Functionalized mesoporous materials as efficient organocatalysts for the syntheses of xanthenes

Article abstract of DOI:10.1016/j.molcata.2012.06.017

Highly efficient and recyclable heterogeneous organocatalysts containing acidic functional groups such as CO₂H and SO₃H have been synthesized by post-synthesis modification of 2D-hexagonal mesoporous silica materials SBA-15 and MCM-4

Full text of DOI:10.1016/j.molcata.2012.06.017

Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
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Journal of Molecular Catalysis A: Chemical
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Functionalized mesoporous materials as efficient organocatalysts for the
syntheses of xanthenes
John Mondala, Mahasweta Nandia,b, Arindam Modaka, Asim Bhaumika,∗
a Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur 700032, India
b Integrated Science Education and Research Centre (ISERC), Visva-Bharati University, Santiniketan, Birbhum, West Bengal 731235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly efficient and recyclable heterogeneous organocatalysts containing acidic functional groups such
as CO2H and SO3H have been synthesized by post-synthesis modification of 2D-hexagonal meso-
porous silica materials SBA-15 and MCM-41, respectively. The CO2H functionalized material is obtained
via Schiff-base condensation of 3-aminopropyl grafted SBA-15 with 4-formylbenzoic acid, whereas
SO3H functionalized material is prepared via partial oxidation of 3-mercaptopropyl grafted MCM-41
with dilute aqueous H2O2. These functionalized mesoporous materials have been thoroughly character-
ized by powder X-ray diffraction (PXRD), nitrogen adsorption/desorption studies, transmission electron
microscopy (TEM) and UV visible diffuse reflectance spectroscopy. Both these materials have highly
ordered 2D-hexagonal mesoporous structures with high surface areas and well-defined pore sizes. These
acid functionalized materials have been used as heterogeneous catalysts for the condensation of aromatic
aldehydes with 2-napthol under mild conditions in the presence or absence of a solvent affording high
yields of the value added xanthenes.
Received 6 February 2012
Received in revised form 22 June 2012
Accepted 30 June 2012
Available online 7 July 2012
Keywords:
Functionalized mesoporous materials
Organocatalyst
Heterogeneous catalysis
Solvent-free conditions
Xanthenes
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
by immobilizing the former over high surface area insoluble solid
supports [9]. In this context functionalized mesoporous silicas can
Heterogeneous catalysts bearing active metal centers have
been studied extensively during the past few decades [1–4].
Although metal catalysis occupies a prominent position in syn-
thetic organic chemistry, owing to the toxic effects associated with
metal-containing catalysts, increasing interests have been paid
for the development of catalysts devoid of metal and these cat-
alytic reactions are broadly known as organocatalysis [5]. Despite
being known for many decades, organocatalysis remained mostly
unrecognized [6]. But in the recent years there has been a rapid
development in this field, attributed to the versatile opportuni-
ties of catalyst design offered by organocatalytic systems. Although
such transformations can be considered as a green process, how-
ever, contrary to metal based catalytic systems, organocatalytic
reactions are mostly homogeneous in nature [7,8]. Thus catalyst
recovery and its reuse is an important drawback associated with
these organocatalytic reactions. Apart from economic advantages
it is also very essential to elude the accumulation of unwanted
waste products to make the processes environmentally benign.
The most extensively practiced technique to avoid this problem
is to transform a homogeneous catalyst to a heterogeneous one
be very promising support due to their exceptionally high surface
area, tunable nanoscale pores and wide range of compositional
variations [10–15]. Design and synthesis of surface functional-
ized ordered mesoporous silica materials have attracted particular
attention for their application in catalysis [16–19], adsorption
[20,21], ion-exchange [22], sensing [23] and so on. Rational design
of these materials using suitable organic functionalities give rise
to smart mesoporous materials [24], which can have potential
applications in many other frontier areas of science. Thus the syn-
theses of organically modified mesoporous materials containing
active functional groups are highly desirable. One of the convenient
approach to synthesize these functionalized mesoporous materials
is by anchoring the desired functionalities on silica matrices with
high specific surface area. In such cases, a covalent interaction could
be established between the organic functional group and the silica
framework, which prevents the leaching of the active centers in the
reaction medium [25].
Xanthenes are a class of compounds, which are hugely recog-
nized in the field of medicine and organic chemistry due to their
antibacterial [26], anti-inflammatory activities [27] and photody-
namic therapy [28]. They are the source of a class of brilliant
fluorescent dyes and are used extensively in laser technology
and pH sensitive fluorescent materials [29]. Various methods
for the syntheses of substituted xanthenes have been achieved
∗
Corresponding author.
E-mail addresses: msab@iacs.res.in, abhaumik68@yahoo.co.in (A. Bhaumik).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.06.017

J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
255
Scheme 1. Schematic diagram for the preparation of CO2H functionalized SBA-15.
including the cycloacylation of carbamates [30], trapping of
benzynes by phenol [31], cyclocondensation between 2-hydroxy
aromatic aldehydes and 2-tetralone [32], and intramolecular
phenyl carbonyl reaction of aldehydes with 2-naphthol by dehy-
dration [33]. Most of these syntheses procedures have been carried
out by metal containing catalysts [34] but the drawbacks associ-
ated with these reactions are long reaction time, low product yield,
vigorous reaction conditions, toxic effect of metals and their leach-
ing during the course of reaction [35]. Absence of the possibility of
metal leaching, low toxicity, simple reaction process and reusabil-
ity make heterogeneous organocatalysts potentially more useful
over conventional metal-based catalysts.
