Visible-light-induced oxidation/[3 + 2] cycloaddition/oxidative aromatization to construct benzo[ a]carbazoles from 1,2,3,4-tetrahydronaphthalene and arylhydrazine hydrochlorides

Article abstract of DOI:10.1021/acs.orglett.9b02939

An efficient synthesis of benzo[a]carbazoles via visible-light-induced tandem oxidation/[3 + 2] cycloaddition/oxidative aromatization reactions was reported. The benzylic C(sp³)-H of tetrahydronaphthalene was activated through visible-light photoredox catalyst with oxygen as the clean oxidant under mild reaction conditions. This protocol proceeds efficiently with broad substrate scope, and the mechanism study was performed.

Full text of DOI:10.1021/acs.orglett.9b02939

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Visible-Light-Induced Oxidation/[3 + 2] Cycloaddition/Oxidative
Aromatization to Construct Benzo[a]carbazoles from 1,2,3,4-
Tetrahydronaphthalene and Arylhydrazine Hydrochlorides
Jiaxuan Shen,* Nannan Li, Yanjiang Yu, and Chunhua Ma*
Collaborative Innovation Center of Henan Province for Green Manufacturing of Fine Chemicals, Key Laboratory of Green
Chemical Media and Reactions, Ministry of Education, School of Chemistry and Chemical Engineering, Henan Normal University,
Xinxiang, Henan 453007, China
S
* Supporting Information
ABSTRACT: An efficient synthesis of benzo[a]carbazoles via visible-light-induced tandem oxidation/[3 + 2] cycloaddition/
oxidative aromatization reactions was reported. The benzylic C(sp3)−H of tetrahydronaphthalene was activated through visible-
light photoredox catalyst with oxygen as the clean oxidant under mild reaction conditions. This protocol proceeds efficiently
with broad substrate scope, and the mechanism study was performed.
more, the C−H amination strategy for B[a]Cs synthesis using
enzo[a]carbazole (B[a]Cs) structural motifs are fre-
Bquently present in a variety of biologically active 2-nitrobiaryls, 2-aminebiaryls, and α-azidobiaryls as the
substrates was also developed.8 However, most of these
reactions needed harsh reactions or complex substrates, so it is
still highly desirable to explore new green and mild conditions
to fit with the diverse structural features of B[a]Cs with
commercially available substrates.
molecules and functional organic materials.1−3 For example,
compound I and compound II exhibit antitumor activity
against various tumor cell lines (Scheme 1).1d,f Compound III
was applied in the field of materials chemistry because of its
optical character.2 Compound IV was utilized in organic light-
emitting diodes (OLEDs).3
Since 2008, visible-light photoredox catalysis has been
widely used in organic synthesis under very mild and clean
conditions.9 The photoinduced intramolecular cyclizations for
the synthesis of B[a]Cs have also been developed.10 The Protti
and Palmieri groups developed the photochemical synthesis of
B[a]Cs from 2-aryl-3-(1-tosylalky)indoles with irradiated
phosphor-coated lamps (366 nm) (Scheme 2, a).10a Yu and
coworkers reported two efficient approaches for constructing
5-hydroxy-benzo[a]carbazoles by employing the photoinduced
intramolecular C−H insertion of 2-aryl-3-(α-diazocarbonyl)-
indoles and ethyl 3-(1-methy-1H-indol-3-yl)-3-oxopropanoates
(Scheme 2, b and c).10b,c Among the various strategies
reported for the synthesis of B[a]Cs, a direct convergent
synthesis from easily available reagents is especially attracti-
ve.5b,7a,b,m More recently, visible-light photoredox catalytic
activation of benzylic C(sp3)−H bonds represents a green and
sustainable process.11−13 Herein, we report a photocatalytic
tandem oxidation/[3 + 2] cycloaddition/oxidative aromatiza-
Due to their importance, a large number of methods for the
synthesis of B[a]Cs have been developed.4 Initially, the classic
B[a]Cs synthesis mainly relies on the Fischer−Borsche
indolization.5 Recently, B[a]Cs synthesis based on metal-
catalyzed reactions employing prefunctionalized benzynes or
indole derivatives has recently been demonstrated.6,7 Futher-
Scheme 1. Some Important Benzo[a]carbazole Derivatives
Received: August 18, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02939
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Photoinduced Synthesis of Benzo[a]carbazoles
Table 1. Optimization of the Reaction Conditions
b
entry catalyst (x mol %)
acid
solvent
CH3CN
yield (%)
1
PC-1 (3)
PC-1 (3)
PC-1 (3)
PC-1 (3)
PC-1 (3)
PC-1 (3)
PC-1 (3)
PC-1 (3)
PC-1 (3)
PC-1 (3)
PC-2 (3)
PC-3 (3)
PC-4 (3)
PC-5 (3)
PC-5 (8)
PC-5 (8)
-
HCl
TFA
TsOH
AcOH
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
HCl
-
18
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
CH3CN
CH3CN
CH3CN
H2O
EtOH
THF
DMSO
DCM
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
CH3CN/H2O (2:1)
10
trace
trace
trace
12
trace
trace
trace
30
trace
14
trace
32
46
75
-
-
-
-
tion sequence to construct B[a]Cs from 1,2,3,4-tetrahydro-
naphthalene and arylhydrazine hydrochlorides using molecular
oxygen as a green oxidant under mild conditions (Scheme 2,
d).
