A New Pathway to 2-Arylbenzoxazoles and 2-Arylbenzothiazoles Via One-Pot Oxidative Cyclization Reactions Under Iron-Organic Framework Catalysis

Article abstract of DOI:10.1007/s10562-019-02747-1

Iron-organic framework MOF-235 was synthesized, and consequently utilized as a productive heterogeneous catalyst for the synthesis of 2-arylbenzoxazoles and 2-arylbenzothiazoles via one-pot oxidative cyclization reactions between 2-aminophenols or 2-aminothiophenols and alcohols. The transformation was considerably controlled by the oxidant and the nature of solvent, and the system of di-tert-butylperoxide with xylene led to best yield of major products. The MOF-235 catalyst presented higher catalytic efficiency for the synthesis of 2-arylbenzoxazoles and 2-arylbenzothiazoles than a number of MOF-based catalysts and established homogeneous catalysts. Recovering and reutilizing the framework catalyst for the cyclization transformation was possible while its catalytic activity was retained. To the best of our knowledge, this iron-catalyzed one-pot oxidative transformation to produce 2-arylbenzoxazoles and 2-arylbenzothiazoles under heterogeneous catalysis conditions was not previously reported in the literature.

Full text of DOI:10.1007/s10562-019-02747-1

Catalysis Letters
https://doi.org/10.1007/s10562-019-02747-1
A New Pathway to 2-Arylbenzoxazoles and 2-Arylbenzothiazoles
Via One-Pot Oxidative Cyclization Reactions Under Iron-Organic
Framework Catalysis
1
1
1
1
1
Son H. Doan · Chau B. Tran · An. L. N. Cao · Nhan T. H. Le · Nam T. S. Phan
Received: 16 January 2019 / Accepted: 11 March 2019
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Iron-organic framework MOF-235 was synthesized, and consequently utilized as a productive heterogeneous catalyst for the
synthesis of 2-arylbenzoxazoles and 2-arylbenzothiazoles via one-pot oxidative cyclization reactions between 2-aminophenols
or 2-aminothiophenols and alcohols. The transformation was considerably controlled by the oxidant and the nature of solvent,
and the system of di-tert-butylperoxide with xylene led to best yield of major products. The MOF-235 catalyst presented
higher catalytic eꢀciency for the synthesis of 2-arylbenzoxazoles and 2-arylbenzothiazoles than a number of MOF-based
catalysts and established homogeneous catalysts. Recovering and reutilizing the framework catalyst for the cyclization trans-
formation was possible while its catalytic activity was retained. To the best of our knowledge, this iron-catalyzed one-pot
oxidative transformation to produce 2-arylbenzoxazoles and 2-arylbenzothiazoles under heterogeneous catalysis conditions
was not previously reported in the literature.
Graphical Abstract
XH
X
MOF-235
NH2
OH
R
+
N
heterogeneous
oxidative cyclization
X: S, O
R
Keywords Iron-organic framework · Benzoxazoles · Benzothiazoles · Heterogeneous catalyst · One-pot
1
Introduction
Benzoxazoles and benzothiazoles are valuable scaꢁolds,
widely occurring in a series of natural products, thera-
peutically useful compounds, and agrochemicals, as well
as in numerous functional polymers [1]. Due to their valu-
able properties, a variety of synthetic protocols have been
investigated for these heterocyclic skeletons [2]. Traditional
approaches to achieve benzoxazole and benzothiazole
structures included the condensation of 2-aminophenols
or 2-aminothiophenols with either aldehydes or carbox-
ylic acids, suꢁering advantages of severe reaction condi-
tions [2–4]. Rubin and co-workers previously reported a
direct ortho-C–H functionalization, followed by Beckman
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02747-1) contains
supplementary material, which is available to authorized users.
*
Nhan T. H. Le
lthnhan@hcmut.edu.vn
*
Nam T. S. Phan
ptsnam@hcmut.edu.vn
1
Faculty of Chemical Engineering, HCMC University
of Technology, VNU-HCM, 268 Ly Thuong Kiet, District
1
0, Ho Chi Minh City, Viet Nam
Vol.:(0
1
123456789)
3

S. H. Doan et al.
rearrangement and intramolecular cyclocondensation to
generate benzoxazoles [5]. Bhanage and co-workers dem-
onstrated a CuCl-catalyzed synthesis of benzoxazoles via a
tandem cyclization of 2-halophenols with amidines [6]. Sun
and co-workers prepared benzoxazoles and benzothiazoles
the organic linkers in the frameworks could serve as active
sites, and therefore oꢁering advantages for catalytic transfor-
mations [21, 30]. Certainly, a broad series of organic reac-
tions have recently been performed under metal–organic
framework catalysis conditions [31–39]. In this work, we
wish to report the synthesis of 2-arylbenzoxazoles and
2-arylbenzothiazoles via one-pot oxidative cyclization reac-
tions between 2-aminophenols or 2-aminothiophenols and
alcohols in the presence of iron-organic framework MOF-
235 as a recyclable catalyst (Scheme 1b). To the best of our
knowledge, this iron-catalyzed one-pot oxidative transforma-
tion to produce 2-arylbenzoxazoles and 2-arylbenzothiazoles
under heterogeneous catalysis conditions was not previously
reported in the literature.
by using one-pot Pd(OAc) -catalyzed cyclization reactions
2
between aryl iodides and 2-hydroxyl/mercapto nitrobenzenes
[
7]. Narender and co-workers synthesized benzothiazoles
from the I -mediated oxidative cyclization of 2-aminothio-
2
phenols with benzylamines [8]. Kempe and co-workers pre-
viously demonstrated an eꢀcient approach to obtain benzi-
midazoles from aromatic diamines and alcohols by iridium
complex-catalyzed acceptorless dehydrogenative alkylation
(
Scheme 1a) [9]. Owing to the signiꢂcant aspects of these
heterocyclic structures, the ꢂeld needs to be expanded, and
heterogeneous catalysts should be targeted.
