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Synthesis of dibe

Department of Chemical Sciences, Tezpu

784028, India. E-mail: ruli@tezu.ernet.in

† Electronic supplementary information (Ethe 1HNMR and 13C NMR spectr1,8-dioxo-decahydroacrdine (4) and dibenspectra. See DOI: 10.1039/c4ra07323a

Cite this: RSC Adv., 2014, 4, 41287

Received 19th July 2014Accepted 27th August 2014

DOI: 10.1039/c4ra07323a

www.rsc.org/advances

This journal is © The Royal Society of C

nzoxanthene and acridinederivatives catalyzed by 1,3-disulfonic acidimidazolium carboxylate ionic liquids†

Arup Kumar Dutta, Pinky Gogoi and Ruli Borah*

New members of 1,3-disulfonic acid imidazolium carboxylate ionic

liquids [DSIM][X] (where X ¼ [CH3COO]�, [CCl3COO]�, [CF3COO]�)

were prepared and characterized by 1H NMR,13C NMR, FT-IR, TGA,UV-

vis and elemental analysis. The more acidic ILs [DISM][CCl3COO] and

[DSIM][CF3COO] were efficiently utilized as recyclable catalysts for the

preparation of 14H-dibenzo[a,j]xanthene and 1,8-dioxo-decahy-

droacridine derivatives in short times under solvent-free conditions at

80–100 �C with excellent yields. The above two ILs could be effec-

tively utilized as catalysts for the synthesis of 1,8-dioxo-decahy-

droacridine in water at the same temperature.

Introduction

The great capability of ionic liquids to replace volatile organicsolvents offers a new and environmentally benign approachtoward modern chemical processes.1 The implementation oftask-specic ionic liquids with special functions furtherenhances the versatility of ionic liquids for those chemicalreactions where both reagents and medium are coupled.2 Byvirtue of the incorporated functional groups, these unique saltscan act not only act as solvents but also as catalysts in an arrayof synthetic, separation and electrochemical applications.3

Their solubility can also be tuned readily depending on thenature of the cations and anions so that they can phase separatefrom organic as well as aqueous media.4 The preparation andapplication of this dual nature of acidic ionic liquids in organicreactions overcomes the common problems of traditionalmolecular solvents and acid catalysts. These ILs are non-am-mable, thermally stable and exhibit negligible vapour pressureand are recyclable. The higher potential of 1,3-disulfonic acidimidazolium ILs in organic synthesis has encouraged us todevelop new members of the [DSIM][X] series where X ¼

r University, Napaam, Tezpur, Assam,

SI) available: This documents containsal data of ILs (1, 2 and 3),zoxanthene (5)with the so copies of

hemistry 2014

[CH3COO]�, [CCl3COO]

�, [CF3COO]�.5,6 In addition, we have

also tried to observe the efficiency of [DSIM][X] ionic liquids forthe one pot synthesis of 1,8-dioxo-decahydroacridine 4 anddibenzoxanthene 5 derivatives in environmentally benignconditions. These heterocyclic compounds have wide applica-tions in medicinal chemistry and material science.7,8 Thetraditional Brønsted or Lewis acid catalyzed synthesis of 1,8-dioxo-decahydroacridine derivatives has been performed inorganic solvents such as HOAc,9a CH3CN9b and DMF.9c Theseconventional acids are oen noxious, corrosive and non-recy-clable, despite their high catalytic activity.9d The use of ionicliquids simplied the reaction conditions in terms of productselectivity, reaction time, recycling of the catalyst and isolationof the product.10 For the synthesis of dibenzoxanthene deriva-tives, a few reports have also utilized ionic liquids as catalyst ormedium.11 This work concerns the synthesis, characterizationand applications of 1,3-disulfonic acid imidazolium carboxylateionic liquids and their applications in the synthesis of bothclasses of heterocyclic compounds.

