Simple and efficient system for the α-bromination of a β-ketoester by using N-bromosuccinimide in the presence of silicasupported NaHCo3 as the heterogeneous catalyst: An environmentally benevolent approach

Article abstract of DOI:10.1080/00397911.2010.530729

Selective and efficient α-bromination of β-ketoesters and cyclic and acyclic ketones is achieved by reaction with N-bromosuccinimide (NBS) catalyzed by silica-supported sodium bicarbonate (NaHCO₃ · SiO₂) under mild reaction conditions and with short reaction times. With 100% selectivity with all substrates, after 45 min at room temperature (20±2 °C), conversions for ethylacetoacetate were 67% and for acetophenone, cyclohexanone, and cycloheptanone were 58, 50, and 55%, respectively. Acetyl acetone recorded 65% conversion with 100% selectivity. Although cyclopentanone and toluene had only 20% conversion, both reactions showed 100% selectivity toward α-bromination. The catalysts exhibit activity and reusability. Copyright Taylor & Francis Group, LLC.

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Simple and Efficient System for the
α-Bromination of a β-Ketoester by
Using N-Bromosuccinimide in the
Presence of Silica-Supported NaHCO3
as the Heterogeneous Catalyst: An
Environmentally Benevolent Approach
Ateeq Rahman a & S. B. Jonnalagadda a
a School of Chemistry, University of KwaZulu-Natal, Westville
Campus, Chiltern Hills, Durban, South Africa
Accepted author version posted online: 28 Oct 2011.Published
online: 22 Dec 2011.
To cite this article: Ateeq Rahman & S. B. Jonnalagadda (2012): Simple and Efficient System for the
α-Bromination of a β-Ketoester by Using N-Bromosuccinimide in the Presence of Silica-Supported
NaHCO3 as the Heterogeneous Catalyst: An Environmentally Benevolent Approach, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
42:8, 1091-1100
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Synthetic Communications1, 42: 1091–1100, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.530729
SIMPLE AND EFFICIENT SYSTEM FOR THE
a-BROMINATION OF A b-KETOESTER BY USING
N-BROMOSUCCINIMIDE IN THE PRESENCE OF SILICA-
SUPPORTED NaHCO3 AS THE HETEROGENEOUS
CATALYST: AN ENVIRONMENTALLY BENEVOLENT
APPROACH
Ateeq Rahman and S. B. Jonnalagadda
School of Chemistry, University of KwaZulu-Natal, Westville Campus,
Chiltern Hills, Durban, South Africa
GRAPHICAL ABSTRACT
Abstract Selective and efficient a-bromination of b-ketoesters and cyclic and acyclic ketones
is achieved by reaction with N-bromosuccinimide (NBS) catalyzed by silica-supported
sodium bicarbonate (NaHCO3 ꢀ SiO2) under mild reaction conditions and with short reac-
tion times. With 100% selectivity with all substrates, after 45 min at room temperature
(20 ꢁ 2 ꢂC), conversions for ethylacetoacetate were 67% and for acetophenone, cyclohexa-
none, and cycloheptanone were 58, 50, and 55%, respectively. Acetyl acetone recorded 65%
conversion with 100% selectivity. Although cyclopentanone and toluene had only 20%
conversion, both reactions showed 100% selectivity toward a-bromination. The catalysts
exhibit activity and reusability.
Keywords Acyclic ketones; a-bromination; b-ketoesters; NaHCO3-SiO2 catalyst; NBS
(N-bromosuccinimide)
INTRODUCTION
a-Monobromination of b-ketoesters and carbonyl compounds is a significant
transformation in organic synthesis as the a -bromintaed products are frequently used
as synthetic intermediates.[1,2] a-Monobromination of b-ketoesters without
a-substituents has been a challenging problem, because the monosubstituted product
Received June 17, 2010.
Address correspondence to S. B. Jonnalagadda, School of Chemistry, University of KwaZulu-
Natal, Westville Campus, Chiltern Hills, Private Bag 54001, Durban 4000, South Africa. E-mail:
jonnalagaddas@ukzn.ac.za
1091

