One-Pot Synthesis of 2-Aminobenzophenones from 2-Alkynyl Arylazides Catalyzed by Pd and Cu Precursors

Article abstract of DOI:10.1002/ejoc.202100772

We describe a novel one-pot three-step reaction of 2-alkynyl arylazides through palladium-catalyzed formation of 3-hydroxy-3-phenylindolin-2-ones followed by hydrolysis of amide bonds and copper-catalyzed decarboxylation to give 2-aminobenzophenones. This synthetic method works well with various 2-alkynyl arylazides and affords the products in moderate to good yields under mild reaction conditions.

Full text of DOI:10.1002/ejoc.202100772

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: One-Pot Synthesis of 2-Aminobenzophenones from 2-Alkynyl
Arylazides Catalyzed by Pd and Cu Precursors
Authors: Fan Yang, Shijie Xu, Hui Fan, Xuechun Zhao, and Xiaoxiang
Zhang
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202100772
Link to VoR: https://doi.org/10.1002/ejoc.202100772
01/2020

10.1002/ejoc.202100772
European Journal of Organic Chemistry
COMMUNICATION
One-Pot Synthesis of 2-Aminobenzophenones from 2-Alkynyl
Arylazides Catalyzed by Pd and Cu Precursors
Fan Yang,[a] Shijie Xu,[a] Hui Fan,[a] Xuechun Zhao[a] and Xiaoxiang Zhang*[a]
[a]
F. Yang, S. Xu, H. Fan, X. Zhao, Prof. Dr. X. Zhang
Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, College of Chemical Engineering, Nanjing Forestry University,
Nanjing 210037, People’s Republic of China
E-mail: zhangxiaoxiang@njfu.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: We describe a novel one-pot three-step reaction of 2-
alkynyl arylazides through palladium-catalyzed formation of 3-
hydroxy-3-phenylindolin-2-ones followed by hydrolysis of amide
bonds and copper-catalyzed decarboxylation to give 2-
aminobenzophenones. This synthetic method woks well with various
2-alkynyl arylazides and affords the products in moderate to good
yields under mild reaction conditions.
arylation of 2-(phenylsulfonamido)benzoic acids generated in situ
from nthranilic acids and arylsulfonyl chlorides (Scheme1, b). [10b]
In the same year, Wu’s group described the formation of 2-
aminobenzophenones by the ortho-C-H bond aroylation of
anilines containing a 4-methoxy-2-pyridinyl directing group, which
[10c]
could be removed after two simple steps (Scheme1, c).
Although much progress has been achieved, the development of
efficient methods for the synthesis of 2-aminobenzophenones
from novel starting materials is still highly desired.
Introduction
The 2-aminobenzophenone skeleton was considered as one of
the most important organic structures, which could be found in
many
pharmaceuticals.
aminobenzophenone moiety such as Nepafenac, [1a] Amfenac, [1a]
bioactive
compounds,
As shown in Figure 1, molecules with 2-
natural
products
and
[1]
[1a]
[1a]
[1b]
Bromfenacum,
Bromodomain inhibitor,
CKD516
and
[1c]
Bradykinin B1 receptor antagonist
have shown excellent
bioactivities. Moreover, this prevalent core structure is widely
used as highly versatile synthetic building blocks for the
preparation of quinolines, [2] indoles [3] and benzodiazepines [4] as
[5]
well as advanced organic materials. Therefore, a variety of
synthetic
strategies
for
the
construction
of
2-
aminobenzophenones have been established. [6-10] Among those
reported methods, 2-aminobenzophenones could be generally
accessed from various starting materials such as anilines, [6] 2-
[8]
[9]
aminobenzonitriles, [7] 2-iodoanilines
and anthranilic acids.
Besides, several novel substrates were also employed for the
preparation of 2-aminobenzophenones. [10] For example, Zhu
and co-workers reported an efficient transformation of 2-
arylindoles to 2-aminobenzophenones via a dearomatic C-C and
C-N bond cleavage in the presence of oxygen (Scheme1, a). [10a]
In 2019, Zhu’s group disclosed a photoredox deoxygenative
Scheme 1. Synthetic methods to 2-aminobenzophenones.
Recently, various transformations of α-imino metal carbenes
generated in situ from 2-alkynyl arylazides have been studied by
us [11] and other groups. [12] In 2020, we reported the synthesis of
imidazoloindolines by palladium-catalyzed one-pot cycloaddition
reactions of thioureas with 3H-indol-3-ones generated in situ from
[11a]
2-alkynyl arylazides.
Later, 3-hydroxy-2-oxindoles were
obtained by our group via acyloin rearrangements of 2-hydroxy-
