Efficient and environmentally friendly Glaser coupling of terminal alkynes catalyzed by multinuclear copper complexes under base-free conditions

Article abstract of DOI:10.1039/c6ra01262k

An efficient catalytic system with dinuclear complex [Cu₂(ophen)₂] 1 and tetranuclear complex [Cu₄(ophen)₄(tp)] 2 as catalysts has been developed for the Glaser coupling reaction, which adopts environmentally fr

Full text of DOI:10.1039/c6ra01262k

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Efficient and environment-friendly Glaser coupling of
terminal alkyne catalyzed by multinuclear copper
complexes under base-free condition †
Cite this: DOI: 10.1039/x0xx00000x
Ling-Juan Zhang, Yan-Hong Wang, Jie Liu, Mei-Chen Xu, Xian-Ming Zhang*
An efficient catalytic system with dinuclear complex [Cu2(ophen)2]
1
and tetranuclear
DOI: 10.1039/x0xx00000x
complex [Cu4(ophen)4(tp)] as catalyst has been developed for the Glaser coupling reaction,
2
which adopts environment-friendly water as solvent at room temperature under air/O2
atmosphere and avoids the utilization of any base. Satisfyingly, due to poor solubility in
www.rsc.org/
common solvents, tetranuclear compound
2 served as a molecular heterogeneous catalyst and
could be reusable five times without loss of activity. Both catalysts could be prepared in high
yield via simple hydrothermal reactions with cheap ligands, which in combination
environment-friendly water media and mild catalytic reaction conditions makes the approach
accords with the concept of green chemistry in synthesis of 1,3-diynes.
preparation process of catalyst,17 hash reaction conditions, including
excess bases (for example, Na2CO3, KOAc, CH3ONa and
Introduction
Et3N) 13,15,24 or high temperature (110°C)14 etc. Therefore, it is still
,
Over the past decades, the concept of green chemistry in organic
synthesis has attracted increasing attention around the world. Hence,
the challenges of effective synthetic method are not only high-
yielding but also environment-friendly.1 All of above request
chemists to find nontoxic/renewable materials, easy bulk preparation
of catalysts and green/free solvent reaction conditions in a green
synthetic strategy. In this work, we describe a green and effective
method towards the construction of symmetrical 1,3-diynes.
necessary to develop more simple, effective, and environmentally
friendly method for the construction of symmetrical 1,3-Diynes
compounds by the Glaser coupling.
In recent years, we have focused on the synthesis of metal
organic complexes with novel structures and exploration of their
application on catalytic organic synthesis.25 Compared with other
copper complexes synthesized in our group,26 metal center of planar
dinuclear complex Cu2(ophen)2 (1, Fig S2) with hydroxylated N-
heterocyclic ligands complex is redox active.27 Driven by our
continuing interest in catalytic application of copper-based
complexes, we envisioned whether the aerobic Glaser-Hay coupling
reaction could occur with compound 1 as catalyst. As expected, we
detected that Cu2(ophen)2 provided access to an effectively active
species for the Glaser coupling reaction using environmentally
friendly water as media and without any base. Satisfyingly,
[Cu4(ophen)4(tp)] (2, Fig S3) with a tetranuclear mixed-valence
dumbbell structure which is the derivative of compound 1 also
showed obviously catalytic activity under similar conditions. It is
noteworthy that compound 2, which is inexpensive and easy
prepared, can serve as a heterogeneous catalyst. In previous reports,
the preparation process of copper-based heterogeneous catalyst were
