Brønsted acidic ionic liquids: Green catalysts for essential organic reactions

Journal of Molecular Liquids 218 (2016) 95–105

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Review

Brønsted acidic ionic liquids: Green catalysts for essential organic reactions Majid Vafaeezadeh, Heshmatollah Alinezhad ⁎ Faculty of Chemistry, University of Mazandaran, P.O. Box 47416-95447, Babolsar, Iran

a r t i c l e

i n f o

Article history: Received 4 September 2015 Received in revised form 31 January 2016 Accepted 5 February 2016 Available online xxxx Keywords: Brønsted acidic ionic liquid Green catalyst Acid-catalyzed organic reactions Homogeneous catalyst Heterogeneous catalyst

a b s t r a c t During recent years, the role of the ionic liquids (ILs) transferred to non-solvent applications such as catalysts. From them, Brønsted acidic ionic liquids (BAILs) have attracted special attention as efficient catalysts for the synthesis of valuable chemicals. The current review deals with the applications of BAILs as clean and versatile replacements for the traditional homogeneous acid catalysts such as H2SO4, HCl and H3PO4. The article covers the applications of BAILs in several essential organic reactions with excellent industrial and synthetic impacts such as: hydration and dehydration; oxidation; biodiesel production via transesterification and esterification; alkylation and biomass transformations. The versatilities and adaptabilities of BAILs in various reactions are predicted as the start of a new era in acid-catalyzed transformations. © 2016 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Reactions involving BAILs as catalyst . . . . . . . . . . . . . . . 2.1. Hydration and dehydration . . . . . . . . . . . . . . . . 2.2. Oxidation . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Biodiesel production via transesterification and esterification . 2.4. Alkylation . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Biomass transformations . . . . . . . . . . . . . . . . . 3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Brønsted acids are widely used in various chemical reactions such as alkylation, dehydration, condensation, esterification and synthesis of pharmaceutical products from both laboratory and industrial scales [1]. Despite widespread applications of homogeneous Brønsted acids (for example the world production of H2SO4 in 2004 was about 180 Mt) [2], serious drawbacks have been encountered by using these chemicals. Homogeneous acids mainly liberate hazardous gas to the environment. Sulfuric acid rapidly reacts with some metals and releases explosive hydrogen gas. Most of the strong homogeneous acids and even concentrated weak acids (such as formic acid and acetic acid) are ⁎ Corresponding author. E-mail address: [email protected] (H. Alinezhad).

http://dx.doi.org/10.1016/j.molliq.2016.02.017 0167-7322/© 2016 Elsevier B.V. All rights reserved.

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corrosive and release the metallic wastes of a reaction reactor and cause cracks within the reactor body and/or pollute the products. In this regard, the replacement of the mentioned homogeneous acid catalysts with safe or less hazardous compounds is necessary. Heterogeneous acid catalysts based on porous materials are a good candidate for this matter [3–6]. In spite of a remarkable improvement in catalyst separation and recovery, they mainly suffer from a “lazy” kinetic since the catalytic centers are fixed on the surface of a support. Other limitations of these materials refer to the weak mass transfer, deactivation of the hydrophilic surface with water and blocking the entrance of the solid support (especially in the case of microporous materials) with organic molecules [7–9]. As “solvent or catalyst”, ionic liquids (ILs) are known as versatile chemicals in diverse fields of the synthetic chemistry [10]. They have unique properties such as low vapor pressure, good thermal stability,

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Scheme 1. The synthetic pathway for the preparation of BAIL 1 used for hydration of phenylacetylene.

