Efficient, one-pot synthesis of xanthene derivatives using boron sulphonic acid as a solid heterogeneous catalyst under solvent-free conditions

Article abstract of DOI:10.1007/s11164-014-1537-5

An efficient, convenient and green method has been introduced for the synthesis of biologically active xanthene derivatives through a one-pot condensation of aldehydes, 2-naphthol, and dimedone/naphthols in the presence of boron sulphonic acid (B(HSO

Full text of DOI:10.1007/s11164-014-1537-5

Res Chem Intermed
DOI 10.1007/s11164-014-1537-5
Efficient, one-pot synthesis of xanthene derivatives
using boron sulphonic acid as a solid heterogeneous
catalyst under solvent-free conditions
•
Hassan Moghanian Akbar Mobinikhaledi
Maryam Deinavizadeh
•
Received: 25 October 2013 / Accepted: 9 January 2014
Ó Springer Science+Business Media Dordrecht 2014
Abstract An efficient, convenient and green method has been introduced for the
synthesis of biologically active xanthene derivatives through a one-pot condensation of
aldehydes, 2-naphthol, and dimedone/naphthols in the presence of boron sulphonic acid
(B(HSO4)3) as an efficient heterogeneous reusable solid acid catalyst under solvent-free
conditions with good to excellent yields. Different types of aromatic and aliphatic
aldehydes were used in the reaction and in all cases the products synthesized success-
fully. In addition, short reaction times, straightforward procedure, non-toxicity, and
reusability of the catalyst are other noteworthy advantages of the present method.
Keywords Xanthenes Á Boron sulphonic acid Á Solid-acid catalyst Á
One-pot reaction Á Solvent-free
Introduction
The synthesis of xanthenes and benzoxanthenes has gained considerable attention in
organic synthesis because of their wide range of therapeutic and biological
properties, such as agricultural bactericide activity [1], anti-inflammatory [2], and
antiviral activity [3]. They have also been used for photodynamic therapy [4], used
as dyes [5], and as fluorescent material for visualization of biomolecules [6] and in
laser technologies [7]. Thus, a broad utility range has made xanthenes prime
synthetic candidates, thereby accentuating the need to develop newer synthetic
routes for scaffold manipulation of xanthene derivatives.
H. Moghanian (&)
Department of Chemistry, Islamic Azad University, Dezful Branch, Dezful, Iran
e-mail: moghanian@gmail.com
A. Mobinikhaledi Á M. Deinavizadeh
Department of Chemistry, Faculty of Science, Ara¯k University, 38156-8-8349 Ara¯k, Iran
123

H. Moghanian et al.
Various methods are available for the construction of xanthenes and benzox-
anthenes involving trapping of benzynes by phenols [8], cyclo-condensation
reaction between 2-hydroxyaromatic aldehydes and 2-tetralone [9], intramolecular
phenyl carbonyl coupling reactions of benzaldehydes and acetophenones [10], and
the reaction of 2-naphthol with 2-naphthol-1-methanol [11] and carbon monoxide
[12]. However, these methods have many disadvantages such as low yields, the need
for a prolonged reaction time, the use of toxic organic solvents, excess reagents, and
harsh reaction conditions. Because of these drawbacks, the reaction has been
improved by mixing aldehydes with 2-naphthol or dimedone in the presence of a
catalyst, such as p-TSA [13, 14], Na?-montmorillonite sulfonic acid [15], molecular
iodine [16], silica sulfuric acid [17], amberlyst-15 [18], wet cyanuric choloride [19],
K12[As2W18Cu3O68]Á30H2O and K12[As2W18U3O74]Á21H2O [20], boric acid [21],
PVPP-BF3 [22], ceric ammonium nitrate (CAN) [23], and InCl3 [24].
The increasing demand for clean and efficient chemical reactions has resulted in
solvent-free reaction conditions, which are of great current interest. Also, economic
and environmental concerns encourage the application of heterogeneous catalysts to
carry out various organic transformations. These catalysts can conveniently be
handled and removed from the reaction mixture, making the experimental procedure
simple and ecofriendly. Recently, B(HSO4)3 was found to be an effective solid acid
catalyst in the synthesis of thiocyanohydrins [25] and anthraquinone derivatives
[26]. In continuation of our studies on the development of practical, safe, and
environmentally friendly procedures for some important transformations [14, 27,
28], we had an opportunity to explore the catalytic activity of B(HSO4)3 towards the
synthesis of xanthenes under solvent-free reaction conditions.
Experimental
General
1
All products were characterized by comparison of their spectral (FT-IR, H NMR,
13C NMR) data or by comparison of their physical and spectroscopic data with those
reported in the literature. Melting points were measured by using capillary tubes on
an electrothermal digital apparatus and are uncorrected. The progress of the
reactions was monitored by thin-layer chromatography (TLC) using n-hexane/
EtOAc as an eluent. The FT-IR spectrum of the samples were recorded on a Unicom
Galaxy Series FT-IR 5000 spectrophotometer in the region 4,000–400 cm-1 using
1
pressed KBr discs. The H and 13C NMR spectra were recorded on a Brucker
Avance spectrometer operating at 300 and 75 MHz for proton and carbon-13,
respectively, in DMSO-d6 or CDCl3 with TMS as an internal standard. Elemental
analysis was performed on a Vario EL III elemental analyzer.
Preparation of catalyst (boron sulfonic acid) [25]
Boric acid (0.62 g, 10 mmol) was charged in a 50-mL flask and chlorosulfonic acid
(3.50 g, 30 mmol) was added dropwise over a period of 1 h at room temperature.
123

