Cobalt-Catalyzed Sustainable Synthesis of Benzimidazoles by Redox-Economical Coupling of o-Nitroanilines and Alcohols

Article abstract of DOI:10.1021/acs.joc.9b02090

This study reveals cobalt-catalyzed sustainable synthesis of benzimidazoles by redox-economical coupling of o-nitroanilines and alcohols. The major advantage of this report is the use of a commercially available cheap cobalt catalyst to produce a wide variety of 2-substituted benzimidazoles by hydrogen autotransfer without using any additional external redox reagent and costly ligand system. A thorough mechanistic insight of the reaction is proposed by performing a series of control experiments.

Full text of DOI:10.1021/acs.joc.9b02090

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Article
Cobalt-Catalyzed Sustainable Synthesis of Benzimidazoles by
Redox-Economical Coupling of o-Nitroanilines and Alcohols
Sanju Das, Samrat Mallick, and Suman De Sarkar
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b02090 • Publication Date (Web): 20 Aug 2019
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The Journal of Organic Chemistry
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Cobalt-Catalyzed Sustainable Synthesis of Benzimidazoles by Re-
dox-Economical Coupling of o-Nitroanilines and Alcohols
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Sanju Das, Samrat Mallick, and Suman De Sarkar*
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur-741246,
West Bengal, India
* Email: sds@iiserkol.ac.in
ABSTRACT: This study reveals cobalt catalyzed sustainable synthesis of benzimidazoles by redox-economical coupling of
o-nitroanilines and alcohols. The major advantage of this report is the use of a commercially available cheap cobalt-catalyst
to produce a wide variety of 2-substituted benzimidazoles by hydrogen autotransfer without using any additional external
redox reagent and costly ligand system. A thorough mechanistic insight of the reaction is proposed by performing a series of
control experiments.
INTRODUCTION
Aza-heterocycles are structurally important building blocks
in pharmaceuticals, agrochemicals and materials chemis-
try.1 Among them, benzimidazole and it’s derivatives are the
crucial structural motif for the pharmaceutical manufactur-
ing, due to their potent bio-activities such as antiviral, anti-
fungal, antiulcer, anticancer, antihelmintic activity etc.2 Nu-
merous essential medicines like omeprazole, astemizole
telmisartan etc. and several important drugs that show
prominent action against the treatment of viruses such as
HIV, or influenza comprise the benzimidazole scaffold in the
molecule (Figure 1).2a In addition, benzimidazoles show
several other applications in bulk material industries for the
preparation of optical brighteners, pigments, thermostable
membranes, etc.3
Figure 1. Therapeutically important benzimidazoles.
Due to the important pharmaceutical activities and other in-
dustrial applications, immense attention has been paid for
the synthesis of substituted benzimidazole molecules.4 Clas-
sically, two general methods are reported which comprise
the reaction of substituted o-phenylenediamines with car-
boxylic acid derivatives at high temperature and under
Scheme 1. Methods for the synthesis of benzimidazoles.
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strong acidic conditions (Scheme 1).5 The second method Table 1. Optimization of the reaction conditionsa
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involves the use of substituted aldehydes or alcohols and
o-phenylenediamines under oxidative reaction conditions.6
Both these approaches, although widely applied, suffer
from the limited functional group tolerance and generation
of stoichiometric quantities of waste, from the leaving
groups, oxidants, additives etc.
To circumvent the issues mentioned above, sustainable pro-
tocols for the synthesis of benzimidazoles from renewable
materials are on high demand. Precious metal catalyzed ac-
tivation of abundantly available alcohols by hydrogen trans-
fer to a suitable hydrogen acceptor7 or acceptorless dehy-
drogenation,8 offers practical route for the synthesis of ben-
zimidazoles (Scheme 1).9 Even though major developments
have been reported in this field using noble metals, the cur-
rent trend has flitted towards the utilization of earth-abun-
dant, non-precious base metals.10 In this context, different
catalysts based on first-row transition metals were also em-
ployed in the synthesis of benzimidazole by acceptorless
dehydrogenation of alcohols.11
Sl
Catalyst
Base
T
[OC]
Solvent
Yield
[%]b
No.
1
2
3
4
Co(acac)2
-
NaOtBu
NaOtBu
NaOtBu
135
135
135
Toluene
Toluene
DMF
60
9
Trace
NR
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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60
Co(acac)2
Co(acac)2 NaOtBu
135 1,4-Di-
oxane
73
5
Co(acac)2
Co(acac)2
KOtBu
135
135
135
135
110
135
135
135
135
135
1,4-Di-
oxane
52
6
K3PO4
1,4-Di-
oxane
Trace
55
7
.
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
NaOtBu
1,4-Di-
oxane
CoCl2
Apart from aromatic diamines which are prone to aerial ox-
idation, more stable o-nitroanilines has been studied as via-
ble starting material for the synthesis of benzimidazole de-
rivatives using different precious metal catalysts.12 Iron cat-
alysts have been used in the redox condensation of o-sub-
stituted nitrobenzenes with alkylamines or alcohols, to
form 2-arylbenzimidazoles by using sulfur derivatives as an
external reductant.13 Cobalt bromide was used as a catalyst
for the synthesis of benzimidazoles by reduction of nitro-
benzenes, however less abundant benzylamines were used
as the reductant cum coupling partner.14 Recently, a cobalt
nanoparticle based heterogenous catalytic system was re-
ported for the synthesis of amines and imines including a
single example of benzimidazole by reductive coupling of
nitroarenes with alcohols.15 It is evident that even though
several protocols based on first-row transition metals are
reported for the preparation of benzimidazoles, but in most
of the cases decorative ligand framework is required, which
makes the overall protocol expensive. Thus, an environ-
mentally benign and economical route to the synthesis of
benzimidazoles is still highly desirable. Herein, we report
the aforesaid reaction, catalyzed by commercially available
Co(acac)2, to selectively produce 2-substituted benzimidaz-
oles by transfer hydrogenation without using any external
redox reagent and costly ligand system.
4H2O
NiCl2
8
1,4-Di-
oxane
Trace
47
9
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
Co(acac)2
1,4-Di-
oxane
10c
11d
12e
13f
14g
1,4-Di-
oxane
40
1,4-Di-
oxane
28
1,4-Di-
oxane
38
1,4-Di-
oxane
24
1,4-Di-
oxane
72
aReaction conditions: 1a (0.5 mmol), 2a (1.0 mmol),
Co(acac)2 (5 mol%), NaOtBu (0.5 mmol) in 1,4-dioxane (2 mL),
under Ar for 24 h. bIsolated yield. cReaction time 16 h. d1 equiv-
alent 2a was used. eReaction performed under air. f0.5 equiva-
lent NaOtBu was used. g1.5 equivalent NaOtBu was used.
