Chiral Crystalline Sponges for the Absolute Structure Determination of Chiral Guests

Article abstract of DOI:10.1021/jacs.7b06607

Chiral crystalline sponges with preinstalled chiral references were synthesized. On the basis of the known configurations of the chiral references, the absolute structures of guest compounds absorbed in the pores of the crystalline sponges can be reliably

Full text of DOI:10.1021/jacs.7b06607

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Communication
Chiral Crystalline Sponges for the Absolute
Structure Determination of Chiral Guests
KaKing Yan, Ritesh Dubey, Tatsuhiko Arai, Yasuhide Inokuma, and Makoto Fujita
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b06607 • Publication Date (Web): 07 Aug 2017
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Chiral Crystalline Sponges for the Absolute Structure
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KaKing Yan,† Ritesh Dubey,† Tatsuhiko Arai, Yasuhide Inokuma, and Makoto Fujita*
Department of Applied Chemistry, School of Engineering, The University of Tokyo, and ACCEL (JST), 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan
ABSTRACT: Chiral crystalline sponges with a pre-installed
chiral reference were synthesized. Based on the known
configurations of the chiral reference, the absolute structures
of guest compounds absorbed in the pores of the crystalline
sponges were reliably determined without crystallization or
chemical modification.
Absolute configuration determination is the most
important, yet probably the most difficult structure analysis of
1
organic small molecules. Single crystal X-ray analysis is one
of the only methods that can directly determine absolute
configurations, without any chiral references, by observing the
1
c
anomalous scattering effect, commonly from heavy atoms
involved in analyte molecules (the Bijvoet method). However,
this absolute method is not always practical because of the
need to prepare the high quality single crystals of analytes and
also for synthetic loads for incorporating heavy atom(s) when
the anomalous scattering from the chiral guest is weak.
Crystalline sponge (CS) method is an emerging X-ray
2
,3
technique that does not require sample crystallization. This
method has been recently utilized to determine the absolute
2
a,d,4,5
Figure 1. (a) Synthesis of chiral CS 1. (b) Top view of the X-ray
crystal structure of (S,S)-1. The host framework is shown in gray.
The chiral reference (S,S)-2 (orange) is embedded in the host
(gray) through columnar stacking. Solvent molecules are omitted
for clarity. (c) Side view of the columnar stacking (inset part of
configuration of chiral compounds.
Since heavy (iodine)
atoms are already installed in the host framework as ZnI
2
metal nodes, incorporation of heavy atoms to guest analytes is
unnecessary. However, to observe effective anomalous
scattering from the host, initial achiral space group (C2/c)
(
b)).
1
must be altered to a chiral space group (C2 or P2 ) through
efficient chirality transfer from the guest to the host. This
requirement limits the method to be applicable only to guests
that show strong host-guest interaction with a high site
occupancy (normally, > 80%).
Chiral
CS,
[(ZnI
2
)
3
(tpt)
2
•(G*)]
n
(1:
tpt
=
2
,4,6-tris(4-pyridyl)-1,3,5-triazine; G* = (S,S)-2 or (R,R)-2),
To overcome this limitation, we designed here a chiral CS,
in which a chiral reference is pre-installed in the host
framework. We show that the absolute configuration of the
guest is easily and reliably determined by observing the
relative configurations of the guest with respect to a chiral
reference pre-installed in the chiral CS. Yaghi and co-workers
have recently reported a chiral MOF available for the absolute
structure determination of post-absorbed chiral guest, but the
chirality of the host is unknown and has to be judged from the
diffraction data. Our chiral CS contains a chiral reference of
known configuration and the guest configuration can be
determined more reliably by simply comparing its relative
configuration with respect to that of the chiral reference.
