A simple, eco-friendly, and recyclable bi-functional acidic ionic liquid catalysts for Beckmann rearrangement

Article abstract of DOI:10.1016/j.molcata.2013.04.021

A library of ionic liquids was prepared by varying the cations and anions. Bi-functional acidic ionic liquids were prepared by the direct combination of ionic liquids and ZnCl₂. Ionic liquids were investigated in the Beckmann rearrangements. A simple, eco-friendly, and recyclable bi-functional acidic ionic liquid based protocol for Beckmann rearrangement is developed, which is based on the fine tuning of the Br?nsted and the Lewis acidity of ionic liquids.

Full text of DOI:10.1016/j.molcata.2013.04.021

Journal of Molecular Catalysis A: Chemical 376 (2013) 90–97
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
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A simple, eco-friendly, and recyclable bi-functional acidic ionic liquid catalysts
for Beckmann rearrangement
∗
Rajkumar Kore, Rajendra Srivastava
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A library of ionic liquids was prepared by varying the cations and anions. Bi-functional acidic ionic liquids
were prepared by the direct combination of ionic liquids and ZnCl2. Ionic liquids were investigated in the
Beckmann rearrangements. A simple, eco-friendly, and recyclable bi-functional acidic ionic liquid based
protocol for Beckmann rearrangement is developed, which is based on the fine tuning of the Brönsted
and the Lewis acidity of ionic liquids.
Received 10 November 2012
Received in revised form 9 April 2013
Accepted 10 April 2013
Available online xxx
©
2013 Elsevier B.V. All rights reserved.
Keywords:
Ionic liquids
Bi-functional acid catalysts
Beckmann rearrangements
Lewis acidity
Brönsted acidity
1
. Introduction
of a library of ILs and on finding their applications in catalysis
and materials synthesis [20–28]. Several ILs based catalysts have
been reported for Beckman rearrangement [29–32]. However, few
of them have suffered from the separation of the desired prod-
uct from the reaction mixture. State of the art for the Beckmann
rearrangement suggests that this reaction can be catalyzed by using
Brönsted or Lewis acidic catalysts [29–33]. However, it is not pre-
cisely known whether Brönsted acid or Lewis acid or a combination
of Brönsted and Lewis acid is good for this reaction. To understand
this phenomenon, varieties of ILs were prepared and their per-
formance was evaluated in the Beckmann rearrangement. In this
study, a comprehensive and systematic study was made to develop
a simple, economical, eco-friendly, and recyclable catalytic protocol
for Beckmann rearrangement.
Beckmann rearrangement is one of the classical and most
popular reactions in the organic chemistry, in which amides are
formed by the acid catalyzed rearrangement of oximes [1,2].
Co-production of large amount of undesired by-product and cor-
rosive phenomenon associated with conventional acid (H SO4 and
SOCl ) based liquid phase protocol provided a challenging task for
chemists to develop alternate methods for this reaction [3]. A vari-
ety of organic acids and inorganic solid acids based alternative
routes were developed [4–12]. However, low selectivity, low reac-
tant to catalyst ratio, corrosive and large volume of solvents, high
operating temperature, and rapid catalyst deactivation are of seri-
ous concern. A green chemical method based on supercritical water
was developed for Beckmann rearrangement, but low conversion
and high temperature condition (646 K) provide its limited scope
2
2
2. Experimental
[
13]. Rising demand of caprolactum production and environmen-
tal concern of the existed industrial process have provided ample
opportunity for the researchers to find eco-friendly and economical
procedure for Beckmann rearrangement.
Ionic liquid attracted significant attention to researchers due
to their favorable physico-chemical properties [14–19]. Ionic liq-
uids (ILs) have been widely investigated in the inter-disciplinary
research areas [14–19]. Our research is focused on the synthesis
2
.1. Synthesis of ILs
[
Bmim][Cl] is commercially available and [Hmim][Cl] was pre-
pared by following the reported procedure (Scheme 1) [25,26].
