Amine- and phosphine-free palladium(II)-catalyzed homocoupling reaction of terminal alkynes

Article abstract of DOI:10.1016/j.tet.2004.12.026

An efficient, amine- and phosphine-free palladium(II)-catalyzed homocoupling of terminal alkynes has been developed. In the presence of PdCl₂, CuI, Me₃NO, and NaOAc, homocoupling of various terminal alkynes underwent smoothly to affo

Full text of DOI:10.1016/j.tet.2004.12.026

Tetrahedron 61 (2005) 1903–1907
Amine- and phosphine-free palladium(II)-catalyzed
homocoupling reaction of terminal alkynes
*
Jin-Heng Li, Yun Liang and Xu-Dong Zhang
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research, College of Chemistry and Chemical Engineering,
Hunan Normal University, Changsha 410081, China
Received 13 July 2004; revised 29 November 2004; accepted 1 December 2004
Available online 22 December 2004
Abstract—An efficient, amine- and phosphine-free palladium(II)-catalyzed homocoupling of terminal alkynes has been developed. In the
presence of PdCl2, CuI, Me3NO, and NaOAc, homocoupling of various terminal alkynes underwent smoothly to afford the corresponding
diynes in moderate to high yields without any phosphine ligands. In contrast, the presence of a phosphine ligand (PPh3) disfavored this
palladium-catalyzed homocoupling procedure. Bases, solvents, and CuI have fundamental influence on the palladium-catalyzed
homocoupling of terminal alkynes.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
report our findings that PdCl2, in combination with CuI,
Me3NO (as the reoxidant), and NaOAc (instead of amines as
the base), was proven to be an extremely effective catalytic
system for the homocoupling of various terminal alkynes.
Diynes are useful building blocks in organic synthesis and a
recurring functional group in many natural products and
bioactive compounds.1–3 As a result, considerable effort has
been directed to the development of new and efficient
methods for the synthesis of diynes since 1869.3–6
Palladium-catalyzed homocoupling reaction of terminal
alkynes transformation represents one of the most attractive
routes to synthesize symmetrical diynes due to their
mildness and efficiency.3g,f,6 This homocoupling method
is generally carried out in the presence of phosphine ligands
(Ph3P) and amines (for example, i-Pr2NH, i-Pr2NEt, Dabco,
and Et3N). For example, Zhang and co-workers6f have
reported an efficient protocol for homocoupling of
alkynes using PdCl2(PPh3)2, CuI, ethyl bromoacetate and
amine (triethylamine or Dabco) as the catalytic system.
Fairlamb and co-workers6g have also described an efficient
PdCl2(MeCN)2 and CuI catalyzed homocoupling of alkynes
procedure in Et3N/MeCN, and more loadings of PPh3 were
required to improve the reaction. Generally, phosphine
ligands are generally sensitive to air and expensive which
puts significant limits on their synthetic applications.7
Amines also have characteristic foul smell and pungent
flavor. For these reasons, the development of an effective
procedure for homocoupling of alkynes under amine- and
phosphine-free conditions would be significant. Here, we
2. Results and discussion
2.1. Palladium-catalyzed homocoupling of phenyl-
acetylene (1a)
To evaluate the efficiency of PdCl2/CuI/Me3NO, homo-
coupling of phenylacetylene (1a) was tested in the absence
of any phosphine ligands, and the results were summarized
in Table 1. The results showed that the combination of
PdCl2, CuI, and Me3NO was an effective catalytic system
for the reaction. Initially, several bases including NaOAc,
Na2CO3, Et3N, and pyridine were examined, and the results
indicated that NaOAc was the most effective (entries 1–7).
Treatment of phenylacetylene (1a) with PdCl2 (5.6 mol%),
CuI (2.5 mol%), and Me3NO (2 equiv) in MeCN at room
temperature, gave the corresponding diyne 2a in only 41%
isolated yield after 14 h when 3 equiv of Et3N was used
(entry 6), whereas the isolated yield of 2 sharply rised to
96% after 10 h when NaOAc (3 equiv) was used (entry 3).
