Efficient and selective copper-grafted nanoporous silica in aqueous conversion of aldehydes to amides

Article abstract of DOI:10.1039/c4cy00963k

The one-pot conversion of aldehydes to their corresponding primary amides has been studied using SBA-15-grafted ethylenediamine copper complexes (En-Cu). The heterogeneous catalyst could be readily isolated from the reaction mixture and reused at least 14 times without significant loss in activity. The influence of reaction parameters was studied and the conditions determining yields and selectivities, including quantity of catalyst, solvent and base, were optimized to afford amides in high yields. The structures of synthesized catalysts were investigated using different characterization techniques including Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), CHN, atomic absorption (AA), scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

Full text of DOI:10.1039/c4cy00963k

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Efficient and selective copper-grafted nanoporous
silica in aqueous conversion of aldehydes to
amides†
Cite this: DOI: 10.1039/c4cy00963k
a
a
b
c
Sadegh Rostamnia,* Nasrin Nouruzi, Hongchuan Xin* and Rafael Luque
The one-pot conversion of aldehydes to their corresponding primary amides has been studied using
SBA-15-grafted ethylenediamine copper complexes (En–Cu). The heterogeneous catalyst could be readily
isolated from the reaction mixture and reused at least 14 times without significant loss in activity. The influ-
ence of reaction parameters was studied and the conditions determining yields and selectivities, including
quantity of catalyst, solvent and base, were optimized to afford amides in high yields. The structures
of synthesized catalysts were investigated using different characterization techniques including Fourier
transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), CHN, atomic absorption (AA), scanning
electron microscopy (SEM) and transmission electron microscopy (TEM).
Received 25th July 2014,
Accepted 4th September 2014
DOI: 10.1039/c4cy00963k
www.rsc.org/catalysis
nitrate and then reduced by H ) were disclosed as alternative
catalytic systems for the conversion of aldoximes to primary
2
Introduction
The formation of C–N bonds via C–H activation processes or
related processes represents an indispensable tool for the
synthesis of important target molecules that have significant
amides in good yields and with good reusability (up to four
5
6
cycles). Ramón, Ganguly and their coworkers later on
utilised CuIJII) salts ijCu(OAc) and CuSO ] as direct catalysts
2
4
1
pharmaceutical impact. Csp2–H bond conversion of aldehydes
for the one-pot transformation of aldehydes to primary amides
via C–H modification of aldehydes. The metal-catalyzed single-
pot conversion of aldehydes into primary amides, in compari-
son with aldoxime rearrangement, is a generally economical,
to the Csp2–N bond of amides can in principle be considered
as a two-step oxidative amidation process in which the cor-
responding oxime forms from aldehydes in the first step.
The second step leads to the formation of primary amides
via nitrile intermediates based on a dehydration/hydration
process. Alternatively, an acid-assisted Beckman rearrangement
6
simple and useful method. Nevertheless, the use of homoge-
neous metal salts requires laborious workup procedures to
separate and isolate pure products. Furthermore, the use of
environmentally unfriendly and biologically harmful solvents
is one of the main issues in these chemical transformations
together with the use of stoichiometric amounts of acids
(as catalysts) and the use of hydroxylamine reagents in large
2
can take place to generate secondary amides (Scheme 1).
Amides are functional groups in organic synthesis that
show a wide range of applications in pharmaceuticals, deter-
3
4a
4b
4c
gents and biologically active compounds. Au, Pd, Ru,
4
d
4e
4,4a–e,5,5a,b,6
Ir, and Rh catalysts have been recently reported to be
effective for the rearrangement of aldoximes into primary
amides. However, Au-, Pd-, Ir-, and Rh-containing catalytic
materials are highly expensive. CuIJII)/CuIJ0) mesoporous silica
materials (unfunctionalized mesoporous impregnated copper
excess.
The design of advanced catalysts with transition metal
active centers on the walls of solid nanoporous material has
7
,8
attracted increasing attention in recent years. SBA-15 sili-
cates have been extensively utilized as mesoporous materials
for a large number of applications (e.g. support for catalysis,
adsorption, drug delivery, etc.) due to their uniform pore
a
Organic and Nano Group (ONG), Department of Chemistry, Faculty of Science,
University of Maragheh, P.O. Box. 55181-83111, Maragheh, Iran.
