Aerobic oxidative synthesis of benzimidazoles from amines catalyzed by 3-methyl-4-oxa-5-azahomoadamantane and iron(III) chloride

Article abstract of DOI:10.1007/s11164-015-2009-2

A simple and efficient catalytic system including 3-methyl-4-oxa-5- azahomoadamantane and FeCl₃ for aerobic oxidative synthesis of benzimidazoles from primary amines and o-phenylenediamine is presented. This process uses O₂ as economic and green oxidant and water as green solvent, tolerates a wide range of substrates, and can afford the target products in moderate to excellent yields.

Full text of DOI:10.1007/s11164-015-2009-2

Res Chem Intermed
DOI 10.1007/s11164-015-2009-2
Aerobic oxidative synthesis of benzimidazoles
from amines catalyzed by 3-methyl-4-oxa-5-
azahomoadamantane and iron(III) chloride
1
1
Jiatao Yu
•
Ming Lu
Received: 29 November 2014 / Accepted: 11 March 2015
Springer Science+Business Media Dordrecht 2015
ꢀ
Abstract A simple and efficient catalytic system including 3-methyl-4-oxa-5-
azahomoadamantane and FeCl for aerobic oxidative synthesis of benzimidazoles
3
from primary amines and o-phenylenediamine is presented. This process uses O as
2
economic and green oxidant and water as green solvent, tolerates a wide range of
substrates, and can afford the target products in moderate to excellent yields.
Keywords Aerobic oxidation Á Benzimidazoles Á Amines Á 3-Methyl-4-oxa-5-
azahomoadamantane Á Iron(III) chloride
Introduction
Five-membered heterocyclic compounds, such as benzimidazoles, are important
building blocks for construction of pharmaceuticals, natural products, functional
materials, and agrochemical compounds. They exhibit significant pharmacological
and biological activities such as anticonvulsant, anticancer, antiulcer, antihyper-
tensive, antibacterial, and antihistaminic properties [1–9]. Consequently, great
efforts have been made to develop efficient methods for preparation of benzimida-
zoles. Classical methods for synthesis of benzimidazoles involve two approaches.
One is condensation of o-phenylenediamine with carboxylic acids or their
derivatives (nitriles, imidates, orthoesters) [2, 10–25]. The second approach is
oxidative cyclo-dehydrogenation of aniline Schiff’s base, which are often generated
Electronic supplementary material The online version of this article (doi:10.1007/s11164-015-2009-2)
contains supplementary material, which is available to authorized users.
&
Ming Lu
luming302@126.com
1
Chemical Engineering College, Nanjing University of Science and Technology,
Nanjing 210094, China
123

J. Yu, M. Lu
in situ by condensation of o-phenylenediamines and aldehydes [26–33]. Although
the reaction is efficiently carried out by the above methods, they suffer from one or
more disadvantages; e.g., the former requires strong acidic conditions and high-
temperature conditions, whereas the latter requires use of stoichiometric or excess
amount of strong oxidant, high-cost catalyst, and odorous and unstable aldehydes,
and generates environmentally hazardous/toxic byproducts. Moreover, these
methods are always conducted in organic solvents. Thus, development of a novel
ecofriendly and economic approach avoiding use of organic solvents is highly
desirable.
The aminoxyl radical TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical) and
its related nitroxyl derivatives have been proven to be powerful promoters for
aerobic oxidation of hydrocarbons, alcohols, and amines over the last decade [34–
4
oxoammonium species, such as NaOCl, Br , trichloroisocyanuric acid, m-
2]. Especially common is application of a co-oxidant to generate reactive
2
chloroperbenzoic acid, Oxone or hypervalent iodine compounds [43–50]. In recent
years, aminoxyl radical-catalyzed oxidative reactions using oxygen as terminal
oxidant have received much attention in view of the concepts of green chemistry
and atom economy. Compared with previous methods, this method has various
advantages, such as water as the only byproduct and low loadings of catalysts.
However, its application in catalytic oxidative synthesis of heterocycles is rarely
reported. Recently, 3-methyl-4-oxa-5-azahomoadamantane, which is a novel
alkoxyamine-type organocatalyst, has been highly efficiently applied in oxidation
of primary and secondary alcohols to their corresponding carbonyl compounds
using NaOCl as terminal oxidant [51]. Based on this work, we wished to extend this
alkoxyamine-type organocatalyst to other oxidations by using oxygen as terminal
oxidant. Herein, we report a novel and efficient approach for aerobic oxidative
synthesis of benzimidazoles directly from primary amines and o-phenylenediamine
catalyzed by 3-methyl-4-oxa-5-azahomoadamantane and iron(III) chloride.
Results and discussion
The reaction conditions were screened by using o-phenylenediamine and benzy-
lamine as starting materials to generate desired 2-phenyl-1H-benzimidazole; the
results are summarized in Table 1. For the initial reaction mixture of 1 mmol o-
phenylenediamine and 1 mmol benzylamine catalyzed by 1 mol% 3-methyl-4-oxa-
5
-azahomoadamantane and 10 mol% FeCl with flow rate of 20 ml/min O bubbled
3 2
into the flask at 100 ꢁC for 12 h, it was discovered that this set of conditions
afforded the desired product in 85 % yield (Table 1, entry 1). Conduction of the
reaction in absence of FeCl or 3-methyl-4-oxa-5-azahomoadamantane afforded
3
trace yield of the desired product, even after 24 h (entries 2, 3). Our investigation
showed that the effect of the amount of o-phenylenediamine on the reaction was
very significant, with 1.2 equivalent being the most suitable (entries 4–6). Other
metal salts, i.e., Fe (SO ) , Fe(NO ) , CuCl , and CoCl , were also screened in the
2
4 3
3 3
2
2
same conditions, among which FeCl provided the best yield (entries 7–10). After
3
screening the amount of Fe salt and catalyst, we found that 10 mol% FeCl and
3
1
23

