Regioselective Mononitration of Aromatic Compounds

Some novel acyclic Brønsted acidic task-specific ionic liquids (TSILs) that bear an alkane sulfonic acid group in an acyclic trialkanylammonium cation were synthesized and their uses as halogen-free catalysts for regioselective mononitration of aromatic compounds were investigated.
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  Regioselective mononitration of aromatic compounds using Brønsted acidicionic liquids as recoverable catalysts Dong Fang, Qun-Rong Shi, Jian Cheng, Kai Gong, Zu-Liang Liu* School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing, PR China 1. Introduction Electrophilicnitrationofaromaticsisafundamentalreactionof great industrial importance, whose products are used as organicintermediates or energetic materials. It is also among the mostimportantunitreactionsinthechemicalindustry.Themechanisticand synthetic aspects of nitration chemistry have been verythoroughly studied over the years. Depending on the reactivity of the nucleophile substrates and the desired chemo- and regios-electivity, many nitrating systems have been developed. Theserange from protic nitrating with mixed acids and superacids tonitryl halides, acyl- and alkyl nitrates, metal nitrates, or nitroniumsalts,aswellassupportednitrationandtransfer-nitrationreagents[1,2].However, the usual commercial process is not environmentallybenign since it results in disposal problems, necessitates regen-eration of used acids, and often provides poor selectivity for thedesired products. Various nitration approaches have thereforebeen explored in order to avoid the problems of the traditionalmixed acid method, which used nitric and sulfuric acids. The newapproaches particularly involve the use of recyclable catalystsincluding lanthanide triflates [3], perfluorinated rare earth metalsalt/fluorous solvent [4,5], or solid acid catalysts such as aperfluorinatedresinsulfonicacid[6],claycop[7],andzeolites[8,9]. Todate,ionicliquidshavebeenusedinaromaticnitration:LaaliandGettwert[9]usedarangeofnitrationagentsinthenitrationof mainly activated substrates; Rajagopal and Srinivasan [10] usedironnitrateinthenitrationofphenols,LancasterandLiopis-Mestre[11]investigatedseveralnitrationsystems,andtheydemonstratedthatHNO 3 /Ac 2 Oinanionicliquidthatcontained[bmpy] + (1-butyl-1-methylpyrrolidinium) cation was the best system. Smith et al.[12] reported regioselective mononitration of simple aromaticcompounds with HNO 3 /Ac 2 O in [bmim]BF 4 , [bmim]PF 6 , and[bdmim]BF 4  systems.Brønsted acidic task-specific ionic liquids (TSILs), since theycombine the advantageous characteristics of solid acids andmineral acids, are designed to replace traditional mineral liquidacids, such as sulfuric acid and hydrochloric acid, in chemicalprocesses[13,14]. Recentyears, some‘‘greener’’ halogen-freeionicliquids that involve phosphate or octyl sulfate anions have beenreported and the effects of the anion and the toxicology have beenstudied [15,16]. Very recently, Qiao et al. [17] reported that a Brønsted acidic ionic liquid was capable of catalyzing the nitrationof aromatic compounds with medium concentration aqueousnitric acid. The use of Brønsted acidic TSILs to catalyze organicreactionsisanareaofongoingactivity;however,developmentandexploring of Brønsted acidic TSILs are currently in the preliminarystage and the number of published examples of Brønsted acidicTSILs is limited. As part of our continuing interest in thedevelopment of clean chemical processes, in particular cleannitration of aromatic compounds [18–20], we synthesized somenovel halogen-free SO 3 H-functional Brønsted-acidic TSILs thatbear an alkane sulfonic acid group in an acyclic trialkanylammo- Applied Catalysis A: General 345 (2008) 158–163 A R T I C L E I N F O  Article history: Received 23 January 2008 Received in revised form 24 April 2008 Accepted 26 April 2008 Available online 3 May 2008 Keywords: Ionic liquidCatalystNitration reaction A B S T R A C T Some novel acyclic Brønsted acidic task-specific ionic liquids (TSILs) that bear an alkane sulfonic acidgroupinanacyclictrialkanylammoniumcationweresynthesizedandtheirusesashalogen-freecatalystsfor regioselective mononitration of aromatic compounds were investigated. The reactions were carriedout at 60–80  8 C with reasonable to good yields and improved para-selectivities for halogenobenzenescompared to those in the absence of the catalysts. In addition, the TSILs could be recovered and reusedwithout noticeably decreasing the catalytic activity.   