Collaborative effect of Mn-porphyrin and mesoporous SBA-15 in the enantioselective epoxidation of olefins with oxygen

Article abstract of DOI:10.1016/j.ica.2017.10.037

The rational design of heterogeneous, low cost transition metal complexes that can catalyze olefin with high enantioselectivity and activity has been a challenging goal for the synthetic chemist. In this study a chiral ion pair strategy was used for the synthesis of a biomimetic efficient manganese-tetrapyridylporphyrin (H₂TPyP) catalyst for the asymmetric epoxidation of olefins with O₂. Complex Mn-TPyP was covalently linked to mesoporous SBA-15 in heme-type environments and its counter ion was replaced by L-tartrate anion (SBA15-[Mn(TPyP)TA]). Chiral and achiral homogeneous analogous of Mn-TPyP were also prepared. The Mn-porphyrin confined in nanoreactors of SBA-15 exhibited enhanced activity (TOF = 652 h^?1) and enantiomeric excess (ee 93%) compared with the value obtained when the same chiral catalyst functioned in homogeneous solution (TOF 97 h^?1 and ee 83%) in the oxidation of 1-decene with O₂/isobutyraldehyde. The high specific surface area, uniformly sized pore channels and site isolated active centers of the catalyst may contribute to the high activity and enantioselectivity. SBA15-[Mn(TPyP)TA] was structurally stable and could be recycled for repeated use. Total turnover number in the oxidation of styrene after five cycles was 47,400 with 86% epoxide selectivity and ee 86%.

Full text of DOI:10.1016/j.ica.2017.10.037

Accepted Manuscript
Research paper
Collaborative effect of Mn-porphyrin and mesoporous SBA-15 in the enantio-
selective epoxidation of olefins with oxygen
Kayhaneh Berijani, Hassan Hosseini-Monfared
PII:
S0020-1693(17)30752-1
https://doi.org/10.1016/j.ica.2017.10.037
ICA 17964
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Reference:
Inorganica Chimica Acta
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28 October 2017
Please cite this article as: K. Berijani, H. Hosseini-Monfared, Collaborative effect of Mn-porphyrin and mesoporous
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doi.org/10.1016/j.ica.2017.10.037
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Collaborative effect of Mn-porphyrin and mesoporous SBA-15 in
the enantioselective epoxidation of olefins with oxygen
Kayhaneh Berijani, Hassan Hosseini-Monfared*
Department of Chemistry, University of Zanjan, 45195-313, Zanjan, Islamic Republic of Iran
Email: monfared@znu.ac.ir
Abstract
The rational design of heterogeneous, low cost transition metal complexes that can catalyze
olefin with high enantioselectivity and activity has been a challenging goal for the synthetic
chemist. In this study a chiral ion pair strategy was used for the synthesis of a biomimetic
efficient manganese-tetrapyridylporphyrin (H2TPyP) catalyst for the asymmetric epoxidation of
olefins with O2. Complex Mn-TPyP was covalently linked to mesoporous SBA-15 in heme-type
environments and its counter ion was replaced by L-tartrate anion (SBA15-[Mn(TPyP)TA]).
Chiral and achiral homogeneous analogous of Mn-TPyP were also prepared. The Mn-porphyrin
confined in nanoreactors of SBA-15 exhibited enhanced activity (TOF =652 h-1) and
enantiomeric excess (ee 93%) compared with the value obtained when the same chiral catalyst
functioned in homogeneous solution (TOF 97 h-1 and ee 83%) in the oxidation of 1-decene with
O2/isobutyraldehyde. The high specific surface area, uniformly sized pore channels and site
* Corresponding author (Hassan Hosseini-Monfared)
E-mail: monfared@znu.ac.ir
1

isolated active centers of the catalyst may contribute to the high activity and enantioselectivity.
SBA15-[Mn(TPyP)TA] was structurally stable and could be recycled for repeated use. Total
turnover number in the oxidation of styrene after five cycles was 47400 with 86% epoxide
selectivity and ee 86%.
Keywords: Chiral Mn(III) porphyrin; Enantioselective; Dioxygen; Asymmetric epoxidation;
mesoporous silica support
2

