RhIII/CuII-cocatalyzed synthesis of 1H-indazoles through C-H amidation and N-N bond formation

Article abstract of DOI:10.1021/ja4033555

Substituted 1H-indazoles can be formed from readily available arylimidates and organo azides by Rh^III-catalyzed C-H activation/C-N bond formation and Cu-catalyzed N-N bond formation. For the first time the N-H-imidates are demonstrated to be go

Full text of DOI:10.1021/ja4033555

Communication
pubs.acs.org/JACS
RhIII/CuII-Cocatalyzed Synthesis of 1H‑Indazoles through C−H
Amidation and N−N Bond Formation
Da-Gang Yu,† Mamta Suri,† and Frank Glorius*
Organisch-Chemisches Institut, Int. NRW Graduate School of Chemistry, Westfalische Wilhelms-Universitat Munster, Corrensstrasse
̈
̈
̈
40, 48149 Munster, Germany
̈
S
* Supporting Information
ABSTRACT: Substituted 1H-indazoles can be formed
from readily available arylimidates and organo azides by
RhIII-catalyzed C−H activation/C−N bond formation and
Cu-catalyzed N−N bond formation. For the first time the
N-H-imidates are demonstrated to be good directing
groups in C−H activation, also capable of undergoing
intramolecular N−N bond formation. The process is
scalable and green, with O2 as the terminal oxidant and N2
and H2O formed as byproducts. Moreover, the products
could be transformed to diverse important derivatives.
Figure 1. Some examples of biologically active 1H-indazoles.
ue to their diverse pharmacological activities, 1H-indazoles
Dare widely used as anti-cancer, anti-inflammatory, anti-
HIV, and anti-microbial drugs (Figure 1).1 The efficient
synthesis of 1H-indazoles has attracted much attention for a
long time.2 Initially, the methods were mainly limited to
transition-metal-free processes, including diazotization or nitro-
sation of o-alkyl-substituted anilines,3 condensation of o-halo- or
mesylate-substituted arylaldehydes or ketones with hydrazines,4
and 1,3-dipolar cycloadditions of diazomethanes with benzynes.5
These methods suffer from several disadvantages, such as harsh
reaction conditions with high temperatures or strong acids,
expensive starting materials, or limited substrate scope.
Figure 2. Novel syntheses of diverse 1H-indazoles.
Transition-metal catalysis has become one of the most
important methods to generate heterocycles, including 1H-
indazoles. For example, many groups have developed efficient
Pd- or Cu-catalyzed intramolecular amination/amidation of o-
halo or o-alkoxy arylhydrazones to synthesize 1H-indazoles.6
Recently, transition-metal-catalyzed C−H activation has become
more and more important and powerful, arising mainly from its
high atom- and step-economy.7 Starting from arylhydrazones,
only three examples of Pd-, Fe-, and Cu-catalysis were reported
to synthesize 1H-indazoles through C−H amidation/amination
(Figure 2A).8 However, carcinogenic organo-hydrazines must be
used, and the substrates are limited to diaryl ketone derivatives,
which suffer from regioselectivity issues. Here we report an
attractive process to synthesize 1H-indazoles from easily
available arylimidates and organo azides with good functional
group tolerance via Rh-catalyzed C−H activation/C−N bond
formation and Cu-catalyzed N−N bond formation (Figure 2D).
Previously, our group developed an efficient synthesis of
substituted pyrazoles from enaminoesters and nitriles through
oxidative C−C/N−N bond formation in the presence of
stoichiometric or catalytic Cu(OAc)2 (Figure 2B).9 Due to the
importance of 1H-indazoles and the limitations of previous
methods, we wished to apply the oxidative N−N bond formation
in the efficient synthesis of such structures without use of toxic
organo-hydrazines.10 Recently, Chang et al. developed an
attractive Rh-catalyzed amidation/amination11,12 of aryl C−H
bond with organo azides13 as the nitrogen source (Figure 2C).
Inspired by these works and other Rh(III)-catalyzed C−H
activations,14,15 we report the development of an efficient
synthesis of 1H-indazoles through Rh-catalyzed directed C−H
amidation/amination with organoazides and following oxidative
N−N bond formation.
There were mainly two challenges: (1) the compatibility of
C−N and N−N bond formation. In Chang’s work,11 the C−N
bond was formed under redox- and pH-neutral reaction
conditions. The need for oxidant and in situ-generated acid in
the N−N bond formation step may inhibit the C−N bond
formation. (2) Proper and stable N−H-containing directing
groups. Several N-containing groups, such as pyridine, quinoline,
Received: April 4, 2013
Published: May 27, 2013
© 2013 American Chemical Society
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Journal of the American Chemical Society
Communication
a
Table 1. Optimization of Reaction Conditions
Scheme 1. Rh/Cu-Cocatalyzed Synthesis of 1H-Indazoles
from 1a with 2
a
b
2a
Cu(OAc)2
(equiv)
2-picolinic
T
yield
entry (equiv)
acid (mol%) (°C) atmosphere
(%)
1
2
3
4
5
6
7
8
9
1.5
2.0
2.0
2.0
2.0
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.1
2.1
−
−
−
−
−
−
5.0
5.0
5.0
5.0
−
−
−
−
−
110
110
90
argon
argon
argon
argon
argon
argon
(62)
67
2.1
30
2.1
130
110
110
110
110
110
110
110
110
110
110
110
110
61
c
2.1
38
a
2.1
(73)
67
Reaction conditions: 1a (0.2−0.25 mmol), 2 (2.5 equiv),
j
0.25
0.25
0.25
0.25
0.25
0.25
0.25
0.10
−
O2
[Cp*RhCl2]2 (2.5 mol %), AgSbF6 (10 mol %), Cu(OAc)2 (25 mol
%), 4 Å MS (150 mg), O2 (1 atm), DCE (1 mL), 110 °C, 24 h.
