Chiral Polymers of Intrinsic Microporosity: Selective Membrane Permeation of Enantiomers

Article abstract of DOI:10.1002/anie.201504934

Following its resolution by diastereomeric complexation, 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spirobisindane (TTSBI) was used to synthesize a chiral ladder polymer, (+)-PIM-CN. (+)-PIM-COOH was also synthesized by the acid hydrolysis of (+)-PIM-CN. Following characterization, both (+)-PIM-CN and (+)-PIM-COOH were solvent cast directly into semipermeable membranes and evaluated for their ability to enable the selective permeation of a range of racemates, including mandelic acid (Man), Fmoc-phenylalanine, 1,1′-bi-2-naphthol (binol), and TTSBI. High ee values were observed for a number of analytes, and both materials exhibited high permeation rates. A selective diffusion–permeation mechanism was consistent with the results obtained with these materials. Their high permeability, processability, and ease of chemical modification offer considerable potential for liquid-phase membrane separations and related separation applications.

Full text of DOI:10.1002/anie.201504934

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Angewandte
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International Edition: DOI: 10.1002/anie.201504934
German Edition: DOI: 10.1002/ange.201504934
Enantioselective Membranes
Chiral Polymers of Intrinsic Microporosity: Selective Membrane
Permeation of Enantiomers
Xilun Weng, JosØ E. Baez, Mariya Khiterer, Madelene Y. Hoe, Zongbi Bao, and Kenneth J. Shea*
Abstract: Following its resolution by diastereomeric complex-
ation, 5,5’,6,6’-tetrahydroxy-3,3,3’,3’-tetramethyl-1,1’-spirobi-
sindane (TTSBI) was used to synthesize a chiral ladder
polymer, (+)-PIM-CN. (+)-PIM-COOH was also synthe-
sized by the acid hydrolysis of (+)-PIM-CN. Following
characterization, both (+)-PIM-CN and (+)-PIM-COOH
were solvent cast directly into semipermeable membranes and
evaluated for their ability to enable the selective permeation of
a range of racemates, including mandelic acid (Man), Fmoc-
phenylalanine, 1,1’-bi-2-naphthol (binol), and TTSBI. High
ee values were observed for a number of analytes, and both
materials exhibited high permeation rates. A selective diffu-
sion–permeation mechanism was consistent with the results
obtained with these materials. Their high permeability, proc-
essability, and ease of chemical modification offer considerable
potential for liquid-phase membrane separations and related
separation applications.
detracts somewhat from the broad application of these
[11]
methods.
Membrane-mediated enantiomer separation offers an
[12]
alternative technology.
Low energy consumption, high
processing capacity, and continuous operation suggest mem-
brane processes have the potential to satisfy many of the
criteria for large-scale enantiomer enrichment. Dense enan-
tioselective membranes can be divided into two classes:
diffusion-selective membranes and sorption-selective mem-
[13,14]
branes.
Diffusion-selective membranes are usually non-
porous, and their proportionality of permeability and perme-
[13]
ation selectivity is inverse, which can limit their application.
Sorption-selective membranes, on the other hand, generally
[14,15]
require a porous support and chiral selectors.
Non-
selective permeation is difficult to avoid with these materi-
[14]
als.
Materials that are intrinsically chiral and have high
pororsity can show both high permeability and high selectiv-
ity. However, the combination of these desirable features is
rare. Furthermore, an ideal enantioselective membrane
should be relatively straightforward to synthesize, possess
satisfactory mechanical properties, and be processable for
direct casting as a membrane.
T
he need for enantiomerically pure compounds creates
incentive for the development of new strategies for the
[
1]
resolution of racemic mixtures. Chromatographic methods,
particularly gas–liquid and solid–liquid chromatography, have
been the standard analytical approach for enantiomer sepa-
One promising class of materials are polymers of intrinsic
[2]
[16,17]
ration. High-performance liquid chromatography (HPLC)
and diastereomeric crystallization remain the most important
methods for the large-scale separation of enantiomers.
However, most current resolution methods have some
limitations. For example, diastereomeric crystallization often
requires the resolution of reagents that are effective only for
microporosity (PIMs).
