Tungsten hexachloride nanoparticles loaded on montmorillonite K-10: a novel solid acid catalyst in the synthesis of symmetrical and unsymmetrical azines

Article abstract of DOI:10.1007/s13738-016-0866-2

In the present investigation, we have developed a novel technique to prepare azines using nano-WCl₆ loaded on Montmorillonite K10 clay as a highly active catalyst. A variety of aldehydes and ketones were efficiently converted to the corresponding azines using catalytic amounts of nanosized WCl₆/Mont. K10 under mild conditions. The nanostructures of WCl₆ loaded on Mont. K10 as solid acid catalyst have been prepared by solid dispersion method. The advantages of this catalyst are rapid completion of the reactions, simplicity of performance, lack of pollution and mild and green reaction conditions. The morphologies, structure, and chemical components of parent and modified clay were successfully characterized using SEM, FT-IR, CV, XRD and EDX measurements.

Full text of DOI:10.1007/s13738-016-0866-2

J IRAN CHEM SOC
DOI 10.1007/s13738-016-0866-2
ORIGINAL PAPER
Tungsten hexachloride nanoparticles loaded on montmorillonite
K‑10: a novel solid acid catalyst in the synthesis of symmetrical
and unsymmetrical azines
1
1
1
Javad Safari · Soheila Gandomi‑Ravandi · Saeedeh Shariat
Received: 21 November 2015 / Accepted: 16 April 2016
©
Iranian Chemical Society 2016
Abstract In the present investigation, we have developed a
cycloaddition reactions [12]. Non-symmetrical azines serve
as educts for the preparation of fused tricyclic heterocy-
clic systems by combined intra-intermolecular criss-cross
cycloaddition [13]. Many methods have been designed
to synthesize azines in the presence of catalysts such as
sulfated anatase–titania [14], TiO –P O [15], tetrakis
novel technique to prepare azines using nano-WCl loaded
6
on Montmorillonite K10 clay as a highly active catalyst.
A variety of aldehydes and ketones were efficiently con-
verted to the corresponding azines using catalytic amounts
of nanosized WCl /Mont. K10 under mild conditions. The
6
2
2
5
III
nanostructures of WCl loaded on Mont. K10 as solid acid
(N-methylpyridyl)porphinatoiron(III) pentachloride ([Fe
6
catalyst have been prepared by solid dispersion method.
The advantages of this catalyst are rapid completion of
the reactions, simplicity of performance, lack of pollution
and mild and green reaction conditions. The morphologies,
structure, and chemical components of parent and modified
clay were successfully characterized using SEM, FT-IR,
CV, XRD and EDX measurements.
TMPyP]Cl ) [16], and triphenylphosphine (PPh ) [17].
5
3
Nanotechnology has gained a considerable importance
in the recent times. Tungsten nanoparticles are attractive
as functional materials [18] for surface metallization [19,
20]. New applications of tungsten hexachloride (WCl6)
as a cost-effective and widely available alternative would
be interesting for us in recent years. To overcome disad-
vantages of Lewis acids, much effort has been devoted to
develop heterogeneous supported catalysts based mont-
morillonite K10 (Mont. K10) clay [21–26]. Montmoril-
lonite K10 is a hydrophilic clay with a three-layered struc-
ture [27] belonging to the group of the smectites with an
ideal chemical formula of (Al2-yMg )Si O (OH) ·nH O
Keywords Montmorillonite K10 · Azine · Tungsten
hexachloride · Clay · Lewis acid · Nanostructure
Introduction
y
4
10
2
2
[
28, 29]. Natural occurrence of both Brønsted and Lewis
Azines are receiving great interest for their ability as
ligands in the synthesis of novel organometallic com-
pounds [1–3], basic dyes and toxic colorants [4, 5], biologi-
cal active compounds [6–8], linear and nonlinear optical
materials [9, 10] and in designing of liquid crystals [11].
