Molecules 2009, 14, 1915-1926; doi:10.3390/molecules14051915
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
Silica Bonded S-Sulfonic Acid: A Recyclable Catalyst for the
Synthesis of Quinoxalines at Room Temperature
Khodabakhsh Niknam *, Dariush Saberi and Maleki Mohagheghnejad
Department of Chemistry, Faculty of Sciences, Persian Gulf University, Bushehr 75169 Iran
* Author to whom correspondence should be addressed; E-mails: niknam@pgu.ac.ir or
khniknam@gmail.com
Received: 26 March 2009; in revised form: 27 April 2009 / Accepted: 30 April 2009 /
Published: 22 May 2009
Abstract: The reaction of 3-mercaptopropylsilica (MPS) and chlorosulfonic acid in
chloroform afforded silica bonded S-sulfonic acid (SBSSA), which was used as a catalyst
for the room temperature synthesis of quinoxaline derivatives from 1,2-diamino
compounds and 1,2-dicarbonyl compounds. The catalyst could be recycled and reused
several times without any loss of efficiency.
Keywords: silica bonded S-sulfonic acid; quinoxalines; 1,2-diamino compounds; 1,2dicarbonyl compounds
1. Introduction
In recent years, the search for environmentally benign chemical processes or methodologies has
received much attention, and the development of heterogeneous catalysts for fine chemical synthesis
has become a major area of research. The potential advantages of these materials over homogeneous
systems (simplified recovery and reusability, the potential for incorporation in continuous reactors and
micro reactors) could lead to novel environmentally benign chemical procedures for academia and
industry [1]. From this viewpoint, a catalytic reaction is a valuable process because the use of
stoichiometric reagents that are often toxic poses inherent limitations regarding product purification
and waste management from both economical and environmental viewpoints [2]. Application of solid
acids in organic transformation has an important role, because they have many advantages such as ease
of handling, decreased reactor and plant corrosion problems, and more environmentally safe waste
Molecules 2009, 14
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disposal procedures [3-8]. It is clear that green chemistry requires the use of environmentally benign
reagents and solvents, and it is very crucial to recover and reuse the catalyst. One way to overcome the
problem of recyclability of the traditional acid catalysts is to chemically anchor their reactive centers
onto an inorganic solid carrier with large surface area to create new organic–inorganic hybrid catalysts
[9]. In this type of solids, the reactive centers are highly mobile, similar to homogeneous catalysts and
at the same time, they have the advantage of recyclability like heterogeneous catalysts. In this view,
several types of solid sulfonic acid functionalized silica (both amorphous and ordered) have been
synthesized and applied as an alternative to traditional sulfonic acid resins and homogeneous acids in
catalyzing chemical transformations [3,10-11]. Despite the attractiveness of these reagents, to the best
of our knowledge, there is no report on the application of these catalysts in quinoxaline synthesis.
Following the line of our studies on the design and application of solid catalysts in chemical
transformations [12-16], we wish to describe herein, the preparation of silica bonded S-sulfonic acid
(SBSSA) as illustrated in Scheme 1, which was then used as catalyst for the synthesis of quinoxaline
derivatives.
Scheme 1. Preparation of silica bonded S-sulfonic acid (SBSSA).
SiO2
SiO2
SiO2
The preparation of quinoxaline and its derivatives plays an important role in organic synthesis
[17,18]. These compounds constitute an important class of benzoheterocycles displaying a broad
spectrum of biological activities, which have made them privileged structures in pharmacologically
active compounds [19,20]. They have also been used as building blocks in the synthesis of organic
semiconductors [21], rigid subunits in macrocyclic receptors or for molecular recognition [22], and as
chemically controllable switches [23]. In general, these compounds can be produced via the
condensation in organic solvents of 1,2-diamines with 1,2-dicarbonyl compounds under refluxing
conditions with 34–85% yields for 2–12 h [24]. However, most of the traditional processes suffer from
a variety of disadvantages such as pollution, high cost, poor chemical yields, requirements for long
reaction times, and tedious work-up procedures, which limit their use as environmentally benign
processes.
