3D Nanoporous FeAl-KIT-5 with a cage type pore structure: A highly efficient and stable catalyst for hydroarylation of styrene and arylacetylenes

Article abstract of DOI:10.1016/j.tetlet.2012.01.040

A novel bimetallic nanoporous FeAl-KIT-5 catalyst with a cage type porous structure and a high surface area has been prepared for the hydroarylation of styrene and arylacetylenes to afford 1,1-diarylalkanes and 1,1-diarylalkenes, respectively. The catalyst was found to be highly active, and selective, affording a high yield of substituted alkanes and alkenes. The catalyst also showed much higher activity as compared to those of other nanoporous catalysts such as AlSBA-15, AlKIT-5, and FeKIT-5, and can be reused several times without much loss of its activity.

Full text of DOI:10.1016/j.tetlet.2012.01.040

Tetrahedron Letters 53 (2012) 1485–1489
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
3D Nanoporous FeAl-KIT-5 with a cage type pore structure: a highly efficient
and stable catalyst for hydroarylation of styrene and arylacetylenes
Shaji Varghese a,b,c, Samuthira Nagarajan a, Mercy R. Benzigar a, Ajayan Mano a, Zeid A. ALOthman d,
George Allen Gnana Raj b,c, Ajayan Vinu a,e,
⇑
a International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Ibaraki 305-0044, Japan
b Research and Development Centre, Bharathiyar University, Coimbatore 641046, India
c Department of Chemistry, Scott Christian College, Nagercoil 629003, India
d Advanced Materials Research Chair, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia
e Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane 4072, QLD, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel bimetallic nanoporous FeAl-KIT-5 catalyst with a cage type porous structure and a high surface
area has been prepared for the hydroarylation of styrene and arylacetylenes to afford 1,1-diarylalkanes
and 1,1-diarylalkenes, respectively. The catalyst was found to be highly active, and selective, affording
a high yield of substituted alkanes and alkenes. The catalyst also showed much higher activity as com-
pared to those of other nanoporous catalysts such as AlSBA-15, AlKIT-5, and FeKIT-5, and can be reused
several times without much loss of its activity.
Received 4 December 2011
Revised 27 December 2011
Accepted 10 January 2012
Available online 17 January 2012
Keywords:
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Hydroarylation
Heterogeneous catalyst
Nanoporous
Catalysis
FeAl-KIT-5
The introduction of C–C bonds to arenes offers a direct method
for the synthesis of various biologically active moieties and phar-
maceuticals.1 Friedel–Crafts alkylation using Lewis acid catalysts
and hydroarylation of olefins via C–H bond activation using metal
catalysts are powerful methods for such C–C bond forming reac-
tions.2–6 Among these reactions, hydroarylation of styrene with
phenols has received much attention because the resulting prod-
ucts contain a diarylmethane motif which is an integral part of a
number of biologically active compounds such as haplopappin,
phenprocoumone, and avrainvilleol (Fig. 1).7 Similarly, hydroaryla-
tion of arylalkynes is also found to be a very attractive protocol in
organic synthesis as it provides 1,1-diarylethylenes, an important
structural moiety often found in biologically active compounds
such as isocombretastatin A-4 (iso CA-4) (Fig. 1).8
Strong Lewis acid catalysts such as TiCl4, BF3ÁOEt2, and AlCl3 and
Brønsted acids such as HF and H2SO4 are generally used for these
organic transformations. Unfortunately, most of these catalysts
are homogeneous in nature, and possess several disadvantages
such as high cost, toxicity, long reaction times, formation of side
products, and difficulty in separation of the products from the reac-
tion mixture. Recently, metals and organometallic complexes have
been used as efficient catalysts for C–C bond forming reactions.4 Of
these metal catalysts, iron catalysts are found to be efficient and
less toxic. However, they often require activating groups or
HO
HO
OMe
Br
HO
O
O
Br
OH
OH
OMe
OH
OH
avrainvilleol
OH
haplopappin
O
O
OH
OMe
O
O
O
phenprocoumone
iso CA-4
⇑
Corresponding author. Tel.: +61 7 3346 4122; fax: +61 7 3346 3973.
E-mail address: a.vinu@uq.edu.au (A. Vinu).
Figure 1. Biologically active and pharmaceutical examples of 1,1-diarylalkanes and
1,1-diarylalkenes.
0040-4039/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.040

