Synthesis and application of modified silica sulfuric acid as a solid acid heterogeneous catalyst in Michael addition reactions

Article abstract of DOI:10.1002/jhet.659

Figure represented. Modified silica sulfuric acid (MSSA) as a new type of silica sulfuric acid was prepared and effectively used in the conjugate addition of indole, pyrrole, and thiols with Michael acceptors under mild conditions at room temperature. Als

Full text of DOI:10.1002/jhet.659

July 2011
Synthesis and Application of Modified Silica Sulfuric Acid as a
Solid Acid Heterogeneous Catalyst in Michael
Addition Reactions
977
Mohammad Ali Zolfigol,a* Hojat Veisi,b* Farajollah Mohanazadeh,c
and Alireza Sedrpoushana,c
aDepartment of Organic Chemistry, Faculty of Chemistry,
Bu-Ali Sina University, Hamedan 6517838683, Iran
bDepartment of Chemistry, Payame Noor University (PNU), Iran
cChemical Industries Department, Iranian Research Organization for Science
and Technology (IROST), Tehran, Iran
*E-mail: zolfi@basu.ac.ir or hojatveisi@yahoo.com
Received August 24, 2010
DOI 10.1002/jhet.659
Published online 21 April 2011 in Wiley Online Library (wileyonlinelibrary.com).
Modified silica sulfuric acid (MSSA) as a new type of silica sulfuric acid was prepared and effec-
tively used in the conjugate addition of indole, pyrrole, and thiols with Michael acceptors under mild
conditions at room temperature. Also, MSSA was used as a catalyst for the synthesis of 1,1,3-tri-indolyl
compounds in good to excellent yield at room temperature.
J. Heterocyclic Chem., 48, 977 (2011).
INTRODUCTION
ditionally catalyzed by strong bases that often lead to
undesirable side reactions and/or products [4]. A variety
of Lewis acids are found to catalyze this reaction, and
these procedures are also not free from disadvantages [5].
Thus, a number of milder reagents and Lewis acids cata-
lysts such as Al2O3 [6], K2CO3 [7], rhodium complex [8],
ruthenium complex [9], clay-supported nickel bromide
[10], quaternary ammonium salt [11], InBr3 [12], InCl3
[13], Bi(NO3)3 [14], I2 [15], Bi(OTf)3 [16], HClO4/SiO2
[17], GaI3 [18], PTSA [19], sulfamic acid [20], Ru(III)
[21], NH4Cl [22], and N-phenyltris(dimethylamino)imino-
phosphorane immobilized on polystyrene resin [23] have
been developed over the past few years. Indeedly, a num-
ber of these procedures have one or the other disadvan-
tages such as longer reaction time, use of excessive
Michael additions promoted by Lewis acid catalysts
are one of the most important carbon–carbon bond-form-
ing reactions in organic synthesis. Addition reactions of
indoles, amines, and thiols to a,b-unsaturated compounds
have received much interest due to a number of their
derivatives occur in nature and possess a variety of bio-
logical activities [1,2]. Thus, the development of facile
and environmentally friendly synthetic methods for the
preparation of these compounds constitutes an active area
of investigation in pharmaceutical and organic synthesis.
The Michael addition is one of the most useful carbon–
carbon bond-forming reactions and has wide synthetic
applications in organic synthesis [3]. This reaction is tra-
C
V
2011 HeteroCorporation

978
M. A. Zolfigol, H. Veisi, F. Mohanazadeh, and A. Sedrpoushan
Vol 48
Scheme 1
Scheme 2
suggested the existence of three distinct groups: silanol
or ‘‘bound water’’ (F), silanol with physically adsorbed
water (G), and dehydrated oxides (H) [30,31]. More
recently, slightly different views of the surface have
arisen. Snyder [32,33] has viewed the silica surface as
containing varying proportions of five groups: geminal
(I), bound and reactive (J), free (F), and siloxane (H).
The amount of each of these groups is dependent on
structural considerations (Scheme 2).
The silica surface is ordinarily considered to be cov-
ered with a monolayer of silanol groups, which arise
from the tendency of each silicon atom on the surface to
maintain tetrahedral coordination [34].
expensive catalyst, harsh reaction conditions, failure to pro-
vide addition product, and tedious experimental procedure.
