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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