Efficient, one-pot synthesis of xanthene derivatives
HCl evolved immediately. After completion of the addition, the mixture was shaken
for 1 h, while the residual HCl was eliminated. The mixture was then washed with
diethyl ether to remove the unreacted chlorosulfonic acid. Finally, the obtained solid
catalyst was dried under vacuum.
General procedure for the preparation of 14-aryl-14H-dibenzo[a,j]xanthenes
(5a–5f)
A mixture of aldehyde (1 mmol), 2-naphthol (2 mmol) and boron sulphonic acid
(0.10 mmol) was stirred at 120 °C for the appropriate time indicated in Table 2. The
progress of the reactions was monitored by TLC (eluent: EtOAc/n-hexane, 1:5).
After completion of the reaction, the reaction mixture was cooled to room
temperature, the crude product was heated in ethanol (30 mL), and the catalyst was
removed by filtration. The pure product was obtained by cooling of the filtrate.
General procedure for the preparation of tetrahydrobenzo[a]xanthen-11-one
derivatives (6a–6h)
A mixture of aldehyde (1 mmol), 2-naphthols (1 mmol), dimedone (1.1 mmol) and
boron sulphonic acid (0.10 mmol) was stirred at 120 °C for the appropriate time
indicated in Table 2. The progress of reactions was monitored by TLC (eluent:
EtOAc/n-hexane, 1:5). After completion of the reaction, the reaction mixture was
cooled to room temperature, the crude product was heated in ethanol (30 mL), and
the catalyst was removed by filtration. The pure product was obtained by cooling of
the filtrate.
Spectral data of new compounds
Compound 5a (Table 2, Entry 1)
IR (KBr): mmax = 3,069, 2,992, 2,936, 2,826, 1,624, 1,589, 1,506, 1,458, 1,418,
1
1,321, 1,240, 1,181, 1,125, 1,080, 1,005, 957, 818, 748, 671, 540 cm-1; H NMR
(300 MHz, CDCl3): d = 8.42 (d, J = 8.52 Hz, 2H), 7.87 (d, J = 8.1 Hz, 2H), 7.82
(d, J = 8.9 Hz, 2H), 7.61 (t, J = 7.1 Hz, 2H), 7.50 (d, J = 8.9 Hz, 2H), 7.45 (t,
J = 7.9 Hz, 2H), 6.72 (s, 2H), 6.46 (s, 1H), 3.73 (s, 6H), 3.67 (s, 3H) ppm; 13C
NMR (75 MHz, CDCl3): d = 153.2, 148.9, 140.6, 136.9, 131.6, 131.2, 128.9,
128.8, 126.7, 124.3, 122.8, 117.9, 117.4, 106.0, 60.7, 56.2, 38.1 ppm; Anal. calcd.
for C30H24O4:C, 80.34; H, 5.39. Found: C, 80.26; H, 5.43.
Compound 5f (Table 2, Entry 6)
IR (KBr): mmax = 3,063, 2,985, 2,934, 2,845, 1,663, 1,595, 1,516, 1,462, 1,391,
1
1,287, 1,244, 1,080, 804, 746 cm-1; H NMR (300 MHz, CDCl3): d = 8.07 (d,
J = 8.4 Hz, 2H), 7.89 (d, J = 8.1 Hz, 2H), 7.80 (d, J = 8.9 Hz, 2H), 7.67 (t,
J = 7.1 Hz, 2H), 7.50 (t, J = 7.8 Hz, 2H), 7.35 (d, J = 8.9 Hz, 2H), 4.64 (s, 2H)
ppm; 13C NMR (75 MHz, CDCl3): d = 147.7, 132.3, 130.4, 128.9, 128.7, 127.4,
123

