Enhanced enantioselective oxidation of olefins catalyzed by Mn-porphyrin immobilized on graphene oxide

Article abstract of DOI:10.1016/j.tet.2018.03.027

An efficient enantioselective heterogeneous catalyst, GO-[Mn(TPyP)tart], was prepared by covalent attachment of Mn(III) complex of H₂TPyP via the propyl linkage to graphene oxide (GO) nanosheet and using chiral tartrate counter ion. The catalyst was characterized by Fourier transform infrared (FT-IR), diffuse reflectance ultraviolet–visible (DR UV–Vis) spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman and thermogravimetric analysis (TGA). The graphene-supported Mn-porphyrin showed higher activity for the enantioselective epoxidation of unfunctionalized olefins with molecular oxygen in the presence of isobutyraldehyde. It could be recovered easily and reused in asymmetric oxidation of styrene precursor in a five-step sequence without any considerable loss of its catalytic activity and selectivity. The obtained optically epoxide selectivities were achieved in 86% to 100%.

Full text of DOI:10.1016/j.tet.2018.03.027

Tetrahedron xxx (2018) 1e9
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Enhanced enantioselective oxidation of olefins catalyzed by Mn-
porphyrin immobilized on graphene oxide
,
Kayhaneh Berijani a, Afsaneh Farokhi a, Hassan Hosseini-Monfared a *, Christoph Janiak b
a Department of Chemistry, University of Zanjan, 45195-313 Zanjan, Iran
b
€
Institut für Anorganische Chemie und Strukturchemie, Heinrich Heine Universitat Düsseldorf, 40204 Düsseldorf, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient enantioselective heterogeneous catalyst, GO-[Mn(TPyP)tart], was prepared by covalent
attachment of Mn(III) complex of H2TPyP via the propyl linkage to graphene oxide (GO) nanosheet and
using chiral tartrate counter ion. The catalyst was characterized by Fourier transform infrared (FT-IR),
diffuse reflectance ultravioletevisible (DR UVeVis) spectroscopy, powder X-ray diffraction (XRD), scan-
ning electron microscopy (SEM), Raman and thermogravimetric analysis (TGA). The graphene-supported
Mn-porphyrin showed higher activity for the enantioselective epoxidation of unfunctionalized olefins
with molecular oxygen in the presence of isobutyraldehyde. It could be recovered easily and reused in
asymmetric oxidation of styrene precursor in a five-step sequence without any considerable loss of its
catalytic activity and selectivity. The obtained optically epoxide selectivities were achieved in 86% to
100%.
Received 22 November 2017
Received in revised form
9 March 2018
Accepted 10 March 2018
Available online xxx
Keywords:
Asymmetric epoxidation
Graphene oxide nanosheets
Manganese-porphyrin
Enantioselectivity
Oxygen
© 2018 Elsevier Ltd. All rights reserved.
Heterogeneous catalyst
1. Introduction
there are a lot of efforts for design and synthesis of efficient het-
erogeneous catalysts especially for asymmetric oxidations.4,5 Ti-
Catalytic oxidation represents the basis of a variety of useful
chemical processes for producing bulk and fine chemicals. One such
process is the oxidation of olefins. Olefins oxidation reactions can
be used for the production of essential precursors in organic syn-
thesis such as alcohols, epoxides, aldehydes. Among these com-
pounds, epoxides particularly chiral epoxides are useful and
valuable intermediates with three membered ether ring structure
that can undergo regioselective ring-opening reactions or trans-
formation of functional groups. Chiral epoxides are used in chem-
icals industry, pharmaceuticals and agrochemicals.1
Asymmetric epoxidation is an important theme to convert ole-
fins to chiral epoxides with high purity. These reactions can be
commonly mediated with homogeneous transition metal com-
plexes. However, wide spread applications of homogeneous cata-
lysts are limited because of their high cost, difficulty in separation
from the reaction mixture.2 These drawbacks can be overcome by
heterogenization of a homogeneous catalyst on solid supports.
Furthermore, the stability, product selectivity and recovery of the
catalyst can be improved in heterogenized form.3 Consequently,
chiral tartrates grafted on MCM-41 catalyze the epoxidation of
styrene by tert-butyl hydroperoxide with reasonable enantiose-
lectivity (55e62% ee).6 Amine-catalyzed epoxidation of
a, b-un-
saturated aldehydes is enhanced by attaching the nanosheets of
layered double hydroxides (LDHs) and the epoxide yield of 76%
(93% ee) was obtained in the asymmetric epoxidation of cinna-
maldehyde.7
A
new family of carbohydrate-based dihy-
droisoquinolinium salts has been as catalysts for the asymmetric
epoxidation of olefins (57% ee).8 New chiral Jacobsen's complexes
attached to alkoxyl-modified ZPS-PVPA are active catalysts in the
asymmetric epoxidations of unfunctionalized olefins with N-
methylmorpholine N-oxide (high enantioselectivity and conver-
sion).9 The Mn complex of a N4-tetradentate tetraamide macrocy-
clic ligand tethered onto Fe3O4@SiO2 efficiently catalyzes the
epoxidation of olefins by O2/RCHO with selectivities 87e100% and
53e100% ee.10 Heterogeneous catalysis is a key process in the
manufacturing of a wide range of chemicals. It is at the heart of
green chemistry and sustainable chemical processes. Among the
used metal compounds, Mn-complexes are of the best catalysts
which are active in the epoxidation of olefins as homogeneous or
heterogeneous due to their low toxicity and high activity.11,12 The
catalysts of Mn are powerful species for the asymmetric oxidation
* Corresponding author.
E-mail address: monfared@znu.ac.ir (H. Hosseini-Monfared).
https://doi.org/10.1016/j.tet.2018.03.027
0040-4020/© 2018 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Berijani K, et al., Enhanced enantioselective oxidation of olefins catalyzed by Mn-porphyrin immobilized on
graphene oxide, Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.03.027

2
K. Berijani et al. / Tetrahedron xxx (2018) 1e9
of un-functionalized olefins.13,14 Synthetic metalloporphyrins
mimic natural cytochrome P-450 enzymes in catalytic proper-
ties.15,16 However, the important problems of metalloporphyrins
that act as cytochrome P-450 models are easy destruction during
the reaction and difficult recovery from the reaction mixture.17 To
overcome the self-decomposition problem of metalloporphyrins
during the oxidation reaction they are usually isolated from each
other by immobilization on solid supports.
to the O-H bonds. It reveals the presence of abundance of hydroxyl
groups in GO. A shoulder at 3611 cmꢀ1 is related to the weak
hydrogen bonded O-H groups of GO. The appearance of new bands
in the region of 2851, 292026 and 295827 cmꢀ1 can be assigned to
the C-H asymmetric and symmetric stretching bonds. The bands at
1737 and 1622 cmꢀ1 are due to the C¼O group of carboxylic acid
and stretching vibration of eC¼C group, respectively.28 The vibra-
tion of C-O of carboxyl groups appears at 1423 cmꢀ1 29
The new
.
