Heterogeneously porous γ-MnO2-catalyzed direct oxidative amination of benzoxazole through C-H activation in the presence of O2

Article abstract of DOI:10.1002/asia.201402057

Oxidative amination of azoles through catalytic C-H bond activation is a very important reaction due to the presence of 2-aminoazoles in several biologically active compounds. However, most of the reported methods are performed under homogeneous reaction conditions using excess reagents and additives. Herein, we report the heterogeneous, porous γ-MnO ₂-catalyzed direct amination of benzoxazole with wide range of primary and secondary amines. The amination was carried under mild reaction conditions and using molecular oxygen as a green oxidant, without any additives. The catalyst can easily be separated by filtration and reused several times without a significant loss of its catalytic performance. Of note, the reaction tolerates a functional group such as alcohol, thus indicating the broad applicability of this reaction.

Full text of DOI:10.1002/asia.201402057

COMMUNICATION
DOI: 10.1002/asia.201402057
Heterogeneously Porous g-MnO2-Catalyzed Direct Oxidative Amination of
À
Benzoxazole through C H Activation in the Presence of O2
Provas Pal, Arnab Kanti Giri, Harshvardhan Singh, Subhash Chandra Ghosh,* and
Asit Baran Panda*[a]
Abstract: Oxidative amination of azoles through catalytic
catalyzed amination of azoles under harsh reaction condi-
tions. Daun et al.[13] also reported the amination of azoles in
the presence of Ni as a catalyst by using acid and organic
peroxide. Miura et al.[14] reported on the amination of azoles
under mild condition using chloramine as a nitrogen source
instead of amine. Wang et al.[15] reported the iron-catalyzed
amination of azoles using DMF as an amine source and stoi-
chiometric amounts of imidazole as an additive. Chang
et al.[16] initially reported the use of a stoichiometric amount
of silver species and acid as an additive for this reaction;
more recently, the same group developed cobalt and manga-
nese-catalyzed amination of azoles in the presence of stoi-
chiometric amounts of T-Hydro (70% tert-butyl hydroperox-
ide in water) as an oxidant and acid as additive.[17] A metal-
free approach for the amination of benzoxazole was report-
ed using iodine and tetrabutylammonium iodide as a catalyst
in the presence of a stoichiometric amount of peroxide as
an oxidant and acid as an additive.[18] However, limitations
of these direct aminations of azoles are harsh reaction con-
ditions and the use of both excess peroxide as an oxidant
and a Brønsted acid; moreover, all reactions were conduct-
ed under homogeneous conditions. Therefore, the develop-
ment of a more efficient, cheap, and reusable catalyst for
amination of azoles is highly desirable.
À
C H bond activation is a very important reaction due to the
presence of 2-aminoazoles in several biologically active
compounds. However, most of the reported methods are
performed under homogeneous reaction conditions using
excess reagents and additives. Herein, we report the hetero-
geneous, porous g-MnO2-catalyzed direct amination of ben-
zoxazole with wide range of primary and secondary amines.
The amination was carried under mild reaction conditions
and using molecular oxygen as a green oxidant, without any
additives. The catalyst can easily be separated by filtration
and reused several times without a significant loss of its cat-
alytic performance. Of note, the reaction tolerates a func-
tional group such as alcohol, thus indicating the broad ap-
plicability of this reaction.
À
The C N bond formation reaction of aromatic and heter-
oaromatic compounds is one of the most important and
vastly used transformations in synthetic organic chemistry.[1]
It is ever demanding simply because of the great interest in
nitrogen-containing heteroaromatic molecules in pharma-
ceutical, biological, and materials sciences.[2] Classically, aryl
halides and pseudo halides with amines or their precursors
À
are used for C N bond formation under transition metal
In view of easy recycling and high catalytic efficiency, het-
erogeneous catalysts have been widely used in recent years.
