602
Acta Chim. Slov. 2016, 63, 602-608
DOI: 10.17344/acsi.2016.2291
Scientific paper
Carbon Nanotube-Supported Butyl 1-Sulfonic Acid Groups as a Novel and Environmentally Compatible Catalyst for the Synthesis of 1,8-Dioxo-octahydroxanthenes
Keveh Parvanak Boroujeni,1'* Zahra Heidari1 and Reza Khalifeh2
1 Department of Chemistry, Shahrekord University, P.O. Box 88186-34141 Shahrekord, Iran
2 Department of Chemistry, Shiraz University of Technology, Shiraz, Iran
* Corresponding author: E-mail: parvanak-ka@sci.sku.ac.ir Tel.: +0098-38-32324401; fax: 0098-38-32324419
Received: 25-01-2016
Abstract
A novel multiwalled carbon nanotube catalyst with -SO3H functional groups was easily prepared from its starting materials and used as an efficient heterogeneous catalyst for one-pot Knoevenagel condensation, Michael addition, and cyclodehydration of 5,5-dimethyl-1,3-cyclohexanedione (dimedone) with various aromatic aldehydes. Using this method 1,8-dioxo-octahydroxanthenes were obtained in excellent yields at room temperature. The present method is superior in terms of reaction temperature, reaction time, easy work-up, high yields, and ease of recovery of catalyst.
Keywords: Nanocatalyst; Heterogeneous catalysis; 1,8-Dioxo-octahydroxanthene; Aldehyde; 5,5-Dimethyl-1,3-cyclo-hexanedione
1. Introduction
Recently, carbon nanotubes (CNTs) have been considered as good supports for homogeneous and heterogeneous catalysts.1-3 When compared to other commonly used supports in heterogeneous catalysis, CNTs present the advantage of extraordinary electrical, thermal, and mechanical strength characteristics, resistance to chemical attack in acidic and basic media, high surface areas, and low cost. They are cylindrically shaped and their surface can be modified with various functional groups, which can be used as building blocks for covalent and noncovalent attachment of catalytic active species.
There is a widespread interest in the synthesis of xanthene derivatives owing to their diverse range of biological and therapeutic properties, such as anti-inflamma-tory,4 antiviral,5 and anticancer activities.6 Also, they were used as antagonists for the paralyzing action of zoxazola-mine,7 fluorescent markers for the visualization of bio-molecules,8 and photostable laser dyes.9 Among various derivatives of xanthene, 1,8-dioxo-octahydroxanthenes have aroused considerable interest. Synthesis of 1,8-dio-xo-octahydroxanthenes is generally achieved by the con-
densation of dimedone with aldehydes. Several types of catalysts were introduced previously for this reaction, such as NaHSO4-SiO2 or silica chloride,10 polyphosphoric acid-SiO2,11 In(OTf)3,12 H2SO4,13 InCl3 or P2O5,14 cerric ammonium nitrate (CAN) under ultrasound irradiation,15 succinimide-N-sulfonic acid,16 CaCl2,17 Fe3O4 nanopartic-les,18 CAN supported HY-zeolite,19 Fe3O4@SiO2-Imid-H3PMo12O40 nanoparticles,20 piperidine,21 Mg-Al hydro-talcite,22 thiourea dioxide,23 and ZnO nanoparticles.24 Although these methods are suitable for certain synthetic conditions, there exist some drawbacks, such as low yields, high reaction temperature, long reaction times, tedious work-up, the formation of 2,2'-aryl-methylene bis(3-hydroxy-2-cyclohexene-1-one) derivatives due to competitive side reactions, and the use of unrecyclable, hazardous or difficult to handle catalysts. In view of this, utilizing eco-friendly and green catalysts for this useful reaction is in demand.
In a continuation of our recent work on synthesis and application of heterogeneous catalysts in organic reactions,25-28 herein we now report the synthesis of mul-tiwalled carbon nanotube-supported butyl 1-sulfonic acid groups (MWCNT-BuSO3H) from the reaction of the salt
Boroujeni et al.: Carbon Nanotube-Supported Butyl 1-Sulfonic Acid Groups
Acta Chim. Slov. 2016, 63, 602-608
603
form hydroxyl functionalized multiwalled carbon nanotu-be (MWCNT-OH) with 1,4-butane sultone followed by the reaction with HCl. MWCNT-BuSO3H was used as a heterogeneous catalyst for one-pot Knoevenagel condensation, Michael addition, and cyclodehydration of dime-done with various aromatic aldehydes at room temperature (Scheme 1).
