A recyclable Mn-porphyrin catalyst for enantioselective epoxidation of unfunctionalized olefins using molecular dioxygen

Article abstract of DOI:10.1039/c6nj00808a

A magnetically separable chiral catalyst, Fe₃O₄/tart/Mn(TCPP)Cl, was prepared by the synthesis and immobilization of manganese-tetra(4-carboxyphenyl)porphyrin, Mn(TCPP)Cl, onto the chiral surface of magnetite nanoparticles (Fe₃O₄/tart-NPs; tart = l-tartaric acid) through the carboxylate groups. The solid catalyst was characterized by a variety of methods including XRD, FE-SEM, FT-IR and UV-Vis spectroscopy. The catalytic activity was tested in the epoxidation reactions of prochiral olefins with molecular oxygen. The reactions were carried out in acetonitrile as solvent with molecular oxygen in the presence of isobutyraldehyde as the stoichiometric oxidant. The catalyst showed consistently high conversion, selectivity and enantioselectivity to epoxide formation in the oxidation of a variety of terminal, cyclic and aromatic olefins. The catalyst showed little deactivation with time and it is easily recovered by magnetic filtration and could be reused four times with little or no loss in activity and selectivity.

Full text of DOI:10.1039/c6nj00808a

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A recyclable Mn-porphyrin catalyst for enantioselective
epoxidation of unfunctionalized olefins using molecular dioxygen
Afsaneh Farokhia and Hassan Hosseini-Monfareda
Received 00th January 20xx,
Accepted 00th January 20xx
A magnetically separable chiral catalyst, Fe3O4/tart/Mn(TCPP)Cl, was prepared by the synthesis and immobilization of
manganese-tetra(4-carboxyphenyl)porphyrin, Mn(TCPP)Cl, onto the chiral surface of magnetite nanoparticles (Fe3O4/tart-
NPs; tart = L-tartaric acid) through the carboxylate groups. The solid catalyst was characterized with a variety of methods
including XRD, FE-SEM, FT-IR and UV-Vis spectroscopy. The catalytic activity was tested in epoxidation reactions of
prochiral olefins with molecular oxygen. The reactions carried out in acetonitrile as solvent with molecular oxygen in the
presence of isobutyraldehyde as the stoichiometric oxidant. The method showed consistently high conversion, epoxide
selectivity and enantioselectivity to epoxide formation for a variety of terminal, cyclic and aromatic olefins oxidation. The
catalyst showed little deactivation with time and is easily recovered by magnetic filtration and could be reused four times
with little or no loss in activity and selectivity.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Numerous asymmetric catalysts have been designed for
producing enantioenriched compounds in the past four
decades18,19,20 and the main emphasis has been on ligand
Introduction
Enantioselective synthesis represents one of the biggest
challenges in synthetic chemistry12,3,4 because of the increasing
trend towards the application of single enantiomers of chiral
compounds especially for pharmaceuticals, agrochemicals,
flavors and fragrances.5,6 Chiral compounds are very important
in our daily life, especially in medicinal chemistry, because
different configurations have different drug activities.7,8
design to control the reactant approach to the active center,
so that the reaction occurs at only one face of the approaching
prochiral substrate. This has led to the design of libraries of
chiral ligands with varying degrees of steric bulk that can
control the enantioselection, for example C2 symmetric
ligands:
salen,
bis(oxazoline)
and
BINAP
(2,2'-
bis(diphenylphosphino)-1,1'-binaphthyl).18 However, their
applications in industrial processes have often been hindered
due to the high costs9 of both the precious metals and chiral
ligands typical of these catalysts, as well as difficulties in
removing trace amounts of toxic metals from the organic
products. Due to these intrinsic shortcomings, many
For producing enantiopure or enantio-enriched
compounds the most elegant approach is enantioselective
catalysis, where prochiral starting materials are transformed
to enantiomerically pure products with the help of chiral
catalysts.9,10,11 The application of enantioselective catalysis to
the fine chemicals industry has great potential both from
economic and ecological points of view.12 Since the early
1980s, numerous catalysts have appeared, the most
recognizable ones being the seminal work of Katsuki and
Sharpless on the asymmetric epoxidation of allylic alcohols13
which followed by the development of manganese–salen
complexes by Katsuki et al.14 and Jacobsen et al.15 for the
epoxidation of certain unfunctionalised olefins. A chiral
epoxide is a valuable intermediate, containing one or two
stereogenic centers, which can be used in further
heterogenization
approaches,
including
adsorption,
encapsulation, tethering using a covalent bond,21,22,23,24,25,26,27
and via cation exchange,28 have been explored to overcome
these problems. However, the heterogenized catalysts
obtained from these methods are often less effective in terms
of both activity and enantioselectivity than their homogeneous
counterparts and their preparations are often difficult. There is
thus a need to develop more efficient and easy to synthesis
immobilization strategies for the heterogenization of
homogeneous asymmetric catalysts.
transformations
reactions.16,17
through
asymmetric
ring
opening
On the other hand, magnetic nanoparticles (MNPs)
application as catalyst supports show unique properties such
as high surface area, good dispersion, superparamagnetic
behavior as well as low toxicity.29 The most attractive feature
of MNP-supported catalysts is that they can be recycled by
simple magnetically driven separation, thereby eliminating the
requirement of catalyst filtration and centrifugation.
Furthermore, improved activity is usually achieved in the
Department of Chemistry, University of Zanjan 45195-313, Zanjan, I.R. Iran.
Corresponding author. Tel.: +98 24 33052576; fax: +98 241 33583203. E-mail
address: monfared@znu.ac.ir (H. Hosseini-Monfared)
† Electronic Supplementary Information (ESI) available:
FT-IR, UV-Vis and NMR spectra, XRD pattern, comparison of the activity of the
catalyst with reported chiral Mn-porphyrin catalysts, particles size distribution.