was prepared by the “plug-in” synthesis, in which a variety of
functional groups can be embedded within the host network by
the plug-in of functionalized triphenylenes (Figure 1a). Layer
6
diffusion of a methanol solution of ZnI into a PhNO /MeOH
2
2
solution of tpt ligand and (S,S)-2 gave yellow needle-like
crystals formulated as {[(ZnI ) (tpt) •(S,S)-2]•(PhNO ) •
2
3
2
2 5
(MeOH)} in 34% yield. Nitrobenzene that fills the pores was
n
completely replaced with less coordinating solvents such as
cyclohexane or n-hexane for facilitating the subsequent guest
5
c
2c
soaking. The solvent-exchanged CS, (S,S)-1, was fully
characterized by elemental analysis, IR, and X-ray diffraction
study (See the Supporting Information; SI). The structure of
(S,S)-1 was solved in the non-centrosymmetric P2 2 2 space
1
1 1
group. (S,S)-1 has two types of 1D channels: triangular and
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rectangular channels (pores A and B in Figure 1b). In a similar
fashion, enantiomeric (R,R)-1 was also synthesized as yellow
needles in 37% yield.
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Dimethyl L-(+)-tartrate (3) was included into the pores of
3
(
S,S)-1 by soaking a single crystal (0.12×0.19×0.20 mm ) of
2 2
(S,S)-1 in a cyclohexane (45 µL)/CH Cl (5 µL) solution of 3
(10 µg). After slow evaporation of the solvent at 50 ºC over 2
d, the resulting inclusion complex (S,S)-1•3 was subjected to
single crystal X-ray analysis. The crystal structure revealed
four guest 3 molecules, one of which was trapped in pores A
with 100% occupancy. Based on the known stereochemistry of
the chiral reference (S,S)-2, the absolute configuration of (+)-3
was confirmed to be 2R,3R (Figure 2). It is worth noting that,
in this case, the absolute configuration of (2R,3R)-3 was also
confirmed from the anomalous scattering (Flack parameter
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(
Parsons) = 0.024(4)).
Figure 3. X-ray crystal structures of (a) (S,S)-1•(1R,2S,5R)-4 and
(b) (S,S)-1•(1S,2R,5S)-4. Samples for the diffraction study were
prepared by the guest soaking of (-)- and (+)-4 enantiomers with
(
S,S)-1, respectively. The conformations of the two enantiomers
are slightly different because of their diastereomeric relationship
to the chiral reference (S,S)-1. Guests are observed both in pores
A
and B in (S,S)-1•(1R,2S,5R)-4 (with 100% and 77%
occupancies respectively), whereas they are in only pore A in
(S,S)-1•(1S,2R,5S)-4 with 100% occupancy. The guest and the
chiral reference are highlighted in color, one pair being presented
as space filling model. Thermal ellipsoids are set at the 50%
probability level.
2
One guest engages with a nearby iodine atom in ZnI with
an O•••I distance of 3.123 Å. In pores B, one slightly
disordered guest with occupancy of 77% was observed in
close proximity to the dioxolane substituent of (S,S)-2 with an
O•••O distance of 2.926 Å. By comparing the relative
stereochemistry with that of reference (S,S)-2, the absolute
configuration of (–)-4 was clearly determined to be
(1R,2S,5R)-4. The enantiomeric (+)-4, was also included in
(S,S)-1 but trapped with a slightly different conformation in
pores A (Figure 3b). Notably, no guest in pores B, nor any
guest interaction with dioxolane substituent of (S,S)-2 was
observed with (+)-4. The observation of different binding
modes in the diastereomeric pairs of (S,S)-1•(–)-4 and
Figure 2. (a) X-ray structure of (S,S)-1•(2R,3R)-3. The analyte
2R,3R)-3 and the chiral reference, (S,S)-1, plugged in the host
framework are highlighted in color. (b) ORTEP drawing of
2R,3R)-3. Thermal ellipsoids are shown at the 50% probability
level. Its R,R configuration was confirmed by comparing the
relative configuration to the chiral reference, (S,S)-1.