In this paper we will discuss about two acid functionalized
organocatalysts supported on mesoporous silica backbone using
two different synthetic approaches. In one approach surface car-
boxylic acid ( CO2H) functionalized mesoporous silica has been
prepared via Schiff-base condensation of 3-aminopropyl grafted
SBA-15 [36]. In the other case sulfonic acid ( SO3H) functional-
ized mesoporous silica has been prepared from 3-mercaptopropyl
grafted MCM-41 via partial oxidation with hydrogen peroxide
(H2O2). Both the acid functionalized materials can be used as
catalyst for the condensation reaction of aromatic aldehyde and
2-naphthol to prepare different value added xanthene derivatives.
The CO2H functionalized mesoporous silica can be used as a cata-
lyst under ambient conditions in the presence of dichloromethane
(DCM) as the solvent, whereas for the SO3H functionalized
material catalysis proceeds under solvent-free conditions at
363 K.
To it 0.5 g F127 is added under stirring, followed by the addition
of 0.6 g TMB. To the homogeneous solution obtained after stir-
ring for 2 h, 2.1 g TEOS is added and the mixture is stirred for
24 h. The resulting synthesis gel is autoclaved at 373 K for 24 h
and the solid product is recovered by filtration, washed several
times with water and dried under vacuum in a lypholyzer. The
template-free mesoporous SBA-15 is obtained by calcination of
the as-synthesized material at 723 K for 6 h in flow of air. This
was further functionalized via condensation with organosilane pre-
cursor 3-aminopropyltriethoxy silane (3-APTES) by stirring 0.1 g
of SBA-15 with 0.18 g 3-APTES in chloroform at room tempera-
ture under nitrogen atmosphere for 12 h. Then this aminopropyl
functionalized mesoporous SBA-15 is refluxed with 0.15 g of 4-
formylbenzoic acid in 20 ml methanol for 6 h to undergo Schiff base
condensation. The final product is collected by filtration and repeat-
edly washed with hot methanol to remove any unreacted acid
precursor.
2.1.2. Synthesis of SO3H functionalized MCM-41 (AFS-2)
Synthesis of SO3H functionalized mesoporous silica, AFS-2
has been carried out by oxidation of thiol group in MCM-41
prepared in situ using a mixture of organosilane precursor 3-
mercaptopropyltrimethoxy silane (3-MPTMS) and TEOS in 1:3
molar ratio as silica precursor (Scheme 2).
In a typical synthesis procedure, 2.73 g of cetyltrimethylam-
monium bromide (CTAB, Loba Chemie) is dissolved in an aqueous
solution of tartaric acid (0.6 g in 30 g H2O) and the resulting mix-
ture is stirred for about 30 min to obtain a clear solution. To it
0.98 g of 3-MPTMS is added, stirred for another 30 min and fol-
lowed by the addition of 3.18 g of TEOS. The reaction mixture is
stirred for another 2 h to obtain a gel and then NaOH (2 M) solution
is added drop wise into it until the pH reaches ca. 12.0. The gel is
aged overnight under continuous stirring, transferred into a Teflon-
lined steel autoclave and kept at 348 K for 72 h. After hydrothermal
treatment the solid product is isolated by filtration, washed several
times with water and dried under vacuum. The CTAB template is
removed from the as-synthesized material by extracting the solid
thrice in a mixture of acid/ethanol. Then the thiol groups ( SH)
of this template-free material are converted into SO3H group by
treating with excess H2O2 at 333 K for 24 h [37]. Then it is filtered,
washed with water and ethanol and acidified with 0.1 M H2SO4 to
obtain the SO3H functionalized MCM-41.
2. Experimental
2.1. Preparation of mesoporous acid functionalized materials
2.1.1. Synthesis of CO2H functionalized SBA-15 (AFS-1)
Synthesis of CO2H functionalized mesoporous silica, AFS-1
has been carried out as described in our previous communication
(Scheme 1) [36].
Mesoporous silica, SBA-15 is synthesized at first through evap-
oration induced self-assembly (EISA) method using non-ionic
pluronic F127 (Sigma–Aldrich) as the structure directing agent.
Trimethyl benzene (TMB) (E-Merck) has been used as the additive
and tetraethyl orthosilicate (TEOS) as the silica source. In a typi-
cal synthesis, 6 g conc. HCl (E-Merck) is added to 25 ml of water.

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J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
Scheme 2. Schematic diagram for the preparation of SO3H functionalized MCM-41.
