Highly efficient synthesis of substituted benzenes in the presence of B(HSO4)3 as a new and reusable catalyst under solvent-free conditions

Article abstract of DOI:10.1080/00397911.2012.696300

A highly efficient and simple procedure for the synthesis of substituted benzenes is described. A broad range of ketones (alkyl-aryl ketones and cyclic ketones) were condensed via a cyclotrimerization reaction in the presence of catalytic amounts of boron sulfonic acid [B(HSO₄)₃], a new, highly efficient, and reusable catalyst, under solvent-free conditions. All reactions completed in short times without formation of any by-products. The catalyst was recovered and reused successfully for 15 cycles of the reaction. Copyright

Full text of DOI:10.1080/00397911.2012.696300

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Highly Efficient Synthesis of Substituted
Benzenes in the Presence of B(HSO4)3
as a New and Reusable Catalyst Under
Solvent-Free Conditions
Hamid Reza Safaei a , Mansooreh Davoodi a & Mohsen Shekouhy a
a Department of Applied Chemistry , Faculty of Science, Shiraz
Branch, Islamic Azad University , Shiraz , Iran
Accepted author version posted online: 05 Mar 2013.Published
online: 03 Jun 2013.
To cite this article: Hamid Reza Safaei , Mansooreh Davoodi & Mohsen Shekouhy (2013)
Highly Efficient Synthesis of Substituted Benzenes in the Presence of B(HSO4)3 as a New and
Reusable Catalyst Under Solvent-Free Conditions, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 43:16, 2178-2190, DOI:
10.1080/00397911.2012.696300
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Synthetic Communications1, 43: 2178–2190, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.696300
HIGHLY EFFICIENT SYNTHESIS OF SUBSTITUTED
BENZENES IN THE PRESENCE OF B(HSO4)3 AS A NEW
AND REUSABLE CATALYST UNDER SOLVENT-FREE
CONDITIONS
Hamid Reza Safaei, Mansooreh Davoodi, and
Mohsen Shekouhy
Department of Applied Chemistry, Faculty of Science, Shiraz Branch, Islamic
Azad University, Shiraz, Iran
GRAPHICAL ABSTRACT
Abstract A highly efficient and simple procedure for the synthesis of substituted benzenes is
described. A broad range of ketones (alkyl–aryl ketones and cyclic ketones) were con-
densed via a cyclotrimerization reaction in the presence of catalytic amounts of boron sul-
fonic acid [B(HSO4)3], a new, highly efficient, and reusable catalyst, under solvent-free
conditions. All reactions completed in short times without formation of any by-products.
The catalyst was recovered and reused successfully for 15 cycles of the reaction.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords B(HSO4)3; cyclotrimerization reaction; self-condensation reactions; solid acid;
solvent-free reaction; substituted benzenes
INTRODUCTION
It is well known that acid catalysts are essential for various chemical reactions
in industrial hydrocarbon chemistry. Traditional liquid acids (such as sulfuric acid or
hydrochloric acid) are highly efficient catalysts but unfortunately are nonrecyclable,
Received March 7, 2012.
Address correspondence to Hamid Reza Safaei, Department of Applied Chemistry,
Faculty of Science, Shiraz Branch, Islamic Azad University, P. O. Box 71993-5, Shiraz, Iran. E-mail:
hrs@iaushiraz.net
2178

