Zn-VCO3 hydrotalcite: A highly efficient and reusable heterogeneous catalyst for the Hantzsch dihydropyridine reaction

Article abstract of DOI:10.1016/j.catcom.2013.11.012

The Zn-VCO₃ hydrotalcite was found to be an excellent solid catalyst for the one-pot synthesis of triphenylpyridine-3,5-dicarboxamide via Hantzsch reaction of acetoacetanilide, ammonium hydroxide and various aromatic aldehydes. The combinatoria

Full text of DOI:10.1016/j.catcom.2013.11.012

Catalysis Communications 45 (2014) 148–152
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Zn-VCO hydrotalcite: A highly efficient and reusable heterogeneous
3
catalyst for the Hantzsch dihydropyridine reaction
Ramakanth Pagadala, Suresh Maddila, Venkata D.B.C. Dasireddy, Sreekantha B. Jonnalagadda ⁎
School of Chemistry & Physics, University of KwaZulu-Natal, Westville Campus, Chiltern Hills, Durban 4000, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
The Zn-VCO hydrotalcite was found to be an excellent solid catalyst for the one-pot synthesis of triphenylpyridine-
3
Received 27 September 2013
Received in revised form 11 November 2013
Accepted 12 November 2013
Available online 21 November 2013
3
,5-dicarboxamide via Hantzsch reaction of acetoacetanilide, ammonium hydroxide and various aromatic
aldehydes. The combinatorial syntheses were achieved for the first time using hydrotalcite as a heterogeneous
catalyst. The catalyst was active for the Hantzsch reaction in water at 60 °C. The products were isolated in
good yields (85–93%) with short reaction times (2–3 h). The resulting substituted dihydropyridines were
characterized and confirmed by 1H and C NMR, FTIR, and HRMS spectral data and the solid catalyst was
characterized by XRD, BET, SEM and TEM. The newly synthesized heterogeneous solid catalyst offers simple
means for recovery and the isolated catalyst was reused for five rounds for the synthesis of compound 4a,
without significant loss of catalytic activity. For all the other reactions carried out with the recycled catalyst,
results were similar to with the fresh catalyst.
13
Keywords:
Multicomponent reaction
One-pot synthesis
Heterogeneous catalysis
Hantzsch reaction
Hydrotalcite
©
2013 Elsevier B.V. All rights reserved.
Green chemistry
1
. Introduction
Few reports are available for the synthesis of dihydropyridines
[29–31] and modifications for Hantzsch reaction were reported, where
Hydrotalcites are layered with anionic species such as hydroxide and
β-diketones, malonic acid esters, β-keto acids, β-aminocrotononitrile,
ethyl cyanoacetate, and cyanoacetamide were used instead of β-keto
esters [32]. Most of the researchers have focused on the modification
and optimization of the Hantzsch reaction to maximize reaction conver-
sion, minimize reaction time and offer high yield of dihydropyridines.
Many of these methods give unsatisfactory yields with other side prod-
ucts and use of large quantity of volatile organic solvents. Thus, the
developments in this area require diversity in selected building blocks
to generate the target libraries possessing different heterocyclic scaf-
folds. Hydrotalcites have received attention as heterogeneous catalysts,
due to the stability and the scope for modification of their surface prop-
erties by incorporating various metal ions in its structure. Sahu et al.
[33,34] investigated one-pot multi-component synthesis in the pres-
ence of hydrotalcite as the catalyst and achieved as an ecologically and
economically good results. Ebitani et al. [35] reported that hydrotalcites
provide a unique acid–base bi-functional surface capable of promoting
the Knoevenagel and Michael reactions of nitriles with carbonyl com-
pounds. In view of the advantages associated with the use of hydrotalcites
in performing synthetic organic chemistry, we report a multicomponent
and simple approach using hydrotalcite as a catalyst. The Hantzsch reac-
tion towards the synthesis of target molecules with good yields in less
reaction time with no need for solvents is described. This is the first report
carbonate located in the interlayer, which have been reported to be
used as ion exchangers [1], drug delivery agents [2] and catalysts or
catalyst supports [3–7]. These materials have been developed and
applied as a heterogeneous catalysts and metal support for highly
efficient liquid-phase organic transformations including alkylations,
isomerizations, condensations, and cycloadditions [8–16]. Their
reactivity under relatively benign conditions is an incentive for the
synthesis of fine chemicals, when ecofriendly and green chemistry
is in great demand.
