Copper nanoparticles catalyzed Se(Te)-Se(Te) bond activation: A straightforward route towards unsymmetrical organochalcogenides from boronic acids

Article abstract of DOI:10.1002/cctc.201402867

A highly porous copper metal-organic framework, [Cu₃(BTC)₂] (BTC=benzene-1,3,5-tricarboxylate) was synthesized and used as a precursor for the synthesis of copper nanoparticles (NPs) and characterized by several techniques, including XRD, SEM, TEM, EDX and BET measurements. The as-synthesized copper nanoparticles were immobilized onto activated charcoal (AC) by means of ultrasonication at room temperature without any pretreatment. The Cu NPs/AC was employed as a heterogeneous catalyst for the cross-coupling of diphenyl diselenide and boronic acids to form diphenyl selenides through Se-Se bond activation under ligand-, base-, and additive-free conditions. The copper NPs/AC, which combines the architecture of MOFs and the high surface area of charcoal, could be an efficient heterogeneous catalytic system that is compatible with a variety of substituents on diphenyl selenides. Its promising catalytic activity relative to that of other homogeneous systems and low catalyst loading for the synthesis of unsymmetrical diaryl selenides is an important application in the area of nanocatalysis. The Cu NPs/AC catalyst, which exhibits excellent catalytic activity and remarkable tolerance to a wide variety of substituents, led to Se sp³-, sp²-, and sp-carbon bond formation by using DMSO as a solvent and atmospheric air as oxidant. This approach can also be extended to the preparation of unsymmetrical organotelluride derivatives. Three-in-one catalyst: Copper nanoparticles on activated charcoal show excellent catalytic activity towards the synthesis of unsymmetrical chalcogenides, starting from boronic acids, in air under ligand-, base-, and additive-free conditions, and resulting in selenium sp³-, sp²-, and sp-carbon bond formation.

Full text of DOI:10.1002/cctc.201402867

DOI: 10.1002/cctc.201402867
Communications
Copper Nanoparticles Catalyzed Se(Te)ÀSe(Te) Bond
Activation: A Straightforward Route Towards
Unsymmetrical Organochalcogenides from Boronic Acids
Balaji Mohan, Chohye Yoon, Seongwan Jang, and Kang Hyun Park*[a]
A highly porous copper metal–organic framework, [Cu3(BTC)2]
(BTC=benzene-1,3,5-tricarboxylate) was synthesized and used
as a precursor for the synthesis of copper nanoparticles (NPs)
and characterized by several techniques, including XRD, SEM,
TEM, EDX and BET measurements. The as-synthesized copper
nanoparticles were immobilized onto activated charcoal (AC)
by means of ultrasonication at room temperature without any
pretreatment. The Cu NPs/AC was employed as a heterogene-
ous catalyst for the cross-coupling of diphenyl diselenide and
boronic acids to form diphenyl selenides through SeÀSe bond
activation under ligand-, base-, and additive-free conditions.
The copper NPs/AC, which combines the architecture of MOFs
and the high surface area of charcoal, could be an efficient
heterogeneous catalytic system that is compatible with a varie-
ty of substituents on diphenyl selenides. Its promising catalytic
activity relative to that of other homogeneous systems and
low catalyst loading for the synthesis of unsymmetrical diaryl
selenides is an important application in the area of nanocataly-
sis. The Cu NPs/AC catalyst, which exhibits excellent catalytic
activity and remarkable tolerance to a wide variety of substitu-
ents, led to Se sp3-, sp2-, and sp-carbon bond formation by
using DMSO as a solvent and atmospheric air as oxidant. This
approach can also be extended to the preparation of unsym-
metrical organotelluride derivatives.
