isoquinoline via iodine-catalyzed Prins and aza-Prins cyclization

Homoallylic alcohols (primary, secondary, or tertiary containing an endocyclic or an exocyclic double bond) react with equimolar amounts of aldehydes (aliphatic or aromatic) and ketones (aliphatic) in the presence of 5 mol % of iodine. This Prins cyclization was used in the preparation of hexahydrobenzo[f]isochromenes and of a 4-hydroxy-tetrahydropyran, in 54-81% yield. The procedure is also efficient for an aza-Prins cyclization of a homoallylic sulfonamide and benzaldehyde, producing a hexahydrobenzo[f]isoquinoline.     

[Rh(μ-Cl)(COD)]2 supported on activated carbons for the hydroformylation of 1-octene: effects of support surface chemistry and solvent

The [Rh(μ-Cl)(COD)]₂ complex has been heterogeneised on activated carbon and used as catalysts for the hydroformylation of 1-octene. The objective is to achieve the effective anchorage of the complex on the carbon surface, keeping or improving the catalytic properties of the complex in the homogeneous process. The effects of surface chemistry of the activated carbon and the solvent used as reaction media (hexane, acetone or methanol) on the activity and selectivity to linear products (alcohols and aldehydes) and on the lixiviation of the complex from support, using different methods of catalyst recovering, are investigated. Gas adsorption, temperature-programmed desorption (TPD), immersion calorimetry and XPS techniques were used for characterisation. Catalytic activity at 353 K and a total pressure of 5 MPa (H₂:CO ratio 1:1) was carried out in a stainless steel stirred tank reactor. Obtained results show that the heterogeneised complex shows a conversion level higher or similar to that of the homogeneous complex and with a higher selectivity to the linear products. Complex lixiviation depends on the support, the solvent and the method used to recover the catalyst from the reactor. The catalyst prepared with the functionalised activated carbon, where the anchorage takes place by ion-exchange, is more stable and active in further catalytic runs using the proper recovering method.     

Efficient homogeneous catalysis of heteropoly acid and its characterization through etherifications of alcohol

A Keggin-type heteropoly acid revealed high catalytic activity for the etherification such as diethylene glycol with ethanol and cyclization of diethylene glycol in the homogeneous liquid. The catalytic activities of the heteropoly acid were much higher than those of conventional acid catalysts such as sulfuric acid, p-toluene sulfonic acid and phosphorous acid at the same catalyst concentrations or the same proton concentrations. On the basis of comparative measurement of electrical conductivity, acidity, and softness of anion for the solutions of acid catalysts, the efficient acid catalysis by heteropoly acid was suggested to due to be the specific properties of the heteropoly anion, which can be characterized by very weak basicity and great softness, together with the large size of the polyhedral structure.