Efficient thioacetalisation of carbonyl compounds

Article abstract of DOI:10.2478/s11696-013-0443-4

The thioacetalisation of a variety of heterocyclic, aromatic, and aliphatic carbonyl compounds (1 mmol) with ethane-1,2-dithiol (1 mmol) using silica sulphuric acid (SSA) is presented as an efficient heterogeneous catalyst under mild and solvent-free cond

Full text of DOI:10.2478/s11696-013-0443-4

Chemical Papers
DOI: 10.2478/s11696-013-0443-4
SHORT COMMUNICATION
Efficient thioacetalisation of carbonyl compounds
a
b
aDavood Habibi*, Payam Rahmani, Ziba Akbaripanah
aDepartment of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina University, Hamedan, 6517838683, Iran
bCentral Laboratory, Jam Petrochemical Company, Assaluyeh, Bushehr, P.O. Box 75391-415, Iran
Received 4 February 2013; Revised 18 April 2013; Accepted 1 May 2013
The thioacetalisation of a variety of heterocyclic, aromatic, and aliphatic carbonyl compounds
(1 mmol) with ethane-1,2-dithiol (1 mmol) using silica sulphuric acid (SSA) is presented as an
efficient heterogeneous catalyst under mild and solvent-free conditions at 60◦C. The thioacetals
were formed within a short reaction time (1–34 min) and isolated with 90–98 % yield following an
extractive procedure and chromatography on silica gel. The competitive protection reaction between
aldehyde and ketone with ethane-1,2-dithiol afforded the protected derivatives of benzaldehyde and
acetophenone with 92 % and 8 % yields, respectively, indicating some selectivity.
c
ꢀ 2013 Institute of Chemistry, Slovak Academy of Sciences
Keywords: silica sulphuric acid (SSA), heterogeneous catalyst, thioacetalisation, chemoselectivity,
carbonyl compounds, 1,2-ethanedithiol
The protection of carbonyl groups as dithioacetals
is a common and popular practice in organic chem-
istry (Wuts & Greene, 2006; Kocienski, 1994), since
they are quite stable under basic or acidic conditions.
Among the different carbonyl-protecting groups, 1,3-
dithianes, 1,3-oxathiolanes, and 1,3-dithiolanes have
long been used as protective groups, and act as
an acyl- anion equivalent in carbon–carbon bond-
forming reactions (Eliel & Morris-Natschke, 1984;
Gro¨bel & Seebach, 1977; Bulman Page et al., 1989).
They are generally prepared by Brønsted or Lewis
acid-catalysed condensation of carbonyl compounds
with thiols or dithiols, in the presence of strong
acid catalysts such as AlCl3 (Ong, 1980), LnCl3
(Garlaschelli & Vidari, 1990), ZnCl2 (Evans et al.,
1977), TiCl4 (Kumar & Dev, 1983), WCl6 (Firouz-
abadi et al., 1998), InCl3 (Muthusamy et al., 2001),
In(OTf)3 (Muthusamy et al., 2002), Sc(OTf)3 (Kamal
& Chouhan, 2002a, 2003), Bi(NO3)3 (Komatsu et al.,
1995), VO(OTf)2 (De, 2005), and Ce(OTf)3 (Kumar
et al., 2010). A number of milder procedures employ-
ing lithium salts (Firouzabadi et al., 1999a, 1999b;
Saraswathy & Sankararaman, 1994; Tietze et al., 2000;
Yadav et al., 2001), NiCl2 (Khan et al., 2003), CoCl2
(De, 2004), NBS (Kamal & Chouhan, 2002b), silica
gel-supported sulphamic acid (Aoyama et al., 2013),
and I2 (Samajdar et al., 2001) have also been re-
ported for this purpose. Many of these protocols suffer
from drawbacks such as requiring harsh reaction con-
ditions and stoichiometric amounts of catalysts, the
use of expensive reagents and/or chlorinated organic
solvents and, in some instances, strong acidic reagents.
Recently, a number of solid-supported reagents have
also been used for the thioacetalisation of various
types of carbonyl compounds, e.g. ZrCl4–SiO2 (Pat-
ney & Margan, 1996), SOCl2–SiO2 (Kamitori et
al., 1986), CoBr2–SiO2 (Patney, 1994), TaCl5–SiO2
(Chandrasekhar et al., 1997), Cu(OTf)2–SiO2 (Anand
et al., 1999), NaHSO4–SiO2 (Das et al., 2005), and
I2–natural phosphate (Zahouily, 2005). Interestingly,
only a few of these methods have demonstrated
chemoselective protection of aldehydes in the presence
of ketones (Ong, 1980; Muthusamy et al., 2001; Ka-
mal & Chouhan, 2002a, 2002b, 2003; Karimi & Seradj,
2000).
However attractive these reagents may be, to the
best of our knowledge there is only one report on the
application of silica sulphuric acid (SSA) in the thioac-
*Corresponding author, e-mail: davood.habibi@gmail.com

