Quinoline-2-carboimine copper complex immobilized on amine functionalized silica coated magnetite nanoparticles: A novel and magnetically retrievable catalyst for the synthesis of carbamates via C-H activation of formamides

Article abstract of DOI:10.1039/c4dt03236e

In the present study, we report the synthesis of a highly efficient and magnetically retrievable catalytic system (Cu-2QC@Am-SiO₂@Fe₃O₄) through the covalent immobilization of quinoline-2-carboxaldehyde (2QC) on an amine functionalized silica coated ferrite nanosupport followed by metallation with copper acetate. The structure of the organic-inorganic hybrid nanomaterial has been confirmed using various physicochemical techniques such as Powder X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-Ray Spectroscopy (EDS), Energy Dispersive X-Ray Fluorescence Spectroscopy (ED-XRF), Atomic Absorption Spectroscopy (AAS), Inductively Coupled Plasma Spectroscopy (ICP) and Vibrating Sample Magnetometry (VSM). The resulting nanocatalyst exhibits a remarkable catalytic efficacy in the synthesis of industrially and pharmaceutically significant carbamates via the C-H activation of formamides under solvent free conditions. The most important attribute of the present methodology is that the catalyst can be recovered simply through an external magnetic force and reused several times without any significant deterioration in its activity. Furthermore, the heterogeneity test has been carried out in order to ensure the intrinsic stability of the nanostructured catalyst. The activity of the Cu-2QC@Am-SiO₂@Fe₃O₄ nanocatalyst has been found to be far more superior in comparison with the literature precedents in terms of the product yield, cost and reusability of the catalyst. Besides, ambient reaction conditions, simple workup procedure, wide substrate scope and cost effectiveness are some of the other outstanding features of this protocol that make it economical and sustainable.

Full text of DOI:10.1039/c4dt03236e

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PAPER
Quinoline-2-carboimine copper complex immobilized on amine
functionalized silica coated magnetite nanoparticles: A novel and
magnetically retrievable catalyst for the synthesis of carbamates via C-H
activation of formamides
5
R. K. Sharma*, Sriparna Dutta and Shivani Sharma
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
In the present study, we report the synthesis of a highly efficient and magnetically retrievable catalytic
system (Cu-2QC@Am-SiO2@Fe3O4) through the covalent immobilization of quinoline-2-carboxaldehyde
10 (2QC) on amine functionalized silica coated ferrite nanosupport followed by metallation with copper
acetate. The structure of the organic-inorganic hybrid nanomaterial has been confirmed using various
physicochemical techniques such as Powder X-Ray Diffraction (XRD), Fourier Transform Infrared
Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM),
Energy Dispersive X-Ray Spectroscopy (EDS), Energy Dispersive X-Ray Fluorescence Spectroscopy
15 (ED-XRF), Atomic Absorption Spectroscopy (AAS), Inductively Coupled Plasma Spectroscopy (ICP)
and Vibrating Sample Magnetometry (VSM). The resultant nanocatalyst exhibits remarkable catalytic
efficacy in the synthesis of industrially and pharmaceutically significant carbamates via the C-H
activation of formamides under solvent free conditions. The most important attribute of the present
methodology is that the catalyst can be recovered simply through an external magnetic force and reused
20 several times without any significant deterioration in its activity. Further, the heterogeneity test has been
carried out in order to ensure the intrinsic stability of the nanostructured catalyst. The activity of the Cu-
2QC@Am-SiO2@Fe3O4 nanocatalyst has been found to be far more superior in comparison to the
literature precedents in terms of product yield, cost and reusability of the catalyst. Besides, ambient
reaction conditions, simple workup procedure, wide substrate scope and cost effectiveness are some of
25 the other outstanding features of this protocol that make it economic and sustainable.
45 atom economy and the preparation of prefunctionalized starting
Introduction
materials necessitate the design of better synthetic methodologies.
Recently, the direct activation of C-H bonds has emerged as a
powerfool tool in streamlining organic synthesis by saving steps,
time and material compared to the other methods that require the
50 introduction and removal of activating or directing groups.8
Inspired by the efficiency of protocols that are based on direct C-
H activations, Reddy et al. for the first time reported a cross
dehydrogenative coupling pathway for obtaining carbamates
through the C-H activation of formamides under oxidative
55 conditions using copper (II) acetate.9 Subsequently, a plethora of
copper derived catalysts have been reported for this
transformation,10 however most of them are homogeneous and
therefore not only cause catalyst separation and recycling
problems, but also inevitably lead to large amounts of metal
60 waste which limit their practical applications on an industrial
scale. Consequently, there is an urgent need to develop other
alternative recyclable catalysts that can surmount the challenges
of homogeneous catalysis for the generation of a library of
carbamates. The immobilization of the existing homogeneous
Organic carbamates have garnered a considerable amount of
attention in synthetic organic chemistry due to their widespread
applications in pharmaceutical, polymer and agrochemical
30 industries as useful drug intermediates, important starting
materials, pesticides etc.1 A large number of organic compounds
containing carbamate moieties have been developed into
marketed drugs in order to treat chronic disorders such as
arrhythmias, asthma, seizures and AIDS due to their bioactive
35 potentials.2 The traditional route involved in the synthesis of
carbamates is based on the incorporation of chloroformates or
isocyanates with phosgene substitutes as starting materials that
are highly toxic because of which they pose deleterious health
and safety threats.3 In order to overcome the issues associated
40 with the harmful phosgene chemistry, some other alternative
strategies have been developed which include the use of carbon
dioxide,4 carbonylation of amines and nitroaromatics,5 transition
metal catalyzed cross coupling reactions,6 rearrangement
reactions7 and many more. But, factors such as high cost, poor
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complexes onto a variety of solid supports has provided an
as such without further purification.