Among the various solid supports, monolayer nanosheets of
graphene oxide (GO) are an excellent candidate that can be func-
tionalized with different materials to support various homoge-
neous catalysts. GO is prepared from abundant and inexpensive
graphite which shows the necessary feature of high surface area for
a catalyst immobilization and site isolation. Two-dimensional
structure of GO allows catalytic species to be dispersed and as a
result the mass transfer facilitated in the reaction processes.18,19
However, there are some reports showing a decrease in the activ-
ity of the immobilized homogeneous catalysts.20 One fascinating
point about GO is the possibility of formation of hole defects during
the GO synthesis with CO2 production which enhance catalytic
activities and also affect the shape or chemo-selectivity.21 The
numerous of porphyrin/graphene oxide nanohybrids have been
synthesized with covalent and noncovalent approaches22 and used
in epoxidation olefins with various oxidants in oxidative reactions.
For environmental considerations, one major goal in the oxidation
reactions is the replacement of classical toxic waste-producing
oxidants such as MnO2 and K2Cr2O7 with environmentally benign
oxidants. Molecular oxygen is an environmentally clean terminal
oxidant with high oxygen content (50%) that is free, easy to handle
reagent with respect to H2O2 and without byproducts.
vibrational signature at about 1284 cmꢀ1 attributed to asymmetric
CꢀOꢀC stretching of the ether and epoxide groups.30,31 The band at
1229 cmꢀ1 represents the C-OH groups.32 The characteristic bands
at 1000 and 1051 cmꢀ1 confirm the presence of the epoxy groups
(C-O-C) complying with the symmetric stretching, asymmetric
stretching, respectively.28,33,34 In GO-Cl, the bands of methylene
and methyl C-H stretchings are seen at 2851, 2920, 2958 cmꢀ1 with
increased intensity with respect to GO. The appearance of two
strong bands at 1033 and 1118 cmꢀ1 are characteristic of the
vibrational stretching of SieOeC and SieOeSi bonds,
respectively.35
The FT-IR spectrum of GO-[Mn(TPyP)tart] showed the C¼N vi-
bration at 1462 cmꢀ1 in Fig. 1. The bands about 1400 cmꢀ1 (1410-
1343) correspond to symmetric vibration of the C¼O groups.36 By
immobilization of the porphyrin onto GO-Cl, a new peak appeared
at~1586 cmꢀ1 due to the C¼C and C¼N vibrations.37 The C¼C
stretching was observed around 1630 cmꢀ1 68
The peaks at 1586
.
and 1630 cmꢀ1 are attributed to the pyridyl C¼C groups.38 In
comparison with GO-[Mn(TPyP)OAc], an increase in the ratio of the
intensity of the peaks at 1729 and 1410-1343 cmꢀ1 to the peak at
1586 cmꢀ1 indicates the replacement of Cl and acetate (OAc) anions
by tartrate anion (tart).67
Herein we report on hybrid catalyst GO-[Mn(TPyP)tart], where
[Mn(TPyP)tart] is manganese-tetra(pyridylporphyrine)tartrate. It
was prepared via the covalent grafting of a Mn-porphyrin complex
onto graphene oxide and its catalytic performance investigated in
the enantioselective epoxidation of various unfunctionalized ole-
fins with O2. The catalyst is recyclable and showed high activity and
enantioselectivity due to the synergistic interactions between GO
and Mn-porphyrin.
2.3. XRD analysis
The X-ray diffraction patterns (Fig. 2) were used to investigate of
the changes in the structure of GO, GO-Cl and GO-[Mn(TPyP)tart].
In diffraction pattern of pristine graphite powder there is an
intense peak (002) at 2
q
¼ 26.37ꢁ (not shown in Fig. 2) corre-
sponding to the d-spacing of 0.33 nm (layer-to-layer distance).39
With the oxidation of graphite to GO, the basal reflection (002)
2. Results and discussion
shifted to lower angle (2
0.80 nm. Decreasing in 2
q
¼ around 10ꢁ) with d-spacing of around
and increasing the d-spacing indicate the
q
2.1. Characterization of the heterogeneous catalyst
formation of oxygen-containing functional groups such as carbox-
ylic acid and hydroxyl on basal plane of graphite moiety (Fig. 2a).40
GO23 and chloropropyl functionalized graphene oxide, GO-Cl,24
were synthesized by following reported procedures. Catalyst GO-
[Mn(TPyP)tart] was synthesized and characterized by FT-IR, XRD,
SEM, Raman, DR UVeVis, TGA and AAS. Graphene oxide shows
great potential as a support with complexation-ability for various
catalysts due to its high specific surface area and accessible ample
oxygenated functional groups. The Mn-porphyrin was attached to
GO-Cl by a covalent bond between the N atom of the pyridyl and C
atom of the propyl chloride groups (Scheme 1). Then, the axial
acetate ligand of the Mn-porphyrin and the chloride counter ion
both were exchanged by chiral tartrate anion. The asymmetric
tartrate anions create a chiral environment around the Mn and
porphyrin pyridyl groups. The resulting GO-[Mn(TPyP)tart] was an
efficient and recyclable catalyst for the aerobic enantioselective
oxidation of olefins.
In GO-Cl, a wide peak with low intensity appeared at 2
q
¼ 15e26ꢁ
which attributed to alkylated GO with amorphous silica (Fig. 2b).
Finally, The XRD pattern of GO-[Mn(TPyP)tart] displayed reduced
peak intensities in comparison with the two other samples Fig. 2c.
The results suggest the preservation of GO nanostructure during
the synthesis of catalyst28 and decreasing the intensity of peaks
confirm the immobilization.
