In situ-generated zinc bromide-catalyzed α-bromination of alkanones in water

Article abstract of DOI:10.1080/00397910600770714

α-Bromination of alkanones with bromine catalyzed by in situ-generated zinc bromide from zinc dust and bromine in water is investigated. Bromination with dioxane-dibromide as a source of bromine does not require zinc dust and provides selectively α-mono and α,α-dibromo products in excellent yields when water is used as solvent. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600770714

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In Situ–Generated Zinc Bromide–Catalyzed
α‐Bromination of Alkanones in Water
Satinder K. Juneja a , Deepak Choudhary a , Satya Paul a & Rajive Gupta a
a Department of Chemistry, University of Jammu, Jammu, India
Version of record first published: 16 Feb 2007.
To cite this article: Satinder K. Juneja , Deepak Choudhary , Satya Paul & Rajive Gupta (2006): In
Situ–Generated Zinc Bromide–Catalyzed α‐Bromination of Alkanones in Water, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:19, 2877-2881
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Synthetic Communicationsw, 36: 2877–2881, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600770714
In Situ–Generated Zinc Bromide–Catalyzed
a-Bromination of Alkanones in Water
Satinder K. Juneja, Deepak Choudhary, Satya Paul,
and Rajive Gupta
Department of Chemistry, University of Jammu, Jammu, India
Abstract: a-Bromination of alkanones with bromine catalyzed by in situ–generated
zinc bromide from zinc dust and bromine in water is investigated. Bromination with
dioxane-dibromide as a source of bromine does not require zinc dust and provides
selectively a-mono and a,a-dibromo products in excellent yields when water is used
as solvent.
Keywords: Alkanones, a-bromination, a,a-dibromination, water, zinc bromide, zinc
dust
INTRODUCTION
The a-bromination of carbonyl compounds[1] is the step for introducing a
heteroatom,[2] generating a stabilized carbon radical or carbanion,[3] and
providing conjugation to carbonyl groups.[4] Various a-bromination
methods for carbonyl compounds[5] have been reported including the basic
method using Br2.[6] N-Bromosuccinimide (NBS) has also been reported as
an efficient and milder reagent for the a-bromination of ketones.[7]
Recently, we reported the a-bromination of alkanones using dioxane-
dibromide[8] and a hexamethylenetetramine-bromine complex[9] under
microwave irradiation.
The use of water as a solvent in organic chemistry was rediscovered
in the 1980s by Breslow,[10] who showed that hydrophobic effects can
strongly enhance the rate of several organic reactions. Previously, the scant
Received in India February 28, 2006
Address correspondence to Satya Paul, Department of Chemistry, University of
Jammu, Jammu 180 006, India. E-mail: paul7@rediffmail.com
2877

2878
S. K. Juneja et al.
solubility of the reactants was the main reason that ruled this solvent out
from studies. Further, reasons that make water unique among other solvents
are that it is cheap, not flammable and more importantly nontoxic. Further,
the workup procedure becomes easy because the products get separated out
upon cooling.
RESULTS AND DISCUSSION
[11]
In 1997, supported ZnBr2
was reported as a highly efficient catalyst for
catalyzing the ring bromination of aromatic substrates. We made our efforts
to generate ZnBr2 in situ from zinc (Zn) dust and bromine, and its catalyzing
action for a-bromination of alkanones was investigated. ZnBr2 in the presence
of water polarizes the Br-Br bond (Fig. 1) and hence a-bromination of
alkanones takes place efficiently. To investigate the polarization of Br-Br
bond by ZnBr2 in water, we carried out the reaction of p-chloroacetophenone
with bromine in water without using Zn dust at room temperature as well as at
708C. We found that monobromination takes place with 75% isolated yield
when stirred for 11 h at room temperature, whereas dibromination takes
place with 65% isolated yield at 708C for 5 h. Thus, quite a longer time is
required to complete the reaction and also lower yields were obtained as
compared to the reaction where Zn dust was used (Table 1, entry 2f
and 3f). This clearly indicates that Zn dust reacts with bromine to
produce ZnBr2, which catalyzes the reaction. Further, to finally confirm
the polarization of Br-Br bond by ZnBr2, we carried out the a-bromination
of p-chloroacetophenone with bromine using ZnBr2 and water as solvent
under the similar conditions where Zn dust was used and obtained similar
results.
To study the effects of the variation of amount of Zn dust and bromine
on the a-bromination of alkanones, we carried out series of reactions using
varying amounts of Zn dust and bromine, taking p-chloroacetophenone as an
alkanone, and noticed that for 5 mmol of p-chloroacetophenone, 2.5 mmol
of Zn dust, 7.5 mmol of bromine, and 5 mL of water, the reaction gave
maximum efficiency in terms of reaction yield and reaction time.
The reaction temperature was an important issue for selectivity. At room
temperature, selective a-bromoalkanones were obtained as the major
Figure 1. Polarization of Br-Br bond by ZnBr2 in combination with water.

