Enantioselective aerobic oxidation of olefins by magnetite nanoparticles at room temperature: A chiral carboxylic acid strategy

Article abstract of DOI:10.1039/c5gc01774b

Asymmetric oxidations of organic compounds are limited in their synthetic scope and by practical factors, such as the use of complex catalyst synthesis. A simple and cheap nanostructured catalyst system comprising magnetite nanoparticles stabilized by l-(+)-tartaric acid (Fe₃O₄/tart-NPs) was successfully synthesized in diethylene glycol. The catalyst was characterized by FT-IR, TGA, ICP-AES, XRPD, SEM and dynamic light scattering (DLS). The catalytic activity of Fe₃O₄/tart-NPs dispersion in acetonitrile in the presence of isobutyraldehyde was studied in selective aerobic oxidation of olefins to form asymmetric epoxide, an important intermediate for the synthesis of biologically active compounds. In addition, the magnetically recoverable nanocatalyst Fe₃O₄/tart-NPs can be conveniently separated and recovered from the reaction system by applying an external magnetic field and reused for five cycles without the loss of activity after each cycle. These results demonstrate that the heterogeneous nanocatalysts possess potential applications for green and sustainable development. As synthesized nanoparticles of Fe₃O₄/tart-NPs are cheap and easy to synthesize asymmetric catalysts which were prepared without the involvement of difficult and cumbersome procedures for the synthesis of complicated asymmetric ligands. Possible reaction mechanisms were outlined.

