Aluminum dodecatungstophosphate (AlPW12O40) as an efficient heterogeneous inorganic catalyst for the chemoselective synthesis of geminal diacetates (acylals) under solvent-free conditions

Article abstract of DOI:10.1016/S0040-4039(03)00811-6

An efficient and chemoselective preparation of acylals from structurally different aldehydes in the presence of AlPW₁₂O₄₀ and acetic anhydride was achieved easily in high yields at room temperature under solvent-free conditions.

Full text of DOI:10.1016/S0040-4039(03)00811-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3951–3954
Aluminum dodecatungstophosphate (AlPW12O40) as an efficient
heterogeneous inorganic catalyst for the chemoselective synthesis
of geminal diacetates (acylals) under solvent-free conditions
Habib Firouzabadi,* Nasser Iranpoor,* Farhad Nowrouzi and Kamal Amani
Department of Chemistry, Shiraz University, Shiraz 71454, Iran
Received 10 February 2003; revised 10 March 2003; accepted 21 March 2003
Abstract—An efficient and chemoselective preparation of acylals from structurally different aldehydes in the presence of
AlPW12O40 and acetic anhydride was achieved easily in high yields at room temperature under solvent-free conditions. © 2003
Elsevier Science Ltd. All rights reserved.
Heteropolyacids (HPAs) as catalysts for fine organic
synthetic processes have been developed for industries
related to fine chemicals, such as the flavors, pharma-
ceuticals and food industries.1 Heteropolyacids are
more active catalysts than conventional inorganic and
organic acids for reactions in solution.2 They are used
as industrial catalysts for several liquid-phase reac-
tions,3–6 including alcohol dehydration,7 alkylation,8
and esterification.9 Among heteropolyacids, poly-
tungstic acids are the most widely used catalysts owing
to their high acid strengths, thermal stabilities, and low
reducibilities. Catalysts based on heteropolyacids have
many advantages over liquid acid catalysts. They are
non-corrosive and environmentally benign and present
fewer disposal problems. Their repeated use is possible
and their separation from liquid products is easier than
for homogeneous catalysts.10
Solvent-free reactions have attracted considerable
attention in chemical processes due to their safety,
economy, easy work up, high yields and usually fast
reaction rates.14 We have recently initiated studies into
new catalytic properties of heteropoly acids, especially
tungstophosphoric acid (H3PW12O40) as
a cheap,
reusable, heterogeneous and easily available catalyst.
We have used catalytic amounts of this compound for
the oxidation of aromatic amines to nitro compounds
with sodium perborate in micellar media,15a for thioac-
etalization and transthioacetalization reactions,15b and
its cesium salt for the regioselective brominating of
aromatic compounds15c and selective O-trimethylsilyla-
tion of a wide variety of alcohols and phenols using
hexamethyldisilazane (HMDS) under solvent-free
conditions15d and the solvent-free oxidation of hydroxy
groups with Fe(NO3)3 activated by H3PW12O40.
Acylals (geminal diacetates) or gem-bis (acyloxy)-
alkanes have been used as protecting groups for car-
bonyl compounds because of their stability in neutral
and basic media,11a as well as towards aqueous acids.11b
Due to the remarkable stability of gem diacetates
towards a variety of reactions, they play an important
role in organic synthesis for the protection of alde-
hydes. In the last decade various catalysts have
reported for their synthesis.12 Reactions of acylals are
also important for the preparation of other compounds
and have been under investigation recently.13
The preparation of aluminum dodecatungstophosphate
(AlPW12O40) was reported in 1982 by Ono using the
reaction of aluminum nitrate and dodecatungstophos-
phoric acid in a quantitative yield.16 We have prepared
this compound by the addition of aluminum nitrate or
by aluminum carbonate to an aqueous solution of
tungstophosphoric acid which on complete evaporation
of water gave the desired compound as a white powder
in a quantitative yield. AlPW12O40 prepared by both
procedures gave satisfactory analytical results within
* Corresponding authors. Tel.: +98-711-2284822; fax: +98-711-
2280926; e-mail: firouzabadi@chem.susc.ac.ir; iranpoor@chem.
susc.ac.ir
Scheme 1.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00811-6

