Rhodium-catalyzed oxidative annulation of hydrazines with alkynes using a nitrobenzene oxidant

Article abstract of DOI:10.1021/ol5030794

Rhodium-catalyzed oxidative annulation of hydrazines with alkynes has been accomplished using 1,3-dinitrobenzene as an oxidant. A variety of hydrazines with alkynes were converted to 1-aminoindole derivatives in good to high yields. Mechanistic investigations support the idea that 1,3-dinitrobenzene serves as the oxidant during C-H activation. This is, to our knowledge, the first report of a nitrobenzene compound used as the oxidant in transition-metal-catalyzed C-H activation.

Full text of DOI:10.1021/ol5030794

Letter
pubs.acs.org/OrgLett
Rhodium-Catalyzed Oxidative Annulation of Hydrazines with
Alkynes Using a Nitrobenzene Oxidant
Deng Yuan Li, Hao Jie Chen, and Pei Nian Liu*
Shanghai Key Laboratory of Functional Materials Chemistry, Key Lab for Advanced Materials and Institute of Fine Chemicals, East
China University of Science and Technology, Meilong Road 130, Shanghai 200237, China
S
* Supporting Information
ABSTRACT: Rhodium-catalyzed oxidative annulation of hydra-
zines with alkynes has been accomplished using 1,3-dinitroben-
zene as an oxidant. A variety of hydrazines with alkynes were
converted to 1-aminoindole derivatives in good to high yields.
Mechanistic investigations support the idea that 1,3-dinitroben-
zene serves as the oxidant during C−H activation. This is, to our
knowledge, the first report of a nitrobenzene compound used as
the oxidant in transition-metal-catalyzed C−H activation.
irect and selective C−H functionalization has emerged as
indazoles, and pyrazoles.12 As an example, Glorius reported the
rhodium-catalyzed hydrazine-directed C−H activation to afford
the indoles in the presence of HOAc.12a In our continuous
effort in heterocycle construction,13 here we report an efficient
rhodium-catalyzed oxidative annulation of hydrazines and
alkynes using a nitrobenzene compound as the sole oxidant
for C−H activation. The reaction affords a variety of 1-
aminoindole derivatives, which are easily deprotected to furnish
1-aminoindoles in good yield.
D
a powerful tool for concise, atom-economical construc-
tion of new carbon−carbon and carbon−heteroatom bonds in
modern synthetic chemistry.1 In particular, oxidative C−H
activation based on transition metal catalytic systems has been
widely explored because of the diverse transformations that it
allows.2 During this activation reaction, the transition metal is
usually reduced to a lower oxidation state. As a result,
stoichiometric or excess amounts of external oxidants must
be added in order to sustain the catalytic cycle. Metal oxidants,3
oxygen or air,4 hypervalent iodine(III) reagents,5 and peroxide6
are the most common additives. Most of these oxidants require
harsh conditions such as high temperature and pressure,
limiting the synthetic usefulness of the overall activation
reaction. Therefore, developing new external oxidants that
function under relatively mild conditions remains a goal of
synthetic chemists.
Nitrobenzene compounds, among the most important
organic materials in modern industrial chemistry,7 can act as
mild oxidants,8 though research into this activity has been
limited primarily to dehydrogenation aromatization.8c−g Re-
cently, Bouwman reported palladium-catalyzed oxidative
carbonylation of methanol using nitrobenzene as an oxidant.9a,b
More recently, Wu reported an elegant cascade reaction with
nitrobenzene as an oxidant involving cross-coupling and in situ
hydrogenation by visible light catalysis.9c To our knowledge,
whether a nitrobenzene compound can act as the sole oxidant
in transition-metal-catalyzed oxidative C−H functionalization
has never been reported.
