SiO2-Cu2O: An efficient and recyclable heterogeneous catalyst for N-benzylation of primary and secondary amines

Article abstract of DOI:10.1016/S1872-2067(14)60009-7

A mild, effective, and selective procedure is reported for the mono N-benzylation and N,N-dibenzylation of primary amines as well as mono N-benzylation of secondary amines using silica-supported copper(I) oxide in water. The silica-supported Cu₂O was generated in situ by the reaction of Fehling solution and glucose at 100 C onto activated silica. The catalyst was filtered, washed with water, and oven-dried, and was characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, and atomic absorption spectroscopy. The prepared Cu₂O-SiO₂ was found to be thermally stable up to 325 C. The copper was uniformly distributed onto the surface of the silica, and the mean particle diameter was 7 nm. The catalyst served as a selective heterogenous catalyst for the N-benzylation of primary and secondary amines. The catalyst is recyclable and was used effectively upto fifth run without a significant loss of catalytic activity. Various reaction solvents including water, acetonitrile, and toluene were screened for N-benzylation of amines, and the success of the aqueous system highlights the low environmental impact of the procedure.

Full text of DOI:10.1016/S1872-2067(14)60009-7

Chinese Journal of Catalysis 35 (2014) 444–450
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
SiO2‐Cu2O: An efficient and recyclable heterogeneous catalyst for
N‐benzylation of primary and secondary amines
Manjulla Gupta, Satya Paul *, Rajive Gupta
Department of Chemistry, University of Jammu, Jammu‐180 006, India
A R T I C L E I N F O
A B S T R A C T
Article history:
A mild, effective, and selective procedure is reported for the mono N‐benzylation and N,N‐diben‐
zylation of primary amines as well as mono N‐benzylation of secondary amines using sili‐
ca‐supported copper(I) oxide in water. The silica‐supported Cu2O was generated in situ by the reac‐
tion of Fehling solution and glucose at 100 °C onto activated silica. The catalyst was filtered, washed
with water, and oven‐dried, and was characterized by Fourier transform infrared spectroscopy,
thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, and
atomic absorption spectroscopy. The prepared Cu2O‐SiO2 was found to be thermally stable up to
325 °C. The copper was uniformly distributed onto the surface of the silica, and the mean particle
diameter was 7 nm. The catalyst served as a selective heterogenous catalyst for the N‐benzylation of
primary and secondary amines. The catalyst is recyclable and was used effectively upto fifth run
without a significant loss of catalytic activity. Various reaction solvents including water, acetonitrile,
and toluene were screened for N‐benzylation of amines, and the success of the aqueous system
highlights the low environmental impact of the procedure.
Received 16 September 2013
Accepted 24 December 2013
Published 20 March 2014
Keywords:
Silica supported copper(I) oxide
Benzylation
Substituted amine
Recyclability
Heterogeneous catalysis
© 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
atom‐economical transformations. Although C–N couplings
using palladium [10,11], ruthenium [4], iridium [12,13], and
In recent years, there has been growing interest in reactions
designed to form carbon–nitrogen bonds, which is a useful
strategy in synthetic organic chemistry. In the past few dec‐
ades, the synthesis of amines has attracted considerable atten‐
tion because of their industrial and commercial importance
[1,2] and also for the synthesis of various dyes. The traditional
synthetic approach to N‐substituted amines is direct N‐alkyla‐
tion [3], but this procedure is problematic owing to over‐alkyl‐
ation, the toxic nature of many alkyl halides [4], and poor selec‐
tivity [5]. Furthermore, various bases including NaOH [6], ce‐
sium hydroxide [7], PhN(n‐Pr)2 [8], and Hunig’s base [9] have
been used for synthesis of N‐substituted amines, but they re‐
quire harsh reaction conditions.