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H. Firouzabadi et al. / Tetrahedron Letters 44 (2003) 3951–3954
Table 1. AlW12O40-catalyzed formation of gem-diacetates
ing diacetates in excellent yields (Table 1, entries 1–10).
Aliphatic as well as a,b-unsaturated aldehydes also
gave the corresponding acylals in good yields under
similar reaction conditions (Table 1, entries 11, 12). We
found that this catalyst was not suitable for the prepa-
ration of acylals from aldehydes carrying amino groups
(Table 1, entry 13). We have also attempted the
acylation of furfural at different temperatures. In all
runs, a dark colored unidentified polymeric material
was obtained (Table 1, entry 14).
from aldehydes
Entry Substrates
Time (min) Yield (%)a Ref. 12
1
2
3
4
5
6
7
8
9
Benzaldehyde
2
1
6
5
4
6
3
89
96
95
91
93
90
l,n,k,i,g,j
j,k,n,l,i,g
i,j,n,k
n
4-Me-Benzaldehyde
4-Cl-Benzaldehyde
2-Cl-Benzaldehyde
3-MeO-benzaldehyde
Anisaldehyde
3-NO2-Benzaldehyde
4-NO2-Benzaldehyde 45
4-CN-Benzaldehyde 20
p
j,g,o,k,j
k,b
88b
89b
88b
95
j,l,i,n
n,b
In order to show the merit of the present work in
comparison with recently reported protocols, we com-
pared the results of the gem-diacetate synthesis from
cinnamaldehyde and 4-methylbenzaldehyde in the pres-
ence of metal triflates InCl3, NBS, LiBF4 and
AlPW12O40 with respect to the amounts of the catalysts
used, reaction times and yields of the products (Table
2). The results show that AlPW12O40 promotes the
reactions more effectively than InCl3, NBS, Cu(OTf)2
and Sc(OTf)3 as far as the amount of catalyst and
reaction times are concerned. Reactions in the presence
of LiBF4 required higher temperatures with longer reac-
tion times using an equal molar ratio. Bi(OTf)3 catalysis
was performed in CH3CN and at room temperature but
the reaction times and also the yields of the products
were lower than those obtained with AlPW12O40.
e
10
2,6-Dichlorobenz-
aldehyde
15
–
11
12
13
Heptanal
Cinnamaldehyde
4-N(Me)2-benzalde- 30
hyde
Furfural
12
5
92
92
0c
l,i
l,g,j
–
14
30
0d
–
a Isolated yields and the purity of the products were determined by
1H NMR and GC.
b Reactions were carried out in 2 mL CH2Cl2.
c Reaction did not occur even under heating.
d An unidentified polymer was produced at 0°C.
e Spectral data are given in Ref. 19.
experimental error. This salt is a water stable and a
non-hygroscopic compound. There is a report available
in the literature, which deals with the conversion of
methanol into hydrocarbons using this compound as a
catalyst.17
In order to show the high selectivity of the method, we
studied competitive reactions for the acylation of alde-
hydes in the presence of ketones using AlPW12O40 as
the catalyst under solvent-free conditions at room tem-
perature. Using this catalytic system, the highly selec-
tive conversion of aldehydes in the presence of ketones
was observed. We also studied the acylation of 4-
methylbenzaldehyde versus 4-nitrobenzaldehyde and 3-
nitrobenzaldehyde in the presence of 4-nitro-
benzaldehyde. These reactions also proceeded with high
selectivity in the presence of this catalyst showing the
importance of electronic effects upon these reactions in
the presence of this catalyst (Table 3).
Very recently we have used AlPW12O40 for the selective
and efficient acetylation of -OH, -NH2, and -SH in the
presence of acetic anhydride.18 Now we report that the
efficient and chemoselective preparation of acylals18
from structurally different aldehydes in the presence of
this catalyst and acetic anhydride was achieved easily at
room temperature under solvent-free conditions19
(Scheme 1).
In conclusion, we have introduced a new mild and
effective catalyst for the preparation of acylals from
aldehydes in the presence of acetic anhydride at room
By using this catalyst, both activated and deactivated
aromatic aldehydes were converted to their correspond-
Table 2. Comparison of the effect of catalysts for gem-diacetate synthesis from cinnamaldehyde and 4-methylbenzaldehyde
Entry
Catalyst
Cat. (mol%)
Timea
Yielda
Timeb
Yieldb
Ref.
1
3
6
InCl3
NBS
LiBF4
10
20
1 h
91
80
86
90
76
76
70
92
1 h
96
93
92
–
98
95
87
96
12n
12j
12l
12o
12i
12g
12k
–
48 h
16 h
4.5 h
3 h
2 h
15 min
5 min
9 he
0.1/16d
5 equiv.
2.5
24 hd
–
2 h
10 min
30 min
1 min
5
2
7
8
Cu(OTf)2
Sc(OTf)3
Bi(OTf)3
2
0.1
0.1
c
AlPW12O40
a Times and yields refer to gem-diacetate synthesis from cinnamaldehyde.
b Times and yields refer to gem-diacetate synthesis from 4-methylbenzaldehyde.
c CH3CN was used as a solvent.
d Reaction was carried out at 40°C.
e 10 mol% of catalyst was used.