indolin-3-ones generated in situ from 2-alkynyl arylazides in the
absence of thioureas. [11b] As a part of our ongoing interest on
transformation of 2-alkynyl arylazides, we envisaged that 2-
aminobenzophenones could be formed through one-pot
hydrolysis / decarboxylation of 3-hydroxy-2-oxindoles generated
in situ from 2-alkynyl arylazides (Scheme 1, d).
Figure 1. Bioactive molecules with 2-aminobenzophenone moiety.
Results and Discussion
1
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10.1002/ejoc.202100772
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COMMUNICATION
At the outset of this study, 1-azido-2-(phenylethynyl)benzene 1a
(1 eq.) and TsOH·H2O 2 (3 eq.) were selected as the model
substrates (Table 1). The reaction containing 1a (0.1 mmol), 2 (3
eq.) and Pd(OAc)2 (5 mol%) was conducted in 1,4-dioxane (1 mL)
at 90℃ for 30 min followed by the addition of Cs2CO3 (3.5 eq.) for
another 30 minutes. Then, CuI (20 mol%) was added to the above
reaction solution and stirred for 18 hours. The desired product (2-
aminophenyl)(phenyl)methanone 3a was obtained in 52% yield
(entry 1). The structure of 3a was then confirmed by the analysis
of 1H and 13C NMR. Later, other copper catalysts were
investigated under the same reaction conditions. It was noted that
CuCl2, Cu(OAc)2, Cu(OTf)2, Cu2O and CuBr2 could afford the
product 3a in either lower yields or trace amounts (entries 2-6).
Considering the solubility of starting materials, DMSO and DMF
instead of 1,4-dioxane were employed as more polar solvents in
this reaction, which gave the corresponding product 3a in yields
of 8% or 9%, respectively (entries 7-8). To our surprise, the
product yield and reaction time were obviously improved when
NaOH (2.0 eq.) was added as an additive, which gave the best
results. The corresponding product 3a was obtained in 70% yield
after 4 hours (entry 9). In this reaction, the addition of NaOH could
accelerate the hydrolysis of 3-hydroxy-2-oxindoles generated in
situ from 1-azido-2-(phenylethynyl)benzene 1a.
an electron-withdrawing group on the aromatic ring such as F, Cl,
and CF3 were all tolerable and provided the corresponding
product 3d-3i in moderate to excellent yields. Next, reactions of -
alkynyl arylazides with substituted groups on the alkyne carbon
(R2 = 4-MePh, 4-EtPh, 4-ClPh, 4-(n-C5H11) Ph and 3-FPh) were
also checked under the standard reaction conditions. The desired
products 3j-3n were obtained in 47-74% yields. The substrates
with substituted groups R1 and R2 were also efficient to give the
corresponding products 3o-3u in yields of 51-71%. The substrate
with a hetero-aromatic group (R2 = 2-thiophen) was also tested,
which led to the formation of 3v in 54% yield. In addition, that
reaction of substrate with an ester functional group resulted in the
generation of 3w in a lower yield of 35%, which might be the
Table 2. Substrates scope of the 2-alkynyl arylazides 1. [a] [b]
Table 1. Optimization of the reaction conditions. [a]
Entry
Catalyst
Time [h]
Solvent
Yield/%[b]
1
CuI
18
18
18
18
18
18
18
18
4
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
DMSO
52
38
trace
37
50
30
8
2
CuCl2
Cu(OAc)2
Cu(OTf)2
Cu2O
CuBr2
CuI
3
4
5
6
7
8
CuI
DMF
9
9[c]
CuI
1,4-dioxane
70
[a] Reaction conditions: 1a (0.1 mmol), Pd(OAc)2 (5 mol%), 2 (3 equiv.), solvent
(1 mL), Cs2CO3 (3.5 equiv.), catalyst (20 mol%). [b] Isolated yield. [c] NaOH (2
equiv.).
When the best optimized reaction conditions obtained in hand,
we next explored the generality of the one-pot reactions of 2-
alkynyl
arylazides
1
for
the
preparation
of
2-
aminobenzophenones 3. As shown in Table 2, various aryl or alkyl
substituted 2-alkynyl arylazides 1a-1x were checked under the
standard reaction conditions. First, reactions of 2-alkynyl
arylazides bearing electron-withdrawing or electron-donating
groups on the aromatic ring (R1) 1a-1w were examined. It was
found that the substrates with an electron-donating group (R1 = 4-
Me or 5-Me) gave the desired products 3b-3c in lower yields of
42% and 38%, respectively. However, the starting materials with
[a] Reaction conditions: 1 (0.1 mmol), Pd(OAc)2 (5 mol%), 2 (3 equiv.), 1,4-
dioxane (1 mL), Cs2CO3 (3.5 equiv.), CuI (20 mol%), NaOH (2 equiv.). [b]
Isolated yield.
2
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10.1002/ejoc.202100772