platinum [14,15] catalysts have been explored, copper‐cata‐
lyzed C–N bond formation has gained more attention because
of its low cost, non‐toxic nature, and excellent selectivity
[16–20]. Heterogenous catalysts have recently gained much
importance because they are more selective, stable at high
temperature, easily separated from the reaction mixture at the
end of the process, and can be reused. These factors favor the
cost effectiveness of what can be regarded as a “green reac‐
tion”. Among the alkylations of amines, benzylation continues
to be one of the most commonly used reactions in pharmaceu‐
tical chemistry [21,22] because of the medicinal importance of
N‐benzylated amines. Here we report a selective procedure for
mono N‐benzylation and N,N‐dibenzylation of anilines (Scheme
1) and N‐benzylation of secondary amines (Scheme 2) using
Transition metal catalysis has proved useful in selective and
* Corresponding author. Tel: +91‐191‐2453969; Fax: +91‐191‐2431365; E‐mail: paul7@rediffmail.com
DOI: 10.1016/S1872‐2067(14)60009‐7 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 3, March 2014

Manjulla Gupta et al. / Chinese Journal of Catalysis 35 (2014) 444–450
silica gel with ethyl acetate/petroleum ether.
Cl
HN
C6H5
NH2
C6H5
N
C6H5
SiO2-Cu2O,
K2CO3
+
+
2.3. General procedure for N‐benzylation of secondary amines
using SiO2‐Cu2O
H2O, 30 oC or
100 oC
R
Scheme 1. N‐Benzylation and N,N‐dibenzylation of amines with benzyl
chloride in water.
A mixture of secondary amine (0.5 mmol), benzyl chloride
(0.127 g, 1 mmol), K2CO3 (0.139 g, 1 mmol), TBAB (0.082 g,
0.25 mmol), and SiO2‐Cu2O (0.2 g, 5 mol% Cu) in water (5 mL)
in a round‐bottom flask (50 mL) was stirred at 100 °C. On com‐
pletion of the reaction (monitored by TLC), the flask was cooled
to room temperature and the mixture filtered. The residue was
washed with water followed by ethyl acetate (3 × 10 mL). The
combined organic extracts were washed with water (3 × 100
mL) and dried over anhydrous Na2SO4. The solvent was re‐
moved under reduced pressure, and the product was obtained
by crystallization from petroleum ether or ethyl acetate/petro‐
leum ether, or by eluting the crude product through a column
of silica gel with ethyl acetate/petroleum ether.
Cl
SiO -Cu O,
C H
2
2
6
5
H
N
K CO
2
3
+
N
R
R'
R
R'
H O, TBAB,
2
o
100 C
Scheme 2. N‐Benzylation of secondary amines with benzyl chloride in
water.
silica‐supported copper(I) oxide in aqueous media.
2. Experimental
2.1. Preparation of silica‐supported copper(I) oxide (SiO2‐Cu2O)
2.4. Selected spectral data
To a round‐bottom flask containing activated silica (10 g),
Fehling solution (50 mL) was added and the reaction mixture
was stirred at 100 °C. After every interval of 30 min, 2 mL of
glucose solution (0.5 g in 96 mL of water) was added up to 24 h.
Then the catalyst was filtered off, washed with water until the
washings were colorless, and dried in an oven at 110 °C. To
remove excess glucose, the catalyst was refluxed in water at
120 °C for 6 h (3 × 2 h). Then the catalyst was dried at 110 °C
for 5 h in an oven.
N‐Benzyl‐3‐methoxyaniline (Table 3). IR (υmax in cm−1, KBr):
1042 (C–O–C symm. str.), 1582 (aromatic C=C str.), 3029 (aro‐
matic C–H str.), 3422 (NH str.). 1H NMR (CDCl3): δ 3.82 (s, 3H,
OCH3), 4.0 (bs, 1H, NH), 4.33 (s, 2H, CH2Ph), 6.85–7.0 (m, 4H,
Ar–H), 7.16–7.32 (m, 5H, Ar–H). MS: m/z 213 (M+).
N‐Benzyl‐4‐methylaniline (Table 3). IR (υmax in cm−1, KBr):
1575 (aromatic C=C str.), 2918 (C–H str.), 3021 (aromatic C–H
str.), 3427 (N–H str.). 1H NMR (CDCl3): δ 2.27 (s, 3H, CH3), 4.02
(bs, 1H, NH), 4.37 (s, 2H, CH2Ph), 6.86–6.96 (d, 2H, Ar–H),
7.06–7.17 (d, 2H, Ar–H), 7.33–7.46 (m, 5H, Ar–H). MS: m/z 197
(M+).
The general preparation procedure for silica‐supported
nano copper(I) oxide (SiO2‐Cu2O) is shown in Scheme 3.
