Chiral Separation of Styrene Oxides Supported by Enantiomeric Tetrahedral Neutral Pd(II) Cages

Article abstract of DOI:10.1021/acs.inorgchem.9b02389

The separation of enantiomers is of considerable importance in the preparation of the compounds of biological interests, catalysis, and drug development. Here, we report a novel enantioseparation of styrene epoxides (SOs) resolved in the presence of a pair of enantio-enriched tetrahedral cages. Chiral neutral cages of formula [(Pd₃X*)₄(C₆O₄Cl₂)₆] ([X*]^3- = RRR- or SSS-[PO(N(*CH(CH₃)Ph)₃]^3-) are constructed from Pd₃ building units supported by tris(imido)phosphate trianions and chloranilate linkers. These cages exhibit considerable enantioselective separation capabilities toward a series of styrene epoxides via a crystallization inclusion method. A highest enantiomeric excess (ee) value of up to 80% is achieved for the (R)-4-fluorostyrene oxide.

Full text of DOI:10.1021/acs.inorgchem.9b02389

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Chiral Separation of Styrene Oxides Supported by Enantiomeric
Tetrahedral Neutral Pd(II) Cages
†
§
∥
∥
Prabhakaran Rajasekar, Swechchha Pandey, Joseph D. Ferrara, Mark Del Campo,
∥
,†,‡
Pierre Le Magueres, and Ramamoorthy Boomishankar*
†
‡
Department of Chemistry and Centre for Energy Science, Indian Institute of Science Education and Research (IISER), Pune, Dr.
Homi Bhabha Road, Pune 411008, India
§
Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
Rigaku Americas Corporation, 9009 New Trails Drive, The Woodlands, Texas 77381, United States
∥
*
S Supporting Information
epoxides are separated by other methods without altering them
to diols. Hence, benign methods based on supramolecular
systems like capsules, cavitands, metal−organic cages, and
frameworks as host materials for the chiral recognition and
separation of epoxides have been investigated.
Recently, our group has reported the synthesis of an oxalate
ABSTRACT: The separation of enantiomers is of
considerable importance in the preparation of the
compounds of biological interests, catalysis, and drug
development. Here, we report a novel enantioseparation
of styrene epoxides (SOs) resolved in the presence of a
pair of enantio-enriched tetrahedral cages. Chiral neutral
11,12
tethered enantiomeric pair of neutral tetrahedral cages 1-R and
3
−
cages of formula [(Pd X*) (C O Cl ) ] ([X*] = RRR-
3−
3
4
6
4
2 6
1-S of formula [(Pd X*) (C O ) ] ([X*] = RRR- or SSS-
3
4
2
4 6
3
−
or SSS-[PO(N(*CH(CH )Ph) ] ) are constructed from
Pd building units supported by tris(imido)phosphate
3−
3
3
[PO(N(*CH(CH )Ph) ] ) that showed a direct separation
3 3
3
of small organic compounds and epoxide such as (±)-
trianions and chloranilate linkers. These cages exhibit
considerable enantioselective separation capabilities to-
ward a series of styrene epoxides via a crystallization
inclusion method. A highest enantiomeric excess (ee)
value of up to 80% is achieved for the (R)-4-fluorostyrene
oxide.
epichlorohydrin via the crystallization inclusion method, albeit
13
with low enantiomeric excess (ee) values. Spurred by this, we
set out to investigate the separation of biochemically important
styrene oxides by utilizing tailor-made Pd(II) cages built on
anilate bound tetrahedral cages. The cage assemblies 2-(R)
all
RRR
and 2-(S) of formulas [(Pd X ) (C O Cl ) ] and
all
3
4
6
4
2 6
SSS
[
(Pd3X ) (C O Cl ) ] were formally formed via the self-
4 6 4 2 6
RRRorSSS
assembly of four chiral Pd3X
units and six bis-bidentate
hiral separation is defined as the process of separating the
individual enantiomers from their racemic pairs.