Baylis–Hillman reaction of aldehydes and electron-deficient al-
kenes.[2–7] Over the past decade, extensive work has been de-
voted to developing new methodologies for the creation of
bonds between sp3-, sp2-, sp-carbon atoms and organochalco-
gens, especially selenium- and tellurium-based compounds. To
synthesize these moieties, a number of synthetic methods
have been reported so far.[8–22] Of these, two methods are gen-
erally used. These methods involve the use of diphenyl disele-
nide (telluride) as starting precursor owing to its stability in air,
environmental friendliness, and commercial availability. Di-
phenyl diselenide can be reacted with either aryl halides or
boronic acids to synthesize organochalcogen derivatives. Orga-
noboronic acids and their esters are commercially available
with a variety of substituents, and they are stable and non-
toxic, whereas aryl halides are difficult to activate because they
require high equivalents of a base and/or reductant as addi-
tives.[23–28]
Therefore, the coupling of diphenyl diselenide and organo-
boronic acids is an easy route to the synthesis of unsymmetri-
cal organochalcogenides. To achieve this, the use of inexpen-
sive and low-toxicity copper catalysts for these transformations
is more desirable in terms of atom economy, cost-effectiveness,
and efficiency. To date, several research groups have produced
aryl selenides by using copper salts in combination with li-
gands and other additives, along with long reaction times,
high temperatures, and high catalyst loadings. Most of these
metal-catalyzed reactions are homogeneous; therefore, the
catalyst cannot be recycled. This factor may increase the cost
and limit the scope of applications. It is highly desirable to de-
velop a new catalytic system that is tolerable wide range of
functional groups in the absence of base, reducing agent, and
other additives.
The field of nanocatalysis[29–31] (i.e., the use of nanoparticles
(NPs) to catalyze reactions) has experienced explosive growth
over the past decade in both homogeneous and heterogene-
ous catalysis. NPs have a larger surface-to-volume ratio than
bulk metals and low-coordinated sites, which enhances their
activity and selectivity for a variety of transformations. In par-
ticular, metal NPs immobilized on solid supports enable easier
recovery and reuse of the catalysts and have higher stability
and better dispersion of the particles. Keeping this in mind, we
decided to use activated charcoal,[32,33] which is commercially
available and cost-effective for industrial purposes, as a solid
support.
Copper catalysts for cross-coupling reactions are an intense
field of research because they are cost-effective and hold
promise for the replacement of expensive metal sources for in-
dustrial purposes. Over the past decade, the construction of
carbonÀselenium bonds has been an intriguing topic for re-
searchers and the number of related publications has in-
creased exponentially to date, mainly owing to the important
therapeutic characteristics of these bonds, such as their antimi-
crobial, antiviral, antitumor, and antioxidant properties. In addi-
tion, chalcogens are important constituents of functional
groups in biomolecules (i.e., those associated with redox
chemistry) in eukaryotic biochemistry.[1] Moreover, selenium-
based ionic liquids have proven to be efficient catalysts in re-
actions such as the carbonylation of aniline, oxidation of thiols,
syntheses of octahydroacridines and thioacetals, and the
[a] B. Mohan, C. Yoon, S. Jang, Prof. K. H. Park
Department of Chemistry and Chemistry Institute for Functional Materials
Pusan National University, Busan 609-735 (Republic of Korea)
E-mail: chemistry@pusan.ac.kr
As a continuation of our ongoing efforts to discover new
nanoparticles[34–37] as heterogeneous catalysts in organic syn-
thesis, we report herein that the copper NPs prepared from
Supporting information for this article is available on the WWW under
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[Cu3(BTC)2] show excellent catalytic activity for the cross-cou-
pling of diphenyl dichalcogenide and a variety of organobor-
onic acids, in terms of yield, short reaction time, and additive-
free conditions, by using DMSO as a solvent under an air at-
mosphere. Metal–organic frameworks (MOFs) have attracted
much attention over the past decade because of their uniform
crystalline structure, high porosity, large surface area, and high
thermal and chemical stability. The application of MOFs in gas
storage, CO2 separation, sensors, and other areas has been
well explored. Of course, there is a particular interest in the
use of MOFs as catalysts. So far, various types of MOFs with dif-
ferent architectures have been reported for catalytic organic
reactions.[38–43] Our current interest lies in a basic copper MOF,
[Cu3(BTC)2], and its application as a catalyst in the preparation
of organochalcogens.[44,45] However, there are some drawbacks
with [Cu3(BTC)2] that must be accounted for; for example, the
direct use of [Cu3(BTC)2] as a catalyst in organic transforma-
tions could require purification of the desired product because
of contamination and solubility issues with the reaction under
thermal conditions.
on charcoal as a catalyst generated diphenyl selenide as the
desired product.
