Cu ion-exchanged and Cu nanoparticles decorated mesoporous ZSM-5 catalysts for the activation and utilization of phenylacetylene in a sustainable chemical synthesis

Article abstract of DOI:10.1039/c6ra19606c

Mesoporous ZSM-5 was synthesized using a 1,4-diazabicyclo[2.2.2]octane based multi-cationic surfactant as a structure directing agent. Cu²⁺ exchanged mesoporous ZSM-5 was prepared by the ion-exchange process. Cu nanoparticles decorated mesoporous ZSM-5 was prepared using NaBH₄ as a reducing agent. Materials were characterized by the complementary combination of X-ray diffraction, N₂-adsorption, UV-visible, and scanning/transmission electron microscopic techniques. For comparative purposes, Cu²⁺ exchanged ZSM-5, HY, and NaY; and Cu nanoparticles decorated conventional ZSM-5, SBA-15, and Al₂O₃ samples were also prepared. A sustainable catalytic process was developed for the selective synthesis of indolizine, chalcone, and triazole derivatives using a mesoporous ZSM-5 based heterogeneous catalyst. A multi-component synthetic strategy is reported here for the selective synthesis of the above mentioned chemicals that involves phenylacetylene as one of the building blocks. Control experiments were performed to ascertain the proposed reaction pathways. Recycling and leaching experiments were performed to demonstrate the sustainability and robustness of the catalytic process. Among these catalysts, Cu nanoparticles decorated mesoporous ZSM-5 exhibited the highest activity in all these reactions. The catalyst was found to be highly stable and it was possible to recycle the catalyst five times with no appreciable loss in the activity. A wide range of indolizine, chalcone, and 1,2,3-triazole derivatives were prepared in high yields using this catalyst.

Full text of DOI:10.1039/c6ra19606c

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Cu ion-exchanged and Cu nanoparticles decorated mesoporous ZSM-5 catalysts for the
activation and utilization of phenylacetylene in the sustainable chemical synthesis
a
b
a
Bhaskar Sarmah , Biswarup Satpati , and Rajendra Srivastava *
a
b
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagarꢀ140001, India
Surface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF,
Bidhannagar, Kolkata 700 064, India
_________________________________________________________________________
_
Abstract
Mesoporous ZSMꢀ5 was synthesized using 1,4ꢀdiazabicyclo[2.2.2]octane based multiꢀ
2
+
cationic surfactant as structure directing agent. Cu exchanged mesoporous ZSMꢀ5 was
prepared by the ionꢀexchange process. Cu nanoparticles decorated mesoporous ZSMꢀ5 was
prepared using NaBH
combination of Xꢀray diffraction, N
electron microscopic techniques. For comparative purpose, Cu exchanged ZSMꢀ5, HY, and
NaY; and Cu nanoparticles decorated conventional ZSMꢀ5, SBAꢀ15, and Al samples were
4
as reducing agent. Materials were characterized by the complementary
2
ꢀadsorption, UVꢀvisible, and scanning/transmission
2+
2
O
3
also prepared. Sustainable catalytic process was developed for the selective synthesis of
indolizine, chalcone, and triazole derivatives using mesoporous ZSMꢀ5 based heterogeneous
catalyst. Multiꢀcomponent synthetic strategy is reported here for the selective synthesis of
above mentioned chemicals that involves phenylacetylene as one of the building blocks.
Control experiments were performed to ascertain the proposed reaction pathways. Recycling
and leaching experiments were performed to demonstrate the sustainability and robustness of
the catalytic process. Among these catalysts, Cu nanoparticles decorated mesoporous ZSMꢀ5
exhibited the highest activity in all these reactions. Catalyst was found to be highly stable and
it was possible to recycle the catalyst five times with no appreciable loss in the activity. A
wide range of indolizine, chalcone, and 1,2,3ꢀtriazole derivatives were prepared in high yields
using this catalyst.
Keywords: Cu nanoparticles; Mesoporous zeolite; Multiꢀcomponent reactions;
Regioselectieve synthesis, Green and sustainable synthetic process.
*
Eꢀmail: rajendra@iitrpr.ac.in;
Phone: +91ꢀ1881ꢀ242175; Fax: +91ꢀ1881ꢀ223395
1

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1
.
Introduction
Microporous zeolites play significant role in the sustainable development of catalytic
1
process for the synthesis of bulk and fine chemicals involving small molecules. These
syntheses are accomplished due to the tuneable acidity of zeolites. Acidity of zeolites can be
2
finely tuned by varying the Si/Al ratio or by the postꢀsynthesis treatment. That is the reason
3
zeolites play crucial role in the petrochemical industry. It is possible to synthesize zeolites
2
having tuneable pore size with wide range of Si/Al ratio. Furthermore, by varying the Si/Al
2
ratio, zeolites with different framework structure can be synthesized. ZSMꢀ5 zeolite rules the
petrochemical industry due to its shape selectivity whereas zeolite Beta rules the fine
3
chemical industry due to its large pore diameter. Though microporous zeolites impart good
activity in many catalytic transformations, however, their role is limited in several
applications due to the following reasons: (1) large molecular organic transformations are
restricted due to small micropore size, (2) applications are mostly based on acid catalyzed
reactions, (3) only a few transition metals can be incorporated in the zeolite framework
structure that limit their wide applications in several important organic transformations. In the
recent time, significant efforts are being made for the development of
hierarchical/nanocrystalline zeolites with inter/intraꢀconnected microporousꢀmesoporous
4
structure. We have contributed in this area and developed a wide range of structure directing
5
agents for the synthesis of mesoporous zeolites. These materials exhibited excellent activity
6
in the acid catalyzed reactions involving large organic molecules. The difficulty to
incorporate transition metals in the zeolite matrix can be taken care up to some extent by the
incorporation of metals as metal ions through ionꢀexchange process or by supporting
metal/metal oxide nanoparticles on the zeolite surface. Due to the recent development of
mesoporous zeolites now it is possible to support highly dispersed metal/metal oxide
7
nanoparticles on the large external surface of nanocrystalline zeolites.
Metal nanoparticles are known to exhibit excellent activity in several important
8
organic transformations. Metal nanoparticles are stabilized by organic molecules known as
capping agent to restrict their aggregation in aqueous or nonꢀaqueous medium. However, it is
difficult to separate and reuse them. Efforts have been made to prepare metal nanoparticles
supported on the surface of heterogeneous catalysts such as metal oxides, silica, zeolite,
9
carbon, etc., for the efficient recovery and reuse. In general, Au, Ag, Ru, Pt, Pd and other
costly metal nanoparticles are found to exhibit good to excellent activity in the organic
10
transformations useful to fine and pharmaceutical industry. Efforts are being made to design
2

