Alkenylation of arenes using disubstituted ethynes by palladium/carboxylic acid catalysis

Article abstract of DOI:10.1246/cl.180143

Palladium/carboxylic acid-catalyzed alkenylation of heteroarenes and electron-deficient arenes using alkynes underwent C(sp²)H bond activation in arene substrates to give the corresponding alkenylarenes straightforwardly. The versatility of this transformation is demonstrated by the double alkenylation of functionalized thiophenes and 5,6-difluorobenzothiadiazole with alkynes and provides a one-step access to vinylenearylene- vinylene units.

Full text of DOI:10.1246/cl.180143

Advance Publication Cover Page
Alkenylation of Arenes using Disubstituted Ethynes by Palladium/Carboxylic Acid Catalysis
Yasunori Minami,* Yuki Furuya, Tatsuro Kodama, and Tamejiro Hiyama*
Advance Publication on the web March 13, 2018
doi:10.1246/cl.180143
© 2018 The Chemical Society of Japan
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Publication manuscripts.

1
Alkenylation of Arenes using Disubstituted Ethynes by Palladium/CarboxylicAcid Catalysis
Yasunori Minami,*1 Yuki Furuya,2 Tatsuro Kodama,2 and Tamejiro Hiyama*1
1 Research and Development Initiative, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551
2 Department of Applied Chemistry, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551
Eꢀmail: yminami@kc.chuoꢀu.ac.jp, thiyama@kc.chuoꢀu.ac.jp
a) Dinuclear palladium-catalyzed alkenylation using excess thiophene (Tsukada)
Palladium/carboxylic acidꢀcatalyzed alkenylation of
heteroarenes and electronꢀdeficient arenes using alkynes
underwent C(sp2)–H bond activation in arene substrates to
give the corresponding alkenylarenes straightforwardly. The
versatility of this transformation is demonstrated by the
double alkenylation of functionalized thiophenes and 5,6ꢀ
difluorobenzothiadiazole with alkynes and provides a oneꢀ
step access to vinyleneꢀaryleneꢀvinylene units.
N
N
R1
R2
H
S
H
Ph2P
PPh2
cat.
Pd
Pd
O
H
excess
+
Me
Me
R2
R1
S
R2
R2
B(n-Bu)3 cat.
b) This work: palladium/carboxylic acid-catalyzed (double) alkenylation
R2
R3
Keywords: alkyne, arene, C–H activation
R2
R3
R2
R3
Pd cat.
RCO2H cat.
H
H
H
R1
H
or
R2
R4
R4
R1
S
Alkenylarene is an important structural motif in
organic chemistry. A promising preparation of such
structural motifs should involve addition of arene C–H
bonds to alkynes, which allows easy access to πꢀconjugated
aryleneꢀvinylene units without byproduct formation.
Presently, arene alkenylation with alkynes via arylꢀC–H
bond activation takes place with arene units containing a
coordinative group (directing group), acidic C–H bonds, or
Nꢀbased heteroaromatics such as pyridine.1 In contrast,
thiophenes and furans, which are important synthetic units
for biological and electronic molecules, are difficult to
participate in the alkenylation reactions with alkynes,
because they do not have coordinating sites toward metal
complexes and Lewis acids.2 A seminal example reported
by Tsukada was made possible only by use of thiophene in
excess (Figure 1a).3 To enhance the versatility of the
alkenylation strategy, a facile and practical procedure is
needed, which is applicable to multiꢀfunctionalized reactants
in a stoichiometric ratio. In addition, double alkenylation of
functional arene units should be possible to efficiently
prepare aryleneꢀvinylene polymers.4 We have recently
S
S
stoichiometric
+
R2
R4
R4
(R1 = H)
R4
R4
— Benzofuran, 1,2,3-triazole, and 5,6-F2-benzothiadiazole as possible substrates
Figure 1. Catalytic C–H alkenylation of thiophenes and others using
alkynes
alkenylation product 3aa in 86% yield as a mixture of
stereoisomers (Table 1, Entry 1). In the absence of the
carboxylic acid, the reaction was drastically delayed.
