Montmorillonite as a heterogeneous catalyst in the efficient, mild and one pot synthesis of Schiff bases under solvent-free conditions

Article abstract of DOI:10.1002/jccs.201100354

A mild and efficient method has been reported for preparation of Schiff base ligands through condensation reaction of various aromatic aldehydes and ketones with different aromatic amines in the presence of montmorillonite as a heterogeneous catalyst under solvent free conditions. The advantages of this friendly, economically, environmentally and mild method are such as; simplicity of the reaction procedure, reusability of catalyst, simple work-up, high product yields and very short reaction times.

Full text of DOI:10.1002/jccs.201100354

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Montmorillonite as a Heterogeneous Catalyst in the Efficient, Mild and One Pot
Synthesis of Schiff Bases under Solvent-Free Conditions
Hossein Naeimi* and Khadijeh Rabiei
Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan 87317, I.R. Iran
(Received Jun. 13, 2011; Accepted Sept. 5, 2011; Published Online Sept. 22, 2011; DOI: 10.1002/jccs.201100354)
Amild and efficient method has been reported for preparation of Schiff base ligands through condensation
reaction of various aromatic aldehydes and ketones with different aromatic amines in the presence of
montmorillonite as a heterogeneous catalyst under solvent free conditions. The advantages of this
friendly, economically, environmentally and mild method are such as; simplicity of the reaction proce-
dure, reusability of catalyst, simple work-up, high product yields and very short reaction times.
Keywords: Montmorillonite; Ketone; Schiff base; Heterogeneous; Solvent free; Aldehyde.
INTRODUCTION
The solvent-free approach to the synthesis of mole-
mina octahedral sheet of alumina, oxygen and hydroxyl,
and replenished with H2O and changeable irons between
these units.7,8
cules is an attractive one since the majority of solvents is ei-
ther toxic or flammable and adds considerably to the cost of
an overall synthesis. In many instances, the solvent-free
approach allows shorter reaction times, improved selec-
tivities and easier separations and purifications than con-
ventional solvents.1,2 Clay minerals are widespread and
abundant in aquatic and terrestrial environments and, being
finely divided, have large surface areas per unit of mass.
Therefore, their surface reactions have strong biogeo-
chemical and ecological effects in soils and waters and are
also exploited in industrial processes, for clinical treat-
ments and for the removal of pollutants.3,4
Schiff bases are an important class of ligands in coor-
dination chemistry and find extensive application in differ-
ent fields. Schiff bases derived from aromatic carbonyl
compounds have been widely studied in connection with
metalloprotein models and asymmetric catalysis, due to the
versatility of their steric and electronic properties. Schiff
bases that have solvent dependent UV-Vis spectra (solvato-
chromicity) can be suitable NLO (non linear optical active)
materials.9,10 Their biologically active complexes have
been often used as chelating ligands in the coordination
chemistry of transition metals as radiopharma-ceuticals for
cancer targeting, agrochemicals, as model systems for bio-
logical macromolecules, as catalysts and as dioxygen carri-
ers.11–13
Adsorption of organic matter by clay minerals is one
of the most widespread reactions in the earth, in the envi-
ronment, agriculture and in various industrial processes. It
includes (i) exchange of initially present inorganic ions by
organic ions and (ii) accumulation of polar and nonpolar
molecules on the surface of the mineral. Organic cations,
anions or molecules are transferred from solutions, liquid
or gaseous states to the clay surface. During adsorption,
physical or chemical bonds (long- or short-range interac-
tions, respectively) are formed between the mineral and the
organic matter. The adsorption products are defined as
organo-clay complexes. The adsorption of organic materi-
als by clay minerals and the different adsorption mecha-
nisms have been widely investigated during the last century
and have been reviewed.5,6
In continuation of our research on to preparation of
Schiff bases,14-21 because of above pointed significant, eco-
nomical and environmental reasons for elimination of sol-
vent, here, we hope to report a procedure using montmoril-
lonite as a new catalyst for synthesis of Schiff bases from
various carbonyl compounds and different primary amine
in the absence of a solvent under mild reaction conditions.