1. Introduction
Epoxidation reactions are among the most challenging reactions in catalysis for the preparation
of fine chemicals [1,2]. An analysis of the various types of industrial processes shows that
epoxidation and dihydroxylation of olefins is the second predominant transformation applied for
industrial processes [3]. Epoxides are reactive intermediates that can be readily involved in
further asymmetric transformations [4]. Limonene epoxide, for example, is a key raw material in
the synthesis of pharmaceuticals, fragrances, perfumes and food additives [5]. Consequently
there is great interest in the design of new heterogeneous catalysts for the enantioselective
oxidation of olefins.
Environmental concerns as well as economic reasons motivate catalysts to be recyclable and
selective with low unwanted waste by-products. Immobilization of a homogeneous catalyst on
inorganic support can enhance its selectivity in addition to heterogenisation. Among the various
supporting materials, mesoporous materials with the pores size in the 2–20 nm range, are suitable
supports for homogeneous catalysts. They show promising features such as large surface area,
uniform pore size distribution and the compatibility between cavity size and substrate/product
size [6,7]. Size and shape of the support cavity effect on the selectivity of hydrocarbon oxidation
has been reported [8]. The desired objectives of immobilizing homogeneous catalysts on the
porous materials are enhancing the activity and enantioselectivity in addition to heterogenization.
Enantioselectivity can be improved by accommodation of chiral catalyst/substrate within the
mesopores of SBA-15 [9]. The support impose a confinement effect through its pores and
surface to increase the catalyst activity and enantioselectivity [10]. The more rigid bonding of the
catalyst provides the higher confinement effect [9].
3

Metalloporphyrins have been extensively studied as catalysts for oxidation reactions [11].
Metalloporphyrins with a core structure similar to that of the iron porphyrin of cytochrome P-450
[12] mimic the functionality of metalloenzymes [13]. They have been extensively studied as
catalysts for oxidation reactions [14,15]. Combination of the catalytic efficiency of
metalloporphyrins with the structural feature of mesoporous silica leads to catalysts displaying
unique features [16].
Herein we are reporting on a highly active and enantioselective Mn-porphyrin supported on
mesoporous SBA-15 (SBA15-[Mn(TPyP)TA]) catalyzed epoxidation of olefins with molecular
oxygen/isobutyraldehyde. L-(+)-tartaric acid (TA) anion was used as chiral auxiliary ion. High
activity and enantioselectivity of SBA15-[Mn(TPyP)TA] proved that it provides active sites very
resemble the prosthetic group of naturally occurring enzymes such as those belonging to the
cytochrome P-450 family. Molecular oxygen is an abundant and environmentally benign oxidant
that plays a central role in the functioning of metalloenzymes [17]. Consequently, there is great
interest in utilizing O2 in synthetic catalytic oxidation systems.
2. Experimental
2.1. Materials and instrumentation
5,10,15,20-Tetra(4-pyridyl)porphyrin (H2TPyP) was purchased from Fluka. SBA-15 and 3-
chloropropyl-functionalized SBA-15 material, SBA15-Cl, were prepared according to our
previously reported procedure [18]. The solvents and other reagents were obtained from Merck
Company and they were used without further purification.
The progress of oxidation reactions were monitored by HP Agilent 6890 gas chromatograph
equipped with an HP-5 capillary column (phenyl methyl siloxane 30 m × 320 µm × 0.25 µm)
4

with flame-ionization detector. The enantiomeric excess (ee) was determined by chiral GC (HP
1
6890-GC) using a SGE-CYDEX-B capillary column (25 m × 0.22 mm ID× 0.25 µm). H NMR
spectra were run on a Bruker 250 MHz spectrometer. UV–Vis spectra of solutions were recorded
by a Shimadzu 160 spectrometer. DR-UV spectra were run on a Shimadzu–2550
spectrophotometer.
FT-IR spectra were obtained with KBr pellets in the range 400-4000 cm-1 using a Perkin-Elmer
597 spectrophotometer. Low angle X-ray diffraction patterns were collectedusing a Bruker,
D8ADVANCE X-ray diffractometer, Cu Kα radiation (λ = 1.5406 Å), voltage: 40 kV, current, 40
mA(Germany). The scanning electron micrographs were taken on a Hitachi F4160 field SEM at
an accelerating voltage of 10 KV. Chemical analysis of the catalyst was recorded by atomic
absorption (Varian spectrometer AAS-110). By standard Brunauer–Emmett–Teller technique
(BET) the specific surface area of the present catalyst was analyzed (Specific Surface Area and
Porosity Analyzer, PHS-1020 (PHSCHINA).
2.2. Synthesis of H2TMPyP
For the synthesis of tetra(4-N-methylpyridyl)porphyrin tetraiodide (H2TMPyP), iodomethane
(426 mg, 3.0 mmol) was added to a solution of H2TPyP (100 mg, 0.15 mmol) dissolved in DMF
(15 mL) and the mixture stirred for 8 h at 135 °C under nitrogen atmosphere. The DMF and
excess of iodomethane were removed with rotary evaporator to afford pure purple H2TMPyP in
1
good yield (142 mg, yield 79%). UV-Vis in CH2Cl2: 421 (Soret), 517, 564, 598, 653 nm. H-
NMR (250 MHz, DMSO-d6): δ = 9.45 (m, 8H, Ho-N-methylpy), 9.16 (m, 8H, Hpyrrole), 8.97 (m, 8H,
13
H
m-py.), 4.68 (s, 12H, N+-CH3), -2.93 (s, 2H, NH) ppm. C-NMR (62.90 MHz, DMSO-d6): δ =
157, 148, 145, 133, 130, 116, 55, 48 ppm
5

