MnO2?Fe3O4 Magnetic Nanoparticles as Efficient and Recyclable Heterogeneous Catalyst for Benzylic sp3 C?H Oxidation

Article abstract of DOI:10.1002/asia.201900810

Herein, we report a highly chemoselective and efficient heterogeneous MnO₂?Fe₃O₄ MNP catalyst for the oxidation of benzylic sp³ C?H group of ethers using TBHP as a green oxidant to afford ester derivatives in high yield under batch/continuous flow module. This catalyst was also effective for the benzylic sp³ C?H group of methylene derivatives to furnish the ketone in high yield which can be easily integrated into continuous flow condition for scale up. The catalyst is fully characterized by spectroscopic techniques and it was found that 0.424 % MnO₂?Fe₃O₄ catalyzes the reaction; the magnetic nanoparticles of this catalyst could be easily recovered from the reaction mixture. The recovered catalyst was recycled for twelve cycles without any loss of the catalytic activity. The advantages of MnO₂?Fe₃O₄ MNP are its catalytic activity, easy preparation, recovery, and recyclability, gram scale synthesis with a TOF of up to 14.93 h^?1 and low metal leaching during the reaction.

Full text of DOI:10.1002/asia.201900810

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: MnO2@Fe3O4 MNP: An Efficient and Recyclable
Heterogeneous Catalyst for Benzylic sp3 C-H Oxidation
Authors: Boopathy Gnanaprakasam, Akanksha M Pandey, Sandip G
Agalave, and Chathakudath P. Vinod
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To be cited as: Chem. Asian J. 10.1002/asia.201900810
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10.1002/asia.201900810
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MnO2@Fe3O4 MNP: An Efficient and Recyclable Heterogeneous
Catalyst for Benzylic sp3 C-H Oxidation
Akanksha M. Pandey,a Sandip G. Agalave,a Chathakudath P. Vinod,b and Boopathy Gnanaprakasama*
Abstract: Herein, we report highly chemoselective and efficient
heterogeneous MnO2@Fe3O4 MNP catalyst for the oxidation of
benzylic sp3 C-H group of ethers using greener TBHP as an oxidant
to afford ester derivatives in high yield under batch/continuous flow
module. This catalyst was also effective for the benzylic sp3 C-H
group of methylene derivatives to the ketone in high yield which can
be easily integrated into continuous flow condition for scale up. The
catalyst is fully characterized by spectroscopic techniques and found
that 0.424% MnO2@Fe3O4 catalyzes the reaction and magnetic
nanoparticle of this catalyst render easy recovery from the reaction
mixture. The recovered catalyst was recycled for twelve cycles
without any loss of the catalytic activity. The advantages of
MnO2@Fe3O4 MNP are its catalytic activity, easy preparation,
recovery, and recyclability, gram scale synthesis with TOF of up to
8670.6 h-1 and low metal leaching during the reaction.
make them less ideal for the synthesis of fine chemicals where
product contamination with heavy metal is highly undesirable
which led to environmental and economic concern in large scale
synthesis.[13] Thus, there is ongoing interest in finding more
sustainable catalysts that assist catalytic transformation to take
place under more realistic and environmentally benign
conditions. Also, there is a report for transition metal free
benzylic sp3 C-H bond oxidation of activated methylene group to
form ketone or ester which requires external additives NaNO2
and HCl.[14] Recently, J. Tan and coworkers[15] showed tert-Butyl
hydrogen peroxide (TBHP) mediated direct oxidation reaction of
benzylic sp3 C-H bonds to ketones, but there is no such report
on the continuous flow benzylic sp3 C-H bond oxidation of ether
to ester.
Heterogenization of the presented homogeneous catalysts,
particularly expensive and/or toxic heavy metal complexes could
resolve these problems. From these perspectives various
materials such as mesoporous silica,[16] activated carbon,[17]
polymers,[18] biomass,[19], etc. have been used as a support to
generate a heterogeneous catalytic system. Recently, Sekar,
et.al. reported the oxidation of alkylarene using binaphthyl
Introduction
Designing a sustainable chemical process for the functional
group transformation is one of the formidable challenges in
organic synthesis.[1] Metal-catalyzed direct conversion of the
C−H bond to carbonyl compounds using oxidant is greatly
increased in the last few decades among the synthetic chemist
due to the molecular economy and reaction greenness. Hence,
oxidation of a saturated sp3 C−H bond can rapidly install an
oxygen atom on a carbon atom and have attracted considerable
attention, given that most of the recognized drugs and natural
products are oxygen-containing compounds.[2] In recent years, a
plethora of methodologies have been developed for direct
benzylic sp3 C-H bond oxyfunctionalization.[3] However, most of
the commonly employed conditions include stoichiometric
quantities of transition metal salts which pose problems in terms
of cost, toxicity and atom economy.[4] Selective benzylic
oxidation using catalytic amount of transition metals such as
Cr,[5] Mn,[6] Co,[7] Ru,[8] Rh,[9] Fe,[10] Re,[11] etc and post-transition
metal Bi[12] has been reported. The reported homogeneous
transition metal catalysts for the direct transformation of
activated methylene group have several advantages to afford
the ketone in moderate to good yield. However, in spite of the
advances, there are limitations exist in this transformation such
as the decomposition of the metal catalysts, the lack of regio-
and stereoselectivity, exothermic reactions, the problems of
catalyst extraction and recycling as well as product purification
stabilized Pt nanoparticles (Pt-BNP) as
a
catalyst.
