Oxidative transformation of N-substituted 2-aminophenols to 2-substituted benzoxazoles catalyzed by polymer-incarcerated and carbon-stabilized platinum nanoclusters

Article abstract of DOI:10.1139/v11-161

The preparation of 2-substituted benzoxazoles was achieved through a sequential aerobic oxidation-enolization-oxidative cyclization reaction of N-substituted 2-aminophenols catalyzed by platinum nanoclusters supported on a polymer/carbon black composite material.

Full text of DOI:10.1139/v11-161

306
Oxidative transformation of N-substituted 2-
aminophenols to 2-substituted benzoxazoles
catalyzed by polymer-incarcerated and carbon-
stabilized platinum nanoclusters
Woo-Jin Yoo, Hao Yuan, Hiroyuki Miyamura, and Shū Kobayashi
Abstract: The preparation of 2-substituted benzoxazoles was achieved through a sequential aerobic oxidation – enolization –
oxidative cyclization reaction of N-substituted 2-aminophenols catalyzed by platinum nanoclusters supported on a polymer /
carbon black composite material.
Key words: nanoparticles, platinum, oxidation, polymers, supported catalysis.
Résumé : La synthèse de benzoxazoles substitués en position 2 a été réalisée à partir de 2-aminophénols N-substitués à
l’aide d’une séquence réactionnelle oxydation aérobie – énolisation – cyclisation oxydante catalysée par des nanoclusters de
platine supportés sur un matériau composite polymère / noir de charbon.
Mots‐clés : nanoparticules, platine, oxydation, polymères, catalyse supportée.
Introduction
clusters were utilized as catalysts for tandem reactions such
as the oxidative esterification of alcohols6 and sequential al-
lylic oxidation – Michael addition reactions between 1,3-
dicarbonyl compounds and allylic alcohols.7
As part of our continuing effort into developing aerobic
oxidative reactions catalyzed by PI metal nanoclusters, we re-
cently reported the use of platinum nanoclusters immobilized
on a polymer / carbon black composite material (PI/CB–Pt) as
an effective catalyst for the oxidative cyclization of phenolic
and thiophenolic imines 1 to 2-substituted benzoxazoles and
benzothiazoles 2 under ambient conditions (Scheme 1,
eq. [1]).8
During the course of the aerobic oxidative cyclization reac-
tion, we found trace amounts of ortho-quinones in the crude
reaction mixture and believed these species to arise from the
hydrolysis of ortho-quinone/thioquinone imines derived from
the aerobic oxidation of 2-aminophenols/thiophenols. In a re-
lated work, Jiang et al.9 described a tert-butyl hydroperoxide
(TBHP) / I2-mediated domino oxidative cyclization reaction
with alkenes 3 and benzyl amines 4 to prepare polysubsti-
tuted oxazoles 5 (Scheme 1, eq. [2]). In their proposed mech-
anism, a-keto imine intermediate 6 was believed to enolize to
7 and was subsequently transformed to their desired oxazole
5 via cyclization and oxidation reactions. Inspired by the
work of Jiang et al., we envisioned a similar mechanistic sce-
nario in which N-substituted aminophenols 8 could be oxida-
tively converted to 2-substituted benzoxazoles 9 (Scheme 1,
eq. [3]). Assuming that the PI/CB–Pt catalyst could oxidize
8 to ortho-quinone imine 10, the enolization to phenolic
The development of novel strategies for the efficient syn-
thesis of compounds of high molecular complexity in an ex-
pedient manner is of great interest to the synthetic organic
community. The utilization of catalysts for sequential reac-
tions circumvents the necessary drain of resources caused by
the act of isolation and purification of intermediates faced by
traditional iterative synthetic methods.1 Furthermore, the abil-
ity to perform one-pot tandem reactions would allow for the
formation and use of highly reactive and (or) unstable inter-
mediates in organic synthetic schemes. Whereas most se-
quential reactions are performed with homogenous catalysts,
the use of heterogeneous catalysts would improve the syn-
thetic utility of cascade reactions by facilitating its recovery
and subsequent reuse. In addition to the ease of separation
from the reaction mixture, heterogeneous systems for multi-
catalytic sequential reactions would allow for the use of mu-
tually incompatible catalysts to coexist because of the site
isolation of the catalytic species caused by the heterogeneous
support.2
With our ongoing interest in the preparation of highly ac-
tive immobilized metal catalysts, we have reported an effi-
cient method for the immobilization of metal catalysts with
styrene-based co-polymers through a process known as the
polymer-incarcerated (PI) method.3 We have demonstrated
that various gold, platinum, and gold–bimetallic nanoclusters
could be stabilized and immobilized by the polymer support
and act as highly active heterogeneous catalysts for the aero-
bic oxidation of alcohols4 and hydroquinones5 under ambient
pressure and temperature. Furthermore, these PI metal nano-
Received 20 June 2011. Accepted 2 October 2011. Published at www.nrcresearchpress.com/cjc on 1 March 2012.
