Formation of C-C, C-S and C-N bonds catalysed by supported copper nanoparticles

Article abstract of DOI:10.1039/c7cy01343d

Transition-metal catalysed cross-coupling reactions are still dominated by palladium chemistry. Within the recent past, copper has gained ground against palladium by virtue of its cheaper price and equivalent function in certain reactions. Four catalysts consisting of copper nanoparticles on zeolite, titania, montmorillonite and activated carbon have been tested in three palladium- and ligand-free cross-coupling reactions to form carbon-carbon, carbon-sulfur and carbon-nitrogen bonds. CuNPs/zeolite has been found to be the best one in the Sonogashira reaction of aryl iodides and arylacetylenes, as well as in the coupling of aryl halides with aryl and alkyl thiols, being reusable in both cases. However, the arylation of nitrogen-containing heterocycles (imidazole, pyrazole, benzimidazole and indole) has been better accomplished with CuNPs/titania, albeit CuNPs/activated carbon showed better recycling properties. The catalytic activity of the nanostructured catalysts has been compared with that of twelve commercial copper catalysts, with the former outperforming the latter in the three types of reactions studied.

Full text of DOI:10.1039/c7cy01343d

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DOI: 10.1039/C7CY01343D
Formation of C-C, C-S and C-N bonds catalysed by supported
copper nanoparticles
Received 00th January 20xx,
Accepted 00th January 20xx
Alexander Yu. Mitrofanov,a Arina V. Murashkina,a Iris Martín-García,b Francisco Alonsob,* and Irina
P. Beletskayaa,*
DOI: 10.1039/x0xx00000x
Transition-metal catalysed cross-coupling reactions are still dominated by palladium chemistry. Within the recent past,
copper has gained ground to palladium by virtue of its cheaper price and equivalent function in certain reactions. Four
catalysts consisting of copper nanoparticles on zeolite, titania, montmorillonite and activated carbon have been tested in
three palladium- and ligand-free cross-coupling reactions to form carbon-carbon, carbon-sulfur and carbon-nitrogen
bonds. CuNPs/zeolite has been found to be the best one in the Sonogashira reaction of aryl iodides and arylacetylenes, as
well as in the coupling of aryl halides with aryl and alkyl thiols, being reusable in both cases. However, the arylation of
nitrogen-containing heterocycles (imidazole, pyrazole, benzimidazole and indole) has been better accomplished with
CuNPs/titania, albeit CuNPs/activated carbon showed better recycling properties. The catalytic activity of the
nanostructured catalysts has been compared with that of twelve commercial copper catalysts, with the former
outperforming the latter in the three types of reactions studied.
www.rsc.org/
their stabilisation and dispersion but separation from the
reaction medium.6b-6f
Introduction
The copper-catalysed formation of carbon-carbon and carbon-
heteroatom bonds is not only a simple renaissance of
Ullmann’s chemistry; it is a new area of transition-metal
catalysed reactions where copper sometimes competes with
palladium and often reveals quite different behaviour.1 There
are many Cu(I) and Cu(II) derivatives, such as salts, oxides, or
complexes with ligands which are widely used in catalysis.
Even though many of these catalytic systems helped to
overcome some of the main drawbacks of the traditional
copper-promoted procedures, the homogeneous nature of the
catalysis hampers the recovery and reuse of these catalysts
and their practical applicability,2 mainly in the synthesis of
drug molecules,3 which must be free of any residual metal.
Consequently, heterogeneous copper catalysis has attracted a
great deal of attention in recent years. There are many
examples in the literature about the utility of heterogeneous
copper-based catalysts in cross-coupling reactions, most of
them based on copper complexes with functionalised ligands
immobilised on different supports.4 Still, these catalytic
systems require the synthesis of specialised ligands,
immobilisation and copper complex formation steps that,
often, may catalyse a narrow scope of reactions. In this sense,
catalytic systems which rely on Cu nanoparticles (CuNPs) may
Immobilised copper nanoparticles obtained by various
methods and on different supports have demonstrated to be
versatile and reusable catalysts for a wide range of reactions,
including cross-coupling reactions forming carbon-carbon and
carbon-heteroatom bonds (С-N, C-O, С-S, C-P),4,7 oxidative
coupling reactions,8 as well as multicomponent reactions.9
In this work, we set out a broad synthetic application of some
copper-based nanocatalysts supported on four different
materials: a carbonaceous material (activated carbon),10
a
ceramic metal oxide (nanosized titania),11 a clay mineral
(montmorillonite-K10)12 and a microporous zeolite (sodium Y
zeolite).13 In particular, we have already demonstrated the
great versatility of activated carbon14 and titania15 as supports
for metal nanoparticles in catalytic organic reactions. Herein,
the support-depending catalytic behaviour has been evaluated
in the Sonogashira-Hagihara reaction and in the coupling of
aryl halides with thiols and azoles. To the best of our
knowledge, such a comparative study involving four supports
of different nature has never been reported for these type of
reactions.
