A Novel Approach to N-Tf 2-Aryl-2,3-Dihydroquinolin- 4(1H)-ones via a Ligand-Free Pd(II)-Catalyzed Oxidative Aza-Michael Cyclization

Article abstract of DOI:10.1002/ejoc.202001425

2-Aryl-2,3-dihydroquinolin-4(1H)-ones have recently been identified as important structures with potent biological activities such as antitumor and antidiabetic effect. Herein, a total of 25 novel N-Tf 2-aryl-2,3-dihydroquinolin-4(1H)-ones were expediently synthesized via the oxidative aza-Michael cyclization of N-Tf-2′-aminodihydrochalcones by ligand-free palladium(II) catalysis. This study presents a new synthetic approach to yield N-Tf 2-aryl-2,3-dihydroquinolin-4(1H)-ones, which can be easily transformed into pharmacologically interesting aza-flavanones and other N-heterocycles, such as quinolines and tetrahydroquinolines, in yields up to 84 %. This methodology has various advantages, which includes short reaction times under mild conditions and suitable functional group tolerance. Furthermore, a plausible mechanism was proposed and demonstrated by kinetic analysis.

Full text of DOI:10.1002/ejoc.202001425

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: A Novel Approach to N-Tf 2-Aryl-2,3-Dihydroquinolin-4(1H)-
ones via a Ligand-Free Pd(II)-Catalyzed Oxidative Aza-Michael
Cyclization
Authors: Young Min Kim, Hyung-Seok Yoo, Seung Hwan Son, Ga
Yeong Kim, Hyu Jeong Jang, Dong Hwan Kim, Soo Dong
Kim, Boyoung Y. Park, and Nam-Jung Kim
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001425
Link to VoR: https://doi.org/10.1002/ejoc.202001425
01/2020

10.1002/ejoc.202001425
European Journal of Organic Chemistry
COMMUNICATION
A Novel Approach to N-Tf 2-Aryl-2,3-Dihydroquinolin-4(1H)-ones
via a Ligand-Free Pd(II)-Catalyzed Oxidative Aza-Michael
Cyclization
Young Min Kim†,[a] Hyung-Seok Yoo†,[a] Seung Hwan Son,[a] Ga Yeong Kim,[a] Hyu Jeong Jang,[a] Dong
Hwan Kim,[a] Soo Dong Kim,[b] Boyoung Y. Park,*[b] and Nam-Jung Kim*[a, b]
[a]
Y. M. Kim, H.-S. Yoo, S. H. Son, G. Y. Kim, H. J. Jang, D. H. Kim, Prof. N.-J. Kim
College of PharmacyDepartment
Kyung Hee University
26 Kyungheedae-ro, Dongdaemun-gu, Seoul 02447, Republic of Korea
E-mail: kimnj@khu.ac.kr
http://chembiol.khu.ac.kr/
[b]
S. D. Kim, Prof. B. Y. Park, Prof. N.-J. Kim
Department of Life and Nanopharmaceutical Sciences
Kyung Hee University
26 Kyungheedae-ro, Dongdaemun-gu, Seoul 02447, Republic of Korea
E-mail: boyoungy.park@khu.ac.kr
[†]
These authors contributed equally.
Supporting information for this article is given via a link at the end of the document.
Abstract: 2-aryl-2,3-dihydroquinolin-4(1H)-ones have recently been
identified as important structures with potent biological activities
such as antitumor and antidiabetic effect. Herein, a total of 25 novel
can be transformed into various nitrogen-bearing heterocycles,
such as tetrahydroquinoline, quinoline and quinolone, which are
key motifs in active pharmaceutical ingredients.6 Thus, several
synthetic transformations have been developed over a number
of years for the prevalence of 2-aryl-2,3-dihydroquinolin-4(1H)-
ones. Intermolecular condensation reactions, such as Mannich
reaction or aldol condensation, between o-aminoacetophenones
and aryl aldehydes have been well known to promote the
formation of 2-aryl-2,3-dihydroquinolin-4(1H)-ones in a one-pot
fashion.7 The majority of these cascade reactions relies upon the
use of organocatalysts or Lewis acid catalysts with relatively
long reaction times. Alternatively, the intramolecular cyclization
of 2′-aminochalcones has also been developed in the presence
of Lewis acid catalysts,8 acids,9 bases,10 or ionic liquids.11 These
Michael addition type of reactions yield in moderate to good
yields but often require harsh reaction conditions. In addition, 2′-
N-Tf
2-aryl-2,3-dihydroquinolin-4(1H)-ones
were
expediently
synthesized via the oxidative aza-Michael cyclization of N-Tf-2′-
aminodihydrochalcones by ligand-free palladium(II) catalysis. This
study presents a new synthetic approach to yield N-Tf 2-aryl-2,3-
dihydroquinolin-4(1H)-ones, which can be easily transformed into
pharmacologically interesting aza-flavanones and other N-
heterocycles, such as quinolines and tetrahydroquinolines, in yields
up to 84%. This methodology has various advantages, which
includes short reaction times under mild conditions and suitable
functional group tolerance. Furthermore, a plausible mechanism was
proposed and demonstrated by kinetic analysis.