Symmetrical azines were used in the preparation of four
fused five-membered rings in intra-molecular criss-cross
acidic catalytic sites available in montmorillonite clay
make it an effective catalyst [30–32]. Mont. K10 with a
surface-active inorganic nature can be applied as efficient
support for various transition metal salts and/or metal ions
[33–38]. Green heterogeneous supports such as montmoril-
lonite K10 clay have been widely used as inexpensive and
noncorrosive solid acid catalyst in organic synthetic meth-
odology and are gaining considerable importance from an
environmental point of view [39, 40]. Montmorillonite K10
has been employed in several composite systems owing to
large surface area, strong acidity, good efficiency, environ-
ment-friendly nature, cation exchange capacity, selectiv-
ity, cheapness, regeneration, and mild reaction conditions
[41, 42]. We have developed new applications of tungsten
*
Javad Safari
safari@kashanu.ac.ir
1
Laboratory of Organic Compound Research, Department
of Organic Chemistry, College of Chemistry, University
of Kashan, P.O. Box: 87317-51167, Kashan,
Islamic Republic of Iran
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J IRAN CHEM SOC
Scheme 1 The synthetic
procedure for the preparation of
symmetrical azines using WCl6/
Mont. K10 catalyst
R
O
R'
NEt3, EtOH
N
N
R'
N2H6.SO4
+
Nano-WCl6/
Mont. K10
R'
R
R
R2
O
H
R2
NEt EtOH
N
N
3
,
H
N H .SO
+
4
+
2
6
H
Nano-WCl6/
Mont. K10
H
R1
R1
O
Scheme 2 The synthetic procedure for the preparation of unsymmetrical azines using WCl /Mont. K10 catalyst
6
hexachloride (WCl ) as either an oxophilic metal chloride
Ag/AgCl/KCl(sat.) reference electrode and a platinum disk
as counter electrode. All potentials are referred to the Ag/
AgCl/KCl(sat.) electrode. All measurements were carried
out at the supporting electrolyte consist of 3 mM potassium
ferrocyanide [K Fe(CN) ].
6
or a powerful Lewis acid. Development of metal nanopar-
ticles of tungsten hexachloride on montmorillonite K10
has not been reported so far. In the present study, we report
green and simple synthesis of azines using WCl /Mont.
6
4
6
K10 nanostructures as solid supported catalyst under mild
reaction conditions (Schemes 1, 2). The production method
of supported catalyst involves in situ generation of highly
corrosive and hazardous homogeneous Lewis acid WCl6
nanoparticles on montmorillonite K10. This is the first
example of the use of this reagent to obtain azines.
General procedure for the preparation of WCl /Mont.
K10 catalyst
6
A simple solid dispersion method was applied to form an
in situ solid product of metal nanoparticles deposited on
Mont. K10. To a solution of montmorillonite K10 clay
(2.7 g) in 50 mL of 2-propanol, 0.081 g of tungsten hex-
Experimental
achloride in 10 mL 2-propanol was added. The reaction
mixture was stirred at room temperature for 4 h and pro-
panol was removed under reduced pressure. The reaction
proceeds cleanly and the work-up is simple, involving fil-
tration of the insoluble catalyst and removal of solvent to
obtain the product in high purity. Then, catalyst was dried
All chemicals and solvents were commercially available
and were used as received without further purification.
Melting points (°C) were determined on an Electrother-
mal MK3 apparatus using open-glass capillary and are
uncorrected. The completion of reactions were monitored
by TLC technique on silica gel plates in the solvent sys-
tem petroleum ether–EtOAc (V/V = 7:3). A Perkin Elmer
FT-IR 550 spectrophotometer was used for recording IR
spectra. Proton NMR spectra were measured on a Bruker
in oven at 110 °C for 3 h. The WCl /Mont. K10 catalyst
6
was obtained as a fine powder. The content of W was deter-
mined to be about 0.61 wt% (Fig. 1).
