Solid superacids as catalysts for the synthesis of acylals from aldehydes and acetic anhydride

Article abstract of DOI:10.1007/s11172-008-0368-1

Sulfated tin, aluminum, and titanium oxides are efficient catalysts for reactions of aldehydes with acetic anhydride leading to acylals.

Full text of DOI:10.1007/s11172-008-0368-1

Russian Chemical Bulletin, International Edition, Vol. 57, No. 12, pp. 2561—2563, December, 2008
2561
Solid superacids as catalysts for the synthesis of acylals
from aldehydes and acetic anhydride
S. A. Lermontov and L. L. Yurkova
Institute of Physiologically Active Compounds, Russian Academy of Sciences,
1
Severnyi pr., 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (496) 524 9508. Eꢀmail: lermon@ipac.ac.ru, yurkova@ipac.ac.ru
Sulfated tin, aluminum, and titanium oxides are efficient catalysts for reactions of aldehydes
with acetic anhydride leading to acylals.
Key words: acetic anhydride, aldehydes, acylals, solid superacids, sulfated oxides, heterogeꢀ
neous catalysis, tin oxide, aluminum oxide, titanium oxide.
9
Acylals 1 are diacetyl derivatives of gemꢀdiols. Acylal
protection is currently used1 in multistep syntheses to
prevent the oxidation of a carbonyl (especially aldehyde)
group (Scheme 1).
densation reactions has also been reported. These cataꢀ
lysts are characterized by an extremely high acidity that
exceeds that of 100% H SO (H = –11.93); this by definiꢀ
—3
2
4
0
4
1
tion makes them superacids. The acid sites are firmly
attached to the surface of heterogeneous catalysts, which
enhances the efficiency of the processes and reduces the
amount of acid waste.
Scheme 1
We found that solid superacids prepared by application
of sulfuric acid or ammonium sulfate to titanium and tin
oxides, as well as iron(III) sulfate applied to aluminum
oxide, effectively catalyze the reaction even at room temꢀ
perature (see Scheme 1). The yields of the target acylals 1
are usually high (Table 1).
The advantages of acylal protection include the high
resistance to oxidation and the ease of introduction: unlike
the synthesis of acetals from aldehydes and alcohols, reacꢀ
tions of aldehydes with carboxylic acid anhydrides are not
accompanied by liberation of water, which shifts the reacꢀ
tion equilibrium to the starting compounds. An acylal
protection can be removed as easily as an acetal one.
The synthesis of acylals from aldehydes and carboxylic
acid anhydrides is usually carried out in the presence of
Addition of a catalyst to a mixture of an aldehyde and
acetic anhydride at room temperature results in an exoꢀ
thermic reaction that is completed in 10—15 min. The reꢀ
covered catalyst (see Table 1, entry 9) is still active. Aromaꢀ
tic and aliphatic aldehydes can be involved in the reaction,
while acetone is substantially less reactive. In most cases, the
high yields of acylals can be achieved even at an equimolar
aldehyde : acetic anhydride ratio; with less reactive 3ꢀniꢀ
trobenzaldehyde, the ratio of the reagents should be 1 : 5
for a higher efficiency of the reaction (see entry 16). Inꢀ
terestingly, nickel sulfate applied to aluminum and tin oxides
did not catalyze the reaction under the conditions studied
4
,5
6
Brønsted and Lewis acids, including heavy metal salts,
7
8
zeolites, and oxidized graphite. Such catalysts are not
ecologically safe (heavy metal salts), not very active (zeoꢀ
lites), or not easy to prepare (graphite has to be oxidized by
permanganate in a nitrating mixture).
1
3
(see entries 1—3) despite its very high acidity (H ≈ –14.5).
0
In the present work, we used for the first time a novel
The factors behind the superacid properties of sulfate
class of solid acids as catalysts for the synthesis of acylals.
anions applied to metal oxides have been investigated earꢀ
These are sulfate anions applied to metal oxides.9
—12
Sulꢀ
lier.
15,16
Presumably, adsorbed sulfate forms cyclic strucꢀ
fate anions are usually applied to aluminum, iron, titaniꢀ
um, and zirconium oxides by treating them with the correꢀ
sponding metal sulfates or by impregnating the oxides with
sulfuric acid or ammonium sulfate. The resulting sulfated
tures at an oxide surface, which brings about electron
deshielding of the adjacent metal atoms and, consequentꢀ
ly, an increase in their Lewis or (after adsorption of water)
Brønsted (proton) acidity (Scheme 2). As a rule, a catalyst
surface bears both types of acid sites.
1
0
oxides are mainly employed in alkane isomerization and
1
1
alkene dimerization. The use of them for dehydration of
However, according to Ref. 17, Lewis sites are domiꢀ
alcohols,1
2,13
dealkylation of cumene,12,13 and some conꢀ
nant on the NiSO ꢀcontaining surface and remain inacꢀ
4
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2510—2512, December, 2008.
066ꢀ5285/08/5712ꢀ2561 © 2008 Springer Science+Business Media, Inc.
1