Further study reveals that KOtBu as a base is less effective,
while the weak phosphate base was inefficient (entries 5
and 6) using the same solvent, which implies that deproto-
nation of the alcohol is necessary for the hydrogen transfer
process. Other cobalt salts like CoCl2.4H2O also catalyzes the
process with lesser yield, but NiCl2 was found to be com-
pletely unproductive (entries 7 and 8). Lowering the tem-
perature or reaction time had a negative impact on the re-
action and lower yield of 4aa was obtained. Second equiva-
lent of alcohol was found to be necessary for the complete
reduction of the nitro group (entry 11). Diminished yield
was obtained when the reaction was performed under air
(entry 12). This is most likely due to the interruption by the
molecular oxygen in the hydrogen transfer from alcohol to
nitro group. Finally, the amount of base was varied and
equimolar quantity with respect to 2-nitroaniline was
found to be optimum.
RESULTS AND DISCUSSION
We began the optimization study by treating 2-nitroaniline
1a (1.0 equiv) and benzyl alcohol 2a (2.0 equiv) in various
solvents at 135 °C for 24 h in presence of different metal
catalysts (5 mol%) and NaOtBu (1.0 equiv) under an argon
atmosphere (Table 1). Reactions were performed in a
closed vial which enforces the oxidation of alcohol and re-
duction of nitro by hydrogen transfer process without using
any external redox reagent. The first reaction in toluene
using Co(acac)2 (3) as catalyst provided 60% isolated yield
of the targeted benzimidazole 4aa (entry 1). The metal cat-
alyst was found to be necessary as only a trace amount of
4aa was detected in the absence of 3. More polar solvents
like DMF was ineffective; however, 1,4-dioxane was found
to be optimum for this reaction (entry 4).
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were observed. 2-Phenyl ethanol provided 2-phenylquinox-
Table 2. Substrate scope using o-nitroaniline and vari-
ous alcoholsa,b
aline 5. Whereas, quinoxalinone derivative (6) was formed
with n-pentanol in moderate yield.
1
2
Next, we studied the scope of several 2-nitroanilines in the
redox-mediated annulation reaction (Table 3). Notably, be-
cause of the fast tautomerization in the benzimidazole 4,
both 4- and 5-substituted 2-nitroaniline derivatives yielded
identical products. A good yield of the benzimidazole 4ba
was obtained with methyl substitution. In the case of halo-
gen substituted nitroanilines moderate yield was observed
(4ca and 4da) without any dehalogenation. Comparatively
lower productivity was noticed in the case of electron rich
benzimidazole 4ea. Disubstituted nitroaniline 1f smoothly
underwent coupling with benzyl alcohol. To verify the
generality, different combinations of coupling partners
were tested in the annulation reaction. The reaction be-
tween 1d and meta-substituted 2e delivered the targeted
product in good yield. Similar efficiency was observed in the
case of 4bl, 4dk, and 4fg. Finally, 4-methyl-2-nitroaniline
was tested in combination with two different alcohols and
moderate to good yields (4bn and 4bo) were obtained.
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Table 3. Substrate scope using o-nitroaniline deriva-
tives and various alcoholsa,b
aReaction conditions: 1a (0.5 mmol), 2 (1.0 mmol), Co(acac)2
(5 mol%), NaOtBu (0.5 mmol) in 1,4-dioxane (2 mL), under Ar
for 24 h. bIsolated yield.
The scope of different alcohols in the annulation reaction
was investivated under optimized reaction condition (Table
2). Electron-rich 4-methyl or 4-methoxy substituted alco-
hols (2b and 2c) underwent smooth conversion to the cor-
responding benzimidazole products in good yields. 4-Chlo-
robenzyl alcohol (2d) was also a competent substrate for
the targeted heterocycle synthesis. Good yields were
achieved for 3-methoxybenzyl alcohol (2e) and
3-chlorobenzyl alcohol (2f). However, comparatively lesser
yield was obtained with 3-fluorobenzyl alcohol (2g). We
were pleased to observe that sterically more demanding 2-
substituted benzyl alcohols were also suitable substrates
for the cyclization reaction. Both 2-methyl and 2-bromo-
benzyl alcohols offered decent yields of the benzimidazole
products (4ah and 4ai) without any detectable dehalogen-
ation. Notably, free phenolic OH group in 2j was also found
to be compatible under the reaction condition. α-Naphthyl
substituted benzimidazole 4ak was synthesized in excellent
yield. Biheteroaryl 4al was synthesized from corresponding
coupling partners in moderate yield. Lastly, the aliphatic al-
cohol 2m underwent annulation with 1a retaining of the
strained cyclopropyl ring. Surprisingly, in case of other ali-
phatic alcohols, formation of six membered heterocycles
aReaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), Co(acac)2
(5 mol%), NaOtBu (0.5 mmol) in 1,4-dioxane (2 mL), under Ar
for 24 h. bIsolated yield.
Several control experiments were performed to inspect the
mechanism of the hydrogen transfer process. o-Phenylene-
diamine (1a’) was treated with benzyl alcohol under stand-
ard reaction condition (Scheme 2a), product formation was
completely shut down in the absence of the hydrogen accep-
tor nitro functionality. Similarly, the nitro reduction didn’t
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occur in the absence of the hydrogen donor alcohol thus
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Yield (%)
Linear Fit of Yield (%)
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4
5
6
7
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didn’t lead to any detectable benzimidazole product when
aldehyde 2a’ was used instead of the alcohol 2a (Scheme
2b). Notably, only a trace amount of 4aa was detected when
pre-reduced 1b’ was reacted with pre-oxidized 2a’ under
standard reaction condition (Scheme 2c). This result indi-
cated the necessity of the hydrogen acceptor nitro group not
only for the oxidation of the alcohol but also for the final ox-
idative aromatization of intermediate 4aa’ to form 4aa
(Scheme 3). Very interestingly, the same diamine 1b’ pro-
duced a significant amount of benzimidazole 4ba in the
presence of 1e which acts as the hydrogen acceptor
(Scheme 2d). After reduction, the corresponding diamine of
1e also takes part in the annulation and generates 4ea. Fi-
nally, the electronic effect in the alcohol partner was tested,
and preference for the electron-rich 2c was observed
(Scheme 2e). This result indicates a faster oxidation rate by
stabilization of the transition state during hydride transfer
in the electron-rich system. Finally, smooth formation of
benzimidazole (4aa) in presence of mercury drop discards
the catalysis by the formation of heterogeneous cobalt na-
noparticles (Scheme 2f).
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Time (hr)
Figure 2. Kinetic plot for the cobalt catalyzed synthesis of
benzimidazole 4ba.
A quantitative analysis on the progress of the reaction was
studied (Fig 2). The time dependent product formation ex-
hibited a linear growth with increasing the reaction time.
Careful analysis of the crude reaction mixture reveals that
apart from the desired benzimidazole, the intermediate
products like o-phenylenediamine 1a’, aldehyde (2a’) and
corresponding imine are present in the mixture. Based on
these studies, the plausible mechanism of the hydrogen
transfer and cyclization process is depicted in scheme 3.