W.-J. Yoo, H. Yuan, H. Miyamura, and S. Kobayashi. Department of Chemistry, School of Science, The University of Tokyo, Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan.
Corresponding author: S. Kobayashi (e-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp).
Can. J. Chem. 90: 306–313 (2012)
doi:10.1139/V11-161
Published by NRC Research Press

Yoo et al.
307
Scheme 1. Oxidative preparation of oxazoles.
Previous work:
XH
N
PI/CB-Pt (1 mol%)
X
N
R2
[1]
K2CO3, CHCl3:H2O (9:1)
O2 (1 atm), 30 oC
R1
R2
R1
1
2
X = O, S
X
XH
X
N
[O]
R2
R2
R1
N
H
R1
R1
N
R2
1
2
Jiang and co-workers:
Ar1
TBHP, I2
O
Ar2
+
Ar1
o
Ar2
[2]
H2N
DMSO, 80 C
N
5
3
4
Ar1
Ar1
O
OH
Ar1
Ar1
O
O
[O]
H
Ar2
Ar2
N
Ar2
N
Ar2
N
H
N
6
7
5
This work:
OH
cat. PI/CB-Pt
O2 (1 atm)
O
R1
R1
R2
[3]
R2
N
H
N
8
9
OH
O
N
OH
O
N
[O]
R1
H
R1
R1
R1
R2
R2
N
H
R2
R2
N
8
10
11
9
imine 11 should be highly favorable because of the rearoma-
tization of the benzene ring.
fully converted and ortho-quinone was observed in the crude
reaction mixture, we attributed this disappointing result to the
relative instability of the reactive ortho-quinone imine inter-
mediate. Thus, by introducing a substituent group to the aro-
matic ring of the aminophenol (8b) to stabilize the
intermediate, 2-substituted benzoxazole 9b was obtained
with a poor yield (Table 1, entry 2). We found that, by dilut-
ing the reaction mixture (Table 1, entry 3) and reducing the
amount of water (Table 1, entry 4), a modest improvement
for the sequential oxidative transformation of 8b was pro-
vided. However, it should be noted that water plays an im-
portant role and excluding water from the system reduces
the ability of PI/CB–Pt to catalyze the aerobic oxidation reac-
tions (Table 1, entry 5).
Next, we examined the scope of the tandem oxidative
transformation reaction of N-substituted 2-aminophenols 8b–
8l (Table 2).
We initially examined the effect of the substituents on the
aromatic ring of the 2-aminophenols and found that, by ad-
justing the position of the methyl group to be para to the
In this paper, we report the preparation of benzoxazoles 9
from N-substituted 2-aminophenols 8 under mild aerobic ox-
idative conditions through a tandem reaction consisting of an
aerobic oxidation – enolization – oxidative cyclization se-
quence catalyzed by polymer-supported platinum nanoclus-
ters.
Results and discussion
We began our studies by preparing PI/CB–Pt catalyst (Pt
particle size = 2.1 0.7 nm; Fig. 1) following our previously
reported procedure (Scheme 2) and attempted to perform the
sequential oxidative transformation of aminophenol 8a to 2-
substituted benzoxazole 9a (Table 1).
However, our initial attempts to synthesize benzoxazole 9a
by applying our previous conditions for the oxidative cycliza-
tion reaction failed, and only trace amounts of the desired
product 9a was observed (Table 1, entry 1). Since 8a was
Published by NRC Research Press

308
Can. J. Chem. Vol. 90, 2012
Fig. 1. Typical STEM image of platinum nanoclusters immobilized
on a polymer / carbon black composite material (PI/CB–Pt).
A tentative mechanism for the sequential oxidative trans-
formation of N-substituted 2-aminophenols 8 to benzoxazoles
9 is shown in Scheme 3.
Based on our previous observations of the conversion of 2-
aminophenols to ortho-quinones, we envisioned a reaction
pathway in which N-substituted 2-aminophenol 8 would be
oxidized to the corresponding ortho-quinone imine 10, cata-
lyzed by PI/CB–Pt. The facile enolization of 10 to phenolic
imine 11 would restore aromaticity to the system and, after
oxidative cyclization, would provide the desired benzoxazole
9. Although the direct oxidation of 8 to 11 is a distinct possi-
bility, this reaction pathway was discounted, since aerobic
oxidation of amines to imines with metal nanoclusters is rel-
atively difficult and requires high reaction temperatures.10
We also examined the possibility of an aerobic oxidation
of 8 in the absence of the platinum catalyst. We performed
control studies in which aminophenol 8d was subjected to
our optimized reaction conditions in the absence of PI/CB–
Pt and also with our hybrid polymer / carbon black support
material without platinum nanoclusters. In both cases, only
1
trace amounts of 10 and 11 were detected by H NMR of
the crude reaction mixture.
