Facile preparation of 2-substituted benzoxazoles and benzothiazoles via aerobic oxidation of phenolic and thiophenolic imines catalyzed by polymer-incarcerated platinum nanoclusters

Article abstract of DOI:10.1002/adsc.201100430

Platinum nanoclusters supported on a polymer/carbon black composite material was found to be an excellent catalyst for the oxidative cyclization of phenolic and thiophenolic Schiff bases to 2-substituted benzoxazoles and benzothiazoles under ambient conditions. Copyright

Full text of DOI:10.1002/adsc.201100430

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DOI: 10.1002/adsc.201100430
Facile Preparation of 2-Substituted Benzoxazoles and Benzothia-
zoles via Aerobic Oxidation of Phenolic and Thiophenolic Imines
Catalyzed by Polymer-Incarcerated Platinum Nanoclusters
Woo-Jin Yoo,a Hao Yuan,a Hiroyuki Miyamura,a and Shu¯ Kobayashia,*
a
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Fax : (+81)-3-5684-0634; e-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
Received: May 25, 2011; Revised: August 20, 2011; Published online: November 16, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100430.
Abstract: Platinum nanoclusters supported on a
polymer/carbon black composite material was
found to be an excellent catalyst for the oxidative
cyclization of phenolic and thiophenolic Schiff
bases to 2-substituted benzoxazoles and benzothia-
zoles under ambient conditions.
could be stabilized and immobilized by the polymer
supports and act as highly active heterogeneous cata-
lysts for the aerobic oxidation of alcohols[12] and hy-
droquinones[13] under ambient pressure and tempera-
ture. In addition, we have demonstrated the viability
of incorporating these PI metal nanocluster catalysts
for tandem reactions such as the oxidative esterifica-
tion of alcohols[14] and sequential allylic oxidation-Mi-
chael addition reactions.[15]
Keywords: nanoparticles; oxidation; platinum;
polymers; supported catalysis
With our ongoing interest in the exploration of
aerobic oxidation reactions catalyzed by PI metal
nanoclusters, we turned our attention to the oxidative
cyclization of phenolic and thiophenolic imines to
Benzoxazoles and benzothiazoles are of great interest prepare 2-subsituted benzoxazoles and benzothia-
due to their occurrence in a wide range of biologically zoles. Herein, we present an efficient synthetic proto-
active natural products and pharmaceutical agents.[1] col for the preparation of these fused 5-membered
One of the most popular strategies towards the prepa- heterocycles using platinum nanoclusters immobilized
ration of 2-substituted benzoxazoles and benzothia- on a polymer/carbon black composite material as cat-
zoles is through the oxidative cyclization of phenolic alyst under ambient conditions.
and thiophenolic Schiff bases, derived from the con-
We began our studies by preparing various poly-
densation of 2-aminophenols/2-aminothiophenols and mer-incarcerated metal nanoclusters (PI-M) following
aldehydes, mediated by the stoichiometric use of our previously reported procedure for PI-Au
strong oxidants.[2] On the other hand, catalytic oxida- (Scheme 1)[12a] and evaluated their ability to catalyze
tive processes are quite limited and have been report- the oxidative cyclization of phenolic imine 2a
ed with ruthenium/iridium hydrogen-transfer cata- (Table 1).
lysts[3] and under aerobic oxidative conditions cata-
lyzed by copper(II) chloride,[4] iron
We initially chose the optimized reaction conditions
AHCTUNGTRENNUNG
palladium(II) acetate,[6] alum,[7] 4-methoxy-TEMPO,[8] PI-Au with a small modification with respect to the
copper nanoclusters,[9] and activated carbon.[10] How- benzotrifluoride (BTF) to water ratio to minimize the
ever, in most cases, relatively high catalyst loading/re- hydrolysis of imine 2a.[12a] We examined various PI-M
action temperatures are required for the preparation catalysts and found that only PI-Pd and PI-Pt provid-
of these 5-membered heterocycles and new catalytic ed the desired benzoxazole 3a with low yields (en-
methodologies to prepare these compounds under tries 2 and 3). Next, we screened many different sol-
mild conditions are highly desirable.
vents and found that chlorinated solvents, especially
Recently, we reported an effective protocol for the chloroform, provided the best results. Through the
immobilization of metal catalysts with styrene-based careful adjustment of the chloroform to water ratio
co-polymers through a process known as the polymer- and the solvent concentration, we were able to im-
incarcerated (PI) method.[11] We have shown that var- prove the yield of 3a to 75% (entry 9).
ious gold, platinum, and gold-bimetallic nanoclusters
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Table 2. Optimization of the reaction conditions with boron-
free catalysts.[a]
Entry
Catalyst
K2CO3 [equiv.]
