3,4-dihalo-2(5H)-furanones: A novel oxidant for the Glaser coupling reaction

Article abstract of DOI:10.1007/s00706-011-0479-5

5-Alkoxy-3,4-dihalo-2(5H)-furanones could be used as a kind of novel oxidant in the Glaser coupling reaction. The screening of reaction conditions showed that both PdCl₂(PPh₃)₂ and 3,4-dichloro-5-methoxy-2(5H)-furanone played crucial roles in the reaction. A possible reaction mechanism was proposed according to the reactivity of 3,4-dihalo-2(5H)-furanones. The new method easily allows the syntheses of alkyl and aryl substituted 1,3-diyne compounds. However, carbyne polymer was unexpectedly obtained when using trimethylsilyl acetylene as the substrate under the Glaser reaction condition.

Full text of DOI:10.1007/s00706-011-0479-5

Monatsh Chem (2011) 142:507–513
DOI 10.1007/s00706-011-0479-5
ORIGINAL PAPER
3,4-Dihalo-2(5H)-furanones: a novel oxidant for the Glaser
coupling reaction
•
Jian-Xiao Li Hao-Ran Liang
•
•
Zhao-Yang Wang Jian-Hua Fu
Received: 24 September 2010 / Accepted: 26 February 2011 / Published online: 22 March 2011
Ó Springer-Verlag 2011
Abstract 5-Alkoxy-3,4-dihalo-2(5H)-furanones could be
used as a kind of novel oxidant in the Glaser coupling
reaction. The screening of reaction conditions showed that
both PdCl2(PPh3)2 and 3,4-dichloro-5-methoxy-2(5H)-
furanone played crucial roles in the reaction. A possible
reaction mechanism was proposed according to the reac-
tivity of 3,4-dihalo-2(5H)-furanones. The new method
easily allows the syntheses of alkyl and aryl substituted
1,3-diyne compounds. However, carbyne polymer was
unexpectedly obtained when using trimethylsilyl acetylene
as the substrate under the Glaser reaction condition.
inhibitors [2–6]. Therefore, in order to synthesize 2(5H)-
furanone derivatives, there has been a continuous interest
in developing efficient and convenient methods [6–9],
including many synthetic methods using some simpler
2(5H)-furanones as organic intermediates [10]. For exam-
ple, in order to use available 3,4-dihalo-2(5H)-furanones as
intermediates, many kinds of reactions, such as the Sono-
gashira reaction [11], Knoevenagel reaction [12], Suzuki
reaction [13], Friedel-Crafts reaction [14], and Michael
addition-elimination reaction [15–17], have been reported
recently.
On the basis of our previous studies on the reaction of
5-alkoxy-3,4-dihalo-2(5H)-furanones [10, 15–17], we hope
to investigate their Sonogashira reaction with terminal
alkynes under the protection of nitrogen atmosphere. Sur-
prisingly, the formation of many 1,3-diyne compounds
along with little expected Sonogashira compounds
was observed. Although the Sonogashira reaction of other
3,4-dihalo-2(5H)-furanones with terminal alkynes has been
reported before [11], the phenomenon of producing many
Glaser coupling compounds was never mentioned to the
best of our knowledge. Moreover, no reports have been
made on the role of 3,4-dihalo-2(5H)-furanones used as the
oxidant in the Glaser coupling reaction before.
Keywords 2(5H)-Furanone chemistry Á
Green chemistry Á Oxidative coupling Á
Alkynes Á Reaction mechanisms
Introduction
As a kind of structural unit, 2(5H)-furanone is frequently
found in many natural products [1, 2], and many com-
pounds containing 2(5H)-furanone structure have been
considered as potential insecticides, bactericides, fungi-
cides, antibiotics, anticancer agents, antiinflammatories,
allergy inhibitors, antisoriasis agents, and cyclooxygenase
As an important carbon-carbon bond formation method,
the Glaser coupling reaction has been extensively used to
synthesize various highly valuable functional molecules
containing a buta-1,3-diyne fragment in natural product
chemistry, material science, and supramolecular chemistry
[18–23]. Considerable efforts have been directed to the
development of the Glaser coupling reaction [24–27]. The
oxidant plays a crucial role in the Glaser coupling reaction,
and its choice has always been a focus of attention. Usu-
ally, air (or oxygen) has been used extensively as the
oxidant because of its cheapness and availability [28–33].
J.-X. Li Á H.-R. Liang Á Z.-Y. Wang (&) Á J.-H. Fu
Department of Chemistry, School of Chemistry and
Environment, South China Normal University,
Guangzhou 510006, People’s Republic of China
e-mail: wangwangzhaoyang@tom.com; wangzy@scnu.edu.cn
H.-R. Liang
College of Chemistry, Sichuan University, Chengdu 610064,
People’s Republic of China
123

