Novel and Efficient Heterogeneous 4-Methylbenzenesulfonic Acid-Based Ionic Liquid Supported on Silica Gel for Greener Fischer Indole Synthesis

Article abstract of DOI:10.1007/s10562-016-1721-x

In this work, a functionalizing active species 4-methylbenzenesulfonic acid-based IL on silica gel (IL-SO₃H-SiO₂) has been prepared, and characterized by FT-IR, XRD, TGA, SEM and EDX spectra. Then, IL-SO₃H-SiO₂ was utilized as an efficient and heterogeneous catalyst for the synthesis of indoles via the one-pot Fischer reaction of phenyl hydrazines with ketones or aldehydes at room temperature. The heterogeneous catalyst could be recovered easily by filtration and reused many times without significant loss of its catalytic activity.

Full text of DOI:10.1007/s10562-016-1721-x

Catal Lett
DOI 10.1007/s10562-016-1721-x
Novel and Efficient Heterogeneous 4-Methylbenzenesulfonic
Acid-Based Ionic Liquid Supported on Silica Gel for Greener
Fischer Indole Synthesis
1
,2
3
1,2
Yu-Lin Hu
•
Dong Fang
•
Dong-Sheng Li
Received: 19 February 2016 / Accepted: 23 February 2016
Ó Springer Science+Business Media New York 2016
Abstract In this work, a functionalizing active species
4
of phenyl hydrazines with ketones or aldehydes at room
temperature. The heterogeneous catalyst could be recov-
ered easily by filtration and reused many times without
significant loss of its catalytic activity.
-methylbenzenesulfonic acid-based IL on silica gel (IL-
SO H-SiO ) has been prepared, and characterized by FT-
2
3
IR, XRD, TGA, SEM and EDX spectra. Then, IL-SO H-
3
SiO was utilized as an efficient and heterogeneous catalyst
2
Graphical Abstract
for the synthesis of indoles via the one-pot Fischer reaction
Keywords Supported catalyst Á Ionic liquid Á Silica gel Á
Fischer indole synthesis Á Heterogeneous catalysis
&
&
Yu-Lin Hu
huyulin1982@163.com
Dong-Sheng Li
lidongsheng1@126.com
1
Introduction
1
2
3
College of Materials and Chemical Engineering, China Three
Gorges University, Yichang 443002, People’s Republic of
China
Indoles are important classes of heterocyclic compounds
used extensively for the synthesis of a variety of fine or
special chemicals such as natural products, drugs, materials,
etc. [1–3]. Therefore, there has been tremendous interest in
developing efficient methods for the synthesis of these
molecules [4–8], and a well known method constitutes the
Fisher indole synthesis [8]. Traditional methods for per-
forming such a transformation generally involve the use of
Key Laboratory of Inorganic Nonmetallic Crystalline and
Energy Conversion Materials, China Three Gorges
University, Yichang 443002, People’s Republic of China
College of Chemistry and Chemical Engineering, Yancheng
Teachers University, Yancheng 224002, People’s Republic of
China
123

Y. Hu et al.
stoichiometric amount of the bronsted or lewis acids (i.e.,
H SO , HCl, polyphosphoric acid, ZnCl , FeCl , AlCl , etc.)
catalytic reactions of ILs could transfer into the heteroge-
neous systems which provide attractive features such as
gaining high catalyst efficiency by using only smaller
amounts of ILs, high system stability, facilitating the cata-
lyst recovery and high reusability [31].
2
4
2
3
3
[
9], and suffer from considerable drawbacks such as low
yield, harsh or delicate reaction condition, and a large
amount of waste byproducts. Recently, a range of catalysts
have been used to promote the reaction such as tartaric acid/
dimethylurea melt [10], Pb(OAc) /ZnCl [11], 2,4,6-tri-
In continuation of our interest on the preparation and
application of supported functional ionic liquids in organic
synthesis, we attempt to design a catalyst functionalizing
active species 4-methylbenzenesulfonic acid-based IL on
silica gel (IL-SO H-SiO ). Here, the prepared material, IL-
2
2
chloro-1,3,5-triazine [12], Ag/SiO –ZnO [13], propylphos-
2
phonic anhydride [14], CH COOH [15], TiCl /t-BuNH
2
3
4
[
16], and other reagents [17–21]. However, these protocols,
3
2
are generally associated with one or more disadvantages,
such as high temperatures, prolonged reaction times, tedious
work-up, problematic side reactions, the use of expensive,
toxic and moisture sensitive reagents, difficulty in separating
the catalyst from the reaction mixture as well as recycling of
it, and environmental concerns associated with the use of
poorly manageable catalysts. Consequently, search for new
and environmentally benign synthetic methodologies for the
synthesis of indoles that address these drawbacks remains to
be of value and interest.
