ZnCl2 loaded TiO2 nanomaterial: An efficient green catalyst to one-pot solvent-free synthesis of propargylamines

Article abstract of DOI:10.1039/c9ra06693d

One-pot green synthesis of propargylamines using ZnCl₂ loaded TiO₂ nanomaterial under solvent-free conditions has been effectively accomplished. The aromatic aldehydes, amines, and phenylacetylene were reacted at 100 °C in the presen

Full text of DOI:10.1039/c9ra06693d

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ZnCl loaded TiO nanomaterial: an efficient green
2
2
catalyst to one-pot solvent-free synthesis of
Cite this: RSC Adv., 2019, 9, 32735
propargylamines†
ab
a
a
a
Digambar B. Bankar, Ranjit R. Hawaldar, Sudhir S. Arbuj,
Mansur H. Moulavi,
d
a
a
c
Santosh T. Shinde, Shrikant P. Takle, Manish D. Shinde, Dinesh P. Amalnerkar
ac
and Kaluram G. Kanade
*
One-pot green synthesis of propargylamines using ZnCl
2
loaded TiO
2
nanomaterial under solvent-free
conditions has been effectively accomplished. The aromatic aldehydes, amines, and phenylacetylene
ꢀ
were reacted at 100 C in the presence of the resultant catalyst to form propargylamines. The
nanocrystalline TiO
and further subjected to ZnCl
revealed the formation of crystalline anatase phase TiO
2
was initially synthesized by a sol–gel method from titanium(IV) isopropoxide (TTIP)
loading by a wet impregnation method. X-ray diffraction (XRD) patterns
. Field emission scanning electron microscopy
FESEM) showed the formation of agglomerated spheroid shaped particles having a size in the range of
2
2
(
2
5–45 nm. Transmission electron microscopy (TEM) validates cubical faceted and nanospheroid-like
sample. Surface loading of ZnCl on
loaded TiO sample. X-ray photoelectron
spectroscopy (XPS) confirmed the presence of Ti and Zn species in the ZnCl loaded TiO catalyst.
Energy-dispersive X-ray (EDS) spectroscopy also confirmed the existence of Ti, O, Zn and Cl elements in
the nanostructured catalyst. 15% ZnCl loaded TiO afforded the highest 97% yield for 3-(1-morpholino-
-phenylprop-2-ynyl)phenol, 2-(1-morpholino-3-phenylprop-2-ynyl)phenol and 4-(1,3-diphenylprop-2-
morphological features with clear faceted edges for the pure TiO
2
2
spheroid TiO nanoparticles is evident in the case of the ZnCl
2
2
2
4+
2+
2
2
2
2
Received 25th August 2019
Accepted 7th October 2019
3
ynyl)morpholine under solvent-free and aerobic conditions. The proposed nanostructure-based
heterogeneous catalytic reaction protocol is sustainable, environment-friendly and offers economic
viability in terms of recyclability of the catalyst.
DOI: 10.1039/c9ra06693d
rsc.li/rsc-advances
play a signicant role in synthetic organic chemistry by
acting as a versatile intermediate for the synthesis of many
1
. Introduction
Organic reactions involving three or more components are biologically active heterocyclic compounds such as 2-ami-
4
5
6
7
becoming increasingly important day by day since they allow noimidazoles, pyrrolidines, pyrroles, phenanthrolines,
8
9
many starting materials to be combined easily in one-pot oxazolidinones, aminoindolizines, etc. Moreover, they are
forming a single compound. One-pot, multi-component the important structural ingredients of therapeutic drug
carbon–carbon and carbon–nitrogen bond forming reac- molecules and also of natural products. For example, rasa-
1
1
0
tions were successfully accomplished for producing molec- giline containing Azilect as a moiety of propargylamine has
2
ular complexity. Three-component coupling of aldehyde, been used for the symptomatic therapy in early Parkinson's
1
1–13
amine, and alkyne is one of the most extensively used disease.
Also, propargylamines are essential intermedi-
methods for the preparation of several propargylamines ates for the preparation of PF960IN as a biologically active
3
14
through C–H bond activation. Propargylamine derivatives drug molecule.
