A green procedure for direct oxidation of organic halides to aldehydes and ketones catalyzed by a molybdate-based catalyst

Article abstract of DOI:10.1039/c4nj01886a

A molybdate-based heterogeneous magnetic catalyst was synthesized and used in direct oxidation of various primary and secondary organic halides to the corresponding aldehydes and ketones using 30% H₂O₂ as an oxidant in ethanol. Various organic halides were oxidized to their corresponding aldehydes and ketones in good to excellent yields. The catalyst system can be easily separated by applying an external magnetic field and reused for 6 runs without any significant loss of catalytic activity. The catalyst was fully characterized by FT-IR, SEM, TEM, EDS, XRD, VSM, ICP-AES techniques, and elemental analysis (CHN).

Full text of DOI:10.1039/c4nj01886a

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ARTICLE TYPE
A Green Procedure for Direct Oxidation of Organic Halides to
Aldehydes and Ketones Catalyzed by Molybdate-Based Catalyst
Ahmad Bayat, Mehdi Shakourian-Fard, Shaghayegh Ramezanpour, Mohammad Mahmoodi Hashemia*
5
Received 20th July 2014, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A molybdateꢀbased heterogeneous magnetic catalyst was synthesized and used in direct oxidation of various primary and secondary
organic halides to corresponding aldehydes and ketones using 30% H2O2 as oxidant in ethanol. Various organic halides were oxidized to
their corresponding aldehydes and ketones in good to excellent yields. The catalyst system can be easily separated by applying an
10 external magnet and reused in 6 run without any significant loss of catalytic activity. The catalyst was fully characterized by techniques
of FTꢀIR, SEM, TEM, EDS, XRD, VSM, ICPꢀAES, and elemental analysis (CHN).
peroxide can be controlled easier than molecular oxygen and air.
Introduction
In recent years, various types of molybdenum based catalyst
systems have been used in oxidation reactions. For example,
Aldehydes and ketones are important classes of chemicals which
15 serve as important precursors and intermediates for many drugs,
vitamins and fragrances [1ꢀ2]. One of the methods for the
synthesis of aldehydes and ketones is direct oxidation of organic
halides [3ꢀ4]. The oxidation of benzyl halides into aldehydes is a
wellꢀknown transformation in organic synthesis, although the
20 oxidation of aliphatic halides to aldehydes is not common and
performed far more difficult than oxidation of benzyl halides [5ꢀ
7].
Up to now, several methods have been developed for direct
oxidation of halides to their corresponding carbonyl compounds.
25 The HassꢀBender [5] and Sommelet [6] reactions are two wellꢀ
known methods for such a conversion. These methods have some
drawbacks. The former method is only suitable for paraꢀ
substituted benzylic halides and the latter is well for active halides
such as benzyl, allyl halides, and αꢀhaloꢀketones.
30 Various reagents have recently been developed as the oxygen
donor for this transformation in decades. Some of these reagents
are MgꢀAl hydrotalcite [8], pyridine Nꢀoxide [9], V2O5 [10],
NaIO4ꢀDMF [11], quinolinium chlorochromate [12], H5IO6 in
[C12mim][FeCl4] [13], amine oxide [14ꢀ16]. However, some of
35 these methods suffer from drawbacks such as long reaction time,
high temperature, low yields, being harmful to environment and
difficult separation of the product from the reaction mixture. From
environmental and green chemistry viewpoint, there is a great
need to develop new and green procedures with a broad substrate
40 scope that address the mentioned drawbacks for the direct
oxidation of organic halides to their corresponding carbonyl
compounds.