HPyr][Cl] was prepared by the similar procedure that was adopted
[
for the synthesis of [Hmim][Cl]. We have already reported the
synthesis of various SO H functionalized ILs used in this work
3
(Scheme 1) [25,26]. However, we did not report the synthesis of
metal anion (especially Zn) containing ILs, which is described as
follows: In a typical synthesis, [C SO Hmim][Cl] was reacted with
various amounts of ZnCl2 under neat condition at 353 K for 12 h to
∗ Corresponding author. Tel.: +91 1881 242175; fax: +91 1881 223395.
E-mail address: rajendra@iitrpr.ac.in (R. Srivastava).
3
3
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.04.021

R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 376 (2013) 90–97
91
spectra were recorded on Analytikjena Specord 250 PLUS spec-
1
13
trophotometer. Nuclear magnetic resonance (NMR) ( H and C)
spectra were recorded on Bruker AM, 400 MHz NMR. Electrospray
ionization-mass spectrometry (ESI-MS) was performed by a Waters
Q-ToF Micro equipped with ESI Source with capillary voltage of
3
000 V and cone voltage 30 V. Analysis was performed in negative
ion mode. Acid values of the BAILs were determined by titration
with alkali solution using phenolphthalein as the indicator.
2.3. Catalytic reaction
In typical procedure, the oxime (1.0 mmol), cata-
a
lyst1 + catalyst2 (catalyst1 = ILs, 0.0–0.1 mmol); catalyst2 = metal
chlorides (0.0–0.125 mmol), where metal = Al, Mn, Fe, Co, Ni,
Cu, Zn, Sn, and Ce) and 4 mL acetonitrile were charged into a
2
5 mL round-bottom flask equipped with a magnetic stirrer and
condenser. The reactions were allowed to proceed for 0.5–4 h at
98–373 K. The reaction mixture was analyzed using gas chro-
2
matography (Yonglin 6100; BP-5; 30 m × 0.25 mm × 0.25 ␮m).
The products were identified by GC–MS (Hewlett-Packard; 30 m
long, 0.25 mm i.d., with a 0.25-␮m-thick capillary HP5column)
and authentic samples obtained from Aldrich.
For the recovery of the ILs after the reaction, reaction mix-
ture was evaporated. Since reactant and products are soluble in
diethyl ether, therefore reaction mixture was washed 4–5 times
with diethyl ether to remove the reactant and products and leaving
behind the catalyst in the reaction flask. Reaction flask containing
ILs were subjected to Rota-evaporation, followed by drying in vac-
uum to remove solvent from the ILs. Reactants were again charged
into the reaction flask containing recovered ILs and reaction was
performed by the above mentioned method.
2.4. Theoretical study details
The minimum-energy geometries of ILs were determined by
performing DFT geometry optimizations at the B3LYP/6-31G level
using the Gaussian09 program [34]. A vibrational analysis was per-
formed to ensure the absence of negative frequencies and verify
the existence of a true minimum.
Scheme 1. Schematic representation for the synthesis of ILs investigated in this
study.
get [C SO Hmim][Cl]–[ZnCl ] as a viscous liquid. With an increase
3
3
2
of the ZnCl2 mass in the reaction, the viscosity of the ionic liquid
3. Results and discussions
(
[C SO Hmim][Cl]–[ZnCl ]) also increased.
3
3
2
[
Hmim][Cl]–[ZnCl ], [HPyr][Cl]–[ZnCl ], and [HCl]–[ZnCl2]
2
2
3.1. Synthesis and characterization of ILs
were synthesized according to the similar procedure that
was adopted for the synthesis of [C SO Hmim][Cl]–[ZnCl ].
3
3
2
A library of ILs was prepared by varying the cations and anions
(Scheme 1). Structure of ILs was characterized using various spec-
troscopic tools such as FT-IR, NMR, and Mass spectrometer. Acidity
of Brönsted acidic ILs (BAILs) investigated in this study was mea-
sured using UV–visible spectrophotometer with a basic indicator by
following the method reported in literature [35,36]. Acidity of BAILs
was investigated in water using 4-nitroanline as indicator. With the
increase of acidity of the BAILs, the absorbance of the unprotonated
form of the basic indicator decreased, whereas the protonated form
of the indicator could not be observed because of its small molar
In
a
typical synthesis, ZnCl2 was reacted with equiva-
lent amount of ([Hmim][Cl]/[HPyr][Cl]/[HCl]) to obtain
[
Hmim][Cl]–[ZnCl ]/[HPyr][Cl]–[ZnCl ]/[HCl][ZnCl ].
