Polystyrene-supported, Al(OTf)3-catalyzed chemoselective synthesis of acylals from aldehydes

Article abstract of DOI:10.1080/00397910903537323

Cross-linked polystyrene-supported aluminium triflate [Ps-Al(OTf)3] has been shown to be a mild, efficient, and chemoselective heterogeneous Lewis acid catalyst for acetylation of aldehydes with acetic anhydride. The catalyst can be recovered simply and reused efficiently at least five times without any noticeable loss of catalytic activity. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910903537323

1
Synthetic Communications , 41: 277–284, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903537323
POLYSTYRENE-SUPPORTED, Al(OTf) -CATALYZED
3
CHEMOSELECTIVE SYNTHESIS OF ACYLALS FROM
ALDEHYDES
Kaveh Parvanak Boroujeni
Department of Chemistry, Shahrekord University, Shahrekord, Iran
3
Cross-linked polystyrene-supported aluminium triflate [Ps-Al(OTf) ] has been shown to
be a mild, efficient, and chemoselective heterogeneous Lewis acid catalyst for acetylation
of aldehydes with acetic anhydride. The catalyst can be recovered simply and reused
efficiently at least five times without any noticeable loss of catalytic activity.
Keywords: Acylal; aldehyde; aluminium triflate; catalysis; polystyrene
INTRODUCTION
The application of heterogeneous catalysts in place of homogeneous catalysts
in organic synthesis is an attractive area of research in the laboratory as well as in the
[1]
industrial context. Such heterogeneous catalysts are as active as their homogeneous
counterparts while having the distinguishing characteristics of being easily separable
from reaction media, ease of handling, stability (toward air and moisture), lack of
corrosion, recyclability, and selectivity in various organic reactions. Although the
use of heterogeneous acid catalysts is of continuing interest, few examples are known
[
2]
for polymer-supported Lewis acid catalysts.
Acylals have attracted great interest in organic synthesis for the protection of
carbonyl groups because of their remarkable stability in basic and neutral reaction
[3]
media as well as toward aqueous acids. In addition, acylals are important synthons
and useful precursors in organic chemistry. They can be used as cross-linking
[4]
reagents for cellulose in cotton and are important starting materials for the syn-
[
thesis of valuable intermediates in Diels–Alder reactions, carbonyl surrogates for
5]
[6]
[7,8]
asymmetric synthesis, and substrates for nucleophilic substitution reactions.
Generally, acylals can be prepared from aldehydes by treatment with acetic anhy-
dride in the presence of a suitable catalyst such as proton acids (e.g., H SO ,
2
4
[
9–12]
CH SO H, NH SO H, and p-toluene sulfonic acid),
3
Lewis acids (e.g., FeCl ,
3
3
2
3
[
13–20]
AlCl , WCl , ZrCl , In(OTf) , Sc(OTf) , Bi(OTf) ꢀ xH O, and RuCl ꢀ xH O),
3
6
4
3
3
3
2
21]
3
2
[
heteropoly acids (e.g., 12-molybdophosphoric acid),
solid acidic catalysts (e.g.,
3
nafion-H, Fe -montmorillonite, amberlyst-15, InCl =Al O , HClO =SiO , and
þ
3
2
3
4
2
Received August 12, 2009.
Address correspondence to Kaveh Parvanak Boroujeni, Department of Chemistry, Shahrekord
University, Shahrekord 115, Iran. E-mail: parvanak-ka@sci.sku.ac.ir
2
77