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Russ.Chem.Bull., Int.Ed., Vol. 57, No. 12, December, 2008
Lermontov and Yurkova
Table 1. Reactions of aldehydes with acetic anhydride (20 °C)
Entry
R
1H NMR,a
Catalystb
RCHO : Yield
Entry
R
1H NMR,a
Catalystb
RCHO : Yield
δ
: Ac O of 1
δ
: Ac O of 1
2
2
(
mol.)
(%)
(mol.)
(%)
2
–
1
2
3
4
5
Ph
Ph
Ph
Ph
Ph
9.2/7.6 NiSO (5%)/Al O
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
0
0
0
100
100
11
4ꢀFC H
4
9.9/7.7 SO4 (0.3 M)/SnO2 1 : 1
89
81
88
91
4
2
3
6
2
–
—
—
—
—
NiSO (5%)/SnO
12 4ꢀMeC H 9.9/7.6 SO4 (0.3 M)/SnO2 1 : 1
6 4
4
2
2
–
NiSO (20%)/Al O
13 4ꢀMeC H 9.9/7.6 SO4 (0.3 M)/SnO2 1 : 5
4
2
3
6
4
2
–
Fe (SO ) (5%)/SnO
14
transꢀ
ꢀPhCH=CH J =
= 8 Hz)/
9.8 (d, SO4 (0.3 M)/SnO2 1 : 1
2
4 3
2
Fe (SO ) (10%)/
2
4 3
/
Al O
2 3
2
–
6
7
8
9
Ph
Ph
Ph
Ph
—
—
—
—
SO /TiO2
1 : 1
1 : 1
1 : 1
1 : 1
100
100
100
100
6.9 (d,
J =
= 17 Hz)
4
2
–
SO4 (0.3 M)/SnO2
SO4 (0.3 M)/SnO2
SO4 (0.3 M)/SnO2
2
–
2
–
15 3ꢀO NC H 10.2/7.8 SO4 (0.3 M)/SnO2 1 : 1c
2–
73
99
2
6
4
2
–
recovered
9.8/6.7 SO42–(0.3 M)/SnO2
16 3ꢀO NC H 10.2/7.8 SO4 (0.3 M)/SnO2 1 : 5
2 6 4
10
Pri
1 : 1
100
17
Me CO
—
SO4 (0.3 M)/SnO2 1 : 1 Traces
2–
2
a
b
The δ(C(O)H)/δ(CH(OAc) ) values are given.
All catalysts were calcined in air at 600 °C for 2 h immediately before use. Pure Al O , TiO , and SnO were catalytically inactive
2
2
3
2
2
(
c
the yields of acylals were 0%).
The reaction mixture was diluted with CH Cl for mixing the reagents.
2
2
1
3
11,12
Scheme 2
chased. Solid superacids (NiSO /Al O , Fe (SO ) /Al O ,
4 2 3 2 4 3 2 3
SO4 /SnO2,
earlier.
2
–
10,18
2–
9
and SO4 /TiO ) were prepared as described
2
Synthesis of acylals (general procedure). A catalyst (1 g) was
calcined at 600 °C for 2 h, cooled in a flow of dry air, and added
to a stirred (with a magnetic stirring bar) mixture (6—7 mL) of an
aldehyde and acetic anhydride. As a rule, the reaction mixture
immediately heated itself to 40—45 °C. After the exothermic
reaction was completed (10—15 min), the catalyst was filtered
off. The composition of the reaction mixture and the yield of the
1
product were determined from the intensity ratio of the H NMR
signals for the C(O)H group in the starting aldehyde and the
CH(OAc)2 fragment in the acylal obtained (see Table 1).
To verify the correctness of this method for determination of the
yield of the target acylal, we conducted reactions with benzaldeꢀ
hyde and cinnamaldehyde (entries 7 and 14) in the presence of
cessible for bulky reagents because of steric hindrance,
which probably makes NiSO inactive in the reaction unꢀ
4
an inert internal standard (0.5 mol of CH Cl2 per mole of
2
der discussion.
the aldehyde) while cooling the reaction mixture with ice water
to prevent evaporation of the standard. We found that the yields
of the products are identical. For recovery of the catalyst, it was
washed with acetone, dried, and calcined in a flow of air like
a freshly prepared sample.
To sum up, we demonstrated that differently sulfated
aluminum, titanium, and tin oxides are efficient cataꢀ
lysts for the synthesis of acylals from aldehydes. Since
the applied sulfate is highly active and can be recovered
and its content is low (e.g., 5% Fe (SO ) applied to one
2
4 3
gram of a substrate contains only 12 mg of sulfur), the
proposed heterogeneous catalysts will allow a substantial
reduction in acid waste. This can be very efficient in comꢀ
mercial use.
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Solid superacids in acylal formation
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Received March 4, 2008;
in revised form August 7, 2008