Sharma 2013
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1463-9262(2010)12:9;1-U
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Cutting-edge research for a greener sustainable future
www.rsc.org/greenchem Volume 12 | Number 9 | September 2010 | Pages 1481–1676
COMMUNICATIONLuque, Varma and BaruwatiMagnetically seperable organocatalyst for homocoupling of arylboronic acids
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Graphical Abstract
Magnetite (Fe3O4) Silica Based Organic-Inorganic Hybrid Copper(II) Nanocatalyst: A
Platform For Aerobic N-alkylation of Amines
R.K. Sharma*, Yukti Monga, Aditi Puri and Garima Gaba
Department of Chemistry, University of Delhi, Delhi, India
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PAPER
This journal is © The Royal Society of Chemistry [year] [journal], [year], [vol], 00–00 | 1
Magnetite (Fe3O4) Silica Based Organic-Inorganic Hybrid Copper(II)
Nanocatalyst: A Platform For Aerobic N-alkylation of Amines
Rakesh K. Sharma,*a Yukti Monga
a, Aditi Puri
a and Garima Gaba
a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x°° 5
A magnetically recoverable, efficient and selective copper based nanocatalyst has been
synthesised via covalent grafting of 2-acetylthiophene on silica coated magnetic nanosupport
followed by metallation with copper acetate. The obtained organic-inorganic hybrid nanomaterial
has been characterized by Electron microscopy techniques (SEM and TEM with
EDS), XRD, VSM, FT-IR and AAS. The catalytic performance of the novel nano-catalyst is 10
evaluated in the active transformation of various aromatic amines to industrially-important
alkylated amines. The nanocomposites afford high turnover frequency and high selectivity for
amines under aerobic condition. Furthermore, the heterogeneous nature of catalyst allows
easy magnetic recovery and regeneration, which makes the present protocol highly beneficial
to address the industrial needs and environmental concerns.15
Introduction
The fusion of green chemistry with nanotechnology has caused a
radical revolution due to its direct implications on human health
and environment. Precisely, this interface has brought radical and
remarkable transformations in synthetic chemical processes and 20
led to the evolution of sustainable and selective nanocatalysts.1
Further advancement in this area has introduced magnetically
retrievable nanocatalysts which posses excellent activity,
selectivity, ease of separation from the reaction mixture, and
recyclability without losing their activity. Apart from these 25
benefits, these nanometric systems provide immense surface area,
by which, the contact between reactants and catalyst enhances
dramatically.2 Even though, the magnetic nanocatalysts have
various advantages but unfortunately, they sometimes show
strong tendency for aggregation and decomposition. In order to 30
resolve this problem, magnetic nanoparticles are coated with
amorphous silica.3 Encapsulation of magnetic nanoparticles with
silica imparts various desirable properties, such as thermal
stability, chemical inertness and ease of functionalization, which
makes it a unique and invincible coating material.4 35
Immobilization of the catalytic centre on silica based
magnetically responsive nanomaterial are nowadays considered
as unbeatable route in the field of nanocatalysis due to the
numerous advantages such as robustness, high stability, potential
recyclability and more catalytic active centres. Therefore, these 40
heterogeneous organic–inorganic nanohybrid catalytic systems
are considered as efficient and inexpensive route with the key
objective of showing high activity and selectivity, low energy
consumption and long lifetime.5
Though, this progression of technology touches every domain but 45
due to the rising environmental and economic concerns, the
development in organic synthesis always has a scope for
improvement. Recently, considerable emphasis is being given on
the “selective synthesis of alkyl substituted amines”. This is due
to their commercial importance in fine chemicals and 50
pharmaceutical industries.6 Also, these compounds perform an
imperative role in the embellishment and composition of
biological and chemical systems.6a&b Some very famous classical
approaches for the synthesis of amines are amination of alkyl/aryl
halides, reductive alkylation with carbonyl compounds7 and 55
hydroamination of olefins and alkynes.8 However, these
procedures are problematic and cannot be commercialized due to
the use of toxic and expensive organic halides, discharge of
corroding organic salt waste, undesired formation of higher
amines and alkylammonium halides and unavailability of the 60
corresponding olefins. Therefore, there is an urgent need to
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develop simple, inexpensive and instant method which can be
applied profitably by the industries. Another efficient and
versatile methodology based on “hydrogen autotransfer” has been
evolved for N-alkylation of amine using alcohol.9 This pathway is
considered to be environmentally benign alternative due to the 5
ubiquitous advantages such as ease of availability; high
selectivity for mono-alkylated products and the removal of non
toxic by-product i.e. water. Although several catalytic methods
have been reported for the present reaction,10 these protocols
suffer from numerous drawbacks such as unfavourable anaerobic 10
dehydrogenative activation of the alcohols, high temperatures
(>150 °C), long durations and use of organic solvents. Also,
unavailability of the expensive metal complexes and inert
atmospheric conditions greatly limits their procedural utility for
the desired organic transformation. In contrast to anaerobic 15
method, Xu et al. first time discovered air promoted
homogeneous copper(II) acetate catalyzed N-alkylation method.11
Though Xu et al. achieved the great accomplishment for the
present work but, the use of homogeneous approach of the
catalyst causes a serious problem of separation from the reaction 20
mixture and is one of the major contributors to the waste in
chemical processes. Therefore, the development of recyclable,
greener, cheaper and easily available heterogeneous metal
catalyst in aerobic environment is desirable in contemporary
organic synthesis of alkylated amines. 25
As a part of our continuing effort12 to synthesize heterogeneous
organic–inorganic hybrid catalytic systems for various organic
transformations, we have also applied upbringing nanotechnology
using silica based magnetically recoverable nano-catalytic
systems for various synthetic applications.13 Considering the use 30
of readily available copper compounds as effective catalyst for N-
alkylation of amines, herein, we report novel silica based
superparamagnetic copper nanocatalyst (Cu-AcTp@Am-Si-
Fe3O4) for the synthesis of various industrially significant amines.