The scaffold of a typical PIM
contains a spiro center, which provides a site of contortion
and inhibits bond rotation of the rigid fused rings. The
combination of both features obstructs the efficient packing
of polymer chains in the solid state, thus giving rise to
microporosity in the material. Porosity originates solely from
[3,4]
a specific system;
enzyme-mediated kinetic resolution
the molecular structure, which is independent of processing
[
5,6]
[16,17]
faces decreased catalytic activity over time; and chromato-
graphic methods present the disadvantage of being discontin-
uous and expensive. Although the development of simulated
moving bed chromatography (SMB) and supercritical-fluid
chromatography (SFC) allows for continuous operation on
history.
These materials have attracted considerable
attention as media for gas separation, gas storage, and the
adsorption of volatile organic compounds, and as supports for
[18–20]
heterogeneous catalysis.
All PIMs reported to date have
been achiral, and all reported studies of materials incorpo-
rating 5,5’,6,6’-tetrahydroxy-3,3,3’,3’-tetramethyl-1,1’-spirobi-
sindane (TTSBI(1)) have utilized the racemic form. Research
on applications of this material for liquid-phase selective
permeation has not been reported. Racemic 1 has also been
used as a building block for siliconate tetraanionic molecular
[7–10]
a preparative scale,
specialized equipment, optimization
studies for each substrate, and the cost of stationary phases
[
21–24]
[
*] X. Weng, Dr. J. E. Baez, Dr. M. Khiterer, M. Y. Hoe, Dr. K. J. Shea
Department of Chemistry, University of California, Irvine
Irvine, CA 92697 (USA)
squares
and tetraanionic organoborate squares, as well as
[24]
for the synthesis of racemic PIMs.
Herein we report the resolution of 1 and the synthesis of
two chiral polymers of intrinsic microporosity derived from
this chiral monomer: chiral, fluorescent ladder polymers
E-mail: kjshea@uci.edu
X. Weng, Dr. Z. Bao
Key Laboratory of Biomass Chemical Engineering of the Ministry of
Education, Department of Chemical and Biological Engineering
Zhejiang University
(+)-PIM-CN (Figure 1) and (+)-PIM-COOH. The organic
soluble polymers were directly cast as porous membranes and
were found to be effective in enabling the selective perme-
ation of a range of enantiomers.
Hangzhou 310027 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504934.
1
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Figure 2. CD spectra of rac-PIM-CN (a), (+)-PIM-CN (b), (+)-PIM-
COOH (c), and (+)-TTSBI (d).
Bisignate Cotton effects arise from exciton coupling of chiral
[
29,30]
ordered chromophores,
thus suggesting that some seg-
Figure 1. a) Condensation polymerization of (+)-1 with 2. b) A com-
puter-modeled structure (Spartan, PM-3) of a random 5-mer.
DMF=N,N-dimethylformamide.
ments of the polymer chain have higher-ordered struc-
[31,32]
ture.
(
+)-PIM-CN can be cast as a free-standing porous thin
film. The (+)-PIM-CN membrane was cast from THF
solutions onto flat-bottomed watch glasses (inside diameter:
2 cm; see the Supporting Information). The membranes were
fitted to a U-tube apparatus (average membrane thickness:
300 mm), and the concentration-driven permeation (CP) of
racemates was evaluated (see Figure S11). The permeation
The synthesis of a chiral PIM requires enantiomerically
pure 1. A number of approaches were explored, and the
formation of diastereomeric complexes with (8S,9R)-(À)-N-
benzylcinchonidinium chloride (3; see Scheme S1 in the
[25]
Supporting Information) proved to be most successful.
À2 À1
Acetone was found to be the most effective solvent of those
screened (see Table S1 in the Supporting Information). When
racemic 1 was heated with 3 at reflux as a solution in acetone,
rate (P (in gmm h )) was estimated from the slope of a plot
À2
of normalized quantity Q (in gmm ) versus permeation time
t (in h). Q is defined by the equation Q = qL/A, in which q is
the quantity of permeated solute, and L and A are the
1
a solid diastereomeric complex was obtained. H NMR
[33]
spectroscopic analysis and elemental analysis revealed that
the precipitate contained the spirobisindane and the alkaloid
in a 1:2 ratio (see Figure S1 and Table S2). Decomposition of
thickness and effective area of the membrane, respectively.
The ee values were determined by HPLC analysis on chiral
stationary phases.
