Conjugate polymer-supported acid catalysts: An efficient and reusable catalyst for the synthesis of geminal diacetates (acylals) under solvent-free conditions

Article abstract of DOI:10.1080/00397910903531946

A comparative study on the catalytic activity of conducting polyaniline (PANI), polypyrrole (PPY), and poly-(3,4-ethylenedioxythiophene) (PEDOT) salts as solid acid catalysts for the synthesis of 1,1-diacetates (acylals) under solvent-free conditions has been presented. Use of the polypyrrole and poly-(3,4-ethylenedioxythiophene) salts as solid acid catalysts is reported for the first time. Preparation, recovery, and reusability of the catalysts were found to be good. Copyright Taylor & Francis Group, LLC.

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Conjugate Polymer-Supported Acid
Catalysts: An Efficient and Reusable
Catalyst for the Synthesis of Geminal
Diacetates (Acylals) Under Solvent-Free
Conditions
Mohammad Reza Nabid a , Seyed Jamal Tabatabaei Rezaei a
Mahvash Abedi a
&
a Department of Chemistry , Shahid Beheshti University , Tehran,
Iran
Published online: 15 Dec 2010.
To cite this article: Mohammad Reza Nabid , Seyed Jamal Tabatabaei Rezaei & Mahvash Abedi (2010)
Conjugate Polymer-Supported Acid Catalysts: An Efficient and Reusable Catalyst for the Synthesis
of Geminal Diacetates (Acylals) Under Solvent-Free Conditions, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 41:2, 191-199, DOI:
10.1080/00397910903531946
To link to this article: http://dx.doi.org/10.1080/00397910903531946
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Synthetic Communications1, 41: 191–199, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903531946
CONJUGATE POLYMER-SUPPORTED ACID
CATALYSTS: AN EFFICIENT AND REUSABLE
CATALYST FOR THE SYNTHESIS OF GEMINAL
DIACETATES (ACYLALS) UNDER SOLVENT-FREE
CONDITIONS
Mohammad Reza Nabid, Seyed Jamal Tabatabaei Rezaei, and
Mahvash Abedi
Department of Chemistry, Shahid Beheshti University, Tehran, Iran
A comparative study on the catalytic activity of conducting polyaniline (PANI), polypyrrole
(PPY), and poly-(3,4-ethylenedioxythiophene) (PEDOT) salts as solid acid catalysts for
the synthesis of 1,1-diacetates (acylals) under solvent-free conditions has been presented.
Use of the polypyrrole and poly-(3,4-ethylenedioxythiophene) salts as solid acid catalysts
is reported for the first time. Preparation, recovery, and reusability of the catalysts were
found to be good.
Keywords: 1,1-Diacetates; ecofriendly catalyst; polymer-supported catalyst; reusable catalyst
INTRODUCTION
To achieve the ‘‘triple bottom line,’’ we need to develop new, more environmen-
tally friendly chemical products and processes.[1] Catalysis, which has played such a
vital role in the success of industry in the 20th century, will also play a very important
role in the greener industry of the century. Catalysis cannot only help to green
chemical processes (e.g., by replacing reagents or by enabling more efficient processes)
but also by demonstrating their value (to reduce the environmental impact of pro-
cesses and reduce the costs of the processes) will catalyze the greening of chemistry.
Accordingly, there is a global effort to replace conventional catalysts with ecofriendly
catalysts.
1,1-Diacetates (acylals) are synthetically useful as protecting groups for
aldehydes because of their stability and easy conversion into parent aldehydes.[2,3]
They are also important building blocks for the synthesis of dienes for Diels–Alder
cycloaddition reaction.[4] In fact, protection of aldehydes is one of the most impor-
tant reactions in organic synthesis.[5] Hence, there are numerous methods for the syn-
thesis of acylals. Some of the catalysts that have been developed for this purpose
include sulfuric acid,[6] triflic acid,[7] PCl3,[8] I2,[9] FeCl3,[10] N-bromosuccinmide
Received October 8, 2009.
Address correspondence to Mohammad Reza Nabid, Department of Chemistry, Shahid Beheshti
University, P. O. Box 1983963113, Tehran, Iran. E-mail: m-nabid@sbu.ac.ir
191