attractive solution to address these problems.11 These solid
supported heterogeneous catalysts not only possess benefits like
ease of product separation, recyclability of catalyst but also
display improved efficiency due to stable active site and better
steric control of the reaction intermediate that makes them an
interesting alternative to the conventional homogeneous and
heterogeneous counterparts. However, the choice of a support
material is extremely important in controlling the catalytic
Instrumentation
Fourier transform infrared (FT-IR) spectra were acquired through
60 a Perkin Elmer Spectrum 2000 FT-IR spectrometer using the KBr
pellet method that was operative in the range of 4000-400 cm-1
under atmospheric conditions with a resolution of 1 cm-1.
Scanning electron microscopy (SEM) images were obtained in
order to derive information about the size, shape and morphology
65 of the nanoparticles using a Tescan Mira 3 FESEM instrument.
The preparative step involved placing a small amount of sample
on a carbon tape and then coating it with a thin layer of gold
using a sputter coater. Energy-dispersive X-ray spectroscopic
(EDS) analysis (equipped with the SEM instrument) was done for
70 the elemental mapping of the nanocomposites. Besides SEM,
Transmission electron microscopy (TEM) was also carried out
using the CM-10, Philips electron microscope. The technique was
utilized in determining the morphology and diameter of the
nanoparticles. Powdered X-ray diffraction (XRD) was performed
75 using a Bruker, D8 Advance (Karlsruhe, bundesland, Germany)
diffractometer equipped with Cu/Kα radiation at a scanning rate
of 4° min-1 in the 2θ range of 5-80° (λ = 0.15405 nm, 40 kV, 40
mA). Anton Paar multiwave 3000 microwave instrument
equipped with temperature and pressure sensor was used for the
80 microwave assisted digestion of the nanocatalyst. Energy
dispersive X-ray fluorescence (ED-XRF) spectroscopy was
conducted using a Fischerscope X-Ray XAN-FAD BC. The
copper content in the catalyst and in the supernatant was
estimated by flame atomic absorption spectroscopy (FAAS) on a
85 LABINDIA AA 7000 atomic absorption spectrometer using an
acetylene flame. The optimum parameters for the copper
measurements are: wavelength = 324.7 nm; lamp current = 2 mA;
slit width = 0.2 nm; fuel flow rate = 0.2 L min-1. Inductively
coupled mass spectroscopy (ICP-MS) was also done in order to
90 confirm the copper loading of the catalyst using ICP-MS (Model
No. 7700e). The nanoparticle aggregates were dispersed in
ethanol using a sonicator (Tekmar Sonic Disruptor model
TM300). A vibrating sample magnetometer (EV-9, Microsense,
ADE) was utilized for obtaining the magnetization curves. The
95 derived products were analyzed through the GC-MS hyphenated
technique that was conducted utilizing the Agilent gas
chromatograph (6850 GC) with a HP-5MS 5% phenyl methyl
siloxane capillary column (30.0 m × 0.25 mm × 0.25 mm) and a
quadrupole mass filter equipped 5975 mass selective detector
100 (MSD) using helium as the carrier gas.
5
10 activity as it can help in holding the active site on its surface
through strong co-ordination during catalysis.
In this context, magnetic nanoparticles have emerged as excellent
and promising solid support material as they are robust, inert,
readily available, non toxic and chemically stable.12 In
15 comparison to the bulk supports, these nanosized particles offer a
large number of potential active sites for the reactants as a result
of large surface area to volume ratio which eventually results in
higher yields. Besides, their magnetic properties allow ready
separation from the reaction mixture via the aid of an external
20 magnet that eliminates the necessity of cumbersome filtration and
centrifugation techniques. The magnetic field-assisted separation
can reduce energy consumption, catalyst loss and save the time in
achieving catalyst recovery. However, despite the large number
of advantages offered by the magnetic nanocatalysts, they show a
25 strong tendency to agglomerate that deteriorates their efficiency.
Recent studies have demonstrated that the coating of the
magnetic core with a layer of amorphous silica around it can
resolve this problem.13 The silica encapsulation works as an
excellent protection strategy that not only prevents the clustering
30 of the magnetic nanoparticles, but also provides a chemically
inert surface for the chemical modification, which renders them
suitable for catalytic applications.14
Engaged in the development of greener and sustainable pathways
for silica based organic-inorganic hybrid nanomaterials and their
35 applications in the field of metal scavengers, sensors and catalysis
for various organic transformations,15 herein we elucidate the
fabrication, characterization and application of magnetic silica
based copper nanocatalyst (Cu-2QC@Am-SiO2@Fe3O4) for the
synthesis of carbamates via the C-H activation of formamides.
40 The newly fabricated magnetic copper nanocatalyst exhibits
remarkable catalytic activity for obtaining a wide range of enol
and O-(2-carbonylphenyl) carbamates. The excellent durability of
the nanostructured catalyst along with its facile separation using
an external magnet make the present catalytic system superior to
45 all the previously reported ones. To the best of our knowledge,
this is the first report wherein a magnetically recoverable
nanocatalyst has been designed for the synthesis of carbamates.