RESULTS AND DISCUSSION
Firstly, in this study, 1 mol aromatic aldehyde was
treated with 1 mol aromatic primary amine in the presence
of montmorillonite as a catalyst via grinding at room tem-
perature under solvent-free conditions (Scheme I). The
yields of products were determined by TLC analyses and
Montmorillonite, a clay mineral is composed of units
made up of two silica tetrahedral sheets with a central alu-
* Corresponding author. E-mail: naeimi@kashanu.ac.ir
208
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CHEMICAL SOCIETY
One Pot Synthesis of Schiff Bases
via isolated amounts of products based on carbonyl com-
pound. The results of the above mentioned reactions are
shown in Table 1. However, condensation of substrates 1a
and 2a was occurred and upon work-up, the corresponding
Schiff base was obtained. As can be seen in Table 1, various
Schiff bases were synthesized using montmorillonite as a
catalyst in solvent-free media. We were pleased to find that
the reaction of aromatic aldehyde with primary amines
were yielded the Schiff base products in high to excellent
yields and in a shorter time by using montmorillonite as
catalyst without any adduct reagent than the some of other
methods which have been occurred in solution me-
dium.22–27
Table 1. Synthesis of various Schiff base ligands in the presence
of montmorillonite under solvent free conditions and
room temperature
Substrate
R1 R2
M.P.
(°C)
Time Yeilda
(min) (%)
Entry
1
Product
N
H
H
H
H
66-68
65-67
61-63
63-65
51-53
54-56
80-82
10
20
25
13
16
25
25
95
80b
77b
95
94
72b
80
82
70b
97
98
98
98
97
96
60
65
N
2
N
3
H
Br
Br
N
N
4
H
Br
Br
Cl
Synthesis of Schiff bases from aromatic al-
dehydes
Scheme I
5
H
Cl
N
6
H
Cl
Cl
N
7
Ome
H
H
MeO
N
N
8
NO2
NO2
141-143 22
138-140 30
NO2
Montmorillonite clay with high surface area between
(220–270 m2/g) have a layered structure and have attracted
considerable research interest as an environmentally be-
nign and reusable catalyst. The presence of both Si4+ and
Al3+ sits on this clay led to activation of carbonyl group of
aromatic aldehydes. Plus, the high surface area of montmo-
rillonite and its structure have been caused Schiff base
products were obtained with excellent yields and short re-
action times. Montmorillonite clay offer several advan-
tages for example non-corrosive properties, cheapness,
mild reaction conditions, high yields and the easy of setting
and working-up.
9
H
NO2
N
N
10
11
12
13
14
15
Cl Cl
Cl Br
138-140
122-124
176-178
128-130
135-137
142-144
6
5
3
4
7
6
Cl
Cl
Cl
Br
N
NO2 Br
NO2 CH3
CH3 Br
Cl CH3
O2N
O2N
Br
N
CH3
N
Also, with attention to this Table, formation of Schiff
base using carbonyl compound containing electron donor
group or primary amine containing electron with drawing
group, occurred in longer reaction time and lower yield
than to other substrates (Table 1, entries 7 and 8). The struc-
ture of products has been assigned by physical and spectro-
scopic data. In the IR spectra, the characteristic Schiff base
C=N stretching frequency is formed in the region between
n = 1618–1634 cm-1 as a signal strong band. In the 1H NMR
spectra, one protons of HC=N have chemical shifts in d =
8.31–8.60 ppm. The signals around d = 7.04–8.40 are as-
signed by protons of CH=CH of aromatic rings.