Finally, we examined the possibility of recovery and reuse
of our PI/CB–Pt catalyst for the sequential oxidative transfor-
mation of 8d, and found that the catalyst could be reused for
five cycles without noticeable loss of catalytic activity
(Scheme 4).
Scheme 2. Synthetic procedure for the preparation of platinum na-
noclusters immobilized on a polymer / carbon black composite mate-
rial (PI/CB–Pt).
Conclusion
In summary, we have developed a sequential aerobic
oxidation – enolization – oxidative cyclization reaction of N-
substituted 2-aminophenols to 2-substituted benzoxazoles cat-
alyzed by platinum nanoclusters supported on a polymer / car-
bon black composite material. The reaction was shown to be
remarkably mild and could be utilized to prepare 2-substiuted
benzoxazoles with aryl, alkenyl, alkynyl, and aliphatic
groups. Further investigations into improving the substrate
scope and exploring additional oxidative methods towards
the preparation of aromatic heterocycles are now in prog-
ress.
NaBH4
Na2PtCl6
coacervation
CB
polymer
diglyme
diglyme
(0 oC)
diglyme
(0 oC to rt)
Et2O
12
extract
wash
wash cross-link
PI/CB-Pt
170 oC
Et2O
dry
H2O, THF H2O, THF,
CH2Cl2
0.19-0.23 mmol/g
of Pt
x
y
z
OH
O
O
4
O
Experimental
12
(x:y:z = 33:38:29)
Preparation method for PI/CB–Pt
Ketjen black (0.3751 g) was added to a solution of poly-
mer 12 (0.3751 g) in diglyme (25 mL). After stirring the het-
erogeneous solution for 15 min, a solution of NaBH4
(95.3 mg, 2.52 mmol) in diglyme (5.4 mL) was added drop-
amine (8c), an improvement of the yield for benzoxazole 9c
(Table 2, entry 2) resulted, whereas the introduction of a
more stabilizing group (t-Bu) lead to the quantitative conver-
sion of 8d to benzoxazole 9d (Table 2, entry 3). Various 2-aryl
benzoxazoles 9e–9h bearing electron-rich and -poor sub-
stituents were prepared through the sequential oxidative
transformation of N-substituted 2-aminophenols 8e–8h with
excellent yields (Table 2, entries 4–7). The synthesis of
benzoxazoles with alkenyl (9i) and alkynyl (9j) substituents
was achieved in moderate yields, but required a stoichio-
metric amount of base (Table 2, entries 8 and 9). Finally,
benzoxazoles that possessed a primary aliphatic substituent
at the 2-position (9k and 9l) were prepared under our opti-
mized reaction conditions with good results (Table 2, en-
tries 10 and 11).
wise, then
a solution of Na2PtCl6⋅6H2O (117.8 mg,
0.21 mmol) in diglyme (15 mL) was added dropwise at 0 °C.
After stirring for 4 h at room temperature, Et2O (200 mL)
was added dropwise. The polymer-coated carbon black was
washed several times with Et2O and dried at room temper-
ature. After light grinding with a motor and pestle, the re-
covered solid powder was heated at 170 °C 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 fil-
tered and then washed with H2O, tetrahydrofuran (THF),
and dichloromethane (DCM), the recovered solid was dried
Published by NRC Research Press

Yoo et al.
309
Table 1. Optimization of reaction conditions.
OH
O
PI/CB-Pt (1 mol%)
Ph
K2CO3 (10 mol%), CHCl3:H2O
O2 (1 atm), 30 °C, 20 h
R
N
H
Ph
N
R
8a-8b
9a-9b
Ratio of
CHCl3–H2O
9:1
9:1
9:1
19:1
1:0
Conc.
(mol/L)
0.25
0.25
0.05
0.05
0.05
Entry
Aminophenol
R
H
Me
Me
Me
Me
Product
9a
9b
9b
9b
Yield (%)a
1
2
3
4
5
8a
8b
8b
8b
8b
<5
28
35
48
28
9b
Note: Aminophenols 8a and 8b (0.25 mmol), K2CO3 (0.025 mmol), and platinum nanoclusters im-
mobilized on a polymer / carbon black composite material (PI/CB–Pt) catalyst (1 mol%) in CHCl3–
H2O at 30 °C under a balloon pressure of oxygen gas.
1
aYield based on 8a and 8b and determined by H NMR analysis of the crude reaction mixture
using 1,1,2,2-tetrachloroethane as an internal standard.
Table 2. Substrate scope of the sequential oxidative transformation of N-substituted 2-aminophenols.
R2
OH
R2
R1
O
N
PI/CB-Pt (1 mol%)
R3
R1
N
H
R3
K2CO3, CHCl3:H2O (19:1)
O2 (1 atm), 30 °C, 20 h
8b-8l
9b-9l
Entry Aminophenol
R1
Me
H
t-Bu
t-Bu
R2
H
Me Ph
H
H
R3
Ph
Product
9b
9c
9d
9e
Yield (%)a
31
65
Quant.