192
M. R. NABID, S. J. T. REZAEI, AND M. ABEDI
(NBS),[11] zeolites,[12] montmorillonite clay,[13] expansive graphite,[14] aluminum
dodecatungstophosphate,[15] zeolite HSZ-360,[16] Cu(OTf)2,[17] Sc(OTf)3,[18]
Bi(OTf)3,[19] Bi(NO3)3 ꢀ 5H2O,[20] Zn (BF4)2,[21] silica sulfuric acid,[22] HClO4 ꢀ SiO2,
[23]
SbCl3,[24] NbCl5,[25] and ZrCl4.[26] Although there is progress for the protection
and deprotection of aldehydes, developing economical and environmentally friendly
procedures remains a challenge. Therefore, we are interested in recyclable, green, and
metal-free catalysts for the production of acylals.
Conducting polymers are a new class of ‘‘synthetic metals’’ that combine the
chemical and mechanical attributes of polymers with the electronic properties of
metals and semiconductors. Among them, polyaniline is one of the most interesting
materials because of its moderately high conductivity upon doping with acids,
well-behaved electrochemistry, easy preparation, possible processability, and good
environmental stability.[27] Among several reusable and heterogeneous catalysts
designed and used in organic reactions, polyaniline salts (PANI salts) are attractive
candidates to explore for supported catalysis.[28–30] The advantages of using conduct-
ing polymer-supported acid catalysts include straightforward synthetic route, easy
preparation in large quantities, easy handling, good stability, easy recovery, recycl-
ability and ecofriendliness.
RESULTS AND DISCUSSION
In continuation of our studies on the application of catalysts for development
of useful new synthetic methodologies,[31–33] herein a simple, efficient, and high-
yielding method for the protection of various aldehydes with acetic anhydride under
solvent-free conditions in the presence of polyaniline, polypyrrole, and poly-(3,4-
ethylenedioxythiophene) salts as catalyst is described (Schemes 1 and 2).
To find the standard experimental procedure, polyaniline salts were used as a
model catalyst. We took benzaldehyde as model substrate and treated it with Ac2O
(under-solvent free conditions) in the presence of different PANI salts, including
PANI hydrochloride, PANI sulfate, and PANI nitrate (Table 1). As a result, we
found that PANI salts can effectively catalyze the reactions. Also, the results in
Table 1 show that among these catalysts, PANI sulfate was the best catalyst of
choice in terms of yield.
Next, we optimized the amount of PANI sulfate in the reaction of benzal-
dehyde and acetic anhydride. In the absence of the catalyst, product was obtained
in poor yield (about 10%) after 4 h, whereas good results were obtained in the pres-
ence of PANI sulfate salt after 15 min. We found that 10 wt% (with respect to alde-
hyde) of PANI sulfate salt could effectively catalyze the reaction for the synthesis of
the desired product. With inclusion of 5 wt% of PANI sulfate salt, the reaction took
Scheme 1. Synthesis of 1,1-diacetates in the presence of polyaniline-sulfate salt.

CONJUGATE POLYMER-SUPPORTED ACID CATALYSTS
193
Scheme 2. Selectivity in 1,1-diacetates formation during inter- and intramolecular competition studies
(based on GC and TLC analysis).
longer time. Using more than 10 wt% PANI sulfate salt had no effect on the yield
and time of the reaction (Table 1).
Also, we investigated the catalytic activity of the polypyrrole and poly-(3,4-
ethylenedioxythiophene) salts as solid acid catalysts under the optimized condition
for the synthesis of diacetates. The results show that poly-(3,4-ethylenedioxythio-
phene)-sulfate and polypyrrole-sulfate have low activity in reactions compared to
PANI sulfate salt. Using more than 10 wt% from these catalysts increased the yield
of the reaction (Table 1). However, PANI sulfate is generally the best catalyst in
terms of yield and time for various derivatives.
Table 1. Preparation of 1,1-diacetoxy-1-(phenyl)methane under various
different reaction conditionsa
Entry
Catalyst (wt%)
Hþ (mmol)
Yieldb (%)
1
2
PANI–HCl(10)
PANI–HNO3(10)
PANI–H2SO4(10)
PANI–H2SO4(20)
PANI–H2SO4(5)
PPY-H2SO4(10)
PPY-H2SO4(20)
PPY-H2SO4(30)
PEDOT-H2SO4(10)
PEDOT-H2SO4(20)
PEDOT-H2SO4(30)
None
0.041
0.026
0.024
0.048
0.012
0.018
0.036
0.054
0.009
0.018
0.027
—
84
86
3
92, 89, 90c
4
93
80
5
6
52, 52, 49c
7
65
8
66
50, 47, 49c
63
9
10
11
12
65
10d
aReaction conditions: acetic anhydride (0.2 mL, 2 mmol), benzaldehyde
(1 mmol), catalyst (1–5.4 mol%), 20 min, at rt.
bIsolated total yield.
cThe yields of three subsequent runs using the same recovered catalyst.
d30 ꢁC for 5 h.

