Copper(II) SBA-15: A reusable catalyst for azide-alkyne cycloaddition

Article abstract of DOI:10.1016/j.molcata.2014.06.003

The azide-alkyne cycloaddition reaction was investigated under catalytic conditions involving a copper(II) loaded silica based mesoporous material. Cu(II) SBA-15 demonstrated a high catalytic effect in 1,4-triazoles synthesis in organic. No additives such

Full text of DOI:10.1016/j.molcata.2014.06.003

Journal of Molecular Catalysis A: Chemical 393 (2014) 56–61
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Copper(II) SBA-15: A reusable catalyst for azide–alkyne cycloaddition
Ibtissem Jlaliaa, Florian Gallierb, Nancy Brodie-Linderb,c,∗, Jacques Uzielb, Jacques Augéb,
Nadège Lubin-Germainb,∗∗
a Laboratoire des Substances Naturelles, INRAP-Pôle Technologique, Sidi Thabet 2020, Tunisia
b Laboratoire de Synthèse Organique Sélective et Chimie bioOrganique, EA 4505, University of Cergy-Pontoise, F-95000 Cergy-Pontoise cedex, France
c Laboratoire Léon Brillouin, CEA Saclay, Bât. 563, F-91191 Gif-sur-Yvette, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The azide–alkyne cycloaddition reaction was investigated under catalytic conditions involving a cop-
per(II) loaded silica based mesoporous material. Cu(II) SBA-15 demonstrated a high catalytic effect in
1,4-triazoles synthesis in organic. No additives such as a base or a reductant are required. Quantitative
yields were obtained and a mere filtration of the mesoporous material which retains copper(II) allows
the recovery of the catalyst. In addition, up to 5 times recycling of the catalyst was achieved without loss
of the activity affording 1,4-triazoles in a yield up to 98%.
Received 26 March 2014
Received in revised form 30 May 2014
Accepted 3 June 2014
Available online 11 June 2014
Keywords:
© 2014 Elsevier B.V. All rights reserved.
Click chemistry
Mesoporous materials
Cycloaddition
Heterogeneous catalysis
Copper(II)
1. Introduction
polymeric backbones [35–37], heteropolyacids [38,39], minerals
[40–43], or biooligomers [44–46] with or without the presence of
The copper-catalyzed azide–alkyne cycloaddition (CuAAC) is
the landmark reaction in click chemistry and originates from the
pioneering works of Sharpless et al. [1] and Meldal et al. [2]. Cop-
per(I) catalysis accelerates dramatically the Huisgen 1,3-dipolar
reaction and leads to a high regioselectivity in favor of the 1,4-
isomer of the triazole product. The copper(I) catalysts generally
originate from copper(I) salts in presence of a base and/or a ligand,
in situ reduction of copper(II) salts usually by ascorbate, and/or
a mixture of copper(0) and copper(II) species [3]. Heterogeneous
copper(I) sources have also been described as they present a great
advantage concerning catalyst removal, recovery and recycling.
Cu(I) salts have been supported on ionic resins [4–7], various
polymeric backbones [8–16], silica [17–21], ionic liquids [22],
montmorillonite [23], zeolites [24–27], or biopolymers [28,29]. The
main drawback is that such solid catalysts frequently suffer from
the thermodynamic instability of Cu(I) which is prone to oxidation
or disproportionation. Therefore, Cu(II) has recently been immo-
bilized on alumina [30,31], silica [32,33], anatase [34], various
a sacrificial reducing agent.
With respect to our work, we have already been concerned
with controlling the regioselectivity of the reaction in microreac-
tors such as micelles [47]. Herein we have explored the usefulness
of mesoporous material and the potential role of the pores in the
catalysis of the process. Indeed copper loaded on silica type mate-
rials as MCM-41 and SBA-15 have been shown to be very useful in
selective catalytic experiments [48–50].
