Amorphous carbon-silica composites bearing sulfonic acid as solid acid catalysts for the chemoselective protection of aldehydes as 1,1-diacetates and for N-, O- and S-acylations

Article abstract of DOI:10.1039/c0gc00900h

Amorphous carbon-silica composites bearing sulfonic acid derived from inexpensive natural organic compounds (glucose, maltose, cellulose, chitosan and starch) were prepared by partial carbonization followed by sulfonation and their catalytic activity was evaluated for the protection of aldehydes as 1,1-diacetates and for N-, O- and S-acylations under solvent-free conditions. Different biomaterials have been chosen, with a view to select the most active solid acid catalyst. Carbon-silica composites were characterized by FTIR, XRD and elemental analysis. Sulfonated carbon-silica composite derived from starch was found to be the most active and could be recycled for several runs without loss of significant activity. It was also characterized by TGA, SEM and TEM.

Full text of DOI:10.1039/c0gc00900h

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PAPER
Amorphous carbon-silica composites bearing sulfonic acid as solid acid
catalysts for the chemoselective protection of aldehydes as 1,1-diacetates
and for N-, O- and S-acylations
Princy Gupta and Satya Paul*
Received 8th December 2010, Accepted 10th June 2011
DOI: 10.1039/c0gc00900h
Amorphous carbon-silica composites bearing sulfonic acid derived from inexpensive natural
organic compounds (glucose, maltose, cellulose, chitosan and starch) were prepared by partial
carbonization followed by sulfonation and their catalytic activity was evaluated for the protection
of aldehydes as 1,1-diacetates and for N-, O- and S-acylations under solvent-free conditions.
Different biomaterials have been chosen, with a view to select the most active solid acid catalyst.
Carbon-silica composites were characterized by FTIR, XRD and elemental analysis. Sulfonated
carbon-silica composite derived from starch was found to be the most active and could be recycled
for several runs without loss of significant activity. It was also characterized by TGA, SEM and
TEM.
biodegradable and naturally derived materials. In recent times,
Introduction
mesoporous carbon functionalized sulfonic acids derived from
Homogeneous acid catalysts such as H2SO4, HF and H3PO4,
carbohydrates have been found to be more effective and selective
are highly efficient catalysts in industrial catalytic processes.
solid acid catalysts.10 More recently, it was found that sulfonated
However, such liquid acid catalysts require special processing in
amorphous carbon when dispersed over silica materials leads
the form of neutralization, which involve costly and inefficient
catalyst separation from products and results in a non-recyclable
to sulfonated carbon-silica composites having more stability,
activity and selectivity. Nakajima et al. have prepared sulfonated
sulfate waste. According to the principles of “green chemistry”
amorphous carbon-silica composites for the selective dimeriza-
and “green technology”, production methods should be refined
so as to minimize adverse effects on the environment or human
health.1 Solid acid catalysts that are easily separable, reusable
tion of a-methylstyrene.11 Sufonated carbon-silica composite
with high hydrothermal stability and amphiphilic property was
studied for the esterification of acetic acid with n-butanol.12
and insoluble acidic solids are most promising candidates for the
Degradation of cellulose into high yield of glucose has also
change from various homogeneous acid catalyst based processes
been reported in the presence of sulfonated silica/carbon
to more environmentally sensible processes. Such solid acids can
nanocomposites.13 Thus, a study based on the preparation of
be readily separated from the products and repeatedly reused,
sulfonated amorphous carbon-silica composites derived from
thereby minimizing the energy consumption for the production
natural organic compounds like glucose, maltose, cellulose,
of chemicals. In this context, solid acids based on sulfated
chitosan and starch is highly desirable to select the appropriate
zirconia,2 Cs-exchanged heteropolyacids,3 acidic polymers,4 and
biomaterial during the preparation of solid acids as catalysts in
zeolites5 have been developed. Among various solid acids, silica
organic synthesis.
