Transition-Metal-Controlled Synthesis of 11 H-Benzo[ a]carbazoles and 6-Alkylidene-6 H-isoindo[2,1- a]indoles via Sequential Intermolecular/Intramolecular Cross-Dehydrogenative Coupling from 2-Phenylindoles

Article abstract of DOI:10.1021/acs.orglett.9b02476

A catalyst-controlled synthesis of 11H-benzo[a]carbazoles and 6-alkylidene-6H-isoindo[2,1-a]indoles is described. Pd(OAc)₂ favored 6-alkylidene-6H-isoindo[2,1-a]indoles via intramolecular C-H/N-H CDC reaction, while [CpRhCl₂]₂

Full text of DOI:10.1021/acs.orglett.9b02476

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Transition-Metal-Controlled Synthesis of 11H‑Benzo[a]carbazoles
and 6‑Alkylidene‑6H‑isoindo[2,1‑a]indoles via Sequential
Intermolecular/Intramolecular Cross-Dehydrogenative Coupling
from 2‑Phenylindoles
Anyi Liu,† Qingshuai Han,† Xiaofeng Zhang,† Buhong Li,‡ and Qiufeng Huang*,†
†Fujian Key Laboratory of Polymer Materials, College of Chemistry & Materials Science, Fujian Normal University, Fuzhou, Fujian
350007, P.R. China
‡MOE Key Laboratory of Optoelectronic Science and Technology for Medicine, Fujian Key Laboratory for Photonics Technology,
Fujian Normal University, Fuzhou, Fujian 350007, P.R. China
S
* Supporting Information
ABSTRACT: A catalyst-controlled synthesis of 11H-benzo[a]-
carbazoles and 6-alkylidene-6H-isoindo[2,1-a]indoles is described.
Pd(OAc)2 favored 6-alkylidene-6H-isoindo[2,1-a]indoles via intra-
molecular C−H/N−H CDC reaction, while [Cp*RhCl2]2 led to 11H-
benzo[a]carbazoles through intramolecular C−H/C−H CDC reac-
tion. Moreover, the synthesis of 11H-benzo[a]carbazoles via
sequential intermolecular ortho C−H/olefin coupling and intra-
molecular C3-H/olefin coupling from 2-phenylindoles and alkenes
can be operated in one pot.
ross-coupling reaction via C−H activation has proved to
be a powerful method for the construction of carbon−
containing indole units are highly appealing to many organic
chemists because they have been found in a broad spectrum of
pharmacologically and biologically active compounds.6 Syn-
thesis of fused polycyclic indoles usually requires multistep
approaches.7 Thus, the development of divergent and more
economical methods for the production of these compounds is
highly promising.8 Recently, we have reported a rhodium-
catalyzed regioselective ortho C−H olefination of 2-phenyl-
indoles.9 For a further study, we disclose our new research in
combination with the intermolecular CDC reaction (C−H/
C−H CDC reaction) and intramolecular CDC reaction (C−
H/C−H or C−H/N−H CDC reaction) from the same
starting materials, 2-phenylindoles, and alkenes. The divergent
products, 11H-benzo[a]carbazoles10 and 6-alkylidene-6H-
isoindo[2,1-a]indoles,11 were obtained in good to excellent
yields with the control of transition-metal catalysts. Notably,
the sequential intermolecular ortho C−H olefination and
intramolecular C3−H olefination can be operated in one pot
(Scheme 2).
C
carbon bonds and carbon−heteroatom bonds.1 In particular,
transition-metal-catalyzed cross-dehydrogenative-coupling
(CDC) is highly attractive because both two coupling partners
do not require prefunctionalization.2 Though intermolecular3
or intramolecular4 CDC reactions have undergone tremendous
development in recent years, methods for developing
intermolecular/intramolecular sequential CDC reactions to
build up polycyclic structures are rare (Scheme 1).5 On the
other hand, the fused-polycyclic molecular frameworks
Scheme 1. Transition-Metal-Catalyzed CDC Reactions
The intramolecular CDC reaction of (E)-ethyl 3-(2-(1H-
indol-2-yl)-3-methylphenyl)acrylate (3a) was initiated as a
model case (Table 1). The first attempt used palladium
acetate, which is the most utilized catalyst in C3−H cross-
Received: July 16, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02476
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
acid (HOAc) was used as a cosolvent (Table 1, entry 2).
Inspired by recent reports using pyridine ligands to enhance
reactivity in Pd-catalyzed Fujiwara−Moritani reactions,14 we
examined Pd(OAc)2 in conjunction with a pyridine ligand.
Indeed, the addition of a catalytic amount of DMAP (10 mol
%) increased the yield of 4a to ca. 82% (Table 1, entry 3).
