Synthesis of dendrimer-type chiral stationary phases based on the selector of (1S,2R)-(+)-2-amino-1,2-diphenylethanol derivate and their enantioseparation evaluation by HPLC

Article abstract of DOI:10.1002/chir.20708

In our recent work, a series of dendritic chiral stationary phases (CSPs) were synthesized, in which the chiral selector was L-2-(p-toluenesulfonamido)-3- phenylpropionyl chloride (selector I), and the CSP derived from three-generation dendrimer showed the best separation ability. To further investigate the influence of the structures of dendrimer and chiral selector on enantioseparation ability, in this work, another series CSPs (CSPs 1-4) were prepared by immobilizing (1S,2R)-1,2-diphenyl-2-(3-phenylureido)ethyl 4-isocyanatophenylcarbamate (selector II) on one- to four-generation dendrimers that were prepared in previous work. CSPs 1 and 4 demonstrated the equivalent enantioseparation ability. CSPs 2 and 3 showed the best and poorest enantioseparation ability respectively. Basically, these two series of CSPs exhibited the equivalent enantioseparation ability although the chiral selectors were different. Considering the enantioseparation ability of the CSP derived from aminated silica gel and selector II is much better than that of the one derived from aminated silica gel and selector I, it is believed that the dendrimer conformation essentially impacts enantioseparation.

Full text of DOI:10.1002/chir.20708

CHIRALITY 22:69–76 (2010)
Synthesis of Dendrimer-Type Chiral Stationary Phases Based on
the Selector of (1S,2R )-(1)-2-Amino-1,2-diphenylethanol
Derivate and Their Enantioseparation Evaluation by HPLC
BAO-JIANG HE,1 CHUAN-QI YIN,1 SHI-RONG LI,2 AND ZHENG-WU BAI1,3
*
1School of Chemical Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan, China
2Hubei Key Laboratory of Biologic Resources Protection and Utilization, Hubei Institute for Nationalities, Enshi, Hubei, China
3Key Laboratory of Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan, China
ABSTRACT
In our recent work, a series of dendritic chiral stationary phases
(CSPs) were synthesized, in which the chiral selector was L-2-(p-toluenesulfonamido)-3-
phenylpropionyl chloride (selector I), and the CSP derived from three-generation den-
drimer showed the best separation ability. To further investigate the influence of the
structures of dendrimer and chiral selector on enantioseparation ability, in this work,
another series CSPs (CSPs 1-4) were prepared by immobilizing (1S,2R)-1,2-diphenyl-2-
(3-phenylureido)ethyl 4-isocyanatophenylcarbamate (selector II) on one- to four-genera-
tion dendrimers that were prepared in previous work. CSPs 1 and 4 demonstrated the
equivalent enantioseparation ability. CSPs 2 and 3 showed the best and poorest enan-
tioseparation ability respectively. Basically, these two series of CSPs exhibited the
equivalent enantioseparation ability although the chiral selectors were different. Consid-
ering the enantioseparation ability of the CSP derived from aminated silica gel and se-
lector II is much better than that of the one derived from aminated silica gel and selec-
tor I, it is believed that the dendrimer conformation essentially impacts enantiosepara-
C
VV
tion. Chirality 22:69–76, 2010.
2009 Wiley-Liss, Inc.
KEY WORDS: (1S,2R)-(1)-2-Amino-1,2-diphenylethanol; dendrimer; chiral stationary
phase; enantioseparation; high-performance liquid chromatography
INTRODUCTION
groups to immobilize selectors, etc.22 To further investi-
gate the influence of the structures of dendrimers and
selectors on the enantioseparation ability, in this work,
another chiral selector that is derivatized from (1S,2R)-
(1)-2-amino-1,2-diphenylethanol is linked to the den-
drimers that are prepared on the silica gel. The compari-
son of enantioseparation ability of the prepared CSPs is
made.
In chiral stationary phase (CSP) pool, there are a few
CSPs belong to the category of dendrimer or dendrimer-
like type.1,2 Dendrimers are monodisperse oligomers and
polymers usually with special function.3–5 They are usually
related to the fields of catalysis,6–9 drug delivery,10–13 liq-
uid crystalline materials14 and contrast media for clinic di-
agnosis.15,16 In a few reported works, dendrimers are used
in chromatography.17–19 In our previous works, we synthe-
sized two kinds of dendrimer-like CSPs. One was prepared
directly on silica gel with chiral low-molecular-weight com-
pounds, where the yielded chiral dendrimers was utilized
as chiral selectors.20 Of this type of CSPs, one-generation
CSP showed the best enantioseparation ability, while the
enantioseparation ability of two- and three-generation
CSPs decreased in turn. Regarding the other kind of
CSPs, the dendrimers were prepared on silica gel with
achiral low-molecular-weight compounds, and then chiral
selectors were immobilized on the prepared dendrimers.21
Three-generation CSP showed the best separation ability.
However, the resolutions of the latter series of CSPs,
in general, are comparatively low. The reason, discussed
in the previous work, is that the amide bonds contained in
the dendrimers significantly impairs the resolutions. In
enantioseparation, there are achiral elements to affect chi-
ral discrimination, such as the spacers and functional
C
MATERIALS AND METHODS
Materials and Chemicals
3-Aminopropyltriethoxysilane was purchased from
Chemical Factory of Wuhan University (China). Phenyl
isocyanate was obtained from Wanke Tech. of Hangzhou
(China). 1,4-Phenylene diisocyanate was from Xinyi Pesti-
cite Factory of Jiangsu (China). (1S,2R)-(1)-2-Amino-1,2-
diphenylethanol was available from Likai Chiral. of
Contract grant sponsor: National Natural Science Foundation of China
(NSFC); Contract grant numbers: 20371037 and 20675061
*Correspondence to: Zheng-Wu Bai, School of Chemical Engineering and
Pharmacy, Wuhan Institute of Technology, Wuhan, Hubei Province
430073, China. E-mail: zhengwu_bai@yahoo.com
Received for publication 23 August 2008; Accepted 19 January 2009
DOI: 10.1002/chir.20708
Published online 24 March 2009 in Wiley InterScience
(www.interscience.wiley.com).
VV
2009 Wiley-Liss, Inc.

