A simple and convenient two-step, one-pot synthesis of hetero-imidazoles from nitroaminoaryls catalyzed by Ytterbium triflate

Article abstract of DOI:10.1016/j.tetlet.2013.05.092

A mild two-step one-pot procedure for the conversion of ortho-nitroamino aromatic heterocycles into corresponding benzo and heteroaromatic fused imidazoles is described. The procedure utilizes iron powder, acetic acid, triethylorthoformate, and a catalytic amount of Ytterbium triflate at 75 C for the nitro group reduction and cyclization reaction. The optimum stoichiometry of each component is highlighted and the broad utility is demonstrated with high compatibility to numerous functional groups.

Full text of DOI:10.1016/j.tetlet.2013.05.092

Tetrahedron Letters 54 (2013) 4054–4057
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A simple and convenient two-step, one-pot synthesis of hetero-
imidazoles from nitroaminoaryls catalyzed by Ytterbium triflate
⇑
Fen Wang, Michelle Tran-Dubé, Stephanie Scales, Sarah Johnson, Indrawan McAlpine, Sacha Ninkovic
Pfizer Inc., World Wide Medicinal Chemistry, 10770 Science Center Drive, San Diego, CA 92121, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A mild two-step one-pot procedure for the conversion of ortho-nitroamino aromatic heterocycles into
corresponding benzo and heteroaromatic fused imidazoles is described. The procedure utilizes iron pow-
der, acetic acid, triethylorthoformate, and a catalytic amount of Ytterbium triflate at 75 °C for the nitro
group reduction and cyclization reaction. The optimum stoichiometry of each component is highlighted
and the broad utility is demonstrated with high compatibility to numerous functional groups.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 1 May 2013
Revised 17 May 2013
Accepted 20 May 2013
Available online 28 May 2013
Keywords:
One-pot
Hetero-imidazole
Benzimidazole
Ytterbium triflate
Triethylorthoformate
Introduction
The traditional method is carried out in a two-step process first
involving the reduction of the nitro group followed by condensation
The imidazole ring system is found in important biological
building blocks and substances that exhibit diverse biological and
pharmaceutical activities.1 The essential amino acid histidine, the
related compound biotin and alkaloids are well known natural
products that have the imidazole moiety.2 Separately, synthetic
imidazole containing derivatives are important antifungal, antithy-
roid, and amoebicidal drugs.2,3 Benzo and heteroaromatic fused
imidazoles such as 1 and 2 (Fig. 1), respectively, represent an
equally important class of bio-active compounds due to their ther-
apeutic and pharmaceutical properties.4,5 Various benzimidazole
scaffolds for example, possess applications in numerous different
therapeutic areas such as antifungal,6 antiviral,4c,7 antihyperten-
sive,8 anticancer,9 and antihistaminic.10 Due to the close similarity
of benzo and various heteroaromatic fused imidazoles to the pur-
ine base adenine, these structural motifs have also been utilized
and incorporated sporadically within protein kinase inhibitors.11
A number of synthetic strategies to access benzo and heterocy-
clic fused imidazoles such as 1 and 2 are well known and described
in the literature.12–14 The most common route involves condensa-
tion of an ortho-diamino substituted aromatic heterocycle with an
appropriate acid or its derivative such as a nitrile, amide or ortho-
ester.13e,14b,15 An ortho-nitroamino aromatic heterocycle can be an-
other useful starting building block which can be converted into the
corresponding benzo and heteroaromatic fused imidazoles.13d,16
with the appropriate acid or orthoester. There have also been some
reports of two-step one-pot processes in which the reduction and
cyclization are performed without isolation of the corresponding
intermediate ortho-diamine. This has been predominantly exempli-
fied in benzimidazole analog synthesis.
During our effort to synthesize 7-bromo-6-methoxy-3H-imi-
dazo[4,5-c]pyridine 5 starting from 3-bromo-2-methoxy-5-nitro-
pyridin-4-amine 3 (Fig. 2), poor yields and long reaction times
were observed using published methods. Our initial attempts re-
lied on well known two-step processes involving reduction of the
nitro group as the first step leading to 4. This was followed by
N
X
Z
N
R
Y
R
N
H
N
H
1
2
Figure 1. Benzimidazole and heteroaromatic fused imidazoles.
H
N
NO2
NH2
NH2
NH2
N
N
N
reduction
cyclization
N
O
O
O
Br
Br
Br
3
4
5
⇑
Corresponding author.
E-mail address: sacha.ninkovic@pfizer.com (S. Ninkovic).
Figure 2. Two-step strategy for azabenzimidazole synthesis.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.05.092