To begin our studies of this reaction, we selected N’-
phenylacetohydrazide (1a) and 1,2-diphenylethyne (2a) as the
model substrates. We conducted the reaction in the presence of
[Cp*RhCl2]2 (5 mol %) and NaOAc (30 mol %) at 60 °C for
24 h under a nitrogen atmosphere. Interestingly, the solvent
MeNO2 led to an unexpected oxidative annulation product, N-
(2,3-diphenyl-1H-indol-1-yl)acetamide (3a), in 51% yield
(Table 1, entry 1). The identity of this product was confirmed
by single-crystal X-ray crystallography (3a). Using the solvent
PhNO2 gave the product 3a in 88% yield (entry 2), whereas
other common solvents such as MeOH, MeCN, THF, 1,2-
DCE, and toluene did not support the formation of 3a. It is
noteworthy that when HOAc (2 equiv) was added to the
reaction with 1,2-DCE as the solvent, 2,3-diphenyl-1H-indole
was obtained in 78% yield (see Supporting Information).12a
We also sought to optimize the choice of oxidant and
additive. Using PhNO2 (2.0 equiv) as the oxidant in the
reaction of N′-phenylacetohydrazide (1a) with 1,2-diphenyle-
thyne (2a) led to the product 3a in 25% yield (entry 3).
Nitrobenzenes carrying electron-donating or -withdrawing
groups on the benzene ring gave the corresponding product
3a in respective yields of 39% and 46% (entries 4 and 5). Using
the dinitro-compound 1,4-dinitrobenzene improved the yield of
3a to 50% (entry 6). To our satisfaction, using 1,3-
Transition-metal-catalyzed oxidative annulation, especially
that involving C−H activation, is synthetically quite valuable
because it can rapidly construct hetero- and carbocyclic
compounds.10 Recently, the external or internal oxidative
annulation of aromatic compounds with directing groups and
internal alkynes have been developed for the indole synthesis.11
More recently, annulation of hydrazines has attracted increasing
interest, leading to the preparation of indoles, 2,3-dihydro-1H-
Received: October 21, 2014
© XXXX American Chemical Society
A
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Letter
Table 1. Optimization of the Oxidative Annulation of
Hydrazine (1a) and Alkyne (2a) Using a Nitrobenzene
Oxidant
Scheme 1. Substrate Scope for Oxidative Annulation
Promoted by 1,3-Dinitrobenzene
a
a
entry
solvent
oxidant
additive
yield (%)
1
MeNO2
PhNO2
−
−
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
AgSbF6
CsOAc
AgOAc
Cu(OAc)2
NaOAc
NaOAc
NaOAc
51
88
25
39
46
50
92
85
71
ND
ND
74
23
26
58
75
78
2
3
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
MeOH
PhNO2
4
p-MePhNO2
5
p-ClPhNO2
6
1,4-dinitrobenzene
1,3-dinitrobenzene
1,3-dinitrobenzene
1,3-dinitrobenzene
1,3-dinitrobenzene
1,3-dinitrobenzene
1,3-dinitrobenzene
1,3-dinitrobenzene
1,3-dinitrobenzene
1,3-dinitrobenzene
1,3-dinitrobenzene
1,3-dinitrobenzene
7
8
9
MeCN
10
11
12
13
14
toluene
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
b
15
c
16
d
17
a
Reaction conditions: 1 (0.3 mmol), 2 (0.2 mmol), [Cp*RhCl2]2
(0.01 mmol), NaOAc (0.06 mmol), and 1,3-dinitrobenzene (0.4
mmol) in 1,2-DCE (1.0 mL) at 60 °C for 24−36 h under a nitrogen
a
Reaction conditions: 1a (0.3 mmol), 2a (0.2 mmol), [Cp*RhCl2]2
(0.01 mmol), additive (0.06 mmol), and oxidant (0.4 mmol) in solvent
(1.0 mL) at 60 °C for 24 h under a nitrogen atmosphere. Isolated
yields are shown. [Cp*RhCl2]2 (0.005 mmol). NaOAc (0.03 mmol)
was used. 1a (7.5 mmol), 2a (5 mmol), [Cp*RhCl2]2 (0.25 mmol),
b
atmosphere. Isolated yields are shown. Reaction time was extended to
48 h. Reaction temperature was increased to 80 °C.
