Using the chemical tunability of ionic liquids to increase sustainability in the electrophilic bromination of unsaturated compounds

Article abstract of DOI:10.1055/s-0030-1260097

Tribromide-based ionic liquids have been synthesized and investigated as efficient bromination agents. The different physicochemical properties of the reaction media, which depend on the cation structure, have been used to improve the efficiency of the bromination process. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1260097

2
392
SPECIAL TOPIC
Using the ‘Chemical Tunability’ of Ionic Liquids to Increase Sustainability in
the Electrophilic Bromination of Unsaturated Compounds
I
C
onic
L
iquids in
t
i
he Elec
n
trophilic Bro
z
mination o
i
f
Alken
a
es Chiappe,* Angelo Sanzone
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, via Risorgimento 35, 56126 Pisa, Italy
Fax +30(50)2219669; E-mail: cinziac@farm.unipi.it.
Received 14 April 2011
Bromine addition to alkenes and alkynes in chlorinated
Abstract: Tribromide-based ionic liquids have been synthesized
and investigated as efficient bromination agents. The different
physicochemical properties of the reaction media, which depend on
solvents represents a fundamental reaction in organic
6
chemistry, reported in all basic textbooks, that gives the
the cation structure, have been used to improve the efficiency of the corresponding addition products in a short time and quan-
7
bromination process.
titative yields. Although the process is still employed for
the production of dibromides in small amounts on a labo-
ratory scale for research purposes, the drawbacks related
to the use of bromine and chlorinated solvents make the
reaction unsuitable for preparations on an industrial scale.
The hazardous nature of bromine means that special care
is required for its storage, transport, and handling, where-
as concerns for worker safety and the environment related
to the use of chlorinated solvents require the use of alter-
native solvents. Actually, the efficiency of the bromine
addition reaction is strictly related to the solvent: in sol-
vents of low polarity (such as hexane, ethers, etc.) radical
processes can ensue, leading to different product distribu-
Key words: electrophilic addition, ionic liquids, stereoselectivity,
bromination, green chemistry
Electrophilic bromination of double and triple bonds or
aromatic compounds is a fundamental reaction in organic
chemistry (Figure 1) providing products that are key inter-
mediates in fine chemicals, pharmaceutical production,
and analytical sciences. The mechanisms and synthetic as-
pects of bromination in molecular solvents have been ex-
tensively investigated over the years and fundamental
features have been discovered through kinetic and product
8
1
tions with respect to chlorinated solvents. On the other
distribution studies. The large number of papers pub-
hand, in more polar nucleophilic solvents (such as alco-
hols, water, MeCN) solvent incorporation products are
predominantly formed.9 Therefore, molecular bromine
addition in molecular solvents is an efficient procedure for
the preparation of dibromides but it is not easy to improve
the sustainability of the process using these media.
lished on this topic can be related to three main classes of
synthetic procedures: (1) direct reaction of unsaturated
,10
1
,2
compounds with molecular bromine; (2) use of carrying
3
agents that support bromine; and (3) in situ generation of
4
bromine using bromide compounds and oxidants. A com-
parison of many bromination techniques taking into ac-
count their resource demands, waste production, and However, many other protocols developed to circumvent
environmental and health and safety aspects has been re- the use of molecular bromine, including many oxidative
5
cently published.
processes, are often based on problematic substances or
they do not give same product distribution that arises from
molecular bromine addition in chlorinated solvents (di-
bromoalkanes, dibromoalkenes, bromoarenes).5
Br
Br
Alternative reagents able to give exclusively dibromo ad-
ducts from alkenes and alkynes with a stereoselectivity
comparable or higher than molecular bromine are tribro-
mide organic salts. These carrying agents of bromine,
however, due to the low stability of the anion in protic me-
dia are traditionally used in chlorinated solvents or in ace-
tonitrile. The advantages arising from the easier handling
and storage of these reagents with respect bromine have
new limits for large-scale applications in the low-atom ef-
ficiency of the processes, the use of toxic solvents, and the
need to remove the residue from the reagent through te-
dious washing procedures.
R
Br
brominating
systems
Br
Br
O
R
CH2Br
Figure 1 Products arising from the bromination of unsaturated
compounds
Recently, it has been shown that it is possible to use ionic
liquids, salts with melting points below 100 °C, as alter-
native solvents in electrophilic halogenations using both
SYNTHESIS 2011, No. 15, pp 2392–2396
x
x.
x
x
.