See DOI: 10.1039/x0xx00000x
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nanometre-sized supported catalyst due to the high surface
area and good dispersion properties. In the past few years,
several transition-metal catalysts have been successfully
immobilized on MNPs with good catalytic activity and
reusability.30,31,32,33
(a)
(b)
Porphyrin-based catalysts show remarkable potential for
the enantioselective epoxidation of unfunctionalized terminal
olefins with high enantiomeric excess (ee) and turnover
numbers.34,35 They give better ee values than those obtained
with other catalytic systems such as the much studied
Mn(salen) derivatives.34 However, several crucial challenges
still remain. First, it is important to develop the synthesis of
the catalysts using inexpensive and easy-to-prepare ligands.
Second, an ultimate achievement would involve the use of
molecular dioxygen. Among various oxidants, molecular
dioxygen is a cheap, clean and readily available oxidant.36,37
Herein we report on a highly efficient chiral catalyst derived
Fig.
1
representation of the immobilized Mn(TCPP)Cl on Fe3O4/tart-NPs,
Representation of (a) Mn(TCPP)Cl structure and (b) schematic
chiral
a
nanocatalyst (see the next section).
The reaction products of the oxidation were determined
and analyzed by HP Agilent 6890 gas chromatograph equipped
from inexpensive raw materials via
immobilization of a manganese-porphyrin on magnetite
nanoparticles stabilized by chiral L-tartaric acid
a
convenient
with an HP-5 capillary column (phenyl methyl siloxane 30 m
320 0.25 m) with flame-ionization detector. The
enantiomeric excess (ee) was determined by chiral GC (HP
6890-GC) using a SGE-CYDEX-B capillary column (25 m 0.22
m). H- and 13C-NMR spectra of the reaction
×
ꢀ
m
×
ꢀ
(Fe3O4/tart/Mn(TCPP)Cl) as shown in Fig. 1. We aimed to
obtain a nanoparticles displaying asymmetric catalytic activity.
The catalytic activity of the synthesized Fe3O4/tart/Mn(TCPP)Cl
was evaluated in the oxidation of prochiral olefins by
molecular dioxygen. We also describe the recovery and reuse
of the heterogeneous catalyst. Supported metalloporphyrins
are provided significant protection under oxidizing conditions,
avoiding the approximation of two porphyrins planes that can
promote a self-destruction of the macrocycle ring or a
×
1
mm ID× 0.25 ꢀ
mixtures (without purification) were run on a Bruker 250 MHz
spectrometer using CDCl3 as solvent. UV–Vis spectra of
solutions were obtained on a Shimadzu 160 spectrometer.
Fourier transform infrared (FT-IR) spectra were recorded using
a Perkin-Elmer 597 spectrophotometer after making pellets
with KBr powder. Powder X-ray diffraction patterns were
collected at the Bruker, D8ADVANCE, Germany, wavelength
dimerization by a ꢀ-oxo bonding between manganese. These
1.5406 Å (Cu Kα), voltage: 40 kV, current, 40 mA. The size and
effects lead to an inactivation of the catalyst behavior and are
commonly reported in homogeneous catalysis studies. In our
previous studies, it was found that Fe3O4/tart-NPs were
efficient catalyst for asymmetric oxidation.30 However, the
Fe3O4/tart-NPs generally showed limited epoxide selectivity for
aromatic olefins.
morphology of solid compounds were recorded by using a
Hitachi F4160 scanning electron microscope (SEM) operated at
an accelerating voltage of 10 KV. The magnetization
measurement was performed at room temperature using a
vibrating sample magnetometer (VSM) device in the
Development Centre of the University of Kashan (Kashan,
Iran).
Experimental section
Synthesis of 5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin
(H2TCPP)
Materials and equipments
4-Carboxybenzaldehyde (99.5%, Merck), propionic acid (99%,
Merck,), nitrobenzene (99%, Merck), pyrrole (97%, Merck),
manganese(II) chloride tetrahydrate (99%, Merck), ferric
chloride hexahydrate (99%, VWR,), urea (99 %, Merck),
diethylene glycol (DEG) (99%, Merck), (2R,3R)(+)-tartaric acid
(L-(+)-tartaric acid, 99%, Merck,) and isobutyrlaldehyde (98%.
Merck,) used as received. Solvents and other reagents of the
highest grade were purchased from Merck and Fluka and used
without further purification.
Manganese(III)
and
metal-free
tetrakis(4-
carboxyphenyl)porphyrin (H2TCPP) were synthesized according
to the methods described in the literature.38,39 Fig. 1
represents the porphyrin structure. To synthesize the metal-
free porphyrin (H2TCPP), pyrrole (0.11 mL, 1.66 mmol) was
added to a mixture of 4-carboxybenzaldehyde (0.25 g, 1.66
mmol), propionic acid (5.8 mL) and nitrobenzene (2.5 mL) with
stirring at room temperature. The mixture was heated at 120
°C for 1 h. After cooling and solvent removal under vacuum,
porphyrin was dissolved in 14 mL of 0.1 M NaOH solution.
H2TCPP was precipitated with a 1 M HCl solution, filtrated,
redissolved in ethanol and recrystallized by solvent
evaporation. Yield 37% (0.12 g). UV-Vis in ethanol: λmax (log ε,
1
1
dm3 mol− cm− ): 418 (4.55), 514 (3.65), 548 (3.53), 588 (3.45)
and 645 (3.38). FT-IR (KBr, cm-1): 3440 (s, br, O-H), 3125 (w, N-
2 | J. Name., 2012, 00, 1-3
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H), 3040 (w), 2921 (w), 2851 (w), 1713 (vs, -CO2), 1606 (m, Catalytic aerobic oxidation of hydrocarbons
C=C).40
The oxidation of hydrocarbons with O2 was performed in a 25-
mL round-bottom flask. The catalyst was used without any
pretreatments. In a typical experiment the flask was charged
with the suspension of Fe3O4/tart/Mn(TCPP)Cl (1.0 mg;
contains 0.17 μmol Mn, 0.01 mol% Mn) in 5 mL acetonitrile,
0.1 g chlorobenzene as internal standard, substrate (2 mmol, 1
mmol for cis-stilbene, 0.5 mmol for trans-stilbene) and 5 mmol
isobutyraldehyde. The flask was conditioned by purging with
oxygen for 5 min and re-filled with oxygen. Dioxygen, as
oxidant, was filled in the system by using an oxygen balloon.
