1. Preparation and use of combustion-derived Bi2O3
for the synthesis of heterocycles with anti-cancer
properties by Suzuki-coupling reactions†
Sebastian Anusha,a
B. S. Anandakumar,a
Chakrabhavi Dhananjaya Mohan,b
G. P. Nagabhushana,a
B. S. Priya,b
Kanchugarakoppal S. Rangappa,b
Basappa*a
and Chandrappa G. T*a
Bismuth oxide was synthesized via simple, rapid and energy efficient solution combustion synthesis (SCS) by
employing sucrose as a fuel. This SCS-Bi2O3 was characterized by analytical techniques such as PXRD, SEM,
EDX, UV-Visible and BET surface area measurement. Using the prepared SCS-Bi2O3, several classes of
heterocyclic compounds were synthesized in good yields via Suzuki-coupling reaction in aqueous
medium. Interestingly, the recovered SCS-Bi2O3 could be reused three times without a significant loss of
catalytic activity. Further, the synthesized compounds were tested for cytotoxic activity against a human
hepatoma cancer cell line (HepG2). Among these, compound 3n effectively inhibited the proliferation of
these cells with an IC50 value of 8.4 mM. Thus, this paper describes the preparation of highly effective
Bi2O3, which can be used for synthesizing various classes of heterocycles, including those having anti-
cancer property.
Introduction
In the past decade a number of methods such as solid-state
reactions,1
sonochemical routes,2
thermolysis,3
hydrothermal
methods4
and solution combustion synthesis (SCS)5,6
have been
developed for the synthesis of Bi2O3. SCS is the simpler route to
develop metal oxides compared to other methods.7,8
The SCS
method has several advantages including rapid synthesis at
ambient atmosphere, low cost with a great potential scale up
and is eco-friendly i.e. complete conversion to non-toxic gases.
SCS is also useful for producing homogeneous, porous and
crystalline ne powders with a large surface area.9
Metal oxides are extensively used in many organic trans-
formations especially in the heteroaryl preparation via cross
coupling reactions. Methods reported for the synthesis of het-
eroaryls have one or more advantages as well as disadvantages
such as involvement of expensive reagents, formation of
byproducts, thermal instability, use of toxic solvents, require-
ment of excess of reagents/catalysts, laborious work up proce-
dure and use of extensive catalyst. To overcome these
drawbacks, the development of an alternate, milder, and
cleaner procedure is highly desirable.10
It should be efficient,
and involve an easy work-up procedure which affords greater
yields of desired products in shorter reaction time.11,12
On the other hand, heteroaryls has been recognized as a
privileged structure by medicinal chemists as they are molec-
ular scaffolds with versatile binding properties, which exhibit
good drug-like properties.13–16
The biaryl motif has shown
activity across a wide range of therapeutic classes which include
antifungal, anti-inammatory, anti-rheumatic, antitumor and
antihypertensive agents.17,18
Herein, we report a combustion derived Bi2O3 by employing
cost effective sucrose as fuel and to the best of our knowledge,
this is the rst report on synthesis of Bi2O3 using sucrose as fuel
via SCS method. However, there are reports on metal oxides
other than Bi2O3 prepared using sucrose19,20
as fuel via SCS. In
this paper, we emphasize not only on the basic characteristics of
Bi2O3 but also on high surface area which makes it a promising
reagent in organic synthesis. The large surface area can provide
a greater number of active sites than the low coordinated oxide
sites (edges, corners) and lattice defects (cation and anion
vacancies).21,22
Also we report the use of SCS-Bi2O3 in combi-
nation with [1,10
-bis(diphenylphosphino)ferrocene]dichlor-
opalladium(II) (Pd(dppf)Cl2) as an effective base–catalyst system
for coupling substituted pyridine halides with boronic acids.
