Silica nanospheres supported diazafluorene iron complex: An efficient and versatile nanocatalyst for the synthesis of propargylamines from terminal alkynes, dihalomethane and amines

Article abstract of DOI:10.1039/c4ra10384j

We present the synthesis of an efficient heterogeneous silica nanosphere supported iron catalyst (SiO₂@APTES@DAFO-Fe) and its catalytic application in a one pot three-component coupling reaction of terminal alkynes, dihalomethane and secondary

Full text of DOI:10.1039/c4ra10384j

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PAPER
Silica nanospheres supported diazafluorene iron
complex: an efficient and versatile nanocatalyst for
the synthesis of propargylamines from terminal
alkynes, dihalomethane and amines†
Cite this: RSC Adv., 2014, 4, 49198
*
R. K. Sharma, Shivani Sharma and Garima Gaba
We present the synthesis of an efficient heterogeneous silica nanosphere supported iron catalyst
(SiO2@APTES@DAFO-Fe) and its catalytic application in a one pot three-component coupling reaction of
terminal alkynes, dihalomethane and secondary amines for the facile synthesis of propargylamines. The
synthesized SiO2@APTES@DAFO-Fe nanocatalyst has been systematically characterized by solid-state
13C CPMAS and 29Si CPMAS NMR spectroscopy, fourier transform infrared spectroscopy (FT-IR),
transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) surface area analysis, scanning
electron microscopy (SEM), atomic absorption spectroscopy (AAS), energy dispersive X-ray fluorescence
spectroscopy (ED-XRF), and elemental analysis. The nanocatalyst exhibits high catalytic performance for
the coupling reaction of aromatic terminal alkynes, dichloromethane and secondary amines to afford
propargylamines. In order to achieve high catalytic efficacy, the effect of various reaction parameters
such as temperature, base, the amount of catalyst, reaction time, type of solvent, substrate molar ratio
etc. have been investigated. Furthermore, the recovery and reuse of the quasi-homogeneous nano-
catalyst have been demonstrated several times without any appreciable loss in its catalytic activity. Apart
from this, FT-IR, SEM and HRTEM techniques are employed in order to prove the stability of the reused
nano-catalyst. The present protocol works under mild reaction conditions with the additional advantages
Received 13th September 2014
Accepted 24th September 2014
of
a simple work-up procedure, high product yield, and easy recovery and reusability of the
nanostructured catalyst. It is noteworthy that this atom economical methodology does not require an
additional co-catalyst or activator. A tentative mechanism is also proposed for the one pot coupling
reaction for the synthesis of propargylamines.
DOI: 10.1039/c4ra10384j
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associated drawbacks of high costs, multi-step preparation and
difficult separation from the reaction mixture.3–6 On the
Introduction
contrary, the economic advantages of heterogeneous catalysis
such as excellent stability, facile catalyst handling and separa-
tion from the reaction mixture have also increased its compe-
tence in industrial organic transformations despite having low
activity and selectivity in comparison to homogeneous catal-
ysis.7 To conquer the problems associated with both homoge-
neous as well as heterogeneous catalysts, chemists and
engineers have investigated a wide range of strategies amongst
which the covalent immobilization of catalytically active species
onto various solid supports appears to be the best logical
solution.8,9 The heterogenization of the existing homogeneous
catalyst can potentially provide novel recyclable and reusable
solid catalysts that have uniform and precisely engineered
active sites similar to their homogeneous counterparts, and
therefore preserve the desirable attributes of both the
systems.10–13 This strategy not only reduces the loss of catalyst,
but also enhances reusability, making the catalyst cost-effective
and promising for wide industrial applications. Over the past
The development of new green and sustainable technology for
various chemical transformations has recently gathered a great
deal of attention because it represents a new strategy to meet
the challenges of energy and sustainability.1,2 One effective way
to surmount these challenges is to employ a highly efficient and
stable catalyst for the synthesis of ne chemicals, especially
under eco-friendly and safe reaction conditions. A homoge-
neous catalytic systems serves as a powerful and versatile tool
for various organic syntheses enabling good control of reactivity
with chemo-, regio- and stereoselectivity as it allows easier and
greater interaction between the components. However, despite
these signicant features, its widespread application in prac-
tical and process chemistry is still hampered due to the
University of Delhi, Department of Chemistry, Delhi, India. E-mail:
rksharmagreenchem@gmail.com; Fax: +91-11-27666250; Tel: +91-11-27666250
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra10384j
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few decades, there have been extensive reports on the hetero- sustainable synthetic pathways for organic–inorganic hybrid
genization of soluble catalysts using various solid support materials, and their applications as metal scavengers, sensors,
materials such as metal oxides, polymers and nano- and catalysts for various organic transformations,46–57 herein we
composites.14–19 In this context, nanomaterials as a solid report the synthesis, characterization and application of novel
support have conquered new horizons in the eld of catalysis silica nanospheres supported diazauorene iron catalyst for
due to their intriguing properties such as large surface area, three-component coupling reaction of aromatic terminal
excellent stability and high complex loading in comparison to alkynes, dichloromethane and secondary amines to afford
bulk materials. It is well evident from literature that the novel propargylamines via C–H and C–Cl bond activations. Notably,
heterogenized catalysts are based on silica nanospheres as a the nanocatalyst exhibits high activity and a wide range of
solid support, primarily because they display some advanta- substrate applicability. Further, this nanocatalyst also displays
geous properties, such as nanometre size, excellent chemical excellent durability and can be reused several times with
and thermal stability, good accessibility, porosity, and large negligible loss of its catalytic activity. Hence, this one pot
surface area to volume ratio.20–31 Besides, the abundant silanol protocol overcomes the traditional drawbacks of the reported
groups on their surfaces make them easily organically func- homogeneous catalysts and has a good prospect for its wide
tionalized by different post-synthetic modications so as to scale application.
