Palladium-catalyzed oxidative homocoupling reaction of terminal acetylenes using trans-bidentatable 1-(2-pyridylethynyl)-2-(2-thienylethynyl)benzene

Article abstract of DOI:10.1007/s11164-012-0654-2

Palladium-catalyzed oxidative homocoupling of terminal acetylenes, in air, occurred in the presence of 1-(2-pyridylethynyl)-2-(2-thienylethynyl)benzene as ligand, affording the corresponding conjugated diynes.

Full text of DOI:10.1007/s11164-012-0654-2

Res Chem Intermed (2013) 39:359–370
DOI 10.1007/s11164-012-0654-2
Palladium-catalyzed oxidative homocoupling reaction
of terminal acetylenes using trans-bidentatable
1-(2-pyridylethynyl)-2-(2-thienylethynyl)benzene
•
Shingo Atobe Motohiro Sonoda Yuki Suzuki
•
•
•
Takuya Yamamoto Haruna Masuno
•
•
Hiroyuki Shinohara Akiya Ogawa
Received: 24 October 2011 / Accepted: 15 December 2011 / Published online: 3 July 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Palladium-catalyzed oxidative homocoupling of terminal acetylenes, in
air, occurred in the presence of 1-(2-pyridylethynyl)-2-(2-thienylethynyl)benzene as
ligand, affording the corresponding conjugated diynes.
Keywords Palladium Á Acetylene Á Glaser coupling Á Bidentate ligand Á Diyne
Introduction
The 1,3-diyne unit is, because of its rigidity and conjugated structure, a useful
building block for the construction of a variety of functional materials [1, 2]; it is
also found in natural products [3]. In addition, 1,3-diyne is regarded as an important
intermediate in synthetic organic chemistry [4–13]. Owing to these advantages, a
variety of methods of synthesis of 1,3-diynes have been developed by coupling of
alkynylmetal species [14, 15] or the coupling of terminal acetylenes with alkynyl
halides [16, 17]. In particular, Cu-mediated or Cu-catalyzed oxidative coupling
reactions of terminal acetylenes, for example Hay coupling [18] and Glaser
coupling [19], are powerful tools for the synthesis of conjugated diynes. There are
few reports of effective methods for synthesis of 1,3-diynes by use of palladium-
catalyzed reactions [20–22], although 1,3-diynes are often found as by-products in
the Sonogashira coupling reactions [23]. Recently, we have reported the copper-free
Sonogashira coupling reaction using 1,2-bis(2-thienylethynyl)benzene (1) as a
trans-spanning ligand (Fig. 1) [24]. In general, trans-spanning ligands, which
This article is published as part of a themed issue in honor of Professor Kazuhiko Mizuno’s research
accomplishments on the occasion of his retirement.
S. Atobe Á M. Sonoda (&) Á Y. Suzuki Á T. Yamamoto Á H. Masuno Á H. Shinohara Á A. Ogawa
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,
1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
e-mail: sonoda@chem.osakafu-u.ac.jp
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S. Atobe et al.
S
S
S
N
1
2
Fig. 1 Rigid trans-spanning ligands 1 and 2
coordinate bidentately at trans-positions of the metal center [25–29], are believed to
inhibit oxidative addition and/or reductive elimination in a catalytic cycle [30].
Despite pessimistic predictions about trans-spanning ligands, we have a great
interest in the effects of their unique coordination structures on reactivity,
selectivity, and/or stability. In the course of our study on trans-spanning
diethynylbenzene ligands, we found that a Pd-catalyzed oxidative coupling reaction
of terminal acetylenes proceeded smoothly when 1-(2-pyridylethynyl)-2-(2-thien-
ylethynyl)benzene (2) was used as the trans-spanning ligand in air. Herein, we
describe the details of this catalytic reaction.
