Synthesis of Thieno[3,2-b]indoles via Halogen Dance and Ligand-Controlled One-Pot Sequential Coupling Reaction

Article abstract of DOI:10.1021/acs.orglett.7b03857

A two-pot synthesis of thieno[3,2-b]indole from 2,5-dibromothiophene is described. A halogen dance of 2,5-dibromothiophene was performed with LDA, and subsequent Negishi coupling was performed with 2-iodoaniline derivatives to provide the corresponding coupling products. The resulting two bromo groups have different reactivities, which were utilized for the one-pot Suzuki-Miyaura coupling/intramolecular Buchwald-Hartwig amination to produce thieno[3,2-b]indole via an assisted tandem catalysis that involved in situ ligand exchange.

Full text of DOI:10.1021/acs.orglett.7b03857

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of Thieno[3,2‑b]indoles via Halogen Dance and Ligand-
Controlled One-Pot Sequential Coupling Reaction
Yuki Hayashi, Kentaro Okano,* and Atsunori Mori
Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
S
* Supporting Information
ABSTRACT: A two-pot synthesis of thieno[3,2-b]indole from
2,5-dibromothiophene is described. A halogen dance of 2,5-
dibromothiophene was performed with LDA, and subsequent
Negishi coupling was performed with 2-iodoaniline derivatives to
provide the corresponding coupling products. The resulting two
bromo groups have different reactivities, which were utilized for
the one-pot Suzuki−Miyaura coupling/intramolecular Buchwald−
Hartwig amination to produce thieno[3,2-b]indole via an assisted
tandem catalysis that involved in situ ligand exchange.
hiophene-fused indoles are an important class of π-
the formation of two chemical bonds in one pot, which reduces
the number of reaction pots. Regardless of this synthetic
potential, the substrate scope has not been fully investigated,
probably owing to the difficulty of controlling the reactivity of
transient thienyl anion species. In this study, we further
explored the synthetic utility of one-pot halogen dance/Negishi
coupling and achieved divergent two-pot synthesis of thieno-
[3,2-b]indoles 1 from 2,5-dibromothiophene (2) via a transient
thienyl lithium species 3 via the development of a one-pot
ligand-controlled Suzuki−Miyaura coupling/Buchwald−Hart-
wig amination that utilizes assisted tandem catalysis (Scheme
1).
T
extended functional molecules and medicinal com-
pounds.1 Among them, thieno[3,2-b]indole is often found in
numerous reports; substantial efforts have been focused on the
development of synthetic methods for these compounds
(Figure 1).1l,n,2 The reported syntheses of thieno[3,2-b]indole
We first examined the effects of substituents on the nitrogen
atom of 2-iodoaniline derivatives for Negishi coupling9 using
the 3,5-dibromothienyl lithium species, which was generated
after the halogen dance of 2,5-dibromothiophene (Table 1). In
a previous report,8 halogen dance was performed with LDA10 at
Scheme 1. Two-Pot Synthesis of Thieno[3,2-b]indoles
Figure 1. Substituted thienoindoles and related compounds.
are categorized by C−N bond formation as follows: (1)
insertion of nitrene via the thermolysis of the azide3 or
reduction of the nitro group,4 (2) amination of thienyl
bromide,5 or (3) copper-catalyzed C−H amination.6 From
the perspective of operational safety, functional group
compatibility, and further functionalization, we focused on
the application of our recent work,7 which included a one-pot
halogen dance/Negishi coupling, which allows regiocontrolled
synthesis of multiple arylated thiophenes or furans. Compared
with conventional stepwise transformations, the LDA (lithium
diisopropylamide)-mediated halogen dance reaction8 leads to
Received: December 10, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03857
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Substituent Effects in Negishi Coupling
Table 2. One-Pot Suzuki−Miyaura Coupling/Intramolecular
a
Buchwald−Hartwig Amination
b
entry
R
product
yield (%)
b
entry
additive
1a (%)
1
2
3
4
5
6
Cbz
Boc
H
4a
4b
4c
4d
4e
4f
16
23
53
69
52
58
1
2
3
4
5
none
0
0
NaOt-Bu (3 equiv)
t-Bu3P·HBF4 (20 mol %), NaOt-Bu
t-Bu3P·HBF4 (10 mol %), NaOt-Bu
t-Bu3P·HBF4 (5 mol %), NaOt-Bu
69
40
19
Ph
n-Bu
Bn
a
a
Reaction conditions: dibromothiophene 4d (0.20 mmol), ArB(OH)2
Reaction conditions: 2,5-dibromothiophene (2) (0.70 mmol, 1.4
(0.24 mmol, 1.2 equiv), PdCl2(dppf)·CH2Cl2 (0.010 mmol, 5 mol %),
K3PO4 (0.40 mmol, 2.0 equiv), 1,4-dioxane, 80 °C, 6 h; t-Bu3P·HBF4
(0.040 mmol, 20 mol %), NaOt-Bu (0.60 mmol, 3.0 equiv), 125 °C, 24
equiv), LDA (0.65 mmol, 1.3 equiv), THF, −78 °C, 5 min; ZnCl2·
TMEDA (0.70 mmol, 1.4 equiv), 0 °C, 15 min; Pd(PPh3)4 (0.025
mmol, 5 mol %), 2-iodoaniline derivative (0.50 mmol, 1.0 equiv), 60
b
b
h (sealed tube). Isolated yield.
