Ionic liquid as the reaction medium

Table 2.5 The N-,0-, and 5 -alkylation using various ionic liquids as the reaction medium... 

The application of ionic liquids as a reaction medium for the copper-catalyzed aerobic oxidation of primary alcohols was reported recently by various groups, in attempts to recycle the relatively expensive oxidant TEMPO [150,151]. A TEMPO/CuCl-based system was employed using [bmim]PF6 (bmim = l-butyl-3-methylimodazolium) as the ionic liquid. At 65 °C a variety of allylic, benzylic, aliphatic primary and secondary alcohols were converted to the respective aldehydes or ketones, with good selectiv-ities [150]. A three-component catalytic system comprised of Cu(C104)2, dimethylaminopyridine (DMAP) and acetamido-TEMPO in the ionic liquid [bmpy]Pp6 (bmpy = l-butyl-4-methylpyridinium) was also applied for the oxidation of benzylic and allylic alcohols as well as selected primary alcohols. Possible recycling of the catalyst system for up to five runs was demonstrated, albeit with significant loss of activity and yields. No reactivity was observed with 1-phenylethanol and cyclohexanol [151]. 

Ionic liquids, being polar and ionic in character, couple to the MW irradiation very efficiently and therefore are ideal microwave absorbing candidates for expediting chemical reactions. An efficient preparation of the 1,3-dialkylimidazolium halides via microwave heating has been described by Varma et al. that reduces the reaction time from several hours to minutes and avoids the use of a large excess of alkyl ha-lides/organic solvents as the reaction medium (Scheme 6.56) [26-28]. 

The preparation of 1,3-dialkylimidazolium halides by conventional heating in solvent under reflux requires several hours to afford reasonable yields and also uses a large excess of alkyl halides and/or organic solvents as the reaction medium. To circumvent these problems Varma and coworkers [106] investigated the preparation of a series of ionic liquids 72 (Scheme 8.74), using microwave irradiation as the energy source, by simple exposure of neat reactants, in open containers, to microwaves by use of an unmodified household MW oven (240 W). 

Abstract The term Lewis acid catalysts generally refers to metal salts like aluminium chloride, titanium chloride and zinc chloride. Their application in asymmetric catalysis can be achieved by the addition of enantiopure ligands to these salts. However, not only metal centers can function as Lewis acids. Compounds containing carbenium, silyl or phosphonium cations display Lewis acid catalytic activity. In addition, hypervalent compounds based on phosphorus and silicon, inherit Lewis acidity. Furthermore, ionic liquids, organic salts with a melting point below 100 °C, have revealed the ability to catalyze a range of reactions either in substoichiometric amount or, if used as the reaction medium, in stoichiometric or even larger quantities. The ionic liquids can often be efficiently recovered. The catalytic activity of the ionic liquid is explained by the Lewis acidic nature of then-cations. This review covers the survey of known classes of metal-free Lewis acids and their application in catalysis. 

An efficient and convenient methodology for the aerobic oxidation of alcohols catalysed by sol-gel trapped perruthenate and promoted by an encapsulated ionic liquid in supercritical carbon dioxide solution has been reported. The reaction is highly selective and useful for substrates otherwise difficult to oxidize.263 A four-component system consisting of acetamido-TEMPO-Cu(C104)2-TMDP-DABCO has been developed for aerobic alcohol oxidation at room temperature. The catalytic system shows excellent selectivity towards the oxidation of benzylic and allylic alcohols and is not deactivated by heteroatom-containing (S, N) compounds. The use of DMSO as the reaction medium allows the catalysts to be recycled and reused for three runs with no significant loss of catalytic activity.264... 

The concept of transesterifications was used for polymerization reactions by Hedrick and colleagues [76]. Various biodegradable polyesters were synthesized with the l,3-dimethylimidazol-2-ylidene carbene in THF at 25 °C. Polymers such as poly(e-caprolactone) were obtained with no need of organometallic catalysts, as in classical methods. Poly(ethylene terephthalate) (PET) 97 was synthesized in the ionic liquid 98, which functions as the reaction medium and, at the same time, as a precatalyst that is activated (99) with KOt-Bu. Dimethyl terephthalate (DMT) 100 was condensed with an excess of ethylene glycol 101 to generate 102. The melt condensation of 102 was performed under vacuum using a heating ramp to 280 °C. 

As can be seen, immobilized CALB efficiently catalyzes the acylation of aU polyhydroxylated compounds in the ionic liquid media used, leading to high conversion yields. The reaction rates for the enzymatic acylations are summarized in Table 9.3. Higher reaction rates were obtained for aU polyhydroxylated compounds tested when [bmim]BF4 was used as the reaction medium. It is interesting to note that the solubilities of esculin, salicin, helicin, naringin, and silybin at 60 C were approximately 68, 23,40,100, and 82 mM, respectively, in [bmim]BF4. On the other hand, in [bmim]PFs and acetone, in which the reactions rates were lower, the solubilities of the aforenamed compounds were 20, 5.5,19.5,1.5, and 7.2 mM and 6.5,... 

