Lewis ionic liquid

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. 

Compositions of Lewis acid-based ionic liquids are generally referred to by the mole frac-... 

Ionic liquids derived from Lewis acids based on Ti, Nb, Sn, Sb [NR3R ]X/SbE5 Atofma, France 2001 25... 

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 field of reaction chemistry in ionic liquids was initially confined to the use of chloroaluminate(III) ionic liquids. With the development of neutral ionic liquids in the mid-1990s, the range of reactions that can be performed has expanded rapidly. In this chapter, reactions in both chloroaluminate(III) ionic liquids and in similar Lewis acidic media are described. In addition, stoichiometric reactions, mostly in neutral ionic liquids, are discussed. Review articles by several authors are available, including Welton [1] (reaction chemistry in ionic liquids), Holbrey [2] (properties and phase behavior), Earle [3] (reaction chemistry in ionic liquids), Pagni [4] (reaction chemistry in molten salts), Rooney [5] (physical properties of ionic liquids), Seddon [6, 7] (chloroaluminate(III) ionic liquids and industrial applications), Wasserscheid [8] (catalysis in ionic liquids), Dupont [9] (catalysis in ionic liquids) and Sheldon [10] (catalysis in ionic liquids). 

A similar study performed by Welton and co-workers studied the rate and selec-tivities of the Diels-Alder reaction between cyclopentadiene and methyl acrylate in a number of neutral ionic liquids [44]. It was found that endo. exo ratios decreased slightly as the reaction proceeded, and were dependent on reagent concentration and ionic liquid type. Subsequently, they went on to demonstrate that the ionic liquids controlled the endo. exo ratios through a hydrogen bond (Lewis acid) interaction with the electron-withdrawing group of the dienophile. 

Deng and Peng have found that certain ionic liquids catalyze the Biginelli reaction [62]. Usually, this reaction is catalyzed by Lewis acids such as InCl3, [Fe(H20)6]Cl3, or BF3.0(C2H5)2, or by acid catalysts such as Nafion-H. The reaction was found to give yields in the 77-99 % range in the ionic liquids [BMIM][PF6] or [BMIM][BF4] for the examples in Scheme 5.1-34. The reaction fails if there is no ionic liquid present or in the presence of tetrabutylammonium chloride. 

Other methods of nitration that Laali investigated were with isoamyl nitrate in combination with a Bronsted or Lewis acid in several ionic liquids, with [EMIM][OTf] giving the best yields (69 %, 1.0 1.0 o p ratio). In the ionic liquid [HNEt( Pr)2] [CE3CO2] (m.p. = 92-93 °C), toluene was nitrated with a mixture of [NH4][N03] and trifluoroacetic acid (TEAH) (Scheme 5.1-37). This gave ammonium trifluoroacetate [NH4][TEA] as a by-product, which could be removed from the reaction vessel by distillation (sublimation). 

Lee found that the reaction gave good yields (70-99 %) in the ionic liquids [BMIM][PF6], [BMIM][OTf, and [BMIM][SbF6] with Lewis acids such as Yb(OTf)3, Sc(OTf)3, Dy(OTf)3, Sm(OTf)3, and InCl3. The reaction was also performed in [BMIM][PF(3] or dichloromethane with Sm(OTf)3 as the catalyst. The ionic liquid reaction gave a yield of 99 %, compared with 70 % for the reaction in dichloromethane [73]. 

The chemical behavior of Franklin acidic chloroaluminate(III) ionic liquids (where X(A1C13) > 0.50) [6] is that of a powerful Lewis acid. As might be expected, it catalyzes reactions that are conventionally catalyzed by aluminium(III) chloride, without suffering the disadvantage of the low solubility of aluminium(III) chloride in many solvents. 

Keim and co-workers have carried out various alkylation reactions of aromatic compounds in ionic liquids substantially free of Lewis acidity [84]. An example is the reaction between benzene and decene in [BMIM][HS04], which was used together with sulfuric acid as the catalyst (Scheme 5.1-54). These authors have also claimed that these acid-ionic liquids systems can be used for esterification reactions. 

