Martin H. G. Prechtl

Carbon-Carbon Cross Coupling Reactions in Ionic Liquids Catalysed by Palladium Metal Nanoparticles

A brief summary of selected pioneering and mechanistic contributions in the field of carbon-carbon cross-coupling reactions with palladium nanoparticles (Pd-NPs) in ionic liquids (ILs) is presented. Five exemplary model systems using the Pd-NPs/ILs approach are presented: Heck, Suzuki, Stille, Sonogashira and Ullmann reactions which all have in common the use of ionic liquids as reaction media and the use of palladium nanoparticles as reservoir for the catalytically active palladium species.

Nanoscale Ru(0) particles: arene hydrogenation catalysts in imidazolium ionic liquids.

The reduction of [Ru(COD)(2-methylallyl) 2] (COD = 1,5-cyclooctadiene) dispersed in various room-temperature ionic liquids (ILs), namely, 1- n-butyl-3-methylimidazolium (BMI) and 1- n-decyl-3-methylimidazolium (DMI), associated with the N-bis(trifluoromethanesulfonyl)imidates (NTf 2) and the corresponding tetrafluoroborates (BF 4) with hydrogen gas (4 bar) at 50 degrees C leads to well-dispersed immobilized nanoparticles. Transmission electron microscopy (TEM) analysis of the particles dispersed in the ionic liquid shows the presence of [Ru(0)] n nanoparticles (Ru-NPs) of 2.1-3.5 nm in diameter. Nanoparticles with a smaller mean diameter were obtained in the ILs containing the less coordinating anion (NTf 2) than that in the tetrafluoroborate analogues. The ruthenium nanoparticles in ionic liquids were used for liquid-liquid biphasic hydrogenation of arenes under mild reaction conditions (50-90 degrees C and 4 bar). The apparent activation energy of E A = 42.0 kJ mol (-1) was estimated for the hydrogenation of toluene in the biphasic liquid-liquid system with Ru-NPs/BMI.NTf 2. TEM analysis of the ionic liquid material after the hydrogenation reactions shows no significant agglomeration of the [Ru(0)] n nanoparticles. The catalyst ionic liquid phase can be reused several times without a significant loss in catalytic activity.

Palladium nanoparticle catalysts in ionic liquids: synthesis, characterisation and selective partial hydrogenation of alkynes to Z-alkenes

The simple heating (120 °C) of Pd(OAc)2 in 1-butyronitrile-3-methylimidazolium-N-bis(trifluoromethane sulfonyl)imide ((BCN)MI·NTf2) under reduced pressure leads to the formation of stable and small-sized Pd(0)-NPs (diameter: 7.3 ± 2.2 nm). These metal nanoparticles were characterised by means of TEM, HRTEM and XPS analysis techniques. Moreover, the potential for partial hydrogenation of alkynes in multiphase systems was evaluated. The hydrogenation of internal alkynes at 25 °C and under 1 bar of hydrogen yields Z-alkenes (up to 98% selectivity). Application of higher hydrogen pressure (4 bar) in these reactions always led to the formation of alkanes without the detection of any alkenes. TOF values were attained up to 1282 h−1 with a good recyclability of the system which does not lose its activity for at least 4 runs.

Imidazolium ionic liquids as promoters and stabilising agents for the preparation of metal(0) nanoparticles by reduction and decomposition of organometallic complexes

The organometallic complexes ([Ru(COD)(2-methylallyl)2] and [Ni(COD)2] (COD=1,5-cyclooctadiene) dissolved in imidazolium ionic liquids (ILs) undergo reduction and decomposition, respectively, to afford stable ruthenium and nickel metal(0) nanoparticles (Ru(0)-NPs and Ni(0)-NPs) in the absence of classical reducing agents. Depending on the case, the reduction/auto-decomposition is promoted by either the cation and/or anion of the neat imidazolium ILs.

Selective and mild hydrogen production using water and formaldehyde

With the increased efforts in finding new energy storage systems for mobile and stationary applications, an intensively studied fuel molecule is dihydrogen owing to its energy content, and the possibility to store it in the form of hydridic and protic hydrogen, for example, in liquid organic hydrogen carriers. Here we show that water in the presence of paraformaldehyde or formaldehyde is suitable for molecular hydrogen storage, as these molecules form stable methanediol, which can be easily and selectively dehydrogenated forming hydrogen and carbon dioxide. In this system, both molecules are hydrogen sources, yielding a theoretical weight efficiency of 8.4% assuming one equivalent of water and one equivalent of formaldehyde. Thus it is potentially higher than formic acid (4.4 wt%), as even when technical aqueous formaldehyde (37 wt%) is used, the diluted methanediol solution has an efficiency of 5.0 wt%. The hydrogen can be efficiently generated in the presence of air using a ruthenium catalyst at low temperature.