2.3. Synthesis of Na-tartrate
The mixture of L-tartaric acid (0.1 g, 0.66 mmol) and CH3OH (10 ml) was stirred at 40 °C for 30
min. The pH of the solution was adjusted to 7 by the dropwise addition of aqueous NaOH (1 M).
The obtained white colored solid was filtrated and thoroughly washed with 3 × 2 ml CH3OH and
dried at 60 °C for 6 h. The identity of Na-tartrate was established by FT-IR. Yield 90% (0.138
g).
2.4. Synthesis of [Mn(TPyP)OAc]
[Mn(TPyP)OAc] was prepared according to the method described by Alder et al. [19]. The free
base porphyrin (H2TPyP) (1.29 mmol) and manganese(II) acetate tetrahydrate (12.9 mmol) were
dissolved in glacial acetic acid (100 mL) and heated to 80 °C for 6 h. The solvent was removed
by rotary evaporator and the resulting dark solid dried in air overnight. For further purification it
was dissolved in deionized water (1000 mL) at 60°C, filtered and precipitated by addition of
dilute sodium acetate solution. Yield 81% (71 mg).
Purple [Mn(TMPyP)OAc] was also synthesized by this method. Yield 72%.
2.5. Synthesis of SBA15-[Mn(TPyP)OAc]
The mixture of [Mn(TPyP)OAc] (0.0137 mmol) and SBA15-Cl (0.1 g) in DMF (30 ml) was
vigorously stirred at 100 ºC for 48 h [20]. Then it was cooled to room temperature and the
resulting solid SBA15-[Mn(TPyP)OAc] was removed by centrifugation. It was washed with
methanol (3 × 2 ml) and ether (2 ml) and dried for 6 h at 60 °C.
6

2.6. Synthesis of SBA15-[Mn(TPyP)TA]
The mixture of SBA15-[Mn(TPyP)OAc] (0.09 g) and Na-tartrate (0.206 mmol) in CH3OH (15
mL) was stirred for 5 h at 40 °C. The resulting solid was washed with methanol (3 × 2 ml) and
DMF (3 × 2 ml) to remove the produced NaCl and NaOAc. Finally, the prepared SBA15-
[Mn(TPyP)TA] was dried at 60 °C for 6 h. Yield 0.12 g. The Mn content of the catalyst was
measured by atomic absorption, which showed value of 21.1 µmol/g in SBA15-[Mn(porph)TA].
For the preparation of [Mn(TMPyP)TA], acetate and iodide ions of [Mn(TMPyP)OAc] was
exchanged by tartrate ion (TA) using the above procedure.
2.7. Enantioselective epoxidation with SBA15-[Mn(TPyP)TA]
In a typical reaction, a 5.0 mL CH3CN suspension containing olefin (2 mmol, 0.5 mmol for cis-
and trans-stilbene), catalyst (10.0 mg), isobutyraldehyde as oxygen acceptor (5 mmol) was
prepared in a flask equipped with condenser and stirrer bar magnet. The atmosphere of oxygen as
oxidant was applied through a balloon. The reaction was proceeded for 8 h at 45 °C under
vigorous stirring. At the end of the reaction the catalyst was separated by centrifugation and the
1
products were evaluated by GC-FID and/or H-NMR. The conversions and yields were
determined by GC using a calibration curve.
3. Results and discussion
3.1. Synthesis and characterization
7

Complex [Mn(TPyP)OAc] was synthesized by treatment of H2TPyP with manganese acetate in
glacial acetic acid and then covalently grafted on chloropropyl functionalized SBA-15 through a
condensation reaction. The counter ion of the pyridinium (chloride) and the Mn-coordinated
acetate were exchanged by L-(+)-tartrate as auxiliary chirality inducer ion (Scheme 1).
< Scheme 1>
The UV–vis spectrum of free base porphyrin H2TPyP shows a Soret band at 417 nm, as well
as four Q bands at 479, 518, 581 and 637 nm. The Soret band of complex Mn(TPyP)OAc
appeared at 459 nm (π→π* electronic transition) and shifted to 464 nm in SBA15-
[Mn(TPyP)TA] (Fig. S1 in the Supplementary and Fig. 1). The red shift of the Soret band
confirmed the grafting of the complex on SBA-15. The Q bands of the complex were shifted
from 560 nm to 570 and 614 nm by immobilization. When the metalloporphyrin is attached to
the support by its meso-substituents, the symmetry of the metalloporphyrin is changed, leading
to a red shift in the Soret band [21].
< Fig. 1>
FT-IT spectra of the synthesized immobiliaed Mn-TPyP and support are shown in Fig. 2. A
very broad peak appeared about 3428 cm-1 corresponds to the stretching O-H vibrations of the
adsorbed water [22]. The highly hygroscopic nature of support SBA-15 causes the significant
peaks due to its silanol groups about 3680 and 3535 cm-1 to be covered by the broad –OH peak
of the adsorbed or hydrogen-bonded water molecule [23]. The width of this broad peak at ~3400
cm-1 for functionalized and grafted samples slightly increased.
The broad peak appeared at 1625 cm-1 is related to the bending vibration (δ(O−H)) of the
adsorbed water [24]. The characteristic band of the C=N groups (pyridyls) in the hybrid
materials overlap by this O-H bending vibration band [25]. The bands centered at around 1625
8

