An efficient method for synthesis of acylals from aldehydes using multi-walled carbon nanotubes functionalized with phosphonic acid (MWCNTs-C-PO3H2)

Article abstract of DOI:10.1016/j.cclet.2014.07.005

MWCNTs-C-PO₃H₂ has been used as an efficient, heterogeneous and reusable nanocatalyst for synthesis of acylals from aldehydes under solvent-free conditions at room temperature. A wide range of aldehydes was studied and corresponding products were obtained in good to excellent yields in short reaction times. Nanocatalyst can be easily recovered by centrifuge and reused for subsequent reactions for at least five times without deterioration in catalytic activity. The major advantages of the present method are high yields, short reaction time, recyclable catalyst and solvent-free reaction conditions at room temperature.

Full text of DOI:10.1016/j.cclet.2014.07.005

G Model
CCLET 3067 1–5
Chinese Chemical Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
2
Original article
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4
5
An efficient method for synthesis of acylals from aldehydes using
multi-walled carbon nanotubes functionalized with phosphonic acid
(MWCNTs-C-PO3H2)
a,b
Q1 Farzaneh Dehghani a, Ali Reza Sardarian a, , Mohammad Mehdi Doroodmand
*
6
7
8
a Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, Iran
b Nanotechnology Research Center, Shiraz University, Shiraz 71545, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
MWCNTs-C-PO3H2 has been used as an efficient, heterogeneous and reusable nanocatalyst for synthesis
of acylals from aldehydes under solvent-free conditions at room temperature. A wide range of aldehydes
was studied and corresponding products were obtained in good to excellent yields in short reaction
times. Nanocatalyst can be easily recovered by centrifuge and reused for subsequent reactions for at least
five times without deterioration in catalytic activity. The major advantages of the present method are
high yields, short reaction time, recyclable catalyst and solvent-free reaction conditions at room
temperature.
Received 11 February 2014
Received in revised form 16 June 2014
Accepted 17 June 2014
Available online xxx
Keywords:
Acylal
ß 2014 Ali Reza Sardarian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
reserved.
MWCNTs-C-PO3H2
Heterogeneous catalyst
Reusable catalyst
Solvent-free condition
9
10
1. Introduction
propyl-diethylene-triamine-N-sulfamic acid (SPDTSA) [19] and 28
sulfonated rice husk ash (RHA-SO3H) [20].
29
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Protection and deprotection of carbonyl groups are extremely
important steps in modern organic chemistry [1]. Acylal formation
is one of the most useful methods to protect the carbonyl group of
aldehydes and ketones and has found wide application in
multistep syntheses due to acylal stability in neutral and basic
The development of heterogeneous catalysts for organic 30
synthesis has become a major area of research. The potential 31
advantages of these materials over homogeneous systems 32
(simplified recovery, reusability and the potential for incorpo- 33
ration in continuous reactors and microreactors) could lead to 34
novel, environmentally benign chemical procedures for academia 35
media [2]. Also, the diacetates of
a,b-unsaturated aldehydes are
important starting materials for Diels–Alder reactions, since
acylals have also been used as cross-linking agents for cellulose
in cotton [3] and are useful intermediates in industries [4].
The conventional method for the preparation of 1,1-diacetates
involves the reaction of aldehydes with acetic anhydride using
Lewis and Brønsted acids, such as FeCl3 [5], Cu(OTf)2 [6], Bi(OTf)3
[7], FeSO4 [8], N-bromosuccinimide [9], montmorillonite clay [10],
zeolites [11], H6P2W18O62Á24H2O [12], methanesulfonic acids [13],
silica sulfuric acid (SSA) [14], P2O5/Al2O3 [15], silica-bonded
S-sulfonic acid (SBSSA) [16], BEA-SO3H (zeolite beta (BEA)) [17],
Fe3O4@SiO2/Schiff base complex of Cr (III) [18], silica-bonded
and industry [21].