Literature study reveals that this type of catalytic system has not 35
been reported so far for present transformation. Therefore, with
the intention to further explore the scope of the aerobic N-
alkylation method, we envisioned the use of silica encapsulated
magnetic nano-core as heterogeneous nanosupport, which makes
the synthetic process for industries more appealing from an 40
environmental and economic view point.
Results and Discussion
Preparation and characterization of Cu-AcTp@Am-Si-Fe3O4
We used magnetite nanoparticles (Fe3O4), of approximately 8-10 45
nm diameter, which were prepared by the co-precipitation
method.14 The particles were subsequently encapsulated with
silica, using tetraethoxysilane (TEOS) and ammonia solution by
sol-gel approach.15 The obtained silica coated magnetic
nanoparticles (Si-Fe3O4) offers binding sites (Si–OH units) for 50
the heterogenization of the molecular catalysts. For achieving the
amine functionalized surface, we have used 3-aminopropyl
triethoxysilane (APTES) as the functionalizing agent (Am-Si-
Fe3O4). It was reacted with acetylthiophene (AcTp) in ethanol to
yield the AcTp@Am-Si-Fe3O4, these obtained nanoparticles were 55
further metallated with copper acetate in acetone to achieve
resulted copper complex grafted magnetically recoverable
nanoparticles (Cu-AcTp@Am-Si-Fe3O4) (Scheme 1).
(HO)3Si(CH2)3NH2
-3H2O
SO
Ethanol, Reflux
TEOS
NH2
N
S
NH2
NH2
NH2
Cu(OAc)2
Stirring
AcTp@Am -Si -Fe3O4Cu -AcTp@Am -Si -Fe3O4
Cu CuS
N
AcO
OAc
S
N
H2NNH2NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
H2N
H2N
NH2H2N
H2N
H2N
H2N
H2N
H2N
H2N
Fe3O4Si -Fe3O4
Am -Si -Fe3O4
60
Scheme 1 A schematic illustration of the formation of the Cu-
AcTp@Am-Si-Fe3O4 core-shell nanocatalyst
The transmission electron microscopy (TEM) images of the
Fe3O4 nanoparticles are shown in Figure 1a. The size distribution
of these nanoparticles is very narrow over the wide range of the 65
TEM grid area (See ESI-S1). Selected area electron diffraction
pattern (SAED) of the particles is shown as an inset in Figure
1a. The white spots as well as the bright diffraction rings indicate
that the nanoparticles produced by the above stated method are
highly crystalline. From HRTEM, the average interfringe 70
distance of obtained nanoparticles was measured to be ∼0.3 nm
which, corresponds to (2 2 0) plane of inverse spinel structured of
Fe3O4 (Figure 1b).The nanoparticles, depicted in Figure 1c after
silica encapsulation step, have a discrete core/shell structure, and
their uniform magnetic core with a diameter of 8-10 nm is 75
surrounded by 3-5 nm thick silica shell. Figure 1d reveals the
grafting of organic polymer (APTES) onto the surface of silica
coated nanoparticles for the functionalization of Si-Fe3O4.
Figure 1 TEM images of the nanoparticles obtained at different stages of 80
synthesis: (a) HR-TEM image of Fe3O4, (b) SAED pattern of Fe3O4, (c)
Si- Fe3O4 and (d) Am-Si- Fe3O4
The morphology of Cu-AcTp@Am-Si- Fe3O4 is characterized by
Scanning electron microscopy (SEM). The SEM images of
magnetite nanoparticles are presented at two different 85
magnifications (Figure 2a & 2b). While after encapsulation,
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smooth surfaces of the Fe3O4 core particles were roughened due
to deposition of silica coating to magnetic nanoparticles (Figure
2c), which formed uniform and continuous shell around them.
Separate silica aggregates are not observed indicating that
precipitation of primary silica nanoparticles was occurred only on 5
the surface of functionalized core particles. Whereas, after
anchoring of organic or organometallic moieties onto the silica
matrix over magnetic support, Figure 2d showed similar particles
appearance. These results mean that silica surfaces efficiently
prohibited the agglomeration of the particles at the selected 10
synthesis conditions.