2. Experimental methods
2.1. Characterization of the mesoporous Cu(II) SBA-15
The SBA-15 material was prepared according to
a well-
established procedure [51]. This mesoporous silica (50 mg) was
then suspended for 10 min at room temperature in 20 mL of a
5 × 10−2 mol/L aqueous solution of Cu(NO3)2·3H2O containing a
particular amount of a solution of 28% (w/w) NH4OH in order to
obtain pH 10.5 [52]. The material was then filtered, washed pro-
fusely with water to ensure the removal of all free Cu(II) ions and
dried in an oven at 60 ◦C for 4 h to give a bluish solid. Control of the
time and pH of impregnation is crucial for the copper loading.
The copper uptake was then determined by UV–visible spec-
troscopy. A sample of Cu(II) SBA-15 (10 mg) is suspended in 5 mL
∗
Corresponding author at: Laboratoire Léon Brillouin, CEA Saclay, Bât. 563,
F-91191 Gif-sur-Yvette, France.
∗∗
Corresponding author. Tel.: +33 1 34 25 70 54; fax: +33 1 34 25 73 78.
E-mail address: nadege.lubin-germain@u-cergy.fr (N. Lubin-Germain).
http://dx.doi.org/10.1016/j.molcata.2014.06.003
1381-1169/© 2014 Elsevier B.V. All rights reserved.

I. Jlalia et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 56–61
57
2% (w/w) aqueous HNO3 and stirred for 15 min at room tempera-
ture, vigorously in order to ensure the best homogeneity possible
in solution and filtered. UV absorbance of the solution was mea-
sured and the copper content was calculated with the standard
calibration curve.
Each batch of Cu(II) SBA-15 is further characterized by IR spec-
troscopy and N2-adsorption measurement. Detailed values of Pore
volume and Pore diameter as well as the BET Surface area for the
freshly prepared Cu(II) SBA-15, the Cu(II) SBA-15 after several runs
(simply filtered after the last run) and the Cu(II) SBA-15 thoroughly
rinsed after the last run can be found in the supporting information.
Extensive characterization of this Cu(II) SBA-15 (elemental anal-
ysis, XRD, SAXS and TEM) has already been published elsewhere
[52].
2.2. General procedure for Cu(II) SBA-15 azide–alkyne
cycloaddition
Alkyne (0.5 mmol), azide (0.55 mmol) and Cu(II) SBA-15 (25 mg,
8 mol% Cu) were stirred in dichloromethane (2 mL) for 18 h. The
mixture was filtered and the solid was washed twice with 2 mL
of dichloromethane. The filtrate was concentrated by evaporation
under vacuum. Excess of azide can be removed after immobilization
on a triphenylphosphine resin. In the case of highly polar substrates,
the solid was washed twice with 2 mL of acetonitrile.
Fig. 1. Cu(II) SBA-15 reusability.
In order to demonstrate the scope of this catalysis, various
alkynes and azides were tested using a moderate amount (8 mol%)
of the catalyst (Table 3). The reaction proceeded efficiently with
ethyl propiolate (entries 1–5), 3,3-diethoxyprop-1-yne (entries
6–9) and 4-bromobut-1-yne (entries 10–14). The isolated yields
are particularly high, sometimes quantitative (entries 1, 3, 12), with
benzyl azides.
2.3. General procedure for catalyst recycling
To a screw-capped vial were successively added ethyl pro-
piolate (50 ␮L; 0.49 mmol), benzyl azide (73.2 mg; 1.15 eq.),
dichloromethane (2 mL) and finally the Cu(II) SBA-15 (25 mg; 7.5%
Cu, w/w; 6 mol%). The reaction was stirred overnight and the cata-
lyst was recovered by filtration over a nylon membrane (0.45 ␮m,
Millipore®) and washed with dichloromethane (2 × 2 mL). The
combined organic solvent was evaporated under vacuo yielding
the desired triazole (97–110 mg; 87–97%). After the 5th run we
recovered 23.9 mg of the catalyst (over the 25 mg used for the 1st
reaction), hence arecoveryof 93%over5runs (99% average recovery
per run).