Some progress on the synthesis of quinoxaline derivatives has been reported in the literature, for
example: the Bi-catalyzed oxidative coupling reaction [25], a tandem oxidation process using
Pd(OAc)2 or RuCl2–(PPh3)3–TEMPO [26], and MnO2 [27], heteroannulation of nitroketene N,S-
Molecules 2009, 14
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arylaminoacetals with POCl3 [28], cyclization of α-arylimino oximes compounds under refluxing
condition in acetic anhydride [29]. Also, there are recent reports on the condensation of o-phenylene
diamines and 1,2-dicarbonyl compounds in the presence of PbO [30], Zn[(L)-proline] in HOAc [31],
copper chloride [32], montmorillonite K-10 [33], ([Hbim]BF4) [34], metal hydrogen sulfates [35], and
oxalic acid [36].
In connection with our studies on the preparation and applications of solid acids in organic
transformations [12-16], we were interested in finding a simple and efficient method for the synthesis
of quinoxaline derivatives under mild conditions using silica bonded S-sulfonic acid (SBSSA) as a
catalyst (Scheme 2).
Scheme 2. Synthesis of quinoxaline derivatives catalyzed by silica bonded S-sulfonic acid.
R1
R1
O
R2
+
H 2N
SiO2
H 2N
O
O
O Si
O
S-SO3H
R1
4
R2
N
rt/ EtOH/H2O [70/30(v/v)]
R1
3
N
5
2. Results and Discussion
To begin this work the condensation reaction between benzil and o-phenylenediamine was
employed as the model reaction to screen the best conditions (Table 1). As shown in the Table,
condensation reaction in a mixture of ethanol/water [70/30 (v/v)] gave the best results in terms of time
and yield, so we chose this solvent system for environmental acceptability.
Table 1. The influence of the solvent on the condensation reaction of o-phenylenediamine
(1 mmol) and 1,2-dibenzylketone (1 mmol) catalyzed by SBSSA (3.4 mol%, 0.1 g).
Entry
Solvent
1
2
3
4
5
6
7
8
9
10
H2O
EtOH
CHCl3
CH2Cl2
EtOH/ H2O [30/ 70 (v/v)]
EtOH/ H2O [50/ 50 (v/v)]
EtOH/ H2O [70/ 30 (v/v)]
EtOH/ H2O [70/ 30 (v/v)]
EtOH/ H2O [70/ 30 (v/v)]
EtOH/ H2O [70/ 30 (v/v)]
The amount of
catalyst (g)
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.03
0.05
0.15
a
Isolated yield.
Time (min)
240
12
60
60
40
30
5
25
20
5
Yield
(%)a
40
97
63
59
88
90
96
75
90
96
Molecules 2009, 14
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The effect of catalyst loading on the condensation reaction between benzil and o-phenylenediamine
was also studied. This results show clearly that SBSSA is an effective catalyst for this condensation.
Although this condensation could be accomplished with a lower catalyst loading 0.03 g of SBSSA,
0.1 g (3.4 mol%) of SBSSA per mmol of dicarbonyl compound was deemed optimum in terms of
reaction time and isolated yield (Table 1, entries 7-10). The scope and generality of the present method
was then further demonstrated by the condensation at room temperature of various 1,2-diaryldiketones
with o-phenylenediamine derivatives using the optimized conditions {3.4 mol%, 0.1 g of SBSSA in
EtOH/H2O [70/30 (v/v)]} and the results are presented in Table 2. The reaction proceeds very cleanly
at this temperature and was free of side products.
Table 2. Synthesis of quinoxaline derivatives catalyzed by SBSSA at room temperature.
Dicarbonyl
Time(min)
Yields(%)a
5
96
5b
8
95
5c
10
90
200
90
15
91
20
90
5g
220
93
5h
30
85
5i
35
85
Entry
compound
Diamine
Product
N
N
5a
NO2
N
N
5d
H2N
5e
Cl
H2N
N
5f
N
Molecules 2009, 14
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Table 2. Cont.
