Solvent-Free Synthesis of Alkylthiazolium-Based Ionic Liquids and their Use as Catalysts in the Intramolecular Stetter Reaction

Abstract

The first synthesis of alkylthiazolium-based ionic liquids under ‘green chemistry’ conditions is described. Thiazolium salts and triethylamine have been found to catalyze efficiently the ­intramolecular Stetter reaction, giving excellent yields within very short reaction times using solvent-free microwave activation conditions.

Key words

solvent-free reaction - microwave activation - ionic liquids - thiazolium salts - Stetter reaction

References and Notes

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12

General Procedure for the Solvent-Free N-Alkylation of Thiazole under Microwave Irradiation: A mixture of thiazole 1 (85 mg, 1 mmol) and 1-iodoalkane 2 (1.5 mmol) was irradiated (CEM Discover reactor) at 150 ˚C for the appropriate time (see Table  [¹] ). The reaction mixture was brought to room temperature and washed with Et₂O (2 × 10 mL). The crude product was dried under reduced pressure to afford a yellow powder which did not need further purification.
1-Butylthiazolium Iodide M.p. 101 ˚C. ^¹H NMR (300 MHz, CDCl₃): δ = 0.99 (3 H, t, J = 7.5 Hz), 1.40-1.47 (2 H, m), 2.00-2.05 (2 H, m), 4.83 (2 H, t, J = 7.5 Hz), 8.28 (1 H, d, J = 2.6 Hz), 8.34 (1 H, d, J = 3.4 Hz), 10.95 (1 H, s). ^¹³C NMR (75 MHz, CDCl₃): δ = 13.9, 19.8, 32.9, 56.5, 127.5, 137.0, 159.6. IR (KBr): 3434, 3020, 2945, 1989, 1829, 1637, 1543, 1434, 1256, 1144, 952, 861, 639 cm^-¹. HRMS (EI): m/z [M⁺] calcd for C₇H₁₂NS: 142.0685; found: 142.0690.
1-Octylthiazolium Iodide M.p. 27 ˚C. ^¹H NMR (300 MHz, CDCl₃): δ =0.74 (3 H, t, J = 7.2 Hz), 1.21-1.26 (10 H, m), 1.91-1.96 (2 H, m), 4.73 (2 H, t, J = 7.1 Hz), 8.4 (1 H, d, J = 3.4 Hz), 8.54 (1 H, d, J = 3.8 Hz), 10.69 (1 H, s). ^¹³C NMR (90 MHz, CDCl₃): δ = 14.0, 22.5, 26.1, 28.8, 28.9, 30.6, 31.6, 56.3, 127.8, 136.9, 158.6. IR (NaCl): 3445, 3046, 2926, 2855, 1621, 1551, 1463, 1422, 1262, 1154, 907, 833, 749, 634 cm^-¹. HRMS (EI): m/z [M⁺] calcd for C₁₁H₂₀NS: 198.1313; found: 198.1316.
1-Decylthiazolium Iodide M.p. 39 ˚C. ^¹H NMR (360 MHz, CDCl₃): δ = 0.79 (3 H, t, J = 6.1 Hz), 1.17-1.28 (14 H, m), 1.96-1.97 (2 H, m), 4.76 (2 H, t, J = 7.0 Hz), 8.42 (1 H, d, J = 1.8 Hz), 8.54 (1 H, d, J = 3.2 Hz), 10.72 (1 H, s). ^¹³C NMR (90 MHz, CDCl₃): δ = 13.7, 22.1, 25.6, 28.5, 28.7, 28.8, 28.9, 30.2, 31.3, 55.8, 127.5, 136.6, 158.2. IR (KBr): 3435, 3078, 2922, 2852, 1555, 1471, 1370, 1264, 1150, 905, 812, 630 cm^-¹. HRMS (EI): m/z [M⁺] calcd for C₁₃H₂₄NS: 226.1625; found: 226.1629.
1-Dodecylthiazolium Iodide M.p. 92 ˚C. ^¹H NMR (300 MHz, CDCl₃): δ = 0.86 (3 H, t, J = 7.0 Hz), 1.27-1.34 (18 H, m), 1.99-2.04 (2 H, m), 4.80 (2 H, t, J = 7.5 Hz), 8.41 (1 H, d, J = 3.4 Hz), 8.47 (1 H, d, J = 3.6 Hz), 10.79 (1 H, s). ^¹³C NMR (90 MHz, CDCl₃): δ = 14.0, 22.6, 26.1, 28.9, 29.2, 29.3, 29.4, 29.5, 29.6, 30.6, 31.8, 56.3, 127.6, 136.6, 159.0. IR (KBr): 3096, 3079, 2916, 2850, 1556, 1472, 1258, 1149, 908, 811 cm^-¹. HRMS (EI): m/z [M⁺] calcd for C₁₅H₂₈NS: 254.1933; found: 254.1942.