Results and discussion

In continuation of our work12 on the development of environ-mentally benign routes for organic synthesis, we synthesisedthree new members of 1,3-disulfonic acid imidazoliumcarboxylate ionic liquids [DSIM][X] where X ¼ [CH3COO]

�,[CCl3COO]

�, [CF3COO]� from the reactions of 1,3-disulfonic

acid imidazolium chloride [DSIM][Cl] and three differentcarboxylic acids (AcOH, TCA and TFA) at room temperaturestirring5a (Scheme 1).The FT-IR spectra (Fig. 1) of these ILsshowed three strong absorption bands at 1178–1192, 1048–1054and 586–587 cm�1 corresponding to S–O asymmetric andsymmetric stretching and bending vibration of –SO3H groups.The peaks around 875–878 cm�1 expressed the N–S stretchingvibration. The carboxylate anions gave strong asymmetricbands around 1709–1751 cm�1 in the same region of imidazo-lium ions. The –OH groups of the ionic liquids indicated broadand strong peaks at 3000–3660 cm�1. In 1H NMR spectra the

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Scheme 1 Synthesis of 1,3-disulfonic acid imidazolium carboxylate ILs.

Fig. 1 FT-IR spectra of 1, 2 and 3 ionic liquids.

Table 1 Values of Hammett function (H�) and pKa for ionic liquids

Entry IL Amaxa [I]% [IH]% H� pKa

1 Blank 1.558 100.0 0 — —2 1 1.07 73.9 26.1 1.44 2.23 2 0.71 45.6 54.4 0.91 1.64 3 0.448 28.8 71.2 0.60 1.2

a Indicator: 4-nitroaniline.

Fig. 2 Hammett plot of ionic liquids using basic indicator4-nitroaniline.

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characteristic acidic protons of two –SO3H groups of the ILsappeared in 14.2 and 11.3–13.4 ppm respectively. The presenceof carbonyl signals at 158.6–172 ppm in 13C NMR spectraconrmed the existence of carboxylate anions in these ionicliquids. The structures were further supported by the elementalanalysis data.

The acidity of the ILs was determined on an UV-Visiblespectrophotometer using 4-nitroaniline as basic indicator byfollowing the Hammett equation that already reported in liter-ature.13 The absorbance of the basic indicator [I] were deter-mined in ionic liquid solutions which decreases with increasingthe acidity of ionic liquids. The protonated form [HI]+ of theindicator never appeared because of lowmolar absorptivity. TheHammett function H� for each ionic liquids was calculatedusing eqn (1) by measuring the absorption differences [I]/[IH]+.

H� ¼ pK(I)aq. + log[I]/[IH]+ (1)

where pK(I)aq. is the pKa value of the basic indicator in aqueoussolution. The relative acidity of the ILs can be obtained bydetermining the values of H� (Table 1). The typical procedureinvolved the mixing of indicator 4-nitroaniline (5 mg L�1, pKa ¼0.99) and ionic liquid (5 mmol L�1) in ethanol with equalconcentration. The maximum absorbance observed at 382 nmin ethanol. The order of acidities of these ionic liquids werefound from the Hammett plot (Fig. 2) in the decreasing order asfollows: [DSIM][CF3COO] > [DSIM][CCl3COO] > [DSIM]-[CH3COO] which was supported by the corresponding pKa

values14 included in Table 1.

41288 | RSC Adv., 2014, 4, 41287–41291

The thermo gravimetric analysis of the three ionic liquidsshowed (Fig. 3) slight weight loss below 100 �C due to absorbedmoisture. All these ionic liquids were thermally stable in thetemperature range of 260–281 �C.