Figure 2 SEM images of (a) Fe3O4 at low magnification (32 KX), (b)
Fe3O4 nanoparticles at high magnification (100 KX), (c) Si-Fe3O4 and (d)
AcTp@Am-Si-Fe3O4 15
To confirm the crystalline nature and surface state of the
nanoparticles (Fe3O4, Si-Fe3O4, AcTp@Am-Si-Fe3O4), powder X-
ray diffraction (XRD) studies were carried out. The XRD patterns
of the native iron oxide particles (Figure 3a) revealed the
reflection peak positions and the relative intensities of the 20
diffraction peaks matched well with the standard XRD data of
Joint Committee on Powder Diffraction Standards (JCPDS) card
number (19-0629) for Fe3O4 crystal with a cubic inverse spinel
structure, which is consistent with the TEM results. The Debye–
Scherrer equation (Dhkl ¼ kl/ βcosθ) was used to estimate an 25
average crystallite size from the XRD patterns, 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 particle, l is the wavelength of
radiation, β is the full width at half maximum (FWHM) in
radians, and θ is the position of the diffraction peak maximum. 30
Here, the mean crystalline sizes of Fe3O4 nanoparticles were
calculated to be 12.9 nm by measuring the (311) peak widths of
the X-ray diffraction lines. After encapsulation with silica, a new
broad peak around 23oθ appears due to the existence of
amorphous silica16 but rest of the pattern remains the same as 35
shown in Figure 3b which, clearly depicts that there is no change
in the topological structure and inherent properties of Fe3O4
before and after the coating with silica. On assessment of the
diffractograms of silica encapsulated and 2-acetylthiophene
grafted nanoparticles, the very distinguishable FCC peaks of 40
magnetite crystal were not changed, which means that these
particles have the phase stability but, there is slight decrease in
intensity with broadening of corresponding peak of silica (Figure
3c). It can be accredited due to the lowering of scattering contrast
between the walls of the silica framework and organic moiety 45
attached over Si-Fe3O4. It also shows that different reaction
conditions during the synthesis, did not affect on crystallinity and
morphology of Fe3O4 nanoparticles throughout the process.
Figure 3 XRD pattern of the (a) Fe3O4 nanoparticles, (b) Si-Fe3O4 and (c) 50
AcTp@ Am-Si-Fe3O4
Figure 4 displays the elemental mapping of the isolated particles
of the Am-Si-Fe3O4 and Cu(II)-AcTp@Am-Si-Fe3O4. Energy-
dispersive X-ray spectroscopy (EDS) analysis carried out with Si-
Fe3O4 showed the presence of silica and iron, which reveals the 55
encapsulation of magnetite core with silica. Whereas, the
coordination behaviour of 2-acetylthiophene complex grafted
over Si-Fe3O4 (AcTp@Am-Si-Fe3O4) for copper ions was also
confirmed with the EDS technique. The presence of iron, silicon,
carbon, oxygen, nitrogen, sulphur and copper components 60
provides a quantitative tool for confirming the immobilization of
2-acetylthiophene ligand, and its further metallation with copper.
The quantitative analysis for copper content in the prepared
nanocatalyst was performed using AAS, and sample digestions
were carried out in microwave at 400 Watt for 15 min. at constant 65
pressure programme with 5 mL aqua regia. The volume of the
filtrate was then adjusted to 50 mL using double deionized water.
Reference solutions for copper measurement were made with
high degree of analytical purity to obtain the calibration curves.
0.135 mmol g−1 copper content in catalyst was quantified using 70
calibration curve in duplicate for each sample.
Fourier transform infrared spectroscopy (FT-IR) seems to be the
best technique to characterize the functionalization and
modification of magnetic nanoparticles. Figure 5a exhibits the 75
characteristic bands of the vibration of the Fe–O bond of the iron
oxide core at 588 cm−1 and broad band around 3124 cm−1 due to
O-H stretching vibrations of adsorbed water.17a Moreover, on
moving from Fe3O4 to Si-Fe3O4, significant reduction of the
intensity of the Fe–O stretching and bending and the O–H 80
stretching and bending vibrations bands is observed.
Additionally, the spectrum also presents (Figure 5b) a band due
to the silica framework related to Si–O–Si asymmetric stretching
(1090 cm−1)17b which revealed the complete encapsulation of the
magnetic cores with silica. Whereas on functionalization in Am-85
Si-Fe3O4, the band at 2925 cm−1 in Figure 5c, corresponds to the
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–CH2 group of amino-propyl group in functionalized Am-Si-
Fe3O4 (which does not appear in the parent Si-Fe3O4 spectrum).
A strong band at 1628 cm−1 corresponding to the scissoring
vibration mode of –NH2 after treating the nanoparticles with 3-
aminopropyltriethoxysilane are also identified. By comparing the 5
spectra of AcTp@Am-Si-Fe3O4 (Figure 5d) with those of
functionalized Am-Si-Fe3O4, is observed that after the Schiff
condensation reaction, the characteristic band of the imine group
(C=N) appears at 1644 cm−1 in Figure 5d.17c On metallation, it is
observed that the absorption at 1644 cm−1 is shifted to 1637 cm−1 10
and also the intensity of this peak has decreased after
immobilization confirming that copper is successfully anchored
onto the surface of Am-Si-Fe3O4 (See ESI-S2).
