Facile Construction of Furanoacenes by a Three-Step Sequence Going through Disilyl-exo-cyclic Dienes

Article abstract of DOI:10.1002/chem.202001119

Facile synthesis of various benzonaphthofurans was achieved by intramolecular hydroarylation of 1,4-disilyl-2-aryloxy-1,3-enynes followed by cycloaddition with arynes or alkenes and finally desilylaromatization. The three-step transformation can be operated sequentially in one-pot, providing with a range of furanoacenes easily and highly effectively.

Full text of DOI:10.1002/chem.202001119

A Journal of
Accepted Article
Title: Facile Construction of Furanoacenes by a Three-step Sequence
Going Through Disilyl-exo-cyclic Dienes
Authors: Yasunori Minami, Yuki Furuya, and Tamejiro Hiyama
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COMMUNICATION
Facile Construction of Furanoacenes by a Three-step Sequence
Going Through Disilyl-exo-cyclic Dienes
Yasunori Minami,*[a,b] Yuki Furuya, and Tamejiro Hiyama*
[c]
[b]
[
a]
Dr. Y. Minami
Interdiscplinary Research Center for Catalytic Chemistry
National Institute of Advanced Industrial Science and Technology
1-1-1, Higashi, Tsukuba, Ibaraki 305-8565, Japan
E-mail: yasu-minami@aist.go.jp
Dr. Y. Minami, Prof. Dr. T. Hiyama
Research and Development Initiative
Chuo University
[
b]
c]
1-13-27, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
E-mail: thiyama@kc.chuo-u.ac.jp
Y. Furuya
[
Department of Applied Chemistry
Chuo University
1-13-27, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
Supporting information for this article is given via a link at the end of the document.
Abstract: Facile synthesis of various benzonaphthofurans was
achieved by intramolecular hydroarylation of 1,4-disilyl-2-aryloxy-1,3-
enynes followed by cycloaddition with arynes or alkenes and finally
desilylaromatization. The three-step transformation can be operated
sequentially in one-pot, providing with a range of furanoacenes
easily and highly effectively.
Cycloaddition reaction is a powerful method for the
construction of condensed cycles. In particular, exo-cyclic
dienes like isobenzofurans are a useful building block for the
construction of various polycondenced acenes through the
cycloaddition with various dienophiles followed by aromatization
(Figure 2a).[ Unlike its simplicity, this particular system is not
directly applicable to the synthesis of heteroacenes such as
furanoacenes. Furthermore, 2,3-bismethylene-2,3-dihydrofurans,
as an ideal building block for furanoacenes, are unstable and
6]
Polycondensed heteroaromatics (PCHA) have become an
important and challenging class of synthetic targets in material
chemistry. PCHA show unique functional properties based on
their own structures. A typical example is thienoacenes that
have a carrier mobility in many instances and are applied to p-
type organic semiconductors for organic field-effect transistors.[1]
Recently, furanoacenes are attracting much interest due to novel
bioactivity and physical properties as functional materials (Figure
[7]
quickly undergo undesired transformations. Thus, for the
construction of heteroacenes, stable but conveniently reactive
building blocks are strongly required. Recently, we reported
intramolecular
exo-hydroarylation
of
enynes
under
palladium/carboxylic acid catalytic system, giving variously
substituted
2,3-bis(silylmethylene)-2,3-dihydrobenzofurans
kinetically-stabilized by two bulky silyl groups.[ We envisaged
that the cyclic bis(silylmethylene) cycles[10] have a potential to
work as building intermediates for various furanoacenes
employing arynes as a representative reaction partner. For
8,9]
1
)[2] and a building block for polyaromatic hydrocarbons.[3]
Accordingly, facile preparative methods of furanoacenes are
disclosed, taking advantage of intramolecular C–O or C–C bond
forming reactions of biaryls or diaryl ethers or annulation with
benzofuran,[4,5] whereas the conventional methods require multi-
step preparation and/or uncontrolled condensations in a late
stage. Thus, invention of convenient synthetic methods for
various kinds of acenes is an urgent research target for the
future material chemistry.
example,
cycloadducts
resulting
from
the
bis(silylmethylene)dihydrobenzofurans and arynes, are expected
to show high solubility and convertibility to a variety of
[11,12,13]
furanoacenes via silyl-promoted aromatization reactions.