Full text of DOI:10.1039/c5gc01774b

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DOI: 10.1039/C5GC01774B
Journal Name
ARTICLE
Enantioselective aerobic oxidation of olefins by magnetite
nanoparticles at room temperature: a chiral carboxylic acid
strategy
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Leila Hadian-Dehkordia and Hassan Hosseini-Monfared*,a
Asymmetric oxidations of organic compounds are limited in their synthetic scope and by practical factors, such as the use
of complex catalyst synthesis. A simple and cheap nanostructured catalyst system comprising magnetite nanoparticles
stabilized by L-(+)-tartaric acid (Fe3O4/tart-NPs) were successfully synthesized in diethylene glycol. The catalyst was
characterized by FT-IR, TGA, ICP-AES, XRPD, SEM and dynamic light scattering (DLS). Catalytic activity of Fe3O4/tart-NPs
dispersion in acetonitrile in the presence of isobutyralhyde was studied in selective aerobic oxidation of olefins to form
asymmetric epoxide, an important intermediate for the synthesis of biologically active compounds. In addition, the
magnetically recoverable nanocatalyst Fe3O4/tart-NPs can be conveniently separated and recovered from the reaction
system by applying an external magnetic field and reused for five cycles without the loss of activity after each cycle. These
results demonstrate that the heterogeneous nanocatalysts possess potential applications for green and sustainable
development. As synthesized nanoparticles of Fe3O4/tart-NPs are a chip and easy to synthesize asymmetric catalyst which
were prepared without involvement of difficult and cumbersome procedure for the synthesis of complicated asymmetric
ligands. Possible reaction mechanisms were outlined.
catalysts to asymmetric transformations.6 There are
difficulties, however, in adapting this type of process for
commercial purposes. The soluble catalysts suffer from
Introduction
Chiral compounds are commonly required in the pharma,
agrochemical, and fine chemical sectors.1 In the synthesis of
chiral compounds, chiral epoxides are one of the most
valuable and versatile building blocks.2 Epoxides containing
one or two stereogenic centers are reactive precursor that can
be readily involved in further asymmetric transformations
through, for example, asymmetric ring-opening reactions.3
Ever since the pioneering work of the Sharpless epoxidation of
allylic alcohols and the Katsuki-Jacobsen epoxidation of
unfunctionalized olefins, there has been great progress in
asymmetric epoxidation over the past decades.4,5
Given the far lower cost, less toxic and greater abundance
of iron over the more precious metals, it is clear that iron
derived catalysts would provide a range of benefits if they
could be made practical, stable, active and selective. In recent
years, significant breakthroughs have been made in the
development and applications of homogeneous iron-based
problems associated with the separation, recovery, and
instability at high temperatures. On the other hand, catalytic
methods which are used routinely in bulk chemical
manufacture, employing heterogeneous catalysts are not
generally applicable for the production of chiral products.7 The
unique combination of magnetic nanoparticles and
catalytically active species presents the opportunity to solve a
range of catalyst recovery problems.8 Organocatalysts
supported by magnetic nanoparticles act under favourable
quasi-homogeneous conditions and can be recycled by simple
magnetic decantation rather than time-consuming and energy-
intensive filtration or centrifugation.9 Magnetically recoverable
nanocatalysts have been used in organic synthesis for a wide
range of catalytic reactions.10,11 Ikenberry and co-workers
achieved monolayer sulfonic acid-functionalized iron oxide
nanoparticles as solid acid catalyst for carbohydrate
hydrolysis.12 Magnetite nanoparticle-supported asymmetric
catalysts based on 4,4′-disubstituted BINAP-Ru-DPEN
complexes were used for enantioselective hydrogenation of
aromatic ketones (BINAP = 2,2'-bis(diphenylphosphino)-1,1'-
binaphthyl; DPEN = diphenylethylenediamine).13 Gawande et
al. summarized reported studies on nano-magnetite (Fe3O4) as
a.Department of Chemistry, University of Zanjan 45195-313, Zanjan, Islamic
Republic of Iran.
Electronic Supplementary Information (ESI) available: FT-IR spectra of the fresh and
used catalyst, TGA of the catalyst, Magnetization, DLS, BETand NMR See
DOI: 10.1039/x0xx00000x
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a support for the immobilization of homogeneous metals, resulting in a remarkable drop of the catalytic capVaibewiliAtryticdleuOenlitnoe
organocatalysts, ligands, N-heterocyclic carbenes and their the decrease of MNPs surface areas.31
DOI: 10.1039/C5GC01774B
applications.14 Zhao et al. studied organic superbase-
functionalized magnetic Fe3O4 as catalyst for the fixation of
Experimental
Materials and instrumentation
CO2 with 2-aminobenzonitriles, resulting in the synthesis of
quinazoline-2,4(1H,3H)-diones.15 Katsuki and Egami reported
iron-catalyzed asymmetric aerobic coupling of 2-naphthols.16
Subramanian et al. achieved magnetite catalysts incorporated
with M (Cu, Ni, Zn, and Co) promoters for high temperature
water gas shift reaction.17 Cano and co-workers performed
cross-alkylation of primary alcohols with an impregnated
iridium on magnetite catalyst.18 Godoi et al. applied Fe3O4
nanoparticles for the synthesis of alkynyl chalcogenides from
terminal acetylenes and diorganyl dichalcogenides.19
L-(+)-Tartaric acid ((2R,3R)-(+)-tartaric acid, >99%, Merck),
iron(III) chloride hexahydrate (FeCl3·6H2O, 98% Fluka),
diethylene glycol (99%, Merck), cyclohexene (99%, Fluka), 1-
methyl-1-cyclohexene (97%, Sigma-Aldrich), alpha-methyl
styrene (>99%, Merck), cis-stilbene (>95%, Merck), trans-
stilbene (96%, Sigma), 1,2,3,4-tetrahydronaphthalin (>98%,
Merck), thioanisole (>99%, Merck) and other reagents were
obtained from commercial sources and were used as received
without further purification.
Asymmetric oxidation using ubiquitous molecular oxygen
as the oxidant and the "free" nanoparticles of abundant iron as
the catalyst has attracted a growing interest.20 Previously, we
have reported the synthesis and catalytic activity of magnetite
nanoparticles (Fe3O4-NPs) in diethylene glycol in the presence
of carboxylic acids.21 Carboxylic acid plays a critical role in
determining the morphology, particle size and/or size
distribution and catalytic activity of the resulting particles. The
resulting nanoparticles can be easily dispersed in aqueous
media and other polar solvents due to coated by a layer of
hydrophilic polyol and carboxylic acid ligands in situ. Easily
prepared Fe3O4/carboxylic acid-NPs showed recyclable and
highly selective catalytic activity for the epoxidation of cyclic
olefins with aqueous 30% H2O2.21 On the other hand, there are
some reports on the effects of carboxylic acid and chiral ester
on asymmetric oxidation. For example, chiral bipyrrolidine
based iron and manganese complexes catalyze the asymmetric
epoxidation of various olefins with H2O2 in the presence of
carboxylic acid additives with high efficiency and selectivity,
and with good to high enantioselectivity.22 The most widely
used protocol for the asymmetric epoxidations of acyclic E-
enones bearing two alkyl substituents uses TBHP in the
presence of a sub-stoichiometric quantity of a magnesium
tartrate catalyst.23 Furthermore, one of the most successful
The reaction products of the oxidation were determined
and analyzed by HP Agilent 6890 gas chromatograph equipped
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
mm ID 0.25
m). 1H NMR spectra of reaction mixture


m





(without purification) were recorded on a Bruker 250 MHz
spectrometer. UV–Vis spectra of solution were recorded on a
Shimadzu 160 spectrometer. Fourier transform infrared (FT-IR)
Perkin-Elmer 597
spectrophotometer after making pellets with KBr powder.
Powder X-ray diffraction patterns were collected at the Bruker,
spectra were recorded using
a
D8ADVANCE, Germany, wavelength 1.5406 Å (Cu K), voltage:
40 kV, current, 40 mA. The size and morphology of solid
compounds were recorded by using a Hitachi F4160 scanning
electron microscope (SEM) operated at an accelerating voltage
of 10 KV. The hydrodynamic diameter of the nanoparticles was
measured using the Zetasizer Nano-ZS3600 (Malvern
Instruments, Malvern, UK) dynamic light scattering (DLS) with
the sonicated nanoparticles in water before measurement.
Magnetization measurement was performed at room
temperature using a vibrating sample magnetometer (VSM)
device, in the Development Center of the University of Kashan
(Kashan, Iran). Textural properties determined from N2
adsorption isotherms measured on a Belsorp mini II (Japan)
instrument.
ways of inducing enantioselectivity in
a heterogeneous
catalytic system is by the adsorption of chiral “modifier”
molecules on the reactive metal surface.24
Considering the above observations and our previous work
motivated us to study the catalytic performances of L-(+)-
tartaric acid stabilized nano-magnetite (Fe3O4/tart-NPs) as a
recyclable and green asymmetric oxidation catalyst under
aerobic and mild reaction conditions. Among various oxidants,
molecular oxygen is a cheap, clean and readily available
oxidant.25,26 Isobutyraldehyde was used as reductant for the
reduction of diogygen at the beginning of the reaction and it is
co-oxidized with substrate. This type of co-oxidation was first
studied by Mukaiyama and co-workers.27 This system
(Fe3O4/tart-NPs) represents the first example of asymmetric
induction from magnetite stabilized by asymmetric tartaric
acid, as well as a rare example of catalytic enantioselective
olefin epoxidation by magnetite nanaoparticles (MNPs) and
dioxygen.28,29,30 Unprotected MNPs are often unstable and
tend to aggregate during the catalytic transformations,
Synthesis of Fe3O4/tart-NPs
Magnetite nanoparticles stabilized with chiral tartaric acid
(Fe3O4/tart-NPs) was synthesized according to our previously
reported procedure.21 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 (Scheme 1).
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Fe3O4-NPs were synthesized similarly without using L- maghemite (
COMMUNICATION

