Organic reaction in Solkane 365 mfc: Homocoupling reaction of terminal alkynes

Article abstract of DOI:10.1039/c1gc15106a

Solkane 365 mfc is proven to be an environmentally benign alternative solvent for the homocoupling reaction of terminal alkynes catalyzed by CuCl/TMEDA under air at room temperature. A variety of terminal alkynes, including aromatic, heteroaromatic and aliphatic alkynes, were cleanly transformed into 1,3-diynes within a short period in high to excellent yields up to 98%. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1gc15106a

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Organic reaction in Solkaneꢀ 365 mfc: homocoupling reaction of terminal
alkynes†
Akihiro Kusuda, Xiu-Hua Xu, Xin Wang, Etsuko Tokunaga and Norio Shibata*
Received 26th January 2011, Accepted 21st February 2011
DOI: 10.1039/c1gc15106a
pressure11a,b,12a or high temperature12a,14b,d are required. In this
context, we came up with an idea to investigate oxidative alkyne–
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Solkaneꢀ 365 mfc is proven to be an environmentally
benign alternative solvent for the homocoupling reaction
of terminal alkynes catalyzed by CuCl/TMEDA under
air at room temperature. A variety of terminal alkynes,
including aromatic, heteroaromatic and aliphatic alkynes,
were cleanly transformed into 1,3-diynes within a short
period in high to excellent yields up to 98%.
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alkyne coupling reactions in commercially available Solkaneꢀ
365 mfc (1,1,1,3,3-pentafluorobutane), developed by Solvay
Fluor GmbH, as an environmentally friendly liquid. In addition
to the potential advantages of fluorinated solvents for oxidation
reactions due to the higher solubility of oxygen,15 Solkaneꢀ
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365 mfc is non-toxic and has no impact whatsoever on the
ozone layer.16 It is used as an insulating and blowing agent for
polyurethane foams, whose main uses are thermal insulation
of residential and industrial buildings as well as cold storage.
Introduction
Conjugate 1,3-diyne derivatives are one of the most versatile
building block materials, particularly for the synthesis of
natural products,1 pharmaceuticals,2 and polymers.3 Since the
first discovery of oxidative coupling of copper acetylides by
Glaser,4 oxidative alkyne–alkyne coupling has become a stan-
dard strategy for the synthesis of 1,3-diyne derivatives.5 Current
research on the Glaser-type coupling reaction is mainly focused
on modifications of Glaser’s original condition to promote
oxidative alkyne–alkyne coupling more effective, including Cu-
mediated conditions,6 Pd/Cu-catalyzed coupling reactions,7 Co-
catalyzed systems8 and the combination of Cu and Ag9a or
Ni9b,c salts. In the 21st century, a fair amount of research has
been geared towards making the Glaser-type coupling reaction
more environmentally benign. The classical synthesis of 1,3-
diynes generally involves organic solvents such as methanol,
acetone, pyridine, methyl cellosolve and toluene. These solvents
have some drawbacks such as toxicity and peroxide formation.
Increasingly tighter restriction of the use of organic solvents in
industrial synthesis10 led us to search for an environmentally
benign alternative solvent for this coupling reaction. Super-
critical fluids,11 water,12 and ionic liquids13 were examined to
minimize using organic solvents in oxidative alkyne–alkyne
coupling. Solvent-free systems14 have also been applied for the
synthesis of 1,3-diynes. However, in these modified procedures,
microwave,14a,b co-catalyst,12c co-solvent,11a,b,12b oxidant,12b,c high
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Solkaneꢀ 365 mfc is now produced in a pilot plant with a
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capacity of several hundred tons per year. Solkaneꢀ 365 mfc
has a flash point below -27 ◦C, but is difficult to ignite. The
minimum ignition energy is around 50 times higher than of
n-pentane and is 10.8 mJ (25 C, 8 vol% in air at 1 bar).16
We have recently reported environmentally benign reaction
◦
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systems with Solkaneꢀ 365 mfc as the medium for inorganic
base-catalyzed trifluoromethylation of carbonyl compounds17a
and organocatalyzed asymmetric Friedel–Crafts alkylation of
indoles.17b As part of our ongoing fluorine chemistry project,17
we report herein a metal-catalyzed homocoupling reaction of
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terminal alkynes in Solkaneꢀ 365 mfc to provide a series of
1,3-diyne derivatives in high yields (Scheme 1). The present
result provides not only for an environmentally benign pro-
cedure for the homocoupling reaction of terminal alkynes,
but also the first example of a metal-catalyzed reaction in
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Solkaneꢀ 365 mfc. These findings reinforce the versatility
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potential of Solkaneꢀ 365 mfc as a medium for organic
reactions.