Results and discussion
be a good alternative to immobilised complexes because of Catalyst preparation and characterisation
their high surface-to-volume ratio, which confers them a
The CuNPs-based catalysts were prepared following the arene-
catalysed16 chemical reduction of metal salts, namely:9b
anhydrous CuCl2 was rapidly reduced with lithium metal and a
catalytic amount of 4,4’-di-tert-butylbiphenyl (DTBB) in THF at
room temperature, followed by the addition of the support.
The resulting mixture was filtered, washed and dried.9b
higher reactivity and selectivity when compared with the bulk
catalysts.5 Although it is difficult to separate nanoparticles by
the practice of standard methods due to their nanometric
size,6a immobilisation on inorganic supports not only favours
The full characterisation of copper nanoparticles on activated
carbon (CuNPs/C)9b and copper nanoparticles on zeolite Y
(CuNPs/ZY)8b was already reported in the literature (see also
the ESI). The copper nanoparticles on titania (CuNPs/TiO2) and
copper nanoparticles on montmorillonite (CuNPs/MK-10)12b
were characterised by means of transmission electron
microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX)
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and X-ray photoelectron spectroscopy (XPS) (see the ESI). In decreasing order of activity: CuNPs/ZY > CuNPs/C > CuNPs/MK-
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general, all catalyst unveiled the presence of well dispersed 10 > CuNPs/TiO2 (Figure 1). The higherDaOcI:t1iv0i.t1y03o9/fCC7CuYN0P13s4/3ZDY
spherical nanoparticles on the supports, with average sizes in and CuNPs/C might be, tentatively, correlated with the larger
the range of 1–6 nm. Analysis by XPS revealed that the surface surface area of these supports and the presence of both Cu(I)
of the CuNPs in all catalysts is oxidised and consists of both and Cu(II) in the catalysts.
Cu(I) and Cu(II) oxides for CuNPs/C and CuNPs/ZY, mainly Cu(I) The most active catalyst (СuNPs/ZY) was deployed in the
oxide for Cu/TiO2 and Cu(II) oxide for CuNPs/MK-10. The coupling reaction of phenylacetylene with different aryl
following copper loadings and BET areas were determined by halides at ca. 4 mol% loading [determined from the Cu content
inductively coupled plasma optical emission spectroscopy (ICP- (3.0 wt%) and the Cu2O/CuO area from XPS (ca. 1:1)] (Table 1).
OES) and adsorption isotherms, respectively, for the different All reaction products were formed in a selective manner with
catalysts: CuNPs/C (3.5 wt%, 1224 m2g‒1), CuNPs/TiO2 (1.9 almost quantitative yields from aryl iodides substituted either
wt%, 119 m2g‒1) CuNPs/ZY (3.0 wt%, 621 m2g‒1) and with electron-withdrawing or electron-donating groups (3aa
-
CuNPs/MK-10 (1.7 wt%, 89 m2g‒1).
3ea). As expected, electron-poor aryl iodides were shown to
be more reactive than those bearing electron-donating groups
in the aromatic ring (compare the reaction times of 1a and 1b
The Sonogashira-Hagihara reaction
The Sonogashira-Hagihara reaction can be considered one of with those of 1c
the most widely practiced strategies to synthesise alkyl and was highly chemoselective towards the C–I bond (3fa
-
1e and 1k). The reaction with haloaryl iodides
3ha).
-
aryl acetylenes as well as conjugated enynes. In this scenario, Unfortunately, this catalyst was confirmed to be less efficient
palladium has always occupied a leading position as the in the coupling of aryl bromides (1i and 1j) under the standard
catalytic metal.17 Latterly, copper has emerged as
a
conditions. The electronic effect on the more reluctant to react
competitive cheaper alternative to palladium, providing in substituted arylacetylenes was also analysed by reaction with
many cases comparable results with simple catalytic systems.18 4-iodobenzonitrile (1c): the lower the electron-rich character
In order to investigate the effect of the nature of the support of the arylacetylene, the better the yield obtained (3cb
in the Cu-catalysed Sonogashira coupling, 4-iodoanisole (1a The heterogeneous nature of CuNPs/ZY facilitated its recovery
and phenylacetylene 2a were chosen as the model by centrifugation and recycling. Indeed, the catalyst showed an
substrates. A preliminary screening which considered the excellent performance when reused in four consecutive cycles,
solvent, base, temperature and catalyst loading as the in the standard reaction of 4-iodoanisole 1a and
-3cd).