Aer calculating the acidity of ILs, we optimized the catalyticactivity for the model synthesis of 1,8-dioxo-decahydroacridine4a and dibenzoxanthene 5a in various conditions (Table 2). As areaction medium the three ionic liquids didn't yield anyproduct at ambient temperature for 4 hours. But at 100 �C theabove reactions produced excellent yields within 10–15 minutesin [DSIM][CF3COO]and [DSIM][CCl3COO] ILs as reactionmedium (Table 2, entries 2, 4, 10, 12) while [DSIM][CH3COO]ionic liquid showed less amount of product (Table 2, entries 1,9). The best catalytic activity observed in solvent-free mediumwith 25 mol% of 3 at 90 �C for the synthesis of dibenzox-anthene(Table 2, entry 14) whereas it was 80 �C for the 1,8-dioxo-decahydroacridine(Table 2, entry 6). Using 25 mol% ofionic liquid 2, the product formation was decreased at 80 �C ascompared to 100 �C under solvent-free condition (Table 2,entries 3, 7). Both ILs were effective for the formation of 4a inwater at 90 �C with the optimized amount in short time (Table 2,entry 8).

Interestingly, the synthesis of 4a was possible only up to 48–60% yields in water at 100 �C along with unreacted 2-naphtholand aldehydes (Table 2, entry 16). It may be due to heteroge-neous phases of 2-naphthol in water medium. The standardizedconditions were extended with various types of aldehydes usingILs 2 and 3 as catalysts for the synthesis of other derivatives of

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Fig. 3 The TG/DTG diagrams of ILs.

Table 2 Ionic liquids catalysed optimization of the reaction conditionsfor the synthesis of 4a and 5a

Entry IL (mol%) T (�C) Time (min) % of yielda

For acridine derivatives 4a1 1(100) 100 25 652 2(100/50) 100 10/10 98/983 2(25/10) 100 10/45 96/654 3(100/50) 100 10/10 100/1005 3(25/10) 100 10/45 100/826 3(25) 90/80/70 10/10/25 100/100/707 2(25) 80/90 20 708 2(25)/3(25) 90 25 93b/98b

For dibenzoxanthene derivatives 5a9 1(100) 100 30 7010 2(100/50) 100 15/30 95/9311 2(25/10) 100 20/45 92/4412 3(100/50) 100 15/15 96/9813 3(25/10) 100 15/45 97/7014 3(25) 90/80/70 15/25/40 97/80/6515 2(25) 80/90 45 5516 2(25)/3(25) 100 1 h 48b/60b

a Isolated yields. b Using 1 mL of water.

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these heterocyclic compounds. All the results were included inTables 3 and 4. The results were observed to be satisfactoryirrespective of the nature of aldehydes except with cinna-maldehyde and furaldehyde in both cases.

The reusability of the ionic liquids 2 and 3 were expressed byusing bar diagram (Fig. 4) for three consecutive runs for thesynthesis of 1,8-dioxo-decahydroacridine 4a and dibenzox-anthene 5a under the optimized reaction conditions in solvent-free medium. Recyclability study indicated the high catalyticactivity of the two ILs even aer the third run with slightincreasing reaction time.

Conclusions

In summary, a new group of 1,3 disulfonic acid imidazoliumcarboxylate ionic liquids was prepared and characterized bydifferent analytical techniques. The acidity order of these ionicliquids in Hammett plot was identical with their catalyticactivity for the one pot synthesis of 1,8-dioxodecahydroacridineand dibenzoxanthene derivatives in solvent-free medium ingood to excellent yields.

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Experimental sectionExperimental procedure

(a) Preparation of 1,3 di-sulfonic acid imidazoliumcarboxylate ionic liquid [DSIM][X] where X ¼ [CH3COO]

�,[CCl3COO]

�, [CF3COO]�. All these ionic liquids were prepared

by mixing of equal amount of (5 mmol) [DSIM]Cl ionic liquidsand carboxylic acids (CH3COOH, CCl3COOH, CF3COOH) in drydichloromethane (12 mL) in a 50 mL two necked round bottomask at room temperature within 30 min. The HCl gas outletwas connected to a vacuum system through water and an alkalitrap. The mixture was continued to stirring for one hour tocomplete the elimination of produced HCl gas and then dilutedwith 10 mL of dry CH2Cl2. The CH2Cl2 layer was decanted andwashed the residue by more of it (3 � 10 mL). The ionic liquidresidue was dried under vacuum to get [DSIM][X] as reddishcolored viscous liquids with 98–100% yields.