These reactions were carried out in water under aerobic
conditions using tetra-n-butylammonium bromide (TBAB) as an
additive.23–25
This system is of due importance as it avoids the
use of any organic solvents and easy separation of product from
the reusable catalyst which greatly facilitates the work up.26
a
Department of Chemistry, Central College Campus, Bangalore University,
Bangalore-560 001, India. E-mail: salundibasappa@gmail.com; Tel: +91 8022961346
b
Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore-
570 006, India. E-mail: gtchandrappa@yahoo.co.in; Tel: +91 8022961350
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra07839j
Cite this: RSC Adv., 2014, 4, 52181
Received 30th July 2014
Accepted 9th October 2014
DOI: 10.1039/c4ra07839j
www.rsc.org/advances
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RSC Advances
PAPER

2. Result and discussion
Characterisation of SCS-Bi2O3
Fig. 1 shows powder X-ray diffraction (PXRD) pattern of
prepared SCS-Bi2O3. All the characteristic peaks in the pattern
can be indexed to a monoclinic phase of a-Bi2O3 with lattice
parameters a ¼ 5.8480, b ¼ 8.1660, c ¼ 7.5100 and b ¼ 113
(JCPDF card no. 00-027-0053). The crystallite size calculated
using Scherer's formula is found to be 65–70 nm.
The scanning electron microscope (SEM) micrograph of
Bi2O3 powders (Fig. 2) shows that the powder is porous and
agglomerated. The pores and voids can be attributed to the
large amount of gas escaping during the combustion. The
energy dispersive X-ray spectroscopy (EDX) spectrum of Bi2O3
synthesised shown in (ESI Fig. 1†) exhibit peaks for Bi and O
elements conrming the presence of Bi2O3. Brunauer–
Emmett–Teller (BET) nitrogen adsorption–desorption
isotherm of SCS-Bi2O3 is shown in (ESI Fig. 2†). The surface
area of SCS-Bi2O3 was calculated by the BET method. SCS-
Bi2O3 has a large surface area, f 24 m2
gÀ1
, compared to
commercial Bi2O3, (8 m2
gÀ1
). During combustion, the larger
surface area attributed to the liberation of gaseous products
such as water, carbon dioxide and gaseous nitrogen. The
band gap energy of SCS-Bi2O3 was calculated (2.88 eV) by
using UV-Visible spectra with Mott–Schottky plot as shown in
Fig. 3 (the absorbance spectra is shown in ESI Fig. 3†).
Optimisation of reaction conditions
The reaction was studied on 2-amino-3-bromopyridine using
4-triuromethylphenylboronic acid (Scheme 1). Efforts were
focussed on screening different palladium catalysts in various
solvents and the results are summarised in (Table 1). It was found
that the ideal system for the cross coupling reaction is combus-
tion derived Bi2O3 (SCS-Bi2O3)/Pd(dppf)Cl2 in water. The reac-
tions with other palladium catalysts were not proceeding in water
and low conversions were observed in 80% dioxane. The most
robust reaction was achieved by the use of 0.8 equivalence of the
SCS-Bi2O3 and 0.03 equivalence of the Pd(dppf)Cl2 in aqueous
medium. These results indicate that the SCS-Bi2O3 is effective in
aqueous medium compared to organic–aqueous medium.
Inuence of SCS-Bi2O3 on Suzuki coupling
SCS-Bi2O3 acts as an effective base in the reaction of various 2-
or 3-halopyridines with a wide range of aryl as well as heteroaryl
boronic acids. The reaction was investigated on both bromides
and iodides under the optimized condition (Scheme 2) and the
results were summarised in Table 2.
Majority of reactions completed within 10 h with product
isolation by simple ltration of the undissolved base–catalyst
from the reaction media. In many cases pure product was
afforded in excellent yields by crystallisation of the crude
product using ethyl acetate and hexane mixture. All the reac-
tions on 2-amino-3-bromopyridine were fast and prolongation
of reaction time was observed in the case of 2-amino-3-bromo-6-
methylpyridine. Reactions of iodides were faster than corre-
sponding bromides but the yields were comparable in most of
the cases. With regard to chemo selectivity this base–catalyst
system is not compatible with chloro substituted pyridines and
is attributed to its reluctance to participate in oxidative addition
as high energy is required for this rst step of the catalytic
cycle.27
We also observed a loss of yield in the case of ortho
substituted and heteroaryl boronic acids28
with no improve-
ment in the reaction conversion on prolonging the reaction
Fig. 1 PXRD pattern of SCS-Bi2O3 powders.