-Fe2O3), and the Raman and IR spectVriaewhAartvicelebOenleinne
tartaric acid.
proved to be an alternative tools.33,34
DOI: 10.1039/C5GC01774B
Catalytic aerobic oxidation of cyclohexene with Fe3O4/tart-NPs
The oxidation reactions were carried out in a glass inlay of a 32
mL steel autoclave. The autoclave was conditioned by
evacuation and re-filling with dioxygen. All autoclave loading
was carried out under air. In a typical experiment, 2.0 mmol of
substrate was added to the reactor with 0.001 g Fe3O4/tart-
NPs, 5.0 mmol isobutyraldehyde and 0.1 g chlorobenzene as
an internal standard. After purging with O2, the reactor was
pressurized to 2 bar. Stirring rate was 375 rpm. At the end of 7
h the reactor was depressurized, the catalyst removed by an
external magnet (1.2 T) and the product mixture was analyzed
1
by H-NMR and gas chromatography. Conversions and yields
were calculated with respect to the starting substrate. The
1
products were identified with authentic samples and H- and
13C-NMR spectroscopic data. The reaction products were
quantified by gas chromatography and identified by
comparison with the retention time and spectral data to those
of an authentic sample. To ensure reproducibility each
catalytic reaction was carried out at least two or three times.
For recycling experiments, after completion of the reaction,
the nanocatalyst was recovered using a magnet, washed with
acetonitrile, dried and reused without further purification. GC
condition with column Hp-5: carrier gas N2 flow = 0.7 mL/min,
inlet temp 250 °C, initial column temp 90 °C, final column temp
190 °C, sleep 10 °C /min. GC condition with column Cydex-B:
carrier gas N2 flow = 0.7 mL/min, inlet temp 200 °C, initial
column temp 50 °C, final column temp 150 °C, sleep 10 °C
/min.
Fig. 1 XRD patterns of the synthesized Fe3O4-NPs and Fe3O4/tart-NPs.
The FT-IR spectra of the tart, Fe3O4-NPs and Fe3O4/tart-NPs
nanoparticles are shown in Fig. 2. The L-tartaric acid spectrum

1
shows a dominant peak at 1732 cm , resulting from the
carbonyl stretch C=O) associated with protonated carboxyl
groups,35 which was reduced in intensity with
adsorption/chemisorption of tartaric acid on Fe3O4 as the
carboxyl groups deprotonated in Fe3O4/tart nanoparticles. As
the intensity of the carbonyl peak waned at Fe3O4/tart, peaks
at 1550 cm and 1422 cm appeared. These two features
1
1


represent the asymmetric
carbon–oxygen stretches
respectively.36,37 These results revealed that L-tartaric acid was
chemisorbed onto the Fe3O4-NPs as a carboxylate.38

as(CO2 ) and symmetric
of carboxylate

s(CO2 )
groups,
Proof of stereochemistry
The presence of both the

(C=O) vibrations of the acid
Prochiral olefins, tetralin and thioanisole were oxidized by
Fe3O4/tart-NPs and isobutyraldehyde/O2. Analysis of the
corresponding chiral products by chiral GC was used to
determine the enantioselectivity of the reaction. The absolute
configuration was established by comparing the GC data with
those observed for R-(+)-limonene.

COOH functionality at 1732 cm and the

s(CO2 ) vibration of
1

1
the carboxylate group at 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. In
addition, the free and intact COOH acid group is held away
from the surface and the lack of downshift in frequency of the

