Sulfonic acid functionalized mesoporous SBA-15 for one-pot synthesis of substituted aryl-14H-dibenzo xanthenes and bis(indolyl) methanes

Article abstract of DOI:10.1016/j.catcom.2010.06.004

The comparative catalytic activity studies of biologically and industrially important compounds such as Xanthenes and bis (indolyl) methanes were evaluated over propylsulfonic acid functionalized SBA-15/SO₃H. The material was characterized via

Full text of DOI:10.1016/j.catcom.2010.06.004

Catalysis Communications 11 (2010) 1148–1153
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Catalysis Communications
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Sulfonic acid functionalized mesoporous SBA-15 for one-pot synthesis of substituted
aryl-14H-dibenzo xanthenes and bis(indolyl) methanes
⁎
Mallari A. Naik, Divya Sachdev, Amit Dubey
Chemistry Group, Birla Institute of Science and Technology, Pilani-333031, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The comparative catalytic activity studies of biologically and industrially important compounds such as
Xanthenes and bis (indolyl) methanes were evaluated over propylsulfonic acid functionalized SBA-15/SO3H.
The material was characterized via standard characterization techniques such as Powder X-ray diffraction,
FTIR, Temperature programmed desorption studies of NH3. The catalytic performance of SBA/SO3H under
environmentally benign conditions results in high yield of xanthenes compared to bis (indolyl) methanes
and finally the mechanism for both the reactions were proposed.
Received 21 April 2010
Received in revised form 31 May 2010
Accepted 4 June 2010
Available online 12 June 2010
Keywords:
Xanthenes
© 2010 Elsevier B.V. All rights reserved.
Bis (indolyl) methanes
SBA-15
Heterogeneous catalysis
1. Introduction
catalyzed cyclisation of polycyclic aryltriflate esters [10], intermolec-
ular trapping of benzynes by phenol [11], reagents like Sulfamic acid
In the current industrial scenario, heterogeneous catalysis plays an
important role in replacing hazardous homogeneous reagents for one
step organic transformations [1]. Many heterogeneous methods have
been developed during last two to three decades to overcome these
difficulties but activity and selectivity of the desired products still
remains a challenging area. Hence, the development of new protocols
in synthesizing products selectively in multi-component syntheses
via heterogeneous systems is strongly encouraged. Xanthenes and bis
(indolyl) methanes are of immense interest because of their versatile
utility in many pharmaceutically derived applications. These com-
pounds are known to exhibit a wide spectrum of therapeutic and
pharmacological properties such as anti-bacterial [2], anti-inflamma-
tory [3] and anti-viral [4]. Particularly, xanthenes have various
applications in industries such as dyes in laser technology [5] and
fluorescent material for visualization of biomolecules [6]. Similarly bis
(indolyl) methanes, also a biologically active compound tremendous-
ly used as an intermediate in various organic syntheses. Bis (indolyl)
methanes such as Vibrindole-A, is gaining prominence in view of their
occurrence in bioactive metabolites [20–22]. Recently, numerous
methods have been developed for the synthesis of Xanthenes and bis
(indolyl)methanes including cyclodehydrations [7–9], palladium
[12], Amberlyst 15 [13], molecular Iodine [14], Alum/H2O [15], BF3/
SiO2 [16], ionic liquids with ultrasound irradiation [17], NaHSO4/SiO2
[18] and Yb(OTf3) [19]. In spite of many methods available for the
synthesis of these compounds, there is still a scope to develop simpler,
recyclable and cost effective processes for one-pot multi-component
transformations. Ordered mesoporous materials (2–50 nm) are
getting tremendous attention because of their outstanding advan-
tages such as large surface area, uniform and tunable pore diameter,
pore volume and easy tailoring of internal surfaces by functionaliza-
tion with acidic, basic and redox moieties. Following the discovery of
SBA-15 silica materials, the organosulphonic acidic moieties were
incorporated in many ways especially during the in-situ synthesis
followed by the oxidation of thiol groups to generate sulphonic acid
groups and achieve maximum acidity. Recently, silica supported
sulphuric acid was used effectively for the synthesis of Xanthenes
[23,24] and bis (indolyl) methanes but the nature of the support was
not described in detail. Moreover, reports on the detailed mechanism
for the synthesis of xanthenes and bis (indolyl) methanes on the
ordered functionalized mesoporous silica materials are scanty.
Therefore, in the present report, we aim to use the sulphonic acid
functionalized SBA-15 silica materials by post synthetic method for
the comparative structure activity studies of xanthenes and bis
(indolyl) methanes. The endeavor of selecting xanthenes and bis
(indolyl) methanes stems from the different nucleophilic nature of
naphthol and indole. Furthermore the inert nature of naphthol and
facile polymerization of indole may be critical to understand the
comparative mechanism of these useful reactions under heteroge-
neous and homogeneous conditions.
⁎
Corresponding author. Tel.: +91 1596 245073*429; fax: +91 1596 244183.
E-mail addresses: amitdubey@bits-pilani.ac.in, amitdubey75@yahoo.com
(A. Dubey).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.06.004

