Synthesis of secondary amides by direct amidation using polymer supported copper(II) complex

Article abstract of DOI:10.1016/j.poly.2018.04.004

A new polymer supported Cu(II) complex has been synthesized and characterized by CHN analyses, IR and UV–Vis spectral studies, ESR and thermogravimetric analyses, ICP-OES, surface area measurements. This complex was screened for their catalytic study towards the direct amidation reaction. The effects of solvents, reaction time, temperature and catalyst amount for the direct formation of amides from aldehydes and benzylamine with the aid of heterogeneous copper complex were reported. The polymer supported Cu(II) catalyst could be reused more than five times without appreciable loss of its initial activity. The plausible reaction mechanism has been proposed. The catalytic activity of the unsupported complex was also compared with the polymer supported Cu(II) complex.

Full text of DOI:10.1016/j.poly.2018.04.004

Accepted Manuscript
Synthesis of secondary amides by direct amidation using polymer supported
copper(II) complex
MK. Renuka, V. Gayathri
PII:
S0277-5387(18)30171-2
https://doi.org/10.1016/j.poly.2018.04.004
POLY 13105
DOI:
Reference:
Polyhedron
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16 February 2018
3 April 2018
Please cite this article as: MK. Renuka, V. Gayathri, Synthesis of secondary amides by direct amidation using
polymer supported copper(II) complex, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.04.004
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Synthesis of secondary amides by direct amidation using polymer supported copper(II)
complex
Renuka M K and Gayathri V*
Department of Post Graduate Studies in Chemistry, Central College Campus, Bangalore
University, Dr. Ambedkar Street, Bangalore 560001, Karnataka, India.
*Corresponding author e-mail: gayathritvr@yahoo.co.in, Phone: 080-22961348
Abstract A new polymer supported Cu(II) complex has been synthesized and characterized by
CHN analyses, IR and UV–Vis spectral studies, ESR and thermogravimetric analyses, ICP-OES,
surface area measurements. This complex was screened for their catalytic study towards the
direct amidation reaction. The effects of solvents, reaction time, temperature and catalyst amount
for the direct formation of amides from aldehydes and benzylamine with the aid of
heterogeneous copper complex were reported. The polymer supported Cu(II) catalyst could be
reused more than five times without appreciable loss of its initial activity. The plausible reaction
mechanism has been proposed. The catalytic activity of the unsupported complex was also
compared with the polymer supported Cu(II) complex.
Keywords Cu(PS-BBMA)Cl2, Direct amidation, Heterogeneous catalysis, Benzyl amine
Introduction
Amide functionality is well known due to its stability, high polarity and conformational diversity
and constitute the most abundant motif in synthesis and medicinal chemistry [1]. They are found
in compounds with anticancer, antihypertensive, antiinflammatory, antiallergic, antimigraine,
and antiviral activities [2-4]. Due to the broad application of amides and sulfonamides, finding
new, efficient and practical methods for their synthesis is desirable. Amides are produced in
enormous amounts every year, thus, environmental friendly and selective methods for their
formation are of great importance.
The most prevalent and straight forward approach to amide bond formation is the condensation
of an amine with a carboxylic acid derivative [5-7]. On the other hand, due to instability of
1