2.4. UVeVis analysis
The UVeVis spectra of the synthesized GO and GO-Cl indicate
that how many degrees remained from the conjugate system.23 The
spectrum of GO indicates a maximum absorbance at 239 nm which
corresponds to
p
/p
* of the aromatic C¼C bonds (Fig. S1 (a)) and a
shoulder at about 300 nm which relates to n/p
* transitions in the
C¼O bonds. In GO-Cl, a new peak appeared at 282 nm due to
2.2. FT-IR analysis
functionalization of GO (Fig. S1(b)).
Considering the insolubility of the GO-anchored Mn-porphyrin
in common organic solvents so solid diffuse reflectance UVeVis
spectrophotometry was used to study electronic transitions. A
typical Soret band with high intensity at 466 nm and two weaker Q
bands appeared at 572 and 616 nm. The red shift of the Soret band
The chemical changes during preparation of the catalyst were
monitored by infrared spectroscopy. Fig. 1 shows the FT-IR spectra
of the starting materials and catalyst. The FT-IR spectrum of GO
(Fig. 1a) shows a broad peak with strong intensity at 3417 cmꢀ1 due
Please cite this article in press as: Berijani K, et al., Enhanced enantioselective oxidation of olefins catalyzed by Mn-porphyrin immobilized on
graphene oxide, Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.03.027

K. Berijani et al. / Tetrahedron xxx (2018) 1e9
3
Scheme 1. The synthesis method for (A) GO-Cl and (B) chiral catalyst GO-[Mn(TPyP)tart]. The nanosheets of graphene oxide with random orientation25 are represented by a
rectangular box for simplicity.
Fig. 1. The FT-IR spectra of (a) GO (b) GO-Cl (c) GO-[Mn(TPyP)OAc] and (d) GO-[Mn(TPyP)tart]. Expanded spectra of (c) and (d) are shown in the right figure.
Fig. 2. XRD patterns of (a) GO (b) GO-Cl and (C) GO-[Mn(TPyP)tart].
Fig. 3. DR UVeVis spectrum of GO-[Mn(TPyP)tart].
with respect to its homogeneous counterpart (459 nm),67 confirms
This finding is a good document which indicates the changes in
the immobilization of [Mn(TPyP)OAc] onto the GO surface (Fig. 3).
conjugation pathway and symmetry of a metalloporphyrin due to
Please cite this article in press as: Berijani K, et al., Enhanced enantioselective oxidation of olefins catalyzed by Mn-porphyrin immobilized on
graphene oxide, Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.03.027

4
K. Berijani et al. / Tetrahedron xxx (2018) 1e9
the attachment on a support.41e43
(38.55%) occurred at temperatures from 97 ꢁC to 212 ꢁC; it was
attributed to the decomposition of the oxygen-containing groups of
GO. The major weight loss from 212 ꢁC to 621 ꢁC was assigned to the
breakdown of the -COOH group in GO.
2.5. Raman analysis
For the GO-Cl curve, the weight loss below 104 ꢁC was assigned
to the loss of residual water and adsorbed solvent. The major
weight loss (25.93%) occurred at the temperatures ranging from
104 ꢁC to 278 ꢁC, and it was attributed to the burning of the residual
oxygen-functioned groups of GO-Cl. This weight loss is about 13%
lower than that of the GO because of the functionalization by
(CH3O)3Si(CH2)3Cl. The third weight loss at 277e560 ꢁC was
ascribed to the decomposition of the chloropropyl groups and
carbon frame of the graphene. Therefore, the loading amount of 3-
chloropropyltrimethoxysilane occupied about 16 wt% of the GO-Cl.
For GO-[Mn(TPyP)tart], the mild losing mass before 100 ꢁC is
evidently related to the adsorbed water. The significant degrada-
tion step at 225 to 308 ꢁC is related to decomposition of labile ox-
ygen functionalities.50,51 The tartrate anions are probably
decomposed in two steps at 308e420 ꢁC and 420e595 ꢁC. It should
be notice that the porphyrin ring is stable up to 634 ꢁC.52 The major
weight loss (19.57%) at the temperatures ranging from 225 to
308 ꢁC is lower than those of GO (38.55%) and GO-Cl (25.93%). In
addition, the maximum temperature of DTG peak is increased from
163 ꢁC (GO) and 201.97 ꢁC (GO-Cl) to 275 ꢁC (GO-[Mn(TPyP)tart]).
Both these findings confirm the immobilization of the Mn-
porphyrin onto GO-Cl.
Raman spectroscopy is a useful and non-destructive character-
ization technique to investigate the ordered/disordered structure in
graphene oxide nanosheets. In the Raman spectrum of pure
graphite, a peak is seen at 1591 cmꢀ1, named G-band that is due to
ordered sp2-bonded carbon atoms but in GO, two well-known and
typical vibration bands are observed.44 In the Raman spectrum of
the catalyst (Fig. 4), D-band appears at 1302 cmꢀ1 that is generally
assigned to the defects at sheet edges or disordered carbon due to
the degradation of the sp2 domains with the extensive oxidation.45
The G-band peak at 1597 cmꢀ1 corresponds to the high-frequency,
Raman-active, E2g mode of the vibrations of sp2 carbons in graphitic
hexagonal lattices.39 The intensity ratio of D and G bands (ID/IG) is a
criterion to assess the disorderness or defects in the sample spe-
cies.46 The ratio of ID/IG was 1.44 for GO-[Mn(TPyP)tart]; which is
showing an increase of the defects concentration and a reduction in
the crystallinity degree of GO.47
Comparison of the G band (1597 cmꢀ1) for GO-[Mn(TPyP)tart]
with that of GO (1575 cmꢀ1 44
suggests an increase in the num-
)
ber of layers by immobilizing Mn-porphyrin on GO. One may
conclude that the porphyrins are covalently attached on both sides
of GO sheets and the linked porphyrins on two monolayer sheets
can interact with weak pep
stacking interactions.48
2.8. XPS analysis
2.6. SEM analysis
The chemical composition of GO-[Mn(TPyP)tart] was charac-
terized by XPS. The wide scan XPS spectrum of the catalyst is shown
in Fig. S2 (in the supplementary). It displayed the sharp peaks at the
binding energy of 285, 530, 101 and 199 eV, which were assigned to
C 1s, O 1s, Si 2p and Cl 2p, respectively. The result indicated the
existence of C, O, Si and Cl in the GO-[Mn(TPyP)tart] catalyst. The
carbon 1s of GO including carbon sp2, epoxy/hydroxyls, carbonyl,
and carboxylates appeared at 285e289 eV.53 The Na peaks are due
to the presence of Na-tartrate or NaCl impurities. Also, in the XPS
spectrum of Fig. S1, the N 1s for Mn-porphyrin located at 399.8 eV,
representing for pyrrolic N.54 All the results from the X-ray
photoelectron spectroscopy (XPS) analysis demonstrated that the
GO-[Mn(TPyP)tart] catalyst was prepared successfully. However,
the Mn peaks are not seen in the XPS spectrum of the catalyst. Very
weak peak for Mn-porphyrin has also been reported in the litera-
ture.55 The analysis by atomic absorption spectroscopy also showed
Fig. 5 shows the selected micrographs for graphite, GO and GO-
[Mn(TPyP)tart]. In Fig. 5a the ordered flakes of graphite are seen.
