Chirally-Modified Graphite Oxide as Chirality Inducing Support for Asymmetric Epoxidation of Olefins with Grafted Manganese Porphyrin

Article abstract of DOI:10.1007/s10562-019-02933-1

Abstract: A chirality inducer was prepared by graphite oxide (GO) functionalization with enantiopure l-tartrate (GO^*) and used as asymmetric support for a covalently-linked manganese porphyrine complex [Mn(TPyP)OAc]. The thereby obtained heterogeneous catalyst, GO^*-[Mn(TPyP)OAc], showed excellent performance and ee-values of 92–99% for the asymmetric epoxidation of prochiral olefins with O₂ as oxidant and isobutyraldehyde as co-reductant in acetonitrile; linear terminal olefins with 54–76% conversion and quantitative conversion of aromatic olefins. The GO^*-[Mn(TPyP)OAc] catalyst is highly active, recyclable, and at the same time simple and inexpensive to prepare with a chiral inducer from the chiral pool. The structure of the catalyst was elucidated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET analysis,?FT-IR, Raman, and photoluminescence spectroscopic methods. Graphic Abstract: Graphite oxide functionalized with an enantiopure group was used as a chirality inducer and asymmetric support for a Mn-porphyrine complex. The thereby obtained heterogeneous catalyst is an excellent enantioselective catalyst for the epoxidation of prochiral olefins.[Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02933-1

Catalysis Letters
https://doi.org/10.1007/s10562-019-02933-1
Chirally‑Modifed Graphite Oxide as Chirality Inducing Support
ꢀ
or Asymmetric Epoxidation oꢀ Olefns with Graꢀted Manganese
Porphyrin
Elahe Ahadi · Hassan Hosseini‑Monfared1,2 · Carsten Schlüsener · Christoph Janiak · Afsaneh Farokhi1
1
3
3
Received: 15 March 2019 / Accepted: 31 August 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
*
A chirality inducer was prepared by graphite oxide (GO) functionalization with enantiopure l-tartrate (GO ) and used as
asymmetric support for a covalently-linked manganese porphyrine complex [Mn(TPyP)OAc]. The thereby obtained het-
*
erogeneous catalyst, GO -[Mn(TPyP)OAc], showed excellent performance and ee-values of 92–99% for the asymmetric
epoxidation of prochiral oleꢀns with O as oxidant and isobutyraldehyde as co-reductant in acetonitrile; linear terminal ole-
2
*
ꢀ
ns with 54–76% conversion and quantitative conversion of aromatic oleꢀns. The GO -[Mn(TPyP)OAc] catalyst is highly
active, recyclable, and at the same time simple and inexpensive to prepare with a chiral inducer from the chiral pool. The
structure of the catalyst was elucidated by scanning electron microscopy (SEM), transmission electron microscopy (TEM),
BET analysis, FT-IR, Raman, and photoluminescence spectroscopic methods.
Graphic Abstract
Graphite oxide functionalized with an enantiopure group was used as a chirality inducer and asymmetric support for a Mn-
porphyrine complex. The thereby obtained heterogeneous catalyst is an excellent enantioselective catalyst for the epoxidation
of prochiral oleꢀns.
Keywords Chiral epoxidation · Oxygen · Manganese porphyrin · Graphite oxide · Heterogeneous catalysis
1
Introduction
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02933-1) contains
supplementary material, which is available to authorized users.
Most biologically important molecules are chiral. Therefore,
preparation of enantiomerically pure chemicals is an impor-
tant topic in many areas of society and science, and as a
result there is a lot of interest on the topic of preparation of
*
Hassan Hosseini-Monfared
hahomonfared@gmail.com
Extended author information available on the last page of the article
Vol.:(0
1
123456789)
3

E. Ahadi et al.
chiral materials [1–4]. The epoxidation reaction is an attrac-
tive C–O bond forming reaction in which an oleꢀn com-
pound is oxidized to an epoxide, which may be subsequently
transformed to a variety of functional groups, such as alco-
hol, carbonyl, diol, azide, etc. In many of these transforma-
tions, the stereogenic centers formed in the addition reaction
are kept, so enantioselective versions of this reaction were
developed on the basis of the use of chiral catalysts. Asym-
metric catalysis is the best method from the atom-economic
point of view to selectively produce single enantiomers in
the pharmaceutical and agrochemical industry [5, 6]. For
separation and reuse of the relatively expensive catalysts
from the reaction mixture at the end of the process, het-
erogenization of chiral catalysts is essential [7, 8]. Hetero-
genization of molecular catalysts sometimes improves their
selectivity compared to the homogeneous ones through site-
isolation, constraint and synergistic eꢁects [9–11]. However,
the use of heterogenized catalysts has usually serious limita-
tions such as low reactivity, cost and the tedious procedure
involved in designing the catalysts [12]. Therefore, there is
a continuing need to develop more eꢂcient and practical
heterogenization methods for chiral catalysts.