The literature survey reveals that the synthesis of prop-
a
argylamines via classical routes involves the addition of alkynyl-
Centre for Materials for Electronics Technology (C-MET), Panchwati, Off Pashan
15,16
metal reagents to the imines
in the separate step. However,
Road, Pune-411008, India. E-mail: kgkanade@yahoo.co.in
b
P. G. Department of Chemistry, R. B. Narayanrao Borawake College, Shrirampur, the addition of moisture sensitive Grignard or Li acetylides
Ahmednagar-413709, India
reagent is tedious. Through direct amination of propargylic
electrophiles like propargyl halides, triates and phosphates,
the formation of propargylamine derivatives also has been
achieved.
c
17
P. G. and Research Centre, Yashavantrao Chavan Institute of Science, Satara-415001,
India
d
P. G. Department of Chemistry, Annasaheb Awate College, Manchar, Pune-410503,
India
In the last few decades, enormous progress has been made
for the C–H bond activation of terminal alkynes. A variety of
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c9ra06693d
1
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1
8
homogeneous transition metallic systems such as Cu(I) salts,
2
. Experimental
19
20
21
22
23
Ag(I) salts, ZnMe , Fe(III) salts, Cu(I) salts, Zr complex,
2
24
25
26
27
2.1. General
Ni(II) salts, InCl3, Ir complex and AgOAc were used for the
C–H bond activation of terminal alkynes. Researchers also used All the chemicals were AR grade and purchased from a local
2
8
29
30
Cu–Ru bimetallic system, InBr
3
,
Co(II) and Au(III) salen dealer. The chemicals were used as received without further
31
complexes as a homogeneous catalytic system for the activa- purication. The progress of reaction was monitored by thin
tion of C–H bond of the terminal alkynes. However, some of layer chromatography (TLC) technique. The crystal structure of
these catalytic systems have certain limitations due to the use of the synthesized catalyst was detected by XRD using a Rigaku
expensive and excess amount of catalyst, toxic solvents, and the MiniFlex-600 diffractometer (Cu Ka radiation, l ¼ 1.5406 A˚ ).
requirement of inert atmospheric conditions.
The surface morphology and particle size were examined using
Diverse heterogeneous catalysts such as layered double eld emission scanning electron microscope (Hitachi S-4800).
3
2
hydroxide-supported gold (LDH-AuCl ) catalyst, polystyrene- Elemental compositions were determined by energy-dispersive
4
3
3
34
supported NHC–Ag(I) catalyst, Fe
3 4
O nanoparticles, Ag X-ray spectroscopy (EDS) analysis. For ne-scale nano-
3
5
nanoparticles/Ni metal–organic framework,
copper(I) structural evaluation, eld emission transmission electron
@SBA-15 (ref. 37) etc. microscopic (FETEM) images of the typical sample were ob-
were used for the synthesis of propargylamines. Nanoscale tained using the JEOL JEM-2200 FS instrument. The same
3
6
3 4
anchored onto MCM-41 silica, Fe O
3
8
metal oxides such as ZnO and In
2 3
O (ref. 39) were also used instrument was also used for obtaining the Energy Dispersive
for the synthesis of propargylamines. Further, copper ferrite Spectroscopy (EDS) and allied elemental mapping information
4
0
41
42
nanoparticles, Ni–Y zeolites, nanosize Co O , Cu nano- using scanning transmission electron microscopy (STEM) in
3
4
4
3
particles on TiO , Au nanoparticles supported on ionic bright eld mode. For the conrmation of chemical composi-
liquid, Au nanoparticles supported on polyacrylamide,
2
4
4
45
tion, X-ray photoelectron spectroscopic (XPS) scans were
etc. were used as hetero- recorded using Al Ka (Monochromatic) anode with 6 mA beam
4
6
47
Zn(II)/HAP/Fe
3
O
4
,
2
Zn(L-proline) ,
geneous catalysts for the three component synthesis of prop- current and 12 kV X-ray source. The as-synthesized propargyl-
argylamines. Besides, a super paramagnetic MCM-41 with amine derivatives were spectrally characterized by Nuclear
4
8
dendrites copper complex was utilized as a recyclable catalyst Magnetic Resonance (NMR) spectroscopy and High-Resolution
1
13
for the propargylamine synthesis. Nonetheless, there are Mass Spectrometry (HRMS) techniques. H and C NMR
disadvantages to some of such catalytic systems viz. require- spectra were recorded using CDCl and DMSO-d solvents on
3
6
ment of high catalyst amount, use of inert gas, use of solvent, 500 and 125 MHz Bruker NMR spectrometer, respectively. The
longer reaction time, etc. Hence, there is a need to nd HRMS was taken in methanol solvent using high-resolution
a
simple and efficient method for the synthesis of mass spectrometer (Bruker Germany, Model: Impact HD, UHR
propargylamines.