70
HE ET AL.
Chengdu (China). Silica gel (LiChrosorb Si100) was from
CSP 4: FTIR (KBr, cm21): 3448 (NꢀꢀH), 1647
Merck (Darmstadt, Germany) with a particle size of 5 lm, (ꢀꢀNHꢀꢀCOꢀꢀ), 1546 (ꢀꢀNHꢀꢀCOꢀꢀ), 1104 (SiꢀꢀO);
2
a pore size of 100 A and a surface area of 300 m g21. Pyri- Solid-state 1H NMR (258C) d: 0.9 (s, SiꢀꢀCH2), 1.4 (s,
˚
dine was refluxed with CaH2 and redistilled. Triethylamine SiꢀꢀCH2ꢀꢀCH2), 2.2–2.7 (m, SiꢀꢀCH2ꢀꢀCH2ꢀꢀCH2), 4.3
(TEA) was dried with P2O5 and redistilled. All other chem- (m, CONHꢀꢀCH2), 5.0–5.3 (m, ꢀꢀCHꢀꢀ), 6.6 (m,
icals for synthesis were of analytic grade and used as CHꢀꢀNH), 6.8–7.7 (m, ArꢀꢀH), 9.0 (m, ArꢀꢀNHꢀꢀCO).
received. All mobile phases were freshly prepared, filtered
and degassed.
CSP 5 was prepared with 3.58 g aminated silica gel,
3.11 g selector I and 3 ml TEA, according to the procedure
described in the previous work that L-2-(p-toluenesulfona-
mido)-3-phenylpropionyl chloride was immobilized on the
dendrimers.21
Instruments and Measurements
Elemental analysis was performed on an Elemental Vari-
oEL III CHNOS apparatus (Germany). IR spectra were
recorded on a Nicolet FTIR instrument with KBr pellets.
Solid-state 1H NMR spectra were recorded on a Varian
Infinity Plus 300 spectrometer, operating at 300 MHz.
The CSPs were packed into stainless steel columns
(250 mm 3 4.6 mm) with an Alltech slurry packer of
model 1666. Enantioseparation was implemented on an
Agilent 1100 chromatographic apparatus consisted of an
Agilent G1365B DAD, an Agilent G1311A Quat Pump, an
Agilent G1379A degasser and an Agilent G1313A ALS
autosampler.
FTIR (KBr, cm21) 3450 (NꢀꢀH), 1648 (ꢀꢀCONHꢀꢀ),
1
1399 (ꢀꢀSO2ꢀꢀNHꢀꢀ), 1104 (SiꢀꢀO); Solid-state H NMR
(258C) d: 0.9 (s, SiꢀꢀCH2), 1.4 (s, SiꢀꢀCH2ꢀꢀCH2), 2.0 (s,
ArSO2ꢀꢀNH), 2.2 (s, ArꢀꢀCH3), 2.8 (m, NꢀꢀCH2), 3.3 (m,
ArꢀꢀCH2), 3.8 (m, CONHꢀꢀCH2), 5.0 (m, ArCH2ꢀꢀCH),
6.5 (s, COꢀꢀNH), 6.8–7.5 (m, ArꢀꢀH).
Column Packing and Enantioseparation Conditions
CSPs 1-5 were, respectively, packed into five columns
through a slurry packing method with chloroform to form
the slurries and hexane as the packing solvent. The enan-
tioseparation on these five CSPs was conducted in various
mobile phase conditions at 258C with 1 ml/min flow rate
except when indicated. The sample solutions (1 mg/ml)
were prepared by dissolving the chiral solutes in methanol
and were filtered before injection. The injection volume is
15 ll.
Preparation of Chiral Stationary Phases
(1S,2R)-1,2-Diphenyl-2-(3-phenylureido)ethyl 4-isocyana-
tophenylcarbamate, L-2-(p-toluenesulfonamido)-3-phenyl-
propionyl chloride, aminated silica gel and one- to four-
generation dendrimers (G1-4) were, respectively, pre-
pared in the same batch of previous works.21,22 The den-
drimers were prepared using aminated silica gel as the
core, and using ethylene diamine and methyl acrylate as
the building blocks.
Selector II (4.0 g) and di-n-butyltin dilaurate (2 ml) were
