[C12mim] Br: A temperature-dependent phase transfer catalyst and its application for aerobic oxidative synthesis of 2-aryl benzimidazoles, benzoxazoles or benzothiozoles catalyzed by TEMPO based ionic liquid

Article abstract of DOI:10.1002/jccs.201300577

The application of [C₁₂mim]Br ionic liquid/o-xylene temperature-dependent biphasic system into the [Imim-TEMPO][Cl]/O ₂-promoted condensation between o-phenylenediamines, o-aminophenol or o-aminothiophenol with aldehydes for preparing benzimidazoles, benzoxazoles or benzothiozoles is described. Several aldehydes and o-phenylenediamines, o-aminophenol or o-aminothiophenol were reacted efficiently to form corresponding products in excellent yields. Both the [Imim-TEMPO][Cl] and [C₁₂mim]Br could be reused at least eight times without significantly decreasing the catalytic activity. The application of [C₁₂mim]Br ionic liquid/o-xylene temperature-dependent biphasic system into the [Imim-TEMPO][Cl]/O₂-promoted condensation between o-phenylenediamines, o-aminophenol or o-aminothiophenol with aldehydes for preparing benzimidazoles, benzoxazoles or benzothiozoles is described. Several aldehydes and o-phenylenediamines, o-aminophenol or o-aminothiophenol were reacted efficiently to form corresponding products in excellent yields. Both the [Imim-TEMPO][Cl] and [C₁₂mim]Br could be reused at least eight times without significantly decreasing the catalytic activity.

Full text of DOI:10.1002/jccs.201300577

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
[C12mim]Br: A Temperature-dependent Phase Transfer Catalyst and Its Application for
Aerobic Oxidative Synthesis of 2-Aryl Benzimidazoles, Benzoxazoles or
Benzothiozoles Catalyzed by TEMPO Based Ionic Liquid
Jiatao Yu and Ming Lu*
School of Chemical Engineer, Nanjing University of Science & Technology, Nanjing 210094, P. R. China
(Received: Nov. 2, 2013; Accepted: Jan. 22, 2014; Published Online: Feb. 17, 2014; DOI: 10.1002/jccs.201300577)
The application of [C12mim]Br ionic liquid/o-xylene temperature-dependent biphasic system into the
[Imim-TEMPO][Cl]/O2-promoted condensation between o-phenylenediamines, o-aminophenol or
o-aminothiophenol with aldehydes for preparing benzimidazoles, benzoxazoles or benzothiozoles is de-
scribed. Several aldehydes and o-phenylenediamines, o-aminophenol or o-aminothiophenol were reacted
efficiently to form corresponding products in excellent yields. Both the [Imim-TEMPO][Cl] and
[C12mim]Br could be reused at least eight times without significantly decreasing the catalytic activity.
Keywords: Benzimidazoles; Benzoxazoles; Benzothiazoles; Aerobic oxidation; Temperature-de-
pendent phase transfer catalyst.
INTRODUCTION
Over the past several decades, Benzimidazoles and
of hydrocarbons, alcohols, and amines owing to its low
toxicity, reversible redox behavior, and high efficiency and
selectivity.18 Chen and co-workers19 reported aerobic oxi-
dative synthesis of 2-substituted benzoxazoles, benzothia-
zoles, and benzimidazoles catalyzed by 4-methoxy-TEMPO.
However, TEMPO is quite expensive. Therefore, its effi-
cient recycling is highly desirable.