Impact II ESI-Q-TOF). All yields refer to the isolated products
In the context of heterogeneous catalysis, nanocrystalline aer purication by column chromatography with ethyl acetate
materials can play signicant role due to their large surface to and n-hexane as an eluent.
volume ratio, easy separation and desirable physico-chemical
4
9
characteristics. Specically, nanocrystalline TiO2 is one of
the most important materials extending widespread applica-
tions in pigments, catalytic systems, inorganic membranes, gas
2.2.
Synthesis of TiO2 nanoparticles was performed by sol–gel
method using titanium(IV) isopropoxide as titanium
precursor. Initially, 80 ml of glacial acetic acid was added drop-
wise to a 40 ml of titanium(IV) isopropoxide (TTIP) in a 500 ml
round bottom ask immersed in an ice bath with constant
stirring. Later, 100 ml isopropyl alcohol was gradually added
with vigorous stirring for a period of 30 min at room tempera-
ture. In the resulting solution, 10 g polyvinyl alcohol (PVA) in
2
Synthesis of TiO nanoparticles
a
50
sensors, support ceramics, etc. Also, non-toxic nature and low
cost of TiO nano-particulate system made it a better catalyst.
The TiO supported with transition metals brings the advantage
of heterogeneous catalysis in the propargylamine synthesis. Fe
doped TiO nanoparticles are used as a heterogeneous catalyst
for the activation of C–H bond of terminal alkynes under
microwave irradiation. Recently, Ag/TiO and Pt/TiO nano-
catalyst have been applied as active recyclable catalysts for the
synthesis of propargylamines under microwave irradiation at
2
2
2
51
2
2
50 ml isopropyl alcohol was added, stirred for 10 min and
subsequently, 10 ml of 1 M HCl was added and the resultant
solution was subjected to constant stirring at room temperature
for 4 h. For natural gel formation, the obtained mixture was
transferred into 500 ml beaker and kept for 5 days. Finally, the
ꢀ
52
1
40 C.
53
Effective use of zinc salts for the propargylamine synthesis
motivated us to perform one-pot synthesis of propargylamine
using ZnCl loaded TiO nanomaterial.
In this report, we have demonstrated a synthetic strategy in
which inexpensive and commercially available ZnCl is loaded
ꢀ
gel was dried at 110 C for 12 h, transferred into a mortar,
2
2
powdered with pestle and further calcined in a muffle furnace at
ꢀ
4
00 C for 4 h.
2
by wet impregnation technique on sol–gel generated TiO
nanoparticles. The as-synthesized nanocrystalline ZnCl loaded
TiO has been used as an efficient and green heterogeneous
catalyst for the synthesis of propargylamines under solvent-free Loading of 15 wt% ZnCl
and aerobic conditions. formed by wet impregnation technique. A mixture of ZnCl
2
2
2 2 2
.3. Synthesis of ZnCl loaded TiO nanomaterial (ZnCl –
2
2
TiO catalyst)
2
2
over TiO
2
nanoparticles was per-
(0.3
2
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2
g) and TiO nanoparticles (2.0 g) was grinded for 10 min and
subsequently transferred into the 250 ml round bottom ask
containing 50 ml ethanol. Further, it was stirred at room
temperature for 1 h and then reuxed for 4 h with persistent
stirring. The excess ethanol was concentrated under vacuum
ꢀ
distillation and obtained solid material was dried at 200 C for
6
1
h. The same procedure was repeated for the loading of 5 wt%,
0 wt% and 20 wt% ZnCl over TiO nanoparticles.
2
2
2.4. General procedure for the synthesis of propargylamine
derivatives
In a typical procedure, aromatic aldehyde (1 mmol), secondary
amine (1.2 mmol) and phenylacetylene (1.5 mmol) were taken
in a clean and oven-dried round bottom ask. The nano-
crystalline ZnCl –TiO catalyst (0.2 mmol) was added into the
2
2
round bottom ask and the resulting reaction mixture was
ꢀ
placed in pre-heated oil bath at 100 C and stirred at the same
temperature for 6 h. Progress of the reaction was checked by
thin layer chromatography (TLC). Aer cooling, ethyl acetate
was added to the reaction mass and stirred at room temperature
for 15 min. Ethyl acetate layer was separated by ltration and
again remaining reaction residue was washed with ethyl
acetate. Combined ethyl acetate washings were mixed with
water. The organic layer was separated by extraction, dried over
Fig. 1 XRD patterns of (a) pure TiO
ZnCl –TiO , (d) 15% ZnCl –TiO and (e) 20% ZnCl
enlarged view of (101) plane.