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and develop economical catalysts based on transition metals. Cu is an interesting metal that
11
can exhibit surprising activity in several organic transformations. In the recent years, efforts
are being made to investigate organic reactions using Cu based catalysts that are generally
1
2
catalyzed by Au, Ag or Ru based catalysts. In general, high surface area materials are
required to support metal nanoparticles. In addition to this, surface functional groups are
important in stabilizing metal nanoparticles on the surface. Due to less external surface area
and less number of surface silanol groups, it is difficult to prepare highly dispersed metal
nanoparticles on the surface of conventional zeolites. Development of mesoporous zeolites
opens up wide opportunity to use them as suitable heterogeneous catalyst and good support
materials in bulkier molecular transformations.
In this manuscript, mesoporous ZSMꢀ5 (hereafter written as MesoꢀZSMꢀ5) with
2+
nanosheet morphology was prepared. Cu ionꢀexchanged MesoꢀZSMꢀ5 and Cu nanoparticles
supported MesoꢀZSMꢀ5 catalysts were prepared and investigated in the green chemical
synthesis of chalcones, indolizines and 1,2,3ꢀtriazoles. Oneꢀpot, multiꢀcomponent synthesis
strategy was adopted to prepare these pharmaceutically important synthetic intermediates.
2+
For comparative purpose, Cu exchanged ZSMꢀ5, HY, and NaY; and Cu nanoparticles
decorated conventional ZSMꢀ5, mesoporous silica SBAꢀ15, and Al
investigated.
2 3
O samples were also
2
.
Experimental
2.1.
Materials
All chemicals were of AR grade and used as received without further purification.
Ludox (Hsꢀ40, 40 % aqueous solution), tetrapropylammonium bromide (TPABr), SiO
Al , and organic substrates used in the catalytic reactions were purchased from Sigma
Aldrich, India. Sodium borohydride (NaBH ) was obtained from Spectrochem Pvt. Ltd.,
India. Copper nitrate trihydrate (Cu(NO .3H O) and Aluminum sulfate (Al (SO . 18H O)
2
,
2
O
3
4
3
)
2
2
2
4
)
3
2
were obtained from Loba Chemie Pvt. Ltd., India. Sodium dodecyl sulfate was obtained from
SD Fine Chemical Ltd., India. Solvent used in this study were procured from Merck India
Pvt. Ltd. HY and NaY were kindly provided by SÜD Chemie India Pvt. Ltd.
2.2.
Catalysts synthesis
6 3
For the synthesis of MesoꢀZSMꢀ5, first structure directing agent (C18ꢀ[DꢀC ] ꢀDꢀC18)
5d
was prepared by following the reported procedure. MesoꢀZSMꢀ5 was synthesized in the
3

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6 3
basic condition (pH ≈ 12) using multiꢀquaternary ammonium C18ꢀ[DꢀC ] ꢀDꢀC18 as structure
directing agent (SDA) by following the molar composition of 6.67 Na O: 0.75 Al O : 30
2
2
3
SiO
2.25 wt% SiO
ꢀDꢀC18 (2.76 g), and distilled water (25.5 g) were mixed to obtain the above
2
6 3 2
: 1.5 C18ꢀ[DꢀC ] ꢀDꢀC18: 2132 H O. In a typical synthesis, sodium silicate (Si/Na=2.25,
1
2
in aqueous sodium silicate solution) (14.7 g), aluminium sulphate (0.50),
C
6 3
18ꢀ[DꢀC ]
mentioned gel composition. The resultant reaction mixture was vigorously stirred using
magnetic stirrer at ambient condition for 10 h. Finally, the reaction mixture was transferred to
a Teflonꢀcoated stainlessꢀsteel autoclave and hydrothermally treated at 423 K for 6 days
under static condition. After the reaction, the product was filtered, washed with distilled
water, and dried at 383 K. Solid product was calcined at 823 K for 6 h under flowing air in
order to remove the organic SDA.
Conventional ZSMꢀ5 was synthesized by following the reported procedure using
13
tetrapropylammonium bromide as structure directing agent. CuO was prepared by following
1
4
the reported procedure. For comparative study, mesoporous silica SBAꢀ15 was also
prepared. The details of synthesis and textural properties of SBAꢀ15 are provided in
supporting information (Fig. S4, Table S1).
2
+
In this study, Cu ionꢀexchanged and Cu nanoparticles supported zeolite materials
were prepared. 2 g of calcined zeolite was added to 50 mL of 1M CuCl aqueous solution and
the reaction mixture was stirred at 343 K for 6 h. After 6 h, solid was filtered, washed with
2
2
+
deionized water, and dried at 373 K for 12 h. This process was carried out three times. Cu
ionꢀexchanged conventional ZSMꢀ5 and MesoꢀZSMꢀ5 are represented as CuꢀZSMꢀ5 and Cuꢀ
MesoꢀZSMꢀ5, respectively.
Cu nanoparticles supported zeolites were prepared by chemical reduction method by
7c
following the reported procedure. In this study, 5 wt % and 10 wt % of Cu nanoparticles
supported zeolites were prepared. Detail for the synthesis of 10 wt% Cu nanoparticles
supported on zeolite is given as an example. First, solution A was prepared by the dissolution
of 380 mg of Cu (NO
minutes. Another solution B was prepared by the dissolution of 620 mg SDS (sodium
dodecyl sulphate) and NaBH (500 mg) in 250 ml of deionized water, which was also kept in
3 2 2
) .3H O in 150 mL of deionized water, which was kept in ice for 10
4
ice for 10 min. Solution A was added dropꢀwise into solution B for 30 minutes at ice cold
condition under stirring condition. Resultant solution was stirred for additional 30 minutes
under iceꢀcold condition. After this, stirring was stopped and the resulting mixture was kept
at ambient temperature under static condition for 45 minutes. Meanwhile, 1 g support
material was dispersed in 50 ml deionized water using ultrasonic bath for 15 min. Finally,
4