SO H-SiO , exhibited advanced chemical behaviors and
3 2
has the advantage of being a solid, which is used in
heterogeneous catalysis and can reach high reaction effi-
ciency for the preparation of indoles via Fischer indoliza-
tion (Scheme 1). Furthermore, it shows that the IL-SO H-
3
SiO catalyst can be easily recovered and reused for the
2
reaction without any significant loss of catalytic activity.
2 Experimental
Recently, ionic liquids (ILs) have become an active area
of research, ILs possess favourable properties such as very
low vapor pressure, wide liquid temperature range, good
ionic conductivity, excellent electrochemical properties,
and strong ability to dissolve many chemicals [22]. There-
fore, they have been widely employed in the field of sepa-
ration, chemical synthesis, electrochemistry, and catalysis
2.1 Apparatus and Reagents
All the chemicals were from commercial sources without
any pretreatment. All reagents were of analytical grade.
The silica functionalized sulfonic acids were synthesized
according to the literature procedures [32, 33]. Fourier
transform infrared spectroscopy (FT-IR) spectra were
obtained on a Nicolet Nexus 470 Fourier transform
infrared spectrometer, using KBr pellets at room temper-
ature. The powder X-ray diffraction (XRD) analysis was
carried out on a Rigaku Ultima IV diffractometer with
high-intensity Cu Ka radiation. Energy dispersive X-ray
analysis (EDX) was used to conduct the elemental anal-
ysis of the IL. Scanning electron microscopy studies were
conducted on a JSM-7500F Scanning Electron Microscope
[
23–25]. Up to now, examples of their application as cata-
lysts in the synthesis of indoles were also reported [26–28].
In spite of high catalytic activity of these ILs presented, they
suffered from serious drawbacks in terms of separation and
recycling of costly ILs, equipment corrosion as well as ILs
toxicity. A useful approach to overcome these limitations is
the development of supported functional ionic liquid cat-
alytic system [29–31]. The supported IL system dispersed
onto a high surface area support. The homogeneous
3 2
Scheme 1 Fischer indole synthesis catalyzed by IL-SO H-SiO
1
23

Novel and Efficient Heterogeneous 4-Methylbenzenesulfonic Acid-Based Ionic Liquid Supported…
3 2
Scheme 2 The synthetic process of IL-SO H-SiO
(
SEM). Thermogravimetric analysis (TGA) were analyzed
(triethoxysilyl)propyl)-1H-imidazole 1 as a viscous liquid.
(iii) Preparation of 2: a mixture of 1 (27.2 g, 0.1 mol),
activated silica (28 g), and toluene (250 mL) was stirred
vigorously in a 500 mL round flask under a nitrogen
atmosphere. After 24 h stirring under reflux conditions, the
white solid 2 was obtained by filtration and washed with
toluene (3 9 20 mL). (iv) Preparation of 3: In this step,
1,4-butane sultone (13.6 g, 0.1 mol), the obtained solid 2,
and toluene (200 mL) were stirred at 100 °C for 8 h under
nitrogen flow, the white solid 3 was obtained by filtration
and washed with toluene (3 9 20 mL). (v) Preparation of
by a Netzsch Thermoanalyzer STA 449 with a heating rate
1
of 10 °C/min under a nitrogen atmosphere. H NMR spectra
were recorded on a Bruker 500-MHz spectrometer using
CDCl as the solvent with tetramethylsilane (TMS) as an
3
internal standard. GC analyses were performed on a Shi-
madzu-14B gas chromatography equipped with HP-1 cap-
illary column (30 m, 0.25 mm, 0.25 mm). Melting points
were recorded on a B u¨ chi B-545 apparatus in open capillary
tubes. Elemental analysis were performed on a Vario EL III
instrument (Elmentar Anlalysensy Teme GmbH, Germany).
