Site-Selective C–H Amidation of Azobenzenes with Dioxazolones under Rhodium Catalysis

Article abstract of DOI:10.1002/ejoc.201601096

The rhodium(III)-catalyzed amidation reaction of azobenzenes with dioxazolones is described. This strategy allows the facile and efficient construction of highly substituted ortho-amidated azobenzenes by direct C–H cleavage approach. A wide range of substrates, excellent levels of chemoselectivity as well as high functional-group tolerance were observed. In addition, this protocol was used to generate an array of ortho-amidated ketazines. Further synthetic transformation of amidated azobenzenes furnished a facile construction of benzimidazole and benzotriazole derivatives.

Full text of DOI:10.1002/ejoc.201601096

A Journal of
Accepted Article
Title: Site-Selective C-H Amidation of Azobenzenes with Dioxazolones Under Rho-
dium Catalysis
Authors: In Su Kim; Neeraj Kumar Mishra; Yongguk Oh; Mijin Jeon; Sangil Han;
Satyasheel Sharma; Sang Hoon Han; Sung Hee Um
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201601096
Link to VoR: http://dx.doi.org/10.1002/ejoc.201601096
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European Journal of Organic Chemistry
10.1002/ejoc.201601096
COMMUNICATION
Site-Selective C–H Amidation of Azobenzenes with Dioxazolones
Under Rhodium Catalysis
[
a]
[a]
[a]
[a]
[a]
Neeraj Kumar Mishra, Yongguk Oh, Mijin Jeon, Sangil Han, Satyasheel Sharma, Sang Hoon
[
a]
[b]
[a]
Han, Sung Hee Um,* and In Su Kim*
Abstract: The rhodium(III)-catalyzed amidation reaction of
azobenzenes with dioxazolones is described. This strategy allows
the facile and efficient construction of highly substituted ortho-
amidated azobenzenes via direct C–H cleavage approach. A range
of substrate scope, excellent levels of chemoselectivity as well as
high functional group tolerance were observed. In addition, this
protocol was used to generate an array of ortho-amidated ketazines.
Further synthetic transformation of amidated azobenzenes furnished
a facile construction of benzimidazole and benzotriazole derivatives.
the direct C−N bond formation on azobenzenes has been rarely
[
8]
explored. Lee reported the efficient synthesis of 2-aryl-2H-
benzotriazoles via the Rh(III)-catalyzed amidation of
azobenzenes with sulfonyl azides followed by oxidation
[
8a]
[8b]
[8c]
reaction.
Jia
and Xu
also described a very similar
protocol, which is the Rh(III)-catalyzed coupling reaction
between azobenzenes and organic azides. Patel demonstrated
a
facile approach for the construction of 2-aryl-2H-
benzotriazoles via the Pd(II)-catalyzed C−N bond formations of
[
8d]
3
azobenzenes with TMSN .
In continuation of our efforts towards the C–H
functionalization of azobenzenes, we herein reported the
rhodium(III)-catalyzed C–H amidation reaction of azobenzenes
Transition-metal-catalyzed C−N bond formation reaction through
C−H bond activation is among the most important topic in
organic synthesis due to the prevalence of nitrogen-containing
bioactive molecules. In this area, various coupling partners,
such as N-carboxylates, N-tosylates, N-
[9]
with
azobenzenes.
dioxazolones
to
afford
various
ortho-amidated
[
1]
[10]
In addition, this protocol can be successfully
applied to the C–H amidation of ketazines. Furthermore,
synthetic transformation of amidated azobenzenes showed
facile access to benzimidazole and benzotriazole derivatives.
fluorobenzenesulfonimide (NFSI), organic azides and etc, have
been used as relevant surrogates for the C−H amination or
[
2]
amidation reactions. Recently, dioxazolones have been also
2
applied to direct C−H amidation reactions of C(sp )−H and
3
[3]
C(sp )−H bonds. For example, Chang described seminal
literatures on the C−H amidation reaction of various arenes with
dioxazolones as new amidating reagents under Rh(III) and
[
3a–c]
Co(III) catalysis.
Li demonstrated the Rh(III)-catalyzed direct
3
amidation of C(sp )−H bonds in 8-methylquinolines using
dioxazolone derivatives.