H. Firouzabadi et al. / Tetrahedron Letters 44 (2003) 3951–3954
3953
Table 3. Competitive acylal formation of aldehydes using Ac2O in the presence of AlPW12O40 at room temperature under
solvent-free conditionsa
temperature in high yields. The method is highly selec-
tive for the sole preparation of acylals from aldehydes
in the presence of ketones. AlPW12O40 can be prepared
from cheap and commercially available chemicals in a
quantitative yield in aqueous media. In contrast to
many other Lewis acids, storage of this compound does
not need special precautions, e.g. it can be stored on a
bench top for months without losing its catalytic activ-
ity. In addition, as a non-hygroscopic, non-corrosive
3. Perspectives in Catalysis; Ono, Y.; Thomas, J. M.; Zama-
raev, K. I., Eds.; Blackwell: London, 1992; p. 341.
4. Kozhevnikov, I. V.; Matveev, K. I. Appl. Catal. 1983, 5,
135.
5. Izumi, Y.; Urabe, K.; Onaka, A. Zeolite, Clay and Het-
eropolyacids in Organic Chemistry; Kodansha: Tokyo-
VCH, Weinheim, 1992; p. 99.
6. Kozhevnikov, I. V. Catal. Rev.-Sci. Eng. 1995, 37, 311.
7. Misono, M.; Noriji, N. Appl. Catal. 1990, 64, 1.
8. Izumi, Y.; Hasebe, R.; Urabe, K. J. Catal. 1983, 84, 402.
9. Soeda, H.; Okuhara, T.; Misono, M. Chem. Lett. 1994,
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and
a water stable compound, the handling of
AlPW12O40 is easy which makes this catalyst suitable
for the large-scale operations.
10. Schwegler, M. A.; Bekkum, H. V.; Munck, N. Appl.
Applications of this new catalytic system to Diels–
Alder, Friedel–Crafts and condensation reactions, and
Beckmann and Fries rearrangements are under study in
our laboratories.
Catal. 1991, 74, 191.
11. (a) Greene, T. W.; Wuts, P. G. M. Protective Groups in
Organic Synthesis, 3rd ed.; John Wiley and Sons: New
York, 1999; p. 306; (b) Kochhar, K. S.; Bal, B. S.;
Deshpande, R. P.; Rajadhyaksha, S. N.; Pinnick, H. W.
J. Org. Chem. 1983, 48, 1765.
Acknowledgements
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Partial support of this work by the Shiraz University
Research Council is highly appreciated.
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2,6-dichlorobenzaldehyde. To
a
solution of 2,6-
dichlorobenzaldehyde (5 mmol, 0.88 g) and freshly dis-
tilled acetic anhydride (5 mmol, 0.46 mL), AlPW12O40
(0.015 g, 0.005 mmol) was added at room temperature.
The reaction mixture was magnetically stirred for 15 min
(monitored by TLC and GC). The resulting mixture was
diluted with Et2O (10 mL) and filtered. The filter cake
was washed with Et2O (2×5 mL) and the ethereal solu-
tions were combined together and washed with a satu-
rated solution of NaHCO3 (10 mL) and water (2×10 mL).
The organic phase was dried over anhydrous Na2SO4,
filtered and the solvent was evaporated to give the pure
desired compound as a white crystalline compound (1.31
1
g, 95%), mp=88.5°C. H NMR (CDCl3, TMS) l (ppm):
2.07 (s, 6H), 7.01 (t, 1H), 7.35 (d, J=7.24 Hz, 2H), 8.62
(s, 1H). 13C NMR l (ppm): 168.56, 135.62, 131.01,
129.37, 87.61, 20.7. IR (KBr): 3080 (C-H (OAc)2), 1772
(CꢀO) and 1257 (O-COMe) cm−1
.