Starch-sulfuric acid as a bio-supported and recyclable solid acid catalyst for rapid synthesis of α,α′-benzylidene bis(4-hydroxycoumarin) derivatives

Article abstract of DOI:10.1007/s11164-013-1272-3

Starch-sulfuric acid-catalyzed, simple, one pot, solvent-free, environmentally benign synthesis of α,α′-benzylidene bis(4-hydroxycoumarin) derivatives has been achieved by reaction of aromatic aldehydes with 4-hydroxycoumarin. The catalyst is reusable and has remarkable activity. This procedure has several advantages including high yields, short reaction times, easy work-up, and use of an inexpensive, moderately acidic, safe catalyst.

Full text of DOI:10.1007/s11164-013-1272-3

Res Chem Intermed
DOI 10.1007/s11164-013-1272-3
Starch–sulfuric acid as a bio-supported and recyclable
0
solid acid catalyst for rapid synthesis of a,a -
benzylidene bis(4-hydroxycoumarin) derivatives
Ramin Rezaei Mohammad Reza Sheikhi
•
Received: 3 November 2012 / Accepted: 10 May 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Starch–sulfuric acid-catalyzed, simple, one pot, solvent-free, environ-
0
mentally benign synthesis of a,a -benzylidene bis(4-hydroxycoumarin) derivatives
has been achieved by reaction of aromatic aldehydes with 4-hydroxycoumarin. The
catalyst is reusable and has remarkable activity. This procedure has several
advantages including high yields, short reaction times, easy work-up, and use of an
inexpensive, moderately acidic, safe catalyst.
Keywords Biscoumarin ꢀ 4-Hydroxycoumarin ꢀ Starch–sulfuric acid ꢀ
Heterogeneous catalyst
Introduction
Coumarin derivatives have important pharmacological properties, for example
anticoagulant, insecticidal, antihelminthic, hypnotic, antifungal, phytoalexin, and
HIV protease inhibition [1–3]. The minimum active pharmacophore is a coumarin
dimer containing an aryl substituent on the central linker methylene [4]. Addition of
4
-hydroxy and 7-hydroxy substituents to the coumarin rings improves the potency
of the compounds [3]. In addition, the coordination ability of the biscoumarins as
ligands has been proved in complexation with lanthanum(III) ions, so this class of
complexes has cytotoxic activity [5]. The most direct procedure for preparation of
bis-coumarins is condensation of an aldehyde with 4-hydroxycoumarin in the
presence of a variety of catalysts, for example molecular iodine [6], 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) [7], MnCl [8], POCl in dry dimethyl-
2
3
formamide (DMF) [9], Et AlCl3 [10], SO H-functionalized ionic liquids [11],
3
2
[
bmim][BF ] [12], tetrabutylammonium bromide (TBAB) [13], Zn(Proline) [14],
4
2
R. Rezaei (&) ꢀ M. R. Sheikhi
Department of Chemistry, Firoozabad Branch, Islamic Azad University, Firoozabad, Iran
e-mail: rezaieramin@yahoo.com
123