Initially, diphenyl diselenide and phenyl boronic acid were
selected as substrates to determine the optimal reaction condi-
tions. The results are summarized in Table 1. The control ex-
Table 1. Optimization of the cross-coupling of diphenyl diselenide with
phenylboronic acid catalyzed by copper nanoparticles on charcoal.[a]
Entry
Solvent[b]
Catalyst
[mol% Cu]
Temp
[8C]
Yield[c]
[%]
1
2
3
4
5
6
7
8
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
–
4
4
3
4
4
4
4
4
4
4
4
4
4
4
4
100
RT
80
5
ND
32
100
100
100
100
100
100
100
reflux
100
100
100
100
reflux
100
100
100
100
100
93
100
traces[d]
97
91
89
95
15
8
3
13
7
ND
ND
84
89
74
100
Therefore, in this report, we demonstrate that the copper
NPs derived from [Cu3(BTC)2] show an excellent catalytic activi-
ty for the synthesis of organochalcogen derivatives. The as-syn-
thesized copper NPs immobilized on activated carbon (AC),
which is a simple, inexpensive and high surface area support
that can serve as an efficient heterogeneous catalyst for the
same transformation. In 2005, Wang et al.[46] first reported
a simple CuI-catalyzed synthesis of diaryl selenides and tellur-
ides from arylboronic acids. Subsequently, Taniguchi[47] used
CuI with the combination of a bipyridyl ligand to prepare a va-
riety of unsymmetrical organochalcogenides. Braga and co-
workers[48] described the use of commercially available CuO
nanopowder as a catalyst to prepare diaryl selenides. In 2009,
Ren et al.[49] explored Lewis acid InBr3-mediated direct cross-
coupling of arylboronic acids with diorganodiselenides and di-
tellurides. Ranu[50] and co-workers reported the coupling of or-
ganoboronic acids and dichalcogenides catalyzed by CuFe2O4.
The Alves group[51] found that glycerol acted as a green sol-
vent to promote the CuI-catalyzed cross-coupling of diaryl dis-
elenides with aryl boronic acids by using DMSO as an additive.
Xu and co-workers[52] demonstrated the copper-salt-catalyzed
preparation of diphenyl selenides in the presence of base and
ligand. Recently, Kumar[53] used copper-salt-catalyzed prepara-
tion of diphenyl chalcogenides in the presence of base, li-
gands, and reducing agents. Bhadra et al.[54] introduced alumi-
na-supported copper-mediated nucleophilic displacement of
phenylselenium bromide as an alternative precursor for the
synthesis of different kinds of aryl selenides. Lenardo and co-
workers[55] disclosed the use of imidazolium ionic liquids for
the synthesis of diaryl selenides from electrophilic selenium
species and nucleophilic boron reagents under a N2 atmos-
phere. Recently, transition-metal-free unsymmetrical diaryl
chalcogenides from arenes and dichalcogenides has been re-
ported by using persulfate oxidant and trifluoroacetic acid sol-
vent.[56]
DMA
NMP
9
10
11
12
13
14
15
16
17
18
19
20
21
DMPU
dioxane
[BMIM][BF4]
glycerol
PEG
EG
DMC
toluene
DMSO
DMSO
DMSO
DMSO
4
CuO hollow spheres[e]
Cu2O cubes[f]
Cu nanoparticles[g]
recovered from
entry 5
22
23
24
25
DMSO
DMSO
DMSO
DMSO
recovered from
entry 21
recovered from
entry 22
recovered from
entry 23
recovered from
entry 24
100
100
100
100
100
89
80
73
[a] Optimized Reaction Conditions: diphenyl diselenide (0.64 mmol), phe-
nylboronic acid (1.41 mmol), 4 mol% of Cu NPs/C with respect to diphen-
yl diselenide, T=1008C, t=3 h; [b] 1 mL solvent was used; [c] conversion
yields were determined from GC-MS analysis; [d] reaction under N2 at-
mosphere; [e–g] copper catalysts prepared according to previous reports.