2
, (b) 5% ZnCl
2
–TiO , (c) 10%
2
–TiO . Inset is
2
2
2
2
2
2
zinc species could not be observed, as expected, due to low
concentration and highly uniform dispersion of Zn species over
2 4
anhydrous Na SO and ltered. The organic solvent was
concentrated under reduced pressure and obtained crude
residue was puried by column chromatography to yield the
desired propargylamine derivative. The pure organic
54
the nanocrystalline TiO2.
The crystallite size of all as-synthesized samples was calcu-
lated by Scherrer's equation using broadening of (101) anatase
1
13
compounds were characterized by H NMR, C NMR, and
peak of TiO
2
. The crystallite size of unloaded TiO
2
is 16 nm,
showed
HRMS spectral techniques (please see ESI†).
whereas 5%, 10%, 15% and 20% ZnCl
2
loaded TiO
2
crystallite size values of 16, 15, 13 and 14 nm respectively. It may
be, however, noted that there is no prominent change in the
overall crystallite size of all the samples under investigation.
3
. Results and discussion
3.1. Catalyst characterization
The miniscule decrease in crystallite size of ZnCl –TiO2 in
2
3
.1.1. XRD analysis. Fig. 1 depicts the XRD patterns of pure comparison to pure TiO
2
as well as the marginal shi in 2q value
on
nanoparticles can be ascribed to the effective surface
8.76 , 70.26 and 75.06 can be indexed as (101), (004), (200), coating of ZnCl over TiO and/or might be due to partial
105), (211), (204), (116), (220) and (215) planes corresponding incorporation of Zn species in the crystalline lattice of TiO
to anatase polymorph of TiO (Fig. 1a). The angular positions
3.1.2. FESEM analysis. The surface morphology of the pure
and relative intensities of all the diffraction peaks are in close TiO and 15% ZnCl –TiO was investigated by FESEM analysis
concurrence with the JCPDS card no. 21-1272 for anatase phase and images are shown in the Fig. 2. Pure TiO sample displays
of TiO
. The XRD peak patterns corresponding to 5%, 10%, 15% the formation of homogeneously distributed spheroidal shaped
and 20% ZnCl loaded TiO samples also exhibit formation of particles having size in the range of 15–30 nm (Fig. 2a and b). In
TiO and ZnCl –TiO nanostructures. The characteristic peaks of (101) plane with increasing loading percentage of ZnCl
2
2
2
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
with 2q values at 25.26 , 37.70 , 48.07 , 53.86 , 55.00 , 62.68 , TiO
2
ꢀ
ꢀ
ꢀ
6
(
2
2
2
.
2
2
2
2
2
2
2
2
anatase phase of TiO (Fig. 1b–e). Furthermore, no additional case of 15% ZnCl
2
–TiO
peaks other than those matching with the crystalline anatase morphology is observed with increased particle size (25–45 nm)
TiO
were found in the XRD, indicating the phase purity of the as compared to pure TiO due to agglomeration of particles and
synthesized materials. It has been observed that 2q values are surface coating/loading of TiO nanoparticles by ZnCl
slightly shied to higher values for ZnCl loaded TiO samples
3.1.3. FETEM analysis. Fig. 3 displays FETEM images cor-
in comparison to pure TiO
implying successful loading of responding to pure TiO and 15% ZnCl loaded TiO nano-
ZnCl over TiO
surface as shown in inset of Fig. 1 for (101) materials. Low and intermediate magnication FETEM images
plane. The 2q values of (101) plane for pure TiO , 5% and 10% (Fig. 3a and b) show cubical faceted and spheroid-like
2
sample (Fig. 2c and d), almost same
2
2
2
2
2
.
2
2
2
2
2
2
2
2
2
ꢀ
ꢀ
ZnCl –TiO samples are almost the same (around 25.26 , 25.28
morphological features with clear faceted edges for pure TiO
and 25.26 , respectively). For higher loading i.e. 15% and 20% sample. Occasionally, nanorods like features are also seen.
ZnCl –TiO
, the 2q values of (101) plane are observed to be Surface coating/loading of spheroid particles of TiO by ZnCl is
sample when compared
2
2
2
ꢀ
2
2
2
2
ꢀ
ꢀ
2 2
around 25.42 and 25.44 , respectively. The XRD peaks due to visible in case of ZnCl loaded TiO
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2 2
Fig. 2 FESEM images of (a and b) Pure TiO and (c and d) 15% ZnCl –
TiO .