The methodology of a Lewis acid dissolved in an ionic liquid has been used for Friedel-Crafts alkylation reactions. Song [85] has reported that scandium(III) tri-flate in [BMIM][PFg] acts as an alkylation catalyst in the reaction between benzene and hex-l-ene (Scheme 5.1-55). 

Ionic liquids formed by treatment of a halide salt with a Lewis acid (such as chloro-aluminate or chlorostannate melts) generally act both as solvent and as co-catalyst in transition metal catalysis. The reason for this is that the Lewis acidity or basicity, which is always present (at least latently), results in strong interactions with the catalyst complex. In many cases, the Lewis acidity of an ionic liquid is used to convert the neutral catalyst precursor into the corresponding cationic active form. The activation of Cp2TiCl2 [26] and (ligand)2NiCl2 [27] in acidic chloroaluminate melts and the activation of (PR3)2PtCl2 in chlorostannate melts [28] are examples of this land of activation (Eqs. 5.2-1, 5.2-2, and 5.2-3). 

As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transition metal complex. The opportunity to optimize the ionic medium used, by variation of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for optimization. However, the choice of these parameters may be restricted by some possible incompatibilities with the feedstock used. Undesired side reactions caused by the Lewis acidity of the ionic liquid or by strong interaction between the Lewis acidic ionic liquid and, for example, some oxygen functionalities in the substrate have to be considered. 

Acidic chloroaluminate ionic liquids have already been described as both solvents and catalysts for reactions conventionally catalyzed by AICI3, such as catalytic Friedel-Crafts alkylation [35] or stoichiometric Friedel-Crafts acylation [36], in Section 5.1. In a very similar manner, Lewis-acidic transition metal complexes can form complex anions by reaction with organic halide salts. Seddon and co-workers, for example, patented a Friedel-Crafts acylation process based on an acidic chloro-ferrate ionic liquid catalyst [37]. 

Flowever, ionic liquids acting as transition metal catalysts are not necessarily based on classical Lewis acids. Dyson et al. recently reported the ionic liquid [BMIM][Co(CO)4] [38]. The system was obtained as an intense blue-green colored liquid by metathesis between [BMIM]C1 and Na[Co(CO)4]. The liquid was used as a catalyst in the debromination of 2-bromoketones to their corresponding ketones. 

Moreover, these experiments reveal some unique properties of the chlorostan-nate ionic liquids. In contrast to other known ionic liquids, the chlorostannate system combine a certain Lewis acidity with high compatibility to functional groups. The first resulted, in the hydroformylation of 1-octene, in the activation of (PPli3)2PtCl2 by a Lewis acid-base reaction with the acidic ionic liquid medium. The high compatibility to functional groups was demonstrated by the catalytic reaction in the presence of CO and hydroformylation products. 

Despite all the advantages of this process, one main limitation is the continuous catalyst carry-over by the products, with the need to deactivate it and to dispose of wastes. One way to optimize catalyst consumption and waste disposal was to operate the reaction in a biphasic system. The first difficulty was to choose a good solvent. N,N -Dialkylimidazolium chloroaluminate ionic liquids proved to be the best candidates. These can easily be prepared on an industrial scale, are liquid at the reaction temperature, and are very poorly miscible with the products. They play the roles both of the catalyst solvent and of the co-catalyst, and their Lewis acidities can be adjusted to obtain the best performances. The solubility of butene in these solvents is high enough to stabilize the active nickel species (Table 5.3-3), the nickel... 

A similar catalytic dimerization system has been investigated [40] in a continuous flow loop reactor in order to study the stability of the ionic liquid solution. The catalyst used is the organometallic nickel(II) complex (Hcod)Ni(hfacac) (Hcod = cyclooct-4-ene-l-yl and hfacac = l,l,l,5,5,5-hexafluoro-2,4-pentanedionato-0,0 ), and the ionic liquid is an acidic chloroaluminate based on the acidic mixture of 1-butyl-4-methylpyridinium chloride and aluminium chloride. No alkylaluminium is added, but an organic Lewis base is added to buffer the acidity of the medium. The ionic catalyst solution is introduced into the reactor loop at the beginning of the reaction and the loop is filled with the reactants (total volume 160 mL). The feed enters continuously into the loop and the products are continuously separated in a settler. The overall activity is 18,000 (TON). The selectivity to dimers is in the 98 % range and the selectivity to linear octenes is 52 %. 

