Copper nanoparticles stabilized on nitrogen-doped carbon nanotubes as efficient and recyclable catalysts for alkyne/aldehyde/cyclic amine A 3-type coupling reactions

Article abstract of DOI:10.1016/j.apcata.2012.04.019

Metallic copper nanoparticles have been efficiently dispersed and stabilized on nitrogen-doped carbon nanotubes. They are about 8-10 nm in diameter and highly resistant against bulk oxidation. Their catalytic activity and recyclability have been investigated in A³-type coupling reactions for the synthesis of propargylamines. It was easily possible to prepare diastereomerically pure derivatives of proline and to efficiently recover and reuse the supported catalyst several times.

Full text of DOI:10.1016/j.apcata.2012.04.019

Applied Catalysis A: General 431–432 (2012) 88–94
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Copper nanoparticles stabilized on nitrogen-doped carbon nanotubes as efficient
and recyclable catalysts for alkyne/aldehyde/cyclic amine A3-type coupling
reactions
Vasanthakumar Ganga Ramua,1, Ankur Bordoloib,1, Tharamani C. Nagaiahc,
Wolfgang Schuhmannc, Martin Muhlerb,∗, Chiara Cabrelea,b,∗∗
a Organic Chemistry I, Faculty of Chemistry and Biochemistry, Ruhr-University Bochum, Universitätsstraße 150, 44780 Bochum, Germany
b Industrial Chemistry, Faculty of Chemistry and Biochemistry, Ruhr-University Bochum, Universitätsstraße 150, 44780 Bochum, Germany
c Analytical Chemistry, Faculty of Chemistry and Biochemistry, Ruhr-University Bochum, Universitätsstraße 150, 44780 Bochum, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Metallic copper nanoparticles have been efficiently dispersed and stabilized on nitrogen-doped carbon
nanotubes. They are about 8–10 nm in diameter and highly resistant against bulk oxidation. Their cat-
alytic activity and recyclability have been investigated in A3-type coupling reactions for the synthesis of
propargylamines. It was easily possible to prepare diastereomerically pure derivatives of proline and to
efficiently recover and reuse the supported catalyst several times.
Received 15 February 2012
Received in revised form 12 April 2012
Accepted 14 April 2012
Available online 20 April 2012
© 2012 Elsevier B.V. All rights reserved.
Keywords:
A3 coupling
Copper catalysis
Copper nanoparticles
Heterogeneous catalysis
Nitrogen-doped carbon nanotubes
1. Introduction
display high surface area, tuneable porosity, extraordinary stability
in acid and basic media as well as at high pressures and tempera-
Heterogeneous catalysis is very attractive because of easy recov-
ery, recycling and enhanced stability of the precious catalytic
species [1,2]. Upon immobilization on solid supports [3–6] also
homogeneous metal catalysts can be made recyclable. However,
the immobilization approach may present some drawbacks such
as the leaching of the active species into the reaction medium,
the poor accessibility of the catalytic site for the substrate, and
the low surface area of the support. To overcome such dif-
ficulties, Reiser and co-workers [7] have recently proposed a
temperature-dependent catch-and-release system, in which a pal-
ladium complex is released from the surface of carbon-coated
cobalt nanoparticles upon heating up to 100 ◦C and caught again by
cooling down the reaction mixture to room temperature. Also the
application of carbon nanotubes (CNTs) as solid support can partly
circumvent the drawbacks of the immobilization. Indeed, CNTs
tures [8]. In particular, nitrogen-doped CNTs (NCNTs) are expected
to be a suitable support for metal catalysts, as the nitrogen atoms
not only can provide strong bonding to the metal, but also behave
as electron donors [9,10].
Copper nanoparticles are very interesting materials due to
their high conductivity, good biocompatibility and low cost, which
makes them attractive for a large variety of applications [11–14].
It has been shown that embedding copper nanoparticles in sup-
ports such as silica improves both the catalytic stability and activity
[15–17]. However, obtaining well dispersed metal nanoparticles
on the surface of the solid support is a challenge, although differ-
ent methods are described in the literature such as impregnation,
adsorption, ion exchange, sol–gel, and co-precipitation [15–21].
