Reductive cleavage of S-S bond by Zn/AlCl3 system: A novel method for the synthesis of sulfides from alkyl tosylates and disulfides

Article abstract of DOI:10.1081/SCC-120030756

Alkyl and aryl disulfides are reduced by zinc powder in the presence of AlCl₃ in aqueous media to yield zinc thiolates. This thiolate anion species then react with alkyl tosylates to give sulfides in high execellent yields.

Full text of DOI:10.1081/SCC-120030756

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Reductive Cleavage of S–S Bond by Zn/AlCl3 System:
A Novel Method for the Synthesis of Sulfides from
Alkyl Tosylates and Disulfides
Barahman Movassagh a & Amir Mossadegh a
a Department of Chemistry, K. N. Toosi University of Technology, P.O. Box 15875‐4416,
Tehran, Iran
Available online: 16 Aug 2006
To cite this article: Barahman Movassagh & Amir Mossadegh (2004): Reductive Cleavage of S–S Bond by Zn/AlCl3 System: A
Novel Method for the Synthesis of Sulfides from Alkyl Tosylates and Disulfides, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 34:9, 1685-1690
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 9, pp. 1685–1690, 2004
Reductive Cleavage of S–S Bond by
Zn/AlCl3 System: A Novel Method
for the Synthesis of Sulfides from
Alkyl Tosylates and Disulfides
*
Barahman Movassagh and Amir Mossadegh
Department of Chemistry, K. N. Toosi University of Technology,
Tehran, Iran
ABSTRACT
Alkyl and aryl disulfides are reduced by zinc powder in the presence of
AlCl3 in aqueous media to yield zinc thiolates. This thiolate anion species
then react with alkyl tosylates to give sulfides in high to excellent yields.
Key Words: Alkyl tosylates; Zn/AlCl3 system; Reductive cleavage of
disulfides; Zinc thiolates; Sulfides.
*
Correspondence: Barahman Movassagh, Department of Chemistry, K. N. Toosi
University of Technology, P.O. Box 15875-4416, Tehran, Iran; Fax: þ98-21-285-
3650; E-mail: movassagh@kntu.ac.ir.
1685
DOI: 10.1081/SCC-120030756
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

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Movassagh and Mossadegh
INTRODUCTION
Historically, the chemistry of sulfides has been relatively well studied.
Nevertheless, the chemistry of sulfides is still important from both theoretical
and practical viewpoints.[1] Sulfides are the starting compounds for the prep-
arations of other sulfur compounds, such as sulfoxides, sulfones, and sulfonium
compounds, etc.[1] Several procedures for the preparation of sulfides have been
reported including, reduction of sulfoxides and sulfones, desulfurization of
disulfides, aryllithium or organocuprate addition to thiocarbonyl com-
pounds;[2,3] treatment of thiols and alkyl halides with DBU;[4] nucleophilic dis-
placement of aryl halides by thiolate ion;[5] the treatment of alkyl halides with
sodium sulfide,[6] thiourea,[7] or thiocarbonate;[8] the use of bis(tributyltin) sul-
fide as a sulfur transfer agent;[9] the reaction of halides with thiosilanes.[10]
Recently, sulfides have been prepared from thiols using montmorillonite-3-ami-
nopropyltriethoxysilane as a catalyst.[11] Also, the synthesis of allyl sulfides
from an organosamarium reagent and disulfides has been reported.[12] Lu
et al.[13] synthesized benzyl sulfides via reaction of benzyl bromide with disul-
fides promoted by Sm/BiCl3 system in aqueous media. Very recently, we intro-
duced Zn/AlCl3 system in aqueous media for the preparation of sulfides from
disulfides and organic halides.[14] Such reactions in aqueous media offer numer-
ous advantages over common reactions in organic solvents.[13]
From a synthetic point of view, there are some major limitations to the
preparation of sulfides by S–S bond cleavage in disulfides, including:
(i) the need for anhydrous organic solvents; (ii) the necessity to prepare an
organometallic complex; (iii) strong base catalysis; and (iv) loss of half unit
of disulfide.[13,15] But, the S–S bond cleavage in disulfides by a Zn/AlCl3
system, circumvents these problems.
As a part of our ongoing work on Zn/AlCl3 system, here, we report that
alkyl tosylates can react with various disulfides promoted by the above system
in CH3CN–H2O mixed solvent at 658C to afford sulfides in high to excellent
yields (Sch. 1).
A series of symmetrical alkyl and aryl disulfides were treated with various
primary alkyl tosylates in the presence of Zn/AlCl3 system with a molar ratio
of disulfide: AlCl3 : tosylate ¼ 0.5 : 1 : 1.5. The presence of aluminum chloride
is quite essential, and in the absence of this Lewis acid, the reaction slows
down considerably. The disappearance of zinc powder during the preliminary
Scheme 1.