2
0
1
1
molecular halogens (Br , BrCl, ICl, or IBr) or trihalide
Advanced online publication: 08.07.2011
DOI: 10.1055/s-0030-1260097; Art ID: C39511SS
Georg Thieme Verlag Stuttgart · New York
2
–
3
–
–
2
– 11–15
salts (Br , BrCl , ICl IBr ).
In ionic liquids, as well
2
2
©

SPECIAL TOPIC
Ionic Liquids in the Electrophilic Bromination of Alkenes
2393
as in aprotic molecular solvents, trihalide anions are stable
and halogenation of double and triple bonds occurs
through attack of the proper anion on the initially formed
halogen–unsaturated compound charge-transfer complex
Br
Br
1
a
2a
2b
(
Scheme 1), practically without the involvement of an
Br
H
ionic intermediate; only the anti-addition products are,
therefore, formed both with alkyl- or aryl-substituted al-
Br
1
1–14
kenes and alkynes.
1
b
Y
X
X
XY2–
+
Y–
Br
+
H
Y
Y–
H
Br
Scheme 1 Electrophilic addition of trihalides to alkenes
2c
+
H
H
However, the replacement of a molecular solvent with an
ionic liquid, despite the improved physicochemical and
solvent properties of these media (low vapor pressure, not
flammable, high solvent power, high chemical stability),16
is not necessarily able to increase the sustainability of the
process or to give significant improvements to the proce-
dure.
1
c
Br
Br
H
H
3
c
Scheme 2 Bromination of various alkenes
Here, we report the possibility of increasing the sustain-
ability of the electrophilic bromination of unsaturated
compounds using selected tribromide-based ionic liquids
as carrying agents and media. Furthermore, we show that
the choice of the ionic liquid cation may favor the recov-
ery of the products as determined by the physicochemical
properties of the system. The optimal conditions for three
representative compounds cyclohexene (1a), styrene (1b),
and cis-stilbene (1c) giving, respectively, a liquid product,
a solid compound soluble in common molecular solvents,
and a solid compound insoluble in most of the widely used
organic solvents, have been examined taking into account
waste production and as well as environmental and health
and safety aspects (Scheme 2).
dazolium bromide ([emim]Br), 1-butyl-3-methyl-
imidazolium bromide ([bmim]Br), 1-octyl-3-methyl-
imidazolium bromide ([omim]Br), and N-butyl-N-meth-
ylpyrrolidinium bromide ([bmpyr]Br) (Figure 2); with the
exception of [omim]Br all the other bromides were solid
1
7,18
at room temperature. As expected,
of the Br anion into the more charge-diffused [Br ] anion
the transformation
–
–
3
resulted in a decrease in water affinity (all five tribromide
salts can be included in the class of water immiscible ionic
liquids) and a significant reduction of the melting point,
the sole exception being [omim][Br ]. More in particular,
3
[
bmim][Br ] and [bmpyr][Br ] appeared as low viscous
3 3
liquids at room temperature and below, [omim][Br ] as a
3
liquid–low melting solid (mp ~28–30 °C) and
cis-Stilbene (1c) was selected since it allows evaluation of
the stereoselectivity of the process; mixtures of anti and
syn adducts (dibromides 2c and 3c) are generally obtained
starting from this olefin in the absence of strong nucleo-
philes (halide anions or nucleophilic solvents).
[
(
emim][Br ] and [mim][Br ] as crystalline compounds
mp 49 and 70 °C, respectively).
3 3
H
R
N
N
X
X
X
In previous works carried out in this laboratory, 1-methyl-
N
N
N
1
7
Me
Bu
3
-butylimidazolium tribromide ([bmim][Br ]) was used
Me
Me
R = Et
3
as a brominating agent both in hydrophilic or hydrophobic
ionic liquids. In all the investigated reactions the products
X = Br; [mim]Br
X = Br3; [mim][Br3]
X = Br; [bmpyr]Br
X = Br3; [bmpyr][Br3]
X = Br; [emim]Br
X = Br3; [emim][Br3]
R = Bu
1
1–14
were recovered
by extraction with diethyl ether. Al-
X = Br; [bmim]Br
X = Br ; [bmim][Br ]
though it was possible to recycle the reaction system, rel-
evant amounts of waste products were generated for each
3
3
R = C8H17
X = Br; [omim]Br
5
kilogram of dibromo adduct.