From these viewpoints, we pursued the facile construction of
furanoacenes by sequential transformations based on the 1,4-
disilylated oxacyclic dienes as a key building intermediate
(Figure 2b).
O
R
R
O
a
Isobenzofuran as a intermediate for acenes
O
O
potent topoisomerase II inhibitor
semiconductor
Ar
O
R
O
O
O
b
This work: Construction of furanoacenes via disilylated precursors
Et3Si
O
R
O
potent antitubercular and
antimycobacterial activities
semiconductor
SiEt3
1
R1
1
R
R
O
O
O
Figure 1. Fused dibenzofuran derivatives in material and medicinal
compounds
SiiPr3
SiiPr3
soluble precursor
furanoacene
Figure 2. Utility of exo-cyclic dienes for the construction of condensed cycles
1
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COMMUNICATION
Scheme 2. Facile synthesis of 4a from 6a
Initially, we examined the reaction of 2-TIPSmethylene-3-
TESmethylene-2,3-dihydrobenzofuran
1a
with
benzyne
generated from 2-trimethylsilylphenyl triflate (2a) and CsF in
acetonitrile at room temperature.[14] To our delight, the desired
We propose the mechanism for the oxidative aromatization
as shown in Scheme 3. By treatment of I , 3 should be initially
converted into iodonium ion 7 via an interaction of the electron-
rich vinyloxy moiety in 3. Subsequent attack of an iodide ion to
disilyl dihydorbenzonaphthofuran 3a was obtained in 74% yield
with 6,11-anti configuration (Scheme 1). The two silyl groups in
2
1a tolerated the presence of CsF due possibly to the steric
the less-bulky SiEt
intermediate 8 and Et
from 8 proceeds through cationic 9 to produce furanoacene 4.
On the other hand, DDQ likely undergoes at first one-electron
oxidation of the vinyloxy moiety in 3 to form 10. The resulting
DDQ anion radical should then nucleophilically attack to the
3
group occurs to generate α-iodo
SiI. Oxygen-assisted release of iPr SiI
crowding without any steric retardation toward the cycloaddition.
3
2
a could be aromatized by treatment with I at room temperature
3
3
to give benzonaphthofuran 4a.[11e] On the other hand, DDQ
oxidatively removed the TES group only to provide TIPS-
substituted benzonaphthofuran 5a.[11g]
3
SiEt group to generate 11, which finally loses the methine
TfO
hydrogen by DDQ-SiEt
3
radical selectively to furnish 5.
SiEt3
Me3Si
SiEt3
2
a (3 eq)
I
CsF (6 eq)
MeCN
rt, 20 h
O
O
I2
SiEt3
SiiPr3
SiiPr3
1a
3a, 74%
4
I
O
O
2
0 h
SiiPr3
I
SiiPr3
O
Et SiI
3
iPr SiI
3
7
8
9
I
SiiPr3
3
4
5
a (X = H), 91% from Conditions A
a (X = SiiPr3), 65% from Conditions B
O
DDQ
Cl
OSiEt3
CN
X
SiEt3
DDQ
Cl
5
O
SiiPr3
Scheme 1. Construction of disilyl precursor 3a and benzonaphthofuran 4a and
a from disilyl exo-oxacyclic diene 1a. Conditions A: I (2 eq) and CH Cl (0.1
H
O
O
CN
DDQHSiEt3
5
2
2
2
10
SiiPr3
11
DDQSiEt3
M) at 25 °C. Conditions B: DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone, 2
eq) and toluene (0.1 M) at 80 °C.
Scheme 3. Proposed mechanism of the formation of 4 and 5
The successful transformation led us to improve this
protocol to an easier one. We first synthesized 3a from disilyl
enyne 6a in one-pot by Pd/acid-catalyzed hydroarylation[15] to
generate 1a and by cycloaddition with in situ generated benzyne,
as shown in Eq. (1) in an overall yield similar to the stepwise
synthesis.[16] Based on this observation, we examined a true
one-pot three-step sequence and obtained 4a in 52% total yield
Scheme 2 upside). This one-pot protocol is quite simple and
effective. However, we worried that reaction mess generated by
the cycloaddition might complicate the last aromatization step.
