Catalysts ionic hquid-water

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 solubihty 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 Hquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibihty with the catalyst or problems with substrate solubility, for example). 

The first application involving a catalytic reaction in an ionic Hquid 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 Hquid [BMIM][PFg]. In the asymmetric hydrogenation of tiglic acid (Scheme 5.4-1), the reaction was carried out in a [BMIM][PF ]/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 Hquid. 

Dong F, Jim L, Zhou X L, et al. Mannich reaction in water using acidic ionic hquid as recoverable and reusable catalyst. Catal. Lett. 

From an economic and environmental perspective, catalytic aerobic alcohol oxidation represents a promising protocol. The use of molecular oxygen as the primary oxidant has several benefits, including low cost, improved safety, abundance, and water as the sole by-product. In this way, many catalytic systems have been used for the aerobic oxidations in ionic hquids as green solvents. Different types of catalysts or catalytic systems useful for the oxidation of alcohols with as terminal oxidant in ionic liquids as solvent will be discussed below. 

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 system [BMIM][PF6]/water/hexane [214]. While the [BMIM]2(PdCl4] catalyst remained in the ionic hquid, the products dissolved in... 

Water is likely to be present in aU practically relevant catalytic applications unless extreme precautions are taken or the system is self-drying (e.g., due to the fact that strong Lewis-acids or metal alkyls are used as co-catalysts). Water will influence the ionic liquid s thermal stability significantly if any part of the ionic liquid is prone to hydrolysis. Apart from the weU-known hydrolysis lability of tetrafluoroborates and hexafluorophosphates, water will thus also affect the stability of ester functionalities in the ionic liquid, e.g. the stability of alkyl sulfate anions. The presence of Bronsted acidity in the reaction system will further promote this kind of thermally induced hydrolysis reaction. Additionally, in strong Lewis-acidic ionic liquids care has to be taken to avoid incompatibilities between oxygen and nitrogen functionalities in the reactants or impurities and the ionic liquid s Lewis acidic group (usually a complex anion). It is for example obvious that the Pd-catalyzed dimerization of methylacrylate caimot be carried out in acidic chloroaluminate ionic hquids since the ionic liquid s anion would decompose in an irreversible reaction with the substrate methylacrylate. 

It is also important to note that in most cases the stationary phase cannot be regarded as an irmocent component of the reaction mixture. The interactions of substrates, catalyst, or CO2 with the stahonary phase can all influence the course of the reaction. In ophmal cases this can be a useful factor, for example in the hydrovinylation system discussed below, where the ionic hquid (I L) acts as a catalyst activator which must otherwise be added separately. In water—CO2 systems, however, the formation of carbonic acid when CO2 is dissolved in water means that the pH of the aqueous phase can be as low as 3. Thus all reagents and catalysts must be stable under acidic conditions when using this approach. 

As mentioned earlier, the most important issue in cellulose hydrolysis is sohd acid operation in water. Cellulose is insoluble in water, but soluble in some ionic Hquids such as l-butyl-3-methyHmidazolium chloride (BMIMCl). The use of ionic hquids thus affords a good accessibihty to dissolved cehulose for conversion at active sites of sohd add catalysts. Amberlyst resins in ionic hquids could effectively depolymerize cellulose (microcrystaUine cehulose and a-cehulose) into ceUo-ohgomers under mild reaction conditions (373 K, <5 h) [147]. 

Supercritical ethanol was used together with ionic hquid [HMim][HS04] catalyst, yielding 97.6% biofuel in only 45 min, and the catalyst was not affected by high pressures, temperatures, or the presence of water, which imphes a sustainable alternative for WFO valorization (Caldas et al., 2016). 

Acyloxyprolines can also be modified with ionic Hquid-tags. The use of the proper ionic liquid-tag enables the use of these catalysts in the presence of water and their recovery and reuse. Indeed, the solubility of ionic Hquid-anchored orga-nocatalysts can be tuned by an accurate choice of cations and anions groups present in their structure, and the solubility of ionic liquids can be readily tuned with different cations and anions. This approach allows phase separation to be realized from organic as well as aqueous media. Following the proper extraction procedure, it could be possible to separate the catalytic molecule from the product and to allow its reuse. 

Examples of type III bearing ionic liquid tags are catalysts 60 and 61 (Figure 24.19). These catalysts were employed in water in l-5mol% (60) or 10mol% (61) and the presence of the ionic hquid tag enabled their recovery [84]. 

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