Research Article | Open Access
Diego M. Ruiz, Juan C. Autino, Nancy Quaranta, Patricia G. Vázquez, Gustavo P. Romanelli, "An Efficient Protocol for the Synthesis of Quinoxaline Derivatives at Room Temperature Using Recyclable Alumina-Supported Heteropolyoxometalates", The Scientific World Journal, vol. 2012, Article ID 174784, 8 pages, 2012. https://doi.org/10.1100/2012/174784
An Efficient Protocol for the Synthesis of Quinoxaline Derivatives at Room Temperature Using Recyclable Alumina-Supported Heteropolyoxometalates
We report a suitable quinoxaline synthesis using molybdophosphovanadates supported on commercial alumina cylinders as catalysts. These catalysts were prepared by incipient wetness impregnation. The catalytic test was performed under different reaction conditions in order to know the performance of the synthesized catalysts. The method shows high yields of quinoxaline derivatives under heterogeneous conditions. Quinoxaline formation was obtained using benzyl, o-phenylenediamine, and toluene as reaction solvent at room temperature. The CuH2PMo11VO40 supported on alumina showed higher activity in the tested reaction. Finally, various quinoxalines were prepared under mild conditions and with excellent yields.
Quinoxaline derivatives are a very important class of nitrogen-containing heterocycles (containing benzene and pyrazine rings in their structure), as they constitute useful intermediates in organic synthesis. This substructure plays an important role as a basic skeleton for the design of a number of heterocyclic compounds with different biological activities, making this type of compounds important in the fields of (a) medicine: antitumor, anticonvulsant, antimalarial, anti-inflammatory, antiamoebic, antioxidant, antidepressant, antiprotozoal, antibacterial, and anti-HIV agents [1–10] and (b) technology: fluorescent dying agents, electroluminescent materials, chemical switches, cavitands, and semiconductors [11–17]. Quinoxalines are important in the pharmaceutical industry, with antibiotics such as echinomycin, levomycin, and actinoleutin having quinoxaline as part of their structure .
A number of synthetic strategies are known for the preparation of substituted quinoxalines. The classic method for quinoxaline preparation is the condensation of a 1,2-dicarbonylic compound with a 1,2-diamino compound. In general, this procedure needs high temperature, the use of a strong acid catalyst, and long reaction times . Other strategies involve oxidative coupling of epoxides and 1,2-diamines , cyclization of aryl amino oximes and α-dicarbonyl compounds , and tandem oxidation of α-hydroxyl ketones .
A variety of catalysts were tested in these reactions such as acetic acid , iodine , CuSO4·5H2O , nickel nanoparticles , gallium(III)triflate , montmorillonite K10 , ionic liquids , Nano-TiO2 , sulfated TiO2 , Pd(OAc)2 , RuCl2-(PPh3)3-2,2,6,6-tetramethylpiperidine 1-oxyl(TEMPO) , MnO2, , Al2O3 , zirconium(IV)-modified silica gel , nanocrystalline CuO , cerium(IV) ammonium nitrate , iron exchanged molybdophosphoric acid , silica-bonded S-sulfonic acid , and sulfamic acid/MeOH . Different reaction media were used to perform this synthesis such as the use of acetonitrile  or DMSO  as solvents, or even cleaner ways as the solvent-free reaction [31, 39], with various ways to give energy to the substrate, such as microwave radiation [31, 39], ultrasound , or even room temperature [30, 36, 37].
The need for greener techniques leads to using different environmentally friendly reaction conditions; among them the replacement of pollutant inorganic acid catalysts, such as sulfuric or hydrochloric acids with reusable solid acids and the use of room temperature avoiding media heating, is yet very necessary. The application of solid acids in organic transformation has an important role, because they have many advantages such as ease of handling, decreased plant corrosion, and more environmentally safe waste disposal procedures .