provide active catalytic centers. Remarkably, these quasi-
homogeneous catalysts are devoid of diffusion problems as they
can be easily dispersed in several solvents which facilitate the
Experimental
Materials and reagents
accessibility of the reactants to the catalytic active sites.
Within this framework a novel silica nanospheres supported
iron catalyst has been prepared for the three component
coupling reaction of aromatic terminal alkynes, dichloro-
methane and secondary amines to furnish propargylamines.
The synthesis of propargylamines is one of the most signicant
transformations from both synthetic and industrial points of
view, since propargylamines are synthetically adaptable key
intermediates for the production of many nitrogen-containing
biologically active compounds such as conformationally
restricted peptides, oxotremorine analogues, b-lactams, iso-
Tetra-ethyl orthosilicate (TEOS) (99.9%) (Sigma Aldrich), 3-
aminopropyltriethoxysilane (APTES) (98%) (Fluka), ferric chlo-
ride (Sisco Research Laboratory) and diazauorene (Alfa Aesar)
were commercially procured, and used as received without
further purication. All other starting materials and reagents
used in the reactions were obtained from Alfa Aesar and Spec-
trochem Pvt. Ltd, India.
Physicochemical characterizations
steres, therapeutic drug molecules, sedative, hypnotic, anti- Fourier transform infrared (FT-IR) spectra of all samples were
spasmodic, analgesic, and anti-inammatory effects.32–37 performed on KBr pellets by using a Perkin Elmer Spectrum
Traditionally, propargylamines synthesis was achieved via three 2000 FT-IR spectrometer in the 4000–400 cmꢀ1 region under
component coupling reaction of aldehydes, alkynes, and atmospheric conditions with a resolution of 1 cmꢀ1. Wide angle
amines catalyzed by various transition metals. However, Contel powder X-ray diffraction (XRD) measurements were conducted
et al. has recently reported gold catalyzed three-component for pre-dried samples by using a Bruker D8 ADVANCE (Karls-
coupling reaction of alkynes, haloalkanes and amines to ruhe, bundesland, Germany) X-ray diffractometer with Cu Ka
furnish propargylamines, wherein new C–C and C–N bonds are radiation and a secondary monochromator. The diffraction
constructed via the activation of C–H and C–halogen bonds.38 patterns were measured in the high angle region (2q scanning
Besides, Cu(I), nano-In2O3, Fe(III), Ni(II) and cobalt have also range from 10ꢁ to 60ꢁ). The structure and morphology of the
been reported to catalyze the concerned coupling reaction.39–44 nanomaterials were veried by high resolution transmission
Unfortunately, these existing synthetic methodologies suffer electron microscopy (HRTEM) (TECNAI G2 T30 microscope)
from numerous shortcomings such as longer reaction times, equipped with X-ray energy dispersive spectroscopy. The
harsh reaction condition, low product yield, lack of atom samples were dispersed in ethanol solution under ultrasonic
economy and tedious workup procedure which render them vibration and one drop of the suspension evaporated onto a
environmentally unsafe and limit their large scale operation. carbon-coated copper grid for HRTEM measurement. Energy-
Moreover, metal contamination of the products is inevitable dispersive X-ray (EDX) analysis (equipped with the TEM
during the process of extraction of the reported homogeneous instrument) was done for detailed composition characterization
catalysts. Therefore, design and development of improved of the nanomaterials. The qualitative identication of free
synthetic method for the preparation of propargylamines is amine groups in APTES@SiO2 was performed by the ninhydrin
highly desirable for industrial application.
test. Digestion of the catalyst was done in an Anton Paar mul-
Considering the need for a better catalytic system that would tiwave 3000 microwave reaction system equipped with a
uniquely combine the best features of both homogeneous and temperature and pressure sensor. Specic surface areas were
heterogeneous catalysts, we have developed silica nanospheres determined by Brunauer–Emmett–Teller (BET) method Gemini-
based palladium catalyst for the oxidative amination of alde- V2.00 instrument (Micromeritics Instrument Corp.). Samples
hydes under solvent free condition using hydrogen peroxide as were outgassed under vaccum at 100 ꢁC for 3 h to evacuate the
a sole environmentally benign oxidant.45 Thus, in continuation physically adsorbed moisture before analysis. CHN elemental
of our ongoing research work on the development of green and analysis was carried out using Elementar Analysensysteme
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GmbH VarioEL V3.00 analyzer. The amount of iron loaded on mixture was diluted with water and the aqueous layer was
silica nanospheres was determined via ame atomic absorption extracted with diethylether. The organic phase thus collected
spectroscopy (FAAS) on a LABINDIA AA 7000 atomic absorption was dried over Na2SO4, ltered, and the solvent was removed
spectrometer using an acetylene ame. The optimum parame- under vacuum. The propargylamine products thus obtained
ters for the measurement of iron content in the sample are: were analyzed and conrmed by GC-MS.