Then the quantity of HOAc was screened. It revealed that the
best ratio for the reaction was 2 equiv of HOAc. The
conversion of 3a decreased if the amount of HOAc changed
(Table 1, entries 5 and 6). On the other hand, an increase of
the amount of DMAP also led to a decreased conversion
(Table 1, entry 7). This transformation can also be performed
in an air atmosphere; the isolated yield of 4a is 80% (Table 1,
entry 8). Intriguingly, when the reaction was carried out using
[Cp*RhCl2]2 as a catalyst, a completely different selectivity
was observed that resulted in C−N bond formation exclusively
leading to 11H-benzo[a]carbazole 5a (Table 1, entry 9). The
structure of 5a was also confirmed by X-ray crystallography
(see the Supporting Information). Further optimization
showed that the use of 16 equiv of HOAc and 10 mol % of
DMAP improved the yield of 5a (Table 1, entries 10 and 11).
It is worth noting that the sequential intermolecular ortho C−
H/olefin coupling and intramolecular C3−H/olefin coupling
leading to 5a can be operated in one pot from 2-phenylindole
and ethyl acrylate as starting materials. The isolated yield of 5a
is 65% (Scheme 3).
Scheme 2. Synthesis of 11H-Benzo[a]carbazoles and 6-
Alkylidene-6H-isoindo[2,1-a]indoles via Sequential
Intermolecular/Intramolecular Cross-Dehydrogenative
Coupling from 2-Phenylindoles
a
Table 1. Optimization of the Reaction Conditions
b
additive
yield (%)
Scheme 3. Synthesis of 11H-Benzo[a]carbazoles via
Double-Successive Oxidative Heck in One Pot
HOAc
(equiv)
DMAP
(mol %)
entry
1
catalyst
4a
5a
Pd(OAc)2
(10 mol %)
Pd(OAc)2
(10 mol %)
Pd(OAc)2
(10 mol %)
Pd(OAc)2
(10 mol %)
Pd(OAc)2
(10 mol %)
Pd(OAc)2
(10 mol %)
Pd(OAc)2
(10 mol %)
Pd(OAc)2
(10 mol %)
0
0
30
45
82
32
49
52
65
2
3
4
5
6
7
8
2
2
0
10
10
10
10
20
10
10
10
20
0
1
4
With the optimized conditions in hand, the generality of the
N−H/C−H CDC reaction was first explored (Table 2).
Regarding the olefin-coupling partner, activated alkenes such
as ethyl acrylate (4a), 2-methoxyethyl acrylate (4b), tert-butyl
acrylate (4c), and n-butyl acrylate (4d) could be employed to
produce the desired products in good yields. Diethyl
vinylphosphonate furnished the product 4e in a synthetically
useful yield (45%). Furthermore, the nonactivated alkene
styrene was not suitable for this transformation (4f). The
scope of the 2-phenylindoles was also examined. A variety of 2-
phenylindole derivatives with substituents at the indole ring or
benzene ring were successfully transformed to desired products
in excellent yields. The CDC reaction of 2-phenyl-1H-indoles
(1) with alkenes (2) can not stop after mono-olefination, and
overalkenylation product was formed. Undoubtedly, diolefina-
tion products from 2-phenylindoles can also be transformed to
annulated products (4m−4r). However, for the sake of
simplicity, one of the two ortho-C−H bonds was blocked
with methyl or chlorine. The experiment results showed that
the electronic and steric properties had no effect on the
reaction efficiency. As seen from Table 2, functional groups
such as chloro (4h, 4l), bromo (4i), fluoro (4r), and ether (4o,
4p) were well tolerated under the reaction conditions.
2
2
81 (80)
c
9
[Cp*RhCl2]2
(2.5 mol %)
[Cp*RhCl2]2
(2.5 mol %)
[Cp*RhCl2]2
(2.5 mol %)
2
33
10
11
16
16
70 (68)
55
a
Reaction conditions: 3a (0.3 mmol), catalyst, AgOAc (2 equiv),
additives, xylene (2 mL), argon atmosphere, 140 °C, 24 h. H NMR
yield on the basis of the amount of 3a used, Number in parentheses is
b
c
isolated yield. The reaction was carried out under air atmosphere.