[20]Advantageously, these supported catalyst systems can be
effectively recycled and reused with the inherent catalytic activity.
Magnetic nanomaterials are found to be more selective
and effective than conventional heterogeneous materials as a
robust, readily available, extremely small size and large surface
area of heterogeneous catalyst support. Further, it has more
volume ratio which allows more reactions to occur at the same
time and consequently speed up the reaction process.[21-22]
Advantageously, they are magnetically separable, which
eliminates the requirement of catalyst filteration or centrifugation
after completion of the reaction. The Fe3O4 MNPs, due to their
unique physicochemical properties becoming increasingly
significant in the field of catalysis, imaging, photonics,
nanoelectronics, sensors, biomaterials, and biomedicine.[23] In
literature, these Fe3O4 nanoparticles are used for the selective
oxidation of benzylic and allylic C–H bonds to carbonyl
compounds using TBHP as an oxidant.[24] Manganese-based
catalyst has attracted considerable research interest because of
its nanostructures have a large surface area and highest
catalytic activity as well as it is cheap, mild and nontoxic
oxidative reagent used for the selective oxidation of various
functional groups.[25] Thus, developing an Mn supported on
Fe3O4 MNPs for benzylic sp3 C-H oxidation of ether and
methylene compounds are highly attractive and desirable
approach in terms of the green chemistry points of view; which
will also allow finding a more economical and environmentally
benign procedure.
[a]
[b]
Akanksha M. Pandey, Sandip G. Agalave, and Boopathy
Gnanaprakasam*
Department of Chemistry, Indian Institute of Science Education and
Research, Pune-411008, India.
E-mail: gnanaprakasam@iiserpune.ac.in
Chathakudath P. Vinod
Herein, we wish to disclose a facile and atom-economical
methodology for the direct oxidation of benzylic sp3 C-H bond
using MnO2@Fe3O4 as an MNP supported catalyst for the
oxidative esterification of benzylic ethers to form various ester
derivatives (Scheme 1).
CSIR-NCL Catalysis and Inorganic Chemistry Division, Pune, India
Supporting information for this article is given via a link at the end of
the document.
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b
a
Scheme 1. General protocol for the benzylic sp3 C-H oxidation using
MnO2@Fe3O4 MNP catalyst.
C
d
The newly developed catalytic system proceeds with high
selectivity and broad substrate scope under mild reaction
conditions with TBHP as the environmentally benign terminal
oxidant. It is also found that the present catalytic system was
effective and versatile for the oxidative benzylic sp3 C-H group to
form various ketone derivatives. This novel and chemoselective
methodology work well in batch and continuous flow module. To
date, to the best of our knowledge, MnO2@Fe3O4 MNP
catalyzed benzylic sp3 C-H group oxidation of ethers and
methylene derivatives using TBHP as an oxidant have not been
reported.
(f) mean size –12.34 nm
(e) mean size –10.48 nm
18
18
16
14
12
10
8
Particle size (nm)
Gaussian fit
Particle size
Gaussian fit
16
14
12
10
8
6
6
4
4
2
2
0
0
-2
-2
4
6
8
10
12
14
16
18
20
22
8
10 12 14 16 18 20 22 24 26 28
Diameter (nm)
Diameter (nm)
Results and Discussion
The synthesis of nano-sized MnO2@Fe3O4 was achieved by the
procedure reported for Fe(OH)3@Fe3O4.[26] The % of Mn on
Fe3O4 support was analyzed by Microwave Plasma Atomic
Emission Spectroscopy (MP-AES) analysis which showed that
catalyst contains 0.424% of Mn.
Figure 2. TEM images of MnO2@Fe3O4 catalyst (a) Fresh catalyst, (b) used
catalyst; Lattice fringes for (c) fresh catalyst, (d) used catalyst; Histograms
generated for (e) fresh, (f) used MnO2@Fe3O4 catalyst.
