Tapioca cellulose based copper nanoparticles for chemoselective N-alkylation

Article abstract of DOI:10.1166/jnn.2017.12930

Biomaterials as a support for catalysts are of prime importance. Tapioca root which is an abundant biopolymer source was used to synthesize cellulose supported bio-heterogeneous poly(hydroxamic acid) copper nanoparticles (CuN@PHA) and was characterized by Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectroscopy (ICP-AES), transmission electron microscopy (TEM) analyses. The tapioca cellulose supported CuN@PHA (50 mol ppm) effectively catalyzed N-alkylation reaction of aliphatic amines with α,β-unsaturated compounds to give the corresponding alkylated products. High yields up to 95% were achieved for the converted products. The reusability of the cellulose supported nanoparticles was found to be excellent with no significant reduction of its catalytic activity over several cycles. The catalyst showed high catalytic activity having turnover number (TON) 18000 and turnover frequency (TOF) 2250 h^-1.

Full text of DOI:10.1166/jnn.2017.12930

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 550–557, 2017
Copyright © 2017 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Tapioca Cellulose Based Copper Nanoparticles for
Chemoselective N-Alkylation
1
1ꢀ2
1ꢀ3
4
Md. Shaharul Islam , Bablu Hira Mandal , Tapan Kumar Biswas , Md. Lutfor Rahman ,
1
1
1ꢀ∗
S. S. Rashid , Suat-Hian Tan , and Shaheen M. Sarkar
Faculty of Industrial Sciences and Technology, University Malaysia Pahang, Gambang 26300, Kuantan, Malaysia
Department of Chemistry, Jessore University of Science and Technology, Jessore 7408, Bangladesh
Department of Chemistry, University of Rajshahi, Rajshahi 6205, Bangladesh
Faculty of Science and Natural Resources, University Malaysia Sabah, Kotakinabalu 88400, Sabah, Malaysia
1
2
3
4
Biomaterials as a support for catalysts are of prime importance. Tapioca root which is an abundant
biopolymer source was used to synthesize cellulose supported bio-heterogeneous poly(hydroxamic
acid) copper nanoparticles (CuN@PHA) and was characterized by Fourier transform infrared
spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV-Vis), field emission scanning electron
microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic
emission spectroscopy (ICP-AES), transmission electron microscopy (TEM) analyses. The tapioca
cellulose supported CuN@PHA (50 mol ppm) effectively catalyzed N-alkylation reaction of aliphatic
amines with ꢀ,ꢁ-unsaturated compounds to give the corresponding alkylated products. High yields
up to 95% were achieved for the converted products. The reusability of the cellulose supported
nanoparticles was found to be excellent with no significant reduction of its catalytic activity over
several cycles. The catalyst showed high catalytic activity having turnover number (TON) 18000
−
1
and turnover frequency (TOF) 2250 h
.
Keywords: N-Alkylation, Tapioca Cellulose, Poly(methyl acrylate), Poly(hydroxamic acid),
Copper Nanoperticles, ꢀ,ꢁ-Unsaturated Compounds.
1
. INTRODUCTION
reduced. Thus, it is essential to develop a heterogeneous
metal catalyst with high catalytic activity, selectivity, sta-
bility, safe and non-toxic reusability for green organic syn-
thesis. Recently a plenty of heterogeneous metal catalysts
have been utilized for the N -alkylation reaction. Namely,
The atom economy N -alkylation reaction so-called Aza-
Michael addition reaction has drawn high attention in
the synthesis of biologically active natural products as
well as the synthetic precursors of antibiotics, phar-
maceutical intermediates, ꢀ-amino alcohols and chi-
2
2
metal organic framework (MOF-199),
polystyrene
2
3
1
–6
supported CuI-imidazole complex,
biomaterial sup-
ral auxiliaries.
