Preparation and characterization of Lewis acid grafted sulfonated carbon@titania composites for the multicomponent synthesis of 4H-pyrimido[2,1-b]benzothiazoles and benzoxanthenones under solvent-free conditions

Article abstract of DOI:10.1016/j.molcata.2015.11.001

A series of novel and highly efficient Lewis acids covalently grafted over sulfonic acid functionalized carbon@titania composites were successfully synthesized via sulfonation of carbon@titania composites followed by treatment with different Lewis acids l

Full text of DOI:10.1016/j.molcata.2015.11.001

Accepted Manuscript
Title: Preparation and characterization of Lewis acid grafted
sulfonated carbon@titania composites for the multicomponent
synthesis of 4H-pyrimido[2,1-b]benzothiazoles and
benzoxanthenones under solvent-free conditions
Author: Manmeet Kour Satya Paul James H. Clark Vivek K.
Gupta Rajni Kant
PII:
S1381-1169(15)30141-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.11.001
MOLCAA 9681
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
11-9-2015
26-10-2015
1-11-2015
Please cite this article as: Manmeet Kour, Satya Paul, James H.Clark, Vivek
K.Gupta, Rajni Kant, Preparation and characterization of Lewis acid grafted sulfonated
carbon@titania composites for the multicomponent synthesis of 4H-pyrimido[2,1-
b]benzothiazoles and benzoxanthenones under solvent-free conditions, Journal of
Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2015.11.001
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Preparation and characterization of Lewis acid grafted sulfonated
carbon@titania composites for the multicomponent synthesis of 4H-
pyrimido[2,1-b]benzothiazoles and benzoxanthenones under solvent-free
conditions
Manmeet Koura, Satya Paul*a,James H. Clarkb, Vivek K. Guptac and Rajni Kantc
aDepartment of Chemistry, University of Jammu, Jammu-180 006, India. Fax: +91-191-
2431365; Tel: +91-191-2453969; E-mail: paul7@rediffmail.com
bGreen Chemistry Centre of Excellence, Department of Chemistry, University of York, York
UK. E-mail: james.clark@york.ac.uk
cX-ray Crystallography Laboratory, Department of Physics and Electronics, University of
Jammu, Jammu-180 006, India.
Corresponding author.

Graphical abstract
A series of Lewis acid covalently grafted over sulfonated carbon@titania composites have been
designed and investigated as sustainable recyclable catalytic system for multicomponent synthesis.

Highlights




Design, synthesis and characterization of C/TiO2-SO3M.
4H-Pyrimido[2,1-b]benzothiazoles and benzoxanthenones were synthesised.
Multicomponent synthesis was effectively carried out under solvent-free conditions.
The catalyst showed excellent recyclability with negligible leaching of Lewis acid.

Abstract
A series of novel and highly efficient Lewis acids covalently grafted over sulfonic acid
functionalized carbon@titania composites were successfully synthesized via sulfonation of
carbon@titania composites followed by treatment with different Lewis acids like AlCl3,
FeCl3, SbCl3, SnCl2, Cu(OAc)2 andBi(NO3)3. The utility of the developed catalysts was
explored for the synthesis of a diverse range of 4H-pyrimido[2,1-b]benzothiazoles and
benzoxanthenones, and among various catalysts, C/TiO2-SO3-SbCl2 was found to be the
most active.We report here the synthesis of five novel compounds and the structure of one of
the compounds has also been confirmed by single-crystal X-ray diffraction. All the five
prepared composites were characterized by FTIR and ICP-AES analysis, whereas the most
active one, C/TiO2-SO3-SbCl2 was further characterized by XRD, EDX, CHNS, SEM, TEM,
HRTEM and TGA. The catalyst can be recovered and reused for atleast five runs without
any significant impact on catalytic activity and selectivity. The high catalytic activity, thermal
stability, simple recovery and reusability, and eco-friendly nature of the catalyst makes the
present method to be particularly attractive from the view point of green chemistry.
Keywords: Lewis acids, sulfonated carbon@titania composites, 4H-pyrimido[2,1-
b]benzothiazoles, benzoxanthenones, solid acid catalyst, recylability.