'■A
+ArCHO
O
MWCNT-BuS03H (0.07 mmol)
EtOH/ r.t.
Ar O
>666<
Scheme 1. Synthesis of 1,8-dioxo-octahydroxanthenes using MW-CNT-BuSOH.
2. Experimental
2. 1. Materials and Methods
Chemicals were either prepared in our laboratory or were purchased from Merck and Fluka. Reaction monitoring and purity determination of the products were accomplished by GLC or TLC on silica-gel polygram SILG/UV254 plates. Gas chromatography was recorded on Shimadzu GC 14-A. IR spectra were obtained by a Shi-madzu model 8300 FT-IR spectrophotometer. NMR spectra were recorded on 400 MHz spectrometer in CDCl3. The Leco sulfur analyzer was used for the measurement of sulfur in the catalyst. TGA was carried out on a Stanton Redcraft STA-780 with a 20 °C/min heating rate. SEM and TEM images were taken with a Hitachi S-3400N scanning electron microscope and a Philips CM10 transmission electron microscope, respectively. Melting points were determined on a Fisher-Jones melting-point apparatus.
2. 2. Synthesis of Multiwalled Carbon Nanotubes
MWCNT and MWCNT-OH were prepared as reported in our previous work.1
2. 3. Synthesis of MWCNT-BuSO3H
In a round bottomed flask (50 mL) equipped with a reflux condenser was added 1 g of the MWCNT-OH to an aqueous solution of sodium hydroxide (1 M, 10 mL) and the mixture was stirred at 60 °C for 12 h, filtered, washed with distilled water (20 mL), and dried at 80 °C overnight to give MWCNT-ONa. Then, 1,4-butane sultone (1.5 mL) was added to the obtained solid and the mixture was stirred at 100 °C for 24 h, filtered, washed with distilled water (20 mL), and dried at 80 °C overnight to give MW-CNT-OBuSO3Na. Afterwards, HCl (3 M, 10 mL) was added to MWCNT-OBuSO3Na and the mixture was stirred at room temperature for 2 h, filtered, washed with distilled
water (20 mL), and dried at 80 °C overnight to give MW-CNT-BuSO3H.
2. 4. General Procedure for
1,8-Dioxo-octahydroxanthene Synthesis
A mixture of an aldehyde (2 mmol), dimedone (0.28 g, 2 mmol), MWCNT-BuSO3H (0.066 g, 0.07 mmol), and ethanol (3 mL) was stirred for an appropriate time at room temperature. After completion of the reaction (monitored by TLC), the catalyst was filtered off and washed with ethanol (2 x 10 mL). Then, the filtrate was concentrated on a rotary evaporator under reduced pressure and the crude product recrystallized from ethanol. All products are known compounds and were identified by comparison of their physical and spectral data with those of the authentic samples.
2. 5. Representative Spectral Data of Some of the Obtained Compounds
3,3,6,6-Tetramethyl-9-phenyl-1,8-dioxo-octahydro-xanthene14 (Table 1, entry 1). 1H NMR (400 MHz, CDCl3) 5 1.01 (s, 6H, 2 CH3), 1.14 (s, 6H, 2 CH3), 2.19-2.48 (m, 8H, 4 CH2), 4.80 (s, 1H, CH), 6.95-7.22 (m, 5H, ArH). IR (KBr) v 2960, 2950, 1663, 1490, 1390, 1250, 850 cm-1.
9-(4'-Nitrophenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-he-xahydro-1ff-xanthene-1,8-(2ff)-dione14 (Table 1, entry 11). 1H NMR (400 MHz, CDCl3) 5 1.05 (s, 6H, 2 CH3), 1.16 (s, 6H, 2 CH3), 2.20-2.50 (m, 8H, 4 CH2), 4.88 (s, 1H, CH), 7.52-7.60 (d, 2H, ArH), 8.09-8.14 (d, 2H, ArH). IR (KBr) v 2966, 2930, 2870, 1730, 1670, 1600, 1350, 1192, 860 cm-1.
