ORIGINAL PAPER

Calixarene ionic liquids: excellent phase transfer catalystsfor nucleophilic substitution reaction in water

Fafu Yang • Hongyu Guo • Ziyu Jiao •

Congcong Li • Jinqi Ye

Received: 25 June 2011 / Accepted: 14 September 2011 / Published online: 3 January 2012

� Iranian Chemical Society 2011

Abstract The first examples of calixarene ionic liquids 3

and 6 with 3D-shaped cavities were obtained in high yields

by reacting calix[4]arene or thiacalix[4]arene with 1,6-

dibromohexane and then refluxing in 1-methylimidazole.

The experiments of phase transfer catalysis in water sug-

gested that they possessed excellent catalytic properties of

aromatic nucleophilic substitution reaction and benzyl

nucleophilic substitution. The optimized yields of product

in catalytic reaction were as high as approximate 97%

under mild reaction conditions. The cavities of calixarene

skeleton played the crucial roles in catalysis and the stable

cone conformation was favorable for catalysis.

Keywords Calix[4]arene � Ionic liquid � Phase transfer

catalysis � Nucleophilic substitution

Introduction

Ionic liquids (ILs) are organic salts with melting point below

100 �C and possess unique properties, such as nonflamma-

bility, negligible vapor pressure, environmental compati-

bility, reusability, high thermal stability and the ability to

dissolve many organic and inorganic materials [1–3]. Sig-

nificant progress has been made regarding their applications,

especially for catalysis in all kinds of organic reaction [3–7].

Recently, ‘‘task-specific’’ ionic liquids have been developed

by tethering functional groups with their cations or anions

[8–11]. However, a survey of the literatures indicated that

almost all of ionic liquids were two dimensional molecules

with one cation and one anion up to now. In theory, if the

structural units of ionic liquids were introduced in a three

dimensional molecular platform with cavity, the synergetic

effect of multiple ionic functional groups and the three

dimensional cavity would result in novel properties, such as

special catalytic abilities for organic reaction in water.

Calixarenes, as the third generation supramolecules after

crown ether and cyclodextrin, possess tuneable 3D-shaped

cavities and exhibit versatile complexation abilities [12].

Many researches were involved in design and syntheses of

different kinds of supramolecules with special properties

using calixarenes as building blocks [13–16]. Lately, some

of calixarene derivatives exhibited good phase transfer

catalysts for some organic reaction in organic solution [17–

19]. On the opinion of the structural analysis, calixarene

ionic liquids might exhibit excellent catalytic abilities for

organic reaction in water due to their good solubility in

water based on ions groups, and good complexation abili-

ties for organic molecules based on the cavities of calixa-

rene skeleton. However, no study on synthesis and catalytic

property of calixarene ionic liquids was presented so far.

Herein, we report the first synthesis of calixarene ionic

liquids and their excellent phase transfer catalysts for

nucleophilic substitution reaction in water.

Experimental

Apparatus and reagents

All chemical reagents were obtained from commercial

suppliers and used without further purification. The other

F. Yang (&) � H. Guo � Z. Jiao � C. Li � J. Ye

College of Chemistry and Materials, Fujian Normal University,

Fuzhou 350007, People’s Republic of China

e-mail: yangfafu@fjnu.edu.cn

F. Yang

Fujian Key Laboratory of Polymer Materials,

Fuzhou 350007, People’s Republic of China

123

J IRAN CHEM SOC (2012) 9:327–332

DOI 10.1007/s13738-011-0027-6

organic solvents and inorganic reagents were purified

according to standard anhydrous methods before use. TLC

analysis was performed using pre-coated glass plates.

Column chromatography was performed using silica gel

(200–300 mesh). IR spectra were recorded on a Perkin-

Elmer PE-983 infrared spectrometer as KBr pellets with

absorption in cm-1. 1H NMR spectra were recorded in

CDCl3 on a Bruker-ARX 600 instrument at 30 �C.

Chemical shifts are reported in ppm, using tetramethylsil-

ane (TMS) as internal standard. ESI–MS spectra were

obtained from DECAX-30000 LCQ Deca XP mass spec-

trometer. Elemental analyses were performed at Vario EL

III Elemental Analyzer.