Catalyst Preparation
Synthesis of amine functionalized silica coated magnetite
nanoparticles (Am-SiO2@Fe3O4)
Magnetite (Fe3O4) nanoparticles were synthesized using the co-
Experimental
105 precipitation technique as previously described in literature.16 In a
typical synthesis, ferric sulphate (6.0 g) and ferrous sulphate (4.2
g) salts were dissolved in double deionized water (250 mL) and
stirred at 60 °C till the solution turned yellow orange. Next, 25%
NH4OH (15 mL) was added dropwise into it to adjust the pH of
110 the solution to 10 and the stirring was continued vigorously for an
hour. The crystalline black precipitate of Fe3O4 nanoparticles
were isolated simply via an external magnetic force, washed
thoroughly with water and ethanol several times until the pH
Materials and Reagents
50 Tetra-ethyl
orthosilicate
(TEOS)
(99.9%),
3-
aminopropyltriethoxysilane (APTES) (98%) and quinoline-2-
carboxaldehyde (2QC) (98%) were procured from Sigma Aldrich,
Fluka and Alfa Aesar respectively. Copper acetate, ferrous
sulphate and ferric sulphate were commercially acquired from
55 Sisco Research Laboratory (SRL). All other starting materials
and reagents were purchased from Spectrochem Pvt Ltd. and used
2
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reached 7 and dried under vacuum for 3 h. Thereafter, the silica
SiO2@Fe3O4 nanocatalyst (Scheme 1).
General procedure for the synthesis of carbamates via the C-
H activation of formamides
A mixture of 2-carbonyl substituted phenol/ β-ketoester (0.5
encapsulated magnetite nanoparticles (SiO2@Fe3O4) were
synthesized using the sol-gel method by the ultrasonic dispersion
of the black precipitate of Fe3O4 (1.0 g) in ethanol (200 mL)
followed by the addition of conc. NH3·H2O (6.0 mL) and TEOS
(2.0 mL) successively. The mixture was then incessantly stirred
for a period of about 24 h at room temperature. The brownish
black precipitate of SiO2@Fe3O4 nanocomposites were separated
magnetically, washed with ethanol and dried under vacuum.17
35 mmol) and formamide (25 mmol) was reacted in the presence of
TBHP (6.0 equiv) as oxidant and Cu-2QC@Am-SiO2@Fe3O4
nanocatalyst (30 mg) under solvent free conditions at 70oC in a
round bottomed flask. After the completion of the reaction (as
monitored by TLC, ethyl acetate-hexane, 1:1), the reaction
40 mixture was allowed to cool to room temperature and the catalyst
was separated magnetically using a bar magnet. Finally, the
resultant solution was extracted with ethyl acetate and dried over
anhydrous Na2SO4. The products were thereafter analyzed and
confirmed by Gas Chromatography Mass Spectroscopic
45 technique (GC-MS).
5
10 Finally, the surface modification of the SiO2@Fe3O4
nanoparticles was accomplished through the introduction of the
amine groups. For obtaining the amine functionalized silica
coated magnetite (Am-SiO2@Fe3O4) nanoparticles, APTES (5.0
mL) was added to a well dispersed solution of SiO2@Fe3O4 (1.0
15 g) in ethanol (200 mL). The resultant mixture was stirred
continuously for 24 h at room temperature. The Am-SiO2@Fe3O4
nanocomposites were separated via magnetic forces, washed
several times with ethanol to eliminate the unreacted silylating
Results and Discussions
Catalyst Characterization
agent and finally dried under vacuum.18
20 Synthesis of the quinoline-2-carboimine immobilized on
amine functionalized silica coated magnetite nanoparticles
(Cu-2QC@Am-SiO2@Fe3O4)
FTIR Spectroscopy
FTIR spectroscopic analysis was done in order to confirm the
50 structures of the Fe3O4, SiO2@Fe3O4, Am-SiO2@Fe3O4,
The final synthesis of the Cu-2QC@Am-SiO2@Fe3O4
2QC@Am-SiO2@Fe3O4
and
Cu-2QC@Am-SiO2@Fe3O4
nanocatalyst was carried out in two steps. Firstly, a mixture of
25 Am-SiO2@Fe3O4 (2.0 g) and 2QC (4.0 mmol) was stirred in 250
mL ethanol solution at room temperature for 24 h.19 Thereafter,
nanoparticles via the identification of appropriate functional
groups (Fig. S1, ESI†). The FT-IR spectrum of Fe3O4 depicts an
intense absorption band at 588 cm-1 that is assigned to the typical
2QC@Am-SiO2@Fe3O4 (1.0 g) was added to the solution 55 Fe-O vibrations of the magnetite structure while a broad band at
containing 1.5 mmol of copper acetate for 9 h. The resultant
nanocomposites were removed magnetically, washed thoroughly
30 with ethanol and dried well to generate Cu-2QC@Am-
314 cm-1 is accredited to the presence of surface OH groups.20 On