1092
A. RAHMAN AND S. B. JONNALAGADDA
is always accompanied by a small amount of disubstituted product. The majority of
reagents include bromine,[3] copper(II) bromide,[4] and N-bromosuccinimide
(NBS).[5] In the presence of protic and Lewis acids, bromine is generally used for
a-bromination of carbonyl compounds.[1] In terms of accessibility and ease of hand-
ling, N-bromosuccinimide is economical and a better brominating reagent. The major
advantage of the use of NBS is that the reaction by-product, succinimide, can be
recovered effortlessly. With NBS, a-bromination of ketones is well known to proceed
via a radical process promoted by initiators such as azo-bis-isobutyronitrile (AIBN)
and dibenzoyl peroxide (BPO) in refluxing CCl4.[6] The Lewis acid, Mg(ClO4)2, is
employed to promote 2-halogenation of 1,3-dicarbonyl compounds.[7] Recently, the
bromination of b-ketoesters using NBS catalyzed by NH4OAc.[8] Amberlyst-15,[9]
and NaHSO4 adsorbed on silica gel[10,11] were reported. Togni et al. reported the
use of the Ti(TADDO-Lato) complex to catalyze enantioselective a-halogenation
of a-substituted b-ketoesters with limited selectivity,[12] where TADDO stands for
tetraaryl-1,3-dioxolane-4,5-dimethanols. While several methods have been developed
for the 2-chlorination[13] and 2-iodination[14] of 1,3-dicarbonyl compounds using
N-chlorosuccinimide and N-iodiosuccinimide, respectively, very few of these methods
were successful in monohalogenation of b-ketoesters. A number of reported methods
were practically inconvenient because of either harsh reaction conditions or the diha-
logenated products formed.[13,14] Researchers have reported 100% selective reactions
using silica-supported catalysts in organic synthesis.[15,16] Earlier, we have reported
use of NiSiO2 catalysts for reduction of various nitro aromatics to aromatic amines
with 100% selectivity and good conversions.[17] In this manuscript, we report a
method for the selective and efficient bromination of b-ketoesters and cyclic and
acyclic ketones, which is achieved via treatment with NBS and catalyzed by NaHCO3
on silica. It provides an alternative route for the fast and highly selective monobromi-
nation of cyclic and acyclic ketones and b-ketoesters with good conversions under
moderate conditions.
EXPERIMENTAL
Materials
All chemicals were synthesis-grade reagents available from Merck.
Preparation of Catalysts
The catalysts were prepared by the impregnation method. This involved dissolv-
ing sodium bicarbonate (2.47 g) in distilled water (20.0 ml) and adding to it silica gel
(5.0 g). The mixture was stirred for 2 h with a magnetic stirrer at room temperature
(20 ꢁ 1 ꢂC) and aged at room temperature overnight. The excess water was removed
by heating the mixture in a water bath and using a rotavapor under mild vacuum to
evaporate the water. The catalyst material was dried in an oven at 100 ꢂC for 12 h.[17]
Typical Reaction Procedure
NBS (1.2 mmol) was added to ethyl acetoacetate (1.0 mmol) together with
10.0 mL ether as solvent and 200 mg of catalyst. The reaction mixture was stirred at