Fig. 2 SEM image of SCS-Bi2O3 powder. Fig. 3 Mott–Schottky plot for SCS-Bi2O3.
52182 | RSC Adv., 2014, 4, 52181–52188 This journal is © The Royal Society of Chemistry 2014
RSC Advances Paper

3. time up to 14 h. (Table 2, entries 3d, 3e, 3p). Thus SCS-Bi2O3 is
tolerant of electronic variations in halopyridines as well as
boronic acid components with cleaner reactions, ease of isola-
tion and better yields.
Re-usability of the base–catalyst system
Experiments were performed to study the recyclability of the
base–catalyst system employing 2-amino-3-bromopyridine
with 4-triuromethylphenylboronic acid to yield the
compound 3a (Scheme 1). We observed the signicant
reduction in the yield of the product aer third run (Table 3).
Aer the completion of the reaction, aqueous layer contain-
ing the product was separated and the base–catalyst
remaining was reused for another run by adding fresh 2-
amino-3-bromopyridine, 4-triuromethylphenylboronic acid
and TBAB. It is important to stress that this system was
readily recyclable for three times and the isolated yields were
above 70%.
Biology
Newly synthesized compounds were evaluated in vitro for their
anticancer property by MTT assay as described earlier.15
It is a
photometric assay that measures the reduction of 3-(4,5-
dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide by
enzyme mitochondrial succinate dehydrogenase. Most
compounds displayed different degree of cytotoxicity. Half
maximal inhibitory concentration (IC50) of the compounds 3a to
3p was summarized in Table 2. Among the newly synthesized
compounds, 3c, 3o and 3n showed signicant cytotoxic effect on
HepG2 cells with the IC50 of 16.6 mM, 20.8 mM and 8.4 mM
respectively, whose activity is comparable to the known anti-
cancer drug Sorafenib. 3f, 3m and 3l are moderately active with
the IC50 of 44.9 mM, 47.2 mM and 35.3 mM respectively. Other
members are less active against HepG2 cells with IC50 higher
than 50 mM.
Experimental section
Materials and method
Bismuth(III) nitrate pentahydrate of the purity of 99%, and
commercial Bi2O3 powder of the purity of 99% were
purchased from Merck Chemicals, India. Commercially
available sucrose was used and all organic chemicals used
were purchased from Sigma-Aldrich. Distilled water was
collected from Merck Millipore Direct-Q 3, 5, 8 Ultrapure
Water Systems. MTT and SDS were purchased from Sigma-
Aldrich (St. Louis, MO). Dulbecco's Modied Eagle Medium
(DMEM), fetal bovine serum (FBS), antibiotic–antimycotic
mixture were obtained from Invitrogen. Powder X-ray
diffraction patterns were obtained by a Philips X'pert PRO
X-ray diffractometer using CuKa radiation
(l ¼ 1.54056 ˚A). Morphologies of combustion derived
powders were examined using a JEOL-JSM-6490 LV scanning
electron microscope (SEM), Nitrogen adsorption–desorption
measurements were carried out at 77 K using a gas sorption
analyzer Quanta chrome corporation NOVA 1000. The surface
area was calculated using the BET method. UV-Vis measure-
ments were performed using a UV-Vis spectrophotometer
(Shimadzu UV-3101). All IR spectra were obtained in KBr disc
on a Shimadzu FT-IR 157 Spectrometer. 1
H and 13
C NMR
spectra were recorded on a Bruker WH-200 (400MZ) spec-
trometer in CDCl3 or DMSO-d6 as solvent, using TMS as an
internal standard and chemical shis are expressed as ppm.
Mass spectra were determined on a Agilent LC-MS and the
elemental analyses were carried out using an Elemental Vario
Cube CHNS rapid Analyzer. The progress of the reaction was
monitored by TLC pre-coated silica gel G plates.