The discovery of metal–organic frameworks (MOFs), a
new family of synthetic crystalline porous materials, has
gained appreciable attention during the last 20 years, since
these structures present huge opportunities for transforming
industrial applications [10–15]. MOFs possess nodes includ-
ing metal cations or polyvalent metal clusters connected
by organic linkers, leading to the generation of numerous
frameworks with diꢁerent architectures and appealing prop-
erties [16]. Both the metals and the functionality as well
as the length of the organic linkers control the shape and
the size of pores, oꢁering massive feasibility to diversify
framework structures [17–20]. A long series of MOFs were
produced, and many of them have been explored for catalytic
applications [21–29]. All metal nodes and functionalities on
2 Experimental
2.1 Catalyst Synthesis
The MOF-235 was prepared by utilizing a literature
approach [40–42]. In a representative experiment, H BDC
2
(H BDC=1,4-benzenedicarboxylic acid; 0.332 g, 2.0 mmol)
2
and FeCl ·6H O (0.541 g, 2.0 mmol) were dissolved in a
3
2
mixture of DMF (DMF=N,N′-dimethylformamide; 40 mL)
and ethanol (40 mL). The mixture was vigorously stirred
to obtain a clear solution, and then equally added to eight
20-mL vials. The vials were carefully capped and conse-
quently heated at 85 °C in an oven for 48 h. Light orange
crystals were produced on the wall of the vials during the
Previous work (9):
H
NH2
OH
+
N
[Ir complex]
R
N
NH2
R
homogeneous
dehydrogenative condensation
(a)
This work:
OH
O
N
MOF-235
R
R
NH2
SH
heterogeneous
OH
+
oxidative cyclization
R
S
N
MOF-235
NH2
(b)
Scheme 1 The diꢁerence between our work and previous work [9]
1
3

A New Pathway to 2-Arylbenzoxazoles and 2-Arylbenzothiazoles Via One-Pot Oxidative Cyclization…
experiment. After cooling the vial to ambient temperature,
the crystals were separated by decantation, and washed
thoroughly with DMF (3 ×10 mL). Solvent exchange was
consequently conducted with ethanol (3×10 mL) at room
temperature. The Fe-MOF product was subsequently dried
under vacuum at 140 °C for 6 h, obtaining 0.306 g of MOF-
catalyst for the synthesis of 1,5-benzodiazepines [43], the
synthesis of α-acyloxy ethers [44], and the derivatization of
indoles [45]. XRD analysis exhibited sharp peaks, indicat-
ing that the Fe-MOF was highly crystalline (Fig. S1). SEM
observation displayed homogeneity with respect to octahe-
dral crystals (Fig. S2). The TEM image veriꢂed that a porous
framework was obtained (Fig. S3). Pore size distribution
analysis conꢂrmed that the Fe-MOF was microporous (Fig.
2
35 in the form of brick red crystals (45% based on H BDC).
2
2
2.2 Catalytic Reactions
S4). Langmuir surface areas of 770 m /g were recorded,
based on nitrogen physisorption measurements (Fig. S5).
TGA results indicated that the framework was thermally
stable up to over 300 °C (Fig. S6). FT-IR data revealed the
presence of carboxylate linkers in the material, being diꢁer-
ent from those of the 1,4-benzenedicarboxylic acid (Fig. S7).
In a representative catalytic experiment, benzyl alcohol
0.135 g, 1 mmol), 2-aminophenol (0.204 mL, 2 mmol),
(
and diphenyl ether (0.1 mL) as an internal standard were
dissolved in xylene (4 mL). The solution was added into
a round bottom ꢃask. The Fe-MOF catalyst with pre-cal-
culated quantity was then introduced to the reactor. The
catalyst amount was worked out regarding the iron/ benzyl
alcohol molar ratio. The reaction mixture was stirred under
magnetic stirring for 5 min to dispense the catalyst in the liq-
uid phase. Subsequently, di-tert-butyl peroxide (tBuOOtBu,
3.2 Catalytic Studies
The iron-based framework was explored as a heterogene-
ous catalyst for the one-pot oxidative cyclization reaction
between 2-aminophenol and benzyl alcohol to produce
2-phenylbenzo[d]oxazole as the major product (Scheme 1 b).
Initially, the impact of temperature on the yield of the prin-
cipal product was addressed. The reaction was conducted at
5 mol% catalyst in xylene for 360 min, with two equivalents
of 2-aminophenol, using 3 equivalents of di-tert-butyl per-
oxide as the oxidant, at room temperature, 80 °C, 100 °C,
120 °C, and 140 °C, respectively (Fig. 1). The cyclization
reaction did not proceed at room temperature, with no evi-
dence of the expected product being noticed after 360 min.