Separation of enantiomers is essential, owing to their unique
biochemical activities, as living systems are chiral in nature.
chloranilate linkers. Further, we demonstrate the ability of
these larger chiral cages for the efficient separation of racemic
styrene derived epoxides by a crystallization inclusion method.
A highest ee value of 80% was obtained for the R-4-
fluorostyrene oxide (+F-SO), and reasonably good ee values
as estimated by gas chromatography (GC) or high-pressure
liquid chromatography (HPLC) were obtained for all the other
investigated styrene epoxides.
C
1
Chiral epoxides are important synthons in synthetic and
pharmaceutical chemistry as the epoxy rings are very reactive
toward nucleophiles and yield various chiral organic function-
alities. Especially, styrene oxides act as excellent precursors for
various biologically relevant compounds. (S)-styrene oxide
The chiral tetrahedral cages, 2-(R) and 2-(S) (henceforth
all
all
14
(
phenyl oxirane) is an important intermediate for the synthesis
shortly referred as 2-R and 2-S), were isolated as brownish
2
3
of nematocide, Levamisole (anticancer agent), and (−)
red solids from the reaction of the respective chiral
4
RRR
SSS
hyperolactone C (anti-HIV agent). The (S)-4-fluorostyrene
phosphoramides, X H and X H , with Pd(OAc) in the
3 3 2
oxide (2-(4-fluorophenyl) oxirane) is a useful starting material
presence of chloranilic acid (LH ) in methanol (Scheme 1).
The MALDI-TOF mass spectrum of both 2-R and 2-S in
chloroform gave peaks centered at 4159 and 4278 correspond-
2
5
,6
for the synthesis of several potential bioactive compounds.
A
number of antiviral agents have been prepared from (S)-4-
7
+
+
chlorostyrene oxide (2-(4-chlorophenyl) oxirane). Among the
ing to the [M + Na] and [M +CHCl
3
+ Na] species,
1
methods available, a well-known protocol for the separation of
epoxides is the hydrolytic or aminolytic kinetic resolution
procedure (HKR) that was first shown to be assisted by
respectively (Figure S1, Supporting Information). The H
NMR spectrum gave well resolved signals due to the peripheral
imido-P(V) trianions, presenting chemical shifts due to the α-
methyl benzylamino group. The C NMR gave peaks
corresponding to the chloranilate linkers at 141.46, 180.59,
and 181.03 ppm, in addition to the peaks due to the imido
1
3
III
Jacobsen’s Co -(salen) type complexes, in which one of the
enantiomers is converted into a chiral diol while the other one
8
,9
is retained as the epoxide. Such HKR reactions have also
been shown to be catalyzed by enzymes such as lipases and
1
0
hydrolases. However, for incorporating a variety of functional
groups on the chiral backbone, it is highly desired that the
Received: August 7, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02389
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Scheme 1. Synthesis of Chiral Neutral Cages 2-R or 2-S
from the Enantiopure XRRRH or X H Ligands
SSS
a
3
3
Figure 2. SCRXD structures of 2-R (left) and 2-S (right). Color code:
Pd, orange; Canilate, gray; N, blue; O, red; Cl, green; P, magenta. The
carbon atoms of the chiral substituents are represented as dark green
and violet colored atoms.
a
Only one out of the six chloranilate linkers along the edges of the
tetrahedron is shown for clarity.
The cages exhibit additional rotational chirality as observed
by the clockwise (in 2-R) or anticlockwise (in 2-S) orientation
of the methyl groups attached to the stereogenic carbon
centers. Their packing structures indicate solvent accessible
voids of volume 3000 Å which amounts to 33% of the total
unit cell volume (Figures S8 and S9, Supporting Information).
ligands (Figures S2−S6, Supporting Information). The
thermogravimetric analysis (TGA) data revealed that the
cages were stable up to 250 °C (Figure S7, Supporting
Information).