Results and discussion
trans-Spanning ligands 1 and 2 were synthesized from 1,2-diiodobenzene,
ethynyltrimethylsilane, and the corresponding 2-haloarenes via repeated Sonogash-
ira coupling reactions, as shown in Scheme 1 and described in the ‘‘Experimental’’
section. An attempt to perform X-ray crystallography of the [PdCl2(2)] complex to
confirm its trans-coordinated structure was not successful. However, DFT
calculations (B3LYP/6-31G*, LanL2DZ) clearly indicate that the trans-[PdCl2(2)]
complex can be formed. Moreover, in the 1H NMR spectrum of a mixture of ligand
2 and an equivalent amount of PdCl2 in CDCl3, prominent downfield shifts for the
aromatic protons adjacent to the nitrogen of the pyridine ring (d 9.01 ppm) and
adjacent to the sulfur of the thiophene ring (d 8.03 ppm) were observed compared
with the corresponding protons for free ligand 2 (d 8.62 and d 7.36 ppm,
respectively). A similar downfield shift was reported in comparison with 1,2-
bis(pyridylethynyl)benzene (3) and the trans-[PdCl2(3)] complex [31]. These results
also support formation of the trans-[PdCl2(2)] complex (Table 1).
When a mixture of 4-ethynyltoluene (4a, 0.5 mmol), triethylamine (0.2 mL,
1.7 mmol), catalytic amounts of PdCl2(PPh3)2 (0.01 mmol), and 2 (0.01 mmol) in
acetonitrile (1.5 mL) was stirred at ambient temperature for 19 h in air, an oxidative
homocoupling product 5a was obtained in 44 % yield (entry 1, Table 2). We
therefore optimized the reaction conditions using 4a as standard substrate. Use of
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Palladium-catalyzed oxidative homocoupling reaction of terminal acetylenes
361
1) Sonogashira
coupling reaction
+
SiMe3
Ar
X
Ar X =
or
Ar
Br
S
2) deprotection
N
Cl
I
I
S
1
Sonogashira
coupling reaction
I
I
N
N
S
2
Sonogashira
coupling reaction
Sonogashira
coupling reaction
I
Scheme 1 Preparation of trans-spanning ligands 1 and 2
Table 1 Oxidative homocoupling reaction under different conditions
catalyst 2 mol%
ligand 2 mol%
base, solvent
rt, 19 h, air
4a
5a
Entry
Catalyst
Ligand
Base
Solvent
Yield/%a
1
PdCl2(PPh3)2
Pd(OAc)2
2
2
2
2
2
2
2
2
1
3
–
2
NEt3
NEt3
K2CO3
NHiPr2
–
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
THF
44
2
20
3
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
PdCl2(PPh3)2
15
4
11
5
0
6
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
42
7
Toluene
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
10
8b
9b
10b
11b
12d
67c
56c
58c
55c
Trace
Conditions: 4a (0.5 mmol), PdCl2(PPh3)2 (0.01 mmol), ligand (0.01 mmol), NEt3 (1.7 mmol), solvent
(1.5 mL), rt, 19 h, in air
a
Determined by 1H NMR
b
40 °C, 40 h
Isolated yield
c
d
Under N2
123

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S. Atobe et al.
Table 2 Oxidative homocoupling reaction of acetylenes
PdCl2(PPh3)2 2 mol%
2 2mol%
R
R
R
NEt3, CH3CN
air
4
5
Entry
-R
Substrate
Time (h)
Temp. (°C)
Yield/%a
Product
Without 2
/with 2
1
2
3
4
5
4a
4b
4c
4d
4e
40
12
40
40
17
40
rt
5a
55/67
5b
5c
5d
5e
54/86
40
rt
(61)/87
40/60
tBu
CF3
OMe
40
(63)/66
6
7
–nC6H13
4f
19
20
rt
rt
5f
56/66
4g
5g
(54)/(66)
8
9
4h
4i
17
40
rt
rt
5h
5i
(48)/(47)
(54)/(53)
OH
HO
10
–(CH2)3CN
4j
21
rt
5j
24/81
Conditions: 4 (0.5 mmol), PdCl2(PPh3)2 (0.01 mmol), 2 (0.01 mmol), NEt3 (1.7 mmol), CH3CN
(1.5 mL)
a
Isolated yield (the values in parentheses were determined by 1H NMR analysis)
Pd(OAc)2 instead of PdCl2(PPh3)2 reduced the yield of 5a to 20 % (entry 2). K2CO3
or NHiPr2 as base was not favorable for this reaction (15 % or 11 % yield,
respectively) (entries 3 and 4). When the reaction was performed without any base,
5a was not obtained (entry 5). When tetrahydrofuran (THF) was used as a solvent,
reactivity was similar to that in acetonitrile (entry 6). A non-polar solvent, for
123

Palladium-catalyzed oxidative homocoupling reaction of terminal acetylenes
363
example toluene, was unfavorable in this system (entry 7). In most cases a
substantial amount of the starting material 4a was recovered. The yield of 5a
increased to 67 % (isolated yield) when the reaction temperature was increased to
40 °C and the reaction time was prolonged, and all the starting material 4a was
consumed (entry 8). As shown in entries 9 and 10, when ligand 1 or 3 was used
instead of 2, 5a was obtained in 56 % or 58 % yield, respectively. The reaction was
performed without 1, 2, or 3 to give 5a in 55 % isolated yield (entry 11). The yield
of 5a decreased drastically under a nitrogen atmosphere (entry 12). Enhancement of
the yield of 5a in the presence of the trans-spanning ligand 2 was apparent, although
1
enhancement was not very large. Moreover, the H NMR spectrum of the resulting
crude mixture obtained from the reaction in the presence of 2 contained no broad
peak attributable to the polymerization of acetylenes, in contrast with the spectrum
of the mixture obtained from the reaction in the absence of 2.