2 2 2
−
1
[
HPyr][Cl]: IR (KBr, ꢀ, cm ) = 615, 679, 752, 926, 1000, 1053,
1
3
1
163, 1198, 1249, 1332, 1385, 1485, 1537, 1611, 1634, 2621, 2947,
1
060, 3400. H NMR (400 MHz, D O): ı (ppm) = 8.8 (d, 2H), 8.7 (t,
2
1
3
H), 8.1 (t, 2H). C NMR (400 MHz, D O): ı (ppm) = 145, 143, 128.
2
Elemental analysis for C5H NCl: Theoretical (%): C 51.97, H 5.23, N
6
1
2.12; Experimental (%): C 51.23, H 5.74, N 12.5.
[
C SO Hmim][Cl]–[ZnCl ]: IR (KBr, ꢀ, cm−1) = 655, 745, 1033,
+
3
3
2
absorptivity and its location. Therefore, the [I]/[IH ] (I represents
1
3
169, 1215, 1367, 1443, 1532, 1739, 2321, 2970, 3022, 3108, 3151,
indicator) ratio can be determined from the differences of measured
absorbance after the addition of BAILs and Hammett function, H0,
can be calculated using Eq. (1). This value can be regarded as the
relative acidity of the BAILs.
1
443. H NMR (400 MHz, D O): ı (ppm) = 8.6 (s, 1H), 7.2 (s, 2H), 4.1
2
13
(
t, 2H), 3.6 (s, 3H), 2.7 (t, 2H), 2.1 (m, 2H). C NMR (400 MHz, D O):
2
−
ı (ppm) = 134, 121, 120, 46, 44, 33, 24. MS (ESI) for [ZnCl ] m/z
1
3
−
−
71, [Zn Cl5] m/z 306.6, and [Zn Cl7] m/z 443.
ꢀ
ꢁ
2
3
I
H = pK(I) + log
(1)
0
aq
+
IH
2.2. Characterization details
Under the same concentration of 4-nitroanline (3 mg/L,
Fourier transform infrared (FT-IR) spectra were recorded on
pK(I)aq = pKa = 0.99) and BAILs (50 mmol/L) in H O, H0 values of all
BAILs were determined. The maximal absorbance of the unproto-
nated form of the indicator was observed at 380 nm in water. When
2
−
1
Bruker Tensor-27 spectrometer in the range of 600–4000 cm
−1
(
spectral resolution = 4 cm
; number of scans = 100). UV–vis

9
2
R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 376 (2013) 90–97
Table 1
Absorbance spectra of 4-nitroaniline in water after addition of BAILs.
+
Entry no.
BAILs
Amax
[I] (%)
[IH ] (%)
H0
Acid value (mg KOH/mol)
1
2
3
4
5
None
0.95
0.80
0.64
0.69
0.71
100
84
67
73
75
–
16
33
27
25
–
–
[C3SO3Hmim][Cl]
[C3SO3Hmim][HSO4]
[C3SO3Hmim][CF3COO]
[C3SO3Hmim][TSO]
1.71
1.30
1.42
1.46
18.9
81.3
39.7
37.2
Indicator: 4-nitroanline.
the BAIL was added, the absorbance of the unprotonated form of
the basic indicator decreased (Fig. 1 and Table 1). Hammett acidity
clearly shows that [C SO Hmim][Cl]–[ZnCl ] exhibited the highest
3 3 2
strength of Lewis acidity, when the complex was formed with 1:1
molar ratio of [C SO Hmim][Cl] and ZnCl ACN FT-IR of various
(
H ) of these BAILs was calculated using Eq. (1). Acid values of the
0
3
3
2.
−
1
BAILs were determined by titration with 0.02 M KOH solution using
phenolphthalein as the indicator (Table 1).