periments showed that only a 5% yield was obtained when
the reaction was performed in the absence of the catalyst
(Table 1, entry 1), and no coupled product was detected when
the reaction was performed at room temperature for 12 h with
4 mol% of the catalyst (Table 1, entry 2). Then, we screened ex-
perimental conditions at a constant catalyst concentration of
4 mol%. Increasing the temperature from room temperature
to 808C for 3 h afforded a 32% yield (Table 1, entry 3), whereas
a lower catalyst loading reduced the product yield (Table 1,
entry 4). The highest yield (i.e., complete disappearance of di-
phenyl diselenide without any side products according to GC-
MS analysis) was achieved with 4 mol% of copper nanoparti-
The cross-coupling reaction between diphenyl diselenide
and phenyl boronic acid in DMSO solvent by using copper NPs
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cles catalyst in DMSO solvent at 1008C within a short time
(Table 1, entry 5). The same experiment in DMSO solvent with
bubbling nitrogen afforded only traces of the product (Table 1,
entry 6); therefore, air was indispensable to the reaction. Other
solvent systems, including N,N’-dimethylformamide (DMF),
N,N’-dimethylacetamide (DMA), N-methyl pyrrolidinone (NMP),
and N,N’-dimethylpropylene urea (DMPU), also provided satis-
factory results (Table 1, entries 7–10). Solvents such as dioxane,
1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4])
ionic liquid, glycerol, polyethylene glycol (PEG), ethylene glycol
(EG), dimethyl carbonate (DMC), and toluene led to diminished
yield of the product (Table 1, entries 11–17). To identify the ad-
vantages of our copper nanoparticles/charcoal catalyst, we
studied the effect of other homogeneous copper sources
(4 mol% copper content), such as CuO hollow spheres,[20] Cu2O
cubes,[21a,b] and copper NPs.[22] These reactions also proceeded
well, but did not deliver the product in as high yield as the
copper NPs synthesized from [Cu3(BTC)2] (Table 1, entries 18–
20) under the same experimental conditions. A reusability
study of the cross-coupling of diphenyl diselenide and phenyl
boronic acid was performed separately by using the optimized
conditions; the results are summarized in Table 1 (entries 21–
25).
Table 2. (Continued)
Entry Boronic acid
Product
Time [h] Yield [%][b]
6
7
6
6
94
99
8
9
6
6
100
96
10
11
6
9
99
100
Next, to further examine the substrate scope, diphenyl disel-
enide, dibenzyl diselenide, and diphenyl ditelluride with vari-
ous substituted boronic acids were examined under optimal
conditions (Table 2, Table 3, and Table 4). The cross-coupling of
diphenyl diselenide was first examined with various substitut-
ed arylboronic acids (Table 2, entries 1–15). Products with the
electron-donating groups methoxy, methyl, and 2,4,6-trimethyl
phenylboronic acids were generated in 100, 100, and 97%
yields, respectively (Table 2, entries 1–3), whereas 2,6-dime-
12
13
6
4
95
100
14
15
16
17
6
6
6
6
99
Table 2. The coupling reactions of diphenyl diselenide with various bor-
100
onic acids catalyzed by Cu NPs/C.[a]
100 (99:1)[d]
99
Entry Boronic acid
Product
Time [h] Yield [%][b]
[a] Reaction conditions: diphenyl diselenide (0.64 mmol), substituted phe-
nylboronic acid (2.2 equiv.), catalyst (4 mol% Cu), DMSO (1 mL), 1008C;
[b] yield was determined from average of two independent runs by GC-
MS. [c] trans-2-phenylvinylboronic acid, Aldrich catalogue No. 473790;
[d] selectivity (E:Z, 99:1) by GC-MS. [e] 2-phenyl-1-ethynylboronic acid pi-
nacol ester, Aldrich catalogue No. 686808.
