Copper-catalyzed C(sp2)-H amidation with azides as amino sources

Article abstract of DOI:10.1021/ol502010g

A copper-catalyzed C-H amidation process, with azides as amino sources under oxidant-free conditions, has been developed. When N-heterocycles were employed as directing groups, sulfonylazide and benzoylazide could be used as amidating reagents to provide corresponding N-arylamides. When amidines or imine were used, tandem C-N/N-N bond formation occurred to afford indazole derivatives in one pot.

Full text of DOI:10.1021/ol502010g

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed C(sp2)−H Amidation with Azides as Amino Sources
Jiangling Peng, Zeqiang Xie, Ming Chen, Jian Wang, and Qiang Zhu*
State Key Laboratory of Respiratory Disease, Guangzhou Institutes of Biomedicine and Health, Chinese Academy of Sciences, 190
Kaiyuan Avenue, Guangzhou 510530, China
S
* Supporting Information
ABSTRACT: A copper-catalyzed C−H amidation process,
with azides as amino sources under oxidant-free conditions,
has been developed. When N-heterocycles were employed as
directing groups, sulfonylazide and benzoylazide could be used
as amidating reagents to provide corresponding N-arylamides.
When amidines or imine were used, tandem C−N/N−N bond
formation occurred to afford indazole derivatives in one pot.
catalyzing sulfonylamidation of indoles with corresponding
sulfonylazides (Scheme 1).19
romatic C−N bond formation is one of the most important
Aresearch topics in organic synthesis due to the ubiquity of
aniline derivatives in numerous molecules of biological
importance.1 Buchwald−Hartwig2 and Ullmann-type3 amina-
tions are among the most reliable and widely used methods for its
formation, which require prefunctionalized reactants. Therefore,
extensive efforts have been devoted to transition-metal-catalyzed
direct C(sp2)−H amination/amidation reactions with higher
step-efficiency and atom-economy. In general, there are two
major strategies for C(sp2)−H activation/C−N bond forma-
tion.4 One employs neutral amines/amides in the presence of
external oxidants,5 while the other uses preactivated amino
sources, such as N-chloroamines,6 N-hydroxycarbamates,7 O-
acylhydroxylamines,8 nitrosobenzenes,9 and NFSI,10 under
oxidant-free conditions. However, in most of these processes,
generation of stoichiometric byproducts derived from either
oxidants or preactivated amino reagents cannot be avoided.
In recent years, azides have emerged as powerful amino
sources in transition-metal-catalyzed direct amination/amidation
of aromatic C−H bonds due to the readily availability, structural
diversity of azides, and environmental benignity of the reaction
byproduct, gaseous N2.11 In 2007, Driver reported a seminal
rhodium-catalyzed intramolecular C−H amination reaction with
vinylazides to construct N-heterocycles.12 Later, the same group
applied a similar catalytic system in cycloamination of C−H
bonds starting from arylazides for carbozole and indole
synthesis.13 In 2012, a breakthrough in intermolecular C−H
amination/amidation with sulfonylazides14 and arylazides15 was
realized by Chang and co-workers using cationic Cp*Rh as a
catalyst assisted by a directing group. Shortly after, they extended
the reaction to benzyl- and alkylazides16 as coupling partners.
Very recently, the work by Chang demonstrated the power of
azides as amino sources in amination of sp2 as well as sp3
hybridized C−H bonds in the presence of Cp*Ir under mild
conditions.17 Many other groups including Ackermann, Sahoo,
and Jiao reported their contributions in C−H amination/
Scheme 1. C(sp2)−H Amination/Amidation with Azides
Copper catalysts have been widely employed in selective C−H
functionalization.20 Given cost and sustainability concerns, the
development of related processes employing less toxic and
inexpensive copper catalysts as surrogates of other transition-
metal catalysts, such as Pd, Rh, and Ir, is still attractive. As a result,
copper mediated/catalyzed dehydrogenative cross-coupling of
amines/amides with aromatic C−H bonds has emerged as a
straightforward strategy for the synthesis of aniline derivatives.21
For example, Shen and co-workers recently reported a copper
catalyzed C−H amidation with phthalimide as an amino
source.22 Herein, we would like to report the capability of
inexpensive copper salt in catalyzing C(sp2)−H amidation with
azides as amino sources.23 N-Arylamides are accessible by using
N-heterocycles as directing groups, while indazole derivatives are
obtained by applying acyclic N−H-containing amidines or imine
as directing groups via tandem C−N/N−N bond formation
(Scheme 1). In this process, a Cu(III) rather than a copper-
nitrenoid24 intermediate is proposed. Acid played an important
role in this catalytic transformation.
