Manganes-Porphyrin as Efficient Enantioselective Catalyst for Aerobic Epoxidation of Olefins

Article abstract of DOI:10.1007/s10562-018-2447-8

A chiral manganese porphyrin, [Mn(TCPP-Ind)Cl], was synthesized using cis-1-amino-2-indanol substituent. It showed remarkable catalytic activity and enantioselectivity in the epoxidation of olefins with O₂/RCHO. Terminal olefins and styrene derivatives were successfully oxidized (> 99% ee). TON of 73,000 was achieved in the epoxidation of α-methylstyrene after five times recycling. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2447-8

Catalysis Letters
https://doi.org/10.1007/s10562-018-2447-8
Manganes-Porphyrin as Efficient Enantioselective Catalyst for Aerobic
Epoxidation of Olefins
1
1
1
Afsaneh Farokhi · Kayhaneh Berijani · Hassan Hosseini‑Monfared
Received: 13 April 2018 / Accepted: 3 June 2018
©
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
A chiral manganese porphyrin, [Mn(TCPP-Ind)Cl], was synthesized using cis-1-amino-2-indanol substituent. It showed
remarkable catalytic activity and enantioselectivity in the epoxidation of olefins with O /RCHO. Terminal olefins and sty-
2
rene derivatives were successfully oxidized (>99% ee). TON of 73,000 was achieved in the epoxidation of α-methylstyrene
after five times recycling.
Graphical Abstract
Keywords Asymmetric catalysis · Epoxidation · Nanoparticles · Magnetic separation · Aerobic oxidation · Hydrogen bond
1
Introduction
good to excellent stereocontrol [5], but the main drawback
is that it is not selective for terminal olefins and requires up
to 15 mol% of catalyst. There are a lot of great efforts for
development of efficient chiral metal catalysts that can per-
form asymmetric epoxidation of olefins. The well-known
Sharpless method applies titanium tartrate complexes for
the epoxidation of hydroxy functionalized olefins [6].
The Jacobsen–Katsuki method uses Mn(salen) complexes
and provides an efficient method for unfunctionalized
and particularly cis olefins [7]. However, these methods
require the use of alkyl hydroperoxides, hypochlorite, or
other nonatom-efficient reagents as the oxidant. Nowadays
environmental constraints encourage new asymmetric oxi-
dation methods to produce epoxides using environmen-
tally benign oxidants with high atom economy and selec-
tive sustainable catalysts [8]. Additionally, there is still a
significant number of olefins that cannot be converted to
Asymmetric epoxides are versatile building blocks and
intermediates in the synthesis of various kinds of func-
tionalized organic molecules [1, 2]. The catalytic asym-
metric epoxidation of olefins by metal catalysts is reliable
and economic transformation in the synthesis of optically
active epoxides [3, 4]. Chiral ketone-derived organocata-
lysts developed by Shi is able to epoxidize olefins with
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-018-2447-8) contains
supplementary material, which is available to authorized users.
*
Hassan Hosseini-Monfared
monfared@znu.ac.ir
1
Department of Chemistry, University of Zanjan, 45195-313,
Zanjan, Islamic Republic of Iran
Vol.:(0
1
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3

A. Farokhi et al.
epoxides with stereochemical control. In particular, the
asymmetric oxidation of terminal nonactivated olefins
remains an area to be developed.
styrene derivatives by H O (ee up to 60%) were carried out
2
2
using chiral iron and manganese porphyrins as catalysts [22].
The artificially synthesized metalloporphyrins can model
the monooxygenase catalytic cycle [23]. Manganese-based
catalytic systems for the asymmetric epoxidation of olefins
have been one of the most developed catalyst systems, rely-
ing on green oxidants. However, developing chiral metal-
loporphyrins which catalyze efficiently the asymmetric
epoxidation of olefins using molecular oxygen remains a
challenge. To overcome the problem of metalloporphyrin
catalyst separation from a reaction medium and increase its
stability against degradation by dimer formation [24], a fast
and clean method is to use magnetic nanoparticles. Magnetic
nanoparticles provide unique properties for immobilization
of a catalyst and solve the catalyst recovery problems of
time-consuming and energy-intensive filtration or centrifu-
gation [25]. Catalysts supported on magnetic nanoparticles
act under favorable quasi-homogeneous conditions without
decreasing the homogenous catalyst activity and can be recy-
cled by simple magnetic decantation [26]. Magnetic separa-
tion of nanoparticles is simple, economical, and promising
for industrial applications [27].
