Preparation of Functionalized Imidazolium Salts under Microwave Irradiation

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Preparation of Functionalized Imidazolium Salts under Microwave Irradiation

Shih-Kang Fu a; Shiuh-Tzung Liu a

a Department of Chemistry, National Taiwan University, Taipei, Taiwan Online Publication Date: 01 June 2006

To cite this Article Fu, Shih-Kang and Liu, Shiuh-Tzung(2006)'Preparation of Functionalized Imidazolium Salts under Microwave Irradiation',Synthetic Communications,36:14,2059 — 2067

To link to this Article: DOI: 10.1080/00397910600634464 URL: http://dx.doi.org/10.1080/00397910600634464

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Preparation of Functionalized Imidazolium

Salts under Microwave Irradiation

Shih-Kang Fu and Shiuh-Tzung Liu

Department of Chemistry, National Taiwan University, Taipei, Taiwan

Abstract:A direct alkylation of a substituted imidazole to prepare the corresponding functionalized ionic liquid has been developed in excellent yields under microwave irradiation.

Keywords:Imidazolium, ionic liquid, microwave irridiation

From an environmental perspective, ionic liquids provide an alternative reaction medium for chemical reactions.[1]Among various ionic liquids, simple alkyl substituted 1,3-dialkyl-imidazolium cations associated with weakly coordinat-ing anions are the most popular used in biphasic catalysis because of their distinct physical – chemical properties.[1,2]More recently, developments of the presence of appropriate functional groups as part of ionic liquid molecules to increase the immobilization of catalysts or facilitate the separation process represent a new area of interest.[3]Thus, a few of this type of ionic liquids, such as imidazolium-based ones with amine,[4]amido,[5]hydroxy,[6] carboxy-late,[7]urea,[8]perfluoro-chains,[9]and nitriles,[3]are known.

Conventional heating in refluxing solvents is a typical way to prepare 1,3-dialkylimidazolium halides. Compared to the time-consuming processes of conventional methods, a microwave-assisted chemical reaction has been applied to synthesize ionic liquids.[10,11]In the past two decades, the use of microwave ovens in chemical synthesis and analysis has grown because of the advantages in reducing reaction times, improving yield, and simplifying procedures. The preparation of functionalized ionic liquids using a solvent-free approach has not reported in literature.[4 – 9]

Received in Japan March 7, 2006

Address correspondence to Shiuh-Tzung Liu, Department of Chemistry, National Taiwan University, Taipei 106, Taiwan. E-mail: [email protected]

Copyright # Taylor & Francis Group, LLC ISSN 0039-7911 print/1532-2432 online DOI: 10.1080/00397910600634464

2059

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RESULTS AND DISCUSSION

The microwave-assisted reaction was conducted in a round-bottom flask using a Discovery microwave heating apparatus with a thermal controller (CEM Corp., Matthews, NC). This apparatus allows the reaction flask to be equipped with condenser fitted into a microwave-irradiation chamber (see the experimental section). Typically, a mixture of equal molar amounts (6.5 mmol) of 1-methylimidazole andg-bromobutyonitrile in a 20-mL flask was irradiated at 180 W at 1808C for 30 s. Upon completion of the reaction, the pale yellow, viscous oil was washed with dichloromethane (20 mL 2) to remove the unreacted starting materials. The residue was dried under vacuum overnight, and the obtained material was analytical pure.

All reactions studied in this work are shown in Scheme 1. As a starting point, we studied the microwave-assisted coupling of 1-methylimidazole with 3-triethoxysilylpropyl chloride. Entries 1 – 5 of Table 1 show the results of this optimization. It appeared that the period of irradiation and reaction temperature were critical for the reaction. The best conditions for the preparation of 1 is irradiation at microwave power of 245 W for 40 s. Either a higher or a lower power of irradiation in this reaction gave decompo-sition or poorer yield. The scale for the reaction is irrelevant to the irradiation period employed. Because of the moisture sensitivity of the triethoxy silyl group, the reaction had to be carried out under a dry nitrogen atmosphere; otherwise the condensation leading to Si-O-Si took place, which caused a decrease in the yield of 1. However, it can be carried out under aerobic

Scheme 1. Preparation of functionalized 1,3-disubstituted imidazolium – based ionic liquids.