Decomposition of Formic Acid Catalyzed by a Phosphine‐Free Ruthenium Complex in a Task‐Specific Ionic Liquid

The dehydrogenation of formic acid is effectively catalyzed by the Ru complex [{RuCl2(p‐cymene)}2] dissolved in the ionic liquid (IL) 1‐(2‐(diethylamino)ethyl)‐3‐methylimidazolium chloride at 80 °C without additional bases. This catalytic system gives TOF values of up to 1540 h−1. Preliminary kinetic insights show formal reaction orders of 0.70(±0.15), 0.78(±0.03) and 2.00(±0.17) for the Ru catalyst, IL 1, and formic acid, respectively. The apparent activation energy of this process is estimated to be (69.1±7.6) kJ mol−1. In addition, dimeric Ru hydride ionic species involved in the reaction, such as [{Ru(p‐cymene)}2{(H)μ‐(H)‐μ‐(HCO2)}]+ and [{Ru(p‐cymene)}2{(H)μ‐(Cl)μ‐(HCO2)}]+, are identified by mass spectrometry. The presence of water in large amounts inhibits higher conversions. Finally, a remarkable catalytic activity is observed during recycles, indicating this system’s potential for hydrogen gas production.

Application of Chiral Ionic Liquids for Asymmetric Induction in Catalysis

Here we present the state-of-the-art for asymmetric catalysis using chiral ionic liquids (CILs) as source of chiral information. The current review covers reactions using typical homogeneous catalysts e.g. organocatalysts, transition metal complexes and solid catalysts for heterogeneous catalysis in solvent-systems with chiral ionic liquids. INTRODUCTION Most of the chemical production processes in industry involve catalysts (>90%) and the majority are heterogeneous processes [1]. This is due to practical aspects like purification of products or intermediates. However, solid catalysts are disadvantageous if asymmetric reactions such as asymmetric hydrogenation need to be carried out. In that case, chiral organometallic complexes or organocatalysts acting in homogeneous manner are clearly superior to the heterogeneous type of catalyst. Further attempts for asymmetric induction in principle use substrates, reagents and auxiliaries bearing the chiral information successfully [2], or chiral reaction media, the latter one less successfully [3]. Other investigations involved asymmetric induction by chiral particles of achiral substances, like chiral quartz (SiO2) or glycine crystals, for example in asymmetric photochemical reactions in the solid state [4, 5]. In the absence of chiral molecules also circularly polarized light can be used as chiral force such as the synthesis of hexahelicene with left or right circularly polarized light [4, 5]. There are reports though, which are describing the immobilization of chiral homogeneous catalyst on solid support [6]. However, in most cases homogeneous catalysts do not fulfill the necessary requirements (catalyst stability, catalyst recycling, separation from product and intermediates) for industrial processes like continuous-flow processes. For years, the industrial adaptation only seemed practical if homogeneous catalysts are immobilized on solid supports or soluble polymers which can be easily separated from solution [6]. Combining the advantages of solid catalysts (stability, recyclable, purification, long life-time) [6, 7] and the advantages of homogeneous catalysts (chemo-, regioand asymmetric reaction controls) [6, 7] lead to chiral ionic liquids (CILs) as an reaction media for the immobilization of the catalyst. Nowadays, CILs seem to be the most adequate *Address correspondence to this author at the Universidade Federal do Rio Grande do Sul (UFRGS), Institute of Chemistry Campus do Vale, Laboratory of Molecular Catalysis, Avenida Bento Gonçalves 9500 (P.O. Box 15003), Porto Alegre RS – Brazil, CEP 91501-970, Brazil; Fax: +49-32124702238; E-mail: martin-prechtl@gmx.net materials potentially fulfilling all the desired requirements for a heterogeneous-type solid catalyst as well as typical molecular complex catalysts or organocatalysts for asymmetric catalysis. Recently, their unique material and solvent properties and the growing interest in a sustainable, “green” chemistry has led to an amazing increase in interest in such salts [8]. Ionic liquids (ILs) are well-organized three-dimensional media. It has been shown that intramolecular C-H••• interactions results in a well-organized 3D structure where ionic channels are formed by cations and anions [9]. This supramolecular arrangement render them in a plethora of applications and opportunities [10]: a media for metal nanoparticles formation and stabilization [11], the formation of organized liquid clathrates [12], and in many different physicochemical processes [13]. There are many different reviews on the subject of ILs that can attend many readers with distinctive needs [14]. Not before 1999 the first CIL, an imidazolium salt with a lactate as chiral anion was published by Seddon, and in the following years most of the CILs had chiral cations derived from the “chiral pool” [15]. Their chirality origin is from axial, central or planar chirality. Additionally, it can be found in both cation, and the anion – the so called doubly chiral IL. As a consequence of a very well organized 3D structure, CILs have an intrinsic potential for enantioselective reactions, especially because in a well organized media it is reasonable to expect some chiral transmission. Whether considering that ILs can participate in the transition state or stabilizing charged (or polar) intermediates, it is expected that chiral induction takes place in that media. The chiral transmission using CILs as the single source of chirality may take place through two different pathways: (1) the CILs indeed participate in the reaction intermediates or transition states [16], e.g. some proline TSCILs (taskspecific CIL) and (2) through ion-pairing formation [17]. The attempts to use chiral solvents as source of chirality goes back to the 1970s, when Seebach and Oei performed electrochemical reduction of ketones in a chiral amino ether which led to a rather low enantioselectivity of ~24% ee [18]. Further investigations in the past decades gave also comparably low asymmetric induction which resulted in the ac1260 Current Organic Chemistry, 2009, Vol. 13, No. 13 Prechtl et al. cepted conclusion that chirality transfer from chiral solvents is rather low [3]. Later on, it was not necessary to wait again several decades for further advantages in catalysis using chiral solvent. This conclusion seemed to be unchanged at least until middle of 2004. Then, the first motivating result was published by Vo-Thanh and coworkers [19]. The middle of May 2006 can be dated as the breakthrough of catalysis using CIL as single source of chirality. Almost parallel Leitner [20], Afonso [21], Malhotra [22], Cheng [23], and all their respectively coworkers presented their contribution to this research field with remarkably high enantiomeric excesses in different reactions. Nowadays, many applications of CILs in different areas of research may be found in the open literature. CILs are used for enantioselective separation of pharmaceutical products by capillary electrophoresis,[24] to tune some kinetics aspects on the asymmetric hydrogenation of some specific compounds [25], chiral stationary phase for the liquid chromatographic resolution [26], enantiomeric recognition properties [27], influence on the excitedstate properties [28], and others. Furthermore, the importance of CILs are reflected in various publications showing simply their syntheses, characterizations and properties [29, 30, 31] Several reviews can be found describing the development in the syntheses and applications of CILs [15]. In the present review we focus on the application of CILs in asymmetric catalysis and not on the synthesis/design of CILs which are more intensively discussed in previous reviews [15]. 1. ORGANOCATALYSIS 1.1. Baylis-Hillman Reaction Vo-Thanh and coworkers presented the pioneering work about a DABCO (1,4-diazabicyclo[2.2.2]octane) catalyzed asymmetric Baylis-Hillman reaction between benzaldehyde and methyl acrylate 3 (Scheme 1, Table 1). Enantioselectivities up to 44% ee were obtained using a chiral cation, based on ephedrine, as source of chirality [19].

