Rhodium-catalyzed direct amination of arenes with nitrosobenzenes: A new route to diarylamines

Article abstract of DOI:10.1002/chem.201400221

A Rh^III-catalyzed addition of aryl C-H bonds to nitrosobenzenes, followed by cleavage of the resulting hydroxylamines in situ, has been reported. Different directing groups, such as N-based heterocycles and ketoximes, can be used in this C-H amination process, providing valuable diarylamines in excellent yields. Most importantly, this process provides a new method for attaching arylamine groups to aromatic rings. A Rh ^III-catalyzed addition of aryl C-H bonds to nitrosobenzenes, followed by cleavage of the resulting hydroxylamines in situ, is reported (see scheme). Different directing groups (DG), such as N-based heterocycles and ketoximes, can be used in this C-H amination process, providing valuable diarylamines in excellent yields. Most importantly, this process provides a new method for attaching arylamine groups to aromatic rings.

Full text of DOI:10.1002/chem.201400221

DOI: 10.1002/chem.201400221
Full Paper
&
CÀH Activation
Rhodium-Catalyzed Direct Amination of Arenes with
Nitrosobenzenes: A New Route to Diarylamines
Juanjuan Du, Yaxi Yang, Huijin Feng, Yuanchao Li,* and Bing Zhou*[a]
Abstract: A RhIII-catalyzed addition of aryl CÀH bonds to ni-
trosobenzenes, followed by cleavage of the resulting hy-
droxylamines in situ, has been reported. Different directing
groups, such as N-based heterocycles and ketoximes, can be
used in this CÀH amination process, providing valuable dia-
rylamines in excellent yields. Most importantly, this process
provides a new method for attaching arylamine groups to
aromatic rings.
Introduction
Diarylamines are ubiquitous structural motifs in numerous bio-
logically active natural products, pharmaceuticals, dyes, agro-
chemicals, and functional materials.[1] Metal-mediated cross-
coupling reactions of aryl (pseudo)halides with anilines has
been developed as a powerful tool for the synthesis of diaryla-
mines.[2] However, these methods require time-consuming and
tedious prefunctionalization of substrates. With the increasing
interest in transition-metal-catalyzed functionalization of CÀH
bonds, the development of a direct intermolecular amination
of arene CÀH bonds with anilines is highly desirable.[3]
Recent studies have demonstrated that two strategies have
been successfully developed for metal-catalyzed direct CÀH
amination. The first strategy is to employ parent amines in the
presence of external oxidants.[4] An alternative strategy is the
use of preactivated amine precursors.[5] However, these reac-
tions are, in most cases, limited to amide or alkylamine cou-
pling partners.[4,5] Only a few examples involving the formation
of diarylamines have been reported, with narrow substrate
scope and low efficiency (Scheme 1a).[4c] Moreover, these reac-
tions usually generate stoichiometric by-products from the ex-
ternal oxidants,[4] or halide salts.[5] Recently, a Rh-catalyzed CÀH
amination of arenes with aromatic azides has been reported to
provide diarylamines (Scheme 1b).[6] Although this approach is
highly promising, there is a safety risk in the handling of
azides, especially when the reactions are carried out on a large
scale. Given the prevalence of diarylamines and the limitation
of previous methods,[4c,6] further development of novel catalyt-
ic arene CÀH amination reactions, to enable the formation of
Scheme 1. The formation of diarylamines by means of CÀH activation.
valuable diarylamines (especially diarylamines that can be syn-
thesized from readily available amine sources), would be
highly desirable and of prime synthetic value.
