Ionic liquids increase the solubility of various coffee ingredients in aqueous solution but little is known about the underlying mechanism. Kirkwood-Buff integrals as well as the potential of mean force indicate that the imidazolium cations are accumulated at the surface of the solutes, removing water molecules from the solute surface. Although hydrogen bonding of the anions to hydroxy groups of the solutes can be detected, their concentration at the surface is less enhanced compared to the cations. The decomposition into solvation shells by Voronoi tessellation reveals that structural features are only observed in the first solvation shell. Nevertheless, the depletion of water and the excess concentration of the ions and, in particular, of the cations are visible in the next solvation shells as well. Therefore, classical arguments of hydrotropic theory fail to explain this behavior.
We investigate the solvation behavior of valuable coffee ingredients in aqueous mixtures of the ionic liquid 1-ethyl-3-methylimidazolium acetate with a particular emphasis on hydrotropic theory and Kirkwood–Buff analysis.
We
report the design and synthesis of novel ion-tagged chiral ligands
for asymmetric transfer hydrogenation (ATH) in aqueous medium. Based
on (R,R)-1,2-diphenylethylene diamine
(DPEN) as structural motif, a straightforward three-step protocol
was developed that gave access to novel chiral ligands with carbamate-substructure
and pyridinium headgroup. The careful optimization of steric and electronic
properties in combination with the adaption of solubility via choice
of the anion gave a set of chiral and water-soluble ligands for use
in ruthenium-catalyzed asymmetric transfer hydrogenations in aqueous
medium. Eventually, a pool of aliphatic and aromatic ketones as well
as two imine substrates were reduced with excellent isolated yields
up to 95% and enantioselectivities >90% ee under environmentally
benign
conditions in the absence of additional surfactants.
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