When propargylamine was used, the reaction proceeded instan-
taneously but the triazole was recovered in very poor yield probably
due to the amino complexation to copper (entry 20). This CuAAC
rate acceleration by amino group complexation has already been
observed [53]. The same observation could be made concerning
alcohol-containing reactants in a less important manner (entries
15–19). Since no reaction was observed with octyne and 2,3,4,6-
tetra-O-benzyl-␤-d-glucopyranosylacetylene (entries 21 and 22),
it is supposed that the catalysis was impeded in these two cases
3. Results and discussion
˚
by the size of the molecules (20 A) compared to that of the pores
˚
(70 A) which contain the copper catalytic species. Such an effect of
3.1. Catalysis
the importance of the size of the pores was not observed with Cu(I)-
zeolites which accept any substrate [25]. Our observation opens the
way to the possibility of tuning the size of the pores in concord-
ance with the substrates. Concerning octyne, we can also take into
account the hydrophilic character of the pores and we can postulate
that the octyne preferred to remain outside in the organic phase.
One of us recently reported the conditions for the controlled
Cu(II) loading in SBA-15 materials from 6 to 20% Cu (w/w) [52]. It
was shown that the amount of Cu(II) loaded on the SBA-15 surface
could be controlled by a reaction in aqueous copper solutions at
ambient temperature in 10 min. It was also shown that good cop-
per dispersion over the surface was achieved. The cycloaddition
between ethyl propiolate and benzyl azide was first investigated
using an increasing amount of Cu(II) SBA-15 in dichloromethane.
The conversion into the 1,4-disubstituted triazole was determined
by 1H-NMR spectroscopy after 2 h of reaction (Table 1). We were
delighted to discover that a copper(II) catalyst could efficiently cat-
alyze at room temperature such a reaction, without further addition
of a reducing agent which would have allowed the existence of
Cu(I) species. The reaction is totally regioselective as in the Cu(I)
cycloaddition.
3.2. Recycling of the Cu(II) SBA-15
The reaction was tested on ethyl propiolate with benzylazide
and we demonstrated that it proceeded with the same yield for at
least five times (Fig. 1). After each run, the catalyst can be almost
quantitatively recovered by a mere filtration over a 0.45 ␮m nylon
membrane (See Section 2.3).
After the last catalytic cycle, a more thorough washing was per-
formed and the material was analyzed in order to control the state
of the catalytic surface. The IR spectra found in Fig. 2 indicates that
no build-up of organic materials on the surface is occurring. This is
also corroborated by N2 volumetric adsorption analysis where only
slight changes in the pore structure can be observed (see Supporting
Information). In addition, the content of copper remained identical
indicating that no leaching occurred.
Next, we have checked the influence of the copper source as well
as the solid support (Table 2). Without any copper source (entry
1–2) no reaction took place. The same goes for Cu(NO3)2·3H2O
(entry 3) which is used for the Cu(II) SBA-15 catalyst preparation.
In this case, as in the case of Cu(II) SBA-15 (entry 4), no reducing
agent or base has been added.

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I. Jlalia et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 56–61
Table 1
Catalyst amount optimization.
Cu(II) SBA-15
(10% Cu w/w)
EtO2C
H
CO2Et
PhCH2N3
+
N
CH2Ph
N
CDCl3, RT, 2h
1a
2a
N
3aa
sole regioisomer
Entry
Cu(II) SBA-15
Conversion*
1
2
3
4
–
Traces
34%
75%
4 mol%
8 mol%
16 mol%
100%
*
Evaluated by 1H-NMR.
Table 2
Solid support and copper source influence.
Br
EtO2C
catalyst
H
PhCH2N3
+
N
CH2Ph
N
CH2Cl2, RT, 2h
1c
2a
N
3ca
sole regioisomer
Entry
Catalyst
Yield (%)
1
2
3
4
–
0
0
0
SBA-15 (20 mg)
CuNO3·3H2O (8 mol%)
Cu(II) SBA-15 (25 mg, 8 mol% Cu)
67
3.3. Mechanistic studies
aqueous medium [54]. Later on, a di-copper(II) substituted silico-
tungstate was shown to catalyze both the oxidative alkyne-alkyne
homocoupling and the Huisgen reaction; the high yield in 1,2,3-
triazoles in that reaction was proven to be due to the preformation
To our knowledge, the first example of the use of Cu(II) as the
reactive catalytic species in the Huisgen reaction was described in
Table 3
Scope and limitations of the catalyzed cycloaddition.
