Efficient three-component coupling catalysed by mesoporous copper-aluminum based nanocomposites

Article abstract of DOI:10.1039/c3gc36607c

Traditional synthesis methods for propargylamines have several drawbacks. A recently developed alternative route is the so-called "A³ coupling" in which an alkyne, an aldehyde, and an amine are coupled together. Typically, these reactions are c

Full text of DOI:10.1039/c3gc36607c

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Efficient three-component coupling catalysed by
mesoporous copper–aluminum based
nanocomposites†
Cite this: DOI: 10.1039/c3gc36607c
a
b
c
Jana Dulle, K. Thirunavukkarasu, Marjo C. Mittelmeijer-Hazeleger,
a
c
c
Daria V. Andreeva,* N. Raveendran Shiju* and Gadi Rothenberg*
Traditional synthesis methods for propargylamines have several drawbacks. A recently developed alterna-
3
tive route is the so-called “A coupling” in which an alkyne, an aldehyde, and an amine are coupled
together. Typically, these reactions are catalysed by homogeneous gold salts, organogold complexes or
silver salts. But these homogeneous catalysts are expensive and their separation is difficult. Here we
Received 16th August 2012,
Accepted 30th January 2013
3
report the discovery that solid Cu/Al/oxide mesoporous “sponges” are excellent A coupling catalysts.
These materials are robust, inexpensive, and easy to make. They give good to excellent yields (87–97%)
for a wide range of substrates. Being heterogeneous, these catalysts are also easy to handle and separate
from the reaction mixture, and can be recycled with no loss of activity.
DOI: 10.1039/c3gc36607c
www.rsc.org/greenchem
1
3
14–17
18
One of the biggest challenges in organic chemistry is mimick- complexes, silver salts,
and Hg Cl . Among these, the
2 2
ing the complexity of biosynthesis in vitro. Nature excels at gold salts show the highest activity. The problem is that rapid
matching multiple simple substrates in one-pot reactions that reduction of cationic gold species to inactive metallic atoms is
1
–4
19–21
give complex molecules with high yields.
We chemists can unavoidable when gold salts activate alkynes/alkenes.
also do this, but typically using a host of stoichiometric Moreover, using homogeneous catalysts brings inherent com-
reagents and protecting groups, and often generating much plications in catalyst separation and reuse. The ideal solution,
waste in the process.
therefore, would be replacing the homogeneous gold salt with
22–35
The synthesis of propargylic amines is a good example. an effective, yet inexpensive, heterogeneous catalyst.
Many
These amines are key intermediates for making drugs and of the heterogeneous catalysts are supported/immobilized gold
5
–8
36
agrochemicals. They are traditionally synthesized by nucleo- nanoparticles. Zhang et al. reported that supported gold is
3
21
philic attack of lithium acetylides or Grignard reagents on active for the A coupling reaction. Also, gold nanoparticles
imines or their derivatives.
9
,10
However, such reagents must be embedded in a mesoporous carbon nitride support was shown
used stoichiometrically, causing large amounts of waste. They to be an efficient heterogeneous catalyst for the synthesis of
are also moisture-sensitive, and require strictly controlled reac- propargylamines, although its catalytic efficiency has been
3
7
tion conditions.
An elegant alternative route is the so-called “A coupling” is gold nanoparticles supported on nanocrystalline MgO.
eqn (1)). This catalytic reaction between an alkyne, an alde- The silver salt of 12-tungstophosphoric acid (Ag 40) was
hyde, and an amine (hence the three A’s) gives water as the also used to catalyse the A -coupling. Gold and copper thin
demonstrated for very few substrates. Another active catalyst
3
38
(
3
PW12O
3
39
7
,11
only by-product.
The reaction is run in a liquid phase, films were used as catalysts in microwave-assisted continuous-
1
2
40
using homogeneous catalysts such as gold salts, organogold flow organic synthesis (MACOS) of propargylamines . Liu
et al. reported gold functionalized IRMOF-3 catalysts for the
4
1
one-pot synthesis of propargylamines. In another report,
gold nanoparticles impregnated on alumina was used as a cata-
lyst for propargylamine synthesis in a flow reactor. Consider-
a
Physical Chemistry II, University of Bayreuth, Universitätsstr. 30, 95447 Bayreuth,
Germany. E-mail: daria.andreeva@uni-bayreuth.de; Fax: +49 (0)921 55 2059;
Tel: +49 (0)921 55 2750
34
b
ing the costs, it would be better to have an active catalyst
containing no precious metals. A recent review by Peshkov
et al. summarised the homogeneous and heterogeneous cata-
National Centre for Catalysis Research, Indian Institute of Technology-Madras,
Chennai-600 036, India
c
Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, Science Park
3
42
9
04, 1098 XH Amsterdam, The Netherlands. E-mail: n.r.shiju@uva.nl,
lysts utilized in A -coupling.
