Organic Reactions in Ionic liquids: N-Alkylation of Phthalimide and Several Nitrogen Heterocycles

Article abstract of DOI:10.1055/s-2003-44383

N-Alkylation of heterocyclic compounds bearing an acidic hydrogen atom attached to nitrogen with alkyl halides is accomplished in ionic liquids ([bmim]BF₄= 1-butyl-3-methylimidazolium te trafluoroborate, [bmim]PF₆ = 1-butyl-3-methylimidazolium hexafluorophosphate, [buPy]BF₄ = butylpyridiniurri tetrafluoroborate) in the presence of potassium hydroxide as a base. In this manner, phthalimide, indole, benzimidazole, succinimide can be successfully alkylated. The procedure is convenient, efficient, and generally affords the N-alkylated product exclusively.

Full text of DOI:10.1055/s-2003-44383

208
PAPER
Organic Reactions in Ionic liquids: N-Alkylation of Phthalimide and Several
Nitrogen Heterocycles
N
-Alkylation of
h
N
itrogen
H
a
nn Ionic Liquig
ds -Gao Le,a,b,c Zhen-Chu Chen,*a,b Yi Hu,b Qin-Guo Zhengd
a
Ningbo Institute of Technology, Zhejiang University, Ningbo, 315100, P. R. China
b
Department of Chemistry, Zhejiang University (xi xi campus), Hangzhou, 310028, P. R. China
E-mail: zhenchuc@mail.hz.zj.cn
c
Department of Applied Chemistry, East China Institute of Technology, Fuzhou, 344000, P. R. China
Pharmaceutical Science Reseach Institute, Aston University, Aston Triangle, Birmingham B4 TET, UK
d
Received 28 July 2003; revised 3 October 2003
ed us to investigate the N-alkylation of phthalimide and
several nitrogen heterocycles in ionic liquids.
Abstract: N-Alkylation of heterocyclic compounds bearing an
acidic hydrogen atom attached to nitrogen with alkyl halides is ac-
complished in ionic liquids ([bmim]BF4 = 1-butyl-3-methylimida-
zolium tetrafluoroborate, [bmim]PF6 = 1-butyl-3-methylimida-
zolium hexafluorophosphate, [buPy]BF4 = butylpyridinium tetra-
fluoroborate) in the presence of potassium hydroxide as a base. In
this manner, phthalimide, indole, benzimidazole, succinimide can
be successfully alkylated. The procedure is convenient, efficient,
and generally affords the N-alkylated product exclusively.
First, we examined the efficacy of different ionic liquids
in the N-ethylation of phthalimide with ethyl bromide
(Scheme 1). We found that the reaction of phthalimide
with ethyl bromide occurred easily in the presence of po-
tassium hydroxide at 40 °C in ionic liquids. The results are
summarized in Table 1. The experimental results show
the reactions of phthalimide with ethyl bromide in ionic
liquids [bmim]BF4, [bmim]PF6, or [buPy]BF4 are all effi-
cient and give excellent results.
Key words: ionic liquids, N-alkylation, phthalimide, heterocycle
Imide derivatives constitute an important class of organic
compounds which have been widely used in biology,1,2
synthetic3,4 and polymer chemistry.5 Despite their wide
applicability, available routes for their synthesis are limit-
ed. Most methods involve direct N-alkylation of phthal-
imide, succinimide, and maleimide with an alkylating
agent. They are generally accomplished by the treatment
of these compounds with an appropriate base such as
NaH,6 KH, Et3N,7 BuLi,8 followed by reaction of the re-
sulting salt with an alkylating agent in various solvents,
for example, acetone, DMSO,9 DMF,10 THF,11 and
HMPT,12 etc. The N-alkylation can be also accomplished
via a phase-transfer catalyst process in which crown ether,
or polyethylene glycols or their dialkyl ether is employed
as the catalyst and t-BuOK or KOH as the base.13 Recent-
ly, Hayat14 et al. reported on the N-alkylation of nitrogen
heterocycles with alkyl halide using CsF/Celite in aceto-
nitrile. However, some of these methods are limited by
harsh reaction conditions, low yields, and use of toxic sol-
vents or catalysts. Therefore, the development of a mild,
efficient, environmentally more benign method of N-
Scheme 1
Table 1 N-Ethylation of Phthalimide with Ethyl Bromide in Differ-
ent Ionic Liquids
Entrya
Ionic Liquid
[bmim]BF4
[bmim]PF6
[buPy]BF4
Yield (%)b
1
2
3
96
95
97
a All reactions were run with phthalimide (3 mmol), EtBr (6 mmol)
and KOH (6 mmol) in ionic liquid (2 mL) at 40 °C for 5 h.