[ZnCl ] containing ILs was investigated. 2294 cm
was shifted to 2319 cm , (2323 and 2340 cm ), and 2329 cm
for [HCl]–[ZnCl ], [HPyr][Cl]–[ZnCl ], and [Hmim][Cl]–[ZnCl ],
peak of ACN
2
−
1
−1
−1
Acetonitrile (ACN) is a weak base and used as a probe molecule
in IR spectroscopy to characterize the Lewis acidity [37]. It is
well known that the C N group can react with Lewis acid to
produce CN–Lewis complex, which shows a new absorption
2
2
2
respectively (Fig. 2c).
Pyridine (Pyr) reacts with the Brönsted acid and Lewis acid sites,
and produce a cation of [PyrH]+ and a complex of Pyr–Lewis acid,
respectively [38]. In general, following are the peak positions of
−1
peak at 2200–2400 cm
in the FT-IR spectra. Upon increasing
the strength of Lewis acid, the absorption peak shifts to higher
various kinds of Brönsted and Lewis acid sites observed using pyri-
−
1
−1
wave number. ACN has two peaks at 2252 and 2294 cm
due
dine IR: Brönsted acid sites = (1540–1550) and (1630–1640) cm
;
−
1
−1
to C N group. Significant shift in 2294 cm
was observed for
strong Lewis acid sites = (1620–1625) and (1450–1455) cm ; weak
−1
metal halide/ACN systems. Here, we first compared the ACN-IR
of AlCl , CoCl , and ZnCl . AlCl /ACN, CoCl /ACN, and ZnCl /ACN
Lewis acid sites = (1480–1490) and (1575–1585) cm ; hydro-
−
1
gen bonded pyridine = (1440–1445) and (1595–1605) cm . Fig. 3
3
2
2
3
2
2
exhibited FT-IR peaks at (2310 and 2331), (2320 and 2331),
represents the Pyr FT-IR spectra of various [ZnCl ] containing
2
−
1
−1
2
321 cm , respectively (Fig. 2a). Peak observed at 2331 cm
BAILs investigated in this study, which confirm the presence of
both, Brönsted and Lewis acid sites based on the peaks observed
(described above) in their Pyr-IR spectra.
in the case of AlCl3 or CoCl , clearly shows the high strength of
2
Lewis acidity. ACN FT-IR of various [C SO Hmim][Cl]–[ZnCl ]
3
3
2
−
1
samples were investigated (Fig. 2b). No shift in 2294 cm
peak was observed for [C SO Hmim][Cl]–[ZnCl ] samples
3
3
2
3.2. Catalytic investigations
(
1
where [C SO Hmim][Cl]:ZnCl molar ratio were 1:0.25 and
3
3
2
−1
−1
for
:0.5). 2294 cm
peaks of ACN was shifted to 2313 cm
First Beckmann rearrangement was investigated using catalytic
amount of [C SO Hmim][Cl] and acetophenone oxime as a model
[
C SO Hmim][Cl]–[ZnCl ] (where, [C SO Hmim][Cl]:ZnCl molar
3 3 2 3 3 2
3
3
−
1
ratio = 1:075) and 2317 cm
for [C SO Hmim][Cl]–[ZnCl ]
3 3 2
substrate. [C SO Hmim][Cl] was found to be inactive in the Beck-
3 3
(
where, [C SO Hmim][Cl]:ZnCl molar ratio = 1:1). FT-IR study
3
3
2
mann rearrangement (Table 2, entry 2). It is known in the literature
that the Beckmann rearrangement takes place, when PTSA is used
along with ZnCl [32]. This gave us an indication to use ZnCl along
2
2
Table 2
Catalytic activity of ILs in the Beckmann rearrangement.
Entry no.
