Magnetically recoverable graphene-based nanocomposite material as an efficient catalyst for the synthesis of propargylamines: Via A3 coupling reaction

Article abstract of DOI:10.1039/c7nj03412a

The magnetically separable Au NPs-Fe₃O₄-rGO catalyst provides a robust and efficient route for the A³ coupling reaction of secondary amines, terminal alkynes and aldehyde with tolerance of diverse functional groups for the

Full text of DOI:10.1039/c7nj03412a

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Magnetically recoverable graphene-based nanocomposite material as an efficient
catalyst for the synthesis of propargylamines via A3 coupling reaction
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Najrul Hussainab* and Manash R. Das ab
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a Materials Sciences & Technology Division, CSIRꢀNorth East Institute of Science and Technology,
Jorhatꢀ785006, Assam, India.
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b Academy of Scientific and Innovative Research, CSIRꢀNEIST Campus, India
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*To whom correspondence should be addressed
Najrul Hussain
Materials Sciences & Technology Division,
CSIR−North East Institute of Science and Technology, Jorhat−785006, Assam, India
Tel: +91ꢀ9864582365,
Fax: +91−376−2370011,
E Mail: najrul0588@gmail.com
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Abstract:
Magnetically separable Au NPsꢀFe3O4ꢀrGO catalyst provides robust, efficient route for A3
coupling reaction of secondary amines, terminal alkynes and aldehyde with tolerance of
diverse functional groups for the synthesis of propargylamine in high yields. Further, the
catalyst can be reused upto several times without lacking of significant activity due to
stability of the NPs onto reduced graphene oxide sheets (rGO). The synthesized catalyst was
characterized by different instrumental techniques before as well as after the catalytic
reaction and found that the catalyst to pertain the size and surface morphology. The
advantages of using Au NPsꢀFe3O4ꢀrGO catalyst for the synthesis of pharmaceutically
important compounds propargylamines over other previous reports includes its magnetically
separable in nature, additiveꢀfree, functional group tolerance and recyclability. The enhanced
catalytic activity of Au NPsꢀFe3O4ꢀrGO composite material is results from the synergistic
effect among Au NPs, Fe3O4 NPs and rGO sheets.
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Keywords: Nanoparticles; reduced graphene oxide; iron oxide nanoparticles; gold
nanoparticles; A3 coupling reaction; propargylamine
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1. Introduction
In the past decade, one pot multicomponent reaction became a powerful synthetic tool for the
synthesis of industrially important compounds. Among them, significant efforts have been
devoted to the one pot A3 coupling reaction of amine, aldehyde and alkyne because of their
coupling products, propargylamine, which have played a remarkable role as major skeleton
for the synthesis of various biologically important compounds like oxazoles, isosters,
oxotremorine analogues, conformational restricted peptides and as a useful intermediate for
the production of natural products such as βꢀlactams, herbicides, fungicides.1ꢀ5
Propargylamines are also important structural elements for various drug molecules in the
treatment of some diseases such as Parkinson’s and Alzheimer’s disease.6,7
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In view of the importance of the propargylamines, there is a continuous efforts by the
researchers in escalating the scope of the A3 coupling reaction by using different types
transition metal based catalysts including iridium complexes,8 Ag(I) salts,9 Cu(I) salts,10
Au(III) salen complexes,11 Au(I) salts,12 Iron(III) salts,13 etc. But all these reported catalysts
have disadvantages like use of homogenous catalyst which results difficulties in separation,
use of large amounts of expensive reagents, the reagents used are highly sensitive to moisture
and prerequisite of exact reaction conditions etc. In order to overcome above drawbacks,
different heterogeneous catalysts such as hydroxyapatiteꢀsupported copper (CuꢀHAP),14 Ag
NPs supported by Ni,15 nanocrystalline CuO,16 LDHꢀAuCl4, silica immobilized CuI,18
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heteropolyacidꢀsupported silver (AgꢀHPA)19 copperꢀzeolites,20 NiꢀYꢀzeolite,21 AgY zeolite,22