b Isolated yield based on phthalimide.
alkylation of heterocyclic compounds bearing an acidic Subsequently, the scope of the reaction of various of alkyl
hydrogen atom attached to nitrogen is still a major chal- halides with phthalimide in ionic liquids [bmim]BF4, or
lenge in organic synthesis.
[bmim]PF6 and [buPy]BF4 was investigated. The results
are summarized in Table 2. The reaction was found to be
general and applicable to primary alkyl iodide or bromide
and activated chloride in various ionic liquids. A lower
yield was obtained in the case of a secondary alkyl halide
as alkylating agent (entry 6). The products were character-
In recent years, room temperature ionic liquids (RTIL) are
attracting increasing interest as environmentally benign
reaction media for synthetic organic chemistry.15 There
are many reports of great improvements in the reaction
yields and rates when using ionic liquids16 which prompt-
1
ized by H NMR, IR spectra and melting points, which
were consistent with the literature data.
SYNTHESIS 2004, No. 2, pp 0208–0212
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Advanced online publication: 15.12.2003
DOI: 10.1055/s-2003-44383; Art ID: F06103SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
N-Alkylation of Nitrogen Hetrocycles in Ionic Liquids
209
Table 2 N-Alkylation of Phthalimide Using KOH in Ionic Liquida
Entry
1
Alkyl Halide
MeI
Ionic Liquid
[bmim]BF4
[bmim]BF4
[bmim]PF6
[buPy]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]PF6
[bmim]BF4
[bmim]BF4
[buPy]BF4
[bmim]BF4
Reaction Conditions Product
Yieldb (%)
91
40 °C, 5 h
40 °C, 5 h
40 °C, 5 h
40 °C, 5 h
60 °C, 5 h
50 °C, 5 h
80 °C, 5 h
80 °C, 5 h
80 °C, 2 h
80 °C, 2 h
80 °C, 2 h
80 °C, 4 h
N-methylphthalimide (1a)
N-ethylphthalimide (1b)
N-ethylphthalimide (1b)
N-ethylphthalimide (1b)
N-propylphthalimide (1c)
N-isopropylphthalimide (1d)
N-butylphthalimide (1e)
N-butylphthalimide (1e)
N-benzylphthalimide (1f)
N-benzylphthalimide (1f)
N-benzylphthalimide (1f)
ethyl N-phthalimidoacetate (1g)
2
EtBr
96
95
97
93
78
94
92
97
96
95
92
3
EtBr
4
EtBr
5
PrBr
6
i-PrBr
BuBr
7
8
BuBr
9
BzBr
10
11
12
BzCl
BzCl
ClCH2CO2Et
a All reactions were run with phthalimide (3 mmol), alkyl halide (6 mmol) and KOH (6 mmol) in ionic liquid (2 mL).
b Isolated yield based on phthalimide.
The ionic liquids can be typically recovered by extracting completed within only 2 hours at 80 °C in ionic liquid
out the product first and drying at vacuum followed by fil- [bmim]BF4 and gave an excellent yield (97%). Also, the
tration of the suspension to remove residual potassium hy- synthesis of N-ethylindole by traditional procedure13 us-
droxide and the formed potassium halide. The recovered ing not only the strong base potassium tert-butoxide, but
ionic liquids can be reused with no appreciable decrease also the 18-crown-6 as phase-transfer catalyst, needed 18
in yield. The representative results are summarized in hours in diethyl ether to give N-ethylindole in 88%. How-
Table 3.
ever, the synthesis was performed only in 3 hours in ionic
liquid [bmim]BF4 at room temperature and gave 94%
yield. Some representative data are summarized in
Table 5.
Table 3 Recycling of [Bmim]BF4 in Ethylation of Phthalimide
Cycle
Product
Yield (%)a
In conclusion, we have demonstrated that the N-alkylation
of phthalimide and several nitrogen heterocycles can ef-
fectively be performed in the ionic liquids [bmim]BF4, or
[bmim]PF6, or [buPy]BF4 which provides a simple and ef-
ficient method for the synthesis of the N-substituted ph-
thalimides and some nitrogen heterocycles.