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well dispersed zeolite suspension was added in one proportion to the above nanoparticles
solution and kept for 5 h at ambient temperature under stirring condition. After 5 h, resulting
mixture was centrifuged and washed several times with ethanol, followed by deionized water
to remove the surfactant. Finally, the solid obtained was dried in vacuum at 353 K for 12 h.
Cu nanoparticles supported on ZSMꢀ5 and MesoꢀZSMꢀ5 samples are designated as
Cu(x%)/ZSMꢀ5 and Cu(x%)/MesoꢀZSMꢀ5, respectively, where x = 5 or 10. Cu(10%)/SBAꢀ
2 3
15 and Cu(10%)/Al O were also prepared by following the above mentioned procedure.
2.3.
Catalyst characterizations
Xꢀray diffraction (XRD) patterns were recorded in the 2θ range of 5–60° with a scan
speed of 2°/min on a PANalytical X’PERT PRO diffractometer using Cu Kα radiation
λ=0.1542 nm, 40 kV, 40 mA) and a proportional counter detector. Nitrogen adsorption
(
measurements were performed at 77 K by Quantachrome Instruments, AutosorbꢀIQ
volumetric adsorption analyzer. Sample was outꢀgassed at 573 K for 3 h in the degas port of
the adsorption apparatus. The specific surface area of the material was calculated from the
adsorption data points obtained at P/P between 0.05–0.3 using the BrunauerꢀEmmettꢀTeller
0
(BET) equation. The pore diameter was estimated using BarrettꢀJoynerꢀHalenda (BJH) and
NLDFT methods. Scanning electron microscopy (SEM) measurements were carried out on a
JEOL JSMꢀ6610LV to investigate the morphology of the zeolites. For deeper understanding
structural analysis were carried out using a FEI, TF30ꢀST transmission electron microscope
(
TEM) operating at 300 kV equipped with a scanning unit and a highꢀangle annular dark field
HAADF) detector from Fischione (model 3000). The compositional analysis was performed
(
using energy dispersive Xꢀray (EDX, EDAX Inc.) spectroscopy attachment on the TF30.
Sample was dispersed in ethanol using ultrasonic bath, and dispersed sample was mounted on
a carbon coated Cu grid, dried, and used for TEM measurement. DRUVꢀvisible spectra were
recorded by Shimadzu (UVꢀ2600) spectrophotometer.
2
.4.
.4.1. Synthesis of chalcones and indolizines
0 ml round bottom flask was charged with 2ꢀpyridinecarboxaldehyde (1.0 mmol),
Procedure of catalytic reactions
2
1
piperidine (1.0 mmol), phenylacetylene (1.0 mmol), dichloromethane (DCM, 4 mL) and
catalyst (40 mg). Then the reaction flask was fitted with a condenser and magnetically stirred
2
in a temperatureꢀcontrolled oil bath at 343 K for 3 h under N atmosphere. After 3 h, catalyst
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was removed by centrifugation and the reaction mixture was monitored by gas
chromatograph (GC) (Yonglin 6100; BPꢀ5; 30 m × 0.25 mm × 0.25 µm). Product was
confirmed using GC–MS (Schimadzu GCMSꢀQP 2010 Ultra; Rtxꢀ5 Sil Ms; 30 m × 0.25 mm
×
0.25 µm).
For the synthesis of chalcone, the reaction was performed in neat condition (in the
absence of solvent) by following the above synthesis procedure for 9 h.
2.4.2. Synthesis of 1,2,3-triazoles from epoxides/benzyl halides
In a typical synthesis, epoxides or benzyl halides (2.0 mmol), sodium azide (2.0
mmol), phenylacetylene (2.0 mmol), water (4 mL), and catalyst (40 mg) were charged into a
0 mL round bottom flask. Then the reaction flask was fitted with a condenser and
magnetically stirred in a temperatureꢀcontrolled oil bath at 343 K for 6 h under N
1
2
atmosphere. After 6 h, reaction mixture was centrifuged and the reaction mixture was
extracted with ethyl acetate. Products were isolated with column chromatography using 100ꢀ
200 mesh silica gel and hexane/ethyl acetate mixture as eluent. Products were confirmed
1
13
using H and C NMR that matched well with the reported NMR data.
3
.
Results and discussion
3.1.
Physico-chemical characterization
MesoꢀZSMꢀ5 exhibits XRD pattern corresponding to a highly crystalline MFI
framework structure with high phase purity (Fig. 1a). Cu(10%)/MesoꢀZSMꢀ5 clearly shows
the diffraction peaks corresponding to both Cu nanoparticles (hereafter represented as
CuNPs) and MesoꢀZSMꢀ5 phases. Three additional diffraction peaks located at 2θ; 43.1,
50.3, and 73.9° are observed in Cu(10%)/MesoꢀZSMꢀ5 when compared with MesoꢀZSMꢀ5,
15
which can be assigned to (111), (200), and (220) planes of CuNPs (Fig. 1a). The XRD
pattern of Cu(5%)/MesoꢀZSMꢀ5 exhibits weak diffraction peaks corresponding to CuNPs
because of low Cu content in Cu(5%)/MesoꢀZSMꢀ5. Cu(10%)/ZSMꢀ5 also exhibits well
resolved diffraction peaks corresponding to both CuNPs and ZSMꢀ5 phases (Figure not
shown). The textural properties of the zeolite material were investigated using nitrogen
adsorptionꢀdesorption measurements. MesoꢀZSMꢀ5 exhibits typeꢀIV isotherm similar to that
of mesoporous materials (Fig. 1b). A sharp increase in the volume of N adsorption in the
2
region 0.4<P/P
0
<0.95 is characteristic of capillary condensation in the inter/intraꢀcrystalline
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mesopore void spaces present in the MesoꢀZSMꢀ5. Both micropores and mesopores
contribute to the porosity (total pore volume) of the material. The difference in the total pore
Cu(10%)/Meso-ZSM-5
(
a)
(
111)
(
200)
(220)
1
1
1
0
0
0
20 30 40 50 60 70 80
Meso-ZSM-5
20 30 40 50 60 70 80
ZSM-5
20 30 40 50 60 70 80
2θ (Degree)
7
6
5
4
3
2
1
00
00
00
00
00
00
00
0
1
1
0
0
0
0
0
.2
.0
.8
.6
.4
.2
.0
(
b)
0
5
10 15 20 25 30
Pore Diameter (nm)
Meso-ZSM-5
Cu(10%)/Meso-ZSM-5
ZSM-5
0
.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
2
Fig. 1. (a) XRD patterns and (b) N ꢀadsorption desorption isotherms (inset shows pore size
distribution) of ZSMꢀ5, MesoꢀZSMꢀ5, and Cu(10%)/MesoꢀZSMꢀ5.
7

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volume of ZSMꢀ5 and MesoꢀZSMꢀ5 materials is due to the mesoporosity present in Mesoꢀ
ZSMꢀ5. Mesopores in MesoꢀZSMꢀ5 are formed due to the void space between the nearest
neighbouring nanosheets. Pore size distribution indicates that the nanosheets maintain local
coherence but assembles in somewhat disordered manner that lead to form widely distributed
pore size (Fig. 1b, inset). Textural properties of zeolite materials investigated in this study are
2
summarized in Table 1. The N ꢀadsorption study confirms that the surface area and total pore
volume decreased after the incorporation of CuNPs on its surface (Table 1). MesoꢀZSMꢀ5 has
large external surface area with large number of surface silanol groups that makes it an ideal
support for the decoration of metal nanoparticles.
Table 1. Physicoꢀchemical properties of ZSMꢀ5 materials investigated in this study.
a
b
Entry
Surface area External surface Mesopore Total
Cu
content
volume (%)
2
area (m /g)
volume
pore
2
SBET (m /g)
(cc/g)
(cc/g)
ZSMꢀ5
302
297
600
ꢀ
71
69
215
ꢀ
0.08
0.08
0.83
ꢀ
0.20
0.20
1.02
ꢀ
ꢀ
CuꢀZSMꢀ5
6.13
ꢀ
MesoꢀZSMꢀ5
CuꢀMesoꢀZSMꢀ5
Cu(5%)/MesoꢀZSMꢀ5
6.52
4.34
8.45
8.40
ꢀ
ꢀ
ꢀ
ꢀ
Cu(10%)/MesoꢀZSMꢀ5 543
195
186
0.76
0.73
0.97
0.95
Cu(10%)/ MesoꢀZSMꢀ 526
c
5
Cu(10%)/ZSMꢀ5
249
64
0.07
0.19
7.38
a
b
S
BET calculated from the adsorption branch in the region of 0.05<P/P
0
≤0.3. Obtained by
c
ICP analysis. Textural properties of the recovered catalyst after the recycling catalyst.
Large micrometer size ZSMꢀ5 particles are observed in the SEM micrograph. Similar
morphology is observed for CuNPs supported ZSMꢀ5 (Fig. 2). Oval shaped flower like
zeolite crystals are observed in the SEM micrograph of MesoꢀZSMꢀ5 that are built with
nanosheet like flower petals (Fig. 2). Cu(10%)/MesoꢀZSMꢀ5 also exhibits the similar
morphology to that of MesoꢀZSMꢀ5 which confirms that NaBH reduction procedure for the
4
synthesis of CuNPs did not disturb the morphology of parent materials (Fig. 2). Details of
8