added to a suspension of one-generation dendrimer (G1,
3.73 g) dispersed in pyridine (16 ml). After stirred at 808C
for 24 h, the resulting mixture was filtered. The collected
solid was extracted with DMF and THF, and dried under
vacuum to give CSP 1 as a yellow solid (4.19 g).
RESULTS AND DISCUSSION
Characterization of Dendritic Stationary Phases
As shown in Figure 1, CSPs 1-4 were prepared by
immobilizing selector II onto one- to four-generation den-
drimers (see Figure 2), which were synthesized in the pre-
vious work.22 The structure of these dendrimers is not per-
fect due to the intramolecular amidation and bulkiness of
dendritic linker to small molecules, i.e. ethylene diamine
and methyl acrylate, during the formation of dendrimers.
The elemental analysis of the CSPs, the increment of car-
bon content of the CSPs and related selector loadings are
tabulated in Table 1. The selector loadings are calculated
CSP 1: FTIR (KBr, cm21): 3454 (NꢀꢀH), 1646
(ꢀꢀNHꢀꢀCOꢀꢀ), 1557 (ꢀꢀNHꢀꢀCOꢀꢀ), 1103 (SiꢀꢀO);
Solid-state 1H NMR (258C) d: 0.9 (s, SiꢀꢀCH2), 1.4 (s,
SiꢀꢀCH2ꢀꢀCH2), 2.4-2.7 (m, SiꢀꢀCH2ꢀꢀCH2ꢀꢀCH2), 4.3
(m, CONHꢀꢀCH2), 5.0–5.5 (m, ꢀꢀCHꢀꢀ), 6.5 (m,
CHꢀꢀNH), 6.9–7.6 (m, ArꢀꢀH), 9.1 (m, ArꢀꢀNHꢀꢀCO).
The above identical procedures were applied for the
preparation of CSPs 2, 3 and 4, with two-, three-, and
four-generation dendrimer respectively.
according to the following formula:
C1ꢀC0
Selector loading ¼
3 104lmol=g[C0 and C1: the
M3n
carbon content of dendrimers (aminated silica gel for CSP
5) and corresponding CSPs; C1-C0: the increment of car-
bon contents of the CSPs. M: the relative atom weight of
carbon; n: the number of carbon atom in each selector].
CSP 2: FTIR (KBr, cm21): 3454 (NꢀꢀH), 1645
(ꢀꢀNHꢀꢀCOꢀꢀ), 1544 (ꢀꢀNHꢀꢀCOꢀꢀ), 1107 (SiꢀꢀO);
Solid-state 1H NMR (258C) d: 0.9 (s, SiꢀꢀCH2), 1.4 (s,
SiꢀꢀCH2ꢀꢀCH2), 2.4–2.7 (m, SiꢀꢀCH2ꢀꢀCH2ꢀꢀCH2), 4.2
Comparison Between Selector Loadings of CSPs 1-4 and
Enantioseparation Ability
The column efficiency of CSPs 1-5 was determined as
(m, CONHꢀꢀCH2), 5.0–5.4 (m, ꢀꢀCHꢀꢀ), 6.5 (m, 16,700, 15,400, 17,300, 20,600 and 32,800 plates per meter
CHꢀꢀNH), 6.5–7.6 (m, ArꢀꢀH), 9.2 (m, ArꢀꢀNHꢀꢀCO).
respectively, with biphenyl as the sample, a mixture of
CSP 3: FTIR (KBr, cm21): 3454 (NꢀꢀH), 1643 hexane and isopropanol (90/10, v/v) as the mobile phase.
(ꢀꢀNHꢀꢀCOꢀꢀ), 1550 (ꢀꢀNHꢀꢀCOꢀꢀ), 1102 (SiꢀꢀO); The enantioseparation ability was investigated with struc-
Solid-state 1H NMR (258C) d: 0.9 (s, SiꢀꢀCH2), 1.4 (s, turally various chiral analytes (see Figure 3). The chro-
SiꢀꢀCH2ꢀꢀCH2), 2.3-2.7 (m, SiꢀꢀCH2ꢀꢀCH2ꢀꢀCH2), 4.5 matographic data are presented in Table 2. CSPs 1-4 sep-
(m, CONHꢀꢀCH2), 4.9–5.5 (m, ꢀꢀCHꢀꢀ), 6.6 (m, arated 8, 13, 6, and 9 chiral compounds respectively. Com-
CHꢀꢀNH), 6.8–7.6 (m, ArꢀꢀH), 9.2 (m, ArꢀꢀNHꢀꢀCO).
paring the selector loadings and the numbers of separated
Chirality DOI 10.1002/chir

