Solvent-free and room temperature synthesis of thiochromans in the presence of a catalytic amount of tungstophosphoric acid

Article abstract of DOI:10.1016/j.tetlet.2005.08.156

Tungstophosphoric acid catalyzed rapid and good yielding reactions of α,β-unsaturated aldehydes with arenethiols to give the corresponding 4-thioaryl-1,2,3,4-tetrahydro-1-benzothiopyrans (thiochromans) under solvent-free and room temperature conditions.

Full text of DOI:10.1016/j.tetlet.2005.08.156

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 7567–7569
Solvent-free and room temperature synthesis of thiochromans in
the presence of a catalytic amount of tungstophosphoric acid
Mohammad Jafarzadeh,* Kamal Amani and Farzad Nikpour
Department of Chemistry, Kurdistan University, Sanandaj, Iran
Received 9 June 2005; revised 16 August 2005; accepted 26 August 2005
Available online 12 September 2005
Abstract—Tungstophosphoric acid catalyzed rapid and good yielding reactions of a,b-unsaturated aldehydes with arenethiols to
give the corresponding 4-thioaryl-1,2,3,4-tetrahydro-1-benzothiopyrans (thiochromans) under solvent-free and room temperature
conditions.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Heteropoly acids (HPAs) have been extensively studied
as acid and oxidation catalysts for many reactions and
have found industrial application in several processes.1
These catalysts are very important for industries associ-
ated with fine chemicals, flavours, pharmaceuticals and
foods2 and are also used as industrial catalysts for sev-
eral liquid-phase reactions,3–6 such as alcohol dehydra-
tion,7 alkylation8 and esterification reactions.9 Poly-
oxometallates have found use in the field of homoge-
neous and heterogeneous catalysis and also in the fields
of medicine, photochemistry, materials and microdevice
technology.10,11 Heteropoly compounds are green cata-
lysts12 that function in a variety of reaction fields and
are efficient bifunctional catalysts, harmless to the envi-
ronment with respect to corrosiveness, safety, quantity
of waste and separability.13 The Keggin-type heteropoly
acids are more active and possess stronger Brønsted
acidity than the usual mineral acids such as H2SO4,
many kinds of reagents; the thiochroman ring system
is also found in many naturally occurring biologically
active compounds.18 The thio-Claisen rearrangement
and its modified version represent highly effective and
general methods for the synthesis of intermolecular
cycloaddition products from a,b-unsaturated com-
pounds. A number of methods for the preparation of
thiochromans have been reported.19 Recently, a method
for selective ring formation through intermolecular
cycloaddition of a,b-unsaturated aldehydes with arene-
thiols in the presence of p-TsOH carried out at 80 ꢁC
for 3 h in anhydrous dichloroethane was reported.20
The synthetic utility of these methods may, however,
be considerably limited because of the use of corrosive
protic acids or expensive Lewis acids, high temperatures,
prolonged reaction times, multi-step reactions, etc.
Here, we report an improved, effective and rapid method
for the intermolecular cycloaddition of a,b-unsaturated
aldehydes and arenethiols to yield thiochromans
using a catalytic amount of tungstophosphoric acid
(H3PW12O40) in good yields under solvent-free and
room temperature conditions (Scheme 1).
14
HCl, HNO3 and conventional solid acids such as
SiO2–Al2O3, H3PO4–SiO2, zeolites including HX, HY,
H-ZSM-5,15,16 Amberlyst-15 and Nafion-H.17 Among
heteropoly acids, polytungstic acids are the most widely
used catalysts owing to their high acid strength, thermal
stabilities, and low reducibilities.
Fused thiochromans are interesting and useful synthetic
intermediates because of their strong activity towards
R
S
CHO
H3PW12O40 (0.01 mmol)
ArSH
+
Solvent-free, rt
10 -15 min
R
SAr
Keywords: Thiochroman; a,b-Unsaturated aldehyde; Arenethiol;
R : Me, Ph
Tungstophosphoric acid; Intermolecular cycloaddition.
Yield = 59 - 76%
*
Corresponding author. Tel.: +98 111 326 8469; fax: +98 111 323
6567; e-mail: mjafarzadeh1027@yahoo.com
Scheme 1.
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.08.156

