1
254
K. J. Borah, R. Borah
Me
O
coumarin product [20]. With the supported catalyst, the
reaction of 4-nitrophenol with acetoacetic ester yielded
only 50% product within a 20-min reaction time (entry 16).
The catalyst was separated by simple filtration, washed
with organic solvent, and reactivated by treatment with
sulfuric acid in diethyl ether; in this manner, the catalyst
can be recycled three times without signifcant loss of cat-
alytic activity (Table 2). The reactivation of the catalyst is
necessary because it loses some amount of sulfuric acid
above 80 °C [32]. The reaction temperature varied within
the range of 65–70 °C at 560-W microwave power
depending on the polarity of the substrate molecules.
2 4
P4VP-H SO
O
O
R'
+
R'
Solvent-free
MW, 65-70 °C
Me
OEt
OH
O
1
2
3
Scheme 1
Results and discussion
In contrast to conventional methods, this report describes
the use of only 0.2 equivalents of P4VP–H SO as acid
2
4
catalyst for the Pechmann reaction under solvent-free
microwave conditions without forming any side product.
The catalyst P4VP–H SO was prepared by a simple wet
2
4
impregnation method [32]. The amount of acid groups
present in the polymeric chain was calculated based on the
weight of supported polymer salt obtained and the weight
of poly(4-vinylpyridine) used. The amount of dopants
present in the P4VP–H SO was found to be 0.652 mol%.
Conclusion
We have successfully demonstrated the catalytic activity of
P4VP–H SO as a simple, eco-friendly, and efficient sup-
2
4
2
4
ported solid acid catalyst for the synthesis of a variety of
coumarins under solvent-free microwave irradiation.
The characterization of the polymer-supported catalyst so
formed was done by Fourier transform infrared spectros-
copy (FT-IR), scanning electron microscopy (SEM),
thermogravimetric analysis (TGA), and energy dispersive
X-ray analysis (EDX) studies [32]. P4VP–H SO is rela-
Experimental
2
4
tively non-toxic, safe to handle, and reused after
reactivation, which makes the synthetic process more
economical and benign.
All chemicals are commercially available and were used
without further purification. The products were identified
1
13
by comparison of their FT-IR, H, and C NMR spectro-
scopic data and CHN analysis data with those of authentic
compounds (prepared by a known method) and literature
data [14–22, 34–38].
To study the feasibility of the P4VP–H SO -catalyzed
2
4
Pechmann reaction, resorcinol (1 mmol) and ethyl aceto-
acetate (1 mmol) were irradiated using microwave energy
in the presence of P4VP–H SO catalyst under solvent-free
2
4
conditions. The best result was obtained with 0.2 equiva-
lents of catalyst at 560-W microwave power (Table 1,
entry 1) at 65 °C (Catalyst System microwave reactor). An
increase in the catalyst to 0.5 equivalents showed no sig-
nificant improvement in yield (entry 2), though a slight
improvement in reaction time was observed. We therefore
selected 0.2 equivalents of acid as the standard amount to
generalize this protocol, and applied these conditions
to structurally diverse monohydric and polyhydric phenols
to obtain the corresponding coumarins in good to moderate
yields (Table 1). The results showed that this method is
effective for both electron-rich and electron-poor phenols.
The reaction of catechol with acetoacetic ester afforded a
good yield of coumarin (entry 4), whereas most of the
reported protocols failed to produce the corresponding
product [14–22]. Heravi et al. [33] carried out the Pech-
mann reaction of catechol at 130 °C during a 1.7-h reaction
period using sodium 30-tungstopentaphosphate as catalyst.
Interestingly, resacetophenone reacts efficiently in the
presence of the supported sulfuric acid catalyst under sol-
vent-free conditions (entry 12), but by the conventional
method using liquid sulfuric acid it did not give any
Preparation of catalyst P4VP–H SO
2
4
3
Poly(4-vinylpyridine) (500 mg) and 0.6 cm sulfuric acid
3
were stirred in 5 cm diethyl ether at room temperature for
15 min. The solid catalyst was filtered and washed with
diethyl ether three times. The catalyst was dried at 70 °C
for 2 h and stored in a vacuum desiccator until use.
Typical procedure for the synthesis of coumarin
derivatives under microwave irradiation
Acetoacetic ester (1 mmol) and phenol (1 mmol) were
mixed thoroughly and 0.2 mmol P4VP–H SO was added.
2
4
The reaction mixture was irradiated in a microwave reactor
(Catalyst System) at a temperature of 65 °C and 560-W
power for the specified time. After completion of the
reaction, as indicated by TLC, the reaction mixture was
cooled to room temperature and ethanol was added in order
to recover the insoluble catalyst by filtration. The product
was purified by column chromatography using ethyl ace-
tate and hexane as solvent system. The recovered catalyst
was dried and reactivated through treatment with sulfuric
1
23
Borah, Kalyan Jyoti
