Bimetallic system for the synthesis of diorganyl selenides and sulfides, chiral β-seleno amines, and seleno- and thioesters

Article abstract of DOI:10.1016/j.tetlet.2011.05.003

The bimetallic reagent Sn(II)/Cu(II) in [bmim]BF₄ was efficiently used for the cleavage of diaryl diselenides and disulfides and reacts with a variety of organic substrates, such as organic halides, acid chlorides, and β-amino mesylates affording the diorganyl selenides and sulfides within very short reaction times, under mild conditions and with excellent yields, using BMIM-BF₄ as a reusable solvent.

Full text of DOI:10.1016/j.tetlet.2011.05.003

Tetrahedron Letters 52 (2011) 3592–3596
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Bimetallic system for the synthesis of diorganyl selenides and sulfides,
chiral b-seleno amines, and seleno- and thioesters
Kashif Gul a, Senthil Narayanaperumal a, Luciano Dornelles a, Oscar E. D. Rodrigues a,
,
⇑
Antonio Luiz Braga a,b,
⇑
a Departamento de Química, Universidade Federal de Santa Maria, 97105-900 Santa Maria, Brazil
b Departamento de Química, Universidade Federal de Santa Catarina, Florianópolis, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
The bimetallic reagent Sn(II)/Cu(II) in [bmim]BF4 was efficiently used for the cleavage of diaryl disele-
nides and disulfides and reacts with a variety of organic substrates, such as organic halides, acid chlo-
rides, and b-amino mesylates affording the diorganyl selenides and sulfides within very short reaction
times, under mild conditions and with excellent yields, using BMIM-BF4 as a reusable solvent.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 7 March 2011
Revised 1 May 2011
Accepted 2 May 2011
Available online 8 May 2011
Keywords:
Ionic liquid
Reusable
Diorganyl selenides and sulfides
Seleno- and thioesters
Tin chloride
Selenium
Interest in organochalcogenides has increased continuously due
to their important role in the areas of heterocyclic, radical chemis-
try, particularly in stereo-controlled processes in asymmetric syn-
thesis.1 For instance, the organo-selenium nucleus is one of the
most abundant structural nucleus found in natural products and
biologically active molecules (e.g., seleno-carbohydrates,2 selenoa-
mino acids,3 and seleno-peptides4). Moreover, organoselenium
compounds have emerged as an exceptional class of structures that
exemplify a role in biochemical processes, serving as important
therapeutic compounds ranging from antiviral and anticancer
agents to a variety of situations where free radicals are involved.5
On the other hand, organo-sulfur compounds have interesting
characteristics in organic chemistry and are often used as effective
reagents or convenient intermediates in organic synthesis.6,7 The
emerging biological importance of organo-sulfur compounds has
led to extensive studies aimed toward the synthesis of this class
of compounds.8
III),12b in photochemical reactions,12c bimetallic Cu(II)/Sn(II) sys-
tem in common organic solvents,13 and others.14 Because of
potential biological, pharmacological and synthetic applications
of organochalcogen compounds, it is considered worthwhile to de-
velop a general and effective method for the construction of com-
pound libraries.
Ionic liquids as versatile and novel reaction media for organic
transformations15,16 have resulted in many diverse and flexible
platforms to establish highly effective and easily separable sys-
tems. The most attractive properties of ionic liquids are very low
vapor pressure, nonflammability, ease of handling, reasonable
thermal stability and the fact that they remain liquid within a wide
range of temperatures.17 From the sustainable chemistry point of
view, there is a need for new methods which are not only very effi-
cient but also high yielding under mild reaction conditions. In this
context, ionic liquids have appeared as suitable solvents for many
organic transformations, showing greater efficiency compared
with the conventional organic solvents.18
The search for efficient, convenient and recyclable reaction
media based on ionic liquids remains a major challenge. Our inter-
est in this area18 and in organochalcogen chemistry19 have
prompted us to explore an efficient protocol, wherein Cu(II)/Sn(II)
is used as a reducing agent for the Y–Y bond (Y = S, Se) to prepare
unsymmetrical diorganyl selenides and sulfides, with very short
reaction times, under mild conditions, in a variety of substrates,
at room temperature and with excellent yields, using an ionic li-
quid as a reusable solvent (Scheme 1).