2
5
the complex with 1n HCl gave (+)-1 (½aŠ5
¼ + 40.18, THF)
Figure 3 shows the CP of a solution of (R,S)-mandelic acid
89nm
À1
in 70% overall yield (based on one enantiomer). HPLC on
a chiral stationary phase established the spirobisindane to be
enantiomerically pure (see Figure S3).
(Man, 2 mgmL ) in methanol with (+)-PIM-CN. (R)-Man
permeates the membrane preferentially, with an ee value of
31.4% at 8 h (Figure 3b). The permeation rate (P) was 0.07 ”
À3
À2 À1
The condensation of enantiomerically pure (+)-1 with
10 gmm
h
(Table 1). We also investigated the perme-
2
,3,5,6-tetrafluorophthalonitrile (2) in DMF with excess
ation of (R,S)-binol. The ee value of material that had
permeated through the (+)-PIM-CN membrane after 3 h
was 53% (Figure 4b). Notably, the selective permeation of
K CO , according to a previously reported procedure for
rac-1 resulted in the formation of (+)-PIM-CN in 96%
yield (Figure 1a). (+)-PIM-CN had a specific rotation of
2
3
18]
[
2
5
5
½
aŠ
¼ + 389.68 in THF. The polymer, a bright-yellow
89nm
fluorescent solid, was characterized by gel permeation
chromatography (GPC) and had a number-average molar
À1
mass, M , of 62239 gmol and a weight-average molar mass,
n
À1
M , of 72778 gmol , with a polydispersity (PDI) of M /M =
w
w
n
1
.2. (+)-PIM-CN was an intrinsically porous polymer with
2
À1
a surface area of 740 m g and an average pore diameter
(
4V/A by BET) of 3.5 nm (see Table S3 and Figure S10).
The specific rotation of ( ++ )-PIM-CN was approximately
1
0 times higher than that of the individual monomer 1.
Furthermore, the CD spectrum of (+)-PIM-CN (Figure 2) in
THF showed a bisignate Cotton effect with a zero crossing at
Figure 3. a) Enantioselective permeation of (R,S)-mandelic acid
through (+)-PIM-CN. b) HPLC chromatogram of the receiver side at
2
40 nm. Although a random structure of the polymer is most
probable, for chiral polymers without helicity, the specific
rotation is usually similar to that of the chiral monomer.
8
h after the start of permeation. c) HPLC chromatogram of racemic
Man. (For HPLC separation conditions, see Table S4).
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Table 1: Permeation rate of racemates through (+)-PIM-CN and
+)-PIM-COOH membranes.
(
3
À2
[a]
Chiral selector
Racemate
10 P [gmm h]
ee [%]
a
(+)-PIM-CN
(+)-PIM-CN
(+)-PIM-CN
(+)-PIM-COOH
Man
binol
TTSBI
Fmoc-Phe
0.07
0.019
1.65
32
1.92
[
b]
b]
[b]
53
87
75
3.3
14.4
[
[b]
Scheme 1. Hydrolysis of (+)-PIM-CN to (+)-PIM-COOH.
0.073
7
[
a] Selectivity factor. [b] The selectivity and ee value were determined at
ing (+)-PIM-CN to acid hydrolysis to produce (+)-PIM-
an early stage of permeation (after 3 h for binol and 1 h for TTSBI).
[
26]
1
13
COOH (Scheme 1).
H NMR, C NMR, and IR spectros-
copy (see Figures S6–S8) showed that hydrolysis proceeded to
over 90% conversion into carboxylic acid groups without
À1
appreciable loss of molecular weight (M = 67578 gmol and
n
M = 123760; some broadening of the PDI occurred, to 1.8).
w
According to the GPC results, the hydrolysis of (+)-PIM-CN
with a strong acid was not destructive to the polymer
[
27,28]
backbone.