HPAs are molecular arrangements with remarkable and diverse electronic and molecular structures, which lead to their application in different areas such as medicine and materials science, among others; among the various possible HPA structures, the Keggin-type primary structure deserves to be mentioned, due to its widely reported applications . As part of a research project to develop environmentally friendly organic reactions, we used different HPAs in various preparative reactions, under greener conditions such as the synthesis of coumarins , flavones and chromones , and N-sulfonyl-1,2,3,4-tetrahydroisoquinolines  among others.
As part of our ongoing research on the development of alternative synthetic procedures for the synthesis of biologically active heterocyclic compounds and the use of green chemistry techniques in organic synthesis, herein we report a simple and efficient method for the preparation of quinoxaline derivatives using heteropolyacids HPMo11VFeO40 and H2PMo11VCuO40 as catalysts.
2. Materials and Methodology
All reagents were purchased from Merck and Aldrich and used without further purification. All the reactions were monitored by TLC on precoated silica gel plates (254 mm). Flash column chromatography was performed with 230–400 mesh silica gel. All the yields were calculated from pure products. All the products were identified by comparison of physical data (mp, TLC, NMR) with those reported. Melting points of the compounds were determined in sealed capillary tubes and are uncorrected. 13C NMR and 1H NMR spectra were recorded at room temperature on Varian-200 spectrometers using TMS as internal standard. Entries and target compounds have the same number.
2.2. Catalyst Preparation
2.2.1. Synthesis of Catalyst
2.2.2. Synthesis of Supported Catalysts
Commercial alumina (Akzo) (specific surface area, 282 m2/g; mean pore diameter, 4.2 nm; pore volume, 0.58 cm3/g) as cylinders was used as support. The support was used without further treatment. The supported catalysts were obtained by incipient wet impregnation of MPV solid using ethanol as solvent. The concentration of the impregnation solution was 120 gMo/l MPV solution. Then, the solids were dried at room temperature. Finally, the catalysts were thermally treated at 200°C for 6 h. The nomenclature is shown in Table 1.
2.3. Catalyst Characterization
In a previous paper, we reported the full characterization of both catalysts by diffuse reflectance spectroscopy (DRS), Fourier transformed infrared spectroscopy (FT-IR), optical and scanning electron microscopies, XRD analyses, and potentiometric titration. Figure 1 shows the copper FT-IR spectra of bulk and alumina-supported catalysts .
2.4. Preparation of Quinoxaline General—Procedure
To a mixture of an o-phenylenediamine (1 mmol, 0.108 g) and 1,2-dicarbonyl compound (1 mmol) in toluene (8 mL), 0.1 g of MoVP catalyst was added and the mixture was stirred at room temperature. The progress of the reaction was monitored by TLC. After completion of the reaction, the insoluble catalyst was separated by filtration. The filtrate was dried over anhydrous Na2SO4. The solvent was evaporated, and the pure product was obtained. The products were purified by recrystallization from ethanol.
2,3-Diphenylquinoxaline (Table 6, entry 1). White needles, 92%, mp 127-128°C (lit. mp 127-128 ). 1H NMR (200 MHz, CDCl3): 8.20–8.10 (m, 2H), 7.82–7.70 (m, 2H), 7.60—7.35 (m, 10H). 13C NMR (50 MHz, CDCl3): 153.6, 141.3, 139.2, 130.0, 129.9, 129.4, 128.9, 128.4.
2.5. Recycling of the Catalyst
After reaction, the catalyst was filtered, washed thoroughly with toluene (), dried under vacuum, and reused for the next cycle, following the procedure described above.
3. Results and Discussion
This work describes the application of a heterogeneous system for the preparation of quinoxalines in the presence of Keggin heteropolyoxometalates (AlCuMoVP and AlFeMoVP) as reusable catalyst. The quinoxaline synthesis involving the reaction of substituted o-phenylenediamines and 1, 2-diketones is illustrated in reaction Scheme 1.