1
3
4
6
100
100
97
2
3
thoxy phenylboronic acid was furnished in a moderate yield
(Table 2, entry 4). Strong electron-withdrawing groups, such as
naphthyl, and groups on the phenylboronic acids, including 4-
acetyl, 4-ethoxycarbonyl, 4-trifluoromethyl, 4-cyano, 4-biphen-
yl, and even 2-formyl, were well-tolerated by our catalytic
system. In the case of halogen-containing phenylboronic acids,
coupling occurred smoothly to afford the corresponding arylat-
ed phenylselenide products (Table 1, entries 12 and 13).
Heterocyclic boronic acids were also generated from their cor-
4
5
6
5
61
100
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boronic acid and 2-phenyl-1-ethynylboronic acid pinacol ester
also gave good yields in a short time under the same experi-
mental conditions as shown in Table 2 (Table 3, entries 2 and
3).
Table 3. Catalyst efficiency studies with dibenzyl diselenide and substi-
tuted boronic acids and esters catalyzed by Cu NPs/C.[a]
Prompted by these outstanding results, we discovered that
our catalyst was applicable to the synthesis of a variety of tel-
luride derivatives at room temperature (Table 4). Firstly, the cat-
alytic activity was tested in a reaction of diphenyl ditelluride
with sterically hindered and electron-withdrawing 2-formylphe-
nylboronic acid at room temperature for 18 h, which afforded
phenyl(2-formylphenyl)tellane with complete conversion and
no side products (Table 4, entry 1). An alternative coupling
partner, trans-2-phenylvinylboronic acid, also provided the cou-
pled product in excellent yield at room temperature with com-
plete trans selectivity (Table 4, entry 2). The yield was reduced
to 30% when 2-phenyl-1-ethynylboronic acid pinacol ester was
reacted at room temperature; however, the yield increased
considerably to 100% when the reaction was performed at
1008C for 6 h (Table 4, entry 3). To the best of our knowledge,
this is the first report of the synthesis of organotellurium deriv-
atives from boronic acids at room temperature without any ad-
ditives.
Entry Boronic acid
Product
Time [h] Yield [%]
1
5
94
2
3
5
6
100 (100)[b]
100
[a] Reaction conditions: same as in Table 2; [b] selectivity (100% E isomer)
by GC-MS.
[Cu3(BTC)2] was synthesized by a solvothermal method ac-
Table 4. Extension of catalytic application to reactions of diphenyl ditel-
cording to
a
literature procedure. The as-synthesized
luride[a]
[Cu3(BTC)2] was used as a precursor for the preparation of
copper NPs by using a chemical-reduction technique. The ob-
tained Cu NPs were characterized by energy-dispersive spec-
trometry (EDS), TEM, XRD, and X-ray photoelectron spectrosco-
py (XPS). Nitrogen adsorption–desorption isotherm measure-
ments were performed on a Tristar II 3020 V1.03 at À1968C.
The BET equation was used to determine the specific surface
area, pore volume, and pore size of the Cu NPs/AC.
Entry Boronic acid
1
Product
Time [h] Yield [%]
The EDS measurement shown in Figure 1 (with inset SEM
image) indicated the presence of a small amount of oxygen as
well as elemental copper in the prepared copper NPs, this find-
ing might indicate an oxidized layer of copper on the surfaces
of the particles. The TEM images show that the surfaces of the
Cu NPs were coated with Cu2O, which is probably because of
oxidation of copper NPs in air. The particles were regular poly-
hedral shape and almost monodisperse, some particles ap-
peared as a Cu core and a Cu2O shell formed from oxidized Cu
NPs (Figure 2a). TEM topography revealed that the particles
18
100
2
3
18
18
100 (100)[b]
30, 100[c]
[a] Reaction conditions: diphenyl ditelluride (0.48 mmol), substituted bor-
onic acid or ester (2.2 equiv.), catalyst (4 mol% Cu) with respect to di-
phenyl ditelluride, DMSO (1 mL), RT, 18 h; [b] Selectivity (100% E isomer);
[c] Reaction at 1008C, 6 h.
responding cross-coupled product in excellent yields (Table 2,
entries 14 and 15). Remarkably, our catalytic system was also
highly compatible with trans-2-phenylvinylboronic acid and
the 2-phenyl-1-ethynylboronic acid pinacol ester afforded (E)
and (Z)-(styryl)(phenyl)selenide (E/Z, 99:1) and phenyl(phenyl-
ethynyl)selane in 99 and 100% yields, respectively (Table 2, en-
tries 16 and 17).