E-mail: rostamnia@maragheh.ac.ir, srostamnia@gmail.com;
Fax: +98(421)2274893; Tel: +98(421)2278001(108)
b
Key Laboratory of Biofuels, Qingdao Institute of Bioenergy and Bioprocess
Technology, Chinese Academy of Sciences, Qingdao 266101, China.
E-mail: xinhc@qibebt.ac.cn
c
Departamento de Quimica Organica, Universidad de Cordoba, Edificio Marie Curie,
Ctra Nnal IV, Km 396, E-14014, Cordoba, Spain. E-mail: q62alsor@uco.es;
Fax: +34 957212066; Tel: +34 957212065
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00963k
Scheme 1 Rearrangements of oximes to amides.
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structures and high surface areas. Nevertheless, purely sili-
ceous SBA-15 is essentially not catalytically active and has
also improvable mechanical and hydrothermal stabilities due
to the high hydrophilicity from its rich silanol groups on the
Results and discussion
Ethylenediamine functionalized SBA-15 materials were previously
reported to have excellent activities in a range of CO capture
as well as aqueous removal of metal ions (e.g. CuIJII)).
1
1a
2
9
11b
surfaces. Organic–inorganic hybrid SBA-15 materials featur-
A
ing organic groups incorporated within the pores of the silica
wall can comparably offer improved hydrothermal stability and
hydrophobicity and consequently improved catalytic properties
for organic reactions. Applications of hybrid SBA-15 materials
in organic processes can provide a more efficient approach of
the reactants to active sites as well as suitable mesochannels
to facilitate mass transfer of products into the bulk solution
similar complex system between grafted ethylenediamine and
copper (SBA-15/En–Cu) was devised and synthesized for the
selective aqueous transformation of aldehydes to amides (Fig. 2).
The solid materials, characterized by a series of techniques
including FT-IR, XRD, SEM, TEM and atomic absorption spec-
trometry (AAS), were found to be mostly composed of supported
copper acetate complexes on SBA-15, which exhibited a well-
ordered hexagonal structure typical of SBA-15 materials.
1
0
for improved reusability.
Herein, we report the design and preparation of an efficient
heterogeneous Cu-containing catalyst for the mild aqueous
conversion of aldehydes into primary amides. The use of widely
available, less toxic and low-cost copper ethylenediamine complex-
based heterogeneous catalysts has not been reported to date in
the synthesis of amides. In this work, we confined copper
complexes inside the channels of ethylenediamine function-
alized silica SBA-15 (Fig. 1) to design highly active and reus-
able nanomaterials in the one-pot synthesis of amides from
aldehydes.
FT-IR spectra of SBA-15 exhibit typical bands at 799 and
−
1
~1100 cm , correlated to (Si–O–Si) bond vibrations, with
−
1
a small band at ca. 960 cm attributed to functionalized
(Si–OH) bonds. SiO–H groups can be visualized in the very
−
1
broad IR absorption band in the 3000–3700 cm region
(Fig. 3). The presence of several bands with low intensity
−
1
in the 1400–1600 cm region corresponds to propyl ethyl-
−
1
enediamine groups, with the band at 2900 cm most possibly
related to the presence of remaining traces of the surfactant
and/or solvent in the material. Importantly, the decrease
−
1
in intensity of the small bands at 850–950 and 1600 cm
in functionalized materials as compared to parent SBA-15
Fig. 3 IR spectra of unfunctionalized SBA-15 (a), SBA-15/En (b) and
SBA-15/En–Cu (c).
Fig. 1 Modification of the aldehydes to amides over SBA-15/En–Cu.
Fig. 2 Schematic representation of the SBA-15/En–Cu preparation.
