Aqueous extract of the pericarp of Sapindus trifoliatus fruits: A novel 'green' catalyst for the aldimine synthesis

Article abstract of DOI:10.1002/cbdv.200900272

The catalytic efficiency in organic synthesis of the aqueous extract of the pericarp of Sapindus trifoliatus fruits was evaluated. The synthesis of a series of aldimines from aromatic aldehydes and amines was successfully catalyzed by the extract, whereas aromatic ketones and amines did not yield ketimines under comparable reaction conditions, indicating the chemoselective catalysis of the extract. The catalytic activity of the extract is due to saponins, which have a common structural skeleton containing a pentacyclic triterpenoid part substituted with different carbohydrate side chains. The mild conditions, high yields, and short reaction times not only make this protocol a valuable alternative to the conventional methods, but it also becomes significant under the roof of environmentally greener and safer processes.

Full text of DOI:10.1002/cbdv.200900272

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CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
Aqueous Extract of the Pericarp of Sapindus trifoliatus Fruits:
A Novel ꢀGreenꢁ Catalyst for the Aldimine Synthesis
by Santosh Pore, Gajanan Rashinkar, Kavita Mote, and Rajeshri Salunkhe*
Department of Chemistry, Shivaji University, Kolhapur 416004, India
(phone: þ91-231-2609166; fax: þ91-231-2691533; e-mail: rss234@rediffmail.com)
The catalytic efficiency in organic synthesis of the aqueous extract of the pericarp of Sapindus
trifoliatus fruits was evaluated. The synthesis of a series of aldimines from aromatic aldehydes and amines
was successfully catalyzed by the extract, whereas aromatic ketones and amines did not yield ketimines
under comparable reaction conditions, indicating the chemoselective catalysis of the extract. The
catalytic activity of the extract is due to saponins, which have a common structural skeleton containing a
pentacyclic triterpenoid part substituted with different carbohydrate side chains. The mild conditions,
high yields, and short reaction times not only make this protocol a valuable alternative to the
conventional methods, but it also becomes significant under the roof of environmentally greener and
safer processes.
Introduction. – Most organic reactions need a catalyst or a promoter either to bring
about the reaction or to achieve better yields. The wide range of commonly used
catalysts includes acids, bases, Lewis acids, clays, microorganisms, etc. Nevertheless, the
need to develop better catalysts is a thrust area in synthetic chemistry and is ever
increasing [1]. This is due to one or several problems associated with prevailing
catalysts, such as their hazardous nature, high cost, tedious work up, requirement of
large quantities of organic solvents during and after the reaction, and, above all, their
detrimental effects on the environment.
In an attempt to overcome some of the shortcomings of catalysts used in organic
synthesis, we looked to nature for help. Nature abounds in the number and variety of
plants, many of which contain chemical constituents, which are pharmacologically,
biologically, or otherwise active [2]. We therefore thought of using aqueous extracts of
plant parts – fruits, flowers, and leaves – as catalysts. To begin with, the fruit of Sapindus
trifoliatus [3] was chosen for experiment. Aqueous extracts of the pericarp of this fruit
have been used as detergent since a long time in India [4]. It is also known to possess
various pharmacological activities [5]. The activity of S. trifoliatus fruits is mainly due
to saponins. The pericarp of this fruit is known to contain 10–11.5% of saponins [6].
The structures of different saponins present in the pericarp of this fruit, viz., saponins
1–6 (Fig.), have been recently established [7]. These saponins have a common
structural skeleton containing a pentacyclic triterpenoid part substituted with different
carbohydrate side chains (Fig.). The aqueous extract of the pericarp of S. trifoliatus
fruits shows an acidic pH (3.8), which is due to the presence of a COOH group in the
triterpenoid part. This acidic nature of the aqueous extract prompted us to use it as a
catalyst in acid-catalyzed reactions, and to our surprise, it worked as an excellent
ꢀ 2010 Verlag Helvetica Chimica Acta AG, Zꢁrich

CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
1797
Figure. Chemical structures of six saponins obtained from the pericarp of Sapindus trifoliatus fruits
homogeneous catalyst. The synthesis of aldimines (Scheme) was the reaction of choice,
because the products of this reaction are important precursors for nucleophilic
additions with organometallic reagents [8] and for cycloadditions [9]. The imines also
provide a facile route to synthesis of therapeutics like lipoxygenase inhibitors, anti-
inflammatory agents [10], anti-cancer agents [11], b-amino esters [12], and b-lactams
[13]. Contemporary methodologies for the imine synthesis make use of catalysts like
ZnCl2 [14], TiCl4 [15], MgSO4/PPTS (pyridinium p-toluenesulfonate) [16], alumina
[17], CuSO4 [18], Mg(ClO4)2 [19], etc. Non-conventional promoters like microwave
[20] and ultrasound irradiation [21] have also been used to synthesize imines with or
without catalyst. However, all these methods have drawbacks like a hazardous catalyst,
a long reaction time, the need of a special apparatus, the use of environmentally
harmful organic solvents, etc.
Scheme
Results and Discussion. – When an attempt was made to synthesize the simple
aldimine N-benzylideneaniline (9a) with an equimolar mixture of benzaldehyde (7a)
and aniline (8a) in the presence of an aqueous extract of the pericarp of S. trifoliatus

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CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
fruits as catalyst, the product was obtained in almost quantitative yield (95%) within
5 min at room temperature (Table 1). This result was stunning, as the same compound
was reported to have formed in 3 h in a H2O suspension [22], although without catalyst.
However, the efficiency of a catalyst to accelerate the imine synthesis is really
challenged, when it comes to catalyze the reaction between 4-methoxybenzaldehyde
(7c), containing a less electrophilic carbonyl C-atom than 7a, and 4-nitroaniline (8d),
containing a much less nucleophilic N-atom than 8a. In the presence of the aqueous
extract of the pericarp of S. trifoliatus fruits, the desired imine 9l, obtained by stirring
equimolar amounts of 7c and 8d, was formed in 1.5 h in over 90% yield (Table 1). The
reaction time and the high yield of 9l confirmed the catalytic efficiency of the extract. A
series of aldimines was then synthesized using different aldehydes and amines
(Table 1).
Table 1. Synthesis of Aldimines Catalyzed by the Aqueous Extract of Sapindus trifoliatus Fruit Pericarp
Aldehyde Ar
Amine Ar’
Producta) Time [min] Yield [%]
7a
7b
7c
7d
7e
7f
7g
7h
7i
7a
7a
7c
7j
Ph
8a
8a
8a
8a
8a
8a
8a
8a
8b
8c
8d
8d
8c
8e
8c
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
9a
9b
9c
9d
9e
9f
9g
9h
5
10
15
20
10
5
10
2
10
2
2
90
2
2
2
95
92
91
93
98
98
92
98
95
98
98
91
98
92
98
95
4-MeꢀC6H4
4-MeOꢀC6H4
4-HO,3-MeOꢀC6H3
PhCH2OꢀC6H4
4-ClꢀC6H4
3,4-Cl2ꢀC6H3
3-NO2ꢀC6H4
3,4-(MeO)2ꢀC6H3
Ph
4-MeOꢀC6H4 9i
4-ClꢀC6H4
4-NO2ꢀC6H4
4-NO2ꢀC6H4
4-ClꢀC6H4
4-HOꢀC6H4
4-ClꢀC6H4
4-ClꢀC6H4
9j
9k
9l
9m
9n
9o
9p
Ph
4-MeOꢀC6H4
Pyridine-3-yl
3,4-(MeO)2ꢀC6H3
2-Hydroxynapht-1-yl
7i
7k
7l
1,3-Benzodioxole-5-yl 8c
2
a) The synthesized aldimines were characterized by 1H-NMR, IR, and MS analyses.
All the aldimines were obtained with yields above 90%, although the reaction times
varied from 2 to 90 min, reflecting the substituent effects. Indeed, benzaldehyde (7a)
reacted with aniline (8a) in 5 min, whereas the electron-donating substituents in 4-
hydroxy-3-methoxybenzaldehyde (7d) increased the reaction time up to 20 min. The
opposite was the case for aldehydes with electron-withdrawing substituents, as for
instance 7h, which reacted with 8a in 2 min. Moreover, electron-withdrawing
substituents in aniline, e.g., amines 8c and 8d, reacting with 7a in 2 min, shorten the
reaction times further (Table 1). Interestingly, ketones like butan-2-one and aceto-
phenone did not react with either of the amines used with aldehydes. No imine
formation was observed, even after stirring for 3 h at room temperature, thus
demonstrating the chemoselectivity of the aqueous S. trifoliatus pericarp extract
towards aldehydes.