)
(
)
(
)
variables, allowed to conclude that reactions carried out in phenylacetylene (2a) (Figure 2), with no discernible variation in
DMF at 120 ºC using K2CO3 as the base and 5 mol% copper the particle size after the fourth cycle (Figure S5, ESI). The hot
loading were appropriate conditions for comparative purposes filtration test after the first run disclosed a leaching of 0.14%
of all catalysts. As expected, the catalytic activity of the of the original copper content (0.01% Cu after the fourth run),
nanoparticles was found to depend on the nature of the as determined by ICP-MS; this leached copper was found to be
support with significant differences and the following
catalytically inactive.
Fig. 2 Recycling of CuNPs/ZY in the synthesis of 3aa.
By comparing the catalytic activity of CuNPs/ZY with that of
other catalysts reported in the literature, we can conclude that
its activity and that reported by Rothenberg et al. using copper
clusters stabilised by tetra-butylammonium acetate (5 mol%
Cu, DMF, 110 ºC, 24 h) are alike,19a e.g., similar reactivity
towards various aryl halides. Commercial nano-CuO19b
manifested lower activity than CuNPs/ZY, given the higher
catalyst loading and temperature required for the coupling of
aryl iodides (10 mol%, DMSO, 160 ºC, 12 h). None of the two
Fig. 1 Sonogashira coupling of 1a and 2a catalysed by CuNPs on different supports; 1a
(0.25 mmol), 2a (1.5 equiv.), catalyst (5 mol% Cu), K2CO3 (2 equiv.), DMF (1 mL), 120 ºC,
4 or 8 h, Ar; yield of 3aa determined by 1H NMR.
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Table 1 The coupling of aryl halides and arylacetylenes catalysed by CuNPs/ZY.a
aforementioned catalysts were reutilised. CuNPs/ZY was not
active in the reaction of aryl chlorides with acetylenes, in
View Article Online
DOI: 10.1039/C7CY01343D
contrast with Cu(0)NPs/Al2O3 which could catalyse this
reaction at room temperature.7a The absence of the oxide film
on the copper surface seems to be crucial for this enhanced
reactivity. Indeed, CuO/Al2O3 not only exhibited relatively
lower activity but also was evinced to be non-recyclable, with a
copper loss of 63% after the first cycle.19c Most importantly,
CuNPs/ZY is a clear alternative to PdNPs stabilised by a tris-
imidazolium salt20a and PdNPs/DNA,20b previously described by
us, though the latter was far more active in the reaction with
aliphatic alkynes under milder conditions.
Aryl halide
t (h)
Product
Yield (%)b
We believe that every laboratory-made catalyst should be
more efficient than the commercial catalysts used for the
same purpose in order to justify the time, materials and
human resources employed during its preparation. With this
principle in mind, we compared the catalytic activity of
CuNPs/ZY with that of a wide variety of commercial copper
sources in the coupling reaction of 4-iodoanisole (1a) and
phenylacetylene
(2a) (Table 2). We were delighted to
demonstrate that our catalyst was distinctly superior to the
commercial catalysts tested. Only CuOTf and Cu(OTf)2, the
most expensive substances in Table 2, furnished the coupling
product in moderate conversion albeit with the concomitant
formation of substantial amounts of the alkyne homocoupling
product (1,4-diphenylbuta-1,3-diyne).21 It is worthwhile
mentioning that in the case of our Sonogashira reactions the
diyne by-product, if present, was formed in negligible
amounts.