The reaction mixture was stirred at 25 °C for 8 h. At
appropriate intervals, aliquots were removed and analyzed by
GC-FID. The products were identified with authentic samples
and/or 1H- and 13C-NMR spectroscopic data. The reaction
products were quantified by gas chromatography using the
following equation:
Synthesis of Mn(TCPP)Cl
The free-base porphyrin was metallated with MnCl2, by
modifying the conventional methodology reported by
Kobayashi, using 0.082 mmol of porphyrin dissolved in 20 mL
of N,N-dimethylformamide (DMF) and an excess of 10 times of
the metal salt.41 The mixture was refluxed for 2 h. DMF was
removed by distillation and Mn(TCPP)Cl precipitated by adding
water. The precipitate was dissolved in 0.1 M NaOH solution
and reprecipitated by adding 1 M HCl solution. Mn(TCPP)Cl
was dissolved in ethanol and recrystallized by solvent
evaporation. Finally, Mn(TCPP)Cl was dried at room
temperature. Yield 80%. Manganese(II) was oxidized to Mn(III)
by air during the reaction.
The porphyrins H2TCPP and Mn(TCPP)Cl have shown the
absorptions by UV-Vis spectroscopy. The porphyrins spectra
are due to
π-π* transitions. For H2TCPP the bands were
obtained at 417 nm (Soret) and 514, 547, 591 and 644 nm (Q-
bands) whereas in Mn(TCPP)Cl the number of bands were
reduced to two (565 and 599 nm) in the visible region and
Soret band appeared at 465 nm. Thus, successful insertion of
metal in porphyrin was confirmed by disappearance of 644 nm
band42 and reduction in number of Q bands as compared to
H2TCPP.
A = peak area in GC chromatogram
Running the reaction when chlorobenzene was used as
substrate resulted to no oxidation product. This finding
confirmed that chlorobenzene as internal standard is innocent
under the oxidation conditions. Control experiments in the
absence and presence of chlorobenzene were also confirmed
that it did not involve in the oxidation of a substrate.
FT-IR (KBr, cm-1): 3440 (s, br, O-H), 2921 (w), 2851 (w),
1713 (shoulder, -CO2), 1606 (s, C=C).40
Synthesis of magnetite nanoparticles with chiral stabilizer
(Fe3O4/tart-NPs)
Catalyst recycling
In order to investigate the possibility of several recycling runs
for Fe3O4/tart/Mn(TCPP)Cl, the solid catalyst was separated
from the reaction mixture at the end of the reaction. In the
presence of a magnetic stirrer bar, Fe3O4/tart/Mn(TCPP)Cl
moved onto the stirrer bar steadily and the reaction mixture
turned clear within 30 s. The catalyst was isolated by simple
decantation. After washing with 2 mL CH3CN and drying at 60
°C for 6 h, the catalyst was reused as fresh catalyst for the
oxidation of 1,2,3,4-tetrahydronaphthalene.
Magnetite nanoparticles stabilized with chiral tartaric acid
(Fe3O4/tart-NPs) was synthesized according to our previously
reported procedures.30,31 In a typical experiment, FeCl3·6H2O
(0.81 g, 3 mmol), L-(+)-tartaric acid (0.075 g, 0.5 mmol) and
urea (1.80 g, 30 mmol) were completely dissolved in
diethylene glycol (30 mL) by vigorous mechanical stirring. The
obtained yellow clear solution was sealed in a Teflon lined
stainless steel autoclave (23 mL capacity) and then heated at
200 °C for 4 h. After cooling down to room temperature, the
black magnetite were separated magnetically and washed with
ethanol for several times to eliminate organic and inorganic
impurities, and then dried at 60 °C for 6 h.
Results and discussion
Synthesis and characterization
Immobilization of Mn(TCPP)Cl on Fe3O4/tart-NPs
In order to heterogenize the Mn-porphyrin for ease of
Mn(TCPP)Cl was adsorbed on Fe3O4-tart NPs surface separation
and
recycling,
we
synthesized
a
(Fe3O4/tart/Mn(TCPP)Cl) by a method similar to the previously tetraphenylporphyrin with the carboxyl functional groups in
reported procedure.43 Typically, 0.25 g Fe3O4/tart-NPs was the p-position of the phenyl rings. The reaction conditions
added to 250 mL of 0.2 mM Mn(TCPP)Cl ethanolic solution. used to prepare the free-base porphyrin H2TCPP were adapted
The mixture was stirred overnight at 60 °C. The solid was from literature procedure38 and metallated it using the
filtered, washed with ethanol in order to remove the manganese(II) salt (Scheme 1). The metallation process was
unadsorbed metalloporphyrin and dried at 60 °C for 6 h.
confirmed by employing UV–Vis and FT-IR spectroscopies (see
Fe3O4/tart/Mn(TCPP)Cl is a stable catalyst without leaching the Experimental section).
of Mn-porphyrin during the oxidation reaction. The
A chiral support of the catalyst (Fe3O4/tart-NPs) was
supernatant solution of the oxidation reaction of tetralin prepared by stabilizing magnetite nanoparticles by L-(+)-
catalyzed by Fe3O4/tart/Mn(TCPP)Cl (see the next section) tartaric acid30 and Mn(TCPP)Cl immobilized on it to obtain
Fe3O4/tart/Mn(TCPP)Cl nanoparticles (Fig. 1b). According to
showed no absorption peak of Mn(TCPP)Cl at 465 nm.
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the UV-Vis analysis,44 the Mn(TCPP)Cl loading in
Journal Name
Fe3O4/tart/Mn(TCPP)Cl was 0.17 ꢀmol/g catalyst.