amidation with azides using Ru-based catalytic systems.18
A
Received: July 9, 2014
recent work by Kanai showed that Cp*Co was also capable of
Published: September 5, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
a
We initiated the study by reacting 2-phenylpyridine 1a with
TsN3 2a in the presence of various copper salts (entries 1−4,
Table 1). To our delight, the desired amidated product 3a was
Scheme 2. Scope of C−H Amidation with Azides
a
Table 1. Optimization of the Reaction Conditions
additive
(equiv)
time
(h)
yield
b
entry catalyst (equiv)
solvent
(%)
1
Cu(TFA)2 (0.2)
Cu(OAc)2 (0.2)
CuTc (0.2)
DCB
DCB
DCB
DCB
DCB
DCB
DCB
DCB
DCE
DMSO
DCB
DCB
30
30
30
30
30
30
30
48
48
48
28
24
36%
nr
2
3
27
nr
4
CuCl2 (0.2)
Cu(TFA)2 (0.2)
CuCl2 (0.5)
CuCl (0.5)
5
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
PivOH (0.4)
PivOH (0.6)
42
38
52
69
54
nr
6
7
8
CuTc (0.2)
9
CuTc (0.2)
10
11
12
CuTc (0.2)
CuTc (0.3)
76
78
CuTc (0.3)
a
Reaction conditions: 1a (0.20 mmol), 2a (0.4 mmol), copper salt,
b
additive, solvent (1 mL), Ar, sealed tube, 110 °C. Yield of isolated 3a,
DCB = 1,2-dichlorobenzene. DCE = 1,2-dichloroethane. CuTc =
copper(I) thiophene-2-carboxylate.
obtained in 36% yield by using Cu(TFA)2·xH2O (20 mol %) as a
catalyst in 1,2-dichlorobenzene under Ar atmosphere at 110 °C.
The presence of PivOH as an additive can improve the yield
slightly to 42% (entry 5). It was further confirmed that PivOH
played an essential role in this amidation process in the case of
CuCl2 as a catalyst (entries 4 and 6). Screening of various
cuprous species in the presence of PivOH revealed that CuTc
was superior to other Cu catalysts, delivering 3a in 69% yield
(entries 5−8). Then, a number of solvents, including DCE, 1,4-
dioxane, MeOH, DMF, and DMSO, were screened, and no
better results were obtained than DCB (entry 9, and the
Supporting Information). Increasing the loading of CuTc to 30
mol % and PivOH to 0.6 equiv further improved the yield of 3a to
78% in a shorter reaction time (24 h, entry 12).
With the optimized reaction conditions in hand, the scope of
substituents on both the phenyl and the pyridyl rings of 2-
arylpyridines 1 was investigated in reactions with TsN3 (Scheme
2). A methyl group on the C5 of the pyridyl moiety had little
influence on the tosylamidation reaction, providing tosylamide
3b in 76% yield. Substituents with varied electronic properties
located at the para-position of the phenyl ring were studied. In
general, electron-donating groups, such as t-Bu (3c), Ph (3d),
and MeO (3e), were preferable to electron-withdrawing ones
including Cl (3g), F (3h), and CF3 (3i) in terms of isolated
yields. When 2-(m-tolyl)pyridine was subjected to the reaction,
the less sterically hindered C−H bond was amidated selectively
to deliver 3j as the sole regioisomer. On the contrary, the reacting
site was switched to the more steric hindered C−H bond
exclusively in the case of 3-(pyridin-2-yl)phenol as a substrate,
giving 3k in 58% yield. The regioselectivity was probably due to
the coordination of the phenolic OH with the copper catalyst.25
Benzo[h]quinoline reacted smoothly with TsN3 to give the
corresponding product 3l in good yield. Other heterocycles
a
Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), CuTc (30 mol %),
PivOH (0.6 equiv), DCB (1.0 mL), Ar, sealed tube, 110 °C, yield of
b
isolated 3. One equiv of Cu(TFA)2 was used.