Base assisted oxidation of alcohol 2a leads to aldehyde 2a’
and [Co]-H intermediate, which reduces the nitroaniline to
diamine 1a’. Condensation between 1a’ and 2a’ produces
the dihydro-benzimidazole 4aa’ which upon oxidative aro-
matization delivered the product 4aa. Notably, three equiv-
alents of hydride are required to reduce the nitro function-
ality. Two of those are generating from the oxidation of ex-
cess alcohol, and the third equivalent is coming from the fi-
nal dehydrogenation of 4aa’ to balance the stoichiometry.
Scheme 2. Mechanistic studies.
Scheme 3. Proposed mechanism.
CONCLUSION
In summary, a cobalt catalyzed practical synthesis of ben-
zimidazoles by the redox-economical coupling of o-nitroan-
ilines and alcohols is reported. The developed hydrogen
autotransfer process allows the reduction of nitro and oxi-
dation of alcohol functionality, thus doesn’t require any ex-
ternal redox reagent. The main highlight of this approach is
the direct use of commercially available cheap Co(acac)2 as
catalyst without any additional ligand system. This proce-
dure exhibits a sustainable and versatile synthetic method
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which has the potential for further process developments
using earth-abundant base metal catalysts.
2-(p-Tolyl)-1H-benzo[d]imidazole (4ab):11a GP was fol-
1
2
3
4
5
6
7
8
lowed using cobalt catalyst (3) (6.5 mg, 5.0 mol%), 2-ni-
troaniline (1a) (69 mg, 0.5 mmol), and p-tolylmethanol (2b)
(122 mg, 1.0 mmol). After 24 h, purification by column chro-
matography using 14% ethyl acetate in hexanes yielded
EXPERIMENTAL SECTION
General Information. Catalytic reactions were performed
under an argon atmosphere using pre-dried glassware and
standard sealed tubes. 1,4-Dioxane was dried with calcium
hydride and freshly distilled under argon. All substituted o-
nitroanilines (1a–1f) and alcohols (2a–2m) were obtained
from commercial sources and were used without further
purification. Yields refer to isolated compounds, estimated
to be >95% pure as determined by 1H NMR. Thin-layer chro-
matography (TLC) was performed on Merck pre-coated sil-
ica gel 60 F254 aluminum sheets with detection under UV
light at 254 nm. Chromatographic separations were carried
out on chempure silica gel (100−200 mesh). Melting points
(mp) were taken on a Labtronics LT-110 capillary melting
point apparatus. Nuclear magnetic resonance (NMR) spec-
troscopy was performed using JEOL 400 MHz and Bruker
500 MHz spectrometers and HRMS was performed on
Bruker Maxis Impact mass spectrometer (TOF). If not oth-
erwise specified, chemical shifts (δ) are provided in ppm.
1
(4ab) (73 mg, 70%) as a white solid (mp 259-261°C). H
NMR (400 MHz, CDCl3) δ 8.11 (d, J = 9.0 Hz, 2H), 7.46 (d, J =
8.2 Hz, 1H), 7.40 – 7.32 (m, 4H), 6.99 (d, J = 8.2 Hz, 1H), 2.40
(s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 151.3, 138.2,
130.5, 130.1, 128.8, 127.0, 123.6, 122.0, 21.1.
9
2-(4-Methoxyphenyl)-1H-benzo[d]imidazole (4ac):11a
GP was followed using cobalt catalyst (3) (6.5 mg, 5.0
mol%), 2-nitroaniline (1a) (69 mg, 0.5 mmol), and (4-meth-
oxyphenyl)methanol (2c) (138 mg, 1.0 mmol). After 24 h,
purification by column chromatography using 18% ethyl
acetate in hexanes yielded (4ac) (79 mg, 71%) as a yellow
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1
liquid. H NMR (400 MHz, CDCl3 + DMSO-d6) δ 8.08 (d, J =
7.9 Hz, 2H), 7.61 – 7.53 (m, 2H), 7.17 (m, 2H), 6.94 (d, J = 8.9
Hz, 2H), 3.82 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3 + DMSO-
d6) δ 161.0, 152.1, 139.0, 128.4, 122.8, 122.2, 115.0, 114.3,
55.4.
2-(4-Chlorophenyl)-1H-benzo[d]imidazole (4ad):11a GP
was followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%),
2-nitroaniline (1a) (69 mg, 0.5 mmol), and (4-chloro-
phenyl)methanol (2d) (143 mg, 1.0 mmol). After 24 h, puri-
fication by column chromatography using 18% ethyl ace-
tate in hexanes yielded (4ad) (77 mg, 67%) as a white solid
(mp. 290-292 °C). 1H NMR (400 MHz, DMSO-d6) δ 12.94 (br
s, 1H), 8.19 (d, J = 8.5 Hz, 2H), 7.75 – 7.46 (m, 4H), 7.22 (dd,
J = 6.0, 3.1 Hz, 2H). 13C{1H} NMR (101 MHz, DMSO-d6) δ
150.7, 135.3, 129.7, 129.1, 128.7, 123.1, 119.3, 112.1.
Procedure for Optimization Study (Table 1):
o-Nitroaniline (1a) (0.50 mmol, 1.0 equiv), phenylmethanol
(2a) (1.0 mmol, 2 equiv), metal catalyst (5 mol%), and base
(0.50 mmol, 1 equiv) were placed in a pre-dried 15 mL
sealed tube. The tube was degassed and purged with argon
three times. Then solvent (2.0 mL) was added, and the mix-
ture was stirred at (110-135) °C in an oil bath for (16-24) h.
At ambient temperature, EtOAc (8 mL) was added, and the
reaction mixture was washed with brine (5 mL), dried over
sodium sulfate, concentrated under a vacuum, and purified
by silica gel column chromatography using 15% ethyl ace-
tate in hexanes to get 2-phenyl-1H-benzo[d]imidazole
(4aa) (40-73)% as an isolated product.
2-(3-Methoxyphenyl)-1H-benzo[d]imidazole (4ae):17
GP was followed using cobalt catalyst (3) (6.5 mg, 5.0
mol%), 2-nitroaniline (1a) (69 mg, 0.5 mmol), and (3-meth-
oxyphenyl)methanol (2e) (138 mg, 1.0 mmol). After 24 h,
purification by column chromatography using 18% ethyl
acetate in hexanes yielded (4ae) (77 mg, 69%) as a brown
General Procedure for Cobalt catalyzed Synthesis of
Benzimidazoles (GP):
1
solid (mp. 200-202 °C). H NMR (400 MHz, CDCl3 + DMSO-
o-Nitroaniline derivatives 1 (0.50 mmol, 1.0 equiv), substi-
tuted benzyl alcohols 2 (1.0 mmol, 2 equiv), Co(acac)2 (3)
(6.5 mg, 5 mol%), and NaOtBu (48 mg, 0.50 mmol, 1 equiv)
were placed in a pre-dried 15 mL sealed tube. The tube was
degassed and purged with argon three times. Then 1,4-di-
oxane (2.0 mL) was added, and the mixture was stirred at
135 °C in an oil bath for 24 h. At ambient temperature,
EtOAc (8 mL) was added, and the reaction mixture was
washed with brine (5 mL), dried over sodium sulfate, con-
centrated under a vacuum, and purified by silica gel column
chromatography using 10−20% ethyl acetate in hexanes to
deliver the benzimidazoles 4.
d6) δ 7.76 – 7.68 (m, 2H), 7.64 – 7.55 (m, 2H), 7.31 (t, J = 8.0
Hz, 1H), 7.18 (dd, J = 6.0, 3.1 Hz, 2H), 6.92 (dd, J = 8.3, 1.9 Hz,
1H), 3.78 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3 + DMSO-d6)
δ 160.0, 151.9, 138.4, 131.6, 129.9, 122.5, 119.2, 116.4,
115.2, 111.6, 55.4.
