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P. Singh et al.
being persuaded worldwide [17–20]. The HPAs are envi-
ronmentally benign solid catalysts which offer several
advantages in terms of catalytic performance, strong acidic
and redox site and selectivity to particular reaction product
by selective stabilization of reaction intermediates. The
HPAs by virtue of their strong acidic site and redox char-
acteristics have been used as catalyst under homogenous as
well as heterogeneous condition.
chromatography using petroleum ether and ethylacetate
with increasing polarity as eluent to yield bis-(4-hydroxy-
coumarin-3-yl)methanes. Structural assignments of the
1
products are based on their H-NMR, 13C-NMR, IR and
Mass analysis. The analysis of complete spectral and
compositional data revealed the formation of bis-(4-hy-
droxycoumarin-3-yl) methanes.
The advantage of using HPAs under homogenous con-
dition lies in their high solubility in polar solvent such as
water, methanol, acetonitrile, etc. After completion of the
catalytic cycles, they can be easily isolated from the
organic reaction media. These used catalysts can subse-
quently be recrystallized and reused for successive cycles.
The concept of ‘‘Green Chemistry’’ has been widely
adopted to meet the fundamental scientific challenges of
protecting human health and environment while simulta-
neously achieving the commercial viability. One of thrust
area for achieving this target is to explore alternative
reaction conditions and reaction media to accomplish the
desired chemical transformation with minimum by prod-
ucts and waste generation as well as eliminating the use of
volatile and toxic organic solvents. It is therefore of utmost
important to evolve simple and effective methodology for
the synthesis of coumarin and its derivatives that cover the
concept of ‘‘Green Chemistry’’. The uses of environmen-
tally benign solvents like water represent green solvent,
being economical and eco-friendly for synthetic transfor-
mations and. However, low solubility of reactant, incom-
patibility of certain intermediate or competition between
the desired reaction and hydrolysis restrict the use of H2O
as a common solvent, although many reactions have been
studied in H2O using different catalyst.
3 Results and Discussion
A new methodology was given for the synthesis of bis-(4-
hydroxucoumarin-3-yl) methanes by the coupling of
4-hydroxycoumarin and aldehydes in water using a phos-
photungstic acid as a catalyst. Phosphotungstic is a green,
non-corrosive catalyst and it is better than iodine which
was used as a catalyst by Kidwai et al. [21] for the syn-
thesis of the bis-(4-hydroxycoumarin-3-yl)methanes.
Along with this iodine is carcinogenic in nature so toxic
and thus we used water as solvent for the synthesis of bis-
(4-hydroxycoumarin-3-yl)methanes, which is green in
nature having no toxicity. The present work shows that
phosphotungstic acid as an excellent catalyst in aqueous
reaction medium. In our continued interest towards green
chemistry, coupled with the benefits of using phospho-
tungstic acid as a catalyst and development of new syn-
thetic methodology, we report here in a very simple and
highly efficient phosphotungstic acid catalysed synthesis of
bis-(4-hydroxycoumarin-3-yl)methanes in aqueous heter-
ogenous system were synthesized as in Scheme 1 and its
mechanism is explained in Scheme 2. Catalyst plays a
major role in synthesis of bis-(4-hydroxycoumarin-3-
yl)methanes by coupling different aromatic aldehydes and
4-hydroxycoumarin at 80 °C with different catalysts
(20 mmol%). After screening different catalysts, phos-
photungstic acid emerges as the best catalyst for the syn-
thesis of bis-(4-hydroxycoumarin-3-yl)methanes (Table 1).
Mineral acids also gives the similar results but they are
corrosive and highly toxic in nature. Thus, we used phos-
photungstic acid as a catalyst in the present work. In order
to elucidate the role of the phosphotungstic acid as catalyst,
a controlled reaction was conducted using 4-hydroxy-
coumarin (20 mmol) and benzaldehyde (10 mmol%) in
water in the absence of catalyst. This resulted in the
2 Experimental {General Procedure for the Synthesis
of Bis-(4-hydroxycoumarin-3-yl)methanes}
In a 50 mL round-bottomed flask, 4-hydroxycoumarin
(20 mmol) and aromatic aldehyde (10 mmol) in water
were taken and the resulting mixture was stirred at 80 °C
for 10 min, after then phosphotungstic acid (15 mmol%)
was added to the reaction mixture. The progress of the
reaction was well monitored by thin layer chromatography
(TLC). After the completion of the reaction, the reaction
mixture was cooled until the solidification appears and then
filtered the solid and washed it water and then the filtrate
was centrifuged at 8,000 rpm for 10 min to pellet out the
catalyst and washed with absolute ethanol to remove all the
organic impurities and then kept at 90 °C for 30 min. The
phosphotungstic acid was reused for evaluating the per-
formance in the next reaction. The isolated products
were subjected to further purification by column
O
O
O
Phosphotungstic acid,
Water, 80
O
O
O
+
°
C
Ar-CHO
OH
OH
OH
Scheme 1 Synthesis of bis-(4-hydroxycoumarin-3-yl)methanes
123