their derivatives, known as an important class of N-con-
taining heterocyclic compounds, have received consider-
able atention in recent years due to its possess significant
pharmacological and biological activities, such as anti-
convulsant, antianxiety, antiulcer, antihypertensive, antivi-
ral, antifungal, anticancer, and antihistamine.1-5 The excel-
lent characterristics of benzimidazole derivatives have pro-
moted extensive studies for their synthesis. Typically, there
are two methods for the synthesis of benzimidazoles. The
first is the condensation of o-phenylenediamine with car-
boxylic acids or their derivatives (nitriles, imidates, ortho-
esters) under harsh dehydrating conditions involving using
strong acids and/or high temperatures.6-10 The second is ox-
idative cyclo-dehydrogenation of aniline Schiff’s base,
which are often generated in situ from the condensation of
o-phenylenediamines and aldehydes. Various oxidative re-
agents such as Pb(OAc)4,11 Na2S2O5,12 (NH4)2S2O8,13
DDQ,14 MnO2,15 Oxone,16 H2O2,17 have recently been re-
ported. However, this method has some drawbacks involv-
ing requiring stoichiometric or excess amount of oxidants
to be used, low yields, long reaction times, harsh reaction
conditions, problematic by-products. In recent years, the
use of molecular oxygen as an economic and green oxidant
for the catalytic oxidative reactions has received much at-
tention. The stable nitroxyl radical 2,2,6,6-tetramethyl-
piperidine-1-oxyl (TEMPO) has attracted considerable at-
tention and been widely used to catalyze oxidative reaction
Recently, several strategies have emerged to conquer
the problem of TEMPO recycling through the design of
various types of supported TEMPO including mesoporous
silica,20 silica solgels,21 cross-linked polystyrene resins,22
which allowed the catalyst to be recycled out of the reac-
tion media by simple filtration. However, owing to the het-
erogeneous nature of the reaction, most of these systems
were far less versatile and efficient than homogeneous
TEMPO. So a homogeneous system that contain TEMPO
is in expectation.
[C12mim]Br is a white powder at room temperature,
and it is nearly insoluble in o-xylene, but miscible at 120
oC. So [C12mim]Br exhibits a temperature-dependent phase
behavior with o-xylene. Moreover, [Imim-TEMPO][Cl]
powder is miscible in [C12mim]Br at high temperature, but
not soluble in o-xylene.
Herein, we report a new temperature-dependent bi-
phasic system, including recoverable 1-dodecyl-3-methyl-
imidazolium bromine([C12mim]Br) ionic liquid and o-xy-
lene, and its application in the synthesis of 2-aryl benzimi-
dazoles, benzoxazoles, and benzothiazoles by o-phenyl-
enediamines, o-aminophenol or o-aminothiophenol and al-
* Corresponding author. E-mail: yjt1989926@163.com, luming302@126.com
578
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Aerobic Oxidative Synthesis of Benzimidazoles, Benzoxazoles and Benzothiozoles
Table 1. Optimization of the reaction conditions[a]
dehydes using [Imim-TEMPO][Cl] as catalyst and molecu-
lar oxygen as oxidant. On the basis of pioneering work, we
explore the catalytic role of the [C12mim]Br/o-xylene bi-
phasic system in the reaction. To our delight, this system
exhibited excellent catalytic activity and stability. In addi-
tion, the catalyst and [C12mim]Br can be successfully re-
covered and reused.
[Imim-TEMPO][Cl]
NH2
N
+
Ph
Ph
O
[Cnmim]Br
solvent
N
H
NH2
N
O
N
N
[Imim-TEMPO][Cl]:
[Cnmim]Br:
Cl
O
N
N
n-2
Br
RESULTS AND DISCUSSION
Catalyst
(mol%)
Ionic liquid
(mmol)
Yield
(%)[b]
Initially, the reaction between o-phenylenediamine
and benzaldehyde was selected for the aerobic oxidative
synthesis of benzimidazole as a model reaction for optimiz-
ing the reaction conditions, and the results are summarized
in Table 1. A variety of [Cnmin]Br ionic liquids were then
investigated in the reaction, showing varying activities in
the reaction, among which [C12min]Br was found to be the
best one, giving the highest yield (entries 1-5). When in the
absence of [C12min]Br, the yield was very poor (entry 6).