the other hand, the spectrum of SiO2@Fe3O4 provides a strong
Ethanol
Ethanol
r.t., 24 h
Fe3O4
Conc NH3.H2O
APTES
Fe3O4
Fe3O4
Fe3O4
Fe3O4
Ethanol
r.t., 24 h
TEOS
r.t., 24 h
O
N
SiO2@Fe3O4
Am-SiO2@Fe3O4
2QC@Am-SiO2@Fe3O4
Cu(OAc)2.H2O
Quinoline-2-carboxaldehyde
(2 QC)
Acetone
Stirring
9 h
O
CH
O
Si
N
Fe3O4
Fe3O4
N
O
Cu
AcO
OAc
Magnetic silica based organic-inorganic hybridcopper nanocatalyst
Cu-2QC@Am-SiO2@Fe3O4
Scheme 1 Schematic description for the synthesis of Cu-2QC@Am-SiO2@Fe3O4 nanocatalyst
60
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evidence of the occurence of silica around the magnetic core as 45 peak between 2ϴ = 20-23o in the XRD pattern of SiO2@Fe3O4
corroborated by the appearance of Si-O-Si symmetric, Si-O
symmetric and Si-O-Si asymmetric stretching modes at 800, 960
and 1099 cm-1 respectively.14c Another distinguishable aspect that
clearly demarcates the bare Fe3O4 from the silica coated Fe3O4
nanocomposites can be accredited to the presence of an
amorphous silica shell around the magnetic core which was also
confirmed by the TEM analysis.17Also, the characteristic bragg’s
peaks are obtained at the same 2ϴ values which indicate that the
5
nanocomposites is the considerable reduction in the intensities of 50 embedded Fe3O4 retains its crystalline structure even after SiO2
the Fe-O and O-H stretching and bending vibrations. Further, on
subsequently moving towards the APTES grafted SiO2@Fe3O4,
coating.
TEM and SEM analysis
10 two new peaks are observed at 2926 cm-1 and 1632 cm-1 that are
attributed to the CH2 and the NH2 groups of the amino-propyl
moiety respectively. The characteristic C=N band at 1642 cm-1
authenticates the immobilization of the ligand over the amine
functionalized magnetite nanoparticles. Notably, a shift in this
15 value is observed in the final Cu-2QC@Am-SiO2@Fe3O4
nanocatalyst suggesting the formation of a co-ordinative linkage
between the metal and the nitrogen atom of the ligand. These
results signify that the copper species effectively anchor onto the
surface of the modified magnetite nanoparticles as a 2QC-copper
20 complex.
The TEM micrograph of the native magnetite nanoparticles
reveals a roughly spherical morphology with an average size
55 lying between 9-12 nm. The presence of the silica coat around the
magnetic core can be visualized through the TEM image of
SiO2@Fe3O4 (Fig. S2, ESI†). It can be seen that the core is
embedded deep within the shell which is an important feature of a
good core shell nanocatalyst. Further, from the TEM image of the
60 recovered nanocatalyst it can be interpreted that the structural
morphology of the nanocatalyst remains unchanged even after 8
consecutive cycles, demonstrating its high catalytic efficacy (Fig.
S2, ESI†). Besides TEM, the Scanning electron microscopic
(SEM) analysis also reveals a considerable amount of information
Powder X-Ray Diffraction Analysis
XRD analysis of Fe3O4 and SiO2@Fe3O4 nanoparticles was done 65 about a sample’s surface topography and composition. The SEM
in order to deduce information about the about the
crystallographic structure, chemical composition and physical
25 properties of the nanocomposites (Fig. 1). The characteristic
Bragg’s diffraction peaks at 30.1o, 35.4o, 43.1o, 53.4o, 57o and
images of Fe3O4 and Cu-2QC@Am-SiO2@Fe3O4 display a
roughly spherical morphology (Fig. 2). The surface roughening of
the final nanocatalyst is apparent as a result of deposition of silica
shell around the magnetic core. Another interesting aspect that
62.6o can be assigned to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 70 needs to be highlighted is that no separate aggregates of silica are
1) and (4 4 0) crystal planes that match well with the database of
magnetite in the Joint Committee on Powder Diffraction
30 Standards [JCPDS] (JCPDS card: 19-629 and reflect the cubic
spinel structure of the magnetite nanoparticles. No other
characteristic peak is observed indicating the high phase purity of
these crystalline nanoparticles. The average crystallite size has
observed in the SEM image of the Cu-2QC@Am-SiO2@Fe3O4
nanocatalyst that suggests that the precipitation of the primary
silica nanoparticles occurs on the surface of the magnetic core.
Compositional analysis
75 The compositional analysis of the nanocomposites was
accomplished using EDS, AAS, ED-XRF, and ICP techniques.