7568
M. Jafarzadeh et al. / Tetrahedron Letters 46 (2005) 7567–7569
Table 1. Reaction of a,b-unsaturated aldehydes with arenethiols in the presence of catalytic amounts of H3PW12O40
Entry
1
Aldehyde
Ar–SH
C6H5
Time (min)
10
Product
Yield (%)a
76b
S
CHO
SPh
S
64b
67b
72b
CHO
CHO
CHO
2
3
4
p-MeC6H4
p-BrC6H4
b-C10H7
10
15
15
Me
Br
SAr
S
SAr
S
SAr
Ph
S
CHO
5
6
7
8
C6H5
10
10
15
15
66c
59b
64c
61c
Ph
Ph
Ph
Ph
SPh
S
Ph
Ph
CHO
CHO
CHO
p-MeC6H4
p-BrC6H4
b-C10H7
Me
Br
SAr
S
SAr
S
Ph
SAr
a Yields refer to isolated pure products. The products were characterized by IR, 1H NMR and 13C NMR spectral data.
b Isolated by preparative thin layer chromatography.
c Isolated by recrystallization.
This reaction in the absence of tungstophosphoric acid
did not proceed, but in the presence of tungstophos-
phoric acid (0.01 mmol, optimized), this conversion
proceeded efficiently to yield the corresponding thio-
chromans. Tungstophosphoric acid can be recovered
and reused easily, by extraction with Et2O from aqueous
solution of acids and dried and dehydrated by heating
at 150–130 ꢁC under vacuum for 1–2 h, without struc-
tural degradation.21 The catalyst can be prepared
according to the published procedure.22 Reactions
carried out under solvent-free conditions have received
immense popularity in recent years.23 The mild reaction
conditions, short reaction times, occasional enhanced
selectivity and clean products are salutary features
of this approach. Representative examples are given in
Table 1.
tion in the benzene ring of the latter. Naphthalene
derivatives also undergo the reaction. Although the de-
tails of the mechanism still remain ambiguous, the reac-
tion may proceed through allylic cationic intermediates
generated in situ.
In conclusion, the present method is a very simple,
one-pot reaction, which is carried out at ambient
temperature under mild and solventless conditions
using tungstophosphoric acid as an effective, heteroge-
neous, readily available and low cost catalyst. This
method is a green protocol for the synthesis of
thiochromans.
Acknowledgements
The results revealed that crotonaldehyde is more effi-
cient than cinnamaldehyde, due to electron delocaliza-
The authors are thankful to the Kurdistan University
Research Council for the partial support of this work.

M. Jafarzadeh et al. / Tetrahedron Letters 46 (2005) 7567–7569
7569
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References and notes
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23. General reaction procedure: The a,b-unsaturated aldehyde
(1 mmol), thiophenol (2.5 mmol) and H3PW12O40
(0.01 mmol, 0.028 g) were mixed together and stirred at
room temperature for 10–15 min. The reaction was
monitored by TLC. NaOH (20%, 5 ml) was then added
and the crude product was extracted with CH2Cl2
(3 · 25 ml). The organic phase was dried over MgSO4
and then purified by preparative thin layer chromatogra-
phy on silica gel (petroleum ether/ethyl acetate ratio was
5:1) or recrystallization from n-hexane or ethyl acetate.
The products were characterized by IR, 1H NMR (250
MHz, CDCl3, TMS) and 13C NMR (50 MHz, CDCl3,
TMS) spectroscopy. Selected spectral data: Entry 5
1
(C21H18S2): H NMR; d (ppm): 3.54 (d, J = 7.3 Hz, 1H),
5.19 (d, J = 9.3 Hz, 1H), 6.22 (dd, J = 1.8, 8.7 Hz, 1H),
6.39 (dd, J = 1.9, 8.9 Hz, 1H), 7.31–7.75 (m, 14H), 13C
NMR; d (ppm): 31.42, 57.00, 59.09, 127.05–136.58; Entry
1
7 (C21H16S2Br2): H NMR; d (ppm): 4.98 (d, J = 7.0 Hz,
1H), 6.18 (d, J = 9.2 Hz, 1H), 6.41 (dd, J = 1.7, 8.5 Hz,
1H), 6.89 (dd, J = 1.6, 8.5 Hz, 1H), 7.21–7.79 (m, 12H),
13C NMR; d (ppm): 31.38, 57.03, 59.06, 123.08–139.04;
Entry
8 d (ppm): 5.12 (d,
(C29H22S2): 1H NMR;
J = 7.3 Hz, 1H), 5.73 (d, J = 9.4 Hz, 1H), 6.21 (dd,
J = 2.1, 9.9 Hz, 1H), 6.78 (dd, J = 2.2, 9.9 Hz, 1H),
7.87–8.48 (m, 18H), 13C NMR; d (ppm): 31.39, 51.63,
56.84, 126.32–133.00.