Rh(III)-catalyzed annulation of azobenzenes and α-Cl ketones toward 3-acyl-2H-indazoles

Article abstract of DOI:10.1016/j.cclet.2020.12.027

Rhodium(III)-catalyzed [4 + 1] cyclization of azobenzenes with α-Cl ketones has been developed. 3-Acyl-2H-indazoles could be easily afforded in up to 97% yields for more than 30 examples. The obtained products are potentially valuable in organic synthesis

Full text of DOI:10.1016/j.cclet.2020.12.027

Journal Pre-proof
Rh(III)-catalyzed annulation of azobenzenes and α-Cl ketones toward
3-acyl-2H-indazoles
Huan Li, Yuxuan Han, Zi Yang, Zhenyu Yao, Lianhui Wang, Xiuling
Cui
PII:
S1001-8417(20)30742-7
https://doi.org/10.1016/j.cclet.2020.12.027
CCLET 6041
DOI:
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Chinese Chemical Letters
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8 October 2020
16 December 2020
17 December 2020
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Communication
Rh(III)-catalyzed annulation of azobenzenes and α-Cl ketones toward 3-acyl-2H-
indazoles
Huan Li, Yuxuan Han, Zi Yang, Zhenyu Yao, Lianhui Wang, Xiuling Cui
Engineering Research Centre of Molecular Medicine of Ministry of Education, Key Laboratory of Fujian Molecular Medi-cine, Key Laboratory of Precision
Medicine and Molecular Diagnosis of Fujian Universities, Key Laboratory of Xiamen Marine and Gene Drugs, School of Biomedical Sciences, Huaqiao
University, Xiamen 361021, China
 Corresponding author.
E-mail address: cuixl@hqu.edu.cn
Graphical Abstract
Rhodium(III)-catalyzed [4 + 1] cyclization of azobenzenes with α-Cl ketones has been developed. 3-Acyl-2H-indazoles could be easily
obtained in up to 97% yields for more than 30 examples. Easily available azobenzenes and α-Cl ketones allow the reaction to proceed
smoothly under the mild reaction conditions with excellent yields and large functional groups tolerance.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Rhodium(III)-catalyzed [4 + 1] cyclization of azobenzenes with α-Cl ketones has been
developed. 3-Acyl-2H-indazoles could be easily afforded in up to 97% yields for more than 30
examples. The obtained products are potentially valuable in organic synthesis and drug
discovery. This protocol featured with high efficiency, extensive functional group tolerance, and
mild reaction conditions. The one-step efficient construction of an anti-inflammatory agent
confirms the practicability of this procedure.
Available online
Keywords:
Rh(III)-catalyzed
C-H activation/cyclization
α-Cl Ketones
3-Acyl-2H-indazoles
Indazole skeleton is favored by medicinal chemists for their
widespread presence in natural products, pharmaceuticals and
bioactive compounds [1]. Particularly, 2H-indazoles have
recently emerged as promising viral polymerase inhibition,
anticancer agents, and anti-inflammatory activities (Fig. 1) [2].
Consequently, numerous efforts have been devoted to construct
such a skeletal motif [3]. However, the known procedures toward
2H-indazoles often suffer from multistep synthetic precursors,
harsh reaction conditions and limited substrate scope. Therefore,
novel efficient strategies to access such a skeleton are highly
desirable.
Fig. 1. Selected examples illustrating the importance of 2H-indazole
moiety.
———