The oxidative homocoupling reactions of different terminal acetylenes were
examined; the results are summarized in Table 2. Phenylacetylene (4b) and 4-tert-
butylphenylacetylene (4c) were found to be good substrates for this reaction, giving
diynes 5b and 5c in 86 % and 87 % yield, respectively. In contrast, the reactions
using 4b and 4c without 2 resulted in moderate yields (entries 2 and 3). The
reactions of 4-substituted phenylacetylenes, for example 4-ethynylbenzotrifluoride
(4d) and 4-methoxyphenylacetylene (4e) also proceeded, and the corresponding
products 5d and 5e were obtained in 60 % and 66 % yields, respectively (entries 4
and 5). For 4-substituted aromatic acetylenes with electron-donating groups, 4a, 4c,
and 4e, the reactions required a temperature of 40 °C to convert the starting
acetylenes completely. Aliphatic acetylenes also reacted to afford the corresponding
conjugated diynes. The reactions of 1-octyne (4f) and 1-ethynylcyclohexene (4g)
were performed at room temperature to give 5f and 5g in moderate yields (entries 6
and 7). With propargyl alcohols, for example 2-methyl-3-butyn-2-ol (4h) and
1-ethynylcyclohexanol (4i), the desired products 5h and 5i were obtained in low to
moderate yields. In these cases, the yields of the products were almost the same as
the yields obtained from the reactions in the absence of ligand 2 (entries 8 and 9).
Presumably, participation of the trans-bidentatable ligand 2 might be prevented by
chelation of the hydroxyacetylene to the metal [32]. In contrast, the yield of 5j
(81 %) from 5-hexynenitrile (4j) was substantially affected by the presence of
ligand 2 (entry 10). Coordination of the terminal nitrile group, which reduced the
catalytic reactivity, might be inhibited by the trans-spanning ligand 2.
The proposed reaction pathway of this Pd-catalyzed homocoupling reaction is
shown in Scheme 2 [33–37]:
1. Formation of a catalytically active species, trans-[Pd(PPh3)2(2)] (I), by reaction
of [PdCl2(PPh3)2], the trans-spanning ligand 2, and 2 equivalents of terminal
acetylene;
2. Oxidative addition of the acetylene to give a [HPd(C:CR)(PPh3)(2)] complex
(II);
3. Ligand-exchange reaction of II to give a trans-[HPd(C:CR)(2)] complex
(III), which preferred to form a trans-structure in the presence of the trans-
bidentatable ligand 2;
123

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S. Atobe et al.
R
R
R
R
(PPh3)2Pd
(PPh3)2Pd
S
PdCl2(PPh3)2
.
.
