Chapter 20
New Cations for Ionic Liquids, Including Chiral Adjuncts with Phosphate and Sulfonylimide Anions 1,
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Robert Engel *, Sharon Lall-Ramnarine , Delroy Coleman , and Marie Thomas
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Department of Chemistry and Biochemistry, Queens College of CUNY, 65-30 Kissena Boulevard, Flushing, NY 11367 and The Graduate School and University Center of CUNY, New York, NY Queensborough Community College of CUNY, Bayside, NY 11364 2
A series of new ionic liquids has been prepared and investigated incorporating a variety of ammonium and polyammonium cationic components. These include several topographies, including linear arrays (strings) with stereogenic sites along the array, and both pyrrolidinium and pyridinium species bearing ether and chiral adjuncts. The associated anions for these systems include the (environmentally friendlier) phosphate and bis(trifluoromethyl)sulfonylimide species. Physical and chemical characteristics of the anhydrous ionic liquids have been investigated.
© 2007 American Chemical Society
In Ionic Liquids in Organic Synthesis; Malhotra, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
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Introduction General
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Prior efforts of this laboratory have been concerned with the preparation and investigation of several varieties of ionic liquids (7-7). In the present effort our particular concern is with the preparation and investigation of chiral ionic liquids, those bearing at least one stereogenic site within the covalent structure of the cationic portion.
Rationale While the potential utility of ionic liquids in general has begun to be realized, that of chiral ionic liquids has yet to be established firmly. We can envision several areas of application for such materials, given that they can be prepared in suitable quantity economically and in an environmentally friendly (green) manner. These potential applications include: • The exhibition of enantiomeric selectivity in reactions using ionic liquids as the reaction media. • The exhibition of enantiomeric selectivity in electrochemical processes. • The exhibition of enantiomeric selectivity in separation processes. Our efforts reported here are concerned with the discovery and exploitation of chiral ionic liquids for these purposes.
Requirements In general, the requirements for chiral ionic liquids to be of value for applications as noted above, in addition to their exhibiting the required enantiomeric selectivity, are quite similar to those of ionic liquids in general. That is, they must be of relatively low viscosity at the appropriate temperature for operation. In addition, they must reasonably be available from precursors without the need for enantiomeric separations and convertible to the proper salt form without compromising the chirality. Finally, for them to be of use as more than a laboratory curiosity, they must exhibit "green" characteristics, including stability of both cation and anion thermally and hydrolytically, and involve minimal energy expense in the preparation of themselves and their precursors. Ideally, the chiral material should be a readily available species from biorenewable sources.
In Ionic Liquids in Organic Synthesis; Malhotra, S.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.
261 Anion Focus With the previously mentioned considerations in mind, activities of the current effort have been directed toward the use of several anions to be associated with the chiral ionic liquids. Overall, as the cationic portions of ionic liquids are generally formed by reactions of amines with organic halides or tosylates, ionic liquids generated bearing those anions (halides or tosylates) involve a minimum of preparative effort and thus have a desirable characteristic. However, at times halide or tosylate anions may not desirable for other required properties of the ionic liquid. Phosphate (P0 ") is an anion with which ammonium ions can often form ionic liquids (1-7) and does not degrade to undesirable substances in aqueous media or under thermal stress. As such, ionic liquids bearing phosphate anions constitute fundamentally "green" materials. The major difficulty with such materials is in their preparation, the conditions of which can often lead to racemization or degradation of the cationic component. Alternatives to the halides and phosphates, not readily hydrolyzable in aqueous media and with reasonable thermal stability are the bis(trifluoromethyl)sulfonylimides [NTff], readily prepared from the halide salts and generally liquids of relatively low viscosity (8, 9). It is with these anion species (halides, tosylates, phosphates and [NTf ]\ that our search for stable, unreactive and facilely generated chiral ionic liquids is directed. 3
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Results and Discussion Our initial efforts toward the preparation and investigation of chiral ionic liquids has been concerned with those bearing the chiral 2,3-dihydroxypropylgroup. While the racemic 2,3-dihydroxy-l-chloropropane was used to establish appropriate reaction conditions for preparation of ionic liquids, the chiral reagents were ultimately used to generate directly the chiral ionic liquid species. Both the (R)- and the (S)-2,3-dihydroxy-l-chloropropane were used in reaction with 4-(dimethylamino)pyridine (acetonitrile solvent, overnight reaction) to generate the enantiomeric chiral ionic liquid chlorides as shown in Figure 1. Determination of water content using the Karl-Fisher titration technique indicated less than 1% residual water. The enantiomeric chloride salts exhibited identical *H and C NMR spectra, in accord with their proposed structures. The resultant (R)- chloride salt was converted to the (R)- [NTf '] salt by the standard procedure (8, 9), the latter was dried under vacuum (
