Synthesis of 2-aryl quinazolinones: Via iron-catalyzed cross-dehydrogenative coupling (CDC) between N-H and C-H bonds

Article abstract of DOI:10.1039/d0ob00866d

Herein, we describe the direct synthesis of quinazolinones via cross-dehydrogenative coupling between methyl arenes and anthranilamides. The C-H functionalization of the benzylic sp3 carbon is achieved by di-t-butyl peroxide under air, and the subsequent amination-aerobic oxidation process completes the annulation process. Iron catalyzed the whole reaction process and various kinds of functional groups were tolerated under the reaction conditions, providing 31 examples of 2-aryl quinazolinones using methyl arene derivatives in yields of 57-95percent. The synthetic potential has been demonstrated by the additional synthesis of aryl-containing heterocycles. This journal is

Full text of DOI:10.1039/d0ob00866d

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Synthesis of 2-aryl quinazolinones via iron-
catalyzed cross-dehydrogenative coupling (CDC)
Cite this: DOI: 10.1039/d0ob00866d
between N–H and C–H bonds†
Yoonkyung Jang, Seok Beom Lee, Junhwa Hong, Simin Chun, Jeeyeon Lee
Suckchang Hong
and
*
Herein, we describe the direct synthesis of quinazolinones via cross-dehydrogenative coupling between
methyl arenes and anthranilamides. The C–H functionalization of the benzylic sp3 carbon is achieved by
di-t-butyl peroxide under air, and the subsequent amination–aerobic oxidation process completes the
annulation process. Iron catalyzed the whole reaction process and various kinds of functional groups
were tolerated under the reaction conditions, providing 31 examples of 2-aryl quinazolinones using
methyl arene derivatives in yields of 57–95%. The synthetic potential has been demonstrated by the
additional synthesis of aryl-containing heterocycles.
Received 25th April 2020,
Accepted 29th June 2020
DOI: 10.1039/d0ob00866d
rsc.li/obc
b). Another synthetic method is palladium-catalyzed carbony-
lative annulation using an aryl halide, with carbon monoxide
Introduction
Quinazolinones, widely found in natural products and syn- as a carbon source (path c).12 Although all of the synthetic
thetic pharmaceuticals,1 are a significant class of annulated methods mentioned above provided various strategies for the
six-membered nitrogen heterocycles. They are key structural synthesis of quinazolinones, they required a specific func-
motifs with a wide range of biological properties, including tional group on the electrophilic carbon synthon.
anticancer,2 antihypertensive,3 anti-inflammatory,4 and anti-
Recently, oxidative cross-dehydrogenative coupling (CDC)
malarial activity.5 Recently, some synthetic drugs based on qui- reactions have become a topic of interest because the coup-
nazolinone such as raltitrexed, ispinesib, and halofuginone lings avoid the tedious and time-consuming prefunctionaliza-
have been used in clinical trials for cancer treatment. tion of substrates.13 In CDC reactions, direct functionalization
Additionally, quinazolinones are important building blocks for of C–H bonds builds C–C or C–heteroatom bonds and offers
the synthesis of bioactive quinazoline derivatives.6
substantial benefits owing to the remarkable potential for
The attractive biological profile and synthetic importance of atom economy and environmental sustainability. Thus, the
quinazolinones have encouraged researchers to develop oxidative CDC reaction is an attractive synthetic tool for estab-
efficient synthetic methods for these heterocycles and their
structural analogs.7 The simplest synthetic approach for pre-
paring 2-aryl quinazolinone would be starting from anthranila-
mide 1a and an electrophilic benzyl carbon synthon.
Depending on the benzyl carbon synthon, various synthetic
methods have already been reported (Fig. 1). The most classi-
cal synthetic approach is oxidative condensation with carbonyl
derivatives (path a).8 Besides the use of carbonyl substrates,
recent studies report the development of various alternative
methods using benzylic carbon as the one-carbon synthon,
attached with hydroxy,9 amine,10 and halogen11 groups (path
Research Institute of Pharmaceutical Sciences, College of Pharmacy, Seoul National
University, 1 Gwanak-ro, Gwanak-gu, Seoul, 08826, Republic of Korea.