R. Rezaei, M. R. Sheikhi
R
OH
H
OSO3H
OH
OH
H
O
O
OSO3H
H
HO3SO
H
H
O
2
RCHO
+
00C, Solvent-free
8
O
O
O
O
O
O
1
2
3
Scheme 1 Synthesis of biscoumarin derivatives over starch–sulfuric acid
sodium dodecyl sulfate (SDS) [15], triethylbenzylammonium chloride (TEBA) [16],
phosphotungstic acid [17], H [NaP W O110])–SiO2 [18], tris(hydrogensulf-
1
4
5
30
ato)boron [19], sulfated titania [20], and ethanol or acetic acid under reflux [21].
The reaction has also been conducted under thermal solvent-free microwave
conditions [22].
Green chemistry emphasizes the development of environmentally benign
chemical processes and technology [23, 24]. Recently, the direction of science
and technology has been shifting toward more eco-friendly, natural product
resources and reusable catalysts. Natural biopolymers are attractive candidates in
the search for solid support catalysts [25, 26]. It was therefore thought worthwhile to
develop a new and mild method using an inexpensive biopolymer-based catalyst
that can be easily separated, reused, and is not contaminated by the products.
0
Herein, we report an efficient one-pot synthesis of a,a -benzylidene bis(4-
hydroxycoumarin) derivatives by reaction of a variety of aldehydes with
4-hydroxycoumarin under solvent-free conditions in the presence of starch–sulfuric
acid (Scheme 1).
Experimental
General
All chemicals and analytical-grade solvents were purchased from Merck or Fluka.
Melting points of all products were determined in open glass capillaries on a Mettler
9
100 melting point apparatus. Infrared (IR) spectra were recorded by use of a 4300
1
Shimadzu FT-IR spectrometer. H NMR spectra were recorded on a Bruker
00-MHz spectrometer. Elemental analysis was performed on a Heraeus CHN
Rapid analyzer. Most of the products were characterized by comparison of their
4
1
melting points and IR and H NMR spectra with those of authentic samples.
Preparation of starch–sulfuric acid
Chlorosulfonic acid (1.0 g, 9 mmol) was added dropwise, at 0 °C, over a period of
2
h, to a magnetically stirred mixture of 5.0 g starch in 20 ml n-hexane. HCl gas
was removed from the reaction vessel immediately. When addition was complete,
the mixture was stirred for 2 h. The mixture was then filtered and washed with
3
0 ml acetonitrile and dried at room temperature to afford 5.25 g of starch–sulfuric
1
23

Starch–sulfuric acid
acid as a white powder. The sulfur content of starch–sulfuric acid samples by
?
conventional elemental analysis was 0.55 mmol/g. The number of H sites on the
starch–SO H, determined by acid–base titration, was 0.50 meq/g. This value
3
corresponds to approximately 90 % of the sulfur content, indicating that most of the
sulfur species on the sample are in the form of the sulfonic acid groups.
0
Typical experimental procedure for synthesis of a,a -benzylidene bis(4-
hydroxycoumarin) derivatives
A mixture of 4-hydroxycoumarin (2 mmol), aldehyde (1 mmol), and starch–sulfuric
acid (0.1 g) was ground and heated to 80 °C for the appropriate time (Table 2).
After completion of the reaction (monitored by TLC), the mixture was cooled to
room temperature and extracted with EtOAc (2 9 5 mL). The solution was
concentrated and the crude product was recrystallized from ethanol yielding the
pure biscoumarin. For reaction of formaldehyde, which needed water as solvent, the
components mentioned above were mixed in 5 mL of water and stirred magnetically
at 80 °C for 15 min. After completion of the reaction, the reaction mixture was
extracted with ethyl acetate. The organic layer was dried over anhydrous sodium
sulfate and evaporated to dryness. The pure product was obtained by crystallization
from ethanol. Most of the biscoumarin derivatives are well known in the literature
and were identified by comparison of their physical and spectral data.
Spectral analysis of compounds (3a–m, 4)
Compound 3a
-
1
1
; H NMR
Yellow crystals, IR (KBr): 3,425, 3,030, 1,660, 1,610, 761 cm
(
s, OH), 11.57 (1H, s, OH); Anal. Calcd for C H O (411.78): C, 72.81; H, 3.91.
400 MHz, CDCl ): d 6.50 (1H, s, CH), 7.22–8.25 (13H, m, 13 9 CH); 11.33 (1H,
3
2
5 16 6
Found: C, 72.69; H, 3.83.
Compound 3b
-
1 1
White crystals, lR (KBr): 3,427, 3,030, 1,670, 1,605, 1,094, 765 cm ; H NMR
(
s, OH), 11.57 (1H, s, OH); Anal. Calcd for C H ClO (445.68): C, 67.20; H, 3.38.
400 MHz, CDCl ): d 6.40 (1H, s, CH), 7.32–8.40 (12H, m, 12 9 CH); 11.33 (1H,
3
2
5
15
6
Found: C, 67.0; H, 3.30.
Compound 3c
-
1 1
Yellow crystals, lR (KBr): 3,435, 2,925, 1,651, 1,618, 1,060, 762 cm ; H NMR
(
OH), 11.63 (H, s, OH), Anal. Calcd for C H ClO (445.68): C, 67.20; H, 3.38.
400 MHz, CDCl ): d 6.14 (1H, s, CH), 7.24–8.03 (12H, m, 12 9 CH); 10.92 (H, s,
3
2
5
15
6
Found: C, 67.15; H, 3.35.
123