in presence of strong acids or bases such as AlCl ,
Usually, this reaction is carried out
2
4
25
7
ported organo-catalyst, silica sulfuric acid, phosphate
3
2
6
27
8
9
10
impregnated titania, nanocrystalline copper(II) oxide,
PtCl · 5H O, InCl /chlorotrimethylsilane, Bi(NO)3,
4
2
3
2
8
1
1
boric acid,12 Ni[(2-
polyaniline supported Cu–I,
polyaniline supported-
samarium iodobinaphtholate,
2
9
30
31
InCl3,
polystyrenesulfonic acid,
Amberlyst-15,
diphenylphosphino)ferrocenyl)ethyl)cyclohexylphosphine)
3
2
33
1
3
ꢀ
cellulose-Cu(0), CeCl · 7H O/NaI/Al O , polystyrene-
(
tetrahydrofuran)](ClO ꢁ , Pd[N ,N -3-phenyl-2-(pyridin-
3
2
2
3
4
2
34 35ꢂ36
1
4
15
AlCl3, enzymes,
potassium fluoride doped natu-
2
-yl)oxazolidine]Cl ,
N -heterocyclic
carbenes,
2
3
7
38
1
6–18
19–21
ral zeolite,
Fe O @SiO @Me&Et-PhSO H,
Mobil
bases
and ionic liquids
have been used and a
3
4
2
3
composition of matter (MCM-41) immobilized het-
number of side products are produced. Due to the changes
of metal oxidation state and leaching of metal ion into the
reaction media make complication in purification and even-
tually overall efficiency of a synthetic process dramatically
3
9
40
eropoly acids, HY zeolite, polystyrene-supported 1,4-
4
1
diazabicyclo[2.2.2]octane, amine functionalized silica
supported Co(acac)2,
silica-bonded imidazolium-sulfonic acid chloride, and
triphenylphosphine functional polystyrene-based porous
monolithic showed good catalytic activity.
4
2
43
cerium-impregnated-MCM-41,
4
4
∗
45
Author to whom correspondence should be addressed.
550
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 1
1533-4880/2017/17/550/008
doi:10.1166/jnn.2017.12930

Islam et al.
Tapioca Cellulose Based Copper Nanoparticles for Chemoselective N-Alkylation
Currently, scientists are looking into low cost environ-
mental friendly and renewable resources as well as sus-
tainable protocol. In this aspect, bio-cellulose could be
the most attractive material due to large abundance in
nature, low-density, bio-renewability and universal avail-
ability, low cost and interesting mechanical and physi-
cal properties. Interestingly, the cellulose backbone can
be chemically modified and suitable chelating ligands
distilled water and was boiled with glacial acetic acid
(600 mL) for 1 h. Again washed with distilled water
and the cellulose was bleached with hydrogen peroxide
(300 mL) and 5% NaOH (500 mL), washed with distilled
ꢁ
water (500 mL) for several times and oven dried at 50 C.
Methyl acrylate monomer purchased from Aldrich and
monomer was passed through a column filled with chro-
matographic grade activated alumina to remove inhibitors.
Other chemicals such as ceric ammonium nitrate (CAN)
(Sigma-Aldrich), methanol (Merck), sulphuric acid (Lab
Scan), metal salts and other analytical grade reagents were
used without purification.
4
6ꢂ47
can be incorporated.
At present, some biopolymers
4
8
49
50
51
like alginate, gelatin, starch and chitosan deriva-
tives have been used as supports for catalytic appli-
cations. Therefore, it highly anticipated to develop a
more general, simple and appropriate method so that it
can be applied to a number of substrates of various
natures in a catalytic process under mild conditions. Here
we report tapioca cellulose supported bio-heterogeneous
poly(hydroxamic acid) copper nanoparticles as an effi-
cient catalyst for chemoselective N -alkylation of aliphatic
amines with ꢃ,ꢀ-unsaturated compounds in methanol at
room temperature. The CuN@PHA showed high catalytic
activity (50 mol ppm) as well as chemoselectivity with
easy recyclability and reusability.
2
.3. Synthesis of Poly(methylacrylate) 1
Tapioca cellulose (3.0 g) was added into 300 mL distilled
water. The reaction was performed in 500 mL three-neck
round bottom flask fixed with stirrer and condenser in a
ꢁ
thermostat water bath. The mixture was heated at 55 C
and 0.1 mL of sulphuric acid (50%) was added to the
mixture. During the grafting reaction, N gas was purged
2
into the flask to remove oxygen. After 5 min, 0.90 g of
CAN (10 mL solution) was added and stirred with N flow.
