Green synthesis of tetraalkylammonium tribromide using cerium(IV) ammonium nitrate (CAN) as oxidant

Article abstract of DOI:10.1080/00397910601163794

Cerium(IV) ammonium nitrate (CAN) mediates oxidation of tetraalkylammonium bromide to tetraalkylammonium tribromide in greener reaction conditions using water as solvent. The procedure is simple, convenient, and environmentally benign for the synthesis of tetraalkylammonium tribromide with great yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910601163794

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Green Synthesis of Tetraalkylammonium
Tribromide using Cerium(IV) Ammonium
Nitrate (CAN) as Oxidant
a
a
Ruli Borah & Ashim Jyoti Thakur
a
Chemical Sciences Department, Tezpur University, Tezpur‐Assam, India
Version of record first published: 24 Feb 2007.
To cite this article: Ruli Borah & Ashim Jyoti Thakur (2007): Green Synthesis of Tetraalkylammonium
Tribromide using Cerium(IV) Ammonium Nitrate (CAN) as Oxidant, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 37:6, 933-939
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Synthetic Communications , 37: 933–939, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910601163794
Green Synthesis of Tetraalkylammonium
Tribromide using Cerium(IV) Ammonium
Nitrate (CAN) as Oxidant
Ruli Borah and Ashim Jyoti Thakur
Chemical Sciences Department, Tezpur University, Tezpur-Assam, India
Abstract: Cerium(IV) ammonium nitrate (CAN) mediates oxidation of tetraalkyl-
ammonium bromide to tetraalkylammonium tribromide in greener reaction conditions
using water as solvent. The procedure is simple, convenient, and environmentally
benign for the synthesis of tetraalkylammonium tribromide with great yields.
Keywords: Bromination, CAN, oxidative, tetraalkylammonium tribromide
Organic ammonium tribromides (OTABs) are useful reagents in organic
synthesis for various transformations,
[1]
especially as mild brominating
agents. The bromination of organic substrates is a very important reaction
for the preparation of synthetic intermediates and pharmaceutical and agricul-
[
2]
tural chemicals.
The general method used to prepare a brominated
[3]
compound has been the use of elemental bromine. However, the bromina-
tion reaction has been associated with environmental hazards with respect
to transport, handling, and particularly storage of bromine. Several crystalline
tetraalkylammonium tribromides constitute a convenient source of bromine
because of their safe handling. These include pyridine hydrobromide
[
4]
[5]
perbromide (PyHBr3),
methyl ammonium tribromide, 1,8-diazabicyclo [5.4.0] tetrabutylammo-
tetramethylammonium tribromide,
6]
phenyltri-
[
[
7]
[8]
nium tribromide
(DBUHBr ), tetrabutylammonium tribromide,
3
and
Received October 7, 2006
Address correspondence to Ruli Borah, Chemical Sciences Department, Tezpur
University, Napaam 784028, Tezpur-Assam, India. E-mail: ruli@tezu.ernet.in
9
33

9
34
R. Borah and A. J. Thakur
cetyltrimethylammonium tribromide. The main drawbacks of these reagents
are their preparation methods using elemental bromine and HBr in some
cases, which lead to heavy environmental pollution. Therefore, development
of an efficient synthetic route for the synthesis of tetraalkylammonium tribro-
[9]
mide would be helpful. Chaudhuri et al. reported an environmentally benign
synthetic protocol for tetraalkylammonium tribromide using vanadium
pentoxide and 30% aqueous hydrogen peroxide as oxidant. In this communi-
cation, a new green synthetic method of tetraalkylammonium tribromide is
reported using CAN as oxidant and water as solvent.
The idea for this work came in continuation of our research on oxidative
10]
[
bromination
of aromatic substrates involving m-diperoxo-vanadium
complexes in aqueous acetonitrile medium. Cerium(IV) ammonium nitrate
is a powerful one-electron oxidant for a variety of substrates and is useful
[11,12]
for various synthetic transformations.
The experimental simplicity,
ease of handling, and nontoxicity combined with its solubility in organic
solvents such as MeOH and MeCN have made CAN attractive in organic
[
13]
synthesis. The mechanism
may proceed through one-electron transfer
with initial formation of a bromine radical to bromine in an aqueous
solution, followed by an another reaction to form the tribromide anion, as
shown in Scheme 1.
SYNTHESIS OF TETRAALKYLAMMONIUM TRIBROMIDE
The brominating agents, tetraalkylammonium tribromides (tetramethyl-
ammonium tribromide 1, tetraethylammonium tribromide 2, tetrabutylammo-
nium tribromide 3, and cetyltrimethylammonium tribromide 4), were prepared
by simply mixing a solution of CAN and a solution of tetraalkylammonium
bromide in water at room temperature (Scheme 2). The stability and reactivity
[
5]
of tetramethylammonium tribromide 1 was studied by Avramoff et al.
14]
[
Chaudhuri et al. have described the stabilities and reactivities of tetrabuty-
lammonium tribromide 3 and cetyltrimethylammonium tribromide 4. All
[
5,8]
these tribromides were reported
to lose bromine gradually on standing to
yield the crystalline tribromides. Also, these remain highly dissociated in
polar organic solvents such as DMF, acetic acid, and alcohol. The ultraviolet
Scheme 1.

