Ionic liquid: An efficient and reusable media for seleno- and thioester synthesis promoted by indium

Article abstract of DOI:10.1016/j.tetlet.2010.08.076

A series of thio and selenoesters were efficiently obtained employing stable diorganyl chalcogenides, acyl chlorides, and In as reducing agent in BMIM·PF₆. Recycling of the ionic liquid was also performed, which was reused three times.

Full text of DOI:10.1016/j.tetlet.2010.08.076

Tetrahedron Letters 51 (2010) 5728–5731
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Tetrahedron Letters
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Ionic liquid: an efficient and reusable media for seleno- and thioester
synthesis promoted by indium
Greice Tabarelli b, Eduardo E. Alberto a, , Anna M. Deobald a, Graciane Marin a, Oscar E. D. Rodrigues a,
⇑
b,
Luciano Dornelles a, , Antonio L. Braga
⇑
⇑
a Departamento de Química, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil
b Departamento de Química, Universidade Federal de Santa Catarina, Florianópolis, SC, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of thio and selenoesters were efficiently obtained employing stable diorganyl chalcogenides, acyl
chlorides, and In as reducing agent in BMIMÁPF6. Recycling of the ionic liquid was also performed, which
was reused three times.
Received 25 March 2010
Revised 20 August 2010
Accepted 25 August 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Selenoesters are attractive intermediates for many organic reac-
tions. They have been extensively applied as mild acyl transfer
agents, both as acyl radicals or anions, to promote the synthesis
of carbonyl compounds.1 On the account of this, they were the
method of choice applied in the acylation step in the synthesis of
many natural products (e.g., Crinipellin A,2 (+)-Geissoschizine,3
Ciguatoxins,4 and (À)-pseudolaric acid B5). This class of com-
pounds has also found application as liquid crystals,6 as precursors
for the synthesis of N-aminoacyl sulfonamides,7 for lactonizations8
and as selenating agents.9
able compounds have been prepared through the activation of
carboxylic acids with diphosgene27 or N-acylbenzotriazoles28 fol-
lowed by the addition of thiol, or by reaction of acyl chlorides with
zinc and thiols.29 They have also been accessed from thiols and car-
bon monoxide by carbonylation of organic substrates catalyzed by
transition metals such as Pt30 or Pd.31 Recently, Alper co-workers
described an interesting procedure for their synthesis through the
thiocarbonylation of iodoarenes catalyzed by palladium in a phos-
phonium salt ionic liquid.32
Nonetheless, the development of new methodologies to accom-
plish the preparation of seleno and thioesters under mild reaction
conditions, employing stable and nonhazardous starting materials
as well as in an efficient way is highly desirable. In this context, a
growing interest has been devoted for the utilization of ionic liq-
uids, which have some interesting properties, including: nonvola-
tility, nonflammability, excellent chemical and thermal stability,
and recyclability, which make them as attractive media for organic
reactions.33 According to the above mentioned, and in connection
with our insights into organochalcogen chemistry,34 in this report
we wish to highlight our results on the synthesis of seleno and thi-
oesters from acyl chlorides promoted by indium in ionic liquid.
Noteworthy characteristics of our protocol are the use of an ionic
liquid as solvent, which is conveniently recyclable, the use of stable
and easily handled diaryl chalcogenides instead of malodorous and
hazardous selenols or thiols, and the use of indium metal as a mild
reducing agent for high yielding and short reaction times.
As a model substrate to determine the best reaction conditions,
we performed a set of experiments using benzoyl chloride, diphe-
nyl diselenide, and one equivalent of indium metal as reducing
agent, Table 1. We evaluated three different imidazolium-based
ionic liquids, which are suitable to our purposes, since they are
liquid in the range of temperature used in this study and are stable
under our reaction conditions, Figure 1.35
Despite several methods have been described in the past,10
nowadays their synthesis under mild reactions conditions is a field
of growing research interest. These compounds have been success-
fully prepared from aldehydes using iBu2AlSePh,11 from chalco-
genoacetylenes,12 by coupling of aryl iodides with CO and
PhSeSnBu3 catalyzed by Pd,13 and most commonly, by the reaction
of acyl chlorides with nucleophilic species of selenium such as
Hg(SePh)2,14 PhSeSnBu3/Pd,15 from reductive cleavage of disele-
nides with InI16 or In17 or by reductive coupling of PhSe)2 and acyl
chloride in an Rh/H2 system.18
On the other hand, it is widely agreed that thioesters are one of
the most useful building blocks for organic transformations. They
have found application in C–C coupling,19 for the synthesis of car-
bonyl compounds,20 in asymmetric aldol reactions21 and more
recently, their a-b-unsaturated analogs have been successfully ap-
plied for asymmetric 1–4 additions, which allow the access to chiral
intermediates for the synthesis of more complex compounds.22 Fur-
thermore, they found application in native chemical ligation for pep-
tide bond formation,23 in natural product synthesis,24 and also
acting as biologically relevant substances, finding application for
in vivo tumor suppression25 and as anti-HIV agents.26 These valu-
Using BMIMÁBF4 at 50 °C for 12 h the product was obtained in
⇑
Corresponding authors. Tel.: +55 48 3721 6427; fax: +55 48 3721 6850 (A.L.B.).
E-mail address: albraga@qmc.ufsc.br (A.L. Braga).
76% yield (entry 1). Under the same reaction conditions, but using
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.08.076