Wrinkling is seen clearly in nanosheets of GO (Fig. 5b). During the
catalyst preparation, GO preserves wrinkled sheet texture nature
without transformation (Fig. 5c). A little changes in the morphology
of the catalyst with respect to that of GO can be related to the
growing of steric hindrance by the covalent linkage of the
porphyrin onto GO.49
2.7. Thermogravimetric analysis
The thermal behaviors of GO, GO-Cl and the catalyst were
characterized by TGA. From the TGA and DTG curves, it can be seen
that GO undergoes decomposition processes at several different
temperatures. Based on the curve of GO (Fig. 6), the initial weight
loss occurred at lower temperatures (below 100 ꢁC) because the
residual water was lost at the beginning. The second weight loss
very low loading of the Mn-porphyrin on GO (20.28
catalyst).
mmol Mn/g
2.9. Catalyst activity
The effectiveness of GO-[Mn(TPyP)tart] nanocomposite as a
chiral catalyst was studied by the oxidation of olefins. Styrene was
selected as model substrate for optimization of the reaction con-
ditions. In this research O2 and isobutyraldehyde were used as an
oxygen source and a co-reactant, respectively.
The effect of the temperature was screened by the oxidation of
styrene at 20, 40 and 60 ꢁC (Table 1, entries 1e3). The temperature
showed high effect on the yield, conversion and enantiomeric
excess (ee). At 20 ꢁC (Table 1, entry 1), the conversion, styrene oxide
yield and ee values were low (17%, 12% and 33%, respectively). By
increasing the temperature to 40 ꢁC, the conversion of styrene was
increased from 17% to 74%, however the reaction did not complete
even after 8 h (Table 1, entry 2). The highest activity (conversion
100%) and selectivity (styrene oxide selectivity 89%, ee 73%) were
achieved at 60 ꢁC and the reaction completed after 2 h (Table 1,
Fig. 4. Raman spectrum of GO-[Mn(TPyP)tart].
Please cite this article in press as: Berijani K, et al., Enhanced enantioselective oxidation of olefins catalyzed by Mn-porphyrin immobilized on
graphene oxide, Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.03.027

K. Berijani et al. / Tetrahedron xxx (2018) 1e9
5
Fig. 5. SEM images of (a) graphite, (b) GO and (C) GO-[Mn(TPyP)tart].
Fig. 6. Thermal gravimetric and DTG analysis of GO, GO-Cl and GO-[Mn(TPyP)tart]..
Table 1
Effect of various parameters on the oxidation of styrene with O2.a
Entry
Catalyst
Solvent
Temp. (ꢁC)
Time (h)
Conv.(%)b
Epoxide selectivity (%)
ee (%) (Conf.)c
1
2
3
4
5
6
7
8
GO-[Mn(TPyP)tart]
GO-[Mn(TPyP)tart]
GO-[Mn(TPyP)tart]
GO-[Mn(TPyP)tart]
GO-[Mn(TPyP)tart]
GO-[Mn(TPyP)tart]
None
GO-[Mn(TPyP)tart] Without PrCHO
GO
GO-Cl
GO-[Mn(TPyP)OAc]
[Mn(TPyP)OAc]
CH3CN
CH3CN
CH3CN
CH3OH
EtOAc
n-hexane
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
20
40
60
60
60
60
60
60
60
60
60
60
8
8
2
2
2
2
2
2
2
2
2
2
17
74
100
0
48
21
0
0
0
0
100
100
71
70
89
33 (S)
54 (S)
73 (S)
54
29
31 (S)
66 (S)
i
9
10
11
12
92
88
a
Reaction conditions: catalyst 10.0 mg, styrene 2 mmol, iPrCHO 5 mmol, solvents (CH3CN, CH3OH, EtOAc and n-hexane) 5 mL, molecular oxygen 1 atm. The optimized
conditions are denoted in underline.
b
Conversions and yields are based on the starting substrate and determined by GC.
Determined by GC on a chiral SGE-CYDEX-B capillary column. Absolute configuration of the epoxide of styrene was determined by comparison with the GC data with those
c
observed for R-(þ)-limonene.
Please cite this article in press as: Berijani K, et al., Enhanced enantioselective oxidation of olefins catalyzed by Mn-porphyrin immobilized on
graphene oxide, Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.03.027

6
K. Berijani et al. / Tetrahedron xxx (2018) 1e9
entry 3).
paracyclophane derivative/NaOCl.62 In addition to the green nature
Solvent nature was also studied for the enantioselective epoxi-
of the present catalytic system (O2), the enantioselectivity
(86e100% ee), recyclability and simplicity of GO-[Mn(TPyP)tart]
catalyst are favored over those of the reported catalysts.6e10 The
formation of trans-epoxide in addition to cis-epoxide in the
oxidation of cis-stilbene (Table 2), suggests a radical chain mecha-
nism for the formation of epoxide by GO-[Mn(TPyP)tart]/iPrCHO/
O2. It is predicted that the oxidation of olefins by this system pro-
ceeds via the formation of an acylperoxy radical upon the reaction
of isopropylaldehyde with the MnIII-porphyrin; this results in the
formation of an acylperoxo-Mn or high valent Mn¼O intermediate
very similar to MnIII(salen)/isobutyraldehyde/O2 system,63,64 which
oxidize olefin.
dation of styrene in different solvents namely acetonitrile, meth-
anol, ethyl acetate and n-hexane (Table 1, entries 3e6). It was found
that acetonitrile is a favored solvent and gave excellent values of
conversion (100%), styrene oxide selectivity (89%) and good ee%
(73%) within 2 h (Table 1, entry 3). Using nonpolar n-hexane, polar
protic (CH3OH) or aprotic polar solvent (ethylacetate) resulted to
lower activities and/or selectivity (Table 1, entries 4e6). These
findings suggest that the polarity,56 low coordinating ability and
aprotic nature of acetonitrile play the main roles in improving the
activity of catalyst GO-[Mn(TPyP)tart].