36
Application of solid acids in organic transformations is 37
important because they have many advantages including ease of 38
handling, decreased reactor and plant corrosion problems and 39
more environmentally safe disposal [22–27].
Catalysis is currently recognized as
40
a
potential field of 41
application for carbon nanotubes (CNTs), and throughout the past 42
decade, the number of publications on this subject has been 43
increasing exponentially [28].
44
As a part of our program aiming at developing efficient and 45
environmentally friendly heterogeneous catalysts for organic 46
synthesis, we have developed multi-walled carbon nanotubes 47
(MWCNTs), functionalized with phosphonic acid (MWCNTs-C- 48
PO3H2) as an efficient, heterogeneous and reusable nanocatalyst 49
for acylation of phenolic hydroxyl groups, different classes of 50
alcohols, aromatic amines and thiols with acetic anhydrides under 51
solvent-free conditions [29]. In this paper, we report a facile and 52
*
Corresponding author.
E-mail addresses: farzanedehghani@gmail.com (F. Dehghani),
sardarian@chem.susc.ac.ir (A.R. Sardarian).
http://dx.doi.org/10.1016/j.cclet.2014.07.005
1001-8417/ß 2014 Ali Reza Sardarian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: F. Dehghani, et al., An efficient method for synthesis of acylals from aldehydes using multi-walled
carbon nanotubes functionalized with phosphonic acid (MWCNTs-C-PO3H2), Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.07.005

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CCLET 3067 1–5
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F. Dehghani et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
product was purified by column chromatography on silica gel
91
with petroleum ether and ethyl acetate (80:20) to give the pure
product.
92
93
Typical procedure for preparation of 1,1-diacetate from 4-
nitrobenzaldehyde: to a mixture of 4-nitrobenzaldehyde (0.151 g,
1 mmol) and acetic anhydride (0.28 mL, 3 mmol) was added
MWCNTs-C-PO3H2 (0.1 mol%, 0.026 g). The reaction mixture was
stirred at room temperature for the appropriate reaction time.
After completion of the reaction, as indicated by TLC, the reaction
mixture was diluted with hot CH2Cl2 (5 mL 2Â) and the resultant
mixture was stirred at room temperature for 10 min. Then, the
catalyst was separated by centrifuge and CH2Cl2 was evaporated
under reduced pressure. The crude product was concentrated and
recrystallized from CH2Cl2 to give the pure product in 92% yield as
a light yellow solid, mp 124 8C.
94
95
96
97
98
99
100
101
102
103
104
105
Scheme 1. Preparation of acylals under solvent-free condition at room temperature
using MWCNTs-C-PO3H2.
53
54
55
efficient method for the preparation of acylals under solvent-free
conditions at room temperature using MWCNTs-C-PO3H2 as an
efficient, heterogeneous and reusable nanocatalyst (Scheme 1).
56
2. Experimental
57
58
59
60
61
62
63
64
65
66
67
68
69
All products were known and their physical and spectroscopic
data were compared to those of authentic samples. Chemicals were
purchased from Fluka or Merck. The purity of the products was
determined by TLC on silica gel polygram SIL G/UV 254 plates. NMR
spectra were recorded on a Bruker Avance DPX-250 (250 MHz for
1H NMR and 62.9 MHz for 13C NMR) spectrometer in pure
deuterated solvents with tetramethylsilane (TMS) as an internal
standard.
The synthesized CNTs were characterized using some electron
microscopic techniques such as scanning electron microscopy
(SEM, XL-30 FEG SEM, Philips, 20 KV), atomic force microscopy
(AFM, DME-SPM, version 2.0.0.9), and also thermogravimetric
analysis.
3. Results and discussion
106
The CVD techniques were used for the synthesis of MWCNTs-C-
PO3H2 (Fig. 1A). For this purpose, acetylene with 99% purity and
ferrocene were used as CNT precursor and catalyst. Iron
nanoparticles were librated in situ from ferrocene and catalyzed
MWCNTs formation in the presence of thiophene as sulfur
precursor for increasing the length of CNT.