1. Introduction
Lewis acid catalysts such as AlCl3, BF3, and transition metal halides find wide applications in
the production of industrially important chemicals, including polymers and pharmaceuticals
[1]. However, these homogeneous catalysts are highly toxic, moisture sensitive, show air
intolerance and expensive. In addition, the recovery and reuse of homogeneous Lewis acid
catalysts is an extremely formidable obstacle. Recently, the use of Lewis acids supported on
‘‘inert’’ carriers has received considerable importance [2-5]. The improved activity, greater
selectivity, ease of handling, enhanced reaction rates, simple workup and recyclability are
other common features that make the use of supported Lewis acids as attractive alternatives
to conventional homogeneous reagents. For this purpose, biomass-derived porous carbons
have been found to be potential supports for the preparation of carbon-supported catalysts
with a wide range of catalytic applications. Such carbonaceous catalysts are environmentally
benign and could provide a cost-competitive advantage as compared to existing
heterogeneous catalysts. Excellent physical properties of carbon materials such as chemically
inert nature, stability against various chemical environments, low cost, hydrophobicity and
tunable surface properties make such catalytic systems compatible with diverse catalysis
reactions [6-9]. Recently, carbon based materials have been dispersed over inorganic oxides
like silica, titania etc. to develop hybrid composite materials that show synergistic effect of
both the organic and inorganic support material on the catalytic performance of the final
catalyst [10-14]. Such hybrid materials show enhanced chemical and thermal stability, higher
catalytic activity and availability of active surface groups for the linkage of the active
catalytic species.
Multicomponent reactions (MCRs) have become powerful tools in organic, combinatorial
and medicinal chemistry, and have attracted much attention from synthetic organic chemists
because they can build complex molecules with a diverse range of complexities from readily

available starting materials [15-18]. 4H-Pyrimido[2,1-b]benzothiazole derivatives are an
important class of fused heterocycles due to their broad range of potential biological
activities, as well as their importance in the preparation of drug molecules and natural
products [19-23]. Xanthenes and benzoxanthenones represent the most common structural
motifs found in several natural products and synthetic bioactive compounds. Over the recent
years, these have attracted considerable attention due to their diverse biological properties as
well as their use as dyes and fluorescent materials [24-27]. Synthetic strategies mediated
through the use of variable catalytic systems have been reported in literature for the synthesis
of 4H-pyrimido[2,1-b]benzothiazoles [28-31] and benzoxanthenones [32-43]. Many of these
synthetic methods, however, have limitations such as harsh reaction conditions, long reaction
time, expensive catalysts, and generation of noticeable amount of side products. Thus, there
is an ample scope for the development of new greener synthetic protocols to assemble such
scaffolds.
In this context, considering the broad applicability of carbon based organic-inorganic hybrid
materials, we have designed and synthesized novel and sustainable catalytic system utilizing
carbon@titania composites functionalized by sulfonic acid as the support material for the
immobilization of different Lewis acids. The catalytic activity of the solid Lewis acid
catalysts has been explored for the one-pot multicomponent synthesis of 4H-pyrimido[2,1-
b]benzothiazoles and benzoxanthenones under solvent-free conditions. The aim of this
protocol is to highlight the synergistic effect of the combined use of multicomponent
coupling reactions under solvent-free conditions and application of solid Lewis acid catalyst
supported on carbon@titania composites for the development of new eco-friendly strategy for
heterocyclic synthesis.

2. Experimental
2.1 Reagents and instrumentation
All starting materials were purchased from commercial sources and used without further
1
purification. The H and 13C NMR data were recorded in CDCl3 or DMSO-d6 or
CDCl3+DMSO-d6 on Bruker Avance III (400 MHz) spectrometer. The FTIR spectra were
recorded on Thermo Nicolet, Avatar 370 spectrophotometer, XRD was recorded in 2θ range
of 10-80° on Bruker AXS D8 Advance and mass spectral data on Bruker Esquires 3000
(ESI). CHNS analysis was recorded on ThermoFinnigan FLASH EA 1112 series. SEM
images were recorded using JEOL Model JSM-6390LVScanning ElectronMicroscope,
Transmission Electron Micrographs (TEM) were recorded on Philips CM-200. EDX analysis
was carried out using JEOL Model JED-2300 and TGA was recorded on Perkin Elmer,
Diamond TG/DTA.
2.2 Preparation and Characterization of Lewis acid grafted sulfonated carbon@titania
composites
Initially, carbon@titania composites were prepared by the partial carbonization of starch in
the presence of nano-titania leading to the formation of amorphous carbon@titania
composites. Thereafter, the functionalization of the as prepared composites with sulfonic
acid group was done using conc. sulfuric acid followed by treatment with different lewis
acids viz AlCl3, FeCl3, SbCl3, SnCl2, Cu(OAc)2, Bi(NO3)3,thereby forming respective Lewis
acid grafted sulfonated carbon@titania composites [C/TiO2-SO3-AlCl2, C/TiO2-SO3-FeCl2,
C/TiO2-SO3-SbCl2, C/TiO2-SO3-SnCl, C/TiO2-SO3-Cu(OAc), C/TiO2-SO3-Bi(NO3)2].
Among the different inorganic oxides available, we have used titanium dioxide because of its
unique surface properties. Besides, titanium dioxide has been shown to exhibit promising
electrochemical properties, high photocatalytic activity, high chemical and thermal stability,
easy commercial availability and non-toxicity[44-45]. Titania and its composites have usually

