Sustainable Synthesis of 2-Arylbenzoxazoles over a cobalt-based nanocomposite catalyst

Article abstract of DOI:10.1021/acs.oprd.6b00168

A new cobalt-based heterogeneous catalyst was prepared by adsorbing in situ produced nitrogen-ligated cobalt(II) acetate complexes on commercially available SiO2 and subsequent pyrolysis at 800 °C for 2 h under N2 atmosphere. By applying this catalyst under the O2 balloon, the aerobic oxidation of phenolic imines proceeded smoothly and gave various 2-arylbenzoxazoles in good yields. Meanwhile, on the basis of the experimental results, a plausible reaction pathway was described to elucidate the reaction mechanism.

Full text of DOI:10.1021/acs.oprd.6b00168

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Full Paper
Sustainable Synthesis of 2-Arylbenzoxazoles
Over a Cobalt-based Nanocomposite Catalyst
Jian He, Fu Lin, Xufang Yang, Di Wang, Xiaohua Tan, and Shujun Zhang
Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.6b00168 • Publication Date (Web): 17 May 2016
Downloaded from http://pubs.acs.org on May 30, 2016
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Sustainable Synthesis of 2-Arylbenzoxazoles
over a Cobalt-based Nanocomposite Catalyst
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a*
b
c
d
b
e*
Jian He, Fu Lin, Xufang Yang, Di Wang, Xiaohua Tan, Shujun Zhang
a. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
China
b. College of Pharmacy, Wenzhou Medical University, Wenzhou 325035, China
c. Mudanjiang Medical University, Mudanjiang 157011, China
d. School of Life Sciences, Jilin University, Changchun 130012, China
e. Harbin Institute of Chemical Engineering, Harbin 150030, China
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Table of Contents:
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ABSTRACT: A new cobalt-based heterogeneous catalyst was prepared by adsorbing
in situ produced nitrogen-ligated cobalt (II) acetate complexes on commercially
available SiO and subsequent pyrolysis at 800 °C for 2 h under N atmosphere. By
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2
2
applying this catalyst under the O
2
balloon, the aerobic oxidation of phenolic imines
proceeded smoothly and gave various 2-arylbenzoxazoles in good yields. Meanwhile,
on the basis of the experimental results, a plausible reaction pathway was described to
elucidate the reaction mechanism.
KEYWORDS: heterogeneous catalysis; cobalt nanoparticles; oxidative cyclization;
benzoxazoles.
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INTRODUCTION
The interaction between central metal with ligand or support is a key technology in
the design of catalyst. The facile control of their interactions has been of
long-standing interest for chemists to exploit catalytic performance. In this context,
many versatile heterogeneous catalysts were mainly developed by controlling the size
and shape of the active metal species, and modifying the composition of the supports.
Despite numerous inherent advantages of heterogeneous catalysts over homogeneous
ones, for example, their reusability and durability, the fine-tuning of metal-ligand
interactions in heterogeneous catalysts is much more difficult than in homogeneous
catalysts, and still a challenging task. Thus, the application of organmetallic
complexes as precursors for the synthesis of stable heterogeneous catalysts is still
need to be well established.
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On the other hand, in the community of fused heterocycles, benzoxazole is common
and has been referred as “core structure” in drug discovery. Benzoxazole derivatives
displayed extensive biological activities such as anti-inflammatory, antitumor,
2
antidepressant, antibiotic and tranquilizer (Scheme 1). In past decades, many
3
strategies for their construction have been reported. Among them, the catalytic
oxidative cyclization of phenolic imines has attracted sizeable attention for its unique
features. Various homogeneous catalysts such as Ruthenium/Iridium complexes,
Pd(OAc)
2
,
RuCl
3
,
Ortho-oiodoxybenzoic acid (IBX), 4-methoxy-TEMPO
4
3 2
(2,2,6,6-tetramethyl-piperidyl-1-oxy TEMPO), FeCl , CuCl , NaCN were employed.