The transformation performed at 80 °C aꢁorded 21% yield
after 360 min. As anticipated, raising the temperature led to
a remarkable improvement in the yield of 2-phenylbenzo[d]
0
.63 mL, 3 mmol) was added dropwise to the reactor. The
mixture was magnetically stirred at 120 °C for 360 min.
Samples were taken at diꢁerent time periods, quenched with
NaCl solution (5% w/w, 1 mL). The organic ingredients were
afterwards extracted into ethyl acetate phase (3 mL), dried
with Na SO , and analyzed by GC concerning the internal
2
4
standard. The desired product, 2-phenylbenzo[d]oxazole,
1
was isolated by silica gel column chromatography. H
1
3
NMR, C NMR, and GC-MS experiments were performed
to verify the product structure. To explore the recyclability,
the iron-based framework was collected by centrifugation,
washed thoroughly with methanol to remove product and
excess reagents, heated under vacuum on a Shlenkline at
1
20 °C for 3 h, and reutilized for new catalytic experiment.
3
Results and Discussion
3.1 Catalyst Characterization
The MOF-235 was prepared from 1,4-benzenedicarboxylic
acid and iron(III) chloride by following a literature approach
[
40, 41]. The material was subsequently characterized using
conventional analysis methods such as XRD, SEM, TEM,
TGA, FT-IR, AAS, and nitrogen physisorption measure-
ments (Figs. S1–S7 in Supporting information). Sheykhi
[
41], Yaghi [42] and co-workers previously demonstrated
that iron atoms in MOF-235 are in trivalent form. Addi-
tionally, it was reported that MOF-235 is constructed from
octahedral iron trimers which are connected through linear
terephthalic acid linkers, forming high symmetric acs topol-
ogy [41, 42]. Possessing high content of trivalent iron in the
framework, MOF-235 have been utilized as a heterogeneous
Fig. 1 Yields of 2-phenylbenzo[d]oxazole versus temperature
1
3

S. H. Doan et al.
oxazole. Certainly, 42% yield was recorded after 360 min for
the experiment executed at 100 °C. Boosting the temperature
to 120 °C, the yield of the desired product was upgraded to
di-tert-butyl peroxide emerged to be the best option for the
reaction, aꢁording 2-phenylbenzo[d]oxazole in 88% yield
after 360 min. Additionally, the amount of di-tert-butyl per-
oxide also exhibited a notable impact on the oxidative cycli-
zation reaction (Fig. 3). Best result was achieved when three
equivalents of oxidant was utilized. Extending the amount of
the oxidant did not improve the reaction yield, while drop-
ping the quantity of the oxidant expressed a negative eꢁect.
It was noted that no 2-phenylbenzo[d]oxazole was detected
in the absence of the oxidant, verifying the requisite of the
oxidant for the formation of the benzoxazole.
8
8% after 360 min. It was noted that extending the reaction
temperature to 140 °C did not intensify the yield considera-
bly, and therefore the catalytic reaction should be performed
at 120 °C.
Comparable to other oxidative reactions, the requirement
of an oxidant in the catalytic cycle should be inevitable for
the cyclization reaction between 2-aminophenol and ben-
zyl alcohol to produce 2-phenylbenzo[d]oxazole. Conse-
quently, the impact of diverse oxidants on the yield of the
major product was investigated, having employed di-tert-
butyl peroxide (DTBP), aqueous tert-butyl hydroperoxide
In numerous organic transformations utilizing hetero-
geneous catalysts, the solvent might display a remarkable
inꢃuence on the reaction rate, being subject to the char-
acteristic of the catalyst. Kempe and co-workers previ-
ously pointed out that diglyme was the best solvent for the
synthesis of benzimidazoles from aromatic diamines and
alcohols by iridium complex-catalyzed acceptorless dehy-
drogenative alkylation [9]. It was accordingly resolute to
survey the impact of various solvents on the cyclization
reaction between 2-aminophenol and benzyl alcohol to pro-
duce 2-phenylbenzo[d]oxazole (Fig. 4). The reaction was
performed at 120 °C for 360 min, with two equivalents of
2-aminophenol, in the presence of 5 mol% catalyst, using
three equivalents of di-tert-butyl peroxide as the oxidant,
in xylene, mesitylene, benzene, chlorobenzene, toluene,
and diglyme, respectively. It should be noted that a pressur-
ized vial reactor was required for solvents with low boiling
point. While diglyme was the best solvent for the synthesis
of benzimidazoles from aromatic diamines and alcohols [9],
it was not appropriate for the formation of 2-phenylbenzo[d]
oxazole with only 21% yield being noted after 360 min.