90
1
2
3
4
8b
8c
8d
8e
Ph
5
6
7
8f
t-Bu
t-Bu
t-Bu
H
H
H
9f
97
8g
8h
9g
9h
87
Quant.
8
8i
t-Bu
t-Bu
t-Bu
H
H
9i
70b
67b
79
9
8j
8k
8l
H
9j
9k
9l
10
11
H
Me
84
Note: Aminophenols 8b–8l (0.25 mmol), K2CO3 (0.025 mmol), and platinum nanoclusters immobilized on a poly-
mer / carbon black composite material (PI/CB–Pt) catalyst (1 mol%) in CHCl3–H2O (19:1, c = 0.05 mol/L) at 30 °C
under a balloon pressure of oxygen gas.
aYield based on 8b–8l and determined by weight of the isolated products 9b–9l.
bOne equivalent of K2CO3 was used. Quant., quantitative.
under vacuum to provide the desired heterogeneous catalyst
(PI/CB–Pt, 0.83 g; Pt loading, 0.2090 mmol/g).
quenched with H2O (20 mL), extracted with CH2Cl2, and the
combined organic layers were washed with a saturated brine
solution. The extracted crude product was dried over MgSO4,
filtered, concentrated under reduced pressure, and then puri-
fied by column chromatography (EtOAc:hexane = 1:4) to
provide the desired product 8d (0.6130 g, 2.40 mmol, 82%)
as a white solid.
General experimental procedure for the preparation of N-
substituted 2-aminophenols via reductive amination
Benzaldehyde (305 µL, 3.00 mmol) was added to a solu-
tion of 2-amino-4-tert-butylphenol (0.4857 g, 2.940 mmol)
in EtOH (15 mL), and the reaction mixture was stirred for
2-(Benzylamino)-4-methylphenol (8b)
4
h at room temperature. Then, NaBH4 (0.1135 g,
mp 108 °C. IR (KBr, cm–1): 3316 (m), 3090 (s), 3061 (s),
3035 (s), 2719 (s), 2342 (w), 1933 (w), 1851 (w), 1601 (s),
3.00 mmol) was added portionwise and was allowed to stir
for 1 h. After the reaction was completed, the reaction was
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310
Can. J. Chem. Vol. 90, 2012
Scheme 3. Tentative mechanism for the platinum nanoclusters immobilized on a polymer / carbon black composite material (PI/CB–Pt)-
catalyzed sequential oxidative transformation of N-substituted 2-aminophenols.
PI/CB-Pt, K2CO3
O2
H2O
OH
OH
N
O
PI/CB-Pt, K2CO3
R1
R1
R1
R2
R2
N
H
R2
N
O2
H2O
8
10
11
O
O
N
PI/CB-Pt, K2CO3
R1
R2
R1
R2
N
H
O2
H2O
9
13
Scheme 4. Recovery and reuse of platinum nanoclusters immobi-
HRMS (m/z) calcd for C17H22N1O1 [(M + H)+]: 256.17014;
lized on a polymer / carbon black composite material (PI/CB–Pt).
OH
found: 256.17079.
O
PI/CB-Pt (1 mol%)
4-(tert-Butyl)-2-((4-methylbenzyl)amino)phenol (8e)
mp 73–74 °C. IR (KBr, cm–1): 3326 (s), 2956 (s), 2900
(s), 1598 (s), 1466 (s), 1409 (s), 1348 (s), 1274 (s), 1183
Ph
K2CO3, CHCl3:H2O (19:1)
tBu
N
H
Ph
tBu
N
O
2 (1 atm), 30 °C, 20 h
8d
9d
>95%
1st to 5th
1
(s), 1044 (s). H NMR (CDCl3, 600 MHz) d: 7.24 (d, J =
7.6 Hz, 2H), 7.10 (d, J = 7.6 Hz, 2H), 6.73–6.58 (m, 3H),
4.25 (s, 2H), 2.31 (s, 3H), 1.23 (s, 9H). 13C NMR (CDCl3,
150 MHz) d: 144.4, 141.6, 136.7, 136.4, 129.6, 129.2,
127.8, 127.2, 114.6, 110.4, 48.8, 33.8, 31.5, 21.0. DART-
HRMS (m/z) calcd for C18H24N1O1 [(M + H)+]: 270.18579;
found: 270.18579.
reuse cycles
1
1362 (m), 1124 (s). H NMR (CDCl3, 600 MHz) d: 7.35–
7.28 (m, 4H), 7.22 (t, J = 7.6 Hz, 1H), 6.48–6.38 (d, J =
6.0 Hz, 3H), 4.75 (s, 1H), 4.27 (s, 2H), 2.19 (s, 3H). 13C
NMR (CDCl3, 150 MHz) d: 141.1, 139.3, 136.8, 131.0,
128.5, 127.3, 127.1, 118.0, 114.3, 113.2, 48.5, 21.1. DART-
HRMS (m/z) calcd for C14H16N1O1 [(M + H)+]: 214.12319;
found: 214.12368.
