OMS-2-Supported Cu Hydroxide-Catalyzed Benzoxazoles Synthesis from Catechols and Amines via Domino Oxidation Process at Room Temperature

Article abstract of DOI:10.1021/acs.joc.7b01119

In the presence of manganese oxide octahedral molecular sieve (OMS-2) supported copper hydroxide Cu(OH)_x/OMS-2, aerobic synthesis of benzoxazoles from catechols and amines via domino oxidation/cyclization at room temperature is achieved. This heterogeneous benzoxazoles synthesis initiated by the efficient oxidation of catechols over Cu(OH)_x/OMS-2 tolerates a variety of substrates, especially amines containing sensitive groups (hydroxyl, cyano, amino, vinyl, ethynyl, ester, and even acetyl groups) and heterocycles, which affords functionalized benzoxazoles in good to excellent yields by employing low catalyst loading (2 mol % Cu). The characterization and plausible catalytic mechanism of Cu(OH)_x/OMS-2 are described. The notable features of our catalytic protocol such as the use of air as the benign oxidant and EtOH as the solvent, mild conditions, ease of product separation, being scalable up to the gram level, and superior reusability of catalyst (up to 10 cycles) make it more practical and environmentally friendly for organic synthesis.

Full text of DOI:10.1021/acs.joc.7b01119

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Article
OMS-2-supported Cu hydroxide-catalyzed benzoxazoles synthesis from
catechols and amines via domino oxidation process at room temperature
Xu Meng, Yanmin Wang, Yuanguang Wang, Baohua
Chen, Zhenqiang Jing, Gexin Chen, and Peiqing Zhao
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017
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OMS-2-supported Cu hydroxide-catalyzed benzoxazoles synthesis
from catechols and amines via domino oxidation process at room
temperature
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Xu Meng,† Yanmin Wang,† Yuanguang Wang,† Baohua Chen,‡ Zhenqiang Jing,§ Gexin Chen† and
Peiqing Zhao*,†
†State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research Institute of LICP,
Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences, Lanzhou 730000, P. R.
China
Corresponding author E-mail: zhaopq@licp.cas.cn
‡State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R.
China
§Suzhou Institute of Nano-Tech and Nano-Bionic (SINANO), Chinese Academy of Sciences, Suzhou
215123, P. R. China
ABSTRACT: In the presence of manganese oxide octahedral molecular sieves
(OMS-2) supported copper hydroxide Cu(OH)x/OMS-2, aerobic synthesis of
benzoxazoles from catechols and amines via domino oxidation/cyclization at room
temperature is achieved. This heterogeneous benzoxazoles synthesis initiated by the
efficient oxidation of catechols over Cu(OH)x/OMS-2 tolerates a variety of substrates,
especially amines containing sensitive groups (hydroxyl, cyano, amino, vinyl, ethynyl,
ester and even acetyl groups) and heterocycles, which affords functionalized
benzoxazoles in good to excellent yields by employing low catalyst loading (2 mol%
Cu). The characterization and plausible catalytic mechanism of Cu(OH)x/OMS-2 are
described. The notable features of our catalytic protocol, such as the use of air as the
benign oxidant and EtOH as the solvent, mild conditions, ease of product separation,
being scalable up to the gram level and superior reusability of catalyst (upto 10
cycles), make it more practical and environmentally friendly for organic synthesis.
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INTRODUCTION:
Oxidation process plays a crucial role in organic synthesis from academic and
industrial points of views, particularly selective oxidative catalysis by the use of green
oxidants, like O2 and H2O2.1,2 In order to realize such green oxidations under mild
conditions, considerable effort has been made to design efficient catalytic systems
inspired by natural occurring redox-active metalloenzymes, like copper-containing
amine oxidases (CuAOs).3 For example, Largeron`s group reported an
iminoquinone/Cu-like co-catalytic system for the oxidative synthesis of imines under
mild conditions, which showed iminoquinone was a redox organocatalyst and copper
salt transferred electrons to terminal oxidant air.4 Similarly, Sthal with co-workers
performed various biomimetic oxidations by the use of o-quinones as redox catalysts
and O2 as oxidant under ambient conditions with assistance of transition metals.5 On
the other hand, Kobayashi and Doris with their co-workers respectively developed a
series of heterogeneous cooperative catalytic systems using noble metal nanoclusters
or nanoparticles as catalysts, catechols as co-catalysts and O2 as the terminal oxidant
for the oxidations under mild conditions.6 By contrast, heterogeneous systems are
more advantageous than homogeneous counterparts because of easy separation and
recyclability of catalysts. As a result, there is an incentive to design bioinspired
heterogeneous redox catalytic process using non-noble metals able to synthesize
value-added organic products under mild conditions.
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Scheme 1. Multistep electron-transfer process over Cu/OMS-2 catalyst.
Recently, we reported a series of mixed valent manganese oxide octahedral
molecular sieves (OMS-2)7 supported copper catalysts for the oxidative synthesis of
heterocycles using air as the oxidant through a multistep redox process (Scheme 1).8,9
From the perspective of redox potential, electrons of an oxidative reaction can quickly
transfer from the substrate to the oxidant under a low-energy barrier path that is
formed by the synergistic interaction between copper and OMS-2.10 This kind of
synergistic catalytic system enhances the catalytic efficiency, decreases the catalyst
loading, and enables air to be terminal oxidant, more importantly, makes oxidations
occur under mild conditions. Interestingly, we quite recently discovered that
OMS-2-supported Cu-based catalysts were highly active in the oxidation of catechol
1a via a double-electron transfer process at room temperature using air as the oxidant
(Scheme 2).11 Cu(OH)x/OMS-2 (Cu: 2 mol%) demonstrated synergistic effect in
ethanol under mild conditions, and o-quinone 1aa was obtained in nearly quantitative
yield from electron-rich catechol 1a that is a general cofactor in CuAOs.3a,5 We
therefore envisaged that using a suitable nucleophile to seize the active o-quinone in
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situ generated from catechol for further oxidative reaction in one-pot manner via a
domino process. With this strategy, amine would be able to react with o-quinone in
situ generated to form 2-substituted benzoxazole via a domino oxidation/cyclization
process over Cu(OH)x/OMS-2 under mild conditions using air as the terminal oxidant.
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Scheme 2. Cu/OMS-2-catalyzed oxidation of catechol 1a.
Benzoxazoles as structural motifs in natural products are frequently used as
building blocks for pharmaceuticals and organic materials.12 Condensation of
2-amino-phenols with carboxylic acid derivatives followed by annulation under acidic
conditions is the traditional method for the synthesis of benzoxazoles.13 Recently,
Ru-based catalysts were applied in the oxidation of alcohols and amines via
hydrogen-transfer and their oxidative products can subsequently react with
2-amino-phenols to offer benzoxazoles.14 Inspired by metalloenzymes, Bäckvall and
co-workers developed a biomimetic electron-transfer process for the synthesis of
benzoxazoles using quinone/Co-salen as ETMs at 120 oC.9f In terms of heterogeneous
catalysis, Kobayashi`s group reported supported nano-Pt-catalyzed oxidative
cyclization of phenolic imines to realize 2-substituted benzoxazoles with assistance of
base.15
Herein, we describe an efficient Cu(OH)x/OMS-2-catalyzed synthesis of
2-substituted benzoxazoles from catechols and amines via domino
oxidation/cyclization process using ambient air as the oxidant at room temperature
(Scheme 3). The heterogeneous catalyst demonstrates favourable synergistic
interaction between copper hydroxide and OMS-2, which promotes the reaction
through a multistep redox process.
t-Bu
t-Bu
Cu(OH)x/OMS-2
O2
EtOH, r.t., air, 2 h
OH
OH
O
N
Cu
Mn
R
+
R
NH2
t-Bu
t-Bu
t-Bu
via
oxidation
cyclization
O
O
H2N
R
t-Bu
nucleophile
electrophile
Scheme 3. Cu(OH)x/OMS-2-catalyzed synthesis of benzoxazoels initiated by catechol oxidations.