Cu(II) SBA-15
R
(8 mol%, 10% Cu w/w)
R
R'N3
+
N
R'
N
CH2Cl2, RT, 16h
1a-g
2a-e
N
3aa-gb
sole regioisomer
Entry
Alkyne 1: R
Azide 2: Rꢀ
Product 3: Yield*
1
2
3
4
5
6
7
8
1a: COOEt
1a: COOEt
1a: COOEt
1a: COOEt
2a: PhCH2
3aa: 98%
3ab: 76%
3ac: 99%
3ad: 87%
3ae: 64%
3ba: 83%
3bb: 66%
3bd: 56%(a)
3be: 35%
3ca: 67%
3cb: 72%
3cc: 98%
3 cd: 57%
3ce: 52%
3 da: 68%
3db: 65%
3dc: 72%
3dd: 46%
3de: 26%
3ea: 17%
3fb:–
2b: EtOCOCH2
2c:p-F-C6H4CH2
2d: 2-oxolan-CH2
2e: HOCH2CH2
2a: PhCH2
2b: EtOCOCH2
2d: 2-oxolan-CH2
2e: HOCH2CH2
2a: PhCH2
2b: EtOCOCH2
2c:p-F-C6H4CH2
2d: 2-oxolan-CH2
2e: HOCH2CH2
2a: PhCH2
2b: EtOCOCH2
2c:p-F-C6H4CH2
2d: 2-oxolan-CH2
2e: HOCH2CH2
2a: PhCH2
1a: COOEt
1b: CH(OEt)2
1b: CH(OEt)2
1b: CH(OEt)2
1b: CH(OEt)2
1c: CH2CH2Br
1c: CH2CH2Br
1c: CH2CH2Br
1c: CH2CH2Br
1c: CH2CH2Br
1d: CH2OH
1d: CH2OH
1d: CH2OH
1d: CH2OH
1d: CH2OH
1e: CH2NH2
1f: n-C6H13
9
10
11
12
13
14
15
16
17
18
19
20
21
22
2b: EtOCOCH2
2b: EtOCOCH2
1 g: 2,3,4,6-tetra-O-benzyl-␤-d-glucopyranosyl
3gb:–
*
Isolated yield.
Isolated as the aldehyde.
a

I. Jlalia et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 56–61
59
before catalysis
after several cycles
0,25
0,20
0,15
0,10
0,05
0,00
4000
3500
3000
2500
2000
1500
1000
-1
Wavenumber (cm
)
Fig. 2. IR spectra of Cu(II) SBA-15 materials before and after 5 catalytic cycles.
Fig. 4. Kinetic experiment.
Fig. 3. Cycloaddition under O2 atmosphere.
by GC–MS and the conversion is complete within 24 h, just as the
classical conditions.
of a Cu(I) acetylide [38]. With Cu(OAc)2 as catalyst, a fast reaction
occurred with organic azides capable of chelation-assisted copper
coordination at the alkylated nitrogen position [55–57].
3.3.2. Kinetic study
In this kinetic study, we are not comparing the direct catalytic
activity between Cu(I) and Cu(II). We are taking into consideration
the particular catalytic system where the reduction of Cu(II) in the
SBA material is not expected to be a minor secondary reaction if at
all. As we want to create a scenario for possible Cu(I) catalysis in
our reaction, as proposed by Sharpless and co-workers [1] or Meldal
and co-workers [2], and in order to have similar reaction conditions
(especially involving a solid catalyst) copper(I) iodide was chosen
as the Cu(I) catalyst and no base was added. Reactions were carried
out in CDCl3 under similar condition using Cu(II) SBA-15 (6 mol%)
and CuI (0.3 mol%) and monitored by GC–MS at given times (Fig. 4).
At this loading of catalyst, the rate of the reaction involving Cu(II)
SBA-15 is much higher than the one of CuI (around 4 times). So if
the copper(I) ions are the only one involved in the catalysis, 20% of
the total amount of the Cu(II) bound to the surface of SBA-15 has
to be reduced to Cu(I), which is highly unlikely to occur. Therefore,
copper(II) is believed to play a crucial role for this reaction.