4
: Then, 4-methylbenzenesulfonic acid (17.2 g, 0.1 mol)
was added into the above silica supported IL 3 in toluene
(200 mL) in 10 min. The final mixture was stirred at 80 °C
for 6 h and by filtration, washed with dichloromethane
(3 9 20 mL) and dried under vacuum at 60 °C to afford
the supported ionic liquid 4 as a white powder.
2
.2 Synthesis of the Supported Ionic Liquid
The preparation process were according to known process
34, 35] and shown in Scheme 2. (i) Silica pretreatment
[
and activation: silica gel (50 g) was activated by standing
in hydrochloric acid solution (10 %, 500 mL) for 12 h at
room temperature and then washed thoroughly with dis-
tilled water. The sample was then dried at 60 °C for 24 h.
2.3 General Procedure for the Synthesis of Indoles
To a solution of phenylhydrazine (10 mmol) and ketone or
aldehyde (10 mmol) in ethanol (15 mL) was added catalyst
IL-SO H-SiO (1.2 g). The mixture was allowed to stirring
(
ii) Preparation of 1: A mixture of imidazole (6.8 g,
0
0
.1 mol), (3-chloropropyl) triethoxysilane (24.1 g,
.1 mol) and toluene (180 mL) were added into a 500 mL
3
2
at room temperature for a period time specified in Table 2.
The reaction was monitored by TLC and GC. After com-
pletion of the reaction, the catalyst was recovered by fil-
tration. Evaporation of the solvent under reduced pressure
gave the crude product. Further purification was achieved
by flash column chromatography on a silica gel to give the
desired product. The recovered catalyst was dried and
reused for the next run. Spectroscopic data for selected
products is as follows.
round flask connected to a reflux condenser, and the mix-
ture was refluxed for 22 h under a nitrogen atmosphere.
Then, triethylamine (10.1 g, 0.1 mol) was added dropwise
to the resulted mixture at 50 °C in 3 min, and stirred for
another 2 h under reflux conditions. After the reaction, the
slurry was filtrated and the filter cake was washed with
toluene (3 9 10 mL). The combined toluene solution was
removed under reduced pressure to afford 1-(3-
123

Y. Hu et al.
2
.4 Spectral Data of Selected Products
7.08–7.25 (m, 3H, Ar–H), 7.54 (m, 1H, Ar–H), 7.72 (s, 1H,
NH) ppm; Anal. Calcd for C H N: C, 83.34; H, 9.13; N,
1
3 17
2
.4.1 3-Isobutyl-2-methyl-1H-indole (Table 2, Entry 4)
7.45. Found: C, 83.37; H, 9.15; N, 7.48.
1
Colourless oil; H NMR: d = 0.98 (d, 6H, CH ), 1.92–2.01
(
2.4.2 6-Methoxy-2,3,4,9-tetrahydro-1H-carbazole
(Table 2, Entry 8)
3
m, 1H, CH), 2.41 (s, 3H, CH ), 2.55 (d, 2H, CH ),
3 2
1
White soild; H NMR: d = 1.71–1.89 (m, 4H, CH CH ),
2
2
2.61–2.72 (m, 4H, CH ), 3.75 (s, 3H, OCH ), 6.64 (m, 1H,
2 3
Fig. 1 FTIR spectra of IL-SO
3
H-SiO
2
3 2
Fig. 3 EDX image of IL-SO H-SiO
3 2
Fig. 2 SEM images of silica gel (a), IL-SO H-SiO (b), the recycled catalyst (c)
1
23

Novel and Efficient Heterogeneous 4-Methylbenzenesulfonic Acid-Based Ionic Liquid Supported…
Ar–H), 6.83 (s, 1H, Ar–H), 7.08 (m, 1H, Ar–H), 10.52 (s,
1H, NH) ppm; Anal. Calcd for C H NO: C, 77.53; H,
13 15
7.49; N, 6.92; O, 7.94. Found: C, 77.58; H, 7.51; N, 6.96;
O, 7.95.
