Ionic liquid/PPh3 promoted cleavage of diphenyl disulfide and diselenide: A straight-forward metal-free one-pot route to the synthesis of unsymmetrical sulfides and selenides

Article abstract of DOI:10.1016/j.tetlet.2012.02.047

A metal-free cleavage of diphenyl disulfide and diphenyl diselenide has been achieved using ionic liquid/triphenyl phosphine (PPh₃) and a convenient protocol for the one-pot synthesis of unsymmetrical sulfides and selenides by condensing 'in situ' generated thiolate or selenate anion with alkyl halides has been developed. In addition, 1,4-conjugate addition of the generated thiolate anions to activated alkenes has also been demonstrated. The ionic liquid, 1-methyl-3-pentyl imidazolium bromide, [pmIm]Br plays a crucial role in promoting the course of the reactions and shows superior activity and selectivity compared to other solvents. The [pmIm]Br has been reused for at least five times without appreciable loss of activity.

Full text of DOI:10.1016/j.tetlet.2012.02.047

Tetrahedron Letters 53 (2012) 2149–2152
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Tetrahedron Letters
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Ionic liquid/PPh3 promoted cleavage of diphenyl disulfide and diselenide:
a straight-forward metal-free one-pot route to the synthesis
of unsymmetrical sulfides and selenides
⇑
Subhash Banerjee, Laksmikanta Adak, Brindaban C. Ranu
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A metal-free cleavage of diphenyl disulfide and diphenyl diselenide has been achieved using ionic liquid/
triphenyl phosphine (PPh3) and a convenient protocol for the one-pot synthesis of unsymmetrical sul-
fides and selenides by condensing ‘in situ’ generated thiolate or selenate anion with alkyl halides has
been developed. In addition, 1,4-conjugate addition of the generated thiolate anions to activated alkenes
has also been demonstrated. The ionic liquid, 1-methyl-3-pentyl imidazolium bromide, [pmIm]Br plays a
crucial role in promoting the course of the reactions and shows superior activity and selectivity compared
to other solvents. The [pmIm]Br has been reused for at least five times without appreciable loss of
activity.
Received 23 December 2011
Revised 10 February 2012
Accepted 11 February 2012
Available online 25 February 2012
Keywords:
Ionic liquid
Triphenyl phosphine
Green synthesis
Sulfides and selenides
1,4-Addition
Ó 2012 Elsevier Ltd. All rights reserved.
Organic sulfides and selenides have been extensively used in or-
ganic synthesis and pharmaceutical industries1a–c and as a food
supplement.1d,e A number of protocols are available in the litera-
ture for the preparation of organic sulfides and selenides.2 In an-
other aspect, conjugate addition of thiols to the activated alkenes
leads to various biologically important organo sulfides. Commonly
used reagents for this conjugate addition reaction are strong
bases,3 Lewis acids4, and ionic liquids, or quaternary ammonium
salts.5 However, with a few exceptions most of these are two-step
procedures and involve malodorous and toxic thiols and handling
of unstable reagents in addition to strong basic or acidic reagents.
Thus, an alternative efficient and milder procedure for the
synthesis of organic sulfides and selenides avoiding toxic and mal-
odorous thiols is highly desirable. One of the straightforward op-
tions is the cleavage of disulfides and diselenides and subsequent
reaction of thiolates or selenates with alkyl halides or conjugated
alkenes in the presence of a mild catalyst. A few reagents have
been developed for the cleavage of disulfide and diselenide, which
include Zn/ZnCl2,6 Zn/RuCl3,7 lanthanum metal,8 indium(I) iodide,9
reduction with NaBH410, or phosphine.11 Although, PPh311c was re-
ported to reduce the disulfide bond the subsequent reaction of the
thiolate anion has not been explored to the best of our knowledge.
Besides being used as the reaction medium,12 ionic liquids also
have the potential to promote the course of a reaction13 by acting
as catalysts.14 Here, we report a simple procedure for a facile cleav-
age of diphenyl disulfide and diselenide mediated by [pmIm]Br/
PPh3 and subsequent reaction with alkyl halides and activated al-
kenes leading to the synthesis of unsymmetrical sulfides, selenides,
and 1,4-addition products, respectively (Scheme 1).
Initially, we have selected the synthesis of benzyl phenyl sulfide
as a model reaction for the optimization of various reaction param-
eters. The results are summarized in Table 1. It was observed that
PPh3 failed to initiate the reaction in the absence of [pmIm]Br,
whereas in other organic solvents the reaction either did not initi-
ate (Table 1, entries 1 and 2) or proceeded marginally (Table 1, en-
tries 3 and 4). When the same reaction was carried out in ethanol
the corresponding benzyl ethyl ether was formed as a major
R
X
RYPh
[pmIm]Br/PPh3
PhYYPh
O
Y = S, Se
O
R = alkyl, acyl; X = Cl, Br, I
PhS
Br
N
N
C5H11
1-pentyl-3-methyl-imidazolium bromide, [pmIm]Br
⇑
Corresponding author.
E-mail address: ocbcr@iacs.res.in (B.C. Ranu).
Scheme 1. PPh3/ionic liquid mediated reaction of diphenyl disulfide and diselenide
with alkyl halide and activated alkene.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.02.047

