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Synlett 2007(1): 0107-0110  
DOI: 10.1055/s-2006-956460
LETTER
© Georg Thieme Verlag Stuttgart · New York

Synthesis of New Lewis Basic Room-Temperature Ionic Liquids by Monoquaternization of 1,4-Diazabicyclo[2.2.2]octane (DABCO)

Angela Wykes, Stephen L. MacNeil*
Department of Chemistry, Wilfrid Laurier University, 75 University Ave. W, Waterloo, ON N2L 3C5, Canada
Fax: +1(519)7460677; e-Mail: smacneil@wlu.ca;
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Publication History

Received 2 October 2006
Publication Date:
20 December 2006 (online)

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Abstract

Sixteen new ionic liquids, including three room temperature ionic liquids, possessing cations derived from monoquaternization of DABCO have been identified through systematic variation of cation alkyl chain length and anion structure. The DABCO salts are obtained in good to excellent yields and show melting point trends, with respect to cation alkyl chain length, consistent with those reported for the imidazolium series of ionic liquids.

Key words

alkyl halides - amines - ionic liquids - green chemistry - heterocycles

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4

A SciFinder® search for ‘ionic liquid’ returns six hits for 1996 and 584 hits for 2005.

7

Based on reported pK a values for protonated DABCO: pK a1 = 2.97; pK a2 = 8.82.

11

Typical Procedure for Synthesis of Halide Salts. A solution of freshly sublimed DABCO (2.01 g, 17.8 mmol) in MeCN (20 mL) was prepared at r.t. under Ar in a flame-dried round-bottomed flask equipped with a reflux condenser. The resulting clear, colorless solution was treated with chloroheptane (1.37 mL, 8.96 mmol), added dropwise via syringe, and the reaction mixture was heated to 80 °C (sand bath temperature) and stirred for 22 h. After being cooled to r.t., the reaction mixture was transferred via cannula into 80 mL of Et2O, yielding a white slurry which eventually separated into two layers. The whole was transferred to a separatory funnel, and the bottom layer was collected, washed repeatedly with Et2O and dried under high vacuum at 60 °C for 16 h to yield 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane chloride (3e, 1.70 g, 77%) as a white glassy solid, mp 110-111.5 °C. 1H NMR (300 MHz, CDCl3): δ = 3.66 (t, J = 7.5 Hz, 6 H), 3.51-3.46 (m, 2 H), 3.24 (t, J = 7.5 Hz, 6 H), 1.75-1.71 (m, 2 H), 1.32-1.22 (m, 8 H), 0.84 (t, J = 6.9 Hz, 3 H). 13C NMR (56.25 MHz, CDCl3): δ = 64.7, 52.6, 45.6, 31.6, 29.0, 26.5, 22.6, 22.3, 14.1. LRMS (ES): positive ion mode, m/z (%) = 211 (100) [C13H27N2]; negative ion mode, m/z (%) = 35 (100), 37 (44).

12

All compounds were characterized by 1H NMR, 13C NMR, 19F NMR [for BF4, PF6 and N(SO2CF3)2 salts] and 31P NMR (for PF6 salts) and LRMS (ES).

14

Typical Procedure for Synthesis of Hexafluorophosphate Salts. A solution of 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane chloride (3e, 0.398 g, 1.61 mmol) in MeCN (1.0 mL), prepared at r.t. under Ar in a flame-dried round-bottomed flask, was transferred via cannula to a slurry of NaPF6 (0.275 g, 1.64 mmol) in MeCN (1.0 mL). A 0.5 mL MeCN rinse was used to ensure complete transfer. A white precipitate formed immediately and the resulting slurry was stirred at r.t. for 20 h. Filtration followed by concentration of the filtrate in vacuo yielded an oily, white solid shown to be contaminated with chloride by a positive AgNO3 test. The crude was partitioned between CH2Cl2 and H2O, and the CH2Cl2 layer was washed repeatedly with H2O until the aqueous layer gave a negative AgNO3 test. The CH2Cl2 extract was dried (Na2SO4), subjected to filtration and concentrated in vacuo. Drying under high vacuum at 60 °C for 5 d yielded 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane hexafluorophosphate (4n, 0.479 g, 83%) as a white solid, mp 151.5-152.5 °C. 1H NMR (300 MHz, CD3CN): δ = 3.17 (t, J = 7.3 Hz, 6 H), 3.10-3.06 (m, 8 H), 1.73-1.62 (m, 2 H), 1.40-1.24 (m, 8 H), 0.89 (t, J = 6.8 Hz, 3 H). 13C NMR (56.25 MHz, CD3CN): δ = 65.6, 53.4, 45.9, 32.3, 29.5, 27.0, 23.3, 22.4, 14.4. 19F NMR (282 MHz, CD3CN): δ = -73.0 (d, J P-F = 707 Hz). 31P NMR (121 MHz, CD3CN): δ = -143.5 (septet, J P-F = 707 Hz). LRMS (ES): positive ion mode, m/z (%) = 211 (100) [C13H27N2]; negative ion mode, m/z (%) = 145 (100) [PF6].

15

Typical Procedure for Synthesis of Bis(trifluoromethylsulfonyl)imide Salts. A solution of LiN(SO2CF3)2 (0.465 g, 1.62 mmol) in MeCN (1.0 mL), prepared at r.t. under Ar in a flame-dried round-bottomed flask, was transferred via cannula to a solution of 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane chloride (3e, 0.396 g, 1.60 mmol) in MeCN (1.5 mL). A 0.5-mL MeCN rinse was used to ensure complete transfer. A white precipitate formed within 5 min and the resulting slurry was stirred at r.t. for 47 h. Filtration followed by concentration of the filtrate in vacuo yielded a colorless liquid shown to be contaminated with chloride by a positive AgNO3 test. The crude was partitioned between CH2Cl2 and H2O, and the CH2Cl2 layer was washed repeatedly with H2O until the aqueous layer gave a negative AgNO3 test. The CH2Cl2 extract was dried (Na2SO4), subjected to filtration and concentrated in vacuo. Drying under high vacuum at 60 °C for 5 d yielded 1-heptyl-4-aza-1-azaniabicyclo[2.2.2]octane bis(trifluoromethylsulfonyl)imide (4o, 0.706 g, 89%) as a colorless liquid, mp 14-15 °C. 1H NMR (300 MHz, CD3CN): δ = 3.19 (t, J = 6.9 Hz, 6 H), 3.12-3.01 (m, 8 H), 1.73-1.63 (m, 2 H), 1.39-1.24 (m, 8 H), 0.89 (t, J = 6.8 Hz, 3 H). 13C NMR (56.25 MHz, CD3CN): δ = 121.0 (q, J = 321 Hz), 65.6 (t, J = 3.3 Hz), 53.41 (t, J = 3.4 Hz), 45.9, 32.3, 29.5, 27.0, 23.3, 22.4, 14.4. 19F NMR (282 MHz, CD3CN): δ = -80.3. LRMS (ES): positive ion mode, m/z (%) = 211 (100) [C13H27N2]; negative ion mode, m/z (%) = 280 (100) [C2F6NS2O4], 282 (9).

16

Yields are unoptimized.

17

All BF4, PF6 and N(SO2CF3)2 salts were deemed halide-free by negative AgNO3 tests.

18

All melting points were determined using samples sealed in evacuated tubes after drying in vacuo at 60 °C overnight and are uncorrected. For salts with melting points <25 °C, the sealed tubes and melting point apparatus were precooled to 5 °C in a cold room prior to melting point determination.