Catalysts ionic liquid-water

Screening of the reaction media showed that, with the use of an ionic liquid/ water mixture (a so-called wet ionic liquid ), recycling of the catalyst could be improved compared to the reaction in ionic liquid without co-solvent. The com-... 

Triphasic systems comprising, for example, an ionic liquid, water and an organic phase in which the catalyst resides in the ionic liquid, the substrate... 

Abbott, A.P., G. Capper, D.L. Davies, R.K. Rasheed and V. Tambyrajah, Quaternary Ammonium Zinc- or Tin-Containing Ionic Liquids Water Insensitive, Recyclable Catalysts for Diels-Alder Reactions, Green Chemistry, 4,24—26 (2002). 

The efficient solvation of water [96,110,112, 113] outlined above can be exploited in condensation reactions, in which water formed during the course of the reaction is deactivated by absorption , which can improve conversion and/or selectivity. For example, in the catalytic oxidation of an alcohol to the aldehyde [140, 141], water is formed as by-product which can undergo further reaction with the aldehyde to yield unwanted carboxylic acid. In ionic liquids, neither a reduced rate of reaction nor the formation of carboxylic acid is observed, even in the presence of up to 1 equiv. of water. However, at higher concentrations of water (absorption from air or accumulated by-product) the selectivity of aldehyde decreases, and the carboxylic acid is formed instead. Upon vigorous drying of the catalyst-ionic liquid mixture, the selectivity of the system is restored [141],... 

The catalyst system can be reused if an ionic liquid-water or ionic liquid-water-tert-butanol solvent system is used [155-157]. Some care has to be taken with the choice of ligand under these conditions, as the presence of an alkene within the ligand itself results in dihydroxylation during the reaction and, as a result, the modified ligand is not extracted from the ionic liquid into the organic layer [158]. Supercritical C02 can be used in place of the organic solvent to extract the product [159, 160]. 

Water and the ionic liquid bmimPF act as powerful reaction media not only for rate acceleration (for adduct 80, in water, conversion = 92-99%, yield = 83-97%, and in bmimPF, conversion = 81-99%, yield = 71-96%) and chemoelectivity enhancement but also for facilitating catalyst recycling in the [0=P(2-py)3W(CO)(NO)2](BF4)2-catalyzed Diels-Alder reaction systems. A key feature of this catalyst-water or catalyst-ionic liquid system is that the catalyst was recycled many times. In addition, the authors illustrated the development of the catalyst by conventional heating or under the action of microwave irradiation, the results of which are summarized in Scheme 11.21. 

The extent of mixing and the distribution of solutes in ionic liquids depend, therefore, on the relative solute-solute and solute-solvent interactions, which can have significant consequences on chemical reactivity and stabihty. In many ionic liquids, water-sensitive catalysts and chemical reactions are less sensitive to water compared with the situation in organic solvents because water dispersed throughout the ionic liquid cannot act like bulk water. 

Abbott AP, Capper G, Davies DL, Rasheed RK, Tambyrajah V (2002) Quaternary ammonium zinc-or tin-containing ionic liquids water insensitive, recyclable catalysts for Diels-Alder reactions. Green Chem 4 24-26... 

Haumann, M., Schonweiz, A., Breitzke, H., Buntkowsky, G., Werner, S. and Szesni, N., Solid-state NMR investigations of supported ionic liquid phase water-gas shift catalysts Ionic liquid film distribution vs. catalyst performance, Chem. Eng. Technol. 35,1421-1426 (2012). 

Palladium-catalyzed carbonylation of aryl halides with nucleophiles such as alcohols, amines, and water can be performed in ionic liquid media. Several systems have been designed so that the ionic phase can be isolated and recycled. Once carbonylation substrates/products form homogeneous mixtures with ionic liquids, the experimental protocols for catalyst/ionic liquid mixture recycling involve separation of the product by either distillation or extraction procedures using organic solvents or supercritical CO2. 

Liao et al. (2005) carried out microwave-assisted allylic substitution with various carbon and heteronucleophiles in an 1-ethyl-3-methylimidazolium tetraborate ionic liquid/water ([emim] BF /H O) containing catalyst system of Pd(OAc)2/TPPTS (3,3 ,3 -phosphanetriyltris(benzene sulfonic acid trisodium salt) in 1 4 ratio. The ionic liquid/water containing catalyst system can be recycled 8 times. 

