Transition-Metal-Free Synthesis of 1,2-diphenyl-1H-benzo[d] Imidazole Derivatives from N-phenylbenzimidamides and Cyclohexanones

Article abstract of DOI:10.1002/adsc.201901161

A transition-metal-free strategy for the formation of 1,2-diphenyl-1H-benzo[d] imidazoles from N-phenylbenzimidamides and cyclohexanones is introduced. This is the first report on the direct synthesis of 1,2-diphenyl-1H-benzo[d] imidazoles from cyclohexanones and N-phenylbenzimidamides via iodine- promoted oxidative cyclization. Non-aromatic cyclohexanones were smoothly dehydrogenated, and acted as an aryl source using oxygen as a green oxidant. The catalytic use of iodine makes this method quite simple, more economical and convenient. Under optimized conditions, various substituted 1,2-diphenyl-1H-benzo[d] imidazoles were smoothly reacted, and the desired substituted imidazoles were generated with moderate to excellent yields. (Figure presented.).

Full text of DOI:10.1002/adsc.201901161

Title: Transition-Metal-Free Synthesis of 1,2-diphenyl-1H-benzo[d]
Imidazole Derivatives from N-phenylbenzimidamides and
cyclohexanones
Authors: guoqiang lu, nan luo, fangpeng hu, zihui ban, zhenzhen zhan,
and Guosheng Huang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901161
Link to VoR: http://dx.doi.org/10.1002/adsc.201901161

