NaBr mediated regioselective synthesis of 3-Selanylindoles

Article abstract of DOI:10.1080/10426507.2018.1424162

A convenient NaBr mediated procedure is developed for the preparation of 3-selanylindoles from indoles and diselenides. In this protocol, NaBr is first oxidized by mCPBA into molecular bromine, which reacts with diselenide to form RSeBr. The in situ generated active electrophilic selenium species, then reacts with indoles, affording a series of 3-selanylindoles with high regioselectivity and in moderate to good yields.

Full text of DOI:10.1080/10426507.2018.1424162

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
NaBr Mediated Regioselective Synthesis of 3-
Selanylindoles
Hongjie Li, Junxing Wang, Xiaolong Wang & Jie Yan
To cite this article: Hongjie Li, Junxing Wang, Xiaolong Wang & Jie Yan (2018): NaBr Mediated
Regioselective Synthesis of 3-Selanylindoles, Phosphorus, Sulfur, and Silicon and the Related
Elements, DOI: 10.1080/10426507.2018.1424162
To link to this article: https://doi.org/10.1080/10426507.2018.1424162
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NaBr Mediated Regioselective Synthesis of 3-Selanylindoles
Hongjie Li, Junxing Wang, Xiaolong Wang and Jie Yan*
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, P. R. of
China
Fax +86(571)88320238; E-mail: jieyan87@zjut.edu.cn
Abstract: A convenient NaBr mediated procedure is developed for the preparation of
3-selanylindoles from indoles and diselenides. In this protocol, NaBr is first oxidized by mCPBA
into molecular bromine, which reacts with diselenide to form RSeBr. The in situ generated active
electrophilic selenium species, then reacts with indoles, affording a series of 3-selanylindoles with
high regioselectivity and in moderate to good yields.
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Graphical Abstract
NaBr Mediated Regioselective Synthesis of 3-Selanylindoles
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Keywards: Selenation, 3-selanylindole, indole, diselenide, NaBr.
Introduction
3-Selanylindoles is a significant class of indole compounds due to their biological activities 1-5 and
potentially applicability as valuable synthetic intermediates.6-11 There are some methods for
synthesis of 3-selanylindoles, the main route is the use of easily available indoles as starting
materials. Recently, using Cu-catalysts to improve the selenation of indoles has become the
important methodology.12-17 More recently, Braga’s group demonstrated a green procedure for the
preparation of 3-selanylindoles, which was catalyzed by K2CO3 in ethanol and gave the
corresponding products in good yields.18 On the other hand, the selenation of indoles with
diselenides in the presence of oxidants is also a convenient method.19-22 Using I2 as catalyst and
DMSO as oxidant, Braga and co-workers reported a microwave (MW) irradiation promoted
reaction of indoles with the electrophilic selenium species PhSeI, which was generated in situ from
diselenides.19 Similarly, treatment of diphenyl diselenide with trichloroisocyanuric acid (TCCA),
the in situ generated electrophilic selenium species PhSeCl reacted easily with indoles to provide
3-selanylindoles.21 However, these above methods, despite being versatile process, the synthesis
of 3-selenylindoles and study on their potential bioactivity have not been extensively explored
compared with the analogous 3-thioindoles.
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We have been interesting in oxidation and functionalization of organic compounds. In our recent
research, we found that some inorganic iodides combined with oxidants can promote the Se-Se
bond cleavage of diselenides, and following an electrophilic reaction was occurred.23-26 Based on
this, to find cheaper and more environment-friendly inorganic salts, we have focused our attention
on bromides and chlorides. Herein, we wish to report an efficient selenation of indoles with
diselenides mediated by NaBr in the presence of oxidant m-chloroperbenzoic acid (mCPBA). To
the best of our knowledge, this method for synthesis of 3-selenylindoles has not been reported
before.
