Highly efficient and recyclable copper based ionic liquid catalysts for amide synthesis

Article abstract of DOI:10.1039/c6nj01711h

A series of Cu based ionic liquids was synthesized by the reaction of 1-butyl-3-methylimidazolium salts and copper(ii) salts. These ionic liquids were investigated as catalysts in the synthesis of amides (primary, secondary and tertiary amides). For easy separation and recycling, the Cu based ionic liquids were decorated on nanocrystalline silicalite. The materials were characterized using powder X-ray diffraction, nitrogen adsorption, scanning and transmission electron microscopy, thermogravimetric analysis, and nuclear magnetic resonance, Fourier transform infrared and UV-visible spectroscopy. The Cu based ionic liquids decorated on nanocrystalline silicalite were found to be efficient and recyclable catalysts for the amide synthesis. The catalysts were found to be stable and show negligible loss of the activity even after five cycles.

Full text of DOI:10.1039/c6nj01711h

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DOI: 10.1039/C6NJ01711H
Highly efficient and recyclable copper based ionic liquid catalysts for amide synthesis
Poonam Rani and Rajendra Srivastava*
Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar-140001, India
____________________________________________________________________________
_
Abstract
A series of Cu based ionic liquids were synthesized by the reaction of 1-butyl-3-
methylimidazolium salts and copper (II) salts. These ionic liquids were investigated as catalyst in
the synthesis of amides (primary, secondary and tertiary amides). For the easy separation and
recycling, Cu based ionic liquids were functionalized on nanocrystalline silicalite. Materials
were characterized by powder X-ray diffraction, nitrogen adsorption, scanning and transmission
electron microscope, thermo gravimetric analysis, nuclear magnetic resonance, Fourier transform
infrared and UV-visible spectroscopy. Cu based ionic liquids functionalized on nanocrystalline
silicalite were found to be efficient and recyclable catalysts in the amides synthesis. Catalysts
were found to be stable and show negligible loss in the activity even after five cycles.
Keywords: Ionic liquid, copper, silicalite, heterogeneous catalyst, amides.
*
Corresponding author: E-mail: rajendra@iitrpr.ac.in, Phone: +91-1881-242175; Fax: +91-1881-
23395
2
1

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1
. Introduction
Ionic liquids (ILs) are organic salts in which the constituent ions are poorly coordinated,
which results in their wide range of melting points varies from room temperature to 300 degree
Celsius (or even more). Initially ILs were developed to replace the conventional organic solvents
1
-6
in chemical reactions. ILs attracted significant attention due to their favorable physico-
chemical properties. Physico-chemical properties of ILs can be finely tuned by varying the
cations and anions. Therefore, it is possible to synthesize wide range of ILs for the required
7
applications. ILs have been widely investigated in the inter-disciplinary research areas such as
-9
1
0,11
12
organometallics, biological uses,
13
14
analytical chemistry, polymer,
15
electrochemistry,
1
6-22
23
material synthesis, etc. We are interested to explore the possibilities to use ILs as
catalysis,
catalyst and structure directing agents for the preparation of catalytic materials of industrial
2
4-26
importance.
ILs based acid catalysts were designed and investigated in several industrially important
2
4-26
reactions by our research group.
We are interested to develop metal based ILs catalysts. Most
of the metal based catalysis are performed in homogeneous medium and shared some common
drawbacks such as difficulty in catalyst/product separation and recyclability of the catalyst. This
problem can be solved when metals becomes the integral part of recyclable system (such as
heterogeneous catalysts). Since ILs can be easily separated from the reaction, therefore various
metals containing ILs can be a good substitute to the homogenous catalyst systems. To make the
process even more user friendly, efforts were made to prepare metal containing ILs supported on
2
7-29
mesoporous materials.
Transition metals and their complexes have been used as active catalysts in many organic
transformations. Especially, ruthenium, palladium, platinum, gold and iridium are the frequently
used transition metals in catalysis. However, to make the catalytic system cost effective, copper
(Cu) has been used instead of the above said metals with comparable efficiency. Redox
chemistry of Cu is very interesting and display excellent catalytic activity ranging from
oxidation, reduction, coupling (C-C, C-N, C-O, and C-S) reactions, amidation etc. to produce
30-32
wide range of fine and pharmaceutically important compounds.
Cu based catalytic systems
are also employed in the bulk chemical synthesis such as methanol synthesis using synthesis gas.
3
3,34
Having fascinated by the Cu chemistry, we have recently shown the application of Cu based
2

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3
metal organic framework in coupling reactions. We have also reported the catalytic reduction
5
3
6
of nitroaromatics using Cu-exchange Nano-ZSM-5 catalyst.
In this study, Cu based ILs (Scheme 1) were prepared and investigated in the synthesis of
primary, secondary and tertiary amides. To achieve the highly recyclable Cu based ILs catalysts,
ILs were supported on high surface area nanocrystalline silicalite.