E-mail: schong17@snu.ac.kr
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0ob00866d
Fig. 1 Diverse syntheses of quinazolinones from anthranilamide (1a).
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Table 1 Screening of catalysts and oxidantsa
lishing quinazolinone structures (path d in Fig. 1 and
Scheme 1). Li et al. reported the oxidative CDC annulation
between 1a and an unfunctionalized benzylic sp3 carbon to
synthesize 2-aryl quinazolinones 3 with the assistance of
p-toluenesulfonic acid.14 The oxidant activated the benzylic C–
H bond, and this carbon synthon participated in annulation
with 1a. Although the authors explored various types of sub-
strates, the substrate scope is limited because of acidic reac-
tion conditions, and low yields are observed with most sub-
strates. Recently, similar transformations have also been
reported by several groups using solid-supported hetero-
geneous catalysts with mild oxidants (air or O2 gas).15 The
advantage of heterogeneous catalysis is obvious, but these cat-
alysts have the drawback of the need to be prepared in
advance. Thus, a more convenient and general method for the
synthesis of quinazolinones using readily available catalysts
and reagents is desirable. In particular, iron is an inexpensive,
nontoxic and environmentally benign transition metal.16
Therefore, an increasing number of reactions using iron metal
have recently been reported. As part of our program related to
the development of a new approach for the synthesis of
N-heterocycles, we wish to develop a synthetic method for
2-aryl quinazolinones 3 via CDC reactions using iron catalysts.
Entry
Catalyst
Oxidant
Yieldb (%)
1c
2c
3
FeCl3
DTBP
DTBP
DTBP
DTBP
DTBP
DTBPe
DTBP
DTBP
DTBP
TBHPf
29
46
88
95
48
60
77
62
FeCl3·6H2O
FeCl3·6H2O
FeCl3·6H2O
No catalyst
FeCl3·6H2O
FeCl2·4H2O
Fe(OTf)3
4d
5
6
7
8
9
10
11
12
CuCl2
31
68
40
FeCl3·6H2O
FeCl3·6H2O
FeCl3·6H2O
g
H2O2
DDQ
No reaction
a Reaction conditions: 1a (0.3 mmol), 2a (18 mmol, 1.9 mL), catalyst
(10 mol%) and oxidant (0.9 mmol) in DMSO (2 mL) at 100 °C under
air for 20 h. b Isolated yield. c The reaction was performed in a sealed
tube. d The reaction vessel was recharged with O2 gas and connected
with an O2 balloon. e 0.6 mmol DTBP was used. f 70% aqueous solu-
tion. g 35% aqueous solution.
amount of the oxidant also resulted in a low yield (entry 6).
Upon screening various types of catalysts and oxidants, we
could not find more efficient ones. Therefore, FeCl3·6H2O and
di-t-butyl peroxide (DTBP) were chosen as the optimal catalyst
and oxidant, respectively.
As shown in Table 2, we further optimized the reaction con-
ditions using FeCl3·6H2O and DTBP. High temperatures over
110 °C were required for sufficient conversion (entries 1–3).
Therefore, we could not use a solvent with a boiling point
Results and discussion
First, we employed FeCl3 as a preliminary catalyst in the reac-
tion of 2-anthranilamide 1a with toluene 2a, and 2-phenyl qui-
nazolinone 3aa was obtained in 29% yield (Table 1, entry 1).
