From propargylic biscarbonate to diaryl[n]dendralenes

Article abstract of DOI:10.1016/j.tetlet.2017.05.016

An efficient cross-coupling reaction using a low cost carbon-supported palladium (Pd/C) catalyst for the synthesis of cross-conjugated compounds, diaryl[n]dendralenes, has been developed. The reaction of a propargylic biscarbonate with phenylboronic acid using Pd/C and phosphine ligand (S-Phos) gave 2,3-diphenyl[2]dendralene in high yield. We found that Pd/C was an effective catalyst for the synthesis of dialyl[n]dendralenes. The synthesis of various dendralenes was successfully achieved under the optimized conditions, giving dialyl [2] and [4] dendralenes in good yields.

Full text of DOI:10.1016/j.tetlet.2017.05.016

Tetrahedron Letters xxx (2017) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
From propargylic biscarbonate to diaryl[n]dendralenes
⇑
Shotaro Hayashi, Masakatsu Kasuya, Junsuke Machida, Toshio Koizumi
Department of Applied Chemistry, National Defense Academy, 1-10-20, Hashirimizu, Yokosuka, Kanagawa 239-8686, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient cross-coupling reaction using a low cost carbon-supported palladium (Pd/C) catalyst for the
synthesis of cross-conjugated compounds, diaryl[n]dendralenes, has been developed. The reaction of a
propargylic biscarbonate with phenylboronic acid using Pd/C and phosphine ligand (S-Phos) gave 2,3-
diphenyl[2]dendralene in high yield. We found that Pd/C was an effective catalyst for the synthesis of dia-
lyl[n]dendralenes. The synthesis of various dendralenes was successfully achieved under the optimized
conditions, giving dialyl [2] and [4] dendralenes in good yields.
Received 13 March 2017
Revised 29 April 2017
Accepted 8 May 2017
Available online xxxx
Keywords:
Pd/C
Ó 2017 Elsevier Ltd. All rights reserved.
Heterogeneous catalyst
Cross-coupling reaction
Cross-conjugated compound
[
n]Dendralene
p
-Conjugated compounds such as polyarylenes, polyenes and
The synthesis of oligomeric dendralenes is a challenging
topic because multi-step reactions are usually requiered. The Pd
1
,2
6
acenes have wide applications in biology and electronics. These
compounds are recognized as model parts of graphene being a
zero-band gap semimetal. Common conjugated compounds,
namely through-conjugated compounds, which have alternating
single and multiple bonds, are composed of unbranched p-electron
(0)-catalyzed Suzuki-Miyaura reaction with 1,3-butadiene-2,3-bis
(pinacolatoborane) and the Stille reaction with 1,3-butadiene-2,3-
bis(tri-n-butyltin) are key processes to access [2–8]dendralene
derivatives. Various 2,3-diaryl[2]dendralenes were also synthesized
3
4
frameworks. Many of such compounds show semiconducting
by Cu, Pd or Ni-catalyzed cross-coupling reaction. Ishino et al.
properties.
reported that the reaction of 1,4-dimethoxy-2-butyne with aryl
Grignard reagents in the presence of CuBr gave 2,3-diaryl[2]den-
On the other hand, cross-conjugated compounds are constructed
by branched
dendralenes and radialenes. In contrast to huge examples of
through-conjugated compounds, the studies on cross-conjugated
compounds have been quite limited so far. In recent years,
however, the cross-conjugated compounds have received
4
h
p
-electron frameworks and the typical examples are
dralenes. The Pd(0)-catalyzed synthesis of diaryl[2]dendralenes
from propargyl carbonates and aryl boronic acids was reported by
4a
4
g
Grigg. We have recently succeeded in the synthesis of cross-
conjugated polymers based on [2]dendralenyl (1,3-butadiene-2,3-
diyl) skeleton via
a
palladium(0)-catalyzed cross-coupling
increasing attention as unique properties such as
p
-electron
[n]Dendralenes are
one of cross-conjugated compounds. The simplest dendralene is
3]dendralene (Scheme 1). 2,3-Diaryl-1,3-butadienes, which are
polycondensation between propargylic biscarbonate 1 and aryl
diboronic acids. The efficient polymerization was achieved by a cat-
communication and charge separation.4
b,4c
7
alytic system composed of Pd
2
(dba)
3
ꢀCHCl
3
as the palladium source
0
0
[
and dicyclohexyl(2 ,6 -dimethoxybiphenyl-2-yl)phosphine (S-Phos)
as the electron-donating ligand. The [2]dendralenyl skeletonisgener-
ated efficiently and selectively from 1. Propargylic biscarbonates are
thus powerful precursors for the synthesis of [2]dendralene
derivatives.