5
94
7
92
6
95
8
92
5n
15
91
5o
20
93
15
91
5j
N
Me
N
5k
O
O
5l
O
O
5m
O
O
O
O
H
5p
H
a
N
Me
N
Me
Isolated yield.
It can easily be seen that the condensation reaction proceeded smoothly in the chosen solvent
system and gave reasonable good to excellent yields, ranging from 85% to 96%. In the case of 1,2diketones, either electron-withdrawing or electron-donating substituents (R1 = OMe, Cl, Br) on the
aromatic ring gave slightly longer reaction times in comparison with R1= H (Table 2, entries 5g-i, 5o).
However, for substituents on o-phenylenediamine (R2), electron-donating substituents (such as Me)
reacted in shorter reaction times in comparison with electron-withdrawing groups (such as NO2). So,
the order of the reactivity for condensation reaction was found to be: H > CH3 > Cl > NO2 (Table 2).
The possibility of recycling the catalyst was examined. For this reason, the room temperature
reaction of o-phenylenediamine and benzyl was studied in EtOH/H2O [70/30 (v/v)] in the presence of
SBSSA. When the reaction was complete, the mixture was filtered, the residue was washed with warm
ethanol and recycled catalyst was reused in the next reaction. No appreciable loss of catalytic activity
was observed after twelve cycles (Figure 1).
Molecules 2009, 14
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Figure 1. Recyclability of SBSSA (0.1 g) in the condensation reaction of ophenylenediamine (1 mmol) with 1,2-dibenzylketone (1 mmol) at room temperature.
Reaction time = 5 min.
100
90
80
70
Yield%
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10
11
12
Run
The proposed mechanism for the condensation reaction of 1,2-diamines with 1,2-dicarbonyl
compounds in the presence of SBSSA is shown in Scheme 3.
Scheme 3. Proposed mechanism for the condensation reaction of 1,2-diamines with 1,2dicarbonyl compounds catalyzed by SBSSA.
R2
NH2
2H+
O
Ar
HO
Ar
+
NH
3
NH2
H
IV
1
O
Ar
I
O
Ar
2
R2
H
N
OH2+
Ar
N
H
Ar
OH2+
+
H
H+
II
R2
N
Ar
H
N
R2
N
Ar
Ar
-2H2O
N
H
+
NH3
R2
Ar
III
O
O Si
O
S-SO3-
O
O Si
O
S-SO3-
=
Molecules 2009, 14
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The reaction is assumed to follow the regular mechanism of acid-catalyzed condensation reactions
[26], with SBSSA acting as an acid in the protonation of the diketone and also playing a role in
promoting the dehydration to give a carbocationic intermediate as shown in Scheme 3: (i) coordination
of a 1,2-dicarbonyl onto acid sites from SBSSA, followed by (ii) the nucleophilic attack on the
carbonyl C providing intermediate I, (iii) dehydration to give a carbocation intermediate and (iv)
elimination of a proton to give the quinoxaline product.
3. Experimental Section
3.1. General
Chemicals were purchased from Fluka, Merck and Aldrich. IR spectra were run on a Shimadzu
Infra Red Spectroscopy FT-IR-8000. The 1H and 13C NMR was run on Bruker Avance (DRX
500 MHz and 400 MHz) instruments in CDCl3. Results are reported in ppm. Melting points were
recorded on a SMP1 Melting Point apparatus in open capillary tubes and are uncorrected. Reaction
progress was followed by TLC using silica gel SILG/UV 254 plates. Measurements of surface area and
pore size distribution were made using the Brunauer–Emmet–Teller (BET) method in a Quanta Sorb
machine. The products were characterized by comparison of their spectral and physical data with those
reported in literature [25-36]. 3-Mercapto-propylsilica (1, MPS) was prepared according to the
previous report [3].