Scheme 1 Synthetic route of the heterogeneous catalyst Fe3O4/tart/Mn(TCPP)Cl
On the basis of our recent studies on the functionalization
of magnetite Fe3O4-NPs with different carboxylic acids,31 we
chose Fe3O4/tart-NPs as a support to immobilize the prepared
Mn(TCPP)Cl. We obtained the Fe3O4/tart-NPs through the
polyol method and used it without any further
functionalization.30 Among the variety of methods for
synthesizing magnetic nanoparticles,45,46,47,48 the polyol
method has been paid more attention.49 The polyols in this
method often serve as reducing agent and stabilizer to prevent
interparticle aggregating. Presence of L-tartaric acid enhances
further the stabilization of Fe3O4 nanoparticles. The carboxyl
group in L-tartaric acid can provide coordination to the
nanoparticles and thereby the NPs are stabilized. It is well-
known that sodium citrate can attach to Au and provides a
Fig. 2 UV–Vis spectra of H2TCPP, Mn(TCPP)Cl and Fe3O4-tart/Mn(TCPP)Cl in ethanol;
Fe3O4-tart/Mn(TCPP)Cl was dispersed in ethanol by using an ultrasound.
XRD analysis was used to investigate the crystalline
structure of the synthesized catalyst. The XRD pattern of
Fe3O4/tart/Mn(TCPP)Cl nanoparticles (Fig. 3) shows a pattern
similar to those of Fe3O4 and Fe3O4/tart nanoparticles which
indicates the stability of the crystalline phase of the magnetic
nanoparticles during the subsequent surface modification.
negatively
charged
surface
for
Au
nanoparticle
stabilization.50,51
UV–Vis absorption spectrum of the H2TCPP in ethanol
showed a typical Soret band at 417 nm and four Q bands at
514, 547, 591 and 644 nm (Fig. 2). The Soret and Q bands are
assigned to constructive and destructive mixing of electron
transitions between the HOMO (a2u, π)/HOMO-1 (a1u, π) and
LUMO (eg, π*) molecular orbitals, respectively.52 The
manganese porphyrin, Mn(TCPP)Cl, was obtained by
metallation of the free ligand H2TCPP with the Mn(II) salt.
During the metallation process, the Mn(II) oxidation took place
and the Mn(III)-porphyrin was obtained. The insertion of
Mn(III) in the porphyrin ring proves a significant modification
of the macrocycle symmetry. Distortions or reduction in the
electronic structure of the porphyrin molecule changes the
frontier orbital symmetry and acts to decrease their free
electron box and raises their energies.53 The free basic
porphyrin with D2h symmetry and four bands in the 500–800
nm region, changes to D4h resulting in a reduction to two
bands (α and β) in this region, and a Soret band red shift for
manganese.54 The UV-Vis spectrum observed for Mn(TCPP)Cl
in ethanol presented the Soret band at 465 nm and, in the
visible region, the bands at 565, 599 nm (Fig. 2). The groups of
the observed bands confirm the metallation process.
Fig.
3 XRD patterns of the synthesized (a) Fe3O4-NPs, (b) Fe3O4/tart-NPs and (c)
Fe3O4/tart/Mn(TCPP)Cl nanoparticles
In the case of Fe3O4/tart/Mn(TCPP)Cl, using intense peak at
2theta = 35.837, FWHM = 0.236, we get crystallite size of 35.3
nm by X’Pert program. Characteristic peaks (2θ = 18.3°, 30.4°,
35.8°, 43.5°, 54.2°, 57.6°, 63.1 and 74.7°) corresponding to
(111), (220), (311), (400), (422), (511), (440) and (533) planes
of cubic inverse spinel Fe3O4 are obtained.55 The positions and
relative intensities of all diffraction peaks for all the Fe3O4-NPs,
Fe3O4/tart-NPs and Fe3O4/tart/Mn(TCPP)Cl-NPs match well
with those expected for Fe3O4. This conclusion was confirmed
by their FT-IR spectra (see below) which proved the mainly
magnetite nature of the catalyst core.56,57
The absorption spectrum of the immobilized Mn(TCPP)Cl
on chiral magnetite nanoparticles is similar to that of
Mn(TCPP)Cl by a small red shift to 469 nm due to the
interaction between the carboxylate groups of the porphyrin
and Fe3O4/tart-NPs.
The FT-IR spectra of Mn(TCPP)Cl, Fe3O4/tart-NPs and
Fe3O4/tart/Mn(TCPP)Cl nanoparticles are shown in Fig. 4.
Strong absorption bands at 587 and 630 cm-1 are related to the
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vibrations of the Fe-O functional group in Fe3O4/tart-NPs and of Mn(TCPP)Cl/Fe3O4/tart are bigger than that of Fe3O4/tart-
Fe3O4/tart/Mn(TCPP)Cl, which shows that the phase of as- NPs with average size about 61 nm.
prepared particles is mainly magnetite.58 The absorption band
at 630 cm-1 might be assigned to the existence of some
amount of oxidized maghemite on the magnetite surface. In
the Fe-O range, fine-grained, synthetic maghemite shows
broad IR bands at 668, 630, 442 cm-1.59,60 The spectrum of
magnetite has broad bands at 587 and 400 cm-1.59 The
stretching vibration of –COOH functional groups of Mn(TCPP)Cl
and Fe3O4/tart-NPs is seen at 1731 cm-1. The presence of both
the ν(C=O) vibrations of the acid COOH functionality at 1731
1
−
cm− and the
ν
s(CO2 ) vibration of the carboxylate group at
1
1422 cm− , reveal that the L-tartaric acid is adsorbed as a
monotartrate species which is bound to the surface via the
deprotonated carboxylate group. L-tartaric acid is adsorbed as
a monotartrate species which is bound to the surface via the
deprotonated carboxylate group and it is not involved in
intermolecular H-bonding interactions with the alcohol groups
of neighboring monotartrate species, as shown in Fig. 1.30,61
The broad characteristic band at 3424 cm-1 could be assigned
to O–H stretching vibration arising from the Fe–OH groups on
nanoparticles and adsorbed (2R,3R)(+)-tartaric acid and water.
The dominant H-O-H bending vibration of water is seen at
1635 cm−1.62 The peaks around 2922 and 2853 cm-1, assignable
to asymmetric/symmetric vibrations of C–H in (2R,3R)(+)-
tartaric acid and intramolecular hydrogen bond derived from
OH can be obviously found. The characteristic functional
groups of Mn(TCPP)Cl are difficult to see in the FT-IR spectrum
of Fe3O4/tart/Mn(TCPP)Cl (Fig. 4c) because of low
concentration.