G. Tabarelli et al. / Tetrahedron Letters 51 (2010) 5728–5731
5729
Table 1
Reaction optimization for the synthesis of selenoesters in ionic liquids promoted by In
Table 2 (continued)
Entry Acyl chloride
Product
Yielda (%)
O
O
(PhSe) (0.5 equiv.)
2
O
Cl
In (1.0 equiv.)
O
Cl
SePh
Cl
7
8
47
53
Ionic Liquid
Se
Yielda (%)
O
O
Entry
IL
T (°C)
Time (h)
O
O
1
2
3
4
5
6
7
8
9b
BMIMÁBF4
BMIMÁPF6
BMIMÁPF6
BMIMÁPF6
BMIMÁPF6
50
50
50
50
50
50
50
25
50
12
12
6
3
1
3
0.5
1
76
100
100
100
98
47
70
17
58
Cl
Cl
Se
Se
BMIMÁN(Tf)2
BMIMÁPF6
BMIMÁPF6
BMIMÁPF6
9
49
38
1
O
O
a
Isolated yields.
With 0.5 equiv of In.
10
Cl
O
b
Se
O
O
11
12
13
17
-
-
-
Cl
BF4
PF6
N(Tf)2
Se
N
N
N
N
N
N
O
Cl
Se
BMIM.BF4
BMIM.PF6
BMIM.N(Tf)2
O
O
Cl
Cl
Cl
Figure 1. Ionic liquids used in this study.
13
14
35
25
Cl
Se
Cl
O
O
Table 2
Synthesis of selenoesters in BMIMÁPF6 promoted by In
O
Cl
O
Se
(R'Se)2 (0.5 equiv.)
O
O
In (1.0 equiv.)
R
Cl
R
SeR'
BMIM.PF6 / 50°C / 1h
a
Yields for isolated pure products.
Entry Acyl chloride
Product
O
Yielda (%)
O
hydrophobic BMIMÁPF6 the product was obtained in quantitative
yield (entry 2). Decreasing the reaction time to 1 h did not have
any significant impact in the reaction efficiency (entries 3–5). Low-
er yields were found by using BMIMÁN(Tf)2, 47% (entry 6) or in
BMIMÁPF6 in 0.5 h at 50 °C or at room temperature (entries 7 and
8). The reduction of indium amount to 0.5 equiv was also evalu-
ated, affording the desired product in 58% yield (entry 9). These re-
sults suggest that the use of ionic liquid to promote this reaction is
suitable and also that the counterion of the IL has an effective role
in modulating the efficiency of the reaction. Comparing our results
with other reports which described the use of other organic sol-
vents and indium metal as a reducing agent, our protocol has the
advantage to be carried out in shorter reaction times or with a low-
er amount of In.17
1
98
76
Cl
Se
O
O
2
3
Cl
Se
O
O
Cl
97
Se
Cl
Cl
Cl
Cl
O
O
O
O
O
With the best conditions in hands36, the scope of the reaction
was extended to a diverse range of acyl chlorides as well as for dia-
ryl diselenides using BMIMÁPF6 at 50 °C for 1 h, Table 2. Good yields
were achieved by using electron donating or electron withdrawing
groups attached to the acyl chloride moiety, both in ortho or para
position (entries 1–4). When substituted diaryl diselenides were
applied, a decrease in the yield was observed (entries 5–9).37 It
showed a trend for all kinds of diselenides, with yields in the range
of 47% and 57% (entries 5–9). A set of experiments was also carried
out with aliphatic acyl chlorides. As expected, a decrease in the
yields was found (entries 10–13).
4
5
84
57
Cl
Cl
Se
Se
Se
OMe
O
Cl
6
52
Notable is the reaction of dichloroacetyl chloride, which has
more than one reactive site, and afforded the desired product in