2-(3-Methoxyphenyl)-1H-benzo[d]imidazole (4af):18 GP
was followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%),
2-nitroaniline (1a) (69 mg, 0.5 mmol), and (3-chloro-
phenyl)methanol (2f) (143 mg, 1.0 mmol). After 24 h, puri-
fication by column chromatography using 18% ethyl ace-
tate in hexanes yielded (4af) (66 mg, 58%) as a white solid.
1
(mp 233-235 °C). H NMR (400 MHz, CDCl3 + DMSO-d6) δ
2-Phenyl-1H-benzo[d]imidazole (4aa):11a,16 GP was fol-
lowed using cobalt catalyst (3) (6.5 mg, 5.0 mol%), 2-ni-
troaniline (1a) (69 mg, 0.5 mmol), and phenylmethanol
(2a) (108 mg, 1.0 mmol). After 24 h, purification by column
chromatography using 15% ethyl acetate in hexanes
yielded (4aa) (71 mg, 73%) as a white solid (mp 288-291
°C). 1H NMR (400 MHz, DMSO-d6) δ 12.93 (br s, 1H), 8.19 (d,
J = 7.2 Hz, 2H), 7.68 (d, J = 7.3 Hz, 1H), 7.59 – 7.41 (m, 4H),
7.32 – 7.12 (m, 2H). 13C{1H} NMR (101 MHz, DMSO-d6) δ
151.6, 139.1, 130.7, 129.8, 129.5, 126.9, 123.0, 115.4.
8.16 (s, 1H), 8.04 (dd, J = 5.6, 2.5 Hz, 1H), 7.57 (dd, J = 5.5,
3.0 Hz, 2H), 7.32 (d, J = 4.7 Hz, 2H), 7.18 (dd, J = 6.0, 3.1 Hz,
2H). 13C{1H} NMR (101 MHz, CDCl3 + DMSO-d6) δ 150.5,
134.7, 132.1, 130.1, 129.6, 126.8, 125.0, 122.7, 115.4.
2-(3-Fluorophenyl)-1H-benzo[d]imidazole (4ag):19 GP
was followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%),
2-nitroaniline (1a) (69 mg, 0.5 mmol), and (3-fluoro-
phenyl)methanol (2g) (126 mg, 1.0 mmol). After 24 h, puri-
fication by column chromatography using 12% ethyl
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acetate in hexanes yielded (4ag) (51 mg, 48%) as a yellow
286 °C). 1H NMR (400 MHz, CDCl3) δ 7.63 (dd, J = 5.9, 3.2 Hz,
liquid. 1H NMR (400 MHz, CDCl3) δ 7.84 – 7.75 (m, 2H), 7.65
(dt, J = 6.5, 3.3 Hz, 2H), 7.44 (td, J = 8.2, 6.0 Hz, 1H), 7.32 –
7.27 (m, 2H), 7.15 (td, J = 8.4, 2.0 Hz, 1H). 13C{1H} NMR (101
MHz, DMSO- d6) δ 162.5 (d, J = 243.5 Hz), 150.0, 149.9,
132.5, 132.42, 131.2 (d, J = 8.4 Hz), 122.6 (d, J = 2.5 Hz),
116.7 (d, J = 21.1 Hz), 113.1 (d, J = 23.4 Hz). 19F NMR (376
MHz, CDCl3) δ -112.6 (dd, J = 13.7, 9.0 Hz).
2H), 7.54 (d, J = 1.3 Hz, 1H), 7.30 – 7.27 (m, 2H), 7.23 (d, J =
3.5 Hz, 1H), 6.58 (dd, J = 3.6, 1.7 Hz, 1H). 13C{1H} NMR (101
MHz, DMSO-d6) δ 151.3, 143.8, 135.0, 130.2, 129.9, 129.0,
126.5, 122.6, 121.7, 118.9, 111.4.
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7
8
2-Cyclopropyl-1H-benzo[d]imidazole (4am):23 GP was
followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%), 2-ni-
troaniline (1a) (69 mg, 0.5 mmol), and cyclopropylmetha-
nol (2m) (72 mg, 1.0 mmol). After 24 h, purification by col-
umn chromatography using 10% ethyl acetate in hexanes
yielded (4am) (47 mg, 59%) as a white solid (mp 227-
229°C). 1H NMR (400 MHz, DMSO-d6) δ 12.12 (br s, 1H), 7.44
– 7.35 (m, 2H), 7.12 – 7.02 (m, 2H), 2.09 (tt, J = 8.1, 5.2 Hz,
1H), 1.03 (ddd, J = 9.2, 5.4, 2.4 Hz, 4H). 13C{1H} NMR (126
MHz, DMSO-d6) δ 156.9, 120.9, 120.8, 114.1, 9.37, 8.6.
2-(o-Tolyl)-1H-benzo[d]imidazole (4ah):20 GP was fol-
lowed using cobalt catalyst (3) (6.5 mg, 5.0 mol%), 2-ni-
troaniline (1a) (69 mg, 0.5 mmol), and o-tolylmethanol (2h)
(122 mg, 1.0 mmol). After 24 h, purification by column chro-
matography using 12% ethyl acetate in hexanes yielded
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(4ah) (65 mg, 62%) as a white solid (mp 223-224 °C). H
NMR (400 MHz, CDCl3 + DMSO-d6) δ 7.58 – 7.47 (m, 3H),
7.30 – 7.21 (m, 2H), 7.20 – 7.11 (m, 3H), 2.50 (s, 3H). 13C{1H}
NMR (101 MHz, CDCl3 + DMSO-d6) δ 152.4, 137.0, 130.7,
130.2, 129.6, 129.1, 125.5, 121.9, 114.9, 20.4.