Fe3O4/tart-NPs; average particles size 19.5 ± 4.2 nm.
Fe3O4/tart/Mn(TCPP)Cl; average particles size 61 ± 16 nm.
Fig. 5 FE-SEM images and average particle size of (a) Fe3O4/tart-NPs, (b)
Fe3O4/tart/Mn(TCPP)Cl.
Fig.
4 An overlay of the FT-IR spectra of (a) Fe3O4/tart, (b) Mn(TCPP)Cl and (c)
Fe3O4/tart/Mn(TCPP)Cl.
The surface morphology of chiral Fe3O4/tart nanoparticles
and Fe3O4/tart/Mn(TCPP)Cl are shown in Fig. 4(a) and (b). The
FE-SEM images of the chiral Fe3O4/tart nanoparticles show
uniformity and spherical morphology with an average
diameter of about 20 nm. The spherical shaped nanoparticles
Fig. 6 Magnetization curve of Fe3O4/tart/Mn(TCPP)Cl at room temperature. The
inset shows the photograph of catalyst separation by a small magnet.
Fig. 6 shows a typical magnetization curve of the
Fe3O4/tar/Mn(TCPP)Cl nanoparticles by vibration sample
magnetometer (VSM) at room temperature. The hysteresis
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loops of the powdered material showed no magnetic tartrate) oxidaized thioanisole with H2O2 in 46% conversion,
hysteresis, with both the magnetization and demagnetization 29% sulfoxide selectivity and 100% ee.27 Chiral sulfoxides are
curves passing through the origin, which clearly indicates the an important class of compounds as chiral auxiliaries in
superparamagnetic nature of the material. Moreover, the asymmetric carbon–carbon bond forming reactions,67,68 as
synthesized heterogeneous nanocatalysts possess strong bioactive ingredients in the pharmaceutical industry69 and
magnetic responsivity due to a high saturation magnetization constitute chiral synthons in organic synthesis for the
value (50.07 emu g−1) which helps the separation of catalyst at preparation of biologically active compounds.70
the end of catalysis by an external magnet.
Table 1 Conversion and selectivity for catalytic oxidation of tetralin with molecular
Catalytic activities
dioxygen a
With the immobilized Mn(TCPP)Cl catalyst in hand, the
oxidation reaction of tetralin was chosen as a model reaction
to examine the catalytic activity and reusability. Tetralin is a
convenient substance to choose for studying the autoxidation
of the CH2 group.63 The oxidation of tetralin was carried out in
acetonitrile at room temperature and in the presence of ≈ 0.01
mol% of catalyst and isobutyraldehyde (iPr-CHO) as co-catalyst
Entry
Catalyst
Conv.
(%) b
0 c
Product(s) /
(Yield %) b
Ee %
with dioxygen as the oxidant. The reaction was evaluated by
gas chromatography after 8 h and the results are summarized
(Conf.)
i
in Table 1. In the absence of Pr-CHO no conversion was
1
2
3
4
5
6
7
Fe3O4/tart/Mn(TCPP)Cl
Mn(TCPP)Cl
observed (Table 1, entry 1). Although the reaction proceeded
very well in the presence of unsupported Mn(TCPP)Cl
(conversion 75%); however, as expected, no enantioselectivity
was observed (entry 2). The L-tartaric acid as capping agent in
Fe3O4/tart-NPs affects the enantioselectivity and sterogenic
centre configuration of the epoxid.30 The support Fe3O4/tart-
NPs catalyzes the oxidation of tetralin, but the conversion is
low 25% (entry 3).30 To our delight immobilization of
Mn(TCPP)Cl on chiral magnetic support Fe3O4/tart-NPs
resulted to chiral 1-tetralol (ee 74%, entry 4). Additionally, the
catalytic activity of Fe3O4/tart was increased from 25 to 69%
and the only product was 1-tetralol (compare entry 3 and
75
25
69
74
0 e
0 f
1-tatralol (75)
none
45 (S)
74 (S)
83 (S)
Fe3O4/tart-NPs
1-tetralol (22)
1-tetralone (3)
1-tetralol (69)
Fe3O4/tart/Mn(TCPP)Cl
Fe3O4/tart/Mn(TCPP)Cld
Fe3O4/tart/Mn(TCPP)Cl
Fe3O4/tart/Mn(TCPP)Cl
1-tetralol (74)
entry
4).
By
increasing
the
concentration
of a Reaction conditions: catalyst 1.0 mg (Fe3O4/tart/Mn(TCPP)Cl contains 0.17 μmol
Mn-porphyrin; Mn(TCPP)Cl 1.14 μmol, 6.7 times higher than
Fe3O4/tart/Mn(TCPP)Cl, tetralin 2 mmol, chlorobenzene 0.1 g, isobutyraldehyde 5
Fe3O4/tart/Mn(TCPP)Cl, the activity increased (conversion
74%) without changing enantiomeric excess. The reaction
proceeded well in acetonitrile and no activity was observed in
methanol or ethyl acetate (entries 6, 7). The possibility of
b
mmol, solvent 5 mL, O2 balloon, reaction temperature 25 °C, time 8 h.
c
Conversions and yields calculated on the basis of the substrate. No co-catalyst
(iPrCHO). d Catalyst 3.0 mg, 0.51 μmol. e In CH3OH as solvent. f In ethyl acetate as
involvement of acetonitrile as co-reactant, through solvent. Reported results are the average of two experiments.
peroxyimidic acid as an oxygen donor, was excluded in the
With the optimized conditions, the substrate scopes were
oxidation reactions, since no MeCONH2 under the catalytic
oxidations was detected.64 The polarity of acetonitrile
(dielectric constant ε/ε0 = 37.5 where ε0 is the electric
permittivity of free space) is higher than methanol (32.7) and
ethyl acetate (6.02).65 Probably, the polarity, low coordinating
ability and aprotic nature of acetonitrile play the main roles in
improving the activity of the catalyst in acetonitrile.