high ion conductivity and simple functionality. Regardless of their environmental compatibility, in most cases the applications of ILs as solvent is not cost-effective. In this regard, a new look at the ILs was introduced with targeted applications. The term “task-specific IL” refers to the nonsolvent applications of the ILs such as catalysts in the organic synthesis and separation of metals or gases [11–19]. The functionalized IL participates in the reactions as a catalyst, mixed catalyst/solvent or in a better description a modified phase transfer catalyst (PTC). Perhaps the most important subcategory of the task-specific ILs is referred to as the Brønsted acidic ionic liquid (BAIL). High thermal stability, high acidity accompanied by a negligible release of hazardous gases, simple separation and reusability for several times are among the advantages of the BAIL. The acidity of BAILs is supplied from two sources: (1) the sulfonic acid specie (− SO3H) which covalently bonded to the IL and (2) a Brønsted acidic counter anion of the IL like hydrogensulfate (HSO− 4 ) or dihydrogen phosphate (H2PO− 4 ). As a catalyst, BAILs in various forms have gained widespread applications in organic chemistry such as in a Mannich reaction [20], Pechmann reaction [21], synthesis of amidoalkyl naphthols [22], εcaprolactam synthesis [23], synthesis of benzoxanthenes [24], diastereoselective synthesis of pyrazolines [25], synthesis of indazolophthalazine-triones and bis-indazolophthalazine-triones [26], Biginelli reaction [27], synthesis of bis-indolylmethanes [28] and protection reactions [29,30]. However, the aim of this article is to review the catalytic applications of the BAIL for some challenging organic reactions with a strong connection to the chemical industries. This review comprises a collection of the BAILs which impart to the reactions in both homogeneous and heterogeneous forms.

2. Reactions involving BAILs as catalyst 2.1. Hydration and dehydration Hydration of alkynes to the carbonyl compounds is one of the most challenging transformations in organic chemistry [31]. The reaction comprises an addition of water to a triple bond to produce carbonyl derivatives followed by tautomerization. The traditional well-known method included the addition of highly toxic mercury as the catalyst for activating the triple bond of an alkyne. To avoid the use of mercury, Brønsted acids such as concentrated sulfuric acid can be employed for this reaction [32]. Unfortunately, in this condition the desired yields could only be obtained by employing excess amounts of H2SO4 (more than 103 equivalents to the alkyne). To overcome the lack of efficient and less hazardous methods, Wang et al. reported the application of the dicationic BAIL 1 with combinations of a small portion of the sulfuric acid for hydration of some alkynes under a relatively mild condition [33]. The structure of the applied BAIL comprises two pyrrolidinium units with two HSO− 4 moieties. The pathway for the preparation of 1 is shown in Scheme 1. BAIL 1 was used for the hydration of various aromatic alkynes with an electron donating and withdrawing substituent at 40 °C. For the aliphatic starting materials, the temperature should be raised up to 60 °C and in most cases the quantitative yields were obtained in less than 1 h even in large-scale synthesis. The IL was simply recycled and reused for ten times without significant loss of efficiency. A noticeable drawback of the above report is attributed to the use of sulfuric acid. Although the amount of H2SO4 was significantly reduced compared to the traditional method (0.5–8 mol equiv. [33] vs. 1500

Scheme 2. Quantitative hydration using BAIL 2 as catalyst.

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Scheme 3. The structure of BAILs 3 and 4 and the mechanistic pathway for the synthesis of the acrolein from glycerol.

equiv. [32]), the process still has some environmental concerns. It was shown that direct incorporation of sulfonic acid specie to the IL is a promising way to increase the acidity of the catalyst for the metal-free

hydration of the alkynes [34]. To solve this problem, a series of \\SO3H functionalized ILs was reported for the hydration of alkyne in the absence of additional sulfuric acid [35]. Quantitative yields of hydra-

Scheme 4. Synthesis of polysubstituted olefin catalyzed by BAIL 5.

Scheme 5. Synthesis of 2-phenylnaphthalenes from styrene oxides using BAIL 6 as catalyst.

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Scheme 6. Dehydration of rac-1-phenyl ethanol to styrene by BAIL 7 as catalyst.