General procedure for the preparation of azine
derivatives
DRX-400 spectrometer operated at 400 MHz in CDCl as
3
solution. X-ray diffraction analysis was carried out on Hol-
land Philips Xpert X-ray diffraction (XRD) diffractometer
0.1 g of WCl /Mont. K10 catalyst in 5 mL absolute ethanol
6
(
CuK, radiation, λ = 0.154056 nm), at a scanning speed
was added to a mixture of hydrazonium sulfate (1 mmol) and
triethylamine (1 mmol). Then, an ethanolic solution contain-
ing of aldehyde or ketone (2 mmol) was added to the above
stirred suspension at 100 °C for the synthesis of symmetri-
cal azines. In order to the synthesis of unsymmetrical azines,
two ethanolic solutions containing the two different precur-
sor aldehydes (1 mmol) were added dropwise and simulta-
neously to a stirred mixture of hydrazonium sulfate/trieth-
ylamine/catalyst/absolute ethanol at 100 °C. Completion
of the reaction was examined by thin layer chromatography
of 2°/min from 10° to 100° (2θ). Scanning electron micros-
copy (SEM) images were obtained using a FEI PhlipsMor-
gagni 268D scanning electron microscope (SEM) operated
at a 100 kV accelerating voltage. Cyclic voltammetry (CV)
measurements were carried out using SAMA 500 elec-
trochemical analyzer. The potential was scanned between
−
0.6 and +0.6 V. The electrochemical cell was assembled
with a conventional three-electrode system, which con-
sisted of a glassy carbon electrode as working electrode, an
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J IRAN CHEM SOC
Fig. 1 Schematic illustration
of the synthesis of WCl /Mont.
6
K10 catalyst
6
WCl Loaded on K10
(
TLC) (ethyl acetate/petroleum ether 3:7). After comple-
Table 1 Comparison of different Lewis acids loaded on Mont. K10
in the model reaction
tion of the reaction, ethyl acetate was added to the solidified
mixture and the insoluble catalyst was separated by filtra-
tion. The solvent was evaporated to get the solid product.
The product was washed with water several times, was col-
lected by suction filtration and dried under vacuum. Finally,
the dried solid was recrystallized from ethanol to give a light
yellow crystalline solid.
Entry
Catalyst
Time (min)
Yield (%)
1
2
3
4
5
6
7
VCl -K10
9
18
11
6
86
79
71
81
89
70
75
3
SbCl -K10
3
MnSO -K10
4
ZrCl -K10
4
WCl -K10
5
6
K10
20
10
WCl6
Results and discussion
Temperature 100 °C, catalyst 0.15 g
Montmorillonite K10 clay acts as a support to increase the
surface area in this heterogeneous reaction. WCl /Mont.
6
K10 catalyst (loading with WCl = 0.61 wt%) showed
the lowest reactivity with the lowest first oxidation poten-
tials. Tungsten hexachloride, a high-potential oxidant is the
strongest Lewis acid in this process.
6
very high activity in this protocol. The WCl nanoparti-
6
cles adsorbed on montmorillonite K-10 as a very inex-
pensive and readily available support, shows much better
performance as compared with the homogeneous catalysts.
Benzaldehyde was selected for reaction with hydrazonium
sulfate as a model reaction. It may be noted from the com-
parison of results (entry nos. 5, 6 and 7 in Table 1) that the
It is necessary to optimize the concentration of the cata-
lyst in the reaction mixture because it significantly influ-
ences the rate of reaction. The effect of catalyst concentra-
tion on the reaction was studied to determine the minimum
amount of catalyst required for maximum formation of
product. It was also observed that the increase in catalyst
concentration increased the rate of product formation and
there was no appreciable change after reaching optimal cat-
alyst concentration. The optimal catalyst concentration was
found to be 0.15 g (Table 2).
equivalent amounts of free WCl or free Mont. K10 showed
6
reduced performance as compared with the WCl loaded on
6
Mont. K10 in the preparation of azines. The results clearly
show that WCl (0.61 W%)/Mont. K10 is a much supe-
6
rior catalyst than the Mont. K10 without WCl . The high
6
catalytic activity of the WCl /Mont. K10 is attributed to its
much higher acidity as compared with that of Mont. K10
The reaction rate is markedly dependent upon tempera-
ture. Therefore, the effect of reaction temperature was stud-
ied in the temperature range from 25 to 110 °C. Results
were reported in Table 3. The reactivity was shown to be
influenced by the temperature of the reaction and increased
with increment in temperature of process. Increasing the
temperature to 100 °C gave a product in 89 % isolated yield
(entry 3).