Thioanisole and cyclohexane were also examined to explore
the scope of the catalytic activity of Fe3O4/tart-NPs. Many
efforts have been made to develop new catalysts to oxidize
cyclohexane under mild conditions.66 The main product in the
oxidation of cyclohexane was cyclohexanol with 88%
selectivity and 25% conversion. This finding shows the
nonradical nature of the active intermediate during the oxygen
transfer. Thioanisole was oxidized to enantioenriched sulfoxide
with 100% selectivity and 88% ee. Immobilized manganese(II)
complex, [{Mn(H2O)2Cl2}2(H2Btar)], on mesoporous support
SBA-15 (H2L = 2,3-O-4-hydroxybenzhydrazidebenzylidene-D-
next explored. In the presence of 0.01 mol% of
Fe3O4/tart/Mn(TCPP)Cl, various olefins including terminal,
cyclic and aromatic olefins were oxidized with molecular
dioxygen to afford mainly the epoxide with high yields and
enantioselectivities (Table 2). Highest enantioselectivity (>
99%) and epoxide selectivity (100%) were obtained in the
oxidations of 1-decene with 95% conversion and 1-octene with
68% conversion (Table 2, entries 1 and 2). For 1-decene and 1-
octene the reaction is almost stopped after the chosen
reaction time. There was not any isomerization of the olefins
for the Mn catalyst. Considering the chain length of 1-octene
and 1-decene, the epoxidation activity of the catalyst is
increased with increasing the chain length of olefin (Fig. S3).
An increase in the olefin chain length resulted in better activity
which can be attributed to the increased steric hindrance at
the catalytic center and facilitating the oxygen transfer in the
active intermediate [Mn-O-CH2-Ċ
HR]‡ (such as Scheme 4). It
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needs additional studies to explore how the steric hindrance of
Furthermore, to probe the activity and selectivity of the
the long chain olefin does not hinder its interaction with the catalyst, we chose the oxidation of cyclohexene because it can
Mn in formation of the intermediate.
lead to several distinct products, where their identities depend
on the combination of catalyst and oxidant used. For example,
Main product in the oxidation of styrene and
β-
methylstyrene was the corresponding epoxides with about radical-mediated oxidation pathways can lead to the allylic-
68% ee; benzaldehyde was also formed with 33% and 37% oxidation products 2-cyclohexen-1-ol and 2-cyclohexen-1-one.
selectivity, respectively (entries 3, 4). Similarly, in addition to Alternatively, the peroxide oxidant can be activated, through
the epoxide, acetophenone (53%) was resulted in the binding to the metal center, to undergo reactions with the
oxidation of
α
lower than those of styrene and
-methyl styrene; enantioselectivity (ee 40%) was cyclohexene double bond, affording cyclohexene oxide. High
-methylstyrene (entry 5). yields of cyclohexene oxide was obtained as the predominant
β
Probably, the stability of the plausible intermediate benzyl product, with small amount of the allylic oxidation product 2-
radical favors breaking of the olefin double bond in the cyclohexen-1-ol (conversion 98%, epoxide selectivity 88%;
oxidation of styrene,
α
- and
β
-methyl styrene. In the case of
α-
Table 2, entry 9). This result is similar to those obtained by
methyl styrene the product results from the breaking of the previous investigators using other metal complexes of
olefin double bond (acetophenone) is high because the double porphyrin and non-porphyrin, as catalysts.74 In the oxidation of
bond is rich in electron and stable (compare entries 3-5).
cyclohexene with O2/iPrCHO, the activity and selectivity of
cis-Stilbene is a frequently used substrate for the study of Fe3O4/tart/Mn(TCPP)Cl is very different from that of
olefin epoxidation mechanism71 because of mechanistic Fe3O4/tart-NPs (conversion 58%, epoxide selectivity 61%) as
information associated with the ratio of cis- and trans-isomers catalyst30 and favoring the epoxide product over the allylic
in the stilbene oxide product. The oxidation of cis-stilbene oxidation (conversion 98%, epoxide selectivity 88%; Table 2,
proceeded to cis- and trans-stilbene oxide with molar ratio of entry 9).
39/61 and mainly S-enantiomer trans-stilbene oxide with 84%
Scientists frequently employ cis-cyclooctene substrate as a
ee (entry 6). This finding shows the important role of steric diagnostic compound in catalytic systems involving
effect in determining the product selectivity by this catalyst. metalloporphyrins. Cyclooctene was converted to its epoxide
The oxidation reaction probably proceeds through a benzyl by 90% after 2 h and 98% after 4 h and the selectivity was 98%
radical intermediate, which is sufficiently stable and allows (entry 10). It is easy to oxidize cis-cyclooctene in the presence
free rotation about the C-C bond axis. The formation of trans- of metalloporphyrins. The low stability of the allyl species
stilbene oxide from cis- and trans-stilbenes as the main formed from this substrate as compared with the allyl species
product via a radical intermediate is expected due to the generated from cyclohexene, favors epoxide production.75
higher thermodynamic stability of the trans in comparison to
The catalytic activity and enantioselectivity of the present
the cis stereoisomer, in which the phenyl groups are located in catalyst are higher than those of complicated and difficult to
anti-position with respect to each other. Our finding is similar synthesis chiral Mn-porphyrins which have been used for the
to the oxidation of cis-stilbene with the O2/isobutyraldehyde epoxidation of olefins with NaOCl, PhIO or H2O2 as oxidant,
catalyzed by iron(II) cyclam which gave trans-stilbene oxide as such as, for example, chiral Mn(III)-meso-tetrakis-[2.2]-p-
the predominant product72 and the system appeared to cyclophanyl-porphyrin and dinorbornabenzene-derived Mn-
proceed by a mechanism different from that of the oxidation porphyrins,76 chiral Mn-meso-tetrakis[(aS)-2´-methoxy-1,1´-
by peracids which gave predominantly cis-stilbene oxide.73 The binaphth-2-yl]porphyrin77 and Mn-tetrakis[(R)-1,1´-binaphth-2-
reaction with Mn(TPP)Cl and O2/isobutyraldehyde give higher yl]porphyrin.78 Some comparisons in the epoxidation of
ratio of cis- to trans-stilbene oxide products, implying that the styrene are summarized in Table S1 (in Supplementary
product ratios were seriously influenced by the types of Information). In addition to the simplicity of the synthesis of
ligands bound to manganese.73 In the reaction of mCPBA (0.8 Fe3O4/tart
mmol) with cis-stilbene (1 mmol) and isobutyraldehyde (1 mL) for enantioselective oxidation is highly remarkable.