Before attempting detailed catalytic work, a noncatalytic reaction between o-phenylenediamine (1 mmol), benzyl (1 mmol), and toluene (7 mL) was examined and it was observed that, under the experimental conditions (25°C, 2 h), no formation of quinoxaline was detected, indicating that from a practical point of view the reaction is not taking place in the absence of a catalyst (Table 2, entry 1). Similarly, no formation of quinoxaline was detected under the same reaction conditions using the support (Al) (Table 2, entry 2).
|Reaction conditions: o-phenylenediamine, 1 mmol; benzyl, 1 mmol; toluene, 10 mL; 100 mg of supported catalyst; 120 min; 25°C.|
Table 1 lists the obtained results for quinoxaline yield using the two different catalysts considered (AlCuMoVP and AlFeMoVP). The experimental conditions were 100 mg of catalyst, 1 mmol of o-phenylenediamine, 1 mmol of benzyl, and 7 mL of toluene, reaction for 2 h at 25°C. Under these conditions, quinoxaline was obtained with a selectivity of 100% for both catalysts. The yields were 92% and 80%, respectively (Table 2, entries 3 and 4). The more active catalyst was used in the next experiments.
Table 3 displays the effect of the amount of catalyst (AlCuMoVP) on the yield of quinoxaline in the reaction. The experimental reaction conditions were o-phenylenediamine, 1 mmol; benzyl, 1 mmol; toluene, 7 mL, 120 min, 25°C, and a variable amount of AlCuMoVP catalyst (10, 50, 100, and 150 mg, resp.). It can be seen that the conversion of yields increased from 85% to 92% when the amount of AlCuMoVP increased from 50 to 100 mg (Table 3, entries 2 and 3). A further increase in the amount of AlCuMoVP (150 mg) caused a very slightly increase in azlactone yields (93%, Table 3, entry 4). Thus, 100 mg of AlCuMoVP is a suitable amount in this reaction.
|Reaction conditions: o-phenylenediamine, 1 mmol; benzyl, 1 mmol; toluene, 10 mL; supported catalyst: AlCuMoVP; 120 min, 25°C.|
Table 4 shows the results for quinoxaline synthesis as a function of reaction time using AlCuMoVP catalyst at a reaction temperature of 25°C. The experimental reaction conditions were o-phenylenediamine, 1 mmol; benzyl, 1 mmol; AlCuMoVP, 100 mg; toluene, 7 mL and 25°C. It can be observed that the yields of azlactone increased with the reaction time up to 120 min and then stayed at a constant level.
|Reaction conditions: o-phenylenediamine, 1 mmol; benzyl, 1 mmol; toluene, 7 mL; supported catalyst: AlCuMoVP, 100 mg; 25°C.|
The possibility of recycling the catalyst was examined. For this reason, the room temperature reaction of o-phenylenediamine and benzyl was studied in toluene in the presence of AlCuMoVP. When the reaction was complete, the mixture was filtered, the residue was washed with toluene and the recycled catalyst was reused in the next reaction. No appreciable loss of catalytic activity was observed after four cycles (Table 5 entry 4).
|Reaction conditions: o-phenylenediamine, 1 mmol; benzyl, 1 mmol; toluene, 10 mL; 100 mg of supported catalyst; 120 min; 25°C.|
|Reaction conditions: molar ratio of substrates: (1 : 1); catalyst: 100 mg. Reactions were run at 25°C. aIsolated yield.|
In order to estimate the possible catalyst solubilization, additional tests were performed. AlCuMoVP sample (100 mg) was stirred in toluene (7 mL) for 5 h, filtered and dried in vacuum till constant weight. Loss of mass was not detected. The refluxed toluene was used as solvent for attempting the reaction without adding the catalyst. After 5 h, quinoxaline was not detected and the starting material was quantitatively recovered.