F. Wang et al. / Tetrahedron Letters 54 (2013) 4054–4057
4055
Table 2
condensation of the resulting 5-bromo-6-methoxypyridine-3,4-
Role of Yb(OTf)3 for the conversion of 3–5
diamine 4 under acidic conditions (Table 1, entries 1–3). Poor
yields were attributed to the unstable nature of the resulting 5-
bromo-6-methoxypyridine-3,4-diamine 4. Therefore, we shifted
our synthetic strategy toward known two-step one-pot procedures
that would enable us to ‘trap’ the unstable compound 4. Entry 4
through 11 in Table 1 describes selected published procedures that
gave unsatisfactory results.
Condition T (°C)
Product conversion (LCMS)
30 min (%) 60 min (%) 3 h (%) 24 h (%)
Yield (%)
A
B
A
B
22
22
75
75
1
20
12
59
3
20
16
81
6
25
30
23a
96
74
N/A
N/A
N/A
86
100
N/A
The unsuccessful attempts in obtaining compound 5 in ade-
quate yields lead us to initiate a new synthetic route from a slightly
modified starting material, 3-bromo-2-chloro-5-nitropyridin-4-
amine 6 (Scheme 1). It was hypothesized that replacing the elec-
tron donating methoxy group in 3 with an electron withdrawing
chloro group as in 6 would help generate a more stable diamino
pyridine intermediate as in compound 7. The most promising con-
ditions, entry 2 in Table 1, were chosen as a starting point.
Gratifyingly, the two-step procedure using Fe, acetic acid in eth-
anol to reduce compound 6 followed by Ytterbium triflate cata-
lyzed condensation with triethylorthoformate resulted in
compound 8 in 87% overall yield. However, attempts to introduce
the methoxy group leading to compound 5 were unsuccessful.
Therefore, our focus shifted back to the original synthetic route
as depicted in Figure 2. Again, we hypothesized that the unstable
nature of 5-bromo-6-methoxypyridine-3,4-diamine 4 limited the
success of these conditions in that case. Because condition 2 in Ta-
ble 1 worked very well when applied in Scheme 1, we attempted to
combine the two-step procedure into a one-pot synthesis for the
Condition A: Fe, AcOH, CH(OEt)3.
Condition B: Fe, AcOH, CH(OEt)3, Yb(OTf)3.
a
A significant unidentified byproduct was observed.
conversion of 3–5, thus avoiding the isolation of 4. Gratifyingly,
treatment of compound 3 with Fe, acetic acid, Yb(OTf)3, and tri-
ethylorthoformate at 75 °C for 3 h was successful in producing de-
sired compound 5 in 86% isolated yield.
We examined the role of Yb(OTf)3 and the results are shown in
Table 2. Both room temperature and 75 °C, conditions not contain-
ing Yb(OTf)3 (condition A) were sluggish whereas those containing
Yb(OTf)3 (condition B) afforded a better reaction profile. It is well
know that rare earth metal triflates such as Yb(OTf)3 are uniquely
strong Lewis acids compatible with polar protic solvents.17
Yb(OTf)3 clearly plays a role in accelerating the overall reaction
time in both the condensation and cyclization steps as well as
improving the isolated yields.