Dihydropyridines represent important and extensively studied
compounds belonging to the class of calcium channel blockers [17–21].
The known Hantzsch reaction that was first reported it in 1882 [22], is
the convenient way to prepare the 1,4-dihydropyridines which exhibit
significant biological activities in the treatment of congestive heart failure
and angina pectoris [23]. 4H-pyran is an isoelectronic displacement ana-
log of dihydropyridines and this pharmacophore is a main structural
motif present in several natural products [24] and several bio-active
compounds [25,26]. More importantly, 4H-pyrans can be transformed
into corresponding pyridines resulting in valuable dihydropyridine type
calcium channel modulators [27,28].
3
of a heterogeneous solid catalyst, Zn-VCO hydrotalcite capable of
catalyzing the reaction to produce dihydropyridines in the presence of
water. Furthermore, this hydrotalcite catalyst system can promote the
aqueous Hantzsch reaction using carbonyl compounds.
⁎
Corresponding author. Tel.: +27 31 260 7325/3090; fax: +27 31 260 3091.
E-mail address: jonnalagaddas@ukzn.ac.za (S.B. Jonnalagadda).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.11.012

R. Pagadala et al. / Catalysis Communications 45 (2014) 148–152
149
2
. Experimental
the reaction, the mixture was cooled to room temperature and extract-
ed with ethyl acetate. After filtering the hydrotalcite solid, the solvent
layer was washed with water, dried with anhydrous sodium sulfate
and the solvent removed to obtain essentially pure product 4a (93%
yield) as off-white solid. The recovered hydrotalcite solid was washed
with dichloromethane and evaporated solvent under reduced pressure
and reused in the next cycles.
2
.1. Preparation of hydrotalcite
The layered double hydroxide Zn-VCO
3
hydrotalcite with a molar
ratio of 1:0.8:0.2 was prepared by co-precipitation method at constant
pH. The mixed solution containing metal nitrates of Zn (Zinc nitrate)
and V (ammonium metavanadate) with desired concentrations,
0
2 3
.35 mol NaOH and 0.15 mol Na CO were added simultaneously
3. Results and discussion
to a 1 L beaker at the rate of 50 mL/h under constant stirring condition
at 35 °C at the pH of 10–11.5. The precipitate formed was aged for 18 h
at 65 °C at the end of which the precipitate was separated by filter. This
A systematic study was carried out to optimize the reaction condi-
tions, including the quantity of catalyst, reaction medium and nature
of catalyst. For the Hantzsch reaction of acetoacetanilide 1, with aromat-
ic aldehydes, 2a–f and ammonium hydroxide, 3 (molar ratio: 2:1:1.5)
was followed by repeated washing with deionized water until free of
−
NO
3
. The wet solid was slowly dried at 80 °C for 12 h to obtain the
hydrotalcite. The catalyst was calcined at 500 °C for 4 h.
(
Scheme 1), the results reflecting the impact of various parameters are
summarized in Table 1. Using NaOH and [Bmim]BF and ethanol,
4
2
.1.1. Characterization of catalyst and textural properties
The Brunauer–Emmett–Teller (BET) surface area, total pore volume
water or dichloromethane (DCM) as solvents, the reactions gave no
expected products (Table 1, entries 3–5). In solvent-free conditions
the desired product 4a was obtained with 55% yield (Table 1, entry 8)
under 60 °C using ionic liquid, [Bmim]BF4 as a catalyst. Best yields
were obtained using hydrotalcite as a heterogeneous catalyst in aqueous
media at 60 °C (Table 1, entry 11).
and average pore size were measured using a Micrometrics TriStar II
surface area and porosity analyzer. Prior to the analysis, the powdered
samples (∼0.180 g) were degassed under N
a Micromeritics FlowPrep 060 instrument. Textural properties of
2
for 12 h at 200 °C using
catalyst sample were measured by N
obtained at −196 °C.