formally [4]dendralenes, have attracted their molecular
structures and electronic features.4
In general, 1,3-butadiene prefers an s-trans conformation, but
Walree and co-workers reported that 2,3-diphenyl[2]dendralene
(
2,3-diphenyl-1,3-butadiene) has an s-gauche conformation mainly
Solid-supported transition-metal catalysts have significant
advantages such as stability, reusability, and reduction of residual
metal catalysts in the products. For these reasons, such catalysts
enable the production of organic materials to become efficient
and cost-effective processes in chemical industry. Recently,
heterogeneous Pd(0)-catalytic systems have accomplished many
important carbon–carbon cross-coupling reactions such as
5
in the crystal state. This [2]dendralene would exist in equilibrium
between two conformational states at room temperature since the
energies of the s-gauche and s-trans conformations are very
8
5
d
similar.
⇑
Corresponding author.
E-mail address: tkoizumi@nda.ac.jp (T. Koizumi).
Mizoroki-Heck,
Suzuki-Miyaura,
Sonogashira, Stille, and
http://dx.doi.org/10.1016/j.tetlet.2017.05.016
040-4039/Ó 2017 Elsevier Ltd. All rights reserved.
0
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2
S. Hayashi et al. / Tetrahedron Letters xxx (2017) xxx–xxx
1
of the products 3 were estimated by H NMR spectroscopy in CDCl
3
using nitromethane as a standard. The products 3 were separated
by silica-gel column chromatography (hexane as an eluent).
The Pd/C-catalyzed cross-coupling reaction of 1 with phenyl-
boronic acid (2a) was conducted under ligand-free conditions to
afford a very low yield of 2,3-diphenyl-1,3-butadiene (diphenyl
[
2]dendralene) 3a (Table 1, Entry 1). Although the reaction with
PPh as a ligand did not give 3a (Entry 2), a 20% yield of 3a was
obtained when P(o-tolyl) was used as a ligand (Entry 3). P(o-
3
Scheme 1. Dendralenes.
3
t
0
0
0
anisoyl)
propyl-1,1 -biphenyl-2-yl)phosphine (X-Phos), and di-tert-butyl
2-(diphenylphosphino)phenyl]phosphine (John-Phos) also gave
a, but the yields were not good (entries 4–8). Among the ligands
3
,
PCy
3 4
ꢀHBF ,
P Bu
3 4
ꢀHBF ,
dicyclohexyl(2 ,4 ,6 -triiso-
0
9
Tsuji-Trost reactions. Especially, palladium on carbon (Pd/C) sup-
plied from commercial sources is one of the most frequently used
catalyst for the cross-coupling reactions with good efficiency.
In general, Pd/C is more stable and cheaper than other
homogeneous Pd catalysts. The handling is also easier. Moreover,
some cases using Pd/C have successfully demonstrated its
reusability. In contrast to the previous works, to the best of our
knowledge, there is no report on the coupling reaction of
propargylic carbonates by the use of Pd/C. It would be a useful
Pd/C-catalyzed synthesis of cross-conjugated ([2]dendralenyl)
compounds via a cross-coupling reaction between propargylic
biscarbonates and aryl boronic acids. In this letter, we report the
synthesis of diaryl[2]dendralenes by a Pd/C-catalyzed reaction of
propargylic biscarbonate 1 with boronic acids.
[
3
9
b–d
t
0
0
tested, P Bu
2
9
2
MeꢀHBF
4
and dicyclohexyl(2 ,6 -dimethoxybiphenyl-
-yl)phosphine (S-Phos) raised the yield of 3a up to 57% (Entries
and 10).
