Efficient reduction of thiopyrylium salts to corresponding 2H- and 4H-thiopyrans under solvent-free condition: Regioselectivity and mechanism

Article abstract of DOI:10.1080/10426500590912682

A solvent-free reduction of 2,4,6-triarylthiopyrylium with NaBH ₄/alumina is described. The regioselectivity of these reductions were compared with those obtained in solution experiments. Copyright Taylor & Francis Inc.

Full text of DOI:10.1080/10426500590912682

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Phosphorus, Sulfur, and Silicon
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Efficient Reduction of
Thiopyrylium Salts to
Corresponding 2H- and 4H-
Thiopyrans Under Solvent-Free
Condition: Regioselectivity and
Mechanism
Arash Mouradzadegun a & Nargess Gheitasvand a
a Department of Chemistry, Faculty of Science ,
Shahid Chamran University , Ahvaz, Iran
Published online: 16 Aug 2006.
To cite this article: Arash Mouradzadegun & Nargess Gheitasvand (2005)
Efficient Reduction of Thiopyrylium Salts to Corresponding 2H- and 4H-
Thiopyrans Under Solvent-Free Condition: Regioselectivity and Mechanism,
Phosphorus, Sulfur, and Silicon and the Related Elements, 180:5-6, 1385-1388, DOI:
10.1080/10426500590912682
To link to this article: http://dx.doi.org/10.1080/10426500590912682
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Phosphorus, Sulfur, and Silicon, 180:1385–1388, 2005
Copyright © Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500590912682
Efficient Reduction of Thiopyrylium Salts to
Corresponding 2H- and 4H-Thiopyrans Under
Solvent-Free Condition: Regioselectivity and Mechanism
Arash Mouradzadegun
Nargess Gheitasvand
Department of Chemistry, Faculty of Science, Shahid Chamran
University, Ahvaz, Iran
A solvent-free reduction of 2,4,6-triarylthiopyrylium with NaBH4/alumina is de-
scribed. The regioselectivity of these reductions were compared with those obtained
in solution experiments.
Keywords Reduction; regioselectivity; sodium borhydride/alumina; solvent free; thiopy-
ran; thiopyrylium
INTRODUCTION
Thiopyrylium cations have been the subject of a wide variety of practi-
cal applications. In particular a large number of technological studies
involving thiopyrylium salts deal with their applications in the prepara-
tion of photosensitive compositions for electrophotographic, photocon-
ductors, and optical recording media. These salts also show biological
activity and find applications in medicine.1
The reduction of these salts to their corresponding 2H- and 4H-
thiopyrans can be accomplished with complex hydrides. Hydride donors
can attack thiopyrylium salts either in the α-position, giving rise to a
corresponding 2H-thiopyran, or in γ -position, leading to 4H-thiopyran.
Received July 9, 2004; accepted October 5, 2004.
We are grateful to H. Pirelahi for his help.
Address correspondence to Arash Mouradzadegun, Department of Chemistry, Faculty
of Science, Shahid Chamran University, Ahvaz, Iran. E-mail: arash-m@cua.ac.ir
1385

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A. Mouradzadegun and N. Gheitasvand
A number of reductions have been carried out with NaBH4. Re-
duction of 2,4,6-triphenylthiopyrylium ions with NaBH4 affords a 3:7
mixture of 2H and 4H isomers in methanol2 and a 1:1 mixture in
acetonitrile.3 2,6-Diphenylthiopyrylium cations have been reduced in
methanol in the ratio 0:100 for (2H:4H). Pentaphenylthiopyrylium ions
undergoe reduction with LiBH4 in THF to yield the corresponding 4H-
thiopyran in 30% yield.4
Thiopyrylium ions can function as catalysts in the equilibrium iso-
merization between the corresponding 2H- and 4H- as shown in the
following equation.
In the course of our studies on synthesis and photoisomerization of
4H-thiopyrans,5,6 and our new interest in using solvent-free conditions
as an efficient media,7 we decided to examine the reduction of some
of triarylthiopyrylium salts under this condition (NaBH4/alumina) and
then compare the results with those obtained in solution experiments.
EXPERIMENTAL
All reagents were used from Merck Co. or Aldrich Co. TLC was per-
1
formed on silica gel plate. HNMR spectra were recorded in CDCl3 on
a Brucker 400 MHz spectometer using TMS as an internal standard.
A domestic microwave used for synthesis of pyrylium precursors. IR
spectra were recorded on IR Shimadzu spectro was photometer.

Reduction of Thiopyrylium Salts
1387
Syntheses
All triarylthiopyrylium perchlorates were synthesized from the corre-
sponding pyrylium salts, which were prepared in our new method with
microwave irradiation,7 using the Ulrich and Wizinger method.8
General Procedure of Reduction
a) Solvent-Free Condition
1.25 mmol of NaBH4 was added to 1 gr of alumina, after homoge-
nizing 1 mmol of each thiopyrylium salt gradually added in portion,
the mixtures were grinded in mortar, the reaction followed by tlc using
hexane:ether (90:10 v/v) until conversion was complete, and then was
worked up with n-hexane.
b) In Methanol
1.25 mmol of NaBH4 was gradually added to a suspension of 1 mmol
of each thiopyrylium salt in 5 ml methanol, maintained at −10 to −15◦C
after evaluating the hydrogen, the solution was worked up by evapo-
rating the solvent.
RESULTS AND DISCUSSION
We investigated the reducing ability of NaBH4/alumina for reduction of
triarylthiopyrylium perchlorate. According to our knowledge this is the
first report for the reduction of thiopyrylium in the solid state. The im-
portant role of alumina is apparent from the fact that only poor yields
were obtained without this support. A simple extraction of the prod-
uct from the solid support affords the corresponding 2H-thiopyrans as
major products. In some cases the reduction of thiopyrylium salts is
complete after few minutes (∼10 min) of grinding. However, the re-
duction of the substrates bearing electron-donoating substituents in
the 2 and 6 positions is relatively slow in comparison with those with
electron-donating groups in 4 position and 2,4,6-triphenylthiopyrylium
1
as model compound. H-NMR study of the product mixture revaeled
that reduction under solvent-free conditions also exhibit higher re-
gioselectivity than those run in methanol, examplified by the case of
2,4,6-triphenylthiopyrylium perchlorate under solvent-free conditions
in which thermodynamically more stable corresponding 2H-thiopyran
was obtained >95% while in methanol a 3:7 mixture of 2H and 4H
isomers was obtained.
In conclusion, we have developed a facile and practical method for
the reduction of triarylthiopyrylium salts with high regioselectivity and

1388
A. Mouradzadegun and N. Gheitasvand
this reveals that the hydride transfer might have occurred in this con-
dition as well as in solvent.
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(1994).
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B31, 496 (1977).
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