Chiral Phosphotungstate Functionalized with (S)-1-Phenylethylamine: Synthesis, Characterization, and Asymmetric Epoxidation of Styrene

Article abstract of DOI:10.1021/acs.inorgchem.1c00636

In the present work, an attempt has been made to induce chirality in copper-substituted phosphotungstate (PW11Cu) by functionalization with (S)-(+)-1-phenylethylamine (S-PEA) via a ligand substitution approach. The formation of a N→Cu dative bond was conf

Full text of DOI:10.1021/acs.inorgchem.1c00636

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Chiral Phosphotungstate Functionalized with (S)‑1-
Phenylethylamine: Synthesis, Characterization, and Asymmetric
Epoxidation of Styrene
Anjali Patel,* Rajesh Sadasivan, and Jay Patel
Cite This: Inorg. Chem. 2021, 60, 10979−10989
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ABSTRACT: In the present work, an attempt has been made to induce chirality in copper-
substituted phosphotungstate (PW11Cu) by functionalization with (S)-(+)-1-phenylethylamine (S-
PEA) via a ligand substitution approach. The formation of a N→Cu dative bond was confirmed by
13C NMR, while 1H NMR, circular dichroism spectroscopy and optical rotation studies confirmed
the introduction of chirality to the Keggin structure. The synthesized material was used as the
heterogeneous catalyst for the asymmetric epoxidation of styrene using various green oxidants to
obtain high enantiomeric excess (ee), and the reaction with molecular oxygen was found to give the
best ee. Regeneration studies were carried out, and the catalyst was found to be suitable for the
same. A probable mechanism is also proposed. A comparison with other copper-based
polyoxometalate catalysts clearly demonstrate the superiority and novelty of the present catalyst
in terms of the reaction conditions as well as the obtained ee.
synthesis and design involving the second approach, are
available on POM-based chiral materials.6−15
INTRODUCTION
■
Chirality is a crucial issue, starting from chemistry to materials
science as well as biology,1 and has been a topic of interest for
a long time, and it is still a hot topic for researchers. Chiral
inorganic−organic hybrid materials based on polyoxometalates
(POMs) have generated much attention because of their
potential applications in the fields of synthetically specialized
materials, enantioselective catalysis, and medicine.2,3 In light of
the advantages of POMs and the importance of chirality,
tremendous effort has been dedicated to the development of
chiral POM-based materials. As a result, this has aroused
increasing interest and become a new growing topic in recent
years.
Chirality can be derived by the introduction of either chiral
ligands or stereogenic side chains in POM/transition-metal-
substituted POM (TMSPOM) structures. Chiral inorganic−
organic materials can be synthesized by mainly two types of
approaches. The first approach is to link suitable discrete chiral
polyoxoanions by simple metal cations or metal−organic
segments. The drawbacks of these enantiomers are that POMs
in acidic solutions are highly unstable, easily hydrolyze, and
racemize rapidly to lose their optical activity. In the second
approach, chiral organic ligands will be linked to achiral POMs
by means of coordination bonding, covalent or electrostatic
interaction.4 The advantages of this technique are that chirality
is transferred from the organic ligand to the symmetric POM
and also the new material possesses unusual characteristics.
Therefore, this is the most common technique to synthesize
chiral POMs.5 Many reviews, especially focusing on the
The materials using the second approach can be designed by
functionalization, and in this context, TMSPOMs can act as a
set of transferable excellent building blocks for construction of
the aforementioned functionally active solids. TMSPOMs can
also be rationally modified on the molecular level including
shape, size, charge density, redox state, and stability. The
exclusive structural diversity of TMSPOMs, along with their
ability to replace their aqua ligand and thereby coordinate with
organic ligands, makes them excellent candidates for
functionalization. They can act as excellent inorganic multi-
dentate oxygen-donor ligands to assemble various inorganic−
organic hybrid compounds with a vast range of structural
properties, combining the features of both the organic and
inorganic components.16,17 The obtained materials have
potential applications in the fields of medicine, magnetism,
optics, biology, catalysis, electro/photo/thermochromic sys-
tems, electrode cathodes, batteries and supercapacitors, solar
energy conversion, molecular recognition, and nonlinear
materials.6,18−32
Received: March 3, 2021
Published: July 16, 2021
https://doi.org/10.1021/acs.inorgchem.1c00636
© 2021 American Chemical Society
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Scheme 1. Functionalization of PW11Cu with S-PEA
Despite the diversity in chiral-POM-based materials, those
based specifically on transition-metal-substituted phosphotung-
states are sparse. In 2005, Belai and Pope synthesized two
cobalt ethylenediamine (en)-incorporated POMs,
Na6.5K5.5[K⊂{Co(en)WO4}WO(H2O)(PW9O34)2]·19H2O
and Na7K5[{Co(en)(μ-OH)2Co(en)}{PW10O37Co(en)}2]·
20H2O, using a coordination competition approach. These
complexes were the first examples of POMs that incorporate
embedded chelated heteroatoms, which open a broad range of
new steric and chiral possibilities for POMs. Of the two, the
former shows mirror symmetry because of the fluxional
behavior of the en ligand, despite an overall C1 symmetry.
