Pd-catalyzed cascade crossover annulation of o-alkynylarylhalides and diarylacetylenes leading to dibenzo[a,e]pentalenes

Article abstract of DOI:10.1021/ja403382d

A novel and selective Pd-catalyzed cascade crossover-annulation of o-alkynylarylhalides and diarylacetylenes for the synthesis of dibenzo[a,e]pentalenes has been reported. Various arylacetylenes with a wide range of functional groups were tolerated, producing the corresponding multisubstituted dibenzopentalenes with the different substituents on the aromatic rings in good to high yields under the optimized reaction conditions. The reaction proceeds through a Pd-catalyzed cascade carbopalladation and C-H activation. The use of the combined DBU and CsOPiv bases is crucial for the successful implementation of the present cross-annulation.

Full text of DOI:10.1021/ja403382d

Communication
pubs.acs.org/JACS
Pd-Catalyzed Cascade Crossover Annulation of o‑Alkynylarylhalides
and Diarylacetylenes Leading to Dibenzo[a,e]pentalenes
Jian Zhao,† Kazuaki Oniwa,† Naoki Asao,† Yoshinori Yamamoto,†,‡ and Tienan Jin*,†
†WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, Sendai 980-8577, Japan
‡State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
S
* Supporting Information
Scheme 1. Catalytic Homoannulations and Crossover
Annulations for the Synthesis of the Multisubstituted
Dibenzo[a,e]pentalene Derivatives
ABSTRACT: A novel and selective Pd-catalyzed cascade
crossover-annulation of o-alkynylarylhalides and diary-
lacetylenes for the synthesis of dibenzo[a,e]pentalenes
has been reported. Various arylacetylenes with a wide
range of functional groups were tolerated, producing the
corresponding multisubstituted dibenzopentalenes with
the different substituents on the aromatic rings in good
to high yields under the optimized reaction conditions.
The reaction proceeds through a Pd-catalyzed cascade
carbopalladation and C−H activation. The use of the
combined DBU and CsOPiv bases is crucial for the
successful implementation of the present cross-annulation.
yclopenta-fused π-conjugated linear acene-type polycyclic
Ccompounds have attracted increasing interest due to their
distinct physical and chemical properties.1 Recent investigations
revealed that those fused polycyclic compounds embedding
cyclopentadiene moieties such as dibenzo-fused s-indacenes and
pentalenes have narrow energy gaps1,2 relative to the linear
acenes such as pentacene,3 which have been utilized as the active
organic semiconductors in optical and electronic device
applications.2 The planar structure of dibenzo[a,e]pentalene
framework possessing two five-membered rings and two external
benzene rings has the increased stability and more extended π-
conjugation than the simple pentalenes,4 and hence the synthetic
methods of its versatile derivatives are of growing recent
interest.5a Especially, the multisubstituted dibenzopentalenes
bearing functional groups at the 5- and 10-positions exhibited
high stability and drastically changed electronic and structure
properties compared to the parent dibenzopentalene.2a,4
Since the first synthesis in 1912, a number of the improved
methods of dibenzopentalene derivatives have been reported.4,5
For example, Yamaguchi et al. reported the reductive cyclization
of o,o′-bis(arylcarbonyl)diphenylacetylenes with lithium naph-
thalenide4c and Saito et al. reported the reductive cyclization of
phenyl(tri-i-propylsilyl)acetylene with lithium followed by
iodine.4a More recently, Tilley and Kawase groups reported
independently the simple synthetic protocols through the Pd- or
Ni-catalyzed C−Br/C−Br homoannulation of o-alkynylarylbro-
mides (Scheme 1a).4b,5h However those reactions provide the
efficient ways for the synthesis of benzopentalenes with
symmetric substituents at 5-, and 10-positions, only a limited
number of approaches for the synthesis of the unsymmetrically
5,10-disubstituted dibenzopentalenes have been reported; for
example, except for the multistep procedures,6 Tilley et al.
reported two examples of the Pd-catalyzed C−Br/C−Sn
crossover-annulation of 1-bromo-2-(phenylethynyl)benzene
and o-(alkynyl)phenyl-tributylstannanes (Scheme 1b).5h How-
ever, it is highly desirable that, if the crossover-annulation
proceeds through the C−H activation7 instead of the C−Sn
activation, which may provide an attractive and general synthetic
method for the construction of the multisubstituted benzopen-
talenes with a wide range of the functional groups on aromatic
rings. Herein, we report a novel Pd-catalyzed crossover-
annulation of o-alkynylarylhalides and diarylacetylenes via C−
Cl(Br)/C−H cascade to afford the corresponding multi-
substituted dibenzo[a,e]pentalenes in good to high yields
(Scheme 1c).