functionalized sulfonic acids have been widely applied in organic
The protection of aldehydes as acylals, acetals or dithioacetals
synthesis,6 but have relatively low –SO3H density and low
is one of the most common functional group transformation
hydrothermal stability in boiling water. However, Je´roˆme et al.7
during multistep synthesis. Numerous methods are available
and Fukuoka et al.8 have developed water tolerant sulfonic acid
for the synthesis of 1,1-diacetates from aldehydes and acetic
functionalized mesoporous silicas.
anhydride.14a–14i Recently, a number of catalysts like conju-
The preparation of porous materials from renewable
gate polymer supported acid catalyst,14j polystyrene supported
biomaterials9 is a relatively new area but new applications
Al(OTf)3,14k BEA-SO3H14l have been used for the protection of
are developing, mainly as a consequence of a demand for
aldehydes as 1,1-diacetates. Although some of these methods
afford good to high yield of corresponding diacetates, majority
suffer from one or more disadvantages such as harsh acidic
Department of Chemistry, University of Jammu, Jammu, 180 006, India.
conditions, long reaction times, need for expensive catalyst
E-mail: paul7@rediffmail.com; Fax: +91-191-2505086;
and stoichiometric or excess amount of reagents. Thus, a
Tel: +91-191-2453841
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mild, cost-effective and green procedure is still required to be
developed.
The N-, O- and S-acylation is one of the most basic
and frequently used processes in organic syntheses.15 A num-
ber of methods using acid catalysts like zinc(II) chloride,16a
scandium(III) triflate,16b bismuth(III) triflate,16c ruthenium(III)
acetylacetonate16d have been reported. In addition solvent-free
acylations using various catalysts have been reported from the
viewpoint of ecofriendly protocol.17a–17e Recently, Mihara et al.17f
have reported iron(III) chloride catalyzed solvent-free O-, S-
and N- acylations and Pasha et al.17g have reported zinc dust
catalyzed acylation of phenols, thiophenol, amines and alcohols
under solvent-free conditions. A practical, mild, cost-effective
and green procedure is still in demand in order to save energy,
reduce the amount of waste produced, and utilize easily available
resources.
Scheme 1 Synthesis of carbon-silica composites by partial carboniza-
tion of inexpensive natural organic compounds.
Keeping in view our interest in the development of mild
procedures for solid acid catalyzed organic transformations,18
herein, we describe the preparation of amorphous carbon-
silica composites from inexpensive natural organic compounds
and their catalytic activity was evaluated for the protection of
aldehydes as 1,1-diacetates and for N-, O- and S-acylations
under solvent-free conditions.
–OH groups in the aromatic rings. The sulfonated carbon-
silica composites also showed the absorption bands from 1450–
1460 cm-1 (due to asymmetric stretching of SO2) and between
1100–1110 cm-1 (due to symmetric stretching of SO2), which
indicated that sulfonated carbon-silica composites possess –
SO3H groups. The broadbandat 2300–2700 cm-1 canbeassigned
to an overtone (Fermi resonance) of the bending mode of –OH—
Results and discussion
O
linked by a strong hydrogen bond, as seen in strong liquid
Preparation and characterization of sulfonated carbon-silica
composites
Brønsted acids such as CF3SO3H,19 suggesting that some –SO3H
groups are in close proximity. Sulfonated carbon-silica compos-
ites prepared from different natural organic compounds showed
similar FTIR spectra. XRD patterns exhibited a weak and broad
C(002) diffraction peak between 2q = 10–30◦, was attributed
to carbon composed of aromatic carbon sheets oriented in a
considerable random fashion (Fig. 1).20 The –SO3H loading was
determined by elemental analysis. The carbon-silica composites
derived from glucose, maltose, cellulose, chitosan and starch
were found to contain 0.48, 0.47, 0.42, 0.40 and 0.50 mmol of
SO3H per gram of carbon-silica composite respectively. Similar
amounts have also been obtained by titration with a standard
NaOH solution using phenolpthalein as an indicator.
