TEMPO-mediated aerobic oxidative synthesis of 2-aryl benzoxazoles via ring-opening of benzoxazoles with benzylamines

Article abstract of DOI:10.1080/00397911.2021.1949476

A simple and efficient TEMPO-mediated system for aerobic oxidative synthesis of 2-aryl benzoxazoles from readily available benzoxazoles and primary benzylic and hetero benzylic amines is presented in one pot. The reaction proceeds through the ring-opening of benzoxazoles and is followed by oxidative condensation with benzylamines. These metal-free, straightforward reactions worked well with a wide range of substrates, yielding moderate to good yields under mild conditions using air as an external green oxidant.

Full text of DOI:10.1080/00397911.2021.1949476

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
TEMPO-mediated aerobic oxidative synthesis
of 2-aryl benzoxazoles via ring-opening of
benzoxazoles with benzylamines
Mugada Sugunakara Rao & Sahid Hussain
To cite this article: Mugada Sugunakara Rao & Sahid Hussain (2021): TEMPO-mediated aerobic
oxidative synthesis of 2-aryl benzoxazoles via ring-opening of benzoxazoles with benzylamines,
Synthetic Communications, DOI: 10.1080/00397911.2021.1949476
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https://doi.org/10.1080/00397911.2021.1949476
TEMPO-mediated aerobic oxidative synthesis of 2-aryl
benzoxazoles via ring-opening of benzoxazoles with
benzylamines
Mugada Sugunakara Rao and Sahid Hussain
Department of Chemistry, Indian Institute of Technology Patna, Patna, India
ABSTRACT
ARTICLE HISTORY
A simple and efficient TEMPO-mediated system for aerobic oxidative
synthesis of 2-aryl benzoxazoles from readily available benzoxazoles
and primary benzylic and hetero benzylic amines is presented in one
pot. The reaction proceeds through the ring-opening of benzoxa-
zoles and is followed by oxidative condensation with benzylamines.
These metal-free, straightforward reactions worked well with a wide
range of substrates, yielding moderate to good yields under mild
conditions using air as an external green oxidant.
Received 21 May 2021
KEYWORDS
Aerobic oxidation; amines;
benzoxazoles; ring-opening;
TEMPO
GRAPHICAL ABSTRACT
N
O
N
TEMPO, air
R1
R2
NH
2
R1
O
R2
Chlorobenzene, 130 °C
14 examples
Introduction
[
1]
Benzoxazole is a privileged structure found in a wide range of natural products, as
well as huge scales of commercially available drugs like Benoxaprofen, Caboxamycin,
[
2]
ERB-041, and Boxazomycins A (Fig. 1). In particular, 2-aryl-substituted benzoxazole
derivatives occupy a special dwelling in pharmacology due to their applications that
include antitumor, antifungal, antibacterial, antimicrobial, anti-HIV, anticancer, antitu-
[
3]
bercular, antiinflammatory, anticonvulsant agents, etc. They have also been used as
[
4]
[5]
[6]
[7]
OLED materials and laser dyes, polymer precursors, drug intermediates, and
[
8]
they are of great importance as enzyme inhibitors.
Because of their great value in terms of synthetic utility, up to the present, many effi-
cient synthetic methods have been developed for the synthesis of 2-aryl benzoxazoles.
The concept of the direct C-H bond activation of benzoxazoles, many catalytic
[
9]
[10]
approaches from benzoxazoles with various aryl sulfonyl, aryl halides,
aryl carbox-
[
11]
[12]
[13]
[14]
[15]
ylic acids,
boronic acids,
aryl silanes,
aryl thioethers,
anilines
and
[
16]
others.
And also reported intramolecular cross-coupling of 2-halo anilides or
CONTACT Sahid Hussain
01106, India.
Supplemental data for this article can be accessed on the publisher’s website.
sahid@iitp.ac.in
Department of Chemistry, Indian Institute of Technology Patna, Patna
8
ß 2021 Taylor & Francis Group, LLC