[
3d]
[3e]
[3f]
In addition, Jiao,
Glorius,
respectively
[
3g,h]
[3i]
[3j]
[3k]
Ackermann,
Li,
Zhu
and Sundararaju
disclosed the C−H amidation reactions with dioxazolones using
Rh(III) and Co(III) catalysts.
Azobenzenes have emerged crucial organic molecules
[
4]
owing to the prevalence in dye industries and material science.
Because of the unique electronic characteristic of conjugated
azo motifs, aromatic azo compounds have been recently used in
supermolecular recognition, photochemical molecular switches,
[
5]
biosensors, pharmaceuticals, and so forth. With a pioneering
work on ortho-C–H chlorination of azobenzenes reported by
[
6]
Fahey in 1971, azobenzenes have been intensively studied in
the transition-metal-catalyzed C–H functionalization. However,
Scheme 1. C–H Amidation Reactions of Azobenzenes.
[
7]
Our investigation was initiated by examining the coupling of
(E)-1,2-diphenyldiazene (1a) to 3-phenyl-1,4,2-dioxazol-5-one
[
[
a]
b]
Dr. N. K. Mishra, Y. Oh, M. Jeon, S. Han, Dr. S. Sharma, S. H. Han,
Prof. Dr. I. S. Kim
School of Pharmacy
Sungkyunkwan University
Suwon 16419, Republic of Korea
E-mail: insukim@skku.edu
(
1
2a) (Table 1). We were delighted to see the coupling between
a and 2a in the presence of [RhCp*Cl and AgSbF in DCE
2
]
2
6
solvent at ambient temperature providing the desired product 3a
in 45% yield (entry 1). Control experiments revealed that cationic
rhodium catalyst and acetate additive is found to be important
for this transformation (entries 2 and 3). Screening of silver
http://pharmasyn.skku.edu/
Prof. Dr. S. H. Um
Department of Molecular Cell Biology
Samsung Biomedical Research Institute, Sungkyunkwan University
School of Medicine
Suwon 16419, Republic of Korea
E-mail: shum@skku.edu
additive and solvents showed that AgNTf
additive and solvent for the formation of our desired product 3a
entries 4–10). Further investigation of acetate additive showed
that NaOAc is found to be superior than KOAc, AgOAc and
2
and DCE are optimal
(
Supporting information for this article is given via a link at the end of
the document.
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European Journal of Organic Chemistry
10.1002/ejoc.201601096
COMMUNICATION
Cu(OAc)
NaOAc to 20 mol-% provided the decreased formation of 3a in
4% yield (entry 14). Cationic Ru(II) and Co(III) catalysts were
found to be ineffective in this coupling reaction (entries 15 and
6). Finally, it should be mentioned that reaction temperature is
quite important to undergo the coupling reaction between 1a and
a in good yield (entries 17 and 18).
2
(entries 11–13). In addition, increasing amount of
substituted azobenzenes 1e–1g with electron-withdrawing
groups was found to be less reactive under the optimal reaction
conditions. For an example, 1e was coupled with 2a to afford 3e
in 17% yield. After further optimization, we were pleased to find
that an increasing amount (3 equiv.) of 2a at 120 C under
otherwise identical conditions displayed significantly increased
4
o
1
2
formation of 3e in 90% yield. Above conditions were applied to
1f and 1g to afford the corresponding products 3f and 3g in 91%
and 54% yields, respectively. Moreover, ortho-substituted
azobenzenes also participated in the C–H amidation reaction to
deliver the corresponding products 3h–3l in moderate go high
yields. This reaction was also compatible with para-substituted
azobenzenes 1m–1s, and all reactions preferentially furnished
mono-amidated products as a single regioisomer.
[
a]
Table 1. Selected Optimization for Reaction Conditions
[
b]
entry
catalyst
additive (mol-%)
solvent
yield
*
1
2
3
4
5
6
7
8
9
[RhCp Cl
2
2
2
2
2
]
]
]
]
]
2
2
2
2
2
AgSbF
6
6
(10)
DCE
DCE
DCE
DCE
DCE
DCE
THF
45
*
[RhCp Cl
AgSbF
(10), NaOAc (10)
61
*
[RhCp Cl
NaOAc (10)
N.R.
