Heteropolyacid encapsulated into mesoporous silica framework for an efficient preparation of 1,1-diacetates from aldehydes under a solvent-free condition

Article abstract of DOI:10.1016/j.tetlet.2006.09.090

Acylals were prepared from aliphatic and aromatic aldehydes using heteropolyacid (SiPW-8) encapsulated into the framework of mesoporous silica as catalysts in a solvent-free procedure. The catalyst was very effective and reusable for the production of 1,1-diacetates from aldehydes under a mild reaction condition.

Full text of DOI:10.1016/j.tetlet.2006.09.090

Tetrahedron Letters 47 (2006) 8309–8312
Heteropolyacid encapsulated into mesoporous
silica framework for an efficient preparation of 1,1-diacetates
from aldehydes under a solvent-free condition
Jianmin Wang,a,b Liang Yan,a Guang Qian,a Keli Yang,a Haitao Liua and Xiaolai Wanga,*
aState key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, China
bGraduate School of the Chinese Academy of Sciences, Beijing 100000, China
Received 13 July 2006; revised 16 September 2006; accepted 19 September 2006
Available online 9 October 2006
Abstract—Acylals were prepared from aliphatic and aromatic aldehydes using heteropolyacid (SiPW-8) encapsulated into the
framework of mesoporous silica as catalysts in a solvent-free procedure. The catalyst was very effective and reusable for the pro-
duction of 1,1-diacetates from aldehydes under a mild reaction condition.
Ó 2006 Elsevier Ltd. All rights reserved.
The use of protecting groups is greatly significant in
organic synthesis,1 and acylal preparation is one of the
most useful methods to protect carbonyl groups due to
the stability of the 1,1-diacetates in neutral and basic
media.1c 1,1-Diacetates are usually prepared from car-
reactions, heteropolyacid catalysts have recently applied
to different reactions. Heteropolyacid catalysts
(H6P2W18O62Æ24H2O) have been also recently applied
to protect aldehydes as acylals.20 Although the sub-
strates could be completely consumed, the separation
of products with catalysts is very difficult. Herein, we
report a simple, convenient and efficient process for
the preparation of 1,1-diacetates from aliphatic and
aromatic aldehydes using heteropolyacids encapsulated
into mesoporous silica frameworks (SiPW-8)21,22 as
solid catalysts under a solvent-free condition.
bonyl compounds with acetic anhydride in the presence
2
´
of Brønsted, Lewis acid catalysts (such as KHSO4,
H2SO4, CH3SO3H, H3PO4,3 NH2SO3H,4 Nafion-7,5
ZnCl2,6 PCl3,7 FeCl3/SiO2,8 I2,9 WCl6,10 LiOTf,11
LiBF4,12 Sc(OTf)3,13 ZrCl414) under neutral conditions
(such as NBS).15 Several other catalysts have also been
employed for acylal preparation, for example, expansive
graphite,16 zeolites,1b,2 tungstosilicic acid, HZSM-5,
Fe3+ on montmorillonite, PVC–FeCl3 complex and zir-
conium sulfophenyl phosphonate.17 However, these
cases were a high cost, rigorous reaction condition,
and not environmentally benign because of the use
of mineral acids, requirement of organic solvents.
Recently, searching for a greener chemical process has
become one of the most important tasks of today’s
chemical researchers.18 There are lots of methods to
achieve this goal such as green solvents, catalysts, and
economic feedbacks.
The results of the solvent-free preparation of acylal from
aliphatic and aromatic aldehydes in the presence of 0.1 g
SiPW-8 at room temperature are shown in Table 1. 1,1-
Diacetates were produced in a nearly 98% yield by the
reaction of benzaldehyde with acetic anhydride in less
than 10 min. Benzaldehyde with electron-donating
groups, that is, 4-methyl-benzaldehyde, were converted
into their corresponding acylal with high yields after a
short reaction time. Compared to that with electron-
donating groups, the one with electron-withdrawing
groups, that is, 3-nitro-benzaldehyde needed a longer
reaction time to form the corresponding acylal.
Aliphatic aldehydes were similarly converted into the
corresponding acylal with good to excellent yields. The
acid sensitive substrate (furfural) also led to the forma-
tion of acylal with a 98% yield without any by-products.
Some aliphatic and aromatic ketones (acetone, butanone,
Heteropolyacids are useful solid catalysts because of
their super acidic properties.19 As a part of a research
project to develop environmentally friendly organic
*
Corresponding author. E-mail: fpierc@lzb.ac.cn
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.09.090

