Novel SO3H-functionalized ionic liquids based on benzimidazolium cation: Efficient and recyclable catalysts for one-pot synthesis of biscoumarin derivatives

Article abstract of DOI:10.1007/s10562-011-0689-9

In this article, a new group of SO₃H-functionalized ionic liquids based on benzimidazolium cation was synthesized and used as environmentally benign catalysts for the one-pot synthesis of biscoumarin derivatives. The ionic liquids showed high catalytic activities and reusabilities with good to excellent yields of the desired products. H ₀ (Hammett function) values and the minimum-energy geometries of SO₃H-functionalized ionic liquids were determined and the results revealed that the acidities and catalytic activities of ionic liquids in the synthesis of biscoumarin derivatives were related to their structures. Graphical Abstract: The simple, more efficient way for synthesis of biscoumarin derivatives has been developed. This new method abandon the traditional hazardous organic liquid and is carried out by using a one-pot reaction of 4-hydroxycoumarin with aromatic aldehydes in the presence of novel SO ₃H-functionalized ionic liquids based on benzimidazolium cation as catalysts. The ionic liquids showed high catalytic activities and reusabilities with good to excellent yields of the desired products. H ₀ (Hammett function) values and the minimum-energy geometries of SO₃H- functionalized ionic liquids were determined and the results revealed that the acidities and catalytic activities of ionic liquids in the synthesis of biscoumarin derivatives were influenced by their structures.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-011-0689-9

Catal Lett (2011) 141:1651–1658
DOI 10.1007/s10562-011-0689-9
Novel SO H-Functionalized Ionic Liquids Based
3
on Benzimidazolium Cation: Efficient and Recyclable Catalysts
for One-Pot Synthesis of Biscoumarin Derivatives
Wei Li
Liyi Dai
•
Yu Wang
•
•
Zhizhong Wang
•
Yuanyuan Wang
Received: 10 July 2011 / Accepted: 18 August 2011 / Published online: 7 September 2011
Ó Springer Science+Business Media, LLC 2011
Abstract In this article, a new group of SO H-function-
3
antibiotics, antitumor drugs, HIV protease inhibitors as
alized ionic liquids based on benzimidazolium cation was
synthesized and used as environmentally benign catalysts
for the one-pot synthesis of biscoumarin derivatives. The
ionic liquids showed high catalytic activities and reus-
abilities with good to excellent yields of the desired
well as additives in food and cosmetics [2–4]. To date,
available synthesis methods of biscoumarins include
Pechmann, Perkin, Knoevenagel, Reformatsky, and Wittig
reactions. Among these, the Knoevenagel reaction is the
most commonly applied one, in which different types of
acid catalysts such as sulfuric acid, phosphorus pentoxide,
aluminum chloride, iodine, trifluoroacetic acid were
employed [5–7]. Most of these previous studies, however,
involved harsh reaction conditions, longer reaction times,
alternative energy source like ultrasound [8], microwave
products. H (Hammett function) values and the minimum-
0
energy geometries of SO H-functionalized ionic liquids
3
were determined and the results revealed that the acidities
and catalytic activities of ionic liquids in the synthesis of
biscoumarin derivatives were related to their structures.
[
9] and corrosive reagents, which are undesirable for
Keywords SO H-functionalized ionic liquids ꢀ
industrialization purposes. Therefore, it was thought
worthwhile to develop an economic and mild condition
method to overcome those drawbacks of traditionally
employed processes. In recent years, ionic liquids (ILs)
are receiving a widespread attention as environmentally
acceptable reaction medium due to their advantageous
properties. The potential of ILs as green ‘‘designer sol-
vents and/or catalysts’’ has become a practical target for
reducing waste and hazards by avoiding traditional vola-
tile organic solvents and corrosive catalysts [10–14].
Previous studies on the preparation of coumarins in ILs
can be found in literatures. Salunkhe et. al. reported
coumarin synthesis in Lewis acidic chloroaluminate IL
via Pechmann condensation [15]. Unfortunately, chloroa-
luminate IL is moisture sensitive and very difficult to
recycle after the reaction. Kumar et. al. studied the syn-
thesis of biscoumarin derivatives in neutral IL 1-butyl-3-
3
Benzimidazolium cation ꢀ Biscoumarin derivatives ꢀ
Minimum-energy geometries ꢀ Ab initio calculation
1
Introduction
Coumarins play an important role in both natural and
synthetic organic chemistry because of their potential
applications in fragrance, pharmaceutical and agrochemi-
cal industries [1]. Coumarin derivatives especially bis-
coumarins have been widely used as anticoagulants,
W. Li
College of Pharmacy, Shandong University of Traditional
Chinese Medicine, Jinan 250014, People’s Republic of China
methyl imidazolium tetrafluoroborate ([bmim][BF ]),
4
which suffered from penalties of long reaction time and
large amount of IL cost [16]. Therefore, if ILs posses
both moisture insensitive and recyclable at the same time,
it will be a potential alternative to be selected for the
synthesis of coumarins.