2-Phenylquinoxaline (5):24 GP was followed using cobalt
catalyst (3) (6.5 mg, 5.0 mol%), 2-nitroaniline (1a) (69 mg,
0.5 mmol), and 2-phenylethan-1-ol (122 mg, 1.0 mmol). Af-
ter 24 h, purification by column chromatography using 5%
ethyl acetate in hexanes yielded (5) (45 mg, 44%) as a yel-
low solid (mp 80-82 °C): 1H NMR (400 MHz,CDCl3) δ 9.34 (s,
1H), 8.22 – 8.08 (m, 4H), 7.85 – 7.73 (m, 2H), 7.65 – 7.48 (m,
3H). 13C{1H} NMR (101 MHz, CDCl3) δ 152.0, 143.5, 142.5,
141.7, 136.9, 130.5, 130.4, 129.8, 129.7, 129.3, 127.7.
2-(2-Bromophenyl)-1H-benzo[d]imidazole (4ai):20 GP
was followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%),
2-nitroaniline (1a) (69 mg, 0.5 mmol), and (2-bromo-
phenyl)methanol (2i) (187 mg, 1.0 mmol). After 24 h, puri-
fication by column chromatography using 12% ethyl ace-
tate in hexanes yielded (4ai) (81 mg, 59%) as a white solid
1
(mp 241-244 °C). H NMR (400 MHz, CDCl3 + DMSO- d6) δ
3-Propylquinoxalin-2(1H)-one (6):25 GP was followed us-
ing cobalt catalyst (3) (6.5 mg, 5.0 mol%), 2-nitroaniline
(1a) (69 mg, 0.5 mmol), and butan-1-ol (75mg, 1.0 mmol).
After 24 h, purification by column chromatography using
10% ethyl acetate in hexanes yielded (6) (34 mg, 36%) as a
white solid (mp 183-185 °C): 1H NMR (400 MHz, CDCl3) δ
11.56 (s, 1H), 7.85 – 7.81 (m, 1H), 7.48 (t, J = 7.4 Hz, 1H),
7.36 – 7.28 (m, 2H), 2.98 – 2.94 (m, 2H), 1.89 – 1.80 (m, 2H),
1.08 (t, J = 7.3 Hz, 3H). 13C NMR{1H} (101 MHz, CDCl3) δ
161.9, 156.4, 133.0, 131.0, 129.8, 128.9, 124.3, 115.6, 35.6,
20.4, 14.2.
7.98 (dd, J = 7.7, 1.4 Hz, 1H), 7.65 – 7.59 (m, 3H), 7.35 (dd, J
= 11.3, 3.8 Hz, 1H), 7.24 (m, 3H). 13C{1H} NMR (101 MHz,
CDCl3 + DMSO-d6) δ 150.3, 140.6, 133.8, 132.7, 131.5, 131.0,
127.8, 122.9, 121.1, 116.4.
2-(1H-Benzo[d]imidazol-2-yl)phenol (4aj):21 GP was fol-
lowed using cobalt catalyst (3) (6.5 mg, 5.0 mol%), 2-ni-
troaniline (1a) (69 mg, 0.5 mmol), and 2-(hydroxyme-
thyl)phenol (2j) (124 mg, 1.0 mmol). After 24 h, purification
by column chromatography using 20% ethyl acetate in hex-
anes yielded (4aj) (66 mg, 63%) as a brown solid (mp 227-
228 °C). 1H NMR (400 MHz, CDCl3 + DMSO-d6) δ 12.59 (br s,
2H), 7.87 (dd, J = 7.7, 1.2 Hz, 1H), 7.54 (d, J = 3.0 Hz, 2H),
7.30 – 7.23 (m, 1H), 7.20 (ddd, J = 5.9, 2.9, 1.4 Hz, 2H), 7.02
(d, J = 8.4 Hz, 1H), 6.88 (dd, J = 11.3, 4.1 Hz, 1H).13C{1H} NMR
(101 MHz, CDCl3 + DMSO-d6) δ 158.8, 152.1, 131.4, 125.8,
122.7, 122.6, 118.8, 118.7, 117.6, 112.9.
6-Methyl-2-phenyl-1H-benzo[d]imidazole (4ba):11a GP
was followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%),
4-methyl-2-nitroaniline (1b) (76 mg, 0.5 mmol), and phe-
nylmethanol (2a) (108 mg, 1.0 mmol). After 24 h, purifica-
tion by column chromatography using 12% ethyl acetate in
hexanes yielded (4ba) (76 mg, 73%) as a pale yellow solid
2-(Naphthalen-1-yl)-1H-benzo[d]imidazole (4ak):11a GP
was followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%),
2-nitroaniline (1a) (69 mg, 0.5 mmol), and naphthalen-1-
ylmethanol (2k) (158 mg, 1.0 mmol). After 24 h, purifica-
tion by column chromatography using 12% ethyl acetate in
hexanes yielded (4ak) (99 mg, 81%) as a white solid (mp.
216-218 °C). 1H NMR (400 MHz, DMSO-d6) δ 12.93 (br s,
1H), 9.11 (d, J = 8.1 Hz, 1H), 8.10 (d, J = 8.1 Hz, 1H), 8.07 –
8.00 (m, 2H), 7.78 (d, J = 7.6 Hz, 1H), 7.73 – 7.56 (m, 4H),
7.32 – 7.21 (m, 2H). 13C{1H} NMR (101 MHz, DMSO-d6) δ
151.4, 143.9, 134.5, 133.6, 130.5, 130.2, 128.4, 127.9, 127.5,
127.1, 126.4, 125.3, 122.7, 121.6, 119.1, 111.4.
(mp 243-244 °C). H NMR (400 MHz, CDCl3 + DMSO-d6) δ
1
8.16 – 7.98 (m, 2H), 7.52 (d, J = 8.2 Hz, 1H), 7.44 – 7.37 (m,
4H), 7.08 (dd, J = 8.3, 0.9 Hz, 1H), 2.45 (s, 3H). 13C{1H} NMR
(101 MHz, CDCl3 + DMSO-d6) δ 152.7, 138.8, 137.4, 131.0,
130.6, 130.0, 129.4, 125.8, 122.2, 115.2, 20.7.
Gram Scale Synthesis of 4ba: 4-methyl-2-nitroaniline
(1b) (1 gm, 6.58 mmol), and phenylmethanol (2a) (1.42 gm,
13.16 mmol) Co(acac)2 (3) (85 mg, 5 mol%), and base (632
mg, 6.58 mmol, 1 equiv) were placed in a pre-dried 150 mL
sealed tube. The tube was degassed and purged with argon
three times. Then solvent (27 mL) was added, and the mix-
ture was stirred at 135 °C in an oil bath for 24 h. After com-
pletion of the reaction the solvent was evaporated and con-
centrated under a vacuum, and purified by silica gel column
chromatography using 15% ethyl acetate in hexanes to ob-
tain 6-methyl-2-phenyl-1H-benzo[d]imidazole (4ba) (780
mg, 57%) as an isolated product.