2.4.3 6-Fluoro-2,3,4,9-tetrahydro-1H-carbazole (Table 2,
Entry 9)
1
White soild; H NMR: d = 1.78–1.85 (m, 4H, CH CH ),
2
2
2.59–2.74 (m, 4H, CH ), 6.85 (m, 1H, Ar–H), 7.09 (m, 1H,
2
Ar–H), 7.22 (m, 1H, Ar–H), 10.79 (s, 1H, NH) ppm; Anal.
Calcd for C H FN: C, 76.17; H, 6.39; F, 10.04; N, 7.40.
1
2 12
Found: C, 76.17; H, 6.39; F, 10.04; N, 7.40.
3 2
Fig. 4 XRD pattern of IL-SO H-SiO
2.4.4 3-Butyl-5-methyl-2-phenyl-1H-indole (Table 2,
Entry 11)
1
Colourless oil; H NMR: d = 0.86 (t, 3H, CH ), 1.35 (m,
3
2H, CH ), 1.62 (m, 2H, CH ), 2.51 (s, 3H, CH ), 2.83 (t,
2 2 3
2H, CH ), 6.89–6.95 (m, 2H, Ar–H), 7.31–7.37 (m, 2H,
2
Ar–H), 7.51-7.56 (m, 2H, Ar–H), 7.59-7.66 (m, 2H, Ar–H),
10.78 (s, 1H, NH) ppm; Anal. Calcd for C H N: C,
19 21
86.62; H, 8.01; N, 5.29. Found: C, 86.65; H, 8.04; N, 5.32.
3
Results and Discussion
The structure of the supported ionic liquid was character-
ized by FT-IR, XRD, TG, EDX, and SEM analysis. The
FTIR spectra of the supported ionic liquid catalyst IL-
SO H-SiO is presented in Fig. 1. Two characteristic peaks
3
2
-
1
at the positions of 1530 and 1625 cm , which were
attributed to the imidazole ring, indicating that imidazole
was successfully grafted on the surface of silica gel.
Fig. 5 TG curve for the supported IL catalyst
Table 1 Optimization of the
Entry
a
Solvent
Catalyst
Amount (g)
Time (h)
Yield (%)
reaction conditions
1
2
3
4
5
6
7
8
9
Solvent-free
Water
IL-SO
IL-SO
IL-SO
IL-SO
IL-SO
IL-SO
IL-SO
IL-SO
IL-SO
IL-SO
3
3
3
3
3
3
3
3
3
3
H-SiO
H-SiO
H-SiO
H-SiO
H-SiO
H-SiO
H-SiO
H-SiO
H-SiO
H-SiO
2
2
2
2
2
2
2
2
2
0.6
0.6
0.6
0.6
0.6
0.6
0.8
1.0
1.2
1.4
1.2
1.2
6
6
6
6
6
8
4
4
4
4
8
8
68
70
75
72
65
50
82
91
94
94
85
72
Ethanol
Methanol
Ethyl acetate
Acetonitrile
Ethanol
Ethanol
Ethanol
10
11
12
Ethanol
2
Ethanol
SiO
2
-Pr-SO
-SO
3
H
Ethanol
SiO
2
3
H
The reactions were carried out with phenyl hydrazine hydrochloride (10 mmol), cyclohexanone (10 mmol),
catalyst, and solvent (15 mL) at room temperature
a
Isolated yield
123

Y. Hu et al.
-
1
Additional bands at 3100, 2970 and 1520 cm are due to
C–H stretching and deformation vibrations of the imida-
zole moiety and alkyl chain. Also, the peaks observed at
of silica gel, which is made up of block structures with a
size of several micrometers. Figure 2b is the SEM image of
the supported IL 4, it clearly shows the supported IL par-
ticles were well-defined and had distinct size owing to the
immobilization of the IL. The particle size of the supported
IL is smaller to that of silica gel, which demonstrates that
the particles of silica gel had a good mechanical stability
during the immobilization step. The change of the parent
structure of the silica to well defined particles in supported
-
1
4
90–870 cm
related to vibrational modes of 4-CH3
PhSO . Moreover, the stretching vibration peaks of Si–O–
3
-
Si (1087 cm ), O–H (1685 cm ) and the stretching
1
-1
-
vibration Si–OH (3478 cm ) were clearly observed.