Sulfonated amorphous carbon-silica composites were prepared
by the partial carbonization of natural organic compounds
(glucose, maltose, cellulose, chitosan and starch) in the presence
of silica followed by sulfonation. In the first step, natural
organic compound and silica which acts as a precursor was
pyrolyzed, followed by dehydration and dissociation of –C–O–
C– linkages, leading to the formation of polycyclic aromatic
carbon rings and the amorphous carbon structure. Sulfonic acid
groups are then introduced into the aromatic carbon rings by
sulfonation with sulfuric acid. The preparation procedure of
sulfonated amorphous carbon-silica composites is represented
in Scheme 1. It has been reported that carbon materials become
harder with increasing carbonization temperature due to plane
growth and stacking of the carbon chains. Carbon materials
carbonized at lower temperatures have smaller carbon sheets and
therefore have higher –SO3H densities since the –SO3H groups
are attached only to the edges of the carbon sheets. Keeping these
facts in view, we carried out the partial carbonization at 523 K
so as to get higher densities of –SO3H. The characterization of
sulfonated carbon-silica composites was carried out with FTIR,
XRD, and elemental analysis. The most active carbon-silica
composites derived from starch was also characterized by TGA,
SEM and TEM.
The most active carbon-silica composite derived from starch
was also characterized by SEM, TEM and thermal analysis.
The microstructure and morphology was studied with Scanning
Electron Microscope (SEM). SEM image (Fig. 2) showed that
composite is a fine homogeneous powder with porous structure.
TEM micrographs (Fig. 3) indicated that materials derived
from starch and silica have composite type of structure in
which sulfonated carbon particles are surrounded by the silica
material. The average size of the composite material is 17 nm
while carbon particles are 7 nm. The silica component (K100
with irregular mesopores) in the composite materials provides
good mechanical and thermal stability to the composite. The
biomaterials can readily be loaded into the mesopores of
silica and then carbonized without aggregation of the carbon
particles in the restricted mesopores. Partial carbonization of
biomaterial loaded onto the walls in mesoporous silica followed
by sulfonation would form small SO3H bearing carbon particles
in the mesopores of silica with large surface area that would
increase the catalytic activity of the catalyst and, also its ability to
The FTIR spectra of sulfonated carbon-silica composites
(CSC-Glu, CSC-Mal, CSC-Cell, CSC-Chit and CSC-Star),
and non-sulfonated carbon-silica composites showed bands at
1625 cm-1 and 1390 cm-1 which were assigned to aromatic C
C
stretching modes in polyaromatic rings. The bands at 2920 cm-1
in non-sulfonated carbon-silica composites and at 2929 cm-1 in
sulfonated carbon-silica composites were due to the phenolic
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Fig. 1 XRD diffraction patterns of CSC-Star.
of the composite. In the DTG curve, peak at 68 ◦C further
indicated the loss of residual solvent and water molecules present
in the composite. Further, weight loss occurred from 275 to
700 ◦C, was due to the loss of organic functionality (Fig. 4).
Thus, the composite is stable up to 275 ◦C and it is safe to
carry out the reaction at 60 ◦C under solvent-free conditions.
Moreover, the sulfonated carbon-silica composites showed quite
good hydrothermal stability as their ordered structure remained
intact after being treated with boiling water for several hours.
The hydrothermal stability of the composites is probably due to
the synergism between the carbon material and the silica (K100,
0.063–0.200 mm).
Catalyst testing for the protection of aldehydes as 1,1-diacetates
Fig. 2 SEM image of CSC-Star.
1,1-Diacetates were synthesized by stirring a mixture of aldehyde
and acetic anhydride at 60 C in the presence of carbon-silica
◦
composites under solvent-free conditions (Scheme 2). To select
the appropriate carbon-silica composite from suitable natural
organic compound (glucose, maltose, cellulose, chitosan and
starch), benzaldehyde was selected as the test substrate and
reaction was carried out with different composite materials
(equivalent to 7.5 mol% SO3H with respect to aldehyde)
◦
using acetic anhydride (1.5 mmol) at 60 C under solvent-free
conditions. The results are represented in Table 1. It was found
that carbon-silica composite derived from starch showed the
Fig. 3 TEM micrographs of CSC-Star.
catalyze hydrophobic acid catalyzed reactions. The TGA curve
◦
showed an initial weight loss up to 100 C, which may be due
Scheme 2 CSC-Star catalyzed chemoselective protection of aldehydes
to loss of residual solvent and water trapped onto the surface
as 1,1-diacetates under solvent-free conditions.