5730
G. Tabarelli et al. / Tetrahedron Letters 51 (2010) 5728–5731
Table 3 (continued)
100
80
60
40
20
0
Entry Acyl chloride
Product
Yielda
(%)
O
O
S
10
43
62
Cl
O
O
11
Cl
S
O
Fresh
98
2nd Run
88
3rd Run
85
4th Run
83
O
Cl
Cl
Cl
Yield (%)
12
13
36
57
Cl
S
Cl
Figure 2. Recycle of BMIMÁPF6.
O
O
reasonable 35% yield (entry 13). In the reaction of Fmoc chloride
with diphenyl diselenide, the product was obtained in 25% yield
(entry 14).
O
Cl
O
S
One of the most attractive features of ionic liquids is that they
frequently can be recycled. In this regard, we performed a set of
experiments aiming to reuse the reaction media, Figure 2. After
the first reaction of diphenyl diselenide and benzoyl chloride in
BMIMÁPF6 the ionic liquid was recovered and subjected to another
run, affording the product in 88% yield. This process was repeated
for two more times, affording the desired product in excellent
yields.38
a
b
c
Yields for isolated pure products.
BMIMÁPF6, 3 h, 50 °C.
BMIMÁPF6, 12 h, 50 °C.
BMIMÁPF6, 12 h, 100 °C.
BMIMÁPF6, 12 h, rt.
d
e
O
O
(PhSe) (0.5 equiv.)
2
In (1.0 equiv.)
Cl
SePh
As a further extension of our protocol, we applied the same reaction
conditions employed for the preparation of selenoesters to prepare
a series of thioesters, Table 3.
BMIM.PF
6
A decrease to 78% in the yield was found in the reaction of ben-
zoyl chloride and diphenyl disulfide (entry 1). Several attempts to
improve the yield of the incoming product failed, even by increas-
ing the reaction time or the temperature (entries 2–5). Then a di-
verse range of acyl chlorides were subjected to the standard
reaction conditions to prepare aryl and alkyl thioesters. Similar
to selenoesters, better yields were found for the reactions involving
acyl chlorides possessing an aryl group attached to the carbonyl
moiety (entries 6–8). When aliphatic acyl chlorides were used, a
decrease in the yield was found, and the formation of the desired
products was in the range of 36–62% yield (entries 9–12). An inter-
esting result was observed for the reaction of FmocCl with diphe-
nyl disulfide, affording the product in 57% yield (entry 13).
To summarize, we described herein an efficient protocol for the
synthesis of seleno and thioesters in ionic liquid. Our results are in
agreement with those previously reported for the synthesis of
these classes of compounds, however in our approach we em-
ployed an ionic liquid as solvent, which is conveniently recyclable,
our reaction media is neutral and mild, and we used stable and eas-
ily handled diaryl chalcogenides instead of malodorous and haz-
ardous selenols or thiols. Moreover, we employed indium metal
which is a suitable reducing agent for high yields and short reac-
tion times.
Table 3
Synthesis of thioesters in BMIMÁPF6 promoted by In
(PhS)2 (0.5 equiv.)
In (1.0 equiv.)
O
O
R
Cl
R
SPh
BMIM.PF6 / 50°C / 1h
Entry Acyl chloride
Product
Yielda
(%)
1
78
80
78
79
23
O
O
S
2b
3c
Cl
4d
5e
O
O
6
7
75
42
Cl
S
O
O
Cl
S
Br
Cl
Br
Cl
Acknowledgments
O
O
8
9
52
44
The authors thank CNPq (INCT-Catálise, INCT_NANOBIOSIMES),
CAPES-Probral for the financial support. E.E.A. is grateful to CNPq
for the PhD fellowship.
Cl
S
O
O
Cl
Supplementary data
S
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.08.076.

G. Tabarelli et al. / Tetrahedron Letters 51 (2010) 5728–5731
5731
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36. General procedure for the synthesis of selenoesters: Under an argon
atmosphere,
a mixture of indium powder (1 mmol, 115 mg), diphenyl
diselenide (0.5 mmol, 156 mg), and benzoyl chloride (1 mmol, 141 mg) in
BMIMÁPF6 (1 mL) was stirred at 50 °C for 1 h. After this time, the mixture was
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a Celite pad to remove the inorganic materials followed by stirring for 2 h with
charcoal to remove the light green color. After this time, the solution was
filtered, concentrated, and subjected to the vacuum for 1 h to eliminate the
moisture and trace of organic solvents. For the following runs the recovered
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(1 mmol, 115 mg), diphenyl diselenide (0.5 mmol, 156 mg), and benzoyl
chloride (1 mmol, 141 mg). Then, the product was extracted as described
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