Akio KAMIMURA1, Nao KANEKO1, Kota MINAMI1, Runa SASAKURA1, Keiya YANAGISAWA1, Takuji KAWAMOTO1, Kenta FUJII1
1Department of Applied Chemistry, Yamaguchi University, Ube, Japan
Ionic liquids are recognized as the third solvent different from water or conventional organic solvent. Although their unique physical properties provided developments of useful reactions, ionic liquids often provoked serious difficulty during separation and/or isolation procedures. They are usually non-volatile and they cannot be removed by evaporation. Thus, liquids-liquids extraction would be a main methodology. This strategy, however, occasionally becomes useless, if ionic liquids and products are both hydrophobic or both hydrophilic. To solve the problem, switching of the solubility of ionic liquids has been suggested, and some special ionic liquids, of which solubility was changed by bubbling carbon dioxide or temperature. Although these switching properties were convenient, however, the transformation was just a temporary one and could not be maintained by subtle change of conditions in the solution. We thought that much robust switching methods would be necessary, and focused on the chemistry of protective group to switch the solubility. With this ideas, we successfully developed a new type of solubility-switchable ionic liquids (SSILs), [PGA][TFSA] and [GA][TFSA] using some ammonium derivatives. In this presentation, we will show full details of preparation, physical properties, and application in chemical reactions of SSILs, and discuss about their future possibility.
A variety of SSILs were prepared commercially available glycelol acetal or glycel aldehyde acetal. We have prepared phosphonium, pyridinium, imidazolium and ammonium derivatives, and all of TFSA salts [PGA][TFSA] prepared were oil at room temperature. The syntheses were simple and easy and large-scale preparation was readily possible. Conversion between acetal derivatives [PGA][TFSA] and diol derivatives [GA][TFSA] was achieved in the presence of small amounts of Nafion as a catalyst with simple and easy manipulation.
The acetal derivatives [PGA][TFSA] were mostly lipophilic and most of part were distributed to organic phase when extracted with water-dichloromethane (DCM) system. On the other hand, some of diol derivatives [GA][TFSA] preferred to be partitioned to aqueous phase as long as extracted with water-DCM or water-ether system. The hydrophilicity depended on the number of carbon in the cationic part as well. The diol-type ionic liquids extracted to organic phase preferably as the number of carbon in the cation part increased.
The SSILs were stable during organic reactions. For example, we performed the Diels-Alder reaction at 160 °C without decomposition of SSIL. We will also discuss on the origin of hydrophilicity in diol-type [GA][TFSA].
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