Magnetite (Fe3O4) silica based organic-inorganic hybrid copper(ii) nanocatalyst: A platform for aerobic N-alkylation of amines

Article abstract of DOI:10.1039/c3gc40818c

A magnetically recoverable, efficient and selective copper based nanocatalyst has been synthesised via covalent grafting of 2-acetylthiophene on a 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 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 conditions. Furthermore, the heterogeneous nature of the catalyst allows easy magnetic recovery and regeneration, which makes the present protocol highly beneficial to address the industrial needs and environmental concerns.

Full text of DOI:10.1039/c3gc40818c

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Graphical Abstract
Magnetite (Fe O ) Silica Based Organic-Inorganic Hybrid Copper(II) Nanocatalyst: A
3
4
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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DOI: 10.1039/C3GC40818C
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PAPER
Magnetite (Fe O ) Silica Based Organic-Inorganic Hybrid Copper(II)
3
4
Nanocatalyst: A Platform For Aerobic N-alkylation of Amines
a
a
a
a
Rakesh K. Sharma,* Yukti Monga , Aditi Puri and Garima Gaba
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
0
5
DOI: 10.1039/b000000x°°
A magnetically recoverable, efficient and selective copper based nanocatalyst has been
synthesised via covalent grafting of 2ꢀacetylthiophene on silica coated magnetic nanosupport
followed by metallation with copper acetate. The obtained organicꢀinorganic hybrid nanomaterial
has been characterized by Electron microscopy techniques (SEM and TEM with
EDS), XRD, VSM, FTꢀIR and AAS. The catalytic performance of the novel nanoꢀcatalyst is
evaluated in the active transformation of various aromatic amines to industriallyꢀimportant
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.
1
1
as unbeatable route in the field of nanocatalysis due to the
Introduction
numerous advantages such as robustness, high stability, potential
recyclability and more catalytic active centres. Therefore, these
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
4
4
5
5
6
0
5
0
5
0
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
2
2
3
3
0
5
0
5
5
consumption and long lifetime.
1
led to the evolution of sustainable and selective nanocatalysts.
Though, this progression of technology touches every domain but
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
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
benefits, these nanometric systems provide immense surface area,
by which, the contact between reactants and catalyst enhances
6
pharmaceutical industries. Also, these compounds perform an
2
dramatically. Even though, the magnetic nanocatalysts have
imperative role in the embellishment and composition of
various advantages but unfortunately, they sometimes show
strong tendency for aggregation and decomposition. In order to
resolve this problem, magnetic nanoparticles are coated with
amorphous silica. Encapsulation of magnetic nanoparticles with
silica imparts various desirable properties, such as thermal
6a&b
biological and chemical systems.
Some very famous classical
approaches for the synthesis of amines are amination of alkyl/aryl
7
halides, reductive alkylation with carbonyl compounds and
3
8
hydroamination of olefins and alkynes. 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
corresponding olefins. Therefore, there is an urgent need to
stability, chemical inertness and ease of functionalization, which
4
makes it
a
unique and invincible coating material.
Immobilization of the catalytic centre on silica based
magnetically responsive nanomaterial are nowadays considered
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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DOI: 10.1039/C3GC40818C
develop simple, inexpensive and instant method which can be
NH
2
H
2
N
NH
2
H
2
N
NH
NH
NH
NH
applied profitably by the industries. Another efficient and
versatile methodology based on “hydrogen autotransfer” has been
2
H
2
N
2
H
2
N
2
(
HO)
3
Si(CH
2 3 2
) NH
9
TEOS
2
evolved for Nꢀalkylation of amine using alcohol. This pathway is
H
2
N
N
-3H
2
O
NH
NH
2
H
2
5
0
5
0
5
0
5
0
considered to be environmentally benign alternative due to the
ubiquitous advantages such as ease of availability; high
selectivity for monoꢀalkylated products and the removal of non
toxic byꢀproduct i.e. water. Although several catalytic methods
Fe O
3 4
2
NH
NH
2
Si Fe O
3
4
H
2
N
H
2
2
N
NH
NH2 NH
NH
2
H
2
N H N
2
2
A m Si Fe O
2
3
4
1
0
O
have been reported for the present reaction, these protocols
suffer from numerous drawbacks such as unfavourable anaerobic
dehydrogenative activation of the alcohols, high temperatures
NH2
NH
S
1
1
2
2
3
3
4
2
Ethanol, Reflux
NH
2
(
>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
method, Xu et al. first time discovered air promoted
NH
2
Ac
O
N
S
N
Cu(OAc)
2
N
Cu
Cu
S
O
Stirring
S
Ac
A Tc p @A m Si Fe O
CuA Tc p @A m Si Fe O
3 4
3
4
11
homogeneous copper(II) acetate catalyzed Nꢀalkylation method.