2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +86 25 84318865; fax: +86 25 84318865. E-mail address: (Z.-L. Liu). Contents lists available at ScienceDirect Applied Catalysis A: General journal homepage: 0926-860X/$ – see front matter    2008 Elsevier B.V. All rights reserved.doi:10.1016/j.apcata.2008.04.037  nium cation (Fig. 1). Their uses as catalysts for aromatic nitration(Scheme 1) have also been explored. 2. Results and discussion  2.1. Preparation of SO  3 H-functional Brønsted-acidic TSILs The synthesis approach is made up of two-step atom economicreactions. The zwitterionic-type precursors (trialkanylammoniumpropane sulfonate and trialkylammonium butane sulfonate) of these TSILs were prepared according to our previous procedure[18] through a one-step direct sulfonation reaction of trialk-anylamine and 1,3-propane or 1,4-butane sulfone. The synthesesof SO 3 H-functional TSILs with different anions are convenientwhen the corresponding acids have more acidity than the alkanesulfonic acid groups bearing in TSILs cations. The zwitterionacidification can be accomplished by mixing of zwitterion withstrong acids, such as CF 3 COOH, H 2 SO 4  (98%, aqueous), H 3 PO 4  (85%,aqueous), and  p -TSA  H 2 O, to convert the pendant sulfonate groupinto an alkane sulfonic acid, trialkanylpropanesulfonic acidammonium hydrogen sulfate, or the like. The result is thetransformationofthezwitterionintoacationbearinganappendedsulfonic acid group, with the conjugate base of the exogenous acidbecoming the TSILs’ anion. Both H 2 SO 4  (98%, aqueous) and H 3 PO 4 (85%, aqueous) can be selected to obtain the greener halogen-freeTSILs; however, in the case of aromatic nitration, the correspond-ing acids should have at least more acidity than nitric acid in orderthat the TSILs’ anion should not endure protolysis. Hence, H 2 SO 4 (98%, aqueous) should be the suitable acid and the TSILs withHSO 4  anions were used as the catalysts in the followingexperiment. The fresh new TSILs with HSO 4  anion are somewhatviscous colorless liquids at room temperature. In keeping with thebehavior of other ionic liquids (IL), neither of the new speciesfumes or manifests any noticeable degree of vapor pressure, thetreatmentofthenewTSILsundervacuumat120  8 Cfor72 hresultsin no loss of mass and verifies that these TSILs are stable at hightemperature unlike strong acids dissolved in conventional IL,which frequently continue to emit noxious vapors and do harm tothe environment. Such stability indicates that the donor acidsbeing fully incorporated into their respective TSILs structures,rather than remaining simply mixtures of added strong acids withdissolved zwitterion, in which case some retention of premixingcharacteristics would be expected.The Brønsted acidities of these TSILs were evaluated for thedetermination of the Hammett acidity functions, using UV–visspectroscopy. For the comparison purposes, the acidities of thesefive TSILs have been examined using 4-nitroanline (Hammettconstant is 0.99) as the indicator (named I) in dichloromethane(nameds),andthentheHammettfunction( H  0 )couldbecalculated(Table 1).All produced TSILs are miscible with water and soluble inmethanol,ethanol,andacetone,andtheyarepartlysolubleinethers,alkanes, and aromatic hydrocarbons according to their structure.The solubility of these five TSILs in organic solvents decreasedas follows: [TBPSA][HSO 4 ] > [TEBSA][HSO 4 ] > [TEPSA][HSO 4 ] > [TMBSA][HSO 4 ] > [TMPSA][HSO 4 ].  2.2. Aromatic nitration with SO  3 H-functional Brønsted-acidic TSILs The electophilic aromatic nitration is one of the typical classicalacid-promoted organic reactions. From either an industrial or anenvironmentalstandpoint,theidealnitrationagentisaqueousnitricacid, which is inexpensive and generates only water as waste.Unfortunately, few catalytic systems that involved using aqueousnitric acid as the nitration agent are reported up to now. Our initialeffort aimed to use TSILs as solvent and aqueous nitric acid as thenitrating reagent so that water would be the only byproduct. Theresults of the nitration of aromatic compounds with 68% nitric acidare listed as below. For the beginning of this study, nitration of  Fig. 1.  