Chirality Transfer in Imidazolium Camphorsulfonate Ionic Liquids through Ion Pairing Effects

Abstract

The paper describes our studies on ion pair interactions in ionic liquids (IL) using an asymmetric hydrogenation reaction as probe. Three different ionic liquids carrying prochiral keto-functionalized cations were hydrogenated in the presence of their chiral, enantiomerically pure counter-ion using an achiral heterogeneous ruthenium catalyst. For the hydrogenation of N-(3'-oxobutyl)-N-methylimidazolium camphorsulfonate (2), N-(3'-oxobutyl)imidazolium camphorsulfonate (4) and N-(5'-oxohexyl)-Nmethylimidazolium camphorsulfonate (6) we found a strong dependency of the enantiomeric excess (ee in the cation) on the polarity of the solvent, the concentration of the IL and the structure of the IL. The highest ee values of up to 94% were found for the hydrogenation of 2 in ethanol. Interestingly, we observed that the ee (and consequently the strength of ion pair interaction) had a pronounced maximum for a certain concentration of the IL in the solvent depending on the nature of the solvent and on the substrate. Remarkably, the concentration leading to the maximum ee could be rationalized by independent determination of the degree of dissociation which was obtained by a combination of diffusion-ordered NMR spectroscopy and conductivity measurements.

Application of Chiral Ionic Liquids for Asymmetric Induction in Catalysis

“…(1) [56]. As noted before, the viscosity of the reaction plays a role, furthermore, the nature of solvent to decrease the viscosity, too.…”

Activation of Small Molecules by P-Ligand—Platinum Metal Complexes in Ionic Liquids

“…That is, the combination of high chemo-, regio-and enantioselectivities, attributed to several transition metal catalyzed homogeneous reactions, and the recyclability of ILs made these systems attractive towards green chemistry. Several recent general reviews on the efficacy of ionic liquids demonstrate their increasing role also in the field of electrochemistry [7,8], conventional organic [9][10][11][12], organometallic synthesis [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] and heterogeneous catalysis [28][29][30]. Interestingly, a steadily increasing importance of nanoparticle catalysis in ILs has also been observed.…”

Aggregation behaviour of chiral ionic liquids

“…Prominent examples are the Diels-Alder reaction, 7-9 the Baylis-Hillman reaction, [10][11][12] Michael additions, 13 and asymmetric hydrogenations. [14][15][16] CILs have also been used as effective organocatalysts. [17][18][19] …”