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RESULTS AND DISCUSSION
Cu(OH)x/OMS-2 was prepared by impregnation-precipitation in water via the
treatment of NaOH solution using CuCl2 as the precursor. X-ray diffraction (XRD)
showed that the diffraction peaks of Cu(OH)x/OMS-2 with increased crystallite size
were the same as that of OMS-2 and no peaks of copper species were observed (Fig.
S1, see ESI), which means that copper was highly dispersed on OMS-2 and the
catalyst was typical cryptomelane material (JCPDS file #29-1020).7b Then, electron
microscope (SEM and TEM) was employed to characterize the morphology of the
catalyst. It is worth noting that the morphology of Cu(OH)x/OMS-2 was significantly
different from support OMS-2 and even CuO/OMS-2 catalyst. Cu(OH)x/OMS-2 was
aggregated micron-sized sphere with decreased BET surface area, while OMS-2 or
CuO/OMS-2 both had a typical nano-rod morphology (Fig. 1). Similarly, TEM
images illustrate aggregated, more uniform and longer nano-rod morphology of
Cu(OH)x/OMS-2 compared with highly dispersed and broken nano-rod morphology
of OMS-2 (Fig. 2). The lattice vibrational behavior of Cu(OH)x/OMS-2 was studied
by FTIR and the results show it had representative Mn-O lattice vibration modes in
MnO6 octahedra (Fig. 3).16 More importantly, the supported copper hydroxide catalyst
possessed large amount of water in tunnels of OMS-2 (1627 cm-2) and hydroxyl group
on the surface of catalyst (3425 cm-2).17 More results of catalyst characterization were
concluded in supporting information.
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Figure 1. SEM images of OMS-2 (a) and Cu(OH)x/OMS-2 (b).
Figure 2. TEM images of OMS-2 (a) and Cu(OH)x/OMS-2 (b).
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Figure 3. FTIR of OMS-2, fresh Cu(OH)x/OMS-2 and retrieved Cu(OH)x/OMS-2.
After investigating distinctive physical properties of Cu(OH)x/OMS-2, the one-pot
domino oxidative synthesis of 2-substituted benzoxazole from catechol 1a and
i-butylamine 2a was studied by the use of different catalysts and solvents at room
temperature (Table 1). When CuO/OMS-2 (20 mg, Cu: 1.3 wt%) and
Cu(OH)x/OMS-2 (20 mg, Cu: 1.48 wt%) were employed in dichloromethane and
ethanol respectively, excellent yields of desired product were observed (Table 1,
entries 1 and 2). Especially, Cu(OH)x/OMS-2 led to nearly quantitative yield of
benzoxazole 3a in EtOH, which makes purification process very simple. Without
column chromatographic isolation, 95% yield of 3a was obtained by filtration of solid
catalyst followed by removal of EtOH and excess i-butylamine under reduced
pressure. More importantly, EtOH is a non-toxic and green solvent to environment
compared with DCM and other organic solvents. Control experiments show that both
copper species and support material were indispensable for the reaction, although
catalytic amounts of Cu(OH)2 demonstrated catalytic activity to some extent (Table 1,
entries 3 and 4). However, physical mixture of Cu(OH)2 and OMS-2 did not show
fruitful results, which means highly dispersed copper species on OMS-2 might be
crucial for the catalysis (Table 1, entry 5). Subsequently, reaction time was examined
and the results indicate that decreased time was unfavourable for the reaction (Table 1,
entry 6). And, the yield of product decreased if less amount of Cu(OH)x/OMS-2 was
used (Table 1, entry 7). Then, different atmospheres were investigated. When the
reaction was carried out under O2, a low yield of benzoxazole 3a was observed
because catechol 1a decomposed to pigment melanin slightly under strong oxidative
conditions (Table 1, entry 8).[18] On the other hand, the reaction proceeded difficultly
under N2, which means air was the terminal oxidant and 9% yield of product might be
resulted from supported Cu(OH)x re-oxidized by OMS-2 (Table 1, entry 9). H2O was
deleterious to the reaction, while the reaction did not proceed at all under solvent-free
conditions (Table 1, entries 10 and 11). Commercial copper catalysts were used in
place of supported catalysts in EtOH at room temperature, and the results illustrate
that supported copper exhibited superior behaviour compared with CuBr and
unsupported CuO, even high catalyst loading (10 mol%) was employed (Table 1,
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entries 1, 2, 12 and 13). As a result of the optimization, we concluded that the best
reaction conditions for synthesis of 2-substituted benzoxazole 3a in ethanol involve
the use of Cu(OH)x/OMS-2 under air at room temperature.
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Table 1. Optimization of the reaction between catechol and aminea
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t-Bu
t-Bu
OH
OH
O
N
catalyst
+
NH2
solvent, r.t., air, 2 h
t-Bu
t-Bu
1a
3a
2a
Entry
1
Catalyst
CuO/OMS-2
Solvent
Yield (%)b
DCM
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
H2O
90
2
Cu(OH)x/OMS-2
OMS-2
>98 (95)
3
Trace
15
18
75
67
87
9
4
5c
6d
7e
8f
9g
10
11h
12
13
Cu(OH)2 (2 mol%)
Cu(OH)2 + OMS-2
Cu(OH)x/OMS-2
Cu(OH)x/OMS-2
Cu(OH)x/OMS-2
Cu(OH)x/OMS-2
Cu(OH)x/OMS-2
Cu(OH)x/OMS-2
CuBr (2 mol%)
CuO (10 mol%)
18
0
-
EtOH
EtOH
5
0
a Reaction condition: 1a (0.2 mmol), 2a (0.24 mmol), catalyst (20 mg), solvent (1 mL), room
temperature, 2 h, under air. b NMR yield using CH2Br2 an internal standard and yield is
given in parenthesis. c Cu(OH)2 (2 mol%) and OMS-2 (20 mg) were used. d For 1 h. e 5 mg of
Cu(OH)x/OMS-2 was used. f Under 1atm O2. g Under N2.. h 1 mL of 2a was used.
According to the initial optimization, it is notable that the combination of copper
and OMS-2 had a superior effect on the reaction. To further investigate this
heterogeneous catalytic system, other supported catalysts were examined under the
standard conditions (Table 2). Because metal hydroxide is hard to prepare for some
metal, we used catalysts containing metal oxide (CuOx or MnOx) for the comparison.
Firstly, CuO/OMS-2 also showed high catalytic activity in EtOH for the reaction,
although it provided lower yield of product than Cu(OH)x/OMS-2 did (Table 2,
entry1). When Al-Ti was used as support, a 33% yield of 3a was obtained. However,
nano-TiO2 and -Al2O3 supported CuO did not show any catalytic activity under the
standard conditions (Table 2, entries 3 and 4). Next, we used Cu-Mn spinel oxide in
the absence of any supports in the reaction, but the result was frustrating (Table 2,
entry 5). Then, we prepared supported CuOx-MnOx catalyst by means of
wet-impregnation using nano-TiO2 and Al-Ti as supports respectively. However, this
kind of bimetallic catalyst was intrinsically inactive for the oxidation and did not
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result in cooperative effect, which means catalytic mechanism of OMS-2-supported
copper catalyst is different from supported Cu-Mn bimetallic catalysts. Eventually, we
made supported CuO catalysts using other manganese oxides, like Mn3O4 and
amorphous MnO2, instead of OMS-2 as supports and they offered desired products in
moderate yields (Table 2, entries 8 and 9). The results indicate Mn-based support is
critical for the catalytic system probably because they are easy to be re-oxidized by
oxygen under mild conditions, while manganese oxide with the feature of mixed
valence is an important role in the catalytic system and improves the catalytic activity
significantly.
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Table 2. Investigation of heterogeneous catalystsa
t-Bu
t-Bu
OH
OH
O
N
heterogeneous catalyst
EtOH, r.t., air, 2 h
+
NH2
t-Bu
t-Bu
1a
3a
2a
Entry
Catalyst
Yield (%)b
1
2
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4
5c
6
7
8
9
CuO/OMS-2
CuO/Al-Ti
93
33
CuO/nano-TiO2
CuO/nano-Al2O3
Trace
Trace
0
Cu-Mn spinel oxide
CuOx-MnOx/nano-TiO2
CuOx-MnOx /Al-Ti
CuO/Mn3O4
0
0
60
CuO/amorphous MnO2
69
a Reaction condition: 1a (0.2 mmol), 2a (0.24 mmol), catalyst (20 mg, theoretical loading of
supported metal is 1.5 wt%), EtOH (1 mL), room temperature, 2 h, under air. b NMR yield
c
using CH2Br2 an internal standard and yield is given in parenthesis. Cu-Mn spinel oxide in
molecular weight ratio of 1:0.25.