Table 2 Comparison of CuNPs/ZY with commercial copper catalysts in the Sonogashira
reaction.a
Entry
1
2
Catalyst
Cu(0)
Cu2O
Conversion (%)b
0
8
3
CuO
0
4
CuCl
5
5
CuCl2
8
6
CuBr
0
7
CuI
4
8
9
10
11
12
13
CuOAc
Cu(OAc)2
CuOTf
Cu(OTf)2
CuBr·SMe2
CuNPs/ZY
9
7
50c
67c
4
99
a
1a (0.25 mmol), 2a (1.5 equiv.), Cu catalyst (4.0 mol%) and K2CO3 (0.5 mmol),
b
DMF (1 mL), 120 ºC, Ar, 8 h. Conversion into 3aa determined by GLC based on
1a. The alkyne homocoupling side product 1,4-diphenylbuta-1,3-diyne and 3aa
c
were obtained in a ca. 1:2 ratio.
a
Aryl halide (1, 0.25 mmol), arylacetylene (2, 1.5 equiv.), CuNPs/ZY (ca. 4 mol%)
and K2CO3 (0.5 mmol), DMF (1 mL), 120 ºC, Ar. 1H NMR yield. c Reaction at 150
b
ºC. d Isolated yield.
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Table 3 The arylation of thiophenol (4a) catalysed by CuNPs/ZY.a
DOI: 10.1039/C7CY01343D
Thiol arylation
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Owing to the paramount importance of thioethers in diverse
disciplines, the transition-metal catalysed formation of C-S
bonds has attracted a great deal of attention lately.5c,22 We
also decided to explore the capability of the supported copper
nanoparticles to catalyse the thiophenol arylation; 4-
iodobenzonitrile (1c) and thiophenol (4a) were first studied as
model substrates. We were pleased to find that, irrespective of
the catalyst, the reaction afforded the coupling product in
almost quantitative yield when was carried out in DMF and
K2CO3 as the base for 2 h at 120 ºC. The presence of air or
absence of solvent had a detrimental effect in the conversion.
A lower temperature (80 ºC) allowed to better bring into
comparison the activity of the catalysts. Even so, all catalysts
manifested quite analogous activity at relatively low copper
loading but, again, CuNPs/ZY was the most active giving the
product in 91% yield after 4 h (Figure 3). The reactivity of the
catalysts followed the order CuNPs/ZY > CuNPs/MK-10 >
CuNPs/TiO2 ≥ CuNPs/C.
Aryl halide
t (h)
Product
Yield (%)b
Fig. 3 The cross coupling of 4-iodobenzonitrile (1c) and thiophenol (4a) catalysed by
CuNPs on different supports; 1c (0.25 mmol), 4a (1.5 equiv.), catalyst (1 mol% Cu),
K2CO3 (2 equiv.), DMF (1 mL), 80 ºC, Ar; yield of 5ca determined by 1H NMR.
Although 5ca was formed in 94% conversion after 4 h at 100
ºC, we decided to conduct the substrate scope at 120 ºC in
order to maximise the yield using CuNPs/ZY at ca. 0.7 mol%
catalyst loading [as determined from the Cu content (3.0 wt%)
and the Cu2O/CuO area from XPS (ca. 1:1)] (Table 3).
Thiophenol (4a) was successfully coupled with a series of aryl
iodides bearing electron-donating and -withdrawing groups as
well as halogens. As expected, 4-iodoanisole (1a) reacted more
sluggishly giving rise to 5aa in moderate yield, whereas
excellent yield was recorded for 4-iodotoluene (1b). Opposite
behaviour was noticed for 4-iodobenzonitrile (1c) which
furnished the expected diaryl thioether 5ac in only 2 h. The
halogenated iodides 1f and 1g reacted chemoselectively
towards the activation of the C–I bond, producing the
a Aryl halide (1, 0.25 mmol), thiophenol (4a, 1.5 equiv.), CuNPs/ZY (0.7 mol%) and
K2CO3 (0.5 mmol) in DMF (1 mL) at 120 ºC under Ar. b 1H NMR yield.
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Table 4 The thiolation of 4-iodobenzonitrile (1c) catalysed by CuNPs/ZY.a
corresponding 4-halogenated thioethers (5fa andVi5ewgaA)rticinle Onnelianer
quantitative yield. The presence of electrDoOn-I:d1o0n.1a0t3i9n/gC7gCrYo0u1p34s3iDn
aryl bromides and chlorides (1l–1j) made them practically
unreactive under these reaction conditions. To our surprise,
aryl chlorides bearing electron-withdrawing substituents at the
4- or 2-positions (1o–1r) participated in this reaction with high
Thiol
Product
Yield (%)b
efficiency, giving the corresponding coupling products (5oa
5ra) in high yields after 2 h.