2
with pure TiO
2 2
sample. The average particle size for TiO and
ZnCl –TiO was found to be in the range of 15–35 nm. The
2
2
lattice fringes with inter-planar spacing (d) of 0.365 nm (Fig. 3c)
and 0.369 nm (Fig. 3g), respectively, for pure and ZnCl –TiO
2
2
samples corresponding to (101) plane of anatase TiO
2
are
55
seen. The lattice fringes corresponding to (105), (103), (004),
200), etc. planes are also noticed in case of both the samples
(
which is an indication of polycrystalline nature of the samples.
The selected area electron diffraction (SAED) patterns as shown
in Fig. 3(d and h) evince nanocrystalline behaviour of the
samples showing presence of ring like patterns, however, such
rings are not continuous but are dominated by the regular
56
2
bright spots. SAED patterns conrm anatase phase TiO in
case of both the samples when compared with XRD diffracto-
grams and allied database (Fig. 1). The overall FETEM study
revealed formation of nanoscale spheroids exhibiting nano-
crystalline nature.
2 2 2
Fig. 3 TEM images: (a and b) pure TiO , (e and f) 15% ZnCl –TiO .
3
.1.4. FETEM-STEM-EDS analysis of ZnCl –TiO2 catalyst. HRTEM images: (c) TiO , (g) 15% ZnCl –TiO . (d and h) are the cor-
2
2 2 2
The energy dispersive X-ray spectroscopy (EDS) analysis was responding SAED patterns of TiO
2 2 2
and 15% ZnCl –TiO .
used for the conrmation and quantication of presence of
elements in the given sample in STEM mode. In our investiga-
tion, weight by weight amount of ZnCl
the wet impregnation method. For TEM-EDS elemental and e, respectively) also overlap with the corresponding electron
composition analysis, we considered 15% ZnCl –TiO
sample image, however, the intensity of the colours ascribable to these
2 2
and TiO
were used in very high. The elemental mapping images for Zn and Cl (Fig. 5d
2
2
and the relevant spectrum with quantitative data (in tabular
form) is reproduced in Fig. 4.
The EDS spectrum discloses that the 15% ZnCl –TiO cata-
2
2
lyst contains 33.36 wt% of oxygen, 6.08 wt% of chlorine,
3.29 wt% of titanium and 17.27 wt% of Zn, even though the
4
XRD pattern did not show any Zn peaks. Thus, EDS spectrum
revealed that the sample is composed of Ti, O, Zn, and Cl which
is in good agreement with results obtained by XPS (Fig. 6).
The EDS-elemental mapping images of 15% ZnCl –TiO
2 2
sample in scanning transmission electron microscopy (STEM)
mode are shown in Fig. 5.
The elemental mapping images due to Ti and O (Fig. 5b and
c, respectively) overlap well with corresponding electron image
Fig. 4 TEM-EDS spectrum exhibiting quantitative elemental compo-
sition data for nanostructured 15% ZnCl –TiO
(Fig. 5a). The intensity of the colours assigned to Ti and O is also
2
2
.
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Fig. 6b) can be assigned to lattice oxygen (O–Ti) at 529.80 eV
and surface-bound hydroxyl group (Ti–OH) at 531.78 eV. The
binding energy peaks at 1022.20 eV and 1045.20 eV can be
associated with Zn 2p3/2 and Zn 2p1/2 energy states, respec-
tively (Fig. 6c). The peak at 1022.20 eV could be assigned to
2
+
58
59
Zn species in ZnO (1021.9 eV), ZnCl
2
(1022.5 eV) or
6
0
Zn(OH)
peak (Fig. 6d) to detect the presence of Zn species among the
ZnO, ZnCl or Zn(OH) . The binding energy at 199.59 eV
2
(1022.6). Subsequently, we studied the chloride XPS
2
+
2
2
signies the Cl 2p3/2 energy state of metal chloride. It appar-
ently conrms the presence of ZnCl2 (ref. 61) conned to
surface in the ZnCl –TiO catalyst (since XPS is surface
2
2
sensitive technique).
The overall appearance of peaks in XPS scans demonstrates
Fig. 5 FETEM-STEM-EDS elemental mapping images of 15% ZnCl
TiO catalyst: (a) Electron image (b) Ti, (c) O, (d) Zn and (e) Cl.
