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Analternative CuClЦpiperidine-catalyzed oxidative homocoupling of terminal alkynes affording 1 3-diynes in air.

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Full Paper
Received: 29 September 2009
Revised: 4 November 2009
Accepted: 11 November 2009
Published online in Wiley Interscience: 22 December 2009
(www.interscience.com) DOI 10.1002/aoc.1604
An alternative CuCl–piperidine-catalyzed
oxidative homocoupling of terminal alkynes
affording 1,3-diynes in air
Qingwei Zhenga, Ruimao Huaa∗ and Youzhi Wanb
CuCl with the use of a catalytic amount of piperidine as additive shows high catalytic activity for the oxidative homocoupling
c 2009 John Wiley & Sons,
reactions of terminal alkynes in toluene at 60 ◦ C in air to afford 1,3-diynes in high yields. Copyright Ltd.
Supporting information may be found in the online version of this article.
Keywords: cuprous chloride; 1,3-diyne; oxidative homocoupling; terminal alkyne
Introduction
1,3-Diynes represent an important class of organic compounds,
and the structural motif of 1,3-diyne has been recognized as an
important functionality in molecular materials. Thus numerous
synthetic approaches have been developed for the synthesis of
1,3-diynes, mainly including oxidative homocoupling reactions of
terminal alkynes catalyzed by palladium,[1 – 8] nickel[9] complexes
with the use of copper(I) as co-catalyst, Cu(I)/(II)-mediated oxidative homocoupling reactions of terminal alkynes[10 – 15] and homocoupling reactions of alkynylsilanes,[16] alkynylboronates[17] and
potassium alkynyltrifluoroborates,[18] and palladium-catalyzed homocoupling reactions of n-butyl alkynyltellurides.[19]
Recently, the use of Cu(I)/(II) salts and complexes as catalysts
in organic synthesis has received increased attention due to
their easy availability, non-toxicity, ease of handling and low
cost.[20 – 29] Although it has been known for decades that CuBr–NH3
(Glaser coupling)[30] and CuCl·TMEDA (TMEDA = N,N,N ,N tetramethylethylenediamine, Hay coupling[31] ) could be used as
a catalyst for the oxidative homocoupling of terminal alkynes in
an atmosphere of oxygen; however, reports on the applications of
Glaser and Hay coupling procedures, as well as the use of Cu(I) alone
as the catalyst in such reactions, are very rare.[32 – 34] Very recently,
Beifuss and co-workers reported on the improved Hay’s procedure,
in which the reaction still requires oxygen as an oxidant and a long
reaction time (24 h in most cases) to achieve high conversion.[35]
Because the Cu(I)-catalyzed oxidative homocoupling of terminal
alkynes is the method of choice to synthesize 1,3-diynes, in this
work attempts have been made to examine the catalytic activity of
simple copper salts as the exclusive metal catalyst in the oxidative
homocoupling reaction of 1-heptyne in air as the model reaction
under different reaction conditions.
DMSO and DMF for the oxidative homocoupling of 1-heptyne,
only a small amount of tetradeca-6,8-diyne (2a) was formed in the
reaction mixture as confirmed by GC-MS (entries 1–6). Although
the addition of Bu3 N (10 mol%) as additive remained inefficient
(entry 7), the addition of Bu2 NH greatly improved the catalytic
activity of CuCl to afford 2a in 31% GC yield (entry 8). Fortunately,
the use of a catalytic amount of piperidine (10 mol%) in DMF
increased the catalytic activity of CuCl significantly to produce
2a in 94% GC yield (entry 9). A similar result was obtained
when toluene was used to replace DMF as the solvent (entry
10). However, when CuCl2 was employed as catalyst to replace
CuCl, only a low yield of 2a was obtained (entry 11), and in the
absence of piperidine, CuCl2 showed no catalytic activity (entry
12). Therefore, it has been confirmed that piperidine, which is
a cheap and commercially available organic base, is capable of
improving the catalytic activity of CuCl as an additive in the
oxidative homocoupling reaction of terminal alkynes under air at
60 ◦ C to afford 1,3-diynes in high yields.