Tuneable Hydrogenation of Nitriles into Imines or Amines with a Ruthenium Pincer Complex under Mild Conditions

The selective hydrogenation of aromatic and aliphatic nitriles into amines and imines is described. Using a ruthenium pincer complex, the selectivity towards amines or imines can be controlled by simple parameter changes. The reactions are conducted under very mild conditions between 50–100 °C at 0.4 MPa H2 pressure without any additives at low catalytic loadings of 0.5–1 mol %, which results in quantitative conversions and high selectivity.

Catalytic CH Bond Activation at Nanoscale Lewis Acidic Aluminium Fluorides: H/D Exchange Reactions at Aromatic and Aliphatic Hydrocarbons

Nanoscopic amorphous Lewis acidic aluminium fluorides, such as aluminium chlorofluoride (ACF) and high-surface aluminium fluoride (HS-AlF(3)), are capable of activating C-H bonds of aliphatic hydrocarbons. H/D exchange reactions are catalysed under mild conditions (40 °C).

New insights into the catalytic cleavage of the lignin β-O-4 linkage in multifunctional ionic liquid media

Ionic liquids are attractive reaction media for the solubilisation and depolymerisation of lignin into value-added products. However, mechanistic insight related to the cleavage of specific linkages relevant to efficient lignin depolymerisation in such solvents is still lacking. This study presents important insight into the scission of the most abundant lignin β-O-4 motif in Bronsted acidic ionic liquids. Using relevant model compounds, cleavage products were identified and undesired side reactions examined carefully. Stabilization of reactive intermediates was achieved in ionic liquids comprising both Bronsted acidic function and stabilized nanoparticles that comprise hydrogenation activity in order to suppress undesired side reactions. Especially, the in situ hydrogenation of the aldehyde intermediate originating from the acid-catalysed cleavage of lignin beta-O-4 model compounds into more stable alcohols was investigated. This is the first time that such products have been systematically targeted in these multifunctional reaction media in relation to lignin depolymerization.