Sulfonylation of aromatic compounds with sulfonic acids using silica gel-supported AlCl3 as a heterogeneous Lewis acid catalyst

Article abstract of DOI:10.1080/17415991003777391

Silica gel-supported aluminum chloride (SiO₂-AlCl₃) has been shown to be a mild, efficient, and chemoselective heterogeneous Lewis acid catalyst for direct conversion of arenes to sulfones using sulfonic acids as sulfonylating agents. The catalyst can be prepared easily with cheap starting materials and is stable (as a bench-top catalyst) and reusable.

Full text of DOI:10.1080/17415991003777391

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Sulfonylation of aromatic compounds
with sulfonic acids using silica gel-
supported AlCl3 as a heterogeneous
Lewis acid catalyst
Kaveh Parvanak Boroujeni a
a Department of Chemistry, Shahrekord University, Shahrekord,
115, Iran
Version of record first published: 26 May 2010.
To cite this article: Kaveh Parvanak Boroujeni (2010): Sulfonylation of aromatic compounds with
sulfonic acids using silica gel-supported AlCl3 as a heterogeneous Lewis acid catalyst, Journal of
Sulfur Chemistry, 31:3, 197-203
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Journal of Sulfur Chemistry
Vol. 31, No. 3, June 2010, 197–203
Sulfonylation of aromatic compounds with sulfonic acids using
silica gel-supported AlCl3 as a heterogeneous Lewis acid catalyst
Kaveh Parvanak Boroujeni*
Department of Chemistry, Shahrekord University, Shahrekord 115, Iran
(Received 18 January 2010; final version received 14 March 2010)
Silica gel-supported aluminum chloride (SiO2–AlCl3) has been shown to be a mild, efficient, and chemos-
elective heterogeneous Lewis acid catalyst for direct conversion of arenes to sulfones using sulfonic acids
as sulfonylating agents. The catalyst can be prepared easily with cheap starting materials and is stable (as
a bench-top catalyst) and reusable.
Keywords: sulfonylation; sulfones; sulfonic acids; aluminum chloride; silica gel
1. Introduction
In recent years, the use of solid-supported catalysts has received considerable attention in different
areas of organic synthesis (1). The activity and selectivity of a catalyst dispersed on the surface of
a support is improved as the effective surface area of the catalyst is increased manifold, and hence
they are expected to perform more effectively than the individual catalysts. Recyclability, easily
separable from the reaction media, moisture and air tolerance, non-toxicity, and easier handling
are other common features that make the use of solid-supported catalysts an attractive alternative
to conventional homogeneous catalysts (2).
Organosulfones are useful intermediates in a wide range of fields such as agrochemicals (3),
pharmaceuticals (4, 5), and polymers (6). In particular, aryl sulfones have received much atten-
tion as powerful anti-HIV-1 agents (7). They also possess high antifungal, antibacterial, and
antitumoral activities (8, 9). The most practical laboratory method of preparing aryl sulfones is
Friedel–Crafts sulfonylation of arenes (10, 11). Typically, sulfonyl chlorides are used as the sul-
fonylating agents and stoichiometric or excess amounts of Lewis acids such as AlCl3 and SbF5
are required (12–22). This procedure has serious drawbacks since sulfonyl chlorides and metallic
oxophilic promoters cause environmental problems due to strongly acidic waste streams. Further,
tedious work-up, highly corrosive conditions, the formation of various side products, and the use
of moisture-sensitive, unrecyclable, or difficult to handle catalysts are other disadvantages of the
classical method. These disadvantages could be avoided by performing the sulfonylation of arenes
*Email: parvanak-ka@sci.sku.ac.ir
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2010 Taylor & Francis
DOI: 10.1080/17415991003777391
http://www.informaworld.com