50 Na2MoO4.2H2O is widely used as
hydrodesulfurization from petroleum [17ꢀ20]. Although using
Na2MoO4.2H2O as catalyst needs homogeneous reaction
condition and therefore the catalyst is difficult to recover and
reuse. Use of recycled catalysts is required for reducing the
55 catalytic cost and green processes. One way to attain this goal is to
immobilize the catalytic system onto a large surface area of solid
carrier.
a
catalyst for
a
Fe3O4 magnetic nanoparticles (MNPs) have been frequently used
as a support for recyclable catalysts because of their unique
60 magnetic property and also their large surface area [21ꢀ23]. Fe3O4
nanoparticles can be easily oxidized in air and dissolved in acidic
solutions. To solve this drawback, Fe3O4 could be coated by other
materials which can protect them from being dissolved in acidic
solutions and oxidation in air. Silica is one of the best materials
65 that can be used for coating of Fe3O4. Silica is chemically inert
and does not affect the redox reaction at the core surface. Also,
nonmagnetic shell can suppress the particleꢀparticle magnetic
bipolar interaction and prevent them from aggregating and
agglomeration [22ꢀ 24].
70 We surmised that molybdateꢀbased magnetic catalyst could affect
the direct oxidation of organic halides to carbonyl compounds.
Therefore, we planned to explore the use of the supported
molybdate anion on Fe3O4@SiO2 magnetic nanoparticles for the
oxidation of organic halides to the carbonyl compounds, wherein
75 H2O2 was used as an oxidant in ethanol as green solvent.
There is no report in which molybdateꢀbased magnetic catalyst
has been directly employed for oxidation of organic halides to
their corresponding carbonyl compounds.
For this target, the use of molecular oxygen or hydrogen peroxide
as a primary oxidant in the presence of a catalyst is particularly
45 attractive. Hydrogen peroxide is a nonꢀtoxic, cheap and effective
oxidizer reagent. In addition, the oxidation reaction by hydrogen
This journal is © The Royal Society of Chemistry [year]
The reaction proceeded well for various organic halides to afford
80 high yields of aldehydes/or ketones. This heterogeneous catalytic
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system can be recycled and reused without any significant loss of 50 trimethoxysilane (AEPTS) was added and heated to reflux for 24
catalytic activity
h. The solid material was separated, washed with methanol and
ethanol, and then dried overnight in an oven at 60 °C. This
product will be referred to as Fe3O4@SiO2ꢀ AEPTS.
Experimental
Mag-Mo catalyst
5
Materials and methods
55 The synthesized Fe3O4@SiO2ꢀ AEPTS (1 g) was reacted with
excess of HCl (1 M, 5 ml) solution for 4 h and then separated by
magnet, washed with water and ethanol, and then dried in vacuum
desicator at 60 °C overnight to afford surface bound ammonium
chloride. Finally, the prepared material (1 g) was added to an
60 aqueous solution of the Na2MoO4 (0.15 g) at room temperature.
The mixture was stirred for 1 h, magnetically separated, washed
several times with water and ethanol, and then dried to afford the
Fe3O4@SiO2ꢀAEPTS supported molybdate anion (Mag-Mo)
(Scheme 1).
FT infrared (FTꢀIR) spectra were recorded on ABB Bomem
MB100 spectrometer with potassium bromide (KBr) pellets.
Scanning electron microscopy (SEM) was performed on KYKYꢀ
10 EM3200. An energy dispersive detector (EDS) coupled to the
microscope was used to indentify chemical elements of the
prepared catalyst. Xꢀray diffraction (XRD) pattern was recorded
on APD 2000, using Cu Kα radiation (50 kV, 150 mA) in the
range 2θ=10ꢀ120°. Transmission electron microscopy (TEM) was
15 performed on Philips CM30.
The roomꢀtemperature magnetization in the applied magnetic field
was performed by a homemade vibrating sample magnetometer
(Meghnatis Daghigh Kavir Company, Iran) from ꢀ10000 to +
10000 Oersted. Inductively coupled plasmaꢀatom emission
20 spectrometer (ICPꢀAES) was performed on PerkinꢀElmer Optima
3300 DV. Elemental analysis (CHN analysis) was performed in
duplicate by a model Perkin Elmer, USA (2400, Series II).