The applicability of this methodology was also extended to
dibenzyl diselenide instead of diphenyl diselenide for the reac-
tion with 4-methoxyphenylboronic acid. Surprisingly, the reac-
tion supplied the cross-coupled product in an excellent yield
(Table 3, entry 1). Similarly, reactions with trans-2-phenylvinyl-
Figure 1. EDS analysis of Cu NPs with an inset SEM image (scale
bar=10 mm).
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Figure 3. XRD pattern of a) Cu NPs synthesized from [Cu3(BTC)2], b,c) corres-
ponds to bulk copper and Cu2O for reference.
Figure 2. a) Cu2O-coated Cu NPs, b) size-distribution graph of nanoparticles,
c) copper nanoparticles immobilized on charcoal d) HRTEM image of Cu
NPs/C.
were approximately 12.5 nm (Figure 2b) and immobilized on
activated charcoal (Figure 2c). The high-magnification TEM
image of Cu NPs/AC (Figure 2d) revealed that the NPs were
well-dispersed on the charcoal. The powder XRD pattern of
these precipitates showed the presence of metallic copper.
The average grain size (D) of the NPs was calculated from the
X-ray diffraction data by using Scherrer’s formula: D=0.9k/
(BcosqB), in which k is the X-ray wavelength used for the dif-
fraction experiments (Cu-Ka =1.5406 ꢁ), B is the full-width at
half-maximum of a particular peak (in radians), and qB is the
peak position. For the (111) peak, we found the grain size
normal to the scattering plane to be approximately 9.6 nm.
The X-ray diffraction pattern shown in Figure 3 reveals that
a Cu2O shell was formed by oxidation of the copper core in air.
The three main peaks at q=43.2, 50.3, and 74.18 were as-
signed to the reflections of the (111), (002), and (022) planes in
the Cu phase (JCPDS No. 04-0836, Figure 3a) together with
small peaks, corresponding to (111) and (220) planes in the
Cu2O phase. Bulk copper (Figure 3b) and Cu2O (JCPDS No. 77-
0199, Figure 3c) are shown for comparison.
Figure 4. XPS spectrum of the copper nanoparticles.
The XPS spectrum of Cu NPs is depicted in Figure 4. As ex-
pected, Cu NPs display two main peaks at 932.8 and 953.7 eV,
which are due to the binding energy of Cu 2p3/2 and Cu 2p1/2,
respectively. The difference between binding energies of Cu
and Cu2O is very small and differs only by 0.1–0.2 eV. Therefore,
the accurate determination and differentiation of Cu and Cu2O
seems to be impossible by XPS analysis.
Figure 5. N2-adsorption–desorption isotherm of copper nanoparticles/AC.
The BET surface area of Cu NPs/AC was found to be
654.0 m2 gÀ1 and the desorption branch of isotherm was used
for the Barrett-Joyner-Halenda (BJH) calculation to measure
pore volume and pore size. The values are 0.268 cm3 gÀ1 and
3.9 nm. The N2-sorption isotherm of the Cu NPs/AC is depicted
in Figure 5. It can be seen that copper nanoparticles on char-
coal exhibit a Langmuir type IV isotherm, which corresponds
to high-surface-area mesoporous materials. The high catalytic
activity of copper nanoparticles on charcoal can be ascribed to
the high surface area, enriched dispersion, and sufficient ad-
sorption of reactants, which leads to the enhancement of cata-
lytic performance.
The heterogeneity of Cu NPs/AC was examined by using
a hot-filtration test and ICP analysis. The reaction between di-
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phenyl diselenide and 2-formylphenylboronic acid was chosen
for the hot-filtration test. Copper leaching into the hot filtrate
was determined to be approximately 0.0005% by inductively
coupled plasma–optical emission spectrometry (ICP-OES) anal-
ysis. The reaction was stopped at 43% conversion and filtered.
