Using enantioselective dispersive liquid–liquid microextraction for the microseparation of trans-cyclohexane-1,2-diamine enantiomers

Article abstract of DOI:10.1016/j.tetasy.2017.02.002

A new chiral separation system effective for the enantioselective extraction of racemic trans-cyclohexane-1,2-diamine is presented. Enantioselective dispersive liquid–liquid microextraction has been used for the chiral microseparation of trans-cyclohexane-1,2-diamine, with a chiral azophenolic crown ether being identified as a versatile chiral selector. The influence of various process conditions on the extraction performance was studied experimentally. It was found that the operational selectivity in one extraction step is mainly related to the type and volume of the solvents, chiral selector concentration, extraction time, temperature of sample solution, and pH. At optimum conditions (300?μL of diethyl ether as the extraction solvent 1?mL of methanol as the disperser solvent, with 5?mmol?L^?1 chiral selector concentration, pH of the sample equal to 4.5, 30?min extraction time and a temperature of 10?°C), the distribution ratio of (R,R)- and (S,S)-trans-cyclohexane-1,2-diamine was 18.3 and 1.8, respectively, while the enantioselectivity value of 10.2 was found at the optimum condition.

Full text of DOI:10.1016/j.tetasy.2017.02.002

Tetrahedron: Asymmetry 28 (2017) 454–459
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Using enantioselective dispersive liquid–liquid microextraction for
the microseparation of trans-cyclohexane-1,2-diamine enantiomers
⇑
Majid Hashemi, Mohammad Reza Hadjmohammadi
Department of Chemistry, University of Mazandaran, Babolsar, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
A new chiral separation system effective for the enantioselective extraction of racemic trans-cyclohex-
ane-1,2-diamine is presented. Enantioselective dispersive liquid–liquid microextraction has been used
for the chiral microseparation of trans-cyclohexane-1,2-diamine, with a chiral azophenolic crown ether
being identified as a versatile chiral selector. The influence of various process conditions on the extraction
performance was studied experimentally. It was found that the operational selectivity in one extraction
step is mainly related to the type and volume of the solvents, chiral selector concentration, extraction
time, temperature of sample solution, and pH. At optimum conditions (3001lL of diethyl ether as the
Received 26 October 2016
Revised 9 January 2017
Accepted 21 January 2017
Available online 8 March 2017
ꢀ
extraction solvent 1 mL of methanol as the disperser solvent, with 5 mmol L chiral selector concentra-
tion, pH of the sample equal to 4.5, 30 min extraction time and a temperature of 10 °C), the distribution
ratio of (R,R)- and (S,S)-trans-cyclohexane-1,2-diamine was 18.3 and 1.8, respectively, while the enantios-
electivity value of 10.2 was found at the optimum condition.
Ó 2017 Elsevier Ltd. All rights reserved.
6
1
. Introduction
non-chromatographic techniques such as polarimetry, nuclear
7
–9
10,11
12
magnetic resonance,
isotopic dilution,
calorimetry, and
1
3
Enantiomerically pure substances play an important role in the
enzyme techniques. The disadvantages of these techniques are
the need for pure samples, and no separation of enantiomers is
involved. Quantification, which does not require pure samples,
and separation of enantiomers can be done simultaneously by
or high performance liquid
chromatography (HPLC). Current chiral HPLC methods are
either direct, utilizing chiral stationary phases and chiral addi-
pharmaceutical and fine chemical industry, as raw materials, inter-
mediates and end products. The importance of chirality with
respect to biological activity has been clearly recognized. An
1
1
3–15
important property of biological systems is their ability to differen-
tiate between enantiomers. The interactions of enantiomers with
organisms are diastereomeric.2 Therefore, the enantiomers of
drugs, pesticides, or waste compounds are recognized by the
organism as different molecules that elicit different physiological
responses. This interest can be attributed largely to a heightened
awareness that enantiomers of a racemic drug may have different
pharmacological activities, as well as different pharmacokinetic
and pharmacodynamic effects.3 Chirality is a major concern in
the modern pharmaceutical industry. In the modern pharmaceuti-
cal industry, especially in anti-cancer drugs, sometimes only one
enantiomer has medicinal properties while the other enantiomer
does not, i.e., one enantiomer is useful for the body while the other
either gas chromatography (GC)
1
6,17
18
1
9
tives in the mobile phase, or indirect, which involves derivatiza-
2
0
tion of samples. Chiral separation is often required as the final
product purification step after asymmetric synthesis. The two main
industrial-scale separation methods each have serious drawbacks.