°C, 24 h. Isolated yield.
−78 °C. The resulting reaction mixture was treated with ZnCl2·
TMEDA11 at 0 °C, and the thienylzinc species that was
generated in situ was subjected to the coupling conditions. In
preliminary experiments, Cbz- or Boc-protected 2-iodoaniline
derivatives were converted to the corresponding products 4a
and 4b in low yields (entries 1 and 2). In both cases, the
iodoaniline derivatives were recovered, which indicated that the
thienylzinc species would be protonated by the relatively acidic
carbamate proton. The same reaction with unprotected 2-
iodoaniline led to the formation of the desired product 4c in
better yield, which also supported the above postulate (entry
3). In contrast to the electron-withdrawing Cbz and Boc
groups, phenyl, butyl, and benzyl groups on the nitrogen
resulted in satisfactory yields of products 4d−f (entries 4−6).
With the dibromothiophenes bearing the 2-aminophenyl
moieties in hand, we focused on the one-pot synthesis of π-
conjugated thienoindoles by Suzuki−Miyaura coupling12 and
intramolecular Buchwald−Hartwig amination.13 In the prelimi-
nary experiments, the amination of dibromothiophene 4d led to
the formation of a complex mixture. We then attempted
arylation at the reactive α-position of thiophene and subsequent
amination of the residual β-bromo group (Scheme 2).
product 1a in 69% yield (entry 3). A reduced amount of the
phosphorus ligand led to lower yields of thienoindole 1a with
recoveries of arylated intermediate 5 (entries 4 and 5). These
results indicate that monodentate t-Bu3P should coordinate to
the palladium with bidentate DPPF to generate the active
catalyst in situ for the intramolecular amination. The one-pot
sequential reaction could be also performed at 125 °C for 5 h
via a one-shot addition that includes all of the reagents to
produce the same thieno[3,2-b]indole 1a in 57% yield (Scheme
3). The established protocol is operationally simple and
provides direct access to compounds 5 and 1a in satisfactory
yields.
Scheme 3. Ligand-Controlled One-Pot Suzuki−Miyaura
Coupling/Intramolecular Buchwald−Hartwig Amination
Scheme 2. Initial Attempt for One-Pot Suzuki−Miyaura
Coupling/Intramolecular Buchwald−Hartwig Amination
Our proposed one-pot assisted tandem catalytic trans-
formation is illustrated in Scheme 4. The first cross coupling
(Suzuki−Miyaura coupling) is catalyzed by the combination of
Pd(0) and DPPF (L1), which exclusively provides the
corresponding arylated thiophene 5. The second reaction
(intramolecular Buchwald−Hartwig amination) is promoted by
Pd(0)−t-Bu3P (L2), which is generated in situ via ligand
exchange. We also performed the one-pot arylation/intra-
molecular Buchwald−Hartwig amination using compound 4d
with Pd(t-Bu3P)2 to provide thienoindole 1a in 16% yield. We
observed neither starting dibromothiophene 4d nor arylated
thiophene 5. These results indicate that both L1 and L2 are
required for achieving the one-pot reaction and support the
assisted tandem catalytic pathway.14
According to our previous report,7a we performed Suzuki−
Miyaura coupling in the presence of 5 mol % of PdCl2(dppf) at
80 °C and obtained compound 5 in 69% yield. Further heating
at 125 °C for 24 h did not produce thienoindole 1a, even with
an increased amount of K2CO3 (4 equiv).