The choice of the oxidant is a keystone for securing
“
green” character of the overall process. Molecular oxygen
as oxidant is very appealing since it is abundant, renewable,
has the highest active oxygen content (50%) and comply-
ing with the current as well as long-term environmental
constraints [9]. However, the direct oxidation of organic
substrates by O is limited due to the high energy barrier
2
for electron transfer from the organic substrate to molecular
oxygen which has a triplet ground state. Following monoox-
ygenases for selective oxygenation via formation of activated
oxygen, electron source such as borohydride [10], hydrogen
and colloidal platinum, zinc or electrons from an electrode
have been used for reductive O activation [11]. Among the
2
commonly used co-reductant reagents, isobutyraldehyde
(
IBA) is one of the most economically viable and efficient
to promote the generation of acylperoxy radicals, recog-
nized as epoxidation intermediates [12]. The side product of
isobutyric acid obtained with co-oxidation of IBA is a useful
chemical which has plastic and polymer industrial appli-
V1
cations [13]. Ruthenium-metalloporphyrin, Ru -(TMP)
The attainment of new efficient and enantioselective
catalyst for the epoxidation of unfunctionalized olefins
with green oxidant has aroused great scientific and indus-
trial interest. In this report we report on the catalytic activ-
ity of a magnetically recoverable hybrid nanocatalyst,
Fe O -[Mn(TCPP-Ind)Cl], for olefin epoxidation reactions
(
O) , catalyzes olefin epoxidations with molecular oxygen,
2
at room temperature and normal pressure without using a
reducing agent [14]. However, its catalytic activity is low:
1
6–45 mol of epoxide are produced per mole of ruthenium
catalyst in 24 h.
3
4
Environmental and economic restrictions drive catalysts
with higher activity and enantioselectivity preferably using
green oxidants to be designed for the asymmetric epoxida-
tion of olefins [15]. Nam and coworkers used water as an
oxygen source for photocatalytic asymmetric epoxidation
of terminal olefins mediated by chiral manganese com-
plexes (up to 60% enantiomeric excess, ee) [16]. Bhadra
et al. examined asymmetric epoxidation of homoallylic
alcohols by a binuclear titanium complex of a chiral 8-quin-
olinol-based ligand and TBHP [epoxidation of primary and
tertiary homoallylic alcohols (up to 98% ee)] [17]. Chiral
salen Mn(III) complexes with phase transfer substituent
were applied for the asymmetric epoxidation of unfunction-
alized olefins with NaClO (67–93% ee) [18]. Remarkable
effect of Brønsted acid was observed in the enantioselective
epoxidation of olefins mediated by manganese complexes of
a tetradentate N4 ligand and aqueous hydrogen [19]. Bach
reported site- and enantioselective epoxidation of diene com-
pounds controlled by hydrogen bonding interaction between
the substrate and Ru-porphyrin catalyst using 2,6-dichloro-
pyridine-N-oxide as a stoichiometric oxidant (60–83% yield,
in the presence of O as the oxidant and IBA as the co-
2
reductant. The Mn-porphyrin contains chiral (1R,2S)-(+)-
cis-1-amino-2-indanol on the porphyrin ligand to induce
enantioselectivity. The use of amino indanols increase enan-
tioselectivity in catalytic reactions because of their rigidity
[28].
2 Experimental Section
2.1 Materials and Instrumentation
Manganese(II) chloride tetrahydrate (99%, Merck),
(1R,2S)-(+)-cis-1-amino-2-indanol (99%, Fluka), (2R,3R)
(+)-tartaric acid (l-(+)-tartaric acid, 99%, Merck) and
isobutyrlaldehyde (IBA) (98%. Merck) were provided com-
mercially. Fe O -NPs stabilized with l-(+)-tartaric acid and
3
4
5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin (H TCPP)
2
were prepared according to our reported procedure [29].
An HP Agilent 6890 gas chromatograph with an HP-5
capillary column (phenyl methyl siloxane 30 m 320 mm
0.25 mm) and a flame-ionization detector was used for
analysis. The enantiomeric excess (ee) was determined by
a chiral GC (HP 6890-GC) using a SGE-CYDEX-B capil-
lary column (25 m 0.22 mm ID 0.25 mm). UV–Vis spectra
8
5–98% ee) [20]. Kawabata developed amino group directed
symmetric epoxidation of dienyl amine derivatives with
H O using catalyst controlled site-selectivity (up to 78%
2
2
yield, up to 90% ee) [21]. The asymmetric epoxidation of
1
3

Manganes-Porphyrin as Efficient Enantioselective Catalyst for Aerobic Epoxidation of Olefins
were run on a Shimadzu 160 spectrometer. Fourier transform
infrared (FT-IR) spectra were achieved using a Perkin-Elmer
0.014 μmol Mn), acetonitrile (5 mL), olefin (0.5 to 2 mmol)
and IBA (5 mmol). It was connected to an O filled balloon
2
5
97 spectrophotometer as pellets made with KBr powder.