Cl
- HCl NEt3
- HCl NEt3
R
N
S
N
2
R
R
S
N
5
R
N
PPh3
PPh3
PPh3
Pd
Ph3P
S
R
R
N
N
I
Pd
Pd
H
Ph3HP
Ph3P
R
II
S
V
S
H2O2
R
R
N
N
R
PPh3
Pd
Pd
S
H2O
+
1/2 O2
+
H
S
HOO
PPh3
O2
III
IV
Scheme 2 The proposed reaction pathway
4. Insertion of oxygen into the H–Pd bond of III to form a trans-
[HOOPd(C:CR)(2)] complex (IV);
5. Ligand-exchange reaction of IV with another acetylene, followed by isomer-
ization to form a cis-[Pd(C:CR)2(PPh3)(2)] complex (V); and
6
Reductive elimination of product 5 from complex V accompanied by
regeneration of I.
Reports have indicated that the trans forms of Pd complexes are important
intermediates in a variety of Pd-catalyzed reactions [36–40]. We also speculated
that a [HPd(C:CR)(2)] complex (III) might be a key intermediate in this oxidative
coupling reaction because the complex III is likely to suppress side reactions caused
by the metal hydride species, for example hydrometalation, hydride migration to an
unsaturated bond, and reverse reaction to the complex I [41–43].
Conclusion
In summary, we have developed a novel palladium-catalyzed oxidative homocou-
pling reaction of terminal acetylene, in air, using the trans-spanning ligand 2. A
variety of aromatic acetylenes and aliphatic acetylenes were converted into the
corresponding symmetrical diynes in moderate to good yields. The trans-palladium
complex, formed in the course of the catalytic reaction, seemed to be an important
intermediate, but the effect of the trans-spanning ligand 2 was not clear. A
mechanistic study is now in progress.
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Palladium-catalyzed oxidative homocoupling reaction of terminal acetylenes
365
Experimental
Instruments
1H NMR spectra were recorded on a Jeol JNM-LA300/WB (300 MHz) or a Jeol
JNM-ECX400 spectrometer (400 MHz) using CDCl3 as solvent and tetramethyl-
silane (TMS) as internal standard (d = 0.0 ppm). 13C NMR spectra were obtained
on the same spectrometers (75 or 100 MHz) using CDCl3 as the solvent. Chemical
shifts in 13C NMR were determined relative to CDCl3 (d = 77 ppm). Infrared
spectra were measured on a Shimadzu FTIR-8400 spectrometer. Melting points
were determined on a Yanagimoto micro melting point apparatus. Conventional
mass spectra were recorded with a Shimadzu GCMS-QP5000 gas chromatograph–
mass spectrometer. High-resolution mass spectra were acquired on Jeol JMS-700
MStation. All reagents were purchased from commercial sources and used without
further purification.
General procedure for the oxidative homocoupling reaction
In air, acetylene 4 (0.50 mmol), PdCl2(PPh3)2 (7.0 mg, 0.010 mmol), 2 (2.9 mg,
0.010 mmol), NEt3 (0.20 mL, 1.7 mmol), and CH3CN (1.5 mL) were placed in a
10-mL two-necked round-bottomed flask and the mixture was stirred at ambient
temperature or 40 °C for 12–40 h. After filtration of the resulting reaction mixture
through a pad of silica gel, the solution was concentrated in vacuo. Purification was
conducted by silica gel column chromatography (eluent hexanes–AcOEt). All
1
products were analyzed by H NMR and GC–MS.
Synthesis of 2-ethynylthiophene
Under an N2 atmosphere, Pd(PPh3)4 (180 mg, 0.16 mmol), CuI (140 mg,
0.74 mmol), NEt3 (9.0 mL), 2-bromothiophene (5.0 g, 31 mmol), and trimethylsi-
lylacetylene (4.0 g, 41 mmol) were placed in a 30-mL three-necked round-
bottomed flask and heated under reflux for 19 h. After filtration of the resulting
mixture through a pad of silica gel the solvent was removed in vacuo and the residue
was purified by distillation under reduced pressure (90 °C/10 mmHg) to give
2-(trimethylsilylethynyl)thiophene [44] as a yellowish oil (4.5 g, 25 mmol, 82 %).
1H NMR (400 MHz, CDCl3, ppm) d 0.25 (s, 9H), 6.95 (dd, J = 4.5, 4.5 Hz, 1H),
7.23 (m, 2 H).