H3C
Br
N
N
N
Cl
CH3
16 c CH3 Br
135-137 90
H3C
Br
17 c
H
H
66-68
120
a Isolated yields based on benzaldehyde
b In the presence of Al2O3/P2O5, Ref. [15]
c In absence of catalyst under solution media
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Article
Naeimi and Rabiei
Table 2. Survey the reaction of aromatic ketone with primary amines in the presence of
montmorillonite at temperature between 100-120 °C
Carbonyl
Reaction
Primary
compound
Entry
condition Time (min) Product (6) Yeilda (%)
amine (R3)
(°C)
R1
R2
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CH3
CH3
CH3
CH3
CH3
CH3
Ph
C6H11
C6H11
100
100
100
100
100
100
120
120
120
30
40
6a
6a
6b
6b
6c
6c
-
75
65b
isopropyl
isopropyl
CH2Ph
CH2Ph
C6H5
25
85
40
70b
88
30
40
70b
115
120
Two days
35
Ph
C6H5
-
10b
15c
CH3
C6H11
6a
a Isolated yields
b In the presence of Al2O3/P2O5 Ref. [15]
c The reaction was refluxed in MeOH solvent
Synthesis of Schiff bases from various
ketons
Scheme II
In this research, we have described a novel and highly
efficient method for synthesis of Schiff bases catalyzed by
montmorillonite under heterogeneous solvent free condi-
tions. The advantages of this friendly, economically and
environmentally benign and safe method include a simple
reaction set-up not requiring specialized equipment, high
product yields, short reaction times, simplicity of work up,
reusability of catalyst and the elimination of solvents.
In order to study the catalytic effect of montmorillo-
nite in this method, a comparison of the reaction of aro-
matic aldehydes with aromatic amines in the absence of
catalyst is given in Table 1 (for example; compare entries 1,
14 versus 16, 17 respectively). As can be seen, the reaction
without a catalyst in methanol solvent under reflux condi-
tions was carried out in low yields and long reaction time.
Whereas, the Schiff bases, as products of this new method
were yielded in excellent yields and short reaction times.
Next, in order to study the development of this method,
treatment of some aromatic ketone compounds with pri-
mary amines have been performed in the presence of mont-
morillonite catalyst at temperature between 100 and 120 °C
(Scheme II). Table 2 was indicated these results. By com-
parison results of the results of Table 1 with Table 2 was
identified and confirmed the lower reactivity of ketones
than aldehyde compounds which may be related to steric
hindrance in ketones.
The aromatic aldehydes, ketones and amines were purified
by standard procedures and purity determined by thin layer
chromatography (TLC).
IR spectra were recorded as KBr pellets on a Perkin-
Elmer 781 spectrophotometer and an Impact 400 Nicolet
FTIR spectrophotometer. 1H NMR and 13C NMR were re-
corded in DMSO/CDCl3 solvents on a Bruker DRX-400
spectrometer with tetramethylsilane as internal reference.
Melting points obtained with a Yanagimoto micro melting
point apparatus are uncorrected. The purity determination
of the substrates and reaction monitoring were accom-
plished by TLC on silica-gel polygram SILG/UV 254 plates
(from Merck Company).
A typical procedure for preparation of Schiff bases
N-phenyl-N-(1-phenylmethylidene)amine
In a typical reaction, a mixture of benzaldehyde (0.21
g, 2 mmol), aniline (0.25 g, 2 mmol) and montmorillonite
(0.5 g,) was ground thoroughly in a mortar. The resulting
mixture was placed in a flask and mechanically stirred for
10 minutes at room temperature conditions. The progress
of the reaction was monitored by TLC. After completion of
the reaction, chloroform (15 mL) was added to the reaction
mixture, filtered off, with vaporization of the filtrate, the
solid product was obtained. The crude product was purified
EXPERIMENTAL
Materials and apparatus
All the materials were of commercial reagent grade.
210
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CHEMICAL SOCIETY
One Pot Synthesis of Schiff Bases
by recrystallization from petroleum ether and the pure
Schiff base 3a was obtained as a white solid in 95% yield.
The Schiff base products were identified by spectroscopic
data.
145.1, 152.1.
N-[1-(4-Nitro-phenyl)-methylidene]-N-phenylamine
(3e)
Yellow solid; m.p. 141-143 °C, reported m.p. 138-
140 °C (lit);15 IR (KBr)/n (cm-1) 3015 (Ar, C–H), 1630 (s,
C=N), 1500, 1593 (Ar, C=C), 1320, 1500 (N=O); 1H NMR
(DMSO, 400 MHz)/d ppm 7.3-7.98 (m, 9H), 8.6 (s, 1H);
13C NMR/(CDCl3, 100 MHz)/d ppm: 119.7, 121.7, 123.0,
128.5, 131.4, 142.3, 149.4, 149.8, 157.8.
