Microwave ionic liquids

Theoretical and applied aspects of microwave heating, as well as the advantages of its application are discussed for the individual analytical processes and also for the sample preparation procedures. Special attention is paid to the various preconcentration techniques, in part, sorption and extraction. Improvement of microwave-assisted solution preconcentration is shown on the example of separation of noble metals from matrix components by complexing sorbents. Advantages of microwave-assisted extraction and principles of choice of appropriate solvent are considered for the extraction of organic contaminants from solutions and solid samples by alcohols and room-temperature ionic liquids (RTILs). 

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. 

Microwave-Enhanced Synthesis Using Functional Ionic Liquid Supports. 115... 

Abstract Current microwave-assisted protocols for reaction on solid-phase and soluble supports are critically reviewed. The compatibility of commercially available polymer supports with the relatively harsh conditions of microwave heating and the possibilities for reaction monitoring are discussed. Instrmnentation available for microwave-assisted solid-phase chemistry is presented. This review also summarizes the recent applications of controlled microwave heating to sohd-phase and SPOT-chemistry, as well as to synthesis on soluble polymers, fluorous phases and functional ionic liquid supports. The presented examples indicate that the combination of microwave dielectric heating with solid- or soluble-polymer supported chemistry techniques provides significant enhancements both at the level of reaction rate and ease of purification compared to conventional procedures. 

In addition to the examples described above, functionalized ionic liquids have been recently introduced as microwave-compatible soluble supports [137,138]. 

Microwave-Assisted Synthesis of a Substituted Pyran Using Ionic Liquid Tagging... 

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. 

However, most of the reactions are reported to be slow, taking up to 12 h for complete conversion of the starting materials. A Diels-Alder reaction of the pyrazinone scaffold with dimethyl acetylenedicarboxylate (DMAD) [57] has been studied in view of investigating the swiftness of this cycloaddition-fragmentation protocol (Scheme 20). The authors investigated the reaction with DMAD (lOequiv) under microwave irradiation at an elevated temperature of 190 °C, using small amounts of ionic liquid (bmimPFe) in... 

Scheme 23 Microwave-enhanced Diels-Alder reactions of alkene-tethered 2(lff)-pyra-zinones in ionic liquid doped solvents...

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. 

Hydroformylation of alkenes can be carried out in a few minutes under microwave activation at a relatively low pressure (2.7 bar) employing the rhodium(I)/XANTPHOS catalyst. The presence of the ionic liquid butyl-methylimdazolium tetrafluoroborate ([bmim][BF4]) was crucial. Unfortu-... 

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. 

Esmaili, M. and Habibi-Yangjeh, A. (2010) Microwave-assisted preparation and characterization of Zri, %Cd S nanoparticles in presence of an ionic liquid and their photocatalytic activities. Journal of Alloys and Compounds, 496 (1-2), 650-655. 

Besides using standard organic solvents in conjunction with microwave synthesis, the use of either water or so-called ionic liquids as alternative reaction media [32] has become increasingly popular in recent years. 

Ionic liquids interact very efficiently with microwaves through the ionic conduction mechanism (see Section 2.2) and are rapidly heated at rates easily exceeding 10 °C s"1 without any significant pressure build-up [52]. Therefore, safety problems arising from over-pressurization of heated sealed reaction vessels are minimized. 

Scheme 4.18 Microwave syntheses in l-alkyl-3-methylimidazolium ionic liquids (see text for references).

The concept of performing microwave synthesis in room temperature ionic liquids (RTIL) as reaction media has been applied to several different organic transformations (Scheme 4.18), such as 1,3-dipolar cycloaddition reactions [54], catalytic transfer hydrogenations [55], ring-closing metathesis [56], the conversion of alcohols to alkyl halides [57, 58], and several others [59-61],... 

Scheme 4.19 Use of ionic liquid-doped toluene as a solvent for microwave synthesis.

Table 4.3 Microwave heating effects of doping organic solvents with ionic liquids (IL) A and B (data from [63]). a ...

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