activated carboxylic acid derivatives, it confines their universal applications and creates
significant challenges [8]. To overcome these challenges, alternative routes like hydrative amide
synthesis with terminal alkynes [9], carbonylation of alkynes [10], alkenes and haloarenes [11],
direct amidation of aldehydes [12] have been used.
Transition metal-catalyzed reactions have emerged in recent years as more atom-economical and
powerful tools for preparing amides. The seminal work in the field oxidative amidation was
performed by Milstein and co-workers, who developed a ruthenium pincer complex that
produces hydrogen gas as by-product and hence avoids the use of external oxidants [13]. A
rhodium complex with hydrogen peroxide as stoichiometric oxidant for the formation of primary
and secondary amides has also been reported for the oxidative amide formation from alcohols
[14]. Palladium complex as catalyst with stoichiometric amounts of an aryl halide as oxidant also
formed amides in good yields [15]. Copper iodide has also been used in catalytic amounts in
combination with tert-butyl hydroperoxide (TBHP) for the oxidative amidation of aldehydes
[16]. In addition, lanthanide-catalyzed Cannizzaro-type protocols have been reported [17, 18].
Furthermore, organocatalytic protocols employing N-heterocyclic carbenes for oxidative
amidation of aldehydes have been developed [19, 20]. In general, formation of imines from
alcohol/aldehyde with amine over heterogeneous catalytic systems is possible [21, 22] but the
current system gave amides from aldehydes and amines in the presence of polymer supported
Cu(II) catalyst.
Direct amidation is the reaction between an aldehyde and an amine, generating the amide
product and hydrogen gas as the only by-product. These reactions attracted considerable
attention due to the high atom economy and easy availability of substrates [23-29]. Amides are
produced in enormous amounts every year, thus, environmentally friendly and selective methods
for their formation are of great importance. In contrast to the most common methods for amide
formation to date, which involve the use of waste-intensive, expensive and often toxic coupling
reagents and also to circumvent the problems of harsh reaction conditions and low atom
economy, catalysis is a promising strategy for amide synthesis. A desired scenario for the amide
synthesis, the direct reaction between readily available aldehydes and amines, is currently very
limited. Herein we describe the direct synthesis of amide compounds from readily accessible
2

benzylamine and aldehyde surrogates. The present method features a much broad scope on
amide synthesis and eliminates the requirement of oxidant. This paper is focused on catalytic
amidation using polymer supported copper complex of substituted benzimidazole Cu(PS-
BBMA)Cl2 under mild reaction conditions to produce secondary amides in good yields. To the
best of our knowledge, there is still no report on using polymer supported copper complex as
catalyst on the direct amidation of aldehydes and alcohols.
Experimental
Materials
Chloromethylated polystyrene divinyl benzene (PS) was obtained as a gift from THERMAX Ltd.
India. The CuCl2.2H2O, iminodiacetic acid, 1,2-diaminobenzene, and substituted aldehydes were
purchased from Merck and used without further purification. Solvents were used after
purification by standard methods.
Catalyst preparation
Bis(benzimidazol-2-ylmethyl)amine (BBMA) was synthesized according to the procedure
described in the literature [30]. Chloromethylated polystyrene (1.0 g; Cl content 6 mmol) was
swelled in DMF for 2 h, and then a solution of BBMA (0.29 g, 0.78 mmol) in DMF (10 mL) was
added, followed by triethylamine (4 mL) and ethyl acetate (30 mL). The mixture was heated at
100 °C for 48 h. After cooling, it was filtered, Soxhlet extracted with ethanol, and dried. 1.0 g of
functionalized polymer (PS-BBMA) was added to ethanol (10 mL) and kept for 1 h. A solution
of CuCl2.2H2O (0.23 g; 1 mmol) in ethanol (20 mL) was added to it and the mixture was stirred
at 70 °C for 48 h. The resultant bluish green coloured beads were filtered off and washed by
Soxhlet extraction and dried at 100 °C (Scheme 1).
3