General Procedure for Solvent-Free ‘One-Pot’ Preparation of Alkylthiazolium 4 from 1 under Microwave Irradiation: A mixture of thiazole 1 (85 mg, 1 mmol), 1-iodooctane (360 mg, 1.5 mmol) and alkaline salt MY (1.5 mmol), was irradiated (CEM Discover reactor) at 150 ˚C for 1.3 h (see Table  [²] ). The reaction mixture was brought to room temperature and CH₂Cl₂ (10 mL) were added. After filtration, the solvent was evaporated. The crude product was washed with Et₂O (2 × 10 mL) and dried under reduced pressure to afford a yellow viscous oil which did not need further purification.
1-Octylthiazolium Trifluoromethanesulfonate ^¹H NMR (250 MHz, CDCl₃): δ = 0.84 (3 H, t, J = 4.8 Hz), 1.23-1.32 (10 H, m), 1.96-2.02 (2 H, m), 4.68 (2 H, t, J = 7.3 Hz), 8.29 (1 H, d, J = 3.8 Hz), 8.38 (1 H, d, J = 3.8 Hz), 10.42 (1 H, s). ^¹³C NMR (90 MHz, CDCl₃): δ = 14.0, 22.5, 26.1, 28.8, 28.9, 30.5, 31.6, 56.1, 127.1, 137.0, 158.2. IR (NaCl): 3500, 3084, 2928, 2858, 1633, 1553, 1468, 1258, 1225, 1162, 1030, 914, 836, 757, 639 cm^-¹. HRMS (EI): m/z [M⁺] calcd for C₁₁H₂₀NS: 198.1308; found: 198.1316.
1-Octylthiazolium Hexafluorophosphate
M.p. 27 ˚C. ^¹H NMR (300 MHz, CDCl₃): δ = 0.83 (3 H, t, J = 6.0 Hz), 1.22-1.31 (10 H, m), 1.98-2.02 (2 H, m), 4.73 (2 H, t, J = 7.5 Hz), 8.36 (1 H, d, J = 2.6 Hz), 8.44 (1 H, d, J = 3.8 Hz), 10.54 (1 H, s). ^¹³C NMR (90 MHz, CDCl₃): δ = 13.9, 22.3, 25.8, 28.7, 28.8, 30.5, 31.4, 56.0, 127.6, 136.7, 158.3. IR (NaCl): 3440, 3084, 2929, 2856, 1602, 1553, 1469, 1174, 1012, 905, 750 cm^-¹. HRMS (EI): m/z [M⁺] calcd for C₁₁H₂₀NS: 198.1309; found: 198.1316. 1-Octylthiazolium Tetrafluoroborate ^¹H NMR (300 MHz, CDCl₃): δ = 0.83 (3 H, t, J = 7.1 Hz), 1.28-1.32 (10 H, m), 1.99-2.03 (2 H, m), 4.79 (2 H, t, J = 7.5 Hz), 8.42 (1 H, d, J = 3.4 Hz), 8.52 (1 H, d, J = 3.4 Hz), 10.57 (1 H, s). ^¹³C NMR (90 MHz, CDCl₃): δ = 14.3, 22.8, 26.4, 29.1, 29.2, 30.8, 31.9, 56.6, 127.2, 137.2, 158.6. IR (NaCl): 3445, 3094, 2929, 2858, 1607, 1553, 1469, 1352, 1194, 1058, 915, 740 cm^-¹. HRMS (EI): m/z [M⁺] calcd for C₁₁H₂₀NS: 198.1308; found: 198.1309.
Solvent-Free Microwave-Promoted ‘Two-Step, One-Pot Sequence’ Preparation of Octylthiazolium Bis(trifluoromethanesulfonyl)imide: A mixture of thiazole 1 (85 mg, 1 mmol) and 1-bromooctane (360 mg, 1.5 mmol) was irradiated (CEM Discover reactor) at 150 ˚C for 1.3 h. Lithium bis(tstrifluoromethanesulfonyl)imide (373 mg, 1.5 mmol) was added and the resulting mixture was then placed under MW irradiation for an additional period of 30 min at 100 ˚C. The reaction mixture was brought to room temperature and CH₂Cl₂ (10 mL) was added. After filtration, the solvent was evaporated. The crude product was washed with Et₂O (2 × 10 mL) and dried under reduced pressure to afford a yellow viscous oil (356 mg, 75%) which did not need further purification.
1-Octylthiazolium Bis(trifluoromethanesulfonyl)imide M.p. 30 ˚C. ^¹H NMR (360 MHz, CDCl₃): δ = 0.84 (3 H, t, J = 6.6 Hz), 1.25-1.35 (10 H, m), 1.97-2.05 (2 H, m), 4.73 (2 H, t, J = 7.5 Hz), 8.35 (1 H, d, J = 3.6 Hz), 8.42 (1 H, d, J = 3.6 Hz), 10.52 (1 H, s). ^¹³C NMR (90 MHz, CDCl₃): δ = 13.4, 21.9, 25.5, 28.2, 28.3, 30.1, 31.0, 55.7, 127.2, 136.4, 158.1. IR (NaCl): 3436, 3064, 2926, 2855, 1622, 1553, 1463, 1267, 1154, 906, 834 cm^-¹. HRMS (EI): m/z [M⁺] calcd for C₁₁H₂₀NS: 198.1314; found: 198.1316.