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
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.
As a part of our continuing effort to synthesize heterogeneous
organic–inorganic hybrid catalytic systems for various organic
transformations, we have also applied upbringing nanotechnology
using silica based magnetically recoverable nanoꢀcatalytic
systems for various synthetic applications. Considering the use
of readily available copper compounds as effective catalyst for Nꢀ
alkylation of amines, herein, we report novel silica based
superparamagnetic copper nanocatalyst (CuꢀAcTp@AmꢀSiꢀ
Fe O ) for the synthesis of various industrially significant amines.
Literature study reveals that this type of catalytic system has not
been reported so far for present transformation. Therefore, with
the intention to further explore the scope of the aerobic Nꢀ
alkylation method, we envisioned the use of silica encapsulated
magnetic nanoꢀcore as heterogeneous nanosupport, which makes
the synthetic process for industries more appealing from an
environmental and economic view point.
60
Scheme 1 A schematic illustration of the formation of the Cuꢀ
AcTp@AmꢀSiꢀFe coreꢀshell nanocatalyst
3
O
4
The transmission electron microscopy (TEM) images of the
Fe O nanoparticles are shown in Figure 1a. The size distribution
of these nanoparticles is very narrow over the wide range of the
TEM grid area (See ESIꢀS1). Selected area electron diffraction
pattern (SAED) of the particles is shown as an inset in Figure
3
4
6
7
7
5
0
5
1
2
1
a. 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
distance of obtained nanoparticles was measured to be ∼0.3 nm
which, corresponds to (2 2 0) plane of inverse spinel structured of
Fe O (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 8ꢀ10 nm is
surrounded by 3ꢀ5 nm thick silica shell. Figure 1d reveals the
grafting of organic polymer (APTES) onto the surface of silica
coated nanoparticles for the functionalization of SiꢀFe O .
1
3
3
4
3
4
3
4
Results and Discussion
Preparation and characterization of Cu-AcTp@Am-Si-Fe O4
3
45
50
55
We used magnetite nanoparticles (Fe O ), of approximately 8ꢀ10
3
4
nm diameter, which were prepared by the coꢀprecipitation
14
method. The particles were subsequently encapsulated with
silica, using tetraethoxysilane (TEOS) and ammonia solution by
1
5
solꢀgel approach.
The obtained silica coated magnetic
nanoparticles (SiꢀFe O ) offers binding sites (Si–OH units) for
3
4
the heterogenization of the molecular catalysts. For achieving the
amine functionalized surface, we have used 3ꢀaminopropyl
triethoxysilane (APTES) as the functionalizing agent (AmꢀSiꢀ
Fe O ). It was reacted with acetylthiophene (AcTp) in ethanol to
80
Figure 1 TEM images of the nanoparticles obtained at different stages of
synthesis: (a) HRꢀTEM image of Fe , (b) SAED pattern of Fe , (c)
Siꢀ Fe O and (d) AmꢀSiꢀ Fe O
3
O
4
3 4
O
3
4
3
4
3
4
The morphology of CuꢀAcTp@AmꢀSiꢀ Fe O is characterized by
yield the AcTp@AmꢀSiꢀFe O , these obtained nanoparticles were
3
4
3
4
Scanning electron microscopy (SEM). The SEM images of
magnetite nanoparticles are presented at two different
magnifications (Figure 2a & 2b). While after encapsulation,
further metallated with copper acetate in acetone to achieve
resulted copper complex grafted magnetically recoverable
nanoparticles (CuꢀAcTp@AmꢀSiꢀFe O ) (Scheme 1).
85
3
4
2
| Journal Name, [year], [vol], 00–00
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smooth surfaces of the Fe O core particles were roughened due
3c). It can be accredited due to the lowering of scattering contrast
to deposition of silica coating to magnetic nanoparticles (Figure 45 between the walls of the silica framework and organic moiety
c), which formed uniform and continuous shell around them. attached over SiꢀFe O . It also shows that different reaction
3
4
2
3
4
Separate silica aggregates are not observed indicating that
precipitation of primary silica nanoparticles was occurred only on
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
synthesis conditions.
conditions during the synthesis, did not affect on crystallinity and
morphology of Fe O nanoparticles throughout the process.