In general, symmetrical and unsymmetrical diorganyl selenides
and sulfides are prepared by the reductive cleavage of Se–Se or S–S
bonds, employing the common reducing agents, such as LiAlH4,9a
NaBH4,9b La,9c InI,9d,e CsOH,9f Na/NH3,9g Yb,10a SmI2,10b Cu/bpy,10c
In,10d,e ArB(OH)2/CuI,10e RhCl(PPh3)/H2,10f Zn,11 Sn/Pd,9g,12a Zn/In(-
⇑
Corresponding authors. Tel.: +55 55 3220 8761 (O.E.D.R.); tel.: +55 48 3721
6427; fax: +55 48 3721 6850 (A.L.B.).
E-mail addresses: roderiguesoed@smail.ufsm.br (O.E.D. Rodrigues), albraga@qm-
c.ufsc.br (A.L. Braga).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.05.003

K. Gul et al. / Tetrahedron Letters 52 (2011) 3592–3596
3593
The amount of CuBr2 and the reaction time (ranging from 180 to
30 min) required to promote the transformation were also evalu-
ated. On analyzing Table 1, it is possible to verify that sulfide affor-
ded the desired compound in a shorter time than selenide and with
better yields (Table 1, entries 1–9). In terms of reaction time, rang-
ing it as depicted in the Table 1 (entries 1 and 8) was possible ver-
ify not considerable modifications in the yield. By changing the
substrate from benzyl chloride to bromide both diselenide and
disulfide were obtained in better yields (Table 1, entry 9). This
can be attributed to the greater leaving group ability of bromide
compared with chloride.
Therefore, the optimum combination for this transformation
was found to be 0.5 equiv of diaryl chalcogenide with 1.1 equiv
of organic halide, which requires 1.2 equiv of SnCl2, 0.1 equiv of
CuBr2, and 0.5 mL of [bmim]BF4 at room temperature, in 60 min
of reaction time to afford the diorganyl selenide or 30 min the dior-
ganyl sulfides. In this way the use of the solvent/[bmim]BF4 yields
diorganyl selenide and sulfides in a short time, at room tempera-
ture, and under neutral and very mild conditions with good to
excellent yields (Table 2). With the optimized results in hand, we
next turned to exploring the versatility of the substitution reaction
for the synthesis of aryl alkylselenides and sulfides using diphenyl
dichalcogenide and a variety of alkyl halides as starting materials.
The results are summarized in Table 2.
SnCl2 / CuBr2
Y
R
X
Y
R
Ionic liquid
rt
Y
R = Bn
X = Cl, Br
Y = S, Se
Scheme 1. General methodology for the synthesis of diorganyl selenides and
sulfides.
To understand the influence of different variables in this reac-
tion, several components were studied to optimize our procedure.
Initially, we investigated the effect of ionic liquids on the reaction
course, using a standard model for the synthesis of diorganyl chalc-
ogenides. To this aim, five different ionic liquids (Fig. 1) were used
for the synthesis of the desired products.
Benzyl phenyl selenide and sulfide were afforded in a standard
protocol using 0.5 equiv of diphenyl diselenide and disulfide in the
presence of 1.2 equiv of SnCl2, 0.2 equiv of CuBr2 and 1.1 equiv of
benzyl chloride as a halide, using different ionic liquids (Table 1).
The changes in the cationic and anionic moieties in the solvent/
ionic liquid have a remarkable effect, as shown in Table 1. Using
[bmim]BF4 the desired products were achieved in good yield, fol-
lowed by [bmim]BF4 and [bmim]PF6. The use of [bmim]NTf2 and
[bpy]BF4 led to a significant decrease in the yield (Table 1, entries
1–6). Using CuCl2, the yield was lower compared with the use of
CuBr2 (Table 1, entries 1 and 2). In the absence of CuBr2 the reac-
tion of benzyl chloride with diphenyl diselenide and disulfide gave
only trace amounts of the corresponding product (Table 1; entry 7).
Thus, a combination of SnCl2 and CuBr2 in ionic liquid is essential
for this transformation.
