Metal-Free Double Csp2-H Bond Functionalization: Strategy for Synthesizing Benzo[a]carbazoles from 2-Arylindoles and Acetophenones/Alkynes

Article abstract of DOI:10.1021/acs.orglett.9b01138

A metal-free strategy for the synthesis of benzocarbazoles from 2-arylindoles and aryl ketones is developed. Various 2-aryl-3-vinyl-indoles were generated in situ through dehydrative condensation of aryl ketones and indoles. These key intermediates could be selectively converted into the corresponding benzo[a]carbazoles via a direct cyclization process between two Csp²-H bonds. Furthermore, terminal alkynes also could be used as the versatile C2 source to afford the corresponding products in good yields.

Full text of DOI:10.1021/acs.orglett.9b01138

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Metal-Free Double Csp2−H Bond Functionalization: Strategy for
Synthesizing Benzo[a]carbazoles from 2‑Arylindoles and
Acetophenones/Alkynes
Penghui Ni, Jing Tan, Wenqi Zhao, Huawen Huang, Fuhong Xiao,* and Guo-Jun Deng*
Key Laboratory for Green Organic Synthesis and Application of Hunan Province, Key Laboratory of Environmentally Friendly
Chemistry and Application of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China
S
* Supporting Information
ABSTRACT: A metal-free strategy for the synthesis of benzocarbazoles from 2-arylindoles and aryl ketones is developed.
Various 2-aryl-3-vinyl-indoles were generated in situ through dehydrative condensation of aryl ketones and indoles. These key
intermediates could be selectively converted into the corresponding benzo[a]carbazoles via a direct cyclization process between
two Csp2−H bonds. Furthermore, terminal alkynes also could be used as the versatile C2 source to afford the corresponding
products in good yields.
s a class of important fused nitrogen-containing hetero-
cycles, carbazole and its derivatives are privileged
substrates for benzo[a]carbazole synthesis via a C−H
amination strategy.12 Recently, the transition metal-catalyzed
benzannulation of 2-arylindole derivatives has emerged as one
of the most powerful tools for the synthesis of benzo[a]-
carbazoles bearing unique substitution patterns. Precious
metals such as Pt or Ru were used as the catalysts to convert
2-(2-alkynylphenyl)indole derivatives into benzo[a]carbazoles
via intramolecular cyclization reaction.13 More importantly,
Tsuchimoto and Shirakawa found that readily accessible 2-
arylindoles could smoothly couple with propargyl ethers
catalyzed by indium to afford annulated benzo[a]carbazole
with ether as the leaving group (Scheme 1a).14 In 2009, Jiao
and co-workers found that less reactive internal alkynes could
be used as the coupling partners when palladium was used as
the catalyst with oxygen as the oxidant (Scheme 1b).15 Very
recently, Gu and co-workers developed bismuth trichloride-
catalyzed formation of a benzo[a]carbazole from 2-arylindoles
and 2-bromoacetaldehydes with Friedel−Crafts alkylation as
the initial step.16 Aryl ketones are easy to handle and
commercially available; therefore, these compounds are ideal
C2 sources for the [2+4] annulation process. However, there is
only one example using acetophenones and 2-(2-bromoaryl)-
indoles as the starting materials (Scheme 1c).17 When less
reactive 2-arylindoles were used, the aryl ketones need to be
initially activated to more reactive α-diazo carbonyls or
A
molecular structures and exhibit a wide range of biological
activities.1 Furthermore, they are also important building
blocks for the synthesis of functional materials such as organic
light-emitting diodes (OLEDs).2 Traditionally, carbazoles are
constructed via the well-known Fisher−Borsche,3 Bucherer,4
Graebe−Ullmann,5 and Cadogan carbazole syntheses.6 In
recent years, carbazole synthesis based on transition metal-
catalyzed methodologies provides a useful and efficient
alternative approach.7 However, a majority of these methods
are targeted to the construction of the five-membered pyrrole
ring in carbazoles. Methods for the construction of the
