Visible-Light-Photocatalyzed Synthesis of Phenanthridinones and Quinolinones via Direct Oxidative C-H Amidation

Article abstract of DOI:10.1021/acs.orglett.7b03600

A straightforward synthetic strategy to construct biologically relevant phenanthridinones and quinolinones was developed via visible-light-promoted direct oxidative C-H amidation. In this photocatalytic system, amidyl radicals can be generated by homolysis of the N-H bond of simple amide precursors via single-electron transfer under blue LED illumination, which leads to oxidative intramolecular C-H amidation. Moreover, an efficient synthetic strategy using a photocascade enabled facile assembly of quinolinone structures through a catalytic sequence involving triplet energy (E_T) transfer-based E/Z olefin isomerization and subsequent photocatalytic generation of amidyl radical intermediates.

Full text of DOI:10.1021/acs.orglett.7b03600

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Visible-Light-Photocatalyzed Synthesis of Phenanthridinones and
Quinolinones via Direct Oxidative C−H Amidation
Yonghoon Moon,†,‡ Eunyoung Jang,†,‡ Soyeon Choi,†,‡ and Sungwoo Hong*,†,‡
†Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea
‡Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS), Daejeon 34141, Korea
S
* Supporting Information
ABSTRACT: A straightforward synthetic strategy to construct bio-
logically relevant phenanthridinones and quinolinones was developed
via visible-light-promoted direct oxidative C−H amidation. In this
photocatalytic system, amidyl radicals can be generated by homolysis of
the N−H bond of simple amide precursors via single-electron transfer
under blue LED illumination, which leads to oxidative intramolecular
C−H amidation. Moreover, an efficient synthetic strategy using a
photocascade enabled facile assembly of quinolinone structures
through a catalytic sequence involving triplet energy (ET) transfer-
based E/Z olefin isomerization and subsequent photocatalytic
generation of amidyl radical intermediates.
Scheme 1. Design of a Visible-Light Photocatalyzed Synthesis
of Phenanthridinones and Quinolinones
uinolinone and phenanthridinone motifs are important
Q structural constituents of numerous naturally occurring
compounds1 and privileged scaffolds in medicinal chemistry that
display a variety of valuable optical properties and biological
activities.2 Accordingly, a number of synthetic strategies have
been investigated to construct these skeletons, most of which rely
on intramolecular lactam ring formation as the key step.
However, these classical approaches require installation of
amino groups to the aryl rings of the precursors. To complement
the existing methodology, a versatile and flexible retrosynthetic
disconnection to construct quinolinone3 and phenanthridinone4
scaffolds under mild reaction conditions without requiring
prefunctionalization is highly desirable; this inspired us to
explore a visible-light photocatalyzed direct C−N bond
formation route.
Recently, visible-light-induced5 direct conversion of an N−H
bond into the corresponding nitrogen-centered radical via
homolytic bond activation by photocatalysis has proven to be a
valuable approach for direct C−N bond formation without
requiring prefunctionalization of the amide.6−8 With this
foundation, we envisioned that a novel synthetic route to
construct phenanthridinone scaffolds could be developed using
intramolecular C−N bond formation with amidyl radicals
generated by visible-light-driven photocatalysis. Furthermore,
we were intrigued by the possibility of a new retrosynthetic
disconnection of the quinolinone moiety from merging a
catalytic sequence involving a triplet energy (ET) transfer-
based E/Z olefin isomerization9 and subsequent photocatalytic
generation of an amidyl radical intermediate, which leads to
oxidative intramolecular C−H amidation to furnish a new C−N
bond (Scheme 1). Our system is based on the understanding that
both the E/Z olefin isomerization and generation of an amidyl
radical could proceed by the action of the a single photocatalyst,
which would set the stage for intramolecular radical cyclization.
To achieve these diverse reactions, the photocatalyst should
meet two critical criteria. First, the photocatalyst needs to possess
a higher triplet state energy (ET) level than that of the excited
state of the (E)-olefin substrate to enable the isomerization
process, and ideally the ET level would be lower than that of the
(Z)-isomer.9b Second, the photocatalyst must have a high redox
potential for homolytic activation of the strong N−H bonds10 in
simple amide precursors. In the latter process, an oxidative
proton-coupled electron transfer (PCET) protocol8 can be a
powerful approach to efficiently activate the N−H bonds of
amides. Herein, we report a visible-light photocatalyzed cascade
strategy to construct valuable phenanthridinone and quinolinone
scaffolds via a sequential (E/Z-isomerization)/amidyl radical
Received: November 20, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03600
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
generation/C−N bond formation/O2-mediated oxidation proc-
ess.
state iridium photocatalyst at 60 °C, albeit with a much lower
efficiency. We next investigated the possible role of the in situ
generated hydroperoxy anion (see the SI and Figure 1 for
details), but no additional quenching was observed in experi-
ments containing the hydroperoxy anion. These Stern−Volmer
studies suggest that the excited-state iridium complex is capable
of directly activating the N−H bond of the N-phenylbenzamide
moiety, and it is at least partly responsible for the generation of
the nitrogen radical in the reactions.