9-(4-Methoxyphenyl)-3,3,6,6-tetramethyl-3,4,5,6,7,9-hexahydro-1ff-xanthene-1,8(2ff)-dione12 (Table 1, entry 4). 1H NMR (400 MHz, CDCl3) 5 1.06 (s, 6H, 2 CH3), 1.14 (s, 6H, 2 CH3), 2.19-2.48 (m, 8H, 4 CH2), 3.80 (s, 3H, OCH3), 4.72 3s, 1H, CH), 6.83-6.91 (d, 2H, ArH), 7.20-7.23 (d, 2H, ArH). IR (KBr) v 2960, 2948, 1668, 1200, 1190, 795 cm-1.
3. Results and Discussion
3. 1. Preparation of MWCNT-BuSO3H
A chemical vapour deposition (CVD) method was used for the synthesis of MWCNT.1 In order to develop hydroxyl groups on the MWCNT surface, the carbon na-nomaterials were submitted to a heat treatment in a synthetic air flow (10 mL/min) at 500 °C for 2 h.1
The synthetic routes for the MWCNT-BuSO3H are shown in Scheme 2. At the first stage, MWCNT-OH was treated with NaOH to form the MWCNT-ONa. In the second step, MWCNT-BuSO3H was prepared from the reac-
Boroujeni et al.: Carbon Nanotube-Supported Butyl 1-Sulfonic Acid Groups
604
Acta Chim. Slov. 2016, 63, 602-608
2 ) HCI / r.t, / 2 h
(MWCNT-BUS03H) Scheme 2. Preparation procedure to MWCNT-BuSO3H.
Figure 1. FT-IR spectra of MWCNT-OH (A) and MWCNT-Bu-SO3H (B).
tion of MWCNT-ONa with 1,4-butane sultone followed by the reaction with HCl. The resulting black solid was analyzed by elemental analysis to quantify the percentage loading of the sulfonic acid groups by measuring the sulfur content, giving 0.98 mmol sulfonic acid moiety per gram. The acidic sites loading in MWCNT-BuSO3H obtained by means of acid-base titration was found to be 1.05 mmol/g.25
3. 2. Characterization of MWCNT-BuSO3H
FT-IR spectra of the MWCNT-OH and MWCNT-BuSO3H are presented in Figure 1. As can be seen in the spectrum of MWCNT-BuSO3H new peaks appeared at 1120, 1150, 1190, and 1230 cm-1, which can be assigned to S=O stretching vibration.25,26
The thermogravimetric analyses (TGA) of MWCNTs, before and after the functionalization processes, are provided in Figure 2. The TGA curves of MWCNT-OH and
Figure 2. TGA curves of MWCNT-OH (A) and MWCNT-Bu-SO3H (B).
Figure 3. SEM photographs of MWCNT-OH (a) and MWCNT-BuSO3H (b).
Boroujeni et al.: Carbon Nanotube-Supported Butyl 1-Sulfonic Acid Groups ...
Acta Chim. Slov. 2016, 63, 602-608
605
Figure 4. TEM photographs of MWCNT-OH (a) and MWCNT-BuSO3H (b).
MWCNT-BuSO3H displayed a weight loss around 100 oC which is corresponding to the loss of the physically adsorbed water. In the case of MWCNT-BuSO3H, the second weight loss started at about 180 oC and is mainly assigned to the decomposition of the alky-sulfonic acid groups. In TGA curves of MWCNT-OH and MWCNT-BuSO3H the last weight losses at about 570-640 °C were likely due to the degradation of MWCNTs.
An attempt was made to investigate the morphology of the MWCNTs using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From SEM photographs of MWCNT-OH and MWC-NT-BuSO3H (Figure 3), it is obvious that the MWCNTs are well distributed and no amorphous carbon is detected. In the TEM photograph of MWCNT-BuSO3H (Figure 4 (B)), it can be seen that the CNTs do not suffer damage after the functionalization and anion-exchange processes and that there are small particles affixed on the surface of MWCNT due to functionalization processes.