tion products of alkyne with propargyl functionalities were obtained when 1-(para-benzylsulfonic)-3-methylamidazolium tosilate 2 was used as catalyst (Scheme 2). Acrolein is the simplest unsaturated aldehyde which uses as a building block for the synthesis of methyl pyridine, methionine and acrylic ester, polymer and super absorbers [36]. In the chemical industries, it is mainly prepared by the oxidation of propene with molecular oxygen. An alternative pathway for the synthesis of this compound is via the acid-catalyzed dehydration of glycerol. Glycerol is the major byproduct of soap industries and biodiesel production [37]. Hence in both environmental and economical points of view, this strategy is great of interest. In 2009, Rane and co-workers synthesized the triphenyl(3-sulfopropyl) phosphonium 4-methylbenzenesulfonate BAIL 3 and supported it onto the surface of a commercially available mesoporous silica gel with 20% loading of the IL [38]. The structure of 3 and the mechanistic pathway for the synthesis of acrolein from glycerol are shown in Scheme 3. In this method, the selectivity for acrolein reached to 58% at 275 °C after 1 h. The reaction byproducts were also screened and found that 2% acetaldehyde and 7% of hydroxyacetone were observed. Imidazolium based BAIL 4 was also

used for the dehydration of glycerol [39]. The authors reported that in the molar ratio of BAIL 4:glycerol = 1:100, 42.3% acrolein, 3.6% hydroxyacetone and 3% acetaldehyde were formed after 100 min at 260 °C. BAIL was used for direct dehydrative coupling of alcohols or alcohols/alkenes without applying metal-catalysts. In 2014, Yan et al. synthesized several BAILs and investigated their applications for the synthesis of polysubstituted olefins via C\\C bond construction reactions [40]. The results showed that when 10 mol% of BAIL 5 was added as catalyst, a broad range of the substrates including benzylic, allylic, propargylic, aliphatic and aromatic olefins could be obtained with high yield under relatively mild condition (Scheme 4). The possibility for the recycling of the catalyst was also explored and found that it can be reused for six reaction runs with high yields. Very recently, BAIL was used for dehydrative synthesis of 2phenylnaphthalenes from styrene oxides by using N-methyl-2-pyrrolidone hydrogensulfate 6 as catalyst (Scheme 5) [41]. The current synthetic method is the first metal and solvent-free protocol for the synthesis of the substituted naphthalene derivatives. Unlike metal-catalyzed synthetic procedures, a major advantage of this

Scheme 7. Route for preparation of long chain multi-SO3H functionalized heteropolyanion-based IL 9.

Scheme 8. The structure of surfactant-like BAIL 10 and synthesis of dual catalytic functions of BAIL 11 used for oxidative cleavage of cyclohexene to adipic acid.

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Scheme 9. The structures of the BAILs used for transesterification of cottonseed oil. n = 1 and 2.

Scheme 10. Synthesis of BAIL 13.

work is attributed to the releasing of water as the reaction by-product. Mechanistic details proposed that the reaction might proceed through the dehydration of a diol intermediate in the presence of 6. One of the industrial methods for the synthesis of styrene is through the dehydration of rac-1-phenyl ethanol. Rhenium (Re) is the most popular and efficient catalyst for this reaction [42,43]. However, rhenium is listed among the most expensive metal catalysts. The catalytic processes based on rhenium have also had the risk of releasing heavy metal waste which can pollute both the reaction products and the environment. Very recently, Marr and co-workers introduced a silica gel supported BAIL for the synthesis of styrene from dehydration of rac-1-phenyl ethanol [44]. BAIL 7 was prepared from the reaction of the triethylamine and 1,3-propanesultone with subsequent treatment by bis(triflamidic) acid (Scheme 6). At 115 °C and in the presence of toluene as solvent, 78% of styrene was obtained from a catalyst/substrate ratio of 1:100. The major byproducts of the reaction are symmetrical ether (8%) and a polymer with high molecular weight (14%). The supported form of BAIL 7 also gave desirable results for the above reaction. The efficiency and selectivity were retained even after six runs of reaction. It is worthy to note that when 1,3-propanesultone and 1,4-butanesultone were used as the precursors of \\SO3H moiety they provided an halogen-free protocol for the synthesis of the BAILs.

Scheme 11. The structure of the BAIL immobilized on poly divinylbenzene (PDVB) 14.