b
c
c
d
NaOAc (1.5 mmol), 1,3-dinitrobenzene (10 mmol), and 1,2-DCE (25
mL) were used.
such as CF3O, CF3, or CN gave substantially lower yields of
products 3f−h. These results suggest that the catalytic reaction
prefers electron-donating and halogen substituents over
electron-withdrawing ones on the benzene ring of N′-
phenylacetohydrazide. Moreover, N′-phenylacetohydrazide
substituted with Me or Cl at the meta position reacted well
with 2a, producing the corresponding product 3i or 3j in a
respective yield of 88% or 86%.
The ortho-F substituted N′-phenylacetohydrazide also gave
the desired product 3k in a slightly lower yield of 50%. This
indicates that steric hindrance at the benzene ring can inhibit
annulation, which was confirmed when N′-phenylaceto-
hydrazide substituted with ortho-Me failed to react. Never-
theless, the reaction tolerated simultaneous Me substitutions at
both meta and para positions, affording product 3l in 55% yield.
Note that heteroaryl hydrazides such as N′-(pyridin-2-yl)aceto-
hydrazide could not react with 2a to give the desired product
under current reaction conditions.
Next we investigated hydrazines with various substitutions at
the R2 position. Placing a long alkyl chain there led to 1-
aminoindole derivatives 3m and 3n in 69% and 68% yield,
whereas a benzyl substituent at that position gave product 3o in
only 66% yield. In contrast, adding a substituted aryl to the R2
position led to no observable product. Adding a t-Bu
substitution to the R2 position similarly failed to give the
desired product. This indicates that steric hindrance at the R2
position of hydrazines can inhibit annulation.
Subsequently, we explored the scope of substituted alkynes 2
that can react with N′-phenylacetohydrazide 1a. Reaction of 1a
with diaryl alkynes bearing electron-donating groups such as
Me or MeO at the para position of the benzene ring gave the
dinitrobenzene dramatically increased the yield of 3a to 92%
(entry 7). This may be because nitrobenzenes with electron-
withdrawing groups are reduced more easily than those with
electron-donating groups. The other common oxidants that we
tested, including PhI(OAc)2, (t-BuO)2, Cu(OAc)2, and
Na2S2O8 did not improve the yield of 3a (see Supporting
Information). When the reaction was carried out under an O2
(1 atm) atmosphere, only a small amount of product 3a was
observed. Screening solvents showed that 1,2-DCE provided a
higher yield of 3a than MeOH, MeCN, and toluene (entries 8−
10). When AgSbF6 was used as the additive, product 3a was not
detected, demonstrating the essential role of the acetate anion
(entry 11). NaOAc proved to be the most effective additive,
affording 3a in 92% yield (entries 12−14). Decreasing the
loading of the catalyst or additive substantially reduced the
product yield (entries 15 and 16). The reaction proceeded
smoothly even on the gram scale, producing 3a (1.2 g) in 78%
yield (entry 17).
After defining the optimal reaction conditions for oxidative
annulation promoted by 1,3-dinitrobenzene, we investigated
the substrate scope (Scheme 1). First, we examined various
substituted hydrazines for their ability to react with 1,2-
diphenylethyne 2a and form 1-aminoindole derivatives. N′-
Phenylacetohydrazides with electron-donating groups such as
Me or MeO at the para position of the benzene ring gave the
corresponding product 3b or 3c in a respective yield of 90% or
93%. N′-Phenylacetohydrazides carrying halogens such as F or
Cl also gave good yields of product 3d or 3e. However, N′-
phenylacetohydrazides bearing electron-withdrawing groups
B
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Letter
corresponding product 3p or 3q in a respective yield of 94% or
83%. Diaryl alkynes with halogens such as F or Cl also gave the
desired product 3r or 3s in a good yield. Similarly, 1,2-
di(thiophen-2-yl)ethyne reacted well with 1a, giving the
product 3t in 83% yield. In addition to symmetric alkynes,
some unsymmetrical alkynes also underwent the transformation
to give the desired products 3u−z in 60−87% yields. For
phenyl acetylenes substituted with various alkyl species,
including alkyl species with OH groups, only one regioisomer
was isolated in their reactions with 1a.