Y. Wang ꢀ Z. Wang ꢀ L. Dai ꢀ Y. Wang (&)
Shanghai Key Laboratory of Green Chemistry and Chemical
Process, Department of Chemistry, East China Normal
University, Shanghai 200062, People’s Republic of China
e-mail: yywang@chem.ecnu.edu.cn
123

1
652
W. Li et al.
The introduction of specific functional groups has led
2 Experimental
to the identification of functionalized ionic liquids (FILs,
or task specific ionic liquids, TSILs): such derivatives
combine an IL-type part (in order to maintain the corre-
sponding physical properties) with an attached extra
function designed for the specific property [17–19]. The
designability of the TSILs gives us more chances to
manipulate the structures of them (with respect to the
organic cation, inorganic anion and the length of the side
chain attached to the organic cation) to achieve fitful IL
of special properties according to the given reactions.
Very interestingly, in our previous work [20], we dis-
covered that change of substituents, specially a replace-
ment of imidazolium by benzimidazolium, dramatically
influences the yield and selectivity of the products in
acetalization of aromatic aldehydes with diols. To extend
more application of TSILs, herein, we designed and
2.1 Chemicals and Instruments
All chemicals involved in the experiments were analytical
1
grade and used as received. H NMR spectra were recorded
on Brucker DRX-500 spectrometer. FT-IR measurements
were performed using KBr disc on a NEXUS 670 FT-IR
infrared spectrometer (Nicolet). UV spectra were recorded
on Agilent 8453 UV–vis spectrophotometer.
2.2 Synthesis and Characterization of ILs
3-(1-Ethylbenzimidazolium-3-yl)
propane-1-sulfonate
(ebim-PS): Under vigorous stirring, amount of 1,3-propane
sultone was dissolved in toluene, equal-mole of 1-ethyl-
benzimidazole was added dropwise into a round bottom
flask of 25 mL capacity and maintained the temperature at
0–5 °C. After the dropping was finished, the mixture was
slowly heated up to room temperature and kept for 2 h with
vigorous stirring then the reaction mixture was filtered to
get the white precipitate. The precipitate was washed with
diethyl ether three times and dried at 100 °C for 5 h, finally
giving ebim-PS as white powder.
synthesized several novel SO H-functionalized ILs based
3
on benzimidazolium cation (Fig. 1) to evidence their
potential uses as catalysts for the synthesis of biscoumarin
derivatives.
The acidity of IL is the important property relevant
to its catalytic activity for acid-catalyzed reactions. In
this work, the acidities of SO H-functionalized ILs
3
based on benzimidazolium cation were determined
using the Hammett method with UV–vis spectroscopy
and their acidity–catalytic activity relationships were
discussed at the molecular level according to their
minimum-energy geometries determined by ab initio
calculations.
1-Ethyl-3-(3-sulfopropyl)-benzimidazolium hydrogen
sulfate ([PSebim][HSO
]), 1-ethyl-3-(3-sulfopropyl)-benz-
PO ]),
4
imidazolium dihydrogen phosphate ([PSebim][H
2
4
1-ethyl-3-(3-sulfopropyl)-benzimidazolium p-toluenesul-
fonate ([PSebim][p-TSA]), 1-ethyl-3-(3-sulfopropyl)-benz-
imidazolium tetrafluoroborate ([PSebim][BF ]), 1-ethyl-3-
4
(
(
3-sulfopropyl)-benzimidazolium trifluoromethanesulfonate
[PSebim][OTf]): Under vigorous stirring, amount of ebim-
O
S
O
S
PS was dissolved in water and equal-mole of sulfuric acid,
phosphoric acid, p-toluenesulfonic acid, tetrafluoroboric
acid or trifluoromethanesulfonic acid was dropped slowly
at room temperature. After the dropping was finished, the
system was slowly heated up to 90 °C and stirred for 2 h,
then the water was removed under vacuum at 70 °C for
3 h, giving [PSebim][HSO ], [PSebim][H PO ], [PSebim]
N
+
N
OH
N
+
N
OH
OH
O
O
P
-
O
O
-
O S OH
O
O
OH
OH
[
PSebim][HSO4]
[PSebim][H PO ]
2
4
O
S
O
4
2
4
S
N
+
N
[p-TSA], [PSebim][BF ] and [PSebim][OTf] as white
4
N
+
N
OH
F
O
S
-
O
powder.