2.2. Oxidation Oxidation of alcohol to the corresponding carbonyl compound is one of the most important functional group conversions in organic synthesis. Several catalytic systems have been developed for this reaction utilizing air, molecular oxygen or hydrogen peroxide as green oxidants. However, the desired yields could only be obtained in the presence of an expensive metal catalyst [45,46]. BAIL was employed as a cocatalyst in the selective oxidation of benzylic alcohols to their corresponding carbonyl compounds [47]. Li and co-workers established a catalytic system including long chain multi-SO3H functionalized heteropolyanion-based ILs by employing 35% H2O2 as a green oxidant [48]. BAIL 9 was prepared from the reaction of a long chain tertiary amine with 1,3-propanesultone to form the zwitterion 8. Then, treatment of 8 by heteropolyacid produces the final BAIL 9 (Scheme 7). The reaction was performed under solvent-free condition and without using any co-solvent or PTC. The roles of the BAILs in this work are to provide efficient miscibility of organic substrates and aqueous oxidant and also bearing tungstate species that are necessary for oxidation. Desirable yields could only be obtained in the acidic media. Designing of conscious catalytic systems based on BAILs is still very challenging since the ILs are expensive reagents for using as solvent. “Engineered” functionalization of the ILs with targeted applications gave the opportunity of reducing the reaction participants and simultaneously the reaction costs. One of these reactions is related to the synthesis of adipic acid. Adipic acid (or 1,6-hexanedioic acid) is among the most important dicarboxylic acids which are used in the chemical industries [49]. The current industrial route for the synthesis of adipic acid is using nitric acid for the oxidation of KA oil (a mixture of cyclohexanone and cyclohexanol). The major disadvantage of this method is attributed to the releasing of greenhouse and acidic N2O gas. Hence, introducing the non-nitric acid catalytic systems seemed to be crucial. Oxidative cleavage of cyclohexene with aqueous hydrogen peroxide (H2O2) is a clean and less hazardous method for the replacement to the nitric acid oxidation procedure which is first reported by Noyori et al. [50]. Generally, a mixture of cyclohexene, sodium tungstate and Brønsted acidic PTC was heated to produce adipic acid. The high yield of adipic acid could only be obtained in the presence of PTC and in the acidic medium. Unfortunately, the applied PTC which was used to provide the acidity of the reaction mixture is a toxic and expensive reagent. To address the application of BAIL in this field, 1,2-dimethyl-3dodecylidazolium hydrogensulfate BAIL 10 (Scheme 8) was introduced

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Scheme 12. Synthesis of BAIL 15.

as the reaction media for the oxidation of cyclohexene to adipic acid in the presence of a catalytic amount of silver tungstate (Ag2WO4) and 30% H2O2 as oxidant [51]. The yield of the adipic acid in this work reached up to 85%. The applied BAIL 10 acts as a surfactant-like Brønsted acid and hence provides a suitable balance between hydrophobicity and hydrophilicity. This led to increasing the overall reaction efficiency via the better miscibility of the organic phase of the starting material and the aqueous oxidant. The acidity of 10 facilitates the formation of an active peroxotungstate specie from the reaction of the oxidant with silver tungstate. To reduce the reaction components, BAIL 11 was prepared with two catalytic functions for the synthesis of adipic acid (85%), glutaric acid (79%) and succinic acid (88%) from the oxidation of their corresponding cyclic olefins [52]. The mentioned IL bears the tungstate (WO2− 4 ) specie which is necessary for oxidation. The acidity of the IL was also supplied from the \\OSO3H group which is directly attached to the IL from the reaction of the\\OH functionality of IL with chlorosulfonic acid. The reusability of BAIL 11 was investigated for three cycles with high efficiency. 2.3. Biodiesel production via transesterification and esterification Fatty acid methyl/ethyl esters are known as biodiesels which have been selected as candidates for the replacement of fossil fuel to diminish the global energy crisis and environmental pollution control. There are two major synthetic methods for biodiesel production: transesterification of triglycerides and esterification reactions of fatty acids. In a transesterification reaction, basic catalysts such as KOH and NaOH are more efficient than acidic. However, the major limitation for this reaction is attributed to the employing of less expensive feedstock (such as waste oil) as the starting material source of the reaction. Unfortunately, the low-cost waste oil, contains a high portion of free fatty acids as an impurity which when in contact with the reaction catalyst (KOH) forms high amounts of soap as the fatty acid neutralization byproduct. Soap formation is an unfavorable side reaction which increases