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evidences a surface functionalization in the silicate. The char-
A preliminary screening of SBA-15/En–Cu in the selected
aqueous transformation of aldehydes to amides, particularly
for the model reaction of benzaldehyde to benzamide under
different conditions, has been included in Table 1. The first
studies of the influence of the [Cu] content in the model reac-
tion were approached by screening a wide range of supported
copper-containing materials (Table 1, entry 1–4). For com-
parative purposes, a propylamine-grafted SBA-15 material
acteristic peak of the C–N bond also experienced a strong
−
1
2+
shift from 1460 to 1420 cm upon Cu incorporation, which
can be attributed to the strong interaction of –NH and –NH
2
2
+
11b
groups with Cu ions.
XRD patterns of the unmodified SBA-15 sample exhibited
three well-resolved peaks at 2θ = 0.90° and 1.5°–2.0°, consis-
tent with (100), (110) and (200) diffraction lines, indicative
of a highly ordered hexagonal SBA-15 mesoporous structure
2
(NH /SBA-15/Cu) was prepared and its reactivity and stability
were compared to those of SBA-15/En–Cu.
1
0
(
see the ESI†). The structure was fully preserved upon
functionalization. The morphology and microstructure of the
catalyst before and after modification as well as metal load-
ing were investigated by SEM and TEM (Fig. 4). TEM images
confirm the presence of well-ordered hexagonal arrays of
mesoporous channels, preserved upon functionalization,
along the direction of the pore axis or in the direction per-
pendicular to the pore axis. TEM images could also confirm
Amine– and ethylenediamine–copper complexes supported
on SBA-15 exhibited superior reactivity. Interestingly, the catalytic
performance of recycled NH /SBA-15/Cu and SBA-15/En–Cu
2
catalysts was also remarkably different. The catalytic activity
of NH –SBA-15/Cu slowly decreased after the third run. Hot
2
filtration test studies revealed the leaching of active species
to be the main reason for the observed gradual deactivation.
Comparably, SBA-15/En–Cu could be successfully reused
14 times in the reaction without any significant decrease in
activity or selectivity under the investigated reaction condi-
tions (Fig. 5 and 6). The observed stability was also confirmed
by hot filtration tests (no Cu leaching detected in solution
after repeated reactions) and TEM (Fig. 4e, the long-range
order and structure were preserved after 14 uses).
0
the absence of Cu nanoparticles in SBA-15/En–Cu.
These results are in good agreement with results from XRD
patterns and confirm that the characteristic pore dimensions
and channel structures of the support materials remained
intact upon organic ethylenediamine and metal complex
functionalization onto SBA-15. Amine contents determined by
−
1
CHN elemental analysis (1.58 mmol N g ) also indicate that
amine moieties can efficiently chelate copper ions (0.74 mmol
of CuIJII) loading per gram of support) in SBA-15/Cu–En. The
thermal stability of SBA-15/Cu–En was subsequently examined
by thermal gravimetric (TG) studies between RT and 800 °C in
a static nitrogen atmosphere (see the ESI†). TG/DTG analysis
shows a remarkable thermal stability for SBA-15/Cu–En up
to 400 °C in which the decomposition/degradation of the
organic part of the material starts to take place.
SBA-15/En–Cu was subsequently utilized as a catalyst in a
number of experiments under various conditions to optimize
the reaction conditions. Various solvents were screened in the
reaction including H O, dioxane, toluene, DMSO and others.
2
Comparing results from Table 1, water was found to be the
optimum solvent for the reaction (Table 1, entry 4–10).
In order to explore the effect of the base, the reaction was
conducted using different bases. No amide product was
Fig. 4 SEM images of (a) SBA-15/En and (b) SBA-15/En–Cu. TEM image of (c) SBA-15/En; (d) SBA-15/En–Cu and (e) SBA-15/En–Cu after 14 reuses
in the aqueous conversion of benzaldehyde to benzamide.
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Table 1 Optimization of reaction conditionsa
b
b
Entry
[Cu]
NH
Solvent
Base
Cu (mol%)
T (°C)
Time (d)
Yield of amide (%)
1
2
3
4
5
6
7
8
9
2
–SBA-15/Cu
MeOH
MeOH
MeOH
MeOH
Dioxane
Toluene
Ethanol
DMSO
Na
Na
Na
Na
Na
Na
Na
Na
Na
Na
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
3
3
3
3
3
5
5
5
5
2
2
2
2
2
2
2
2
2
2
1
80
80
80
80
80
80
60
110
80
80
80
80
80
80
80
80
110
80
r.t.
