I2O5-mediated bromohydroxylation and dibromination of olefins using KBr in water

Article abstract of DOI:10.1016/j.tetlet.2014.03.064

An efficient and green bromohydroxylation and dibromination of various olefins mediated by I₂O₅ has been developed in this work. A series of olefins gave the corresponding α-bromo-alcohols and dibromides as the major products using KBr as the brominating reagent in aqueous medium at room temperature. The diastereo- and regio-selectivities are extremely high.

Full text of DOI:10.1016/j.tetlet.2014.03.064

Tetrahedron Letters 55 (2014) 2734–2737
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
I2O5-mediated bromohydroxylation and dibromination of olefins
using KBr in water
Yajun Wang a, Jinxi Wang b, Yun Xiong c, Zhong-Quan Liu c,
⇑
a School of Civil Engineering, Lanzhou University of Technology, Lanzhou 730050, China
b School of Chemical Engineering, Lanzhou University of Arts and Science, Lanzhou 730000, China
c State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou, Gansu 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and green bromohydroxylation and dibromination of various olefins mediated by I2O5 has
Received 5 December 2013
Revised 7 March 2014
Accepted 13 March 2014
Available online 20 March 2014
been developed in this work. A series of olefins gave the corresponding a-bromo-alcohols and dibromides
as the major products using KBr as the brominating reagent in aqueous medium at room temperature.
The diastereo- and regio-selectivities are extremely high.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Bromohydroxylation
Dibromination
Alkene
Highly chemoselective difunctionalization of alkenes represents
a very important strategy in organic synthetic chemistry. Among
them, regioselective hydroxybromination and dibromination of
olefins is attractive.1 During the past decades, various systems
using hydrogen bromide, metal bromides, as well as N-bromosuc-
cinimide (NBS) etc. had been developed.2–4 Although these pio-
neering studies, along with their modifications provide helpful
strategies for this type of reactions,5–8 most of these systems suffer
from some disadvantages such as using of hazardous elemental
bromine and HBr, metals, and relatively expensive N-bromoimide
etc. Hence, more efficient and environmentally benign systems
are highly desirable. Recently, a series of efficient dibromination
and hydroxybromination as well as bromoamination of olefins
using sodium bromide, ammonium bromide, and NBS as the bro-
mine source have been developed by Wang,9 Nama,10 Liang, and
Zhao et al.11 As our continuous studies on exploring green oxida-
tion systems using more safe and environmentally friendly
oxidants such as molecular oxygen, HIO3 (iodic acid, IA), and its
anhydride I2O5 (iodine pentoxide, IP),12 we hypothesize that mer-
ger of IA or IP and KBr would selectively realize hydroxybromina-
tion and/or dibromination of olefins. Fortunately, we successfully
accomplished a facile I2O5-mediated hydroxybromination and
dibromination of alkenes with very high regioselectivities and iso-
lated yields in mild conditions (Scheme 1).
OH
Ar2
O
O
Br
O
Ar1
+
Ar1
Ar2
Ar1
Ar
Ar2
Br
2 eq. I2O5
Br
Br
K
+
(
)
( )
R
H O
CH3CN/
RT
2
Br
2.5 eq.
R
1 eq.
Ar
OH
(
)
Scheme 1. I2O5-mediated hydroxybromination and dibromination of olefins.
Initially, we chose chalcone as the model substrate to optimize
suitable conditions for this reaction. It was found that the solvent
and the quantity of oxidant and KBr critically affect the reaction
efficiency (Fig. 1, see also the Supporting information). A mixture
of acetonitrile (CH3CN)/H2O (Fig. 1b) is proved to be a better
solvent than others such as Tetrahydrofuran (THF)/H2O (Fig. 1a),
N,N-Dimethylmethanamide (DMF)/H2O (Fig. 1c), and acetone/
water (Fig. 1d). In addition, a series of experiments have been
taken out to modify the quantities of oxidant and KBr. Finally,
the products (mixture of bromohydrins and dibromides) were iso-
lated up to 90% yields under the following conditions: 1 equiv of
olefin, 2 equiv of I2O5, 2.5 equiv of KBr, CH3CN/H2O (4:1), rt.
To examine the scope of this system, the hydroxybromination
and dibromination of various olefins under the typical reaction
conditions were studied (Table 1). Alkenes bearing electron-with-
drawing groups such as substituted chalcones and (E)-4-phenyl-
but-3-en-2-one gave a mixture of bromohydrins and dibromides
⇑
Corresponding author. Tel.: +86 931 8912280; fax: +86 931 8915557.
E-mail address: liuzhq@lzu.edu.cn (Z.-Q. Liu).
http://dx.doi.org/10.1016/j.tetlet.2014.03.064
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