Flavonoids are a common class of natural products and their
synthetic analogs, frequently serving as pharmaceutical agents
with numerous biological activities.1 Recently, 2-aryl-2,3-
dihydroquinolin-4(1H)-ones which are also known as aza-
flavanones in which the N is substituted for an O in the
flavanone structure, have been considered novel privileged
structures found in a variety of antitumor2 and antidiabetic3
agents, cross-species microRNA inhibitors,4 and PARP inhibitors
(Figure 1).5 Furthermore, 2-aryl-2,3-dihydroquinolin-4(1H)-ones
Figure 1. Examples of biologically active 2-aryl-2,3-dihydroquinolin-4(1H)-
ones.
Scheme 1. Synthetic approach to N-Tf 2-aryl-2,3-dihydroquinolin-4(1H)-ones.
1
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10.1002/ejoc.202001425
European Journal of Organic Chemistry
COMMUNICATION
sequence for the synthesis of flavanones (Scheme 1).12 Based
on our previous study, we herein report a novel approach to
synthesize N-Tf 2-aryl-2,3-dihydroquinolin-4(1H)-ones from
manageable N-Tf-2′-aminodihydrochalcones via Pd(II)-catalyzed
dehydrogenation followed by intramolecular aza-Michael
addition. In particular, the attractive features of this method
include the in situ formation of 2′-aminochalcones for 1,4-
addition using Pd(TFA)2 without ligands under mild conditions.
Initial experiments involving the reaction of N-protected-1-(2-
aminophenyl)-3-phenylpropanone (N-PG-1) were inspired by the
established conditions for palladium(II)-catalyzed β-arylation of
chromanones in our previous report. The experiment with 1-(2-
aminophenyl)-3-phenylpropanone 1 failed ligand in DMSO under
an O2 atmosphere because of the potential to form 2-phenyl-2,3-
dihydroquinolin-4(1H)-ones 2 in the presence of Pd(TFA)2 as the
catalyst and 5-NO2-phen as the coordination between free
amine and the Pd(II) catalyst to suppress aminopalladation
(Table 1, entry 1).13 Thus, Ms group was introduced as a
protecting group to prevent the coordination effect of the free
amine in 1. Several reactions involving N-Ms-1 as a model
compound were conducted with the use of various ligands, such
as 5-NO2-phen, bpy, pyridine, and DMAP (entries 2–5).
Table 1. Optimization experiments for the selective synthesis of N-Tf 2-phenyl-
2,3-dihydroquinolin-4(1H)-ones 2.[a]
Yield (%)[b]
Temp
(°C)
Time
(h)
Entry
PG
Ligand
Solvent
2
3
1
-
5-NO2-phen
DMSO
100
100
100
100
100
100
80
48
48
48
48
48
2
n.r
48
26
43
50
62
65
83
20
n.r
n.r
n.r
65
20
70
61
13
11
28
70
n.r
9
2
Ms
Ms
Ms
Ms
Tf
Tf
Tf
Tf
Tf
Tf
Tf
Tf
Tf
Tf
Tf
Tf
Tf
Tf
Tf
5-NO2-phen
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
3
bpy
7
4
pyridine
19
28
12
6
5
DMAP
6
DMAP
7
DMAP
2
Table 2. Scope for the synthesis of N-Tf 2-aryl-2,3-dihydroquinolin-4(1H)-ones
2.[a],[b]
8
-
-
-
-
-
-
-
-
-
-
-
-
-
80
2
7
9
80
2
6
10
11
12
13[c]
14[d]
15[e]
16
17[f]
18[g]
19[h]
20[i]
1,4-dioxane
n-BuOH
toluene
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
80
2
n.r
n.r
n.r
3
80
2
80
2
80
2
80
2
1
80
2
13
15
0
80
24
2
80
80
2
2
80
2
4
80
2
4
[a] Reaction conditions: 1 (0.1 mmol), Pd(TFA)2 (10 mol %), ligand (20 mol %),
and solvent (0.3 mL) under O2 for 2–48 h. [b] Isolated yield. [c] Pd(TFA)2 (5
mol %). [d] Pd(OPiv)2 was used instead of Pd(TFA)2. [e] Pd(OAc)2 was used
instead of Pd(TFA)2. [f] Cu(OAc)2 (0.1 mmol, 1 equiv) was used instead of O2
under argon. [g] Cu(OAc)2 (0.2 mmol, 2 equiv) was used instead of O2 under
argon. [h] MnO2 (0.1 mmol, 1 equiv) was used instead of O2. [i] AgOAc (0.1
mmol, 1 equiv) was used instead of O2.