1
The shape and surface morphology of the IL-SO H-SiO
3
2
were studied by SEM (Fig. 2). Figure 2a is the SEM image
IL indicated that IL-SO H had been immobilized on the
3
surface of the silica particles. EDX of the supported IL
showed the presence of the expected elements in its
structure (Fig. 3). Figure 4 displayed the wide-angle XRD
pattern of the supported IL. As can be seen in Fig. 4, the
catalyst had obvious characteristic peak at 2h & 22°,
which was attributed to the amorphous nature of the SiO2
support.
1
00
8
6
4
2
0
0
0
0
0
9
4
93
93
92
92
91
The stability of supported IL was determined by ther-
mogravimetric analysis (Fig. 5), the observed weight loss
was associated with the loss of the organic components
attached to the surface. The TG curve indicates no weight
loss occurred before 300 °C, 2.8 % initial weight loss
occurred from 300 to 400 °C, complete loss of the IL
1
2
3
4
5
6
Run
Fig. 6 The repeating reactions of phenyl hydrazine hydrochloride
10 mmol), cyclohexanone (10 mmol) and ethanol (15 mL) in the
(
3 2
presence of the recycled IL-SO H-SiO
O
H
NH-N
1
NHNH . HCl
NH-NH2
2
2
SO -H
3
N
N
N
H
H C
SO3
3
H
HCl
H
H
NH Cl
4
NH2
NH
NH2
N
5
3
H
H
NH2
NH2
4
Scheme 3 Possible mechanism for the Fischer indole synthesis
1
23

Novel and Efficient Heterogeneous 4-Methylbenzenesulfonic Acid-Based Ionic Liquid Supported…
Table 2 Catalytic synthesis of indoles in the presence of IL-SO H-SiO
Ketone/Aldehyde
3
2
a
Entry
1
Phenyl hydrazine
Product
Time (h)
4
Yield (%)
94
2
3
4
6
95
89
4
5
4
4
91
96
6
7
4
90
6
4
90
95
8
9
7
4
84
96
1
0
1
1
6
5
87
86
1
2
Unless otherwise noted, the reactions were carried out with phenylhydrazine (10 mmol), ketone or aldehyde (10 mmol), IL-SO
and ethanol (15 mL) at room temperature
3 2
H-SiO (1.2 g),
a
Isolated yield
123

Y. Hu et al.
covalently grafted onto the silica is seen in the temperature
range 400–500 °C, and the amount of organic moiety was
about 10 % against the total solid catalyst. From the curve
depicted, one can see that IL-SO H-SiO is thermally
the reaction of phenyl hydrazine hydrochloride and
cyclohexanone as an example, a possible mechanism is
proposed (Scheme 3). At first, the substrates phenyl
hydrazine hydrochloride and cyclohexanone are activated
by acidic group of IL-SO H-SiO to forms a phenylhy-
3
2
stable below about 300 °C. The good thermal stability of
the silica gel supported IL may be beneficial to the cat-
alytic experiments.
3
2
drazone 1 which isomerizes to the respective enamine 2.
After protonation, a cyclic [3,3]-sigmatropic rearrange-
ment occurs producing an imine 3. The resulting imine
forms a cyclicaminoacetal 4. 4 is then very rapidly form
The initial study was carried out a model reaction
between phenyl hydrazine hydrochloride and cyclohex-
anone for the synthesis of tetrahydrocarbazole to optimize
the reaction conditions, and the results are summarized in
Table 1. The effect of various solvents on the yield of
product is given in Table 1, entries 1–6. The reaction does
not progress effectively in acetonitrile (Table 1, entry 6).
The reaction in water, ethanol, methanol, ethyl acetate as
well as solvent-free conditions resulted in 65–75 % yield
of product (Table 1, entries 1–5). The results show that
ethanol is a better solvent than the other solvents tested
into 5, which under acid catalysis eliminates NH , result-
3
ing in the energetically favorable product.
