Fe/S-Catalyzed synthesis of 2-benzoylbenzoxazoles and 2-quinolylbenzoxazolesviaredox condensation ofo-nitrophenols with acetophenones and methylquinolines

Article abstract of DOI:10.1039/d1ob00976a

An Fe/S catalyst generatedin situfrom FeCl₂·4H₂O and elemental sulfur S₈in the presence of a tertiary amine as a base was found to catalyze efficiently a 6e^?redox condensation ofo-nitrophenols with acetophenones and methylquinolines. The condensed products 2-benzoylbenzoxazoles and 2-quinolylbenzoxazoles were obtained in reasonable yields with water as the only byproduct at a temperature as low as 80 °C.

Full text of DOI:10.1039/d1ob00976a

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Fe/S-Catalyzed synthesis of 2-benzoylbenzoxazoles
and 2-quinolylbenzoxazoles via redox condensation
of o-nitrophenols with acetophenones and
methylquinolines†
Cite this: Org. Biomol. Chem., 2021,
19, 6015
Received 20th May 2021,
Accepted 12th June 2021
Le Anh Nguyen,‡a,b Thi Thu Tram Nguyen,‡c Quoc Anh Ngo*a,b and
DOI: 10.1039/d1ob00976a
d
rsc.li/obc
Thanh Binh Nguyen
*
An Fe/S catalyst generated in situ from FeCl2·4H2O and elemental redox condensation of o-nitrosophenols which act as oxidant
sulfur S8 in the presence of a tertiary amine as a base was found to components with 2-bromoacetophenones as reducing com-
catalyze efficiently a 6e− redox condensation of o-nitrophenols ponents (Scheme 2).10 This base-promoted reaction proceeded
with acetophenones and methylquinolines. The condensed pro- without any added reducing or oxidizing agent. However, the
ducts 2-benzoylbenzoxazoles and 2-quinolylbenzoxazoles were main drawbacks of this approach are limited availability of
obtained in reasonable yields with water as the only byproduct at a both starting substrates, especially o-nitrosophenols, and the
temperature as low as 80 °C.
lachrymatory nature of phenacyl bromides.
In general, all known methods for the synthesis of 2-ben-
zoylbenzoxazoles require substrates that are more or less func-
tionalized with activating or protecting groups; the efficiency
of the global processes in terms of atom, step, and redox econ-
omies could be further improved. For example, while 2-amino-
phenols are the precursors of choice for most benzoxazoles,
they are generally obtained by 6e− reduction of readily avail-
able o-nitrophenols 1. In an ideal scenario, the synthesis of 3
via redox condensation with 1 as an oxidizing partner would
involve acetophenones 2 as a reducing partner. Such a shortcut
approach would produce two molecules of water as the only
byproduct.
2-Benzoylbenzoxazoles1 and 2-quinolylbenzoxazoles represent
versatile building blocks for building diverse molecular
scaffolds, including bioactive molecules I–III2 and fluorescent
probes IV and V.3 In fact, I and III were identified to be anti-
proliferative agents,2d while II was found to be a potent inhibi-
tor of fatty acid amide hydrolase (Scheme 1).2a
Consequently, a number of synthetic methods have been
developed. Benzoxazole functionalization allows access to
complex derivatives from simpler starting materials such as
2H-benzoxazoles,4 benzoxazoles 2-substituted5 with suitable
activating groups or 2-benzylbenzoxazoles.6 The benzoxazole
ring could be formed from 2-aminophenols via non-redox con-
densation,7 oxidative coupling8 or cyclization9 reactions.