and the mixture stirred at 25 °C for 8 h. Then, the catalyst
was removed by using an external magnet and analyzed by
1
13
For H- and C-NMR spectra a Bruker 250 MHz spec-
trometer used. Measurements X-ray diffraction (XRD) data
were collected by using a Bruker-D8ADVANCE (Germany),
λ=1.5406 Å (Cu Kα), voltage: 40 kV, and current, 40 mA.
The electronic images of compounds were recorded by using
a Hitachi F4160 scanning electron microscope (SEM) oper-
ated at an accelerating voltage of 10 kV. The magnetic stud-
ies was performed using a vibrating sample magnetometer
1
13
GC and H- and C-NMR spectroscopic methods. For GC-
analysis, 0.2 μL of the reaction mixture was used together
with chlorobenzene as internal standard.
3 Results and Discussion
(
VSM) device in the Development Centre of the University
3.1 Synthesis and Characterization
of Kashan (Kashan, Iran).
Synthesis of the chiral Mn-porphyrin and its immobilization
on Fe O -NPs were achieved according to Scheme 1.
2
.2 Synthesis of Manganese(III)
3
4
5
1
‑[4‑N‑(2‑hydroxy‑indan‑1‑yl)benzamide],
0,15,20‑tris(4‑carboxy phenyl)porphyrin
Catalyst [Mn(TCPP-Ind)Cl] was synthesized by the reac-
tion of the carbodiimide-activated carboxylic side chain of
Mn(TCPP)Cl with the amino function of (1R,2S)-(+)-cis-
chloride [Mn(TCPP‑Ind)Cl]
1
-amino-2-indanol [32]. It was immobilized on magnetite
Following the methodology reported by Kobayashi [30], the
metallation of H TCPP with MnCl provided Mn(TCPP)Cl.
nanoparticles through the carboxylate group to obtain hybrid
Fe O -[Mn(TCPP-Ind)Cl]. The Mn content of the catalyst
2
2
3
4
For adding the chiral substituent, the solution of (1R,2S)-
was measured by atomic absorption, which showed value of
−
1
(
+)-cis-1-amino-2-indanol (8.30 mg, 0.056 mmol) and dicy-
13.71 µmol g in Fe O -[Mn(TCPP-Ind)Cl].
3 4
clohexylcarbodiimide (DCHC) (11.0 mg, 0.056 mmol) in
The free base porphyrin H TCPP shows the Soret band
2
1
0 mL of N,N-dimethylformamide (DMF) was added drop-
at 417 nm and four Q bands at 514, 547, 591 and 644 nm
in UV–vis spectrum. The Soret band is seen at 466 nm and
only two peaks appeared at 565 and 599 nm for Q bands in
the UV–vis spectrum of [Mn(TCPP-Ind)Cl] (Fig. 1). These
findings proved the Mn(III) ion insertion into the free base
porphyrin [33]. The absorption spectrum of the immobilized
[Mn(TCPP-Ind)Cl] on magnetite nanoparticles is similar to
that of [Mn(TCPP-Ind)Cl] by a small red shift to 469 nm
due to the interaction between the carboxylate groups of the
porphyrin and Fe O -NPs (Fig. 1).
wise to the solution of Mn(TCPP)Cl (0.05 g, 0.056 mmol)
in DMF (5 mL). The mixture was refluxed at 140 °C for
8
h. Following the removal of the solvent under vacuum,
the precipitate dissolved in 20 mL of 0.1 M NaOH solution
and precipitated by adding 1 M HCl solution. The product
was recrystallized from ethanol. The pure [Mn(TCPP-Ind)
Cl] was dried at room temperature. Yield 72% (0.041 g).
3
−1
−1
UV–Vis in ethanol: λ
(log ε, dm mol cm ): 466
max
−
1
(
(
(
4.69), 565 (3.82), 599 (3.67). FT-IR (KBr, cm ): 3424
3
4
s, br, O–H), 2926 (S), 2853 (m), 1712 (vs, –CO ), 1629
Figure 2 shows FT-IR spectra at various stages of the syn-
2
−
1
s, N–C=O), 1608 (m, C=C), 1541 (m, N–H). Anal. Calcd
thesis. Appearance of a new band at 1542 cm with respect
to [Mn(TCPP)Cl], due to the bending vibration of the amide
N–H group [34], and the absorption bands which are seen
for C H ClMnN O ([Mn(TCPP-Ind)Cl]·4DMF·H O): C,
69
67
9
13
2
6
2.75; H, 5.11; N, 9.54. Found C, 62.60; H, 6.59; N, 9.53.