R. Rezaei, M. R. Sheikhi
Compound 3d
-1 1
Yellow crystals, IR (KBr): 3,450, 3,035, 1,657, 1,615, 1,348, 760 cm ; H NMR
(
s, OH), 11.57 (1H, s, OH); Anal. Calcd for C H NO (458.43): C, 65.65; H, 3.31;
400 MHz, CDCl ): d 6.63 (1H, s, CH), 7.42–8.35 (12H, m, 12 9 CH); 11.44 (1H,
3
2
5
15
8
N, 3.06. Found: C, 65.78; H, 3.34; N, 2.83.
Compound 3e
-1 1
Yellow crystals, lR (KBr): 3,420, 2,926, 1,656, 1,616, 1,335, 765 cm ; H NMR
(
s, 2 9 OH), Anal. Calcd for C H NO (458.43): C, 65.65; H, 3.31; N, 3.06.
400 MHz, CDCl ): d 6.13 (1H, s, CH), 7.26–8.17 (12H, m, 12 9 CH); 11.58 (2H,
3
2
5
15
8
Found: C, 65.69; H, 3.30, N, 2.90.
Compound 3f
-
1 1
White crystals, IR (KBr): 3,386, 3,035, 1,668, 1,607, 1,258, 769 cm ; H NMR
(
400 MHz, CDCl ): d 3.72 (3H, s, OCH ), 6.45 (1H, s, CH), 7.12–8.18 (12H, m,
3
3
1
2 9 CH), 11.34 (1H, s, OH), 11.52 (1H, s, OH); Anal. Calcd for C H O
26 18 7
(
443.56): C, 70.58; H, 4.10. Found: C, 70.75; H, 3.96.
Compound 3g
-
1 1
White crystals, IR (KBr): 3,410, 3,075, 1,670, 1,614, 1,350, 764 cm ; H NMR
(
400 MHz, CDCl ): d 2.36 (3H, s, CH ), 6.12 (1H, s, CH), 7.11–8.07 (12H, m,
3
3
1
2 9 CH), 11.32 (1H, s, OH), 11.57 (1H, s, OH); Anal. Calcd for C H O
26 18 6
(
427.50): C, 73.23; H, 4.25. Found: C, 73.15; H, 4.43.
Compound 3h
-1 1
White crystals, IR (KBr): 3,335, 3,040, 1,668, 1,607, 1,325, 766 cm ; H NMR
(
OH), 11.33 (1H, s, OH), 11.68 (1H, s, OH); Anal. Calcd for C H O (429.82): C,
400 MHz, CDCl ): d 6.44 (1H, s, CH), 7.15–8.13 (12H, m, 12 9 CH), 9.82 (1H, s,
3
2
5 16 7
7
0.09; H, 3.76. Found: C, 70.69; H, 4.08.
Compound 3i
-1 1
Yellow crystals, IR (KBr): 3,435, 2,915, 1,645, 1,610, 1,450, 925 cm ; H NMR
(
s, OH), 11.58 (1H, s, OH); Anal. Calcd for C H O Br: (491.29): C, 61.12; H, 3.08.
400 MHz, CDCl ): d 6.33 (1H, s, CH), 7.15–7.88 (12H, m, 12 9 CH), 11.44 (1H,
3
2
5 15 6
Found: C, 61.25; H, 3.11.
Compound 3j
-
1 1
Yellow crystals, IR (KBr): 3,325, 3,025, 1,725, 1,670, 1,614, 760 cm ; H NMR
400 MHz, CDCl ): d 6.56 (1H, d, CH), 6.68 (1H, d, CH =), 6.75 (1H, d, CH=),
(
3
1
23

