Cu-Al mixed oxide catalysts for azide-alkyne 1,3-cycloaddition in ethanol-water

Article abstract of DOI:10.1039/c6ra10097j

Cu(Al)O mixed oxides, which are obtained by the calcination of Cu-Al layered double hydroxide (LDH), promote the formation of 1,2,3-triazoles from an alkyne-azide cycloaddition reaction (Huisgen-type reaction) with excellent yields using an EtOH-H₂O mixture as the solvent under microwave heating. The yield of the reaction is the result of both heterogeneous and homogeneous catalytic processes, as a consequence of capturing Cu(ii) from the material by sodium ascorbate. Although the appropriate reaction conditions were employed (EtOH-H₂O, 80 °C, MW, 10 min), the reconstruction of LDH by the so-called memory effect was not observed.

Full text of DOI:10.1039/c6ra10097j

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Cu–Al mixed oxide catalysts for azide–alkyne 1,3-
cycloaddition in ethanol–water†
Cite this: RSC Adv., 2016, 6, 63660
a
b
R. Gonzalez-Olvera, C. I. Urquiza-Castro, G. E. Negron-Silva, a D. A´ngeles-Beltran,a
*
´
´
´
L. Lomas-Romero, b A. Gutierrez-Carrillo,b V. H. Lara,b R. Santillanc and J. A. Morales-
´
*
Sernab
Cu(Al)O mixed oxides, which are obtained by the calcination of Cu–Al layered double hydroxide (LDH),
promote the formation of 1,2,3-triazoles from an alkyne–azide cycloaddition reaction (Huisgen-type
reaction) with excellent yields using an EtOH–H2O mixture as the solvent under microwave heating. The
yield of the reaction is the result of both heterogeneous and homogeneous catalytic processes, as
a consequence of capturing Cu(II) from the material by sodium ascorbate. Although the appropriate
reaction conditions were employed (EtOH–H2O, 80 ꢀC, MW, 10 min), the reconstruction of LDH by the
so-called memory effect was not observed.
Received 19th April 2016
Accepted 27th June 2016
DOI: 10.1039/c6ra10097j
www.rsc.org/advances
1. Introduction
Layered double hydroxides (LDHs) are a class of materials that
are considered hydrotalcite-like compounds1 because the
brucite-type octahedral layer in their structure is similar to the
naturally occurring Mg–Al hydrotalcite mineral.2 In both cases,
Scheme 1 Representation of as-synthesized LDHs, Cu(Al)O mixed
oxide and reconstructed LDHs.
the general formula is [M1ꢁx2+Mx3+(OH)2]x+[Anꢁx/n$mH2O]xꢁ
,
where the excess of positive charges from the substitution of
M2+ (Mg, Cu) by M3+ (Al, Fe) is compensated by carbonate
anions in the interlayer space.3 As in brucite [Mg(OH)2], the
layers are built up by the condensation of octahedral MO6 units
(M2+ or M3+) to form anionic clays in which the OH groups point
towards the interlayer region and are shared by three octahedral
the rehydration reaction generates Brønsted base sites7 between
the new layers where carbonate anions are substituted by
hydroxyl anions.8
Layered double hydroxides have been increasingly explored
as heterogeneous catalysts, exploiting their unique properties to
create more efficient, and ultimately more eco-friendly,
processes.9 Recently, Cu–Al LDHs have been synthesized and
used in alkyne–azide cycloaddition reactions (Huisgen-type
reaction)10–12 with different substrates, demonstrating that the
Cu(II) (M2+) present in the material is able to participate in the
organic reaction.13 Those studies were carried out in the pres-
ence of the as-synthesized Cu–Al LDH but not in the presence of
the calcined material. In this context, as part of our research into
the use of LDHs in organic reactions,14 we focus on the structural
effect of as-synthesized15 and calcined (mixed oxide)16 Cu–Al
LDHs on the same alkyne–azide cycloaddition reaction to obtain
1,2,3-triazoles, considering that the different morphologies and
textural properties could affect the development of the reaction.
