Facile and efficient method for preparation of Schiff bases catalyzed by P2O5/SiO2 under free Solvent conditions

Article abstract of DOI:10.1002/hc.20383

A simple and convenient method for the preparation of Schiff bases is developed. In this method, the Schiff base compounds were prepared from the reaction of carbonyl compounds with primary amines. In this reaction, P ₂O₅/SiO₂ have catalyzed the Schiff base formation in dry media under solvent-free conditions. Advantages of this method are use of an inexpensive and efficient catalyst, high yields of products, short reaction times, and simplicity of the reaction and workup.

Full text of DOI:10.1002/hc.20383

Heteroatom Chemistry
Volume 19, Number 1, 2008
Facile and Efficient Method for Preparation of
Schiff Bases Catalyzed By P O /SiO under
5
2
2
Free Solvent Conditions
Hossein Naeimi,1 Hashem Sharghi,2 Fariba Salimi,1
and Khadijeh Rabiei1
1Department of Chemistry, Faculty of Sciences, Kashan University, Kashan, 87317,
Islamic Republic of Iran
2Department of Chemistry, Faculty of Sciences, Shiraz University, Shiraz, 71454,
Islamic Republic of Iran
Received 8 April 2006; revised 18 November 2006
These ligands represent one of the most widely
ABSTRACT: A simple and convenient method for
the preparation of Schiff bases is developed. In this
method, the Schiff base compounds were prepared
from the reaction of carbonyl compounds with pri-
mary amines. In this reaction, P2O5/SiO2 have cat-
alyzed the Schiff base formation in dry media un-
der solvent-free conditions. Advantages of this method
are use of an inexpensive and efficient catalyst, high
yields of products, short reaction times, and simpli-
utilized classes of ligand in metal coordination
chemistry [2]. The Schiff base compounds and their
complexes are widely applied in enantioselective cy-
clopropanation of styrenes [3], asymmetric addition
of cyanide to aldehydes [4], asymmetric aziridina-
tion of olefins [5], enantioselective epoxidation [5,6],
regio-selective ring opening of epoxides [7], and as a
membrane in ion selective electrode [8–12].
Schiff base compounds containing imine group
( RC N ) are usually formed by the condensation
of a primary amine and diamine with an active car-
bonyl compound in a methanol solvent [13]. These
methods have not been entirely satisfactory owing
to a number of drawbacks such as low yields, long
reaction time [14], and tedious workup.
ꢀ
C
city of the reaction and workup. 2008 Wiley Periodi-
cals, Inc. Heteroatom Chem 19:43–47, 2008; Published on-
line in Wiley InterScience (www.interscience.wiley.com).
DOI 10.1002/hc.20383
Over the past few years, significant research
has been directed toward the development of new
technologies for environmentally benign processes
(green chemistry) [15], which are both economically
and technologically feasible [16,17]. An important
area of green chemistry deals with solvent minimiza-
tion. A solvent-free or solid-state reaction may be
carried out by using the reactants alone or incor-
porating them in clays, zeolites, silica, alumina, or
other matrices [18]. In recent years, β-zeolite [19],
montmorillonite clays [20], SiO2, and Al2O3 were
employed as catalysts for this purpose to obtain rel-
atively better results.
INTRODUCTION
The Schiff base ligands and their complexes have sig-
nificant importance in chemistry, and every year a
number of reports are published on the preparation
of these compounds and their application in chemi-
cal reactions [1].
Correspondence to: Hossein Naeimi; e-mail: naeimi@kashanu.
ac.ir.
Contract grant sponsor: University of Kashan Research Council.
c
ꢀ
2008 Wiley Periodicals, Inc.
43

44 Naeimi et al.
SCHEME 1
SCHEME 2
SCHEME 3
Although some of these methods represent a con-
venient procedure with good to high product yields,
we hope to report a procedure using P2O5/SiO2 as
a catalyst for the preparation of Schiff bases in the
absence of a solvent under mild conditions.
RESULTS AND DISCUSSION
P2O5/SiO2 as a catalyst (Scheme 2 and Scheme 3).
The results of such studies are in Table 2 and
Table 3.