Figure 4 EDS Pattern of (a) Si-Fe3O4 and (b) Cu-AcTp@Am-Si-Fe3O4 15
The magnetic properties of the synthesized Fe3O4 nanoparticles,
Si-Fe3O4 and Cu-AcTp@Am-Si-Fe3O4 were analyzed by
vibrating sample magnetometry (VSM). The field-dependent
magnetization curves shown in Figure 6 indicates the
magnetization as a function of applied magnetic field, measured 20
at room temperature (293 K) with the field sweeping from
−20,000 to 20,000 Oe. The saturation magnetization, Ms, of bulk
magnetite (92 emu g−1)18a was reduced to 69 emu g−1 for
magnetite nanoparticles. It is known that the magnetization of a
magnetic particle in an external field is proportional to its size 25
value. Therefore, a smaller saturation magnetization value for the
magnetite nanoparticles compared to the bulk material is
reasonable. The saturation magnetic moments of the silica coated
Fe3O4 nanoparticles and Cu-acetythiophenyl complex
immobilized Fe3O4 nanoparticles reached up-to 53 emu g−1 and 30
28 emu g−1 respectively. In spite of these low magnetization
values with respect to magnetization of pure Fe3O4 nanoparticles,
which was owing to decrease in the surface moments of the
magnetite nanoparticles by diamagnetic silica coating18b over
Fe3O4 nanoparticles and grafting of metal-ligand complex over 35
Am-Si-Fe3O4. But, it is still sufficient for magnetic separation by
a conventional magnet. The above showed TEM images also
confirmed that the encapsulation and grafting of nonmagnetic
SiO2 and organic layer over Fe3O4 nanoparticles. Another
important parameter for practical applications of nanoparticles is 40
revealed from the enlarge VSM curve shown in Figures 6d. The
hysteresis loops of powdered materials showed almost negligible
magnetic hysteresis with both the magnetization and
demagnetization curves passing through the origin, which clearly
indicates the superparamagnetic nature of the materials. This also 45
means that the magnetic material can only be aligned under an
applied magnetic field but, will not retain any residual magnetism
upon removal of the field. Thus, the above discussed Fe3O4
nanoparticles appear to be suitable as the support for catalyst.
50
Figure 5 FT-IR spectra of (a) Fe3O4 (b) Si-Fe3O4 (c) Am-Si-Fe3O4 (d)
AcTp@Am-Si-Fe3O4 and (e) Cu-AcTp@Am-Si-Fe3O4
Figure 6 Magnetization curves obtained by VSM at room temperature for 55
(a) Fe3O4 (b) Si-Fe3O4, (c) Cu-AcTp@Am-Si-Fe3O4 and (d) inset:
enlarged image near the coercive field
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Catalytic activity of Cu-AcTp@Am-Si-Fe3O4 for N-alkylation of aniline with benzyl alcohol
The optimization of catalytic conditions was carried out by
employing the alkylation of aniline with benzyl alcohol under 5
aerobic conditions. Aniline was chosen as the test substrate
because it is electronically (+R effect or –R effect) and sterically
deactivated. Hence, gives the accurate assessment of catalytic
activity of nanocatalyst.
10
A control experiment of alkylation of aniline without any catalyst
showed negligible conversion even after 24 h of reaction (Table
1). Similarly, the experiment of oxidation in similar conditions
using Am-Si-Fe3O4 also did not undergo any product formation
(Table 1, entry 2). Also, the use of copper directly attached to 15
amine functionalized silica coated Fe3O4 nanoparticles i.e.
Cu@Am-Si-Fe3O4 (without being complexed to acetylthiophene)
(Table 1, entry 3) resulted in the formation of secondary amine
with lower conversion (59%). However, this indicated that the
reaction is catalyzed by copper ions. The use of Cu-20
Acetylthiophenyl complex grafted nanoparticles over Am-Si-
Fe3O4 i.e. Cu-AcTp@Am-Si-Fe3O4 for reaction (Table 1, entry
4), gave product with highest conversion and selectivity. It
clearly showed that though, the reaction is catalyzed by Cu ions
but the role of ligand is significant for the transformation. In this 25
case, acetylthiophene with strong σ donation by O and S atoms
acted as a co-catalyst and also bound to the metal ion effectively.
The quantitative analysis of the nanocatalyst was also performed
and it was found that the yield of the product increased from 59% 30
to 98% with increasing amount of catalyst from 10-25 mg
respectively. This could be mainly due to the availability of large
number of active sites on the surface of the catalyst, which
increases with the amount of the catalyst (Table 1, entries 4-7).
In previous literature, various alcohol activation protocols with 35
different oxidants are documented but reaction driven under air is
clearly found to be the greenest and most advantageous protocol
regarding convenience, efficiency, economy, environmental
considerations, etc 11&19. This was again confirmed by carrying
out a reaction under different conditions (Table 1, entries 7-9). 40
Reaction carried with TEMPO (Table 1, entry 8), produce
considerable amounts of benzaldehyde (49%) and imine (24%)
with low yield of the target product (27%) at 100 °C. This result
agrees well with the findings that the aerobic condition is more
effective and appropriate alternative for alcohol activation. To 45
demonstrate the suitability of aerobic environment, reaction under
anaerobic condition (under inert atmosphere of nitrogen) was also
scrutinized (Table 1, entry 9).
Table 1 Screening of copper catalysts for N-alkylation of aminesa
Entry Catalyst Conditio
n
Time
(h)
Conv.