65
Preparations
Fe3O4
70
25 Fe3O4 nanoparticles were synthesized according to previous
published method [25]. Briefly, FeCl2 (5.4 g) and FeCl3 (2 g) were
dissolved in 25 ml of 2 M hydrochloric acid at room temperature
under mechanical stirrer until salts dissolved completely. 40 ml of
25% aqueous ammonia was added dropwise into the mixture at
30 room temperature under argon atmosphere. Then, the black
precipitate of magnetic nanoparticles (MNPs) was formed
immediately and followed by stirring about 1 h with mechanical
stirrer. The Fe3O4 nanoparticles were separated by an external
magnet and washed with the double distilled water (five times),
35 then vacuumꢀdried at 50 °C overnight.
75
Scheme 1. Preparation of magnetically recoverable heterogeneous
80 nanocatalyst MagꢀMo
Fe3O4@SiO2
The synthesized Fe3O4 magnetic nanoparticles were
homogeneously dispersed in ethanol (35 ml) and deionized water
(6 ml). 1.5 ml of tetraethyl orthosilicate (TEOS) was added slowly
40 to the mixture and sonicated for 15 min. 1 ml of 25% aqueous
ammonia was added slowly over 10 min under mechanical stirrer
and then the mixture was heated at 50 °C for 6 h. The magnetic
nanoparticles with a thin layer of silica (Fe3O4@SiO2) were
separated by an external magnet and washed three times with
45 ethanol and dried under vacuum.
General procedure for oxidation of organic halides
A mixture of organic halide (1 mmol), 30% H2O2 (4 equiv), Mag-
Mo catalyst (73 mg) were stirred in ethanol (5 ml) under reflux
85 condition. Completion of the reaction was indicated by thin layer
chromatography (TLC) (nꢀhexane/ethylacetate 10:1) and gas
chromatography (GC). After completion of the reaction, the
reaction mixture was cooled, and the catalyst was separated by an
external magnet. Then, 10 ml of water was added and the product
90 was extracted by ether (3 × 5 ml). After drying with anhydrous
sodium sulfate, the organic residue was analyzed by GC and then
purified by column chromatography on silica gel with ethyl
acetate/hexane (1:10) to afford the desired product. The separated
catalyst was washed with EtOH and used directly for a subsequent
Fe3O4@SiO2-AEPTS
1 g of dry Fe3O4@SiO2 was added to 25 ml of dry toluene and
sonicated for 10 min to produce a homogeneously mixed solution.
Then,
1 ml of 3ꢀ[2ꢀ(2ꢀAminoethylamino)ethylamino]propylꢀ
2
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round of reaction without further purification. All reaction
products were identified by GC chromatogram, IR spectra and
melting point as compared with authentic samples.
lower than the unsupported Fe3O4 due to the nonꢀmagnetic
properties of silica and the presence of AEPTS and molybdate
anion moieties in the prepared catalyst. This saturation
50 magnetization of Mag-Mo catalyst shows that the catalyst could
be easily separated by applying an external magnet that minimizes
the loss of the catalyst during the separation stage.
The IR spectrum of Mag-Mo shows peaks corresponding to
molybdate anion, which clearly differ from that of Fe3O4@SiO2ꢀ
55 AEPTS. The spectrum of Mag-Mo catalyst (Fig. 3b) shows the
peaks at 886 cmꢀ1, which could be assigned to the Mo=O stretch
mode of the characteristic vibrations of molybdate anion [27ꢀ 28].
Moreover, absorption band at 2926 cmꢀ1 is attributed to CꢀH
stretching vibration of CH2 groups in AEPTS. As shown in Fig.
60 3b, peaks at 580 cm−1 and 1104 cmꢀ1 are related to stretching
vibration of FeꢀO and SiꢀO bonds, respectively. The bands at 464
cm−1 and 798 cm−1 are due to the deformation of SiꢀO bond [29].
Results and discussion
5
Preparation and characterization of Mag-Mo
catalyst
The first step, the bare Fe3O4 was prepared by using a Fe+2/Fe+3
mixture via the conventional coprecipitation method (Scheme 1).
Then, the SiO2 was used to protect the magnetic core material
10 against
agglomeration
and
oxidation.