The carbonyl compound was treated with the pri-
mary amine in the presence of P2O5 supported on
SiO2 as a catalyst without a solvent by grinding. For
the first time, in this reaction, 1 mol benzaldehyde
was reacted with 1 mol aliphatic or aromatic primary
amine in the presence of P2O5/SiO2 at room temper-
ature under solvent-free conditions (Scheme 1).
The results are indicated in Table 1. As shown
in Table 1, a series of Schiff bases were synthesized
using P2O5/SiO2 as a catalyst in solvent-free media.
We were pleased to find that the reaction of different
amines with benzaldehyde was obtained as the cor-
responding Schiff bases in good to excellent yields
in a shorter time, using P2O5/SiO2 than some of the
previously reported methods [21–27]. In compari-
son, the reaction of benzaldehyde with 4-nitroaniline
in the absence of a catalyst is given at entry 12 in
Table 1. As can be seen, the yield of reaction without
a catalyst was very low and the reaction time was
very long.
As shown in Table 2, the yields of the reactions
with this new method for the preparation of vari-
ous Schiff bases are quite fair, and the reaction time
is very short. At the entry 5, Table 2, the reaction
of benzophenone with p-bromo-aniline occurred in
low yield and long reaction time. This low reactivity
may be related to steric hindrance in the carbonyl
compound and electron withdrawing of a bromine
atom from the aromatic ring of amine. It is of great
importance that the reaction is largely affected by
the P2O5/SiO2 catalyst.
To study the effect of catalyst in this method,
the reaction in the presence of P2O5/SiO2 catalyst
with the same one without a catalyst has been com-
pared (e.g., compare entries 4, 5, and 6 vs. 7, 8, and
9, respectively, in Table 3). However, the reaction
products were obtained in very low yield in the ab-
sence of a catalyst in methanol solvent under reflux
condition, even after extending the reaction time to
several days.
Then, to study the development of this method,
various carbonyl compounds with diamines and
monoamines were treated in the presence of
TABLE 1 The Reaction of Benzaldehyde with Various Amines Catalyzed by P2O5/SiO2 at Room Temperature
Entry
Amine Substrate (2) R
Time (min)
Product (3)
Yield (%)a
1
2
3
4
5
6
7
8
C6H11
CH(CH3)2
Allyl
20
25
25
30
35
30
40
42
45
a
b
c
d
e
f
g
h
i
85
80
90
80
82
85
79
77
70
75
78
10c
Ph
3-Me-C6H4
4-Me-C6H4
3-Cl-C6H4
4-Cl-C6H4
3-NO2-C6H4
4-NO2-C6H4
4-Br-C6H4
4-NO2-C6H4
9
10
11
12b
45
40
>180
j
k
–
aIsolated yield based on benzaldehyde.
bIn absence of catalyst.
cYield obtained by 1H NMR.
Heteroatom Chemistry DOI 10.1002/hc

Facile and Efficient Method for Preparation of Schiff Bases Catalyzed by P2O5/ SiO2 under Free Solvent Conditions 45
TABLE 2 Preparation of Schiff Bases from Various Carbonyl Compounds and Primary Amines
Carbonyl Compound (4)
Primary Amine (5)
Entry
R1
R2
R3
Reaction Condition (◦C) Time (min) Product (6) Yield (%)a
1
2
3
4
5
4-NO2-C6H4
H
CH2Ph
CH2Ph
C6H11
CH(CH3)2
4-BrC6H4
RT
100
100
100
120
45
60
60
60
120
a
b
c
d
e
75
70
65
Ph
Ph
Ph
Ph
CH3
CH3
CH3
Ph
70
10b
aIsolated yield based on carbonyl compound.
bYield obtained by 1H NMR.
TABLE 3 Preparation of Schiff Bases from Various Carbonyl Compounds and Diamines
Carbonyl compound (4)
Diamine (7)
Entry
R1
R2
Y
Reaction Condition (◦C) Time (min)
Product (8) Yield (%)a
1
2
3
4
5
6
7
8
9
2-OH-4-NO2-C6H3
2-OH-4-NO2-C6H3
2-OH-4-NO2-C6H3
2-OH-C6H4
2-OH-C6H4
2-OH-C6H4
2-OH-C6H4
2-OH-C6H4
2-OH-C6H4
H
H
H
CH3
CH3
CH3
CH3
CH3
CH3
C6H4-CH2-C6H4
C6H4-O-C6H4
C6H4-SO2-C6H4
C6H4-CH2-C6H4
C6H4-O-C6H4
C6H4-SO2-C6H4
C6H4-CH2-C6H4
C6H4-O-C6H4
RT
RT
RT
50
50
50
20
20
20
50
50
50
a
b
c
d
e
f
g
h
i
95
85
92
78
75
78
–
b
c
–
–
–
–
b
c
–
–
b
c
C6H4-SO2-C6H4
–
–
aIsolated yield based on carbonyl compound.
bThe reaction was refluxed in methanol solvent.
cSeveral days.
