Ionic liquid-promoted stereoselective synthesis of (Z)-vinyl bromides by [bmIm]OH under organic solvent-free conditions: A green approach

Article abstract of DOI:10.1080/00397910701471386

A simple and easily accessible ionic liquid, 1-butyl-3-methylimidazolium hydroxide ([bmIm]OH), has been demonstrated as an efficient promoter and reaction medium for the highly stereoselective synthesis of (Z)-vinyl bromides by the debrominative decraboxylation of dibrominated α,β-unsaturated carboxylic acids in high yields. The reaction does not require either a conventional base or an organic solvent. Copyright Taylor & Francis Group, LLC.

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Ionic Liquid–Promoted Stereoselective
Synthesis of (Z)‐Vinyl Bromides by [bmIm]OH
under Organic Solvent–Free Conditions: A
Green Approach
Brindaban C. Ranu a , Subhash Banerjee a & Jhinuk Gupta a
a Department of Organic Chemistry , Indian Association for the Cultivation
of Science , Jadavpur, Kolkata, India
Published online: 30 Aug 2007.
To cite this article: Brindaban C. Ranu , Subhash Banerjee & Jhinuk Gupta (2007) Ionic Liquid–Promoted
Stereoselective Synthesis of (Z)‐Vinyl Bromides by [bmIm]OH under Organic Solvent–Free Conditions: A Green
Approach, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 37:17, 2869-2876, DOI: 10.1080/00397910701471386
To link to this article: http://dx.doi.org/10.1080/00397910701471386
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Synthetic Communicationsw, 37: 2869–2876, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701471386
Ionic Liquid–Promoted Stereoselective
Synthesis of (Z)-Vinyl Bromides by
[bmIm]OH under Organic Solvent–Free
Conditions: A Green Approach
Brindaban C. Ranu, Subhash Banerjee, and Jhinuk Gupta
Department of Organic Chemistry, Indian Association for the Cultivation
of Science, Jadavpur, Kolkata, India
Abstract: A simple and easily accessible ionic liquid, 1-butyl-3-methylimidazolium
hydroxide ([bmIm]OH), has been demonstrated as an efficient promoter and reaction
medium for the highly stereoselective synthesis of (Z)-vinyl bromides by the debromi-
native decraboxylation of dibrominated a,b-unsaturated carboxylic acids in high
yields. The reaction does not require either a conventional base or an organic solvent.
Keywords: debrominative decarboxylation, green synthesis, ionic liquid, (Z)-vinyl
bromides
INTRODUCTION
The vinyl bromides are very useful intermediates in organic synthesis, and
thus development of procedures for their stereoselective synthesis is
important. One of the classical methods involves Wittig olefination of an
aldehyde with bromomethylene triphenylphosphorane,[1] leading to
primarily (Z)-vinyl bromides. However, this procedure is not very convenient
as it entails a large amount of triphenyl phosphine oxides as waste, and in
addition, preparation of the ylide is rather complicated. Thus, several
methods have been developed for this transformation.[2] The most common
Received in the USA January 19, 2007
Address correspondence to Brindaban C. Ranu, Department of Organic Chemistry,
Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India.
E-mail: ocbcr@iacs.res.in
2869

2870
B. C. Ranu, S. Banerjee, and J. Gupta
methods include metal-halogen exchange of metalloalkenes,[2a] hydroboration
of haloalkynes,[2b] reduction of gem-dibromides,[2c,2f] and debrominative dec-
arboxylation of dibrominated a,b-unsaturated carboxylic acids using a variety
of bases in different organic solvents.[2g] However, several of these methods
produce a mixture of (E)- and (Z)- bromoalkenes, and isolation of one pure
isomer is often very tedious. Thus, development of a simpler highly stereo-
selective method in an environment-friendly reaction condition is appreciated.
During past few years we have been actively engaged in exploring ionic
liquids[3] as effective catalysts and regents for useful chemical transformations
in organic solvent free conditions.[4] As a part of this activity, we recently
introduced a task-specific ionic liquid [bmIm]OH for Michael addition[4h]
and Knoevenagel condensation[4l] and report here another novel application
of this ionic liquid for debrominative decarboxylation of dibrominated a,b-
unsaturated carboxylic acid to (Z)-vinyl bromides (Scheme 1).
RESULTS AND DISCUSSION
The experimental procedure is very simple. A mixture of 2,3-dibromoalkanoic
acid and [bmIm]OH was stirred at room temperature for a certain period of
time (monitored by thin-layer chromatography, TLC). The product was then
isolated by direct distillation from the reaction vessel under vacuum. The
product obtained was of high purity, and no further purification was necessary.
A variety of 2,3-dibromoalkanoic acids underwent debrominative decar-
boxylations to produce the corresponding (Z)-alkenyl bromides in high yields.
The results are summarized in Table 1. Both aromatic and aliphatic dibro-
moalkanoic acids were responsive to this reaction. The aromatic substrates
bearing electron-withdrawing as well as electron-donating groups underwent
reactions without any difficulty. As (Z) alkenyl bromides were produced in
very high proportion from all substrates except p methoxy-substituted
cinnamic acid dibromide where a (3:1) mixture of (Z)- and (E)- alkenyl
bromides were obtained (entry 4), it is speculated that most of the reactions
proceed via trans-b-elimination followed by simultaneous loss of CO2 and
Br ions to give (Z)-alkenyl bromide (Scheme 2). However, in the case of
p-methoxy-substituted substrate, an unimolecular elimination process may
Scheme 1.