A plausible mechanism is rationalized in Scheme 2. As proposed by Niknam and Coworkers , the reaction follows a mechanism of acid-catalyzed condensation reactions, in our case with AlMoVP acting as a Brønsted acid, (1) coordination of a diketone to acid sites of AlMoVP, (2) the nucleophilic attack on the carbonyl intermediate, (3) dehydration to give a carbocation intermediate, and (4) elimination of a proton to give the quinoxaline product.
Encouraged by the remarkable results obtained with the above reaction and in order to show the generality and scope of this new protocol, we used various substituted 1,2-phenylenediamines and two diketones under the optimized conditions at room temperature (o-phenylenediamine: 1 mmol, benzyl: 1 mmol, toluene: 7–12 mL, and catalyst (AlCuMoVA): 100 mg); the results obtained are summarized in Table 6. All the reactions proceeded very cleanly at room temperature and no undesirable side reactions were observed, although the reaction time for a 100% conversion of substrates and reaction yields of products were highly dependent on the substituent. Results in Table 6 show that electron-donating groups at the phenyl ring of 1,2-diamine favored product formation (Table 6, entry 2). In contrast, electron-withdrawing groups such as chlorine and bromine slightly lowered the yields (Table 6, entries 3, 4, 8, and 9) with longer reaction times. 4-Nitro-2,3-diaminobenzene and 2,3-diaminopyridine also gave moderate yields (Table 6, entries 5 and 6).
In conclusion, supported molybdophosphovanadates, which can simply be prepared from commercially available and relatively cheap starting materials, are an efficient, thermally stable, and recoverable catalyst for the silylation of phenols and alcohols in toluene at ambient temperature. The present protocol provides a novel, efficient, and recyclable methodology for the preparation of quinoxalines in high yields with an easy workup procedure; the catalyst can be recovered and reused over several reaction cycles without considerable loss of reactivity. Moreover, this methodology introduces a practical and viable green technology for quinoxaline preparation. We are currently exploring further applications of this solid to other types of heterocycles.
The authors thank L. Osiglio and G. Valle for their experimental contribution and CONICET, ANPCyT, and National University of La Plata for the financial contribution.
- S. D. Undevia, F. Innocenti, J. Ramirez et al., “A phase I and pharmacokinetic study of the quinoxaline antitumour Agent R(+)XK469 in patients with advanced solid tumours,” European Journal of Cancer, vol. 44, no. 12, pp. 1684–1692, 2008.
- P. Corona, A. Carta, M. Loriga, G. Vitale, and G. Paglietti, “Synthesis and in vitro antitumor activity of new quinoxaline derivatives,” European Journal of Medicinal Chemistry, vol. 44, no. 4, pp. 1579–1591, 2009.
- C. Urquiola, D. Gambino, M. Cabrera et al., “New copper-based complexes with quinoxaline N1,N4-dioxide derivatives, potential antitumoral agents,” Journal of Inorganic Biochemistry, vol. 102, no. 1, pp. 119–126, 2008.
- Q. Weng, D. Wang, P. Guo et al., “Q39, a novel synthetic Quinoxaline 1,4-Di-N-oxide compound with anti-cancer activity in hypoxia,” European Journal of Pharmacology, vol. 581, no. 3, pp. 262–269, 2008.
- S. Wagle, A. V. Adhikari, and N. S. Kumari, “Synthesis of some new 4-styryltetrazolo[1,5-a]quinoxaline and 1-substituted-4-styryl[1,2,4]triazolo[4,3-a]quinoxaline derivatives as potent anticonvulsants,” European Journal of Medicinal Chemistry, vol. 44, no. 3, pp. 1135–1143, 2009.
- E. Vicente, L. M. Lima, E. Bongard et al., “Synthesis and structure-activity relationship of 3-phenylquinoxaline 1,4-di-N-oxide derivatives as antimalarial agents,” European Journal of Medicinal Chemistry, vol. 43, no. 9, pp. 1903–1910, 2008.