Then phosphorous atom was doped on carbon nanostructures
by the reaction of MWCNTs with triphenylphosphineoxide at
1300 8C under the atmosphere of hydrogen and argon in a quartz
tube. Then hydrogen peroxide oxidized the produced phosphor-
doped carbon nanostructures to obtain MWCNs-C-PO3H2 and also
simultaneously purified it from any amorphous carbon as
graphically shown in Fig. 1B. The MWCNTs-C-PO3H2 catalyst has
been fully characterized using some different microscopic and
spectroscopic techniques [29].
Due to stability and mild acidity of MWCNTs-C-PO3H2, we
decided to investigate its capability for the preparation of acylals.
Therefore we needed to determine the best reaction conditions for
this transformation.
For this purpose, the reaction of p-nitrobenzaldehyde (1 mmol)
with acetic anhydride (3 mmol) in the presence of MWCNTs-C-
PO3H2 was chosen as a model reaction. The model reaction was
investigated in various solvents such as H2O, EtOH, CH2Cl2, CHCl3,
CH3CN and also under solvent-free condition at room temperature
in the presence of various amount of catalyst. The results are
summarized in Table 1.
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
70
2.1. Preparation of MWCNTs-C-PO3H2
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
According to Ref. [28], acetylene gas with a flow rate of
ꢀ50 mL minÀ1 was bubbled into a solution containing ferrocene
(0.30 g), triphenylphosphine (3.0 g), thiophene (0.7 mL) in benzene
(25 mL). This was mixed with hydrogen and argon with flow rates
to 0.5 and 800 mL minÀ1, respectively, followed by introduction
into a quartz tube passed through a 80 cm tubing furnace set at
1300 8C. The produced phosphor-doped carbon nanostructures
were then directly purified from any amorphous carbon via
purging oxygen and aerosols of hydrogen peroxide into the
production line, followed by on-line activation using ultraviolet
(UV) and microwave irradiators.
General procedure for preparation of 1,1-diacetates: to a
mixture of aldehyde (1 mmol) and acetic anhydride (3 mmol)
was added MWCNTs-C-PO3H2 (0.1 mol%, 0.026 g). The reaction
mixture was stirred at room temperature for the appropriate
reaction time. After completion of the reaction, as indicated by
TLC, the reaction mixture was diluted with hot CH2Cl2 (5 mL 2Â)
and the resultant mixture was stirred at room temperature for
10 min. Then, the catalyst was separated by centrifuge and
CH2Cl2 was evaporated under reduced pressure. The crude
As it is shown in Table 1, the yield of the reaction under solvent-
free conditions in the presence of 0.1 mol% catalyst was the highest
and the reaction time was the shortest. Aprotic solvents, such as
CHCl3, CH2Cl2 and CH3CN, afforded the desired product in lower
yields and longer reaction times (Table 1, entries 1–3). In protic
solvents, such as water and ethanol (Table 1, entries 4–5), this
protection reaction proceeded with longer reaction times and very
Fig. 1. Schematic of CVD-synthesized MWCNTs-C-PO3H2 catalyst (A), schematic representing the CVD-synthesis of MWCNT-C-PO3H2 (B).
Please cite this article in press as: F. Dehghani, et al., An efficient method for synthesis of acylals from aldehydes using multi-walled
carbon nanotubes functionalized with phosphonic acid (MWCNTs-C-PO3H2), Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.07.005

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F. Dehghani et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
3
Table 1
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141
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145
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147
148
149
150
151
152
153
154
155
156
157
poor yields, which may be related to the instability of acetic
anhydride and corresponding product in protic solvents.
Therefore, we employed the optimized conditions for the
conversion of various aldehydes into the corresponding acylals.
The results of the preparation of acylals from aromatic aldehydes
in the presence of MWCNTs-C-PO3H2 at room temperature are
shown in Table 2.
