Copper-Catalyzed Synthesis of Indolyl Benzo[b]carbazoles and Their Photoluminescence Property

Article abstract of DOI:10.1021/acs.orglett.1c01659

A copper-catalyzed cascade cyclization of dihydroisobenzofurans with indoles for the rapid construction of indoly benzo[b]carbazoles has been reported, providing the desired products in moderate to good yields under mild conditions along with a broad substrate scope and good functional group tolerance. The photoluminescence property of these indoly benzo[b]carbazoles has also been investigated.

Full text of DOI:10.1021/acs.orglett.1c01659

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Letter
Copper-Catalyzed Synthesis of Indolyl Benzo[b]carbazoles and Their
Photoluminescence Property
Tonggang Hao, Long Huang, Yin Wei, and Min Shi*
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ABSTRACT: A copper-catalyzed cascade cyclization of dihydroisobenzofurans with indoles for the rapid construction of indoly
benzo[b]carbazoles has been reported, providing the desired products in moderate to good yields under mild conditions along with a
broad substrate scope and good functional group tolerance. The photoluminescence property of these indoly benzo[b]carbazoles has
also been investigated.
arbazole, as a unique fused tricyclic structure, was first
1
Cseparated from coal tar by Graebe and Glazer in 1872.
Since then, many organic chemists and material scientists have
been conducting research on this peculiar compound and have
discovered a variety of diversified carbazole derivatives with
pivotal physiological and photophysical properties or existing
in numerous naturally occurring alkaloids.2 In particular,
carbazole derivatives have recently exhibited key applications
in the construction of bioactive organic molecules3 and the
Figure 1. Some useful carbazole derivatives.
synthesis of novel organic electroluminescent materials.4 For
example, the indolocarbazole drug midostaurin has been
carbazole could be realized by the transition-metal-catalyzed
dehydrogenative cross-coupling reaction of 2-arylindole with
alkyne or alkene, as shown in Scheme 1a.9 For benzo[b]-
carbazole, it could be achieved through the intramolecular
carbon−hydrogen amination upon transition-metal catalysis
(Scheme 1a).10 With regard to this aspect, in 2019, we
reported a Pd(OAc)2-catalyzed intramolecular amination for
the effective synthesis of benzo[b]carbazole derivatives.11
Herein we wish to disclose a novel copper-catalyzed
intermolecular cascade cyclization for the rapid construction
of indolyl benzo[b]carbazole compounds in a simple
approved by the FDA for the treatment of acute myeloid
leukemia mutations as a highly effective broad-spectrum
protein kinase inhibitor.5 TCTA and CBP6 containing three/
two carbazole units have higher triplet energy and excellent
hole conductivity, allowing their wide use as host materials in
organic light-emitting diodes (OLEDs).6 4CzlPN, as a donor−
acceptor fluorescent photosensitizer, has good photocatalytic
activity and has been extensively applied in many visible-light-
mediated photochemical reactions (Figure 1).7
Moreover, the conjugated system of the carbazole skeleton
could be upgraded by fusing another benzene ring into its
backbone scaffold with the enhancement of its photochemical
properties.8 Therefore, the benzocarbazole compound is a very
important class of carbazole derivatives, and these compounds
have key research prospects in the field of organic electro-
luminescent materials. Thus far, several synthetic method-
ologies have been developed to obtain these interesting organic
compounds. The typical synthetic protocol of benzo[a]-
Received: May 16, 2021
Published: June 18, 2021
https://doi.org/10.1021/acs.orglett.1c01659
© 2021 American Chemical Society
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triflate salts such as Y(OTf)3, Yb(OTf)3, or Tm(OTf)3, the
yield of 3a significantly decreased, ranging from 18 to 56%
(Table 1, entries 5−7). Reducing the catalyst loading to 5 mol
%, we observed that the yield of 3a dropped to 51% (Table 1,
entry 8). The examination of solvent effects revealed that the
utility of ethyl acetate or toluene did not enhance the yield of
3a in this reaction (Table 1, entries 9 and 10), and no reaction
occurred in THF or DMSO under identical conditions (Table
1, entries 11 and 12). It should be emphasized here that the
reaction worked less efficiently under an ambient atmosphere
using the commercially available DCM as a solvent, giving 3a
in 62% yield (Table 1, entry 13). The control experiment
indicated that no 3a could be formed in the absence of
Cu(OTf)2 (Table 1, entry 14). (For more information on the
optimization of the reaction conditions, see Table S1 in the
Supporting Information.)
Scheme 1. Previous Synthetic Approaches and This Work
After the optimal conditions were established, the substrate
scope of this copper-catalyzed intermolecular cyclization
process was then evaluated through the variation of N-
substituted indoles under the optimized conditions, and the
results are summarized in Scheme 2. First, with the utilization
manipulation under mild conditions and will also disclose their
extremely high-fluorescence luminescence efficiency (Scheme
1b).