including pyrazole and cyclic amidine could also act as directing
groups in this Cu-catalyzed C−H amidation reaction, furnishing
3m and 3n in 58% and 51% yields, respectively. The scope of
arylsulfonylazide was then briefly tested. Similar yields were
obtained for NO2 (3p) and OMe (3q) substituted arylsulfony-
lazides, suggesting that a broad range of arylsulfonylazides could
be used as amino sources in the current tosylamidation reaction.
It was notable that alkylsulfonylazide was an excellent class of
sulfonylamidating reagent, as exemplified by the high-yielding
formation of 3r. Other organic azides were also tested.
Optimized yield (62%) of 4-methyl-N-(2-(pyridin-2-yl)phenyl)-
benzamide 3s was obtained by reacting 4-methylbenzoyl azide
with 1a under modified conditions. However, phenylazide and
benylazide failed to deliver the corresponding amination
products (3t and 3v).
Cu-catalyzed oxidative coupling of two NH groups to form a
new N−N bond, either intermolecularly or intramolecularly, has
been well documented.26 It is hypothesized that when N−H-
containing moieties replace N-heterocycles as directing groups in
the aforementioned C−H amidation reactions with azides, the
preexisted NH will couple with the introduced NH intra-
molecularly in the presence of copper catalyst. During this
process, new C−N and N−N bonds will be formed sequentially
in one pot starting from simple substrates, providing an efficient
approach to N-heterocycles. To test the hypothesis, a one-pot
reaction between N-(tert-butyl)benzimidamide 4a and tosylazide
2a was studied. Satisfyingly, the desired heterocyclic product, N-
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dx.doi.org/10.1021/ol502010g | Org. Lett. 2014, 16, 4702−4705

Organic Letters
Letter
(tert-butyl)-1-tosyl-1H-indazol-3-amine 5a, was generated in
61% yield under slightly modified conditions (CuTc (30 mol %),
HFIP (1.0 equiv) in DCB at 115 °C under argon) (for details, see
the Supporting Information). In this case, HFIP acted as a
superior acidic additive to PivOH. Recently, Glorius and Jiao
reported one-pot tandem C−N/N−N bond-forming reactions
for the synthesis of 1H-indazoles and pyrido[1,2-b]indazoles,
respectively.27
Scheme 4. Mechanistic Studies
Next, the generality of this Cu-catalyzed tandem C−N/N−N
bond-forming method was tested (Scheme 3). N-tert-Butylar-
Scheme 3. Cu-Catalyzed Synthesis of 1H-Indazoles with
a
TsN3
Scheme 5. Plausible Mechanism
a
Reaction conditions: 4 (0.2 mmol), TsN3 (0.3 mmol), CuTc (30 mol
%), HFIP (1.0 equiv), DCB (1.5 mL), Ar, sealed tube, 115 °C, yield of
isolated 5. HFIP = 1,1,1,3,3,3-hexafluoro-2-propanol. nr = no reaction.
protonated azide with concurrent releasing of N2. The following
C−H bond activation and C−Cu bond formation yields a Cu(III)
intermediate C. When N-heterocycles are used as directing
groups, the corresponding amidated products 3 and Cu(I) are
generated upon reductive elimination. In the case of R1 = H,
intermediate D chelated with Cu(I) can be oxidized again by
another molecule of TsN3 in the presence of acid. Reductive
elimination of the Cu(III) intermediate E delivers 1H-indazole 5
and Cu(I) as a result of intramolecular NH/NH oxidative
coupling.