So [C12min]Br was essential for the reaction. The screen of
the loadings of [C12mim]Br was then carried out, the result
revealed that 2 mmol was the most suitable proportion,
more loading could not enhance the product yield (entries
5, 7-9). Moreover, the loading of catalyst ([Imim-TEMPO]
[Cl]) was also tested, we found that 5 mol% was the most
suitable proportion (entries 5, 10-11). The effect of solvent
was also screened by using several solvents which could
formed thermoregulated phase-transfer system with
[C12min]Br. We discovered that nonpolar aromatic solvents
trate (entries 5, 12-13). In addition, temperature signifi-
cantly affected the reaction, the phase transformation tem-
perature is about 100 oC; when the temperature was lower
Entry
Solvent T/
Gas
1
5
5
5
5
5
5
5
5
5
0
10
5
5
5
5
5
5
[C4min]Br (2) o-xylene 120 O2
[C6min]Br (2) o-xylene 120 O2
[C8min]Br (2) o-xylene 120 O2
[C10min]Br (2) o-xylene 120 O2
[C12min]Br (2) o-xylene 120 O2
[C12min]Br (0) o-xylene 120 O2
[C12min]Br (0.5) o-xylene 120 O2
[C12min]Br (1) o-xylene 120 O2
[C12min]Br (3) o-xylene 120 O2
[C12min]Br (2) o-xylene 120 O2
[C12min]Br (2) o-xylene 120 O2
75
79
84
91
97
19
65
85
97
15
97
0
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
[C12min]Br (2) EtOAc
[C12min]Br (2) toluene
77
90
O2
O2
O2
O2
52
45
63
73
[C12min]Br (2) o-xylene 60
[C12min]Br (2) o-xylene 80
[C12min]Br (2) o-xylene 100 O2
[C12min]Br (2) o-xylene 120 N2 trace
[a] Reaction conditions: o-phenylenediamines (5 mmol) and
benzaldehyde (5 mmol) were placed in a 30 mL three-necked
flask containing 15 mL o-xylene. The reaction mixture was
stirred at 120 oC for 0.5 h. Then [Imim-TEMPO][Cl] and
[Cnmin]Br were then added, and stirred at corresponding
temperature under atmosphere for 4 h.
[b] Isolated yield.
o
than 80 C, the low yield was obtained due to the system
was not totally homogeneous, and it reached a maximum at
120 oC. These results show that the moderate temperature
is 120 oC (entries 5, 14-16). Finally, we also tested the in-
fluence of the gas, and we found that the O2 is significant
for the reaction. When the reaction conducted under N2, the
reaction could not occurred (entries 5, 17).
higher yields than did those bearing electron-rich aromatic
rings (entries 2-9). The reason may be that the strong elec-
tron-withdrawing groups (NO2) enhanced the reaction rate
due to the strong withdrawing ability of the group increase
in electrophilicity of the carbonyl carbon (entries 6-8). The
sterical hindrance of the substituted benzaldehydes also in-
fluenced the reaction. For example, the more sterically hin-
dered ortho-chlorobenzyl benzaldehydes which had reac-
tion with o-phenylenediamine furnished the target product
in lower yield than para-chlorobenzyl benzaldehydes (en-
tries 3, 4). Heteroaryl amines such as ortho-furyl aldehydes
were used in the reaction, and the yields were satisfactory
(entries 10, 11). In addition, 4-nitro-o-phenylenediamine
was also employed in the reaction, and to our delight, it re-
Under the optimized conditions, the scope of the
present transformation was extended to o-phenylenedi-
amine with various substituted aromatic aldehydes, and the
results are shown in Table 2. In most cases, o-phenylene-
diamine reacted with various aldehydes smoothly to give
the corresponding products in excellent yields, regardless
of the presence of electron-donating or electron-withdraw-
ing functionalities. It seemed that benzaldehydes bearing
electron-deficient aromatic rings at the para-position gave
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Yu and Lu
Table 2. Synthesis of 2-substituted benzimidazoles from various
aromatic aldehydes[a]
Table 3. Aerobic oxidative synthesis of benzoxazoles and
benzothiazoles[a]
[Imim-TEMPO][Cl]
[Imim-TEMPO][Cl]
N
X
NH2
+
NH2
+
N
R
R2
R
O
R1
R1
R2
O
[C12mim]Br/ o-xylene
O2,120oC
[C12mim]Br/ o-xylene
O2,120oC
N
H
XH
X=O,S
NH2
Reaction time
(h)
Reaction time
(h)
Yield (%)[b]
Entry
X
R
Yield (%)[b]
Entry
R1
R2
1
H
H
C6H5
4
3
97
95
95
91
93
96
94
90
93
89
88
93
1
2
3
4
5
6
O
O
O
S
Ph
4
88
93
91
85
87
83
2
4-BrC6H4
4-ClC6H4
2-ClC6H4
2, 4-ClC6H3
4-NO2C6H4
4-MeOC6H4
4-ClC6H4
4-CH3OC6H4
Ph
4
4.5
4
3
H
3
4
H
3.5
3.5
3
5
H
S
4-ClC6H4
4-NO2C6H4
4
6
H
S
3.5
7
H
3
[a] Reaction conditions: 2-aminophenol or 2-amino-thiophenole
(5 mmol) and benzaldehyde (5 mmol) were placed in a 30 mL
three-necked flask containing 15 mL o-xylene. The reaction
mixture was stirred at 120 oC for 0.5 h. Then 5 mol% [Imim-
TEMPO][Cl] and 2 mmol [C12min]Br were then added, and
stirred at 120 oC under O2 atmosphere for several hours.