We initiated our investigation by examining various base
additives in the reaction of 1-bromo-2-((4-methoxyphenyl)-
ethynyl)benzene (1a′) and 1,2-diphenylacetylene (2a) (5 equiv)
in the presence of Pd(OAc)2 (5 mol %) and PtBu3 (15 mol %) in
1,4-dioxane (0.2 M) at 120 °C for 24 h (Table 1 and Supporting
Information, Table S1). Among the inorganic bases, such as
Cs2CO3, CsOPiv, and K2CO3 tested, only a small amount of the
corresponding product 3a was obtained (entries 1−3) with
decomposition of 1a′. The use of organic bases, such as DBU
Received: April 11, 2013
Published: June 28, 2013
© 2013 American Chemical Society
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a
toluene as a solvent gave a slightly lower yield of 3a than that of
1,4-dioxane, whereas the polar solvents, such as N,N-
dimethylformamide or N,N-dimethylacetamide gave very low
yields of 3a (see Table S1). The reaction with other palladium
catalysts, such as Pd2(dba)3 (dba = diphenylbenzylideneacetone)
and Pd(PtBu3)2 (without additional PtBu3) were also active,
affording 3a in 69% and 73% yields, respectively, while
Pd(PPh3)4 catalyst was inactive (see Table S1). The yield of 3a
was further increased to 81% yield when 1,4-dioxane was diluted
to 0.1 M at the higher temperature of 140 °C (entry 16). The use
of a lower catalytic loading of Pd(OAc)2/PtBu3 (2/6 mol %) did
not affect the yield of 3a (entry 17). The yield of 3a was
decreased slightly when 0.5 equiv of CsOPiv was used (entry 18).
Overall, the use of Pd(OAc)2/PtBu3 catalyst systems with a
mixture of DBU and CsOPiv bases has been proved to be the
most efficient and selective condition for the formation of the
crossover-annulation product.
Table 1. Optimization of Reaction Conditions
b
entry
X
ligand (15 mol %)
base (3 equiv)
yield (%)
1
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
PtBu3
PtBu3
PtBu3
PtBu3
PtBu3
PtBu3
PtBu3
PtBu3
PtBu3
PtBu3
PiBu3
PiPr3
Cs2CO3
CsOPiv
38
2
7
3
K2CO3
trace
28
4
DBU
5
DABCO
Pyridine
Cs2CO3/PivOH
trace
trace
24
6
c
7
c
8
DBU/Cs2CO3
19
Under optimized conditions, the scope of Pd-catalyzed
crossover-annulation of various o-alkynylarylhalides and diary-
lacetylenes was examined (Table 2). In each case, a very small
c
9
DBU/CsOPiv
78 (66)
81 (75)
trace
trace
trace
19
d
d
d
d
d
d
d
d
d
c
10
11
12
13
14
15
16
17
18
DBU/CsOPiv
c
DBU/CsOPiv
c
DBU/CsOPiv
c
Table 2. Pd-catalyzed Crossover Annulation of Various o-
PCy3
P(Tol)3
dppf
PtBu3
PtBu3
PtBu3
DBU/CsOPiv
Alkynylarylchlorides with 1,2-Diarylacetylenesa
c
DBU/CsOPiv
c
DBU/CsOPiv
40
,
,
,
e
e
e
c
DBU/CsOPiv
87 (81)
87 (82)
70
,
,
f
f
c
DBU/CsOPiv
g
DBU/CsOPiv
a
Reaction conditions: 1a or 1a′ (0.4 mmol), 2a (5 equiv), Pd(OAc)2
(5 mol %), PtBu3 (15 mol %), base (3 equiv), solvent (0.2 M), under
b
Ar atmosphere, 120 °C for 24 h. 1HNMR yield determined using
CH2Br2 as an internal standard. The isolated yield is shown in
parentheses. An amount of 1.1 equivalent was used. Reaction time is
b
entry
1
2
3
yield (%)
c
d
1
1b R1 = NMe2, R2 = H
1c R1 = n-C4H9, R2 = H
1d R1 = CO2Me, R2 = H
2a R3 = H
2a R3 = H
2a R3 = H
2a R3 = H
2a R3 = H
2a R3 = H
2a R3 = H
2b R3 = Me
2b R3 = Me
3b
3c
3d
3e
3f
74
83
e
48 h, Reaction temperature is 140 °C and concentration is 0.1 M.
f
Concentrations of 2 mol % of Pd(OAc)2 and 6 mol % of P(tBu)3 were
2
c
g
3
58
used. CsOPiv (0.5 equiv) was used.