To further explore the flexibility of oxidative annulation
promoted by 1,3-dinitrobenzene, we tried to react N′-
phenylacetohydrazide 1a with various substituted 1,3-diynes.
Diaryl- or dialkyl-substituted 1,3-diynes reacted well to give the
corresponding products 3a′−e′ in good yields. Asymmetrical
1,3-diynes such as 5-phenylpenta-2,4-diyn-1-ol also gave the
desired product 3f′ in 62% yield. Single-crystal X-ray diffraction
analysis of 3c′ confirmed the regioselectivity of the oxidative
annulation of 1,3-diynes.14
into traceable 3-nitroaniline and other complicated products.
Moreover, when PhNO (3 equiv) was used instead of 1,3-
dinitrobenzene, the annulaiton product 3a was almost
undetectable even after 48 h. These results suggest that
nitrosobenzene is not a suitable oxidant for this annulation.
They also suggest that 1,3-dinitrobenzene might be reduced to
1-nitro-3-nitrosobenzene during the reaction, which is in turn
converted into complicated products such as 3-nitroaniline.
On the basis of the above experiments and previous studies
of hydrazines with alkynes that undergo reactions catalyzed by
[Cp*RhCl2]2,12 we propose a tentative mechanism for
oxidative annulation promoted by 1,3-dinitrobenzene (Scheme
4). First, the active catalyst [Cp*Rh(OAc)2] forms from the
Scheme 4. Proposed Mechanism of Oxidative Annulation
Promoted by 1,3-Dinitrobenzene
1-Aminoindoles are an important subfamily of indoles,
displaying a broad spectrum of biological activities, and their
synthesis has attracted increasing attention in recent years.15
Using simple deprotection of the acetyl group under acidic
conditions, we easily converted the oxidative annulation
products 3a and 3a′ to 1-aminoindoles 4 and 5 in respective
yields of 95% and 94%. This demonstrates the usefulness of our
approach for synthesizing 1-aminoindoles (Scheme 2, eq 1).12c
Scheme 2. Deprotection of Oxidative Annulation Products
To Afford 1-Aminoindoles
To clarify the role of 1,3-dinitrobenzene in this annulation,
we tried to capture byproducts formed from 1,3-dinitroben-
zene. When hydrazine 1a and alkyne 2a reacted under optimal
reaction conditions, the product of 1,3-dinitrobenzene
reduction, 3-nitroaniline, was isolated in 29% yield (Scheme
3, 1). When the same reaction was performed in the absence of
[Cp*RhCl2]2/NaOAc catalytic system, and it directly promotes
C−H activation of hydrazine 1a. The resulting arene rhodation
intermediate A undergoes alkyne coordination and insertion
with alkyne 2a, giving the seven-membered rhodacycle B. The
metallacycle rearranges to a more stable six-membered rhodium
complex C, which subsequently undergoes reductive elimi-
nation to release the annulation product 3a and [Cp*Rh(I)]. In
contrast, rhodium complex C could also undergo the internal
oxidation to release the 2,3-diphenyl-1H-indole by N−N bond
cleavage in the presence of a stoichiometric amount of HOA.12a
Finally, external oxidant 1,3-dinitrobenzene reoxidizes [Cp*Rh-
(I)] to regenerate the active catalyst [Cp*Rh(OAc)2].