Under the optimized conditions indicated in entry 10 of Table 1,
the scope of the present CuCl–piperidine-catalyzed oxidative
homocoupling reactions of a variety of simple and functionalized
alkynes in air was examined (Table 2). Both aliphatic and aromatic
alkynes underwent oxidative homocoupling reactions to give the
desired 1,3-diynes in high isolated yields. Functional groups such
as vinyl-, hydroxy-, chloro- and cyano- bound to aliphatic alkynes
were tolerated (entries 2–5). The reactions of aromatic alkynes
with both electron-donating groups (Me, MeO) (entries 7–9) and
electron-withdrawing groups (CF3 , phenyl) (entries 10 and 11)
gave the corresponding 1,3-diynes in high yields. In addition, 2thienylacetylene also underwent the oxidative coupling reaction
∗
Results and Discussion
314
First, solvent effects on the catalytic activity of CuCl and CuI were
investigated. As shown in Table 1, at 60 ◦ C, CuCl and CuI (2 mol%)
showed no catalytic activity in toluene, 1,4-dioxane, Cl2 CHCHCl2 ,
Appl. Organometal. Chem. 2010, 24, 314–316
Correspondence to: Ruimao Hua, Department of Chemistry, Tsinghua University, Beijing 100084, China. E-mail: ruimao@mail.tsinghua.edu.cn
a Department of Chemistry, Tsinghua University, Beijing 100084, China
b Department of Applied Chemistry, College of Science, Beijing University of
Chemical Technology, Beijing 100029, China
c 2009 John Wiley & Sons, Ltd.
Copyright Alternative CuCl–piperidine-catalyzed oxidative homocoupling of terminal alkynes
Table 1. Copper-catalyzed oxidative homocoupling of 1-heptyne
under different conditionsa
cat. (2.0 mol%)
additive (10 mol%)
n-C5H11
solvent, in air
60 °C, 5 h
1a
Entry
1
2
3
4
5
6
7
8
9
10
11
12
Catalyst
CuCl
CuCl
CuCl
CuCl
CuCl
Cul
CuCl
CuCl
CuCl
CuCl
CuCl2
CuCl2
Solvent
Toluene
1,4-Dioxane
Cl2 CHCHCl2
DMSO
DMF
DMF
DMF
DMF
DMF
Toluene
Toluene
Toluene
n-C5H11
n-C5H11
2a
Yield (%)b
Additive
Bu3 N
Bu2 NH
Piperidine
Piperidine
Piperidine
<5
<5
<5
<5
<5
<5
<5
31
94
93 (89)
34
<5
Table 2. CuCl-catalyzed
alkynesa
oxidative
homocoupling of
CuCl (2.0 mol%), piperidine (10 mol%)
R
(5 ~ 20 mmol) toluene, in air, 60 °C, 5 h or 8 h
1
Entry
terminal
R
R
2
Isolated yield(%)b
R
1
2
1b
1c
n-C6 H13
2b
2c
94
90
3
4
5
6
7
8
9
10
11
1d
1e
1f
1g
1h
1i
1j
1k
1l
Me2 (HO)C
Cl(CH2 )3
NC(CH2 )3
C6 H5
p-MeC6 H4
o-MeC6 H4
p-MeOC6 H4
o-F3 CC6 H4
2d
2e
2f
2g
2h
2i
2j
2k
2l
77
90
87
96
89
88
89
94
96
12
1m
2m
92
a
Reactions were carried out using 5.0 mmol of 1a, 0.5 mmol of additive
(if used) and 0.1 mmol of catalyst in 4.0 ml of solvent for 5 h.
b
Yield according to GC based on 1a used. Number in parentheses is
isolated yield.
smoothly. Therefore, the present catalytic method is general and
encompasses a variety of functional groups.
It should be noted that neither dimerization nor trimerization of
terminal alkynes were observed under the reaction conditions as
confirmed by GC and GC-MS of reaction mixtures. In addition,
the asymmetrical 1,4-disubstituted 1,3-diynes could be also
synthesized under similar reaction conditions through the crossoxidative coupling of two different terminal alkynes. As shown in
Scheme 1, performing the reaction of 1g or 1-naphthyl acetylene
(1n) with an excess amount of 1c resulted in the formation of 2n
and 2o in 78 and 50% isolated yields, respectively. These reactions
were accompanied by the formation of 2c.
