Carbon nanotube-copper ferrite-catalyzed aqueous 1,3-dipolar cycloaddition of: In situ -generated organic azides with alkynes

Article abstract of DOI:10.1039/c8cc00231b

A novel nanohybrid catalyst was developed by assembling copper ferrite nanoparticles on carbon nanotubes. The supramolecular catalyst was applied to the one-pot azidation/1,3-dipolar cycloaddition of various substrates, at room temperature, and in an aqueous medium. The nanohybrid could also be recycled and reused by means of magnetic recovery.

Full text of DOI:10.1039/c8cc00231b

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Carbon nanotube-copper ferrite-catalyzed aqueous 1,3-dipolar
cycloaddition of in situ-generated organic azides with alkynes
Received 00th January 20xx,
Accepted 00th January 20xx
a,†
a,b,†
c
d
Praveen Prakash, Ramar Arun Kumar,
Frédéric Miserque, Valérie Geertsen, Edmond
a
a
Gravel,* and Eric Doris*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel nanohybrid catalyst was developped by assembling
copper ferrite nanoparticles on carbon nanotubes. The and synthesis of a novel heterogeneous catalyst that can
supramolecular catalyst was applied to the one pot azidation/1,3- efficiently promote the aqueous one-pot
With these features in mind, we report here, the design
dipolar cycloaddition of various substrates, at room temperature, condensation/cycloaddition of in situ generated organic azides
5
and in an aqueous medium. The nanohybrid could also be recycled with terminal alkynes. This carbon nanotube-based assembly
and reused by means of magnetic recovery.
also incorporates a hemi-micellar domain for concentration of
the reagents at the vicinity of the active catalytic copper
species.
1
a,b
The Huisgen 1,3-dipolar cycloaddition,
involving alkyne and
azide derivatives, is a widely employed click reaction in
research fields as diverse as chemical biology or material
The supramolecular catalyst was prepared according to a
strategy inspired by our previous work on metal nanoparticles
1
c-e
6
NPs) supported on carbon nanotubes (CNTs). The process
-8
chemistry.
The reaction classically requires a cuprous salt
(
2
catalyst to trigger smooth and regioselective transformations.
However, copper salts residues are rather toxic and difficult to
eliminate from reaction mixtures, thus prompting the search
for copper-free alternatives. This led to recent development by
relies on a layer-by-layer approach using the self-assembly
properties of some amphiphiles to provide highly functional
nanotubes. In short, multi-walled carbon nanotubes were
sonicated in an aqueous solution of amphiphilic units made of
a lipophilic diacetylene (DA) chain and a nitrilotriacetate polar
head (NTA). The resulting CNT suspension is the upshot of the
self-assembly of the amphiphiles at the surface of the
nanotubes, giving rise to hemi-micellar structures also referred
to as nanorings (Figure 1).
3
Bertozzi and co-workers of strained cyclooctynes which are
reactive in the absence of any copper catalyst but are
somehow complex to synthesize and lack of chemoselectivity.
These shortcomings associated to the 1,3-dipolar cycloaddition
of organic azides with alkynes could likely be overcome by
heterogenization of the active catalyst as, besides easy
removal from the reaction mixture, supporting the metal
should also allow for its recycling and reuse, thus contributing
to the sustainability and greening of the overall process. As
regards the latter point, the replacement of conventional
organic solvents with water could also constitute a major
improvement. However, the use of water is not always suited
due to the insolubility of the reagents which can be
4
circumvented, in some cases, by using surfactant micelles that
are acting as dispersant and nano-reactors where the reagents
are concentrated.
a.
Service de Chimie Bioorganique et de Marquage (SCBM), CEA, Université Paris-
Saclay, 91191 Gif-sur-Yvette, France.
E-mail: edmond.gravel@cea.fr; eric.doris@cea.fr
SRM Research Institute, Department of Chemistry, SRM Institute of Science and
b.
Technology, Kattankulathur, 603203 Chennai, India.
NIMBE, CEA, CNRS, Université Paris-Saclay, 91191, Gif-sur-Yvette, France.
c.
d.
Service d’Etude de Corrosion et du Comportement des Matériaux dans leur
Environnement (SCCME), CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette,
France.
Figure 1. a) Structure of DANTA; b) Structure of PDADMAC; c) Schematic representation
†
These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: assembly procedure of the
CuFe CNT nanohybrid and copies of NMR spectra. See DOI: 10.1039/x0xx00000x
of the assembly process.