2.2. General procedure for mono N‐benzylation and
N,N‐dibenzylation of anilines in the presence of SiO2‐Cu2O
N‐Benzyl‐4‐chloroaniline (Table 3). IR (υmax in cm−1, KBr):
740 (C–Cl str.), 1580 (aromatic C=C str.), 3029 (aromatic C–H
str.), 3420 (N–H str.). 1H NMR (CDCl3): δ 4.13 (bs, 1H, NH), 4.29
(s, 2H, CH2Ph), 6.89–6.97 (d, 2H, Ar–H), 7.21–7.34 (m, 5H,
Ar–H), 7.50–7.58 (d, 2H, Ar–H). MS: m/z 217 (M+), 219 (M+2).
N‐Benzyl‐4‐nitroaniline (Table 3). IR (υmax in cm−1, KBr):
865 (C–NO2 str.), 1530 (NO2 str.), 1585 (aromatic C=C str.),
A mixture of aniline (0.5 mmol), benzyl chloride (0.5 mmol
for mono N‐benzylation and 2 mmol for N,N‐dibenzylation),
K2CO3 (0.5 mmol for mono N‐benzylation and 1 mmol for
N,N‐dibenzylation), SiO2‐Cu2O (0.1 g, 2.5 mol% Cu for mono
N‐benzylation and 0.2 g, 5 mol% Cu for N,N‐dibenzylation), and
tetra‐n‐butylammonium bromide (TBAB, 0.082 g, 0.25 mmol,
only in the case of N,N‐dibenzylation) in water (5 mL) in a
round‐bottom flask (50 mL) was stirred at 30 °C for mono
N‐benzylation and 100 °C for N,N‐dibenzylation. On completion
of the reaction monitored by thin‐layer chromatography (TLC),
the reaction mixture was filtered, and the residue was washed
with water followed by ethyl acetate (3 × 10 mL). The com‐
bined organic extracts were washed with water (3 × 100 mL)
and dried over anhydrous Na2SO4. The solvent was removed
under reduced pressure, and the product was obtained by
crystallization from petroleum ether or ethyl acetate/petrole‐
um ether, or by eluting the crude product through a column of
1
3039 (aromatic C–H str.), 3460 (N–H str.). H NMR (CDCl3): δ
4.10 (bs, 1H, NH), 4.25 (s, 2H, CH2Ph), 6.94–7.03 (d, 2H, Ar–H),
7.46–7.59 (m, 5H, Ar–H), 8.01–8.12 (d, 2H, Ar–H). MS: m/z 228
(M+).
N,N‐Dibenzyl‐3‐methoxyaniline (Table 3). IR (υmax in cm−1,
KBr): 1052 (C–O–C symm. str.), 1579 (aromatic C=C str.), 3029
(aromatic C–H str.). 1H NMR (CDCl3): δ 3.83 (s, 3H, OCH3), 4.35
(s, 4H, 2 × CH2Ph), 6.87–7.20 (m, 4H, Ar–H), 7.29–7.34 (m, 10H,
Ar–H). MS: m/z 303 (M+).
N,N‐Dibenzyl‐4‐methylaniline (Table 3). IR (υmax in cm−1,
KBr): 1572 (aromatic C=C str.), 2925 (C–H str.), 3017 (aromatic
C–H str.). 1H NMR (CDCl3): δ 2.28 (s, 3H, CH3), 4.42 (s, 4H, 2 ×
CH2Ph), 6.89–6.97 (d, 2H, Ar–H), 7.08–7.18 (d, 2H, Ar–H),
7.29–7.46 (m, 10H, Ar–H). MS: m/z 287 (M+).
Cu2O
Cu2O
OH
OH
OH
Glucose
100 oC, reflux, 24 h
N,N‐Dibenzyl‐4‐chloroaniline (Table 3). IR (υmax in cm−1,
Fehling solution
+
OHOH
OH
SiO2
KBr): 743 (C–Cl str.), 1582 (aromatic C=C str.), 3030 (aromatic
1
C–H str.), 3422 (N–H str.). H NMR (CDCl3): δ 4.13 (s, 4H, 2 ×
CH2Ph), 6.93–7.02 (d, 2H, Ar–H), 7.25–7.40 (m, 10H, Ar–H),
Scheme 3. General procedure for the preparation of SiO2‐Cu2O.