(%)b
1. No aerobic 24 Trace
2. Am-Si-Fe3O4 aerobic 24 Trace
3. Cu@Am-Si-Fe3O4 aerobic 10 59
4. Cu-AcTp@ Am-Si-Fe3O4
(10 mg)
aerobic 10 67
5. Cu-AcTp@ Am-Si-Fe3O4 (15 mg)
aerobic 10 82
6. Cu-AcTp@ Am-Si-Fe3O4
(20 mg)
aerobic 10 98
7. Cu-AcTp@ Am-Si-Fe3O4
(25 mg)
aerobic 10 98
8. Cu-AcTp@ Am-Si-Fe3O4 (25 mg)
TEMPO 10 27
9. Cu-AcTp@ Am-Si- Fe3O4
(25 mg) anaerobic 10 Trace
aAniline (1 mmol); Benzyl alcohol (2 mmol); Temp. 100oC 50
bConversion was determined by GC
The promotional effect of base has been examined by carrying a
base blank reaction. The desired amine product was not detected,
indicating that its presence is helpful in the deprotonation of the 55
primary alcohol to form an alkoxide and also facilitate the
removal of a hydride from it. To examine the effect of different
base on the catalytic activity of Cu-AcTp@Am-Si-Fe3O4, the
various bases (KOH, K2CO3, NaOH, t-BuOK) have been
employed using series of solvents such as dioxane, toluene, H2O, 60
o-xylene. Moreover, the solventless conditions were also tested
for desired catalytic reaction. Among various conditions tested as
shown in Figure 7, it was found that present catalyst (Cu-
AcTp@Am-Si-Fe3O4) under solvent free conditions gave
maximum conversion (96%) with KOH as base. Whereas, weaker 65
substitute of base K2CO3 also showed good conversion for the
reaction, but provided less selectivity for amine products. Since
reagent-grade KOH (RG-KOH) is known to have transition metal
contaminant, hence, semiconductor-grade KOH (SG-KOH,
99.99% pure based on trace metal analysis) obtained from Alfa 70
Aesar was also examined as a base in the reaction (Figure 7).
The experiment revealed that the change of grade (reagent Vs
semiconductor grade) does not affect the product yield.
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Figure 7 Effect of base and solvent on N-alkylation of amines (Reaction
conditions: Amines (1 mmol); Alcohols (1.5 mmol); Catalyst (20 mg);
Temp. 100oC; Time 10 h)
To study the effect of temperature on the activity of the catalyst
(Figure 8), model reaction was carried out under the diverse 5
range of temperature (25-120 °C). Unexpectedly, when the
temperature was low (< 60 °C), the imine was obtained instead of
the targeted amine. However, on further raising the temperature,
desired amine formation started and its maximum yield was
obtained when the reaction was carried out at 100 °C. On the 10
other hand, the further increase in temperature did not have any
effect on the yield.
Figure 8 Effect of temperature on selectivity and conversion for N-
alkylation of amines (Reaction conditions: Amines (1 mmol); Alcohols 15
(1.5 mmol); Catalyst (20 mg); Temp. 100oC; Time 10 h)
A variety of substrates were taken to explore the scope and
limitations of the reaction under optimized reaction conditions.
All reactions were performed in same (1 mmol) scale and the
isolated yields of the products are summarized in Table 2. The 20
reaction of various aniline derivatives with electron donating
(entries 2-4) and electron-withdrawing (Table 2, entry 5)
substituent’s proceeded smoothly to afford the high yield of
corresponding product. The nature and the position of
substitution in the aromatic ring did not have much effect on the 25
reaction. Like para- and meta- substituted substrates, the
sterically more bulky ortho substituted also gave good results
(Table 2, entries 5-6). The reaction of trans-cinnamyl alcohol
with aniline derivatives yielded trans-cinnamyl amine
compounds selectively and formation of cis-cinnamyl amine 30
compounds and regioisomers were not detected. Moreover, this
magnetically recoverable copper(II) nanocatalyst catalyzed
aerobic alkylation method is not limited to only aromatic amines.
Also, the nanocatalyst was found to be efficient for alkylation of
aliphatic amine such as 1-butanol, 2-butanol and methanol (Table 35
2, entries 12-14).
It is evident from the Table 3 that Cu-AcTp@Am-Si-Fe3O4
nanocatalyst is highly efficient in catalyzing the aerobic N-
alkylation of amines and gave products in good yields with high 40
turnover number (TON) values in comparison to the previous
literature reports.19 Hence, catalytic efficiency of the present
catalytic system is remarkable in terms of mild reaction
conditions, short reaction time and easy recovery of the catalyst.
Table 2 Scope of catalytic activity of the Cu-AcTp@Am-Si-Fe3O4 in aerobic N-alkylation of amines with different alcoholsa 45
S.No.
R1NH2
R2OH
Conv.b
(%)
Product Selectivity c
Applications
TONd
1. NH2
OH
98 N-benzylbenzenamine (100 %)
Pharmaceutical intermediate 362
2. NH2
OH
91 N-cinnamylbenzenamine
(100 %)
Herbicide 337
3. NH2
Cl
OH
93 N-benzyl-4-chlorobenzenamine
(99 %)
Pesticides, Herbicidal 344
4. NH2
No2
OH
88 N-benzyl-4-nitrobenzenamine (100 %)
Intermediate for dyestuffs 307
5.
