Alternative solvents ionic liquids

In microwave-assisted synthesis, a homogeneous mixture is preferred to obtain a uniform heating pattern. For this reason, silica gel is used for solvent-free (open-vessel) reactions or, in sealed containers, dipolar solvents of the DMSO type. Welton (1999), in a review, recommends ionic liquids as novel alternatives to the dipolar solvents. Ionic liquids are environmentally friendly and recyclable. They have excellent dielectric properties and absorb microwave irradiation in a very effective manner. They exhibit a very low vapor pressure that is not seriously enhanced during microwave heating. This makes the process not so dangerous as compared to conventional dipolar solvents. The polar participants of organic ion-radical reactions are perfectly soluble in polar ionic liquids. 

In this chapter alternative acidic ionic liquids systems will be briefly presented that have been recently developed as alternatives to chloroaluminate ionic liquids. In spite of this selection, it should be noted that chloroaluminate ionic liquids may still be attractive catalyst phases in reactions where their tuneable acidity and solubility properties offer advantage over AICI3 in organic solvents. 

Abbott AP, Boothby D, Capper G et al (2004) Deep eutectic solvents formed between choline chloride and carboxylic acids versatile alternatives to ionic liquids. J Am Chem Soc 126 9142-9147... 

One of the main efforts towards the development of sustainable chemistiy is the reduction in the generation of waste. The largest amount of waste produced by a reaction is typically associated with the solvent employed as it is the component added in the largest quantity and, usually not incorporated into the final product but rather removed and disposed of at the end of the process. For these reasons, many efforts are being devoted to the development of catalytic systems that can operate under solvent-free conditions. When the use of a solvent is mandatory, the efforts are directed towards the use of sustainable solvents. In this context, water, which is the only natural solvent, is the preferred choice. Other green alternatives are ionic liquids or supercritical carbon dioxide. 

As an alternative to ionic liquids, "deep eutectic solvents" (DES) have been investigated. DES are mixtures of mefal or ammonium salts witii hydrogen-bond donors they show low melting points and are cheap, more biocompatible, and regarded as safe [389,390]. Also, liquid pol5uners have been used in combination with supercritical carbon dioxide [391]. 

Interest in using ionic liquid (IL) media as alternatives to traditional organic solvents in synthesis [1 ], in liquid/liquid separations from aqueous solutions [5-9], and as liquid electrolytes for electrochemical processes, including electrosynthesis, primarily focus on the unique combination of properties exhibited by ILs that differentiate them from molecular solvents. 

An alternative avenue for the exploration of the polarity of a solvent is by investigation of its effect on a chemical reaction. Since the purpose of this book is to review the potential application of ionic liquids in synthesis, this could be the most productive way of discussing ionic liquid polarity. Again, the field is in its infancy, but some interesting results are beginning to appear. 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

However, ionic liquids and SCCO2 are not competing concepts for the same applications. While ionic liquids can be considered as alternatives for polar organic solvents, the use of SCCO2 can cover those applications in which non-polar solvents are usually used. 

We are far here from aiming to advise anybody about future research projects. The only message that we would like to communicate is that a chemical reaction is not necessarily surprising or important because it somehow works as well in an ionic liquid. One should look for those applications in which the specific properties of the ionic liquids may allow one to achieve something special that has not been possible in traditional solvents. If the reaction can be performed better (whatever you may mean by that) in another solvent, then use that solvent. In order to be able to make that judgement, it is imperative that we all include comparisons with molecular solvents in our studies, and not only those that we loiow are bad, but those that are the best alternatives. 

Room temperature ionic liquids arc currently receiving considerable attention as environmentally friendly alternatives to conventional organic solvents in a variety of contexts.144 The ionic liquids have this reputation because of their high stability, inertness and, most importantly, extremely low vapor pressures. Because they are ionic and non-conducting they also possess other unique properties that can influence the yield and outcome of organic transformations. Polymerization in ionic liquids has been reviewed by Kubisa.145 Commonly used ionic liquids are tetra-alkylammonium, tetra-alkylphosphonium, 3-alkyl-l-methylimidazolium (16) or alkyl pyridinium salts (17). Counter-ions are typically PF6 and BF4 though many others are known. 

As outlined above, immobilization in a fluorinated liquid phase demands the functionahzation of the ligand with perfluoroalkyl chains and, even then, the solubihty is strongly influenced by the nature of the complex. Ionic hquids of the alkylmethyhmidazolium type (Fig. 4) have been recently developed as alternative solvents for organometallic catalysis and have the practical advantage of using directly the commercially available chiral hgands and complexes. 

Before leaving ionic liquids it is worth mentioning their potential value in separation processes. Organic solvents are frequently used in multiphase extraction processes and pose the same problems in terms of VOC containment and recovery as they do in syntheses, hence ionic liquids could offer a more benign alternative. Interesting applications along this line which have been studied include separation of spent nuclear fuel from other nuclear waste and extraction of the antibiotic erythromycin-A. 

Significant progress has been made in the application of ionic liquids (ILs) as alternative solvents to C02 capture because of their unique properties such as very low vapour pressure, a broad range of liquid temperatures, excellent thermal and chemical stabilities and selective dissolution of certain organic and inorganic materials. ILs are liquid organic salts at ambient conditions with a cationic part and an anionic part. 

Besides using standard organic solvents in conjunction with microwave synthesis, the use of either water or so-called ionic liquids as alternative reaction media [32] has become increasingly popular in recent years. 

Ionic liquids are particularly applicable for use in microwave-mediated liquid-phase reactions as they efficiently couple with microwaves resulting in very rapid heating profiles (see Section 4.3.3.2). Furthermore, ionic liquids are immiscible with a wide range of organic solvents and provide an alternative non-aqueous two-phase system. An interesting approach involves the attachment of ionic liquid components to organic substrates. 

These alternative processes can be divided into two main categories, those that involve insoluble (Chapter 3) or soluble (Chapter 4) supports coupled with continuous flow operation or filtration on the macro - nano scale, and those in which the catalyst is immobilised in a separate phase from the product. These chapters are introduced by a discussion of aqueous biphasic systems (Chapter 5), which have already been commercialised. Other chapters then discuss newer approaches involving fluorous solvents (Chapter 6), ionic liquids (Chapter 7) and supercritical fluids (Chapter 8). 

There are many good reasons for applying ionic liquids as alternative solvents in transition metal catalysed reactions. Besides their very low vapour pressure and then-good thermal stability [33], an important advantage is the possibility of tuning then-solubility [34] and acidity/coordination properties [35] by varying the nature of the anions and cations systematically. 

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