H. Moghanian et al.
124.9, 123.5, 117.8, 111.7, 22.3 ppm; Anal. calcd. for C21H14O:C, 89.34; H, 5.00.
Found: C, 89.39; H, 5.05.
Compound 6a (Table 2, Entry 7)
IR (KBr): mmax = 3,183, 3,055, 3,003, 2,955, 2,841, 1,626, 1,514, 1,451, 1,379,
1
1,233, 1,138, 1,028, 829, 756 cm-1; H NMR (300 MHz, DMSO-d6): d = 9.47 (s,
1H), 7.64 (d, J = 8.8 Hz,1H), 7.61 (d, J = 8.8 Hz, 1H),7.29 (s, 1H), 7.09 (d,
J = 8.8 Hz, 1H), 6.97 (s, 1H), 6.95 (d, J = 8.6 Hz, 1H), 6.72 (d, J = 8.2 Hz, 1H),
6.65 (d, J = 8.2 Hz, 1H), 5.38 (s, 1H), 3.78 (s, 3H), 3.71 (s,3H), 2.57 and 2.56 (AB
system, J = 4.4 Hz, 2H), 2.18 and 2.29 (AB system, J = 16.3 Hz, 2H), 1.12 (s,
3H), 0.96 (s, 3H) ppm; 13C NMR (75 MHz, DMSO-d6): d = 196.4, 164.1, 156.9,
148.7, 148.1, 147.7, 137.8, 133.1, 130.6, 129.0, 125.9, 120.6, 117.6, 116.2, 113.9,
112.9, 112.1, 105.8, 56.0, 55.8, 50.6, 34.1, 32.2, 29.3, 26.6 ppm; Anal. calcd. for
C27H26O5:C, 75.33; H, 6.09. Found: C, 75.25; H, 6.12.
Compound 6b (Table 2, Entry 8)
IR (KBr): mmax = 3,227, 3,081, 2,961, 2,891, 1,630, 1,518, 1,451, 1,381, 1,238,
1,182, 1,030, 841, 698, 617, 530, 442 cm-1 1H NMR (300 MHz, DMSO-d6):
;
d = 9.86 (s, 1H), 8.08 (d, J = 7.2 Hz, 2H), 7.80 (d, J = 8.8 Hz, 1H), 7.75 (d,
J = 8.9 Hz, 1H), 7.45 (d, J = 7.3 Hz, 2H), 7.21 (d, J = 8.8 Hz, 1H), 7.15 (s, 1H),
6.97 (d, J = 8.8 Hz, 1H), 5.5 (s, 1H), 2.58 and 2.69 (AB system, J = 17.6 Hz, 2H),
2.13 and 2.34 (AB system, J = 16.3 Hz, 2H), 1.06 (s, 3H), 0.86 (s, 3H) ppm; 13C
NMR (75 MHz, DMSO-d6): d = 196.4, 164.9, 157.1, 152.5, 148.1, 146.3, 132.8,
130.8, 129.9, 129.8, 125.9, 123.8, 117.8, 114.5, 114.0, 112.6, 105.5, 50.4, 34.9,
32.2, 29.2, 26.6 ppm; Anal. calcd. for C25H21NO5:C, 72.28; H, 5.10; N, 3.37.
Found: C, 72.34; H, 5.13; N, 3.44.
Compound 6e (Table 2, Entry 11)
IR (KBr): mmax = 3,187, 2,959, 2,932, 2,870, 1,624, 1,595, 1,451, 1,397, 1,235, 1,175,
1,150, 1,028, 833 cm-1; 1H NMR (300 MHz, CDCl3): d = 9.12 (s, 1H), 7.83 (s, 1H),
7.73 (d, J = 8.8 Hz, 1H), 7.63 (d, J = 8.8 Hz, 1H), 7.14 (d, J = 8.8 Hz, 1H), 7.02 (d,
J = 8.8 Hz, 1H), 4.81 (s, 1H), 2.60 (s, 2H), 2.51 (s, 2H), 1.91 (q, J = 5.7 Hz, 2H),
1.24–1.20 (s, 6H), 1.07 (m, 4H), 0.90 (m, 2H), 0.69 (t, J = 6.5 Hz, 3H) ppm; 13C NMR
(75 MHz, CDCl3): d = 200.1, 168.2, 156.3, 148.7, 132.9, 130.3, 127.9, 126.4, 117.3,
116.4, 113.5, 112.8, 105.6, 50.6, 41.5, 34.3, 32.3, 31.9, 29.5, 28.3, 27.2, 24.6, 22.5,
13.9 ppm; Anal. calcd. for C24H28O3:C, 79.09; H, 7.74. Found: C, 79.03; H, 7.80.
Results and discussion
In this paper, the condensation of 2-naphthols, various aldehydes, and 2-naphthol/
dimedone in the presence of a heterogeneous solid acid catalyst of B(HSO4)3 for the
preparation of xanthenes derivatives has been studied (Scheme 1).
123

