Lewis acids chloroaluminate ionic liquids

Following up this initial work a large number of reactions have been published by academic and industrial groups wherein a Lewis-acidic chloroaluminate ionic liquid is used as the acidic catalyst. A comprehensive overview on these research activities can be found in several reviews on this topic and the literature cited therein [18, 22, 37]. 

Highly Lewis-acidic chloroaluminate ionic liquids (ILs) are well known to be both versatile solvents and effective catalysts for Friedel-Crafts reactions [1,2]. Tailoring the physical and chemical properties of the ILs to the needs of a specific reaction allows for a high diversity of applications [3,4]. We could show that immobilising these ILs on inorganic supports yields very active catalysts for alkylation reactions. The immobilisation of ionic liquids leads to novel Lewis-acidic catalysts (NLACs). The methods presented include the method of incipient wetness (method 1, further on called NLAC I), which has been introduced in detail by Hoelderich et al. f5], but focus of this presentation lies on the methods 2 (NLAC II) and 3 (NLAC III). 

Potdar, M. K., Mobile, S. S., Salunkhe, M. M. Coumarin syntheses via Pechmann condensation in Lewis acidic chloroaluminate ionic liquid. Tetrahedron Lett. 2001, 42, 9285-9287. 

Hard Lewis acid chloroaluminate ionic liquids show intense catalytic activity in the Friedel-Crafts acylation reaction however, they suffer from the same issues as anhydrous aluminum chloride. i Of particular interest to these reactions, aluminum chloride may be replaced by indium trichloride to form chloroindate(III) ionic liquids. The advantage of using indium trichloride compared with aluminum chloride is represented by its hydrolytic stability and reduced oxophilicity. Chloroindate(III) ionic liquids are synthesized by mixing l-butyl-3-methylimidazolium chloride [C4mim]Cl with anhydrous indium trichloride at 80°C. In the benzoyla-tion of anisole with benzoic anhydride (BAN) at 80°C, the best yield of... 

Scheme 5.6-1 Preparation of supported Lewis acidic chloroaluminate ionic liquids catalysts by impregnation with [BMIM][(AICl3)jfCl] (top), by grafting of l-(triethoxysilylpropyl)-3-methylimidazolium chloride followed by AICI3 addition (middle), and by sol-gel formation followed byAICis addition (bottom) [73].

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]. 

Because of the extensive amount of waste generated in traditional Friedel-Crafts reactions, it is not surprising that this reaction has been studied in RTIL. Early examples included the use of catalytic chloroaluminate ionic liquids. However, the moisture sensitivity of such systems was a drawback. Therefore, water-stable rare-earth Lewis acids, such as Sc(CF3S03)3, have come to be used for these reactions.The same Lewis acid has also been used to catalyse Diels-Alder reactions in RTILs.Interestingly, in this example, the RTIL not only provided a means for recycling the catalyst but also accelerated the rate and improved selectivity. It has also been demonstrated that a moisture stable, Lewis acidic, catalytic ionic liquid could be prepared from choline chloride and zinc dichloride, and that this was an excellent medium for the Diels-Alder reaction. Yields of 90% or more were achieved in reaction times of between 8 min and 5h for a range of dienes and dienophiles. 

The cationic nickel complex [ /3-allylNi(PR3)]+, already described by Wilke etal. [21], as an efficient catalyst precursor for alkene dimerization when dissolved in chlorinated organic solvents. It proved to be very active in acidic chloroaluminate ionic liquids. In spite of the strong potential Lewis acidity of the medium, a similar phosphine effect is observed. Biphasic regioselective dimerization of propylene into 2,3-dimethylbutenes can then be achieved in chloroaluminates. However, there is a competition for the phosphine between the soft nickel complex and the hard aluminum chloride coming from the dissociation of polynuclear chloroaluminate anions. Aromatic hydrocarbons, when added to the system, can act as competitive bases thus preventing the de-coordination of phosphine ligand from the nickel complex [22 b]. Performed in a continuous way, in IFP pilot plant facilities, dimerization of propene and/or butenes with this biphasic system (Difasol process) compares... 

Lewis acid-containing ionic liquids such as 1-butyl-3-methylimida-zolium (bmim) chloroaluminate, butylpyridinium chloro-aluminate, and FeCls-doped l-butyl-3-methylimidazolium triflimide have likewise proven effective promoters for the Pechmann condensation. [Bmim]Cl 2AlCl3 has proven to be one of the most reactive ionic liquid promoters, effecting the Pechmann condensation of ethyl acetoacetate and a range of electron-rich phenols in 10—45 min at 30 °C. ... 

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... 

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 presence of Lewis acidic species in chloroaluminate ionic liquids has also been used to bring about various acid catalysed transformations that do not require additional catalysts. For example, acidic ionic liquids are ideally suited to Friedel-Crafts acylation reactions. In a traditional Friedel-Crafts acylation an acylium ion is generated by reaction between acyl chloride and A1C13 or FeCL as shown in Scheme 10.7. 

The anions, on the other hand, determine to a large extent the chemical properties of the system. For example, the main anions present in chloroaluminate ionic liquid systems such as (emim)Cl-AlCl3 are CT, which is a Lewis base (AICI4) , which is neither acidic nor basic and the Lewis acid (AbCb). The concentration of each anion, and therefore the Lewis acidity of the system, varies depending on the relative amounts of AICI3 and (emim)Cl added to the system. 

Tab. 2. Examples for non-chloroaluminate ionic liquids formed by the reaction of a halide salt with a Lewis acid.

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