A Tandem Aerobic Photocatalytic Synthesis of Benzimidazoles by Cobalt Ascorbic Acid Complex Coated on TiO2 Nanoparticles Under Visible Light

Article abstract of DOI:10.1007/s10562-017-2232-0

Abstract: In this study, we developed methods for the one pot environmentally benign synthesis of benzimidazoles by cobalt ascorbic acid complex coated on TiO₂ nanoparticles via aerobic photooxidative cyclization reactions. Easy work-up procedure, reusability of the catalyst and scalable to the multi-mole scale, which is valuable for an industrial process make these catalytic systems highly attractive. Also, the combination of photocatalytic and catalytic reactions presented here may help to develop a new strategy towards the development of photocatalysis-based organic synthesis. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2232-0

Catal Lett
DOI 10.1007/s10562-017-2232-0
A Tandem Aerobic Photocatalytic Synthesis of Benzimidazoles
by Cobalt Ascorbic Acid Complex Coated on TiO2 Nanoparticles
Under Visible Light
Fahimeh Feizpour1 · Maasoumeh Jafarpour1 · Abdolreza Rezaeifard1
Received: 7 June 2017 / Accepted: 25 October 2017
© Springer Science+Business Media, LLC 2017
Abstract In this study, we developed methods for the one
pot environmentally benign synthesis of benzimidazoles by
cobalt ascorbic acid complex coated on TiO2 nanoparticles
via aerobic photooxidative cyclization reactions. Easy work-
up procedure, reusability of the catalyst and scalable to the
multi-mole scale, which is valuable for an industrial pro-
cess make these catalytic systems highly attractive. Also,
the combination of photocatalytic and catalytic reactions
presented here may help to develop a new strategy towards
the development of photocatalysis-based organic synthesis.
Graphical Abstract
Keywords Benzimidazole · Cobalt nanocatalyst ·
One-pot tandem synthesis · Photocatalysis · Aerobic
photooxidation
1 Introduction
Electronic supplementary material The online version of
this article (https://doi.org/10.1007/s10562-017-2232-0) contains
supplementary material, which is available to authorized users.
Achieving high synthetic efficiency and atom economy has
been an important goal for development of synthetic organic
chemists. Tandem reactions which complete several chemi-
cal transformations in one vessel, are a powerful strategy in
which not only reduce the number of reaction steps, energy
consumption and waste but also minimize the use of solvents
and reagents [1]. So, these cascade reactions are sustainable
green processes that illustrate the concepts of efficiency and
atom economy.
* Maasoumeh Jafarpour
mjafarpour@birjand.ac.ir
* Abdolreza Rezaeifard
rrezaeifard@birjand.ac.ir; rrezaeifard@gmail.com
1
Catalysis Research Laboratory, Department
of Chemistry, Faculty of Science, University of Birjand,
Birjand 97179-414, Iran
Vol.:(0123456789)
1 3

