Ionic liquids requirements

The Ni-catalyzed oligomerization of olefins in ionic liquids requires a careful choice of the ionic liquid s acidity. In basic melts (Table 5.2-2, entry (a)), no dimerization activity is observed. FFere, the basic chloride ions prevent the formation of free coordination sites on the nickel catalyst. In acidic chloroaluminate melts, an oligomerization reaction takes place even in the absence of a nickel catalyst (entry (b)). FFowever, no dimers are produced, but a mixture of different oligomers is... 

A rather new concept for biphasic reactions with ionic liquids is the supported ionic liquid phase (SILP) concept [115]. The SILP catalyst consists of a dissolved homogeneous catalyst in ionic liquid, which covers a highly porous support material (Fig. 41.13). Based on the surface area of the solid support and the amount of the ionic liquid medium, an average ionic liquid layer thickness of between 2 and 10 A can be estimated. This means that the mass transfer limitations in the fluid/ionic liquid system are greatly reduced. Furthermore, the amount of ionic liquid required in these systems is very small, and the reaction can be carried in classical fixed-bed reactors. 

Many properties of ionic liquids make them more desirable solvents than organic solvents. For example, ionic liquids require a lot of energy to change their state—they remain as liquids even at temperatures of 200°C. 

As a new class of materials, ionic liquids require special analytical methods. In the case of imidazolium halides and similar compounds the most common impurities are amines, alkyl halides and of course water. Seddon et al. described a method for the detection of residual amines using the strong UV absorbance of copper tetramine complexes. These complexes are readily formed by the addition of Cu2+ ions [24]. The detection of both amines and alkyl halides is possible by NMR spectroscopy but with limited resolution [25]. By far the most powerful analytical method is liquid chromatography combined with UV detection. This sensitive method allows the detection of traces of amines and halides [26]. Unreacted amines can be also detected by ion chromatography combined with a suppressor module. In this case detection is achieved using a continuous flow conductivity cell since amines are protonated and thus detectable. For traces of other ionic impurities ion chromatography is also the most powerful analytical tool [27]. Finally, residual water can be quantified using Karl Fischer titration or coulometry [28]. 

Electrochemical synthesis of electroconducting polymers such as polyarene [28— 31], polypyrrole [32-34], polythiophene [35], and polyaniline [36, 37] has been carried out in moisture sensitive chloroaluminate ionic liquids. However, the polymer hlms are decomposed rapidly by the corrosive products like HCl generated by hydrolysis of the ionic liquids. In addition the treatment of the chloroalminate ionic liquids requires a special equipment such as glove box. 

Process related studies of transition metal catalyzed hydroformylation in ionic hq-uids mainly focused on a better utilization of the amount of ionic hquid and thus on a reduction of the amount of ionic liquids required. A key aspect in this context is to enhance the solubility of the reactants CO, H2 and olefin in the ionic hquid and to enhance the mass transfer between the gas phase/organic fluid and the ionic liquid phase. 

The theoretical simulation of ionic liquids requires a combined approach that involves the use of more than one techniques. It is customary to perform a series of preliminary ab-initio calculations before moving to molecular dynamics simulations. The ab-initio simulations are dictated by the necessity of obtaining the "true" charge distribution in order to check whether it matches the partial atomic charges of the force field and, if necessary, to provide new partial atomic charges. 

The alkylation process possesses the advantages that (a) a wide range of cheap haloalkanes are available, and (b) the substitution reactions generally occur smoothly at reasonable temperatures. Furthermore, the halide salts formed can easily be converted into salts with other anions. Although this section will concentrate on the reactions between simple haloalkanes and the amine, more complex side chains may be added, as discussed later in this chapter. The quaternization of amines and phosphines with haloalkanes has been loiown for many years, but the development of ionic liquids has resulted in several recent developments in the experimental techniques used for the reaction. In general, the reaction may be carried out with chloroalkanes, bromoalkanes, and iodoalkanes, with the reaction conditions required becoming steadily more gentle in the order Cl Br I, as expected for nucleophilic substitution reactions. Fluoride salts cannot be formed in this manner. 

The anion-exchange reactions of ionic liquids can really be divided into two distinct categories direct treatment of halide salts with Lewis acids, and the formation of ionic liquids by anion metathesis. These two approaches are dealt with separately, as quite different experimental methods are required for each. 

From Section 2.1 it has become very clear that the synthesis of an ionic liquid is in general quite simple organic chemistry, while the preparation of an ionic liquid of a certain quality requires some know-how and experience. Since neither distillation nor crystallization can be used to purify ionic liquids after their synthesis (due to their nonvolatility and low melting points), maximum care has to be taken before and during the ionic liquid synthesis to obtain the desired quality. 