ꢀ
cations. Heating the material to 500–600 C yields a homoge-
neous mixed oxide with very small crystals, a large surface area
and good thermal stability.4 Exposing the mixed oxide to
aqueous solutions or humid atmospheres can lead to the
reconstruction of the original LDH by the so-called memory
effect (Scheme 1).5 The reconstructed layered material exhibits
different morphologies and textural properties depending on
whether the reconstruction process has been carried out in the
presence of CO2 or carbonates.6 Absence of those compounds in
aDepartamento
de
Ciencias
Universidad
´
Basicas,
´
Autonoma
´
Metropolitana-Azcapotzalco, Av. San Pablo No. 180, Ciudad de Mexico, C.P. 02200,
Mexico. E-mail: gns@correo.azc.uam.mx; Fax: +52-55-5318-9000 ext. 2169; Tel:
+52-55-5318-9593
b
´
´
Departamento de Quımica, Universidad Autonoma Metropolitana-Iztapalapa, Av. San
´
Rafael Atlixco No. 186, Ciudad de Mexico, C.P. 09340, Mexico. E-mail: llr@xanum.
uam.mx; Fax: +52-55-5804-4666; Tel: +52-55-5804-4913
2. Results and discussion
2.1. Material synthesis
The LDH investigated in the present study Cu2+/Al3+ in a ratio of
3 : 1 were synthesized by the co-precipitation technique and
c
´
´
Departamento de Quımica, Centro de Investigacion y de Estudios Avanzados del
´
´
Instituto Politecnico Nacional, Apartado Postal 14-740, Ciudad de Mexico, 07000,
Mexico
† Electronic supplementary information (ESI) available: Synthetic procedures and
NMR spectra. See DOI: 10.1039/c6ra10097j
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Fig. 1 X-ray diffraction patterns of (A) as-synthesized LDH and (B) the
Cu(Al)O mixed oxide.
Fig. 2 Nitrogen adsorption–desorption isotherms of as-synthesized
LDH and Cu(Al)O mixed oxide.
microwave-hydrothermal crystallization.17 The rapid synthesis
method used in this work facilitated a homogeneous and crys-
talline material, which was characterized by X-ray powder
diffraction (XRD), Brunauer–Emmett–Teller (BET) method,
scanning electron microscopy (SEM) and Fourier transform
infrared spectroscopy (FTIR). The obtained XRD patterns are
shown in Fig. 1. The as-synthesized LDHs exhibited Cu–Al
reections associated with the layered double hydroxide crystal
structure. The maxima correspond to typical diffraction by
planes (1 1 0), (0 0 2), (1 1 1), (1 1 2), (0 2 0), (1 0 1) and (1 1 3)
(Fig. 1, black line). These planes are similar to those shown by
malachite [Cu2CO3(OH)2] because of the presence of Cu in the
material.18 Calcining the material yields a Cu(Al)O mixed oxide19
with a periclase-like structure with (110), (111), (202), (022),
(113), (311) and (220) plane reections, which are typical of CuO
(Fig. 1, green line).
The adsorption of N2 (the BET method) was used to quantify
the surface area and pore size of materials (Table 1). The as-
synthesized LDH sample revealed lower porosity and a higher
surface area than the calcined material [Cu(Al)O mixed oxide].
The isotherms of both materials closely resemble a type II
isotherm. Nitrogen uptake monotonically increases with p/po
values due to sorption in the LDH mesoporous, and the
hysteresis loops belong to type H III, which are due to a different
adsorption and desorption behaviour and are usually found in
solids consisting of agglomerates or aggregates of slit-shaped
particles (Fig. 2 and 3).
adsorbed and interlayer water molecules, the second at 337 ꢀC is
attributed to the dehydroxylation and decarbonization of the
brucite-like material,18 and the last at 945 C is the loss asso-
ꢀ
ciated with the collapse of the structure. The calcined sample
exhibited one signicant peak aer 800 ꢀC due to structural
collapse.