the separation cost and causes difficulties for biodiesel separation. Many pretreatment methods have been proposed for reducing the high free fatty acid content, including steam distillation, extraction by alcohol, and esterification by acid catalysis [53]. Acid-catalyzed esterification of the free fatty acid is a promising method which has been introduced for decreasing the free fatty acid of the waste oil. After reducing the amounts of free fatty acids to an acceptable level, the remaining purified starting material is suitable for transesterification reaction by employing basic catalyst and/or acid-catalyzed transesterification with the initial Brønsted acid catalyst [54]. As mentioned above, acid-catalyzed transesterification reaction is routinely performed at high temperature. Hence, most of the traditional inorganic acids are not suitable for this issue. In 2007, Wu and coworkers introduced water-stable BAILs for transesterification of cottonseed oil and explored the effects of the reaction conditions on biodiesel production efficiency [55]. The structure of the BAILs included both covalently bonded \\SO3H moiety and the counter anion of HSO− 4 (Scheme 9). The best catalytic activity was obtained when pyridinium BAIL 12 was used as catalyst. The catalyst activity is comparable to that of concentrated sulfuric acid. The effect of the reaction temperature was investigated in the range of 150–180 °C. Despite traditional methods, the applied catalytic system comprises using facile recyclable catalyst and simple product isolation methodology. Heteropolyacid based-pyridinium BAIL has been shown as a versatile class of Brønsted acids for transesterification. In 2011, Yan et al. reported that the pyridinium BAIL based on heteropolyanion 13 could efficiently catalyze the transesterification of methyl oleate and trimethylolpropane [56]. The preparation procedure for the synthesis of BAIL 13 is shown in Scheme 10. The results showed that at 120 °C, 92% of the transesterification product was obtained. An interesting feature of this protocol was attributed to the separation step of the catalyst. At the reaction temperature, the process was conducted in the homogeneous phase. However, at the end of the

Scheme 13. The structures of BAILs 16 and 17.

Scheme 14. The structures of BAILs 18 and 19.

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Scheme 15. The structures of BAIL precursors 20 and 21 used for covalent attachment on the surface of porous materials.

Scheme 16. The structure of the magnetic nanoparticle contains dual BAIL 22.

reaction and after cooling the mixture to room temperature, the BAIL was solidified and thus could be separated as a heterogeneous catalyst. One-step synthesis of biodiesel from waste oil which contains high amounts of free fatty acids was performed by Liang using BAIL immobilized on poly divinylbenzene (PDVB) 14 as catalyst (Scheme 11) [57]. The authors reported that the solid acid has high activities for both the esterification of the free fatty acid and transesterification of triglyceride with a total yield of up to 99%. The catalyst could be reused for several times with retention of its high activity. The catalyst has a desirable activity at 70 °C. However, the high yield of product in a short reaction time was obtained at 140 °C. Although the reaction temperature is high, it can readily be conducted under atmospheric pressure. For example, in the case of both heterogeneous sulfonic acid catalyst as well as H2SO4, the high yields only could be obtained in the pressurize reactor with 12 and 18 bar, respectively [58]. Consequently, the reaction over the BAIL can be

Scheme 17. The proposed mechanism for the enhancement of the catalytic activity of SBA15-propyl-SO3H@BAIL.