S.-K. Fu and S.-T. Liu 2060

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Table 1. Results for the reactions of 1-methylimidazole with various alkyl halidesa

Entry

Quantity of

reactant (mmol) Product

Power of

MW (W) T (s)

Temp.

(8C) Atm. Yieldb Physical property Ref.

1 6.5 1 245 40 245 N2 95% Pale yellow liquid [12]

2 60 1 245 40 245 air 49% Pale yellow liquid

3 6.5 1 245 25 140 N2 trace —

4 6.5 1 185 40 145 N2 trace —

5 6.5 1 245 40 185 N2 93% Pale yellow liquid

6 11.3 2 185 25 180 air 90% Pale yellow liquid [13]

8 6.5 4 100 25 175 air 92% Lemon yellow liquid [14]

9 6.5 5 100 35 180 air 90% Colorless liquid [15]

11 4.6 7 180 30 180 air 81% Lemon yellow liquid

12 5.0 8 250 90 185 air 90% Colorless liquid

13 2.9 9 250 90 185 air 85% Colorless liquid

14 1.5 10 30 20 110 air 45% White powder solid

15 1.5 10 100 20 170 air dec. —

16 1.5 10 5 20 170 air 0 —

a

Reaction conditions: an equal molar amount of 1-methylimidazole and RX in flask without using any solvent. b

Isolated yield and characterized by1H NMR.

Imidazolium

Salts

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conditions for other substrates. All products were characterized by1H NMR and elemental analysis. The optimum reaction conditions for reactions of 1-methylimidazole with various alkyl halides are summarized in Table 1.

As shown in Table 1, various alkyl halides were able to react with imidazole to give the corresponding ionic liquid in excellent yields (entries 6 – 13). For example, the production of 10 was achieved by adopting 30 W at 1108C, whereas the compound 8 was with 250 W at 1858C. For the acetyl-ated glucose derivative, the reaction provided a moderate yield by using 30 W of microwave at 1108C. Attempts to improve the yield by varying the microwave powers and temperatures failed, indicating an intrinsic property of this substrate.

All functionalized imidazolium halides except 10 are in liquid phase at room temperature. Compound 10 is in a solid form, which becomes liquid at temperatures greater than 90 – 938C. As for solubility, those containing 22OH of polyether groups are water- and methanol-soluble materials. In fact, compound 8 is completely miscible with water. Of course, the halide anions of this series of ionic liquids also increase their solubility toward water. Similarly, the alkylation of 1-propyl-2-methylimidazole proceeded smoothly to give the corresponding ionic liquid in excellent yield (Scheme 2). The reaction conditions employed are comparable to the preparation of the related 1,3-disubstituted imidazolium-based ionic liquids (Table 2).

Reaction of 1-bromo-3-chloropropane with 1-methylimidazole under microwave irradiation yielded a mixture of 14, 15,[17]and 16[18](Scheme 3). It shows that the poor selectivity of nucleophilic substitution of imidazole toward alkyl bromide versus chloride under microwave-assisted conditions even with a lower power irradiation. Even under mild conditions with lower power (30 W), this reaction still provided a mixture of products. However, the reaction proceeded poorly at the temperature lower than 1008C. All these compounds were not able to be obtained in pure form because of the anion.

3-Aminopropyl chloride was also tested as a substrate for the alkylation. A mixture of an equalmolar amount of 3-aminopropyl chloride hydrochloride

Scheme 2. Preparation of functionalized 1,2,3-trisubstituted imidazolium – based ionic liquids.

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salt, 1-methylimidazole, and triethylamine was irradiated at 1708C with the power setting of 180 W for 90 s to provide the desired ionic liquid 17 in 65% yield accompanied by the formation of azetidine Eq. (1). This outcome shows that the alkylation takes place with imidazole nitrogen superior to the amine site. However, the use of excess 3-aminopropyl chloride salt can provide 17 in a high yield.