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J. Wang et al. / Tetrahedron Letters 47 (2006) 8309–8312
Table 1. Catalytic conversion of aldehydes to acylals using SiPW-8
Table 2. Comparison of the effect of catalysts for gem-diacetate
synthesis from 4-methylbenzaldehyde
Entry
Substrate
Products
Yield (%)
Entry Catalyst
Yield Time
Ref.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Benzaldehyde
A
B
C
D
E
F
G
H
I
95
98
98
96
93
93
92
89
94
92
98
—
—
—
—
—
—
4-Methyl-benzaldehyde
4-Methoxy-benzaldehyde
4-Chloro-benzaldehyde
2-Nitro-benzaldehyde
3-Nitro-benzaldehyde
4-Nitro-benzaldehyde
Butyl aldehyde
Heptyl aldehyde
Cinnamaldehyde
Furfural
Acetophenone
Bezophenone
Ethyl n-butyl ketone
Butanone
Acetone
1a
2a
3a
4a
6
SiPW-8
96
99
97
96
95
92
93
6 min
H3PW12O40Æ12.5H2O
H6P2W18O62Æ24H2O
AlPW12O40
Sc(OTf)3
10 min
10 min
5 min
10 min 16
24 h
9 h
8
LiBF4
15
18
9
NBS
10
Zr(CH3PO3)1.2(O3PC6H4SO3H)0.8 75
12 min 20
J
a Reaction condition: catalyst 0.1 g, alcohol 5 mmol, acetic anhydride
15 mmol, room temperature.
K
L
M
N
O
P
flates and zirconium sulfophenyl phosphonate at the
same reaction conditions. Reaction with LiBF4 as the
catalyst required a higher temperature and longer reac-
tion time. The preparation of acylals with zirconium
sulfophenyl phosphonate and metal triflates as catalysts
was performed in solvents such as CH3CN and at room
temperature with longer reaction times and lower yields.
It is interesting that the catalytic activity of the silica-
included H3PW12O40 (SiPW-8) was higher than that of
H3PW12O40Æ12.5H2O, AlPW12O40 and H6P2W18O62Æ
24H2O for the preparation of acylals, which suggested
that H3PW12O40 formed a highly concentrated aqueous
solution in the matrix.
Heptanone
Q
Reactions condition: reaction temperature 20 °C, reaction time 30 min,
catalyst amount 0.1 g, yields are expressed from crystallized products
(see text).
acetophenone, and ethyl n-butyl ketone) studied for the
reaction were not reactive under the described experi-
mental conditions.
The results of the gem-diacetate synthesized from
4-methylbenzaldehyde in the presence of H3PW12O40Æ
12.5H2O, AlPW12O40, H6P2W18O62Æ24H2O, Sc(OTf)3,
LiBF4, NBS and Zr (CH3PO3)1.2(O3PC6H4SO3H)0.8
are listed in Table 2. It showed that SiPW-8 catalyzed
the reactions more effectively than did NBS, metal tri-
We studied the competitive reaction for the acylation of
aldehydes in the presence of ketones using SiPW-8 as the
catalyst at room temperature. In the presence of
Table 3. Competitive acylal formation of aldehydes using Ac2O in the presence of SiPW-8 at room temperature
SiPW-8, 0.1 g
RCH(OAc)2+ R'CH(OAc)2
RCHO + R'CHO + Ac2O
R, R' = alkyl, aryl
room temp. 5 min
Entry
1
Substrate
Product (selectivity %)
H
O
O
H
OAc
OAc
OAc
OAc
O
O
(0%)
(100%)
H
H
OAc
OAc
OAc
2
3
OAc
(0%)
(100%)
H
O
H
H
OAc
OAc
H
OAc
OAc
O2N
O2N
O2N
(96%)
O2N
(4%)
H
H
O
H
H
OAc
OAc
OAc
OAc
4
5
O
CH3O
CH3O
O2N
O2N
(95%)
(5%)
H
H
OAc
OAc
O
H
H
OAc
OAc
NO2
NO2
Reaction condition: Each substrate 2 mmol, acetic anhydride 2 mmol, catalyst amount 0.1 g, room temperature, time 5 min.

