Synthesis and characterization of chiral ionic liquids based on quinine, L-proline and L-valine for enantiomeric recognition

Article abstract of DOI:10.1016/j.molliq.2019.03.084

The separation of enantiomers remains a major challenge for the pharmaceutical industry. In this work, eight chiral ionic liquids (CILs) directly derived from the ‘chiral pool’ were synthesized and characterized in order to develop enantioselective systems, for the chiral resolution. According to their chiral cations, three different groups of CILs were prepared, namely based on quinine, L-proline and L-valine, and their enantiomeric recognition ability evaluated. For that purpose the diastereomeric interactions between a racemic mixture of Mosher's acid sodium salt and each CIL were studied using ¹⁹F NMR spectroscopy. The remarkable chemical shift dispersion induced by some CILs demonstrates their potential application in chiral resolution. Additionally the optical rotation, thermophysical properties and ecotoxicity against the marine bacteria Aliivibrio fischeri of these chiral ionic liquids were addressed.

Full text of DOI:10.1016/j.molliq.2019.03.084

Accepted Manuscript
Synthesis and characterization of chiral ionic liquids based on
quinine, l-proline and l-valine for enantiomeric recognition
Tânia E. Sintra, Mikhail G. Gantman, Sónia P.M. Ventura, João
A.P. Coutinho, Peter Wasserscheid, Peter S. Schulz
PII:
S0167-7322(18)36299-8
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2019.03.084
MOLLIQ 10625
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
3 December 2018
5 February 2019
Please cite this article as: T.E. Sintra, M.G. Gantman, S.P.M. Ventura, et al., Synthesis
and characterization of chiral ionic liquids based on quinine, l-proline and l-valine for
enantiomeric recognition, Journal of Molecular Liquids, https://doi.org/10.1016/
j.molliq.2019.03.084
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ACCEPTED MANUSCRIPT
Synthesis and characterization of chiral ionic liquids based on quinine, L-
proline and L-valine for enantiomeric recognition
Tânia E. Sintra,[a] Mikhail G. Gantman,[b] Sꢀnia P. M. Ventura,[a] Joꢁo A. P. Coutinho,[a]
Peter Wasserscheid[b] and Peter S. Schulz*[b]
[a]
CICECO - Aveiro Institute of Materials, Department of Chemistry, University of Aveiro,
Campus Universitário de Santiago 3810-193 Aveiro, Portugal
[b]
Department of Chemical and Biological Engineering (CBI), Institute of Chemical
Reaction Engineering, University of Erlangen-Nuremberg, Egerlandstraße 3, 91058 Erlangen,
Germany
*Corresponding author: e-mail address: peter.schulz@fau.de
Abstract: The separation of enantiomers remains a major challenge for the pharmaceutical
industry. In this work, eight chiral ionic liquids (CILs) directly derived from the ‘chiral pool’
were synthesized and characterized in order to develop enantioselective systems, for the
chiral resolution. According to their chiral cations, three different groups of CILs were
prepared, namely based on quinine, L-proline and L-valine, and their enantiomeric
recognition ability evaluated. For that purpose the diastereomeric interactions between a
racemic mixture of Mosher’s acid sodium salt and each CIL were studied using 19F-NMR
spectroscopy. The remarkable chemical shift dispersion induced by some CILs demonstrates
their potential application in chiral resolution. Additionally the optical rotation,
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thermophysical properties and ecotoxicity against the marine bacteria Aliivibrio fischeri of
these chiral ionic liquids were addressed.
Keywords: chiral ionic liquids, enantioselectivity, chiral resolution, Mosher’s acid,
thermophysical properties, ecotoxicity.