2
2
–
the successful loading of ZnCl
nanoparticles.
2 2
on the surface of TiO
2
two elements is low which is quite obvious as ZnCl is used only
for the purpose of surface loading. FETEM images correspond
well with FESEM images and endorse the surface loading of
3
.2. Catalytic activity of ZnCl loaded TiO nanomaterial
2 2
2 2
Aer successful synthesis and characterization of ZnCl –TiO
catalysts, it is employed as heterogeneous catalyst in multi-
component synthesis of propargylamine. To optimize the
reaction conditions, several parameters such as catalyst,
temperature and time were studied and corresponding results
are summarized in Table 1. To achieve sustainable green
chemistry approach, prior reaction was set under solvent-free
and aerobic conditions. In the initial optimizations, 3-hydrox-
ybenzaldehyde, morpholine, and phenylacetylene were used as
substrates for the model reaction (Scheme 1).
ZnCl
.1.5. XPS analysis. XPS scans of the nanostructured
ZnCl –TiO were acquired to determine the elemental compo-
sition and the electronic state of the Ti, O, Zn and Cl elements.
Fig. 6 shows the high resolution XPS scans corresponding to
2 2
over TiO nanoparticles.
3
2
2
2 2
15% ZnCl –TiO sample. The binding energies were corrected
with respect to the peak for adventitious C 1s at 284.8 eV. High
resolution scans of core level Ti 2p, Zn 2p and Cl 2p exhibit
symmetrical single peaks, while core level O 1s exhibits asym-
metry due to ensemble of two peaks as noticed in its deconvo-
lution spectrum.
ꢀ
When the reaction was performed at 30 C in the presence of
commercial ZnCl , no product formation was observed even aer
2
6
h (Table 1, entry 1). However, when the reaction was carried out
The Ti binding energy peaks observed at 458.72 and
ꢀ
at 70 C, low yield of product was obtained in 6 h (Table 1, entry
2). In addition, an increase in yield (76%) was observed as the
temperature was raised to 100 C (Table 1, entry 3). This indicates
4
64.35 eV are related to Ti 2p and Ti 2p1/2, in close resem-
3/2
4
+
57
2
blance with that of Ti in TiO . The two O 1s peaks (inset of
ꢀ
that the temperature plays an important role in this reaction. The
lower yield using commercial ZnCl can be due to its hygroscopic
2
Table 1 Optimization of catalyst, temperature and time for model
reaction between 3-hydroxybenzaldehyde, morpholine and
a
phenylacetylene
ꢀ
b
Entry
Catalyst
Temp. ( C)
Time (h)
Yield (%)
ꢀ
1
2
3
4
5
6
7
8
9
ZnCl
ZnCl
ZnCl
Pure TiO
5% ZnCl
10% ZnCl
10% ZnCl
15% ZnCl
15% ZnCl
15% ZnCl
15% ZnCl
20% ZnCl
2
2
2
30 C
6.0
6.0
6.0
6.0
6.0
3.0
6.0
6.0
6.0
3.0
6.0
6.0
—
53
76
38
76
57
86
—
74
68
97
97
ꢀ
70 C
ꢀ
ꢀ
ꢀ
ꢀ
100 C
100 C
100 C
100 C
2
2
–TiO
2
2
2
2
2
2
2
2
–TiO
2
2
2
2
2
2
2
ꢀ
–TiO
–TiO
–TiO
–TiO
–TiO
–TiO
100 C
ꢀ
30 C
ꢀ
70 C
ꢀ
ꢀ
ꢀ
10
11
12
100 C
100 C
100 C
a
Fig. 6 High resolution XPS scans of the respective elements corre-
Reaction conditions: 3-hydroxybenzaldehyde (1 mmol), morpholine
sponding to 15% ZnCl
deconvoluted spectrum.
2
–TiO
2
sample. The inset of (b) displays (1.2 mmol), phenylacetylene (1.5 mmol), catalyst (0.2 mmol), temp.,
b
time, solvent-free condition. Isolated yield.
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catalyst. In solvent, reactants are dispersed so it delays the
contact with the catalyst. Under solvent-free conditions, reac-
tants and catalyst can quickly come in intimate contact with
each other which, in turn, can accelerate the reaction on the
surface of the catalyst. Thus, best results could be observed for
2 2
15% ZnCl loaded TiO nanoparticle catalyst with 97% yield
Scheme 1 Model reaction.
under solvent-free conditions.