(C=O) vibration of this group suggests that it is not involved in
intermolecular H-bonding interactions with the alcohol groups
of neighbouring monotartrate species, which is in contrast to
the adsorption of L-tartaric acid on a Cu(1 1 0) surface at 300
K.24,39 IR bands can be used to distinguish the type of the
interaction between the carboxylate group and the metal
atom. Carboxylate groups which are chelating or coordinating
with each O atom to a metal atom exhibit differences between
Results and discussion
The crystalline nature of the as-synthesized L-(+)-tartaric acid
protected iron oxide nanoparticles was determined by wide
angle X-ray powder diffraction (XRD). Fig. 1 shows the XRD
patterns of Fe3O4 and Fe3O4/tart nanocrystals. The Intensity of
the peaks decrease by functionalization of Fe3O4. In the case of
Fe3O4/tart-NPs, using intense peak at 2theta = 35.423, FWH =
0.590, we get crystallite size of 14.1 nm by X’Pert program. Six
characteristic peaks (2θ = 30.3°, 35.4°, 43.2°, 53.5°, 57.2° and
62.9°) corresponding to (220), (311), (400), (422), (511) and
(440) planes of cubic inverse spinel Fe3O4 are obtained.32 The
positions and relative intensities of all diffraction peaks for
both Fe3O4-NPs and Fe3O4/tart-NPs match well with those
the asymmetric and symmetric stretching frequencies, which
1
are less than the ionic value (Δ
ion).36 For Fe3O4/tart-NPs Δ

= 164 cm for the acetate

= 1550–1422 = 128 cm1 suggests
a chelate action of each tartaric acid carboxylate group toward
the iron oxide nanoparticles, as shown in Scheme 1.
Fig. 2a and Fig. 2c clearly show the characteristic lattice

vibration of magnetite at 576 cm and 420 cm . These
1
1 40
expected for Fe3O4 rather than for -Fe2O3. XRD is relatively
insensitive to the difference between magnetite (Fe3O4) and
results are consistent with the as-synthesized nanocrystals
being Fe3O4 rather than -Fe2O3. In the Fe–O range, synthetic
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
1 40
maghemite shows broad IR bands at 668, 630, 442 cm . The
broad characteristic band at 3441 cm-1 could be assigned to O-
H stretching vibration arising from the Fe–OH groups on
nanoparticles, the carboxylic groups of tartaric acid, and
adsorbed water.41 In addition, for Fe3O4-NPs and Fe3O4/tart-
NPs the dominant H-O-H bending vibration of water is seen at
View Article Online
DOI: 10.1039/C5GC01774B