2
After 20 min, 12 mL methyl acrylate purified monomer
was added into the reaction and stirred for further 3 h with
2
. EXPERIMENTAL DETAILS
N flow. Then, the mixture was cooled under running tap
2
2
.1. General Information
water and the grafting product was washed with aqueous
All manipulations were performed under atmospheric
conditions otherwise noted. Reagents and solvents were
obtained from commercial suppliers and used without fur-
ther purification. Water was deionized with a Millipore
system as a Milli-Q grade. CuSO · 5H O was pur-
methanol (methanol:water = 4:1). The product was finally
ꢁ
52
oven dried at 50 C to a constant weight.
2
.4. Determination of Grafting Fraction
4
2
1
The homopolymer was extracted from the grafting product
using Soxhlet purification with DMF for 12 h. The purified
chased from Aldrich Chemical Industries, Ltd. H NMR
1
3
(
500 MHz), C NMR (125 MHz) spectra were mea-
ꢁ
grafted copolymer was dried at 50 C to a constant weight.
sured with a BRUKER-500 spectrometer, central labora-
tory, University Malaysia Pahang (UMP). The H NMR
chemical shifts were reported relative to tetramethylsi-
lane (TMS, 0.0 ppm). The C NMR chemical shifts
1
The percentage of grafting (Gp) was determined using the
following Eq. (1).
1
3
W
2
Grafting percentage (Gp) = W ×100
(1)
were reported relative to CDCl (77.0 ppm). Inductively
3
1
coupled plasma atomic emission spectrometry (ICP-AES)
was performed on Shimadzu ICPS-8100 equipment by
the central laboratory, UMP. FTIR spectra were measured
with a Perkin-Elmer 670 spectrometer equipped with an
ATR device (ZnSe crystal), UMP. XPS spectra were mea-
sured with a Scanning X-ray Microprobe PHI Quantera II,
MIMOS, Kuala Lumpur. FE-SEM was measured with
JSM-7800F, central laboratory, UMP. TEM were measured
with HT-7700, Hi-Tech Instruments SDN BHD, Puchong,
Malaysia. TLC analysis was performed on Merck silica
gel 60 F254. Column chromatography was carried out on
silica gel (Wakogel C-200).
where, W is the weight of parent polymer (cellulose) and
1
W is the weight of grafted polymer.
2
2.5. Synthesis of Poly(Hydroxamic Acid) Ligand 2
Hydroxylamine hydrochloride (20 g, NH OH · HCl) was
2
dissolved in 500 mL of aqueous methanol. An aque-
ous solution of NaOH (50%) was slowly added into the
ꢁ
NH OH·HCl solution at 20 C until the pH of the solution
2
was reached to pH 11 and the precipitate was removed by
filtration. Then poly(methyl acrylate) grafted tapioca cel-
lulose 1 (9 g) was added into a two-neck round bottom
flask fixed with a stirrer, condenser and thermostat water
bath. Then hydroxylamine solution with pH 11 was added
into the flask and the reaction was continued for 6 h at
2
.2. Materials
Tapioca root was procured from the local market at Kuan-
tan city, Pahang, Malaysia. The tapioca was cut into small
pieces (∼1 cm) and 100 g sample was boiled with 10%
NaOH (800 mL) for 4 h. The product was washed with
ꢁ
70 C. After completion of reaction, the polymer ligand
was isolated from hydroxylamine solution by filtration and
washed with aqueous methanol. The polymeric ligand was
J. Nanosci. Nanotechnol. 17, 550–557, 2017
551

Tapioca Cellulose Based Copper Nanoparticles for Chemoselective N-Alkylation
Islam et al.
again treated with 100 mL of 0.1 M HCl in methanol for 5
min to obtain H-form of the ligand. The ligand was further
filtered and washed several times with methanol and dried
ꢁ
52ꢂ53
at 50 C to get a constant weight.
2
.6. Preparation of Poly(hydroxamic acid)
Copper Complex 3
To a stirred suspension of 2 (2 g) in 50 mL water was
added an aqueous solution of CuSO · 5H O (492 mg, in
4
2
3
0 mL H O) at room temperature. The blue CuSO was
2 4
immediately turned into green colour copper complex and
the mixture was stirred for 2 h at room temperature. The
poly(hydroxamic acid) copper complex 3 was filtered and
washed several times with excess amount of ammonium
ꢁ
chloride, water, MeOH and dried at 60 C for 2 h. The
5
3
ICP-AES analysis showed that 0.5 mmol/g of copper was
adsorbed by the poly(hydroxamic acid) ligand.