Representative Procedure for Microwave-Assisted Intramolecular Stetter Reaction: A mixture of methyl 4-(2-formylphenoxy)but-2-enoate 6 (Z = H; 0.11 g, 0.5 mmol), Et₃N (51 mg, 0.5 mmol) and octylthiazolium iodide (25 mg, 15% mol) was irradiated at 100 ˚C for 20 minutes. The reaction was quenched with 0.1 N HCl and extracted with CH₂Cl₂. The organic phase was washed with H₂O, dried over MgSO₄, filtered and concentrated in vacuum to afford a pale-orange oil 7 (107 mg, 97% yield).


Methyl 2-(3,4-Dihydro-4-oxo-2 H -chromen-3-yl)acetate ^¹H NMR (250 MHz, CDCl₃): δ = 2.42 (1 H, dd, J = 8.2, 16.8 Hz), 2.92 (1 H, dd, J = 5.1, 17.1 Hz), 3.28-3.36 (1 H, m), 3.71 (3 H, s), 4. 28 (1 H, t, J = 11.7 Hz), 4.58 (1 H, dd, J = 5.4, 11.1 Hz), 6.99 (1 H, dd, J = 8.8, 15.5 Hz), 7.46 (1 H, t, J = 8.2 Hz), 7.86 (1 H, d, J = 7.9 Hz). ^¹³C NMR (90 MHz, CDCl₃): δ = 29.9, 42.4, 51.9, 70.1, 117.7, 121.4, 135.9, 161.4, 171.7, 192.4. IR (NaCl): 3583, 2953, 1738, 1694, 1606, 1580, 1480, 1324, 1301, 1215, 1014, 870, 760 cm^-¹. HRMS (EI): m/z [M + Na⁺] calcd for C₁₂H₁₂O₄Na: 243.0627; found: 243.0633.