3.2. Catalyst Preparation
To a magnetically stirred mixture of 3-mercaptopropylsilica (1, 5 g) in CHCl3 (20 mL),
chlorosulfonic acid (1.00 g, 9 mmol) was added dropwise at 0 ˚C during 2 h. After the addition was
complete, the mixture was stirred for 2 h until all HCl was removed from reaction vessel. The mixture
was then filtered, washed with methanol (30 mL) and dried at room temperature to give the silica
bonded functionalized sulfonic acid 2 (SBSSA) as a cream powder (5.22 g). Sulfur content of the
samples by conventional elemental analysis was 16.12%. Typically a loading at ca. 0.35 mmol/g is
obtained. On the other hand, when the washed SBSSA was placed in an aqueous NaCl solution, the
solution pH dropped virtually instantaneously to pH ≈ 1.85, as ion exchange occurred between protons
and sodium ions (proton exchange capacity: 0.34 mmol/g of SBSSA. Also, a BET surface area of
435 m2g-1 and a total pore volume 0.63 cm3g-1 were measured for the catalyst.
3.3. General procedure for the synthesis of quinoxalines
To a stirred solution of 1,2-phenylenediamine (1 mmol) and dicarbonyl compound (1 mmol) in
EtOH/H2O (70/30 v/v) (2 mL) was added SBSSA (3.4 mol%, 0.1 g) and stirred at room temperature.
The reaction was followed by TLC. After completion of the reaction, the mixture was filtered, and the
remaining was washed with warm ethanol in order to separate catalyst. Then water (20 mL) was added
to the filtrate, and was allowed to stand at room temperature for 1 h. During this time, crystals of the
Molecules 2009, 14
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pure product were formed which were collected by filtration and dried. For further purification if
needed, the products recrystallized from hot ethanol.
2,3-Diphenylquinoxaline (5a): mp 126-127 ˚C (lit.[31] 128-129 ˚C); 1H-NMR (500 MHz) δ: 7.40 (m,
6H), 7.58 (m, 4H), 7.83 (m, 2H), 8.23 (d, 2H, J= 8.0 Hz); 13C-NMR (125 MHz) δ: 128.70, 129.23,
129.65, 130.28, 130.37, 139.54, 141.68, 153.91.
6-Methyl-2,3-diphenylquinoxaline (5b): mp 116-117 ˚C (lit.[31] 117-118 ˚C); 1H-NMR (500 MHz) δ:
2.64 (s, 3H), 7.35 (m, 6H), 7.54 (m, 4H), 7.63 (d, 1H, J= 8.5 Hz), 8.03 (s, 1H), 8.14 (d, 1H, J= 8.5 Hz);
13
C-NMR (125 MHz) δ: 22.01, 127.69, 128.31, 128.52, 128.88, 128.98, 129.89, 129.94, 132.68,
138.55, 138.68, 139.48, 140.81, 141.02, 152.47, 153.07.
6,7-Dimethyl-2,3-diphenylquinoxaline (5c): mp 172-174 ˚C (lit.[37] 172 ˚C); 1H-NMR (500 MHz) δ:
2.52 (s, 6H), 7.31-7.33 (m, 6H), 7.50 (dd, 4H, J1= 7.6 Hz, J2= 1.6 Hz), 7.92 (s, 2H); 13C-NMR
(125 MHz) δ: 20.84, 128.61, 128.66, 128.93, 130.27, 139.84, 140.66, 140.92, 152.93.
6-Nitro-2,3-diphenylquinoxaline (5d): mp 192-193 ˚C (lit.[31] 193-194 ˚C); 1H-NMR (400 MHz) δ:
7.42 (m, 6H), 7.58 (m, 4H), 8.32 (d, 1H, J= 8.0 Hz), 8.57 (m, 1H), 9.10 (s, 1H); 13C-NMR (100 MHz)
δ: 124.41, 126.74, 129.54, 129.61, 130.80, 130.93, 130.99, 131.07, 131.90, 139.17, 139.23, 141.07,
144.70, 148.96, 156.80, 157.43.