[
(
TBHP (water)], tert-butyl hydroperoxide in decade [TBHP
decade)], cumyl hydroperoxide (CHP), hydrogen peroxide,
and potassium peroxodisulfate, respectively, for the trans-
formation (Fig. 2). The reaction was conducted at 120 °C in
xylene for 360 min, with 2 equivalents of 2-aminophenol,
in the presence of 5 mol% catalyst, using three equivalents
of the oxidant. Potassium peroxodisulfate was found to be
inappropriate for this transformation, and no evidence of
2
-phenylbenzo[d]oxazole was detected after 360 min. The
reaction using hydrogen peroxide aꢁorded only 15% yield
after 360 min, while 24% yield was recorded for that using
cumyl hydroperoxide. Tert-butyl hydroperoxide displayed
better performance, in which using the oxidant in water led
to 49% yield after 360 min, and using the one in decane
resulted in 70% yield after 360 min. In this series of oxidants,
Fig. 2 Yields of 2-phenylbenzo[d]oxazole versus oxidant
Fig. 3 Yields of 2-phenylbenzo[d]oxazole versusoxidant quantity
1
3

A New Pathway to 2-Arylbenzoxazoles and 2-Arylbenzothiazoles Via One-Pot Oxidative Cyclization…
Fig. 4 Yields of 2-phenylbenzo[d]oxazole versus solvent
Fig. 5 Yields of 2-phenylbenzo[d]oxazole versus catalyst amount
Benzene, toluene, and mesitylene exhibited poor perfor-
mance, producing the desired product in 24%, 34%, and 42%
yields, respectively after 360 min. The cyclization reaction
was accelerated when conducted in chlorobenzene, and
oxidant, in the presence of 5 mol% catalyst, with 1, 1.5,
2, and 2.5 equivalents of 2-aminophenol, respectively. The
reaction using 1 equivalent of benzyl alcohol aꢁorded only
43% yield after 360 min, while this value was upgraded to
63% for that employing 1.5 equivalents of benzyl alcohol.
Nevertheless, utilizing more than two equivalents of benzyl
alcohol did not lead to a remarkable improvement in the
yield of 2-phenylbenzo[d]oxazole.
5
3% yield was recorded after 360 min. Among this solvent
series, xylene emerged as the best solvent for the synthesis
of 2-phenylbenzo[d]oxazole via the cyclization transforma-
tion, achieving 88% yield after 360 min.
One more factor that must be explored for the cycliza-
tion reaction between 2-aminophenol and benzyl alcohol to
produce 2-phenylbenzo[d]oxazole is the required catalyst
amount (Fig. 5). The reaction was conducted in xylene at
Since the cyclization reaction between 2-aminophenol
and benzyl alcohol to produce 2-phenylbenzo[d]oxazole
1
20 °C for 360 min, with 2 equivalents of 2-aminophenol,
using three equivalents of di-tert-butyl peroxide as the
oxidant, in the presence of 1 mol%, 3 mol%, 5 mol%, and
7
mol% catalyst, respectively. It was noted that 5% yield
of 2-phenylbenzo[d]oxazole was recorded after 360 min,
verifying that the catalyst was obligatory for the cycliza-
tion reaction. The yield of the expected product was sub-
stantially accelerated in the presence of the iron-organic
framework catalyst. The transformation employing 1 mol%
catalyst managed to reach 57% yield after 360 min. Extend-
ing the catalyst amount to 3 mol% led to the production of
2
-phenylbenzo[d]oxazole in 72% yield after 360 min. As
previously noted, the reaction continued to 88% yield after
3
60 min when 5 mol% catalyst was utilized. Expanding the
catalyst range to 7 mol% slightly improved the reaction yield
to 91% after 360 min. Furthermore, the inꢃuence of ben-
zyl acohol:2-aminophenol molar ratio to the generation of
2
-phenylbenzo[d]oxazole should be considered (Fig. 6). The
reaction was implemented in xylene at 120 °C for 360 min,
Fig. 6 Yields of 2-phenylbenzo[d]oxazole versus benzyl acohol:2-
using three equivalents of di-tert-butyl peroxide as the
aminophenol molar ratio
1
3

S. H. Doan et al.
employing the Fe-MOF catalyst was implemented in solu-
tion phase, the leaching analysis must be conducted. In a
number of situations, despite the fact that a solid catalyst
was applied, the reaction did not progress under real hetero-
geneous catalysis circumstances attributable to the leaching
matter. In order to test if iron species migrated from the
solid iron-organic framework donated noticeably to the gen-
eration of 2-phenylbenzo[d]oxazole though the cyclization
reaction, a control experiment was executed (Fig. 7). The
reaction was conducted in xylene at 120 °C for 360 min,
with 2 equivalents of 2-aminophenol, using three equivalents
of di-tert-butyl peroxide as the oxidant, in the presence of
5
mol% catalyst. Succeeding to the ꢂrst 60 min with 48%
yield being noted, the solid iron-based framework catalyst
was removed. The liquid phase was afterwards transferred
to a fresh reactor, and heated at 120 °C for an additional
3
00 min. The yield of 2-phenylbenzo[d]oxazole during this
period was assessed by GC analysis as previously described.
It was noted that virtually no further product produced by
leached iron species, if any, was probed during this experi-
ment. These data would aꢀrm that the cyclization reaction
between 2-aminophenol and benzyl alcohol to produce
Fig. 8 Yields of 2-phenylbenzo[d]oxazole versus catalyst poison
was carried out in xylene at 120 °C for 360 min, with 2
equivalents of 2-aminophenol, using three equivalents of
di-tert-butyl peroxide as the oxidant, in the presence of
5 mol% catalyst. Succeeding to the ꢂrst 60 min with 48%
yield being detected, pyridine was added to the reactor. The
reaction mixture was subsequently heated at 120 °C for
an extra 300 min. Almost no additional 2-phenylbenzo[d]
oxazole was detected when pyridine as the catalyst poi-
son was present in the reaction mixture. This observation
aꢀrmed that the interaction between pyridine as a Lewis
base and iron as an Lewis acid resulted in the deactivation
of the Fe-MOF catalyst. In another experiment series, in
order to substantiate the prerequisite of the oxidant for this
cyclization reaction, curcumin, ((1E,6E)-1,7-bis(4-hydroxy-
2
-phenylbenzo[d]oxazole was able to progress only with the
solid iron-based framework, and the contribution of homo-
geneous catalysis to the production of 2-phenylbenzo[d]
oxazole was inconsequential.