Results and discussion
Initially, we utilized indole 1a and diphenyl diselenide 2a as model substrates to investigate the
selenation of indoles at room temperature. It was found that on simple stirring of the mixture of 1.0
equiv of 1a, 1.1 equiv of 2a and 1.2 equiv of mCPBA with 0.5 equiv of NaBr in MeCN for 24
hours at room temperature in air, the expected product, 3-(phenylselanyl)-1H-indole 3a was
obtained in 74% yield (Table 1, entry 1). Then, the selenation of 1.0 equiv of 1a with 2a in the
presence of NaBr and mCPBA at room temperature was optimized. Several solvents were first
evaluated, in which MeCN had the best effect to the reaction (entries 1-9). Similar to NaBr, other
inorganic salts such as KBr, NH4Br, KCl and NaCl, all can promote the reaction and resulted in 3a
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in the yields ranging from 52%-63% (entries 10-13). With MeCN as the suitable solvent, the
optimized amount of NaBr was investigated, and 0.5 equiv proved to be the best choice (entries 1,
14-18). Notably, when the amount of 2a was reduced to 1.0 equiv, the yield was decreased to 59%
(entry 19). Following, the suitable amount of oxidant mCPBA was examined, and with 1.2 equiv
of it, the best yield of 74% was reached (entries 1, 20-22). Other oxidants, such as KHSO5 was also
active in the reaction in place of mCPBA, giving a moderate yield; but TBHP was not effective in
the reaction (entries 23 and 24). As the control experiment, 3a was not determined in the absence
of mCPBA or NaBr (entries 25 and 26). Finally, the reaction time was also optimized and the
results showed that in 24 hours the reaction was completed (entries 1, 27-30).
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Table 1 Optimization of the selenation of indole (1a) with diselenide (2a) mediated by NaBr
(PhSe)2
(equiv)
1.1
NaBr
(equiv)
0.5
mCPBA
(equiv)
1.2
Time
(h)
24
Yield
(%)a
74
46
53
59
37
68
7
Entry
Solvent
1
2
3
4
5
6
7
8
MeCN
MeOH
EtOH
1.1
0.5
1.2
24
1.1
0.5
1.2
24
1.1
0.5
1.2
CH2Cl2
CH2ClCH2Cl
EtOAc
THF
24
1.1
0.5
1.2
24
1.1
0.5
1.2
24
1.1
0.5
1.2
24
1.1
0.5
1.2
DMF
24
0
1.1
0.5
1.2
MeCN:MeOH
(1:1)
24
9
39
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.0
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
KBr (0.5)
1.2
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
6
58
55
63
52
31
41
55
66
70
59
39
70
74
52
7
NH4Br (0.5)
1.2
KCl (0.5)
NaCl (0.5)
0.1
1.2
1.2
1.2
0.2
1.2
0.4
1.2
0.6
1.2
1.0
1.2
0.5
1.2
0.5
0.5
0.5
1.0
0.5
1.5
0.5
KHSO5 (1.2)
0.5
TBHP (1.2)
0.5
-
0
-
1.2
1.2
1.2
0
0.5
55
63
0.5
12
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29
30
1.1
1.1
0.5
0.5
1.2
1.2
MeCN
MeCN
18
36
64
74
aIsolated yields.
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With these optimized conditions in hands, a detailed study was performed with different indoles
and diselenides, showing the generality of this method (Table 2). First, the effect of methyl group
on the indole nucleus was evaluated through the reaction of various methyl indoles from 1b to 1g
with 2a. It is obvious that most methyl indoles reacted with 2a smoothly, resulting in the
corresponding 3-selanylindoles 3b, 3c, 3e, 3f and 3g in moderate to good yields (Table 2, entries
2-3, 5-7). However, 3-methylindole 1d reacted with 2a difficultly and slowly, a much poor yield of
23% was determined for 2-selanylindole 3d, meaning this protocol is not suitable for the
preparation of 2-selanylindole (entry 4). The electronic character of both electron-donating and
electron–withdrawing substituents on the indole nucleus was also checked from several
5-substituented indoles, which usually had no significant influence on their reactivity (entries
7-10). Next, the scope of diselenide was examined. Dibenzyl diselenide 2b, similar to 2a, an
aliphatic diselenide was also effective in the reaction, affording the corresponding products 3k-3o
in 54-77% yields (entries 12-15). From Table 2, it is clear that except 1d, all indoles provided the
corresponding products were 3-selanylindoles, which indicates this selenation of indoles has a
high regioselectivity.