1. Introduction
The differences in the pharmacological activities of enantiomers may result in serious
problems in the treatment of diseases using racemates.[1] Regulatory authorities therefore
require a complete pharmacological and toxicological characterization of each enantiomer
separately, even when the drug is commercialized as a racemate.[2,3] Furthermore, the
commercialization of the therapeutically active isomer should be prioritized. In order to get
single enantiomer drugs, there are three approaches to produce them: chiral pool synthesis
(limited by the availability of precursors from natural sources), asymmetric synthesis
(requiring specific expensive catalysts) and chiral resolution. Taking into consideration the
chiral resolution, crystallization is still one of the most used technique, due to its simplicity of
operation and cost‐efficiency.[4] Nevertheless, the direct crystallization of an enantiomer
from a racemic solution is only achievable when the enantiomers in their mixtures form
separate but pure crystals, which only represent 5 to 10ꢂ% of the racemates. On the other hand,
enantioselective chromatographic methods have been widely used for resolving racemates.
However, large-scale chromatographic processes are expensive and require careful design
and optimization.[4] The enantioselective liquid-liquid extraction has been considered as an
attractive technology to get isolated enantiomers from racemic mixtures in a continuous mode,
being easy to scale-up, to use in continuous operation, and of low cost.[5] The chiral selector
plays a key role in this extraction process, and the chiral recognition mechanism follows the
“three-point rule” by which chiral recognition requires a minimum of three simultaneous
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interactions between the chiral selector and the enantiomers, at least one of these interactions
being stereochemically dependent.[6] The most used chiral selectors are cyclodextrin
derivatives, tartrate derivatives, crown ethers and metal complexes.[5]
Ionic liquids (ILs) are a class of solvents that, due to their unique properties, have been
proposed in the past few years as alternatives to volatile organic compounds for many
applications.[7,8] Chiral ionic liquids (CILs) are a subclass of ILs with a chiral moiety at the
cation, anion or both. The first CIL reported was the 1-butyl-3-methylimidazolium lactate, by
Seddon and co-workers.[9] Afterwards, the synthesis of imidazolium-CILs based on chiral
amines (D-α-phenylethylamine) or amino acids (L-alanine, L-leucine, and L-valine) was
described by Bao.[10] Since then many examples of CILs were reported and explored for
various applications, namely in separation processes where CILs have been used as chiral
selectors, background electrolyte additives, chiral ligands and chiral stationary phases in
chromatographic and electrophoretic techniques.[11–17] Some of the CILs known for their
chiral recognition ability have been used as NMR chiral shift reagents for the discrimination
of racemates.[18–24] Recently, chiral imidazolium-based ILs naturally derived from carvone[25]
and D-xylose[26] have demonstrated excellent enantioselective discrimination of the racemic
Mosher's acid salt. In 2015, aqueous biphasic systems based on CILs and salts were proposed
for the enantiomeric separation of amino acids.[27,28] In this work, CIL is not only a
constituent of the biphasic system as well as the chiral selector.[27,28] These preliminary
results show still limited enantioselectivities.
In this work we prepared eight novel CILs for chiral resolution directly derived from the
‘chiral pool’. The diastereomeric interactions between a racemic mixture of Mosher’s acid
19
sodium salt and each CIL were studied using F-NMR spectroscopy and additionally they
were characterized regarding their optical rotation, thermophysical properties and ecotoxicity
against the marine bacteria Aliivibrio fischeri (A. fischeri).