Yield [%][b]
1
2
3
4
PI-Pt (B-free)
PI/CB-Pt
PI/CB-Pt
3
3
0.1
0
>95
>95
>95[c]
<5
PI/CB-Pt
[a]
Phenolic imine 2a (0.25 mmol) and PI-M catalyst
(1 mol%) in CHCl3:H2O (9:1, C=0.25M) at 308C under
a balloon pressure of oxygen gas.
Scheme 1. Preparation of polymer-incarcerated metal nano-
clusters.
[b]
[c]
1
Yield based on 2a and determined by H NMR analysis
Table 1. Optimization of the reaction conditions.[a]
using 1,1,2,2-tetrachloroethane as an internal standard.
No leaching of Pt was confirmed by ICP analysis (detec-
tion limit: 0.13 ppm; no peak of Pt was observed).
Entry M Solvent Ratio
[solvent:H2O]
Conc. [M] Yield [%][b]
1
2
3
4
5
6
7
8
9
10
Au BTF
Pd BTF
Pt BTF
Pt CH2Cl2 19:1
Pt CHCl3 19:1
Pt CHCl3 14:1
Pt CHCl3 9:1
Pt CHCl3 4:1
Pt CHCl3 9:1
Pt CHCl3 9:1
19:1
19:1
19:1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.25
0.5
n.d.
<5
17
34
49
58
63
53
75
55
Scheme 2. Preparation of a platinum nanocluster catalyst on
a polymer-carbon black composite material.
ogeneous material to be an excellent catalyst for the
aerobic oxidative cyclization reaction (entry 2). Final-
ly, we determined that the amount of the base could
be decreased to catalytic amounts without sacrificing
the yield of 3a (entry 3). In addition, no leaching of
platinum was observed after filtration of PI/CB-Pt
from the crude reaction mixture.
With the optimized conditions in hand, we exam-
ined the scope of the aerobic oxidative cyclization re-
action of phenolic imines 2a–o (Table 3).
[a]
Phenolic imine 2a (0.25 mmol), K2CO3 (0.75 mmol), and
PI-M catalyst (1 mol%) in solvent:H2O at 308C under a
balloon pressure of oxygen gas.
Yield based on 2a and determined by H NMR analysis
using 1,1,2,2-tetrachloroethane as an internal standard.
[b]
1
Based on our recent discovery on the influence of
We found that phenolic imines 2a–f, derived from
boron for the aerobic allylic oxidation reaction,[16] we the condensation of 2-aminophenols and various aro-
examined the boron content for PI-Pt and found that matic aldehydes, were easy to handle solids that read-
0.4681 mmolgÀ1 of boron remained after our washing ily converted to the desired 2-substituted benzoxa-
protocol with dichloromethane and water. To com- zoles 3a–f with excellent yields under our optimized
pletely remove the boron, PI-Pt was subjected to an reaction conditions (entries 1–6). On the other hand,
extraction procedure in which the heterogeneous cat- not all imines studied were easily isolable solids and
alyst was stirred in THF:H2O (1:1) overnight and in those cases, the Schiff bases were in situ prepared
then washed successively with water, THF, and (entries 7, 9–11, 13). We then examined the oxidative
CH2Cl2. When the boron-free PI-Pt catalyst was uti- cyclization reaction of phenolic imines bearing non-
lized for the oxidative cyclization of 2a, the desired aromatic substituent groups 2g–k and found that
benzoxazole 3a was obtained in quantitative yield slight modification of our optimized reaction condi-
(Table 2, entry 1).
tions were necessary in order to obtain the best re-
We also prepared a platinum nanocluster catalyst sults. For the preparation of 2-furylbenzoxazole 3g, a
on a polymer-carbon black composite material (PI/ slight increase in catalyst loading was required
CB-Pt, Scheme 2)[17] and found this boron-free heter- (entry 7), while Schiff bases with an alkene or an
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Facile Preparation of 2-Substituted Benzoxazoles and Benzothiazoles via Aerobic Oxidation
Table 3. Scope of the PI/CB-Pt-catalyzed aerobic oxidation of phenolic imines 2a–o.[a]
Entry
Phenolic Imine
R1
R2
Benzoxazole
Yield [%][b]
1
2
3
4
5
6
7
2a
2b
2c
2d
2e
2f
2g
H
H
H
H
H
H
H
Ph
3a
3b
3c
3d
3e
3f
3g
96
93
quant.
quant.