d
e
4
1e R1 = COMe, R2 = H
70
e
5
1f R1 = CN, R2 = H
76
(1,8-diazabicycloundec-7-ene), DABCO (1,4-diazabicyclo-
[2.2.2]octane), and pyridine also produced very low yields of
3a (entries 4−6). However, it should be noted that, in contrast to
the inorganic bases, the strong organic base DBU produced 3a in
28% yield with a 72% yield of the recovered 1a′ (entry 4),
indicating the good compatibility of arylbromide substrates with
DBU base. The combination of Cs2CO3 base with PivOH
(pivalic acid) (1.1 equiv) or DBU with Cs2CO3 (1.1 equiv) did
not increase the yield of 3a (entries 7 and 8). To our delight,
DBU combined with CsOPiv (1.1 equiv) showed a high activity,
affording the crossover-annulation product 3a in 66% isolated
yield together with the homoannulation product of 1a′ in less
than 10% yield, implying the involvement of the pivalate-assisted
C−H bond cleavage in this reaction (entry 9).8 We thought that
the use of arylbromide as a substrate might be favorable for the
formation of a homoannulation product due to the ease of the
intramolecular oxidative addition of the in situ formed vinyl-
palladium to arylbromide. As expected, when 1-chloro-2-((4-
methoxyphenyl)ethynyl)benzene (1a) was used as a substrate
instead of arylbromide 1a′, the yield of 3a was increased to 75%
isolated yield with about 2−3% yield of homoannulation
product, although a prolonged reaction time (48 h) was needed
(entry 10). The reaction with other ligands, such as PiBu3, PiPr3,
and PCy3 were almost completely inactive (entries 11−13).
Although other ligands, such as P(Tol)3 and dppf [bis-
(diphenylphosphino)ferrocene] were active, the yields of 3a
were lower than 40% (entries 14 and 15). Moreover, the use of
c,e
6
1g R1 = OMe, R2 = F
1h R1 = n-C4H9, R2 = OMe
1a R1 = OMe, R2 = H
1c R1 = n-C4H9, R2 = H
1a R1 = OMe, R2 = H
1c R1 = n-C4H9, R2 = H
1i R1 = F, R2 = H
3g
3h
3i
68
89
75
86
73
74
76
71
7
f
f
8
9
3j
f
f
f
10
11
12
13
2c R3 = OMe
2c R3 = OMe
2c R3 = OMe
3k
3l
e
e
3m
3n
f
1a R1 = OMe, R2 = H
2d R3 = CN
a
Reaction conditions: 1 (0.4 mmol), 2a (2 mmol), Pd(OAc)2 (2 mol
%), PtBu3 (6 mol %), DBU (1.2 mmol), CsOPiv (0.44 mmol), 1,4-
b
dioxane (4 mL), Ar atmosphere, 140 °C, 48 h. Isolated yield.
c
d
Reaction time was 3 days. 1-(4-((2-Bromophenyl)ethynyl)phenyl)-
e
ethanone (1e) was used as arylhalide. Pd(OAc)2 (4 mol %) and PtBu3
f
(12 mol %) were used. Diarylacetylenes 2b,c (2 equiv) was used.
amount of homoannulation products (∼2%) and side-products
derived from the decomposition of both two alkynes were
observed, but they were separated by silica gel chromatography.
A variety of functional groups, such as N,N-dimethyl, n-butyl,
ester, ketone, and cyano groups at R1, were tolerated, affording
the crossover-annulated products 3b−f in good to high yields
(entries 1−5). Relative to the good reactivity of the arylchlorides
1a−c with an electron-donating group at R1, the arylhalides 1d−f
bearing an electron-withdrawing group at R1 required a longer
reaction time or higher catalytic loading. As a representative
determination of the dibenzo[a,e]pentalene products, the
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structure of 3e was unambiguously confirmed by X-ray crystal-
structure analysis (Figure 1).9
Table 3. Pd-Catalyzed Crossover Annulation of
Heteroaromatic Alkynes with 1,2-Diphenylacetylene
a
Figure 1. ORTEP drawing of the crossover-annulation product 3e.
Hydrogen atoms are omitted for clarify. Red: oxygen atom.
The reaction was further extended to the synthesis of the
multisubstituted debenzopentalene derivatives. The reaction
using arylchloride 1g having an electron-withdrawing group at
R2, such as fluorine group, furnished the corresponding product
3g in 68% yield, whereas the arylchloride 1h bearing an electron-
donating group at R2, such as methoxy group, gave the
multisubstituted product 3h in 89% yield (entries 6 and 7).