–
The same procedure at a slightly lower temperature (100 ºC)
was extended to thiols other than thiophenol, including other
aromatic (4b-4d), heteroaromatic (4e-4f) and benzylic (4g)
thiols; 4-iodobenzonitrile (1c) was selected as a common
partner to furnish the expected thioethers in moderate-to-
excellent yields in relatively low reaction time (4 h) (Table 4).
Furthermore, thiolation was feasible for all types of aliphatic
thiols, i.e., linear-alkyl (4h), branched (4i) and cyclic thiols (4j);
a stronger base (KOH) is, by any means, recommended for
upgrading the yields. It is noteworthy that 2,2-dimethyl-
ethanethiol (4i) reacted quantitatively towards the alkylthio
benzonitrile 5ci under milder conditions (70 ºC), whereas the
corresponding amide (5si) was selectively formed at 120 ºC.
Notwithstanding the considerable amount of studies on CuNPs
as catalysts for C–S bond formation,7d,23 they are generally
applied to the coupling of thiols with aryl iodides. The coupling
with aryl chlorides is limited to a few examples,7d,23f,23g
normally, chloroaromatics bearing electron-withdrawing
groups which are coupled with thiophenols.7,23f In other cases,
the coupling with electron-rich iodides (e.g., 4-iodoanisole)
was documented to be troublesome,7,23a whereas very seldom
the catalyst was not reusable.23b In our case, the nature of the
CuNPs/ZY differs from that in the literature examples and
leads to a catalytic activity between that of Cu(0)/Cu(I)NPs7d
and CuONPs,23f,g albeit the performance with aliphatic thiols is
unknown in some cases.7d,23f
The recycling studies in the coupling of 4-iodobenzonitrile (1c
)
and thiophenol (4a) followed a pattern resembling that in the
Sonogashira reaction, i.e., the catalyst could be reused in four
consecutive runs with no apparent decrease in the catalytic
activity (Figure 4). The hot filtration test brought forth 0.12%
and 0.02% copper leaching after the first and fourth runs,
respectively, which are very close to those observed in the
Sonogashira reaction and were also catalytically inert.
a 4-Iodobenzonitrile (1c, 0.5 mmol), thiol (4, 1.5 equiv.), CuNPs/ZY (0.7 mol%) and
K2CO3 (1.0 mmol) in DMF (2 mL) at 100 ºC for 4 h under Ar, unless otherwise
b
c
d
stated. Isolated yield. Reaction using KOH as the base (1.0 mmol) at 120 ºC.
Reaction using KOH as the base (1.0 mmol) at 70 ºC.
Fig. 4 Recycling of CuNPs/ZY in the synthesis of sulfide 5ca.
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Apparently, the nanoparticle-support interaction in CuNPs/ZY heterocycles25a,c,f but also a transformation applicaVibewleArttoiclelaOrngliene-
3b,25d
DOI: 10.1039/C7CY01343D
is independent of the type of reaction implemented, even scale production in the pharmaceutical industry.
Although
when they are so different. As occurred in the Sonogashira most of the research has been focused on homogeneous
reaction, particle agglomeration was not noticeable after reuse catalytic heterogeneous copper-catalysed
systems,25e
(Figure S5, ESI).
processes, including copper nanoparticles,5a,c have gained
Recently, in an interesting research, Ananikov et al. have more attention in recent times.
revealed that the unsupported copper oxide-catalysed Recently, the preparation and use of CuNPs/MagSilica in the N-
coupling of aryl halides and thiols takes place through leaching arylation of imidazole has been reported.26 This catalyst
from the surface involving the formation of a copper thiolate.24 successfully catalysed the N-(hetero)arylation of imidazole
In contrast with our study, the leached copper species were with (hetero)aryl bromides and iodides but efforts to apply
found to be catalytically active.
CuNPs/MagSilica to the arylation of other azoles (pyrazole,
The catalytic activity of CuNPs/ZY was compared with that of benzotriazole and indole) were unfruitful. Herein, we have
Cu(0) and an array of commercial Cu(I) and Cu(II) catalysts in deployed CuNPs on other supports in order to arylate a set of
the reaction of 4-iodobenzonitrile (1c) and thiophenol (4a
)
azoles [imidazole (6a), pyrazole (6b), benzimidazole (6c) and
(Table 5). The heterogeneous catalysts, Cu(0), Cu2O and CuO indole (6d)] and compare their catalytic activity.