Recently, we developed a Rh-catalyzed addition of aryl CÀH
bonds to electron-withdrawing nitrosobenzenes to give N,N-
diaryl hydroxyl amines, which could be reduced in the pres-
ence of Zn/AcOH to afford diarylamines (Scheme 1c).[7a] Very
recently, we disclosed a RhIII-catalyzed aryl CÀH amidation with
N-hydroxycarbamates under oxidative conditions, giving
access to N-carbamate-protected arylamines.[7b] Inspired by
these results, and other RhIII-catalyzed aryl CÀH bond function-
alizations,[8–12] we herein report a Rh-catalyzed aryl CÀH addi-
tion to nitrosobenzenes, under an air atmosphere, followed by
cleavage of the resulting hydroxylamines in situ to produce
valuable and versatile diarylamines in excellent yields
(Scheme 1d). Significantly, different directing groups, such as
pyridine, pyrimidine, pyrazole, and ketoxime, can be employed
in this CÀH amination reaction, and various synthetically im-
[a] Dr. J. Du, Dr. Y. Yang, H. Feng, Dr. Y. Li, Dr. B. Zhou
Shanghai Institute of Materia Medica
Chinese Academy of Sciences
555 Zu Chong Zhi Road, Shanghai 201203 (China)
Fax: (+86)21-50807288
E-mail: ycli@mail.shcnc.ac.cn
zhoubing2012@hotmail.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201400221.
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portant functional groups (e.g., halogen, ester, and aldehyde
moieties) are tolerated.
and 1-AdCO2H proved to be particularly effective (entry 9). Of
the different solvents tested (entries 9–12) PhCl was found to
be the best solvent for the reaction (entries 9). Control experi-
ments revealed that the RhIII complex is essential and that the
addition of water did not affect the yield (entries 13–14).
With the optimized reaction conditions in hand, the sub-
strate scope, with respect to 2-arylpyridines, was subsequently
explored and the results are summarized in Scheme 2. 2-Aryl-
Results and Discussion
2-Phenylpyridine (1a) and methyl 4-nitrosobenzoate (2a) were
chosen as the model substrates (Table 1). When 1a was treated
with [RhCp*(CH3CN)3](SbF6)2 (2.5 mol%, Cp*=pentamethylcy-
clopentadienyl and 2a in PhCl, at 1008C for 10 h under air, the
Table 1. Optimization of Rh-catalyzed CÀH amination reaction.[a]
Entry
Catalyst
Additive
Solvent
3
4
[2.5 mol%]
[%] [%]
1
2
3
4
5
6
7
8
(Cp*RhCl2)2
(Cp*RhCl2)2/AgSbF6
[Cp*Rh(CH3CN)3](SbF6)2
–
–
–
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
DCE
0
15
35
0
0
5
4
0
0
4
0
3
0
4
4
5
0
0
[Cp*Rh(CH3CN)3](SbF6)2 Cs2CO3
[Cp*Rh(CH3CN)3](SbF6)2 PivOK
[Cp*Rh(CH3CN)3](SbF6)2 Cu(OAc)2
[Cp*Rh(CH3CN)3](SbF6)2 AcOH
[Cp*Rh(CH3CN)3](SbF6)2 PivOH
[Cp*Rh(CH3CN)3](SbF6)2 1-AdCO2H
[Cp*Rh(CH3CN)3](SbF6)2 1-AdCO2H
[Cp*Rh(CH3CN)3](SbF6)2 1-AdCO2H
[Cp*Rh(CH3CN)3](SbF6)2 1-AdCO2H
0
30
45
70
86
50
45
49
0
9
10
11
12
13
14
THF
CH3OH
PhCl
PhCl
–
1-AdCO2H
[Cp*Rh(CH3CN)3](SbF6)2 1-AdCO2H
H2O (5 equiv)
85
[a] 1a (0.2 mmol), 2a (0.24 mmol), catalyst (0.005 mmol), additive
(0.2 mmol), and solvent (1 mL) at 1008C for 10 h. Yield of isolated prod-
uct. 1-AdCO2H=1-Adamantane carboxylic acid; PivOH=pivalic acid;
DCE=1,2-dichloroethane.