Scheme 1. Preparation of the polymer anchored Cu(II) complex, Cu(PS-BBMA)Cl2.
Characterization of Cu(PS-BBMA)Cl2
Elemental analyses were carried out using an Elementar Vario Micro Cube CHNS analyser.
Diffuse reflectance spectra were recorded as BaSO4 disks on a Shimadzu UV-Vis-NIR model-
3101P spectrophotometer. Infrared spectra were recorded on Shimadzu 8,400 s FTIR
spectrometer in the range of 4000-400 cm−1 using KBr disks. Thermogravimetric analyses was
carried out with a TA instrument, SDT analyser model Q600. ESR measurements were carried
out in the solid state at room temperature in a standard X band JES - FA200 spectrometer. The
percentage of Cu in the supported complex was determined by using AAS, Avanta Pm, GF 3000.
Nuclear magnetic resonance spectra were recorded on 300 and 600 MHz Bruker Avance
spectrometer using TMS as standard in CDCl3 solvent. Spin multiplicities are described as s
(singlet), d (doublet), t (triplet), q (quartet), m (multiplet), dd (doublet of doublet) and br s (broad
singlet).
General procedure for direct amidation reaction
In a typical reaction 60 mg of Cu(PS-BBMA)Cl2 was taken in 5 mL acetonitrile, benzaldehyde
(1.0 mmol) and benzylamine (1.2 mmol) were added to it. The mixture was stirred at 60 °C for 6
h (monitored by TLC). The catalyst was separated by filtration and the solution was concentrated
in vacuo. The resulting residue was purified by column chromatography on silica gel with ethyl
acetate/petroleum ether (1:8) as elutant to afford the amide product.
4

Scheme 2. Synthesis of amide (Table 2, entry 8) catalyzed by Cu(PS-BBMA)Cl2.
Results and Discussion
Catalyst Characterization
CHN and FTIR analyses
The elemental analyses and IR spectral data of polymer support, ligand, functionalized polymer
and supported complex are depicted in Table 1. The % of Cu present in the polymer supported
complex was 2.3 %.
Table 1
Elemental analyses and IR spectral data (cm-1)
CHN analyses (%)
IR spectral data
Compound
BBMA
PS
PS-BBAMA 90.7 5.2 4.1 -
Cu(PS-
C
N
H
Cu ν(CH2-Cl) ν(C-Cl) ν(ArN-H) ν(N-H) ν(C=N)
70.1 23.1 6.8 -
97.8 2.2 -
-
-
3056
-
3010
1583
-
-
1267 (s)
1267 (w)
829 (s)
-
-
-
829 (w) 2924
- 2923
1580
1508
90.1 5.6 4.3 2.3 -
BBMA)Cl2
s = strong; w = weak band
The BBMA exhibited (ArN-H) ν(N-H) and ν(C=N) peaks at 3056, 3010 and 1583 cm−1
respectively. The functionalization of polymer beads was confirmed by comparing the IR
spectrum of PS-BBMA with polymer beads and BBMA. The PS-BBMA showed a peak at 1580
cm−1 corresponding to ν(C=N) which was absent in polymer beads while latter displayed peaks at
1267 and 829 cm−1 ascribed to ν(CH2-Cl), ν(C-Cl) respectively decreased in intensity was an
evidence for anchoring of BBMA on to polymer and the peak at 3010 cm-1 due to aliphatic N-H
in BBMA was absent in PS-BBMA, it confirmed that anchoring of aliphatic nitrogen of BBMA
5

to the polymer. Cu(PS-BBMA)Cl2 exhibited a peak at 1508 cm−1 due to ν(C=N) which was
shifted by 72 cm-1 on complexation indicating the coordination of copper to aliphatic secondary
nitrogen of benzimidazole moieties (Table 1).
Electronic spectral studies
330
(a)
(b)
208
(c)
337
200
300
400
500
Wavelength (nm)
Fig. 1. Electronic spectra of (a) PS, (b) PS-BBMA and (c) BBMA
225
315
352
692
838
431
200
400
600
Wavelength (nm)
800
1000
Fig. 2. Electronic spectrum of Cu(PS-BBMA)Cl2
6