d
e
j
O2
58
j
O2
j
11
f
10
11
O2
25
Scheme 2. Rh/Cu-Cocatalyzed Synthesis of 1H-Indazoles
from 1 with 2a
j
a
O2
(73)
55
g
j
12
13
14
15
O2
h
j
O2
<5
j
O2
56
argon
<5
i
j
16
0.25
−
O2
(69)
a
1a (0.2 mmol), [Cp*RhCl2]2 (2.5 mol%), AgSbF6 (10 mol%), 4 Å
b
MS (150 mg), DCE (1.0 mL), 24 h. GC yields are given with
mesitylene as an internal standard, and isolated yields are given in
c
d
parentheses. Without 4 Å MS. Pivalic acid (25 mol%) was added.
e
f
g
CsOPiv (25 mol%) was added. Without AgSbF6. [Cp*RhCl2]2 (1.0
h
i
mol%) was used. Without [Cp*RhCl2]2. 7.0 mmol scale (1.0 g of
1a), 48 h. 1 atm.
j
pyrazole, oxime, and amides, have been demonstrated to show
good ability to direct C−H amination/amidation.11 However, in
these cases it is difficult to successively generate a N−N bond.
For example, when various amide directing groups were explored
at the initial stage of this project, only amidation but no
successive N−N bond formation occurred. We hypothesized that
increasing the electron density of the N-atom might enhance the
oxidative process. After testing N-H-benzophenone imine with
only limited success (see Supporting Information (SI)), we
tested the ethyl phenylimidate 1a, easily synthesized from
benzonitrile and ethanol,16 as the starting material in the
presence of RhIII complex as the catalyst and stoichiometric
Cu(OAc)2 as the oxidant. To our delight, the corresponding 1H-
indazole 3aa was obtained in 62% isolated yield (Table 1, entry
1). To our best knowledge, this is the first example of the use of
N-H-imidates as substrates in transition-metal-catalyzed C−H
activation and also in a cascade with N−N bond formation.17
Further optimization study indicated that the use of molecular
sieves was important to improve the efficiency (Table 1, entries 2
and 5). Temperature showed a great impact on this reaction. The
imidate 1a showed very low reactivity at 90 °C (entry 3), and
higher reaction temperature did not improve the yield (entry 4).
Using slightly more of tosyl azide 2a afforded the product in 73%
isolated yield (entry 6). To make the process greener, we further
investigated the possibility of using Cu(OAc)2 as the catalyst and
O2 as the oxidant.18 In the presence of 25 mol% of Cu(OAc)2 and
5 mol% of picolinic acid as the additive, similar yield could be
achieved under an atmosphere of O2 (entry 7). Under such
reaction conditions, adding base or acid induced lower yields
(entries 8 and 9). To our surprise, higher yield could be obtained
in the absence of picolinic acid (entry 11). Lower loading of Rh
(1.0 mol%) or Cu catalyst (10 mol%) still could give synthetically
a
For reaction conditions, see Scheme 1. The crude NMR yield is given
in parentheses when 2.1 equiv of Cu(OAc)2 was used under argon.
useful yields (entries 12 and 14). It is important to note that only
a trace amount of 3aa was detected in the absence of either Rh or
Cu catalyst (entries 13 and 15), indicating the importance of
these two catalysts. Moreover, such a reaction could be carried
out on a gram scale with similar yields and efficiency (entry 16).
With the optimized reaction conditions in hand, we first
investigated the scope of organo azides 2 (Scheme 1). A variety
of arylsulfonyl azides with different functional groups were
tested. The azides with electron-donating groups showed better
reactivity than those with electron-withdrawing groups. Various
kinds of functional groups, such as OMe, NO2, C−F, C−Cl, and
even C−Br, were well tolerated. Besides arylsulfonyl azides,
alkylsulfonyl azides (2h, 2i) also showed reactivity in this
transformation with synthetically useful yields. However, aryl
azides, which showed good reactivity in previous work,11b gave
the desired product in very low yields only.