Initially, tetrahydronaphthalene (1a) and phenylhydrazine
hydrochloride (2a) were chosen as model substrates to
optimize the reaction conditions (Table 1). We were pleased
to find that using 9-mesityl-10-methylacridinium perchlorate
(PC-1) as a photoredox catalyst, which had already been used
in several oxidative benzylic C(sp3)−H activations12 and
dehydrogenations of tetrahydronaphthalenes,14 HCl as the
acid, and O2 as the terminal oxidant in CH3CN under 3 W
blue LED irradiation for 24 h, 11H-benzo[a]carbazole (3a)
was obtained in 18% yield (entry 1). Evaluation of different
acids showed that HCl is the best acid for this process (entries
2−4). The reaction proceeded less efficiently in other solvents,
including H2O, EtOH, THF, DMSO, and DCM (entries 5−9).
To our delight, CH3CN/H2O (V:V = 2:1) gave better results
than CH3CN (entry 10), probably because phenylhydrazine
hydrochloride can dissolve well in this cosolvent mixture.
Other photocatalysts such as 10-(3,5-dimethoxyphenyl)-9-
mesityl-1,3,6,8-tetramethoxyacridin-10-ium tetrafluoroborate
(PC-2), 2,4,6-triphenylpyrylium tetrafluoroborate (PC-3),
and Ru(bpy)3Cl2·6H2O (PC-4) did not increase the yield of
the desired product (entries 11−13). When 10-phenyl-9-
(2,4,6-trimethylphenyl) acridinium tetrafluoroborate (PC-5)
was used as the catalyst, the yield increased by a small amount
(entry 14). Increasing the catalyst loading to 8 mol %, the yield
of 3a could be improved to 46% (entry 15). Finally, increasing
the reaction time from 24 to 48 h afforded 3a in a yield of 75%
(entry 16). Control experiments showed that in the absence of
photocatalyst (entry 17), O2 (entry 18), HCl (entry 19), or
light (entry 20) gave no desired product (for a complete list of
conditions, see the Supporting Information).
c
d
e
PC-5 (8)
PC-5 (8)
PC-5 (8)
HCl
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), photocatalyst
(x mol %), HCl (37% aq.), O2 (balloon), solvent (1.0 mL), 3W blue
b
c
d
e
LEDs for 24 h. Isolated yield. 48 h. N2. In the dark.
Scheme 3. Scope of Phenylhydrazine Hydrochlorides
With the optimized reaction conditions in hand, the scope
and the generality of this reaction were explored. First, with
tetrahydronaphthalene (1a) as a model substrate, several
substituted phenylhydrazine hydrochlorides (2) were explored,
and the results were presented in Scheme 3. In general, the
reaction with phenylhydrazines bearing electron-donating
a
Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol), PC-5 (8 mol %),
HCl (37% aq., 5 equiv), O2 (balloon), CH3CN/H2O (2/1, 1.0 mL), 3
W blue LEDs for 48 h.
B
DOI: 10.1021/acs.orglett.9b02939
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
groups such as methyl, ethyl, isopropyl, and t-butyl were well
tolerated under the optimal reaction conditions (3b−3g, 52−
78% yield). Notably, the substrates containing halogen, such as
fluorine, chloride, or bromide on the aromatic ring, also
provided desired products in moderate to good yields (3h−3l,
48−62% yield), providing the potential chance for further
chemical transformation. The meta-Br-substituted phenyl-
hydrazine resulted in a low yield (3m, 35% yield). In addition,
when 4-OCF3-phenylhydrazine was employed in the reaction,
the desired product 3n was also obtained in 48% yield. It was
noted that the disubstituted phenylhydrazines containing two
methyl or fluorine groups also could undergo this process well
and furnish the target products 3o−3r. A mixture of isomers
was obtained in 80% combination yield when 3-methylphe-
nylhydrazine was used (3s and 3s′, C1:C2 = 1:1.4). However,
no desired product was obtained when electron-withdrawing
groups, such as NO2 or CF3, were present on the para-position
of the aromatic ring. A possible reason is that the presence of
strongly deactivating groups precludes efficient Fischer [3 + 2]
cyclization.15
morpholine, could afford the desired product 4 (Scheme 4,
4i−4t, 30−56% yield).