In theory, volatile impurities can easily be removed from the nonvolatile ionic liquid by simple evaporation. However, this process can sometimes take a considerable time. Factors that influence the time required for the removal of all volatiles from an ionic liquid (at a given temperature and pressure) are a) the amount of volatiles, b) their boiling points, c) their interactions with the ionic liquid, d) the viscosity of the ionic liquid, and e) the surface of the ionic liquid. 

In this context it is important to note that the detection of this land of alkali cation impurity in ionic liquids is not easy with traditional methods for reaction monitoring in ionic liquid synthesis (such as conventional NMR spectroscopy). More specialized procedures are required to quantify the amount of alkali ions in the ionic liquid or the quantitative ratio of organic cation to anion. Quantitative ion chromatography is probably the most powerful tool for this kind of quality analysis. 

Without a doubt, tetrafluoroborate and hexafluorophosphate ionic liquids have shortcomings for larger-scale technical application. The relatively high cost of their anions, their insufficient stability to hydrolysis for long-term application in contact with water (formation of corrosive and toxic HF during hydrolysis ), and problems related to their disposal have to be mentioned here. New families of ionic liquid that should meet industrial requirements in a much better way are therefore being developed. FFowever, these new systems will probably be protected by state of matter patents. 

Transition metal catalysis in liquid/liquid biphasic systems principally requires sufficient solubility and immobilization of the catalysts in the IL phase relative to the extraction phase. Solubilization of metal ions in ILs can be separated into processes, involving the dissolution of simple metal salts (often through coordination with anions from the ionic liquid) and the dissolution of metal coordination complexes, in which the metal coordination sphere remains intact. 

The most common measure of polarity used by chemists in general is that of dielectric constant. It has been measured for most molecular liquids and is widely available in reference texts. However, direct measurement, which requires a nonconducting medium, is not available for ionic liquids. Other methods to determine the polarities of ionic liquids have been used and are the subject of this chapter. However, these are early days and little has been reported on ionic liquids themselves. I have therefore included the literature on higher melting point organic salts, which has proven to be very informative. 

The description of electronic distribution and molecular structure requires quantum mechanics, for which there is no substitute. Solution of the time-independent Schrodinger equation, Hip = Eip, is a prerequisite for the description of the electronic distribution within a molecule or ion. In modern computational chemistry, there are numerous approaches that lend themselves to a reasonable description of ionic liquids. An outline of these approaches is given in Scheme 4.2-1 [1] ... 

From the molecular point of view, the self-diffusion coefficient is more important than the mutual diffusion coefficient, because the different self-diffusion coefficients give a more detailed description of the single chemical species than the mutual diffusion coefficient, which characterizes the system with only one coefficient. Owing to its cooperative nature, a theoretical description of mutual diffusion is expected to be more complex than one of self-diffusion [5]. Besides that, self-diffusion measurements are determinable in pure ionic liquids, while mutual diffusion measurements require mixtures of liquids. 

Ionic liquids have been described as designer solvents [11]. Properties such as solubility, density, refractive index, and viscosity can be adjusted to suit requirements simply by making changes to the structure of either the anion, or the cation, or both [12, 13]. This degree of control can be of substantial benefit when carrying out solvent extractions or product separations, as the relative solubilities of the ionic and extraction phases can be adjusted to assist with the separation [14]. Also, separation of the products can be achieved by other means such as, distillation (usually under vacuum), steam distillation, and supercritical fluid extraction (CO2). 

As a demonstration of the complete synthesis of a pharmaceutical in an ionic liquid, Pravadoline was selected, as the synthesis combines a Friedel-Crafts reaction and a nucleophilic displacement reaction (Scheme 5.1-24) [53]. The allcylation of 2-methylindole with l-(N-morpholino)-2-chloroethane occurs readily in [BMIM][PF6] and [BMMIM][PF6] (BMMIM = l-butyl-2,3-dimethylimida2olium), in 95-99 % yields, with potassium hydroxide as the base. The Friedel-Crafts acylation step in [BMIM][PF6] at 150 °C occurs in 95 % yield and requires no catalyst. 

Reactions between aldehydes and alkynes to give propargyl alcohols are also described in Kitazume and Kasai s paper [55]. Here, various aldehydes such as benzaldehyde or 4-fluorobenzaldehyde were treated with alkynes such as phenylethyne or pent-l-yne in three ionic liquids [EDBU][OTf], [BMIM][PFg], and [BMIM][BF4] (Scheme 5.1-27). A base (DBU) and Zn(OTf)2 were required for the reaction to be effective the yields were in the 50-70 % range. The best ionic liquid for this reaction depended on the individual reaction. 

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