The reactions of various arylchlorides 1a, 1c, and 1i also worked
well with the diarylacetylenes 2b−d having two symmetric
electron-donating or electron-withdrawing groups at R3,
affording the expected products 3i−n in good to high yields
(entries 8−13), while 1i or 2d with a fluorine group at R2 or two
cyano groups at R3 required a higher catalytic loading (entries 12
and 13). It was noted that the reaction of 1a with the
unsymmetric diarylacetylenes, such as 1-fluoro-4-((4-
methoxyphenyl)ethynyl)benzene, gave the desired product as a
mixture of two regioisomers (see Supporting Information). The
alkyl-substituted arylchloride, such as 1-chloro-2-(dec-1-yn-1-
yl)benzene, was not compatible for the present reaction, giving a
mixture of the unidentified products.
a
Reaction conditions: 1 (0.4 mmol), 2a (2.0 mmol), Pd(OAc)2, PtBu3,
DBU (1.2 mmol), CsOPiv (0.44 mmol), 1,4-dioxane (4 mL), Ar
b
atmosphere, 120 °C, 48 h. Pd(OAc)2 (4 mol %) and PtBu3 (12 mol
c
%). Pd(OAc)2 (5 mol %) and PtBu3 (10 mol %) at 120 °C.
d
Pd(OAc)2 (10 mol %) and PtBu3 (20 mol %) at 140 °C.
transition state.8 Moreover, the use of DBU as a strong and non-
nucleophilic base is crucial for removing the pivalic acid to form
the six-membered palladacycle D. Reductive elimination of D
produces the corresponding products 3.
Scheme 2. Proposed Reaction Mechanism
The increasing interests of heteroacenes and heterocycle-fused
polyaromatic compounds in electronic device applications as
organic semiconducting materials,10 led us to examine the
construction of heterocycle-fused pentalene derivatives (Table
3). The reaction of 3-bromo-2-alkynylthiophene 1o with
diphenylacetylene 2a afforded the corresponding product 3o in
moderate yield because of the low stability of the substrate 1o
which has no substituent at 2′-position of thiophene moiety. In
contrast, 3-bromo-2-alkynylbenzothiophene 1p showed a very
high stability under the present conditions, which furnished the
corresponding product 3p in 94% yield. Although the 3-chloro-2-
alkynylpyridine 1q showed a low stability, we were able to obtain
the crossover-annulated product 3q in 39% yield.
On the basis of these observations, the present crossover-
annulation mechanism is proposed as shown in Scheme 1.
Initially, the oxidative addition of the strong electron rich Pd(0)/
Ln to arylhalide 1 forms arylpalladium intermediate A. The
intermolecular alkyne carbopalladation of the intermediate A,11
followed by the intramolecular 5-exo-dig carbopalladation of the
vinylpalladium intermediate B affords the second vinylpalladium
intermediate C. The electron-donating groups at R1, R2, and R3
in the substrates 1 and 2 not only facilitate the smooth
carbopalladation paths but also stabilize the two vinypalladium
species in intermediates B and C, which resulted in high chemical
yields of the desired products. The result that only CsOPiv
combined with DBU showed the significantly increased yields of
the products 3 (Table 1, entry 9), strongly suggests that the
reaction involves the pivalate-promoted C(sp2)-H activation
The UV−vis absorption spectra of the dibenzopentalene
products were measured in chloroform (Supporting Informa-
tion, Figure S1). Dibenzopentalene derivatives with various
functional groups exhibit similar absorption pattern in the 250−
380 nm and 400−600 nm regions, where the compounds having
an electron-donating group on an aromatic ring showed a red-
shifted absorption maximum (λmax) compared to that of
dibenzopentalenes having an electron-withdrawing group on
an aromatic ring. Solution/thin-film absorption spectra of the
selected dibenzopentalenes with a longer wavelength are shown
in Figure 2. In CHCl3 (Figure 2a), 3b with a N,N-dimethylamino
group at R1 showed a most red-shifted λmax of 472 nm. 3n having
a methoxy and two cyano groups, and pentalene derivatives fused
with thienyl (3o) or benzothienyl (3p) moiety also exhibited an
enhanced λmax compared to 3a having a methoxy group at R1.
Thin film optical absorptions of the present compounds were
recorded on spin-coated films on the quartz plates (Figure 2b).
The film absorption maxima are red-shifted by ∼15−30 nm
(from 472 to 502 nm for 3b) versus the corresponding solution
absorption maxima, indicating the existence of the intermolec-
ular π−π stacking in the solid state. The highest occupied
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MEXT, Japan. This work is dedicated to Professor Irina Petrovna
Beletskaya for her contribution to metal-catalyzed reactions.
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
tjin@m.tohoku.ac.jp
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a Scientific Research (B) from Japan
Society for Promotion of Science (JSPS) (No. 25288043), and
World Premier International Research Center Initiative (WPI),
10225
dx.doi.org/10.1021/ja403382d | J. Am. Chem. Soc. 2013, 135, 10222−10225