2
adsorption–desorption isotherms
To establish the optimal amount of catalyst, the title reaction was
run with different initial amounts of the hydrotalcite (10, 20 and
30 mg) and the product yields were 60, 93 and 95% respectively
(Table 1, entries 11–13). The results suggest that for chosen reaction
conditions 20 mg of hydrotalcite catalyst was adequate to carry out
the reaction efficiently.
We next investigated the scope of reuse the hydrotalcite catalyst.
After completion of the reaction, task-specific hydrotalcite catalyst
was recovered and subjected to another run, affording the product in
93% yield. This process was repeated four more times, affording the de-
sired product in good yields, with undiminishing efficiency (Table 2).
The simple experimental and product isolation procedures combined
with the ease of recovery and reuse of the reaction medium are expected
to contribute to the development of a green strategy for the Hantzsch re-
actions. Using five different substrates, the formation of dihydropyridine
(4a–f) has occurred in good yields spontaneously and exclusively. Nota-
bly, even sterically hindered substrate also participated effectively to
afford the dihydropyridine derivative (Table 3, entry 6).
Based on the results of this study, it's clear that the hydrotalcite as
catalyst improves the yields of products significantly. The zinc and vana-
dium loading on hydrotalcite provided scope for altering the properties
like acidity, pore size and surface area resulting in appropriate surface
characteristics suited for its performance as good catalyst. The Hantzsch
reaction mechanism may cautiously be visualized to occur via a tandem
sequence of the reactions depicted in the reaction scheme in Mechanism
1. Possibly, the catalyzed reaction proceeds through a cyclic transition
state in the interlamellar space of the catalyst, which is helped by its high-
ly effective acid–base bi-functional surface character capable of mediating
the Hantzsch reaction and Michael addition of carbonyl compounds. We
propose, the surface Lewis base species abstracts active hydrogen of
acetoacetanilide as a pronucleophile in the aqueous phase to generate a
carbanion intermediate, which can be paired with the cationic surface
Metal oxide phases in the catalyst were observed using powder X-
ray diffraction (XRD) performed on a Bruker D8 Advance instrument,
equipped with an Anton Paar XRK 900 reaction chamber, a TCU 750
temperature control unit and a CuKα radiation source with a wave-
length of 1.5406 nm at 40 kV and 40 mA. Diffractogram was recorded
over the range 15–90 °C with a step size of 0.5 per second.
The Transmission Electron Microscopy (TEM) images were viewed
on a Jeol JEM-1010 electron microscope. The images were captured
and analyzed by using iTEM software. High resolution TEM images
were recorded by using a Jeol JEM 2100 electron microscope. The
Scanning Electron Microscopy (SEM) measurements were carried
out using a JEOL JSM-6100 microscope equipped with an energy-
dispersive X-ray analyzer (EDX). The images were taken with an
emission current = 100 μA by a Tungsten (W) filament and an acceler-
ator voltage = 12 kV. The catalysts were secured onto brass stubs with
carbon conductive tape, sputter coated with gold and viewed in a JEOL
JSM-6100 microscope. The pre-treatment of the samples consisted of
coating with an evaporated Au film in a Polaron SC 500 Sputter Coater
metallizator to increase the catalyst electric conductivity.
2
.2. General experimental procedure for the synthesis of dihydropyridine
derivatives
A mixture of freshly distilled benzaldehyde (1.0 mol) in water
2 mL) at room temperature, acetoacetanilide (2.0 mol), ammonium
(
hydroxide (1.5 mol) and hydrotalcite (20.0 mg) were added to a
round bottom flask equipped with a magnetic bar and condenser.
Then, the reaction mixture was heated at 60 °C for appropriate time as
shown in Table 3. The reaction progress was monitored by Thin Layer
Chromatography (TLC) (EtOAc/hexane = 5:5). After completion of
R
O
O
O
O
N
H
O
R
H
1
O
O
hydrotalcite
N
H
N
H
o
N
H
Water, 60 C
N
H
4a-f
NH OH
4
1
2
3a-f
Scheme 1. Multicomponent reaction catalyzed by hydrotalcite.