Diastereomeric crystallization is inflexible and involves solid phase
21
22
handling, while simulated moving bed chromatography is very
expensive and produces diluted product streams. Therefore, there
is a need for a flexible cost-effective separation method suitable
for commercial production scale. Extraction methods are good
methods to achieve this target. Chiral selectors play a very impor-
tant role in the separation efficiency of all enantiomer separation
4,5
one is harmful or has no effect on. Therefore, separation of a
racemic mixture is desirable, and hence there is a great need to
develop the technology for analysis and separation of racemic
drugs. Current methods of enantiomeric analysis include such
2
3
techniques. In the extraction methods, the enantioselective chiral
selector is generally present in the organic solvent because chiral
recognition tends to be more selective in
environment.
a
hydrophobic
The enantiomer that forms a complex most
2
4
strongly with the chiral selector is recovered from the
extract and the other enantiomer ends up in the raffinate. In
⇑
Corresponding author. Tel./fax: +98 1125342350.
E-mail address: hadjmr@umz.ac.ir (M.R. Hadjmohammadi).
http://dx.doi.org/10.1016/j.tetasy.2017.02.002
957-4166/Ó 2017 Elsevier Ltd. All rights reserved.
0

M. Hashemi, M. R. Hadjmohammadi / Tetrahedron: Asymmetry 28 (2017) 454–459
455
enantioselective liquid–liquid extraction, an enantiopure host is
2.2. Instrumentation and operating condition
The resolved diamine was derivatized as the m-toluoyl bisamide
based on the work of Walsh. Derivatization allows detection of the
enantiomers as they are eluted from the column by UV–vis spec-
troscopy (k = 254 nm). The derivatization of the racemic and
resolved diamine as the bisamides with m-toluoyl chloride is easily
accomplished under basic conditions. Chromatographic measure-
ments were carried out using a HPLC system equipped with a series
10-LC pump, UV detector model LC-95 set at 254 nm and model
used as a chiral selector to bind enantiospecifically and reversibly
with a racemic substrate.25 If the host is confined to one phase in a
biphasic system, enantiomeric separation of the substrate can take
place between the two phases in a single step. If the separation is
imperfect, a fractional extraction series is required.2 Dispersive
liquid–liquid microextraction is a mode of liquid–liquid extraction
3
2
6
2
7
on a smaller scale.
Dispersive liquid–liquid microextraction
employs a mixture of an extracting solvent and water miscible
polar disperser solvent. In dispersive liquid–liquid microextrac-
tion, after a rapid injection of an appropriate mixture, containing
the extracting and disperser solvents, into the aqueous sample, a
cloudy state is formed. The contact area between the extracting
solvent and the sample solution is very large. Thus the extraction
equilibrium is achieved rapidly. After centrifugation, the extracted
7725i manual injector with a 20
lL sample loop (Perkin–Elmer, Nor-
walk, CT, USA). Column used was Pirkle
L
-Leucine (250 ꢁ 4.6 mm)
and mobile phase was 90:10 hexane/isopropyl alcohol at a flow rate
of 1 mL/min. typical chromatogram of trans-cyclohexane-1,2-dia-
mine racemic mixture has shown in Fig. 2. The pH of the solutions
was measured by a 3030 Jenway pH meter (Leeds, UK).
2
8,29
phase settles at the bottom or top of the conical test tube.
If we
use a disperser solvent in the extraction process, extracting the sol-
vent can capture the target analyte in less time and higher recov-
ery. Therefore, we can use a chiral selector in dispersive liquid–
liquid microextraction for the extraction and separation of enan-
tiomers. In this method, the chiral selector is mixed with the
extracting and disperser solvents and then this mixture is injected
into the sample solution. Herein we have used enantioselective
dispersive liquid–liquid extraction with an azophenolic crown
ether (Fig. 1) as the chiral selector for the microseparation of
trans-cyclohexane-1,2-diamine enantiomers. trans-Cyclohexane-
2
.3. Enantioselective dispersive liquid–liquid microextraction
procedure
A 10 mL sample solution containing 1.0 mg/L of trans-cyclohex-
ane-1,2-diamine was placed in a centrifuge tube with narrow neck
ꢂ4 mm i.d.), which was specially designed for ease of removing
the supernatant phase. A mixture of 1 mL disperser solvent and
(
ꢀ1
3
00 lL extracting solvent with 2 mmol L of chiral selector was
rapidly injected into the sample solution using a 5.0 mL syringe,
and mixed by vortex mixer at 500 rpm stirring rate for 20 min,
so that a cloudy solution was formed. The cloudy solution was cen-
trifuged for 5 min at 3500 rpm, and the extraction product (super-
natant phase) was collected in the neck of the tube. Finally, this
supernatant phase was derivatized and injected into the HPLC.
