Palladium-catalyzed domino C-H/N-H functionalization: An efficient approach to nitrogen-bridged heteroacenes

Article abstract of DOI:10.1002/chem.201500897

Palladium-catalyzed domino C-H/N-H functionalization for the synthesis of novel nitrogen-bridged thienoacenes and 10H-benzo[4,5]thieno[3,2-b]indole derivatives from dihaloarene is reported. This domino sequence consists of initial C-H functionalization of the benzo[b]thiophene moiety, followed by Buchwald-Hartwig coupling. This transformation is also useful for the synthesis of highly π-extended compounds. Extended heteroacenes: A twofold arylation protocol for the efficient synthesis of 9H-benzo[4,5]thieno[3,2-b]thieno[3,4-d]pyrroles and 10H-benzo[4,5]thieno[3,2-b]indoles was developed (see scheme). The selectivity of this novel sequence allows the construction of highly π-extended heteroacenes.

Full text of DOI:10.1002/chem.201500897

DOI: 10.1002/chem.201500897
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&
Conjugated Heterocycles
Palladium-Catalyzed Domino CÀH/NÀH Functionalization:
An Efficient Approach to Nitrogen-Bridged Heteroacenes
Natsuyo Kamimoto,[a, b] Dieter Schollmeyer,[a] Koichi Mitsudo,[b] Seiji Suga,[b] and
Siegfried R. Waldvogel*[a]
Abstract: Palladium-catalyzed domino CÀH/NÀH functionali-
zation for the synthesis of novel nitrogen-bridged thieno-
acenes and 10H-benzo[4,5]thieno[3,2-b]indole derivatives
from dihaloarene is reported. This domino sequence consists
of initial CÀH functionalization of the benzo[b]thiophene
moiety, followed by Buchwald–Hartwig coupling. This trans-
formation is also useful for the synthesis of highly p-extend-
ed compounds.
Introduction
Over the last few decades, heteroacene derivatives have re-
ceived much attention because of their potential application in
devices such as organic field-effect transistors (OFEDs) and or-
ganic light emitting diodes (OLEDs).[1] Most recently, nitrogen-
bridged thienoacene compounds[2,3] have shown particular
promise in materials for OFEDs, OLEDs, and they have also dis-
played biological activity as antitumor agents.[4] Palladium-cata-
lyzed aryl CÀH functionalization[5,6] has been studied by many
researchers and this approach has turned out to be a powerful
methodology with which to construct several organic com-
pounds. Among them, Teulade–Fichou and co-workers have
developed NÀH/CÀH functionalization[7] of an aniline derivative
with o-dibromobenzene to form the carbazole moiety. Howev-
er, the yield was very low.[7d] Subsequently, Ackermann and co-
workers developed an elegant domino NÀH/CÀH functionaliza-
tion by using dichloroarenes as substrates (Scheme 1, a),[7b,c]
which consists of an initial Buchwald–Hartwig amination[8] fol-
lowed by CÀH functionalization of aniline derivatives. In con-
trast, a domino type CÀH/NÀH functionalization of heteroarene
has not been developed; this method can be an efficient ap-
proach with which to synthesize nitrogen-bridged heteroa-
Scheme 1. Palladium-catalyzed domino reaction for the synthesis of annulat-
ed heterocycles.
cenes. Herein, we report the first domino CÀH/NÀH functionali-
zation for the synthesis of nitrogen-bridged thienoacenes
(Scheme 1b). Our reaction differs significantly from the Acker-
mann-type conversion in regard to the domino sequence.