In order to comprehend the pathway of the cyclization
reaction between 2-aminophenol and benzyl alcohol to
produce 2-phenylbenzo[d]oxazole, more control experi-
ments were performed. In the ꢂrst experiment series, pyri-
dine was utilized as catalyst poison (Fig. 8). The reaction
3
-methoxyphenyl) -1,6-heptadiene-3,5-dione), and TEMPO
(2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) were utilized
as the antioxidant (Fig. 9). Following the ꢂrst 60 min with
8% yield being noted, curcumin was added to the reactor,
(
4
and the reaction mixture was subsequently heated at 120 °C
for an extra 300 min. It was conꢂrmed that the cycliza-
tion reaction did not continue in the presence of curcumin.
Similarly, the presence of TEMPO in the reaction mixture
restrained the generation of 2-phenylbenzo[d]oxazole. These
data veriꢂed that curcumin or TEMPO trapped the radicals
generated in the catalytic cycle, accordingly discontinuing
the cyclization.
Based on previous reports [46–49] and the above results,
a plausible reaction pathway was proposed (Scheme 2). The
reaction may proceed via following steps. Initially, tert-
butoxide anion and tert-butoxy radical would be formed
through the cleavage of DTBP with the assistance of Fe(II).
Hydrogen abstraction of benzaldehyde by the tert-butoxy
Fig. 7 2-Phenylbenzo[d]oxazole was only produced with the solid
iron-based framework
1
3

A New Pathway to 2-Arylbenzoxazoles and 2-Arylbenzothiazoles Via One-Pot Oxidative Cyclization…
radical would form an acyl radical. At the same time, the
reaction between 2-aminophenol and tert-butoxide anion
would result in anion intermediate 5. The key C–O bond
formation step would occur through the radical addition
between the acyl radical and intermediate 5 to provide
the radical anion 6. Subsequently, single electron transfer
between 6 and Fe(III) would form 2-aminophenyl benzo-
ate and regenerate Fe(II), which would continue the cata-
lytic cycle. Indeed, GC-MS analysis indicated the presence
of 2-aminophenyl benzoate in the reaction mixture (Fig.
S26). At the final step, 2-aminophenyl benzoate would
undergo the catalyst-free condensation process to generate
2
-phenylbenzo[d]oxazole. Additionally, it should be noted
that xylene was not oxidized under the present experimen-
tal conditions, with no oxidized products being detected by
GC-MS.
To insist the remarkable points of this work, the cata-
lytic activity of MOF-235 in the cyclization reaction
between 2-aminophenol and benzyl alcohol to produce
Fig. 9 Yields of 2-phenylbenzo[d]oxazole versus antioxidant
2
-phenylbenzo[d]oxazole was correlated with that of
Scheme 2 Plausible reaction pathway
1
3

S. H. Doan et al.
MOF-based catalysts, including Fe O(BPDC) , Fe-BTC,
3
3
Cu (BDC) (DABCO), Cu (OBA) (BPY), Cu (BTC) ,
2
2
2
2
3
2
Co (BDC) (DABCO), and Ni (BDC) (DABCO) (Fig. 11).
2
2
2
2
The reaction was conducted in xylene at 120 °C for 360 min,
with two equivalents of 2-aminophenol, using three equiva-
lents of di-tert-butyl peroxide as the oxidant, in the presence
of 5 mol% catalyst. Ni (BDC) (DABCO) was noted to be
2
2
unsuitable for the synthesis of 2-phenylbenzo[d]oxazole,
reaching only 27% yield after 360 min. In the same way,
Co (BDC) (DABCO) displayed low eꢀciency in the cycli-
2
2
zation reaction, providing the desired product in 35% yield
after 360 min. Copper-based frameworks also presented low
activity in the cyclization transformation. The reaction using
Cu (BTC) catalyst provided 39% yield of 2-phenylbenzo[d]
3
2
oxazole after 360 min, while 42% yield was observed for
the case of Cu (OBA) (BPY). Similarly, in the presence of
2
2
Cu (BDC) (DABCO) as catalyst, the reaction progressed to
2
2
4
8% yield after 360 min. Fe-BTC emerged to have lower
catalytic eꢀciency in the synthesis of 2-phenylbenzo[d]
oxazole as other iron-organic frameworks, although the
transformation reached 49% yield after 360 min under this
condition. The yield of the major benzoxazole product was
amended to 77% after 360 min when Fe O(BPDC) was
Fig. 10 Yields of 2-phenylbenzo[d]oxazole versus homogeneous iron
catalysts
3
3
employed as catalyst. In this series of MOF-based catalysts,
MOF-235 emerged as the most appropriate candidate, pro-
viding the principal product in 88% yield after 360 min.