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microꢀnano structure is provided here for the highly active CuNPs(10%)/MesoꢀZSMꢀ5 and
MesoꢀZSMꢀ5 which was obtained from TEM investigation. TEM images confirm the oval
like morphology of MesoꢀZSMꢀ5 particles in the nanocomposite material (Fig. 3). Some
irregular zeolite particles can also be seen in the TEM images (Fig. 3). High resolution TEM
(HRTEM) image shows that oval or irregular shape particles are built with nanosheet like
crystal morphology (Fig. 3). TEM also reveals that zeolite growth has taken place in one
direction and these uniꢀdirectional grown zeolite nanosheets selfꢀassembled to form oval
shape nanoarchitecture. Selected area diffraction pattern indicates that material is highly
ordered in the micropores length scale (Fig. 3e, inset). Further examination of HRTEM
images reveals that CuNPs are adhered to the surface of zeolite nanosheets. Highly dispersed,
well defined spherical CuNPs of the dimension of 2ꢀ3 nm can be seen in the HRTEM images
of CuNPs(10%)/MesoꢀZSMꢀ5 (Figs. 3g, h). HRTEM image also shows lattice fringes, which
further confirms the highly crystalline nature of the material (Fig. S1a, ESI). To confirm the
presence of Si, Al, O, and Cu elements in the material, EDX analysis was performed. EDX
spectrum (Fig. S1b, ESI) clearly shows the incorporation of CuNPs on the surface of Mesoꢀ
ZSMꢀ5.
Formation of CuNPs on the surface of MesoꢀZSMꢀ5 was also investigated using
DRUVꢀvisible spectroscopy (Fig. S2, ESI). In general, absorptions in the range of 250ꢀ450
nm are ascribed to charge transfer bands, whereas absorptions above 620 nm are ascribed to
1
6
dꢀd transition. In addition, absorption above 500 nm belongs to plasmonic resonance of Cu
n
16
nanoparticles. In this study, no absorption in the range of 400ꢀ1000 nm is observed in
MesoꢀZSMꢀ5. However, it is clearly observed in Cu(10%)/MesoꢀZSMꢀ5. The visible range
n
absorption can be assigned to the plasmonic resonance produced by Cu nanoparticles.
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ZSM-5
Cu(10%)/ZSM-5
1
0 ꢀm
ꢀm
5
ꢀm
5
10 ꢀm
Meso-ZSM-5
Meso-ZSM-5
1
ꢀm
1
ꢀm
Cu(10%)/Meso-ZSM-5
1
ꢀm
Fig. 2. SEM micrographs of parent and CuNPs decorated ZSMꢀ5 and MesoꢀZSMꢀ5 samples.
1
0

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(
b)
(
a)
c)
Meso-ZSM-5
(
Cu(10%)/Meso-ZSM-5
Cu(10%)/Meso-ZSM-5
(d)
(
e)
(f)
Fig. 3. TEM images of (a,b) MesoꢀZSMꢀ5 and (cꢀf) Cu(10%)/MesoꢀZSMꢀ5. Inset (d) shows
the diffraction patterns obtained from the domain represented in (d).
3.2.
Catalytic investigation
In this manuscript, efforts have been made to demonstrate the application of Cu ionꢀ
2+
exchanged and Cu nanoparticles decorated mesoporous zeolites in the synthesis of large
organic molecules such as indolizines, chalcones, and 1,2,3ꢀtriazoles by using
phenylacetylene as one of the building blocks.
1
1

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Table 2. Catalytic activity data obtained in the synthesis of indolizine in dichloromethane
over various catalysts investigated in this study.
N
Catalyst
H
N
N
CHO
N
H
N
DCM, 3 h, 343 K
O
Chalcone
Indolizine
E.No Catalyst
2ꢀPyridinecarboxaldehyde Product selectivity (%)
conv.(%)
Indolizine Chalcone
1
2
3
4
5
6
7
8
9
1
1
1
CuO
65.2
55.3
98.1
84.2
77.5
60.6
38.2
60.5
46.1
76.3
91.8
93.6
60.4
100
18.2
69.7
100
100
100
100
70.6
100
100
100
39.6
CuꢀMesoꢀZSMꢀ5
CuꢀHY
0
81.8
30.3
0
CuꢀNaY
Cu(10%)/MesoꢀZSMꢀ5
Cu(5%)/MesoꢀZSMꢀ5
Cu(10%)/ZSMꢀ5
0
0
Cu(10%)/SBAꢀ15
0
Cu(10%)/Al
2
O
3
29.4
0
a
b
c
0
1
2
Cu(10%)/MesoꢀZSMꢀ5
Cu(10%)/MesoꢀZSMꢀ5
Cu(10%)/MesoꢀZSMꢀ5
0
0
Reaction condition: 2ꢀPyridinecarboxaldehyde (1.0 mmol), piperidine (1.0 mmol),
phenylacetylene (1.0 mmol), catalyst (40 mg), time (3 h), dichloromethane (DCM, 4 mL),
reaction temp (343 K).
a
Catalytic data obtained after the fifth recycles.
Catalytic data obtained after the 4 h.
Catalytic data obtained using 60 mg of catalyst.
b
c
Indolizine can be prepared by the conventional transition metal catalyzed CꢀN bond
17
forming reaction. In the recent time, efforts have been made to synthesize indolizines using
18
Cu based homogeneous and heterogeneous catalysts. In this study, effort is made to prepare
indolizines by oneꢀpot multiꢀcomponent CꢀC coupling synthesis strategy which allows
structural diversity in the sustainable atomꢀefficient, oneꢀstep synthesis route. We started our
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2