2
. Experimental Section
2.1. Chemicals
All chemicals used in the study were of AR grade and used as received without further
purification. Copper(II) chloride (CuCl .2H O) copper acetate (Cu(OAc) . H O), sodium acetate
2
2
,
2
2
(NaOAc), sodium chloride (NaCl), sulphuric acid (H SO ), calcium carbonate (CaCO ) and
2 4 3
hydroxylamine hydrochloride (HONH ·HCl) were purchased from Loba Chemie, Mumbai,
2
India. 2-Pyrrolidone, tripotassium phosphate (K PO ) tert-butyl hydroperoxide (TBHP) (70 %
3 4 ,
aqueous solution), tetraethylorthosilicate (TEOS, 98%), n-propyltriethoxysilane, 3-
chloropropyltriethoxysilane, benzaldehyde derivatives, and tetrapropylammonium hydroxide
(TPAOH, 1 M aq. solution) were obtained from Sigma-Aldrich, India. 1-Methylimidazole,
benzaldehyde, sodium carbonate (Na CO ), and iodobenzene were purchased from Spectrochem
2
3
Pvt. Ltd., Mumbai, India. Dimethyl formamide (DMF) and cuprous chloride (CuCl) were
obtained from SD Fine Chem. Ltd., Mumbai. 1-Chlorobutane was purchased from Alfa Aesar,
India. Organic solvents used in this study were obtained from Merck India Pvt. Ltd.
2.2. Catalyst preparation
Reaction steps adopted for the preparation of Cu based ILs catalysts are shown in Scheme 1.
Changes in the color observed during the synthesis of ILs are presented in Scheme S1
(Supporting information).
3

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N
CuCl
Cl .CuCl
N
N
N
N
Bu
Cl
+
Cl
[
Bmim][Cl]-[CuCl]
N
1-MethyImidazole 1-Butylchloride
Bu
CuCl .2H O
2 2
N
[
Bmim][Cl]
Cl .CuCl2
N
NaOAc
Bu
[
Bmim][Cl]-[CuCl2]
N
N
Cu(OAc) .H O
2
2
OAc
OAc
N
N
Cu(OAc)2
Bu
Bu
[Bmim][OAc]-[Cu(OAc)2]
[
Bmim][OAc]
Scheme 1. Schematic representation for the synthesis of Cu based ILs.
2.2.1. Synthesis of [Bmim][Cl]-[CuCl] and [Bmim][Cl]-[CuCl2]
1
-butyl-3-methylimidazolium chloride ([Bmim][Cl]) was prepared by following the
3
7
reported procedure. In a typical synthesis, equivalent amounts of 1-methylimidazole and 1-
chlorobutane were magnetically stirred at 343 K for 48ꢀh.
[
Bmim][Cl] was reacted with equivalent amount of cuprous chloride in neat condition.
The reaction mixture was magnetically stirred at 353 K for 12 h to obtain [Bmim][Cl]-[CuCl].
[
[
Bmim][Cl]-[CuCl ] was prepared by the reaction of equivalent amounts of cupric chloride and
2
Bmim][Cl] at 353 K for 12 h.
2.2.2. Synthesis of [Bmim][OAc]-[Cu(OAc)2]
For the synthesis of [Bmim][OAc]-[Cu(OAc) ], first 1-butyl-3-methylimidazolium
2
3
acetate ([Bmim][OAc]) was prepared by following the reported procedure. [Bmim][Cl] (2.61 g,
8
15 mmol) and NaOAc (1.39 g, 17 mmol) were dissolved in ethanol (15 mL) at ambient condition
and the reaction mixture was stirred at 313 K for 12 h. After the reaction, NaCl was precipitated,
which was removed by filtration. Reaction mixture was evaporated under reduced pressure to
4

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remove the ethanol. Reaction mixture was kept in refrigerator at 273 K for 24 h to precipitate
remaining amount of NaOAc and NaCl. Finally, the precipitate was removed by filtration to
obtain the [Bmim][OAc].
[
Bmim][OAc] was reacted with equivalent amount of copper acetate in neat condition.
3
9
The reaction mixture was stirred at 303 K for 12 h to get [Bmim][OAc]-[Cu(OAc)2].
2
.2.3. Synthesis of nanocrystalline silicalite-1
.26 g of TPAOH (1 M aq. Solution), 0.4 g PrTES, and 6.24 g of deionised water was
mixed and stirred for 15 minutes at ambient temperature, until a clear solution was formed.
.92 g of TEOS was added to the clear solution and the reaction mixture was stirred for 6 h. The
6
4
molar composition of the synthesis gel was 92.5 TEOS/7.5 PrTES/25 TPAOH/2500 H O. Gel
2
was transferred to a Teflon-lined stainless steel autoclave, and hydrothermally treated at 443 K
for 3 days under static conditions. The product was then washed with deionised water and dried
at 373 K for 10 h. Material was calcined at 823 K for 6 h in air to obtain nanocrystalline
silicalite-1 (hereafter represented as Nano-S-1).