DENDRIMER-TYPE CHIRAL STATIONARY PHASES
71
Fig. 1. The synthetic scheme of CSPs 1-5.
chiral compounds, the CSP of highest selector loading, loading. CSP 2 has comparatively optimal selector loading
such as CSP 1, does not have most powerful separation in view of this series of CSPs, where selector and den-
ability. In previous work, the CSP, which is prepared by drimer are not so crowded and the selector loading is suf-
immobilizing selector II on 3-aminopropyl silica gel shows ficient for chiral recognition. Therefore, it shows best sep-
excellent enantioseparation ability.22 This enantiosepara- aration ability. In our ongoing work, where the chiral se-
tion ability difference corresponds to the different struc- lector is a derivative of tartaric acid, it is found that
tures of these two CSPs. For CSP 1 in this work, the chi- enantioseparation ability is related to selector loading. Fur-
ral selector is anchored to the first-generation dendrimer; thermore, at the same separation conditions, the CSP of
and in previous work, it is immobilized on 3-aminopropyl higher selector loading exhibit lower overall enantiosepa-
silica gel that is the core of the dendrimer. There are ration ability. Together with the observations in this work,
many amide bonds in the dendrimer. Thus, it is suggested it is concluded that there should be an optimal selector
that the hydrogen bonding inside the dendrimer and the loading for a CSP to demonstrate its enantioseparation
jam between chiral selector and dendrimer reduce the ability.
enantioseparation ability. CSP 3 has lowest enantiosepara-
tion ability due to its too low selector loading. CSP 4
shows equivalent separation ability comparing to CSP 1,
The Relationship Between Enantioseparation and the
Structure of CSPs 1-4 and Chiral Analytes
although its selector loading is much lower than that of
The structures of chiral compounds separated by CSPs
CSP 1. It is believed that the selector and the dendrimer 1-4 can be divided into two classes. One class is featured
in CSP 4 are not crowded because of its lower selector by the functional groups that can interact with selector
Chirality DOI 10.1002/chir