O
-
F
B F
F
-1
O
[PSebim][HSO ]: IR (KBr, m/cm ): 3421, 3121, 3021,
4
O
2951, 2924, 2854, 1620, 1567, 1457, 1378, 1347, 1215,
1
[
PSebim][p-TSA]
[PSebim][BF ]
4
1
100, 708, 626, 487. H NMR (500 MHz, D O) d 1.57
2
O
S
(t, 3H, CH ), 2.39 (m, 2H, CH CH CH ), 2.95 (t, 2H,
3 2 2 2
N
+
N
OH
CH CH ), 4.47 (m, 2H, NCH ), 4.61 (m, 2H, CH SO H),
2 3 2 2 3
F
C
F
O
S
O
O
7
.62 (m, 2H, Ph), 7.84 (m, 2H, Ph), 9.30 (s, 1H, NCHN).
Anal. calcd. for C H N S O : C, 39.32; H, 4.95; N, 7.65;
-
F
1
2 18 2 2 7
O
found: C, 39.25; H, 4.89; N, 7.72.
[
PSebim][OTf]
-1
[
PSebim][H PO ]: IR (KBr, m/cm ): 3426, 3124, 3021,
2
4
2
954, 2930, 2859, 1623, 1567, 1442, 1378, 1346, 1216,
Fig. 1 Structures of SO
dazolium cation
3
H-functionalized ILs based on benzimi-
1
1193, 1008, 1092, 706, 623, 483. H NMR (500 MHz,
1
23

Novel SO
3
H-Functionalized Ionic Liquids
1653
D O) d 1.57 (t, 3H, CH ), 2.38 (m, 2H, CH CH CH ), 2.95
reused in the next run after heat treatment under vacuum at
2
3
2
2
2
(
t, 2H, CH CH ), 4.47 (m, 2H, NCH ), 4.61 (m, 2H,
2
70 °C for 2 h.
3
2
CH SO H), 7.62 (m, 2H, Ph), 7.83 (m, 2H, Ph), 9.27 (s,
2
3
1
H, NCHN). Anal. calcd. for C H N O PS (366.33): C,
2 19 2 7
2.5 Computational Methods
1
3
9.34; H, 5.23; N, 7.65; found: C, 39.28; H, 5.27; N, 7.71.
-
1
[
PSebim][p-TSA]: IR (KBr, m/cm ): 3418, 3120, 3025,
953, 2927, 2857, 1624, 1565, 1450, 1372, 1342, 1216,
The minimum-energy geometries of SO H-functionalized
3
?
ILs and isolated [PSebim] cation were determined by
2
1
3
1
092, 706, 621, 488. H NMR (500 MHz, D O) d 1.56 (t,
performing ab initio geometry optimizations at the RHF/
6-31(d, p) level using the Gaussian 03 program [22]. A
vibrational analysis was performed to ensure the absence of
negative frequencies and verify the existence of a true
minimum.
2
H, CH3), 2.02 (s, 3H, CH -Ph), 2.36 (m, 2H,
3
CH CH CH ), 2.92 (t, 2H, CH CH ), 4.46 (m, 2H, NCH ),
2
2
2
2
2
3
4
.60 (m, 2H, CH SO H), 7.45-7.61 (m, 4H, Ph), 7.76-7.83
2 3
(
m, 4H, Ph), 9.25 (s, 1H, NCHN). Anal. calcd. for
C H N O S (426.52): C, 50.69; H, 5.20; N, 6.57; found:
1
8 22 2 6 2
C, 50.65; H, 5.24; N, 6.62.
PSebim][BF ]: IR (KBr, m/cm ): 3439, 3126, 3017,
-
1
[
4
2
8
3
995, 2929, 2862, 1630, 1413, 1378, 1342, 1219, 1087,
1
3 Results and Discussion
35, 708, 617, 491. H NMR (500 MHz, D O) d1.54 (t,
2
0
H, CH ), 2.37 (m, 2H, CH CH CH ), 2.92 (t, 2H,
3
3.1 Synthesis of 3,3 -Benzylidene-bis-(4-
2
2
2
CH CH ), 4.45 (m, 2H, NCH ), 4.62 (m, 2H, CH SO H),
3
Hydroxycoumarin) Catalyzed by Various ILs
2
3
2
2
7
.62 (m, 2H, Ph), 7.84 (m, 2H, Ph), 9.27 (s, 1H, NCHN).