6
or WCl alone (Table 1). Meanwhile, the above experiment
6
was carried out with several Lewis acids/Mont. K10, and it
was found that WCl /Mont. K10 is superior to other solid
6
acids with respect to both the chemical yield and reac-
tion time. The increasing order of reactivity is as follows:
WCl > ZrCl > VCl > SbCl > MnSO . MnSO exhibits
6
4
3
3
4
4
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J IRAN CHEM SOC
Table 2 Optimization of amount of WCl /Mont. K10 as catalyst
K10 and their corresponding symmetrical and unsymmetri-
cal azines were obtained in good to excellent yields. The
influence of the substituents on the yields of the products
is also not significant. In addition, ketones have shown less
reactivity than aldehydes for this reaction. It is worth noting
6
Entry
Catalyst (g)
Time (min)
Yield (%)
1
2
3
4
5
–
25
20
10
5
70
74
80
89
89
0.05
0.1
0.15
0.2
that WCl /Mont. K10 as an inexpensive catalyst gave good
6
yields with short reaction times for this process (Tables 4,
1
5
5
). The analyses of products were confirmed by their H
NMR, UV–vis and FT-IR spectroscopies.
Temperature 100 °C
Since WCl /Mont. K10 acts as a solid acid catalyst, acid
6
catalyzed mechanism has been proposed for this reaction
(
Scheme 3). After production of hydrazone from conden-
Table 3 Study of temperature effect on the reaction yield
sation of aldehyde and hydrazine, the mechanism includes
the protonation of hydrazone with acidic WCl /Mont. K10.
Entry
Temperature (°C)
Time (min)
Yield (%)
6
Then, protonated hydrazone (I) condenses with enolic form
of the ketone to afford an intermediate (II) which on dehy-
dration and deprotonation produces the product azine (III).
1
2
3
4
25
80
5
5
5
5
65
77
89
89
100
110
Solid acid catalyst WCl /Mont. K10 promotes dehydration
6
and deprotonation processes.
Catalyst 0.15 g
The structure and surface morphology of the catalyst
are very important parameters so that they influence on the
catalytic activity. SEM images of the K10 clay and WCl6/
Mont. K10 clay catalyst have been presented in Fig. 2.
These images confirm deposition of tungsten nanoparti-
cles on the surface of Mont. K10 when was compared with
In order to explore the scope and generality of this pro-
cess, different types of carbonyl compounds were reacted
with hydrazonium sulfate in the presence of WCl /Mont.
6
Table 4 Catalytic synthesis
of symmetrical azines in the
Entry
R
R′
Product
Mp (°C)
Obs.
Time (min)
Yield (%)
presence of WCl /Mont.K10
Lit.
6
under mild conditions
1
2
3
4
5
6
7
8
9
H
H
3a
3b
3c
3d
3e
3f
93–94
92–93 [43]
213 [44]
10
6
89
90
91
81
75
78
97
92
78
75
4-Cl
H
210–213
213–214
159–160
214–215
169–170
134–135
214–217
134–135
197–199
2-OH
4-Me
4-NMe2
4-OMe
2-F
H
216 [45]
7
H
159–161 [45]
215 [46]
13
25
20
5
H
H
171 [47]
H
3g
3h
3i
132 [48]
2,4-Cl2
4-Cl
H
213 [48]
5
Me
Me
133–134 [49]
196–198 [49]
20
10
1
0
2-OH
3j
Catalyst 0.15 g, Temperature 100 °C
Table 5 Catalytic synthesis
of unsymmetrical azines in the
R1
R2
Entry
Product
Mp (°C)
Obs.
Yield (%)
presence of WCl /Mont. K10
Obs.