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investigation to find which form of Cu containing species would be more suitable for the
synthesis of indolizine. Experiments show that CuO is active for the synthesis of indolizine
but the reaction is not very selective towards the formation of indolizine (Table 2). Chalcone
is formed as side product. When the similar reaction is performed with CuꢀMesoꢀZSMꢀ5 or
Cu(10%)/MesoꢀZSMꢀ5 then indolizine is obtained as selective product (Table 2). The
reactivity of Cu(10%)/MesoꢀZSMꢀ5 is found to be more when compared to CuꢀMesoꢀZSMꢀ5
under the identical reaction condition. If one compares the Cu content in both these samples,
then also the reactivity of Cu(10%)/MesoꢀZSMꢀ5 is more when compared to CuꢀMesoꢀZSMꢀ
5
(Table 1 and Table 2). It may be noted that amount of Cu ionꢀexchange depends on the
2
+
amount of exchangeable cations present in the zeolite sample. To investigate the role of Cu
concentration in the zeolite sample, CuꢀHY and CuꢀNaY were also investigated. It is found
that using these catalysts, in addition to indolizine, chalcone is also formed as side product. It
is interesting to note that reaction is found to be more selective for chalcone when HY is used
when compared to NaY. This provides evidence that acid sites present in the catalyst would
be favorable for the synthesis of chalcone. The role of CuNPs is to activate the phenyl
acetylene terminal acidic proton and form Cuꢀacetylide, which is one of the key steps in the
multiꢀcomponent synthesis of indolizine. As shown in Table 1 that the synthesis of indolizine
is favoured if the support is basic or even neutral. Since CuO is somewhat acidic in nature,
therefore it would result in the formation of substantial amount of chalcone as we obtained in
the case of acidic CuNPs/HY. Since amount of Cu nanoparticles can be finely tailored and
indolizine can be selectively prepared, therefore Cu nanoparticles supported MesoꢀZSMꢀ5
would be better for the selective synthesis of indolizine. Under the optimized reaction
condition, Cu(10%)/ZSMꢀ5 exhibits less product yield (Table 2). Catalytic investigations
show that the yield of indolizine can be easily improved by increasing the reaction time or
amount of the catalyst (Table 2). This provides evidence that large external surface area is
beneficial for the synthesis of highly dispersed Cu nanoparticles that facilitates this reaction.
Using Cu(10%)/MesoꢀZSMꢀ5, more number of active sites are accessible to the reactants that
facilitate the multiꢀcomponent reaction. To further evaluate the superiority of this catalytic
system, CuꢀMesoꢀZSMꢀ5 and Cu(10%)/MesoꢀZSMꢀ5 were recycled for the synthesis of
indolizine. Progressive decrease in the activity is observed in CuꢀMesoꢀZSMꢀ5 whereas no
significant change in the activity is observed in the case of Cu(10%)/MesoꢀZSMꢀ5 (Fig. S3,
2+
ESI). This provides evidence that Cu leach to solution and catalyzes the reaction. Leaching
experiments further confirm that conversion increases even after the removal of the catalyst.
Based on the recycling and leaching experiments one can conclude that Cu(10%)/ Mesoꢀ
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ZSMꢀ5 is found to be sustainable and robust catalyst for the synthesis of indolizine (Table 2).
Physicoꢀchemical characterization shows that the textural properties and Cu content of the
recovered catalyst are almost similar to that of fresh catalyst (Table 1).
Based on the results obtained in this work, one can conclude that highly dispersed
CuNPs supported on Na form of ZSMꢀ5 nanosheet is best suited for this reaction. CuꢀMesoꢀ
ZSMꢀ5 exhibits higher selectivity when compared with CuꢀNaY, which clearly shows that
reaction is favourable when support is basic and contains less Al in its framework. Since NaY
has very large Al content (Si/Al = 2.5) in its framework, therefore, it will impart some acidity
even in the Na form, therefore it exhibits low selectivity for indolizine. This can also be
2 3
correlated from the results obtained using Cu(10%)/SBAꢀ15 and Cu(10%)/Al O . SBAꢀ15
has large surface area (even somewhat higher than MesoꢀZSMꢀ5) but it possess small acidity
that is not favourable to obtain high yield of indolizine. However, it may be noted that in the
case of SBAꢀ15, only indolizine is obtained as selective product. It may be noted that SBAꢀ
15 is prepared in acidic medium and it will exhibit very low acidity even in the absence of Al
in the amorphous mesoporous framework structure. Less surface area and biꢀfunctional
nature (acidic and basic sites) of the Al O are responsible for the low conversion and
2
3
selectivity for indolizine. Based on the results obtained one can clearly say that favourable
metal support interaction between CuNPs and Na form of MesoꢀZSMꢀ5 nanosheet leads to
provide interesting catalytic properties. In order to validate this conclusion, CuNPs supported
5
a
2
Naꢀform of NanoꢀBeta (Si/Al = 19) (Surface area = 672 m /g) and CuNPs supported Mesoꢀ
2
ZSMꢀ5 (with Si/Al = 50) (Surface area = 608 m /g) were prepared and investigated. The
activity of Cu(10%)/NanoꢀBeta (Conversion = 77.9 %, Selectivity = 100 %) and
Cu(10%)/MesoꢀZSMꢀ5 (Si/Al = 50) (Conversion = 76.7 %, Selectivity = 100 %) were found
to be similar. These results clearly demonstrate that the optimum basicity of support, large
surface area, favourable metalꢀsupport interactions, highly dispersed active metal centres, all
are important to achieve high activity and selectivity.
Influence of solvent in the synthesis of indolizine is investigated and data is presented
in Fig. 4. Reaction did not proceed in toluene. Very low yield of indolizine is obtained in the
protic solvent such as methanol. However, only indolizine is obtained as selective product in
methanol. Polar aprotic solvent such as dichloromethane (DCM) and acetonitrile are
investigated. Catalytic data shows that DCM is more effective and produced indolizine as
selective product in excellent yield. However, using acetonitrile, not only the 2ꢀpyridine
carboxaldehyde conversion is low but the indolizine selectivity is also low. In order to see the
general applicability of this protocol for wide range of amine derivatives, primary, secondary
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and tertiary amines were investigated (Table 3). Tertiary amine such as triethyl amine is
found to be inactive under the optimized reaction condition (Table 3). When primary amine is
reacted under the optimized reaction condition, indolizine is not obtained. In this case,
condensation product of 2ꢀPyridine carboxaldehyde and nꢀbutyl amine is obtained (Table 3).
Reaction proceeds well with various secondary amines such as piperidine, pyrrolidine, and
diethyl amine. In all these cases corresponding indolizine product is obtained (Table 3).
However, reactivity varies by varying the amines. Diethyl amine is found to be the most
reactive under the present reaction condition (Table 3). This difference in the reactivity can
be attributed to the ring strain and steric factor. Wide range of phenylacetylene and secondary
amines can be converted to corresponding indolizine with good to excellent yield (Table 3). It
may be noted that the catalytic activity data presented in Table 3 are under the optimized
reaction condition. Product yield can be easily improved by increasing the amount of catalyst
or reaction time.
Table 3. Synthesis of indolizines using Cu(10%)/MesoꢀZSMꢀ5
Base
Phenylacetylene
derivatives
2ꢀPyridine
carboxaldehyde
conv. (%)
Product selectivity
(%)
7
6.1
(100%)
N
H
96.0
(100%)
0
_
9
9.8
(100%)
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6
7.2
3.5
(100%)
6
(100%)
7
7.9
(100%)
Reaction condition: As mentioned in Table 2.
%
Conv.
1
00
Indolizine
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
DCM
ACN
MeOH Toluene
Different Solvents
Fig. 4. Influence of solvent in the synthesis of indolizine using Cu(10%)/MesoꢀZSMꢀ5.
Based on the results obtained, following reaction mechanism is presented for the
synthesis of indolizine (Scheme 1). In this multiꢀcomponent synthesis methodology iminium
1
6