gave the lowest conversions into the thioether 5ca with the The reactions were implemented using 4-iodobenzonitrile (1c
)
concurrent and abundant formation of the corresponding as a common coupling partner, under equivalent conditions to
disulfide (Table 5, entries 1–3). CuCl2 (our copper source to those for the previous N-arylation of imidazole:26 catalyst (5
generate the CuNPs) was the best one within the copper mol% Cu), DMF, K2CO3 as the base at 120 ºC (instead of 152
halides, though the conversion was only moderate (Table 5, ºC); the product yields were determined after 16 hours. The
entries 4–7). However, good-to-high conversions were reactions proceeded in a selective manner, i.e., the aryl iodide
recorded for Cu(OAc)2, CuOTf, Cu(OTf)2 and CuBr·SMe2 (Table was exclusively converted into the product with no side
5, entries 9–12). Nonetheless, CuNPs/ZY can be considered the reactions (Figure 5). The four catalysts were proven to be
best choice for this reaction because it led to the highest active in the N-arylation of all azoles but exhibiting different
conversion and the catalyst is recyclable (Table 5, entry 13).
activity. Contrary to the copper-catalysed Sonogashira reaction
In general, the thiol arylation seems to be less dependent on and thiol arylation, in this case, CuNPs/TiO2 was the most
the support, oxidation state and source of Cu (Figure 3 and active catalyst for all the studied azoles. Their general catalytic
Table 5) when put alongside of the Sonogashira reaction activity follows the sequence Cu/TiO2 > Cu/C ≈ Cu/MK-10 >
(Figure 1 and Table 2).
Cu/ZY.
Table
5 Comparison of CuNPs/ZY with commercial copper catalysts in the thiol
arylation.a
Entry
1
2
3
4
5
6
7
8
Catalyst
Cu(0)
Cu2O
CuO
CuCl
CuCl2
CuBr
CuI
CuOAc
Cu(OAc)2
CuOTf
Cu(OTf)2
CuBr·SMe2
CuNPs/ZY
Conversion (%)b
25
23
32
54
64
38
50
63
91
77
82
77
94
9
10
11
12
13
a 1c (0.25 mmol), 4a (1.5 equiv.), Cu catalyst (1 mol%) and K2CO3 (0.5 mmol), DMF
(1 mL), 100 ºC, Ar, 4 h. b Conversion into 5ca determined by GLC based on 1c.
Arylation of azoles
The formation of C(aryl)-N bonds by the coupling of aryl
halides and nitrogen nucleophiles has become one of the most
studied copper-catalysed reactions in the last ten years.2,25
This is not only a valuable tool for the synthesis of
Fig.
5 Copper-catalysed coupling of nitrogen-containing heterocycles with 4-
iodobenzonitrile (1c); 1c (0.25 mmol), 6 (1.2 equiv.), catalyst (5 mol% Cu) and K2CO3
(0.5 mmol), DMF (1 mL), 120 ºC, Ar, 16 h; 1H NMR yield.
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Pyrazole (6b) was found to be most reactive azole, producing An attempt to recycle of the most active catalyst V(iСewuNArPticsle/TOinOlin2e)
yields in the range of 75–82%, whereas indole was the least in the coupling of imidazole (6a) with 4D-iOoId: 1o0b.1e0n3z9o/Cn7iCtrYil0e13(413cD
)
reactive one (36–59%). It is worthy of note that CuNPs/TiO2 was unsuccessful: a significant decrease in the product yield
clearly outmatched the other catalysts in the reaction with was observed when the catalyst was reused with K2CO3 as the
imidazole (6a); taking into account that it is mainly composed base (Figure 6). As aforesaid, Cs2CO3 usage increased the
of Cu2O, the real catalyst loading was ca. 1.6 mol%. Therefore, product yield in the first cycle, but a decrease after recycling
the nature of the support seems to exert an influence not only also took place (from 99 to 83%). A similar trend was observed
on the activity of CuNPs but also on the reactivity of the azoles. for ZY and MK-10 (65 and 78% in the second run, respectively);
Notably, quantitative yields of the N-arylated imidazole were the support seems to influence not only the catalytic activity
reached when employing Cs2CO3 as a base instead of K2CO3, but also the possibility of recycling. It was gratifying, however,
irrespective of the catalyst utilised (Figure 6).
to check that CuNPs/C led to a quantitative yield that was
The arylation of azoles with aryl halides has been effected with preserved in three cycles; a yield decrease was observed only
copper nanoparticles in different oxidation states in the fourth cycle (61%) (Figure 6).
