A green Hunsdiecker reaction: Synthesis of β-bromostyrenes from the reaction of α,β-unsaturated aromatic carboxylic acids with KBr and H2O2 catalysed by Na2MoO4·2H2O in aqueous medium

Article abstract of DOI:10.1039/b104540g

Reaction of α,β-unsaturated aromatic carboxylic acids with KBr and H₂O₂ in the presence of Na₂MoO₄·2H₂O in aqueous medium affords β-bromo alkenes in high yield.

Full text of DOI:10.1039/b104540g

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A green Hunsdiecker reaction: synthesis of b-bromostyrenes from the
reaction of a,b-unsaturated aromatic carboxylic acids with KBr and
H O catalysed by Na MoO ·2H O in aqueous medium
2
2
2
4
2
a
a
b
a
Joy Sinha, Suman Layek, Gagan C. Mandal and Manish Bhattacharjee*
a
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India.
E-mail: mxb@chem.iitkgp.ernet.in; b_manish@hotmail.com; Fax: +91-3222-82252
Department of Chemistry, Kharagpur College, Kharagpur 721 305, India
b
Received (in Cambridge, UK) 22nd May 2001, Accepted 16th August 2001
First published as an Advance Article on the web 13th September 2001
Reaction of a,b-unsaturated aromatic carboxylic acids with
KBr and H in the presence of Na MoO ·2H O in aqueous
medium affords b-bromo alkenes in high yield.
acid yield the corresponding b-bromo alkenes in 85 and 65%
yield respectively. It may be noted that for 3-(4-chlor-
ophenyl)prop-2-enoic acid a larger amount of Na MoO (acid–
2
O
2
2
4
2
2
4
Mo, 10+1) is required for better yield. The unsubstituted acid
also affords b-bromostyrene in good yield (65%). In the case of
an acid bearing substituents in 2-positions, for example,
3-(2-chlorophenyl)prop-2-enoic acid and 3-(2-methylphenyl)-
prop-2-enoic acid, the yield of the corresponding b-bromostyr-
ene is very poor (5 and 15%, respectively). The evolution of
The classical Hunsdiecker reaction and its later modifications
1
are efficient for the synthesis of organic halides. However,
these reactions have major limitations, such as toxicity and
hazard involving use of elemental bromine, salts of Hg(II), Tl(
I
),
Pb(IV), Ag(
I
). Apart from this, very poor yields of the products
are obtained in the cases of a,b-unsaturated aromatic carboxylic
acids. Recently, some of these difficulties have been overcome
by the use of lithium acetate as the catalyst and N-bromosuccini-
mide as the bromine source, the reaction medium being aqueous
2
CO was detected by passing the outcoming gas through
limewater. It may be noted that this is the first report of isolation
of Hunsdiecker products in water. All the earlier reports deal
with the reactions in non-aqueous medium.
2
acetonitrile. However, low yields were obtained in the cases of
The solution of Na
2
MoO
4 2
(0.001 mol) and KBr in H O (5
3
a,b-unsaturated aromatic carboxylic acids bearing electron-
cm ) is basic in nature (pH 10). Upon addition of solid acid, the
3
withdrawing groups in the aromatic ring.
2 2
pH of the slurry changes to 4.5. When H O (15 cm ) is added
Transition metal peroxo compounds have been shown to be
to the reaction mixture, the measured pH is 3.9. The pH of the
reaction changes to 9 upon completion of the reaction. It has
been shown by Rothenberg and Clark that a proton source is
required for oxyhalogenation.10 Thus in these reactions the
proton source is the acid itself.
3–6
efficient oxidising agents for various organic substrates.
Some marine organisms use a vanadium dependant enzyme,
bromoperoxidase, for the bromination of organic compounds
6
using inorganic bromides and hydrogen peroxide. We have
2
2
been investigating bromination of organic compounds using
It has been shown earlier that, MoO
peroxo species in solution in the presence of H
2 2
NMR spectra of a solution of Na MoO (0.001 M) in H O
4
forms a number of
9
5
vanadium(
V
) or molybdenum(VI), potassium bromide, and
2
O
2
. The Mo
(15
7–9
hydrogen peroxide in aqueous medium. It was observed that
2
4
3
the transition metal peroxo complexes can oxidise inorganic
bromide and produce Br
HOBr and Br
cm ) shows a major signal at 2336 ppm and a relatively low
intensity signal at 2509 ppm. These are due to the formation of
2
3
species which is in equilibrium with
22
22
11
2
.
[MoO(O
2
)
3
]
and [Mo(O
2
)
4
]
species, respectively. Upon
2
We thought that this biomimetic system could be utilised as
an alternative route to the Hunsdiecker reaction. Herein we
report the initial results of our studies on the reaction of a,b-
addition of Br both the signals at 2509 and 2336 ppm
disappear and two signals at 2222 and 237 ppm appear. The
2
2 11
signal at 2222 ppm can be assigned to [MoO(O
2
)
2
]
.
The
unsaturated aromatic carboxylic acids with KBr and H
2
O
2
in
signal at 237 ppm may be assigned to a molybdate species
95
aqueous medium in the presence of Na