3ꢀ[2ꢀ(2ꢀ
Aminoethylamino)ethylamino]propylꢀtrimethoxysilane (AEPTS)
was employed to modify the surface of Fe3O4@SiO2 to yield the
aminoꢀmodified MNPs (Fe3O4@SiO2ꢀAEPTS). Finally, these
synthesized magnetic materials were treated with HCl and then
15 Na2MoO4 to generate MagꢀMo catalyst.
Fig. 1 shows the XRD result of Mag-Mo catalyst. The six main
peaks in XRD at 2θ= 30.4, 35.9, 43.4, 53.8, 57.4, and 62.9
correspond to (220), (311), (400), (422), (551), and (440)
diffraction planes of Fe3O4 MNPs which are in good agreement
20 with the standard XRD data for the cubic Fe3O4 phase of inverse
spinel crystal structure (JCPDS Card number 89ꢀ43191) [26]. The
XRD result of Mag-Mo catalyst shows that the main peaks
associated to Fe3O4 MNPs do not change after the attachment of
AEPTS organic functional group and immobilization of
25 molybdate anion on the Fe3O4 surface, indicating no change in the
crystal structure of the Fe3O4 core during the functionalization
process. The average size of the particles was estimated to be 29
nm for Mag-Mo using the DebyeꢀScherrer equation according to
the (311) reflection.
30
35
Fig. 1 XRD pattern of Mag-Mo catalyst
The magnetic property of uncoated Fe3O4 nanoparticles,
Fe3O4@SiO2, and Mag-Mo catalyst were characterized by using
40 the vibrating sample magnetometer (VSM). Fig. 2 shows the
magnetic hysteresis loops of Fe3O4 (a), Fe3O4@SiO2 (b), and
Mag-Mo catalyst (c) in an applied field of ꢀ10 000 to 10 000 Oe
at 298 K. The hysteresis loops of all of the magnetic particles
exhibited superparamagnetic behavior and had little hysteresis,
45 remanence and coercivity. The saturation magnetization of the
Mag-Mo catalyst is about 42 emu gꢀ1 (Fig. 2) which is relatively
Fig. 2 Magnetization curves of Fe3O4 (a), Fe3O4@SiO2 (b), and
65 Mag-Mo catalyst (c). The inset is a photograph of Mag-Mo
catalyst under an external magnetic field.
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Finally, the broad peak at 3477 cmꢀ1 and the band around 1629
cmꢀ1 are due to the OꢀH vibration and the bending vibration of
water molecules adsorbed on the surface, respectively [27ꢀ 28].
Fig. 4 TEM (a), SEM (b) and EDS (c) images of Mag-Mo
catalyst.
The stretching vibration of –NꢀH bonds in Mag-Mo catalyst may 25 was calculated using quantitative elemental analysis (CHN),
5
have an overlap with the OꢀH vibration of water molecules
adsorbed on the surface at 3477 cmꢀ1.
In order to investigate the morphology and particle size of Mag-
Mo catalyst, scanning electron microscopy (SEM) and transition
electron microscopy (TEM) images of Mag-Mo are shown in Fig. 30 about 0.41 mmol.gꢀ1.
which approximately confirmed a loading of 0.37 mmol.gꢀ1.
Inductively coupled plasma atomic emission analysis (ICPꢀAES)
was used to obtain an insight into the accurate amount of Mo in
the Mag-Mo catalyst. ICPꢀAES indicated that the Mo content is
10 4. These images indicate that the size of nanoparticles is between
30ꢀ40 nm, and the particles are also semiꢀspherical and uniform.
As seen from EDS analysis in Fig. 4c, the molybdenum peak
clearly confirms the presence of a molybdate anion in Mag-Mo
catalyst. The loading of AEPTS group on Fe3O4@SiO2 surface
15
Catalytic activity of Mag-Mo catalyst for direct
oxidation of organic halides to aldehydes and
ketones
35
To investigate the catalyst activity of Mag-Mo catalyst, we
have chosen the oxidation of organic halides to aldehydes and
ketones with H2O2 catalyzed by Mag-Mo catalyst. In order to get
an insight into the optimum catalytic conditions for the oxidation
process, the oxidation reaction of benzyl chloride was used as a
40 model substrate. The effect of solvent, temperature and the
amount of Mag-Mo catalyst on the oxidation reaction was
investigated in order to improve the efficiency of the reaction.
