Synthesis of Task-specific Ionic Liquids

Despite the utility of chloroaluminate systems as combinations of solvent and catalysts in electrophilic reactions, subsequent research on the development of newer ionic liquid compositions focused largely on the creation of liquid salts that were water-stable [4]. To this end, new ionic liquids that incorporated tetrafluoroborate, hexafiuorophosphate, and bis (trifluoromethyl) sulfonamide anions were introduced. While these new anions generally imparted a high degree of water-stability to the ionic liquid, the functional capacity inherent in the IL due to the chloroaluminate anion was lost. Nevertheless, it is these water-stable ionic liquids that have become the de rigueur choices as solvents for contemporary studies of reactions and processes in these media [5]. 

The incorporation of functionality into an ion slated for use in formulation of an ionic liquid is a usually a multi-step process. Consequently, a number of issues must be considered in planning the synthesis of the ion. The first of these is the choice of the cationic core. The core of a TSIL cation may be as simple as a single 

The second element of general importance in the synthesis of a task-specific ionic liquid is the source of the functional group that is to be incorporated. Key to success here is the identification of a substrate containing two functional groups with different reactivities, one of which allows the attachment of the substrate to the core, and the other of which either is the functional group of interest or is modifiable to the group of interest. Functionalized alkyl halides are commonly used in this capacity, although the trifiate esters of functionalized alcohols work as well. 

The choice of reaction solvent is also of concern in the synthesis of new TSILs. Toluene and acetonitrile are the most widely used solvents, the choice in any given synthesis being dictated by the relative solubilities of the starting materials and products. The use of volatile organic solvents in the synthesis of ionic liquids is decidedly the least green aspect of their chemistry. Notably, recent developments in the area of the solventless synthesis of ionic Hquids promise to improve this situation [10]. 

The choice of the anion ultimately intended to be an element of the ionic liquid is of particular importance. Perhaps more than any other single factor, it appears that the anion of the ionic Hquid exercises a significant degree of control over the molecular solvents (water, ether, etc.) with which the IL wiU form two-phase systems. Nitrate salts, for example, are typically water-miscible while those of hexaflu-orophosphate are not those of tetrafluoroborate may or may not be, depending on the nature of the cation. Certain anions such as hexafluorophosphate are subject to hydrolysis at higher temperatures, while those such as bis(trifluoromethane)sul-fonamide are not, but are extremely expensive. Additionally, the cation of the salt used to perform any anion metathesis is important While salts of potassium, sodium, and silver are routinely used for this purpose, the use of ammonium salts in acetone is frequently the most convenient and least expensive approach. 

Imizuka, T. Nakashima, JP 2002003478 (to Foundation for Scientific Technology Promotion, Japan), 2002 [Chem. Abstr. 2002, 136, 85811], 

Ignatyev, V. Hilarius, EP 1162204 (to Merck Patent GmbH, Germany), 2001 [Ghem. Abstr. 2001, 136, 20157], V. Hilarius, U. Heider, M. Schmidt EP 1160249 (to Merck Patent GmbH, Germany) 2001 [Ghem. Abstr. 2001, 136, 6139], 

Boesmann, WO 0155060 (to Solvent Innovation GmbH, Germany) 2001 [Ghem. Abstr. 2001, 135, 152789], 

Bahrmann, H. Bohnen, DE 19919494 (to Gelanese Ghemicals Europe GmbH, Germany) 2000 [Ghem. Abstr. 2000, 133, 321998], 

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Scheme 2.3-1 General synthesis of task-specific ionic liquids from l-alkylimidazoles.

J. H. Davis, Jr., Synthesis of task-specific ionic liquids, in Ionic Liquids in Synthesis (Eds. Peter Wasserscheid, T. Welton), VCH, Weinheim, 2002, p. 33 - 40. 

Scheme 23-1 General synthesis of task specific ionic liquids with functionalized cations from 1-alkylimidazoles. The preparation of functionalized pyridinium, phosphonium, etc. cations may be accomplished in like fashion.

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