95
4-Me-C6H4
2-Me-C6H4
4-MeO-C6H4
4-Cl-C6H4
4-CN-C6H4
2-furyl
96
84[c,d]
8
2h
H
3h
84[d,e]
9
2i
H
3i
65[c,d,e]
10
11
12
13
14
15
2j
2k
2l
2m
2n
2o
H
H
Me
OMe
Cl
t-Bu
c-hexyl
Ph
Ph
Ph
3j
3k
3l
3m
3n
3o
95[c]
62[c]
93
quant.[c]
70[e]
58
COOEt
Ph
[a]
Phenolic imines 2a–o (0.50 mmol), K2CO3 (0.05 mmol) and PI/CB-Pt catalyst (1 mol%) in CHCl3:H2O (9:1, C=0.25M)
at 308C under a balloon pressure of oxygen gas.
Yield based on 2a–o and determined by weight of the isolated product 3a–o.
Phenolic imines were prepared in situ.
Using 2 mol% of PI/CB-Pt.
1 equivalent of K2CO3 was used.
[b]
[c]
[d]
[e]
alkyne substituent required a stoichiometric amount
of base to ensure full conversion of the starting mate-
rials (entries 8 and 9). Phenolic imines derived from
aliphatic aldehydes 1j–k were also examined and sur-
prisingly we found that 2-substituted benzoxazole
with a non-tertiary aliphatic group could be prepared
with our catalyst system. While only a moderate yield
of benzoxazole 3k was obtained, this result represents
a significant improvement for the catalytic oxidative
cyclization of phenolic Schiff bases with aliphatic
groups. Finally, we varied the substituents on the 2-
aminophenol ring. We found that electron-rich groups
(entries 12 and 13) were found to be good substrates,
while electron-poor moieties diminished the oxidative
cyclization reaction (entries 14 and 15).
With the successful development of a synthetic pro-
tocol for the preparation of 2-substituted benzoxa-
zoles under mild conditions, we turned our attention
to the oxidative cyclization of thiophenolic imines 4a–
i to benzothiazoles 5a–i (Table 4).
In general, we found that the oxidative cyclization
of the thiophenolic Schiff bases 4a–i to be more slug-
gish and often required a slightly higher loading of
the heterogeneous platinum nanocluster catalyst.
While imines with electron-poor aromatic groups
could be converted to the desired benzothiazoles 5b
Table 4. Scope of the PI/CB-Pt-catalyzed aerobic oxidation
of thiophenolic imines 4a–i.[a]
Entry Imine
R
Benzothiazole Yield [%][b]
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
4g
4h
4i
Ph
5a
5b
5c
5d
83[c]
82[c,d]
75[c]
60
29
83
4-Cl-C6H4
4-CN-C6H4
4-Me-C6H4
4-MeO-C6H4 5e
2-furyl
t-Bu
c-hexyl
Et
5f
5g
5h
5i
90
94[c]
73
[a]
Thiophenolic imines, prepared in situ from aldehydes and
2-aminothiophenols, 4a–i (0.50 mmol), K2CO3
(0.05 mmol) and PI/CB-Pt catalyst (2 mol%) in
CHCl3:H2O (9:1, C=0.25M) at 308C under a balloon
pressure of oxygen gas.
Yield based on 4a–i and determined by weight of the iso-
lated product 5a–i.
[b]
[c]
[d]
Using 1 mol% of PI/CB-Pt.
1 equivalent of K2CO3 was used.
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Scheme 3. Proposed reaction mechanism for the PI/CB-Pt-catalyzed aerobic oxidative synthesis of 2-substituted benzoxa-
zoles and benzothiazoles.
and 5c (entries 2 and 3), electron-rich substituents
were found to be inferior substrates (entries 4 and 5).
While the electron-richness of the aromatic moiety
seems to diminish the ability of the thiophenolic
imine to undergo oxidative cyclization, 2-furylbenzo-
thiazole 4f was synthesized with
a high yield
(entry 6). We also examined thiophenolic imines de-
rived from aliphatic aldehydes (4g–i) and found the
aerobic oxidative cyclization reaction to proceed ex-
tremely well under our optimized reaction conditions
Scheme 4. Recovery and reuse of PI/CB-Pt for the aerobic
oxidative cyclization of phenolic Schiff base 2a.