SYNTHESIS OF SUBSTITUTED BENZENES
2179
and it is costly and difficult to separate them from the homogeneous reaction mix-
ture, which makes them unsuitable reagents in fine chemical synthesis. The principles
of ‘‘green chemistry’’ emphasize the usage of environmentally benign catalysts and
chemicals to have minimal unwholesome effects on the environment.[1] One of the
most suitable strategies to design environmentally benign catalytic systems is the
preparation of solid acid catalysts with strong acidic sites. It is obvious from the
literature that the application of solid acid catalysts has become more prominent
in organic transformations because of their unique properties and advantages[2] over
liquid acids,[3] such as facile handling, no need for workup procedures, easy separ-
ation of products, and easy recovery and reusability of the catalyst. Up to now, sev-
eral solid acids have been reported such as high silica zeolites,[4] ion exchange
resins,[5] and mesoporous materials.[6] Boron sulfonic acid is a new solid acid catalyst
that was prepared by Kiasat et al. for the regioselective conversion of epoxides to
thiocyanohydrins under solvent-free conditions.[7]
In parallel with the application of solid acids, solvent-free reactions promise to
be an essential facet of green chemistry. Solvent-free reactions make it possible to
reduce the consumption of hazardous organic solvents and use scaled-down reaction
vessels. Moreover, the use of a catalyst for solvent-free reactions can achieve energy
savings.
Polyarylated benzene propellers have attracted a great deal of enthusiasm in
leading-edge carbon nanotechnology for developing new efficient molecular rotors
and new electroluminescent materials for flat-panel displays.[8] These organic com-
pounds have unique chemical, photophysical, and optical properties that make them
useful as building blocks for material and biological sciences.[9] Moreover, these are
important for understanding the concepts of aromaticity, chemical reactivity, and
reaction kinetics.[10] There are many reports about the application of p-conjugated
polyaromatics, dendrimers,[11] and their derivatives in liquid crystals,[12] laser
dyes,[13] electronic and opto-electronic devices,[14] and molecular wires.[8c,15] In
addition, biaryls and teraryls are often present as subunits in numerous biologically
active natural products and pharmaceuticals.[9,16] Beside these, 1,2,3-triarylbenzenes
are potential compounds for fabricating thermally stable blue organic light-emitting
diodes.[17] Because of these widespread applications, numerous synthetic methodol-
ogies to highly functionalized aromatic p systems have been reported in the literature
such as cyclotrimerization of alkynes in the presence of organometallic reagents[18]
and palladium- or nickel-catalyzed multifold sequential Suzuki–Miyaura reaction
between oligohaloarenes with organometallic partners.[19] Unfortunately most of
these procedures are often complicated by harsh reaction conditions,[20] the lack of
regioselectivity, formation of undesirable 1,2,4-trisubstituted by-products, and
the requirement of expensive or specialized organometal catalysts.[21] A simple
method for the preparation of these compounds is cyclotrimerization of carbonyl
compounds possessing a reactive a-methylene group and a variety of acid catalysts
such as hydrochloric acid,[22] sodium or potassium pyrosulfate in H2SO4,[23]
aniline hydrochloride,[24] titanium tetrachloride (TiCl4),[25] SiCl4,[26] SmCl3,[27]
CuCl2,[28] Cp2ZrCl2,[29] Cp2Zr(OSO2C8F17)2,[30] p-toluenesulfinic acid–SnCl4,[31]
TsOH.H2O,[32] Bi(OTf)3 ꢀ 4H2O,[33] ethylenediamine, trifluroacetic acid (TFA),[34]
BMImPF6,[35] and Amberlyst 15[36] have been employed for the modification of this
reaction. Most of these catalytic systems suffer from some drawbacks such as very