194
M. R. NABID, S. J. T. REZAEI, AND M. ABEDI
To evaluate the efficiency of this methodology, various aromatic, heteroaro-
matic, and aliphatic aldehydes were subjected to 1,1-diacetate formation under the
catalytic influence of PANI sulfate (Table 2). Excellent results were obtained in
10–45 min. In general, the products obtained after usual workup were clean
(spectral analyses) and did not require any further purification. The reaction was
compatible with other functionality such as nitro, cyano, and ether. In the case
of hydroxybenzaldehyde, the corresponding triacetates were formed (Table 2, entry
11). No diacetate formation was observed for acetophenone (at rt for 24 h or at
80 ꢁC for 10 h). In the case of aromatic aldehydes, it was possible to monitor the
reaction visually; a clear solution was obtained after mixing the aldehyde, Ac2O,
and the catalyst, and the formation of a solid precipitate (after stirring the reaction
mixture for a few minutes) indicated the completion of the reaction. For aliphatic
aldehydes, an exothermic reaction took place after addition of Ac2O to the mag-
netically stirred mixture of the aldehyde and the catalyst, indicating the completion
of the reaction.
The difference in the reactivity of aldehydes and ketones for 1,1-diacetate for-
mation encouraged us to test the efficiency of PANI sulfate for selective 1,1-diacetate
formation during inter- and intramolecular competitions between an aldehyde and a
ketone. Thus, a mixture of benzaldehyde (1 mmol) and acetophenone (1 mmol) was
treated with Ac2O (1.5 mmol) for 15 min at room temperature in the presence of
PANI sulfate (2.4 mol%) under solvent-free conditions (Scheme 2). Benzaldehyde
1,1-diacetate was obtained in quantitative yields, and no significant formation of
the 1,1-diacetate of acetophenone was observed (NMR, GC). The treatment of
4-acetylbenzaldehyde (1 mmol) with Ac2O (1.5 mmol) for 10 min under similar
conditions resulted in exclusive formation of the 1,1-diacetate of the aldehyde group
(Scheme 2).
Table 2. Preparation of acylals in the presence of polyaniline-sulfate salt
under solvent-free conditions at rt
Entry
Substrate
Time (min)
Yielda (%)
1
2
Benzaldehyde
15
10
10
15
15
12
12
15
15
20
40
10
12
15
15
45
92
96
96
93
94
92
93
90
94
91
89
94
92
90
87
87
3-Nitrobenzaldehyde
4-Nitrobenzaldehyde
3-Chlorobenzaldehyde
4-Chlorobenzaldehyde
3-Bromobenzaldehyde
4-Bromobenzaldehyde
4-Cyanobenzaldehyde
4-Methylbenzaldehyde
4-Methoxybenzaldehyde
2-Hydroxybenzaldehyde
Cinnamaldehyde
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Isobutyraldehyde
Hexanal
Furfural
Vanillin
aIsolated yields.

CONJUGATE POLYMER-SUPPORTED ACID CATALYSTS
195
Figure 1. FTIR spectrum of polyaniline sulfate salt in KBr medium (a) as synthesized and (b) after
three uses.
One of the advantages of solid acid catalysts is their ability to perform as a
recyclable reaction media. The feasibility of recyclable properties of the PANI sulfate
and other polymer-supported acid catalysts was also examined through a series of
sequential reactions of benzaldehyde and Ac2O as model substrates. In a typical
reaction, the catalysts were simply filtered from the reaction and reused for three
cycles (Table 1, entries 3, 6, and 9). The reaction proceeded smoothly with a good
yield, and this result indicates that the catalysts do not lose activity and can be
reused. After the third trial, the catalyst was recovered and characterized by infrared
(IR), the amount of acid present in the polymer, and density measurements. A
similar IR pattern was observed for the PANI salt as prepared and the sample after
reaction (Fig. 1). Similarly, the value of the amount of acid group, density, and
conductivity for the polymer salt samples (before and after reaction) are almost
the same (see Table 3).
The suggested mechanism for PANI sulfate salt–catalyzed acetylation of
aldehydes is shown in Scheme 3.
Table 3. Physical and electrical properties of polymer salts
Before reaction
After the fourth trial
Conductivity
(S=cm)
Dopant per Density Conductivity
monomeric unit (g=cm3)
(S=cm)
Dopant per
Density
Polymer salt
monomeric unit (g=cm3)
PANI–HCl
0.5 ꢂ 10ꢃ4
2.0 ꢂ 10ꢃ2
1.3 ꢂ 10ꢃ2
1.2 ꢂ 10ꢃ2
0.45
0.30
0.33
0.14
0.13
1.22
1.22
1.21
1.42
1.47
0.46 ꢂ 10ꢃ4
1.95 ꢂ 10ꢃ2
1.28 ꢂ 10ꢃ2
1.11 ꢂ 10ꢃ2
0.72 ꢂ 10ꢃ2
0.43
0.28
0.31
0.11
0.10
1.22
1.23
1.22
1.45
1.48
PANI–H2SO4
PANI–HNO3
PPY-H2SO4
PEDOT-H2SO4 0.8 ꢂ 10ꢃ2