ii
D. Habibi et al./Chemical Papers
Table 1. Thioacetalisation of carbonyl compounds in the presence of SSA
Time/min
Yield/%
Entry
R1
R2
Aa
Bb
Aa
Bb
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Phenyl
4-Bromophenyl
4-Chlorophenyl
4-Nitrophenyl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
5
2
4
3
3
1
2
12
7
1
3
3
3
1
3
12
3
34
20
3
–
–
14
16
4
5
–
6
5
–
–
–
–
17
–
4
10
–
95
98
97
98
94
98
98
85
90
95
90
70
96
90
70
90
98
90
98
96
–
–
74
76
97
94
–
94
94
–
–
–
–
98
–
3-Nitrophenyl
4-Methoxyphenyl
2-Methoxyphenyl
3-Bromophenyl
4-Methylphenyl
4-Hydroxyphenyl
4-Dimethylaminophenyl
4-Formylphenyl
1-Hydroxy-2,6-dimethoxyphenyl
Thiophen-2-yl
2-Furyl
3-Indolyl
2-Phenylethen-1-yl
Phenyl
95
5
–
CH3
CH3
Benzyl
a) Reaction conditions in present work (A): carbonyl compound (1.0 mmol), ethane-1,2-dithiol (1.0 mmol), SSA (0.1 g), 60 ◦C; b)
reported (Pourmousavi & Kazemi, 2012) reaction conditions (B): carbonyl compound (2.0 mmol), ethane-1,2-dithiol (2.4 mmol),
SSA (0.02 g), ambient temperature.
and ethane-1,2-dithiol (0.094 g, 1.0 mmol) and the
reaction mixture was stirred at 60◦C under solvent-
free conditions. After the appropriate time (disap-
pearance of starting carbonyl compound was mon-
itored by TLC), the reaction mixture was diluted
Fig. 1. Thioacetalisation of carbonyl compounds. Reaction
with CH2Cl2 (DCM, 20 mL), filtered and the fil-
trate washed with water (15 mL). The organic layer
was dried with anhydrous MgSO4, filtered and con-
centrated under reduced pressure affording the crude
product which was purified by chromatography on a
conditions: i) SSA, 60◦C; for R1 and R2, see Table 1.
etalisation of carbonyl compounds (Pourmousavi &
Kazemi, 2012). As a continuation of our studies on
the application of SSA in organic synthesis (Habibi
et al., 2013a, 2013b, 2013c), the utilisation of SSA
as a heterogeneous acid catalyst is here reported for
the highly chemoselective thioacetalisation of a variety
of carbonyl compounds using ethane-1,2-dithiol under
solvent-free conditions at 60◦C (Fig. 1). The results
are compared with those obtained by Pourmousavi
and Kazemi (2012) for analogous thioacetalisation of
several identical carbonyl compounds at ambient tem-
perature.
Solvents and reagents were obtained from commer-
cial sources (Aldrich, Merck) and used without pu-
rification. Column chromatography was performed on
silica gel (60–120 mesh). SSA was prepared accord-
ing to the method previously reported (Zolfigol, 2001;
Pourmousavi & Kazemi, 2012).
short silica gel column using EtOAc/hexane (ϕr
=
1 : 4) as the eluent to afford the corresponding thioac-
etal (II ). The reaction was carried out with various
aldehydes such as benzaldehyde (Table 1, entry 1),
aromatic aldehydes containing electron-withdrawing
and electron-donating substituents (entries 2–13), het-
erocyclic aldehydes (entries 14–16), α,β-unsaturated
aldehydes (entry 17), and ketones (1-phenylethan-1-
one (acetophenone), entry 18; 1-phenylpropan-2-one,
entry 19). The products were characterised by their
physical and spectral data and a comparison with au-
thentic samples.
First, the solvent effect and catalytic activity of
SSA for the thioacetalisation of 4-chlorobenzaldehyde
with ethane-1,2-dithiol at 60◦C were investigated. It
was found that the products under solvent-free condi-
tions were very clean and the reactions were completed
much more rapidly than reactions using solvents. The
relevant results are summarised in Table 2. It can
be seen that, under solvent-free reaction conditions,
4-chlorobenzaldehyde afforded 2-(4-chlorophenyl)-1,3-
dithiolane (Table 2, entry 6) within 4 min with a 97 %
General procedure for synthesis of 2-R -2-R -
1,3-dithiolanes (II)
SSA (0.1 g) was added to a mixture of correspond-
1
2
—
ing carbonyl compound (R R C O, I ) (1.0 mmol)
—