1
2
3
N-ethylphthalimide
N-ethylphthalimide
N-ethylphthalimide
96
94
95
a Isolated yield based on phthalimide.
Melting points were determined on digital melting point apparatus
and are not corrected. IR spectra were recorded on a Vector 22
(Bruker) spectrophotometer, NMR spectra were recorded on
Avance DMX 400 (Bruker) spectrometer. The ionic liquids
[bmim][BF4], [bmim]PF6 and [buPy]BF4 were synthesized accord-
ing to reported procedures.18 The other materials are commercially
available and were used without further purification.
On the basis of these results, we exploited our method to
indole,17 benzimidazole, and succinimide. We found that
the N-alkylation of these nitrogen heterocycles can also be
performed with high yield in ionic liquids [bmim]BF4, or
[bmim]PF6 and [buPy]BF4. The results are summarized in
Table 4.
N-Alkylation of Hetercyclic Compounds; General Procedure
KOH (0.336 g, 6 mmol) and heterocycle (3 mmol) were added to
The present method has many obvious advantages, com-
pared to those reported in the literature, including being ionic liquid (2 mL), and the mixture was stirred magnetically for 5
min. Then, the appropriate alkylating agent (6 mmol) was intro-
environmentally more benign, the generality, simplicity
duced in a single portion, the stirring was continued for 2–5 h (for
of the methodology, the ease of product isolation, the
reaction conditions, see Tables 2 and 4). After the reaction was
higher yield and potential for recycling of ionic liquids.
complete, the product was extracted with Et2O (3 × 10 mL). The
For example, the reaction of N-benzylation of phthalimide
combined ethereal phases were evaporated under reduced pressure
by recent reported method14 using CsF/Celite as base
needed reflux for two days in acetonitrile to give N-ben-
zylphthalimide in 86% yield, but the same reaction was
and the crude product was purified by recrystallization or prepara-
tive TLC (silica gel). After isolation of the product, the remainder
of the ionic liquid was recovered by drying at vacuum first and fil-
Synthesis 2004, No. 2, 208–212 © Thieme Stuttgart · New York

210
Z.-G. Le et al.
PAPER
Table 4 N-Alkylation of Nitrogen Heterocycles Using KOH in Ionic Liquida
Entry
1
Substrate
indole
Alkyl Halide
MeI
Ionic Liquid
[bmim]BF4
[bmim]BF4
[bmim]PF6
[bmim]BF4
[bmim]BF4
[buPy]BF4
[bmim]BF4
[bmim]PF6
[buPy]BF4
[bmim]BF4
[bmim]PF6
[buPy]BF4
[bmim]BF4
[bmim]PF6
[buPy]BF4
[bmim]BF4
[bmim]BF4
Reaction Conditions Product
N-methylindole (2a)
Yieldb (%)b
20 °C, 3 h
20 °C, 3 h
20 °C, 3 h
20 °C, 3 h
20 °C, 3 h
20 °C, 3 h
20 °C, 3 h
20 °C, 3 h
20 °C, 3 h
20 °C, 4 h
20 °C, 4 h
20 °C, 4 h
20 °C, 3 h
20 °C, 3 h
20 °C, 3 h
80 °C, 5 h
80 °C, 4 h
93
94
9217
92
94
88
95
96
90
97
94
96
2
EtBr
N-ethylindole (2b)
3
EtBr
N-ethylindole (2b)
4
PrBr
N-propylindole (2c)
N-butylindole (2d)
5
BuBr
BuBr
BzCl
BzCl
BzCl
BuBr
BuBr
BuBr
BzCl
BzCl
BzCl
BuBr
BzCl
6
N-butylindole (2d)
7
N-benzylindole (2e)
N-benzylindole (2e)
N-benzylindole (2e)
N-butyl benzimidazole (3a)
N-butyl benzimidazole (3a)
N-butyl benzimidazole (3a)
8
9
10
11
12
13
14
15
16
17
benzimidazole
N-benzylbenzimidazole (3b) 93
N-benzylbenzimidazole (3b) 96
N-benzylbenzimidazole (3b) 97
succinimide
N-butylsuccinimide (4a)
N-benzylsuccinimide (4b)
90
93
a All reactions were run with heterocycle (3 mmol), alkyl halide (6 mmol) and KOH (6 mmol) in ionic liquid (2 mL).
b Isolated yield based on indole, benzimidazole, and succinimide, respectively.
tering the suspension to remove the residual KOH and the formed
potassium halide.
