Annulative π-Extension (APEX) of Heteroarenes with Dibenzosiloles and Dibenzogermoles by Palladium/o-Chloranil Catalysis

Article abstract of DOI:10.1021/acs.orglett.7b00684

Annulative π-extension (APEX) reactions of heteroarenes are described herein. A catalytic system comprising a cationic palladium species and o-chloranil using dimethyldibenzosiloles as π-extending agents enabled the extension of the π-system of benzo[b]thiophenes. π-Extended dibenzofurans and carbazoles could also be obtained from benzofuran and N-tosylindole, respectively, with dimethyldibenzogermole as a germanium-based π-extending agent. Mechanistic investigations indicated two possible reaction pathways involving carbopalladation-based double C-H arylation of benzothiophene or formal cycloaddition/oxidation cascades.

Full text of DOI:10.1021/acs.orglett.7b00684

Letter
pubs.acs.org/OrgLett
Annulative π‑Extension (APEX) of Heteroarenes with Dibenzosiloles
and Dibenzogermoles by Palladium/o‑Chloranil Catalysis
†
†
,†
,†,‡,#
Kyohei Ozaki, Wataru Matsuoka, Hideto Ito,* and Kenichiro Itami*
†
Graduate School of Science, Nagoya University, Chikusa, Nagoya 464-8602, Japan
‡
JST-ERATO, Itami Molecular Nanocarbon Project, Nagoya University, Chikusa, Nagoya 464-8602, Japan
Institute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa, Nagoya 464-8602, Japan
#
*
S Supporting Information
ABSTRACT: Annulative π-extension (APEX) reactions of
heteroarenes are described herein. A catalytic system
comprising a cationic palladium species and o-chloranil using
dimethyldibenzosiloles as π-extending agents enabled the
extension of the π-system of benzo[b]thiophenes. π-Extended
dibenzofurans and carbazoles could also be obtained from
benzofuran and N-tosylindole, respectively, with dimethyldi-
benzogermole as a germanium-based π-extending agent.
Mechanistic investigations indicated two possible reaction
pathways involving carbopalladation-based double C−H
arylation of benzothiophene or formal cycloaddition/oxidation
cascades.
3
uring the past few decades, fused π-conjugation systems
alcohols, amines, and sulfides, (ii) intramolecular carbon−
4
D
containing heteroatoms have been recognized as quintes-
carbon bond formation of diaryl heteroethers, (iii) stepwise
functionalization and π-extension of heteroarenes, and (iv)
single-step annulative π-extension (APEX) of unfunctionalized
1
5
sential structural motifs in the field of optoelectronics. The
electronic structure of heteroarenes can be easily controlled by
the replacement of heteroatoms with other ones, changing the
substitution patterns, and the extension of fused π-conjugation,
thereby manifesting unique and kaleidoscopic photophysical
6
heteroarenes. Although synthetic methods (i), (ii), and (iii)
reliably give π-extended fused heteroaromatics, stepwise
reactions including the prefunctionalization of starting com-
pounds always decrease the synthetic utility and total yields. In
stark contrast, APEX reactions do not require the prefunction-
alization of template heteroarenes, thereby providing consid-
erable synthetic benefits from the viewpoint of convenience and
step/atom economy. Thus, a variety of APEX reactions of
unfunctionalized heteroarenes have recently garnered consid-
2
properties.
As shown in Figure 1a, mainstream synthetic approaches
toward heteroatom-containing fused π-systems include (i)
intramolecular carbon−heteroatom bond formation of biaryl
6
−9
erable attention.
While a number of APEX reactions of
pyrroles and indoles have been reported using Brønsted and
Lewis acids as well as transition metal catalysts, APEX reaction7s
of thiophenes and benzothiophenes are not well established.
The development of thiophene-based APEX reactions would
offer easy access to attractive π-extended fused thiophenes, which
2
are promising compounds in materials science. Herein, we
report a new APEX reaction of heteroarenes, especially
benzothiophenes, with dibenzosiloles and dibenzogermole as
π-extending agents (Figure 1b).
During the course of our studies on APEX reactions of
6a
6e
polycyclic aromatic hydrocarbons (PAHs) and alkynes with
dibenzosiloles, we found that a palladium/o-chloranil catalytic
system promoted APEX reactions at the most reactive, carbon−
carbon multiple bond such as the K-region (most olefinic
Figure 1. (a) Classical synthetic methods for heteroatom-containing
fused π-systems. (b) Pd-catalyzed annulative π-extension (APEX) of
heteroarenes with dibenzosilole and dibenzogermole.
Received: March 14, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00684
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
armchair convex region) on PAHs or triple bonds on alkynes.
Based on this site selectivity, we envisioned that other aromatic
templates with reactive C−C multiple bonds could be suitable
candidates for novel APEX reactions.