wavelength ¼ 248.3 nm; lamp current ¼ 2 mA; slit width ¼ 0.2
nm; fuel ow rate ¼ 0.2 L minꢀ1. Field-emission scanning
Results and discussion
Catalyst characterizations
electron microscopy (FESEM) images were collected on a Tescan
Mira 3 FESEM microscope; samples were coated with a gold
lm. GC-MS analyses of the products were performed on an
Fourier-transform infrared spectroscopy. The FT-IR spec-
Agilent gas chromatograph (6850 GC) with HP-5MS 5% phenyl trum of SiO2 exhibits characteristic vibrations of the silica
methyl siloxane capillary column (30.0 m ꢂ 250 mm ꢂ 0.25 mm) materials related to Si–O–Si asymmetric stretching (1087 cmꢀ1),
and a quadrupole mass lter equipped 5975 mass selective stretching vibrations of Si–OH (955 cmꢀ1), Si–O–Si symmetric
detector (MSD) using helium as the carrier gas (ratꢁe 0.9 mL minꢀ1). stretching (802 cmꢀ1) and Si–O–Si bending vibrations (475 cmꢀ1).
The temperature of the injection port was 250 C. The temper- Moreover, the spectrum also shows H–O–H bending vibrations
ature program of the GC column was set to an initial oven of physically adsorbed water at 1628 cmꢀ1 and a broad band
temperature of 60 ꢁC which increased at a rate of 10 ꢁC minꢀ1 to centered around 3423 cmꢀ1 due to O–H stretching vibrations of
ꢁ
ꢁ
250 C, and the oven was held at 250 C for 10 min.
hydrogen bonded surface silanol groups and physically adsor-
bed water. In the FTIR spectrum of the amine functionalized
silica nanospheres, besides the absorption bands originated by
Si–O vibrations, there are two weak bands at 2928 cmꢀ1 and
2858 cmꢀ1 assigned to aliphatic –CH2 stretching vibrations of
the propyl chain of the silylating agent (APTES) which conrm
the presence of APTES onto silica surface. These results are in
good agreement with the data reported in the literature.60
Furthermore, ongoing from SiO2 to SiO2@APTES, a signicant
reduction in the intensity of the peak from Si–OH stretching
vibrations (at 955 cmꢀ1) is observed. This is because of a reac-
tion between the surface Si–OH groups of SiO2 and ethoxy
groups of APTES during the surface modication. The spectrum
of nanocatalyst shows a band at 1623 cmꢀ1 indicative of the
imine group resulting from Schiff condensation between the
NH2 group of SiO2@APTES and the carbonyl group of the ligand
further provide the evidence of the graing of the ligand onto
amine functionalized silica nanospheres. FT-IR spectrum of
recovered SiO2@APTES@DAFO-Fe catalyst aer reaction was
also recorded in the region 4000–400 cmꢀ1. The spectrum of
SiO2@APTES@DAFO-Fe aer coupling reaction shows identical
IR absorption bands indicating that originality of the catalyst is
still retained. The recovered SiO2@APTES@DAFO-Fe catalyst
does not show any IR bands corresponding to the organic
materials suggesting that the catalyst is not poisoned during the
reaction (Fig. 1).
Fabrication of the silica nanospheres supported diazauorene
iron nano-catalyst (SiO2@APTES@DAFO-Fe)
Firstly, monodisperse silica nanospheres (SiO2) were synthe-
sized via stober's method by hydrolysis of TEOS in ethanol
medium using ammonium hydroxide as catalyst.58 In a typical
synthesis, 1 mL of TEOS was added into 10 mL of ethanol.
Subsequently, 10 mL of 25% ammonium hydroxide and 10 mL
of ethanol were also introduced to the reaction mixture
synchronously under sonication which was further continued
for 60 minutes to get a white turbid suspension. The reaction
mixture was centrifuged in order to separate silica nanospheres
which were then washed with water and ethanol respectively.
Aer that, silica nanospheres were functionalized with APTES
by the addition of 1 mL of APTES to 50 mL of a vigorously stirred
dispersion of SiO2 in ethanol, and the mixture was stirred
overnight at room temperature.59 The resulting amine func-
tionalized silica nanospheres (SiO2@APTES) were separated by
centrifugation and re-dispersed in ethanol which was repeated
three times. Next, the mixture of SiO2@APTES and diaza-
uorene (DAFO) was stirred for 24 h in ethanol solution at 60 ꢁC
and the product (SiO2@APTES@DAFO) was centrifuged and
washed with ethanol thoroughly to remove unreacted diaza-
uorene which was dried in a vacuum oven. Finally, SiO2@
APTES@DAFO was reacted with the solution of ferric chloride in
acetone (Scheme 1) and the resulting mixture was stirred for 12
h at room temperature. The nanocatalyst (SiO2@APTES@DAFO-
Fe) was centrifuged, washed thoroughly with acetone and dried
in a vacuum.