1486
S. Varghese et al. / Tetrahedron Letters 53 (2012) 1485–1489
protection and deprotection steps.4,6 Thus, it is important to devel-
op a simple, facile, and environmentally benign method for direct
metal-catalyzed C–C bond forming reactions.
OH
OH
FeAl-KIT-5
Recently, nanoporous materials have emerged as important and
efficient catalysts for organic transformations owing to their excel-
lent physical characteristics including high surface area, large pore
volume, and uniform pore size distribution, and their simple work-
ups and recyclability.9–13 Among various nanoporous catalysts,
materials with 3D porous structures have many advantages over
catalysts with 1D nanoporous structures for various acid- or
base-catalyzed organic transformations. It has been found that
catalysts with a 3D porous structure enable easy diffusion of reac-
tant molecules which can avoid pore blocking and provide much
better physical parameters than those of catalysts with 1D porous
structures. Very recently, we reported a series of mono-metal
substituted nanoporous catalysts (AlKIT-5) with 3D cage type por-
ous structures and Fm3 m symmetry, and their application in var-
ious organic transformations.9,10,12,14 For example, AlKIT-5 showed
high selectivity for monoacylated products in the acylation of fer-
rocence with bulky acylating agents.14 Here we try to incorporate
both aluminum and iron in the silica framework of KIT-5 (FeAl-
KIT-5). It is expected that the inclusion of iron in the nanoporous
aluminosilicate with well ordered structural characteristics includ-
ing 3D pores, high Brønsted acidity, and high surface area may cre-
ate strong Lewis acid sites in the pore channels and enhance the
efficiency of the catalyst in C–C bond forming reactions. Unlike
homogeneous iron salts or complexes which can contaminate the
final product by leaching out, here the iron atoms in FeAl-KIT-5
are strongly bonded with either silica or alumina and stabilize
the structure which rules out leaching problems during the reac-
tion. Further, it is assumed that the bimetallic reagent helps to
avoid the excess use of catalyst and limits the side products.12
Considering all these advantages, herein we report on the prep-
aration of bimetallic nanoporous silica catalysts with 3D structures
and cage type pores, high surface areas and large pore volumes,
and their application for the synthesis of 1,1-diarylalkanes and
1,1-diarylalkenes. The catalytic activity of these novel catalysts is
also compared with mono metal substituted nanoporous catalysts.
Nanoporous FeAl-KIT-5 was prepared by using a simple self-
assembly of the polymeric surfactant Pluronic F127 and inorganic
sources such as tetraethyl orthosilicate, ferric nitrate and alumi-
num isopropoxide, which supply Si, Fe, and Al, respectively, at a
temperature of 100 °C in a highly acidic medium.15 The materials
prepared are labeled FeAl-KIT-5. The nSi/nFe and nSi/nAl ratios were
calculated to be 20 and 10, respectively. For comparison, FeKIT-5,
AlKIT-5, and AlSBA-15 were also prepared by the procedure re-
ported elsewhere.11 XRD and nitrogen adsorption analysis revealed
that the FeAl-KIT-5 sample exhibited a well-ordered 3D structure
with cage type pores (Figs. 1S and 2S). The specific surface area,
specific pore volume, and the pore diameter of the sample are