M.A. Naik et al. / Catalysis Communications 11 (2010) 1148–1153
3. Results and discussions
1149
2. Experimental
2.1. Preparation of SBA-15/SO3H catalysts
3.1. Catalyst characterization
All the chemicals were procured from Sigma Aldrich. The synthesis
of SBA-15 was carried out in accordance to the earlier reports [25–27]
using Pluronic (P123) (EO20PO70EO20, MW=5800, Aldrich) as
surfactant and TEOS (ACROS, 98%) as the silica source. In a typical
synthesis batch, 3 g of P123 was dissolved in 100 g of distilled water
and 5.9 g of conc. HCl (35%). After stirring for 3 h, 6.5 g of TEOS was
added together at 35 °C maintaining the molar ratio of P123:H2O:HCl:
TEOS as 1:5562.9:86.29:42.51 respectively. The mixture was left
under stirring for 24 h at 35 °C, and subsequently heated for 24 h at
100 °C under static conditions in a closed polypropylene bottle. The
solid product obtained after the hydrothermal treatment was filtered
and dried at 80 °C. The surfactant was removed by extraction in an
ethanol–HCl mixture and finally by calcining at 550 °C for 6 h. Surface
functionalization of SBA-15 was carried out by post grafting method
wherein 2 g of SBA-15 with 0.49 g of 3-mercaptopropyltrimethox-
ysilane (MPTMS) was stirred in dry toluene (50 ml) at 120 °C for 24 h
to obtain 30% of acidic sites on SBA-15. The –SH groups present were
converted to –SO3H groups by mild oxidation with H2O2 by
continuous stirring for 24 h at 60 °C. The modified SBA-15 was
filtered and washed in soxhelet apparatus with ethanol for 10 h.
Similarly SBA/SO3H catalysts with different amount of MPTMS loading
were synthesized with similar reaction conditions.
PXRD at low angles showed the spectrum similar to the SBA-15
suggesting that the ordered structure of SBA-15 is retained after
introducing the propylsulfonic acid group (Fig. 1). The FTIR for SBA-15
and SBA-15/SO3H (10%) (Fig. S1) exhibits a broad intense signal in the
1300–1000 cm−1 region corresponding to the characteristic Si–O–Si
stretching adsorption. The –CH aliphatic stretch (of the propyl chain)
at 2942 cm−1, and peaks around 1450 and 1375 cm−1 arising from –
SO3H group in SBA-15/SO3H sample clearly indicates incorporation of
sulfonic group in SBA-15. Total number of acidic sites was determined
by NH3/TPD (Fig. S2). The amount of acidic sites increases with
increase in the loading of SO3H in SBA-15. The acidic values for 5, 10,
20 and 30% loading of SO3H in SBA-15 are 26, 110, 330 and 439 μmol/g
respectively.
3.2. Catalytic studies
Synthesis of xanthenes and bis (indolyl) methanes were optimized
on their respective model reactions (Scheme 1, R=Ph and Scheme 2,
R=Ph). Various parameters were studied to obtain the suitable
conditions for the synthesis of xanthenes and bis (indolyl) methanes.
Variation of different solvents showed that dichloroethane is the best
solvent for both the conversion under our experimental conditions
(Table S1). The optimized conditions were further extended for the
synthesis of substituted xanthenes and bis (indolyl) methanes
(Tables 1 and 2).
2.2. Characterization techniques
PXRD pattern was recorded on Hecus X-Ray Systems S3 Model
from 0 to 10°. The IR spectra of samples (as KBr pellets) were recorded
using a Shimadzu FTIR spectrophotometer in the range of 400–
4000 cm −1. The number of acidic sites on the surface was determined
by ammonia TPD method. Melting points were determined using SRS
Melting Point apparatus and are uncorrected. 1H NMR spectra were
recorded on Bruker 400 MHz NMR spectrometer. All the reactions
were monitored by TLC technique on a 0.2 mm silica gel F-254 plates.
All products were characterized by comparing their IR, 1HNMR and
melting point with those reported in literature.
3.2.1. Effect of SO3H loading on SBA-15
Fig. 2 shows the effect of the percentage loading of SO3H on SBA-15
on the catalytic activity of both the reactions. The catalytic conversion
as well as the yield for both xanthenes and bis (indolyl) methanes
increased with increasing concentration of acidic contents (SO3H
loading), however maximum yield (95%) of xanthenes was achieved
with only 5% loading of the active centers (SO3H groups) compared to
(70%) yield of bis (indolyl) methanes for the same loading. The lower
yield of bis (indolyl) methanes can be attributed to the polymeriza-
tion of indole (reaction mixture turns red) with the progress of the
reaction. As a result, the degree of polymerization of indole over
different temperatures was verified under similar reaction conditions.
It was observed that the rate of polymerization increases at higher
temperature which was later confirmed by verifying the yield of the
product and unreacted starting materials.
2.3. Catalytic studies
2.3.1. Synthesis of aryl-14H-dibenzo [a, j] xanthenes
In a typical synthesis batch, 2 mmol of 2-Napthol, 1 mmol of
benzaldehyde and (10–100 mg) of the SBA-15/SO3H (catalyst) in
10 ml of solvent dichloroethane were stirred and refluxed at 90–95 °C
for 8 h in a glass reactor. The reaction mixture was periodically
monitored with TLC and catalyst was filtered after the completion of
the reaction. The filtrate obtained was dried over silica gel in vacuum
and purified by column chromatography using 10% polar mobile
phase (hexane: ethylacetate=9:1) to isolate the pure product, aryl-
14H-dibenzo [a, j] xanthenes.
2.3.2. Synthesis of bis (indolyl) methane
A mixture of benzaldehyde (1 mmol), indole (2 mmol) and the
desired amount of catalyst (10–100 mg), in 10 ml of acetonitrile
was stirred at 60 °C for 6 h. The reaction was monitored periodically
and completion of the reaction was confirmed by TLC. The mixture
was treated with aq. Na2S2O3 solution (5%, 10 ml) and the product
was extracted with ethyl acetate (3×5 ml). The ethyl acetate was
dried over anhydrous sodium sulphate, and the product was
separated by column chromatography (petroleum ether: ethyl
acetate=1:9).
Fig. 1. Powder XRD pattern of (a) SBA-15 & (b)SBA-15/SO3H(10%).