To demonstrate the utility of our methodology, the gram-
scale synthesis and synthetic application were performed
(Scheme 5). Benzo[a]carbazole 3a was obtained in moderate
Scheme 5. Gram-Scale Synthesis and Synthetic Application
Next, the reactions of different substituted tetrahydronaph-
thalenes were studied. The substrates containing halogen on
the aromatic ring were well tolerated, and the desired products
4 were obtained in moderate yields (Scheme 4, 4a−4f, 45−
yield at the 5 mmol scale (55% yield) (Scheme 5, a). This
result provides a possibility for the usage of preparative
synthesis. Furthermore, the desired anticancer activity
compounds 4u and 4v were successfully obtained in 50%
and 59% yields (Scheme 5, b). Notably, the previously
reported synthesis of 4u from 2-(4-bromophenyl)acetic acid
required 12 steps.1d In contrast, using our strategy, 4u was
elegantly constructed in only three steps from 1,2,3,4-
tetrahydronaphthalene-1-carboxylic acid (48% overall yield)
(Scheme 5, c).
a
Scheme 4. Scope of Tetrahydronaphthalenes
To gain some insight into the mechanism, we then
performed a series of control experiments (Scheme 6). Upon
addition of 2.0 equiv of radical trapping agent TEMPO or
BHT, the reaction was completely inhibited, which indicates
the reaction may proceed through a radical pathway. We used
cyclohexane (5) instead of 1a to perform the reactions, and
there was no reaction occurring (Scheme 6, 2). When
tetrahydronaphthalene (1a) was added independently under
the standard conditions, 3,4-dihydronaphthalen-1(2H)-one
(7) was obtained in 60% yield (Scheme 6, 3). We used 3,4-
dihydronaphthalen-1(2H)-one (7) instead of 1a to perform
the reactions. The desired product 3a was obtained with 52%
yield, and 28% of 6,11-dihydro-5H-benzo[a]carbazole (8) was
detected (Scheme 6, 4). However, when cyclohexanone (9)
was used in the optimized conditions, only 2,3,4,9-tetrahydro-
1H-carbazole (10) was obtained, and 9H-carbazole (6) was
not detected (Scheme 6, 5). Meanwhile, when 8 was used to
replace 1a and 2a to perform the photoredox-catalyzed
reaction, we obtained 3a with 69% yield (Scheme 6, 6).
Based on our studies and the literature report,12 a plausible
mechanism is proposed in Scheme 7. The photocatalyst is
excited by visible-light irradiation to generate [acridinium
catalyst]*, which then undergoes a single electron transfer
(SET) process with the benzylic C(sp3)−H compound to
generate radical cation A and the [acridinium catalyst] radical
anion. The [acridinium catalyst] radical anion can be oxidized
by O2 to finish the photocatalytic cycle. The generated radical
cation A loses one proton to generate the radical B, followed
by the reaction with O2 or the O2 radical anion to generate the
a
Reaction conditions: 1 (0.2 mmol), 2a (0.3 mmol), PC-5 (8 mol %),
HCl (37% aq., 5 equiv), O2 (balloon), CH3CN/H2O (2/1, 1.0 mL), 3
W blue LEDs for 48 h.
56% yield). It is worth noting that the structure of 4c was
unambiguously determined by X-ray diffraction analysis (see
the Supporting Information). In addition, 6-CF3-tetrahydro-
naphthalene and 6-phenyl-tetrahydronaphthalene were also
suitable for this reaction to give 4g and 4h in 48% and 28%
yields. Moreover, 1-substituted tetrahydronaphthalene, includ-
ing methyl, ethyl, aryl, amide, pyrrolidine, piperidine, and
C
DOI: 10.1021/acs.orglett.9b02939
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Letter
Scheme 6. Control Experiments
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.9b02939.
Experimental details, compound characterization data,
and NMR spectra (PDF)
Accession Codes
CCDC 1920539 contains 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: shenjiaxuan@htu.edu.cn.
*E-mail: xiaoma3625@126.com.
ORCID
Jiaxuan Shen: 0000-0002-3113-973X
Notes
The authors declare no competing financial interest.
Scheme 7. Proposed Mechanism
ACKNOWLEDGMENTS
■
We would like to thank Dr. Dachang Bai (HTU) for his
assistance with this article. This work was supported by the
High-end Foreign Expert Project (GDW20186300346),
He’nan Province Natural Science Foundation
(162300410182), the Foundation of He’nan Educational
Committee (17A350008), and the Doctoral Scientific
Research Foundation of He’nan Normal University.
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DOI: 10.1021/acs.orglett.9b02939
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