Syntheses of compound 2

Compound 1 (0.65 g, 1 mmol) and 1, 6-dibromohexane

(1.22 g, 5 mmol) with K2CO3 (0.97 g, 7 mmol) were

refluxed in 50 mL anhydrous acetonitrile for overnight

purged with N2. After reaction, the mixture was filtered and

evaporated to dryness by rota-vapor. Then the mixture was

further purified by column chromatography (SiO2 100–200

mesh, petroleum ether/CH2Cl2 (4:1, V/V) as eluant). The

compound 2 was obtained as white solid in yield of 65%.

Compound 2: m.p. = 216–218 �C; 1H NMR (600 MHz,

CDCl3) dppm: 0.99 (s, 18 H, C(CH3)3), 1.30 (s, 18 H,

C(CH3)3), 1.52 * 2.08 (m, 16 H, CH2), 3.31 (d, 4 H,

J = 12.6 Hz, ArCH2Ar), 3.47 (t, 4 H, J = 6.6 Hz, OCH2),

3.99 (t, 4 H, J = 6.6 Hz, BrCH2), 4.28 (d, 4 H, J = 12.6 Hz,

ArCH2Ar), 6.83 (s, 4 H, ArH), 7.04 (s, 4 H, ArH), 7.62(s, 2 H,

OH); MS m/z (%): 975.22 (M?, 100). Anal. calcd for

C56H78O4Br2: C 69.05, H 8.07; found C 69.01, H 8.02%.

Synthesis of calix[4]arene ionic liquid 3

The mixture of compound 2 (0.49 g, 0.05 mmol) and

1-methylimidazole (2 mL) were stirred at 110 �C for 24 h

purged with N2. After reaction, the excess solvent was

evaporated under vacuum line at 110 �C. Then the residue

was treated in desiccator under reduced pressure for a week

to dryness. The ionic liquid of calix[4]-1,3-methylimida-

zolium bromide 3 was obtained in almost 100% yield as

soft straw yellow solid. Compound 3: m.p. = 95–98 �C;1H

NMR (600 MHz, CDCl3) dppm: 0.93 (s, 18 H, C(CH3)3),

1.29 (s, 18 H, C(CH3)3), 1.49 * 2.08 (m, 16 H, CH2), 3.31

(d, 4 H, J = 13.2 Hz, ArCH2Ar), 3.91 (t, 4 H, J = 6.6 Hz,

OCH2), 4.06 (s, 6 H, NCH3), 4.21 (d, 4 H, J = 13.2 Hz,

ArCH2Ar), 4.51 (t, 4 H, J = 6.6 Hz, NCH2), 6.77 (s, 4 H,

ArH), 7.07 (s, 4 H, ArH), 7.41(s, 2 H, ImH), 7.45 (s, 2 H,

ImH), 7.71(s, 2H, OH), 10.29 (s, 2H, ImH); IR/cm-1:

3410, 2956, 2864, 1569, 1482, 1361, 1298, 1201, 1116;

Anal. calcd for C64H90O4N4Br2: C 67.54, H 7.97; found C

67.52, H 7.90%.

Synthesis of compound 5

Compound 4 (0.72 g, 1 mmol) and 1,6-dibromohexane

(2.44 g, 10 mmol) with K2CO3 (0.97 g, 7 mmol) were

refluxed in 50 mL anhydrous acetonitrile for overnight

purged with N2. After reaction, the mixture was filtered and

evaporated to dryness by rota-vapor. Then the mixture was

further purified by recrystallization in CH3OH/CHCl3. The

compound 5 was obtained as white solid in yield of 80%.

Compound 5: m.p. = 189–192 �C; 1H NMR (600 MHz,

CDCl3) dppm: 1.02 (m, 8 H, CH2), 1.15 (m, 8 H, CH2),

1.28 (s, 36 H, C(CH3)3), 1.32 (m, 8 H, CH2), 1.80 (m, 8 H,

CH2), 3.38 (t, 8 H, J = 6.6 Hz, OCH2), 3.82 (t, 8 H,

J = 7.8 Hz, BrCH2), 7.30 (s, 8 H, ArH); MS m/z (%):

1371.64 (M?, 100). Anal. calcd for C64H92O4Br4S4: C

55.98, H 6.75; found C 55.91, H 6.71%.