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ions and Cuꢀacetylide would form first. Iminium ion would form by the condensation of
amine (piperidine) and aldehyde (pyridine 2ꢀcarboxaldeyde). Furthermore, metal
nanoparticles are known to activate the phenylacetylene acidic proton by the coordination to
CꢀC triple bond to form Cuꢀacetylide. These two reactive components would cycloisomerize
and aromatize in the presence of Cu catalyst to produce indolizine as the selective product.
1
8a
The presented mechanism is in accordance with the previous literature report.
Ph
Acidic Hydrogen
Ph
H
R2
N
R1
H
N
R2
R1
N
CHO
N
H
R2
R2
R1
R2
N
R1
R1
N
H
N
H
N
N
N
Ph
.
.
Ph
Ph
Scheme 1. Plausible mechanism for the synthesis of indolizine by the multiꢀcomponent
reaction of 2ꢀpyridine carboxaldehyde, amine and phenylacetylene.
Having shown the selective synthesis of indolizine, we were interested to find the
suitable reaction condition for the selective synthesis of chalcone from 2ꢀPyridine
carboxaldehyde and phenylacetylene under our experimental setꢀup. During the selective
synthesis of indolizine, we found that solvent and catalyst both were important for the
selective synthesis of indolizine. Similar results were obtained in one of the recent
18a
manuscript. Reaction investigations reveal that chalcone can be selectively obtained when
the reaction is performed in neat condition using Cu(10%)/MesoꢀZSMꢀ5 (Table 4). Various
other catalysts are also investigated under neat condition in this study (Table 4). Using CuO,
in addition to chalcone, 2, 2’ꢀbipiperidine is also formed. It is interesting to note that in this
case, CuꢀMesoꢀZSMꢀ5 or CuꢀNaY is found to be inactive whereas CuꢀHY is found to be
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active (Table 4). However, CuꢀHY produced both, chalcone and indolizine, under the
2+
optimized reaction condition. These results confirm that Cu is not suitable for this reaction,
whereas Cu nanoparticle is found to be active and selective for the synthesis of chalcone. In
this case also, the activity of Cu(10%)/ZSMꢀ5 is found to be low when compared to
Cu(10%)/MesoꢀZSMꢀ5 (Table 4).
Table 4. Selective synthesis of chalcone in neat condition using Cu catalysts investigated in
this study.
Catalyst
H
Base
N
N
CHO
Neat, 9 h, 343 K
O
Chalcone
E.No Catalyst
2ꢀPyridine
Product selectivity (%)
carboxaldehyde
conv. (%)
Chalcone Indolizine Other
1
2
3
4
5
6
7
8
9
1
CuO
98.2
0
74.2
-
25.8
CuꢀMesoꢀZSMꢀ5
CuꢀHY
-
-
-
-
-
-
-
-
53.1
0
55.4
-
44.6
CuꢀNaY
-
Cu(10%)/MesoꢀZSMꢀ5
Cu(5%)/MesoꢀZSMꢀ5
Cu(10%)/ZSMꢀ5
Cu(10%)/SBAꢀ15
100
76.4
54.3
64.8
46.7
100
100
100
100
100
77.5
100
-
-
-
-
Cu(10%)/Al
2
O
3
22.5
a
0
Cu(10%)/MesoꢀZSMꢀ5
-
-
Reaction condition: 2ꢀPyridine carboxaldehyde (1.0 mmol), piperidine (1.0 mmol),
phenylacetylene (1.0 mmol), catalyst (40 mg), reaction time (9 h), reaction temp (343 K).
a
Catalytic data obtained after the fifth recycles.
Influence of bases used in the synthesis of chalcone is also investigated (Table 5).
Triethylamine is found to be inactive for this reaction, whereas primary amine such as butyl
amine produced chalcone in low yield (Table 5). In addition to chalcone, condensation
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product of butyl amine and pyridine 2ꢀcarboxaldehyde is also obtained in large amount.
Secondary amine is found to be excellent in assisting this reaction to form chalcone as
selective product (Table 5). Almost quantitative yield of chalcone is obtained using various
secondary amines such as piperidine, pyrrolidine, and diethylamine (Table 5). Cu(10%)/
MesoꢀZSMꢀ5 is recycled in the synthesis of chalcone. Recycling study shows that no
significant change in the activity and selectivity is obtained even after five recycles, justifying
robustness of the catalyst system (Table 4). In this case also optimum metalꢀsupport
interaction between CuNPs and Na form of MesoꢀZSMꢀ5 nanosheet is responsible for the
high catalytic activity as explained above in the case of indolizine.
Table 5. Role of amines in the selective synthesis of chalcone using Cu(10%)/MesoꢀZSMꢀ5.
R1
R2
N
H
H
N
N
CHO
O
Chalcone
Base
2ꢀPyridine
carboxaldehyde conv. (%)
Products Selectivity
%)
(
98.9
(100%)
99.7
(
100%)
̶
0
99.8
(
74.7 %)
25.3 %)
(
Reaction condition: 2ꢀPyridinecarboxaldehyde (1.0 mmol), base (1.0 mmol), phenylacetylene
1.0 mmol), catalyst (40 mg), reaction time 9 h, reaction temp (343 K).
(
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Results obtained using primary amine provides evidence that iminium ions produced
by the condensation of pyridine 2ꢀcarboxaldeyde and secondary amine certainly participate in
the reaction process. Considering this aspect, following mechanism is presented (Scheme 2).
In this case, secondary amine has two roles to play: first, it acts as a reactant for the formation
of iminium ion and second, it acts as a good base in this reaction. First few steps are the same
as outlined in the mechanism for the synthesis of indolizine. In this case, reaction takes place
in the absence of solvent, therefore the basicity of secondary amine would predominate and
reacts with the intermediate formed by the reaction between iminium ion and Cuꢀacetylide to
form allenyl amine. The negative inductive effect produced by the electron withdrawing
group increases the acidity of proton attached to propargyl hydrogen atom and stabilizes the
negative charge produced after the deprotonation of propargylꢀallenyl amine intermediate.
Hydrolysis of the propargylꢀallenyl amine intermediate takes place (by the H
2
O formed in the
initial condensation step) to form chalcone. The presented mechanism is in accordance with
1
8a
the previous literature report. Catalytic activity of Cu(10%)/MesoꢀZSMꢀ5 in the synthesis
of indolizine/chalcone using phenylacetylene is either similar or higher when compared to
1
8
reported literature.
As outline in the mechanism of chalcone synthesis that basicity of amine is very
important. Only secondary amine is able to produce chalcone. Basicity of amine plays
decisive role in the synthesis of chalcone. Based on the results one can say that the
unsolvated diethylamine is acting as more efficient base when compared to solvated diethyl
amine. Since the acidity and basicity is highly depended on the reaction medium, one can say
that the basicity of secondary amine is found to be the optimum in the synthesis of chalcone
under neat condition. Based on the result, one can say that DCM favors the formation of
indolizine through cycloisomerization of pyridinyl propargylamine as intermediate. In DCM,
rearrangement of the pyridinyl propargylamine intermediate takes place to form the product.
Whereas under neat condition, pyridinyl propargylamine does not undergo rearrangement and
reaction follows different pathway due to the high basic strength of secondary amine in neat
condition. In this case the abstraction of proton associated to pyridinyl propargylamine takes
place that leads to form allenyl amine intermediate, which upon hydrolysis leads to form
chalcone.
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0

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Scheme 2. Plausible mechanism for the synthesis of chalcone by the multiꢀcomponent
reaction of 2ꢀpyridine carboxaldehyde, amine, and phenylacetylene.
These reactions involve large organic molecules that cannot be able to diffuse into the
micropores of the materials. It may be noted that reaction proceeds even with CuO or
2 3
Cu/Al O . These results clearly show that zeolite microporosity has no role to play in these
reactions. In these reactions, oxidation state of Cu and dispersion of CuNPs at the large
external surface of MesoꢀZSMꢀ5 are more important when compared to the micropores and
inter/intra crystalline mesopores of MesoꢀZSMꢀ5. Since CuNPs (Cu(0)) could activate the
phenyl acetylene terminal acidic proton easily when compared to CuO (Cu(II)), therefore
CuNPs supported catalysts exhibited higher activity. Cu(10%)/MesoꢀZSMꢀ5 exhibited
higher product yield when compared to Cu(10%)/ZSMꢀ5. This clearly shows that the highly
dispersed active CuNPs are present at the large external surface of MesoꢀZSMꢀ5, which
enhances the accessibility of large number of the reactant molecules when compared to
Cu(10%)/ZSMꢀ5 that are responsible for the high activity of Cu(10%)/MesoꢀZSMꢀ5.
2
1