−
1
at 1630–1600 cm by increasing the intensity due to the
C=O group of the amide (N–C=O) demonstrate the forma-
tion of [Mn(TCPP-Ind)Cl]. In addition, intensity increasing
2
.3 Immobilization of [Mn(TCPP‑Ind)Cl]
on Fe O ‑NPs
3
4
−
1
of the peaks at 2853 and 2922 cm in [Mn(TCPP-Ind)Cl]
[
Mn(TCPP-Ind)Cl] was immobilized on magnetite nanopar-
rather than Mn(TCPP)Cl ascribed to vibrations of the -CH
2
ticles (Fe O -NPs) surface to get Fe O -[Mn(TCPP-Ind)Cl]
groups in (1R,2S)-(+)-cis-1-amino-2-indanol which can be
3
4
3
4
[
31]. Typically, Fe O -NPs (0.25 g) were added to a 250 mL
observed.
3 4
−
3
ethanolic solution of [Mn(TCPP-Ind)Cl] (0.2 mmol dm ).
The mixture was stirred overnight at 60 °C. It was then fil-
tered, washed with ethanol and dried at 60 °C for 6 h.
Fe O -[Mn(TCPP-Ind)Cl] particles show strong absorp-
3
4
−
1
tion at 500–600 cm corresponding to characteristic vibra-
tion of Fe–O in the magnetite lattice [35]. Furthermore,
−
1
−1
the bands at 3230 cm (O–H) and 1731 cm (C=O) can
be assigned to the vibrational absorptions of the COOH
groups of [Mn(TCPP-Ind)Cl] [36]. The bands at 3425 and
2.4 Conditions for Catalytic Aerobic Oxidation
−
1
In a typical experiment a flask was charged with the sus-
pension of Fe O -[Mn(TCPP-Ind)Cl] (1.0 mg, contains
1636 cm are assignable to the ν(O–H) and δ(O–H) mode
[37] of the adsorbed water, respectively. The amide N–H
3
4
1
3

A. Farokhi et al.
CO2H
NH2
N
N
Cl
N
N
O
DCHC, reflux
40 °C, 8 h
HO2C
Mn
C
[Mn(TCPP)Cl]
+
OH
NH
1
HO
[Mn(TCPP-Ind)Cl]
CO2H
6
2
0 o
4 h
C
OH
C
H O (S)
O
HN (R)
OH
C
O
Cl
C
O
N
N
N
Mn
H
O (S)
Fe O
O
3
4
N
O
N
Cl
HN
N
(R)
Mn
O
C
O
or
N
O
N
O
Fe O
3
4
O
Fe3O4-[Mn(TCPP-Ind)Cl]
HO2C
Scheme 1 Synthetic route of heterogeneous catalyst Fe O -[Mn(TCPP-Ind)Cl]
3
4
Fig. 1 UV–vis spectra of
Mn(TCPP-Ind)Cl] (left) and
Fe O -[Mn(TCPP-Ind)Cl] in
[
3
4
ethanol. Fe O -[Mn(TCPP-Ind)
3
4
Cl] was dispersed in ethanol by
using an ultrasound
−
1
−1
bending of the compound appears about 1630 cm which
is overlapping with the water O–H bending mode. The
characteristic functional groups of [Mn(TCPP-Ind)Cl] in
and 1731 cm suggesting the successful attachment of
[Mn(TCPP-Ind)Cl] onto the magnetite nanoparticles.
The position and relative intensity of all diffraction peaks
in the XRD pattern of Fe O -[Mn(TCPP-Ind)Cl] (Fig. 3)
−
1
the range 1500–750 cm are seen as weak peaks in the
FT-IR spectrum of Fe O -[Mn(TCPP-Ind)Cl] because of
3
4
match well with those of the commercial magnetite powder
(Ref. Code 96-900-5843). The average particle diameter was
estimated using Scherrer’s formula [38] about 9.0 nm, which
3
4
low concentration (Fig. 2c). The most characteristic car-
boxylic acid of the porphyrin and indanol bands at 3230
1
3

Manganes-Porphyrin as Efficient Enantioselective Catalyst for Aerobic Epoxidation of Olefins
3.2 Catalytic Asymmetric Oxidation
To test the performance of Fe O -[Mn(TCPP-Ind)Cl] as a
3
4
heterogeneous catalyst in the epoxidation of olefins, initial
reactions were carried out using α-methyl styrene. In the
presence of 0.7 mol% catalyst Fe O -[Mn(TCPP-Ind)Cl]
3
4
and 1 bar O at 25 °C for 8 h, the reaction could offer 100%
2
conversion of α-methyl styrene with 78% epoxide selectiv-
ity and enantiomeric excess (ee) 100% (Table 1, entry 1).