In summary, we have developed a Cu-based reaction system
for the conversion of aromatic C−H to C−N bonds with azides
as amino sources in the presence of acid. Several N-heterocycles
can act as chelation groups to assist the key C−H activation step
with sulfonylazides or acylazide, forming secondary anilides
containing an ortho-N-heterocycle under oxidant-free conditions.
The synthesis of substituted 1H-indazoles from easily available
arylimidines/imines with tosylazide via Cu-catalyzed tandem C−
N and N−N bond formation is also developed. A plausible
reaction mechanism involving oxidation of Cu(I) to Cu(III) by
azide in the presence of acid is also proposed. The exploration of
related transformations and detailed mechanistic studies are
currently underway in our laboratories.
ylamidine substrates 4 bearing Me, MeO, Cl, or Br reacted well
with TsN3 to provide corresponding N-tert-butyl-3-amino-
indazoles 5b−5f in 34−61% yields. Unfortunately, both tosyl-
protected and free benzimidamide did not produce the desired
product under the current conditions. 3-Phenylated 1H-indazole
5g was obtained in a synthetically useful yield with
diphenylmethanimine as the substrate. It was notable that
increasing the amount of TsN3 to 2.0 equiv deteriorated the yield
of 5, probably due to decomposition of the amidine substrates by
the reaction byproduct, TsNH2.
In the process of exploring the scope for the synthesis of 1H-
indazoles, an unexpected formation of 2-(p-tolyl)-1H-benzo[d]-
imidazole 7 was observed by employing 4-methyl-N-phenyl-
benzimidamide 6 as a substrate in inert atmosphere (a, Scheme
4). Furthermore, when N-(3-(pyridin-2-yl)phenyl)acetamide 8
was subjected to the standard reaction conditions, 2-methyl-7-
(pyridin-2-yl)benzo[d]oxazole 9 was formed without any
formation of the desired tosylamidated product. The preferred
intramolecular C−N or C−O bond formation suggested that a
Cu(III) species rather than a reactive copper-nitrenoid inter-
mediate might be involved in these processes.28 TsN3 probably
acted as an oxidant of Cu(I) in the presence of acid, and TsNH2
was isolated as a byproduct in both cases.
ASSOCIATED CONTENT
* Supporting Information
On the basis of mechanistic studies and literature reports,29
a
■
S
possible reaction mechanism was proposed in Scheme 5. First, in
the presence of acid, a higher valent Cu(III) intermediate B is
generated through two electrons transfering from Cu(I) to
Experimental procedures and full analytical data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Organic Letters
Letter
(13) (a) Shen, M.; Leslie, B. E.; Driver, T. G. Angew. Chem., Int. Ed.
2008, 47, 5056. (b) Stokes, B. J.; Jovanovic, B.; Dong, H.; Richert, K. J.;
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(14) Kim, J.-Y.; Park, S.-H.; Ryu, J.; Cho, S. H.; Kim, S. H.; Chang, S. J.
Am. Chem. Soc. 2012, 134, 9110.
AUTHOR INFORMATION
Corresponding Author
■
*(Q.Z.) E-mail: zhu_qiang@gibh.ac.cn
Notes
(15) Ryu, J.; Shin, K.; Park, S. H.; Kim, J. Y.; Chang, S. Angew. Chem.,
Int. Ed. 2012, 51, 9904.
The authors declare no competing financial interest.
(16) Shin, K.; Baek, Y.; Chang, S. Angew. Chem., Int. Ed 2013, 52, 8031.
(17) (a) Ryu, J.; Kwak, J.; Shin, K.; Lee, D.; Chang, S. J. Am. Chem. Soc.
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ACKNOWLEDGMENTS
We are grateful to the National Science Foundation of China
(21202167) for financial support of this work.
■
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