F. Feizpour et al.
The photocatalysis of organic synthesis has become an
important research area in photochemistry in the past two
decades [2–4]. The photocatalytic reaction demonstrates the
possibilities of converting light energy into chemical energy
providing a new synthetic route [5, 6].
Various catalytic systems that enable one-pot organic
transformations have been reported [39–48], but one-pot
systems that combine photocatalysis and catalysis are lim-
ited [55, 56] Meanwhile, these systems use a homogeneous
photocatalyst (photoredox catalyst) and an organocatalyst to
effect the one-pot transformations.
The title method in comparison to the commercially used
synthetic methods, which involve long reaction times, expen-
sive and hazardous reagents, and side product formation, is
cost-effective, highly selective, rapid, and eco-friendly.
Benzimidazoles, which are known as important building
blocks for the construction of pharmaceuticals, natural prod-
ucts, functional materials, and agrochemical compounds,
have attracted much attention in organic synthesis in recent
decades [7–48].
2 Experimental
2.1 General Remarks
All chemicals were purchased from Merck and Fluka Chem-
ical Companies. The FT-IR spectra were recorded on NICO-
LET system. TEM images were obtained by TEM instru-
mentation (Philips CM 10). Progresses of the reactions were
monitored by TLC using silica-gel SIL G/UV 254 plates.
NMR spectra were recorded on a Brucker Avance DPX 250
and 400 MHz instruments.
Among reported methods, the use of alcohols as start-
ing materials is suitable alternatives for integrates consid-
erations of economic viability and environmental integrity
because of wide availability of alcohols [39–48].
In view of the above and during our research on develop-
ment of heterogeneous nanophotocatalysts performance for
organic synthesis in green conditions [49–54], recently we
reported the selective aerobic benzylic C–H oxidation of
alcohols and hydrocarbons by TiO2/AA/Co nanohybrid in
ethyl acetate [52]. In a continuous study, we envision a direct
and green method for benzimidazols synthesis by aerobic
oxidative cyclocondensation of alcohols and o-aminoani-
lines in visible light (Scheme 1).
2.2 Preparation of TiO2/AA/Co Nanohybrid (See
Supporting Information) [52]
2.2.1 General Procedure for Synthesis of Benzimidazole
from Benzyl Alcohols and o‑Phenylenediamines
To a mixture of benzyl alcohol (1.2 mmol), 1,2-phenylen-
ediamine (1 mmol) and NHPI (5 mol%) in a glass test tube
(10 cm tall×1 cm diameter) was added TiO2/AA/Co nano-
catalyst (0.06 mol%) and the reaction mixture was heated
at 70 °C under air, visible light and solvent free conditions.
The reaction progress was monitored by TLC. After 3 h,
ethanol (5 mL) was added to the mixture and then TiO2/AA/
Co nanocatalyst (solid phase) was separated by centrifuging
followed by decantation (3×5 mL ethanol). Desired product
(liquid phase) was extracted by plate chromatography eluted
with n-hexane/EtOAc (10/2). Assignments of products were
made by 1H NMR spectral data in comparison with authentic
samples.
This catalyst system involves photooxidation of alcohols
to aldehydes followed by cyclocondensation with 1,2-phe-
nylenediamine to afford benzimidazoles in one-pot operation
with good to excellent yield.
H
N
OH
N
O
O
O
O
Dehydrogenation
O
O
O
O
O
O
O
O
O
TiO2
O
O
O
H
N
2.2.2 General Procedure for Synthesis of Benzimidazole
from Benzaldehydes and o‑Phenylenediamines
O
O
O
O
N
H
O
O
O
O
To a mixture of benzaldehyde (1.2 mmol), 1,2-phenylen-
ediamine (1 mmol) in ethanol (0.8 mL) in a glass test tube
(10 cm tall×1 cm diameter), was added TiO2/AA/Co nano-
catalyst (0.02 mol%) and the reaction mixture was heated at
60 °C under air and visible light conditions for the required
time. The reaction progress was monitored by TLC. After
completion of the reaction, ethanol was added to the reaction
mixture (3 mL). TiO2/AA/Co nanocatalyst (solid phase) was
separated by centrifuging followed by decantation (3×5 mL
ethanol). Desired product (liquid phase) was purified by
Cyclization
NH2
O
Condensation
N
NH2
NH2
H
2O
Scheme 1 Aerobic oxidative cyclocondensation of alcohols and
o-aminoanilines catalyzed by TiO2/AA/Co nanohybrid in visible light
1 3

