Microwave irradiation and ionic liquids

The combined merits of microwave irradiation and ionic liquid make the three-component condensation with safe operation, low pollution, and rapid access to... 

Reactions Using Microwave Irradiation and Ionic Liquids as Solvents and Reagents... 

Nitriles are versatile and important components of a range of dyes, natural products, and pharmaceuticals. Aryl nitriles can be synthesized from aryl halides by direct reaction between aryl halides and copper cyanide, known as the Rosemund von Braun reaction [36]. These cyanation reactions can have several disadvantages, in particular the long reaction times required. Ren and coworkers showed that 1,3-dialkylimidazolium halide-based ionic liquids can be used as solvents in the Rosemund von Braun reaction [37]. Complete conversion, based on GC-MS analysis, was achieved after 24 h at 90 °C. When using microwave irradiation and ionic liquid as a solvent, Leadbeater and coworkers showed the reaction times could be reduced to between 3 and 10 min [38]. Under the optimized reaction conditions, 2 equiv. CuCN and 1 equiv. aryl halide were rapidly heated to 200 °C in [i-PrMIM]Br as solvent. Representative results are collected in Table 7.3. The microwave method works as well as the conventional method for a range of aryl iodide and aryl bro-... 

Other new methods used in heterogeneous catalytic reactions include the use of ionic liquids as catalysts and simultaneous applications of ultrasound and microwave irradiation. An ionic liquid as an acidic catalyst has been used for synthesis of cou-marins [78], in esterification of acetic acid with benzyl alcohol [79], in catalytic esterification at room temperature [80], in alkylation of a-methylnaphthalene with long-chain alkenes [81], and in hydrogenation of benzene [82]. 

Lopez I, Silvero G, Arevalo MJ, Babiano R, Palacious JC, Bravo JL (2007) Enhanced Diels-Alder reactions on the role of mineral catalyst and microwave irradiation in ionic liquids as recyclable media. Tetrahedron 63 2901-2906... 

Due to its lack of toxicity, potassium hexacyanoferrate(II) has also been explored as a cyanide source. Li and co-workers (Entry 6) utilised this reagent in their palladium-catalysed protocol, involving microwave irradiation in ionic liquid [27]. Most recently Saha et al. reported a hydroxyapatite-supported copper(I)-catalysed methodology (Entry 7) [28]. Whilst moderate to good yields were obtained, in both of these cases examples were restricted solely to styrenyl bromide substrates. 

This chapter covers the recent advances in amidocarbonylations, cyclohydrocarbonylations, aminocarbonylations, cascade carbonylative cyclizations, carbonylative ring-expansion reactions, thiocarbonylations, and related reactions from 1993 to early 2005. In addition, technical development in carbonylation processes with the use of microwave irradiation as well as new reaction media such as supercritical carbon dioxide and ionic liquids are also discussed. These carbonylation reactions provide efficient and powerful methods for the syntheses of a variety of carbonyl compounds, amino acids, heterocycles, and carbocycles. 

In this chapter, the recent advances in amidocarbonylations, cyclohydrocarbonylations, aminocarbonylations, cascade carbonylative cyclizations, carbonylative ring-expansion reactions, thiocarbonylations, and related reactions are reviewed and the scope and mechanisms of these reactions are discussed. It is clear that these carbonylation reactions play important roles in synthetic organic chemistry as well as organometallic chemistry. Some of the reactions have already been used in industrial processes and many others have high potential to become commercial processes in the future. The use of microwave irradiation and substitutes of carbon monoxide has made carbonylation processes suitable for combinatorial chemistry and laboratory syntheses without using carbon monoxide gas. The use of non-conventional reaction media such as SCCO2 and ionic liquids makes product separation and catalyst recovery/reuse easier. Thus, these processes can be operated in an environmentally friendly manner. Judging from the innovative developments in various carbonylations in the last decade, it is easy to anticipate that newer and creative advances will be made in the next decade in carbonylation reactions and processes. 

Lee, J. K., Kim, D.-C., Song, C. E., Lee, S.-g. Thermal behaviors of ionic liquids under microwave irradiation and their application on microwave-assisted catalytic Beckmann rearrangement of ketoximes. Synth. Commun. 2003, 33, 2301-2307. 