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Table 2 Synthesis of 3-selanylindoles 3
Entry
1
Indoles 1
Diselenides 2
R2 = Ph, 2a
3-Selanylindoles 3
Yield
(%)a
R1 = H, 1a
74
51
3a
2
R1 = 1-Me, 1b
2a
3b
3c
3
4
5
R1 = 2-Me, 1c
R1 = 3-Me, 1d
R1 = 4-Me, 1e
2a
2a
2a
63
23b
3d
71
3e
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6
R1 = 7-Me, 1f
R1 = 5-Me, 1g
R1 = 5-MeO, 1h
R1 = 5-F, 1i
R1 = 5-Br, 1j
1a
2a
61
61
63
57
87
54
57
3f
7
2a
3g
3h
8
2a
9
2a
3i
3j
10
11
12
2a
R2 = PhCH2, 2b
2b
3k
1g
3l
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13
14
15
1h
2b
2b
2b
67
56
77
3m
1i
3n
1j
3o
aIsolated yield. b 2-Selanylindole 3d was obtained.
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Based on the above results and control experiments, a plausible mechanism is depicted in Scheme
1 for the NaBr mediated selenation of indoles: NaBr is first oxidized by mCPBA into molecular
Br2, which reacts smoothly with diselenide 2 to form PhSeBr via a rapid cleavage of Se-Se
bond.27,28 The in situ generated active electrophilic selenium species then reacts with indole to
form an unstable intermediate A, and after removing a proton the desired product 3-selanylindole
3 is obtained with regioselectivity. In the cycle, bromine anion can be oxidized by mCPBA into
molecular Br2 again.
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Scheme 1 Proposed mechanism for the selenation of indole mediated by NaBr
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Conclusions
We have developed a novel and efficient NaBr mediated processor for synthesis of
3-selanylindoles. This method has some advantages such as mild reaction conditions and simple
procedure, which provides a series of corresponding compounds with high regioselectivity and in
moderate to good yields. Furthermore, this reaction will extend the application scope of inorganic
salts in organic synthesis.
Experimental Section
General. IR spectra were recorded on a Thermo-Nicolet 6700 instrument, 1H NMR and 13C NMR
spectra were measured on a Bruker-AVANCE Ⅲ(500 MHz) spectrometer, Mass spectra were
determined on Waters-GCT Premier, Thermo-DECAX-60000 LCQ Deca XP and Thermo-ITQ
1100 mass spectrometers. Indoles, diselenides, NaBr, mCPBA and solvents were commercially
available.
A typical procedure for the preparation of 3-selenylindoles:
Indoles 1 (0.2 mmol), diselenides 2 (0.11 mmol), NaBr (0.1 mmol) and mCPBA (0.24 mmol) were
added successively to MeCN (3 mL). The suspension was vigorously stirred at r. t. for 24 h. Upon
completion, the reaction was quenched by addition of sat. aq. Na2S2O3 (2 mL), and then sat. aq.
Na2CO3 (8 mL) and H2O (5 mL) were added, respectively. The mixture was extracted with CH2Cl2
(3×5 mL) and the combined organic phase was dried over anhydrous Na2SO4, filtered, and
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concentrated under reduced pressure. The residue was finally purified on a silica gel plate (5:1
petroleum ether-ethyl acetate) to furnish products 3.
3-(Phenylselanyl)-1H-indole (3a)19
1H-NMR: 8.42 (br s, 1H), 7.67 (d, J = 8.0 Hz, 1H), 7.50 (d, J = 2.5 Hz, 1H), 7.46 (d, J = 8.2 Hz,
1H), 7.32-7.24 (m, 3H), 7.23-7.09 (m, 4H).
13C-NMR: 136.4, 133.8, 131.2, 130.0, 129.0, 128.7, 125.6, 122.9, 120.9, 120.4, 111.4, 98.1.
1-Methyl-3-(phenylselanyl)-1H-indole (3b)19
1H-NMR: 7.67 (dd, J = 8.0, 0.8 Hz, 1H), 7.41 (d, J = 8.2 Hz, 1H), 7.34-7.31 (m, 2H), 7.29-7.25 (m,
1H), 7.24-7.11 (m, 5H), 3.87 (s, 3H).
13C-NMR: 137.5, 135.6, 134.2, 130.7, 128.9, 128.6, 125.5, 122.4, 120.4, 109.5, 109.4, 33.0.
2-Methyl-3-(phenylselanyl)-1H-indole (3c)19
1H-NMR: 8.29 (br s, 1H), 7.59 (d, J = 7.9 Hz, 1H), 7.36 (d, J = 8.0 Hz, 1H), 7.24-7.09 (m, 7H),
2.57 (s, 3H).