been studied for their photocatalytic properties[46-49], whereas in the present work titania
based carbon composites have been studied for catalytic activity.
In a typical procedure, a mixture of starch and nano-titania (nanopowder, <100nm particle
size (BET), >14.0 m2/g; 10 g) in the ratio of 1: 1.2 was taken in a round-bottom flask and
heated at 353 K for 10 h. The incomplete carbonization of resulting mixture was done by
heating strongly at 673 K under N2 atmosphere for 10 h leading to the formation of
amorphous carbon@titania composites. Afterwards, the sulfonation of the as prepared
composites was done by adding concentrated sulfuric acid (>96 wt%) and further heated at
423 K for 10 h under N2 atmosphere. The sulfonated composite obtained was then washed
repeatedly with hot distilled water (>353 K) until sulfate anions were no longer detected in
the filtered water. Sulfonated carbon@titania composites were dried in an oven at 373 K for 2
h (7 g) and then converted into lewis acid grafted sulfonated carbon@titania composites
through anion metathesis. The solid acid (2 g) was treated with different lewis acids (0.5
mmol) viz AlCl3, FeCl3, SbCl3, SnCl2, Cu(OAc)2 or Bi(NO3)3 in acetonitrile (10 mL) at
reflux temperature for 24 h. After cooling to room temperature, the mixture was filtered,
washed with acetonitrile and dried under vacuum and finally kept at 90 ˚C overnight. The
catalysts were conditioned by refluxing for 12 h each in xylene at 130 ˚C (2×2 h), ethanol at
78 ˚C (2×2 h) and acetonitrile at 80 ˚C (2×2 h) and finally dried in oven at 90 ˚C overnight.
The general procedure for the synthesis of Lewis acid grafted sulfonated carbon@titania
composites has been shown in Scheme 1.All the solid Lewis acid catalysts were
characterized by FTIR and ICP-AES analysis. The most active catalyst, C/TiO2-SO3-SbCl2
was further characterized by XRD, CHNS, SEM, TEM, HRTEM and TGA.

FTIR
Successful functionalization and modification of the carbon @titania composites can be
inferred from the FTIR analysis. The FTIR spectrum of Lewis acid grafted sulfonated
carbon@titania composites shows the presence of a broad band in the range 3412-3472 cm-1
due to phenolic OH groups of the polycyclic aromatic carbon rings. The strong absorptions in
the range 1700-1720 and 1606-1649 cm-1 were assigned to C=O and C=C stretching
frequencies respectively. The C=O groups were produced by the oxidation of carbon by small
amounts of sulfuric acid used for sulfonation.In addition, the presence of sulfonic acid group
was confirmed by characteristic peaks at 1152-1180 and 1031-1137 cm-1 due to O=S=O
asymmetric and symmetric stretching vibrations. The presence of a well defined band in the
range 526-563 cm-1 is characteristic of TiO2 attributable to Ti-O stretching frequency. It is
also worth noting that there are no significant changes observed in the spectra of sulfonated
carbon@titania composite and Lewis acid grafted sulfonated carbon@titania composites as
the linkage of Lewis acid to SO3H of the catalyst has only a slight decreasing effect on the
S=O stretching frequency (Fig. S1, ESI and Fig. 1). Also there is no significant difference in
the FTIR spectra of different Lewis acid catalysts as is clear from the Table 1.
ICP-AES
The amount of metal loaded onto the solid Lewis acid catalysts was determined by
inductively coupled plasma atomic emission spectroscopy and it was found that 0.28 wt%
Al, 0.58 wt% Fe, 1.26 wt% Sb, 1.23 wt% Sn, 0.60 wt% Cu and 2.17 wt% Bi were present in
C/TiO2-SO3-AlCl2, C/TiO2-SO3-FeCl2, C/TiO2-SO3-SbCl2, C/TiO2-SO3-SnCl, C/TiO2-SO3-
Cu(OAc) and C/TiO2-SO3-Bi(NO3)2 respectively.