Generally, the drawbacks of these methods are either agglomera-
tion of the metal nanoparticles or poor metal–support interaction.
The application of NCNTs as supports should avoid these problems
due to the fact that the pyridine nitrogen atoms not only may pro-
vide the main initial nucleation sites for the formation of small
and highly dispersed metal nanoparticles, but may also reduce the
leaching of the metal nanoparticles during catalysis due to strong
nitrogen metal bonding.
∗
Corresponding author. Tel.: +49 234 3228754.
Corresponding author. Current address: Dept. of Molecular Biology, University
∗∗
of Salzburg, Billrothstraße 11, 5020 Salzburg, Austria. Tel.: +43 662 80447200.
E-mail addresses: martin.muhler@rub.de (M. Muhler), chiara.cabrele@rub.de
(C. Cabrele).
In this work we have combined the use of NCNTs as solid
supports with that of copper nanoparticles to provide novel
1
These authors contributed equally to this work.
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.04.019

V.G. Ramu et al. / Applied Catalysis A: General 431–432 (2012) 88–94
89
Scheme 1. A3-type coupling reaction catalyzed by Cu nanoparticles stabilized on nitrogen-doped multi-walled CNTs (Cu/NCNTs).
heterogeneous catalysts, which have been tested in an A3-type cou-
pling reaction for the synthesis of propargylamines. The latter are
important intermediates for the synthesis of natural products and
play a key role in medicinal chemistry and chemical biology. In
particular, propargylamine-based compounds have been shown to
possess neuroprotective properties and to be effective in the treat-
ment of Parkinson’s and Alzheimer’s diseases [22].
In the last decade many efforts have been done to develop new
methodologies for the A3-type coupling [23,24], which include the
use of copper (I) catalysts in combination with (a) microwave irra-
diation [25,26], (b) Quinap for enantioselective synthesis [27], (c)
ionic liquids [28], or polyethylene glycol (PEG) [29]. Recently, sil-
ver nanoparticles supported by Ni-MOFs [30], gold nanoparticles
stabilized on mesoporous carbon nitride [31], gold and copper as
nanoparticles [32–34] or thin films [35], copper (I) immobilized
on modified silica [36–39], as well as molecular sieve [40], zeo-
lite [41] and magnetite [42] modified with copper (I) or (II) have
been also used as catalysts for A3-type reactions. However, in some
cases harsh conditions or expensive and non-recyclable catalysts
are necessary. Here, we propose the synthesis of propargylamines
under mild reaction conditions by using metallic copper nanoparti-
overall energy resolution better than 0.5 eV. Charging effects were
compensated by applying a flood gun. The binding energies were
calibrated based on the graphite C 1s peak at 284.5 eV. The CASA
XPS program with a Gaussian–Lorentzian mix function and Shirley
background subtraction was used to deconvolute the XP spectra.
The peak positions were reproducible along with the fixed Lorentz
to Gaussian ratio and FWHM and the surface atomic concentration
of C, N and O was calculated from the corresponding spectra.
TEM images were obtained in bright field mode with two dif-
ferent electron microscopes – a Philips CM12 and a JEOL 1011
equipped with a numeric camera – operating both at low accelerat-
ing voltage (60 kV) to avoid organic phase damage under the beam.
Both imaging and diffraction modes were employed for character-
ization. Specimens were dispersed on 3 mm carbon-coated copper
grids by deposition of a drop of a diluted suspension of particles in
ethanol, after sonication. SEM observations were performed with a
JEOL JSM 6100 (tungsten filament) operating at 20 kV, with a work-
ing distance of 10 mm. Prior to observation the samples were fixed
with glutaraldehyde, dehydrated in acetone and dried with a crit-
ical point dryer BAL-TEC CPD 030 with liquid CO2 (critical point
31 ◦C, 73.8 bar). Imaging was also performed with a high voltage
Secondary Electron Detector.