When we used the hydrated catalyst FeCl3·6H2O, an improved
yield was observed (entry 2). Interestingly, exposure of the reac-
tion mixture to air was a crucial factor for a high yield (entry
3). We supposed that oxygen gas plays an important role in
this reaction. As expected, the desired product was obtained in
a high yield of 95% with an O2 balloon (entry 4). However,
because we obtained an adequate yield without an O2 balloon,
we focused on optimizing our reaction under aerobic con-
ditions. In the absence of the iron catalyst, the yield was sig-
nificantly decreased (entry 5). An attempt to reduce the
Table 2 Optimization of the reaction conditionsa
T
(°C)
Time
(h)
Yieldb
(%)
Entry 2a/solvent (mL)
1
2
3
4
5
6
7
8
2a/DMSO (1.9/2)
2a/DMSO (1.9/2)
2a/DMSO (1.9/2)
2a/DMF (1.9/2)
2a/HMPA (1.9/2)
2a/DMSO (1.9/1)
2a/DMSO (1.9/0.5)
2a/DMSO (1.9/0.2)
2a/DMSO (0.95/0.5)
2a/DMSO (0.95/0.5)
2a/diphenyl sulfoxide (0.95/
0.5)
110
100
90
20
20
20
20
20
40
40
40
40
40
40
88
30
9
110
110
110
110
110
110
120
120
12
Trace
90
95
64
74
93
68
9
10
11
a Reaction conditions: 1a (0.3 mmol), FeCl3·6H2O (10 mol%) and DTBP
(0.9 mmol) in 2a/solvent under air. b Isolated yield.
Scheme 1 Synthesis of 2-aryl quinazolinones 3 via the CDC strategy.
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lower than 110 °C in the open-air reaction system. Among the 2a (Table 3). Under the developed conditions, 2-phenyl quina-
screened solvents, DMSO gave the best results (entries 1, 4 and zolinones 3 bearing N-substituents were successfully syn-
5). In the reaction system, side product 4a was always observed. thesized from the corresponding 2-amino N-substituted benza-
However, we could not calculate the exact mass of 4a at this mides (3ba–3ma). Not only a sterically hindered amide substi-
stage because of the low solubility of 4a in organic solvents. 4a tuted with an i-propyl group (3ea) but also less nucleophilic
might be formed through the cleavage of the sp3 C–hetero- amides substituted with an aryl group (3ja–3ma) afforded
atom bond of the solvent followed by the formation of a C–N good yields. The N-substituted homo-allyl group was tolerated
bond with 1a. With this hypothesis, we tried to reduce the under the reaction conditions (3da). In addition to benzyl,
amount of the solvent to increase the 2a/solvent ratio. The other heteroarene methyl substituted amides also provided the
increased amount of 2a caused more substrate 1a to convert desired products in good yields (3ga–3ia). Moreover, there was
into the desired product 3aa (entries 1, 6 and 7), although a no significant difference with regard to the substituents on the
longer reaction time was needed. We expected a decrease in aromatic ring of the substrate (3na–3sa). Bromide and iodide
the formation of 4a to result in this tendency. However, a were also retained in the annulated product, representing
minimum amount of DMSO was necessary to achieve clean efficient functional groups for further transformation (3qa and
conversion (entries 7 and 8). We could reduce the number of 3ra).
equivalents of 2a by half at increased temperature to obtain a
Next, we applied various types of methyl arenes 2 to syn-
sufficient yield (entry 10). We also employed diphenyl sulfox- thesize 2-aryl quinazolinones under the same conditions
ide to suppress the formation of 4a. Although 4a was not (Table 4). Because of the low solubility of 2-phenyl quinazoli-
observed in the reaction mixture as we expected, these solvents none 3aa in organic solvents, we used 2-amino N-methyl benz-
did not afford better yields than DMSO (entries 7 and 11).
amide 1b as a starting material. A diverse array of methyl
After the optimization of the reaction conditions, a wide arenes bearing electron-withdrawing and electron-donating
range of anthranilamides 1 were employed for annulation with groups could smoothly react with 1b, and the corresponding
quinazolinones could be obtained in moderate to good yields.
Generally, better results were achieved with methyl arenes with
electron-donating groups, such as methyl groups, than those
Table 3 Scope of 2-anthranilamide analog substrates
with electron-withdrawing groups (3bb–3be vs. 3bf–3bj), except
for the methoxy group (3bk). The steric effect slightly influ-
enced the formation of the desired product, depending on the
Table 4 Scope of methyl arene analog substrates
All of the presented yields are isolated yields.
All of the presented yields are isolated yields.
