Nitration of Aromatics in Ionic Liquid Solvents
J . Org. Chem., Vol. 66, No. 1, 2001 39
(a ) Syn th esis of New Im id a zoliu m Ion ic Liqu id s by
Cou n ter ion Exch a n ge. (a 1) 1-Eth yl-3-m eth yl-1H-im id a -
zoliu m Tr iflu or oa ceta te. To a solution of [emim]Cl (1.47 g,
10.0 mmol) in acetonitrile/ether (15 mL, 2:1) was added a
solution of sodium trifluoroacetate (1.36 g, 10.0 mmol) in ether/
acetone (6 mL, 1:1), whereupon a white solid continually
formed. The mixture was allowed to stir for 1 h to ensure
complete reaction. The solution was separated from the
precipitate, and the solvents were removed, leaving 2.24 g
(100%) of a pale yellow oil: IR (film) ν 3080 (s, br), 2260 (br),
1780 (s, br), 1575 (s) cm-1; 1H NMR (acetone-d6, 300.14 MHz)
δ 1.56 (t, 3H, J ) 7.2, CH3CH2), 4.10 (s, 3H, CH3), 4.46 (q, 2H,
J ) 7.2, CH3CH2), 7.86 (s, 1H, CH), 7.95 (s, 1H, CH), 9.77 (s,
1H, N2CH); 13C {1H} NMR (acetone-d6, 75.47 MHz) δ 15.84
(s), 36.62 (s), 45.54 (s), 116.39 (q, J ) 288, CF3), 123.02 (s),
124.61 (s,), 137.89 (s), 158.93 (q, J ) 39, CO).
possibly with minor amounts of AlF3 or AlClxFy) (see also
further). The suspension was stirred for 2 h and extracted with
ether and with CH2Cl2. Work up similar to b1. Yield: 228 mg
(47%) yellow oil. Nitrotoluene isomer distribution: 54.9%
ortho, 3.2% meta, 41.9% para. Extraction of the white pre-
cipitate with CH3CN yielded [emim][BF4], which was identified
by 1H NMR. Formation of [emim][BF4] by counterion exchange
(>95%) was further confirmed in an independent run via 19F
NMR in comparison with an authentic sample, showing that
most of [BF4] ends up in the recovered ionic liquid).
(b3) [em im ]Cl/2 AlCl3 (em im Al2Cl7). Mixing [emim]Cl
(439 mg, 3.0 mmol) and commercial “anhydrous” AlCl3 (800
mg, 6.0 mmol) gave a gray liquid to which [NO2][BF4] (393
mg, 3.0 mmol) was added (a green suspension). Addition of
toluene (6 mL) gave a dark-red solution. After 10 min a white
precipitate and a yellow liquid were formed. The suspension
was stirred for 2 h and extracted with ether and CH2Cl2. Work
up similar to b1. Removal of the solvent left a yellow solid
which was extracted again with ether. Product weight was
131 mg (32%). Nitrotoluene isomer distribution: 53.8% ortho,
3.2% meta, 43.0% para. Extraction of the white precipitate
with CH3CN yielded [emim][BF4] (1H NMR).
(a 2) 1-Eth yl-3-m eth yl-1H-im id a zoliu m p-Tolu en e-
su lfon a te (Tosyla te). To a solution of [emim]Cl (293 mg, 2.0
mmol) in acetonitrile (10 mL) was added silver p-toluene-
sulfonate (594 mg, 2.0 mmol), and the above procedure was
followed: yield 559 mg (99%) of a pale yellow oil; IR (film) ν
3120 (m), 3080 (s), 2960 (m), 1760 (s), 1480 (w), 1440 (m), 1380,
1480 (w) cm-1 1H NMR (acetone-d6, 300.14 MHz) δ 1.40 (t,
;
(b 4) [em im ][BF 4]. Addition of [NO2][BF4] (500 mg, 3.76
mmol) to [emim][BF4] (606 mg, 3.06 mmol) gave an orange
viscous oil. After 1 h toluene (6 mL) was added and the mixture
was stirred overnight. Work up analogous to b1). Product
weight was 21 mg (5%). Nitrotoluene isomer distribution:
3H, J ) 7.2, CH3CH2), 2.29 (s, 3H, CH3), 3.93 (s, 3H, NCH3),
4.27 (q, 2H, J ) 7.2, CH3CH2), 7.13 (d, 2H, J ) 7.9, CCH),
7.67 (d, 2H, J ) 7.9, CCH), 7.73 (s, 1H, NCH), 7.82 (s, 1H,
NCH), 9.48 (s, 1H, N2CH); 13C {1H} NMR (acetone-d6, 75.47
MHz) δ 15.83, 21.22, 36.38, 45.30, 122.95, 124.54, 126.66,
129.05, 138.10, 139.01, 146.95.