IL
Acetophenone
N-
oxime conversion
phenylacetamide
selectivity (%)
(
%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
–
0
0
2
–
–
a
[C3SO3Hmim][Cl]
[C3SO3Hmim][Cl]
b
92
97
95
94
97
84
98
25
90
97
–
[C3SO3Hmim][Cl]c
8
d
[C3SO3Hmim][Cl]
[C3SO3Hmim][Cl]
21
48
46
6
e
[C3SO3Hmim][Cl]
[C3SO3Hmim][HSO4]
[C3SO3Hmim][TFA]
[C3SO3Hmim][TSO]
[SO3Hmim][Cl]
[Benz-SO3Hmim][Cl]
[Bmim][Cl]
[Benz-mim][Cl]
[Hmim][Cl]
[HPyr][Cl]
31
1
1
1
1
1
1
1
1
1
100
25
16
0
0
–
100
73
16
98
93
86
0
a
HCl
HCl
85
Reaction and conditions: IL (0.10 mmol), ZnCl2 (0.10 mmol), acetophenone oxime
(
1.0 mmol), CH3CN (4 mL), reaction temperature (353 K), and reaction time (4 h).
a
ZnCl2 = 0.
ZnCl2 = 0.025 mmol.
ZnCl2 = 0.050 mmol.
ZnCl2 = 0.075 mmol.
b
c
d
e
ZnCl2 = 0.125 mmol.
Fig. 1. Absorbance spectra of 4-nitroaniline in water after addition of BAILs.

R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 376 (2013) 90–97
93
Fig. 2. ACN-FT-IR spectra of (a) metal halides (metal halide:ACN = 2:1); (b) [C3SO3Hmim][Cl]–[ZnCl2] samples with different ZnCl2 contents
[C3SO3Hmim][Cl]–[ZnCl2]:ACN = 2:1); (c) ZnCl2 containing ILs (ILs:ACN = 2:1).
(
with [C SO Hmim][Cl]. ZnCl alone was found to be inactive under
However, catalytic activity data is just reverse of the acidity
trends observed using Hammett acidity and acid value (Table 2).
Based on these observations, it can be said that the Beckmann
rearrangement does not depend solely on the acidity of BAILs.
It may be noted that ZnCl2 alone and BAILs alone are inactive,
therefore the catalytic species are generated in situ by the reac-
3
3
2
our reaction condition (Table 2). Reaction was found to be occur-
ring when both [C SO Hmim][Cl] and ZnCl were used together in
3
3
2
the reaction. It was found that by increasing the amount of ZnCl2
with respect to [C SO Hmim][Cl]), the activity of the catalytic sys-
(
3
3
tem increased (Table 2). Based on the catalytic data obtained, one
can conclude that 1:1 molar ratio of [C SO Hmim][Cl]:ZnCl was
tion of BAILs and ZnCl . Based on the catalytic activity data of
3
3
2
2
found to be optimum for this reaction.
[C SO Hmim] cation and various anions (Entries 7–10, Table 2),
3
3
Next, the influence of anions and location of SO H in the
it can be concluded that the catalytic active species formed by
the reaction of [C SO Hmim][Cl] and [ZnCl ] is more active than
3
structure of BAILs were investigated. To study the influence of
anion, [C SO Hmim] cation based BAILs were investigated. Cat-
3
3
2
[C SO Hmim][HSO ]/[C SO Hmim][CF COO]/[C SO Hmim][TSO]
3
3
3
3
4
3
3
3
3
3
alytic activity data confirmed that among the anions investigated;
Cl] was found to be the best anion (Table 2). At one instance, if
and [ZnCl ]. It is known that ZnCl2 reacts with [C SO Hmim][Cl]
and form [C SO Hmim][Cl]–[ZnCl ] or similar species [19], which
3 3 2
2
3
3
[
we assume that this reaction is an acid catalyzed reaction, then
with increase in the acidity, the catalytic activity should increase.
Hammett acidity and acid value for [C SO Hmim][HSO ] was
is the active species for the Beckmann rearrangement investigated
in this study. Based on this observation, it can be concluded that
[C SO Hmim][Cl]–[ZnCl ] is the most active species for the Beck-
3
3
4
3
3
2
found to be the maximum (Table 1), therefore, its activity should
found to be maximum among this series of BAILs investigated.
mann rearrangement compared to other [BAILs]–[ZnCl ] species.
In order to validate this observation, several control experiments
2

9
4
R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 376 (2013) 90–97
Fig. 3. Pyridine FT-IR spectra of (a) [C3SO3Hmim][Cl]–[ZnCl2], (b) [Hmim][Cl]–[ZnCl2], (c) [HPyr][Cl]–[ZnCl2], (d) [HCl]–[ZnCl2]. In these studies ILs and Pyr were taken in
:1.