Therefore, we modified the protocol to the one involving the first
two-step one-pot sequence, filtration through Celite, and the
We next examined the scope of the furanoacene synthesis
by the modified sequence (Figure 3). Non-substituted enyne 6b
gave 4b without any problem in 53% total yield. Variously
substituted enynes 6c-6j having a methoxy, methyl, phenyl, or
trifluoromethyl group, or fluorine at para position of the 2-
phenoxy group were converted into furanoacenes 4c-4j in good
yields. Worthy of note is that the site selectivity of the
hydroarylation permitted to control the structure of furanoacenes
(
4.
Specifically, the hydroarylation of meta-fluorinated 6i
proceeded via the cleavage of C–H bond ortho to fluorine and
provided 4i having fluorine at C1 position exclusively. meta-
Trifluoromethylated 6j was converted into 4j substituted at C3
position via less-hindered C–H bond activation. Next, we
examined various arynes as shown in Figure 4. The reaction of
symmetrical 4,5-dimethoxy benzyne and 2,3-naphthalyne (from
treatment with I
2% (Scheme 2 lower equation), showing this protocol is highly
effective to construct the furanoacenes via 1.
2
. As a result, yield of 4a was enhanced up to
6
2
b and 2c) gave the corresponding acenes in good yields.
SiEt3
Pd2(dba)3 (2.5 mol%)
PCy3 (5 mol%)
tBuCO2H (20 mol%) CsF (6 eq)
Unsymmetrical arynes such as 1,2-naphthalyne and 3-methyl
benzyne (from 2d and 2e) which are slightly biased sterically,
gave the corresponding cycloadducts (3n-3p) as an inseparable
isomeric mixture. Final aromatization led to furanoacenes 4n-4p
in good yields although with low regioselectivity. On the other
hand, 3-methoxybenzyne (from 2f) produced furanoacene 4q
and 4r with high regioselectivity (6.8:1, 6.7:1). The observed
selectivity can be attributed to the nucleophilic 3-
2a (3 eq)
H
O
3
a
(1)
4-octyne (10 mol%) toluene/MeCN
7
0%
toluene (0.25 M)
SiiPr3 100 °C, 20 h
(1:1)
rt, 20 h
6a
one-pot
Pd/acid cat.
2a, CsF
I2
5
2% (total)
3
-step one-pot sequence
[
8]
SiEt3
TESmethylidene carbon in 1 which attacks to the distal C1-
H
O
position of the highly distorted 3-methoxybenzyne (Scheme
).
4 [
17]
3-Morpholinobenzyne (from 2g),[18] which have the
O
character similar to 3-methoxy benzyne,
gave 7-
6
a
SiiPr3
Pd/acid cat.
-step one-pot sequence
4a
2
CsF
a
aminobenzonaphthofuran 4s with perfect regioselectivity. Thus,
in addition to the site-selective C–H cleavage in the
hydroarylation, the difference in the nucleophilicity of the two
filtration
I2
62% (total)
2
2
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Chemistry - A European Journal
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COMMUNICATION
methylidene carbons in 1 also contributes to the site-selective
enhancing the potential of the present method (Scheme 5).
synthesis of furanoacenes. These features suggest advantages
Methyl α-chloroacrylate (12) could be used for the synthesis of
of the present enyne 6-based sequence.[19]
.
unsymmetrical dibenzofuran 13 by the modified sequence with
20]
a.[ 1,4-Epoxy-1,4-dihydronaphthalene (14) was used in the
6
two-step sequence formed cycloadduct 15 stereoselectively.
SiEt3
1
. Pd cat., tBuCO2H cat.
then, 2, CsF
R'
Subsequent treatment with TfOH[21] gave anthracene-fused
4
H
O
Rn
benzofuran 16 in 71% yield.
R
2
3. I2
. filtration
3
O
2
4
SiiPr3
1
2
. Pd cat.
tBuCO2H cat.
6
6
b (R = H), 6c (R = 4-OMe), 6d (R = 4-Me), 6e (R = 4-Ph), 6f (R = 4-F)
g (R = 4-CF3), 6h (R = 2-Ph), 6i (R = 3-F), 6j (R = 3-CF3)
filtration
I2 (1 eq)
CH2Cl2
CO2Me
. Cl
CO2Me
(0.1 M)
O
F
R
2
25 °C, 20 h
1
1
8
2 (1.7 eq)
0 °C, 20 h
13, 31%
O
4
O
O
6
a
Ph
4
4
4
4
4
4
b (R = H), 53%
c (R = MeO), 54%
d (R = Me), 60%
e (R = Ph), 51%
f (R = F), 54%
4h, 40%
4i, 50%
1
. Pd cat.