1
39.5% ortho, 33.5% meta, 27.0% para. The H NMR spectrum
of the ionic liquid showed [emim][BF4] and two isomeric nitro-
imidazolium salts with nitration taking place at the 4 and 5
position of the imidazole ring. Additional [NO2][BF4] (359 mg,
2.7 mmol) was added to bring about complete [NO2-emim][BF4]
formation which was washed with ether and CH2Cl2 to give a
yellow viscous oil (4-NO2 and 5-NO2-[emim][BF4]): IR (film) ν
3140 (s), 3000 (w), 1740 (m), 1690 (m), 1650 (m), 1590 (s), 1525
(a3) 1-Eth yl-3-m eth yl-1H-im idazoliu m Flu or osu lfon ate.
To a solution of [emim]Cl (146 mg, 1.0 mmol) in acetone/
acetonitrile (1:1; 6 mL) was added potassium fluorosulfate (138
mg, 1.0 mmol), and the same procedure as above was fol-
lowed: yield 210 mg (100%) of a yellow viscous oil; IR (film) ν
3080 (m), 2940 (s), 2840 (m), 1715 (m), 1560 (m), 1375 (w),
1270 (s) cm-1; 1H NMR (acetone-d6, 300.14 MHz) δ 1.56 (t, 3H,
J ) 7.3, CH3CH2), 4.08 (s, 3H, CH3), 4.43 (q, 2H, J ) 7.2,
CH3CH2), 7.77 (s, 1H, CH), 7.84 (s, 1H, CH), 9.73 (s, 1H,
N2CH); 13C {1H} NMR (MeCN-d3, 75.47 MHz) δ 15.60, 36.68,
45.64, 122.82, 124.54, 137.25.
(a 4) 1-Bu tyl-3-m eth yl-1H-im id a zoliu m Tr iflu or oa ce-
ta te. To a solution of [bmim]Cl (3.49 g, 20.0 mmol) in CH3CN
(10 mL) was added a solution of sodium trifluoroacetate (2.72
g, 20.0 mmol) in ether/acetone (1:1; 6 mL), whereby a white
solid was continually formed. The mixture was allowed to stir
for 1 h to ensure complete reaction. The solution was separated
from the precipitate, and the solvent was removed leaving 4.79
g (95%) of a yellow liquid: IR (film) ν 3060 (s), 2960 (s), 2860
(m), 2700-1850 (br), 1760 (m), 1660 (s, br), 1465 (m) cm-1; 1H
NMR (acetone-d6; 300.14 MHz) δ 0.88 (t, 3H, J ) 7.5, CH3-
CH2), 1.31 (m, 2H, CH2), 1.89 (m, 2H,CH2), 4.10 (s, 3H, NCH3),
4.42 (t, 2H, J ) 7.3, NCH2), 7.97 (pt, 1H, J ) 1.7, CH), 8.06
(pt, 1H, J ) 1.7, CH), 9.90 (s, 1H, N2CH); 13C {1H} NMR
(acetone-d6; 75.47 MHz) δ 13.86, 19.86, 36.68, 49.88, 123.34,
124.56, 138.07.