On the other hand, in the second complex, [PW10O37Co-
(en)]6− is chiral in nature and functions as a bidentate ligand
between two Co(en) groups.33
In 2012, Patel and Patel synthesized two chiral inorganic−
organic hybrid molecules by functionalizing monomanganese-
substituted phosphotungstate with (S)-(+)-sec-butylamine and
(R)-(−)-1-cyclohexylethylamine by a ligand substitution
approach and described their detailed characterization.34,35
More recently, the same group reported the synthesis of a
monoruthenium-substituted phosphotungstate functionalized
with (R)-(−)-1-cyclohexylethylamine using the same approach
and its catalytic activity for the aerobic asymmetric oxidation of
styrene.36
PW11Cu using a simple chiral molecule, and (S)-1-phenyl-
ethylamine (S-PEA) was chosen as the model ligand. The
material was synthesized by a ligand substitution method,
characterized by various techniques, and then evaluated for its
catalytic activity for the asymmetric oxidation of styrene.
Studies were also carried out using different green oxidants
with an effort to obtain maximum ee of the chiral product, and
the best oxidant was used to optimize various reaction
parameters for high conversion as well as maximum ee.
EXPERIMENTAL SECTION
■
Material. All chemicals were of analytical reagant grade and were
used as received. 12-Tungstophosphoric acid, copper chloride
dihydrate, cesium chloride, sodium hydroxide, styrene, 70% tert-
butyl hydroperoxide (TBHP), and dichloromethane were obtained
from Merck. (S)-(+)-1-Phenylethylamine (S-PEA) was obtained from
TCI Chemicals, Chennai, India, while NaHCO3 was obtained from
SRL, Mumbai, India.
Synthesis of a Chiral-Functionalized Material (PW11Cu-S-
PEA). The synthesis of PW11Cu-S-PEA was carried out in two steps.
(i) Synthesis of PW11Cu: PW11Cu was synthesized by the one-pot
method reported previously by our group.52 A total of 2.88 g (1
mmol) of 12-tungstophosphoric acid was dissolved in a minimum
amount of water, its pH was adjusted to 4.8 using saturated sodium
hydroxide, and the solution was heated to 90 °C for 1 h. A total of
0.17 g (1 mmol) of copper chloride dissolved in water was slowly
added to the hot solution, and then the mixture was refluxed for 1.5 h,
followed by filtration. Solid cesium chloride (0.5 g) was immediately
added, and the resulting greenish-blue precipitates were filtered, dried
at room temperature, and designated as PW11Cu.
(ii) Functionalization of PW11Cu with S-PEA: A total of 0.35g (0.1
mmol) of PW11Cu was dissolved in 10 mL of water by heating, and 50
μL (0.1 mmol) of S-PEA was dissolved 10 mL of methanol. The
methanolic solution of S-PEA was added dropwise to the hot, clear
solution of PW11Cu with stirring, and the pH was adjusted to 6.4
using dilute sodium hydroxide. The resultant mixture was refluxed for
24 h at 90 °C, cooled, filtered, and dried at 100 °C, and the light-
green solid obtained was designated as PW11Cu-S-PEA (Scheme 1).
Synthesis of a physical mixture of PW11Cu and S-PEA (PW11Cu+S-
PEA): The physical mixture was synthesized by mixing an equimolar
amount of cesium salt of PW11Cu (0.35g, 0.1 mmol) and S-PEA (50
μL, 0.1 mmol). The mixture was ground properly in a mortar and
pestle until it became homogeneous in nature. The light-green
mixture obtained was designated as PW11Cu+S-PEA.
