Applications of Ionic Liquids in Synthesis and Catalysis by Paul J. Dyson and Tilmann J. Geldbach he “modern ionic liquids” part of the product, just something of cyclopentadiene to methyl acrylate described in the first article in added prior to the reaction and then in chloroaluminate ionic liquids Tthis issue by Keith Johnson have later removed; thus elimination takes place with very high rates certainly captured the imagination should be as facile as possible. How and selectivities (endo:exo ratio = of chemists across the globe, and then can non-volatile ionic liquids 15), unfortunately the ionic liquids without at doubt, nowhere more be used—presumably the rotary cannot be reused.9 Chloroaluminate than in synthesis and catalysis.1 The evaporator can be thrown away?2 ionic liquids have also been shown to reasons for such phenomenal interest While removal of the solvent by promote selectivities toward different in this area are due to several factors, evaporation is no longer an option products depending on the mol notably: the reverse, distillation of the product fraction of aluminum(III)chloride 1. synthetic chemists are limited may become possible instead, if employed.10 However, despite the by the available molecular it is sufficiently volatile. Other, considerable potential for Diels- solvents in which they can well-established techniques for Alder reactions due to the variety of conduct chemistry; the isolation of a product include cation-anion combinations that are 2. increasingly the solvents extraction with supercritical CO2 or available, it does not appear that the chemists like to use are simple phase separation, provided benefits of using ionic liquids are at banned by international that the product is immiscible in the present significant enough to warrant protocols determined to reduce ionic liquid. Such approaches are their frequent or large scale use. pollution, of which volatile not new: biphasic (or multiphasic) organic compounds represent a processes where a catalyst Catalyzed Reactions significant part; and immobilized in an aqueous phase 3. the prospect of discovering together with water-soluble substrates It was known for many years that new chemistry or the ability to leads to the formation of a second ionic liquids could dissolve transition design a solvent that facilitates/ phase comprised of the reaction metal complexes and support allows a specific reaction to product, have been used industrially organic chemistry. Chauvin and occur. for many years.3 However, water is Osteryoung independently combined The reasons stated above are all somewhat limited as a solvent; hence these ideas with Chauvin showing important and many papers in the the attractiveness of ionic liquids and that nickel complexes dissolved literature respond directly to such some specific strategies and examples in acidic chloroaluminate ionic challenges. However, there are a large are outlined below. liquids catalyze the dimerization number of papers that justify their of alkenes11 while Osteryoung used existence merely to the fact that ionic Stoichiometric Organic Synthesis Ziegler-Natta catalysts in a similar liquids are perceived as “green”— solvent to polymerize ethylene.12 But since they do not evaporate under Many organic reactions have been it was Zaworotko’s water-stable ionic ambient conditions—and apart from studied in ionic liquids, mostly with liquids that contain tetrafluoroborate, this aspect, little novelty or insights the expectation that the ionic solvent hexafluorophosphate, nitrate, are reported. In the remainder of this will accelerate product formation or sulfate and acetate anions that really article we have selected examples alter the selectively of the reaction moved things forward. Using these from the literature that demonstrate and many notable and impressive ionic liquids Chauvin and Dupont the potential of ionic liquids in examples have been reported.4 In this demonstrated their potential in synthetic applications. These not respect the Diels-Alder reaction is a hydrogenation catalysis13 and showed only offer new opportunities on how particularly interesting case as higher that reaction rates and selectivities to perform known reactions, but reaction rates and selectivities are could be enhanced. can even be tailored to meet specific obtained in polar solvents compared It is impossible to describe all the synthetic needs. to non-polar solvents,5 and endo:exo catalyzed reactions that have been product selectivities are often higher reported in ionic liquids, although Ionic Liquids as Solvents in aqueous solution.6 These effects just about every important reaction have been attributed to enhanced has been performed with varying How do chemists use ionic liquids hydrogen bonding between the