Brønsted acid ionic liquid [Et3NH][HSO4] as efficient and reusable catalyst
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halogen-free ionic liquid [Et3NH][HSO4] as an efficient and reusable catalyst
(Scheme 1).
Experimental
All chemicals were commercial products. Melting points were determined on a
melting point apparatus and were uncorrected. FT-IR spectra were obtained on a
Nexus 470 spectrophotometer. 1H NMR spectra were recorded on a Bruker Avance
III 400 with TMS as internal standard. Mass spectra were recorded on Agilent 6120
using electrospray ionization technique. [Et3NH][HSO4] was prepared according to
the literature method [38].
General procedure for the synthesis of 2,4,5-triaryl-1H-imidazoles
A mixture of aromatic aldehyde (10 mmol), benzil (10 mmol), ammonium acetate
(24 mmol), and [Et3NH][HSO4] (10 mol %) was stirred at 130 °C for the
appropriate time indicated in Table 2 (below). The progress of the reaction was
monitored by thin-layer chromatography (ethyl acetate/petroleum ether = 1/5 as
eluent). After completion of the reaction, the reaction mixture was cooled to room
temperature and diluted with water. The separated solid was suction filtered, washed
with water, dried and recrystallized from ethanol to get the corresponding 2,4,5-
triaryl-1H-imidazoles. All products obtained are known compounds and were
identified by comparing their physical and spectra data with the reported ones.
Spectroscopic data of some representative compounds are given below.
2,4,5-Triphenyl-1H-imidazole (Table 2, Entry 1): IR (KBr, cm-1): 3,424, 3,041,
2,972, 1,601, 1,490, 1,461, 1,126, 1,072, 769, 733, 697. 1H NMR (400 MHz,
CDCl3/DMSO-d6) d: 7.29–7.63 (m, 13H), 8.12 (d, J = 7.6 Hz, 2H). 13C NMR
(100 MHz, CDCl3/DMSO-d6) d: 125.9, 127.6, 128.4, 128.7, 128.8, 129.1, 131.1,
146.3. MS (ESI): m/z = 297.2 [M ? H]?.
2-(4-Methoxyphenyl)-4,5-diphenyl-1H-imidazole (Table 2, Entry 3): IR (KBr,
cm-1): 3,418, 3,056, 2,963, 1,611, 1,496, 1,446, 1,290, 1,248, 1,177, 1,130, 1,032,
1
969, 832, 768, 730, 697. H NMR (400 MHz, CDCl3/DMSO-d6) d: 3.85 (s, 3H),
6.97 (d, J = 8.4 Hz, 2H), 7.27–7.76 (m, 10H), 8.06 (d, J = 8.4 Hz, 2H). 13C NMR
(100 MHz, CDCl3/DMSO-d6) d: 55.7, 114.5, 123.9, 127.4, 128.4, 128.8, 134.0,
146.4, 160.1. MS (ESI): m/z = 327.2 [M ? H]?.
2-(2-Chlorophenyl)-4,5-diphenyl-1H-imidazole (Table 2, Entry 4): IR (KBr,
cm-1): 3,426, 3,061, 2,955, 1,601, 1,478, 1,446, 1,126, 1,034, 971, 763, 735, 695.
1H NMR (400 MHz, CDCl3) d: 7.31–7.58 (m, 13H), 8.45 (d, J = 7.6 Hz, 1H). 13
C
NMR (100 MHz, CDCl3) d: 127.4, 127.5, 127.8, 128.1, 128.6, 129.5, 129.8, 130.4,
130.9, 143.2. MS (ESI): m/z = 331.1 [M ? H]?.
2-(4-Fluorophenyl)-4,5-diphenyl-1H-imidazole (Table 2, Entry 5): IR (KBr,
cm-1): 3,425, 3,061, 2,955, 1,602, 1,479, 1,446, 1,119, 1,073, 972, 873, 765, 735,
700. 1H NMR (400 MHz, CDCl3) d: 7.31–7.58 (m, 12H), 8.45 (d, J = 7.6 Hz, 2H).
13C NMR (100 MHz, CDCl3/DMSO-d6) d: 127.3, 127.4, 127.8, 128.3, 128.5, 129.5,
130.0, 130.3, 131.0, 143.2. MS (ESI): m/z = 315.1 [M ? H]?.
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