3. 3. Catalytic Activity of MWCNT-BuSO3H
In order to explore the catalytic activity of MWC-NT-BuSO3H, we studied the synthesis of 1,8-dioxo-oc-tahydroxanthenes by the reaction of aldehydes with dime-done. Initially, to optimize the reaction conditions, we tried to convert benzaldehyde to 3,3,6,6-tetramethyl-9-phenyl-1,8-dioxo-octahydroxanthene with dimedone at different conditions and various molar ratios of substrates. The best results were obtained at room temperature and a molar ratio of benzaldehyde:dimedone:MWCNT -BuSO3H of 1:2:0.07. Then, under optimal conditions, a wide variety of substituted benzaldehydes (containing both electron withdrawing and donating groups) and 1-naphthaldehyde were treated with dimedone to give the corresponding products in high to excellent yields (Table 1, entries 1-13). Acid sensitive substrates, such as thiop-hene-2-carbaldehyde and cinnamaldehyde gave the cor-
responding products without generation of polymeric byproducts under the present reaction conditions (entries 14,15). In the case of substituted benzaldehydes, the 2-substituted isomer (entries 8,9,12) was less reactive than the 4-substitued isomer, probably due to the increased ste-ric hindrance. It is noteworthy that no competitive side reactions such as the formation of 2,2'-aryl-methylene bis(3-hydroxy-2-cyclohexene-1-one) derivatives were observed in these transformations.10,17
To the best of our knowledge synthesis of 1,8-dioxo-octahydroxanthenes from the reaction of aldehydes with dimedone at room temperature is rare. Most of the reported methods need high temperatures or the use of an additional energy (ultrasound or microwave).29,30
Following these results, we further investigated the potential of MWCNT-BuSO3H for the synthesis of te-trahydrobenzo[a]xanthen-11-ones through condensation of aldehydes, dimedone, and 2-naphtol at room temperature with ethanol as the solvent. We observed that te-trahydrobenzo[a]xanthen-11-ones were obtained in moderate yields after long reaction times. However, when the reactions were carried out in refluxing ethanol the desired products were obtained in high yields at very short reaction times in the presence of 0.05 mmol of catalyst (Scheme 3). In comparison with the other catalysts employed for the synthesis of tetrahydrobenzo[a]xanthen-11-ones,3132 MWCNT-BuSO3H showed a higher catalytic activity in terms of shorter reaction time and higher yields.
As shown in Table 1 (entries 10,11), the aromatic aldehydes with electron withdrawing groups reacted very well at faster rate compared with aromatic aldehydes substituted with electron releasing groups. This observation can be rationalized on the basis the mechanistic details of the reaction (Scheme 4). The aldehyde is first activated by MWCNT-BuSO3H. Nucleophilic addition of dimedone to the activated aldehyde followed by the loss of H2O generates intermediate I, which is further activated by MW-
Boroujeni et al.: Carbon Nanotube-Supported Butyl 1-Sulfonic Acid Groups
606
Acta Chim. Slov. 2016, 63, 602-608
Scheme 3. Synthesis of tetrahydrobenzo[a]xanthen-11-ones using MWCNT-BuSO3H.
CNT-BuSO3H. Then, the 1,4-nucleophilic addition of a second molecule of dimedone on the activated intermediate I, in the Michael addition fashion, affords the intermediate II, which undergoes intramolecular cyclodehy-dration to give the 1,8-dioxo-octahydroxanthene. The electron withdrawing groups present on the aromatic al-
dehyde in the intermediate I increase the rate of 1,4-nucleophilic addition reaction because the alkene LUMO is at lower energy in their presence compared with the aldehydes possessing electron donating groups.33
The reusability of the MWCNT-BuSO3H was also determined. MWCNT-BuSOH recovered after the reac-
Table 1: Synthesis of 1,8-dioxo-octahydroxanthenes.
