Mild and efficient one-pot synthesis of chiral β-chalcogen amides via 2-oxazoline ring-opening reaction mediated by indium metal

Article abstract of DOI:10.1016/j.jorganchem.2008.08.031

A simple and efficient procedure for the synthesis of β-seleno and β-thio amides via the ring-opening reaction of chiral 2-oxazolines in the presence of indium metal has been developed. Features of this method include the following: (i) easily and accessi

Full text of DOI:10.1016/j.jorganchem.2008.08.031

Journal of Organometallic Chemistry 693 (2008) 3563–3566
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Mild and efficient one-pot synthesis of chiral b-chalcogen amides via
2-oxazoline ring-opening reaction mediated by indium metal
a
a
Antonio L. Braga a, , Fábio Z. Galetto , Paulo S. Taube , Márcio W. Paixão a,b, Claudio C Silveira a,
*
Devender Singh a, Fabrício Vargas a,b
a Departamento de Química, Universidade Federal de Santa Maria, 97105-900 Santa Maria, RS, Brazil
b Instituto de Química, Universidade de São Paulo, São Paulo, SP 05508-900, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 June 2008
Received in revised form 25 August 2008
Accepted 25 August 2008
Available online 31 August 2008
A simple and efficient procedure for the synthesis of b-seleno and b-thio amides via the ring-opening
reaction of chiral 2-oxazolines in the presence of indium metal has been developed. Features of this
method include the following: (i) easily and accessible starting materials; (ii) indium metal is more stable
and less expensive then its respective salts; (iii) useful to excellent yields of b-chalcogen amides
derivatives.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
b-Chalcogen amides
2-Oxazolines
Ring-opening reaction
Indium metal
1. Introduction
selenoesters [8], b-hydroxyl selenides [9], selenocysteine deriva-
tives [10] with special attention given to unsymmetrical diorganyl
Organoselenium and sulfur compounds have been employed as
very useful reagents in organic synthesis [1]. They allow the che-
mo-, regio- and stereoselective introduction of new functional
groups into complex organic substrates under mild experimental
conditions. In addition, these compounds are gaining contempo-
rary interest due to their applications as powerful ligands in asym-
metric catalysis [2] as well as for the design, synthesis and
investigation of new molecular materials, especially for conducting
or superconducting materials and for liquid crystals [3].
Besides, the biological and medicinal role of selenium and orga-
noselenium compounds has also become increasingly esteemed,
mainly due to their antioxidant, anti-tumor, antimicrobial, and
antiviral properties [4]. Therefore, the development of new meth-
ods for the introduction of selenium-containing groups into organ-
selenydes [11].
In this context, we recently reported an indium(I) protocol for
the preparation a wide range of useful chiral b-seleno amides
and selenocysteine derivatives via 2-oxazolines ring-opening reac-
tion promoted by in situ generated bis(organoseleno)iodoindi-
um(III) [12]. This method provided
a practical and concise
synthesis of a structurally diverse organochalcogen compounds
in a straightforward and flexible strategy in the absence of a Lewis
acid [13]. Attempting to disclose further extension of these previ-
ous reports, we describe herein a simple experimental procedure
which uses metallic indium it is easy to handle, less expensive
when compared to indium(I) protocols and active enough to pro-
mote the heterocycle ring-opening reaction (Scheme 1). The study
resulted in an efficient synthesis of b-chalcogen amides 2 under
mild and neutral conditions.
ic molecules [5], particularly in
remains a significant challenge.
a stereocontrolled manner,
It is well known that the selenium anions are generated in situ
via chemical Se–Se bonds cleavage to avoid handling unstable re-
agents such as selenols. Reduction of Se–Se bonds, especially cleav-
age of diaryl diselenides were performed with reducing agents such
as NaBH4, Na/NH3, Bu3SnH and LiAlH4 [6]. In recent years, some
protocols with indium(I) iodide-mediating cleavage of diorganoyl
diselenides have been developed to prepare vinylic selenides [7],
2. Results and discussion
Aiming to determine the optimum conditions, we performed
studies about effects that can influence this reaction, such as: (i)
loading and structure of organoyl halide; (ii) loading of indium me-
tal; (iii) temperature and; (iv) solvent. We first investigated the
present reaction under the following conditions: oxazoline 1a,
PhSeSePh, indium powder in refluxing 1,4-dioxane on varying the
halide R3-X (Table 1). When a tert-alkyl, benzylic or allylic halide
were employed under the above conditions, 2a was obtained in
* Corresponding author. Tel.: +55 55 3220 8761; fax: +55 55 3220 8998.