With the optimized conditions in hand, the scope of
benzo[b]thiophene derivatives 1 and dibenzosiloles 2 was
investigated (Scheme 1). tert-Butyl and methyl substituted
Optimal conditions for the APEX of benzo[b]thiophene (1a)
Scheme 1. Substrate Scope of APEX Reaction of
Benzo[b]thiophenes
a
were established using Pd(OAc) as a catalyst, dimethyldibenzo-
2
silole (2a) as a π-extending agent, a silver salt, and o-chloranil in
1,2-dichloroethane at 70 °C (Table 1). The use of 1.0 equiv of
Table 1. Screening of Reaction Conditions for Pd-Catalyzed
APEX Reactions of Benzo[b]thiophene (1a) with 9,9-
a
Dimethyldibenzosilole (2a)
b
entry
1
Ag salt
AgPF6
oxidant
yield
63%
o-chloranil
o-chloranil
o-chloranil
o-chloranil
o-chloranil
o-chloranil
o-chloranil
o-chloranil
DDQ
c
2
AgPF6
35%
46%
34%
27%
38%
40%
0%
3
4
5
6
7
8
AgPF (50 mol %)
6
AgOTf
AgBF4
AgSbF6
AgNTf2
none
a
Reaction conditions: 1 (0.20 mmol), 2 (3.0 equiv), Pd(OAc) (5 mol
2
9
AgPF6
AgPF6
AgPF6
AgPF6
18%
0%
%), AgPF
6
(1.0 equiv), o-chloranil (2.0 equiv), 1,2-dichloroethane (1.5
b
d
mL), 70 °C, 16 h. Pd(OAc) (10 mol %).
1
1
1
0
1
2
DTBQ
2
BQ
0%
e
f
o-chloranil
69% (68%)
a
benzo[b]thiophenes 1b and 1c provided desired APEX products
Reaction conditions: benzo[b]thiophene (1a: 0.20 mmol), 9,9-
dimethyldibenzosilole (2a: 3.0 equiv), Pd(OAc) (5 mol %), AgPF6
3ba and 3ca in moderate yields. The reaction of electron-
2
deficient benzo[b]thiophenes, such as 5-bromo, 5-chloro, and 5-
fluorobenzo[b]thiophenes 1d, 1e, and 1f, provided the desired
products 3da, 3ea, and 3fa without concomitant loss of the
halogen atoms, which are useful for further functionalization by
classical cross-coupling protocols. Additionally, the reaction of
benzo[b]thiophene (1a) with 3,7-di-tert-butyldibenzosilole (2b)
afforded the corresponding APEX product 3ab in 28% yield.
When unsymmetrical dibenzosilole 3-chlorodimethyldibenzosi-
lole (2c) was used as the π-extending agent, regioisomers 3ac and
3ac′ were obtained in 38% combined yield as a 78:22 mixture.
Other heteroarene templates were also investigated (Scheme
2). Under the optimized conditions, 2,3-diphenylthiophene (1g)
and 2-aryl-3-methoxythiophene 1h were transformed into
phenanthro[9,10-b]thiophenes 3ga and 3ha in 50% and 36%
yields, respectively. However, the standard conditions with
dibenzosilole 2a were not applicable to the APEX of N-
tosylindole (5). Further optimization revealed that the
combination of dimethyldibenzogermole (2d) and 30 mol %
of AgOTf provided dibenzocarbazole 6, albeit in a low yield (30%
yield). When benzofuran (7) was used in the APEX reaction with
2.0 equiv of dibenzogermole 2d, desired benzonaphthofuran 8
was obtained in 19% yield, along with cis-dehydrogenated 8′ in
23% yield. Increasing the amount of o-chloranil afforded 8 as the
only product. Notably, 8′ was readily oxidized to 8 in the
presence of 2.0 equiv of o-chloranil in 1,2-dichloroethane at room
temperature. Although these phenomena may only occur in the
(
1.0 equiv), o-chloranil (2.0 equiv), 1,2-dichloroethane (2.0 mL), 70
b
°
C, 16 h. GC or NMR yield. GC yield was determined using n-
decane as an internal standard. NMR yield was determined using
c
d
CH Br2 as an internal standard. 2.0 equiv of 2a was used. β-
Biphenylbenzo[b]thiophene (4) was obtained in 7% GC yield. 1.5
mL of 1,2-dichloroethane was used. Isolated yield.