2180
H. R. SAFAEI, M. DAVOODI, AND M. SHEKOUHY
Scheme 1. Solvent-free synthesis of highly substituted benzenes via cyclotrimerization of carbonyl com-
pounds possessing a reactive a-methylene group in the presence of B(HSO4)3 as a new and reusable solid
acid catalyst.
long reaction times,[25,26,28] poor yields,[22,25,29] application of toxic reagents and
catalyst,[22,25,27] application of hazardous organic solvents,[22–26,28–30,33,34] lack of
generality,[22,25,26,28,31] tedious workup, generation of by-products,[22,23,28,30,35] and
nonrecyclability of the catalyst.[22,23,25,26,31,32]
So, based on these facts and as a part of our research program to develop selec-
tive and efficient methods in organic synthesis, we report here the application of
B(HSO4)3 as a new, highly efficient, and reusable solid acid catalyst for the synthesis
of highly substituted benzenes under solvent-free conditions (Scheme 1).
RESULTS AND DISCUSSION
At first B(HSO4)3 was prepared via the reaction of chlorosulfonic acid and
boric acid.[7] The catalyst structure was demonstrated with infrared (IR) spec-
troscopy. Figure 1 presents the Fourier transform (FT)–IR spectra of starting boric
acid [B(OH)3] and B(HSO4)3. As seen in Fig. 1 the spectrum of B(HSO4)3 is different
from that of boric acid. In the spectrum of B(HSO4)3 absorption bands at 1400,
1200, and 650 cmꢁ1 correspond to t S O asymmetric stretching, t S O symmetric
stretching, and t S-O stretching respectively. Moreover a broad band from 3400
to 2700 cmꢁ1 corresponds to acidic O-H stretching.
=
=
In the next step, the cyclotrimerization reaction of acetophenone (1) (18 mmol)
was selected as a model reaction (Scheme 2).
To evaluate the efficacy of B(HSO4)3 in the preparation of highly substituted
benzenes, we examined the model reaction in the presence of B(HSO4)3 at 100 ꢂC
under solvent-free conditions. The optimized reaction conditions were screened by
conducting the reaction using different amounts of the catalyst in various solvents
and in the presence of different catalytic systems. The procedure was optimized by
varying reaction time, molar ratio of catalyst, temperature, and solvent for this reac-
tion (Table 1).
As is clear from Table 1, the best results were obtained in the presence of
B(HSO4)3 (20 mol%) under solvent-free conditions at 100 ꢂC. The reaction was also
checked without the catalyst, in which case the reaction did not proceed even after
48 h. These observations established the crucial role of B(HSO4)3 for the improve-
ment of the reaction time and the product yield.
In the next step, scope and efficiency of the catalyst was explored under the
optimized reaction conditions. For this purpose, a broad range of structurally

SYNTHESIS OF SUBSTITUTED BENZENES
2181
Figure 1. IR spectra of B(HSO4)3 (a) and B(OH)3(b).
diverse ketones (methyl–aryl ketones as well as cyclic ketones) were condensed to
furnish the corresponding products in good yield, and the obtained results are dis-
played in Table 2.
As shown in Table 2, electron-withdrawing substituents in the aromatic ring of
aryl-alkyl ketones accelerated the reaction rate (entry 12), whereas electron-releasing
substituents reduced the reaction rate (entry 11) but the nature of substituents does
not affect the yield of the reaction.
To assess the capability of the present method with respect to the reported
methods for the preparation of the titled compounds, the synthesis of compound
2a was compared with the reported methods (Table 3). As is clear from Table 3,
the present method is more efficient.
Scheme 2. Cyclotrimerization of acetophenone under various reaction conditions.