Aer optimization of the reaction parameters, we have
extended the scope of reaction to the different aromatic alde-
, but hydes, cyclic secondary amines (pyrrolidine and morpholine),
2
nature. We also performed the reaction using pure TiO
observed only 38% yield (Table 1, entry 4).
and phenylacetylene using 15% ZnCl –TiO (Scheme 2 and
2 2
Subsequently, the catalytic activities of 5%, 10%, 15% and Table 3, entries 1–12).
0% ZnCl loaded TiO nanoparticles (nanocrystalline ZnCl
The results in Table 3 indicate that the aromatic aldehydes
TiO catalysts) were examined. For 5% and 10% ZnCl
sup- with electron-donating and halogen substituents showed
2
2
2
2
–
2
2
ported TiO catalysts, yields of 76% and 86%, respectively, were high reactivity and produced the corresponding propargyl-
2
obtained (Table 1, entries 5 and 7). This means that as the amines in better yields. Aryl aldehydes with a hydroxyl group
loading of ZnCl on TiO increases, an increase in product yield at meta or ortho positions (Table 3, entries 1, 4 and 5), afforded
2
2
is observed under a constant mmol of catalyst (0.2 mmol). excellent yields as compared to aldehydes having methoxy
Therefore, we studied the reaction with 15% ZnCl
No product formation was detected at 30 C using 15% ZnCl
2
loaded TiO
2
.
group at para position (Table 3, entries 2 and 3). The reactivity
of 2-chloro and 4-chlorobenzaldehydes (Table 3, entries 9–11)
ꢀ
2
loaded TiO
7
3
2
catalyst (Table 1, entry 8), 74% yield obtained at was found to be slightly more than 4-ourobenzaldehyde
0 C in 6 h (Table 1, entry 9), 68% yield obtained at 100 C in (Table 3, entry 8) and 4-bromobenzaldehyde (Table 3, entry
ꢀ
ꢀ
h (Table 1, entry 10). However, when the reaction was per- 12). Benzaldehyde afforded the coupling products in excellent
ꢀ
formed at 100 C for 6 h, 97% yield of the desired product was yield (Table 3, entries 6 and 7). We observed slightly higher
observed (Table 1, entry 11). 15% ZnCl loading over TiO was yields of the products using morpholine as compared to pyr-
2
2
necessary as the optimum amount of ZnCl
2
loading and rolidine. Thus, 15% ZnCl –TiO catalyst worked as an efficient
2 2
increasing the loading of ZnCl
Table 1, entry 12).
2
(20%) did not improve the yield catalyst for the synthesis of propargylamines under solvent-
free conditions. It may be noted that, the as-synthesized
(
It is clearly noticed that the nanostructured ZnCl
2
–TiO
2
propargylamines are all chiral compounds and spectral data
system exhibits excellent catalytic activity aer loading ZnCl
2
of all products conrmed that they are single compounds and
over TiO nanoparticles. The excellent catalytic activity of the not mixture of isomers.
2
nanostructured ZnCl –TiO can be attributed to the greater
The reusability of the ZnCl
ined for the reaction between 3-hydroxybenzaldehyde, mor-
It is very interesting to see the effect of solvents on catalytic pholine, and phenylacetylene under the optimized reaction
activity of 15% ZnCl –TiO
catalyst. Therefore, we investigated conditions (Table 3, entry 1). Aer the rst run, the solid
the role of various solvents such as toluene, CH
and H O.
2 2
–TiO catalyst was also exam-
2
2
degree of crystallinity at nanoscale.
2
2
CN, DMF, THF, catalyst was separated from the reaction mixture by ltration,
3
ꢀ
washed with the ethyl acetate and dried at 120 C for 4 h. The
2
From Table 2, is very clear that catalyst exhibits poor activity dried catalyst was then subjected to the next run of the reac-
in polar solvents (CH CN, DMF, H O and THF). Non-polar tion under optimized conditions and afforded 94% yield.
3
2
solvent (toluene) shows good catalytic activity, while under Similar procedure was followed for next runs. The relevant
solvent-free conditions, catalyst shows excellent catalytic results are presented in Table 4. As can be seen, the catalyst
activity. The reason for lowering the yield of propargylamine could be recycled for ve successive runs without signicant
and hence catalytic activity lies in solvation of reactants and loss of catalytic activity.