2
e
f
AgPF , 3.0 equiv of 2a, and 2.0 equiv of o-chloranil (relative to
1
(
resulted in lower yields (entries 2 and 3). While the reactions
with other silver salts such as AgOTf, AgBF , AgSbF , and
6
a) gave the best result affording APEX product 3aa in 63% yield
Table 1, entry 1). Decreasing the amount of 2a or AgPF6
4
6
AgNTf also provided 3aa in 27−40% yields, the reactions did
2
not exceed the yield obtained with AgPF (entries 4−7). The use
6
of neutral palladium acetate without any silver salt did not afford
3
aa at all (entry 8). Although 5,6-dichloro-2,3-dicyanobenzo-
quinone (DDQ) instead of o-chloranil afforded the APEX
product in 18% GC yield (entry 9), 3,5-di-tert-butylbenzoqui-
none (DTBQ) only gave nonfused β-biphenylbenzo[b]-
thiophene (4) in 7% GC yield (entry 10). p-Benzoquinone
(
BQ) was not an effective oxidant (entry 11). Finally, the best
result was obtained under a marginally higher concentration of
a (0.13 M), which gave 3aa in 68% isolated yield (entry 12),
along with a byproduct derived from the hydrofluorination of 2a
1
1
0
(
see SI for details).
B
DOI: 10.1021/acs.orglett.7b00684
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. APEX of Thiophene Derivatives (1g and 1h),
Benzofuran (5), and N-Tosylindole (7)
Figure 2. Two possible reaction pathways in the carbopalladation-
triggered APEX reaction of benzoheteroles.
carbopalladation events in monoarylation product C to give
the trans-fused precursor of APEX product D. Then, a second β-
H elimination/demetalation occurs to give APEX product 3aa.
As an alternative mechanism for the reaction of benzofuran (7)
with dibenzogermole 2d, path B involving formal cycloaddition/
oxidation cascades is also possible, where the intramolecular
transmetalation of intermediate B is followed by reductive
elimination from palladacycle E to give cis-dihydro adduct 8′,
which would be oxidized by o-chloranil to provide APEX product
8.
In summary, APEX reactions of heteroarene templates such as
thiophenes, benzofuran, and N-tosylindole in the presence of a
palladium catalyst, silver salts, and o-chloranil have been
established. The combination of π-extending agents such as
dimethyldibenzosilole and dimethyldibenzogermole with silver
hexafluorophosphate or silver triflate enabled the synthesis of a
variety of π-extended fused heteroarenes in one-step. The
developed APEX methodology has obvious advantages over
existing stepwise π-extension methods in terms of the step- and
atom-economy.
reaction with benzofuran, these results provided mechanistic
insights into the APEX reaction (vide infra).
6a
In light of previous studies on the APEX of PAHs and
11
oxidative C−H arylation of thiophenes, we surmised that the
current heterocycle APEX reaction of benzo[b]thiophene (1a)
might be initiated by the oxidative β-arylation of benzo[b]-
thiophene via carbopalladation (vide infra). In fact, when
trimethylsilylbenzene was used as a coupling partner under the
influence of the Pd(OAc) /AgOTf/o-chloranil system, C−H
2
arylation of 1a gave α- and β-arylated products 9a and 9b in 27%
combined yield with 93% β-selectivity (Scheme 3). Thus, β-
Scheme 3. (a) β-Selective C−H Arylation of
Benzo[b]thiophene (1a) with Trimethylsilylbenzene; (b)
Intramolecular C−H Arylation of Silylated 3-
Biphenyllbenzothiophene 10
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b00684.
Experimental procedures and characterization data for all
compounds, general procedures, and ORTEP drawings
(
PDF)
X-ray crystallographic data for 3da (CIF)
X-ray crystallographic data for 8′ (CIF)
AUTHOR INFORMATION
Corresponding Authors
■
*
E-mail: ito.hideto@g.mbox.nagoya-u.ac.jp (H.I.)
E-mail: itami@chem.nagoya-u.ac.jp (K.I.)
1
1g
*
arylation of 1a might be the first step in the current APEX with
dibenzosilole. Furthermore, palladium-catalyzed intramolecular
C−H arylation of quasi-intermediate 10 under the standard
conditions gave 3aa in 51% NMR yield.
ORCID
Hideto Ito: 0000-0002-4034-6247
Kenichiro Itami: 0000-0001-5227-7894
Notes
Based on the aforementioned mechanistic insights, two
possible reaction pathways can be assumed (Paths A and B,
Figure 2). Both pathways are initiated from the formation of
biphenyl palladium intermediate A by the transmetalation of a
cationic palladium species with dibenzosilole or dibenzogermole.
As described in the literature, regioselective carbopalladation
can occur with benzoheteroles to form β-biphenyl-α-palladated
adduct B. Path A involves β-H elimination/demetalation from
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
11g
This work was supported by the ERATO program from JST (to
K.I.), JSPS KAKENHI Grant Numbers JP26810057 and
JP16H00907, and a grant from SHOWA DENKO Award in
Synthetic Organic Chemistry, Japan (to H.I.). We thank Dr.
1
0
intermediate B, followed by sequential transmetalation/
C
DOI: 10.1021/acs.orglett.7b00684
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Yasutomo Segawa and Dr. Takao Fujikawa for X-ray diffraction
analyses. ITbM is supported by the World Premier International
Research Center (WPI) Initiative, Japan.
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̈
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DOI: 10.1021/acs.orglett.7b00684
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