cm−1 in the spectra of the hybrid materials (SBA15-Cl, SBA15-[Mn(TPyP)OAc] and SBA15-
[Mn(TPyP)TA]) shifted slightly to higher wavenumbers and increased in intensity, also
suggesting immobilization of the Mn(TPyP) complex on the support material. The C-N
stretching vibration is normally observed in the wavenumbers range 1000-1200 cm-1. However,
this peak was not resolved due to the overlay with the IR absorptions of Si-O-Si in the range
1130-1000 cm-1 and of Si-CH2-R in the range 1250-1200 cm-1. The characteristic bands of
support SBA-15 were seen at 1212 and 1084 cm-1 due to νas(Si-O-Si) [26] and νs(Si-O-Si) mode
at 807 cm−1. The bending vibration band of O-Si-O unit [27] was found at 462 cm−1. The peaks
at 2976 and 2853 [28] in SBA15-Cl were assigned to the presence of 3-chloropropyl group. The
peaks appeared in the range 1438-1343 cm-1 (Fig. 2d) results from the tartrate groups as counter
ion and axial ligand of the immobilized [Mn(TPyP)TA] [29,30]. The UV-Vis and FT-IR spectra
of H2TMPyP, [Mn(TMPyP)OAc] and [Mn(TMPyP)TA] confirmed their synthesis (Fig. S2 and
Fig. S3).
< Fig. 2>
The low-angle X-ray diffraction patterns of SBA-15, SBA15-Cl and SBA15-[Mn(TPyP)TA]
are shown in Fig. 3. They showed reflections in the 2Ɵ range of 0.8–2 ° (Fig. 3) attributable to
2D hexagonal p6mm symmetry [31]. Functionalization and grafting of the complex on SBA-15
do not change its mesoporous nature. Mesoporous long range ordering is evident from the well-
resolved (110) and (200) reflections. In comparison to the support, the grafted [Mn(TPyP)TA]
shows a decrease in the relative intensity of (100) peak which suggest the reduction in the
ordering of the mesoporous structure with immobilization. The relative intensities of the (110)
and (200) peaks in SBA15-[Mn(TPyP)TA] pattern are lower than those in the SBA15-Cl. These
changes are due to structural differences between the silicate framework and organic moieties
9

which are located inside the channels of SBA-15; confirming the immobilization of the Mn-
porphyrin complex inside the channels of SBA-15 [32].
< Fig. 3>
SEM analyses were employed to illustrate the surface morphology of the SBA-15 and
SBA15-[Mn(TPyP)TA], Fig. 4. Particles of SBA-15 present a curved cylinders form. The
particles of SBA-15 have a diameter of about 500–700 nm and length up to 1–2 µm, which is
typical morphology for SBA-15 mesoporous materials [33]. As seen from SEM images, the
surface of the supported catalyst becomes denser when compared with pure silica. This result
agreed well with the XRD patterns and indicated that the silica surface kept its structure intact
after the complex immobilization.
< Fig. 4>
The isotherm curves of the samples SBA-15, fresh and used SBA15-[Mn(TPyP)TA] are
shown in Fig. 5. The parameters in Table 1 were calculated from nitrogen adsorption data using
the BJH method. Clearly, they exhibited typical type IV isotherms [34]. According to IUPAC
classification, type IV isotherms relate to capillary condensation steps, characteristic of the
mesoporous materials [35], with completely reversible nature, uniformly sized mesoporous.
Surface area of SBA has reduced from 2643 to 1824 m2/g upon anchoring the Mn-porphyrin. It
has been reported that immobilization of homogenous catalysts on porous supports decreases its
specific surface area [36]. The structural properties of the synthesized SBA-15 are not in
agreement with the reported values for a SBA-15 [37,38]. These differences may be resulted
from the removal of the surfactant (Pluronic P123) by washing before final calcination in the
synthesis of silica SBA-15. A reduction of solvent volume and a controlled washing protocol
allow the synthesis of high surface area SBA-15 [39].
10