X = Br3; [omim][Br3]
Considering that immobilization of bromine in a halide
based ionic liquid can give a halogen carrier with different
physicochemical properties depending on the cation, we
have prepared a series of tribromide-based ionic liquids
by bromine addition to selected bromide salts, 1-meth-
ylimidazolium bromide ([mim]Br), 1-ethyl-3-methylimi-
Figure 2 Investigated ionic liquids in bromination reactions
However, it should be noted that the cation structure af-
fects not the only physicochemical properties of these
salts but also their environmental impact. In the case of
19
bromide salts, it has been shown that toxicity towards
Synthesis 2011, No. 15, 2392–2396 © Thieme Stuttgart · New York

2
394
C. Chiappe, A. Sanzone
SPECIAL TOPIC
the aquatic environment increases on going from pyrroli- Table 1 Bromination of Alkenes 1a–c with Tribromide-Based Ionic
dinium to imidazolium salts and, in the case of these latter
compounds, toxicity increases on increasing the alkyl
chain length; the environmental impact of [bmpyr]Br and
Liquids
Alkene Ionic liquid
Product
Yield (%)
98a
Selectivity (%)
[
mim]Br is therefore lower than that of all the other salts 1a
[mim][Br3]
[emim][Br
2a
98
99
99
99
99
a
here investigated.
3
]
93
92
93
a
[
[
bmim][Br3]
omim][Br3]
To test the efficiency of the selected tribromide-based ion-
ic liquids, the reactions were carried out at room tempera-
ture or at the lowest temperature at which the systems
were liquid (ranging from 40 to 70 °C) by addition of the
three target alkenes to an equimolar amount of the ionic
liquid with stirring (Table 1). The alkene addition always
resulted in the rapid disappearance of the color and the
formation of a solid phase due to the transformation of the
tribromide salt into the corresponding bromide or to the
formation of a solid dibromo adduct or to both phenome-
na. When the reaction product was a liquid, as in the case
of cyclohexene, the formation of a biphasic system at
room temperature allowed an easy and efficient separa-
tion of the reaction product from the formed bromide
salts. The latter could be eventually washed with the min-
imal amount of a nonpolar solvent (toluene or hexane) and
recycled after transformation into the corresponding tri-
bromides. The workup procedure for alkenes giving a sol-
id product was more complicated and more medium-
dependent. In this case the most effective ionic liquid
a
a
[bmpyr][Br3]
97
b
c
1
1
b
c
[mim][Br3]
2b
2c
92, 95
93
95
94
98
99
99
99
99
b
[emim][Br3]
[bmim][Br3]
[omim][Br3]
b
c
[bmpyr][Br3]
98
[mim][Br3]
90c
98
99
99
99
99
c
[emim][Br3]
[bmim][Br3]
[omim][Br3]
92
90
90
98
c
c
c
[bmpyr][Br3]
a
b
c
Workup procedure A.
Workup procedure B.
Workup procedure C.
tions practically under solvent-free conditions; products
can be recovered by simple phase separation or by filtra-
tion after addition of a very small amount of ethanol or
water. The exhausted ionic liquids can be easily recycled
reducing the problems of the low-atom economy related
to the application of bromine carriers. Among the investi-
gated ionic liquids, which cannot be considered exhaus-
[
bmpyr][Br ] gave a semi-solid mixture that allowed effi-
3
cient stirring until the end of the process, even at room
temperature. Then, the solid dibromides could be isolated
from the reaction mixture by extraction with diethyl ether
or by filtration after dissolution of the thus formed
[
(
suitable for dibromides, such as those arising from stil-
benes, which have very low solubility in organic solvents
or water. The solid dibromides, recovered in quantitative
yield, were pure by NMR analysis. On the other hand, af-
ter solvent removal the bromide salts could be converted
into the corresponding tribromides and reused repeatedly.