conducted under significantly milder condition. Moreover, it was found that by introducing a basic IL, the transesterification was performed at a relatively milder thermal condition (95% at 60 °C) [59]. However, this method does not have compatibility with the starting materials containing free fatty acid. The first example of direct esterification using BAILs was reported in 2002 by Davis et al. [60]. They synthesized the BAILs from the reaction of N-butyl imidazole or triphenylphosphine with 1,4-butane- or 1,3propanesultone, respectively. Trifluoromethanesulfonic acid and ptoluenesulfonic acid were selected to convert the zwitterions to the corresponding BAILs. The same protocol was applied to the synthesis of BAILs with propylsulfonic acid attached to a pyridinium cation core − − [61]. The counter anion of these BAILs comprises BF− 4 , H2PO4 , HSO4 − and p-CH3(C6H4)SO3 . From them, the best results for the esterification of benzoic acid and some short chain alcohols were obtained when the BAIL with the counter anion HSO− 4 was utilized. To address the efficiency of the catalyst, the authors proposed that the partial immiscibility of TSILs with the produced esters facilitates the shifting of the esterification reaction equilibrium to the product side. In 2007, Zhou prepared N-methyl-2-pyrrolidonium methylsulfonate BAIL 15 through the direct reaction of N-methyl-2-pyrrolidone as a source of cation and methanesulfonic acid (Scheme 12) [62]. The catalyst was found to be highly efficient for various esterifications such as reaction of oleic acid with methanol at room temperature to produce a corresponding methyl ester with 93% yield after 10 h. The mentioned BAIL 15 could be reused for six reaction runs with high conversion and selectivity. Increasing the reaction temperature did not have a significant increase for efficiency. Moreover, they found that by increasing the reaction temperature, small amounts of ether byproduct were also observed. The solubility of the BAILs is a key factor for reactivity during the esterification reaction. Considering BAILs 16 and 17 (Scheme 13) [63], 16 has shown a remarkably higher activity for esterification. The presence of electron withdrawing fluorine atoms made 17 a higher Brønsted acid compound. However, the poor solubility of 17 in the applied alcohol significantly reduced the yield of ester. Consequently, the stronger acidity of the BAILs is not the sole factor for the higher efficiency. Dicationic BAILs are another class of acid catalyst with high catalytic efficiency [64]. In 2011, Fang et al. synthesized BAIL 18 with four acidic species and used this catalyst for biodiesel production from some fatty acid including oleic, stearic, myristic and palmitic acid with ethanol at

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Scheme 18. Schematic illustration for the physical confinement of BAIL 23 on the surface of silica.

mild thermal condition (Scheme 14) [65]. BAIL 18 was used several times without a significant loss of activity. Another advantage of this catalyst refers to the simple workup procedure. During the reaction, both of the starting materials and the catalyst formed a homogeneous solution. However, at the end of the reaction, 18 and the biodiesel product formed a “two-phase” system. In this situation the products could easily be separated and the catalyst was subjected to another reaction without a special workup methodology. Simple product isolation from the catalyst by a liquid–liquid biphasic system was also reported using BAIL 19 as catalyst (Scheme 14) [66]. Immobilization of ILs is an interesting feature in supported catalysts which mainly performs to facilitate the work-up process. In this case, the ILs are supported on the surface of solid materials such as porous silica gel via both covalent and non-covalent (or physisorption) interactions. The examples of techniques which were developed for the covalent attachments comprise direct grafting of the precursor BAIL 20 on the surface of silica gel which was used for esterification of oleic acid (Scheme 15) [67] or radical initiator in one-pot sol–gel condensation of the host support. For example, precursor 21 is prepared from the reaction of vinyl imidazole and 1,3-propanesultone followed by immobilization via a thiol precursor on the surface of silica gel as shown in Scheme 15 [68].

A useful method for immobilization of BAIL is using magnetic silica gel. In 2014, Wan and colleagues synthesized a magnetic nanoparticle supported by dual BAIL 22 on the surface of Fe3O4@SiO2 using 3chloropropyltrimethoxysilane as the linker (Scheme 16) [69]. They used the mentioned catalyst for the synthesis of biodiesel with oleic acid and ethanol with nearly a 93% yield. The catalyst also showed good activity in transesterification of soybean oil. It was simply separated by magnetic force and reused for eight times with small reduction in catalytic activity. Physical confinement of the ILs on the surface of the various materials is a useful method to perform reactions on a supported liquid layer. In this method, the contact surface area of the BAILs can be significantly increased which highly improves the catalyst efficiency and separation. In 2012, Karimi and a coworker used this technique to immobilize hydrophobic 1-methyl-3-octylimidazolium hydrogensulfate onto the mesochannels of the SBA-15 material bearing propylsulfonic acid moieties [70]. The catalyst showed high efficiency for various esterification reactions as well as fatty acid esterification with ethanol at room temperature without special methodology. The catalyst had higher activity compared to the solely BAIL or SBA-15@propyl-SO3H catalyst. To explain the extraordinary catalytic activity, the authors proposed that the catalytic species were surrounded by the hydrophobic N-octyl substituent

Scheme 19. Possible products which can form from isopropylation of m-cresol in the presence of BAIL 24 as catalyst.