Table 5 N-Alkylation of Phthalimide and Indole under Different
Reaction Conditions
N-Methylphthalimide (1a)
Product
This Work Literature Methods, Yield (%)
Yield (%)
Colorless needles; mp 133–134 °C (Lit.19 mp 132–134 °C).
IR (KBr): 1771, 1720, 1466, 1434, 1009, 795, 717 cm–1.
1H NMR (CDCl3): = 7.86 (dd, J = 5.6, 3.2 Hz, 2 H), 7.73 (dd,
J = 5.6, 3.2 Hz, 2 H), 3.19 (s, 3 H).
Aa
–
Bb
88
80
72
–
Cc
–
N-ethylphthalimide
96
N-isopropylphthalimide 78
51
86
–
–
N-Ethylphthalimide (1b)
Colorless needles; mp 77–78 °C (Lit.10a mp 71–72 °C).
N-benzylphthalimide
N-ethylindole
97
94
95
–
IR (KBr): 3065, 2942, 1773, 1717, 1466, 1444, 1396, 1345, 1333,
1035, 875, 720 cm–1.
1H NMR (CDCl3): = 7.86 (dd, J = 5.6, 3.2 Hz, 2 H), 7.73 (dd,
J = 5.6, 3.2 Hz, 2 H), 3.76 (q, J = 6.8 Hz, 2 H), 1.29 (t, J = 6.8 Hz,
3 H).
88
90
N-benzylindole
92
–
a Lit.14: yields for reaction with CsF/Celite as base, stirring at reflux
in MeCN for 2 d.
b Lit.10a: yields for reaction with potassium phthalimide and alkyl ha-
lides, stirring at reflux in DMF.
N-Propylphthalimide (1c)
Colorless plates; mp 65–66 °C (Lit.10a mp 64 °C).
c Lit13: yields for reaction with 18-crown-6 as phase-transfer catalyst,
t-BuOK as base for 1–18 h.
IR (KBr): 2970, 2937, 1768, 1710, 1467, 1398, 1368, 1049, 902,
725 cm–1.
1H NMR (CDCl3): = 7.86 (dd, J = 5.6, 3.2 Hz, 2 H), 7.73 (dd,
J = 5.6, 3.2 Hz, 2 H), 3.67 (q, J = 7.2 Hz, 2 H), 1.72 (m, 2 H), 0.97
(t, J = 7.9 Hz, 3 H).
Synthesis 2004, No. 2, 208–212 © Thieme Stuttgart · New York

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N-Alkylation of Nitrogen Hetrocycles in Ionic Liquids
211
N-Isopropylphthalimide (1d)
J = 5.6, 2.4 Hz, 1 H), 4.19 (t, J = 7.6 Hz, 2 H), 1.90 (m, 2 H), 1.44
(m, 2 H), 1.04 (t, J = 7.4 Hz, 3 H).
Colorless needles; mp 85 °C (Lit.10a mp 84–86 °C).
IR (KBr): 3064, 2914, 1774, 1715, 1646, 1617, 1508, 1352, 1223,
1053, 767, 738 cm–1.
1H NMR (CDCl3): = 7.86 (dd, J = 5.6, 3.2 Hz, 2 H), 7.74 (dd,
J = 5.6, 3.2 Hz, 2 H), 4.52 (m, 1 H), 1.49 (d, J = 6.9 Hz, 6 H).
N-Benzylindole (2e)
Colorless solid; mp 41–42 °C (Lit14 mp 41–43 °C).
IR (KBr): 3002, 2982, 2142, 1465, 1342, 1203, 1095, 1024, 765,
735, 642 cm–1.
N-Butylphthalimide (1e)
1H NMR (CDCl3): = 7.63–7.37 (m, 4 H), 7.31–7.16 (m, 5 H), 6.96
(d, J = 2.8 Hz, 1 H), 6.65 (d, J = 2.8 Hz, 1 H), 5.31 (s, 2 H).
Colorless needles; mp 32 °C (Lit.20 mp 31–32 °C).
