©
2011 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 84, No. 11, 12611266 (2011) 1261
A Recyclable Solid Acid Catalyst Sulfated Titania for
Easy Synthesis of Quinoxaline and Dipyridophenazine
Derivatives under Microwave Irradiation
Balu Krishnakumar and Meenakshisundaram Swaminathan*
Department of Chemistry, Annamalai University, Annamalainagar 608 002, India
Received May 24, 2011; E-mail: chemres50@gmail.com
TiO2SO42 , prepared by solgel method has been used for the synthesis of quinoxaline and dipyridophenizine
derivatives under microwave irradiation. High-resolution transmission electron microscope (HR-TEM) and atomic force
microscope (AFM) images reveal the corrosion of TiO2 particles by sulfuric acid, which causes an increase in the acidity
of the catalyst. Sulfate loading by H2SO4 increases catalytic activity of TiO2. This catalyst gives an excellent yield with
less reaction time and is an inexpensive, easily recyclable and efficient catalyst for this reaction.
¹
Green chemistry is a rapidly developing new field since it
implies the decrease in crystalline size of TiO2. Both size
provides a proactive avenue for the sustainable development of
future science and technologies.1 Green chemical synthesis
uses highly efficient and environmentally benign synthetic
protocols to deliver life saving medicines, accelerating lead
optimization processes in drug discovery. For green synthesis it
is desirable to avoid any organic solvents as a reaction medium
and to use green catalysts.
reduction and retardation of aggregation result in increase in
2¹
surface area of the catalyst. Morphology of TiO SO
has
2
4
been further analyzed by high-resolution transmission electron
microscope (HR-TEM) and atomic force microscope (AFM).
The use of microwaves in organic synthesis has attracted
considerable attention in recent years, as this method shortens
the time, enhances reaction rates, and improves product
2
527
Quinoxaline and its derivatives are an important class of
yields.
So, we tried the synthesis of quinoxaline and
2
benzoheterocycles displaying a broad spectrum of biological
dipyridopyridine derivatives under irradiation with micro-
waves. Recently, we reported the condensation reaction of
o-phenylenediamine (1a) and benzil (2a) in the presence of
3
,4
activities which has made them privileged structures in
5
combinatorial drug discovery libraries. Dipyridophenazines
have been used as a metal ligand for the formation of ligand
complexes with attractive features. A number of synthetic
sulfated TiO with different concentrations of sulfate at room
2
6
23,28
temperature for 5 min in ethanol medium.
It was found that
strategies have been developed for the preparation of substi-
tuted quinoxalines and dipyridophenazines.6
Though a number of catalysts such as Ga(OTf)3, mont-
this reaction with 5 wt % sulfated TiO could be completed in
2
9
5 min to give 99% yield of quinoxaline. Interestingly we found
that this reaction when performed with the same catalyst under
microwave irradiation in dry media (Scheme 1) could be
completed in 1 min giving a yield of 99% of quinoxaline.
Some of the solid super acid catalysts prepared using
sulfuric acid and metal oxides were reported for the alkylation
1
0
1
1
12
13
morillonite K-10, sulfamic acid, CuSO ¢5H O, Zn(L-
4
2
1
4
15,16
17
proline), I ,
Ni nanoparticles, cellulose sulfuric acid
CSA), silica sulfuric acid (SSA), Zn K-10 clay, acidic
alumina, and MeOH/AcOH under microwave irradiation
2
1
8
19
2+
20
(
2
1
22
2
9,30
have been used for this condensation reaction, they have a few
constraints like the use of strong acids and high reaction
temperatures. As the aforesaid methods are not compatible
with heat or acid sensitive substrates, there is a need to develop
an effective synthesis of quinoxalines employing more eco-
friendly catalysts. Hence, recent research has been focused on
finding new methods to improve the yield of this condensation
reactions.
Herein we report a recyclable, easily separable,
eco-friendly, and effective solid acid catalyst sulfated TiO for
2
the solvent free synthesis of quinoxaline and dipyridophena-
zine derivatives under microwave irradiation.
Aqueous sulfuric acid used in the preparation of the catalyst
protonates titania hydroxyls by an acidbase reaction and hence
3
1,32
the catalyst acts as a strong Lewis acid (Scheme 2).
Surface
2¹
reaction. Characterization of TiO SO4 by FT-IR, SEM,
acidity was determined spectrophotometrically on the basis of
2
EDS, XRD, and BET surface area measurements were reported
irreversible adsorption of an organic base pyridine. The amount
2
3,24
2¹
2¹
earlier.
The SEM image of TiO SO4 shows that the
of pyridine adsorbed by 1 g of TiO and TiO SO
4
are 1600
2
2
2
2¹
2¹
particles are uniformly distributed in spherical shape. SO4
and 2200 ¯g, respectively. This reveals that TiO SO
has
2
4
2
3
modification retarded the aggregation. XRD peaks exactly
more acidic sites than TiO2.
match with the anatase phase of TiO , and sulfate modification
2
2¹
Experimental
o-Phenylenediamine, substituted diamines,
and benzil (Aldrich). AnalaR grade titanium tetraisopropoxide
does not change the phase. TiO SO4 has similar 2ª values
2
to TiO but the peak intensities differ. The FWHM of TiO
Materials.
2
2
2¹
SO4 peaks are higher than TiO and this broadening of peaks
2