CeO2ꢀ or ZrO2ꢀstabilized Au(III),23 Au@HSꢀMCM,24 Supported Au/MILꢀ5325 are reported
for the A3 coupling reaction. However, these catalytic systems still suffers some serious
issues like use of toxic solvents, need of different additives, prolonged reaction time, tedious
catalyst separation etc.17,26,27 Therefore, the development of an efficient, environmentally
benign and easily recoverable heterogeneous nanocatalyst for the A3 coupling reaction is
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highly desirable. Fe3O4 NPs have attracted worldwide attention due to its extra ordinary
magnetic behaviour which results separation of the catalyst using magnet after performing the
reaction and it is typically more effective than centrifugation as well as filtration.28ꢀ30 Zeng et
al. recently developed robust, safe and magnetically recoverable Fe3O4 NPs for A3ꢀcoupling
of aldehyde, terminal alkyne and secondary amine.31 Kaur and his coꢀworkers have also
developed superparamagnetic Fe3O4 NPs for this coupling reaction.32 However, the direct use
Fe3O4 NPs without support or modification results agglomeration of the particles which
results low surface to volume ratio of the material which in turn results decrease in the
catalytic activity. Also, the drawback associated with these reports is that there is no details
study of morphology as well as the stability of the Fe3O4 NPs after performing the A3ꢀ
coupling reaction. In this regard, Fe3O4 NPs are decorated on some support materials to
increase the surface to volume ratio of the material for their efficient use in heterogeneous
catalysis.33ꢀ35Among different support materials, graphene is particularly important material
as a support for Fe3O4 NPs due to its extraordinary properties such as high electron mobility,
exceptional thermal conductivity and large surface area.36ꢀ40 Mandal and his coꢀworkers
developed Fe3O4–graphene composites for their catalytic application in the A3 coupling
reaction.41 Huo et al. developed graphene–Fe3O4 nanocomposite material by adopting thermal
decomposition method and the resulted nanocomposite material showed high catalytic
efficiency in this reaction.42 In addition to Fe3O4 NPs, Au NPs supported on graphene also
exhibit excellent catalytic activity towards the A3 coupling reaction since Au NPs can act as
soft Lewis acid which in turn helps to activate unsaturated functionalities to form C–C
bond.43,44 Dabiri et al. reported the synthesis of Au NPs on ionic liquid functionalized
graphene oxide and the synthesized catalyst was found to exhibit good catalytic activity in the
A3 coupling reaction of aldehyde, amine and benzyl alcohols.45 Movahed and his coꢀworkers
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reported the synthesis of composite material comprises of Au NPs and HSꢀfunctionalized
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In view of the advantages of both Au and Fe3O4 NPs as discussed above, herein, we have
synthesized Au NPs onto the surface of Fe3O4ꢀrGO nanocomposite material which includes
both the advantages of using Au and Fe3O4 nanoparticles to enhance the reaction yield for A3
coupling reaction as well as for effective magnetic separation. The noble metal like Au NPs
supported on magneticꢀgraphene composite material should be expected to exhibit high
catalytic efficiency due to numerous favourable aspects such as the size and shape of overall
composite materials, the support effect, oxidation state of the metal and the synergistic
effect.47 The synthesized Au NPsꢀFe3O4ꢀrGO composite material showed outstanding
catalytic activity for the A3 coupling reaction of aldehyde, alkyne and amine and can be
simply separated by using an external magnet after performing the reaction. Therefore, this
new heterogeneous catalytic system provides an easy, robust and effective catalytic activity
towards A3 coupling reaction.
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2. Experimental
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2.1. Materials and Methods
Graphite powder (<20ꢁm, SigmaꢀAldrich), H2SO4 (AR grade, Qualigens, India), HCl (AR
grade, Qualigens, India), H2O2 (30%, Qualigens, India), KMnO4 (>99 %, EꢀMerck, India),
FeCl2.4H2O (>99%, SigmaꢀAldrich, India), HAuCl4 (Sigma Aldrich, USA), hydrazine
hydrate (80%, EꢀMerck) and all substrates for organic reactions were bought from Sigma
Aldrich, USA.