The results also demonstrated that the loadings of NaOAc
affected the reaction, and 3 equiv of NaOAc provided the
highest yields (entries 1–4).
Keywords: Phosphine-free; Palladium; Homecoupling reaction; Terminal
alkynes.
Oxidative reagents were then investigated (entries 3 and
8–10). The results indicated that oxidative reagents have
* Corresponding author. Tel.: C86 731 8872530; fax: C86 731 8872531;
e-mail: jhli@mail.hunnu.edu.cn
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.12.026

1904
J.-H. Li et al. / Tetrahedron 61 (2005) 1903–1907
Table 1. Palladium-catalyzed homocoupling of phenylacetylene (1a)a
Entry
Base (equiv)
Solvent
Time (h)
Conversion (%)b
Yield (%)c
1
2
3
4
5
6
NaOAc (1)
NaOAc (2)
NaOAc (3)
NaOAc (4)
Na2CO3 (3)
NEt3 (3)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
EtOH
10
10
10
10
14
14
14
24
10
10
12
12
12
12
12
12
12
12
16
12
23
17
100
100
100
100
9
47
57
100
100
20
31
38
20
55
47
36
39
32
68
75
96
90
7
41
51
!5
84
18
28
34
16
49
45
34
33
28
Trace
95
70
60
7
Pyridine (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
NaOAc (3)
8d
9e
10f
11
12
13
14
15
16g
17h
18i
19j
20k
21l
22m
MeOH
C6H6
THF
Dioxane
CH3CN/H2O
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
!5
100
100
100
a Reaction conditions: 1a (1 mmol), PdCl2 (5.6 mol%), CuI (2.5 mol%), Me3NO$2H2O (2 equiv), and solvent (5 mL) at room temperature under nitrogen.
b Conversion of 1a was determined by GC analysis.
c Isolated yield.
d CuCl2 (2 equiv) instead of Me3NO$2H2O. Red oil was obtained as the major product.
e Me3NO$2H2O (1 equiv).
Without Me3NO$2H2O.
f
g CH3CN/H2OZ4:1.
h Without CuI.
i
Without both CuI and Me3NO$2H2O.
Without PdCl2.
j
k Pd(OAc)2 (5.6 mol%) instead of PdCl2.
l
PdCl2(PPh3)2 (5.6 mol%) instead of PdCl2.
mPPh3 (11.2 mol%) was added.
fundamental influence on the reaction. Without any
oxidative reagent, a low yield was isolated after 10 h in
the presence of PdCl2 (5.6 mol%), CuI (2.5 mol%), and
NaOAc (3 equiv), whereas the present of 2 equiv of Me3NO
resulted in 96% yield of 2. But the yield was decreased to
84% with decreasing the loadings of Me3NO to 1 equiv.
CuCl2 also served as an oxidant for the conversion of 1a to
2a catalyzed by PdCl2, but was inferior to Me3NO (entry
8).8 A series of solvents including MeCN, EtOH, MeOH,
C6H6, THF, dioxane, and MeCN/H2O were also examined,
and MeCN was found to be the most effective solvent for the
homocoupling reaction (entries 3 and 11–16).
1a to 2a (entries 3, 21 and 22). This observation suggested
that the presence of PPh3, a phosphine ligand, disfavored the
reaction.
2.2. Palladium-catalyzed homocoupling of terminal
alkynes 1b–l
As summarized in Table 2, homocoupling of various
terminal alkynes were carried out smoothly to afford the
corresponding diynes in moderate to good yields under the
optimum reaction conditions. The results showed that
the palladium-catalyzed homocoupling reaction tolerated
a variety of functional groups, and the yields and rates
depended upon the substrates. For homocoupling of
aromatic alkynes 1b–e, the aromatic alkynes 1b and 1c
bearing electron-donating groups proved to be more
effective. In the presence of PdCl2 (5.6 mol%), Me3NO$
2H2O (2 equiv), CuI (2.5 mol%), and NaOAc (3 equiv),
aromatic alkynes 1b–e were coupled to afford the
corresponding diynes 2b–e in 98, 90, 65 and 78% yields,
respectively (entries 1–4 in Table 2). Interestingly, for
homocoupling of aliphatic alkynes 1f–h, the corresponding
products 2f–h were also obtained in moderate to good
yields (70, 81 and 100% yields, respectively, entries 5–7).