The FTIR spectra of as-synthesized LDH and the Cu(Al)O
mixed oxide are shown in Fig. 5. The bands in the wave-number
range of 500–800 cmꢁ1 correspond to the vibration of metal
oxygen bonds in both materials. The absence of the band at
1321 and 1397 cmꢁ1 (CO32ꢁ) in the Cu(Al)O mixed oxide FTIR
spectrum conrms that the calcination evolved favourably. The
wide bands in the wave region of 1625 and 3200 cmꢁ1 (O–H)
demonstrates both the presence of water molecules in the
interlayer space of as-synthesized LDH and the presence of the
hydroxyl vibration from the hydrogen-bonded metal hydroxide
sheets in both materials (Fig. 5).
The SEM images of the an as-synthesized LDH sample
(Fig. 6A) and the Cu(Al)O mixed oxide (Fig. 6B) showed micro-
particles similar to akes and agglomerates.20
2.2. Catalytic activity
To study the catalytic activity of as-synthesized and calcined
(mixed oxides) Cu–Al LDH in the alkyne–azide cycloaddition
The thermal decomposition behaviour of the as-synthesized
and calcined hydrotalcite was observed by TGA in a nitrogen
atmosphere. The proles in Fig. 4 correspond to the as-
synthesized (yellow) and calcined (blue) samples. The as
synthesized sample showed three steps. The rst one at
approximately 100 ꢀC is assigned to the loss of physically
Table 1 The textual properties of materials obtained by nitrogen
adsorption–desorption analysis
As-synthesized
LDH
Cu(Al)O mixed
oxide
Entry
Parameters
1
2
3
SBET (m2 gꢁ1
)
74
0.44
161
63
0.42
174
Pore volume (cm3 gꢁ1
)
Fig. 3 Nitrogen adsorption–desorption isotherms of as-synthesized
LDH and Cu(Al)O mixed oxide.
˚
Pore size (A)
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Fig. 4 Thermograms of as-synthesized LDH (blue) and Cu(Al)O mixed
oxide (red).
reaction to obtain 1,2,3-triazoles, we chose benzyl chloride,
phenylacetylene and sodium azide as a model system. In our
initial experiment, the multicomponent reaction (MCR)21–23 was
conducted in the presence of as-synthesized or calcined Cu–Al
LDH, with sodium ascorbate and ethanol–water as the solvent
(Table 2, entries 1 and 4). The highest yield was obtained when
the reaction was carried out with the Cu(Al)O mixed oxide and
sodium ascorbate (98%, Table 2, entry 4). In contrast, a poor
yield (30%) was obtained in the absence of sodium ascorbate
(Table 2, entry 3). The use of as-synthesized Cu–Al LDH allowed
obtaining the desired product with a 75% yield. That yield
decreased in the absence of sodium ascorbate (Table 2, entries 1
and 2). The mixture of CuO and sodium ascorbate catalysed the
reaction with a 40% yield, and only trace amounts of the triazole
was isolated when the reaction was performed with only CuO. It
is noteworthy, that aer a long reaction time (8 h), the yield does
not increase in all cases.
Fig. 6 SEM images of as-synthesized LDH (A) and the Cu(Al)O mixed
oxide (B).
In this case, the pyrimidine derivatives uracil 1b and thymine 1c
were used as the alkyne substrates.24 In both cases, the reaction
evolved with an excellent yield if the calcined LDH and sodium
ascorbate were used as the catalytic system (Scheme 2).
Finally, the catalytic system was tested with steroidal azide
derivatives of bile acids: lithocholic 4b, deoxycholic 4d, cheno-
deoxycholic acids 4f and cholic 4h. The reactions were carried
out in the presence of uracil 1d and thymine 1e25 to generate the
pyrimidine–steroid conjugate systems 5a–5f (Table 4). In
general, the reactions evolved with excellent yields; however,
when the steroidal azide derivatives had a free hydroxyl group
(4d), the yield decreased notably. This loss in the yield can be
avoided by using an acetyl protective group (4f and 4h), which
may be added and removed with near quantitative yield.
Attempts to use mesylate derivatives of the same bile acids (4a,
4c, 4e, and 4g) and NaN3 in the multicomponent reaction
resulted products 5a–5f with moderate yields (Table 4). In those
cases, the long reaction time and high temperatures did not
change the yield. Analysis of the supernatant of the reactions by
1H NMR demonstrated the presence of a free hydroxyl group at
C-3 on the steroids as a consequence of the hydrolysis of
mesylate derivatives.