2.2.4. Synthesis of Nano-S-1 supported Cu based ionic liquids
Reaction steps adopted for the preparation of supported ILs catalysts are shown in
Scheme 2. Equivalent amount of N-methyl imidazole was reacted with 3-chloropropyltriethoxy
2
silane by following the reported procedure. Resulted compound was reacted with Nano-S-1
9
2
9
pre-activated) by following the reported procedure. The resulted compound was converted to
(
Nano-S-1-[Bmim][Cl]-[CuCl], Nano-S-1-[Bmim][Cl]-[CuCl ] and Nano-S-1-[Bmim][OAc]-
2
[
Cu(OAc) ] by following the procedure discussed above in the Section 2.2.1 and 2.2.2.
2
Materials were subjected to elemental analysis to determine the amount of ILs in the sample.
Furthermore, atomic absorption spectroscopy (AAS) analysis was performed to determine the Cu
content in the sample (Table S1).
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OEt
Si
Cl
OEt
OEt
OEt
N
N
Si
N
N
OEt
OEt
Cl
1
-Methylimidazole
Nano-S-1
CuCl .2H O
2
O
O
2
N
Si
N
OEt
Cl. CuCl2
O
O
N
Nano-S-1-[Bmim][Cl]-[CuCl2]
Si
N
OEt
Cl
Nano-S-1-[Bmim][Cl]
NaOAc
O
CuCl
N
Si
N
O
OEt
Cl. CuCl
Nano-S-1-[Bmim][Cl]-[CuCl]
O
Cu(OAc) .H O
2
2
O
N
N
Si
Si
N
N
O
O
OEt
OEt
OAc
OAc. Cu(OAc)2
Nano-S-1-[Bmim][OAc]
Nano-S-1-[Bmim][OAc]-[Cu(OAc)2]
Scheme 2. Schematic representations for the synthesis of Nano-S-1 grafted Cu based ILs.
2.3. Catalyst characterization
X-ray diffraction (XRD) patterns were recorded in the 2θ range of 5–60° with a scan
speed of 2°/min on a PANalytical X’PERT PRO diffractometer, using Cu Kα radiation
λ=0.1542 nm, 40 kV, 40 mA) and a proportional counter detector. Nitrogen adsorption
(
measurements were performed at 77 K by Quantachrome Instruments, Autosorb-IQ volumetric
adsorption analyzer. Sample was out-gassed at 473 K for 3 h in the degas port of the adsorption
apparatus. The specific surface area was calculated from the adsorption data points obtained at
P/P between 0.05–0.3 using the Brunauer-Emmett-Teller (BET) equation. The pore diameter
0
was estimated using the Barret–Joyner–Halenda (BJH) method. Scanning electron microscopy
6

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(SEM) measurements were carried out on a JEOL JSM-6610LV, Japan to investigate the
morphology of the materials. The nanostructure was investigated using TEM on a JEM-2100,
JEOL-Japan at an accelerating voltage of 200 kV. Fourier transform infrared (FTIR) spectra
-1
were recorded on a Bruker spectrophotometer in the region 400–4000 cm (spectral resolution =
-1
4
cm ; number of scans = 100). Diffused reflectance UV-visible (DR UV-visible) spectra were
recorded on Shimadzu UV-2600 spectrophotometer. BaSO was used as reference material.
4
Thermo gravimetric analysis (TGA) was performed in the range of 298-873 K on a TGA/DSC 1
e
STAR SYSTEM from Mettler Toledo, Switzerland, with temperature increments of 10 K/min in
1 13
N atmosphere. H, C, COSY and HETCOR NMR spectra were recorded on Jeol AM-400
2
MHz NMR.
2.4. Procedure for catalytic reactions
2.4.1. Synthesis of primary amide
A mixture of aromatic aldehyde (1.0 mmol), hydroxylamine hydrochloride (0.07 g, 1
mmol), sodium carbonate (0.12 g, 1.1 mmol), ILs (2 mol %) and water (3 mL) was stirred at 383
K for 2 days in Teflon-lined stainless steel autoclave. After the reaction, autoclave was cooled to
ambient temperature and diluted with ethyl acetate. Organic portion was extracted and analyzed
by using gas chromatography (GC) (Yonglin 6100; BP-5; 30 m × 0.25 mm × 0.25 µm). Reactant
conversion and product selectivity were obtained using GC. Product was confirmed using GC–
MS (Schimadzu GCMS-QP 2010 Ultra; Rtx-5 Sil Ms; 30 m × 0.25 mm × 0.25 µm). Product was
also confirmed by the authentic sample obtained from Aldrich (based on the retention time of the
authentic sample in the GC analysis).