3
The circular dichroism (CD) spectra confirm the enanti-
enrichment of 2-R and 2-S in solution. Thus, both 2-R and 2-S
gave mirror image peaks at 249, 341, and 396 nm, with the
respective positive and negative Cotton effects. While the first
peak is due the ligand centric π−π* transitions, the second and
third ones are due to metal to ligand charge transfer (MLCT)
Notably, both 2-R and 2-S exhibit a tighter packing than the
3
previously reported 1-R and 1-S (13051 Å ; 51% of the unit
cell volume), though the intrinsic void volumes of these new
3
15,16
cages are higher at 224 Å per cage.
A closer analysis of the
2
void structures in 2-R and 2-S indicates that they exhibit a
remarkable 3D-net of Sodalite topology for channels running
along the exterior cavities of the cages, which are in turn
connected to the intrinsic cavities of the individual cages
transitions (Figure 1). The absolute configurations ([α] D) of
(
Figure S10, Supporting Information).
Inspired by the host−guest capabilities of the earlier
reported cages based on such imido-Pd motifs, we examined
3
the preliminary guest encapsulation studies of these chiral
cages for CCl and toluene. Thus, the CCl and toluene treated
4
4
samples of 2-S gave the respective peaks at m/z = 4310 and
+
+
4
256 corresponding to the [CCl ⊂2-S] and [C H ⊂2-S]
4
7
8
ions (Figures S12 and S13, Supporting Information). A similar
set of experiments yielded the same results for 2-R as well.
Furthermore, due to the charge neutral nature of these chiral
cages as well as due to the presence of quinoid linkers, we
investigated the suitability of these cages for the chiral
separation of racemic styrene oxides (SOs). A family of
styrene oxides, phenyl oxirane (SO), 2-(4-fluorophenyl)-
oxirane (F-SO), 2-(4-chlorophenyl)oxirane (Cl-SO), and 2-
(4-bromophenyl)oxirane (Br-SO), were probed for the
enantioselective adsorption experiments with 2-R and 2-S.
For the separation experiments, the chloroform solutions of
the host cages were mixed with the racemic mixture of the
styrene oxides in excess and left for evaporation at 298 K. The
obtained solids were washed with cold n-pentane in order to
remove the remaining guest molecules. The resultant solids
were soaked in n-hexane to remove the enantio-enriched
styrene oxides, and the filtrates were subsequently injected for
the HPLC analysis.
The highest ee value of 80% is recorded for the R-isomer of
F-SO resolved by cage 2-R. A similarly high ee value of 78% is
again obtained for the +F-SO desorbed from 2-S as well. For
SO, the ee values of 7 and 39% for the R-isomer as resolved by
2-R and 2-S, respectively, were obtained from HPLC analysis.
The HPLC resolved ee values were found to be intermediate at
57 and 56% for the R- isomer of Cl-SO using the cages 2-R and
2-S, respectively (Table 1). The obtained ee values were
Figure 1. Circular dichroism spectra of the chiral cages 2-R and 2-S.
2
-R and 2-S were determined by optical rotation measure-
2
ments in 0.1 M solution of CHCl . The observed [α] D values
3
of 2-R (1322°) and 2-S (−1340°) were higher than the
R
S
corresponding phosphoramide ligands X H (+23) and X H
3
3
(
−25).
Further, the enantiopure structures of these cages were
confirmed by their structural determination. Crystals of 2-R
and 2-S suitable for single crystal X-ray diffraction (SCXRD)
analysis were grown from slow evaporation of their
corresponding DCM solutions. Both the cages were crystal-
lized in chiral cubic space group I23 signifying the T-symmetry
of these cages (Figure 2). The asymmetric unit consists of one-
twelfth of the cage molecule. Each corner of the tetrahedron
consists of the trigonal Pd X* chiral units that are built via the
3
cis-coordination of the tridentate tris(imido)phosphate trian-
ions. The edges of the tetrahedron consist of chloranilate
linkers which offer a pair of wide-angle O,O- chelation to the
9
0° cisoidal sites at each Pd(II) center.
