P2O5/SiO2 as a new, efficient and reusable catalyst for preparation of 4,4'-epoxydicoumarins under solvent-free conditions

Article abstract of DOI:10.1155/2011/849795

An efficient solvent-free procedure for the preparation of 4,4'-epoxydicoumarins via the ondensation of 4-hydroxycoumarin with aldehydes in the presence of catalytic amount of phosphorus pentoxide/silica gel at 110 °C is described. The advantages of this

Full text of DOI:10.1155/2011/849795

ISSN: 0973-4945; CODEN ECJHAO
E-Journal of Chemistry
2011, 8(4), 1626-1631
http://www.e-journals.net
P2O5/SiO2 as a New, Efficient and Reusable
Catalyst for Preparation of 4,4′-Epoxydicoumarins
Under Solvent-free Conditions
LIQIANG WU* and XIAO WANG
School of Pharmacy, Xinxiang Medical University
Xinxiang, Henan 453003, P. R. China
wliq870@163.com
Received 27 December 2010; Accepted 28 February 2011
Abstract: An efficient solvent-free procedure for the preparation of
4,4′-epoxydicoumarins via the ondensation of 4-hydroxycoumarin with
aldehydes in the presence of catalytic amount of phosphorus pentoxide/silica gel
at 110 °C is described. The advantages of this method are generality, high yields,
short reaction times, ease of product isolation, low cost and ecologically friendly.
Keywords: 4,4′-Epoxydicoumarins, 4-Hydroxycoumarin, P2O5/SiO2, Solvent-free
Introduction
The development of simple, efficient and general synthetic methods forwidely used organic
compounds fromreadily available reagents is one of the major challenges in organic
synthesis. Recently, a variety of reports regarding synthetic studies on 4,4′-epoxydicoumarin
derivatives has been presented, as these compounds were documented to exhibit a wide
range of biological activities1-3. The compounds can be synthesized by a two-step process
(Scheme 1). However, the method has not been entirely satisfactory, with disadvantages
such as low yields, long reaction times, harsh reaction conditions and toxic reagents or
media. Therefore, to avoid these limitations, the discovery of a new and efficient process for
the synthesis of epoxydicoumarins is of prime interest.
Phosphorus pentoxide/silica gel (P2O5/SiO2) is inexpensive, green, commercially
available and heterogeneous catalytic system which has been used in several organic
transformations, such as synthesis of N-sulfonyl imines4, condensation of indoles with
carbonyl compounds5, esterification6, oxidation of sulfides to sulfoxides7, schmidt reaction8,
Fries rearrangement9, direct sulfonylation of aromatic rings10, oxime preparation11,
conversion of aldehydes to acylals12, selective deprotection of 1,1-diacetals13 and
acetalization of carbonyl compounds14.

1627
LIQIANG WU et al.
Solvent-free organic reactions have been applied as useful protocol in organic synthesis.
Solvent-free conditions often lead to shorter reaction times, increased yields, easier workup,
matches with green chemistry protocols and may enhance the regio- and stereoselectivity of
reactions15.
OH
EtOH
Ac2O, C5H5N
OH OH
Scheme 1
Considering the above facts and also in extension of our previous studies on green
organic synthesis16, herein we report an efficient, green and simple method for the synthesis
of 4,4′-epoxydicoumarins by the condensation of 4-hydroxycoumarin with aldehydes in the
presence of catalytic amount of P2O5/SiO2 at 110 ºC under solvent-free conditions (Scheme 2).
Heat, 110 0C
Scheme 2
Experimental
NMR spectra were determined on Bruker AV-400 spectrometer at room temperature using
TMS as internal standard, coupling constants (J) were measured in Hz; Elemental analysis
were performed by a Vario-III elemental analyzer; Melting points were determined on a
XT-4 binocular microscope and were uncorrected; COmmercially available reagents were
used throughout without further purification unless otherwise stated.
Preparation of P2O5/SiO2 catalytic system
A mixture of SiO2 (2 g) and P2O5 (1 mmol, 0.142 g) was ground vigorously to give
P2O5/SiO2 catalytic system as a white powder (2.142 g).
General procedure for synthesis of 3
To a mixture of compounds consisting of aldehyde (1 mmol)and 4-hydroxycoumarin
(2 mmol) in a 10 mL round-bottomed flask connected to a reflux condenser was added
P2O5/SiO2 (1.07 g) and the resulting mixture was stirred in an oil-bath (110 °C) for the
times reported in Table 3. Afterward, the reaction mixture was cooled to room temperature
and was suspended in warm CHCl3 (50 mL), filtered and the filtrate was washed with
saturated solution of Na2CO3 (2×50 mL) and water (2×50 mL) and dried with MgSO4. The
solvent was evaporated and the crude product was purified by recrystallization from ethyl
alcohol. Due to very low solubility of the products 3a, 3h, we cannot report the 13C NMR
data for these products.
3,3′-Benzylidene-4,4′-epoxydicoumarin (3a)
o
White powder, m.p. 386-388 C; IR (cm-1): 1730, 1718, 1666, 1609, 1456, 1365, 1336,
1
1178, 1062, 1042, 888, 766, 713; H NMR (CDCl3, 400 MHz) δ: 8.11 (d, 2H, J = 8.0 Hz),
7.69-7.64 (m, 2H), 7.50-7.40 (m, 8H), 7.24-7.20 (m, 1H), 5.19 (s, 1H); Anal. calcd for
C25H14O5: C 76.14, H 3.58; found: C 76.20, H 3.52.