2
2
2
2
2
3
2
3
3
2
2
2
2
2
2
2
2
3
3
66
33
38
74
83
62
51
49
20
95
72
93
33
>99
66
47
50
c
SiO
2
/Cu
c
SBA-15/Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
SBA-15/En–Cu
DMSO/H
2
O (2/1)
10
11
12
13
14
15
16
17
18
19
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
NaHCO
CO
NaOAc
Cs CO
3
K
2
3
2
3
Na
Na
Na
—
2
CO
2
CO
2
CO
3
3
3
0.5
0.5
2
Trace
Trace
Na
2
CO
3
2
a
b
c
Reaction conditions: benzaldehyde (1 mmol), NH
via impregnation, ref. 5.
2
OH·HCl (1 mmol), base (1.1 mmol) in 3 mL of solvent. Isolated yield. Catalyst
3 2
synthesized using CuIJNO )
Fig. 6 Reusability of SBA-15/En–Cu in the conversion of benzaldehyde
to benzamine (14th run performed during 82 h). Reaction conditions:
Fig. 5 Investigation of Cu leaching into solution in the conversion
of benzaldehyde to benzamine. Reaction conditions: 1 mmol of
benzaldehyde, 1 mmol of hydroxylamine hydrochloride, 1.1 mmol of
1
1
mmol of benzaldehyde, 1 mmol of hydroxylamine hydrochloride,
.1 mmol of Na CO , 2 mol% SBA-15/En–Cu, 3 mL of water.
2
3
2 3
Na CO , 2 mol% SBA-15/En–Cu, 3 mL of water.
(
Table 1, entry 10), while a further reduction (<1 mol%) led to
obtained under base-free conditions. Cs
most effective to provide quantitative yields to benzamide
Table 1, entry 14). These optimum results for cesium carbon-
2
CO
3
was found to be
remarkably reduced amide yields. Interestingly, reactions
conducted with 0.5 mol% catalyst could provide good yields
of oximes (intermediate products) which did not undergo the
final reaction step to amides (Table 1, entry 17).
With these results in hand, the optimized reaction condi-
tions comprised the utilization of 1 mmol of aldehyde, 1 mmol
of hydroxylamine hydrochloride, 2 mol% SBA-15/En–Cu,
(
ate are in good agreement with previously reported results
4
using homogeneous Pd catalysts. However, no significant
differences were observed for the use of other bases in the
reaction which provided excellent yields of products (93% to
9
5%), as shown in Table 1 (entries 10 and 12). Na
2
CO
3
was
1.1 mmol of Na CO and 3 mL of water as solvent. Optimum
2 3
consequently selected as a suitable base for optimization
studies based on price and availability.
The amount of the catalyst had a critical effect on the
yields of isolated amides. Catalyst quantities could be reduced
to 2 mol% without a significant influence on product yields
conditions were subsequently extended for a range of aryl
aldehydes to assess the scope of the reaction. Good to excel-
lent yields of products were obtained using aromatic alde-
hydes bearing electron-withdrawing (Table 2, entry 2–4) or
electron-donating (Table 2, entry 5–10) functional groups.
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Ortho-substituted aryl aldehydes (Table 2, entry 4–7) gave com-
parably lower isolated yields than those of para-substituted
compounds, which can be attributed to steric effects. Similar
results were obtained with the reactions of heteroaromatic
aldehydes including thiophene-2-carbaldehyde (Table 2, 14).
A hot filtration test was subsequently performed to
determine the stability in terms of the amount of Cu leached
in SBA-15/En–Cu under the investigated reaction conditions.
The solid catalyst was removed after 5 h reaction (~17%
conversion) and amide formation followed, maintaining the
same reaction conditions for a further 90 h. No further
increase in product yield was observed upon catalyst removal.