In parallel, a condensation between two starting materials
proceeding with the change in oxidation states and removal of
small molecules, i.e. redox condensation, is an interesting
strategy to form benzoxazole cores. Beifuss suggested an orig-
inal approach to synthesize 2-benzoylbenzoxazoles based on
In living systems, redox transformations are frequently
orchestrated by metalloenzymes.11 Among the most represen-
tative examples, iron–sulfur clusters, ubiquitous in cellular
aInstitute of Chemistry, Vietnam Academy of Science and Technology, 18 Hoang Quoc
Viet, Cau Giay, Hanoi, Vietnam. E-mail: ngoqanh@ich.vast.vn
bGraduate University of Science and Technology, Vietnam Academy of Science and
Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam
cDepartment of Chemistry, Faculty of Science, Can Tho University of Medicine and
Pharmacy, Vietnam
dInstitut de Chimie des Substances Naturelles, CNRS UPR 2301, Université Paris-
Sud, Université Paris-Saclay, 1, av de la Terrasse, 91198 Gif-sur-Yvette, France.
E-mail: thanh-binh.nguyen@cnrs.fr
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1ob00976a
‡These authors contributed equally to this work.
Scheme 1 2-Benzoyl and 2-quinolyl benzoxazoles.
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similar result (entry 8), which could be understood by the
reversibility of Fe2+/Fe3+ interconversion in the presence of
both oxidizing and reducing agents. The reaction occurred
even with a lower iron or sulfur catalyst loading (entries 9–12)
but was found to be totally inhibited in the absence of either
the iron salt or sulfur (entries 13 and 14). Interestingly, the
reaction was found to proceed at a temperature as low as 60 °C
albeit with a low yield (entry 15). A lower yield was observed at
a higher reaction temperature (entry 16), possibly due to the
loss of both starting materials by evaporation. Under the opti-
mized conditions of catalysts and the base additive, we found
that pyridine as a second additive was shown to be beneficial
(entry 17 vs. entry 9). Although further investigation is needed
to understand the role of pyridine, this base could act as a sta-
bilizing ligand for the Fe/S complex and thus enhance the cata-
lytic activity.
Finally, for comparison purpose, Co(OAc)2·4H2O and Ni
(OAc)2·4H2O were intendedly used in place of the iron salt.
While the Co salt showed weak catalytic activity, the Ni salt
was shown to be completely inefficient; the reaction resulted
in both unchanged starting materials.
Encouraged by this initial result, we then explored the gen-
erality of the Fe/S-catalyzed redox condensation. Pleasingly, the
optimized conditions for the model reaction in Table 1, entry
17, could be applied for the redox condensation of 22 different
methyl ketones 2b–2w with o-nitrophenol 1a. In all cases,
Scheme 2 Redox condensation to benzoxazoles 3.
proteins, are best known for their involvement in a wide range
of oxidation–reduction reactions.12
The last few decades have seen a tremendous development
in the understanding of the mechanism of these enzymes.13
However, their application in organic synthesis has so far not
been adequately explored due to both structural and electronic
complexity of these systems. Recently, bioinspired Fe/S-based
catalytic systems were developed for the redox condensation
between nitro groups with different reducing substituents.14,15
Using iron sources such as metallic iron or simple iron salts
and elemental sulfur as the sulfur source for the in situ for-
mation of Fe–S catalytic species, we and others have succeeded
in condensing aromatic nitro compounds with methyl-
hetarenes,14a amines,14b,c phenylacetic acids,14d alcohols,14e
Table 1 Screening of redox condensation conditions
Entrya
x
y
Additive
T (°C)
Yieldb (%)
1
2
3
4
5
6
10
10
10
10
10
10
10
10
5
2
1
5
0
1
5
5
5
5
5
1
1
1
1
1
1
1
1
1
1
1
0.5
1
0
1
1
1
1
1
N-Methylpiperidine
DIPEA
NEt3
80
80
80
80
80
80
80
80
80
80
80
80
80
80
60
100
80
80
80
50
63
46
40
0c
NPr3
N-Methylmorpholine
Pyridine
—
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
0c
benzyl
cyanides,14f
dibenzyl
disulfides15a
and
7
0c
8d
9
52
67
57
45
55
0
p-methylphenols,15b–d enabling the synthesis of a wide range
of nitrogen heterocycles.
10
11
12
13
14
15
16
17
18f
19g
In continuation of our work, we report here our extension
to 6e− redox condensation of 1 and 2 to 3.