In general, the diorganyl sulfides were obtained in better yield
than the diorganyl selenides. Although the selenolate intermediate
is more reactive than the respective thiolate, the lower stability of
the selenium species can explain their lower efficiency in the sele-
nide synthesis. Initially, the experiments were carried out with al-
kyl halides, with different chain lengths and halides. From Table 2
X=PF6 - [bmim]PF6
-
N
BF4
N
N
N
N
X=BF4 - [bmim]BF4
X-
-
BF4
[bpy]BF4
[bmim]BF4
X=NTf2 - [bmim]NTf2)
Figure 1. Room temperature ionic liquids.
Table 1
Optimization of reaction time and temperature
SnCl2/CuX2
Y
R
X
Y
R
Y
[bmim]BF4
Y = S, Se
X = Cl,Br; R = Bz
SnCl2 (mmol)
Catalysta (mmol)
Ionic liquidb
Time (min)
Yieldc (%)
Y = Se
Y = S
Y = Se
Y = S
1
2
3
4
5
6
7
8
9d
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
A
B
[bmim]BF4
[bmim]BF4
[bmim]PF6
[bmim]NTf2
[bmmim]BF4
[bpy]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
180
180
180
180
180
180
180
60
120
120
120
120
120
120
120
30
85
79
58
28
63
34
88
83
65
37
70
38
A
A
A
A
—
A
A
Traces
81
88
85
92
60
30
a
b
c
Catalyst A = CuBr2, B = CuCl2.
Ionic liquids were prepared using a procedure available in the literature [Ref. 16c] and were subjected to vacuum before use.
Yields refer to pure isolated products.
X = Br.
d

3594
K. Gul et al. / Tetrahedron Letters 52 (2011) 3592–3596
Table 2
benzylic systems were studied in the chalcogenide synthesis. A
good result was obtained in the reaction of 4-chloro benzyl chlo-
ride, with diphenyl diselenide and disulfide (Table 2; entry 11).
Notably, a steric effect by the aromatic substituents could be ob-
served in the course of the reaction. For instance, with the more
hindered o-methyl substituent the respective selenide and sulfide
were obtained in lower yield comparing with the corresponding
m- and p-methyl substituents (Table 2, entries 12–14).
Synthesis of diorganyl selenides and sulfides using Sn(II)/Cu(II) in [bmim]BF4
SnCl2 / CuBr2
Y
R
X
Y
R
[bmim]BF4
X = Cl, Br rt
Y
Y = S, Se
In order to extend the scope of our methodology, we attempted
to synthesize interesting functionalities, such as seleno- and thio-
esters and chiral b-aminochalcogenides. Selenoesters have been
extensively applied as mild acyl transfer agents, both as acyl radi-
cals or anions, to promote the synthesis of carbonyl compounds.20
On account of this, they have been the method of choice applied in
the acylation step in the synthesis of many natural products.21 This
class of compounds has also found application as liquid crystals,22
as precursors for the synthesis of N-aminoacyl sulfonamides, for
lactonizations and as selenating agents.23 There are a number of
methods reported in the literature to synthesize selenoesters using
different metals, including palladium complexes (such as
Pd(PPh3)4), Sm, In, InI, Hg(SePh)2, PhSeSnBu3/Pd, and Rh/H2
systems.9d,9e,24
Entrya
RX
Yield (%)b
Y = Sc
Y = Sed
Br
1
2
3
4
5
6
7
75
80
79
85
65
82
89
68
75
74
80
58
80
84
I
Cl
Br
OMs
Cl
Br
Br
8
77
72
10
9
99
97
98
93
I
On the other hand, thioesters are one of the most useful build-
ing blocks for organic transformations. They have found applica-
tion in C–C coupling,25 for the synthesis of carbonyl
compounds,26 in asymmetric aldol reactions27 and more recently,
10
Br
Cl
11
12
92
75
83
67
Cl
Br
Br
their
a–b unsaturated analogs have been successfully applied for
asymmetric 1–4 additions, which allow access to chiral intermedi-
ates for the synthesis of more complex compounds.28 Furthermore,
they have been applied in the natural product synthesis and can
act as biologically relevant substances for in vivo tumor suppres-
sion and as anti-HIV agents.29 Many methods have been described
in the literature for the synthesis of this valuable class of com-
pounds.30 Employing our standard reaction conditions, seleno-
and thioesters were synthesized and the results are summarized
in Figure 2.
13
14
84
98
79
94
Br
a
b
c
Ionic liquids were subjected to vacuum before use.
Yields refer to pure isolated products.