A number of other alkynes with different functional groups
such as 1-ethynyl cyclohexene (1i), 1-ethynyl cyclohexanol
It is noteworthy that both PdCl2 and CuI played crucial roles
in the reaction (entries 3 and 17–19). Without the aid of CuI,
only 33% yield of 2 was isolated after 12 h in the presence
of PdCl2 (5.6 mol%), Me3NO (2 equiv), and NaOAc
(3 equiv). Similarly, trace amount of 2 was observed in
the absence of PdCl2.
Finally, various palladium catalytic systems were evaluated
(entries 3 and 19–22). In addition to PdCl2, Pd(OAc)2
served as an effective catalyst for the homocoupling
reaction of alkyne 1a to form diyne 2a (entries 3 and 20).
In contrast, neither PdCl2(PPh3)2 nor PdCl2/PPh3 as the
catalytic system was effective as PdCl2 for the conversion of

J.-H. Li et al. / Tetrahedron 61 (2005) 1903–1907
Table 2. Palladium-catalyzed homocoupling of alkynesa
1905
Entry
1
Alkyne
Time (h)
12
Yield (%)b
98
(1b)
2
3
4
20
12
48
90
65
78
(1c)
(1d)
(1e)
5
6
7
8
24
24
9
70
81
(1f)
(1g)
(1h)
100
82
16
(1i)
9
13
88
(1j)
10
11
15
12
68
67
(1k)
(1l)
a Reaction conditions: 1 (1 mmol), PdCl2 (5.6 mol%), CuI (2.5 mol%), Me3NO$2H2O (2 equiv), NaOAc (3 equiv), and MeCN (5 mL) at room temperature.
b Isolated yield.
(1j), 3,3,5-trimethyl hex-1-yn-3-ol (1k), and propargyl
acetate (1l) underwent the homocoupling reaction smoothly
to afford the corresponding diynes 2i–l in 82, 88, 68 and
67% yields, respectively (entries 8–11).
4 and regenerate the active Cu(I) species. Reductive
elimination of the dialkynylpalladium(II) intermediate 4
could undergo to form the desired diyne 2 and Pd(0).
Finally, Pd(0) was oxidated by Me3NO to regenerate the
active Pd(II) species leading to a new catalytic cycle.
2.3. Mechanism
It should be noted that: (1) the presence of both amines and
phosphine ligands were disfavored to the present palladium-
catalyzed homocoupling reaction. The reason might be that
reduction of Pd(II) to Pd(0) by amines or phosphine ligands
occurs to slow the reaction;9 (2) without the aid of CuI, there
are 33% isolated yield of 2 after 12 h in the presence of
PdCl2 (5.6 mol%), Me3NO (2 equiv), and NaOAc (3 equiv),
whereas without PdCl2, trace amount of 2 was observed
in the presence of CuI (2.5 mol%), Me3NO (2 equiv), and
NaOAc (3 equiv). This observation suggested that with the
aid of base, the dialkynylpalladium(II) intermediate 4 could
be formed from either direct attack of Pd(II) to 1 (Pathway
II) or replacement of Pd(II) with Cu(I) of intermediate 3
(Pathway I), and the latter (Pathway I) is faster. This result
also demonstrated that Me3NO was not an effective oxidant
for the Cu(I)-catalyzed Glaser coupling reaction.11
As outlined in Scheme 1, we have formulated a working
mechanism for the palladium-catalyzed homocoupling of
terminal alkynes based on the proposed mechanism of the
previous reports and our results.6 With the aid of a base,
the reaction of 1 with CuI afforded intermediate 3 readily.
Then the replacement reaction of Pd(II) with intermediate 3
would occur to form a dialkynylpalladium(II) intermediate
3. Conclusion
In summary, we have developed a mild and efficient
protocol for homocoupling of various terminal alkynes in
the presence of PdCl2, CuI, Me3NO and NaOAc. This
Pd(II)-catalyzed procedure not only tolerates a range of
functional groups, but also does not require any phosphine
Scheme 1.