During the course of our studies on the catalytic activities of
our developed lotus-shaped porous g-MnO2,[19] we anticipat-
catalysis.[3] The strategies towards the synthesis of C N
À
bond are palladium-catalysed Buchwald–Hartwig coupling,[4]
and copper-catalyzed Ullman and Goldberg coupling.[5] Sev-
eral groups have also developed Pd-,[6] Cu-,[7] and Rh[8]-cata-
lyzed C N bond formation with proper ligands. Alternative-
ly, amination of arenes and heteroarenes through catalytic
À
À
ed that C H bond activation followed by oxidative C N
bond formation of heteroarenes with amine could be ach-
ieved with our catalyst in the presence of a suitable oxidant.
À
À
À
C H bond activation is developing very fast in recent
We have made a conscious effort to develop C N bond for-
years.[9] By contrast, limited examples have been reported
for oxidative amination of azoles. Mori et al.,[10] Schreiber
et al.,[11] and Li et al.[12] independently reported the copper
mation of benzoxazole with an amine. Herein, we report the
direct oxidative amination of benzoxazole using a catalytic
amount of g-MnO2 and molecular oxygen as an oxidant
(Scheme 1). To the best of our knowledge, this is the first
example of a heterogeneous catalytic method for the direct
oxidative amination of benzoxazole under very mild condi-
tions.
[a] P. Pal, A. K. Giri, H. Singh, Dr. S. C. Ghosh, Dr. A. B. Panda
Discipline of Inorganic Materials and Catalysis, & Academy of Scien-
tific and Innovative Research
Central Salt and Marine Chemicals Research Institute (CSIR)
G.B. Marg, Bhavnagar-364002, Gujarat (India)
Fax : (+91)278-2567562
E-mail: abpanda@csmcri.org
scghosh@csmcri.org
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402057.
Scheme 1. Amination of benzoxazole with an amine.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Subhash Chandra Ghosh, Asit Baran Panda et al.
À
The synthesis of the catalyst was performed by following
a reported procedure in which MnCO3 is generated under
hydrothermal conditions using aqueous MnCl2, ammonium
carbonate, and citric acid followed by calcination at 3508C
in air (see the Supporting Information for details).[19] The
XRD pattern of calcined materials was indexed to hexago-
nal g-MnO2 (JCPDS Card 14-644), as a binary mixture of
hexagonal akhtenskite (e-MnO2) (~60%) and intergrowth
of tetragonal rutile-type-structured pyrolusite (b-MnO2,
~15%) and orthorhombic ramsdellite (~25%) with differ-
ent crystalline domain sizes (Figure 1a).[20] Scanning elec-
sorption of surface (OÀ and O2 ) and labile oxygen spe-
cies,[21] and second step with a peak at 4208C (Figure S1,
Supporting Information) is associated with the evolution of
lattice oxygen during the transformation of MnO2 to the
Mn2O3 phase.[22] In the first step the material consumed
a reasonably high amount of H2 originating from its texture
and surface area; thus, it was expected to display good oxi-
dation properties at low temperature. Analysis by X-ray
photoelectron spectroscopy (XPS) revealed the presence of
Mn+3 species; it is well known that a mixed valency of man-
ganese ions gives rise to oxidation properties.[23] The oxygen
adsorption on the surface of the catalyst increased and the
À
formation of OÀ, O2 , and of labile oxygen increased due to
the presence of the mixed valency (Figure S2, Supporting
Information). This excess amount of oxygen facilitates the
oxidation reaction. As this material contains an excess of
low-temperature reducible surface oxygen species, we ex-
À
pected that oxidative C N bond formation of heteroarenes
could be achieved. Accordingly, we began our study with
the model reaction of 5-methyl benzoxazole with morpho-
line to afford 2-(4-morpholinyl)-5-methylbenzoxazole
(Table 1).
In an initial attempt, a catalytic amount of g-MnO2 to-
gether with molecular oxygen as an oxidant and acetic acid
as an additive was used in acetonitrile at 508C to yield the
desired amino benzoxazole product in 84% yield (entry 1,
Table 1); however, most of the heterogeneous MnO2 catalyst
was leached out as MnACHTUNTRGNEUNG(OAc)2 after reaction with acetic
acid. To prevent the leaching of MnO2, we replaced acetic
acid with a heterogeneous solid acid, that is, mesoporous zir-
conium phosphate,[24] and a comparable reactivity was ob-
Table 1. Optimization of the reaction conditions.[a]
Figure 1. Analysis of the synthesized g-MnO2 catalyst by XRD (a), SEM
(b), and TEM (c). d) Nitrogen sorption isotherms and BET surface area
of the g-MnO2 catalyst.