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2. Experimental section
2.1. Materials. Eight CILs based on chiral selector were synthesized, namely
[C1Qui]I, 1-methyl quininium iodide; [C1Qui][C1SO4], 1-methyl quininium methylsulfate;
[C1Qui][NTf2], 1-methyl quininium bis(trifluoromethylsulfonyl)imide; [C1C1C1Pro]I, N,N-
dimethyl-L-proline methyl ester iodide; [C1C1C1Pro][C1SO4], N,N-dimethyl-L-proline methyl
ester methylsulfate; [C2C2C2Pro]Br, N,N-diethyl-L-proline ethyl ester bromide;
[C1,C1,C1Val]I N,N,N-trimethyl-L-valinolium iodide and [C1,C1,C1Val][C1SO4], N,N,N-
trimethyl-L-valinolium methylsulfate. Quinine (98 wt % of purity), iodomethane (99 wt % of
purity), dimethyl sulfate (99 wt % of purity), bis(trifluoromethane)sulfonimide lithium salt
(99 wt % of purity), dichloromethane anhydrous (99.8 wt % of purity), ethanol (99.8 wt % of
purity), acetone (HPLC grade), potassium carbonate (99 wt % of purity), L-proline (99 wt %
of purity), bromoethane (98 wt % of purity), acetonitrile (99.8 wt % of purity), chloroform
(99 wt % of purity), L-valine (98 wt % of purity), tetrahydrofuran anhydrous (99.9 wt % of
purity), sodium borohydride (99 wt % of purity), sulfuric acid (99.9 wt% of purity), methanol
(99 wt % of purity), ethyl acetate (99.8 wt % of purity), potassium hydroxide (90 wt % of
purity), formic acid (98 wt % of purity), formaldehyde (37 wt % in water solution),
hydrochloric acid (37 wt % in water solution) and Mosher’s acid (97 wt % of purity) were
acquired from Sigma-Aldrich®.
2.2. Synthesis and Characterization of CILs. Considering the chiral cation, three
different groups of CILs were prepared: (I) quinine-, (II) L-proline- and (III) L-valine-based
CILs (Scheme 1, 2 and 3). The water content of the CILs was determined by coulometric
Karl Fischer titration and was verified to be less than 0.1wt% in all samples. The chemical
structures and acronym of all CILs synthesized are depicted in Figure 1. The structure of all
1
compounds synthesized was confirmed by H and 13C NMR spectroscopy, and when
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1
1
appropriate, by the 2D H-13C HSQC and H-1H Cosy NMR sequences, showing a high
purity level of all the ionic structures after their synthesis, as reported in the Supporting
Information (Figures S1-S19).
2.2.1. Quinine-based CILs
1-Methyl quininium iodide, [C1Qui]I, was prepared by the dropwise addition of 1.4 mL of
iodomethane (22.3 mmol), in a dichloromethane solution, to a solution of quinine (6.9 g, 21.3
mmol) in dichloromethane, at 0 C and under inert atmosphere. The reaction mixture was
stirred overnight at room temperature, under inert atmosphere. The obtained solid was
filtrated, washed with acetone, and recrystallized in ethanol. Finally, the residual solvent was
removed under reduced pressure and the obtained compound was dried under vacuum (10-2
mbar, 313 K) for at least 48 h, affording [C1Qui]I as a pale yellow solid (6.7 g, 68% yield). 1-
Methyl quininium methylsulfate, [C1Qui][C1SO4], was obtained in a very similar manner as
[C1Qui]I, using the dimethyl sulfate as the alkylating agent. [C1Qui][C1SO4] was obtained as
a white solid (66% yield). 1-Methyl quininium bis(trifluoromethylsulfonyl)imide,
[C1Qui][NTf2], was prepared by adding an aqueous solution of 8.0 g (17.7 mmol)
[C1Qui][C1SO4] to an aqueous solution of 5.3 g (18.6 mmol) Li[NTf2] leading to the
precipitation of [C1Qui][NTf2]. The final IL was washed three times with 40 mL water.
Finally, the residual water was removed under vacuum for at least 48 h, affording
[C1Qui][NTf2] as a white solid (8.7 g, 79% yield).
2.2.2. L-Proline-based CILs
In order to prepare N,N-dimethyl-L-proline methyl ester iodide, [C1C1C1Pro]I, potassium
carbonate (14.4 g, 104.2 mmol) was added into the mixture of L-proline (12.0 g, 104.2 mmol)
and acetonitrile (150 ml). After stirring the mixture for 1h at room temperature, 20 mL of
iodomethane (321.0 mmol) was added dropwise at 0 C, under inert atmosphere. The reaction
mixture was stirred overnight at room temperature, under inert atmosphere. Then, the solid
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was filtered off, and the resulting liquid was concentrated under reduced pressure. The light
yellow solid crude was washed with chloroform, filtered, and the liquid phase concentrated
under reduced pressure, obtaining a viscous yellow oil. The obtained oil was solubilized and
crystallized in ethanol. Finally, the residual solvent was removed under vacuum for at least
48 h, affording [C1C1C1Pro]I as a white solid (4.4 g, 15% yield). To prepare N,N-dimethyl-L-
proline methyl ester methylsulfate, [C1C1C1Pro][C1SO4], potassium carbonate (4.8 g, 34.7
mmol) was added to the mixture of L-proline (4.0 g, 34.7 mmol) and acetonitrile (70 ml).