Morphology of the NCNTs and Cu/NCNTs was investigated using
SEM (FEI Quanta 3D). Raman spectroscopy measurements were
performed using a modular system (iHR-550, Horiba) with a green
Ar+ laser (532 nm) with adjustable output power. The objective
of the Raman spectrometer was 10× and the spectra acquisition
time was 20 s 4×. The laser power was set at 79.4 mW in all mea-
surements. The TG/DTG analysis has been performed with a CAHN
TG-2131 coupled with Qministar Q mass spectrometer (in synthetic
air).
cles that are highly dispersed and stabilized on NCNTs for the C
bond activation of terminal alkynes (Scheme 1).
H
2. Experimental
2.1. Materials and methods
All chemicals were purchased by Sigma–Aldrich and used as
received. Copper content in the catalyst materials was estimated
by inductively coupled plasma atomic emission spectrometry
(ICP-AES). Nitrogen adsorption and desorption isotherms were
measured at −196 ◦C on a Quantachrome Autosorb 1 sorption
analyzer. The samples were out gassed for 3 h at 200 ◦C under
vacuum in the degas port of the adsorption analyzer. The spe-
cific surface area was calculated using the BET model. The pore
size distributions were obtained from the adsorption branch of
the nitrogen isotherms by the BJH method. XRD analysis of Cu-
based catalysts was carried out using X-ray powder diffractometer
with Cu K␣ radiation at room temperature. XPS measurements
were performed in an ultra-high vacuum (UHV) set-up equipped
with a monochromatic Al K␣ X-ray source (1486.6 eV) and a high
resolution Gammadata-Scienta SES 2002 analyzer. The base pres-
sure in the measurement chamber was about 7 × 10−10 mbar. XP
spectra were recorded in the fixed transmission mode with an
The A3-type coupling reactions were monitored by gas chro-
matography (GC) (Shimadzu 2014B) using a RTX Volatiles column
with a length of 60 m, inner diameter of 0.53 mm and film thick-
ness of 22.0 ␮m. The reaction products we characterized by GC–MS,
chiral HPLC and 1H- and 13C-NMR spectroscopy.
2.2. Functionalization of CNTs [43,44]
Multi-walled CNTs with inner diameters of 20–50 nm and outer
diameters of 70–200 nm were obtained from Applied Sciences Inc.
(OH, USA) and used as received. The CNTs were first thermally
treated under flowing He for 1 h at 800 ◦C to remove polyaromatic
impurities at the surface. The thermally treated CNTs were then
oxidized via a HNO3 vapor treatment in the gas phase at 200 ◦C

90
V.G. Ramu et al. / Applied Catalysis A: General 431–432 (2012) 88–94
for 48 h. After cooling, the CNTs were dried at 60 ◦C overnight. In
order to introduce N-containing functional groups, 200 mg of the
oxidized CNTs (OCNTs) were loaded into a tubular quartz reactor
with an inner diameter of 20 mm. The nitrogen functionalized CNTs
(NCNTs) were then obtained by treatment with ammonia with a
total flow rate of 25 mL/min at four different temperatures (200,
400, 600 and 800 ◦C) for 6 h.
2.3. Deposition of Cu nanoparticles on NCNTs
The copper catalyst was prepared by impregnating the NCNTs
high surface area with an aqueous solution of copper acetate with
a copper loading of 10 wt.% under sonication for 3 h. The impreg-
nated NCNTs were then dried by rotaevaporator followed by drying
in oven at 80 ◦C for about 2 h. The resulting impregnated NCNTs
were placed in quartz boats in a tubular furnace, submitted to an
additional drying step under He atmosphere at 100 ◦C for 1 h and
the reduction of metal precursors was carried out at 230 ◦C under
a mixture of 1:1 He/H2 for 3 h. The resulting catalyst is referred
to as Cu/NCNTs. The loading has been further confirmed with ICP
analysis.
Fig. 1. Wide-angle X-ray diffraction pattern of Cu/NCNTs.
even after 30 days under ambient conditions, as confirmed by XRD
(Fig. S2).
The specific surface area and pore volume of the NCNTs and
Cu/NCNTs were found to be similar (for NCNT: 47 m2/g and
0.18 cm3/g; for Cu/NCNT 48 m2/g and 0.17 cm3/g). SEM and TEM
images show homogenous Cu/NCNTs (Fig. 2), in which the cop-
per nanoparticles appear as evenly distributed bright spots on the
NCNT surfaces. Their size is estimated to be 8–10 nm (Fig. 2b, inset),
whereas the NCNTs have diameters of 70–200 nm. The rather small
dimensions of the copper nanoparticles compared to the NCNT
diameter may explain, why the specific surface area and pore vol-
ume are similar in the NCNTs and Cu/NCNTs.