2-(Trimethylsilylethynyl)thiophene (4.5 g, 25 mmol), CH3OH (20 mL), and
powdered KOH (2.3 g) were placed in a 50-mL two-necked round-bottomed flask,
and the mixture was heated under reflux for 38 h. After filtration of the resulting
mixture through a pad of silica gel the solvent was removed in vacuo and the residue
was purified by distillation under reduced pressure (70 °C/20 mmHg) to give
1
2-ethynylthiophene [45] as a yellowish oil (1.3 g, 44 %). H NMR (400 MHz,
CDCl3, ppm) d 3.33 (s, 1H), 6.97 (dd, J = 4.4, 4.4 Hz, 1H), 7.25–7.29 (m, 2 H).
123

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S. Atobe et al.
Synthesis of 2-ethynylpyridine
Under an N2 atmosphere, Pd(PPh3)4 (700 mg, 0.61 mmol), CuI (290 mg,
1.5 mmol), NEt3 (20 mL), 2-chloropyridine (3.4 g, 30 mmol), and trimethylsilyl-
acetylene (3.1 g, 32 mmol) were placed in a 50-mL two-necked round-bottomed
flask and heated under reflux for 46 h. After filtration through a pad of silica gel and
removal of solvent, the residue was purified by distillation under reduced pressure to
give 2-(trimethylsilylethynyl)pyridine [46] as a yellowish oil (4.4 g, 25 mmol,
83 %). 1H NMR (400 MHz, CDCl3, ppm) d 0.27 (s, 9H), 7.22 (dd, J = 7.8, 4.7 Hz,
1H), 7.45 (d, J = 7.8 Hz, 1H), 7.64 (dd, J = 7.8, 7.8 Hz, 1H), 8.57 (d, J = 4.7 Hz,
1H).
A mixture of CH3OH (40 mL), Na2CO3 (1.5 g), and 2-(trimethylsilylethy-
nyl)pyridine (2.2 g, 13 mmol) was placed in a 50-mL two-necked round-bottomed
flask and stirred for 2 h at room temperature. The mixture was concentrated in
vacuo and extracted with ether and H2O. The organic layer was dried over
anhydrous MgSO4 and the solvent was evaporated in vacuo to give 2-ethynylpyr-
1
idine [47] as a colorless oil (1.0 g, 9.8 mmol, 75 %). H NMR (300 MHz, CDCl3,
ppm) d 3.15 (s, 1H), 7.27 (dd, J = 7.8, 4.5 Hz, 1H), 7.49 (d, J = 7.8 Hz, 1H), 7.66
(dd, J = 7.8, 7.8 Hz, 1H), 8.63 (d, J = 4.5 Hz, 1H).
Synthesis of 1,2-bis(2-thienylethynyl)benzene (1) [48]
A mixture of 2-ethynylthiophene (940 mg, 8.7 mmol), 1,2-diiodobenzene (1.2 g,
3.6 mmol), Pd(PPh3)4 (43 mg, 0.037 mmol), CuI (22 mg, 0.12 mmol), NEt3
(5.0 mL), and benzene (4.0 mL) was placed in a 50-mL three-necked round-
bottomed flask and the mixture was heated under reflux for 17 h under N2. After
cooling to room temperature, the reaction mixture was filtered through a pad of
silica gel and concentrated in vacuo. The resulting mixture was purified by silica gel
column chromatography (eluent hexanes–AcOEt 15:1), to give 1 as a brown solid
(650 mg, 61 %).
1
Brown solid; m.p. 117–119 °C; H NMR (400 MHz, CDCl3, ppm) d 7.03 (dd,
J = 5.0, 3.7 Hz, 2H), 7.27–7.35 (m, 6H), 7.53 (dd, J = 6.0, 3.2 Hz, 2H); 13C NMR
(100 MHz, CDCl3, ppm) d 87.0, 91.9, 123.2, 125.3, 127.2, 127.6, 128.1, 131.4,
132.2; IR (KBr) 3101, 2199, 1967, 1512, 1474, 1420, 1211, 1074, 1034, 833, 764,
710 cm-1; MS (EI) m/z (% relative intensity) 290 (M?, 100); HRMS (EI) calcd for
C18H10S2 (M?) 290.0224, found 290.0211.