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Fig. 4 TGA of CSC-Star.
Table 1 Effect of the carbon-silica composites on the synthesis of 1,1-
diacetate of benzaldehyde and acetanilide of aniline at 60 ◦C under
solvent-free conditions
tions milder, the reaction in the case of test substrate was also
carried out at room temperature under similar conditions, it was
found that the reaction was quite slow and complete conversion
of benzaldehyde to its 1,1-diacetate could be obtained in 20 h.
Thus, 60 ◦C was selected as the optimum reaction temperature.
Further, for 1 mmol of aldehyde, 1.5 mmol of acetic anhydride
was required to get the optimum yield and 7.5 mol% SO3H
with respect to aldehyde was the optimum CSC-Star amount.
Chemoselectivity of the developed protocol was demonstrated
by the fact that in the presence of cyclic (cyclohexanone)
and acyclic (acetophenone) ketones, exclusively 1,1-diacetate
of benzaldehyde was formed (Scheme 2). To demonstrate the
generality of the developed protocol, aldehydes substituted with
electron-donating (Table 2, entries 2, 3, 4 and 5) and electron-
withdrawing (Table 2, entries 8, 9 and 10) groups were chosen
and excellent results were obtained (Table 2). Furthermore,
heterocyclic aldehydes (entry 13) also undergo 1,1-diacetate
formation efficiently.
1,1-Diacetateb
Acetanilidec
Carbon-silica
Entry compositea
Time (h) Yieldd (%) Time (h) Yieldd (%)
1
2
3
4
5
CSC-Glu
CSC-Mal
CSC-Cell
CSC-Chit
CSC-Star
5
6
9.5
10
2.5
85
80
80
82
96
6
85
90
90
80
95
6.5
10
10.5
2
a CSC-Glu: carbon-silica composite from glucose; CSC-Mal: carbon-
silica composite from maltose; CSC-Cell: carbon-silica composite from
cellulose; CSC-Chit: carbon-silica composite from chitosan; CSC-Star:
carbon-silica composite from starch. b Reaction was carried out by
stirring a mixture of benzaldehyde (1 mmol), acetic anhydride (1.5 mmol)
and carbon-silica composite (7.5 mol% SO3H) at 60 ◦C under solvent-
free conditions. c Reaction was carried out by stirring a mixture of
aniline (1 mmol), acetic anhydride (1 mmol) and carbon-silica composite
(7.5 mol% SO3H) at 60 ◦C under solvent-free conditions. d Isolated
yields.
Catalyst testing for the N-, O- and S-acylations
The catalytic activity of carbon-silica composites was also
studied for N-, O- and S-acylations (Scheme 3). Aniline was
best results. Thus, out of various natural organic compounds
(glucose, maltose, cellulose, chitosan and starch), starch could be
the best choice for the preparation of carbon-silica composites
for acid catalysis. The more activity of sulfonated amorphous
carbon-silica composite derived from starch might be due to the
presence of amylopectin which leads to the formation of small
polycyclic aromatic carbon rings that provided anchoring points
for SO3H groups. In order to further make the reaction condi-
Scheme 3 CSC-Star catalyzed N-, O- and S-acylations under solvent-
free conditions.
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Table 2 CSC-Star catalyzed protection of aldehydes as 1,1-diacetatesa
excellent results were obtained (Table 4). In order to compare
the activity of CSC-Star with silica, amorphous carbon, non-
sulfonated carbon-silica composite and sulfonated amorphous
carbon, 2-nitrobenzaldehyde (for 1,1-diacetate formation) and
2-nitroaniline (for acylation) were selected as test substrates. The
results are presented in Table 5. It was found that CSC-Star was
superior to all of these materials used for its preparation.