Reports are available on the synthesis and characterizations
of copper-based inorganic−organic hybrid POMs having
copper complexes as a countercations37−51 consisting of
phosphotungstates37−41 and silicotungstates.39,42−51 However,
it should be highlighted that, out of the cited references, very
few reports are available where the oxidation of styrene has
been carried out. In 2017, the Qiuxia group reported the use of
synthesized of L- or D-pyrrolidine-2-ylimidazole-functionalized
copper phosphotungstate (CuW−PYI1) for styrene oxidation
using tert-butyl hydroperoxide (TBHP).41 In 2008, Mirkhani
et al. reported Cu(salen)-silicotungstate for styrene oxidation
using hydrogen peroxide (H2O2) in acetonitrile (CH3CN).47
A
year later (2009), Zheng et al. described the use of {[Cu2(4,4′-
bpy)(4,4′-Hbpy)4(H2O)4](SiW12O40)2(H2O)4}n, {[Cu2(4,4′-
bpy)(4,4′-Hbpy)6(SiW12O40)3](4,4′-Hbpy)2(H2O)7}n,
{[Cu2(μ2-H2O)2(4,4′-bpy)3(SiW12O40)](H2O)6}n, and
{[Cu2(μ2-H2O)2(4,4′-bpy)3(SiW12O40)](H2O)6}n as catalysts
for styrene oxidation using TBHP in an CH3CN solvent.49
After that, in 2011, He et al. reported the synthesis of (i)
Characterization Techniques. Elemental analysis as well as
thermogravimetric analysis (TGA)−differential thermal analysis was
carried out using a PerkinElmer Optima-3300 RL ICP spectrometer
and JSM 5610 LV EDX-SEM analyzer and a Mettler Toledo Star SW
7.01 up to 550 °C, respectively. Fourier transform infrared (FT-IR)
spectra of the sample were obtained using KBr pellets on a Shimatzu
IRAffinity-1S instrument. Solid-state 13C magic-angle-spinning
(MAS) NMR was recorded in a JEOL ECX 400 MHz high-resolution
[CuII (C5H5NCOO)2(4-bpo)2(H2O)2] SiW12O40·H2O, (ii)
2
[CuI4(4-bpo)6]SiW12O40·3H2O, and (iii) [CuI4(3-bpo)4]-
SiW12O40·3H2O and its use for the said reaction using
TBHP in CH3CN.50 It is worth noting that all reported
catalysts do not give enantiomeric excess (ee).
1
multinuclear FT-NMR spectrometer for solids. H NMR was carried
A literature survey shows that monocopper-substituted
phosphotungstate (PW11Cu) has never been functionalized
using a chiral organic ligand. In the present work, an attempt
has been made to execute this by the functionalization of
out in deuterated water as a dispersion solvent after sonication of the
sample on a Bruker AVANCE 400 MHz instrument. Electron-spin
resonance (ESR) spectra were recorded on a Varian E-line Century
series X-band ESR spectrometer at liquid-nitrogen temperature and
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scanned from 2000 to 3000 G. Powder X-ray diffraction (XRD)
patterns were obtained using a Philips PW-1830 instrument. The
conditions were as follows: Cu Ka radiation (1.54 Å); scanning angle
from 0° to 60°. Circular dichroism (CD) spectroscopy was carried out
in a Jasco J 815 instrument.
Catalytic Activity. The oxidation of styrene was carried out with
stirring using PW11Cu-S-PEA as the catalyst in a 50 mL batch reactor
attached to a double-walled air condenser on a magnetic hot plate.
Styrene (10 mmol), the catalyst, and 70% TBHP as an oxidant were
added to the substrate. Because of the absence of any aqueous media,
the catalyst remained heterogeneous. In the case of reaction with
molecular oxygen (O2 as an oxidant), 100 mmol (10 mL) of styrene,
the catalyst, and a small amount of TBHP as the initiator (∼0.2 mL)
were taken in a 50 mL two-necked round-bottom flask. O2 gas was
steadily bubbled at 1 atm of pressure with continuous stirring. After
the reaction was completed, the obtained mixture was extracted with
dichloromethane and analyzed using a gas chromatograph (Shimatzu-
2014), having a flame ionization detector with a chiral capillary
column (Rt-bDEXsm) using nitrogen as the carrier gas. The products
were identified by a comparison with the authentic samples of
benzaldehyde and (R) and (S)-styrene epoxide, and % ee was
calculated as follows:
Figure 2. FT-IR spectra of (a) PW11Cu (b) S-PEA, and (c) PW11Cu-
S-PEA.
TGA, the molecular formula of the synthesized material is
proposed to be Cs5[PW11O39Cu(C8H11N)]·H2O.