benzene ring are less developed. Recently, an indole-to-
carbazole strategy has become a very powerful synthetic
approach for the construction of a new benzene ring in
carbazoles because many substituted indole derivatives are
commercially available.8 Furthermore, 2,3-unsubstituted in-
doles can be directly used as the starting materials for carbazole
construction via the C−H functionalization protocol.9
As a subclass of carbazoles, benzo[a]carbazoles have also
gained considerable attention from both synthetic and
medicinal chemists due to their wide range of applications in
medicinal and material chemistry.10 Because these compounds
are rarely found in nature, a number of synthetic methods for
the preparation of benzo[a]carbazoles have been developed
and most of them involve transition metal-catalyzed cyclization
of functionalized alkynes.11 Furthermore, 2-nitrobiaryls, 2-
aminebiaryls, and α-azidobiaryls also could be used as the
Received: April 1, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01138
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Benzo[a]carbazole Synthesis Based on 2-
Table 1. Screening the Reaction Conditions
Arylindoles
b
entry
additive
solvent
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
chlorobenzene
o-xylene
m-xylene
mesitylene
nitrobenzene
DMF
NMP
DMSO
chlorobenzene
yield (%)
1
2
3
4
5
NH4I
KI
I2
nd
nd
29
27
46
55
38
62
76
47
51
64
19
nd
nd
nd
nd
HI
ICl
IBr
ICl3
IBr
IBr
IBr
IBr
IBr
IBr
IBr
IBr
IBr
c
6
7
8
9
10
11
12
13
14
15
16
17
sulfoxonium ylides using expensive rhodium as the catalyst
(Scheme 1d).18 In our recent research, we found that 3-vinyl-
indoles could be generated in situ from acetophenones and
indoles promoted by an iodide-containing additive.9j We
envisioned that CDC reaction (cross-dehydrogenative cou-
pling) may occur between the vinyl group and the phenyl
group in 3-vinyl-indoles, thus leading to the desired benzo-
[a]carbazole products, although CDC reaction between two
Csp2−H bonds under metal-free conditions is quite challeng-
ing. As part of our continuing interest in the preparation of
indole-based heterocycles,19 we herein report an efficient
synthesis of benzo[a]carbazoles from 2-arylindoles and
acetophenones under metal-free conditions. Furthermore,
terminal alkynes also could be used as the versatile C2 source
in this kind of reaction (Scheme 1e).
With this hypothesis in mind, we started our investigation by
using 2-phenyl-1H-indole (1a) and acetophenone (2a) as the
model reagents. Unfortunately, no product was obtained when
NH4I or KI was added as the additive using toluene as the
solvent (Table 1, entries 1 and 2). Gratifyingly, the reaction
proceeded smoothly in the promotion of molecular iodine to
give desired product 3aa in 29% yield (entry 3). Inspired by
this finding, we investigated several iodide-containing additives
to further improve the reaction yield, and among them, IBr
showed the best efficiency to afford the product in 55% yield
(entries 4−7). The solvent has an obvious effect on the yield of
the reaction. In general, a better yield was achieved in less
polar solvents such as benzene, chlorobenzene, o-xylene, and
mesitylene, whereas no product could be observed in polar
solvents such as DMF, NMP, and DMSO (entries 8−16).
Chlorobenzene was found to be the best solvent giving desired
product 3aa in 76% yield (entry 9). However, no other
byproducts were observed by GC−MS. A control experiment
showed that the addition of IBr is indispensable for this kind of
transformation (entry 17). After extensive screening of
different parameters, optimized reaction conditions for
construction of benzo[a]carbazoles were established: IBr (1
equiv), PhCl (0.5 mL), 130 °C, 4 h under an air atmosphere.
a
Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), additive (0.2
mmol), solvent (0.5 mL), 130 °C, 4 h, under air. Abbreviations: nd,
not detected; DMF, N,N-dimethylformamide; NMP, N-methylpyrro-
b
lidone; DMSO, dimethyl sulfoxide. Isolated yield based on 2a.
c
Caution: The IBr is a highly toxic compound. Please be careful.