NH2
OH
83 N-cinnamyl-2,6-dimethylbenzenamine
(95 %)
Fine organic and custom intermediate 340
6.
NH2
OH
92 N-benzyl-2,6-
dimethylbenzenamine (97 %)
Herbicide 340
7. NH2
H3CO
OH
93 N-benzyl-4-methoxybenzenamine
( 100%)
A potent Fungicidal 344
8. NH2
OH
86 (Z)-N-benzyl-3-phenylprop-2-
en-1-amine
(97 %)
Herbicide 318
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9. NH2
OH
96 Dibenzylamine (100 %)
Pharmaceutically acceptable a therapeutic agent for arteriosclerosis
355
10. NH2
OH
82 N-cinnamylnaphthalen-1amine
(96.3 %)
A potent fungicidal 303
11. NH2
OH
88 N-benzylnaphthalen-1-amine
(100 %)
Antiparasitic or in combination with
fungicidal and antibacterial
325
12. NH2
CH3-OH 90 N-methylbenzenamine
(98%)
Pharmaceutical intermediate 333
13. NH2
OH
92 N-methyl-N-
propylbenzenamine (99%)
Pharmaceutically intermediate for
treating Amyloid Disease
340
14. NH2
No2
OH
87 N-methyl-4-nitro-N-
propylbenzenamine (97%)
Intermediate for the production of
agricultural chemicals
322
aReaction conditions: Amines (1 mmol); Alcohols (1.5 mmol); Catalyst (20 mg); Temp. 100 oC; Time 10 h
b&c Conversion and selectivity were determined by GC & GC-MS
c Selectivity = (GC area of amine product)/(GC area of all products)×100
dTON= Calculated using the 0.135 mmol/g Copper (Obtained by AAS for Cu-AcTp@Am-Si-Fe3O4)
Table 3 A comparisons of the results of the present system with the 5
recently published aerobic catalytic systems for the N-alkylation of
amines.
S.No. Substrate Catalytic system
Reaction conditions Yield Ref.
1 PhSO2NH2 + PhCH2OH
MnO2 K2CO3, 120 oC, Air, 24 h
96% 19a
2 PhSO2NH2 +
PhCH2OH
RhCl(PPh3)3 K2CO3, 150 oC,
Toluene, 24 h
92% 19b
3 PhCH2CH2NH2 +
PhCH2OH
Pd/AlO(OH) 90 oC, Heptane, 20
h
94% 19c
4 PhNH2 + PhCH2OH Ru(OH)x/Al2O3 132 oC, 11 h, Mesitylene
98% 19d
5 PhSO2NH2 +
PhCH2OH
Cu(OAc)2.H2O K2CO3, 120 oC, 12
h
97% 11c
6 PhNH2 + PhCH2OH Our catalyst Solventless, 10 h,
KOH,100 oC, Air
99% This
work
10
Hot Filtration Test
Hot-filtration based leaching test was conducted to exclude any
homogeneous catalytic contribution or lixiviation of catalytic
species in the catalyzed reaction. First, AAS analysis of the post
reaction mixture after catalyst separation was conducted and the 15
results revealed that concentration of Cu(II) ions in the
supernatant correspond to the negligible catalyst leaching (< 0.01
ppm). Another reaction was carried out at 100 °C for 5 h with the
procured catalyst from previous cycle. After the catalyst was
separated using external magnet and the supernatant was again 20
poured back into the reactor and the reaction was continued for
additional 5 h. It was found that there was almost no further
conversion after separation of the catalyst (Figure 9). It
corroborated that the copper has not been leached out during the
course of the reaction which further signifies the stability and 25
heterogeneity of prepared nanocatalyst. To further intensify the
fact, catalyst recovered after this run was subjected to digestion
using microwave irradiation and metal content analysis using
AAS. The result showed that there was barely any change in the
amount of Cu compared with the fresh catalyst. 30
Figure 9 Percent conversion versus reaction time in a leaching
experiment. The green arrow indicates the time the catalyst was filtered
and separated from the reaction mixture and supernatant was then run by
itself afterward 35
Reusability of the catalyst
The recycling and recovery of used catalyst is one of the most
important criteria of industrial based catalyst system, which gives
useful information about the immobilization process and catalytic
stability along the catalytic cycles. To address the concern, after 40
each catalytic reaction, the spent catalyst Cu-AcTp@Am-Si-
Fe3O4 was trapped with simple magnet, and washed with
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8 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]
degassed DCM (5×10 mL) and dried under vacuum. As shown in
Figure 10, the catalyst could be reused 10-fold with no
significant detrimental effect on the chemical yield obtained. The
structure and morphology of the recycled nanocatalyst (after 10th
reaction) was observed through SEM and VSM. As shown by 5
VSM curve (See ESI-S3), interestingly after ten cycles of
catalytic reaction, apparent magnetic saturation value decreased
very less in contrast to the fresh catalyst, which means the
efficiency of the catalyst remains unaltered during the runs.