g.rothenberg@uva.nl; Fax: +31 (0)20525 5604; Tel: +31 (0)20525 6515
http://www.hims.uva.nl.hcsc
†
We now report the discovery that copper/aluminum (Cu/Al)
3
based mesoporous nanocomposites are excellent A coupling
Electronic supplementary information (ESI) available: Pore-size distribution of
1
catalyst and H NMR spectral data. See DOI: 10.1039/c3gc36607c
catalysts. These materials are robust, inexpensive, and easy to
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make. They are highly active in the coupling reaction of alde- Table 1
hydes, amines, and alkynes, giving propargylamines selectively
with water as the only by-product.
A coupling catalyzed by Cu/Al
The catalysts are essentially RANEY®-type materials,
though our method for making them differs from the RANEY®
4
3
process. We create first a Cu/Al alloy, and then use intense
sonication for fragmenting and partially oxidising the particles
a
R
R′
(amine)
2
NH
Yield
(GC/isolated)
4
4–47
b
(see the Experimental section for detailed procedures).
Entry
(aldehyde)
R′′ (alkyne)
1
2
3
4
5
6
7
8
9
Ph
4-CH
Cyclohexyl
3-ClC
3-OHC
3-NO
Ph
Ph
Ph
Ph
Ph
4-OCH
Ph
Ph
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Morpholine
Pyrrolidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
99/94
93/87
97/90
96
92
88
94/88
90
92
95
90
3
C
6
H
4
ð1Þ
6
H
4
6
H
4
C
2 6
H
4
This simple and waste-free route is based on our recent
finding that ultrasound can cause re-structuring of metal
alloys due to cavitation induced microphase separation (Fig. 1).
During sonication the metal alloy particles undergo the sono-
Hexyl
1
1
1
0
1
2
4-CH
4-CH
Ph
Ph
Ph
3
C
6
H
4
3
OC
6
H
4
3
6
C H
4
94
45
92
81
c
mechanical and sonochemical effects stimulated by cavitation. 13
d
1
1
4
5
These effects lead to fragmentation, surface reactions, and for-
mation of pores. The metals in the alloy have different chemical
standard electrode potentials) and physical (melting points
and hardness of the oxide layer) properties. Thus, the rates of
the above processes are different for the alloy components. In
e
Ph
Ph
a
(
Reaction conditions: aldehyde (1.0 mmol), amine (1.2 mmol), and
alkyne (1.3 mmol), Cu/Al (0.12 mmol Cu), toluene (1.7 ml), 100 °C,
b
3
2
2 h; mol% of A coupling product yield based on aldehyde starting
c
2
material (entries 1–12, complete conversion monitored by GC); Cu O
d
e
the case of Cu/Al, this gives a mesoporous aluminum-based catalyst, 75% conv.; Cu/Al, 90 °C, 22 h (GC conv. 92%); Cu/Al, 70 °C,
2
2 h (GC conv. 82%).
framework, with homogeneously distributed oxidized copper
particles. This morphology of the Cu/Al particles has several
attractive features for heterogeneous catalysis: increased surface
area, mesopores, and unique textural stability.
Benzaldehyde, piperidine, and phenylacetylene were used
as model substrates to study the catalytic activity of the Cu/Al
coupling, under the same conditions. Homocoupling of the
alkyne was also observed in this case and selectivity to the A
coupled product was only 60%. Indeed, homogeneous copper
salts were also reported to be active as catalysts, but again
these gave only moderate conversions and selectivities.
3
3
catalyst in A coupling. Our catalyst gave high benzaldehyde
conversion (∼99%), as well as a high yield of the coupling
product (see Table 1). Control experiments confirmed that no
conversion was found in the absence of catalyst under other-
48–51
Moreover, difficulties in recovering the catalyst from the reac-
tion mixture limit their use.
wise identical conditions. Compared with commercial Cu O
3
2
To examine the scope of the A coupling reaction, we
+
(
Cu is predominant in our catalyst; see below the XPS charac-
studied different combinations of aldehydes, amines, and
alkynes (see Table 1). Aromatic aldehydes as well as cyclohexane-
carboxaldehyde (entry 3) gave high yields. The reactions with
morpholine and pyrrolidine (entries 7 and 8) also gave >90%
3
terization) the sample was not as active and selective to A
3
yields of the A coupled product.