The recycled ZnCl –TiO catalyst was further characterized
2
2
by XRD which conrms retention of anatase phase of TiO
(Fig. S1, ESI†). It is, thus, noted that the crystal structure of the
2
a
Table 2 Solvent study for model reaction
b
Entry
Solvent
Yield (%)
1
2
3
4
5
6
Toluene
76
68
40
36
97
44
CH
3
CN
DMF
H O
2
Solvent-free
THF
a
Reaction conditions: 3-hydroxybenzaldehyde (1 mmol), morpholine
–TiO (0.2 mmol),
00 C, 6.0 h, solvent. Isolated yield.
(1.2 mmol), phenylacetylene (1.5 mmol), ZnCl
2
2
ꢀ
b
1
Scheme 2 General reaction for propargylamine synthesis.
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Table 3 Propargylamine synthesis by three-component coupling of Table 3 (Contd.)
aromatic aldehyde, secondary amines and phenylacetylene catalysed
a
b
by ZnCl
2
–TiO
2
NPs
Entry
Aldehyde
Product
Yield (%)
b
Entry
Aldehyde
Product
Yield (%)
1
1
95
91
1
2
97
88
12
a
Reaction conditions: aromatic aldehyde (1 mmol), amine (1.2 mmol),
ꢀ
3
4
90
94
phenylacetylene (1.5 mmol), ZnCl –TiO (0.2 mmol), 100 C, 6.0 h,
solvent-free condition. Isolated yield.
2
2
b
recovered catalyst has not changed. The sharp peaks signify that
the nature of recovered catalyst is crystalline.
The transition metals are generally used to activate the
C–H bond of terminal alkynes in carbon–carbon coupling
reactions. The transition metal supported on solid support
5
6
97
97
such as TiO
heterogeneous catalysis in C–C bond forming reactions. In
the proposed reaction, the ZnCl loaded TiO nanomaterial
ZnCl –TiO ) can also lead to activation of C–H bond of
2
nanoparticles might bring the advantage of
5
1
2
2
(
2
2
terminal alkyne. A tentative mechanism for the one-pot
propargylamine synthesis under solvent-free condition
using ZnCl loaded TiO nanomaterial is pictorially demon-
2
2
strated in Scheme 3. The reaction initially involves activation
of C–H bond of phenylacetylene (3) by ZnCl from the nano-
structured ZnCl –TiO catalyst. The generated phenyl-
acetylide–ZnCl –TiO intermediate (a) is then reacted with
2
2
2
2
2
7
95
iminium ion (b) (which is produced in situ from aldehyde 1
and amine 2) in order to offer the desired product of prop-
argylamine 4. Here, ZnCl in the ZnCl –TiO nanomaterial is
2
2
2
the main species to activate the C–H bond of terminal alkynes
and anatase TiO2 provides solid support for ZnCl . Due to
2
2 2
their synergistic effect, nanostructured ZnCl –TiO can work
as an efficient heterogeneous catalyst for the synthesis of
propargylamines.
8
9
90
92
94
a
2 2
Table 4 Recyclability study of ZnCl –TiO
b
Run
Yield (%)
1
2
3
4
5
97
94
90
88
84
1
0
a
Reaction conditions: 3-hydroxybenzaldehyde (1 mmol), morpholine
–TiO catalyst,
00 C, 6.0 h, solvent-free. Isolated yield.
(1.2 mmol), phenylacetylene (1.5 mmol), recycled ZnCl
2
2
ꢀ
b
1
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We have reported a simple and efficient route for the synthesis
of propargylamine using small amount of nanocrystalline
ZnCl –TiO as a heterogeneous catalyst. The role of catalyst is
2 2
signicantly observed for alkyne C–H bond activation. The re-
ported technique is solvent-free and required mild reaction
2 2
conditions. The synthesized 15% ZnCl loaded TiO nano-
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of diverse propargylamine derivatives. We believe that our
present protocol is eco-friendly for green approach and can
become an alternative to conventional homogeneous catalysis
for the synthesis of propargylamine.
1
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2
2
Conflicts of interest
2
There are no conicts to declare.
2
Acknowledgements
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Chem., Int. Ed., 2003, 42, 4244–4247.
D. B. Bankar is obliged to Dr B. B. Kale, Director of C-MET,
Pune, India for providing research facility. He is also indebted
to the Principal and Head of the Chemistry Department of R. B.
Narayanrao Borawake College, Shrirampur, India for support to
this research work. Dr K. G. Kanade is thankful to RUSA, India
for nancial and the Rayat Shikshan Sanstha, Satara, for
administrative support. Authors are grateful to the Director of
NCL, Pune for providing XPS facility.
2
2
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