CHNS
The amount of sulfonic acid group grafted onto the surface ofC/TiO2-SO3-SbCl2 was
determined by elemental analysis. The loading of 1.5 wt% sulfur indicated that 0.32 mmole
of -SO3H groups were grafted per gram of C/TiO2-SO3-SbCl2.
XRD
The XRD pattern of C/TiO2-SO3-SbCl2exhibited diffraction peaks corresponding to
combination of both rutile and anatase phase of TiO2. The peaks at 25.1°, 41.0°, 48.2°,
54.1°and 68.8°correspond to the (101), (004), (200), (211) and (116) reflections of anatase
TiO2 whereas peaks at 27.2°, 35.8° and 56.4° correspond to the (110), (101) and (211) planes
of rutile phase [50-51]. The observed diffraction peaks agree well with the tetragonal
structure of TiO2 (Fig. 2). This factindicates that the crystalline structure of titania
nanoparticles is conserved even after the composite formation with carbon. The average
crystallite size of titania nanoparticles was determined by Scherrer’s equation and found to be
24 nm.
Energy dispersive X-ray analysis (EDX)
The elemental composition of C/TiO2-SO3-SbCl2 was determined from energy dispersive X-
Ray (EDX) analysis. The EDX spectrum, displayed in Fig. 3, clearly reveals the presence of
all the expected main elements C, S, O, Ti, Sb and Cl in C/TiO2-SO3-SbCl2.
Scanning electron microscopy (SEM)
The shape and surface morphology of the C/TiO2-SO3-SbCl2 was investigated by scanning
electron microscopy (Fig. 4). The SEM micrographs of the composite material exhibited

irregular massivecongeries with relatively rough surface depicting the amorphous nature of
the composite formed.
Transmission electron microscopy (TEM)
To provide more accurate information on the particle size and fine structure of the catalyst,
transmission electron microscopy (TEM) was carried out. It can be seen from the TEM
micrographs that the nanoparticles of TiO2 with spherical morphology are partially
agglomerated on the surface of the carbon material (Fig. 5a, b). The sulfonated
carbonmaterial with immobilized antimony chloride appears as light grey spherical particles
over which black coloured spots of titania can be visualised. The average size of titania
nanoparticles as determined from TEM analysis was found to be 26 nm which is in
accordance with the results obtained from XRD data (Fig. 5c). The TEM image of the
C/TiO2-SO3-SbCl2 does not show any significant change from the TEM micrographs of the
sulfonated carbon@titania composites (C/TiO2-SO3H) depicting that the microstructure of the
catalyst is preserved even after its modification by treatment with Lewis acid (Fig. 5d). The
HRTEM image of the C/TiO2-SO3-SbCl2 clearly shows the lattice fringes with d-spacing of
0.32 nm and 0.35 nm corresponding to (110) plane of rutile TiO2 and (101) plane of anatase
TiO2 respectively (Fig. 5e), thereby confirming the presence of highly crystalline TiO2 on the
carbon material of the composite. In addition, the TEM micrographs were also taken for the
recovered catalyst. Interestingly, it was observed that the shape and size of the particles
remain unchanged which ultimately provides evidence that the morphology of the catalyst
remains the same even after several catalytic runs.
Thermal gravimetric analysis (TGA)
The thermal stability of C/TiO2-SO3-SbCl2 was investigated by carrying out the thermal
analysis in the temperature range of 40-700 °C in a static atmosphere of nitrogen (Fig. 6).