at 60 ◦C under solvent-free conditions
EntryR
Time (h)Yieldb (%) m.p/lit. m.p (◦C)
1
2
3
4
5
6
7
8
C6H5
2.5
2
2
2
7
2.5
4
5
5
4
95
95
94
96
95
96
96
95
95
96
94
96
44–45/4514i
4-(OCH3)C6H4
4-(CH3)C6H4
2-(OH)C6H4
4-(OH)-3-(OCH3)C6H3
4-ClC6H4
2,4-DiClC6H3
2-(NO2)C6H4
3-(NO2)C6H4
4-(NO2)C6H4
C6H5CH CH
2-Furyl
59–60/63—6421a
77–78/81–8214i
102–103/103-10421c
88–89/90–9121b
80–81/82–8314i
100–101/101–10221b
87–89/9021a
Recyclability
In the case of solid acid catalysis, the most important point
is the deactivation and recyclability of the catalyst. To test
the deactivation and recyclability of CSC-Star, a series of
7 consecutive runs for the synthesis of 1,1-diacetate from
benzaldehyde (entry 1, Table 2) and N-acylation of aniline (entry
2, Table 4) were carried out and results are represented in Fig. 5,
which demonstrate that CSC-Star is highly active and selective,
stable and recyclable up to the 7th run without loss of significant
activity. The elemental analysis of the CSC-Star after 7th run
indicated negligible change in the sulfur content. The slight loss
of activity may be due to the microscopic changes on the surface
of the catalyst.
9
65–66/64–6614i
124–125/125–12621a
84–85/86–8721a
52–53/52–5421a
10
11
12
4
3.5
a Reaction conditions: aldehyde (1 mmol), acetic anhydride (1.5 mmol),
CSC-Star (0.15 g, 7.5 mol% SO3H) at 60 ◦C under solvent-free
conditions. b Isolated yields.
selected as the test substrate and acylation was carried out with
acetic anhydride using CSC-Glu, CSC-Mal, CSC-Cell, CSC-
Chit and CSC-Star under the similar conditions developed for
the chemoselective protection of aldehydes as 1,1-diacetates. The
results are presented in Table 1, which indicated that out of
various carbon-silica composites, CSC-Star exhibited highest
activity. The optimum conditions selected for acylation are:
substrate (1 mmol), Ac2O (1 mmol), CSC-Star (0.15 g, 7.5 mol%
SO3H with respect to the substrate) and 60 ◦C as the reac-
tion temperature under solvent-free conditions. The acylation
reaction was also carried out using different acylating agents
selecting 2-nitroaniline as the test substrate for N-acylation and
phenol for O-acylation. The results are presented in Table 3. The
results in Table 3 indicated that among various acylating agents,
acetyl chloride was the best for N-acylations, whereas for O-
acylations, acetic anhydride was the best. The acetic acid was
found to be ineffective for N-acylations, but for O-acylations,
65% isolated yield of phenyl acetate was obtained. Maleic and
benzoic anhydrides were also found to be successful acylating
agents for N- and O-acylations (Table 3). To demonstrate the
generality of the developed protocol, various amines (aliphatic,
aromatic and heterocyclic); phenols and thiols were subjected
to acylation with both acetyl chloride and acetic anhydride
using CSC-Star under solvent-free conditions at 60 ◦C and
Fig. 5 Recyclability of CSC-Star. Reaction conditions – 1,1-diacetate:
benzaldehyde (1 mmol), Ac2O (1.5 mmol), CSC-Star (0.15 g, 7.5 mol%
SO3H) at 60 ◦C for 3 h; Acetanilide: aniline (1 mmol), Ac2O (1 mmol),
CSC-Star (0.15 g, 7.5 mol% SO3H) at 60 ◦C for 2 h.
Table 3 Effect of different acylating agents in CSC-Star catalyzed N-
acylation of 2-nitroanilinea and O-acylation of phenola
Entry Substrate
Acylating agent
Time (h) Yieldb (%)
Experimental
1
2
2-Nitroaniline AcCl
0.75
10
2.5
3.5
3.5
3.5
6
3
4.5
4
95
NRc
95
85
90
88
65
90
74
82
General
AcOH
Ac2O
The chemicals used were either prepared in our laboratories
or purchased from Aldrich chemical company and Merck. The
products were characterized by comparison of their physical
data with those of known samples or by their spectral data. The
1H NMR data were recorded in CDCl3 or CDCl3+DMSO-d6
on Bruker DPX 200 (200 MHz) spectrometer using TMS as an
internal standard. The FTIR spectra were recorded on Perkin–
Elmer FTIR spectrophotometer using KBr windows and mass
spectral data were recorded on Bruker Esquires 3000 (ESI).