(entries 7–9). The improved reactivity of thiophenolic clusters immobilized on a polymer/carbon black com-
imines bearing non-tertiary aliphatic substituents (4h– posite material. This work represents a rare example
i) over phenolic imine 2k could be explained based on of a catalytic oxidative synthesis of benzoxazoles and
the proposed reaction mechanism (Scheme 3).
benzothiazoles under ambient temperature and pres-
The oxidative cyclization of phenolic imines 2/4 sure. The ability to perform the oxidative cyclization
could be envisioned by the initial addition of the nu- reaction under mild conditions allowed for the forma-
cleophilic heteroatom to the imine to form heterocy- tion of 2-substituted benzoxazoles and benzothiazoles
cle 6 which then proceeds to undergo PI/CB-Pt-cata- with aliphatic groups with moderate to good yields.
lyzed aerobic oxidation to generate the desired prod- Additional investigations into the substrate scope and
uct 3/5. Conversely, 2/4 could be hydrolyzed by water application of PI-M catalysts for the oxidative prepa-
to regenerate 2-aminophenol/thiophenol and this aro- ration of aromatic heterocycles are now in progress.
matic substrate could be oxidized to its corresponding
ortho-quinone/thioquinone imine 7. Although we did
not detect 7, indirect evidence for their formation was
Experimental Section
confirmed by the presence of ortho-quinone in the
crude reaction mixture. Unlike phenolic Schiff bases
with aryl substituents, imines bearing aliphatic groups
are relatively unstable and these imines and aliphatic
aldehydes could undergo undesirable side-reactions
under basic conditions. On the other hand, the tauto-
Preparation Method for PI/CB-Pt
To a solution of polymer 1 (0.3751 g) in diglyme (25 mL)
was added ketjen black (0.3751 g). After stirring the hetero-
geneous solution for 15 min, a solution of NaBH4 (95.3 mg,
meric equilibrium of thiophenolic imines 4 lies almost 2.52 mmol) in diglyme (5.4 mL) was added dropwise, then a
exclusively in its ring form.[18] Thus, the opportunity
solution of Na2PtCl6·6H2O (117.8 mg, 0.21 mmol) in diglyme
(15 mL) was added dropwise at 08C. After stirring for 4 h at
to undergo side reactions is nearly eliminated.
room temperature, Et2O (200 mL) was added dropwise. The
Finally, we performed recovery and reuse studies
polymer-coated carbon black was washed several times with
with our PI/CB-Pt catalyst for the oxidative cycliza-
Et2O and dried at room temperature. After light grinding
tion reaction of Schiff base 2a and found that the cat-
with a mortar and pestle, the recovered solid powder was
alyst could be reused for seven cycles without noticea-
heated at 1708C for 4 h under a balloon of argon. After
cooling to room temperature, the cross-linked polymer-
coated carbon black was stirred in a THF:H2O (1:1, 40 mL)
mixture overnight. After being filtered and then washed
ble loss of catalytic activity (Scheme 4).[19]
In conclusion, we have developed a facile synthetic
protocol for the preparation of 2-substituted benzoxa-
zoles and benzothiazoles catalyzed by platinum nano- with H2O, THF, and DCM, the recovered solid was dried
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Adv. Synth. Catal. 2011, 353, 3085 – 3089

Facile Preparation of 2-Substituted Benzoxazoles and Benzothiazoles via Aerobic Oxidation
under vacuum to provide the desired heterogeneous catalyst
(PI/CB-Pt, 0.83 g, Pt loading: 0.2090 mmolgÀ1, B loading:
0 mmolgÀ1).
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Rec. 2010, 10, 271–290.
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General Experiment Procedure for the Aerobic
Oxidation of Phenolic Imines
In a screw-cap glass test tube was charged PI/CB-Pt
(0.0210 g, 0.005 mmol, 1 mol%), phenolic imine 2a (0.0986 g,
0.5 mmol), K2CO3 (0.007 g, 0.05 mmol), and the mixture was
partially dissolved in CHCl3 (1.8 mL) and H2O (0.2 mL).
Then the reaction mixture was allowed to stir for 20 h at
308C under an O2 balloon. After the completion of the reac-
tion, the heterogenous catalyst was removed by filtration
and washed with DCM. To the resulting filtrate, MgSO4
(0.02 g) was added and the dried crude reaction mixture was
filtered again and then concentrated under reduced pres-
sure. The crude product was purified by column chromatog-
raphy (EtOAc:hexane=1:9) to provide 3a as a white solid;
yield: 0.0936 g (0.479 mmol, 96%).
Acknowledgements
This work was partially supported by a Grant-in-Aid for Sci-
entific Research from the Japan Society for the Promotion of
Science (JSPS), Global COE Program, The University of
Tokyo, MEXT (Japan), and NEDO. W.-J.Y. thank JSPS for
the JSPS Postdoctoral Fellowship for Foreign Researchers.
We also thank Mr. Noriaki Kuramitsu (The University of
Tokyo) for STEM analysis.
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Adv. Synth. Catal. 2011, 353, 3085 – 3089
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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