59
*
[RhCp Cl
AgPF
6
4
(10) , NaOAc (10)
(10), NaOAc (10)
*
[RhCp Cl
AgBF
58
*
[RhCp Cl
2
]
2
AgNTf
2
(10), NaOAc (10)
(10), NaOAc (10)
(10), NaOAc (10)
(10), NaOAc (10)
(10), NaOAc (10)
(10), KOAc (10)
(10), AgOAc (10)
70
*
[RhCp Cl
2
]
]
]
]
]
]
]
]
2
AgNTf
AgNTf
AgNTf
AgNTf
AgNTf
AgNTf
AgNTf
AgNTf
AgNTf
AgNTf
AgNTf
AgNTf
2
20
*
[RhCp Cl
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
MeCN
EtOAc
MeOH
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
trace
34
*
[RhCp Cl
*
10
11
12
13
14
15
16
17
18
[RhCp Cl
8
*
[RhCp Cl
58
*
[RhCp Cl
64
Scheme 2. Scope of Symmetrical Azobenzenes. [a] Reaction conditions: 1b–
*
[RhCp Cl
(10), Cu(OAc)
2
(10)
60
*
1
s (0.2 mmol), 2a (0.3 mmol), [RhCp Cl
]
2 2
(2.5 mol-%), AgNTf
2
(10 mol-%),
NaOAc (10 mol-%), DCE (1 mL) under air at room temperature for 12 h in
*
[RhCp Cl
(10), NaOAc (20)
(10), NaOAc (10)
(10), NaOAc (10)
(10), NaOAc (10)
(10), NaOAc (10)
44
pressure tubes. [b] Isolated yield by flash column chromatography. [c] 2a (0.6
o
mmol, 3 equiv.) was used at 120 C.
[
[
[
[
c]
d]
e]
f]
[Ru(p-Cy)Cl
]
2 2
N.R.
N.R.
62
2
[CoCp*(CO)I ]
*
[RhCp Cl
2
]
2
2
*
[RhCp Cl
2
]
55
[
a] Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), catalyst (2.5 mol-%),
additive (quantity noted), solvent (1 mL) under air at room temperature for 12 h
in pressure tubes. [b] Isolated yield (%) by flash column chromatography. [c] p-
o
o
Cy = p-cymene. [d] 5 mol-% of cobalt catalyst was used. [e] 50 C. [f] 80 C.
With the optimized reaction conditions in hand, the substrate
scope of symmetrical azobenzenes was examined (Scheme 2).
The coupling of 3-phenyl-1,4,2-dioxazol-5-one (2a) and meta-
substituted azobenzenes 1b–1d with electron-donating groups
Scheme 3. Scope of Unsymmetrical Azobenzenes. [a] Reaction conditions:
(
Me, Et and OMe) was found to smoothly undergo our desired
*
1t–1v (0.2 mmol), 2a (0.3 mmol), [RhCp Cl
]
2 2
(2.5 mol-%), AgNTf
2
(10 mol-%),
products 3b–3d in good to high yields. However, meta-
NaOAc (10 mol-%), DCE (1 mL) under air at room temperature for 12 h in
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European Journal of Organic Chemistry
10.1002/ejoc.201601096
COMMUNICATION
o
pressure tubes. [b] 2a (0.6 mmol, 1
3 equiv.) was used at 120 C. [c]
Finally, we were delighted that 3-alkyl-substituted dioxazolones
Regioisomeric raio was determined by H NMR analysis of crude mixture.
2l–2n were also coupled with 1c to give alkyl amides 4l–4n.
To extend the scope of our protocol, we envisioned the C–H
amidation reaction of other N−N bond containing compounds, i.e.
Next, the coupling reaction of unsymmetrical azobenzenes
t and 1u containing electron-donating and electron-withdrawing
ketazines, which are recently used in the C–H functionalization
1
[11]
events.
Gratifyingly, ketazine 5a was smoothly reacted with
groups (Me and CF ) with 2a was investigated under standard
3
dioxazolones to furnish ortho-amidated ketazines 6a–6c in high
reaction conditions, as shown in Scheme 3. The results revealed
that the C–H amidation reaction predominantly occurred on the
electron-rich aromatic ring to provide 3tb and 3ua as major
products, respectively. In addition, when azobenzene 1v
containing sterically different aromatic ring was subjected under
standard reaction conditions, a single product 3v was obtained
in 50% yield. This data suggests that the steric environment of
azobenzene is also important to tune the regioselectivity of C–H
functionalization.
yields (Scheme 5).