508
J.-X. Li et al.
However, in order to develop novel catalytic systems or
avoid other side reactions caused by oxygen [34, 35], more
and more other oxidants, including chloroacetone [36],
iodine [37], benzoquinone [34], trimethylamine oxide [38],
sodium percarbonate [39], ethyl bromoacetate [35, 40], and
nitrobenzene [41], have been explored and used in inert
argon (or nitrogen) atmosphere to generate 1,3-diyne
compounds.
The reaction time also affected the reaction signifi-
cantly. Shortening the reaction time lowered the yield
(Table 1, entries 4, 7–10), and the suitable time should be
72 h (entry 4). However, without the oxidative reagent 2c,
only a little product could be isolated (entry 11). This
control experiment showed that 2c was indispensable for
the reaction system after the degassing step, and it was the
oxidant indeed.
Even so, these oxidants also have many disadvantages.
For example, iodine may take part in the side reaction and
lead to homopolymerization of the substrate diyne benzene
[34]. More importantly, many of the above oxidants, such
as chloroacetone, ethyl bromoacetate, trimethylamine
oxide, and nitrobenzene, are irritating, corrosive, or highly
toxic. To the best of our knowledge, now only sodium
percarbonate is an environment-friendly oxidant with easy
handling and storage [39]. Thus, it is still urgent to develop
safe, effective, and convenient oxidants for the Glaser
coupling reaction in inert atmosphere. Herein, we hope to
report that 3,4-dihalo-2(5H)-furanones 2 can be novel
effective and green oxidants for the Glaser coupling reac-
tion (Scheme 1), especially 3,4-dichloro-5-methoxy-
2(5H)-furanone (2d; the preparation method is shown in
Scheme 2).
When the reaction was carried out for the same time
(60 h), the influences of different palladium catalysts were
evaluated for the new homocoupling reaction system
(Table 1, entries 10, 12, 13). When PdCl2(PPh3)2 was used
as the catalyst, a satisfactory yield could also be obtained
(entry 10). If the catalyst was replaced by Pd(OAc)2 or
PdCl2, the coupling reaction afforded the product in mod-
erate yields (entries 12 and 13). This observation suggested
that the presence of PPh3 as phosphine ligand could pro-
mote the reaction. The investigation of the influences of
PdCl2(PPh3)2 dosage (entries 9, 14–16) showed that the
suitable palladium catalyst dosage was 1.0 mol% of the
substrate phenylacetylene (entry 16).
Thus, choosing phenylacetylene as the model substrate,
1.0 equiv. 3,4-dibromo-5-methoxy-2(5H)-furanone (2c) as
the oxidant, 2.50 mol% CuI as cocatalyst, and acetonitrile
as solvent, the optimized reaction conditions could be
summarized in the following: 50 °C, 72 h, 2.0 equiv.
K2CO3 as the base, and 1.0 mol% PdCl2(PPh3)2 as the
main catalyst.
Results and discussion
Optimization of reaction conditions
Influences of different oxidants
The unexpected result of the Sonogashira reaction showed
that the reaction conditions, including the reaction tem-
perature and time, the base, the kinds and dosage of the
main catalyst, affected the Glaser coupling reaction.
Therefore, choosing phenylacetylene as the model sub-
strate, we first investigated the optimization of reaction
conditions (Table 1).
Under the above-optimized conditions, the influences of
different 3,4-dihalo-2(5H)-furanones as oxidant on the
formation of 1,4-diphenylbuta-1,3-diyne (3a) in N2 atmo-
sphere were then investigated (Table 2). The results
indicated that the oxidative reagents have an important
influence on the reaction.
The influences of reaction temperature on the conver-
sion of 1a showed that the lower the temperature, the lower
the conversion (Table 1, entries 1–4). When the reaction
was carried out at 50 °C, diyne 3a could be obtained in a
conversion of 100% (entry 4). The influences of some
common bases, such as K2CO3, Et3N, and KF, were also
examined (entries 4–6), and the results indicated that
K2CO3 was the most effective base and the isolated yield
of 3a was 80% (entry 4).
It could be seen that 3,4-dichloro-2(5H)-furanones
(Table 2, entries 2, 4, 6) usually had better oxidation results
than 3,4-dibromo-2(5H)-furanones (entries 1, 3, 5). This
may be related to the less steric hindrance of chlorine as
compared to bromine [15–17]. Meanwhile, for different
5-substituents of 3,4-dihalo-2(5H)-furanones, the best was
5-methoxy, and the worst was 5-hydroxy. Because of the
isomerism, 3,4-dihalo-5-hydroxy-2(5H)-furanones 2a, 2b
had a certain acidity (Scheme 3) [12, 42, 43], which
Scheme 1
123