X-ray diffraction studies. In order to conrm the structural
characteristics of SiO2, SiO2@APTES and SiO2@APTES@DAFO-
Fe, XRD studies were carried out, and resultant patterns are
presented in Fig. 2. All silica based nanomaterials exhibit a
single broad diffraction peak at 2q ¼ 23ꢁ which is characteristic
of the amorphous nature of the Stober's silica nanospheres. The
results show that there is no remarkable change observed in the
topological structure of silica nanospheres even aer surface
functionalization reactions. Hence, it may be concluded that
General procedure of silica nanospheres supported iron
nanocatalyst (SiO2@APTES@DAFO-Fe) mediated synthesis of
propargylamines
A mixture of aromatic alkynes (1 mmol), dichloromethane (2 the bare silica nanospheres are stable enough to experience the
mmol), secondary amine (2 mmol), DBU (1 mmol), 2 mL CH3CN chemical modication reactions starting from the surface
and SiO2@APTES@DAFO-Fe nanocatalyst (30 mg) were reuxed functionalization to the binding of iron. However, upon
ꢁ
at 70 C for 10 h. At the end of the reaction, the catalyst was comparison of the diffraction patterns of these samples,
recovered by centrifugation and washed with diethylether. The decrease in XRD peak intensities of SiO2@APTES and
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Scheme 1 Preparation of silica nanospheres supported diazafluorene iron nano-catalyst (SiO2@APTES@DAFO-Fe).
Fig. 2 XRD diffraction patterns of (a) SiO2, (b) SiO2@APTES and (c)
SiO2@APTES@DAFO-Fe.
Fig. 1 FT-IR spectra of (a) SiO2, (b) SiO2@APTES and (c) SiO2@
APTES@DAFO-Fe (d) recovered SiO2@APTES@DAFO-Fe.
TEM and SEM analysis. The shape, size and morphologies of
the synthesized nanomaterials were examined by scanning
electron microscopy (SEM) and transmission electron micros-
copy (TEM). SEM images of functionalized as well as non-
functionalized silica nanospheres reveal that they are
monodispere having a smooth surface (Fig. 3). TEM micro-
graphs further conrm that these particles are spherical in
shape having uniform size with an average diameter of 250 nm.
SiO2@APTES@DAFO-Fe is observed with the successive
anchoring of APTES and iron complex onto silica nanospheres.
On the other hand, XRD analysis of nanomaterials does not
show any novel diffraction peak. This gives a clear indication
that these species existed on the surface of silica nanospheres in
the form of non-crystalline state.
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Fig. 3 TEM micrographs of (a) SiO2 and (b) SiO2@APTES@DAFO-Fe. (c) Recovered SiO2@APTES@DAFO-Fe. (d) EDX spectrum of SiO2@APTES. (e)
SEM image of SiO2@APTES@DAFO-Fe. (f) Recovered SiO2@APTES@DAFO-Fe.
It is also evident that the morphological homogeneity that nanospheres. Besides, TEM and SEM images of the recovered
characterized these amorphous SiO2 nanospheres is main- nano-catalyst aer the completion of reaction have been
tained even aer functionalization of organic moieties. provided which indicate that the shape and morphology of the
Furthermore, these particles do not agglomerate into clusters particles remain unchanged. Hence, the catalyst is stable under
and hence leave enough surface area available on the nano- the reaction conditions employed in the present work and
particle for catalytic activity. EDX mapping results of SiO2@ retains catalytic activity even aer recycling.
APTES show the presence of nitrogen species which is consis-
tent with the presence of APTES at the surface of the SiO2 29Si and 13C CPMAS solid-state NMR spectroscopy provides
Solid state 13C CPMAS and 29Si CPMAS NMR spectroscopy.
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useful information regarding the presence of organic moiety in values aer successive immobilization of organic moieties.
the hybrid framework since the spectra of these nuclei are These reductions in surface areas are likely to be the result of
sensitive monitors of the silica surface and the reactants. blocking of the access of nitrogen gas molecules aer the
Hence, insight into the successful modication of the SiO2 with functionalization of silica nanospheres. Hence, these data
APTES is gained by 13C CPMAS NMR spectrum of APTES@SiO2 provide a strong evidence of the successive anchoring of the
as shown in Fig. 4(a). In the 13C CPMAS spectrum of the APTES and iron complex onto the surface of silica nano-
APTES@SiO2, one can observe signals stemming from the alkyl spheres.62 Further, ninhydrin test was also carried out in order
chain of APTES (10.7, 23.8, 44.2 ppm) and additionally two to discover the free amino groups in the SiO2@APTES which are
peaks of un-reacted ethoxy groups (16.8 and 58.1 ppm) which at the surface of solid support. Upon addition of the ninhydrin
further conrms the covalent bonding between the silane solution to the nanomaterial (SiO2@APTES), change in colour
linking group and silica nanospheres. The 13C CPMAS results from white to blue was observed, this provides clear evidence
are in good agreement with the values reported in the literature for the presence of the accessible amine groups (Fig. S1, ESI†).