ligand complexes to GO. Furthermore, the synergic interac-
tions between the complex and GO can improve the complex
activity [21]. Additionally, the required large speciꢀc surface
area of the resulting hybrid materials for catalysis is obtained
by exfoliation of GO sheets during the complex-grafting
reaction, with the anchored complexes then preventing a new
aggregation [22]. Based on the above-mentioned studies,
*
we have set out to prepare a chiral graphite oxide (GO ) as
chirality-inducing support for the asymmetric epoxidation
of oleꢀns with grafted manganese porphyrin and molecular
oxygen. In comparison with other oxidants such as NaIO /
4
imidazole, NaClO/4-phenylpyridine N-oxide (PPNO) and
m-CPBA/N-methylmorpholine N-oxide (NMO), molecular
oxygen is an ideal oxidant in the epoxidation of oleꢀns to
the corresponding epoxides catalyzed by metalloporphyrins
because of its low cost and environmentally friendly nature
[23].
2 Experimental Section
2.1 Materials
For a heterogeneous chiral catalyst, a very success-
ful strategy to induce chirality relied on the use of chiral
ligands immobilized onto the support material [13]. Various
methods of noncovalent immobilization techniques such as
entrapment, ion exchange or adsorption have been used to
prevent the loss of the optimal geometry of the catalyst or
chemical change of the chiral ligand [13, 14], however, they
were not completely successful. Alternatively, in an inter-
esting study, cellulose nanocrystals were used as enantiose-
lective support for the immobilization of Pd patches in the
hydrogenation of prochiral ketones with high enantiomeric
5,10,15,20-Tetra(4-pyridyl)porphyrin (H TPyP) (95%) was
2
obtained from Fluka, (2R,3R)-(+)-tartaric acid, (l-(+)-tar-
taric acid, 99%) from Merck. The other reagents and chemi-
cals used for experiments were purchased from Merck with
highest purity. Deionized water was used for all experiments.
For analysis of the reaction products an HP Agilent 6890
gas chromatograph equipped with an HP-5 capillary col-
umn (phenyl methyl siloxane 30 m × 320 µm × 0.25 µm)
with ꢃame-ionization detector was used. The enantiomeric
excess (ee) was determined by an HP 6890-GC using a
chiral SGE-CYDEX-B capillary column (25 m×0.22 mm
excess in water at 4 bar H [15]. In another study, the mixed-
2
1
ligand strategy, the usage of a chiral N-carbamylglutamate
and an achiral bispyridine ligand, was applied to prepare
homochiral neutral layered coordination polymers [16].
Adsorption of chiral modiꢀers onto non-chiral metal sur-
faces was also investigated to create eꢁective heterogeneous
catalysts [17, 18]. Gellman and coworkers have summarized
the latest report on chiral surface chemistry which was per-
formed on model single crystalline surfaces [19]. It has been
suggested that the energetic diꢁerences in the interaction of
prochiral substrates with a chiral surface results in enanti-
oselectivity for the products.
ID×0.25 µm). H-NMR spectra were recorded on a Bruker
250 MHz spectrometer using CDCl as solvent, with the
3
residual proton solvent peak as reference and calibrated
against TMS.
UV–Vis spectra of the solutions were run on a Shimadzu
160 spectrometer. Fourier transform infrared (FT-IR) spec-
tra were taken using a Perkin-Elmer 597 spectrophotom-
eter. Powder X-ray diꢁraction patterns were collected at a
Bruker D8 ADVANCE, wavelength 1.5406 Å (Cu Kα), volt-
age 40 kV, current 40 mA. Scanning electron microscope
(SEM) were acquired using a Hitachi F4160 SEM operated
at an accelerating voltage of 10 kV or a JEOL JSM-6510
Advanced electron microscope (Jeol, Akishima, Japan) with
Graphene is a single-layer sheet from graphite, consist-
2
ing of carbon atoms with sp hybridization and honeycomb
configuration. It possesses a high thermal conductivity
a LaB cathode at 5–20 keV. The microscope was equipped
6
2
−1
and a high surface area of 2630 m g [20] that makes it
a superior candidate, in comparison to SBA-15, MCM-41
and conventional MOFs, to be used as a support for the het-
erogenization of catalysts. Oxygen-containing functional
groups of graphite oxide (GO) help to anchor metal–organic
with an Xꢃash 410 (Bruker) silicon drift detector for energy-
dispersive X-ray spectroscopic (EDX) elemental analysis.