3,4-Dihalo-2(5H)-furanones: a novel oxidant for the Glaser coupling reaction
509
Scheme 2
Table 1 Optimization of reaction conditions for the Glaser coupling reaction of 3a
Entrya
Pd catalyst (mol%)
Base
t (h)
72
T (°C)
Yield (%)
1
PdCl2(PPh3)2 (1.25)
PdCl2(PPh3)2 (1.25)
PdCl2(PPh3)2 (1.25)
PdCl2(PPh3)2 (1.25)
PdCl2(PPh3)2 (1.25)
PdCl2(PPh3)2 (1.25)
PdCl2(PPh3)2 (1.25)
PdCl2(PPh3)2 (1.25)
PdCl2(PPh3)2 (1.25)
PdCl2(PPh3)2 (1.25)
PdCl2(PPh3)2 (1.25)
Pd(OAc)2 (1.25)
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
r.t.
30
40
50
50
50
50
50
50
50
50
50
50
50
50
50
51b
73b
84b
80 (100b)
62
2
72
72
72
72
72
24
36
48
60
72
60
60
48
48
48
3
4
5
6
KF
78
7
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
41
8
48
9
63
10
11
12
13
14
15
16
a
76
2c
58
PdCl2 (1.25)
63
PdCl2(PPh3)2 (0.50)
PdCl2(PPh3)2 (0.75)
PdCl2(PPh3)2 (1.00)
46
62
63
Reaction conditions: phenylacetylene (1 mmol), CuI (2.50 mol%), 5-methoxy-3,4-dibromo-2(5H)-furanone (2c, 1 equiv.), CH3CN (5 cm3),
base (2 equiv.), and nitrogen atmosphere
b
Conversion of 3a was determined by GC analysis, and in other cases the yield was the isolated yield of 3a
c
As a control experiment, there is no 2c in the reaction system after the degassing step
Table 2 Influences of different oxidants on the reaction
Entrya
Oxidant
X
5-Substituent
Yield (%)b
1
2
3
4
5
6
a
2a
2b
2c
2d
2e
2f
Br
Cl
Br
Cl
Br
Cl
HO
59
70
80
90
66
77
HO
CH3O
CH3O
Menthoxy
Menthoxy
Reaction conditions. Phenylacetylene (1 mmol), PdCl2(PPh3)2 (1.0 mol%), CuI (2.50 mol%), 3,4-dihalo-2(5H)-furanone 2 (1 equiv.), K2CO3
(2 equiv.), and 5 cm3 CH3CN
b
Isolated yield
123