Whereas, SEM micrograph (See ESI-S4) also showed no 10
agglomeration between particles which shows high colloidal
stability and re-dispersibility which might come from the strong
electrostatic repulsion and steric hindrance among hydrophobic
silica layer over magnetic support. Additionally, the conversion
during each run was found to be >96.5%. Therefore, Cu-AcTp@ 15
Am-Si-Fe3O4 nanocomposites have proved as an efficient
nanocatalyst in terms of recoverability and recyclability.
Figure 10 Catalyst recycling test for successive ten runs of N-alkylation
of aniline 20
Proposed catalytic mechanism
For the present transformation, reaction has not occurred in the
absence of catalyst under aerobic condition, which indicates that
the lewis acidity of the catalyst plays significant role in the
transformation. The reaction in presence of Cu-AcTp@Am-Si-25
Fe3O4 not only gave product with highest conversion and
selectivity but also provides ease of recyclability. The probable
mechanism has been illustrated in Scheme 2. In anaerobic
atmosphere, the efficiency of the catalyst to abstract hydrogen
from the alcohol to yield aldehyde and to provide a hydridometal 30
species i.e. the formation of Cu(I) from Cu(II) is greatly reduced
thus, making it the rate limiting step for the reaction. Whereas,
metal-catalyzed reaction carried in aerobic conditions shows that
the presence of air facilitates the formation of aldehyde. This
aldehyde readily reacts with a starting amine to form the 35
corresponding imine via condensation reaction in Step-2. In Step-
3, imine receives new hydrogen in presence of base from alcohol
(shown in blue) and oxidizes it to aldehyde. Whereas, after the
transfer of hydrogen, imine changes itself to corresponding
desired product. This step is found to be analogous to a relay race 40
game with the “handing off” of hydrogen atoms.11b&c Finally, in
Step-4, reaction for the next cycle proceeds with the aldehyde
obtained in the previous step.
R1 OH R1 O R1 O
Cu
Cu
R1 NR2
R1 OH
R1 NH
R2
Cu(II) Cu(I)
R2NH2
H2O
Air(1/2 O2)H2O
Base
Step-1
Step-2
Step-4
Step-3
Cu Cu-AcTp@ASMPs
Fast under air
Slow under nitrogen
Scheme 2 Proposed reaction mechanism 45
Conclusions
In conclusion, we have developed novel, efficient and reusable
silica based organic-inorganic hybrid copper nanocatalyst for
aerobic N-alkylation of amines using alcohols under less 50
demanding conditions. This unique environmentally benign
protocol provides unique advantages such as the use of
magnetically recoverable nanocatalyst and alcohol as green
alkylating agent in safe and less-corrosive conditions, which
leads to zero effluent discharge to the environment. In addition to 55
this broad substrate scope, high conversion, short reaction time,
good catalytic turnover number, easy recoverability and
reusability make it a valuable economical system as compared to
the other catalysts reported earlier.
60
Experimental Details
General remarks
TEOS (tetraethoxyorthosilicate) were procured from Sigma
Aldrich. 3-Aminopropyltriethoxysilane (APTES) was obtained
from Fluka. Copper(II) acetate, Ferric(III) sulphate and 65
Ferrous(II) sulphate were purchased from Sisco Research
Laboratory (SRL). Ethanol, Dichloromethane (DCM) and Ethyl
acetate (EtOAc) were procured from Merck and purified before
use. All other reagents used were of analytical grade and
commercially available. Double-distilled water was utilized 70
throughout the studies
X-ray diffraction (XRD) was performed using a Bruker, D8
Advance, (Karlsruhe, bundesland, Germany) diffractometer
equipped with Cu/Kα radiation at a scanning rate of 4°/min in the
2θ range of 5–80° (λ = 0.15405 nm, 40 kV, 40 mA). 75
Transmission electron microscopy (TEM) images were acquired
using a Jeol, 2010, Japan, Transmission Electron Microscope
operated at 300 kV by dispersing samples on a lacy amorphous
carbon support film and the “ImageJ” software was used for
image processing and analysis. The mean particle size of the 80
nanoparticles was determined by image analysis of at least 150
colloidal aggregates. Energy dispersive spectroscopy (EDS)
analysis was performed using an adjacent Oxford INCA system.