Efficient, one-pot synthesis of xanthene derivatives
X
X
OH
R
3
B(HSO4)3
O
5
X= H
O
X
OH
RCHO
2
+
R
O
1
O
4
X= H, OH
B(HSO4)3
O
6
X= H, OH
Scheme 1 Synthesis of xanthene derivatives
Table 1 Optimization of reaction conditions
Entry
Solvent
Conditions
B(HSO4)3 (mol%)
Time (min)
Yield (%)a
1
2
3
4
5
6
7
a
CHCl3
Reflux
Reflux
Reflux
Reflux
120 °C
120 °C
120 °C
10
10
10
10
10
–
180
180
180
180
10
65
43
31
72
91
\5
91
C2H5OH
CH3CN
THF
Solvent-free
Solvent-free
Solvent-free
60
20
10
Isolated yields
Initially, in order to optimize the reaction parameters, we investigated the
catalytic activity of B(HSO4)3 in the synthesis of xanthene derivatives under
different reaction conditions, using the condensation reaction of benzaldehyde and
2-naphthol as a model reaction. As shown in Table 1, among the tested solvents,
such as ethanol, CH3CN, THF, CHCl3, and a solvent-free system, the best result was
obtained after 10 min under solvent-free conditions in excellent yield (91 %). When
the same reaction was performed in the absence of the catalyst, the corresponding
product was obtained in only \5 % yield, whereas, B(HSO4)3 in excess of 0.2
equival. did not improve the yield to a greater extent (Table 1, entry 7).
After optimizing the conditions, we next examined the scope and generality of
this method with respect to various aromatic aldehydes with electron-withdrawing
and electron-donating substituents as well as aliphatic aldehydes to prepare a series
of dibenzoxanthene derivatives (Table 2). a,b-Unsaturated aldehyde such as
cinnamaldehyde was also examined but its yield was not satisfactory even after
1 h. The present methodology afforded good to high yields of the products within
short times (10–40 min).
To further show the applicability of this method, the preparation of tetra-
hydrobenzo[a]-xanthene-11-one derivative was also investigated. A variety of
aromatic aldehydes bearing electron-withdrawing groups (such as nitro and halide),
electron-donating groups (such as methyl and methoxy), and dimedone were treated
123