[R] − [S]
[R] + [S]
% ee =
= % R − % S
Figure 2 shows the FT-IR spectra of PW11Cu, S-PEA, and
PW11Cu-S-PEA. The FI-IR spectrum of PW11Cu shows
characteristic bands at 1103 and 1068, 964, 887, and 821
and 488 cm−1 corresponding to P−O, WO, W−O−W, and
Cu−O bonds, respectively. The FT-IR spectrum of pure S-
PEA shows characteristic bands of −NH stretching vibrations
at 3433 cm−1, aromatic ring stretching at 3062 and 3032 cm−1,
methyl stretching at 2924 cm−1, and CC stretching at 1490
and 1450 cm−1. The FT-IR spectrum of PW11Cu-S-PEA shows
bands of both PW11Cu and S-PEA but with a slight shift in all
of the bands, indicating functionalization of the POM with an
organic ligand. Along with this, an additional band is observed
at 516 cm−1, attributed to Cu−N stretching vibrations.
RESULTS AND DISCUSSION
Elemental analysis of PW11Cu and PW11Cu-S-PEA was carried
out using inductively coupled plasma atomic emission
■
1
To confirm the functionalization, 13C MAS and H NMR
were recorded. The chemical shift values of pure and
functionalized S-PEA are shown in Table 1, while the spectra
are presented in Figure 3. The characteristic peaks of S-PEA at
126.9, 128.5, and 147.9 ppm correspond to aromatic carbons,
that at 25.6 ppm corresponds to methyl carbon, and that at
51.3 ppm corresponds to chiral carbon.
The 13C NMR spectrum of PW11Cu-S-PEA (Figure 3a)
shows all of the characteristic peaks with shifting, indicating the
intact structure of the ligand as well as successful
functionalization. The chiral carbon shows a downfield shift
from 51.3 to 74.9 ppm. Similarly, the chemical shift of the
methyl carbon also is highly deshielded, giving a peak at 66.4
ppm. The drastic shift of δ values toward the downfield region
is attributed to the loss of electronegativity around the adjacent
nitrogen, indicating that the lone pair of electrons is used up
for the formation of a N→Cu dative bond. On the other hand,
the aromatic carbons displaying an upfield shift may be due to
the electron-rich POM unit, which shields the aromatic
carbons. Further, the inducement of S-PEA is confirmed by
1H NMR spectra (Figure 3b,c).
Figure 1. TGA of (a) PW11Cu and (b) PW11Cu-S-PEA.
spectrometry as well as energy-disperive X-ray spectroscopy
and the obtained results were in good agreement with the
theoretical ones. Theoretical for PW11Cu: Cs, 19.95; W, 57.36;
P, 0.95; Cu, 1.83; O, 19.88. Experimental: Cs, 20.01; W, 57.43;
P, 0.97: Cu, 1.80; O, 19.78.
Theoretical for PW11Cu-S-PEA: P, 0.87; W, 57.05; Cs,
19.29; Cu, 1.79; O, 17.89; C, 2.71; N, 0.40. Experimental: P,
0.76; W, 56.65; Cs, 19.45; Cu, 1.81; O, 17.45; C, 2.91; N, 0.51.
TGA of S-PEA shows 99.2% weight loss up to 150 °C. TGA
of PW11Cu-S-PEA (Figure 1) shows an initial weight loss of
1.8% up to 120 °C attributed to the loss of adsorbed water as
well as partial decomposition of the organic ligand. A further
weight loss of 1.7% up to 370 °C is attributed to complete
decomposition of SPEA. The total number of calculated water
molecules is found to be 1. From elemental analysis as well as
The 1H NMR spectrum of S-PEA (Figure 3b) shows a broad
methyl group as a singlet at 1.191 ppm, a chiral proton was
observed at 3.872 ppm, and a phenyl ring proton was observed
1
at 7.236 ppm. The H NMR spectrum of PW11Cu-S-PEA
(Figure 3c) shows a downfield shift with a broad peak at 7.37
ppm of an aromatic proton of S-PEA and a peak at 1.419 ppm
of a methyl group proton. This confirms the formation of a
N→Cu dative bond, which results in a decrease in the electron
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Table 1. 13C MAS NMR Chemical Shifts of S-PEA and PW11Cu-S-PEA
1
Figure 3. (a) 13C MAS NMR spectrum of PW11Cu-S-PEA and H NMR spectra of (b) S-PEA and (c) PW11Cu-S-PEA
density around the protons, making them more deshielded
compared to the protons of S-PEA.
structure of the Keggin unit. Further, a slight shift in the 2θ
value of copper in the case of PW11Cu-S-PEA may be due to
chemical interaction between nitrogen and copper.