1 42
1632 cm .
Scheme 1 Preparation of Fe3O4/tart nanoparticles
Fig. 2 FT-IR spectra of (a) Fe3O4-NPs, (b) L-(+)-tartaric acid (tart) and (c) Fe3O4/tart-NPs.
The thermal stability of Fe3O4/tart-NPs was examined by
means of TGA, and the result is shown in Fig. S1 (in Supporting
information). It is seen that the sample exhibits continuous
weight loss of ~17% from 25 to 141 °C due to the removing of
adsorbed H2O and the O-H functional groups from the surface
of magnetite. The weight loss of 6% in the temperature range
of 141-362 °C should be mainly due to the decomposition of
the supporting tartaric acid and the content of Fe3O4 is 77%.
The weight loss of ~6% of the sample is observed above 141
°C, indicating the existence of enough supporting ligand. The
supporting tartaric acid (C4H6O6) functional groups might play
an important role in stabilizing magnetite nanoparticles,
promoting the magnetite catalytic activity and inducing
chirality in the oxidation products (vide infra). The content of
Fe3O4 in Fe3O4/tart-NPs sample was confirmed with measuring
iron of the sample by ICP technique (51% iron).
The size, shape and size distribution of the nanoparticles of
Fe3O4/tart-NPs were examined by SEM (Fig. 3). SEM images
show that the magnetite particles obtained in the presence of
L-(+)-tartaric acid have nearly spherical shape and uniform size
distribution with an average size of 19.5
 4.2 nm. The
synthesized Fe3O4-NPs without using L-tartaric acid show the
particles with average size of 373 ± 88 nm (Fig. 3c). By using
the carboxylic acid the resulting nanoparticles became more
uniform and the average diameter decreased dramatically.
Carboxylic acid affects the morphology, particle size and size
distribution of the magnetite particles.21 The nanoparticles of
Fe3O4/tart-NPs are stable and preserve their shape and
uniform distribution after catalysis (vide infra). After four times
recycle in the catalytic oxidation of cyclohexene, the
nanoparticles are well separated and their size increased to
239.3
 66.3 nm (Fig. 3; b).
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DOI: 10.1039/C5GC01774B
Fig. 3 SEM images and particle size distribution of (a) Fe3O4/tart-NPs as-synthesized, (b) recovered Fe3O4/tart-NPs after use (see the next section) and (c) Fe3O4-Nps.
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ARTICLE
Dried powder of the present sample was easily redispersed in
The ground state of molecular oxygen is a triplet with two
neutral water solution and its hydrodynamic size was unpaired electrons having parallel spins. Therefore, the direct
determined by dynamic light scattering (DLS) to be 74 nm (Fig. reaction of molecular oxygen with a singlet organic molecules
S2) and demonstrate the monodispersity of the nanoparticles. is a spin-forbidden process.48 With regard to aerobic oxidation
Usually, the particles size in solution is approximately three catalyzed by transition-metal complexes, several reactions
times of their values in the solid state because of hydration. involving the combined use of molecular oxygen with reducing
According to DLVO theory,43 NPs are stabilized by
a
agents such as NaBH4, 2-propanol and aldehyde have been
combination of van der Waals interactions and double-layer studied.49,50 Co-oxidation of cyclohexene and isobutyraldehyde
forces. In the system described herein, van der Waals under the investigated conditions leads to the formation of
interactions presumably originate from the carboxylic group cyclohexene oxide (main product) and isobutyric acid. The
providing electrostatic stabilization.
oxidation of cyclohexene was not occurred by using
a
Analysis of the magnetic properties of the synthesized substochiometric
amount of isobutyraldehyde
Fe3O4/tart-NPs indicated a superparamagnetic nature of the (isobutyraldehyde/substrate ratio = 0.2, 0.5), Table 1, entry 8.
material (Fig. S3). The magnetization value of Fe3O4/tart-NPs is In order to be sure that the oxidation of the substrates
approximately the same with the value of Fe3O4-NPs and proceeds up to its maximum extent, a large stoichiometric
higher than the magnetization of core-shell Fe3O4/SiO2-NPs.44 excess of isobutyraldehyde with respect to the substrate (2.5
This finding shows the negligible effect of diamagnetic tartaric times of substrate) was used.
acid coating over the Fe3O4-NPs and a favorable property for
magnetic separation by a conventional magnet.
Catalytic oxidation
Having synthesized and characterized the Fe3O4/tart
nanocomposite, its role as a heterogeneous catalyst was then
evaluated for the oxidation of olefins. We first examined the
reaction using magnetite nanoparticles as a catalyst in air
oxidation of cyclohexene at 25 °C (Table 1). Cyclohexene is
more prone to both epoxidation and allylic oxidation45 and
oxygenation of cyclohexene is a good probe to provide
evidence for or against a non-radical mechanism and to
evaluate the catalyst selectivity. While isobutyraldehyde,
tart/isobutyraldehyde or Fe3O4-NPs without tartaric acid
showed poor oxidation catalysis (Table 1, entries 1-3) and
using the mixture of FeCl3·6H2O + L-tartaric acid gave no
epoxide (entry 4), only Fe3O4/tart-NPs bearing tartaric acid as
stabilizer exhibited oxidation catalysis (entry 7). Presence of
co-oxidant isobutyraldehyde is essential for activation of
dioxygen and acetonitrile as solvent provide better medium
than methanol (entries 5 and 6). In fact, the catalyst does not
work in methanol (entry 6), however its activity in acetonitrile
is remarkable. The possibility of involvement of acetonitrile as
co-reactant, through peroxyimidic acid as an oxygen donor,
was excluded in the oxidation reactions, since no MeCONH2
under the catalytic oxidations was detected.46 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).47 Probably, the polarity, low coordinating ability and
aprotic nature of acetonitrile play the main roles in improving
the activity of the catalyst in acetonitrile.
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Stilbene is a frequently used substrate for the stVuiedwyArotifcleoOlenfliinne
Table 1 optimization of carboxylic acid mediated olefin oxidation
epoxidation mechanism51 because of mechanistic information
DOI: 10.1039/C5GC01774B
associated with the ratio of cis- and trans-isomers in the