Therein, the NÀH functionalization proceeded faster than the
modification at CÀH. The CÀH functionalization at the benzene
moiety is accelerated in an intramolecular fashion by formation
of a NÀC bond. In our reaction, by contrast, the benzo[b]thio-
phene ring, which has much higher nucleophilicity than a ben-
zene, facilitates the initial CÀH functionalization step. We calcu-
lated HOMO and LUMO levels by density functional theory
(DFT) methods and selected the 9H-benzo-[4,5]thieno[3,2-
b]thieno[3,4-d]pyrrole skeleton (1C) as a target, because inter-
esting electrochemical and optical properties resulting from its
narrow HOMO–LUMO band gap should be expected among
their structural isomers (Figure 1).[9]
[a] N. Kamimoto, Dr. D. Schollmeyer, Prof.Dr. S. R. Waldvogel
Institut fꢀr Organische Chemie
Johannes Gutenberg-Universitꢁt Mainz
Duesbergweg 10–14
55128 Mainz (Germany)
Fax: (+49)61313926777
E-mail: waldvogel@uni-mainz.de
Homepage: http://www.chemie.uni-mainz.de/OC/AK-Waldvogel/
[b] N. Kamimoto, Dr. K. Mitsudo, Prof.Dr. S. Suga
Division of Chemistry and Biotechnology
Graduate School of Natural Science and Technology
Okayama University
Results and Discussion
The reaction conditions were screened and the results are
summarized in Table 1. In the presence of a catalytic amount
of Pd(dba)2 (dba=dibenzylideneacetone), RuPhos, and NaOtBu
(2.4 equiv), N-phenyl-benzo[b]thien-3-yl-amine (1a; 1.5 equiv)
reacted with 3,4-dibromothiophene (2) in toluene at 1008C,
but the reaction did not proceed at all (entry 1). Under the
3-1-1 Tsuhsima-naka, Kita-ku
Okayama 700-8530 (Japan)
Homepage: http://achem.okayama-u.ac.jp/reacteng/lang/en
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500897.
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(entry 10). By using more air-stable PtBu3·HBF4 salt as the
ligand precursor, the product was only obtained in moderate
yield (entry 11). We also tested the reaction with 20 mol%
PtBu3·HBF4 (in analogy to entry 10), and by using 2 mol%
Pd(PtBu3)2 as a catalyst instead of the combination of 2 mol%
Pd(dba)2 and 20 mol% PtBu3. Under these catalysis conditions
the reaction mixtures became highly complex. When applying
4 or 5 mol% Pd(PtBu3)2, the product was obtained in moderate
yields but the efficiency did not improve on that reported in
entry 10 (entries 12 and 13).
The structure of product 3a was confirmed by X-ray analysis
of a suitable single crystal (Figure 2a).[10] The optimized reac-
Figure 1. Comparison of HOMO and LUMO levels by DFT calculation (B3LYP/
6-31G(d)).
Figure 2. X-ray analysis of a) 3a and b) 3a’.
Table 1. Screening of reaction conditions.
tion conditions were applied to the conversion of several
amines with 2 (Scheme 2). The reaction of p-hexylphenyl sub-
stituted amine 1b with 2 proceeded and the product 3b was
obtained in moderate yield. Amines with either an electron-
withdrawing (p-fluorophenyl) or an electron-donating (p-me-
thoxyphenyl) group were suitable for use in this reaction se-
quence, and the corresponding products were obtained in 60
and 72% isolated yields, respectively. Furthermore, naphthyl
amines could be used and these gave the corresponding prod-
Entry
Ligand [mol%]
T [8C]
Yield of 3a[%][a]
1
2
3
4
5
6
7
8
RuPhos (8)
JohnPhos (8)
SPhos (8)
DavePhos (8)
PCy3 (8)
PtBu3 (8)
rac-BINAP (8)
dppf (8)
100
100
100
100
100
100
100
100
120
120
120
120
120
0
0
0
0
5
21
4
0
21
84
55
60
73
9
PtBu3 (8)
10
11
12
13
PtBu3 (20)
PtBu3·HBF4 (8)
[b]
–
–
[c]
[a] GC yield using undecane as internal standard. [b] Pd(PtBu3)2 (4 mol%)
was used instead of Pd(dba)2. [c] Pd(PtBu3)2 (5 mol%) was used as a cata-
lyst instead of Pd(dba)2.
same conditions, JohnPhos, SPhos, DavePhos, and 1,1’-bis(di-
phenylphosphino)ferrocene (dppf) did not facilitate this reac-
tion (entries 2–4 and 8). When using PCy3 or racemic BINAP as
ligands, the reaction proceeded albeit in very low yield (en-
tries 5 and 7). Improvement was found when using PtBu3; with
this ligand, the yield of the product was increased to 21%
(entry 6). When the reaction temperature was elevated to
1208C, a comparable result was obtained (entry 9). When the
amount of PtBu3 was increased to 20 mol%, the yield of the re-
action increased dramatically to give the product in 84% yield
Scheme 2. Scope of the reaction of amines with 3,4-dibromothiophene (2)
with isolated yields given. [a] Reaction was conducted with 1 (2.0 equiv) on
a 1.0 mmol scale of 2.