J. Wang et al. / Tetrahedron Letters 47 (2006) 8309–8312
8311
ketones, the highly selective conversion of aldehydes was
observed. The acylation of 4-methylbenzaldehyde versus
4-nitro-benzaldehyde and 3-nitro-benzaldehyde also
showed a high selectivity in the presence of this
catalyst, which indicated the importance of electronic
effects upon these reactions (Table 3).
CH); IR (KBr, v/cmÀ1): 3063, 3032, 2930, 1760, 1512,
1446, 1253, 1225, 1011.
B: White solid, melting point: 81 °C, 1H NMR d 2.13 (s,
6H, 2COCH3), 2.40 (s, 3H, CH3), 7.28 (d, 2H,
J = 8.2 Hz, Ar–H), 7.65 (d, 2H, J = 7.9 Hz, Ar–H),
7.80 (s, 1H, CH); IR (KBr, v/cmÀ1): 2950, 1778, 1719,
1520, 1400, 1250, 1220, 1015, 956, 910.
One of the main aims of using SiPW-8 as the catalyst
was to study the possibility of catalyst separation and
recycle. We found that the catalyst SiPW-8 could be
easily separated from the reaction system by a simple
procedure. Recycling experiments results of benzyl alde-
hydes to acylals are listed in Table 4. It showed that the
catalyst was easily separated from the reaction mixture
and was reusable without a significant loss of activity
and selectivity for the preparation of 1,1-diacetates.
1
D: White solid, melting point: 81 °C, H NMR, d 2.14
(s, 6H, 2COCH3), 7.30–7.41 (m, 2H, Ar–H), 7.45–7.50
(m, 2H, Ar–H), 7.60 (s, 1H, CH); IR (KBr, v/cmÀ1):
1772, 1590, 1348, 1235, 986.
1
F: White solid, melt point: 64 °C, H NMR, d 2.12 (s,
6H, 2COCH3), 7.60–7.70 (m, 1H), 7.71 (s, 1H, Ar–H),
7.75–7.84 (m, 1H, Ar–H), 8.22–8.23 (m, 1H, Ar–H),
8.35 (m, 1H, CH); IR (KBr, v/cmÀ1): 1774, 1540,
1439, 1351, 1012.
General procedure for the protection of aldehydes: A mix-
ture of aldehyde (10 mmol), acetic anhydride (20 mmol)
and SiPW-8 catalyst (0.1 g) in a flask was stirred at room
temperature for 30 min and filtered, and then diethyl
ether (10 mL) was added to the filtrate. The resulting
solution was successively washed with 1 M NaOH and
water, solvent evaporated to get the crude product,
which was then dried over anhydrous Na2SO4. Further
purification was performed by column chromatography
on silica gel using petroleum ether/ethyl acetate as the
eluent to afford the pure product in good to excellent
yields. All products are known compounds, which were
satisfactorily characterized by spectral data.2,3,8,14,23
1
I: Colorless liquid; H NMR d 0.98 (t, 3H, J = 6.8 Hz,
CH3), 1.22–1.40 (m, 8H, 4CH2), 1.60–1.80 (m, 2H,
CH2O), 2.07 (s, 6H, 2COCH3), 6.77 (t, 1H, CH); IR
(KBr, v/cmÀ1): 3030, 2963, 1762, 1465, 1378, 1250,
1214, 1112, 1015, 968.
References and notes
1. (a) Greene, T. W.; Wuts, P. G. M. Protection for the
Carbonyl Group. Protective Groups in Organic Synthesis;
2nd ed.; John Wiley: New York, 1991; p 184; (b) Pereira,
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Pe’rot, G.; Guisnet, M. Synthesis 1995, 1077; (c) Kochhar,
K. S.; Bal, B. S.; Deshpande, R. P.; Rajadhyaksha, S. N.;
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2. Heravi, M. M.; Bakhtiari, K.; Taheri, S.; Oskooie, H. A.
Green Chem. 2005, 7, 867.
Recycling of the catalyst: A mixture of aldehyde
(10 mmol), acetic anhydride (20 mmol) and SiPW-8 cat-
alyst (0.1 g) in a flask was stirred at room temperature
for 30 min and filtered. The solid was successively
washed with water and dried for recycling. The next
run was performed by adding fresh benzyl aldehyde
(10 mmol), and acetic anhydride (20 mmol) and the
washed SiPW-8 catalyst into the flask under the same
experimental conditions.
3. Freeman, I.; Karcherski, E. M. J. Chem. Eng. Data 1977,
22, 355.
4. Jin, T. S.; Sun, G.; Li, Y. W.; Li, T. S. Green Chem. 2002,
4, 255.
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In conclusion, a very simple and convenient catalytic
method was developed for the preparation of acylal
from a variety of aldehydes under solvent-free condi-
tions. The method has advantages of low environmen-
tal impact, high yields, high selectivity, short reaction
time, and the catalyst can be recycled. It is believed
that this protocol would be
methodology.
a useful synthetic
Spectra data of selected compounds:
A: White solid, melting point: 44 °C, 1H NMR d 2.14 (s,
6H, 2COCH3), 7.38–7.55 (m, 5H, Ar–H), 7.71 (s, 1H,
14. Smitha, G.; Reddy, Ch. S. Tetrahedron 2003, 59, 9571.
15. Karimi, B.; Seradj, H.; Ebrahimian, G. R. Synth. Lett.
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Table 4. Catalytic conversion of benzyl aldehydes to acylals using
SiPW-8 as catalyst
16. Jin, T.-S.; Ma, Y.-R.; Zhang, Z.-H.; Li, T.-S. Synth.
Commun. 1997, 27, 3379.
17. (a) Li, Y.-Q. Synth. Commun. 2000, 30, 3913; (b) Joshi,
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4665; (d) Curini, M.; Epifano, F.; Marcotullio, M. C.;
Run
Yield (%)
1
95
2
96
3
95
4
94
Reactions condition: Reaction temperature 20 °C, reaction time
30 min, catalyst amount 0.1 g, yields are expressed from crystallized
products.