(
(
(
S,S)-1•(+)-4 clearly show that the chirality of (–)-4 and (+)-4
is discriminated by the chiral pores of 1. For further
confirmation of this chiral discrimination, we analyzed the
structure of (+)-4 with (R,R)-1 host (see SI). As expected,
Since CS 1 is chiral, the two enantiomers of any chiral
guests should show different absorption behavior because of
their diastereomeric relationship with respect to the chiral
reference. Thus, the two enantiomers of menthol (4) were
(
(
(
R,R)-1•(+)-4 revealed the exact mirror-image structure of
S,S)-1•(–)-4. In all cases, reasonable Flack parameter values
Parsons) were obtained (< 0.06) and the absolute
7
stereochemistry of the guests was also confirmed by the
Bijvoet method.
subjected to chiral CS analysis. When (–)-4 was included in
(
S,S)-1, the guests were observed in both pores A and pores B
Furthermore, we applied this method to the absolute
configuration analysis of sugar derivative 5, as sugars are one
of the most difficult classes of compounds to be
crystallographically analyzed. Empirical configuration
by X-ray diffraction study (Figure 3a). Two guest molecules in
pores A were found in an asymmetric unit with occupancy of
1
00% and 64%.
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method relayed to the known configuration of the chiral
analysis by Mösher method is hardly applied to sugars because
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of signal overlapping in the NMR analysis. After protecting
2,3- and 4,5-diol groups, D-fructopyranose was subjected to
chiral CS analysis with (S,S)-1 and its 2S,3S,4R,5R
configuration was confirmed (Figure 4).
reference, the present chiral CS is expected to be one of the
best tools for absolute configuration determination.
(a)
(b)
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Figure 4. X-ray crystal structure (2S,3S,4R,5R)-5 determined by
the chiral CS analysis (occupancy: 54%). Thermal ellipsoids are
shown at the 50% probability level.
(c)
OH
OH
R
N3
N3
S
(
S)-6
(R)-6
In combination with chiral HPLC analysis, chiral CS 1
provide a means of chiral LC–SCD (liquid chromatography–
single crystal diffraction) analysis for facile and reliable
absolute structure determination. A prochiral ketone, phenacyl
azide,
was
reduced
to
a
racemic
pair
of
2-azido-1-phenylethanol (rac-6) by NaBH
4
reduction. When
the crude reaction product, rac-6 (100 µg), was subjected to
chiral HPLC analysis, the two enantiomers were separated
with retention times of 32.3 and 34.6 min (column:
CHIRALPAK IC, solvent: hexane/iPrOH = 99/1, flow rate:
(d)
OH
OH
R
S
0
.5 mL/min at 25 °C). The first and second fractions showed
positive and negative CD signals, respectively, at 254 nm
Figure 5a). The first fraction was examined by chiral CS
Br
Br
(
R)-7
(S)-7
(
analysis with (S,S)-1. In the asymmetric unit, four guest
molecules of 6 were observed, two of which directly interact
with (S,S)-2 through a hydrogen bond with HO•••O distance of
1
.94 and 2.41 Å (Figure 5b). The absolute configuration of the
first fraction was thus reliably determined to be R (Figure 5c,
left). The S-configuration for the second fraction was also
confirmed by X-ray analysis using (R,R)-1 (Figure 5c, right).
In a similar way, a racemic pair of 4-bromo-a-methylbenzyl
Figure 5. (a) HPLC chromatogram of the racemic pair of 6
(black: UV absorption at 210 nm; red: CD signal at 254 nm). (b)
X-ray structure of (S,S)-1•(R)-6. A sample for the diffraction
study was prepared by the guest soaking of the first fraction in the
HPLC purification into (S,S)-1. The guest (two molecules) and the
chiral reference in one pore are highlighted in color. (c) Left:
ORTEP drawing of (R)-6 found in the (S,S)-1•(R)-6 structure
alcohol (rac-7), obtained from NaBH
4
reduction of
4’-bromoacetophenone, was analyzed by chiral HPLC and the
first fraction collected (tret = 58.3 min) was deduced to be
(
(
(
occupancy: 100%); right: ORTEP drawing of (S)-6 found in the
R,R)-1•(S)-6 (occupancy: 94%). (d) Left: ORTEP drawing of
R)-7 found in the (S,S)-1•(R)-7 structure (occupancy: 82%). The
(R)-7 using (S,S)-1 as the chiral CS and second fraction (tret
4.0 min) was assigned to be (S)-7 using (R,R)-1 (Figure 5d).