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ucts in 32 and 59% yields, respectively. In contrast, the reac-
tion of 2 with amines 1g and 1h, bearing highly electron-defi-
cient pyridyl moieties, did not proceed at all. The electron-defi-
cient substituent at the benzothiophene decreases its nucleo-
philicity, and the efficiency of nucleophilic attack or CÀH metal-
ation at the 2-position of benzo[b]thiophene moiety might be
suppressed (see below, Figure 3). Inhibition of the catalytic ac-
tivity by coordination of pyridine to the Pd catalyst might also
be possible. We also studied N-alkyl substituted benzo[b]thien-
3-yl-amines, but no desired products were obtained.
The elaborated transformation was also effective for the
construction of the 10H-benzo[4,5]thieno[3,2-b]indole[11] skele-
ton (5) (Table 2). The reaction of phenyl substituted 1a with o-
Table 2. Scope of the reaction with amines and o-dibromobenzene
(4a).[a]
Scheme 3. Scope of o-dibromoarenes with isolated yields given.
Entry
1
R
5
Yield[%][b]
1
2
3
4
5
6
1a
1b
1c
1d
1e
1f
Ph
5aa
5ba
5ca
5da
5ea
5fa
85
71
83
86
84
85
p-hexyl-C6H4
p-F-C6H4
p-MeO-C6H4
1-naphthyl
2-naphthyl
[a] Reaction conditions: aryl-substituted amines 1 (0.75 mmol), o-dibro-
mobenzene (4a) (0.5 mmol), Pd(dba)2 (2 mol%), PtBu3 (20 mol%), NaOtBu
(1.2 mmol), toluene (2.5 mL), 1208C, 24 h. [b] Isolated yield.
Scheme 4. Study for mechanistic rationale.
dibromobenzene (4a) proceeded smoothly to give the product
5aa in 85% isolated yield (entry 1). In analogy to the reaction
presented in Scheme 2, the reaction of 1b–d with 4a provided
the corresponding products in good to high yield (entries 2–4).
1-Naphthyl- and 2-naphthyl-substituted amines 1e and 1 f
were good substrates and the products 5ea and 5 fa, respec-
tively, were obtained in good yields (entries 5 and 6).
same conditions (Scheme 4). The reaction led to a single prod-
uct 3a[10] in good yield (Figure 2b). We conclude that this
domino reaction consists of initial CÀH functionalization[12] and
subsequent NÀH functionalization; a plausible rationale is de-
picted in Scheme 4.
Reactions with other o-dibromoarene derivatives were car-
ried out to determine the scope of the method (Scheme 3).
With difluorobenzene derivative 4b as the substrate, 5ab was
obtained in 54% yield. When switching to the electron-rich di-
methoxybenzene derivative 4c, the isolated amount of prod-
uct increased to 84% yield. Fortunately, p-extended dibro-
moarenes such as 2,3-dibromonaphthalene (4d) and 5,6-dibro-
mobenzo[c][1,2,5]thiadiazole (4e) were also successfully con-
verted into the highly p-extended products 5ad and 5af in 82
and 75% yields, respectively. Therefore, this sequence is useful
not only for construction of the 9H-benzo[4,5]thieno[3,2-
b]thieno[3,4-d]pyrrole skeleton but also for the 10H-benzo[4,5]-
thieno[3,2-b]indole core.
By using this reaction selectivity, in addition, some bromoio-
doarenes were applied to the reaction with 1a (Table 3). With
4-bromo-3-iodotoluene (6a), the reaction proceeded to com-
pletion and only one product was obtained (89% yield;
entry 1). The molecular structure of 7aa was also confirmed by
X-ray analysis (Figure S5 in the Supporting Information).[13] In
a similar way, the reaction with 1-bromo-4-fluoro-2-iodoben-
zene (6b) gave a single product in 87% yield (entry 2), and 3-
bromo-2-iodothiophene (6c) could be used to provide the cor-
responding product (entry 3).