To better understand the importance of each component, the
stoichiometry was explored (Table 3). All reactions were performed
in 1.0 M of acetic acid. As a preliminary reaction we tested the con-
version of compounds 3–5 varying the number of equivalents of tri-
ethylorthoformate in the presence of 3 equiv of Fe and 0.01 equiv of
Yb(OTf)3 (conditions 1–3). Increasing the number of equivalents of
triethylorthoformate significantly improved the reaction profile
resulting in a better isolated yield (conditions 2 and 3). An improve-
ment of yield is also observed when the amount of iron is increased
to 5 equiv (condition 4 vs condition 2 and condition 5 vs condition
3). Further increase of iron to 7 equiv did not affect the reaction pro-
file (conditions 7 and 8). When Yb(OTf)3 is reduced to 0.005 equiv,
the reaction profile was not affected and therefore these conditions
(entry 6) were considered most desirable.
Table 1
Conditions for the synthesis of compound 5 via Figure 2
Entry
1
Conditions
Yield (%)
15
(a) Pd/C, H2, EtOAc 22 °C, 16 h
(b) HCNNH2ÁHOAc, HCOOH, EtOH, reflux, 24 h
(a) Fe, HOAc, EtOH reflux, 2.5 h
(b) CH(EtO)3, Yb(OTf)3, CH3CN, reflux, 0.5 h
(a) Pd/C, H2, EtOAc 22 °C, 16 h
2
3
23
45
(b) CH(EtO)3, Yb(OTf)3, CH3CN , reflux, 0.5 h
Fe, NH4Cl, HCOOH, 80 °C, 1 h
Fe, HCOOH 100 °C, 16 h
Pd/C, CH(EtO)3, H2, MeOH, HOAc, rt, 16 h
SnCl2, HCOOH, Microwave 130 oC, 20 min
SnCl2, HCOOH 75 °C, 24 h
SnCl2, HCOOH 100 °C, 16 h
Pt/C, HCOOH, H2O 95 oC, 1h
a
a
b
4
5
6
7
8
9
10
11
30c
5c
Having established the optimal reaction conditions, we turned
our attention to the scope of the reaction. First, we examined the
formation of benzimidazoles from ortho-nitroanilines (Table 4,
Scheme 2).
Utilizing our optimum conditions, all reactions were completed
within 3 h in excellent isolated yields ranging from 82% to 99% (Ta-
ble 4). Reactions in which the phenyl ring was substituted with
electron-withdrawing groups (entries 4 and 5) performed as well
as reactions in which the phenyl group was substituted with an
d
e
b
Pd/C, HCOOH 100 °C, 66 h
a
b
c
Formylated intermediate 4.
Des Bromo 3 and unidentified side products.
By LCMS.
Demethylated product.
Starting material.
d
e
NH2
NO2
Table 3
N
Fe, AcOH, EtOH
reflux, 2 hr, 98%
N
Reaction stoichiometry for the conversion of 3–5
Cl
NH2
Cl
NH2
Condition
Fe (equiv)
(EtO)3CH (equiv)
Yb(OTf)3 (equiv)
Yield (%)
Br
Br
a
1
2
3
4
5
6
7
8
3
3
3
5
5
5
7
7
1
3
5
3
5
5
3
5
0.01
0.01
0.01
0.01
0.01
0.005
0.01
0.01
6
7
54
64
63
84
(EtO)3CH, Yb(OTf)3
CH3CN, reflux 1.5 h
89%
86
H
N
b
N
c
N
Cl
Reactions were performed with compound 3 (150 mg, 1 equiv) in 1.0 M AcOH at
75 °C.
Br
a
Trace of product observed by LCMS with intermediate remaining.
Reaction profile by LCMS similar to condition 4. Product not isolated.
8
b
c
Reaction profile by LCMS similar to condition 6. Product not isolated.
Scheme 1. Two-step route to compound 8.

