Iron-catalyzed bromination of aryl azides by N-bromosuccinimide: Efficient method for the synthesis of brominated aryl azides

Article abstract of DOI:10.1016/j.cclet.2010.10.024

An efficient and mild protocol for bromination of aryl azides with N-bromosuccinimide (NBS) under FeCl₃ catalysis in 1,2-dichloroethane was developed. It is proved to be an efficient method for obtaining brominated aryl azides.

Full text of DOI:10.1016/j.cclet.2010.10.024

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 310–313
www.elsevier.com/locate/cclet
Iron-catalyzed bromination of aryl azides by N-bromosuccinimide:
Efficient method for the synthesis of brominated aryl azides
a
Hui Jin a, Zhen Dong Huang b, Chun Xiang Kuang a, , Xiao Kun Wang
*
a Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, China
b Zhejiang Citrus Research Institute, Taizhou 318020, China
Received 29 June 2010
Available online 22 December 2010
Abstract
An efficient and mild protocol for bromination of aryl azides with N-bromosuccinimide (NBS) under FeCl3 catalysis in 1,2-
dichloroethane was developed. It is proved to be an efficient method for obtaining brominated aryl azides.
# 2010 Chun Xiang Kuang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Iron-catalyzed; Aryl azides; N-Bromosuccinimide; Bromination
Aryl azides are versatile intermediates with various applications in organic and bioorganic chemistry [1]. They are
extensively employed in the synthesis of aryl nitrenes [2], aryl anilines [3], particularly nitrogen-containing five-
membered heterocycles upon 1,3-dipolar cycloaddition [4]. In addition, aryl azides are well known for their use as
photoaffinity labeling agents for proteins [5]. However, perhaps because of their partly less attractive properties
(explosiveness, toxicity), very few bromination systems in the literature has been published for their bromination.
Smith et al. [6] reported a method for bromination of phenyl azide with Br2, the reaction gives the para-bromination
product in low yield. Latyshev et al. [7] reported bromination of 1,3-diazido-2-methylbenzene using 1,4-dioxane
dibromide affords the monobromination product in moderate yield, and the reaction requires long reaction time and
the substrate scope is limited to electron-rich aryl azides. Thus, we still consider it highly desirable to develop milder
and more efficient bromination method of aryl azides.
In recent years, iron as one of the transition metal catalysts was revealed to be of high interest in term of cost and
efficiency [8]. To the best of our knowledge, although iron catalyzed the reaction of aromatic ring has been previously
reported [9], iron-catalyzed bromination of aryl azides has been never studied.
Herein, we report an iron-catalyzed bromination reaction of aryl azides using NBS, which afford the corresponding
brominated aryl azides 2 in moderate to good yield (Scheme 1).
To screen suitable reaction conditions, 1-azido-4-chlorobenzene 1a has been used as model substrate. Initially, the
reaction was carried out in the presence of FeCl3 (20 mol%) and NBS (1.1 equiv.) in 1,2-dichloroethane (3 mL) at
room temperature for 12 h, 49% isolated yield of 1-azido-2-bromo-4-chlorobenzene 2a was obtained (Table 1, entry
1). Then we examined other catalysts, FeCl3 proved to be superior to Fe2(SO4)3 and FeCl2 (Table 1, entries 1–3).
* Corresponding author.
E-mail address: kuangcx@tongji.edu.cn (C.X. Kuang).
1001-8417/$ – see front matter # 2010 Chun Xiang Kuang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.10.024