10.1002/adsc.201901161
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Transition-Metal-Free Synthesis of 1,2-diphenyl-1H-benzo[d]
Imidazole Derivatives from N-phenylbenzimidamides and
cyclohexanones
Guoqiang Lu, Nan Luo, Fangpeng Hu, Zihui Ban, Zhenzhen Zhan and Guo-Sheng
Huang*
State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Nonferrous Metal Chemistry and Resources
Utilization of Gansu Province, Department of Chemistry, Lanzhou University, Lanzhou 730000, China.
Fax: 86-931-8912596; E-mail: hgs@lzu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract.
A transition-metal-free strategy for the
formation of 1,2-diphenyl-1H-benzo[d] imidazoles from
N-phenylbenzimidamides and cyclohexanones is
introduced. This is the first report on the direct synthesis
of
1,2-diphenyl-1H-benzo[d]
imidazoles
from
cyclohexanones and N-phenylbenzimidamides via
iodine- promoted oxidative cyclization. Non-aromatic
cyclohexanones were smoothly dehydrogenated, and
acted as an aryl source using oxygen as a green oxidant.
The catalytic use of iodine makes this method quite
simple, more economical and convenient. Under
optimized conditions, various substituted 1,2-diphenyl-
1H-benzo[d] imidazoles were smoothly reacted, and the
desired substituted imidazoles were generated with
moderate to excellent yields.
Keywords:
Imidazoles;
N-phenylbenzimidamides;
Cyclohexanones; Oxidative cyclization; Iodine
Benzimidazole is one of the oldest known nitrogen
heterocycles and was first synthesized by Hoebrecker
and later by Ladenberg and Wundt during 1872-
1878.[1] In nature, N-ribosyl dimethylbenzimidazole
serves as an axial ligand for cobalt ions in vitamin
B12.[2] Over years of active research, benzimidazole
has evolved as an important heterocyclic system due
to its presence in a wide range of bioactive
compounds, such as antiparasitics, analgesics,
antihypertensives, antivirals and anticancers.[3] For
example, telmisartan and candesartan are used as
antihypertensives, omeprazole is used as a proton
pump inhibitor, and as-temizole, clemizole, and
bilastine are used as anti-histaminic agents.[4]
Therefore, intensive attention has been paid to the
development of
benzimidazoles. Traditional synthesis relies on the
condensation of o-phenylenediamines with
carboxylic acids or their derivatives under harsh
dehydrating conditions by oxidative coupling with
aldehydes.[5] Meanwhile, aerobic catalytic oxidative
Scheme 1. Different strategies transformations for 1,2-
diphenyl-1H-benzo[d] imidazoles.
cross-coupling reactions employing either alcohols or
amines as substrates have also been reported for the
synthesis of benzazoles.[6] Considering these
literature precedents, and drawbacks, such as the
competitive formation of 2-substituted[7] and 1,2-
disubstituted benzimidazoles,[8] and the need for
expensive catalysts/ reagents/ starting materials, the
development of facile and practical methods for the
formation of 1,2-diphenyl-1H-benzo[d] imidazoles is
a
method for preparing
1
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10.1002/adsc.201901161
Advanced Synthesis & Catalysis
Table 1. Optimization of reaction conditions.a
the
formation
of
1,2-diphenyl-1H-benzo[d]
imidazoles using oxygen as a hydrogen acceptor.
Initially, we attempted N-phenylbenzimidamide (1a)
with cyclohexanone (2a) in the absence of metal
catalyst under an oxygen atmosphere (Table 1). When
the reaction mixture was heated at 140 °C in the
absence of the iodide-containing catalyst, no desired
3a was formed, as determined by the 1HNMR method
(entry 1). Fortunately, the desired product (3a) was
obtained with only a 15% yield in the presence of KI
(Table 1, entry 2). The yield increased to 27% when
NH4I was used as the additive (Table 1, entry 3).
Following the investigation of several iodide-
containing chemicals, iodine proved to be the most
efficient (Table 1, entry 4). Then we tested different
amounts of I2, on increasing the amount of iodine to
0.2 ,0.5 and 1.0 equivalents, the reaction yields
increased to 71%, 85% and 77% (Table 1, entries 5-7).
Based on this excellent result, with 0.5 equiv found to
be the optimal loading. In addition to 1,1,2,2-
tetrachloroethane, a lower yield was obtained when
the solvent was changed to chlorobenzene and NMP,
and no desired product was detected when DMSO
was used (Table 1, entries 8-10). Interestingly, o-DCB
gave a similar yield with 1,1,2,2-tetrachloroethane
(Table 1, entry 11). It was found that the catalyst and
solvent critically affect the reaction efficiency.
Notably, slightly lower yield (71%) of 3a was
obtained when 0.15 mmol of 2a was used (Table 1,
entry 12). When the temperature was decreased to
120 °C and the yield of the desired product 3a
showed a corresponding drop to 34% (Table 1, entry
13). In addition, reducing the time led to a decrease in
the yield of 3a (Table 1, entry 14). When the reaction
time prolonged to 36 h, the yield of product showed
almost no change (Table 1, entry 15). A much lower
yield was obtained when the reaction was carried out
in air (Table 1, entry 16). After several attempts, the
suitable reaction conditions were identified as follows：
1a (0.1 mmol), 2a (0.2 mmol) and I2 (50 mol%) in
1,1,2,2-tetrachloroethane (1 mL) at 140 °C for 24 h
under oxygen.
With the optimized reaction conditions in hand, we
investigated the substrate scope of the amidines
derivatives (1) with (2a) as a coupling partner (Table
2). As shown in Table 2, both electron-rich and
electron-withdrawing groups on the N-aryl ring (R1)
of amidines could be used smoothly to afford 1,2-
diphenyl-1H-benzo[d] imidazoles in excellent yields
(3b-3m). Amidine substituted with 2-isopropyl at R1
performed well in this reaction under the standard
conditions, and the desired product was isolated with
a yield of 68% (3k). Gratifyingly, the amidine
substituent with an electron-withdrawing group 4-
trifluoromethyl and 4-cyano were tolerated to product
3l and 3m in 69% and 64% yield. To our
disappointment, we failed to find the target product
when the benzene was changed to a tine ring (3n)
group. While1-naphthyl and 2-naphthyl amidines
gave products 3o (81% yield) and 3p (83% yield)
with remarkable yields. The desired products
Entry
Catalyst
Solvent
Yield
(%)b
0