Synthesis of 1-(2-pyridylethynyl)-2-(2-thienylethynyl)benzene (2)
CuI (41 mg, 0.21 mmol), Pd(PPh3)4 (68 mg, 0.059 mmol), NEt3 (10 mL), 1,2-
diiodobenzene (2.6 g, 7.9 mmol), and 2-ethynylpyridine (850 mg, 8.2 mmol) were
placed in a 30-mL three-necked round-bottomed flask and the mixture was stirred at
20 °C for 19 h under N2. After filtration of the resulting reaction mixture through a
pad of silica gel with AcOEt, the extract was concentrated under reduced pressure.
Purification was conducted by silica gel column chromatography (eluent hexanes–
AcOEt 1:1) to give 1-iodo-2-(2-pyridylethynyl)benzene [49] as a yellowish oil
123

Palladium-catalyzed oxidative homocoupling reaction of terminal acetylenes
367
(1.0 g, 42 %). 1H NMR (300 MHz, CDCl3, ppm) d 7.05 (dd, J = 7.8, 7.8 Hz, 1H),
7.26 (ddd, J = 7.5, 5.0, 1.3 Hz, 1H), 7.35 (ddd, J = 7.5, 7.5, 1.1 Hz, 1H),
7.59–7.64 (m, 2H), 7.70 (dd, J = 7.8, 7.8 Hz, 1H), 7.88 (d, J = 7.8 Hz, 1H), 8.65
(d, J = 5.0 Hz, 1H).
Under an N2 atmosphere, PdCl2(PPh3)2 (31 mg, 0.044 mmol), CuI (24 mg,
0.13 mmol), NEt3 (15 mL), 1-iodo-2-(2-pyridylethynyl)benzene (1.0 g, 3.3 mmol),
and 2-ethynylthiophene (1.1 g, 10 mmol) were placed in a 50-mL three-necked
round-bottomed flask and the mixture was stirred at 50 °C for 19 h. After filtration
of the resulting reaction mixture through a pad of silica gel, the solution was
concentrated in vacuo. Purification was conducted by silica gel column chroma-
tography (eluent hexanes–AcOEt 9:1–1:2) to give 2 as a brown solid (850 mg,
92 %).
1
Brown solid; m.p. 83–87 °C; H NMR (300 MHz, CDCl3, ppm) d 7.02 (dd,
J = 5.0, 3.6 Hz, 1H), 7.24 (ddd, J = 7.3, 5.0, 1.5 Hz, 1H), 7.29–7.37 (m, 4H),
7.52–7.56 (m, 1H), 7.60–7.70 (m, 3 H), 8.63 (d, J = 4.8 Hz, 1H); 13C NMR
(75 MHz, CDCl3, ppm) d 87.0, 87.8, 91.8, 92.7, 122.8, 123.1, 124.6, 126.0, 127.2,
127.4, 127.6, 128.1, 128.7, 131.4, 132.3 (overlapped), 136.0, 143.4, 150.1; IR (KBr)
3109, 3055, 2198, 1951, 1573, 1481, 1452, 1427, 1211, 756, 694 cm-1; MS (EI)
m/z (% relative intensity) 285 (M?, 100); HRMS (EI) calcd for C19H11NS 285.0612,
found 285.0613.
Synthesis of 1,2-bis(2-pyridylethynyl)benzene (3) [50]
CuI (36 mg, 0.19 mmol), Pd(PPh3)4 (110 mg, 0.095 mmol), NEt3 (10 mL), 1,2-
diiodobenzene (630 mg, 1.9 mmol), and 2-ethynylpyridine (320 mg, 3.1 mmol)
were placed in a 50-mL two-necked round-bottomed flask under N2 and the mixture
was stirred at 45 °C for 17 h. After filtration of the resulting reaction mixture
through a pad of silica gel, the solution was concentrated in vacuo. The residue was
purified by silica gel column chromatography (eluent hexanes–AcOEt 5:1) to give
1,2-bis(2-pyridylethynyl)benzene (3) as a brown solid (210 mg, 48 %).
Brown solid; 1H NMR (400 MHz, CDCl3, ppm) d 7.25–7.27 (m, 2H), 7.36–7.39
(m, 2H), 7.64–7.70 (m, 6H), 8.65 (dd, J = 4.4, 1.0 Hz, 2H); 13C NMR (100 MHz,
CDCl3, ppm) d 87.7, 92.9, 122.8, 125.4, 127.6, 128.7, 132.2, 136.1, 143.5, 150.0;
MS (EI) m/z (% relative intensity) 280 (M?, 100).