usually complicated.28
1,3-Diynes have obtained widespread applications in the
synthesis of material chemistry, natural products, pharmaceuticals,2
such as polymers,2a,d polyradical,2b Ivorenolide A,2f Debilisone C,2e,g
phosphoiodyns A2h etc. In contrast to the homocoupling of various
alkynyl
organometallics3
(organotin,3a
organosilicon,3b
organolithium,3c organoboron,3d organomagnesium,3e compounds),
alkynyltellurides4 or iodoalkynes,5 the homocoupling of terminal
alkynes is widely believed to be an economic and practical method
to construct 1,3-diynes, for instance, Glaser-Hay coupling6 and
Cadiot-Chodkiewicz coupling.7
Since 1869 year, Glaser firstly found the oxidative coupling of
copper acetylides to access diynes, numerous metal calalytic systems
like CoBr2,8 palladium reagents,9 [Ru(dppp)2(CH3CN)Cl][BPh4],10
Au nanoparticles,11 Au(I)/Au(III)12 etc have been studied on this
reaction. However, most of their widely applications are limited
because of their higher price or unstability in air. Copper-catalyzed
aerobic homocoupling of terminal alkynes remains the most popular
due to copper being an abundant and inexpensive metal. Numerous
modifications, like CuI/iodine,13 CuI-USY,14 CuNPs/MagSilica,15
CuAl-LDH,16 {Cu2+-M3+-Cu2+}17 etc have been demonstrated to
exhibit good catalytic activity. In addition, looking for more green
solvents such as PEG,18 ionic liquid,19 fluorous media,20 ScCO2,21
water22 or solvent-free23 conditions also have been attracted much
attention. In spite of the significant progress offered by these
methods, there are still certain limitations because of complicated
Herein, we wish to report a novel and mild Cu-promoted direct
Csp-Csp coupling reaction through aerobic homocoupling of
terminal alkynes. The protocol has the advantages of broad
substrates scope, green reaction conditions, easy operation and good
yields (Scheme 1).
Scheme 1. Homocoupling of terminal alkynes.
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substrate (Table 1, entries 7-8). Adding TBAB (5 mol%) as the
phase transfer catalyst, the reaction time can be obviously shortened
using water as solvent (Table 1, entry 9). Therefore, the optimal
base-free system under air for this reaction involves Cu2(ophen)2 (5
mol%) and TBAB (5 mol%) in greener water at room temperature.
Experimental section
Materials and physical measurements
[Cu2(ophen)2] (1) and [Cu4(ophen)4(tp)] (2) were synthesized
according to our previous methods.27 Other chemicals were
analytically pure from commercial sources and used without further
purification. The catalyst was dried at room temperature before the
catalytic reactions. The conversion of terminal alkynes was obtained
by GC or 1HNMR. Gas chromatography (GC) analyses were
obtained by Agilent 7820A (FID from AGILENT 7820) equipped
with a cross-linked (95%)-dimethyl-(5%)- diphenylpolysil-oxane
column (HP-5, 30 m * 0.32 mm * 0.25 µm), helium, injector
temperature 250°C, detector temperature 300°C, and oven
temperature program 45°C (3 min)-20°C/min-280°C (2 min). 1H and
13C NMR spectras were recorded with ASCend TM 600 MHz in
CDCl3 with TMS as the internal standard. Powder X-ray diffraction
(XPRD) datas were collected in a Rigaku Ultima IV diffractometer.
Table 1. Optimization of reaction conditions a
Temp
(°C)
Time
(h)
Conv.
(%) b
Entry
Cat(mol %)
base
Sol.
1
2
5
5
5
1
3
5
5
5
5
5
5
-
r.t.
60
DMF-1
DMF-1
DMF-1
DMF-1
DMF-1
DMF-1
EtOH-1
H2O-1
10
8
99.6
99
99.5
56
94
-
3
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
10
20
20
20
12
20
10
4
-
5
-
6c
-
General procedure for the Cu2(ophen)2 catalyzed homocoupling
for Table 2.