for APTES in siliceous supports.61 Further, Fig. 4(b) represents Next, for the quantitative estimation of the organic functional-
solid state 29Si NMR spectrum of SiO2@APTES@DAFO-Fe ities present on the surface of silica nanospheres, CHN micro-
catalyst which exhibits four peaks. The rst of them, at ꢀ55 analysis was performed (Table S1, ESI†). The observed carbon
ppm, may be assigned to the silicon atom of the silylating agent and nitrogen contents of SiO2@APTES are 5.43% and 1.83%
bound to one hydroxyl group forming the structure C– respectively. Hence, calculated C/N ratio of SiO2@APTES is
Si(OSi)2(OH). This signal is usually referred to as the T2 signal. found to be to be ꢃ3 which matches the theoretical value,
The second peak, at ꢀ66 ppm, is assigned to C–Si(OSi)3 and thereby conrming the APTES graing onto the surface of SiO2.
corresponds to the T3 signal. The presence of T2 and T3 signals The nitrogen loading of SiO2@APTES was found to be 1.31
conrms that the organic groups are covalently bound to the mmol gꢀ1. Lastly, in order to determine the iron content in the
SiO2 nanospheres. The other two less intense peaks, at ꢀ101.7 SiO2@APTES@DAFO-Fe catalyst, quantitative analysis was per-
and ꢀ110.5 ppm, are attributed to inorganic polymeric struc- formed using AAS aer it was digested in nitric acid and cor-
ture of silica nanospheres and are assigned to Si(OSi)3OH group responding iron loading was found to be 0.126 mmol per gram
(Q3) signal and free silanol group of Si(OSi)4 (Q4).
of catalyst. These observations were further supported by well
Elemental and BET surface area analysis. The Brunauer– resolved peak of iron observed in the ED-XRF spectrum of
Emmett–Teller (BET) surface area measurements were also SiO2@APTES@DAFO-Fe catalyst (Fig. 5).
performed in order to analyze the graing reactions and results
Catalytic activity tests. The catalytic efficacy of silica nano-
are summarized in Table S1 (ESI†). The surface area of bare spheres supported diazauorene iron complex (SiO2@
silica nanospheres was measured as 241.75 m2 gꢀ1. However, APTES@DAFO-Fe) was investigated in three component
the results reveal that there is decrease in the surface area coupling reaction of terminal alkynes, halomethanes and
secondary amines for the facile synthesis of propargylic amines.
Initial catalytic tests were carried out in order to obtain an
optimum reaction prole for coupling by varying different
parameters, such as temperature, amount of catalyst, reaction
time, substrate molar ratio, type of solvent and effect of bases.
For that, phenyl acetylene, dichloromethane and diethylamine
were chosen as the test substrates.
Screening of various catalysts. Initially, to compare the
catalytic performance of the SiO2@APTES@DAFO-Fe
Fig. 4 (a) 13C CPMAS NMR spectrum of SiO2@APTES and (b) 29Si
Fig. 5 ED-XRF spectrum of nanocatalyst (SiO2@APTES@DAFO-Fe).
CPMAS NMR spectrum of SiO2@APTES@DAFO-Fe.
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silica nanospheres can easily be dispersed in several solvents
and hence can facilitate the accessibility of the reactants to the
catalytic active sites. Further, the effect of amount of the
SiO2@APTES@DAFO-Fe nanocatalyst on the conversion was
also studied. The results revealed that the percentage conver-
sion increased with an increase in the amount of the catalyst
upto 30 mg. This suggests that the transition metal functions as
an active site for the coupling reaction and hence with an
increase in the amount of the active species the reaction
becomes fast, which favours the formation of propargylic
amines. A further increase in the amount of catalyst did not
produce any considerable increase in the conversion because of
the hindrance of the mass transfer between the active sites and
reagent caused by the low dispersity of the excess catalyst in the
reaction media. Thus, 30 mg of the SiO2@APTES@DAFO-Fe
nanocatalyst was found to be optimum in order to obtain
maximum conversion.
Fig. 6 Effect of temperature on model reaction (reaction conditions:
phenyl acetylene (1 mmol), dichloromethane (2 mmol), diethylamine
(2 mmol), DBU (1 mmol), acetonitrile (2 mL), catalyst (30 mg), time 10 h).
nanocatalyst, the model reaction was performed using wide
range of iron sources (Table S2, ESI†). It can be seen that the
reaction did not proceed in the absence of catalyst. Similarly,
the reaction did not occur with silica nanospheres alone. All of
the reported homogeneous catalysts exhibited moderate
activity. These homogeneous catalysts decomposed immedi-
ately aer the reaction and could not be reused. On the other
hand, the developed SiO2@APTES@DAFO-Fe nanocatalyst
acquired the advantages of a heterogeneous catalytic system so
that it could be easily recovered and reused for several catalytic
cycles. In addition, the heterogeneous SiO2@APTES@DAFO-Fe
nanocatalyst was devoid of the diffusion problems because the
Fig. 8 Effect of substrate molar ratio on one pot three component
coupling reaction (reaction conditions: phenyl acetylene (a mmol),
dichloromethane (b mmol), diethylamine (c mmol), DBU (1 mmol),
solvent (2 mL), catalyst (30 mg), temp. 70 ꢁC, time 10 h).