Atomic absorption (AAS) analysis only showed 4.8 ppm
copper in solution, demonstrating that essentially no Cu
leached out into the reaction mixture (Fig. 5, Cu content >1 mg
in 2 mol% catalyst per reaction). These studies showed that
copperIJII) ions are strongly coordinated with ethylenediamine
and that the trace amounts of copper leached into solution
are unable to further catalyze the reaction. Comparatively, the
use of the recovered catalyst in another reaction run with
fresh substrates provided identical results to those obtained
for fresh SBA-15/En–Cu (Table 2). These findings support the
truly heterogeneous nature of the catalytic system under the
investigated reaction conditions.
1
2
Table 2 Aqueous conversion of various aldehydes to primary amides using SBA-15/En–Cu
Entry
1
Product
Time [d]
2
Yield [%]
95
Entry
8
Product
Time [d]
3
Yield [%]
86
2
2
95
9
2.5
89
3
4
5
6
7
2
96
71
73
80
81
10
11
12
13
14
2
2
2
2
2
89
92
93
92
85
2.5
3
3
2.5
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The recyclability of SBA-15/En–Cu was further investigated
Conclusions
in the model reaction. Results showed that apart from being
highly stable (in terms of Cu leaching) under the investigated
reaction conditions, the catalyst could be reused at least
A highly active heterogeneous SBA-15/En–Cu catalyst was pre-
pared by grafting copper acetate complexes onto diamine-
modified SBA-15. Fully characterized SBA-15/En–Cu exhibited
excellent activity as a heterogeneous catalyst in the conver-
sion of aldehydes to amides. The catalyst could be reused at
least 14 times without a significant loss in catalytic activity.
The present method features several advantages including
simplicity and one-pot protocol requiring no activation or
modification of intermediate products. We envisaged that
this catalytic system can be potentially extended further to
C–H activation and C–C and C–heteroatom coupling chemistries
that will be reported in due course.
1
4 times without a significant loss of catalytic activity and
selectivity. After the 14th run, the catalytic activity considerably
decreased, although amide yields could be slightly increased
by prolonging the time of reaction (Fig. 6). A plausible mech-
anistic explanation for the one-pot preparation of primary
amides from aldehydes using SBA-15/En–Cu has been pro-
posed in Scheme 2.
From a mechanistic point of view, two different metal-
4
promoted reaction pathways have been previously reported.
The first one involves a metal-containing five-membered
cyclic intermediate, which decomposes into the final amide
product (pathway A), while a dehydration/hydration pathway
Experimental
Preparation of SBA-15, SBA-15/En and SBA-15/En–Cu
(
pathway B) followed by subsequent rearrangement to the
4
amide product was also proposed. We believe that pathway
B (dehydration/hydration) seems to be the most plausible
on the basis of obtained results and previously reported
Mesoporous silica SBA-15 was prepared using the hydrothermal
method according to Zhao et al. Diamine group-grafted SBA-15
materialswereobtainedbyrefluxing1gofSBA-15silicain250mL
17
1
3
Cu activities in chemical processes.
The catalytic activity of SBA-15/En–Cu was finally com-
pared with previously reported catalytic protocols. Interest-
ingly, the reported activities in this reaction using the
designed SBA-15/En–Cu system were comparable to those
of optimum homogeneous copper catalysts reported in the
literature, with remarkable differences in terms of stability
and reusability (Table 3).
oftoluenewithN-IJ2-aminoethyl)-3-aminopropyltrimethoxysilane
10d
ij(MeO) SiPrEn] for 20 h, based on our previous report.
3
Complexation of the diamine-modified SBA-15 (SBA-15/En) with
copper is depicted in Fig. S1 (see the ESI†).
In a typical synthesis, 1 g of SBA-15/En was activated at
60 °C for 5 h and then dispersed in MeOH/H
2
O (25 mL : 25 mL).
1
mmol of copper acetate was separately dissolved in 10 mL
of H O and subsequently added dropwise to the dispersion
2
of SBA-15/En under continuous stirring at RT for 24 h. The
solvent was then removed by filtration and the resulting solid
was thoroughly washed with H O (×2 with 5 min bath sonication),
2
ethanol and dichloromethane several times, followed by oven
vacuum drying at 60 °C.