The primary weight loss from the catalyst up to 100 °C is attributed to the elimination of the
physically adsorbed solvent and chiefly surface adsorbed water, whereas weight loss at
o
higher temperatures from 240-700 Ccould be largely attributed to thermal decomposition of
the organic moieties on the surface of the composite. Thus, from the TGA analysis, it can be
concluded that the catalyst is thermally stable upto 240 °C, confirming that it could be safely
used in organic reactions at 100 °C
2.3 General procedure for the C/TiO2-SO3-SbCl2catalyzed synthesis of 4H-
pyrimido[2,1-b]benzothiazoles
To a mixture of ethylacetoacetate (1.0 mmol), aldehyde (1.0 mmol) and 2-
aminobenzothiazole (1.0 mmol) in a round bottom flask (25 mL), C/TiO2-SO3H-SbCl2 (0.1g)
o
was added and the reaction mixture was heated at 90 C for the appropriate time. On
completion of the reaction (monitored by TLC), hot ethanol (2×10 mL) was added and the
catalyst was separated by simple filteration. Removal of the solvent under reduced pressure
afforded the product, which was further purified by crystallization from ethanol.
2.4 General procedure for the C/TiO2-SO3-SbCl2 catalyzed synthesis of
benzoxanthenones
A mixture of α- or β-naphthol (1.0 mmol), aldehyde (1.0 mmol), dimedone (1.0 mmol) and
C/TiO2-SO3H-SbCl2 (0.1g) was taken in a round bottom flask (25 mL) and heated at 100°C
under solvent-free conditions for the appropriate time. After completion of the reaction as
monitored by TLC, hot ethanol (2×10 mL) was added and catalyst was separated by simple
filteration. The filtrate was concentrated to get the crude product, which was crystallized
from ethanol. The catalyst was washed with double distilled water (3×10 mL) followed by
CH2Cl2 (2×10 mL). It was dried at 90 oC for 2 h and reused for subsequent reactions.

13
All the products were characterized by 1H, C NMR and mass spectral data and comparison
with authentic samples obtained commercially or prepared according to the literature
methods. The structure of compound 5lwas also confirmed by single-crystal X-ray
diffraction.
3. Results and Discussion
3.1Catalytic testing for the one-pot synthesis of 4H-pyrimido[2,1-b]benzothiazoles
Initially, to select the most appropriate solid Lewis acid catalyst for the one-pot
multicomponent synthesis of 4H-pyrimido[2,1-b]benzothiazoles, the reaction of 2-
aminobenzothiazole (1.0 mmol), 3-nitrobenzadehyde (1.0 mmol) and ethylacetoacetate (1.0
mmol) was investigated as the model reaction. It was found that C/TiO2-SO3-SbCl2 was the
most effective catalyst in terms of reaction time and yield among the different lewis acid
catalysts screened (Table 2, entry 3). The high activity of C/TiO2-SO3-SbCl2 may be due to
the mild nature of the Lewis acid grafted on the sulfonated composite. Strong Lewis acids
like AlCl3 and FeCl3 lead to undesirable side reactions and thus, reduces the desired product
formation. Whereas, otherweak Lewis acids tend to be less active and thus need longer
reaction time. Further, the effect of catalyst amount on the model reaction was also examined
by varying the catalyst amount to 0.05 g (0.5 mol% Sb), 0.1 g (1 mol% Sb), 0.2 g (2 mol%
Sb) and 0.3 g (3 mol% Sb). It was found that 0.1 g (1 mol% Sb) of C/TiO2-SO3-SbCl2 was
sufficient to get the optimum product yield and further increasing the catalyst amount did not
increase the yields considerably (Table 3, entry 5, 6, 9, 10). To optimize the reaction
conditions, the effect of different solvents and reaction temperature on the model reaction
was studied. The model reaction was examined in different solvents such as water, ethanol,
acetonitrile as well as under solvent-free conditions. It was found that higher product yields
were obtained when the reaction was carried out under solvent-free conditions in comparison

to liquid phase conditions (Table 3). The highest catalytic activity under solvent-free
conditions may be due to the good dispersion of active reagent sites which facilitates better
contact between reactant molecules and the catalyst [53]. Moreover, due to the absence of
any solvent (as medium), there is no dilution effect and the heat needed for energy of
activation is directly available to the reactant molecules. Further, to select the optimum
reaction temperature, the model reaction was carried out under solvent-free conditions at 80,
°
°
90, 100 and 120 C, and 90 C was choosen to be the optimum reaction temperature
considering the product yield and reaction time (Table 3, entry 6). Subsequently, with
optimal conditions in hand, to study the scope and limitations of the present protocol, a wide
range of substituted aldehydes were allowed to undergo three component condensation in the
presence of C/TiO2-SO3-SbCl2 under solvent-free conditions and the desired products were
obtained in quantitative yields (Table 4, products 5a-m). The single crystal X-ray
crystallography confirmed the formation of ethyl 2-methyl-4-(3-methoxyphenyl)-4H-
pyrimido[2,1-b][1,3]benzothiazole-3-carboxylate (Fig. 7, S3, ESI). The scope of the reaction
was further extended using dimedone as the 1,3-dicarbonyl compound for the
multicomponent synthesis under the optimized conditions. The reaction using dimedone also
proceeded smoothly with different aldehydes leading to the formation of fused thiazolo[2,3-
b]quinazolinones in good to excellent yields (Table 4, 6a-e). Therefore, the present protocol
has general applicability accommodating a variety of substitution patterns.
3.2 Catalytic testing for the one-pot synthesis of benzoxanthenones
The catalytic activity of different Lewis acid catalysts has been evaluated for the one-pot
synthesis of benzoxanthenones using aldehyde (1 mmol), dimedone (1 mmol) and β-naphthol
(1 mmol). C/TiO2-SO3-SbCl2 was again found to be the most active catalyst for the one-pot
multicomponent synthesis of benzoxanthenones among the different Lewis acid catalysts
screened (Table 2, entry 3). To derive the optimal reaction parameters such as the amount of