Fig. 7 Effect of solvents and bases on catalytic efficacy of SiO2@
APTES@DAFO-Fe in one pot three component coupling reaction Fig. 9 Effect of reaction time on one pot three component coupling
(reaction conditions: phenyl acetylene (1 mmol), dichloromethane reaction (reaction conditions: phenyl acetylene (1 mmol), dichloro-
(2 mmol), diethylamine (2 mmol), base (1 mmol), solvent (2 mL), methane (2 mmol), diethylamine (2 mmol), DBU (1 mmol), solvent (2
catalyst (30 mg), temp. 70 ꢁC, time 10 h).
mL), catalyst (30 mg), temp. 70 ꢁC).
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Scheme 2 Probable reaction mechanism of SiO2@APTES@DAFO-Fe catalyzed one pot three component coupling of terminal alkynes,
dichloromethane and secondary amines.
DBU, DABCO and triethylamine showed better performance
than inorganic bases, probably due to their better solubility in
the reaction system. Remarkably, DBU gave maximum conver-
sion percentage. Moreover, base free conditions could not
catalyze the reaction. The coupling reaction worked well in
various organic solvents, but MeCN appeared to be the best
choice amongst them. Negligible amount of product was
formed when the reaction carried out in THF or hexane.
Effect of substrate molar ratio. Having established acetoni-
trile as the best solvent for three component coupling reaction,
we then investigated the inuence of substrate ratio and the
results are summarized in Fig. 8.
Different molar ratios of phenylacetylene, dichloromethane
and diethylamine (1 : 1 : 1, 1 : 2 : 2, and 1 : 2 : 3) were consid-
ered while keeping the other parameters xed. Upon utilizing
an equal molar ratio of the three substrates moderate conver-
sion was observed. With further increase in the molar ratio of
dichloromethane and diethylamine to 1 : 2 : 2, the reaction
time was shortened and maximum conversion was observed.
Hence, 1 : 2 : 2 molar ratio was the minimum requirement for
the effective one pot synthesis of propargylamine. It was also
Effect of temperature. Temperature is a crucial factor for the
one pot three component coupling reaction. Thus, in order to
determine the optimum temperature, the model reaction was
ꢁ
performed at diverse range of temperatures (30–100 C) while
keeping the other parameters xed and the results are pre-
sented in Fig. 6. It can be seen that the conversion increased
with increasing temperature from 30 to 70 ꢁC. Further increase
in temperature resulted in decrease in conversion percentage
probably due to the formation of small amount of dimerization
product of phenyl acetylene. At 70 ꢁC highest conversion was
observed and the dimerization product was suppressed. Hence,
the optimum temperature for the one pot synthesis of propragyl
amines was 70 ꢁC and further studies were carried out at 70 ꢁC
in the presence of 30 mg of SiO2@APTES@DAFO-Fe catalyst.
Effect of various solvents and bases. Noting the importance
of base in the reaction, various organic as well as inorganic
bases were screened such as 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), triethylamine (TEA) and 1,4-diazabicyclo[2.2.2]octane
(DABCO), K2CO3 and K3PO4 using series of solvents such as
dioxane, toluene, H2O, DMF, MeCN, and DCM. The results are
presented in Fig. 7. It was found that organic bases such as
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Table 1 SiO2@APTES@DAFO-Fe catalyzed one pot coupling reaction of terminal alkyne, dichloromethane and aminesa
S. No.
1
Alkyne
Dihalomethane
CH2Cl2
Amines
Time (h)
10
Conversionb (%)
Yield (%)
98
100
69
2
3
CH2Cl2
CH2Cl2
10
10
62
93
100
4
5
6
CH2Cl2
CH2Cl2
CH2Cl2
10
12
10
100
100
100
88
84
89
7
8
CH2Cl2
CH2Cl2
15
10
14
10
96
100
9
CH2Cl2
CH2Cl2
10
10
100
100
89
83
10
11
12
CH2Cl2
CH2Cl2
10
15
100
0
87
0
13
CH2Cl2
10
95
90
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Table 1 (Contd.)
S. No.
14
Alkyne
Dihalomethane
CH2Cl2
Amines
Time (h)
10
Conversionb (%)
Yield (%)
89
95
0
15
16
CH2Cl2
CH2Cl2
10
10
0
40
37
17
18
19
CH2Cl2
CH2Cl2
CH2Cl2
10
10
10
100
29
94
23
49
56
20
21
CH2Cl2
CH2Cl2
10
10
95
0
82
0
22
CH2Cl2
10
0
0
a
Reaction conditions: terminal acetylene (1 mmol), dichloromethane (2 mmol), secondary amine (2 mmol), acetonitrile (2 mL), DBU (1 mmol),
catalyst (30 mg), temp. 70 ꢁC. b Conversions were determined by GC-MS.
observed that further increase in the molar ratio did not have
any appreciable effect on the reaction conversion.
