Solvent-free catalytic preparation of 1,1-diacetates using a silica-supported functional ionic liquid as catalyst

Article abstract of DOI:10.1007/s00706-012-0777-6

An efficient and convenient preparation of acylals from aldehydes and acetic anhydride, under solvent-free conditions, in the presence of a silica-supported functional ionic liquid, Si-[SbSipim][PF₆], is reported. Si-[SbSipim][PF₆] acts as a catalyst and can be recovered and reused four times without apparent loss of its catalytic activity.

Full text of DOI:10.1007/s00706-012-0777-6

Monatsh Chem (2013) 144:247–249
DOI 10.1007/s00706-012-0777-6
ORIGINAL PAPER
Solvent-free catalytic preparation of 1,1-diacetates
using a silica-supported functional ionic liquid as catalyst
•
Li Q. Kang Yue Q. Cai Lin Cheng
•
Received: 11 October 2011 / Accepted: 22 April 2012 / Published online: 22 May 2012
Ó Springer-Verlag 2012
Abstract An efficient and convenient preparation of
acylals from aldehydes and acetic anhydride, under solvent-
free conditions, in the presence of a silica-supported
functional ionic liquid, Si–[SbSipim][PF6], is reported.
Si–[SbSipim][PF6] acts as a catalyst and can be recovered
and reused four times without apparent loss of its catalytic
activity.
temperatures and an excess of costly dehydrating reagents,
limit the applicability of the reaction [2]. Acylals are
synthetically useful protecting groups for aldehydes
because of their remarkable stability in basic and neutral
reaction media and toward aqueous acids [3]. Acylals are
usually prepared from aldehydes and acid anhydrides by
use of sulfuric acid, Nafion-H, or CH3SO3H [4–6] or FeCl3,
ZnCl2, or PCl3 [7–9]. Heterogeneous catalysts, for example
K10 clay, HZSM-5, zeolite Y, and Zr(HSO4)4, and het-
eropolyacids have also been developed for use as catalysts
in the synthesis of acylals [10–17].
Keywords Support Á Ionic liquid Á Solvent-free Á
Acylal
Ionic liquids, a new class of solvents and catalysts, have
several interesting properties, especially their lack of vapor
pressure and ease of reuse. Use of an ionic liquid as cata-
lyst in the synthesis of 1,1-diacetates has been reported
[18, 19]. Our group also has reported the use of ionic liquids
as catalysts in the synthesis of other organic compounds
[20–22]. Despite successful use of these ionic liquid-based
catalysts, repetitious extraction from the ionic liquids is
necessary after the reaction, because the products are often
partially soluble in the ionic liquid. Highly efficient tech-
niques for separation of catalyst and chemical product are
therefore desirable. For this reason, solvent-free reactions
and supported ionic liquid catalysis have attracted much
attention in chemical processes, because of their safety,
economy, and easy work up [23–25]. Our group has
reported use of a supported functional ionic liquid as
scavenger [26] for organic synthesis. The functional ionic
liquid is attached to silica gel by chemical bonding. It is
easy to separate the supported ionic liquid from the product
by filtration. In continuation of our ongoing program to
develop environmentally benign methods using supported
ionic liquids as novel promoters, we report herein an effi-
cient, convenient, mild synthesis of 1,1-diacetates under
solvent-free conditions using a silica-supported sulfonic
Introduction
Formation of 1,1-diacetates is one of the most versatile
acylal-formation reactions in modern organic synthesis; it
has attracted substantial attention in the past decade for
protection of aldehydes. Acetal formation is also used
for protection of aldehydes, and deprotection is readily
achieved by use of an aqueous acid [1]. The condensation
of amines and hydrazines with carbonyl compounds is
another means of protection of aldehydes, but disadvan-
tages, for example the requirement for high reaction
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-012-0777-6) contains supplementary
material, which is available to authorized users.
L. Q. Kang (&) Á L. Cheng
School of Chemical and Environmental Engineering,
Shanghai Institute of Technology, Shanghai 200235, China
e-mail: klq@sit.edu.cn
Y. Q. Cai
School of Chemistry and Molecular Engineering,
East China University of Science and Technology,
Shanghai 200237, China
123