1906
J.-H. Li et al. / Tetrahedron 61 (2005) 1903–1907
ligands. Currently, further efforts to study the mechanism
and apply the new approach in organic synthesis are
underway in our laboratory.
13.9. MS m/z (%): 190 (MC, 1), 161 (15), 105 (57), 91
(100).
4.2.7. Eicosa-9,11-diyne (2g).6 Colorless oil; 1H NMR
(400 MHz, CDCl3) d: 2.27 (t, JZ7.2 Hz, 4H), 1.57–1.50 (m,
4H), 1.43–1.38 (m, 4H), 1.34–1.20 (m, 16H), 0.90 (t, JZ
6.4 Hz, 6H); 13C NMR (100 MHz, CDCl3) d: 77.5, 65.2,
31.8, 29.1, 29.0, 28.8, 28.3, 22.6, 19.2, 14.1. MS m/z (%):
245 (MCKC2H5, 2), 161 (23), 147 (25), 133 (30), 119 (42),
105 (56), 91 (100).
4. Experimental
4.1. General methods
1H and 13C NMR spectra were recorded on an INOVA-400
(Varian) spectrometer with CDCl3 as the solvent. All
reagents were directly used as obtained commercially.
4.2.8. 2,2,7,7-Tetramethyl octa-3,5-diyne (2h).6,10g White
solid, mp 128–130 8C (lit.10g 130–130.5 8C); 1H NMR
(400 MHz, CDCl3) d: 1.23 (s, 18H); 13C NMR (100 MHz,
CDCl3) d: 86.3, 63.6, 30.6, 28.0. MS m/z (%): 162 (MC, 8),
161 (25), 133 (37), 119 (55), 91 (100).
4.2. Typical experimental procedure for the palladium-
catalyzed homocoupling of alkynes
A mixture of alkyne 1 (1 mmol), PdCl2 (5.6 mol%), CuI
(2.5 mol%), Me3NO$2H2O (2 equiv), NaOAc (3 equiv),
and MeCN (5 mL) was stirred under N2 at room temperature
until complete consumption of starting material as judged
by TLC and GC analysis. After the mixture was filtered and
evaporated, the residue was purified by flash column
chromatography to afford 2 (hexane or hexane/ethyl
acetate). All products 2 are known and all the melting
points are uncorrected.6,10
4.2.9. 1,4-Bis(cyclohex-1-enyl) buta-1,3-diyne (2i).6f
White solid, mp 63–65 8C; H NMR (400 MHz, CDCl3) d:
6.25 (t, JZ4.0 Hz, 2H), 2.12–2.10 (m, 8H), 1.66–1.57 (m,
8H); 13C NMR (100 MHz, CDCl3) d: 138.0, 119.9, 82.7,
71.5, 28.6, 25.8, 22.1, 21.3. MS m/z (%): 210 (MC, 3), 111
(20), 85 (71), 71 (95), 57 (100).
1
4.2.10. 1,4-Bis(1-hydroxycyclohexyl) buta-1,3-diyne
1
(2j).6f White solid, mp 173–175 8C; H NMR (400 MHz,
4.2.1. 1,4-Diphenyl buta-1,3-diyne (2a).6,10 White solid,
mp 86–88 8C (lit.10a 88 8C); 1H NMR (400 MHz, CDCl3) d:
7.54–7.52 (m, 4H), 7.38–7.34 (m, 6H); 13C NMR
(100 MHz, CDCl3) d: 132.5, 129.2, 128.5, 121.8, 81.5,
73.9. MS m/z (%): 202 (MC, 100).
CDCl3) d: 1.95–1.91 (m, 4H), 1.82 (s, 2H), 1.71 (t, JZ
8.0 Hz, 4H), 1.63–1.53 (m, 8H), 1.26–1.20 (m, 4H); 13C
NMR (100 MHz, CDCl3) d: 83.0, 69.2, 68.3, 39.7, 25.0,
23.1. MS m/z (%): 246 (MC, 1), 210 (100).
4.2.11. 2,4,9,11-Tetramethyl dodeca-5,7-diyne-4,9-diol
1
(2k).10h White solid, mp 78–80 8C; H NMR (400 MHz,
4.2.2. 1,4-Bis(p-methylphenyl) buta-1,3-diyne (2b).6,10a,e
White solid, mp 138–140 8C; H NMR (400 MHz, CDCl3)
d: 7.41 (d, JZ8.0 Hz, 4H), 7.14 (d, JZ8.0 Hz, 4H), 2.36 (s,
6H); 13C NMR (100 MHz, CDCl3) 139.5, 132.4, 129.2,
118.8, 81.5, 73.4, 21.6. MS m/z (%): 230 (MC, 100).
1
CDCl3) d: 4.82 (s, 2H), 1.95–1.56 (m, 2H), 1.61 (d, JZ
6.4 Hz, 4H), 1.51 (s, 6H), 1.02 (d, JZ5.2 Hz, 6H), 1.00 (d,
JZ5.2 Hz, 6H); 13C NMR (100 MHz, CDCl3) d: 83.4, 68.6,
67.7, 51.5, 34.6, 25.1, 24.2, 24.0. MS m/z (%): 250 (MC, 1),
232 (1), 85 (51), 71 (100).
4.2.3. 1,4-Bis(p-methoxyphenyl) buta-1,3-diyne (2c).6,10
White solid, mp 140–142 8C; H NMR (400 MHz, CDCl3)
d: 7.46 (d, JZ8.8 Hz, 4H), 6.85 (d, JZ8.8 Hz, 4H), 3.82 (s,
6H); 13C NMR (100 MHz, CDCl3) d: 160.2, 134.0, 114.1,
113.9, 81.2, 72.9, 55.3. MS m/z (%): 262 (MC, 100)
4.2.12. Acetic acid 6-acetoxy hexa-2,4-diynyl ester (2l).6g
1
1
Colorless oil; H NMR (400 MHz, CDCl3) d: 4.74 (s, 4H),
2.10 (s, 6H); 13C NMR (100 MHz, CDCl3) d: 169.4, 73.6,
70.3, 52.8, 20.6. MS m/z (%): 194 (MC, 5), 135 (3), 76
(100).
4.2.4. 1,4-Bis(trifloromethylphenyl) buta-1,3-diyne
(2d).10e White solid, mp 166–168 8C; H NMR (400 MHz,
1
Acknowledgements
CDCl3) d: 7.65 (d, JZ8.4 Hz, 4H), 7.62 (d, JZ8.4 Hz, 4H);
13C NMR (100 MHz, CDCl3) d: 132.8, 131.2, 130.9, 125.5,
125.4, 125.2, 125.0, 122.3, 81.0, 75.6. MS m/z (%): 338
(MC, 100).
We thank the National Natural Science Foundation of China
(No. 20202002), Education Department of Hunan Province
(No. 02C211) and Hunan Normal University (2001) for the
financial support.
4.2.5. 1,4-Bis(2-pyridine) buta-1,3-diyne (2e).10e White
solid, mp 116–118 8C (lit.10f 118–119 8C); 1H NMR
(400 MHz, CDCl3) d: 8.63 (d, JZ4.4 Hz, 2H), 7.71 (t, JZ
7.6 Hz, 2H), 7.56 (d, JZ8.4 Hz, 2H), 7.31 (t, JZ4.8 Hz,
2H); 13C NMR (100 MHz, CDCl3) d: 150.3, 141.8, 134.3,
128.4, 123.8, 80.8, 73.2. MS m/z (%): 204 (MC, 100).
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4.2.6. Tetradeca-6,8-diyne (2f).10e Colorless oil; H NMR
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4H), 1.39–1.28 (m, 8H), 0.89 (t, JZ7.2 Hz, 6H); 13C NMR
(100 MHz, CDCl3) d: 77.5, 65.2, 31.0, 28.0, 22.2, 19.2,
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