Recyclable ZnO nano particles: Economical and green catalyst for the synthesis of A3 coupling of propargylamines under solvent free conditions

Article abstract of DOI:10.1016/j.catcom.2012.03.031

Recyclable ZnO nano particles (ZnO NPs) provide an efficient and economic route for synthesis of multi component A³ coupling reaction of propargylamines under solvent free condition. It was found that without using co-catalyst and any other act

Full text of DOI:10.1016/j.catcom.2012.03.031

Catalysis Communications 25 (2012) 50–53
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Recyclable ZnO nano particles: Economical and green catalyst for the synthesis of A3
coupling of propargylamines under solvent free conditions
a
a
a
b
b,
K.V.V. Satyanarayana , P. Atchuta Ramaiah , Y.L.N. Murty , M. Ravi Chandra , S.V.N. Pammi ⁎
a
Department of Organic Chemistry and Food, Drugs & Water, College of Science and Technology, Andhra University, Visakhapatnam, 530003, India
Advanced Analytical Laboratory, DST-PURSE Programme, Andhra University, Visakhapatnam, 530003, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Recyclable ZnO nano particles (ZnO NPs) provide an efficient and economic route for synthesis of multi com-
ponent A coupling reaction of propargylamines under solvent free condition. It was found that without using
co-catalyst and any other activator, high catalytic activity was obtained in short reaction time in case of zinc
oxide when compared to other metal ion and/or metal oxide catalysts. In addition, ZnO NPs catalyst can be
easily recycled and reused for ten times without any significant loss of catalytic activity with excellent yields.
ZnO NPs were synthesized by a simple, economic and elegant chemical bath deposition technique.
Received 7 December 2011
Received in revised form 16 March 2012
Accepted 23 March 2012
Available online 1 April 2012
3
Keywords:
Zinc oxide
©
2012 Elsevier B.V. All rights reserved.
Catalyst
Excellent yields
Propargylamines
1
. Introduction
Especially zinc oxide nano particles (ZnO NPs) have great potential as a
catalyst for variety of organic and inorganic reactions due to their high
surface-to-volume ratio, whereas nano-sized particles increase the ex-
posed surface area of the active component of the catalyst, thereby
enhancing the contact between reactants and catalyst. Zinc oxide has
been widely used as a catalyst in the photo degradation of many organic
pollutants and also proved as inexpensive and highly efficient catalyst
in various organic reactions [18–20]. In addition, nano-ZnO is an eco-
friendly material which is non-toxic for human bodies and widely
used as antibacterial agent in biomedical applications [21,22]. However
there was no report on the usage of ZnO nano particles as a catalyst for
the synthesis of propargylamines by three component one-pot coupling
Propargylamines and its derivatives have received much attention
because of providing access to versatile synthetic intermediates for
biologically active compounds and natural products i.e., β-lactams,
herbicides, fungicides and isosteres [1–3]. Some of them have been
tested and approved as drug molecules in the treatment of Parkinson's
and Alzheimer's diseases [4,5]. Propargylamines were synthesized by
3
three component one-pot coupling reaction (A coupling) of aldehydes,
alkynes and amines. One-pot multi component coupling reactions
(
MCRs) seem to be attractive strategy in organic synthesis and are high-
ly valued among synthesis methodologies, as several elements of diver-
sity can be introduced into a molecule in a single synthetic step [6,7].
In the past few years, various transition metal complexes and com-
pounds of Ag, Au, Cu, Fe and Zn etc., have been investigated as homoge-
3
reaction (A coupling) of aldehydes, alkynes and amines under solvent
free conditions.
In this study, we are reporting the synthesis and characterization
of zinc oxide nano particles along with catalytic activity of nano ZnO
for the A3 coupling reaction to generate propargylamines under sol-
vent free condition in the absence of any co-catalyst and/or additive
(Scheme 1).
3
neous and/heterogeneous catalysts for the A -reaction [8–10]. From the
literature, the use of these catalysts have some disadvantages such as
expensiveness, non-recyclability, prolonged reaction time to get higher
conversion yield and their separation from the reaction mixture is
tedious [10–16].
Metal oxide and/or metal composites are gaining tremendous im-
portance due to their distinct catalytic activities for various organic
transformations because they were active adsorbents for gases and
destruction of hazardous chemicals [17]. Among various metal oxides,
ZnO is a magic material because of its wide area of applications and flex-
ibility of preparation in different morphologies with different properties.
1.1. Preparation and characterization of zinc oxide nano particles
ZnO nano particles were synthesized by a simple chemical bath de-
position method. ZnO nano particles were prepared from a solution
of analytical grade with high purity of Zn (CH
Zn
3
COO)
ion source and urea in an alkaline solution of ammonia. Aqueous
solutions of analytical grade with high purity 0.2 M zinc acetate
Zn(CH COO) ), 0.6 M urea (CO(NH ), and complexing agent (ammo-
2
(zinc acetate), a
++
(
3
2
2 2
)
nia, 70% hydrazine hydrate or 0.2×2/3 M tri-sodium citrate) were used
to prepare ZnO nano particles. Initially, 10 ml of zinc acetate solution
⁎
Corresponding author. Tel.: +91 8985211665; fax: +91 891 2734821.
E-mail address: sreepammi@gmail.com (S.V.N. Pammi).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.03.031