Recrystallization from CH2Cl2 and hexane and neutral silica
gel chromatography allowed isolation of major double synꢀ
adduct EEꢀ3aa, whose structure was confirmed by NMR
and Xꢀray crystallographic analysis.8 The EEꢀisomer of 3aa
was sensitive to acidic conditions even on silica gel, and
caused isomerization to the EZ- and ZZꢀforms.
Next, double alkenylation of arenes was examined
using various thiophene and alkyne derivatives.
Diarylalkynes 2bꢀ2f possessing electronꢀdonating and ꢀ
withdrawing groups at the 3ꢀ or 4ꢀposition reacted with 1a
to form products 3abꢀ3af (Table 1, Entries 2ꢀ6). Electronꢀ
withdrawing fluorine and trifluoromethyl group enhanced
the EEꢀselectivity. Due to poor solubility, reaction with 2c
required lower concentration (Entry 3). Thieno[3,4ꢀ
c]pyrroleꢀ4,6ꢀdione 1b, an effective electronꢀaccepting unit
for e.g. organic solar cells,9 underwent reaction smoothly
with 2a to form 3ba with high EE selectivity (Entry 7). In
contrast, 2,2’ꢀbithiophene 1c gave 5,5’ꢀbisalkenylated
product 3ca with lower stereoselectivity (Entry 8),
indicating that sterically bulky arenes affected the
stereoselectivity by preventing repulsion from the aryl rings
at the βꢀalkenyl carbon. Of note, in the case of 1c, no 1:2
cyclization form, i.e. a dibenzothiophene form, was
observed as a putative byproduct.10 Thieno[3,2ꢀb]thiophene
(1d) and electronꢀdeficient benzodithiopheneꢀ4,8ꢀdione (1e)
also could be applied to the double alkenylation (Entries 9
and 10).
disclosed that
a combination of alkynes with dual
palladium/carboxylic acid catalytic system can activate C–H
bonds close to the alkynyl groups, thereby undergoing
intramolecular C–H alkenylation reactions selectively.5,6 For
example, a (hetero)aryl alkynyl silanes underwent antiꢀ
insertion to form condensed siloles.5c We postulated that this
catalytic system would be applicable to intermolecular
reactions as well. The present report describes a solution for
the intermolecular alkenylation of alkynes with a variety of
heteroaromatics and functionalized electronꢀdeficient arenes
that proceeds via siteꢀselective C–H bond activation to
produce various alkenylarenes (Figure 1b).
First, 3,4ꢀethylenedioxythiophene (1a) was used
because of its versatility as an πꢀconjugated material
compound.7 After various catalytic conditions examined, 1a
was found to react with diphenylacetylene (2a) using
Pd{dba(OMe)}2, PCy3, and 2,2ꢀdimethylbutyric acid in
catalytic amounts in 1,4ꢀdioxane at 120 °C to give double

2
Table 1. Double alkenylation of 1a
the present reaction is applicable to other heteroaromatics.
Benzofuran (4e) could also be used in the alkenylation
reaction. Reaction with 1,4ꢀdiphenyltriazole (4f), caffeine
Pd{dba(OMe)}2 (5 mol%)
PCy3 (10 mol%)
t-AmylCO2H (10 mol%)
R
R
Ar
R
R
H
H
+
(4g), and pentoxifylline as
a drug for intermittent
Ar
Ar
1,4-dioxane (1 M)
120 °C
claudication (4h)12 proceeded at the triazole and diazole ring
C–H bonds to give 5fa, 5gf, and 5hf, respectively.
Particularly, the C–H bond on the triazole ring in 4f was
selectively activated and alkenylated over cleavage of both
orthoꢀC–H bonds on N1ꢀ and C4ꢀphenyl groups directed by
the nitrogen atoms in the triazole ring.13 This is the first
example of straightforward alkenylation of a triazole at the
5ꢀposition using alkynes.14 In addition to heteroarenes,
pentafluorobenzene (4i) reacted with 2a to give addition
product 5ia.