As shown in Scheme 2, the substrate scope of the reaction
with respect to various aromatic ketones (2) with 2-phenyl-
1H-indole (1a) was investigated under the optimized
conditions. First, a series of para-substituted acetophenones
bearing electron-donating groups (Me and butyl) were
smoothly converted into the corresponding products in good
yields (3ab−3ad). When 4-acetylbiphenyl (2e) was used, 3ae
was obtained in 88% yield. To our delight, aromatic ketones
with electron-withdrawing groups (CO2CH3, OCF3, NO2, and
SO2CH3) were also reactive to involve this kind of reaction
(3af, 3ag, 3al, and 3am). Halogen-substituted substrates,
including relatively reactive 4′-iodoacetophenone (2k), were
compatible to give products 3ah−3ak in good yields. 3aj could
be achieved in good yield when a gram-scale reaction was
carried out. Furthermore, meta-substituted acetophenones
were suitable reactants to give the desired products in good
to high yields (3an−3as). A steric hindrance effect was
observed when halogen substituents were located at the ortho
position in acetophenone, and moderate yields were obtained
(3at and 3au). This kind of steric effect is more pronounced in
the acetonaphthones. For example, 3aw could be obtained in
78% yield when 2′-acetonaphthone (2w) was used. However,
in stark contrast, 3av could be achieved in only 40% yield.
Bulky aromatic ketones such as 2-acetylphenanthrene (2x) and
1-acetylpyrene (2y) also could be used as the substrates for
this kind of conversion. Furthermore, hetero-aromatic ketones
such as 2-acetylbenzo[b]thiophene (2aa) underwent the
reaction to give the corresponding product 3aaa in 45%
yield. Aliphatic ketones such as 1-cyclohexylethanone and 3-
methylbutan-2-one failed to afford the desired product under
the current conditions.
B
DOI: 10.1021/acs.orglett.9b01138
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Organic Letters
Letter
a
a
Scheme 2. Substrate Scope with Respect to the Ketones
Scheme 3. Substrate Scope with Respect to the Indoles
a
Conditions: 1 (0.3 mmol), 2a (0.2 mmol), IBr (0.2 mmol), PhCl
(0.5 mL), 130 °C, 4 h, under an air atmosphere. Yields of isolated
a
Conditions: 1a (0.3 mmol), 2 (0.2 mmol), IBr (0.2 mmol), PhCl
(0.5 mL), 130 °C, 4 h, under an air atmosphere. Yields of isolated
products.
b
products. On a 6 mmol scale.
Scheme 4. Terminal Alkyne Involved in Benzo[a]carbazole
a
Formation
Subsequently, the generality and limitation of 2-arylindole
components were studied (Scheme 3). First, different
substituents on the 2-phenyl ring were surveyed for the
reaction with acetophenone (3ba−3ea). The corresponding
product 3ea was obtained in 67% yield even when the chloro
substituent was presented at the ortho position. In addition, the
substituent positions on the indole ring significantly affected
the reaction. 2-Phenylindoles with substituents (methyl,
chloro, and bromo) located at the C5 position were effectively
involved in this reaction to give the products in reasonable
yields (3fa−3ha). Besides free NH 2-arylindoles, other N-
substituted indoles are also involved in this kind of reaction
(3ia−3la). Finally, a mixture of isomers was obtained in 61%
combination yield when a fluoro group was located at the meta
position in the phenyl ring. A mixture of two isomers in 49%
yield (1:6 3na:3na′) was obtained when (m-tolyl)-1H-indole
was used as the substrate.