Xꢀray diffraction (XRD) measurement for the materials were carried out from 5ꢀ100o with 3
deg·min−1 scanning rate using ULTIMA IV, Rigaku, Japan Xꢀray diffractometer with a Cu
Kα Xꢀray source (λ = 1.54056 Å). The diffuse reflectance infrared Fourier transform
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(DRIFT) spectral analyses of the samples were made in the frequency range of 400 – 4000
cm−1 by IR Affinityꢀ1, Shimadzu, Japan FTIR spectrophotometer. The thermal stability of
the composite materials was examined by Thermal Analyzer of model No: Netzsch STA
449F3 from 20 °C to 1000 °C with a heating rate 10 °C min−1 under inert atmosphere. The
morphology of the composites were examined by Transmission electron microscopy (TEM)
and high resolution transmission electron microscopy (HRTEM) images with the help of
JEOL JEMꢀ2011 electron transmission microscope, Japan. SEMꢀEDS analysis was
performed by using Carl ZEISS Field Emission SEM with Oxford EDS to determine the
element present on the composite material. Room temperature vibrating sample
magnetometer analysis was carried to measure the magnetic parameters of the composite
material using VSM, Lake Shore 7410, USA. Atomic absorption spectroscopy (AAS)
analysis was carried using Perkin Elmer (ModelꢀAnalystꢀ700) spectrometer to determine the
metal content of the composite material. All NMR spectra for confirmation of the organic
products were obtained on Bruker Advance DPX 500 MHz NMR spectrometer. Chemical
shifts are given as ppm on the δ scale downfield from tetramethylsilane. NMR spectra of
compounds are reordered with a deuterated solvent with residual solvent signal peaks at
δ=7.26 ppm (1H) and δ=77 ppm (13C). (NMR data are given in supporting information).
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2.2. Synthesis of the Graphene oxide (GO)
The synthesis and characterization of GO was discussed in our previous publication.48,49
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2.3. Synthesis of Au NPs-rGO composite material
The synthesis procedure of Au NPsꢀrGO composite material and its characterization was
reported in our previous publication.50
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2.4. Synthesis of Fe3O4-rGO composite material
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The synthesis procedure of Fe3O4ꢀrGO composite material and its characterization was
mentioned in our previous publication.51,52
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2.5. Synthesis of Au NPs-Fe3O4-rGO composite material
250 mg of Fe3O4ꢀrGO composite material is dispersed in 25 mL deionized water followed by
addition of aqueous HAuCl4 solution (2 mL, 0.01M). The solution was then stirred for 15
min. Then hydrazine hydrate (5 mL + 10 mL water) was added on the above reaction mixture
and then heated at 75 °C for 20 min. After that 40 mg of NaOH is added to the above mixture
and then the reaction mixture was heated to 85 °C with constant mechanical stirring. After
that the solid product was separated by centrifugation and washed with ethanol followed by
water. Finally the solid residue was dried at 60 °C.
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2.6. General experimental procedure for the synthesis of propargylamine
Aldehyde (1 mmol), amine (1.2 mmol) and alkyne (1.5 mmol) were taken in round bottom
flask containing the suspension of the Au NPsꢀFe3O4ꢀrGO composite material (25 mg in 3
mL DMSO). The whole reaction mixture was ultrasonicated for 5 min followed by heating
with continuous stirring at 90 °C for desired time. After the completion of the reaction, the
product was extracted with ethyl acetate followed by drying over Na2SO4. The crude products
were purified by using column chromatography to get the desired product.
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2.7. Reusability experiment
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After completion of the reaction, the catalyst was separated using an external magnet without
any filtration or centrifugation. Then, the catalyst was washed 2ꢀ3 times with acetone
followed by water and dried in air oven at 65 °C. Then the dried catalyst was further used in
the A3 coupling reaction without any prior activation or additives. To study the reusability of
the catalyst, 4ꢀchlorobenzaldehyde (1 mmol), piperidine (1.2 mmol) and phenylacetylene (1.5
mmol) were taken as model substrates for the A3 coupling reaction.
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3. Results and Discussion
3.1.Characterization of the synthesized composite material
The crystalline nature of the synthesized Au NPsꢀFe3O4ꢀrGO composite material was
characterized by XRD. (Fig. 1) The observed peaks at 30.1°, 35.5°, 43.2°, 53.5°, 57.0° and
62.8° corresponding to the (220), (311), (400), (422), (511) and (440) planes of face cantered
cubic structure of Fe3O4 NPs (JCPDS card no. 00–003–863). Other characteristic peaks of 2θ
value at 38.0°, 44.4°, 64.5° and 77.4° corresponding to the (111), (200), (220) and (311)
planes of Au NPs. The XRD peaks are well matched with the JCPDS card No 004 – 0784.