XRD diffraction patterns were determined on Bruker AXSD8
Maleic anhydride
Benzoic anhydride
AcCl
Phenol
AcOH
Ac2O
Maleic anhydride
Benzoic anhydride
a Reaction conditions: Substrate (1 mmol), acylating agent (1 mmol),
CSC-Star (0.15 g, 7.5 mol% SO3H) at 60 ◦C under solvent-free
conditions. b Isolated yields. c NR: No reaction.
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Table 4 CSC-Star catalyzed N-, O- and S- acylationsa at 60 ◦C under solvent-free conditions
Entry
1
Reactant
Acylating agent
Product
Time (h)
Yieldb (%)
m.p/lit. m.p. (◦ C)
228–229/22922
110–111/113–522
90–92/9422
Butylamine
Aniline
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
AcCl
Ac2O
N-Butylacetamide
Acetanilide
0.35
2
0.5
2
0.75
2.5
0.55
3
0.5
3
0.4
4
0.45
2
0.75
2
3.5
3
6
4
4.5
4
4
3.5
6
4
3
3
91
90
96
94
95
95
94
95
94
94
94
94
96
95
94
92
88
90
89
96
91
96
91
94
86
94
90
95
92
96
92
95
90
94
86
90
2
3
2-Nitroaniline
3-Nitroaniline
4-Nitroaniline
Morpholine
Diphenylamine
2-Nitroacetanilide
3-Nitroacetanilide
4-Nitroacetanilide
N-acetylmorpholine
N-acetyldiphenylamine
4
154–155/15522
215–216/21622
13–14/1422
5
6
7
102–103/10323
209–210/208–922
195–196/19622
38–40/4122
8
2-Amino-4-phenyl
thiazole
Phenol
2-(N-acetylamino)- 4-
phenylthiazole
Phenylacetate
9
10
11
12
13
14
15
16
17
18
2-Nitrophenol
4-Nitrophenol
1-Naphthol
2-Nitrophenylacetate
4-Nitrophenylacetate
1-Acetoxynaphthalene
2-Acetoxynaphthalene
1,3-Diacetoxybenzene
1,2-Diacetoxybenzene
1,4-Diacetoxybenzene
Cyclohexyl acetate
80–81/8323
47–48/43–4622
69–70/69–7022
145–146/146–722
97–98/9823
2-Naphthol
Resorcinol
Catechol
4.5
4
6.5
5
4
3
Hydroquinone
Cyclohexanol
2-Aminobenzene thiol
119–120/12122
76–77/7723
1-Thioacetoxy-2-N-
acetylaminobenzene
8
7
133–134/13522
a Reaction conditions: substrate (1 mmol), acetyl chloride or acetic anhydride (1 mmol), CSC-Star (0.15 g, 7.5 mol% SO3H) at 60 ◦C under solvent-free
conditions. b Isolated yields.
Table 5 Comparison of activity of CSC-Star with silica, amorphous
carbon, non-sulfonated carbon-silica composite and sulfonated amor-
phous carbon for diacetate formationa and acylationa using Ac2O at
60 ◦C under solvent-free conditions
Micrographs (TEM) were recorded on H7500 Hitachi. The
amount of sulfur in carbon-silica composites was determined by
elemental analysis on Elementar Analysensyteme GmbH Vari-
oEL. Thermal analysis was carried out on DTG-60 Shimadzu
make thermal analyzer. All yields refer to isolated yields.