Entry	Aldehyde	Time (min)	Yield (%)a'b	mp (°C) (lit.)ref-
1	Benzaldehyde	30	95	201-203 (204-205)14
2	4-Methylbenzaldehyde	35	94	210-215 (213-215)14
3	4-Isopropylbenzaldehyde	36	95	201-203 (203-206)18
4	4-Methoxybenzaldehyde	37	96	240-243 (242-244)12
5	3-Methoxybenzaldehyde	36	93	163-165 (162-165)11
6	4-Hydroxybenzaldehyde	40	93	243-245(249-251)14
7	4-Chlorobenzaldehyde	30	95	228-231 (231-233)20
8	2-Chlorobenzaldehyde	35	91	226-228 (224-226)23
9	2,4-Dichlorobenzaldehyde	35	92	249-253 (248-250)11
10	4-Cyanobenzaldehyde	26	95	222-225 (218-220)23
11	4-Nitrobenzaldehyde	25	96	225-227 (223-224)14
12	2-Nitrobenzaldehyde	28	91	260-263 (259-261)12
13	1-Naphthaldehyde	40	95	235-237 (232-234)12
14	Thiophene-2-carbaldehyde	35	94	162-164 (161-162)14
15	Cinnamaldehyde	36	93	179-181 (177-178)22
a Isolated yield, b All products are known compounds and were identified by comparison of their melting points and 'H NMR and FT-IR data with those of the authentic samples.
Scheme 4. Suggested mechanism for the preparation of 1,8-dioxo-octahydroxanthenes.
Boroujeni et al.: Carbon Nanotube-Supported Butyl 1-Sulfonic Acid Groups ...
Acta Chim. Slov. 2016, 63, 602-608
607
tion can be washed with EtOH and used again at least six times without any noticeable loss of catalytic activity (Figure 5).
Figure 5. Recyclability of MWCNT-BuSO3H (0.07 mmol) in the reaction of benzaldehyde (1 mmol) with dimedone (2 mmol) at room temperature. Reaction time 30 min.
To show the merit of the present work in comparison with the other results reported in the literature, we compared results of MWCNT-BuSO3H with selected previously known protocols in the synthesis of 1,8-dioxo-octahydro-xanthenes (Table 2). As can be seen in addition to having the general advantages attributed to the solid catalysts, MWCNT-BuSO3H has a good efficiency compared to many of other reported catalysts in the synthesis of 1,8-dioxo-octahydroxanthenes.
4. Conclusion
In conclusion, we synthesized a novel multiwalled carbon nanotube catalyst with -SO3H functional groups.
This reusable heterogeneous acid catalyst preserved high catalytic activity for the synthesis of 1,8-dioxo-octahydro-xanthenes. Easy preparation and handling of the catalyst, easy workup, high selectivity, excellent yields, short reaction times, and mild reaction conditions are the obvious advantages of the present method.
5. Acknowledgement
We gratefully acknowledge the partial support of this study by the Shahrekord University and the Shiraz University of Technology Research Council, Iran.
6. References
1.	H. Sharghi, M. H. Beyzavi, A. Safavi, M. M. Doroodmand, R. Khalifeh, Adv. Synth. Catal. 2009, 351, 2391-2410. http://dx.doi.org/10.1002/adsc.200900353
2.	M. S. Hoogenraad, R. A. G. M. M Van Leeuwarden, G. J. B. Van Breda-Vriesman, A. Broersma, A. J. Van Dillen, J. W. Geus, in: G. Poncelet, J. Martens, B. Delmon, P. A. Jacobs, P. Grange (Ed.): Metal Catalysts Supported on a Novel Carbon Support, Elsevier, Amsterdam, 1995. http://dx.doi.org/10.1016/s0167-2991(06)81762-3
3.	X. Pan, Z. Fan, W. Chen, Y. Ding, H. Luo, X. Bao, Nature 2007, 6, 507-511. http://dx.doi.org/10.1038/nmat1916
4.	J. P. Poupelin, G. Saint-Ruf, O. Foussard-Blanpin, G. Narcisse, G. Uchida-Ernouf, R. Lacroix, Eur. J. Med. Chem. 1978, 13, 67-71.
5.	R. W. Lambert, J. A. Martin, J. H. Merrett, K. E. B. Parkes, G. J. Thomas, PCT Int. Appl. WO 9706178; 1997. Chem. Abstr. 1997, 126, P212377y.