When a suspension of a,b-unsaturated aromatic carboxylic
acid (0.02 mol) is stirred with KBr (0.04 mol) and Na
2
MoO
4
·2H
2
O.
coordinated to bromide. The slight shift in the observed Mo
NMR spectra from those of reported ones11 may be due to the
effect of pH as well as extent of hydration.
2
-
3
3
MoO
4
·2H
2
O (0.001 mol) in water (5 cm ) and H
2
O
2
(15 cm ,
A blank reaction, which is a similar reaction without the
addition of Na MoO , did not afford the brominated products.
2 4
0
.13 mol) is added drop-wise at room temperature (30 °C), a
rapid reaction takes place and the temperature rises to 80 °C
within 20 min. The reaction mixture was stirred for further 20
min and the product as well as the unreacted acid was extracted
with diethyl ether. The corresponding b-bromostyrene was
separated from the starting material by column chromatography
in good yield (Scheme 1). The yield of the styrene is very good
in the cases of a,b-unsaturated aromatic carboxylic acid bearing
a substitutent in the 4-position. For example, 3-(4-methox-
yphenyl)prop-2-enoic acid and 3-(4-chlorophenyl)prop-2-enoic
Thus, it is clear that bromide is oxidised by the peroxo-
molybdate species formed in solution.
2
3
The electronic spectrum of a solution (10 M) of Na
2
MoO
4
2
2
in H
nm (e = 1620 mol
2 2
O (2 3 10 M) shows bands at 445 (e = 162) and at 293
2
1
3
21
dm cm ). These are due to the
1
1
tetraperoxo and triperoxomolybdate species. When KBr is
added to this solution an absorbance increase is observed in the
region 700 to 290 nm. However, no new band appears. The
molar extinction coefficient of the 445 nm band increases to 320
and the band at 293 nm shifts to 298 nm with increase in the
molar extinction coefficient (e = 1833). It may be noted that the
2
2
oxidised bromide species, Br
2
, Br
3
and OBr absorb in the
1
2
region, 450 to 300 nm. Meister and Butler and our earlier
2
work on the oxidation of Br by H
2
O
2
in the presence of
9
Mo(VI) have shown that Mo(VI) first forms peroxomolybdate
2
2
which then oxidises Br to form an equilibrium mixture of Br ,
2
2
Br
3
and OBr . Thus it clear form the earlier work as well as
Scheme 1
from the present evidence that the peroxomolybdates generated
1916
Chem. Commun., 2001, 1916–1917
DOI: 10.1039/b104540g
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products in the cases of ortho substituted aromatic acids may be
due to steric crowding of the intermediate bromonium species.
The reactions described here show that the Hunsdiecker
reaction can be brought about in aqueous medium, contrary to
earlier reports, and also this is the greenest route to Hunsdiecker
products.
The authors would like to thank Dr Sujit Roy, Department of
Chemistry, Indian Institute of Technology, Kharagpur, India for
helpful discussions and suggestions.
Notes and references
1
2
R. G. Johnson and K. R. Ingham, Chem. Rev., 1956, 56, 219.
D. Naskar, S. Chowdhury and S. Roy, Tetrahedron Lett., 1998, 39,
6
99.
Scheme 2
3
4
5
H. Mimoun, M. Mignard, P. Brechot and L. Saussine, J. Am. Chem.
Soc., 1986, 108, 3711.
H. Mimoun, L. Saussine, E. Daire, M. Postal, J. Fischer and R. Weiss,
J. Am. Chem. Soc., 1983, 105, 3101.
M. Bhattacharjee, S. K. Chettri, M. K. Chaudhuri, N. S. Islam and S.
Roy Barman, J. Mol. Catal., 1993, 78, 143.
22
react with Br2 to
from the reaction of H
2
O
2
and MoO
4
+
produce the equivalent of the Br species, which then react with
the unsaturated acid to give bromo alkene.
In order to identify the brominating species, reactions of Br
2
2
in aqueous KOH (HOBr–OBr ) (pH = 10) as well as Br
2
in
2
3
) with the acid in aqueous medium were
6 J. Rudolph, K. L. Reddy, J. P. Chiang and K. B. Sharpless, J. Am. Chem.
Soc., 1997, 119, 6189.
7 M. Bhattacharjee, Polyhedron, 1992, 11, 2817.
8
aqueous KBr (Br
carried out. But in both the cases we could only isolate the
starting material. Thus, it is clear that the Mo center plays an
important role, and probably the coordination of the acid is
required for the conversion as observed in the case of vanadium
bromoperoxidase.13
M. Bhattacharjee, S. Ganguly and J. Mukherjee, J. Chem. Res., (S).,
1
995, 80.
9
M. Bhattacharjee and J. Mukherjee, J. Chem. Res., (S), 1995, 238.
1
1
0 G. Rothenberg and J. H. Clark, Green Chem., 2000, 2, 248.
1 V. Nardello, J. Marko, G. Vermeersch and J. M. Aubry, Inorg. Chem.,
From the above spectral evidence, it may be inferred that, the
reaction proceeds through an ionic mechanism (Scheme 2). It
may be noted that a similar mechanism has been suggested by
1
995, 34, 4950.
1
2 G. E. Meister and A. Butler, Inorg. Chem., 1994, 33, 3269.
13 R. A. Tschirret-Guth and A. Butler, J. Am. Chem. Soc., 1994, 116,
2
Roy et al. earlier. The reason behind the low yield of the
411.
Chem. Commun., 2001, 1916–1917
1917