Manjulla Gupta et al. / Chinese Journal of Catalysis 35 (2014) 444–450
7.48–7.57 (d, 2H, Ar–H). MS: m/z 307 (M+), 309 (M+2).
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
N,N‐Dibenzyl‐3‐nitroaniline (Table 3). IR (υmax in cm−1, KBr):
860 (C–NO2 str.), 1530 (NO2 str.), 1585 (aromatic C=C str.),
1
3047 (aromatic C–H str.). H NMR (CDCl3): δ 4.33 (s, 4H, 2 ×
CH2Ph), 7.13–7.21 (m, 2H, Ar–H), 7.53–7.63 (m, 10H, Ar–H),
8.16–8.27 (m, 2H, Ar–H). MS: m/z 318 (M+).
N‐Benzylmorpholine (Entry 1, Table 4). IR (υmax in cm−1,
KBr): 1050 (C–O–C str.), 1340 (C–N str.), 1565 (C=C aromatic
1
str.), 3070 (C–H symm. str.). H NMR (CDCl3): δ 2.43 (t, 4H,
CH2), 4.92 (s, 2H, CH2Ph), 3.70 (t, 4H, CH2), 7.04–7.16 (m, 5H,
Ar–H). MS: m/z 177 (M+).
3750 3250 2750 2250 1750 1250
750
Wavenumber (cm1)
N‐Benzylpiperidine (Entry 2, Table 4). IR (υmax in cm−1, KBr):
1310 (C–N str.), 1560 (C=C aromatic str.), 3085 (C–H aromatic
str.). H NMR (CDCl3): δ 1.40 (m, 6H, CH2), 2.24 (m, 4H, CH2),
Fig. 2. FTIR spectrum of SiO2‐Cu2O.
1
4.90 (s, 2H, CH2Ph), 7.12–7.23 (m, 5H, Ar–H). MS: m/z 175 (M+).
50
40
30
20
10
0
2
0
25.2 oC -0.7%
Heat flow (smoothed)
3. Results and discussion
-2
-4
3.1. Characterization of SiO2‐Cu2O
-6
116.1 oC -7.8%
dM-rel
Prepared SiO2‐Cu2O was characterized by Fourier transform
infrared (FTIR) spectroscopy, thermogravimetric analysis
(TGA), X‐ray diffractometry (XRD), X‐ray photoelectron spec‐
troscopy (XPS), scanning electron microscopy (SEM), transmis‐
sion electron microscopy (TEM), and atomic absorption spec‐
troscopy (AAS). In the FTIR spectra, the activated silica showed
absorption peaks at 459, 800, 964, 1344, 1384, 1400, 1598,
2341, 2362, 2856, and 2929 cm−1, and wide absorption bands
were observed at 1078 and 3396 cm−1 (Fig. 1). For SiO2‐Cu2O,
an additional peak observed at 669 cm−1 was attributed to Cu2O
(Fig. 2).
The stability of the SiO2‐Cu2O was determined by TGA (Fig.
3). The curve showed an initial weight loss up to 116 °C, which
was attributed to the loss of residual water trapped onto the
surface of the silica. This was followed by a slight weight loss
up to 325 °C, which was considered to be caused by the loss of
unreacted glucose or components of Fehling solution. Further
continuous weight loss occurred up to 648 °C. This indicates
that the catalyst is stable up to 325 °C and is safe to use in reac‐
tions at 100 °C.
-8
325.5 oC -10.3%
-10
-12
-14
471.1 oC -12.3%
648.9 oC -13.6%
-10
-20
100
200
300
400
500
600
700
Temperature (oC)
Fig. 3. TGA of SiO2‐Cu2O.
found corresponding to 2θ = 36.3° and 42.2°, which were at‐
tributed to Cu2O. The absence of a peak at 2θ = 38.7° excluded
the possibility of CuO formation (Fig. 4).
The oxidation state of copper in the SiO2‐Cu2O was deter‐
mined by measurements of binding energies using XPS. The
spectrum of Cu 2p3/2 photoelectrons is shown in Fig. 5. The
intense and broad peak at 933 eV corresponds to Cu(I) and is in
accordance with the literature values [23,24].
The amount of copper loaded onto the silica surface was
determined by AAS analysis. The catalyst was stirred in dilute
HNO3 solution and then subjected to AAS analysis. The
SiO2‐Cu2O contained 0.0158 g of copper per gram of catalyst.