6.	N. Mulakayala, P. V. N. S. Murthy, D. Rambabu, M. Aeluri,
Table 2: Comparison of the efficiencies of a number of different reported catalysts with that of MWCNT-BuSO3H in the reaction of benzaldehyde with dimedone.
Entry	Reaction conditions	Time (min)	Yield (%)a
1	Silica chloride, MeCN, reflux	360	9310
2	Polyphosphoric acid-SiO2, neat, reflux	30	9211
3	In(OTF)3, toluene, reflux	240	8512
4	H2SO4, water, 70-80 °C	120	9013
5	InCl3, solvent-free, 100 °C	36	8314
6	CAN under ultrasound irradiation, 2-propanol, 50 °C	35	9815
7	Succinimide-N-sulfonic acid, solvent-free, 80 °C	35	9216
8	CaCl2, DMSO, 85-90 °C	240	8517
9	Fe3O4 nanoparticles, solvent-free, 100 °C	30	8918
10	CAN supported HY-zeolite, solvent-free, 80 °C	90	8819
11	Fe3O4@SiO2-Imid-H3PMo12O40, EtOH, reflux	150	8220
12	Mg-Al hydrotalcite, H2O or EtOH, reflux	180	8522
13	Thiourea dioxide, H2O, 50-60 °C	45	9623
14	ZnO, H2O, reflux	20	9424
15	MWCNT-BuSO3H, EtOH, r.t.	30	95
"Isolated yield.
Boroujeni et al.: Carbon Nanotube-Supported Butyl 1-Sulfonic Acid Groups
608
Acta Chim. Slov. 2016, 63, 602-608
R. Adepu, G. R. Krishna, C. M. Reddy, K. R. S. Prasad, M. Chaitanya, C. S. Kumar, M. V. B. Rao, M. Pal, Bioorg. Med. Chem. Lett. 2012, 22, 2186-2191. http://dx.doi.Org/10.1016/j.bmcl.2012.01.126
7.	G. Saint-Ruf, H. T. Hieu, J. P. Poupelin, Naturwissenschaften 1975, 62, 584-585. http://dx.doi.org/10.1007/BF01166986
8.	C. G. Knight, T. Stephens, Biochem. J. 1989, 258, 683-687. http://dx.doi.org/10.1042/bj2580683
9.	M. Ahmad, T. A. King, D. K. Ko, B. H. Cha, J. Lee, J. Phys. D: Appl. Phys. 2002, 35, 1473-1476. http://dx.doi.org/10.1088/0022-3727/35/13/303
10.	B. Das, P. Thirupathi, K. Ravinder Reddy, B. Ravikanth, L. Nagarapu, Catal. Commun. 2007, 8, 535-538. http://dx.doi.org/10.1016Zj.catcom.2006.02.023
11.	S. Kantevari, R. Bantu, L. Nagarapu, J. Mol. Catal. A: Chem. 2007, 269, 53-57.
http://dx.doi.org/10.1016/j.molcata.2006.12.039
12.	D. Hwan Jung, Y. Rok Lee, S. Hong Kim, W. Seok Lyoo, Bull. Korean Chem. Soc. 2009, 30, 1989-1995. http://dx.doi.org/10.5012/bkcs.2009.30.9.1989
13.	R. H. Wang, B. Cui, W. T. Zhang, G. F. Han, Synth. Commun. 2010, 40, 1867-1872. http://dx.doi.org/10.1080/00397910903162825
14.	G. K. Verma, K. Raghuvanshi, R. K. Verma, P. Dwivedi, M. S. Singh, Tetrahedron 2011, 67, 3698-3704. http://dx.doi.org/10.1016/j.tet.2011.03.078
15.	N. Mulakayala, G. Pavan Kumar, D. Rambabu, M. Aeluri, M. V. Basaveswara Rao, M. Pal, Tetrahedron Lett. 2012, 53, 6923-6926. http://dx.doi.org/10.1016/j.tetlet.2012.10.024
16.	F. Shirini, N. Ghaffari Khaligh, Dyes Pigm. 2012, 95, 789794. http://dx.doi.org/10.1016/j.dyepig.2012.06.022
17.	A. Ilangovan, S. Muralidharan, P. Sakthivel, S. Malayappa-samy, S. Karuppusamy, M. P. Kaushik, Tetrahedron Lett. 2013, 54, 491-494.