Transition-metal-catalyzed directly intermolecular cyclization
via C−H bond activation has been established as
in the reaction, the co-additive was replaced with an equal
amount of single copper acetate to obtain 3aa in 79% yield (entry
18). Reducing the loading of copper acetate to 2 equiv. resulted
in 3aa in 82% yield (entry 19 and 20). Among other substituted
acetophenones (entries 21 and 22), α-Br acetophenone led to a
lower yield of 31% and α-OTs gained 3aa in 68% yield. Finally,
the optimized reaction conditions were identified as follows:
azobenzene 1a (0.20 mmol), α-Cl acetophenone 2a (0.40 mmol),
[Cp*RhCl2]2 (5.0 mol%), AgNTf2 (20 mol%), and Cu(OAc)2 (2.0
equiv.) in DCE at 110 °C for 24 h under nitrogen.
a
straightforward method to construct heterocycles [4]. Among
which, the rhodium complex as a catalyst with excellent activity
plays a vital role in organic synthesis [5]. As the same time, the
azobenzene derivatives are the most classical and conventional
compounds in C−H activation/annulation under metal catalysts.
Since 2013, the C−H functionalization of azobenzenes as a
directing group for constructing various bioactive molecules
have been reported using Pd, Co, Rh or Re as catalysis [6]. In
these reactions, sp2- or sp-carbons, such as aldehydes, alkynes,
alkenes, sulfoxonium ylides and diazo compounds, were usually
used as the key synthons for the C−H bond activation/cyclization
process. However, sp3-carbon synthons have been rarely
reported. To solve this limitation, Glorius’s group [7] reported a
pioneering example of rhodium(III)-catalyzed [4 + 2] cyclization
to synthesize C3-monosubstituted N-heterocycle based on C(sp3)
Table 1
Optimization of the reaction conditions.a
Entry
Additive
Solvent
Yield (%)b
electrophiles
of
α-Cl/MsO/TsO
substituted
ketones,
complementing the previously reported mode of π-bond reaction.
Subsequently, Li and co-workers [8] disclosed the synthesis of
1
2
3
4
Cu(OAc)2 (50)
Zn(OAc)2 (50)
NaOAc (50)
MeOH
MeOH
MeOH
9
N.R
N.R
46
C−H/N−H
functionalized
isoquinolines
catalyzed
by
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) MeOH
rhodium(III) starting from α-Cl/MsO/TsO ketones. In these
procedures, α-halos or pseudohalogens are usually used as C2
synthons to construct these useful frameworks [7-9]. Very
recently, Liu and colleagues [10] pioneered the coupling of
rhodium(III)-catalyzed aromatic C(sp2)-H with α-Cl ketones as a
C1 synthon [11] for the synthesis of 3-acylindoles. To build
versatile heterocycles as well as with our continuing interest in
the clean transition-metal-catalyzed C−H bond functionalization,
[12] we embark on rhodium(III) catalyzed [4 + 1] cyclization of
azo compounds with α-Cl ketones for the synthesis of 3-acyl-2H-
indazoles. Cheap and easily available azobenzenes and α-Cl
ketones allow the reaction to proceed smoothly under the mild
reaction conditions with excellent yields and large functional
groups tolerance. Additional, the azo group not only participates
as a directing group but also as key component in the core
skeleton.
5
6
7
8
Cu(OAc)2 (50), HOAc (100)
Cu(OAc)2 (50), LiCO3 (100)
Cu(OAc)2 (50), K2S2O8 (100)
MeOH
MeOH
MeOH
36
N.R
N.R
57
N.R
49
N.R
40
62
44
65
70
76
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) DCE
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) CHCN3
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) TFE
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) THF
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) PhCF3
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) DCE
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) DCE
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) DCE
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) DCE
Cu(OAc)2 (50), CuCO3·Cu(OH)2 (100) DCE
Cu(OAc)2 (250)
Cu(OAc)2 (200)
Cu(OAc)2 (100)
Cu(OAc)2 (200)
Cu(OAc)2 (200)
9
10
11
12
13c
14d
15e
16f
17g
18g
19g
20g
21g,h
22g,i
DCE
DCE
DCE
DCE
DCE
79
82
52
31
68
THF: tetrahydrofuran, TFE: trifluoroethanol.
a
Reaction conditions: 1a (0.20 mmol), 2a (0.30 mmol), X = Cl,
additive, [Cp*RhCl2]2 (5.0 mol%), AgSbF6 (20 mol%), solvent (2.0
We initially started our investigation by employing
azobenzene 1a and α-Cl acetophenone 2a as the model substrates
to screen various reaction parameters (Table 1). The desired 2-
phenyl-3-benzoyl-2H-indazol 3aa was obtained in an isolated
9% yield when [Cp*RhCl2]2 (5.0 mol%)/AgSbF6 (20 mol%) was
used as a catalyst and Cu(OAc)2 (0.50 equiv.) as an additive in
MeOH at 110 °C under air atmosphere (entry 1). Meanwhile, no
desired conversion was observed using other acetate salts,
mL) at 110 °C under air for 24 h.
b Isolated yield.
c AgNTf2.
d AgBF4.
e AgNTf2, oxygen atmosphere.
f AgNTf2, nitrogen atmosphere.
g 1a (0.20 mmol), 2a (0.40 mmol), AgNTf2, nitrogen atmosphere.
h X= Br.
i X= OTs.
including Zn(OAc)2, NaOAc (entries
2 and 3). When
CuCO3·Cu(OH)2 (1.0 equiv.) was used as a co-additive, the yield
of 3aa was significantly improved to 46% (entry 4). Other co-
additives, such as HOAc (36%), K2S2O8 (N.R) and LiCO3 (N.R),
were not as effective as CuCO3·Cu(OH)2 (entries 5-7).
Subsequently, the effect of solvent was evaluated and 1,2-
dichloroethane (DCE) was found to be the most effective (entries
8-12). In addition, other silver additives were found to promote
this coupling reaction (entries 13 and 14) and AgNTf2 gave the
best yield of 62%. Atmosphere also has a great impact on this
reaction (entries 15-17) and the yield was superior to the air and
oxygen atmosphere under nitrogen atmosphere. When the ratio
of reactants was 1:2 (1a:2a), a yield of 76% was given.
Considering that the divalent copper salt might act as an oxidant
Under the optimized conditions (Table 1, entry 19), we first
studied the scope and generality of various azobenzenes as
shown in Scheme 1. Azobenzenes bearing various electron-
donating/withdrawing groups and halides at the para position all
coupled smoothly with 2a to afford the target products in
generally good yields (3aa-3ja). Among them, the reaction of
various alkyl or alkoxy or halogen-substituted azobenzenes with
2a gained up to 89% good yields. Electron-withdrawing group
led to the moderate yields (3ia-3ja). The ortho-substituted
azobenzenes gave the annulated products 3ka and 3la in 47%
and 44% yields. In the case of meta-substituted substrates, the C-
H activation occurred selectively and consistently at less