The purity of ionic liquids is a key parameter, especially when they are used as solvents for transition metal complexes (see Section 5.2). The presence of impurities arising from their mode of preparation can change their physical and chemical properties. Even trace amounts of impurities (e.g., Lewis bases, water, chloride anion) can poison the active catalyst, due to its generally low concentration in the solvent. The control of ionic liquid quality is thus of utmost importance. 

Jacobsen subsequently reported a practical and efficient method for promoting the highly enantioselective addition of TMSN3 to meso-epoxides (Scheme 7.3) [4]. The chiral (salen)Cl-Cl catalyst 2 is available commercially and is bench-stable. Other practical advantages of the system include the mild reaction conditions, tolerance of some Lewis basic functional groups, catalyst recyclability (up to 10 times at 1 mol% with no loss in activity or enantioselectivity), and amenability to use under solvent-free conditions. Song later demonstrated that the reaction could be performed in room temperature ionic liquids, such as l-butyl-3-methylimidazo-lium salts. Extraction of the product mixture with hexane allowed catalyst recycling and product isolation without recourse to distillation (Scheme 7.4) [5]. 

The use of Lewis acids (ZnU, BF3 Et20) in ionic liquids, tested in the cycloaddition of but-3-en-2-one with isoprene, increases both the rate and selectivity of the reaction. The ionic liquid remains catalytically active after the work-up and can be reused. 

Chloroaluminate ionic liquids (typically a mixture of a quaternary ammonium salt with aluminum chloride see Table 6.9) exhibit at room temperature variable Lewis acidity and have been successfully used as solvent/catalyst for Diels-Alder reactions [57]. The composition of chloroaluminate ionic liquids can vary from basic ([FMIM]C1 or [BP]C1 in excess) to acidic (AICI3 in excess) and this fact can be used to affect the reactivity and selectivity of the reaction. The reaction of cyclopentadiene with methyl acrylate is an example (Scheme 6.31). 

The modest endo/exo ratio observed when the reaction was carried out in basic chloroaluminate ionic liquids is ascribable to the polarity of the medium, while the high diastereoselectivity found in the acidic mixture is due to the increase of Lewis/Bronsted acidity of the medium. The rates of the reactions performed in basic and acidic chloroaluminates ([EMIMJCl AlCh, [BPJCl AlCh) are seven times slower and ten times faster, respectively, than those observed when the reactions were carried out in water [57]. 

The types of ionic liquids shown in Figure 5.4 have been most extensively studied, especially ones based on chloroaluminate. Whilst these chloroaluminate materials also display useful Lewis acid properties they are highly air and moisture sensitive, which renders them relatively commercially unattractive. Newer ionic liquids containing C104 and NOa anions, for example, which are less air and moisture sensitive, are now being more widely studied, but these are less catalytically active. Other than lack of vapour pressure and catalytic properties there are several other features common to most ionic liquids that make them attractive reaction solvents. These include ... 

Scheme 5.16. In some instances, e.g. the aza-Diels-Alder reaction illustrated, Lewis acid catalysts are additionally required but use of ionic liquids greatly enhanees their ease of recovery and recycle.

Some 2,3-diazabicyclo[2.2.1]heptene derivatives, for example, 175, with an aryl iodide and allyltributyltin in the presence of [Pd(allyl)Cl]2 in toluene provide unexpected products 176. It is interesting to note that aryl iodide is recovered almost completely but no reaction can be observed in its absence. When the aryl iodide is replaced by a Lewis acid, good yields of 176 are obtained. The reaction is very slow in toluene, but in ionic liquid [bmim]PF6 the reaction rate is significantly enhanced (Equation 21) <2005SL2273>. 

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