We initially investigated the reaction outcome by using N-
methylindole 1a (1.0 equiv) and dihydroisobenzofuran 2a
(0.75 equiv) as substrates in the presence of Cu(OTf)2 (10
mol %) as the catalyst at ambient temperature for 2 h. It was
found that the reaction proceeded smoothly to give the desired
product 3a in 98% NMR yield along with 94% isolated yield
under an argon atmosphere for 2 h (Table 1, entry 1). The
structure of 3a was unequivocally determined by X-ray
diffraction. The ORTEP drawing is shown in Table 1. With
the replacement of Cu(OTf)2 with BF3·OEt2, In(OTf)3, or
Sc(OTf)3, the yield of 3a slightly decreased (Table 1, entries
2−4). However, when the catalyst was replaced by other
a
Scheme 2. Substrate Scope of Indoles 2
Table 1. Optimization of the Reaction Conditions for the
Construction of Indolyl Benzo[b]carbazole
a
entry
catalyst
solvent
yield of 3a (%)
a
Reactions were carried out with 1a (0.3 mmol), 2 (0.4 mmol), and
b
1
2
3
4
5
6
7
8
Cu(OTf)2
BF3·OEt2
In(OTf)3
Sc(OTf)3
Y(OTf)3
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
EA
98 (94)
Cu(OTf)2 (10 mol %) in DCM (2 mL) at ambient temperature.
Yields were determined by isolated products. Use BF3·OEt2 (50 mol
%) as a catalyst.
89
88
90
18
33
56
51
26
54
b
of N-methyl indoles 1b−1k as substrates, in which R could be
a diversified substituent, whether an electron-donating or an
electron-withdrawing one, the desired products 3b−3k were
delivered in moderate to high yields ranging from 41 to 96%.
In the case of substrate 1k, the corresponding product 3k was
obtained as a pair of atropisomeric mixtures with a 1:1 ratio,
and one of atropisomers has been unambiguously assigned by
X-ray diffraction. Its ORTEP drawing is indicated in Scheme 2.
A screen of N-protecting groups of indole revealed that the use
of Cu(OTf)2 as the catalyst gave the desired products in low
yields, but using BF3·OEt2 (50 mol %) as the catalyst afforded
the corresponding products 3l−3n in 32−58% yields for ethyl,
benzyl, or the isopropyl protecting group, presumably due to
the steric effect. None of the desired product could be
obtained if using N-Boc-protected indole as the substrate. For
the N-unprotected indole, the reaction also proceeded
Yb(OTf)3
Tm(OTf)3
c
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
9
10
11
12
13
14
toluene
THF
DMSO
d
DCM
62
DCM
a
Reactions were carried out with 1a (0.3 mmol), 2a (0.4 mmol), and
catalyst (10 mol %) in solvents (2 mL) at ambient temperature for 2
h. Yields were determined by NMR spectroscopy using CH2Br2 as an
internal standard. Yield of isolated product. Reaction was performed
with 5 mol % Cu(OTf)2. Using untreated solvent without Ar-purged
b
c
d
treatment.
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smoothly, delivering the target product 3o in 58% yield.
Furthermore, replacing N-methyl indole with N-methyl
pyrrole, the desired compound 3p could be obtained in 16%
yield.
Scheme 4. Substrate Scope of Phenyl
Dihydroisobenzofurans 1
a
On the contrary, the scope of dihydroisobenzofuran has
been examined as well, and the results are shown in Scheme 3.
a
Scheme 3. Substrate Scope of Dihydroisobenzofurans 1
a
Reactions were carried out with 1 (0.3 mmol), 2a (0.4 mmol), and
Cu(OTf)2 (10 mol %) in DCM (2 mL) at ambient temperature.
Yields were determined by isolated products.
Scheme 5. Gram-Scale Synthesis and a Crossover
Preparation of 6
a
Reactions were carried out with 1 (0.3 mmol), 2a (0.4 mmol), and
Cu(OTf)2 (10 mol %) in DCM (2 mL) at ambient temperature.
Yields were determined by isolated products.
As for dihydroisobenzofurans 1, no matter whether a methyl,
methoxy group, or halogen atom was introduced on the
benzene ring, the desired products 4a−4e were obtained in
moderate to good yields ranging from 40 to 93%. In the case of
product 4d, it was produced in only 40% isolated yield,
presumably due to its poor solubility in a variety of organic
solvents. Notably, to further enhance the conjugation system,
we introduced and fused an additional benzene ring and into
the dihydroisobenzofuran as substrate 1f, producing the
desired product 4f in 70% yield.
To further extend the substrate scope of this synthetic
method, we examined the substituent on the styrene moiety of
dihydroisobenzofuran under the standard conditions, and the
results are outlined in Scheme 4. As can be seen, a variety of
aromatic rings as well as the 1-naphthyl group could be
introduced at the terminal position of styrene, giving the
desired products 5a−5e in 47 to 84% yields.