- A. Burguete, E. Pontiki, V. D. Hadjipavlou-Litina et al., “Synthesis and anti-inflammatory/antioxidant activities of some new ring substituted 3-phenyl-1-(1,4-di-N-oxide quinoxalin-2-yl)-2-propen-1-one derivatives and of their 4,5-dihydro-(1H)-pyrazole analogues,” Bioorganic and Medicinal Chemistry Letters, vol. 17, no. 23, pp. 6439–6443, 2007.
- A. Budakoti, A. R. Bhat, and A. Azam, “Synthesis of new 2-(5-substituted-3-phenyl-2-pyrazolinyl)-1,3-thiazolino[5,4-b]quinoxaline derivatives and evaluation of their antiamoebic activity,” European Journal of Medicinal Chemistry, vol. 44, no. 3, pp. 1317–1325, 2009.
- W. He, M. R. Myers, B. Hanney et al., “Potent quinoxaline-based inhibitors of PDGF receptor tyrosine kinase activity. Part 2: the synthesis and biological activities of RPR127963 an orally bioavailable inhibitor,” Bioorganic and Medicinal Chemistry Letters, vol. 13, no. 18, pp. 3097–3100, 2003.
- Y. B. Kim, Y. H. Kim, J. Y. Park, and S. K. Kim, “Synthesis and biological activity of new quinoxaline antibiotics of echinomycin analogues,” Bioorganic and Medicinal Chemistry Letters, vol. 14, no. 2, pp. 541–544, 2004.
- J. Y. Jaung, “Synthesis and halochromism of new quinoxaline fluorescent dyes,” Dyes and Pigments, vol. 71, no. 3, pp. 245–250, 2006.
- Q. Y. Zhang, B. K. Liu, W. Q. Chen, Q. Wu, and X. F. Lin, “A green protocol for synthesis of benzo-fused N,S-, N,O- and N,N-heterocycles in water,” Green Chemistry, vol. 10, no. 9, pp. 972–977, 2008.
- K. R. J. Thomas, M. Velusamy, T. Lin Jiann, C. H. Chuen, and Y. T. Tao, “Chromophore-labeled quinoxaline derivatives as efficient electroluminescent materials,” Chemistry of Materials, vol. 17, no. 7, pp. 1860–1866, 2005.
- M. J. Crossley and L. A. Johnston, “Laterally-extended porphyrin systems incorporating a switchable unit,” Chemical Communications, no. 10, pp. 1122–1123, 2002.
- S. Dailey, W. J. Feast, R. J. Peace, I. C. Sage, S. Till, and E. L. Wood, “Synthesis and device characterisation of side-chain polymer electron transport materials for organic semiconductor applications,” Journal of Materials Chemistry, vol. 11, no. 9, pp. 2238–2243, 2001.
- A. Katoh, T. Yoshida, and J. Ohkanda, “Synthesis of quinoxaline derivatives bearing the styryl and phenylethynyl groups and application to a fluorescence derivatization reagent,” Heterocycles, vol. 52, no. 2, pp. 911–920, 2000.
- J. L. Sessler, H. Maeda, T. Mizuno, V. M. Lynch, and H. Furuta, “Quinoxaline-bridged porphyrinoids,” Journal of the American Chemical Society, vol. 124, no. 45, pp. 13474–13479, 2002.
- S. A. Raw, C. D. Wilfred, and R. J. K. Taylor, “Tandem oxidation processes for the preparation of nitrogen-containing heteroaromatic and heterocyclic compounds,” Organic and Biomolecular Chemistry, vol. 2, no. 5, pp. 788–796, 2004.
- D. J. Brown, “Quinoxalines,” in The Chemistry of Heterocyclic Compounds, E. C. Taylor and P. Wipf, Eds., vol. 61, pp. 1–510, John Wiley & Sons, Hoboken, NJ, USA, 2004.