2150
S. Banerjee et al. / Tetrahedron Letters 53 (2012) 2149–2152
Table 1
Table 2
Optimization of reaction conditions
Synthesis of alkyl phenyl sulfides, selenides, and alkyl thiobenzoates, selenobenzoates
in a one-pot reaction
PPh3, Solvent
75 oC, 8 h
+
PhCH2Br
PhSSPh
PhCH2SPh
R
X
+
PhYYPh
RYPh
Yielda (%)
Entry Solvent
PPh3
Product
Yielda (%)
Entry
1
R
X
Y
Time (h)
Ref.
9b
6a
9b
6a
9b
6a
PhCH2
Cl
(a) S
(b) Se
(a) S
(b) Se
(a) S
(b) Se
2.5
4.0
1.5
3.5
1.5
3.0
80
78
84
80
88
82
1
2
3
4
5
Toluene
1.1 equiv
1.1 equiv
1.1 equiv PhCH2SPh
1.1 equiv PhCH2SPh
—
—
0
0
20
40
CH2Cl2
CH3CN
THF
2
3
PhCH2
PhCH2
Br
I
EtOH
1.1 equiv PhCH2SPh + PhCH2OEt 60
(10:90)
1.1 equiv PhCH2SPh + PhCH2OH 75
(15:85)
84
9b
9a
CH2
CH2
(a) S
(b) Se
2.0
4.0
82
74
6
7
Dioxan + H2O (1:1)
4
Br
MeO
O
[pmIm]Br
(40 mol %)
[pmIm]Br
(1 mmol)
[pmIm]Br
(40 mol %)
[pmIm]Br
(20 mol %)
1.1 equiv PhCH2SPh
1.1 equiv PhCH2SPh
0.5 equiv PhCH2SPh
1.1 equiv PhCH2SPh
9b
9a
(a) S
(b) Se
2.5
3.0
78
70
5
6
Br
Br
8
90
35
25
O
4h
9a
4h
PhCH2CH2
(a) S
(b) Se
S
4.0
5.5
4.0
80
75
75
9
7
8
PhCH2CH2
I
10
9b
Cl
(a) S
3.0
76
CH2
9b
9b
a
(a) S
(b) Se
3.0
5.0
83
72
Isolated yield. Amount of PPh3 and [pmIm]Br was calculated with respect to
9
Br
CH2
alkyl bromide.
9b
10
11
I
S
2.5
84
CH2
(CH3)2CH
2f
Br
(a) S
(b) Se
(a) S
5.5
6.0
4.5
5.0
70
72
75
70
product along with 10% of the desired product (Table 1, entry 5). As
PPh3 was reported11c to cleave the disulfide bond in dioxane–water
solvent mixture we have tried this condition for our condensation
reaction and we found that benzyl alcohol was produced as the
major product (Table 1, entry 6).
6c
4h
9b
12
13
PhCH(Me)
Br
Br
(b) Se
16
9a
(a) S
(b) Se
5.0
6.0
76
73
It was found that PPh3 mediated cleavage of diphenyl disulfide
and the subsequent condensation of the thiolate anion proceeded
efficiently in [pmIm]Br (40 mol %) at 75 °C and no other side prod-
uct was isolated. The diphenyl diselenide was also found to pro-
duce benzyl phenyl selenide by the same sequence of reactions
in excellent yield. The above studies clearly demonstrate the cru-
cial role of [pmIm]Br in the cleavage of diphenyl disulfide and sub-
sequent reaction of thiolate/selenate ion.
To explore the general applicability and scope of this procedure,
the reaction of ‘in situ’ generated phenyl thiolate and selenate an-
ion was carried out with various alkyl halides, allyl halides, and
acid chlorides under the optimized reaction conditions. Briefly, in
a simple experimental procedure15 PPh3 was added to a mixture
of diphenyl disulfide or diselenide, and alkyl halide in [pmIm]Br
and the reaction mixture was heated at 75 °C till completion of
reaction (TLC). Extraction of the product with diethyl ether fol-
lowed by evaporation of solvent lead to a crude product, which
was purified by column chromatography to provide pure unsym-