150
R. Pagadala et al. / Catalysis Communications 45 (2014) 148–152
Table 1
Table 3
Optimization of reaction conditions of the Hantzsch synthesis.
3
Hydrotalcite (Zn-V/CO ) catalyzed synthesis of the dihydropyridine (4a–f) derivatives.
Entry Product Catalyst
no.
Amount
Solvent Temp (°C)
Time Yielda
Entry
Product no.
R
1
Time (h)
Yielda (%)
(h)
(%)
1
2
3
4
5
6
4a
4b
4c
4d
4e
4f
H
2.0
2.0
3.0
2.5
3.0
3.0
93.0
90.0
92.0
90.0
89.0
85.0
b
1
2
3
4
5
6
7
8
9
1
1
1
1
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
–
–
EtOH
EtOH
Room temp 12.0
80
4-Br
3 2
4-N(CH )
4-OCH
4-OH
–
–
12.0
6.0
25
c
NaOH
[Bmim]BF
[Bmim]BF
[Bmim]BF
[Bmim]BF
[Bmim]BF
[Bmim]BF
HT
1.0 (equiv.)
6 drops
6 drops
6 drops
6 drops
6 drops
6 drops
10.0 mg
10.0 mg
20.0 mg
30.0 mg
2
H O
Room temp
3
4
4
4
4
4
4
EtOH
DCM
Room temp 10.0
60 8.0
Room temp 10.0
b
b
b
2-NO
2
H O
2
–
a
Isolated yields.
Room temp
5.0
5.0
6.0
5.0
3.0
2.0
2.0
b
–
60
55
20
b
60
93
95
2
H O
2
H O
2
H O
2
H O
2
H O
60
for the V-doped carbonate mesoporous was quite narrow and mono
modal. The adsorption–desorption curves showed hysteresis loops in
the relative-pressure (P/P ) range of 0.5–0.9, indicating good homoge-
0
neity and fairly small pore size [37]. This indicated that the average
pore size increased with an increase in the amount of carbonate loaded.
The specific surface area (13.4 m /g), pore size (0.044 cm /g) and total
pore volume (131 A°). The sample had large surface areas and high pore
volumes and the pore size distribution was very narrow.
0
1
2
3
Room temp
HT
HT
HT
60
60
60
a
b
c
Isolated yields.
Products were not found.
Trace.
2
3
of the hydrotalcite (Pathway B). In Pathway A, protonated carbonyl is
attacked by the amine from ammonium hydroxide to form the enamine.
The reaction is typically run under Lewis acidic conditions, which can be
paired with the metal cationic surface of the hydrotalcite. The resulting
intermediates followed by Michael reaction. We sense that ammonia pro-
duced from ammonium hydroxide in water is well stabilized by hydrogen
bond formation in 60 °C, which will be a rare phenomenon in the
solvent media. The structures of all the dihydropyridine derivatives
have been characterized and confirmed by 1H NMR, CNMR, FTIR
and HRMS spectra (Supplementary data).