- S. Antoniotti and E. Duñach, “Direct and catalytic synthesis of quinoxaline derivatives from epoxides and ene-1,2-diamines,” Tetrahedron Letters, vol. 43, no. 22, pp. 3971–3973, 2002.
- N. P. Xekoukoulotakis, C. P. Hadjiantoniou-Maroulis, and A. J. Maroulis, “Synthesis of quinoxalines by cyclization of α-arylimino oximes of α-dicarbonyl compounds,” Tetrahedron Letters, vol. 41, no. 52, pp. 10299–10302, 2000.
- M. R. Islami and Z. Hassani, “One-pot and efficient protocol for synthesis of quinoxaline derivatives,” Arkivoc, vol. 2008, no. 15, pp. 280–287, 2008.
- S. V. More, M. N. V. Sastry, C. C. Wang, and Y. Ching-Fa, “Molecular iodine: a powerful catalyst for the easy and efficient synthesis of quinoxalines,” Tetrahedron Letters, vol. 46, no. 37, pp. 6345–6348, 2005.
- M. M. Heravi, S. Taheri, K. Bakhtiari, and H. A. Oskooie, “On water: a practical and efficient synthesis of quinoxaline derivatives catalyzed by CuSO4· 5H2O,” Catalysis Communications, vol. 8, no. 2, pp. 211–214, 2007.
- A. Kumar, S. kumar, A. Saxena, A. De, and S. Mozumdar, “Ni-nanoparticles: an efficient catalyst for the synthesis of quinoxalines,” Catalysis Communications, vol. 9, no. 5, pp. 778–784, 2008.
- J. J. Cai, J. P. Zou, X. Q. Pan, and W. Zhang, “Gallium(III) triflate-catalyzed synthesis of quinoxaline derivatives,” Tetrahedron Letters, vol. 49, no. 52, pp. 7386–7390, 2008.
- T. K. Huang, R. Wang, L. Shi, and X. X. Lu, “Montmorillonite K-10: an efficient and reusable catalyst for the synthesis of quinoxaline derivatives in water,” Catalysis Communications, vol. 9, no. 6, pp. 1143–1147, 2008.
- F. Dong, K. Gong, Z. Fei, X. Zhou, and Z. Liu, “A practical and efficient synthesis of quinoxaline derivatives catalyzed by task-specific ionic liquid,” Catalysis Communications, vol. 9, no. 2, pp. 317–320, 2008.
- B. B. F. Mirjalili and A. Akbari, “Nano-TiO2: an eco-friendly alternative for the synthesis of quinoxalines,” Chinese Chemical Letters, vol. 22, no. 6, pp. 753–756, 2011.
- B. Krishnakumar, R. Velmurugan, S. Jothivel, and M. Swaminathan, “An efficient protocol for the green synthesis of quinoxaline and dipyridophenazine derivatives at room temperature using sulfated titania,” Catalysis Communications, vol. 11, no. 12, pp. 997–1002, 2010.
- M. Jafarpour, A. Rezaeifard, and M. Danehchin, “Easy access to quinoxaline derivatives using alumina as an effective and reusable catalyst under solvent-free conditions,” Applied Catalysis A: General, vol. 394, no. 1-2, pp. 48–51, 2011.
- R. K. Sharma and C. Sharma, “Zirconium(IV)-modified silica gel: preparation, characterization and catalytic activity in the synthesis of some biologically important molecules,” Catalysis Communications, vol. 12, no. 5, pp. 327–331, 2011.
- S. Sadjadi, S. Sadjadi, and R. Hekmatshoar, “Ultrasound-promoted greener synthesis of benzoheterocycle derivatives catalyzed by nanocrystalline copper(II) oxide,” Ultrasonics Sonochemistry, vol. 17, no. 5, pp. 764–767, 2010.
- S. V. More, M. N. V. Sastry, and C. F. Yao, “Cerium (IV) ammonium nitrate (CAN) as a catalyst in tap water: a simple, proficient and green approach for the synthesis of quinoxalines,” Green Chemistry, vol. 8, no. 1, pp. 91–95, 2006.
