Generic placeholder image

Current Organic Synthesis

Author(s): Suzaimi Johari, Mohd Rafie Johan and Nader Ghaffari Khaligh*

DOI: 10.2174/0115701794268766231108110816

DownloadDownload PDF Flyer Cite As
Organocatalytic Synthesis of (Hetero)arylidene Malononitriles Using a More Sustainable, Greener, and Scalable Strategy

Page: [704 - 716] Pages: 13

  • * (Excluding Mailing and Handling)

Abstract

Aim and Objective: The establishment of a green and sustainable Knoevenagel condensation reaction in organic chemistry is still crucial. This work aimed to provide a newly developed metal-free and halogen-free catalytic methodology for the synthesis of CS and (hetero-) arylidene malononitriles in the laboratory and industrial scale. The Knoevenagel condensation reaction of various carbonyl groups with malononitrile was investigated in ethanol, an ecofriendly medium, in the presence of seven nitrogen-based organocatalysts.

Materials and Methods: A comparative study was conducted using two as-obtained and four commercially available nitrogen-based organocatalysts in Knoevenagel condensation reactions. The synthesis of CS gas (2-chlorobenzylidene malononitrile) using a closed catalytic system was optimized based on their efficiency and greener approach.

Results: The conversion of 100% and excellent yields were obtained in a short time. The products could be crystallized directly from the reaction mixture. After separating pure products, the residue solution was employed directly in the next run without any concentration, activation, purification, or separation. Furthermore, the synthesis of 2-chlorobenzylidenemahmonitrile (CS) was carried out on a large scale using imidazole as a selected nitrogen-based catalyst, afforded crystalline products with 95 ± 2% yield in five consecutive runs.

Conclusion: Energy efficiency, cost saving, greener conditions, using only 5 mol% of organocatalyst, high recyclability of catalyst, prevention of waste, recycling extractant by a rotary evaporator for non-crystallized products, demonstrated the potential commercial production of CS using imidazole in ethanol as an efficient and highly recyclable catalytic system.