CHEMISTRY & BIODIVERSITY – Vol. 7 (2010)
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Furthermore, aqueous extracts or fresh juices of some other plants were tested for
their ability to catalyze the synthesis of N-benzylideneaniline (9a) using an equimolar
mixture of 7a and 8a. The results of these investigations are summarized in Table 2.
Fresh juice of Solanum lycopersicum (tomato) and Emblica officinalis (amla) were
found to be effective in catalyzing the reaction, however, with longer reaction times and
lower yields of 9a than observed for the S. trifoliatus pericarp extract. Nevertheless, the
fact that such easily available, cheap ꢂgreen materialsꢃ act as catalysts in organic
synthesis is heartening.
Table 2. Catalytic Activity of Various Plant Extracts on the Synthesis of Aldimine 9a
Plant
Part used
Reaction pH
Reaction time [h]
Yield [%]
Zingiber officinale
Solanum lycopersicum
Emblica officinalis
Cyperus rotundus
Terminalia chebula
Rhizome
Ripe fruits
Ripe fruits
Tubers
4.91
4.18
3.10
4.86
3.72
6.0
2.5
6.0
6.0
6.0
No reaction
50
55
No reaction
No reaction
Ripe fruits
Conclusions. – The protocol developed for the aldimine synthesis uses an efficient
green catalyst, the aqueous extract of the pericarp of S. trifoliatus fruits, which is easily
available and of low cost ($2 per kg). The need for organic solvent is reduced to nil,
because only in a few cases EtOH was added. The reaction times have been
considerably reduced and that at room temperature. There is no need to be cautious
while using this catalyst, as it is absolutely harmless to humans and the environment.
Moreover, it has opened new avenues for catalysis in organic synthesis. Studies for a
further exploitation of this extract in organic synthesis are underway in our laboratory.
S. P. thanks the University Grants Commission, New Delhi, India for the award of a Teacher
Fellowship under the F. I. P. of the Xth Plan, Dr. P. P. Wadgaonkar, Scientist, NCL, Pune, and Mr. N. D.
Kokare, New Drug Discovery, Wockhardt Research Centre, Aurangabad, India for valuable suggestions
during the preparation of the manuscript.
Experimental Part
Plant Material and Aqueous Extract. Dried fruits of Sapindus trifoliatus were purchased from local
markets and identified by the Department of Botany, Shivaji University, Kolhapur, India. The dry
pericarp of the fruits (100 g) was soaked in H2O (400 ml) for 12 h. The material was then macerated with
the H2O in which it was soaked and filtered. The filtrate, i.e., the aq. extract, was kept below 58 and used
as catalyst for 15 d.
Typical Procedure of Imine Synthesis. 4-Methoxybenzaldehyde (7c; 1.36 g, 10 mmol) was stirred with
4-nitroaniline (8d; 1.38 g, 10 mmol) in presence of 8 ml of aq. S. trifoliatus extract at r.t. for 1.5 h (until
completion as monitored by TLC). The mixture was then filtered to obtain the crude product, which, on
recrystallization from EtOH, afforded the desired product 9l (2.328 g, 91%). IR (KBr): 3478, 3365, 1674,
1630, 1590, 1512, 1340, 1108, 1025, 840. 1H-NMR (CDCl3, 300 MHz,): 3.88 (s, 3 H); 7.23 (d, J¼8.5, 2 H);
7.31 (d, J¼8.1, 2 H); 7.80 (d, J¼8.2, 2 H); 8.26 (d, J¼8.2, 2 H); 8.38 (s, 1 H). EI-MS: 256 (Mþ ).

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Received September 2, 2009