6
under mild conditions
1
2
3
4
5
3-NO2
3-NO2
2-F
4-NMe2
4-OCH3
4-CH3
4a
4b
4c
4d
4e
131–133 133–135 [50]
191–193 192–194 [50]
98
97
95
88
88
91–93
90–92 [50]
4-Cl
4-CH3
110–111 112–113 [50]
112–114 113 [50]
3-Cl
3-CH3
Catalyst 0.15 g, Temperature 100 °C
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J IRAN CHEM SOC
O-
S
OH
H N NH2+
2
N
H N NH2
2
N+
HSO4-
H
O
O
H
+
+
+
O
OH
H
NH2
N
H
1
)
H N NH2
2
R1
R1
Acidic Sites
OH2
OH
NH2
NH2
N
H
N
2
)
W-K10
H
W-K10
R1
R1
OH2
NH2
NH2
N
N
H
3
)
- H O
2
R1
R1
Acidic Sites
NH2
NH3
N
N
4
)
W-K10
W-K10
R1
R1
I
O
OH
CH2
CH3
5
)
R2
R2
R2
OH
CH2
H
N
H
OH
NH3
N
N
6
)
CH3
R1
R1
II
R2
R2
R2
H
N
H
OH2
OH
N
H
N
N
7
8
)
CH3
CH3
R1
R1
Acidic Sites
R2
R2
OH2
CH3
N
N
H
W-K10
N
N
)
W-K10
CH3
III
R1
R1
Scheme 3 Possible mechanistic route for the synthesis of azines in the presence of WCl /Mont. K10 catalyst
6
pure montmorillonite K10. Figure 2d, e shows SEM micro-
graphs of tungsten hexachloride nanoparticles that were
support. It is clear that layered nature prevails after depo-
sition of tungsten into Mont. K10 and the surface texture
loaded on montmorillonite K10. WCl has tendency to
of WCl /Mont. K10 was slightly rougher than the parent
6
6
agglomerate into large particles during processing. In order
Mont. K10. The uniform dispersion of the WCl nanopar-
6
to obtain better dispersion, the as-received WCl was incor-
ticles within the montmorillonite K10 support plays a key
role in the efficiency of catalyst. The presence of metal
nanoparticles deposits on the surface of Mont. K10 clay
suggests that W is not deposited between the Mont. K10
clay layers. This is in agreement with the XRD data where
6
porated in the montmorillonite K10 using a simple solid
dispersion method. It reveals from Fig. 2d, e that uniform
distribution of nanoparticles occurred and the metal nano-
particles are widely sprayed over the surface of Mont. K10
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J IRAN CHEM SOC
Fig. 2 Scanning electron microscopy (SEM) images of a–c pure montmorillonite K10 and d, e WCl /Mont. K10
6
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J IRAN CHEM SOC
4
3
2
1
0
-
-
-
1
2
3
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Potential / V
Fig. 3 Cyclic voltammetry of pure montmorillonite K10
3
2
1
0
Fig. 5 X-ray diffraction patterns of a pure montmorillonite K10 and
b WCl /Mont. K10
6
ethanol solution for 1 h in an ultrasonic bath to form a stable
suspension. The catalyst was deposited on the glassy carbon
electrode by droplet evaporation for 15 min and then dry-
ing in nitrogen atmosphere for 20 min. Bare montmorillon-
ite K10 did not give any anodic potential (Fig. 3) but WCl6/
Mont. K10 gave anodic potential (Fig. 4). The increase in
anodic peak potential (and decrease in cathodic peak poten-
tial) is indicative of presence of “W” in the catalyst. This
increase in anodic potential and decrease in cathodic poten-
tial is the result of the electron transport in the direct oxida-
-
-
-
1
2
3
-
0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Potential / V
6
+
4+
tion and reduction of Tungsten (W ↔ W ).
Figure 5 shows the crystallinity and the phase purity
Fig. 4 Cyclic voltammetry of tungsten hexachloride (WCl ) loaded
on montmorillonite K10
6
of montmorillonite K10 and WCl /Mont. K10 catalyst in
6
XRD patterns. The montmorillonite K10 clay shows a crys-
talline structure. The broad 2θ peaks of montmorillonite
K10 were observed at about 20.10, 35.17, 54.80 and 62.01
which are assigned to the diffractions of the (110), (105),
(210) and (300) reflections, respectively [51]. Furthermore,
a sharp peak at 2θ = 26.88 belongs to quartz [52]. There is
no difference between the XRD pattern of the Mont. K10
no modification of basal spacing was observed (10 A°). As
it shown in Fig. 2, the WCl nanoparticles were immobi-
6
lized on the surface of Mont. K10 in nanosized dimension
with average size of 7.9 nm. SEM micrographs of WCl6/
Mont. K10 composite (Fig. 2c, d) confirm the presence of
WCl nanoparticles on the surface of Mont. K10.
clay and the WCl /Mont. K10 indicating the clay structure
6
6
Tungsten hexachloride (WCl ) loaded on montmoril-
is retained after the metal loading. The main reason for the
6
lonite K10 has been characterized by CV measurements
in Figs. 3 and 4. In cyclic voltammetry measurements,
disappearance of WCl in the catalyst was due to the low
6
content of the W. Basically, after the metal nanoparticles
loading, montmorillonite K10 became more strongly acidic
in nature but its structure was not destroyed as was con-
firmed from its surface properties and XRD results. Due to
pure montmorillonite clay K10 and WCl /Mont. K10 were
6
used in the redox reaction of 3 mM potassium ferrocya-
nide [K Fe(CN) ] as an electroactive probe. This measure-
4
6
ment is applied to find out the electrical conductivity of
the catalyst. The cyclic voltammograms were recorded by
cycling the potential between −0.6 and +0.6 V. A predeter-
mined amount of catalyst was dispersed in a 0.1 % nafion in
this reason, WCl /Mont. K10 showed very high catalytic
activity.