Electronic spectra of PS, PS-BBMA and its complex are displayed in figures 1 and 2. The bands
in the range of 200 to 293 nm were attributed to π → π* and n → π* transitions and bands at
315-352 nm due to charge transfer. In addition to ligand peaks, polymer supported complex
exhibited bands at 431 (2Eg → 2B1g), 692 (2B2g → 2B1g) and 838 nm (2A1g → 2B1g) attributed to
d → d transitions (Fig 2) indicating the square pyramidal environment around Cu(II) ion.
ESR spectral and magnetic moment measurements
Room temperature magnetic moment of Cu(PS-BBP)Cl2 was measured using vibrating sample
magnetometer (VSM). It exhibited a value of 1.71 BM indicating the presence of one unpaired
electron.
X-band ESR spectrum of polymer supported complex at room temperature showed a well-
defined hyperfine structure with four line splitting due to interaction of electron with the nuclear
spin of copper(II) (I = 3/2) (Fig. 3). ESR of Cu(PS-BBMA)Cl2 exhibited axial symmetric g-
tensor parameters with g║ ˃ g┴ ˃ 2.0023 (g║ = 2.24, g┴ = 2.06, A|| = 19 mT, A┴ = 27 mT)
indicating that the copper site has a dx2-y2 ground state characteristic of five coordinated square
pyramidal stereochemistry.
1000
2.18
2.29
500
2.41
0
-500
2.06
-1000
-1500
-2000
250
300
350
400
Magnet field value (mT)
Fig. 3. X-band ESR spectrum at room temperature for Cu(PS-BBMA)Cl2
7

TGA measurements
The polymer support was stable up to 275 ºC. Cu(PS-BBMA)Cl2 complex showed the first stage
decomposition in the range 184-216 ºC with mass loss of ~2.5 % attributed to the loss of
chlorides. In the second stage loss of BBMA ligand occurred in the temperature range 216-310
ºC with ~9.3 % weight loss, while the third step (>301 ºC) could be attributed to the polymer
decomposition (Fig. 4).
184 oC
275 oC
~ 2.5% weight loss
216 oC
90
~ 9.3% weight loss
301 oC
80
367 oC
70
60
410 oC
~ 68.3% weight loss
50
40
30
528 oC
500
100
200
300
400
Temperature (oC)
Fig. 4. Thermograms of (a) PS and (b) Cu(PS-BBMA)Cl2
Catalytic activity studies
Effect of catalyst free and oxidant free conditions
The synthesized Cu(II) polymer supported complex was used to investigate the optimized
reaction conditions. Several parameters such as solvents, reaction temperature, time and catalyst
amount were varied by employing benzaldehyde and benzylamine as the model reaction. The
amidation reaction was carried out at 60 °C for 6 h in the absence of catalyst wherein only 20 %
yield occurred. The reaction performed in presene and absence of oxidant TBHP, the yield
obtained was same, and the reaction was performed in O2 as oxidant to confirm whether the
8

atmospheric air helps in the amidation, it confirmed that O2 is not involved in the amide
formation. Hence the amidation reactions were carried out in the absence of oxidant.
Effect of solvents
To explore the effect of reaction media on the transformation yield, reaction was carried out in
water, thf, methanol, and acetonitrile. Though the reaction proceeded in all the solvents, the yield
was maximum in acetonitrile. Hence, it was chosen for the subsequent investigations (Table 2,
entry 3).
Effect of temperature
This amidation was found to be sensitive to the reaction temperature. At lower temperatures
(RT-40 °C) only low yield was obtained (Table 2, entries 8 and 9; 10-29 % yield). A reaction
temperature of 60 °C was found to be optimal for the model reaction to give 90 % yield whereas
at higher temperature yield decreased. Therfore 60 °C was considered as optimum temperature.
Effect of catalyst amount
Effect of concentration of catalyst in amidation reaction was evaluated using different amounts
of catalyst. On increasing the Cu(PS-BBMA)Cl2 amount from 30-60 mg, the amide yeild was
increased from 40 to 90 % and for 90 mg there was no increase in yield was observed (Table 2,
entries 3, 5, 6). As 60 mg of catalyst showed maximum yield it was choosen as optimum
concentration.
Thus, the optimized reaction conditions for amidation of benzaldehyde (1 mmol) with
benzylamine (1.2 mmol) were the 60 mg of Cu(PS-BBMA)Cl2 in 4 mL acetonitrile at 60◦C for 6
h (entry 3, Table 2).
Comparision of catalutic activity with unsupported Cu(II) complex
The unsupported Cu(II) complex Cu(BBMA)Cl2 was prepared according to the literature [31].
Under optimized reaction conditions, Cu(BBMA)Cl2 showed 40 % yield (entry 12). The results
indicated that polymer supported Cu(II) complex showed good catalytic activity than the
9

