Solvent-dependent metal-free chemoselective synthesis of benzimidazoles and 1,3,5-triarylbenzenes from 2-amino anilines and aryl alkyl ketones catalyzed by I2

Article abstract of DOI:10.1016/j.tetlet.2021.153016

A solvent-dependent I₂-catalyzed chemoselective reaction was developed for the synthesis of benzimidazoles and 1,3,5-triarylbenzenes via the annulation of 2-amino anilines and aryl alkyl ketones or the cyclization of aryl alkyl ketones, respectively. With 1,4-dioxane as the solvent, sequential C[sbnd]N bond formation followed by C(CO)-C(CH₃) bond cleavage leads to the formation of benzimidazoles in a highly selective manner while aldol-type self-condensation of aryl alkyl ketones predominates using PhNO₂ or PhCl as the solvent to afford 1,3,5-triarylbenzenes.

Full text of DOI:10.1016/j.tetlet.2021.153016

Tetrahedron Letters 70 (2021) 153016
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Solvent-dependent metal-free chemoselective synthesis of
benzimidazoles and 1,3,5-triarylbenzenes from 2-amino anilines and
aryl alkyl ketones catalyzed by I2
Yuxin Ding a,b, Renchao Ma a, Yongmin Ma a,b,
⇑
a Institute of Advanced Studies and School of Pharmaceutical and Chemical Engineering, Taizhou University, Taizhou 318000, PR China
b School of Pharmaceutical Science, Zhejiang Chinese Medical University, Hangzhou 310053, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A solvent-dependent I2-catalyzed chemoselective reaction was developed for the synthesis of benzimida-
zoles and 1,3,5-triarylbenzenes via the annulation of 2-amino anilines and aryl alkyl ketones or the
cyclization of aryl alkyl ketones, respectively. With 1,4-dioxane as the solvent, sequential CAN bond for-
mation followed by C(CO)-C(CH3) bond cleavage leads to the formation of benzimidazoles in a highly
selective manner while aldol-type self-condensation of aryl alkyl ketones predominates using PhNO2
or PhCl as the solvent to afford 1,3,5-triarylbenzenes.
Received 28 January 2021
Revised 11 March 2021
Accepted 16 March 2021
Available online 20 March 2021
Keywords:
Ó 2021 Elsevier Ltd. All rights reserved.
Solvent-dependent
Benzimidazole
Annulation
Iodine
Chemoselectivity
Benzimidazoles represent an important and abundant class of
fused heterocycles which possess various biological activities,
including cardiovascular, antihypertensive, anticonvulsant, anti-
cancer, antihistamine, antiviral, antiulcer, anti-inflammatory, anti-
fungal and antimicrobial.[1] Representative benzimidazole-based
marketed drugs such as Thiabendazole,[2] Cambendazole,[3]
Albendazole[4] and Mebendazole[5] illustrate the importance of
such building blocks (Fig. 1). Due to their broad applications, vari-
ous efficient and practical synthetic methods have been developed
to construct these important fused heterocycles.
Conventional methods for synthesis of the benzimidazole scaf-
fold typically involve the annulation reaction of 1,2-diamines with
aryl aldehydes[6] or carboxylic acid derivatives (acids,[7] acyl chlo-
rides,[8] esters,[9] anhydrides,[10] nitriles[11]) with the aid of an
oxidant under acidic or basic conditions. Benzyl alcohols,[12] ami-
nes,[13] thiols,[14] dibenzyl disulfides[15] and methylarenes[16]
have also been employed. However, harsh reaction conditions
and expensive transition metal complexes (Au,[16c] Ru,[17] Mo,
[12a] Cu,[12c,18] Co[19]) were required for the oxidation/annula-
tion. Recently, the cyclization of 2-amino anilines with aryl alkyl
ketones mediated by I2/DMSO under metal-free conditions[20] or
oxidized by SeO2[21] has been reported (Scheme 1a). This involves
sequential CAN bond formation followed by C(CO)-C(alkyl) bond
cleavage. A similar cleavage was also applied to the reaction of
2-aminobenzamides to produce quinazolin-4(3H)-ones.[22] The
I2/DMSO mediated system has been widely used by Wu’s group
for the synthesis of several heterocyclic compounds from aryl
methyl ketones via the formation of phenylglyoxal as a key inter-
mediate[23].
1,3,5-Triarylbenzenes are very useful compounds with applica-
tions as electroluminescent materials, electrode devices, resistance
materials and conducting polymers.[24] Their synthesis typically
involves the acid-catalyzed triple condensation of aryl methyl
ketones (Scheme 1b).[25] To the best of our knowledge, the base-
catalyzed self-condensation of aryl ketones has not been reported
in the literature.
Very recently, we found that when the I2/DMSO and I2/PhNO2
systems were employed for the condensation of aryl methyl
ketones with 2-aminobenzenethiols, two different products were
afforded selectively (Scheme 1c)[26]. Intrigued by this finding,
we attempted to investigate the I2/PhNO2 system for the reaction
of 2-amino anilines with aryl alkyl ketones. Interestingly, benzim-
idazoles were afforded with a catalytic amount of I2 in the presence
of PhNO2 (1 equiv.) while 1,3,5-triarylbenzenes were formed using
PhNO2 or PhCl as the solvent (Scheme 1d). Herein, we report our
preliminary results in this area.
Initially, 40-methoxyacetophenone (1c, 1 mmol) and 2-amino
⇑
Corresponding author.
E-mail address: yongmin.ma@tzc.edu.cn (Y. Ma).
aniline (2a,
1 mmol) were selected as model substrates to
https://doi.org/10.1016/j.tetlet.2021.153016
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