All of the experiments were carried out in triplicate and the aver-
age of the result was reported.
1
,2-diamine is required for oxaliplatin (anti-cancer drug) synthe-
3
0
sis, where only the (R,R)-isomer is effective, meaning that separa-
tion of the two isomers before the synthesis is necessary.
Parameters affecting this process, such as selection type and vol-
ume of extracting and disperser solvents, chiral selector concentra-
tion, pH of sample solution, temperature and extraction time were
optimized.
2
.4. Calculation of distribution ratio and selectivity
O
O
O
O
O
The extent of extraction is characterized by the distribution
3
3,34
ratios D
R
and D
S
for each enantiomer
:
OH
½Rꢃorg; allforms
D
R
¼
¼
ð1Þ
ð2Þ
½
Rꢃaq; allforms
O N
2
½
sꢃorg; allforms
D
S
½
sꢃaq; allforms
N
N
NO2
The concentrations of the enantiomers in the organic phase
Figure 1. Azophenolic crown ether, the chiral selector used in this study.
were determined from HPLC peak area while the amount of residue
of trans-cyclohexane-1,2-diamine in the initial phase were found
by subtracting the initial amount from the amount of trans-cyclo-
hexane-1,2-diamine in the organic phase. The operational selectiv-
2
2
. Experimental
ity aop is defined by the ratio of these distribution ratios. Its upper
limit is the intrinsic selectivity
plexation constants:
aint which is the ratio of the com-
.1. General
trans-Cyclohexane-1,2-diamine and m-toluoyl chloride were
purchased from Sigma–Aldrich (Steinheim, Germany). HPLC grade
methanol, isopropyl alcohol, hexane, acetonitrile and acetone),
sodium hydroxide, hydrochloric acid and chloride were obtained
from Merck (Darmstadt, Germany). Xylene, diethyl ether, dode-
cane, toluene, octanol and cyclohexane were obtained from Aldrich
D
D
R
a
op
¼
¼
Assuming D
R
> D
S
ð3Þ
ð4Þ
S
(
K
K
R
S
a
int
(
Milwaukee, WI, USA). The azophenolic crown ether was synthe-
K
R
S
and K are complexation constants for the complexation
31
sised by Syncom BV (Groningen, The Netherlands). The water
used was double distilled deionized. A stock solution of trans-
cyclohexane-1,2-diamine (10.0 g/L) was prepared in methanol
and stored in the dark at 4 °C. Working standard solutions were
diluted with deionized double distilled water at concentration of
between the crown ether and trans-cyclohexane-1,2-diamine enan-
tiomers. As a result, even if one of the two enantiomers is much less
valuable than the other one, the enantiomeric purity of both enan-
tiomers should be high in their appropriate exit streams, to ensure a
good yield of the desired enantiomer. From calculations based on
3
5
1
.0 mg/L whenever needed.
the Kremser equation, it is clear that the operational selectivity

4
56
M. Hashemi, M. R. Hadjmohammadi / Tetrahedron: Asymmetry 28 (2017) 454–459
Figure 2. Typical HPLC chromatogram for trans-cyclohexane-1,2-diamine enantiomers, mobile phase: 90:10 hexane/isopropyl alcohol; flow rate: 1.0 mL/min; column: Pirkle
-Leucine (250 ꢁ 4.6 mm); k = 254 nm.
