Ionic Liquids as Green Solvents - American Chemical Society

Chapter 7

Expeditious Synthesis of Ionic Liquids Using Ultrasound and Microwave Irradiation

Downloaded by UNIV OF ARIZONA on July 26, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch007

Rajender S. Varma Clean Processes Branch, National Risk Management Research Laboratory, U.S. Environmental Protection Agency, MS 443, 26 West Martin Luther King Drive, Cincinnati, O H 45268

Environmentally friendlier preparations of ionic liquids have been developed that proceed expeditiously under the influence of microwave (MW) or ultrasound irradiation conditions using neat reactants, alkylimidazoles and alkyl halides. A number of useful ionic liquids have been prepared and the efficiency of this solvent-free approach has been extended to the preparation of other ionic salts bearing tetrafluoroborate anions that involves exposing N, N'-dialkylimidazolium chloride and ammonium tetrafluoroborate salt to MW irradiation. These novel solventless alternatives circumvent the common drawbacks in widely used preparative methods for ionic liquids that employ volatile organic solvents and relatively much longer reaction time. The utilization of these novel synthetic methods for ionic liquids generation may facilitate truly 'greener' applications of ionic liquids.

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U.S. government work. Published 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

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Introduction Industrial chemistry in the new millennium is adopting the concept of green chemistry to meet the fundamental scientific challenges of protecting the human health and environment while maintaining commercial viability. The emerging area of green chemistry envisages minimum hazard as the performance criteria while designing new chemical processes. The reduction or replacement of volatile organic solvents from the reaction medium is of utmost importance with possible substitutions by nonvolatile or recyclable alternatives. One of the thrust areas for achieving this target is to explore alternative reaction media and optimize reaction conditions to accomplish the desired chemical transformations with minimized by-products or waste generation as well as eliminating the use of conventional organic solvents wherever possible. Among the emerging important tools the use of microwave (MW) as alternative energy source is becoming an attractive alternative especially under the solvent-free conditions (1-4). To address the challenges posed by the waste minimization and pollution prevention efforts, several strategies have been developed. A significant development in the last few years has been the discovery and the use of ionic liquids in synthetic organic chemistry (5). Room temperature ionic liquids (RTIL's) comprise a wide range of anions that form low melting salts with several organic cations such as imidazolium, pyridinium, phosphonium and ammonium etc. although the most widely studied examples in the literature encompass 1,3-dialkylimidazolium cations. The second generation of ionic liquids predominantly comprise tetrafluoroborate and hexafluorophosphate as anions although several others have appeared recently that contain CF - or similar fluorinated alkyl groups (6) and some others that are completely halogen free. Their potential for recyclability, ability to dissolve a variety of materials, and importantly their non-volatile nature with barely measurable vapor pressure are some of their unique attributes responsible for newly found popularity. Although originally used in electrochemical applications (7), these room temperature ionic liquids are currently being explored as environmentally benign solvent substitutes for conventional volatile solvents in a variety of applications such as chemical synthesis (5), liquid/liquid separations (8), extractions (9), dissolution (10), catalysis (11), biocatalysis (12) and polymerization (13). 3

Synthesis of Ionic Liquids Using Microwave Irradiation The preparation of the 1,3-dialkylimidazolium halides via conventional heating method in refluxing solvents requires several hours to afford reasonable

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


84 yields and also uses a large excess of alkyl halides/organic solvents as the reaction medium (14). In view of the emerging importance of the ionic liquids as reaction media (5), the high propensity of the polar reactants to couple to microwaves and our general interest in microwave-assisted chemical processes (1-4), we decided to explore the synthesis of ionic liquids using microwave (MW) irradiation under solvent-free conditions. To further simplify the preparation, we experimented by exposing neat reactants in open glass containers to microwaves in a household MW oven. In an unmodified household MW oven, however, it is not possible to effectively adjust the M W power. The reduction in power level simply means that the oven operates at its full power but for a reduced period of time. A recently introduced household MW oven (Panasonic) equipped with inverter technology provides a realistic control of the microwave power to a desirable level and was used throughout in our preparations. Initially, the effect of microwave power on a set of reactions using alkyl halides and 1-methyl imidazole (MIM) as reactants was examined and it was found that operating the MW oven at reduced power level (240 Watts) afforded the highest yields. Upon microwave irradiation, the ionic liquid starts forming which increases the polarity of the reaction medium thereby increasing the rate of microwave absorption. It was observed that, at elevated power levels, the evaporation of alkyl halide and partial decomposition/charring of the ionic liquid occurs possibly due to the localized overheating of ionic liquid, which eventually results in lower yields. To circumvent this problem, the reactions were conducted with intermittent heating and mixing at a moderate power level to obtain better yields and cleaner ionic liquid formation. After the first irradiation for 30 sec at 240 Watts (-bulk temperature 70-100 °C), the homogeneity of the reaction mixture changed due to formation of a small amount of ionic liquid. The reaction mixture was then taken out, mixed for 10 sec on a vortex mixer, and then heated at same power level for additional 15 sec. This process was repeated until the formation of a clear single phase was obtained. Thus, the formation of ionic liquid could be monitored visibly in the reaction as it turned from clear solution to opaque and finally clear. Any unreacted starting materials were removed by washing with ether and the final product was dried under vacuum at 80 °C. This solventless approach (75) requires only a few minutes of reaction time in contrast to several hours required by procedures using conventional heating, which uses an excess of reactants. A general schematic representation for the preparation of mono (I) and dicationic (II) 1,3-dialkylimidazolium halides is depicted in Figure 1. This method is ideally suited for the preparation of ionic liquids with longer alkyl chains or with higher boiling points. Comparison of a series of ionic liquids prepared by microwave heating and similar preparation using conventional heating (oil bath at 80 °C) is summarized in Table 1. Most of the