Table 1. Synthesis of (Z)-vinyl btomides by [bmlm]OH
Entry
1
R
Product(s)
Time (h)
1.0
Yield (%)a
90
(Z/E) ratiob
100/0
Ref.
2g
2
3
1.5
1.5
88
90
99/1
2g
—
100/0
4
5
2.0
1.5
85
82
75/25
99/1
2g
—
(continued)

Table 1. Continued
Entry
6
R
Product(s)
Time (h)
1.5
Yield (%)a
86
(Z/E) ratiob
98/2
Ref.
2g
7
1.5
88
97/3
2g
8
9
2.0
1.5
1.0
85
80
85
98/2
99/1
2g
2g
2g
10
C7H15
100/0
1
aYields refer to those of pure isolated products characterized by spectroscopic (IR, H NMR, and 13C NMR) data.
1
bThe ratio was determined by H NMR.

Stereoselective Synthesis of (Z)-Vinyl Bromides
2873
Scheme 2.
Scheme 3.
be involved to give relatively stable carbocations, followed by elimination of
CO2 to give a mixture of (Z)- and (E)-isomers (Scheme 3).
In general, the reactions are very clean and reasonably fast. The reaction
conditions are mild to tolerate a variety of functional groups such as Cl, Br,
OMe, NO2, and OCH2O- present in the substrates. The (Z)-isomers were
1
easily identified from the coupling constant (J ¼ 7.9–8.4 Hz) in H NMR
spectra of these compounds. The ionic liquid works here as promoter as well
as reaction medium, and thus no additives or organic solvents are required.
CONCLUSION
In conclusion, the present procedure using an easily accessible basic ionic
liquid, [bmIm]OH,[4h] as an effective promoter, and reaction medium
provides an efficient methodology for the synthesis of (Z)-vinyl bromides.
The significant advantages offered by this method are a) operational simplicity,
b) remarkable stereoselectivity, and c) environmentally friendly procedure that
avoids toxic catalyst and hazardous organic solvent. We are not aware of any
such use of ionic liquid for the debrominative decarboxylation reaction.
Moreover, this procedure demonstrates the potential of ionic liquids and
provides great promise for further useful applications in organic synthesis.

2874
B. C. Ranu, S. Banerjee, and J. Gupta
EXPERIMENTAL
General
1H NMR spectra were recorded at 300 MHz in CDCl3 solutions with
trimethylsilane as internal standard. 13C NMR spectra were run at 75 MHz
in CDCl3 solutions. IR spectra were taken as neat for liquids and as KBr
pellets for solids on a Shimadzu FT-8300 spectrometer. Elemental analyses
were done by a Perkin-Elmer autoanalyzer. Column chromatography was
performed on silica gel (60–120 mesh, SRL, India).
General Procedure for the Debrominative Decarboxylation of
Dibrominated a,b-Unsaturated Carboxylic acids: Preparation of
(Z)-b-Bromostyrene (Entry 1, Table 1)
A mixture of anti-3-phenyl-2,3-dibromopropionic acid (915 mg, 3 mmol) and
[bmIm]OH (312 mg, 2 mmol) was stirred for 1 h at room temperature until
completion of the reaction (TLC). The product was then directly distilled
from the reaction vessel under vacuum to provide a colorless liquid
(516 mg, 90%), which was identified as (Z) b-bromostyrene because its spec-
troscopic data (IR, 1H NMR, and 13C NMR) are in good agreement with those
reported.[2g] This procedure is followed for all reactions listed in Table 1. Most
of the products are known compounds except two (entries 3 and 5). The known
compounds were easily identified by comparison of their spectroscopic data
with the reported values.[2g] The unknown compounds were characterized
by their spectroscopic data and elemental analysis.
Data
1-Bromo-3-(2-bromovinyl)benzene (Entry 3, Table 1)
Colorless oil; IR (neat): 1640, 1450, 960 cm21; 1H NMR (300 MHz, CDCl3)
d 6.48 (d, J ¼ 8.1 Hz, 1H), 6.99 (d, J ¼ 8.1 Hz, 1H), 7.17–7.23 (m, 1H),
7.43–7.46 (m, 1H), 7.58–7.62 (m, 1H), 7.81 (s, 1H); 13C NMR (75 MHz,
CDCl3) d 107.8, 122.1, 127.4, 129.6, 130.9, 131.1, 136.7. Anal. calcd. for
C8H6Br2: C, 36.68; H, 2.31. Found: C, 36.45; H, 2.10.
1-(2-Bromovinyl)-3-methoxybenzene (Entry 5, Table 1)
1
Colorless liquid; IR (neat): 1645, 1451, 940 cm21; H NMR d 3.82 (s, 3H),
6.57 (d, J ¼ 8.4 Hz, 1H), 6.75–6.79 (m, 2H), 7.18 (d, J ¼ 8.4 Hz, 1H),
7.37–7.38 (m, 1H), 7.46–7.49 (m, 1H); 13C NMR d 55.1, 106.4, 114.2,

Stereoselective Synthesis of (Z)-Vinyl Bromides
2875
121.5, 129.1, 132.1, 136.0, 159.2. Anal. calcd. for C9H9BrO: C, 50.73, H,
4.26. Found: C, 50.54; H, 4.05.
If the reaction was carried out in a smaller scale (,1 mmol) or a solid
product was formed, the product can be isolated by extraction with ethyl
acetate followed by purification over column chromatography.
ACKNOWLEDGMENTS
This investigation has enjoyed financial support from Council of Scientific and
Industrial Research (CSIR), New Delhi [Grant No. 01(1936)/04]. S. B. also
thanks CSIR for this fellowship.
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