The ESR spectra of PW11Cu and PW11Cu-S-PEA are shown
in Figure 4. It is well-known that PW11Cu gives a five-line
hyperfine spectrum characteristic of copper(II) in octahedral
or distorted octahedral geometry. PW11Cu-S-PEA shows a
similar spectrum, indicating that copper exists in the 2+ state
even after functionalization with retention of the geometry.
The decrease in the intensity of the hyperfine lines is attributed
to interactions of the copper center with the ligand, as
expected.
Wide-angle powder XRD spectra of PW11Cu and PW11Cu-S-
PEA are shown in Figure 5. PW11Cu shows characteristic peaks
of PW11 from 25 to 35° 2θ values,52 indicating the presence of
PW11 in the synthesized complex. In addition, it also shows
peaks at 48 and 50° 2θ, conforming the presence of
copper(II).53 The powder XRD pattern of PW11Cu-S-PEA is
similar to that of unfunctionalized PW11Cu, indicating an intact
Optical rotation studies were carried out using a pulse
polarimeter to get an idea about the presence of chirality in the
synthesized catalyst. A stock solution containing 5 mg of
PW11Cu-S-PEA in 10 mL of double-distilled water was
prepared. After a blank run using distilled water, the stock
solution was placed in a sample holder and the relative optical
rotation was recorded. While the pure ligand showed an optical
rotation ranging from [α]2D0 −37 to −410, the synthesized
material gave a relative optical rotation value of −0.74710.
This confirmed that the synthesized material showed chirality;
however, the very small value compared to that of pure ligand
was attributed to two reasons: (a) the sparingly soluble nature
of the material and (b) a much lower concentration of the
chiral ligand in the synthesized material (5.64 × 10−6 mmol).
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Figure 6. CD spectra of PW11Cu-S-PEA.
Figure 4. ESR spectra of (a) PW11Cu and (b) PW11Cu-S-PEA.
Figure 7. Effect of the catalyst amount (styrene, 10 mmol; TBHP, 2
mL; time, 16 h; temperature, 60 °C).
Figure 5. Powder XRD spectra of (a) PW11Cu and (b) PW11Cu-S-
PEA.
The presence of chirality is further confirmed by CD
spectroscopy.
The CD spectra of PW11Cu-S-PEA (Figure 6) shows only a
single strong Cotton effect at around 250 nm, which is the
fingerprint region for O → W charge transfer in Keggin
structures.54 Figure 6 also indicates that optical rotation
activity has been induced in the POM moiety because of
chemical interaction with the chiral ligand, confirming the
observations of the polarimetry experiment. The intensity of
the induced chirality seems to be much less, which may be
because of the much lower concentration of the chiral ligand
(5.64 × 10−6 mmol) as well as the poor solubility of the
synthesized complex, as explained previously.
Asymmetric Oxidation of Styrene. The oxidation of
styrene was carried out using TBHP as the oxidant, and various
parameters were optimized with the aim of obtaining
Figure 8. Effect of time (catalyst amount, 20 mg; styrene, 10 mmol;
TBHP, 2 mL; temperature, 60 °C).
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reduced. Hence, 12 h was considered to be the optimum time
for the reaction.
Table 2. Effect of the Oxidants
% selectivity
With a view to obtain better ee, the reaction was carried out
using different green oxidants, and the results are presented in
Table 2. The reactions using H2O2 and molecular O2 were
carried out at 80 °C; however, a few drops of TBHP (∼0.2
mL) was added as the initiator in the case of molecular O2. It is
interesting to note that, despite the lower selectivity of the
epoxide, the reaction with molecular O2 gave a higher ee of 16.
This can be explained as follows.
It is known that in olefinic solvents, such as styrene, TBHP
undergoes decomposition to give styrene oxide.55 The
obtained result, a racemic mixture (Table 2, entry 1) with
TBHP in the absence of catalyst, is as expected. On the other
hand, TBHP in the presence of chiral catalyst gives almost a
similar conversion, but the epoxide is no longer racemic (ee =
6). This may be due to the introduction of chirality. Further, it
can be assumed that the majority of the TBHP molecules
undergo self-disproportionation, while a small number of
molecules interact with the catalyst, thereby giving a small ee.