Control experiments proved that the oxidation of styrene by
O2/iPrCHO is catalytic since in the absence of GO-[Mn(TPyP)tart] no
reaction was ocurred (Table 1, entry 7). In the absence of iso-
butyraldehyde the reaction was not successful as well (Table 1,
entry 8). Supporting agent GO and GO-Cl did not show catalytic
activity (Table 1, entries 9 and 10). The oxidation of styrene in the
presence of homogeneous [Mn(TpyP)OAc] and heterogeneous GO-
[Mn(TPyP)OAc] catalysts, where the axial ligand is acetate anion
and there is no tartrate anion as counter ion, resulted in complete
conversion with high epoxide selectivities. However, there were no
enantiomeric preferences between two enantiomers of the epoxide
(Table 1, entries 11 and 12).
Catalyst GO-[Mn(TPyP)tart] showed noticeable potential for the
epoxidation of aromatic, cyclic and linear terminal olefins with
excellent epoxide selectivity (86e100%) and good enantiose-
lectivity (58e100%) with O2/RCHO (Table 2). The oxidation of aryl
olefins were 100%. Trans-stilbene was oxidized to trans-stilbene
oxide with retention of configuration and 58% ee. However, the
oxidation of cis-stilbene gave cis-stilbene and trans-epoxide
epoxide in the ratio of 20:80 (Table 2, entry 1). This finding suggests
the involvement of a relatively stable radical intermediate that
permits rotation around the C-C bond and conversion of cis-isomer
to transeisomer during the oxidation.57 The completion of the
epoxidation reaction at low time might be related to synergic effect
between Mn(III)-Porphyrin and the GO support.58
2.10. Catalyst reusability
For the development of advanced, cost-effective and mild in-
dustrial processes, catalyst stability in reusable experiments are
crucial. To study the recyclability and stability of GO-[Mn(TPyP)
tart], the oxidation of styrene was investigated in five reaction
runs under the optimized conditions (Fig. 7). At the end of each
reaction run, the catalyst was completely separated from reaction
mixture by centrifugation, washed thoroughly with acetonitrile to
get rid of the organic reactant molecules and dried at 60 ꢁC. The
catalyst was reused in repeated epoxidation reactions of fresh re-
actants. Through the examining the filtrate by AAS and UVeVis, it
was found that there is no manganese, manganese-porphyrin or
free base porphyrin in each run (Fig. S3). Consecutive conversions
of styrene were 100%. GO-[Mn(TPyP)tart] showed no loss of activ-
ity, decreasing of epoxide selectivity or ee even after five consec-
utive catalytic cycles. The results confirmed that GO-[Mn(TPyP)tart]
is a stable and recyclable heterogeneous catalyst. The SEM and FT-
IR did not show any significant difference in the structure of the
used catalyst versus the fresh one (Figs. S4 and S5). A decrease in
conversion was observed after six times recycle.
3. Conclusions
In comparison to styrene, steric and electron withdrawing ef-
fects of the second phenyl on the olefinic C¼C group increased the
rate and epoxide selectivity (100%) in the oxidation of cis- and
trans-stilbenes (Table 2, entries 1e3). The electron donor methyl
group slightly decreases the epoxide selectivity (86%) and benzal-
In summary, we have developed an efficient system with high
activity (TOF up to 14792) for the aerobic enantioselective epoxi-
dation of olefins by using chiral catalyst GO-[Mn(TPyP)tart] under
mild reaction conditions. The catalyst was stable and could be
easily recycled. Various aromatic and aliphatic epoxides were
produced from the corresponding olefins with excellent selectivity,
and high yields. Synergic effect of the GO and catalyst play critical
role in enhancing the activity of [Mn(TPyP)tart].58
dehyde yield in the oxidation of
styrene (Table 2, entry 4). However, involvement of steric effect
around the olefin double of bond -methyl styrene cannot be
excluded. TOF of 4930 was obtained for both styrene and -methyl
a-methyl styrene with respect to
a
a
styrene. 1-Phenyl-1-cyclohexene was converted to the corre-
sponding epoxide with 100% selectivity and 69% ee (Table 2, entry
5).
4. Experimental section
Interestingly, the catalyst could oxidize even the linear terminal
olefins, 1-octene and 1-decene, which have the lowest activity. The
conversions were reasonable with high epoxide selectivity and
enantioselectivity (Table 2, entries 6 and 7). The moderate yield of
1-octene is not surprising since 1-octene is less prone to epoxida-
tion.59 Generally, terminal aliphatic olefins need longer reaction
time for complete conversion.60 The important point is that in the
enantioselective epoxidation of the two used terminal olefins, the
epoxide was obtained with an excellent selectivity (100%) and ee
(>91%). For 1-octene TOF was 837 (entry 6). Their ee values can be
attributed to proximity and easy access to chiral centers in the
catalyst.
4.1. General
Graphite powder, (2R,3R)-(þ)-tartaric acid (L-(þ)-tartaric acid),
5,10,15,20-tetra(4-pyridyl)porphyrin (H2TPyP) and the other re-
agents were purchased from Fluka and Aldrich companies and used
as received.