The very broad peak at around 24.2° is due to the rGO sheets. The XRD confirm the presence
of both Au as well as Fe3O4 NPs on to the rGO sheets.
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The morphology of the Au NPsꢀFe3O4ꢀrGO composite material was examined by TEM and
HRTEM analysis. The TEM images displayed the rGO sheets which were in micron size as
indicated by arrow mark in the Fig. 2 and Au NPs as well as Fe3O4 NPs are uniformly
distributed onto those sheets. Fig. 2(aꢀc) shows that high densities of Au NPs and Fe3O4 NPs
in the size range of 20 to 45 nm are decorated uniformly onto rGO sheets. The Au NPs and
Fe3O4 NPs can be clearly differentiated on the basis of lattice fringe in the HRTEM images as
shown in Fig. 2(d, e). The measured lattice spacing value of 0.25 nm and 0.23 nm are
consigned to the interplanar distances of (311) plane in the Fe3O4 lattice and (111) plane of
Au lattice, respectively. The diffraction dots seen in the selected area of electron
diffraction image proves the presence of crystallinity in the Au NPsꢀFe3O4ꢀrGO composite
material (Fig. 2(f)).
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The surface morphology as well as the chemical composition present on Au NPsꢀFe3O4ꢀrGO
composite material was studied by SEMꢀEDS analysis (Fig. 3 (aꢀc)). The crumpled structure
of GO is partially destroyed after the synthesis of the composite material due to the reduction
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of oxygen containing functional groups. The EDS analysis as shown in Fig. 3(d) clearly
confirms the presence of Au, Fe, C and O in the composite material which evident deposit of
Au NPs onto Fe3O4ꢀrGO composite surfaces.
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The FTIR spectra of GO and Au NPsꢀFe3O4ꢀrGO composite material shows the prominent
peaks of GO at 3131 and 1728 cm−1 which are due to the O−H and C=O stretching vibration
[53] (Fig. 4). The other peaks observed at 1436 and 1042 cm−1 are due to the C−O−H and
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C−O−C, stretching vibrations, respectively.54 The intensities of these stretching vibration
observed in GO are diminished in the synthesized Au NPsꢀFe3O4ꢀrGO composite material
due to the deposition of Au NPs and Fe3O4 NPs on the GO surface. Further, the prominent
peak observed at 583 cm−1 is due to the FeꢀO bending vibration of the Au NPsꢀFe3O4ꢀrGO
composite material.55
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The magnetic properties of the synthesized composite material Au NPsꢀFe3O4ꢀrGO were
measured at room temperature as shown in Fig. 5. The magnetic parameters such as
saturation magnetization (Ms), the coercivity (Hc) and remanent magnetization (Mr) are listed
inside the Fig. 5. The saturation magnetization is found to be 42.72 emu/gwhich is very much
sufficient for easy and effective separation of the catalyst from the reaction mixture using
external magnet. The Remanence magnetization (Mr) and the coercivity (Hc) of the material
are found to be 3.26 emu/g and 59.98 Oe, respectively. The hysteresis behaviour as well as
the magnetic parameters found clearly reveals the ferromagnetic behaviour of Au NPsꢀFe3O4ꢀ
rGO composite material. The magnetic property of the composite material is results from the
Fe3O4 species. Mahmood et al. investigated the effect of Au on the magnetic properties of
Fe3O4 species with and without doping Au.56 It was observed that the Fe3O4 species exhibit
ferromagnetic properties in both presence and absence of Au. Thus, our synthesized Au NPsꢀ
Fe3O4ꢀrGO composite material does not have any effect of Au NPs on the magnetic
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properties of the resulted composite materials. The excellent ferromagnetic behaviour of Au
NPsꢀFe3O4ꢀrGO composite material results magnetic separation of the catalyst after
performing the A3 coupling reaction which is much more efficient method than the normal
centrifugation or filtration.
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Thermogravimetric analysis (TGA) evident that the major weight loss of GO of about
37.72% occurred at 200 °C resulting from the decomposition of the labile functional groups
present in GO as shown in Fig. 6. The weight loss decreases from 37.72% to 8.29% after
formation of Fe3O4ꢀrGO composites material because the oxygen containing functional
groups present in GO such as hydroxyl, epoxy, carbonyl and carboxyl groups are reduced.