Y. Ding, R. Ma and Y. Ma
Tetrahedron Letters 70 (2021) 153016
were afforded (see ESI, Table S1). We then altered the reaction con-
ditions by using the I2/PhNO2 system. After screening different sol-
vents (DMSO, DMF, PhCl, PhCH3, 1,4-dioxane), we found that the
reaction in 1,4-dioxane at 120 °C gave the highest yield (Entry 5
vs 1–4). The yield of 3ca was improved by increasing the reaction
temperature from 120 °C to 140 °C (Entry 6). However, increasing
the amount of I2 gradually decreased the yield of 3ca and only
trace amounts of 3ca was observed when the amount of I2 reached
1 equiv. (Entries 7–9). On the other hand, increasing the amount of
PhNO2 to 3 equiv. had no marked influence on the yield of 3ca
(Entry 10). Interestingly, when PhNO2 was employed as the sol-
vent, the desired benzimidazole (3ca) was afforded in only 5% yield
and 1,3,5-triarylbenzene (4c) was the main product (Entry 11).
Replacing PhNO2 with PhCl gave a similar yield (Entry 12). Since
PhNO2 has a high boiling point and is difficult to evaporate, we pre-
ferred PhCl as the solvent. In contrast to PhCl, other solvents such
as DMF, DMSO, MeCN or toluene failed to facilitate the reaction or
provided much lower yields of 4c (Entries 13–16). It is worth not-
ing that although Bathula’s group reported that benzimidazoles (3)
were synthesized from acetophenones and 2-amino anilines in
moderate to good yields when promoted by the I2/DMSO system,
[20] only trace amounts of 3ca was obtained in our case and 4c
was the major product (Entry 14). Although 2-amino aniline (2a)
was not incorporated into the structure of 4c, decreasing the
amount of 2a dramatically influenced the yield (see ESI,
Table S2). Upon prolonging the reaction time to 72 h, a moderate
yield of 4c was achieved even in the presence of 0.1 equiv. of 2a
(Table S2). Interestingly, replacing 2a with other organic or inor-
ganic bases such as aniline, ethylenediamine, meta-phenylenedi-
amine, para-phenylenediamine, 2-aminophenol, NaOH or Na2CO3,
resulted in the reaction failing to provide the 1,3,5-triarylbenzene
(Entry 17). On the other hand, in the absenceof I2, the reaction also
failed (Entry 18). These results indicated that both 2a and I2 played
Fig. 1. Selected benzimidazole-based marketed drugs.
Scheme 1. Literature reports and our work.
investigate the optimal reaction conditions (Table 1). After screen-
ing the additives and the amount of substrate 2a, only low yields of
the expected benzimidazole (3ca) or 1,3,5-triarylbenzene (4c)
Table 1
Optimization of the reaction conditions.a
Entry
Additive 1 (equiv.)
Additive 2 (equiv.)
Solvent
Time (h)
T (oC)
Yield (%)
3ca
4c
1
2
3
4
5
6
7
8
I2 (0.05)
I2 (0.05)
I2 (0.05)
I2 (0.05)
I2 (0.05)
I2 (0.05)
I2 (0.1)
I2 (0.2)
I2 (1)
I2 (0.05)
I2 (0.05)
I2 (0.05)
I2 (0.05)
I2 (0.05)
I2 (0.05)
I2 (0.05)
I2 (0.05)
–
PhNO2 (1)
PhNO2 (1)
PhNO2 (1)
PhNO2 (1)
PhNO2 (1)
PhNO2 (1)
PhNO2 (1)
PhNO2 (1)
DMSO
DMF
PhCl
PhCH3
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
PhNO2
PhCl
DMF
DMSO
MeCN
toluene
PhCl
PhCl
PhCl
PhCl
PhCl
20
20
20
20
20
20
20
20
20
20
12
12
12
12
12
12
12
12
12
12
12
120
120
120
120
120
140
140
140
140
140
120
120
120
120
120
120
120
120
80
36
trace
17
trace
52
67
63
49
trace
66
5
trace
trace
trace
0
trace
0
0
0
0
trace
trace
trace
trace
trace
trace
13
18
0
trace
73
75
trace
37
trace
trace
0
0
66
0
57
9
PhNO2 (1)
PhNO2 (3)
10
11
12
13
14
15
16
17b
18
19
20
21
–
–
–
–
–
–
–
–
–
–
–
I2 (0.05)
I2 (0.05)
I2 (0.05)
50
140
trace
a
Reagents and conditions: 1c (1 mmol), 2a (1 mmol), solvent (3 mL) and additives in the indicated amounts in a sealed tube at the indicated reaction time and
temperatures; isolated yield.
b
1,2-diaminobenzene (1 equiv.) was replaced with 1 equiv. of either aniline, ethylenediamine, meta-phenylenediamine, para-phenylenediamine, 2-aminophenol, NaOH or
Na2CO3.
2