CF3SO3H
(10 eq)
Et3Si
F3C 3
H
tBuCO H cat.
2
O
O
g (R = CF3), 52%
2.
CHCl3
4
j, 30%
O
(0.1 M)
70 °C, 16 h
O
H
O
OMe
iPr Si
3
1
2
14 (2 eq)
1
20 °C, 20 h
15, 72%
16, 71%
Rn
OMe
3
O
O
Scheme 5. Sequence using alkene partners
4
4
k (R = 1,3-Me2), 70%
l (R = 2-Me), 55%
4m, 54%
1
2
Rn
In conclusion, we have disclosed a facile reaction sequence
for the modular synthesis of various furanoacenes from aryloxy-
disilylenynes. This sequence involves catalytic intramolecular
hydroarylation, cycloaddition with a dienophile like arynes, α-
chloroacrylate, and 1,4-epoxy-1,4-dihydronaphthalene, and
oxidative desilylaromatization, without isolation of intermediates.
The two silyl groups act as the versatile roles in among
transformations, endowing high stability and solubility for the
diene intermediates, high regioselectivity for the second
cycloaddition, and the key functional groups for easy
desilylaromatization. As a result, all reactions proceed smoothly,
giving the targeted furanoacenes in high yields with excellent
site-selectivity. In view that the starting materials are readily
accessible, the reaction sequence reported here will find a
variety of applications in the smooth preparation of a diverse
range of furanoacenes.
3
O
O
b
b
4
4
n (R = 1,3-Me2), 56% (1.3:1)
o (R = 2-Me), 52% (1.4:1)
4p, 48% (1.5:1)
b
1
2
Rn
3
O
OMe
O
N
b
4q (R = 1,3-Me2), 49% (6.8:1)
4s, 41%
b
4r (R = 2-Me), 46% (6.7:1)
O
Figure 3. Synthesis of 4. A mixture of 6, Pd
2
3 3
(dba) (2.5 mol%), PCy (5 mol%),
tBuCO
2
H (20 mol%), 4-octyne (10 mol%), and toluene was stirred at 100 °C
for 20 h. Then, 2 (3 eq), CsF (6 eq), and MeCN were added and the solution
was stirred at 25 °C for 20 h. After filtration through Celite, a mixture of crude 3,
I
2
(1.6 eq), and CH
2
Cl
2
(0.1 M) was stirred at 25 °C for 20 h. Isolated total yield
b
based on 6. Ratio of regioisomers. The displayed structure is the main
isomer.
TfO
TfO
OMe TfO
OMe TMS
TfO
TfO
Acknowledgements
TMS
TMS
TMS
R
TMS
R = OMe, 2f
R = Mor, 2g
This research was supported financially by Grants-in-Aid for
Scientific Research (C) and Challenging Research (Exploratory)
(19K05481 and 17K19127 to Y.M.) from the JSPS, the
Tokuyama Science Foundation, and the UBE Foundation. T.H.
also acknowledges the Sumitomo Chemicals. The authors would
like to thank Ms. Megumi Sakai for her contribution of the initial
stage of this research.
2
b
2c
2d
2e
Figure 4. Aryne precursor 2
SiEt3
3
SiEt3
1
R
R
I2
+
4
O
SiiPr3
X
O
X
iPr3Si
X = OMe, Mor
Keywords: addition • arene • aromatization • alkyne • diene
Scheme 4. Regioselective cycloaddition with arynes (from 2f and 2g)
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3
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Chemistry - A European Journal
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COMMUNICATION
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2
gave bi(benzonaphthofuran)s as a mixture. The details are shown in
the Supporting Information.
[20] A plausible intermediate after the cycloaddition is discussed in the
supplementary information.
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Facile synthesis of various benzonaphthofurans was achieved from 1,4-disilyl-2-aryloxy-1,3-enynes via hydroarylation, cycloaddition
with arynes or alkenes, and desilylated aromatization. These three reactions could be operated sequentially in one-pot, being highly
effective and providing a range of furanoacene products with ease.
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