E-mail address: albraga@quimica.ufsm.br (A.L. Braga).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.08.031

3564
A.L. Braga et al. / Journal of Organometallic Chemistry 693 (2008) 3563–3566
R1
(Table 1, entries 6–7). These results are consistent with respect to
R1
HN
In0/R3-X
YR2
Ph
the stability and reactivity under radical reaction conditions, read-
ily generated from organoyl halides and indium metal [14].
When the reaction was carried out in the absence or in a lower
amount of indium metal, the formation of the product was not ob-
served or it was obtained in lower yields (Table 1, entries 8 and 9).
The influence of reaction temperature and loading of the corre-
sponding halide were also investigated in the present ring-opening
reaction. We could observe a drastic decrease in the yield of the
product when the reaction was performed at room temperature,
probably due to a decreasing in the efficiency of the ring-opening
step in a less energetic system (Table 1, entry 10). The reaction
was also evaluated in the presence of 0.8 equivalent of p-
ClPhCH2Cl. However, a significant decrease in the product yield
was observed (Table 1, entry 11).
O
+
R2YYR2
N
Solvent, reflux, 24 h
Ph
O
1
2
Scheme 1. General procedure for the synthesis of b-chalcogen amides mediated by
indium metal.
Table 1
Ring-opening reaction of 2-oxazoline 1a in the presence of structurally diverse R3-X
In0(1 equiv)/R3-X
SePh
Ph
+
PhSeSePh
O
With these results in hand, the efficiency of the solvent to pro-
mote the present process was examined (Table 2). When the reac-
tion was carried out in dichloromethane, quite disappointing
results were observed (Table 2, entry 2). By increasing the polarity
HN
N
1,4-dioxane, reflux
24h
Ph
O
1a
2a
Entry
R3-X
In0 (equiv)
Yield (%)a
Table 3
1
2
3
4
5
6
7
8
t-BuCl
BnCl
BnBr
AllylBr
p-ClPhCH2Cl
MeI
PhBr
p-ClPhCH2Cl
p-ClPhCH2Cl
p-ClPhCH2Cl
p-ClPhCH2Cl
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0
70
45
90
62
98
_
_
_
46
15
76
Ring-opening reaction of 2-oxazolines
R1
In0/
p-ClPhCH2Cl
R1
HN
+
R2SeSeR2
SeR2
Ph
O
N
1,4-dioxane, reflux
24h
Ph
O
1
9
0.5
1.0
1.0
2
10b
11c
Entry
Oxazoline
R1
R2
2, Yield (%)a
a
Yields refer to those pure isolated products characterized by spectroscopic
methods.
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1a
1a
1a
1a
1a
1a
iPr
Bn
Ph
iBu
iPr
iPr
iPr
iPr
iPr
iPr
Ph
Ph
Ph
Ph
p-ClPh
p-MeOPh
o-MePh
Bn
nBu
Et
2a, 98
2b, 94
2c, 86
2d, 81
2e, 97
2f, 94
2g, 83
2h, 88
2i, 44
2j, 58
b
c
Reaction was carried out at room temperature.
0.8 equiv. of p-ClPhCH2Cl was used.
Table 2
Ring-opening reaction of 2-oxazoline 1a in the presence of different solvents
10
In0/p-ClPhCH2Cl
SePh
Ph
a
Yields refer to those pure isolated products characterized by spectroscopic
+
PhSeSePh
O
HN
N
methods.
solvent, reflux
24h
Ph
O
1a
2a
Table 4
Ring-opening reaction of 2-oxazolines in the presence of sulfur and tellurium
nucleophiles
Entry
Solvent
Yield (%)a
1
2
3
4
5
6
7
8b
1,4-Dioxane
DCM
THF
Acetonitrile
DMF
EtOH
98
15
78
93
48
34
86
23
R1
1
R
In0/
p-ClPhCH2Cl
YR2
Ph
+
R2YYR2
O
HN
N
1,4-dioxane, reflux
24h
Ph
O
Toluene
1,4-Dioxane/H2O
3a-f, 4
1
a
Yields refer to those pure isolated products characterized by spectroscopic
Entry
Oxazoline
R1
R2
Y
Product, yield (%)a
methods.
b
1,4-Dioxane/H2O = 2:1.