248
L. Q. Kang et al.
O
O
giving 1,1-diacetates in good to high yields. The reaction is
selective for aromatic aldehydes; aliphatic aldehydes did
not furnish the desired products. It is worth mentioning that
in the reaction of 4-hydroxybenzaldehyde, 2-hydroxy-
benzaldehyde, and 3-methoxy-4-hydroxybenzaldehyde
with acetic anhydride under the above conditions, both
carbonyl and phenolic OH groups were acylated (Table 1,
entries 9, 10, 12). The acid sensitive substrate furfural was
also protected as a gem-diacetate in good yield, possibly
because of the mild acidic conditions. 4-(Dimethyl-
amino)benzaldehyde failed to give the 1,1-diacetate under
the same conditions, possibly because of strong electron
donation by the dimethylamino group (Table 1, entry 15)
[27]. In addition, the sterically hindered substrate piperonal
was also efficiently converted into its corresponding acylal.
In conclusion, we describe a simple and efficient prep-
aration of 1,1-diacetates from aldehydes and acetic
anhydride in air, under mild conditions, using a moisture-
stable silica-supported functional ionic liquid as catalyst.
The catalyst can be recovered and reused at least four times
without apparent loss of activity. Use of a supported ionic
liquid for this transformation has several advantages—it
avoids use of either strongly acidic or basic conditions and
the reaction is solvent-free. Simple work up, ease of
recovery, and reuse of this catalyst are expected to con-
tribute to the development of a green strategy for preparation
of 1,1-diacetates.
O
R
CH3
CH3
O
R
CH3
CH3
Si-[SbSipim][PF6]
CHO
HC
+
O
O
O
1
2
3a-3n
OH
O
-
PF6
OMe
Si
SO3H
N
N
O
Si-[SbSipim][PF6]
Scheme 1
acid-functional ionic liquid (Si–[SbSipim][PF6]) [26] as
catalyst (Scheme 1).
Results and discussion
The reactions were conducted by dissolving 1 mmol
aldehyde in 3 mmol acetic anhydride followed by addition
of 0.1 g silica-supported functional ionic liquid Si–[SbSi-
pim][PF6]. The reaction mixture was then stirred and
heated for appropriate time at 30 °C (Table 1). After the
reaction Si–[SbSipim][PF6] was isolated by simple filtra-
tion. Si–[SbSipim][PF6] could be recovered easily by
washing with ethyl acetate and drying at 80 °C for 2 h. The
catalyst can be reused for several experiments. The reac-
tion in entry 1 (Table 1) has been repeated four times with
the following yields: 90, 89, 89, and 88 %. The product
was isolated by extraction with ethyl acetate, a relatively
non-toxic and industry-friendly solvent.
Experimental
Melting points are corrected and were measured on a
WRS-IB digital melting point instrument. 1H NMR spectra
As shown in Table 1, a wide range of aromatic alde-
hydes underwent smooth reaction with acetic anhydride
Table 1 Formation of acylals
using a supported functional
ionic liquid as catalyst under
solvent-free conditions
Entry Substrate R-CHO Product R-CH(OAc)2
Time/h Yield/% MP/°C
Found
Reported
1
Ph
Ph (3a)
3
4
90
88
89
91
89
87
89
88
86
85
85
84
85
76
0
44.3–44.6
82.4–83.3
65.0–65.4
81.1–81.8
78.9–80.1
52.8–53.6
61.7–62.4
44–46 [27]
83 [28]
2
3-Br-Ph
3-NO2-Ph
4-CH3-Ph
4-Cl-Ph
4-F-Ph
3-Br-Ph (3b)
3-NO2-Ph (3c)
4-CH3-Ph (3d)
4-Cl-Ph (3e)
4-F-Ph (3f)
3
6
65–66 [27]
82–83 [29]
80–81 [29]
50–51 [29]
64–65 [29]
4
3
5
4
6
4
7
4-CH3O-Ph
2,4-Cl2-Ph
4-OH-Ph
2-OH-Ph
2-Cl-Ph
4-CH3O-Ph (3g)
2,4-Cl2-Ph (3h)
4-AcO-Ph (3i)
2-AcO-Ph (3j)
2-Cl-Ph (3k)
4
8
5
100.0–100.3 101–102 [27]
89.1–90.2 89–90 [30]
100.2–100.3 101–103 [30]
9
4
10
11
12
13
14
15
4
4
55.7–56.1
88.3–89.2
78.1–78.5
51.2–51.9
–
56–57 [31]
90–91 [29]
78–79 [29]
50–51 [29]
–
4-OH-3-CH3O-Ph 4-AcO-3-CH3O-Ph (3l)
5
Products were characterized by
1H NMR and GC–MS, and by
comparison of their physical
properties with those reported in
the literature
3,4-(OCH2O)-Ph
2-Furanyl
3,4-(OCH2O)-Ph (3m)
5
2-Furanyl (3n)
4
4-(CH3)2N-Ph
–
10
123

Solvent-free catalytic preparation of 1,1-diacetates
249
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To a stirred suspension of aldehydes (1 mmol) and acetic
anhydride (3 mmol), 0.1 g silica-supported ionic liquid,
Si–[SbSipim][PF6], was added at room temperature. The
mixture was stirred at 30 °C for the appropriate time
(Table 1), with progress of the reaction monitored by TLC.
After the reaction the catalyst was isolated by simple fil-
tration and washed with ethyl acetate. The filtrate was
washed twice with a 10 % solution of sodium bicarbonate,
and the organic layer was dried over anhydrous sodium
sulfate and evaporated under reduced pressure. The residue
was purified by crystallization or by flash column chro-
matography and afforded the corresponding 1,1-diacetates
in 76–91 % yield. Si–[SbSipim][PF6] was reused imme-
diately after washing with ethyl acetate and drying at 80 °C
1
for 2 h. All compounds were characterized by H NMR,
GC–MS, and use of physical data (see Supplementary
Material).
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Acknowledgments We are grateful to the Science and Technology
Foundation of Shanghai Institute of Technology (no. KJ 2010-08) for
financial support.
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