5
3
4
1
0
50
3 4 3 4
Figure 3 XRD pattern of the (a) Fe O nanoparticles, (b) SiꢀFe O and (c)
AcTp@ AmꢀSiꢀFe
3
O
4
Figure 4 displays the elemental mapping of the isolated particles
of the AmꢀSiꢀFe O4 and Cu(II)ꢀAcTp@AmꢀSiꢀFe O . Energyꢀ
3
3
4
dispersive Xꢀray spectroscopy (EDS) analysis carried out with Siꢀ
Fe O showed the presence of silica and iron, which reveals the
55
3
4
Figure 2 SEM images of (a) Fe
Fe nanoparticles at high magnification (100 KX), (c) SiꢀFe
AcTp@AmꢀSiꢀFe
3
O
4
at low magnification (32 KX), (b)
encapsulation of magnetite core with silica. Whereas, the
coordination behaviour of 2ꢀacetylthiophene complex grafted
over SiꢀFe O (AcTp@AmꢀSiꢀFe O ) for copper ions was also
3
O
4
3 4
O
and (d)
1
5
3 4
O
3
4
3
4
To confirm the crystalline nature and surface state of the
nanoparticles (Fe O SiꢀFe O AcTp@AmꢀSiꢀFe O ), powder Xꢀ 60 carbon, oxygen, nitrogen, sulphur and copper components
confirmed with the EDS technique. The presence of iron, silicon,
3
4,
3
4,
3
4
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
diffraction peaks matched well with the standard XRD data of
provides a quantitative tool for confirming the immobilization of
2ꢀacetylthiophene ligand, and its further metallation with copper.
The quantitative analysis for copper content in the prepared
nanocatalyst was performed using AAS, and sample digestions
2
2
3
3
4
0
5
0
5
0
Joint Committee on Powder Diffraction Standards (JCPDS) card 65 were carried out in microwave at 400 Watt for 15 min. at constant
number (19ꢀ0629) for Fe O crystal with a cubic inverse spinel
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.
3
4
structure, which is consistent with the TEM results. The Debye–
Scherrer equation (Dhkl ¼ kl/ βcosθ) was used to estimate an
average crystallite size from the XRD patterns, where D is the
−
1
size of the axis parallel to the (hkl) plane, k is a constant with a 70 0.135 mmol g copper content in catalyst was quantified using
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.
Here, the mean crystalline sizes of Fe O nanoparticles were
calibration curve in duplicate for each sample.
Fourier transform infrared spectroscopy (FTꢀIR) seems to be the
best technique to characterize the functionalization and
calculated to be 12.9 nm by measuring the (311) peak widths of 75 modification of magnetic nanoparticles. Figure 5a exhibits the
3
4
the Xꢀray diffraction lines. After encapsulation with silica, a new
characteristic bands of the vibration of the Fe–O bond of the iron
− −
o
1
1
broad peak around 23 θ appears due to the existence of
oxide core at 588 cm and broad band around 3124 cm due to
1
6
17a
amorphous silica but rest of the pattern remains the same as
OꢀH stretching vibrations of adsorbed water. Moreover, on
shown in Figure 3b which, clearly depicts that there is no change
moving from Fe O to SiꢀFe O , significant reduction of the
3 4 3 4
in the topological structure and inherent properties of Fe O4 80 intensity of the Fe–O stretching and bending and the O–H
3
before and after the coating with silica. On assessment of the
diffractograms of silica encapsulated and 2ꢀacetylthiophene
grafted nanoparticles, the very distinguishable FCC peaks of
magnetite crystal were not changed, which means that these
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
−
1
17b
(1090 cm ) which revealed the complete encapsulation of the
particles have the phase stability but, there is slight decrease in 85 magnetic cores with silica. Whereas on functionalization in Amꢀ
−1
intensity with broadening of corresponding peak of silica (Figure
SiꢀFe
3
O
4
, the band at 2925 cm in Figure 5c, corresponds to the
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 |
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–
CH2 group of aminoꢀpropyl group in functionalized AmꢀSiꢀ
hysteresis loops of powdered materials showed almost negligible
magnetic hysteresis with both the magnetization and
demagnetization curves passing through the origin, which clearly
Fe O (which does not appear in the parent SiꢀFe O spectrum).