the catalyst required, reaction temperature, and the influence of the solvent for the C/TiO2-
SO3-SbCl2 catalyzed benzoxanthenone formation, the model reaction was performed under
different variations of these parameters (Table 3). The optimum amount of catalyst for the
model reaction was found to be 0.1 g (1 mol% Sb) in order to obtain the best results. The
influence of the reaction medium was evaluated for the model reaction in protic and aprotic
solvents as well as under solvent free conditions (Table 3) in the presence of catalytic
amount of C/TiO2-SO3-SbCl2 and again best results were obtained under solvent-free
conditions (Table 3 entry 7). Further, the model reaction was carried out by varying the
reaction temperature and the optimum reaction temperature was found to be 100 °C, as
increase of the reaction temperature to 120 °C did not show a significant increase in the
product yield (Table 3, entry 8), which however, decreased to 80% and 50% on lowering the
reaction temperatures to 90 and 80 °C, respectively (Table 3, entries 4 and 5). Using the
optimized conditions and to evaluate the scope of the developed method, the present reaction
was further extended to a broader range of substituted aldehydes and found that the reaction
works well for both electron-donating and electron-withdrawing groups (Table 5, 11a-j). In
order to demonstrate generality and scope of this new efficient methodology, the optimized
reaction conditions were investigated using 1-naphthol also and the results are summarized in
Table 5. Again, almost quantitative yields were obtained with various substituted aldehydes
for the desired products (Table 5, 12a–j) under the optimized conditions. Finally, the scope
of the present protocol was further investigated using other 1,3-dicarbonyl compounds like
ethylacetoacetate and acetylacetone instead of dimedone for quantitative preparation of
benzoxanthenones. However, the three component reaction of aldehyde, 2-naphthol and
etylacetoacetate/acetylacetone afforded aryl-14H-dibenzo[a,j]xanthenes (13a-d) instead of
expected formation of benzoxanthenones. The higher reactivity of aldehydes with 2-naphthol
in comparison to ethylacetoacetate and acetylacetone lead to the formation of benzoxanthenes

instead of the expected product. Thereafter, the reaction was also tried with other substituted
aldehydes and the products were obtained in almost quantitative yields (Table 5, 13a-d)
under the optimized conditions.
To establish the catalytic performance of the present catalyst for the one-pot multicomponent
synthesis of 4H-pyrimido[2,1-b]benzothiazoles and benzoxanthenones, the test reaction in
each case was carried out in the presence of nano-titania, amorphous carbon, non-sulfonated
carbon@titania, sulfonated amorphous carbon, antimony chloride grafted sulfonated carbon,
anhyd. SbCl3 and without using any catalyst under the same reaction conditions (Table 6). As
is clear from Table 6, the hybrid composite (C/TiO2-SO3-SbCl2) showed an enhanced
catalytic performance compared with nano-titania, amorphous carbon, non-sulfonated
carbon@titania, sulfonated amorphous carbon and antimony chloride grafted sulfonated
carbon catalyst. The amorphous carbon provided a large specific surface area with high
density of catalytically active sites, allowing high loading of sulfonic acid functionality, and
thus facilitating the linkage of Lewis acid. Whereas, the nano-titania in the composite
provides good mechanical and thermal stability to the composite. In addition, nano-titania
also provides large surface, on which biomaterial is well dispersed and carbonized without
aggregation of the carbon particles. Hence, due to the synergistic effect of nano-titania and
amorphous carbon, the composite material exhibited enhanced catalytic activity. It is
noteworthy to mention that the use of homogeneous anhyd. SbCl3 as catalyst gave the desired
product in shorter reaction times in comparison to the heterogeneous C/TiO2-SO3-SbCl2
catalyst, but the homogeneous Lewis acid catalyst get decomposed during the work-
upprocedure whereas heterogeneous C/TiO2-SO3-SbCl2 could be easily recovered and reused
for several catalytic cycles and also gave higher conversion. Thus, C/TiO2-SO3-SbCl2 was
found to be superior and thus selected as the heterogeneous catalyst to carry out the present
multicomponent synthesis.