stilbene oxide product. trans-Stilbene oxide was formed as the
main product of the both oxidation, but epoxide selectivity for
cis-stilbene was much higher (entries 8 and 9). This finding
shows the important role of steric effect in determining the
product selectivity by this catalyst. The side product of
benzaldehyde was formed from the breaking of the olefin
double bond which is comparable in the case of the trans-
stilbene (44%). The oxidation reaction probably proceeds
through a benzyl radical intermediate, which is sufficiently
stable and allows free rotation about the C-C bond axis. The
formation of trans-stilbene oxide from cis- and trans-stilbenes
as the main product via a radical intermediate is expected due
to the higher thermodynamic stability of the trans in
comparison to the cis stereoisomer, in which the phenyl
groups are located in anti-position with respect to each other.
Interestingly, the catalyst was also found to be useful for the
enantioselective sulfoxidation of thioanisole and hydroxylation
of tetralin (entries 10 and 11). Thioanisol was oxidized to the
sulfoxide in 100% selectivity, 36% conversion and 78% ee. This
activity is close to the reported studies by H2O2. Bolm reported
on the use of an asymmetric Schiff base/Fe(acac)3/H2O2 which
catalyzed the formation of sulfoxides in up to 90% ee, albeit in
low moderate yields.52 This was improved in later work
through the use of a lithium carboxylate additive to the
sulfoxide in 63% yield and 90% ee.53 Immobilized
Entry
Catalyst
Reductant Solvent
Conv.
(%) b
Epoxide
selectivity
(%)b
0
0
1
2
3
none
tart
Fe3O4-NPs
(no tart)
FeCl3·6H2O
+ tart c
Fe3O4/tart-NPs
Fe3O4/tart-NPs
Fe3O4/tart-NPs
Fe3O4/tart-NPs
iPrCHO
iPrCHO
iPrCHO
CH3CN
CH3CN
CH3CN
0
0
3.2
99
4
iPrCHO
CH3CN
56
0 d
5
6
7
8
None
iPrCHO
iPrCHO
iPrCHO f
CH3CN
CH3OH
CH3CN
CH3CN
0
0
0
0
617 e
0
584e
0
a Reaction conditions: Fe3O4/tart-NPs 1.0 mg, isobutyraldehyde (iPrCHO) 5 mmol,
cyclohexene 2 mmol, solvent 3 ml, chlorobenzene (internal standard) 0.1 g, O2
bar, room temperature, reaction time 7 h. b As compared to chlorobenzene as an
internal standard. Conversion and selectivity were determined by GC and is an
average of at least two runs. c FeCl3·6H2O 0.12 mmol, tart 0.02 mmol; the ratio of
2
d
tart/Fe is equal to the ratio used for Fe3O4/tart-NPs synthesis. the other
products were cyclohex-2-en-1-ol 49 % and cyclohex-2-en-1-one 7 %. e Average of
6 runs; the other products were cyclohex-2-en-1-ol 38  8 % and cyclohex-2-en-1-
one 1%. f isobutyraldehyde 1 mmol.
Under the optimized conditions, aerobic oxidation of
various olefins was studied (Table 2). Increasing the reaction
time up to 13 h gave almost the same results. Thereby, the
minimum reaction time which results to the highest
conversion and yield was chosen. Tetralin and thioanisole were
also examined to explore the scope of the catalytic activity of
Fe3O4/tart-NPs. Highest enantioselectivity (99%) and epoxide
selectivity (100%) were obtained in the oxidations of 1-decene
with 77% conversion and 1-octene with 48% conversion (Table
2, entries 1 and 2). The epoxide selectivity for 1-methyl-1-
cyclohexene was also 100%, however enantioselectivity
decreased to 62% (entry 3). The oxidation of styrene, alpha-
and beta-methylstyrene were occurred by breaking the double
bond and mainly benzaldehde and acetophenone obtained
(entries 4, 5 and 7). Probably, the stability of the plausible
intermediate benzyl radical favours breaking of the olefin
double bond. In spite of the lower epoxide selectivity in the
oxidation of alpha-methylstyrene by Fe3O4/tart-NPs, the
remarkable enantioselectivity of 33% was obtained (entry 5).
The L-tartaric acid as capping agent in Fe3O4/tart-NPs affects
the enantioselectivity and sterogenic centre configuration of
the epoxid (compare entries of 5 and 6). Although the mixture
of (FeCl3·6H2O + L-tartaric acid) catalysed the oxidation of
alpha-methylstyrene with 74% epoxide selectivity, its
enantioselectivity was very low (5%) and the configuration of
the epoxide was opposite to that obtained by Fe3O4/tart-NPs
(entry 6). Catalyst Fe3O4/tart-NPs was also tested for the
oxidation of cis- and trans-stilbene (entries 8 and 9). cis-
manganese(II)
mesoporous
complex,
support
[{Mn(H2O)2Cl2}2(H2Btar)],
on
SBA-15
(H2L
=
2,3-O-4-
hydroxybenzhydrazidebenzylidene-D-tartrate)
oxidaized
thioanisole in 46% conversion, 29% sulfoxide selectivity and
100% ee.54 Chiral sulfoxides are an important class of
compounds as chiral auxiliaries in asymmetric carbon–carbon
bond forming reactions,55 as bioactive ingredients in the
pharmaceutical industry56 and constitute chiral synthons in
organic synthesis for the preparation of biologically active
compounds.57 Tetralin was oxidized by O2/isobutyraldehyde in
the presence of Fe3O4/tart-NPs to 1-tetralol (22%) and 1-
tetralone (3%) with 45% ee for 1-tetralol (entry 11). Tetralin is
a
convenient substance to choose for studying the
autoxidation of the CH2 group.58 The intermediate radicals are,
like the benzyl radical, resonance-stabilized systems on
account of the presence in the molecule of the aromatic ring,
which promotes exclusive attack at the alpha positions of
tetralin in the reduced ring. Indene was not oxidized by
Fe3O4/tart-NPs under the optimized condition and at 40 °C
even after 24 h.
The catalytic activity and/or enantioselectivity of the
present Fe3O4/tart-NPs are comparable and sometimes better
than chiral Fe(III) complexes with complicated and difficult to
synthesis ligands. Some comparisons are summarized in Table
S1.
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Table 2 Asymmetric oxidation of olefins, thioanisole and tetralin by Fe3O4/tart-NPs a
Entry
Sub.
Conv.
(%) b
Product(s)
(Yield%) b
Epoxide
Selectivity
(%)
Ee (%) c
(Config.)
1
2
3
1-Decene
1-Octened
77
48
37
R-Epoxide (77)
R-Epoxide (48)
S-Epoxide (7)
100
99
(R)
100
100
99
(R)
62
(R)
R-Epoxide (30)
(5)
4 d
23
32
S-Epoxide (3)
S-Epoxide (4)
78
38
67
(R)
R-Epoxide (15)
R-Epoxide (8)
5
33
(R)
(20)
(26)
6 e
100
76
S-Epoxide (39)
S-Epoxide (28)
74
55
5
(S)
R-Epoxide (35)
7 d
33
(S)
R-Epoxide (14)
(34)
8 f
9 f
100 g
100 g
Benzaldehyde (44)
trans-Epoxide (48)
cis-Epoxide (8}
cis-Epoxide (27)
56
92
Nd h
Nd h
Benzaldehyde (8)
trans-Epoxide (65)
78
(R)
10
11