6
The FTIR spectra of the Mont. K10 and WCl /Mont.
6
K10 are presented in Fig. 6. The differences of FTIR
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J IRAN CHEM SOC
SiKα
W Mα
8
7
6
5
4
3
2
1
000
000
000
000
000
000
000
000
0
AlKα
O Kα
AuMβ
AuMα
NaKα
AuLα
ClKβ
ClKα
AuLl
W Ll W Lα
W Lβ2
W Lβ
keV
0
5
10
Fig. 7 EDX analysis of WCl /Mont. K10 catalyst
6
which has the additional benefit of improving selectivity
in the process. The other benefits of this clay catalyst are
justified by its unique interlayer spacing and highly stable
layer structure to afford economically the desired product
in a green protocol. There is a great scope to develop green
chemical processes and environmentally friendly methods
Fig. 6 FT-IR spectroscopy of a pure montmorillonite K10 and b
WCl /Mont. K10
6
spectra between Mont. K10 and WCl /Mont. K10 are obvi-
6
ous. In the Mont. K10 spectrum, two peak are attributed to
−1
OH stretching vibrations. The shoulder near 3626 cm is
owing to OH stretching vibrations coordinated to octahe-
dral cations (mainly Al, but also Fe and Mg) and the band at
using WCl /Mont. K10 clay as the heterogeneous catalyst
6
for our present studies. This approach can be successfully
applied for the synthesis of a various range of compounds.
The substituents on each aromatic aldehyde were varied to
yield differently substituted azines. The yields were found
to be best when aldehydes were employed with an electron-
withdrawing substituent.
−
1
1
633 cm is related to OH deformation (interlayer H–O–H
−1
molecules) [53, 54]. The broad band near 3427 cm is cor-
responded to the stretching H–O–H vibrations (physisorbed
interlayer water) [53]. In the low energy region, an intense
and broad band is observed at 1050 cm , which is attrib-
uted to Si–O stretching vibrations of the tetrahedral layer.
In addition, appeared band as a shoulder near 800 cm was
associated with bending modes of structural Al–OH–Al and
a band at 527 cm corresponding to Si–O–Al (octahedral
−1
−1
Conclusions
−1
In the present study, tungsten hexachloride (WCl ) nano-
6
−1
Al) deformation [53, 54]. The bending vibration at 467 cm
particles were successfully loaded on montmorillonite K10
clay by a simple solid dispersion method. The heterogene-
ous solid acid catalyst presented high performance for the
synthesis of azine derivatives. All the azines were obtained in
high yields under green, mild and simple experimental con-
ditions. The simplicity of performance, the environmental
safety of the process, high yields of the products, use of inex-
pensive and non-corrosive catalyst, and rapid completion of
the reactions are noteworthy advantages of this process.
is corresponded to Si–O–Si deformation. Comparing the IR
spectrum of WCl /Mont. K10 nanocomposite (Fig. 6b) with
6
that of the pure Mont. K10 indicates the appearance of new
−1
adsorption peak at wavenumber 470 cm assigned to W–O
stretching vibration.
The energy-dispersive analysis of X-ray (EDAX) of the
prepared WCl /Mont. K10 catalyst has been illustrated in
6
Fig. 7. It can be obviously seen that the WCl /Mont. K10
6
contained main elements O (42.53 W%), Si (16.22 W%),
Al (12.74 W%), Na (2.22 W%) and W (0.61 W%). These
Acknowledgments We thank University of Kashan Research Coun-
cil for support of this work (Grant No. 363022/22).
results indicate that WCl nanoparticles were successfully
6
loaded on the Mont. K10 clay support.
We studied the treatment of aromatic aldehyde/ketone
derivatives with hydrazonium sulfate under mild conditions
and found that the anticipated azines were formed smoothly
in excellent yields. This method has utilized a relatively
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6
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J IRAN CHEM SOC
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