Entry Catalyst
Time (h) Yieldb (%)
1
2
3
4
5
6
7
No Catalyst
Silica
Amorphous carbon
Non-sulfonated carbon-silica composite
Sulfonated amorphous carbon
CSC-Star
No Catalyst
Silica
Amorphous carbon
5
5
5
5
5
5
2.5
2.5
2.5
NRc
NRc
NRc
NRc
45
General procedure for the synthesis of sulfonated carbon-silica
composites
Carbon-silica composites were prepared by drying the mixture
of silica (K100, 0.063–0.200 mm) which acts as a precursor
and natural organic compound (glucose, maltose, cellulose,
chitosan or starch) in the ratio of 1 : 1.2 in a round-bottom
flask (25 mL) at 353 K for 15 h, followed by incomplete
carbonization at 673 K under nitrogen for 10 h. During partial
carbonization, white color get changed to black, indicating that
carbon-silica composites are formed. These were then sulfonated
in concentrated sulfuric acid (>96 w%) on heating at 423 K for
10 h under N2 atmosphere. The composite material obtained
was then washed repeatedly with hot distilled water (>353 K)
until sulfate anions were no longer detected in the filtered water.
Sulfonated carbon-silica composites were finally dried in an oven
at 373 K for 2 h.
95
NRc
40
8
9
45
10
11
12
Non-sulfonated carbon-silica composite 2.5
62
65
95
Sulfonated amorphous carbon
2.5
2.5
CSC-Star
a Reaction conditions: 2-nitrobenzaldehyde (for diacetate formation,
entries 1–6) and 2-nitroaniline (for acylation, entries 7–12) (1 mmol),
acetic anhydride (1.5 mmol for diacetate formation and 1 mmol for
acylation), catalyst (0.15 g) at 60 ◦C under solvent-free conditions.
b Isolated yields. c NR: No reaction.
X-ray diffraction spectrometer and SEM using Jeol make
T-300 Scanning electron Microscope. Transmission Electron
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General procedure for the protection of aldehydes as
1,1-diacetates
Notes and references
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To a mixture of aldehyde (1 mmol) and acetic anhydride
(1.5 mmol) in a round-bottom flask (10 mL), CSC-Star (0.15 g,
7.5 m◦ol% SO3H) was added and reaction mixture was stirred
at 60 C under solvent-free conditions for an appropriate time
(Table 2). On completion (monitored by TLC), reaction mixture
was triturated with EtOAc (5 mL) and filtered. The filtrate was
washed with distilled water (2 ¥ 5 mL) and dried over anhyd.
Na2SO4. The 1,1-diacetate was obtained after removal of the
solvent under reduced pressure followed by crystallization with
ethanol. The residue was washed with EtOAc (3 ¥ 5 mL) followed
by double distilled water (3 ¥ 5 mL). It was dried at 100 ◦C for
2 h and reused for subsequent reactions.
General procedure for N-, O- and S-acylations under
solvent-free conditions
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(1 mmol) or AcCl (1 mmol) in a round-bottom flask (10 mL),
CSC-Star (0.15 g, 7.5 mol% SO3H) was added and reaction
mixture was stirred at 60 ◦C under solvent-free conditions
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(7 mL) and filtered. The filtrate was washed with distilled
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The products were confirmed by IR, 1H NMR, mass spectral
data and comparison with authentic samples obtained commer-
cially or prepared according to literature methods.
Conclusions
We have reported a simple and efficient method for the prepa-
ration of carbon-silica composites derived from inexpensive
natural organic materials (glucose, maltose, cellulose, chitosan
and starch) and their catalytic activity was evaluated for the
protection of aldehydes as 1,1-diacetates and for N-, O- and S-
acylations under solvent-free conditions with a view to select
the appropriate inexpensive natural organic material for the
preparation of carbon-silica composites. The main advantages
of carbon-silica composites include (i) high yield of products
with high purity; (ii) greater selectivity; (iii) simple work-up
procedure; and (iv) recyclability up to several runs with high
activity.
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Acknowledgements
We thank the Director, IIIM Jammu for spectral and library fa-
cilities; Head, Sophisticated Analytical Instrumentation facility,
Punjab University Chandigarh for XRD, SEM and TEM; and
Head, RSIC, IIT Roorkee for thermal analysis.
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