Y. Ding, R. Ma and Y. Ma
Tetrahedron Letters 70 (2021) 153016
a crucial role in this transformation. Similar to the production of
3ca catalyzed by I2, increasing the amount of I2 decreased the yield
of 4c dramatically (see ESI, Table S3). Preliminary investigation of
the reaction temperatures showed that 120 °C was optimal for this
transformation (Entries 19–21). Based on these experiments, the
optimal reaction conditions for the production of 3ca are as
described in Entry 6 and the optimal reaction conditions for the
production of 4c are as described in Entry 12.
Scheme 2. Synthesis of 3aa from 1a’ and 1b’.
With the optimal reaction conditions in hand, we explored the
scope and limitations of this transformation using various substi-
tuted aryl methyl ketones (1) and 2-amino anilines (2) (Table 2).
We first investigated the scope of different substituted aryl methyl
ketones 1 reacting with 2-amino aniline 2a. Generally, aryl methyl
ketones bearing either an electron-donating group (EDG) such as
methyl and methoxy or an electron-withdrawing group (EWG)
such as fluoro, chloro and bromo at the ortho-, meta- or para-posi-
tion of the ring gave the desired products 3aa-oa in moderate to
good yields. It was found that the positions of both the EDG and
EWG on the aromatic ring had no appreciable influence on the
yield. It is worth noting that an aryl methyl ketone bearing a
hydroxyl group was well tolerated and gave the desired product
(3ga) in 57% yield. In addition, di-substituted aryl methyl ketones
such as 3,4-dichloro and 3,5-dimethoxy substituted ketones 1r and
1s were also well tolerated and afforded the expected products in
77% and 72% yield, respectively. Furthermore, 1-acetonaphthone
(1t), 2-acetonaphthone (1u) and heterocyclic ketones (1v–x) were
also investigated; except for 1w and 1x gave the desired products
(3ta–va) in moderate yields. However, aliphatic ketones such as
pinacolone and acetone did not afford the desired products (3ya
and 3za). Next, a series of substituted 2-amino anilines 2 were
investigated for their reaction with acetophenone 1a under the
optimized conditions (Table 2). All of the investigated substrates
(2b-e) gave moderate yields of the corresponding benzimidazoles
(3ab-ae), regardless of the electronic nature and the position of
the substituted group. To further explore the universality of the
reaction, propiophenone (1a’) and 2-phenylacetophenone (1b’)
were reacted with 2-amino aniline (2a) under the optimized **
reaction conditions and similar results to that of 1a were obtained
(Scheme 2). In this case, the ethyl and benzyl group of the substrate
1 were eliminated.
Having examined the synthesis of benzimidazoles, we then
turned our attention to exploring the scope and limitations of
the synthesis of 1,3,5-triarylbenzenes using various substituted
aryl methyl ketones. A range of substituted aryl methyl ketones
1 containing both EDG and EWG were examined (Table 3). Aryl
methyl ketones bearing either EDG or EWG at the para- or meta-
position gave the desired products 4a-l in good yields. It is
worth noting that aryl methyl ketones bearing an EDG gave
higher yields than those bearing an EWG. Among the ortho-sub-
stituted aryl methyl ketones (1 m-q) examined, only 1 m pro-
