Catalysts ionic liquids

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. 

A related study used the air- and moisture-stable ionic liquids [RMIM][PFg] (R = butyl-decyl) as solvents for the oligomerization of ethylene to higher a-olefins [49]. The reaction used the cationic nickel complex 2 (Figure 7.4-1) under biphasic conditions to give oligomers of up to nine repeat units, with better selectivity and reactivity than obtained in conventional solvents. Recycling of the catalyst/ionic liquid solution was possible with little change in selectivity, and only a small drop in activity was observed. 

A butoxylcarbonylation reaction was conducted in a liquid-liquid biphasic system under process conditions, but the removal of the product was conducted in a liquid-solid biphasic system at a lower temperature (84). lodobenzene or 4-bromoacetophenone reacted with CO at a pressure of 1-8 atm in the presence of a palladium-benzothiazole complex catalyst in the ionic liquid [TBA]Br (m.p. = 110°C) in the presence of Et3N base. The catalyst/ionic liquid system was recycled by extractive removal of the butyl ester product with diethyl ether. The solid residue, containing the catalyst, [TBA]Br, and Et3N.HBr, remained effective in subsequent carbonylation tests. After each cycle, the yields were still close to the initial value. A slight decrease in yield was attributed to a loss of catalyst during handling. 

Two-phase systems in which an insoluble organic substrate is reacted with a catalyst dissolved in an ionic liquid show promise for commercial use (Chauvin and Helene, 1995 Freemantle, 1998). One such system is a nickel-catalyzed olefin dimerization. The dimers are produced selectively and decanted from the ionic liquid. The catalyst/ ionic liquid phase is recycled without loss of activity. Other reactions investigated include ... 

R1 = aryl, heteroaryl R2 = aryl allyltributyltin no catalyst ionic liquid 80-93 133... 

A chiral pyridine-bisoxazoline ( PYBOX ) ligand has been combined with indium (III) triflate to produce an enantioselective catalyst for allylation of a wide variety of aldehydes in ionic liquids 183 ees of >90% were obtained, and extraction and reuse of the catalyst-ionic liquid combination saw this figure hold up to >80% on the fourth recycle. 

It is probably in asymmetric dihydroxylation, where the use of ionic liquids appears to be most promising. The decreased acute toxicity of osmium tetroxide due to its suppressed volatility certainly represents a great benefit for those who work with this reagent and its derivatives. Furthermore, high cost of both the osmium catalyst, as well as the chiral ligand, make recycling of the catalyst-ionic liquid particularly attractive. On the other hand, disposal of osmium-contaminated ionic liquids in an environmentally benign manner has yet to be addressed. 

Summary The use of an ionic liquid in hydrosilylation reactions enables standard hydrosilylation catalysts to be easily recovered and subsequently reused after separation from the product at the end of the reaction. Remarkably, the recovered catalyst/ionic liquid solution does not need to be purified or treated before its reuse. Employing this method, a variety of organomodified polydiraethylsiloxanes were synthesized. 

Typically, the reaction is performed in a liquid-liquid biphasic system where the substrates and products (upper phase) are not miscible with the catalyst/ionic liquid solution (lower phase). The SiH-functional polydimethylsiloxane and the olefin are placed in the reaction vessel and heated up to 90 °C. Then the precious metal catalyst (20 ppm) and the ionic liquid (1 %) are added. After complete SiH conversion, the reaction mixture is cooled to room temperature and the products are removed from the reaction mixture by either simple decantation or filtration (in case of non-room-temperature ionic liquids). The recovered catalyst/ionic liquid solution can be reused several times without any significant change in catalytic activity. A treatment or workup of the ionic liquid-catalyst solution after each reaction cycle is not necessary. The metal content of the products was analyzed by ICP-OES (Inductively coupled plasma optical emission spectroscopy) and the chemical identity of the organomodified polydimethylsiloxane was verified by NMR spectroscopy. 

It turned out that for successful recovery of the catalyst and its reusability it is crucial to find an appropriate combination of a catalyst and an ionic liquid, which has to be harmonized with the hydrophilicity/hydrophobicity of the product. First of all, not every catalyst is soluble in every ionic liquid, and secondly, not every ionic liquid separates as readily as desired from the product phase. However, we were able to identify at least one suitable catalyst/ionic liquid combination for the synthesis of each polyethersiloxane (Table 1). 

In some cases, more than one catalyst/ionic liquid combination gave good results (see entries Nos. 3 and 4, Table 1). It is noteworthy that all of the polyethersiloxanes synthesized in this way exhibit very different polarities. 

A novel transition metal-catalyzed hydrosilylation process is described. The use of an ionic liquid in this process allows for the immobilization, heterogenization, and recovery of the expensive precious metal catalyst as well as its direct reuse in a subsequent hydrosilylation reaction. From an economic and ecological point of view, this process perfectly fits in the concept of "Sustainable Chemistry". Future research activities will aim at the prolongation of the catalyst life-time. For this, it is necessary to gain a deeper understanding of the catalytically active species in the catalyst/ionic liquid solution. 

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],... 

Entry Catalyst Ionic liquid Run Proton source Conv. (%) ee (%)... 

From above, it is evident that ionic liquids used as unconventional green solvents may promote alkylation reactions and allow the recycling of homogeneous catalysts. Ionic liquids have exhibited wide prospects on their applications in alkylation. 

A green catalyst, ionic liquid [EtPy][CF3COO], was first time used for the synthesis of amino acid esters including unnatural amino acid esters. The results show that under mild reaction conditions, satisfactory conversion can be achieved for the formation of amino acid esters. 

A very recent addition to the already powerful range of microwave cycloaddition chemistry is the development of a general procedure applying a catalyst/ionic liquid system [19]. Several studies in this area have used ionic liquids, or mixtures of ionic liquids and other solvents, as reaction media in several important microwave-heated organic syntheses [20], including Diels-Alder reactions [21, 22] and 1,3-dipolar cycloaddition reactions [23]. 

A key feature of this catalyst/ionic liquid system is its recyclability [21c, 25]. Because ionic liquids can be very costly to use as solvents, several research groups use them instead as doping agents for microwave heating of otherwise nonpolar solvents, for example hexane, toluene, THF, or dioxane. This technique, first introduced by Ley et al. in 2001 [26] is becoming increasingly popular, as demonstrated by many recently published examples [21b, 24, 27]. 

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. 

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