More importantly, terminal aromatic alkynes also could be
used as the versatile C2 source for construction of benzo[a]-
carbazole derivatives without the aid of a transition metal
catalyst. As shown in Scheme 4, the reaction showed good
regioselectivity when terminal alkynes were used to give the
aryl group at the C6 position in the benzo[a]carbazole moiety.
In general, moderate to good yields could be obtained when
electron-donating substituted phenylacetylenes were used
(3ab−5ac). Halogen functional groups (3ah−3aj and 3aq−
3at). A lower yield was obtained when the Cl group was
located at the ortho position (3at vs 3ai). Alkyl alkynes such as
oct-1-yne and ethyl propiolate were not suitable substrates.
Using phenylacetylene as the condensation partner, several
a
Conditions: 1 (0.3 mmol), 4 (0.2 mmol), IBr (0.2 mmol), PhCl (0.5
mL), 130 °C, 6 h, under an air atmosphere. Yields of isolated
products.
indole derivatives were also screened for this transformation. In
general, good yields were obtained whether the substituents
were on the 2-phenyl ring or in the indole skeleton (3ba−3ga).
C
DOI: 10.1021/acs.orglett.9b01138
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Organic Letters
Letter
Again, besides free NH indoles, N-methyl- and N-ethylindoles
were also smoothly engaged in this type of cyclization process
(3ia and 3ja).
Several control experiments were designed to obtain more
information about the mechanism (Scheme 5). The key
C. Further single-electron oxidation and deprotonation of C
form intermediate D. The final product 3aa can be obtained by
a deprotonation step.
In summary, we have developed a versatile strategy for the
synthesis of benzo[a]carbazoles from readily available starting
materials. The key 3-vinyl-indole intermediates were generated
in situ through dehydrative condensation of aryl ketones and 2-
arylindoles under metal-free conditions. This kind of
intermediate could further convert into the final products via
CDC reaction between two Csp2−H bonds without the aid of
any metal catalyst. Furthermore, terminal aromatic alkynes also
could be used as the condensation partners under these metal-
free conditions. This method provides an efficient approach for
the versatile construction of various substituted benzo[a]-
carbazoles from readily available raw materials.
Scheme 5. Control Experiments under Various Conditions
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.9b01138.
1
Experimental procedures, characterization data, and H
and 13C NMR spectra for all new products (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: fhxiao@xtu.edu.cn.
*E-mail: gjdeng@xtu.edu.cn.
ORCID
intermediate 3-vinyl-indole 3a could be obtained in 19% yield
in 30 min under the standard conditions (Scheme 5a). Isolated
3a could be smoothly converted into 3aa in 57% yield in an
IBr/air system (Scheme 5b). Inhibition experiments were
conducted by addition of butylated hydroxytoluene (BHT),
ethene-1,1-diyldibenzene, and 2,2,4,4-tetramethyl-1-piperidiny-
loxy (TEMPO) to the standard reaction mixture (Scheme 5c),
and the results showed that a radical cyclization pathway was
possibly involved because the reaction was completely
suppressed by 1 equiv of TEMPO.
On the basis of the experimental observations and related
references presented above,20 we proposed a plausible
mechanism in Scheme 6. Dehydrative condensation of 2-
phenyl-1H-indole (1a) and acetophenone (2a) generates
intermediate 3a. A radical intermediate A is generated via
single-electron transfer promoted by an IBr/air oxidation
system. This intermediate can further convert into vinyl radical
intermediate B via an isomerization step. Radical abstraction of
the proton on the phenyl ring generates cyclized intermediate
Huawen Huang: 0000-0001-7079-1299
Guo-Jun Deng: 0000-0003-2759-0314
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21871226 and 21572194), the
Collaborative Innovation Center of New Chemical Technol-
ogies for Environmental Benignity and Efficient Resource
Utilization, and the Hunan Provincial Innovative Foundation
for Postgraduate (CX2018B362).
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