H
n H
S
n
S
Ar
Ar
Ar
1
2
3
time
(h)
Entry
1
2 (Ar’)
3 (%) (EE:EZ:ZZ)b
O
O
H
H
S
R
R
O
O
R
R
S
R
1
1a
1a
1a
1a
1a
2a, R = H
2b, R = OMe 24
2c, R = CN
2d, R = F
2e, R = CF3
OMe
20
3aa, R = H, 86 (84:16:<1)
3ab, R = OMe, 73 (78:22:0)
3ac, R = CN, 72 (61:38:1)
3ad, R = F, 79 (94:6:0)
2
3c
4
24
20
12
Table 2. Mono alkenylation of 4a
Pd{dba(OMe)}2 (5 mol%)
PCy3 (10 mol%)
5
3ae, R = CF3, 86 (96:4:0)
H
Ar
Ar
O
O
t-AmylCO2H (10 mol%)
H
H
H
H
+
Ar
Ar
1,4-dioxane (1 M)
120 °C
m-An
m-An
m-An
Ar
6
7
1a
15
Ar
S
4
2
5
m-An
2f
3af, 86 (72:28:0)
O
H
O
H
O
nOct
O
O
nOct
O
N
m-An
S
O
H
O
N
SiEt3
SiEt3
O
O
S
Et3Si
S
H
S
Et3Si
S
m-An
Et3Si
2a
24
Ph
Ph
S
S
5ag, 57% (94:6)b (72 h)
Ph
Ph
5af, 89% (90:10) (48 h)
5ah, 70% (67:33) (24 h)
1b
3ba, 77 (95:5:0)
H
H
Ph
H
H
H
m-An
S
m-An
S
C6H4-F-p
Ph
S
8
9
2a
2f
24
24
S
Ph
S
m-An
Ph
H
S
m-An
S
C6H4-F-p
TMS
1c
3ca, 80 (71:29:0)
H
5bf, 81% (63:37) (12 h)
5cf, 73% (51:49) (35 h)
5dd, 64% (78:22) (24 h)
S
O
O
m-An
m-An
S
S
S
m-An
m-An
O
H
1d
H
H
3df, 75 (80:20:0)
Ph
m-An
N
N
N
m-An
N
O
H
H
N
m-An
m-An
m-An
N
Ph
S
S
m-An
O
N
O
m-An
10
2f
16
S
S
m-An
O
5ef, 56% (69:31) (24 h)
5fa, 70% (>99:1) (24 h)
5gf, 72% (84:16) (14 h)
3ef, 77 (91:9:0)
1e
a Unless otherwise noted, a mixture of 1, 2 {2.2 equiv. or entries 4, 5, 7,
and 8 (3 equiv.)}, Pd{dba(OMe)}2 (5 mol%), PCy3 (10 mol%), t-
AmylCO2H (10 mol%), and 1,4ꢀdioxane (1.0 M) was stirred at 120 °C.
H
O
O
N
F
F
F
H
Ph
m-An
N
N
N
F
Ph
F
m-An
O
b
c
Isolated yield. 1,4ꢀdioxane (0.1 M). dba(OMe) = (1E,4E)ꢀ1,5ꢀ
5hf, 78% (86:14) (24 h)
5ia, 50% (>99:1) (38 h)
bis(3,5ꢀdimethoxyphenyl)ꢀ1,4ꢀpentadienꢀ3ꢀone, Cy = cyclohexyl, tꢀ
Amyl = CMe2CH2CH3, mꢀAn = 3ꢀMeOꢀC6H4.
a
Unless otherwise noted, a mixture of arene 4, 2 (1.2ꢀ1.5 equiv.),
Pd{dba(OMe)}2 (5 mol%), PCy3 (10 mol%), t-AmylCO2H (10 mol%),
and 1,4ꢀdioxane (1.0 M) was stirred at 120 °C. Yields and ratios of E:Z
of isolated products are given. b 1,4ꢀdioxane (0.4 M).