To prove the viability of the proposed reactions, the feasibility
of the photocatalytic C−H amidation of amide 1a was
investigated in a model system. The addition of a suitable acid
or base could promote direct homolytic bond activations by
activating chemical bonds or modulating redox potentials.11
After extensive screening of potential catalytic systems,
illuminating the reaction mixture with a blue LED in the
presence of 2.5 mol % of Ir complex provided significant amounts
of the corresponding phenanthridinone product 2a under basic
conditions, which confirmed that the overall process was
operating effectively (Table 1). Various light sources were
As illustrated in Scheme 2, the optimal conditions were then
applied to a wide range of substrates to demonstrate the utility
Scheme 2. Substrate Scope of Photocatalytic Synthetic Route
a
to Phenanthridinone
a
Table 1. Optimization of the Reaction Conditions
b
photocat
(2.5 mol %)
temp
(°C)
yield
(%)
entry
additive (0.5 equiv)
1
Ru(bpy)3(PF6)2
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
K3PO4
25
25
25
25
40
60
60
60
60
60
60
60
60
60
2
Ir(ppy)3
Eosin Y
[Ir]
3
4
58
66
85
77
45
39
35
5
[Ir]
6
[Ir]
7
[Ir]
8
[Ir]
NaHCO3
9
[Ir]
10
11
[Ir]
TFA
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
c
12
[Ir]
[Ir]
[Ir]
d
13
e
14
trace
a
Reactions were performed using 1a (0.1 mmol, 1.0 equiv),
photocatalyst (2.5 mol %), base (0.5 equiv), and 1,2-dichloroethane
(3.0 mL) upon irradiation with blue LEDs for 20 h under air balloon.
b
c
d
Yield was determined by 1H NMR. N2 purged. The reaction was
e
conducted in the dark. TEMPO (1 equiv) was used as additive. [Ir]:
[Ir(dFCF3ppy)2bpy]PF6. DCE = 1,2-dichloroethane.
screened, and the use of a blue LED resulted in an improved
yield. Among the solvents screened for this reaction, 1,2-DCE
was most efficient, and the desired product was obtained in 85%
yield at 60 °C (entry 5). Control experiments confirmed that the
visible light and the photocatalyst were required for the
amidation process (entries 11 and 13) and molecular oxygen
can be used as the terminal oxidant (entry 12). Complete
suppression of the reaction in the presence of 2,2,6,6-
tetramethylpiperidine-1-oxyl (TEMPO) (entry 14) supports a
radical mechanism being operative. Intriguingly, the reactions
run in the absence of a base resulted in significant conversion of
the amide starting material (39%, entry 9). Moreover, in the
presence of TFA, the product was obtained in 35% yield (entry
10). To gain mechanistic insight, we designed a series of
competitive luminescence quenching experiments, and solutions
containing 1a and a constant amount of tetrabutyl ammonium
dibutyl phosphate resulted in a large decrease in the emission
intensity in concentration-dependent luminescence quenching
(see the Supporting Information for details). Remarkably, we
observed that N-phenylbenzamide 1a in the absence of an
external base could affect the emission intensity of the excited
a
Reactions were performed using 1 (0.1 mmol, 1.0 equiv),
[Ir(dFCF3ppy)2bpy]PF6 (2.5 mol %), NMeBu3OP(O)(OBu)2 (0.5
equiv) and 1,2-dichloroethane (3.0 mL) upon irradiation with blue
LEDs for 20−24 h at 60 °C under air balloon. Yields of isolated
product.
and generality of this method. With this catalytic method,
phenanthridinone derivatives bearing a wide variety of
substituents could easily be synthesized in good yields. Notably,
halides such as fluoro, chloro, bromo, and iodo substituents were
tolerated under the reaction conditions. The corresponding
desired products were provided, which enable further function-
alization at these positions (2g, 2h, 2o, and 2q). Considering the
facile deprotection of p-methoxyphenyl (PMP) group under
mild conditions, we subsequently assessed the applicability of our
method with respect to substrates bearing PMP groups on the
amide nitrogen, which worked well in the optimized system and
provided the desired products. Substrates containing methoxy,
tert-butyl, chloro, or trifluoromethyl substituents at the para
B
DOI: 10.1021/acs.orglett.7b03600
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
position (2i-2m) were well tolerated and provided the
corresponding products under the standard conditions. Use of
meta-substituted substrate resulted in C−N bond formation,
exclusively at the more sterically accessible C−H bond of the aryl
ring to afford only one regioisomer of 2n.