1
2
3
4
5
6
7
1a
1b
1a
1a
1a
1a
1a
iPr
Bn
iPr
iPr
iPr
iPr
iPr
Ph
Ph
p-ClPh
p-MeOPh
Bn
S
S
S
S
S
S
Te
3a, 79
3b, 71
3c, 92
3d, 94
3e, 50
3f, 40
4, 12
yields ranging from 45% to 98% (Table 1, entries 1–5) and the most
efficient ring-opening process was achieved when p-chloro benzyl
chloride was used in the ring-opening process (Table 1, entry 5).
On the other hand, no product was observed when the reaction
was performed in the presence of a primary alkyl or an aryl halide
Et
Ph
a
Yields refer to those pure isolated products characterized by spectroscopic
methods.

A.L. Braga et al. / Journal of Organometallic Chemistry 693 (2008) 3563–3566
3565
(I)
In Cl
3
R3-Cl
+
In0
R
.
+
PhSeSePh
(III)
In Cl
PhSe
PhSe
R1
R1
R1
HN
R1
N
SePh
SePh
Ph
5
O
O
N
N
In SePh
Cl
O
Ph
Ph
O
Ph
1
2
Scheme 2. Ring-opening reaction of 2-oxazoline mediated by indium metal.
and boiling point of the solvent system in comparison with DCM,
better yields of 2a were achieved (Table 2, entries 3–6), probably
due to a higher efficiency in the ring-opening step. Toluene was
also investigated affording 2a in very good yield (Table 2, entry
7). However, the best reaction system was achieved using refluxing
1,4-dioxane (Table 2, entry 1) under the standard conditions as
established in Table 1. Concerning the high stability of organic in-
dium compounds in water compared with other metals, we also
evaluated the reaction in a mixture of dioxane/H2O, but unfortu-
nately the product was obtained in very low yield (Table 2, entry
8).
Se–Se bond [15] to generate the complex bis(organoylseleno)chlor-
oindium-(III) 5, responsible for the nucleophilic selenium transfer-
ring step. At this point, this indium selenolate promotes the
oxazoline ring-opening reaction by the coordination of the indiu-
m(III) complex with the nitrogen on the oxazoline moiety given
the charged nature of the second resonance structure [16], acting
itself as a Lewis acid [10,12]. Because of the double bond character
in both C–O and C–N bonds in oxazoline ring, we believe that the
intermediate formed between indium (III) complex and oxazoline
is non-planar, and this geometric aspect allow the selenium trans-
ferring step. By this way, the preparation of b-seleno-amides pro-
ceeds through the regio- and chemoselective nucleophilic attack
of the phenyl selenide anion at C(5) position of the ring, leads to
the C(5)–O(1) bond cleavage and furnishes the desired product,
without any loss of enantiomeric purity, as determined by chiral
HPLC.
After determine the optimum conditions, the present reaction
was further expanded to a broader range of oxazolines and diorga-
noyl dichalcogenides in order to evaluate the scope and limitations
of the reaction procedure.
As delineated in Table 3, all the b-seleno amides were obtained
in good to excellent yields for the oxazolines studied. The ring-
opening reaction permits a series of different lipophilic groups R1
attached to the oxazoline ring, since the respective products were
obtained in up to 98% yield (Table 3, entries 1–4). The present
method also tolerates organodiselenides bearing electron-with-
drawing (R2 = p-ClPh) and electron-donating groups (R2 = p-
MeOPh) as well as substituents attached to different positions of
the ring (R2 = o-MePh), furnishing the corresponding chiral organo-
selenides 2e–g in excellent yields (Table 3, entries 5–7). Moreover,
the reaction was performed with dialkyl diselenides as nucleo-
philic source of selenium, furnishing the products 2h–j in moder-
ate to good yields (Table 3, entries 8–10).
3. Conclusions
In summary, a simple efficient and broadly applicable general
method with metallic indium system for the preparation of modu-
lar chiral b-chalcogen amides were developed. 2-Oxazolines was
used as inexpensive and easily accessible chiral pool starting mate-
rials. Features of this method include the following: (i) easily
accessible reagents; (ii) stability of indium metal towards air and
moisture, its nontoxic, available in pure form and less expensive
than indium(I); (iii) excellent yields of b-chalcogen amides were
obtained. Studies dealing with the application of this methodology
in the synthesis of selenocysteine derivatives and other biologi-
cally relevant selenium compounds are currently in progress in
our laboratory.
