Scope and reaction mechanism of an aerobic oxidative alkyne homocoupling catalyzed by a di-copper-substituted silicotungstate

Article abstract of DOI:10.1016/j.cattod.2010.02.073

The di-copper-substituted γ-Keggin-type silicotungstate TBA ₄[γ-H₂SiW₁₀O₃₆Cu ₂(μ-1,1-N₃)₂] (I, TBA = tetra-n- butylammonium) could act as an efficient reusable homogeneous catalyst for the aerobic oxidative alkyne homocoupling. Various kinds of structurally diverse terminal alkynes including aromatic, heteroaromatic, aliphatic, double bond-containing, silylacetylene, propargylic alcohol, and propargylic amine derivatives could selectively be converted into the corresponding diynes in the presence of I. The catalytic activity of I was much higher than those of the mono-copper-substituted silicotungstate, monomeric copper complexes, and simple copper salts, showing that the di-copper core in I plays an important role in the present alkyne homocoupling. The reaction mechanism involving the formation of the di-copper(II)-alkynyl intermediate, reductive elimination of a diyne, and re-oxidation of reduced copper species by O₂ has been proposed.

Full text of DOI:10.1016/j.cattod.2010.02.073

Catalysis Today 157 (2010) 359–363
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Scope and reaction mechanism of an aerobic oxidative alkyne homocoupling
catalyzed by a di-copper-substituted silicotungstate
Noritaka Mizunoa,b,∗, Keigo Kamataa,b, Yoshinao Nakagawaa, Takamichi Oishia, Kazuya Yamaguchia,b
a Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
b Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 24 April 2010
The di-copper-substituted ␥-Keggin-type silicotungstate TBA4[␥-H2SiW10O36Cu2(␮-1,1-N3)2] (I,
TBA = tetra-n-butylammonium) could act as an efficient reusable homogeneous catalyst for the aerobic
oxidative alkyne homocoupling. Various kinds of structurally diverse terminal alkynes including
aromatic, heteroaromatic, aliphatic, double bond-containing, silylacetylene, propargylic alcohol, and
propargylic amine derivatives could selectively be converted into the corresponding diynes in the
presence of I. The catalytic activity of I was much higher than those of the mono-copper-substituted
silicotungstate, monomeric copper complexes, and simple copper salts, showing that the di-copper core
in I plays an important role in the present alkyne homocoupling. The reaction mechanism involving the
formation of the di-copper(II)-alkynyl intermediate, reductive elimination of a diyne, and re-oxidation
of reduced copper species by O2 has been proposed.
Keywords:
Alkyne
Copper
Homocoupling
Molecular oxygen
Polyoxometalate
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
the di-copper(II)-alkynyl intermediate, i.e., {Cu2(␮-C CR)2}, which
collapses directly to the diyne products with the formation of cop-
Diyne derivatives are a very important class of compounds and
have been used as key structural elements for natural product syn-
thesis, polymer chemistry, supramolecular chemistry, and material
science [1]. Therefore, much attention has been paid to the devel-
opment of efficient synthetic procedures for the diyne derivatives.
An alkyne homocoupling is one of the most widely used procedures
for the synthesis of symmetrical diyne derivatives. Classically, sto-
ichiometric amounts of copper salts have been used for the alkyne
homocoupling (Glaser coupling) [2]. In Hay’s [3] and Campbell and
Eglinton’s [4] systems, catalytic amounts of copper salts or com-
plexes with appropriate nitrogen bases (as ligands or solvents)
and O2 (as an oxidant of reduced copper species) have been uti-
lized [5–8]. Although the Hay’s and Eglinton’s systems show high
catalytic performance for the homocoupling of aromatic alkynes,
yields of aliphatic diynes are generally still low to moderate for
aliphatic alkynes [3–8]. Palladium salts and complexes have been
reported to be active for the homocoupling of aliphatic alkynes.
However, these systems have disadvantages of the use of expensive
palladium catalysts and the need for bases and copper co-catalysts
[9,10].
per(I) species [11]. The reduced copper species can be re-oxidized
by O2. The reaction mechanism proposed by Bohlmann and co-
workers has widely been accepted [1]. Although it is expected
that the alkyne homocoupling should efficiently be promoted by
the presence of catalysts with di-copper(II) sites on the basis of
Bohlmann’s mechanism, an alkyne homocoupling catalyzed by
complexes with di-copper(II) sites has been as yet unknown.