General Procedure for Preparation of Schiff bases
6a-e
A mixture of ketone compound (1 mmol), amine (1
mmol) and montmorillonite (0.5 g) was ground thoroughly
in mortar under temperature between 100-120 °C. The re-
sulting mixture was poured in to a flask and stirred for ap-
propriate time under thermal conditions. The progress of
the reaction was monitored by TLC. After completion of
the reaction, chloroform was added to the reaction mixture,
filtered off, with vaporization of the filtrate, desire product
was obtained. The crude product was purified by recrys-
tallization from petroleum ether and the pure Schiff bases
were obtained as a white solid in 95% yield. The Schiff
base products were identified by spectroscopic data.
N-Phenyl-N-(1-phenylmethylidene)amine (3a)
White solid; m.p. 66-68 °C, reported m.p. 65-67
(lit);15 IR (KBr)/n (cm-1) 3052 (Ar, C–H), 1623 (s, C=N),
1405, 1560 (Ar, C=C); 1H NMR (CDCl3, 400 MHz)/d ppm:
7.21-7.78 (m, 10H), 8.31 (s, 1H); 13C NMR/(CDCl3, 100
MHz)/d ppm: 119.0, 120.1, 125.1, 126.1, 127.1, 128.3,
134.2, 140.1, 152.1.
N-[1-(4-Chlor-phenyl)-methylidene]-N-[4-Chloro-
phenyl]amine (3f)
Pale yellow solid; m.p. 138-140 °C; IR (KBr)/n
(cm-1) 3041 (Ar, C–H), 1630 (s, C=N), 1495, 1589 (Ar,
C=C); 1H NMR (CDCl3, 400 MHz)/d ppm: 7.15-7.85 (m,
8H), 8.40 (s, 1H); 13C NMR/(CDCl3, 100 MHz)/d ppm:
122.3, 129.1, 129.3, 130.1, 131.8, 134.5, 137.6, 150.2,
159.8.
N-[1-(4-Bromo-phenyl)-methylidene]-N-[4-Chloro-
phenyl]amine (3g)
Pale yellow solid; m.p. 122-124 °C; IR (KBr)/n
(cm-1) 3056 (Ar, C–H), 1629 (s, C=N), 1455, 1565 (Ar,
C=C); 1H NMR (CDCl3, 400 MHz)/d ppm: 7.18-7.95 (m,
8H), 8.45 (s, 1H); 13C NMR/(CDCl3, 100 MHz)/d ppm:
121.4, 129.2, 129.3, 130.0, 131.8, 135.0, 137.3, 150.1,
159.6.
N-[1-(4-Bromo-phenyl)-methylidene]-N-[4-nitro-
phenyl]amine (3h)
N-[1-(4-Bromo-phenyl)-methylidene]-N-phenylamine
(3b)
Pale yellow solid; m.p. 176-178 °C; IR (KBr)/n
(cm-1) 3056 (Ar, C–H), 1632 (s, C=N), 1465, 1570 (Ar,
C=C); 1H NMR (CDCl3, 400 MHz)/d ppm 7.14-8.36 (m,
8H), 8.59 (s, 1H); 13C NMR/(CDCl3, 100 MHz)/d ppm:
120.7, 122.7, 124.0, 129.5, 132.4, 141.3, 149.4, 149.8,
157.8.
Pale yellow solid; m.p. 63-65, reported m.p. 61-63 °C
(lit);15 IR (KBr)/n (cm-1) 3048 (Ar, C–H), 1625 (s, C=N),
1445, 1535 (Ar, C=C); 1H NMR (CDCl3, 400 MHz)/d ppm:
7.20-8.41 (m, 9H), 8.60 (s, 1H); 13C NMR/(CDCl3, 100
MHz)/d ppm: 119.4, 122.8, 128.9, 129.0, 132.0, 136.0,
150.7, 160.6.
N-[1-(4-Methyl-phenyl)-methylidene]-N-[4-nitro-
N-[1-(4-Chloro-phenyl)-methylidene]-N-phenylamine
(3c)
phenyl]amine (3i)
Yellow solid; m.p. 128-130 °C; IR (KBr)/n (cm-1)
3050 (Ar, C–H), 1634 (s, C=N), 1451, 1560 (Ar, C=C); 1H
NMR (CDCl3, 400 MHz)/d ppm: 2.40 (s, 3H), 7.11-8.34
(m, 8H), 8.58 (s, 1H);13C NMR/(CDCl3, 100 MHz)/d ppm:
21.1, 121.1, 123.9, 129.2, 129.7, 137.3, 141.2, 148.2,
156.3.