Based on the successful approach developed in the preparation
of a wide range of chiral b-selenium amides by ring-opening reac-
tion, we decided to extend our studies in order to prepare a series
of chiral organochalcogenide compounds using sulfur and tellu-
rium nucleophilic species (Table 4).
Chiral b-thio amides 3a–f were obtained in yields ranging from
40% to 94% (Table 4, entries 1–6). Once again, the ring-opening
reaction tolerates the presence of different R1 groups presented
at the oxazoline ring (Table 4, entries 1 and 2) as well as substitu-
ents attached at the aromatic ring of the organosulfur moiety (Ta-
ble 4, entries 3 and 4) or even the use of dialkyl disulfides as
nucleophilic source of sulfur (Table 4, entries 5 and 6). At despite
of observed for sulfur and selenium derivatives, b-telluro amide 4
was obtained in very unsatisfactory yield (entry 7).
The reaction pathway for the formation of the b-seleno amides
(Scheme 2) is believed to occur first through the single-electron
transfer (SET) from indium14a–d to alkyl halide with generation of
an alkyl radical and indium(I) chloride. The indium salt thus
formed reacts with PhSeSePh through oxidative insertion into the
4. Experimental
4.1. General procedures
Melting points are uncorrected. Optical rotations were recorded
on a Perkin–Elmer 341 Polarimeter.
1H and 13C NMR spectra were
recorded at 400 and 100 MHz, respectively, with tetramethylsilane
as internal standard. High-resolution mass spectra were recorded
on a Bruker BioApex 70 eV spectrometer. Column chromatography
was performed using Merck Silica Gel (230–400 mesh) following
the methods described by Still [17]. Thin layer chromatography
(TLC) was performed using Merck Silica Gel GF254, 0.25 mm thick-
ness. For visualization, TLC plates were either placed under
ultraviolet light, or stained with iodine vapor, or acidic vanillin.

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A.L. Braga et al. / Journal of Organometallic Chemistry 693 (2008) 3563–3566
(g) A.L. Braga, F.Z. Galetto, O.E.D. Rodrigues, C.C. Silveira, M.W. Paixão,
Chirality 20 (2008) 839.
[3] (a) G. Heppke, J. Martens, K. Praefcke, H. Simon, Angew. Chem., Int. Ed. Engl. 16
(1977) 318;
THF was dried over sodium benzophenone ketyl and distilled prior
to use. Dichloromethane, 1,4-dioxane and acetonitrile were dis-
tilled from phosphorus pentoxide. All other solvents were ACS or
HPLC grade unless otherwise noted. Air- and moisture-sensitive
reactions were conducted in flame-dried or oven dried glassware
equipped with tightly fitted rubber septa and under a positive
atmosphere of dry argon. Reagents and solvents were handled
using standard syringe techniques. Temperatures above room tem-
perature were maintained by use of a mineral oil bath with an elec-
trically heated coil connected to a Variac controller. Oxazolines
were prepared according to the procedures described in the litera-
ture [18].
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Bottega, Synthesis (2002) 2338;
4.2. General procedure for the synthesis of chiral b-chalcogen amides
Indium powder (0.5 mmol, 57.5 mg), p-chloro benzyl chloride
(0.5 mmol, 80.5 mg), appropriate dichalcogenide (0.5 mmol),
appropriate oxazoline (0.5 mmol) and dry 1,4-dioxane (4 mL) were
placed in a 25 mL two-necked flask. The resulting suspension was
heated at reflux and stirred for 24 h. After cooled to r.t., the mixture
was quenched with HCl 1 M, extracted with CH2Cl2 and the com-
bined organic fractions were collected, dried over MgSO4 and fil-
tered; the solvent was then removed in vacuo yielding crude b-
chalcogen amides which were purified by flash chromatography.
Spectroscopic data were in good agreement with literature [12,13].
(b) A.L. Braga, M.W. Paixão, D.S. Lüdtke, C.C. Silveira, O.E.D. Rodrigues, Org.
