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1463-9262(2010)12:9;1-U

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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

Page 1 of 11 Green Chemistry

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View Article OnlineDOI: 10.1039/C3GC40818C


Green Chemistry

Cite this: DOI: 10.1039/c0xx00000x

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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

Page 2 of 11Green Chemistry

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2 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]

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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This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 3

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


(20 mg)

aerobic 10 98


(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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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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