Rh(III)-Catalyzed [5 + 1] Annulation of Indole-enaminones with Diazo Compounds to Form Highly Functionalized Carbazoles

Article abstract of DOI:10.1021/acs.orglett.1c01341

A novel Rh(III)-catalyzed C-H activation/annulation cascade of indole-enaminones with diazo compounds was reported to construct diversely functionalized carbazole frameworks. The most notable characteristic is that this transformation could smoothly furnish a novel [5 + 1] cyclization product with good to excellent yields (up to 95%), accompanied by the thorough removal of acetyl and N,N-dimethyl groups of two substrates from the target products, rather than the normally expected [4 + 2] cyclization products.

Full text of DOI:10.1021/acs.orglett.1c01341

pubs.acs.org/OrgLett
Letter
Rh(III)-Catalyzed [5 + 1] Annulation of Indole-enaminones with
Diazo Compounds To Form Highly Functionalized Carbazoles
Zhidong Jiang, Jianhui Zhou, Haoran Zhu, Hong Liu,* and Yu Zhou*
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ABSTRACT: A novel Rh(III)-catalyzed C−H activation/annulation cascade of indole-enaminones with diazo compounds was
reported to construct diversely functionalized carbazole frameworks. The most notable characteristic is that this transformation
could smoothly furnish a novel [5 + 1] cyclization product with good to excellent yields (up to 95%), accompanied by the thorough
removal of acetyl and N,N-dimethyl groups of two substrates from the target products, rather than the normally expected [4 + 2]
cyclization products.
Fisher−Borsche synthesis, Graebe−Ullmann synthesis, biar-
ylnitrenes−Cadogan cyclization, and iron-mediated synthesis
had been developed to build these intriguing core scaffolds, but
they usually required harsh reaction conditions and had many
limitations.2a,3 Although some new strategies that originated
from easily available indole templates as a starting point have
recently been reported,3h,4 more facile and effective synthetic
strategies are still highly desired.
In the past several years, N,N-dimethyl enaminones have
been widely used as building blocks to build a variety of
heterocyclic scaffolds,5 in which the key enaminone moiety can
play different roles to fulfill the selective annulation pattern
owing to its unique chemical properties with both an electron-
donating amino group and an electron-withdrawing carbonyl
group. In 2016, Zhu’s group reported a pioneering Rh(III)-
catalyzed C−H functionalization of the unique conjugated
O=C−CC−N system of enaminone with either alkynes or
α-diazo-β-ketoesters to synthesize naphthalenes (Scheme 1a).6
The reaction took advantage of the directing capability of
ketone groups of enaminones and this unique reactivity pattern
to fulfill the separation of directing and cyclization sites of
arbazoles are very important privileged scaffolds and
Cfrequently reported in natural products, medicinal
chemistry, and agricultural chemistry,1 displaying a wide
range of biological potencies such as anticancer,1b antipsycho-
tic,1e antituberculosis, and antioxidative activities.1d Among
them, Midostaurin and Carvediol with a carbazole core have
been approved by the FDA for therapy for tumor and
congestive heart failure, respectively (Figure 1).2 A large
number of classical or nonclassical synthetic methods including
Received: April 19, 2021
Published: May 21, 2021
Figure 1. Related bioactive products.
