Highly efficient and chemoselective thioacetalization of carbonyl compounds catalyzed with aluminum trifluromethanesulfonate [Al(OTf)3]

Article abstract of DOI:10.1081/SCC-120015573

Aluminum trifluromethanesulfonate [Al(OTf)₃] is a highly efficient and chemoselective catalyst for the thioacetalization of carbonyl compounds.

Full text of DOI:10.1081/SCC-120015573

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Highly Efficient and Chemoselective Thioacetalization
of Carbonyl Compounds Catalyzed with Aluminum
Trifluromethanesulfonate [Al(OTf) ]
3
a b
a
a
H. Firouzabadi
, N. Iranpoor & G. Kohmareh
a
Department of Chemistry , College of Sciences , Shiraz University , Shiraz, Iran
Department of Chemistry , College of Sciences , Shiraz University , Shiraz 71454, Iran
b
Published online: 29 Jun 2011.
To cite this article: H. Firouzabadi , N. Iranpoor & G. Kohmareh (2003) Highly Efficient and Chemoselective Thioacetalization
of Carbonyl Compounds Catalyzed with Aluminum Trifluromethanesulfonate [Al(OTf) ], Synthetic Communications: An
3
International Journal for Rapid Communication of Synthetic Organic Chemistry, 33:1, 167-173, DOI: 10.1081/SCC-120015573
To link to this article: http://dx.doi.org/10.1081/SCC-120015573
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Õ
SYNTHETIC COMMUNICATIONS
Vol. 33, No. 1, pp. 167–173, 2003
Highly Efficient and Chemoselective
Thioacetalization of Carbonyl Compounds
Catalyzed with Aluminum
Trifluromethanesulfonate [Al(OTf )3]
H. Firouzabadi,* N. Iranpoor, and G. Kohmareh
Department of Chemistry, College of Sciences,
Shiraz University, Shiraz, Iran
ABSTRACT
3
Aluminum trifluromethanesulfonate [Al(OTf ) ] is a highly efficient
and chemoselective catalyst for the thioacetalization of carbonyl
compounds.
Key Words: Aluminum trifluromethanesulfonate; Carbonyl com-
pounds; Catalyst; Chemoselective; Solvent-free; Thioacetalization.
*
Correspondence: H. Firouzabadi and N. Iranpoor, Department of Chemistry,
College of Sciences, Shiraz University, Shiraz 71454, Iran. Fax: þ98-711-2280926;
E-mail: firouzabadi@chem.susc.ac.ir.
1
67
DOI: 10.1081/SCC-120015573
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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68
Firouzabadi, Iranpoor, and Kohmareh
INTRODUCTION
Dithioacetals have long been used as protection groups for the
carbonyl functions, while the carbanion forming capacity of cyclic
dithioacetals have brought out numbers of useful applications in
[
1,2]
organic synthesis as the nucleophilic acyl equivalents.
In addition,
they often serve as intermediates for the conversion of carbonyl com-
pounds to parent hydrocarbons. Dithioacetalization of carbonyl com-
[
3]
[4]
pounds is usually catalyzed by Lewis acids, protic and solid acids.
We have recently reported efficient thioacetalization reactions catalyzed
[
5a]
[5b]
by molecular iodine,
acids.
trichloroisocyanoric acid
Preparation of dithioacetals has been reviewed recently.
Now in this report, we wish to introduce aluminum trifluromethanesul-
and heteropoly
[5c]
[5d]
[6]
fonate [Al(OTf )3] as a stable, easily prepared compound which is
weakly hygroscopic in comparison with AlCl , can be used in catalytic
3
amounts as an efficient catalyst for the highly chemoselective dithioa-
cetalization of aryl aldehydes and ketones in solution and also under
solvent-free conditions (Sch. 1).
RESULTS AND DISCUSSION
Various types of aromatic and aliphatic aldehydes were efficiently
converted to the corresponding dithianes in the presence of equimolar
amounts of 1,3-propanedithiol and Al(OTf ) (5 mol%) under solvent-
3
free conditions. In the absence of solvent, aliphatic and aromatic ketones
were also converted to their corresponding dithianes in high yield in the
presence of Al(OTf ) (5–20 mol%) with short reaction times (Table 1).
3
Scheme 1.