[()TD$FIG]
H. Jin et al. / Chinese Chemical Letters 22 (2011) 310–313
311
[Fe] Cat.
NBS
conditions
R
N3
R
N3
Br
2
1
Scheme 1. Iron-catalyzed bromination of aryl azides 1 by NBS.
Table 1
Optimization of bromination of 1-azido-4-chlorobenzene 1a to 1-azido-2-bromo-4-chlorobenzene 2a.
Entry
Catalyst (0.2 equiv)
Solvent (3 mL)
Temperature (8C)
Time (h)
Yield of 2a (%)a
1
FeCl3
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
MeCN
25
25
25
25
25
25
25
25
60
80
80
12
12
12
12
12
12
12
12
12
12
24
49
23
27
9
2
Fe2(SO4)3
FeCl2
3
4
Fe2O3
Fe(acac)3
none
5
5
6
Trace
43
0
7
FeCl3
8
FeCl3
1,4-Dioxane
9
FeCl3
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
65
72
85
10
11
FeCl3
FeCl3
a
Isolated yields.
Fe2O3 and Fe(acac)3 proved to be inefficient (Table 1, entries 4, 5), and the absence of iron catalyst gives trace product
(Table 1, entry 6). The reaction also showed a strong solvent dependence. Except 1,2-dichloroethane, other solvents
such as MeCN, dioxane proved to be inappropriate (Table 1, entries 7, 8). After that we examined temperature and
reaction time influence in the reaction. To our delight, the yield of 2a could be improved when the temperature was
increased to 80 8C and the reaction time was extended to 24 h. On the basis of these results, the optimal conditions
involved the following parameters: FeCl3 as catalyst, 1,2-dichloroethane as solvent, with reaction temperature at
80 8C for 24 h.
Under the optimized conditions, the substrate scope of this iron-catalyzed bromination reaction of aryl azides was
investigated and these newly developed conditions appeared to be general for a spectrum of aryl azides (Table 2). The
aryl azides were obtained from the anilines employing standard diazotization conditions [10]. The reaction conditions
are compatible with various functionalities such as chloro (2a), bromo (2b), ethers (2c, 2d), methoxy (2e) and methyl
(2f, 2f0). It was observed that azidobenzene could also proceed smoothly with good yield (2g). The substitution on the
aromatic ring had almost no influence on the yield of the reaction, as systems substituted with electron-withdrawing
(2a–2d), electron-donating groups (2e, 2f, 2f0) provided similar results. But the electronic factors had shown some
effect on the reaction time. In general, electron-rich aryl azides (2e, 2f, 2f0) gave shorter reaction time than electron-
deficient counterpart (2a–2d).
The reaction of heterocyclic azide such as thiazole (2h) proceeded with moderate yield 77% and gives good
regioselectivity. Bromination of naphthalene (2i) gives disubstituted product, bromine atom was added to both ortho
and para positions of azide group simultaneously in yield 67%.
In summary, we have successfully employed FeCl3 as catalyst for the direct bromination aryl azides to
corresponding brominated derivatives in moderate to good yield. This is a versatile method to accomplish the synthesis
of a series of brominated aryl azides from aryl azides in a single-step operation.
1. Experimental
1H and 13C NMR spectra were recorded using a Bruker AM-500 spectrometer. High resolution mass spectra were
determined using a Finnigan-NAT GC/MS/DS 8430 spectrometer.
To a sealed tube were added 1,2-dichloroethane 3 mL, aryl azide (0.5 mmol), FeCl3 (16 mg, 0.1 mmol), NBS
(97 mg, 0.55 mmol). The mixture was stirred at 80 8C and the reaction was monitored by TLC. After reaction
completed, the system was extracted with Et2O (3 ꢀ 30 mL). The organic layer was separated, washed with water and

312
H. Jin et al. / Chinese Chemical Letters 22 (2011) 310–313
Table 2
FeCl3-catalyzed bromination of aryl azides 1 to brominated aryl azides 2.
Entry
1
Azide 1
Product 2
Time (h)
24
Yield (%)a
84
Ref.
2a
2b
2c
2d
[
Cl
N3
N3
Cl
N3
N3
[DT$LINE]
Br
Br
2
3
4
24
24
24
78
81
80
[11]
[12]
[13]
Br
[TD$LINE]
Br
[DT$LINE]
[
EtO2C
N3
EtO2C
N3
[DT$LINE]
Br
[TD$LINE]
MeO2C
N3
MeO2C
N3
[DT$LINE]
Br
N3
5
6
2e
4
4
86
[14]
[TD$LINE]
MeO
N3
MeO
[DT$LINE]
Br
2f
43
40
[11]
[15]
Me
N3
[TD$LINE]
Me
N3
[DT$LINE]
Br
2f0
N3
N3
[D$LINE]
MT e
Br
7
8
2g
2h
4
4
89
77
[16]
[17]
[
[TD$LINE]
Br
N3
N
S
N
N3
[TD$LINE]
N3
[DT$LINE]
S
Br
9b
2i
4
67
[18]
N3
N3
Br
[TD$LINE]
[DT$LINE]
Br
a
Isolated yields.
b
2.2 equiv. NBS was added.
saturated brine, and dried over anhydrous Na2SO4. Evaporation of the solvent gave the crude product, which was
subjected to column chromatography (silica gel, EtOAc–petroleum ether) to afford brominated aryl azides 2. The
structure of these compounds has been elucidated by spectral (1H NMR, 13C NMR) data [19].
Acknowledgments
The work was supported by National Natural Science Foundation of China (No. 30873153), the Key Projects of
Shanghai in Biomedical (No. 08431902700) and the Scientific Research Foundation of State Education Ministry for
the Returned Overseas Chinese Scholars. We would like to thank the Center for Instrumental Analysis, Tongji
University, China.
References
[1] (a) E.F.V. Scriven, K. Turnbull, Chem. Rev. 88 (1988) 297;
¨
(b) S. Brase, C. Gil, K. Knepper, et al. Angew. Chem. Int. Ed. 44 (2005) 5188.
[2] (a) N.P. Mankad, P. Mu¨ller, J.C. Peters, et al. J. Am. Chem. Soc. 132 (2010) 4083;
(b) K.N. Crabtree, K.J. Hostetler, T.E. Munsch, et al. J. Org. Chem. 73 (2008) 3441.
[3] Z.G. Liu, X.Q. Niu, W. Yu, et al. Chin. Chem. Lett. 19 (2008) 885.
[4] (a) R. Huisgen, Angew. Chem. Int. Ed. 2 (1963) 565;
(b) C.W. Tornoe, C. Christensen, M. Meldal, et al. J. Org. Chem. 67 (2002) 3057.