Department of Frontier Materials, Graduate School of Engineering,
Nagoya Institute of Technology, Gokiso, Showa-ku, Nagoya, 466-8555,
Japan. E-mail: nozshiba@nitech.ac.jp; Fax: +81 52-735-5442
† Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/c1gc15106a
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Scheme 1 Homocoupling reaction of terminal alkynes in Solkaneꢀ
365 mfc.
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Table 1 Optimization of homocoupling reaction conditions in
Solkaneꢀ 365 mfc
Table 2 Homocoupling reaction of various terminal alkynes in
Solkaneꢀ 365 mfc
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R
Entry
CuCl (mol%)
TMEDA (mol%)
Time (h)
Yield (%)a
Entry
1
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Time (h)
2
Yield (%)a
1
2
3
4
5
6
7
20
10
5
10
10
5
20
20
20
10
5
5
5
1.0
1.0
1.0
1.0
1.0
1.0
24.0
91
88
73
93
82
68
86
1
1a
1b
1b
1c
1d
1e
1f
1g
1h
1i
Phenyl
1.0
2.0
2.5
3.0
1.0
3.0
2.0
4.5
1.0
5.0
4.0
5.5
6.0
6.0
3.0
3.0
2.5
24.0
2a
2b
2b
2c
2d
2e
2f
93
33
90
87
89
91
89
86
82
85
84
2
2-CH3C6H4
2-CH3C6H4
3-CH3C6H4
4-CH3C6H4
2-FC6H4
3-FC6H4
4-FC6H4
3,5-F2C6H3
4-CH3OC6H4
4-CF3OC6H4
4-n-C5H11C6H4
C6H5CH2CH2
3b
4b
5
6
7
8
9
5
2g
2h
2i
a Isolated yield by silica-gel column chromatography.
10b
11
12b
13b
14
15
16
17
18
1j
1k
1l
2j
2k 98
2l
95
Results and discussion
1m Pyridine-2-yl
2m 92
1n
1o
1p
1q
Thiophen-3-yl
HOC(CH3)2
(1-Hydroxy)-c-pentyl
(1-Hydroxy)-c-hexyl
2n
2o
2p
2q
89
88
96
91
In 1962, Hay improved the Glaser coupling reaction by us-
ing a CuCl-TMEDA (N,N,N¢,N¢-tetramethylethylenediamine)
complex as an effective catalyst under oxygen or air.6c The
Hay reaction was since the most commonly used protocol
for oxidative homocoupling of alkynes. Although Pd-assisted
Glaser-type coupling reactions are efficient and proceed under
mild reaction conditions, palladium reagents are far more
expensive than Cu salts, and air-sensitive and poisonous ligands
are often required.7 So we decided to choose Hay’s procedure
a Isolated yield by silica-gel column chromatography. b DBU (1.0 eq.)
was used as an additive.
(entries 12–13). For heterocyclic aromatic alkynes 1m and 1n, the
yields were almost the same as aromatic alkynes (entries 14–15).
The reaction of the alkynes based on propargylic alcohols (1o–
1q) also gave the corresponding diynes in good yields (entries
16–18). Most of the reactions were completed within 1 to 6 h, but
the coupling reaction of 1-ethynylcyclohexanol (1q) required 24
h to be completed (entry 18). This is mainly due to the
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for the homocoupling of terminal alkynes in Solkaneꢀ 365 mfc.
Phenylacetylene (1a) was initially chosen as the test substrate.
The homocoupling of 1a was carried out with a different amount
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of CuCl and TMEDA in Solkaneꢀ 365 mfc under air at room
temperature (Table 1). The best ratio of substrate, CuCl, and
TMEDA was 1 : 0.1 : 0.1 (entry 4). The reaction was completely
finished in 1 h, giving the desired homocoupling product (2a)
in 93% yield. It should be mentioned that this reaction time was
solubility
mfc.
problem
of
1q
into
Solkaneꢀ
365
What is more, this catalytic system can be applied in the
synthesis of binuclear phthalocyanine (Pc; Scheme 2). Pcs
have drawn substantial interest as molecular materials, which
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much shorter in Solkaneꢀ 365 mfc (1 h) than in organic solvents
such as methanol (12 h) and ionic liquids (4.5 h).13a This is
presumably due to the higher solubility profile of oxygen into
fluorous solvents.15 When a smaller amount of CuCl or TMEDA
was used, the yield was somewhat decreased (Table 1, entries 3,
5 and 6), although higher yield was achieved after 24 h (Table 1,
entry 7).
Under the optimized reaction conditions, the homocoupling
reactions of various terminal alkynes (1a–1q) were examined
to see the reaction scope in terms of the substrates (Table 2).