2
M. S. RAO AND S. HUSSAIN
[
17]
[18]
benzanilides
and some other methods.
Especially, these substances can also be
prepared include the reactions of 2-aminophenols with simple aryl precursors such as
[
19]
[20]
[21]
[22]
aldehydes,
tones,
carboxylic acids,
benzyl alcohols,
benzoyl chlorides,
b-dike-
[
23]
[24]
[25]
[26]
phenyl glyoxylic acid,
nyl orthoesters,
compounds,
aryl methyl chlorides,
aryl methyl thiols,
a,a,a-trihalomethyl aromatic
phe-
[
27]
[28]
[29]
[30]
nitriles,
alkynes,
gem-di-bromomethyl arenes,
aryl ketones,
[
31]
[32]
[33]
and methyl arenes.
Besides this, the
preparation of benzoxazoles has been reported by several research groups from benzyl-
[
34,35]
amines with 2-aminophenols using different protocols.
Consequently, the development of an alternative protocol for the rapid construction
of 2-aryl-substituted benzoxazoles demands great attention to drug discovery. In recent
years, very few methodologies for the synthesis of 2-aryl benzoxazoles by mixing ben-
[
36]
[37]
[38]
zoxazoles with aromatic aldehydes,
acids
acyl chloride,
aryl nitriles,
phenyl glyoxylic
[
39]
have been reported. In terms of step economy, these methods of ring-opening
benzoxazoles and condensation steps in a one-pot reaction are more effective than trad-
itional condensation. For instance, Zhang et al. reported I protocol for the construction
2
[
36b]
of 2-aryl benzoxazoles using benzoxazoles and aromatic aldehydes (Scheme 1a).
In addition, Cheng et al. reported base-promoted protocol from benzoxazoles and acyl
[
37a]
chlorides as starting materials (Scheme 1b).
Recently, Zhang et al. developed a base-
mediated protocol for the synthesis of 2-aryl benzoxazoles from benzoxazoles and aryl
[
38]
nitriles (Scheme 1c).
HO
O
OH
F
O
NH
2
HO
HO
HO
N
O
N
O
HO
N
N
OH
N
O
Cl
O
N
O
HO
Me
Benoxaprofen2a
Antiinflammatory activity
Caboxamycin2b
Antibio activity
ERB-0412c
Antiinflmmatory activity
2d
Boxazomycins A
PTP1B-inhibitor
N
Me
NH
N
O
F
O
O
R
BF-168
OLED material4a
4b
(Senile plaque-selective fluorescent probe)
Figure 1. Examples of some important 2-aryl substituted benzoxazoles.
Previous work
Ar-CHO (a)
I2, PhCl/DMF, 130 °C
N
O
(
b)
N
O
Ar-COCl
K CO , PhCl/H O, 140 °C
2
3
2
Ar-CN
(c)
Cs CO , DMF, 160 °C
2
3
Our work
N
O
TEMPO, air
Chlorobenzene, 130 °C
N
O
NH2
+
Scheme 1. (a–c) Approaches to access 2-aryl benzoxazoles.