Kantam et al. showed that CuO nanoparticles gave a good
3
yield for A coupling, while commercially available bulk Cu
2
O
3
5
and bulk CuO gave poor yields. The yield was 82% using benz-
aldehyde, piperidine and phenylacetylene using toluene as the
solvent. The yield varied from 8% to 84% when the substrates
were changed. Recently, Albaladejo et al. reported that Cu O
2
3
52
on titania is a good catalyst for A coupling. The catalyst was
prepared by supporting copper nanoparticles, made by the
addition of CuCl to a suspension of lithium and 4,4′-di-tert-
2
butylbiphenyl in THF, on TiO . They obtained 32% yield
2
(
using toluene as the solvent) and 98% yield (using neat sub-
strate) starting from benzaldehyde, piperidine and phenyl-
3
acetylene. For other substrates, the yield of the A coupled
Fig. 1 Sonochemical formation of mesoporous Cu/Al based particles. Intensive
sonication of the initial alloy particles leads to fragmentation, porosity, and
surface oxidation.
products varied, from 52% to 98%. Notwithstanding these
3
results, our Cu/Al catalyst gives complete conversion to the A
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Fig. 2 Scanning electron micrograph (a), 3D confocal microscopy reconstruc-
tion of the particle loaded with rhodamine B (picture taken in fluorescent
mode) showing the porosity throughout the catalyst particle (b), transmission
electron micrograph (c), and nitrogen adsorption isotherm (d) of the Cu/Al cata-
lyst, showing the morphology and porous structure.
3
Scheme 1 Tentative mechanism of the A coupling. The Cu(I) species activates
the C–H bond of the alkyne to give a copper acetylide intermediate (a), which
reacts with the immonium ion (b) generated in situ from the aldehyde and sec-
ondary amine to give the corresponding propargylamine (c).
surface area to 34 m g−1. The BET isotherm is type II.53 We
see that the monolayer coverage is completed and multilayer
N2 adsorption starts at a relative pressure of ca. 0.03. This
2
product in toluene, as well as high activity for different sub- shows that the material has both micropores and mesopores.
2
−1
2
strates as well, with 88%–99% yield. This matches and in The meso and micropore areas were 11.2 m g and 22.8 m
−
1
3
−1
some cases outperforms gold catalysts published elsewhere.
g
and the meso and micropore volumes were 0.03 cm g
3
−1
Thus, Wei et al. reported that AuCl, AuI, AuBr , and AuCl
and 0.01 cm g (pore size distribution is given in ESI†).
We also examined the inner structure of the material by
3
3
3
12
showed good activities for A coupling. The yield of propargyl-
amines ranged from 53% to 99%, depending on the sub- monitoring the spatial distribution of a fluorescent dye inside
3
strates. Zhang et al. showed that Au/CeO catalyses A coupling the particles. Fig. 2b shows a 3D reconstruction of confocal
2
and the yield varies from 25%–99%, again depending on the scanning fluorescence microscopy (CSFM) images of a catalyst
2
1
substrates. Another report shows that gold nanoparticles particle loaded with rhodamine B. Here, too, we can see the
3
embedded in a mesoporous carbon nitride catalyses the A
coupling, yielding 51% after 12 h and 62% after 24 h.
porous inner structure.
37
The powder X-ray diffraction (PXRD) pattern of the initial
We believe that the reaction mechanism (Scheme 1) alloy showed intense peaks for Al and CuAl
(Fig. 3). After soni-
2
involves the formation of copper acetylide, as was proposed for cation, the PXRD reveals the peaks corresponding to highly
3
A
coupling with cationic gold under homogeneous con- ordered Al(OH)
3
(bayerite) crystallites and CuO, thus, partial
1
2,28
3
ditions.
Recently, Albaladejo et al. demonstrated that A
oxidation of the alloy. The different Cu/Al mixed phases also
coupling using heterogeneous Cu/TiO also involves copper can be distinguished in the PXRD pattern of the modified par-
2
5
2
acetylide formation. The same mechanism can be invoked in ticle, indicating an increased interaction between the two
our system. Thus, the C–H bond of the alkyne is activated by a metals.
