Cerium(III)-catalyzed synthesis of schiff bases: A green approach

Article abstract of DOI:10.1080/00397910903370725

The reaction of primary aromatic amines with aryl aldehydes is found to be catalyzed by cerium chloride heptahydrate under solvent-free conditions to give the corresponding Schiff bases in good yields. Copyright Taylor & Francis Group, LLC.

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Cerium(III)-Catalyzed Synthesis of Schiff
Bases: A Green Approach
Lakshmy Ravishankar a , Siddhartha A. Patwe a , Nehal Gosarani a &
Aparajita Roy a
a Department of Chemistry, V. G. Vaze College of Arts, Science, and
Commerce, Mumbai, Maharashtra, India
Published online: 04 Oct 2010.
To cite this article: Lakshmy Ravishankar , Siddhartha A. Patwe , Nehal Gosarani & Aparajita Roy
(2010): Cerium(III)-Catalyzed Synthesis of Schiff Bases: A Green Approach, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 40:21, 3177-3180
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Synthetic Communications1, 40: 3177–3180, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903370725
CERIUM(III)-CATALYZED SYNTHESIS OF SCHIFF
BASES: A GREEN APPROACH
Lakshmy Ravishankar, Siddhartha A. Patwe, Nehal Gosarani,
and Aparajita Roy
Department of Chemistry, V. G. Vaze College of Arts, Science, and
Commerce, Mumbai, Maharashtra, India
The reaction of primary aromatic amines with aryl aldehydes is found to be catalyzed
by cerium chloride heptahydrate under solvent-free conditions to give the corresponding
Schiff bases in good yields.
Keywords: CeCl3 ꢀ 7H2O; green chemistry; Schiff bases; solvent free
INTRODUCTION
Aromatic imines or Schiff bases are nitrogen-containing organic compounds
that are reported to exhibit analgesic, antimicrobial, antibacterial, and anti-
inflammatory activity.[1] They are conventionally prepared by condensation of an
aldehyde=ketone with a primary aromatic amine and with the simultaneous removal
of water using molecular sieves or a Dean–Stark apparatus.[2] Several modified meth-
ods for synthesis of Schiff bases have been reported in the literature, including use of
catalysts such as ZnCl2,[3] TiCl4,[4] alumina,[5] P2O5,[6] and Mg(ClO4)2,[7] besides acti-
vation of the reaction mixture by infrared (IR) irradiation.[8] Environmentally
friendly methods for the synthesis of imines have been reported in the literature.
Touchelte[9] has reported the solventless synthesis of imines by grinding o-vanillin
and p-toluidine, resulting in quantitative yields of the product. Varma et al.[10] reported
the solvent-free synthesis of imines under microwave conditions using montmorillonite
K10 as a solid support. Recently, Gopalakrishnan et al.[11] reported the synthesis of imi-
nes catalyzed by CaO under microwave conditions. Ethyl lactate as a tunable solvent
recently has been found to be useful in the synthesis of aryl aldimines.[12]
RESULTS AND DISCUSSION
Several Schiff bases were synthesized both in the presence and absence of
CeCl3 ꢀ 7H2O, and the results are summarized in Scheme 1 and Table 1. In a typical
Received June 26, 2009.
Address correspondence to Lakshmy Ravishankar, V. G. Vaze College of Arts, Science, and
Commerce, Mithagar Road, Mulund (East), Mumbai 400 081, Maharashtra, India. E-mail: lakshravi@
yahoo.com
3177

3178
L. RAVISHANKAR ET AL.
Scheme 1. Synthesis of Schiff bases 1–6.
procedure, an equimolar mixture of aldehyde (10 mmol) and amine (10 mmol) were
taken in a porcelain dish and mixed thoroughly. CeCl3 ꢀ 7H2O (1 mmol) was added
and the entire reaction mixture was mixed vigorously. The progress of the reaction
was monitored by thin-layer chromatography (TLC). The mixture melted initially
and then solidified, indicating formation of the imine. The product was recrystallized
using absolute alcohol. The procedure for the reaction in the absence of
CeCl3 ꢀ 7H2O was the same as before. The products were characterized by IR
(C¼N stretching between 1610 and 1630 cmꢁ1) and comparison of their melting
points with the reported data.
In case of deactivated amines (entries 1 and 2), the reaction does not proceed in
the absence of CeCl3 ꢀ 7H2O. However, addition of catalytic amounts of
CeCl3 ꢀ 7H2O results in the formation of the Schiff base in a very short time.
Similarly, deactivated aldehydes having electron-releasing substituents (entry 3)
underwent the reaction faster in the presence of the catalyst, probably because of
coordination of the carbonyl oxygen with cerium, which enhanced the electrophili-
city of the carbon. In the case of reactions using anisaldehyde, it was observed that
addition of catalytic amounts of the catalyst reduced the reaction time considerably
from 2 h to a few minutes (entries 4–6). Todd[13] had reported the synthesis of various
Schiff bases derived from anisaldehyde by refluxing amines with aldehydes in etha-
nol. The melting points of the products obtained in the present work compare well
with those reported in the literature. The CeCl3 ꢀ 7H2O-catalyzed reaction was found
Table 1. Synthesis of Schiff bases in the presence and absence of CeCl3 ꢀ 7H2O
No.
1
R1
R2
CeCl3
Time (min)
Melting point (ꢂC)a
Yieldb (%)
H
4-NO2
—
1:1:0.1
—
120
30
—
144 (138–140)[6]
No reaction
41
2
3
4
5
6
H
3-NO2
H
120
10
—
66 (66–68)[6]
47 (47–49)[15]
47
59 (57–62)[13]
59
No reaction
88
1:1:0.1
—
2-OH
120
30
68
91
1:1:0.1
—
4-OCH3
4-OCH3
4-OCH3
H
120
5
50
85
1:1:0.1
—
4-Cl
3-NO2
60
5
92 (93)[13]
92
—
82
1:1:0.1
—
1:1:0.1
89
No reaction
80
120
50
109 (109–110)[13]
aFigures in parentheses correspond to reported melting points.
bYields of recrystallized product.

CERIUM(III)-CATALYZED SYNTHESIS OF SCHIFF BASES
3179
Scheme 2. Synthesis of 2,3-diphenylquinoxaline.
to be effective in the synthesis of 2,3-diphenylquinoxaline, by condensation of
o-phenylenediamine and benzil (Scheme 2). In the absence of the catalyst, the reac-
tion requires refluxing in ethanol for more than 30 min.[14] However, addition of
catalytic amounts of the catalyst avoided the use of solvent and refluxing conditions
and gave good yields of the product, albeit in 2 h.
CONCLUSION
In conclusion, we have developed a simple, solvent-free procedure for the
synthesis of Schiff bases, catalyzed by CeCl3 ꢀ 7H2O. The method works well with
deactivated amines and aldehydes to give the desired product in good yields. The
present method is advantageous over the conventional procedure, in that it does
not require azeotropic removal of water by using excess of aromatic solvents.
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