The structure of products was assigned by the
spectroscopic data. In the IR spectra, the charac-
teristic Schiff base C N stretching frequency is ob-
served in the region between υ 1600 and 1700 cm−1
as a strong signal band. The stretching vibration of
C H in the alkyl groups appears at region between
Germany in high purity. IR spectra were recorded as
KBr pellets on a Perkin-Elmer 781 spectrophotome-
ter and an Impact 400 Nickolet FTIR spectropho-
1
tometer. H NMR spectra were recorded in CDCl3
and DMSO with (400 MHz) spectrometer using TMS
as an internal reference. Melting points were ob-
tained with a Yanagimoto micromelting point ap-
paratus and are uncorrected. The purity of the sub-
strates and reactions was determined by TLC on
silica-gel polygram SILG/UV 254 plates.
1
υ 2800 and 2900 cm−1. In the H NMR spectra, one
proton of CH N has chemical shifts at δ = 8.25–
8.35 ppm. The signals around δ = 6.5–8.4 are as-
signed by protons of CH CH of the aromatic rings.
CONCLUSION
General Procedure for Preparation of Schiff
Bases
Schiff base ligands have been prepared from car-
bonyl compounds with high yields and short reac-
tion times, catalyzed by P2O5/SiO2 under solvent-free
conditions. The other advantages of this method-
ology are simple reaction procedures, simplicity of
workup, and mild reaction condition at room to low
temperature.
In a typical reaction, a mixture of benzaldehyde
(0.29 g,
2
mmol), amine (0.25 g,
2
mmol),
and P2O5/SiO2 mixture (0.4 g,
2
mol% of
P2O5/benzaldehyde) was grounded thoroughly in a
mortar. The resulting mixture was placed in a flask
and mechanically stirred for 30 min. The progress
of the reaction was monitored by TLC. After com-
pletion of the reaction, methanol (50 mL) was added
to the reaction mixture, filtered off the solid product
and washed with cold methanol. The crude product
was purified by recrystallization in petroleum ether,
EXPERIMENTAL
Chemicals were purchased from the Merck Chemi-
cal Company Frankfurter Str. 250 64293 Darmstadt,
Heteroatom Chemistry DOI 10.1002/hc

46 Naeimi et al.
and the pure Schiff base was obtained in 80% yield.
The Schiff base products were identified and con-
firmed by the spectroscopic data and physical meth-
ods and were consistent with the previously reported
data [21–27].
N-(4-Nitrobenzylidene)aniline
(3j). Yellow
solid; mp 138◦C–140◦C; IR (KBr) υ (cm−1): 3010 (Ar,
C H), 1625 (s, C N), 1480, 1593 (Ar, C C), 1310,
1500 (N O); 1H NMR/(DMSO), δ (ppm): 8.5 (s, 1H),
7.3–7.93 (m, 9H).
N-(4-Bromobenzylidene)aniline (3k). Pale yel-
low solid; mp 61◦C–63◦C IR (KBr) υ (cm−1): 3056
(Ar, C H), 1625 (s, C N), 1445, 1535 (Ar, C C), 1135
(C Br); 1H NMR/(CDCl3) δ (ppm): 8.6 (s, 1H), 7.2–8.4
(m, 9H).
Benzylidene Cyclohexylamine (3a). Yellow liq-
uid (lit.) [21]; H NMR (CDCl3), δ (ppm): 1.10–1.80
(m, 10H), 3.21 (m, 1H), 7.39–7.67 (m, 5H), 8.38
(s, 1H); 13C NMR (CDCl3), δ (ppm): 24.8, 25.7, 34.5,
70.1, 136.7, 158.5.