European Journal of Organic Chemistry
COMMUNICATION
reason of partial hydrolysis of ester group under the basic reaction
conditions. Unfortunately, repeating this reaction with a steric
hindered group installed to the substrate (R2 = t-Bu), did not give
the desired product 3x. And only the acyloin rearrangement
The amide bond of I was then hydrolysed to generate J. After a
copper-catalyzed decarboxylation of J , the active copper species
K was formed, which was then oxidized to product 3a and
regenerated the copper catalyst. [18] In this proposed mecahnism,
product 3-(tert-butyl)-3-hydroxyindolin-2-one was obtained in 76% the water involved in the hydrolysis process might come from the
yield.
In order to understand the reaction mechanism, several control
crystal water of TsOH·H2O and/or neutralization reaction.
experiments were then examined as depicted in Scheme 2. First,
the reaction of 1-azido-2-(phenylethynyl)benzene 1a was tested
under
a nitrogen atmosphere, which did not afford the
corresponding product 3a (Scheme 2a). Next, a radical scavenger
such as TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) was used
to repeat the reaction. It was found that the product 3a could be
still obtained in 23% yield, which was dramatically affected by the
radical scavenger (Scheme 2b). Therefore, a radical process
might be involved in the reaction. Replacing the weak base
Cs2CO3 with NaOH led to the formation of product 3a in quite low
yield of 39% (Scheme 2c). When the reaction of 3-hydroxy-3-
phenylindolin-2-one 4a was proceeded in the presence of NaOH
(2 eq.) and CuI (20 mol%), the product 3a was also obtained in
44% yield (Scheme 2d).
Scheme 3. Proposed mechanism.
Conclusion
In conclusion, we have disclosed a novel one-pot, three steps
reaction from 2-alkynyl arylazides for the preparation of 2-
aminobenzophenones. The corresponding products were
obtained in moderate to good yields under mild reaction
conditions. In this reaction, 2-alkynyl arylazides are used as
simple starting materials, which can be easily accessed. Further
applications of this strategy are currently underway and will be
reported in future.
Acknowledgements
Scheme 2. Control Experiments.
This work was supported by the National Natural Science
Foundation of China (21302096), the Natural Science Foundation
of Jiangsu Province (BK20171449), the Qing Lan Project, the
National First-class Disciplines (PNFD), China, and the Priority
Academic Program Development of Jiangsu Higher Education
Institutions (PAPD), China. We are also grateful for support from
the advanced analysis and testing center of Nanjing Forestry
University.
Based on the above experimental results and our previous work,
[11] we tentatively propose a possible mechanism in Scheme 3 for
the transformation of 1a to (2-aminophenyl)(phenyl)methanone
3a, although it is highly speculative. The 1-azido-2-
(phenylethynyl)benzene 1a was initially activated by palladium
catalyst to generate α-imino Pd carbene C, [13] which could be
attacked by TsOH·H2O to give complex D. Next, a rearrangement
of D led to intemediate E [11d] and the palladium catalyst was
regenerated. In the presence of Cs2CO3, E was converted to F
after a 1,3-Ts shift process.[15] Although F was unstable, it was still
Keywords: Azides • 2-Aminobenzophenones • Decarboxylation •
Hydrolysis • One-pot
detected by TLC and LC-MS in our previous work.[11f]
[16]
Subsequently, a reductive desulfonation of F
afforded 2-
phenyl-3H-indol-3-one G. [11f,14] Then, a hydrolysis of C=N double
bond of G led to the formation of 2-hydroxy-2-phenylindolin-3-one
H, which was followed by an acyloin rearrangement[17] to give I.[11e]
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4
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COMMUNICATION
Entry for the Table of Contents
A novel one-pot three-step synthetic method to 2-aminobenzophenones from 2-alkynyl arylazides has been disclosed. This reaction is
catalyzed by palladium to form 3-hydroxy-3-phenylindolin-2-ones, which is followed by hydrolysis of amide bonds and copper-catalyzed
decarboxylation to generate 2-aminobenzophenones. The desired products are afforded in moderate to good yields under mild reaction
conditions.
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