Expeditious and efficient synthesis of benzoxazoles, benzothiazoles, benzimidazoles catalyzed by Ga(OTf)3 under solvent-free conditions

Article abstract of DOI:10.1002/cjoc.201180310

A new and efficient method for the synthesis of benzoxazoles, benzothiazoles, benzimidazoles from reactions of o-substituted aminoaromatics with orthoesters in the presence of catalytic amounts of Ga(OTf)₃ under solvent-free conditions is presented. The remarkable features of this new protocol are high conversion, very short reaction times, cleaner reaction profiles under solvent-free conditions, straight forward procedure, and use of relatively non-toxic catalysts.

Full text of DOI:10.1002/cjoc.201180310

FULL PAPER
Expeditious and Efficient Synthesis of Benzoxazoles,
Benzothiazoles, Benzimidazoles Catalyzed by Ga(OTf)3 under
Solvent-Free Conditions
Liu, Juyan*(刘巨艳)
Liu, Qian(刘倩)
Xu, Wei(徐玮)
Wang, Weilu(王玮璐)
College of Chemistry, Tianjin Key Laboratory of Structure and Performance for Functional Molecule,
Tianjin Normal University, Tianjin, China
A new and efficient method for the synthesis of benzoxazoles, benzothiazoles, benzimidazoles from reactions of
o-substituted aminoaromatics with orthoesters in the presence of catalytic amounts of Ga(OTf)3 under solvent-free
conditions is presented. The remarkable features of this new protocol are high conversion, very short reaction times,
cleaner reaction profiles under solvent-free conditions, straight forward procedure, and use of relatively non-toxic
catalysts.
Keywords benzoxazoles, benzothiazoles, benzimidazoles, solvent-free
Introduction
lysts,17a,17b special apparatus and harsh reaction condi-
tions.17c,17e,17l Due to the importance of these com-
pounds as intermediates in organic synthesis, the de-
velopment of convenient, environmental benign,
high-yielding and clean approaches is highly desirable.
In recent years, gallium(III) triflate has received in-
creasing attention as a novel type of water-tolerant
green Lewis acid catalyst for organic synthesis.24 As
part of our continuing interest in the development of
new synthetic methodologies,25 we report herein an ef-
ficient, convenient, and facile synthesis of benzoxazoles,
benzothiazoles, and benzimidazoles from o-substituted
aminoaromatics and orthoesters using a catalytic
amount of Ga(OTf)3 under solvent-free conditions at
room temperature (Eq. 1).
The benzoxazoles, benzothiazoles, benzimidazoles
have attracted a great deal of interest in diverse fields of
chemistry.1 These heterocycle derivatives have been
shown to possess a broad spectrum of biological activi-
ties such as antiviral,2 antimicrobial,3 anticancer,4 anti-
parkinson,5 antifungal,6 antibiotic activities.7 They have
been also utilized as advanced materials including or-
ganic light-emitting diodes (OLED),8 non-linear optics
(NLO),9 liquid crystals,10 H2-antagonists,11 elastase in-
hibitors.12 It is therefore not surprising that many syn-
thetic methods have been reported for the preparation of
these compounds. A wide range of methods have been
developed for the synthesis of these heterocycles which
include the reaction between N-ethoxycarbonylthioa-
mides with 1,2-diamines,13 conversion of esters using an
aluminum reagent,14 condensation of an appropriate
1,2-phenylenediamine with carboxylic acid and its de-
rivative,15 nitriles,16 or orthoesters.17 Recently, several
approaches
have
been
introduced,
where
o-dinitrobenzene,18 aldehydes,19 acid chloride,20
Gold’reagent,21 and 2-nitroanilines22 are used as the
starting materials for this synthesis. Some methods em-
ploying transition metal catalyzed coupling reactions to
construct the benzimidazole nucleus have also been re-
ported. Those involved a palladium-catalyzed in-
tramolecular aryl-amination chemistry.23 Although these
methods are suitable for certain synthetic conditions,
however, many of these procedures are associated with
one or more disadvantages such as long reaction
time,17c,17e low yield,17d use of volatile organic sol-
vents,17m,17f requirement of excess of reagents or cata-
Results and discussion
The condensation reaction of o-phenylenediamine (1
mmol) with triethyl orthoformate (1.2 mmol) was car-
ried out at room temperature. In the study of catalyst
loading, 0, 1, 5, 10, and 20 mol% of Ga(OTf)3 were
*
E-mail: hxxyljy@mail.tjnu.edu.cn.; Tel.: 0086-022-23766531
Received January 19, 2011; revised March 6, 2011; accepted April 12, 2011.
Project supported by Research Foundation for the Doctoral Program of Tianjin Normal University (No. 5RL091)..
Chin. J. Chem. 2011, 29, 1739— 1744 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Liu et al.
FULL PAPER
tested. Reactions with both 10% and 20% catalysts both
gave quantitative yields in 5 min (Table 1, Entries 4 and
5), whereas 1 mol% catalyst loading only gave 85%
yield even after a long reaction time of 30 min. In the
absence of catalyst, the reaction did not yield any prod-
uct at room temperature even after a long reaction time.
This result suggests that a Lewis acid catalyst plays the
critical role in this reaction.
investigated the scope and limitation of this reaction
under optimized conditions (10 mol% of Ga(OTf)3, r.t.),
and the results are summarized in Table 2. As shown in
Table 2, a variety of structurally diverse o-substituted
aminoaromatics and orthoesters underwent the conden-
sation reaction smoothly to afford the corresponding
benzoxazoles, benzothiazoles, and benzimidazoles in
moderate to excellent yields. The presence of elec-
tron-donating and electron-withdrawing groups on the
aromatic ring of o-substituted aminoaromatics makes an
obvious difference to the yields under current reaction
conditions. In the case of benzimidazole, when R repre-
sented the electron-withdrawing groups such as chloro
(Table 2, Entries m—n) and nitro (Table 2, Entries d,
r—s), the yields and purities of the products were obvi-
ously worse, and long reaction times were required. All
of the 2-substituted benzoxazole, benzothiazole, and
benzimidazole derivatives have been characterized by
1H NMR, and IR spectra, and the known compounds
were confirmed by comparison of their spectral data and
melting points with those reported in the literature.
Table 1 Ga(OTf)3-catalyzed reactions of o-phenylenediamine
with triethyl orthoformatea
Entry
Ga(OTf)3/mol% Time/min
Yieldb/%
1
2
3
4
5
0
1
120
30
8
0
85
96
98
98
5
10
5
20
5
a Room temperature. b Isolated yield.
To evaluate the generality of this method, we then
Table 2 Ga(OTf)3-catalyzed synthesis of benzoxazoles, benzothiazoles, and benzimidazoles
Entry
a
Product
Time/min
Yield/%
m.p./℃ (lit.)[ref]
171—172 (172—173)17g
5
98
b
c
8
7
98
98
Oil17e
112—114 (115—116)17h
d
e
f
35
15
11
65
95
93
204—205 (206—208)17a
Oil17c
175—176 (175—176)17b
g
10
94
196—198 (198—200)18
h
i
13
16
30
90
92
90
Oil17e
Oil17c
Oil17h
j
1740
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© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 1739—1744