Y = S reaction time 30 min.
d
Y = Se reaction time 60 min.
The use of benzoyl chloride, which reacts with diphenyl disele-
nide, gave the respective selenoester in 79% yield whereas with di-
phenyl disulfide afforded the thioester in 84% yield (Fig. 2, entry 2).
A drastic decrease in yield was obtained for reactions with ali-
phatic acyl chloride (Fig. 2, entry 4). Electron withdrawing groups
attached to the acyl chloride moiety afford better yields when
compared with the electron donating groups (Fig. 2, entries 1
and 3). This can be rationalized due to the highest electrophilicity
in the carbonyl center to electron withdrawing groups. A more
complex challenge in organochalcogenium chemistry is the devel-
opment of new methods for the introduction of selenium or sulfur-
containing groups into organic molecules, particularly in a stereo-
controlled manner. Synthesis using bimetallic reagents in ionic
it is possible to verify that, in all cases, bromides furnished the
respective selenides and sulfides in higher yields than the corre-
sponding chlorides and mesylates (Table 2, entries 3–7) and the
best leaving group ability could be evidenced as an important fac-
tor in the reaction yield. This protocol shows high tolerance in
terms of chain length and the corresponding chalocogenides could
be prepared in good yield with 2–12 carbon atoms in the organic
chain (Table 2, entries 1–8). The use of more reactive allyl iodide
allowed near quantitative conversion (Table 2; entry 9) with a
similar result found for allyl bromide (Table 2; entry 10) showing
the influence of the substrate on the reaction yield. Substituted
(PhY)2
SnCl2/CuBr2
O
O
Ph
R
Cl
R
Y
[bmim]BF4, rt
Y = S, Se
O
O
Y
O
Y
O
Y
Y
Y
S
Br
Y
Y
Y
Se
S
Se
S
Se
S
Se
Yielda,b (%)
Entries
17 10
60 52
84 79
82 75
1
4
3
2
Figure 2. Synthesis of seleno- and thioesters using Sn(II)/Cu(II) [bmim]BF4. aIonic liquids were subjected to vacuum before use. bYields refer to pure isolated products. cY = S
reaction time 30 min. dY= Se reaction time 60 min.

K. Gul et al. / Tetrahedron Letters 52 (2011) 3592–3596
3595
Ed. Jovem Pesquisador em Nanotecnologia), Capes and FAPERGS
for their financial support.
Ph
X
Ph
Se
NHBoc
NHBoc
Ph
S
NHBoc
SnCl2 / CuBr2
[bmim]BF4/rt
PhY)2
Supplementary data
X = OMs; Time: 180 min, 69% 15b
X = OMs, Time: 120 min, 75% 15a
X = Br; Time: 120 min, 71% 15a
X = Br;
Time: 180 min, 62% 15b
Y = S, Se
Supplementary data (synthetic procedures and compounds
characterisation data) associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2011.05.003.
Figure 3. Synthesis of chiral b-sulfur and seleno amines.
References and notes
SnCl2/CuBr2
[bmim]BF4
Y
R
X
Y
1. For selenium-containing chiral ligands, see: (a) Wirth, T. Organoselenium
Chemistry–Modern Developments in Organic Synthesis. In Topics in Current
Chemistry 208; Springer: Heidelberg, Germany, 2000; (b) Wirth, T. Angew.
Chem., Int. Ed. 2000, 39, 3740–3749; (c) Braga, A. L.; Lüdtke, D. S.; Vargas, F.
Curr. Org. Chem. 2006, 10, 1921–1938.
2. (a) Mukherjee, C.; Tiwari, P.; Misra, A. K. Tetrahedron Lett. 2006, 47, 441–445;
(b) Tiwari, P.; Misra, A. K. Tetrahedron Lett. 2006, 47, 2345–2348.
3. (a) Phadnis, P. P.; Mugesh, G. Org. Biomol. Chem. 2005, 3, 2476–2481; (b) Braga,
A. L.; Schneider, P. H.; Paixão, M. W.; Deobald, A. M.; Peppe, C.; Bottega, D. P. J.
Org. Chem. 2006, 71, 4305–4307.