4-(tert-Butyl)-2-((4-methoxybenzyl)amino)phenol (8f)
mp 87 °C. IR (KBr, cm–1): 3513 (w), 3330 (s), 2953 (s),
2904 (s), 2867 (s), 1603 (s), 1520 (s), 1467 (m), 1362 (m),
1
1254 (s), 1184 (s), 1131 (s), 1041 (s). H NMR (CDCl3,
600 MHz) d: 7.32 (d, J = 8.2 Hz, 2H), 6.90–6.63 (m, 5H),
4.64–4.20 (m, 3H), 3.80 (s, 3H), 1.27 (s, 9H). 13C NMR
(CDCl3, 150 MHz) d: 158.7, 144.4, 141.4, 136.2, 131.6,
129.0, 114.6, 113.9, 110.5, 55.3, 48.4, 33.7, 31.5. DART-
HRMS (m/z) calcd for C18H24N1O2 [(M + H)+]: 286.18070;
found: 286.18094.
2-(Benzylamino)-5-methylphenol (8c)
mp 102 °C. IR (KBr, cm–1): 3312 (s), 3032 (s), 2918 (s),
2865 (s), 1934 (w), 1842 (w), 1722 (w), 1615 (m), 1524 (s),
1
1451 (s), 1123 (s). H NMR (CDCl3, 600 MHz) d: 7.35–7.29
(m, 4H), 7.25–7.22 (t, J = 6.0 Hz, 1H), 6.59 (s, 2H), 6.46 (s,
1H), 4.67 (s, 1H), 4.26 (s, 2H), 2.16 (s, 3H). 13C NMR
(CDCl3, 150 MHz) d: 144.1, 139.4, 134.1, 128.5, 128.3,
127.6, 127.1, 121.6, 115.4, 113.5, 49.2, 20.5. DART-HRMS
(m/z) calcd for C14H16N1O1 [(M + H)+]: 214.12319; found:
214.12326.
4-(tert-Butyl)-2-((4-chlorobenzyl)amino)phenol (8g)
mp 83 °C. IR (KBr, cm–1): 3319 (s), 2961 (s), 2900 (s),
2864 (s), 1869 (w), 1739 (w), 1599 (s), 1518 (s), 1347 (m),
1
1185 (s), 1128 (s), 1048 (s). H NMR (CDCl3, 600 MHz) d:
7.10 (d, J = 2.1 Hz, 4H), 6.56–6.39 (m, 3H), 4.91 (s, 1H),
4.12 (s, 2H), 1.10 (s, 9H). 13C NMR (CDCl3, 150 MHz) d:
144.5, 141.3, 137.8, 135.8, 132.7, 128.9, 128.6, 114.9,
114.2, 110.5, 48.1, 34.2, 31.4. DART-HRMS (m/z) calcd for
C17H21N1O1Cl1 [(M + H)+]: 290.12117; found: 290.13109.
2-(Benzylamino)-4-(tert-butyl)phenol (8d)
mp 99 °C. IR (KBr, cm–1): 3460 (w), 3335 (m), 3207 (s),
2958 (s), 2870 (m), 1953 (w), 1852 (w), 1816 (w), 1604 (s),
1
1524 (s), 1410 (m), 1268 (s), 1132 (s). H NMR (CDCl3,
600 MHz) d: 7.48–7.34 (m, 5H), 7.85–6.69 (d, J = 9.1 Hz,
3H), 5.27 (s, 1H), 4.41 (s, 2H), 1.35 (s, 9H). 13C NMR
(CDCl3, 150 MHz) d: 144.3, 141.4, 139.2, 136.1, 128.5,
127.7, 127.1, 114.7, 114.2, 110.5, 48.9, 33.9, 31.5. DART-
4-(((5-(tert-Butyl)-2-hydroxyphenyl)amino)methyl)
benzonitrile (8h)
mp 128 °C. IR (KBr, cm–1): 3393 (s), 2961 (s), 2869 (s),
Published by NRC Research Press

Yoo et al.
311
1
2232 (s), 1924 (w), 1705 (w), 1608 (s), 1523 (s), 1417 (s),
1275 (s), 1129 (s), 1012 (w). H NMR (CDCl3, 600 MHz)
1417 (s), 1312 (m), 1208 (s), 1106 (m). H NMR (CDCl3,
600 MHz) d: 7.34–7.21 (m, 6H), 6.74 (s, 2H), 2.77 (s, 2H),
2.00 (s, 2H), 1.31 (s, 9H). 13C NMR (CDCl3, 150 MHz) d:
141.7, 128.4, 125.9, 33.3, 31.6. DART-HRMS (m/z) calcd
for C19H26N1O1 [(M + H)+]: 284.20144; found: 284.20186.