Consequently, there is a need for a catalytically effi-
cient method for these transformations, which might
work under mild and more economical conditions.
Any reduction in the amount of sulfuric acid needed
and/or any simplification in handling procedures is
required for risk reduction, economic advantage, and
environment protection. In addition, during the recent
years, the use of reusable heterogeneous catalysts has
received considerable importance in organic synthesis
because of their environmental, economical, and indus-
trial aspects. The development of efficient methods
using recoverable and reusable catalysts is an important
goal in organic synthesis. Very recently, we have pub-
lished a comprehensive review related of the synthesis
and applications of bis- and tris-indolylmethanes [24].
The dehydration of silica has been also a much argued
topic. A number of authors [31,35] have suggested the follow-
ing: below 150ꢀC loss of physically adsorbed water; around
150 to 600ꢀC evolution of bound water without appreciable
structural deformation; and above 600ꢀC, an increased
possibility of internal alterations. On the other hand,
Unger [36] has recently shown evidence to indicate that
the hydroxyl group, silanol, concentration decreases
only slightly up to 300ꢀC, and that a pronounced
decrease in silanol concentration occurs between about
300 and 500ꢀC. This decrease is attributed to condensa-
tion of bound or paired hydroxyl groups. The condensa-
tion of free hydroxyl groups occurs only above 600ꢀC.
When the silica surface is heated above 150ꢀC, partial
removal of the ‘‘bound water’’ occurs, very slowly up to
ꢁ300 to 350ꢀC at which point a pronounced decrease
occurs to about 500 to 550ꢀC, leaving a dehydrated oxide
condition, which will not be chemically active to DMCS.
These conditions may be described by the following reac-
tion (Scheme 3).
RESULTS AND DISCUSSION
In continuation of our studies on the synthesis [25]
and application of silica sulfuric acid (SSA) [25–29] as
a solid acid heterogeneous catalyst, we wish to reported
the synthesis of modified silica sulfuric acid (MSSA) as
a new type of SSA in organic reaction. Although our
first report of SSA has been widely used and cited by
international researchers [25], we think that MSSA is
superior due to its more stability in the presence of mois-
ture. MSSA is easily prepared from the silanization of
activated silica gel with dimethyldichlorosilane (DMCS)
followed by treatment with water and then adding chloro-
sulfonic acid at room temperature (Scheme 1).
Thus, by controlling the prereaction dehydration,
varying amounts of DMCS may be chemically bonded
to the porous silica surface. It has been suggested in the
literature [37,38] that two possibilities exist for the reac-
tion of DMCS with the silica surface. These possibilities
are illustrated in the following equations (Scheme 4).
Scheme 3
The extent to which chemical reactions occur with a
given porous’ silica surface are controlled by the types
and reactivity of chemical groups present and the steric
availability of these groups. Much disagreement and
confusion has arisen over these points. Earlier workers
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Synthesis and Application of MSSA
979
dried in a dry flowing nitrogen stream at 120ꢀC for 24 h
to maintain controlled moisture conditions. In continua-
tion, reaction of silanol groups with chlorosulfonic acid
at room temperature was caring out. It is interesting to
note that the reaction is easy and clean without any
work-up procedure, because HCl gas is evolved from
the reaction vessel immediately (Scheme 1).
Scheme 4
A study of the stability of bonded sulfonic acid surfa-
ces in SSA and MSSA toward water was made. We
added 1 g of SSA and MSSA in the separate column
and then washed them with 20-mL water. After wash-
ing, we removed the solid and on titration of the
sulfonic-coated silica surface with a solution of NaOH
(0.1 N). The surface-bonded sulfonic acid in SSA was
reduced to 1 s of MSSA. This would indicate that the
stability of MSSA as a solid acid heterogeneous catalyst
toward water and hydrolysis condition. This stability
may be because of steric effects of methyl groups in
MSSA toward water attack.
If reaction 1 is the reaction path, then the chlorine
and carbon bound to the surface should have a 1: 2 ratio
and be directly related to the number of OH groups
available for reaction. In the event that reaction 2 is the
controlling process, no chlorine will be detectable, and
the ratio of bound carbon to initially available hydroxyl
groups should have a one to one correspondence. How-
ever, in view of these reactions, a third complicating
alternative must be considered, the possibility that both
reactions 1 and 2 occur simultaneously to some degree.
