Novel Metal- and Mineral-Acid-Free Synthesis of Organic Ammonium Tribromides and Application of Ethylenephenanthrolium Bistribromide for Bromination of Active Methylene Group of 1,3-Diketones and β-Ketoesters

Article abstract of DOI:10.1080/00397911.2014.979509

A novel procedure for the preparation of organic ammonium tribromides (OATBs) is described from their corresponding bromides. Quaternary ammonium bromides (QABs) and a N,N′-heterocyclic dibromide are efficiently oxidized to their corresponding monotribromides and bistribromide by m-chloroperbenzoic acid (MCPBA) in the presence of 2 and 4 equiv of KBr, respectively. The reactions are carried out in an aqueous medium without the use of any mineral acid or metal catalyst/promoters. A variety of tribromides are synthesized in very good yields including a hitherto unknown reagent, 1,10-(ethane-1, 2-diyl)phenanthrolinediium bistribromide (EPDBT). EPDBT is investigated as brominating agent and found to be highly effective for selective bromination of active methylene groups of a variety of 1,3-diketones and β-ketoesters. GRAPHICAL ABSTRACT.

Full text of DOI:10.1080/00397911.2014.979509

A Novel Metal- and Mineral-acid free Synthesis of Organic Ammonium Tribromides
and Application of Ethylenephenanthrolium Bistribromide for Bromination of
Active Methylene Group of 1, 3-Diketones and β-Ketoesters
1
1
1
Rupa Rani Dey , Bappi Paul , Siddhartha Sankar Dhar
1
Department of Chemistry, National Institute of Technology Silchar, Assam, India
Corresponding Author Contact Information: Email: ssd_iitg@hotmail.com,
siddharthashankardhar@gmail.com
Abstract
A novel procedure for the preparation of organic ammonium tribromides (OATBs) is
described from their corresponding bromides. Quaternary ammonium bromides (QABs)
and a N, Nʹ -heterocyclic dibromide are efficiently oxidized to their corresponding
mono-tribromides and bistribromide by m-chloroperbenzoic acid (MCPBA) in presence
of 2 and 4 equiv of KBr, respectively. The reactions are carried out in aq. medium
without the use of any mineral acid and metal catalyst/promoters. A variety of
tribromides are synthesized in very high yields including a hitherto unknown reagent viz.
1
, 10-(ethane-1, 2-diyl)phenanthrolinediium bistribromide (EPDBT). EPDBT is
investigated as brominating agent and found to be highly effective for selective
bromination of active methylene groups of a variety of 1, 3-diketones and β-ketoesters.
1

KEYWORDS: MCPBA, bistribromide, bromination, 1,3-diketones, β-ketoesters, metal-
free
INTRODUCTION
Over the past two decades or so, sustainable and eco-friendly organic preparations have
attracted huge attention of synthetic organic chemists across the globe due to
[
1]
environmental issues with most of the traditional synthetic methodologies. A few
components that contribute heavily towards environmentally clean reactions are solvent-
free or aq medium reactions, use of non-toxic reagents, obtaining higher yields in lesser
[
2]
time etc. In recent past, Quaternary Ammonium Tribromides (QATBs) and N-
Heterocyclic Ammonium Tribromides (NHATBs) have garnered significant interest
[
3]
owing to their versatile utility in synthetic organic chemistry. Conventionally, QATBs
and NHATBs are prepared using potentially dangerous liquid Br and/or HBr and thus
2
[
4]
considered to be not favorable from environmental perspective. Over the past few
years, many improved methods of synthesis of OATBs were reported that involve
-
-
oxidation of Br to Br in efficient and environmentally benign manner. A few of them
3
[5]
2-
4
+ [5]
-
+ [6]
+ [7]
worth mentioning are V O /H O , MoO /H O /H , MnO /H , (NH ) S O /H ,
2
5
2
2
2
2
4
4 2
2
8
+
[8]
[9]
[10]
NaOCl/H , H IO , oxone etc. In spite of several advantages, many of such methods
5
6
usually involve metals and mineral acids which still cause environmental concerns.
Therefore, development of newer methods which can do away with metals and mineral
acids is still a challenge for synthetic chemists. Organic oxidants are often considered
superior to inorganic ones because of their mildness and efficacy. Moreover, on many
occasions they don’t require any mineral acids to carry out the oxidative transformations.
2