Table 2. Results for preparation of 1,2,3-trisubstituted imidazolium – based ionic liquids

Product Power (W) T (s) Temp. (8C) Yield (%) Physical property

11 235 40 185 75 Pale yellow liquid 12 180 50 180 89 Pale yellow liquid 13 105 30 180 85 Pale yellow liquid

Scheme 3. Reaction of imidazole with 3-chloro-1-bromopropane.

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In summary, we have demonstrated a much faster and more efficient method for the preparation of functionalized imidazolium-based ionic liquid in high yields. Work on these functionalized ionic liquids continues and is expected to illustrate interesting new results.

EXPERIMENTAL

Nuclear magnetic resonance spectra were recorded in CDCl3or acetone-d6on

either a Bruker AM-300 or Avance 400 spectrometer. Chemical shifts are given in parts per million relative to Me4S for 1H and 13C NMR. Infrared

spectra were measured on a Nicolet Magna-IR 550 spectrometer (series-II) as KBr pallets, unless otherwise noted. Microwave irradiation was carried out in a Discovery microwave heating apparatus with temperature controller (CEM Corp., Matthews, NC). The reaction temperature reported was based on this readout. Chemicals and solvents were of analytical grade and used as received unless otherwise stated. a-Bromotetraacetyl-L-glucose was prepared from the earlier reported procedures.[19]

General Procedure

An equal molar amount of 1-methylimidazole and alkyl halides in a 20-mL flask was set up as shown in Fig. 1 and was irradiated at a certain level of power with programming the temperature for a certain period of reaction time. Upon completion of the reaction, the pale yellow, viscous oil was washed with dichloromethane (20 mL 2) to remove the unreacted starting materials and was dried under vacuum overnight. Spectroscopic data of the obtained compounds 1 – 6 are essentially identical to the literature reported.[3,12 – 16]

Figure 1. Microwave-assisted heating apparatus.

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Data Compound 7.ymaxKBr/cm 21 3164, 2952, 2866, 1726 (yC55O), 1653 (yC55C), 1573;1H NMR (300 MHz, CDCl3):d9.99 (s, 1H), 7.53 (s, 1H), 7.47 (s, 1H), 4.20 (t, 2H, J ¼ 7.31 Hz), 3.93 (s, 3H), 3.91 (q, 2H, J ¼ 7.05 Hz), 2.16 (t, J ¼ 7.29 Hz), 1.80 (m, 2H), 1.50 (m, 2H), 1.24 (m, 2H), 1.07 (t, 3H, J ¼ 7.18 Hz). MS found [M-Br]þ 239 for C13H23N2O2; anal. calcd. for

C13H23BrN2O2: C, 48.91; H, 7.26; N, 8.78. Found for C, 45.45; H, 7.15;

N, 9.46.

Compound 8.ymaxKBr/cm213552 (yO22H), 3164, 2872, 1653 (yC55C), 1587,

1116;1H NMR (300 MHz, D2O):d 8.75 (s, 1H), 7.52 (s, 1H), 7.45 (s, 1H),

4.39 (t, 2H, J ¼ 4.6 Hz), 3.91 (s, 3H), 3.89 (m, 2H), 3.72 (m, 6H), 3.60 (m, 2H). MS found [M-Cl]þ 215 for C10H19N2O3; anal. calcd. for

C10H19ClN2O3: C, 47.9; H, 7.64; N, 11.19. Found: C, 47.21; H, 7.83; N, 11.37. Compound 9. ymax KBr/cm 21 3164, 2873, 1646, 1566, 1103; 1H NMR (300 MHz, CDCl3):d 9.81 (s, 1H), 7.66 (s, 1H), 7.47 (s, 1H), 3.97 (s, 3H), 3.79 (t, 2H, J ¼ 4.58 Hz), 3.52 (m, 8H), 3.25 (s, 3H). MS found [M-Br]þ 229 for C11H21N2O3; anal. calcd. for C11H21BrN2O3: C, 42.73; H, 6.85; N,