After stirring the mixture for 1h at room temperature, 10 mL of dimethyl sulfate (106.0
mmol) were added dropwise at 0 C, under inert atmosphere. The reaction mixture was
stirred overnight at room temperature, under inert atmosphere. Then, the solid was filtered off,
and the resulting liquid was concentrated under reduced pressure. Then, the obtained pale
yellow liquid was washed with ethyl acetate (3 x 10 mL). Finally, the residual ethyl acetate
was removed under reduced pressure, followed by high vacuum for at least 48 h, affording
[C1C1C1Pro][C1SO4] as pale yellow liquid (7.2 g, 76% yield). In order to obtain N,N-diethyl-
L-proline ethyl ester bromide, [C2C2C2Pro]Br, potassium carbonate (4.8 g, 34.7 mmol) was
added into the mixture of L-proline (4.0 g, 34.7 mmol) and acetonitrile (70 ml). After stirring
the mixture for 1h at room temperature, 8 mL of bromoethane (107.7 mmol) was added
dropwise at 0C, under inert atmosphere. The reaction mixture was stirred at 70 °C for 2 days
in an inert atmosphere. After filtering, the resulting liquid was concentrated under reduced
pressure. Then, the obtained pale yellow solid was washed with ethyl acetate (3 x 10 mL).
Finally, the residual ethyl acetate was removed under reduced pressure, followed by high
vacuum for at least 48 h, affording [C2C2C2Pro]Br as a white solid (3.3 g, 34% yield).
2.2.3. L-Valine-based CILs
L-Valinol, L-2-amino-3-methyl-1-butanol, was obtained by reduction of L-valine, as
described in literature.[29] L-Valine (31.0 g, 264.6 mmol) was added to a stirred suspension of
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sodium borohydride (25.0 g, 661.5 mmol) in tetrahydrofuran (250 mL). The flask was
immersed in an ice-water bath, and a solution of concentrated sulfuric acid (17.5 mL, 330.8
mmol) in ether was added dropwise at such a rate as to maintain the reaction mixture below
20 C (addition time approximately 3h). The reaction mixture was stirred at room
temperature overnight, and then 50 mL of methanol were added carefully to destroy the BH3
in excess. The mixture was concentrated to c.a. 100 mL and 5 N of potassium hydroxide (250
mL) was added. After removing the tetrahydrofuran and methanol under reduced pressure,
the mixture was heated at reflux for 3 h (110 C). The turbid aqueous mixture was cooled and
filtered. The filtrate was diluted with additional water (50 mL). The dichloromethane
extraction (4 x 250 mL) followed by evaporation of the solvent left a yellow liquid, which
was distilled to yield 15.5 g (57%) of a colourless solid. The N,N-dimethylvalinol was
synthesized by reductive alkylation of primary amine of L-valinol using the well-known
Eschweiler-Clark reaction.[30,31] For that, 30 mL of formic acid (795.0 mmol) were slowly
added to an aqueous solution of L-valinol (15.5 g, 150.3 mmol) at 0 C. The formaldehyde
solution (35 mL of 37% wt in a water solution, 470.1 mmol) was added to the resulting
solution. The flask was connected to a reflux condenser and heated to 95 C. A vigorous
evolution of CO2 begins after 2-3 minutes, at which time the flask was removed from the oil
bath until the gas evolution notably subsides (15 – 20 min) and then heated at 100 C
overnight. After the solution has been cooled, 70 mL of 4 N hydrochloric acid was added and
the solution evaporated to dryness under reduced pressure. The pale yellow liquid was
dissolved in 30 mL of water, and the organic base was liberated by the addition of 70 mL of 9
N of potassium hydroxide. The upper organic phase was separated, and the aqueous phase
(lower phase) was extracted with dichloromethane (3 x 50 mL). The organic base and the
dichloromethane extracts were combined, followed by evaporation of the solvent that left a
pale yellow liquid, which was distilled to yield 12.8 g (65%) of a colourless liquid. N,N,N-
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Trimethyl-L-valinolium iodide, [C1C1C1Val]I,[31] was prepared by the dropwise addition of
3.0 mL of iodomethane (48.0 mmol), in a dichloromethane solution, to a solution of N,N-
dimethylvalinol (6.0 g, 45.7 mmol) in dichloromethane, at 0 C and under inert atmosphere.