1
N-(4-Nitrobenzylidene)benzylamine
(6a). mp
Benzylidene Isopropylamine (3b). Yellow liquid
(lit.) [22]; H NMR (CDCl3), δ (ppm): 1.20 (d, 6H),
56◦C–58◦C (lit.) [24]; 1H NMR (CDCl3) δ (ppm): 4.84
1
(s, 2H), 7.30–8.07 (m, 9H), 8.42 (s, 1H).
3.40–3.55 (m, 1H), 7.29–7.75 (m, 5H), 8.38 (s, 1H);
13C NMR (CDCl3), δ (ppm): 24.3, 62.1, 136.7, 158.7.
Benzylidene Benzylamine-1-methyl (6b). Yellow
1
solid; mp 56◦C–58◦C (lit.) [25]; H NMR/(CDCl3) δ
Benzylidene Allylamine (3c). Yellow liquid (lit.)
[23]; 1H NMR (CDCl3), δ (ppm): 4.21 (dd, 2H), 5.08–
5.25 (dd, 2H), 5.80–6.12 (m, 1H); 13C NMR (CDCl3),
δ (ppm): 63.5, 116.1, 135.9, 136.2, 162.1
(ppm): 2.25 (s, 3H), 4.75 (s, 2H), 7.10–7.80 (m, 10H);
13C NMR/(CDCl3) δ (ppm): 15.9, 57.6, 140.6, 141.1,
165.1.
Benzylidene Cyclohexylamine-1-methyl (6c).
Liquid (lit.) [26]; H NMR/(CDCl3) δ (ppm): 1.10–
1.80 (m, 10H), 2.15 (s, 3H), 3.20 (m, 1H), 7.10–8.0
(m, 5H); 13C NMR/(CDCl3) δ (ppm): 15.2, 24.9, 25.2,
33.6, 59.8, 133.1, 137.1, 141.8, 162.2.
Benzylidene Aniline (3d). White solid; mp 65◦C–
67◦C; IR (KBr), υ (cm−1): 3052 (Ar, C H), 1623 (s,
C N), 1405, 1560 (Ar, C C); 1H NMR/(CDCl3), δ
(ppm): 8.3 (s, 1H), 7.2–7.78 (m, 10H)
1
Benzylidene Isopropylamine-1-methyl (6d). Liq-
uid (lit.) [27]; 1H NMR/(CDCl3) δ (ppm): 1.20 (d, 6H),
3.20 (m, 1H), 7.1–7.8 (m, 5H); 13C NMR/(CDCl3) δ
(ppm): 15.3, 23.6, 51.2, 134.5, 142.3, 162.8.
N-(3-Methylbenzylidene)aniline (3e). Brown oil;
IR (KBr) υ (cm−1) 3027 (Ar, C H), 1630 (s, C N),
1435, 1500 (Ar, C C), 2900 (C H); 1H NMR/(CDCl3),
δ (ppm): 8.65 (s, 1H), 2.6 (s, 3H), 7.2–8.2 (m, 9H).
N,Nꢁ-bis(4-Nitrosalicylidene)-4,4ꢁ-diaminodiphen-
ylmethane (8a). Yellow solid; mp 193◦C–195◦C
IR/(KBr) υ (cm−1): 3420–3500 (OH), 3006 (Ar, C H),
2903 (C H), 1635 (s, C N), (1605, 1475 C C),
1335–1535 (s, N O), 1080 (s, C O), 940, 730, 670;
1H NMR/(DMSO) δ (ppm): 14.5–15) s, 1H), 8.95 (s,
2H), 8.4 (d, 2H), 8.0 (d, 2H), 6.4–7.3 (m, 10H), 3.7
(s, 2H).
N-(4-Methylbenzylidene)aniline (3f). Brown oil;
IR (KBr) υ (cm−1) 3027 (Ar, C H), 1630 (s, C N),
1435, 1500 (Ar, C C), 2900 (C H); 1H NMR/(CDCl3),
δ (ppm) 8.25 (s, 1H), 2.2(s, 3H), 6.8–7.7 (m, 9H).
N-(3-Chlorobenzylidene)aniline (3g). Brown oil;
IR (KBr)/υ (cm−1): 3060 (Ar, C H), 1623 (s, C N),
1
1435, 1560 (Ar, C C), 1050 (C–Cl); H NMR/CDCl3,
δ (ppm): 8.3 (s, 1H), 6.8–7.8 (m, 9H).