To verify whether the catalysis was derived from Cu(OH)x/OMS-2 or leached
copper species in solution, the reaction between catechol 1a and i-butylamine 2a
catalyzed by Cu(OH)x/OMS-2 under present conditions was performed. After the
reaction carried out for 30 minutes, the catalyst was removed by filtration from the
reaction solution and the NMR yield of desired product was approximately 68%.
Meanwhile, the filtrate was kept reacting without addition of fresh Cu(OH)x/OMS-2
for further 2 h, and no more product 3a was detected (Fig. S7, see ESI). The filtrate
was analysed by ICP-AES, which shows copper and manganese species were both
hardly detected in filtrate. These observations prove that copper and manganese did
not leach from the solid catalyst and the observed catalysis was genuinely
heterogeneous.
On the base of above research, the recyclability of Cu(OH)x/OMS-2 was studied.
Generally, the catalyst was easily separated from the reaction solution by filtration at
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the end of each run. The retrieved Cu(OH)x/OMS-2 was washed by water and EtOH,
and reused after drying in vacuum oven at 50 oC for overnight. Although the retrieved
catalyst maintained the morphology and structure of cryptomelane very well (Fig.
S2-S4, see ESI), it resulted in a decreased yield of product in 80% (Table 3). After
analysing of retrieved catalyst by FTIR, it was found that the catalyst lost the
hydroxyl group on the surface and water in the tunnel of OMS-2 (Fig. 3), which likely
causes a low efficiency of the catalytic system. To our delight, the catalytic activity of
retrieved Cu(OH)x/OMS-2 could be regenerated by the treatment of NaOH solution
(for detailed procedure and characterization of regenerated catalyst, see ESI). The
recycling experiments show that the regenerated catalyst could be reused at least 9
times without loss of catalytic activity and provided 3a in excellent yields (Table 3).
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Table 3. Recycling of Cu(OH)x/OMS-2a
t-Bu
t-Bu
OH
OH
O
N
Cu(OH)x/OMS-2 (20 mg)
EtOH, r.t., air, 2 h
+
NH2
t-Bu
t-Bu
1a
3a
2a
Cycle
1
2
3
4
5
6
7
8
9
10
92
Yield (%)
95 96 (80b)
94
95
92
93
95
94
94
a
Reaction condition: 1a (0.2 mmol), 2a (0.24 mmol), Cu(OH)x/OMS-2 (20 mg), EtOH (1 mL),
r.t., 2 h, under air. b The retrieved catalyst without NaOH treatment was used directly.
Next, with the optimized reaction conditions in hand, we investigated the substrate
scope of the domino oxidation by the use of substituted benzylamines (Table 4). In
general, benzylamines with electron-withdrawing and electron-donating groups all
reacted with catechol 1a smoothly and offered desired benzoxazoles in very good
yields. Moreover, benzylamines with quite sensitive groups, like hydroxyl, cyano and
amino groups, participated the reactions very well and good to excellent yields of
products were obtained under mild conditions (Table 4, entries 6-9). On the other
hand, more electron-rich catechols were successfully tolerated in the domino
oxidations, and corresponding products were obtained in excellent yields (table 4,
entries 10-12). Notably, homogeneous CuBr2 catalytic system only gave amino
group-functionalized benzoxazoles other than desired ones.21 Unfortunately,
electron-neutral and -poor catechols were not applicable because they were hardly
oxidized to corresponding o-quinones under the present conditions.
Table 4. The scope of reactions between catechols and aryl benzylamines.a
OH
O
Cu(OH)x/OMS-2 (Cu: 2 mol%)
H2N
R1
R
R
+
R1
N
EtOH, air, r.t., 2 h
OH
1
3b-m
2
Entry
R =
R1 =
H
Yields (%)
1
2
3,5-di-t-Bu
3,5-di-t-Bu
88
96
4-Br
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3
3,5-di-t-Bu
3,5-di-t-Bu
3,5-di-t-Bu
3,5-di-t-Bu
3,5-di-t-Bu
3,5-di-t-Bu
3,5-di-t-Bu
4-Me
4-Cl
4-F
95
4
82
80
86
63
95
70
91
97
93
5
4-CF3
4-OH
4-CN
2-NH2
3-NH2
H
6
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8
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4-t-Bu
H
3-Me
H
a Reaction conditions: 1 (0.2 mmol), 2 (0.24 mmol), Cu(OH)x/OMS-2 (20 mg), EtOH (1
mL), r.t., 2 h, under air.
For the further investigation of reaction scope, aromatic heterocycles were suitable
substrates and corresponding heteroatom-containing benzoxazoles were obtained in
excellent yields (Table 5, 4a-4c). Afterward, amines substituted by aliphatic
heterocycles, like morpholine, pyrrolidine, piperidine and tetrahydrofuran, reacted
with 1a fruitfully, while amines with unsaturated compounds, like
2-(1-cyclohexenyl)ethylamine and phenethylamine, gave corresponding products in
good yields as well (Table 5, 4d-4l). Simple linear alkyl amines, even
trifluoroethylamine, were successful reaction partners, although longer reaction time
was needed in the case of ammonia (Table 5, 4m-4q). 2-Vinylbenzoxazole and
2-ethynylbenzoxazole were synthesized in good yields from 1a and allylamine or
propargylamine, and they are good raw for further organic transformations (Table 5,
4r and 4s). Eventually, aliphatic amines with various sensitive groups were tolerated
in the domino oxidation and benzoxazoles with hydroxyl, eater and secondary or
tertiary amino groups were isolated in excellent yields (Table 5, 4t-4w).
Table 5. The scope of the reactions between catechol 1a and substituted aminesa
t-Bu
t-Bu
OH
OH
O
N
Cu(OH)x/OMS-2 (Cu: 2 mol%)
EtOH, air, r.t., 2-10 h
R
+
H2N
R
t-Bu
t-Bu
4a-w
2
1a
t-Bu
t-Bu
t-Bu
O
O
O
N
N
N
S
N
O
t-Bu
t-Bu
t-Bu
t-Bu
4b, 85%
4a, 95%
4c, 89%
t-Bu
t-Bu
t-Bu
O
N
O
N
O
N
(CH2
N
)
2
N
N
t-Bu
t-Bu
t-Bu
O
O
4e, 89%
4f, 91%
4d, 98%
t-Bu
t-Bu
t-Bu
O
O
O
N
N
O
N
N
t-Bu
t-Bu
4i, 97%
4g, 87%
4h, 83%
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t-Bu
t-Bu
t-Bu
Ph
Ph
O
N
O
N
O
N
Ph
Ph
t-Bu
t-Bu
t-Bu
6
7
8
9
4k, 82%
4j, 72%
t-Bu
4l, 75%
t-Bu
t-Bu
O
O
N
Et
O
N
Me
t-Bu
t-Bu
N
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t-Bu
4m,e 55%
4o, 93%
4n, 59%
t-Bu
t-Bu
t-Bu
Me
(CH2)6
O
O
O
CF3
N
N
N
t-Bu
t-Bu
t-Bu
4r,b 60%
4q, 42%
4p, 94%
t-Bu
t-Bu
t-Bu
O
O
N
OH
O
O
O
Et
t-Bu
N
N
t-Bu
t-Bu
4u,c 45%
4t, 96%
4s, 82%
t-Bu
t-Bu
O
(CH2)2
N
Me
O
N
t-Bu
Me
N
HN Et
t-Bu
4w,d 95%
4v, 95%
a Reaction conditions: 1a (0.2 mmol), 2 (0.24 mmol), Cu(OH)x/OMS-2 (20 mg), EtOH
(1 mL), r.t., 2 h, under air, yields. For 4 h. c For 6 h. d For 10 h. 0.48 mmol of
ammonia was used and the reaction was run at 40 oC for 20 h.
b
e
Encouraged by the great tolerance of the heterogeneous catalytic system,
p-xylylenediamine 2aa was employed in the reaction for the synthesis of dicyclized
product.
