Microwaves and Ionic Liquids

There are two mechanisms by which microwaves interact with reaction mixtures [7]. Polarization of dielectric material arises when the distribution of an electron cloud is distorted or physical rotation of molecular dipoles occurs. For generation of heat on irradiation with microwaves, at least one component of a reaction mixture must have a dipole moment. Compounds with high dipole moments also have large dielectric constants, e. The selectivity of microwave irradiation is clear when comparing the heating of water and hexane. Water, a polar solvent, has a high dielectric constant and therefore heats rapidly on microwave irradiation whereas hexane, a nonpolar solvent, heats very slowly. 

Microwave activation also occurs via a conduction mechanism. In a solution that contains ionic material, the ions start to move under the influence of the electric field of the microwave irradiation. This results in expenditure of energy because of an increased collision rate, thus converting kinetic energy into heat. This effect is particularly important at higher temperatures. 

Issues arise when chemists want to use nonpolar reagents in conjunction with less polar solvents such as hexane (e = 1.88), toluene (s = 2.38), tetrahydrofuran (fi = 7.58), and dioxane (e = 2.21). Because of their lower dielectric constants, these 

Microwaves in Organic Synthesis, Second edition. Edited by A. Loupy Copyright 2006 WILEY-VCH Veriag GmbH Co. KGaA, Weinheim ISBN 3-527-31452-0 

Ionic liquids are interesting materials which have found new functions in chemistry [8]. Their unique properties and ionic structure has made them useful in synthesis and catalysis [9]. Because of their low toxicity, low vapor pressure and recyclability these compounds have a green reputation. 

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Microwaves and ionic liquids have been used to induce acetal formation, hut the methods have not been broadly tested on significant substrates. 

Microwave and Ionic Liquid-assisted Beckmann Rearrangement... 

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]. 

G. Keglevich, R. Kovacs, L. Drahos, Diels-Alder cycloadditions of 1,2-dihydrophosphinine oxides and fi agmentation-related phosphorylations with 2-phosphabicyclo[2.2.2]octadiene oxides under green chemical conditions - the role of microwave and ionic liquids. Phosphorus Sulfur Sfflcon 186 (2011) 2172-2179. 

Cao, S.W. and Zhu, Y.J. (2009) Iron oxide hollow spheres microwave-hydrothermal ionic liquid preparation, formation mechanism, crystal phase and morphology control and properties. Acta Materialia, 57 (7), 2154-2165. 

Cao, S.W., Zhu, Y.J., Cheng, G.F. and Huang, Y.H. (2009) ZnFe204 nanopartides microwave—hydrothermal ionic liquid synthesis and photocatalytic property over phenol. Journal of Hazardous materials, 171 (1—3), 431—435. 

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. 

Apart from the traditional solid supports (see above), several publications also report the successful use of microwave enhancement for supported transformations involving soluble polymers49-54, fluorous phase conditions55,56, and ionic liquids grafted onto polymeric supports57,58. 

Leveque, J. and G. Cravatto. 2006. Microwaves power ultrasound, and ionic liquids. A new synergy in green organic synthesis. Chimia 60 313-320. 

In the last few years numerous reports have been published in the field of microwave-promoted aryl halide cyanation, utilizing nickel [71], palladium [72,73] and copper [74,75] catalysis. Even water [75] and ionic liquids [76] have proven useful as solvents in these processes. Srivastava and Collibee have exemplified a swift and dynamic procedure using polymer-supported triphenyl phosphine to enable easy subsequent removal through filtration [72]. As shown in Scheme 19, both bromides and iodides could be activated using palladium catalysis in DMF. Even without optimization of the individual reaction times, the overall process time involving simple filtration and extraction for compound isolation appears to be short. 

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

The above examples clearly demonstrate the potential of microwaves in ionic liquids as solvent and/or catalyst. Many well-known and new reactions have been discovered by this technology with evident advantages in most of the cases. The synergism arising from the combined use has enormous potential to meet the increasing demand for environmentally benign chemical processes. 

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-... 

Scheme 7.10. Conversion of alcohols into halides by use of microwaves with ionic liquids as reagents and solvents.

The examples provided above from the recent literature on IMDA and IMHDA reactions, typically illustrate the significant advantages of the use of solvent and ionic liquids under microwave conditions in closed or open vessels compared with classical heating procedures. 

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