(b) General procedure for the synthesis of 1,8-dioxo-deca-hydroacrdine derivatives 4. A mixture of dimedone (2 mmol),aldehydes (1 mmol), ammonium chloride (1 mmol) and ionicliquid (25 mol%) was heated in an oil bath for the speciedtemperature in absence of any solvent (or in 1 mL of water).Aer completion of the reaction as monitored by TLC, theproduct was extracted from the ionic liquid phases using dry

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Table 3 Synthesis of 1,8-dioxo-decahydroacridines using ionic liquids 2 and 3 as catalysts

Entry R

Time (min) % of yielda,b

Mp (�C)Solvent free In water Solvent free(A/B) In water(C/D)

1 a 10 25 96/100 4a 93/98 4a 287 (ref. 15)2 b 10 30 96/100 4b 92/96 4b 286 (ref. 16)3 c 15 35 94/98 4c 90/94 4c 279 (ref. 15)4 d 15 35 96/97 4d 93/94 4d 300 (ref. 15)5 e 15 35 95/99 4e 90/96 4e 280 (ref. 15)6 f 20 40 83/85 4f 80/83 4f 1567 g 15 40 95/97 4g 92/93 4g 2048 h 15 35 94/97 4h 90/94 4h 215

a Method A: using 25 mol% of ionic liquid 2 at 100 �C; B: using 25 mol% of ionic liquid 3 at 80 �C; C: using 25 mol% of ionic liquids 2 at 90 �C; D:using 25 mol% of ionic liquid 3 at 90 �C. b All the products were characterized by 1NMR, 13CNMR, FT-IR and CHN analysis.

Table 4 Synthesis of dibenzoxanthene derivatives 5 using ionic liquids2and 3 as catalysts

Entry R Time (min) (A/B)a % of yieldb Mp (�C)

1 a 20/15 92/97 5a 186 (ref. 17)2 b 25/20 88/94 5b 312 (ref. 17)3 c 20/15 90/93 5c 228 (ref. 17)4 d 25/20 85/94 5d 288 (ref. 17)5 e 20/15 92/97 5e 204 (ref. 17)6 f 30/35 78/85 5f 1357 g 20/15 90/96 5g 196–1978 h 25/20 91/97 5h 181 (ref. 18)

a Method A: using 25 mol% of ionic liquid 2 at 100 �C; B: using 25 mol%of ionic liquid 3 at 90 �C. b All the products were characterized by 1NMR,13CNMR, FT-IR and CHN analysis.

Fig. 4 Recycling of the ILs during the synthesis of 4a and 5a.

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dichloromethane (3 � 3 mL) as solvent. The product was iso-lated through distillation of dichloromethane solution underreduced pressure and the ionic liquid medium was again usedfor next cycle of reaction. The solid product was puried byrecrystallization in aqueous ethanol (15%) to get analyticallypure product.

(C) General procedure for the synthesis of dibenzox-anthene derivatives 5. A mixture of 2-naphthol (2 mmol), alde-hydes (1 mmol) and ionic liquid (25 mol%) was heated in an oil

41290 | RSC Adv., 2014, 4, 41287–41291

bath for the specied temperature in absence of any solvent.Aer completion of the reaction as monitored by TLC, theproduct was extracted from the ionic liquid phases using drydichloromethane (3 � 3 mL) as solvent. The product was iso-lated through distillation of dichloromethane solution underreduced pressure and the ionic liquid medium was again usedfor next cycle of reaction. The crude product, thus isolated wassubjected to further purication by recrystallization in aqueousethanol (15%) to get analytically pure product.

Acknowledgements

The authors are thankful to Sophisticated Analytical Instru-mentation Centre, Tezpur University, for analyses of varioussamples for this work and also to CSIR, New Delhi, India forgranting a research project no. 02(0067)/12/EMR-II to R.B.

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