Scheme 5. Amidation of Ketazines with Dioxazolones.
To show the practicality of this transformation, we performed
a scale-up experiment using 3 mmol of 1c, and obtained 0.66 g
of 3c in 62% isolated yield (Scheme 6). To highlight the
synthetic utility of amidated azobenzenes, hydrolysis conditions
were first subjected to 3a and 3c affording 2-amino-
azobenzenes 7a and 7b in high yields. Further transformation of
synthesized amino products 7a and 7b was carried out.
Benzimidazole derivative 8a was successfully formed by use of
formaldehyde in the presence of AcOH. In addition, treatment of
7b with PhI(OAc)
2
provided 2-aryl-2H-benzotriazole 8b in 52%
yield. Moreover, 7b was readily converted to bis-azobenzene 8c
[
12]
in 92% yield.
Scheme 4. Scope of Dioxazolones. [a] Reaction conditions: 1c (0.2 mmol),
*
2
b–2n (0.3 mmol), [RhCp Cl
]
2 2
(2.5 mol-%), AgNTf
2
(10 mol-%), NaOAc (10
mol-%), DCE (1 mL) under air at room temperature for 12 h in pressure tubes.
o
[
b] Isolated yield by flash column chromatography. [c] 100 C.
To further examine the substrate scope of this process, a
range of dioxazolones was screened to couple with azobenzene
1
2
c (Scheme 4). The reactions between 1c and dioxazolones
b–2j with either electron-rich or electron-deficient groups at
para- and meta-positions of phenyl ring were found to be
favored in the amidation reaction to afford the corresponding
products 4b–4j in good to high yields. Particularly noteworthy
was the tolerance of nitro or bromo groups as versatile
functionalities for further transformation. It should be mentioned
that the second C–H amidation on benzamide ring of products
4b–4f were not detected. This observation is in contrast to a
previous literature for the second C–H amidation occurring on
[
3d]
para-substituted benzamides reported by Li.
In addition, this
Scheme 6. Scale-up Experiment and Transformation.
transformation was compatible with heterocycle-containing
dioxazolone 2k to give our desired product 4k in 90% yield.
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European Journal of Organic Chemistry
10.1002/ejoc.201601096
COMMUNICATION
To get mechanistic insight, parallel reactions of 1a and
deuterio-1a with 2a under standard reaction conditions were
performed, which provided the kinetic isotope effect of 1.07
ketazines. Further transformation of amidated azobenzenes led
to facile access to benzimidazole and benzotriazole derivatives.
(
Scheme 7), thus suggesting that C−H bond cleavage might be
[
13]
not involved in the rate-determining step.
Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP)
(
Nos. 2016R1A4A1011189 and 2015R1A2A1A15053033).
Keywords: amidation • azobenzenes • C–H functionalization •
dioxazolones • rhodium
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[
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14]
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In conclusion, we have disclosed the rhodium(III)-catalyzed
C–H amidation reaction of azobenzenes with dioxazolones. This
protocol has been applied to a wide range of substrates, and
typically proceeds with excellent levels of chemoselectivity as
well as with high functional group tolerance. Additionally, this
protocol allows the generation of an array of ortho-amidated
[7]
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[
[
3
a
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European Journal of Organic Chemistry
10.1002/ejoc.201601096
COMMUNICATION
Entry for the Table of Contents
Key Topic: C–H Amidation
COMMUNICATION
Neeraj Kumar Mishra, Yongguk Oh,
Mijin Jeon, Sangil Han, Satyasheel
Sharma, Sang Hoon Han, Sung Hee
Um,* and In Su Kim*
Page No. – Page No.
Site-Selective C–H Amidation of
Azobenzenes with Dioxazolones
Under Rhodium Catalysis
The rhodium(III)-catalyzed amidation reaction of azobenzenes with dioxazolones is
described. This strategy allows the facile and efficient construction of highly
substituted ortho-amidated azobenzenes via direct C–H cleavage approach.
This article is protected by copyright. All rights reserved