Aldehydes with electron-donating or electron-withdrawing
groups were converted into the corresponding acylals in high
yields after short reaction times. The acid-sensitive substrate,
thiophene-2-carbaldehyde, gave the expected acylal in 80% yield
without any by-product formation (Table 2, entry 11). We also
investigated the reactions of 4-hydroxybenzaldehyde under the
above-mentioned conditions and observed after 90 min, both the
carbonyl and phenolic groups were acylated (Table 2, entries 9).
Several aliphatic and aromatic ketones, including cyclohexanone
and acetophenone, were not reactive under the described
experimental conditions even after 2 h (Table 2, entries 14 and 15).
Conversion of p-nitrobenzaldehyde to the corresponding acylal in the presence of
MWCNTs-C-PO3H2 in different conditions.a
O
CH(OAc)
2
Reaction conditions
H
Ac O
2
O N
2
O N
2
Entry
Solvent/catalyst (mol%)
Time (min)
Yields (%)b
1
2
3
4
5
7
8
9
CH2Cl2/0.1
20
20
20
60
60
5
87
75
75
25
10
92
87
92
CH3CN/0.1
CHCl3/0.1
C2H5OH/0.1
H2O/0.1
Solvent-free/0.1
Solvent-free/0.05
Solvent-free/0.2
5
5
a
The reaction was carried out with 3 mmol of Ac2O at r.t.
Isolated yield.
b
Table 2
Preparation of various acylals in the presence of MWCNTs-C-PO3H2 under solvent-free conditions at room temperature.a
Q3
Entry
1
Aldehyde
Product
CH(OAc)
Time (min)
5
Yields (%)b
90
Mp (8C) found (reported) [reference]
O
44–46 (45–46) [16]
2
H
H
2
5
92
64 (64–65) [16]
O
CH(OAc)
2
NO
2
NO2
3
4
5
5
92
90
124 (124) [30]
CH(OAc)
O
2
H
O N
2
O N
2
90–91 (90–93) [16]
O
CH (OAc)2
H
Br
Br
5
6
7
O
10
5
87
91
91
60 (58–60) [16]
82–84 (81–83) [16]
64–65 (65) [31]
CH(OAc)
2
H
Cl
Cl
O
CH(OAc)
2
H
Cl
Cl
O
CH(OAc)
5
2
H
H CO
3
H3CO
8
CH(OAc)
10
85
75–76 (76) [31]
O
2
H
OCH3
OCH
3
9c
90
70
90
93–95 (94–96) [16]
80–81 (79–81) [16]
O
CH(OAc)
2
H
AcO
H3C
HO
10
CH(OAc)2
5
O
H
H C
3
Please cite this article in press as: F. Dehghani, et al., An efficient method for synthesis of acylals from aldehydes using multi-walled
carbon nanotubes functionalized with phosphonic acid (MWCNTs-C-PO3H2), Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.07.005

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F. Dehghani et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
Table 2 (Continued )
Entry
11
Aldehyde
Product
Time (min)
20
Yields (%)b
Mp (8C) found (reported) [reference]
O
H
80
66–68 (67–68) [16]
S
S
CH(OAc)
2
12
13
14
O
O
5
5
87
101–103 (101) [31]
83–85 (84) [31]
CH(OAc)
2
H
H
CH(OAc)
88
2
O
120
120
15
O
a
Reaction conditions: aldehyde (1 mmol), acetic anhydride (3 mmol), catalyst (0.1 mol %), at r.t.
Isolated yield.
b
c
The reaction was carried out with 4 mmol of Ac2O at r.t.
Scheme 2. Competitive acylal formation of aldehydes in the presence of ketones using MWCNTs-C-PO3H2 under solvent-free conditions. Reaction conditions: substrate
(1 mmol each), acetic anhydride (3 mmol), MWCNTs-C-PO3H2 (0.1 mol %), 5 min at r.t.
158
159
160
161
162
163
164
165
166
167
168
169
170
Next, we studied the competitive acylation reactions of
aromatic aldehydes in the presence of ketones using MWCNTs-
C-PO3H2. Under these conditions exclusive acylation of the
aldehyde functions was observed. The chemoselective acylations
of p-chlorobenzaldehyde in the presence of acetophenone and
cyclohexanone are shown in Scheme 2.