Kinetic studies proved that the reaction proceeds monotoni-
cally and up to 4 h the only product is the epoxide (Fig. 5).
No reaction was occurred in the absence of co-reductant,
IBA (Table 1, entry 2). IBA is one of the most economi-
cally viable and efficient co-reductant reagents to promote
the generation of acylperoxy radicals, recognized as epoxi-
dation intermediates [39]. In the presence of homogeneous
[Mn(TCPP-Ind)Cl], the reaction achieved quantitatively
and with high epoxide selectivity (88%). However, its enan-
tiomeric excess (95% ee) was lower than the immobilized
Fig. 2 The FT-IR spectra of (a) [Mn(TCPP)Cl], (b) [Mn(TCPP-Ind)
Cl] and (c) Fe O -[Mn(TCPP-Ind)Cl]
3
4
[
(
Mn(TCPP-Ind)Cl] (Table 1, entry 3). The catalyst support
Fe O -NPs stabilized with l-(+)-tartaric acid) showed
3
4
lower activity (conversion 32%) and 32% ee (Table 1, entry
4
) [40]. This finding suggests that there is a synergic effect
between the magnetite and the Mn-porphyrin, but the main
catalytic activation of the olefin is achieved by [Mn(TCPP-
Ind)Cl. Methanol, ethylacetate and n-hexane as solvent was
examined (Table 1, entries 5–7). Acetonitrile provides a suit-
able medium without involvement as the co-reactant [41].
3.3 Substrate Scope
Results of the epoxidation of olefins with this catalyst and O
2
in acetonitrile are presented in Table 2. Remarkable activity
and excellent enantioselectivity were found in the oxidation
of various olefins including terminal, cyclic and aromatic
olefins. Isobutanoic acid was also formed as co-oxidation
product in all reactions. What is most striking being the
result obtained for 1-octene and 1-decene. For this sim-
ple nonactivated olefins excellent enantioenriched epoxide
could be attained. Highest enantioselectivity (>99%) and
epoxide selectivity (100%) were achieved in the oxidation
of 1-octene with 85% conversion (Table 2, entry 1). The
conversion and enantiomeric excess for 1-decene were 57
and 96%, respectively (Table 2, entry 2).
Fig. 3 XRD pattern of Fe O -[Mn(TCPP-Ind)Cl]
3
4
is different from the 60-nm average size determined by sta-
tistical analysis of the SEM images. This indicates that each
individual particle is not a single crystal.
Figure 4 shows the shape, size and size distribution
of the prepared hybrid by SEM. The nanoparticles of
Fe O -[Mn(TCPP-Ind)Cl] are spherical with slight aggre-
The oxidation of styrene, α- and β-methyl styrene were
achieved completely within 8 h and the corresponding epox-
ides was only one of the enantiomers (Table 2, entries 3–5).
In addition to the epoxides which were the main product,
they produced benzaldehyde as minor product by break-
ing the double bond. The stability of the plausible inter-
mediate benzyl radical favors breaking of the olefin dou-
ble bond in styrene derivatives. In comparison to styrene,
α- and β-methyl substituted styrene increased the epoxide
3
4
gation. The average diameter of the particles was measured
to be 58.6± 1.9 nm based on the counting of over 50 NPs.
The synthesized nanocomposite Fe O -[Mn(TCPP-Ind)
3
4
Cl] showed a high saturation magnetization value of 49.28
−
1
emu g . This magnetic responsivity is strong enough
for magnetic decantation in catalysis (Fig. S1 in the
Supplementary).