Entry Catalyst [mg] Oxidant Additive T [8C] Conversion[b] [%]
tron microscopy (SEM) images revealed the formation of
bunches of uniform lotus flowers made of spindle-shaped
petals with about 6 mm length and 2 mm width in the middle
(Figure 1b). The petals are porous and the pores were gen-
erated due to the evolution of CO2 during the decomposi-
tion of MnCO3 (Figure 1c). The electron diffraction ring
patterns and distinct lattice fringes in the high-resolution
transmission electron microscopy (HR-TEM) image con-
firmed that the materials are polycrystalline and highly crys-
talline (Figure S1, Supporting information). The nitrogen
sorption isotherm corresponding to the type IV and H2-type
hysteresis loop confirmed the formation of mesopores in the
synthesized porous lotus-shaped g-MnO2 samples.
The synthesized material showed a reasonably high total
surface area of 145 m2 gÀ1 with a narrow pore-size distribu-
tion of 4.2 nm (Figure 1d, inset). It showed two H2 con-
sumption steps during H2-TPR (TPR=temperature-pro-
grammed reduction) in the temperature range of 150 to
4508C. The first step with a peak at 3048C is due to the de-
(yield%)[c]
1
2
3
4
5
6
7
8
25
25
25
25
25
10
50
25
25
25
25
25
25
25
O2
O2
O2
O2
O2
O2
O2
air
O2
O2
O2
O2
O2
O2
AcOH
m-ZrP
50
50
50
80
25
50
50
50
50
50
50
50
50
50
92 (84)
89 (82)
86 (81)
88 (82)
20
–
–
–
–
–
–
–
–
–
–
–
45
88
18
9
89[d]
68[e]
42[f]
10
11
12
13
14
88[g]
18[h]
0[i]
[a] Unless otherwise stated, all reactions were carried out under the fol-
lowing conditions: acetonitrile (2 mL), 1a (0.5 mmol), 2a (1.2 equiv), cat-
alyst g-MnO2 (25 mg), oxygen atmosphere, 508C, 24 h. [b] NMR conver-
sion. [c] Isolated yield. [d] 48 h. [e] 12 h. [f] 4 h. [g] Bubbling with molecu-
lar oxygen. [h] Commercial MnO2. [i] Synthesized Mn2O3. m-ZrP=meso-
porous zirconium phosphate.
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Subhash Chandra Ghosh, Asit Baran Panda et al.
served (entry 2). Leaching of manganese did not occur as
confirmed by inductively coupled plasma-atomic emission
spectrometry (ICP-AES) measurements of the reaction mix-
ture. To examine the role of the solid acid, we have carried
out the same reaction in the absence of zirconium phos-
phate. To our delight, a similar conversion and yield were
obtained (entry 3). Next, we varied the reaction tempera-
ture. A similar conversion was observed in refluxing acetoni-
trile, while the yield was significantly decreased at room
temperature (258C, entries 4–5). Subsequently, variation of
catalyst loading was tested (entry 6–7), and it was found that
an amount of 25 mg of catalyst is optimal for this reaction.
When we used air as an oxidant a significant drop in yield
was observed (entry 8). Next, we carried out reactions at dif-
ferent reaction times (entries 9–11), and we found that
a minimum of 24 h is required for the reaction to reach the
highest conversion. Screening a range of solvents revealed
that acetonitrile led to better results as compared to those
obtained with solvents such as THF, DMSO, toluene, ethyl
acetate, and dichloromethane (see the Supporting Informa-
tion). Bubbling of oxygen into reaction mixture did not im-
prove the yield significantly (entry 12). When we used com-
mercial MnO2 and synthesized Mn2O3 instead of our synthe-
sized g-MnO2, the desired aminated benzoxazole product
was obtained in a yield of only 18% and 0%, respectively
(entries 13–14).