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Reductive Cleavage of S–S Bond by Zn/AlCl3 System
1687
treatment of disulfides with Zn/AlCl3 is attributed to the formation of zinc
thiolate intermediate[14] which further undergoes nucleophilic displacement
with alkyl tosylates in the presence of AlCl3 to afford the sulfide.
The reaction of diaryl disulfides with benzyl tosylates (entries 1–3, Table 1)
were rather fast and gave the corresponding benzyl-aryl sulfides in excellent yields.
The results also show the need for relatively longer reaction times and sometimes
lower yields for symmetrical alkyl disulfides (entries 4, 8, and 9, Table 1).
In conclusion, the present method introduces an alternative with shorter
reaction times to the ones reported previously.[14] Also, it offers some advan-
tages over earlier reported procedures, in that, it avoids the need to apply thiol,
the manipulation of air-sensitive transition metal complexes, and the use of
dry solvent, as well as being carried out under mild conditions.
EXPERIMENTAL
Disulfides and alkyl tosylates were readily prepared according to the
procedures given by Fristad[22] and Choudary,[23] respectively. All products
Table 1. Reaction conditions used and yields of sulfides.
M.p. (8C)(obs.)/
M.p. or B.p.
Reaction Yielda
time (hr) (%)
Entry
R
R0
(8C, mm) (lit.)
1
2
3
4
5
6
7
Ph
PhCH2
PhCH2
PhCH2
PhCH2
4
94
91
96
88
90
86
96
(41–42)/40–42[13]
(45.5)/45–45.5[14]
(52)/53[11]
p-MeC6H4
p-ClC6H4
PhCH2
3.5
4
8
(52)/50[16]
CH3(CH2)2CH2 PhCH2
Ph
p-MeC6H4
6
11
(Oil)/130, 5 mm[14]
(Oil)/117, 15 mm[17]
(Oil)/80–82,
1 mm[14]
(Oil)/130, 5 mm[14]
(Oil)/187[16]
CH3(CH2)2CH2
CH3(CH2)2CH2
9
8
9
PhCH2 CH3(CH2)2CH2
CH3(CH2)2CH2 CH3(CH2)2CH2
9
15
5
94
84
91
10
p-ClC6H4
CH3(CH2)2CH2
(Oil)/136–7,
10 mm[18]
11
Ph
CH3(CH2)3CH2
8
99
(Oil)/122–4,
12 mm[19]
12
13
14
p-ClC6H4
p-MeC6H4
Ph
CH3(CH2)4CH2
CH3(CH2)4CH2
CH3(CH2)4CH2
8
9
8
91
86
89
(Oil)/175, 12 mm[14]
(Oil)/oil[20]
(Oil)/oil[21]
aYields based on pure isolated products.

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Movassagh and Mossadegh
were identified by comparison of their spectra and physical data with those of
the known samples. IR spectra were recorded using a Shimadzu IR-408 instru-
ment. 1H-NMR spectra were determined by a Bruker DRX-500 Avance
500 MHz spectrometer. Melting points were obtained by an Electrothermal
9100 apparatus.
General Procedure for Preparation of Sulfides
In a 50 mL two-neck flask fitted with a reflux condenser, were placed
AlCl3 (1 mmol), zinc powder (2 mmol), acetonitrile (10 mL), and disulfide
(0.5 mmol). The mixture was stirred at 658C for about 2 hr until the zinc
powder disappeared; alkyl tosylate (1.5 mmol), and water (2.5 mL) were then
added at once to the solution and stirring was continued at that temperature for
3.5–15 hr in air atmosphere. Progress of the reaction was monitored by TLC
(eluent: Petroleum ether/EtOAc, 2/1). After completion of the reaction, the
solution was filtered and washed with ether. The combined ether/CH3CN
solution was then washed with water and dried over anhydrous sodium sulfate.
The solvent was evaporated in vacuo to give the corresponding sulfide which
was purified either by column chromatography using silica gel (Petroleum
ether/EtOAc, 2/1), or recrystallized from ethanol.
Selected 1H NMR (in CDCl3) spectral data for product 1: d 3.65 (s, 2H),
7.20–7.50 (m, 10H). 6: d 0.86 (t, 3H, J ¼ 7.5 Hz), 1.22–1.29 (m, 2H), 1.40–
1.46 (m, 2H), 2.87 (t, 2H, J ¼ 7.8 Hz), 7.21–7.34 (m, 5H). 10: d 0.91 (t, 3H,
J ¼ 7.4 Hz), 1.37–1.48 (m, 4H), 2.90 (t, 2H, J ¼ 7.6 Hz), 7.33 (d, 2H,
J ¼ 8.0 Hz), 7.49 (d, 2H, J ¼ 8.0 Hz). 12: d 0.88 (t, 3H, J ¼ 7.5 Hz), 1.21–
1.49 (m, 6H), 1.52–1.79 (m, 2H), 2.91 (t, 2H, J ¼ 7.6 Hz), 7.34 (d, 2H,
J ¼ 8.0 Hz), 7.44 (d, 2H, J ¼ 8.0 Hz). 14: d 0.87 (t, 3H, J ¼ 7.5 Hz), 1.20–
1.48 (m, 6H), 1.50–1.77 (m, 2H), 2.91 (t, 2H, J ¼ 7.8 Hz), 7.23–7.38 (m, 5H).
ACKNOWLEDGMENT
We thank the K. N. Toosi University of Technology Research Council for
financial support.
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Received in India November 27, 2003

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