6-Chloro-2,3-diphenylquinoxaline [17] (5e): mp 115-116 ˚C; 1H-NMR (500 MHz) δ: 7.36-7.44 (m,
6H), 7.56-7.57 (m, 4H), 7.74 (dd, 1H, J1= 8.9 Hz, J2= 2.3 Hz), 8.14 (d, 1H, J= 8.9 Hz), 8.22 (d, 1H,
J= 2.3 Hz); 13C-NMR (125 MHz) δ: 128.52, 128.76, 129.46, 129.54, 130.25, 130.29, 130.87, 131.37,
136.08, 139.11, 139.18, 140.15, 141.92, 154.03, 154.71.
2,3-Diphenyl-4a,5,6,7,8,8a-hexahydroquinoxaline (5f): mp 167-169 ˚C (lit.[37] 167 ˚C); 1H-NMR
(500 MHz) δ: 1.46-1.50 (m, 2H), 1.67-1.70 (m, 2H), 1.93-1.96 (m, 2H), 2.54-2.57 (m, 2H), 2.88-2.90
(m, 2H), 7.25-7.29 (m, 4H), 7.31-7.34 (m, 2H), 7.42-7.44 (m, 4H); 13C-NMR (125 MHz) δ: 25.87,
33.956, 59.95, 128.45, 128.55, 129.88, 138.23, 160.10.
2,3-Bis(4-chlorophenyl)quinoxaline (5g): mp 195-196 ˚C (lit.[31] 195-196 ˚C); 1H-NMR (500 MHz) δ:
7.39 (dt, 4H, J1= 8.5 Hz, J2= 2.0 Hz), 7.51 (dt, 4H, J1= 8.5 Hz, J2= 2.0 Hz), 7.83 (dd, 2H, J1= 6.3 Hz,
J2= 3.4 Hz), 8.20 (dd, 2H, J1= 6.3 Hz, J2= 3.4 Hz); 13C NMR (125 MHz) δ: 129.15, 129.63, 130.79,
131.61, 135.78, 137.69, 141.67, 152.34.
2,3-Bis(4-methoxyphenyl)quinoxaline (5h): mp 148-150 ˚C (lit.[36] 148-150 ˚C); 1H-NMR (400 MHz)
δ: 3.84 (s, 6H), 6.85 (d, 4H, J= 8.6 Hz), 7.47 (d, 4H, J= 8.6 Hz), 7.68 (m, 2H), 8.19 (m, 2H).
2,3-Bis(4-methoxyphenyl)-6-methylquinoxaline (5i): mp 129-131 ˚C (lit.[36] 129-131 ˚C); 1H-NMR
(400 MHz) δ: 1.58 (s, 3H), 3.88 (s, 6H), 6.65 (d, 4H, J= 8.6 Hz), 7.05 (d, 4H, J= 8.6 Hz), 7.61 (d, 1H,
J= 8.5 Hz), 7.90 (s, 1H), 8.09 (d, 1H, J= 8.5 Hz).
Molecules 2009, 14
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Dibenzo[a,c]phenazine (5j): mp 223-225 ˚C (lit.[38] 224.8-225.7 ˚C); 1H-NMR (400 MHz) δ: 7.517.66 (m, 6H), 8.10-8.13 (m, 2H), 8.33 (d, 2H, J= 8.0 Hz), 9.18 (d, 2H, J= 8.1 Hz); 13C-NMR (100
MHz) δ: 124.01, 127.35, 129.01, 130.51, 130.81, 131.41, 133.23, 143.23, 143.51.
11-Methyldibenzo[a,c]phenazine [35] (5k): mp 208-210 ˚C; 1H-NMR (400 MHz) δ: 2.46 (s, 3H), 7.467.58 (m, 5H), 7.85 (s, 1H), 7.97 (d, 1H, J= 8.0 Hz), 8.32 (d, 2H, J= 8.0 Hz), 9.13-9.16 (m, 2H);
13
C-NMR (100 MHz) δ: 23.20, 123.95, 127.15, 127.29, 128.92, 129.10, 130.01, 131.07, 131.20,
131.45, 131.49, 132.87, 133.06, 133.45, 141.41, 141.81, 142.72, 143.27, 143.29.