A strong band at 1628 cm corresponding to the scissoring
3
4
3
4
−1
vibration mode of –NH after treating the nanoparticles with 3ꢀ 45 indicates the superparamagnetic nature of the materials. This also
2
5
aminopropyltriethoxysilane are also identified. By comparing the
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 Fe O
spectra of AcTp@AmꢀSiꢀFe O4 (Figure 5d) with those of
3
functionalized AmꢀSiꢀFe O , is observed that after the Schiff
3
4
3
4
condensation reaction, the characteristic band of the imine group
nanoparticles appear to be suitable as the support for catalyst.
−1
17c
(
C=N) appears at 1644 cm in Figure 5d. On metallation, it is
50
−1
−1
10
observed that the absorption at 1644 cm is shifted to 1637 cm
and also the intensity of this peak has decreased after
immobilization confirming that copper is successfully anchored
onto the surface of AmꢀSiꢀFe O (See ESIꢀS2).
3
4
15
3 4 3 4
Figure 4 EDS Pattern of (a) SiꢀFe O and (b) CuꢀAcTp@AmꢀSiꢀFe O
The magnetic properties of the synthesized Fe O nanoparticles,
3
4
Figure 5 FTꢀIR spectra of (a) Fe
3
O
4
(b) SiꢀFe
3
O
4
(c) AmꢀSiꢀFe
3
O
4
(d)
SiꢀFe O and CuꢀAcTp@AmꢀSiꢀFe O were analyzed by
AcTp@AmꢀSiꢀFe O and (e) CuꢀAcTp@AmꢀSiꢀFe O
3
4
3
4
3
4
3
4
vibrating sample magnetometry (VSM). The fieldꢀdependent
magnetization curves shown in Figure indicates the
6
20
25
30
35
40
magnetization as a function of applied magnetic field, measured
at room temperature (293 K) with the field sweeping from
−
20,000 to 20,000 Oe. The saturation magnetization, Ms, of bulk
−1 18a
−1
magnetite (92 emu g )
was reduced to 69 emu g for
magnetite nanoparticles. It is known that the magnetization of a
magnetic particle in an external field is proportional to its size
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
Fe O
nanoparticles
and
Cuꢀacetythiophenyl
complex
3
4
−1
immobilized Fe O nanoparticles reached upꢀto 53 emu g and
3
4
−1
2
8 emu g respectively. In spite of these low magnetization
values with respect to magnetization of pure Fe O nanoparticles,
3
4
which was owing to decrease in the surface moments of the
18b
magnetite nanoparticles by diamagnetic silica coating
over
Fe O nanoparticles and grafting of metalꢀligand complex over
3
4
AmꢀSiꢀFe O . But, it is still sufficient for magnetic separation by
3
4
a conventional magnet. The above showed TEM images also
confirmed that the encapsulation and grafting of nonmagnetic
SiO2 and organic layer over Fe O nanoparticles. Another
3
4
55 Figure 6 Magnetization curves obtained by VSM at room temperature for
(a) Fe (b) SiꢀFe , (c) CuꢀAcTp@AmꢀSiꢀFe and (d) inset:
enlarged image near the coercive field
important parameter for practical applications of nanoparticles is
revealed from the enlarge VSM curve shown in Figures 6d. The
3
O
4
3
O
4
3 4
O
4
| Journal Name, [year], [vol], 00–00
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a
Table 1 Screening of copper catalysts for Nꢀalkylation of amines
Catalytic activity of Cu-AcTp@Am-Si-Fe O for N-alkylation
of aniline with benzyl alcohol
Entry
Catalyst
No
Conditio
n
aerobic
Time
(h)
24
Conv.
(%)
Trace
3
4
b
1
.
The optimization of catalytic conditions was carried out by
employing the alkylation of aniline with benzyl alcohol under
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.
5
0
5
0
5
0
5
0
5
2.
AmꢀSiꢀFe
Cu@AmꢀSiꢀFe
3
O
3
4
aerobic
aerobic
24
10
Trace
59
3
.
O
4
4
.
CuꢀAcTp@ AmꢀSiꢀFe
10 mg)
CuꢀAcTp@ AmꢀSiꢀFe
15 mg)
CuꢀAcTp@ AmꢀSiꢀFe
20 mg)
3
3
3
3
3
O
O
O
O
O
4
4
4
4
4
aerobic
aerobic
10
10
10
10
10
10
67
82
(
5.
(
1
1
2
2
3
3
4
4
6
7
8
.
.