Interestingly,
expected
5a
dicyclized
isolated,
symmetric
meanwhile
1,4-bis(benzoxazol-2-yl)benzene
was
4-(benzoxazol-2-yl)benzaldehyde 5b was observed in the reaction through a
competitive reaction (Scheme 4). The results indicate that Cu(OH)x/OMS-2 was
bifunctional because it not only catalyzed the regular oxidative synthesis of
di-benzoxazole 5a, but also hydrolyzed intermediate 3aa into 5b.19 This
transformation can produce synthetically useful formyl group-functionalized
benzoxazoles from simple substrates in one-pot, which was not observed previously.
t-Bu
t-Bu
O
N
O
N
t-Bu
t-Bu
t-Bu
cat. Cu(OH)x/OMS-2
EtOH, r.t., air, 2 h
OH
OH
5a, 30% yield
NH2
+
H2N
t-Bu
t-Bu
O
N
O
2aa, 0.2 mmol
1a, 4 equiv.
t-Bu
via:
5b, 32% yield
t-Bu
O
N
NH2
t-Bu
intermediate 3aa
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Scheme 4. The domino oxidation between catechol 1a and p-xylylenediamine 2aa. Reaction
conditions: 1a (0.8 mmol), 2aa (0.2 mmol), Cu(OH)x/OMS-2 (20 mg), EtOH (1 mL), r.t., air, 2 h.
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7
8
9
To make our catalytic system more attractive, more challenging substrate
2-aminoacetophenone 2bb was employed to react with catechol 1a (Scheme 5).
Delightedly, the desired di-heteroaryl ketone 6a was isolated in 74% yield at room
temperature using air as terminal oxidant, while homogeneous catalysts, like CuBr2,
Cu(OAc)2 and CuI, cannot provide desired product due to quick decomposition of
2bb in these systems. Di-heteroaryl ketones are generally synthesized by noble
metal-catalyzed carbonylation with CO.20 Take into account safety and price issues,
the recyclable Cu(OH)x/OMS-2/air system for the synthesis of di-heteroaryl ketones is
more favourable and greener.
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t-Bu
t-Bu
O
O
O
N
cat. Cu(OH)x/OMS-2
EtOH, r.t., air, 2 h
OH
OH
H2N
+
t-Bu
t-Bu
6a, 74% yield
1a
2bb
Scheme 5. The domino oxidation between catechol 1a and 2-aminoacetophenone 2bb. Reaction
conditions: 1a (0.2 mmol), 2bb (0.24 mmol), Cu(OH)x/OMS-2 (20 mg), EtOH (1 mL), r.t., air, 2
h.
Finally, attempts to realize the synthesis of benzoxazole 3a on a gram-scale were
carried out. The gram-scale reaction led to 94% yield of desired product without using
column purification, which illustrates our heterogeneous catalytic system is practical
and can be a potential option for industrial application (Scheme 6).
Scheme 6. Gram-scale reaction.
To gain insight into the catalytic mechanism of the domino oxidation, some control
experiments were performed. First of all, o-quinone 1aa produced from oxidation of
catechol 1a was directly used as starting material to react with i-butylamine 2a, and
product 3a was isolated in nearly quantitative yield (Scheme 7, a). And, it is found
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that benzoxazole 3a could be obtained even without Cu(OH)x/OMS-2 in a low yield
by used of 1aa as the raw directly, while unoxidized intermediate 3a` was not
detected no matter whether the catalyst was used (Scheme 7, a). Then, another
possible intermediate 7a gave desired product 2-phenylbenzoxazole 7b smoothly
under Cu(OH)x/OMS-2/O2 system at room temperature (Scheme 7, b). These
observations indicate that amine was able to react with o-quinone in situ generated
from catechol via nucleophilic addition/dehydration/tautomerization to give
intermediate substituted 2-(benzylideneamino)phenol which is further transferred to
benzoxazole through oxidative cyclization. On the other hand, Yin`s group reported
homogeneous CuBr2-catalyzed the synthesis of benzoxazoles from catechols and
amines, which shows amine is a necessity for the copper-catalyzed oxidation of
catechol and the entire reaction proceeds via a synergistic catalysis between catechol
and amine.21 However, in our catalytic system, the homo-coupling of amine 2b did
not occur in the presence of different amounts of catechol or o-quinone, which means
catechol in our system did not play a role as cofactor (Scheme 7, c). Furthermore, we
used GC-MS to analyze the reaction mixture of catechol 1a and 3-phenylpropylamine
2l, and there is no corresponding aldehyde formed in the reaction under our catalytic
system (Fig. S8, see ESI). More importantly, catechol 1a could be oxidized efficiently
by Cu(OH)x/OMS-2 without the involvement of amine at all (Scheme 2). These
experimental facts indicate that the competitive oxidation and synergistic effect
between catechol and amine did not exist in our catalytic system, however, the
synergistic effect from Cu(OH)x/OMS-2 played the vital role for the catalysis. As a
consequence, amine can quickly trap the in situ generated o-quinone species to
perform the further transformation.
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Scheme 7. Control experiments a-c.
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CONCLUSION
In summary, we have developed that the combination of heterogeneous catalyst
Cu(OH)x/OMS-2 and ambient air as an effective system for the aerobic oxidative
synthesis of 2-substituted benzoxazoles from catechols and amines under mild
conditions. The domino reaction initiated by the oxidation of catechols proceeds
through oxidation/cyclization in one-pot manner and tolerates a great number of
substrates. The use of air as the terminal oxidant, mild reaction conditions, along with
catalyst reusability and easy isolation of the products make the catalytic protocol
practical and environmentally friendly. Mechanistic studies suggest that copper
hydroxide and OMS-2 work cooperatively to facilitate the electron transfer process of
the domino oxidation. This catalytic system should be useful for other organic
transformation involving other nucleophiles as the reaction partners.
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EXPERIMENTAL SECTION
General information. Unless otherwise noted, all reagents were purchased from
commercial suppliers and used without further purification. Metal salts were
commercially available and were used directly. All experiments were carried out
under air. Flash chromatography was carried out with Merck silica gel 60 (200-300
mesh). Analytical TLC was performed with Merck silica gel 60 F254 plates, and the
products were visualized by UV detection. 1H NMR and 13C NMR (400 and 100
MHz respectively) spectra were recorded in CDCl3. Chemical shifts (δ) are reported
in ppm using TMS as internal standard, and spin-spin coupling constants (J) are given
in Hz. The electron ionization (EI) method was used as the ionization method for the
HRMS measurement, and the mass analyzer type is TOF for EI. GC mass
spectrometry (GC-MS) spectra were recorded on a Shimadzu GCMS-QP2010
equipped with an InertCap-5 capillary column at an ionization voltage of 70 eV. All
supported catalysts are synthesized by wet impregnation in deionized water and
Cu(OH)x/OMS-2 is made by deposition-precipitation in water. The crystal phase and
composition were determined by power X-ray diffraction using a X-Pert PRO X-ray
diffractometer with Cu Ka radiation in the 2θ range of 10–90o. Infrared spectra of the
materials were recorded on calcined powders dispersed in KBr (2 mg sample in 300
mg KBr) using a Perkin-Elmer One FTIR spectrometer with a resolution of 4 cm-1
operating in the range 500-2000 cm-1 with 4 scans per spectrum. The morphologies
of the samples were characterized by a TF20 transmission electron microscope and
SM-5600LV or Quanta 250 FEG scanning electron microscope. Nitrogen
adsorption-desorption measurements were performed at 76 K using an ASAP 2020M
analyzer utilizing the BET model for the calculation of specific surface areas.The
reducibility of the catalysts was measured by the hydrogen temperature-programmed
reduction (H2-TPR) technique. A 50 mg of OMS-2, Cu(OH)x/OMS-2 was placed in a
quartz reactor that was connected to a TPR apparatus and the reactor was heated from
r.t. to 550 oC with a heating rate of 10 oC/min. The reducing atmosphere was the
mixture of H2 and N2 with a total flow rate of 30 mL/min and the amount of H2
uptake during the reduction was measured by a thermal conductivity detector (TCD).