L
in one stage mainly determines the number of stages required for a
certain product purity: the larger the selectivity, the lower the num-
ber of stages required. The capacity of extraction (expressed as dis-
tribution ratio) influences the required solvent-to-feed ratio and
wash-to-feed ratio, and thus the productivity. This effect is reflected
in the extraction factors. In a fractional extractor, the extraction fac-
tors in the wash section are defined by (for enantiomer ‘R’ and ‘S’):
selection of an appropriate solvent is a major factor for the enan-
tioselective dispersive liquid–liquid extraction process. Herein we
used solvents lighter than water. The extracting solvent has to
meet two properties: it must have less density than water and
have low solubility in water. Hence, xylene, n-hexane, diethyl
ether, dodecane, toluene, octanol and cyclohexane were tested
for this purpose. In order to choose which disperser solvent should
be used in the enantioselective dispersive liquid–liquid extraction,
the miscibility of it in the organic phase (extracting solvent) and
the aqueous phase (sample solution) were key factors. Acetonitrile,
acetone and methanol were examined as the disperser solvent in
the extraction of trans-cyclohexane-1,2-diamine. To obtain a high
S
W
S
W
E
R
¼ D
R
and
E
S
¼ D
S
ð5Þ
For the extraction and separation processes, we have to find the
best selectivity between the two enantiomers. It can be said that
one enantiomer has to stay in the initial phase while the other
enantiomer has to move to the extraction phase with a chiral selec-
tor. Based on these statements we can evaluate the selectivity or
separation factor between the two enantiomers to determine the
optimum conditions.
selectivity, all combinations using 250
l
L of extraction solvents
ꢀ1
(
2 mmol L
chiral selector) with 400 lL of dispersive solvents
were examined. The results in Table 1 indicate that diethyl ether
as the extraction solvent and methanol as the disperser solvent
had the best selectivity of approximately 7. Therefore, this diethyl
ether-methanol combination was selected for subsequent
experiments.
3
. Results and discussion
In order to achieve the highest selectivity between the two
3.2. Effect of extracting solvent volume on the distribution
ratios of the trans-cyclohexane-1,2-diamine enantiomers and
selectivity
enantiomers, the chiral selector experimental parameters, which
influence the extraction and selectivity of the enantioselective dis-
persive liquid–liquid extraction procedure, including the extract-
ing and disperser solvents as well as their volume, pH value of
the sample solution, chiral selector concentration, temperature
and extraction time were optimized using a one variable-at-a-time
optimization method.
In order to examine the effect of the extraction solvent volume,
experiments involving different volumes of diethyl ether were
used for the microseparation of trans-cyclohexane-1,2-diamine
enantiomers from standard sample solutions with 400
methanol as the disperser solvent. When the extraction solvent
volumes were lower than 50 L, no supernatant organic phase
lL of
3
.1. Selection of extracting and disperser solvents
l
was collected on the top of aqueous phase. Table 2 shows the dis-
tribution ratio of the enantiomers and the selectivity factor with
the extraction solvent volume. The results in Table 2 show that
In order to obtain a good selectivity for the enantioselective dis-
persive liquid–liquid extraction of trans-cyclohexane-1,2-diamine,
Table 1
The effect of different types of extraction and disperser solvents on the distribution ratios of the trans-cyclohexane-1,2-diamine enantiomers and selectivity. Extraction
ꢀ
1
conditions: aqueous sample volume, 10 mL (1.0 mg/L of trans-cyclohexane-1,2-diamine); extracting solvent volume, 300 lL with 2 mmol L of chiral selector; disperser solvent
volume, 1 mL
Solvents
Without dispersive solvent
Methanol
Acetone
Acetonitrile
D
R
D
S
a
D
R
D
S
a
D
R
D
S
a
D
R
D
S
a
Xylene
4.3
5.3
6.5
5.1
5.2
6.5
4.7
1.7
1.5
1.9
1.5
1.8
1.7
1.4
2.5
3.5
3.4
3.4
2.8
3.8
3.4
8.2
8.6
13.2
9.2
10.6
12.1
9.3
2.1
2.4
1.8
2.3
2.5
2.8
2.0
3.9
3.5
7.3
4.0
4.2
4.3
6.6
7.4
8.1
11.2
9.1
9.8
9.6
2.3
2.1
1.9
1.8
1.9
1.9
2.0
3.2
3.9
5.9
5.1
5.1
5.0
3.6
7.3
9.5
1.9
2.1
2.2
2.5
3.6
6.6
2.2
3.8
4.5
5.0
4.2
2.9
1.7
3.6
n-Hexane
Diethyl ether
Dodecane
Toluene
Octanol
Cyclohexane
11.1
10.5
10.4
11.2
8.0
7.2

M. Hashemi, M. R. Hadjmohammadi / Tetrahedron: Asymmetry 28 (2017) 454–459
457
Table 2
The effect of the extraction solvent (diethyl ether) volume on the distribution ratios of the trans-cyclohexane-1,2-diamine enantiomers and the selectivity. Extraction conditions:
as Table 1. With different volume of extraction solvent
Extraction solvent volume (
l
L)
50
100
200
250
300
350
400
D
D
a
R
S
9.2
1.4
6.6
10.8
1.6
6.8
12.4
1.7
7.2
13.2
1.8
7.3
15.0
2.0
7.5
15.2
3.1
4.9
15.9
4.5
3.5
3
00
l
L of the extraction solvent produced the highest selectivity
the selectivity decreases at high chiral selector concentration. This
concentration was chosen for the optimum concentration.