vided the desired product 4 m in a moderate yield. Although
the other ortho-substituted aryl ketones were completely con-
sumed, none of the expected products were detected by GC–
MS and only unidentified complex mixtures were afforded. In
addition, 2-acetonaphthone (1u), heterocyclic ketones (1v ꢀ x),
pinacolone (1y), propiophenone (1a’) and 2-phenylacetophenone
(1b’) were examined but only 3-acetylthiophene (1v) provided a
moderate yield of the desired product 1,3,5-triarylbenzene (4v)
(Table 3).
In order to demonstrate the suitability of this new synthetic
method on an increased scale, a gram-scale preparation of 3ba
and 4b was investigated. The 10 mmol reaction of 1b and 2a under
both sets of conditions afforded the corresponding products 3ba
and 4b in 75% and 78% yield, respectively (Scheme 3), without a
significant loss of efficiency compared to the 1 mmol scale (76%
and 81%, respectively).
At the outset of our studies, the mechanism for the formation of
1,3,5-triarylbenzenes 4 was not well understood. The synthesis of 4
usually involves the triple condensation of aryl methyl ketones
without an oxidation process.[25] However, we found that the
yield of 4 was significantly decreased when the reaction was car-
ried out under a N2 atmosphere, indicating that the reaction pro-
ceeds via an oxidation pathway. As shown in Table 1, both 2a
and I2 were essential in this transformation and the presence of
2a (from 0.1 to 1 equiv.) could accelerate the reaction (see ESI,
Table S2). Further experiments are still required to clarify the
mechanism. On the other hand, based on our previous report,[26]
a plausible reaction mechanism for the production of benzimida-
zoles is proposed in Scheme 4. Initially, the condensation of 1
and 2a generates the corresponding imine A, which undergoes
cyclization to afford intermediate B. Oxidation of the methyl group
by I2 generates intermediate C. In this process, the HI by-product
could be oxidized by nitrobenzene to regenerate I2. Subsequently,
intermediate C undergoes oxidation with PhNO2 to form oxo
derivative D. Aromatization of intermediate D produces the final
product 3.
Table 2
Substrate scope for the synthesis of 3.a
In conclusion, we have developed an I2-catalyzed annulation of
aromatic alkyl ketones and 2-amino anilines under metal-free con-
ditions. This versatile synthetic approach can selectively produce
either benzimidazoles or 1,3,5-triarylbenzenes by altering the sol-
vent. Using 1,4-dioxane as the solvent, 2-aryl benzimidazoles were
selectively obtained while the aldol-type self-condensation of aro-
matic alkyl ketones predominates using PhNO2 or PhCl as the sol-
vent to afford 1,3,5-triarylbenzenes. Further studies utilizing this
strategy are currently ongoing in our laboratory.
a
Reagents and conditions: 1 (1 mmol), 2 (1 mmol), PhNO2 (1 mmol), 1,4-dioxane
(3 mL), I2 (5 mol%), 140 °C, 20 h.
3

Y. Ding, R. Ma and Y. Ma
Tetrahedron Letters 70 (2021) 153016
Table 3
Substrate scope for the synthesis of 4.a
a
Reagents and conditions: 1 (1 mmol), 2a (1 mmol), PhCl (3 mL), I2 (5 mol%), 120 °C, 12 h.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.153016.
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