3.1.3. XRD (X-ray diffractogram)
Powder XRD of calcined catalyst (Supplementary data Fig. 3) is in
agreement in the standard hydrotalcite peaks, which indexes are corre-
lating with the reported hydrotalcites [38]. The value of the c-axis calcu-
lated corresponds to the three times the thickness of the ‘sandwich’
formed between the two consecutive brucite like structures and the
interlayer where carbonate ions together with the water molecules
exist. The thickness calculated from the position of the diffraction
peak corresponding to the (0 0 3) plane, was recorded in the range of
13
7
.3–7.6 A° for carbonate precursors. Other main peaks correspond to
3
.1. Characterization of hydrotalcite
the plane (0 0 6), recorded in the range of 3.5–3.8 A° for carbonate pre-
cursors. These values coincide with those reported in the literature for
carbonate hydrotalcite. After calcination due to the removal of water
from the hydrotalcite structure, mixed metal oxides of hydrotalcite pre-
cursors are formed. Powder diffractograms and the d-spacing values of
3
.1.1. SEM (Scanning Electron Microscopy), TEM (Transmission Electron
Microscopy) and & ICP-OES
ICP-OES analysis showed that zinc to vanadium metal ratio is 1:3
which a stoichiometric metal ratio of the hydrotalcite. The SEM micro-
graph (Supplementary data Fig. 1) shows that agglomeration of the
metal oxide particles which is caused by the calcination of the hydrotal-
cite precursor and these aggregates are in the size ranging from 0.86 μm
to 1.73 μm. EDS analysis of this catalyst showed that Zn and V are
homogenously distributed in the catalyst, and the metal ratio (Zn:V) is
also in agreement with the ICP elemental analysis. The catalyst
morphologies as indicated by the SEM image clearly point out the
homogeneity in shapes for the sample and high crystallinity. TEM
micrograph showed in (Supplementary data Fig. 2) that calcined
catalyst has a square plated like structures, TEM micrographs showed
the square plate like structure, which is one, the typical vanadate struc-
tures. These square planar structures are in the size of 25 ± 3 nm. The
selected area diffractions showed that catalyst is in polycrystalline in
nature which is further observed by the XRD-diffractogram.
mixed metal oxide precursor for Zn-VCO are in correlation with the
ICDD file no. 34-378.
3
4. Conclusions
In summary, we have developed a highly efficient and green one-pot
methodology for the synthesis of a wide range of dihydropyridine
derivatives via Hantzsch reaction using hydrotalcite as a heterogeneous
catalyst. Solvent-free conditions and non-toxic reusable heterogeneous
catalyst make this method simple, convenient, cost-effective and
3
.1.2. BET (Brunauer–Emmett–Teller)
Fig. 1 shows the N adsorption–desorption isotherms for the V-
doped carbonate mesoporous catalyst loaded with Zn. The isotherms
for the sample were of type-IV with a H hysteresis loop [36]. This was
2
2
symptomatic of their mesoporous structure. The pore size distribution
Table 2
Recyclability of hydrotalcite catalyst tested for compound 4a.
Entry
Catalyst
Time (h)
Yielda (%)
1
2
3
4
5
Fresh
2.0
2.0
2.0
2.0
2.0
93.0
93.0
91.0
91.0
90.0
2nd run
3rd run
4th run
5th run
a
Isolated yields.
2 3
Fig. 1. N adsorption–desorption isotherm of calcined Zn-VCO hydrotalcite sample.

R. Pagadala et al. / Catalysis Communications 45 (2014) 148–152
151
O
O
O
O
O
Ar
O
N
H
N
H
Pathway D
Pathway C
N
H
N
H
N
H
H
yd
N
H
r
o
ta
Hydrotalcite
Pathway A
(1)
Pathway B
Hydrotalcite
Hydrotalcite -H O Ar-CHO
lci
te
O O
NH3
O
-
H O + NH3
2
Hydrotalcite
Ar-CHO
2
Hydrotalcite
NH2
O
O
O
O
Ar
O
N
OH
N
H
H
Hydrotalcite
Ar-CHO
N
H
N
H
O
Ar
Ar
N
N
H
N
OH
H
NH
O
O
-
H O
Michael addition
NH2
2
N
H
HO
Ar
Ring closure
O
Ar
O
O
Ar
N
O
-
H O
2
N
H
N
H
N
H
N
H
O
Ar
O
O
Ar
O
NH
2
N
N
H
N
Michael addition
H
H
NH
Ring closure
N
Ring closure
HO
H
OH
-
H O
2
O
Ar
O
O
Ar
O
N
H
N
H
N
H
N
H
N
H
N
OH
Isomerization
-
H O
2
O
Ar
O
N
N
H
H
N
H
4a-f
Mechanism 1. Reaction pathways for the Hantzsch synthesis.
environmentally friendly in character, which will have an advantage
over other existing methodologies. The present method significantly
expands the scope of the multicomponent reaction by studying the
reactivity of substituted aromatic aldehydes.
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