9.06. Found: C, 42.59; H, 7.19; N, 8.98. Compound 10. Mp: 90 – 938C; ymax KBr/cm21 3171, 2939, 2879, 1726 (yC55O), 1653, 1587, 1089; 1H NMR (300 MHz, CDCl3): d 10.05 (s, 1H), 7.38 (s, 1H), 7.31 (s, 1H), 5.15 (t, 1H, J ¼ 9.6 Hz), 5.02 (t, 1H, J ¼ 9.9 Hz), 4.85 (t, 1H, J ¼ 8.0 Hz), 4.50 (d, 1H, J ¼ 8.0 Hz), 4.39 (m, 2H), 4.20 (m, 1H), 4.06 (s, 3H), 3.71 (m, 2H), 3.62 (m, 2H), 2.20 (m, 2H), 2.05 (s, 1H), 2.02 (s, 1H), 2.00 (s, 1H), 1.95 (s, 1H). MS found [M-Br]þ472 for C20H29N2O10; anal. calcd. for C20H29BrN2O10: C, 44.70; H, 5.44; N, 5.21.

Found: C, 44.26; H, 5.45; N, 4.99. Compound 11.ymaxKBr/cm 21 3144, 2980, 2933, 2849, 1640, 1586, 1076; 1_{H NMR (300 MHz, CDCl} 3): d 7.66 (s, 1H), 7.51 (s, 1H), 4.27 (t, 2H, J ¼ 7.3 Hz), 4.08 (t, 6H, J ¼ 7.3 Hz), 3.92 (q, 2H, J ¼ 7.2 Hz), 2.65 (s, 3H), 2.34 (t, 2H, J ¼ 7.2 Hz), 1.97 (m, 2H), 1.72 (m, 2H), 0.82 (t, 9H, J ¼ 7.3 Hz). MS found [M-Cl]þ 287 for C13H27N2O3Si; anal. calcd. for

C13H27ClN2O3Si: C, 48.35; H, 8.43; N, 8.68. Found: C, 47.21; H, 7.83; N, 11.37. Compound 12.ymaxKBr/cm213403 (yO22H), 3144, 2952, 2886, 1653, 1593, 1129;1H NMR (300 MHz, CDCl3):d7.81 (s, 1H), 7.49 (s, 1H), 4.43 (t, 2H, J ¼ 4.5 Hz), 4.10 (t, 2H, J ¼ 7.2 Hz), 3.81 (t, 2H, J ¼ 4.5 Hz), 3.52 (m, 8H), 2.65 (s, 3H), 1.75 (m, 2H), 0.88 (t, 3H, J ¼ 5.2 Hz). MS found [M-Cl]þ 257 for C13H25ClN2O3; anal. calcd. for C13H25ClN2O3: C, 53.33;

H, 8.61; N, 9.57. Found: C, 53.18; H, 8.98; N, 9.44.

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Compound 13. ymax KBr/cm21 3139, 2974, 2883, 1727, 1640, 1587; 1_{H NMR (300 MHz, CDCl}

3): d 7.65 (s, 1H), 7.54 (s, 1H), 4.19 (t, 2H,

J ¼ 11.2 Hz), 3.43 (s, 9H), 2.72 (s, 3H), 1.79 (m, 2H), 0.84 (t, 3H, J ¼ 18.4 Hz), 0.54 (t, 3H, J ¼ 12.6 Hz). MS found [M-Br]þ 239 for C13H23BrN2O2; anal. calcd. for C13H23BrN2O2: C, 48.91; H, 7.26; N, 8.78.

Found for C, 48.96; H, 7.93; N, 8.96.

Compound 17.ymaxKBr/cm213455 (yN22H), 3157, 2963, 2753, 1634, 1579; 1_{H NMR (300 MHz, D}

2O): d 8.65 (s, 1H), 7.39 (s, 1H), 7.32 (s, 1H), 4.19

(t, 2 H, J ¼ 7.3 Hz), 3.75 (s, 3H), 2.93 (m, 2H), 2.16 (m, 2H). MS found [M – Cl]þ140 for C7H14N3; anal. calcd. for C7H14ClN3: C, 47.86; H, 8.03.

Found for C, 47.66; H, 7.83.

ACKNOWLEDGMENT

This research was supported by the National Science Council (NSC93-2113-M002-004), Taiwan, China.

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