Scheme 2. Substrate scope. Reaction conditions: [RhCp*(CH3CN)3](SbF6)2
(2.5 mol%), substrate 1 (0.2 mmol), 2a (0.24 mmol), 1-AdCO2H (0.2 mmol),
and PhCl (1 mL), for 10 h at 1008C. Yields of isolated product are shown.
desired diarylamine, 3, was obtained in 35% yield, possibly re-
sulting from cleavage of N,N-diaryl hydroxylamine
4
(entry 3).[13] The structure of 3 was confirmed by X-ray crystal-
lography (Figure 1).[14] No product was observed with the addi-
tion of a base (entries 4 and 5). In contrast, the addition of an
acid gave a dramatic increase in the reaction yield (entries 7–9)
pyridines bearing both electron-donating and electron-with-
drawing groups were viable substrates for this amination reac-
tion, giving the corresponding
diarylamine products (5–14) in
good to excellent yields. meta-
Substituted derivatives under-
went this amination reaction
only at the sterically more acces-
sible CÀH bond (11–13). In par-
ticular, the 2-methyl derivative
smoothly afforded the corre-
sponding product (14) in excel-
lent yield, thus indicating high
steric tolerance of this system.
Moreover, the tolerance of vari-
Figure 1. X-ray crystallographic structure of 3.
ous functional groups, such as
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ester (7), chloro (8 and 13), bromo (9), and nitrile (10) groups,
afforded a great opportunity for further functionalization. In
addition, we were delighted to find that heterocyclic sub-
strates (15) proved to be reactive.
We next evaluated various pyridinyl directing groups. Elec-
tron-rich and electron-poor pyridine rings were well tolerated
(16–20) and the influence of the steric bulk on the pyridine
ring was minor for this CÀH amination reaction (16–18 and
20). Notably, benzo[h]quinolone (21) and other heterocycles,
such as pyrimidine (22 and 23) and pyrazole (24), also worked
well as effective directing groups.
high to excellent yields, emphasizing the generality of this
method. In a similar manner to the reactions of 2-arylpyridines,
a variety of functional groups (e.g., ester (3), halogen (30, 31,
and 35), and even the electrophilic carboxaldehyde (32)
groups) were compatible with this amination reaction, allow-
ing for high diversity in the synthesis of diarylamines.
Several experiments have been carried out to probe the
mechanism of this reaction. Firstly, a cyclometalated RhIII com-
plex, 36, can catalyze this amination reaction with an efficiency
that is comparable to the method described herein, suggesting
the relevancy of CÀH activation [Eq. (1)]. Next, a series of con-
trol experiments were carried out on the conversion of hydrox-
ylamine 4 into 3 [Eqs. (2), (3), and (4)]. Firstly, compound 4 was
heated under air for 10 h and afforded 3 with 25% conversion
[Eq. (2)]. Addition of 1-AdCO2H did not improve the conver-
sion, suggesting an acid-promoted reaction mechanism is
likely irrelevant [Eq. (3)]. A dramatic increase in reaction yield
(90%) was observed when [Cp*Rh(CH3CN)3](SbF6)2 was added,
suggesting that a RhIII complex promoted the conversion
[Eq. (4)].
Notably, in addition to N-based heterocycles, more common
and synthetically useful ketoximes also reacted well as a direct-
ing group to facilitate the amination reaction (Scheme 3). Inter-
Scheme 3. Ketoxime substrate scope. Reaction conditions: [RhCp*(CH3CN)3]-
(SbF6)2 (2.5 mol%), ketoxime (0.2 mmol), 2a (0.24 mmol), 1-AdCO2H
(0.2 mmol), and PhCl (1 mL), for 10 h at 1008C. Yields of isolated product are
shown.
estingly, on using an oxime group as the directing group, an
anilino unit was selectively inserted at the ortho-position of the
oxime group, followed by in situ deprotection to afford 2-anili-
noacetophenones (25–28).