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position of the substituent (3bb–3bd). Based on these results, step depending on the substrate. In the case of 2-amino thio-
we assumed that the activation process of the benzylic C–H phen-3-amide 7, the annulation and oxidation processes were
bond usually would be the rate-determining step instead of easily achieved under the standard conditions. We could
the annulation process. The electron-donating group could control the final oxidation step depending on the reaction
stabilize the benzyl cationic charge that is generated during time with 2-amino benzenesulfonamide 8. A long reaction
benzylic carbon activation by the oxidant. In the case of the time gave the fully oxidized product 8a, and a short reaction
methoxy-substituted methyl arene, the reaction proceeded time gave the unoxidized annulation product 8a′. Quinazoline
smoothly until annulation with 1b, but the final oxidation to derivative 9a was also well synthesized using the developed
afford the quinazolinone moiety was quite slow. Additionally, conditions, even though more oxidant and solvent were
naphthalene and thiophene could be substituted at the 2-posi- needed. Under the previous Li’s conditions, basic nitrogen
tion of the quinazolinones (3bl and 3bm).
containing substrates (5a and 9) failed to afford the corres-
As shown in Table 5, we also explored other nucleophiles ponding annulated products (the failure examples are given in
that have structural similarity with 2-anthranilamide to obtain the ESI of ref. 14). Moreover, only 8a′ was reported as a
other aryl-containing heterocycles. First, nicotinamide sub- product which could be obtained from sulfonamide substrate
strates 5 and 6, which have a pyridine moiety instead of the 8 under Li’s conditions. These examples showed that our con-
phenyl group of 1, were subjected to the reaction conditions, ditions are more efficient for the CDC annulation with methyl
and the desired product was obtained in a high yield. arene.
However, similar to the case of 3bk shown in Table 4, the final
Then, we carried out several control experiments to under-
oxidation process to generate 5a and 6a was slow. We assumed stand the mechanism (Scheme 2). We focused on the for-
that a longer reaction time is needed for the final oxidation mation of benzaldehyde during the reaction because the
corresponding benzaldehyde could be observed in most reac-
tions (see the details in the ESI†). This observation contrasts
with the previous mechanistic study, although the transform-
Table 5 Screening of other types of nucleophiles
ation and reaction conditions were similar.14 In the compe-
tition reaction with toluene and toluene-d8, a significant
kinetic isotope effect (KIE, kH/kD = 6/1) was observed, so the
activation of the C–H bond was suggested to be the rate-deter-
mining step (a). A well-known radical scavenger, TEMPO
(2,2,6,6-tetramethylpiperidine N-oxide), quenched the reaction.
Benzylated TEMPO was observed during the reaction, so we
Nucleophile
Product
Time (h)
65
Yielda (%)
>99
supposed that the reaction involved a benzyl radical intermedi-
65
76
40
65
69
92
20
40
59
54b
a Isolated yield. b DMSO (0.7 mL) and DTBP (1.2 mmol).
Scheme 2 Investigation of the reaction mechanism.
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ate (b). On comparing the experiments using 2-amino species. After the formation of benzaldehyde A, it reacted with
N-benzylbenzamide 1g and 2-(benzylamino) benzamide 1t in 2-anthranilamide 1a, and imine B was generated with the
the absence of toluene, 1g is supposed to be an intermediate assistance of the iron catalyst. Subsequently, annulation of
in the reaction, so the activated benzylic carbon coupled with imine B through intramolecular nucleophilic amination fol-
the anilinic amine prior to coupling with the amide (c). We lowed by aerobic oxidation gave the final product 3aa.