H. Moghanian et al.
Table 2 Synthesis of xanthene derivatives in presence of boron sulphonic acid under solvent-free
conditions
Entry
R (aldehyde)
X
Pro.
Time
(min)
Yeild
(%)a
MP (°C)
Found
Reported [lit.]
1
3,4,5-(MeO)3C6H2
C6H5
H
5a
5b
5c
5d
5e
5f
25
10
15
35
40
30
25
15
20
15
60
30
10
25
85
91
85
76
73
39
89
93
91
89
78
86
87
83
192–193
186–187
213–214
154–156
114–115
205–207
228–230
266–267
302–303
280–281
167–169
195–197
156–157
178–180
–
2
H
184–185 [29]
214–216 [29]
150–152 [30]
112–113 [31]
–
3
2-ClC6H4
H
4
CH3CH2
H
5
(CH3)2CHCH2
2-OH-1-naphthyl
3,4-(MeO)2C6H3
4-NO2C6H4
4-MeC6H4
H
6
H
7
OH
OH
OH
OH
OH
H
6a
6b
6c
6d
6e
6f
–
8
–
9
303–305 [32]
282–283 [32]
–
10
11
12
13
14
a
2-ClC6H4
CH3(CH2)3CH2
4-MeOC6H4
C6H5
198–202 [21]
151–154 [21]
173–175 [21]
H
6g
6h
4-MeC6H5
H
Isolated yields
Table 3 The comparison efficiency of B(HSO4)3 with other reported catalysts for synthesis of 4b and 4c
Entry
Catalyst
Conditions
Time (min)/yield (%)a
Reference
5b
5c
1
2
3
4
5
6
a
B(HSO4)3
Solvent-free/120 °C
Solvent-free/100 °C
Solvent-free/125 °C
Solvent-free/110 °C
Solvent-free/100 °C
Solvent-free/125 °C
10/91
90/78
120/94
90/95
55/94
10/95
15/93
90/91
20/91
120/93
57/92
10/88
This method
[33]
Dowex-50 W
Amberlyst-15
Cellulose sulfuric acid
P(4-VPH)HSO4
HClO4–SiO2
[18]
[34]
[29]
[19]
Isolated yields
with 2-naphthol or 2,7-naphthalenediol under the same experimental conditions, and
the corresponding products were obtained in good to high yields without any
difficulties (Table 2). Interestingly, aliphatic aldehydes such as hexanal also gave
the expected xanthene derivatives in good yield.
In order to assess the efficiency and generality of this methodology, the obtained
result from the reaction of benzaldehyde and 2-chlorobenzaldehyde with 2-naphthol
and the reaction of benzaldehyde and 4-methoxybenzaldehyde with 2-naphthol and
dimedone by this method have been compared with those of the previously reported
methods using inorganic or organic catalysts (Tables 3, 4). It was found that the
123

Efficient, one-pot synthesis of xanthene derivatives
Table 4 The comparison efficiency of B(HSO4)3 with other reported catalysts for synthesis of 5f and 5g
Entry
Catalyst
Conditions
Time (min)/yield (%)a
Reference
6f
6g
1
2
3
4
5
6
a
B(HSO4)3
InCl3
Solvent-free/120 °C
Solvent-free/120 °C
Solvent-free/120 °C
Solvent-free/80 °C
Solvent-free/100 °C
Solvent-free/120 °C
30/86
45/76
55/71
70/86
15/83
20/75
10/87
30/84
40/76
50/90
15/89
30/73
This method
[24]
P2O5
[24]
TCT
[35]
H4SiW12O40
CCl3COOH
[36]
[21]
Isolated yields
Table 5 Reuse of the catalyst
for the synthesis of 5b (Table 2,
entry 2)
Run
Time (min)
Yield (%)a
1
2
3
4
5
10
10
10
10
10
91
90
88
82
75
All reactions carried out at
120 °C
a
Isolated yields
B(HSO4)3 is a fairly good reagent for this reaction with respect to reaction time,
yield of product, and amount of the catalyst.
For investigation of the reusability and recycling of B(HSO4)3 as a catalyst, we
stirred a mixture of benzaldehyde (1 mmol) and 2-naphthol (2 mmol) in the
presence of B(HSO4)3 (10 mol%) at 120 °C for 10 min. At the end of the reaction,
the B(HSO4)3 was easily separated by simple filtration and washed with hot ethanol
and the recovered catalyst was reused for at least five runs without significant
degradation in catalytic activity and performance (Table 5).
Conclusion
In conclusion, this paper describes a convenient and efficient process for the
synthesis of tetrahydrobenzo[a]xanthen-11-one and dibenzo[a,j]xanthene deriva-
tives in the presence of boron sulphonic acid [B(HSO4)3] under solvent-free
conditions at 120 °C. Some advantages of this environmentally safe and benign
protocol include a simple reaction set-up, high products yields, short reaction times,
and elimination of toxic solvents. The catalyst is readily prepared and inexpensive,
and can conveniently be handled and removed from the reaction mixture.
Acknowledgment We gratefully acknowledge the financial support from the Research Council of Arak
University.
123