8312
J. Wang et al. / Tetrahedron Letters 47 (2006) 8309–8312
Rosati, O.; Nocchetti, M. Tetrahedron Lett. 2002, 43,
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and finally dried in vacuo (<10À2 Pa) at 300 °C for 5 h.
The final products were designated as SiPW-8, where 8
stands for the Si/W molar ratio in the initial gel. The final
products have been intensively characterized by various
techniques, such as XRD, TEM, N2 adsorption isotherm
analysis, and by IR, UV/visible, and 31P magic angle
spinning (MAS) NMR spectroscopy. Characterization
results suggest that the sample shows very ordered
hexagonal meso-structures.
18. (a) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory
and Practice; Oxford University Press, New York, 1998;
(b) Anastas, P. T.; Williamson, T. C. Green Chemistry-
Frontiers in Benign Chemical Syntheses and Processes;
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J. Eur. J. Inorg. Chem. 2005, 4801.
21. The catalyst was synthesized according to the literature22
as follows: (1) Pluronic P123 (EO20PO70EO20) (4.0 g) was
dissolved in a solution of deionized water (100 mL)
containing hydrochloric acid (20 mL, 36 wt %), followed
by the addition of TEOS (10 mL). The mixture was stirred
at 40 °C for 3 h. (2) A requisite amount of Na2WO4 and
Na2HPO4 was added simultaneously to the above mixture
to form a white precipitate. (3) The mixture was further
stirred at 40 °C for 20 h and then transferred to a stainless
steel autoclave for crystallization at 100 °C for 48 h. The
obtained products were collected and dried in vacuo
(<10À2 Pa) at 150 °C for 6 h and washed with an ethanol
solution containing HCl and distilled water three times,
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