2182
H. R. SAFAEI, M. DAVOODI, AND M. SHEKOUHY
Table 1. Cyclotrimerization of acetophenone under various reaction conditions
Entry
Catalyst
Reaction conditions
Time (h)
Yield (%)a
1b
2c
3d
4
SiO2 ꢀ P2O5
POCl3-n(SiO2)n
SiO2-OSO3H
Ca(HSO4)2
Mg(HSO4)2
Al(HSO4)3
B(HSO4)3
—
3 g = solvent-free = 100 ꢂC
3 g = solvent-free = 100 ꢂC
3 g = solvent-free = 100 ꢂC
20 mol% = solvent-free = 100 ꢂC
20 mol% = solvent-free = 100 ꢂC
20 mol% = solvent-free = 100 ꢂC
20 mol% = solvent-free = 100 ꢂC
Solvent-free = 100 ꢂC
10 mol% = solvent-free = 100 ꢂC
30 mol% = solvent-free = 100 ꢂC
20 mol% = EtOH = reflux
20 mol% = DMSO = 100 ꢂC
20 mol% = DMF = 100 ꢂC
20 mol% = THF = reflux
48
48
48
48
48
48
5
22
28
36
Trace
26
33
5
6
7
94
8
48
48
5
NR
73
9
B(HSO4)3
B(HSO4)3
B(HSO4)3
B(HSO4)3
B(HSO4)3
B(HSO4)3
B(HSO4)3
B(HSO4)3
B(HSO4)3
B(HSO4)3
10
11
12
13
14
15
16
17
18
95
48
48
48
48
48
48
48
5
Trace
72
61
67
20 mol% = solvent-free = r.t.
20 mol% = solvent-free = 60 ꢂC
20 mol% = solvent-free = 80 ꢂC
20 mol% = solvent-free = 120 ꢂC
NR
43
82
91
aIsolated yield.
bFor more informations see Ref. 38.
cFor more informations see Ref. 39.
dFor more informations see Ref. 40.
One of the noteworthy features of our method is that electron-withdrawing
substituted acetophenones, especially p-nitro acetophenone, which are difficult to
cycloterimerize by other methods, could be easily converted into the corresponding
arylated benzenes (Table 4).
The experimental procedure is remarkably simple. After the completion of
the reaction, hot ethanol was added to the reaction mixture and the insoluble
catalyst was isolated by simple filtration and reused. The filtrate was concen-
trated under reduced pressure, and the obtained crude products were recrysta-
lized from EtOH=H2O 3:1. To establish the reusability of B(HSO4)3 in
cyclotrimerization of ketons, the model reaction was studied in the presence
of recovered catalyst and the obtained results are summarized in Graph 1.
As shown in Graph 1, smooth loss of catalytic activity was observed from
the 10th reuse.
In summary, we have reported a highly efficient and green method for the
synthesis of highly substituted benzenes via cyclotrimerization reaction of ketones
in the presence of B(HSO4)3, a new, highly efficient, and reusable solid acid cata-
lyst, under solvent-free conditions. The reaction is experimentally simple, pro-
ceeding well without protection from atmospheric air and generating virtually
no by-products. Equally important is the good selectivity and good isolated yields
of this transformation. No additives and=or toxic reagent(s) and=or by-product(s)
were involved and no laborious purifications were necessary. The method avoids
isolation and handling of in situ–generated intermediates, which reduces the
reaction time and improves the overall yield. Water was the only by-product in

SYNTHESIS OF SUBSTITUTED BENZENES
2183
Table 2. Cyclotrimerization of carbonyl compounds in the presence of B(HSO4)3 under solvent-free con-
ditions at 100 ꢂC
Time
(h)
Yield
(%)a
Entry
Reactant
Product
Mp (ꢂC) (lit.)
172–173 (174.1–
174.7)[33]
1
5
8
94
91
176–178 (175.7–
176.9)[33]
2
3
167–169 (167.5–
168.1)[33]
8
92
116–117 (116.8–
118.1)[33]
4
5
7
5
90
92
246–247 (244.1–
244.9)[33]
261–262 (260.2–
260.9)[33]
6
5
93
7
8
4
6
92
90
236–237 (236.5–238)[33]
263–265 (264.6–
265.9)[33]
(Continued )

2184
H. R. SAFAEI, M. DAVOODI, AND M. SHEKOUHY
Table 2. Continued
Time
(h)
Yield
(%)a
Entry
Reactant
Product
Mp (ꢂC) (lit.)
9
5
8
92
91
171–172 (172.4–173)[33]
165–166 (165.1–
165.5)[33]
10
11
10
86
141–143 (143)[31]
12
3
92
151–153 (153)[31]
13
6
90
235–237 (236.2–238)[33]
14
6
91
>250 (>250)[36]
(Continued )