General procedure for amide synthesis catalyzed by
SBA-15/En–Cu
In a typical run, benzaldehyde (1.0 mmol), hydroxylamine
hydrochloride (1 mmol), sodium carbonate (1.1 mmol) and
SBA-15/En–Cu (0.027 g, 2 mol%) were added to 3 mL of H O.
2
The reaction mixture was stirred at 80 °C for 2 days. Upon
reaction completion, the catalyst was separated by centrifuga-
tion and the solution was quenched with dichloromethane.
The organic phase was evaporated under reduced pressure.
Scheme 2 Proposed mechanisms for the catalytic conversion of
aldehydes to amides.
4
Table 3 Comparison of catalytic performance of different systems in the conversion of benzaldehyde to benzamide
Catalyst
Cat. (mol%)
Yield (%)
Reaction conditions
100 °C/DMSO–H
Ref.
PdIJOAc)
2
5
2
2
2
1
1
5
2
98
31
99
24
95
88
95
95
2
O
4b
6a
6a
14
15
4e
16
CoIJOAc)
CuIJOAc)
RuIJDMSO)
ijRuCl(CO)IJPPh
TerpyRuIJPPh )Cl
CuSO ·5H
2
110 °C/H
110 °C/H
2
O
O
2
2
4
Cl
2
110 °C/toluene
110 °C/toluene
110 °C/toluene
3
)IJTAC)]
2
3
4
2
O
110 °C/solventless
80 °C/H
SBA-15/En–Cu (heterogeneous)
2
O
This work
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Evaporation of the organic phase was followed by purification
on column chromatography using silica gel to provide the
desired products (see the ESI†).
(b) H. Peng, D. Wang, L. Xu and P. Wu, Chin. J. Catal., 2013,
34, 2057.
6 (a) A. Martínez-Asencio, M. Yus and D. J. Ramón, Tetrahedron,
2012, 68, 3948; (b) N. C. Ganguly, S. Roy and P. Mondal,
Tetrahedron Lett., 2012, 53, 1413.
Once the reaction was complete, the catalyst was recovered
(
by filtration or centrifugation), washed with methanol and
dried at 60 °C. The catalyst was transferred to a flask and then
fresh reagents including the base (1.1 mmol), benzaldehyde
7 N. Linares, E. Serrano, M. Rico, A. M. Balu, E. Losada,
R. Luque and J. García-Martínez, Chem. Commun., 2011,
47, 9024.
(
1 mmol) and hydroxylamine hydrochloride (1 mmol) were
added and the mixture was again heated under stirring for a
certain time. Products were separated following the proce-
dure described in the first cycle and the catalyst was again
collected and reused in another reaction run.
8 R. Luque and J. Garcia-Martinez, ChemCatChem, 2013,
5, 827–829.
9 (a) F. Rajabi, S. Naserian, A. Primo and R. Luque, Adv. Synth.
Catal., 2011, 353, 2060; (b) F. Rajabi, N. Karimi, M. R. Saidi,
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354, 1707.
2
-Thiophenecarboxamide
White crystalline powder. MP: 182–184 °C. H NMR (400 MHz,
DMSO-d ): δ = 8.00 (1H, s, 1H of NH), 7.75–7.74 (2H, m, 2H of
CH aromatic), 7.40 (1H, s, 1H of NH), 7.14 (1H, t, 1H of
1
10 (a) B. Karimi and M. Khorasani, ACS Catal., 2013, 3, 1657;
(
(
b) B. Karimi and M. Vafaeezadeh, RSC Adv., 2013, 3, 23207;
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6
3
13
S. Rostamnia and H. Xin, Appl. Organomet. Chem., 2013, 27,
48; (e) S. Rostamnia, H. Xin, X. Liu and K. Lamei, J. Mol.
CH aromatic, J
= 4.2 Hz); C NMR (100 MHz; DMSO-d6):
HH
3
d = 163.3 (CONH
2 2
), 140.68 IJArCCONH ), 131.47 (ArCH), 129.19
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F. Pourhassan, Chin. Chem. Lett., 2013, 24, 401.
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(
ArCH), 128.39 (ArCH).
1
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