Cell lines
HepG2 was a kind gi of Dr Gautam Sethi at the National
University of Singapore. HepG2 cells were cultured in Dulbec-
co's Modied Eagle Medium (DMEM) containing 1Â antibiotic–
antimycotic solution with 10% FBS.
Scheme 1 Schematic representation for optimising of reaction
conditions of suzuki coupling.
Table 1 The optimization of Pd catalyst, and medium for the Suzuki-
coupling reactiona
Catalyst Solvent Yield (%)
Pd(dppf)Cl2 80% dioxane 68b
Pd(dppf)Cl2 Water 88
Pd(PPh3)4 80% dioxane 55b
Pd(PPh3)4 Water NR
Pd2dba3 80% dioxane NR
Pd2dba3 Water NR
Pd(PPh3)2Cl2 80% dioxane 61b
Pd(PPh3)2Cl2 Water NR
Pd(OAc)2 80% dioxane NR
Pd(OAc)2 Water NR
a
Conditions: 2-amino-3-bromopyridine : 4-triuromethylphenyl-
boronic acid : SCS-Bi2O3 : Pd catalyst : TBAB ¼ 1 : 1.05 : 0.8 : 0.03 : 0.5
(mmol), solvent ¼ 10 mL per mmol substrate, 110 degrees, 8 h.
b
Isolated yield aer column chromatography; SCS-combustion
derived, NR – no reaction.
Scheme 2 Schematic representation for the synthesis of
heterocycles.
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Paper RSC Advances

4. Table 2 The physical parameters and cytotoxicity of the newly synthesized heterocyclic compounds via Suzuki-coupling reactionsa
Entry Halopyridine (1a–1h) Boronic acid (2a–2j) Product (3a–3r) Time (h) Yield (%) IC50 (mM)
3a 5 88 >50
3b 5 80 >50
3c 4 86 16.6 Æ 1.9
3d
6 68b
>50
4 69b
>50
3e 8 70b
>50
52184 | RSC Adv., 2014, 4, 52181–52188 This journal is © The Royal Society of Chemistry 2014
RSC Advances Paper

5. Table 2 (Contd.)
Entry Halopyridine (1a–1h) Boronic acid (2a–2j) Product (3a–3r) Time (h) Yield (%) IC50 (mM)
4 72b
>50
3f 4 89 44.9 Æ 3.6
3g 4 87 >50
3h 3 78 >50
3i 4 83 >50
3j 4 81 >50
This journal is © The Royal Society of Chemistry 2014 RSC Adv., 2014, 4, 52181–52188 | 52185
Paper RSC Advances

6. Table 2 (Contd.)
Entry Halopyridine (1a–1h) Boronic acid (2a–2j) Product (3a–3r) Time (h) Yield (%) IC50 (mM)
3k 4 79 >50
3l 5 84 35.3 Æ 2.3
3m 8 87 47.2 Æ 2.1
3n 9 81 8.4 Æ 2.8
3o 9 76 20.8 Æ 2.5
3p 9 66b
>50
52186 | RSC Adv., 2014, 4, 52181–52188 This journal is © The Royal Society of Chemistry 2014
RSC Advances Paper

7. Synthesis of SCS-Bi2O3
In a petridish, solution of Bi(NO3)3$5H2O was prepared by dis-
solving 5 g Bi(NO3)3$5H2O in 4 mL of 6 M HNO3 and the excess
acid was removed by heating on a hot plate to this sucrose
solution was added by dissolving 1.106 g of sugar in 25 mL of
distilled water. An aqueous solution containing the above
mixture of Bi(NO3)3$5H2O as an oxidizer (O) and sugar (F)
(corresponding F/O ratio Ø ¼ 1 : 1)29,30
was taken in a petridish.
Excess water was allowed to evaporate by heating the solution
on a hot plate until the formation of a yellow viscous gel. Then
the petridish was introduced into a muffle furnace maintained
at 400 Æ 10
C. Initially, the viscous gel underwent dehydration
and commenced smoldering combustion, which appeared at
one end and propagated through the mass within one minute. A
voluminous and porous nanocrystalline product was obtained.