At the outset of this study, we chose o-nitrophenol 1a and
acetophenone 2a as the model substrates. Their reaction in the
presence of FeCl2·4H2O (10 mol%) and sulfur16 (0.5 mmol
based on atomic sulfur) was found to provide the expected
2-benzoylbenzoxazole 3aa in a moderate yield in the presence
of N-methylpiperidine (1 equiv.) as a base at 80 °C (entry 1).
While other bases such as DIPEA, NEt3 and NPr3 were found
to be efficient as an additive (entries 2–4, respectively), no reac-
tion was observed with weaker nitrogen bases such as
N-methylmorpholine (entry 5) or pyridine (entry 6) or in the
absence of a basic additive (entry 7). FeCl3·6H2O also showed a
DIPEA
DIPEA
DIPEA
0
28
65
72
27
0c
DIPEA : pyridinee
DIPEA : pyridinee
DIPEA : pyridinee
a Reaction conditions: o-nitrophenol 1a (1 mmol), acetophenone 2a
(1.2 mmol), FeCl2·4H2O (x mol %), sulfur (y mmol, 32 mg mmol−1),
additive (1 mmol), 80 °C, and 16 h. b Isolated yield. c Determined by 1H
NMR of the crude mixture. d FeCl3·6H2O was used in place of
FeCl2·4H2O. e DIPEA (1 mmol) and pyridine (1 mmol). f Co(OAc)2·4H2O
was used in place of FeCl2·4H2O. g Ni(OAc)2·4H2O was used in place of
FeCl2·4H2O.
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2-acylbenzoxazole derivatives 3ab–3aw were obtained in mod-
Next, we focused our attention on the reactions of acetophe-
erate to high yields (Scheme 3). Substituents such as methyl, none 2a with different substituted o-nitrophenols 1b–1g
methoxy, and halogens were found to be suitable to deliver the (Scheme 4). In general, our reaction conditions were found to
expected products 3ab–3al in comparable yields to that of be nicely applicable to substrates bearing various functional
unsubstituted acetophenone 2a. It should be noted that the groups including alkyl 1b and 1c, alkoxy 1d and 1e, halogen
reactions with sterically demanding ketone such as 2c or with 1f–1h, ester 1i and pyridine analogues 1j. It is noteworthy that
sparingly soluble ketone 2g required heating at higher temp- unlike o-aminophenols,8f the corresponding o-nitrophenol
eratures (100 °C and 90 °C, respectively). We emphasized that substrates were commercially available and readily obtained by
ketone 3af, which was previously found to exhibit anti-prolif- direct nitration of the corresponding phenols.
erative properties (compound I, Scheme 1), could be syn-
Finally, we extended our reactions to 2-methylquinoline
thesized directly by our method.
and its analogues 4 as stable methyl iminic substrates
Ketones bearing electron-withdrawing groups such as tri- (Scheme 5). Since they all contain a pyridine ring, their reac-
fluoromethyl, CN, NO2, and methyl ester were also competent tion involved DIPEA as the only additive. 2-Methylquinoline 4a
substrates. Acetyl derivatives of polyaromatic or heteroaromatic was found to be an excellent substrate capable of reacting with
substrates such as 2-naphthyl 2t, thiophene 2u, furan 2v and a wide range of o-nitrophenols 1a–1k. Among the resulting pro-
benzofuran 2w exhibited similar reactivity, providing ketones ducts, 5da and 5ga represent the direct precursors or core
3at–3aw in reasonable yields.
structures of fluorescent probes V and IV (Scheme 1).
Compared to our previous catalytic system based on metallic
iron and sulfur which required a high temperature up to
150 °C,14a the present system using DIPEA as a base and
FeCl2·4H2O as an iron source was found to be more reactive,
allowing the reaction to proceed at only 80 °C.
Other isomers of 4a bearing an active methyl group such as
4-methylquinoline 4b and 1-methylisoquinoline 4c were also
competent substrates. On the other hand, 2-methylquinoxaline
4d reacted sluggishly under the standard conditions but pro-
vided the expected product 5ad, which is a core structure of
compound III presented in Scheme 1 as an anti-proliferative
compound.