Scheme 20. Reductive Friedel–Crafts alkylation of indoles and cyclohexanone using BAIL 25 as catalyst.

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Scheme 21. Dehydration of fructose to 5-hydroxymethylfurfural in the presence of BAIL 26.

of the BAIL. In this situation, the water by-product could expel out of the catalytic centers. The approaching of the water to the surface of the catalyst was inhibited due to the hydrophobic repulsion. This phenomenon which was accompanied by a cooperative effect between acidic species leads to observing a significant improvement of the supported BAIL catalyst (Scheme 17). Very recently, an improvement in the catalytic activity was illustrated through the immobilization of BAIL 23 on the surface of the commercially available silica gel for biodiesel production from the esterification of some fatty acids with ethanol at 70 °C [71]. The catalyst also showed fair activity in water as solvent. Density functional theory (DFT) modeling of the catalyst structure showed that the ΔHacidity of the “silica-supported BAIL 23” catalyst in this model is 321.8 kcal/mol which is comparable to the reported value for the methanesulfonic acid (319 kcal/mol). The same calculation for the “homogeneous BAIL 23” gave the value of ΔHacidity to be 345.4 kcal/mol. It should be noted here that the smaller values of ΔHacidity refer to higher acidity. The catalyst was simply prepared from dropwise addition of the solution of 23 and ethanol to the silica gel which was followed by evaporation of solvent and drying the BAIL supported catalyst (Scheme 18).

can activate the aromatic molecules. For example, Liu et al. performed a comprehensive study for the liquid phase isopropylation of m-cresol with isopropyl alcohol using BAIL 24 as catalyst [73]. They found that the catalyst had good activity in the isopropylation of m-cresol with 77.7% conversion of m-cresol and 48.7% selectivity for 2-isopropyl-5methyl phenol (2I-5MP) at 190 °C. Possible products in this reaction are shown in Scheme 19. In spite of solid acid catalysts that undergo rapid deactivation, 24 could still be reused for eight cycles with relatively constant efficiency and selectivity. In an example, the monoalkylation of phenol and anisole was reported by using imidazolium-based Brønsted acidic triflate ILs containing the\\SO3H group [74]. Very recently, Gu et al. used BAIL 25 for reductive Friedel–Crafts alkylation of indoles and cyclic ketones without using an external reductant (Scheme 20) [75]. The reactions were performed under a solventfree condition and the water that was generated at the initial stage of the reaction acted as a rendering agent. BAIL 25 was found to be the most efficient catalyst among the several acid catalysts.

2.5. Biomass transformations 2.4. Alkylation In the chemical industries, alkylation reactions are catalyzed by highly acidic and superacidic catalysts. The major concerns about these catalysts are attributed to the release of hazardous waste and difficulties in catalyst and product isolation. There are examples of using BAIL for the alkylation reaction with high yield and improving the selectivity. For instance, a mixture of H2SO4 and a catalytic amount of 1-octyl-3-methylimidazolium tetrakis(hydrogensulfato)-borate ([omim][B(HSO4)4]) yielded 90% of monoalkylbenzene product in the reaction of benzene and 1-decene [72]. The most challenging issue for the alkylation of an aromatic is the formation of various isomers. Moreover, after the first step, the product is more reactive for alkylation since the electron donating alkyl group

Among the literature reports around the applications of BAILs, biomass transformations are the most interesting and challenging subject of study. In both economical and environmental points of view, biomass resources have been found to be renewable sources and sustainable supplies for the production of chemicals and biodiesel productions [76–78]. Application of the BAILs in green biomass transformations is relatively younger than those in other reactions. In 2010, Tong and Li performed selective dehydration of fructose to 5-hydroxymethylfurfural using N-methyl-2-pyrrolidonium methylsulfonate 26 as catalyst (Scheme 21) [79]. In their catalytic system, the yield reached to 72% and up to 87% selectivity at 90 °C. Among the solvents which were tested for this reaction,

Scheme 22. The structures of the symmetrical and unsymmetrical BAILs used for the conversion of fructose to 5-hydroxymethyl furfural.