Starch–sulfuric acid
7.14–8.05 (12H, m, 12 9 CH); 11.42 (1H, s, OH), 11.58 (1H, s, OH); Anal. Calcd
for C H O (437.55): C, 73.97; H, 4.14. Found: C, 73.78; H, 4.08.
2
7 18 6
Compound 3k
White crystals, IR (KBr): 3,330, 3,073, 1,670, 1,614, 1,330, 765 cm ; H NMR
-
1 1
(
400 MHz, CDCl ): d 3.76 (3H, s, CH O), 3.89 (3H, s, CH O), 6.10 (1H, s, CH),
3
3
3
6.73–8.09 (11H, m, 11 9 CH), 11.32 (1H, s, OH), 11.55 (1H, s, OH); Anal. Calcd
for C H O : (473.75): C, 68.43; H, 4.25. Found: C, 68.21; H, 4.12.
2
7 20 8
Compound 3l
-1 1
Yellow crystals, IR (KBr): 3,338, 3,083, 1,662, 1,610, 1,337, 765 cm ; H NMR
(
m, 12 9 CH); Anal. Calcd for C H NO (456.47): C, 66.53; H, 4.34; N, 2.87.
400 MHz, DMSO-d ): d 3.20 (6H, s, 2 9 CH ), 6.31 (1H, s, CH), 7.23–7.84 (12H,
6
3
2
7
21
6
Found: C, 65.29; H, 3.24; N, 2.98.
Compound 3m
-1 1
Black amorphous solid, IR (KBr): 3,030, 1,657, 1,604, 1,325, 765 cm ; H NMR
(
m, 8 9 CH); Anal. Calcd for C H O (402.42): C, 68.66; H, 3.51. Found: C,
400 MHz, CDCl ): d 6.0 (1H, s, CH), 6.32–6.58 (3H, m, 3 9 CH), 7.33–8.35 (8H,
3
2
3 14 7
6
8.46; H, 3.48.
Compound 3n
-
1
1
Cream crystals, IR (KBr): 3,443, 2,926, 1,650, 1,600, 762 cm
;
H NMR
(
s, 2 9 OH); Anal. Calcd for C H O (336.29): C, 67.86; H, 3.60. Found: C, 67.82;
400 MHz, DMSO-d ): d 3.85 (2H, s, CH ), 7.26–8.01 (8H, m, 8 9 CH), 11.32 (2H,
6
2
1
9 12 6
H, 3.63.
Compound 4
Yellow crystals, mp: 300–302 °C; IR (KBr): 3,436, 3,074, 1,662, 1,603, 1,315,
-
1 1
7
62 cm ; H NMR (400 MHz, DMSO-d ): d 6.35 (2H, s, 2 9 CH), 7.05–7.92
6
(20H, m, 20 9 CH); Anal. Calcd for C H O (745.24): C, 70.60; H, 3.75. Found:
44 26 12
C, 70.35; H, 3.66
Results and discussion
Starch–sulfuric acid (SSA) is readily prepared by dropwise addition of chlorosul-
fonic acid to a mixture of starch in n-hexane at 0 °C. It is important to note that this
reaction is easy and clean without any work-up procedure because HCl gas is
evolved from the reaction vessel immediately. This white homogeneous, nonhy-
groscopic solid acid is stable under the reaction conditions (Scheme 2).
123

R. Rezaei, M. R. Sheikhi
Scheme 2 Synthesis of starch–
sulfuric acid
OH
S
O
O
OH
Cl
+
OH
O
S
O
-
HCl
0
n-hexane, 0 C
O
Starch
Starch
0
To evaluate the feasibility of SSA as a catalyst for synthesis of a,a -benzylidene
bis(4-hydroxycoumarin) derivatives (Scheme 1), reaction of a 1:2 molar ratio of
0
benzaldehyde and 4-hydroxycoumarin, respectively, to give a,a -(benzylidene)-bis-
(
4-hydroxycoumarin), was conducted under different conditions both in the absence
and presence of SSA; the results are listed in Table 1. In the absence of SSA only
5 % yield of the product was obtained, with recovery of starting material, even
after heating at 80 °C for 2 h (entry 1, Table 1) whereas in the presence of SSA
0.05 g), under the same conditions the yield increased to 60 % (entry 2, Table 1).
2
(
On the basis of this result, further studies were conducted and it was found that 0.1 g
SSA was optimum for this reaction, and gave the product in 90 % yield in just
1
0 min (entry 3, Table 1). The reaction was also examined in EtOH, H O, CHCl ,
2 3
and toluene as solvents. In the presence of solvents the reaction was sluggish;
solvent selection was critical and had a significant effect on product yields. When
ethanol or water was replaced by chloroform and toluene, there was a significant
increase in the product yield (entries 4–7, Table 1). Reaction temperature was also
optimized. Below 80 °C the reaction proceeded slowly giving a relatively low yield
and no improvement was observed above 80 °C. All further studies were carried out
under solvent-free conditions with 0.1 g catalyst at 80 °C.
The scope and generality of this procedure were demonstrated by applying it to a
broad range of aromatic aldehydes carrying either electron-donating or electron-
0
Table 1 Effect of different amounts of starch–sulfuric acid and solvents on formation of a,a -(benzyl-
idene)-bis-(4-hydroxycoumarin)
a
Entry
Starch–sulfuric acid (g)
Condition/solvent
Time (min)
Yield (%)
1
2
3
4
5
6
7
0
80 °C/solvent free
80 °C/solvent free
80 °C/solvent free
Reflux/ethanol
120
120
10
25
60
90
60
70
55
40
0.05
0.10
0.10
0.10
0.10
0.10
30
Reflux/water
30
Reflux/chloroform
Reflux/toluene
60
60
Reaction conditions: benzaldehyde (1 mmol); 4-hydroxycoumarin (2 mmol)
a
isolated yield
1
23