36%
25
R (33)
S (4)
(3)
45
(S)
S (16), R (6)
a Fe3O4/tart-NPs 1.0 mg, isobutyraldehyde 5 mmol, substrate 2 mmol, CH3CN 3 ml, internal standard (chlorobanzene) 0.1 g, O2 (2 bar), room temperature, time 7 h. b
Conversion and yield were determined by GC and is an average of at least two runs. c The enantiomeric excess (ee) values were determined by GC analysis on a chiral
stationary phase (see the Experimental section). d at 40 °C. e.Catalyst: FeCl3·6H2O 0.12 mmol + tart 0.02 mmol. f 0.5 mmol substrate. g Evaluated from 1H NMR data. h
Not determined
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Catalyst recycling
In order to investigate the possibility of several recycling runs
for Fe3O4/tart-NPs, the solid catalyst was separated from the
reaction mixture by an external magnet (Fig. 4). It was washed
two times by acetonitrile and used again in a fresh reaction.
The catalyst was recycled four times for cyclohexene
oxidations (Fig. S4) and five times for tetralin oxidation (Fig. 5).
FT-IR spectrum of the used and recovered Fe3O4/tart-NPs
after first and fifth times are the same with that of the fresh
one (Fig. S5). Therefore, recycling is possible in the case of
Fe3O4/tart-NPs. As shown in Fig. 5, the increased activity of
Fe3O4/tart-NPs is evident after each cycle. Enantiomeric
excess (ee) increased in the first two recycles and then
remained almost constant. Although, the particles size
increases during the recycling process, the improvement of
the catalyst efficiency after each use can be assigned to the
well separation of Fe3O4/tart-NPs and reduction of the NPs
aggregation which lead to higher surface area for the catalyst
Fig. 5 Reuse of the catalyst in the aerobic oxidation of tetralin. Conditions: catalyst
Fe3O4/tart-NPs 1.0 mg, isobutyraldehyde 5 mmol, substrate 2 mmol, CH3CN 3 ml,
chlorobanzene 0.1 g, oxygen 2 bar, time 7 h at 25 °C.
and its higher activity. This point is clearly seen in the SEM The possible oxidation mechanism
image of the used Fe3O4/tart nanoparticles (compare Fig. 3a
As a matter of fact, trace of the allyl ketone and 38% allyl
and 3b). Reduction of the nanoparticles aggregation was also
confirmed by an increase in the specific surface area of the
catalyst Fe3O4/tart-NPs from 85 m2 g1 to 108 m2 g1 after two
times recycle (Fig. S6). Increasing the particles size of the
catalyst is partially due to the uncapping of the catalyst. Since
presence of the tartaric acid is seen even after the 5th recycle
of the catalysis (Fig. S5), separation of the capping ligand
probably is occurred only to some extent. This conclusion was
also confirmed by the occurrence of the enantioselective
oxidation of tetralin. The homogeneous mixture of iron salt
and L-tartaric acid shows very low enantioselectivity (Table 2,
entry 6).
alcohol were detected for the epoxidation of cyclohexene
which is typically regarded as a good substrate to check for
competition of olefin epoxidation vs. allylic oxidation (Table
1). This observation suggests that typical free radical
intermediates are directly involved as potential oxidizing
agents. Additional evidence for this was obtained from
stereochemical investigations of the epoxidation of cis/trans
stilbene (Table 2). Furthermore, electron-rich 1-methyl-1-
cyclohexene with three substituent displayed
a lower
reactivity than cyclohexene with two electron donor
substituent. This reflects the non-electrophilic nature of the
oxygen transfer from the plausible intermediate to the olefin.
The higher reactivity of cyclohexene relative to 1-methyl-1-
cycohexene might be due to the greater steric effect around
the double bond in the later. In the oxidation of cis- and trans-
stilbene the steric effects were also distinct. The epoxide
selectivity for the sterically demanding trans-stilbene was less
than the cis isomer. On the other hand, the behaviour of R,R-
tartaric (L-tartaric) acid adsorption on a Cu(110) surface using
high-resolution surface analytical techniques has been
studied.24 Under certain conditions, the 2-dimensional order
of the R,R-tartaric acid adlayer destroys all symmetry
elements at the surface of Cu(111), leading to the creation of
extended chiral surfaces. Such chiral surfaces may be an
important factor in defining the active site in heterogeneous
Fig. 4 Separation of the dispersed catalyst Fe3O4/tart-NPs in acetonitrile (left) by using
an external magnet (right).
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enantioselective reactions. By considering all these
information we may guess the probable mechanism for the
aerobic enantioselective epoxidation by Fe3O4/tart-NPs.
View Article Online
DOI: 10.1039/C5GC01774B
The oxidation of cyclohexene with oxygen does not
proceed without the presence of Fe3O4/tart-NPs nor without
isobutyraldehyde (Table 1, entries 1, 5), which indicates that
the chain reaction is initiated by interaction of
isobutyraldehyde with Fe3O4/tart-NPs containing trivalent
iron. The proposed mechanism is presented in Scheme 2, in
which the conversions of an olefin and isobutyraldehyde
observed in their co-oxidation in the presence of Fe3O4/tart-
NPs are plotted. The initiation starts with the conversion of
the aldehyde to the corresponding acyl radical (Scheme 2, A)
catalysed by the nanocatalyst. Subsequently this radical reacts
with O2 producing an acylperoxy radical (Scheme 2, B) which is
the intermediate, responsible for the transfer of oxygen from
its molecular form in the gas phase to the products of olefin
oxidation. This observation and the mechanism were similar
to
that
of
CuO@Ag
nanowires59
and
cobalt-
Fig. 6 The proposed intermediate for the adsorbed olefin (exemplified by 1-
decene) on Fe3O4/tart-NPs
porphyrin/isobutyraldehyde/O2 system60 catalysts, which
were used as the catalysts for stilbene oxidation.
The olefin coordinates to the Fe3+ of Fe3O4/tart-NPs surface
(Scheme 2, C). The reaction then proceeds on the surface of
magnetite nanoparticles by attacking of the acylperoxy radical
on the coordinated olefin (Scheme 2, D), which results finally to
Coordination of the acylperoxy radical intermediate
species to the transition metal–
been reported by Nolte et al.61 It was found that the metal
catalyst ( -diketonate–transition metal complexes) is not only
-diketonate complexes has