- K. T. V. Rao, P. S. S. Prasad, and N. Lingaiah, “Iron exchanged molybdophosphoric acid as an efficient heterogeneous catalyst for the synthesis of quinoxalines,” Journal of Molecular Catalysis A: Chemical, vol. 312, no. 1-2, pp. 65–69, 2009.
- K. Niknam, D. Saberi, and M. Mohagheghnejad, “Silica bonded S-sulfonic acid: a recyclable catalyst for the synthesis of quinoxalines at room temperature,” Molecules, vol. 14, no. 5, pp. 1915–1926, 2009.
- H. R. Darabi, S. Mohandessi, K. Aghapoor, and F. Mohsenzadeh, “A recyclable and highly effective sulfamic acid/MeOH catalytic system for the synthesis of quinoxalines at room temperature,” Catalysis Communications, vol. 8, no. 3, pp. 389–392, 2007.
- S. B. Rajesh, S. R. Swapnil, S. A. Suresh, N. J. Wamanrao, R. B. Sudhakar, and P. P. Rajendra, “An efficient protocol for the synthesis of quinoxaline derivatives at room temperature using molecular iodine as the catalyst,” Tetrahedron Letters, vol. 46, no. 42, pp. 7183–7186, 2005.
- J. F. Zhou, G. X. Gong, K. B. Shi, and S. J. Zhi, “Catalyst-free and solvent-free method for the synthesis of quinoxalines under microwave irradiation,” Chinese Chemical Letters, vol. 20, no. 6, pp. 672–675, 2009.
- M. N. Timofeeva, “Acid catalysis by heteropoly acids,” Applied Catalysis A: General, vol. 256, no. 1-2, pp. 19–35, 2003.
- G. P. Romanelli, D. Bennardi, D. M. Ruiz, G. Baronetti, H. J. Thomas, and J. C. Autino, “A solvent-free synthesis of coumarins using a Wells-Dawson heteropolyacid as catalyst,” Tetrahedron Letters, vol. 45, no. 48, pp. 8935–8939, 2004.
- D. O. Bennardi, D. M. Ruiz, G. P. Romanelli, G. T. Baronetti, H. J. Thomas, and J. C. Autino, “Efficient microwave solvent-free synthesis of Flavones, Chromones, Coumarins and Dihydrocoumarins,” Letters in Organic Chemistry, vol. 5, no. 8, pp. 607–615, 2009.
- G. P. Romanelli, D. M. Ruiz, J. C. Autino, and H. E. Giaccio, “A suitable preparation of N-sulfonyl-1,2,3,4 tetrahydroisoquinolines and their ring homologues with a reusable preyssler heteropolyacid as catalyst,” Molecular Diversity, vol. 14, no. 4, pp. 803–807, 2010.
- G. Romanelli, P. Vázquez, L. Pizzio et al., “Phenol tetrahydropyranylation catalyzed by silica-alumina supported heteropolyacids with Keggin structure,” Applied Catalysis A: General, vol. 261, no. 2, pp. 163–170, 2004.
- P. Villabrille, G. Romanelli, P. Vázquez, and C. Cáceres, “Vanadium-substituted Keggin heteropolycompounds as catalysts for ecofriendly liquid phase oxidation of 2,6-dimethylphenol to 2,6-dimethyl-1,4- benzoquinone,” Applied Catalysis A: General, vol. 270, no. 1-2, pp. 101–111, 2004.
- P. Villabrille, G. Romanelli, N. Quaranta, and P. Vázquez, “An efficient catalytic route for the preparation of silyl ethers using alumina-supported heteropolyoxometalates,” Applied Catalysis B: Environmental, vol. 96, no. 3-4, pp. 379–386, 2010.
Copyright © 2012 Diego M. Ruiz et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.