2.4. A3-type coupling reaction
In a standard experiment Cu/NCNTs (25 mg, 0.04 mmol Cu) were
added to a reaction mixture consisting of aldehyde (1.0 mmol),
alkyne (1.2 mmol), and preneutralized (in the case of hydrochloride
salts) cyclic amine (1.3 mmol) in dry THF (5.0 mL). The combined
mixture was refluxed at 70 ◦C for 4–6 h under nitrogen atmo-
sphere. After complete conversion of the aldehyde, which was
monitored by TLC and GC, the mixture was left to cool down to room
temperature and filtered through silica crucibles. The filtrate was
evaporated to dryness and the desired product was isolated by sil-
ica gel flash column chromatography with a mixture of petroleum
ether/ethyl acetate as eluent. Propargylamine derivatives were
characterized by 1H/13C NMR, GC–MS, and chiral HPLC.
No agglomerates or free background particles are observed
(Fig. S3), which supports the strong interaction of copper with the
NCNT functionalities.
3. Results and discussion
Due to their hydrophobic properties, the CNTs are not suitable
for adhesion of most metal nanoparticles. Therefore, in this work
nitrogen-doped multi-walled CNTs (NCNTs) were prepared and
characterized as described previously [43,44]. Briefly, the multi-
walled CNTs were oxidized under HNO3 vapor at 200 ◦C. The
resulting oxygen-doped CNTs (OCNTs) were successively converted
to NCNTs by treatment with ammonia at 400 ◦C. Based on the quan-
titative analysis of the XP spectrum, the NCNT surface composition
was as follows: 89% carbon, 5% oxygen and 6% nitrogen [43]. The
different types of nitrogen species were present in the following
percentages: 52% pyridinic (N1), 35% pyrrolic (N2), 13% quater-
nary (N3) nitrogen [43]. The high amount of pyridinic sites should
be suitable for high loading capacity as well as dispersion. These
NCNTs were thus subjected to impregnation-decomposition of cop-
per acetate under reducing conditions (He/H2, 1:1, atmosphere).
The wide-angle X-ray diffraction pattern of the copper nanopar-
ticles supported on NCNTs (Fig. 1) shows four major peaks. The
graphite peak at 26.45◦ originates from the CNT walls, whereas
the other peaks at 43.3◦, 50.4◦, and 74.1◦ correspond to the metal-
lic copper (1 1 1), (2 0 0), and (2 2 0) reflections, respectively. These
characteristic peaks confirm the formation of a face-centered cubic
(FCC) copper phase without significant oxides or other impurity
phases. The presence of copper has been additionally confirmed by
energy-dispersive X-ray analysis with dot mapping (Fig. S1).
Usually, metal nanoparticles are prone to air oxidation due to
their high surface area. In contrast, the copper nanoparticles sup-
ported on NCNTs have shown high stability against bulk oxidation
Fig. 2. (a) SEM and (b) TEM images of Cu/NCNTs (inset: histogram depicting the size
distribution of Cu nanoparticles on NCNTs).

V.G. Ramu et al. / Applied Catalysis A: General 431–432 (2012) 88–94
91
Fig. 4. XP Cu 2p spectrum of Cu/NCNTs.
The Cu/NCNTs were further characterized by Raman spec-
troscopy and TG analysis. The Raman bands for amorphous
(D-band, 1340 cm−1) and graphitic (G-band, 1545 cm−1) carbon
were in about 1:1 ratio (data not shown). The DTG curve of the
Cu/NCNTs is characterized by two peaks that correspond, respec-
tively, to the decomposition of oxygen and nitrogen functionalities,
and to the total oxidation of the CNTs to CO and CO2 (Fig. 5).