2.2. Catalysis
2.3. Instrumentation
2.2.1. Synthesis of xanthene derivatives catalyzed by acid
functionalized mesoporous organocatalysts
Powder X-ray diffraction patterns of the samples are recorded
Bruker AXS D8 Advanced diffractometer using Cu K␣
on
a
˚
In a typical procedure (Scheme 3), a mixture of 2 mmol ␤-
naphthol and 1 mmol aromatic aldehyde is well stirred with
20 mg of AFS-1 organocatalyst in the presence of 5 ml dry
dichloromethane (DCM) at room temperature for suitable time.
After completion of reaction, the solid catalyst is filtered of from
the reaction mixture and washed thoroughly with DCM. Then the
filtrate is evaporated to dryness under reduced pressure to give the
solid product. The crude product is crystallized from ethanol and
characterized by 1H and 13C NMR spectroscopy.
(ꢀ = 1.5406 A) radiation. TEM images are recorded in a Jeol JEM
2010 transmission electron microscope. N2 adsorption/desorption
isotherms of the sample have been recorded on a Beckman Coulter
SA 3100 automatic gas adsorption apparatus, at 77 K. Prior to the
measurement, the samples are degassed at 373 K for 6 h under high
vacuum. UV–visible diffuse reflectance spectra have been obtained
by using a Shimadzu UV 2401PC spectrophotometer with an inte-
grating sphere attachment. BaSO4 pellet is used as background
standard. Thermogravimetry (TG) and differential thermal analy-
sis (DTA) of the samples are carried out on a TA instrument Q600
DSC/TGA thermal analyzer. The solid NMR spectra of the samples
are measured using Bruker MSL 500 spectrometer. Liquid state
NMR spectra of the products obtained during catalysis have been
measured on a Bruker Avance DPX 300 NMR (300 MHz) spectrom-
eter using TMS as the internal standard.
2.2.2. Synthesis of xanthene derivatives catalyzed by SO3H
functionalized mesoporous organocatalyst under solvent free
condition
In a typical catalytic cycle (Scheme 4), a mixture of 2 mmol ␤-
naphthol and 1 mmol of aromatic aldehyde is heated at 353 K in
the presence of 20 mg of AFS-2 organocatalyst in an oil bath for
the necessary time (determined by product analysis using thin
layer chromatography (TLC)). Then the reaction mixture is cooled,
diluted with 10 ml DCM and the catalyst is separated from the reac-
tion mixture by filtration followed by thorough washing with DCM.
Then the filtrate is evaporated to dryness under reduced pressure
to get the crude product which is then crystallized from ethanol.
The crystallized products are characterized by 1H and 13C NMR
spectroscopy.
3. Results and discussion
The small-angle powder X-ray diffraction (XRD) patterns of
mesoporous SBA-15, carboxylic acid functionalized SBA-15 before
and after using it as a catalyst are shown in Fig. 1. For SBA-15
(Fig. 1a) four well-resolved diffraction peaks in the 2ꢁ region of
0.6–2.07 can be observed which can be indexed to the 1 0 0, 1 1 0,
2 0 0 and 2 1 0 reflections [10] corresponding to a two-dimensional
hexagonal mesostructure. When the silica framework is grafted
with 3-APTES followed by its functionalization with 4-formyl-
benzoic acid a decrease in the intensity of the peaks is observed
(Fig. 1b), however the 2D-hexagonal ordering is retained. This
decrease in the peak intensities is attributed to the lowering of local
order, viz. variations in the wall thickness or reduction of scattering
contrast between the channel walls of the silicate framework [38].
The PXRD pattern of AFS-1 after its use as a catalyst for six reaction
cycles (Fig. 1c) shows that it retains its mesoporous structure,
although the ordering of the pores becomes quite disordered.
Further, as seen from Fig. 1c that peak has been significantly
shifted. Possibly during six repetitive reaction cycles contraction of
2.2.3. Recovery of the catalyst
After the reaction is over, the reaction mixture is separated
by filtration and washed four times with DCM followed by three
times with diethyl ether. Then the catalyst is dried at 348 K for
6 h and used for further reaction cycles. The COOH and SO3H
functionalized organocatalyst have been recycled for six consecu-
tive times for the condensation reaction between ␤-naphthol and
4-chlorobenzaldehyde.

J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
257
Scheme 3. General procedure for the synthesis of xanthene over CO2H functionalized SBA-15.
Scheme 4. General procedure for xanthene synthesis over SO3H functionalized MCM-41.
Scheme 5. Reaction of thiophene 2,5-dialdehyde and 2-naphthol over CO2H functionalized SBA-15.
pore-wall occurs, which could be responsible in decrease in
d-spacing. Similar results can be seen for the powder X-ray diffrac-
tion of the MCM-41 based materials. 3-MPTMS-MCM-41 shows
a highly order two-dimensional mesoporous structure (Fig. 1d),
whereas the material after treatment with hydrogen peroxide
to give the SO3H functionalized MCM-41, losses some of its
ordering (Fig. 1e). The PXRD pattern of this material (Fig. 1f) after
six reaction cycles shows that the material retains its mesoporous
structure after repeated use. Fig. 2 depicts the representative
transmission electron microscopy (TEM) images of mesoporous
SBA-15 (Fig. 2a) and AFS-1 (Fig. 2b). The uniform and long range
ordering of the large mesopores are clearly seen throughout the
respective specimens. The corresponding FFT diffractogram for
the mesoporous SBA-15 material has been shown in the inset of
Fig. 2a which also indicates the presence of hexagonally arranged
pores [39,40]. The TEM images of 3-MPTMS-MCM-41 (Fig. 2c) and
AFS-2 (Fig. 2d), also show similar hexagonally ordered mesoporous
structure, but the size of the pores here are smaller (ca. 2 nm)
compared to the SBA-15 analogs. The hexagonal arrangement
of pores can also be confirmed from the FFT diffractogram of
3-MPTMS-MCM-41 given in the inset of Fig. 2c.