aminochalcones, traditional intermediates or substrates of these
reactions are sometimes unmanageable due to their reactive
α,β-unsaturated carbonyls. Therefore, there has been a growing
need to develop an efficient approach to synthesize 2-aryl-2,3-
dihydroquinolin-4(1H)-ones
from
chemically
compatible
substrates equipped with structural diversity under mild reaction
conditions.
Recently, we developed the palladium(II)-catalyzed one-pot β-
arylation of chromanones, a kind of simple and tolerable ketones,
[a] Reaction conditions: 1 (0.1 mmol), Pd(TFA)2 (10 mol %), and DMSO (0.3
mL) at 80 °C under O2 for 2 h. [b] Isolated yield.
with arylboronic acids via
a
dehydrogenation/conjugate
2
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10.1002/ejoc.202001425
European Journal of Organic Chemistry
COMMUNICATION
Consequently, a mixture of N-Ms 2-phenyl-2,3-dihydroquinolin-
4(1H)-one 2a and N-Ms 2-phenylquinolin-4(1H)-one 3a was
obtained. Upon use of N-Tf-1, it was observed that the second
dehydrogenation pathway that occurred via β-hydride elimination
was suppressed, which might have resulted from the relatively
unstable transition state induced by increased electron
withdrawing effect of Tf compared to Ms, delivering the desired
product, N-Tf-2 (2b) and undesired product, N-Tf-3 (3b), in 62%
and 12% isolated yields, respectively (entry 6). Remarkably, the
ligand-free system enables the reaction to afford the desired 2-
phenyl-2,3-dihydroquinolin-4(1H)-one 2b in the highest yield of
83% (entries 7 and 8) with a lower reaction temperature (80 °C),
and a shorter reaction time (2 h) than those reactions under
other conditions. Further variation in the solvent (entries 9–12),
catalyst loading amount (entry 13), type of catalysts (entries 14
and 15), reaction time (entry 16), and oxidants (entries 17–20)
did not provide additional improvement.
Under the optimal conditions, the reactions of a variety of A
ring derivatives of N-Tf-2′-aminodihydrochalcones
1 were
performed to investigate the functional group compatibility of the
reaction (Table 2, top section). A series of substrates with
electroneutral (1b), electron donating (1c–1d and 1g–1h), and
electron withdrawing (1e–1f and 1i) groups on the A ring were
successfully transformed into the desired products 2b–2i,
respectively, in moderate to good yields. The scaled-up
cyclization reaction was also successfully performed on a 2.0
mmol scale and 2b was obtained in a yield of 61%. Furthermore,
it was found that heteroaryl A ring such as 1,3-benzodioxole (1j)
successfully engaged in C–N bond formation in 84% isolated
yield. However, the reaction was relatively inefficient for the
Figure 2. Mechanism study by kinetic analysis
OEt, OBn, and 1,3-benzodioxole) or electron-withdrawing (CF3
and halogens) groups were explored, and the corresponding
products 2l–2x were obtained in 37 to 75% yield. Relatively, C–
preparation of 2k, which included
a
dimethoxy group.
N
bond
formation
of
N-Tf-1-(2-aminophenyl)-3-(4-
Subsequently, to further broaden the scope of this method, the
reactions of the N-Tf-2′-aminodihydrochalcones 1 with a variety
of aryl or heteroaryl B rings were also performed under the
optimized reaction conditions (Table 2, bottom section). Various
aryl B rings containing either electron-donating (alkyl, aryl, OMe,
bromophenyl)propanone 1v was less productive because the 4-
bromophenyl group would presumably participate in oxidative
addition in the presence of the Pd(0) catalyst species. Moreover,
the reactions with a heteroaryl B ring (thiophene and furan)
delivered the desired adducts 2y–2z in 56% and 29% yield,
respectively.