The scope of the multicomponent reaction was studied with
benzyl halides 2b–2e, and in all cases the yield was excellent
when the reaction was performed in the presence of both the
Cu(Al)O mixed oxide and sodium ascorbate (Table 3). Then, the
reaction was tested with other alkynes to conrm these results.
The results above demonstrate that the presence of sodium
ascorbate was important for very high yields. Therefore,
Fig. 5 FTIR spectra for as-synthesized LDH (black) and Cu(Al)O mixed
oxide (blue).
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Table 2 Catalyst screening and reaction condition optimizationa
dissolved in the mixture EtOH–H2O. In the case of as-
synthesized LDH, the material lost 4% Cu. On the other hand,
0.4% Cu was quantied from the reaction carried out with the
Cu(Al)O mixed oxide.
Once the in situ degradation of material occurs, the redox
process then occurs in the solution, and Cu1+ catalyses the
azide–alkyne 1,3-cycloaddition. Thus, we assume that both
heterogeneous and homogeneous processes take place during
the organic reaction. The homogeneous process was demon-
strated when the reaction of cycloaddition between 1a, 2a and
NaN3 was carried out using the recovered sodium ascorbate–
Cu2+/EtOH–H2O mixture (40% yield). Finally, the difference in
the yield between as-synthesized LDH and the Cu(Al)O mixed
oxide could be attributed to the major catalytic activity shown
for Cu(Al)O mixed oxide, as expected in this type of materi-
als.2a,26 Mixed oxide resistance to rehydration.
Entry
Catalyst
Yieldb (%)
1
As-synthesized Cu–Al LDH
As-synthesized Cu–Al LDH
in the presence of sodium
ascorbate
65
75
2c
3
Cu–Al mixed oxide
30
98
4c
Cu–Al mixed oxide in the
presence of sodium ascorbate
CuO
CuO in the presence of sodium
ascorbate
5d
5
45
6c,d
An XRD analysis of the material aer the organic reaction
demonstrates that the structure of the Cu(Al)O mixed oxide was
not modied, which means that under the organic reaction
a
Reagents: alkyne 1a (1 mmol), benzyl chloride 2a (1.2 mmol), NaN3 (1.2
b
mmol), catalyst (10 mg) and EtOH–H20 (2 mL, 3 : 1). Yield of isolated
c
ꢀ
conditions (EtOH–H2O, 80 C, MW, 10 min), the original LDH
product aer purication by crystallization. The reaction was
performed in the presence of sodium ascorbate (10 mg). d CuO 10 mg.
was not obtained by the reconstruction process (Fig. 7). We tried
to reconstruct the original structure in using various conditions
described in the literature.27 However, the reconstruction in the
presence of H2O or an EtOH–H2O mixture was not possible even
at high temperatures (150 ꢀC) and long reconstruction times (48
h). In all cases, Cu(Al)O mixed oxide was identied by XRD as
the only structure.
This behaviour demonstrates the stability of the Cu(Al)O
mixed oxide under conditions that are not anhydrous, which is
an advantage from material view point, given that this type of
mixed oxide, which is highly catalytically active, cannot be
stored for a long time. However, in this case the material has
been stored for one year without the need for special conditions
and without a loss of catalytic activity.
understanding the role of sodium ascorbate in the catalytic
process is evidently necessary. The difference between the yields
when the reactions were carried out in the presence or absence
of this salt suggests that a redox process occurred during the
azide–alkyne 1,3-cycloaddition. Cu2+ is known to catalyse
Huisgen-type reactions, and Cu2+ can be reduced to Cu1+ in the
presence of sodium ascorbate to make the cycloaddition more
efficient. However, in this case, how can Cu2+ into Cu1+ be
reduced into a rigid structure?