A plausible mechanism for the present domino reaction is
outlined in Figure 3. This reaction mechanism consists of CÀH
and NÀH functionalization. The first CÀH activation (the top di-
agram) involves three steps; 1) oxidative addition, 2) nucleo-
To gain insights into the reaction mechanism, the reaction of
1a was carried out with 3-bromothiophene (2’) under the
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Table 3. Application for nonsymmetric substrates using bromoiodoarene-
s.[a]
Entry
1
6
Product
Yield of 7 [%][b]
89
2
3
87
51
[a] Reaction conditions: aryl-substituted amines 1 (0.75 mmol), bromoio-
doarene (0.5 mmol), Pd(dba)2 (2 mol%), PtBu3 (20 mol%), NaOtBu
6
(1.2 mmol), toluene (2.5 mL), 1208C, 24 h. [b] Isolated yield.
philic addition to PdII species (Path A) or direct metalation
(Path B), and 3) reductive elimination. Since a higher nucleo-
philicity of benzothiophene should be responsible for the first
CÀH functionalization, Path A is considered to be more reason-
able. The second NÀH functionalization proceeded though the
widely known Buchwald–Hartwig amination mechanism de-
picted in the bottom diagram.
Figure 3. Plausible mechanism of the domino reaction.
Finally, this catalytic sequence was applied for the construc-
tion of a highly p-extended compound (Scheme 5). Three
equivalents of 1b reacted with 1,4-dibromo-2,5-diiodobenzene
(6d) to provide the desired product 8 selectively in 51% yield.
An original goal in our studies was the formation of N-arylated
products, which should serve as substrates for the oxidative
coupling reaction by MoV reagents.[14] Most pleasing, the palla-
dium-catalyzed shortcut provides fast access to such com-
pounds.
Conclusion
Scheme 5. Application for construction of highly p-extended derivative.
We have established a palladium-catalyzed domino CÀH/NÀH
functionalization method by using aryl-substituted secondary
amines with dihaloarenes. This sequence consists of an initial
CÀH functionalization that is followed by NÀH derivatization
(Buchwald–Hartwig amination). Higher nucleophilic character
of the benzo[b]thiophene ring seems to promote CÀH func-
tionalization. This transformation is useful to construct not
only 9H-benzo[4,5]thieno[3,2-b]thieno[3,4-d]pyrrole derivatives
and 10H-benzo[4,5]thieno[3,2-b]indole derivatives but also
more highly p-extended compounds, which are of particular
interest for molecular electronics.[15] The described method
closes the gap to previous methods and provides a rapid
access to these heteroacenes. The use of appropriate thio-
phene substrates has provided insights into the mechanistic
rationale.
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Experimental Section
General procedure for domino reaction of aryl substituted
amine with dihaloarenes: A suspension of 1 (0.75 mmol), dihaloar-
ene 2 or 4 (0.5 mmol), Pd(dba)2 (2 mol%), PtBu3 (20 mol%), and
NaOtBu (1.2 mmol) in distilled toluene (2.5 mL) was stirred at
1208C for 24 h under an argon atmosphere in a sealed tube. After
the reaction, the reaction mixture was cooled to RT, EtOAc (10 mL)
was added and the mixture was washed with H2O (3ꢁ20 mL) and
brine (20 mL). The organic fraction was dried with MgSO4, filtered,
and the solvents were evaporated to give the crude product. The
corresponding product was isolated by flash column chromatogra-
phy (cyclohexane/EtOAc, 1–2% Et3N).
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Acknowledgements
This work was supported greatly by the Graduate School of Ex-
cellence Materials Science in Mainz (MAINZ, GSC 266)
Keywords: amination · CÀH activation · domino reactions ·
heterocycles · palladium
[9] For details of calculations, see the Supporting Information.
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Received: March 6, 2015
Published online on && &&, 0000
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
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Conjugated Heterocycles
N. Kamimoto, D. Schollmeyer, K. Mitsudo,
S. Suga, S. R. Waldvogel*
&& – &&
Palladium-Catalyzed Domino CÀH/NÀ
H Functionalization: An Efficient
Approach to Nitrogen-Bridged
Heteroacenes
Extended heteroacenes: A twofold ary-
lation protocol for the efficient synthesis
of 9H-benzo[4,5]thieno[3,2-b]thieno[3,4-
d]pyrroles and 10H-benzo[4,5]thie-
no[3,2-b]indoles was developed (see
scheme). The selectivity of this novel se-
quence allows the construction of
highly p-extended heteroacenes.
&
&
Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!