Very recently, we have successfully synthesized the silico-
tungstate I with the di-copper core and used it for the aerobic
oxidative alkyne homocoupling [12,13]. As we expected, various
kinds of alkynes could be converted into the corresponding diynes
in high yields in the presence of I and the catalytic activity of I
was much higher than those of the mono-copper-substituted sil-
icotungstate, monomeric copper complexes, and simple copper
salts [12,13]. In this paper, we extend the scope of the previously
reported I-catalyzed alkyne homocoupling (Fig. 1) and discuss the
reaction mechanism in more detail on the basis of the catalyst
effects, spectroscopic analyses, kinetics, and DFT calculations. In
addition, the applicability of I to the 1,3-dipolar cycloaddition reac-
tions is also mentioned.
Bohlmann and co-workers proposed that the copper(II)-
catalyzed alkyne homocoupling proceeds through the formation of
2. Experimental
2.1. General
∗
Corresponding author at: Department of Applied Chemistry, School of Engineer-
ing, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
Tel.: +81 3 5841 7272; fax: +81 3 5841 7220.
Copper salts and complexes were obtained from Wako,
Kanto, or Alfa Aesar (reagent grade) and used as received. Sol-
E-mail address: tmizuno@mail.ecc.u-tokyo.ac.jp (N. Mizuno).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.02.073

360
N. Mizuno et al. / Catalysis Today 157 (2010) 359–363
culations were performed with the Gaussian03 program package
[18].
3. Results and discussion
3.1. Effects of solvents, additives, and catalysts on the alkyne
homocoupling
The effects of solvents on the I-catalyzed oxidative homocou-
pling of phenylacetylene (1a) to 1,4-diphenyl-1,3-butadiyne (1b)
were investigated (Table 1). Among the solvents examined with-
out any additives, benzonitrile gave the highest yield of 1b (entry
1). Non-polar and protic polar solvents such as 1,2-dichloroethane,
toluene, 2-propanol, and acetic acid gave low yields of 1b (entries
10–14). The present homocoupling was strongly inhibited by the
presence of water (entry 5 vs. entry 2). In the case of benzonitrile,
equimolar amounts of benzamide (formed through the hydration
of benzonitrile) with respect to the corresponding diyne 1b (water
produced) were formed as a co-product. When the homocoupling
of 1a in DMF was carried out in the presence of benzonitrile, 88%
yield of 1b was obtained (entry 4). In contrast, 63% yield of 1b was
obtained without benzonitrile (entry 3). These results suggest that
benzonitrile can act as a water scavenger in the present I-catalyzed
homocoupling. Furthermore, the homocoupling in DMF and DMSO
with Molecular Sieve 4A (MS 4A) pretreated at 150 ◦C gave 1b in
high yields (entries 6 and 8). Therefore, benzonitrile is not a spe-
cial solvent and various solvents can be used for the I-catalyzed
homocoupling reaction with appropriate water scavengers.
Next, the homocoupling of 1a in benzonitrile was carried
out with various catalysts (Table 1). In the absence of cata-
lysts, or the presence of copper salts and complexes, the reaction
hardly proceeded (entries 18–23). The mono-copper-substituted
silicotungstate II (entry 15), the non-copper-substituted lacunary
silicotungstate III (entry 16), and a mixture of catalyst precursors
III and CuCl2 (entry 17) were almost inactive, suggesting that the
di-copper(II) core {Cu2(␮-1,1-N3)2} in I plays an important role in
the present alkyne homocoupling (see Section 3.3).
Fig. 1. Various kinds of alkynes used.
vents and alkynes were obtained from Tokyo Kasei or Aldrich
(reagent grade) and purified prior to the use [14]. The di-
copper-substituted silicotungstate I, mono-copper-substituted
silicotungstate TBA4[␣-H2SiW11CuO39
] (II), and lacunary sil-
icotungstate TBA4[␥-SiW10O34(H2O)2] (III) were synthesized
according to the previously reported procedures [12,13,15].