an efficient initiator of the reaction, but is also believed to
enhance the reactivity of acylperoxy radical intermediate
species in the oxidation process by allowing these to co-
ordinate to the metal center. Even if the exact role of the
metal catalyst in the epoxidation of olefins by dioxygen with
co-oxidation of aldehydes is still unclear, studies by Valentine
and coworkers62 demonstrate that it coordinates to the
acylperoxy radicals generated in the autooxidation of the
aldehyde forming a metal-acylperoxo complex. That being so,
iron(IV)–acylperoxo or iron(V)–oxo species derived by oxygen–
oxygen bond cleavage of the acylperoxo group, probably play
the role of active epoxidizing agents. High-valent Fe=O species
are suggested as the actual active oxygen transfer agents in
biological mono-oxygenations in both heme and non-heme
iron enzymes and model systems.63 However, rationalization
of the Fe3O4/tart-NPs enantioselectivity by coordination of the
acylperoxy radical to the Fe is difficult.
the enantiomeric epoxide and isobutyric acid. The
observed
enantioselectivity can be rationalized by the nucleophlic
coordination of the olefin to the Fe3+ of the Fe3O4/tart-NPs
surface (Scheme 2, C). Although detailed considerations on the
transition state are not yet clear, the stereochemistry of all the
products indicates that Fe3O4/tart-NPs 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 coordinated to
the nanocatalyst surface. Product analyses of the various olefin
epoxidation (Table 2) proves that the R-face of the prochiral
olefin is mainly preferred for oxygenation, thereby the
adsorption of olefin is through pro-S face, as shown in Fig. 6-(I)
for 1-decene. Facial selectivity in the addition of the olefin to
Fe3O4/tart 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 (III) in
Fig. 6 for 1-decene). Similar intermediate has been proposed
for the asymmetry achieved in the osmium tetroxide oxidation
of olefins by employing chiral amines.28,29
10 | J. Name., 2012, 00, 1-3
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Scheme 2 The tentative mechanism for the oxidation of prochiral olefins by oxygen in the presence of Fe3O4/tart-NPs and isobutyraldehyde.
magnetization value (~60 emu g−1). After completion of the
reaction, the catalysts can be collected by simple magnetic
decantation. In addition, the heterogeneous nanocatalysts are
relatively stable and can be reused five times with increasing
catalytic after each use in the catalytic process. Because of its
simple recyclability, the catalyst is well-suited for continuous
processes. Also, the method exhibits a number of highly
favourable practical characteristics: (1) very good
enantioselectivity in the oxidation of unfunctionalized olefins,
thioaniol and tetralin, (2) oxygen can be used as the oxidant,
(3) acetonitrile, a standard organic solvent which is not as
harmful as halogenated compounds like CH2Cl2 or CHCl3, is the
reaction medium, (4) room temperature reaction and (5) all of
the catalyst components (FeCl3 salt, L-(+)-tartaric acid) are
inexpensive, stable, and commercially available reagents.
Conclusions
A chiral carboxylic acid stabilized magnetite nanoparticles
have been shown to be an active, stable and good
enantioselective catalyst for the production of asymmetric
epoxide, sulfoxide and 1-tetralol using oxygen and
isobutyraldehyde at room temperature. This oxidation system
provides a general and ecofriendly method for the synthesis of
chiral epoxides, which are one of the most valuable and
versatile building blocks in the synthesis of chiral compounds.
To the best of our knowledge, this is the first report on iron-
catalyzed asymmetric aerobic oxidation by an inexpensive and
easily prepared asymmetric magnetite nanoparticles and O2,
allowing for general epoxidation of a relatively wide variety of
olefins in good yield and ee values up to greater than 99%.
Moreover, the synthesized heterogeneous nanocatalysts
possess strong magnetic responsivity due to a high saturation
Acknowledgements
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We would like to gratefully thank the University of Zanjan and
the Iran National Science Foundation under Grant No. INSF
92036382.
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DOI: 10.1039/C5GC01774B
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Green Chemistry
Page 14 of 14
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DOI: 10.1039/C5GC01774B
Magnetite nanoparticles stabilized by L-(+)-tartaric acid show high to excellent enantioselectivity in the
aerobic epoxidation of olefins