Conclusion
In conclusion, we have demonstrated that CuCl with the use of a
catalytic amount of piperidine as additive exhibits high catalytic
activity in the oxidative homocoupling of terminal alkynes in air
to afford 1,3-diynes with high yields. The following advantages of
the present procedure are worth noting: (i) CuCl is the exclusive
metal precatalyst, which is cheap and easily available; (ii) the
addition of piperidine very efficiently promotes the catalytic
activity of CuCl under mild reaction conditions in air; (iii) neither
complicated ligands nor excess amount of organic base is required;
and (iv) a wide variety of functional groups are tolerated in
this procedure. These advantages make the present catalytic
procedure an attractive option for the synthesis of 1,3-diynes.
Experimental Section
General Methods
Appl. Organometal. Chem. 2010, 24, 314–316
a
Reactions were carried out using 5.0 ∼ 20 mmol of alkyne, 0.1 equiv
of piperidine and 0.02 equiv of CuCl in toluene (1.25 M) at 60 ◦ C for 5 h
(5.0 mmol of alkyne used) or 8 h (>5.0 mmol of alkyne used).
b Isolated yield based on alkyne used.
Ar
CuCl (2.0 mol%)
piperidine (10 mol%)
+ 1c
toluene, air, 60 °C, 5 h
Ar
5.0 equiv
Ar = Ph (1g)
α-naphthyl (1n)
Ar = ph (2n): 78% (2c: 21%)
α-naphthyl (2o): 50% (2c: 51%)
Scheme 1. Synthesis of asymmetric 1,3-diynes.
obtained from Joel JNM-ECA300 spectrometers at 300 and 75 MHz,
respectively. 1 H chemical shifts (δ) were referenced to TMS, and
13
C NMR chemical shifts (δ) were referenced to internal solvent
resonance. GC analyses of organic compounds were performed
on an Agilent Technologies 1790 GC (with an SGE-OV1701 25m
capillary column) instrument. Mass spectra were obtained on
a Shimadzu GCMS-QP2010S. High-resolution mass spectra were
obtained with a ZAB-HS mass spectrometer from the Department
of Chemistry of Beijing University. Element analyses were obtained
with a Flash EA 1112 Element Analyzer in the Institute of Chemistry,
Chinese Academy of Sciences.
Typical Experimental Procedure for the Oxidative Homocoupling of Heptyne (1a) to Afford Tetradeca-6,8-diyne (2a)
(Table 1, Entry 10)
A mixture of heptyne (1a) (481.0 mg, 5.0 mmol), CuCl (10.0 mg,
0.1 mmol), piperidine (45.0 mg, 0.52 mmol) and toluene (4.0 ml)
in an atmosphere of air (opened to air) was heated with stirring
at 60 ◦ C for 5 h. After cooling, the reaction mixture was diluted
with CH2 Cl2 to 10.0 ml and n-hexadecane (46.3 mg, 0.20 mmol)
was added as internal standard for GC analysis. After GC and GCMS
c 2009 John Wiley & Sons, Ltd.
Copyright www.interscience.wiley.com/journal/aoc
315
All organic starting materials were analytically pure and were
used without further purification. 1 H and 13 C NMR spectra were
S
Q. Zheng, R. Hua and Y. Wan
analyses, the solvents and volatiles were removed under vacuum,
and the residue was then subjected to column chromatography
isolation on silica gel using cyclohexane as eluent. Compound 2a
was obtained in 423.4 mg (2.23 mmol, 89%) as a pale yellow oil.
The results of GC analysis of the reaction mixture revealed that 2a
was formed in 93% yield.
A larger-scale reaction required a longer reaction time (8 h)
to give a satisfactory yield. For example, the reactions of 1a
(30.0 mmol), or 1g (20.0 mmol) at 60 ◦ C for 8 h afforded 2a and 2g
in 91 and 96% isolated yields, respectively.
Compounds 2n and 2o are new compounds, which were
characterized by 1 H, 13 C-NMR, mass spectra and elemental analysis
or HRMS. Other homocoupling products are known compounds
and were characterized by 1 H, 13 C-NMR and mass spectra. The
spectroscopic data of 2n and 2o are reported below.
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Supporting information
Supporting information may be found in the online version of this
article.
316
www.interscience.wiley.com/journal/aoc
c 2009 John Wiley & Sons, Ltd.
Copyright Appl. Organometal. Chem. 2010, 24, 314–316
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