Aerobic oxidative synthesis of benzimidazoles from amines…
Table 1 Optimization of aerobic oxidative synthesis of 2-phenyl-1H-benzimidazole
Entry
Diamine
equiv.)
Metal salt
(mol%)
Catalyst loading
(mol%)
T (ꢁC)
Gas
Time
(h)
Yield
a
(%)
(
1
2
3
4
5
6
7
8
9
1
FeCl
3
(10)
1
1
0
1
1
1
1
1
1
1
1
1
2
1
1
1
1
1
100
100
100
100
100
100
100
100
100
100
100
100
100
80
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
12
24
24
12
12
12
12
12
12
12
12
12
12
12
12
24
12
24
85
1
–
Trace
Trace
79
1
FeCl
FeCl
FeCl
FeCl
3
3
3
3
(10)
(10)
(10)
(10)
0.8
1.2
1.5
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
90
88
Fe
2
(SO
4
)
3
(10)
(10)
78
Fe(NO
3
)
3
87
CuCl
CoCl
2
2
(10)
(10)
(5)
66
10
11
12
13
14
15
16
17
18
59
FeCl
FeCl
FeCl
FeCl
FeCl
FeCl
FeCl
FeCl
3
75
3
(20)
(10)
(10)
(10)
(10)
(10)
(10)
87
3
3
3
3
3
3
90
73
50
45
RT
Trace
82
100
100
Air
N
2
Trace
Reaction conditions: o-phenylenediamine, benzylamine, metal salt, 3-methyl-4-oxa-5-azahomoadaman-
tane were mixed in a three-necked flask, then stirred rapidly for several hours. Reaction progress was
monitored by TLC
a
Isolated yield
1
mol% 3-methyl-4-oxa-5-azahomoadamantane was the most appropriate propor-
tion (entries 2, 3, 11–13). To determine the optimal reaction condition, various
temperatures were tested in follow-up temperature screening experiments (entries 5,
14–16). Results showed that temperature of 100 ꢁC afforded the best yield (90 %),
whereas reducing the temperature resulted in a large decrease of yield. Further study
of the gas for the reaction indicated that the reaction did not occur in N atmosphere.
2
Besides, when using air in the reaction, the yield was reduced slightly (entries 17,
1
8).
Encouraged by the good activity of the 3-methyl-4-oxa-5-azahomoadamantane/
FeCl catalytic system, we expanded the scope of the present transformation to a
3
series of amine and diamine derivatives to obtain the corresponding 2-aryl and
heteroaryl substituted benzimidazoles in good to excellent yields. Electron-donating
and electron-withdrawing group on the aromatic ring of benzylamines were well
123