factor. Thus this volume was selected as the optimum volume for
the extraction solvent.
3
.5. Effect of sample pH on the distribution ratios of the trans-
3
.3. Effect of disperser solvent volume on the distribution ratios
cyclohexane-1,2-diamine enantiomers and selectivity
of the trans-cyclohexane-1,2-diamine enantiomers and
selectivity
In enantioselective dispersive liquid–liquid extraction, the pH
of the sample solution is a key factor for the extraction of acidic
and basic compounds. In order to obtain high selectivity for acidic
compounds, the sample solution should be acidified to deionize the
analyte and consequently increase their transfer from the sample
solution into the organic phase. Thus, the pH of the sample solution
should be adjusted to make neutral the molecular forms of the ana-
lytes prior to the microextraction step, as the pH of the sample
might effect the complexation constant between the chiral selector
and the two enantiomers. For this purpose, the effect of the pH of
the sample solution on the selectivity of trans-cyclohexane-1,2-
diamine was investigated in the range of 3–8. Values of pH higher
than 8.0 were not examined, because trans-cyclohexane-1,2-dia-
mine can be hydrolyzed at basic pH. Table 5 shows that the highest
selectivity was obtained with pH = 4.5 and so subsequent experi-
ments were performed at this pH. This observation can be
explained by the fact that in higher or lower pH, the trans-cyclo-
hexane-1,2-diamine molecules are protonated or not and so the
complexation constant of the enantiomers changes.
In order to determine the best volume of the disperser solvent,
the extractions were carried out by changing the volume of metha-
nol in the range of 100–2000 lL. The results in Table 3, show that
when increasing the volume up to 1 mL methanol, the selectivity
increased and decreased after this volume. The lower selectivity
at volume of methanol less than 1 mL can be attributed to the fact
that the cloudy state was not well formed and the extracting sol-
vent (diethyl ether) could not be well dispersed among aqueous
solution in the form of very little droplets. Hence for the following
experiments, 1 mL of methanol was used as the optimal disperser
solvent volume.
3
.4. Effect of the chiral selector concentration on the
distribution ratios of the trans-cyclohexane-1,2-diamine
enantiomers and selectivity
The type and concentration of the chiral selectors are very
important factors in this method. The choice of a good chiral selec-
tor can give high selectivity factor. An azophenolic crown was used
in the chiral separation of amine compounds.36 The influence of the
crown ether concentration on the extraction performance has been
studied in diethyl ether for trans-cyclohexane-1,2-diamine enan-
tiomers. In Table 4, the observed distribution ratios D and the oper-
ational selectivity are given for trans-cyclohexane-1,2-diamine
microseparation. It can be seen that a very good operational selec-
tivity is achieved for the trans-cyclohexane-1,2-diamine enan-
3.6. Effect of extraction time on the distribution ratios of the
trans-cyclohexane-1,2-diamine enantiomers and the selectivity
In enantioselective dispersive liquid–liquid extraction, the
extraction time is defined as the gap time between the injection
of the mixture of the disperser and extraction solvent with a chiral
selector into the aqueous sample and the start of the centrifuga-
tion. Herein, the effect of the extraction time was examined from
1 to 120 min. The obtained results showed that the selectivity var-
ied greatly depending on the extraction time. This observation can
be explained by the fact that the rate at which the complex forms
between the chiral selector and trans-cyclohexane-1,2-diamine
enantiomers is slow. The results in Table 6 show increasing the
tiomers with
a relatively low chiral selector concentration
ꢀ1
(
ꢂ5 mmol L ). For two enantiomers, the distribution ratios
increased when increasing the chiral selector concentration, but