1
d: 7.59 (d, J = 7.6 Hz, 2H), 7.49 (d, J = 7.6 Hz, 2H), 6.67–
6.64 (m, 2H), 6.53 (s, 1H), 4.44 (s, 2H), 1.26 (s, 9H). 13C
NMR (CDCl3, 150 MHz) d: 145.7, 144.2, 141.1, 135.5,
132.3, 127.9, 118.8, 114.5, 113.8, 110.3, 109.6, 48.0, 34.1,
31.4. DART-HRMS (m/z) calcd for C18H21N2O1 [(M + H)+]:
281.16539; found: 281.16648.
General experimental procedure for the sequential
oxidative transformations of N-substituted 2-
4-(tert-Butyl)-2-(cinnamylamino)phenol (8i)
aminophenols
mp 98–99 °C. IR (KBr, cm–1): 3317 (s), 3061 (m), 2957
(s), 2861 (m), 1601 (s), 1521 (s), 1415 (s), 1364 (m), 1196
(s), 1133 (s), 1047 (w). 1H NMR (CDCl3, 600 MHz) d:
7.45–7.31 (m, 5H), 6.97 (s, 1H), 6.77–6.70 (m, 3H), 6.44–
6.40 (m, 1H), 5.62 (s, 1H), 4.31 (d, J = 5.5 Hz, 2H), 1.68
(s, 9H). 13C NMR (CDCl3, 150 MHz) d: 144.1, 141.8,
136.8, 136.0, 131.8, 128.4, 127.3, 127.0, 126.3, 115.0,
114.3, 110.7, 46.9, 34.2, 31.6, 31.5. DART-HRMS (m/z)
calcd for C19H24N1O1 [(M + H)+]: 282.18579; found:
282.18705.
In a screw cap glass test tube, PI/CB–Pt (0.0210 g,
0.0025 mmol, 1 mol%), 8d (0.0638 g, 0.250 mmol), and
K2CO3 (0.0025 g, 0.025 mmol) were added and then parti-
ally dissolved in CHCl3 (4.75 mL) and H2O (0.25 mL). The
reaction mixture was allowed to stir for 20 h at 30 °C under
an O2 balloon. After the completion of the reaction, the het-
erogeneous catalyst was removed by filtration and washed
with CH2Cl2. MgSO4 (0.02 g) was added to the resulting fil-
trate, and the dried crude reaction mixture was filtered again
and then concentrated under reduced pressure. The crude
product was purified by preparative thin-layer chromatogra-
phy (EtOAc:hexane = 1:9) to provide 9d (0.0626 g,
0.249 mmol, 100%) as a white solid.
4-(tert-Butyl)-2-((3-phenylprop-2-yn-1-yl)amino)phenol (8j)
IR (NaCl, film, cm–1): 3061 (m), 2959 (s), 2905 (s), 2868
(s), 2362 (w), 2337 (w), 1951 (w), 1885 (w), 1733 (s), 1604
1
5-Methyl-2-phenylbenzooxazole (9b)
(s), 1523 (s), 1489 (s), 1420 (s), 1250 (s), 1197 (s). H NMR
1H NMR (CDCl3, 600 MHz) d: 8.24–8.22 (m, 2H), 7.54–
7.50 (m, 4H), 7.44 (d, J = 8.2 Hz, 1H), 7.14 (d, J = 8.2 Hz,
1H), 2.47 (s, 3H). 13C NMR (CDCl3, 150 MHz) d 163.1,
149.0, 142.2, 134.4, 131.4, 128.9, 127.5, 127.3, 126.2,
119.9, 109.9, 21.5. This is a known compound and the spec-
tral data are identical to those reported in the literature.11
(CDCl3, 600 MHz) d: 7.30–7.21 (m, 2H), 7.16–7.09 (m, 3H),
6.83 (s, 1H), 6.57–6.54 (m, 2H), 4.13–3.97 (m, 2H), 1.19 (s,
9H). 13C NMR (CDCl3, 150 MHz) d: 143.8, 142.4, 135.0,
128.1, 127.9, 122.9, 115.6, 114.0, 111.3, 86.6, 83.3, 34.9,
31.3, 20.9, 14.0. DART-HRMS (m/z) calcd for C19H22N1O1
[(M + H)+]: 280.17014; found: 280.17017.
6-Methyl-2-phenylbenzooxazole (9c)
5-Methyl-2-((3-phenylpropyl)amino)phenol (8l)
1H NMR (CDCl3, 600 MHz) d: 8.16 (s, 2H), 7.57 (d, J =
7.6 Hz, 1H), 7.45 (s, 1H), 7.09 (d, J = 8.2 Hz, 1H), 2.43 (s,
3H). 13C NMR (CDCl3, 150 MHz) d: 162.5, 151.0, 139.8,
135.5, 131.3, 128.8, 127.4, 127.3, 125.8, 119.3, 110.7, 21.8.