Scheme 3. Substrate Scope of Photocatalytic Synthetic Route
to Quinolinone
a
Recently, photoisomerization of (E)-olefins was used to
efficiently and selectively synthesize (Z)-olefins.9 We envisaged
that the (Z)-cinnamamide readily accessed in situ as a result of
olefin isomerization could be further utilized for C−N bond
formation in the same photocatalytic system. In line with our
hypothesis, we began our studies by investigating triplet energy
(ET) transfer-based isomerization. Both geometric isomers (E)-
and (Z)-N-phenylcinnamamides were independently subjected
to the above Ir photocatalytic conditions in the absence of base.
Importantly, we observed that the E → Z isomerization of both
amides proceeded efficiently under blue LED irradiation, and
analysis of the crude mixtures revealed Z/E ratios of 74:26
(Figure S6 in the SI).
Encouraged by the (Z)-selectivity of the isomerization, we
next turned our attention to the proposed cascade process for the
construction of valuable quinolinone derivatives. After extensive
screening, we were delighted to observe that the photocascade
involving sequential E/Z-isomerization, amidyl radical gener-
ation, C−N bond formation, and O2-mediated oxidation
proceeded well, thus allowing the synthesis of quinolinone 4a
under optimized conditions (70%) in Table 2. Notably, unlike
a
a
Table 2. Optimization of the Reaction Conditions
Reactions were carried out using 3 (0.1 mmol, 1.0 equiv),
[Ir(dFCF3ppy)2bpy]PF6 (2.5 mol %), and NMeBu3OP(O)(OBu)2
(0.5 equiv) in 1,2-dichloroethane (3.0 mL) upon irradiation with blue
LEDs for 20−24 h at 60 °C under air balloon. Yields of isolated
products.
b
photocat
(2.5 mol %)
yield
(%)
entry
base (0.5 equiv)
solvent
observe that a variety of substrate patterns were well tolerated in
this cascade process and were successfully converted into the
corresponding products (4b−d and 4n−r). Importantly,
substrates bearing β-substituents from methyl (4e, 4g, and 4h),
ethyl (4f), benzene propyl (4i), cyclopropyl (4j), aryl (4k and
4m), and pyridyl (4l) groups reacted well to provide the desired
products; this shows the considerable advantages in both
simplicity and efficiency of this protocol.
A plausible mechanism for the current transformation is
illustrated in Figure 1. Irradiation of IrIII with blue LEDs will
generate an excited-state *IrIII species which can play a dual role
in the reaction; catalyzing ET transfer for E/Z olefin isomer-
ization and serving as a photooxidant to facilitate the direct
homolytic bond activations of the N−H bonds of the amides.
1
2
3
4
5
6
7
8
[Ir]
DCE
DCE
DCE
DCE
DCE
CH3CN
PhCl
DCE
trace
70
13
5
[Ir]
[Ir]
[Ir]
[Ir]
[Ir]
[Ir]
NMeBu3OP(O)(OBu)2
K3PO4
NaHCO3
K2CO3
6
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
34
37
35
Ir(dFCF3ppy)
(dtbbpy)PF6
9
Ru(bpy)3(PF6)2
[Ir]
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
NMeBu3OP(O)(OBu)2
DCE
DCE
DCE
c
10
11
a
Reactions were carried out using 3a (0.1 mmol, 1.0 equiv),
photocatalyst (2.5 mol %), base (0.5 equiv) and 1,2-dichloroethane
(3.0 mL) upon irradiation with blue LEDs for 20 h at 60 °C under air
b
c
balloon. Yield was determined by 1H NMR. N2 purged. [Ir]:
[Ir(dFCF3ppy)2bpy]PF6.
the reaction conditions for phenanthridinone formation, only a
mixture of E/Z-olefin amide substrates remained and the desired
product was not obtained in the absence of an external base
(entry 1). This result indicates that a concerted PCET event
involving the excited-state Ir photocatalyst and a phosphate base
appears to be necessary at the beginning of the reaction to
homolytically cleave the N−H bond of the amide substrate for
amidyl radical formation.
After successfully achieving the visible light-induced trans-
formation of N-phenylcinnamamide 3a into quinolinone 4a, we
then conducted a series of experiments to investigate the scope of
amide substrates. As shown in Scheme 3, we were delighted to
Figure 1. Proposed mechanistic pathway.
C
DOI: 10.1021/acs.orglett.7b03600
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Organic Letters
Letter
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Experimental procedure and characterization of new
compounds (1H and 13C NMR spectra) (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hongorg@kaist.ac.kr.
ORCID
Sungwoo Hong: 0000-0001-9371-1730
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This research was supported financially by the Institute for Basic
Science (IBS-R010-G1).
■
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DOI: 10.1021/acs.orglett.7b03600
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