A mixture of benzaldehyde (1.0 mmol), hydroxylamine hydrochloride (0.10 g, 1 mmol),
sodium carbonate (0.12 g, 1.1 mmol), solid catalyst (2 mol % with respect to ILs) and water (3
mL) was stirred at 383 K for 2 days in Teflon-lined stainless steel autoclave. After the reaction,
autoclave was cooled to ambient temperature and catalyst was separated by centrifugation.
Reaction mixture was diluted with ethyl acetate. Organic portion was extracted and analyzed by
GC. Recovered catalyst was washed thoroughly with ethyl acetate, dried at 353 K for 10 h, and
then reused in next cycle.
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2.4.2. Synthesis of secondary amide
To a mixture of ILs (5.0 mol %), glycine ethyl ester hydrochloride (0.16 g, 1.2 mmol),
calcium carbonate (0.11 g, 1.1 mmol) and acetonitrile (0.2 mL); benzaldehyde (0.10 g, 1 mmol)
and TBHP (0.15 g, 1.1 mmol) were added and stirred at ambient temperature for 5 min under
nitrogen atmosphere. Temperature of the reaction mixture was raised to 333 K and the reaction
mixture was stirred for 6 h under nitrogen atmosphere. After the reaction, reaction mixture was
diluted with diethyl ether and water. Organic portion was extracted and analyzed by GC-MS.
2.4.3. Synthesis of tertiary amide
To a mixture of ILs (5.0 mol %), secondary amide/secondary amine (1.2 mmol), and
K PO (0.63 g, 3.0 mmol); iodobenzene (0.20 g, 1.0 mmol) and dimethyl formamide (2.0 mL)
3
4
were added at ambient temperature under nitrogen atmosphere. Temperature of the reaction
mixture was raised to 383 K and the reaction mixture was stirred for 24 h under nitrogen
atmosphere. After the reaction, solvent was evaporated and reaction mixture was diluted with
ethyl acetate and water. Organic portion was extracted and analyzed by GC-MS.
3
. Results and discussion
3.1. Physico-chemical characterization
Synthesis procedure followed for the preparation of Cu based ILs catalysts is presented in
Scheme 1. ILs and their precursor were characterized by NMR. In this study, in-depth NMR
investigation is made for [Bmim][OAc]-[Cu(OAc) ] and its precursor compound. Brief
2
description of NMR investigation along with NMR spectra is provided in Supporting information
section (Fig. S1, S2). FT-IR spectra of [Bmim][Cl]-[CuCl], [Bmim][Cl]-[CuCl ], [Bmim][OAc]-
2
[
Cu(OAc) ] and their precursors are shown in Fig. S3. TGA analysis provides evidence for the
2
formation of addition complex with Cu salts (Fig. S4). The decomposition temperatures for
Bmim][Cl] and [Bmim][OAc] were found to be 475 K and 496 K, respectively. Upon addition
[
of Cu salt, the decomposition temperature of Cu based ILs was shifted. The decomposition
temperatures for [Bmim][Cl]-[CuCl], [Bmim][Cl]-[CuCl ] and [Bmim][OAc]-[Cu(OAc) ] were
2
2
found to be 534 K, 600 K, and 509 K, respectively.
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Nano-S-1-[Bmim][OAc]-
[
Cu(OAc) ]
2
Nano-S-1
10
20
30
40
50
2
θ
(degree)
Fig. 1: XRD patterns of Nano-S-1 and Nano-S-1-[Bmim][OAc]-[Cu(OAc)2] .
In this study nanocrystalline silicalite (Nano-S-1) was chosen as support material to
functionalize the ILs. Synthesis procedure followed for the preparation of Cu based ILs
supported on Nano-S-1 is shown in Scheme 2. Nano-S-1 exhibited XRD pattern similar to the
4
0
XRD pattern of silicalite-1 reported in literature (Fig. 1). XRD pattern of Nano-S-1
corresponds to MFI framework structure of silicalite-1. The N -adsorption isotherm of Nano-S-1
2
exhibited a type-IV isotherm similar to mesoporous materials (Fig. S5). A sharp increase in the
volume of N adsorption in the region 0.4<P/Po<0.9 is characteristic of the capillary
2
condensation within the intercrystalline mesopore void spaces. The mesopores for Nano-S-1
showed a pore size distribution in the range of 2–10 nm. Total surface area, external surface area,
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Fig. 2: SEM images of Nano-S-1 and Nano-S-1-[Bmim][OAc]-[Cu(OAc)2].
2
3
and total pore volume of Nano-S-1 were found to be 498 m²/g, 245 m /g, and 0.57 cm /g,
respectively. SEM image shows that Nano-S-1 exhibits spheroid morphology with particle size
in the range of 300-500 nm (Fig. 2). TEM image clearly shows that these particles are built with
small zeolite nanocrystals of the order of 15-20 nm (Fig. S6). Nano-S-1 was prepared in the
presence of PrTES as additive. Nano-S-1 was obtained by the addition of PrTES to the synthesis
1
0

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composition of conventional silicalite. PrTES contains three hydrolyzable moieties and one
hydrophobic propyl group. The hydrophobic propyl group is unfavorable for the formation of
extended tetrahedral SiO linkages. Consequently, the zeolite growth is significantly retarded at
2
the organic and inorganic interfaces, resulting in the formation of nanocrystalline zeolites.