unsupported complex.
Table 2
Results of screening the reaction conditionsa
No. Solvents Catalyst
amount (mg)
Oxidant Temperature Yeild (%)b
(°C)
1
2
3
4
5
6
7
8
9
THF
H2O
60
60
-
-
-
-
-
-
60
60
60
60
60
60
60
RT
40
80
60
60
72
62
90
42
40
88
90
10
29
80
20
40
CH3CN 60
CH3OH 60
CH3CN 30
CH3CN 90
CH3CN 60
CH3CN 60
CH3CN 60
TBHP
-
-
-
10 CH3CN 60
11 CH3CN Blank reaction -
12 CH3CN Cu(BBMA)Cl2
-
a All the reactions were run with benzaldehyde (1 mmol), benzylamine (1.2 mmol) with catalyst
in 4 mL solvent for 6 h. b Isolated yield.
Catalytic amidation reaction of aldehyde surrogates
To examine the scope for this heterogeneous copper-catalyzed direct amidation reaction, a series
of reactions were carried out by employing a wide range of aromatic, aliphatic and hetero-
aromatic aldehydes under the optimized reaction conditions and the results are summarized in
Table 3. All the reactions proceeded smoothly and efficiently in acetonitrile to generate the
desired product amides in moderate to excellent yields up to 96 %. Diverse functionalities
including nitro, methoxy, halo, methyl, and trifluoro methyl groups were tolerated. Remarkably,
the coupling of less electrophilic aldehydes also gave the desired products, albeit in low yields.
The results summarized in Table 3 indicated that the electron-withdrawing substituent on
aromatic rings of aldehydes had positive effects on the transformation (entries 1-4, 7, 9, 11) and
for those with no electron withdrawing substituent(s), poor to moderate yields were obtained
(entry 10, Table 3). The aliphatic aldehydes gave good yields (entry 6 and 12).
10

Table 3
% yield of various benzamides using Cu(PS-BBMA)Cl2
No. Aldehydes
1
Product (Amides)
Yield (%)
90
2
3
90
92
4
5
6
87
90
92
7
8
96
90
11

9
85
80
10
11
96
12
13
94
93
14
92
aldehyde surrogates (1 mmol), benzylamine (1.2 mmol), 60 mg Cu(PS-BBMA)Cl2, CH3CN (4
mL), at 60 °C for 6 h.
Plausible reaction mechanism catalyzed by Cu(PS-BBMA)Cl2
To find out whether reaction followed free radical mechanism or not, radical scavenger BHT
(Butylated Hydroxy Toluene) was added to the reaction system wherein there was reaction
proceeded in the presence of scavenger showed non free radical mechanism.
12

The hemi-aminal was formed by nucleophilic addition of amine with aldehyde, undergoes
dehydrogenation to give corresponding amide product [32, 33].
Scheme 3. Plausible mechanism for coupling between benzylamine and benzaldehyde catalyzed
by Cu(PS-BBMA)Cl2.
1H NMR spectral data of amides
1. N-benzyl-4-nitrobenzamide
1H NMR (300 MHz, CDCl3): δ 4.72 (d, 2H, J = 6 Hz), δ 6.64 (bs, 1H), δ 7.36 (m, 5H), δ 7.96 (d,
2H, J = 9.0 H), δ 8.28 (d, 2H, J = 9.0 Hz).
2. N-benzyl-4-methoxybenzamide
13