https://doi.org/10.1021/acs.orglett.1c01341
© 2021 American Chemical Society
Org. Lett. 2021, 23, 4406−4410
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hydroxy-9H-carbazole-1-carboxylate (3aa) could be obtained
in 75% yield (Table 1, entry 1), and its precise structure was
Scheme 1. Transition-Metal-Catalyzed of Enaminone
a
Table 1. Optimization of the Reaction Conditions
b
entry
1
Ag salt
AgSbF6
AgSbF6
AgSbF6
AgSbF6
additive
HOAc
HOAc
HOAc
HOAc
solvent
DCE
DCE
DCE
DCE
yield (%)
75
49
68
0
c
2
d
3
e
4
5
6
7
8
−
−
−
−
−
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
MeCN
MeOH
HFIP
25
72
75
68
65
75
40
86
<10
80
0
85
75
0
65
90
90
90
90
0
AgSbF6
CF3COOAg
AgOTf
9
Ag(OPiv)
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
CF3COOAg
−
HOAc
Na2CO3
succinic acid
PivOH
adipic acid
oxalic acid
succinic acid
succinic acid
succinic acid
succinic acid
succinic acid
succinic acid
succinic acid
succinic acid
succinic acid
succinic acid
substrates, in which the two inherently reactive groups
(aldehyde and hydroxyl) have been fused into the newly
formed naphthalene framework. Similarly, Wang’s group
reported a Rh(III)-catalyzed C−H activation and annulation
between enaminones and sulfoxonium ylides to build a series
of polysubstituted naphthalenes.7 However, when introducing
dioxazolone8 or α-diazo-α-phosphonoacetate9 as a coupling
partner, a two-step cascade transformation was needed to form
intriguing 4-quinolones and 4-hydroxy-1-naphthoates, respec-
tively.8,9 Recently, the Loh and Jiang’s group reported an
interesting Rh(III)-catalyzed enaminone-directed C−H bond
activation and subsequent [5 + 1] annulation of enaminones
with vinyl esters to construct amino/formyl substituted
polyaromatic heterocycles with a broad range of functional
groups (Scheme 1b).10
10
11
12
13
14
15
16
17
18
19
20
THF
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
f
21
22
23
24
g
h
i
Within our continuous studies on developing rhodium(III)-
catalyzed C−H activation/functionalization methods to build
drug-like heterocyclic skeletons11 and the importance of
carbazole scaffolds, we introduced this enaminone as a
directing group into the indole scaffold to form a series of
indole-3-enaminones, and tried to further construct highly
functionalized carbazole scaffolds. To our surprise, we found
that treating indole-3-enaminones with α-diazo-β-ketoesters
could smoothly furnish a novel [5 + 1] cyclization product,
ethyl 4-hydroxy-9H-carbazole-1-carboxylate, rather than the
expected [4 + 2] cyclization product (Scheme 1c). Especially,
this new finding is significantly different from the reported
methods in which the reaction goes through a new reaction
course, requiring that acetyl and N,N-dimethyl groups of
substrates are thoroughly removed from the target products.
We speculate that the difference in the electron-rich properties
of indolyl relative to the phenyl substrates may have
contributed to the different results of our article. The
electron-rich indolyl substrates undergo another process to
give the novel [5 + 1] cycloaddition products.4b Meanwhile,
good to excellent yields (up to 95%) can be obtained at room
temperature for 2 h along with broad generality and versatility.
Therefore, we report these results here.
j
25
76
a
General reaction conditions: 1a (0.3 mmol), 2a (0.45 mmol),
[Cp*RhCl2]2 (5 mol %), Ag salt (20 mol %), and additive (0.6 mmol)
b
in solvent (2 mL) at 80 °C in oil bath, under air, for 12 h. Isolated
c
d
yield. [Ru(p-cymene)Cl2]2 (5 mol %). [Cp*IrCl2]2 (5 mol %).
e
f
g
Cp*Co(CO)I2 (5 mol %). At 100 °C in oil bath for 12 h. At rt for
h
i
j
12 h. At rt for 2 h. Without [Cp*RhCl2]2 Additive (5 mol %) DCE:
dichloroethane. THF: tetrahydrofuran. HFIP: hexafluoroisopropanol.
unambiguously verified by X-ray crystallography (CCDC
2076731; see the Supporting Information for details). Inspired
by this result, we have implemented a comprehensive
investigation for the reaction parameters of this transformation.
We first screened different metal catalysts, such as [RuCl2(p-
cymene)]2, [Cp*IrCl2]2, and Cp*Co(CO)I2, but the results
indicated that [Cp*RhCl2]2 was still the best catalyst for this
transformation (Table 1, entries 2−4). Then we investigated
the necessity of Ag salts and additives. The results showed that
removal of the Ag salt and additive led to a significant decrease
in yield, but only removal of the additive slightly decreased the
yield (Table 1, entries 5 and 6). Further screening of Ag salts
revealed CF3COOAg was the best choice with a yield of 75%
(Table 1, entries 7−9). In order to improve the yield, we have
further explored the effects of different additives including
HOAc, Na2CO3, succinic acid, PivOH, adipic acid, and oxalic
acid. Surprisingly, succinic acid could give a better yield for
target product 3aa (up to 86%) (Table 1, entries 10−15).