With the optimized conditions (entry 3) in hand, we ex-
plored the substrate scope of this reaction. Under the opti-
mized conditions, amination of benzoxazole with various
primary and secondary amine was performed (Scheme 2).
These reactions afforded the corresponding 2-amino ben-
zaoxazole products in moderate to good yields. The reaction
of 5-methyl benzoxazole with cyclic secondary amines like
morpholine, piperidine, and 1-methylpiperazine afforded the
corresponding amination products 3a–3c in high yields.
Acyclic secondary amines like diallyl amine, N-methyl
benzyl amine, and N-methyl aniline also reacted very
smoothly to afford corresponding amination products 3d–3 f
in high yields. However, sterically hindered acyclic secon-
dary amines like diisobutyl amine led to the corresponding
product 3g with a much lower yield (30%). Benzoxazole
bearing an electron-withdrawing chloride group and unsub-
stituted benzoxazole reacted smoothly with morpholine and
other seconday amines to produce the corresponding ami-
nated products 3h–3k. We next examined the scope of the
amination of 5-methyl benzoxazole with primary amines. To
our delight, amination of benzoxazole with a variety of pri-
mary amines such as cyclohexyl amine, n-pentyl amine, 2-
phenylethyl amine, and benzyl amine also worked well, af-
fording the products 3l–3o in yields of 66–83%. Of note,
propargyl amine could also be used as a facile substrate for
the amination reaction to give product 3p. Amination with
pyridine-3-methylamine worked smoothly to yield 3q in
56%, and 3-amino pyridine gave a promising yield of 45%
of the aminated product 3r at an elevated temperature. It is
important to note that a functional group such as alcohol is
tolerated to give 3s in a moderate yield, thus indicating
Scheme 2. Reaction scope of the amination of benzoxazole. Reactions
were carried out with benzoxazole (0.5 mmol), amine (1.2 equiv), g-
MnO2 (25 mg) in acetonitrile (1 mL) at 508C for 24 h under oxygen at-
mosphere. All yields reported are isolated yields. [a] At 1008C.
a broad applicability of this reaction. We sought to extend
our facile amination method to the synthesis of primary
amines using several ammonia sources such as ammonium
hydroxide, ammonium chloride, and ammonium carbonate,
but to our disappointment no reaction product 3t could be
obtained. We have tested other substrates like N-ethylbenzi-
midazole, benzothiazole, and benzofuran under the opti-
mized conditions and also at higher temperature (1258C)
and increased catalyst loading; however, no or very poor
yield of aminated product was obtained (see Table S2, Sup-
porting Information). This might be due to the higher pKa
À
values of C2 H of the other substrates compared to that of
benzoxazole. Catalyst g-MnO2 showed promising recyclabili-
ty for a model reaction, that is, amination of 5-methyl ben-
zoxazole with morpholine (Table S3, Supporting Informa-
tion). For the first two runs, the catalytic activity remained
the same, while for further cycles a decrease in catalytic ac-
tivity was observed. We observed breaking of petals in the
catalyst by SEM after the fourth cycle (Figure S3, Support-
3
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Subhash Chandra Ghosh, Asit Baran Panda et al.
ing Information). Furthermore, a decrease in surface area
Experimental Section
was observed (SBET =125 m2 gÀ1). This could be the main
reason for the gradual deactivation of the catalyst. After the
reaction, the catalyst was separated through simple filtra-
tion. Notably, negligible amounts (<1 ppm) of leached Mn
was observed, as assessed from the ICP-AES analysis of the
filtrate. To examine if the leached catalyst is responsible for
the observed catalytic activity, we have carried out the ami-
nation reaction after removal of the solid catalyst by filtra-
tion. No appreciable amination was observed after 24 h,
thus suggesting that heterogeneous porous g-MnO2 is the
true active catalyst (Table S3, Supporting Information).
While the reaction mechanism has not been elucidated
yet, we have carried out some experiments to gain some in-
sight into the potential mechanism. When we carried out
the experiments in the presence of a radical scavenger
(TEMPO)[25] under the standard reaction conditions, the de-
sired product was still obtained with a similar yield (79%).