K.V.V. Satyanarayana et al. / Catalysis Communications 25 (2012) 50–53
51
Scheme 1. ZnO nano particles catalyzed three component reactions of aldehyde, amine and alkyne.
and 10 ml of tri-sodium citrate were taken in a 100 ml beaker, and stir-
red for several minutes till the solution becomes clear and homogenous.
Under continuous stirring, 10 ml urea solution was added to the above
solution and finally made up to volume (50 ml) with deionized water.
The pH of the resultant solution was adjusted to about 10 by adding am-
monia solution. The solution was continuously stirred at a constant
speed during the growth time with the help of AC motor. The bath tem-
perature was raised to a maximum of 80± 5 °C from room temperature
using temperature controller. The ZnO nano particles were washed with
DI water and methanol ultrasonically to remove the loosely adhered
ZnO particles and finally dried in vacuum at 200 °C to evaporate solvents
in ZnO nano powder. The morphological, structural and chemical com-
position of ZnO nano particles were analyzed with SEM-EDAX (JEOL-
JSM-6610), XRD (PANalytical), and, TEM (JEOL-2100 F, HR) techniques.
nano particles. The presence of some larger particles attributes aggre-
gating or overlapping of smaller particles. For further investigation
on size of ZnO nano particles were analyzed by TEM and it shows
(Fig. 1(c)) the nano particles were composed with 10–20 nm diameter.
The interplanar spacing is about 0.32 nm from HRTEM image (Fig. 1(d))
represents (101) lattice plane of hexagonal wurtzite structure of ZnO
nano particles.
The chemical composition of ZnO nano particles was analyzed by
EDX, from this the atomic percentage of Zn and oxygen were found
to be 52.69% and 47.31% respectively Fig. 2(a). This result indicates
that product was pure ZnO and rich in Zn content compared to oxy-
gen content.
Fig. 2(b) shows XRD pattern of zinc oxide nano particles exhibiting
hexagonal wurtzite structure (JCPDS CARD NO 36-1451) with preferred
orientation (101) along with (100), (002), (102), (110) and (103) di-
rections. No other metal phases were detected by XRD. The average
grain size of zinc oxide nano particles was calculated using the Scherrer
formula and was found to be about 15 nm indicating nano crystalline
nature.
2
. Results and discussions
The morphology and size of the ZnO nano particles were analyzed
by scanning electron microscopy (SEM) and transmission electron
microscopy (TEM) as shown in Fig. 1. The low magnification SEM im-
ages (Fig. 1(a) and (b)) shows small nano sized grains having spherical
and hexagonal morphology with a narrow size distribution ranging
from 90 to 120 nm, which indicates the nano crystalline nature of ZnO
After characterization of ZnO NPs catalyst, we have examined its
catalytic activity along with various zinc based catalysts (10 mol% as stan-
3
dard) in acetonitrile for the effective A coupling between benzaldehyde,
piperidene, and phenyl acetylene for the synthesis of propargylamines
Fig. 1. (a) & (b) SEM, (c) TEM and (d) HRTEM images of zinc oxide nano particles.