Anal. calcd. for C H N SO BF : C, 40.47; H, 4.81; N,
Kumar et. al. reported the synthesis of biscoumarins in
large amount of neutral IL [bmim][BF ] (Table 1, entry 1)
4
12
17
2
3
4
7
.87; found: C, 40.41; H, 4.79; N, 7.84.
-
1
[
PSebim][OTf]: IR (KBr, m/cm ): 3410, 3121, 3018,
952, 2926, 2856, 1624, 1567, 1453, 1380, 1343, 1217,
[16]. For comparison, we investigated the synthesis of
0
2
1
1
3,3 -benzylidene-bis-(4-hydroxycoumarin) with different
1
145, 1083, 625, 706, 487. H NMR (500 MHz, D O) d
ILs. The results demonstrate that the catalytic performances
2
.54 (t, 3H, CH ), 2.36 (m, 2H, CH CH CH ), 2.92 (t, 2H,
3
of SO H-functionalized ILs based on benzimidazolium
3
2
2
2
CH CH ), 4.44 (m, 2H, NCH ), 4.61 (m, 2H, CH SO H),
3
cation (Table 1, entries 3–7) are much better than that of
neutral IL (Table 1, entry 2) under the same experimental
2
3
2
2
7.62 (m, 2H, Ph), 7.84 (m, 2H, Ph), 9.31 (s, 1H, NCHN).
Anal. calcd. for C H F N O S (418.43): C, 37.31; H,
conditions. On one hand, SO H-functionalized IL, as it
3
1
3
17
3
2
6
2
4
2
.10; N, 6.70; found: C, 37.27; H, 4.14; N, 6.75.
behaves as an acidic catalyst, is capable of increasing the
reactivity of benzaldehyde; on the other hand, the substrates
have good solubility in benzimidazolium IL than imidazo-
lium IL due to the similar structure, which facilitate the shift
of the equilibrium to the aldo condensation direction. Among
.3 UV–Vis Acidity Evaluation
Dichloromethane solutions of the ILs were prepared with
all SO H-functionalized ILs, we found that [PSebim][OTf]
3
0
dried CH Cl and the ILs (dried under vacuum at 70 °C for
is the best one to catalyze the synthesis of 3,3 -benzylidene-
2
2
2
h). All spectra were obtained with an Agilent B453
bis-(4-hydroxycoumarin) with 95% yield.
spectrophotometer.
Table 1 Synthesis of biscoumarins catalyzed by various ILs
c
Entry
ILs
Yield (%)
2
.4 General Procedure for Synthesis of Biscoumarin
Derivatives
a
1
2
3
4
5
6
7
a
[bmim][BF
[bmim][BF
4
]
]
84
63
92
81
84
89
95
b
4
b
[PSebim][HSO
[PSebim][H PO
[PSebim][p-TSA]
4
]
A mixture of aromatic aldehyde (20 mmol), 4-hydroxy-
coumarin (40 mmol) and IL (10 mol%) was added into a
round bottom flask and stirred at 70 °C for the appropriate
time mentioned in Tables 1 and 4. The reaction was
monitored by TLC, after completion of the reaction the
mixture was cooled down to room temperature. The crude
solid product was extracted with diethyl ether, and then
washed with water and dried over anhydrous MgSO4,
respectively. The IL phase, which contained some water
produced in the reaction, could be easily recovered and
b
2
4
]
b
b
4
[PSebim][BF ]
b
[PSebim][OTf]
Benzaldehyde (20 mmol), 4-hydroxycoumarin (40 mmol),
[bmim][BF ] (80 mmol), T = 70 °C, t = 2.0 h [16]
Benzaldehyde (20 mmol), 4-hydroxycoumarin (40 mmol), IL
(10 mol%), T = 70 °C, t = 2.0 h
4
b
c
Yield refers to isolated pure products
123

1
654
W. Li et al.
3
.2 Determination of H Values
0
observed in the synthesis of biscoumarin derivatives. The
acidities of the ILs depended both on the characteristics of
the cations and anions. When the cations of the ILs were the
same, the acidities of the ILs significantly depended on
anions. And the acidities of the ILs with the conjugate base of
of SO H-Functionalized ILs
3
The acidic scale of these SO H-functionalized ILs was
3
measured on an Agilent 8453 UV–vis spectrophotometer
with a basic indicator according to the previously literature
-
[OTf] as the anion reached the highest (Table 2, entry 6).