The EDS spectra of the SiO2@Fe3O4 and Cu-2QC@Am-
Fe3O4@SiO2 nanocomposites have been depicted in Fig. 3. The
well defined peaks of iron, oxygen and silica in the EDS
80 spectrum of SiO2@Fe3O4 confirm the presence of silica around
the magnetic core. Besides these three key elements, the peaks of
nitrogen, carbon and copper substantiate the incorporation of the
amine groups, ligand and the metal on the surface of the silica
(b)SiO2@Fe3O4
(a)
Fe3O4
10
20
30
40
50
60
70
80
2 (degree)
35
Fig. 1 Powdered XRD curves of (a) Fe3O4 and (b) SiO2@Fe3O4
been determined using the Debye Scherrer equation (Dhkl = kλl/β
cos ϴ) where D is the size of the axis parallel to the (hkl) plane, k
is a constant with a typical value of 0.89 for spherical particles, λ
is the wavelength of radiation, β is the full width at half maxima
40 in radians and ϴ is the position of diffraction maxima of the peak
with the highest intensity from the XRD pattern of the Fe3O4
nanoparticles. The (311) peak is chosen as the reference peak and
the mean crystalline size is calculated to be 11.9 nm according to
the above equation. Further, the appearance of a strong broad
85 Fig. 3 EDX spectra of (a) SiO2@Fe3O4 and (b) Cu-2QC@Am-SiO2@Fe3O4
4
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Dalton Transactions
Fig. 2 TEM images of (a) Fe3O4 (b) Cu-2QC@Am-SiO2@Fe3O4 and SEM images of (c) Fe3O4 (d) Cu-2QC@Am-SiO2@Fe3O4
coated ferrite support of Cu-2QC@Am-SiO2@Fe3O4. Further, the 30 the application of an external magnetic force. Besides, an
5
presence of copper in the final nanocatalyst is also confirmed by
the ED-XRF spectrum of Cu-2QC@Am-SiO2@Fe3O4 (Fig. S3,
ESI†). Finally, in order to quantify the amount of copper in the
nanocatalyst, it was subjected to AAS determination. The
corresponding copper loading was found to be 0.132 mmol g-1. In
important evidence for the superparamagnetic nature of these
nanoparticles is provided by the inset that shows negligible
coercivity and remanence as both the magnetization as well as
demagnetization curves pass through origin.
80
10 addition, to support the above observation, the sample was also
analysed through ICP and it was found that the copper content
matched well with the AAS result.
40
30
60
40
(d)
20
10
Vibrating Sample Magnetometry (VSM)
0
-10
-20
-30
-40
Magnetization behaviour of the Fe3O4, Fe3O4@SiO2 and Cu-
15 2QC@Am-SiO2@Fe3O4 nanoparticles under the applied
magnetic field has been depicted in Fig. 4. The curves exhibit an
extremely interesting phenomenon showing a decrease in the
values of the saturation magnetization (Ms) from the Fe3O4
nanoparticles (60.5 emu/g) to Fe3O4@SiO2 (49.5 emu/g) to the
20 final Cu-2QC@Am-SiO2@Fe3O4 nanocatalyst (29.2 emu/g)
which can be attributed to the presence of silica in the core–shell
structured materials. Further, in comparison to the bulk magnetite
nanomaterials that typically show a saturation magnetization
value of 92 emu/g, the Ms value of the Fe3O4 nanoparticles was
25 found to be much lower. It could be reasoned that magnetization
of a magnetic particle in an external field is a function of its size
that causes a reduction in the Ms value.21 However, the net
magnetism exhibited by the final nanocatalyst is sufficiently good
for an effective separation from the solution medium through
20
0
-400
-200
0
200
400
(a)
(b)
(c)
-20
-40
-60
-80
-30000
-20000
-10000
0
10000
20000
30000
Magnetic Field (Oe)
35
Fig. 4 Magnetization curves obtained by VSM at room temperature for
(a) Fe3O4 (b) SiO2@Fe3O4 (c) Cu-2QC@Am-SiO2@Fe3O4 (d) inset: enlarged
image near the coercive field.
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DOI: 10.1039/C4DT03236E
Catalytic Activity Tests
60
The catalytic efficacy of the newly fabricated Cu-2QC@Am-
SiO2@Fe3O4 nanocatalyst was investigated in the synthesis of
carbamates via the C-H activation of formamides by
100
5
comprehensively
choosing
salicylaldehyde
and
N,N-
Dimethylformamide as the test substrates (model reaction). The
effect of various important parameters such as amount of catalyst,
time, temperature, solvent, oxidant and oxidant concentration
were investigated thoroughly in order to obtain an optimum
80
60
10 reaction profile for the cross dehydrogenative coupling reaction.
Screening of various catalysts
40
20
The catalytic screening was done by performing the model
reaction in the presence of a series of copper based catalysts.
Initially, a control experiment was conducted for the considered
15 reaction in the absence of any catalyst. It was observed that no
product formation occurred in this case signifying the need for a
catalyst to afford the synthesis of carbamates via the cross
dehydrogenative coupling pathway. Further, the results revealed
that the presence of a copper based source is extremely
20 imperative to catalyze the cross coupling between carbonyl
moiety and N,N-Dimethylformamide (Table S1, ESI†). The
highest conversion percentage was obtained in case of the Cu-
2QC@Am-SiO2@Fe3O4 nanocatalyst suggesting the efficiency of
the nanostructured copper based magnetite nanocomposites in the
25 cross dehydrogenative coupling reaction.
0
80
70
15
o
C)
60
12
50
ture (
9
T
ime (
40
6
min)
30
3
Tempera
Fig. 5 Effect of time and temperature on the synthesis of carbamates via
C-H activation of formamides [Reaction conditions: Salicylaldehyde (0.5
mmol), N, N- Dimethylformamide (25 mmol), catalyst (30 mg), TBHP (70
wt% in water, 6.0 equiv)]
65
reaction. In order to obtain the best catalytic conversion, the
effect of different solvents on the cross dehydrogenative coupling
of salicylaldehyde and N,N-Dimethylformamide was tested.