Chromatographic experiments were performed on an HP Agi-
lent 6890 gas chromatograph equipped with an HP-5 capillary
column (phenyl methyl siloxane 30 m ꢂ 320
m
m ꢂ 0.25
mm) with
flame-ionization detector. The enantiomeric excess (ee%) was
determined by a GC (HP 6890-GC) using a SGE-CYDEX-B capillary
column (25 m ꢂ 0.22 mm ID ꢂ 0.25
m
m). 1H NMR spectra of the
Epoxide selectivity and enantioselectivity of GO-[Mn(TPyP)tar-
t]/iPrCHO/O2 in the oxidation of styrene is higher than Mn(chiral-
salen)Cl/NaOCl/pyridine N-oxide,61 GO-Mn(chiral-salen)Cl/NaOCl/
pyridine N-oxide61 and Mn-porphyrin with chiral [2.2]
reaction mixture were collected on a Bruker 250 MHz spectrom-
eter. The UVeVisible absorption spectra were recorded on a Shi-
madzu 160 spectrometer. DR-UV spectra were run on
Shimadzue2550 spectrophotometer. The structure of the
a
Please cite this article in press as: Berijani K, et al., Enhanced enantioselective oxidation of olefins catalyzed by Mn-porphyrin immobilized on
graphene oxide, Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.03.027

K. Berijani et al. / Tetrahedron xxx (2018) 1e9
7
Table 2
Enantioselective epoxidation of different olefins with O2/RCHO and GO-[Mn(TPyP)tart].a
d
No.
1
Substrate
Conv.(%)b/time
100/10 min
Product(s)/(Yield%)b
Epoxide selectivity (%)
100
ee(%)c (Conf.)
TOF (hꢀ1
14792
)
cis-stilbene
60 (R,R)
2
trans-stilbene
100/10 min
100
58 (R,R)
14792
3
4
Styrene
100/2 h
100/2 h
89
86
73 (S)
58 (S)
4930
4930
a
-methyl styrene
5
1-Phenyl-1-cyclohexene
90/8 h
100
69 (S,S)
1109
6
7
1-Octene
1-Decene
68/8 h
23/8 h
100
100
100 (R)
91 (R)
837
283
a
Reaction conditions: GO-[Mn(TPyP)tart] 10.0 mg (20.28 mmol Mn/g catalyst), substrates 2.0 mmol (except cis- and trans-stilbene 0.5 mmol), isobutyraldehyde 5 mmol, O2
balloon 1 bar, CH3CN 5 mL, 60 ꢁC. Each experiment was repeated at least two times.
b
All products conversion and yield were characterized through the GC analysis.
The enantiomeric excess (ee) values were determined by the GC analysis on a chiral stationary phase (see the experimental section). The enantiomeric configuration of the
c
major isomer was determined by comparing the GC data with those observed for R-(þ)-limonene.
d
TOF (turnover frequency) ¼ (number of reacted molecules)/(active sites) (reaction time).
morphology chemical composition of the prepared nanocomposite.
Raman spectra were run on a RIGAKU spectrometer with laser
power and wavelength 490 mW and 1064 nm, respectively. Ther-
mogravimetric analysis (TGA) of the catalyst was investigated by
Perkin Elmer Pyris Diamond. Manganese assay in the compounds
were performed by a Varian spectrometer AAS-110.
X-ray photoelectron spectroscopy, XPS-(ESCA), measurements
were performed with a Fisons/VG Scientific ESCALAB 200X spec-
trometer, operating at room temperature at
a pressure of
1.0 ꢂ 10ꢀ8 bar and a sample angle of 30ꢁ.
4.2. Synthesis of [Mn(TPyP)OAc]
Complex [Mn(TPyP)OAc] was obtained from the metalation of
H2TPyP by Alder's method.65 Briefly, 12.9 mmol of Mn(OAc)2$4H2O
and 1.29 mmol H2TPyP were dissolved in 100 mL of glacial acetic
acid and heated at 80 ꢁC for 6 h. After the completion of the reac-
tion, the mixture was concentrated to dryness by a rotary evapo-
rator. The residue was dissolved in 1000 mL of D.I. water at 60 ꢁC.
Then it was filtered and precipitated by the addition of sodium
acetate solution (2 M). Yield: 81%.
Fig. 7. The reusability of GO-[Mn(TPyP)tart] in the oxidation of styrene using
O2/iPrCHO.
synthesized compounds was studied using Fourier transform
infrared (FT-IR, Perkin-Elmer 597) spectrophotometer in the 4000-
400 cmꢀ1 frequency range using the prepared compounds diluted
in KBr pellets. The X-ray powder profile (XRD) was recorded using
UVeVis (Fig. S6) in methanol: lmax
(
3
(Soret, 85829), 559 (9598), 598 (2914). IR (KBr) spectrum (Fig. S7)
showed the disappearing of ligand H2TPyP bands at 3306 cmꢀ1 due
to N-H (pyrrole) and 1466 cmꢀ1 due to C¼N (pyridine and
pyrrole).66,67
Cu-K
a
radiation, la ¼ 1.5406 Å, voltage: 40 kV, 40 mA, Bruker
D8ADVANCE diffractometer, Germany. Scanning electron micro-
scope (SEM, Hitachi F4160, voltage ¼ 10 KV) was used to investigate
Please cite this article in press as: Berijani K, et al., Enhanced enantioselective oxidation of olefins catalyzed by Mn-porphyrin immobilized on
graphene oxide, Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.03.027

8
K. Berijani et al. / Tetrahedron xxx (2018) 1e9
4.3. Synthesis of graphene oxide (GO)
centrifugation. The substrate conversion and yields of the products
were quantitatively determined by GC-FID using a calibration
curve. Enantioselectivity was determined using a chiral capillary
column SGE-CYDEX-B. The configuration of the chiral products was
assigned by comparison of the retention times in chiral GC with
that of R-(þ)-limonene.
GO was synthesized from graphite powder by using the modi-
fied Hummers method.23 In a typical synthesis, a 10:1 mixture of
concentrated H2SO4/H3PO4 (60:6 mL) was added to a mixture of
graphite powder (0.5 g) and KMnO4 (3 g). The reaction mixture was
kept in an ice bath and stirred for 30 min and then heated at 70 ꢁC
for 24 h. Then, ice (60 g) and 35% H2O2 were added to the mixture.
The resulting yellow suspension was washed consecutively with
deionized water (30 mL), 30% HCl solution (30 mL), EtOH (200 mL)
and diethylether (30 mL). The obtained GO was dried in the air.