In Situ–Generated ZnBr2
2879
Table 1. a-Bromination and a,a-dibromination of alkanones/cycloalkanones
(5 mmol) using bromine (7.5 mmol for 2 and 12.5 mmol for 3) and Zn dust
(2.5 mmol) in water (5 mL)
Producta (%)
R
R0
Time (h)
Yieldb
2a
2b
2c
2d
2e
2f
C6H5
H
H
H
H
H
H
H
H
0.75
1.75
0.5
0.25
0.1
4.5
2
93
90
80
94
88
97
90
79
89
84
88
89
87
88
75
77
80
93
88
77
75
74
4-MeC6H4
4-(MeO)C6H4
4-(HO)C6H4
2-(HO)C6H4
4-ClC6H4
4-BrC6H4
4-(NO2)C6H4
-(CH2)4-
2g
2h
2i
8
0.75
0.40
1
2j
-(CH2)5-
-(CH2)6-
2k
2l
-(CH2)7-
C6H5
1
3a
3b
3c
3d
3e
3f
H
H
H
H
H
H
H
H
3
4-MeC6H4
4-(MeO)C6H4
4-(HO)C6H4
2-(HO)C6H4
4-ClC6H4
4-BrC6H4
4-(NO2)C6H4
-(CH2)4-
1
0.5
1.5
1.25
2
3g
3h
3i
0.5
5
2.25
1.5
3j
-(CH2)5-
aFor products 2a–l, all reactions were carried out at room temperature, and for 3a–j,
all reactions were carried out at 708C.
bIsolated yield from three runs.
products; however, at 708C using 12.5 mmol of bromine, 2.5 mmol of Zn
dust, and 5 mL of water, a,a-dibromoalkanones were obtained as major
products.
To demonstrate the versatility of this method, a series of alkanones
substituted with electron-releasing and electron-withdrawing groups were
brominated. The results are summarized in Table 1. We then tried the
a-bromination of alkanones with dioxane-dibromide using water as solvent;
a-bromo and a,a-dibromoalkanones could be isolated by monitoring the
reaction temperature.

2880
S. K. Juneja et al.
EXPERIMENTAL
General
Melting points were determined on a Buchi melting-point apparatus and are
uncorrected. All reagents obtained from the chemical companies were used as
such. 1H NMR spectra were obtained on a Bruker DPX200 NMR spectrometer
(200 MHz) in CDCl3 using tetramethylsilane as an internal standard. The IR
spectra were recorded using KBr discs on a Hitachi 270-30 spectrophotometer.
The reactions were monitored by thin-layer chromatography (TLC).
General Procedure for the Synthesis of a-Bromo and
a,a-Dibromoalkanones using Zn Dust, Bromine, and Water
To a mixture of alkanone (5 mmol), Zn dust (2.5 mmol), and distilled water
(5 mL) in a round-bottomed flask (50 mL), bromine (7.5 mmol for 2 and
12.5 mmol for 3) was added. The reaction mixture was stirred at room temp-
erature (in the case of 2) and 708C (in the case of 3) for the appropriate time
(Table 1, monitored by TLC). On cooling, the solid obtained was filtered,
washed with water, and dried. Finally, it was purified by crystallization
from petroleum ether–ethyl acetate. In the case of cycloalkanones, the
2-bromoproducts were obtained by passing through a column of neutral
alumina and elution with petroleum ether.
General Procedure for the Synthesis of a-Bromo and
a,a-Dibromoalkanones using Dioxane-Dibromide and Water
To a mixture of alkanone (5 mmol) and distilled water (5 mL) in a round-
bottomed flask (50 mL), dioxane-dibromide (7.5 mmol in the case of 2 and
12.5 mmol in the case of 3) was added. The reaction mixture was stirred at
room temperature for 0.25–4 h (in the case of 2) and 708C for 0.5–5 h (in
the case of 3). The reaction was monitored by TLC. The products were
obtained by the same workup procedure as described previously.
1
The structure of the products was confirmed by H NMR, IR, and com-
parison with authentic samples obtained commercially or prepared by
reported methods.
ACKNOWLEDGMENT
The authors thank the Jammu and Kashmir State Council, Department of
Science and Technology, Jammu and Kashmir government, for providing
financial assistance and awarding a project fellowship to Satinder K. Juneja.

In Situ–Generated ZnBr2
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