The weight loss in this region further decreases upto 1.37% in case of Au NPsꢀFe3O4ꢀrGO
composite material is because of the further reduction of the remaining functional groups
when hydrazine hydrate is used for the reduction of Au3+ to Auo NPs onto the surface of
Fe3O4ꢀrGO composite.
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3.2. Establishment of optimized reaction condition system for the A3 coupling reaction for the
synthesis of propargylamine
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To find the most effective catalyst among the Fe3O4ꢀrGO, Au NPsꢀrGO and Au NPsꢀFe3O4ꢀ
rGO composite material, we have first examined the catalytic activity of these catalysts in the
A3 coupling reaction of aryl aldehydes, secondary amines and terminal alkynes as shown in
Table 1. It is observed that the yield of the coupling products are found to be 87% and 76%
for the A3 coupling reaction of 4–ethynyltoluene, benzaldehyde and piperidine using Au
NPs–rGO and Fe3O4–rGO as catalyst, respectively. Whereas it is worth to mention that Au
NPs–Fe3O4–rGO showed remarkably greater catalytic activity than that of Au NPs–rGO and
Fe3O4–rGO and it produce 96% yield of the desired product. Further, Au NPs–Fe3O4–rGO
composite material notably possessed higher catalytic activity and gave the desired product in
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excellent yield than that of Au NPs–rGO and Fe3O4–rGO in case of other substrates also as
shown in Table 1. The enhanced catalytic efficiency of Au NPs–Fe3O4–rGO is expected to
come from the synergistic effect between Fe3O4 NPs and Au NPs in the composite material.
The synergistic effect of Fe3O4 NPs and Au NPs was also reported in many previous
literatures.57,58 For example, Au@Fe3O4–graphene composite material shows enhanced
catalytic activity towards the reduction of 4ꢀnitrophenol as compared to Fe3O4–graphene due
to their synergistic effect.59 In another report, synergistic effect of Au and Fe3O4 NPs was
observed in the oxidative degradation of dye molecules.60
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Table 1. Comparison of catalytic activity of synthesized composite materials a
Entry
Catalysts
R1
R2
Yield b (%)
1
2
3
4
H
Me
87
H
Me
Cl
H
H
H
H
Me
H
H
H
Me
H
H
H
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81
89
76
74
68
79
96
94
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97
Au NPs-rGO
Fe3O4 -rGO
1
2
3
4
1
2
3
4
H
Me
Cl
H
H
Me
Cl
Au NPs-Fe3O4-rGO
a Reaction condition: Aldehyde(1 mmol), Amine (1.2 mmol), Alkyne (1.5 mmol), Catalyst (25 mg);
b Isolated Yield
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Further, we have investigated the synergistic effect of the Au NPsꢀFe3O4ꢀrGO composite
materials by tuning the wt% of Au and Fe3O4 in the composite materials and evaluated their
turn over number (TON) by taking benzaldehyde, piperidine and phenylacetylene as the
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coupling partners as shown in Table 2. The results revealed that the TON of the Au NPsꢀ
Fe3O4ꢀrGO composite material increased by almost two times as compared to Au NPsꢀrGO
as well as Fe3O4 ꢀrGO. Further, both Au NPsꢀrGO and Fe3O4 –rGO were mixed physically
and evaluated their catalytic efficiency in this aforementioned reaction. It is found that the
catalytic activity is almost similar to that of individual Au NPsꢀrGO and Fe3O4 –rGO catalyst.
These results clearly confirmed the presence of synergistic effect between Au and Fe3O4 in
the Au NPsꢀFe3O4ꢀrGO composite catalyst and as a result of which highest catalytic activity
in the A3 coupling reaction is achieved using Au NPsꢀFe3O4ꢀrGO as catalyst.
Table 2. Synergistic effect of the composite materials a
Entry
Catalysts
Au/Fe3O4
Yield c
(%)
TON
wt % ratio b
1
2
3
4
5
rGO
0:0
4.8:0
0:28.6
2.8: 20.4
ꢀ
none
none
Au NPsꢀrGO
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94
89
139
Fe3O4 ꢀrGO
24
258d, 44e
ꢀ
Au NPsꢀFe3O4ꢀrGO
Au NPsꢀrGO, Fe3O4 ꢀrGO
b
a Reaction condition: Benzaldehyde(1 mmol), piperidine (1.2 mmol), Phenylacetylene (1.5 mmol), Catalyst (25 mg); The
c
d
weight% (wt%) of metals were determined from AAS analysis.