White solid; m.p. 51-53, reported m.p. 53–56 °C
(lit);15 IR (KBr)/n (cm-1) 3060 (Ar, C–H), 1618 (s, C=N),
1475, 1567 (Ar, C=C); 1H NMR (CDCl3, 400 MHz)/d ppm:
7.20-8.40 (m, 9H), 8.60 (s, 1H); 13C NMR/(CDCl3, 100
MHz)/d ppm: 119.4, 122.8, 128.9, 130.0, 132.1, 133.0,
137.1, 160.7.
N-[1-(4-Methyl-phenyl)-methylidene]-N-[4-boromo-
N-Phenyl-N-(4-methoxy-phenylmethylidene)amine (3d)
Pail yellow solid; m.p. 80-82 °C; IR (KBr)/n (cm-1)
3062 (Ar, C–H), 1627 (s, C=N), 1405, 1560 (Ar, C=C); 1H
NMR (CDCl3, 400 MHz)/d ppm: 4.21 (s, 3H, OCH3), 7.11-
7.78 (m, 9H), 8.30 (s, 1H); 13C NMR/(CDCl3, 100 MHz)/d
ppm: 67, 118.0, 121.1, 124.1, 125.1, 126.1, 128.3, 134.2,
phenyl]amine (3j)
White solid; m.p. 135-137 °C; IR (KBr)/n (cm-1)
3050 (Ar, C–H), 1629 (s, C=N), 1449, 1555 (Ar, C=C); 1H
NMR (CDCl3, 400 MHz)/d ppm: 2.41 (s, 3H), 7.04-7.81
(m, 8H), 8.44 (s, 1H);13C NMR/(CDCl3, 100 MHz)/d ppm:
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White solid; m.p. 140-142 °C; IR (KBr)/n (cm-1)
3050 (Ar, C–H), 1630 (s, C=N), 1450, 1560 (Ar, C=C); 1H
NMR (CDCl3, 400 MHz)/d ppm: 2.39 (s, 3H), 7.14-7.86
(m, 8H), 8.44 (s, 1H);13C NMR/(CDCl3, 100 MHz)/d ppm:
20.0, 120.6, 120.9, 129.1, 129.8, 129.9, 134.9, 136.1,
137.2, 149.1, 157.9.
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Liquid (lit);28 IR (neat)/n (cm-1) 2950, 3090 (Ar,
1
C–H), 1620 (s, C=N), 1460, 1550 (Ar, C=C); H NMR
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(CDCl3, 400 MHz)/d ppm 1.10-1.80 (m, 10H), 2.15 (s, 3H),
3.20 (m, 1H), 7.10-8.0 (m, 5H); 13C NMR (CDCl3, 100
MHz)/d ppm 15.2, 24.9, 25.2, 33.6, 59.8, 133.1, 137.1,
141.8, 162.2.
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Benzylidene isopropylamine-1-methyl (6b)
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(CDCl3, 400 MHz)/d ppm 1.20 (d, 6H, J = 7.1 Hz), 2.18 (s,
3H), 3.20 (m, 1H), 7.1-7.8 (m, 5H); 13C NMR (CDCl3, 100
MHz)/d ppm 15.3, 23.6, 51.2, 134.5, 142.3, 162.8.
Benzylidene benzylamine-1-methyl (6c)
Yellow solid; m.p. 56-58 °C (lit);29 IR (KBr)/n (cm-1)
3020 (Ar, C–H), 1630 (s, C=N), 1460, 1550 (Ar, C=C); 1H
NMR (CDCl3, 400 MHz)/d ppm 2.25 (s, 3H), 4.75 (s, 2H),
7.10-7.80 (m, 10H); 13C NMR (CDCl3, 100 MHz)/d ppm
15.9, 57.6, 140.6, 141.1, 165.1.
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We are grateful to the University of Kashan Research
Council for the partial support of this work.
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