Catalysts, also ionic liquids

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]. 

In general, most of the methods used to analyze the chemical nature of the ionic liquid itself, as described in Chapter 4, should also be applicable, in some more sophisticated form, to study the nature of a catalyst dissolved in the ionic liquid. For attempts to apply spectroscopic methods to the analysis of active catalysts in ionic liquids, however, it is important to consider three aspects a) as with catalysis in conventional media, the lifetime of the catalytically active species will be very short, making it difficult to observe, b) in a realistic catalytic scenario the concentration of the catalyst in the ionic liquid will be very low, and c) the presence and concentration of the substrate will influence the catalyst/ionic liquid interaction. These three concerns alone clearly show that an ionic liquid/substrate/catalyst system is quite complex and may be not easy to study by spectroscopic methods. 

Many organometallic catalysts are soluble in ionic liquids, especially including ionic compounds. Neutral species, such as Wilkinson s catalyst, are also soluble to some extent in ionic liquids (169). There are numerous examples illustrating the dispersion and isolation of organometallic catalysts in ionic liquids a list of examples is given in a recent review (/). 

MTO has also been successfully applied as an alkene epoxidation catalyst in ionic liquids [43, 44]. The values of the rate constants for the formation of the peroxo complexes of MTO have been found to be highly dependent on the concentration of water in the solvent. Ionic liquids can behave like organic solvents and aqueous solutions of high salt concentrations. 

In the reaction using the Rh-sulfoxantphos catalyst without ionic liquid, the activity and selectivity decreased sharply after 5-10 h on stream, while the SILP system (a = 0.1) reached its maximum activity only after 30 h and maintained this level stable up to 60 h (i.e. TON > 2400) along with a high selectivity corresponding to a n/iso ratio of 21-23. It can also be deduced that the apparently negative ionic liquid effect on the catalyst activity measmed in the short time reactions can be explained by a delayed formation of the catalytically active species. 

MTO has also been successfully applied as an olefin epoxidation catalyst in ionic liquids ([44, 45] see also Chapter 5). 

Ionic liquids can efficiently immobilize transition metal complexes used as catalysts for epoxidation. This phenomenon is encountered not only when specially designed, polar ligands are used, but also for unmodified catalysts. For instance, several unsuccessful attempts have been made to immobilize Jacobsen s chiral Mn(iii)salen epoxidation catalyst 81. Ionic liquids are the first medium to immobilize this complex efficiently without requiring additional modification of the ligand. 

Silica gels have been used in recent years as supports for the immobihzation of ILs in order to improve their applicability and reusabiHty in industrially important catalytic processes. In particular, there have also been numerous studies in which silica gels are used as inorganic supports for the development of recyclable and efficient catalytic systems based on supported ILs, namely, supported ionic hquid phase (SILP) catalysts [57-59], supported ionic liquid catalysts (SILCs) [60-62], and solid catalysts with ionic liquid layers (SCILL) [63]. 

Later, Yamamoto reported that inexpensive and air-stable Cu(I) catalyst in DMF could also catalyze this transformation (Scheme 8.39) [156]. Recently, Liang discovered that a robust air-stable and recyclable Bu4N[AuCl4[-catalyst in ionic liquids [157] could be employed for analogous cyclization of alkynyl alkenones 93 into furans 94 in high yields [158]. 

Also ionic liquids have to be much superior to conventional organic solvents where activity, selectivity and stability of the catalyst are concerned besides being environmentally benign [Song et al., 2001]. However, we have to keep in mind that organic solvents can be recycled and purified by distillation, while it is difficult to recycle ILs because immiscible organic solvents are required to separate the products [Fioroni et al., 2003 Hassan et al., 2006]. 

Interestingly, we found that the CuBr-catalyzed aza-Heniy-type CDC reaction also proceeds well in ionic liquids under an ojygen atmosphere. The catalyst-containing ionic liquid can be re-used without loss of reactivity after extraction of the product with diethyl ether. Since ionic liquids are highly polar and excellent media for conducting electricity, we subsequently demonstrated that the CDC of Af-phenyltetrahydroisoquinoline with nitro-methane is also feasible under electrochemical conditions. The reaction... 

We had no good way to predict if they would be liquid, but we were lucky that many were. The class of cations that were the most attractive candidates was that of the dialkylimidazolium salts, and our particular favorite was l-ethyl-3-methylimid-azolium [EMIM]. [EMIMJCl mixed with AICI3 made ionic liquids with melting temperatures below room temperature over a wide range of compositions [8]. We determined chemical and physical properties once again, and demonstrated some new battery concepts based on this well behaved new electrolyte. We and others also tried some organic reactions, such as Eriedel-Crafts chemistry, and found the ionic liquids to be excellent both as solvents and as catalysts [9]. It appeared to act like acetonitrile, except that is was totally ionic and nonvolatile. 

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 Friedel-Crafts acylation reaction has also been performed in iron(III) chloride ionic liquids, by Seddon and co-workers [96]. An example is the acetylation of benzene (Scheme 5.1-66). Ionic liquids of the type [EMIM]Cl/FeCl3 (0.50 < X(FeCl3) < 0.62) are good acylation catalysts, with the added benefit that the ketone product of the reaction can be separated from the ionic liquid by solvent extraction, provided that X(FeCl3) is in the range 0.51-0.55. 

The ability of iron(III) chloride genuinely to catalyze Friedel-Crafts acylation reactions has also been recognized by Holderich and co-workers [97]. By immobilizing the ionic liquid [BMIM]Cl/FeCl3 on a solid support, Holderich was able to acetylate mesitylene, anisole, and m-xylene with acetyl chloride in excellent yield. The performance of the iron-based ionic liquid was then compared with that of the corresponding chlorostannate(II) and chloroaluminate(III) ionic liquids. The results are given in Scheme 5.1-67 and Table 5.1-5. As can be seen, the iron catalyst gave superior results to the aluminium- or tin-based catalysts. The reactions were also carried out in the gas phase at between 200 and 300 °C. The acetylation reac-... 

A number of enantioselective hydrogenation reactions in ionic liquids have also been described. In all cases reported so far, the role of the ionic liquid was mainly to open up a new, facile way to recycle the expensive chiral metal complex used as the hydrogenation catalyst. 

One of the key factors controlling the reaction rate in multiphasic processes (for reactions talcing place in the bulk catalyst phase) is the reactant solubility in the catalyst phase. Thanks to their tunable solubility characteristics, the use of ionic liquids as catalyst solvents can be a solution to the extension of aqueous two-phase catalysis to organic substrates presenting a lack of solubility in water, and also to moisture-sensitive reactants and catalysts. With the different examples presented below, we show how ionic liquids can have advantageous effects on reaction rate and on the selectivity of homogeneous catalyzed reactions. 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

The use of acidic chloroaluminates as alternative liquid acid catalysts for the allcy-lation of light olefins with isobutane, for the production of high octane number gasoline blending components, is also a challenge. This reaction has been performed in a continuous flow pilot plant operation at IFP [44] in a reactor vessel similar to that used for dimerization. The feed, a mixture of olefin and isobutane, is pumped continuously into the well stirred reactor containing the ionic liquid catalyst. In the case of ethene, which is less reactive than butene, [pyridinium]Cl/AlCl3 (1 2 molar ratio) ionic liquid proved to be the best candidate (Table 5.3-4). 

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