<Fig. 5>
<Table 1>
3.2. Catalytic activity
Immobilization on SBA-15 provides distribution and site isolation of the complex in addition to
the advantages of its heterogenization. Table 2 summarizes the catalytic activity obtained with
SBA15-[Mn(TPyP)TA] catalyst for the oxidation of styrene. The corresponding blank reactions
were carried out in the absence of the catalyst. The oxidation of styrene produced styrene oxide
and benzaldehyde. Isobutyric acid was also produced due to co-oxidation of oxygen acceptor
isobutyraldehyde. Benzaldehyde is used as a flavoring and fragrance in food, cosmetics,
pharmaceuticals, and soap and is "generally regarded as safe" by the US FDA [40]. Effects of the
solvent and temperature were screened (Table 2, entries 1-5).
The reaction solvent showed remarkable effect on the rate and products. The oxidation of
styrene was performed in different solvents, namely acetonitrile, ethyl acetate, methanol, and n-
hexane (Table 2, entries 2-5). The styrene conversion catalyzed by SBA15-[Mn(TPyP)TA] was
in order CH3CN > ethyl acetate > n-hexane > CH3OH. The best activity (100%), epoxide
selectivity (86%) and enantioselectivity (81%) were obtained in acetonitrile with a relative
dielectric constant of ε/ε0 = 37.5. Medium-polarity, low coordinating ability and aprotic nature of
acetonitrile are favorable for this reaction [41].
Interestingly, in the absence of tartrate ion (TA), as axial ligand and counter ion, conversion
(93%) and epoxide selectivity (74%) decreased (entry 6). Therefore, tartrate ion as axial ligand
affects the activity and selectivity of the Mn-porphyrin in addition to induction chirality feature
11

in the product. Negligible or no or amounts of epoxide was formed when the catalytic oxidation
reactions were performed in the absence of SBA15-[Mn(TPyP)TA], iPrCHO, or [Mn(TPyP)TA];
indicating that all of these components are required for the formation of the epoxide (entries 7-
10).
Use of non-immobilized [Mn(TPyP)OAc] as catalyst proved that it is active for the oxidation
reaction (entry 11). However, as expected, no enantioselectivity was observed. L-tartaric acid as
chiral inducing agent induce the enantioselectivity and stereogenic center in the epoxide [42,43].
< Table 2>
Catalyst SBA15-[Mn(TPyP)TA] is an efficient catalyst for the epoxidation of various types
olefins. Reaction conditions are based on our previous findings and the results of the control
experiments (Table 2). Linear terminal, cyclic and aromatic olefins were oxidized catalytically to
the corresponding epoxides with high epoxide selectivity (86-100%) and enantioselectivity
(Tables 3); the highest yield (100%) was obtained for indene with ee 87 10 % (Table 3, entry
8), whereas the highest enantioselectivity (94 1% ee) but with a lower yield (48 7%) was
obtained for 1-decene (Table 3, entry 5). Enantioselectivity of SBA15-[Mn(TPyP)TA] in the
oxidation of indene is much higher than Fe3O4/tartaric acid/Mn-porphyrin catalyst [44]. Chiral
indene oxide can be converted to cis-aminoindanol with retention of stereogenic center
configuration. cis-Aminoindanol is currently produced on large scale by stereospecific catalysis
of chiral indene epoxide [45,46]. cis-Aminoindanol is an important precursor to the HIV protease
inhibitor CRIXIVAN [47].
<Table 3>
In order to evaluate the effect of the support on the enantioselectivity, tetra(4-N-
methylpyridyl)porphyrin tetraiodide (H2TMPyP) was synthesized. Asymmetric complex
12

[Mn(TMPyP)TA] was obtained by the metalation of H2TMPyP and then replacing iodide counter
ions and Mn-porphyrin axial ligand (acetate) by chiral L-(+)-tartrate anion (Fig. 6).
< Fig. 6>
The results of the oxidation of styrene and 1-decene by O2/iPrCHO catalyzed by heterogeneous
SBA15-[Mn(TPyP)TA] and homogeneous [Mn(TPyP)TA] and [Mn(TMPyP)TA] are
summarized in Table 4.
The oxidation of styrene, an aromatic olefin, is easier than linear terminal olefin 1-decene; Table
4, entries 1-3 and 4-6. The rate of the oxidation by homogeneous catalysts [Mn(TPyP)OAc] and
[Mn(TMPyP)TA] is higher than SBA15-[Mn(TPyP)TA] due to the limitation access to the
catalytic sites located at the SBA-15 structure. In the oxidation of styrene, SBA15-
[Mn(TPyP)TA] showed the highest activity (TOF = 2370 h-1) but its ee was slightly lower than
homogeneous [Mn(TMPyP)TA]. Difference in the enantioselectivity is partially due to the
presence of higher number of chiral L-(+)-tartrate ion (TA) in the later; there are two TA ions in
SBA15-[Mn(TPyP)TA] while five TA ions in [Mn(TMPyP)TA] are present. Despite the several
advantages of the homogeneous catalyst, its heterogeneous counterpart is preferred because of
practicality, due to its easier separation from the reaction media and high enantioselectivity for
linear olefin (1-decene). Although SBA 15 displays an open porous structure with pore sizes in
the range of mesopores, which facilitates the diffusion of bulk molecules inside the material and
minimizes mass transfer hindrances, the rate of the bulk substrates (for example, styrene) is low
because of the limited access to the catalytic sites located at the SBA-15 structure.
The great effect of the support was observed in the oxidation of 1-decene. The oxidation rate
by homogeneous catalysts was higher than the heterogenous SBA15-[Mn(TPyP)TA], however
enough time was considered to achieve the highest possible conversion. The reaction times and
13