The reaction mixture was stirred overnight at room temperature, under inert atmosphere. The
obtained solid was filtered and the residual solvent removed under high vacuum for at least
48 h, affording [C1C1C1Val]I as white solid (11.3 g, 90% yield). N,N,N-Trimethyl-L-
valinolium methylsulfate, [C1C1C1Val][C1SO4], was prepared by dropwise addition of 4.0
mL of dimethyl sulfate (43.2 mmol), in a dichloromethane solution, to a solution of N,N-
dimethylvalinol (5.4 g, 41.1 mmol) in dichloromethane, at 0 C and under inert atmosphere.
The reaction mixture was stirred overnight at room temperature, under inert atmosphere.
Dichloromethane was removed under reduced pressure and the obtained colorless liquid was
dried under high vacuum for at least 48 h, affording [C1C1C1Val][C1SO4] as colorless liquid
(9.3 g, 88% yield).
2.3. Thermogravimetric Analysis. The onset temperature of decomposition was
determined by TGA. TGA was conducted on a Setsys Evolution 1750 (SETARAM)
instrument. The sample was heated in an alumina crucible, under nitrogen flow, over a
temperature range of 300−1000 K, and with a heating rate of 5 K∙min−1.
2.4. Differential Scanning Calorimetry. The melting temperatures were measured
using a differential scanning calorimetry (DSC), Hitachi DSC7000X model, working at
atmosphere pressure. The equipment was previously calibrated with several standards with
weight fraction purities higher than 99%. Each sample (5 mg) was submitted to three cycles
of cooling and heating at 2 K∙min-1. The thermal transitions temperatures were taken as the
peak temperature. The temperature uncertainty calculated through the average of the standard
deviation of several consecutive measurements was better than ± 0.1 K.
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2.5. Optical rotation. The optical rotation of the synthesized CILs was carried out
at 589 nm using a polarimeter JASCO P-2000 and a cylindrical glass cell CG3-100 3.5 x 100
mm (1mL), at 298 K.
2.6. Microtox Assay. To evaluate the ecotoxicity of the CILs synthesized, the
Standard Microtox liquid-phase assay was applied. Microtox is a bioluminescence inhibition
method based on the bacterium A. fischeri (strain NRRL B-11177) luminescence after its
exposure to each sample solution at 288.15 K. In this work, the standard 81.9% test protocol
was followed.[32] The microorganism was exposed to a range of diluted aqueous solutions of
each compound (from 0 to 81.9 wt %), where 100% corresponds to a previously prepared
stock solution, with a known concentration. After 5, 15, and 30 min of exposure to each
aqueous solution, the bioluminescence emission of A. fischeri was measured and compared
with the bioluminescence emission of a blank control sample. Thus, the corresponding 5, 15,
and 30 min EC50 values (EC50 being the estimated concentration yielding a 50% of inhibition
effect), plus the corresponding 95% confidence intervals, were estimated for each compound
tested by nonlinear regression, using the least-squares method to fit the data to the logistic
equation.