1,4-Bis(4-tolyl)-1,3-butadiyne (5a) [51]
1H NMR (400 MHz, CDCl3, ppm) d 2.36 (s, 6H), 7.12 (d, J = 8.0 Hz, 4H), 7.40 (d,
J = 8.0 Hz, 4H); MS (EI) m/z (% relative intensity) 230 (M?, 100).
1,4-Diphenyl-1,3-butadiyne (5b) [51]
1H NMR (400 MHz, CDCl3, ppm) d 7.31–7.39 (m, 6H), 7.52 (dd, J = 8.0, 1.6 Hz,
4H); MS (EI) m/z (% relative intensity) 202 (M?, 100).
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S. Atobe et al.
1,4-Bis(4-tert-butylphenyl)-1,3-butadiyne (5c) [20]
1H NMR (400 MHz, CDCl3, ppm) d 1.31 (s, 18H), 7.35 (d, J = 8.0 Hz, 4H), 7.45
(d, J = 8.0 Hz, 4H); MS (EI) m/z (% relative intensity) 314 (M?, 67), 299 (100).
1,4-Bis(4-trifluoromethylphenyl)-1,3-butadiyne (5d) [51]
1H NMR (400 MHz, CDCl3, ppm) d 7.61 (d, J = 8.7 Hz, 4H), 7.64 (d, J = 8.7 Hz,
4H); MS (EI) m/z (% relative intensity) 338 (M?, 100).
1,4-Bis(4-methoxyphenyl)-1,3-butadiyne (5e) [51]
1H NMR (400 MHz, CDCl3, ppm) d 3.81 (s, 6H), 6.84 (d, J = 8.0 Hz, 4H), 7.45
(d, J = 8.0 Hz, 4H); MS (EI) m/z (% relative intensity) 262 (M?, 100).
7,9-Hexadecadiyne (5f) [52]
1H NMR (300 MHz, CDCl3, ppm) d 0.88 (t, J = 7.0 Hz, 6H), 1.20–1.43 (m, 12H),
1.51 (quint, J = 7.1 Hz, 4H), 2.24 (t, J = 7.0 Hz, 4H); MS (EI) m/z (% relative
intensity) 218 (M?, 0.1), 91 (100).
1,4-Bis(1-cyclohexenyl)-1,3-butadiyne (5g) [51]
1H NMR (400 MHz, CDCl3, ppm) d 1.56–1.64 (m, 8H), 2.10–2.13 (m, 8H), 6.24
(t, J = 8.4, 2H); MS (EI) m/z (% relative intensity) 210 (M?, 100).
2,7-Dimethylocta-3,5-diyne-2,7-diol (5h) [52]
1H NMR (400 MHz, CDCl3, ppm) d 1.34 (s, 12H), 1.94 (brs, 2H); MS (EI) m/z (%
relative intensity) 166 (M?, 5), 105 (100).
1,4-Bis(1-hydroxycyclohexyl)-1,3-butadiyne (5i) [51]
1H NMR (400 MHz, CDCl3, ppm) d 1.21–1.30 (m, 4H), 1.49–1.62 (m, 8H),
1.64–1.80 (m, 6H), 1.90–1.95 (m, 4H); MS (EI) m/z (% relative intensity) 246 (M?,
0.03), 55 (100).
1,10-Dicyano-4,6-decadiyne (5j) [53]
1H NMR (400 MHz, CDCl3, ppm) d 1.90 (quint, J = 6.9 Hz, 4H), 2.46
(t, J = 6.4 Hz, 4H), 2.50 (t, J = 7.2 Hz, 4H); MS (EI) m/z (% relative intensity)
184 (M?, 36), 51 (100).
Acknowledgments This work was supported in part by the Grant-in-Aid for Young Scientists (B) from
The Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan (no. 19750082),
123

Palladium-catalyzed oxidative homocoupling reaction of terminal acetylenes
369
and by a Shionogi Pharmaceuticals Inc. Award in Synthetic Organic Chemistry, Japan, through The
Society of Synthetic Organic Chemistry, Japan, granted to M.S.
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