.
aerobic
98
A control experiment of alkylation of aniline without any catalyst
showed negligible conversion even after 24 h of reaction (Table
(
CuꢀAcTp@ AmꢀSiꢀFe
(25 mg)
CuꢀAcTp@ AmꢀSiꢀFe
aerobic
98
1
). Similarly, the experiment of oxidation in similar conditions
TEMPO
anaerobic
27
using AmꢀSiꢀFe O also did not undergo any product formation
3
4
(25 mg)
(Table 1, entry 2). Also, the use of copper directly attached to
amine functionalized silica coated Fe O nanoparticles i.e.
9.
CuꢀAcTp@ AmꢀSiꢀ Fe
(25 mg)
3
O
4
Trace
3
4
a
o
Cu@AmꢀSiꢀFe O (without being complexed to acetylthiophene) 50 Aniline (1 mmol); Benzyl alcohol (2 mmol); Temp. 100 C
3
4
b
Conversion was determined by GC
(
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 Cuꢀ
Acetylthiophenyl complex grafted nanoparticles over AmꢀSiꢀ
The promotional effect of base has been examined by carrying a
base blank reaction. The desired amine product was not detected,
Fe O i.e. CuꢀAcTp@AmꢀSiꢀFe O for reaction (Table 1, entry 55 indicating that its presence is helpful in the deprotonation of the
3
4
3
4
4
), gave product with highest conversion and selectivity. It
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 CuꢀAcTp@AmꢀSiꢀFe
various bases (KOH, K CO NaOH, tꢀBuOK) have been
60 employed using series of solvents such as dioxane, toluene, H O,
oꢀxylene. 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 (Cuꢀ
clearly showed that though, the reaction is catalyzed by Cu ions
but the role of ligand is significant for the transformation. In this
case, acetylthiophene with strong σ donation by O and S atoms
acted as a coꢀcatalyst and also bound to the metal ion effectively.
O , the
4
3
2
3
,
2
The quantitative analysis of the nanocatalyst was also performed
and it was found that the yield of the product increased from 59%
to 98% with increasing amount of catalyst from 10ꢀ25 mg
AcTp@AmꢀSiꢀFe O ) under solvent free conditions gave
3 4
respectively. This could be mainly due to the availability of large 65 maximum conversion (96%) with KOH as base. Whereas, weaker
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
different oxidants are documented but reaction driven under air is
substitute of base K
2
CO also showed good conversion for the
3
reaction, but provided less selectivity for amine products. Since
reagentꢀgrade KOH (RGꢀKOH) is known to have transition metal
contaminant, hence, semiconductorꢀgrade KOH (SGꢀKOH,
clearly found to be the greenest and most advantageous protocol 70 99.99% pure based on trace metal analysis) obtained from Alfa
regarding convenience, efficiency, economy, environmental
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.
1
1&19
considerations, etc
. This was again confirmed by carrying
out a reaction under different conditions (Table 1, entries 7-9).
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
demonstrate the suitability of aerobic environment, reaction under
anaerobic condition (under inert atmosphere of nitrogen) was also
scrutinized (Table 1, entry 9).
This journal is © The Royal Society of Chemistry [year]
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Page 7 of 11
Green Chemistry
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DOI: 10.1039/C3GC40818C
Figure 7 Effect of base and solvent on Nꢀalkylation of amines (Reaction
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
reaction of various aniline derivatives with electron donating
conditions: Amines (1 mmol); Alcohols (1.5 mmol); Catalyst (20 mg);
o
Temp. 100 C; Time 10 h)
2
2
3
3
4
0
5
0
5
0
To study the effect of temperature on the activity of the catalyst
(Figure 8), model reaction was carried out under the diverse
range of temperature (25ꢀ120 °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
other hand, the further increase in temperature did not have any
effect on the yield.
5
(entries 2-4) and electronꢀwithdrawing (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
reaction. Like paraꢀ and metaꢀ substituted substrates, the
sterically more bulky ortho substituted also gave good results
10
(
Table 2, entries 5-6). The reaction of transꢀcinnamyl alcohol
with aniline derivatives yielded transꢀcinnamyl amine
compounds selectively and formation of cisꢀcinnamyl amine
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 1ꢀbutanol, 2ꢀbutanol and methanol (Table
2
, entries 12-14).
It is evident from the Table 3 that CuꢀAcTp@AmꢀSiꢀFe O
3
4
nanocatalyst is highly efficient in catalyzing the aerobic Nꢀ
alkylation of amines and gave products in good yields with high
turnover number (TON) values in comparison to the previous
19
literature reports. 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.