2.7. Chiral discrimination ability. After the successful synthesis of the target CILs,
the chiral discrimination ability was evaluated by studying the diastereomeric interaction
between each CIL and the racemic Mosher's acid sodium salt. For that, each CIL was
dissolved in 0.5 mL CD2Cl2 to obtain a 0.5 M solution and stirred with 0.2 mL of a saturated
aqueous solution of racemic Mosher's acid sodium salt (2.0 M). After stirring, the mixture
was allowed to equilibrate overnight at room temperature, aiming at the complete separation
of the coexisting phases. At this point, the lower organic rich-phase was carefully separated
19
and analyzed using F NMR spectroscopy. Duplicate measurements were carried out. The
differentiation of the diastereomers was easily visible via the different shift of the signal of
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the CF3 group in Mosher’s acid carboxylate. Racemic Mosher’s acid sodium salt was
synthesized by stirring racemic Mosher’s acid (1.0 g, 4.3 mmol) with an equimolar amount of
NaOH (170.8 mg, 4.3 mmol) in 50 ml H2O for 1 h and subsequent evaporation of the solvent.
3. Results and Discussion
In this work, a series of CILs naturally derived from chiral compounds, namely quinine, L-
proline and L-valine, were prepared and characterized. Their acronym and chemical structure
are depicted in Figure 1. All CILs were obtained with high purity levels and yield, cf. the
Experimental Section.
The adoption by the industry of renewable natural sources as starting materials has become a
topic of increasing importance. Due to its recognized application as chiral selector,[33–38]
quinine, a natural alkaloid, was selected to incorporate three CILs here studied. The synthesis
of [C1Qui]I was already described in literature;[39] however to the best of our knowledge,
[C1Qui][C1SO4] and [C1Qui][NTf2] have not been reported hitherto. Natural amino acids and
their derivatives provide the most abundant renewable natural chiral pool, and can form an
efficient, practical, and facile precursor for the preparation of chiral compounds. Since amino
acids have both a carboxylic acid residue and an amino group in a single molecule, they can
be used as either anions or cations. In the present work, three L-proline- and two L-valine-
based CILs were prepared and characterized. L-Proline is a representative amino acid with
the chiral centre in the pyrrole ring, so the CILs derived from L-proline cannot be racemized
easily.[21] While the synthesis of [C1C1C1Pro]I and [C2C2C2Pro]Br reported in literature
comprises two steps, esterification and N-alkylation,[21] the here proposed synthetic route is
simpler, being a one-pot synthesis. The here reported L-valine-based ILs were readily
obtained from this branched-chain amino acid in a three step synthesis: reduction of L-valine,
Eschweiler-Clark reaction, followed by N-alkylation.[40] Zhao et al.[41] have already reported
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the preparation of [C1C1C1Val]I by using a similar procedure. To the best of our knowledge,
the synthesis of [C1C1C1Pro][C1SO4] and [C1C1C1Val][C1SO4] have not been reported
hitherto.
For all the obtained CILs their optical rotation, melting and decomposition temperatures, and
ecotoxicity were estimated. The specific rotations,[]2퐷5, of the CILs and respective chiral
precursor are indicated in Table S1. While L-proline-based CILs present optical rotations
with lower magnitude than L-proline, quinine and L-valine based CILs show higher optical
activity when compared with respective precursors.
Decomposition and melting temperatures of the CILs were measured by TGA and DSC,
respectively, and are presented in Table 1. From the TGA profiles depicted in the Supporting
Information (Figure S20), as well as from the Td values, it is possible to conclude that all
CILs studied present a high thermal stability, at least up to 445 K. Nevertheless,
[C1C1C1Val]I decomposes immediately after its melting temperature is reached.
Table 1. Temperature of fusion (Tfus) and temperature of decomposition (Td) of the obtained
CILs.