P2O5/SiO2 as a New, Efficient and Reusable Catalyst
1628
3,3′-(4-Chlorobenzylidene)-4,4′-epoxydicoumarin (3b)
o
White powder, m.p. 364-365 C; IR (cm-1): 1730, 1667, 1611, 1488, 1456, 1368, 1216,
1
1180, 1058, 1015, 889, 763; H NMR (CDCl3, 400 MHz) δ: 8.12 (d, 2H, J = 7.6 Hz),
8.02-7.98 (m, 2H), 7.90-7.87 (m, 2H), 7.69 (t, 2H, J = 7.2 Hz), 7.52-7.34 (m, 4H), 5.13
(s, 1H); 13C NMR (CDCl3, 100 MHz) δ: 160.0, 153.6, 152.8, 139.5, 132.9, 130.3, 128.7,
124.7, 122.3, 117.2, 113.3, 105.9, 34.4; Anal. calcd for C25H13ClO5: C 70.02, H 3.06; found:
C 70.13, H 3.00.
3,3′-(4-Methoxybenzylidene)-4,4′-epoxydicoumarin (3c)
White powder, m.p. 293-295 oC; IR (cm-1): 1732, 1668, 1610, 1520, 1482, 1366, 1253, 1178,
1
1045, 889, 768; H NMR (CDCl3, 400 MHz) δ: 8.11 (d, 2H, J = 8.0 Hz),7.68-7.62 (m, 2H),
7.49-7.37 (m, 6H), 6.88-6.81 (m, 2H), 5.13 (s, 1H), 3.73 (s, 3H); 13C NMR (CDCl3, 100
MHz) δ: 160.1, 159.0, 153.3, 152.7, 133.2, 132.7, 130.0, 124.5, 124.3, 122.3, 117.1, 113.9,
113.5, 106.5, 55.2, 34.1; Anal. calcd for C26H16O6: C 73.58, H 3.80; found: C 73.62, H 3.75.
3,3′-(4-Methybenzylidene)-4,4′-epoxydicoumarin (3d)
o
White powder, m.p. 293-295 C; IR (cm-1): 1740, 1667, 1609, 1469, 1365, 1284, 1177,
1063, 887, 767; 1H NMR (CDCl3, 400 MHz) δ: 8.11 (d, 2H, J = 8.0 Hz),7.68-7.64 (m, 2H),
7.49-7.34 (m, 6H), 7.10 (d, 2H, J = 8.0 Hz), 5.15 (s, 1H), 2.28 (s, 3H); 13C NMR (CDCl3,
100 MHz) δ: 160.1, 153.4, 152.7, 138.1, 137.4, 132.7, 129.2, 128.7, 124.5, 122.3, 117.1,
113.5, 106.5, 34.5, 21.1; Anal. calcd for C26H16O5: C 76.46, H 3.95; found: C 76.53, H 4.02..
3,3′-(4-Nitrobenzylidene)-4,4′-epoxydicoumarin (3e)
o
White powder, m.p. 356-358 C; IR (cm-1): 1726, 1668, 1610, 1511, 1456, 1368, 1348,
1180, 1069, 889, 763; 1H NMR (CDCl3, 400 MHz) δ: 8.18-8.13 (m, 4H), 7.73-7.65 (m, 4H),
7.51 (t, 2H, J = 7.6 Hz), 7.44 (t, 2H, J = 8.4 Hz), 5.28 (s, 1H); 13C NMR (CDCl3, 100 MHz)
δ: 159.9, 154.0, 152.8, 148.0, 147.3, 133.3, 130.0, 124.9, 123.7, 122.5, 117.3, 113.0, 105.0,
35.1; Anal. calcd for C25H13NO7: C 68.34, H 2.98, N 3.19; found: C 68.29, H 3.00, N 3.14.
3,3′-(3-Nitrobenzylidene)-4,4′-epoxydicoumarin (3f)
o
White powder, m.p. 348-349 C; IR (cm-1): 1723, 1668, 1610, 1530, 1456, 1366, 1306,
1
1243, 1180, 1063, 888, 758, 717; H NMR (CDCl3, 400 MHz) δ: 8.16-8.11 (m, 4H), 8.03
(d, 1H, J = 7.6 Hz), 7.71 (t, 2H, J = 7.6 Hz), 7.55-7.50 (m, 3H), 7.44 (d, 2H, J = 8.4 Hz),
5.28 (s, 1H); 13C NMR (CDCl3, 100 MHz) δ: 160.0, 154.1, 152.9, 148.5, 143.1, 136.3,
133.3, 129.3, 124.9, 123.1, 122.9, 122.6, 117.3, 113.1, 105.0, 35.0; Anal. calcd for
C25H13NO7: C 68.34, H 2.98, N 3.19; found: C 68.25, H 3.04, N 3.11.
3,3′-(4-Florobenzylidene)-4,4′-epoxydicoumarin (3g)
o
White powder, m.p. 352-354 C; IR (cm-1): 1725, 1667, 1609, 1532, 1457, 1367, 1221,
1
1179, 1061, 888, 761, 560; H NMR (CDCl3, 400 MHz) δ: 8.12-8.10 (m, 2H), 7.70-7.65
(m, 2H), 7.50-7.41 (m, 6H), 6.98 (t, 2H, J = 8.8 Hz), 5.16 (s, 1H); Anal. calcd for
C25H13FO5: C 72.82, H 3.18; found: C 72.78, H 3.20.
Results and Discussion
In order to get the best experimental reaction condition, the reaction of 4-hydroxycoumarin 1
and benzaldehyde 2a (in 2:1 molar ratio) in the presence of 50 mol% of P2O5/SiO2 under
solvent-free conditions has been considered as a standard model reaction. We have
investigated the model reaction at ambient temperature, and we did not get the product.