More recently, the development of easily recoverable and recyclable heterogeneous
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catalysts that can solve the problem of the homogeneous systems has captured special
attention, including activated carbon, copper nanoparticles, platinum nanoclusters,
ZnO nanoparticles and organic photocatalyst eosin Y bis (tetrabutylammonium salt)
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(
TBA-eosin Y). However, in most cases, tedious procedure for the catalyst synthesis,
high reaction temperatures, noble metals and/or additives are involved. Therefore, a
new sustainable catalytic system to synthesis these compounds under a mild condition
is highly desirable.
Scheme 1. Prevalent benzoxazole functionality in pharmaceuticals
O
N
COOH
Cl
Cl
F
O
N
O
COOH
HOOC
N
HO
Flunoxaprofen
Caboxamycin
Tafamidis
In 2013, Beller and co-workers carried out pioneering studies on heterogenized cobalt
oxide catalysts for the selective hydrogenation of nitroarenes by pyrolysis of
21
organometallic complexes. Subsequently, they designed a series of Co- and Fe-oxide
6
supported catalysts for the selective reduction and oxidative reactions. Inspired by
these findings, we became interested in applying these catalysts in the aerobic
oxidation of phenolic imines. Herein, we wish to report a protocol to prepare a
CoO
x
-NGr/SiO
2
nanocomposite catalyst through pyrolysis of organometallic
complexes. The aerobic oxidation of phenolic imines proceeded smoothly promoted
by the catalyst and the corresponding benzoxazoles were obtained in good to excellent
yields. Depending on ligands we used, we modified the catalyst structure and notable
differences in catalyst activity were observed. Therefore, the ligand indirectly controls
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the selectivity and activity of the title reaction. Furthermore, silica supported catalysts
are hopeful for academic and industrial applications due to their high mechanical and
thermal stabilities, noncorrosive and easy separation feature of the catalyst from the
reaction mixture.
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RESULTS AND DISCUSSION
We began our studies by preparing various cobalt-based catalysts from different
cobalt salts and nitrogenous ligands (Table 1) by adsorbing a solution of the
respective cobalt complex on SiO
evaluated their performance in catalyzing the oxidative cyclization of phenolic imine
a. Cobalt oxide particles were expected to form which were surrounded by
nitrogen-doped graphene layers (NGr) loaded on SiO surface. Table 1 summarized
2 2
and subsequent pyrolysis under N , and then we
1
2
some results obtained during the optimization of the reaction. Among the different
candidates nitrogenous ligands, 1, 10-phenanthroline (L5) was considered to be the
most active catalytic system, and 2-phenylbenzoxazole 2a was formed in a yield of
87% (Table 1, entry 6). The other similar nitrogenous ligands such as 2,2'-bipyridine,
pyridine etc. gave no product at all (Table 1, entries 1-5). Therefore, comparably small
changes in the organic ligand caused wide loss of selectivity and reactivity. The title
2 2
reaction did not occur in the presence of pyrolyzed SiO -supported Co(OAc) , and L5
(Table 1, entries 7-8). Next, the pyrolysis of Co(OAc) -L5 on Al O , TiO , and
2
2
3
2
activated carbon supports gave also active catalysts which delivered
2-phenylbenzoxazole in 45-79% yields(Table 1, entries 9-11). All these results
2
showed that the Co-L5 supported on SiO will be the most active catalyst. It is
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demonstrated in the Table 1 (Table 1, entry 12) that O
2
is crucial for the oxidative
process.
Table 1. Optimization of the Aerobic Oxidation of Phenolic Imine 1a
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pyrolysis
yield
a
entry support
ligand
cobalt salt
none
o
b
[
-
C, h, gas]
[%]
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9
10
none
SiO
SiO2
none
trace
N.R.
N.R.
N.R.
N.R.