[
21]. With increasing the acidic scale of the acidic ILs, the
absorbance of the unprotonated form of the basic indicator
decreased, whereas, the protonated form of the indicator
could not be observed because of its small molar absorp-
3
.3 The Molecular Geometries of TSILs
The minimum-energy geometries of TSILs were deter-
mined by performing ab initio geometry optimizations at
the RHF/6-31(d, p) level. The fully optimized geometries of
these ILs are presented in Fig. 3; Table 3. It is obvious that
?
tivity and its location, so the [I]/[IH ] (I represents indi-
cator) ratio could be determined from the differences of
measured absorbance after the addition of Brønsted acidic
ILs and Hammett function, H , and could be calculated by
0
a strong hydrogen bond network is presented in the SO H-
3
using Eq. 1. This value could be regarded as the relative
functionalized ILs. Taking IL [PSebim][HSO ] for exam-
4
acidity of the ILs.
ple, the anion is located on the side of the propyl sulfonic
acid group, and interacts with the propyl alkyl sulfonic acid
group and benzimidazolium ring in the form of C–HꢀꢀꢀO or
O–HꢀꢀꢀO(F) type (hydrogen bonds). There are six hydrogen
þ
H0 ¼ pKðIÞ þ logð½Iꢁ=½IH ꢁÞ
ð1Þ
aq
Under the same concentration of 4-nitroanline (10 mg/L,
pk = 0.99) and ILs (5 mmol/L) in dichloromethane, we
a
bonds in IL [PSebim][HSO ], which are shorter than the
4
determined the H values of these SO H-functionalized
3
0
˚
Van der Waals distance of 2.67 A [23]. These include the
ILs. The maximal absorbance of the unprotonated form of
the indicator observed in CH Cl was 350 nm. Up on the
˚
hydrogen bonds C –H ꢀꢀꢀO (bond distance is 2.2421 A)
5
9
39
2
2
between the anion and benzimidazolium ring; the hydrogen
addition of IL, the absorbance of the unprotonated form of
the basic indicator decreased.
bonds C –H ꢀꢀꢀO , C –H ꢀꢀꢀO (bond distances are
2
3
24
39
28
29
39
˚
.4129, 2.2738 A) between the anion and alkyl sulfonic
2
As shown in Fig. 2, the absorbance of the unprotonated
form of the indicator on five TSILs decreased as following
order: [PSebim][H PO ] [ [PSebim][p-TSA] [ [PSebim]
acid chain; the hydrogen bond C –H ꢀꢀꢀO (bond dis-
1
5
17
32
˚
tance is 2.3301 A) between benzimidazolium ring and alkyl
2
4
sulfonic acid chain; the hydrogen bond C –H ꢀꢀꢀO (bond
2
8
29
34
[
BF ] [ [PSebim][HSO ] [ [PSebim][OTf]. After the cal-
4
4
˚
distance is 2.6615 A) inside the alkyl sulfonic acid chain;
the most importantly, the strongest interaction between the
anion and the sulfonic acid proton was O –H ꢀꢀꢀO (bond
culation, we obtained the acidity order of the nine ILs
with the following H values (Table 2): [PSebim][OTf]
0
3
4
35
40
(
-0.09) [[PSebim][HSO ] (0.05) [[PSebim][BF ] (0.19) [
4
4
˚
distance is 1.6459 A). The other four SO H-functionalized
3
[
PSebim][p-TSA] (0.42) [ [PSebim][H PO ] (0.64), which
2
4
ILs also exhibit similar strong interactions between the
anions and the sulfonic acid groups, which make the H–O
bond of the sulfonic acid group to be lengthened with dif-
ferent levels compared with that of in the isolated cation.
When the cations of the ILs are the same, the H–O bond
is consistent with the sequence of catalytic activity
3.0
2.5
2.0
1.5
1.0
0.5
0.0
distance of SO H-functionalized ILs decreased as following
3
a
a: blank
b: [PSebim][H PO]
2
4
c: [PSebim][pTSA]
d: [PSebim][BF4]
Table 2 Calculation and comparison of H0 values of different
SO H-functionalized ILs in CH Cl
e: [PSebim][HSO4]
3
2
2
f: [PSebim][OTf]
?
[IH ](%)
Entry
ILs
A
max
[I](%)
H
0
b
c
1
2
3
4
5
6
–
2.546
0.266
0.793
0.538
0.336
0.193
100.0
10.4
31.1
21.1
13.2
7.6
0
–
d
e
[PSebim][HSO
4
]
89.6
68.9
78.9
82.6
92.4
0.05
0.64
0.42
0.19
f
[PSebim][H
2
PO
4
]
[PSebim][p-TSA]
[PSebim][BF
[PSebim][OTf]
300
400
500
600
700
800
4
]
Wavelength (nm)
-0.09
Fig. 2 Absorbance spectra of 4-nitroaniline for various SO
functionalized ILs in CH Cl
3
H-
Indicator 4-nitroanline
2
2
1
23

Novel SO
3
H-Functionalized Ionic Liquids
1655
Fig. 3 Molecular structures of
?
isolated [PSebim] cation and
?