2-(Furan-2-yl)-1H-benzo[d]imidazole (4al):22 GP was
followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%), 2-ni-
troaniline (1a) (69 mg, 0.5 mmol), and furan-2-ylmethanol
(2l) (98 mg, 1.0 mmol). After 24 h, purification by column
chromatography using 12% ethyl acetate in hexanes
yielded (4al) (42 mg, 46%) as a pale yellow solid (mp. 284-
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The Journal of Organic Chemistry
6-Bromo-2-phenyl-1H-benzo[d]imidazole (4ca):22 GP
acetate in hexanes yielded (4bl) (51 mg, 52%) as a white
solid (mp 188- 192°C). 1H NMR (400 MHz, DMSO-d6) δ
12.42 ( br s, 1H), 7.92 (d, J = 1.3 Hz, 1H), 7.44 (d, J = 8.2 Hz,
1H), 7.34 (s, 1H), 7.16 (d, J = 3.3 Hz, 1H), 7.02 (dd, J = 8.3, 1.0
Hz, 1H), 6.71 (dd, J = 3.3, 1.8 Hz, 1H), 2.41 (s, 3H). 13C{1H}
NMR (101 MHz, DMSO-d6) δ 145.7, 144.4, 143.3, 138.4,
131.5, 131.4, 123.7, 115.0, 114.0, 112.3, 110.2, 21.3.
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was followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%),
4-bromo-2-nitroaniline (1c) (108 mg, 0.5 mmol), and phe-
nylmethanol (2a) (108 mg, 1.0 mmol). After 24 h, purifica-
tion by column chromatography using 10% ethyl acetate in
1
hexanes yielded (4ca) (74 mg, 54%) as a liquid. H NMR
(400 MHz, CDCl3 + DMSO-d6) δ 8.11 (dd, J = 4.2, 2.6 Hz, 2H),
7.68 (s, 1H), 7.46 – 7.36 (m, 4H), 7.31 – 7.18 (m, 1H). 13C{1H}
NMR (101 MHz, CDCl3 + DMSO-d6) δ 153.2, 140.4, 138.5,
130.3, 129.7, 129.0, 127.0, 125.7, 117.9, 116.5, 115.5.
6-Chloro-2-(naphthalen-1-yl)-1H-benzo[d]imidazole
(4dk):29 GP was followed using cobalt catalyst (3) (6.5 mg,
5.0 mol%), 4-chloro-2-nitroaniline (1d) (86 mg, 0.5 mmol),
and naphthalen-1-ylmethanol (2k) (158 mg, 1.0 mmol) Af-
ter 24 h, purification by column chromatography using 20%
ethyl acetate in hexanes yielded (4dk) (99 mg, 71%) as a
9
6-Chloro-2-phenyl-1H-benzo[d]imidazole (4da):26 GP
was followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%),
4-chloro-2-nitroaniline (1d) (86 mg, 0.5 mmol), and phe-
nylmethanol (2a) (108 mg, 1.0 mmol). After 24 h, purifica-
tion by as a solid (mp 212-214 °C). column chromatography
using 10% ethyl acetate in hexanes yielded (4da) (60 mg,
52%).1H NMR (400 MHz, CDCl3 + DMSO-d6) δ 8.10 (dd, J =
6.5, 2.5 Hz, 2H), 7.53 (d, J = 1.3 Hz, 1H), 7.47 (d, J = 8.6 Hz,
1H), 7.42 – 7.36 (m, 3H), 7.13 (dd, J = 8.5, 1.6 Hz, 1H).13C{1H}
NMR (101 MHz, CDCl3 + DMSO-d6) δ 153.2, 130.2, 129.8,
129.2, 128.9, 128.7, 127.9, 126.9, 123.0, 116.1, 114.8.
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pale yellow liquid. H NMR (400 MHz, CDCl3 + DMSO-d6) δ
8.51 (dd, J = 9.8, 6.6 Hz, 1H), 7.79 – 7.67 (m, 2H), 7.62 (t, J =
7.2 Hz, 1H), 7.44 – 7.23 (m, 5H), 7.02 (dt, J = 8.8, 4.2 Hz, 1H).
13C{1H} NMR (101 MHz, CDCl3 + DMSO-d6) δ 152.8, 139.7,
137.7, 133.4, 130.8, 130.1, 128.0, 127.9, 127.6, 127.3, 126.8,
126.0, 125.7, 124.6, 122.4, 115.8, 114.7.
2-(3-Fluorophenyl)-5,6-dimethyl-1H-benzo[d]imidaz-
ole (4fg): GP was followed using cobalt catalyst (3) (6.5 mg,
5.0 mol%), 4,5-dimethyl-2-nitroaniline (1f) (83 mg, 0.5
mmol), and (3-fluorophenyl)methanol (2g) (126 mg, 1.0
mmol). After 24 h, purification by column chromatography
using 12% ethyl acetate in hexanes yielded (4fg) (54 mg,
45%). 1H NMR (400 MHz, DMSO-d6) δ 12.70 ( br s, 1H), 7.94
(d, J = 7.8 Hz, 1H), 7.87 (d, J = 10.2 Hz, 1H), 7.53 (dd, J = 14.1,
8.0 Hz, 1H), 7.39 (s, 1H), 7.25 (td, J = 8.6, 2.4 Hz, 2H), 2.28 (s,
6H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 163.0 (d, J = 243.3
Hz), 149.6, 134.0, 133.3 (d, J = 8.4 Hz), 131.6 (d, J = 8.5 Hz),
122.8 (d, J = 2.3 Hz), 119.5, 116.7 (d, J = 21.2 Hz), 113.2 (d, J
= 23.6 Hz), 112, 20.5. 19F NMR (376 MHz, DMSO-d6) δ -112.4
(td, J = 9.4, 6.6 Hz). HRMS (ESI) m/z calculated for
C15H13FN2H [M+H]+ 241.1136, found 241.1139.
6-Methoxy-2-phenyl-1H-benzo[d]imidazole (4ea):22 GP
was followed using cobalt catalyst (3) (6.5 mg, 5.0 mol%),
4-methoxy-2-nitroaniline (1e) (84 mg, 0.5 mmol), and phe-
nylmethanol (2a) (108 mg, 1.0 mmol). After 24 h, purifica-
tion by column chromatography using 20% ethyl acetate in
hexanes yielded (4ea) (44 mg, 39%) as a white solid. (mp
146-147°C). 1H NMR (400 MHz, CDCl3) δ 8.07 (dd, J = 6.7, 3.0
Hz, 2H), 7.46 (d, J = 8.7 Hz, 1H), 7.38 – 7.34 (m, 3H), 6.99 (d,
J = 2.3 Hz, 1H), 6.85 (dd, J = 8.9, 2.4 Hz, 1H), 3.73 (s, 3H).
13C{1H} NMR (101 MHz, CDCl3) δ 156.9, 151.8, 139.1, 134.1,
130.1, 129.8, 129.2, 126.7, 116.2, 112.8, 97.5, 55.9.