7
-
99
99
99
A solution of the corresponding terminal alkynes (1.0 mmol),
Cu2(ophen)2 (6.5 mg, 5 mol%) , TBAB (8 mg, 5 mol %) and water
(1.0 mL) in 5 mL round-bottomed flask was stirred under air and the
reaction was monitored by TLC. After the substrate was consumed,
the reaction conversions were determined by gas chromatography
(GC) analysis. The resulting mixture was poured into brine (10 mL),
and extracted with dichloromethane (3 * 10 mL). The organic layer
was washed with brine, dried over Mg2SO4, the residue was
chromatographed via a short column of silica gel and evaporated
8
-
9d
-
H2O-1
a Reaction conditions: phenylacetylene (1 mmol; 55 µL), catalyst (5 mol%;
b
d
6.5 mg), solvent (1 mL), air; determined by GC; c Nitrogen atmosphere;
TBAB (5 mol%, 8 mg).
Encouraged by the efficiency and environmental friendliness of
the reaction protocol described above, we investigated the substrate
scope under optimized conditions. The terminal alkynes bearing
electron-deficient and electron-rich aryl groups proceeded
homocoupling reactions smoothly and gave the corresponding 1,3-
diyne derivatives 2a-j in good to excellent yields (Table 2). It
seemed that the electronic and steric effects of substituents on
benzene ring had no obvious effect on the Csp-Csp homocoupling
reaction (Table 2, entries 1-10). The heteroaromatic alkynes 1k-1l
were also compatible substrates and gave the target products 2k-2l in
98% and 99% yields respectively (Table 2, entries 11-12).
Interestingly, we successfully expanded the scope of substrates to
aliphatic alkynes and the yields of the obtained products being 97-
99% (Table 2, entries 13-15). It was worth noting that the reaction
condition of aliphatic alkynes didn’t need to be strengthened and the
reaction rate was faster than aromatic alkynes under same condition.
For instance, the homocoupling of aliphatic alkynes 1m-o can be
completed in only 5h (Table 2,entries 13-15).
1
under reduced pressure. The products were determined by HNMR
and 13CNMR spectroscopy.
General procedure for Table 3.
A suspension of terminal alkynes (1.0 mmol), [Cu4(ophen)4(tp)]
(7.5 mg, 5 mol%) with TBAB (8 mg) in H2O (1 mL) was stirred at
60°C under O2 (1 atm). After the substrate was consumed, the
reaction conversions were determined by GC analysis.
Recycle Experiment
At the end of the reaction, the catalyst was centrifuged and
washed with ethanol and water (5 * 10 mL) respectively and drying
in air. It was reused directly without further purification. The
recovered catalyst was used in the next run, and almost consistent
activity was observed for 5 consecutive cycles.
Table 2. homocoupling of various terminal alkynes a
Results and discussion
The copper-catalyzed alkyne coupling reaction was initially
evaluated using phenylacetylene (1a) as substrate for optimization of
reaction conditions. Firstly, the reaction was carried out in DMF,
with Cu2(ophen)2 (1) (5 mol%) as catalyst and KF (5 mol%) as base,
under air at different temperatures. It was found that the reaction
proceeded perfectly and the conversions had no significant
differences at r.t. or 60°C (Table 1, entries 1-2). Fortunately, we
found that the homocoupling reaction still performed well in 99.5%
conversion under base-free condition (Table 1, entry 3). Decreasing
the amount of catalyst resulted in decrease of conversions and
extension of reaction times (Table 1, entries 4-5). A control
experiment conducted in N2 atmosphere showed that the reaction did
not occur (Table 1, entry 6). When the reaction was conducted in
EtOH and H2O respectively, the high conversion was obtained
although it took a long time in H2O because of the poor solubility of
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desired 1,3-diynes in excellent conversions under the condition of
heterogeneous catalysis (Scheme 2). Broad tolerance for various
substrates proves that complex 2 is also an efficient catalyst for the
Glaser coupling reaction.
Scheme 2. [Cu4(ophen)4tp] catalyzed homocoupling of terminal alkynes.