This is a known compound and the spectral data are identical
to those reported in the literature.12
mp 69–71 °C. IR (KBr, cm–1): 3412 (w), 3285 (m), 2955
(s), 2852 (s), 2663 (s), 1901 (w), 1818 (w), 1746 (w), 1585
(s), 1478 (s), 1445 (s), 1371 (s), 1180 (s), 1084 (m). 1H
NMR (CDCl3, 600 MHz) d: 7.35–7.22 (m, 5H), 6.96 (t, J =
7.6 Hz, 1H), 6.80 (d, J = 8.2 Hz, 1H), 6.71 (d, J = 7.6 Hz),
2.92 (t, J = 6.8 Hz, 2H), 2.76 (t, J = 7.6 Hz, 2H), 2.29 (s,
3H), 2.00–1.94 (m, 2H). 13C NMR (CDCl3, 150 MHz) d:
152.2, 141.5, 133.5, 132.8, 128.4, 128.3, 125.9, 125.4,
121.6, 111.8, 48.7, 33.4, 32.3, 17.3. DART-HRMS (m/z)
calcd for C16H20N1O1 [(M + H)+]: 242.15449; found:
242.15469.
5-(tert-Butyl)-2-phenylbenzooxazole (9d)
1H NMR (CDCl3, 600 MHz) d: 8.25–8.22 (m, 2H), 7.79
(d, J = 2 Hz, 1H), 7.52–7.47 (m, 4H), 7.41–7.39 (dd, J =
2 Hz, 1H), 1.38 (s, 3H). 13C NMR (CDCl3, 150 MHz) d:
163.1, 148.7, 148.1, 142.0, 131.3, 128.9, 127.5, 127.3,
122.8, 116.5, 109.7, 34.9, 31.7. This is a known compound
and the spectral data are identical to those reported in the lit-
erature.13
Experimental procedure for the preparation of 4-(tert-
butyl)-2-((3-phenylpropyl)amino)phenol (8k)
1-Bromo-3-phenylpropane (3.0 mL, 20 mmol) was added
to a heterogeneous mixture of 2-amino-4-tert-butylphenol
(6.573 g, 39.78 mmol) and KHCO3 (8.763 g, 87.52 mmol)
in DMF (10 mL), and the reaction mixture was stirred over-
night at room temperature. The reaction mixture was diluted
with EtOAc (30 mL) and H2O (30 mL), extracted with
EtOAc, and the combined organic layers were washed with a
saturated brine solution. The extracted crude product was
dried over MgSO4, filtered, concentrated under reduced pres-
sure, and then purified by column chromatography (EtOAc:
hexane = 1:5) to provide the desired product 8k (5.617 g,
19.82 mmol, 95%) as a white solid, mp 107–108 °C. IR
(KBr, cm–1): 3414 (s), 3330 (s), 3235 (s), 2959 (s), 2863 (s),
1940 (w), 1870 (w), 1806 (w), 1598 (s), 1522 (s), 1475 (s),
2-(4-Methylphenyl)-5-tert-butylbenzooxazole (9e)
1H NMR (CDCl3, 600 MHz) d: 8.12 (d, J = 8.2 Hz, 2H),
7.78 (d, J = 1.4 Hz, 1H), 7.45 (d, J = 8.2 Hz, 1H), 7.39–
7.36 (dd, J = 2 Hz, 1H), 7.29 (d, J = 8.3 Hz, 2H), 2.41 (s,
3H), 1.38 (s, 9H). 13C NMR (CDCl3, 150 MHz) d: 163.4,
148.7, 148.0, 142.0, 141.8, 129.6, 127.4, 124.6, 122.5,
116.3, 109.5, 34.9, 31.7, 21.6. This is a known compound
and the spectral data are identical to those reported in the lit-
erature.14
2-(4-Methoxylphenyl)-5-tert-butylbenzooxazole (9f)
mp 134.8 °C. IR (KBr, cm–1): 3069 (m), 3003 (w), 2957
(s), 2372 (w), 2043 (w), 1898 (w), 1609 (s), 1499 (s), 1471
Published by NRC Research Press

312
Can. J. Chem. Vol. 90, 2012
1
(m), 1337 (m), 1256 (s), 1170 (m), 1055 (m), 1026 (s). H
NMR (CDCl3, 500 MHz) d: 8.15 (d, J = 8.5 Hz, 2H), 7.75
(d, J = 1.7 Hz, 1H), 7.43 (d, J = 8.5 Hz, 1H), 7.34 (d, J =
8.5 Hz, 1H), 6.98 (d, J = 8.5 Hz, 2H), 3.84 (s, 3H), 1.37 (s,
9H). 13C NMR (CDCl3, 125 MHz) d: 163.2, 162.1, 148.6,
147.8, 142.1, 129.2, 122.2, 119.8, 116.1, 114.2, 109.4, 55.3,
34.8, 31.7. DART-HRMS calcd for C18H20N1O2 [(M + H)+]:
282.14940; found: 282.14963.