metrical sulfides and selenides in good yields. The results are sum-
marized in Table 2. All the reactions were successfully carried out
under the reaction conditions, giving alkyl aryl sulfides, selenides,
and alkyl thiobenzoates and selenobenzoates in high yields (70–
86%) at a reasonable time period (1.5–6 h).
Encouraged by the reactivity and effectiveness of this
[pmIm]Br/PPh3 reagent system toward the synthesis of unsymmet-
rical sulfides and selenides, we have attempted the 1,4-conjugate
addition of phenyl thiolate and phenyl selenate anion with acti-
vated alkenes.
We have observed that, the phenyl thiolate anion generated by
the cleavage of diphenyl disulfide also underwent clean conjugate
addition with conjugated alkenes under similar reaction conditions
to provide the corresponding 1,4-addition products in high yields.
The results for thio-Michael addition are presented in Table 3. A
variety of conjugated alkenes (e.g., aldehydes, ketones, esters,
and nitriles) took part in this reaction producing the corresponding
1,4-addition products.
CH2
17
6c
14
15
Br
Br
S
3.5
5.0
80
80
CH2
Se
6a
9b
18
9b
9a
9b
8a
9b
9b
16
17
CH2CO2Et
MeCO
Br
Cl
Se
4.5
3.0
4.5
2.0
3.0
2.0
3.5
2.5
4.0
86
80
74
85
84
82
80
82
85
(a) S
(b) Se
(a) S
(b) Se
(a) S
(b) Se
(a) S
(b) Se
18
19
20
MeCO
Cl
Cl
Cl
PhCO
cHex-CO
9b
19
(a) S
(b) Se
3.0
3.5
84
80
O
21
Cl
MeO
A
mixture of substrate, diphenyl diselenide or diphenyl disulfide, (1 mmol),
[pmIm]Br (0.4 mmol), and PPh3 (0.7 mmol) was heated at 75 °C.
a
Yields refer to those of pure isolated products.
tion without any difficulty. However, this reagent system is unable
to initiate the 1,4-conjugate addition of phenyl selenate anion to
activated alkenes under similar reaction conditions.
In general, the cleavage of both diphenyl disulfide and diphenyl
diselenide by triphenyl phosphine in ionic liquid is very clean and
considerably fast. This protocol offered a one-pot synthesis of
unsymmetrical sulfides and selenides by condensing ‘in situ’ gen-
erated thiolate or selenate anion with alkyl halides. Moreover gen-
erated thiolate anion underwent addition reaction with various
conjugated alkenes to produce 1,4-addition products in high yields.
The results are summarized in Table 3. All the products are ob-
tained in high purity and were characterized by their spectroscopic
(IR, 1H NMR and 13C NMR) data. The PPh3 here acts as a catalyst.
Without PPh3 the cleavage reaction did not proceed at all.
The ionic liquid was reused for five times without appreciable
loss of activity. The mechanism of this reaction is interesting since
phosphorus, being a second-row element, has the potential for va-
We observed that
a,b-unsaturated ketones with substitution at
b-position (Table 3, entries 3 and 7) also participated in this reac-