Synthesis of calix[4]arene ionic liquid 6

The mixture of compound 5 (0.65 g, 0.5 mmol) and

1-methylimidazole (2 mL) were stirred at 110 �C for 24 h

purged with N2. After reaction, the excess solvent was

evaporated under vacuum line at 110 �C. Then the residue

was treated in desiccator under reduced pressure for a week

to dryness. The ionic liquid of thiacalix[4]-1,3-methylimi-

dazolium bromide 6 was obtained in almost 100% yield as

soft straw yellow solid. Compound 6: m.p. = 84–87 �C;1H NMR (600 MHz, CDCl3) dppm: 1.01 * 2.05 (m, 68 H,

CH2 and C(CH3)3), 3.67 * 4.45 (m, 28 H, NCH3, OCH2

and NCH2), 6.91 * 7.33 (m, 16 H, ArH and ImH),

10.19 * 10.59 (m, 4 H, ImH); IR/cm-1: 3,427, 3,065,

2,945, 2,863, 1,631, 1,569, 1,445, 1,361, 1,266, 1,168,

1,089, 998, 876, 763, 649; Anal. calcd for

C80H116N8S4Br4O6: C 56.46, H 6.87; found C 56.42, H

6.81%.

Synthesis of compound 8

Compound 7 (0.75 g, 5 mmol) and 1,6-dibromohexane

(4.88 g, 20 mmol) with K2CO3 (2.76 g, 20 mmol) were

refluxed in 50 mL anhydrous acetonitrile for overnight

purged with N2. After reaction, the mixture was filtered and

evaporated to dryness by rota-vapor. Then the mixture was

further purified by column chromatography (SiO2 100–200

mesh, petroleum ether/CH2Cl2 (5:1, V/V) as eluant). The

compound 8 was obtained as white solid in yield of 84%.

Compound 8: m.p. = 76–78 �C; 1H NMR (600 MHz,

CDCl3) dppm: 1.17 (m, 4 H, CH2), 1.25 (s, 9 H, C(CH3)3),

1.55 (m, 2 H, CH2), 1.63(m, 2 H, CH2), 3.53 (t, 2 H,

J = 6.6 Hz, OCH2), 3.78 (t, 2 H, J = 6.6 Hz, BrCH2),

7.06 (d, 2 H, J = 9.6 Hz, ArH), 7.39 (d, 2 H, J = 9.6 Hz,

ArH); MS m/z (%): 313.02 (M?, 100). Anal. calcd for

C16H25OBr: C 61.41, H 8.05; found C 61.33, H 8.01%.

328 J IRAN CHEM SOC (2012) 9:327–332

123

Synthesis of ionic liquid 9

The mixture of compound 8 (0.32 g, 1 mmol) and

1-methylimidazole (2 mL) were stirred at 110 �C for 10 h

purged with N2. After reaction, the excess solvent was

evaporated under vacuum line at 110 �C. Then the residue

was treated in desiccator under reduced pressure for a week

to dryness. The ionic liquid 9 was obtained in almost 100%

yield as soft white solid. Compound 9: m.p. = 41–43 �C;1H NMR (600 MHz, CDCl3) dppm: 1.26 (s, 9 H, C(CH3)3),

1.42 (m, 2 H, CH2), 1.37 (m, 2 H, CH2), 1.63 (m, 2 H,

CH2), 1.78 (m, 2 H, CH2), 3.78 (t, 2 H, J = 6.6 Hz,

OCH2), 4.15 (s, 3 H, CH3), 4.23 (t, 2 H, J = 6.6 Hz,

NCH2), 7.07 (d, 2 H, J = 9.6 Hz, ArH), 7.38 (d, 2 H,

J = 9.6 Hz, ArH); 7.53 (s, 2 H, ImH), 10.17 (s, 1H, ImH);

MS m/z (%): 394.89 (M?, 100). Anal. calcd for

C20H31ON2Br: C 60.82, H 7.89; found C 60.75, H 7.84%.