TEM images were recorded with a Zeiss 902A electron
microscope. The samples were prepared by suspending
the samples in acetonitrile under short ultrasonication and
1
3

Chirally‑Modifed Graphite Oxide as Chirality Inducing Support ꢀor Asymmetric Epoxidation…
dripping the suspension onto a carbon-coated copper grid
and letting it dry. BET method was employed to perform the
speciꢀc surface area analysis, using a Micromeritics ASAP
3-chloropropyltrimethoxysilane (0.86 mL) in 3.5 mL of dry
toluene, the mixture was reꢃuxed for 24 h under nitrogen
atmosphere. The black powdery sample was separated by
ꢀltration after cooling to room temperature and washed sev-
eral times with toluene (3×2 mL) and EtOH (3×2 mL). The
2
020 analyzer. Alternatively, nitrogen (purity 99.9990%)
physisorption isotherms were carried out on an Autosorb-6
from Quantachrome at 77 K. The manganese content of the
catalyst was measured by a Varian spectrometer AAS-110.
resulting product GO(NH )(Cl) was dried at 70 °C for 6 h.
2
Yield: 0. 14 g.
2.2 Graphite Oxide Synthesis
2.6 Synthesis of [Mn(TPyP)OAc]
Graphite powder was oxidized using the improved synthesis
method of graphite oxide by Marcano et al. [24]. Typically,
For the metalation of H TPyP, 0.80 g (1.29 mmol) of H TPyP
2
2
and 3.16 g (12.9 mmol, 10 equiv.) of Mn(OAc) ·4H O were
2
2
to a mixture of graphite powder (0.5 g) and KMnO (3.0 g),
dissolved at room temperature in 100 mL of glacial acetic
acid and heated at 80 °C for 6 h. From the resulting solution
the solvent was removed in a rotary evaporator and the solid
residue dissolved in 1000 mL of water at 60 °C. The solution
was ꢀltered and the metalloporphyrin was precipitated by
the addition of 100 mL of sodium acetate solution (2 mol/L).
Yield: 0.76 g, 81%.
4
the mixture of concentrated H SO /H PO (60:6 mL) was
2
4
3
4
added dropwise under ice-cooling. The resulting suspension
was stirred for 30 min in an ice bath, then it was heated at
7
0 °C for 24 h. After cooling to RT 1.5 mL of 35% H O
2 2
in ice was added to remove the unreacted KMnO reagent.
4
Finally, the obtained yellow solid was washed consecutively
with deionized water (30 mL), 30% HCl solution (30 mL),
EtOH (200 mL) and diethyl ether (30 mL). Yield: 0.40 g.
2.7 Immobilization of [Mn(TPyP)OAc] on GO(NH2)
(Cl)
2
.3 Synthesis of Amine‑Functionalized Graphite
Oxide, GO(NH2)
The amino- and chloro-functionalized graphite oxide
GO(NH )(Cl) (0.15 g) and [Mn(TPyP)OAc] (15 mg,
2
Following the reported procedure by Bhanja et al. the
hydroxyl and carboxyl groups of the GO surfaces were used
for grafting 3-aminopropyl groups [25]. First GO (0.66 g)
was sonicated in dry toluene (46 mL) for 20 min, then a
solution of 3-aminopropyltriethoxysilane (2 mL) in dry tolu-
ene (13 mL) was added slowly into the GO/toluene suspen-
sion and the combined mixture was reꢃuxed under nitrogen
atmosphere for 24 h. The resulting amine-functionalized
0.0205 mmol) were dispersed in DMF (30 mL) and the
suspension was heated at 100 °C for 48 h under vigorous
stirring. The obtained black precipitate was washed with
methanol and ether. The resulting GO(NH )-[Mn(TPyP)
2
+
−
OAc] Cl was dried at 60 °C for 6 h. Yield: 99 mg.
+
−
2.8 Functionalization GO(NH )‑[Mn(TPyP)OAc] Cl
2
by Tartrate Ester
product, GO(NH ), was ꢀltered and washed thoroughly with
2
dry toluene (3×2 mL) to remove unreacted 3-aminopropyl-
Diethyl-2,3-O-benzylidene-l-tartrate was synthesized
following a procedure developed by our group [26].
The dispersion of diethyl-2,3-O-benzylidene-l-tar-
trimethoxysilane. The dark black solid was dried at 70 °C for
6
4
h. Yield: 0. 56 g. CHN analysis of GO(NH ): C 36.83%, H
2
.69%, N 6.26%, molar C/N ratio=6.82.
trate (1.7 mg, 0.0058 mmol) and GO(NH )-[Mn(TPyP)
2
+
−
OAc] Cl (26.4 mg) in methanol (10 mL) was reꢃuxed
2
.4 Synthesis of GO‑tart (GO*)
for 72 h. After cooling to room temperature, the resulting
*
+
−
product GO -[Mn(TPyP)OAc] Cl was washed with water
(10 mL) and dried in air [27]. Atomic absorption meas-
urement showed 26 μmol Mn or 1.4 mg Mn per gram of
The mixture of diethyl-2,3-O-benzylidene-l-tartrate (12 mg,
0
.041 mmol) and amine-functionalized graphene (GO(NH ),
2
*
+
−
0
.18 g) in methanol (5 mL) was reꢃuxed for 2 h and the
GO -[Mn(TPyP)OAc] Cl , corresponding to 0.14 wt% Mn.