The microstructure and morphology of the SiO2‐Cu2O was
studied by SEM and TEM. The SEM image revealed a fine ho‐
The XRD diffraction patterns for SiO2‐Cu2O showed peaks
that were indexed on the basis of crystallographic data for the
known structure of silica. In addition, reflection patterns were
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
(111)
(200)
3750 3250 2750 2250 1750 1250
Wavenumber (cm1)
750
10
20
30
40
50
60
70
2/(o )
Fig. 1. FTIR spectrum of activated silica.
Fig. 4. XRD pattern of SiO2‐Cu2O.

Manjulla Gupta et al. / Chinese Journal of Catalysis 35 (2014) 444–450
Table 1
Cu 2p3/2
Cu 2p1/2
Effect of solvent on the SiO2‐Cu2O catalyzed N‐benzylation of 4‐nitroan‐
iline in water.
Solvent
Yield a (%)
953.1
Acetonitrile
Toluene b
Water
90
60
95
956.0
962.8
Reaction conditions: 4‐nitroaniline (0.069 g, 0.5 mmol), benzyl chloride
(0.0635 g, 0.5 mmol), SiO2‐Cu2O (0.1 g, 2.5 mol% Cu), K2CO3 (0.069 g,
0.5 mmol), solvent (5 mL), 30 °C, 8 h, air atmosphere.
943.1
944
a Based on separation by column chromatography.
b Along with N‐benzyl‐4‐nitroaniline, N,N‐dibenzyl‐4‐nitroaniline was
also formed together with unreacted amine.
976
960
928
Binding energy (eV)
3.2. Catalyst testing for selective N‐benzylation of primary and
secondary amines
Fig. 5. XPS spectra of SiO2‐Cu2O.
mogenous powder with a porous structure (Fig. 6).
The reaction conditions for selective mono N‐benzylation
were optimized by using 4‐nitroaniline as the test substrate
and benzyl chloride as the benzylating agent. The reaction was
carried out under different conditions with respect to solvent,
temperature, and molar ratios of SiO2‐Cu2O. After carrying out a
series of reactions, 2.5 mol% of Cu in SiO2‐Cu2O was found to be
sufficient to complete the reaction selectively and give maxi‐
mum yield of N‐benzyl‐4‐nitroaniline. Solvents of different
polarities were also tried, and the results are presented in Ta‐
ble 1.
The TEM micrographs of the catalyst showed that copper
was uniformly distributed on the surface of silica (Fig. 7). The
mean particle diameter was found to be 7 nm. Furthermore, no
bulk aggregation of the metal was observed, indicating that the
copper was evenly dispersed onto the surface of the silica.
(a)
(b)
The best results were obtained when water was used as
solvent, with consideration to environmental impact, reaction
time, yield, and selectivity. The reaction temperature also plays
a crucial role in the N‐benzylation of anilines; therefore, the
reaction of the test substrate was also conducted at different
temperatures, and the yield of mono N‐benzyl‐4‐nitroaniline
was determined (Table 2).
Fig. 6. SEM images of SiO2‐Cu2O. (a)  500; (b)  3500.
For N,N‐dibenzylation, 4‐chloroaniline was selected as the
test substrate, and benzyl chloride was used as the benzylating
agent. At room temperature, no dibenzylated product was
formed, so the reaction temperature was increased to 100 °C.
The dibenzylated product formed as the major product with a
small amount of mono benzylated product. Further addition of
TBAB increased the amount of dibenzylated product. With
4‐toluidine, 2‐toluidine, 4‐chloroaniline, or 4‐bromoaniline as
the substrates, the N,N‐dibenzylated product was formed ex‐
clusively. However, for 2‐chloroaniline and 2‐nitroaniline, only
the N‐benzylated product was formed, and no N,N‐dibenzylated
(a)
100 nm
(b)
4.76 nm
6.67 nm
Table 2
10.3 nm
7.99 nm
5.76 nm
Effect of temperature on the SiO2‐Cu2O catalyzed N‐benzylation of 4‐
nitroaniline in water.