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L.A. Wessjohann, Tetrahedron Lett. 43 (2002) 7329;
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11664;
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N.P.E. Vermeulen, J. Med. Chem. 39 (1996) 2040;
4.2.1. (S)-N-(3-methyl-1-(phenylselanyl)butan-2-yl)benzamide (2a)
The enantiomeric purity was determined by HPLC analysis (col-
umn Chiralcel-OD, eluent hexane/isopropanol 90:10, flow rate
1.0 ml minꢀ1, R isomer, tR 9.33 min, S isomer, tR 13.47 min and
found to be 99.9%; yield 0,322 g (93%); white solid; m.p. 101–
(b) M. Sakakibara, K. Katsumata, Y. Watanabe, T. Toru, Y. Ueno, Synthesis
(1992) 377;
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Tetrahedron Lett. 43 (2002) 7921;
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Mello, Synlett (2005) 3091.
[8] B.C. Ranu, A. Das, Adv. Synth. Catal. 347 (2005) 712.
[9] O.S. do Rego Barros, A.B. de Carvalho, E.S. Lang, C. Peppe, Lett. Org. Chem. 1
(2004) 43.
[10] A.L. Braga, P.H. Schneider, M.W. Paixão, A.M. Deobald, C. Peppe, D.P. Bottega, J.
Org. Chem. 71 (2006) 4305.
[11] (a) B.C. Ranu, T. Mandal, S. Samanta, Org. Lett. 5 (2003) 1439;
(b) B.C. Ranu, T. Mandal, J. Org. Chem. 69 (2004) 5793;
(c) B.C. Ranu, Eur. J. Org. Chem. (2000) 2343;
103 °C; ½a 2D0
¼ þ210 (c = 1.0, CH2Cl2); IR (KBr) 3313, 2965, 1633,
ꢁ
1533, 1470, 1178, 733, 693; 1H NMR (CDCl3, 400 MHz) d 7.60–
7.35 (m, 7H); 7.25–7.19 (m, 3H); 6,29 (d, J = 8.4, 1H); 4.22 (m,
1H); 3.25–3.23 (m, 2H); 2.05–2.00 (m, 1H); 0.98–0.96 (m, 6H);
13C NMR (CDCl3, 100 MHz) d 167.04; 134.58; 132.73; 131.17;
129.98; 129.11; 128.32; 126.99; 126.77; 54.74; 31,73; 31.69;
19.33; 18.50; 77Se NMR (CDCl3) d 251,54; HRMS-ESI m/z calc. for
C18H21NOSe + Na+ 370.0680, found 370.0677.
Acknowledgements
(d) A.L. Braga, P.H. Schneider, M.W. Paixão, A.M. Deobald, Tetrahedron Lett. 47
(2006) 7195.
[12] A.L. Braga, F. Vargas, F.Z. Galetto, M.W. Paixão, R.S. Schwab, P.S. Taube, Eur. J.
Org. Chem. (2007) 5327.
[13] (a) A.L. Braga, F. Vargas, J.A. Sehnem, R.C. Braga, J. Org. Chem. 70 (2005) 9021;
(b) F. Vargas, J.A. Sehnem, F.Z. Galetto, A.L. Braga, Tetrahedron 64 (2008) 392.
[14] (a) P. Cintas, Synlett (1995) 1087;
The authors gratefully acknowledge CNPq, CAPES and FAPERGS
for financial support. F.Z.G. thanks CAPES for a Ph.D. fellowship.
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[18] (a) T.G. Gant, A.I. Meyers, Tetrahedron 50 (1994) 2297;
(b) A.L. Braga, R.M. Rubim, H.S. Schrekker, L.A. Wessjohann, M.W.G. de Bolster,
G. Zeni, J.A. Sehnem, Tetrahedron: Asymmetr. 14 (2003) 3291.
(d) . For organosulfur compounds, see:S.R. Dabbaka, P. Vogel, Angew. Chem.,
Int. Ed. Engl. 44 (2005) 7674. and references therein.
[2] (a) For selenium-containing chiral ligands, see: T. Wirth, Angew. Chem., Int.
Ed. Engl. 39 (2000) 3740;
(b) A.L. Braga, D.S. Lüdtke, F. Vargas, Curr. Org. Chem. 10 (2006) 1921;
(c) A.L. Braga, D.S. Lüdtke, F. Vargas, R.C. Braga, Synlett (2006) 1453;
(d) . For sulfur-containing chiral ligands, see:J.C. Anderson, R. Cubbon, M.
Harding, D.S. James, Tetrahedron: Asymmetr. 9 (1998) 3461;
(e) M. Mellah, A. Voituriez, E. Schulz, Chem. Rev. 107 (2007) 5133;
(f) H. Pellissier, Tetrahedron 63 (2007) 1297;