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halides used in this study have higher boiling points and are converted efficiently to ionic liquids under microwave irradiation conditions. The relatively less reactive and low boiling reactants such as 1-bromobutane incurred losses due to evaporation and therefore required the use of more than stoichiometric amounts for best results. The reactivity trend of halides was found to be in the order Γ> Br > CF. Highly reactive iodides afforded excellent yield of ionic liquids in all cases with minimum exposure time (15). In contrast, the conventional methods reported in the literature generally use a large excess of alkyl halide or tetrahydrofuran as solvents.

(I)

(10

Where R « Alkyl group and X » Cl, Br and ! Figure 1. Microwave-assisted Preparation of Ionic Liquids

The butyl and hexyl dicationic salts are solids at room temperature. The corresponding octyl analogues bearing bromide/chloride as the anions are viscous liquids whereas the iodo compound is a solid. The NMR data analysis indicated that the dicationic salts generated from chloro and bromoalkanes were slightly contaminated with the corresponding monocationic intermediate ( Br> C l . The alkyl chlorides are relatively less reactive and required longer irradiation time and some heating except for reactive entities such as ethyl chloroacetate (entry 20). The higher reactivity of bromides and iodides provided excellent yields with minimum sonication time and the reactions were essentially completed at room temperature. For the preparation of chloride salts, the use of a probe system (Sonic & Materials Inc.) was explored wherein an ultrasonic field was produced directly in the reaction vessel. A number of chloride salts could be easily prepared via this modified approach (entries 2, 6, 10 and 17, Table 3). During sonication of 1-chlorobutane with MIM, thetemperatureof the reaction mixture rose from 22 °C to 80 °C and became constant at 80 °C. Upon formation of the ionic liquid, due to exothermicity of the reaction, the temperature increased to US °C. Similarly, in the case of reaction between 1-chlorooctane and MIM the temperature reached 100 °C and upon formation of the ionic liquid it further increased to 125 °C. Butyl, hexyl and octyl dicationic salts (entries 13-19, Table 3) were also prepared efficiently by this methodology. The diiodo and dibromoalkanes, being more reactive, rapidly afforded pure products. The in-situ generation of ionic liquids and their subsequent utilization as reaction media by sequential addition of reactants in the same pot renders this an ideal approach for chemical transformation where higher viscosity of the ionic liquids impedes the efficient mixing of the reactants and catalysts. This ultrasound-accelerated method is adaptable for efficiently accomplishing a onepot palladium acetate catalyzed Suzuki reaction of iodobenzene with 4-fluorophenylboronic acid at room temperature by subsequent addition of reactants to in situ generated ionic liquid. The ionic liquid and catalyst can be recycled 4 times without the loss of any reactivity (91 %). In brief, the developed sonochemical protocol provides the clean synthesis of ambient temperature ionic liquids that occurs nearly at room temperature.


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Conclusion Efficient, expeditious and environmentally friendlier chemical processes have been detailed for the preparation of ionic liquids that precludes the excessive use of volatile organic solvents and utilizes common microwave oven or the ultrasonic bath. Ionic liquids, being polar and ionic in character, couple to the microwave irradiation very efficiently and therefore are ideal microwave absorbing candidates (22). The surge of interest continues with this class of solvents especially under MW irradiation conditions where potential recycling of the catalyst is enhanced as demonstrated for a high-speed Heck reaction in ionic liquid media (23) and for expediting hetero Diels-Alder reactions of 2(1/?)pyrazinones in ionic liquid doped solvents (24). Further investigations on the use of such in situ generated ionic liquids via microwave or ultrasound (19c,d) activation should promote the utility and applications of ionic liquids in chemical transformations and syntheses.

Acknowledgement The author thanks Dr. V. V. Namboodiri for his efforts in this research.

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Ionic Liquids as Green Solvents - American Chemical Society

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