Starch–sulfuric acid
Table 2 Starch–sulfuric acid-catalyzed synthesis of biscoumarin derivatives
a
Entry
R
Product
Time (min)
Yield (%)
m.p. (°C)
Found
Reported
6, 13, 15, 17, 20]
[
1
2
3
4
5
6
7
8
9
C
6
H
5
3a
3b
3c
3d
3e
3f
10
7
90
90
80
95
90
85
90
80
85
85
80
88
83
80
228–230
254–256
218–220
232–234
120–122
242–244
266–268
218–220
280–282
228–230
260–262
205–207
198–200
290–292
233–235
258–260
–
4-ClC
6
6
H
H
4
4
2-ClC
10
5
4-NO
2
C
6
H
H
4
4
238–240
122–124
248–250
270–272
222–224
285–287
230–232
265–267
210–212
202–204
–
3-NO
2
C
6
5
4-CH
3
3
OC
6
H
4
8
4-CH
C
6
H
4
3g
3h
3i
8
4-HOC
3-BrC
–CH=CH–C
3,4-(CH O)
4-N(CH
6
H
4
12
8
6 4
H
1
1
1
1
1
0
1
2
3
4
6
H
4
3j
8
3
2
C
6
H
3
3k
3l
10
8
3
)
2
C
6
H
4
2-Furanyl
H
3m
3n
10
15
Reaction conditions: aldehyde (1 mmol); 4-hydroxycoumarin (2 mmol); SSA (0.10 g); 80 °C
a
Isolated yields
withdrawing substituents. All the aromatic aldehydes reacted very well under the
optimized reaction conditions, giving good-to-excellent yields of the desired products
without formation of any side products, and all the reactions were complete within
5
–12 min (entries 1–13, Table 2). Variation of the structure the aromatic aldehydes
had no significant effect on the yield, and with aldehydes bearing sensitive functional
groups, for example Cl, NO , OCH , and CH=CH, the reaction proceeded smoothly to
2
3
afford the corresponding products in excellent yields (entries 2–6 and 10, Table 2).
The reaction proceeded more quickly with aldehydes containing para and meta nitro
0
groups and required only 5 min to give the corresponding a,a -benzylidene bis(4-
hydroxycoumarins) in excellent yields (entries 4 and 5, Table 2). A longer reaction
time (12 min) was needed for reaction of p-hydroxybenzaldehyde (entry 8, Table 2).
SSA also worked well even with the acid-sensitive aldehyde furfural, without leading
to formation of any side products (entry 13, Table 2). The reaction worked very well
for aromatic aldehydes but poorly for aliphatic aldehydes, because of their low boiling
points under solvent-free conditions. For aliphatic aldehydes the reaction was
therefore conducted in water as solvent (entry 14, Table 2).
The reaction was also found to be applicable to dialdehydes, as exemplified by
use of 1,4-benzenedialdehyde. Two carbonyl groups from the dialdehyde were
reacted with four molecules of 4-hydroxycoumarin to afford the product 4
(
Scheme 3) in 80 % yield.
With regard to the mechanism, the transformation involves a Knoevenagel
condensation of 4-hydroxycoumarin with the aldehydes followed by a Michael
addition with another molecule of 4-hydroxycoumarin (Scheme 4).
123