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SYNTHETIC COMMUNICATIONS
3
As a result of these reported works of literature, the development of new effective
and straightforward reactions for their synthesis from simple starting materials is highly
desirable. TEMPO being a stable radical, could initiate reactions or capture free radicals
in the reaction system. It was also found useful as an oxidant or catalyst in a variety of
[
40]
organic reactions to synthesize valuable compounds.
Herein, a one-pot method for
aerobic oxidative synthesis of 2-aryl benzoxazoles directly from benzoxazoles and
amines through a domino process mediated by TEMPO with oxygen as the terminal
oxidant in chlorobenzene is reported.
Results and discussion
In search of the optimal reaction conditions, benzoxazole (1a) and benzylamines (2a) were
chosen as model substrates for the reactions. Initially, the reaction was tried with
PhI(OAc) (2 equiv. relative to the amount of 1a) as an oxidant, and chlorobenzene as a
2
ꢀ
solvent, the desired product 3a was isolated in only 40% yield after stirring at 130 C for
1
2 h under an air atmosphere (Table 1, entry 1). Then, various oxidants such as K S O ,
2 2 8
(
NH ) S O , or TBHP proved to be ineffective, which led to low yields (Table 1, entries
4
2 2 8
2–4). Further investigation has shown that the reaction proceeds well in the presence of
TEMPO gave a 74% yield (Table 1, entry 5). Screening of the solvents revealed that the sol-
vents like 1,2-dichlorobenzene (DCB), toluene, 1,4-dioxane, DMF, and DMSO are not suit-
able for this reaction (Table 1, entries 6–10). Further monitoring of the TEMPO loading
and reaction temperature produce lower yields or no product at all (Table 2, entry 11–14).
When the reaction was conducted under the N atmosphere, it did not give the desired
2
a
Table 1. Optimization of reaction parameters for the synthesis of 2-aryl benzoxazoles .
N
N
NH2
Oxidant
Solvent, temperature
O
O
1a
2a
3a
ꢀ
Entry
Oxidant
Solvent
Temp. ( C)
Time (h)
Yield (%)b
1
2
3
4
5
6
7
8
9
PhI(OAc)
2
(2 equiv.)
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
1,2-Dichloro benzene
Toluene
1,4-Dioxane
DMSO
DMF
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
Chlorobenzene
130
130
130
120
130
130
130
130
130
130
130
130
100
RT
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
12
16
40
58
43
31
74
60
22
27
36
29
75
44
15
-
K
2
S
2
O
8
(2 equiv.)
(NH ) S O (2 equiv.)
4 2 2 8
TBHP (2 equiv.)
TEMPO (2 equiv.)
TEMPO (2 equiv.)
TEMPO (2 equiv.)
TEMPO (2 equiv.)
TEMPO (2 equiv.)
TEMPO (2 equiv.)
TEMPO (3 equiv.)
TEMPO (1 equiv.)
TEMPO (2 equiv.)
TEMPO (2 equiv.)
TEMPO (2 equiv.)
TEMPO (2 equiv.)
-
1
1
1
1
1
1
1
1
a
0
1
2
3
4
5
6
7
c
130
130
130
Trace
74
-
d
Reaction conditions: benzoxazole (0.5 mmol), benzylamine (1 mmol), TEMPO (x equiv. with respect to 1a), solvent, open
ꢀ
b
c
d
2 2
to air, rt-130 C for 12–16 h. Isolated yield; N balloon; O balloon.

4
M. S. RAO AND S. HUSSAIN
a
Table 2. Substrate scope .
b
Entry
Benzoxazole
Amine
Product
Yield [%]
N
O
N
O
NH
2
1
74
3a
MeO
OMe
N
O
N
O
2
NH2
72
3
b
N
O
N
O
NH
2
OMe
3
4
80
76
MeO
3
c
N
N
O
NH
2
Me
F
O
3d
Me
F
N
O
N
O
NH
2
5
6
70
71
3
N
O
e
N
O
NH
2
Cl
Cl
Cl
3f
NH
2
N
O
N
7
68
66
O
3g
Cl
Cl
N
O
N
O
NH
2
8
9
Cl
3
h
N
O
N
O
NH
2
NO2
65
58
O
2
N
3i
N
N
O
1
1
0
1
O
3j
O
Me
N
Me
Me
Me
Cl
N
O
NH
2
81
83
78
O
k
3
Me
N
N
O
NH2
OMe
1
1
2
3
O
MeO
Cl
3
l
Me
N
N
O
NH2
Cl
O
3
m
Cl
N
N
O
NH
2
1
1
4
5
72
-
O
3
n
H
3
C
C
NH
2
N
O
CH
3
N
O
CH
3
H
3
C
CH3
H3
3o
a
Reaction conditions: Benzoxazole (0.5 mmol), amine (1 mmol), TEMPO (2 equiv. with respect to benzoxazole), chloroben-
zene, open to air, 130 C for 12 h. Isolated yield.
ꢀ
b

