Organic ionic liquids

Chloroaluminate(III) ionic liquid systems are perhaps the best established and have been most extensively studied in the development of low-melting organic ionic liquids with particular emphasis on electrochemical and electrodeposition applications, transition metal coordination chemistry, and in applications as liquid Lewis acid catalysts in organic synthesis. Variable and tunable acidity, from basic through neutral to acidic, allows for some very subtle changes in transition metal coordination chemistry. The melting points of [EMIM]C1/A1C13 mixtures can be as low as -90 °C, and the upper liquid limit almost 300 °C [4, 6]. 

The term Supported Ionic Liquid Phase (SILP) catalysis has recently been introduced into the literature to describe the heterogenisation of a homogeneous catalyst system by confining an ionic liquid solution of catalytically active complexes on a solid support [68], In comparison to the conventional liquid-liquid biphasic catalysis in organic-ionic liquid mixtures, the concept of SILP-catalysis offers very efficient use of the ionic liquid. Figure 7.10 exemplifies the concept for the Rh-catalysed hydroformylation. 

TABLE 1.1 Basic Characteristics of Organic ionic Liquids... 

Oriented structures of self-organized ionic liquids can be preserved in solid films... 

Homogeneous catalysts used for polyolefin degradation have mostly been classical Lewis acids such as AICI3, metal tetrachloroaluminates melts and more recently, new catalytic systems based on organic ionic liquids. 

Upon treating certain (but not all) aromatic aldehydes or glyoxals (a-keto aldehydes) with cyanide ion (CN ), benzoins (a-hydroxy-ketones or acyloins) are produced in a reaction called the benzoin condensation. The reverse process is called the retro-benzoin condensation, and it is frequently used for the preparation of ketones. The condensation involves the addition of one molecule of aldehyde to the C=0 group of another. One of the aldehydes serves as the donor and the other serves as the acceptor. Some aldehydes can only be donors (e.g. p-dimethylaminobenzaldehyde) or acceptors, so they are not able to self-condense, while other aldehydes (benzaldehyde) can perform both functions and are capable of self-condensation. Certain thiazolium salts can also catalyze the reaction in the presence of a mild base. This version of the benzoin condensation is more synthetically useful than the original procedure because it works with enolizable and non-enolizable aldehydes and asymmetric catalysts may be used. Aliphatic aldehydes can also be used and mixtures of aliphatic and aromatic aldehydes give mixed benzoins. Recently, it was also shown that thiazolium-ion based organic ionic liquids (Oils) promote the benzoin condensation in the presence of small amounts of triethylamine. The stereoselective synthesis of benzoins has been achieved using chiral thiazolium salts as catalysts. 

Davis, J. H., Jr., Forrester, K. J. Thiazolium-ion based organic ionic liquids (OILs). Novel OILs which promote the benzoin condensation. Tetrahedron Lett. 1999, 40,1621-1622. 

Boschetti A, Montagnaro F, Rienzo C, Santoro L (2007) A preliminary investigation on the use of organic ionic liquids as green solvents for acylation and oxidation reaction. J Qeaner Prod 15 1797-1805... 

Organic Ionic Liquids Ultimate Green Solvents in Organic Synthesis... 

Fig. 4 shows the example of cyclic voltammetiy curves for an activated carbon at 5 mV s" using different electrolyte (aqueous, organic, ionic liquid), where it is well visible that the voltage range is imposed by the stability vrfndow of the electrolyte. These almost rectangular box like shape curves are characteristic of an ideal EDL capacitor, with low ESR. 

Figure 6.2 Schematic representation of continnons organic-ionic liquid hiphasic catalysis - general principle Qefti and process flow scheme (right).

Apart from acidic catalysis, ionic liquids have been intensively tested in the last two decades for the immobilisation of homogeneously dissolved transition metal catalysts. Successful catalyst immobilisation techniques are essential for industrial homogeneous catalysis to solve the problem of catalyst/product separation and to recover and recycle the often very expensive dissolved transition metal complexes. Different immobilisation concepts applying ionic liquids have been developed, including the use of organic-ionic liquid multiphase reaction systems and the use of SILP catalysis. These concepts will be described in the following sections. 

A schematic representation of a continuous liquid-liquid biphasic catalytic process is shown for the case of an organic-ionic liquid system in Figure 6.2. Over the last decade, hundreds of successful exanples ionic liquid-based liquid-liquid biphasic catalysis have been reported, far too numerous to mention here. The reader more interested in a conplete picture of this huge research activity is referred to the large number of excellent reviews on the topic, with recent... 

Davis and Forrester prepared thiazolium based organic ionic liquids (29, scheme-8) and used them for catalyzing the benzoin condensation (Davis Forrester 1999). 

Davis and Forrester have studied the benzoin condensation promoted with a small ( 5 mol %) quantity of triethylamine and thiazolium salts (160). The reaction was accomplished when the thiazolium based organic ionic liquid (OIL) was stirred under nitrogen as a clearly heterogeneous mixture with a toluene solution of benzaldehyde. The reaction gave about 80% conversion to benzoin (163) (scheme-40) (Davis Forrester 1999). 

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