Having established optimum conditions, a series of
indoles was synthesized by reacting various phenyl
hydrazines with ketones/aldehydes in order to showcase the
broad scope and generality of this method. It is clear that
various types of phenyl hydrazines with ketones or alde-
hydes in the presence of a catalytic amount of IL-SO H-
3
SiO at room temperature, can be efficiently converted to
2
(
Table 1, entry 3). To investigate the effect of catalyst
the corresponding indoles in good to high yields (Table 2).
In general, electron-rich aromatic amines afforded a higher
yield than the electron-deficient aromatic amines, and it
was observed that electron-donating groups on the phenyl
ring of aromatic amines favoured the formation of the
corresponding products in excellent yields (Table 2, entries
8, 10 and 11). In contrast, electron-withdrawing groups
associated with aromatic amines slightly decreased the
reactivity of the substrate (Table 2, entries 9 and 12).
Surprisingly, the Fischer reaction of aliphatic ketones or
aldehydes (Table 2, entries 1, 2, 4–6, 8–10 and 12) to the
corresponding indoles is faster than aromatic ketones or
aldehydes (Table 2, entries 3, 7 and 11).
concentration, systematic studies were carried out in the
presence of various amounts of the catalyst in ethanol,
affording tetrahydrocarbazole in 75–94 % isolated yields,
respectively (Table 1, entries 3, 7–9). Thus, the best yield
is obtained in the presence of just 1.2 g of IL-SO H-SiO
3
2
(
Table 1, entry 9). The use of a greater amount of catalyst
does not improve the result to an appreciable extent
Table 1, entry 10). Besides IL-SO H-SiO , silica func-
(
3
2
tionalized sulfonic acids, such as SiO -Pr-SO H and SiO -
2
3
2
SO H, were tested as heterogeneous catalysts in this model
3
reaction (Table 1, entries 11 and 12), and the results
showed that the supported IL IL-SO H-SiO demonstrated
3
2
the best performance in terms of yield and reaction rate.
Therefore, IL-SO H-SiO is a very efficient catalyst for
3
2
Fischer indole synthesis.
4 Conclusion
The reusability of the IL-SO H-SiO catalyst was also
3
2
studied, and the results are summarized in Fig. 6. It was
found that the catalyst could be conveniently recovered at
the end of reaction, and could be readily reused for the next
cycle. The catalyst could be typically recovered and reused
for subsequent reactions with no appreciable decrease in
yields and reaction rates. An SEM observation of the
recovered catalyst after five runs was made, and there was
no obvious change in the morphology and size in com-
parison with fresh catalyst (Fig. 2c). The recycling process
involved the removal of the product from the catalyst by a
simple filtration. Fresh substrates were then recharged to
the recovered catalyst and the mixture was heated to react
once again.
In conclusion, we have shown that 4-methylbenzenesul-
fonic acid-based IL on silica gel can act as a novel,
effective and heterogeneous catalyst for the one-pot syn-
thesis of indoles from commercially available starting
materials. Various phenyl hydrazines were reacted with
ketones or aldehydes to give the corresponding products in
good to excellent yields. Mild reaction conditions, sim-
plicity of operation, high yields, easy isolation of products,
and excellent recyclability of the catalyst are the attractive
features of this methodology. The scope and synthetic
application of this reaction are currently under study in our
laboratory.
Acknowledgments The authors would like to acknowledge finan-
cial support provided by the National Natural Science Foundation
of China (No. 21506115) and the Natural Science Foundation of
Jiangsu Province (No. BK20140460) for carrying out this research.