epoxide selectivity were the same for all the catalysts. SBA15-[Mn(TPyP)TA] showed the
highest activity (TOF= 652 h-1) and enantioselectivity (94 1%) with respect its homogeneous
analogous. These findings (Table 4) suggest that by providing the time required for the diffusion
of olefin into the mesoporous of SBA-15, ee will increase because the enantioselectivity is very
sensitive to the surface/pore environment of the immobilized catalysts. Remarkable confinement
effect of the support on the catalysis is observed in the oxidation of 1-decene and styrene.
The results of Tables 3 and 4 show that the steric effect of the olefin is an important
factor in determining the catalyst activity. Presence of three substituent in 1-phenyl-1-
cyclohexene as well as the long alkyl chain substituent in 1-decene and 1-octene, decreases the
activity of SBA15-[Mn(TPyP)TA] (Table 3). By considering the results, it is predicted that the
oxygenation of olefins with SBA15-[Mn(TPyP)TA]/O2/isobutyraldehyde is achieved by a
mechanism proposed to the oxidation with O2/aldehydes in the presence of metal complexes
[48]. Acyl radical is formed by the interaction of iPrCHO with the catalyst and then converted to
acylperoxy radical to carry a chain mechanism. The active intermediate is formed by the
interaction of acylperoxy radical and Mn-porphyrin [44]. Preference one face of the olefin
(chirality induction) is induced by the chiral L-tartrate when the prochiral olefin is closing the
intermediate since the L-tartrate ion lies in the vicinity of the Mn-porphyrin core. Exact
determination of the mechanism of oxidation of olefins by SBA15-[Mn(TPyP)TA]/RCHO/O2
needs further studies. However, the present studies suggest that collaborative effect of the L-
tartrate and confinement effect of mesoporous SBA-15 place the olefin in appropriate
configuration to enhance activity and enantioselectivity of the catalyst. The weak interactions
including van der Waals forces, hydrogen bonding, and physical adsorption possibly involved in
the chiral reactions in pores or on surfaces of SBA-15 [16].
14

<Table 4>
3.3. Catalyst recycling
Catalyst SBA15-[Mn(TPyP)TA] was stable under the reaction conditions and could be separated
at the end of the reaction by centrifugation, washed two times with acetonitrile, dried at 60 °C
and reused such as fresh catalyst. As shown in Fig. 7, the catalyst activity and epoxide selectivity
did not change for five runs, while enantioselectivity increased progressively from ee 81% for
the fresh catalyst to 86% in the 5th recycle. Progressive increase in the enantioselectivity may be
resulted from the enhancement in motion restriction and confinement effect in the nanochannels
of the SBA-15; movement of the solvent in mesoporous channels of the SBA-15 during the
reaction makes slight shrinking of the hybrid framework after each recycle (Table 1, compare
properties of the fresh and used catalyst).
The lack of Mn leaching into the reaction mixture was confirmed by examining the reaction
filtrates after each run by UV-Vis spectroscopy. The UV-Vis spectrum of the reaction solution
after each recycle step did not show any absorption peaks characteristic of [Mn(TPyP)TA],
indicate that the leaching of [Mn(TPyP)TA] or Mn ion do not happen during the course of the
oxidation reactions. Stability of the Mn-porphyrin on SBA-15 results from its strong covalent
linkage through the covalent bond. Analyses of the recovered catalyst after 5 times use by FE-
SEM (Fig. 8A), FT-IR (Fig. 8B) and BET (Fig. 8C) methods also confirmed the stability and
lack of the catalyst structural change.
< Fig. 7>
15