The halogen-free ionic liquids TSILs. Scheme 1.  Nitration of aromatic compounds. D. Fang et al./Applied Catalysis A: General 345 (2008) 158–163  159  halogenobenzene with 68% nitric acid in synthesized TSILs as thecatalysts was explored, and the results are listed in Tables 2–4.Nitrofluorobenzeneswereobtainedintheyieldsrangingfrom62to72%,witha  para to ortho ratioof12–13.InthepresenceofTSILs,thepara-selectivity was significantly higher than that nitrated in theabsence of TSILs (Table 2). In case of chlorobenzene (Table 3) and bromobenzene (Table 4), the use of all five TSILs was advantageousfrom the points of view of the reaction yield and selectivity incomparison to the reaction without the catalysts.As seen from the results of  Tables 2–4, different yields wereobtained in these TSILs. The order of catalytic activity is notconsistent with the sequence of ionic liquid acidity (Table 2) but isconsistent with the sequence of solubility in organic solvents.Reactions were monitored by GC over a period of time. The resultsobtained even with 3 equiv of nitric acid (68%) at 80  8 C were notideal in the absence of TSILs. This indicated that the catalyst wasabsolutely necessary for improved para-selectivities of thehalogenobenzenes nitration.In view of the success of the new reactions, it was decided toapply them to a range of simple substrates, and the results areshown in Table 5.  Table 1 H  0  values of the TSILs in dichloromethane a TSILs Absorbance (AU) [I] (%) [IH + ] (%)  H  0 [TMPSA][HSO 4 ] 0.37 12.3 87.7 0.14[TEPSA][HSO 4 ] 0.40 13.3 86.7 0.18[TBPSA][HSO 4 ] 0.76 25.3 74.7 0.52[TMBSA][HSO 4 ] 0.39 13.0 87.0 0.16[TEBSA][HSO 4 ] 0.90 30.0 70.0 0.62 H  0  = p K  (I) aq  + log([I] s /[IH + ] s ). a Concentration: 10 mmol/L; indicator: 4-nitroanline.  Table 2 Result for nitration of fluorobenzene with different TSILs a Entry TSILs Selectivity (%) b Yield (%) c Product isomers (%) d  para / ortho e ortho meta para 1 [TMPSA][HSO 4 ] 99 62 7.7 0 92.3 122 [TEPSA][HSO 4 ] 99 67 7.6 0 92.4 123 [TBPSA][HSO 4 ] 99 72 7.2 0 92.8 134 [TMBSA][HSO 4 ] 99 70 7.6 0 92.4 125 [TEBSA][HSO 4 ] 99 71 7.3 0 92.7 136 None 99 13 15.7 0 84.3 5.4 a Reaction conditions: 20 mmol fluorobenzy, 60 mmol 68% nitric acid, TSILs 0.4 mmol, 80  8 C. b Selectivity of mono-nitrating. c Based on crude product. d By quantitative GC analysis and qualitative GC/MS. e Ratio of   para / ortho  calculated from srcinal GC data.  Table 3 Result for nitration of chlorobenzene with different TSILs a Entry TSILs Selectivity (%) b Yield (%) c Product isomers (%) d  para / ortho e ortho meta para 1 [TMPSA][HSO 4 ] 99 46 24.1 0.7 75.2 3.12 [TEPSA][HSO 4 ] 99 52 23.6 0.7 75.7 3.23 [TBPSA][HSO 4 ] 99 65 22.2 0.3 77.5 3.54 [TMBSA][HSO 4 ] 99 53 23.4 0.4 76.2 3.15 [TEBSA][HSO 4 ] 99 55 23.2 0.6 76.2 3.36 None 99 10 35.2 1.0 63.8 1.8 a Reaction conditions: 20 mmol chlorobenzy, 60 mmol 68% nitric acid, TSILs 0.4 mmol, 80  8 C. b Selectivity of mono-nitrating. c Based on crude product. d By quantitative GC analysis and qualitative GC/MS. e Ratio of   para / ortho  calculated from srcinal GC data.  Table 4 Result for nitration of bromobenzene with different TSILs a Entry TSILs Selectivity (%) b Yield (%) c Product isomers (%) d  para / ortho e ortho meta para 1 [TMPSA][HSO 4 ] 99 42 25.1 0 74.9 3.02 [TEPSA][HSO 4 ] 99 50 23.9 0 76.1 3.23 [TBPSA][HSO 4 ] 99 55 23.8 0 76.2 3.24 [TMBSA][HSO 4 ] 99 51 25.0 0 75.0 3.05 [TEBSA][HSO 4 ] 99 52 24.8 0 75.2 3.06 None 99 12 47.6 0.4 52.0 1.1 a Reaction conditions: 20 mmol bromobenzy, 60 mmol 68% nitric acid, TSILs 0.4 mmol, 80  8 C. b Selectivity of mono-nitrating. c Based on crude product. d By quantitative Waters 600E/2487 HPLC analysis. e Ratio of   para / ortho  calculated from srcinal HPLC data. D. Fang et al./Applied Catalysis A: General 345 (2008) 158–163 160  As seen from the results of the above tables, although thereactions of fluorobenzene, chlorobenzene, and bromobenzenewere more para-selective in the presence of TSILs than that in theabsence of TSILs, this trend was not apparent for some activatedaromaticcompounds.Inthenitrationofactivatedsubstrates,therewas relatively