In the case of molecular O2, self-disproportionation does not
occur, and hence the entire oxidant has to interact with the
catalyst to form an intermediate, which leads to the formation
of epoxide with a high value of ee. The addition of a few drops
of TBHP is expected to initiate the reaction, i.e., to convert
molecular O2 to a superoxo radical (a detailed explanation is
given in the Probable Reaction Mechanism section). Further
%
(R)-styrene
(S)-styrene
oxidant conversion benzaldehyde
oxide
oxide
ee
a
TBHP
TBHP
36
35
N.A.
43
63
40
21
b
25
13
28
18
6
c
H2O2
d
O2
63
16
a
Without catalyst; styrene, 10 mmol; TBHP, 2 mL; time, 12 h;
b
temperature, 60 °C. Catalyst amount, 20 mg; styrene, 10 mmol;
c
TBHP, 2 mL; time, 12 h; temperature, 60 °C. Catalyst amount, 20
mg, styrene, 10 mmol; H2O2, 30 mmol; time, 12 h; temperature, 80
d
°C. Catalyst amount, 20 mg, styrene, 100 mmol; O2, 1 atm; TBHP,
0.2 mL; time, 12 h; temperature, 80 °C.
maximum ee. The reaction was carried out at different catalyst
amounts from 10 to 20 mg (Figure 7), and a steady increase in
the conversion and selectivity of styrene oxide were observed.
As expected, a higher selectivity of the S isomer was obtained,
but a maximum ee of only 4 was attained. From the results
obtained, 20 mg of catalyst was considered to be optimum.
Next, when the reaction time was varied from 8 to 12 h,
there was an increase in the conversion and selectivity of
epoxide along with its ee (Figure 8). With a further increase in
the reaction time, no increase in the conversion was observed,
but the selectivity and ee of the S isomer of epoxide were
Figure 9. Various reaction parameters: (a) time (catalyst amount, 20 mg; styrene, 100 mmol; O2, 1 atm; TBHP, 0.2 mL; temperature, 80 °C); (b)
catalyst amount (styrene, 100 mmol; O2, 1 atm; TBHP, 0.2 mL; time, 12 h; temperature, 80 °C); (c) temperature (catalyst amount, 20 mg; styrene,
100 mmol; oxidant O2, 1 atm; TBHP, 0.2 mL; time, 12 h).
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Table 3. Effect of S-PEA
% selectivity
entry
catalyst
% conversion
benzaldehyde
(R)-styrene oxide
(S)-styrene oxide
ee
a
1
2
3
4
5
PW11Cu
S-PEA
13
0.3
43
22
15.2
99
100
63
80
72.6
b
c
PW11Cu-S-PEA
PW11Cu-S-PEA
PW11Cu+S-PEA
13.2
8
18.3
9
16
6
d
d
13.5
13.4
0.25
a
b
c
Catalyst amount: 19.2 mg. Catalyst amount: 0.8 mg. Catalyst amount, 20 mg, styrene, 100 mmol; O2, 1 atm; TBHP, 0.2 mL; time, 12 h;
d
temperature, −80 °C. Catalyst amount, 20 mg, styrene, 100 mmol; O2, 1 atm; TBHP, 0.2 mL; time, 4 h; temperature, −80 °C.
Table 4. Effect of the Oxidant and Scavenger
% selectivity
(R)-
styrene
oxide
(S)-
styrene
oxide
%
catalyst
conversion benzaldehyde
ee
a
O2
1
1.7
22
23
100
62.5
80
80
b
TBHP
TBHP + O2
TBHP + O2 +
scavenger
18.1
8
18.7
9
1.6
6
c
8
9
6
d
a
Catalyst amount, 20 mg; styrene, 100 mmol; O2, 1 atm; time, 4 h;
b
temperature, 80 °C. Catalyst amount, 20 mg; styrene, 100 mmol;
c
TBHP, 0.2 mL; time, 4 h; temperature, 80 °C. Catalyst amount, 20
mg; styrene, 100 mmol; O2, 1 atm; TBHP, 0.2 mL; time, 4 h;
d
temperature, 80 °C. Catalyst amount, 20 mg; styrene, 100 mmol; O2,
1 atm; TBHP, 0.2 mL; time, 5 h (4 h + 1 h); temperature, 80 °C.
Figure 10. Heterogeneity test.
Scheme 2. Mechanistic Evaluation of Styrene Epoxidation
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a
temperatures, and the results are shown in Figure 9c. As
expected, the conversion and ee toward the S isomer increase
with an increase in the temperature from 60 to 80 °C. Beyond
80 °C, polymerization of styrene was observed, and, hence, the
temperature for the reaction was optimized at 80 °C.