Some organic oxidants like DDQ (2,3-Dichloro-5,6-dicyano-1,4-benzoquinone), t-
BuOOH, MCPBA in particular, is highly useful oxidant in organic syntheses as it
possesses several advantages such as cheap, non-hazardous, highly efficient and often
requires no catalyst to generate products in high yields. An exhaustive literature survey
-
-
reveals that MCPBA has never been used in the conversion of Br to Br leading to the
3
synthesis of tribromide reagents. Among the tribromides, QATBs are reported to be more
useful as reagents or catalysts in a variety of organic reactions. Relatively less is known
about N-heterocyclic tribromides, especially N, Nʹ -heterocyclic bistribromides.
Recently, Patel et al. have prepared 1, 1´-(ethane-1, 2-diyl)dipyridinium bistribromide as
[
10]
stable bistribromide by using oxone. The compound is found to have vast utility in
organic reactions like bromination of organic substrates (alkene, alcohols and amines)
[
11]
and synthesis of heterocyclic compounds. A few more bistribromides particularly
[
12]
[13]
DABCO-Bromine and ethylenebis(N-methylimidazolium)ditribromide are known
in the literature, which also have high utilities as reagents in organic synthesis.
In the context of application of tribromides as brominating agent, attention may be drawn
to the importance of regio-selective bromination of 1, 3-dicarbonyls and β-ketoesters at
[
14]
α-position due to their versatility as intermediates in organic synthesis. Such
brominated products often serve as useful building blocks in the synthesis of natural and
[
15]
non-natural products. Most commonly used reagents to access those monobrominated
[
16]
[17]
[18]
[14a]
products include Br2, H O -HBr, NBS, CuBr -Koser’s reagent,
NaOBr in
2
2
2
[
19]
[20]
[21]
acetone/AcOH, NH Br-oxone, bromodimethylsulfonium bromide (BDMS) etc.
4
Though, these methodologies generate good yield of products, but most of them suffer
3

from drawbacks such as harsh reaction conditions, difficult work-up technique etc.
Moreover, selectivity is an issue with some of these methodologies which lead to
[
14a,22]
dibrominated products as well.
In the backdrop of discussion made above on the importance of tribromides and their
applications, it becomes evident that developing newer methodologies for such reagents
that are efficient and environmentally acceptable, would still pose a challenge to
6
–9
synthetic organic chemists. As a part of our constant endeavor over the past couple
years to develop newer synthetic strategies for tribromide reagents and their applications,
we would like to report herein, a new procedure for synthesis of tribromides that include
a variety of QATBs, viz tetramethylammonium tribromide (TMATB),
tetraethylammonium tribromide (TEATB), tetrabutylammonium tribromide (TBATB),
cetyltrimethylammonium tribromide (CTMATB) and benzyltrimethylammonium
´
tribromide (BTMATB) and a N, N -heterocyclic bistribromide. The bistribromide, viz. 1,
1
0-(ethane-1, 2-diyl)phenanthrolinediium bistribromide (EPDBT) is hitherto unknown
and thus it makes new addition to the family of tribromide reagents. With the successful
synthesis of EPDBT as new reagent, it is considered a worthwhile task to explore the
efficacy of this reagent in α-bromination of 1, 3-dicarbonyls and β-ketoesters.
RESULTS AND DISCUSSION
-
All the QATBs have been prepared very efficiently by carrying out oxidation of Br to
-
Br by using MCPBA as oxidizing agent. A variety of tribromides,
3
tetramethylammonium tribromide (TMATB), tetraethylammonium tribromide (TEATB),
4

