Chemoselective Protection of Carbonyl Compounds as Dithioacetals Using Silica Gel Supported Aluminium Chloride

Article abstract of DOI:10.1081/SCC-120026854

Silica gel supported aluminium chloride is shown to be a mild chemoselective catalyst for dithioacetalization of carbonyl compounds.

Full text of DOI:10.1081/SCC-120026854

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Chemoselective Protection of Carbonyl Compounds
as Dithioacetals Using Silica Gel Supported Aluminium
Chloride
B. Tamami a & K. Parvanak Borujeny a
a Department of Chemistry , College of Sciences , Shiraz University , Shiraz, Iran
Published online: 21 Aug 2006.
To cite this article: B. Tamami & K. Parvanak Borujeny (2003) Chemoselective Protection of Carbonyl Compounds as
Dithioacetals Using Silica Gel Supported Aluminium Chloride, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 33:24, 4253-4258, DOI: 10.1081/SCC-120026854
To link to this article: http://dx.doi.org/10.1081/SCC-120026854
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 24, pp. 4253–4258, 2003
Chemoselective Protection of Carbonyl
Compounds as Dithioacetals Using Silica
Gel Supported Aluminium Chloride
*
B. Tamami and K. Parvanak Borujeny
Department of Chemistry, College of Sciences,
Shiraz University, Shiraz, Iran
ABSTRACT
Silica gel supported aluminium chloride is shown to be a mild che-
moselective catalyst for dithioacetalization of carbonyl compounds.
Key Words: Dithioacetalization; Carbonyl compound; Aluminium
chloride; Silica gel.
INTRODUCTION
Heterogenization of inorganic reagents and catalysts that are useful
in organic reactions is an important goal in clean technology and much
*Correspondence: B. Tamami, Department of Chemistry, College of Sciences,
Shiraz University, Shiraz 71454, Iran; E-mail: tamami@chem.susc.ac.ir.
4253
DOI: 10.1081/SCC-120026854
Copyright & 2003 by Marcel Dekker, Inc.
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4254
Tamami and Parvanak Borujeny
research has been focused on it.[1] Aluminium chloride is one of the most
widely used inorganic catalysts in organic chemistry and its drawbacks,
such as its corrosivity, hygroscopicity, difficulties in handling, and the
large volumes of enviromentally hazardous waste associated with its use,
make it a prime target for heterogenation.[1b] For this purpose, several
heterogeneous catalysts based on AlCl3 have been developed.[2,3]
Dithioacetalization of carbonyl compouds is an important transfor-
mation in organic synthesis.[4] The reaction is usually catalyzed by Lewis
acids.[5] Among many of the recently developed catalysts, solid supported
catalysts are of the choice for this purpose due to easier handling,
mildness of the reaction conditions, and simple work-up procedures.[6]
However, literature survey shows that only a few methods are available
for selective thioacetalization of aldehydes in the presence of ketones
specially by using a heterogeneous catalyst.[6b,c,e] Furthermore, some of
these methods suffer from a few drawbacks. For example, Hojo[6b]
reported utilization of thionyl chloride impregnated silica gel as a
chemoselective catalyst for thioacetalization but a large amount of HCl
and SO2 were released upon mixing thionyl chloride and silica gel, and in
addition its handling is difficult. Amberlyst-15 catalyst has also been
reported by Perni[6c] for thioacetalization but reaction times were long
and due to acidic nature of the catalyst it may be incompatible with acid
sensitive substrates. Thus, it seems that convenient and general methods
for thioacetalization using heterogeneous catalysts are still being
demanded. In this communication, we described the preparation and
application of immobilized AlCl3 on silica gel, SiO2-AlCl3, as an efficient
and chemoselective heterogeneous catalyst for dithioacetalization (Sch. 1).
RESULTS AND DISCUSSION
SiO2-AlCl3 is prepared by treating silica gel with aluminium chloride
in refluxing carbon tetrachloride.[3a] Thioacetalization with this reagent
was carried out in 1,2-dichloroethane, dichloromethane, chloroform, and
benzene. 1,2-Dichloroethane proved to be the best. This catalyst was stable
O
_
S
S
SiO2
AlCl 3
+
C
C
SH SH
ClH CCH Cl
R1
R2
/
2
2
R1
R2
Scheme 1.