198 K. Parvanak Boroujeni
O
S
ArSO3H
Ar: ph, tolyl
Ar
SiO2–AlCl3
80oC
Y: H, OMe, Me,
ph, Cl, Br
O
Y
Y
Scheme 1. Sulfonylation of arenes with sulfonic acids using SiO2–AlCl3.
with sulfonic acids as sulfonylating agents over recoverable solid acid catalysts. Sulfonic acids are
probably the best sulfonylating reagents since they are common precursors of sulfonyl chlorides
and their reactions produce water as the only by-product, which meets the recent requirement for
environmentally benign processes. In addition, in this method the products are purer than those
obtained by the Friedel–Crafts method and the isolation of products is simple with no need of acid
removal. Sulfonylation involving sulfonic acids has been studied, although these procedures are
not quite successful as practical and general synthetic methods (23–31). For instance, Graybill
reported polyphosphoric acid as a catalyst for the direct sulfonylation of arenes (23). However,
polyphosphoric acid is a viscous liquid and is not easy to handle, and the reaction suffers from
low yields and long reaction times. MeSO3H/P2O5 (as a dehydrating agent) has been studied for
the synthesis of sulfones using sulfonic acids (26). This method has certain disadvantages such
as being time-consuming, the thermal decomposition of the reagent, and using a 10-fold excess
of methane–sulfonic acid. Recently, it was also shown that Fe3+-montmorillonite, Nafion-H, and
P2O5/Al2O3 promote the sulfonylation of arenes using sulfonic acids (13, 27, 30). Although these
methods are environmentally safe, the reactions have some limitations such as unsatisfactory
yields, long reaction times, and reflux conditions. Sulfonylation of arenes using sulfonic acids
without the need of any catalyst has been reported but in this method the reactions occur in the
presence of 1.5 molar equivalents of Tf2O in CH3NO2 which results in a substantial amount of
waste and in corrosion problems (31). In view of this, a reliable method for this useful reaction
involving heterogeneous catalysts is in demand.
In a continuation of recent work on the use of heterogeneous Lewis acid catalysts, a report has
been published on silica gel-supported aluminum chloride (SiO2–AlCl3) as an effective catalyst
for the sulfonylation of aromatic compounds with sulfonyl chlorides (32).Along this line, I further
wish to report that SiO2–AlCl3 (33–35) is also a highly efficient and chemoselective catalyst for the
sulfonylation of arenes using p-toluenesulfonic acid (TsOH) and benzene sulfonic acid (PhSO3H)
in solvent-free conditions (Scheme 1).
2. Results and discussion
SiO2–AlCl3 was prepared by treating silica gel with AlCl3 in refluxing carbon tetrachloride. As
shown in Table 1, different kinds of aromatic compounds underwent sulfonylation with PhSO3H
and TsOH as sulfonylating agents effectively to afford sulfones at 80 ◦C. It was observed that the
electron donating groups such as alkyl and OMe on the aromatic compound gave high to excellent
yields of the product (Entries 2–7). Also, weakly deactivated arenes such as chlorobenzene and
bromobenzene underwent sulfonylation with high yields (Entries 8–9). However, sulfonylation of
highly deactivated arenes such as nitrobenzene and 1,2-dichlorobenzene failed. The methodology
showed excellent positional selectivity as the para-substituted product was formed exclusively.
Naphthalene was converted to the corresponding sulfones with high regioselectivity in high yields
(Entry 10). The sulfonylation of indole with sulfonic acids in the presence of SiO2–AlCl3 was also
studied and the corresponding 3-arylsulfonyl indoles were obtained in 90–92% yields (Entry 11).
No N-substituted products were observed under these reaction conditions. The optimum molar

Journal of Sulfur Chemistry 199
Table 1. Sulfonylation of arenes with sulfonic acids using SiO2–AlCl3.a
Yield (%)
(o:m:p)b
m.p. (◦C)
(reported)
Entry
1
Arene
Ar
Ph
Product
Time (h)
1.5
Ref.
SO2C6H5
Benzene
90
90
125–127 (123)
(23)
SO2C6H4Me-4
SO2C6H5
Tolyl
Ph
1.5
1.4
128 (125)
(23)
(14)
2
Toluene
91 (6:4:90)
92 (6:4:90)
90 (6:0:94)c
79–80 (78–79)
Me
SO2C6H4Me-4
Tolyl
Ph
1.4
1.5
147–150 (146–149)
147–150 (148)
(14)
(23)
Me
SO2C6H5
3
4
5
Biphenyl
o-Xylene
m-Xylene
Ph
SO2C6H4Me-4
Tolyl
Ph
1.5
1.5
1.4
91 (5:0:95)c
91 (93:7)d
92 (92:8)d
173–175 (178)
88–90 (86–88)
(23)
(14)
(14)
Ph
SO2C6H5
Me
Me
SO2C6H4Me-4
Tolyl
117–119 (113–115)
Me
Me
Me
Ph
1.5
91
91–94 (92–94)
(14)
Me
SO2C6H5
Me
Tolyl
Ph
1.4
1.5
1.5
93
92
93
50–52 (49)
103–105 (101–103)
107–109 (104)
(26)
(14)
(13)
Me
SO2C6H4Me-4
Me
SO2C6H5
6
p-Xylene
Me
Me
SO2C6H4Me-4
Tolyl
Me
(Continued)