the slope of the distribution ratio increasing for (S,S)-trans-cyclo-
hexane-1,2-diamine was more than the other enantiomer, and so
Table 3
Effect of disperser solvent (methanol) volume on the distribution ratios of the trans-cyclohexane-1,2-diamine enantiomers and the selectivity. Extraction conditions: extracting
solvent (diethyl ether) volume, 300
l
L. Other conditions as Table 2
Disperser solvent volume ( L)
l
100
400
900
1000
1100
2000
D
D
a
R
S
10.2
1.5
6.8
14.4
1.7
8.4
15.2
1.8
8.4
16.1
1.9
8.5
16.2
2.4
6.7
16.9
2.8
6.0
Table 4
Effect of chiral selector concentration on the distribution ratios of the trans-cyclohexane-1,2-diamine enantiomers and the selectivity. Extraction conditions: disperser solvent
(methanol) volume, 1 mL. Other conditions as Table 3
Chiral selector concentration (mmol Lꢀ1)
1
4
5
6
10
D
D
a
R
S
8.1
1.5
5.4
15.5
1.6
9.7
16.9
1.7
9.9
17.0
4.6
3.6
17.5
6.1
2.8

4
58
M. Hashemi, M. R. Hadjmohammadi / Tetrahedron: Asymmetry 28 (2017) 454–459
Table 5
Effect of pH of sample solution on the distribution ratios of the trans-cyclohexane-1,2-diamine enantiomers and the selectivity. Extraction conditions: chiral selector
ꢀ
1
concentration, 5 mmol L . Other conditions as Table 4
pH of sample solution
3
3.5
4
4.5
5.0
7.0
8.0
D
D
a
R
10.2
1.2
8.5
12.5
1.4
8.9
14.4
1.5
9.6
17.0
1.7
10.0
15.5
1.7
9.1
12.2
1.3
9.3
7.0
1.0
7.0
S
Table 6
Effect of Extraction time on the distribution ratios of the trans-cyclohexane-1,2-diamine enantiomers and the selectivity. Extraction conditions: pH of sample solution, 4.5. Other
conditions as Table 5
Extraction time (min)
1
10
20
30
40
60
120
D
D
a
R
8.2
1.5
5.5
10.8
1.6
6.7
15.2
1.8
8.6
18.2
1.8
10.1
18.1
1.9
9.5
18.1
2.4
7.5
18.3
2.8
6.5
S
Table 7
Effect of Temperature on the distribution ratios of the trans-cyclohexane-1,2-diamine enantiomers and the selectivity. Extraction conditions: Extraction time, 30 min. Other
conditions as Table 6
Temperature (°C)
5
8
10
15
25
37
D
D
a
R
10.2
1.2
8.5
14.8
1.5
9.8
18.3
1.8
10.2
15.2
1.8
8.4
14.2
1.8
7.8
11.2
1.7
6.6
S
Figure 3. HPLC chromatogram for extraction phase at optimum conditions.
time up to 30 min increased the selectivity factor, after which the
selectivity factor decreased. Hence 30 min was chosen for the
extraction time.
or the complexation between the enantiomers and the chiral selec-
tor did not occur. However, below T = 10 °C, extraction became too
difficult due to an increase in the viscosity of the solution and a
decrease in chiral selector solubility (Table 7). Therefore, in this
case, T = 10 °C was considered to be the optimal temperature for
enantioselective dispersive liquid–liquid extraction.
3
.7. Effect of temperature on the distribution ratios of the
trans-cyclohexane-1,2-diamine enantiomers and the selectivity
Some researchers have reported that the temperature of the
sample solution has a beneficial effect on the selectivity in chiral
4
. Conclusions
1
5,16
extraction procedures.
It is well known that increasing the
The extraction and microseparation of trans-cyclohexane-1,2-
temperature of aqueous sample can improve the selectivity. As
can be seen in Table 1, the temperature has a strong influence on
the absolute values of the selectivity. Over the temperature range
diamine enantiomers in an aqueous sample solution have been
performed using enantioselective dispersive liquid–liquid
extraction and HPLC. The results show that enantioselective
dispersive liquid–liquid extraction is an effective method for the
microseparation of trans-cyclohexane-1,2-diamine enantiomers.
5
–37 °C, the selectivity increased with increasing temperature up
to 10 °C while after this temperature, the selectivity strongly
decreased. Optimal selectivity was achieved at lower temperatures
because at high temperatures, the chiral selector was demolished
The azophenolic crown ether is
enantioselective compound for
a
versatile and highly
chiral microseparation

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