Proposed reaction mechanism. A plausible mechanism for
this coupling reaction has been proposed as shown in Scheme
2. Initially, precatalyst SiO2@APTES@DAFO-Fe might get
reduced to a low-valent Fe(I) state in the presence of alkyne and
base. The catalytic cycle starts via insertion of the C–H bond of
the terminal alkyne promoted by Fe(I) species A in combination
with DBU which leads to generation of a Fe–acetylide interme-
diate.63,64 The intermediate A thus formed acts as active nucle-
ophilic species and reacts with CH2Cl2 to form the Fe(III) species
Effect of reaction time. Further, the percentage conversion of
the model reaction was monitored at different reaction times. It
can be seen from Fig. 9, that with an increase in the reaction
time, the percent conversion also increased upto 10 h. When the
reaction was allowed to continue further, no signicant change
in the conversion was observed. So, the reaction time was
optimized as 10 h.
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Table 2 Conversion percentage for coupling reaction of terminal
alkyne, dichloromethane and amines under optimized conditions
B. Further, Fe(III) species B subsequently undergo reductive
elimination to afford the propargylchloride C which upon
reaction with a secondary amine yields corresponding prop-
argylamine as product.39,40,65–67
SiO2@APTES@DAFO-Fe catalyzed coupling reaction. In an
effort to establish the scope of the protocol, we have assessed
the catalytic activity of SiO2@APTES@DAFO-Fe under the opti-
mized conditions for the synthesis of variety of propargylamine
derivatives. For that, various terminal alkynes and secondary
amine derivatives were subjected to coupling reaction to give a
wide range of substituted propargylamines and results are
summarized in Table 1. Both electron withdrawing and electron
donating aromatic terminal alkynes afforded corresponding
propargylamine products in good to excellent conversion
percentage. However, in the case of heteroaromatic acetylene
derivative such as ethynyl pyridine, negligible amount of
product was obtained (Table 1, entry 21 & 22). Further, both
cyclic and acyclic secondary amines such as piperidine, pyrro-
lidine, morpholine, diisopropylamine and dibutylamine were
efficiently converted into the corresponding coupling products.
Among these, dibutylamine required more time to complete the
reaction (Table 1, entry 5 & 11). There was no reaction observed
for bulky amines, such as dioctylamine which may be attributed
to extra steric hindrance and lower nucleophilicity (Table 1,
entry 7 & 12). The prominent feature of the present methodology
was that this system had very good functional group tolerance
for aromatic substituted alkynes and secondary amine
derivatives.
Heterogeneity test. It is a well-known fact that stability and
reusability are two important parameters based upon which a
catalyst nds potential application in industry. In order to
determine the true heterogeneous nature of Fe species in our
catalytic system, standard leaching experiment was performed.
For that, the model reaction was carried out under optimized
reaction conditions employing SiO2@APTES@DAFO-Fe as
catalyst. Aer 5 h, the reaction mixture was centrifuged to
discharge SiO2@APTES@DAFO-Fe catalyst and subsequently
permitted the remaining liquor to react for another 10 h. Then,
GC-MS analysis of the supernatant was done and the results
revealed that the conversion did not change with the increase in
time, suggesting that the catalytic reactivity by the leaching iron
metal could be excluded in the present coupling reaction (Table
2). Further, to ensure the absence of iron in the supernatant,
UV-vis spectroscopic analysis was carried out and there was no
characteristic peak of iron metal in the UV-vis spectrum of the
supernatant. This was further conrmed by atomic adsorption
analysis of the ltrate solution that showed no traces of iron in
the supernatant aer removal of the catalyst. Hence, it may be
affirmed that silica nanospheres bind the iron metal very rmly
and thus minimize the deterioration of the catalyst by reducing
metal leaching which eventually facilitate efficient catalyst
recycling.
Entry
Time (h)
Conversion (%)
TON (TOF)
1a
2b
5
10
85
85
225 (49)
225 (49)
a
Reaction conditions: phenyl acetylene (1 mmol), dichloromethane
(2 mmol), diethylamine (2 mmol), DBU (1 mmol) acetonitrile (2 mL),
b
catalyst (30 mg), temp. 70 ꢁC. GC-MS analysis of supernatant aer
the removal of nano-catalyst.
the optimized reaction conditions (Fig. 10). Upon completion of
the reaction, the catalyst was recovered, rinsed with ethyl
acetate and dried under vacuum to eliminate any water trapped
from moisture. Further, the recovered catalyst was redispersed
in a freshly prepared reaction solution with other compositions
identical to the initial run. Such recovery/reinitiation cycles
were repeated nine times. No remarkable decrease was observed
in the catalytic activity aer being used repetitively seven times,
revealing its superiority over the corresponding homogeneous
catalyst for reducing cost and diminishing environmental
pollution from heavy metal ions. TEM and SEM image of the
recovered catalyst did not show signicant change in the
morphology or in the size which indicates that the nano-
architecture of the catalyst largely survived aer reaction and
recovery. Further, XRD patterns of recovered SiO2@
APTES@DAFO-Fe were also compared with that of the fresh
catalyst and it was found that both samples exhibited similar
Reusability of catalyst. The easy catalyst recovery is a salient
advantage of heterogeneous catalysis, allowing the metal
reprocessing and even sometimes the catalyst recycling. To test
the catalyst reusability, the reaction was carried out in the
Fig. 10 Catalytic reusability test for one pot three component
coupling reaction (reaction conditions: phenyl acetylene (1 mmol),
dichloromethane (2 mmol), diethylamine (2 mmol), DBU (1 mmol),
presence of a catalytic amount of SiO2@APTES@DAFO-Fe under acetonitrile (2 mL), catalyst (30 mg), temp. 70 ꢁC, time 10 h).