in CH3CN (5 mL) under Ar, only the cis-stilbene oxide and not Enantioselectivity of Fe3O4/tart Mn(TCPP)Cl in the epoxidation
trans-stilbene oxide has been detected in the products.73 of olefins with O2/isobutyraldehyde is comparable to the
trans-Stilbene was oxidized quantitatively by enantioselctivity of homogeneous threitol-strapped
Fe3O4/tart/Mn(TCPP)Cl to the corresponding epoxide with manganese-porphyrin/1,5-dicyclohexylimidazole in the
100% selectivity (entry 7). oxidation of olefins which oxidize styrene with iodosylbenzene
/Mn(TCPP)Cl, its potential in using molecular oxygen
/
Indene was oxidized successfully to indene oxide with to styrene oxide in 86% yield and ee 69%.79
conversion 82% and selectivity to epoxide was 100%, but ee
Recycling of the catalyst
20% was low (entry 8). This result again elucidates importance
of the steric effect of the substrate during the chirality The recyclability of Fe3O4/tart/Mn(TCPP)Cl was examined using
induction step of the reaction. Indene is not oxidized by the oxidation of tetralin. The only product was 1-tetralol with
Fe3O4/tart-NPs even after 24 h.30 Epoxide selectivity and S-configuration and ee about 83%. After each run, the solution
enantioselectivity (ee) of catalyst Fe3O4/tart/Mn(TCPP)Cl are was simply decanted with the assistance of a small magnet.
much better than the support (Fe3O4/tart-NPs) activity as The catalyst was quickly concentrated to the side wall of the
catalyst.30
reaction flask once a magnet is placed nearby. After a simple
wash by CH3CN and subsequent removal of residual solvent by
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drying at 60 °C for 6 h, the catalyst was reused for the next
run. Washing the catalyst and, specially, drying at 60 °C for 6 h
is essential for achieving high efficiency. When the separated
catalyst was washed and reused without drying, the activity
decreased significantly. To our delight, the obtained catalyst by
the adsorption of Mn(TCPP)Cl on magnetic chiral support
(Fe3O4/tart-NPs) showed excellent activity and reusability for 4
recycles. The yield and enantioselectivity was maintained at
similar level after 4 cycles (Fig. 7), however, activity of the
catalyst decreased somewhat in the fifth run from 72% to 62%
without any reduction in its enantioselectivity. The spectra of
the filtrates after each recycle are shown in Fig. S10. It is
obviously seen that there is no Mn-porphyrin leaching up to
four cycle, but porphyrin is leached during the fifth run. These
data are consistent with results of the catalyst activity shown
in the recycle experiments (Fig. 7). Probably, the carboxylate
groups of the immobilized Mn(TCPP)Cl is gradually protonated
during each recycle by the produced isobutyric acid and in the
fifth run the protonation is enough to begin the Mn(TCPP)Cl
separation. UV-Vis spectrum of the filtrate after 5th run shows
the spectrum of H2TCPP instead of Mn(TCPP)Cl. This finding
also confirms our prediction concerning the protonation of the
immobilized Mn(TCPP)Cl.
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Table 2 Aerobic asymmetric oxidation olefins catalyzed by Fe3O4/tart/Mn(TCPP)Cl a
Entry
1
Substrate
Conv.
(%) b
95
Product(s)/Yield (%) b
(configuration)
Epoxide
Select. (%)
100
ee (%)
(config.) c
100 (S)
2
3
68
100
67
100 (S)
67 (S)
100
4
100
63
68 S)
5
6
7
100
100c
100 d
47
40 (R)
84 (S)
ND
Ph
100
100
Ph
O
cis-epoxide
(39)
8
9
82
98
100
88
20 (S,S)
50 (S)
10
98 e
98
O
96
a Reaction conditions: catalyst Fe3O4/tart/Mn(TCPP)Cl 1.0 mg (0.17 μmol Mn-porphyrin), substrate 2 mmol, chlorobenzene 0.1 g, isobutyraldehyde 5 mmol, CH3CN 5
mL, O2 balloon, reaction temperature 25 °C, reaction time 8 h. b Conversions and yields calculated on the basis of the substrate. c cis-stilbene 1 mmol. d trans-stilbene
0.5 mmol. e After 4 h; after 2 h the conversion was 90%. All data were reproduced at least twice and reported as an average.
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The oxidation mechanism
The key role of Fe3O4/tart/Mn(TCPP)Cl catalysis is evident in
the oxidation of hydrocarbons. In the absence of the catalyst
no reaction occurred under our mild operating conditions with
O2/isobutyraldehyde,
while
an
increase
in
Fe3O4/tart/Mn(TCPP)Cl concentration led to higher conversion
in the oxidation of tetralin (Table 1, entry 5). In addition,
producing the epoxide (80%) as main product (Table 2, entry 9)
suggests that a normal free radical is not the predominant
pathway
in
the
oxidation
of
cyclohexene
by
Fe3O4/tart/Mn(TCPP)Cl and O2/aldehyde. It has often been
assumed that high-valent metal oxo species are the most likely
active intermediates when olefins are selectively oxidized to
the corresponding epoxides in metal complex-catalyzed
oxygen atom transfer reactions.80,81,74 It has been also
reported that species such as metal-OOR complexes (R = H,
CR´, C(O)R´),82 which are the precursors to the high-valent
metal oxo complexes, are sometimes themselves competent
as epoxidizing agents that can react directly with olefins. We
suppose that the oxygenation of substrates with
Fe3O4/tart/Mn(TCPP)Cl and O2/isobutyraldehyde occurs by a
mechanism proposed to the oxidation with O2/aldehydes
catalyzed by metal complexes as is given in Scheme 2.83 In this
mechanism, the supported Mn-porphyrin complex ([MnIII]) is
assumed to play two roles. First, the Mn complex reacts with
the aldehyde to generate an acyl radical (RC(O)•). The acyl
radical then reacts with dioxygen to give an acylperoxy radical
(RC(O)OO•). The acylperoxy radical acts as a carrier in a chain
mechanism by reacting with another aldehyde molecule to
give the peroxyacid, thereby generating another acyl radical.