We next examined the ratio of phosphine ligand to Pd. Although
9
a
t
changing the ratio of Pd to P Bu
2
MeꢀHBF
4
from 1:2 to 1:3 did not
increase the yield of 3a (Table 1, Entries 9 and 11), the Pd/S-Phos
ratio of 1:3 raised the yield to 73% (Entry 12). We reported that
Pd
pling reaction with 1. To compare with the results with Pd/C, the
ꢀCHCl
reaction of 1 with 2a was carried out using Pd (dba) instead
of Pd/C. The yield (74%) of 3a was similar to the case with Pd/C
Entry 14). The results show that Pd/C as well as Pd (dba)
3
2
(dba)
3
ꢀCHCl
3
is an effective palladium source for the cross-cou-
7
2
3
3
(
2
3
ꢀCHCl
The cross-coupling reaction of propargylic biscarbonate 1 with
arylboronic acids 2 was carried out with Pd/C, 10% Palladium on
carbon (wetted with 55% water) supplied by Tokyo Chemical
Industry (TCI) Corporation, and 2 M K CO aq. at 100 °C. The Pd/C
2 3
used was easily removed by filtration after the reaction. The yields
is highly effective for the reaction. The kind of solvent affected
the reaction largely. THF and DMF were not suitable (Entries 16
and 17). The higher Pd/C catalyst loading (5.0 mol%) did not
increase the yield of 3a (Entries 18 and 19). Thus, the optimized
reaction conditions were found to be as follows: 1 (1.0 mmol), 2
(
(
2 3
2.0 mmol), Pd/C (2.5 mol%), S-Phos (7.5 mol%), 2 M K CO aq.
1.0 mL), toluene (2.0 mL) at 100 °C.
Table 1
Cross-coupling of 1 with Phenylboronic Acid 2a using Palladium on Carbon as
Catalyst.a
It is known that a solid-supported metal catalyst acts as a sol-
uble metal source by leaching of the metal into solution. 9
a,10
In
order to learn whether the Pd/C used plays a role as a heteroge-
neous catalyst, the reaction of 1 with 2a was carried out under
the optimized conditions in the presence of an equimolar amount
of 3-mercaptopropyl-functionalized silica gel (2.5 mol% of SH),
1
0
which is known to be a good palladium scavenger, to Pd/C. A dra-
matic decrease of yield (9%) of 3a was observed (Table 1, Entry 20).
The leached Pd species would be captured by the scavenger. This
result suggests that the Pd/C used would play a homogeneous cat-
alyst under our optimized conditions because the phosphine ligand
leads to leaching of Pd atoms on carbon to form the phosphine-
ligated palladium species in the bulk solution. The reusability
was tested by the use of the used Pd/C. The yield of 3a dropped
to 3%, probably due to the decrease of amount of the palladium
atoms supported on the carbon surface (Entries 12 and 21).
With the optimized conditions in hand, the coupling reaction of 1
with a variety of aryl boronic acids 2 was investigated by using
Entry
Phosphine Ligand
–
Pd/L
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
9
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:3
1:3
1:4
1:3
1:3
1:3
1:3
1:3
1:3
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
THF
2
0
PPh
P(o-Tolyl)
P(o-Anisoyl)
PCy
ꢀHBF
ꢀHBF
X-Phos
John-Phos
3
3
20
33
33
20
15
8
57
57
58
73
45
74 (70)d
0
3
3
4
9
a
t
P Bu
3
4
t
P Bu
2
MeꢀHBF
4
4
10
11
12
13
14
16
17
18
19
20
21
S-Phos
t
P Bu
2
MeꢀHBF
S-Phos
S-Phos
S-Phos
S-Phos
S-Phos
S-Phos
S-Phos
S-Phos
S-Phos
1
2,13
Pd/C or Pd
2b) and 3,5-dimethylphenylboronic acid (2c) gave dendralenes 3b
and 3c in good yields (Entries 1 and 2). In the reaction using
Pd (dba) as a catalyst, 4-methoxyphenylboronic acid (2d)
2
(dba)
3
ꢀCHCl
3
(Table 2).