In this event, the number of C1 and OH groups neither
show direct agreement nor the surface be completely
devoid of C1. Also, the bound carbon would not show a
2:l or 1:l ratio with initially available hydroxyl groups
as in the case of purely reactions 1 or 2, respectively,
but some intermediate value [39].
As a part of our ongoing research program to develop
new synthetic methodologies [40–42], we found that
MSSA as a stable solid acid heterogeneous catalyst,
nonvolatile, inexpensive, and safe reagent for conjugate
addition of indoles, pyrroles, and thiols to Michael
acceptors under solvent conditions (Scheme 5).
First, we studied the reaction of chalcon 1a with
indole (1:1 molar ratios) to optimize the reaction condi-
tions with respect to temperature, time, and the ratio of
MSSA to the substrate. We found that 200 mg of
MSSA was sufficient to obtain the desired Michael
adduct in 98% yield within 90 min at room temperature
in CH3CN using chalcon 1a (Table 1, Entry 1).
Under the optimized reaction conditions, a variety of
a,b-unsaturated ketones and electron-deficient olefins
were tested in acetonitrile catalyzed by 200-mg MSSA
through the reaction of indole with these Michael
acceptors under mild conditions. The reactions pro-
ceeded easily, and the products were isolated with
Thus, after activation of silica gel surface with HCl
(0.05 N), to avoid the reaction of DMCS with adjacent
hydroxyl groups to form a dimmer, we used excess
amount of DMCS in reaction medium (Scheme 1).
Then, for conversion of SiACl to SiAOH groups after
silanization and drying, we added water to solid product,
and stirring it for 10 min at room temperature. The
amount of Cl groups in this step was determined by
titration with AgNO3 or with ion selective electrode
(ISE). After completion of all reactions, samples were
Scheme 5
Journal of Heterocyclic Chemistry
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980
M. A. Zolfigol, H. Veisi, F. Mohanazadeh, and A. Sedrpoushan
Vol 48
Table 1
Michael addition of indole, pyrrole, and thioles to Michael acceptors catalyzed by MSSA.
Entries
Nucleophile
Electrophile
Productsa
Time (h)
1.5
Yield (%)b
98
Refs.
[19]
1
2
1.3
1.6
1.1
2
96
92
98
95
92
96
90
98
96
90
85
92
[19]
[19]
[19]
[48]
[19]
[46]
[46]
[41]
[41]
[48]
[19]
[16]
3
4
5
6
1.8
1.5
1.1
1.2
1.6
1.5
2.1
2.5
7
8
9
10
11
12
13
(Continued)
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July 2011
Synthesis and Application of MSSA
981
Table 1
(Continued)
Entries
14
Nucleophile
Electrophile
Productsa
Time (h)
1.4
Yield (%)b
85
Refs.
[16]
[16]
[41]
[47]
[41]
[47]
[47]
[17]
[16]
[17]
[16]
[16]
[17]
15
16
17
18
19
20
21
22
23
24
25
26
1.3
2
92
90
85
92
90
75
85
95
90
75
92
95
2.5
2.1
2.5
3
0.25
0.084
0.5
0.25
0.17
0.084
a Products were characterized by 1H NMR, 13C NMR, IR, and comparison with reported data.
b Isolated yields.
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M. A. Zolfigol, H. Veisi, F. Mohanazadeh, and A. Sedrpoushan
Vol 48
Scheme 6
comparable yields in short reaction times. The results
are summarized in Table 1 (entries 1–15). The reactions
were clean, and the products were obtained in high
yields without the formation of any side products, such
as N-alkylation product.
On the basis of the results of above study, we planned
to synthesis of 1,1,3-triindolyl compounds using MSSA
as a solid acid heterogeneous catalyst under room tem-
perature and reflux conditions through the tandem
Michael addition and Friedel–Crafts reaction of a,b-un-
saturated aldehydes or ketones and indoles (Scheme 6).
In initial experiments, indole 2a and crotonaldehyde
(5a) were used as model substrates to evaluate suitable
reaction conditions for the preparation of tris-indole 5b
(Table 2). The reaction to form 1,1,3-tri(1H-indol-3-
yl)butane (5b) in 96–98% yield was completed in 10 h
at room temperature and 20 min at reflux condition in
CH3CN when 250-mg MSSA was used as the catalyst
(Table 2, entry 1).