2.2. Procedures for the catalytic reactions
The alkyne homocoupling was carried out as follows. Into a glass
tube reactor were successively placed I (2.2 mol%), alkyne (1 mmol),
and benzonitrile (1 mL). The reaction mixture was stirred at 100 ◦C
under O2 atmosphere. After the reaction was completed, diethyl
ether (20 mL) was added to the solution. The precipitated catalyst
was recovered by filtration (ca. 95% recovery), washed with diethyl
ether (ca. 50 mL), and dried in vacuo prior to the recycling experi-
ments. The isolation of diyne products were carried out by column
chromatography on silica gel using n-hexane as an eluent. The 1,3-
dipolar cycloaddition reactions were carried out according to the
similar procedures to those under Ar atmosphere.
3.2. Scope and limitation of the I-catalyzed reactions
The scope of the present I-catalyzed oxidative homocoupling
was examined (Table 2). The homocoupling reactions of aro-
matic alkynes with electron-donating as well as -withdrawing
substituents on phenyl rings efficiently proceeded to afford the
corresponding diphenylbutadiyne derivatives (entries 1–13). The
position of substituents on phenyl rings did not affect the reac-
tion; the reaction rates of 2-, 3-, and 4-methylphenylacetylenes
were almost the same (entries 5–7), for example. The reaction of
the nitrogen- and sulfur-containing heteroaromatic alkynes also
smoothly proceeded to give the corresponding diynes (entries
14–16). Notably, less reactive aliphatic C6–C16 alkynes were effec-
tively converted into the corresponding aliphatic diynes (entries
17–21). The reaction of the double bond-containing alkyne also
smoothly proceeded to give the corresponding enyne product
(entry 23). The present system could be applied to the homo-
coupling of the silylacetylene derivative, giving the corresponding
silyl diyne (entry 24). Interestingly, propargylic alcohols and amine
also gave the corresponding hydroxy and amino group-containing
diynes, respectively (entries 25–28). Although the homocoupling
of terminal alkynes efficiently proceeded, no reactions of internal
alkynes such as 2-octyne and diphenylacetylene proceeded in the
presence of I. This suggests that the copper species in I is terminally
bound to an alkyne (see Section 3.3).
2.3. Density functional theory (DFT) calculations
The DFT calculations were carried out at the B3LYP level the-
ory [16] with 6-31G* basis sets for H, C, N, O, and Si atoms,
and the double-␰ quality basis sets with effective core poten-
tials proposed by Hay and Wadt [17] for Cu and W atoms. The
entire structure of the I anion was used as a model for the
calculations. Methylacetylene was used as a model alkyne sub-
strate for the calculation. In order to minimize the differences in
the solvent effect among the intermediates and transition states,
protons were added to the polyanion framework and the total
charge of the system was kept −4 throughout the calculations.
Transition state structures were searched by numerically esti-
mating the matrix of second-order energy derivatives at every
optimization step and by requiring exactly one eigenvalue of this
matrix to be negative. For the transition states, the frequency
analysis was conducted at the same level at the final geometry.
The zero-point vibrational energies were not included. All cal-
After the reaction was completed, the catalyst could easily be
recovered by addition of an excess of diethyl ether (ca. 95% recov-

N. Mizuno et al. / Catalysis Today 157 (2010) 359–363
361
Table 1
Effects of solvents, additives, and catalysts on the oxidative homocoupling of 1aa.
Entry
Catalyst
Solvent
Additive
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16c
17d
18
19
20
21
22
23
24
I
I
I
I
I
I
I
I
I
I
I
I
I
I
II
III
Benzonitrile
DMF
DMF
DMF
DMF
DMF
DMSO
DMSO
Acetonitrile
1,2-Dichloroethane
Nitrobenzene
Toluene
2-Propanol
Acetic acid
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
Benzonitrile
–
–
–
3
3
24
24
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
91
39
63
88
9
90
39
86
15
4
3
2
2
2
2
<1
5
10
4
7
Benzonitrile (0.5 mmol)
Water (1 mmol)
MS 4A (200 mg)
–
MS 4A (200 mg)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
III + CuCl2
Cu(OAc)2
CuCl2
CuCl
CuI
[Cu(CH3CN)4]PF6
[Cu(I)(C CPh)]n
None
2
<1
<1
<1
a
Reaction conditions: Catalyst (Cu: 4.4 mol% with respect to 1a), 1a (1 mmol), solvent (1 mL), 100 ◦C, O2 (1 atm).