The catalytic properties of the Cu/NCNTs were investigated in
A3-type coupling reactions with phenyl acetylene, benzaldehyde
(or 2-methylbenzaldehyde) and a series of achiral (piperidine and
isonipecotic acid methyl ester), chiral (methyl ester of S-proline),
and racemic (methyl esters of R/S-pipecolic and R/S-nipecotic acid)
cyclic amines. The optimal reaction conditions with respect to
reagents ratio, catalyst amount, temperature and time were inves-
tigated (Table S1). The reaction progress was monitored by GC
analysis of the conversion of the aldehyde component. We observed
that a slight excess of secondary amine (up to 1.3 equivalents) and
phenylacetylene (up to 1.2 equivalents) with respect to benzalde-
hyde affected positively the yield of the product, whereas reaction
times longer than 6 h had no effect on the yield. This suggests
that the reaction proceeds via iminium ion formation, followed
by the nucleophilic attack of the in situ generated Cu-acetylide,
as generally proposed for Cu(I)-catalyzed A3 coupling reactions
[25]. Kidwai et al. [34] have reported on the use of metallic copper
nanoparticles for this type of reaction under nitrogen atmosphere,
proposing a free-radical mechanism in which the metal cluster
is assumed to behave as a redox catalyst [45]. Such mechanism
would be applicable also for the Cu/NCNTs. However, despite con-
ducting the reaction under inert gas atmosphere, the presence of
minor amounts of the air-oxidized copper species cannot be fully
excluded.
Fig. 3. (a) XPS survey scan, (b) deconvoluted XP O 1s and (c) N 1s spectra of the
Cu/NCNTs.
The surface atomic composition of the NCNTs after deposition
of the copper nanoparticles was determined by high-resolution
XPS. The survey spectrum of the Cu/NCNTs shows the C 1s peak
at 284.5 eV, those of N and O 1s at about 400 eV and 530 eV,
respectively, and the two peaks of Cu 2p at about 932 and 952 eV
(Fig. 3a). The quantitative XPS analysis provided the following sur-
face atomic composition: 87.5% C, 5.5% N, 4.5% O, 2.5% Cu. The
deconvoluted XP O 1s spectrum shows the three peaks corre-
sponding to oxygen (a) double-bonded to carbon (531.7 eV), (b)
single-bonded to carbon (533.2 eV), and (c) in physisorbed water
(534.0 eV) (Fig. 3b). The deconvoluted XP N 1s spectrum of the
Cu/NCNTs reported in Fig. 3c confirms the presence of pyridinic,
pyrrolic and quaternary nitrogen groups, the pyridinic ones being
the predominant species.
Fig. 5. TG (black) and DTG (blue) curves of the Cu/NCNTs with 0.06 mmol Cu loading.
The DTG curve is characterized by two peaks that correspond, respectively, to the
decomposition of oxygen and nitrogen functionalities, and to the total oxidation of
the CNTs to CO and CO2. (For interpretation of the references to color in this figure
caption, the reader is referred to the web version of the article.)
The XP Cu 2p spectrum shows the presence of the 2p3/2 and
2p1/2 states at 932.75 and 952.55 eV, respectively (Fig. 4). The Cu
2p1/2 peak is broader than the Cu 2p3/2 one, which is attributed to
metallic copper.

92
V.G. Ramu et al. / Applied Catalysis A: General 431–432 (2012) 88–94
Table 1
Cu/NCNTs catalyzed A3-type coupling reaction for the synthesis of propargylamines.
Product no.a
Aldehyde
Yield (%)c
dr (%)d
conversion (%)b
4a
4b
4c
4d
4e
4f
4g
4h
4i
99
95
95
93
96
96
94
96
93
85
82
84
81
89
84
83
80
81
–
–
–
–
98:2
97:3
80:20
65:35
55:45
a
Reaction conditions: aldehyde/alkyne/amine 1.0:1.2:1.3 mmol, 25 mg
Cu/NCNTs (0.04 mmol Cu) in THF, 70 ◦C, 4–6 h.
b
Determined by GC analysis of the aldehyde peak.
c
Obtained after reaction work-up.
Fig. 6. Time-dependent conversion of benzaldehyde in an A3-coupling reaction cat-
alyzed by Cu/NCNTs under the conditions reported in Table 1. Aldehyde conversion
runs to completion over 6 h in the presence of the supported catalyst (blue curve)
or stops by 65% after 2 h upon its removal (red curve). (For interpretation of the
references to color in this figure caption, the reader is referred to the web version
of the article.)
d
Diastereomeric ratio (dr), as determined by 1H NMR.