Oxygenation of substrate is assumed by this mechanism to
occur via a reactive high-valent Mn-oxo intermediate, which is
produced by the reaction of the peroxyacid with the Mn-
porphyrin of Fe3O4/tart/Mn(TCPP)Cl and which then reacts
with the substrate (olefin) to give the oxygenated product.
Fig. 7 Recycling experiments. Reaction conditions: Fe3O4/tart/Mn(TCPP)Cl 3.0 mg (0.51
μmol), tetralin (2 mmol), CH3CN 5 mL, isobutyraldehyde 5 mmol, 25 °C and time 8 h. ee
(%) Determined by chiral GC. Averages of duplicate experiments and error bars
represent the standard deviations.
UV-Vis spectrum of the recycled catalyst (Fig. 8) shows the
presence of the Mn(TCPP)Cl on the recycled catalyst. The FE-
SEM image of the catalyst after recycle indicated that the
catalyst is quite robust and no significant aggregation was
found (Fig. 8).
Scheme
2
The suggested mechanism for the oxidation of olefins by
Fe3O4/tart/Mn(TCPP)Cl and O2/isobtyraldehyde.
It has been shown that the acylperoxy radical generated in the
radical reaction of dioxygen and aldehyde is a competent
oxidizing agent for olefin epoxidations.84 The acylperoxy
radical has a greater tendency to yield epoxides by addition to
the double bond of olefins rather than to abstract an allylic
hydrogen atom of the olefins to give allylic oxidation products,
whereas hydroxyl and alkylperoxy radicals tend to abstract an
allylic hydrogen.85 However, the high enantioselectivity of
Fe3O4/tart/Mn(TCPP)Cl catalyst shows that direct involvement
of acylperoxy radical in the epoxidation is probably not
Average particles size 55.9 ± 13.6 nm after 4 times recycle.
Fig. 8 UV-Vis spectra of fresh and used Fe3O4/tart/Mn(TCPP)Cl (above) and FE-SEM of
the recovered Fe3O4/tart/Mn(TCPP)Cl (bottom).
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favorable in this system. Another possible epoxidizing agent in
this system is percarboxylic acids, which is a potential
epoxidizing intermediate since such peracids can be formed
from reaction of acylperoxy radicals with aldehydes (see
Scheme 2). Peroxy acids are known to epoxidize cis-stilbene
stereoselectively to give only the cis-oxide as product. The
oxidation of cis-stilbene by [MnIII]/O2/isobutyraldehyde gave
trans-stilbene oxide as the predominant product (Table 2,
entry 6). Therefore, we can conclude that the epoxidizing
intermediate is not the peroxyacid, presumably because the
rate of the reaction of the acylperoxy radical with [MnIII] is
faster than that of its reaction with aldehyde under our
reaction conditions.
Fig. 9 The proposed intermediate for the olefin epoxidation by Fe3O4/tart/Mn(TCPP)Cl
Conclusion
In summary, we have successfully developed, for the first time,
Additional evidence for this was obtained from
stereochemical investigations of the epoxidation of cis/trans
stilbene (Table 2). The epoxidation of trans-stilbene proceeded
stereospecifically with retention of configuration at the double
bonds (Table 2, entry 7). In oxidation of cis-stilbene a mixture
of cis- and trans-stilbene oxide with 1:1.56, molar ratio was
obtained (Table 2, entry 6). This result showed that oxygen
transfer to olefin is not concerted and that it goes through a
radical intermediate. The radical intermediate can be
transferred to cis-epoxide via direct collapse or there is a
rotation of the radical intermediate which then collapses to
trans-epoxide as shown in Scheme 3.86
a
novel magnetically separable catalytic system by
immobilization of Mn-porphyrin on chiral Fe3O4/tart
nanoparticles stabilized by a chiral stabilizer. The immobilized
Mn-porphyrin catalyst showed excellent catalytic activities and
enantioselectivities for the asymmetric epoxidation of various
terminal, cyclic and aromatic olefins by O2/aldehyde and could
be reused for at least 5 times. Asymmetric induction is done
through the supported L-(+)-tartaric acid and anchoring
Mn(TCPP)Cl on magnetite helps easy separation of the catalyst
by magnetic decantation. This strategy work shows that
valuable chiral epoxide can be synthesized without using any
complicated and expensive metal catalysts which have been
proved difficult to prepare. The generality of this catalyst-
chiral inducer immobilization strategy should allow the design
of many highly active and enantioselective heterogeneous
asymmetric catalysts which are one of the most promising
solutions to the problems presented by homogeneous
catalysts.
Acknowledgments
We would like to gratefully thank the University of Zanjan and
the Iran National Science Foundation under Grant No. INSF
92036382.
Scheme 3 The suggested intermediate for the oxidation of cis-stilbene.
Although detailed considerations on the transition state
which results in enantioselectivity are not yet clear, the
stereochemistry of all the products indicates that
Fe3O4/tart/Mn(TCPP)Cl approaches the carbon-carbon double
bond preferentially from the pro-S face of the olefin plane
consisting of RCH=CHR´ group. Chirality is induced by the chiral
L-tartaric acid when the olefin is closing the nanocatalyst
surface. Product analyses of the various olefin epoxidation
(Table 2) proves that the S-face of the prochiral olefin is mainly
preferred for oxygenation, thereby the approching of olefin is
through pro-S face, as shown in Fig. 9. Facial selectivity in
addition of the olefin to Fe3O4/tart/Mn(TCPP)Cl would be
established by hydrogen bond interaction between the
hydrogen atom on the olefin double bond and the hydroxyl
group of the tartrate (suggested transition state in Fig. 9).
Similar intermediate has been proposed for the asymmetry
achieved in the osmium tetroxide oxidation of olefins by
employing chiral amines.87,88
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For Table of Contents Graphic and Synopsis
A highly efficient green process for the asymmetric epoxidation of olefins by an easy to synthesis Mn-
porphyrin and O2/aldehyde is reported.