Ionic Liquids as Solvents for Catalyzed Oxidations of Organic Compounds

This review summarizes the oxidation methods of organic compounds carried out in the presence of a catalyst using ionic liquids or molten ammonium salts as solvents. From an ecological point of view, these procedures are particularly interesting when the catalyst is efficiently immobilized in the solvent, thus leading to a reusable system.     

Liquid‐Liquid Biphasic,Platinum‐Catalyzed Hydrosilylation of Allyl Chloride with Trichlorosilane using an Ionic Liquid Catalyst Phase in a Continuous Loop Reactor

The platinum‐catalyzed hydrosilylation of allyl chloride with trichlorosilane was investigated in an ionic liquid‐organic biphasic reaction mode. After an ionic liquid screening and repetitive batch mode experiments, the process was realized in a continuous mode using a loop reactor concept with integrated continuous separation and recycling of the ionic liquid catalyst phase. The continuous reactor could be operated for 48 h at constant activity and selectivity without addition of platinum indicating that platinum leaching into the product phase was far below 1 ppm. Enhanced selectivity for the product trichloro(3‐chloropropyl)silane (compared to the state‐of‐the art) and the possibility to use simple platinum tetrachloride (PtCl₄) as platinum source are further attractive features of this new ionic liquid‐based process concept.     

Mixtures of Ionic Liquids and Water as a Medium for Efficient Enantioselective Hydrogenation and Catalyst Recycling

In a screening of ligands, ionic liquids and reaction conditions in the Rh‐catalyzed hydrogenation of enamides, a novel multi‐phase reaction system consisting of an ionic liquid (IL) and water (wet ILs) was found to give the most promising results. In many cases such IL/water combinations were superior compared to conventional organic solvents and biphasic ILs/organic co‐solvents media with respect to catalytic performance as well as to catalyst separation and recycling. So far, the best results were obtained with Rh‐ferrocenyl‐diphosphine catalysts (>99% ee). Generally, somewhat lower ees were observed at higher pressure. However, this effect was less pronounced with wet ILs than with conventional solvents. It is shown that IL/water combination allow repeated catalyst recycling without significant loss of activity and that industrially relevant turnover numbers of >10,000 can be obtained.     

Recyclable Selective Palladium‐Catalyzed Synthesis of Five‐, Six‐ or Seven‐Membered Ring Lactones and Lactams by Cyclocarbonylation in Ionic Liquids

The ionic liquids, BMIM PF₆ or BMIM NTf₂, are used successfully for the palladium‐catalyzed cyclocarbonylation of 2‐allylphenols and anilines, 2‐vinylphenols, and 2‐aminostyrenes. The reaction proceeds cleanly and efficiently to afford high yields of lactones or lactams with good or excellent selectivity for one isomer. The ionic liquid containing the palladium catalyst, and ligand, is recyclable in all cases.     

Synthesis of Bipyridine‐Stabilized Rhodium Nanoparticles in Non‐Aqueous Ionic Liquids: A New Efficient Approach for Arene Hydrogenation with Nanocatalysts

A new approach to stabilize metal nanoparticles with polynitrogen ligands in ionic liquids (ILs) is described. Zerovalent metal nanospecies in the size range of 2.0 nm were easily prepared in various ionic liquids by chemical reduction of a rhodium salt with an excess amount of sodium borohydride (NaBH₄) and efficiently stabilized by 2,2′‐bipyridine. The influence of the bipyridine ratio in various ILs according to the nature of the cation‐anion association was investigated. These nanocatalysts were evaluated in the hydrogenation of aromatic compounds in ILs under various catalytic conditions (P=1–40 bar, T=20–80 °C).     

Highly Enantioselective Synthesis of Chiral Tetrahydroquinolines and Tetrahydroisoquinolines by Ruthenium‐Catalyzed Asymmetric Hydrogenation in Ionic Liquid

Asymmetric hydrogenation reactions of quinolines and 3,4‐dihydroisoquinolines using the chiral cationic ruthenium complex Ru(TsDPEN) [TsDPEN=N‐(p‐toluenesulfonyl)‐1,2‐diphenylethylenediamine] as catalyst in neat imidazolium ionic liquids have been investigated. The catalytic performance was influenced by the anion of the ionic liquids for both substrate classes. A range of 2‐alkyl‐substituted 1,2,3,4‐tetrahydroquinolines and 1‐alkyl‐substituted 1,2,3,4‐tetrahydroisoquinolines was obtained in high yields with up to >99% ee. Interestingly, the hydrogenation of quinoline derivatives bearing a carbonyl group was selective for the CN (quinoline) over the CO (ketone) bonds, while such a unique chemoselectivity was not observed in methanol. Furthermore, the ruthenium catalysts could be easily recycled at least 5 times in the asymmetric hydrogenation of 3,4‐dihydroisoquinoline by solvent extraction. To further facilitate the recovery of catalyst and reduce the use of organic solvent, a thin film of ionic liquid containing Ru(TsDPEN) was supported on silica gels. This supported ionic liquid‐phase catalyst was effective in the asymmetric hydrogenation of quinoline, and could be recycled at least 6 times by simple filtration.