The potential synthetic practicability of this novel protocol
on the gram scale was next assessed. As shown in Scheme 5, 3a
could be acquired in 60% yield (1.121 g) in the reaction of 1a
(1.216 g, 7.5 mmol) with 2a (1.313 g, 10.0 mmol) under the
optimal reaction conditions due to its poor solubility in the
large-scale synthesis. Moreover, the use of 1a (0.811 g, 5.0
mmol) and 2o (1.757 g, 15 mmol) as substrates could deliver
the desired product 3o in 58% yield (1.034 g). In addition, the
reaction of 1f (63 mg, 0.3 mmol) and 2j (71 mg, 0.4 mmol)
produced the corresponding new crossover product 6 in 26%
yield (26 mg).
were measured. As shown in Figure 2, we found that these
products barely emit in the range of 254−380 nm but exhibit
strong blue fluorescence emission from 390 to 530 nm in the
visible light range, implying the possibility of these compounds
being used as highly promising candidates of OLEDs, as we
conjectured. The substituent on the aromatic ring can adjust
the emission wavelength only slightly, suggesting that it would
As we previously mentioned, benzocarbazole holds special
biological activities and fluorescence properties. Thus the UV−
vis absorption spectroscopy and fluorescence emission spec-
troscopy of benzo[b]carbazoles 3a, 3g, 3j, 3k, 3o, 4e, and 4f
Figure 2. UV−vis absorption spectroscopy (left) and fluorescence
emission (right) spectroscopy of products.
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Letter
just enhance its chemical diversity and handling properties. As
compared with 3a (λmax = 433 nm), the maximum emission
wavelength blue shift takes place for 3o (λmax = 421 nm), but it
has the largest fluorescence quantum yield (Φf = 0.80) upon
the removal of the N-methyl protecting group from the indole
(Table 2). The largest maximum emission wavelength was
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01659.
■
sı
Experimental procedures and characterization data for
all compounds (PDF)
a
Table 2. Fluorescence Quantum Yield
Accession Codes
CCDC 2082031 and 908539 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.
compound
Φf
compound
Φf
3a
3j
3o
4f
0.46
0.66
0.80
0.49
3g
3k
4e
0.47
0.45
0.43
a
Samples were measured in CH2Cl2, c = 0.3 mM.
observed at 444 nm for 3j having a strongly electron-donating
substituent on the indole moiety. For 4f, its λmax is at 434 nm
along with a slightly improved Φf (0.49) as compared with 3a.
It was found that the introduction of a fluorine atom as a
substituent at different moieties of benzo[b]carbazole dis-
tinctively affected the maximum emission wavelength. As
compared with 3a, introducing a fluorine atom on
dihydroisobenzofuran such as product 4e could cause a red
shift of the maximum fluorescence wavelength to 441 nm;
however, when a fluorine atom was introduced at an indole,
such as product 3g, it could lead to a blue shift to 424 nm.
Moreover, as shown in Table 2, introducing a strongly
electron-donating substituent on the indole moiety can also
enhance the fluorescence quantum yield (FF).
AUTHOR INFORMATION
Corresponding Author
■
Min Shi − State Key Laboratory of Organometallic Chemistry,
Center for Excellence in Molecular Synthesis, University of
Chinese Academy of Science, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032,
China; orcid.org/0000-0003-0016-5211; Email: mshi@
mail.sioc.ac.cn; Fax: 86-21-64166128
Authors
Tonggang Hao − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
University of Chinese Academy of Science, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
The proposed reaction mechanism is outlined in Scheme 6
using 1a and 2a as model substrates. The reaction is initiated
Long Huang − State Key Laboratory of Organometallic
Chemistry, Center for Excellence in Molecular Synthesis,
University of Chinese Academy of Science, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
Scheme 6. Proposed Mechanism
Yin Wei − State Key Laboratory of Organometallic Chemistry,
Center for Excellence in Molecular Synthesis, University of
Chinese Academy of Science, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032,
China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01659
Notes
by a copper-catalyzed elimination of a methoxy group from 1a
forming an oxonium ion I in the presence of Cu(OTf)2, which
undergoes a nucleophilic attack by 2a to afford the indole-
substituted dihydroisobenzofuran II. Then, it again undergoes
a nucleophilic attack by another indole under copper catalysis
to acquire bis(indoly)acetophenone III. Finally, an intra-
molecular Friedel−Crafts reaction takes place along with the
release of H2O to deliver the final product 3a.
In summary, we have accomplished a copper-catalyzed
intermolecular cascade cyclization of indoles and dihydroiso-
benzofurans for the rapid construction of indolyl benzo[b]-
carbazoles. This synthetic method exhibits excellent functional
group tolerance and a broad substrate scope and can also be
successfully adapted to a scale-up application. These indolyl
benzo[b]carbazole compounds demonstrated interesting fluo-
rescence emission character. Further application of these newly
synthesized compounds as organic photonics materials is
currently under investigation.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the Strategic Priority
Research Program of the Chinese Academy of Sciences (grant
no. XDB20000000) and the National Natural Science
Foundation of China (nos. 21372250, 21121062, 21302203,
20732008, 21772037, 21772226, 21861132014, and
91956115).
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