To obtain suitable solvent for the oxidation reaction, the reaction
was carried out in various solvents (Table 1). The results for
45 various solvents indicated that not only conversion of benzyl
chloride to benzaldehyde in ethanol is high, but also selectivity
towards benzaldehyde is also good.
After choosing ethanol as the best solvent, the effect of
temperature and the amount of Mag-Mo catalyst on the oxidation
50 reaction was investigated. As seen from Table 1, in the absence of
Mag-Mo catalyst (entry 8), benzaldehyde was obtained only 39%
in 24 h. When the catalyst (1 mol% (24 mg)) was added to the
reaction mixture (entry 3), the yield of reaction increased from
39% to 82% and the time and selectivity of reaction deceased to 8
55 h and 98%, respectively. It was found that the amount of catalyst
could remarkably promote conversion of benzyl chloride to
benzaldehyde and decrease the time and selectivity of reaction. As
seen from Table 1, with the increase of Mag-Mo catalyst amount
from 1% to 4%, the selectivity of the reaction decreased generally
60 depending on the catalyst value used in the oxidation reaction. For
example, with the increase of catalyst value from 3% to 4%, the
selectivity decreased from 97% to 94%. Thus, we performed the
reaction in the presence of 3% catalyst to get the highest yield and
selectivity values.
Fig. 3 IR spectra of Fe3O4@SiO2ꢀAEPTS (a) and Mag-Mo
catalyst (b)
20
65
Table 1. Optimization of the reaction conditions for benzyl
chloride oxidation to benzaldehydea
70
Entry
Solvent
Catalyst mol% (g) Time Yield Selectivity
(h)
(%)b
53
(%)
1
2
3
THF
H2O
EtOH
1 mol%(0.024)
1 mol%(0.024)
1 mol%(0.024)
8
8
8
95
88
98
60
82
4
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4
5
6
7
8
MeOH
CH3CN
nꢀHexane
Toluene
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
1 mol%(0.024)
1 mol%(0.024)
1 mol%(0.024)
1 mol%(0.024)
ꢀ
1 mol%(0.002)
2 mol% (0.048)
3 mol% (0.073)
4 mol% (0.097)
3 mol% (0.073)
8
8
8
8
24
8
4
4
4
4
76
79
37
40
39
79c
89
94
94
38d
97
98
99
98
100
96
99
97
94
99
aldehydes and ketones under the optimized conditions. As seen
15 from Table 2, a wide variety of functionalized benzyl halides with
electronꢀdonating and withdrawing substituents was used in the
oxidation reaction. Generally, benzyl halides with electronꢀ
donating substituents produced good to excellent yields, while
benzyl halides containing withdrawing groups exhibited slightly
20 lower conversions. Over oxidation was also seen in the oxidation
of organic halides to their corresponding carbonyl compounds.
For example, the primary benzyl halides were oxidized to their
corresponding carboxylic acids. The selectivity values for all
substrates are summarized in Table 2. As seen from Table 2, the
25 selectivity values for primary benzyl halides are lower than other
substrates. Our results indicate that benzyl halides could be easily
converted to their corresponding aldehydes in good to excellent
yield, although conversion of aliphatic halides to aldehydes is far
more difficult than oxidation of benzylic halides. For example, 1ꢀ
30 bromo hexane was converted to corresponding aldehyde with only
62% yield in 7 h (entry 12). The primary allyl chloride was also
converted into the oxidation product in a relatively good yield
(entry 13). When these conditions were applied to cyclohexyl
bromide, the corresponding ketone was obtained as the product
9
10
11
12
13
aReaction conditions: benzyl chloride (1 mmol), 30% H2O2 (4
equiv), solvent (5 ml), under reflux condition. bIsolated yield.
cNa2MoO4 was added. dRoom temperature.
5
As seen in Table 1, temperature has remarkably effect on the yield
of benzaldehyde. With increasing temperature from room
temperature to reflux condition, the yield of benzaldehyde
increased from 38% (entry 13) to 94% (entry 11). Moreover,
overꢀoxidation of benzaldehyde to benzoic acid occurred with the
10 increase of time to 10 h, and the selectivity of the desired product
was reduced.