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M.A. Naik et al. / Catalysis Communications 11 (2010) 1148–1153
not dependent on the generation of acylium ion (by the catalyst) but on
the nucleophilicity of the reagent (naphthol or indole, Scheme 2). In all
the catalyst, the selectivity of the product was found to be more than
98%. These results are quite beneficial industrially and indicate the
potential use of this simple catalyst. Theoptimum catalyst 5% SO3H/SBA-
15 is selected for further detailed catalytic studies for both xanthenes
and bis (indolyl) methanes.
3.2.2. Effect of catalyst weight
The effect of catalyst weight on the conversion of xanthene
showed (Fig. 3) that the catalytic conversion increased with increase
in the weight of the catalyst up to 30 mg and then became constant
with further increase in the weight. However, the conversion of bis
(indolyl) methanes increased linearly with the catalyst weight up to
80 mg and then became constant. These results indicate that different
magnitude of acidic contents is required for both the conversions.
Scheme 1. (a) Strategy for synthesis of xanthenes and (b) bis(indolyl)methanes.
3.2.3. Effect of the reaction temperature and time on stream studies
Contrasting results were observed in the variation of temperature
in both the reactions. The increase in the catalytic activity of
xanthenes was observed with increase in temperature up to 120 °C
where as opposite results were obtained in the case of bis (indolyl)
methanes. The catalytic activity for bis (indolyl) methanes initially
increased with reaction temperature up to 80 °C and then starts
decreasing with further increase in the reaction temperature (Fig. 4).
This can be ascribed to the polymerization of the indole in acidic
conditions at higher temperatures. The best reaction temperature
70 °C was chosen to carry out bis (indolyl) methanes and xanthenes
reaction.
In addition our interest also emerged to compare the catalytic
activity as well the degree of polymerization under homogeneous
(p-toluene sulphonic acid) and SBA/SO3H catalysts under our
experimental conditions. The catalytic activity for bis (indolyl)
methanes dropped significantly in presence of p-toluene sulphonic
acid at the reaction temperature corroborating the endeavor of using
heterogeneous catalysts. We believe that in homogeneous conditions
the timeof contact for indole to react with acylium ion is not enough and
results in increasedamountof polymerized product than heterogeneous
medium, where indole gets sufficient time to react with acylium ion and
finally result in bis (indolyl) methanes (Scheme 2). It is also to be
mentioned here that no catalytic activity was observed with SBA-15
indicating that the minimum acidic centers are essentially required to
drive these reactions. These results further show that the reactions are
The time on stream studies (Fig. 4) for xanthenes showed that the
reaction is completed within 6–12 h of the reaction time and no
further increase in the yield was observed with increase in reaction
Scheme 2. Proposed reaction mechanism.