Figure 8 Effect of temperature on selectivity and conversion for Nꢀ
1
5
5
alkylation of amines (Reaction conditions: Amines (1 mmol); Alcohols
o
(
1.5 mmol); Catalyst (20 mg); Temp. 100 C; Time 10 h)
a
4
Table 2 Scope of catalytic activity of the CuꢀAcTp@AmꢀSiꢀFe
3
O
4
in aerobic Nꢀalkylation of amines with different alcohols
b
c
d
S.No.
R
1
NH
2
R
2
OH
Conv.
%)
Product Selectivity
Applications
TON
(
NH
NH
2
2
1
2
3
.
.
.
98
91
93
Nꢀbenzylbenzenamine
Pharmaceutical intermediate
362
OH
(100 %)
Nꢀcinnamylbenzenamine
100 %)
Herbicide
337
344
OH
(
NH
2
Nꢀbenzylꢀ4ꢀchlorobenzenamine
99 %)
Pesticides, Herbicidal
OH
OH
(
Cl
NH
2
4
5
.
.
88
83
Nꢀbenzylꢀ4ꢀnitrobenzenamine
100 %)
Intermediate for dyestuffs
307
340
(
No
2
Nꢀcinnamylꢀ2,6ꢀ
Fine organic and custom intermediate
OH
NH
2
dimethylbenzenamine
(95 %)
6
.
92
Nꢀbenzylꢀ2,6ꢀ
dimethylbenzenamine
Herbicide
340
OH
OH
NH
2
(97 %)
NH
2
7
8
.
.
93
86
Nꢀbenzylꢀ4ꢀ
methoxybenzenamine
A potent Fungicidal
Herbicide
344
318
(
100%)
3
H CO
NH2
(Z)ꢀNꢀbenzylꢀ3ꢀphenylpropꢀ2ꢀ
enꢀ1ꢀamine
OH
(97 %)
6
| Journal Name, [year], [vol], 00–00
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Green Chemistry
Page 8 of 11
View Article Online
DOI: 10.1039/C3GC40818C
9
.
NH2
96
82
88
90
Dibenzylamine
Pharmaceutically acceptable a
therapeutic agent for arteriosclerosis
355
303
325
333
OH
(100 %)
NH
NH
2
1
1
1
0.
1.
2.
Nꢀcinnamylnaphthalenꢀ1amine
96.3 %)
A potent fungicidal
OH
(
2
Nꢀbenzylnaphthalenꢀ1ꢀamine
100 %)
Antiparasitic or in combination with
fungicidal and antibacterial
OH
(
NH
NH
2
CH
3
ꢀOH
Nꢀmethylbenzenamine
98%)
Pharmaceutical intermediate
(
2
OH
92
NꢀmethylꢀNꢀ
propylbenzenamine
Pharmaceutically intermediate for
treating Amyloid Disease
340
322
1
3.
(99%)
NH
2
OH
1
4.
87
Nꢀmethylꢀ4ꢀnitroꢀNꢀ
propylbenzenamine
Intermediate for the production of
agricultural chemicals
(97%)
No
2
a
o
Reaction conditions: Amines (1 mmol); Alcohols (1.5 mmol); Catalyst (20 mg); Temp. 100 C; Time 10 h
b&c
Conversion and selectivity were determined by GC & GCꢀMS
c
Selectivity = (GC area of amine product)/(GC area of all products)×100
d
3 4
TON= Calculated using the 0.135 mmol/g Copper (Obtained by AAS for CuꢀAcTp@AmꢀSiꢀFe O )
conversion after separation of the catalyst (Figure 9). It
corroborated that the copper has not been leached out during the
25 course of the reaction which further signifies the stability and
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
5
Table 3 A comparisons of the results of the present system with the
recently published aerobic catalytic systems for the Nꢀalkylation of
amines.
S.No.