510
J.-X. Li et al.
To our surprise, when we treated trimethylsilylacetylene
(1g) under the same optimized conditions, no Glaser cou-
pling diyne 3g was obtained (Scheme 4), but a dark powder
was isolated. Moreover, the dark powder was amorphous
and insoluble in any known solvent. According to the lit-
erature, these properties and its FT-IR characterization
(Fig. 1, the IR test is one of the main characterization
methods for the carbyne polymer because of its insolubility
in most solvents) [46–53] indicated that the product should
be carbyne (Scheme 4), especially b-carbyne, as the
absorptions at 1,611 and 1,395 cm-1 were attributed to
cumulative double bonds [46, 49–53].
Scheme 3
lowered the alkalinity of the reaction systems. Therefore,
for 3,4-dibromo-5-hydroxy-2(5H)-furanone (2a), the yield
was the lowest of all because of the above two reasons
(entry 2).
For 3,4-dihalo-5-menthoxy-2(5H)-furanones 2e, 2f,
perhaps the larger steric hindrance of the 5-substituent was
disadvantageous for the reaction. Therefore, because of the
less steric hindrance of both chlorine and methoxy, 3,4-
dichloro-5-methoxy-2(5H)-furanone (2d) was the most
efficient oxidant for the homocoupling reaction, and the
isolated yield of 3a was 90% (Table 2, entry 4). Thus,
using 2d as the oxidant, the reaction of more terminal
alkynes was investigated.
There are two possible reasons for the unexpected for-
mation of carbyne under the Glaser coupling reaction
condition. Firstly, the trimethylsilyl group is often
employed as a protective group for terminal alkynes and
could easily be removed to give –C:CH under the basic
condition [54–56], so trimethylsilylacetylene (1g) or 1,4-
bis(trimethylsilyl)-buta-1,3-diyne (3g) was unstable. Of
course, the coupling reaction of alkynylsilanes (1g or 3g)
also may be mediated by Cu(I) under oxidative conditions
[57]. Secondly, the b-carbyne could be formed via the
Cu(I)-catalyzed coupling of the –C:CH group [48]. The
two reasons affected each other, which led to the formation
of carbyne under the Glaser coupling reaction condition.
Carbyne can be utilized in many fields, such as bio-
medicine (used as surgical sutures and organ replacement)
[58], and carbon material science [59–61] (for example,
used as the material of the conversion into diamond [62] or
fullerene [63]). Therefore, it is notable that the formation of
b-carbyne under the Glaser coupling reaction condition
may be a novel mild and convenient method for the syn-
thesis of carbyne compared with the previous reported
routes [46–53] once the reaction conditions are further
optimized.
Application scope of the novel oxidant
Under the above-optimized conditions, the results of dif-
ferent substrates showed that the combination of
PdCl2(PPh3)2, CuI, K2CO3, and 3,4-dichloro-5-methoxy-
2(5H)-furanone (2d) was an effective catalytic system for
the Glaser coupling reaction. Not only the homocoupling of
aromatic alkyne gave diyne 3a in good yield (90%), but
also the aliphatic alkynes gave the products in yields of
76–88% (Table 3).
Usually, terminal aliphatic alkynes are sluggish in
undergoing Glaser dimerization because of the weaker
acidity of the acetylenic proton [44, 45]. However, in our
reaction system, the homocoupling was smoothly carried
out to afford the corresponding diynes 3b–3e in good to
excellent yields (mostly over 80%). At the same time, some
functional groups on the alkynes, e.g., hydroxy, also had no
effects on the reaction under the performed conditions, and
the yield of 3f was 86% (Table 3, Entry 6).
Plausible mechanism of 3,4-dihalo-2(5H)-furanones
as oxidant
Compared with other oxidants used in the Glaser coupling
reaction, such as trimethylamine oxide (Me3NO) [38],
Table 3 The yields and physical constants of the compounds 3a–3f
Entrya
Compound
Appearance
Yieldb/%
M.p./°C (lit. data)
1
2
3
4
5
6
a
3a
3b
3c
3d
3e
3f
White solid
Colorless oil
Colorless oil
Colorless oil
White solid
White needle
90
80
76
85
88
86
85.0–86.4 (86–88 [65])
— ([65])
— ([65])
— ([65])
127.4–129.3 (128–130 [38])
128.0–130.0 (131–133 [66])
Reaction conditions. Alkynes 3 (1 mmol), PdCl2(PPh3)2 (1.00 mol%), CuI (2.50 mol%), 5-methoxy-3,4-dichloro-2(5H)-furanone 2d
b
(1 equiv.), K2CO3 (2 equiv.), and CH3CN (5 cm3); Isolated yield
123