This low Mn content was in agreement with its EDX analysis
(see Supplementary Information).
resulting product was washed with water (10 mL) and dried
in air. Yield: 0.17 g. CHN analysis of GO-tart: C 38.96%, H
4
.73%, N 6.18%, molar C/N ratio=7.00.
2
.9 Catalytic Oxidation of Styrene with Molecular
2
.5 Functionalization of GO(NH ) by Chloropropyl,
Oxygen
2
GO(NH )(Cl)
2
Styrene was used as a representative prochiral oleꢀn. The
*
+
−
GO(NH ) (0.16 g) was sonicated in dry toluene
catalytic activity of GO -[Mn(TPyP)OAc] Cl was investi-
gated for the oxidation of styrene in a 25 mL round bottom
2
(
13 mL) for 20 min. After addition of a solution of
1
3

E. Ahadi et al.
ꢃask equipped with a magnetic stirring bar and a reꢃux
condenser at the determined temperature (20 °C, 40 °C and
chloropropyl unit and the pyridyl nitrogen atom in the com-
+
−
pound GO(NH )-[Mn(TPyP)OAc] Cl , Scheme 1b. Finally,
2
6
0 °C). The oleꢀn substrate (1 mmol), catalyst (1.0 mg) and
functionalization of GO through the aminopropyl groups
isobutyraldehyde (3 mmol) were dispersed in 3 mL of sol-
vent (n-hexane, acetonitrile, and ethyl acetate) and oxygen
was provided by a balloon. The reaction was continued for a
determined time (2 h, 4 h, 6 h). During the reaction, aliquots
of the reaction mixture (2 μL) were withdrawn and ana-
lysed by GC. Authentic samples and spectroscopic methods
were used for the identiꢀcation of the products. For GC with
HP-5 capillary column: injection temperature 250 °C, detec-
by a chiral, enantiopure l-tartrate derivative led to chiral
*
+
−
GO -[Mn(TPyP)OAc] Cl , Scheme 1c. For the synthesis
of the chiral tartrate unit, (2R,3R)-tartaric acid was treated
with ethanol to produce the corresponding ester and then its
dihydroxyl groups were protected by benzaldehyde in the
presence of p-toluenesulfonic acid [26].
The speciꢀc surface areas were calculated according
to the Brunauer–Emmett–Teller (BET) method from N
2
tor temperature 250 °C, carrier gas N ꢃow rate 0.70 mL/
adsorption isotherms at 77 K for GO(NH )-[Mn(TPyP)
2
2
+
−
*
+
−
2
min; the oven temperature was held at 90 °C for 0.1 min,
OAc] Cl and GO -[Mn(TPyP)OAc] Cl to 50 m /g and
−
1
2
then heated to 190 °C at a rate of 10 °C min and kept for
44 m /g, respectively. The two surface areas are within
5
min. For chiral GC with SGE-CYDEX-B capillary col-
experimental error to each other. In comparison to GO,
the development of a porous structure contributed to the
−
1
umn, a column ꢃow of 0.70 mL min was applied. The
detector and injection temperatures were 200 °C. The oven
temperature was held at 50 °C for 0.1 min, then the tem-
increase in the BET surface area of GO(NH )-[Mn(TPyP)
2
+
−
*
+
−
OAc] Cl and GO -[Mn(TPyP)OAc] Cl , and this has been
−
1
perature was increased to 150 °C at a rate of 7 °C min and
kept for 15 min. Moreover, the reusability and time stability
conꢀrmed by N adsorption–desorption analysis (Table 1).
2
+
−
The pore volume of GO(NH )-[Mn(TPyP)OAc] Cl do not
2
*
+
−
of catalyst GO -[Mn(TPyP)OAc] Cl were examined under
the same reaction conditions as that of the ꢀrst catalytic run.
For the manganese leaching test in the oxidation of
change after functionalization with enantiopure l-tartrate.
The lower results observed for the GO sample could be
attributed to the incomplete re-stacking of GO material lay-
ers and nearby blocked pores. GO is prone to re-stacking
owing to active π–π interactions and van der Waals forces
connecting the nearly planar planes of GO sheets [33].
*
+
−
styrene, the solid catalyst GO -[Mn(TPyP)OAc] Cl was
removed from the reaction mixture after 6 h by centrifuga-
tion. The supernatant was decanted, 1 mL of concentrated
H SO added and the mixture was reꢃuxed for 1 h. The
2
4
resulting solution was investigated by UV–Vis absorption
spectroscopy and atomic absorption spectrometry.