The scope of nitrosobenzenes was then examined
(Scheme 4). In particular, not only electron-poor nitrosoben-
zenes (29–32), but also electron-neutral (33) and even elec-
tron-rich (34) nitrosobenzenes gave the desired diarylamines in
In view of the results presented above, we tentatively de-
duced the following mechanism (Scheme 5). The reaction of 2-
phenylpyridine with the rhodium catalyst, and subsequent
ortho-directed CÀH bond activation, results in rhodacycle A.[15]
Then, coordination and subsequent nucleophilic addition (or
N=O insertion) takes place to afford Rh species C. Species C is
protonated to produce the hydroxylamine 4 and the RhIII cata-
lyst. As previously reported,[13] nitroxide D is generated from 4
and oxygen. This transformation proceeds more quickly in the
presence of the RhIII complex.[16] Nitroxide D may undergo di-
merization and subsequent fragmentation to give two aminyl
radicals E and molecular oxygen, in a similar manner to the di-
merization of peroxyl radicals. Finally, hydrogen abstraction
from the solvent, or from 4, delivers diarylamine 3.
Scheme 4. Nitrosobenzene substrate scope. Reaction conditions: [RhCp*-
(CH3CN)3](SbF6)2 (2.5 mol%), substrate 1a (0.2 mmol), 2 (0.24 mmol), 1-
AdCO2H (0.2 mmol), and PhCl (1 mL), for 10 h at 1008C. Yields of isolated
product are shown. [a] [RhCp*(CH3CN)3](SbF6)2 (5 mol%) and PhCl (0.4 mL)
were used at 1308C for 15 h.
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The reaction mixture was stirred at 1008C for 10 h, then EtOAc
(6 mL) was added. Volatiles were removed under reduced pressure
and the crude product was purified by column chromatography
on silica gel.
Acknowledgements
Support of this work by the National Natural Science Founda-
tion of China (21302200) is gratefully acknowledged.
Keywords: CÀH activation
nitrosobenzenes · rhodium
·
diarylamines
·
isatin
·
Scheme 5. Proposed mechanism (Ar=4-COOMe-phenyl).
In order to demonstrate the strength of our methodology,
an experiment was carried out on a larger scale [Eq. (5)]. Satis-
fyingly, by using only 1 mol% catalyst loading, the present
amination reaction could be carried out with excellent efficien-
cy to provide a diarylamine product, which can be used in flu-
orescent dyes.[17] In addition, diarylamines accessible by this CÀ
H amination reaction have other synthetic utilities.[18] For ex-
ample, 2-anilinoacetophenone (25) can be easily converted
into isatin (37) and acridone (38) in one step [Eq. (6)].
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Conclusion
In summary, we have developed an unprecedented Rh-cata-
lyzed intermolecular addition of aryl CÀH bonds to nitrosoben-
zenes, followed by cleavage of the resulting hydroxylamines in
situ, to produce valuable and versatile diarylamines, which
have versatile synthetic and medicinal utility. This reaction is
compatible with air and water and can be carried out in the
absence of external oxidants, thus providing an environmental-
ly benign and straightforward synthesis of diarylamines that
can be readily scaled-up. More importantly, this process may
provide a new direction for attaching arylamine groups to aro-
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tion
[Cp*Rh(MeCN)3](SbF6)2 (2.5 mol%), 1-adamantane carboxylic acid
(0.2 mmol), 2-phenylpyridine 1a (0.2 mmol), methyl p-nitrosoben-
zoate (0.24 mmol), and PhCl (1 mL) were added to a sealed tube.
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Received: January 19, 2014
Published online on && &&, 0000
Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
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CÀH Activation
J. Du, Y. Yang, H. Feng, Y. Li,* B. Zhou*
&& – &&
Rhodium-Catalyzed Direct Amination
of Arenes with Nitrosobenzenes: A
New Route to Diarylamines
A RhIII-catalyzed addition of aryl CÀH
bonds to nitrosobenzenes, followed by
cleavage of the resulting hydroxyla-
mines in situ, is reported (see scheme).
Different directing groups, such as N-
based heterocycles and ketoximes, can
be used in this CÀH amination process,
providing valuable diarylamines in ex-
cellent yields. Most importantly, this
process provides a new method for at-
taching arylamine groups to aromatic
rings.
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Chem. Eur. J. 2014, 20, 1 – 6
www.chemeurj.org
6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!