4-Methylphenylboronic acid
c
(
DMF
0
e
f
Toluene
Toluene
Toluene
Toluene
12
67
9
2
3
ꢀCHCl
3
and 3-acetylphenylboronic acid (2e) afforded 3d and 3e in good
yields, but the use of Pd/C catalyst results in lower yields of 3d and
g
h
g
1:3
3
3
e (Entries 3 and 4). Although 4-chlorophenyl-substituted den-
a
11
Conditions: 1 (1.0 mmol), 2a (2.0 mmol), Pd/C (2.5 mol%), ligand (5.0–10.0
mol%), 2 M K CO aq. (1.0 mL), solvent (2.0 mL), 100°C, under argon.
NMR yield of 3a.
Pd (dba) (1.25 mol%).
ꢀCHCl
Isolated yield.
dralene 3f was obtained from 4-chlorophenylboronic acid (2f),
the yields were not good in the cases using the both palladium
sources (Entry 5). The reaction of 1 with 2-thienylboronic acid (2g)
gave 3g in 41% (Entry 6). 3-Thienylboronic acid (2h) was a much bet-
ter reaction partner than 2g, and gave 3h in 89% (Entry 7). The yield
of 3h was higher in the reaction with Pd/C. Potassium(4-pyridinyl)
trifluoroborate was also used, but the product was not formed at all.
We next tried to synthesize [4]dendralenes from 1 and vinyl-
boronic acids under the optimized conditions. The reaction of 1
2
3
b
c
d
e
f
2
3
3
Pd/C (1.25 mol%), Ligand (3.75 mol%).
Pd/C (5.0 mol%), Ligand (15.0 mol%).
An equimolar amount of 3-mercaptopropyl silica gel (ca. 0.5 mmol/g, TCI) to
Pd/C was added to the reaction media.
The Pd/C collected from the reaction mixture (Entry 12) was used.
g
h
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S. Hayashi et al. / Tetrahedron Letters xxx (2017) xxx–xxx
3
Table 2
Pd/C-Catalyzed Cross-coupling of 1 with Various Boronic Acids 2.
a
Entry
1
Products, 3
Yieldb
Pd/C
66
%
Pd
2
(dba)
3
ꢀCHCl
3
71
2
3
76
31
–
Fig. 1. UV–vis absorption spectra of styrene, 3a and 3i in chloroform (a) and 3a (i)
and 3i (ii) in various solvents (b).
61
Interestingly, both 3a and 3i showed solvatochromic behavior.
Red-shifts were observed changing from polar to non-polar
solvents (Fig. 1b). The conformational change by rotating the C–C
single bonds probably would take place in the solvents with
different polarities. In both 3a and 3i, each spectrum was shifted
to the highest energy region in methanol.
In conclusion, we have successfully achieved a Pd/C-catalyzed
synthesis of various diaryl[n]dendralenes (n = 2 and 4) from
propargylic biscarbonate and aryl boronic acids. The UV–vis
absorption spectra of diphenyl[4]dendralene revealed a more
blue-shifted absorption maximum than that of diphenyl[2]dendra-
lene. Further study on the reactions and properties of the obtained
dendralenes is now under investigation.
4
5
29
28
57
35
6
28
56
Acknowledgments
7
8
89
63
70
53
The authors thank Grant-in-Aid for Scientific Research (C)
2
6410141 and Grant-in Aid for Scientific Research on Innovative
Areas ‘‘ -figuration” 17H05171 for financial support. This work
was partly performed under the Cooperative Research Program of
‘Network Joint Research Center for Materials and Devices.”
p
‘
a
Conditions: 1 (1.0 mmol), 2 (2.0 mmol), Pd/C (2.5 mol%), S-Phos (7.5 mol%), 2 M
CO aq. (1.0 mL), solvent (2.0 mL), 100 °C, under argon.
Isolated yield.
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Please cite this article in press as: Hayashi S., et al. Tetrahedron Lett. (2017), http://dx.doi.org/10.1016/j.tetlet.2017.05.016

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: C, 66.02; H, 4.62. Found: C, 66.27; H, 4.55.
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Pd/C (53.3 mg, 0.025 mmol), S-Phos (30.6 mg, 0.075 mmol) were dissolved in
3
3
13
2
.0 mL of toluene under argon. To the solution were added K
2
CO
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(aq) (2.0 M, 1
3
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C H S
filtered, washed with methanol, and concentrated under reduced pressure. The
Please cite this article in press as: Hayashi S., et al. Tetrahedron Lett. (2017), http://dx.doi.org/10.1016/j.tetlet.2017.05.016