Besides employing a wide variety of solvents such as chloroform,
70 methanol, acetonitrile, dimethyl sulphoxide (DMSO) and water in
a 1:1 ratio with DMF, the reaction was also carried out under
solvent free conditions since from the perspective of green
chemistry, a solvent free organic synthesis results in a drastic
reduction in generation of wastes. It was found that the reaction
75 mixture that involved no additional solvent underwent the
maximum conversion (Fig. 6), highlighting the essence of a
green-“solvent free protocol.” Therefore, further catalytic studies
were carried out in absence of any solvent.
Effect of the variation in the catalytic amounts
After establishing the vital importance of the Cu-2QC@Am-
SiO2@Fe3O4 nanocatalyst in the synthesis of carbamates, the
effect of variation in the amount of the catalyst was tested for the
30 cross dehydrogenative coupling reaction by performing the model
reaction. The results displayed a significant increase in the
conversion percentage on increasing the amount of the catalyst
upto 30 mg due to an increase in the number of potential active
sites. However, a further increase in the catalytic amount did not
35 result in any considerable increase in the conversion (Fig. S4,
ESI†). The reason could be accredited to the exhaustion of all the
active sites present on the surface of the catalyst. The finest
results were obtained with the use of 30 mg of the nanocatalyst
since it resulted in the maximum conversion and hence for the
40 rest of the experiments the amount of the catalyst was fixed to 30
mg.
No solvent
DMSO
Effect of time and temperature
Time and temperature play a crucial role in affecting the kinetics
of a reaction significantly. Hence, in order to examine the effect
45 of these two parameters, the model reaction was performed under
Methanol
Chloroform
Acetonitrile
Water
o
a diverse range of temperatures (30 to 80 C) for different time
periods (3 to 15 minutes). The conversion percentages as a
function of time and temperature have been represented (Fig. 5).
Detailed examination of the results revealed that on raising the
50 temperature, an enhancement in the conversion percentages
occured and the best results were obtained at 70oC when the
reaction was continued for 12 minutes. Above this temperature a
noticeable decrease in the conversion of salicylaldehyde could be
attributed to the self decomposition of the oxidant TBHP.
55 Effect of solvents
Solvents drastically affect various physical and chemical aspects
of a reaction such as solubility, stability and the reactivity rates.
Therefore, the choice of an appropriate solvent is imperative as it
can allow thermodynamic and kinetic control over a chemical
0
20
40
60
80
100
Conversion (%)
80 Fig. 6 Effect of various solvents on the the synthesis of carbamates via C-
H activation of formamides [Reaction conditions: Salicylaldehyde (0.5
mmol), N, N-Dimethylformamide (25 mmol), catalyst (30 mg), TBHP (70
wt% in water, 6.0 equiv), reflux at 70 oC]
85
6
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Dalton Transactions
Effect of different oxidants
Cross dehydrogenative coupling (CDC) protocols require the
incorporation of stoichiometric amounts of strong oxidants for
high catalytic turn over numbers. Considering the critical role of
oxidants in such reactions, the oxidizing capability of various
oxidants was evaluated in the synthesis of carbamates (Table S2,
ESI†). The use of H2O2 resulted in trace amounts of salicylic
acid, whereas the rest of the oxidants like m-CPBA, I2 and
benzoquinone failed to produce the desired carbamates. The
5
10 success of TBHP as the oxidant could be evaluated from the
highest conversion percentage.
Effect of oxidant concentration
Further, the effect of concentration of TBHP was examined by
varying the equivalents of it from one to seven. It was observed
15 that initially the conversion percentage increased on increasing
the amount of the oxidant and six equivalents were sufficient
enough to result in the complete conversion of the reactants (Fig.
7). Thus, for the rest of the catalytic experiments, the amount of
the oxidant was fixed to 6 equivalents.
20 Synthesis of carbamates via C-H activation of formamides
using Cu-2QC@Am-Si@Fe3O4 nanocatalyst
Fig. 7 Effect of oxidant concentration on the synthesis of carbamates via
the C-H activation of formamides [Reaction conditions: Salicylaldehyde
(0.5 mmol), N, N-Dimethylformamide (25 mmol), catalyst (30 mg), TBHP
(70% in water), reflux at 70oC].
30
substituted phenol carbamates and enol carbamates were obtained
with very high turnover numbers (TONs). No profound effect on
the conversion of the reactants due to the presence of electron
withdrawing or electron donating group on either the β-keto
35 esters or 2-carbonyl substituted phenols was observed, as all these
substrates underwent the reaction smoothly, displaying the broad
functional group tolerance and versatility of the protocol.
However, the time required for the completion of
To evaluate the versatility of this methodology, series of 2-
carbonyl substituted phenols and β-ketoesters were reacted with
N,N substituted formamides under the optimized reaction
25 conditions (Table 1). In general, the desired 2-carbonyl
OH
O
Fe3O4
H
TBHP HOAc
+
Cu(OAc)2
(i)
HOAc
(iv)
R
Fe3O4
Cu(OAc)
O
N
Fe3O4
R
Mechanism
O
Cu
O
H
O
H
OAc
(iii)
O
Complex A
O
Fe3O4
Cu
(ii)
Carbamate
O
OAc
O
H
R
N
R
R
H
R
N
R
R
TBHP
N
Complex B
O
O
40
Scheme 2 Proposed mechanism for the synthesis of carbamates via C-H activation of formamides
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7

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DOI: 10.1039/C4DT03236E
Table 1 Cu-2QC@Am-SiO2@Fe3O4 catalyzed synthesis of carbamates via C-H activation of formamidesa
N
R
Fe3O4
OH
O
O
O
O
N
O
R
H
TBHP
70oC
Solvent free
Carbamate
2 carbonyl susbtituted phenol/
N,N substituted
formamide
-ketoester
Entry
N,N substituted
Formamide
Phenol/ β-ketoester
Time (min)
Conversion (%)b
TONc
379
OH
O
H
H
N
1.