a-BROMINATION OF b-KETOESTER
1093
room temperature, and the reaction progress was monitored by thin-layer chromato-
graphy (TLC) and gas chromatography (GC). At the end of the reaction, the reaction
mixture was filtered, washed with sodium metabisulfite solution, and extracted with
ethyl acetate. The ethyl acetate extract was washed twice with deionized water. The
extract was dried over Na2SO4, and the solvent was evaporated on a rotavapor to give
the crude product of monosubstituted bromo ester, which was then subjected to
column chromatography to afford the pure product. The final product was character-
ized by IR (neat): 1718, 1462, 1227 cmꢃ1; 1H NMR (400 MHz, CDCl3): d 4.67 (1H, s),
4.28 (2H, q, J ¼ 7.0 Hz), 2.43 (3H, s), 1.32 (3H, t, J ¼ 7.0 Hz); 13C NMR (50 MHz,
CDCl3): d 197.6, 167.3, 6.8, 54.6, 27.1, 14.2. 1H and 13C NMR (400 MHz, Bruker).
RESULTS AND DISCUSSION
The solid catalysts offer many advantages in organic synthesis, such as oper-
ational simplicity, environmental compatibility, nontoxicity, reusability, and low cost.
Hence, suitable heterogeneous catalysts are in great demand both in academic research
and industry. Several heterogeneous materials are thus finding increasing applications
in the field of catalysis.[13,14] A tremendous upsurge of interest in various chemical
transformation processes by catalysts under heterogeneous conditions has occurred.
The factors that influence the physicochemical, surface, and catalytic properties of
the catalytic material include the preparation conditions, thermal treatment, and nat-
ure of the support, doping with certain foreign oxides, and treatment with ionizing
radiation. The properties of the support material depend also on its preparation
method, concentration of reagents, precursor of the support, and the nature of gas
atmosphere used in calcination. All these parameters affect the behavior of the catalytic
material toward certain catalytic reactions. The synthesis of a-monobromo products of
a-unsubstituted b-ketoesters is difficult because bromo compounds are unstable and
undergo disproportionation to dibrominated and debrominated products.
Initially, we explored the possibility of using the Ni-loaded silica catalysts for
selective a-monobromination of b-ketoesters using NBS. Choosing the ethylacetoace-
tate as a model substrate, the activity of 5%, 10%, and 15% Ni loaded SiO2 catalysts
were studied. Although even after 120 min of reaction time, the conversions were poor
but interesting enough the selectivity was 100%. While 5% Ni silica gave 45% conver-
sion, 10% Ni gave 50% and 15% Ni gave 55%. Ni-supported silica catalysts were
observed to be less active because of the weak interaction between Ni and silica,
multilayer nickel on the support, or formation of the thick Ni particles.
As long reaction durations were demanded with the nickel silica catalysts, we
investigated the scope of use of NaHCO3 on SiO2 support as catalyst for the title reac-
tions, and inspiring results were obtained. Table 1 presents the results for the bromin-
ation of a b-ketoester as the model substrate with a series of catalysts. The 15%
NaHCO3 loaded SiO2 exhibited greater activity relative to the 5% and 10% NaHCO3
loadings. With the 5% NaHCO3 on SiO2, conversion was 58%, while with 10% and
15% NaHCO3 loadings, the conversions increased to 60% and 67% respectively.
Furthermore, the reaction duration decreased with increased NaHCO3 loading on sil-
ica. With reactions with great loading of NaHCO3 (i.e., with 20% NaHCO3 on SiO2),
interestingly the conversion decreased from 67% to 55%. The observed lower activity
of the catalyst with higher bicarbonate loading could be due to excessive loading of

1094
A. RAHMAN AND S. B. JONNALAGADDA
a
Table 1. Bromination of b-ketoester using N-bromosuccinimide and supported catalyst in ether or CCl4
Support=loading
Loading (%)
Reaction time (min)
Conversion (%)
Selectivity (%)
SiO2-Ni
5
10
15
5
120
120
120
60
45
50
55
58
60
67
100
100
100
100
100
100
SiO2-NaHCO3
10
15
50
45
Notes. Ethylacetoacetate, 1.0 mmol; NBS, 1.2 mmol; catalyst, 200 mg. Temperature, 20 ꢁ 2 ꢂC.
aMean of duplicate runs.
NaHCO3 on the silica surface, restricting the simultaneous access for reactants to
active sites on silica.
Thus, identifying 15% of bicarbonate loading on silica as the optimal, the
efficiency of the catalyst material in transformation of selected keto compounds
was investigated. Of the three cyclic ketones, namely cyclohexanone, cyclopentanone,
and cycloheptanone, all had 100% selectivity. After 45 min, cyclohexanone was con-
verted to 2-bromocylcohexanone with 55% conversion, while cycloheptanone had
50% conversion and cyclopentanone had 20%. Acetophenone, the aromatic ketone,
had 58% conversion, and acetylacetone, an aliphatic ketone, showed 65% conversion.
Toluene, an alkyl aromatic gave poor conversion to 2-bromotoluene (20%) but still
retained 100% selectivity. Table 2 summarizes the reaction conditions, conversion,
and product selectivity of the different substrates studied. Sodium bicarbonate loaded
on silica proved to be an extremely active catalyst for the quick and selective bromin-
ation of the selected ketones with good conversions under moderate reaction
conditions, because of the mild basic nature of the reaction on the metal surface.
Neither silica nor sodium bicarbonate alone could give any conversions under
otherwise identical conditions.
Catalyst Characterization: 15% NaHCO3
Silica catalysts were characterized by x-ray diffraction (XRD), infrared (IR)
techniques, and Si NMR spectroscopy. The XRD spectra (Phillip-PW 1830XRD
Table 2. Bromination of b-ketoester and cyclic ketones using N-bromosuccinimide and 15% NaHCO3-
SiO2 in Ether or CCl4.a
No
Substrate
Product
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
Ethylacetoacetate
Acetophenone
Cyclohexanone
Cycloheptanone
Cyclopentanone
Acetyl acetone
Toluene
2-Bromo ethyl Acetoacetate
1-Bromo-methylene Acetophenone
1-Bromo cyclohexanone
1-Bromo cycloheptanone
1-Bromo cyclopentanone
2-Bromoacetyl acetone
2-Bromotoluene
67
58
55
50
20
65
20
100
100
100
100
100
100
100
Notes. Substrate, 1.0 mmol; NBS, 1.2 mmol, and catalyst, 200 mg. Reaction duration, 45 min. Tempera-
ture, 20 ꢁ 2 ꢂC.
aMean duplicate runs.