H. Moghanian et al.
References
1. T. Hideu, In Jpn. Tokkyo Koho JP 56005480, Chem. Abstr., 95, p. 80922b (1981)
2. J.M. Jamison, K. Krabill, A. Hatwalkar, Cell Biol. Int. Rep. 14, 1075 (1990)
3. R.M. Ion, D. Frackowiak, K. Wiktorowicz, Acta Biochim. Pol. 45, 833 (1998)
4. J.P. Poupelin, G. Saint-Ruf, O. Foussard-Blanpin, G. Narcisse, G. Uchida-Emouf, R. Lacroix, Eur.
J. Med. Chem. 13, 67 (1978)
5. B.B. Bhowmik, P. Ganguly, Spectrochim. Acta A 61, 1997 (2005)
6. C.G. Knight, T. Stephens, Biochem. J. 258, 683 (1989)
7. O. Sirkecioglu, N. Talinli, A. Akar, J. Chem. Res. (S) 502 (1995)
8. D.W. Knight, P.B. Little, Synlett. 1141 (1998)
9. A. Jha, J. Beal, Tetrahedron Lett. 45, 8999 (2004)
10. C.W. Kuo, J.M. Fang, Synth. Commun. 31, 877 (2001)
11. R.N. Sen, N. Sarkar, J. Am. Chem. Soc. 47, 1079 (1925)
12. K. Ota, T. Kito, Bull. Chem. Soc. Jpn 49, 1167 (1976)
13. A.R. Khosropour, M.M. Khodaei, H. Moghannian, Synlett. 955 (2005)
14. A. Mobinikhaledi, H. Moghanian, M. Deinavizadeh, C. R. Chimie 16, 1035 (2013)
15. F. Shirini, M. Mamaghani, S.V. Atghia, J. Iran. Chem. Soc. 10, 415 (2013)
16. B. Das, B. Ravikanth, R. Ramu, K. Laxminarayana, B.V. Rao, J. Mol. Catal. A 255, 74 (2006)
17. M. Seyyedhamzeh, P. Mirzaei, A. Bazgir, Dyes Pigm. 76, 836 (2008)
18. S. Ko, C.-F. Yao, Tedraherdon Lett. 47, 8827 (2006)
19. M.A. Bigdeli, M.M. Heravi, G.H. Mahdavinia, Catal. Commun. 8, 1595 (2007)
20. S. Sheshmani, J. Chem. Sci. 125, 345 (2013)
21. Z. Karimi-Jaberi, S.Z. Abbasi, B. Pooladian, M. Jokar, E-J. Chem. 8, 1895 (2011)
22. M. Mokhtary, S. Refahati, Dyes Pigm. 99, 378 (2013)
23. A. Kumar, S. Sharma, R.A. Maurya, J. Sarkar, J. Comb. Chem. 12, 20 (2010)
24. G.C. Nandi, S. Samai, R. Kumar, M.S. Singh, Tetrahedron 65, 7129 (2009)
25. A.R. Kiasat, M. Fallah-Mehrjardi, J. Braz. Chem. Soc. 19, 1595 (2008)
26. B.R. Madje, M.B. Ubale, J.V. Bharad, M.S. Shingare, Bull. Catal. Soc. India 9, 19 (2011)
27. N. Foroughifar, A. Mobinikhaledi, H. Moghanian, Chem. Lett. 39, 180 (2010)
28. H. Moghanian, M. Shabanian, H.C.R. Jafari, Chimie 15, 346 (2012)
29. N. Ghaffari Khaligh, Ultrason. Sonochem. 19, 736 (2012)
30. M. Hong, C. Cai, J. Fluorine Chem. 130, 989 (2009)
31. M. Dabiri, M. Baghbanzadeh, M.S. Nikcheh, E. Arzroomchilar, Bioorg. Med. Chem. Lett. 18(1), 436
(2008)
32. N. Foroughifar, A. Mobinikhaledi, H. Moghanian, R. Mozafari, H.R.M. Esfahani, Synth. Commun.
41, 2663 (2011)
33. G.I. Shakibaei, P. Mirzaei, A. Bazgir, Appl. Catal. A 325, 188 (2007)
34. J.V. Madhav, Y.T. Reddy, P.N. Reddy, M.N. Reddy, S. Kuarm, P.A. Crooks, B. Rajitha, J. Mol.
Catal. A 304, 85 (2009)
35. Z.-H. Zhang, P. Zhang, S.-H. Yang, H.-J. Wang, J. Deng, J. Chem. Sci. 122, 427 (2010)
36. A. Hassankhani, E. Mosaddegh, S.Y. Ebrahimipour, E-J. Chem. 9, 786 (2012)
123