Cu(I) species to give a copper acetylide intermediate (a), which
Using XPS, we observed mixed oxidation states for Cu on
reacts with the immonium ion (b) generated in situ from the the catalyst surface (Fig. 4 and Table 2). The majority (72%) of
+
aldehyde and secondary amine to give the corresponding pro- the copper on the surface was Cu , with a binding energy (BE)
2
+
pargylamine (c) and regenerate the catalytically active site.
of 932.5 eV. The rest is Cu (BE = 933.7 eV). Indeed, low inten-
2
+
Understanding the structure and the morphology of the cata- sity satellite features (exclusively due to Cu ) support the
lyst surface is the key to understanding its activity. Therefore, minor contribution from the latter. However, because Cu and
we carried out a series of characterisation experiments. Elec- Cu states have the same BE, we also recorded the X-ray-
0
+
tron microscopy (Fig. 2a and 2c) showed that the initial par- excited Auger electron spectroscopy (XAES) for the Cu LMM
ticles of ∼120 μm are broken into ∼40 μm pieces after 60 min region. The Cu LMM peak can be resolved into two distinct
of sonication. The SEM and TEM images show a porous inter- peaks (see Fig. 4, inset) at 914.2 eV (kinetic energy, KE) and
+
2+
facial layer. Combining this with N
Fig. 2d) we confirm the formation of a porous outer surface presence of Cu was further confirmed by the satellite peaks
and modification of the inner structure, increasing the specific (denoted by arrows in Fig. 4). The absence of an ∼918.5 eV KE
2
adsorption studies 917.7 eV. These correspond to Cu and Cu , respectively. The
2
+
(
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Fig. 5 Catalyst recycling studies in the coupling of phenyl acetylene, benz-
aldehyde and piperidine. Reaction conditions: aldehyde (1.0 mmol), amine
(1.2 mmol), and alkyne (1.3 mmol), Cu/Al (0.12 mmol Cu), toluene (1.7 ml),
100 °C, 22 h. After each cycle, the catalyst was filtered, washed with acetone
Fig. 3 PXRD patterns of the initial particles (a) and particles sonicated for
and water and dried at 403 K.
6
0 min (b).
2
show peaks corresponding to Cu O, it is either amorphous or
highly dispersed on the Al surface.
To test catalyst reusability, we ran five consecutive reaction
cycles. After each cycle, the catalyst was filtered, washed with
acetone and water and dried at 403 K. The results were similar
to the original reaction and no significant deactivation was
observed, confirming the reusability of our catalyst (Fig. 5).
Conclusions
3
We showed here that the A coupling reaction can run in the
presence of a solid catalyst that contains only copper, alumi-
num, and oxygen. This catalyst is stable. It is readily recovered
after the reaction and can be recycled several times without
deactivation. The process is simple and general, giving propar-
gylamines in good to excellent yields for a variety of substrates.
Finally, the fact that our catalyst contains neither noble nor
heavy or toxic metals opens true opportunities for practical
application.
Fig. 4 Cu 2p XP spectrum of Cu/Al; the inset shows the Cu LMM XAE spectrum
tabulated data are provided in Table 2).
(
Table 2 XPS data of the Cu/Al sample
Core level
Binding energy (eV)
FWHM (eV)
Al 2p
O 1s
Cu 2p (Cu )
73.7
531.3
932.5 (914.2 )
933.7 (917.7 )
1.87
2.55
2.18
2.83
Experimental section
Materials and instrumentation
+
a
2
+
a
Cu 2p (Cu
)
a
Aluminium shot (irregular, <15 mm, 99.9%; metal basis) and
copper beads (2–8 mm, 99.9995%; trace metals basis) were
purchased from Alfa Aesar and Sigma Aldrich, respectively,
Kinetic energy values of the Cu LMM XAE spectra.
0
feature in the spectra shows that there is no Cu in the and used as received. Water was purified using a three-stage
5
4,55
sample.
core level (73.7 eV) compared with the Al 2p of the Al
support alone (∼74 eV) indicates a metal-to-support electron
Moreover, the negative shift in the BE of the Al 2p Millipore Milli-Q Plus 185 purification system (final resistivity
2
O
3
>18.2 MΩ cm).