2
4
6
8
10
d
e
f
Nitrogen sorption at 77 K over
CO2H functionalized
mesoporous SBA-15 (not shown here) shows type IV
adsorption–desorption isotherms with very large H2 type hystere-
sis loop in the 0.5–0.8 P/P0 range. Such type of isotherm suggests
the presence of large uniform mesopores connected by windows
of small cage-like pores and confirms the mesoporous nature of
the sample [39]. The BET (Brunauer–Emmett–Teller) surface area
a
b
c
and pore volume of the material are 250 m2 g−1 and 0.199 cm3 g−1
,
1
2
3
4
respectively. The values of P/P0 near the inflection point are closely
related to the pore-widths, which lie in the mesopore range and the
sharpness of these steps indicates the uniform size distribution of
the mesopores [39]. Pore size of the sample estimated employing
the Barett–Joyner–Halenda (BJH) method is ca. 9.16 nm, which
is in good agreement with the pore widths depicted by the TEM
2 in degree
Fig. 1. Small angle powder XRD patterns of SBA-15 (a), CO2H functionalized SBA-
15 (b), CO2H functionalized SBA-15 after six reaction cycles (c), 3-MPTMS-MCM-
41 (d), SO3H functionalized MCM-41 (e) and SO3H functionalized MCM-41 after
six reaction cycles (f).

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J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
100
90
80
70
60
50
image analysis and XRD studies. The nitrogen sorption isotherm
for AFS-2 is shown in Fig. 3. The sharp increase in N2 uptake for
adsorption is observed at a lower P/P0 ca. 0.05–0.2 compared to
the AFS-1 sample based on MCM-41 structure. The surface area
for this material is 320 m2 g−1 and the pore volume 0.135 cm3 g−1
.
0.015
0.010
0.005
0.000
The pore size distribution plot for this material obtained by using
non-local density functional theory (NLDFT) is given in the inset
of Fig. 3. The average pore width of this material was ca. 1.84 nm,
which is also supported by the PXRD and TEM analyses.
1.84 nm
IR spectroscopy has been employed to characterize the presence
of CO2H and SO3H groups in the functionalized mesoporous
organocatalysts. In Fig. 4 FTIR spectra of AFS-1 and 2 samples
are shown. As seen from the figure that the weak absorbance
at 1470 and 1420 cm−1 is observed with the introduction of
3-mercaptopropyltrimethoxysilane functionality in AFS-2. These
peaks could be assigned to the bending vibrations of methylene
groups. Further, absorbance in the range of 2850–2930 cm−1 corre-
sponding to the methylene stretching vibrations of the propyl chain
appeared, indicating the incorporation of the organic moiety in AFS-
2. Before the oxidation of the mercaptopropyl group a very weak
peak at 2567 cm−1 (Fig. 4a, inset) was observed. This peak could
be assigned to the S H stretching, which disappeared upon H2O2
treatment, demonstrating that the thiol group gets completely oxi-
dized into SO3H group (Fig. 4b) [41]. The peaks corresponding to
the S O stretching vibrations of sulfonic acid are normally observed
in the range of 1000–1200 cm−1. However, these peaks cannot be
1.5
2.0
2.5
3.0
Pore width (nm)
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Fig. 3. Nitrogen adsorption (filled symbol)/desorption (empty symbol) isotherm of
AFS-2 at 77 K. Inset: corresponding pore size distribution using NLDFT method.
groups. The bands at ca. 1200, 1100 and 800 cm−1 are the char-
acteristic bands of SiO2 assigned to the Si O stretching vibrations
while the bands at 950 and 470 cm−1 are assigned to the bend-
ing vibrations of the surface silanols and Si O, respectively. For
the CO2H functionalized organocatalyst (AFS-1) the peaks in the
range 1693 cm−1 are attributed for the C O stretching frequency
(Fig. 4c). On the other hand the peaks at 1394 cm−1 and 3400 cm−1
resolved due to their overlap with the absorbance of the Si
O
Si
R
stretch in the 1000–1130 cm−1 range and that of the Si CH2
stretching in the 1200–1250 cm−1 range (Fig. 4b). The spectra for
silica show peaks in the range of 3600–3200 cm−1 attributed to
the hydroxyl stretching of the hydrogen bonded internal silanol
Fig. 2. Transmission electron microscopy images of SBA-15 (a), CO2H functionalized SBA-15 (b), 3-MPTMS-MCM-41 (c) and SO3H functionalized MCM-41 (d).