Yields were determined by GC using naphthalene as an internal standard.
2.2 mol%.
b
c
d
A mixture of III (2.2 mol%) and CuCl2 (4.4 mol%) was used as a catalyst.
ery, precipitation method, see Section 2.2). The recovered catalyst
to afford the corresponding 1,2,3-triazole (1d) and five-membered
heterocyclic compound (1f), respectively.
could be recycled at least five times without significant loss of the
catalytic activity; 94, 82, 88, 80, and 78% yields of 1b for the first,
second, third, forth, and fifth recycling experiments, respectively.
In a larger-scale reaction of 1a (reaction conditions: I (0.2 mol%),
1a (20 mmol), benzonitrile (20 mL), O2 (1 atm), 100 ◦C, 18 h), the
turnover number (TON = amounts of 1a consumed/amounts of I)
reached up to 468. This value was about one order of magnitude
higher than those reported for the copper-based oxidative alkyne
homocoupling reactions (TON: 1–50) [2–10].
As above-mentioned, an alkyne was converted into the corre-
sponding diyne in the presence of I. In other words, an alkyne itself
can act as a reducing agent to produce copper(I) species (see Section
3.3). Therefore, it is expected that the cycloadditions of 1,3-dipolar
compounds with alkynes, in which copper(I) species is reported
to be active [19,20], can be promoted with I in the absence of O2
(anaerobic conditions). Indeed, the 1,3-dipolar cycloadditions of an
organic azide (1c) to an alkyne (1a) (Fig. 2(a)) [21] and an azome-
thine imine (1e) to an alkyne (29a) (Fig. 2(b)) efficiently proceeded
3.3. Mechanistic studies
The reaction profiles for the I-catalyzed homocoupling of 1a
showed an induction period (approximately 5 min under the con-
ditions described in Table 2). The length of the induction period
depended on the concentrations of I and 1a and the decrease of the
concentrations of I and 1a resulted in a prolongation of the induc-
tion period. The induction period disappeared upon pretreatment
of I with 1a under Ar atmosphere at 100 ◦C. These results suggest
that the induction period is caused by the reaction of I with an
alkyne to form the catalytically active species.
Compound 1a (50 equivalents with respect to I) was added to
the benzonitrile solution of I (4 mM, 1 mL) and the resulting solu-
tion was heated to 100 ◦C under Ar atmosphere. Then, the UV–vis
spectra of the solution and the production of 1b were monitored.
An approximately 50 min-induction period was observed for the
Table 2
Scope of the I-catalyzed oxidative homocouplinga.
Entry
Alkyne
Time (h)
Yield (%)b
Entry
Alkyne
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1a
2a
3a
4a
5a
6a
7a
8a
3
3
3
3
3
3
3
3
91(88)
87
91(90)
97(82)
95
97(93)
95(97)
96(64)
96(89)
90(93)
94(90)
90
15
16
17
18
19
20
21
22
23
24
25
26
27
28
15a
16a
17a
18a
19a
20a
21a
22a
23a
24a
25a
26a
27a
28a
2
4
5
5
7
8
5
7
6
4
18
2
4
4
83
92
91(80)
91(86)
92(89)
76
90
76
94
84
9a
3
10a
11a
12a
13a
14a
3.5
4.5
5
4
2
85
>99(99)
90(82)
91
93
95(82)
a
Reaction conditions: I (2.2 mol% with respect to an alkyne), alkyne (1 mmol), benzonitrile (1 mL), 100 ◦C, O2 (1 atm).