The widest application of ionic liquids in microwave chemistry has been as polar solvents. They heat very rapidly under the action of microwave irradiation and have excellent solvating properties. Many chemical reactions benefit from this combination of rapid heating and an ionic reaction medium. 

Ionic liquids can also be advantageously used as solvents or co-solvents in conjunction with microwave irradiation for catalytic reactions [67]. Small amoimts of I Ls are sufficient to reduce the heating time of nonpolar solvents such as toluene or cyclohexane imder microwave conditions. In recent literature there are many examples in which microwaves and ionic liquids are associated to accelerate catalytic reactions (Heck [68], metathesis [69]). More recently, ILs have been used in sonochemical accelerations of Heck and Suzuki cross-coupling reactions [70, 71]. 

The use of ionic liquids in Mizoroki-Heck reactions has been described in several reviews [9, 12, 24-26]. Modern trends of Mizoroki-Heck reactions in ionic liquids are the application of new ligand types, like A-heterocycfic carbene (NHC) ligands, supported catalysts, combination with ultrasound irradiation, microwave irradiation and multiphase catalysis. The combination of ionic liquids and activation methods is summarized in the representative chapters. 

In 2008, Yu and coworkers [31 ] reported the Au nanoparticle-supported Pd(II) microwave-assisted tilkyne cyclotrimerization reaction in ionic liquids. The Pd complexes were immobilized onto Au nanoparticles via chelation to the surface-bound dipyridyls. The catalysis was performed in bmimPF under microwave irradiation and the recovered catalysts could be recycled many times. The conversions were excellent, the regioselectivity was good, and the scope was broad. 

Abstract Some innovative S3mthetic methods in organic chemistry are concisely presented, multicomponent reactions, specifically the Ugi multicomponent reaction, parallel syntheses and combinatorial chemistry, mechanochemically promoted organic reactions, organic reactions promoted by microwave irradiation and syntheses in ionic liquids. Examples of chemoselective or as3mimetric syntheses completed by one of the presented specific methods are presented for the antihy-pertensive drug nifedipine, the alkaloid tropinone, the local anesthetic xylocaine and the HIV inhibitor tipranavir. 

Lee SE, Ko JW, Ko WB. Synthesb of gold nanoparticles in presence of ionic liquid [Bmin] [CF3SO3] with humic acid rmder microwave irradiation and its application as a nanocatalyst. Asian J Chem. 2013 25 9949-9995. 

An improvement in Diels-Alder irreversible cycloaddition of 1,3-cyclohexadi-ene, 3-carbomethoxy-2-pyrone and 2-methoxythiophene with acetylenic compound was observed (Loupy et al., 2004) while a combined effect of microwave irradiation and solid supports in ionic liquids on the Diels-Alder reaction of 1,3-cyclopentadi-ene and different nucleophiles was investigated by Lopez et al. (2007). 

Finally, dissolution of non-activated cellulose in LiCl/DMAc, and in ionic liquids has been accelerated by microwave irradiation [72,103,104], although the effect of microwave heating on the DP of the polymer has not been investigated. This last point is relevant in view of the fact that ILs are heated with exceptional efficiency by microwaves [105], so that care must be taken to avoid excessive localized heating that can induce chain degradation of the polymer during its dissolution. 

Scheme 78) [89]. Aryl chlorides with activating as well as deactivating substituents could also be coupled under the same conditions in high yields, ranging from 60% to 95%, within 30-60 min of microwave irradiation. The process does not require an inert atmosphere. The increased conversion observed with the addition of the ionic liquid reveals that it might have an additional function besides simply acting as a molecular irradiator . It cannot be excluded for instance that carbene palladium complexes are formed in situ and implicated in the catalytic cycle. 

Based on the properties of ionic hquids in high-temperature microwave-enhanced reactions, the authors chose l-butyl-3-methylimidazolium tetraflu-orophosphate ([bmimjPFe) as the suitable ionic liquid (Scheme 23). The addition of 0.15 mmol of [bmimjPFe to a reaction in 2.0 mL of DCF was found to increase the reaction rate dramatically and a set-temperature of 190 °C was reached in a mere 1 min, while the reactions programmed at 190 °C, in the absence of the ionic liquid, reached only 170 °C in 10 min. The reactions were finished in a mere 18-25 min of irradiation time, including the hydrolysis of the sensitive imidoyl chloride moiety with water. The formed bis-lactams were isolated in good yield and purity. 

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