3
2
.7. Preparation of Poly(hydroxamic Acid) Copper
Nanoparticles, CuN@PHA
Scheme 1. Synthesis of tapioca cellulose-supported CuN@PHA.
Poly(hydroxamic acid) copper complex 3 (1 g) was dis-
copper was adsorbed with the poly(hydroxamic acid) lig-
persed in 100 mL deionized water and then hydrazine
and 2. The copper complex 3 was then reacted with
5
4
hydrate 1 mL was added. The resulting dark brown color
CuN@PHA materials (Ø = 7.5± 3 nm) were collected by
filtration, washed with methanol, and dried under vacuum
5
4
hydrazine hydrate to give dark brown color CuN@PHA
(
Ø = 7.5 ± 3 nm).
The tailoring of cellulose backbone, generation of
ꢁ
at 100 C for 3 h and stored under nitrogen atmosphere.
poly(hydroxamic acid) ligand as well as addition of cop-
per ion into the ligand were verified by IR analysis. Fresh
2
.8. General Procedure for N-Alkylation Reaction
In a typical experiment, a mixture of aliphatic amine
10 mmol), ꢃ,ꢀ-unsaturated compound (11 mmol), tapioca
cellulose supported CuN@PHA (1 mg, 50 mol ppm) in
.5 mL MeOH was stirred at room temperature for 8 h.
tapioca cellulose showed IR adsorption bands at 3430 and
−1
2
915 cm which referred to O–H and C–H stretching
(
−1
respectively (Fig. 1). The band at 1655 cm is due to
OH and a smaller peak at 1450 cm was assigned for
−1
4
CH symmetric stretching while the bands at 1385 and
2
The reaction progress was monitored by GC analysis.
After completion of the reaction, CuN@PHA was filtered,
washed with MeOH (3 × 5 mL), dried, and reused in the
next run under the same reaction conditions. The filtrate
was concentrated to give the crude product which was
purified by column chromatography (hexane/ethyl acetate)
to give the corresponding N -alkylated products.
−1
1
162 cm support the stretching frequency of O–H and
C–O respectively. A strong band at 1085 cm indicates
C–O–C stretching in the pyranose ring moiety. A small
5
5
−1
−1
sharp peak at 908 cm corresponded to the glycosidic
3
. RESULTS AND DISCUSSION
The tapioca cellulose was obtained after cutting tapioca
root into small pieces with successive boiling with 10%
NaOH and glacial acetic acid respectively. The result-
ing cellulose was washed with water and bleached with
hydrogen peroxide. The obtained white color cellulose
was graft polymerized with methyl acrylate using ceric
ammonium nitrate as an initiator to give cellulose sup-
ported poly(methyl acrylate) 1 which was further treated
with hydroxylamine to convert into poly(hydroxamic acid)
chelating ligand 2. The poly(hydroxamic acid) function-
alized cellulose was then treated with aqueous cop-
per sulphate at room temperature to give green color
poly(hydroxamic acid) copper complex 3 (Scheme 1).
The ICP-AES analysis revealed that 0.5 mmol/g of
Figure 1. IR spectra of cellulose, poly(methyl acrylate) 1,
poly(hydroxamic acid) 2 and cellulose-supported copper complex 3
respectively.
552
J. Nanosci. Nanotechnol. 17, 550–557, 2017

Islam et al.
Tapioca Cellulose Based Copper Nanoparticles for Chemoselective N-Alkylation
C –H deformation with ring vibration contribution and OH
poly(methyl acrylate) grafted cellulose 1 also showed
wooden stick like cellulosic structure having rough sur-
face surrounding the stick which displayed distinguish-
able grafting occurred on the surface (Fig. 2(b)). The
poly(hydroxamic acid) chelating ligand 2 exhibited distinct
spherical morphology which is completely different from
1 indicating the successful production of graft copolymer
onto the cellulose surface (Fig. 2(c)). The SEM image
of 3 displayed bigger spherical morphology which proved
the aggregation of ligand 2 and was coordinated with
copper(II) (Fig. 2(d)).