[b] Isolated yield.
8
H
4-Me2NC6H4
3,4,5-MeOC6H2
2-Furyl
4
9
H
4
10
11
12
H
5
H
5-Methyl-2-Furyl
Ph
5
4-NO2
5
[a] Reaction conditions: o-phenylenediamines (5 mmol) and
benzaldehyde (5 mmol) were placed in a 30 mL three-necked
flask containing 15 mL o-xylene. The reaction mixture was
stirred at 120 oC for 0.5 h. Then 5 mol% [Imim-TEMPO][Cl] and
2 mmol [C12min]Br were then added, and stirred at 120 oC under
O2 atmosphere for several hours.
A possible mechanism for synthesis of 2-
arylbenzimidazoles catalyzed by [Imim-
TEMPO][Cl] in [C12mim]Br/o-xylene sys-
tem
Scheme 1
[b] Isolated yield.
sulted in good yields (entry 12).
Having successfully achieved the synthesis of benz-
imidazoles, the [C12mim]Br/o-xylene system was then ex-
panded to catalyze the synthesis of benzoxazoles and
benzothiazoles using o-aminophenol and o-aminothiophe-
nol respecttively as the starting materials. As summarized
in Table 3, benzoxazoles and benzothiazoles were also
been converted from corresponding o-aminophenol and o-
aminothiophenol in high yields.
Based on these results and the literature reports about
the synthesis of benzimidazoles and TEMPO-catalyzed
aerobic oxidative reactions previous studies,19,25-26 a possi-
ble mechanism for the aerobic oxidative synthesis of 2-sub-
stituted benzimidazole in this system was proposed as
shown in Scheme 1. First, the condensation of o-phenylene-
diamine with aldehyde takes place to form imine 1, and
then the imine further reacted with another amine group of
o-phenylenediamine resulting in the formation of benzimi-
dazoline 2. The nitroxyl radical of [Imim-TEMPO][Cl] ox-
idized cyclo-dehydrogenation of 2 to afford the final prod-
uct benzimidazole and was turned into hydroxylamine
([Imim-TEMPOH][Cl]) itself. The oxidation of [Imim-
TEMPOH][Cl] into [Imim-TEMPO][Cl] could proceed
smoot-hly with the help of O2.
The recyclability of the catalytic system was also
investtigated. After completion of the reaction, the mixture
was allowed to cool to room temperature, the organic phase
was separated by decantation and the residual consist of
580
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Aerobic Oxidative Synthesis of Benzimidazoles, Benzoxazoles and Benzothiozoles
[Imim-TEMPO][Cl] and [C12mim]Br, concentrated to re-
move generated water and reused without further purifica-
tion. Fresh substrates, solvent were heated to 120 oC for 0.5
h with stirring, then the residual [Imim-TEMPO][Cl] and
[C12mim]Br were recharged to the mixture to react once
again. The procedure was successfully repeated for at least
8 times without great loss of catalytic activity (Scheme 2).
The yield of the reaction was decreased obviously after 8
times of recycle.
TEMPO][Cl] was prepared by the procedure given in the litera-
ture.24
Typical Procedure for synthesis of benzimidazoles:
o-phenylenediamine (0.54 g, 5 mmol) and benzaldehyde (0.53 g,
5 mmol) were placed in a 30 mL three-necked flask containing
o-xylene (15 mL). The reaction mixture was stirred at 120 oC for
0.5 h. [C12mim]Br (0.67 g, 2 mmol) and [Imim-TEMPO][Cl]
(0.0826 g, 0.25 mmol) were then added, and stirred at 120 oC un-
der oxygen atmosphere for several hours till the starting materials
were completely disappeared as determined by TLC. The organic
phase was separated, and then ionic liquid phase washed with
o-xylene and combined with the organic solvent, concentrated
under vacuum and the product was purified by using silica gel
column chromatography (n-hexane/EtOAc = 7:3) to afford the
corresponding pure product.
Repeating reactions using recovered
[Imim-TEMPO][Cl] and [C12mim]Br
Scheme 2
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