     

Proline‐Catalyzed Asymmetric Aldol Reaction in Guanidine‐ Derived Ionic Liquids

A remarkable improvement of both the chemical yield (from 6% to 82%) and the enantiomeric excess (up to >99%), of (S)‐proline catalyzed direct aldol reactions of a wide range of aldehydes with acetone was found when hexasubstituted or pentasubstituted guanidinium salts were added as ionic liquids. Effects of temperature, amount of proline and the type of guandidinium salts on the outcome of the reaction were investigated.     

Improved Stability and Catalytic Activity of Palladium Nanoparticle Catalysts using Phosphine‐Functionalized Imidazolium Ionic Liquids

Palladium nanoparticles (Pd NPs) stabilized by 6 different phosphine‐functionalized ionic liquids (PFILs) were synthesized in imidazolium‐based ionic liquids (ILs) using H_2(g) (4 bar) as a reductant. Characterization showed well‐dispersed particles of ∼3 nm (TEM) and confirmed the PFIL stabilization of the NPs (XPS). The PFILs were composed of an imidazolium functionality separated from the phosphine group by a propyl or undecyl chain. The counter anions for both FILs and IL solvents were chosen from N‐bis(trifluoromethanesulfonyl)imide (Tf₂N⁻), trifluoromethanesulfonate (TfO⁻) or hexafluorophosphate (PF₆⁻). Colloidal suspensions of the Pd NPs were employed as biphasic hydrogenation catalysts for the reduction of the olefinic bond in styrene under mild conditions (50 °C, 4 bar H_2(g), 1.5 h). The PFIL‐stabilized Pd NPs were effective hydrogenation catalysts and showed superior activity and recyclability over NPs synthesized in the absence of PFILs. Poisoning tests of the Pd NP catalysts and characterization of the electronic properties of the phosphine were also performed.     

Josiphos Ligands with an Imidazolium Tag and their Application for the Enantioselective Hydrogenation in Ionic Liquids

Josiphos ligands bearing an imidazolium moiety show high efficiency and reusability for the rhodium‐catalyzed enantioselective hydrogenation of methyl acetamidoacrylate (MAA) and dimethyl itaconate (DMI) in biphasic co‐solvent/ionic liquid combinations.     

Stereoselective Palladium‐Catalyzed Alkenylation and Alkynylation of Thioglycosides

An efficient and unprecedented palladium‐catalyzed S‐glycosylation reaction of a range of alkenyl and alkynyl halides by using thiosugars as nucleophile partners has been established. With palladium diacetate in combination with Xantphos as the catalytic system, a variety of β‐alkenylthioglycosides as well as β‐alkynylthioglycosides can be prepared in good to excellent yields. The efficiency of this general protocol was well‐demonstrated by the formal synthesis of a leaf‐closure β‐glucosidase inhibitor.

     

Selective Hydrogenation of 1,3‐Butadiene to 1‐Butene by Pd(0) Nanoparticles Embedded in Imidazolium Ionic Liquids

The reduction of Pd(acac)₂ (acac=acetylacetonate), dissolved in 1‐n‐butyl‐3‐methylimidazolium hexafluorophosphate (BMI⋅PF₆) or tetrafluoroborate (BMI⋅BF₄) ionic liquids, by molecular hydrogen (4 atm) at 75 °C affords stable, nanoscale Pd(0) particles with sizes of 4.9±0.8 nm. Inasmuch as 1,3‐butadiene is at least four times more soluble in the BMI⋅BF₄ than butenes, the selective partial hydrogenation could be performed by Pd(0) nanoparticles embedded in the ionic liquid. Thus, the isolated nanoparticles promote the hydrogenation of 1,3‐butadiene to butenes under solventless or multiphase conditions. Selectivities up to 97% in butenes were observed in the hydrogenation of 1,3‐butadiene by Pd(0) nanoparticles embedded in BMI⋅BF₄ under mild reaction conditions (40 °C and 4 atm of hydrogen at constant pressure). Selectivities up to 72% in 1‐butene were achieved at 99% 1,3‐butadiene conversion, 40 °C and 4 atm of constant pressure of hydrogen. The amounts of butane (fully hydrogenated 1,3‐butadiene) and cis‐2‐butene products are marginal and the butenes do not undergo isomerisation process, indicating that the soluble Pd(0) nanoparticles possess a pronounced surface‐like rather than homogeneous‐like catalytic properties.     

离子液体稳定的金属纳米粒子制备及其催化作用研究进展

离子液体作为一种新型的绿色溶剂和稳定剂,已成功应用于制备金属纳米粒子.与传统的溶剂和稳定剂相比,它具有许多独特的物理化学性质和鲜明的优势,已引起越来越多研究者的关注.本文对近年来国内外在离子液体中制备金属纳米粒子及其催化作用的研究进行综述,并对该领域的研究前景作了展望.     