The reaction mixture was divided into two portions. Continua-
tion of the reaction in two portions under the same conditions
at 1008C for a further 6 h resulted in 99% (GC-MS) conversion
of the portion containing Cu NPs/AC; in the absence of cata-
lyst the conversion reached only 49% (from the original 43%).
Thus, the catalytic reaction should be considered to be pre-
dominantly heterogeneous.
Figure 7. EDS spectrum of the intermediate.
To confirm the necessity and role of copper in promoting
the cross-coupling, we reacted dibenzyl diselenide with excess
CuNPs/AC (1.2 equiv.) in the absence of phenylboronic acid
under the same experimental conditions. During the reaction,
a solid intermediate, [(PhCH2Se)2-Cu)], precipitated and was
isolated by centrifugation using ethanol several times. The GC-
MS analysis of the combined supernatant solution (after centri-
fugation) revealed the complete absence of dibenzyl disele-
nide starting material. XPS analysis of the intermediate, as
shown in Figure 6, suggested the presence of a bond between
Scheme 1. Coupling of intermediate with phenylboronic acid.
Figure 8. Recycle performance of copper nanoparticles in the cross-coupling
reaction of diphenyl diselenide with phenylboronic acid.
Figure 6. XPS spectrum of the intermediate.
dropped slightly in the subsequent runs (Table 1, entries 23–
25). The decreased catalytic activity was because of oxidation
of copper NPs in air, which was indispensable in the reaction
because it acted as an oxidant. It should be noted that the re-
action flushed with nitrogen rendered only traces of the cou-
pled product (Table 1, entry 6). In addition, a TEM image of Cu
NPs/AC after three uses is shown in Figure 9. The image shows
severe aggregation of the Cu nanoparticles at high reaction
temperature, suggesting that the decreased catalytic activity
may largely be due to the agglomeration of copper nanoparti-
cles into larger colloids.
copper (932.7, 952.7 eV) and selenium (53.6 eV). The intermedi-
ate was insoluble in all solvents, which inhibited the additional
characterization and separation of the solid intermediate from
unreacted CuNPs/AC. The EDS analysis further confirmed that
the intermediate contained a CuÀSe bond (Figure 7). Surpris-
ingly, further reaction of the solid intermediate with phenylbor-
onic acid in DMSO solvent provided only 30% of the cross-
coupled product (benzyl phenyl selenide, Scheme 1). The same
observation for sulfides was noticed and reported by Taniguchi
in 2007; he proposed that the yield was reduced because of in-
sufficient oxidant or poor solubility and could be improved, to
some extent, through the use of additives.[47]
In conclusion, highly catalytically active copper NPs were
synthesized without the use of surfactants and stabilizers by
the reaction of [Cu3(BTC)2] and NaBH4 by a simple chemical-re-
duction method and characterized by XRD, TEM, SEM, and XPS
analyses. These inexpensive nanoparticles were used as an effi-
cient heterogeneous catalyst for the cross-coupling reaction of
diphenyl dichalcogenides with boronic acids to generate multi-
purpose metalÀcarbon bonds in the absence of additives. The
The recyclability of the catalyst was assessed for the cou-
pling reaction between diphenyl diselenide and phenylboronic
acid to produce diphenylselenide at 1008C with 4 mol% of Cu
(Figure 8). The first two runs of the reaction provided complete
conversion (Table 1, entries 21 and 22), but the catalytic activity
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with diethyl ether, and filtered through a Celite bed, the filtrate
was washed with water and KOH solution (to remove excess phe-
nylboronic acid). The solution was extracted with ether (and
washed with brine wash when necessary); the combined ether
layer was then dried over anhydrous magnesium sulfate, filtered,
evaporated by using a rotary evaporator, and dried under high
vacuum. The crude product was subjected to GC-MS analysis.
Synthesis of [Cu3(BTC)2]
[Cu3(BTC)2] was synthesized by using a solvothermal method from
slightly modified literature procedure.[61] Typically, a mixture of
Cu(NO3)2·2.5H2O (10 g, 43 mmol) and H3BTC (5 g, 23 mmol) was
dissolved in a mixture of solvents DMF/EtOH/H2O (250 mL, 1:1:1)
and stirred for 10 min. The mixture was heated at 858C for 20 h
and, after cooling to RT, underwent centrifugation for 20 min at
10000 rpm. Then, the solid product was removed by decanting the
mother liquor and washed with DMF, followed by dichloromethane
three times in three days. The product was then dried under high
vacuum to yield blue crystals.