J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
259
- a- CO2H
- b- SO3H
a
b
100
80
956
(SiCH R)
1243
2
(Si-OH)
(Si-O-Si) 1093
1693
60
40
20
0
(C=O)
c
1646
1393
3400
(-OH)
(C=N)
(-COH)
2500 2525 2550 2575 2600 2625
Wavenumber in cm-1
4000
3000
2000
1000
1
2
3
4
5
6
Wavenumber in cm-1
Number of catalytic cycles
Fig. 4. FTIR spectra of SH functionalized mesoporous organosilica (a), SO3H
functionalized organocatalyst AFS-2 (b) and CO2H functionalized organocatalyst
AFS-1 (c). S-H signal at 2567 cm−1 has been expanded in the inset.
Fig. 5. Recycling efficiency of functionalized mesoporous organocatalysts AFS-1 (a)
and AFS-2 (b).
yield of the products and time taken for maximum conversion of
the substrates in each case, are listed in Table 1.
(broad) correspond to the C
O H bending and OH stretching
vibrations, respectively. Further, the peak at 1646 cm−1 could be
assigned to the C N stretching frequency of AFS-1 (Fig. 4c). Thus
FTIR spectroscopic results suggested the grafting of the respective
organic functionalities in AFS-1 and -2 materials (Schemes 1 and 2).
Quantitative determinations of the amount of the organic moi-
eties have been estimated to calculate the total number of acid sites
present in these materials. It is directly related to the active sites
participating in the catalytic reactions and thus we could estimate
the turn over frequency (TOF) of the catalytic reactions based on
this acidity data. The results of the titrimetric analyses show that
the loading of the CO2H and SO3H groups in these acid function-
alized mesoporous materials is 4.5 and 3.7 mmol g−1, respectively.
The reaction when performed with 1-naphthol instead of 2-
naphthol, keeping the other conditions same, did not produce
any xanthene product. The aldehydes with both electron donat-
ing (Table 1, entries 3 and 4) and electron withdrawing groups
(Table 1, entries 2, 5, 6, 7, 8, 9 and 12) participated in the con-
densation reaction with equal efficiency. Thus, the nature and
position of substitution in the aromatic ring did not affect the reac-
tions much. The meta-substituted aromatic aldehydes (Table 1,
entry 5, entry 7) as well as sterically hindered ortho-substituted
aromatic aldehydes (Table 1, entries 6 and 12), both undergo con-
densation reaction without any difficulty. When fluoro substituted
aromatic aldehyde is taken (Table 1, entry 5) the fluoro groups
are found to remain inert during the course of condensation reac-
tion. Several sensitive functional groups such as Cl, Br, and
NO2 attached with the aromatic ring are also compatible for this
reaction. The turn over number (TON) for different reactions is
moderately good, which confirms the high catalytic efficiency of
the acid functionalized mesoporous silica materials in these reac-
tions. When the reaction is performed at 313 K under identical
conditions, the yield of the product (Table 1, entry 11) increases
slightly, suggesting that the condensation reaction becomes more
facile at higher temperature. When the condensation reaction is
performed with thiophene 2,5-dialdehyde (Scheme 5) one of the
CHO group takes part in the reaction, while the other remains
intact. 14-(4-Formylphenyl)-14H-dibenzo [a, j] xanthenes, which is
obtained via the condensation reaction of terephthaldehyde and 2-
naphthol under refluxing condition (Scheme 6) is highly useful for
the synthesis of different target organic molecules. To confirm the
catalytic role of CO2H functionalized SBA-15, the condensation
reaction between 4-methylbenzaldehyde and 2-naphtol has been
carried out in the absence of catalyst (Table 1, entry 10). It is found
that even after continuing the reaction for a prolonged time no
corresponding xanthene product is observed. We have also carried
out the reaction in the presence of pure silica mesoporous SBA-15
(Table 1, entry 13) and NH2 functionalized SBA-15 (Table 1, entry
14) for 4-chlorobenzaldehyde. However, no xanthenes derivative
was obtained under these conditions too. This data clearly suggests
that acid sites present at the catalyst surface are crucial to carry out
this condensation reaction.
3.1. Catalysis: synthesis of xanthenes
The acid functionalized mesoporous materials have been used
as catalysts for the synthesis of xanthene via condensation reaction
between aromatic aldehydes and 2-naphthol. The condensation
reaction of various aromatic aldehydes (1a–j) with 2-naphthol (2)
in the presence of optimized amount of CO2H functionalized
organocatalyst under mild reaction condition gave 14-aryl-14H-
dibenzo [a, j] xanthenes (3a–j) (Scheme 1) [42]. It has been found
that 0.02 g of CO2H functionalized SBA-15 is the optimum amount
of catalyst required to carry out the reactions. Increasing the
amount of catalyst does not improve the yield of the products
any further, whereas decreasing the amount of catalyst leads to
decrease in the product yield. Under optimized reaction conditions
The condensation reaction between 4-chlorobenzaldehyde
and 2-naphthol has been performed using CO2H functionalized
Scheme 6. Reaction of terephthaldehyde and 2-naphthol over CO2H functional-
ized SBA-15.