278
K. P. BOROUJENI
2 3
Scheme 1. Acetylation of aldehydes with Ac O using Ps-Al(OTf) .
[
22–27]
[28]
and ionic liquids. How-
H PW O -supported MCM-41 molecular sieves),
3 12 40
ever, most of these methods suffer from one or more disadvantages such as long
reaction times, high reaction temperature, strongly acidic or oxidizing conditions,
poor selectivity, tedious workup, disturbance to other functional groups, environ-
mental pollution, use of stoichiometric or excess amounts of catalyst, or the use of
moisture-sensitive, expensive, or unreusable catalysts. In view of these, the search
for finding a cost-effective, mild, and simple chemoselective protocol for the syn-
thesis of acylals is still relevant.
In continuation of our ongoing program to develop environmentally benign
[
29–31]
methods using heterogeneous catalysts,
meric Lewis acid catalyst for dithioacetalization of carbonyl compounds and trans-
we reported Ps-Al(OTf) as a new poly-
3
[32]
dithioacetalization of acetals. Along this line, we now report that Ps-Al(OTf) is
3
also an effective and highly chemoselective heterogeneous catalyst for acetylation
of aldehydes with acetic anhydride under mild reaction conditions (Scheme 1).
RESULTS AND DISCUSSION
Ps-Al(OTf)3 was prepared by the exchange reaction between cross-linked
polystyrene-supported aluminium chloride (Ps-AlCl ) and triflic acid in freon 113
3
under reflux conditions. The capacity of the catalyst obtained by the gravimetric
method and checked by atomic absorption was 0.41 mmol Al(OTf) per gram. Using
3
this catalyst, various aldehydes (aromatic, aliphatic, and a,b-unsaturated aldehydes)
were converted into their corresponding acylals in good to excellent yields at room
temperature in dichloromethane, which proved to be the solvent of choice among
other organic solvents (Table 1, entries 1–10). The optimum molar ratio of
Ps-Al(OTf) to aldehyde was found to be 0.1:1. In the case of 2-hydroxybenzalde-
3
hyde and vanillin, both carbonyl and phenolic-OH groups were acetylated when
excess acetic anhydride was used (entries 1i and 3). The acid-sensitive substrate fur-
fural also led to the formation of the corresponding acylal in 94% yield without the
formation of any side products, which are normally encountered under acidic con-
ditions (entry 5). 4-(Dimethylamino) benzaldehyde failed to give the corresponding
acylal under the same reaction conditions, possibly due to tautomerization and the
[
23]
existence of the quininoid structure (entry 1j). However, ketones did not produce
the corresponding diacetates under the same reaction conditions (entries 11–13).
Ps-Al(OTf) is stable under the reaction conditions, and there is no leaching of acid
3
moieties during reactions.
To evaluate the selectivity of this method, we investigated competitive reactions
using Ps-Al(OTf) , Al(OTf) , and AlCl for acetylation of aldehydes in the presence
3
3
3

SYNTHESIS OF ACYLALS
279
a,b
Table 1. Conversion of aldehydes into acylals catalyzed by Ps-Al(OTf)
3
ꢁ
ꢁ
Mp ( C) or Bp ( C=torr)
Entry
1
Aldehyde
Time (h)
Yield (%)
Found
Reported
[
[
[
14]
23]
14]
a. X ¼ H
1
1
94
94
94
94
95
94
93
95
94
—
44–46
82–83
65–67
80
43–44
81–82
67–68
b. X ¼ 4-CH
3
c. X ¼ 4-OCH
3
1
[
27]
d. X ¼ 4-Cl
1.1
1.3
1.25
1.1
1.5
2.1
6
81
62–64
[
14]
e. X ¼ 3-NO
2
63–65
125
[14]
124
f. X ¼ 4-NO
g. X ¼ 4-Ph
2
[
16]
26]
127–129
107–109
105
128–130
[
h. X ¼ 4-OCH
i. X ¼ 2-OH
2
Ph
108–110
[27]
104
—
j. X ¼ 4-N(CH
3
)
2
—
[27]
52
2
3
0.9
2.1
95
90
53–55
[26]
90–91
91
[
17]
23]
4
5
1.1
92
94
78–79
50–51
77–78
50–51
[
1.15
6
[27]
110
[16]
133–135
a. X ¼ H
b. X ¼ OCH
2.2
2.1
91
92
109
134–136
3
[13]
185–190=0.7
7
8
1.5
89
Oil
3 2 n
CH (CH ) CHO
[26]
a. n ¼ 4
b. n ¼ 5
c. n ¼ 6
1.15
1.5
91
90
90
Oil
135–137=2
Oil
Oil
136–137=2
[14]
[17]
1.6
Oil
(
Continued )

280
K. P. BOROUJENI
Table 1. Continued
ꢁ
ꢁ
Mp ( C) or Bp ( C=torr)
Entry
9
Aldehyde
Time (h)
Yield (%)
Found
Reported
[20]
83–85
2
91
90
84–86
[14]
90–91=15
1
1
0
1
CH
3
CH=CHCHO
1.8
91–93=15
8
8
—
—
—
—
—
1
1
2
3
—
—
8
—
—
a
The molar ratio of the aldehyde=Ac
ratio is 1:2.5:0.12.
2
O=cat. is 1:1.5:0.1 except for entries 1i and 3, in which the molar
b
Yields refer to pure isolated products, and all acylals prepared are known compounds and were char-
acterized by comparison of their physical and spectral data with those of authentic samples.
of ketones (Table 2, entries 1 and 2) and more important, selective acetylation of
benzaldehyde and 4-methoxybenzaldehyde versus 3-nitrobenzaldehyde (entries 3
and 4). From the results, it is clear that very high chemoselectivity was observed in
the presence of Ps-Al(OTf) and that this catalyst shows greater chemoselectivity than
3
Al(OTf) and AlCl in acetylation reactions. Moreover, Al(OTf) and AlCl might be
3
3
3
3
required in reagent quantities as a consequence of their ability to strongly complex
Lewis base products. The observed high chemoselectivity of acetylation reactions is
presumably a result of mild catalytic activity of Ps-Al(OTf) , difference in steric bulki-
3
ness of the carbonyl compounds, and also the influence of electronic effects upon
these reactions in the presence of Ps-Al(OTf) . The present chemoselectivities can
3
be suitable for acetylation of aldehydes for the synthesis of complex molecules by
multistep processes.
It is important to note that Ps-Al(OTf) shows greater catalytic activity than
3
Ps-AlCl in acetylation reactions (Scheme 2).
3
In conclusion, Ps-Al(OTf) is an efficient and convenient polymer-supported
3
Lewis acid catalyst for acetylation of aldehydes. This catalyst has comparable
activity with AlCl and Al(OTf) , but it has added advantages involving its ability
3
3
to be recycled, nontoxicity, noncorrosiveness, ease of handling (as a benchtop cata-
lyst), and high chemoselectivity. Good yields, green and mild reaction conditions,
facile workup, and short reaction times are other notable features of this protocol.
Further investigations to apply Ps-Al(OTf) are now in progress.
3