600 MHz) d: 7.62 (d, J = 1.4 Hz, 1H), 7.32–7.26 (m, 2H),
7.23–7.11 (m, 5H), 3.13 (s, 4H), 1.29 (s, 9H). 13C NMR
(CDCl3, 150 MHz) d: 166.3, 148.7, 147.6, 141.2, 140.1,
128.2, 126.4, 122.1, 116.1, 109.4, 34.8, 32.8, 31.7, 30.4.
DART-HRMS calcd for C19H22N1O1 [(M
280.17014; found: 280.17142.
+
H)+]:
6-Methyl-2-phenethylbenzooxazole (9l)
IR (KBr, cm–1): 3061 (w), 3029 (m), 2925 (m), 2860 (w),
2362 (w), 1905 (w), 1608 (s), 1577 (s), 1495 (m), 1451 (m),
2-(4-Chlorophenyl)-5-tert-butylbenzooxazole (9g)
1
mp 167.1 °C. IR (KBr, cm–1): 3061 (m), 2962 (s), 2905
(w), 2869 (w), 2369 (w), 1891 (s), 1600 (m), 1479 (s), 1399
(m), 1362 (m), 1275 (s), 1200 (m), 1088 (s), 1050 (s), 1009
1242 (s), 1148 (m), 1037 (m). H NMR (CDCl3, 600 MHz)
d: 7.22 (t, J = 6.9 Hz, 3H), 7.17 (t, J = 7.6 Hz, 2H), 7.16–
7.09 (m, 2H), 7.02 (d, J = 7.6 Hz, 1H), 3.19–3.10 (m, 4H),
2.53 (s, 3H). 13C NMR (CDCl3, 150 MHz) d: 165.4, 150.5,
140.4, 140.2, 130.0, 128.6, 128.3, 126.4, 124.7, 124.2,
107.6, 33.1, 30.7, 16.5. DART-HRMS calcd for C17H16N1O1
[(M + H)+]: 238.12319; found: 238.12273.
1
(m). H NMR (CDCl3, 600 MHz) d: 8.14 (d, J = 8.2 Hz,
2H), 7.77 (s, 1H), 7.47–7.44 (t, J = 8.2 Hz, 3H), 7.39 (d,
J = 2 Hz, 1H), 1.38 (s, 9H). 13C NMR (CDCl3, 150 MHz)
d: 162.1, 148.7, 148.3, 141.9, 137.5, 129.2, 128.7, 125.8,
123.1, 116.5, 109.7, 35.0, 31.7. DART-HRMS calcd for
C17H17N1O1Cl1 [(M + H)+]: 286.09987; found: 286.09902.
Acknowledgements
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Japan Society for the Promotion
of Science (JSPS), the Global Centers of Excellence (COE)
Program, The University of Tokyo, the Ministry of Educa-
tion, Culture, Sports, Science and Technology - Japan
(MEXT), and the New Energy and Industrial Technology De-
velopment Organization (NEDO). W.-J.Y. thanks JSPS for
the JSPS Postdoctoral Fellowship for Foreign Researchers.
We also thank Mr. N. Kuramitsu (The University of Tokyo)
for STEM analysis and Dr. J.-F. Soulé for assistance during
manuscript preparation.
2-(4-Cyanophenyl)-5-tert-butylbenzooxazole (9h)
mp 207.2 °C. IR (KBr, cm–1): 3061 (m), 2968 (s), 2905
(w), 2875 (m), 2225 (m), 1899 (w), 1746 (w), 1616 (m),
1573 (m), 1549 (m), 1479 (s), 1404 (s), 1367 (s), 1332 (s),
1
1278 (s), 1200 (m), 1133 (w), 1052 (s), 1013 (w). H NMR
(CDCl3, 600 MHz) d: 8.24 (d, J = 8.3 Hz, 2H), 7.74–7.70 (t,
J = 8.3 Hz, 3H), 7.44–7.38 (m, 2H), 1.32 (s, 9H). 13C NMR
(CDCl3, 150 MHz) d: 160.9, 148.8, 148.7, 141.7, 132.6,
131.2, 127.7, 124.0, 118.2, 116.9, 114.4, 109.9, 34.9, 31.7.
DART-HRMS calcd for C18H17N2O1 [(M
277.13409; found: 277.13372.
+
H)+]:
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1
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34.9, 31.7. DART-HRMS calcd for C19H18N1O1 [(M + H)+]:
276.13884; found: 276.13927.
5-(tert-Butyl)-2-phenethylbenzooxazole (9k)
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2867 (w), 2492 (w), 2371 (w), 1618 (m), 1576 (s), 1477 (m),
1452 (m), 1365 (m), 1266 (m), 1226 (m), 1202 (w), 1161
(m), 1178 (m), 1077 (m), 1032 (w). 1H NMR (CDCl3,
Published by NRC Research Press

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