Hydroaminomethylation of n‐Alkenes in a Biphasic Ionic Liquid System

Hydroaminomethylation reactions were performed successfully in an imidazolium‐based ionic liquid using a rhodium/sulfoxantphos system by reacting piperidine with different n‐alkenes, affording yields higher than 95 % of the resulting amine with turnover frequencies of up to 16,000 h⁻¹, along with high regioselectivity for the linear amines with l/b ratios up to 78. Additionally, facile quantitative catalyst recovery was accomplished and recycling of the catalyst and product separation was achieved by a fast phase separation after the reaction. The product distribution was monitored over time at different temperatures both in an organic solvent and in the ionic liquid in order to investigate and compare the course of the formation of (side) products and intermediates in these reactions. Furthermore, it was shown that the nature of the rhodium precatalyst has a profound effect on the activity and selectivity. Protic organic solvents and ionic liquids containing a C H acidic bond in the imidazolium part have a beneficial effect on the hydrogenation activity of the catalyst systems.     

Kinetic Model of Two‐Phase Epoxidation with Ionic Liquids as Micellar Catalysts

The biphasic catalytic epoxidation of cyclooctene using the ionic liquid (IL) 1,2‐dimethyl‐3‐octyl‐imidazolium perrhenate ([OMMIM]ReO₄) as micellar catalyst and H₂O₂ as oxidant was investigated. Kinetic experiments were carried out in the intrinsic kinetic regime as proved by variation of stirring rate and temperature. Variation of catalyst concentration allowed for determination of the critical micellar concentration (CMC) of the catalytic IL. The effect of substrate concentrations on the reaction rate was also assessed. Based on the experiments, a kinetic model adapted from enzyme catalysis was proposed to account for the micellar reaction environment. The model takes into account the onset of micelle formation at the CMC. The application of the kinetic model illustrated the good agreement with the experimental data. The model will be applied to other micellar epoxidation reactions and for the design of an appropriate reaction setup in the future.     

Sulfonic Acid‐Functionalized Ionic Liquids as Metal‐Free,Efficient and Reusable Catalysts for Direct Amination of Alcohols

A series of sulfonic acid‐functionalized (SO₃H‐functionalized) ionic liquids was synthesized and used as metal‐free, highly selective and efficient catalysts for the direct amination of alcohols. Notably, the activities of the series of SO₃H‐functionalized ionic liquids were compared and a 92% isolated yield was obtained using 3‐tetradecyl‐1‐(butyl‐4‐ sulfonyl)imidazolium trifluoromethanesulfonate ([BsTdIM][OTf]) as the catalyst. Importantly, the catalytic system has wide substrate scope including benzylic, allyl, propargylic, aliphatic alcohols with sulfonamide, amide, carbamate, aromatic amine and N‐heterocyclic compounds. Interestingly, the system was also suitable for a multi‐gram scale direct amination of alcohols. Additionally, the reusable nature of [BsTdIM][OTf] makes this protocol more attractive and avoids the disposal and neutralization of acidic catalysts. Moreover, preliminary experiments indicated that this reaction should proceed via an S_N1 pathway.     

Synergistic Effect of a Palladium‐on‐Carbon/Platinum‐on‐Carbon Mixed Catalyst in Hydrogen/Deuterium Exchange Reactions of Alkyl‐Substituted Aromatic Compounds

We have found a synergistic effect in the H‐D exchange reaction of alkyl‐substituted aromatic compounds using the Pd/C‐Pt/C‐D₂O‐H₂ system. This system would lead to fully H‐D exchange results even on the sterically hindered sites which were only low‐deuterium incorporated by Pd/C or Pt/C independently. Since the reaction was general for a variety of aromatic compounds, it could be applied to the deuteration of dianiline derivatives as raw materials for polyimides.     

New Carbohydrate‐Based Phosphite‐Oxazoline Ligands as Highly Versatile Ligands for Palladium‐Catalyzed Allylic Substitution Reactions

We have designed and synthesized a new family of readily available phosphite‐oxazoline ligands for Pd‐catalyzed asymmetric allylic substitution reactions. These ligands can be tuned in two regions to explore their effect on catalytic performance. By carefully selecting the ligand components, we obtained high enantioselectivities in the Pd‐catalyzed allylic substitution in substrates with different steric properties     

Ionic Liquids and their Polymers in Lithium‐Sulfur Batteries

Future optimized lithium‐sulfur batteries may promise higher energy densities than the current standard. However, there are many barriers which hinder their commercialization. In this review we describe how ionic liquids (ILs) and their polymers are utilized in different components of the battery to address some of these issues. For example, IL‐based electrolytes have the potential to reduce the solubility of polysulfides compared to conventional organic electrolytes. Polymerizing ILs directly on the surface of the Li‐metal anode is suggested as an approach to protect the surface of this electrode. Finally, using poly(ionic liquids) (PILs) as binders for the cathode active material may increase the performance of the cathode as compared to polyvinylidene difluoride (PVdF) and could inhibit swelling‐induced degradation. These results demonstrate the advantages of ILs and their polymers for improving the performance of Li?S batteries.     

Palladium‐Catalyzed Asymmetric Hydrogenation of Simple Ketimines Using a Brønsted Acid as Activator

Using a catalytic amount of a Brønsted acid as activator of simple imines, the highly enantioselective homogeneous palladium‐catalyzed asymmetric hydrogenation of simple ketimines was successfully developed with up to 95% ee.