Almost complete conversion of the aldehyde (93–99%) and
excellent product selectivity (95–98%) were obtained by using
4 mol% copper catalyst and running the reaction at 70 ◦C under
nitrogen atmosphere for 4–6 h (Table 1). In contrast, no reaction
occurred in the absence of the Cu/NCNTs (Fig. 6) or in the presence
of the NCNTs alone, indicating that the catalytic activity is exerted
only by the supported copper nanoparticles.
A3-type coupling reactions involving proline methyl ester have
been already reported and shown to be diastereoselective [25]. Also
in our case the reactions with S-proline methyl ester are diastereos-
elective (dr >97:3, as determined for compounds 4e,f by 1H NMR).
Fig. 7. SEM images (a and c) and EDX spectra (b and d) of the Cu/NCNTs recovered after the first (a and b) and fifth (c and d) reaction runs.

V.G. Ramu et al. / Applied Catalysis A: General 431–432 (2012) 88–94
93
Interestingly, the dr value decreases to 80:20 with pipecolic acid
methyl ester, the higher homolog of proline: this observation is
in agreement with the already observed superior property of pro-
line as chiral inductor compared to pipecolic acid [46,47]. Further
increase in the distance of the chirality center from the amino
group, like in the case of nipecotic acid methyl ester, leads to almost
complete (dr of 65:35 with 2-methylbenzaldehyde) or complete (dr
of 55:45 with benzaldehyde) loss of diastereoselectivity.
for recording the TEM images, and to the technical staff at the
Anorganische Chemie I, Ruhr-Universität Bochum, for the GC–MS
analyses. Dr. T. Reinecke, Dr. M. Sanchez and Dr. R. Kar are gratefully
acknowledged for their technical help.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2012.04.019.
One of the main advantages of using a heterogeneous catalyst in
a liquid-phase reaction is the ease of separation and reuse in sev-
eral runs. To test the recyclability of the Cu/NCNTs, we filtered off
the catalyst after a reaction run, washed and dried it at 100 ◦C for
1 h, then reused it for four repeated runs. The catalyst performance
was comparable until the fourth run, leading to aldehyde conver-
sions from 99% in the first run to 95% in the fourth one (Table S2).
Moreover, ICP analyses of copper in the reaction filtrates did not
detect the metal in any of the four runs, indicating the absence of
metal leaching. In contrast, in the fifth repeated run about 2% metal
leaching was observed, which was accompanied by a decrease in
aldehyde conversion (below 80%). Accordingly, the SEM image and
the EDX spectrum of the Cu/NCNTs recovered after the first run
show no chemical and morphological alterations, whereas those of
the Cu/NCNTs recovered after the fifth run reveal partial cracking
of the support (Fig. 7). The latter might be related to the possible
redox behavior of the catalyst in a free-radical-based mechanism, as
CNTs have recently been shown to be good free-radical scavengers
[48]. Moreover, due to prolonged air exposure during the recovery
steps, the presence of oxidized besides metallic copper cannot be
fully excluded.
References
[1] D.E. De Vos, M. Dams, B.F. Sels, P.A. Jacobs, Chem. Rev. 102 (2002)
3615–3640.
[2] M.J. Climent, A. Corma, S. Iborra, Chem. Rev. 111 (2011) 1072–1133.
[3] C. Bianchini, P. Barbaro, Top. Catal. 19 (2002) 17–32.
[4] D.J. Cole-Hamilton, Science 299 (2003) 1702–1706.
[5] M. Heitbaum, F. Glorius, I. Escher, Angew. Chem. Int. Ed. 45 (2006)
4732–4762.
[6] T. Matsumoto, M. Ueno, N. Wang, S. Kobayashi, Chem. Asian J. 3 (2008)
196–214.
[7] S. Wittmann, A. Schätz, R.N. Grass, W.J. Stark, O. Reiser, Angew. Chem. Int. Ed.
49 (2010) 1867–1870.