Isolated Yield; TON was calculated on the basis of
total Au content in the Au NPsꢀFe3O4ꢀrGO composite materials; e TON was calculated on the basis of total Fe3O4 content
in the Au NPsꢀFe3O4ꢀrGO composite materials
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After optimizing the suitable catalyst, then we have optimized the solvent and the reaction
temperature for the aforementioned reaction as shown in Table 3. 4ꢀchlorobenzaldehyde,
piperidine and phenylacetylene were taken as standard substrates in order to optimize the
solvent in the Au NPsꢀFe3O4ꢀrGO catalysed A3 coupling reaction. Various solvents such as
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DMF, NMP, toluene, DMSO, acetonitrile, THF etc. were screened for this A3 coupling
reaction. It is found that solvents like DMF, NMP and acetonitrile afforded the coupling
product in moderate yields (Table 3, Entry 5, 11, 13). However, low yield of the coupling
product was found with THF and toluene as solvents (Table 3, entry 15 and 16). Among
these, DMSO is found as the best solvent under this catalytic system and it gave 97% yield of
the desired product (Table 3, Entry 9). Additionally, we have performed A3 coupling
reaction at different temperatures and the optimum reaction temperature is found to be 90 °C
using Au NPsꢀFe3O4ꢀrGO composite catalyst.
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Table 3. Effect of solvent and temperature on the A3 coupling reaction under Au NPs-
Fe3O4-rGO catalyst system a
Entry
Solvent
Temperature (°C)
Time (h)
Yield (%) b
1
H2O
35
6
No reaction
2
3
4
5
6
7
8
9
H2O
H2O
DMF
DMF
100
140
80
90
110
80
80
90
110
90
110
80
80
110
80
15
48
6
6
10
6
10
6
6
6
10
6
10
10
10
No reaction
No reaction
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70
83
90
94
97
97
61
80
54
62
39
24
DMF
DMSO
DMSO
DMSO
DMSO
NMP
10
11
12
13
14
15
16
NMP
Acetonitrile
Acetonitrile
Toluene
THF
a Reaction condition: 4ꢀchlorobenzaldehyde (1 mmol), Piperdine (1.2 mmol), phenylacetylene (1.5 mmol), Catalyst (25 mg); b Isolated Yield
290
291
3.3. Substrate scope towards the A3 coupling reaction under Au NPs-Fe3O4-rGO catalyst
system
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317
After having the optimized condition with the suitable catalyst in hand, the substrates scope
of the A3 coupling reaction was examined by taking a series of different substituted
aldehydes, secondary amines and terminal alkynes as shown in Table 4. Different types of
aromatic aldehydes underwent A3 coupling with phenylacetylene, piperidine as well as
morpholine to afford the desired prapargylamines with excellent yields. It is noticed that
aldehydes containing electron withdrawing substituent such as chloro produces higher yield
of the coupling product (97%) than that of aldehydes containing electron donating group like
methyl as well as unꢀsubstituted ones (Table 4; entry 1, 2). It is worth to be mention that the
aliphatic aldehydes such as cyclohexanecarboxaldehyde, formaldehyde, pentanal also
provided good yields of the product along with the aryl aldehydes. Also it is mentioned that
the alkynes containing electron donating group like methyl exhibited high catalytic activity
than that of unꢀsubstituted alkynes under optimized reaction condition (Table 4; entry 3, 4).