Sodium ascorbate is able to capture different metals, and in
this case, it is a chelating agent that degrades the as-synthesized
LDH and Cu(Al)O mixed oxide. The degradation is minimal,19
but it is enough to decisively increase the reaction yield (Table 1,
entries 2 and 4). This hypothesis is supported by the post-
reaction identication and quantication by UV of Cu2+
2.3. Recyclability
To screen the recyclability of the Cu(Al)O mixed oxide catalyst,
two reactivation processes were applied to the material. In the
rst one, aer separating the material from the cycloaddition
reaction mixture between 1a, 2a and NaN3, the material was
Table 3 Scope of the reaction with various benzyl halidesa
dried at 100 C for 24 h. The second process includes calcina-
ꢀ
ꢀ
tion at 500 C for 5 h in the ambient atmosphere, without the
need for N2. Thus, if the used catalyst was reactivated at high
temperature (500 ꢀC), the reactions still proceeded well, and
high yields were obtained at the end of the reaction (Fig. 8). The
Cu(Al)O mixed oxide can be recycled ten times without
Entry
Benzyl halide
X
R
Product
Yieldb (%)
1
2
3
4
2b
2c
2d
2e
Cl
Cl
Br
Br
F
3b
3c
3d
3e
90
95
85
90
Cl
Br
I
a
Reagents: alkyne 1a (1 mmol), benzyl halide 2a–e (1.2 mmol), NaN3
(1.2 mmol), catalyst (10 mg), sodium ascorbate (10 mg) and EtOH–
b
H20 (2 mL, 3 : 1). Yield of the isolated product aer purication by
crystallization.
Scheme 2 Scope of the reaction with pyrimidine derivatives.
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Table 4 Scope of the reaction with steroidal derivativesabc
Fig. 7 X-ray diffraction patterns of Cu(Al)O mixed oxide after the
organic reaction.
Fig. 8 Reuse of Cu(Al)O mixed oxide dry at 100 ꢀC and calcined at
500 ꢀC.
3. Conclusions
In summary, the present study demonstrates the capacity of as-
synthesized LDH and Cu–Al mixed oxides to produce 1,2,3-tri-
azoles via an alkyne–azide cycloaddition reaction (Huisgen-type
reaction) using an EtOH–H2O mixture as the solvent. The yield
of the reactions depends primarily on the surface area of the
material, which is large in the case of Cu(Al)O mixed oxide.
However, the formation of 1,2,3-triazoles notably increased
when the reaction was performed in the presence of sodium
ascorbate, as a consequence of both heterogeneous and
homogeneous processes. In addition, this study demonstrated
that the reconstruction of Cu–Al LDH from Cu(Al)O mixed
oxides by the so-called memory effect was not possible, despite
the use of severe reaction conditions.
a
Reagents: alkyne 1 (1 mmol), 4 (1.2 mmol), catalyst (10 mg), sodium
b
ascorbate (10 mg) and EtOH–H2O (2 mL, 3 : 1). When Z ¼ OMs,
c
NaN3 (1.2 mmol) was necessary. Yield of the isolated product aer
chromatographic purication.
signicant losses in its catalytic activity. In all cases, the crys-
talline structure of the material was monitored by XRD. That
analysis revealed that the structure of the catalyst remained
unchanged, even aer ten cycles. The full characterization of
material aer ten catalytic cycles is showed in the ESI.†
This result contrasts with the results using the Cu–Al dry
material at 100 ꢀC, which demonstrated a loss of catalytic
Acknowledgements
activity through reuse, despite maintaining the structure of The authors would like to thank Consejo Nacional de Ciencia y
´
a mixed oxide. The reduced activity of the Cu(Al)O mixed oxide Tecnologıa, CONACyT (project 181448) for nancial support.
is probably due to the presence of organic material on the RGO, GENS, DAB, LLR, AGC, LVH, RS and JAMS wish to
surface, which is eliminated aer calcination at 500 ꢀC, thus acknowledge the SNI (Sistema Nacional de Investigadores) for
recovering catalytic properties of the material (Fig. 8, red the distinction of their membership and their stipend. CIUC is
column, yield 80%).
grateful to CONACyT for her student fellowship. We also wish to
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´
thank Maria Eugenia Ochoa, Rebeca Yepez and Geiser Cuellar 12 For recent examples about Huisgen reaction, see: (a)
for the technical assistance. Thanks to Laboratorio Divisional
H. Koganei, S. Tachikawa, M. E. El-Zaria and
´
´
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