The procedures of phase transfer catalysis

of nucleophilic substitution reaction

Typical nucleophilic substitution reaction was conducted

by mixing reactant and corresponding inorganic salts in

10 mL of distilled water. Corresponding amount of catalyst

was added and the reaction mixture was stirred (800 rpm)

for a stipulated time period at corresponding temperature

(Tables 1, 2). After reaction, the flask was allowed to cool

to room temperature. 10 mL of cyclohexane was added to

the reaction mixture and induced phase separation. Then

the cyclohexane layer was separated and subjected to GC–

MS analysis (Varian 450-GC/240-MS) with the internal

standard method to calculate the amount of products.

Results and discussion

Syntheses, structures and conformations

The synthetic route was showed in Scheme 1. By refluxing

calix[4]arene 1 with excess 1,6-dibromohexane in K2CO3/

MeCN system, 1, 3-bromohexoxyl-calix[4]arene 2 was

prepared in 65% yield after column chromatography.

Subsequently, compound 2 was reacted with 1-methylim-

idazole at 110 �C for 24 h. After reaction, the excess sol-

vent was evaporated under vacuum line and the residue

was treated in desiccator under reduced pressure for a week

to dryness. The ionic liquid of calix[4]-1,3-methylimida-

zolium bromide 3 was obtained in almost 100% yield as

soft straw yellow solid. We had tried to synthesize the

tetra-substituted bromohexoxyl-calix[4]arene by reacting

calix[4]arene 1 with far excess 1,6-dibromohexane and

prolonging reaction time, but compound 2 was still the

main product in all kinds of reaction condition.

In order to study the influence of structure and confor-

mation on the properties, thiacalix[4]arene, bridged by S

groups with more flexible conformation reversal, was also

reacted to prepare thiacalix[4]arene ionic liquid. Due to the

different reaction activities of calix[4]arene and thiaca-

lix[4]arene, no di-bromohexoxyl-substituted thiacalix[4]

arene but tetra-bromohexoxyl- substituted thiacalix[4]arene

5 was obtained as main product under same reaction condi-

tion. The yield of compound 5 was as high as 80% yield after

recrystallization in CH3OH/CHCl3. Then by refluxing

compound 5 in 1-methylimidazole solution, the ionic

liquid of thiacalix[4]-tetra-methylimidazolium bromide 6

was obtained in almost 100% yield as soft straw yellow

solid. To the best of our knowledge, compounds 3 and 6

were the first examples of calixarene-based ionic liquids.

Also, ionic liquid 4-t-butyl phenoxylimidazolium bromide

Table 1 Catalytic investigation of compounds 3, 6 and 9 in aromatic

nucleophilic reaction

Cl

KF+Catalyst

F

NO2 NO2

Water

Entrya Catalyst Run

time (h)

Temperature

(oC)

Catalyst

mol%

Yield

(%)b

1 9 12 30 3.0 4.5

2 9 12 50 3.0 7.4

3 9 12 80 3.0 8.3

4 9 12 80 5.0 9.2

5 3 12 30 2.0 68.8

6 3 12 30 3.0 88.9

7 3 12 50 3.0 96.6

8 3 12 50 5.0 94.2

9 3 12 80 3.0 96.7

10 3 6 50 3.0 90.4

11 3 18 50 3.0 97.2

12 6 12 30 3.0 77.3

13 6 12 50 3.0 82.5

14 6 12 80 3.0 84.3

15 6 18 50 3.0 83.4

16 6 12 50 5.0 85.6

17 3 12 50 3.0 84.3

18 3 12 50 3.0 72.5

19 3 12 50 3.0 63.8

20 None 12 50 None 0.5

a Reaction condition: p-nitrochlorobenzene (5 mmol), potassium

fluoride (20 mmol for entries 1–16, 15 mmol for entry 17, 10 mmol

for entry 18, 5 mmol for entry 19), water as solvent (10 mL)b Yields were determined by GC–MS analysis using internal standard

method

J IRAN CHEM SOC (2012) 9:327–332 329

123

9, which was the monomer of compounds 3 and 6, was

synthesized as contrastive compound for property study

(Scheme 2).