As can be seen in the Table, the reaction proceeded cleanly
to give high to excellent yields of conjugate 1,3-diynes. When
the reaction was slow (entry 2), the addition of a base,6r
such as DBU (1,8-diazabicycloundec-7-ene) accelerated the
reaction and increased the product yield (entry 3). The catalytic
oxidative homocoupling reactions of ethynylbenzene derivatives
(1a–k) with electron-donating as well as electron-withdrawing
substituents proceeded to afford the corresponding aromatic
1,4-disubstituted-1,3-diyne derivatives (2a–k) in high yields
(entries 1–12). Notably, the reaction of fluorine-containing
alkynes 1e–h also proceeded efficiently (entries 6–9). Less
reactive aliphatic linear alkynes 1k and 1l were converted into
their corresponding diynes in the presence of DBU in high yields
Scheme 2 Synthesis of trifluoroethoxy-coated binuclear phthalocya-
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nine 4 in Solkaneꢀ 365 mfc.
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Table 3 Homocoupling reaction of various terminal alkynes in
Solkaneꢀ 365/227
under reduced pressure. The residue was purified by column
chromatography on silica-gel (n-hexane) to give 2a as a white
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solid (47.1 mg, 93%).
Acknowledgements
This study was financially supported in part by Grants-in-
Aid for Scientific Research (B21390030, 22106515), and Chubu
Bureau of Economy Trade and Industry. We thank Dr Max
Braun and Mr Yoshitaka Toyofuku, Solvay Fluor GmbH for
Entry
1
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Time (h)
2
Yield (%)a
1
1a
1c
1g
1m
1o
Phenyl
3-CH3C6H4
4-FC6H4
Pyridine-2-yl
HOC(CH3)2
4.0
5.0
9.0
9.0
3.0
2a
2c
2g
2m
2o
87
93
76
87
93
2b
3
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the generous gift of Solkaneꢀ 365 mfc and Solkaneꢀ 365/227.
4
5
a Isolated yield by silica-gel column chromatography. b DBU (1.0 eq.)
was used as an additive.
Notes and references
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Naturally Occurring Acetylenes, Academic Press, London, 1973;
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Justus Liebigs Ann. Chem., 1870, 154, 137.
5 For a review, see: P. Siemsen, R. C. Livingston and F. Diederich,
Angew. Chem., Int. Ed., 2000, 39, 2633.
can be widely applied in electrochromic devices, information
storage systems and liquid crystal colour displays, and more.18
The homocoupling reaction of trifluoroethoxy-coated terminal
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acetylene 3 in Solkaneꢀ 365 mfc at room temperature gave
the desired binuclear Pc 4 in moderate yield. The yield was
comparable to that when the coupling reaction proceeded in
pyridine.19
Finally, we examined the homocoupling reactions of terminal
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alkynes in commercially available Solkaneꢀ 365/227 blend
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solvent (Solkaneꢀ 227: 1,1,1,2,3,3,3-heptafluoropropane). It is
disclosed that Solkaneꢀ 365 mfc has a flash point £-27 ◦C;
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however, its blend form (93/7 mixture of Solkaneꢀ 365 mfc
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and Solkaneꢀ 227) as Solkaneꢀ 365/227 is known to have
no flash point (non-flammable) and could provide the benefits
16b
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of Solkaneꢀ 365 mfc. As shown in Table 3, both aromatic
and aliphatic terminal alkynes underwent the coupling reaction
cleanly to produce corresponding 1,3-diynes in high to excellent
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yields in Solkaneꢀ 365/227.
Conclusions
We have reported the Cu-catalyzed Glaser-type coupling reac-
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tion with Solkaneꢀ 365 mfc as the medium. Good to excellent
yields were obtained with a large variety of alkynes, including
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phthalocyanine. Even in Solkaneꢀ 365/227, a non-flammable
solvent, the coupling reaction also proceeded efficiently. We
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hope that Solkaneꢀ 365mfc and Solkaneꢀ 365/227 would be
green organic solvents of choice next to water and ionic liquids
in industrial process chemistry. Extension to other ubiquitous
organic reactions with Solkaneꢀ 365 mfc as a medium is
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currently under investigation.
Experimental
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Typical procedure for homocoupling in Solkaneꢀ 365 mfc
A mixture of phenylacetylene 1a (54.9 mL, 0.50 mmol), TMEDA
(7.5 mL, 0.050 mmol) and CuCl (5.0 mg, 0.050 mmol) in
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Solkaneꢀ 365 mfc (1.0 mL) was stirred under air at room tem-
perature. The resulting mixture was stirred for 1 h. The reaction
was diluted with saturated aqueous NH4Cl. The aqueous layer
was extracted with CH2Cl2. The combined organic layer was
then washed with brine, dried over Na2SO4 and concentrated
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The Royal Society of Chemistry 2011
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16 (a) See Solvay Solkaneꢀ 365 mfc brochure, http://www.so-
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(accessed February 2011). These web addresses may not be accessible
or could be changed in the future. If this were to happened, this
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846 | Green Chem., 2011, 13, 843–846
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The Royal Society of Chemistry 2011
©