Figure
1
represents the XRD pattern of 0.424%
The morphology of the fresh and used MnO2@Fe3O4 catalyst
was evaluated by TEM analysis (Figure 2). From TEM analysis it
is found that, the average particle size of fresh catalyst is 10.48
nm, whereas average particle size of used catalyst is 12.34 nm.
The N2 adsorption-desorption isotherms and pore size
distribution of fresh MnO2@Fe3O4 catalyst is shown in Figure 3.
From figure 3, it is clear that pure MnO2@Fe3O4 catalyst
exhibited isotherm of type IV, which is a typical characteristic of
the mesoporous material.[27] Specific surface area (SBET) was
calculated by employing the Brunauer–Emmett–Teller (BET)
method and the pore size distribution is obtained by means of
Barrett–Joyner–Halenda (BJH) method equation using the
adsorption isotherm branch. Figure 3 shows that the volume
adsorbed increases with increasing relative pressures for all
isotherms which are due to the volume filling of micropores in
Fe3O4 membrane. The BET specific surface area and the pore
diameter of MnO2@Fe3O4 were found to be 13.19 m2/g and
0.059 cm3/g respectively.
MnO2@Fe3O4 nanocomposites, showing that the peaks of both
Fe3O4 and MnO2 appear along with enhanced peak intensity
which is caused by overlapping of both the peaks. As revealed,
diffraction peaks are completely corresponding to the standard
pattern characteristic peaks of the magnetite cubic inverse
spinel structure (JCPDS 01-074-2435).
4000
(c) MnO2@Fe3O4
3000
2000
(b) MnO
2
1000
(a) Fe
O
Thermal gravimetric analysis (TGA) of the MnO2@Fe3O4
composite nanoparticles were also performed at the range of 25
to 800 C, with a temperature ramp rate of 10 C min-1 under a
nitrogen atmosphere (Figure 4). As shown in Figure 4, the first
weight loss stage (below 150 C) can be attributed to the
evaporation of water and solvent molecules onto the surface of
3
4
0
20
30
40
50
60
70
80
2 Theta
Figure 1. PXRD data of (a) Fe3O4; (b) MnO2; (c) MnO2@Fe3O4
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80
60
40
20
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Figure 3. BET isotherm of MnO2@Fe3O4 catalyst
Figure 5. (a) FESEM images of MnO2@Fe3O4 catalyst; (b) EDAX of
MnO2@Fe3O4 catalyst
a
100
b
(b)
(a)
90
80
100
200
300
400
500
o
600
700
800
Temeperature ( C)
(c)
Figure 4. Thermogravimetric analysis of (a) Fe3O4; (b) MnO2@Fe3O4 catalyst
the catalyst. The weight loss of nanocomposites is about 3.3 %
at 300-550 C, corresponding to the thermal decomposition of
the crystal phase transformation of Fe3O4 to γ-Fe2O3. Thus, the
results indicate that MnO2@Fe3O4 catalyst has excellent stability
at temperatures as high as 800 °C. Surface morphology of fresh
MnO2@Fe3O4 nanocomposite was determined by FESEM
(Figure 5). The FESEM image of MnO2@Fe3O4 showing the
formation of spherical particles with an average size 14-23 nm
range. The EDAX analysis reveals that Fe, O, and Mn are the
main elements present with Fe being the most abundant in the
selected field.
To confirm the XRD results, the compositions and the
valence states of fresh MnO2@Fe3O4 MNP were further
characterized with XPS and the results are shown in Figure 6.
The Mn peak is analyzed in figure 6a, which shows that each
broad peak can be classified into two parts at 641.52 and 643.06
eV for Mn2p3/2 and 653.58 and 656.34 for Mn2p1/2. The peaks at
641.52 and 643.06 eV are the characteristics of Mn3+ while
Figure 6. XPS for fresh MnO2@Fe3O4 MNP catalyst of (a) Mn 2p3/2 and 2p1/2
(b) Fe 2p3/2 and 2p1/2 ; (c) O 1s
;
those at 653.58 and 656.34 eV are the characteristics of Mn4+.
[28-29]
Moreover, the Fe spectrum is depicted in Fig. 6b, and two
dominant peaks located at 711.0 and 724.23 eV are in good
accordance with Fe 2p3/2 and Fe 2p1/2 spin orbit peaks along
with other peaks which are consistent with the standard Fe3O4
XPS spectrum in which Fe is present in the form of Fe2+ and
Fe3+. For the O1s XPS spectrum (Figure 6c), the spectrum
contains three main peaks located at 530.09, 530.70, and
531.98 eV.[30] The XPS results for used MnO2@Fe3O4 MNP is in
ESI.
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Catalytic studies
MnO2@Fe3O4 MNP afforded 55% of 2a (Table 1, entry 5).