tetrabutylammonium tribromide (TBATB), cetyltrimethylammonium tribromide
CTMATB) and benzyltrimethylammonium tribromide (BTMATB) have been prepared
(
by mixing corresponding bromide with 2 equiv KBr and oxidizing the whole with
MCPBA (Scheme 1). The resulting QATBs have been obtained in nearly quantitative
yields.
Similar methodology has been adopted to prepare EPDBT (Scheme 2). The first step
involves conversion of 1, 10-phenanthroline into a dibromide intermediate (1,10-ethane-
[
23]
1
,2-diyl phenanthrolinediium dibromide) by the literature method. The dibromide thus
formed was isolated and characterized by spectroscopic techniques. Four equiv of oxidant
MCPBA) and 4 equiv KBr have been added to the intermediate to convert it to its
(
corresponding bistribromide. Excess of MCPBA, if any, was destroyed by treating the
reaction mixture with aq solution of 10% NaHCO which, subsequently, converts
3
remaining MCPBA to sodium chlorobenzoate, H O and CO . The tribromides were
2
2
finally extracted with minimum volume of EtOAc. Removal of solvent afforded the pure
product. It should be pointed out that H O or molecular oxygen are perhaps more
2
2
environmentally friendly than MCPBA but our experience and survey of literature reveal
-
-
that such oxidants alone cannot effectively carry out transformation of Br to Br . They
3
5
b
often require transition metals as activator and/ or mineral acid such as H SO . The use
2
4
of transition metals in stoichiometric amount or mineral acid in a reasonable
concentration may pose environmental issues in spite of highly friendly character of
molecular O or H O . MCPBA has its inherent acidity and very effective in oxidizing
2
2
2
5

-
-
Br to Br without a metal activator which is why the present methodology is believed to
3
be environmentally safe.
1
13
All the QATBs and EPDBT were characterized by UV-Visible, FT-IR, H and C NMR
spectroscopy. The QATBs are all known compounds and their analytical data consistent
with literature reports. A intense band appeared at 269 nm in the UV-Vis spectrum
-
5a
(
Figure 1) for EPDBT is characteristic for tribromide ion (Br ). The FT-IR spectra of
3
-1
the compound (Figure 2) shows characteristic peak at 196 cm indicating the presence of
-
[24]
1
Br moieties in the compound. In the H NMR spectrum of EPDBT, it can be
3
observed that the peak at δ value 2.20 is certainly for the ethylene protons attached to the
1
N-atoms. However no such peak may be observed in the H NMR spectrum of
phenanthroline.
After getting pure EPDBT, we performed the bromination reaction taking methyl
acetoacetate (1a) as model substrate. Ethyl acetate was found to be the suitable solvent
for the bromination of β-carbonyl compounds. Initially, 0.6 mmol of EPDBT was
dissolved in ~ 5 mL of EtOAc followed by the addition of 1a. The reaction mixture was
stirred for several min in a magnetic stirrer maintaining ice-cold condition. TLC was run
to monitor the reaction (10% EtOAc/hexane). It took ~ 25 min to complete the reaction
with 96% yield. No trace of dibrominated product was detected even with excess of
EPDBT. After getting success with 1a, we tried α-monobromination of some selectively
chosen 1, 3-diketone and β-ketoester substrates (2a-14a). All of these substrates were
brominated by pursuing similar reaction condition mentioned for 1a affording products in
6

8
0-96% and in a short reaction time, (Table 1). In most of the cases, the predicted α-
monobrominated product was formed. But in case of some particular substrates likely,
a-7a, 9a, 13a-14a, a trace amount of ring bromination was also observed. It may be
5
[
25]
added that, according to literature survey, bromination is also possible at the C-1
position of 1, 3-diketones viz. 3a, 5a or 11a. However, no such bromination was observed
at that position as EPDBT as brominating agent. It is apparent from table 1 that ß-
ketoesters requires relatively longer time as compared to 1, 3-diketones which can be
explained in terms of the P value of –CH – protons. For example, yield of 2b is 89 % in
ka
2
~
24 min which is higher than that of 3b for similar reaction condition. It may be further
mentioned that the bromination was also tried taking other solvents like CH Cl and
2
2
MeCN which led to good yield of products. But those solvents were avoided because of
[
26]
environmental issues. Scheme 3 shows the bromination of diketone and β-ketoesters
by EPDBT.
A plausible mechanism of 1, 3-diketone bromination by tribromide may be explained by
scheme 4. Initially diketone undergoes tautomerism and transform itself to enol-form. In
the next step, the bromine molecule generated from EPDBT extracts the acidic proton
from enolated diketone as HBr giving the desired product.
CONCLUSION
In conclusion, we have come up with a novel synthetic protocol for the synthesis of
organic ammonium tribromides and a N, N-heterocyclic bistribromide, EPDBT,
-
-
employing m-chloroperbenzoic acid as oxidant for transformation of Br to Br . EPDBT
3
7