With strong interest in the synthesis of unsymmetric 1,3-diyne
type compounds[30] and to expand the application of this
methodology, we also examined the cross-coupling reaction of two
different terminal alkynes under the above conditions. Medium
yields of unsymmetric 1,3-diynes were observed when using two
different terminal alkynes as the substrates, while the corresponding
homocoupling products were obtained unavoidably (Table 4).
Table 4. cross-coupling reaction of two different terminal alkynes a
a
Reaction conditions: alkynes (1 mmol), Cu2(ophen)2 (5 mol%, 6.5 mg),
TBAB (5 mol%), H2O (1 mL), r.t., 10 h; 1m-1o:5h; b Isolated yield.
Our group found that [Cu4(ophen)4(tp)] (2) can be obtained from
dinuclear 1 in the presence of terephthalic acid as ligands under
hydrothermal conditions.27 Mixed-valence copper(I,II) complex 2,
which consists of a pair of [Cu2(ophen)2]+ fragment bridged by a
dicarboxylate ligand, has a neutral tetranuclear dumbbell structure. It
is generally known that the change of ligand has subtle influence on
properties of metal complexes. To see this point, we explored the
catalytic activity of compound 2 to the homocoupling of terminal
alkynes. It was found that the conversion of phenylacetylene
obviously rose when the temperature was increased from r.t. to 60°C
as well as under oxygen atmosphere. The result indicated that the
catalytic activity of complex 2 for homocoupling of alkynes was
lower than compound 1 because of the change of metal complex
structure based change of ligand (Table 3, entries 1-3). However,
satisfyingly, catalyst 2 could be served as a heterogeneous catalyst29
and still keep high activity after recycled five times (Table 3, entries
4-5). Comparison of the powder XRD patterns of the fresh and five
times reused compound 2 were performed (Fig. S4), which showed
similar powder XRD patterns and clearly demonstrated the practical
recyclability of catalyst 2 and its remarkable stability.
31
On the basis of related reports and our early work,28,
the
possible dimetallic mechanism of the oxidative homocoupling
reaction is proposed as shown in Scheme 3. In the initial step, the
molecular oxygen is activated by Cu(I) of catalyst 1 in the presence
of water, meanwhile, Cu(II) intermediate A is formed along with
shrink of the Cu-Cu distance.27, 31a Tansmetallation of the terminal
alkynes with copper(II) complex A to give a dimetallic intermediate
B, while release of byproduct H2O through attack of the hydroxyl
ligand to the hydrogen of alkyne. Finally, the desired diyne product
is formed by reduction from B and elimination from C, with
concomitant regeneration of catalyst.
Table 3. Optimization of reaction conditions with compound 2 as catalyst a
Entry Cat (mol %) Temp (°C) Conv. (%) b
1
5
r.t.
60
60
60
60
70
80
98
94
90
2
5
3c
4c
5c
5
5 (run 3rd)
5 (run 5th)
a Reaction conditions: phenylacetylene (1 mmol), Cu4(ophen)4(tp) (5 mol%,
b
c
7.5 mg), TBAB (5 mol%), H2O (1 mL), air; determined by GC; oxygen
atmosphere (1 atm).
Scheme 3. A plausible mechanism of the homocoupling reaction.
Similarly, we then focused on our attention to substrate
generality using the optimized reaction conditions (Table 3, entry 3).
The substrates with typical electronic effect of substitute group were
investigated. Both aromatic alkynes and aliphatic alkynes gave
Conclusions
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A facile, effective, economical and environmental-friendly
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coupling of terminal alkynes to synthesize 1,3-diynes has been
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functional group tolerance and good conversion under base-free
condition. Remarkably, the multinuclear copper complexes exhibit
excellent performance in neat water under an atmospheric pressure
of air/O2. Cheapness, easy bulk preparation of catalysts, mild
reaction conditions and heterogeneous catalysis make the approach
match well with the concept of green chemistry and be attractive in
industrial preparation of symmetric 1,3-diynes.
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Financial support for this research provided by the NSFC (Grant
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