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Table 6. Synthesis of 1,4ꢀdisubstituted, 1,2,3ꢀtriazole using benzyl/aliphatic halides as
reactant using Cu nanoparticles supported ZSMꢀ5 catalysts.
Cu (10%)/Meso-ZSM-5
N
N
N
R
X
H
NaN3
R
H O/333 K
2
E.No.
Catalyst
Product (%Yield)
1
Cu(10%)/MesoꢀZSMꢀ5
(96.4)
2
3
4
Cu(5%)/MesoꢀZSMꢀ5
Cu(10%)/ZSMꢀ5
(
(
62.1)
50.4)
Cu(10%)/MesoꢀZSMꢀ5
(
80.3)
5
6
7
Cu(10%)/MesoꢀZSMꢀ5
Cu(10%)/MesoꢀZSMꢀ5
Cu(10%)/MesoꢀZSMꢀ5
Nil
Nil
(55.2)
a
8
9
Cu(10%)/MesoꢀZSMꢀ5
Cu(10%)/MesoꢀZSMꢀ5
(94.3)
b
(93.2)
Reaction condition: Benzyl chloride (2 mmol), sodium azide (2 mmol), phenylacetylene (2
mmol), H O (4 mL), catalyst amount (40 mg), reaction temperature (343 K), reaction time (6
2
a
b
h). Catalytic data obtained after the 14 h. Catalytic data obtained after the fifth recycles.
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Having shown the application of this robust catalyst system in the synthesis of
indolizine and chalcone, it is important to show the application of the catalyst in other multiꢀ
component reactions where phenylacetylene can be used as building block. We were
interested to explore the possibility to use phenylacetylene as building block for the
construction of other functionalized organic substructures, which are precursors for the
synthesis of many industrial products, molecular materials and bioactive compounds. Multiꢀ
component reaction strategy can be used to synthesize 1,2,3ꢀtriazole derivatives using
1
9
phenylacetylene as one of the reactant. In general, for the synthesis of 1,2,3ꢀtriazole,
19
aromatic azides are chosen as one of the reactant. However, aromatic azides are costly.
Therefore attempts are made in this study to use sodium azide as economical azide precursor
2
0
for the synthesis of 1ꢀbenzylꢀ4ꢀphenylꢀ1Hꢀ1,2,3ꢀtraizole. In recent time, efforts have been
made to synthesize triazole derivatives using Cu based homogeneous and heterogeneous
20
catalysts. Herein, we have explored the possibility to use inꢀsitu generated aromatic azides
from suitable precursors) in one pot synthesis of 1,2,3ꢀtriazoles. In this manuscript benzyl
(
halide and epoxides are chosen to obtain functional 1,4ꢀdisubstituted 1,2,3ꢀtriazoles. First
reaction condition was optimized using benzyl bromide, phenylacetylene, and sodium azide
2
in H O as green reaction medium with Cu(10%)/MesoꢀZSMꢀ5 catalyst. Reaction proceeds
well using this catalyst and produced 1,4ꢀdisubstituted, 1,2,3ꢀtriazole compound as selective
product (Table 6). Product was confirmed with NMR spectroscopy that matches well with the
reported literature. The reaction also proceeds with Cu(10%)/ZSMꢀ5 but low yield of the
product is obtained. This provides further evidence that the MesoꢀZSMꢀ5 support is ideal for
the synthesis of large molecules such as substituted 1,2,3ꢀtriazoles. The reaction proceeds
faster when benzyl bromide is used as reactant when compared to benzyl chloride (Table 6).
Reaction also takes place when aliphatic bromide such as bromohexane is reacted (Table 6).
However, it requires longer reaction time to afford good yield. It may be noted that reaction
did not proceed with iodobenzene or bromobenzene. The reusability of Cu(10%)/MesoꢀZSMꢀ
5
was examined by the model reaction of phenylacetylene, benzyl bromide, and sodium azide
and the results are given in Table 6. It is observed that after five recycles, the product yield is
only marginally lower when compared to the fresh catalyst, which indicates that
Cu(10%)/MesoꢀZSMꢀ5 exhibits high catalytic activity and stability under our experimental
condition. To present the mechanism of this reaction, two control experiments were
performed. In the first control experiment, first benzyl bromide is reacted with sodium azide
followed by the reaction with phenylacetylene to afford the desired product (Scheme 3). The
yield (91 %) of this two step reaction is somewhat lower when compared to oneꢀpot multiꢀ
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component reaction of the present investigation. In second control reaction, first
phenylacetylene is reacted with sodium azide followed by the reaction with benzyl bromide.
In this twoꢀstep process, desired product 1ꢀbenzylꢀ4ꢀphenylꢀ1Hꢀ1,2,3ꢀtraizole is not obtained.
Based on these observations, reaction mechanism is presented (Scheme 4). As mentioned
above in the synthesis of indolizine, in this case also, Cu nanoparticles would react with
phenylacetylene and form Cuꢀacetylide (Intermediate I). Meanwhile, benzyl bromide would
react with sodium azide to form benzyl azide (Intermediate II). Inꢀsitu formed benzyl azide
+
would react with Cuꢀacetylide to form Intermediate III, which abstracts H (that was
generated in the first step of the reaction) and lead to form desired product and the catalyst is
regenerated.
Scheme 3. Exploration of reaction pathways involved in the synthesis of 1,4ꢀdisubstituted
1,2,3ꢀtriazoles.
R
H
N
N
N
H
H
R
N
N
N
H
(III)
(I)
R
N N N
II)
R
X
NaN3
(
Scheme 4. Plausible mechanism for the synthesis of 1,4ꢀdisubstituted 1,2,3ꢀtriazoles by the
multiꢀcomponent reaction of phenylacetylene, benzyl/aliphatic halides and sodium azide.
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O
HO
H
N
a
Yield = 94.6 %
N
N
b
=
54.4 %
Major
O
N
N
N
OH
Catalyst
H C
3
H
NaN3
a
Yield = 80.2 %
CH3
b
=
47.8 %
H
Major
O
N
N
Cl
N
N
OH
N
N
a
Major
Yield = 68.4 %
b
=
26.6 %
N
N
N
aCu(10%)/Meso-ZSM-5
bCu(10%)/ZSM-5
a
Yield = 30.1 %
O
b
=
36.3 %
Minor
Scheme 5. Influence of epoxides in the product selectivity during the multiꢀcomponent
reaction of phenylacetylene, sodium azide and epoxide over CuNPs decorated ZSMꢀ5
nanosheets.
Reaction condition: Epoxides (2.0 mmol), sodium azide (2.0 mmol),
phenylacetylene (2.0 mmol), water (4 mL), catalyst (40 mg), reaction time (6 h), reaction
temp (343 K).
Having successful in the synthesis of 1,4ꢀdisubstituted 1,2,3ꢀtriazole using halide
reactant, aromatic and aliphatic epoxides were reacted under the same condition to obtain βꢀ
hydroxy, 1,4ꢀdisubstituted 1,2,3ꢀtriazoles (Scheme 5). This reaction depends on the nature of
epoxide and can be grouped in three categories: aromatic epoxide such as styrene oxide and
its derivative, aliphatic epoxide such as propylene oxide or other aliphatic linear chain based
epoxides and halogen based epoxide such as epichlorohydrin. In all three cases, three
different types of products are obtained (Scheme 5). When styrene oxide is reacted it forms
primary alcohol based βꢀhydroxy, 1,4ꢀdisubstituted 1,2,3ꢀtriazoles [2ꢀPhenylꢀ2ꢀ(4ꢀphenylꢀ