Scheme 3. Mechanistic Investigation of Oxidative
Annulation Promoted by 1,3-Dinitrobenzene
In summary, we have achieved an efficient rhodium-catalyzed
oxidative annulation of hydrazines with alkynes under mild
conditions, using 1,3-dinitrobenzene as a new oxidant in C−H
activation. The catalytic reactions afforded 1-aminoindole
derivatives in good to excellent yields, and these products
could be further deprotected to generate 1-aminoindoles.
Mechanistic investigation demonstrated that 1,3-dinitrobenzene
served as the oxidant during the reaction, consuming the
leaving hydrogen atoms. This protocol may open the door for
using nitrobenzene oxidants in transition-metal-catalyzed C−H
functionalization.
2a, 1,3-dinitrobenzene was recovered intact and no 3-
nitroaniline was detected, excluding the possibility that
hydrazine 1a reduces 1,3-dinitrobenzene and further confirming
that the 1,3-dinitrobenzene was the oxidant for the annulation
to produce 3a. Consistent with this interpretation, reacting 1a
and 2a under optimal reaction conditions using 1-nitro-3-
nitrosobenzene as the oxidant gave a lower 38% yield of 3a
(Scheme 3, 2). Reacting 1a with 1-nitro-3-nitrosobenzene
without 2a led to the conversion of 1-nitro-3-nitrosobenzene
C
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Organic Letters
Letter
(g) Yang, F.; Rauch, K.; Kettelhoit, K.; Ackermann, L. Angew. Chem.,
Int. Ed. 2014, 53, 11285.
(6) (a) Leskinen, M. V.; Madaras
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, characterization data, H and 13C
NMR spectra for all compounds, and X-ray crystallographic
data for 3a and 3c′. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
́
́
z, A.; Yip, K.-T.; Vuorinen, A.;
1
Papai, I.; Neuvonen, A. J.; Pihko, P. M. J. Am. Chem. Soc. 2014, 136,
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6453. (b) Girard, S. A.; Knauber, T.; Li, C.-J. Angew. Chem., Int. Ed.
2014, 53, 74. (c) Duan, P.; Yang, Y.; Ben, R.; Yan, Y.; Dai, L.; Hong,
M.; Wu, Y.-D.; Wang, D.; Zhang, X.; Zhao, J. Chem. Sci. 2014, 5, 1574.
(d) Tang, H.; Qian, C.; Lin, D.; Jiang, H.; Zeng, W. Adv. Synth. Catal.
2014, 356, 519. (e) Zhang, L.; Qiu, R.; Xue, X.; Pan, Y.; Xu, C.; Wang,
D.; Wang, X.; Xu, L.; Li, H. Chem. Commun. 2014, 50, 12385.
(7) (a) Hunger, K. Industry Dyes: Chemistry, Properties and
Applications; Wiley-VCH: Weinheim, 2003. (b) Vogt, P. F.; Gerulis,
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: liupn@ecust.edu.cn.
Notes
(8) (a) Smith, L. T.; Lyons, R. E. J. Am. Chem. Soc. 1926, 48, 3165.
(b) Ong, J. H.; Castro, C. E. J. Am. Chem. Soc. 1977, 99, 6740.
(c) Charris, J.; Camacho, J.; Ferrer, R.; Lobo, G.; Barazarte, A.;
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
Gamboa, N.; Rodrigues, J.; Lopez, S. J. Chem. Res. 2006, 2006, 769.
This work was supported by NSFC/China (Project Nos.
21172069, 21372072, 21421004, and 21190033), NCET
(NCET-13-0798), the Basic Research Program of the Shanghai
Committee of Sci. & Tech. (Project No. 13NM1400802), and
the Fundamental Research Funds for the Central Universities.
(d) Tojo, M.; Fukuoka, S.; Tsukube, H. J. Mol. Catal. A: Chem. 2011,
337, 89. (e) Li, J.; Jiao, C.; Huang, K.-W.; Wu, J. Chem.Eur. J. 2011,
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