Y. Wang et al. / Tetrahedron Letters 55 (2014) 2734–2737
2735
100
80
60
40
20
0
Table 1 (continued)
Entry Product A and yieldb
Product B and yieldb
A
B
C
D
Br
O
OH
O
8
9
Br
Br
(8a( ), 26%, 24 h)
(8b( ), 72%, 24 h)
O
OH
Trace
Br
O
(9b( ), 98%, 5 h)
0.0
0.5
1.0
1.5
2.0
time/h
2.5
3.0
3.5
4.0
OH
Br
10
11
Trace
Trace
Figure 1. Solvent effect of the reaction.
(10b, 68%, 7 h)
OH
Table 1
I2O5-mediated hydroxybromination and dibromination of olefinsa
Br
Br
OH
Ar2
O
Cl
O
Br
O
(11b, 76%, 7 h)
Ar1
+
Ar1
Ar2
Ar1
Ar
Ar2
OH
Br
2 eq. I2O5
Br
(
Br
K
+
(
)
)
R
H O
CH3CN/
RT
2
12
13
Trace
Trace
Br
2.5 eq.
R
1 eq.
(12b, 98%, 10 h)
Ar
OH
OH
(
)
Entry Product A and yieldb
Br
Product B and yieldb
OH
Br
O
O
(13b( ), 59%, 10 h)
OH
Ph
1
Br
(1b( ), 65%, 15 h)
OH
Br
(1a( ), 25%, 15 h)
Br
14
Trace
O
O
Br
(14b( ), 79%, 5 h)
O
2
Br
(2a( ), 46%, 20 h)
Br
Br
(2b( ), 43%, 20 h)
OH
Cl
F
Cl
Br
O
3
3
O
O
OH
Cl
15b
(15a, 35%, 22 h)
3
4
15
O
Br
Br
O
F
(3a( ), 41%, 23 h)
OH
(3b( ), 58%, 23 h)
O
Br
O
3
O
Br
O
OH
Br
Br
Cl
15b'
Cl
(15b/15b0, 33% (1.4/1), 22 h)
Br
Br
Br
O
Br
(4b( ), 50%, 24 h)
(4a( ), 50%, 24 h)
O
Br
3
3
O
Br
O
OH
OH
Br
Br
16b
5
6
Br
Br
(16a, 36%, 22 h)
16
O
Cl
O
Cl
O
O
(5b( ), 49%, 23 h)
OH
(5a( ), 50%, 23 h)
O
OH
Br
O
3
O
Br
O
Br
Br
16b'
Br
(16b/16b0, 31% (3.5/1), 22 h)
Br
Br
a
(6b( ), 75%, 10 h)
(6a( ), 21%, 10 h)
Reaction conditions: alkene (0.5 mmol, 1 equiv), KBr (1.25 mmol, 2.5 equiv),
I2O5 (1.0 mmol, 2 equiv), CH3CN/H2O (4/1, 10 mL), rt.
O
Br
O
OH
b
Isolated yield.
Br
(7a( ), 42%, 23 h)
7
O
Br
O
in very high overall isolated yields (Table 1, entries 1–8). However,
(E)-3-(4-methoxyphenyl)-1-phenylprop-2-en-1-one gave the cor-
responding bromohydrin as the only product in nearly quantitative
(7b( ), 57%, 23 h)

2736
Y. Wang et al. / Tetrahedron Letters 55 (2014) 2734–2737
O
Br
)
Br
MeO
Cl
(
5a
O
OH
)
Br
(
6b
Figure 2. X-ray analysis of 5a( ) and 6b( ).
O
O
Br
for the Central Universities (lzujbky-2013-197), the Specialized Re-
search Fund for the Doctoral Program of Higher Education
(20110211120015), the Gansu Province Natural Science Founda-
tion (1212RJZA005) and the scientific and technological project
of construction of Gansu Province (JK2010-67).
Br-
O
R1
R2
(
)
O
R1
R2
Br
I2O5
Br
Br
KBr
Br2
R1
R2
OH
R2
R1
(
)
H2O
Supplementary data
Scheme 2. Possible reaction mechanism.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.03.
064.
yields (Table 1, entry 9). Similarly, styrene and its derivatives all
gave moderate to very high yields of the corresponding bromohy-
drins as the major products (Table 1, entries 10–14). In addition,
linear aliphatic alkenes are also amenable to this new strategy (Ta-
ble 1, entries 15 and 16). However, a mixture of dibromides and
bromohydrins were obtained. In the cases of styrenes, the diaste-
reo- and regio-selectivities are extremely high and no syn-isomer
and regioselective isomer are observed in either bromohydrins or
dibromides, which can be seen from Table 1. All of the products
were carefully characterized by 1H and 13C NMR, MS, HRMS, and
X-ray (Fig. 2, X-ray analysis of 5a( ) and 6b( )). Although various
methods have been developed to prepare these compounds, the
features of transition-metal free system, ambient conditions, KBr
as the brominating reagent, and very high diastereo- and regio-
selectivities make this strategy attractive to organic synthesis.
The possible mechanism is depicted in Scheme 2. The elemental
bromide would be formed from KBr by oxidation of I2O5. Then
reaction of alkene with Br2 would generate the bromonium ion
intermediate,12 which is subsequently attacked by bromide anion
and water to give the corresponding dibromide and bromohydrin
(Scheme 2).
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In summary, we have developed an efficient I2O5-mediated
dibromination and bromohydroxylation of olefins by using KBr as
the brominating reagent in aqueous media at room temperature.
Various electron-deficient alkenes give a mixture of dibromides
and bromohydrins, and relatively electron-rich styrenes give bro-
mohydrins as the major products in high yields and extremely high
diastereo- and regio-selectivities. Although simple aliphatic al-
kenes give a mixture of dibromides and bromohydrins with mod-
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of KBr as the bromide source, and mild condition make this method
attractive. Further studies on this system are ongoing in our
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Acknowledgments
This project is supported by the National Science Foundation of
China (No. 21002045, 21272096), the Fundamental Research Funds

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