Apparently, the pyrrole Michael addition was also
done in the same fashion as indole Michael addition.
Based on the results obtained, the reaction was extended
to pyrrole and was found that MSSA can also efficiently
catalyzed the reaction of pyrrole with different a,b-un-
saturated ketones, affording 2-substituted pyrrole deriva-
tives with comparable yields (Table 1, entries 16–20).
Thia-Michael addition products of a,b-unsaturated car-
bonyl compounds are very important building blocks for
the synthesis of bioactive compounds [43], heterocycles
[44], and are also used as chiral auxiliary for the synthesis
of optically active a-hydroxy aldehydes [45]. Therefore,
the development of an efficient and selective catalyst for
the construction of carbon–sulfur bond is of interest in or-
ganic synthesis. In continuation of our ongoing research
program to develop better and newer synthetic methodolo-
gies [24–29,40–42], we perceived that MSSA might be a
very useful catalyst for thia-Michael reaction (Scheme 5).
In a set of initial experiments, 1,3-diphenylpropenone
was allowed to react with thiophenol in an equimolar ra-
tio in the presence of a varied quantity of MSSA and
various conditions. After a series of experimentations, it
was observed that excellent yield of the Michael adduct
can be achieved by reacting a mixture of 1,3-diphenyl-
propenone (1 equiv.), thiophenol (1.1 equiv.), and
MSSA (100 mg) in acetonitrile (3 mL) at room tempera-
ture. Aliphatic and aromatic thiols can be used in the
optimized procedure with numerous cyclic and acyclic
a,b-unsaturated carbonyl compounds. Products were iso-
lated with comparable yields, and the results were sum-
marized in Table 1 (entries, 21–26). It was noticed that
the reactions of thiols with enones yielded the desired
thia-Michael addition products rapidly (5–30 min) and
in an excellent yields (75–95%).
Encouraged by this initial success, the reactions were
then performed between a variety of a,b-unsaturated
compounds with different indoles under similar condi-
tions, at room and reflux temperature. The results sum-
marized in Table 2 highlight the general applicability of
this reaction.
The mechanism for this three indole incorporating
reaction is shown in Scheme 7. The 1,4- and 1,2-addi-
tions of indole to a,b-unsaturated ketone take place
sequentially to give intermediate K in the presence of
MSSA (Scheme 7). The dehydration of K gives another
intermediate L, which is further activated by MSSA and
serves as an electrophile to react with a third molecule of
indole, affording intermediate M. The corresponding
three indole-incorporated product is subsequently formed
from intermediate M. It is thought that MSSA promotes
the reaction by increasing the electrophilic character of
the enal.
The reusability of the catalysts is one of the most im-
portant benefits and makes them useful for commercial
applications. Therefore, we investigated the recovery
and reusability of MSSA catalyst. The catalyst can be
easily separated by simple filtration and reused after
washing with anhydrous CHCl3 and drying at 60ꢀC. The
reusability of the catalyst was checked in the Michael
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Synthesis and Application of MSSA
983
Table 2
Synthesis of 1,1,3-triindolyl compounds catalyzed by MSSA.
Method A
Method B
Entries
1
Nucleophile
Electrophile
Productsa
Time (h)
10
Yield (%)
96
Time (min)
Yield (%)
98
Refs.
20
60
60
35
30
65
45
50
75
[43]
[43]
[43]
[43]
[43]
[43]
[43]
[43]
[43]
2
3
4
5
6
7
8
9
13
12.5
12
85
80
92
90
75
80
75
70
96
92
96
98
96
98
98
97
10
12
10
10
12
(Continued)
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M. A. Zolfigol, H. Veisi, F. Mohanazadeh, and A. Sedrpoushan
Vol 48
Table 2
(Continued)
Method A
Method B
Entries
Nucleophile
Electrophile
Productsa
Time (h)
15
Yield (%)
65
Time (min)
Yield (%)
90
Refs.