3.2 FT‑IR spectra
FT-IR analysis proved the presence of [Mn(TPyP)
+
−
*
OAc] Cl on the surface of the GO sample. Upon func-
tionalization of GO, significant changes were observed
in the FT-IR data with respect to the GO spectrum which
indicated an eꢁective functionalization. Figure 1 compares
the FT-IR spectra recorded on GO, GO(NH ), GO(NH )
3
Results and Discussion
3.1 Synthesis of Catalyst
2
2
+
−
*
Graphite oxide is an attractive support material since it is
thermally, chemically, and mechanically stable during the
reaction process and has a large surface area as well. GO
consists of oxygen-containing functional groups such as
hydroxyl, carbonyl, and epoxy on the basal graphite plane
(Cl), GO-[Mn(TPyP)OAc] Cl and GO -[Mn(TPyP)
+
−
−1
OAc] Cl samples from 650 to 2000 cm . Many of the
spectral features in the ꢀngerprint region between 700 and
−1
1400 cm decrease in intensity when going from neat GO to
the functionalized GO samples. Vibrations in this ꢀngerprint
−
1
[
28], and it can be well-dispersed in polar solvents such as
region mainly correspond to ν
(~854 and 1227 cm ),
C–O–C
−
1
−1
water. Furthermore, GO is known to be a good substrate
for the dispersion of catalytically active nanoparticles
δ
(~1379 cm ) and ν
(~1053 cm ), i.e. originating
OH
C–OH
from the epoxy, alcohol and carbonyl groups from GO [34].
The elimination of these groups upon functionalization was
conꢀrmed by their disappearance in GO-derived samples.
[
29] and for the stabilization of the immobilized metal-
complexes [20, 30–32]. The chiral heterogeneous catalyst
*
+
−
−1
−1
GO -[Mn(TPyP)OAc] Cl was synthesized as follows.
First, the prepared GO was functionalized with aminopro-
Additionally, the peaks at ~1820 cm and 1735 cm are
seen only in the pristine GO sample. These peaks are related
pyl and chloropropyl to produce GO(NH )(Cl), Scheme 1a.
to ν
from carbonyl groups [34]. The two diꢁerent peaks
C=O
2
−
1
Second, the manganese–tetrapyridylporphyrin complex was
can be related to isolated (absorbing at 1820 cm ) carbonyl
groups and C=O which is H-bonded to nearby OH groups or
covalently attached to GO(NH )(Cl) through quaternization
2
−1
of the pyridyl nitrogen atom of [Mn(TPyP)OAc] by reac-
tion with the formation of a pyridinium chloride moiety
with a covalent link between the carbon atom of the former
water molecules (peak at 1735 cm ). The carboxylic groups
−
1
also contribute at the peak 1735 cm . While the peak at
−
1
1735 cm appeared with decreased intensity in the sample
1
3

Chirally‑Modifed Graphite Oxide as Chirality Inducing Support ꢀor Asymmetric Epoxidation…
(a)
(
(
b)
c)
*
+
−
Scheme 1 Synthesis of chiral heterogeneous catalyst GO -[Mn(TPyP)OAc] Cl
Table 1 Surface area and pore volume of GO, GO(NH )-[Mn(TPyP)
GO [34]. Although the changes in the FT-IR spectrum of
2
+
−
*
+
−
*
+
−
OAc] Cl and GO -[Mn(TPyP)OAc] Cl
GO -[Mn(TPyP)OAc] Cl with respect to those of GO-
+
−
[
Mn(TPyP)OAc] Cl and GO(NH )(Cl) are small, ꢃuores-
2
Sample
BET surface area Pore volume
Pore
2
−1
3
−1
(
m g
)
(cm g
)
diameter
cence studies (see the next section) conꢀrmed the grafting
(
nm)
*
of Mn-porphyrin onto GO .
GO
3
50
0.010
0.084
7
3
.3 Photoluminescence Property
GO(NH )-
43
44
2
[
Mn(TPyP)
+
−
OAc] Cl
Porphyrins and GO show ꢃuorescence because of hav-
ing multiple conjugated double bonds with high degree
of resonance stability. They have delocalized π-electrons
that can be placed in low-lying excited singlet states [35].
*
GO -[Mn(TPyP)
44
0.089
+
−
OAc] Cl
*
The solid-state luminescent properties of GO -[Mn(TPyP)
−
1
+
−
*
GO(NH ), the peak at 1820 cm completely disappeared
OAc] Cl together with free GO and GO were investi-
gated at room temperature. From Fig. 2, it can be seen that
2
on all functionalized samples. This conꢀrms that most of
*
the carbonyl groups are eliminated with GO silylation. The
a broad emission band is centered at 483 nm for GO, GO
peak centered at~1623 cm− is attributed to ν
1
on
*
+
−
and GO -[Mn(TPyP)OAc] Cl (λ = 405 nm). While GO
ex
C–C(aromatic)
1
3

E. Ahadi et al.
+
−
*
+
−
Fig. 1 FT-IR spectra of GO, GO(NH ), GO(NH )(Cl), GO-[Mn(TPyP)OAc] Cl and GO -[Mn(TPyP)OAc] Cl
2
2
3.4 Thermogravimetric Analysis
Typical thermogravimetric (TGA) curves of GO(NH ),
2
*
+
−
GO-tart and GO -[Mn(TPyP)OAc] Cl are shown in Fig. 3
to analyze the thermal stabilities through the temperature-
dependent weight loss of the materials. Weight loss is
observed for GO in several steps (Fig. S1 in Supplementary
Information). The residual water is lost by heating to 100 °C.