1
Hꢀ1,2,3ꢀtriazolꢀ1ꢀyl)ethanol] (isolated yield = 94.6%) selectively, whereas when propylene
oxide is reacted, then secondary alcohol based βꢀhydroxy, 1,4ꢀdisubstituted 1,2,3ꢀtriazole [1ꢀ
4ꢀPhenylꢀ1Hꢀ(4ꢀphenylꢀ1,2,3ꢀtriazolꢀ1ꢀyl) propanꢀ2ꢀol] (isolated yield = 80.2%) is
(
selectively obtained. Thin layer chromatography investigation shows that, in these cases trace
amount of other regioisomer is also formed during the reaction, which is somewhat difficult
to isolate using column chromatography. Mechanism of this reaction can be explained based
on the ring opening of epoxide with sodium azide. Azide ion opens the oxirane ring
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differently due to the formation of more stable species as illustrated in the proposed
mechanism that leads to form two different regioisomers in these two cases. Cu nanoparticles
ꢀ
activate the epoxide ring and N
3
opens the oxirane ring to form azido alcohol intermediate (I)
2
1
(
Scheme 6). In the mean time, a πꢀcomplex is simultaneously formed between
phenylacetylene and Cu nanoparticles to afford the copper(II) acetylide intermediate (II).
Then, 1,3ꢀdipolar cycloaddition between these two intermediates (I) and (II) would take place
to form copper(II) βꢀhydroxytriazolide intermediate (III), which rearranges to intermediate
(IV). Protonolysis of intermediate (IV) takes place to afford corresponding βꢀhydroxyꢀ1,2,3ꢀ
1
13
triazole selectively in high yield. H and C NMR of the isolated products confirmed the
formation of two different regioisomers obtained using styrene oxide and propylene oxide
(
(
(
Figs. S5ꢀS8, ESI). When the same reaction is performed with epichlorohydrin, bisꢀtriazole
1,3ꢀbis(5ꢀphenylꢀ1Hꢀ1,2,3ꢀtriazolꢀ1ꢀyl) propanꢀ2ꢀol] is obtained as selective product
1
13
1
13
isolated yield = 68.4%), which was confirmed with H, C, and Hꢀ C HETCOR NMR
ꢀ
investigations (Figs. S9ꢀS12, ESI). This product is formed because, Cl is a good leaving
group that reacts with sodium azide to form azide derivative of epoxide similar to the
proposed reaction for benzyl bromide/chloride with sodium azide (Intermediate I, Scheme 7).
Azide derivative of epoxide reacts with phenylacetylene to form Intermediate II (Scheme 7).
This intermediate is isolated as minor product (isolated yield = 30.1%) during the reaction
(Scheme 5) (Fig. S12, ESI). Ring opening of Intermediate II would take place to form
Intermediate III (Scheme 7), which would react with phenylacetylene to form bisꢀtriazole as
major product (Scheme 7). The activity of Cu(10%)/ZSMꢀ5 is significantly low when
compared to Cu(10%)/MesoꢀZSMꢀ5 in this reaction, which confirms that MesoꢀZSMꢀ5 is
more suitable for this reaction (Scheme 5). Cu(10%)/MesoꢀZSMꢀ5 was easily recycled five
times with no appreciable loss in the activity during the multiꢀcomponent reactions of
phenylacetylene, styrene oxide and sodium azide. Upon completion of the reaction, the
catalyst was recovered from the reaction mixture by simple filtration, washed with ethyl
acetate followed by distilled water, dried at 373 K in an oven for 6 h, and was then used in
the next cycle. The recovered catalyst exhibits remarkable consistent catalytic activity during
five cycles (Fig. S13, ESI). Catalytic activity of Cu(10%)/MesoꢀZSMꢀ5 in the synthesis of
1,4ꢀdisubstituted 1,2,3ꢀtriazoles using phenylacetylene is either similar or higher when
20
compared to reported literature.
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Ph
Same pathway
as shown for a
N3
Ph
HO
N
N
b path
N
a
N3
O
b
O
Ph
Ph
Minor (trace) Ph
Ph
HO
N
a path
N
N
Major
H
Ph
HO
N
Ph
N
N
N
Ph
N
Ph
Ph
(IV)
N3
Ph
O
N
OH
III)
(
I)
(
+
H
Ph
Ph
(
II)
-H
H
Scheme 6. Plausible mechanism involved during the multiꢀcomponent reaction of
phenylacetylene, sodium azide and aromatic/aliphatic epoxide over CuNPs decorated ZSMꢀ5
catalysts.
N
N
H
N
Cu (10%)/Nano-ZSM-5
N3
Cl
O
O
NaN3
O
(
I)
(
II)
NaN3
N
N
N
HO
N3
(
III)
H
N
N
N
N
OH
N
N
Scheme 7. Proposed reaction steps involved in the synthesis of bisꢀtriazole by the multiꢀ
component reaction of epichlorohydrin, phenylacetylene, and sodium azide.
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4
.
Conclusion
Cu nanoparticles decorated ZSMꢀ5 nanosheet was successfully prepared.
Physicochemical characterization reveals that highly dispersed 2ꢀ3 nm Cu nanoparticles were
supported on the large external surface of MesoꢀZSMꢀ5. Cu nanoparticles supported Mesoꢀ
ZSMꢀ5 nanosheet exhibited excellent activity in the synthesis of large organic molecules
when compared to Cu nanoparticles decorated microporous ZSMꢀ5 zeolite. Catalyst
exhibited excellent activity in the sustainable synthesis of indolizines, chalcones, and
substituted triazoles by oneꢀpot multiꢀcomponent synthesis. Medium of reaction plays crucial
role in the selective synthesis of indolizine/chalcone. When DCM was used as solvent,
indolizine was selectivity obtained whereas in neat condition chalcone was selectively
obtained. Cu nanoparticles decorated MesoꢀZSMꢀ5 exhibited better activity when compared
2
+
to Cu ionꢀexchanged MesoꢀZSMꢀ5. Catalyst can be easily recycled and exhibited no loss in
the activity even after five recycles. Regiospecific 1,4ꢀdisubstituted 1,2,3ꢀtriazole can be
prepared using Cu nanoparticles decorated MesoꢀZSMꢀ5. Regiospecificity was found to be
dependent on the nature of epoxide. Aromatic and aliphatic epoxides offered different
regioselectivity. Furthermore, epichlorohydrin afforded bisꢀtraizole as selective product under
the optimized reaction condition. High reactivity of Cu nanoparticles decorated MesoꢀZSMꢀ5
can be attributed to the optimum metalꢀsupport interaction, highly dispersed Cu nanoparticles
that could be easily accessible to large reactant molecules, and the facile diffusion of product
molecules from the active centres through inter/intraꢀcrystalline mesopores of MesoꢀZSMꢀ5.
Large external surface and mesopores enable the active sites to be available for further
reaction, which makes this catalyst recyclable and sustainable. Further work in this direction
is underway to develop economical metal nanoparticles based sustainable catalytic process
for selective synthesis of unique molecules of therapeutic importance.
Acknowledgements
Authors are thankful to CSIR, New Delhi for funding (01/(2802)/14/EMRꢀII). BS is
grateful to UGC, New Delhi for fellowship. Authors are thankful to Director, IIT Ropar for
constant encouragements.
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