We then examined additives for achieving the one-pot
Suzuki−Miyaura coupling/intramolecular Buchwald−Hartwig
amination (Table 2). NaOt-Bu proved ineffective in this case,
even with a prolonged reaction time (24 h) at 125 °C (entries 1
13e
and 2). Additional monodentate t-Bu3P·HBF4 (20 mol %)
significantly promoted the amination to produce the desired
B
DOI: 10.1021/acs.orglett.7b03857
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Plausible Assisted Tandem Catalytic Pathway for
the One-Pot Suzuki−Miyaura Coupling/Intramolecular
Buchwald−Hartwig Amination
Table 3. Scope of the One-Pot Suzuki−Miyaura Coupling/
a
Intramolecular Buchwald−Hartwig Amination
a
Reaction conditions A: dibromothiophene 4d (0.20 mmol), ArB-
(OH)2 (0.24 mmol, 1.2 equiv), PdCl2(dppf)·CH2Cl2 (0.010 mmol, 5
mol %), t-Bu3P·HBF4 (0.040 mmol, 20 mol %), NaOt-Bu (0.60 mmol,
3.0 equiv), K3PO4 (0.40 mmol, 2.0 equiv), 1,4-dioxane, 125 °C, 5 h
Scheme 3 demonstrates that the Pd−DPPF complex was
converted to another catalyst via the addition of the second
phosphorus ligand t-Bu3P (Scheme 5). We cannot exclude the
b
c
(sealed tube). Isolated yield. Reaction conditions B: dibromothio-
phene 4d (0.20 mmol), ArB(OH)2 (0.24 mmol, 1.2 equiv), Pd(PPh3)4
(0.020 mmol, 10 mol %), t-Bu3P·HBF4 (0.080 mmol, 40 mol %),
NaOt-Bu (0.60 mmol, 3.0 equiv), K3PO4 (1.40 mmol, 7.0 equiv), 1,4-
Scheme 5. Mechanistic Implications for the Active Catalyst
in the Intramolecular Buchwald−Hartwig Amination
d
dioxane/H2O (4:1), 125 °C, 22 h (sealed tube). Not detected.
group was reduced under heating conditions,15 and the nitrene
intermediate reacted with the proximal thiophene to produce
616 in a moderate yield.
Scheme 6. Synthesis of Unsymmetrical Indolothienoindole
possibility that bidentate DPPF was exchanged by two
molecules of t-Bu3P. The results in Table 2 indicate that t-
Bu3P is crucial for the intramolecular amination. Either A or B,
or both, could catalyze the amination to provide thieno[3,2-
b]indole 1a.
Subsequently, we investigated the scope and limitations of
the one-pot Suzuki−Miyaura coupling/Buchwald−Hartwig
amination (Table 3). Phenyl and 4-tert-butylphenyl groups
were introduced in comparable yields with the p-tolyl group, as
shown in Scheme 3. The electron-donating 4-(dimethylamino)-
phenyl group was introduced to provide the corresponding
thienoindole 1d in 14% yield; however, 4-acetylphenylboronic
acid was not reactive, and the desired product 1e was obtained
in 29% yield with concomitant generation of unidentified
byproducts. A similar tendency was observed for meta-
substituted phenylboronic acids. The reaction conditions were
not applied to 4-nitrophenylboronic acid due to its low
solubility in 1,4-dioxane; alternative reaction conditions B
including water as a cosolvent, which proved effective for
producing the corresponding product 1h. The reaction
conditions B were also effective for improving the yield of
products 1b, 1e, 1g, and 1i.
In conclusion, we described the two-pot synthesis of
thieno[3,2-b]indole from 2,5-dibromothiophene. Halogen
dance and subsequent Negishi coupling with 2-iodoaniline
derivatives provided the coupling product, which underwent
ligand-controlled one-pot Suzuki−Miyaura coupling/intramo-
lecular Buchwald−Hartwig amination to produce thieno[3,2-
b]indoles.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03857.
Experimental procedure, compound characterization
data, and 1H and 13C NMR spectra for all new
compounds (PDF)
Thienoindole 1h bearing a 2-nitrophenyl group was
confirmed to be a potential synthetic intermediate for an
unsymmetrical indolothienoindole (Scheme 6). The nitro
C
DOI: 10.1021/acs.orglett.7b03857
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: okano@harbor.kobe-u.ac.jp.
ORCID
Kentaro Okano: 0000-0003-2029-8505
Notes
(5) Huang, Y.; Wu, D.; Huang, J.; Guo, Q.; Li, J.; You, J. Angew.
Chem., Int. Ed. 2014, 53, 12158.
(6) Takamatsu, K.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett. 2014,
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The authors declare no competing financial interest.
(7) (a) Okano, K.; Sunahara, K.; Yamane, Y.; Hayashi, Y.; Mori, A.
Chem. - Eur. J. 2016, 22, 16450. (b) Miyagawa, N.; Murase, Y.; Okano,
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ACKNOWLEDGMENTS
■
This work was financially supported by JSPS KAKENHI, Grant
Nos. JP16K05774 in Scientific Research (C), JP16H01153 in
the Middle Molecular Strategy, Creation of Innovation Centers
for Advanced Interdisciplinary Research Areas (Innovative
Bioproduction Kobe), and Kawanishi Memorial ShinMaywa
Education Foundation. This work was performed under the
Cooperative Research Program of “Network Joint Research
Center for Materials and Devices”.
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