72
HE ET AL.
Fig. 2. The structures of generation-various dendrimers. G 1, G 2, G 3, and G 4 represent one to four-generation dendrimer respectively.
through hydrogen bonding, such as compounds 1, 4, 9, which only one oxygen atom contributes hydrogen bond-
14, 16, 22, and 23, which were all separated by CSPs 1-4. ing interaction because of the lower electronegativity of
As for the enantioseparation of compounds 3 and 4, it is sulfur. In compound 4, two oxygen atoms in the lactone
believed that the difference attributes to the slight change can interact with proton, and as a result compound 4 was
in their structures. There is a sulfur atom in the lactone, in separated better by CSPs 1-4 than compound 3. Another
Chirality DOI 10.1002/chir

DENDRIMER-TYPE CHIRAL STATIONARY PHASES
73
TABLE 1. Elemental analysis data and chiral selector
loadings for CSPs
phase mode. Despite of the same selectors on CSPs 1-4,
the favorable mobile phases are different. CSP 1 shows
its enantioseparation ability preferably in methanol-con-
tained mobile phases; and CSPs 2-4 preferably in acetoni-
trile-contained mobile phases. This difference is mainly
owing to the generation-different dendrimers and selector
loadings. CSP 1 was prepared from one-generation den-
drimer with highest selector loading. In this case, the se-
lector and dendrimer crowd each other, making it incon-
venient for chiral analytes to interact with the amide
bonds. Thus, methanol participates in the diastereoisomer
formation, by bridging chiral analytes and amide bonds
through hydrogen bonding. Because CSPs 2-4 are
derived from two-, three-, and four-generation dendrimers
respectively, where the selectors are not so congested, the
amide bonds in selectors and in dendrimers are exposed
to chiral analytes for interaction. Hereby, methanol is not
needed to be involved in the diastereoisomer formation.
Elemental
analysis
CSP 1 CSP 2 CSP 3 CSP 4 CSP 5
C%
H%
N%
21.41
3.38
5.14
7.86
23.96
3.97
6.87
5.77
25.60
4.36
7.92
2.44
29.49
5.09
9.93
2.86
17.98
3.42
4.35
12.59
C content
Increment (%)a
Selector loading,
lmol/g
226
166
70
82
656
aThe difference of carbon contents of CSPs 1–5 and corresponding sup-
ports are given as italic values. The elemental analysis of generation-various
dendrimers and aminated silica gel is same as those in previous work.21
one class is featured by bearing the structure of p-acidity in
chemical property, such as compounds 5, 14, 20, 21, 24, and
25 etc. There are aromatic amide bonds contained in selec-
tors, which are p-basic. p-Acids readily interact with p-bases
to form temporary diastereoisomers between chiral analytes
and selectors. The enantioseparation is caused due to the
stability difference of these diastereoisomers. The similar
instance occurs for the enantioseparation resulted from the
stability difference of these diastereoisomers formed by
hydrogen bonding interaction. For compounds 13 and 14,
there is a chlorine atom at para position of the phenyl in
compound 14, and this phenyl bears strong p-acidity
because the chlorine atom withdraws electron. Therefore,
compound 14 is chirally recognized by CSPs 1 and 2, while
compound 13 is only recognized by CSP 4. Forasmuch
compound 7 shows p-basicity because the amino at para
position of the phenyl donates electron, and compound 8
shows p-neutrality, they are very similar in enantiosepara-
tion. In addition, the size and shape of chiral analytes prob-
ably impact the formation of temporary diastereoisomers
and subsequent enantioseparation.
The influence of achiral elements on enantioseparation
has been reported in a few works.23–25 CSPs 1-4 are
derived from the same selector; however, the chiral analy-
tes that are separated by CSPs 1-4 are not completely the
same in structures. Therefore, the chiral recognition is