Substrate
Catalytic Reaction conditionsYield Ref.
system
o
1
2
3
4
5
6
PhSO
PhCH
2
NH
2
+
MnO
2
K
2
CO
Air, 24 h
3
, 120 C, 96% 19a
2
OH
30
amount of Cu compared with the fresh catalyst.
o
PhSO NH
PhCH OH
PhCH CH NH
PhCH OH
PhNH + PhCH
2
2
+
RhCl(PPh
3
)
3
2 3
K CO , 150 C, 92% 19b
2
Toluene, 24 h
o
2
2
2
+
Pd/AlO(OH) 90 C, Heptane, 20 94% 19c
2
h
o
2
2
OHRu(OH)x/Al
2
O
3
132 C, 11 h,
Mesitylene
o
98% 19d
PhSO
2
NH
2
+
Cu(OAc) .H
2
2
O K
2
CO
3
, 120 C, 12 97% 11c
h
PhCH
2
OH
PhNH + PhCH OH Our catalyst Solventless, 10 h, 99% This
2
2
o
KOH,100 C, Air
work
1
0
Hot Filtration Test
Hotꢀfiltration 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
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
poured back into the reactor and the reaction was continued for
additional 5 h. It was found that there was almost no further
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
3
5
1
5
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
each catalytic reaction, the spent catalyst CuꢀAcTp@AmꢀSiꢀ
2
0
40
Fe O4 was trapped with simple magnet, and washed with
3
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Green Chemistry
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DOI: 10.1039/C3GC40818C
degassed DCM (5×10 mL) and dried under vacuum. As shown in
Figure 10, the catalyst could be reused 10ꢀfold with no
Fast under air
2
H O
2
Air(1/2 O )
Step-4
significant detrimental effect on the chemical yield obtained. The
structure and morphology of the recycled nanocatalyst (after 10
Cu
th
R
1
OH
R1
O
R
1
O
5
0
5
reaction) was observed through SEM and VSM. As shown by
VSM curve (See ESIꢀS3), 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 ESIꢀS4) also showed no
agglomeration between particles which shows high colloidal
stability and reꢀdispersibility 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, CuꢀAcTp@
AmꢀSiꢀFe O nanocomposites have proved as an efficient
Step-1
R
2
R
1
N
H
Cu(II)
Slow under nitrogen
R NH
Cu(I)
Cu
Step-3
Step-2
2
2
Base
1
R
1
OH
R
1
H
2
O
NR2
Cu
Cu-AcTp@ASMPs
45
Scheme 2 Proposed reaction mechanism
1
3
4
nanocatalyst in terms of recoverability and recyclability.
Conclusions
In conclusion, we have developed novel, efficient and reusable
silica based organicꢀinorganic hybrid copper nanocatalyst for
aerobic Nꢀalkylation of amines using alcohols under less
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 lessꢀcorrosive conditions, which
leads to zero effluent discharge to the environment. In addition to
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.
5
5
6
0
5
0
Figure 10 Catalyst recycling test for successive ten runs of Nꢀalkylation
20
of aniline
Experimental Details
Proposed catalytic mechanism
General remarks
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 CuꢀAcTp@AmꢀSiꢀ
Fe O 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
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,
metalꢀcatalyzed 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
corresponding imine via condensation reaction in Stepꢀ2. In Stepꢀ
TEOS (tetraethoxyorthosilicate) were procured from Sigma
Aldrich. 3ꢀAminopropyltriethoxysilane (APTES) was obtained
from Fluka. Copper(II) acetate, Ferric(III) sulphate and
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. Doubleꢀdistilled water was utilized
throughout the studies
Xꢀray 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).
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
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
on chosen spots of the analyzed samples after exclusion of
6
7
7
8
8
5
0
5
0
5
25
30
35
40
3
4
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
1
1b&c
game with the “handing off” of hydrogen atoms.
Finally, in
Stepꢀ4, reaction for the next cycle proceeds with the aldehyde
obtained in the previous step.
8
| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C3GC40818C
extraordinary high or low values (not exceeding a twoꢀfold
amount). Size and morphology analysis of prepared
nanocomposites was also performed using Gemini Ultra 55
Synthesis of acetylthiophene functionalized silica based
organic-inorganic hybrid Cu(II) nanocatalyst
60
For covalent grafting of 2ꢀacetylthiophene (AcTp) on AmꢀSiꢀ
Fe O , AmꢀSiꢀFe O (2 g) and 2ꢀacetylthiophene (4.0 mmol) in
(Zeiss) scanning electron microscope (SEM). Samples were
3
4
3
4
5
0
5
0
5
0
prepared by first applying doubleꢀsided tape to metal stubs. The
sample was then placed onto the tape. Excess sample was
removed using pressurized air. The sample was then sputterꢀ
coated with a 10ꢀnm 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
aggregates in the magnetiteꢀethanol dispersions. The
magnetization curve was obtained by a vibrating sample
magnetometer (EVꢀ9, Microsense, ADE). Fourier transform
infrared spectra (FTꢀIR) were recorded using PerkinꢀElmer
Spectrum 2000. Digestions were performed in Anton Paar
multiwave 3000 microwave reaction system equipped with
temperature and pressure sensor. The amount of copper in the
ethanol (250 mL) were refluxed for 3 h. Then, the resultant
grafted AcTp@AmꢀSiꢀFe O (1 g) were stirred with solution of
3
4
6
5
1.5 mmol of copper acetate in acetone for 4 h. The resulted
copper complex grafted nanoparticles (CuꢀAcTp@AmꢀSiꢀFe O )
3
4
were separated magnetically, washed thoroughly with DCM and
then with water and dried in vacuum oven.