52
K.V.V. Satyanarayana et al. / Catalysis Communications 25 (2012) 50–53
Fig. 2. (a) & (b) EDX and XRD analysis of zinc oxide nano particles respectively.
by heating at a temperature of 90± 5 °C under continuous stirring.
Detailed results have been presented in Table 1. Zn based salts i.e.,
(Table 3). From these studies, ZnO NPs have been proved as efficient
reusable catalyst without any compensation in yield in the synthesis
of propargylamines.
Zn dust, Zn acetate, ZnBr
2 2
, ZnCl , and ZnO (commercial) (entries
1
–5) catalysts show moderate yields, where as bulk ZnO (commer-
After optimization of reaction condition, to expand the scope and
understand the generality of the ZnO NPs promoted A coupling reac-
3
cially) shows poor yield (entry 6). The increased catalytic activity
of commercial nano-ZnO over bulk ZnO may be attributed to the
higher surface area, thus resulting in higher surface concentration
of the reactive sites. In addition, their enhanced reactivity was attrib-
uted to the morphological differences. It can be noted that ZnSO
Zn metal and Zn (NO (entries 7–9) did not promote the reaction
tion, we have chosen different aldehydes and amines possessing wide
range of functional groups. We opted a variety of aromatic aldehydes
for coupling with piperidine/or morpholine and phenylacetylene and
total of 10 compounds were prepared (Table 4). It was found that
para based aryl aldehydes (Table 4, entries 3,4,5,6,7,9 and 10) afforded
excellent yields with short reaction time than that of ortho based aryl
aldehydes (Table 4, entries 2). The order of reactivity of aldehydes
in terms of yields and reaction time was bromo > chloro > methoxy >
nitro benzaldehydes. It was found that piperidine gives better results
in term of yield and reaction time than morpholine. The formation of
zinc-acetylide intermediate I, and its further reaction with iminium
ion, which was generated from the aldehyde and the amine II, was
noticed during the formation of propargylamine III (Fig. 3).
In conclusion, ZnO nano particles provide an efficient, economic and
greener method for synthesis of single pot multi component A3 cou-
pling reaction of aldehydes, alkynes and amines under solvent free con-
ditions. Easy recoverability and reusability of the catalyst without any
further purification and without any significant loss of ZnO NPs proved
as a potential candidate for organic reactions. The catalyst was efficient
for the proposed reaction without need of additive or co-catalyst and
gives high yield in short reaction time.
4
,
3 2
)
instead giving unidentified products. ZnO nano particles prepared
by chemical bath deposition show superior catalytic properties in
terms of reaction time (4 hr.) and yields (91%) when compared to
other Zn based catalysts. Moreover the synthesis of ZnO-nano parti-
cles is simple and follows an economical method. In order to investi-
gate the effect of solvent and catalyst concentrations, we carried out
the condensation of benzaldehyde, piperidene, and phenyl acetylene in
various organic solvents and without solvent at 90± 5 °C using 10 mol%
ZnO NPs as catalyst. Ultimately, solvent free conditions were preferable
in terms of high yield and short reaction times (entry 1–7, Table 2).
While evaluating catalyst concentration, the best yield was observed
in the presence of 20 mol% ZnO NPs (entry 9) with short reaction
time. It was noted that there is no improvement in terms of yield and
reaction time when the amount of catalyst is greater than 20 mol%.
The catalyst was recycled and reused with only slight drop in activity
till ten runs. The catalyst was separated by centrifugation and was
reused without any significant loss of catalyst activity up to ten runs
Table 2
Table 1
Optimization of the ZnO-NP-catalyzed model reaction for synthesis of propargylamines
with various solvents and catalyst loading.
Comparison of catalytic activity of ZnO-NPs with Zn-based commercial catalysts.
Entry
Catalysta
Time (h)
Yield (%)b
Entry
Solventa
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
Zn dust
Zn acetate
10
10
10
10
10
10
12
12
12
4
85
86
80
77
25
74
0-Traces
0-Traces
0-Traces
91
1
2
3
4
5
6
6
7
8
9
Acetonitrile
Methanol
THF
Acetone
Toluene
Dioxane
240
240
240
240
240
240
240
180
120
90
91
71
52
67
59
38
26
89
95
97
98
Zn Br
ZnCl
2
2
Bulk ZnO (commercial)
ZnO nano (commercial)
ZnSO
Zn metal
Zn(NO
ZnO NP
4
2
H O
Neat-ZnO NPs (10%)
Neat-ZnO NPs (15%)
Neat-ZnO NPs (20%)
Neat-ZnO NPs (30%)
3
)
2
10
10
90
a
Reaction conditions: benzaldehyde (1 mmol), piperidene (1.1 m mol), and phenyl
a
acetylene (1.2 mmol), catalyst (10 mol%), solvent acetonitrile, temperature 90 °C,
Reaction conditions: benzaldehyde (1 m mol), piperidene (1.1 m mol), and phenyl
reflux.
acetylene (1.2 m mol), catalyst (10 mol%), solvent 3 mL, temperature 90 °C, reflux.
b
b
Based on isolated yields.
Based on isolated yields.

K.V.V. Satyanarayana et al. / Catalysis Communications 25 (2012) 50–53
53
Table 3
Recycling of ZnO-nano particles.
No. of cycles a
Fresh
Run 3
Run 5
Run 7
Run 10
Yield (%)b
Time (min)
98
90
96
100
94
120
91
150
88
180
a
Reaction conditions: benzaldehyde (1 mmol), piperidene (1.1 mmol), and phenyl
acetylene (1.2 mmol), catalyst (10 mol%), temperature 90 °C.
b
Based on isolated yields.
Table 4
Three component coupling of aldehydes, amines and alkynes by ZnO nano particles
as catalyst.
a
Yield (%)b
Entry
R
1
Amine(R
2
,R
3
)
R
4
Time(min)
1
2
3
4
5
6
7
8
9
C
6
H
5
Piperidene
Piperidene
Piperidene
Piperidene
Piperidene
Piperidene
Piperidene
Morpholine
Morpholine
Morpholine
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
90
150
90
110
90
120
90
120
150
240
94
84
97
88
96
89
98
89
82
45
2-ClC
4-ClC
4-NO
4-CH
4-CH
4-BrC
6
H
H
4
6
4
2
C
6
H
4
3
O-C
6
H
4
3
C
6
H
4
6
H
4
C
6
H
5
4-ClC
4-CH O-C H
6 4
H
Fig. 3. Mechanism of ZnO nano particles catalyzed three component coupling.
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a
Reaction conditions: benzaldehyde (1 mmol), piperidene (1.1 mmol), and phenyl
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b
Based on isolated yields.
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1
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1
Supplementary data related to this article can be found online at
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Acknowledgements
[
4
We are thankful to the DST-PURSE Programme, Analytical Labora-
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for ZnO NPs catalyst in this research work.
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