857 m2 gÀ1, 0.79 cm3 gÀ1, and 5.7 nm, respectively. These excellent
characteristics are very important for catalytic applications and
facilitate the adsorption and interaction of the reactant molecules
within the nanochannels.
+
cyclohexane
OMe
OMe
3a
1
2
Scheme 1. Hydroarylation of mequinol with styrene.
Table 1
Effect of solvent and catalyst on the synthesis of 1,1-diarylmethane 3aa
Entry
Catalyst
Solvent
Yielda,b (%)
1
2
3
4
5
6
7
8
9
FeAl-KIT-5c
FeAl-KIT-5d
FeAl-KIT-5d
FeAl-KIT-5d
FeAl-KIT-5d
FeAl-KIT-5d
FeKIT-5
AlKIT-5
AlSBA-15
FeAl-SBA-15
Cyclohexane
Cyclohexane
Dichloroethane
Toluene
Nitromethane
Acetonitrile
Cyclohexane
Cyclohexane
Cyclohexane
Cyclohexane
80
89
84
79
70
0
31
78
73
74
10
a
Reaction conditions: styrene (1 equiv), mequinol (2 equiv), 40 mg catalyst,
85 °C, 4 h.
b
Isolated yield after column purification.
nSi/nAl ratio: 10, nSi/nFe ratio: 15.
nSi/nAl ratio: 10, nSi/nFe ratio: 20.
c
d
nanoporous catalysts such as AlKIT-5, FeKIT-5, and AlSBA-15.
Among the catalysts, FeAl-KIT-5 showed superior performance. This
strongly confirms the role of the bimetal in the pore channels of
FeAl-KIT-5. It is assumed that the combination of the strong Lewis
acid sites generated by the tetrahedrally-coordinated iron atoms
in the silica framework and the Brønsted acid sites, originating from
the aluminum atoms, activate the reactant molecules in the pore
channels whereas the large cages in the catalyst allow the formation
of large reaction intermediates. These factors trigger the formation
of the desired 1,1-diarylalkane in high yield. To the best of our
knowledge, this is the first report on the preparation and the activity
of FeAl-KIT-5 catalyst for this type of reaction.
We also investigated the effect of the catalyst loading on the
above reaction. The yield of the product increased from 77% to
89% on increasing the amount of catalyst from 20 to 40 mg, with-
out much affect on the selectivity. This could be due to the higher
number of iron active sites in the sample which facilitates adsorp-
tion and activation of the reactant molecule and significantly
enhances interactions between the reactant molecules. However,
when the quantity of the catalyst was increased to 80 mg, a signif-
icant reduction in the yield was observed (80%).
The presence of a large number of active sites supports the non-
uniform adsorption of the reactant molecules in the pore channels
and further encourages the reaction between the reactants as well
Initially, we investigated the catalytic properties of the novel
FeAl-KIT-5 catalyst (40 mg) in the hydroarylation of mequinol (1)
(2 mmol) with styrene (2) (1 mmol) in cyclohexane (5 mL) under re-
flux (Scheme 1). The catalyst was found to be highly active, affording
a ca. 89% yield of the desired product 3a within 4 h. When the reac-
tion temperature was decreased to lower than the reflux tempera-
ture of cyclohexane, the activity of the catalyst decreased
significantly. Thus, it was decided to maintain the reaction temper-
ature at 85 °C for further reactions. When the reaction was carried
out without any catalyst, no trace of the desired product was
formed, confirming the active role of the FeAl-KIT-5 catalyst. The
reaction was also attempted with different mono-metal substituted
OH
OH
FeAl-KIT-5
+
cyclohexane
OMe
3h
OMe
1
4
Scheme 2. Hydroarylation of mequinol with phenylacetylene.