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The X-ray photoelectron spectroscopy (XPS) measurements were performed on a
Kratos AXIS Ultra DLD high performance electron spectrometer using
nonmonochromatized Al Kα excitation source (hv = 1486.6 eV). Binding energies
were calibrated by using the contaminant carbon (C 1s = 284.6 eV).
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Preparation of OMS-2. KMnO4 (5.89 g, 37 mmol) in 100 mL of deionized water
was added to a solution of MnSO4-H2O (8.8 g, 52 mmol) in 30 mL of deionized water
o
and 3 mL concentrated HNO3. The solution was refluxed at 100 C for 24 h, and the
product was filtered, washed, and dried at 120 oC for 8 hours. Finally, the dry OMS-2
o
was calcined in a muffle furnace at 350 C for 2 h. Then, the black powder OMS-2
was obtained.
Preparation of Cu(OH)x/OMS-2. Support OMS-2 (2 g, 2.7 mmol) was added to a
100 mL round-bottom flask. A solution of CuCl2 (0.085 g, 0.6 mmol) in deionized
water (60 mL) was added to OMS-2. Then the mixture was stirred for 1 h at room
temperature and quickly adjusted the pH of it to 12 by the addition of an aqueous
solution of NaOH (1.0 M). After that, the above solution was stirred for 24 h at room
temperature. Then, the solid in the solution was filtered off, washed with 1 L of water
and dried in oven at 50 oC for overnight. Eventually, the dried black solid was dried in
o
vacuo oven at 50 C for another 10 h to afford about 2 g Cu(OH)x/OMS-2 catalyst.
The black powder Cu(OH)x/OMS-2 (1.5 wt.% theoretical loading, actual loading is
1.48 wt.%) was characterized by ICP-AES, BET, XRD, FTIR, TEM, SEM, H2-TPR
and XPS techniques.
General procedure for Cu(OH)x/OMS-2 2-substituted benzoxazoles synthesis.
Cu(OH)x/OMS-2 (20 mg, 2 mol% for Cu), catechol (0.2 mmol), amine (0.24 mmol)
and EtOH (1 mL) were added to a vial with a bar. The mixture was stirred at ambient
temperature for 2 h under air. Then, the mixture was diluted with ethyl acetate and
filtered. The filtrate was removed under reduced pressure to get the crude product or
pure product for some cases, and the crude product was further purified by
chromatography (petroleum/ethyl acetate = 40/1 as eluent) to yield corresponding
product if it is necessary.
General procedure for catalyst regeneration. Once the reaction was finished, the
mixture was diluted with10 mL of EtOH. The solid catalyst was filtrated off via filter
paper, washed by water and EtOH. Then, retrieved catalyst was stirred in 1 M NaOH
solution for 2 h at room temperature. After that, the catalyst was filtrated off, washed
by 1 L water and dried in oven at 50 oC for overnight. And, the dried catalyst was
further dried in vacuo oven at 50 oC for another 10 h to afford about regenerated
Cu(OH)x/OMS-2 catalyst for the next run. This procedure was repeated for every
cycle and the yield of the reaction was determined by 1H NMR using Br2CH2 as
internal standard.
5,7-di-tert-butyl-2-isopropylbenzo[d]oxazole (3a).21
Brown liquid, yield 95%, 51 mg. 1H NMR (400 MHz, CDCl3): δ 7.50 (s, 1H), 7.16 (s,
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1H), 3.24-3.09 (m, 1H), 1.40 (s, 9H), 1.37 (s, 6H), 1.29 (s, 9H). C{1H}NMR (100
MHz, CDCl3): δ 170.6, 147.1, 146.9, 141.3, 133.4, 118.6, 113.8, 34.9, 34.3, 31.8, 29.9,
28.8, 20.3.
5,7-di-tert-butyl-2-phenylbenzo[d]oxazole (3b).21
Yellow liquid, yield 88%, 54 mg. 1H NMR (400 MHz, CDCl3): δ 8.27 (s, 2H), 7.68 (s,
1H), 7.53 (s, 3H), 7.32 (s, 1H), 1.57 (s, 9H), 1.41 (s, 9H). C{1H}NMR (100 MHz,
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CDCl3): δ 162.4, 147.8, 146.9, 142.3, 133.7, 131.2, 128.9, 127.5, 127.4, 119.6, 114.2,
35.1, 34.5, 31.9, 30.1.
2-(4-bromophenyl)-5,7-di-tert-butylbenzo[d]oxazole (3c).21
Colorless liquid, yield 96%, 73 mg. 1H NMR (400 MHz, CDCl3): δ 8.11 (d, J = 7.8 Hz,
13
2H), 7.66 (d, J = 7.5 Hz, 3H), 7.33 (s, 1H), 1.55 (s, 9H), 1.40 (s, 9H). C{1H}NMR
(100 MHz, CDCl3): δ 161.6, 147.9, 146.9, 142.2, 133.8, 132.2, 128.8, 126.5, 125.8,
119.9, 114.3, 35.1, 34.5, 31.9, 30.1.
5,7-di-tert-butyl-2-(4-chlorophenyl)benzo[d]oxazole (3d).21
Pale yellow liquid, yield 95%, 65 mg. 1H NMR (400 MHz, CDCl3): δ 8.18 (d, J = 8.0
Hz, 2H), 7.65 (s, 1H), 7.50 (d, J = 8.0 Hz, 2H), 7.32 (s, 1H), 1.55 (s, 9H), 1.40 (s, 9H).
13C{1H}NMR (100 MHz, CDCl3): δ 161.5, 147.9, 146.9, 142.1, 137.3, 133.7, 129.2,
128.6, 125.9, 119.8, 114.2, 35.1, 34.5, 31.8, 29.9.
5,7-di-tert-butyl-2-(4-fluorophenyl)benzo[d]oxazole (3e).21
Colorless liquid, yield 82%, 53 mg. 1H NMR (400 MHz, CDCl3): δ 8.25 (dd, J = 8.8,
5.3 Hz, 2H), 7.65 (d, J = 1.8 Hz, 1H), 7.32 (d, J = 1.8 Hz, 1H), 7.21 (t, J = 8.6 Hz,
13
2H), 1.56 (s, 9H), 1.40 (s, 9H). C{1H}NMR (100 MHz, CDCl3): δ 165.9, 163.3,
161.6, 147.8, 146.9, 142.2, 133.7, 129.6, 129.5, 123.9, 123.8, 119.6, 116.2, 116.1,
114.1, 35.1, 34.5, 31.8, 30.1.
5,7-di-tert-butyl-2-(4-(trifluoromethyl)phenyl)benzo[d]oxazole (3f).21
1
Colorless liquid, yield 80%, 60 mg. H NMR (400 MHz, CDCl3): δ 8.38 (d, J = 7.3
Hz, 2H), 7.80 (d, J = 7.4 Hz, 2H), 7.69 (s, 1H), 7.37 (s, 1H), 1.57 (s, 9H), 1.41 (s, 9H).
13C{1H}NMR (100 MHz, CDCl3): δ 160.9, 148.2, 147.1, 142.1, 133.9, 132.8, 132.5,
130.7, 127.8, 127.6, 125.9, 125.8, 125.1, 122.4, 120.3, 114.5, 35.1, 34.5, 31.8, 29.9.
4-(5,7-di-tert-butylbenzo[d]oxazol-2-yl)phenol (3g).21
Colorless liquid, yield 86%, 55 mg. 1H NMR (400 MHz, CDCl3): δ 9.82 (s, 1H), 8.08
(d, J = 8.0 Hz, 2H), 7.58 (s, 1H), 7.30 (s, 1H), 7.00 (d, J = 7.9 Hz, 2H), 1.55 (s, 9H),
13
1.36 (s, 9H). C{1H}NMR (100 MHz, CDCl3): δ 163.2, 160.4, 148.1, 146.4, 141.1,
133.7, 129.6, 119.4, 118.2, 116.4, 113.2, 35.1, 34.4, 31.8, 29.9.