Mesopores are created in Nano-S-1 because of the crystal packing of these nano size zeolite
particles. Large external surface area and surface silanol groups are important for the surface
functionalization. Furthermore, Nano-S-1 donot contain any Al in the framework, therefore the
catalytic activity demonstrated in this study is only due to Cu based ILs functionalized on the
surface of Nano-S-1.
XRD patterns of Nano-S-1-[Bmim][Cl]-[CuCl], Nano-S-1-[Bmim][Cl]-[CuCl ] and
2
Nano-S-1-[Bmim][OAc]-[Cu(OAc) ] were found to be similar to Nano-S-1, which confirms that
2
the functionalization of ILs did not disturb the framework structure of the material (Fig. 1). Upon
surface functionalization, surface area and pore volume of the material were reduced. For
example, the total surface area, external surface area, and total pore volume of Nano-S-1-
2
3
[
Bmim][OAc]-[Cu(OAc) ] were found to be 394 m²/g, 150 m /g, and 0.44 cm /g, respectively.
2
DR-UV visible study provides further information about the incorporation of metal salts in the
ILs structure (Fig. S7). TGA analysis was performed for Nano-S-1 supported Cu based ILs
samples (Fig. S4). The decomposition temperatures for Nano-S-1-[Bmim][Cl] and Nano-S-1-
[
Bmim][OAc] were found to be 494 K and 556 K, respectively. Upon addition of Cu salt, the
decomposition temperature was shifted. The decomposition temperatures for Nano-S-1-
Bmim][Cl]-[CuCl], Nano-S-1-[Bmim][Cl]-[CuCl ] and Nano-S-1-[BMIM][OAc]-[Cu(OAc) ]
[
2
2
were found to be 558 K, 560 K, and 598 K, respectively. The amount of the ILs anchored on
Nano-S-1 was calculated by elemental analysis and TGA analysis. The amounts of [Bmim][Cl]-
[
CuCl], [Bmim][Cl]-[CuCl ] and [Bmim][OAc]-[Cu(OAc) ] anchored on Nano-S-1 are found to
2 2
be 0.55 mmol/g, 0.56 mmol/g, and 0.49 mmol/g by TGA, which are consistent with the
elemental analysis of these materials (Table S1). Incorporation of Cu based ILs on the external
surface of Nano-S-1 was also confirmed by the EDX spectrum obtained from the SEM
investigation. Morphology of Nano-S-1-[Bmim][OAc]-[Cu(OAc) ] is similar to the morphology
2
of Nano-S-1 (Fig. 2). EDX analysis clearly shows that Cu is not present in the parent Nano-S-1
but it is present in the EDX spectrum of Nano-S-1-[Bmim][OAc]-[Cu(OAc) ] along with other
2
constituent elements of Nano-S-1 (Fig. 3).
1
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Fig. 3: EDX spectra (right panel) obtained from the SEM images (Left panel) of Nano-S-1 and
Nano-S-1-[Bmim][OAc]-[Cu(OAc)2].
3.2.
Catalytic activity
Recent literature suggests that efforts are being made for the synthesis of amides using a
4
1-49
variety of catalysts.
Cu based ILs prepared in this study was investigated in the synthesis of
wide range of aromatic amides. In-depth investigation was made for the synthesis of primary
amide with Cu based ILs and supported ILs catalysts. First, reaction condition was optimized
with benzaldehyde as a reactant and [Bmim][OAc]-[Cu(OAc) ] as a catalyst (Table 1). Influence
2
of the base was investigated. Among the base investigated in this study, Cs CO exhibited the
2
3
highest activity. However, the activities of Na CO and K CO were also found to be very good.