dehydrogenative Heck reaction for indoles.12 To our surprise,
it did not afford 11H-benzo[a]carbazole (5a) via the C3−H/
C−H (olefin partner) CDC reaction but gave 6-alkylidene-6H-
isoindo[2,1-a]indole (4a) via N1−H/C−H (olefin partner)
CDC reaction exclusively in 30% yield (Table 1, entry 1),
whose structure was confirmed by X-ray crystal diffraction (see
the Supporting Information). HOAc can be employed to
enhance the electrophilicity ability of the transition-metal
ion.13 We were able to increase the yield to 45% when acetic
B
DOI: 10.1021/acs.orglett.9b02476
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 2. Substrate Scope for the Synthesis of 6-Alkylidene-
6H-isoindo[2,1-a]indole s
Table 3. Substrate Scope for the Synthesis of 11H-
Benzo[a]carbazoles
a
a
a
Reaction conditions: 1 (0.3 mmol), 2 (0.3 mmol), [Cp*RhCl2]2 (2.5
mol %), AgOAc (2 equiv), DMAP (10 mol %), HOAc (16 equiv),
xylene (2 mL), argon atmosphere, 140 °C, 24 h. Isolated yield.
a
Reaction conditions: 3 (0.3 mmol), Pd(OAc)2 (10 mol %), AgOAc
(2 equiv), DMAP (10 mol %), HOAc (2 equiv), xylene (2 mL), air
atmosphere, 140 °C, 24 h. Isolated yield.
Scheme 4. Synthesis of Fused-Pentacyclic Ring via
Quadruple CDC Reaction from 2-Phenylindole
Next, we examined the one-pot, 2-fold oxidative Heck
reaction of 2-phenyl-1H-indole derivatives with alkenes (Table
3). When 2-phenylindole was employed as the substrate,
competitive intermolecular diolefination of two ortho-C−H
bonds and subsequent intramolecular C3−H/C−H (olefin)
coupling would give rise to a product which is difficult to
separate from the complicate mixture. Only 38% yield of 5l
was obtained after careful isolation. Therefore, we turned our
attention to investigating the reaction of 2-(o-tolyl)-1H-indole
with alkenes. Various acrylates performed smoothly to give
corresponding desired products 5a−5d in good yields (55−
65%). In contrast to palladium-catalyzed intramolecular C−H/
N−H CDC, rhodium-catalyzed intramolecular C−H/C−H
CDC reaction could occur with styrenes delivering 6-phenyl-
11H-benzo[a]carbazoles (5e, 5f), although the yields need to
be further improved. We suspect that a large conjugated
structure of corresponding product is beneficial for this
transformation. A variety of 2-(o-tolyl)-1H-indoles reacted
with ethyl acrylate to provide functionalized carbazoles in 50−
55% yield (5g−5j). Blockage one of the ortho positions with
chlorine atom was also acceptable, as demonstrated by the
formation of ethyl 1-chloro-11H-benzo[a]carbazole-6-carbox-
ylate (5k). Additionally, a fused-pentacyclic ring 6a could be
efficiently synthesized in three steps with an overall yield of
54% from 2-phenyl-1H-indole via quadruple CDC reactions
(Scheme 4). In addition, when compound 4a was subjected to
the Rh-catalyzed reaction conditions [[Cp*RhCl2]2 (2.5 mol
%), AgOAc (2 equiv), DMAP (10 mol %), HOAc (16 equiv),
xylene (2 mL)] no reaction occurred, and compound 4a were
recovered. This result likely indicates that the intramolecular
C−H/N−H cross-dehydrogenative-coupling reaction is irre-
versible.
In conclusion, a catalyst-controlled switch of regioselectivity
in the intramolecular CDC reaction of ortho-alkenylated 2-
phenylindole was developed. Pd(OAc)2 catalyst favored 6-
alkylidene-6H-isoindo[2,1-a]indoles via intramolecular C−H/
N−H CDC reaction, while [Cp*RhCl2]2 led to 11H-
benzo[a]carbazoles through an intramolecular C−H/C−H
CDC reaction. Moreover, a variety of 11H-benzo[a]carbazoles
can be assembled in two C−H/C−H CDC steps from 2-
phenylindoles and alkenes in one pot. Further investigation on
the application of the developed methods is currently
underway in our laboratory.
C
DOI: 10.1021/acs.orglett.9b02476
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
W.; Yuan, D.; Wang, G.; Zhao, Y.; Xie, J.; Li, S.; Zhu, C. J. Am. Chem.
Soc. 2019, 141, 3187.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02476.
■
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Experimental procedures and spectral data (PDF)
Accession Codes
CCDC 1938416 and 1938457 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: qiufenghuang@fjnu.edu.cn.
ORCID
Qiufeng Huang: 0000-0002-0166-2140
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We Thank Dr. Zizhu Yao and Prof. Shengchang Xiang (Fujian
Normal University) for obtaining X-ray crystal diffraction
data.Financial support from NSFC (Grant Nos. 21872028 and
61520106015), Natural Science Foundation of Fujian Province
(Grant No. 2017J01572), the Foundation of Fujian Educa-
tional Committee (Grant No.JZ160424), and the Fujian
Province University Fund for New Century Excellent Talents
for Financial Support.
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