Subsequently, we screened the reaction solvents and the results
Initially, the (E)-3-(dimethylamino)-1-(1H-indol-3-yl)prop-
2-en-1-one (1a, 0.3 mmol) and ethyl 2-diazo-3-oxobutanoate
(2a, 0.45 mmol) were selected as the model substrates and
treated with [Cp*RhCl2]2 (5 mol %), AgSbF6 (20 mol %), and
HOAc (0.6 mmol) in 1,2-dichloroethane (DCE) at 80 °C for
12 h, and the results showed that the desired product ethyl 4-
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showed that when 1,4-dioxane was used as the solvent, the
yield could slightly increase to 90% (Table 1, entries 16−20).
Additionally, the influence of time and temperature on this
reaction were also explored (Table 1, entries 21−23). We were
pleased to find that the reaction could proceed smoothly at
room temperature with a 90% yield in only 2 h (Table 1, entry
23). The control reaction showed that the product 3aa could
not be detected in the absence of [Cp*RhCl2]2 (Table 1, entry
24). When the amount of additives was reduced to 0.05 equiv,
the target product was obtained with a yield of 76% (Table 1,
entry 25).
Scheme 3. Substrate Scope of Diazo Compounds
With the optimized reaction conditions in hand, we further
investigated the scope of indole-3-enaminones (Scheme 2).
Scheme 2. Substrate Scope of Indole-3-enaminones
and the corresponding ketone products 3af and 3hf were
obtained with yields of 71% and 85%, respectively.
Intrigued by the diverse carbazole derivatives, we next
investigated its synthetic utility and late-stage functionalization.
First, we performed a gram-scale reaction and observed the
product 3aa could be obtained at 79% isolated yield (Scheme
4a). It seems that the ester group exhibits greater potential to
Scheme 4. Gram-Scale Preparation and Conversion of 3aa
When the substituents were located at the 5-position or 6-
position of the indole, regardless if the substituent was an
electron-withdrawing group, an electron-donating group, or a
halogen atom, all of them could smoothly deliver the products
with good yields, some up to a 95% yield (3ba−3ka). It
seemed that the ester group was harmful to this transformation
and resulted in a moderate yield (3ca and 3ia). Subsequently,
we investigated different influences from the substituents at the
4-position or 7-position of indole, and all substituents were
tolerated (3la−3oa), with 4-fluoro substituted indole giving
the best yield with a 95% yield (3pa). Meanwhile, disubstituted
indole-3-enaminone (3qa) could also complete this trans-
formation in excellent yield (up to 95%). However,
replacement of the N-atom position of indole with a methyl
group resulted in a slight decrease of the yield; only a 62%
yield could be obtained (3ra).
We next investigated the scope of the other coupling partner
diazo compounds (Scheme 3). The results showed that
methoxy, isopropoxy, and tert-butoxy at the R2 position
could be well tolerated and delivered the desired products in
good yields (3ab−3hd). Moreover, when the alkyl group in
diazo compounds was replaced with a benzyl group, the
reaction can also proceed smoothly with good yields (3ae−
3he). Surprisingly, when the coupling partner ethyl 2-diazo-3-
oxobutanoate (2a) was changed to symmetric diketone partner
3-diazopentane-2,4-dione (2f), the reaction could still proceed
perform the late-stage functionalization for this scaffold; we
thus first prepared the hydrolyzed product 4 in 80% yield, and
the resulting carboxyl group provided a further modification
opportunity. Therefore, by treating 4 with diphenylacetylene in
the presence of [Cp*RhCl2]2 and Cu(OAc)2·H2O in DMF at
120 °C for 12 h (Scheme 4b),12 we obtained an intriguing
polyaromatic product 5 containing an isochromanone scaffold.
Besides, the hydroxyl group of 3aa also provides convenience
for later modification. We further synthesized another
intriguing fused aromatic compound, pyrano[3,2-c]carbazole
6, which was reported to have antiproliferative activity and
antioxidant activity in vitro (Scheme 4c).13
In order to understand the possible mechanism, several
mechanistic experiments were carried out. First, we performed
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an H/D exchange experiment of 1a under standard reaction
conditions with D2O for 2 h (Scheme 5a). Four hydrogen/
rhodium species. Meanwhile, the N,N-dimethylamine released
from the rhodium complex further condensed with V to give
VI via a nucleophilic addition of an acetyl group. Finally, an
aromatization process prompted the formation of the desired
product 3aa, accompanied by the release of N,N-dimethylace-
tamide.