1
3

A. Farokhi et al.
Fig. 4 SEM image and the particles size distribution of Fe O -[Mn(TCPP-Ind)Cl]
3
4
Table 1 Conversion and selectivity for catalytic oxidation of α-methyl styrene with molecular oxygen
O
+
O
CHO
Cat, O2
iPrCHO, CH3CN
+
S-E
R-E
B
a
Product(s)/yield %a
Ee % (conf.)b
Entry
Catalyst
Conv. (%)
E–S
E–R
B
–
1
2
3
4
5
6
7
Fe O -[Mn(TCPP-Ind)Cl]
100
0
100
32
0
0
0
78
22
100 (S)
3
4
Fe O -[Mn(TCPP-Ind)Cl] No IBA
3
4
[Mn(TCPP-Ind)Cl]
Fe O -NPs
86
4
2
8
12
20
95 (S)
33 (R)
3
4
Fe O -[Mn(TCPP-Ind)Cl] in CH OH
3
4
3
Fe O -[Mn(TCPP-Ind)Cl] in ethylacetate
3
4
Fe O -[Mn(TCPP-Ind)Cl] in n-hexane
3
4
Reaction conditions: catalyst 1.0 mg (Fe O -[Mn(TCPP-Ind)Cl] contains 0.014 μmol Mn-porphyrin), α-methyl styrene 2 mmol, IBA 5 mmol,
3
4
solvent 5 mL, O filled balloon, reaction temperature 25 °C, time 8 h
2
a
Conversions and yields are based on the starting substrate and determined by GC
Determined by GC on a chiral SGE-CYDEX-B capillary column. Reported results are the average of two experiments
b
selectivity. These findings suggest the importance of steric
hindrance effect during the oxygen transfer step and elec-
trophilic nature of the active oxygen transfer intermediate.
Resulting epoxides from the epoxidation of cis- and
trans-stilbene by Fe O -[Mn(TCPP-Ind)Cl]/O /IBA/CH CN
reactive than trans olefins toward porphyrin-catalyzed epoxi-
dation by iodosylbenzene [42]. The degree of this selectivity
has been shown to be sensitive to relatively small changes in
the steric environment of the porphyrin.
The steric effect around olefin double bond was also
observed in the oxidation of 1-phenyl-1-cyclohexene, an
olefin with three substituents. Although enantiomer selec-
tivity is reasonable (65% ee), the lowest conversion (34%)
was obtained for 1-phenyl-1-cyclohexene (entry 8).
In addition to the green and sustainable nature of
Fe O -[Mn(TCPP-Ind)Cl]/O /RCHO system, comparison
3
4
2
3
system were stable enough that no diol formation or break-
ing of the double bond were observed; epoxide selectivities
were 100% (entries 6 and 7). While enantiomeric excess of
7
6% was obtained for the oxidation of trans-stilbene, the
enantiomeric excess for the doxidation of cis-stilbene was
5%. A quarter of cis-stilbene is oxidized with retention of
configuration. Similar to this system, cis olefins are more
6
3
4
2
of the results with previously reported studies demonstrates
1
3

Manganes-Porphyrin as Efficient Enantioselective Catalyst for Aerobic Epoxidation of Olefins
reaction conditions. At the end of the reaction, catalyst
Fe O -[Mn(TCPP-Ind)Cl] was isolated with magnetic
3
4
decantation, washed with CH CN and dried at 60 °C for
3
6
h. Recovered catalyst was committed to another oxida-
tion of α-methylstyrene which resulted to an enantiomeric
excess of >99. The conversion and enantioselectivity were
maintained at a similar level after five cycles (Fig. 6). The
UV–Vis absorption (Fig. 7) and FE-SEM (Fig. 8) image of
the catalyst after recycling indicate that the catalyst is quite
robust and no significant aggregation was found.
Stability the catalyst against leaching Mn ion or Mn-
porphyrin through Fe O -[Mn(TCPP-Ind)Cl] during the
3
4
oxidation reaction was confirmed by the following studies:
(
i) the UV–Vis spectrum of the recycled catalyst (Fig. 6)
showed presence of the Mn-porphyrin, (ii) the supernatant
solution of the oxidation reaction suggested no absorption
peak for the Mn-porphyrin at 466 nm, (iii) No detectable
traces of the porphyrin or Mn-porphyrin were found in the
reaction filtrate by employing atomic absorption and UV–vis
spectroscopy. These findings confirmed the heterogeneous
nature of catalyst Fe O -[Mn(TCPP-Ind)Cl].
Fig. 5 Effect of reaction time on the autoxidation of α-methylstyrene.