Expeditious and Efficient Synthesis of Benzoxazoles, Benzothiazoles, Benzimidazoles
Continued
Entry
k
Product
Time/min
33
Yield/%
90
m.p./℃ (lit.)[ref]
Oil17e
l
m
n
o
p
q
r
23
25
20
6
91
86
88
99
95
92
75
71
92
90
90
164—167 (166—167)17g
197—199 (199—200)17b
124-126(123-126)17b
205—206 (203—205)17c
238—240 (239—240)17c
117—118 (115—116)17g
226—227 (218—221)17a
137—139 (138—140)17g
160—162 (163—165)15d
165—167 (163—164)17a
158—160 (160—163)18
11
18
70
90
19
21
16
s
t
u
v
This reaction was further explored for the synthesis
3,3'-diaminobenzidine 4 and 2 equiv. of triethyl or-
of bis-benzimidazole compound 5 by the reaction of
thovalerate under similar conditions (Eq. 2). Compound
Chin. J. Chem. 2011, 29, 1739—1744
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1741

Liu et al.
FULL PAPER
5 was obtained in excellent yield.
by TLC, 2 mL of ethanol was added. The crude product
was collected by filtration, and Ga(OTf)3 was left in
filtrate. The crude product was subjected to column
chromatography over silica gel using petroleum
ether/ethyl acetate as eluent to obtain pure product.
Reducing the amount of catalyst and making it re-
usable are two important aspects of green chemistry.
We developed a catalyst recycle protocol, and also
tested the the activity of recovered Ga(OTf)3. The pro-
cedure is as follows: a mixture of o-substituted ami-
noaromatics and orthoesters (1 mmol each) and
Ga(OTf)3 (0.1 mmol) was stirred at room temperature
for 5—90 min. After completion of the reaction (moni-
tored by TLC), 2 mL of ethanol was added. The crude
product was collected by filtration, and Ga(OTf)3 was
left in filtrate.
In comparison with other catalysts such as
ZrOCl2•8H2O, ZrCl4, sulfamic acid (SA), I2, KSF clay,
H2SO4, which were recently used for the synthesis of
benzoxazoles, benzothiazoles, and benzimidazoles,
Ga(OTf)3 employed here exhibits more effective cata-
lytic activity than those previously reported in terms of
the reaction conditions, yields and reaction time (Table
3).
Analyses and spectral data
Selected spectral data for compounds 3a—3v, and 5.
1
3a: H NMR (CDCl3, 400 MHz) δ: 7.31—7.34 (m,
2H), 7.67—7.70 (m, 2H), 8.19 (s, 1H).
1
3b: H NMR (CDCl3, 400 MHz) δ: 7.44—7.47 (m,
2H), 7.71—7.75 (m, 2H), 8.09 (s, 1H).
3c: 1H NMR (CDCl3, 400 MHz) δ: 2.47 (s, 3H), 7.13
(d, J＝7.2 Hz, 1H), 7.45 (s, 1H), 7.56 (d, J＝7.2 Hz,
1H), 8.06 (s, 1H), 9.04 (s, 1H).
1
3d: H NMR (CDCl3, 400 MHz) δ: 7.69 (d, J＝7.2
Hz, 1H), 8.18 (d, J＝7.2 Hz, 1H), 8.21 (s, 1H), 8.41 (s,
1H), 9.09 (s, 1H).
1
3e: H NMR (CDCl3, 400 MHz) δ: 7.55—7.59 (m,
2H), 8.18—8.23 (m, 2H), 9.26 (s, 1H).