Scheme 5. Preliminary Mechanism Studies
In summary, we have reported a novel synthetic strategy for
the carbazole skeleton with important pharmacological effects
by an unexpected [5 + 1] cycloaddition reaction from indole-
enaminones with α-diazo-β-ketoesters in good to excellent
yields. What is noteworthy is that the transformation
underwent a different course and delivered novel [5 + 1]
annulation products, accompanied by the thorough removal of
acetyl and N,N-dimethyl groups of substrates from the target
products. The reaction exhibits good functional group
tolerance and provides an effective pathway to construct useful
carbazole derivatives under simple and facile reaction
conditions. The gram-scale reaction could also maintain high
transformation efficiency. Further studies of the synthetic
utility indicate that the resulting carbazole derivatives could be
facilely converted into intriguing polyaromatic scaffolds.
deuterium scrambling sites were identified, such as 1,2,4-
positions of indole, and the β carbon of the carbonyl group,
indicating that the ketone group is the directing group. Then,
the competition experiment between the electron-withdrawing
substituted 1d and electron-donating substituted 1b was
carried out (Scheme 5b). The results showed that products
3ba and 3da were obtained at a ratio of 0.66:1, suggesting that
the electron-deficient substrate has a slightly higher reactivity.
According to our mechanistic investigations and previous
studies,4b,6,10 a preliminary mechanism was proposed as shown
in Scheme 6. First, [Cp*RhCl2]2 was exchanged with a silver
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01341.
Detailed experimental procedures, X-ray crystallographic
data of 3aa, spectral data (1H, 13C and HRMS) for all
new compounds (PDF)
Scheme 6. Proposed Catalytic Cycle
Accession Codes
CCDC 2076731 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
■
Hong Liu − State Key Laboratory of Drug Research, Shanghai
Institute of Materia Medica, Chinese Academy of Sciences,
Shanghai 201203, China; University of Chinese Academy of
Sciences, Beijing 100049, China; School of Pharmaceutical
Science and Technology, Hangzhou Institute for Advanced
Study, University of Chinese Academy of Sciences, Hangzhou
310024, China; orcid.org/0000-0003-3685-6268;
Email: hliu@simm.ac.cn
Yu Zhou − State Key Laboratory of Drug Research, Shanghai
Institute of Materia Medica, Chinese Academy of Sciences,
Shanghai 201203, China; University of Chinese Academy of
Sciences, Beijing 100049, China; School of Pharmaceutical
Science and Technology, Hangzhou Institute for Advanced
Study, University of Chinese Academy of Sciences, Hangzhou
310024, China; orcid.org/0000-0002-7684-2939;
Email: zhouyu@simm.ac.cn
salt and additives to form an active catalyst, and then
coordinated with indole-enaminone via a C−H activation
process to form species I. Then species I coordinated with α-
diazo-β-ketoester 2a to obtain rhodium carbene species II,
followed by an intramolecular migratory insertion and a
subsequent intramolecular coordination with the olefinic bond
of substrate to form species III. Next, a six-membered cyclic
intermediate IV was generated through another migratory
insertion and protonolysis, and then an elimination process
furnishes the intermediate V along with a release of the
Authors
Zhidong Jiang − State Key Laboratory of Drug Research,
Shanghai Institute of Materia Medica, Chinese Academy of
Sciences, Shanghai 201203, China; University of Chinese
Academy of Sciences, Beijing 100049, China
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Jianhui Zhou − State Key Laboratory of Drug Research,
Shanghai Institute of Materia Medica, Chinese Academy of
Sciences, Shanghai 201203, China; College of Pharmacy,
Nanjing University of Chinese Medicine, Nanjing 210023,
China
Haoran Zhu − State Key Laboratory of Drug Research,
Shanghai Institute of Materia Medica, Chinese Academy of
Sciences, Shanghai 201203, China; University of Chinese
Academy of Sciences, Beijing 100049, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01341
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (21977106, 21632008), Shanghai Science and Tech-
nology Development Funds (Grant No. 19431901000,
19431901200), Science and Technology Commission of
Shanghai Municipality (18JC1411304), Youth Innovation
Promotion Association CAS for their financial support.
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