Graphical Abstract

[1]
How communities have defined zero waste. Available from: https://www.epa.gov/transforming-waste-tool/how-communities-have-defined-zero-waste (Accessed on: September 19, 2023).
[2]
In support of municipal zero waste principles and a hierarchy of materials management. Available from: https://www.usmayors.org/the-conference/resolutions/?category=b83aReso050&meeting=83rd
[3]
12 Principles of Green Chemistry-American Chemical Society. Available from: https://www.acs.org/greenchemistry/principles/12-principles-of-green-chemistry.html (Accessed on: September 19, 2023).
[4]
Lapkin, A.; Constable, D.J.C. Green chemistry metrics: Measuring and monitoring sustainable processes; Green Chem Metrics Meas Monit Sustain Process, 2009, pp. 1-324.
[http://dx.doi.org/10.1002/9781444305432]
[5]
The principles of sustainable chemistry-the science blog. Available from: https://www.reagent.co.uk/blog/the-principles-of-sustainable-chemistry/ (Accessed on: September 19, 2023).
[6]
Tan, D.; García, F. Main group mechanochemistry: From curiosity to established protocols. Chem. Soc. Rev., 2019, 48(8), 2274-2292.
[http://dx.doi.org/10.1039/C7CS00813A] [PMID: 30806391]
[7]
Burke, A.J. Asymmetric organocatalysis in drug discovery and development for active pharmaceutical ingredients. Expert Opin. Drug Discov., 2023, 18(1), 37-46.
[http://dx.doi.org/10.1080/17460441.2023.2160437] [PMID: 36527181]
[8]
Kloss, F.; Neuwirth, T.; Haensch, V.G.; Hertweck, C. Metal-free synthesis of pharmaceutically important biaryls by photosplicing. Angew. Chem. Int. Ed., 2018, 57(44), 14476-14481.
[http://dx.doi.org/10.1002/anie.201805961] [PMID: 30001481]
[9]
Wagay, S.A.; Rather, I.A.; Ali, R. Unraveling the potential role of green chemistry in carrying out typical condensation reactions of organic chemistry. Nanoparticles Green Org. Synth; Elsevier, 2023, pp. 317-349.
[http://dx.doi.org/10.1016/B978-0-323-95921-6.00011-1]
[10]
Thorat, B.R.; Mali, S.N.; Wavhal, S.S.; Bhagat, D.S.; Borade, R.M.; Chapolikar, A.; Gandhi, A.; Shinde, P. L-Proline: A versatile organo-catalyst in organic chemistry. Comb. Chem. High Throughput Screen., 2023, 26(6), 1108-1140.
[http://dx.doi.org/10.2174/1386207325666220720105845] [PMID: 35864793]
[11]
Diksha, D.; Naresh, K. Recent developments in Knoevenagel condensation reaction: A Review. Int. J. Adv. Sci. Res., 2022, 13(5), 17-25.
[http://dx.doi.org/10.55218/JASR.202213502]
[12]
Johari, S.; Johan, M.R.; Khaligh, N.G. An overview of metal-free sustainable nitrogen-based catalytic knoevenagel condensation reaction. Org. Biomol. Chem., 2022, 20(11), 2164-2186.
[http://dx.doi.org/10.1039/D2OB00135G] [PMID: 35225313]
[13]
Maity, T.K.; Paul, A.; Maji, A.; Sarkar, A.; Saha, S.; Janah, P. Recent approaches in the synthesis of 5-arylidene-2,4-thiazolidinedione derivatives using Knoevenagel condensation. Mini Rev. Org. Chem., 2023, 20(1), 5-34.
[http://dx.doi.org/10.2174/1570193X19666220331155705]
[14]
Laskar, K.; Bhattacharjee, P.; Gohain, M.; Deka, D.; Bora, U. Application of bio-based green heterogeneous catalyst for the synthesis of arylidinemalononitriles. Sustain. Chem. Pharm., 2019, 14, 100181.
[http://dx.doi.org/10.1016/j.scp.2019.100181]
[15]
Gad, S.C. Riot control agents (RCAs). Ref. Modul. Biomed. Sci; Elsevier, 2023.
[http://dx.doi.org/10.1016/B978-0-12-824315-2.00932-5]
[16]
Uddin, K.M.; Sakib, M.; Siraji, S.; Uddin, R.; Rahman, S.; Alodhayb, A.; Alibrahim, K.A.; Kumer, A.; Matin, M.M.; Bhuiyan, M.M.H. Synthesis of new derivatives of benzylidinemalononitrile and ethyl 2-cyano-3-phenylacrylate: In silico anticancer evaluation. ACS Omega, 2023, 8(29), 25817-25831.
[http://dx.doi.org/10.1021/acsomega.3c01123] [PMID: 37521603]
[17]
Shivamurthy Harisha, A.; Nagarajan, K.; Saravanan, S.; Manohar, V.; Thomas, S.P.; Narasingarow Guru Row, T. A new finding in the old Knoevenagel condensation reaction. Results Chem., 2022, 4, 100376.
[http://dx.doi.org/10.1016/j.rechem.2022.100376]
[18]
Appaturi, J.N.; Ratti, R.; Phoon, B.L.; Batagarawa, S.M.; Din, I.U.; Selvaraj, M.; Ramalingam, R.J. A review of the recent progress on heterogeneous catalysts for Knoevenagel condensation. Dalton Trans., 2021, 50(13), 4445-4469.
[http://dx.doi.org/10.1039/D1DT00456E] [PMID: 33720238]
[19]
Ortiz-Bustos, J.; Cruz, P.; Pérez, Y.; Hierro, I. Prolinate-based heterogeneous catalyst for Knoevenagel condensation reaction: Insights into mechanism reaction using solid-state electrochemical studies. Molecular Catalysis, 2022, 524, 112328.
[http://dx.doi.org/10.1016/j.mcat.2022.112328]
[20]
Farzaneh, F.; Aghabali, S.; Azarkamanzad, Z. Polyamine-functionalized carbon dots as active catalyst for Knoevenagel condensation reactions. React. Kinet. Mech. Catal., 2020, 130(2), 1009-1025.
[http://dx.doi.org/10.1007/s11144-020-01826-4]
[21]
Tokala, R.; Bora, D.; Shankaraiah, N. Contribution of Knoevenagel condensation products toward the development of anticancer agents: An updated review. ChemMedChem, 2022, 17(8), e202100736.
[http://dx.doi.org/10.1002/cmdc.202100736] [PMID: 35226798]
[22]
Kumar, S.; Saroha, B.; Kumar, G.; Lathwal, E.; Kumar, S.; Parshad, B.; Kumari, M.; Kumar, N.; Mphahlele-Makgwane, M.M.; Makgwane, P.R. Recent developments in nanocatalyzed green synthetic protocols of biologically potent diverse O-heterocycles—A review. Catalysts, 2022, 12(6), 657.
[http://dx.doi.org/10.3390/catal12060657]
[23]
Sonali Anantha, I.S.; Kerru, N.; Maddila, S.; Jonnalagadda, S.B. Recent progresses in the multicomponent synthesis of dihydropyridines by applying sustainable catalysts under green conditions. Front Chem., 2021, 9, 800236.
[http://dx.doi.org/10.3389/fchem.2021.800236] [PMID: 34993177]
[24]
Sarmah, D.; Borah, K.K.; Bora, U. Aqueous extracts of biomass ash as an alternative class of green solvents for organic transformations: A review update. Sustain. Chem. Pharm., 2021, 24, 100551.
[http://dx.doi.org/10.1016/j.scp.2021.100551]
[25]
Johari, S.; Halim, S.N.A.; Johan, M.R.; Khaligh, N.G. Synthesis and characterization of 1,4-di(1H-imidazol-1-yl) butane dihydrate and 1,4-di(1H-2-methylimidazol-1-yl) butane tetrahydrate: A study of the methyl group effect on spectroscopic data, thermal properties, and the crystal structures. J. Mol. Struct., 2022, 1269, 133823.
[http://dx.doi.org/10.1016/j.molstruc.2022.133823]
[26]
Olmstead, W.N.; Bordwell, F.G. Ion-pair association constants in dimethyl sulfoxide. J. Org. Chem., 1980, 45(16), 3299-3305.
[http://dx.doi.org/10.1021/jo01304a033]
[27]
Bordwell, F.G.; Fried, H.E. Acidities of the hydrogen-carbon protons in carboxylic esters, amides, and nitriles. J. Org. Chem., 1981, 46(22), 4327-4331.
[http://dx.doi.org/10.1021/jo00335a001]
[28]
Hall, H.K., Jr Correlation of the base strengths of amines 1. J. Am. Chem. Soc., 1957, 79(20), 5441-5444.
[http://dx.doi.org/10.1021/ja01577a030]
[29]
Tshepelevitsh, S.; Kütt, A.; Lõkov, M.; Kaljurand, I.; Saame, J.; Heering, A.; Plieger, P.G.; Vianello, R.; Leito, I. On the basicity of organic bases in different media. Eur. J. Org. Chem., 2019, 2019(40), 6735-6748.
[http://dx.doi.org/10.1002/ejoc.201900956]
[30]
Edsall, J.T.; Felsenfeld, G.; Goodman, D.S.; Gurd, F.R.N. The association of imidazole with the ions of zinc and cupric copper 1a,b,c. J. Am. Chem. Soc., 1954, 76(11), 3054-3061.
[http://dx.doi.org/10.1021/ja01640a068]
[31]
Lenarcik, B.; Ojczenasz, P. The influence of the size and position of the alkyl groups in alkylimidazole molecules on their acid-base properties. J. Heterocycl. Chem., 2002, 39(2), 287-290.
[http://dx.doi.org/10.1002/jhet.5570390206]
[32]
Khalili, F.; Henni, A.; East, A.L.L. pKa values of some piperazines at (298, 303, 313, and 323). K. J. Chem. Eng. Data, 2009, 54(10), 2914-2917.
[http://dx.doi.org/10.1021/je900005c]
[33]
1,3-Bis(4-piperidyl)propane-16898-52-5. Available from: https://www.chemicalbook.com/ChemicalProductProperty_EN_CB0344188.htm (Accessed on: September 19, 2023).
[34]
Snyder, L.R. Classification of the solvent properties of common liquids. J. Chromatogr. A, 1974, 92(2), 223-230.
[http://dx.doi.org/10.1016/S0021-9673(00)85732-5]
[35]
Siodła, T.; Ozimiński, W.P.; Hoffmann, M.; Koroniak, H.; Krygowski, T.M. Toward a physical interpretation of substituent effects: The case of fluorine and trifluoromethyl groups. J. Org. Chem., 2014, 79(16), 7321-7331.
[http://dx.doi.org/10.1021/jo501013p] [PMID: 25046196]
[36]
Benedetto Tiz, D.; Bagnoli, L.; Rosati, O.; Marini, F.; Sancineto, L.; Santi, C. New Halogen-containing drugs approved by FDA in 2021: An overview on their syntheses and pharmaceutical use. Molecules, 2022, 27(5), 1643.
[http://dx.doi.org/10.3390/molecules27051643] [PMID: 35268744]
[37]
Khaligh, N.G.; Abbo, H.; Titinchi, S.J.J.; Johan, M.R. An overview of recent advances in biological and pharmaceutical developments of fluoro-containing drugs. Curr. Org. Chem., 2020, 23(26), 2916-2944.
[http://dx.doi.org/10.2174/1385272824666191213123930]
[38]
Wang, X.; Liu, D.; Shen, L.; Li, F.; Li, Y.; Yang, L.; Xu, T.; Tao, H.; Yao, D.; Wu, L.; Hirata, K.; Bohn, L.M.; Makriyannis, A.; Liu, X.; Hua, T.; Liu, Z.J.; Wang, J. A genetically encoded F-19 NMR probe reveals the allosteric modulation mechanism of cannabinoid receptor 1. J. Am. Chem. Soc., 2021, 143(40), 16320-16325.
[http://dx.doi.org/10.1021/jacs.1c06847] [PMID: 34596399]
[39]
Yang, K.; Song, M.; Ali, A.I.M.; Mudassir, S.M.; Ge, H. Recent advances in the application of selectfluor as a “Fluorine‐free” functional reagent in organic synthesis. Chem. Asian J., 2020, 15(6), 729-741.
[http://dx.doi.org/10.1002/asia.202000011] [PMID: 32068956]
[40]
Zhang, R.; Ma, R.; Fu, Q.; Chen, R.; Wang, Z.; Wang, L.; Ma, Y. Selective electrophilic di- and monofluorinations for the synthesis of 4-difluoromethyl and 4-fluoromethyl quinazolin(thi)ones by a selectfluor-triggered multi-component reaction. Org. Chem. Front., 2022, 9(6), 1567-1573.
[http://dx.doi.org/10.1039/D1QO01728D]
[41]
Cotman, A.E.; Guérin, T.; Kovačević, I.; Benedetto Tiz, D.; Durcik, M.; Fulgheri, F.; Možina, Š.; Secci, D.; Sterle, M.; Ilaš, J.; Zega, A.; Zidar, N.; Mašič, L.P.; Tomašič, T.; Leroux, F.R.; Hanquet, G.; Kikelj, D. Practical synthesis and application of halogen-doped pyrrole building blocks. ACS Omega, 2021, 6(14), 9723-9730.
[http://dx.doi.org/10.1021/acsomega.1c00331] [PMID: 33869952]
[42]
Schönherr, H.; Cernak, T. Profound methyl effects in drug discovery and a call for new C-H methylation reactions. Angew. Chem. Int. Ed., 2013, 52(47), 12256-12267.
[http://dx.doi.org/10.1002/anie.201303207] [PMID: 24151256]
[43]
Tarasi, S.; Tehrani, A.A.; Morsali, A. The effect of methyl group functionality on the host-guest interaction and sensor behavior in metal-organic frameworks. Sens. Actuators B Chem., 2020, 305, 127341.
[http://dx.doi.org/10.1016/j.snb.2019.127341]
[44]
Barchi, J.J., Jr; Strain, C.N. The effect of a methyl group on structure and function: Serine vs. threonine glycosylation and phosphorylation. Front. Mol. Biosci., 2023, 10, 1117850.
[http://dx.doi.org/10.3389/fmolb.2023.1117850] [PMID: 36845552]
[45]
Liepouri, F.; Foukaraki, E.; Deligeorgiev, T.G.; Katerinopoulos, H.E. Iminocoumarin-based low affinity fluorescent Ca2+ indicators excited with visible light. Cell Calcium, 2001, 30(5), 331-335.
[http://dx.doi.org/10.1054/ceca.2001.0240] [PMID: 11733939]
[46]
Komatsu, K.; Urano, Y.; Kojima, H.; Nagano, T. Development of an iminocoumarin-based zinc sensor suitable for ratiometric fluorescence imaging of neuronal zinc. J. Am. Chem. Soc., 2007, 129(44), 13447-13454.
[http://dx.doi.org/10.1021/ja072432g] [PMID: 17927174]
[47]
Bouattour, A.; Fakhfakh, M.; El-Gharbi, S.A.; Abid, M.; Paquin, L.; Le Guevel, R.; Charlier, T.; Ammar, H.; Bazureau, J.P. 3-(Tetrazol-5-yl)-2-imino-coumarins derivatives: Synthesis, characterization, and evaluation on tumor cell lines. Int. J. Org. Chem., 2021, 11(1), 24-34.
[http://dx.doi.org/10.4236/ijoc.2021.111003]
[48]
Volmajer, J.; Toplak, R.; Leban, I.; Marechal, A.M.L. Synthesis of new iminocoumarins and their transformations into N-chloro and hydrazono compounds. Tetrahedron, 2005, 61(29), 7012-7021.
[http://dx.doi.org/10.1016/j.tet.2005.05.020]
[49]
Yang, G.; Luo, C.; Mu, X.; Wang, T.; Liu, X.Y. Highly efficient enantioselective three-component synthesis of 2-amino-4H-chromenes catalysed by chiral tertiary amine-thioureas. Chem. Commun., 2012, 48(47), 5880-5882.
[http://dx.doi.org/10.1039/c2cc30731f] [PMID: 22572702]
[50]
Sakurai, A.; Motomura, Y.; Midorikawa, H. Substituted benzopyranopyridopyrimidine ring syntheses by the ternary condensation of malononitrile, salicylaldehyde, and aromatic ketones in the presence of ammonium acetate. J. Org. Chem., 1972, 37(10), 1523-1526.
[http://dx.doi.org/10.1021/jo00975a013]
[51]
Junek, H. Synthesen mit Nitrilen, 7. Mitt.: Darstellung und umsetzungen von 3-cyancumarinen. Monatsh. Chem., 1964, 95(1), 234-241.
[http://dx.doi.org/10.1007/BF00909275]
[52]
O’Callaghan, C.N.; McMurry, T.B.H.; O’Brien, J.E.; Draper, S.M.; Wilcock, D.J. Formation of polyheterocyclic systems by reaction of 2-imino-4-methyl-2H-1-benzopyran-3-carbonitrile with active methylene compounds. J. Chem. Soc., Perkin Trans. 1, 1996, 1067(10), 1067.
[http://dx.doi.org/10.1039/p19960001067]
[53]
Schroeder, C.H.; Link, K.P. The synthesis of some 3-substituted-4-methylcoumarins 1. J. Am. Chem. Soc., 1953, 75(8), 1886-1888.
[http://dx.doi.org/10.1021/ja01104a032]
[54]
Loh, C.C.J.; Schmid, M.; Peters, B.; Fang, X.; Lautens, M. Exploiting distal reactivity of coumarins: A rhodium-catalyzed vinylogous asymmetric ring-opening reaction. Angew. Chem. Int. Ed., 2016, 55(14), 4600-4604.
[http://dx.doi.org/10.1002/anie.201600654] [PMID: 26946053]
[55]
Brimioulle, R.; Guo, H.; Bach, T. Enantioselective intramolecular [2+2] photocycloaddition reactions of 4-substituted coumarins catalyzed by a chiral Lewis acid. Chemistry, 2012, 18(24), 7552-7560.
[http://dx.doi.org/10.1002/chem.201104032] [PMID: 22539275]
[56]
Dalessandro, E.V.; Collin, H.P.; Valle, M.S.; Pliego, J.R. Mechanism and free energy profile of base-catalyzed Knoevenagel condensation reaction. RSC Advances, 2016, 6(63), 57803-57810.
[http://dx.doi.org/10.1039/C6RA10393F]
[57]
Dalessandro, E.V.; Collin, H.P.; Guimarães, L.G.L.; Valle, M.S.; Pliego, J.R., Jr Mechanism of the piperidine-catalyzed Knoevenagel condensation reaction in methanol: The role of iminium and enolate ions. J. Phys. Chem. B, 2017, 121(20), 5300-5307.
[http://dx.doi.org/10.1021/acs.jpcb.7b03191] [PMID: 28471675]
[58]
Li, J.P.H.; Adesina, A.A.; Kennedy, E.M.; Stockenhuber, M. A mechanistic study of the Knoevenagel condensation reaction: new insights into the influence of acid and base properties of mixed metal oxide catalysts on the catalytic activity. Phys. Chem. Chem. Phys., 2017, 19(39), 26630-26644.
[http://dx.doi.org/10.1039/C7CP04743F] [PMID: 28956036]
[59]
Anh Tran, V.; Nhu Quynh, L.T.; Thi Vo, T.T.; Nguyen, P.A.; Don, T.N.; Vasseghian, Y.; Phan, H.; Lee, S.W. Experimental and computational investigation of a green Knoevenagel condensation catalyzed by zeolitic imidazolate framework-8. Environ. Res., 2022, 204(Pt D), 112364.
[http://dx.doi.org/10.1016/j.envres.2021.112364] [PMID: 34767819]
[60]
Lukton, A. Participation of imidazole in intramolecular hydrogen bonding. Nature, 1961, 192(4801), 422-424.
[http://dx.doi.org/10.1038/192422a0]
[61]
Gougoula, E.; Cole, D.J.; Walker, N.R. Bifunctional hydrogen bonding of imidazole with water explored by rotational spectroscopy and DFT calculations. J. Phys. Chem. A, 2020, 124(13), 2649-2659.
[http://dx.doi.org/10.1021/acs.jpca.0c00544] [PMID: 32141751]
[62]
Pande, A.; Ganesan, K.; Jain, A.K.; Gupta, P.K.; Malhotra, R.C. A novel eco-friendly process for the synthesis of 2-chlorobenzylidenemalononitrile and ITS analogues using water as a solvent. Org. Process Res. Dev., 2005, 9(2), 133-136.
[http://dx.doi.org/10.1021/op0498262]
[63]
Heravi, M.M.; Tehrani, M.H.; Bakhtiari, K.; Oskooie, H.A. A practical Knoevenagel condensation catalysed by imidazole. J. Chem. Res., 2006, 2006(9), 561-562.
[http://dx.doi.org/10.3184/030823406778521329]
[64]
van Beurden, K.; de Koning, S.; Molendijk, D.; van Schijndel, J. The Knoevenagel reaction: A review of the unfinished treasure map to forming carbon–carbon bonds. Green Chem. Lett. Rev., 2020, 13(4), 349-364.
[http://dx.doi.org/10.1080/17518253.2020.1851398]
[65]
Qin, H.; Zhou, Y.; Zeng, Q.; Cheng, H.; Chen, L.; Zhang, B.; Qi, Z. Efficient Knoevenagel condensation catalyzed by imidazole-based halogen-free deep eutectic solvent at room temperature. Green Energy & Environment, 2020, 5(2), 124-129.
[http://dx.doi.org/10.1016/j.gee.2019.11.002]
[66]
Chen, Y.; Mu, T. Revisiting greenness of ionic liquids and deep eutectic solvents. Green Chem. Eng., 2021, 2(2), 174-186.
[http://dx.doi.org/10.1016/j.gce.2021.01.004]
[67]
Wang, Y.; Wang, L.; Liu, C.; Wang, R. Benzimidazole-containing porous organic polymers as highly active heterogeneous solid-base catalysts. ChemCatChem, 2015, 7(10), 1559-1565.
[http://dx.doi.org/10.1002/cctc.201500244]
[68]
Sakthivel, B.; Dhakshinamoorthy, A. Chitosan as a reusable solid base catalyst for Knoevenagel condensation reaction. J. Colloid Interface Sci., 2017, 485, 75-80.
[http://dx.doi.org/10.1016/j.jcis.2016.09.020] [PMID: 27649093]
[69]
Haferkamp, S.; Fischer, F.; Kraus, W.; Emmerling, F. Mechanochemical Knoevenagel condensation investigated in situ. Beilstein J. Org. Chem., 2017, 13, 2010-2014.
[http://dx.doi.org/10.3762/bjoc.13.197] [PMID: 29062421]
[70]
Haferkamp, S.; Kraus, W.; Emmerling, F. Studies on the mechanochemical Knoevenagel condensation of fluorinated benzaldehyde derivates. J. Mater. Sci., 2018, 53(19), 13713-13718.
[http://dx.doi.org/10.1007/s10853-018-2492-0]
[71]
Sharifi, Z.; Daneshvar, N.; Langarudi, M.S.N.; Shirini, F. Comparison of the efficiency of two imidazole-based dicationic ionic liquids as the catalysts in the synthesis of pyran derivatives and Knoevenagel condensations. Res. Chem. Intermed., 2019, 45(10), 4941-4958.
[http://dx.doi.org/10.1007/s11164-019-03874-5]