The second weight loss (39%) occurs at about 100 °C to
2
00 °C, probably due to the decomposition of the oxygen-
containing functional groups of GO. The weight loss from
about 200 °C to 620 °C is assigned to the breakdown of
the –COOH group in GO. These oxygen-functional groups
make GO more hydrophilic compared to graphite, as a result
a signiꢀcant amount of water molecules is incorporated into
the stacked structure of GO. Compared to the behavior of
GO, the 3-chloropropyl-, 3-aminopropyl- and [Mn(TPyP)
+
−
Fig. 2 Solid-state emission spectra (upon excitation at λ=405 nm) of
OAc] Cl -modiꢀed GO sheets exhibit diꢁerent weight loss
curves. When the temperature is less than 300 °C, only a
slight weight loss is observed, probably still due to the ther-
mal degradation of the unreacted oxygen functional groups
of GO. The evident weight loss occurs in the range from
210 to 400 °C, which might be caused by the degradation
of 3-chloropropyl and 3-aminopropyl chains attached onto
the GO surface (see Supplementary Information). The
reduction of weight loss at low temperatures results from
the changes in the surface properties of GO by the replace-
*
*
+
−
(
a) GO, (b) GO (GO-tart) and (c) GO -[Mn(TPyP)OAc] Cl ; 1.0 mg
sample dispersed in 3 mL of N,N-dimethylformaide
*
*
and GO have similar emission spectra, GO -[Mn(TPyP)
+
−
OAc] Cl feature two additional emission bands at 652
and 718 nm. The observation above suggests that its
luminescent mechanism possibly originates from the
[
Mn(TPyP)OAc]-centered emission. In addition, the
ꢃuorescence emission conꢀrms the presence and immo-
ment of its oxygen functional groups with –NH , –tart,
2
*
bilization of [Mn(TPyP)OAc] on GO . The ꢃuorescence
and –Mn(porph) units. For GO(NH ) and GO-tart (=GO*)
2
bands in the 600–800-nm range of the emission spectra
of metalloporphyrins result from S1 → S0 transitions; the
individual bands correspond to the (0,0), (0,1) and (0,2)
transitions with respect to vibrational states [36].
only a slight weigh loss of 10% occurs up 450 °C. Even up
to 900 °C the two intermediates lose only about 20%. The
*
+
−
material GO -[Mn(TPyP)OAc] Cl that possesses all of the
aforementioned attached groups shows a higher temperature
1
3

Chirally‑Modifed Graphite Oxide as Chirality Inducing Support ꢀor Asymmetric Epoxidation…
Fig. 3 Thermogravimet-
ric (TGA) and DTG curves
of GO(NH ), GO-tart and
2
*
+
−
GO -[Mn(TPyP)OAc] Cl
*
for the maximum weight loss. For GO -[Mn(TPyP)tart], the
mass loss before 100 °C is evidently related to the adsorbed
water. The signiꢀcant degradation step (28%) at 100 to
n-hexane, ethyl acetate) and reaction time (2 h, 4 h, 6 h)
was achieved using the Box-Behnken method [39]. Maxi-
mum conversion and selectivity were obtained when the
4
60 °C is related to decomposition of labile oxygen func-
reaction was carried out in CH CN at 52 °C for 6 h. The
3
tionalities and the tartrate. The major weight loss of 32% at
the temperatures ranging from 460 to 700 °C is attributed to
the decomposition of Mn-porphyrin due to stability of the
porphyrin ring up to higher than 600 °C [37].
homogeneous catalyst [Mn(TPyP)OAc] was more active
*
+
−
than GO -[Mn(TPyP)OAc] Cl and could catalyze the
oxidation of styrene to styrene oxide in 4 h under the
similar reaction conditions. However, the product was not
asymmetric and the homogeneous catalyst could not be
separated easily from the reaction mixture.
3.5 SEM and TEM Studies
Various aromatic oleꢀns and terminal linear oleꢀns
were successfully oxygenated under these optimized con-
ditions with high enantioselectivity in the presence of
The morphology and microstructure of the GO sheets
were assessed by scanning electron microscopy, SEM.