H. Jin et al. / Chinese Chemical Letters 22 (2011) 310–313
[5] (a) A. Fishier, A. Mann, R.L. Johnson, et al. J. Med. Chem. 49 (2006) 307;
313
(b) C.A. Gartner, Curr. Med. Chem. 10 (2003) 671.
[6] P.A.S. Smith, J.H. Hall, R.O. Kan, J. Am. Chem. Soc. 84 (1962) 485.
[7] G.V. Latyshev, M.S. Baranov, A.V. Kazantsev, et al. Synthesis (2009) 2605.
[8] (a) B.D. Sherry, A. Fu¨rstner, Acc. Chem. Res. 41 (2008) 1500;
(b) J. Norinder, A. Matsumoto, N. Yoshikai, et al. J. Am. Chem. Soc. 130 (2008) 5858.
[9] K. Tanemura, T. Suzuki, Y. Nishida, et al. Chem. Lett. 32 (2003) 932.
[10] F. Pagliai, T. Pirali, G.C. Tron, et al. J. Med. Chem. 49 (2006) 467.
¨
[11] K. Knepper, M.E.P. Lormann, S. Brase, J. Comb. Chem. 6 (2004) 460.
[12] C.-Y. Liu, P. Knochel, J. Org. Chem. 72 (2007) 7106.
[13] B.R. de Costa, A. Lewin, K.C. Rice, Heterocycles 32 (1992) 2343.
[14] K. Odlo, J. Hentzen, T.V. Hansen, et al. Bioorg. Med. Chem. 16 (2008) 4829.
[15] J.Q. Li, M.Y. Hu, S.Q. Yao, Org. Lett. 11 (2009) 3008.
[16] Q. Liu, Y. Tor, Org. Lett. 5 (2003) 2571.
[17] R. Faure, J.P. Galy, J. Elguero, et al. Can. J. Chem. 56 (1978) 46.
[18] G. Mitchell, C.W. Rees, J. Chem. Soc. (1987) 403.
1
[19] Spectroscopic data. 1-Azido-2-bromo-4-chlorobenzene 2a: H NMR (500 MHz, CDCl3): d 7.10 (d, 1H, J = 8.6 Hz), 7.32 (dd, 1H, J = 8.6,
2.3 Hz), 7.56 (d, 1H, J = 2.3 Hz). 13C NMR (125 MHz, CDCl3): d 119.1, 120.1, 128.7, 130.6, 133.5, 137.6. HRMS: m/z calcd. for C6H3BrClN3
230.9199, found 230.9198. 1-Azido-2,4-dibromobenzene 2b: 1H NMR (500 MHz, CDCl3): d 7.04 (d, 1H, J = 8.5 Hz), 7.46 (dd, 1H, J = 8.5,
2.1 Hz), 7.70 (d, 1H, J = 2.1 Hz). Ethyl 4-azido-3-bromobenzoate 2c: 1H NMR (500 MHz, CDCl3) d 1.40 (t, 3H, J = 7 Hz), 4.38 (q, 2H,
J = 7 Hz), 7.21 (d, 1H, J = 8.5 Hz), 8.02 (dd, 1H, J = 8.5, 1.5 Hz), 8.23 (d, 1H, J = 1.5 Hz). Methyl 4-azido-3-bromobenzoate 2d: 1H NMR
(500 MHz, CDCl3): d 3.92 (s, 3H), 7.22 (d, 1H, J = 8.5 Hz), 8.01 (dd, 1H, J = 8.5, 1.9 Hz), 8.23 (d, 1H, J = 1.9 Hz). 4-Azido-2-bromo-1-
methoxybenzene 2e: 1H NMR (500 MHz, CDCl3): d 3.88 (s, 3H), 6.87 (d, 1H, J = 8.8 Hz), 6.94 (dd, 1H, J = 8.8, 2.7 Hz), 7.23 (d, 1H,
J = 2.7 Hz). 1-Azido-2-bromo-4-methylbenzene 2f: 1H NMR (500 MHz, CDCl3): d 2.31 (s, 3H), 7.06 (d, 1H, J = 8.0 Hz), 7.14 (dd, 1H, J = 8.0,
1.0 Hz), 7.37 (s, 1H). 4-Azido-2-bromo-1-methylbenzene 2f’: 1H NMR (500 MHz, CDCl3): d 2.37 (s, 3H), 6.88 (dd, 1H, J = 8.2, 2.3 Hz), 7.19
1
(d, 1H, J = 8.5 Hz), 7.21 (d, 1H, J = 2.5 Hz). 1-Azido-4-bromobenzene 2g: H NMR (500 MHz, CDCl3): d 6.92ꢁ6.94 (d, 2H, J = 8.8 Hz),
7.47ꢁ7.49 (d, 2H, J = 8.8 Hz). 2-Azido-5-bromothiazole 2h: 1H NMR (500 MHz, CDCl3): d 7.36 (s, 1H), 1-Azido-2,4-dibromonaphthalene 2i:
1H NMR (500 MHz, CDCl3): d 7.61ꢁ7.68 (m, 2H), 7.91 (s, 1H), 8.18 (d, 1H, J = 7.8 Hz), 8.26.(d, 1H, J = 8.5 Hz).