Increasing the temperature up to 100 C has no influence on the
outcome of the reaction (Table 1, entry 6). Performing this
reaction with other solvents such as dichloroethane (DCE),
dichloromethane (DCM), chlorobenzene, diethyl carbonate
(DEC) and dimethylsulfoxide (DMSO) has not improved the yield
(Table 1, entry 7–11). However, no phenyl benzoate was
detected when 1,4-dioxane, dimethoxyethane (DME) and
acetone were employed as a solvent (Table 1, entry 12–14).
Initially, the conversion of (benzyloxy)benzene 1a into phenyl
benzoate 2a was chosen as a model reaction using 7 mmol of
TBHP (5-6 M in decane) as an oxidant in acetonitrile (ACN)
solvent at room temperature for 24 hrs. To identify the
importance
(benzyloxy)benzene 1a,
performed (Table 1, entry 1), and no phenyl benzoate 2a was
detected. Heating of 1a in ACN by using mol
of
the
catalyst
for
the
oxidation
of
a
blank control experiment was
5
%
Mn(OAc)3.2H2O gave 23% of phenyl benzoate 2a along with the
recovery of starting material (Table 1, entry 2). In another control
experiment only with Fe3O4, 33% yield of phenyl benzoate 2a
was observed when the reaction carried out at 80 C (Table 1,
entry 3).
Table 2. Optimization of the reaction conditions (oxidant/additive) for the
benzylic sp3 C-H groupa
Table 1. Optimization of the reaction conditions (solvent) for the benzylic sp3
C-H group
entry
catalyst
(50 mg)
oxidant
TBHP
Additive
/ligand
yield
(%)
1
2
MnO2@Fe3O4
MnO2@Fe3O4
-
-
55
-
4-Methyl
pyridine-N-
oxide
entry
catalyst (50 mg)
solvent
temp
(C)
yield
(%)
3
4
5
6
7
8
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
K2S2O8
TEMPO
NHPI
-
-
-
-
1
-
ACN
80
80
80
rt
nd
23
33
38
55
55
48
30
50
50
55
nd
nd
nd
-
30
-
2
Mn(OAc)3.2H2O
Fe3O4
ACN
TBHP
TBHP
NEt3
3
ACN
Pyridine
Pyridine
66
-
4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
ACN
4-Methyl
pyridine-N-
oxide
5
ACN
80
100
80
80
80
80
80
80
80
80
6
ACN
9
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
Mn@Al2O3
K2S2O8
TEMPO
NHPI
Pyridine
-
7
DCE
10
11
12
13
14
Pyridine
-
8
DCM
Pyridine
60
80
75
72
9
Chlorobenzene
DEC
TBHP
TBHP
TBHP
2,2’-bipyridine
2,2’-bipyridine
2,2’-bipyridine
10
11
12
13
14
DMSO
1,4-Dioxane
DME
Ru@Fe3O4
aReaction conditions: (benzyloxy)benzene 1a (1mmol), oxidant (7 mmol),
additive (10mol%) and 50 mg of catalyst were stirred at 80 C (see table 1) for
24 hrs.
Acetone
Furthermore, we have also optimized the effect of various
oxidants and additives for the oxidation reaction. Oxidants such
as 4-methyl pyridine-N-oxide, K2S2O8, TEMPO, and NHPI are
failed to deliver or giving less yield of phenyl benzoate 2a (Table
2, entries 2-5). Based on the previous studies,[31] the role of the
nitrogen-containing ligand was pivotal in the benzylic sp3 C-H
oxidation reaction. This is because that nitrogen-containing
Reaction conditions: (benzyloxy)benzene 1a (1mmol), TBHP in decane (7
mmol), and 50 mg of catalyst were stirred at various temperature (see table 1)
for 24 hrs.
Whereas 0.424% MnO2@Fe3O4 catalyst at room temperature
condition afforded 1a in 38% yield (Table 1, entry 4). Further,
heating of 1a in ACN solvent using 50 mg of 0.424 %
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ligand such as triethylamine, pyridine, 2,2’-bipyridine could
behave as a hydrogen bond acceptor with TBHP to speed up
the formation of t-butoxyl radical. This radical could abstract the
hydrogen from a benzylic carbon atom of the substrate.[32]There
is no (benzyloxy)benzene 2a formation when triethylamine was
used as a additive (Table 2, entry-6). Moreover, the good
conversion of about 66% was obtained when 10 mol% of
pyridine used as a additive (Table 2, entry 7). Other oxidants
have no effect on the conversion of (benzyloxy)benzene when
pyridine used as a base (Table 2, entries 8-10). The reactions
proceeded smoothly when the reaction carried out by using
TBHP as the oxidizing agent and 2,2’-bipyridine as an additive in
ACN as the solvent, afforded the desired (benzyloxy)benzene 2a
in 80% yield (Table 2, entry 12). Furthermore, other supported
catalysts such as Mn@Al2O3 and Ru@Fe3O4 under similar
reaction conditions are also afforded esters 2a in good yield
(Table 2, entries 13-14). From the results of optimization, the
best
reaction condition was
obtained by reacting
(benzyloxy)benzene 1a (1 mmol), 50 mg of 0.424%
MnO2@Fe3O4 MNP as a catalyst, 10 mol% 2,2’-bipyridine as an
additive and 5-6 M TBHP in decane (7 mmol) as an oxidant in
ACN solvent (2.0 mL) at 80 C for 24 hrs.