1629
LIQIANG WU et al.
(Table 1, Entry 1). In the next step, we carried out the reaction at 80 ºC, 90 ºC, 100 ºC,
110 ºC, 120 ºC and 130 ºC, the best result was obtained at 110 ºC and the product was
obtained within 1.5 min in 86% yield (Table 1, Entry 5).
Table 1. Temperature optimization for the synthesis of 3,3′-benzylidene-4,4′-epoxydicoumarina
Entry
Temperature / °C Time/ h
Yield/ %b
1
2
3
4
5
6
7
25
80
4
0
2.5
2.5
2
35
52
76
86
86
85
90
100
110
120
130
1.5
1.5
1.5
a Reaction conditions: 4-hydroxycoumarin (2 mmol); benzaldehyde (1 mmol); P2O5/SiO2 (0.5 mmol);
neat. bIsolated yield
To determine the appropriate concentration of the catalyst P2O5/SiO2, we
investigated the model reaction at different concentrations of catalyst like 0, 10, 20,
30, 40, 50, 60, 70 and 80 mol% under solvent-free conditions. The product formed in
0, 59, 64, 69, 79, 86, 86, 85 and 84% yield respectively. This indicates that 50 mol%
of P2O5/SiO2 is sufficient for the best result (Table 2, Entry 6). The model reaction
was also tested in the presence of only P2O5 or only SiO2 at 110 °C under solvent-free
conditions; however, these reagents were not efficient separately. P2O5 afforded the
product in 56% yield after 3 h and SiO2 gave the product in 34 % after 6 h (Table 2,
entries 10 and 11). Thus, it is necessary to support P2O5 on SiO2.
Table 2. The amounts of catalyst optimization for the synthesis of 3,3′-benzylidene- 4,4′-
epoxydicoumarina
Entry
Catalyst
0
Time/ min
Yield/ %b
1
2
6
3
<10
68
80
0
P2O5/SiO2, 0.1 mmol
P2O5/SiO2, 0.2 mmol
P2O5/SiO2, 0.3 mmol
P2O5/SiO2, 0.4 mmol
P2O5/SiO2, 0.5 mmol
P2O5/SiO2, 0.6 mmol
P2O5/SiO2, 0.7 mmol
P2O5/SiO2, 0.8 mmol
P2O5, 0.5 mmol
SiO2, 1 g
3
2
4
2
5
1.5
1.5
1.5
1.5
1.5
3
46
57
78
96
95
56
34
6
7
8
9
10
11
6
aReaction conditions: 4-hydroxycoumarin (2 mmol); benzaldehyde (1 mmol); 110 °C ; neat. bIsolated yield
After optimization of the reaction conditions, the reaction was examined with structurally
diverse aldehydes and 4-hydroxycoumarin. The results are summarized in Table 3. As it is clear
from Table 3, all reaction proceeded efficiently and the desired epoxydicoumarins were produced
in high to excellent yields. Various substituted aryl aldehydes were used for the synthesis of
epoxydicoumarin having different substituents such as -Cl, -F, -NO2, -Me, -OMe. It was found
that; electron donating substituent requires longer time whereas electron withdrawing substituent
requires shorter time for the completion of reaction.