ILs based on [PSebim] cation
Table 3 The geometry parameters of isolated cations and SO
3
H-functionalized ILs
PO [PSebim][p-TSA]
35–O34 = 0.9936
Cation
[PSebim][HSO
4
]
[PSebim][H
2
4
]
[PSebim][BF
4
]
[PSebim][OTf]
H–O bond
distance (A)
35
H –
O34 = 0.9491
H35–O34 = 0.9784
H
H
H
35–O34 = 0.9903
35ꢀꢀꢀO40 = 1.4478
H35–O34 = 0.9844
H
35–O34 = 0.9702
35ꢀꢀꢀO40 = 1.6946
˚
Distance
H35ꢀꢀꢀO40 = 1.6459
35ꢀꢀꢀO40 = 1.2937
H
H
35ꢀꢀꢀF36 = 1.5362
35ꢀꢀꢀF40 = 2.5300
H
between the
anion and
sulfonic acid
H
˚
group (A)
Other
H
H
H
H
H
17ꢀꢀꢀO32 = 2.3301
24ꢀꢀꢀO39 = 2.4129
29ꢀꢀꢀO34 = 2.6615
29ꢀꢀꢀO39 = 2.2738
H
H
H
H
36ꢀꢀꢀO32 = 1.3757
H
H
H
H
30ꢀꢀꢀO36 = 2.1878
H
H
H
H
30ꢀꢀꢀF40 = 2.1851
H
H
H
H
30ꢀꢀꢀO39 = 1.7740
hydrogen
bonds in ILs
˚
9
ꢀꢀꢀO41 = 1.8727
22ꢀꢀꢀO36 = 2.0690
9
ꢀꢀꢀF40 = 2.0386
22ꢀꢀꢀO39 = 2.4436
24ꢀꢀꢀO41 = 2.1107
25ꢀꢀꢀO32 = 2.2638
9
ꢀꢀꢀO36 = 2.2778
22ꢀꢀꢀF40 = 2.1424
10ꢀꢀꢀF39 = 2.4253
9
ꢀꢀꢀO39 = 2.4700
(
A)
10ꢀꢀꢀO39 = 2.4514
10ꢀꢀꢀO36 = 2.2382
9
ꢀꢀꢀO39 = 2.2421
order: [PSebim][H PO ] (H –O = 0.9936) [ [PSebim]
[PSebim][OTf] (H –O = 0.9702), which is contrary to
35 34
2
4
35
34
[
p-TSA] (H –O = 0.9903) [ [PSebim][BF4] (H –
their acidity sequence. The formation of hydrogen bond
was a process of sharing lone electron pair of O or F atom
3
5
34
35
O = 0.9844) [ [PSebim][HSO ] (H –O = 0.9784) [
3
4
4
35
34
123

1
656
W. Li et al.
Table 4 Synthesis of various biscoumarin derivatives catalyzed by IL [PSebim][OTf]
a
Entry
Substrate
Product
Time (h)
2.0
Yield (%)
OH
OH
O
CHO
1
95
O
O O
Cl
CHO
OH
O
OH
O
2
3
4
5
2.0
2.0
2.0
2.0
96
94
95
94
Cl
O O
Cl
CHO
OH
O
OH
Cl
O
O O
Br
CHO
OH
O
OH
O
Br
O O
Br
CHO
OH
O
OH
Br
O
O O
NO2
CHO
OH
O
OH
O
6
2.0
96
2
O N
O O
CH3
CHO
OH
O
OH
O
7
2.5
93
H C
3
O O
OCH3
CHO
OH
O
OH
O
8
2.5
93
H CO
3
O O
1
23

Novel SO
3
H-Functionalized Ionic Liquids
1657
Table 4 continued
a
Entry
Substrate
Product
Time (h)
Yield (%)
CH =CH
2
2
CHO
OH
O
OH
O
9
3.0
92
H C=H C
2
2
O O
a
Yield refers to isolated pure products: aromatic aldehyde (20 mmol), 4-hydroxycoumarin (40 mmol), IL (10 mol%), T = 70 °C
with H atom. When the hydrogen bond was strong, the lone
Table 5 Recycling of the IL [PSebim][OTf]
electron pair was shared in deep degree, and cause an
a
a
Recycling times
Yield (%) /H
was removed
2
O
2
Yield (%) /H O
increase in the electron density and stability of H atom. As a
result, the acidity of H atom decreased.