In summary, chiral CS with a pre-installed chiral reference
=
6
analyte (R)-7 was obtained as the first fraction of the chiral HPLC
purification; right: ORTEP drawing of (S)-7 found in the
(R,R)-1•(S)-7 (occupancy: 86%). The analyte (S)-7 was obtained
as the second fraction of the chiral HPLC purification. Thermal
ellipsoids are shown at the 50% probability level.
has been developed and successfully used for the
determination of the absolute configuration of chiral
compounds by comparing the relative configurations between
the reference and the analyte compound. The only requirement
of our method is the concomitant observation of both chiral
molecules (the reference and the analyte) in the crystal
structure. This represents a great advantage over other
commonly used relative methods, such as Mosher’s empirical
8a-c
method, co-crystallization method, or sample derivatization
8d-e
with a chiral reference,
in which close contact or efficient
ASSOCIATED CONTENT
chiral interaction between the analyte and the reference is
essential, yet does not always promise successful
determination of the absolute configuration. Because of the
crystallization-free protocol, applicability for trace-amount
analysis, and, more importantly, high reliability of the relative
Supporting Information
This material is available free of charge via the Internet at
http://pubs.acs.org.
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Materials and methods; experimental procedures and
Lett. 2016, 57, 4633–4636. (e) Brkljača, R.; Schneider, B.;
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characterizations; X-ray crystallographic data.
Hidalgo, W.; Otálvaro, F.; Ospina, F.; Lee, S.; Hoshino, M.;
Fujita, M.; Urban, S. Molecules 2017, 22, 211–219. (f)
Vinogradova, E. V.; Müller, P.; Buchwald, S. L. Angew. Chem.
Int. Ed. 2014, 53, 3125–3128. (g) Kamimura, D.; Urabe, D.;
Nagatomo, M.; Inoue, M. Org. Lett. 2013, 15, 5122–5125. (h) G.
O’Brien, A.; Maruyama, A.; Inokuma, Y.; Fujita, M.; Baran, P.
S.; Blackmond, D. G. Angew. Chem. Int. Ed. 2014, 53, 11868–
Crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
1
1871.
mfujita@appchem.t.u-tokyo.ac.jp
(
4) Absolute structure determination using Zn-tpt based crystalline
sponge: (a) Yoshioka, S.; Inokuma, Y.; Hoshino, M.; Sato, T.;
Fujita, M. Chem. Sci. 2015, 6, 3765–3768. (b) Takizawa, S.;
Kishi, K.; Yoshida, Y.; Mader, S.; Arteaga, F. A.; Lee, S.
Hoshino, M.; Rueping, M.; Fujita, M.; Sasai, H. Angew. Chem.
Int. Ed. 2015, 54, 15511–15515. (c) Urban, S.; Brkljača, R.;
Hoshino, M.; Lee, S.; Fujita, M. Angew. Chem. Int. Ed. 2016,
Author Contributions
†K.Y. and R.D. contributed equally.
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Notes
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5, 2678–2682. (d) Inokuma, Y.; Ukegawa, T.; Hoshino, M.;
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Fujita, M. Chem. Sci. 2016, 7, 3910–3913. (e) Matsuda, Y.;
Mitsuhashi, T.; Lee, S.; Hoshino, M.; Mori, T.; Okada, M.;
Zhang, H.; Hayashi, F.; Fujita, M.; Abe, I. Angew. Chem. Int.
Ed. 2016, 55, 5785–5788. (f) Lee, S.; Hoshino, M.; Fujita, M.;
Urban, S. Chem. Sci. 2017, 8, 1547–1550. (g) Sairenji, S.;
Kikuchi, T.; Abozeid, M. A.; Takizawa, S.; Sasai, H.; Ando, Y.;
Ohmatsu, K.; Ooi, T.; Fujita, M. Chem. Sci. 2017, DOI:
T.A. is grateful for a JSPS Research Fellowship for Young
Scientists.
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5
ACS Paragon Plus Environment