[43]
10
11
12
90
24
24
25
35
150
120
75
75
[43]
[43]
addition of chalcon 1a with indole in the presence of
MSSA. As can be seen, the catalyst could be used at
least four successive times without any decrease in its
T3 activity (Table 3).
and Friedel-Crafts reaction of a,b-unsaturated aldehydes
or ketones, in the presence of MSSA in CH3CN. This
method has advantages, such as ease of workup, mild
condition, high yields, and short reaction time and is a
novel method for the synthesis of important 3-substi-
tuted indole derivatives.
CONCLUSION
In conclusion, MSSA as a new type of SSA and as a
good proton source was prepared. Also, we have devel-
oped an efficient and cost-effective method for Michael
addition of indole, pyrroles, and thiols to Michael
acceptors and preparation of 1,1,3-triindolyl compounds
in excellent yields through the tandem Michael addition
EXPERIMENTAL
All commercially available chemicals were obtained from
Merck and Fluka companies, and used without further purifica-
tions unless otherwise stated. 1H NMR spectra were recorded
on a Jeol 90 MHz FT NMR spectrometer using TMS as inter-
nal standard, and chemical shifts are in d (ppm). Infrared (IR)
Scheme 7
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Synthesis and Application of MSSA
985
Table 3
General procedure for the preparation of 1,1,3-triindolyl
compounds using MSSA as a solid acid heterogeneous
catalyst. To a stirring solution of a,b-unsaturated carbonyl
compound (1 mmol) and indole (4 mmol) in CH3CN (5 mL),
MSSA (200 mg) was added at room temperature or reflux con-
ditions. After completion of the reaction and filtration, the sol-
vent was evaporated under reduced pressure. The product was
purified by flash column chromatography using n-hexane-ethyl
acetate (8:2) as eluent to give pure adducts.
Reusability of MSSA catalyst in the Michael addition
of chalcon 1a with indole.
Run
Time (h)
Yield (%)
1
2
3
4
1.5
1.5
1.5
1.5
98
96
90
80
Acknowledgments. The authors are thankful to Bu-Ali Sina
University, Center of Excellence and Development of Chemical
Methods (CEDCM) for financial support, and Payame Noor Uni-
versity (PNU).
was conducted on a Perkin Elmer GX FT-IR spectrometer. All
yields refer to isolated products.
Preparation of MSSA. Fifty gram of silica gel A (SiO2,
mesh 35–70, 675 m2/gr, Merk) was added to 150-mL HCl
(0.05 M) in a suitable vessel and stirred for 30 min at room
temperature. After 30 min, the solid was filtrated and washed
with deionized water for several times to get the activated
silica gel (B). The activated SiO2 (B) was dried under vacuum
(60 mm/Hg) in 70ꢀC for 3 h.
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Activated SiO2 (50 g) was dispersed in dry toluene (70 mL)
under nitrogen condition and then DMCS (50 g) was added to the
dispersion and stirred for 6 h. After that, filterated and washed
with dry toluene (100 mL) and Et2O (150 mL) to get the chlorodi-
methysililated silica (C). Next step for conversion of SiACl (C) to
SiAOH (D) groups, 200-mL water was added to the reaction ves-
sel of C for 10 min stirred at room temperature. Afterward, the
solid was separated by filtration and washed by 100-mL water and
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urated carbonyl compounds in the presence of MSSA as
a solid acid heterogeneous catalyst. A mixture of indole
or pyrrole (1 mmol), a,b-unsaturated carbonyl compound
(1 mmol), MSSA (200 mg), and CH3CN (3 mL) was stirred at
room temperature for the appropriate time (Table 1). After
completion of the reaction (monitored by TLC, hexane/acetone
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and filtrated. Evaporation of the solvent followed by recrystal-
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adducts, which were characterized by spectral methods.
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a,b-unsaturated carbonyl compounds in the presence of
MSSA as a solid acid heterogeneous catalyst. To a mixture
of thiol (1.1 mmol), a,b-unsaturated carbonyl compound
(1 mmol) in CH3CN (3 mL) and MSSA (100 mg) was added.
The mixture was allowed to stir at room temperature for a pe-
riod time specified in Table 1. The reaction was monitored by
TLC (3:1 n-hexane/acetone). After completion of the reaction,
the reaction mixture was diluted with CH3CN (10 mL) and fil-
trated. Evaporation of the solvent followed by short column
chromatography over silica gel (petroleum ether/ethyl acetate,
95:5, v/v) afforded pure thia-Michael adducts, which were
characterized by spectral methods.
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet