Bronsted acid ionic liquids

In the BASF BASIL process that utilizes A-methylimidazole to scavenge HCl byproduct, the acidic ionic liquid A-methylimidazolium chloride [HMIMJCl was formed, with a melting point of 75°C (13,102). Recently, the group of Bronsted acidic ionic liquids with the same cation was extended to include other anions, such as BFF, TfO , and TsO . The melting point of the salt is between 30 and 109°C. Strong hydrogen bonding in the tosylate salt was characterized by IR spectroscopy. 

Bronsted acidic ionic liquids [HMIM]BF4, [HMIM]TfO, and [HMIM]TsO are effective for proton- and metal-assisted catalytic processes such as the dimerization of methyl acrylate and ring-closure metathesis (Scheme 3) (72J). For example, the acidic [HBIM]BF4 in association with the [Ru( = C = C = CPh2)(/ -cymene) (PCy3)Cl][OTf catalyst afforded a dramatic increase in the activity (100%) and selectivity (100%) for the ring-closure metathesis of A,A-diallyltosylamide. 

Cole, A.C. et al.. Novel Bronsted acid ionic liquids and their use as dual solvent-catalysts, /. Am. Chem. Soc., 124,5962,2002. 

The easiest way to create a Bronsted acidic ionic liquid is to dissolve a strong Bronsted acid in an ionic liquid. Already in 1989, Smith and coworkers described that mixtures of HC1 and acidic chloroaluminate ionic liquids result in the formation of superacidic Bronsted acids (more acidic than 100% sulfuric acid). This is due to the reaction of HCl with the acidic anions (e.g. [AI2CI7]-) of the melt forming a proton with extremely low solvation and therefore very high acidity [46],... 

Li et al. [93] have used l-ethylimidazolium trifluoroacetate, which is a Bronsted acidic ionic liquid, as a medium for the electropolymerization of aniline. They report that in this ionic liquid the oxidation potential of aniline is lower (0.58 V compared to 0.83 V in 0.5 M H2SO4) and that the growth rate of the polymer is increased. Further, the resultant films are smooth, strongly adhered to the Pt working electrode and are very electrochemically stable. Similar results have been reported by Liu et al. [92], who found that this was the best ionic liquid for the polymerization of aniline, compared to the unsatisfactory results observed in other protic ionic liquids 1-butylimidazolium tetrafluoroborate, 1-butylimidazolium nitrate and 1-butylimidazolium p-toluenesulfonate, as well as the l-butyl-3-methylimidazolium hydrogen sulfate and l-butyl-3-methyimidazolium dihydrogen phosphate. 

Kondamudi K, Elavarasan P, Dyson PJ et al (2010) Alkylation of p-cresol with /erf-butyl alcohol using benign Bronsted acidic ionic liquid catalyst. J Mol Catal A Chem 321 34 1... 

Recently, Zolflgol et al. reported that the ionic liquid, 3-methyl-l-sulfonic acid imidazolium chloride ([msim][Cl]), as a new Bronsted acidic ionic liquid, can be easily... 

The results showed that the catalyst can be employed four times, although the activity of the catalyst gradually decreased. This indicated that the Bronsted acidic ionic liquid ([TEBS A][HSOJ) as a catalyst for the preparation of amidoalkyl naph-thols was recyclable. 

The advantages of this method, in which a relatively nontoxic (halogen-free) and reusable Bronsted acidic ionic liquid is employed as an effective catalyst, are high catalytic efficiency, short reaction times, high yields, a straightforward work-up, and environmental benignancy. 

Hajipour AR, Ghayeb Y, Sheikhan N, Ruoho AE (2009) Bronsted acidic ionic liquid as an efficient and reusable catalyst for one-pot synthesis of 1-amidoalkyl 2-naphthols under solvent-free conditions. Tetrahedron Lett 50 5649-5651... 

Acid-catalysed esterification reactions in ILs have been extensively studied, and will be the main focus of this section. In 2001, Deng et al. first reported the synthesis of allq l acetate esters in an IL with concentrated sulfuric acid as the acid catalyst (Scheme 3.7). The majority of subsequent studies, however, have switched away from an IL with an added acid catalyst and towards Bronsted acid ionic liquids (BAILs) - a type of task-specific ionic liquid. BAILS incorporate an acidic moiety (typically either a sulfonic acid or a protonated nitrogen) on the cation allowing the BAILS to have dual functionality as both a solvent and a catalyst. There are several different classes of BAILs that have been applied to esterifications such as imidazo-lium 1, imidazolium sulfonic 2, phosphonium sulfonic 3, pyridinium sulfonic 4, quaternary ammonium 5, quaternary ammonium sulfonic 6 and lactam 7 based-BAILs (Scheme 3.8). 

Scheme 3.8 Examples of different classes of Bronsted acid ionic liquids (BAILs).

Shaikh, J. S., R. C. Pawar, R. S. Devan et al. 2011. Synthesis and characterization of Ru doped CuO thin films for supercapacitor based on Bronsted acidic ionic liquid. Electrochimica Acta 56 2127-2134. 

Tao, D. J., Wu, Y. T., Zhou, Z., Geng, J. A., Hu, X. B., Zhang, Z. B. (2011). Kinetics for the esterification reaction of n-butanol with acetic acid catalyzed by noncorrosive bronsted acidic ionic liquids. Industrial Engineering Chemistry Research, 50, 1989—1996. 

Joseph, T., S. Sahoo S. B. Halligudi (2005) Bronsted acidic ionic liquids A green, efficient and reusable catalyst system and reaction medium for Fischer esterification. Journal of Molecular CatalysisA Chemical, 234,107-110,ISSN 13811169. 

The caprolactam based Bronsted acidic ionic liquids such as [NHC][BP4], [NHC][CF3C00] and [NHC][N03] have been used to catalyze the rearrangement of cyclohexanone oxime to caprolactam and was found that with a 3 1 mole ratio of ionic liquid oxime, the conversion increased to... 

Beheshtiha YS, Heravi MM et al (2010) Efficient and green synthesis of 1,2-disubstitut-ed benzimidazoles and quinoxalines using bronsted acid ionic liquid, [(CH ) SO HMIM] [HSO ], in water at room temperature. Synth Commun 40 1216-1223... 

Banothu J, Gali R, Velpula R, Bavantula R (2013) Bronsted acidic ionic liquid catalyzed an efficient and eco-friendly protocol for the synthesis of 2,4,5-trisubstituted-l H-imidazoles under solvent-free conditions. Arabian J Chem (Article in Press http //dx.doi.0rg/lO.lOl6/j. arabjc.2013.10.022)... 

J. Ebrahimi, A. Mohammadi, V. Pakjoo, E. Bahramzade, A. Habibi, Highly efficient solvent-free synthesis of pyranopyrazoles by a Bronsted-acidic ionic liquid as a green and reusable catalyst, J. Chem. Sci. 124 (2012) 1013-1017. 

S. Safaei, I. Mohammadpoor-Baltork, A.R. Khosropour, M. Moghadam, S. Tangestaninejad, V. Mirkhani, R. BCi, Application of a multi-SOsH Bronsted acidic ionic liquid in water a highly efficient and reusable catalyst for the regioselective and scaled-up synthesis of pyrazoles under mild conditions, RSC Adv. 2 (2012) 5610-5616. 

X. Deng, Z. Zhou, A. Zhang, G. Xie, Bronsted acid ionic liquid [EtsNHUI-ISOJ as an efficient and reusable catalyst for the synthesis of 2,4,5-triaryl-lH-imidazoles, Res. Chem. Intermediate 39 (2013) 1101-1108. 

H.R. Shaterian, M. Ranjbar, An environmental friendly approach for the synthesis of highly substituted imidazoles using Bronsted acidic ionic liquid, N-methyl-2-pyrrolidonium hydrogen sulfate, as reusable catalyst, J. Mol. Liq. 160 (2011) 40-49. 

A.R. Hajipour, M. Seddighi, Pyridinium-based Bronsted acidic ionic liquid as a highly efficient catalyst for one-pot synthesis of dihydropy-rimidinones, Syn. Commun. 42 (2012) 227-235. 

A.R. Hajipour, L. Khazdooz, A. Zarei, Bronsted acidic ionic liquid-catalyzed one-pot synthesis of 3,4-dihydropyrimidin-2(lfl)-ones and thiones under solvent-Free conditions, Syn. Commrm. 41 (2011) 2200-2208. 

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