Acenaphtho[1,2-b]quinoxaline (5l): mp 238-240 ˚C (lit.[38] 239.5-241.3 °C); 1H-NMR (400 MHz) δ:
7.55-7.65 (m, 4H), 7.89 (d, 2H, J= 8.4 Hz), 8.00-8.20 (m, 2H), 8.21 (d, 2H, J= 6.8 Hz); 13C-NMR (100
MHz) δ: 122.96, 129.78, 130.36, 130.59, 130.74, 131.10, 132.92, 137.60, 142.39, 155.19.
9-Methylacenaphtho[1,2-b]quinoxaline [35] (5m): mp >300 ˚C; 1H-NMR (500 MHz) δ: 2.60 (s, 3H),
7.55 (d, 1H, J= 8.25 Hz), 7.79 (t, 2H, J= 7.5 Hz), 7.95 (s, 1H), 8.03-8.07 (m, 3H), 8.35 (t, 2H,
J = 6.3 Hz); 13C-NMR (125 MHz) δ: 22.19, 121.97, 122.12, 129.01, 129.03, 129.21, 129.53, 129.60,
129.75, 130.39, 131.72, 132.44, 136.68, 140.06, 140.10, 141.73, 153.76, 154.48.
9,10-Dimethylacenaphtho[1,2-b]quinoxaline (5n): mp 304-306 ˚C (lit.[36] 304-306 ˚C); 1H-NMR
(400 MHz) δ: 2.51 (s, 6H), 7.78 (m, 2H), 7.89 (s, 2H), 8.03 (m, 2H), 8.34 (m, 2H); 13C-NMR
(100 MHz) δ: 20.3, 121.5, 127.8, 128.0, 128.6, 128.9, 129.1, 139.5, 140.0, 148.5, 153.3.
2,3-Bis(4-bromophenyl)quinoxaline (5o): mp 192-194 ˚C (lit.[34] 194-195 ˚C); 1H-NMR (500 MHz) δ:
7.41 (dt, 4H, J1= 8.5 Hz, J2= 2.0 Hz), 7.51 (dt, 4H, J1= 8.5 Hz, J2= 2.0 Hz), 7.79 (dd, 2H, J1= 6.5 Hz,
J2= 3.5 Hz), 8.15 (dd, 2H, J1= 6.5 Hz, J2= 3.5 Hz); 13C-NMR (125 MHz) δ: 124.13, 129.64, 130.84,
131.86, 132.12, 138.12, 141.68, 152.34.
6,7-Dimethyl-2-phenylquinoxaline (5p): mp 127-129 ˚C (lit.[32] 128-129 ˚C); 1H-NMR (500 MHz) δ:
2.55 (s, 6H), 7.55 (t, 1H, J= 7.1 Hz), 7.59 (t, 2H, J= 7.1 Hz), 7.89 (s, 1H), 7.95 (s, 1H), 8.21 (d, 2H,
J = 7.2 Hz), 9.26 (s, 1H); 13C-NMR (125 MHz) δ: 20.75, 20.78, 127.80, 128.59, 129.10, 129.48,
130.24, 137.58, 140.50, 141.02, 141.18, 141.66, 142.83, 151.42.
4. Conclusions
In summary, this work shows that silica bonded S-sulfonic acid, prepared from commercially
available and relatively cheap starting materials by a simple transformation, efficiently catalyzes the
room temperature condensation reactions between 1,2-diamino compounds and 1,2-dicarbonyl
compounds to give quinoxaline derivatives in good to excellent yield. It could also be recovered and
reused for more than twelve reaction cycles without noticeable loss of reactivity.
Molecules 2009, 14
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Acknowledgements
We are thankful to Persian Gulf University Research Council for partial support of this work and
Dr. Mohammad Reza Shamsaddini for his helpful comments, and also, Dr Mohammad Reza Hormozi
Nezhad for running and explanation of BET.
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Sample Availability: Samples of the compounds are available from the authors.
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