has been added as a new tribromide reagent to the host of already existing tribromide
based compounds. Although quite a few environmentally benign methods for synthesis of
tribromides are known in the literature, the new procedure reported herein will certainly
be one of the effective tools for accessing such reagents. This new tribromide is found to
be a reagent of choice for selective bromination of active methylene group of diketones
and ketoesters leading to nearly quantitative yield of α-monobrominated products. The
notable advantage of this method is mildness, region-selectivity and very good yields.
Moreover, the process requires no bases or lewis acid or other catalyst to get products.
The methodology has the potential to make a value addition to the existing protocols for
monobromination of diketones and ketoesters.
EXPERIMENTAL
General Procedure For The Synthesis Of Qatbs By MCPBA
The general procedure involves the mixing of 1 equiv of quaternary ammonium bromide
(1 mmol), 2 equiv of KBr (2 mmol, 0.238 g) and 2 equiv of MCPBA (2 mmol, 0.346 g)
in 10 mL of water and stirring of reaction mixture for several min. An orange coloured
product formed was washed with NaHCO solution (10 % solution) for several times to
3
remove unreacted substrate. After that the crude product was again washed with water to
remove by-products. After that the crude product was again washed with water to remove
by-products and extracted with EtOAc. Removal of solvent afforded the pure product.
Synthesis Of EPDBT By MCPBA And Its Characterization
8

Firstly, 1,10-(ethane-1,2-diyl)phenanthrolinediium dibromide was prepared by refluxing
phenanthroline (10 mmol, 1.96 g) and 1,2-dibromoethane (10 mmol, 1 mL) in acetone (6
o
mL) in a round bottom flask for 0.5 h at temperature of 90 C. The solid product of
dibromide precipitated was filtered, followed by washing with Et O (2 X 10 mL), dried in
2
vacuum and then recrystallized from EtOH-water (2:5) mixture.
Now 1 equiv of dibromide, thus synthesized (1 mmol, 0.354 g), 4 equiv of KBr (4 mmol,
0
.476 g) and 4 equiv of MCPBA (4 mmol, 0.692 g) were mixed in 10 mL of water and
stirred for ca. 5 min. The orange coloured product formed was washed with NaHCO3
solution (10 %) for several times to remove unreacted MCPBA. After that the crude
product was again washed with water to remove by-products and extracted with EtOAc.
Removal of solvent afforded the pure product.
o
EPDBT: Orange compd; mp 97 C; Yield: 90 % (0.610 g); UV-Vis: 269 nm; FT-IR (KBr,
-1
1
cm ): 3383, 2968, 1466, 779, 196; H NMR δ: 9.18 (t, J = 1.8 Hz, 2H), 8.24-8.22 (t, J =
`
2
.8 Hz, 2H), 7.77-7.57 (dd, J = 3.5 Hz, J = 1.5 Hz, 2H), 7.62 (s, 2H), 2.20-1.96 (m, 4H);
1
3
C NMR δ: 77, 123, 126, 129, 136, 146, 150; Anal., C H N Br (Mol Wt 687.66):
14
12
2
6
Calcd C 24.45, H 1.76, N 4.07, Br 69.71; Found C 24.60, H 1.69, N 3.99, Br 69.65.
Bromination Of 1, 3-Diketones And Β-Ketoesters By EPDBT With Spectral And
Analytical Data Of Some Products
0
.6 mmol of EPDTB (0.818 g) was dissolved in 5 mL of EtOAc followed by the addition
of 1 mmol of 1, 3-diketone/β-ketoester to the reaction mixture. The above mixture was
9