Reaction conditions: catalyst 1.0 mg (Fe O -[Mn(TCPP-Ind)Cl]
3
4
contains 0.014 μmol Mn-porphyrin), α-methyl styrene 2 mmol, IBA
5
mmol, solvent 5 mL, O filled balloon, reaction temperature 25°C
2
3
4
the outstanding enantioselectivity of this catalyst. It has
been reported that the oxidation of 1-octene with H O /
3.5 Mechanism of the Oxidation
2
2
dimethylbutyric acid mediated by manganese complex of
chiral N,N′-bis(2-pyridylmethyl) N,N′-dimethyl-trans-1,2-
diaminocyclohexane results in the corresponding epoxide
with 91% yield and 31% ee [43]. This catalytic system is also
able to oxidize styrene (epoxide selectivity 84%, ee 78%),
trans-β-MethylStyrene (71%, ee 2%), cis-stilbene (67%, ee
In spite of Jacobsen-type complexes of cobalt(II) in the pres-
ence of some carboxylic acids [49], Fe O -[Mn(TCPP-Ind)
3
4
Cl]/O /IBA system is selective for epoxide and the products
2
are optically active 65–100% consistent with epoxidation at
the manganese complex rather than radical epoxidation in
solution. Significant catalytic asymmetric epoxidations are
achieved for a series of olefins that can only have nonbonded
interactions with the catalyst. Changes in the steric environ-
ment of the olefin had a pronounced effect on asymmetric
induction. Substitution of the double bond as in stilbenes
and 1-phenyl-1-cyclohexene, resulted in significantly lower
enantiomeric excesses. These changes are not as expected
since increasing the size of substituent groups should have
increased nonbonded interactions between the catalyst and
the substrate.
2
1
8%), and trans-stilbene (85%, ee 52%). The conversion of
00% and 32% ee were reported in the oxidation of styrene
with m-CPBA-NMO and poly(Mn-salen-norbornene) [44].
Gómez et al. achieved the epoxide yield of 80% and 40%
II
ee in the epoxidation of styrene by Λ-[Mn (CF SO ) (α-
3
3 2
(
[
N ,N ,S,S)-MCPP)]/AcOOH [45]. Mn(III)-meso-tetrakis-
R
R
2.2]-p-cyclophanyl porphyrin/NaOCl system could oxidize
styrene to yield epoxide 65% enantiomeric excess of 25%
III
[
46]. By using catalytic system Mn TBNPCl/PhIO for the
epoxidation of styrene 37% yield and 8% ee were achieved,
The system operates in monooxygenase fashion [23]:
where TBNPH is 5α,10β,15α,20β-tetrakis[(R)-l,l’-binaphth-
one oxygen atom of O is incorporated into the epoxide, the
2
2
-yl]-porphyrin [47]. Threitol-strapped manganese porphy-
other atom transferring to the externally added co-reductant
(IBA). Molecular oxygen is activated through the oxidation
of the aldehyde to the corresponding peroxycarboxylic acid
[50], which further interacted with the manganese-porphyrin
to form the active species. The reaction proceeds by the oxy-
genation of the olefin via the interaction of the active spe-
rin/PhIO oxidized styrene with 39% ee (yield 64%) and
trans-β-methylstyrene with 17% ee [48]. We have studied
Fe O /tart/manganese–tetra(4-carboxyphenyl)porphyrin/O -
3
4
2
RCHO which converted styrene to epoxide with 67% ee [38].
III
3.4 The Catalyst Reuse
cies, the manganese(III) acylperoxo (Mn -OOCOR) [51] or
V
the manganese(V) oxo (Mn =O) [52, 53] complex, with the
The synthesized nanocomposite is a stable heterogeneous
catalyst that can be separated and reused several times.
The catalyst was tested for its reusability in the epoxida-
olefin to give the oxygenated product (Scheme 2).
The enantiometric excesses observed with
Fe O -[Mn(TCPP-Ind)Cl] appear to be induced by
3
4
tion of α-methyl styrene with O /IBA under the optimized
the conformational mobility of the chiral appendage
2
1
3

A. Farokhi et al.
Table 2 Asymmetric oxidation of olefins with O mediated by Fe O -[Mn(TCPP-Ind)Cl].
2
3
4
Conv. (%) a
Product(s)/(Yield %) (configuration)
a
Epoxide
Ee (%) (config.)b
Entry
Substrate
select. (%)
1
1-Octene
85
57
100
100 (S)
2
3
1-Decene
Styrene
100
96 (S)
100
63
100 (S)
4
5
β-methyl styrene
α-methyl styrene
100
100
75
78
100 (S,S)
100 (S)
6c
cis-stilbene
100
100
65 (R,R)
7d
8
trans-stilbene
100
34
100
100
76 (R,R)
65 (S,S)
1-Phenyl-1-cyclohexene
For the reaction conditions see Table 1. Each experiment was repeated at least two times
a
Conversions and yields are based on the starting substrate and determined by GC
Determined by GC on a chiral SGE-CYDEX-B capillary column
cis-Stilbene 1 mmol
trans-Stilbene 0.5 mmol
b
c
d
(
1R,2S)-(+)-cis-1-amino-2-indanol. Among chiral auxil-
within the hydrogen bonded aryl units, and does not par-
ticipate in the π-conjugation resonance of the porphyrin
ring, enforced to lie perpendicular to the MnN4 core and
parallel to the axial ligand chloride (Fig. 9). The suggested
strongly distorted Mn-porphyrin structure is very similar
to the Collman’s picket fence metalloporphyrins [49].