3,4-Dihalo-2(5H)-furanones: a novel oxidant for the Glaser coupling reaction
511
Scheme 4
mechanism of 3,4-dihalo-2(5H)-furanones as an oxidant
based on the previous reports on the mechanism of the
Glaser coupling reaction [29, 35, 38, 64] and the reactivity
of 3,4-dihalo-2(5H)-furanones [3, 4, 11].
As outlined in Scheme 5, firstly, with the aid of a base
the reaction of terminal alkynes 1 with CuI afforded the
intermediate A readily. Then, the replacement of Pd(II)
with intermediate A would occur to form a dialkynylpal-
ladium(II) intermediate B and regenerate the active
Cu(I) species. Reductive elimination of the dialkynylpal-
ladium(II) intermediate B could form the desired diyne 3
and Pd(0). Finally, Pd(0) was oxidized by 5-alkoxy-3,4-
dihalo-2(5H)-furanones 2 to regenerate the active Pd(II)
species leading to a new catalytic cycle.
In summary, using 5-alkoxy-3,4-dihalo-2(5H)-furanones
as the oxidant, we have successfully developed a novel
pathway for the palladium-catalyzed homocoupling reac-
tion. This oxidant with higher thermal stability is more
environmentally friendly and convenient than most previ-
ously reported oxidants. These unexpected results and
systematic investigations are beneficial to further enrich the
chemistry of 2(5H)-furanones.
Fig. 1 FT-IR spectrum of the product of trimethylsilylacetylene (1g)
ethyl bromoacetate [35, 40], and chloroacetone [36], 3,4-
dihalo-2(5H)-furanones are a kind of easily available oxi-
dant (Scheme 2) [15–17] with higher thermal stability and
better oxidation resistance, especially better environmental
friendliness. In order to further expand their applications
in organic synthesis, we proposed a possible reaction
Scheme 5
123

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J.-X. Li et al.
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Experimental
All reagents and solvents were obtained commercially and
used without further purification. All melting points were
1
determined on an X-5 digital melting points apparatus. H
and 13C NMR spectra were obtained in CDCl3 or DMSO-
d6 on a Varian DRX-400 MHz spectrometer using tetra-
methylsilane (TMS) as an internal standard. The mass
spectra were recorded on a Finnigan Trace DSQ at an
ionization voltage of 70 eV. Using 2a, 2b, natural menthol,
and methanol as starting materials, 2c–2f could be easily
prepared according to the literature [15–17] (but the reso-
lution step for 2e and 2f was omitted in this case,
Scheme 2).
General procedure for the Glaser coupling reaction
After degassing,
a
mixture of alkyne (1 mmol),
PdCl2(PPh3)2 (1.0 mol%), CuI (2.50 mol%), K2CO3 (2
equiv.), 3,4-dichloro-5-methoxy-2(5H)-furanone (2d, 1
equiv.), and 5 cm3 MeCN was stirred under N2 at 50 °C for
72 h. Once the reaction was complete, the reaction mixture
was diluted with 5 cm3 water and then extracted with ethyl
acetate (10 cm3 9 2). The combined organic layers were
dried with magnesium sulfate and concentrated under a
vacuum to give a crude product, which was purified by
column chromatography on silica gel with gradient eluent
of mixtures of n-hexane and dichloromethane to afford
compounds 3a–3f (Scheme 1) for analysis. IR and 1H
NMR spectra were found to be identical with the data
described in the literature [38, 65, 66], and some products
selected at random were further confirmed by MS.
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Acknowledgments We are grateful to the National Natural Science
Foundation of China (no. 20772035) and the Natural Science Foun-
dation of Guangdong Province (no. 5300082) for financial support.
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