1
1
2
2
3
General procedure for Cu-AcTp@Am-Si-Fe O nanocatalyst
3
4
70
mediated N-alkylation of amines
Typically, the Nꢀalkylation 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
catalyst and in the supernatant was estimated by Atomic 75 regular time intervals and analyzed by gas chromatography (GC).
absorption spectroscopy (AAS) on LABINDIA AA 7000 Atomic
Absorption Spectrometer using an acetylene flame. The optimum
parameters for Cu measurements are: wavelength= 324.7 nm;
lamp current=2 mA; slit width=0.2 nm; fuel flow rate=0.2 L
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%
−1
min . The derived products were analyzed and confirmed by 80 NaHCO
and water solution, dried and concentrated to give
3
using Agilent gas chromatography (6850 GC) with a HPꢀ5MS
5% phenyl methyl siloxane capillary column (30.0 m × 0.25 mm
products. The structure elucidations of the products were
confirmed by GCꢀMS. 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).
×
0.25 mm) and a quadrupole mass filter equipped 5975 mass
selective detector (MSD) using helium as carrier gas. The carrier
−1
gas was helium (rate 1.0 mL min ) and the temperature of the
85
Recycling procedure
o
injection port was 250 C. The temperature program of the
column was set to an initial oven temperature of 100 °C and was
After completion of the oxidation reaction, the catalyst was
separated by an external magnet followed by washing with
−1
increased at a rate of 10 °C min to 250 °C, and the oven was
held at 250 °C for 10 min.
Ethanol and H
O. The catalyst was then used directly for next
2
round of reaction without further purification.
Synthesis of nano-support composites
Magnetite (Fe O4) nanoparticles were synthesized using coꢀ
precipitation method. Ferric sulphate (6.0 g) and ferrous
3
90 Acknowledgement
1
4
3
4
4
5
5
5
0
5
0
5
Y. Monga thanks the DST (Department of science and
technology), New Delhi, India, for awarding the Inspire
Fellowship. Also, due thanks to USICꢀCLF, University of Delhi,
Delhi, India for HRꢀXRD and HRꢀTEM and AIRF, JNU, Delhi,
sulphate (4.2 g) were dissolved in water (250 mL) and stirred at
o
6
0 C to give yellowishꢀorange solution. Then, 25% NH OH (15
4
mL) was added with vigorous mechanical stirring, with which
colour of the bulk solution changed to black. Stirring was
9
5
India for SEM analysis
continued for another 30 min. The precipitated Fe O4 were
3
separated magnetically and washed several times with deionized
water and ethanol. Silica coating of these Fe O nanoparticles
Notes and references
3
4
1
5
*
was performed via solꢀgel approach. Dispersed solution of
activated Fe O with 0.1 M HCl (2.2 mL) was prepared in
Prof. R. K. Sharma
Green Chemistry Network Centre, Department of Chemistry,
University of Delhi, Delhi-110007, India. Tel/Fax +91-011-27666250
3
4
mixture of ethanol (200 mL) and water (50 mL) under sonication.
1
1
00 E-mail: rksharmagreenchem@hotmail.com
Electronic Supplementary Information (ESI) available: [details of any
Then, 25% NH OH (5 mL) was added to the suspension at room
4
†
temperature followed by the addition of TEOS (1 mL). Further,
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
05 to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
o
the mixture was kept for stirring at temperature of 60 C for 6 h.
The obtained silica coated magnetic nanoparticles (SiꢀFe O )
3
4
were separated magnetically, washed with ethanol and dried
under vacuum.
Finally, in order to introduce the amine groups to the silica
1
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3
4
resulting mixture was stirred for 6 h at 50 °C. Devised
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