tive, the best systems appeared to be [mim][Br ] and
3
[
bmpyr][Br ]; the first is associated with relatively low
3
bmpyr]Br in a minimal amount of water or alcohol
MeOH or EtOH). This latter procedure is particularly
toxicity towards the aquatic environment and extremely
easy procedure of synthesis (it is obtained by neutraliza-
tion of N-methylimidazole by HBr in aqueous medium),
the second is associated with relatively low environmental
impact and it has the properties of a reagent liquid at room
temperature (i.e. a more efficient stirring of the reaction
mixture even at r.t., a higher solvent power). Finally, it is
to note that these ionic liquids can be used for several oth-
1
4
NMR and GC-MS analysis of the reaction products con- er organic transformations previously investigated: aro-
firmed that the reaction occurs in all investigated ionic liq- matic bromination and carbonyl functionalization.
uids in a stereospecific way, giving exclusively the anti-
addition products. Nevertheless, should be noted that the
expected dibromo adducts were practically the sole reac- trometer. GC-MS were performed with a gas-chromatograph
1H and 13C NMR spectra were recorded on a Bruker 250 MHz spec-
tion products also when the protic [mim][Br ] was used as equipped with a DB-5 capillary column (30 m × 0.25 mm; coating
3
thickness 0.25mm) and an ion trap mass detector. Analytical condi-
brominating agent: only traces (<2%) of the HBr addition
tions: injector and transfer line temperatures 220 °C and 240 °C, re-
product were detected in the GC-MS chromatogram of the
spectively; oven temperature programmed from 60 °C to 240 °C at
reaction product arising from bromination of styrene in
3
°C min; carrier gas He at 1 mL/min; injection of 0.2 mL (10% hex-
this medium.
ane soln); split ratio 1:30. Identification of the constituents was
In conclusion, considering that tribromide salts are one of based on comparison of the retention times with those of authentic
samples, and matching against commercial (NIST 98 and ADAMS)
the most efficient systems to obtain dibromo derivatives
and home-made mass spectra library built up from pure substances
with high stereo- and chemoselectivity, these data show
and MS literature data. 1-Methylimidazolium bromide ([mim]Br),
that it is possible to use the ‘chemical tunability’ of ionic
1
-ethyl-3-methylimidazolium bromide ([emim]Br), 1-butyl-3-me-
liquids to modulate the properties of the system and to in-
thylimidazolium bromide ([bmim]Br), 1-octyl-3-methylimidazoli-
crease the sustainability of the process. Using tribromide- um bromide ([omim]Br), and N-butyl-N-methylpyrrolidinium
based ionic liquids, it is possible to carry out these reac- bromide ([bmpyr]Br) were prepared following literature proce-
1
2–14
dures.
Synthesis 2011, No. 15, 2392–2396 © Thieme Stuttgart · New York

SPECIAL TOPIC
Ionic Liquids in the Electrophilic Bromination of Alkenes
2395
Synthesis of Tribromide-Based Ionic Liquids
To an appropriate amount of the starting ionic liquid (bromide or
Recycling Procedure
After product recovery, traces of organic solvents or water were
eliminated from the collected ionic liquids at reduce pressure and
the salt was reused after transformation into the corresponding tri-
chloride) an equimolar amount of Br was added slowly under stir-
2
ring. Attention was paid to avoid an excess of halogen which, at
2
0
least in the case of Br , could be eventually removed under vacuum.
bromide-based ionic liquids.
2
1
-Methylimidazolium Tribromide [mim][Br3]
1
Acknowledgment
H NMR (DMSO-d ): d = 3.87 (s, 3 H, CH ), 7.66 (s, 1 H, CH), 7.68
6
3
(
s, 1 H, CH), 9.04 (s, 1 H, CH).
We thank University of Pisa for the financial support.
1
3
C NMR (DMSO-d ): d = 35.6 (CH ), 119.8 (CH), 123.2 (CH),
6
3
1
35.9 (CH).
References
1
-Ethyl-3-methylimidazolium Tribromide [emim][Br3]
(
1) (a) Brown, R. S. Acc. Chem. Res. 1997, 30, 131.
1
H NMR (DMSO-d ): d = 1.41 (t, J = 7.5 Hz, 3 H, CH ), 3.83 (s, 3
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6
3
(
(
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2
1
H, CH), 9.07 (s, 1 H, CH).
(
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C NMR (DMSO-d ): d = 15.2 (CH ), 35.8 (CH ), 44.3 (CH ),
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6
3
3
2
1
12.0 (CH), 123.6 (CH), 136.3 (CH).