12
100
O
OH
O
H
N
2.
3.
15
30
98
92
371
349
CH3
O
OH
O
H
N
CH3
O
H
O
O
4.
5.
N
15
20
100
88
379
334
O
O
H
O
O
N
O
O
H
O
O
N
6.
7.
40
84
318
ClH2C
O
O
H
O
O
N
O
O
30
40
95
77
360
292
O
H
O
N
8.
9.
O
O
O
O
H
N
50
81
307
F3C
O
O
8
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Dalton Transactions
H
O
O
10.
N
30
71
269
O
O
H
O
O
11.
12.
N
N
O
35
45
72
70
273
265
O
H
O
O
ClH2C
O
O
O
H
O
13.
N
O
50
63
239
O
H
O
OH
O
N
14.
15.
16.
H
60
60
65
66
62
60
250
235
227
H
O
OH
O
N
N
N
CH3
H
O
OH
O
CH3
H
O
O
O
17.
18.
60
30
67
99
254
375
O
OH
O
N
H
O
H
O
O
19.
N
35
89
337
O
O
H
H
O
O
20.
21.
N
N
60
80
77
92
292
349
ClH2C
O
O
H
O
O
F3C
O
O
a Reaction Conditions: Phenol/ β-ketoester (0.5 mmol), N,N substituted formamide (25 mmol), TBHP (70 wt% in water, 6.0 equiv), catalyst (30 mg),
reflux at 70oC, b Conversion (%) was determined using GC-MS, c TON is the number of moles of product per mol of catalyst
the reaction was noted to be considerably higher in case of ethyl-
4, 4, 4 trifluoroacetoacetate due to the presence of three highly
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DOI: 10.1039/C4DPT0a3g23e6E10 of 12
deactivating fluoro groups (Table 1, entries 9 and 21). Further, in 60 runs without any significant deterioration in its activity under the
case of the cyclic and bulky formamides, a slight decrease in the
conversion percentages could be attributed to the steric hindrance
factor (Table 1, entries 10, 12, 13, 14, 15, 16 and 17). The most
distinguishing aspects of this methodology were the wide
substrate applicability and high conversion percentages.
Plausible Reaction Mechanism
A plausible reaction mechanism has been proposed for the
synthesis of carbamates via the C-H activation of formamides
10 (Scheme 2). It is believed that a radical mechanistic pathway is
operative in this case as the addition of TEMPO (2,2,6,6-
similar experimental conditions (Fig. S5, ESI†) indicating that the
catalyst is stable and can be regenerated for repeated use.
Comparison of the catalytic activity of the Cu-2QC@Am-
SiO2@Fe3O4 nanocatalyst with the literature precedants
65 The supremacy of the present methodology could be established
5
by comparing the results obtained with those of the previously
reported protocols. The Cu-2QC@Am-SiO2@Fe3O4 nanocatalyst
showed promising results in terms of product yield and reaction
time in comparison to the literature precedents (Table 2) which
70 could be primarily ascribed to the nanometer size of the catalyst
that provided a large number of catalytically active sites where
the substrates could bind, react and result in the formation of the
desired products. Additionally, all the homogeneous catalysts
reported for the synthesis of carbamates underwent
75 decomposition immediately after the completion of the reaction
and could not be reused for further cycles, whereas the magnetic
silica based copper nanocatalyst could be recovered easily
through magnetic attraction and recycled for eight consecutive
cycles without any loss in its activity. Besides, in comparison to
80 the only heterogeneous catalyst10b reported for the cross
dehydrogenative coupling reaction, the Cu-2QC@Am-
SiO2@Fe3O4 nano-catalyst was found to be far more superior in
terms of product yield, reaction time and catalytic recyclability.
