S. Kumar et al. / Tetrahedron Letters 55 (2014) 3802–3804
3803
Literature survey reveals that [bmim]HSO
4
has been used in a
Table 1
9
The [bmim]HSO4 catalysed synthesis of aryl/heteroaryl/alkyl nitriles to corresponding
carboxylic acid
variety of organic reactions, such as prins reaction, synthesis of
1
3
14
15
16
coumarins, xanthenes, xanthenedione, nitration of phenol,
1
7
halogenation of alcohols, acetalization and thioacetalization of
S. No. Nitriles (a)
Carboxylic Acids (b)
Time (h) Yield (%)
1
8
carbonyl compounds and their subsequent deprotection, pyrany-
1
9
20
1
2
3
4
CN
COOH
3.00
2.20
2.20
2.45
94
92
95
90
lation of diols, Friedel–Crafts alkylation, synthesis of amid-
1
0
11
oalkyl napthols, formylation of alcohols, conversion of inulin
1
2
Cl
Br
CN
CN
Cl
Br
COOH
COOH
COOH
COOH
into fructose etc.
4
The wide spectrum utilization of [bmim]HSO led us to check
that if its applicability can be further extended and exploited as a
new and more efficient method for the synthesis of acids from
H
F
3
C
CN
CN
H
F
C
3
nitriles. It is well established that the C-2 proton of the 1,3-dialky-
limidazolium cation is acidic,2
1–26
Ionic liquids are known to
7
5
6
2.45
2.40
92
90
contain small amount of water which may act as nucleophile.
To prove one more unique application of ionic liquid in organic
transformations, we carried out several reactions using different
substrates, and found high yield with maximum purity. The scope
of ionic liquid-mediated conversion of nitriles into carboxylic acids
was investigated by selecting five ionic liquids, such as
Br
Br
NO
2
NO
2
NC
NH
HOOC
HOOC
NH
2
O
H
3
C
[
bmim]HSO
4
, [bmim]BF
4
, [bmim]Br, [bmim]Cl and [bmim]CH3-
showed promis-
Br
Br
COO. Out of these ionic liquids, only [bmim]HSO
4
7
8
2.25
2.50
90
92
NC
F
OH
OH
ing result, while the other ionic liquids did not work in this
transformation and thus were eliminated from our present study.
CN
F
COOH
F
On the basis of this observation, we may conclude that counter
À
F
F
ion HSO
4
of [bmim]HSO
4
is playing an important role in addition
F
to C-2 proton of imidazolium ion.
CN
Cl
COOH
To establish the utility of the ionic liquid for the conversion of
nitriles into acids in single step, preliminary experiment was per-
formed using simple benzonitrile which on treatment with
9
2.50
95
Cl
Cl
Cl
[
4
bmim]HSO at 65 °C for 3 h, yielded benzoic acid up to 94% as
CN
Cl
COOH
Cl
2
7
10
11
2.50
2.45
95
95
white solid with high purity (Table 1, entry 1b). This interesting
result encouraged us to extend the same on a series of both ali-
phatic as well as aromatic nitriles having various substituents
NC
HOOC
(
Scheme 1). The nitriles and their respective products, yield and
NC
Cl
NO
2
HOOC
N
NO2
time required for reactions are summarized in Table 1. It is obvious
from Table 1 that aromatic nitriles (Table 1, entries 1a–12a), as
well as aliphatic nitriles (Table 1, entries 13a–18a) were equally
sensible towards transformation in the ionic liquid, and final prod-
uct yields were found more than 90%.
The electron withdrawing substituents on the benzene ring at
ortho/para position were found to enhance the reactivity. The reac-
tivity of hetero-aromatic nitrile (Table 1, entry 12) was also found
to be equally sensible and the transformation proceeded with a
substantial yield. The idea was further extended using dinitrile
compound (Table 1, entry 11), which on ionic liquid treatment
gave corresponding dicarboxylic acids with significantly higher
yields. It is also evident from this transformation that steric crowd-
ing has no considerable effect on the hydrolysis of nitriles (Table 1,
entries 9 and 10).
Most conversions of acids into nitriles were highly effective
with yields in the range of 90–96%, indicating the utility of new
approach. The purity and authenticity of the final products were
established by melting point measurement and various analytical
techniques such as IR, H and C NMR. The methodology was fur-
ther authenticated by solving X-ray diffraction data of 15b (Fig. 2).
Further, to test whether the ionic liquid can be used as a highly
recyclable solvent/catalyst system, we examined turnover during
the conversion reaction. Our analysis and results explicitly reveal
that ionic liquid can be recycled and used multiple times (five to
six times) without a significant loss in its activity (Fig. 3).
N
12
2.40
1.40
92
94
CN
Cl
COOH
COOH
13
14
CN
1.25
1.25
94
H C
CN
H
3
C
C
COOH
3
1
1
1
5
6
7
96
95
95
CN
COOH
COOH
H
H
3
3
C
C
H
H
3
3
C
1.20
1.40
1.10
CN
CN
CN
COOH
COOH
Cl
Cl
Cl
Cl
18
96
1
13
Nitriles (a) (2 mmol) were dissolved in 5 ml of [bmim]HSO
mixture was heated to 60–65 °C for 1–3 h.
4
and the reaction
[bmim]HSO4
60-65°C, 1-2 h
CN
COOH
COOH
R
R
R
[
bmim]HSO4
0-65°C, 2-3 h
Till date, no nitrile hydrolysis mechanism by ionic liquid is
reported. It is estimated that ionic liquids contain approximately
2
CN
6
R
00–600 ppm of water.7 We proposed that imidazolium salt may
À
trap water molecule on the C-2 position. The HSO
4
anion of
Scheme 1. Synthesis of various carboxylic acids from nitriles.