196
M. R. NABID, S. J. T. REZAEI, AND M. ABEDI
Scheme 3. A mechanistic proposal.
CONCLUSION
In conclusion, PANI sulfate is a new and highly efficient polymer-supported
proton source catalyst for 1,1-diacetate formation from aldehydes. The advantages
include recyclability of the catalyst with no loss in its activity, simple experimental
procedure, use of very small quantities of catalyst, nontoxicity, noncorrosivity, short
reaction times, inexpensive solid acid catalyst, operation at room temperature, excel-
lent yields, and good selectivity. With the increasing tight legislation on the release of
waste and use of toxic substances as a measure to control environmental pollution,[34]
the use of solvent-free reaction conditions employed in the present method makes it
environmentally friendly and suitable for large-scale synthesis.
EXPERIMENTAL
A polymer sample in the form of a cylindrical pellet (10 mm in diameter, 1–1.5 mm
thick) was obtained by subjecting the sample to a pressure of 400 MPa. Resistance
measurement of the pellet was carried out using a Keithley 213 Quad voltage source.
Pellet density was measured from mass per unit volume of the pressed pellet. The
amount of acid group present in the polymer chain was calculated based on the weight
of redoped polymer salt and the weight of the used polymerbase. Melting points were
measured on an Electrothermal 9200 apparatus and are uncorrected. 1H and 13C NMR
spectra were recorded on a Bruker DRX-300 Avance spectrometer at 300.13 and
75.47 MHz, respectively. IR spectra were recorded on a Shimadzu IR-470 spectrometer.
General Procedure for the Preparation of Catalysts
In a 2-L round-bottomed flask, 100 ml of water were taken, and 20 ml (1.0 N)
of acid solution were added slowly with stirring. To this mixture, 10 mmol of

CONJUGATE POLYMER-SUPPORTED ACID CATALYSTS
197
monomer was added, and the solution was kept under constant stirring at room tem-
perature. To this solution, 25 ml aqueous solution containing ammonium persulfate
(2.28 g) were added for 15–20 min. The reaction was allowed to continue for 4 h at
room temperature (except for 3,4-ethylenedioxythiophene; 10 h). The precipitated
polymer salt was recovered from the polymerization vessel by filtration and then
washed with distilled water until the washing liquid was colorless. To remove oligo-
mers and other organic by-products, the precipitate was washed with methanol until
the methanol solution was colorless. Finally, the resulting polymer salt was washed
twice with acetone and subsequently dried at 100 ꢁC until it achieved a constant
mass. The physical and electrical properties of polymer salts are shown in Table 3.
General Procedure for the Preparation of Acylals
Polymer-supported catalyst (10 wt% with respect to aldehyde) was added to a
stirred solution of the respective aldehyde (1 mmol) in freshly distilled Ac2O (0.2 mL,
2 mmol), and the mixture was stirred at room temperature for the time specified in
Table 2. The reaction was followed by thin-layer chromatography (TLC; n-hexane–
EtOAc, 9:1). After completion of the reaction, the mixture was diluted with Et2O
and filtered. The organic layer was washed with aqueous 10% NaHCO3 and satu-
rated NaHSO3 solution and dried (Na2SO4). The solvent was evaporated under
reduced pressure to give the corresponding pure 1,1-diacetate (Scheme 1 and
Table 2). The spectral data of some selected 1,1-diacetates are listed.
All the products were characterized by spectral methods as well as by compari-
son of their spectral data with those reported earlier.[8–25]
Selected Spectral Data of the Products
1,1-Diacetoxy-1-(4-chlorophenyl)methane (Entry 5, Table 2). Mp 80–
82 ꢁC; IR (KBr): 3060, 3020, 1760, 1605, 1482, 1210, 1011, 780, 620 cmꢃ1 1H
.
NMR (300.13 MHz, CDCl3): d 7.55 (s, 1 H), 7.3–7.2 (s, 4 H), 2.1 (s, 6 H).
1,1-Diacetoxy-1-(2-acetoxyphenyl)methane (Entry 11, Table 2). Mp
100–101 ꢁC; IR (KBr): 3050, 2985, 1760, 1750, 1610, 1530, 1492, 1380, 1250, 1205,
1
1160, 1070, 990, 950, 810, 750 cmꢃ1. H NMR (300.13 MHz, CDCl3): d 7.75 (s,
1 H), 7.25–7.15 (m, 2 H), 7.1–7.0 (m, 2 H), 2.18 (s, 3 H), 1.94 (s, 6 H).
ACKNOWLEDGMENT
We gratefully acknowledge the financial support from the Research Council of
Shahid Beheshti University.
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