P2O5/SiO2 as a New, Efficient and Reusable Catalyst
1630
a
Table 3. Preparation of 4,4′-epoxydicoumarins catalyzed by P2O5/SiO2
Entry
R
Time/ h Product
Yield/ %b
m.p. / °C
387-388
364-366
293-295
316-317
356-357
348-349
350-352
1
2
3
4
5
6
7
1.5
86 (84, 80, 76)c
3a
C6H5
1
2.5
2
88
84
82
89
87
91
3b
3c
3d
3e
3f
4-Cl-C6H4
4-MeO-C6H4
4-Me-C6H4
4-NO2-C6H4
3-NO2-C6H4
4-F-C6H4
1
1.5
1
3g
o
aReaction conditions: 4-hydroxycoumarin (2 mmol); aldehyde (1 mmol); P2O5/SiO2 (0.5 mol); 110 C;
neat. bIsolated yield. cYields after three times of catalyst recovery
P2O5/SiO2 is immiscible with non-polar organic compounds or solvents. Thus, the
recovery of catalyst is convenient when a non-polar solvent is used as a medium. In this
protocol, P2O5/SiO2 is a heterogeneous acid catalyst. Because it is insoluble in the reaction
medium, (CH2Cl2). The catalyst was recovered after reaction by simple filtration. To rule out
the possibility of catalyst leaching, the activity of the recovered catalyst in each reaction was
investigated carefully. The experimental results revealed that P2O5/SiO2 could be recovered
in more than 85% of the cases. In the synthesis of 3,3′-benzylidene-4,4′-epoxydicoumarin,
the catalyst could be reused for three times without significant loss of activity (Table 3, entry 1).
In order to show the merit of P2O5/SiO2 in comparison with other catalysts used for the
similar reaction, we have tabulated some of the results in Table 4. As it is evidence from the
results, P2O5/SiO2 found to be effective catalyst for the synthesis of 4,4′-epoxydicoumarins.
Table 4. Effect of catalysts on the reaction of benzaldehyde and 4-hydroxycoumarina
Catalyst
p-TsOH
Time/ h
Yield/ %b
Entry
2
4
72
25
18
29
73
56
86
1
2
3
4
5
6
7
H2SO4
H3PO4
PPA3
5
7
I2
2
ZnCl2
P2O5/SiO2
5
1.5
a
Reaction conditions: 4-hydroxycoumarin (2 mmol); benzaldehyde (1 mmol); Catalyst (0.5 mmol);
110 o C; neat. bIsolated yield
Conclusion
In conclusion, P2O5/SiO2 is an easily available, inexpensive, efficient and safe catalyst for
the synthesis of epoxydicoumarins from various aryl aldehydes in the presence of catalytic
amount of P2O5/SiO2 at 110 ºC under solvent-free conditions. The remarkable advantages
offered by this method are simple experimental procedure, solvent-free reaction conditions,
short reaction times, high yields and easiness of product isolations.
Acknowledgment
We are pleased to acknowledge the financial support from Xinxiang Medical University.

1631
LIQIANG WU et al.
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