1
bending indicating ꢃ-glycosidic linkages between glucose
5
5
units in cellulose. The IR spectrum of poly(methyl acry-
late) grafted cellulose 1 showed new absorption bands at
−
1
1
742, 1475, 1402 cm due to C O, scissoring and wag-
5
6
ging stretching of methyl group respectively. The peak
−
1
3
at 2980 and 2870 cm represent C–H (sp ꢁ stretching of
methyl group. The poly(hydroxamic acid) 2 showed new
−1
absorption bands at 1690 and 1651 cm corresponding
to the C O stretching and N–H bending modes. Addi-
−
1
tionally, another new broad band at 3185 cm for N–H
−1
stretching and 1410 cm for OH bending were observed.
It was found that the C O band at 1742 cm in 1 shifted
to 1690 cm which confirmed the successful production
of hydroxamic acid ligand onto the tapioca cellulose. After
incorporation of copper salt with poly(hydroxamic acid),
the color of 2 changed to green color and a new absorp-
The zero oxidation state of copper in the cellulose sup-
ported poly(hydroxamic acid) copper nanoparticles was
confirmed by XPS analysis. A single Cu 2p3/2 peak at
932.5 eV in the XPS spectrum of CuN@PHA (Fig. 3(a))
−1
−1
5
4ꢂ59
is assigned for the binding energy of Cu(0).
UV-vis
spectrum of 3 (Fig. 3(b)) exhibited a single absorption
−
1
tion band at 1738 cm was observed which corresponding
at 694 nm which demonstrated the presence of Cu–N
5
7ꢂ58
59
to C O stretching for hydroxamic acid.
A new peak
coordination. The TEM micrograph showed the pres-
−1
at 1618, 1450 and 1162 cm were assigned for C
and C–O stretching respectively. A peak at 615 cm con-
firmed the successful incorporation of copper with the
N
ence of copper nanoparticles onto the cellulose backbone
(Fig. 3(c)). These results ensured the successful produc-
tion of the tapioca cellulose poly(hydroxamic acid) copper
complex 3 as well as the formation of copper nanoparticles
on to the cellulose.
−1
5
7
poly(hydroxamic acid) ligand 2.
The FE-SEM analysis also complies with the result
of IR. FE-SEM micrograph of tapioca cellulose revealed
wooden stick like morphology (Fig. 2(a)) whereas
N -alkylation of amines by ꢃ,ꢀ-unsaturated com-
pounds was considered for the investigation of the
(
a)
(b)
(
c)
(d)
Figure 2. (a) FE-SEM image of tapioca cellulose, (b) FE-SEM image of 1, (c) FE-SEM image of 2, (d) FE-SEM image of 3.
J. Nanosci. Nanotechnol. 17, 550–557, 2017
553

Tapioca Cellulose Based Copper Nanoparticles for Chemoselective N-Alkylation
a)
Islam et al.
(
Poly(hydroxamic acid) Cu(II) 3
Poly(hydroxamic acid) 2
1
1
0
0
0
.6
.2
.8
.4
.0
(b)
300
400
500
600
700
800
900
Wavelength (nm)
(
c)
(d)
Figure 3. (a) XPS of CuN@PHA, (b) UV-vis of 2 and 3, (c) TEM image of CuN@PHA, (d) TEM image of 3rd reuse CuN@PHA.
catalytic performance of the tapioca cellulose supported
CuN@PHA catalyst. The catalyst was found workworthy
and optimum reaction condition was determined. For the
optimization, addition reaction between morpholine and
butyl acrylate was chosen as a model reaction to evaluate
the catalytic activity of CuN@PHA in N -alkylation reac-
tion. Table I presents the results of the effect of catalyst
dosages and reaction time. The initial reaction was carried
out using 1000 mol ppm (0.1 mol%) of CuN@PHA in
methanol at room temperature. The CuN@PHA efficiently
forwarded the reaction to give the corresponding addition
product 4g with 95% yield within 1.5 h (entry 1). The
reaction completion time was found to be increased to 3 h
when 500 mol ppm of CuN@PHA was used (entry 2).