4.9. Reusability test
The recycling experiments were achieved by repetitive use of
GO-[Mn(TPyP)tart] in the epoxidation of styrene. After completion
of the reaction and collecting the catalyst from the reaction mixture
by centrifugation, the catalyst was washed thoroughly with
2 ꢂ 2 ml CH3CN to remove the excess of organic compounds and
dried at 60 ꢁC for 6 h. It was reused for five consecutive runs. The
probable leaching of Mn, Mn-porphyrin or porphyrin ligand was
screened by employing atomic absorption and UVeVis spectro-
photometry. No remarkable leaching of the catalyst was detected in
the reaction mixture. The nature of the recovered catalyst was
investigated by FE-SEM and FT-IR.
4.4. Synthesis of GO-Cl
GO sheets were functionalized with propyl chloride following
the reported procedure.24 GO (0.33 g) was dispersed in dry toluene
(23 ml) by sonication for 20 min. Then, the diluted 3-chlor-
opropyltrimethoxysilane (1 mL in 6.5 mL dry toluene) was slowly
added to the dispersion and heated under reflux for 24 h under
nitrogen atmosphere. After cooling, the black powdery sample was
filtered and washed for several times with toluene and EtOH to
remove any impurities. The resulting GO functionalized by propyl
chloride (shown as GO-Cl) was dried at 70 ꢁC for 6 h.68 Yield: 0.24 g.
Acknowledgments
The authors are grateful for the financial support of this study by
4.5. Immobilization of [Mn(TPyP)OAc] on GO
the University of Zanjan.
The mixture of GO-Cl (0.1 g) and [Mn(TPyP)OAc] (0.01 g,
0.0137 mmol) in DMF (30 mL) was heated at 100 ꢁC for 48 h under
vigorous stirring.69 After cooling the mixture to room temperature,
the obtained black precipitate was washed with methanol
(3 ꢂ 2 ml) and ether (2 ml). The resulting GO-[Mn(TPyP)OAc] was
dried at 60 ꢁC for 6 h. Yield: 96 mg.
Appendix A. Supplementary data
Supplementary data related to this article can be found at
https://doi.org/10.1016/j.tet.2018.03.027.
References
1. Labinger JA. J Mol Catal A Chem. 2004;220:27e35.
2. Zhang A, Li L, Li J, Zhang Y, Gao S. Catal Commun. 2011;12:1183e1187.
3. Meunier B. Chem Rev. 1992;92:1411e1456.
4. Schrader I, Neumann S, Sulce A, Schmidt F, Azov V, Kunz S. ACS Catal. 2017;7:
3979e3987.
5. Rodríguez-García L, Hungerbühler K, Baiker A, Meemken F. J Am Chem Soc.
2015;137:12121e12130.
6. Fadhli M, Khedher I, Fraile JM. J Mol Cat A Chem. 2016;420:282e289.
7. Zhe HL, He AJ. Mol Cat. 2017;443:69e77.
8. Philip C, Page P, Chan Y, Liddle J, Elsegood MRJ. Tetrahedron. 2014;70:
7283e7305.
9. Huang J, Luo Y, Cai J, Chen X. J Organomet Chem. 2016;819:20e26.
10. Hadian-Dehkordi L, Hosseini-Monfared H, Aleshkevych P. Inorg Chim Acta.
2017;462:142e151.
4.6. Synthesis of Na-tartrate
ꢀ
For the synthesizing of the sodium tartrate salt, L-tartaric acid
(0.1 g, 0.66 mmol) was stirred in 10 ml MeOH at 40 ꢁC for 30 min.
The pH of the solution was adjusted to 7 by dropwise addition of
NaOH solution (1 M). The obtained white solid was filtered and
thoroughly washed with MeOH (3 ꢂ 2 ml) and dried at 60 ꢁC for 6 h.
The identity of Na-tart was established by FT-IR. Yield: 90%
(138 mg).
4.7. Synthesis of GO-[Mn(TPyP)tart]
11. Chatel G, Goux-Henry C, Mirabaud A, et al. J Catal. 2012;291:127e132.
12. Wang X, Miao C, Wang S, Xia C, Sun W. ChemCatChem. 2013;5:2489e2494.
13. Ito YN, Katsuki T. Tetrahedron Lett. 1998;39:4325e4328.
14. Jacobsen EN, Wu HM. In: Jacobsen EN, Pfaltz A, Yamamoto H, eds. Compre-
hensive Asymmetric Catalysis. vol. 2. Berlin: Springer; 1999:649.
15. Ishihara S, Labuta J, Van Rossom W, et al. Phys Chem Chem Phys. 2014;16:
9713e9746.
A mixture of GO-[Mn(TPyP)OAc] (33 mg) and sodium-tartrate
(50 mg, 0.206 mmol) in 15 mL methanol was stirred for 5 h at
40 ꢁC. The resulting solid was exhaustively washed with methanol
(3 ꢂ 2 ml) and DMF (3 ꢂ 2 ml) to remove the formed NaCl and
NaOAc. At the end, the prepared GO-[Mn(TPyP)tart] was dried at
60 ꢁC for 6 h.
The Mn-porphyrin loading on GO-[Mn(TPyP)tart] catalyst was
determined by measuring the amount of Mn with atomic absorp-
tion spectrometry (20.28 mmol Mn/g catalyst).
€
16. Heier P, Forster C, Schollmeyer D, Boscher N, Choquet P, Heinze K. Dalton Trans.
2013;42:906e917.
17. Meunier B, de Visser SP, Shaik S. Chem Rev. 2004;104:3947e3980.
ꢁ
ꢁ
ꢁ
ꢁ ꢁ
18. Szabo T, Tombacz E, Illes E, Dekany I. Carbon. 2006;44:537e545.
19. Zhao Q, Bai C, Zhang W, et al. Ind Eng Chem Res. 2014;53:4232e4238.
20. Zheng W, Tan R, Zhao L, Chen Y, Xiong C, Yin D. RSC Adv. 2014;4:11732e11739.
21. Su C, Acik M, Takai K, et al. Nat Commun. 2012;3:1e9.
22. Xu Y, Zhao L, Bai H, Hong W, Li C, Shi G.
J Am Chem Soc. 2009;131:
13490e13497.
4.8. Catalytic studies
23. Marcano DC, Kosynkin DV, Berlin JM, et al. ACS Nano. 2010;8:4806e4814.
24. Mungse HP, Verma S, Kumar N, Sain B, Khatri OP. J Mater Chem. 2012;22:
5427e5433.