a-BROMINATION OF b-KETOESTER
1095
Figure 1. XRD data of NaHCO3-SiO2 catalysts: A, 15%; B, 10%; C, 5%; and D, 20%.
diffraction spectrometer) showed strong reflections at 2h ꢃ 25 ꢂ, which may be
associated with those usually observed in amorphous silicates between 24 ꢂ and
31 ꢂ in 5%, 10%, 15%, and 20% NaHCO3-SiO2.[18,19] (See Fig. 1.) The smaller broad
peak occurred at 2h ¼ 12.0 ꢂ for this sample. This peak may be associated with an
interlayer spacing in silicates, broadened by stacking disorder observed in 5%, 10%,
and 20% NaHCO3-SiO2, whereas with 15% NaHCO3-SiO2, the broad peak is less
observed. Similar basal reflections were already observed.[20] There are also narrow
reflections, indicating the presence of some crystalline phases. The majority of these
reflections were attributed to sodium sesquicarbonate.[21,22] Occassionally, some traces
Figure 2. IR spectra of 15% NaHCO3-silica catalysts.

1096
A. RAHMAN AND S. B. JONNALAGADDA
of trydimite or quartz may appear in some samples. During the identification process
of these crystallike reflections other carbonates, such as Na2CO3, its hydrates, and
Na2SiO3 ꢀ 5H2O, were considered.
The examination of IR spectra (Perkin-Elmer FT-IR spectrometer) of 15%
NaHCO3-SiO2 fresh catalysts showed that the observed bands agree with the litera-
ture data. (See Fig. 2.) Many bands are associated with the amorphous silica: Asym-
metric stretching Si-O (1154 and 1037 cmꢃ1 respectively), stretching of Si-O-X, where
x ¼ K or Na (953 cmꢃ1), symmetric stretching of O-Si-O (457–600 cmꢃ1).[23–28] The
presence of molecular water is revealed by the band from 1642 to 1660 cmꢃ1 (bending
of H2O). Also, the broad band extending from 2300 to 3700 cmꢃ1 is associated with
stretching of X-OH, with xþH or Si, indicating the presence of a substantial amount
of OH- groups in the silica matrix.[29,30] On the other hand, the band at 1470 cmꢃ1
corresponds to CO23ꢃ anions, indicating the presence of carbonates in these samples
in agreement with XRD experiments.[31–34] This shows that the NaHCO3 is as active
species in association with silica for the activation of n-bromosuccinimide, which
initiated the bromination of b-ketoester and cyclic ketones. These results conclusively
demonstrate that 15% NaHCO3–silica catalyst is ideally suited for the selective bro-
mination of b-ketoester and cyclic ketones and acetophenone 1-bromo compounds
with good conversion and selectivity.
Si NMR Spectroscopy
The 29Si MAS-NMR spectra of the sample shows few broad peaks, a typical
distribution of isotropic chemical shift values caused by structural disordered
distribution. These can be identified with the reported literature data for different tetra-
hedral Qn environments for Si in silicates.[35] As usual, the symbol, Qn identifies the con-
nectivity of silicate tetrahedron, n being the number of bridging oxygens (Si-O-Si). The
population of each Qnspecies observed in the 29Si NMR spectrum of sample can be
obtained from the least-square fitting of Gaussian function observed peaks. In the
analysis, the Gaussian distributions centered about ꢃ79 ppm, ꢃ88ppm, ꢃ97 ppm and
ꢃ106 ppm were attributed to Q1, Q2, Q3, and Q4 units respectively. The most shielded
resonance centered at an isotropic shifts of ꢃ106 ppm can be attributed unambiguously
to Q4 units, while the others show a remarkably close resemblance with the progressive
10 ppm deshielding expected from each additional non-bridging oxygen.[36] All the
observed lines, with the exception of the narrow Q4 line centered at ꢃ107 ppm in sample
D, correspond to silicon sites in the disordered sample. The chemical shift of additional
Q4 line in sample agrees well with the crystalline quartz (ꢃ107.4 ppm,)[37] indicating the
presence of nonreactive, or at least less reactive, silica in this kind of samples.
Effect of Solvent Medium
Further, to understand the influence of solvent on conversion efficiency, reac-
tions were conducted in different solvent systems, employing ethyl acetoacetate as sub-
strate and 15% NaHCO3-SiO2 as catalyst. While all solvents retained the 100%
selectivity, both CCl4 and ether could give 67% conversion after 45 min. Reactions were
slow with the other solvents. After 180 min, while CHCl3 gave 60% conversion, CH2Cl2
(50%), CH3CN (45%), and MeOH recorded only 20% conversions. Thus, both ether