little difference with or without the TSILs. Forexample,inthenitrationoftoluene,67%yieldcouldbeachievedinthe absence of catalysts with near identical regioselectivity (entry11). This suggested that a correlation exists between the degree of para-selectivity and the ability of electron-withdrawing substi-tuents to induce an electrostatic attraction between substrate andTSILs, causing greater hindrance at positions  ortho  to suchsubstituents [10]. The limit of the system appears to be reachedin the nitration of nitrobenzene. The reaction was sluggish and noproduct could be detected by GC analysis.Generally, the use of these TSILs was advantageous from thepoints of view of the reaction yield and selectivity in comparisonthe reactions without the catalysts. There appear to be severalpossible explanations for the difference between the reaction inthe present of and in the absence of the TSILs. The first is that thereagent and the substrate are both charge neutral, but react toform a charged intermediate. Indeed, the outcomes of bothelectrophilic and nucleophilic substitutions can be predictedusing the Hughes–Ingold rules for ionic liquids. The same rulesshow that the rate of reaction would be increased by using theionic liquids [11].Second, the nitric acid system can be thought to be a molecularnitrating agent. We propose that a self-catalytic reaction occurs asshown (Fig. 2). The compound may be a nitronium ion carrier,which could not dissociate before reacting with aromaticcompounds. The absorbance of nitronium ion could not be foundby Raman spectroscopy in the mixture of nitric acid and TSILs.There is an additional consideration, based on the properties of theionicliquidcationsandanions.TheTSILsusedinthisworkhavestructures similar to those of tetrabutyl ammonium bromide(TBAB) or tetrabutyl ammonium chloride (TBAC), so they could beused as phase transfer catalysts [21]. It should facilitate the bettersolvation of a charged intermediate electrophilic species, NO 2+ -carrier,inthechargedmorehighlypolarTSILs.[TBPSA][HSO 4 ]gavea higher rate enhancement than the others. It is possible that thelipophilicity of the TSILs increases as the length of alkyl chainincreases;thesolubilityvaluesoftheTSILsinaromaticcompoundswouldincreaseasthelipophilicityincreasesandwouldpromptthenitration reaction.It is important that the TSILs contain no aromatic rings, thusthey are superior to imidazolium cation-based ionic liquids,because the imidazolium core itself can be nitrated in competitionin some cases. Within the literature, the nitration of imidazoliumsalts is often reported [7,9]. Therefore, the concentrated nitric acid( > 68%)andtheothernitratingagents(notsuitabletoimidazoliumsalts) might be promising in this procedure, and further work isunderway.  Table 5 Result for nitration of substituted benzenes with different TSILs a Entry R TSILs  T   ( 8 C) Yield (%) b Product isomers (%) c  para / ortho d ortho meta para 1 H [TMPSA][HSO 4 ] 70 62 n/a e 2 H [TEPSA][HSO 4 ] 70 62 n/a e 3 H [TBPSA][HSO 4 ] 70 66 n/a e 4 H [TMBSA][HSO 4 ] 70 63 n/a e 5 H [TEBSA][HSO 4 ] 70 63 n/a e 6 Me [TMPSA][HSO 4 ] 60 82 57.0 5.5 37.5 0.667 Me [TEPSA][HSO 4 ] 60 85 56.0 6.0 38.0 0.688 Me [TBPSA][HSO 4 ] 60 88 55.3 5.5 39.2 0.719 Me [TMBSA][HSO 4 ] 60 84 56.9 5.6 37.5 0.6610 Me [TEBSA][HSO 4 ] 60 85 56.0 5.5 38.5 0.6611 Me None 60 67 58.3 5.5 36.2 0.6212 Et [TMPSA][HSO 4 ] 60 65 44.1 2.0 53.9 1.213 Et [TEPSA][HSO 4 ] 60 67 44.0 2.0 54.0 1.214 Et [TBPSA][HSO 4 ] 60 72 42.6 2.0 55.4 1.315 Et [TMBSA][HSO 4 ] 60 70 44.1 2.2 53.7 1.216 Et [TEBSA][HSO 4 ] 60 75 42.5 2.0 55.5 1.317  t  -Bu [TMPSA][HSO 4 ] 60 82 10.4 6.0 83.6 8.018  t  -Bu [TEPSA][HSO 4 ] 60 82 10.3 6.2 83.5 8.119  t  -Bu [TBPSA][HSO 4 ] 60 85 9.8 6.5 83.7 8.520  t  -Bu [TMBSA][HSO 4 ] 60 82 10.3 6.1 83.6 8.021  t  -Bu [TEBSA][HSO 4 ] 60 86 10.2 6.4 83.4 8.222 NO 2  [TBPSA][HSO 4 ] 80 0 n/a da Reaction conditions: 20 mmol aromatic compounds, 60 mmol 68% nitric acid, TSILs 0.4 mmol. b Based on crude product. c By quantitative GC analysis and qualitative GC/MS. d Ratio of   para / ortho  calculated from srcinal GC data. e No isomer was detected. Fig. 2.  Formation of nitronium ion carrier. D. Fang et al./Applied Catalysis A: General 345 (2008) 158–163  161
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