S. Banerjee et al. / Tetrahedron Letters 53 (2012) 2149–2152
2151
Table 3
[pmIm]Br/PPh3-promoted cleavage of diphenyl disulfide followed by 1,4-addition to conjugated alkenes
Entry
Michael acceptor
Product
Time (h)
4.0
Yielda (%)
80
Ref.
PhS
4a
1
O
O
O
O
5b
2
3
5.0
6.0
78
75
SPh
O
O
20
SPh
OCH3
PhS
CO2Me
5c
5c
4
5
4.5
7.0
80
68
O
PhS
PhS
CN
H
CN
H
5a
6
7
8
6.0
6.5
7.0
72
75
70
O
O
PhS
PhS
5a
5b
O
O
Ph
Ph
Ph
O
O
Ph
O
SPh
O
4d
4d
Ph
Ph
9
7.0
6.5
72
76
MeO
Cl
MeO
Cl
O
SPh
O
Ph
Ph
10
A mixture of conjugated alkene (1 mmol), diphenyl disulfide (0.7 mmol), [pmIm]Br (0.4 mmol) was heated at 75 °C.
a
Yields refer to those of pure isolated products characterized by spectroscopic data (IR, 1H and 13C NMR).
lence expansion. Following a reported reaction pathway,11c an
addition–elimination mechanism has been proposed as outlined
in Scheme 2. The nucleophile, PPh3 attacks the sulfur or selenium
atom to produce an intermediate 1 which underwent a facile cleav-
age to release thiolate or selenate anion. These anions underwent
subsequent reactions to provide unsymmetrical sulfides, selenides,
and 1,4-addition products. As the reaction did not proceed in the
absence of ionic liquid it has a vital role in this reaction. Although
we do not have any experimental data regarding the precise func-
tion of ionic liquid we believe that the ionic liquid acts as solvent,
promoter, and stabilizer for the intermediates 1, 2, and other
charged species via involvement of its imidazolium and bromide
ions. It is likely that because of the hygroscopic nature of the ionic
liquid, the moisture absorbed from air releases thiolate anion from
Ph3P+-SPh (2),11c which then reacts with other electrophiles to
yield the final product. In the course of the reaction PPh3 is con-
verted into triphenyl phosphine oxide and thus 1 equiv of PPh3 is
required to carry out the reaction.
In summary, we have demonstrated a simple metal-free one-
pot procedure for the synthesis of unsymmetrical sulfides, sele-
nides, and 1,4-thio-adducts by the [pmIm]Br/PPh3 mediated cleav-
age of diphenyl disulfide and diphenyl diselenide and subsequent
condensation. Thus, this protocol avoided the use of expensive me-
tal catalysts usually used for this reaction, volatile organic solvents
and toxic thiols. Certainly, the present protocol offers a novel and
green alternative to the existing procedures.
Acknowledgments
We are grateful for the financial support from the DST, New
Delhi under JC Bose National Fellowship to BCR [Grant No. SR/S2/
JCB-11/2008] for this investigation. S. B. and L. A. are thankful to
the CSIR, New Delhi for their fellowships.
Ph
S
Ph
δ
δ
Ph3P +
SPh
Supplementary data
S
1
SPh
Ph3P
Supplementary data (general experimental procedure and char-
acterization data of all products listed in Tables 2 and 3) associated
with this article can be found, in the online version, at doi:10.1016/
j.tetlet.2012.02.047.
R
X
RSPh
+
Ph3P SPh
SPh
2
H2O
References and notes
R
X
+
Ph3P O
2H
PhS +
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subsequent reaction.

2152
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