also affected by achiral elements for CSPs 1-4. There are
a lot of amide bonds in the dendrimers that can interact
with chiral analytes by hydrogen bonding. Chiral analytes
simultaneously interact with the amide bonds and selec-
tors to form temporary diastereoisomers, whose stability
difference results in enantioseparation. For example,
although the selector loading of CSP 4 is quite low, its
enantioseparation ability is equivalent to that of CSP 1,
because chiral analytes interact with the amide bonds in
the dendrimer in addition to with the selector to form dia-
stereoisomers that probably lead enantioseparation.
The Influence of Selector Structure on Enantioseparation
The dendrimers in this work were prepared in the same
batch in our previous work,21 and the difference of these
two series of CSPs only lies on their chiral selectors. Gen-
erally, the enantioseparation ability of the CSPs in this
work is not improved in comparison with the previous
work, where the chiral selector is L-2-(p-toluenesulfona-
mido)-3-phenylpropionyl chloride (see Figure 1). There
are one chiral center and two phenyls in selector I; while
there are two chiral centers and four phenyls in selector
II. Relative to selector II, the molecular size of selector I is
smaller, and its steric hindrance for the interaction with
chiral analytes is less; whereas, appropriate steric hin-
drance is a necessary element for chiral recognition. To
compare the enantioseparation ability of chiral selectors of
L-2-(p-toluenesulfonamido)-3-phenylpropionyl chloride and
(1S,2R)-1,2-diphenyl-2-(3-phenylureido)ethyl 4-isocyanato-
phenylcarbamate, the former selector was immobilized on
3-aminopropyl silica gel by the reported method to give
CSP 5.21 Its enantioseparation ability has also been eval-
uated (Table 2), and is lower than that of the CSP of the
latter selector.22 No improvement in enantioseparation
ability attributes to the support of the CSPs of this work,
which is functionalized with generation-various den-
drimers. There are more amide bonds in selector II, and
they are desired to interact with chiral analytes. However,
these amide bonds also interact with other amide bonds in
the dendrimers. Wherefore, the possibility of the interac-
tion between the selector and the chiral analytes does not
increase as expected.
CONCLUSIONS
In comparison with the previous work, the prepared
CSPs, whose chiral selector bears one more chiral center,
more amide bonds and more phenyls, do not demonstrate
improved enantioseparation ability. The CSPs prepared
The Influence of Mobile Phase Composition on
Enantioseparation of CSPs 1-4
Mobile phases complicatedly impact separation not only from one- and four-generation dendrimers have the equiva-
by eluting chiral nanlytes out, being involved in the forma- lent enantioseparation ability due to the huddle of the se-
tion of temporary diasetereoisomers but also. As a whole, lector and the dendrimer in CSP 1 and the comparative
CSPs 1-4 show their enantioseparation ability in reversed sparsity of selector in CSP 4. The CSP prepared from
Chirality DOI 10.1002/chir

74
HE ET AL.
Fig. 3. The chiral analytes separated by CSPs 1-5.
three-generation dendrimer has the poorest enantiosepara- tor loading. CSP 1 shows enantioseparation ability pref-
tion ability due to its lowest selector loading; while the erably in methanol-contained mobile phases, because its
CSP prepared from two-generation dendrimer exhibits the selector and the dendrimer jam each other; and the amide
strongest enantioseparation ability due to its optimal selec- bonds are shielded to chiral analytes, leading the necessity
Chirality DOI 10.1002/chir

DENDRIMER-TYPE CHIRAL STATIONARY PHASES
75
Chirality DOI 10.1002/chir

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Chirality DOI 10.1002/chir