http://dx.doi.org/10.1016/j.tetlet.2012.11.058
18.	M. A. Ghasemzadeh, J. Safaei-ghomi, S. Zahedi, J. Serb. Chem. Soc. 2013, 78, 769-779.
19.	P. Sivaguru, A. Lalitha, Chin. Chem. Lett. 2014, 25, 321323. http://dx.doi.Org/10.1016/j.cclet.2013.11.043
20.	M. Esmaeilpour, J. Javidi, F. Dehghani, F. Nowroozi Dodeji, New J. Chem. 2014, 38, 5453-5461. http://dx.doi.org/10.1039/C4NJ00961D
21.	A. M. J. Reeve, J. Chem. Educ. 2015, 92, 582-585. http://dx.doi.org/10.1021/ed400457c
22.	R. Gupta, S. Ladage, L. Ravishankar, Chem. J. 2015, 1, 1-4. http://dx.doi.org/10.2174/1874842201502010001
23.	P. S. Bhale, S. B. Dongare, Y. B. Mule, Chem. Sci Trans. 2015, 4, 246-250.
24.	A. Gharib, L. Vojdani Fard, N. Noroozi Pesyan, M. Roshani, Chem. J. 2015, 1, 58-67.
25.	K. Parvanak Boroujeni, P. Ghasemi, Catal. Commun. 2013, 37, 50-54. http://dx.doi.org/10.1016/jxatcom.2013.03.025
26.	K. Parvanak Boroujeni, P. Shojaei, Turk. J. Chem. 2013, 37, 756-764. http://dx.doi.org/10.3906/kim-1206-28
27.	K. Parvanak Boroujeni, P. Ghasemi, Z. Rafienia, Monatsh. Chem. 2014, 145, 1023-1026. http://dx.doi.org/10.1007/s00706-014-1156-2
28.	H. Sharghi, A. Khoshnood, M. M. Doroodmand, R. Khali-feh, J. Heterocycl. Chem. 2016, 53, 164-174. http://dx.doi.org/10.1002/jhet.1811
29.	T.-S. Jin, J.-S. Zhang, J.-C. Xiao, A.-Q. Wang, T.-S. Li, Ultrason. Sonochem. 2006, 13, 220-224. http://dx.doi.org/10.1016Zj.ultsonch.2005.04.002
30.	K. Venkatesan, S. S. Pujari, R. J. Lahoti, K. V. Srinivasan, Ultrason. Sonochem. 2008, 15, 548-553. http://dx.doi.org/10.1016/j.ultsonch.2007.06.001
31.	F. Shirini, N. Ghaffari Khaligh, Dyes and Pigm. 2012, 95, 789-794. http://dx.doi.org/10.1016/j.dyepig.2012.06.022
32.	M. A. Ghasemzadeh, Acta Chim. Slov. 2015, 62, 977-985. http://dx.doi.org/10.17344/acsi.2015.1501
33.	E. V. Anslyn, D. A. Dougherty, Modern Physical Organic Chemistry, University Science Books, California, 2006.
Povzetek
Iz ustreznih izhodnih snovi smo enostavno pripravili nov katalizator sestavljen iz večstenskih ogljikovih nanocevk z -SO3H funkcionalnimi skupinami. Uporabili smo ga kot učinkovit heterogeni katalizator za enolončno Knoevenaglovo kondenzacijo, Michaelovo adicijo in ciklodehidracijo 5,5-dimetil-1,3-cikloheksandiona (dimedona) z različnimi aro-matskimi aldehidi. S pomočjo te metode smo z odličnimi izkoristki pri sobni temperaturi pripravili serijo 1,8-diokso-ok-tahidroksantenov. Predstavljena metoda je boljša od že znanih glede na mnoge reakcijske parametre: reakcijsko temperaturo, reakcijski čas, postopek izolacije, izkoristek in ponovno uporabo katalizatorja.
Boroujeni et al.: Carbon Nanotube-Supported Butyl 1-Sulfonic Acid Groups ...