4-(5,7-di-tert-butylbenzo[d]oxazol-2-yl)benzonitrile (3h).
1
Colorless liquid, yield 63%, 41 mg. H NMR (400 MHz, CDCl3): δ 8.34 (d, J = 8.4
Hz, 2H), 7.81 (d, J = 8.4 Hz, 2H), 7.67 (d, J = 1.8 Hz, 1H), 7.37 (d, J = 1.8 Hz, 1H),
13
1.55 (s, 9H), 1.40 (s, 9H). C{1H}NMR (100 MHz, CDCl3): δ 160.3, 148.4, 147.1,
142.1, 134.0, 132.6, 131.4, 127.7, 120.7, 118.2, 114.6, 114.3, 35.1, 34.5, 31.7, 29.9.
HRMS (EI): calcd for C22H24N2O: 332.1976; found: 332.1998.
2-(5,7-di-tert-butylbenzo[d]oxazol-2-yl)aniline (3i).
1
Colorless liquid, yield 95%, 61 mg. H NMR (400 MHz, CDCl3): δ 8.12 (d, J = 7.2
Hz, 1H), 7.65 (d, J = 1.7 Hz, 1H), 7.38-7.25 (m, 2H), 6.89-6.77 (m, 2H), 6.24 (s, 2H),
1.61 (s, 9H), 1.45 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ 171.1, 162.5, 147.6,
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147.3, 145.2, 141.9, 133.3, 131.9, 128.5, 119.1, 116.6, 116.1, 113.6, 108.9, 35.0, 34.4,
31.8, 29.9. HRMS (EI): calcd for C21H26N2O: 322.2048; found: 322.2076.
3-(5,7-di-tert-butylbenzo[d]oxazol-2-yl)aniline (3j).
Colorless liquid, yield 70%, 45 mg. 1H NMR (400 MHz, CDCl3): δ 7.67-7.62 (m, 2H),
7.59-7.56 (m, 1H), 7.33-7.27 (m, 2H), 6.83 (ddd, J = 8.0, 2.4, 0.9 Hz, 1H), 3.88 (s,
2H), 1.55 (s, 9H), 1.40 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ 162.7, 147.6,
146.8, 146.8, 142.2, 133.6, 129.8, 128.3, 119.4, 117.9, 117.7, 114.1, 113.5, 35.0, 34.4,
31.8, 29.9. HRMS (EI): calcd for C21H26N2O: 322.2048; found: 322.2076.
5-methyl-2-phenylbenzo[d]oxazole (3k).15a
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White solid, yield 91%, 38 mg. 1H NMR (400 MHz, CDCl3): δ 8.24 (dd, J = 6.6, 2.8
Hz, 2H), 7.55 (d, J = 7.0 Hz, 1H), 7.54-7.49 (m, 3H), 7.45 (d, J = 8.3 Hz, 1H), 7.15 (d,
J = 8.3 Hz, 1H), 2.48 (s, 3H). 13C{1H}NMR (100 MHz, CDCl3): δ 163.0, 148.9, 142.2,
134.3, 131.3, 128.8, 127.5, 127.2, 126.2, 119.8, 109.9, 21.5.
5-(tert-butyl)-2-phenylbenzo[d]oxazole (3l).15a
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White solid, yield 97%, 48 mg. H NMR (400 MHz, CDCl3) δ 8.15 (dd, J = 6.6, 2.9
Hz, 2H), 7.72 (d, J = 1.5 Hz, 1H), 7.45 – 7.38 (m, 4H), 7.31 (dd, J = 8.6, 1.7 Hz, 1H),
1.30 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3) δ 163.0, 148.6, 148.0, 141.9, 131.2,
128.8, 127.4, 127.2, 122.8, 116.4, 109.6, 34.8, 31.7.
4-methyl-2-phenylbenzo[d]oxazole (3m).15a
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White solid, yield 93%, 39 mg. H NMR (400 MHz, CDCl3) δ 8.17 (dd, J = 6.6, 3.0
Hz, 2H), 7.49 – 7.37 (m, 3H), 7.30 (d, J = 8.0 Hz, 1H), 7.14 (t, J = 7.7 Hz, 1H), 7.04
(d, J = 7.5 Hz, 1H), 2.59 (s, 3H). 13C{1H}NMR (100 MHz, CDCl3) δ 162.2, 150.4,
141.3, 131.2, 130.5, 128.8, 128.7, 128.7, 127.5, 127.3, 124.9, 124.6, 107.8, 16.5.
5,7-di-tert-butyl-2-(pyridin-2-yl)benzo[d]oxazole (4a).21
Colorless liquid, yield 95%, 58 mg. 1H NMR (400 MHz, CDCl3): δ 8.83 (dd, J = 4.7,
0.7 Hz, 1H), 8.28 (d, J = 7.9 Hz, 1H), 7.86 (td, J = 7.8, 1.7 Hz, 1H), 7.69 (d, J = 1.9
Hz, 1H), 7.41 (ddd, J = 7.6, 4.8, 1.1 Hz, 1H), 7.36 (d, J = 1.8 Hz, 1H), 1.56 (s, 9H),
13
1.40 (s, 9H). C{1H}NMR (100 MHz, CDCl3): δ 161.1, 150.4, 148.0, 147.3, 146.4,
142.1, 136.8, 134.1, 125.1, 123.1, 120.4, 114.7, 35.1, 34.5, 31.8, 30.1.
5,7-di-tert-butyl-2-(thiophen-2-yl)benzo[d]oxazole (4b).21
Colorless liquid, yield 85%, 53 mg. 1H NMR (400 MHz, CDCl3): δ 7.81 (s, 1H), 7.54
(s, 1H), 7.44 (s, 1H), 7.21 (s, 1H), 7.09 (s, 1H), 1.45 (s, 9H), 1.30 (s, 9H).
13C{1H}NMR (100 MHz, CDCl3): δ 158.4, 147.8, 146.5, 142.1, 133.5, 130.1, 129.7,
129.3, 128.1, 119.5, 114.0, 35.0, 34.4, 31.8, 29.9.
5,7-di-tert-butyl-2-(furan-2-yl)benzo[d]oxazole (4c).21
1
Colorless liquid, yield 89%, 52 mg. H NMR (400 MHz, CDCl3) δ 7.56 (d, J = 15.4
Hz, 2H), 7.27 – 7.09 (m, 2H), 6.53 (s, 1H), 1.45 (s, 9H), 1.31 (s, 9H). 13C{1H}NMR
(100 MHz, CDCl3) δ 154.8, 147.9, 146.3, 145.3, 142.9, 141.8, 133.7, 119.6, 114.3,
113.5, 112.1, 35.0, 34.4, 31.8, 29.9.
5,7-di-tert-butyl-2-(morpholinomethyl)benzo[d]oxazole (4d).
Pale yellow oil, yield 98%, 64 mg. 1H NMR (400 MHz, CDCl3): δ 7.57 (s, 1H), 7.28
(d, J = 1.5 Hz, 1H), 3.88 (s, 2H), 3.76-3.70 (m, 4H), 2.67 (d, J = 4.1 Hz, 4H), 1.48 (s,
13
9H), 1.36 (s, 9H). C{1H}NMR (100 MHz, CDCl3): δ 162.1, 147.5, 141.2, 133.7,
119.3, 114.1, 66.8, 55.3, 53.0, 34.9, 34.3, 31.8, 29.9. HRMS (EI): calcd for
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C20H30N2O2: 330.2223; found: 330.2248.
5,7-di-tert-butyl-2-(2-morpholinoethyl)benzo[d]oxazole (4e).
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Brown liquid, yield 89%, 61 mg. 1H NMR (400 MHz, CDCl3): δ 7.53 (d, J = 1.6 Hz,
1H), 7.25 (d, J = 1.6 Hz, 1H), 3.73 – 3.68 (m, 4H), 3.13 (t, J = 7.4 Hz, 2H), 2.93 (t, J
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= 7.4 Hz, 2H), 2.54 (s, 4H), 1.47 (s, 9H), 1.36 (s, 9H). C{1H}NMR (100 MHz,
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CDCl3): δ 164.8, 147.2, 146.9, 141.4, 133.5, 118.8, 113.7, 66.87, 55.6, 53.4, 34.9,
34.4, 31.8, 29.9, 26.5. HRMS (EI): calcd for C21H32N2O2: 344.2554; found: 344.2532.