was accumulated
1
2
3
4
5
6
a
95
93
93
92
92
91
92
87
84
82
81
78
3
.4 The Generality of SO H-Functionalized ILs
3
Towards the Synthesis of Various Biscoumarin
Derivatives
The generality of the SO H-functionalized ILs to the var-
3
ious aromatic aldehydes was investigated in Table 4, in
Yield refers to isolated pure products: Benzaldehyde (20 mmol),
(40 mmol),
which different functional groups such as Cl, Br, NO , Me,
2
4-hydroxycoumarin
T = 70 °C, t = 2.0 h
[PSebim][OTf]
(10 mol%),
OMe were selected to react with 4-hydroxycoumarin in the
presence of 10 mol% of IL [PSebim][OTf]. The reactivities
of aromatic aldehydes with electron donating (entries 7–9,
Table 4) as well as with electron withdrawing groups
was removed at 70 °C upon completion in each run,
the activity of [PSebim][OTf] maintained even better.
After six runs, no obvious loss of catalytic activity was
observed as shown in Table 5. This indicated that the IL
(
entries 2–6, Table 4) are almost the same with bis-
coumarins in good to excellent yields. Effect of substitute
position (entries 2–5, Table 4) on the reactivity of aryl ring
is not significant.
[PSebim][OTf] was a high efficient and recyclable
catalyst for aldo condensation of 4-hydroxycoumarin with
4
3
-nitrobenzaldehyde.
3
.5 Reusability of [PSebim][OTf] as Catalyst
0
.6 Plausible Mechanism for the Catalytic Activity
for the Synthesis of 3,3 -Benzylidene-Bis-(4-
of SO H-Functionalized ILs
3
Hydroxycoumarin)
The plausible reaction mechanism for the synthesis of
biscoumarin derivatives is expected to follow the gen-
eral scheme shown in Fig. 4. Aldo condensation of
Compared with traditional acid catalysts, easy recycling is
an attractive property of the TSILs. Consequently, we
investigated the catalytic activity of recycled IL [PSebim]
0
4-hydroxycoumarin with aromatic aldehyde under mild
[
OTf] in the synthesis of 3,3 -benzylidene-bis-(4-hydroxy-
condition can proceed by the acid-catalyzed mechanism.
coumarin). Upon completion in each run, the reaction
solution was cooled to room temperature. The substrates
and the products were extracted with diethyl ether. The left
SO H-functionalized ILs is capable of binding with the
3
carbonyl oxygen increasing the reactivities of parent car-
bonyl as it behaves as a acidic catalyst. As shown in Fig. 4,
first IL activates carbonyl group of aromatic aldehyde to
give IL-aldehyde complex I and thus increases the elec-
trophilicity carbonyl carbon of aldehyde. Nuleophilic
addition of 4-hydroxycoumarin to I to give II and followed
IL phase contained some H O which make it recovery
2
easily and can be used directly without further treatment. It
was found that IL [PSebim][OTf] with some H O can be
2
reused for at least two subsequent reactions without sig-
nificant loss of activity, however, for the next four runs, it
showed lower efficiency in terms of products yields
by loss of H O from II to afford III, which is further activated
2
by acidic IL. Another molecule of 4-hydroxycoumarin is
(
Table 5). On the other hand, if the formed byproduct H O
2
123

1
658
W. Li et al.
N
N
SO H
Academy of Sciences, Shanghai, China for providing the Gaussian 03
program and helpful guidance. And we are thankful to the National
Natural Science Foundation of China (No. 21003049, 21073064).