4.70 nm
Yield a (%)
Temperature (°C)
Mono
95
80
75
40
Di
0
5.27 nm
5.30 nm
30
40
60
80
100
8.66 nm
10
12
35
50
4.25 nm
30
Reaction conditions: 4‐nitroaniline (0.069 g, 0.5 mmol), benzyl chloride
(0.0635 g, 0.5 mmol), SiO2‐Cu2O (0.1 g, 2.5 mol% Cu), K2CO3 (0.069 g,
0.5 mmol), H2O (5 mL), 8 h, air atmosphere.
20 nm
a Based on separation by column chromatography.
Fig. 7. TEM images of SiO2‐Cu2O.

Manjulla Gupta et al. / Chinese Journal of Catalysis 35 (2014) 444–450
Table 3
SiO2‐Cu2O catalyzed selective N‐benzylation and N,N‐dibenzylation of anilines in water.
N‐Benzylation of anilinesa
N,N‐Dibenzylation of anilinesb
Yieldc (%)
Amine/Mono/Di
40/50/0
15/78/0
15/75/5
15/55/10
—
Yieldc (%)
Entry
Amine
Time (h)
MP or BP/Ref. (°C)
Time (h)
MP or BP/Ref. (°C)
Di/Mono
80/14
—
1
2
3‐Anisidine
2‐Anisidine
5
1.5
4
15
—
4
Liq/183–187 [25]
Oil/oil [26]
17
—
10
17
9
55–56/56 [32]
—
3
4‐Toluidine
243–245/244–245 [27]
166–168/160–170 [28]
—
36–38/38–39 [29]
46–47/46–48 [30]
38–40/38–39 [26]
50–51/51–52 [30]
73–74/72–74 [31]
104–106/106–107 [29]
—
97/0
82/11
97/0
—
96/0
0/96
93/0
65/20
80/10
0/95
62–63/63–64 [33]
74–75/75–76 [34]
42–43/42 [35]
4
3‐Toluidine
5
2‐Toluidine
6
7
8
4‐Fluoroaniline
4‐Chloroaniline
2‐Chloroanilined
4‐Bromoaniline
4‐Nitroaniline
3‐Nitroaniline
2‐Nitroaniline
0/60/30
0/80/10
20/75/0
10/82/0
0/95/0
—
11
1.5
16
17
16
18
—
3
104–106/104–105 [36]
38–39/38–39 [31]
124–125/124–126 [37]
86–87/84–86 [32]
73–75/73–74 [38]
65–66/66–67 [39]
12
0.75
8
10
—
9
10
11
12
20/70/0
—
a Reaction conditions: amine (0.5 mmol), benzyl chloride (0.0635 g, 0.5 mmol), SiO2‐Cu2O (0.1 g, 2.5 mol% Cu), K2CO3 (0.069 g, 0.5 mmol), H2O (5 mL),
30 °C.
b Reaction conditions: amine (0.5 mmol), benzyl chloride (0.254 g, 2 mmol), TBAB (0.082 g, 0.25 mmol), K2CO3 (0.139 g, 1 mmol), SiO2‐Cu2O (0.2 g, 5
mol % Cu), water (5 mL), 100 °C.
c Isolated yield based on separation through column chromatography or crystallization.
d Benzyl chloride (0.127 g, 1 mmol) was used for for N,N‐dibenzylation of anilines.
product was observed. For N‐benzylation of secondary amines,
morpholine was used as the test substrate and benzyl chloride
as the benzylating agent. After carrying out a series of reac‐
tions, 5 mol% of Cu in SiO2‐Cu2O was found to be sufficient to
achieve good yields. Being benign and environmentally friend‐
ly, water was used as the solvent. Furthermore, the addition of
base was found to have a profound effect on the rate of the
reaction. Various bases were screened for the test reaction, and
K2CO3 was selected because of its wide availability and low
cost. A reaction temperature of 100 °C was found to be the op‐
timum reaction temperature. Addition of TBAB as phase trans‐
fer catalyst was also found to enhance the reaction rate.
of catalyst. The reaction was observed to proceed very slowly
in the absence of catalyst, confirming the catalytic effect of
SiO2‐Cu2O.
The mechanism for the N‐benzylation of amines catalyzed
by SiO2‐Cu2O is considered to proceed through a recyclable
coordination process via oxidative addition followed by reduc‐
tive elimination (Scheme 4). First, the oxidative addition of
amine to copper is thought to occur in the presence of base to
give a copper complex (I), followed by the addition of benzyl
chloride to form the second intermediate (II). Intermediate II
then undergoes reductive elimination to give the product.