2
were performed. When only HCl was used as catalyst under the
optimized reaction condition, it did not form any desired prod-
uct (acetophenone oxime conversion = 16%, product selectivity
system ([Hpyr][Cl] + [ZnCl ]) but the highest activity was observed
2
for ([Hmim][Cl] + [ZnCl ]) catalytic system (Table 2).
2
Having found [Cl] as the best anion, the influence of the location
of SO H in the structure of BAILs [direct N SO H and N-spacer
(
%) = N-phenylacetamide: acetophenone = 0: 100). It may further
3
3
be noted that in the presence of only [C SO Hmim][Cl] (Entry no.
SO H (where spacer = propyl or benzyl)] was also investigated
3
3
3
2
, Table 2) or PTSA [31], no reaction took place, which confirms
(Scheme 1). Catalytic activity data shows that among the SO H
3
that only Brönsted acidic catalyst (HCl or [C SO Hmim][Cl] or
containing BAILs investigated, [C SO Hmim] was found to be the
3
3
3
3
PTSA) is not suitable for Beckman rearrangement. Based on our
experiment, we can conclude that HCl is not the catalytic active
species for the Beckman rearrangement (in case, if it is formed in
best cation investigated in this study (Table 2). Above section shows
that the catalytic species are generated in situ by the reaction of [Cl]
containing BAILs and [ZnCl ]. Since in these cases, only [Cl] anion
2
situ by the reaction of IL and ZnCl ). HCl and a variety of [Cl] con-
taining ILs along with ZnCl2 were investigated (Table 2). Catalytic
investigation and FT-IR investigations reveals that Brönsted as
is present, then, in-principle the activity should be similar. This is
possible at only one condition, when availability of [Cl] in BAILs to
make complex with ZnCl2 is the same. If all the [Cl] is not available
to make complex then its activity will be less. Based on the catalytic
data, one can say that availability of [Cl] to form active species is
2
well as Lewis acidity provided by the ZnCl based ILs are important
2
for this reaction. Though a good activity was observed for catalyst

R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 376 (2013) 90–97
95
Fig. 4. Optimized structure of BAILs calculated at B3LYP/6-31G.
Table 3
The geometry parameters of BAILs calculated at B3LYP/6-31G.
Observed parameters
[C3SO3Hmim][Cl]
[Benz-SO3Hmim][Cl]
[SO3Hmim][Cl]
H
(
O bond distance of sulfonic acid ( A˚ )
N)2C H
H23 O22 = 0.988
C3 H7 = 1.088
H30 O29 = 0.989
C3 H7 = 1.112
H17 O16 = 1.542
C3 H7 = 1.077
Other hydrogen bonds in BAILs ( A˚ )
H7· · ·Cl12 = 2.520
H7· · ·Cl31 = 2.067
H10· · ·Cl31 = 2.559
H22· · ·Cl31 = 2.604
H17· · ·Cl18 = 1.005
H15· · ·Cl12 = 2.674
H7· · ·Cl18 = 2.596
different for different BAILs investigated in this study. To confirm
[C SO Hmim][Cl] and ZnCl (Table 4). ACN-IR also confirmed
3 3 2
our hypothesis, structure of SO H containing BAILs were geomet-
rically optimized using B3LYP/6-31G (Fig. 4). In order to confirm
that the Lewis acid strength is found to be the highest for
[C SO Hmim][Cl]–[ZnCl ] (where [C SO Hmim][Cl]:ZnCl = 1:1)
3
3
3
2
3
3
2
the availability of [Cl] to make complex with ZnCl , the bonding
(Fig. 2c). The catalytic activity of [Hpyr][Cl]–[ZnCl ] was found
2
2
of Cl with its neighboring groups were analyzed. Since the bond
to be better than the [Hmim][Cl]–[ZnCl ]. Though the com-
2
length of (N) C H· · ·Cl in [C SO Hmim][Cl] was found to be max-
plete conversion of acetophenone oxime was observed using
2
3
3
imum (Table 3), hence it is most easily available to make complex
[Hpyr][Cl]–[ZnCl ], but the product selectivity was less (82%). High
2
with ZnCl , therefore its activity was found to be the maximum in
product selectivity (98%) with good reactant conversion (72%) was
2
this series.