The scope of monoꢀalkenylation was explored (Table
2).
smoothly underwent the reaction with 2f, bisꢀ2ꢀ
naphthylacetylene (2g), and bis{2ꢀ(5ꢀTESꢀ
2ꢀTriethylsilylꢀ3,4ꢀethylenedioxythiophene
(4a)
Electronically biased diarylethyne 2i reacted with 4a to
give a mixture of 5ai and 5’ai in 33% and 25% yield,
respectively (eq 1), suggesting that electronꢀrich aryl groups
are preferentially incorporated at the αꢀposition of the
thiophene group due to the polarization of the triple bond.15
Similar selectivity was observed for 1ꢀ(4ꢀcyanophenyl)ꢀ2ꢀ
(2ꢀnaphthyl)ꢀacetylene (2j), which yielded 5aj with a 2ꢀ
naphthyl group at the αꢀposition in 57% yield. In contrast,
orthoꢀanisylꢀsubstituted alkyne 2k showed the opposite
regioselectivity, giving 5’ak as the major product due
possibly to steric repulsion between an orthoꢀanisyl group
and palladium complex during an insertion event. When the
thienyl)}acetylene (2h) to give the corresponding adducts
5af, 5ag, and 5ah. The TES group on the thiophene ring
was tolerated in the reaction, which provided an opportunity
for subsequent crossꢀcoupling of the ipsoꢀTES positions.11
2ꢀPhenyl and 2ꢀtrimethylsilylthiophenes (4b and 4c) and
benzothiophene (4d) were converted into products 5bf, 5cf,
and 5dd, respectively, in high yields. On the other hand, 2ꢀ
methylbenzothiophene did not participate in the reaction,
suggesting that the reaction conditions proceed selectively at
the electronꢀrich site. In addition to thiophene derivatives,

3
electronically and sterically biased alkynyl ether 2l was used,
5al was obtained with complete regioꢀ and stereoselectivity
in a manner similar to that from the previously reported
hydroalkylation (eq 2).5a
Pd{dba(OMe)}2 (5 mol%)
PCy3 (10 mol%)
t-AmylCO2H (10 mol%)
O
O
1a
H
(4)
+
+
EE-3af
m-An
1,4-dioxane (1 M)
120 °C, 5 h
2f
(1 eq)
S
20%
m-An
Ar2
O
O
optimized
conditions
H
H
E-8af, 29%
O
Ar2
Ar1
O
(1)
+
+
4a
Ar1
2, 1.2-1.5 eq
Ar1
Ar2
S
S
TES
TES
2i Ar1 = 4-OMe-C6H4
Ar2 = 4-CF3-C6H4
2j Ar1 = 2-Naphthyl
Ar2 = 4-CN-C6H4
48 h
20 h
20 h
5ai, 33% (94: 6)
5aj, 57% (75:25)
5ak, 23% (86:14)
+
5'ai, 25% (94:6)
5'aj, 22% (97:3)
5'ak, 54% (91:9)
+
2k Ar1 = 2-OMe-C6H4
Ar2 = 4-CF3-C6H4
+
SiiPr3
O
H
optimized
conditions
O
SiiPr3
3,5-Xyl
4a
+
(2)
Figure 2. Time course of double alkenylation using 1a, 2a (3 equiv),
Pd(dba)2 (5 mol%), PCy3 (10 mol%), t-BuCO2H (10 mol%), and 1,4ꢀ
dioxane (1.0 M) at 120 °C.
24 h
O
S
O
TES
3,5-Xyl
2l (3.0 eq)
5al, 72%
Based on results from a previous study,5 we propose a
reaction mechanism shown in Figure 3. Coordination of
alkynes to palladium(0) complex forms I followed by the
reaction with carboxylic acid to generate vinylpalladium
carboxylate II.20 Subsequent C–H bond activation of 1
proceeds via a CMD pathway or an SEAr mechanism to give
vinylarylpalladium III.21 Reductive elimination produces
the addition product and regenerates the palladium(0)
complex. This mechanism indicates that addition of a C–H
bond to alkynes appears formally as metalꢀcatalyzed direct
coupling of R–X with R’–H bonds.22,23
Difluorobenzothiadiazole 6, an efficient electronꢀ
accepting structural unit, was applicable to the double
alkenylation reaction. Double synꢀaddition product EEꢀ7
was selectively obtained analyzed by 19F NMR, although
twice the amount of 2d and the catalysts were required due
to low reactivity of 6 (eq 3).16 This is the first example of
straightforward C–H alkenylation of benzothiadiazoles
using alkynes.17 Although EEꢀ7 was formed selectively in
the crude mixture, stereoisomerization of EEꢀ7 occurred
during the purification, indicating that isolation of pure EEꢀ
7 is absolutely difficult.