To study the generality of the developed protocol for mono
N‐benzylation and N,N‐dibenzylation, various anilines substi‐
tuted with electron‐withdrawing and electron‐releasing groups
were chosen. The results are presented in Table 3. For
N‐benzylation of secondary amines, various secondary amines
were chosen and products were obtained in almost quantita‐
tive yields (Table 4). To confirm the role of the catalyst for
N‐benzylation of primary and secondary amines, the test reac‐
tion was also carried out with the test substrate in the absence
3.3. Recyclability of SiO2‐Cu2O for N‐benzylation of amines
In the use of heterogeneous catalysts, the recyclability of the
catalyst is an important factor. To determine the recyclability, a
NH2
PhH2C
Base
+
SiO2-Cu(I)
NH
R
Base-H
Table 4
R
SiO2‐Cu2O catalyzed N‐benzylation of secondary amines in water at 100
°C.
Time (h) Yielda (%)
MP/Ref. (°C)
194–195/194 [40]
182–183/181–182 [41]
85–87/86–87 [42]
70–72/71–73 [43]
SiO2-Cu CH2Ph
NH
Entry
Amine
Morpholine
Piperidine
Diphenylamineb
Imidazolec
1
2
3
4
0.5
0.5
93
90
80
80
SiO2-Cu
NH
12
12
R
II
Reaction conditions: amine (0.5 mmol), benzyl chloride (0.127 g, 1
mmol), K2CO3 (0.139 g, 1 mmol), TBAB (0.082 g, 0.25 mmol), SiO2‐Cu2O
(0.2 g, 5 mol% Cu), water (5 mL), 100 °C.
R
I
a Isolated yield.
Cl-
PhCH2Cl
b Rest being the starting material.
c Product was obtained after passing through column of silica gel and
elution with ethyl acetate/petroleum ether.
Scheme 4. Proposed mechanism for N‐benzylation of amines using
SiO2‐Cu2O.

Manjulla Gupta et al. / Chinese Journal of Catalysis 35 (2014) 444–450
the selective mono N‐benzylation and N,N‐dibenzylation of
Primary amine
Secondary amine
100
80
60
40
20
0
primary aromatic amines. The catalytic system has also been
used for the N‐benzylation of secondary amines. The environ‐
mentally friendly nature of the reaction is underscored by the
use of water as the reaction solvent.
Acknowledgements
We are grateful to Director, SAIF, Punjab University,
Chandigarh for SEM, TEM, and XRD and also extend our sincere
thanks to UGC, New Delhi for financial support to purchase
FTIR; awarding Major Research Project (F 41‐281/2012 (SR)),
and Prof. R. K. Bamezai, Department of Chemistry, University of
Jammu for recording TGA.
1
2
3
4
5
Run
Fig. 8. Recyclability of SiO2‐Cu2O. Reaction conditions for primary
amine: 2‐chloroaniline (0.5 mmol), benzyl chloride (0.127 g, 1 mmol),
TBAB (0.082 g, 0.25 mmol), K2CO3 (0.139 g, 1 mmol), SiO2‐Cu2O (0.2 g, 5
mol% Cu), water (5 mL), 100 °C. Reaction conditions for secondary
amine: morpholine (0.5 mmol), benzyl chloride (0.127 g, 1 mmol),
K2CO3 (0.139 g, 1 mmol), TBAB (0.082 g, 0.25 mmol), SiO2‐Cu2O (0.2 g, 5
mol% Cu), water (5 mL), 100 °C.
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Graphical Abstract
Chin. J. Catal., 2014, 35: 444–450 doi: 10.1016/S1872‐2067(14)60009‐7
Glucose
Solution
SiO2‐Cu2O: An efficient and recyclable heterogeneous catalyst
for N‐benzylation of primary and secondary amines
Manjulla Gupta, Satya Paul*, Rajive Gupta
University of Jammu, India
Fehling
Solution
Cu2O
N
(1) C H CH Cl
6 5 2
SiO2-Cu2O
NH
(2) K CO
2
3
SILICA
Silica‐supported Cu2O was prepared in situ using inexpensive and
widely available starting materials and used as a catalyst for
N‐benzylation and N,N‐dibenzylation of primary and secondary
amines.
NH2
C6H5
C6H5
C6H5
+

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