R
Y
Y = S, Se
X = Cl,Br; R = Bz rt
S
100
90
80
70
60
50
40
30
20
10
0
92
90
Se
88
87
86
84
82
80
77
73
4. (a) Braga, A. L.; Lüdtke, D. S.; Paixão, M. W.; Alberto, E. E.; Stefani, H. A.; Juliano,
L. Eur. J. Org. Chem. 2005, 20, 4260–4264; (b) Schwab, R. S.; Soares, L. C.;
Dornelles, L.; Rodrigues, O. E. D.; Paixão, M. W.; Godoi, M.; Braga, A. L. Eur. J.
Org. Chem. 2010, 3574–3578.
5. (a) Mugesh, G.; Du Mont, W. W.; Sies, H. Chem. Rev. 2001, 101, 2125–2180; (b)
Nogueira, C. W.; Zeni, G.; Rocha, J. B. T. Chem. Rev. 2004, 104, 6255–6286; (c)
Alberto, E. E.; Nascimento, V.; Braga, A. L. J. Braz. Chem. Soc. 2010, 21, 2032–
2041.
6. (a)Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon
Press Ltd.: New York, 1991. Vol. 6; (b) Metzner, P.; Thuillier, A. Sulfur Reagents
in Organic Synthesis In Katritzky, A. R., Meth-Cohn, O., Rees, C. W., Eds.;
Academic Press: San Diego, CA, 1994.
Fresh
2nd Run
3rd Run
4th Run
5th Run
Figure 4. Recyclability of [bmim]BF4.
7. For general reviews on sulfides, see: (a) Jones, D. N. In Comprehensive Organic
Chemistry; Barton, D. H. R., Ollis, W. D., Eds.; Pergamon Press: Oxford, 1979. pp
33–103, Vol 3; (b)Organic Sulfur Chemistry Structure and Mechanism; Oae, S.,
Ed.; CRC Press: Boca Raton, FL, 1991.
liquids is a highly versatile way to create C–Se and C–S bonds in a
stereo-controlled manner.3,19 Hence, we focused our attention on
the synthesis of chiral b-sulfur and seleno amines which can be ob-
tained from the reaction of b-amino mesylate and bromo deriva-
tive31 with diphenyl diselenide/disulfide to give the
corresponding b-amino sulfur and seleno derivatives, as depicted
in Figure 3.
Using our standard reaction conditions it was possible to verify
the versatility of the methodology, allowing the synthesis of di-
verse organochalcogenium compounds from different functional-
ities. The results revealed the same behavior affording sulfide
derivatives in higher yield as compared with selenides.
To further explore the scope of our method, and in an effort to
obtain an environmentally benign protocol, we examined the pos-
sibility of reusing the reaction media. Accordingly, after the work-
up (see Supplementary data) the ionic liquid was recovered and
then used in additional runs. In a positive response, the yield
was found to be similar to that obtained in the first run (Fig. 4;
run 2). This operation was repeated three more times as depicted
in Figure 4.
In summary, herein we describe an efficient methodology for
the preparation of diorganyl selenides and sulfides from the corre-
sponding alkyl and aryl halides. Some important aspects of this
methodology are the high reactivity in the preparation of the dif-
ferent organochalcogen compounds, with very short reaction
times, mild reaction conditions, room temperature, and excellent
yields using an ionic liquid as a reusable solvent. The methodology
shows a wide versatility, allowing the synthesis of different classes
of organochalcogen compounds. Ongoing investigations into the
application of this methodology are underway in our laboratory.
8. (a) Angelici, R. J. Acc. Chem. Res. 1988, 21, 387–394; (b) Herradura, P. S.;
Pendola, K. A.; Guy, R. K. Org. Lett. 2000, 2, 2019–2022. and references therein.
9. (a) Yoshimatsu, M.; Sato, T.; Shimizu, H.; Hori, M.; Kataoka, T. J. Org. Chem.
1994, 59, 1011–1019; (b) Andreadou, I.; Menge, W. M. P. B.; Commandeur, J. N.
M.; Worthington, E. A.; Vermeulen, N. P. E. J. Med. Chem. 1996, 39, 2040–2046;
(c) Nishino, T.; Okada, M.; Kuroki, T.; Watanabe, T.; Nishiyama, Y.; Sonoda, N. J.
Org. Chem. 2002, 67, 8696–8698; (d) Ranu, B. C.; Mandal, T.; Samanta, S. Org.