Further, the A3 coupling proceeded smoothly with different secondary amines like piperidine,
morpholine to afford the desired product in high yields using Au NPsꢀFe3O4ꢀrGO composite
catalyst. In addition to secondary amines, we have also investigated the primary amines such
as benzylamine, 4ꢀfluorobenzylamine and 4ꢀmethoxybenzylamine in the A3 coupling
reaction. However, the present reported catalytic system was unable to produce the desired
coupling product. It is found in the literature that A3 coupling reaction with primary amines is
difficult to perform because primary amine generally produces imine instead of iminium
species, which are very less efficient as compared to iminium species to initiate further
reaction with terminal alkyne. 61ꢀ63
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Table 4. Au NPs-Fe3O4-rGO catalysed A3 coupling reaction of various aldehydes,
amines and alkynes a
Entry
Aldehyde
1(a-f)
Amine
2(a-b)
Alkyne
3(a-b)
Products
4(a-n)
Yields b
(%)
1
97
97
96
2
3
4
1a
2a
3b
3b
95
1b
2b
5
91
94
2b
3a
6
2a
3a
1b
15

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94
7
8
1b
2b
3a
95
3b
2b
93
93
92
9
1d
1d
3a
3a
3b
2b
10
11
2a
2b
91
91
12
13
2a
2b
3a
3a
1e
1e
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O
N
89
14
4n
a Reaction condition: Aldehyde (1 mmol), Amine (1.2 mmol), Alkyne (1.5 mmol), Au NPsꢀFe3O4ꢀrGO
(25 mg); DMSO (3 mL); b Isolated Yield
318
319
320
321
322
323
324
325
The catalytic performances of the Au NPs–Fe3O4–rGO composite catalyst was compared
with other previously reported catalyst for the synthesis of propargylamine as shown in Table
5. The results as shown in the Table 5 evident that the Au NPs–Fe3O4–rGO composite
catalyst remarkably presented noticeable advantage over other catalyst reported in the
literature towards the synthesis of propargylamine via A3 coupling reaction in terms of yields
and reaction times.
Table 5. Comparison of catalytic activity for the synthesis of propargylamine
Catalyst
Reaction Conditions
Time
20 h
24 h
Yield (%) Ref
Ag(I)−K10–
montmorillonite
Pd@MOF
H2O:DMSO (1:1), 100 °C
75–86
38–72
64
Cs2CO3, Toluene, 90 °C
CH2Cl2,
65
CuꢀMPTA
12–24 h 54–93
66
CuSBAꢀ15(42)
Agꢀdiatomite
IRMOFꢀ3ꢀGlꢀCu
NAPꢀMgꢀAu(0)
Au NPs–Fe3O4–rGO
Toluene, 110 °C
MeCN, 80 °C, N2
CHCl3, 40 °C
Toluene, 100 °C
DMSO, 90 °C
6 h
6–12 h
6 h
24 h
6 h
50–95
47ꢀ82
74ꢀ91
45ꢀ96
89ꢀ97
67
15
68
69
This
work
326
327
328
329
330
3.4. Reusability study
Reusability and stability of a heterogeneous catalyst are an essential parameter for the
sustainability of the catalyst. The reusability of the Au NPsꢀFe3O4ꢀrGO catalyst in this A3
coupling reaction was investigated by taking 4ꢀchlorobenzaldehyde, piperidine and
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332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
phenylacetylene as coupling partners. After performing the reaction, the catalyst was
separated magnetically using an external magnet without any filtration or centrifugation and
washed with acetone and water two to three times and then dried. The magnetically
separation of the catalyst is results from the ferromagnetic behaviour of the materials as
discussed earlier. The catalyst can be used up to five times without any further activation of
the catalyst as shown in Fig. 7. The catalyst has the potential to offer 85% yield of the
coupling product even after using fourth time of the catalyst in the reaction. Furthermore, to
examine the leaching of the active species after the reaction, hot filtration test was performed
by taking benzaldehyde, morpholine and phenylacetylene as the coupling partners for this
coupling reaction. After continuation of the reaction for 3 h, the catalyst was separated and
the yield of the propargylamine was found to be 67% and the filtrate part was allowed to
progress for another 5 h under same reaction condition. It is found that there is no further
progress of the reaction when the catalyst was separated which confirmed that no active
species were leached out from the composite material. The stability and the surface
morphology of the catalyst were examined by XRD and TEM after performing the coupling
reaction. The XRD patterns of reused Au NPs–Fe3O4–rGO composite material as shown in
the Fig. 8(a) established that same crystallographic planes corresponding to Au and Fe3O4
NPs were observed as in the fresh catalyst. Moreover, Au NPs–Fe3O4–rGO catalyst after
performing the A3 coupling reaction was again characterized by TEM and HRTEM analysis
to investigate the size, shape and morphology of the catalyst (Fig. 8(bꢀd)). It is observed from
the TEM images that both Au NPs and Fe3O4 NPs are more or less distributed over the entire
surface of rGO sheets. The HRTEM images shows two different types of lattice fringe
corresponding to Au and Fe3O4 NPs as shown in Fig. 8(c). Furthermore, the diffraction dots
seen at the SAED pattern evident that the crystallinity of the composite material remains
same even after performing the coupling reaction (Fig. 8 (d)).