When the loading of CuN@PHA was further decreased
(200–100 mol ppm), it took little longer time to complete
the reaction (entries 3, 4). The catalyst loading can be
reduced to 50 mol ppm (1 mg, 0.005 mol%, 50 mol ppm)
which also efficiently promoted this addition reaction. For
each entry, the TON and TOF of the catalyst were cal-
culated and highest value was obtained when 50 mol
ppm CuN@PHA was used (entry 5). As far as we know,
these are the highest TON and TOF obtained for cellulose
supported CuN@PHA catalyzed N -alkylation reaction. It
should be noted that the N -alkylation reaction was also
carried out using 2 and 3 which gave only 12–19% of the
product (entry 6, 7).
Since the acceptability of the catalyst depends on its
wide range of application, we focused our attention to
extend the wide relevancy of CuN@PHA using 50 mol
ppm of copper at room temperature. The ꢃ,ꢀ-unsaturated
carbonyl compound, methyl acrylate was found to be for-
warded smoothly N -alkylation reaction with secondary
aliphatic amine such as dibutylamine, piperidine, morpho-
line and even sterically hindered secondary dibenzylamine
to give the corresponding products in up to 94% yields
(Tables II, 4a–e).
a
Table I. Optimization of N-alkylation reaction
CuN@PHA
CO Bu
2
NH +
CO Bu
O
2
O
N
MeOH,rt
Entry
1
Catalyst
1000 (ppm)
CuN@PHA
500 (ppm)
CuN@PHA
200 (ppm)
CuN@PHA
100 (ppm)
CuN@PHA
50 (ppm, 1 mg)
CuN@PHA
2 (1 mg)
Time (h)
TON
950
TOF (h 1ꢁ Yield (%)
−
1.5
3
633
620
95
93
92
93
90
2
3
4
5
1860
4600
4.5
5.5
8
1022
1690
2250
9300
18000
6
7
8
8
−
−
−
−
12
19
3 (1 mg)
Notes: aReactions were carried using 10 mmol of morpholine, 11 mmol of butyl
acrylate in 4.5 mL MeOH at room temperature. Each reaction has been done at
least three times.
554
J. Nanosci. Nanotechnol. 17, 550–557, 2017

Islam et al.
Tapioca Cellulose Based Copper Nanoparticles for Chemoselective N-Alkylation
a
Table II. N-alkylation reaction of aliphatic amines
R2
CuN@PHA
H
N
(50 mol ppm)
R2
N
+
R1
R1
MeOH, rt, 8 h
R1
R1
4
Amine
Olefin
Product
Yield (%)
CO Me
2
Bu NH
2
CO
2
Me
93
94
Bu N
4a
2
CO Me
2
NH
CO Me
N
2
4
b
CO Me
2
O
NH
CO Me
O
N
N
2
94
94
4
c
CO Me
Ph
NH
2
Ph
CO Me
2
4
d
Ph
Ph
N
CO Me
2
NH
2
CO Me
2
88
4e
CO Me
2
CO Bu
2
NH
NH
CO Bu
N
2
92
93
91
4
f
CO Bu
2
O
CO Bu
O
N
2
4
g
CO Bu
2
NH
CO Bu
N
2
4
h
CO Bu
Ph
NH
2
Ph
N
CO Bu
95
93
2
4
i
Ph
Bu NH
Ph
CO
2
Bu
CO Bu
2
2
Bu N
2
4j
BuO C
CO
2
Bu
2
H NCH CH NH
CO Bu
2
2
2
2
2
BuO C
NCH CH N
CO Bu
87
2
2
2
2
4
k
CN
CN
Bu NH
2
CN
CN
Bu N
90
88
2
4l
NH
N
4
m
CN
CN
Ph
O
NH
Ph
N
CN
CN
CN
93
92
90
Ph
Ph
4n
NH
O
N
N
4o
CN
NH
4
p
NC
CN
CN
H NCH CH NH
C
N
2
2
2
2
N
C
N H2 H N
C C
2
88
4
q
Note: aReactions were carried out using 10 mmol of amine, 11 mmol of ꢃ,ꢀ-unsaturated compound and CuN@PHA (1 mg, 50 mol ppm) in 4.5 mL MeOH at room
temperature for 8 h.
J. Nanosci. Nanotechnol. 17, 550–557, 2017
555

Tapioca Cellulose Based Copper Nanoparticles for Chemoselective N-Alkylation
Islam et al.