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J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
Table 1
CO2H functionalized SBA-15 catalyzed synthesis of 14-aryl-14H-dibenzo [a, j] xanthenes 3.a
Entry
1
Aldehyde
Time (h)
Yieldb (%)
80
Product
3a
TONc
8.8
CHO
6.0
Br
CHO
CHO
CHO
2
3
4
4.0
5.0
5.0
75
80
78
3b
3c
3d
8.3
8.9
8.7
HO
H3C
F
5
6
7
5.0
4.0
4.0
80
86
82
3e
8.8
9.5
9.1
CHO
CHO
NO 2
3f
Br
3g
CHO
O2N
Cl
CHO
8
9
4.0
4.5
82
75
–
3h
3i
–
9.1
8.3
–
CHO
CHO
10d
11e
16.0
4.0
Cl
H3C
CHO
84
3d
9.3
CHO
Cl
12
4.0
86
3j
9.5

J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
261
Table 1 (Continued)
Entry
Aldehyde
Time (h)
10.0
Yieldb (%)
Product
–
TONc
13f
–
–
–
Cl
CHO
CHO
14g
10.0
–
–
Cl
a
Reaction conditions: Ar-CHO: 1.0 equiv., 2-naphthol: 2.0 equiv., solvent: dichloromethane, 5 ml, reaction temperature: 298 K, catalyst: 20 mg.
Isolated yield of pure product.
Turn over number (TON) = moles of substrate converted per mole of active site.
Reaction carried out in the absence of catalyst.
Reaction carried out at 313 K.
Reaction carried out with mesoporous SBA-15.
b
c
d
e
f
g
Reaction carried out with amine functionalized SBA-15.
SBA-15 catalyst in different solvents; the results are given in Table 2.
can be seen that the catalyst can be recycled and reused effec-
tively without significant loss of its catalytic activity as well as
mesoporous structure (Fig. 1c).
All these condensation reactions are carried out at the same tem-
perature (333 K) and the yield are reported after same reaction
time (6 h). The best conversion takes place when dichloromethane
is chosen as the solvent, whereas in solvents like tetrahydro-
furan or water, the conversion is almost negligible. In case of
solvents such as acetonitrile, dimethylformamide, ethanol and
toluene, moderate conversion takes place, with yields ca. 40–50%.
The condensation reaction between 4-chlorobenzaldehyde
and 2-naphthol has been taken as a representative case to
The condensation reaction of aromatic aldehydes (4a–f) and 2-
naphthol (5) at 353 K under solvent-free condition to synthesize
xanthene derivatives (6a–f) (Scheme 6) have been studied in the
presence of SO3H functionalized MCM-41 as the catalyst. The
results (Table 3) suggest that the reactions proceed very smoothly
under solvent-free condition and give 14-aryl-14H-dibenzo [a, j]
xanthenes in good yield within a short period of time. The TONs
for all the reactions are very high, which indicates the high effi-
ciency of the SO3H functionalized catalyst. The reactions are not
affected by the nature of substituents (electron withdrawing and
electron donating) attached with the aromatic ring as well as the
check the recycling efficiency of the
CO2H functionalized
SBA-15 organocatalyst. The recovered catalyst (as mentioned
previously) after each reaction cycle is recycled further for six
consecutive times and the results have been plotted in Fig. 5a. It
Fig. 6. Proposed catalytic cycle for condensation reaction of aromatic aldehyde and 2-naphthol in the presence of CO2H functionalized SBA-15.

262
J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
Table 2
position of substitution. The catalyst has been reused for six con-
secutive cycles for the condensation reaction between 2-naphthol
and 4-chlorobenzaldehyde and the results are given in Fig. 5b.
A control experiment with the nonporous sulfonic acid function-
alized catalyst prepared via copolymerization of styrene-divinyl
benzene followed by sulphonation with the H2SO4 to introduce
SO3H group was performed (Table 3, entry 8). Internal diffusional
is the inherent property for the porous solid catalyst which becomes
limited for the nonporous solid catalyst. From the above data it is
clear that pores play a decisive role in our mesoporous organocat-
alyst for the diffusion of organic molecules, which facilitates the
condensation reaction and as result rate of the catalytic reaction
becomes greater than the nonporous catalyst.
Synthesis of xanthene derivative over CO2H functionalized SBA-15 using different
solvents.
Entry
Solvent
Yielda (%)
1
2
3
4
5
6
7
Dichloromethane
Tetrahydrofuran
Acetonitrile
Dimethylformamide
Ethanol
80.0b
Trace
45.0
50.0
30.0
Water
Toluene
Trace
25.0
a
Reaction carried out at 333 K for 6 h.
Reaction carried out at 313 K for 4 h as dichloromethane solvent has boiling
b
point 313 K.