R. Rezaei, M. R. Sheikhi
O
O
O
O
OH
H
OSO3H
H
O
OH
OH
O
OH
OH
O
O
OSO3H
H
HO3SO
H
H
O
OHC
CHO
+
4
8
00C, Solvent-free
O
O
O
O
4
0
00 000
0
Scheme 3 Synthesis of a,a ,a ,a -(2,2 -(1,4-phenylene)bis(propane-3,2,1 triyl) tetrakis(4 hydroxy-2H-
chromen-2-one) over starch–sulfuric acid
Starch–sulfuric acid was recovered byfiltration after addition of ethanol to the stirred
reaction mixture. Conventional elemental analysis showed the presence of sulfur,
indicating sulfur had not leached out. For reusability experiments, recovered catalyst
was dried in an oven at 80 °C for 1 h prior before use. The results, given in Table 3,
indicate the catalyst was reusable five times without any significant loss of activity.
To show the merit of this work, we compared our results with those from synthesis
0
of a,a -benzylidene bis(4-hydroxycoumarin) derivatives by use of previously
reported catalysts. The yield of products in the presence of starch–sulfuric acid is
comparable with that reported for other catalysts. However, reaction in the presence
of most catalysts required longer reaction times than in this work (Table 4).
To compare the efficiency of SSA with that of simple silica gel and hydrated
silica gel, the reaction was conducted in the presence these materials under the same
reaction conditions. In the presence of silica gel 75 % yield of the product was
Starch
Starch
O
O
S
O
S
O
O
O
O
H
H
H
O
O
O
O
O
O
O
H
O
R
-
H2O
O
R
H
O
O
R
R
OH
OH
OH
O
O
H
O
O
O
O
O
O
O
Scheme 4 Plausible mechanism for the catalytic activity of starch–sulfuric acid
1
23

Starch–sulfuric acid
Table 3 Reusability study of
starch–sulfuric acid catalyst in
the condensation of
a
Yield (%)
Run
1
2
3
4
5
95
92
90
87
85
benzaldehyde and
4-hydroxycoumarin under
optimum conditions
a
Isolated yield
Table 4 Comparison results obtained with starch–sulfuric acid with those obtained with other catalysts
reported in the literature
Entry Conditions
Catalyst
Time
min)
Yield
(%)
Ref.
(
1
2
3
4
5
6
H
2
O, reflux
I
2
(10 mol%),
] (4 mmol)
20–32
91–97 [6]
Solvent-free, 60–70 °C [bmim][BF
4
120–180 77–91 [12]
25–40 82–95 [13]
150–180 80–96 [15]
H
H
H
2
O, reflux
2
O, 60 °C
2
O, 80 °C
TBAB (10 mol%)
SDS (20 mol%)
Phosphotungstic acid (15 mol%)
14–25
20–30
90–98 [17]
90–98 [18]
EtOH, 25 °C
H
14[NaP
0.3 mol%)
B(HSO (0.3 mmol)
5 30 2
W O110])–SiO
(
7
8
9
1
1:1 H
2
O–EtOH, 70 °C
4
)
3
3–6
12–30
8–10
5–12
81–98 [19]
2
4
-
H
H
2
O, 80 °C
TiO
–
2
/SO
(10 mol%)
82–96 [20]
2
O, MW
84–95 [22]
0
Solvent-free, 80 °C
SSA (5 mol%)
80–95 This work
obtained after heating for 13 min. Reaction with hydrated silica gel required less
time (10 min) and 80 % of the product was obtained.
Conclusions
We have described a simple and highly efficient procedure for preparation of
biscoumarins using starch–sulfuric acid as catalyst. The attractive features of the
procedure are that it is environmentally benign because the reaction is solvent-free
and the biodegradable acid catalyst can be recycled. Mild reaction conditions and
short reaction times lead to high yields.
Acknowledgments The authors are grateful to Firoozabad University Research Council for the partial
financial support.
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123

R. Rezaei, M. R. Sheikhi
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