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-Butyl-3-methylimidazolium Tribromide [bmim][Br3]
1
H NMR (DMSO-d ): d = 0.89 (t, J = 7.5 Hz, 3 H, CH ), 1.25 (sext,
J = 7.5 Hz, 2 H, CH ), 1.76 (quint, J = 7.5 Hz, 2 H, CH ), 3.84 (s, 3
6
3
2
2
H, CH ), 4.15 (t, J = 7.5 Hz, 2 H, CH ), 7.67 (s, 1 H, CH), 7.74 (s,
1
3
2
H, CH), 9.08 (s, 1 H, CH).
1
3
C NMR (DMSO-d ): d = 13.4 (CH ), 15.9 (CH ), 31.4 (CH ), 35.9
6
3
2
2
(
g) Djerassi, C.; Scholz, C. R. J. Am. Chem. Soc. 1948, 70,
(
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3
2, 193.
1
-Octyl-3-methylimidazolium Tribromide [omim][Br3]
1
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H, CH ), 1.76 (m, 2 H, CH ), 3.84 (s, 3 H, CH ), 4.13 (t, J = 7.3 Hz,
2
6
3
2
2
3
H, CH ), 7.67 (s, 1 H, CH), 7.74 (s, 1 H, CH), 9.07 (s, 1 H, CH).
2
1
3
C NMR (DMSO-d ): d = 14.1 (CH ), 22.2 (CH ), 25.7 (CH ), 28.5
6
3
2
2
(
CH ), 28.6 (CH ), 29.6 (CH ), 31.3 (CH ), 35.9 (CH ), 48.9 (CH ),
2 2 2 2 3 2
1
12.4 (CH), 123.7 (CH), 136.5 (CH).
2
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3
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H NMR (DMSO-d ): d = 0.92 (t, J = 7.2 Hz, 3 H, CH ), 1.31 (m, 2
6
3
H, CH ), 1.67 (m, 2 H, CH ), 2.08 (m, 4 H, 2 CH ), 2.97 (s, 3 H,
2
2
2
CH ), 3.30 (m, 2 H, CH ), 3.45 (m, 4 H, 2 CH ).
3
2
2
1
3
C NMR (DMSO-d ): d = 13.7 (CH ), 19.4 (CH ), 21.2 (CH ), 25.9
6
3
2
2
(
CH ), 47.6 (CH ), 63.1 (CH ), 63.6 (CH ).
2 3 3 2
(
l) Safaiee, M. Synlett 2006, 2513. (m) Stavber, S.; Zupan,
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To a round-bottom flask equipped with a magnetic stirrer and con-
taining the halogenating ionic liquid (3–4 g, ca. 10 mmol) (eventu-
ally warmed to the minimal temperature to obtain a liquid system)
was added the unsaturated compound (1 equiv). The mixture was
stirred at the same temperature until disappearance of the color and
subjected to the most suitable workup procedure.
(
7) (a) Bellucci, G.; Chiappe, C.; Lo Moro, G. J. Org. Chem.
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Workup A: formation of a liquid product: After cooling to r.t. the
liquid product was separated by decantation and the residue was
washed with a small amount of toluene (1 mL). The collected prod-
uct was analyzed by GC-MS.
1
1
(
Workup B: formation of a solid product: After cooling to r.t., the re-
action product was extracted with Et O (3 × 3 mL). The combined
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2
extracts were analyzed by GC-MS and by NMR.
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1
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Workup C: formation of a solid product: After cooling to r.t., a pro-
(
(
tic solvent (0.5 mL) was added (EtOH or H O) and the insoluble re-
2
5
action product was separated by filtration through a sintered funnel.
The product was analyzed by NMR.
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GC-MS and NMR spectra of compounds 2a–c were identical to
those reported in the literature.
Synthesis 2011, No. 15, 2392–2396 © Thieme Stuttgart · New York

2
396
C. Chiappe, A. Sanzone
SPECIAL TOPIC
(
12) Bortolini, O.; Bottai, M.; Chiappe, C.; Conte, V.; Pieraccini,
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(20) In this work tribromide based ionic liquids have been
prepared by bromine addition to the corresponding
bromides, however, it is possible to obtain this
D. Green Chem. 2002, 4, 621.
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