3.3.1. Oxidation state of copper
In these cases, the authors suggest that Cu(II) undergoes
reduction to Cu(I) via alcohol (solvent) oxidation and/or alkyne
homocoupling; they gave experimental evidence of the formation
of a mixed-valency dinuclear copper (I/II) species. They proposed
that the copper centers function as a Lewis acid to enhance the
electrophilicity of the azido group, whereas the other one is a two-
electron reducing agent in oxidative metallacycle formation [58].
At this stage of discussion, we must remember that, as early
as 2002, Sharpless et al. noted that even Cu(0) could be used as
a source of the catalytic process, although these reactions took
longer to proceed to completion [1,59]. Recently Cu(0) nanopar-
ticules as nanocatalysts were proved to be particularly active in
the process [59]. Copper (II) has been reported to be the reactive
catalytic species in other cases as well [60].
Our copper source was loaded onto a solid support which ham-
pers direct observation of the oxidation state; therefore indirect
measurement might give information about the mechanism. No
spectroscopic evidences concerning the formation of the homo-
coupling 1,3-diyne were found, either by NMR or GC–MS. Thus,
reduction to Cu(I) seems unlikely.
In order to probe the oxidation state of the copper ions, we
studied the influence of an atmosphere of molecular oxygen. Air
oxidation of Cu(I) to Cu(II) is generally fast (thus explaining the
instability of Cu(I) ions) and has been proven here when copper(I)
iodide was introduced into vials that were successively flushed
(for 5 min) and kept (for ½ h) separately under either O2 or Ar
atmosphere. Complexation by addition of NH4OH solution occurs
readily in the case of oxygen producing a blue solution (character-
istic for CuII(NH3)n ions). In the case of argon, this blue solution
was observed only after few minutes. We have then performed
the cycloaddition under O2 atmosphere and shaded from the light
(Fig. 3). In that case, if Cu(I) happened to be formed then it would
instantly be oxidized to Cu(II) again. This reaction was monitored
3.3.3. Acetylide copper intermediate
Deuterium exchange experiments can also give us insights into
the reaction mechanism. We have first prepared ethyl deuteropro-
piolate under basic classical conditions. GC–MS showed deuterium
incorporation over 99% (Fig. 5). This deuterated alkyne was then
submitted to the usual cycloaddition conditions. No deuterium was
incorporated into the triazole ring (confirmed either by 1H-NMR
and GC–MS) thus indicating the cleavage of the C D bond and the
formation of a copper carbon bond can be postulated. Therefore,
Lewis acidic mechanism (as exemplified with zeolites [26]) can be
ruled out.
3.3.4. A dinuclear mechanism
Although copper(II) reduction to Cu(I) by
a hydrogen-
abstraction/proton-delivery/electron-gain mechanism is a remote
possibility [60], the previous set of experiments led us to
hypothesize the formation of the triazole through the following
mechanism involving only Cu(II) species (Fig. 6). The complete

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I. Jlalia et al. / Journal of Molecular Catalysis A: Chemical 393 (2014) 56–61
In addition, the pore size may have a great effect on the reactivity
as the reaction is believed to occur inside the pores (as corroborated
by N2 volumetric adsorption of the catalyst without washing). The
influence of these parameters is currently under investigation and
will be reported in due time.
4. Conclusion
We have shown herein that our Cu(II) SBA-15 material can be
used successfully as a reusable catalyst for an azide–alkyne cycload-
dition reaction without degradation. The conversion can be as high
as 99% and a total regioselectivity is obtained. To our knowledge,
this is the first time that a copper(II) SBA-15 material has been
shown to be active as a reusable catalyst for azide–alkyne cycload-
dition reactions.
Fig. 5. Deuteration experiment.
Acknowledgements
Nicolas Pasternak is gratefully acknowledge for providing some
Cu(II) SBA-15 samples and for performing N2-adsoprtion analysis.
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.molcata.2014.06.003.
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