Likewise, butyl acrylate efficiently combined with dif-
1
00
ferent amines to produce their corresponding N -alkylated
products in up to 95% yields (4f–k). It was observed
that the ꢃ,ꢀ-unsaturated acrylonitrile also underwent the
addition reaction to produce the monoalkylated products
8
6
4
0
0
0
(
4l–q).
Subsequently, we attempted to synthesize multi alky-
9
5
93
93
9
2
90
8
8
85
lated products and CuN@PHA impressively stimu-
lated N -alkylation reaction of cyclohexylamine and
ethylenediamine with ꢃ,ꢀ-unsaturated compounds (2.2 to
20
0
4
.4 mol equiv) to give the poly-alkylated products 4e, 4k,
1
2
3
4
5
6
7
and 4q in up to 88% yield.
Run
Figure 4. N-alkylation reaction by recycled CuN@PHA.
Chemoselectivity of catalysts play an important role in
transformation reactions. To investigate the chemoselec-
tivity of CuN@PHA was considered and a mixture of
aniline and dibenzylamine were subjected to butyl acry-
late (4 mol equiv) using 100 mol ppm of CuN@PHA.
It was fascinating that aniline did not undergo this reac-
tion rather only the N -alkylation adduct of dibenzylamine
was obtained exclusively (Scheme 2). It definitely demon-
strated the chemoselectivity of aliphatic amines versus aro-
matic amines towards CuN@PHA. The less reactivity of
aromatic amines is the cause of this selectivity and there-
fore, this selectivity could be functional to differentiate
between the two types of amines for synthetic applications.
On the ground of green chemistry, using heterogeneous
catalysts for organic synthesis, the ease of separation,
deactivation and reusability of the catalyst are important
issues. The catalyst is best suited when it can be recov-
ered easily and reused several times before it finally gets
deactivated totally. Thus, CuN@PHA catalyst was con-
sidered for recyclability and reusability for N -alkylation
reactions over seven successive runs through repeated sep-
aration of CuN@PHA from reaction mixture, washing it
and then reusing it. After initial reaction, the reaction mix-
ture was diluted with EtOAc and filtered. The solid dark
brown color CuN@PHA catalyst was washed with MeOH
large-scale production with no notable loss of its catalytic
activity. The TEM image (Fig. 3(b)) of reused CuN@PHA
exhibited similarity (Ø = 7.6 ± 6 nm) with the original
CuN@PHA (Fig. 3(a)).
4
. CONCLUSION
In this study, we have successfully tailored tapioca cel-
lulose backbone and synthesized tapioca cellulose sup-
ported bio-heterogeneous poly(hydroxamic acid) copper
nanoparticles as well as characterized by different tech-
niques including FTIR, UV-vis, FE-SEM, XPS, TEM
and ICP-AES analyses. The CuN@PHA was effectively
applied in the N -alkylation of aliphatic amines with con-
jugate ꢃ,ꢀ-unsaturated compounds at room temperature in
methanol and outstanding conversion was achieved under
mild condition. The catalyst (50 mol ppm) displayed sig-
nificant catalytic activity and chemoselectivity both in
mono-N -alkylation and poly-N -alkylation. Our catalyst
can be separated by simple filtration and reused for several
cycles with no unusual degradation of catalytic activity.
Thus, N -alkylation reaction could proceed in the presence
of solid tapioca cellulose supported CuN@PHA and there
was no leaching of active copper species in the liquid
phase. Hence, it could be used in practical applications.
ꢁ
and dried at 80 C under vacuum and then applied it to
the next cycle without changing the reaction conditions.
Seven cycles were performed and CuN@PHA was found
not to lose its catalytic activity in any remarkable extent
Acknowledgments: This work is supported by Min-
istry of Education Malaysia, fund no. RDU 140343. We
are grateful to Nor Hafizah Bt Zainal Abidin, scientific
officer, and Central lab University Malaysia Pahang.
(
Fig. 4). Only slight loss of catalytic activity was observed
under the same reaction conditions as for initial run, it
is supposed to be the loss of CuN@PHA during filtra-
tion process. The ICP-AES analysis also indicated that no
copper species was leached out into the reaction mixture.
Thus, it is very logical to claim that the tapioca cellu-
lose supported CuN@PHA could be used repeatedly for
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Received: 26 February 2016. Accepted: 15 March 2016.
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