The modified material (+)-PIM-CN had
25
a specific rotation of ½aŠ5
¼ + 378.38 in THF and exhibited
89nm
a CD spectrum very similar to that of (+)-PIM-CN. (+)-PIM-
COOH showed reduced solubility in solvents such as chloro-
form, dichloromethane, and DMSO, but greater solubility in
THF. Whereas (+)-PIM-CN was fluorescent yellow, the solid
(+)-PIM-COOH was dark-brown in color. Furthermore, as
compared to (+)-PIM-CN, (+)-PIM-COOH films were
Figure 4. Enantioselective permeation of (R,S)-binol through (+)-PIM-
CN. The inset shows the results at an early stage of permeation.
b,c) HPLC chromatogram of the receiver side at 3 and 7 h, respectively,
after the start of permeation. d) HPLC chromatogram of racemic binol.
slightly more brittle, probably as a result of interchain
[27,28]
interactions between the carboxylic acid groups.
A
decrease in the water contact angle of ( ++ )-PIM-CN as
compared to that of (+)-PIM-COOH from 1008 to 308
suggests that the (+)-PIM-COOH membrane is considerably
more hydrophilic, a property that could be advantageous for
the permeation of more-polar analytes (see Figure S9).
The carboxylate derivative (+)-PIM-COOH was also
evaluated for enantioselective permeation. Although insuffi-
cient examples have been explored to generate guidelines for
choosing the most suitable membrane for separation, the
(+)-PIM-COOH membrane provided superior selective per-
meation of N-(9-fluorenylmethoxycarbonyl)phenylalanine
(Fmoc-Phe) racemates as compared to (+)-PIM-CN.
During the initial 1 h, 75% ee was observed (Table 1 and
Figure 5b). The increased hydrogen-bonding interactions of
(+)-PIM-COOH may contribute to the difference in perfor-
mance.
spirobisindane (Æ)-TTSBI was also observed with (+)-PIM-
CN: After 1 h, the ee value was 87% (see Figure S12). Thus,
(+)-PIM-CN membranes have the potential to produce
starting material for the replication of chiral membranes.
Significantly, the permeation rates of the (+)-PIM mem-
branes are higher than or comparable to those of previously
reported nonporous solid (À)-OMPS/PMMA silicone chiral
[34]
membranes
and a chiral, highly porous metal–organic
[35]
framework (MOF) membrane (Table 1; see also Table S8).
We suggest that the significantly higher permeation is due to
the intrinsic porosity of (+)-PIM polymers, thus emphasizing
the significance of this class of enantiomerically pure materi-
als for chiral resolution. Unlike chiral membranes based on
[
36]
[35,37,38]
polysaccharides
or chiral MOFs (CMOFs),
which
Regarding the mechanism of permeation, we observed
selective adsorption (68% ee) and permeation (75% ee) of
(S)-Fmoc-Phe with (+)-PIM-COOH (see Figure S12; Fig-
ure 5d). That is, the isomer selectively absorbed and perme-
ated was identical. In contrast to (+)-PIM-CN, selective
sorption occurs within (+)-PIM-COOH; however, since the
selectively absorbed enantiomer is also selectively perme-
ated, the mechanism of the (+)-PIM-COOH material may
often require the development of coating techniques on
porous substrates, the solution processability of high-molec-
ular weight (+)-PIMs facilitates the direct fabrication of
porous, mechanically robust membranes with highly selective
permeability.
Some insight into the origin of the highly selective
permeation of Man, binol, and TTSBI by (+)-PIM-CN was
obtained by measuring the adsorption of racemates by this
material. Interestingly, within the analytical precision of the
experiment, the adsorption of these compounds was not
selective (see Table S5). We conclude that permeation
through the ( ++ )-PIM-CN membrane is based on a selective
[13,14]
still be classified as selective diffusion.
Solid membranes through which enantiomers are selec-
tively permeated on the basis of selective diffusion typically
exhibit maximum enantioselectivity during the initial stage of
permeation; selectivity generally decreases with time owing
to competing nonselective diffusion of the more weakly
[33]
diffusion–permeation mechanism.
+)-PIM-CN can be chemically modified to expand the
[39]
(
binding enantiomer. The (+)-PIM membranes in this study
also exhibited a decrease in enantioselectivity as a function of
time (see Figure S13). Besides a mechanism-based contribu-
possible range of separation targets. To demonstrate this
possibility, we prepared a carboxylated derivative by subject-
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How to cite: Angew. Chem. Int. Ed. 2015, 54, 11214–11218
Angew. Chem. 2015, 127, 11366–11370
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Keywords: chirality · enantioselectivity · membranes ·
permeation · polymers of intrinsic microporosity
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Published online: July 31, 2015
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