B
DOI: 10.1021/acs.inorgchem.9b02389
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Table 1. Racemic Epoxides Employed in the Chiral
Separation Studies Resolved by 2-R and 2-S along with
Their HPLC-Determined Separation Results
structural cage exhibited prominent encapsulation of the guest
13
molecules at their external cavities. Similar transformations
involving cubic P23 to rhombohedral R3 have been observed
in the family of chiral tetrahedral organic cages reported by
17
Cooper and co-workers.
In summary, this work illustrates a reasonably good enantio-
separation of styrene epoxides by a crystallization inclusion
method using supramolecular ensembles. The tetrahedral cages
2
-R and 2-S were built in a facile strategy by using trinuclear
Pd(II) clusters containing chiral tris(imido)phosphate trian-
ions and chloranilate linkers. A highest enantioseparation of
8
0% has been observed for the +F-SO compound using the 2-
R cage. For the other SOs, the ee values were found to range
from 7 to 78%, indicating the variations in the noncovalent
interactions between the host and guest molecules play a
crucial role in the enantioseparation process. These findings
pave the way for the search of novel supramolecular systems
that can facilitate the efficient separation of epoxides and other
sensitive organic racemates.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02389.
consistently reproducible for three independent separation
experiments (Tables S3 and Figures S14−S22, Supporting
Information). Remarkably, the host cages can be regenerated
after each separation cycle by crystallizing them from DCM.
From the above observations, the obtained enantioselectivities
fall in the order of F-SO > Cl-SO > SO with respect to the
cages 2-R and 2-S. The Br-SO did not show any selectivity at
all, probably due to its larger size being incompatible with the
guest binding sites at the external cavities of the cages. Control
■
*
S
NMR and additional figures pertaining to crystal
structures, tables of bond lengths and bond angles,
TGA (PDF)
R
S
Accession Codes
experiments showed that the ligands X H and X H alone
3
3
CCDC 1898032−1898033 and 1957460−1957461 contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
could not resolve the above mentioned styrene oxides under
identical conditions. The previously reported cages 1-R and 1-
S did not show any selectivity as well. This indicates that the
tight association of the present cages, containing planar metal-
coordinated electron deficient chloranilate dianion surfaces,
with the oxirane guests plays a vital role in their
enantioselective separation.
1223 336033.
To track the nature of interaction of these racemic styrene
oxides with the chiral cages, H NMR and MALDI-TOF mass
1
AUTHOR INFORMATION
Corresponding Author
*E-mail: boomi@iiserpune.ac.in.
■
spectral analyses were performed on the host−guest mixtures
in solution. Since the size of ±SO is similar to that of toluene,
we presumed that it might exhibit encapsulation within the
intrinsic cavities of the cage. However, the MALDI-TOF mass
ORCID
Ramamoorthy Boomishankar: 0000-0002-5933-4414
1
1
spectrometry and H and H-2D-DOSY NMR of the ±SO did
not show any encapsulation at the intrinsic cavities (Figures
S23−S28, Supporting Information). Also, no encapsulation
and enantioselective separation were observed for the other
oxiranes such as p-cyano and p-nitro styrene oxides and other
chiral molecules such as 2-butanol and 1-phenyl ethanol.
Furthermore, we obtained crystals of both 2-R and 2-S when
treated with either ±SO or ±F-SO. However, higher quality
data sets from these crystals did not show the presence of these
guest molecules in their structures, albeit these cages were
found to crystallize in the rhombohedral space group of R3.
Formation of these new polymorphs could presumably be
driven by the template effects of the guest molecules which
resulted in the efficient packing of the cages in the R3 phase
compared to that in the I23 phase. These observations suggest
that the guest molecules would seemingly be recognized at the
chiral centers located at the exteriors of the cages via any one
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by SERB, India through Grant No.
EMR/2016/000614 (R.B.). P.R. thanks the UGC, India for the
fellowship. We thank Dr. S. Chikkali for providing access to
GC and HPLC measurements. We thank Dr. M. Benson for
helpful suggestions.
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DOI: 10.1021/acs.inorgchem.9b02389
Inorg. Chem. XXXX, XXX, XXX−XXX

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