After the optimal conditions established, the scope of this
reaction was investigated with various benzylic ethers and the
results are summarized in Scheme 2. This oxidative reaction is
well tolerated with various electron donating (2b-2d). It is
noteworthy that, the reaction conditions were compatible with
the presence of halogenated groups which can extend the
possibility for further functionalization. Interestingly, reaction with
benzylic aliphatic ether 1k-n is smoothly converted into
corresponding ester 2k-n in excellent yield. This reaction is
highly chemoselective for benzylic ethers giving corresponding
esters in excellent yields; however, there is no ester formation
with alkylated phenol 1o and allylic ether 1p.
In order to demonstrate the general utility of this synthetic
strategy, we carried out gram scale reactions in batch (Scheme
2). Two representative benzyloxy benzene, one with a methyl
substituted 1b and the other with a halogen-substituted 1g, were
chosen to react with TBHP in presence of supported
MnO2@Fe3O4 MNP catalyst under optimized reaction condition.
The reactions were performed in 10 mmol scales using 0.500 g
of MnO2@Fe3O4 at 80 oC for 24 hrs. after which 2b and 2g were
obtained in 86% and 80% yields, with TON = 208095.7; TOF =
8670.0 h-1 and TON = 208095.7; TOF = 8670.6 h-1 respectively.
To expand this reaction for sustainable continuous flow
process, the optimized batch reaction condition was transferred
to a continuous flow reactor. In continuous flow, the reaction and
separation of the MnO2@Fe3O4 MNP can be performed
simultaneously which avoid the mechanical degradation of the
supported catalyst. Developing a continuous flow process for sp3
C-H oxidation reaction using 0.424% MnO2@Fe3O4 MNP
catalyst can improve efficiency and safety for the TBHP
mediated reaction. Hence, continuous flow reactions were
performed using Holmarc syringe pump (Model no.-HO-SPLF-
2D) and by preparing the catalyst bed in the Omnifit column
reactor (Scheme 3).
Scheme 2. Substrate scope for ester synthesis in batch condition
Table 3. Optimization of the reaction conditions for the benzylic sp3 C-H group
in continuous flowa
entry
catalyst/
additiveb
substrate
(1a):
TBHP
flow rate
(ml/min)
temp
(C)
tR
(min)/
cycle
yield
(%)c
1
2
3
4
5
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
0.05:0.25
0.05:0.35
0.05:0.35
0.05:0.35
0.05:0.45
0.1
0.1
0.1
0.1
0.1
rt
17.10/1
17.10/1
17.10/2
17.10/2
17.10/2
10
10
50
50
50
rt
80
100
80
aReaction conditions: 0.05 M solution of 1a + 0.25-0.45 M solution of TBHP
(5.0−6.0 M in decane) prepared from in 20 mL ACN and flown on 0.424%
MnO2@Fe3O4 catalyst loaded bed reactor with the help of 1.7 mL syringe
pump (Model no.-HO-SPLF-2D). b0.1 mmol of 2,2′-bipyridine ligand used, tR
residence time. cIsolated yields.
=
For our initial study, we chose (benzyloxy)benzene 1a as a
model substrate and a set of reaction conditions was attempted
(Table 3). Initially, 0.05:0.25 M solution of 1a:TBHP in presence
of 0.10 mmol of 2,2′-bipyridine as a ligand in acetonitrile solvent
was reacted by flowing it through the Omnifit fixed bed reactor
containing 0.424% MnO2@Fe3O4 catalyst (1.0 g; void volume
1.7 mL; flow rate 0.1 mL/min) at room temperature it furnished
10 % of product 2a after one cycle (Table 3, entry 1). Increase in
the molar concentration of TBHP up to 0.35 does not improve
the outcome of the reaction (Table 3, entry 2). Effect of
temperature has a great influence on the esterification of ether
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1a. A best-optimized condition was obtained by increasing
temperature up to 80 C, to afford the product 2a in 50% yield
after two cycles (Table 3, entry 3). This reaction required only
17.10 min. to afford the final product of ester 2a. Further
increase in the temperature and molar concentration of TBHP
has no effect on the formation of product 2a (Table 3, entries 4
and 5).