Estimated loading of CO2H and SO3H groups in AFS-1 and
2 materials are 4.5 and 3.7 mmol g−1, respectively. Although the
loading of SO3H group and thus total acid strength of the cata-
lyst AFS-2 is less than that of CO2H group of AFS-1, the reaction
proceeds very smoothly for the SO3H functionalized catalyst. This
result proves that the acid strength of the SO3H group is greater
than that of CO2H group. Since, in this acid catalyzed condensa-
tion reaction is facilitated by the increase of electrophilicity of the
aldehyde group, stronger acid site promotes this reaction. Further,
the surface areas of AFS-1 and -2 are 250 m2 g−1 and 320 m2 g−1
,
respectively. Surface area also plays a crucial role in the rate of the
Table 3
SO3H functionalized MCM-41 catalyzed synthesis of 14-aryl-14H-dibenzo [a, j] xanthenes (6) under solvent free conditions.a
Entry
1
Aldehyde
Time (h)
Yieldb (%)
Product
3a
TONc
12.1
CHO
1
90
H3C
CHO
CHO
2
3
2
2
92
90
3b
3c
12.4
12.1
Br
F
4
5
2.5
88
95
3d
3e
11.8
12.8
CHO
Cl
CHO
CHO
1.5
2
O2N
6
88
–
3f
–
11.8
7d
16
–
CHO
CHO
8e
2
30
3a
2.7
a
Reaction conditions: Ar-CHO: 1.0 equiv., 2-naphthol: 2.0 equiv., reaction temperature: 353 K, catalyst: 20 mg.
Isolated yield of pure product.
Turn over number (TON) = moles of substrate converted per mole of active site.
Reaction carried out in the absence of catalyst.
b
c
d
e
In the presence of nonoporous polymer containing SO3H group as catalyst.

J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
263
catalytic reaction, as higher surface area helps favorable diffusion
and thus facilitates the reaction. Due to the larger surface area of
AFS-2 there is favorable diffusion of organic molecules in the pore
channels and as a result the rate of the catalytic reaction is quite
high for AFS-2 compared to that of AFS-1. Thus due to higher sur-
face area and stronger acidity the reaction becomes quite facile over
SO3H functionalized material AFS-2.
It can be observed that the SO3H functionalized organocatalyst
can be repeatedly used for many reaction cycles without apprecia-
ble loss of activity.
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Plausible reaction pathway for the condensation reaction over
CO2H functionalized SBA-15, as a representative case, is shown
in Fig. 6. The acid functionalized organocatalyst acts as a protonic
acid which increases the electrophilicity of the carbonyl carbon of
aromatic aldehyde that undergoes 1,4-addition reaction with 2-
naphthol, leading to the formation of the corresponding product,
A. A on further reaction with another molecule of 2-naphthol in the
presence of the acid catalyst forms B, which undergoes cyclization
with the elimination of water molecule to give the xanthene deriva-
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in case of the SO3H functionalized AFS-2 catalyst. Since the acid
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4. Conclusions
In conclusion, we have developed two metal-free acid cata-
lysts based on mesoporous silica SBA-15 and MCM-41. The CO2H
functionalized SBA-15 has been prepared via Schiff-base conden-
sation of 3-aminopropyl grafted SBA-15 with 4-formyl benzoic
acid, whereas the SO3H functionalized catalyst has been syn-
thesized via in situ incorporation of mercaptopropyl group into
mesoporous silica, MCM-41, followed by the oxidation of SH
to SO3H. Both these materials are highly ordered 2D-hexagonal
structure, mesoporosity and retained the organic functionalities at
their surfaces. These materials can be used for the condensation
reaction of aromatic aldehyde and 2-naphthol for the synthesis of
various xanthene derivatives in good yield, under mild conditions
in the presence or absence of solvents. These catalytic reactions
proceed in the absence of any other external metal co-catalyst and
hence they are designated as environmentally benign organocat-
alytic pathways. Both these organocatalysts can be recovered easily
after each catalytic cycle and show high recycling efficiency, which
makes them highly attractive for their potential use in large scale
synthesis of value added organic fine chemicals.