CIL
Tfus / K
497.55
Td / K
510.23
[C1Qui]I
[C1Qui][C1SO4]
[C1Qui][NTf2]
[C1C1C1Pro]I
[C1C1C1Pro][C1SO4]
[C2C2C2Pro]Bra
[C1C1C1Val]Ib
[C1C1C1Val][C1SO4]
Quinine
464.25
404.31
377.00
230.38
376.00
495.25
321.76
448.15c
501.15c
568.15c
b
514.08
525.48
491.09
548.19
451.07
503.99
485.65
L-Proline
L-Valine
a Solid-solid transition temperature = 349.89 K. Solid-solid transition temperature = 379.19
K. c ChemSpider database (http://www.chemspider.com; at October 11th, 2017).
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The ecotoxicological impact of the CILs studied was evaluated using the standard Microtox
acute assay. Althougth ILs have been considered as an innovative approach to sustainable
chemistry, their solubility in water allows their easy access to the aquatic compartment,
which makes them potentially hazardous compounds to aquatic organisms.[42] The standard
assay using the luminescent marine bacteria A. fischeri, the Microtox® bioassay, is today one
of the most widespread toxicological bioassays due to its quick response, simplicity and cost-
effective implementation. Indeed, this assay has been widely used to evaluate the toxicity of
various ionic liquid families.[43–47] Thus, EC50 values (mg∙L-1), the estimated concentration
yielding a 50% of luminescence inhibition of the bacteria A. fischeri, were determined for
each CIL after 5, 15, and 30 min of exposure to the marine bacteria, and reported in the
Supporting Information, Table S2. In general, the exposure time had little or no impact on the
ecotoxicity of the studied compounds. In order to contemplate the entire toxic effect, only the
EC50 values obtained after 30 min of exposure were considered for further discussion. With
the exception of quinine-based CILs which belongs to the category “acute 3” (10 mg∙L-1 <
EC50 < 100 mg∙L-1), the remaining CILs can be classified as non-hazardous substances (EC50
> 100 mg∙L-1), according to the European Legislation for the aquatic ecosystems.[48]
According to Passino’s classification,[49] the CILs here investigated can be categorized as:
“moderately toxic” (quinine-based CILs, with 10 mg∙L-1 < EC50 < 100 mg∙L-1), “practically
harmless” ([C1C1C1Pro][C1SO4], with 100 mg∙L-1 < EC50 < 1000 mg∙L-1) and “harmless” (L-
valine-based CILs, [C1C1C1Pro]I and [C2C2C2Pro]Br, with EC50 > 1000 mg∙L-1). Figure 2
shows the EC50 data of the CILs in mM, being possible to rank their ecotoxicity according to
the following tendency (30 min of exposure): [C1C1C1Val]I < [C1C1C1Pro]I <
[C1C1C1Val][C1SO4] < [C2C2C2Pro]Br < [C1C1C1Pro][C1SO4] < [C1Qui]I < [C1Qui][C1SO4]
< [C1Qui][NTf2]. Considering the anion impact, the results obtained suggest that the
replacement of an iodide by a methylsulfate anion increases the toxicity of the CILs against A.
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fischeri. Additionally, their toxicity increases with the chiral cation nature, following the
trend: [C1C1C1Val]+ < [C1C1C1Pro]+ < [C1Qui]+. This trend may be related with the
enhancement of the ILs hydrophobic/lipophilic character, defined in several works by the
octanol-water partition coefficients (Kow),[50,51] which is in accordance with the log Kow of
their precursors (-0.01, 0.05 and 2.51 for L-valinol, L-proline methyl ester and quinine,
respectively).[52]
After their synthesis and characterization, the enantiomeric recognition ability of CILs was
evaluated. For that, the diastereomeric interactions between a racemic mixture of Mosher’s
19
acid sodium salt and each CILs were studied using F-NMR spectroscopy. The split of the
signal related to the CF3-group of the racemic Mosher's acid sodium salt indicates the chiral
discrimination properties of CILs. The chemical shift difference of the CF3 signals of racemic
Mosher's acid sodium salt in presence of CILs are summarized in Table 2. As presented in
Figure 3, quinine- and L-valine-based CILs and [C1C1C1Pro]I show good splitting of the CF3
signal of racemic Mosher's acid sodium salt, while in the case of [C1C1C1Pro][C1SO4] and
[C2C2C2Pro]Br no splitting was observed. Considering the quinine-based CILs, the split of
the signal related to the CF3-group of the racemic Mosher's acid sodium salt increases with
the anion nature, following the trend: I- < [C1SO4]- < [NTf2]-. This trend may be related with
the enhancement of the counter anion hydrophobic/lipophilic character, which is in
agreement with previous data.[24] The same trend was observed for the L-valine-based CILs.
