Ionic liquids, as recyclable solvents

Gong and Zhao et al. have independently presented the application of pyridinium-based ionic liquid as recyclable solvent for the DABCO-catalyzed MBH reaction. Compared with the commonly used imidazolium-based ionic liquids [bmimlPFg] and [bdmim][PF6], which evidently react with aldehydes under basic conditions, the pyridinium-based ionic liquid [EPyJpFJ and [BuPy][N03] are inert and the MBH reactions of aldehydes with methyl acrylate, MVK or acrylonitrile proceed quickly in good yields. Zhao et al. also examined hexamethyl-enetetramine (HMTA)-catalyzed MBH reactions by using the ionic liquid [EPy][BF4] as a reaction media short reaction times and good to excellent yields was obtained (Scheme 1.120). ° ... 

Ionic Liquids as Recyclable Solvents for Asymmetric Organocatalytic Reactions... 

Among these in situ protocols are those using ionic liquids as the solvent, or as both the solvent and the ligand. It was shown that the use of PdCOAc) in imidazolium-based ionic liquids forms in situ NHC-Pd(II) species [42], The use of methylene-bridged bis-imidazolium salt ionic liquids to form chelated complexes has also been reported [43], although better results have been obtained when Bu NBr is used as the solvent [44] and imidazolium salts were added together with PdCl in catalytic amounts [45]. Other related catalytic species such as bis-NHC complexes of silica-hybrid materials have been tested as recyclable catalysts [46,47]. 

The proposed mechanisms, however, have not yet been clearly established and more detailed studies under catalytic conditions [46-47] would be welcome. These ruthenium clusters have also been shown to be active for arene hydrogenation in a biphasic medium using ionic liquids as the solvent, where the catalysts is retained, allowing its easy recovery and recycling [48]. 

Yadav JS, Reddy BVS, Basak AK, Narsaiah AV (2004) Recyclable 2nd generation ionic liquids as green solvents for the oxidation of alcohols with hypervalent iodine reagents. Tetrahedron 60 2131-2135... 

The palladium-catalyzed cyclocarbonylation reaction of o-iodoanilines with allenes and CO in l-butyl-3-methylimidazolium hexafluorophosphate [bmimJPFg afforded 3-methylene-2,3-dihydro-lH-quinolin-4-ones in yields of up to 90% under a low pressure (5 atm) of CO (Scheme 17.2). The ionic liquid, as the solvent and promoter, enhanced the efficiency of the cyclocarbonylation reaction. In this work. Ye and group demonstrated the recyclability of the system of ionic liquids and their use as catalysts [44]. 

In the first publication describing the preparative use of an enzymatic reaction in ionic liquids, Erbeldinger et al. reported the use of the protease thermolysin for the synthesis of the dipeptide Z-aspartame (Entry 6) [34]. The reaction rates were comparable to those found in conventional organic solvents such as ethyl acetate. Additionally, the enzyme stability was increased in the ionic liquid. The ionic liquid was recycled several times after the removal of non-converted substrates by extraction with water and product precipitation. Recycling of the enzyme has not been reported. It should be noted, however, that according to the log P concept described in the previous section, ethyl acetate - with a value of 0.68 - may interfere with the pro-... 

In microwave-assisted synthesis, a homogeneous mixture is preferred to obtain a uniform heating pattern. For this reason, silica gel is used for solvent-free (open-vessel) reactions or, in sealed containers, dipolar solvents of the DMSO type. Welton (1999), in a review, recommends ionic liquids as novel alternatives to the dipolar solvents. Ionic liquids are environmentally friendly and recyclable. They have excellent dielectric properties and absorb microwave irradiation in a very effective manner. They exhibit a very low vapor pressure that is not seriously enhanced during microwave heating. This makes the process not so dangerous as compared to conventional dipolar solvents. The polar participants of organic ion-radical reactions are perfectly soluble in polar ionic liquids. 

Rather than modifying the workup conditions, other groups have changed the reaction solvent or catalyst structure. The use of ionic liquids as a reaction solvent allows for easy catalyst and product separation as well as catalyst recycling.79 Alternatively, others have attached the various metathesis catalysts onto solid supports,80 82 which again allows for easy workup and catalyst recycle. 

Examples of applying biphasic systems to catalyzed reactions, such as phase-transfer catalysis, overpower the stoichiometric reactions. In a typical catalytic biphasic system, one phase contains the catalyst, while the other phase contains the substrate. In some systems, the catalyst and substrates are in the same phase, while the product produced is transferred to the second phase. In a typical reaction, when the two phases are mixed during the reaction and after completion, the catalyst remains in one phase ready for recycling while the product can be isolated from the second phase. The most common solvent combination consists of an organic solvent combined with another immiscible solvent that, in most applications, is water. However, there are few examples of suitable water-soluble and stable catalysts, and therefore various applications are limited to some extent [192]. Immiscible solvents other than water are recently becoming more applicable in biphasic catalysis because of the better solubility and stability of various catalysts in such solvents. For example, ionic liquids and fluorous solvents have many successful applications in liquid-liquid... 

Recently, Yao showed that osmium tetroxide could be immobilized in an ionic liquid. The recyclability of osmium tetroxide was improved by the addition of DMAP. Both the catalyst and the ionic liquid were reused in six consecutive runs without significant reduction in yield [47]. Dihydroxylations in a solvent mixture of l-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), tert-butanol, and water with 0s04 (2 mol%), DMAP (2.4 mol%), and NMO (1.1 equivalents) as cooxidant afforded diols in good yield (73-99% depending... 

Chapter 7 addresses another key topic in the context of green chemistry the replacement of traditional, environmentally unattractive organic solvents by greener alternative reaction media such as water, supercritical carbon dioxide, ionic liquids and perfluorous solvents. The use of liquid/liquid biphasic systems provides the additional benefit of facile catalyst recovery and recycling. 

CjCqmHBFJ Pd(PPh3)4 Na2C03 110 °C. Solid-phase reaction coupling of 4-iodophenol immobilised on a polystyrene-Wang resin with arylboronic acids DMF as co-solvent acceleration in the presence of the ionic liquid catalyst recycling not practical. [102]... 

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