2
3
2
3
Considering the economy of the process, Na CO was chosen as suitable base for further
2
3
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optimization. Reactions were performed to investigate the influence of solvent in the production
of benzamide from benzaldehyde. Among the solvent investigated, H O was found to be ideally
2
suited for this reaction. It was found that high selectivity of benzamide was obtained when the
reaction was performed at 383 K. Reaction also took place at low temperature (353 K), however,
it lead to the formation of oxime as side product. Having optimized the reaction condition,
several Cu based ILs were investigated in this reaction. All Cu based catalysts investigated in
this study were found to be active and produced benzamide as selective product. The order of
catalytic activity was found to be [Bmim][OAc]-[Cu(OAc) ]>[Bmim][Cl]-[CuCl ] ≈
2
2
[
Bmim][Cl]-[CuCl]. Having found the best catalyst and optimized reaction condition, it was
important to investigate the reusability of catalyst system. After the reaction, two separate phases
were obtained. Aqueous phase containing [Bmim][OAc]-[Cu(OAc) ] and base were easily
2
separated from the organic portion containing products and un-reacted benzaldehyde. Aqueous
portion was extracted several times with ethyl acetate to separate the product/reactant. Water
fraction was evaporated under reduced pressure and then diluted with CH Cl . CH Cl fraction
2
2
2
2
was collected and evaporated to recover the [Bmim][OAc]-[Cu(OAc) ]. During this process
2
approximately 8 wt % of ILs was lost; therefore it was not possible to recover 100% ILs by this
process. However, recovered ILs was found to be reusable. Catalytic activity data demonstrated
that only a negligible decrease in the catalytic activity was observed after first cycle.
1
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Table 1. Optimization of the reaction condition for the synthesis of benzamide from
benzaldehyde and hydroxyl amine hydrochloride salts using Cu based ILs catalyst.
O
O
H
Catalyst
NH2
+
NH OH. HCl
2
E.
no.
Benzaldehyde
conv. (%)
Catalyst
Base
Solvent
Temp. (K)
1
2
3
4
5
6
7
8
9
[Bmim][OAc]-[Cu(OAc)2]
[Bmim][OAc]-[Cu(OAc)2]
[Bmim][OAc]-[Cu(OAc)2]
[Bmim][OAc]-[Cu(OAc)2]
[Bmim][OAc]-[Cu(OAc)2]
[Bmim][OAc]-[Cu(OAc)2]
[Bmim][OAc]-[Cu(OAc)2]
[Bmim][OAc]-[Cu(OAc)2]
Na CO
2
H O
2
383
383
383
383
383
383
383
383
353
373
383
383
92
93
100
70
60
42
35
49
83
88
81
82
3
K CO
2
H O
2
3
Cs CO
2
H O
2
3
NaHCO3
Na CO
H O
2
CH OH
3
2
3
3
3
3
3
3
3
3
Na CO
2
C H OH
2 5
Na CO
2
Toluene
Na CO
2
CH CN
3
a
[Bmim][OAc]-[Cu(OAc)2]
[Bmim][OAc]-[Cu(OAc)2]
[Bmim][Cl]-[CuCl]
Na CO
2
H O
2
10
11
12
Na CO
2
H O
2
Na CO
2
H O
2
[Bmim][Cl]-[CuCl2]
Na CO
2
H O
2
b
13
[Bmim][OAc]-[Cu(OAc)2]
Na CO
2
H O
2
383
89
3
Reaction condition: Benzaldehyde (1.0 mmol), hydroxylamine hydrochloride (1.0 mmol),
catalyst (2.0 mol %), base (1.1 mmol), solvent (3 mL), reaction time (2 d).
a
80 % selectivity for the benzamide and 20 % selectivity of oxime were obtained.
Data obtained after first recycle.
b
We were interested to develop recyclable and user-friendly catalyst system based on ILs
for this process. To achieve this goal, ILs was functionalized on the surface of Nano-S-1.
Through this study, we would like to demonstrate the functionalization ability of Nano-S-1,
which is rarely reported in the literature. Functionalized ILs based heterogeneous catalysts were
investigated in the synthesis of benzamide from benzaldehyde under the optimized reaction
condition (Table 2). In this case also, [Bmim][OAc]-[Cu(OAc) ] functionalized Nano-S-1
2
exhibited the highest activity. After the reaction, catalyst was easily centrifuged, washed with
1
4

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ethyl acetate, dried at 353 K for 10 h, and reused in the next cycle. Recovered catalyst was
recycled at least five times with no appreciable decrease in the catalytic activity. To study the
leaching phenomenon, hot filtration test was performed. The solid catalyst was removed after 6 h
of reaction (Benzaldeyde conversion = 25.1 %) and the reaction was extended for 2d. No further
increase in the benzaldehyde conversion (= 25.4 %) was observed after the catalyst removal.
Atomic absorption (AAS) analysis only showed 0.3 ppm copper in the solution, demonstrating
that essentially no Cu was leached into the solution. These studies show that [Cu(OAc) ] are
2
strongly bonded with [Bmim][OAC] and Nano-S-1 and that the trace amount of copper leached
into solution was unable to catalyze the reaction. These investigations confirmed the
heterogeneous nature of the catalytic system under the optimized reaction condition.
Scope of this catalytic protocol was extended for the wide range of aromatic aldehyde.