(s), 1460 (m), 1400 (s), 1320 (s) cm-1 1H NMR (acetone-d6;
;
300.14 MHz) δ 1.62 (q, 6H, J ) 7.3, CH3CH2, both isomers),
4.11 (s, 3H, NCH3, isomer 1), 4.27 (s, 3H, NCH3, isomer 2),
4.47 (q, 2H, J ) 7.3, CH3CH2, isomer 2), 4.73 (q, 2H, J ) 7.2,
CH3CH2, isomer 1), 8.78 (s, 1H, CH, isomer 1), 8.85 (s, 1H,
CH, isomer 2), 9.15 (s, 2H, N2CH, isomer 1 + 2).
Isomer ratio (1H NMR: NCH3 group): 2/1 ) 1.1.
(b 5) [NO2-em im ][BF 4]. Addition of [NO2][BF4] (500 mg,
3.76 mmol) to [NO2-emim][BF4] (729 mg, 3.0 mmol) gave a
yellow suspension to which toluene (5 mL) was slowly added.
After the mixture was stirred overnight, the products were
separated from the ionic liquid phase by addition of ether and
CH2Cl2. Removal of solvent gave a dark-green oil which was
rinsed with ether. Removal of ether left 366 mg (71%) of
isomeric nitrotoluene (56.8% ortho, 3.2% meta, 40.0% para).
NMR assay of the recovered [NO2-emim][BF4] salt (a viscous
oil) showed it to be almost pure.
(b6) [em im ][P F 6]. A colorless oil (containing some undis-
solved nitronium salt) was formed when [emim][PF6] (623 mg,
2.43 mmol) and [NO2][BF4] (995 mg, 7.49 mmol) were mixed.
Slow addition of toluene (3 mL) led to a vigorous reaction. After
stirring overnight the mixture was extracted with ether. GC
analysis indicated a 45% yield of nitrotoluene isomers (59.2%
ortho, 2.6% meta, 38.2% para). The ionic liquid phase was a
yellow, highly viscous oil (with greenish precipitate inside).
1H NMR showed imidazolium ring nitration and counterion
exchange.
(b1) Rea ction of [NO2][BF 4] w ith Ion ic Liqu id s a n d
Nitr a tion of Tolu en e. (b1) [em im ]Cl. [NO2][BF4] (414 mg,
3.1 mmol) was added to [emim]Cl (440 mg, 3.0 mmol) at -50
°C, and the solid mixture was allowed to warm to around -16
°C whereby a yellow liquid was formed (minor gas evolution
was noted; NO2Cl!). After the liquid was cooled to -50 °C, cold
toluene (5 mL) was added. The mixture was stirred at ca. -50
°C for 30 min, followed by 2 h at rt. The toluene phase was
separated by extraction with ether and with CH2Cl2. Removal
of solvent left 91 mg (21%) of nitrotoluene isomers. GC
(b7) [em im ][OTf]. A hot liquid was formed when [emim]-
[OTf] (520 mg, 2.0 mmol) and [NO2][BF4] (323 mg, 2.43 mmol)
were mixed. After the liquid was cooled to rt, toluene (5 mL)
was added. Extraction with ether and removal of unreacted
toluene left 63 mg (19%) of nitrotoluene isomers (GC analy-
1
analysis: 58.7% ortho, 2.4% meta, 38.9% para. The H NMR
spectrum of the recovered ionic liquid (in CD3CN) showed
complete counterion exchange (f[emim][BF4]).
1
sis: 59.4% ortho, 3.0% meta, 37.6% para). H NMR assay of
(b2) [em im ]Cl/AlCl3 (em im AlCl4). Mixing [emim]Cl (439
mg, 3.0 mmol) and commercial “anhydrous” AlCl3 (400 mg, 3.0
mmol) (not sublimed) gave a pale yellow oil to which [NO2]-
[BF4] (466 mg, 3.5 mmol) was added, whereby a slow reaction
ensued inside the yellow suspension (gas evolution). After 5
min, toluene (6 mL) was slowly added whereby an orange fluid
and a white precipitate were formed (the latter is mostly AlCl3
the ionic liquid phase showed that metathesis took place to
give [emim][BF4].
(c) Nitr a tion w ith Isoa m yl Nitr a te. Gen er a l P r oce-
d u r e. The aromatic compound (4.0 mmol) was added to a
solution of the ionic liquid (3-7 mmol) and isoamyl nitrate
(266 mg, 2.0 mmol).