1H NMR (300 MHz, CDCl3): δ 3.90 (s, 3H), δ 4.68 (d, 2H, J = 6 Hz), δ 6.41 (bs, 2H, J = 9.0 Hz),
δ 7.28-7.38 (m, 5H), δ.78 (d, 2H, J = 9.0 Hz).
3. N-benzyl-4-fluorobenzamide
1H NMR (400 MHz, CDCl3):3 δ 4.65 (d, 2H, J = 6 Hz), δ 6.48 (bs, 1H), δ 7.07-7.14 (m, 2H), δ
7.28- 7.36 (m, 5H), δ 7.78 – 7.85 (m, 2H).
4. N-benzyl-4-chlorobenzamide
1H NMR (400 MHz, CDCl3): δ 4.59 (d, 2H, J = 4.0 Hz), δ 6.96 (br s, 1H), δ 7.29-7.36 (m, 5H), δ
7.52 (d, 2H, J = 8.0 Hz), 7.66 (d, 2H, J = 8.0 Hz).
5. N-benzylcyclohexanecarboxamide
1H NMR (400 MHz, CDCl3): δ 1.20-1.32 (m, 3H), δ 1.48 (q, 2H, J = 12.0Hz), δ 1.69 (s, 1H), δ
1.79-1.81 (m, 2H), δ 1.87-1.90 (m, 2H), δ 2.13 (t, 1H, J = 12.0Hz), δ 4.42 (d, 2H, J = 8.0Hz), δ
6.04 (br s, 1H), δ 7.25-7.35 (m, 5H).
6. N-benzylpentanamide
1H-NMR (CDCl3): δ 0.91 (t, 3H, J = 7.26 Hz), δ 1.62 (m, 2H), δ 1.34 (m, 2H), δ 2.20 (t, 2H, J =
7.52 Hz), δ 4.41 (d, 2H, J = 5.77 Hz), δ 5.97 (bs, 1H), δ 7.22-7.36 (m, 5H).
7. N-benzyl-4-(trifluoromethyl)benzamides
1H NMR (300 MHz, CDCl3): δ 4.66 (d, 2H, J = 3 Hz), δ 6.58 (bs, 1H), δ 7.28-7,36 (m, 5H), δ
7.70 (d, 2H, J = 9 Hz), δ 7.91 (d, 2H, J = 6 Hz).
8. N-benzylbenzamide
1H NMR (300 MHz, CDCl3): δ 4.65 (d, 2H, J = 6 Hz), δ 6.44 (bs, 1H), δ 7.28-7,36 (m, 5H), δ
7.40-7.45 (m, 2H), δ 7.48-7.52 (m,1H), δ 7.78-7.81 (m, 2H).
9. N-benzyl-4-iodobenzamide
1H NMR (300 MHz, CDCl3): δ 4.63 (d, 2H, J = 3 Hz), δ 6.48 (bs, 1H), δ 7.28-7.38 (m, 5H), δ
7.52 (d, 2H, J = 9.0 Hz), δ 7.78 (d, 2H, J = 9.0 Hz).
14