13C-NMR: 140.8, 135.8, 133.9, 131.3, 128.9, 128.3, 125.4, 122.1, 120.6, 119.8, 110.5, 96.3, 13.2.
3-Methyl-2-(phenylselanyl)-1H-indole (3d)29
1H-NMR: 8.08 (br s, 1H), 7.64 (d, J = 1.8 Hz, 1H), 7.62 (d, J = 1.5 Hz, 1H), 7.35-7.32 (m, 1H),
7.28 (d, J = 1.6 Hz, 1H), 7.22-7.21 (m, 1H), 7.21 (s, 1H), 7.21-7.20 (m, 1H), 7.19-7.18 (m, 1H),
7.17 (d, J = 0.8 Hz, 1H), 2.44 (s, 3H).
13C-NMR: 134.6, 134.0, 131.5, 130.3, 129.0, 128.4, 125.5, 124.8, 124.6, 120.5, 119.8, 111.1, 21.4.
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4-Methyl-3-(phenylselanyl)-1H-indole (3e)12
1H-NMR: 8.40 (br s, 1H), 7.46 (d, J = 2.6 Hz, 1H), 7.32 (d, J = 8.2 Hz, 1H), 7.26-7.22 (m, 2H),
7.20-7.11 (m, 4H), 6.95-6.92 (m, 1H), 2.74 (s, 3H).
13C-NMR: 136.9, 136.1, 132.4, 132.2, 129.0, 128.1, 127.3, 125.4, 123.0, 122.4, 109.4, 96.9, 18.8.
7-Methyl-3-(phenylselanyl)-1H-indole (3f)12
1H-NMR: 8.37 (br s, 1H), 7.52 (d, J = 8.3 Hz, 1H), 7.50 (s, 1H), 7.28-7.24 (m, 2H), 7.16-7.07 (m,
5H), 2.56 (s, 3H).
13C-NMR: 136.0, 133.9, 130.9, 129.6, 128.9, 128.7, 125.6, 123.5, 121.0, 120.5, 118.1, 98.7, 16.5.
5-Methyl-3-(phenylselanyl)-1H-indole (3g)19
1H-NMR: 8.31 (br s, 1H), 7.48 (s, 1H), 7.44 (d, J = 2.6 Hz, 1H), 7.35 (d, J = 8.3 Hz, 1H), 7.29-7.25
(m, 2H), 7.21-7.11 (m, 4H), 2.47 (s, 3H).
13C-NMR: 134.7, 134.0, 131.5, 130.3, 130.2, 129.0, 128.5, 125.5, 124.6, 119.8, 111.0, 97.3, 21.4.
5-Methoxy-3-(phenylselanyl)-1H-indole (3h)19
1H-NMR: 8.41 (br s, 1H), 7.45 (d, J = 2.6 Hz, 1H), 7.33 (d, J = 8.8 Hz, 1H), 7.27 (d, J = 1.6 Hz,
1H), 7.26 (d, J = 0.6 Hz, 1H), 7.19-7.09 (m, 4H), 6.95 (dd, J = 8.8, 2.4 Hz, 1H), 3.83 (s, 3H).
13C-NMR: 155.1, 133.9, 131.9, 131.3, 130.7, 129.0, 128.5, 125.5, 113.4, 112.2, 101.5, 97.5, 55.8.
5-Fluoro-3-(phenylselanyl)-1H-indole (3i)12
1H-NMR: 8.46 (br s, 1H), 7.54 (d, J = 2.6 Hz, 1H), 7.37 (dd, J = 8.8, 4.2 Hz, 1H), 7.35-7.28 (m,
2H), 7.26-7.22 (m, 2H) 7.17-7.13 (m, 2H), 7.03-6.95 (m, 1H).
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13C-NMR: 159.7, 157.8, 133.4, 132.9, 130.2, 129.1, 128.8, 125.9, 112.3, 112.2, 111.7, 111.5,
105.5, 105.3.
5-Bromo-3-(phenylselanyl)-1H-indole (3j)19
1H-NMR: 8.48 (br s, 1H), 7.80 (d, J = 1.7 Hz, 1H), 7.49 (d, J = 2.5 Hz, 1H), 7.36 (dd, J = 8.6, 1.8
Hz, 1H), 7.32 (d, J = 8.6 Hz, 1H), 7.25-7.22 (m, 2H), 7.20-7.14 (m, 3H).