At this stage, although the detailed mechanism was not
fully elucidated, we were eager to investigate some control reac-
tions for a better understanding of the reaction pathway
(Scheme 6).
Since the phenol function was found to complex with Fe3+,
we carried out experiments with a range of nitro substrates
Scheme 3 Scope of methyl aryl ketones 2.
Scheme 4 Scope of o-nitrophenols 1.
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Scheme 5 Scope of methylquinolines 4.
without an o-phenol function such as nitrophenol, o-methoxy-
nitrophenol, and p-nitrophenol with acetophenone 2a (eqn
(1)). In all the cases tested, the expected redox condensed pro-
ducts were not formed in detectable or significant amounts.
This suggests the central role of the o-hydroxy group in the
reaction, allowing the reaction to occur at a temperature as low
as 80 °C.
Scheme 6 Control experiments and the proposed mechanism.
Second, our optimized conditions were not applicable to
aliphatic methyl ketones such as pinacolone, isopropyl methyl
ketone and 2-pentanone (eqn (2)). It is strongly possible that
the reaction occurs via an enol tautomer of the methyl ketone
derivative B and such an intermediate is stabilized more effec-
tively by an aryl group than by an alkyl one.
Third, the redox condensation of 1a and 2a also failed
when monosubstituted sulfide precursors such as dimethyl,
dibenzyl or diphenyl disulfide and their thiol parents were
used in place of elemental sulfur (eqn (3)). It should be noted
that, unlike these sulfur derivatives, elemental sulfur is
capable of forming Fe/S clusters, suggesting the involvement
of this kind of catalyst in the reactions.
Based on these observations and on our previous experi-
ences on similar redox condensation, we tentatively proposed
a reaction mechanism starting with the formation of Fe/S clus-
ters A (in a simplified form without charge on iron atoms for
clarity purposes) from the iron salt and sulfur. Although the
exact structures of such catalytic species could not be deter-
Complexation of o-nitrophenolate and 2-mercaptoacetophe-
none (generated in situ from sulfur and acetophenone 2a) with
iron to form B followed by a cascade of intramolecular conden-
sation of the methylene group to the nitro group would lead to
nitrone D via C. Extrusion of benzoyl nitrone E from D will
regenerate catalyst A. Deoxygenation of E with H2S will provide
the expected product 3aa.
The reaction of 2-nitroaniline with acetophenone resulted
in complex mixtures from which we could identify the pres-
ence of 2-phenylquinoxaline. The reaction of 2,2′-dinitrodiphe-
nyl disulfide, a stable analog of 2-nitrothiophenol, with aceto-
phenone could proceed without the iron catalyst to provide
2-benzoylbenzothiazole.17
Conclusions
In conclusion, we have developed a direct access to 2-benzoyl-
mined in the reaction mixtures, the key features are that they benzoxazoles from 2-nitrophenols and acetophenones. Iron–
contain many iron atoms capable of existing in a wide range of sulfur catalysts generated in situ from simple iron salts such as
easily interchangeable oxidation states. These complexes are
FeCl2·4H2O and elemental sulfur S8 were found to be highly
stabilized by both sulfur bridges and external sulfide ligands. reactive, allowing the reactions to proceed at temperatures as
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low as 80 °C with high functional group tolerance for both sub-
strates. Moreover, o-nitrophenols, acetophenones and methyl-
quinoline analogues are inexpensive and readily available
starting materials with a high degree of structural diversity.
Compared to known methods, our developed strategy is inar-
guably the cheapest and most convenient method to provide a
library of 2-benzoylbenzoxazoles and 2-quinolylbenzoxazoles
in a green manner since water is the only by-product. We
strongly believe that with suitable modifications, Fe/S catalysts
will find wider applications with temperatures approaching
that of physiological conditions. Such efforts are being
pursued in our laboratory.
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Acknowledgements
We thank the Institut de Chimie des Substances Naturelles
and the Vietnam Academy of Science and Technology
(KHCBHH.01/18-20) for financial support.
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