To show the advantage of MWCNTs-C-PO3H2 over some of the
reported catalysts in the literature, we compared the reaction of
3-nitrobenzaldehyde with acetic anhydride, in the presence of
MWCNTs-C-PO3H2 and different catalysts reported in the litera-
ture (Table 3). As evident from the results, the required ratio for the
some catalysts, used for this purpose, is a large amount and also the
required reaction times are much longer in comparison with
MWCNTs-C-PO3H2, and also excess amounts of Ac2O are required.
We also observed, some of these catalysts resulted in shorter
reaction time in comparison with MWCNTs-C-PO3H2 (Table 3,
entries 2 and 5).
For checking the reusability of the catalyst, we performed the
reaction in the following way, the reaction of p-nitrobenzaldehyde
(1 mmol) with acetic anhydride (3 mmol) in the presence of
MWCNTs-C-PO3H2 (0.1 mol%) was chosen in a 10 mmol scale. After
completion of the reaction, hot CH2Cl2 (20 mL) was added and the
catalyst was recovered by centrifuging, dried under air and then
reused for the next cycle. The recovered catalyst was reused for five
runs in the preparation of acylals from aldehyde and acetic
anhydride. The results are summarized in Table 4.
171
172
173
174
175
176
177
178
179
180
181
182
183
Table 3
Comparison between different acid catalysts with MWCNTs-C-PO3H2 in the reaction of 3-nitrobenzaldehyde with acetic anhydride.
Entry
Catalyst (g)
Molar ratio (aldehyde:aceticanhydride)
Time (min)
Yield (%)
Ref.
1
2
3
4
5
6
7
Fe3O4@SiO2/Schiff base complex of Cr (III) (0.035)
BEA-SO3H (0.015)
1:3
1:15
1:3
20
4
92
85
88
90
87
93
92
[18]
[17]
Bi(OTf)3 (0.031)
270
45
2
[7]
P2O5/Al2O3 (0.05)
1:1
[15]
SBSSA (0.02)
1:15
1:2
[16]
H6P2W18O62Á24H2O (0.1)
MWCNTs-C-PO3H2 (0.026)
30
5
[12]
1:3
This work
Please cite this article in press as: F. Dehghani, et al., An efficient method for synthesis of acylals from aldehydes using multi-walled
carbon nanotubes functionalized with phosphonic acid (MWCNTs-C-PO3H2), Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.07.005

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F. Dehghani et al. / Chinese Chemical Letters xxx (2014) xxx–xxx
5
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Table 4
a
Reusability of MWCNTs-C-PO3H2 in the reaction of p-nitrobenzaldehyde and acetic
anhydride.a
227
229
Runs
Fresh
92
1
2
3
4
5
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aldehydes using environmentally benign solid acid catalyst under solvent-free 231
Yieldsb (%)
92
91
91
91
91
a
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232
Reaction conditions: MWCNTs-C-PO3H2 (0.1 mol %), p- nitrobenzaldehyde
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b
Isolated yields.
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236
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4. Conclusion
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185
186
187
188
189
190
191
192
In conclusion, we report a mild and efficient method for the
preparation of 1,1-diacetates from aldehydes in the presence of
acetic anhydride under solvent-free conditions at room tempera-
ture using MWCNTs-C-PO3H2. This method is selective for the
preparation of 1,1-diacetates from aldehydes in the presence of
ketones. Good yields were obtained within short reaction times;
reusability of the catalyst and mild conditions are some of the
notable features of this protocol.
(2009) 4058–4062.
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an efficient and chemoselective acid catalyst, J. Mol. Catal. A: Chem. 335 (2011) 244
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359–367.
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Please cite this article in press as: F. Dehghani, et al., An efficient method for synthesis of acylals from aldehydes using multi-walled
carbon nanotubes functionalized with phosphonic acid (MWCNTs-C-PO3H2), Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/
j.cclet.2014.07.005