Reactions in Acidic Ionic Liquids

This was confirmed by taking a sample of 9-acetylanthracene and allowing it to isomerize in the ionic liquid. This gave a mixture of anthracene, 1,5-diacetylan-thracene and 1,8-diacetylanthracene. It should be noted that a proton source was needed for this reaction to occur, implying an acid-catalyzed mechanism (Scheme 5.1-65) [95]. 

The sulfonation of aromatic compounds occurs readily in ionic liquids, with the simplest case being the direct sulfonation of aromatic compounds with sulfur trioxide to give the aryl sulfonic acid [113]. Ionic liquids such as triflate or triflimide ionic liquids were found to enhance the reaction rate. In the reaction of chloro-sulfuric acid with aromatic compounds, the reaction in the ionic liquid gave a 

Scheme 5.2-114 The Mannich reaction in acidic ionic liquids [259].

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 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). 

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). 

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. 

Wilkes and co-workers have investigated the chlorination of benzene in both acidic and basic chloroaluminate(III) ionic liquids [66]. In the acidic ionic liquid [EMIM]C1/A1C13 (X(A1C13) > 0.5), the chlorination reaction initially gave chlorobenzene, which in turn reacted with a second molecule of chlorine to give dichlorobenzenes. In the basic ionic liquid, the reaction was more complex. In addition to the 

BP patented the synthesis of allq lated aromatics by the reaction of an olefin with benzene in acidic ionic liquids [19]. Advantages of this process conpared to the reaction with aluminium(in) chloride in organic solvents are the sinple product separation, catalyst recycling and higher selectivities to alleviation products. The alkylation of benzene with linear olefins (Ciq—is largely used industrially to produce alkylbenzenes. They serve as 

The oxidative coupling of acetone with monosubstituted benzenes, promoted by manganese(m) acetate in three ionic liquids proceeded using ultrasound irradiation [278]. The reactions in the ionic liquids gave far better yields and improved selectivity for the pora-isomer than similar reactions in acetic acid. This is shown in Scheme 5.2-128. 

Diketones condense to furans or pyrroles in the presence of a primary amine and an acidic catalyst. Yadav has carried out numerous such reactions in the ionic liquid [BM1M][BF4], using bismuth triflate as a catalyst [256]. The reaction typically gave 80-90% yield and the ionic hquid/catalyst was easily recycled (Scheme 5.2-112). With the addition of a primary amine to a 1,4-diketone, pyrroles can be formed. 

This type of co-catalytic influence is well loiown in heterogeneous catalysis, in which for some reactions an acidic support will activate a metal catalyst more efficiently than a neutral support. In this respect, the acidic ionic liquid can be considered as a liquid acidic support for the transition metal catalysts dissolved in it. 

Electrophilic substitution " and other reactions of naphthalenes (alkylation, acylation, condensation and migration in acidic ionic liquids have been reported. Anthra- 

Boon et al. have reported the alkylation of benzene with a wide number of alkyl halides in acidic chloroaluminate(lll) ionic liquids and general organic reactions in low melting chloroaluminate ionic liquids have also been described, which include chlorinations and nitrations in acidic ionic liquids.  

The synthesis of tctrahydropyranols from the reaction between an aldehyde and a homoallylic alcohol with catalytic amounts of Ce(OTf)3 immobilised in [C4Ciim][PF6] affords the desired product in moderate yield after 12 hours at room temperature, see Scheme 9.18.[64] Addition of one equivalent of benzoic acid as co-catalyst was found to increase the isolated yield, but its role is not yet clear. A beneficial effect of performing the reaction in the ionic liquid is that, contrary to the reaction in chloroform, no ether derivatives were observed. 

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). 

An example of attaching a carbon nucleophile to an o, j8-unsaturated ketone is the Sakurai reaction. This involves the reaction of allyltrimethylsilane with an a,fi-unsaturated ketone to form a -unsaturated ketone in the presence of a Lewis acid. Howarth used indium(iii) chloride to catalyze this reaction in the ionic liquids [BMIM][BF4] and [BMIM][PF6] [207]. An example ofthis is shovm in Scheme 5.2-83. 

A Mannich type reaction was used in the three-component synthesis of isoquino-lic acids in [BMIM][BF4] or [BMIM][PF6] [260] (Scheme 5.2-115). These reactions are normally catalyzed by Lewis acids such as BF3-OEt2, TiQ4 or SnCU. The reaction in the ionic liquids does not require a catalyst to be added, although addition of 5 mol% of indium(iii) chloride does marginally improve the yield (87-95%). 

Indoles can be 3-alkylated by allyl alcohols in the presence of lithium perchlorate and acetic acid 101 is an example (Scheme 42). Pyrrole -alkylation can be achieved with simple alkyl halides [1-bromopentadecane, l-(bromomethyl)-, l-(3-chloropropyl)- and l-(3-iodopropyl)benzenes, 2-(2-bromoethyl)- and 2-(3-bromopropyl)naphthalenes] and mesylates [3-phenylpropyl-, l-methyl-3-phenylpropyl-, 2-(2-naphthyl)ethyl- and 3-(2-naphthyl)propyl methanesulfonates] selectively at C(2) and C(5) positions via reaction in various ionic liquids (e.g., Scheme 43) <20050L1231>. 

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. 

---