After identifying the best BAILs and BAILs:ZnCl2 ratio, the next
aim was to find the best metal halide for this reaction. A series of
commercially available metal halides along with [C SO Hmim][Cl]
3
3
were investigated in the Beckmann rearrangement under the opti-
mized condition and the results were analyzed (Fig. 5). Based on
the catalytic activity data, one can conclude that ZnCl2 was found
to be the best among the metal halides investigated (based on the
selectivity). Based on the catalytic activity and ACN FT-IR, it can be
concluded that for the Beckmann rearrangement, moderate Lewis
acidity would be more appropriate for high selectivity of amide
product.
It is described above that the Beckmann rearrangement occurs
efficiently, when along with ILs having [Cl] anion, ZnCl2 is also
added in the equimolar ratio (with respect to ILs) in the reaction. It
is well known that a catalytic process can be efficient, if the catalyst
can be recycled with negligible loss in catalytic activity. Since the
above mentioned protocol involves the use of combination of ILs
and ZnCl , separation of these two catalysts from the reaction mix-
2
ture after the reaction is very difficult. Hence, it was thought that
if a catalyst is designed in which both ILs and ZnCl2 were present
concomitantly in the same structure then it would be one catalyst
system and its separation and reusability would become an easy
affair. Keeping that in mind, using direct combination of ILs with
ZnCl2, four bi-functional catalysts [C SO Hmim][Cl]–[ZnCl ],
3
3
2
[
Hmim][Cl]–[ZnCl ],
[HPyr][Cl]–[ZnCl ],
and
[HCl]–[ZnCl2]
2
2
were prepared (Scheme 1) and investigated in the Beck-
mann rearrangement (Table 4). Influence of the amount of
ZnCl2 in the bi-functional ionic liquid was investigated using
[
C SO Hmim][Cl]–[ZnCl ]. Based on the catalytic activity data,
3 3 2
one can conclude that with an increase in the amount of ZnCl2
in the bi-functional ionic liquid, catalytic activity increases, and
it levels off with highest catalytic activity at equimolar ratio of
Fig. 5. The Beckmann rearrangement of acetophenone oxime catalyzed by (IL+
metal halides; where IL = [C SO Hmim][Cl]).
3
3

9
6
R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 376 (2013) 90–97
Table 4
Catalytic activity of [ZnCl2] containing ILs in the Beckmann rearrangement.
Entry No.
Catalyst
Acetophenone oxime conversion (%)
N-phenylacetamide selectivity (%)
1
2
3
4
5
6
7
8
[C3SO3Hmim][Cl]–[ZnCl2]a
3
12
37
72(66)
14
70(65)
100(92)
95(0)
100
99
99
98(95)
0
90(85)
82(74)
89(0)
b
[C3SO3Hmim][Cl]–[ZnCl2]
[C3SO3Hmim][Cl]–[ZnCl2]c
d
e
e
[C3SO3Hmim][Cl]–[ZnCl2]
f
[C3SO3Hmim][Cl]–[ZnCl2]
d
e
e
[Hmim][Cl]–[ZnCl2]
d
e
e
[HPyr][Cl]–[ZnCl2]
d
e
e
[HCl]–[ZnCl2]
Reaction conditions: ILs (0.10 mmol), acetophenone oxime (1.0 mmol), CH3CN (4 mL), reaction temperature (353 K), and reaction time (4 h).
a
0
0
0
1
.25 equiv. of ZnCl2 in ILs.
.5 equiv. of ZnCl2 in ILs.
.75 equiv. of ZnCl2 in ILs.
.0 equiv. of ZnCl2 in ILs.
b
c
d
e
f
Catalytic activity data after third recycle.
Reaction was performed in the absence of any solvent and using 1.0 equiv. of ZnCl2 in ILs.
Table 5
Catalytic activity of [C3SO3Hmim][Cl]–[ZnCl2] in the Beckmann rearrangement.