hindered site, furnishing the products in 64%-85% yields (3ma-
3qa). It is noteworthy that meta-fluoro substituent resulted in a
mixture of two regioisomers (3ra and 3rb) in 61% combined
yield with 3.3:1 ratio, favoring functionalization at the less
hindered position. In addition, the disubstituted azobenzenes
gave the target products in 33% and 52% yields (3sa-3ta).
However, 3ua did not get the corresponding target product,
perhaps due to the large steric hindrance. In this case of
unsymmetrical azobenzenes, the (E)-1-(4-methoxyphenyl)-2-(4-
nitrophenyl)diazene led to the indazole products in 56% total
yield with 1.65:1 regioselectivity. Simultaneously, 1-(3-
methoxyphenyl)-2-phenyldiazene was coupled with 2-chloro-1-
(3-hydroxy phenyl)ethan-1-one to obtain a single isomer 3wn
with high regioselectivity and excellent yield (82%), which was
used as an anti-inflammatory agent (Scheme 1) [2b]. Alkyl or
heterocyclic azo compounds did not give the target product the
under standard conditions (3xa-3ya).
Meanwhile, the scope of α-Cl ketones was studied, as shown
in Scheme 2. First, diverse monosubstituted α-Cl ketones (2b-2j)
and disubstituted α-Cl ketones (2k) were selected as the
substrates for this reaction, furnishing the desired products 3ab-
3ak in moderate to high yields. These results revealed that the
electron density of the benzene ring had effect on the coupling
reaction, in which strong electron-withdrawing groups (3ae and
3ak) appear to be beneficial for this transformation. The
coupling reaction also exhibited better functional group tolerance
(F, Cl, Br, Ph, NO2) in 78%-97% yields. Significantly, 3aj could
be obtained in 89% yield when the reaction was enlarged as the
scale of 1.0 mmol. Furthermore, alkenyl- and naphthyl-
substituted analogues also worked well under the standard
reaction conditions, leading to the corresponding 3-acyl-2H-
indazoles in 47% (3al) and 73% (3am) yields. However, 2-
chloro-1-(1H-pyrrol-2-yl)ethan-1-one did not undergo the
annulation (3an).
Scheme 1. Scope of azobenzenes. Reaction conditions: 1 (0.20 mmol), 2a (0.40 mmol), [Cp*RhCl2]2 (5.0 mol%), AgNTf2 (20 mol%), and
Cu(OAc)2 (2.0 equiv.) in DCE (2.0 mL) at 110 °C for 24 h under nitrogen. Isolated yields.
Scheme 2. Scope of α-Cl ketones. Reaction conditions: 1a (0.20
mmol), 2 (0.40 mmol), [Cp*RhCl2]2 (5.0 mol%), AgNTf2 (20
mol%), and Cu(OAc)2 (2.0 equiv.) in DCE (2.0 mL) at 110 °C for 24
h under nitrogen, Isolated yields.
To clarify the reaction mechanism, some control experiments
were conducted. When azobenzene 1a reacted with MeOD,
azobenzene was recovered in 97% yield (Scheme 3a). 50% H/D
exchange was found, which revealed that the C−H activation was
reversible. A kinetic isotope (kH/kD = 2.0) was revealed in
deuterium competition experiment (see Supporting inforamtion),
implying that the C-H bond activation process was probably
involved as the rate-determining step (Scheme 3b). Morever, α-
aryl-ketones species D could be converted to 3aa in the absence
of the catalyst (Scheme 3c). It revealed that D might be an key
intermediate in the cyclization process (see Supporting
inforamtion). Furthermore,
a radical trapping experiment
suggested that radical process could be ruled out (see Supporting
inforamtion).

The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (No. 21572072), 111 Project (No. BC
2018061), and Xiamen Southern Oceanographic Center (No.
15PYY052SF01).
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