Ionic liquids as alternative solvents

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). 

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. 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in synthetic organic chemistry and particularly in catalytic reactions. Besides the engineering... 

Thanks to the extensive studies in the last years on ionic liquids as alternative solvents for homogeneous catalysis, the recourse to liquid-liquid biphasic catalysis is now a common way to heterogenisation. The result is that the catalyst and the product are confined into two separate and immiscible liquid phases, for example an aqueous phase" or a molecular organic solvent, and an ionic liquid. Tire catalyst is dissolved in the IL phase, and the substrate resides in the other phase. During reactions, the two layers are vigorously stirred, thus allowing suitable interaction of catalyst... 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal catalyzed reactions. Besides the engineering advantage of their exhemely low volatility, 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 allows a systematic optimization of the... 

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. 

G. Meseguer, A. B. deHaan, Ionic Liquids as Alternatives to Organic Solvents in Liquid-Liquid Extraction oFAromatics, in Ionic Liquids IV Fundamentals, Progress, Challenges, and Opportunities Rogers, R. D., Seddon, K. R. (Eds.), ACS Symposium Series 902, 2005, pp. 57-71. 

Abstract Separation and recycling of rare-earth metals are attracting continuous attention worldwide. Liquid-liquid extraction is a conventionally employed technique for the separation of rare-earth metals. In recent years, growing attention has focused on room temperature ionic liquids as alternatives to conventional organic solvents. 

Nakashima K, Kubota F, Maruyama T, Goto M (2005) Feasibility of ionic liquids as alternative separation media fra- industrial solvent extraction processes. Ind Eng Chem Res 44 4368 372... 

Marszalkowska, B. Regel-Rosocka, M. Nowak, L. Wisniewski, M. (2010). Quaternary Phosphonium Salts as Effective Extractants of Zinc(II) and Iron(III) Ions from Acidic Pickling Solutions. Pol.. Chem. Technol, Vol.l2, No.4, pp. 1-5 Nakashima, K Kubota, F. Maruyama, T. Goto, M (2003). Ionic Liquids as a Novel Solvent for Lanthanide Extraction. Anal Set, Vol.l9, pp. 1097-1098 Nakashima, K. Kubota, F. Maruyama, T. Goto, M. (2005). Feasibility of Ionic Liquids as Alternative Separation Media for Industrial Solvent Extraction Processes. Ind. Eng. Chem. Res., Vol.44, p>p. 4368-4372... 

Visser, A.E. Holbrey, J.D. Rogers, R.D. (2002a). Room Temperature Ionic Liquids as Alternatives to Traditional Organic Solvents in Solvent Extraction. Proceedings of Solvent Extraction Conference, ISBN 1-919783-24-5, Cape Town, March 2002... 

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. 

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. 

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. 

Various solid-supported perruthenate reagents have been designed for the oxidation of alcohols.Solid-supported NMO has also been used. A number of perruthenate systems employing O2 as the terminal oxidant have also been reported. The use of ionic liquids based upon substituted imidazolium cations as alternative solvent media for the selective oxidation of alcohols to aldehydes and ketones has also been investigated. ... 

Rather than modifying the workup conditions, other groups have changed the reaction solvent or catalyst structure. The use of ionic liquids as a reaction solvent allows for easy catalyst and product separation as well as catalyst recycling.79 Alternatively, others have attached the various metathesis catalysts onto solid supports,80 82 which again allows for easy workup and catalyst recycle. 

Chapter 7 addresses another key topic in the context of green chemistry the replacement of traditional, environmentally unattractive organic solvents by greener alternative reaction media such as water, supercritical carbon dioxide, ionic liquids and perfluorous solvents. The use of liquid/liquid biphasic systems provides the additional benefit of facile catalyst recovery and recycling. 

Ionic liquids continue to receive increasing interest as alternative solvents for catalysis and the historical developments in the field are described in Section 1.4. 

The fact that ionic liquids are solvents composed entirely of ions creates a new set of challenges to chemists and chemical engineers who are not used to handling such materials in the capacity of a solvent. At present, the most popular solvents are those which are volatile, since solvents are added to facilitate reactions, but ultimately they must be removed from the product. Accordingly, alternative techniques are required for the isolation of reaction products from ionic liquids, as well as for their disposal. Furthermore, if catalysed reactions are to be understood in ionic liquids, then new procedures may be required to delineate reaction mechanisms and identify catalytic intermediates. With respect to this latter goal, some simple solutions have already been devised. For example, by using labelled... 

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