Entry no.
Reactant
Product
Conversion (%)
72
Selectivity (%)
98
1
2
3
4
69
65
35
98
94
91
Reaction and conditions: [C3SO3Hmim][Cl]–[ZnCl2] (0.1 mmol), oxime (1.0 mmol), CH3CN (4 mL), reaction temperature (353 K), and reaction time (4 h).
obtained using [C SO Hmim][Cl]–[ZnCl ]. Pyridine IR confirms
compared to other two catalysts investigated for the reusability
(Table 4). The result suggests that a marginal decrease in the activity
3
3
2
the presence of Brönsted and Lewis acidity and ACN-IR confirms
the presence of Lewis acidity in [C SO Hmim][Cl]–[ZnCl ],
was observed using [C SO Hmim][Cl]–[ZnCl ] after third recy-
3
3
2
3
3
2
[
Hmim][Cl]–[ZnCl ], [HPyr][Cl]–[ZnCl ], and [HCl]–[ZnCl ].
cles. Having found the optimized reaction condition, applicability
of [C SO Hmim][Cl]–[ZnCl ] was investigated in the wide variety
2
2
2
Brönsted/Lewis acid ratio for [C SO Hmim][Cl]–[ZnCl ],
3
3
2
3
3
2
[
Hmim][Cl]–[ZnCl ], [HPyr][Cl]–[ZnCl ], and [HCl]–[ZnCl ] were
of commercially available oximes. Beckmann rearrangement pro-
ceeded well, delivering good product yields, and accommodated a
wide range of oximes (Table 5). Both aromatic and aliphatic oximes
could be converted to amides under the given conditions.
2
2
2
found to be 1.48, 0.54, 0.85, and 0.87, respectively. Therefore,
based on FT-IR and catalytic activity data, it can be concluded
that an optimum Brönsted and Lewis acidity is required for the
catalyst to exhibit the highest activity and selectivity. Such a
fine tuned optimum Brönsted and Lewis acidity is provided by
4. Conclusion
[
C SO Hmim][Cl]–[ZnCl ].
3 3 2
[
ZnCl ] containing BAILs were subjected to reusability up to
A highly efficient, simple, eco-friendly, economical, and recy-
clable bi-functional acidic ionic liquids were developed for the
Beckmann rearrangement for wide variety of ketoximes. It was pos-
sible to find a highly active and selective catalyst by fine tuning
the Brönsted and the Lewis acidity of ionic liquids by systematic
variation of cations and anions. Structure activity relationship was
established by using theoretical studies and acidity measurements.
2
3
times (Table 4). [HCl]–[ZnCl ] was found to be soluble in the
2
reaction medium and other extracting solvents and therefore it is
not possible to reuse (Table 4). Catalysts were recovered by evap-
orating solvent and washing the reaction mixture with diethyl
ether. Among other three catalysts investigated for reusability,
[
C SO Hmim][Cl]–[ZnCl ] exhibited excellent reusability profile
3 3 2

R. Kore, R. Srivastava / Journal of Molecular Catalysis A: Chemical 376 (2013) 90–97
97
Acknowledgements
[21] R. Kore, R. Srivastava, RSC Adv. 2 (2012) 10072–10084.
[
[
[
22] R. Kore, R. Sridharkrishna, R. Srivastava, RSC Adv. 3 (2013) 1317–1322.
23] R. Kore, M. Tumma, R. Srivastava, Catal. Today 198 (2012) 189–196.
24] R. Kore, R. Srivastava, J. Mol. Catal. A: Chem. 345 (2011) 117–126.
Authors are grateful to Department of Science and Technology
(
SR/S1/PC/31/2009) and Director IIT Ropar for financial assistance.
[25] R. Kore, T.J. Dhilip Kumar, R. Srivastava, J. Mol. Catal. A: Chem. 360 (2012) 61–70.
[
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27] R. Kore, R. Srivastava, Catal. Commun. 18 (2012) 11–15.
28] R. Kore, R. Srivastava, Catal. Commun. 12 (2011) 1420–1424.
RK thank CSIR, New Delhi for fellowship.
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