All new compounds were well characterized by melting

point, elemental analyses, IR, 1H NMR, ESI–MS spectra,

etc. The melting points of ionic liquids 3, 6 and 9 were

below 100 �C. The ESI–MS spectra of compounds 2 and 5

showed clearly molecular ion peak at 975.22 and 1,371.64,

respectively. The 1H NMR spectra of compounds 2 and 3

showed two singlets (1:1) for the tert-butyl groups, one pair

of doublets (1:1) for the methylene bridges of the

calix[4]arene skeleton and two singlets (1:1) for the ArH

groups, which certainly indicated they adopted cone con-

formations (the 1H NMR of ionic liquid 3 was showed in

Scheme 3). The 1H NMR spectrum of compound 5 was in

accordance with the reported 1,3-alternate conformation

[20, 21]. However, the overlapped and complicated 1H

NMR spectrum of thiacalix[4]arene ionic liquid 6 might

indicate that it had unstable or mixed conformation due to

Table 2 Catalytic investigation of compounds 3, 6 and 9 in benzyl

nucleophilic substitution

KCN+Catalyst

Cl CN

R R

Water

Entry R Catalyst Yield (%)a

1 H 9 7.8

2 H 3 96.6

3 H 6 84.5

4 CH3 9 3.8

5 CH3 3 95.1

6 CH3 6 73.4

7 C(CH3)3 9 0.6

8 C(CH3)3 3 94.7

9 C(CH3)3 6 62.4

Reaction condition: benzyl chloride or its derivatives (5 mmol),

potassium cyanide (20 mmol), catalyst (3.0 mol%), water as solvent

(10 mL), reaction time (12 h), reaction temperature (50 �C)a Yields were determined by GC–MS analysis using internal standard

method

HOHO HOOH

BrBr

K2CO3, MeCNHOHO OO

Br Br

N NHOHO OO

N N

N NBr-

Br-12

3

+ +

+

SOO

OOS SS

OH* S *

4

BrBr

K2CO3, MeCN

Br Br

BrBr

N NS

OO

OOS SS

N

N

N

N

N

N

N

N

Br-

Br- Br-

4

56

Br- +

+ +

Scheme 1 The synthesis of

calixarene ionic liquids 3 and 6

+

OH

BrBr

K2CO3, MeCN

N N

7 8 9

O

Br

N

N

Br-

O

Scheme 2 The synthesis of contrastive ionic liquid 9

330 J IRAN CHEM SOC (2012) 9:327–332

123

thiacalix[4]arene skeleton bridged by S groups with more

flexible conformation reversal.

The phase transfer catalysis of nucleophilic substitution

reaction in water

The experiments of solubility indicated that calixarene ionic

liquids 3 and 6 possessed good soluble capabilities in water.

