Organic Letters
Letter
believe that this work may open the way for the use of chiral
ionic liquids in enantioseparation.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
Figure 5. Disassembly of (RCIL, Ra)-12 and the recycle of (R)-CIL-3.
Experimental details, characterization of new com-
pounds, and NMR spectra (PDF)
Due to the insolubility of the precipitate in solvents, FTIR
spectra, DFT calculations (at the B3LYP/3-21G* level using
the Gaussian 03 program package16), and 1H NMR spectra are
used to establish possible structures of the coordination
complex. From the FTIR spectrum of (RCIL,Ra)-13, CIL-3,
and rac-9 (Figure S2), the stretching vibration of the carbamido
group red shifts from 1667 to 1558 cm−1. The stretching
vibration of CN also red shifts. The stretching vibration of
C−O blue shifts from 1132 to 1205 cm−1. It indicates the
coordination bond among −OH and ureido of (R)-CIL-3 and
CN and −COOH of serine.
AUTHOR INFORMATION
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Corresponding Author
ORCID
Notes
The authors declare no competing financial interest.
As shown in Figure 6, we display energy-optimized structures
of (RCIL,Ra)-13−H2O* and (RCIL,Sa)-13−H2O*. In (RCIL,Ra)-
ACKNOWLEDGMENTS
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We are grateful to the National Natural Science Foundation of
China (Grant Nos. 21327010 and 81230080) for its financial
support.
REFERENCES
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Figure 6. Energy-optimized structures of (a) (RCIL,Ra)-13−H2O* and
(b) (RCIL,Sa)-13−H2O* obtained by DFT calculations. Dotted lines
indicate hydrogen bonds.
13−H2O*, multiple hydrogen bonds around the hydrone and
−OH belonging to (R)-CIL-3 and serine promote stability of
the coordinative complex. In contrast, the intermolecular
hydrogen bond between −NH of ureido and −OH of Schiff
base product restricts the freedom of the CN bond. It could
bring enhanced steric hindrance between the indole ring and
pyridinium cation, especially in (RCIL,Sa)-13−H2O*. As a result,
the bond distance of coordination bonds formed with Cu(II)
and CN bond is much different (1.322 vs 1.554 Å).
Calculations predict that the coordination bond of (RCIL,Ra)-
13−H2O* is more stable than that of (RCIL,Sa)-13−H2O* by
7.3 kcal/mol. Furthermore, optimized structures of (RCIL,Sa)-14
and (RCIL,Ra)-14 are also obtained by DFT calculations (Figure
S3). Unfortunately, DFT calculations do not perform
reasonable energy-optimized structure of (RCIL,Ra)-12. Alter-
natively, as shown in Figure S4B, partial 1H NMR signals of the
pyridinium cation moiety up-shift 0.18 ppm once the the
product is formed. It verified the electrostatic interaction
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To conclude, this work demonstrates an approach to an
enantioselective precipitate via multicomponent self-assemblies.
Namely, the product of Schiff base coordinated with the novel
chiral ionic liquid in the presence of metal ions. (R)-CIL-3
shows high enantioselectivities and a wide scope toward
amines, amino alcohols, and amino acids. The whole process
could be realized even on a large scale (5 g). Above all, we
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