S. Varghese et al. / Tetrahedron Letters 53 (2012) 1485–1489
1487
Table 2
Synthesis of diarylmethanes and diarylethylenes
Entry
Reactants
Producta
Time (h)
Yieldb (%)
OH
OH
1
4
89
91
OMe
3a
OMe
OH
OH
2
4
3b
OH
OH
3
4
5
6
86
79
3c
OH
OH
OH
3d
OH
5
4.5
81
3e
OH
OH
6
7
3.5
95
78
3f
OH
OH
4
3g
OH
OH
8
9
5
87
84
3h
OMe
OMe
OH
OH
5.5
3i
OH
OH
10
6
84
3j
OMe
OMe
(continued on next page)

1488
S. Varghese et al. / Tetrahedron Letters 53 (2012) 1485–1489
Table 2 (continued)
Entry
Reactants
Producta
Time (h)
Yieldb (%)
OH
OH
11
6
73
3k
OH
OH
12
4
93
3l
a
The products were characterized by 1H, 13C NMR, IR, and mass spectroscopy.
Yield refers to pure products after chromatography.
b
as the reactants and the products. As a result, a significant reduc-
tion in the yield of the main product was observed. As a catalyst
loading of 40 mg was found to be the best, we decided to use that
amount for further experiments.
mequinol. The catalyst was separated by filtration, washed with
dichloromethane, and reused after activation at 500 °C for 4 h.
The yield of 1,1-diarylmethane 3a was 89%, 84%, and 80% over
three cycles. There was a slight decrease in the activity after the
third cycle which was not considered as being due to degradation
of the catalyst structure, but due to the presence of adsorbed
product on the active sites which caused a slight reduction in the
catalyst activity. We also investigated the structure of the catalyst
after the third cycle by powder X-ray diffraction and nitrogen
adsorption analysis. However, we found no difference in the struc-
ture even after the third cycle, which further confirms that the
structure is stable and the catalyst is highly robust.
In conclusion, we have demonstrated the hydroarylation of sty-
rene and arylacetylenes with substituted phenols for the first time
using a novel bimetallic nanoporous FeAl-KIT5 catalyst possessing
a cage type porous structure and a high surface area to afford 1,1-
diarylalkanes and 1,1-diarylalkenes. The catalyst showed excellent
performance for the hydroarylation of styrene and phenylacety-
lene affording very high yields in short reaction times and worked
well with different substituted phenols to afford substituted 1,1-
diarylalkanes and alkenes. In addition, the catalyst displayed much
higher activity than other nanoporous catalysts such as AlSBA-15,
AlKIT-5, and FeKIT-5, and can be reused several times without
much loss of activity. We believe that this novel catalyst could
be used for the synthesis of organic molecules via Friedel–Crafts
alkylation and arylation.
The effect of solvents on the activity of FeAl-KIT-5 for the hydro-
arylation of styrene was studied. Solvents such as cyclohexane,
dichloroethane, acetonitrile, toluene, and nitromethane were used.
It should be mentioned that the catalyst was not effective when
MeCN was used as the solvent. However, more than 70% yields of
product 3a were observed when other solvents were used. Of
these, cyclohexane afforded a high yield of product within a short
reaction time (Table 1). We surmise that cyclohexane favors the
adsorption and/or the interaction between the reactant molecules
on the catalyst active sites located inside the nanocages. It is also
interesting to note that the reaction can proceed under mild condi-
tions without adding any strong acid or base or toxic reagent
which makes this system simple and environmentally friendly.
As we obtained excellent results on the hydroarylation of
styrene with mequinol, we decided to further study the hydroary-
lation of mequinol with arylacetylene (Scheme 2) using the FeAl-
KIT-5 catalyst to give a 1,1-diarylethylene. Initially, the reaction
of mequinol (1) with phenylacetylene (4) was studied to optimize
the hydroarylation. A mixture of mequinol (1) (2 mmol), phenyl-
acetylene (4) (1 mmol) and FeAl-KIT-5 (40 mg) in cyclohexane
(5 mL) was stirred at 85 °C for the specified amount of time (Table
2). We found that the catalyst was also highly active and selective
for this reaction and afforded the desired 1,1-diarylethylene 3h in
an 87% yield in 5 h after column chromatographic purification.
To assess the generality of the catalytic process, we carried out
reactions of a variety of phenols with styrene and arylacetylenes
under similar conditions and the results are displayed in Table
2.16 To our surprise, the reaction proceeded smoothly with phenol
and different substituted phenols and gave products in good to
excellent yield. It should be mentioned that the catalyst gave a
much higher yield with 2-naphthol (Table 2, entries 6 and 12),
but a lower yield with 1-naphthol (Table 2, entries 7 and 11).
Mequinol underwent smooth hydroarylation with 2-methylphen-
ylacetylene (Table 2, entry 10). The hydroarylation of mono, di-al-
kyl, and tert-butyl substituted phenols with styrene was also
studied. The catalyst was found to work well and afforded excel-
lent yields (81–91%) irrespective of the presence of substituents
on the phenolic ring. These results reveal that our catalyst is highly
active and suitable for the hydroarylation of styrenes and arylacet-
ylenes, which is mainly due to the high surface area, large pore vol-
ume, and cages together with the combination of both the Lewis
and Brønsted acid sites.
Acknowledgements
This work was financially supported by the Ministry of Educa-
tion, Culture, Sports, Science, and Technology (MEXT) under the
Strategic Program for Building an Asian Science and Technology
Community Scheme and World Premier International Research
Center (WPI) Initiative on Materials Nanoarchitectonics, MEXT,
Japan.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.01.040.
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General procedure for the synthesis of 1,1-diarylalkenes 3h–l: A mixture of
phenol (2 mmol), arylacetylene (1 mmol), and FeAl-KIT-5 (40 mg) was stirred
in cyclohexane at reflux temperature. The reaction was monitored by thin layer
chromatography. After completion, the catalyst was recovered by filtration.
The organic layer was concentrated and the crude product purified by silica gel
column chromatography using EtOAc–n-hexane (1:9) as eluent to afford the
product 3h–l.