tetramethylpiperidine-1-oxyl), which is basically
a radical
scavenger, quenches the reaction completely. GC analysis clearly
proves that no product is formed when the reaction is carried out
15 in presence of TEMPO. The first step involves the generation of a
co-ordination complex (A) between the enol tautomer of the
dicarbonyl moiety and the transition metal. In the next step,
hydrogen atom is abstracted from formamide by the reagent
TBHP that functions both as an oxidant as well as a radical
20 initiator generating the radical (R2NCO.). Recent literature
supports the decomposition of TBHP to tert-butoxy radical which
is capable of hydrogen abstraction.22 Further, the free radical thus
generated reacts with complex A to produce complex B which
eventually eliminates the carbamate product by giving Cu (I)
25 species. Finally, Cu (I) gets oxidized back to Cu (II) in the
presence of TBHP that sustains the catalytic cycle.10c,23
Concluding remarks
Hot filtration test
85 In conclusion, we have successfully fabricated a novel, efficient
and an economically viable magnetically separable nanocatalyst
through the covalent anchoring of quinoline-2-carboimine copper
complex onto the APTES functionalized silica coated magnetite
nanoparticles for the synthesis of carbamates via the C-H
90 activation of formamides. The design of the nanocatalyst
involved a logical approach and was accomplished in a stepwise
manner through the successive surface modifications of the
magnetite nanoparticles. The presence of a rich magnetic core
facilitated the catalyst separation process while a layer of silica
95 around it prevented the agglomeration of the nanoparticles. The
resultant nanocatalyst afforded a diverse range of industrially and
pharmaceutically important carbamates with excellent conversion
percentage and high turnover number. The hot filtration test
provided a concrete evidence for the heterogeneous nature of the
100 nanocatalyst by clearly ruling out the possibility of leaching of
the active metal species from the solid support. On the other
hand, the recyclability test demonstrated that the catalyst could be
reused at least eight times without any significant degradation in
its activity. In addition, the involvement of a green oxidant
105 (TBHP), inexpensive precursors, mild and solvent free
conditions, less reaction time, broad substrate scope, easy
catalytic recoverability and reusability made the protocol a
potential candidate for addressing the challenges of sustainability.
We speculate that this novel magnetic nanocomposite system
110 would find applications in several other industrially significant
catalytic processes as well as the general synthetic organic
transformations.
In order to ensure that the newly synthesized Cu-2QC@Am-
SiO2@Fe3O4 nanocatalyst is devoid of leaching problems, a
30 standard hot filtration test was performed using the model
reaction. After continuing the stirring of the reaction mixture for
about half the reaction time, the catalyst was removed
magnetically and the conversion percentage was determined to be
30% by the GC-MS analysis. The resulting supernantant was
35 subjected to further heating for 30 minutes. The GC results of the
supernatant revealed that no progress in the reaction occurred
after the removal of the nanocatalyst. This fact suggested that
there was no loss of the catalytic components during the course of
the reaction which was further supported by Atomic Absorption
40 Spectroscopy (AAS) and UV-visible spectroscopic studies. No
absorption peak characteristic of the copper metal in the UV-
visible spectra of the supernatant ruled out the possibility of
involvement of any homogeneous contribution. Also, the AAS
technique showed the absence of any traces of copper. All the
45 above observations provided a thorough evidence for the
heterogeneous nature of the catalyst.
Recovery and reuse of the catalyst
Recovery and reusability of the catalyst are two essential factors
that need to be evaluated, considering the stringent economical
50 and ecological demands for sustainability. Thus, the recyclability
of the Cu-2QC@Am-SiO2@Fe3O4 nanocatalyst was examined in
the synthesis of phenol carbamate under the optimal reaction
conditions using the test substrates. After the completion of the
reaction, the nanocatalyst was recovered magnetically with the
55 aid of an external magnetic force, washed well with acetone for
the removal of traces of the previous reaction mixture and dried
under vacuum. The oven dried, recovered catalyst was used in the
subsequent runs of the same reaction. Interestingly, it was
observed that the catalyst could be reused for eight successive
Acknowledgements
One of the authors, Sriparna Dutta, expresses her gratitude to the
10 | Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C4DT03236E
Page 11 of 12
Dalton Transactions
University Grant Commission, Delhi, India for the award of
Junior Research Fellowship. Also the authors thank USIC-CLF,
DU for FTIR, XRD and VSM analysis.
5 Table 2 Comparison of the catalytic activity of the Cu-2QC@Am-SiO2@Fe3O4 nano-catalyst with other catalysts reported in literature for the synthesis of
carbamates via C-H activation of formamides
Entry
Formamide
Phenols/β-ketoesters
Reaction Conditions
Yield (%)
Reference
H
O
O
1.
CuBr2 (5 mol %) / TBHP (1.5 equiv) /80oC/3
h
69
[9]
N
O
O
OH
O
H
2.
3.
4.
Cu(OAc)2 (5 mol %)/ TBHP (1.5 equiv) /
80oC/3 h
81
85
99
[9]
CH3
N
O
H
O
O
CuCl (1 mol %) / TBHP/
70oC/ 15-30 min
[10a]
[10a]
N
O
O
H
OH
O
CuCl(1 mol %) /TBHP/
70oC/ 15-30 min
N
CH3
O
H
OH
O
5.
CuCl(1-2mol%)/TBHP/ 80oC/ 15 min
96
[10c]
N
H
O
H
OH
O
6.
7.
Cu2(BPDC)2(BPY)/TBHP/ 100oC/ 120 min
(n-hexane as an internal standard)
88
98
[10b]
N
H
O
H
O
O
Cu-2QC@Am-SiO2@Fe3O4
Present study
N
nanocatalyst/TBHP/ reflux at 70oC/ 15 min
O
O
H
OH
O
N
8.
9.
Cu-2QC@Am-SiO2@Fe3O4
CH3
nanocatalyst/TBHP/ reflux at 70oC/ 15 min
97
99
Present study
Present study
O
H
OH
O
Cu-2QC@Am-SiO2@Fe3O4
N
nanocatalyst/TBHP/ reflux at 70oC/ 12 min
H
O
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Green Chemistry Network Centre, Department of Chemistry
20
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University of Delhi, New Delhi-110007, India. Fax: +91-011-27666250;
10 Tel: 011-27666250; E-mail: rksharmagreenchem@hotmail.com
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
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