5,6-Dimethyl-2-phenyl-1H-benzo[d]imidazole (4fa):22
GP was followed using cobalt catalyst (3) (6.5 mg, 5.0
mol%), 4,5-dimethyl-2-nitroaniline (1f) (83 mg, 0.5 mmol),
and phenylmethanol (2a) (108 mg, 1.0 mmol). After 24 h,
purification by column chromatography using 20% ethyl
acetate in hexanes yielded (4fa) (67 mg, 60%) as a white
2-(4-Fluorophenyl)-6-methyl-1H-benzo[d]imidazole
(4bn):30 GP was followed using cobalt catalyst (3) (6.5 mg,
5.0 mol%), 4-methyl-2-nitroaniline (1b) (76 mg, 0.5 mmol),
and (4-fluorophenyl)methanol (2n) (126 mg, 1.0 mmol)Af-
ter 24 h, purification by column chromatography using 20%
ethyl acetate in hexanes yielded (4bn) (52 mg, 46%) as a
1
solid. (mp 249-252°C). H NMR (400 MHz, CDCl3 + DMSO-
d6) δ 8.07 (d, J = 6.9 Hz, 2H), 7.42 – 7.26 (m, 5H), 2.28 (s, 6H).
13C{1H} NMR (101 MHz, CDCl3 + DMSO-d6) δ 151.1, 145.9,
138.3, 131.3, 130.5, 129.4, 128.7, 126.6, 20.4.
1
white solid (mp 180-182 °C). H NMR (400 MHz, CDCl3 +
DMSO-d6) δ 8.08 (dd, J = 8.3, 5.5 Hz, 2H), 7.44 (d, J = 8.2 Hz,
1H), 7.31 (s, 1H), 7.13 – 6.84 (m, 3H), 2.39 (s, 3H). 13C{1H}
NMR (101 MHz, CDCl3 + DMSO- d6) δ 163.6 (d, J = 249.8 Hz),
151.0, 132.4, 128.6 (d, J = 8.4 Hz), 126.7, 126.6, 124.1, 115.8
(d, J = 21.8 Hz), 115.1, 114.4, 99.9, 21.7. 19F NMR (376 MHz,
CDCl3) δ -110.8 (m).
6-Chloro-2-(3-methoxyphenyl)-1H-benzo[d]imidazole
(4de):27 GP was followed using cobalt catalyst (3) (6.5 mg,
5.0 mol%), 4-chloro-2-nitroaniline (1d) (86 mg, 0.5 mmol),
and (4-methoxyphenyl)methanol (2e) (138 mg, 1.0 mmol).
After 24 h, purification by column chromatography using
20% ethyl acetate in hexanes yielded (4de) (76 mg, 59%)
as a liquid. 1H NMR (400 MHz, CDCl3) δ 7.64 – 7.61 (m, 1H),
7.59 (d, J = 7.7 Hz, 1H), 7.53 (d, J = 1.9 Hz, 1H), 7.48 (d, J =
8.6 Hz, 1H), 7.30 (dd, J = 10.1, 5.8 Hz, 1H), 7.19 (dd, J = 8.6,
1.8 Hz, 1H), 6.96 (dd, J = 8.3, 2.2 Hz, 1H), 3.68 (s, 3H).13C{1H}
NMR (101 MHz, CDCl3) δ 160.3, 153.1, 139.6, 137.9, 130.6,
130.4, 128.8, 123.8, 119.0, 117.2, 116.2, 115.0, 111.7, 55.4.
2-(2,4-Dichlorophenyl)-6-methyl-1H-benzo[d]imidaz-
ole (4bo):31 GP was followed using cobalt catalyst (3) (6.5
mg, 5.0 mol%), 4-methyl-2-nitroaniline (1b) (76 mg, 0.5
mmol), and (2,4-dichlorophenyl)methanol (2o) (177 mg,
1.0 mmol). After 24 h, purification by column chromatog-
raphy using 12% ethyl acetate in hexanes yielded (4bo) (80
mg, 58%) as a white solid (mp 103-105 °C). 1H NMR (400
MHz, CDCl3) δ 10.01 ( br s, 1H), 7.47 (d, J = 8.3 Hz, 1H), 7.38
(s, 1H), 7.35 (s, 1H), 7.14 (d, J = 3.5 Hz, 1H), 7.03 (d, J = 8.6
Hz, 1H), 2.40 (s, 3H). 13C{1H} NMR (101 MHz, CDCl3) δ 145.3,
143.9, 138.1, 137.0, 133.1, 130.4, 124.7, 122.7, 120.7, 115.1,
114.6, 112.4, 111.0, 21.8.
2-(Furan-2-yl)-6-methyl-1H-benzo[d]imidazole (4bl):28
GP was followed using cobalt catalyst (3) (6.5 mg, 5.0
mol%), 4-methyl-2-nitroaniline (1b) (76 mg, 0.5 mmol),
and furan-2-ylmethanol (2l) (98 mg, 1.0 mmol). After 24 h,
purification by column chromatography using 12% ethyl
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Procedure of Control Experiments (Scheme 2):
mixture was stirred at 135 °C in an oil bath for 24 h. At am-
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3
4
5
6
7
8
bient temperature, EtOAc (8 mL) was added, and the reac-
tion mixture was washed with brine (5 mL), dried over so-
dium sulfate, concentrated under a vacuum, and careful
analysis of the crude mixture by 1H NMR revealed the ratio
of 4ac:4an = 1.6:1.0.
Procedure of Scheme 2a: o-Phenylenediamine (1a’) (0.50
mmol, 1.0 equiv), phenylmethanol (2a) (0.5 mmol, 1 equiv),
cobalt catalyst (3) (6.5 mg, 5 mol%), and NaOtBu (48 mg,
0.50 mmol, 1 equiv) were placed in a pre-dried 15 mL sealed
tube. The tube was degassed and purged with argon three
times. Then 1,4-dioxane (2.0 mL) was added, and the mix-
ture was stirred at 135 °C in an oil bath for 24 h. At ambient
temperature, EtOAc (8 mL) was added, and the reaction
mixture was washed with brine (5 mL), dried over sodium
sulfate, concentrated under a vacuum, and the crude mix-
Procedure of Scheme 2f (Mercury Drop Test): o-Nitroan-
iline (1a) (0.50 mmol, 1.0 equiv), phenylmethanol (2a) (1.0
mmol, 2 equiv), Co(acac)2 (3) (6.5 mg, 5 mol%), and NaOtBu
(48 mg, 0.50 mmol, 1 equiv) were placed in a pre-dried 15
mL sealed tube. The tube was degassed and purged with ar-
gon three times. Then a drop of mercury and 1,4-dioxane
(2.0 mL) were added in the reaction medium under a gentle
stream of argon and the mixture was stirred at 135 °C in an
oil bath for 24 h. At ambient temperature, EtOAc (8 mL) was
added, and the reaction mixture was filtered through a
short celite pad and filtrate was washed with brine (5 mL),
dried over sodium sulfate, concentrated under a vacuum,
and purified by silica gel column chromatography using
20% ethyl acetate in hexanes to deliver the 2-phenyl-1H-
benzo[d]imidazole (4aa) (69 mg, 71%).