Typical procedure for the synthesis of novel heterocyclic
compounds
To a solution of halopyridine (1a–1h) (0.3 mmol) in 10 mL of
distilled water in a sealed tube was added boronic acid (2a–2j)
(0.32 mmol), SCS-Bi2O3 (0.24 mmol), TBAB (0.15 mmol) and
[1,10
-bis(diphenylphosphino)ferrocene]dichloropalladium(II)
(0.009 mmol). The reaction mixture was heated to 110
C for the
required hours. The reaction was then cooled to room temper-
ature and ltered to remove the base and catalyst. The product
was extracted from the water layer by 3 Â 5 mL ethyl acetate,
dried with magnesium sulfate, ltered, and concentrated in
vacuum. The crude product was puried by recrystallisation
using ethyl acetate : hexane. All new compounds exhibited
spectral properties consistent with the assigned structures and
were fully characterized by their spectroscopic data (1
H, IR,
Mass, elemental and 13
C NMR analysis) (ESI-4 to ESI-19†).
MTT assay
The anti-proliferative activity of compounds (3a to 3p) against
HCC cells was determined by the MTT dye uptake method as
described previously.31
Briey, the cells (2.5 Â 104
per mL) were
incubated in triplicate in a 96-well plate in the presence or
absence of different concentrations of compounds in a nal
volume of 0.2 mL for indicated time intervals at 37
C. There-
aer, 20 mL MTT solution (5 mg mLÀ1
in PBS) was added to each
well. Aer a 2 h incubation at 37
C, 0.1 mL lysis buffer
(20% SDS, 50% dimethyl-formamide) was added; incubation
was continued overnight at 37
C; and then the optical density
(OD) at 570 nm was measured by Tecan plate reader.
Conclusion
Combustion derived Bi2O3 prepared and used for the synthesis
of heteroaryls using Suzuki coupling reactions in aqueous
medium. The base–catalyst system can also be recovered and
reused up to three times with no signicant loss of catalytic
activity. Furthermore, the newly synthesized compounds were
tested for its cytotoxic effects against human hepatoma cancer
cells (HepG2) which revealed that the described application is
effective in developing reagents for the chemical biology
programme.
Table 2 (Contd.)
Entry Halopyridine (1a–1h) Boronic acid (2a–2j) Product (3a–3r) Time (h) Yield (%) IC50 (mM)
3q No reaction
3r No reaction
a
Conditions: substrate : boronic acid : SCS-Bi2O3 : Pd(dppf)Cl2 : TBAB ¼ 1 : 1.05 : 0.8 : 0.03 : 0.5 (mmol), H2O ¼ 10 mL per mmol substrate, 110
degrees. b
Isolated yield aer column chromatography.
Table 3 Evaluation of the re-use of SCS-Bi2O3 for the Suzuki-
coupling experiment
Runsa
1 2 3
Yieldb
89 83 75
a
Conditions: 2-amino-3-bromopyridine : 4-triuromethylphenylboronic
acid : SCS-Bi2O3 : Pd(dppf)Cl2 : TBAB ¼ 1 : 1.05 : 0.8 : 0.05 : 0.5 (mmol),
H2O ¼ 10 mL per mmol substrate, 110 degrees, 8 h. b
Yield aer
chromatographic separation.
This journal is © The Royal Society of Chemistry 2014 RSC Adv., 2014, 4, 52181–52188 | 52187
Paper RSC Advances

8. Acknowledgements
This research was supported by University Grants Commission
(41-257-2012-SR), Vision Group Science and Technology,
Department of Science and Technology (no. SR/FT/LS-142/2012)
to Basappa. AS, CDM and B thank, UGC, DST-INSPIRE and
Pavate fellowships respectively. The authors are thankful to
prof. Y. S. Bhat, Bangalore Institute of Technology for surface
area measurement.
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RSC Advances Paper