[Cu(0)NPs,27a-d CuINPs,27e Cu2ONPs27f,g and CuONPs27h-l]. The leaching issue was assessed for CuNPs/TiO2 and CuNPs/C
Nevertheless, some mechanistic studies consider that Cu(I) in the reaction of 4-iodobenzonitrile (1c) and imidazole (6a).
species are implicated in the first steps of the catalytic cycles.28 Using CuNPs/TiO2 as catalyst, 44% and 47% conversions were
This could be a reason whereby CuNPs/TiO2, mainly composed noted in the first run after 4 h (with catalyst) and 16 h (prior
of Cu2O, displayed better catalytic activity than the other catalyst removal by hot filtration after 4 h), respectively. The
catalysts, made of mixtures of Cu2O and CuO or of CuO.
copper content in the filtrate was determined to be 0.02 wt%
of the original amount. In the case of CuNPs/C, the leaching
was also marginal in both the second (0.005 wt%) and fourth
cycles (0.05 wt%). The negligible, catalytically inactive, leaching
detected with CuNPs/TiO2 reveals a quite strong metal-support
interaction in the catalyst, with no notable change in the latter
after reuse (Figures S5, S6, ESI). These facts point to a possible
poisoning effect as the main reason for partial deactivation
and yield depletion upon recycling CuNPs/TiO2. Metal oxides
possess surface acid-base properties, which can facilitate the
adsorption and accumulation of heteroatom-containing
species. Conversely, CuNPs/C, with the less reactive charcoal
surface, must interact more weakly with those species,
allowing its efficient reuse in several cycles until certain
saturation occurs, with the concomitant yield attenuation.
XPS analysis on reused CuNPs/TiO2 at the N 1s level brought
into view two peaks at 398.6 and 400.2 eV (Figure 7). These
peaks are consistent with those displayed by fresh CuNPs/TiO2
impregnated with 4-iodobenzonitrile (398.8 and 400.4 eV)
(Figure S7, ESI) and with that described for imidazole (400.2
eV).29 This reinforces the hypothesis of the starting materials
being strongly adsorbed on the TiO2 surface and their
poisoning effect upon reuse.
CuNPs/C
Fig. 6 Recycling of CuNPs/support in the synthesis of 7ca.
Fig. 7 XPS spectrum at the N 1s level of reused CuNPs/TiO2.
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As in the previous coupling reactions, we also compared the conversion in the coupling of 4-iodobenzonitrile with
View Article Online
catalytic activity of CuNPs/TiO2 with that of the same imidazole, pyrazole, benzimidazole and iDnOdIo: l1e0,.1u03s9in/Cg7KC2YC0O13343aDs
commercial copper catalysts as above in the arylation of the base. Anyhow, all catalysts reached quantitative yield
imidazole (6a) with 4-iodobenzonitrile (1c) (Table 6). It is worth when changing K2CO3 into Cs2CO3 as the base.
noting that Cu2O and CuO, which were rather inactive in both The comparative study has been extended to a collection of
the Sonogashira and thiol arylation reactions, led to twelve commercial copper catalysts: only CuOTf and Cu(OTf)2
conversions around 75% (Table 6, entries 2 and 3); the maintained a moderate-to-good activity in the three reactions
behaviour of the Cu(I) and Cu(II) triflates was akin to that of examined (50-82% conversion). The rest of the catalysts failed
the Cu oxides (Table 6, entries 10 and 11). Still, once more, the in the Sonogashira reaction (< 9% conversion), whereas
nanostructured catalyst got the highest performance with a CuBr·SMe2 and Cu(OAc)2 in the thiol arylation, and Cu2O and
quantitative conversion (Table 6, entry 13).
CuO in the azole arylation gave comparable results to those
attained with the copper(I) and (II) triflates. At any rate, in
general, the nanoparticulate supported catalysts are markedly
superior to the commercial catalysts in terms of catalytic
activity and reusability: they can be reused in four
(Sonogashira and thiol arylation) and three (azole arylation)
cycles with no loss of activity. The negative filtration test and
insignificant leaching lend weight to the argument that the
catalysis is heterogeneous, taking place at the nanoparticle
surface; the possibility of leached Cu from the CuNPs which get
into a homogeneous catalytic cycle can, practically, be ruled
out. Hence, taking into account that the catalysts are easily
prepared, the protocols introduced in this report are an
attractive alternative to the utilisation of the more expensive
palladium catalysts and the commercial (non-reusable) copper
catalysts.