The applicability of this protocol was studied by oxidation of
primary and secondary organic halides to their corresponding
35 without formation of cyclohexene
.
Table 2. Oxidation of organic halides to aldehydes and ketones.a
Entry
Substrate
Product
Time (h)
Yield (%)b
Selectivity
96
CHO
NO2
Br
NO2
1
8
87
CHO
Br
2
3
9
4
83
93
97
94
O2N
O2N
CHO
Br
Cl
Cl
CHO
CHO
Br
Br
4
5
4
4
96
95
92
95
MeO
Me
MeO
Me
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CHO
Br
6
4
92
96
Br
Br
CHO
Cl
Cl
7
8
4
91
72
97
99
Cl
Cl
CHO
7
Cl
O
9
6
89
70
92
97
99
Br
O
10
11
8
Br
O
6
99>
12
13
7
62
90
99
93
Br
O
Cl
Cl
5
6
CHO
CHO
14
15
60
65
99
Br
O
8
4
99>
Br
CHO
16
96
97
b
aReaction conditions: organic halide (1 mmol), catalyst (3 mol%, 73 mg), H2O2(4 equiv), EtOH (5 mL) under reflux condition. Isolated
yield.
Table 3. Comparison of the activity of various catalysts in the oxidation reaction of benzyl chloride.
Entry
Catalyst
H5IO6
Na2MoO4
Solvent
[C12mim][FeCl4]
H2O
Temp (°C)
Time (h)
Yield %
94
[ref]
[13]
[30]
1
2
30
90
2
18
72
6
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3
4
5
V2O5
Mg–Al hydrotalcites
H5IO6/V2O5
H2O
DMSO
[bmpy][PF6]
EtOH
100
140
50
6
10
3
70
80
92
94
[10]
[8]
[7]
ꢀ
This work
MagꢀMo
78
4
The efficiency of Mag-Mo catalyst was compared with other
catalyst systems in literature for the oxidation reaction of benzyl
chloride (Table 3). Some of these procedures (entries 2ꢀ4) are
associated with disadvantages such as long reaction time, high
temperature, and low yields. In comparing our work and the
reactions done in ionic liquid solvents (entries 1 and 5), the
magnetic property of Mag-Mo heterogeneous catalyst provides
the easy separation of catalyst from reaction mixture only by
Acknowledgement We gratefully acknowledge financial
support from the Research Council of Sharif University of
45 Technology.
5
Notes and references
aDepartment of Chemistry, Sharif University of Technology, PO
Box 11465-9516, Tehran, Iran, E-mail: mhashemi@sharif.edu;
[1] M. Hudlicky, Oxidation in Organic Chemistry, American
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Tetrahedron Lett., 2013, 54, 1983.
10 applying an external magnetic field, although the catalyst
separation, recycling and reuse is difficult in ionic liquids.
Catalyst recycling
One of the issues in using heterogeneous catalyst is its
15 recyclability and reusability. In with this line, recyclability of the
catalyst was investigated in the oxidation of benzyl chloride. After
each reaction cycle, the Mag-Mo catalyst was separated with an
external magnet, washed with ethanol, and used in subsequent
reaction. According to Fig. 5, only a slight reduction in the yield
20 of benzaldehyde was observed after 6 consecutive runs.
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Fig. 5 The recycling experiment of Mag-Mo catalyst in the
oxidation reaction of benzyl chloride to benzaldehyde under
30 optimized conditions. The oxidation reaction was quenched after 4
h at each step.
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Conclusion
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In this study, a molybdateꢀbase magnetic catalyst (Mag-Mo) was
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35 aldehydes and ketones using 30% H2O2 as oxidant in ethanol. In
these conditions, a wide range of organic halides with electronꢀ
donating and withdrawing substituents could be oxidized to
corresponding products with good conversions. This catalytic
system is very easy to handle and can be recycled and reused in 6
40 run without any significant loss of catalytic activity. Moreover,
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