200 K. Parvanak Boroujeni
Table 1. Continued.
Yield (%)
(o:m:p)b
m.p. (◦C)
(reported)
Entry
7
Arene
Ar
Ph
Product
Time (h)
1.3
Ref.
SO2C6H5
Anisole
93 (8:3:89)
94 (7:3:90)
76–78 (79–80)
101–104 (104)
(14)
OMe
SO2C6H4Me-4
Tolyl
1.3
(26)
OMe
SO2C6H5
8
Chlorobenzene
Ph
Tolyl
Ph
1.9
1.9
1.8
88 (9:5:86)
88 (8:3:89)
89 (6:3:91)
79–81 (77–78)
118–120 (123)
91–94 (92–94)
(14)
(26)
(14)
Cl
SO2C6H4Me-4
Cl
SO2C6H5
9
Bromobenzene
Br
SO2C6H4Me-4
Tolyl
Ph
1.8
2.6
2.7
90 (4:5:91)
85 (10:90)e
90 (9:91)e
133–135 (136–137)
70–73 (69–71)
(15)
(14)
(14)
Br
SO2C6H5
10
Naphthalene
SO2C6H4Me-4
Tolyl
103–106 (102–104)
SO2C6H5
11
Indole
Ph
2.3
2.3
90c
145–147 (147)
(36)
(37)
N
H
SO2C6H4Me-4
Tolyl
92c
173–176 (172–173)
N
H
Notes: aAll reactions were carried out at 80 ◦C in the presence of SiO2–AlCl3. The molar ratio of the sulfonic acid: SiO2–AlCl3 is: 1:0.1.
bYield refers to the mixture of isomeric products. Isomer distribution based on 1H NMR study of crude sulfones. cThe reaction was
performed in acetonitrile. dIsomer distribution of 3,4-dimethylphenyl isomer to 2,3-dimethylphenyl isomer. eα : β ratio. The reaction was
performed in acetonitrile.

Journal of Sulfur Chemistry 201
O
S
PhSO3H (1.5 mmol)
CH3
CH3
SiO2–AlCl3 (0.15 mmol)
O
(2 mmol)
°
80 C / 1.4 h
Usea
1
2
3
4
Yield (%)
91
91
90
90
Note: aRecovered catalyst was used successively (Use 2,3,…).
Scheme 2. Sulfonylation of toluene using recovered SiO2–AlCl3.
ratio of SiO2–AlCl3 to sulfonylating agent was 0.1:1. Side product formation was not observed
in the reactions we have studied.
In order to show the role of the silica gel in the sulfonylation reactions, we performed sul-
fonylation of anisole with TsOH in the presence of AlCl3 (1.5 equiv.) at 80 ◦C in the absence
of silica gel. We have observed that the corresponding sulfone was produced with 40% yield
after 3.5 h, whereas the desired sulfone was isolated with 94% yield after 1.3 h in the presence of
SiO2–AlCl3 (Table 1, Entry 7). Presumably, the lower catalytic activity ofAlCl3 than SiO2–AlCl3
in sulfonylation of arenes with sulfonic acids is due to its ability to strongly complex Lewis base
products. Unlike AlCl3, SiO2–AlCl3 is a milder catalyst which forms no stable complex with
starting materials and/or products. Further, the presence of silica makes the present method even
more effective, as it acts as a water scavenger. It should be noted that in the absence of silica,
the reaction required forcing conditions, azeotropic removal of water using a Dean–Stark trap
(13, 27).
To find out whether the reaction takes place in the solid matrix of SiO2–AlCl3 or whether
AlCl3 simply released in the reaction medium is responsible for the sulfonylation reaction, SiO2–
AlCl3 was added to anisole and the mixture was stirred at room temperature for 2 h. Then, the
catalyst was filtered off and the filtrate was analyzed for its aluminum content, which showed a
negligible release of AlCl3. The filtrate was found to be inactive for the sulfonylation reaction.
These observations indicate that SiO2–AlCl3 is stable under the reaction conditions and there is
no leaching of acid moieties during reactions due to being chemically bonded to the support.
The recyclability of the catalyst wasalso studied.The spent catalyst from sulfonylation reactions
was washed with ether and used again without further drying. The activity of the catalyst was
unaffected even after five cycles (Scheme 2). The capacity of the catalyst after five uses was
1.3 mmol AlCl3 per gram.
3. Experimental
3.1. General
Chemicals were either prepared in our laboratory or were purchased from Merck and Fluka.
The capacity of the catalyst was determined by the Mohr titration method and atomic absorption
technique using a Philips atomic absorption instrument. Reaction monitoring and purity determi-
nation of the products were accomplished by GLC or TLC on silica gel polygram SILG/UV254
plates. Gas chromatographic analysis was performed on Shimadzu GC. 14-A. IR spectra were run
on a Shimadzu model 8300 FT-IR spectrophotometer. NMR spectra were recorded on a Bruker
Advance DPX-300 spectrometer.