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Scheme 23. Synthesis of furfural from one-pot hydrolysis and dehydration reactions using BAIL 28 as catalyst under microwave heating (150–180 °C).

dimethylsulfoxide (DMSO) was found to be the best for giving high catalytic performance in the dehydration of D-fructose and glucose. In another example, Kotadia and Soni used some symmetrical and unsymmetrical BAILs for the conversion of fructose to 5hydroxymethyl furfural [80]. Similar to the above protocol, in this catalytic system DMSO was used as the solvent of the reaction. Among the BAILs which were prepared for this reaction (Scheme 22), BAIL 27 gave 72.8% yield with 93% fructose conversion at 80 °C after 1 h. Water is the best solvent in chemistry due to low cost, safety, and environmental concerns [81]. Incompatibility of water with most of the organic reactants reduces the yields or even leads to decomposition of the reagents. In this regard, in addition to use as a catalyst, it is necessary to add PTC to gain the desired catalytic activity. BAILs gave desirable opportunities to perform the reactions in water [82–84]. Dehydration of xylose to furfural utilizing some BAIL in water was reported by Serrano-Ruiz and co-workers under microwave heating (Scheme 23) [85]. According to their findings, BAIL 28 gave the best result with up to 95% conversion of xylose and 85% furfural yield in the mixture of water and THF. The observation around the cation and anion screening of the several BAILs showed that the ILs with the pyridinium cation and BF− 4 anion gave the best results. Moreover, in this protocol direct one-pot hydrolysis and dehydration of an agricultural lignocellulosic waste-derived sample containing C5 oligomers to furfural were also successfully performed at the optimized reaction condition. Bisphenol A is an important chemical which is widely used in the polycarbonate and epoxy resin industries. The synthetic route to bisphenol A is via the condensation of acetone and phenol using concentrated HCl as catalyst. Recently diphenolic acid (γ,γ-bishydroxyphenyl valeric acid, (DPA)) was reported to be an alternative replacement for bisphenol A. This compound can be prepared by the reaction of levulinic acid (derived from carbohydrates) with phenol. Despite using renewable biomass for the synthesis of DPA, the structure of DPA can be simply

modified via the manipulation of a carboxyl group. In 2013, Deng et al. prepared several BAILs and investigated their catalytic activity toward the synthesis of DPA (Scheme 24) [86]. They reported that by using BAIL 29 and catalytic amounts of thiol-containing BAIL 30, a high yield of DPA (over 90%) and an excellent ratio of the p,p′-DPA/o,p′-DPA isomer were obtained at 60 °C within 48 h of the reaction. Moreover, the catalyst could retain its activity for five runs. Furanic compounds are known as a new class of precursors for biodiesel production. After the deoxygenation of these compounds, saturated hydrocarbons with 11 to 25 carbon atoms are obtained which can be used as biodiesel [87–90]. For this proposal, in an interesting research, Bell and co-workers developed both homogeneous and heterogeneous BAIL to synthesize some furanic derivatives using starting materials which can be derived from lignocellulosic biomass resources [91]. They discovered that the BAIL with a hydrophobic anion and side chain N-butyl functionality had higher catalytic activity. For example, BAIL 31 was used as catalyst for the synthesis of 5,5′-bis(2methylfuranyl)furan-2-ylmethane 32 with a high yield from the reaction of furfural and 2-methylfuran at 65 °C after 2 h (Scheme 25).

3. Conclusion In this review, the application of BAILs for several challenging syntheses was investigated. Various BAILs were introduced as highly efficient catalyst as practical replacements for the current acid-catalyzed reactions. In most cases, the BAILs could be prepared with short reaction steps by employing commercially available starting materials. The BAILs have adaptability with different reaction conditions such as high temperature, microwave condition, various organic solvents and even aqueous media. Moreover, they can be utilized in the form of homogeneous, physical and covalent attachments on silica, magnetic silica and polymers. The relatively lower costing of the BAILs accompanied with simple

Scheme 24. Possible isomer distributions of DPA.

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105

Scheme 25. Synthesis of furanic biodiesel precursor 32.

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