5,7-di-tert-butyl-2-(piperidin-1-ylmethyl)benzo[d]oxazole (4f).
Brown liquid, yield 91%, 59 mg. 1H NMR (400 MHz, CDCl3): δ 7.58 (s, 1H), 7.27 (d,
J = 1.6 Hz, 1H), 3.90 (s, 2H), 2.60 (s, 4H), 1.62 (dt, J = 11.0, 5.6 Hz, 4H), 1.48 (s, 9H),
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1.43-1.34 (m, 11H). C{1H}NMR (100 MHz, CDCl3): δ 162.8, 147.3, 146.9, 141.3,
133.7, 119.1, 114.1, 55.65, 53.8, 34.9, 34.4, 31.8, 29.9, 25.9, 23.9. HRMS (EI): calcd
for C21H32N2O: 328.2509; found: 322.2526.
5,7-di-tert-butyl-2-(pyrrolidin-1-ylmethyl)benzo[d]oxazole (4g).
Yellow oil, yield 87%, 54 mg. 1H NMR (400 MHz, CDCl3): δ 7.59 (d, J = 1.8 Hz, 1H),
7.29 (d, J = 1.8 Hz, 1H), 4.03 (s, 2H), 2.77 (s, 4H), 1.84 (dt, J = 6.4, 3.1 Hz, 4H), 1.50
(s, 9H), 1.39 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ 171.1, 163.2, 147.3, 146.9,
141.3, 133.6, 119.1, 114.1, 60.4, 53.7, 51.9, 34.9, 34.4, 31.8, 29.9. HRMS (EI): calcd
for
C20H30N2O:
314.2446;
found:
322.2471.
5,7-di-tert-butyl-2-(tetrahydrofuran-2-yl)benzo[d]oxazole (4h).
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Pale green liquid, yield 83%, 50 mg. H NMR (400 MHz, CDCl3): δ 7.51 (s, 1H),
7.20 (s, 1H), 5.15 (s, 1H), 4.05 (d, J = 6.2 Hz, 1H), 3.95 (s, 1H), 2.31 (d, J = 6.6 Hz,
2H), 2.12 (s, 1H), 1.98 (s, 2H), 1.40 (s, 9H), 1.29 (s, 9H). 13C{1H}NMR (100 MHz,
CDCl3): δ 165.8, 147.5, 147.1, 140.9, 133.8, 119.4, 114.3, 73.8, 69.2, 34.9, 34.4, 31.8,
30.8, 29.9, 29.7. HRMS (EI): calcd for C19H27NO2: 301.2009; found: 301.2031.
5,7-di-tert-butyl-2-(cyclohex-1-en-1-ylmethyl)benzo[d]oxazole (4i).
Colorless liquid, yield 97%, 63 mg. 1H NMR (400 MHz, CDCl3): δ 7.60 (s, 1H), 7.29
(s, 1H), 5.72 (s, 1H), 3.61 (s, 2H), 2.09 (d, J = 4.0 Hz, 4H), 1.68 (d, J = 3.9 Hz, 2H),
1.61 (d, J = 4.5 Hz, 2H), 1.52 (s, 9H), 1.42 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3):
δ 164.9, 147.0, 141.6, 133.4, 132.0, 125.4, 118.7, 113.8, 37.6, 34.9, 34.3, 31.8, 29.8,
28.3, 25.3, 22.7, 21.9. HRMS (EI): calcd for C22H31NO: 325.2498; found: 325.2466.
2-benzyl-5,7-di-tert-butylbenzo[d]oxazole (4j).21
Brown liquid, yield 72%, 46 mg. 1H NMR (400 MHz, CDCl3): δ 7.55 (s, 1H), 7.38 (s,
2H), 7.32 (s, 2H), 7.24 (s, 2H), 4.26 (s, 2H), 1.44 (s, 9H), 1.35 (s, 9H). 13C{1H}NMR
(100 MHz, CDCl3): δ 164.6, 147.2, 141.5, 135.2, 133.5, 128.8, 128.6, 127.1, 118.9,
113.9, 35.3, 34.9, 34.3, 31.7, 29.8.
2-benzhydryl-5,7-di-tert-butylbenzo[d]oxazole (4k).
1
Pale yellow liquid, yield 82%, 65 mg. H NMR (400 MHz, CDCl3) δ 7.21 (m, 8H),
7.06 (m, 4H), 5.68 (s, 1H), 1.33 (s, 9H), 1.28 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3)
δ 165.9, 147.3, 141.3, 139.6, 133.7, 128.7, 128.5, 128.3, 127.2, 119.1, 114.3, 51.5,
35.0, 34.3, 31.7, 29.8. HRMS (EI): calcd for C28H31NO: 397.2424; found: 972.2431.
5,7-di-tert-butyl-2-phenethylbenzo[d]oxazole (4l).21
Brown liquid, yield 75%, 50 mg. 1H NMR (400 MHz, CDCl3): δ 7.57 (d, J = 1.9 Hz,
1H), 7.34-7.25 (m, 5H), 7.25 – 7.19 (m, 1H), 3.26 (dd, J = 8.1, 4.4 Hz, 4H), 1.48 (s,
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9H), 1.40 (s, 9H). C{1H}NMR (100 MHz, CDCl3): δ 165.5, 147.1, 146.9, 141.4,
140.1, 133.4, 128.5, 128.2, 126.3, 118.7, 113.7, 34.9, 34.3, 32.9, 31.8, 30.3, 29.8.
5,7-di-tert-butylbenzo[d]oxazole (4m).21
Colorless liquid, yield 55%, 25 mg. 1H NMR (400 MHz, CDCl3): δ 8.08 (s, 1H), 7.65
(s, 1H), 7.34 (s, 1H), 1.49 (s, 9H), 1.39 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ
147.7, 134.1, 119.8, 114.6, 35.0, 34.4, 31.8, 29.8.
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5,7-di-tert-butyl-2-methylbenzo[d]oxazole (4n).21
Brown liquid, yield 59%, 28 mg. 1H NMR (400 MHz, CDCl3): δ 7.43 (s, 1H), 7.16 (s,
1H), 2.56 (s, 3H), 1.39 (s, 9H), 1.29 (s, 9H). C{1H}NMR (100 MHz, CDCl3): δ
13
163.2, 147.1, 141.7, 133.3, 118.6, 113.6, 34.9, 34.3, 31.8, 29.9, 14.6.
5,7-di-tert-butyl-2-ethylbenzo[d]oxazole (4o).21
Colorless crystal, yield 93%, 48 mg. 1H NMR (400 MHz, CDCl3): δ 7.55 (s, 1H), 7.24
(s, 1H), 2.96 (q, J = 7.6 Hz, 2H), 1.47 (s, 9H), 1.45 (s, 3H), 1.37 (s, 9H).
13C{1H}NMR (100 MHz, CDCl3): δ 147.0, 141.4, 133.3, 118.6, 113.6, 34.9, 34.3,
31.8, 29.8, 22.1, 11.0.
5,7-di-tert-butyl-2-heptylbenzo[d]oxazole (4p).21
Brown liquid, yield 94%, 62 mg. 1H NMR (400 MHz, CDCl3): δ 7.46 (s, 1H), 7.16 (s,
1H), 2.84 (s, 2H), 1.80 (s, 2H), 1.39 (s, 9H), 1.29 (s, 9H), 1.21 (s, 7H), 0.80 (s, 3H).
13C{1H}NMR (100 MHz, CDCl3): δ 166.7, 147.0, 146.9, 141.4, 133.3, 118.5, 113.7,
34.9, 34.3, 31.8, 31.6, 29.8, 29.6, 29.0, 28.8, 28.6, 26.9, 22.5, 14.0.