Table 5. Recycle Study
% selectivity
(R)-
styrene
oxide
%
(S)-styrene
oxide
catalyst
conversion benzaldehyde
ee
The optimized conditions for 43% conversion and 16 ee are
as follows: catalyst amount, 20 mg (the active amount of S-
PEA is 0.68 mg, 5.64 × 10−6 mmol); styrene, 100 mmol; O2, 1
atm; TBHP, 0.2 mL; time, 12 h; temperature, 80 °C.
PW11Cu-S-
43
42
42
63
64
63
13.2
18.3
16
PEA
R1-PW11Cu-
S-PEA
R2-PW11Cu-
S-PEA
13
18
16
16
13
18
Control Experiments: Effect of S-PEA. In order to study
the role of the organic ligand as well as to confirm chemical
interaction, experiments were carried out with PW11Cu, S-
PEA, and PW11Cu+S-PEA (physical mixture) under optimized
conditions, and the results are shown in Table 3. The reaction
does not proceed (negligible conversion) in the presence of an
organic ligand alone, while the single product, benzaldehyde
without any epoxide in the case of PW11Cu, can be attributed
to the absence of chirality in the reaction system. However, it
can be clearly seen that the functionalized material shows a
substantial increase in conversion with significant ee. This may
be attributed to the synergic effect between PW11Cu and S-
PEA, where PW11Cu stabilized the chirality of S-PEA.
a
Catalyst amount, 20 mg; styrene, 100 mmol; O2, 1 atm; TBHP, 0.2
mL; time, 12 h; temperature, 80 °C.
The catalytic activity of both the functionalized material
(entry 4) and physical mixture (entry 5) was carried out under
optimized conditions but for 4 h (Table 3). If any chemical
interaction in the physical mixture exists, an almost similar
result should be obtained for entries 4 and 5, but the negligible
ee clearly confirms that either the S-PEA moiety is almost free
or it is attached with PW11Cu via electrostatic interaction only.
Probable Reaction Mechanism. In order to understand
the mechanism, the reactions were carried out only in the
presence of a small amount of oxygen and TBHP separately
(entries 1 and 2 in Table 4), and no significant conversion was
obtained with either one. The obtained results indicate that
TBHP is required to initiate the reaction in terms of the
activation of molecular O2 in a superoxo anion radical. The
formed superoxo anion radical will bind to copper, in place of
one of the surface oxygen atoms, in the form of an end-on
superoxo (it should be noted that, in PW11Cu, copper is
coordinated to three bridging and two surface oxygen atoms).
Here we assume (i) unlike WO bonds, which are covalent,
Cu−O are coordinated bonds and (ii) in general, copper forms
a stronger coordinate bond with amine nitrogen than oxygen.
Hence, superoxide can temporarily substitute for one of the
Figure 11. FT-IR spectra of (a) PW11Cu-S-PEA and (b) R-PW11Cu-
S-PEA.
optimization of the reaction was carried out using molecular
O2.
The reaction was monitored at different time intervals, and
the results are presented in Figure 9a. The highest ee was
obtained at 12 h, while beyond 12 h, polymerization of styrene
was observed. Hence, 12 h was considered to be the optimum
time for the reaction. Next, the reaction was carried out over
different catalyst amounts (Figure 9b). It was observed that,
although the change in conversion was negligible, there was a
significant change in the ee. An amount of 20 mg gave the best
ee and hence was considered to be the optimum amount of
catalyst. Finally, the reaction was carried out at different
Table 6
% selectivity
series
time (h)/
%
styrene
oxide
no.