87
2
pyridine
Co(OAc)
2
•4H
2
O 800, 2, N
2
2
2
2
2
2
quinoline
Co(OAc) •4H O 800, 2, N
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2
2
2
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SiO
SiO
SiO
SiO
2
2
2
2
quinolin-8-ol
Co(OAc)
Co(OAc)
Co(OAc)
Co(OAc)
none
•4H
•4H
•4H
•4H
O 800, 2, N
O 800, 2, N
O 800, 2, N
O 800, 2, N
2,2’-bipyridine
1,10-phenanthroline
none
trace
c
SiO2
Al
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
800, 2, N2
N.R.
45
2
O
3
Co(OAc)
Co(OAc)
Co(OAc)
Co(OAc)
2
2
2
2
•4H
•4H
•4H
•4H
2
2
2
2
O 800, 2, N
O 800, 2, N
O 800, 2, N
O 800, 2, N
2
2
2
2
TiO
C
2
53
11
79
d
12
SiO
2
trace
a
Unless otherwise noted, reactions were carried out with 1a (0.5 mmol), cobalt
o
catalyst (3 mol% Co) in 1,4-dioxane (1.0 mL) at 65 C for 24 h under O balloon.
2
b
Isolated yields.
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c
N.R. = no reaction
In the presence of N
d
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balloon.
Characterization of the metal species in the active catalyst (CoO
x 2
-N-Gr/SiO ) by XRD,
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HRTEM, XPS demonstrated the preferential formation of smallCoO particles of
x
2
0-80 nm in size on a carbon-nitrogen-based support surface. On the other hand, a
few larger particles (20-80 nm) are also present. XRD patterns (Figure 1(a)) of
o
samples pyrolysed at 800 C showed that they contained Co(0), CoO
x
and CoN
samples reveal the
presence of Si, O, C, N as well as Co without any other impurities, confirming that
x
species. As shown in Fig. 1b), the survey spectra of Co-N-Gr/SiO
2
2
nitrogen and cobalt were successfully incorporated into the SiO .
Figure 1. Catalyst Characterization
Based on the optimized catalytic system already in hand (Table 1, entry 6), we
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explored the scope of the present aerobic oxidation of phenolic imines. As shown in
Table 2, the results revealed that electronic nature and steric bulk of the substituents
on the aromatic imines had a slight influence on the catalytic performance (Table 2,
entries 1-9). Furthermore, important functional groups, for example nitro, chloro,
bromo, allyl, were tolerable under mild reaction conditions, thus giving the
functionalized benzoxazole derivatives with 87-99% yields (Table 2, entries 6-10). In
the case of imines derived from 2-naphthaldehyde, 92% yield could also be obtained
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(
Table 2, entry 11). Meanwhile, imines derived from heteroaromatic aldehydes
2-pyridinyl and 2-furyl) were also suitable substrates to afford the desired adducts
(
with 86-95% yields (Table 2, entries 12-13). However, by using aliphatic aldehyde,
the corresponding product was produced in a lower yield (Table 2, entry 14). Because
imines bearing aliphatic groups are less unstable than aryl ones, and these imines may
undergo undesirable side-reactions under our catalytic conditions. In order to show
the synthetic utility of the catalyst system, the gram-scaled reaction was carried out
with a gram of imine 1a. The reaction was carried out smoothly and we obtained the
desired product 2a with 93% yield.