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Silica Gel Supported Aluminium Chloride
4255
Table 1. Dithioacetalization of carbonyl compounds with 1,2-ethanedithiol
catalyzed with SiO2-AlCl3 in 1,2-dichloroethane.
Entry
R1
R2
Time(h)
1,3-Dithiolane (%)b
1
2
3
4
5
Ph
H
H
H
H
H
1.2
1.4
1.3
1.1
1.4
95
94
94
91
93
p-MeC6H4
p-Cl C6H4
p-NO2C6H4
PhCH¼CH
6
H
1.2
92
O
7
8
PhCH2CH2
PhCH2
n-hexyl
H
H
H
H
1.8
1.5
1.6
1.8
1.7
2.5
2.1
3
91
93
90
90
90
92
87
29
17
8
9
10
11
12
13
14
15
16
n-heptyl
-(CH2)5-
PhCH2
n-Butyl
Ph
C2H5
CH3
CH3
Ph
Ph
n-Propyl
Ph
3
3
aThe molar ratio of substrate/thiol is: 1:1.1–1.2. The reactions were carried out in
1,2-dichloroethane under reflux condition using SiO2-AlCl3 (0.3 mmol).
bIsolated and GC yield. Products were GC pure and were identified by com-
parison of their physical and spectral data with those of the authentic samples.
under the reaction conditions, and during reactions no leaching of acid
moieties was observed due to being chemically bonded to the support.[3]
1,3-Dithiolanes were obtained in excellent yields in the presence of
SiO2-AlCl3 from variouse aliphatic and aromatic aldehydes containing
electron donating and withdrawing groups using 1,2-ethanedithiol
(Table 1, Entries 1–10). Aliphatic ketones were also thioacetalized in
the presence of SiO2-AlCl3 (Entries 11–13). As shown in Table 1 under
the same conditions aromatic ketones were dithioacetalized very slowly
(Entries 14–16). This striking difference in reactivity prompted us to
investigate application of the present method to chemoselective dithio-
acetalization. Therefore equimolar mixtures of aldehydes and ketones
were allowed to react with 1,2-ethanedithiol. As shown in Table 2 very
high chemoselectivity was observed in each case. The observed chemo-
selectivity of SiO2-AlCl3 is suitable for selective protection of aldehydes
in the presence of keto carbonyl functions and can be used in synthesis of

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4256
Tamami and Parvanak Borujeny
Table 2. Dithioacetalization of mixture of aldehyde and ketone using
SiO2-AlCl3.a,b
H
H3
C
S
S
S
S
C
H3
C
CHO
H3
C
+
COCH3
C
+
4%
93%
Et
H
S
S
S
S
CH2COEt
+
C
CH2
H3
C
CHO
+
H3C
C
7%
93%
H
Et
CH2
S
S
CH2COEt
CHO
+
5
C
5
C
+
9%
87%
S
S
H3
C
S
Et
CH2 C
S
COCH3
+
CH2COEt
+
C
S
S
92%
3%
aMolar ratio of Sub.1/Sub.2 (1/1 mmol), 1,2-ethanedithiol (1.1–1.2 mmol), and
SiO2-AlCl3 (0.3 mmol) in 1,2-dichloroethane under reflux condition for 3 h.
bYields based on GC and NMR.
complex molecules by multistep processes. Chemoselectivity of SiO2-
AlCl3 toward aldehydes and ketones is presumably due to its mild
catalytic activity and also to the difference in steric bulkiness of the
carbonyl comounds.
In conclusion, the advantages of the present method may be
summarized as, useful chemoselectivities accompanied by high yields of
the desired products, easy work-up procedures, and mild reaction condi-
tions. In addition, the use of this solid acid has resulted in a reduction in
the unwanted and hazardous waste that is produced during conventional
homogeneous processes. This catalyst is reusable and after each reaction,
can be reused up to three times with negligible loss in its activity. We
believe that the efficiency of SiO2-AlCl3 is probably attributed to its
strong affinity for carbonyl oxygen facilitating the formation of the
intermediate hemithioacetal and also to water trapping nature of silica
gel. Further synthetic investigations of this catalyst system are currently
in progress and will be reported in due course.
EXPERIMENTAL
Material and Techniques
Starting materials were obtained from Fluka Company. Products
were characterized by comparison of their physical data with those of

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Silica Gel Supported Aluminium Chloride
4257
known samples. Gas chromatography was recorded on Shimadzu GC.
1
14-A. Infrared spectra recorded on Perkin–Elmer 781 spectrometer. H
and 13C NMR spectra were recorded on Bruker Advance DPX FT
250 MHz instrument.
Preparation of SiO2-AlCl3
This catalyst was prepared as reported in the Lit.[3a] The loading of
aluminium chloride determined was 1.3 mmol/g.
General Dithioacetalization Procedure
To a mixture of carbonyl compound (1 mmol), 1,2 ethanedithiol
(1.1–1.2 mmol) in 1,2-dichloroethane (10 mL) was added SiO2-AlCl3
(0.3 mmol), and the resulting mixture was magnetically stirred under
reflux condition. Progress of the reaction was monitored by TLC and
GC. After completion of the reaction , the catalyst was filtered off and
washed with dichloromethane (2 ꢀ 15 mL). The filtrate was treated with
an aqueous solution of NaOH (10%, 25 mL), and the mixture extracted
with dichloromethane. The organic layer was washed with water and
dried over anhydrous (Na2SO4). Evaporation of solvent gave the pure
product. After each reaction catalyst was washed with diethyl ether and
used again without further drying.
ACKNOWLEDGMENT
We are thankful to Shiraz University Research Council for partial
supported this work.
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4258
Tamami and Parvanak Borujeny
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Received in the UK July 4, 2003