SYNTHESIS OF SUBSTITUTED BENZENES
2185
Table 2. Continued
Time
(h)
Yield
(%)a
Entry
Reactant
Product
Mp (ꢂC) (lit.)
15
6
9
91
90
221–223 (222–223)[34]
16
113–114 (112–114)[36]
17
18
12
12
85
89
97–99 (96–97)[28]
214–216 (216)[31]
aIsolated yield.
the whole process, which added to its attractiveness. In addition, this safe and
clean procedure is ideally suited to automated applications because the entire
synthetic sequence can be carried out in the presence of a reusable catalyst. These
conditions are also cost-effective, environmentally friendly, and possess high
generality, which make our methodology suitable as a valid contribution to the
existing processes in the field of highly substituted benzenes synthesis.
Table 3. Comparison of the self-condensation of acetophenone for the synthesis of compound 2a using
the reported methods versus the present method
Entry
Reagents and conditions
Time (h) Yield (%)a
Ref.
1
2
TiCl4 (200 mol%), toluene, reflux
24
5
65
80
76
60
87
80
82
87
91
94
[25a]
[25b]
[28]
TiCl3(OTf) ꢀ 4H2O (2 mol%), toluene, reflux
CuCl2 (8 mol%), solvent-free, 155 ꢂC
Amberlyst 15 (30 mol%), toluene, reflux
3
4
4
10
5
[36]
[37]
5
Phosphomomolybdic acid (5 mol%), EtOH, reflux
Bi(OTf)3 ꢀ 4H2O (2 mol%), solvent-free, 140 ꢂC
p-TSOH, SnCl4 (5 mol%), 1-pentanol, reflux
Ethylenediamine (20 mol%), TFA (40 mol%), CH3NO2, reflux
p-TSOH ꢀ H2O (10 mol%), solvent free, 130 ꢂC
B(HSO4)3 (20 mol%), solvent-free, 100 ꢂC
6
5
[33]
[31]
7
4
8
48
10
5
[34]
[32]
9
10
This work
aIsolated yield.

2186
H. R. SAFAEI, M. DAVOODI, AND M. SHEKOUHY
Table 4. Comparison of the cyclotrimerization of p-nitro acetophenone for the syn-
thesis of compound 2 l using the reported methods versus the present method
Entry
Reagents and conditions
Yield (%)a
Ref.
1
2
3
4
Phosphomolybdic acid (5 mol%), EtOH, reflux
p-TSOH, SnCl4 (5 mol%), 1-pentanol, reflux
30
<30
[37]
[31]
p-TSOH ꢀ H2O (10 mol%), solvent-free, 140 ꢂC No reaction
[32]
This work
B(HSO4)3 (20 mol%), solvent-free, 100 ꢂC
92
aIsolated yield.
Graph 1. Cyclotrimerization of acetophenone (1a) in the presence of recovered B(HSO4)3 under
solvent-free conditions at 100 ꢂC. (Figure is provided in color online.)
EXPERIMENTAL
Chemicals and Apparatus
All chemicals were purchased from Merck or Fluka Chemical Companies.
B(HSO4)3 was prepared by reported method.[7] All synthesized compounds are
known and were identified by comparison of their melting points and 1H NMR data
with those in the authentic samples. The 1H NMR (500 MHz) and 13C NMR
(125 MHz) were run on a Bruker Avance DPX-250, FT-NMR spectrometer. Micro-
analysis was performed on a Perkin-Elmer 240-B microanalyzer. Melting points were
recorded on a Stuart Scientific Apparatus SMP3 (UK) in open capillary tubes.
General Procedure for the Synthesis of Highly Substituted Benzenes
In a typical example, B(HSO4)3 (3.2 mmol, 20 mol%) and ketone (18 mmol)
were added in a 50-mL round-bottomed flask, and the resulting mixture was stirred
at 100 ꢂC. The completion of the reaction was followed by thin-layer chromato-
graphy (TLC) using n-hexane = ethyl acetate 3:1 as an eluent. After completion,
the reaction mixture was allowed to cool to room temperature, hot ethanol