Table 4. Optimization table for sp3 benzylic oxidation of ethyl benzenea
Having established the optimal reaction conditions, various
benzylic ethers (1a-n) were tested in a continuous flow to
explore the generality of this oxidative transformation. By
screening different substrate in a continuous flow, and we found
that the present catalytic system was not compatible in a
continuous flow and afforded moderate yields (15 to 50%) of the
ester product after 2 runs (Scheme 3).
entry
catalyst
solvent
temp
(C)
time
(h)
yield
(%)
1b
2
Mn(OAc)3.2H2O
Fe3O4
ACN
ACN
ACN
80
80
80
7
7
7
72
48
83
3
0.424% Mn@Fe3O4
Reaction conditions: aEthylbenzene 3a (1mmol), TBHP in decane (5 mmol),
b
and 50 mg of catalyst were stirred at 80 C (see table 4) for 7 hrs. 5 mol%
catalyst was used
The conversions were 72
%
and 48% when using
Mn(OAc)3.2H2O and Fe3O4 respectively. 0.424% MnO2@Fe3O4
was found to be superior to the others and produced the desired
product acetophenone 4a in 83 % in isolated yield. The present
catalytic system has been optimized again in a continuous flow
to acclimatize the benzylic sp3 C-H oxidation reaction of
ethylbenzene 3a. The reaction solvents such as toluene and
methanol led poor yield of desired product 4a. The best-
optimized condition was obtained by a flowing mixture of a
0.05M solution of ethylbenzene 3a and 0.25M solution of TBHP
(5-6 M in decane) by keeping a flow rate of 0.1 mL/min. affording
95 % of desired product 4a in residence time of tR = 17.10 min
(Table 5).
Having established the optimal reaction conditions in batch
and in a continuous flow, a series of substrates were used to
examine the generality of substrates and the catalytic activity of
the MnO2@Fe3O4 MNP catalyst (Scheme 4). The reactions
proceeded smoothly in acetonitrile as the solvent and using
TBHP as the oxidizing agent afforded the desired oxidative
product 4a in excellent yields in both batch and continuous flow.
The yields were generally comparable to the batch reaction, but
reactions are completed in a shorter duration of time in
continuous flow (17.10 minutes) as compared to the batch
condition (7 hrs.).
Scheme 3. Continuous flow setup and substrate scope for ester synthesis
The catalytic selective oxidation of benzylic sp3 C-H group
to the corresponding carbonyl compounds is one of the mainly
significant reactions in the synthesis of fine chemicals and
pharmaceutically important ingredients.[1] We also investigated
the catalytic activity of MnO2@Fe3O4 catalyst in batch as well as
in continuous flow reactor for ketone synthesis using
ethylbenzene 3a as a model substrate and using TBHP as an
oxidant Table 4. In batch, the oxidation of ethylbenzene (1mmol)
3a for the generation of acetophenone 4a was carried out using
5 mmol of TBHP (5-6 M in decane) in 2.0 mL acetonitrile at 80
Besides model substrate, other substrates such as 9H-xanthene,
diphenylmethane, 1,2,3,4-tetrahydronaphthalene, 2,3-dihydro-
1H-indene, 9H-fluorene, and 2-ethylnaphthalene afforded the
desired ketones (4b-g) in good to excellent yield under batch
and continuous flow
module. Substituted ethylbenzene
containing an electron-donating functional group also afforded
respective ketone products (4h-i) in good to excellent yield (72-
90%). On the other hand, methylene groups adjacent to a
heterocyclic ring in 1H-oxindole, and 1-methyl oxindole were
also converted into the corresponding ketones (4j-k) in excellent
yield.
C for
7 h (Table 4). Control catalytic experiments of
ethylbenzene 3a were performed on Mn(OAc)3.2H2O and Fe3O4
(Table 4, entries 1-2) and under the same reaction conditions.
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Table 5. Continuous flow setup and optimization of the reaction conditions for benzylic sp3 C-H oxidation of ethyl benzenea
entry
catalyst
substrate
(3b):TBHP
solvent
temp (C)
flow rate
(ml/min)
tR
Yield of 4b
(min)/cycle
(%)
20
55
nd
95
95
1
2
3
4
5
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
MnO2@Fe3O4
0.05:0.25
0.05:0.25
0.05:0.25
0.05:0.25
0.05:0.25
toluene
toluene
methanol
ACN
rt
0.1
0.1
0.1
0.1
0.1
17.10/1
17.10/1
17.10/1
17.10/1
17.10/1
80
80
80
100
ACN
aReaction conditions: 0.05 M solution of 3a + 0.25 M solution of TBHP (5.0−6.0 M in decane) was flown on 0.424% MnO2@Fe3O4 catalyst loaded on Omnifit
fixed bed reactor with the help of 1.7 mL syringe pump (Model no.-HO-SPLF-2D).