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4. Conclusion
357
358
359
360
361
362
363
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365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
In conclusion, a novel and efficient Au NPs–Fe3O4–rGO composite material has been
developed by simple solution chemistry approach. The synthesized composite material was
characterized by numerous sophisticated instrumental techniques such as XRD, FTIR, TGA,
VSM, TEM, HRTEM, SEM–EDS etc. The synthesized composite material was applied as an
efficient catalyst in the A3 coupling reaction for the synthesis of biologically important
compound propargylamine. The Au NPs–Fe3O4–rGO catalyst could promote this reaction to
obtain up to 97% yield of the coupling product under optimized condition. The broad
substrate scope and excellent functional group tolerability were demonstrated by the reported
catalyst for this aforementioned reaction. The enhanced catalytic efficiency of Au NPs–
Fe3O4–rGO towards A3 coupling reaction than that of Au NPs–rGO and Fe3O4–rGO is a
result of the synergistic effect between Au NPs and Fe3O4 NPs onto surface of rGO sheets.
The presence of synergistic effect between Au and Fe3O4 NPs was proved by determining the
TON of the catalysts. The developed catalytic system offers the advantages of separation of
the catalyst magnetically after performing the catalytic reaction which is much more effective
than the filtration or centrifugation. The catalyst could be reused up to 4–5 times and it offers
87% yield of the desired product even after using forth time of the catalyst. Furthermore, the
catalyst was again characterized to investigate the morphology of the catalyst after
performing the coupling reaction which confirms the well stability of the catalyst and the
catalyst can be used up to five times without any further activation of the catalyst. Moreover,
the catalytic activity of Au NPs–Fe3O4–rGO catalyst was compared with other previously
reported catalysts for the synthesis of propargylamines. The results revealed that the Au NPs–
Fe3O4–rGO catalyst is superior over the other catalysts in terms of yield, reaction times and
other reaction conditions. Therefore, the reported protocol provides an important platform for
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381
382
383
the synthesis of efficient magnetically recoverable heterogeneous catalyst for its application
in the synthesis of biologically important compounds.
Acknowledgements
384
385
386
387
We acknowledge the DST, New Delhi for the funding (Project No. INT/RUS/RFBR/Pꢀ193).
We are also grateful to the Director, CSIR−North East Institute of Science & Technology,
Jorhat for his attention in this work. We acknowledge SAIF NEHU, Shillong for the TEM and
HRTEM facility. NH acknowledges UGC, New Delhi for SRF grant.
388
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Figures
Fig. 1. Powder XRD pattern of Au NPs-Fe3O4-rGO composite material
a
d
b
c
f
e
0.23 nm
Au (111)
0.25 nm
Fe3O4 (311)
Fig. 2. TEM and HRTEM images of Au NPs-Fe3O4-rGO composite material: TEM images
(a-c), HRTEM images along with fringe spacing of Au and Fe3O4 NPs (d-e), SAED pattern
(f)

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B
a
d
c
Fig. 3. SEM images of Au NPs-Fe3O4-rGO composite material (a-c); EDS analysis of
selected area of the SEM image C (d)
Fig. 4. FTIR spectra of GO and Au NPs-Fe3O4-rGO composite material

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Fig. 5. Variation of magnetization (M) with magnetic field (H) at room temperature
for Au NPs-Fe3O4-rGO composite material
Fig. 6. TGA curve of GO, Fe3O4-rGO and Au NPs-Fe3O4-rGO composite material

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Fig. 7. Reusability of the Au NPs-Fe3O4-rGO catalyst in the A3 coupling reaction
a b
c
d
Fig. 8. Characterization of the Au NPs-Fe3O4-rGO composite material after
performing the reaction: (a) XRD pattern; (b) TEM image; (c)HRTEM image;
(d) SAED pattern

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Graphical Abstract
0.23 nm
Au (111)
Au NPs
Fe3O4 NPs
0.25 nm
Fe3O4 (311)