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Table 3 Comparison of the catalytic activity of SiO2@APTES@DAFO-Fe nano-catalyst with other catalysts reported in literature for one pot
coupling reaction of terminal alkyne, dihalomethane and amines
Reaction
conditions
Time
(h)
Yield
(%)
Entry Acetylene
Halomethane Amines
CH2Cl2
Reusability
Reference
39
1
Ni(Py)4Cl2/bipyridine/MeCN, 70 ꢁ
Ni(Py)4Cl2/bipyridine/MeCN, 70 ꢁ
C
C
28
28
—
92
90
2
3
4
5
6
7
8
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
—
—
—
—
—
—
—
39
40
40
41
41
42
42
CoBr2, DBU, MeCN, 80 ꢁ
CoBr2, DBU, MeCN, 80 ꢁ
C
C
30
30
12
12
14
14
85
88
67
88
95
94
FeCl3, TMG, MeCN, 100 ꢁ
FeCl3, TMG, MeCN, 100 ꢁ
C
C
CuCl, DBU, 60 ꢁ
CuCl, DBU, 60 ꢁ
C
C
Nano In2O3, DABCO,
DMSO, 65 ꢁC
Upto 3 catalytic
cycles
9
CH2Cl2
20
72
43
43
Nano In2O3, DABCO,
Upto 3 catalytic
cycles
10
11
CH2Cl2
CH2Cl2
16
10
80
98
DMSO, 65 ꢁ
C
DBU, acetonitrile,
Upto 7 catalytic
cycles
Present
study
SiO2@APTES@DAFO-Fe, 70 ꢁC
DBU, acetonitrile,
Upto 7 catalytic
cycles
Present
study
12
CH2Cl2
10
96
SiO2@APTES@DAFO-Fe, 70 ꢁC
XRD patterns indicating that the catalyst was stable and could compared with that of the earlier reported catalytic systems for
be regenerated for repeated use (Fig. S2, ESI†). Hence, silica coupling reaction of terminal alkyne, dihalomethane and
nanospheres supported iron catalyst was proved to be a prom- amines. As evident from Table 3, the SiO2@APTES@DAFO-Fe
ising catalyst for one pot three component coupling reaction.
catalyst exhibited much better results in terms of catalytic
Comparison of the catalytic activity of the SiO2@ activity, reaction conditions and superior reusability in
APTES@DAFO-Fe nano-catalyst with the literature precedents. comparison to the literature precedents. This may be due to the
The catalytic performance of the SiO2@APTES@DAFO-Fe was high surface to volume ratio of the nano-catalyst in comparison
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to the bulk catalytic systems. It should also be noted that most
of the reported catalyst were homogeneous and hence decom-
posed immediately aer the reaction and could not be reused.
Moreover, it is necessary to remove toxic metals from the nal
product as metal contamination is highly regulated, especially
in the pharmaceutical industry. There is only one report in
literature wherein expensive indium oxide was used as catalyst
for the concerned reaction and could be repeatedly used for
three consecutive cycles. On the other hand, the reusability of
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In conclusion, a novel silica nanospheres supported iron cata-
lyst has been synthesized via graing of diazauorene ligand
onto amine functionalized silica nanospheres followed by
metallation with ferric chloride. The prepared catalyst has been
found to be highly efficient for the one pot synthesis of prop-
argylamines from alkynes, aldehydes, and amines. The meth-
odology is quite simple and allows the synthesis of a diverse
range of propargylamines in good to excellent yields. The
activity of the nano-catalyst is found to be superior in terms of
reaction time, yield, cost, selectivity and reusability as
compared to literature precedents. This may be ascribed to the
nanometre size of silica nanospheres which enhances the
dispersion of the catalytic active sites in the reaction medium
and thus improving their accessibility to the substrate and base
species. Further, the nanocatalyst can be easily recovered by
centrifugation and reused several times without any signicant
decay in its activity, thereby making this protocol environ-
mentally benign. The heterogeneity test provides an excellent
evidence for the absence of leaching of the active catalytic
species which results in excellent durability of the catalyst
under present experimental conditions. The synthetic protocol
is straightforward, safe, environmentally clean, and free from
any other additives and thus abides by a number of principles of
green chemistry. Some of the other advantages of the present
methodology are ease of preparation of the catalyst from
commercially available starting materials, simple work-up
procedure, high product yield, easy recovery and reusability of
the catalyst for several cycles with unaltered activity and selec-
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for the synthesis of propargylamines using silica nanospheres
supported iron catalyst for the rst time under mild reaction
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