The excellent results of IL-SO H-SiO2 suggest the
3
reaction has a particular reaction mechanism. On the basis
of previous reports [9–12] and the experiment result, taking
1
23

Novel and Efficient Heterogeneous 4-Methylbenzenesulfonic Acid-Based Ionic Liquid Supported…
1
8. Chitra S, Paul N, Muthusubramanian S, Manisankar P (2011)
Green Chem 13:2777
References
1
2
9. Yasui E, Wada M, Takamura N (2006) Tetrahedron Lett 47:743
0. Rosenbaum C, Katzka C, Marzinzik A, Waldmann H (2003)
Chem Commun 1822
1. Porcheddu A, Mura MG, Luca LD, Pizzetti M, Taddei M (2012)
Org Lett 14:6112
2. Tariq M, Freire MG, Saramago B, Coutinho JAP, Lopes JNC,
Rebelo LPN (2012) Chem Soc Rev 41:829
3. Rostamizadeh S, Nojavan M, Aryan R, Azad M (2014) Catal Lett
1
2
3
4
. Inman M, Moody CJ (2013) Chem Sci 4:29
. Vicente R (2011) Org Biomol Chem 9:6469
. DiPoto MC, Hughes RP, Wu J (2015) J Am Chem Soc 137:14861
. Li H, Chen C, Nguyen H, Cohen R, Maligres PE, Yasuda N,
Mangion I, Zavialov I, Reibarkh M, Chung JYL (2014) J Org
Chem 79:8533
2
2
2
5
6
7
8
9
. Abe T, Nakamura S, Yanada R, Choshi T, Hibino S, Ishikura M
(2013) Org Lett 15:3622
1
44:1772
. Alorati AD, Gibb AD, Mullens PR, Stewart GW (2012) Org
Process Res Dev 16:1947
. Li H, Zhu Z, Zhang F, Xie S, Li H, Li P, Zhou X (2011) ACS
Catal 1:1604
. Wei W, Dong X, Nie S, Chen Y, Zhang X, Yan M (2013) Org
Lett 15:6018
. Platon M, Amardeil R, Djakovitch L, Hierso JC (2012) Chem Soc
Rev 41:3929
2
2
2
2
4. Upadhyay P, Srivastava V (2016) Catal Lett 146:12
5. Mart ´ı nez-Palou R, Luque R (2014) Energy Environ Sci 7:2414
6. Li BL, Xu DQ, Zhong AG (2012) J Fluor Chem 144:45
7. Xu DQ, Wu J, Luo SP, Zhang JX, Wu JY, Du XH, Xu ZY (2009)
Green Chem 11:1239
2
8. Yi FP, Sun HY, Pan XH, Xu Y, Li JZ (2009) Chin Chem Lett
2
0:275
2
3
9. Li H, Bhadury PS, Song B, Yang S (2012) RSC Adv 2:12525
0. Feh e´ r C, Kriv a´ n E, Hancs o´ k J, Skoda-F o¨ ldes R (2012) Green
Chem 14:403
1. Candu N, Rizescu C, Podolean I, Tudorache M, Parvulescu VI,
Coman SM (2015) Catal Sci Technol 5:729
2. Nakajima K, Okamura M, Kondo JN, Domen K, Tatsumi T,
Hayashi S, Hara M (2009) Chem Mater 21:186
3. Gholamzadeh P, Ziarani GM, Lashgari N, Badiei A, Asadiatouei
P (2014) J Mol Catal A: Chem 391:208
4. Lazarin AM, Gushikem Y, de Castr SC (2000) J Mater Chem
1
1
1
0. Gore S, Baskaran S, K o¨ nig B (2012) Org Lett 14:4568
1. Lim BY, Jung BE, Cho CG (2014) Org Lett 16:4492
2. Siddalingamurthy E, Mahadevan KM, Masagalli JN, Harishku-
mar HN (2013) Tetrahedron Lett 54:5591
3. Liu D, Huo X, Liu J, Shi L, Sun Q (2010) React Kinet Mech
Catal 99:335
4. Desroses M, Wieckowski K, Stevens M, Odell LR (2011)
Tetrahedron Lett 52:4417
5. Srinivasa A, Mahadevan KM, Sureshakumara TH, Hulikal V
3
3
3
3
3
1
1
1
(2008) Monatsh Chem 139:1475
10:2526
5. Vafaeezadeh M, Hashemi MM (2014) Chem Eng J 250:35
1
1
6. Ackermann L, Born R (2004) Tetrahedron Lett 45:9541
7. Mun HS, Ham WH, Jeong JH (2005) J Comb Chem 7:30
123