Thus, we are interested in study on their abilities of phase

transfer catalysis in water, which have the comprehensive

prospects in green synthesis. Considering that aromatic

fluorine compounds have important application in insecti-

cidal and refined chemicals [22], the replacement of aro-

matic Cl by F in nucleophilic substitution reaction of p-

nitrochlorobenzene were chose as reaction system to eval-

uate the phase transfer catalytic abilities of compounds 3, 6

and 9. The experiment results were showed in Table 1. It

could be seen that the yield of product was only 0.5% when

no catalyst was added and yields were less than 10% when

the reaction were performed with contrastive ionic liquid 9

as catalyst (entries 1–4 and 20). However, when calix[4]-

arene ionic liquid 3 or 6 was used as catalyst, the yields

were increased greatly. The yields of product were as high

as 96.7% and 97.2% in experiment of entries 9 and 11,

respectively. Also, the results of experiments under differ-

ent temperatures and times (entries 5–16) suggested that

50 �C was the appropriate temperature and 3.0 mol% was

ideal amount for catalysts 3 and 6. When reducing the molar

ratio of reactants from 3:1 to 1:1(entries 17–19), the yields

decreased greatly, which implied the positive and reverse

reaction were catalyzed synchronously. Comparing with the

structures of contrastive ionic liquid 9 and calix[4]arene

ionic liquids 3 and 6, it could be concluded that the high

catalytic abilities of 3 and 6 were attributed to the effect of

cavities of calix[4]arene skeleton. The proposal catalytic

mechanism was that p-nitrochlorobenzene was binded in

the cavities of calix[4]arene skeleton, which improved the

solubility of p-nitrochlorobenzene in water greatly and

promoted the nucleophilic substitution reaction in water (as

showed in Fig. 1). On the other hand, it could be seen that

the catalytic abilities of compound 3 were higher than that

of compound 6, which indicated that the stable cone con-

formation of compound 3 was more favorable for binding p-

nitrochlorobenzene and then promoting catalysis more

efficiently than the unstable and mixed conformation of

compound 6.

Also, the replacement of aliphatic Cl by nitrile group in

benzyl nucleophilic reaction of benzyl chloride or its

derivatives were studied using compounds 3, 6 and 9 as

phase transfer catalyst. Referring to the optimized reaction

condition in Table 1, these catalytic reactions were carried

out under temperature of 50 �C, reaction time of 12 h, 4:1

molar ratio of reactants and 3.0 mol% amount of catalyst.

The experiment results were showed in Table 2, which were

Scheme 3 the 1H NMR of

calix[4]arene ionic liquid 3

NO2

ClF-N+ N+

calixarene skeleton

Fig. 1 The proposal catalytic mechanismfor calixarene ionic liquid 3

J IRAN CHEM SOC (2012) 9:327–332 331

123

similar to the results in Table 1. The yields of products were

greatly increased from 10 to 90% when catalysts of calix-

arene ionic liquids 3 and 6 were added and ionic liquid 3

showed higher catalytic abilities than that of ionic liquid 6.

Moreover, when the R groups on p-position of benzyl

chloride is CH3 or C(CH3)3, the catalytic abilities of ionic

liquid 9 decreased distinctly to zero and the catalytic abil-

ities of ionic liquid 6 decreased markedly from 84.5% to

62.4%. However, the catalytic abilities of compound 3 were

almost not influenced by R groups and the yields of product

were still more than 90%. These catalytic results also could

be explained by that stable cone conformation of ionic

liquid 3 binded benzyl chloride or its derivatives effectively

and maintain high catalytic abilities, but the unstable and

mixed conformation of ionic liquid 6 was not favorable for

binding benzyl derivatives with big R group on p-position.

All these catalytic results in Tables 1 and 2 indicated that

calixarene ionic liquids were excellent phase transfer cat-

alyst for nucleophilic substitution reaction and the stable

cone conformation was more favorable for catalysis. It was

worthy of noting that these effective catalysis of nucleo-

philic substitution in water exhibits potential applied pros-

pects in green chemistry.

Conclusions

By reacting calix[4]arene or thiacalix[4]arene with 1,6-

dibromohexane, and then with 1-methylimidazole, the first

examples of calixrene ionic liquids 3 and 6 were obtained

in high yields. Compound 3 and 6 possessed stable cone

conformation and unstable conformation, respectively. The

experiments of phase transfer catalysis in water showed

they possessed excellent catalytic properties for aromatic

nucleophilic substitution reaction and benzyl nucleophilic

substitution. The excellent catalytic abilities were attrib-

uted to the synergetic effect of methylimidazole cations

and cavities of calixarene skeleton. The stable cone con-

formation of calixarene skeleton was more favorable for

catalysis than the unstable conformation. The yields of

product in catalytic reaction were as high as approximate

97% in optimized condition.

Acknowledgments Financial support from the National Natural

Science Foundation of China (No. 20402002), Fujian Natural Science

Foundation of China (No. 2011J01031), Program for Excellent young

researchers in University of Fujian province (JA10056) and Project of

Fujian Provincial Department of Education (JA11044) were greatly

acknowledged.

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