S
S
N
N
N
N
H
H
R
R
R'CO2H
H
Ar
R
R
H
R
R
H
R
R
H
conditions
Ar
Ar
+
2d
(3)
H
H
Pd
1,4-dioxane (1 M)
120 °C, 24 + 24 h
Pd
Pd
Ar
Ar
Ar
OC(O)R'
Ar
F
F
F
F
R'CO2H
Pd(0)
Ar = 4-F-C6H4
6
I
II
III
7, 83% (NMR, EE only)
62% (isolated, EE:EZ:ZZ = 61:30:9)
Figure 3. Proposed reaction mechanism. Pd = Pd(PCy3)n
conditions: 2d (2.2 eq), Pd{dba(OMe)} (5 mol%), PCy (10 mol%), t-AmylCO H (20 mol%).
2
3
2
After 24h, 2d (1.0 eq), Pd{dba(OMe)} (5 mol%), PCy (10 mol%), t-AmylCO H (20 mol%).
2
3
2
In conclusion, a simple palladium/carboxylic acidꢀ
catalyzed alkenylation of arenes with alkynes is made
possible, which is considered to proceed via C–H bond
activation to give monoꢀ or doubleꢀalkenylated arenes. A
combination catalyst of palladium(0) and carboxylic acid
was effective for the reaction. This method allows the use of
various arenes including 5,6ꢀdifluorobenzothiadiazole and
various functionalized thiophenes as electronꢀdonating or ꢀ
accepting units on a stoichiometric basis. Current efforts are
directed toward the elucidation of the reaction mechanism,
extension of the scope of the reaction using unactivated
arenes and terminal alkynes, and application to the synthesis
To gain an insight into the double alkenylation
mechanism, we performed the reaction of 1a with 1 equiv of
2f to compare the rate of first alkenylation with that of the
second; products Eꢀ8af and EEꢀ3af were selectively formed
in similar yields (eq 4). Then, a kinetic study was carried out
using 1a with 3 equiv of 2a18 and yields of products double
adduct 3aa and mono adduct 8aa were plotted as a function
of reaction time (Figure 2). After 1 h, the major products
were monoꢀsynꢀadduct Eꢀ8aa and doubleꢀsynꢀadduct EEꢀ
3aa in 40% and 24% yields, respectively.19 The alkenylation
was nearly completed within 3 h but the stereoselectivity of
3aa was greatly reduced. Further heating to 9 h resulted in
complete comsuption of 8aa and major formation of EEꢀ3aa.
These results suggest that synꢀaddition proceeds and the
stereoselectivity of adducts is due to a thermodynamic
factor.
of
aryleneꢀvinyleneꢀbased
organic
materials
and
functionalized polyaromatic hydrocarbons.
This research was supported financially by JST, ACTꢀC
(JPMJCR12Z1 to Y.M., T.H.). Y.M. also acknowledges a
GrantꢀinꢀAid for Challenging Research (Exploratory)

4
11
a) T. Komiyama, Y. Minami, T. Hiyama, Angew. Chem. Int. Ed.
2016, 55, 15787. b) T. Komiyama, Y. Minami, Y. Furuya, T.
Hiyama, Angew. Chem. Int. Ed. 2018, 57, 1987.
K. Salhiyyah, R. Forster, E. Senanayake, M. AbdelꢀHadi, A.
Booth, J. A. Michaels, Cochrane Database Syst. Rev. 2015.
For examples of triazoleꢀdirected C–H alkenylation using
alkynes, see: a) W. Zhou, Y. Yang, Z. Wang, G.ꢀJ. Deng, Org.
Biomol. Chem. 2014, 12, 251. b) X. G. Li, K. Liu, G. Zou, P. N.
Liu, Eur. J. Org. Chem. 2014, 7878. c) S. Zhao, R. Yu, W. Chen,
M. Liu, H. Wu, Org. Lett. 2015, 17, 2828.
For examples of C–H functionalization on triazole ring, see: a) M.