Furthermore, the reactivity of different arylimidates 1 was
tested with 2a as the reaction partner (Scheme 2). First, the
impact of the alkyl in the alkoxy group was investigated. We
found that the ethylimidate (1a) showed better reactivity than
methyl (1b) and isopropyl (1c) derivatives. Furthermore,
different ethyl arylimidates were applied in this reaction. It was
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Journal of the American Chemical Society
Communication
Scheme 3. Mechanistic Hypothesis
found that the reaction was sensitive to the steric demand of the
arenes. Both 3-CF3 (1d) and 3-Me (1g) derivatives showed
excellent selectivity to the C−H bond with less steric hindrance.
The imidate with o-F (1j) also afforded the desired product in
52% yield. In contrast with electron-rich arylimidates, the
electron-poor imidates showed better reactivities. For example,
4-CF3 (1e) and 4-PhO (1m) derivatives gave the desired product
in 76% and 73% yield, respectively, while 4-EtO (1n) only
afforded the product in 62% yield. These reaction conditions
tolerated various kinds of functional groups on arene, including
EtO, PhO, ester, NO2, CF3, C−F, C−Cl, and even C−Br bonds,
which could be further transformed to other functionalities.
Besides the phenyl ring, naphthylimidate also showed good
reactivity in this reaction to give the product 3pa in good yield
and excellent regioselectivity.
Based on the experiments and previous mechanistic studies,
we propose the following mechanism (Scheme 3). In the
presence of AgSbF6, cationic [Cp*RhIII] (I) is generated in situ
as the active catalyst, which coordinates to imidate and further
undergoes C−H activation to afford rhodacyclic complex II.
(The KIE of 2.1 indicated that the C−H activation might be
involved in the rate-determining step.) Coordination of azide to
rhodium and further migratory insertion generate the RhIII
amido species IV with release of N2. The complex may undergo
protonation to regenerate active [Cp*RhIII] catalyst I and give
the amidated product V (see SI), which coordinate to Cu(OAc)2
or be directly transmetalated to generate complex VII. Higher
valent CuIII complex VIII might be generated through oxidation
by another Cu(OAc)2 or O2 (path A). Further N−N bond
formation through reductive elimination afforded the desired
product 3aa and CuI complex IX, which could be oxidized by O2
in the presence of acid to regenerate Cu(OAc)2 (VI). Another
pathway (path B) is also possible to form a N−N bond via double
single-electron transfer. To test this hypothesis, we tested several
radical scavengers (such as BHT, hydroquinone, and TEMPO)
and radical clock (diallyl ether) in the reaction with
stoichiometric Cu(OAc)2 (see SI). All the reactions were
significantly inhibited, which indicated that radicals might be
involved in the N−N bond formation.19
Grignard reagents generated various 3-aryl-1H-indazoles (4ca−
4cd) and 4ce in moderate to good yields (eq 4). In these cases, in
situ detosylation was followed by arylation.22 Notably, 2-
mesitylmagnesium bromide could afford the desired product in
good yield. Arylation reagents with both electron-donating
groups and electron-withdrawing groups worked well in the N-1
or C-3 position, and many functional groups, such as OMe, CF3,
and C−Cl bond, were well tolerated. The plethora of
postsynthetic modifications of the formed 1H-indazoles
significantly enhances the value of this method.
In conclusion, we have developed a novel synthesis of
substituted 1H-indazoles from easily available arylimidates and
organo azides via RhIII-catalyzed C−H activation/C−N bond
formation and Cu-catalyzed N−N bond formation. N-H-
Imidates were applied as versatile directing groups in RhIII-
catalyzed C−H activation, delivering substrates that undergo
intramolecular N−N bond formation. The process is scalable and
green, with O2 being used as the terminal oxidant and only N2
and H2O formed as the byproducts. The corresponding 1H-
indazoles were obtained in moderate to high yields with good
functional group tolerance. Moreover, the products could also be
further transformed to diverse important derivatives. Further
mechanistic studies, synthesis of drug candidates, and other
novel transformations of arylimidates are underway in our
laboratory.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
glorius@uni-muenster.de
■
Author Contributions
†D.-G.Y. and M.S. contributed equally to this work.
Notes
To demonstrate the utility of the products, we further
investigated transformations of 3aa. First, detosylation of 3aa
occurred smoothly under basic reaction conditions, giving 4a in
66% yield (eq 1).20 Moreover, in situ detosylation and further
benzylation (eq 1) and arylation (eq 2) could afford the products
in good efficiency, which gave an alternative method to construct
such structures. Second, selective cleavage of the C−O bond was
realized in different reaction conditions in the absence of
transition metals. For example, selective cleavage of the sp3 C−O
bond gave 4b in high yield under acidic reaction conditions21 (eq
3), while arylation and allylation of the sp2 C−O bond with
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Alexander von Humboldt Foundation (D.-G.Y.),
the International NRW Graduate School of Chemistry (M.S.),
the European Research Council (ERC) under the European
Community’s Seventh Framework Program (FP7 2007-2013)/
ERC grant agreement no. 25936, and the Alfried Krupp von
Bohlen und Halbach Foundation for generous financial support
(F.G.). We thank Dr. Honggen Wang, Dr. Zhuangzhi Shi,
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8805
dx.doi.org/10.1021/ja4033555 | J. Am. Chem. Soc. 2013, 135, 8802−8805