Lett. 2003, 5, 1439–1441; (e) Ranu, B. C.; Mandal, T. J. Org. Chem. 2004, 69,
5793–5795; (f) Cohen, R. J.; Fox, D. L.; Salvatore, R. N. J. Org. Chem. 2004, 69,
4265–4268; (g) Bonaterra, M.; Martín, S. E.; Rossi, R. A. Tetrahedron Lett. 2006,
47, 3511–3515.
10. (a) Dowsland, J.; McKerlie, F.; Procter, D. J. Tetrahedron Lett. 2000, 41, 4923–
4927; (b) Su, W.; Gao, N.; Zhang, Y. J. Chem. Research, Synopses 2002, 4, 168–
169; (c) Taniguchi, N.; Onami, T. J. Org. Chem. 2004, 69, 915–920; (d) de
Andrade, F. M.; Massa, W.; Peppe, C.; Uhl, W. J. Organomet. Chem. 2005, 690,
1294–1299; (e) Wang, L.; Wang, M.; Huang, F. Synlett 2005, 2007–2010; (f)
Ajiki, K.; Hirano, M.; Tanaka, K. Org. Lett. 2005, 7, 4193–4195.
11. (a) Krief, A.; Derock, M.; Lacroixa, D. Synlett 2005, 2832–2834; (b) Santi, C.;
Santoro, S.; Battistelli, B.; Testaferri, L.; Tiecco, M. Eur. J. Org. Chem. 2008, 5387–
5390; (c) Santoro, S.; Battistelli, B.; Testaferri, L.; Tiecco, M.; Santi, C. Eur. J. Org.
Chem. 2009, 4921–4925; (d) Narayanaperumal, S.; Alberto, E. E.; Gul, K.;
Rodrigues, O. E. D.; Braga, A. L. J. Org. Chem. 2010, 75, 3886–3889.
12. (a) Nishiyama, Y.; Tokunaga, K.; Sonoda, N. Org. Lett. 1999, 1, 1725–1727; (b)
Braga, A. L.; Schneider, P. H.; Paixão, M. W.; Deobald, A. M. Tetrahedron Lett.
2006, 47, 7195–7198; (c) Ouchi, A.; Liu, S.; Li, Z.; Kumar, S. A.; Suzuki, T.;
Hyugano, T.; Kitahara, H. J. Org. Chem. 2007, 72, 8700–8706.
13. (a) Kundu, A.; Prabhakar, S.; Vairamani, M.; Roy, S. Organometallics 1997, 16,
4796–4799; (b) Kundu, A.; Roy, S. Organometallics 2000, 19, 105–107; (c) Sinha,
P.; Kundu, A.; Roy, S.; Prabhakar, S.; Vairamani, M.; Ravi Sankar, A.; Kunwar, A.
C. Organometallics 2001, 20, 157–162.
14. (a) Tang, R.; Zhong, P.; Lin, Q. Synthesis 2007, 1, 85–91; (b) Sureshkumar, D.;
Ganesh, V.; Vidyarini, R. S.; Chandrasekaran, S. J. Org. Chem. 2009, 74, 7958–
7961.
15. (a) Wulff, G. Chem. Rev. 2002, 102, 1–27; (b)Ionic Liquids in Synthesis;
Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2002.
16. For a comprehensive review about ionic liquids see: (a) Wasserscheid, P.; Keim,
W. Angew. Chem., Int. Ed. 2000, 39, 3772–3789; (b) Dupont, J.; de Souza, R. F.;
Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667–3692; (c) Cassol, C. C.; Ebeling, G.;
Ferrera, B.; Dupont, J. Adv. Synth. Catal. 2006, 348, 243–248.
17. (a) Welton, T. Chem. Rev. 1999, 99, 2071–2084; (b) Earle, M. J.; Seddon, K. R.
Pure Appl. Chem. 2000, 72, 1391–1398; (c) Sheldon, R. A. Chem. Commun.
(Cambridge) 2001, 2399–2407.