Aromatic aldheydes were converted to aromatic amides in good to excellent yield by this
protocol. Reactivity of aromatic aldehyde varies by varying the substituent (Table 2). Electron
donating substituted (-OCH ) aromatic aldehyde exhibited higher product yield when compared
3
to electron withdrawing substituted (-NO ) aromatic aldehyde. Based on the results obtained one
2
can conclude that electron donating group enhances the rate of reaction whereas electron
withdrawing group retards the reaction rate. This protocol can also be applied to cinnamaldehyde
and aliphatic aldehydes. In these cases product yield was comparatively low when compared to
benzaldehyde. To achieve good product yield, rate of the reaction need to be enhanced. This can
be achieved just by increasing the amount of catalyst. Here we have shown that product yield can
be easily improved by increasing the amount of catalyst (Table 2). Mechanism of the formation
of amide from aldehyde is presented in Scheme 3. Dehydration/hydration followed by
4
2
rearrangement lead to form the amide. This mechanism is consistent with the literature report.
Furthermore; oxime obtained at low temperature also advocates the proposed mechanism.
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Table 2. Synthesis of benzamide and its derivative using Cu based ILs catalysts
E.
No.
Catalyst
Reactant
Product (Yield %)
1
Nano-S-1-[Bmim][OAc]-[Cu(OAc)2]
(82)
Nano-S-1-[Bmim][Cl]-[CuCl2]
Nano-S-1-[Bmim][Cl]-[CuCl]
2
3
(
77)
O
NH2
(66)
O
Nano-S-1-[Bmim][OAc]-
a
4
5
NH2
[
Cu(OAc)2]
(80)
[Bmim][OAc]-[Cu(OAc)2]
(84)
6
7
[Bmim][OAc]-[Cu(OAc)2]
[Bmim][OAc]-[Cu(OAc)2]
(
67)
65)
(
1
6

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8
[Bmim][OAc]-[Cu(OAc)2]
(
62)
92)
b
9
[Bmim][OAc]-[Cu(OAc)2]
(
Reaction condition: Aldehyde (1.0 mmol), hydroxylamine hydrochloride (1.0 mmol), catalyst
2.0 mol %), Na CO (1.1 mmol), H O (3 mL), reaction temperature (383 K), time (2 d).
(
2
3
2
a
Data obtained after fifth recycle.
b
Catalyst (3 mol %).
[
IL-Cu]
R
N
OH
H
H
O
N
N
IL-Cu
N R
R
R
H
R
R
O
O
(C)
(
D)
NH OH.HCl
2
(
A)
OH
[
IL-Cu]
OH
N
[IL-Cu]
N
R
N
[IL-Cu]
-
H O
2
H O
2
H
R
H
R
NH2
(A)
(C)
(B)
Scheme 3. Proposed mechanism for the conversion of aldehyde to primary amide using Cu
based ILs
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We were interested to show the applicability of this catalyst system in the synthesis
of
wide range of amides, such as secondary amides and tertiary amides. Cu based ILs and supported
ILs catalysts were investigated in the synthesis of secondary amide (Table 3). Model reaction of
benzaldehyde and glycine ethyl ester hydrochloride to form ethyl 2-benzamido acetate was
chosen to optimize the reaction condition using [Bmim][Oac]-[Cu(Oac) ]. Wide range of bases
2
such as Na CO , K CO and CaCO were investigates. CaCO exhibited the highest product yield
2
3
2
3
3
3
(90%) when compared to other bases (Na CO (61%), K CO (69%)) investigated in this study
2 3 2 3
using [Bmim][OAc]-[Cu(OAc) ]. Influence of solvent show that reaction was facilitated in the
2
presence of acetonitrile when compared to CH Cl , toluene, and H O. TBHP was found to be the
2
2
2
best among various oxidants investigated in this study. Reaction proceeded very well at ambient
temperature. Having optimized the reaction condition, various Cu based ILs and supported ILs
were investigated in the synthesis of ethyl 2-benzamido acetate from benzaldehyde. All cu based
catalysts exhibited almost similar activity in this reaction. The activity of Nano-S-1-
[
(
Bmim][Oac]-[Cu(Oac) ] was somewhat less when compared to [Bmim][Oac]-[Cu(Oac) ]
2 2
Table 3). Scope of this catalytic protocol was extended for the wide range of aromatic
aldehydes. Aromatic aldheydes were converted to aromatic amides in good to excellent yield by
this protocol. Reactivity of aromatic aldehyde varies by varying the substituent (Table 3).
Electron donating substituted (-OCH ) aromatic aldehyde exhibited higher product yield when
3
compared to electron withdrawing substituted (-NO ) aromatic aldehyde. This protocol can also
2
be applied to aliphatic aldehydes. In the case of heptanal, product yield was comparatively low
when compared to benzaldehyde. The reaction proceeds via a hemiaminal intermediate that is
oxidized to amide via a free-radical pathway (Scheme 4).
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Table 3. Synthesis of secondary amide by the reaction of aldehyde and glycine ethyl ester
hydrochloride using Cu based ILs catalyst.
E.
No.