2 4
O
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These structures arise from hydrophobic interactions between
the lipidic chains of the amphiphiles and the graphitic surface
of the CNTs, while the polar head groups are interfacing with
the aqueous environment. The nanorings are then
consolidated through a photopolymerization process that
involves the diacetylene motifs. In fact, UV irradiation at 254
nm triggers a topochemical 1,4-addition process allowing
covalent attachment of neighboring diacetylene groups. The
polymerized nanorings located on the CNTs build a continuous
domain containing a hydrophobic region that is likely to
accommodate organic molecules and behave as nano-reactor.
This domain also displays high density of negative charges at
its surface. The presence of these charges were thus exploited
to electrostatically adsorb a second cationic layer made of
poly(diallyldimethyl ammonium chloride) (PDADMAC). The
cationic polymer creates a tridimensional network that is
adapted to the stabilization and robust anchoring of metal
nanoparticles. Hence, a colloid suspension of preformed
2 4
Table 2. Scope of the CuFe O CNT-catalyzed Huisgen reaction of alkynes with
[
a]
bromides.
yield
(%)
entry
bromide 1
alkyne 2
[b]
1
1a
2a
90
2
3
1a
1a
2b
2c
92
82
9
4
1a
2d
87
copper ferrite nanoparticles was added to the doubly-coated
carbon nanotubes. This led to dense and homogeneous
covering of CNT surface with copper nanoparticles embedded
in the polymer layer, as evidenced by transmission electron
microscopy (TEM, Figure 2a). The nanoparticles were shown to
be of spherical shape with an average diameter of 4.8 nm (see
size distribution, Figure 2b). The hybrid catalyst was kept in an
aqueous suspension and used as such for further experiments.
5
6
7
8
1a
1a
1b
1c
2e
2f
84
65
88
78
The copper content of the CuFe O CNT suspension was
2
4
2a
2a
measured by inductively coupled plasma mass spectroscopy
ICP-MS) which indicated a [Cu] = 0.8 mM. The hybrid was
further characterized by X-ray photoelectron spectroscopy
(
(
XPS) which confirmed the binary nature of the metal particles
see the Cu2p and Fe2p regions in Figure 2c and 2d,
(
9
1c
2d
84
respectively).
1
1
0
1
1d
1e
2a
2a
81
traces
3 2 4
[a] 1 (0.1 mmol), 2 (0.1 mmol), NaN (0.11 mmol), CuFe O CNT (0.4 mol %),
H
2
O/Ethanol 4:1 (1 mL), room temp., 24 h. [b] Isolated yields.
The newly assembled catalyst was then tested for the
promotion of the Huisgen 1,3-cycloaddition through the
reaction of terminal alkynes with in situ generated organic
azide. This one pot sequence (R-X
3
→ R-N → “click”) offers the
advantage of a straightforward process without the need of
pre-synthesizing the azido partner, as opposed to most of
1
0
nanoparticle-catalyzed click reactions. The reaction was
carried out at room temperature, under air, using 0.4 mol% of
‡
the catalyst and in water/EtOH 4:1. The catalytic system
proved efficient on a variety of substrates (Table 1) and was
active on different terminal alkynes such as phenylacetylene
derivatives (plain phenylacetylene 2a
–
Entry
Entry
–
Entry 1, 4-
methoxyphenylacetylene
chlorophenylacetylene
2b
2d
–
2,
4,
Entry
3-
4-
5,
–
2 4
Figure 2. a) TEM image of the final CuFe O CNT hybrid; b) size distribution of the
trifluoromethylphenylacetylene
2e
CuFe
2 4
O nanoparticles (obtained from the measurement of 250 particles); c) Cu2p
region and d) Fe2p region of the XPS spectra of the nanohybrid.
phenethylacetylene 2f – Entry 6), and propargyl alcohol 2c
2
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Entry 3). On the other hand, different brominated
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(
counterparts were also successfully employed in the
transformation, e.g. fluorinated benzyl bromide derivative 1b
(Entry 7), ethyl bromoacetate 1c (Entries 8–9), and allyl
bromide 1d (Entry 10), giving rise to the expected triazoles in
good to excellent yields (Entries 1–10). However, 1-
bromooctane 1e displayed very poor reactivity under our
reaction conditions and only afforded trace amounts of the
anticipated cycloaddition product (Entry 11). This lack of
reactivity of the aliphatic halide was attributed to the initial
nucleophilic displacement of the bromine atom by the azido
group which is here the rate limiting step. Indeed, a simple
control experiment conducted with authentic 1-azidooctane,
Figure 3. Left: nanohybrid catalyst suspension. Right: magnetic separation of
CuFe CNT (picture taken 15 seconds after the positioning of a permanent magnet).