This could be attributed to the diꢁerences in the structure
of these Fe-MOFs. The structure of MOF-235 is decorated
numerous iron homogeneous catalysts, including FeCl ,
2
FeBr , Fe(OAc) , FeCl , and Fe (SO ) (Fig. 10). The reac-
2
2
3
2
4 3
tion was implemented in xylene at 120 °C for 360 min, with
two equivalents of 2-aminophenol, using 3 equivalents of di-
tert-butyl peroxide as the oxidant, in the presence of 5 mol%
catalyst. It was noted that the nature of anions in these iron
salts displayed an extraordinary eꢁect on the catalytic eꢀ-
ciency. The cyclization reaction progressed steadily in the
presence of Fe (SO ) catalyst, providing only 15% yield
2
4 3
of the expected product after 360 min. Similarly, Fe(OAc)
2
catalyst was noticed to be inappropriate for the synthesis of
2
-phenylbenzo[d]oxazole, with 23% yield being recorded
after 360 min. FeCl oꢁered higher catalytic activity than
3
Fe (SO ) and Fe(OAc) , and the FeCl -catalyzed cycliza-
2
4 3
2
3
tion reaction aꢁorded 43% yield of 2-phenylbenzo[d]oxa-
zole after 360 min. The yield of the desired product was
improved to 52% after 360 min when FeCl was utilized as
2
catalyst. FeBr emerged to be more active than FeCl for
3
2
the production of 2-phenylbenzo[d]oxazole, and 67% yield
was obtained after 360 min. As previously noted, the reac-
tion employing MOF-235 catalyst progressed to 88% yield
of 2-phenylbenzo[d]oxazole under similar conditions, thus
exhibiting superior aspects than homogeneous iron-based
catalysts.
To additionally highlight the beneꢂt of MOF-235 for
the cyclization reaction between 2-aminophenol and
benzyl alcohol to produce 2-phenylbenzo[d]oxazole,
its performance was differentiated with that of other
Fig. 11 Yields of 2-phenylbenzo[d]oxazole versus heterogeneous iron
catalysts
1
3

A New Pathway to 2-Arylbenzoxazoles and 2-Arylbenzothiazoles Via One-Pot Oxidative Cyclization…
and expanded form of the acs net with ꢃexible links, in
which each six-coordinate vertex is substituted by a trigonal
prism and separated from each other by the two-coordinated
organic links [40–42]. In comparison, both Fe-BTC [50] and
Fe O(BPDC) [51, 52] possess a rigid structure in the frame-
reused MOF-235 were 16.8% (wt/wt) and 16.5% (wt/wt),
respectively. Moreover, the structure of the iron-based was
retained under these reaction conditions, as demonstrated by
XRD (Fig. 13) as well as FT-IR (Fig. 14) analysis results.
With all of these results in hand, we additionally inves-
tigated the synthesis of 2-arylbenzoxazoles and 2-arylben-
zothiazoles via the one-pot oxidative cyclization reaction
employing the iron-based framework catalyst (Table 1). In
the ꢂrst experiment series, the reaction was implemented in
xylene at 120 °C for 360 min, with 2 equivalents of 2-ami-
nophenol, using three equivalents of di-tert-butyl peroxide
as the oxidant, in the presence of 5 mol% catalyst. The prod-
uct was isolated on silica gel column chromatography. Fol-
lowing this protocol, 2-phenylbenzo[d]oxazole was achieved
in 84% isolated yield via the cyclization reaction between
2-aminophenol and benzyl alcohol (Entry 1). It was noticed
that benzyl alcohol possessing substituent reacted readily
with 2-aminophenol under these condition. Certainly, 72% of
2-(4-bromophenyl)benzo[d]oxazole (Entry 2) was obtained
for the reaction of 4-bromobenzyl alcohol, while 85% yield
3
3
works, thus making active sites less accessible to reactants.
As previously noted, the MOF-235 displayed higher eꢀ-
ciency than a number of homogeneous and heterogeneous
catalysts in the cyclization reaction between 2-aminophe-
nol and benzyl alcohol to produce 2-phenylbenzo[d]oxa-
zole. To additionally underline the extraordinary features
of using this iron-organic framework, one important aspect
that must be explored is the readiness of reusability after
each catalytic run. For organic transformations conducted
with heterogeneous catalysts, it would be demanded that the
catalyst could be recovered and reutilized for numerous runs.
The iron-organic framework was accordingly considered for
reusability in the synthesis of 2-phenylbenzo[d]oxazole. The
reaction was performed in xylene at 120 °C for 360 min,
with 2 equivalents of 2-aminophenol, using three equiva-
lents of di-tert-butyl peroxide as the oxidant, in the presence
of 5 mol% catalyst. Following the ꢂrst run with 88% yield
of 2-phenylbenzo[d]oxazole being recorded, the iron-based
catalyst was collected, washed thoroughly with methanol to
remove product and excess reagents, heated under vacuum
on a Shlenkline at 120 °C for 3 h, and reutilized for new
catalytic experiment. Experimental data revealed that it was
possible to reutilize the catalyst for the cyclization reaction
between 2-aminophenol and benzyl alcohol to produce
2
-phenylbenzo[d]oxazole. In the 6th catalytic run, 85% yield
of the major product was detected (Fig. 12). ICP-OES analy-
sis results indicated that the amount of iron in the fresh and
Fig. 13 XRD results of the new (a) and reutilized (b) catalyst
Fig. 12 Catalyst reutilizing investigation
Fig. 14 FT-IR results of the new (a) and reutilized (b) catalyst
1
3