Finally, to examine the efficiency of the present protocol for the synthesis of 4H-
pyrimido[2,1-b]benzothiazoles and benzoxanthenones, it has been compared with some of the
previously reported methods. The results summarized in Table 7, show that the present
protocol is indeed superior to several of the others in terms of high TOFs which clearly
indicate the high catalytic activity of the present catalytic system with such low amounts of
lewis acid loading onto the sulfonated composite material. Besides, high product yields, low,
reaction time and elimination of solvent are other added advantages of the present protocol.
3.4 Recyclability
In case of solid Lewis acids, the study of catalytic activity is an important aspect. To
investigate this, the catalyst was recovered from the reaction mixture by simple filtration,
o
dried in vacuum oven at 100 C and was reused as such for subsequent experiments under
similar reaction conditions (Table 4, 5i, Table 5, 11a). The activity of the recovered catalyst
did not decrease appreciably even after five consecutive runs, and no significant loss of
conversion was observed, depicting the high stability of the catalyst under the reaction
conditions (Fig. 8). In addition, no quantifiable amount of leached antimony was detected in
the filtrate by ICP-AES analysis, hence it can be concluded that the catalytic process is truly
heterogeneous.
Reaction conditions: 2-aminobenzothiazole (1mmol), 3-nitrobenzaldehyde (1mmol), ethylacetoacetate
o
(1mmol), C/TiO2-SO3-SbCl2 (0.1g, 1 mol% Sb) at 90 C under solvent-free conditions (Table 4, 5i).
Reaction conditions: 2-naphthol (1 mmol), benzaldehyde (1 mmol), dimedone (1mmol), C/TiO2-SO3-
SbCl2 (0.1 g, 1mol% Sb) at 100 oC under solvent-free conditions (Table 5, 11a)
4. Conclusion
In conclusion, the present work describes the synthesis and characterization of novel
heterogeneous Lewis acid catalyst that exhibits excellent catalytic activity, high thermal and

chemical stability, thus increasing the applicability of Lewis acids in different reaction
conditions that are otherwise highly sensitive to moisture and air. The developed catalyst is
air-stable, strongly acidic and showed high catalytic potential for the eco-friendly
multicomponent synthesis of 4H-pyrimido[2,1-b]benzothiazoles and benzoxanthenones. High
TOFs, short reaction time, high product yield, mild reaction conditions, facile preparation and
reusability of the catalyst are the noteworthy advantages of the present catalytic protocol.
5. Acknowledgements
The authors gratefully acknowledge The Head, SAIF, IIT Bombay for CHNS, TEM, HRTEM
and ICP-AES studies; Head SAIF Kochi for FTIR, XRD, SEM, EDX and TGA analysis.
Financial support from UGC, New Delhi (Major Research Project, F 41-281/2012 (SR)) is
highly acknowledged. One of the authors (MK) is thankful to UGC for project fellowship.
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Fig. 1 FTIR spectra of C/TiO2-SO3-SbCl2.

Fig. 2 XRD of C/TiO2-SO3-SbCl2.

Fig. 3 EDX spectra of C/TiO2-SO3-SbCl2.

Fig. 4SEM images of C/TiO2-SO3-SbCl2.

Fig. 5(a, b)TEM images of C/TiO2-SO3-SbCl2; (c) Particle size histogram; (d)TEM image of
C/TiO2-SO3H; (e, f)HRTEM images of C/TiO2-SO3-SbCl2.

Fig. 6 TGA of C/TiO2-SO3-SbCl2.

Fig.
7
ORTEP view of Ethyl 2-methyl-4-(3-methoxyphenyl)-4H-pyrimido[2,1-
b][1,3]benzothiazole-3-carboxylate (CCDC No. 1409951).