[8] D. Tasis, N. Tagmatarchis, A. Bianco, M. Prato, Chem. Rev. 106 (2006) 1105–1136
(and references therein).
[9] R. Czerw, M. Terrones, J.-C. Charlier, X. Blase, B. Foley, R. Kamalakaran, N.
Grobert, H. Terrones, D. Tekleab, P.M. Ajayan, W. Blau, M. Rühle, D.L. Carroll,
Nano Letters 1 (2001) 457–460.
[10] H.J. Burch, J.A. Davies, E. Brown, L. Hao, S. Antoranz Contera, N. Grobert, J.F.
Ryan, Appl. Phys. Lett. 89 (2006) 143110.
[11] G. Evano, N. Blanchard, M. Toumi, Chem. Rev. 108 (2008) 3054–3131.
[12] T. Mitsudome, Y. Mikami, K. Ebata, T. Mizugaki, K. Jitsukawa, K. Kaneda, Chem.
Commun. (2008) 4804–4806.
[13] A. Sarkar, T. Mukherjee, S. Kapoor, J. Phys. Chem. C 112 (2008) 3334–3340.
[14] M. Kidwai, N.K. Mishra, S. Bhardwaj, A. Jahan, A. Kumar, S. Mozumdar, Chem-
CatChem 2 (2010) 1312–1317.
4. Conclusions
[15] K. Yoshida, C. Gonzalez-Arellano, R. Luque, P.L. Gai, Appl. Catal. A: Gen. 379
(2010) 38–44.
[16] Z. Huang, F. Cui, H. Kang, J. Chen, X. Zhang, C. Xia, Chem. Mater. 20 (2008)
5090–5099.
[17] C. Gonzalez-Arellano, R. Luque, D.J. Macquarrie, Chem. Commun. (2009)
1410–1412.
[18] R. Takahashi, S. Sato, T. Sodesawa, M. Kato, S. Yoshida, J. Sol-Gel Sci. Technol.
19 (2000) 715–718.
[19] G.G. Wildgoose, C.E. Banks, R.G. Compton, Small 2 (2006) 182–193.
[20] M. Behrens, A. Furche, I. Kasatkin, A. Trunschke, W. Busser, M. Muhler, B. Kniep,
R. Fischer, R. Schlögl, ChemCatChem 2 (2010) 816–818.
[21] R.J. White, R. Luque, V.L. Budarin, J.H. Clark, D.J. Macquarrie, Chem. Soc. Rev. 38
(2009) 481–494.
[22] O. Bar-Am, T. Amit, O. Weinreb, M.B.H. Youdim, S. Mandel, J. Alzeihmer’s Dis.
21 (2010) 361–371.
[23] W.-J. Yoo, L. Zhao, C.-J. Li, Aldrichim. Acta 44 (2011) 43–51.
[24] V.A. Peshkov, O.P. Pereshivko, E.V. Van der Eycken, Chem. Soc. Rev. (2012),
http://dx.doi.org/10.1039/C2CS15356D.
[25] L. Shi, Y.Q. Tu, M. Wang, F.M. Zhang, C.A. Fan, Org. Lett. 6 (2004) 1001–1003.
[26] O.P. Pereshivko, V.A. Peshkov, E.V. Van der Eycken, Org. Lett. 12 (2010)
2638–2641.
[27] N. Gommermann, P. Knochel, Chem. Commun. (2004) 2324–2325.
[28] S.B. Park, H. Alper, Chem. Commun. (2005) 1315–1317.
[29] Q. Zhang, J.-X. Chen, W.-X. Gao, J.-C. Ding, H.-Y. Wu, Appl. Organometal. Chem.
24 (2010) 809–812.
[30] S. Wang, X. He, L. Song, Z. Wang, Synlett (2009) 447–450.
[31] K.K.R. Datta, B.V.S. Reddy, K. Ariga, A. Vinu, Angew. Chem. Int. Ed. 49 (2010)
5961–5965.
[32] X. Zhang, A. Corma, Angew. Chem. Int. Ed. 47 (2008) 4358–4361.
[33] M. Kidwai, V. Bansal, A. Kumar, S. Mozumdar, Green Chem. 9 (2007) 742–745.