SYNTHESIS OF SUBSTITUTED BENZENES
2187
(20 mL) was added, and the mixture was stirred for 2 min. After this, the insoluble
catalyst was filtered and washed with hot ethanol (5 mL two times), dried, and
reused. The filtrate was concentrated under reduced pressure and the obtained crude
products were recrystallized from EtOH=H2O 3:1 to afford the desired products in
good to excellent yield.
1,3,5-Tris(3-methylphenyl)benzene (2d)
White solid, mp 116–117 ꢂC (lit. 116.8–118.1 ꢂC[33]); 1H NMR (500 MHz,
CDCl3): d ¼ 2.44 (s, 9 H), 7.20 (d, J ¼ 7.7 Hz, 3 H), 7.39 (t, J ¼ 7.7 Hz, 3 H), 7.51
(d, J ¼ 7.7 Hz, 3 H), 7.54 (s, 3 H), 7.75 (s, 3 H). 13C NMR (125 MHz, CDCl3):
d ¼ 21.5, 124.6, 125.0, 128.1, 128.2, 128.6, 138.4, 141.2, 142.4. Anal. calcd. for
C27H24: C, 93.06; H, 6.94%. Found: C, 93.02; H, 6.95%.
1,3,5-Tris(4-fluorophenyl)benzene (2g)
White solid, mp 236–237 ꢂC (lit. 236.5-238.0 ꢂC[33]); 1H NMR (500 MHz,
CDCl3): d ¼ 7.14 (t, J ¼ 8.5 Hz, 6 H), 7.62–7.67 (m, 6H), 7.69 (s, 3 H). 13C NMR
(125 MHz, CDCl3): d ¼ 115.5, 124.6, 128.9, 137.1, 141.3, 162.5. Anal. calcd. for
C24H15F3: C, 79.99; H, 4.20%. Found: C, 79.91; H, 4.22%.
1,3,5-Tris(2-chlorophenyl)benzene (2j)
White solid, mp 165–166 ꢂC (lit. 165.1–165.5 ꢂC[33]); 1H NMR (500 MHz,
CDCl3): d ¼ 7.27–7.38 (m, 6 H), 7.44–7.52 (m, 6 H), 7.57 (s, 3 H). 13C NMR
(125 MHz, CDCl3): d ¼ 126.8, 128.7, 129.7, 130.1, 131.6, 132.6, 138.8, 140.0. Anal.
calcd. for C24H15Cl3: C, 70.35; H, 3.69%. Found: C, 70.39; H, 3.74%.
1,3,5-Tri(4’-bromobiphenyl)benzene (2n)
White solid, mp > 250 ꢂC (lit. > 250 ꢂC[36]); 1H NMR (500 MHz, CDCl3)
d ¼ 7.52 (d, J ¼ 8.5 Hz, 6H), 7.63 (d, J ¼ 8.5 Hz, 6H), 7.72 (d, J ¼ 8.5 Hz, 6H, 7.80
(d, J ¼ 8.5 Hz, 6H, ArH), 7.89 (s, 3H). Anal. calcd. for C42H27Br3: C, 65.40; H,
3.53%. Found: C, 65.49; H, 3.41%.
1,3,5-Tris(biphenylmethyl)benzene (2p)
1
White solid, mp 113–114 ꢂC (lit. 112–114 ꢂC[36]); H NMR (500 MHz, CDCl3)
d ¼ 5.27 (s, 3H), 6.63–7.10 (m, 33H). Anal. calcd. for C45H36: C, 93.71; H, 6.29%.
Found: C, 93.68; H, 6.33%.
Trindan (2q)
White solid, mp 97–99 ꢂC (lit. 96–97 ꢂC[28]); 1H NMR (500 MHz, CDCl3):
d ¼ 1.92–2.51 (m, 6H), 2.49–3.21 (t, J ¼ 7.3 Hz, 12 H). 13C NMR (125 MHz, CDCl3):
d ¼ 25.6, 31.2, 137.6. Anal. calcd. for C15H18: C, 90.85; H, 9.15%, Found: C, 90.71;
H, 9.10%.

2188
H. R. SAFAEI, M. DAVOODI, AND M. SHEKOUHY
Complete experimental details can be found online in the Supplemental
Material.
ACKNOWLEDGMENT
We thank the Islamic Azad University (Shiraz Branch) Research Council for
the financial support of this work.
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