< Fig. 8.>
4. Conclusions
A recyclable heterogeneous catalyst SBA15-[Mn(TPyP)TA] was synthesized by covalent
grafting of Mn-tetra(4-pyridyl)porphyrin complex on mesoporous SBA-15 and using L-tartrate
counter ion as chirality inducing agent. It was used for asymmetric epoxidation of olefins with
oxygen. Linear, cyclic and aromatic olefins were oxidized successfully by SBA15-
[Mn(TPyP)TA]/O2/RCHO with high conversion and selectivity. The catalyst oxidized
quantitatively indene to epoxide with 100% selectivity and ee 87 10%. This catalyst was found
superior to its homogeneous analogous Mn(TPyP)OAc and [Mn(TMPyP)TA] in terms of activity
and enantioselectivty for the selective oxidation of 1-decene to 1-decene oxide. There were
remarkable effects of the support on the catalyst activity and enantioselectivity. Total turnover
number in the oxidation of styrene after five cycle was 47400 with 86% epoxide selectivity and
ee 86%.
Acknowledgments
Financial support from the University of Zanjan and the Iran National Science Foundation under
Grant No. INSF 95820950 is gratefully acknowledged.
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Figs., Schemes, Tables captions
Scheme 1 Synthesis of chloropropyl functionalized SBA-15 (above) [18]; nucleophilic displacement of
chloride of SBA15-Cl with [Mn(TPyP)OAc] and finally the exchange of Cl− and OAc− by tartrate
(TA−) resulted to the formation of heterogenized complex SBA15-[Mn(TPyP)TA] (below).
Fig. 1. UV-vis spectrum of (a) [Mn(TPyP)OAc] in methanol and (b) DR UV-Vis spectrum of
SBA15-[Mn(TPyP)TA].
Fig. 2. FT-IR spectra of (a) SBA-15, (b) SBA15-Cl, (c) SBA15-[Mn(TPyP)OAc] and (d)
SBA15-[Mn(TPyP)TA].
19

Fig. 3. Low angle powder XRD patterns of (a) SBA-15 (b) SBA15-Cl and (c) SBA15-
[Mn(porph)TA].
Fig. 4. SEM images of (a) SBA-15 and fresh SBA15-[Mn(TPyP)TA].
Fig. 5. Nitrogen adsorption–desorption isotherms of (a) SBA-15,(b) SBA15-[Mn(TPyP)TA] and
(c) SBA15-Cl
Fig. 6. Asymmetric catalysts used for evaluation of the support effect.
Fig. 7. Catalyst recyclability study for the oxidation of styrene. Reaction conditions: SBA15-
[Mn(TPyP)TA] 10.0 mg, styrene 2 mmol, MeCN 5 mL, iPrCHO 5 mmol, at 45 °C for 4 h. Total
TON for five cycle was 47400.
Fig. 8. (A) SEM images of recovered SBA15-[Mn(TPyP)TA] after catalysis, (B) IR spectra of the
(a) fresh and (b) recovered SBA15-[Mn(TPyP)TA] after 5 cycles and (C) The isotherms of the (a) fresh
and (b) recovered SBA15-[Mn(TPyP)TA] after 5 cycles.
Table 1 Structural properties of the support and catalyst characterized by N2 adsorption.
a
Table 2 Catalytic oxidation of styrene with O2
Table 3 Enantioselective epoxidation of olefins catalyzed by SBA15-[Mn(TPyP)TA].a
Table 4 Catalytic oxidation of styrene and 1-decene by molecular oxygen. a
20

Scheme 1 Synthesis of chloropropyl functionalized SBA-15 (above) [18]; nucleophilic displacement of
chloride of SBA15-Cl with [Mn(TPyP)OAc] and finally the exchange of Cl− and OAc− by tartrate (TA−)
resulted to the formation of heterogenized complex SBA15-[Mn(TPyP)TA] (below).
21

Fig. 1. UV-vis spectrum of (a) [Mn(TPyP)OAc] in methanol and (b) DR UV-Vis spectrum of
SBA15-[Mn(TPyP)TA].
Fig. 2. FT-IR spectra of (a) SBA-15, (b) SBA15-Cl, (c) SBA15-[Mn(TPyP)OAc] and (d)
SBA15-[Mn(TPyP)TA].
22

Fig. 3. Low angle powder XRD patterns of (a) SBA-15 (b) SBA15-Cl and (c) SBA15-
[Mn(porph)TA].
Fig. 4. SEM images of (a) SBA-15 and fresh SBA15-[Mn(TPyP)TA].
23

Fig. 5. Nitrogen adsorption–desorption isotherms of (a) SBA-15,(b) SBA15-[Mn(TPyP)TA] and
(c) SBA15-Cl.
Fig. 6. Asymmetric catalysts used for evaluation of the support effect.
24

Fig. 7. Catalyst recyclability study for the oxidation of styrene. Reaction conditions: SBA15-
[Mn(TPyP)TA] 10.0 mg, styrene 2 mmol, MeCN 5 mL, iPrCHO 5 mmol, at 45 °C for 4 h. Total
TON for five cycle was 47400.
A
B
25