The presented metal contents were calculated from the EDS
results and the averaged values are based on 3−5 measurements 85
on chosen spots of the analyzed samples after exclusion of
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extraordinary high or low values (not exceeding a two-fold
amount). Size and morphology analysis of prepared
nanocomposites was also performed using Gemini Ultra 55
(Zeiss) scanning electron microscope (SEM). Samples were
prepared by first applying double-sided tape to metal stubs. The 5
sample was then placed onto the tape. Excess sample was
removed using pressurized air. The sample was then sputter-
coated with a 10-nm gold layer using a Cressington 208 HR
Sputter Coater. A Tekmar Sonic Disruptor model TM300
(sonicator) fitted with a microtip was used to disrupt nanoparticle 10
aggregates in the magnetite-ethanol dispersions. The
magnetization curve was obtained by a vibrating sample
magnetometer (EV-9, Microsense, ADE). Fourier transform
infrared spectra (FT-IR) were recorded using Perkin-Elmer
Spectrum 2000. Digestions were performed in Anton Paar 15
multiwave 3000 microwave reaction system equipped with
temperature and pressure sensor. The amount of copper in the
catalyst and in the supernatant was estimated by Atomic
absorption spectroscopy (AAS) on LABINDIA AA 7000 Atomic
Absorption Spectrometer using an acetylene flame. The optimum 20
parameters for Cu measurements are: wavelength= 324.7 nm;
lamp current=2 mA; slit width=0.2 nm; fuel flow rate=0.2 L
min−1. The derived products were analyzed and confirmed by
using Agilent gas chromatography (6850 GC) with a HP-5MS
5% phenyl methyl siloxane capillary column (30.0 m × 0.25 mm 25
× 0.25 mm) and a quadrupole mass filter equipped 5975 mass
selective detector (MSD) using helium as carrier gas. The carrier
gas was helium (rate 1.0 mL min−1) and the temperature of the
injection port was 250 oC. The temperature program of the
column was set to an initial oven temperature of 100 °C and was 30
increased at a rate of 10 °C min−1 to 250 °C, and the oven was
held at 250 °C for 10 min.
Synthesis of nano-support composites
Magnetite (Fe3O4) nanoparticles were synthesized using co-
precipitation method.14 Ferric sulphate (6.0 g) and ferrous 35
sulphate (4.2 g) were dissolved in water (250 mL) and stirred at
60 oC to give yellowish-orange solution. Then, 25% NH4OH (15
mL) was added with vigorous mechanical stirring, with which
colour of the bulk solution changed to black. Stirring was
continued for another 30 min. The precipitated Fe3O4 were 40
separated magnetically and washed several times with deionized
water and ethanol. Silica coating of these Fe3O4 nanoparticles
was performed via sol-gel approach.15 Dispersed solution of
activated Fe3O4 with 0.1 M HCl (2.2 mL) was prepared in
mixture of ethanol (200 mL) and water (50 mL) under sonication. 45
Then, 25% NH4OH (5 mL) was added to the suspension at room
temperature followed by the addition of TEOS (1 mL). Further,
the mixture was kept for stirring at temperature of 60 oC for 6 h.
The obtained silica coated magnetic nanoparticles (Si-Fe3O4)
were separated magnetically, washed with ethanol and dried 50
under vacuum.
Finally, in order to introduce the amine groups to the silica
surface of the nanoparticles, APTES (0.5 mL) was added to the
dispersed solution of Si-Fe3O4 (0.1 g) in ethanol (100 mL) and
resulting mixture was stirred for 6 h at 50 °C. Devised 55
aminopropylated Si-Fe3O4 (Am-Si-Fe3O4) were separated and
washed several times with ethanol to remove the unreacted
silylating agent.
Synthesis of acetylthiophene functionalized silica based
organic-inorganic hybrid Cu(II) nanocatalyst 60
For covalent grafting of 2-acetylthiophene (AcTp) on Am-Si-
Fe3O4, Am-Si-Fe3O4 (2 g) and 2-acetylthiophene (4.0 mmol) in
ethanol (250 mL) were refluxed for 3 h. Then, the resultant
grafted AcTp@Am-Si-Fe3O4 (1 g) were stirred with solution of
1.5 mmol of copper acetate in acetone for 4 h. The resulted 65
copper complex grafted nanoparticles (Cu-AcTp@Am-Si-Fe3O4)
were separated magnetically, washed thoroughly with DCM and
then with water and dried in vacuum oven.
General procedure for Cu-AcTp@Am-Si-Fe3O4 nanocatalyst mediated N-alkylation of amines 70
Typically, the N-alkylation of aniline (1 mmol) with benzyl
alcohol (1.5 mmol) was performed in a 25 mL round bottom flask
equipped with a condenser. After heating to 100 °C, 20 mg of
catalyst and 0.1 g of base were added. Aliquots were removed at
regular time intervals and analyzed by gas chromatography (GC). 75
After completion of reaction, as indicated by GC, the reaction
mixture was cooled, catalyst was allowed to gather at the side of
the vessel using external magnet. Rest of the solution was taken
out with pipette and extracted with EtOAc, washed with 10%
NaHCO3 and water solution, dried and concentrated to give 80
products. The structure elucidations of the products were
confirmed by GC-MS. The data was collected by using extracted
ion chromatograms of marker m/z values for each molecule from
the total ion chromatograms (TIC) (See ESI).
Recycling procedure 85
After completion of the oxidation reaction, the catalyst was
separated by an external magnet followed by washing with
Ethanol and H2O. The catalyst was then used directly for next
round of reaction without further purification.
Acknowledgement 90
Y. Monga thanks the DST (Department of science and
technology), New Delhi, India, for awarding the Inspire
Fellowship. Also, due thanks to USIC-CLF, University of Delhi,
Delhi, India for HR-XRD and HR-TEM and AIRF, JNU, Delhi,
India for SEM analysis 95
Notes and references
* Prof. R. K. Sharma
Green Chemistry Network Centre, Department of Chemistry,
University of Delhi, Delhi-110007, India. Tel/Fax +91-011-27666250
E-mail: [email protected] 100
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and 105
spectral data, and crystallographic data.
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View Article OnlineDOI: 10.1039/C3GC40818C