Yields were determined by GC using naphthalene as an internal standard. The values in the parentheses were the isolated yields.
b

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N. Mizuno et al. / Catalysis Today 157 (2010) 359–363
Fig. 2. The I-catalyzed 1,3-dipolar cycloaddition reactions.
formation of 1b (Fig. 3(a)). During the induction period, the inten-
sity of the absorption band at around 360 nm assignable to the
N3− → Cu(II) ligand to metal charge transfer (LMCT) [22,23] of I
was gradually decreased and the LMCT band almost disappeared
Fig. 4. A possible reaction mechanism for the present I-catalyzed alkyne homocou-
6−
pling. Polyoxotungstate frameworks of [␥-H2SiW10O36
]
are omitted for clarity.
after approximately 50 min (Fig. 2(b)). As mentioned in Section 3.1,
the dicopper core in I plays an important role in the present alkyne
homocoupling. Thus, it is likely that the present homocoupling pro-
ceeds through the formation of the di-copper(II)-alkynyl species
−
[1,11] formed by the ligand exchange between the N3 ligands in I
and alkynyl groups and that the induction period corresponds to the
formation of the catalytically active di-copper(II)-alkynyl species
(step 1 in Fig. 4).1 The formation of the di-copper species during
the catalysis is supported by the fact that the reaction is first-order
dependent on the concentration of I.
After the induction period, almost equimolar amounts of 1a
(8.8 1.0 mM) with respect to the copper(II) species in I (8.0 mM)
was converted into the corresponding diyne 1b with a simultane-
ous decrease in the absorption band at around 700 nm assignable
to the d–d transition of the copper(II) species in I (Fig. 3(c)) [24].
These results suggest that the copper(II) species in I is reduced to
copper(I) species (step 2 in Fig. 4). The d–d transition band of the
copper(II) species again appeared by the introduction of O2 (1 atm)
to the solution, indicating that the reduced copper(I) species in I
can be re-oxidized to copper(II) species by O2 (step 3 in Fig. 4).
On the basis of the above-mentioned experimental results, we
here proposed the possible reaction mechanism for the present
I-catalyzed alkyne homocoupling, as shown in Fig. 4. The step 1
corresponds to the induction period for the present homocoupling.
Monitoring the formation of water (in this case the formation of
benzamide, see Section 3.1) for the I-catalyzed homocoupling of 1a
revealed that the amount of water produced was the same as that of
1b. The measurement of O2 uptake during the homocoupling of 1a
was also carried out and the amount of O2 consumed was half that
of 1b produced. These respective 1:1 (water formed:diyne formed)
1
While it has been reported that the absorption bands of copper alkynyl species
Fig. 3. Reaction profiles for the formation of 1b under Ar atmosphere (a), and the
profiles of the absorbance at 360 nm (b) and 700 nm (c). Reaction conditions: I (4 mM),
1a (200 mM), benzonitrile (1 mL), 100 ◦C, Ar (1 atm).
are observed around 370 nm, the absorption band could not be detected because of
the overlap with the very intense absorption bands of I, solvent, and products.

N. Mizuno et al. / Catalysis Today 157 (2010) 359–363
363
singlet potential energy surfaces would proceed during the trans-
formation from I-a to I-b. The activation energy from di-copper(II)
intermediate I-a to di-copper(I) species (I-d) via the transition state
I-c was calculated to be 38 kJ mol−1. The re-oxidation of I-d with
O2 (step 3, Eq. (5) in Fig. 5) was a downhill reaction and calcu-
lated to be exothermic by 56 kJ mol−1. These results are consistent
with the experimental ones that the formation of a diyne from the
di-copper(II)-alkynyl species is the rate-determining step.
4. Conclusions
The di-copper(II)-substituted silicotungstate I exhibited high
catalytic activity for the aerobic oxidative homocoupling. In addi-
tion, the 1,3-dipolar cycloaddition reactions efficiently proceeded
with I. The catalyst effect, spectroscopic analyses, kinetics, and DFT
calculations show that the di-copper(II) core on I is the active site
for the present oxidative homocoupling.
Acknowledgment
This work was supported in part by the Core Research for Evo-
lutional Science and Technology (CREST) program of the Japan
Science and Technology Agency (JST), the Global COE Program
(Chemistry Innovation through Cooperation of Science and Engi-
neering), and Grants-in-Aid for Scientific Researches from Ministry
of Education, Culture, Sports, Science and Technology.
Fig. 5. (a) The calculated energy changes for each step and (b) detailed energy dia-
gram in the step 2 (Eqs. (2)–(5)) for the oxidative homocoupling of 30a in the gas
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1740.
to the triplet state of I-a were calculated to be 53 and 21 kJ mol−1
respectively. Thus, the spin interconversion between the triplet and
,
[24] B.J. Hathaway, D.E. Billing, Coord. Chem. Rev. 5 (1970) 143.