13C-NMR: 135.0, 133.3, 132.4, 131.9, 129.1, 128.7, 126.0, 125.9, 123.0, 114.4, 112.9, 97.9.
3-(Benzylselenyl)-1H-indole (3k)18
1H-NMR: 8.27 (s, 1H), 7.70 (d, J = 7.9 Hz, 1H), 7.41 (d, J = 8.1 Hz, 1H), 7.30-7.26 (m, 2H),
7.26-7.17 (m, 3H), 7.07-7.04 (m, 3H), 3.90 (s, 2H).
13C-NMR: 139.7, 136.2, 130.7, 130.1, 128.8, 128.2, 126.5, 122.6, 120.5, 120.1, 111.2, 98.9, 32.3.
3-(Benzylselenyl)-5-methyl-1H-indole (3l)
Yellow viscous liquid.
IR (film): 3411, 3293, 3059, 3025, 1573, 1453, 1319, 1195, 1123, 907, 795, 727, 696.
1H-NMR: 8.09 (br s, 1H), 7.47 (s, 1H), 7.29 (d, J = 8.3 Hz, 1H), 7.24-7.20 (m, 3H), 7.12 (d, J = 1.3
Hz, 1H), 7.11-7.07 (m, 2H), 7.01 (d, J = 2.5 Hz, 1H), 3.91 (s, 2H), 2.53 (s, 3H).
13C-NMR: 139.9, 134.5, 130.8, 130.3, 129.8, 128.8, 128.1, 126.4, 124.2, 119.8, 110.9, 98.3, 32.2,
21.5.
HR-MS: calcd for C16H15NSe [M]+: 301.0370; found: 301.0361.
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3-(Benzylselenyl)-5-methoxy-1H-indole (3m)
Yellow viscous liquid.
1H-NMR: 8.19 (br s, 1H), 7.30-7.26 (m, 1H), 7.20-7.16 (m, 3H), 7.08-7.03 (m, 4H), 6.90 (dd, J =
8.7, 2.4 Hz, 1H), 3.87 (s, 2H), 3.87 (s, 3H).
13C-NMR: 154.9, 134.0, 131.4, 131.1, 131.0, 128.8, 128.2, 126.5, 113.1, 112.0, 101.5, 98.6, 55.8,
32.4.
HR-MS: calcd for C16H15NOSe [M]+: 317.0319; found: 317.0311.
3-(Benzylselenyl)-5-fluoro-1H-indole (3n)
Yellow viscous liquid.
IR (film): 3409, 3300, 3060, 3026, 1573, 1452, 1399, 1199, 1176, 907, 795, 726, 697.
1H-NMR: 8.27 (br s, 1H), 7.33-7.26 (m, 3H), 7.21-7.17 (m, 2H), 7.08 (d, J = 2.6 Hz, 1H),
7.05-7.02 (m, 2H), 7.00 (d, J = 2.5 Hz, 1H), 3.87 (s, 2H).
13C-NMR: 139.5, 132.5, 130.1, 128.8, 128.2, 126.6, 112.0, 111.9, 111.1, 110.9, 105.2, 105.0, 32.3.
HR-MS: calcd for C15H12FNSe [M]+: 305.0119; found: 305.0110.
3-(Benzylselenyl)-5-bromo-1H-indole (3o)
Yellow viscous liquid.
IR (film): 3408, 3285, 3059, 3025, 1586, 1451, 1396, 1199, 1126, 906, 795, 753, 697.
1H-NMR: 8.32 (br s, 1H), 7.71 (d, J = 1.8 Hz, 1H), 7.32 (dd, J = 8.8, 1.9 Hz, 1H), 7.24 (d, J = 1.7
Hz, 2H), 7.19 (dd, J = 5.0, 1.9 Hz, 2H), 7.04-7.00 (m, 3H), 3.86 (s, 2H).
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13C-NMR: 139.5, 134.7, 131.8, 129.6, 128.8, 128.2, 126.6, 126.1, 125.5, 123.7, 122.8, 114.0,
112.7, 98.4, 32.4.
HR-MS: calcd for C15H12BrNSe [M]+: 364.9318; found: 364.9289.
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