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Aluminum Trifluromethanesulfonate
169
Table 1. Thioacetalization of carbonyl compounds with 1,3-propanedithiol and
Al(OTf ) at roomtemperature in solution or under non-solvent conditions.
3
Time Yield
a
(min) (%)
1
2
Entry
R
R
Solvent
None
Sub./Thiol/Al(OTf )
3
b
1
2
3
4
5
6
7
8
9
Ph
4-(CH )C H
H
H
H
H
H
H
CH
CH
1/1.1/0.05
1/1.1/0.05
1/1.1/0.05
1/1.1/0.05
1/1.1/0.05
1/1.1/0.05
1/1.5/0.15
1/1.5/0.15
1/1.7/0.2
5
5
97
95
b
b
b
b
b
b
None
None
None
None
None
None
None
3
6
4
2-(Cl)C
6
H
4
10 98
3-(CH
PhCH¼CH
CH (CH
3
O)C
6
H
4
5
5
95
94
10 92
60 93
3 h 95
4 h 94
3
2
)
5
PhCH
Ph
2
CH
2
3
b,c
b,c
3
Ph
Ph None
b
1
0
None
1/1.2/0.1
25 96
b,c
1
1
None
None
1/2/0.2
14 h 50
b
1
2
1/2/0.2
2 h 85
13
14
15
16
Ph
PhCH¼CH
H
H
CH
ClCH CH Cl
1/1.1/0.05
1/1.1/0.05
1/1.5/0.15
1/1.7/0.15
15 95
20 94
7 h 85
2
2
2
2
2
2
ClCH
ClCH
CH
CH
Cl
Cl
PhCH
Ph
2
CH
2
3
CH3 ClCH CH Cl
2
24 h
—
2
1
7
ClCH
2
CH
2
Cl
1/1.2/0.1
45 95
a
Isolated yields.
b
Under neat conditions.
NMR yield.
c
Dithioacetalization of hindered ketones such as benzophenone is a low
yield and a time-consuming process. However, with the present method,
benzophenone can easily be converted to its corresponding dithioacetal in
95% yield within 4 h at roomte mp erature. The catalytic activity of
Al(OTf ) for dithioacetalization in solution has also been studied. 1,2-
3
Dichloroethane (ClCH CH Cl) is a good solvent for this purpose and the
2
2

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70
Firouzabadi, Iranpoor, and Kohmareh
protection of aldehydes and aliphatic ketones proceeded well and their
corresponding dithianes were obtained in high yields. Sterically hindered
ketones, in solution, resist towards protection by this catalyst (Entry 16;
Table 1).
The selectivity of the present method is demonstrated by competition
experiments using structurally differing carbonyl compounds in the
absence of solvent and also in solution. Our observation shows that
selectivity of the catalyst is more pronounced in solution (Table 2) than
in the absence of the solvent (Table 3).
Table 2. Selective dithioacetalization of carbonyl compounds catalyzed with
3
Al(OTf ) in 1,2-dichloroethane.
Sub. 1/Sub. 2/
Thiol/Cat
Time
(min)
Yield
a
(%)
Entry
1
Substrate
Product
1/1/1.1/0.05
1/1/1.1/0.05
1/1/1.1/0.05
1/1/1.2/0.1
15
15
15
45
100
0
2
3
4
95
5
80
2
0
35
6
5
5
1/1/1.2/0.1
45
80
2
0
a
Yields based on GC and NMR.

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Aluminum Trifluromethanesulfonate
171
Table 3. Selective dithioacetalization of carbonyl compounds catalyzed with
Al(OTf ) in the absence of solvent.
3
Sub. 1/Sub. 2/
Thiol/Cat
Time
(min)
Yield
a
(%)
Entry
1
Substrate
Product
1/1/1.1/0.05
5
98
2
2
3
1/1/1.1/0.05
1/1/1.5/0.15
5
70
3
0
60
70
3
0
4
1/1/1.1/0.05
5
92
8
a
Yields based on GC and NMR.
In conclusion, Al(OTf) which is a weakly hygroscopic compound in
3
comparison with AlCl and its handling is also easier, can be used in
3
catalytic amounts for selective protection of carbonyl groups in the
absence of solvent and also in solution. Efficient protection of hindered
ketones proceeded well with Al(OTf ) whereas, the other recently
3
[
3i]
[3j]
[3l]
reported catalysts such as WCl6, LiBr,
failed or the reaction proceeded very sluggishly.
and LiOTf
completely
General Procedure
To a mixture of carbonyl compound (5 mmol) and 1,3-propane-
dithiol (5.5–10 mmol) was added Al(OTf ) (0.25–1 mmol), and the result-
ing mixture was magnetically agitated at room temperature. The progress
3

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Firouzabadi, Iranpoor, and Kohmareh
of the reaction was monitored by TLC or GC. After completion of the
reaction, a cold aqueous solution of NaOH (10%, 25 mL) was added to
the mixture and stirred for a few minutes. The resulting mixture was
extracted with Et O (3 ꢀ 40 mL) and the etheral solution was washed
2
with water (20 mL) and dried over anhydrous Na SO . Evaporation
4
2
of the solvent gave almost pure product(s). Further purification, if
necessary, was proceeded by column chromatography by an appropriate
solvent. Procedure for the reactions in solution is similar to non-solvent
procedure.
ACKNOWLEDGMENT
We thank Shiraz University Research Council for partial support
of this work.
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Aluminum Trifluromethanesulfonate
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Received in the Netherlands December 12, 2001

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