[34] M. Kidwai, V. Bansal, N.K. Mishra, A. Kumar, S. Mozumdar, Synlett (2007)
1581–1584.
[35] G. Shore, W.J. Yoo, C.J. Li, M.G. Organ, Chem. Eur. J. 16 (2010) 126–133.
[36] P. Li, L. Wang, Tetrahedron 63 (2007) 5455–5459.
[37] P.R. Likhar, S. Roy, M. Roy, M.S. Subhas, M.L. Kantam, R.L. De, Synlett (2007)
2301–2303.
[38] M. Wang, P. Li, L. Wang, Eur. J. Org. Chem. (2008) 2255–2261.
[39] B. Sreedhar, P.S. Reddy, C.S.V. Krishna, P.V. Babu, Tetrahedron Lett. 48 (2007)
7882–7886.
We have prepared highly dispersed and catalytically active
metallic copper nanoparticles stabilized on NCNTs. The strong
interaction between the NCNTs and the copper nanoparticles
together with the protective effect of the solid support against
metal oxidation guarantees for high stability, recyclability and no
metal leaching at least for four runs. The latter aspect is par-
ticularly important for the development of synthetic methods
aiming at the synthesis of pharmaceutically relevant compounds
with no metal contamination. The immobilization procedure of
the copper nanoparticles on the NCNTs is straightforward and
clean, and it does not alter the catalytic activity of the metal, as
shown here for the A3-type coupling reaction. In comparison to the
previously reported protocol that utilizes non-supported metal-
lic copper nanoparticles [34], the presented Cu/NCNT-based one
allows reducing the amount of metal catalyst (4 mol% vs. 15 mol%)
and lowering the reaction temperature (70 ◦C vs. 100–110 ◦C). In
contrast, recent protocols based on the combination of copper (I)
salts with ionic liquids [28] or PEG [29] require temperatures above
70 ◦C (100–120 ◦C). Concerning the recyclability of the catalytic
system, copper ions immobilized on SiO2-CHDA [36], SiO2-NHC
[38], or on magnetite [42] could be reused in more cycles (up to
10–15) than when combined with ionic liquids [28], PEG [29], zeo-
lites [41], molecular sieve [40] or SiO2-Py [37] (3–5 cycles). The
recyclability of our NCNT-supported metallic copper nanoparti-
cles is comparable to that reported for non-supported ones [34]
(up to 5 cycles). Altogether, the catalytic and recycling features of
the Cu/NCNTs makes them an interesting tool for heterogeneous
copper-catalyzed organic reactions.
[40] A. Fodor, A. Kiss, N. Debreczeni, Z. Hell, I. Gresits, Org. Biomol. Chem. 8 (2010)
4575–4581.
Acknowledgements
[41] M.K. Patil, M. Keller, B.M. Reddy, P. Pale, J. Sommer, Eur. J. Org. Chem. (2008)
4440–4445.
[42] M.J. Aliaga, D.J. Ramon, M. Yus, Org. Biomol. Chem. 8 (2010) 43–46.
AB and TCN acknowledge the AvH foundation for postdoctoral
fellowship. We are grateful to Dr. B. Thomas (UPMC, Paris, France)

94
V.G. Ramu et al. / Applied Catalysis A: General 431–432 (2012) 88–94
[43] S. Kundu, W. Xia, W. Busser, M. Becker, D.A. Schmidt, M. Havenith, M. Muhler,
Phys. Chem. Chem. Phys. 12 (2010) 4351–4359.
[46] K. Sakthivel, W. Notz, T. Bui, C.F. Barbas III, J. Am. Chem. Soc. 123 (2001)
5260–5267.
[44] T.C. Nagaiah, S. Kundu, M. Bron, M. Muhler, W. Schuhmann, Electrochem. Com-
mun. 12 (2010) 338–341.
[47] P.H.-Y. Cheong, H. Zhang, R. Thayumanavan, F. Tanaka, K.N. Houk, C.F. Barbas
III, Org. Lett. 8 (2006) 811–814.
[45] K. Mallick, M. Witcomb, M. Scurrell, Mater. Chem. Phys. 97 (2006) 283–287.
[48] A. Galano, Nanoscale 2 (2010) 373–380.