4056
F. Wang et al. / Tetrahedron Letters 54 (2013) 4054–4057
Table 4
Table 5
Formation of substituted benzimidazoles from ortho-nitroanilines
Formation of heteroaromatic-fused imidazoles from ortho-nitroamino aromatic
heterocycles
Entry
1
Substrate
NO2
Product
Yield (%)
94
Entry
Substrate
Product
Yield (%)
N
NO2
N
N
N
N
H
NH2
NO2
1
92
N
H
NH2
NO2
N
N
N
N
2
3
4
82
85
87
N
H
2
3
87
NH2
NO2
N
H
N
Cl
Cl
NH2
NO2
N
N
N
100a
N
H
O
NH2
NO2
O
N
H
NH2
NC
NC
N
NO2
NH2
H
N
N
N
N
H
N
NH2
NO2
O
N
N
O
4
83
Br
Br
5
6
87
96
O
N
H
F3C
Cl
O
F3C
Cl
NH2
NO2
NO2
NH2
N
N
N
5
6
7
32
36
78
N
H
N
N
N
H
NH2
NO2
Cl
Br
Cl
Br
Br
Br
NO2
NH2
N
N
N
N
N
N
H
N
N
NH
N
7
99
Br
Br
N
Cl
N
NO2
NH2
N
N
Cl
N
H
N
N
Br
NO2
Br
N
Fe (5 eq.)
(EtO)3CH (5 eq.)
H
N
8
9
90
87
85
NO2
NH2
N
H
N
NH2
NO2
N
R
R
I
I
Yb(OTf)3 (0.005 eq.)
75 °C, 3h
N
N
N
H
N
N
NH2
NO2
Scheme 2. One-pot conversion of nitro, amino aromatic heterocycles.
Cl
Cl
N
Cl
Cl
N
N
N
10
N
H
N
NH2
NO2
NH2
electron-donating group (entry 3). The presence of halogens was
very well tolerated (entry 6). We were particularly pleased with
the result described in entry 7 because of the potentially sensitive
and reactive nature of both the starting material and product.
Our focus then shifted to the more complex systems testing the
scope of reaction with the formation of various heteroaromatic-
fused imidazoles from ortho-nitroamino aromatic heterocycles (Ta-
ble 5). The majority of starting materials in Table 5 come from our
internal collections and are highly engineered. Both starting mate-
rials and expected products are characterized by the presence of
various sensitive moieties for which traditional two-step reduction
and cyclization conditions either failed to give the desired product
or gave very low yields. Unsubstituted nitro aminopyridine gave an
excellent yield of desired aza benzimidazole (entry 1). When our
optimized Yb(OTf)3 catalyzed conditions were applied to various
2-substituted 4-nitro-5-aminopyridines, yields varied from moder-
ately low to excellent. 2-Chloro, 2-cyclopropyl and 2-ether substi-
tuted pyridines worked equally well (entries 2–4). However, 2-
amino-substituted pyridines, such as pyrrolidine and azetidine ana-
logs (entries 5 and 6) did not perform well giving somewhat lower
yields and significant debrominated side product. When various 2-
amino-3-nitropyridines (entries 7–9) were treated with our condi-
tions, yields were again excellent demonstrating the versatility of
the one pot reduction/cyclization reaction with Yb(OTf)3. The utility
of these conditions was further expanded with 2-amino-3-nitro-
pyrazines and 4-nitro-3-aminopyridone leading to desired prod-
ucts with very good yields in both cases (entries 10 and 11).
Replacing the fused 6-membered aromatic ring with a 5-mem-
bered heterocyclic ring is known to be challenging when forming
O
O
N
N
N
11
87
N
H
NO2
S
N
S
12
13
53
54
N
H
NH2
NO2
S
N
S
N
H
NH2
NO2
N
S
S
N
H
14
61
20
NH2
O
O
O
O
O
NO2
b
N
H2N
N
15
O
NH2
N
H
a
Crude yield.
Structure was confirmed by X-ray.
b
heteroimidazole compounds.13d We were therefore very pleased
when the desired thienoimidazoles were obtained in moderate
yields (entries 12–14). In our hands, previously published methods
utilizing Palladium as a catalyst failed in these cases, presumably
due to sulfur poisoning. Finally, 3-methyl-4-nitroisoxazol-5-amine
(entry 15) is converted into 5-methyl-1H-imidazole-4-carboxam-
ide in 20% yield under these conditions. We believe that the N–O

F. Wang et al. / Tetrahedron Letters 54 (2013) 4054–4057
Barluenga, J.,(Eds.,) Wiley-VCH Verlag GmbH
4057
&
Co. KGaA, Weinheim,
O
Germany. doi: 10.1002/9783527637737.ch10.
NO2
NH2
N
H2N
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studies.
Conclusion
In conclusion, a simple two-step one-pot conversion of ortho-
nitroamino aromatic heterocycles into corresponding benzo and
heteroaromatic fused imidazoles has been optimized. The proce-
dure is quick and utilizes catalytic amounts of Ytterbium triflate
in combination with triethylorthoformate, iron as the reducing
agent and acetic acid. The conditions are mild and highly compat-
ible with a wide range of sensitive groups and therefore should
find its usefulness in organic synthesis.
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Acknowledgments
We would like to thank Professor Arnold Rheingold and Dr. Cur-
tis Moore from the UCSD Crystallography Lab for providing crystal
structures. We would also like to thank Jason Ewanicki for NMR
support as well as Alex Yanovsky.
Supplementary data
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092.
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