o
stirred maintaining cool condition (5-10 C) in a magnetic stirrer for several minutes until
the solution becomes colourless. The progress of the reaction was monitored by doing
TLC (10 % EtOAc/hexane). After the completion of reaction, the mixture was washed
with water (2 X 5 mL). The organic layer was dried over anhyd Na SO after separating
2
4
from aq layer. The excess solvent was removed by evaporation in a rotary evaporator to
get the crude product.
o
Methyl 2-bromo-3-oxobutanoate (1b): Oily liquid; bp 215 C; Yield: 96 %; FT-IR:
-1 1
3
493, 2986, 1792, 1718, 869 cm ; H NMR δ: 4.19 (s, 1H), 3.07 (s, 3H), 2.18 (s, 3H);
C NMR δ: 27, 51, 67, 164, 201; Anal., C H O Br (Mol Wt 195.01): Calcd C 30.79, H
1
3
5
7
3
3
.62; Found C 30.73, H 3.58.
o
Ethyl 2-bromo-3-oxobutanoate (2b): Liquid; bp 230-232 C; Yield: 89 %; FT-IR: 1745,
-1 1
1
718 cm ; H NMR δ: 4.70 (s, 1H), 3.38 (q, J = 7.1 Hz, 2H), 2.45 (t, J = 7.1 Hz, 3H),
1
3
1
.27 (s, 3H); C NMR δ: 15, 26.3, 55, 67, 163, 198; Anal., C H O Br (Mol Wt 209.03):
6 9 3
Calcd C 34.47, H 4.34; Found C 34.50, H 4.37.
-1 1
3
-Bromopentane-2,4-dione (3b): Liquid; Yield: 88 %; FT-IR: 1743, 1721 cm ; H
1
3
NMR δ: 4.17 (s, 6H), 2.07 (s, 1H); C NMR δ: 27, 67, 195; Anal., C H O Br (Mol Wt
5
7
2
1
79.01): Calcd C 33.52, H 3.94; Found C 33.47, H 3.90.
o
t-Butyl 2-Bromo-3-oxobutanoate (4b): Liquid; bp 240 C; Yield: 92 %; FT-IR: 1732,
-1
1
13
1
718 cm ; H NMR δ: 4.10 (s, 1H), 2.02 (s, 3H), 1.23 (s, 9H); C NMR δ: 27, 31, 67,
1
0

8
4, 164, 206; Anal., C H O Br (Mol Wt 237.09): Calcd C 40.53, H 5.53; Found C 40.41,
8
13
3
H 5.57.
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
ACKNOWLEDGEMENT
The authors are thankful to Sophisticated Analytical Instrument Facility, IIT Madras and
IIT Bombay for providing the spectral analyses of the compounds. The authors also
acknowledge the Director, NIT Silchar for financial assistance and other facilities.
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4
1
5

Table 1. Monobromination of 1, 3-diketones and β-ketoesters by EPDBT
Entry Reactants Products Reaction Time
Min)
Yield Ref.
(
1
25
96
27
2
3
25
20
89
88
21(a)
13
4
5
30
15
92
80
10
25
6
7
30
15
82
85
18
17
8
20
86
18
1
6

9
1
30
20
92
80
10
10
0
1
1
20
25
81
85
18
28
1
2
1
1
3
4
15
15
81
80
-
-
[
a]
o
The reaction was conducted in temp 5-10 C using reagent to substrate molar ratio 1:0.6
[b]
and taking EtOAc as the solvent. All the products were first checked in TLC and then
[
c]
identified by FT-IR and NMR spectral data. Isolated Yield.
1
7

Scheme 1. Reaction showing the synthesis of QATBs from QABs by MCPBA.
1
8

Scheme 2. Strategy for the preparation of EPDBT.
1
9

Scheme 3. Bromination of 1,3-diketones and β-ketoesters by EPDBT.
2
0

Scheme 4. A plausible mechanism for the bromination of 1, 3-diketones.
2
1

Figure 1. UV-Visible Spectra of EPDBT
2
2

Figure 2. FT-IR spectra of EPDBT
2
3