1
3f: H NMR (CDCl3, 400 MHz) δ: 2.65 (s, 3H),
7.21—7.24 (m, 2H), 7.52—7.56 (m, 2H), 7.81 (s, 1H).
3g: 1H NMR (CDCl3, 400 MHz) δ: 2.44 (s, 3H), 2.61
(s, 3H), 7.04 (d, J＝8.4 Hz, 1H), 7.32 (s, 1H), 7.44 (d,
J＝8.4 Hz, 1H), 9.41 (s, 1H).
Experimental
Material and instrumentation
All chemicals were purchased from Aldrich Co. and
used as received. All products were identified by their
spectra and physical data. Melting points were measured
using an X-4 apparatus and are uncorrected. IR spectra
were obtained using Shimadzu FTIR-8900 spectrometer
instrument. NMR spectra were taken with a Varian 400
spectrometer. Mass spectra were performed on a Ther-
moFinnigan LCQ Advantage instrument with an ESI
source (4.5 keV). Elemental analyses were carried out
on MT-3 analyzer. All of the obtained quinoxalines are
known compounds and were well characterized by
comparison with TLC, spectral and physical data with
data of authentic samples.15-18
1
3h: H NMR (CDCl3, 400 MHz) δ: 2.64 (s, 3H),
7.41—7.46 (m, 2H), 7.76—7.80 (m, 2H).
1
3i: H NMR (CDCl3, 400 MHz) δ: 2.81 (s, 3H),
7.54—7.58 (m, 2H), 8.06—8.21 (m, 2H).
3j: 1H NMR (CDCl3, 400 MHz) δ: 2.35 (s, 3H), 2.64
(s, 3H), 7.23 (d, J＝7.2 Hz, 1H), 7.56 (s, 1H), 7.68 (d,
J＝7.2 Hz, 1H).
1
3k: H NMR (CDCl3, 400 MHz) δ: 1.41 (t, J＝7.6
Hz, 3H), 2.94 (q, J＝7.6 Hz, 2H), 7.29—7.34 (m, 2H),
7.65—7.68 (m, 2H).
3l: 1H NMR (CDCl3, 400 MHz) δ: 1.43 (t, J＝7.6 Hz,
3H), 2.61 (s, 3H), 3.09 (q, J＝7.6 Hz, 2H), 7.07 (d, J＝
8.0 Hz, 1H), 7.17 (t, J＝8.0 Hz, 1H), 7.44 (d, J＝8.0 Hz,
1H), 10.01 (s, 1H).
General procedure for the synthesis of benzoxazoles,
benzothiazoles, and benzimidazoles under solvent-
free conditions
1
3m: H NMR (CDCl3, 400 MHz) δ: 2.63 (s, 3H),
7.17 (d, J＝8.4 Hz, 1H), 7.44 (d, J＝8.4 Hz, 1H), 7.51
A mixture of o-substituted aminoaromatics (1 mmol),
triethyl orthoformate (1 mmol) and Ga(OTf)3 (0.1 mmol)
was stirred at room temperature for an appropriate time
(Table 2). After completion of the reaction, as indicated
(s, 1H), 8.49 (s, 1H).
1
3n: H NMR (CDCl3, 400 MHz) δ: 7.18 (d, J＝8.4
Hz, 1H), 7.49 (CDCl3, 400 MHz), 7.58 (s, 1H), 8.15 (s,
1H), 12.25 (s, 1H).
Table 3 Comparison of the effect of catalysts for the synthesis of benzimidazole (3a)
Solvent Reaction conditions Time Yield/%
r.t. 9 min 86
Catalyst/solvent
ZrOCl2•8H2O
ZrCl4
Ref.
17 h
17 f
—
EtOH
MeOH
MeCN
Toluene
—
r.t.
2 h
95
96
98
78
77
98
Sulfamic acid (SA)
I2
r.t.
r.t.
1 h
17 m
17 g
17 c
17 e
15 min
12 h
2 h
KSF clay
Reflux, under N2
175—178 ℃
r.t.
H2SO4
Ga(OTf)3
—
5 min
1742
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Chin. J. Chem. 2011, 29, 1739—1744