18. (a) Narayanaperumal, S.; Alberto, E. E.; de Andrade, F. M.; Lenardão, E. J.; Taube,
P. S.; Braga, A. L. Org. Biomol. Chem. 2009, 7, 4647–4650; (b) Braga, A. L.; Vargas,
F.; Zeni, G.; Silveira, C. C.; Andrade, L. H. Tetrahedron Lett. 2002, 43, 4012–4399;
Acknowledgments
Kashif and Senthil are recipients of TWAS-CNPq doctoral fel-
lowships and cordially acknowledge their financial support. We
are also obliged to CNPq (INCT-Catálise, INCT-NANOBIOSIMES,

3596
K. Gul et al. / Tetrahedron Letters 52 (2011) 3592–3596
(c) Singh, D.; Narayanaperumal, S.; Gul, K.; Godoi, M.; Rodrigues, O. E. D.; Braga,
A. L. Green Chem. 2010, 6, 957–960.
19. For selected examples see: (a) Braga, A. L.; Lüdtke, D. S.; Paixão, M. W.;
Rodrigues, O. E. D. Org. Lett. 2003, 5, 2635–2638; (b) Braga, A. L.; Zeni, G.;
Andrade, L. H.; Silveira, C. C. Synlett 1997, 595–596.
26. (a) Mukaiyama, T.; Araki, M.; Takei, H. J. Am. Chem. Soc. 1973, 95, 4763–4765;
(b) Anderson, R. J.; Henrick, C. A.; Rosenblum, L. D. J. Am. Chem. Soc. 1974, 96,
3654–3655.
27. McGarvey, G. J.; Williams, J. M.; Hiner, R. N.; Matsubara, Y.; Oh, T. J. Am. Chem.
Soc. 1986, 108, 4943–4952.
20. (a) Kozikowski, A. P.; Ames, A. J. Org. Chem. 1978, 43, 2735–2737; (b) Boger, D.
L.; Mathvink, R. J. J. Org. Chem. 1988, 53, 3377–3379.
21. (a) Schwartz, C. E.; Curran, D. P. J. Am. Chem. Soc. 1990, 112, 9272–9284; (b)
Martin, S. F.; Chen, K. X.; Eary, C. T. Org. Lett. 1999, 1, 79–82.
22. Heppke, G.; Martens, J.; Praefcke, K.; Simon, H. Angew. Chem., Int. Ed. Engl. 1977,
16, 318–319.
23. (a) Ogawa, A.; Kuniyasu, H.; Sonoda, N.; Hirao, T. J. Org. Chem. 1997, 62, 8361–
8365; (b) Kawai, Y.; Ando, H.; Ozeki, H.; Koketsu, M.; Ishihara, H. Org. Lett.
2005, 7, 4653–4656.
28. (a) Mazery, R. D.; Pullez, M.; López, F.; Harutyunyan, S. R.; Minnaard, A. J.;
Feringa, B. L. J. Am. Chem. Soc. 2005, 127, 9966–9967; (b) Horst, B.; Feringa, B. L.;
Minnaard, A. J. Org. Lett. 2007, 9, 3013–3015.
29. (a) Fukuyama, T.; Lin, S. C.; Li, L. J. Am. Chem. Soc. 1990, 112, 7050–7051; (b)
Hondal, R. J.; Nilsson, B. L.; Raines, R. T. J. Am. Chem. Soc. 2001, 123, 5140–5141;
(c) Macmillan, D. Angew. Chem., Int. Ed. 2006, 45, 7668–7672.
30. (a) Xiao, W.; Alper, H. J. Org. Chem. 1997, 62, 3422–3423; (b) Xiao, W.; Alper, H.
J. Org. Chem. 2001, 66, 6229–6233; (c) Cao, H.; McNamee, L.; Alper, H. J. Org.
Chem. 2008, 73, 3530–3534.
24. (a) Nishiyama, Y.; Kawamatsu, H.; Funato, S.; Tokunaga, K.; Sonoda, N. J. Org.
Chem. 2003, 68, 3599–3602; (b) Marin, G.; Braga, A. L.; Rosa, A. S.; Galetto, F. Z.;
Burrowa, R. A.; Gallardo, H.; Paixão, M. W. Tetrahedron 2009, 65, 4614–4618.
25. (a) Prokopcová, H.; Kappe, C. O. Angew. Chem., Int. Ed. 2008, 47, 3674–3676.
31. Synthetic procedures for the bromo ester derivative see: Stocking, E. M.;
Schwarz, J. N.; Senn, H.; Salzmann, M.; Silks, L. A. J. Chem. Soc., Perkin Trans. 1
1997, 2443–2447.