N,Nꢁ-bis(4-nitrosalicylidene)-4,4ꢁ-diaminodiphe-
nylether (8b). Yellow solid; mp 233◦C–236◦C;
IR/(KBr) υ (cm−1) 3420–3500 (br, OH), 30569 (Ar,
C H), 1618 (s, C N), 1475, 1570 (C C), 1345–1570
(s, N O), 1090 (s, C O), 940, 740; 1H NMR/(DMSO),
δ ppm 13.5–14.5 (s, 1H), 8.95 (s, 2H), 8.5 (d, 2H), 8.1
(dd, 2H), 6.6–7.4 (m, 10H).
N-(4-Chlorobenzylidene)aniline
(3h). White
solid; mp 53◦C–56◦C IR(KBr) υ (cm−1): 3060 (Ar,
C H), 1618 (s, C N), 1475, 1567 (Ar, C C), 1086
(C–Cl); 1H NMR/(CDCl3), δ (ppm): 8.6 (s, 1H),
7.2–8.4 (m, 9H).
N-(3-Nitrobenzylidene)aniline
(3i). Yellow
N,Nꢁ-bis(4-Nitrosalicylidene)-4,4ꢁ-diaminodiphe-
nylsulphone (8c). Yellow solid; mp 281◦C–248◦C;
IR/(KBr) υ (cm−1): 3420–3500 (br, OH), 3056 (Ar,
C H), 1618 (C N), 1475, 1570 (Ar, C C), 1345, 1530
solid; mp 66◦C–68◦C IR(KBr) υ (cm−1): 3010 (Ar,
C H), 1625 (s, C N), 1470, 1582 (Ar, C C), 1320,
1517 (N O); 1H NMR/(DMSO), δ (ppm): 8.5 (s, 1H),
7.3–7.93 (m, 9H).
1
(s, N O), 1090 (s, C O) 940, 740; H NMR/(DMSO)
Heteroatom Chemistry DOI 10.1002/hc

Facile and Efficient Method for Preparation of Schiff Bases Catalyzed by P2O5/ SiO2 under Free Solvent Conditions 47
[6] Jacobsen, E. J.; Zhang, W.; Guler, M. L. J Am Chem
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δ (ppm): 13.5–14.5 (s, 1H), 8.8 (s, 2H), 8.4 (d, 2H),
8.0 (dd, 2H), 7.8 (m, 4H), 6.9–7.3. (m, 6H). 13C
NMR/(DMSO) δ (ppm): 177.4, 168.8, 159, 135.1,
136.1, 133.2, 1341.6, 128.2, 123.1, 122.8, 115.8.
N,Nꢁ-bis(1-Salicylethylidene)-4,4ꢁ-diaminodiph-
enylmethane (8d). Pale yellow solid; mp 128◦C–
130◦C; IR (KBr) υ (cm−1): 3420–3500 (br, OH), 3060
(Ar, C H), 2895 (C H), 1608 (s, C N), 1465, 1565
1
(Ar, C C), 1160 (C O); H NMR/(DMSO) δ (ppm):
14.5–15 (s, 2OH), 6–8 (m, 16H), 3.3 (s, 2H), 2.6 (6H).
N,Nꢁ-bis(1-Salicylethylidene)-4,4ꢁ-diaminodiph-
enylether (8e). Pale yellow solid; mp 210◦C–212◦C;
IR (KBr) υ (cm−1): 3300–3500 (br, OH), 3056 (Ar,
C H), 1608 (s, C N), 1475, 1570 (Ar, C C), 1250
(C O); 1H NMR (DMSO) δ (ppm): 14.5–15 (s, 2OH),
6–8 (m, 16H), 3.7 (s, 6H).
N,Nꢁ-bis(1-Salicylethylidene)-4,4ꢁ-diaminodiph-
enylsulfone (8f). White solid; mp 165◦C–167◦C; IR
(KBr) υ (cm−1): 3300–3500 (br, OH), 3056 (Ar, C H),
2903 (C H), 1629 (s, C N), 1450, 1570 (Ar, C C),
1142, 1265 (s, S=O), 1101 (C O); 1H NMR/(DMSO)
δ (ppm): 14.5–15 (s, 2OH), 6–8 (m, 16H), 3.6 (s, 6H).
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Heteroatom Chemistry DOI 10.1002/hc