A plausible mechanism involving the dehydrogenation/aza-
Michael cyclization sequence for the synthesis of N-Tf 2-phenyl-
2,3-dihydroquinolin-4(1H)-one 2b is illustrated in Scheme 2.
Based on our prior mechanistic studies,12 it is plausible that N-
Tf-2′-aminodihydrochalcone 1b might react with Pd(II)X2 to form
Pd(II) complex I via C–H palladation.14 β-Hydride elimination of I
delivers
a
key intermediate, N-Tf-2′-aminochalcone 4b,
accompanied by HPd(II)X which undergoes reductive elimination
to afford the Pd(0) species.15 In the Pd(II)-catalyzed oxidation
condition, benzylic carbon could be easily oxidized and
transformed into a carbocation which is enabled to make the
cyclization (for 2b) or E1 type elimination (for 4b).16 To elucidate
whether the reaction occurs via β-hydride elimination or benzylic
oxidation, we performed a reaction of the substrate without
terminal aryl substituent, 1aa, in the optimized condition. It
provided the anticipated product 2aa with good yield, suggesting
that the reaction might be progressed via β-hydride elimination
step (Supporting Information, p. 57). Finally, the [O] process
generates Pd(II)X2 in the presence of O2 as an oxidant to
regenerate the Pd(II)-catalyzed dehydrogenation cycle.17
Subsequently, key intermediate 4b converts to the desired
product 2b through aza-Michael cyclization.18 In the case of the
formation of N-Tf 2-phenylquinolin-4(1H)-one 3b, 2b would
Scheme 2. A plausible mechanism of the reaction.
3
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10.1002/ejoc.202001425
European Journal of Organic Chemistry
COMMUNICATION
additionally undergo C–H palladation followed by β-hydride
elimination in an undesired pathway.19 Thus, in order to
selectively synthesize 2b in good yield, it is likely that second
dehydrogenation should be suppressed.
To corroborate the proposed mechanism, kinetic analysis of
the reaction was performed by a time-dependent conversion
experiment of 1b under the optimized reaction conditions (Figure
2). As anticipated, gradual formation of key intermediate 4b and
subsequent conversion into the desired 2b was observed by
HPLC analysis over time. After 1 h, 1b was completely
consumed, and the undesired 3b began to be generated. Thus,
based on these experiments, short reaction time is required to
prevent the formation of 3b.
dihydroquinolin-4(1H)-one derivatives, which could potentially be
converted into pharmacologically interesting aza-flavanones and
other N-heterocycles such as quinolines, in up to 84% yield
under mild conditions. In addition, this reaction has several
advantageous features, including being ligand-free and atom-
economic, and offering good compatibility with a wide scope of
substrates. Mechanistic studies were successfully conducted by
HPLC-based kinetic analysis. Further studies will focus on the
biological studies using the compounds synthesized by our
methodology and the development of another novel
methodology for the synthesis of flavonoids and related
compounds.
Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIT)
(NRF - 2019R1H1A2079819 and 2020R1F1A1064755).
Scheme 3. Control experiment for the intramolecular aza-Michael cyclization.
Conflict of interest
In order to confirm whether the Pd(II) catalyst is significantly
involved in the 1,4-addition, further intramolecular 1,4-addition
experiments were performed in the absence of Pd(TFA)2
(Scheme 3). Consequently, 2b was obtained in 86% isolated
yield from 4b with 1 h, while undesired product 3b was not
observed. This control experiment indicated that the 1,4-addition
could occur through aza-Michael addition without Pd(TFA)2 and
that catalysis might be more crucial for the dehydrogenation
process than for the aza-Michael cyclization.
The authors declare no conflict of interest.
Keywords: 2-aryl-2,3-dihydroquinolin-4(1H)-one •
dehydrogenation • Michael addition • palladium • quinoline
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4
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5
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The one-pot ligand-free palladium-catalyzed α,β-dehydrogenation of ketone and sequential aza-Micahel addition was developed,
which provides N-Tf 2-aryl-2,3-dihydroquinolin-4(1H)-ones in moderate to good yields. This catalytic reaction provides simple and
mild reaction conditions, a wide scope of substrates to biologically interesting N-heterocycles.
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