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[42] 1H and 13C NMR chemical shifts for different xanthene derivatives reported in
Table 1. 14-(Phenyl)-14H-dibenzo [a, j] xanthene (Table 1, entry 1): 1H NMR
(300 MHz, DMSO-d6) ı 8.69 (2H, d, J = 9Hz), 7.93 (4H, d, J = 9 Hz), 7.64 (6H, m),
7.47 (2H, t), 7.16 (2H, t), 6.98 (1H, t), 6.71 (1H, s); 13C NMR (300 MHz, DMSO-
d6) ı 147.9, 145.4, 130.8, 130.6, 128.9, 128.5, 128.3, 127.9, 126.8, 126.1, 124.4,
123.4, 117.6, 117.3, 36.5; EIMS, 70 Ev, m/z: 358.32 (M+). 14-(4-Bromophenyl)-
14H-dibenzo [a, j] xanthene (Table 1, entry 2): 1H NMR (300 MHz, DMSO-d6)
ı 8.65 (2H, d, J = 9 Hz), 7.94 (4H, d, J = 9 Hz), 7.64–7.53 (6H, m), 7.48 (2H, t), 7.34
(2H, d, J = 9 Hz), 6.72 (1H, s); 13C NMR (300 MHz, DMSO-d6) ı 147.8, 144.8, 131.2,
130.7, 130.6, 130.0, 129.2, 128.5, 126.9, 124.5, 123.3, 119.4, 116.8, 38.6; EIMS,
70 Ev, m/z: 436.79 (M+). 14-(4-Hydroxyphenyl)-14H-dibenzo [a, j] xanthene
(Table 1, entry 3): 1H NMR (300 MHz, DMSO-d6) ı 9.69 (1H, s), 8.43 (1H, d,
J = 9 Hz), 7.57 (1H, d, J = 9 Hz), 7.49–7.42 (7H, m), 7.27–7.19 (5H, t), 6.94 (1H,
d, J = 6 Hz), 6.69 (1H, d, J = 9 Hz), 6.45 (1H, s); 13C NMR (300 MHz, DMSO-d6) ı
154.2, 149.2, 137.9, 131.9, 131.4, 129.9, 129.3, 129.2, 127.6, 124.6, 123.3, 118.4,
117.6, 115.8, 37.5; EIMS, 70 Ev, m/z: 374.34 (M+). 14-(4-Methylphenyl)-14H-
dibenzo [a, j] xanthene (Table 1, entry 4): 1H NMR (300 MHz, CDCl3) ı 8.44
(2H, d, J = 9 Hz), 7.86–7.79 (4H, m), 7.64–7.58 (8H, m), 7.00 (2H, d, J = 9 Hz), 6.48
(1H, s), 2.1 (3H, s); 13C NMR (300 MHz, CDCl3) ı 148.7, 142.2, 135.9, 131.5, 131.1,
129.2, 128.8, 128.1, 126.8, 124.2, 122.7, 118.0, 117.5, 37.7, 20.9; EIMS, 70 Ev,
m/z: 372 (M+). 14-(3-Fluorophenyl)-14H-dibenzo [a, j] xanthene (Table 1,
entry 5): 1H NMR (300 MHz, CDCl3) ı 6.51(s, 1H), 6.72–8.35 (m, 16H); 13C NMR
(75 Hz, CDCl3) ı 38.2, 113.8 and 90, 114.0 (JC-F 21.5 Hz), 115.4 and 115.8 (JC-F
21.5 Hz), 117.2, 118.2, 122.7, 124.31, and 124.32 (JC-F 2.8 Hz) 124.7, 127.3, 129.3,
129.5, 130.1and130.2 (JC-F 8.3 Hz), 131.4, 131.7 (JC-F 19.4 Hz), 147.8, 147.9 (JC-F
6.2 Hz), 149.2, 161.7, 165.1; EIMS, 70 Ev, m/z: 376 (M+). 14-(2-Nitrophenyl)-
14H-dibenzo [a, j] xanthene (Table 1, entry 6): 1H NMR (300 MHz, DMSO-d6)
ı 10.28 (1H, d, J = 6 Hz), 9.76 (1H, d, J = 9 Hz), 7.97–7.66 (4H, m), 7.40–7.01 (10H,
m), 5.75 (1H, s); 13C NMR (300 MHz, DMSO-d6) ı 32.7, 118.0, 118.3, 123.2, 124.5,
125.1, 125.3, 127.7, 128.0, 129.3, 129.6, 129.9, 130.6, 132.2, 132.6, 134.4, 141.2,
147.5, 149.8; EIMS, 70 Ev, m/z: 425.25 (M+) + Na+. 14-(3-Bromophenyl)-14H-
dibenzo [a, j] xanthene (Table 1, entry 7): 1H NMR (300 MHz, DMSO-d6) ı
8.71 (2H, d, J = 9 Hz), 7.94 (4H, d, J = 9 Hz), 7.81 (1H, s), 7.69–7.67 (3H, m), 7.58
(2H, d, J = 9 Hz), 7.48 (2H, t), 7.18–7.10 (2H, m), 6.76 (1H, s); 13C NMR (300 MHz,
DMSO-d6) ı 155.8, 153.1, 148.6, 148.5, 135.1, 134.6, 131.3, 131.2, 130.9, 129.8,
129.2, 129.1, 128.2,127.6, 125.2, 123.8, 122.3, 120.2, 118.2, 117.3, 109.2, 36.6;
EIMS, 70 Ev, m/z: 436 (M+). 14-(4-Nitrophenyl)-14H-dibenzo [a, j] xanthene
Acknowledgements
JM and AM thank CSIR, New Delhi for their respective senior
research fellowships. This work is partly funded by Nanoscience
and Nanotechnology Initiative of DST, New Delhi.
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264
J. Mondal et al. / Journal of Molecular Catalysis A: Chemical 363–364 (2012) 254–264
(Table 1, entry 8): 1H NMR (300 MHz, CDCl3) ı 8.30–8.46 (4H, m), 7.66–7.72 7.58 (2H, t), 7.45–7.33 (5H, m), 7.22 (1H, s), 6.89–6.81 (2H, m), 6.75 (1H, s); 13
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153.0, 135.7, 134.2, 130.5, 129.4, 129.2, 128.2, 128.1, 126.6, 124.3, 123.1, 119.4,
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