25. Zahed B, Hosseini-Monfared H. Appl Surf Sci. 2015;328:536e547.
26. Shao G, Lu Y, Wu F, Yang C, Zeng F, Wu Q. J Mater Sci. 2012;47:4400e4409.
27. Zhang Y, Fei D, Xin G, Cho UR. J Compos Mater. 2015;0:1e13.
28. Bhanja P, Kanti Das S, Patra AK, Bhaumik A. RSC Adv. 2016;6:72055e72068.
29. Acik M, Lee G, Mattevi C, et al. J Phys Chem C. 2011;115:19761e19781.
30. Acik M, Lee G, Mattevi C, Chhowalla M, Cho K, Chabal YJ. Nat Mater. 2010;9:
840e845.
The catalytic activity of GO-[Mn(TPyP)tart] was screened in the
epoxidation of the selected unfunctionalized olefins with O2 which
was provided by an oxygen balloon (O2,1 atm).
Typically, in a round bottom flask placed in an oil bath, the
mixture of styrene (2 mmol), GO-[Mn(TPyP)tart] (10.0 mg) and
isobutyraldehyde (5 mmol, 0.46 ml) as co-catalyst in acetonitrile
(5 mL) was stirred for required time at determined temperature.
After completion of the reaction, the catalyst was separated by
31. Acik M, Mattevi C, Gong C, et al. ACS Nano. 2010;4:5861e5868.
32. Pan D, Wang S, Zhao B, et al. Chem Mater. 2009;21:3136e3142.
Please cite this article in press as: Berijani K, et al., Enhanced enantioselective oxidation of olefins catalyzed by Mn-porphyrin immobilized on
graphene oxide, Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.03.027

K. Berijani et al. / Tetrahedron xxx (2018) 1e9
33. Oh J, Lee JH, Koo JC, et al. J Mater Chem. 2010;20:9200e9204.
9
52. Lee KY, Lee YS, Kim S, et al. CrystEngComm. 2013;15:9360e9363.
53. Stankovich S, Dikin DA, Piner RD, et al. Carbon. 2007;45:1558e1565.
54. Liu L, Su L, Lang J, Hu B, Xu S, Yan X. J Mater Chem A. 2017;5:5523e5531.
55. Huang G, Yuan RX, Peng Y, et al. RSC Adv. 2016;6:48571e48579.
56. Gutmann V. Coord Chem Rev. 1976;18:225e255.
€
34. Lee DW, De Los Santos V, Seo JW, et al. J Phys Chem B. 2010;114:5723e5728.
35. Gao Y, Zhang Y, Qiu C, Zhao J. Appl Organomet Chem. 2011;25:54e60.
ꢁ
36. Gonzalez-Mendoza AL, Cabrera-Lara LI. J Mex Chem Soc. 2015;59:119e129.
37. Khatri PK, Choudhary S, Singh R, Jain SL, Khatri OP. Dalton Trans. 2014;43:
8054e8061.
57. Adam W, Roschmann KJ, Saha-Moller CR, Seebach D. J Am Chem Soc. 2002;124:
5068e5073.
58. Berijani K, Hosseini-Monfared H. Inorg Chim Acta. 2018;471:113e120.
59. Betz D, Raith A, Cokoja M, Kühn FE. ChemSusChem. 2010;3:559e562.
38. Zhu Y, Yu L, Wang X, Zhou Y, Ye H. Catal Commun. 2013;40:98e102.
39. Liu F, Sun J, Zhu L, Meng X, Qi C, Xiao F-S. J Mater Chem. 2012;22:5495e5502.
40. Zhang S, Zhu L, Song H, et al. J Mater Chem. 2012;22:22150e22154.
41. Li Z, Xia CG, Zhang XM. J Mol Catal A Chem. 2002;185:47e56.
42. Giovannetti R. In: Uddin J, ed. Macro to Nano Spectroscopy. vol. 1. Rijeka,
Croatia: InTech; 2012:448.
43. Gouterman M. J Mol Spectrosc. 1961;6:138e163.
44. Bala Murali Krishna M, Venkatramaiah N, Venkatesan R, Narayana Rao D.
J Mater Chem. 2012;22:3059e3068.
45. Stankovich S, Dikin DA, Piner RD, et al. Carbon. 2007;45:1558e1565.
46. Fu C, Zhao G, Zhang H, Li S. Int J Electrochem Sci. 2013;8:6269e6280.
47. Sunil M, Udaya Bhat K, Rahman MR, Jayalakshmi M. Am J Mater Sci. 2015;5:
12e18.
48. Liu Y, Zhou J, Zhang X, et al. Carbon. 2009;47:3113e3121.
49. Chen L, Guo X, Guo B, Cheng S, Wang F. J Electroanal Chem. 2016;760:105e112.
50. Su H, Li Z, Huo Q, Guan J, Kan Q. RSC Adv. 2014;4:9990e9996.
51. Salavati-Niasari M, Bazarganipour M. Appl Surf Sci. 2008;255:2963e2970.
€
60. Klawonn M, Bhor S, Mehltretter G, Dobler C, Fischer C, Beller M. Adv Synth
Catal. 2003;345:389e392.
61. Zheng W, Rong T, Yin S, et al. Catal Sci Technol. 2015;5:2092e2102.
62. Banfi S, Manfredi A, Montanari F, Pozzi G, Quici S. J Mol Catal A Chem. 1996;113:
77e86.
63. Bryliakov KP, Khavrutskii IV, Talsi EP, Kholdeeva OA. React Kinet Catal Lett.
2000;71:183e191.
64. Katsuki T. J Mol Catal A Chem. 1996;113:87e107.
65. Adler AD, Longo FR, Kampas F, Kim J. J Inorg Nucl Chem. 1970;32:2443e2445.
66. Harriman A, Porter G. J Chem Soc Faraday Trans. 1979;75:1532e1542.
67. Berijani K, Hosseini-Monfared H. Mol Catal. 2017;433:136e144.
68. Nasseri MA, Allahresani A, Raissi H. RSC Adv. 2014;4:26087e26093.
69. Saeedi MS, Tangestaninejad S, Moghadam M, Mirkhani V, Mohammadpoor-
Baltork I, Khosropour AR. Polyhedron. 2013;49:158e166.
Please cite this article in press as: Berijani K, et al., Enhanced enantioselective oxidation of olefins catalyzed by Mn-porphyrin immobilized on
graphene oxide, Tetrahedron (2018), https://doi.org/10.1016/j.tet.2018.03.027