a-BROMINATION OF b-KETOESTER
1097
and carbon tetrachloride proved to be the ideal solvents. While nickel loaded on silica
was a good and selective catalyst for a-monobromination of various ketones, bicarbon-
ate-supported silica proved to be a superior basic catalyst for this transformation. The
NaHCO3 silica catalyst could be easily separated by simple filtration, and the recovered
catalyst could be reused. The NaHCO3 on silica catalysts can be prepared with ease
from the environmentally friendly reagents, NaHCO3 and silica gel. The bromination
of b-keto esters is known to pass through an enol formation, and such transformation
is accelerated by the availability of suitable acid and base proton donors and acceptors.
A literature survey shows that cocatalyst loaded on silica can promote the acidic nature
of silanol groups on a silica surface behaving as proton donors.[32,33]
Mechanism
A probable mechanism involves the enolization of ketone through an initial hydro-
gen bonding to silanol group. Subsequently, because of its activation by the presence of a
cocatalyst, bicarbonate in its vicinity, Si-OH provides the proton to substrate. Possibly
the reactivity of NBS also gets enhanced by the acquired acid character of silanol groups,
which leads to the desired product. The bicarbonate on the silica surface readily accepts
the proton from the enol, facilitating the reaction. Hence, it can be concluded that
combination of both bicarbonate and silica activities lead to increased efficiency in con-
version. Even distribution of bicarbonate on silica surface manifested by the character-
ization of the catalyst using XRD and IR studies support the hypothesis.
SiO2 is an amorphous solid, and its surface has the aggregated primary parti-
cles, which are noncrystalline (hence the term silica gel). The most reactive groups
are the -OH groups (silanol groups) that terminate the primary particles; these
results are confirmed by measurements of the number of adsorbed pyridiunium ions,
which are acidic in character (Brønsted acid sites).[39] The NaHCO3, which is sup-
ported on SiO2, is basic in character. In conclusion, we can say that bicarbonate
and silica provide the combination of basic and acidic properties needed for the
efficient catalytic process.[39]
The comparison of efficiency of Ni and NaHCO3 supported on silica shows
that NaHCO3 has superior activity. Loading of bicarbonate on the silica support
has a positive effect on the rate of bromination reaction compared to Ni on silica.
Lower conversion efficiencies observed with 5%, 10%, and 20% of sodium bicarbon-
ate on SiO2 points to 15% loading as ideal for good conversions. The swift reactions
achieved at 15% of NaHCO3 loading can be attributed to the need for surface OH
groups to provide the necessary Brønsted acid site needed for fast reactions. These
results show the variables available in the design of solid catalysts (loading) can
be used for selective bromination of b-ketoester initiated by surface sites.
CONCLUSIONS
In conclusion, an efficient and highly selective method for a-monobromination
of b-ketoesters by treatment with NBS and catalyzed by NaHCO3 ꢀ SiO2 is
developed. This method demonstrates simplicity, high selectivity, shorter reaction
times ,and good yields with recyclable and economically viable catalyst. The charac-
terization of NaHCO3-SiO2 catalysts with XRD, IR, and 29Si NMR conclusively

1098
A. RAHMAN AND S. B. JONNALAGADDA
demonstrates that silica and carbonate species are present on the matrix of the sup-
ported system and are active species for a-bromination of b-ketoesters and cyclic
ketones. The simple experimental procedure, good yields of monobrominated
products, atom economy, ecofreindly nature, reusability, short reaction times, and
application of heterogeneous catalysts are advantages of the proposed procedure.
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