*
+
−
*
+
−
The images of GO, GO -[Mn(TPyP)OAc] Cl and used
GO -[Mn(TPyP)OAc] Cl . Activity and selectivity of
the catalyst were substrate dependent. Styrene, α-methyl
styrene and trans-stilbene were completely converted to
the corresponding epoxide with enantioselectivities of
more than 94% (Table 2, GC-chromatogram in the Sup-
plementary Information). Presence of the benzylic posi-
tions improve the activity of styrene derivatives. It is
interesting to note that even linear terminal oleꢀns such as
1-octene (conversion 76%) and 1-decene (conversion 54%)
were oxidized with enantiomeric excess values of 92%
and 93%, respectively. In agreement with reported stud-
ies [32], the activity of terminal linear oleꢀns was lower
than those of the activated styrene derivatives (Table 2).
Linear terminal oleꢀns are electron deꢀcient compounds
with a long alkyl chain which could be the reason for their
lower activity. These ꢀndings suggest the importance of a
steric hindrance eꢁect during the oxygen transfer from the
active intermediate to the oleꢀn. The limited conversion
of terminal oleꢀns has also been reported in the oxidation
catalyzed by peroxygenases [40] and P450 models [41].
The lower conversion of terminal oleꢀns is attributed to
*
+
−
GO -[Mn(TPyP)OAc] Cl after the fourth recycling are
shown in Fig. 4. In Fig. 4a the typical wrinkling is clearly
seen in the nanosheets of GO. During the catalyst prepara-
tion, GO preserves its wrinkled sheet texture without any
transformation (Figs. 4b, 5). Some changes in the mor-
*
+
−
phology of the catalyst GO -[Mn(TPyP)OAc] Cl with
respect to that of GO result from the increase of the steric
hindrance by the covalent linkage of the porphyrin onto GO
[
38]. The TEM images reveal the well-distributed and sepa-
+
−
rated [Mn(TPyP)OAc] Cl species. The agglomeration on
*
+
−
the planes of GO -[Mn(TPyP)OAc] Cl is seen after ꢀve
times use and recovering in the oxidation reaction of styrene
(
Fig. 4c).
3.6 Catalyst Activity
Styrene as a representative prochiral oleꢀn was used for
*
evaluation of the catalytic activity of GO -[Mn(TPyP)
+
−
OAc] Cl . The optimization of the relevant parameters,
namely temperature (20, 40 and 60 °C), solvent (CH CN,
3
1
3

E. Ahadi et al.
Fig. 4 SEM images of GO (a),
*
fresh catalyst GO -[Mn(TPyP)
+
−
OAc] Cl (b) and used catalyst
after fourth recycling (c) at dif-
ferent magniꢀcations
1
3

Chirally‑Modifed Graphite Oxide as Chirality Inducing Support ꢀor Asymmetric Epoxidation…
+
−
*
+
−
Fig. 5 TEM images of GO(NH )-[Mn(TPyP)OAc] Cl (above) and GO -[Mn(TPyP)OAc] Cl (bottom). The scale bar is 100 nm for left and
2
middle ꢀgures and 500 nm for the two right ꢀgures
the covalently bound tartrate onto GO in inducing chiral-
Table 2 Enantioselective epoxidation of prochiral oleꢀns
ity onto the catalytically active manganese-porphyrin unit.
Entry
Substrate
Conv. (%)
ee (%) (Conf.)
To examine the recyclability of the catalyst, styrene
1.0 mmol), isobutyraldehyde (3.0 mmol) and the catalyst
(
(
1
2
3
4
5
Styrene
100
100
100
76
99 (R)
94 (R)
100 (R,R)
92 (R)
93 (R)
1.0 mg) were added to CH CN (3.0 mL) and the reaction
3
α-Methylstyrene
trans-Stilbene
1-octene
was followed under 1 atm of oxygen at 52 °C for 6 h by
GC analysis. At the end, the heterogeneous catalyst was
separated from the reaction mixture by centrifugation. The
composition of the supernatant solution was studied by GC
analysis. The remaining solid catalyst was washed with ace-
tonitrile and centrifuged, then used for the next run after
addition of styrene (1.0 mmol), isobutyraldehyde (3.0 mmol)
1-decene
54
*
+
−
Reaction conditions: catalyst GO -[Mn(TPyP)OAc] Cl 1.0 mg, ole-
ꢀ
3
n 1.0 mmol, oxygen balloon, isobutyraldehyde 3.0 mmol, CH CN
3
mL, temperature 52 °C, time 6 h
and CH CN (3.0 mL). The conversion remained at almost
3
1
00% for the ꢀrst four runs, and started only to decrease in
the heme alkylation by 1-alkenes that leads to the inactiva-
tion of the enzyme.
the 5th run (Fig. 6). A reduction of 4% in styrene conver-
sion was observed after ꢀve times recovery and reuse of the
catalyst. This decrease in the activity of catalyst is partially
due to its loss during recovery by sequential centrifuga-
tion and washing. In addition, reduction of conversion to
95.7% in the fourth run probably is due to agglomeration
*
The high enantioselectivity of GO -[Mn(TPyP)
+
−
OAc] Cl is much better than that of the catalyst GO-
+
−
[
Mn(TPyP)OAc] tart (ee 73% for styrene and ee 58%
for α-methyl styrene and trans-stilbene) where chirality is
induced by a chiral, enantiopure l-tartrate (tart) counter
ion [10]. These ꢀndings indicate the superior potential of
*
+
−
of the planes of GO -[Mn(TPyP)OAc] Cl (see Fig. 4c).