Increase in steric hindrance around the olefin dou-
ble bond (compare styrene with stilbene and 1-phenyl-
cyclohexene), slightly decrease differences in the enantio-
meric transition state energies for si and re approach and
results to lower enantiomeric excess (65–76% ee, Table 2).
iaries, indanol is a more attractive building unit because of
its rigid, planar and π-conjugated structure [54]. In com-
parison to reported chiral catalysts, the striking enantiose-
lectivity of Fe O -[Mn(TCPP-Ind)Cl] for terminal olefins
3
4
and styrene derivatives is mainly boosted by the structural
conformation of the immobilized [Mn(TCPP-Ind)Cl]. The
Mn-porphyrin is highly distorted due to the strong intra-
molecular hydrogen bond between the –OH group of the
chiral cis-aminoindanol and –CO H group of the nearby
2
aryl unit which strongly distorts the nearly planar Mn-
porphyrin core. As a result, the pyrrol unit which is placed
1
3

Manganes-Porphyrin as Efficient Enantioselective Catalyst for Aerobic Epoxidation of Olefins
Fig. 8 FE-SEM image of the recovered Fe O -[Mn(TCPP-Ind)Cl];
3
4
average particles size 56± 1.7 nm after five times recycle
MnIII] + RCO3H
V
III
(1)
[
[Mn =O] (or [Mn -OOCOR]) + RCO H
2
Fig. 6 Catalyst Fe O -[Mn(TCPP-Ind)Cl] recyclability study for
V
3
4
[MnIII] + product(O)
[
Mn =O] + Substrate
model reaction. Reaction conditions: catalyst 1.0 mg, α-methyl sty-
rene (2 mmol), CH CN 5 mL, IBA 5 mmol, 25 °C and time 8 h
3
Scheme 2 The suggested mechanism for the oxidation of olefins by
Fe O -[Mn(TCPP-Ind)Cl] and O /IBA
3
4
2
4
Conclusions
In conclusion, we were able to develop a sustainable cata-
lyst for green and economic synthesis of epoxides of linear,
cyclic and aromatic olefins. The catalyst is highly active
for epoxidation of unfunctionalized olefins with molecular
oxygen as the oxidant in the presence of isobutyraldehyde.
The striking enantioselectivity of catalyst manganese(III)-
porphyrin with chiral aminoindanol substituent immobilized
on magnetite nanoparticles, Fe O -[Mn(TCPP-Ind)Cl], is
3
4
related to its favored conformation which is fixed by an intra-
molecular hydrogen bond. It is noteworthy that this catalyst
showed high stability during the recycling experiment for
five runs with conversions 100% as well as extraordinary
enantioselectivity without any significantly loss in its reac-
tivity. The promise of this approach lies in its high degree of
simplification, robustness and enantioselectivity.
Fig. 7 UV–Vis spectra of fresh and used Fe O -[Mn(TCPP-Ind)Cl]
3
4
1
3

A. Farokhi et al.
Fig. 9 Intramolecular hydro-
gen bond (confirmed by MM2
calculation) between the –OH
group of the indanol and the
H
O
O
H
H
H
H
H
O
H
H
–
CO H group of the next aryl
H
H
C
O
2
provides a suitable conforma-
tion which makes the prefer-
ence to one of the faces of
the interacting olefin with the
Mn-porphyrin
N
Cl
H
H
H
H
Mn
H
N
N
H
N
H
H
H
H
H
H
H
H
H
H
O
H
H
H
O
N
O
H
H
O
H
H
H
H
H
H
Acknowledgements We thank Professor M. Vahedpour for helpful
advice and discussions. The authors are grateful for financial support
from the University of Zanjan.
19. Miao C, Wang B, Wang Y, Xia C, Lee YM, Nam W, Sun W (2016)
J Am Chem Soc 138:936
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Compliance with Ethical Standards
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4. Ucoski GM, Nunes FS, DeFreitas-Silva G, Idemori YM, Nakagaki
S (2013) Appl Catal A 459:121
Conflict of interest The authors declare that they have no conflict of
interest.
2
2
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