5,7-di-tert-butyl-2-(trifluoromethyl)benzo[d]oxazole (4q).21
Brown liquid, yield 42%, 25 mg. 1H NMR (400 MHz, CDCl3): δ 7.71 (s, 1H), 7.47 (s,
1H), 1.50 (s, 9H), 1.39 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ 149.3, 147.0,
139.7, 134.9, 122.2, 115.6, 35.1, 34.5, 31.6, 29.7.
5,7-di-tert-butyl-2-vinylbenzo[d]oxazole (4r).21
Colorless liquid, yield 60%, 30 mg. 1H NMR (400 MHz, CDCl3): δ 7.57 (s, 1H), 7.29
(s, 1H), 6.76 (dd, J = 17.1, 11.2 Hz, 1H), 6.44 (d, J = 17.5 Hz, 1H), 5.82 (d, J = 10.7
Hz, 1H), 1.50 (s, 9H), 1.37 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ 161.5, 147.6,
141.9, 133.6, 124.4, 124.3, 124.1, 119.8, 114.2, 35.0, 34.3, 31.7, 29.9.
5,7-di-tert-butyl-2-ethynylbenzo[d]oxazole (4s).21
Brown liquid, yield 82%, 41 mg. 1H NMR (400 MHz, CDCl3): δ 7.59 (s, 1H), 7.37 (s,
1H), 3.36 (s, 1H), 1.48 (s, 9H), 1.37 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ
155.9, 148.4, 147.0, 140.7, 134.0, 121.2, 114.6, 81.0, 72.1, 35.0, 34.4, 31.7, 29.8.
(5,7-di-tert-butylbenzo[d]oxazol-2-yl)methanol (4t).21
Colorless liquid, yield 96%, 50 mg. 1H NMR (400 MHz, CDCl3): δ 7.47 (s, 1H), 7.20
(s, 1H), 5.26 (s, 1H), 4.89 (s, 2H), 1.37 (s, 9H), 1.28 (s, 9H). 13C{1H}NMR (100 MHz,
CDCl3): δ 168.6, 147.7, 146.9, 140.6, 133.9, 119.5, 113.7, 57.7, 35.0, 34.3, 31.7, 29.8.
ethyl 5,7-di-tert-butylbenzo[d]oxazole-2-carboxylate (4u).21
Colorless liquid, yield 45%, 27 mg. 1H NMR (400 MHz, CDCl3): δ 7.70 (s, 1H), 7.45
(s, 1H), 4.54 (s, 2H), 1.51 (s, 13H), 1.38 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ
156.7, 149.0, 140.9, 134.9, 122.5, 115.7, 62.9, 35.1, 34.5, 31.7, 29.9, 14.2.
N-((5,7-di-tert-butylbenzo[d]oxazol-2-yl)methyl)ethanamine (4v).21
Colorless liquid, yield 95%, 54 mg. 1H NMR (400 MHz, CDCl3): δ 7.49 (s, 1H), 7.19
(s, 1H), 4.02 (s, 2H), 2.70 (d, J = 6.0 Hz, 2H), 1.40 (s, 9H), 1.29 (s, 9H), 1.08 (s, 3H).
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13C{1H}NMR (100 MHz, CDCl3): δ 147.1, 146.8, 143.9, 140.9, 133.6, 133.4, 118.9,
113.7, 34.7, 34.1, 31.5, 31.3, 29.6, 29.5, 14.9.
2-(5,7-di-tert-butylbenzo[d]oxazol-2-yl)-N,N-dimethylethanamine (4w).21
Colorless liquid, yield 95%, 57 mg. 1H NMR (400 MHz, CDCl3): δ 7.55 (s, 1H), 7.25
(s, 1H), 3.12 (s, 2H), 2.89 (s, 2H), 2.33 (s, 6H), 1.49 (s, 9H), 1.38 (s, 9H).
13C{1H}NMR (100 MHz, CDCl3): δ 164.9, 147.1, 147.0, 141.5, 133.5, 118.8, 113.8,
56.41, 45.2, 35.0, 34.3, 31.8, 29.9, 27.3.
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1,4-bis(5,7-di-tert-butylbenzo[d]oxazol-2-yl)benzene (5a).
1
White solid, yield 30%, 32 mg. H NMR (400 MHz, CDCl3): δ 8.41 (s, 4H), 7.69 (s,
13
2H), 7.35 (s, 2H), 1.58 (s, 18H), 1.41 (s, 18H). C{1H}NMR (100 MHz, CDCl3): δ
161.5, 148.0, 147.0, 142.2, 133.8, 129.6, 127.8, 120.0, 114.3, 35.1, 34.5, 31.8, 30.0.
HRMS (EI): calcd for C36H44N2O2: 536.3471; found: 536.3480.
4-(5,7-di-tert-butylbenzo[d]oxazol-2-yl)benzaldehyde (5b).
White solid, yield 32%, 21 mg. 1H NMR (400 MHz, CDCl3): δ 10.11 (s, 1H), 8.41 (d,
J = 7.3 Hz, 2H), 8.04 (d, J = 7.3 Hz, 2H), 7.68 (s, 1H), 7.36 (s, 1H), 1.56 (s, 9H), 1.40
(s, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ 191.5, 161.0, 159.3, 148.2, 142.2, 137.7,
133.9, 132.6, 130.1, 127.8, 120.4, 114.5, 35.1, 34.5, 31.7, 30.0. HRMS (EI): calcd for
C22H25NO2:
335.1965;
found:
335.1958.
(5,7-di-tert-butylbenzo[d]oxazol-2-yl)(phenyl)methanone (6a).20
White solid, yield 74%, 49 mg. 1H NMR (400 MHz, CDCl3): δ 8.52 (d, J = 7.3 Hz,
2H), 7.79 (d, J = 1.8 Hz, 1H), 7.68 (t, J = 7.4 Hz, 1H), 7.57 (t, J = 7.7 Hz, 2H), 7.50
(d, J = 1.8 Hz, 1H), 1.55 (s, 9H), 1.41 (s, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ
180.6, 149.0, 146.9, 141.1, 135.4, 135.1, 134.0, 130.9, 128.5, 122.9, 116.0, 35.2, 34.6,
31.7, 29.9.
3,5-di-tert-butylcyclohexa-3,5-diene-1,2-dione (1aa).21
Colorless liquid, yield 98%, 43 mg. 1H NMR (400 MHz, CDCl3): δ 6.91 (s, 1H), 6.18
13
(s, 1H), 1.24 (s, 9H), 1.20 (s, 9H). C{1H}NMR (100 MHz, CDCl3): δ 181.0, 179.9,
163.2, 149.8, 133.4, 121.9, 35.9, 35.3, 29.1, 27.7.
(E)-2-(benzylideneamino)phenol (7a).15a
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Yellow solid. H NMR (400 MHz, CDCl3): δ 8.71 (s, 1H), 7.97-7.89 (m, 2H),
7.54-7.45 (m, 3H), 7.34-7.26 (m, 2H), 7.21 (t, J = 7.5 Hz, 1H), 7.03 (d, J = 8.1 Hz,
1H), 6.92 (t, J = 7.6 Hz, 1H). 13C{1H}NMR (100 MHz, CDCl3): δ 157.0, 152.2, 135.7,
135.3, 131.6, 128.9, 128.8, 128.8, 120.0, 115.8, 114.9.
2-phenylbenzo[d]oxazole (7b).15a
Colorless solid, yield 79%, 31 mg. 1H NMR (400 MHz, CDCl3): δ 8.38-8.24 (m, 2H),
7.83 (dd, J = 5.3, 3.7 Hz, 1H), 7.67-7.54 (m, 4H), 7.44-7.37 (m, 2H). 13C{1H}NMR
(100 MHz, CDCl3): δ 162.9, 150.6, 142.0, 131.5, 128.8, 127.6, 127.0, 125.0, 124.5,
119.9, 110.6.
ACKNOWLWDGMENTS
We gratefully acknowledge the National Natural Science Foundation of China (Grant
Nos. 21403256, 21573261, 21372102) and the Suzhou Industrial Technology and
Innovation Project (Grant No. SYG201531).
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Supporting Information. Characterization of catalysts, hot filtration experiment and
copies of NMR Spectra.
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REFERENCES:
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