After each recycle experiment, the possibility of leaching
1
3

E. Ahadi et al.
manganese or porphyrine were investigated by analyzing
the ꢀltrate using atomic absorption spectroscopy (AAS)
at wavelength 279.5 nm and UV–Vis spectroscopy in the
range 200–800 nm, respectively. No traces of manga-
nese (< 0.02 ppm), Mn-porphyrin or free porphyrin were
detected.
3
.7 Plausible Mechanism for the Epoxidation
i
*
with PrCHO/O Catalyzed by GO ‑[Mn(TPyP)
2
+
−
OAc] Cl
Many reports conꢀrm the formation of catalytically active
high-valent manganese(IV or V) oxo porphyrins in the
Mn-porphyrin catalyzed oxygenation of organic sub-
strates [42–44]. It is well-known that a free radical reac-
tion results in the formation of manganese oxo porphyrins
in the presence of molecular oxygen and aldehyde as co-
reductant [45]. Based on the observed results (Table 2) and
other reported studies [46], a plausible mechanism can be
proposed as given in Scheme 2 for the asymmetric aerobic
*
+
−
Fig. 6 The reusability of GO -[Mn(TPyP)OAc] Cl in the oxidation
of styrene with oxygen
(a)
(b)
*
+
−
Scheme 2 Mechanism proposed for the asymmetric epoxidation of styrene (oleꢀns) catalyzed by GO -[Mn(TPyP)OAc] Cl using O as the
2
oxidant and isobutyraldehyde as a co-reductant
1
3

Chirally‑Modifed Graphite Oxide as Chirality Inducing Support ꢀor Asymmetric Epoxidation…
epoxidation of styrene as a representative oleꢀn in the
an eꢂcient and enantioselective recyclable catalyst for the
*
+
−
presence of GO -[Mn(TPyP)OAc] Cl and isobutyralde-
asymmetric oxygenation of oleꢀns with oxygen/isobutyral-
dehyde under mild reaction conditions. Non-covalent inter-
hyde. First a peracid (RCO H) is formed by the treatment
3
*
of O and isobutyraldeyde (RCHO) which is catalyzed by
actions between GO and the oleꢀn facilitate the adsorp-
2
the Mn-porphyrin via the involvement of an acyl peroxy
radical (Scheme 2a). The subsequent reaction between
the acyl peroxy acid and the Mn-porphyrin produces a
catalytically active high-valent manganese oxo species,
either a Mn(IV) oxo or Mn(V) oxo complex, which can
readily oxidize an oleꢀn to an epoxide while it is being
reduced itself to a manganese(III) complex. This process
is repeated many times before decomposition of manga-
nese oxo species to μ-oxo dimeric manganese porphyrin or
other catalytically inactive species [47]. Enantioselectivity
tion and interaction of the prochiral oleꢀn preferentially
through its si-face with the probable reactive intermediate
V
[O=Mn (porphyrin)] which then leads to the enantiopure
epoxide.
Acknowledgements The University of Zanjan is acknowledged for
ꢀ
nancial support. We thank the Center for Advanced Imaging (CAi)
and Sophia Köhler of the Institut für Kolloide und Nanooptik of the
Heinrich-Heine-Universität Düsseldorf for their support in obtaining
the TEM images.
*
+
−
of the catalyst GO -[Mn(TPyP)OAc] Cl probably origi-
nates from enantioselective adsorption via non-covalent
interaction of the prochiral oleꢀn (styrene) with the chi-
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Chirally‑Modifed Graphite Oxide as Chirality Inducing Support ꢀor Asymmetric Epoxidation…
Aꢀliations
Elahe Ahadi · Hassan Hosseini‑Monfared1,2 · Carsten Schlüsener · Christoph Janiak · Afsaneh Farokhi1
1
3
3
2
Christoph Janiak
janiak@hhu.de
Department of Chemistry, Amirkabir University
of Technology, Tehran, Iran
3
Institut für Anorganische Chemie und Strukturchemie,
Heinrich-Heine-Universität, 40204 Düsseldorf, Germany
1
Department of Chemistry, University of Zanjan,
Zanjan 45195-313, Iran
1
3