Iwasaki, H. Yorimitsu, K. Oshima, Chem. Asian J. 2007, 2, 1430.
b) L. Ackermann, R. Vicente, R. Born, Adv. Synth. Catal. 2008,
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Chem. Int. Ed. 2009, 48, 201. d) H. Jiang, Z. Feng, A. Wang, X.
Liu, Z. Chen, Eur. J. Org. Chem. 2010, 1227. e) L. Ackermann,
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(17K19127) from the JSPS and the Sumitomo Foundation
(160752). We express our thanks to Prof. Fukuzawa (Chuo
Univ.) for the supply of 1,4ꢀdiphenyltriazole.
12
13
Supporting
Information
is
available
on
http://dx.doi.org/10.1246/cl.******.
References and Notes
1
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14
2
Direct C–H alkenylation of thiophenes using alkenes was
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3
4
N. Tsukada, K. Murata, Y. Inoue, Tetrahedron Lett. 2005, 46,
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16
M. Rubin, A. Trofimov, V. Gevorgyan, J. Am. Chem. Soc. 2005,
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Attempted
reaction
of
6
with
2a
under
the
Ni(cod)2/tricyclopentylphosphine catalytic conditions resulted in
no product formation. See: Y. Nakao, N. Kashihara, K. S.
Kanyiva, T. Hiyama, J. Am. Chem. Soc. 2008, 130, 16170.
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To avoid the steric and electronic effect on stereoselectivity, we
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These observed yields indicate a possibility that the double
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17
18
19
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a
direct allylation of carbon nucleophiles using
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We examined the reaction of deuterated 4a with 2f in the
presence of stoichiometric amounts of Pd{dba(OMe)}2, PCy3,
and acetic acidꢀd4 in toluene or 1,4ꢀdioxane and obtained nonꢀ
deuterated product 5af whereas the same reaction in tolueneꢀd8
afforded 5af deuterated at the alkenyl carbon. These results show
H/D scrambling takes place readily with solvent, making assay
and discussion of deuteriumꢀlabeling experiments difficult.
23
8
9
In a manner similar to EEꢀ3aa, products EEꢀ3ba, EEꢀ3ca, Eꢀ5fa,
and Eꢀ5aj were unambiguously determined by Xꢀray
crystallographic analysis.
For recent examples, see: a) Y. Lin, P. Cheng, Y. Liu, X. Zhao,
D. Li, J. Tan, W. Hu, Y. Li, X. Zhan, Sol. Energ. Mater. Sol.
2012, 99, 301. b) A. Najari, P. Berrouard, C. Ottone, M. Boivin,
Y. Zou, D. Gendron, W.ꢀO. Caron, P. Legros, C. N. Allen, S.
Sadki, M. Leclerc, Macromolecules 2012, 45, 1833. c) Q. Feng,
X. Lu, G. Zhou, Z.ꢀS. Wang, Phys. Chem. Chem. Phys. 2012, 14,
7993. d) Y. J. Kim, J. Y. Baek, J. Ha, D. S. Chung, S.ꢀK. Kwon,
C. E. Park, Y.ꢀH. Kim, J. Mater. Chem. C 2014, 2, 4937.
10
For recent examples, see: a) Y. Lin, P. Cheng, Y. Liu, X. Zhao,
D. Li, J. Tan, W. Hu, Y. Li, X. Zhan, Sol. Energ. Mater. Sol.
2012, 99, 301. b) A. Najari, P. Berrouard, C. Ottone, M. Boivin,
Y. Zou, D. Gendron, W.ꢀO. Caron, P. Legros, C. N. Allen, S.
Sadki, M. Leclerc, Macromolecules 2012, 45, 1833. c) Q. Feng,
X. Lu, G. Zhou, Z.ꢀS. Wang, Phys. Chem. Chem. Phys. 2012, 14,
7993. d) Y. J. Kim, J. Y. Baek, J. Ha, D. S. Chung, S.ꢀK. Kwon,
C. E. Park, Y.ꢀH. Kim, J. Mater. Chem. C 2014, 2, 4937.