Transmission and scanning electron microscopes (TEM,
transfer process. The interaction between the Cu and the Zeiss 922 EFTEM operating at 200 kV and Zeiss 1530 FE-SEM
support and consequent modification of the electronic respectively) in combination with an ultra-microtome (Ultracut
environment may be responsible for the high selectivity of the E Reichert Jung, thickness 50 nm) were applied to characterise
catalyst, compared to the bulk Cu O. Since the XRD did not the optical response, structure, and size of the Cu/Al powder.
2
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Powder X-ray diffraction data were collected on a Stoe (VB-1 column, FID), and/or purified by column chromatography
1
STADI P X-ray diffractometer using Cu Kα radiation at room (silica gel; hexane : EtOAc 4 : 1) and analysed using H NMR.
1
1
temperature. The N
measured at 77 K on a vacuum gas sorption apparatus Surfer scopy and mass spectrometry.
Thermo Scientific), after evacuation at 300 °C for 24 h. The Example: N-(1,3-Diphenyl-2-propynyl)piperidine (Table 1,
2
adsorption–desorption isotherms were Product identity and purity was confirmed by H NMR spectro-
(
surface area was calculated by the BET method. X-ray entry 1): Benzaldehyde (1.0 mmol, 106.1 mg), piperidine
photoelectron spectroscopy (XPS) measurements were carried (1.2 mmol, 102.2 mg), phenylacetylene (1.3 mmol, 132.8 mg),
out using a multiprobe system (Omicron Nanotechnology, Cu/Al catalyst (0.12 mmol based on Cu, 30 mg), and toluene
Germany) equipped with a dual Mg/Al X-ray source and a (1.7 ml) were reacted and analysed as above (258 mg, 94%
1
hemispherical analyzer operating in constant analyzer energy yield). H NMR δ = 7.00–7.65 (m, 10H), 4.80 (s, 1H), 2.40–2.60
(
5
CAE) mode. The spectra were obtained with a pass energy of (m, 4H), 1.56–1.69 (m, 4H), 1.30–1.50 (m, 2H); MS m/z (%) 275
0 eV for the survey scan and 20 eV for individual scans. A Mg (M , 20), 274 (10), 191 (100), 198 (79), 192 (24), 189 (24), 115
+
Kα X-ray source was operated at 300 W and 15 kV. The base (14), 165 (10), 232 (8).
pressure in the analyzing chamber was maintained at
−10
1
× 10
mbar. The data were processed with the Casa XPS
program (Casa Software Ltd, UK), and calibrated with reference
to the adventitious carbon peak (284.9 eV) in the sample. Peak
Acknowledgements
areas were determined by integration employing a Shirley-type We thank Dr C.S. Gopinath (National Chemical Laboratory,
background. Peaks were considered to be a 70 : 30 mix of Gaus- Pune) for his suggestions on XPS measurements and B. Putz
sian and Lorentzian functions. The relative sensitivity factors (University of Bayreuth) for the PXRD measurements. J. D. and
(RSF) provided by the manufacturer were used for quantifying D. A. thank SFB 840 for financial support.
elements.
Procedure for catalyst preparation
Notes and references
Aluminium and copper were first alloyed using an AM arc
melter (Edmund Bühler GmbH, Germany) with a melt stream
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−
5
of 300 A. The reactor was first evacuated to 10 mbar and
then pressurised to 500 mbar argon. For homogenization, the
sample was overturned three times and fused each time. The
bulk material was cut into pieces and ground using a rotary
mill (Pulverisette 14, Fritsch GmbH, Germany) with a sieve
ring of 0.12 mm. The resulting powder (typically 30 g alloy,
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2
5.0 wt% Cu) was then sieved by a 150 μm sieve.
Five grams of this Cu/Al alloy were then dispersed in 50 ml
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purified water and sonicated for 60 min with an ultrasound tip
VIP1000hd, Hielscher Ultrasonics GmbH, Germany). The
(
device was operated at 20 kHz with a maximum output power
of 1000 W by an ultrasonic horn BS2d22 (head area of
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3
.8 cm ). It was equipped with a booster B2–1.8 for 60 min.
−2
The maximum intensity was calculated to be 140 W cm at a
mechanical amplitude of 106 μm. To avoid the temperature
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20 °C for 5 h.
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The amine (1.2 mmol), aldehyde (1.0 mmol), alkyne
(
(
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the reaction, the mixture was cooled to ambient temperature
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