J. Yu, M. Lu
tolerated in this tandem reaction and gave the desired benzimidazoles in good to
excellent yields (entries 1–16). The electronic effect of the substituent on the benzyl
ring was investigated with respect to the yield of this transformation. It is obvious
that electron-withdrawing group (entries 2–4, 7) is superior to electron-donating
group (entries 8–10) at the para-position. The reason may be that strong electron-
withdrawing groups enhanced the reaction rate. Additionally, the steric hindrance of
the substituted benzylamines also influenced the reaction. When the benzylamines
bear the same substituent, the yield of benzylamine with substituent at
Table 2 Synthesis of 2-substituted benzimidazoles from aromatic amines and diamines
1
2
6
a
Entry
X
R
R
Reaction time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
O
H
C
H
5
12
10
10
10
12
11
8
90
92
92
93
87
88
95
89
85
87
84
84
83
86
85
89
89
88
86
87
82
88
90
92
H
4-FC
4-ClC
4-BrC
2-ClC
2,4-ClC
4-NO
4-MeOC
6 4
H
H
6 4
H
H
6 4
H
H
6 4
H
H
6 3
H
H
2 6 4
C H
H
6
H
4
12
12
12
14
14
14
16
16
12
10
12
16
14
18
16
14
12
H
4-Me
2
2
NC
6
H
4
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
H
4-Me
CHC
6
H
4
H
3,4,5-MeOC
6
H
2
H
2-Furyl
H
5-Methyl-2-furyl
Pyridin-2-yl
H
4-NO
4-Cl
H
2
C
C
6
6
H
5
5
H
1-Naphthyl
Styryl
H
H
6 5
C H
O
H
4-ClC
6
H
4
6
O
H
4-MeOC
H
4
S
H
6 5
C H
S
H
4-ClC
4-NO
6
H
4
S
H
2
C
6
H
4
Reaction conditions: 1.2 mmol diamine and 1 mmol aromatic amine, 0.01 mmol 3-methyl-4-oxa-5-
O, 100 ꢁC. Reaction progress was monitored by TLC
azahomoadamantane, 0.1 mmol FeCl , O
3
2
, 5 ml H
2
a
Isolated yield
1
23

Aerobic oxidative synthesis of benzimidazoles from amines…
ortho-position was lower than those with substituent at para-position (entries 3, 5).
We also subjected heterocyclic primary amines to the employed reaction conditions.
To our surprise, heterocyclic primary amines showed high reactivity to give the
corresponding benzimidazoles in good yields (entries 12–14). Subsequently,
substituted o-phenylenediamine, including 4-chloro-o-phenylenediamine and 4-ni-
tro-o-phenylenediamine, were also employed in the reaction, resulting in good
yields (entries 15, 16). Furthermore, 1-naphthalenemethylamine and 3-phenyl-2-
propylene amine were subjected to the standard conditions; to our delight, good
yields successfully resulted (entries 17, 18). In light of our above successful results,
we focused the scope of this protocol toward synthesis of benzoxazole and
benzothiazole. For this purpose, o-aminophenol and o-aminothiophenol were
replaced with o-phenylenediamine as starting materials and reacted with benzy-
lamine under similar reaction conditions, which were optimized for benzimidazoles.
Benzoxazoles and benzothiazoles were also converted from corresponding o-
aminophenol and o-aminothiophenol in high yields (entries 19–24) (Table 2).
Preliminary mechanistic studies of the reaction were also conducted. In the
reaction conditions in absence of FeCl or 3-methyl-4-oxa-5-azahomoadamantane,
3
the reaction could not be conducted (Table 1, entries 2, 3). Besides, when the
reaction was conducted in N2 atmosphere, the yield was also trace (Table 1,
entry 18). Thus, FeCl , 3-methyl-4-oxa-5-azahomoadamantane, and O were
3
2
essential for this reaction.
Besides, to provide mechanistic insight into the 3-methyl-4-oxa-5-azaho-
moadamantane-catalyzed oxidation, attempts were undertaken to identify the active
species generated. We conducted aerobic oxidation of o-phenylenediamine
(
6 mmol) and benzylamine (5 mmol) catalyzed by FeCl (0.5 mmol), 3-methyl-4-
3
oxa-5-azahomoadamantane (0.05 mmol) in water at refluxing condition for 12 h.
After cooling to room temperature, we obtained the residue after extraction and
evaporation. Then, the crude product was purified by chromatography to give the
product (2-phenyl-1H-benzimidazole) and 1-Me-AZADO in 65 % (based on the
equiv. of 3-methyl-4-oxa-5-azahomoadamantane) yield (Scheme 1).
Based on these results and literature reports about 3-methyl-4-oxa-5-azaho-
moadamantane in alcohol oxidation [51], a possible catalytic mechanism of
3
-methyl-4-oxa-5-azahomoadamantane/FeCl3 in aerobic oxidative synthesis of
Scheme 1 Aerobic oxidative synthesis of 2-phenyl-1H-benzimidazole from o-phenylenediamine and
benzylamine catalyzed by 3-methyl-4-oxa-5-azahomoadamantane/FeCl
3
123