degrees of success.14 What has become in place of organic solvents? It solvent and the transition state, as apparent is that frequently used may seem like the answer to this well as to enforced hydrophobic and well-established homogeneous question is very simple, perhaps interactions, essentially creating a catalysts need to be modified in entirely obvious, i.e. “just dissolve hydrophobic pocket, which facilitates order to work effectively in ionic your compounds in the ionic liquid alignment of the substrates.7 The liquids. For example, it was shown and get on with it,” but the reality first example of a Diels-Alder that activation of a pre-catalyst can is somewhat more complicated. reaction involving an ionic liquid, be thermodynamically hindered in Synthetic chemists have geared up viz. [EtNH3][NO3], was published an ionic liquid, affording no catalysis virtually everything of what they by Jaeger and Tucker in 19898 with at all if reaction conditions are do to being conducted in volatile results comparable with those not adapted properly.15 In addition solvents. After all, the solvent is not obtained in water. The cycloaddition to catalyst activation, sufficient 50 The Electrochemical Society Interface • Spring 2007 PF PF Cl 6 Cl 6 O Ru L O Ru L Cl Cl 2 PF6 H H N N N N N N O N N N N 2+ 2+ N Ph N O Ru N Ph Cl Me N (CH2)4 O S N NH Me H 2 O NH3 TsHN Ph Ph Cl O 3 BF4 N N N N V N N O O tBu Ph2P PPh2 O N N N Rh N S tBu tBu (cod) FIG. 1. Imidazolium tagged ligands/catalysts that enhance immobilization in ionic liquids and improve catalyst recycling/reuse. retention of the catalyst in the the majority of catalyzed reactions appropriate functional groups ionic liquid is important in order to reported to date have been carried out attached to the cation and a charged provide good recycling. The most in imidazolium-based ionic liquids. catalyst then resides within the ionic versatile approach used to anchor Other approaches have been matrix. The concept is illustrated in homogeneous catalysts in ionic liquid used to facilitate catalyst reuse and Fig. 2 for the racemic epoxidation phases is to attach charged groups in the context of continuous flow of olefins using a peroxotungstenate 2– to the ligands bound to the metal processes supported ionic liquid catalyst, [{W(=O)(O2)2(H2O)}2(µ- O)] , catalyst centre; some recent examples phase (SILP) catalysis has been quite supported on ionic liquid of ligands/catalysts modified in extensively studied.16 The general modified silica.17 The solid this manner are shown in Fig. 1. concept involves the immobilization support was reacted with 1-octyl- Imidazolium groups have been of imidazolium and other ionic 3-(3-triethoxysilylpropyl)-4,5- widely used for this purpose, since fragments onto solid supports using dihydroimidazolium, affording a SiO2 (continued on next page) Olefin Epoxide Ionic Liquid Film Silica support FIG. 2. A tungstenate catalyst immobilized on ionic liquid modified silica. The Electrochemical Society Interface • Spring 2007 51 surface on which the ionic liquid is ionic liquid soluble nanoparticles. As an example, nitrile covalently bound. This heterogeneous It has also been demonstrated that functionalized ionic liquids were catalyst was successfully used to nanoparticles stabilized by an ionic developed with a view to improve epoxidise olefins using H2O2 as liquid polymer can be efficiently catalyst retention in reactions oxidant and reaction rates were transferred between phases via employing simple palladium salts comparable to those observed under anion exchange,24 which could have as catalyst precursors, as it was homogeneous conditions. important applications in catalysis envisaged that the nitrile group Ionic liquids have further with respect to product separation. could weakly coordinate to the metal proven to be excellent solvents center thereby anchoring it in the to both immobilize and stabilize Ionic Liquids by Design ionic liquid phase. Such ionic liquids nanoparticle catalysts. Nanoparticles have been utilized in Suzuki and were first identified in ionic liquids The application of ionic liquids in Stille coupling reactions and they as species formed during Heck catalysis is not without problems, one exhibit excellent recycling properties reactions using Pd(II) compounds as being that yields and selectivities can relative to other ionic liquids.28 The catalyst precursors.18 Dupont et al. vary according to the batch of ionic superior activity observed in the reported the controlled preparation liquid used. Impurities such as halides, nitrile system was not only the result of transition metal nanoparticles and to lesser extent organic compounds of better catalyst retention in the in ionic liquids by reduction of of low volatility, have been implicated