10. N-benzyl-4-methylbenzamide
1H NMR (400 MHz, CDCl3): δ 2.36 (s, 3H), δ 4.58 (d, 2H, J = 4.0Hz), δ 6.74 (br s, 1H), δ 7.17
(d, J = 8.0 Hz, 2H), δ 7.26-7.30 (m, 5H), δ 7.69 (d, 2H, J = 8.0Hz).
11. N-benzyl-3,5-dimethoxybenzamide
1H NMR (400 MHz, CDCl3): δ 3.80 (s, 6H), δ 4.61 (d, 2H, J = 4.0 Hz), δ 6.58 (s, 1H), δ 6.75 (br
s, 1H), δ 6.95 (s, 2H), δ 7.29-7.34 (m, 5H).
12. N-benzylbutyramide
1H NMR (400 MHz, CDCl3): δ 0.91 (t, 3H, J = 8.0 Hz), δ 1.58-1.65 (m, 2H), δ 2.14 (t, 2H, J =
8.0 Hz), δ 4.34 (d, 2H, J = 8.0 Hz), δ 6.81 (br s, 1H), δ 7.21-7.28 (m, 5H).
13. N-benzyl-2-phenylacetamide
1H-NMR (400 MHz, CDCl3) δ 3.64 (s, H), δ 4.42 (d, 2H, J = 5.85 Hz), δ 5.66 (bs, 1H), δ 7.15-
7.20 (m, 2H), δ 7.22-7.38 (m, 8H).
14. N-benzyl-2-(thiophen-3-yl)acetamide
1H-NMR (400 MHz, CDCl3): δ 3.65 (s, 2H), δ 4.42 (d, 2H, J = 5.79 Hz), δ 5.81 (bs, 1H), δ 7.01
(dd, 1H, J = 4.90), δ 7.13-7.16 (m, 1H), δ 7.17-7.22 (m, 2H), δ 7.23-7.36 (m, 4H).
Comparision with literature
Table 4
Comarision of present catalyst Cu(PS-BBMA)Cl2 with literature methods
No. Catalyst
Oxidant Base/Additive Yield (%) Reference
1
2
3
5
FeCl2
TBHP
-
90
76
95
96
[34]
[35]
[36]
[(COD)2Rh]BF4 + BINAP Styrene CsOAc
[RhCl(PPh3)(TAC)]
Present catalyst
-
-
NaHCO3
-
BINAP = 2,2'-Bis(diphenylphosphino)-1,1'-binaphthyl; TAC = 2-(2-thiazolylazo)cresol
Catalyst recycling and heterogeniety test
15

Recycling efficiency of Cu(PS-BBMA)Cl2 was investigated by carrying out the amidation
reaction between benzaldehyde and benzylamine. The catalyst was separated from the reaction
mixture after each run by filtration, washed with solvent and dried before using it in the
subsequent run. The catalyst showed same activity for three runs and slight decrease was
observed in subsequent runs. Esimation of Cu present in the recycled catalyst indicated that no
leaching of the metal occurred confirmed by ICP-OES. The IR spectrum of the recycled catalyst
(after fourth cycle) was similar to that of fresh catalyst. Catalyst recycling studies is depicted in
Fig. 5.
The heterogeneity of the catalyst was established by hot filtration method. For this, one reaction
set was quenched after 2 h and product was isolated (25 % yield) at optimized conditions. In the
other set, the catalyst was separated after 2 h by filtration and the reaction was allowed to
continue for 6 h, which resulted in no appreciable conversion (27 %). The solution was tested for
metal content and absence of metal suggested no leaching of the metal complex during the
reaction, indicating the heterogeneity of the catalyst.
100
80
60
40
20
0
Cycle 1 Cycle 2 Cycle 3 Cycle 4 Cycle 5
Fig. 5. Catalyst recycling studies using Cu(PS-BBMA)Cl2 as catalyst for the amidation
Conclusions
The polymer-supported bis(benzimidazolylmethyl)amine copper complex was synthesized and
characterized successfully and investigated its catalytic activity towards direct amidation
reaction. The formation of complex was evidenced from elemental, AAS, and thermogravimetric
16

analyses, FT-IR, UV-Vis, and ESR spectral techniques and magnetic moment measurement. The
yield of the amide was dependent on the various parameters such as solvent, reaction time,
Cu(PS-BBMA)Cl2 concentrataion and reaction temperature. The amidation reactions also
proceeded in water with slight lower yield. The electronic effect of the substituents can also
perturb the amidation reaction. This procedure provides an efficient and oxidant-free method for
direct amidation. The catalyst was recyclable and showed excellent activity. A tentative reaction
mechanism was proposed.
Acknowledgements Authors are thankful to Bangalore University for providing research
facilities. Thermax Ltd. India is gratefully acknowledged for providing polymer beads. Renuka
M K acknowledges the University Grants Commission (UGC), New Delhi, India, for awarding
UGC-JRF fellowship.
Authors declare ‘no conflict of interest’.
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Graphical abstract
R
PS
R
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