supposed that first, t-butoxy benzyl ether was generated from However, we could not exclude direct amination of 1a by
toluene, and then, it was converted to benzaldehyde in the radical species Rad-I or Rad-II. We supposed that exposure to
presence of DTBP. Imine formation from benzaldehyde and air accelerated the formation of benzaldehyde A through
amine 1 would be catalyzed by an iron catalyst, and the quina- another pathway. In the presence of oxygen gas, benzyl hydro-
zolinone would be formed via an intramolecular amination– gen peroxide was generated via the coupling of Rad-I with O2,
oxidation process. As expected, when t-butoxy benzyl ether was followed by the abstraction of H• from the solvent (2a or
reacted with 1a, annulated product 3aa was obtained under DMSO). Then, benzyl hydrogen peroxide could be converted to
the standard conditions (left side of (d)). However, without benzaldehyde A under the reaction conditions. Through this
DTBP, only a trace amount of 3aa was detected by TLC. Based pathway, not only the formation of aldehydes but also the
on this result, we could exclude the direct nucleophilic attack overall reaction was accelerated by oxygen gas.
of the amine on the benzyl ether intermediate. After the annu-
As mentioned in Table 2, we always observed another
lation between 1a and benzaldehyde, the ensuing oxidation annulated quinazolinone 4a as a side product under the
process readily occurred under aerobic conditions. Therefore, reaction conditions. To investigate the significant formation
the quinazolinone structure of 3aa could be adequately con- of 4a, we carried out several control experiments (Scheme 4).
structed in the absence of DTBP (right side of (d)).
We assumed that one carbon atom comes from the solvent
Based on the results of the control experiments, we based on a related research study.17 The methyl carbon adja-
suggested a possible mechanism (Scheme 3). First, t-butoxy cent to a heteroatom is activated by an oxidant or radical
radicals were generated by the homolysis of DTBP under the intermediate, and this activated carbon reacts with the
reaction conditions. The benzyl radical Rad-I was formed by amine group of 1 instead of benzaldehyde A. After sequen-
the abstraction of H• from the benzylic carbon of toluene by tial cleavage of the C–heteroatom bond, annulated product
the t-butoxy radical. The coupling of these two radicals gave 4a was formed following a similar process as that shown in
t-butoxy benzyl ether 2a-OtBu as an intermediate. Another Scheme 3. Due to the solubility problem of 4a, we used the
t-butoxy radical abstracted H• from 2a-OtBu, and its sub- methyl-substituted amide substrate 1b. Based on the results
sequent autolysis produced benzaldehyde A. The iron(III) salt shown in Table 2, we supposed that the reactivity of DMSO
might be involved in the oxidation process of 2a through was lower than that of toluene. Therefore, the reaction was
single-electron transfer, especially in the formation of radical carried out in the absence of toluene and side product 4b
was obtained in 48% yield. Among the similar types of sol-
vents, we found that dimethylacetamide (DMA) is the most
efficient solvent to serve as a methyl carbon source. To verify
that the carbon atom comes from the solvent, diethyl-
acetamide, which has an ethyl substituent, was used under
the same conditions. As a result, 4b-Me was obtained as a
major product (4b : 4b-Me = 1 : 20), which means that the
solvent serves as the carbon source (see the details in the
ESI†). The trace amount of 4b might be attributed to the
methyl radicals of DTBP.18
Scheme 3 Proposed mechanism.
Scheme 4 Annulation of 1b without toluene.
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Conclusions
In summary, we have developed an efficient synthetic method
for the synthesis of 2-aryl quinazolinones from unreactive
methyl arenes via an iron-catalyzed CDC reaction. During the
reaction, C–H bond activation on a benzylic sp3 carbon occurs
in the presence of DTBP, and the annulated product is
obtained followed by dual amination with anthranilamides 1
with the assistance of an iron catalyst. The reaction was per-
formed under air, and we found that oxygen gas plays a crucial
role in the oxidative process of the reaction. Compared with
the previous CDC conditions, our conditions improved the
yields with most substrates and provided a broad substrate
scope to obtain various N-heterocycles. The developed method
also tolerates various functional groups allowing further
functionalization. And all of the reagents and catalysts used
are inexpensive and readily available. Based on the results and
control experiments, we suggested a possible radical mecha-
nism, with benzaldehyde as a key intermediate. Further exten-
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N-heterocycles is under investigation in our research group.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korean government
(2018R1C1B6005607 and 2018R1A4A1021703) and the
Creative-Pioneering Researchers Program through Seoul
National University (SNU).
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