Task-specific ionic liquids (TSILs) represent a more advanced approach to the design of ionic liquids to be exploited in catalytic applications. Indeed, the structure of one partner of the ion pair is tailored in such a way to incorporate a functional group able to stabilise the catalytically active metal. A range of different functional groups have been installed on the cation moiety of onium salts, for example nitrile, thiok alcohol, acid, amide,52 urea. etc. Thus, the ionic liquid plays a double role, acting both as the reaction medium and as a stabilising ligand for the catalytically active species. Such a stabilisation, hopefully, should improve the catalyst half-life and afford better results compared to unfunctionalised ionic liquids. 

Particular highlights include the first structurally authenticated example of a phosphonium diylide and the use of ionic liquids as a novel medium for performing Wittig reactions. Ylides, particularly their iminophosphorane counterparts, continue to provide a strikingly rich avenue of coordination chemistry research. A useful study by Pandolfo et al. reports the solid state NMR spectra of several ylides and their complexes, showing that this technique can provide valuable structural information in situations where it is difiicult to obtain solution data due to solubility problems or decomposition. 

The ionic liquids, as the green solvents, have been extensively applied to the C-, 0-, N-, and 5 -acylations. It is obvious that the use of ionic liquids as replacement to the conventional organic solvents has been eco-friendly to the environment. Besides, the ionic liquids as the unusual reaction medium could promote the reaction and increase the conversion and selectivity as well. Moreover, the immobilization of homogeneous catalysts in the ionic liquids to form composite catalytic systems could make the catalysts to be more easily separation and reuse. 

The ionic liquids as the green solvents for the 0-acylation of 2-octanol with succinic anhydride brought good results for the reactions [122], Water-tolerant ionic liquid [BMlM][PFg] as the clean reaction medium was appUed to the acylation of... 

Diarylketones are also the important fine chemical intermediates, which could be prepared by the acylation of aromatic hydrocarbon with benzoyl chloride and benzoic anhydride. The ionic liquids have also been used to catalyze the synthesis of diarylke-tone. Earle et al. [105] have reported the benzoylation of benzene, derivants of benzene (toluene, anisole, isobutyl benzene, phenyl chloride and fluoride) to synthesize the diarylketone by use of chloroindate (III) ionic liquids as the green dual catalysts and solvents. As a result, good yields (75-96%) were obtained under proper conditions with the ionic liquids as the clean reaction medium and recyclable catalysts. 

The ionic liquids as the eco-friendly catalysts and clean reaction medium have also been used for the acylation of polynuclear aromatics. Chen [145-147] and Yuan [148] have investigated the Friedel-Ciafts acylation reactions of anthracene with oxalyl chloride by use of the chloroaluminate ionic Uquids as the catalysts. In these reactions, the ionic liquids exhibited excellent performance as the efficient clean catalysts and reaction media Almost 100% selectivity with 90% yield of... 

The easy work-up, the absence of a catalyst, and short reaction times when nonvolatile ionic liquid is used as the reaction medium make the method amenable for scale-up operations. 

Intermolecular Diels-Alder or hetero Diels-Alder reactions have been greatly improved by using microwave technology - again with higher reaction rates and improved yields [3j]. Remarkable improvements in rate acceleration and selectivity enhancement for a variety of intermolecular Diels-Alder reactions have also been accomplished in the past two decades by application of catalysts such as Lewis acids. Recently, many such examples have been reported under microwave conditions in polar solvents or ionic liquids as energy-transfer medium. These reactions have also been developed in open vessels by adsorption of the reactants on mineral solid supports or using neat reactants. 

An early account of the role of Pd nanoparticles in the Heck reaction was published by Srinivasan and coworkers in 2001 [283]. They carried out Heck reactions of dif ferent aryl iodides with acrylates and styrene at ambient temperature and observed a significant rate enhancement by combining ultrasonic irradiation with the use of the ionic liquid 1,3-dibutylimidazolium tetrafiuoroborate as the reaction medium. Under the sonochemical conditions the formation of Pd-biscarbene complexes was observed and these later transformed into Pd nanoparticles. 

Epoxidation of enone can be carried out in the absence of a metal catalyst in ionic liquid. A basic aqueous solution of hydrogen peroxide behaves as an efficient oxidant both in [bmim][PF6], which is immiscible in water, and in hydrophilic [bmim] [BF4]. Quantitative yields of the corresponding epoxides were obtained after very short reaction times (Scheme 27). The use of H2O2 in ionic liquids is a viable alternative to using water as the solvent. There is a significant rate acceleration when these salts are used as the reaction medium. This strategy has been successfully applied to the epoxidation of chromones and flavonoids in all cases, the reaction was dramatically improved in [bmim] [BF4] compared with standard classical organic sol vents. 

An easy recycling method involving both catalyst and reaction medium was achieved in a Mizoroki-Heck arylation reaction of acrylic acid, using a fluorous carbene complex (prepared in situ fl om a fluorous ionic liquid and palladium acetate) as the catalyst and a fluorous ether solvent (F-626) as the reaction medium. Because of the very low solubility of arylated carboxylic acids in F-626, the products precipitated during the course of the reaction. After separation of the products and amine salts by filtration, the filtrate, which contained the fluorous Pd catalyst, could be recycled for several runs (Scheme 13). The Mizoroki-Heck reaction was effectively promoted by a fluorous SCS pincer palladium, which is discussed in Section 3.4.5. 

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