SYNTHESIS OF ACYLALS
281
, Al(OTf)
Table 2. Competitive acylal formation reactions using Ac
a,b
2
O in the presence of Ps-Al(OTf)
3
3
,
and AlCl
3
c
c
Entry
Substrate 1
Substrate 2
Product 1 yield (%)
Product 2 yield (%)
Time (h)
1
2
1.1
1.2
3
1.15
4
1.1
a
The molar ratio of substrate 1 to substrate 2 to Ac
2
O is 1:1:1.2 for entries 1 and 2 and 1:1:1 for entries 3
and 4.
b
1
at room temperature in the presence of Ps-Al(OTf)
All reactions were carried out in CH
2
0.1 mmol), Al(OTf)
2
Cl
2
3
3
(0.2 mmol), or AlCl
(
3
3
(0.28 mmol).
c
GC yield.
3 3
Scheme 2. Catalyst a: Ps-AlCl (0.35 mmol); b: Ps-Al(OTf) (0.1 mmol).

282
K. P. BOROUJENI
EXPERIMENTAL
Materials and Techniques
Chemicals were either prepared in our laboratory or were purchased from
Merck and Fluka. Polystyrene (8% divinylbenzene, prepared via suspension poly-
merization, polyvinylpyrrolidone 90 K as suspension agent, grain size range:
0
.25–0.6 mm) was obtained from Iran Polymer and Petrochemical Institute. Infrared
IR) spectra were run on a Shimadzu model 8300 Fourier transform (FT)–IR spec-
(
trophotometer. H NMR spectra were recorded on a Bruker DPX-300 (300-MHz)
1
spectrometer using CDCl as solvent and tetramethylsilane (TMS) as the internal
3
standard. Capacity of the catalyst was determined by the gravimetric method and
atomic absorption technique using a Philips atomic absorption instrument. Melting
points were determined on a Fisher-Jones melting-point apparatus and are uncor-
rected. Reaction monitoring and purity determination of the products were
accomplished by GLC or thin-layer chromatography (TLC) on commercial plates
of silica gel GF254
.
Preparation of Ps-Al(OTf)3
In a two-necked round-bottomed flask (100 mL) equipped with a reflux con-
[
denser, a solution of Ps-AlCl (3 g, 8% DVB) in freon 113 (50 mL) was prepared.
31]
3
To the stirred solution, 5 mL of triflic acid were added dropwise over a period of 2 h,
while maintaining a gentle reflux. The mixture was then refluxed for 12 h. The poly-
mer beads were filtered, washed with CH Cl (50 mL) and then with Et O (50 mL),
2
2
2
and dried in a vacuum oven overnight to give Ps-Al(OTf) (3.41 g).
3
Typical Procedure for the Preparation of Acylals
Ps-Al(OTf)3 (0.1 mmol) was added to a solution of 4-nitrobenzaldehyde
1 mmol) and Ac O (1.5 mmol) in CH Cl (10 mL), and the resulting mixture was
(
2
2
2
stirred using a magnetic stirrer at room temperature. Progress of the reaction was
monitored by TLC. After completion of the reaction, the catalyst was filtered off
and washed with CH Cl (15 mL). The filtrate was washed with water (10 mL),
2
2
and then the organic layer was dried over anhydrous sodium sulfate and concen-
trated on a rotary evaporator under reduced pressure to give the desired pure pro-
ꢁ
[14]
ꢁ
duct: Yield (94%), mp 125 C (lit.
1
2
124 C); IR (KBr): 2948, 1755, 1620, 1530,
ꢂ1
1
359, 1242, 1210, 999 cm ; H NMR (300 MHz, CDCl ): d ¼ 8.4 (d, J ¼ 7.9 Hz,
3
H), 7.7 (d, J ¼ 7.9 Hz, 2H), 7.5 (s, 1H), 2.1 (s, 6H). After each reaction, the spent
polymeric catalyst from different experiments was washed with Et O, dried over-
2
night in a vacuum oven, and reused.
ACKNOWLEDGMENT
The author thanks the Research Council of Shahrekord University for partial
support of this work.

SYNTHESIS OF ACYLALS
283
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