15
27
39
71
85
77
0
13
15
73
71
34
69
84
23
1
2
3
4
5
6
7
8
9
1,1,2,2-Tetrachloroethane
1,1,2,2-Tetrachloroethane
1,1,2,2-Tetrachloroethane
1,1,2,2-Tetrachloroethane
1,1,2,2-Tetrachloroethane
1,1,2,2-Tetrachloroethane
1,1,2,2-Tetrachloroethane
DMSO
KI (0.1)
NH4I (0.1)
I2 (0.1)
I2 (0.2)
I2 (0.5)
I2 (1.0)
I2 (0.5)
I2 (0.5)
I2 (0.5)
I2 (0.5)
I2 (0.5)
I2 (0.5)
I2 (0.5)
I2 (0.5)
I2 (0.5)
Chorobenzene
10
11
12c
13d
14e
15f
NMP
o-DCB
1,1,2,2-Tetrachloroethane
1,1,2,2-Tetrachloroethane
1,1,2,2-Tetrachloroethane
1,1,2,2-Tetrachloroethane
1,1,2,2-Tetrachloroethane
16g
a
Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol),
solvent (1 mL), 140 ℃ under oxygen for 24 h, b Isolated
yield. c 0.15mmol 2a was used. d At 120 ℃. e 12 h under
f
g
oxygen. 36 h under oxygen. 12 h under air. DMSO:
dimethylsulfoxide, NMP: N-methylpyrrolidone, o-DCB:
1,2-dichlorobenzene.
still highly desirable. Although there are other
methods available for the construction of 1,2-
diphenyl-1H-benzo[d] imidazoles, the utilization of
cheap and non-aromatic coupling partners under
transition-metal-free conditions is rare and
challenging. Cyclohexanones are commercially
available, stable and widely used as raw materials to
prepare many important bulk chemicals.[9] In 2011, S.
You et al. disclosed a palladium-catalysed cascade
amination that allows regiospecific and modular
synthesis of a library of structurally diverse 1,2-
disubstituted (hetero)aryl fused imidazoles (Scheme
1a).[10] Significantly, the group of Largeron has
developed a direct method for the synthesis of 1,2-
diphenyl-1H-benzo[d] imidazoles by aerobic
oxidative C-H functionalization of primary aliphatic
amines (Scheme 1b).[11] Very recently, Isao et al.
reported
a one-pot method for the selective
construction of 1,2-diphenyl-1H-benzo[d] imidazoles
involving the dehydro-genation of benzyl alcohols by
a π-benzylpalladium(II) system (Scheme 1c).[12] The
construction of C–C,[13] C–N,[14] C–O,[15] C–S[16]
bonds as well as heterocycles has been carried out
under oxidative conditions in the Deng group. In
these transformations, cyclohexanones were used as a
versatile aryl source via
a
dehydrogenation
tautomerization sequence. Inspired by reports about
the use of cyclohexanone for the construction of
heterocyclic compounds and our experiences in the
formation of 1,2-diphenyl-1H-benzo[d] imidazoles,
herein, we describe a transition-metal-free method for
2
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10.1002/adsc.201901161
Advanced Synthesis & Catalysis
Table 2. Scope of the reaction with amidines.a
Table 3. Scope of the reaction with cyclohexanones.a
a Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol), I2 (50
o
mol%) and solvent (1 mL), 140 C under oxygen for 24 h.
b Isolated yield.
treated with N-phenylbenzimidamides (1a) under
optimized conditions. When a methyl group located
at the ortho position of cyclohexanone was employed
for this reaction, only a trace amount of the desired
product (3ae) was detected, we suspected that the 2-
methylcyclohexanone had a strong steric effect that
was not conducive to the reaction. Within the above
scope for the cyclohexanones, this result indicates
that the present method provides an alternative
pathway for the synthesis of 1,2-diphenyl-1H-
benzo[d] imidazoles from cyclohexanones.
Gram-scale
applicability of
the
present
methodology was also demonstrated (Scheme 2). A
detailed complete synthesis of 3a at the 5 mmol scale
was performed. N-phenylbenzimidamide 1a (5 mmol,
0.98 g) reacted smoothly with cyclohexanone 2a (10
mmol, 0.98 g) under the optimized reaction
conditions, giving the corresponding 1,2-diphenyl-
1H-benzo[d] imidazoles 3a with a yield of 69% (0.93
g).
a Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol), I2 (50
o
mol%) and solvent (1 mL), 140 C under oxygen for 24 h.
b Isolated yield.
(3r-3u) were obtained in moderate to excellent yields
(74- 84%) when another aryl ring (R2) was added.
Finally, it was found that disubstituted or
trisubstitutedamidines were a good substrate for this
kind of transformation to obtain the desired products
(3q, 3v-3y).
Scheme 2. Practical applicability of the present protocol.
To further explore the possible reaction mechanism,
a set of control experiments was performed. The
Next, the substrate scope for various
cyclohexanones were also investigated using (1a) as a
coupling partner (Table 3). Various 4-alkyl
cyclohexanones smoothly reacted with N-
phenylbenzimidamides (1a) to give the corresponding
products in good yields (3aa-3ab). As a challenging
reaction
of
acetophenone
with
N-
phenylbenzimidamide was performed under the
standard conditions in the presence of the radical
scavenger
2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) (Scheme 2a). The desired product 3a was
obtained with 81% yield, the yield was just slightly
decreased, which might exclude the possibility of a
radical mechanism. When 2-chlorocyclohexanone
was employed as the substrate, and the product 3a
was obtained in 74% yield (Scheme 3b), which
substrate, cyclohexanones with
a
tert-butyl
substituent were successfully applied to the reaction
(3ac). The desired product (3ad) was obtained with a
yield of 73% when 4-phenylcy-clohexanone was
3
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10.1002/adsc.201901161
Advanced Synthesis & Catalysis
Solvent was removed, and the residue was separated
by column chromatography to give a pure sample by
using mixed petroleum ether/ethyl acetate 8:1 (v/v) as
an eluent to afford the desired product 3a. The
remaining substituted imidazoles were prepared in a
similar manner.
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4
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10.1002/adsc.201901161
Advanced Synthesis & Catalysis
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5
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10.1002/adsc.201901161
Advanced Synthesis & Catalysis
UPDATE
Transition-Metal-Free Synthesis of 1,2-diphenyl-
1H-benzo[d] Imidazole Derivatives from N-
phenylbenzimidamides and cyclohexanones
Adv. Synth. Catal. Year, Volume, Page – Page
Guoqiang Lu, Nan Luo, Fangpeng Hu, Zihui Ban,
Zhenzhen Zhan and Guo-Sheng Huang*
6
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