Table 6 Comparison of CuNPs/ZY with commercial copper catalysts in the arylation of
azoles.a
Entry
1
Catalyst
Cu(0)
Conversion (%)b
28
74
78
44
46
18
62
25
52
78
67
55
99
2
Cu2O
3
CuO
4
CuCl
5
CuCl2
6
CuBr
Acknowledgements
7
CuI
8
9
10
11
12
13
CuOAc
Cu(OAc)2
CuOTf
Cu(OTf)2
CuBr·SMe2
CuNPs/TiO2
I. P. Beletskaya thanks the Russian Science Foundation (RSF,
grant no. 14-23-00186 P) and A. Yu. Mitrofanov thanks the
Russian Foundation for Basic Research (grant no. 16-33-60207)
for their financial support. This work was also generously
supported by the Spanish Ministerio de Economía
y
Competitividad (MINECO; grant no. CTQ-2015-66624-P) and
the Institute of Organic Synthesis (ISO). I. M.-G. thanks the ISO
and the Vicerrectorado de Investigación y Transferencia del
Conocimiento of the Universidad de Alicante for pre-doctoral
grants (no. UAFPU2016-034).
a
1c (0.25 mmol), 6a (1.2 equiv. mmol), catalyst (1.6 mol%) and Cs2CO3 (0.5
b
mmol), DMF (1 mL), 120 ºC, Ar, 16 h. Conversion into 7ca determined by GLC
based on 1c.
Conclusions
Experimental
We have presented herein a comparative survey on the
catalytic activity of four catalysts, comprised of copper
nanoparticles on different supports (zeolite Y, titania,
montmorillonite K-10 and activated carbon), in three types of
reactions: the Sonogashira reaction, the arylation of thiols and
the arylation of azoles. CuNPs/ZY was the most effective
catalyst for the Sonogashira reaction (4 mol% catalyst, K2CO3,
DMF, 120 ºC), being applicable to aryl iodides bearing either
electron-donating or –withdrawing groups; aryl bromides
reacted more sluggishly. The same catalyst at lower loading
(0.7 mol%) allowed the coupling of thiophenol with aryl
iodides of different electronic character, but also with
electron-poor aryl chlorides in near quantitative yields.
Moreover, alkyl thiols were successfully coupled when KOH
was used as the base instead of K2CO3. As regards the azole
arylation, CuNPs/TiO2 (1.6 mol%) achieved the highest
General procedure for the preparation of the catalysts. All the
supported copper catalysts handled in this work were
prepared by adding the support (titania,8a zeolite Y,8b activated
carbon14b or montmorillonite K-10) to a newly prepared
suspension of the CuNPs readily generated, in turn, by the
chemical reduction of copper(II) chloride with lithium metal
and a catalytic amount of 4,4′-di-tert-butylbiphenyl (DTBB) as
an electron carrier. In a general procedure: anhydrous
copper(II) chloride (134 mg, 1 mmol) was added to a
suspension of lithium (14 mg, 2 mmol) and DTBB (27 mg, 0.1
mmol) in THF (2 mL) at room temperature under an argon
atmosphere. The reaction mixture, which was initially dark
blue, rapidly changed to black, indicating that the suspension
of copper nanoparticles was formed. This suspension was
diluted with THF (18 mL) followed by the addition of the
8 | J. Name., 2012, 00, 1-3
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support (1.28 g). The resulting mixture was stirred for 1 h at phase was subjected to solvent evaporation under vacuum
View Article Online
1
DOI: 10.1039/C7CY01343D
room temperature, filtered, and the solid successively washed and H NMR analysis (mesitylene as the internal standard).
with water (20 mL), THF (20 mL), and dried under vacuum. The Notes and references
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mmol) and DMF (1 mL) were added to a reactor tube. The
mixture was warmed to 120 ºC under Ar and stirred for the
specified time in Table 1. The reaction crude was diluted with
EtOAc (3 mL) and filtered through a pad with Celite, followed
by extraction of the filtrate with water (3 × 3 mL) to remove
the DMF, washing with brine (4 mL) and drying with anhydrous
MgSO4. The resulting organic phase was subjected to solvent
evaporation under vacuum and to 1H NMR analysis
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K2CO3 (138 mg, 1.0 mmol) or KOH (1.0 mmol) and DMF (2 mL)
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mL) to remove the DMF and washing with brine (8 mL). The
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Page 11 of 11
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C7CY01343D
Graphical abstract
Copper nanoparticles on different supports are effective reusable catalysts for the
palladium- and ligand-free coupling of aryl halides with alkynes, thiols and azoles.