3
Anion
OH
O
O
C
CHO
N
N
SO H
H
O
3
+
(
IL)
Anion
R
R
References
(
I )
1
. Hinman JW, Hoeksema H, Caron EL, Jackson WG (1956) J Am
Chem Soc 78:1072
N
N
SO H
N
N
SO H
3
3
Anion
OH
Anion
2. Manolov L, Raleva S, Genova P, Savov A, Froloshka L,
Dundarova D, Argirova R (2006) Bioinorg Chem Appl: 1
OH
O
O
O
O
O
3
. Manolov I, Maichle-Moessmer C, Nicolova I, Danchev N (2006)
Arch Pharm (Weinheim) 339:319
-
H O
O
2
O
O
4. Khan KM, Iqbal S, Lodhi MA, Maharvi GM, Ullah Z, Choudhary
MI, Rahman AU, Perveen S (2004) Bioorg Med Chem 12:1963
R
R
(
II )
( III )
5
. Brufola G, Fringuelli F, Piermatti O, Pizzo F (1996) Heterocycles
3:1257
4
N
SO H
3
6. Yadav JS, Reddy BVS, Basak AK, Visali B, Narsaiah AV,
Nagaiah K (2004) Eur J Org Chem: 546–551
R
Anion
R
N
7
8
. Narsaiah AV, Nagaiah K (2003) Synth Commun 21:3825
. Zhang XR, Chao W, Chuai YT, Ma Y, Hao R, Zou DC, Wei YG,
Wang Y (2006) Org Lett 8:2563
OH
O
O
O
OH
OH
O
H
-
IL
O
9. Radi M, Bernardo V, Bechi B, Castagnolo D, Pagano M, Botta M
2009) Tetrahedron Lett 47:6572
0. Welton T (1999) Chem Rev 99:2071
1. Parvulescu VI, Hardacre C (2007) Chem Rev 107:2615
2. Wasserscheid P (2006) Nature 439:797
3. Vieira RC, Falvey DE (2008) J Am Chem Soc 130:1552
4. Saha D, Saha A, Ranu BC (2009) Green Chem 11:733
5. Potdar MK, Mohile SS, Salunkhe MM (2001) Tetrahedron Lett
42:9285
OO
OO
(
(
IV )
1
1
1
1
1
1
3
Fig. 4 Plausible mechanism for the catalytic activity of SO H-
functionalized IL
added to III to give IV and IL, which can catalyze reaction
in a catalytic manner.
1
1
1
6. Khurana JM, Kumar S (2010) Monatsh Chem 141:561
7. Li XZ, Lin Q, Ma L (2010) Ultrason Sonochem 5:752
8. Akbari J, Heydari A, Kalhor HR, Kohan SA (2010) J Comb
Chem 1:137
4
Conclusion
1
9. Olivier JH, Camerel F, Selb J, Retailleau P, Ziessel R (2009)
Chem Commun 9:1133
In conclusion, the present procedure using easily available
2
2
0. Wang YY, Jiang D, Dai LY (2008) Catal Commun 9:2475
1. Wang YY, Gong XX, Wang ZZ, Dai LY (2010) J Mol Catal A
novel SO H-functionalized ILs based on benzimidazolium
3
cation provides a very simple and efficient methodology
for the synthesis of biscoumarin derivatives by a one-pot
condensation of 4-hydroxycoumarin with aromatic alde-
3
22:7
22. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Montgomery JA, Vreven JrT, Kudin KN, Burant
JC, Millam JM, Iyengar SS, Tomasi J, Barone V, Mennucci B,
Cossi M, Scalmani G, Rega N, Petersson GA, Nakatsuji H, Hada
M, Ehara M, Toyota K, Fukuda R, Hasegawa J, Ishida M, Nak-
ajima T, Honda Y, Kitao O, Nakai H, Klene M, Li X, Knox J E,
Hratchian H P, Cross J B, Adamo C, Jaramillo J, Gomperts R,
Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C,
Ochterski JW, Ayala PY, Morokuma K, Voth GA, Salvador P,
Dannenberg JJ, Zakrzewski VG, Dapprich S, Daniels AD, Strain
MC, Farkas O, Malick DK, Rabuck AD, Raghavachari K,
Foresman JB, Ortiz JV, Cui Q, Baboul AG, Clifford S, Cio-
slowski J, Stefanov BB, Liu G, Liashenko A, Piskorz P, Ko-
maromi I, Martin RL, Fox DJ, Keith T, Al-Laham MA, Peng CY,
Nanayakkara A, Challacombe M, Gill PM W, Johnson B, Chen
W, Wong MW, Gonzalez C, Pople JA, Gaussian 03, Revision
C.02, Gaussian, Inc., Wallingford CT, 2004
hydes. Furthermore, this series of SO H-functionalized ILs
3
have higher catalytic activities under mild reaction condi-
tion without the addition of any other organic solvents or
catalysts. The results got from the experiments and the
minimum-energy geometries of SO H-functionalized ILs
3
revealed that the acidities and catalytic activities of ILs in
aldo condensation were related to their structures. The IL
with shorter H–O bond distance has stronger acidity and
higher catalytic activity in the synthesis of biscoumarin
derivatives.
Acknowledgments The authors gratefully acknowledge Prof. Hua
Liang Jiang at Shanghai Institute of Materia Medica, Chinese
¨
23. Meng Z, Dolle A, Caper WR (2002) J Mol Strut (Theo) 585: 119
1
23