J. Yu, M. Lu
Scheme 2 Possible reaction mechanism for formation of benzimidazoles from primary amines and
o-phenylenediamine catalyzed by FeCl and 3-methyl-4-oxa-5-azahomoadamantane
3
benzimidazoles from primary amines and diamines is proposed in Scheme 2. The
3
?
first step of the reaction involves Fe
moadamantane 1 to form its radical 2 accompanied by reduction to Fe . Then,
oxidating 3-methyl-4-oxa-5-azaho-
2
?
1
23

Aerobic oxidative synthesis of benzimidazoles from amines…
Scheme 3 Preparation of 3-methyl-4-oxa-5-azahomoadamantane
heterolytic cleavage of 2 gives the nitro-alkene 3 as a transient intermediate, which
immediately generates oxoammonium 4; this oxidizes an amine to give the
corresponding benzyl imine, and 4 is reduced to form 5. Then, 5 is oxidated to its
3
?
2?
radical 6 by recycling of Fe to Fe assisted by O . Subsequently, radical 6
2
oxidated primary amines to form benzyl imine, while 6 transforms to 5 at the same
time. Benzyl imine reacted with o-phenylenediamine to form imine, which is then
transformed to the intermediary benzimidazoline. The final step is oxidative cyclo-
dehydrogenation of benzimidazoline to afford the product benzimidazole.
Conclusions
We have developed a simple and efficient catalytic system including FeCl and
3
3
-methyl-4-oxa-5-azahomoadamantane with broad substrate scope for aerobic
oxidative synthesis of benzimidazoles from primary amines. This process uses O as
2
economic and green oxidant and water as green solvent, tolerates a wide range of
substrates, and can afford the target products in moderate to excellent yields.
Experimental
All starting materials were purchased from commercial sources and used without
further treatment. Melting points were determined on a Thomas Hoover capillary
apparatus and are uncorrected. All known compounds were identified by appropriate
1
technique such as H nuclear magnetic resonance (NMR) and compared with
1
previously reported data. H NMR (500 MHz) was recorded on a Bruker 500
spectrometer with tetramethylsilane (TMS) as internal standard. Mass spectra were
recorded on an Agilent Technologies 6110 quadrupole LC/MS equipped with an
electrospray ionization (ESI) probe operating in positive ion mode. Analytical thin-
layer chromatography (TLC) was performed on precoated silica gel 60 F254 plates.
Yields refer to isolated yields of products after purification by silica gel column
123

J. Yu, M. Lu
chromatography (300 mesh). The spectra are presented in the Electronic Supple-
mentary Material.
3
-Methyl-4-oxa-5-azahomoadamantane is a known compound, prepared by the
1
literature procedure [51, 52] as shown in Scheme 3. H NMR (500 MHz, CDCl ): d
3
(
ppm) 4.68 (br s, 1H), 3.58 (s, 1H), 2.07–1.97 (m, 2H), 1.95–1.68 (m, 8H),
1
3
1
.61–1.47 (m, 2H), 1.23 (s, 3H); C NMR (125.72 MHz, CDCl ): d (ppm) 80.7,
3
?
5.0, 42.0, 36.8, 34.3, 29.8, 26.3; MS m/z: 168 (M ).
5
Typical procedure for oxidative synthesis of benzimidazoles, benzoxazoles
or benzothiazoles
A mixture of 1.2 mmol o-phenylenediamine, 2-aminophenol or 2-aminothiophenol
and 1 mmol primary amine, 10 mol% FeCl , 1 mol% 3-methyl-4-oxa-5-azaho-
3
moadamantane, 5 ml H O were mixed in a 10-ml three-necked flask, then O was
2
2
bubbled into the flask at flow rate of 20 ml/min. The reaction mixture was stirred at
100 ꢁC for several hours, and reaction progress was monitored by TLC. The final
reaction mixture was cooled to room temperature and extracted with ethyl acetate.
The organic layer was washed with brine, dried over MgSO , and concentrated
4
under reduced pressure. The residue was directly purified by column chromatog-
raphy on silica gel using hexane/ethyl acetate (7:3) as eluent to afford the pure
product.
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