catalyst
oxidant
TBHP
solvent
temp (°C) conversion BzOH
ee
ref
1
2
3
CuW−PYI1
Cu(salen)-POM
120/40
4/80
10/70
10/70
71.9
84
48.5
44.9
36.7
95.2 4.8
87 12.2
41
47
CH3CN
CH3CN
CH3CN
{[Cu2(4,4′-bpy)(4,4′-Hbpy)4(H2O)4](SiW12O40)2(H2O)4}n
{[Cu2(4,4′-bpy)(4,4′-Hbpy)6(SiW12O40)3](4,4′-
TBHP
TBHP
90.2 8.5
Hbpy)2(H2O)7}n
{[Cu2(μ2H2O)2(4,4′-bpy)3(SiW12O40)](H2O)6}n
TBHP
TBHP
CH3CN
CH3CN
10/70
10/70
79.5
74.4
43.1 55.9
55.7 44
{[Cu2(μ2-OH)(4,4′-bpy)3(SiW12O40)(H2O)][Cu2(μ2-O)(4,4′-
bpy)4(H2O)2]0.5(H2O)3}n
4
5
[CuII2(C5H5NCOO)2(4-bpo)2(H2O)2] SiW12O40·H2O
[CuI4(4-bpo)6]SiW12O40·3H2O
TBHP
TBHP
TBHP
O2
CH3CN
CH3CN
CH3CN
10/70
7/70
5/70
90.6
93.8
56
33.7 44.7
35.4 35.6
43.1 49.5
50
[CuI4(3-bpo)4]SiW12O40·3H2O
PW11Cu-S-PEA (present work)
12/80
43
63
13 (R)/18 16
(S)
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Article
surface oxygen atoms, and thus the formed end-on superoxo
species is responsible for the epoxidation of the substrate held
in the vicinity of copper and chiral ligands (Scheme 2). In
other words, S-PEA acta as a directing group, orienting the
surface in such a way that it results in the formation of a
specific stereoisomer. The catalyst is recovered after
completion of the reaction, and FT-IR was taken in order to
confirm no degradation/loss of the organic ligand. The
participation of a superoxo radical is further supported by
carrying out the experiment in the presence of a radical
scavenger (entry 4 in Table 4), and almost the same results
indicate that the reaction proceeds via the formation of a
superoxo radical. The slight increase in the conversion may be
due to the reaction of styrene with the reactive species that is
already present in the reaction mixture.
Finally, on the basis of two well-known factors, (a) TBHP is
a well-known radical initiator and (b) styrene oxide is formed
as an intermediate during styrene oxidation only when the
radical mechanism is followed, we are quite confident that the
reaction follows a radical mechanism. However, more work is
required to confirm the said results.
AUTHOR INFORMATION
Corresponding Author
■
Anjali Patel − Polyoxometalates and Catalysis Laboratory,
Department of Chemistry, Faculty of Science, The Maharaja
Sayajirao University of Baroda, Vadodara 39002 Gujarat,
India; orcid.org/0000-0003-0177-3956;
Email: anjali.patel-chem@msubaroda.ac.in
Authors
Rajesh Sadasivan − Polyoxometalates and Catalysis
Laboratory, Department of Chemistry, Faculty of Science,
The Maharaja Sayajirao University of Baroda, Vadodara
39002 Gujarat, India
Jay Patel − Polyoxometalates and Catalysis Laboratory,
Department of Chemistry, Faculty of Science, The Maharaja
Sayajirao University of Baroda, Vadodara 39002 Gujarat,
India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00636
Author Contributions
Heterogeneity Test. In order to prove the heterogeneous
nature of the catalyst, the reaction was run for 8 h, after which
the catalyst was filtered out and the reaction was continued for
a further of 2 h. The reaction mixtures after 8 h as well as after
completion were analyzed, and the results are presented in
Figure 10. No changes in % conversion and selectivity indicate
that the catalyst is truly heterogeneous in nature.
A.P.: conceptualization, writing (review and editing), and
supervision. R.S.: methodology, software, and writing (original
draft). J.P.: graphical abstract, reproduction of the results, and
writing (review and editing).
Notes
The authors declare no competing financial interest.
Regeneration Study. Because of its heterogeneous nature,
the catalyst was easily regenerated by a simple centrifugation,
followed by washing with methanol and air drying. This was
used for the same reaction under optimized conditions. As
seen in Table 5, the catalyst gave almost the same activity up to
two cycles, indicating that it can be reused multiple times. A
slight decrease in conversion may be attributed to the loss
incurred during the process of regeneration.
In FT-IR, all of the bands of the fresh catalyst were
replicated in the recycled one, indicating that the catalyst
remains stable during the course of the reaction and also after
recycling (Figure 11).
Comparison Study. From Table 6, it is seen that, for all of
the reported copper-based inorganic−organic hybrid catalysts,
styrene epoxidation has been carried out in the presence of
TBHP (except one case) as the oxidant in CH3CN as the
solvent. It is also seen that all reported systems are inactive
toward enantiomer selectivity. The superiority of the present
catalyst lies in terms of ee using molecular O2 as a greener
oxidant under solvent-free conditions.
ACKNOWLEDGMENTS
■
A.P. and J.P. thank the Science and Engineering Research
Board (New Delhi, India; Project EMR/2016/005718) for
financial support. The authors also thank the Department of
Chemistry, The Maharaja Sayajirao University of Baroda,
1
Vadodara, India, for H NMR analysis.
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CONCLUSION
■
PW11Cu was successfully functionalized with a chiral organic
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