C
Fig. 8. (A) SEM images of recovered SBA15-[Mn(TPyP)TA] after catalysis, (B) IR spectra of the
(a) fresh and (b) recovered SBA15-[Mn(TPyP)TA] after 5 cycles and (C) The isotherms of the (a) fresh
and (b) recovered SBA15-[Mn(TPyP)TA] after 5 cycles.
Table 1 Structural properties of the support and catalyst characterized by N2 adsorption.
Compound
SBET
(m2g-1)
Vtotal
(cm3g-1)
6.406
Dav
(nm)
2.375
SBA-15
2643.09
Fresh SBA15-[Mn(TPyP)TA]
Used SBA15-[Mn(TPyP)TA]
1823.69
1149.79
4.326
3.024
2.375
2.406
a
Table 2 Catalytic oxidation of styrene with O2
Epoxide
Selectivity
(%)
Conv.
(%) b
Ee (%)
(Conf.) c
No.
Catalyst/cocatalyst/oxidant
Solvent
26

SBA15-[Mn(TPyP)TA]/iPrCHO/O2
at 25 °C after 8 h
1
2
CH3CN
CH3CN
70
50
86
25 (S)
81 (S)
SBA15-[Mn(TPyP)TA]/iPrCHO/O2
100
3
SBA15-[Mn(TPyP)TA]/iPrCHO/O2
SBA15-[Mn(TPyP)TA]/iPrCHO/O2
SBA15-[Mn(TPyP)TA]/iPrCHO/O2
0
CH3OH
EtOAc
4
30
22
93
47
50
74
43 (S)
63 (S)
5
n-hexane
6 d
{SBA-15}-[Mn(TPyP)OAc]/iPrCHO/O2 CH3CN
SBA15-[Mn(TPyP)TA]/no
CH3CN
7
0
cocatalyst/O2
8
No catalyst/iPrCHO/O2
SBA-15/iPrCHO/O2
CH3CN
CH3CN
CH3CN
19
13
8
100
69
9
10
SBA15-Cl/iPrCHO/O2
[Mn(TPyP)OAc]/iPrCHO/O2
0.001 g, in 150 min
38
11
CH3CN
100
92
a
i
Reaction conditions: catalyst 10.0 mg, styrene 2 mmol, PrCHO (isobutyraldehyde) 5 mmol, solvent 5 mL, O2 1
atm, at 45 °C, time 4 h.
b Conversions and yields are based on the starting substrate and determined by GC.
c 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 observed for R-(+)-limonene.
d
In {SBA-15}-[Mn(TPyP)OAc] the axial ligand (acetate) and chloride (pyridinium counter ion) ions were not
replaced with L-tartrate ion.
Table 3 Enantioselective epoxidation of olefins catalyzed by SBA15-[Mn(TPyP)TA].a
Conv.
(%)b
Epoxide
Ee(%)c
No.
Substrate
Product(s)/(Yield%)b
TOF
Select.(%)
(Config.)
1
cis-stilbene
100 d
100 e
100
100
66 1 (R) 2370
66 2 (R) 1185
2
trans-stilbene
27

3
4
Styrene
100
100
86
80
78 4 (S)
2370
2370
α-Methyl
74 4 (S)
styrene
48 7 f
94 1 (R)
94 2 (S)
652
664
5
6
1-Decene
1-Octene
100
100
56 1 f
1-Phenyl-1-
cyclohexene
45 10
7
8
89 3 f
100
100
1055
1185
(S)
87 10
Indene
100 f
.
(S,R)
a
Reaction conditions: SBA15-[Mn(TPyP)TA] 10.0 mg (contains 21.1 µmol Mn/g catalyst), substrate 2.0 mmol
(except cis- and trans-stilbene 0.5 mmol), isobutyraldehyde 5 mmol, O2 balloon, CH3CN 5 mL, 4 h, 45 °C.
b All products conversion and yield were characterized through the GC analysis.
c
The enantiomeric excess (ee) values were determined by the GC analysis on a chiral stationary phase (see the
experimental section). The absolute configuration of the major isomer was determined by comparing the GC data
with those observed for R-(+)-limonene. Evaluated from 1H NMR data.
d after 1 h. e after 2 h. f after 8 h.
f TOF (turnover frequency) = Total number of moles product / (mole catalyst)(time)
Table 4 Catalytic oxidation of styrene and 1-decene by molecular oxygen. a
Epoxide
Select.
(%)
Conv.
(%)
Ee(%)
(Config.)
Time
(h)
No.
Catalyst
Substrate
TOF
28

1
2
3
SBA15-[Mn(TPyP)TA]
[Mn(TPyP)OAc]
Styrene
Styrene
Styrene
100
100
100
86
92
85
78 4 (S)
none
4
2370
584
2.5
2.5
[Mn(TMPyP)TA]
88 (S)
1290
4
5
6
SBA15-[Mn(TPyP)TA]
[Mn(TPyP)OAc]
1-Decene
1-Decene
1-Decene
48
11
24
7
100
100
100
94 1 (R)
none
8
8
8
652
8
[Mn(TMPyP)TA]
83 (R)
97
a Reaction conditions such as Table 3. SBA15-[Mn(TPyP)TA] 10.0 mg, [Mn(TPyP)OAc] 1.0 mg, [Mn(TMPyP)TA]
1.0 mg.
29