Recyclability of the catalyst
The recyclability of
a heterogeneous catalyst is an
important application in industry. The reusability and recyclability
of the MnO2@Fe3O4 MNP catalyst were studied for the benzylic
oxidation of compound 1b (Figure 7) and 3b (Figure 8) for the
ten and twelve catalytic cycles respectively in batch condition.
Furthermore, this catalyst is still active for the further oxidation
reaction. After completion of each cycle, the catalyst was
separated from the reaction mixture simply with the help of a
magnetic needle retriever, washed with acetonitrile and ethyl
acetate three times, dried at 100 °C for 2 hrs and then the
catalyst was used directly for the next cycle without any
additional treatment. The yield of the product remained the same
without any extension of reaction time. This result clearly
indicates the efficiency of the catalyst which was not lost over a
prolonged reaction.
Scheme 4. Substrate scope for ketone synthesis in a batch as well as in a
continuous flow module
To check the stability and productivity of the
heterogeneous MnO2@Fe3O4 MNP catalyst, we have performed
a long time experiment in continuous flow. For this, substrate 3a
was chosen as a model substrate. For instance, 11 mmol of 3a
was pumped continuously for 12 hrs with a flow rate of 0.1
mL/min to afford 10.43 mmol of product 4a. The progress of
product 4a formation was monitored by using 1H-NMR which
clearly indicates that the present catalyst is highly efficient and
productive.
Figure 7. Recyclability of MnO2@Fe3O4 MNP for the synthesis of phenyl 4-
methylbenzoate (2b)
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(CAM) stain followed by heating. The iron (III) chloride (product number:
44939) was purchased from Sigma Aldrich. All the experiments were
carried out without maintaining the inert condition. The flow chemistry
experiments were carried on Holmarc syringe pump (Model no.-HO-
SPLF-2D). 1H and 13C NMR spectra were recorded on 400 and 100
MHz, respectively, using
a Bruker 400 MHz or JEOL 400 MHz
spectrometers. Abbreviations used in the NMR follow-up experiments: b,
broad; s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet. High-
resolution mass spectra were recorded with Waters-synapt G2 using
electrospray ionization (ESI-TOF). Fourier-transform infrared (FTIR)
spectra were obtained with a Bruker Alpha-E Fourier transform infrared
spectrometer. Powder X-ray diffraction (PXRD)patterns were measured
on Bruker D8 Advanced X-ray diffractometer at room temperature using
Cu Kαradiation (λ= 1.5406 Å) with a scan speed of 0.5° min−1 and a step
size of 0.01° in 2θ. BET was recorded on Quantachrome Instruments.
Thermogravimetric analysis was recorded on a Perkin Elmer STA 6000,
TGA analyzer under air atmosphere with a heating rate of 10 °C/min. The
Hi-Resolution Transmission Electron Microscopy (HRTEM) imaging was
performed using Jeol JEM2200FS (200 kV) HRTEM instrument. The
XPS was collected using Thermo Scientific Kalpha+ spectrometer using
a monochromated Al Kalpha (1486.6 eV) source. The base pressure of
the spectrometer was always better than 5x 10-9 mbar.The electron flood
gun was on during acquisition for charge neutralization. The wide area
spectrum was collected using 200 eV pass energy and individual core-
levels at 50 eV.
Figure 8. Recyclability of MnO2@Fe3O4 MNP for the synthesis of 9H-xanthen-
9-one (4b)
Hot filtration test
In order to check the heterogeneity of the catalyst, a hot filtration
test was performed for the benzylic sp3 C-H oxidation of 1a to
investigate whether the reaction proceeded in a heterogeneous
or a homogeneous approach. After continuing the reaction for 16
h, the catalyst was separated by magnetic retriever and found
that 60% of the product 2a. No further enhancement in the
product 2a formation was observed after the separation of the
MNP catalyst. Moreover, Microwave Plasma Atomic Emission
Spectroscopy measurement of the filtrate showed the absence
of Mn in the supernatant solution of the reaction mixture. This
study clearly shows that Mn was intact to a great extent with the
heterogeneous support and no leaching occurred during the
catalytic reaction.
1. General procedure for the synthesis MnO2@Fe3O4 MNP catalyst
A mixture of FeCl3.6H2O (4.32 g, 16 mmol) and FeCl2.4H2O (1.60 g, 8
mmol) was dissolved in 40 mL deionized water. The resultant solution
was left to be stirred for 30 min at 80° C. Then ammonia solution (25%
(w/w)) was added in a drop-wise manner over 5 min to the stirring
mixture to maintain the reaction pH about 11. The resulting black
dispersion was stirred vigorously for 1h at room temperature and then
was refluxed for 1h. The black magnetite Fe3O4 nanoparticles were
isolated by magnetic decantation, washed several times with deionized
water and then dried at 80 °C for 4 h. To introduce reactive Mn on the
surface of the magnetic nanoparticle (MNP), 0.6 g of dried Fe3O4
nanoparticles were suspended in a mixture of 50 mL ethanol and then,
0.6 g of Mn(OAc)3.2H2O was ultrasonically dispersed. After complete
dissolution and dispersion, the nanoparticles were separated from the
ethanol solution by magnetic decantation and dried at 80 °C for 4h.