The remarkable chemical shift dispersion induced by some of the CILs demonstrates their
potential application in chiral resolution.
Table 2. Chemical shift difference of the CF3 signals of racemic Mosher's acid sodium salt in
the presence of each CIL under study.
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R/S ± σ / Hz
CIL
[C1Qui]I
21.0 ± 1.6
34.5 ± 1.8
54.8 ± 0.6
7.8 ± 1.2
NS
NS
8.4 ± 0.5
10.3 ± 0.2
[C1Qui][C1SO4]
[C1Qui][NTf2]
[C1C1C1Pro]I
[C1C1C1Pro][C1SO4]
[C2C2C2Pro]Br
[C1C1C1Val]I
[C1C1C1Val][C1SO4]
NS - no splitting observed for the CF3 signal.
4. Conclusion
Herein eight CILs were efficiently synthesized from naturally occurring enantiopure
compounds (L-valine, L-proline and quinine) and characterized regarding their optical
rotation, thermophysical properties and ecotoxicity against the marine bacteria A. fischeri. All
CILs synthesized exhibit a high thermal stability, at least up to 445 K. Furthermore, with the
exception of quinine-based CILs, they present low ecotoxicity being in general considered as
“practically harmless” or even “harmless” for the bacteria analyzed. Among all the CILs
investigated, quinine-based compounds were particularly promising for the future use in
enantiomeric separations, as outstanding shift differences were observed in case of Mosher’s
acid carboxylate as a racemic probe.
Acknowledgements
This work was developed within the scope of the project CICECO-Aveiro Institute of
Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID /CTM /50011/2013), financed by
national funds through the FCT/MEC and when appropriate co-financed by FEDER under the
PT2020 Partnership Agreement. The authors acknowledge the FCT funding through the
projects “Multi-purpose strategies for broadband agro-forest and fisheries by-products
valorization: a step forward for a truly integrated biorefinery (POCI-01-0145-FEDER
016403) and “ChiResus-Chiral resolution of drugs in centrifugal partition chromatography
14

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using enantioselective aqueous biphasic systems (POCI-01-0145-FEDER-030750). T.E.
Sintra acknowledges COST Action CM1206 (EXIL – Exchange on Ionic Liquids) for the
STSM grant (COST-STSM-CM1206-34926). S.P.M. Ventura acknowledges FCT/MEC for a
contract under Investigador FCT 2015 contract number IF/00402/2015.
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Graphical Abstract
Highlights
Chiral ionic liquids derived from valine, proline and quinine are successfully prepared.
In general, these chiral compounds exhibit high thermal stability and low ecotoxicity.
Outstanding shift difference of racemic Mosher’s acid carboxylate are observed.
Quinine-based compounds seem to be particularly promising for chiral resolution.
Schemes and Figures
Scheme 1. Synthesis scheme followed to prepare the quinine-based CILs.
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Scheme 2. Synthesis scheme followed to prepare the L-proline-based CILs.
Scheme 3. Synthesis scheme followed to prepare the L-valine-based CILs.
Figure 1. Chemical structures and acronym of all ILs with chiral cation here synthesized.
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Figure 2. EC50 values (mM) determined after 5, 15, and 30 minutes of V. fischeri exposure.
The error bars correspond to 95% confidence level limits.
19
Figure 3. The partial F NMR spectra of CILs under study and the racemic Mosher's acid
salt complex.
22