D. Habibi et al./Chemical Papers
iii
Table 2. Solvent effect on thioacetalisation of 4-chlorobenz-
ing SSA-like solvent-free versus solvent conditions (en-
tries 2–4, 6, 9–11), short versus longer reaction time
(entries 4, 6, 9-11), equimolar versus higher amounts
of reactants (entries 2–11), no toxicity (entries 5, 8),
availability, preparation, recycling, and easy storage of
the catalyst compared with other methods are sum-
marised in Table 3.
In conclusion, it was found that SSA efficiently
catalyses the dithioacetalisation of a variety of car-
bonyl compounds under solvent-free conditions. The
simplicity, mild reaction conditions and chemical se-
lectivity represent several advantages over many ex-
isting methods.
aldehydea
Entry
Solvent
Time/min
Conversion/%
1
2
3
4
5
6
n-hexane
CH3CN
CH2Cl2
CHCl3
MeOH
no solvent
33
15
7
5
5
70
90
90
80
65
97
4
a) Reaction conditions: 4-chlorobenzaldehyde (1.0 mmol),
ethane-1,2-dithiol (1.0 mmol), SSA (0.1 g), 60◦C.
Acknowledgements. The financial support for this work re-
ceived from the Bu-Ali Sina University, Hamedan 6517838683,
Iran, is gratefully acknowledged.
yield. In this respect, other starting aldehydes and ke-
tones were allowed to react under the solvent-free con-
ditions.
To examine the selectivity of thioacetalisation in
the competitive protection of aldehyde and ketone,
a mixture of benzaldehyde (1 mmol), acetophenone
(1 mmol) and ethane-1,2-dithiol (1 mmol) in the pres-
ence of SSA (0.1 g) was allowed to react at 60◦C for
5 min. Examination of the isolated and purified re-
action products revealed that 2-phenyl-1,3-dithiolane
(protected benzaldehyde) and 2-methyl-2-phenyl-1,3-
dithiolane (protected acetophenone) were formed with
92 % and 8 % yields, respectively. This observation
indicates that this method is potentially applicable in
the chemoselective conversion of aldehydes to the cor-
responding thioacetals in the presence of ketones.
It should also be noted that the thioacetalisation
of a conjugated aldehyde takes place chemoselectively
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