Expeditious and Efficient Synthesis of Benzoxazoles, Benzothiazoles, Benzimidazoles
3o: 1H NMR (CDCl3, 400 MHz) δ: 2.38 (s, 6H), 7.43
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8
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3p: H NMR (CDCl3, 400 MHz) δ: 2.32 (s, 6H),
2.68 (s, 3H), 7.36 (s, 2H).
1
3q: H NMR (CDCl3, 400 MHz) δ: 0.86 (t, J＝7.6
Hz, 3H), 1.37 (sext, J＝7.6 Hz, 2H) 1.81 (quin, J＝7.6
Hz, 2H), 2.33 (s, 6H), 2.92 (t, J＝7.6 Hz, 2H), 7.33 (s,
2H), 10.49 (s, 1H).
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3r: 1H NMR (CDCl3, 400 MHz) δ: 2.71 (s, 3H), 7.79
(d, J＝8.4 Hz, 1H), 8.18 (d, J＝8.4 Hz, 1H), 8.49 (s,
1H).
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1
3s: H NMR (CDCl3, 400 MHz) δ: 0.88 (t, J＝7.2
Hz, 6H), 1.43 (sext, J＝7.2 Hz, 4H), 1.88 (quin, J＝7.2
Hz, 4H), 3.03 (t, J＝7.2 Hz, 4H), 7.62 (d, J＝8.4 Hz,
4H), 8.16 (d, J＝8.4 Hz, 4H), 8.48 (s, 2H) 11.04 (s, 2H).
3t: 1H NMR (CDCl3, 400 MHz) δ: 1.44 (t, J＝7.6 Hz,
3H), 2.98 (q, J＝7.6 Hz, 2H) 7.21—7.25 (m, 2H),
7.54—7.57 (m, 2H), 9.99 (s, 1H).
1
3u: H NMR (CDCl3, 400 MHz) δ: 0.91 (t, J＝7.2
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Hz, 3H), 1.38 (sext, J＝7.2 Hz, 2H) 1.85 (quin, J＝7.2
Hz, 2H), 2.93 (t, J＝7.2 Hz, 2H), 7.18—7.22 (m, 2H),
7.52—7.55 (m, 2H), 9.02 (s, 1H).
1
3v: H NMR (CDCl3, 400 MHz) δ: 1.41 (t, J＝7.6
Hz, 3H), 2.46 (s, 3H), 2.96 (q, J＝7.6 Hz, 2H), 7.06 (d,
J＝8.4 Hz, 1H), 7.32 (s, 1H), 7.46 (d, J＝8.4 Hz, 1H),
8.42 (s, 1H).
5: 1H NMR (CDCl3, 400 MHz) δ: 0.98 (t, J＝7.2 Hz,
6H), 1.43 (sext, J＝7.2 Hz, 4H), 1.80 (quin, J＝7.2 Hz,
4H), 2.87 (t, J＝7.2 Hz, 4H), 7.47 (d, J＝8.4 Hz, 2H),
7.54 (d, J＝8.4 Hz, 2H), 7.69 (s, 2H).
Conclusions
(i) Lu, J.; Yang, B. Q.; Bai, Y. J. Synth. Commun. 2002, 32,
3703.
In conclusion, we have developed an expeditious,
novel and efficient method for the synthesis of ben-
zoxazole, benzothiazole, and benzimidazole derivatives
from o-substituted aminoaromatics with orthoesters us-
ing a catalytic amount of Ga(OTf)3 at room temperature.
The advantages of current protocol include high effi-
ciency, good substrate generality, environmental benign
catalyst, mild reaction conditions, and experimentally
operational ease, all of which make it a useful and at-
tractive strategy for the preparation of various ben-
zoxazole, benzothiazole, and benzimidazole derivatives
simply by changing different substrates.
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