Water in an ionic liquid may be a problem for some applications, but not for others. However, one should in all cases know the approximate amount of water present in the ionic liquid used. Moreover, one should be aware of the fact that water in the ionic liquid may not be inert and, furthermore, that the presence of water can have significant influence on the physicochemical properties of the ionic liquid, on its stability (some wet ionic liquids may undergo hydrolysis with formation of protic impurities), and on the reactivity of catalysts dissolved in the ionic liquid. 

Despite the utility of chloroaluminate systems as combinations of solvent and catalysts in electrophilic reactions, subsequent research on the development of newer ionic liquid compositions focused largely on the creation of liquid salts that were water-stable [4], To this end, new ionic liquids that incorporated tetrafiuoroborate, hexafiuorophosphate, and bis (trifiuoromethyl) sulfonamide anions were introduced. While these new anions generally imparted a high degree of water-stability to the ionic liquid, the functional capacity inherent in the IL due to the chloroaluminate anion was lost. Nevertheless, it is these water-stable ionic liquids that have become the de rigueur choices as solvents for contemporary studies of reactions and processes in these media [5],... 

Friedel-Crafts acylation reactions usually involve the interaction of an aromatic compound with an acyl halide or anhydride in the presence of a catalyst, to form a carbon-carbon bond [74, 75]. As the product of an acylation reaction is less reactive than its starting material, monoacylation usually occurs. The catalyst in the reaction is not a true catalyst, as it is often (but not always) required in stoichiometric quantities. For Friedel-Crafts acylation reactions in chloroaluminate(III) ionic liquids or molten salts, the ketone product of an acylation reaction forms a strong complex with the ionic liquid, and separation of the product from the ionic liquid can be extremely difficult. The products are usually isolated by quenching the ionic liquid in water. Current research is moving towards finding genuine catalysts for this reaction, some of which are described in this section. 

Temperature-dependent phase behavior was first applied to separate products from an ionic liquid/catalyst solution by de Souza and Dupont in the telomerization of butadiene and water [34]. This concept is especially attractive if one of the substrates shows limited solubility in the ionic liquid solvent. 

Seddon s group described the option of carrying out Heck reactions in ionic liquids that do not completely mix with water. These authors studied different Heck reactions in the triphasic [BMIM][PFg]/water/hexane system [91]. While the [BMIM]2[PdCl4] catalyst used remains in the ionic liquid, the products dissolve in the organic layer, with the salt formed as a by-product of the reaction ([H-base]X) being extracted into the aqueous phase. 

It is noteworthy that the best results could be obtained only with very pure ionic liquids and by use of an optimized reactor set-up. The contents of halide ions and water in the ionic liquid were found to be crucial parameters, since both impurities poisoned the cationic catalyst. Furthermore, the catalytic results proved to be highly dependent on all modifications influencing mass transfer of ethylene into the ionic catalyst layer. A 150 ml autoclave stirred from the top with a special stirrer... 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

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 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. 

In comparison with classical processes involving thermal separation, biphasic techniques offer simplified process schemes and no thermal stress for the organometal-lic catalyst. The concept requires that the catalyst and the product phases separate rapidly, to achieve a practical approach to the recovery and recycling of the catalyst. Thanks to their tunable solubility characteristics, ionic liquids have proven to be good candidates for multiphasic techniques. They extend the applications of aqueous biphasic systems to a broader range of organic hydrophobic substrates and water-sensitive catalysts [48-50]. 

The first application involving a catalytic reaction in an ionic liquid and a subsequent extraction step with SCCO2 was reported by Jessop et al. in 2001 [9]. These authors described two different asymmetric hydrogenation reactions using [Ru(OAc)2(tolBINAP)] as catalyst dissolved in the ionic liquid [BMIM][PFg]. In the asymmetric hydrogenation of tiglic acid (Scheme 5.4-1), the reaction was carried out in a [BMIM][PF6]/water biphasic mixture with excellent yield and selectivity. When the reaction was complete, the product was isolated by SCCO2 extraction without contamination either by catalyst or by ionic liquid. 

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