2 4
O
[
a]
0
.4 mol% of CuFe O CNT, and phenylacetylene 2a led to the Table 3. recyclability of the CuFe
2 4
O CNT catalyst.
2
4
clean formation of the expected triazole product 3ea. It is to
be noted that, for all the above examples, the CuFe O CNT-
2
4
catalyzed
[3+2] cycloaddition reaction proceeded in a
regioselective fashion affording solely the 1,4-substituted
triazole. The system also proved efficient with chlorinated
substrates (Table 2). In fact, benzyl chloride 4a afforded as well
the corresponding cycloadducts (triazoles) in satisfactory to
[b]
entry
catalyst
time (h)
yield (%)
1
2
fresh
24
24
24
24
90
89
85
87
good yields upon reaction with NaN and phenylacetylene 2a
3
st
1
2
3
reuse
reuse
reuse
(Entry 1), 4-methoxyphenylacetylene 2b (Entry 2), or 4-
nd
rd
trifluoromethylphenylacetylene 2e (Entry 3). The same
comment applies to allyl chloride 4d which led to triazole 3da
3
in 76% yield upon reaction with NaN and phenylacetylene 2a
3
(
Entry 4).
One of the key advantages provided by our one-pot
nanohybrid-catalyzed azide−alkyne 1,3-dipolar cycloaddition
CuAAC) is the mildness of the operating conditions. In fact, A second advantage provided by the CuFe O CNT nanohybrid
[a] 1a (0.1 mmol), 2a (0.1 mmol), NaN
3 2 4
(0.11 mmol), CuFe O CNT (0.4 mol %),
H
2
O/Ethanol 4:1 (1 mL), room temp., 24 h. [b] Isolated yields.
(
2
4
while other analogous systems require elevated is the combination of catalytically active copper with
1
1
12
temperatures,
1
high catalytic loadings,
pure organic magnetically active iron. The latter key feature was exploited
to proceed for easy magnetic recovery and recycling of the catalyst. In
3
14
solvents,
and/or oxygen free conditions
satisfactorily, our CNT-based nanohybrid operates at room fact, after each cycle, the nanohybrid was separated
temperature with as little as 0.4 mol% Cu and in a binary magnetically from the mixture by use of a permanent magnet
aqueous phase.
that was placed on the walls of the reactor (Figure 3). The
product was collected using a pipette, the catalyst rinsed with
Table 2. Scope of the CuFe
2 4
O
CNT-catalyzed Huisgen reaction of alkynes with EtOH and redispersed in liquid media before fresh reagents
[
a]
chlorides.
(benzyl bromide, phenylacetylene and NaN ) were introduced
3
to the reactor. This process was repeated three times with no
significant decrease of the catalytic activity as similar yields of
triazole were obtained in each case after 24 h (Table 3). In
addition, the mother liquors of each run were analysed by ICP-
[
b]
entry
1
chloride 4
alkyne 2
yield (%)
MS which showed negligible leaching of copper species during
the process (< 2 ppm). This recycling experiment suggests no
alteration of the active metals and strong interaction with the
CNT-based multi-layer assembly.
4a
2a
77
In conclusion, an efficient and straightforward method for
the 1,3-dipolar cycloaddition of in situ generated organic
azides with alkynes was developed. The process is catalyzed by
a newly developed heterogeneous copper catalyst supported
on carbon nanotubes which operates in an aqueous solvent
system, with low catalytic loadings (0.4 mol%), at room
temperature, and under air. The assembly offers three key
advantages over known heterogeneous catalysts, i.e. i) the
presence of a lipophilic domain behaving as a nano-reactor
combined with ii) the close vicinity of the active catalytic
2
4a
2b
57
3
4
4a
4d
2e
2a
72
76
[a] 4 (0.1 mmol), 2 (0.1 mmol), NaN
3 2 4
(0.11 mmol), CuFe O CNT (0.4 mol %),
H
2
O/Ethanol 4:1 (1 mL), room temp., 24 h. [b] Isolated yields.
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species, and iii) the ease of recycling by simple magnetic
recovery of the bimetallic nanohybrid.
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Acknowledgements
P. P. and R. A. K. thank the Enhanced Eurotalents program for
support. The “Service de Chimie Bioorganique et de
Marquage” belongs to the Laboratory of Excellence in
Research on Medication and Innovative Therapeutics (ANR-10-
LABX-0033-LERMIT).
1
1
Conflicts of interest
There are no conflicts of interest to declare.
Notes and references
‡
First attempts were carried out in pure water but only led to
moderate conversion yields.
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