Scheme 2. The Substrate Scope of 2-Arylbenzoxazoles
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The cobalt catalyst was filtered off from the reaction mixture after 36% conversion of
imine 1a. Continued stirring of the filtrate under identical reaction conditions for 12 h
did not promote the reaction to proceed any more. ICP-AES analysis of the filtrate
showed only negligible amount of Co (0.28 ppm corresponding to 1.7 wt % of initial
charge) was leached into the solution. We also carried out recovery and reuse studies
with CoO -NGr/SiO catalyst for the oxidative cyclization reaction of Schiff base 1a
x
2
and we found that the catalyst could be reused five cycles without noticeable loss of
catalytic activity (for details see Supporting Information). These results confirmed
that the oxidative coupling reaction was indeed catalyzed by the heterogeneous
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catalyst. A possible oxidative cyclization mechanism for this reaction has been
depicted in Scheme 3. Initially, the intramolecular nucleophilic addition of hydroxyl
group to the C=N double bond provides hemiaminal A. Notably, this intermediate is
unstable and immediately undergoes Co-catalyzed aerobic cyclization to generate the
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desired product 2. Conversely, 1 could be hydrolyzed by H
-aminophenol and aldehydes, these reactants could be oxidized to their
corresponding byproducts ortho-quinones and acids.
2
O to regenerate
2
Scheme 3. The mechanism for the CoO
synthesis 2-aryl benzoxazole
x
-NGr/SiO
2
catalyzed aerobic oxidative
CONCLUSIONS
In summary, we have designed a CoO
x
-NGr/SiO
2
nanocomposite catalyst for the
aerobic oxidation of phenolic imines to give various benzoxazoles in good to
excellent yields. The cobalt catalytic system could be easily recovered from the
reaction mixture and reused five times without any loss of catalytic activity. By
altering the ligands, several heterogeneous cobalt catalysts were produced and led to
the development of high efficiency catalysts. We believe that this strategy is attractive
for the design of other heterogeneous metal catalysts. Further extension application of
this catalytic system for the preparation of some other kind of useful heterocyclic
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compounds is underway in our laboratory.
EXPERIMENTAL
Materials. Unless otherwise stated, all the reagents and reactants were purchased
from commercial suppliers without further purification. General part including
instruments, synthesis and characterization of various substrates are provided in the
Supporting Information.
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Preparation of CoO -NGr/SiO catalyst.
x
2
Cobalt (II) acetate tetrahydrate (249 mg, 1.0 mmol) and ligand (L) (2.0 mmol) were
o
combined in EtOH (100 mL). The mixture was stirred for 1 h at 25 C. Then, SiO
2
(200-300 mesh, 1.40 g) was added and the whole reaction mixture was refluxed for 4
h. The reaction mixture was cooled to room temperature. The ethanol was removed in
vacuum. The solid sample obtained was dried at 80 °C for 12 h. Then, the grinded
powder was transferred into a ceramic crucible and placed in the oven. In order to
exclude air, N was constantly passed through the oven for 30 min. Then, the oven
2
was heated at 10 °C/min until 800 °C, and was held at 800 °C for 2 h under N
2
(Scheme 4). After the heating was finished, the oven cooled back to room temperature
2
under N . Accordingly, the catalyst was obtained. Elemental analysis of catalyst
Co-L5 (wt%): Si = 28.40, C = 4.51, N = 2.10, Co = 3.42, H = 0.36, O = 61.21.
x 2
Scheme 4. General Procedure for Preparation of CoO -NGr/SiO Catalyst
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General procedure for aerobic oxidation of phenolic imines.
In a glass test tube was added CoO
x
-NGr/SiO
2
(25.0 mg), imine 2 (0.5 mmol), and 1,
o
4-dioxine (1.0 mL). The reaction mixture was stirred for 24 h at 65 C under a O
2
balloon. The crude product was directly purified by column chromatography (EA: PE
1:10) to provide the corresponding 2-substituted benzoxazoles 3. The identity of the
=
1
3
1
product was confirmed by C and H NMR spectroscopic analysis.
ASSOCIATED CONTENT
Supporting Information
General procedures and spectral characterization of compounds 2a-2m are included.
This material is available on http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*
E-mail: jih003@dicp.ac.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This work was supported by the scholarship from China Scholarship Council (No
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010617090), Wenzhou Science and Technology Bureau Research Foundation
(Y20140740) and National Science Foundation of China (81500519) and Wenzhou
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Medical University Research Foundation (QTJ14014).
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