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1
ture was carefully analyzed by H NMR using 1,1,2,2-tetra-
chloroethane as an internal standard (4aa yield <5%).
Procedure of Scheme 2b: o-Nitroaniline (1a) (0.50 mmol,
1.0 equiv), benzaldehyde (2a’) (0.5 mmol, 1.0 equiv), cobalt
catalyst (3) (6.5 mg, 5 mol%), and NaOtBu (48 mg, 0.50
mmol, 1 equiv) were placed in a pre-dried 15 mL sealed
tube. The tube was degassed and purged with argon three
times. Then 1,4-dioxane (2.0 mL) was added, and the mix-
ture was stirred at 135 °C in an oil bath for 24 h. At ambient
temperature, EtOAc (8 mL) was added, and the reaction
mixture was washed with brine (5 mL), dried over sodium
sulfate, concentrated under a vacuum, and the crude mix-
Kinetic Study of Product Formation (yield vs time): To
determine the time dependent progress of the reaction,
5 sets of reactions containing 4-methyl-2-nitroaniline (1b)
(38 mg, 0.25 mmol), phenylmethanol (2a) (55 gm, 0.5
mmol), Co(acac)2 (3) (3.3 mg, 5 mol%), and NaOtBu (24 mg,
0.25 mmol, 1 equiv) were placed in pre-dried 15 mL sealed
tubes. The tubes were degassed and purged with argon
three times. Then 1,4-dioxane (2 mL) was added in each
tube, and stirred at 135 °C in an oil bath for (3-15 h). At reg-
ular interval of 3 hours, the reaction vessels were removed
one by one and quenched with EtOAc (8 mL) and the solvent
was evaporated and concentrated under a vacuum, and
passed through a short pad silica using 20% ethyl acetate in
hexanes and concentrated under a vacuum. Careful analysis
of the crude mixture by 1H NMR (using 1,1,2,2-tetrachloro-
ethane as an internal standard) revealed the time depend-
ent yield of 6-methyl-2-phenyl-1H-benzo[d]imidazole
(4ba).
1
ture was carefully analyzed by H NMR using 1,1,2,2-tetra-
chloroethane as an internal standard (4aa yield <5%).
Procedure of Scheme 2c: 4-methylbenzene-1,2-diamine
(1b’) (0.50 mmol, 1.0 equiv), benzaldehyde (2a’) (0.5 mmol,
1.0 equiv), cobalt catalyst (3) (6.5 mg, 5 mol%), and NaOtBu
(48 mg, 0.50 mmol, 1 equiv) were placed in a pre-dried 15
mL sealed tube. The tube was degassed and purged with ar-
gon three times. Then 1,4-dioxane (2.0 mL) was added, and
the mixture was stirred at 135 °C in an oil bath for 24 h. At
ambient temperature, EtOAc (8 mL) was added, and the re-
action mixture was washed with brine (5 mL), dried over
sodium sulfate, concentrated under a vacuum, and the
crude mixture was carefully analyzed by 1H NMR using
1,1,2,2-tetrachloroethane as an internal standard (4ba
yield <5%).
Procedure of Scheme 2d: 4-methylbenzene-1,2-diamine
(1b’) (0.50 mmol, 1.0 equiv), 5-methoxy-2-nitroaniline (1e)
(0.50 mmol, 1.0 equiv) and benzaldehyde (2a’) (0.5 mmol,
1.0 equiv), cobalt catalyst (3) (6.5 mg, 5 mol%), and NaOtBu
(48 mg, 0.50 mmol, 1 equiv) were placed in a pre-dried 15
mL sealed tube. The tube was degassed and purged with ar-
gon three times. Then 1,4-dioxane (2.0 mL) was added, and
the mixture was stirred at 135 °C in an oil bath for 24 h. At
ambient temperature, EtOAc (8 mL) was added, and the re-
action mixture was washed with brine (5 mL), dried over
sodium sulfate, concentrated under a vacuum, and careful
analysis of the crude mixture by 1H NMR (using 1,1,2,2-tet-
rachloroethane as an internal standard) revealed yield of
4ea = 9% and 4ba = 20%.
Sl No.
Time (hour)
Yield (%)
1
2
3
4
5
3
6
19
27
33
42
48
9
12
15
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: Copies of 1H and 13C{1H} NMR
spectra for all compounds (PDF).
Procedure of Scheme 2e: o-Nitroaniline (1a) (0.50 mmol,
1.0 equiv), (4-methoxyphenyl)methanol (2c) (0.5 mmol, 1.0
equiv) and (4-fluorophenyl)methanol (2n) (0.5 mmol, 1.0
equiv), cobalt catalyst (3) (6.5 mg, 5 mol%), and NaOtBu (48
mg, 0.50 mmol, 1 equiv) were placed in a pre-dried 15 mL
sealed tube. The tube was degassed and purged with argon
three times. Then 1,4-dioxane (2.0 mL) was added, and the
AUTHOR INFORMATION
Corresponding Author
* Email: sds@iiserkol.ac.in
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Page 9 of 10
The Journal of Organic Chemistry
Catalysis. Org. Lett. 2009, 11, 2039. (b) Shiraishi, Y.; Sugano, Y.;
Notes
Tanaka, S.; Hirai, T. One-pot Synthesis of Benzimidazoles by
Simultaneous Photocatalytic and Catalytic Reactions on Pt@TiO2
Nanoparticles. Angew. Chem. Int. Ed. 2010, 49, 1656. (c) Kuo, H.-Y.;
Liu, Y.-H.; Peng, S.-M.; Liu, S.-T. N,N′-Dialkylation Catalyzed by
1
2
3
The authors declare no competing financial interest.
ACKNOWLEDGMENT
4
5
6
7
8
9
Bimetallic Iridium Complexes Containing
a Saturated Bis-N-
Generous support by the CSIR [Fellowship to SD], SERB, DST,
India [ECR/2016/000225 to SDS] and IISER Kolkata [Fellow-
ship to SM and infrastructure] is gratefully acknowledged.
Heterocyclic Carbene (NHC) Ligand. Organometallics 2012, 31,
7248. (d) Luo, Q.; Dai, Z.; Cong, H.; Li, R.; Peng, T.; Zhang, J. Oxidant-
free Synthesis of Benzimidazoles from Alcohols and Aromatic
Diamines Catalysed By New Ru(II)-PNS(O) Pincer Complexes.
Dalton. Trans. 2017, 46, 15012. (e) Li, L.; Luo, Q.; Cui, H.; Li, R.;
Zhang, J.; Peng, T. Air-stable Ruthenium(II)-NNN Pincer Complexes
for the Efficient Coupling of Aromatic Diamines and Alcohols to 1H-
benzo[d]imidazoles with the Liberation of H2. ChemCatChem 2018,
10, 1607. (f) Guan, Q.; Sun, Q.; Wen, L.; Zha, Z.; Yang, Y.; Wang, Z.
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