Tetraalkylammonium Tribromide
935
Scheme 2.
(UV) spectrum of TEATB 2 (Fig. 1) shows an intense electronic absorption
peak centered at 273 nm, the lmax typical
[15]
2
for the Br3 anion. As far as
stability of the TEATB is concerned, it loses bromine upon standing in the
presence of light. (the color changes from bright orange to light yellow).
The present procedure is simple, convenient, and environmentally benign
for the synthesis of tetraalkylammonium tribromide reagents with high yields.
[1]
There are several articles related to the synthesis and reactivities of tetraalk-
ylammonium tribromides 1,3,4 available, but studies on tetraethylammonuim
tribromide (TEATB) 2 as brominating agent have not been performed exten-
sively. That is why we became interested in the synthesis and exploiting the
synthetic utility of TEATB 2 in bromination of some aromatic compounds.
BROMINATION OF AROMATIC COMPOUNDS USING
TETRAETHYLAMMONIUM TRIBROMIDE
After the successful synthesis of CAN-mediated tetraalkylammonium tribro-
mide, we checked the efficacy of the TEATB thus synthesized in the bromina-
tion of selected aromatic substrates under mild reaction condition. These
experimental results are summarized in Table 1.
From these results, we observe that TEATB rapidly brominates activated
aromatics with better yield than nonactivated aromatic compounds. Acetani-
lide (entry 1) yielded 65% of 4-bromoacetanilide on reaction with TEATB
Figure 1. UV spectrum of tetraethylammonium tribromide 2.

Table 1. Bromination of aromatic compound with TEATB (2)
a
Reaction
time (h)
Molar ratio
(1/substrate)
b,c
Entry
Substance
Solvent
Product
Yield (%)
1
2
3
4
5
6
7
8
Acetanilide
Aniline
Aniline
Water–DMF (1:1)
Water–DMF (1:1)
Water–DMF (1:1)
Water–DMF (1:1)
Acetic acid
1
1:1
1:1
3:1
3:1
1:1
1:1.2
1:1
3:1
4-Bromoacetanilide
p-Bromoaniline
65
62
63
55
60
54
46
45
1.5
1.5
1.5
2
2,4,6-Tribromoaniline
2,4,6-Tribromophenol
9-Bromoanthracene
1-Bromonaphthalene
2-Bromo-4-nitroaniline
2,4,5-Tribromoimidazole
Phenol
Anthracene
Naphthalene
4-Nitroaniline
Imidazole
Acetic acid
Acetonitrile
2
1.5
2
CH
(
2
Cl
2
/MeOH
1:1), CaCO
3
a
Reactions were monitored by TLC.
b
Products were characterized by comparison of their physical data and IR and NMR spectra with authentic pure samples.
Yields refer to pure isolated product.
c

Tetraalkylammonium Tribromide
937
at room temperature in aqueous DMF within 1 h. Similarly, this reagent
selectively converted aniline to p-bromoaniline and 2,4,6-tribromoaniline
respectively in moderate yields depending upon the molar ratios of the
reagent used with longer reaction time. 4-Nitroaniline was also less reactive
as compared to aniline with the same molar ratios. Polycyclic aromatic
compounds, such as naphthalene and anthracene, undergo bromination with
TEATB in acetic acid at room temperature with moderate yields. Imidazole
in CH Cl –MeOH (1:1) solution was brominated at room temperature
2
2
within 2 h to 2,4,5-tribromoimidazole with 45% yield, which is generally
sensitive to normal bromination involving elemental bromine.
These investigations indicate that TEATB is moderately reactive for
nuclear bromination of activated aromatic compounds in polar solvents
such as aqueous DMF, acetonitrile, and acetic acid under mild reaction
conditions.
In conclusion, we have developed a new synthetic route for the synthesis
of tetraalkylammonium tribromide using CAN as oxidant in an environmen-
tally benign condition. Interestingly, the reagent TEATB 2 formed with this
new method is found to be effective in nuclear bromination of aromatic
compounds with longer reaction times.
EXPERIMENTAL
General Procedure for the Synthesis of Tetraalkylammonium
Tribromide
The brominating agents, tetraalkylammonium tribromides, were prepared by
simply mixing a 10-ml solution of CAN (1 mmol) and a 10-ml solution of tet-
raalkylammonium bromide (1.5 mmol) in water at room temperature. In a
typical procedure, an orange-colored compound precipitated immediately
for tetraethylammonium tribromide (TEATB, 2). Stirring continued for
1
5 min after the addition of the CAN solution. The resulting orange-yellow
product was filtered off and washed with water. The product was dried in
air or by quickly placing between the folds of filter paper. The isolated
yield was 80% (mp 858C) for tetraethylammonium tribromide 2. The yields
were approximately 70–80% for other types of tetraalkylammonium
tribromides, which include tetramethylammonium tribromide 1 (76%),
tetrabutylammonium tribromide 3 (78%), and cetyltrimethylammonium tri-
bromide 4 (70%).
General Procedure for Bromination of Aromatic Compounds with
TEATB
TEATB (1 mmol) was added to a solution of the aromatic substrate (1 mmol)
in 50% aqueous DMF (5 ml) at room temperature. The mixture was stirred

9
38
R. Borah and A. J. Thakur
vigorously for the specified reaction period and diluted with water (15 ml) for
complete precipitation of the product. The product was filtered in vacuum and
washed with water. Similarly, treatment of 1 mmol of imidazole with 3 mmol
of TEATB in a mixture of 2 mmol of CaCO and CH Cl –MeOH as solvent
3
2
2
afforded 2,4,5-tribomoimidazole in 45% yield.
In the case of a polycyclic aromatic compound, TEATB was added to the
solution of aromatic compound (5 ml) in acetic acid with a suitable proportion
of molar ratio. The reaction mixture was stirred at room temperature for the
specified time, during which the orange color of the mixture disappeared,
and was then diluted with water. The aqueous solution was extracted with
ether. The organic layer was separated, washed with water and aqueous
NaHCO solution, and dried over anhydrous sodium sulfate. The solvent
3
ether was evaporated under reduced pressure. For anthracene, products were
precipitated from the reaction mixture on dilution with water and then
isolated by filtration.
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