IR (KBr): 3030, 2953, 2936, 1773, 1699, 1614, 1464, 1400, 1364,
1049, 935, 865, 718 cm–1.
1H NMR (CDCl3): = 7.83 (dd, J = 5.2, 3.2 Hz, 2 H), 7.71 (dd,
J = 5.6, 3.0 Hz, 2 H), 3.68 (q, J = 7.8 Hz, 2 H), 1.66 (m, 2 H), 1.36
(m, 2 H), 0.94 (t, J = 7.8 Hz, 3 H).
N-Butylbenzimidazole (3a)
Oil.
IR (neat): 3056, 2960, 1673, 1615, 1496, 1459, 1366, 1286, 1203,
743 cm–1.
1H NMR (CDCl3): = 7.91 (s, 1 H), 7.82 (dd, J = 6.0, 1.6 Hz, 1 H),
7.42 (dd, J = 4.6, 0.8 Hz, 1 H), 7.28–7.34 (m, 2 H), 4.19 (t, J = 7.6
Hz, 2 H), 1.90 (m, 2 H), 1.39 (m, 2 H), 0.97 (t, J = 7.4 Hz, 3 H).
N-Benzylphthalimide (1f)
Colorless needles; mp 114–115 °C (Lit10a mp 113–114 °C).
IR (KBr): 3060, 2947, 1765, 1715, 1611, 1432, 1391, 1332, 1063,
938, 718 cm–1.
1H NMR (CDCl3): = 7.86 (dd, J = 5.6, 3.2 Hz, 2 H), 7.73 (dd,
J = 5.6, 3.2 Hz, 2 H), 7.46 (m, 2 H), 7.35 (m, 3 H), 4.87 (s, 2 H).
N-Benzylbenzimidazole (3b)
Colorless solid; mp 116–117 °C (Lit.10b mp 116–118 °C).
IR (KBr): 3006, 2942, 2154, 1609, 1466, 1184, 1075, 756, 722, 694
cm–1.
Ethyl N-Phthalimidoacetate (1g)
1H NMR (CDCl3): = 7.97 (s, 1 H), 7.66–7.26 (m, 4 H), 7.21–7.0 7
(m, 5 H), 5.52 (s, 2 H).
Colorless needles; mp 112–113 °C (Lit20 mp 111–112 °C).
IR (KBr): 2988, 2945, 1775, 1741, 1722, 1419, 1389, 1375, 1232,
1114, 957, 745, 713 cm–1.
1H NMR (CDCl3): = 7.90 (dd, J = 5.6, 3.2 Hz, 2 H), 7.76 (dd,
J = 5.6, 3.2 Hz, 2 H), 4.45 (s, 2 H), 4.24 (q, J = 7.2 Hz, 2 H), 1.30
(t, J = 7.0 Hz, 3 H).
N-Butylsuccinimide (4a)
Oil.
IR (neat): 2954,1774, 1697, 1401,880, 718 cm–1.
1H NMR (CDCl3): = 3.50 (t, J = 7.6 Hz, 2 H), 2.70 (s, 4 H), 1.55
(m, 2 H), 1.30 (m, 2 H), 0.91 (t, J = 7.4 Hz, 3 H).
N-Methylindole (2a)
Oil.
N-Benzylsuccinimide (4b)
Colorless solid; mp 97–99 °C (Lit.21 mp 97–99 °C).
IR (neat): 3054, 2942, 1513, 1477, 1330, 1316, 1242, 763, 740, 712
cm–1.
1H NMR (CDCl3): = 7.67 (dd, J = 8.4, 1.0 Hz, 1 H), 7.37 (dd,
J = 8.4, 0.8 Hz, 1 H), 7.26 (m, 1 H), 7.14 (dd, J = 7.6, 1.2 Hz, 1 H),
7.08 (d, J = 3.6 Hz, 1 H), 6.52 (dd, J = 3.2, 0.8 Hz, 1 H), 3.83 (s, 3
H).
IR (KBr): 3072, 3036, 2950,1768 1698, 1586, 1495, 1431, 1403,
1164, 883, 832, 717, 698 cm–1.
1H NMR (CDCl3): = 7.41 (dd, J = 4.8, 1.6 Hz, 2 H), 7.30–7.34 (m,
3 H), 4.67 (s, 2 H), 2.71 (s, 4 H).
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