S. H. Doan et al.
Table 1 The synthesis of
Entry
1
Reactant 1
Reactant 2
Product
Isolated
yield (%)
84
2
2
-arylbenzoxazoles and
-arylbenzothiazoles via the
one-pot oxidative cyclization
reaction
OH
O
N
OH
NH2
OH
2
3
4
5
O
72
85
86
85
OH
Br
Cl
O
N
Br
Cl
NH2
OH
O
N
OH
OH
NH2
OH
O
N
O
NH2
OH
OH
O
N
NH2
OH
6
7
8
9
O
87
82
69
80
OH
N
NH2
OH
O
N
OH
NH2
SH
S
OH
Br
N
Br
NH2
SH
S
OH
NO2
N
O N
2
NH2
of 2-(4-chlorophenyl)benzo[d]oxazole was recorded for the
case of 4-chlorobenzyl alcohol (Entry 3). Benzyl alcohol
with electron-donating substituent was reactive towards
the cyclization reaction with 2-aminophenol, generating
between 2-aminothiophenol and 4-bromobenzyl alcohol.
4-Nitrobenzyl alcohol was reactive towards the cylization
reaction, forming 2-(4-nitrophenyl)benzo[d]thiazole in 80%
yield (Entry 9).
2
4
-(4-methoxyphenyl)benzo[d]oxazole in 86% yield (Entry
). In the same way, 2-o-tolylbenzo[d]oxazole was pro-
duced in 85% yield for the case of 2-methyl benzyl alcohol
4 Conclusions
(
Entry 5), and 2-p-tolylbenzo[d]oxazole was achieved in
8
6
7% yield for the case of 4-methyl benzyl alcohol (Entry
). 2-Amino-4-methylphenol was also reactive towards the
Iron-organic framework MOF-235 was synthesized, and
consequently utilized as an eꢁective heterogeneous catalyst
for the synthesis of 2-arylbenzoxazoles and 2-arylbenzothia-
zoles via one-pot oxidative cyclization reactions between
2-aminophenols or 2-aminothiophenols and alcohols. The
solvent character expressed a considerable impact on the
cyclization reaction, and xylene was the most appropriate
transformation, generating 5-methyl-2-phenylbenzo[d]oxa-
zole in 82% yield (Entry 7). Next, the transformation was
expanded to the cyclization of 2-aminothiophenol. Employ-
ing the same approach, 69% yield of 2-(4-bromophenyl)
benzo[d]thiazole (Entry 8) was achieved via the reaction
1
3

A New Pathway to 2-Arylbenzoxazoles and 2-Arylbenzothiazoles Via One-Pot Oxidative Cyclization…
solvent for this synthetic protocol. The transformation
required the presence of an oxidant, and the system of di-
tert-butylperoxide with xylene led to best yield of major
products. The MOF-235 catalyst presented higher catalytic
eꢁectiveness for the synthesis of 2-arylbenzoxazoles and
20. Song B-Q, Chen D-Q, Ji Z, Tang J, Wang X-L, Zang H-Y et al
(
2017) Chem Commun 53:1892–1895
2
1. Rogge SMJ, Bavykina A, Hajek J, Garcia H, Olivos-Suarez AI,
Sepúlveda-Escribano A et al (2017) Chem Soc Rev 46:3134–3184
22. Sudarsanam P, Zhong R, Van den Bosch S, Coman SM, Par-
vulescu VI, Sels BF (2018) Chem Soc Rev 47:8349–8402
2
3. Dhakshinamoorthy A, Li Z, Garcia H (2018) Chem Soc Rev
2-arylbenzothiazoles than a number of MOF-based catalysts
4
7:8134–8172
and established homogeneous catalysts. The cyclization
reaction was able to progress only with the solid iron-based
framework, and the contribution of homogeneous catalysis
to the production of major products was inconsequential. It
was feasible to recover and reutilize the framework catalyst
for the cyclization transformation various times while its
catalytic eꢀciency was retained. The beneꢂts that 2-arylb-
enzoxazoles and 2-arylbenzothiazoles could be synthesized
through one-pot oxidative cyclization reactions between
2
4. Zhu L, Liu X-Q, Jiang H-L, Sun L-B (2017) Chem Rev
117:8129–8176
25. Xiong G, Yu B, Dong J, Shi Y, Zhao B, He L-N (2017) Chem
Commun 53:6013–6016
2
6. Cirujano FG, López-Maya E, Rodríguez-Albelo M, Barea E, Nav-
arro JAR, Vos DED (2017) ChemCatChem 9:4019–4023
27. Shao Z, Mengjia Liu JD, Huang C, Xu W, Jie Wu, and Hongwei
Hou. Inorg Chem.57:10224–10231
2
8. Xu Z, Meng W, Li H, Hou H, Fan Y (2014) Inorg Chem
5
3:3260–3262
2
9. Wu Q, Han Y, Shao Z, Li J, Hou H (2018) Dalton Trans
47:8063–8069
2
-aminophenols or 2-aminothiophenols and alcohols uti-
lizing a recyclable catalyst would provide advantages over
classical homogeneous approaches, and therefore would be
engaging to the chemical industry.