MnO2@Fe3O4 magnetic nanoparticles were obtained by drop-wise
addition of aqueous ammonia (25% (w/w)) to the dried brown
nanoparticles under vigorous stirring. Finally, the MnO2@Fe3O4 MNP
were magnetically separated, washed with water, and dried in an oven at
100 oC for overnight.
Conclusions
In summary, we demonstrated that MnO2@Fe3O4 MNP is an
efficient heterogeneous catalyst for the direct benzylic sp3 C-H
oxidation of ethers by using TBHP as an oxidant to afford the
ester in higher yield. This process was also applicable to
benzylic sp3 C-H oxidation of methylene compounds to furnish
ketone derivatives under batch and continuous flow module in
higher yield. The main advantages of this procedure are mild
reaction conditions, a demonstration in batch as well as in
continuous flow module, scalable synthesis with high TOF
values, easy and quick isolation of products, and recyclable of
catalyst for more than 12 cycles. Hence, we believe that the
present catalyst will find a wide range of applications in organic
synthesis as well as in industry.
2. General procedure for the synthesis of the esters from sp3-CH
oxidation of (benzyloxy)benzene derivatives in batch
In a 20 mL glass seal tube, catalyst (25 mg), (benzyloxy)benzene
derivatives (0.5 mmol, 1 equiv) in ACN (2 mL) were added TBHP (5-6M
in decane, 3.5 mmol, 7 equiv), 0.05 mmol of 2,2′-bipyridine and the tube
was sealed by using a crimper. The mixture was stirred at 80 °C for 24
hrs. After completion, the reaction mixture was allowed to cool to room
temperature. It was then diluted with EtOAc and the catalyst was
separated with an external magnet and washing twice with EtOAc. The
mixture was extracted with EtOAc, the volatiles were removed under
reduced pressure, and the crude product was purified by column
chromatography to afford the ester products. All of the esters were
identified by spectral comparison with literature data.
Experimental Section
Materials and characterization: All the chemicals are purchased Sigma
Aldrich or Alfa-Aesar. Deuterated solvents were used as received. All the
solvents used were dry grade. Column chromatographic separations
performed over 100−200 Silica-gel. Visualization was accomplished with
UV light and phosphomolybdic acid (PMA), Ceric ammonium molybdate
3. General procedure for the synthesis of the esters from sp3-CH
oxidation of (benzyloxy)benzene derivatives in a continuous flow
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0.05 M solution of the substrate and 0.35 M of 5.0-6.0 M TBHP in decane
and 0.1 mmol of 2,2′-bipyridine in 20 ml of ACN solvent was pumped
through syringe pump packed with 1.5 g of 0.424% MnO2@Fe3O4 (up to
5cm) is heated at 80 °C with the flow rate of 0.1 mL min-1. A 3.5 to 3.8
psi back pressure regulator was placed after the reactor. The collected
organic layer was concentrated under reduced pressure and the crude
product was purified by column chromatography using ethyl acetate:
hexane as an eluent to afford the ketone product.
This research was supported by the Council of Scientific and
Industrial Research [02(0296/17/EMR-II)], India. A. P. thanks,
IISER-Pune for the research support. S.G.A. thanks SERB-
NPDF (PDF/2017/001286) for the research fellowship. B. G.
thanks CSIR for the research support.
Keywords: MnO2@Fe3O4 MNP • Benzylic sp3 C-H Oxidation •
TBHP • Continuous flow • Carbonyl Compounds
4. General procedure for oxidation of benzylic sp3 C-H group of
methylene derivatives to the ketone in batch
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removed under reduced pressure, and the crude product was purified by
column chromatography to afford the desired product.
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magnet and washed with acetonitrile and ethyl acetate for three times,
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Acknowledgements
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Akanksha M. Pandey, Sandip G.
Agalave, Chathakudath P. Vinod, and
Boopathy Gnanaprakasam*
Page No. – Page No.
MnO2@Fe3O4 MNP: An Efficient and
Recyclable Heterogeneous Catalyst
for Benzylic sp3 C-H Oxidation
Highly chemoselective and efficient heterogeneous MnO2@Fe3O4 MNP catalyst for the oxidation of benzylic sp3 C-H group to afford
ester and ketone derivatives in high yield under batch and continuous flow module was reported.
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