Figure 9. TEM analysis of copper nanoparticles/AC after three cycles.
nanoparticles were three-in-one catalysts owing to the forma-
tion of Se(Te)-sp3, Se(Te)-sp2, and Se(Te)-sp bonds. The copper
nanoparticles from Cu-MOF offered higher catalytic activity
than CuO hollow spheres, Cu2O nanocubes, and Cu NPs. It
should be noted that the cross-coupling reaction of diphenyl
dichalcogenide and boronic acids could only proceed in the
presence of the solid copper nanoparticles catalyst, and there
was no participation from leached copper species present in
the reaction solution. The protocol offers significant advantag-
es, such as simple synthesis of the catalyst, mild reaction con-
ditions, compatibility for various substrates, and reusability of
the catalysts.
Synthesis of copper nanoparticles/AC
Freshly prepared NaBH4 (0.312 g, 82.66 mmol) in water (5 mL) was
added dropwise to [Cu3(BTC)2] (0.5 g, 0.8266 mmol) in water
(20 mL) under cooling and stirring for 10 min. Stirring was contin-
ued for another 30 min, followed by centrifugation at 10000 rpm
for 20 min. After decanting the mother liquor, the obtained solid
black precipitate was copiously washed with deionized water and
then ethanol, dried under high vacuum to afford black powder.
The black powder was added to the mixture of ethanol and acti-
vated charcoal, followed by ultrasonication at RT for 30 min and
then heated at reflux temperature for 6 h under vigorous magnetic
stirring. Next, the particles/AC was centrifuged three times with
ethanol at 10000 rpm for 20 min. The immobilized particles on ac-
tivated charcoal were dried under vacuum. The copper loading on
activated charcoal was approximately 33.1% measured by ICP-OES
analysis.
Experimental Section
Materials and instrumentation
All commercial reagents and starting materials were purchased
from Sigma–Aldrich and TCI and used without further purification.
The products reported are known and their mass spectra were
confirmed by using Shimadzu GCMS-QP 2010 Ultra (Pusan National
University). TEM samples (FEI, Tecnai F30 Super-Twin, National
Nanofab Center, South Korea) were prepared by placing a few
drops of the corresponding colloidal solution on carbon-coated
copper grids (200 mesh, F/C coated, Ted Pella Inc., Redding, CA,
USA). SEM was performed by using a SEM (VEGA3, TESCAN). The
elemental analysis was performed by using EDS (550i, IXRF Sys-
tems, Inc.). The XRD patterns were recorded by a Rigaku GDX-
11P3A diffractometer (Pusan National University). N2-adsorption iso-
therm was measured at À1968C on a Tristar II 3020 V1.03 machine.
ICP-OES analysis was determined by using Thermoscientific icap
6300 ICP spectrometer.
Acknowledgements
This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Science, ICT
&
Future Planning
(No.2013R1A1A1A05006634) and the Human Resource Training
Project for Regional Innovation (No. 2012H1B8A2026225). K. H. P
thanks the TJ Park Junior Faculty Fellowship and LG Yonam
Foundation.
Keywords: copper · cross-coupling · diphenyl dichalcogenide ·
heterogeneous catalysis · nanoparticles
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Published online on && &&, 0000
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B. Mohan, C. Yoon, S. Jang, K. H. Park*
&& – &&
Copper Nanoparticles Catalyzed
Se(Te)ÀSe(Te) Bond Activation: A
Straightforward Route Towards
Unsymmetrical Organochalcogenides
from Boronic Acids
Three-in-one catalyst: Copper nanopar-
ticles on activated charcoal show excel-
lent catalytic activity towards the syn-
thesis of unsymmetrical chalcogenides,
starting from boronic acids, in air under
ligand-, base-, and additive-free condi-
tions, and resulting in selenium sp3-,
sp2-, and sp-carbon bond formation.
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