30. Huang Y-B, Liang J, Wang X-S, Cao R (2017) Chem Soc Rev
4
6:126–157
1. Chughtai HA, Ahmad N, Younus HA, Laypkov A, Verpoort F
2015) Chem Soc Rev 44:6804–6849
3
(
3
2. Drake T, Ji P, Lin W (2018) Acc Chem Res 51:2129–2138
Acknowledgements The Viet Nam National University—Ho Chi Minh
City (VNU-HCM) is acknowledged for ꢂnancial support via project
No. NV2019-20-02.
33. Oar-Arteta L, Wezendonk T, Sun X, Kapteijn F, Gascon J (2017)
Mater Chem Front 1:1709–1745
34. Chen Y-Z, Zhang R, Jiao L, Jiang H-L (2018) Coord Chem Rev
3
62:1–23
3
5. Wen Y, Zhang J, Xu Q, Wu X-T, Zhu Q-L (2018) Coord Chem
Rev 376:248–276
References
3
3
6. Liang J, Huang Y-B, Cao R (2019) Coord Chem Rev 378:32–65
7. Dhakshinamoorthy A, Garcia H (2014) Chem Soc Rev
43:5750–5765
1
2
3
4
5
.
.
.
.
.
Gong J, Huang L, Deng Q, Jie K, Wang Y, Guo S et al (2017) Org
Chem Front 4:1781–1784
38. Dhakshinamoorthy A, Asiri AM, Garcia H (2019) ACS Catal
9:1081–1102
Prajapati NP, Vekariya RH, Borad MA, Patel HD (2014) RSC Adv
4
:60176–60208
39. Dhakshinamoorthy A, Asiri AM, Garcia H (2015) Chem Soc Rev
44:1922–1947
Bougrin K, Loupy A, Soufiaoui M (1998) Tetrahedron
5
4:8055–8064
40. Haque E, Jun JW, Jhung SH (2011) J Hazard Mater 185:507–511
41. Anbia M, Hoseini V, Sheykhi S (2012) J Ind Eng Chem
18:1149–1152
Wu F, Zhang J, Wei Q, Liu P, Xie J, Jiang H et al (2014) Org
Biomol Chem 12:9696–9701
Aksenov NA, Aksenov AV, Nadein ON, Aksenov DA, Smirnov
AN, Rubin M (2015) RSC Adv 5:71620–71626
42. Sudik AC, Côté AP, Yaghi OM (2005) Inorg Chem 44:2998–3000
43. Le TD, Nguyen KD, Nguyen VT, Truong T, Phan NTS (2016) J
Catal 333:94–101
6
7
.
.
Tiwari AR, Bhanage BM (2016) Org Biomol Chem 14:7920–7926
Lin W-H, Wu W-C, Selvaraju M, Sun C-M (2017) Org Chem
Front 4:392–397
44. Ha PTM, Le TD, Doan SH, Nguyen TT, Le NTH, Phan NTS
(2017) Tetrahedron 73:5883–5891
8
9
.
.
Naresh G, Kant R, Narender T (2014) J Org Chem 79:3821–3829
Toni Hille T, Irrgang KR, (2014) Chem Eur J 20:5569–5572
45. Doan SH, Nguyen KD, Huynh PT, Nguyen TT, Phan NTS (2016)
J Mol Catal A 423:433–440
1
1
1
0. Lustig WP, Mukherjee S, Rudd ND, Desai AV, Li J, Ghosh SK
46. Li Z, Fan F, Yang J, Liu Z-Q (2014) Org Lett 16:3396–3399
47. Liu Q, Jackstell R, Beller M (2013) Angew Chem Int Ed
52:13871–13873
(
2017) Chem Soc Rev 46:3242–3285
1. Bao Z, Chang G, Xing H, Krishna R, Ren Q, Chen B (2016)
Energy Environ Sci 9:3612–3641
48. Liu C, Liu D, Lei A (2014) Acc Chem Res 47:3459–3470
49. Teng F, Sun S, Jiang Y, Yu J-T, Cheng J (2015) Chem Commun
51:5902–5905
2. Zou F, Chao SL, Wang YX, Wang YL, Guan QX, Li W (2017)
Environ Sci 4:46–51
1
1
3. Qin L, Zheng H-G (2017) CrystEngComm 19:745–757
4. Stassen I, Burtch N, Talin A, Falcaro P, Allendorf M, Ameloot R
50. Horcajada P, Surblé S, Serre C, Hong D-Y, Seo Y-K, Chang J-S
et al. (2007) Chem Commun. https://doi.org/10.1039/b704325b
51. Dan-Hardi M, Chevreau H, Devic T, Horcajada P, Maurin G,
Férey G et al (2012) Chem Mater 24:2486–2492
52. Baburin IA, Blatov VA, Carlucci L, Ciani G, Proserpio DM (2005)
J Solid State Chem 178:2452–2474
(
2017) Chem Soc Rev 46:3185–3241
1
1
5. Julien PA, Mottillo C, Friščić T (2017) Green Chem 19:2729–2747
6. Li P, Cheng F-F, Xiong W-W, Zhang Q (2018) Inorg Chem Front
5
:2693–2708
1
1
1
7. Lv X-X, Shi L-L, Li K, Li B-L, Li H-Y (2017) Chem Commun
3:1860–1863
8. Al-Ghoul M, Issa R, Hmadeh M (2017) CrystEngComm
5
Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional aꢀliations.
1
9:608–612
9. Qin J-S, Yuan S, Wang Q, Alsalme A, Zhou H-C (2017) J Mater
Chem A 5:4280–4291
1
3