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SYNTHETIC COMMUNICATIONS
5
product (Table 2, entry 15). However, using O in the reaction, the same yield was found
2
as in the case of air (Table 2, entry 16). As economic consideration, the air was chosen as
the oxidant. However, the reaction does not proceed at all in the absence of TEMPO
(
Table 1, entry 17). Altogether, the optimal reaction conditions for the synthesis of 2-aryl
ꢀ
benzoxazoles consist of TEMPO as an oxidant at 130 C under open-air conditions.
With the optimized reaction conditions in hand, the scope of the reactions of different
benzoxazoles 1 with a variety of benzylamines 2 was investigated (Table 2). Initially, for
the scope of the reaction, benzoxazoles were employed as the substrates to react with vari-
ous substituted benzylamines 2a. The electronic nature of substituents on the phenyl ring
of benzylamines was found to have a significant impact on product yields, as shown in
Table 2. The benzylamines with electron-donating groups such as (o-OMe, p-OMe, p-Me)
afforded noticeably high yields compared to electron-withdrawing groups such as F, Cl,
and NO at various position (Table 2, entries 2–9). It was pleasing to find that the reaction
2
of heterocyclic methylamines with benzoxazole smoothly affording the corresponding prod-
uct 3j in 58% yields (Table 2, entry 10). To further expand the scope of the reaction, vari-
ous substituted benzoxazoles such as methyl and chloro at 5-position were employed as the
substrates to react with substituted benzylamines also tolerated in 72–83% yields (Table 2,
entries 11–14). However, aliphatic amines such as 2,2-dimethyl propylamine were not suit-
able substrates for such transformations under these conditions (Table 2, entry 15).
Column chromatography was used to purify all of the compounds. By comparing
spectroscopic data to literature values, the structure of all products was confirmed.
The structure of 2-Furan-2-yl-benzoxazole (3j) was unambiguously proven by X-ray sin-
gle-crystal studies (Fig. 2).
The hypothesized reaction followed a sequential ring-opening and closing route, as
indicated in Scheme 2, based on the foregoing observations and published studies con-
[
41]
cerning the aerobic oxidation of primary amines to imines
tem.
and the TEMPO sys-
[
42]
To begin with, it was believed that the possible mechanism involves the
oxidation of benzylamines by TEMPO/O to the corresponding imine 2a , as well as the
0
2
oxidative opening of the oxazole ring of benzoxazole in the reaction mixture to create
0
[43]
0
which reacts with imine 2a to produce the intermediate 3ab.
2
-amino phenol 1a ,
[
44]
After that, the TEMPO radical interacts with 2-hydroxybezoimine 3ab
the reaction by abstracting hydrogen from the phenol moiety’s,
and initiates
resulting in the for-
[
45]
mation of phenoxyl radical 3ac, and then undergoes intramolecular cycloaddition to the
imine to form the corresponding aminyl radical 3ad. Finally, the second hydrogen
abstraction takes place between the aminyl radical 3ad and TEMPO/or oxygen yielding
the target compound benzoxazole 3a. Although the mechanism is unclear the process
Figure 2. Single-crystal XRD structure of 3j (CCDC 2083123).

6
M. S. RAO AND S. HUSSAIN
O
2
NH
2a
NH
2
N
OH
N
H
2
O
N
O
C
o
2a'
O
n
d
it
io
1a
n
s
NH
2
OH
1a'
N
H
N
NH
2
NH
3
OH
OH
3ab
HN
3aa
N
O
N
2
H O
2
a'
N
O
OH
H
2
O
O
2
O
2
N
OH
N
O
or O2
N
N
O
Cyclization
O
3ad
3
a
3
ac
N
N
O
OH
O
2
2
H O
Scheme 2. A proposed mechanism for TEMPO-mediated aerobic oxidative synthesis of 2-aryl
benzoxazoles.
has several benefits, including a transitional metal-free protocol and the ability to work
with a variety of substrates under mild conditions. The application of this reaction, as
well as a thorough mechanistic analysis, are currently underway in our lab.
Conclusions
In conclusion, an efficient TEMPO mediated aerobic oxidative domino protocol for the
synthesis of 2-aryl benzoxazoles is reported. The reaction proceeds via ring-opening of
benzoxazoles, followed by oxidative condensation with benzylamines has been reported
in the presence of TEMPO in chlorobenzene in one-pot. Advantages of this method
include the use of commercially available starting materials, mild reaction conditions,
good functional group compatibility, and air as the external green oxidant.
Experimental section
General procedure for the synthesis of 2-aryl benzoxazoles (3)
A solution of benzoxazole (0.5 mmol), benzylamine (1 mmol), and TEMPO (2 equiv.
with respect to benzoxazole) in chlorobenzene (2 mL) was added at room temperature.

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SYNTHETIC COMMUNICATIONS
7
ꢀ
The temperature of the oil bath was increased up to 130 C and the mixture was stirred
at the same temperature for 12 h. The mixture was then diluted with ethyl acetate,
washed with water and a saturated NaCl aqueous solution, dried over anhydrous
sodium sulfate, and the solvent was evaporated under reduced pressure to get crude
product which was purified by column chromatography.
Acknowledgments
M. S. Rao is thankful to IIT Patna for his research fellowship and research support. The authors
are thankful to SAIF (IIT Patna) for the single crystal XRD facility.
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