Product
yield (%)
Catalyst
Reactant
1
2
3
4
[Bmim][OAc]-[Cu(OAc)2]
90
91
94
82
[
Bmim][Cl]-[CuCl]
[
Bmim][Cl]-[CuCl2]
Nano-S-1-[Bmim][OAc]-[Cu(OAc)2]
[
Bmim][OAc]-[Cu(OAc)2]
5
93
[
[
Bmim][OAc]-[Cu(OAc)2]
Bmim][OAc]-[Cu(OAc)2]
6
7
72
45
O
H
Reaction condition: Catalyst (5.0 mol %), aldehyde (1.0 mmol), glycine ethyl ester hydrochloride
1.2 mmol), calcium carbonate (1.1 mmol), tert-butyl hydroperoxide (1.1 mmol), acetonitrile (0.2
mL), reaction temperature (333 K), time (6 h).
(
1
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Scheme 4. Proposed mechanism for the synthesis of secondary amide from aldehyde using Cu
based ILs.
Tertiary amide can be synthesized by the reaction of haloarene and secondary amide. In
this study, a model reaction between 2-pyrrolidinone and iodobenzene leads to the formation of
tertiary amide (Table 4). Wide range of bases such as Na CO , K CO , CsCO and K PO were
2
3
2
3
3,
3
4
investigates. K PO exhibited the highest product yield (79 %) when compared to other bases
4
3
(Na CO (12 %), K CO (15 %), and CsCO (32 %) investigated in this study using [Bmim][Cl]-
2 3 2 3 3
[
CuCl ]. Influence of solvent show that reaction was facilitated in the presence of N,N’-
2
dimethylformamide (DMF) when compared to toluene and xylene. Having optimized the
reaction condition, various Cu based neat and supported ILs catalysts were investigated. Among
the catalysts investigated, [Bmim][Cl]-[CuCl ] exhibited higher activity. The activity of Nano-S-
2
1-[Bmim][Cl]-[CuCl ] was somewhat low when compared to [Bmim][Cl]-[CuCl ]. Reaction
2 2
with bromobenzene and chlorobenzene was not facilitated. However, a wide range of secondary
amides and amines were able to convert to tertiary amides and amines with this protocol. Some
of these representative examples are given in Table 4. Mechanism for the formation of amides
and amines is provided in Scheme 5. This mechanism is consistent with the Ullmann mechanism,
in which nucleophile coordination, oxidative addition, followed by reductive elimination lead to
form the product.
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Table 4. Synthesis of tertiary amides/amines by the reaction of iodobenzene and secondary
amides/secondary amines using Cu based ILs catalyst
.
E.
Product
yield (%)
Catalyst
no.
Amine/Amide
1
2
3
4
[Bmim][Cl]-[CuCl
2
]
79
58
57
65
[Bmim][Cl]-[CuCl]
[Bmim][OAc]-[Cu(OAc)
]
2
Nano-S-1-[Bmim][Cl]-[CuCl
Nano-S-1-[Bmim][Cl]-[CuCl
]
]
2
5
6
2
78
42
Nano-S-1-[Bmim][Cl]-[CuCl
]
2
Reaction condition: Catalyst (5.0 mol %), iodobenzene (1.0 mmol), secondary amide/amine (1.2
mmol), DMF (2 mL), K PO (3.0 mmol), reaction temperature (383 K), time (24 h).
3
4
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R
R
Base
+
Ph N
HN
R'
R'
Cu-IL
Base-H
R
R
N R'
IL-Cu
IL-Cu
N
R'
Ph
Ph I
I
Scheme 5. Proposed mechanism for Cu based ILs catalyzed conversion of secondary
amide/amine to tertiary amide/amine.
4
. Conclusion
Copper based ionic liquid catalysts were found to be excellent catalysts for the synthesis
of amides. Wide range of amides such as benzamide, 2-benzamido acetate, tertiary amides and
their derivatives were successfully prepared in good to excellent yield. For easy recovery and
recyclability, nanocrystalline silicalite supported Copper based ionic liquids were successfully
prepared. Large external surface area and abundant surface silanol groups of nanocrystalline
silicalite were important for the preparation of supported catalyst. Heterogenized Copper based
ionic liquids were found to be efficient catalysts for these reactions. Supported Copper based
ionic liquid catalysts exhibited no loss in the activity even after five recycles in the synthesis of
benzamide. Hot filtration test further confirmed that catalyst was not leached during the reaction.
Acknowledgements
Authors thank Council of Scientific and Industrial Research, New Delhi for financial assistance
(CSIR grant 01(2802)/14/EMR-II). Authors are also thankful to Director IIT Ropar for his
constant encouragement.
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Supporting Information
The Supporting Information contains additional characterization results.
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3

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Graphical Abstract
Highly efficient and recyclable copper based ionic liquid catalysts for amide synthesis
Poonam Rani and Rajendra Srivastava*
Copper based ionic liquids have been successfully prepared for the synthesis of primary,
secondary and tertiary amides under mild reaction condition.