202 K. Parvanak Boroujeni
3.2. Preparation of SiO2 − AlCl3 (32, 38)
AnhydrousAlCl3 (5.1 g) was added to silica gel (Merck grade 60, 230–400, washed with 1 M HCl
and dried under vacuum at 80 ◦C for 72 h, 10.2 g) in carbon tetrachloride (30 ml). The mixture was
stirred using a magnetic stirrer under reflux condition for 2 days under N2 atmosphere and filtered
and washed with 50 ml of dry CCl4, and then dried under vacuum at 60 ◦C for 3 h. The loading of
AlCl3 determined was 1.3 mmol/g. (One gram of solid catalyst was fused with Na metal, extracted
with 10 ml of water and filtered. The aluminum and chloride content of the filtrate was determined
by the Mohr titration method [for the procedure refer to (39)]).
3.3. Typical experimental procedure
In a round-bottomed flask (25 ml) equipped with a condenser and a magnetic stirrer, a solution of
m-xylene (10 mmol) and TsOH (7.5 mmol) was prepared. SiO2–AlCl3 (0.75 mmol) was added to
the solution and the reaction mixture was stirred magnetically at 80 ◦C. Progress of the reaction
was followed by TLC and GC. After completion of the reaction, the catalyst was filtered off
and washed with CH2Cl2 or ether (2 × 15 ml) and the filtrate was washed with 10% NaHCO3
solution (2 × 10 ml) and water (10 ml). The combined organic layers were dried over anhydrous
Na2SO4, concentrated in vacuo, and the residue was recrystallized from CH2Cl2/n-hexane to
give (2,4-dimethylphenyl)tolylsulfone as a white crystalline solid, yield 93%. All products are
known compounds and were identified by comparing their physical and spectral data with those
of the authentic samples (13, 14, 25). Representative examples of spectroscopic data are given
below.
1
Di(p-tolyl) sulfones (Table 1, Entry 2): H NMR (300 MHz; CDCl3) δ 2.30 (s, 5.40H, para
isomer), 2.36 (s, 0.24H, meta isomer), 2.41 (s, 0.36H, ortho isomer), 7.26 (d, J = 8.0 Hz, 4H,),
7.79 (d, J = 8.0 Hz, 4H); IR (KBr) 1160, 1315 cm−1
.
1
(2,4-Dimethylphenyl)p-tolylsulfone (Table 1, Entry 5): H NMR (300 MHz; CDCl3) δ 2.40
(s, 3H), 2.41 (s, 3H), 2.45 (s, 3H), 7.12 (s, 1H), 7.24 (d, 1H), 7.80 (2H, d), 7.78 (2H, d), 8.09 (d,
1H); IR (KBr) 1157, 1315 cm−1
.
1
Napthyltolylsulfone (Table 1, Entry 10): H NMR (300 MHz; CDCl3) δ 2.26 (s, 2.73H, β-
isomer), 2.28 (s, 0.27H, α-isomer), 7.28 (m, 2H), 7.5–7.64 (m, 2H), 7.80–8.16 (m, 5H), 8.55 (d,
J = 8.6 Hz, 1H), 8.71 (d, J = 8.5 Hz, 1H).
3-(4-Methylphenylsulfonyl)-1H-indole (Table 1, Entry 11): 1H NMR (300 MHz, CDCl3): δ
2.25 (s, 3H), 6.81–6.90 (m, 4H), 7.07–7.19 (m, 2H), 7.27–7.31 (m, 2H), 7.49 (d, 1H, J = 8.0 Hz),
8.19 (brs, NH); IR (KBr) 3330, 1582, 1450, 1388, 1299, 1230, 1147, 1097, 758 cm−1
.
4. Conclusion
In summary, SiO2–AlCl3 is an effective and chemoselective heterogeneous Lewis acid catalyst
for the direct sulfonylation of arenes using sulfonic acids, with possible application in “green
chemistry” scenarios by virtue of its activity under solventless conditions and its ability to be
recycled. Mild reaction conditions, easy workup, short reaction times, and high to excellent yields
are other obvious advantages of the present method.
Acknowledgements
The author thanks the Research Council of Shahrekord University for the partial support of this work.

Journal of Sulfur Chemistry 203
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