This result indicates that the reaction does not proceed
through a free-radical pathway. Unlike Chang et al.,[17] we
have not observed a ring-opened amidine intermediate in
Synthesis of lotus-shaped porous g-MnO2
In a typical synthetic procedure, a solution of (NH4)2CO3 (4 g, 25 mmol)
in H2O (20 mL) was added to 20 mL of a pre-mixed solution of MnCl2
(analytical reagent grade, 2 g, 10.1 mmol) and citric acid (2 g, 10.4 mmol).
The clear solution was then transferred to a 60 mL Teflon-lined stainless
steel autoclave, sealed, and heated for 12 h at 1508C. After that, the mix-
ture was allowed to cool to room temperature and the resultant precipi-
tates were collected, thoroughly washed with water and ethanol, and fi-
nally dried at 908C. Finally, the dried powder was calcined at 3508C for
6 h. The catalyst was characterized by XRD, SEM, TEM, BET surface
area, and H2-TPR.
Representative procedure for the amination of benzoxazole
A two-neck, 10 mL round-bottomed flask fitted with a reflux condenser
was charged with 5-methylbenzoxazole (0.5 mmol, 1.0 equiv) and catalyst
g-MnO2 (25 mg). The reaction vessel was then evacuated and refilled
with oxygen. Subsequently, morpholine (0.6 mmol, 1.2 equiv) and aceto-
nitrile (1.0 mL) were added under oxygen atmosphere, and an oxygen
balloon was fitted at the top of the condenser. The reaction mixture was
then stirred for 24 h at 508C. The crude mixture was filtered through
a plug of celite and washed with EtOAc (25 mL). The organic layer was
washed with water and brine, dried over MgSO4, and then concentrated
under reduced pressure. The resulting residue was purified by column
chromatography on silica gel (n-hexane/EtOAc=10:1) to afford 2-(4-
morpholinyl)-5-methylbenzoxazole (3a, 81%) as a white solid.
1
the H NMR spectrum of the crude reaction mixture during
the reaction. Based on previously reported mechanistic stud-
ies on the amination of azoles and our observations during
the course of the present study, we propose a tentative
mechanism for this reaction (Scheme 3), in which the amine
as a nucleophile first reacts with benzoxazole to form 2-
amino benzoxazolidine, which then re-aromatizes in the
presence of oxygen as an oxidant and g-MnO2 as a cata-
lyst.[16]
In summary, we have developed the first heterogeneous
catalytic system for the direct amination of benzoxazole
with amines using molecular oxygen as an oxidant. A wide
range of both primary and secondary amines were reacted
Acknowledgements
CSIR-CSMCRI Communication No. 156/2013. The authors acknowledge
the Department of Science and Technology (DST), India, (SR/S1/IC-33/
2011) for financial support. The authors also acknowledge the Analytical
Discipline and Centralized Instrumental Facility of CSMCRI for materi-
als characterization. PP,AKG and HS are thankful to CSIR and UGC for
research fellowships. PP, AKG and HS are also thankful to AcSIR for
providing the PhD degree.
À
Keywords: amination · benzoxazole · C H activation ·
heterogeneous catalysis · manganese
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Received: February 14, 2014
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COMMUNICATION
Amination
Sorry for being so direct: The hetero-
geneously catalyzed direct amination
of benzoxazole with a wide range of
primary and secondary amines, using
molecular oxygen as a green oxidant
under mild reaction conditions, is
reported. The catalyst can easily be
separated by filtration and reused. The
2-aminobenzoxazole products are
important skeletons in several biologi-
cally active compounds.
Provas Pal, Arnab Kanti Giri,
Harshvardhan Singh,
Subhash Chandra Ghosh,*
Asit Baran Panda*
&&&& — &&&&
Heterogeneously Porous g-MnO2-
Catalyzed Direct Oxidative Amination
of Benzoxazole through C H Activa-
tion in the Presence of O2
À
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Chem. Asian J. 2014, 00, 0 – 0
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!