M.A. Naik et al. / Catalysis Communications 11 (2010) 1148–1153
1151
Table 1
Table 2
Synthesis of substituted benzoxanthenes.
Synthesis of substituted bis(indolyl)methanes.
S no.
1a.
R-CHO
Yield (%)
95[8]
S no.
1b.
R-CHO
Yield (%)
52[28]
2a.
83[12]
2b.
3b.
63[28]
70[28]
3a.
4a.
5a.
86[12]
4b.
60[28]
91[8]
5b.
6b.
62[29]
79[12]
65[28]
6a.
7a.
87[12]
89[12]
7b.
8b.
68[29]
8a.
78[12]
82[8]
85[2]
52[28]
Reaction conditions: indole:benzaldehyde (2:1, molar ratio); solvent: CCl4, catalyst:
SBA-15/SO3H (70%), time: 24 h, catalyst amt: 100 mg.
[28,29] references correspond to the melting point of the compounds.
9a.
10a.
and water. The catalyst was dried at 120 °C and subjected to fresh
reaction. No difference in the activity and selectivity was noted.
Subsequently the catalyst was repeated for 3 cycles without signifi-
cant loss in the activity indicating the non-leaching behavior of acidic
contents from the catalyst during the course of reaction. The non-
leaching behavior of the acidic contents was also checked by doing
several blank experiments (supporting information). The optimized
conditions for the synthesis of substituted xanthenes and bis (indolyl)
methanes were also carried out (Tables 1 and 2).
Reaction conditions: 2-napthol:benzaldehyde (2:1, molar ratio); solvent: dichloroethane;
catalyst: SBA-15/SO3H (diff loading); time: 24 h; temperature: 85 °C.
[2,8,12] references correspond to the melting point of the compounds.
time indicating the product formed is stable under the reaction
conditions. This shows the requirement of lower acidity in former and
higher in the latter. However, different trend was observed for bis
(indolyl) methanes. The yield of the reaction increased with increase
in time up to 6 h but decreased further with increase in the time. The
decrease in the conversion is due to the polymerization of indole
thereby inhibiting the conversion. In order to check the advantages of
heterogeneous systems, the reaction was also monitored under
homogeneous conditions (p-toluene sulphonic acid). It was noted
that the yield was dropped significantly under homogeneous
conditions compared to heterogeneously derived reactions indicating
the promising use of SBA/SO3H catalysts (Fig. 5).
3.2.4. Plausible mechanism of the reaction
The interaction of benzaldehyde with the catalyst surface
generates the electrophilic carbon centre (acylium cation) followed
by the nucleophilic attack of the indole or 2-naphthol to generate the
product. The catalytic activity of both the reactions is controlled by the
strength of the nucleophile which is higher in 2-naphthol compared
to indole (Scheme 2). The solid support (heterogeneous catalyst)
helps to promote the reaction preferentially via K1 route compared to
K2 route which is more facile under homogeneous conditions.
3.2.5. Reusability of the catalyst
In order to see the reusability of the catalysts, the catalyst after the
reaction was filtered, washed thoroughly first with methanol, acetone
Fig. 2. Effect of catalyst loading (conditions as in Tables 1, 2).

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M.A. Naik et al. / Catalysis Communications 11 (2010) 1148–1153
Acknowledgement
AD thanks the Birla Institute of Science and Technology and DST
(Department of Science and Technology-New Delhi, Govt. of India) for
financial support. M. Naik and D. Sachdev thank CSIR (Council of Scientific
and Industrial Research) for senior research fellowship and IIT-Kanpur for
PXRD and SAIF-Punjab University-Chandigarh for NMR analysis.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.catcom.2010.06.004.
Fig. 3. Effect of catalyst weight (conditions as in Tables 1, 2).
Fig. 4. Time on stream and effect of temperature (Conditions as in Tables 1, 2).
Fig. 5. Catalytic activity studies of xanthenes and bis(indolyl)methanes in p-toluene sulphonic acid and SBA-15/SO3H catalysis.
4 .
Conclusions
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