A Tandem Aerobic Photocatalytic Synthesis of Benzimidazoles by Cobalt Ascorbic Acid Complex…
plate chromatography eluted with n-hexane/EtOAc (10/2).
Assignments of products were made by 1H NMR spectral
data in comparison with authentic samples.
3 h. To achieve a new condition for efficient condensation
of benzaldehydes and o-phenylenediamines different factors
were screened (Fig. S1). The reaction of 4-chlorobenzal-
dehyde (1.2 mmol) with o-phenylenediamine (1 mmol) in
ethanol (0.8 ml) containing 0.02 mol% TiO2/AA/Co nano-
hybrid proceeded quantitatively within 30 min under aerobic
conditions at 60 °C with no need to NHPI.
3 Results and Discussion
3.1 Synthesis of Benzimidazoles using Aromatic
Diamines and Benzylic Alcohols (Method A)
Various aryl aldehydes were allowed to react with
o-phenylenediamines. The corresponding benzimidazoles
produced in high yields, except for electron-deficient ones
which formed moderately (Table 2, entries 4, 5, 13 and 17).
Initially, we tried to prepare benzimidazole derivatives using
benzene-1,2-diamines and benzylic alcohols. The observa-
tions for model reaction of phenylenediamine (1 mmol) and
benzyl alcohol (1.2 mmol) have been summarized in Fig. 1.
It was found that to trigger the reaction, both TiO2/AA/Co
nanohybrid and NHPI should be present in the reaction in
an open vessel (in air). Moreover, the reaction reached to the
highest conversion under solvent-free conditions at 70 °C
(Fig. 1i, ii). Raising the temperature up to 70 °C decreased
the yield (Fig. 1ii). To achieve the quantitative yield of prod-
uct, the reaction needs to 0.06 and 5 mol% of catalyst and
NHPI, respectively. (Fig. 1iii and vi). The role of NHPI as a
free radical oxidation co-catalyst in combination with cobalt
complexes under aerobic conditions has been well docu-
mented (Fig. S2) [52].
3.3 Photocatalytic Activity
To evaluate the photocatalytic function of TiO2 at the core
of title nanocomposite, the model reactions under conditions
used in methods A and B (aerobic oxidative condensation of
phenylenediamine and benzyl alcohol as well as phenylen-
ediamine and benzaldehyde) subjected under UV and visible
light as well as in dark (Table 3). Increasing the reaction
rate under light radiation particularly UV light revealed the
involving of the photocatalytic activity of TiO2 core on the
catalytic efficiency of TiO2/AA/Co nanohybride (entries 3
and 6) (see proposed mechanism in SI). A good evidence
for this claim may be the smaller band-gap of 2.8 eV for
TiO2/AA/Co nanohybrid than that of bare TiO2 nanoparti-
cles (3.2 eV) [52].
Besides, when the other common oxidants such as O2,
H2O2, TBHP, UHP, Oxone® and tetra-n-butylammonium
Oxone® (TBAOX) were used under optimized conditions,
the yields of benzimidazole reduced (Fig. 1v).
The superior catalytic performance of TiO2/AA/Co
nanoparticles in both methods A and B was also affirmed
by replacing the other nanoparticulate metal oxides such
as MoO3 [57], monoclinic ZrO2 [58], TiO2 as well as their
nanocomposites, such as TiO2/AA, AA/Co, γ-Fe2O3/AA/Co,
and MoO3/AA/Co. The product yield and selectivity reduced
markedly under the same conditions (Table 4).
The scope and generality of the reaction with various aryl
alcohols and 1,2-diaminoarenes discovered under the above
mentioned optimized conditions using air as oxidant and vis-
ible light as energy source (Table 1). According to the results
summarized in Table 1, the benzimidazole derivatives were
successfully produced by the visible light irradiation of aryl
alcohols solutions containing o-arylenediamines and cata-
lytic amount of TiO2/AA/Co (Table 1). Inspection of the
results in Table 1 revealed that the reaction rate affected by
electronic demands of substrates. Benzyl alcohols bearing
electron-donating groups were efficiently converted to the
pertinent benzimidazole derivatives (Table 1, entries 5 and
7). Nevertheless, strong electron-withdrawing nitro group
on the phenyl ring of both alcohol and amine molecules,
significantly retarded the reaction (Table 1, entry 4, 10).
In continued, the dependence of the catalytic activity on
the irradiation wavelength was illustrated in Fig. 2 (Method
B). A series of optical low-pass filters were employed to
block light below a specific cut-off wavelength. For exam-
ple, the 450 nm optical filter blocks the wavelength below
450 nm and over 800 nm, in other words, the light irradiat-
ing the reactor has a wavelength range from 450 to 800 nm.
Without any filters, the irradiation of the light with wave-
lengths ranging from 400 to 800 nm gives a 2-[4-chlorophe-
nyl] benzimidazole yield of 96%. The yield decreases to 73,
46 and 34% when the wavelength range of the irradiation
is 450–800, 550–800, and 600–800 nm, respectively. Since
the yield of 2-[4-chlorophenyl] benzimidazole in the dark
is 20%, the contribution of 400–450 nm light accounts for
about 30% ((76–53)/76 × 100%) in the total light-induced
yield. Similarly, the light in the wavelength range of
450–550, 550–600 and 600–800 nm, respectively accounts
for 36, 16 and 18% of the light-induced yield (Fig. 2A).
These values agree well to the UV–visible absorption
3.2 Synthesis of Benzimidazoles Using Aromatic
Diamines and Benzaldehydes (Method B)
Next, the catalytic system was employed for condensation
of benzaldehydes with o-phenylenediamine. Performing the
condensation reaction of 4-chlorobenzaldehyde with o-phe-
nylenediamine under conditions used for benzyl alcohols
(Table 1) led to 60% of the pertinent benzimidazole after
1 3

F. Feizpour et al.
Fig. 1 The screening of the (i) solvent nature (ii) temperature (iii) NHPI amount (iv) catalyst amount (v) various oxidant on synthesis of ben-
zimidazole from benzyl alcohol (1.2 mmol) and benzene-1,2-diamine (1.0 mmol) catalyzed by TiO2/AA/Co after 3 h under air and visible light
spectrum of the TiO2/AA/Co catalyst (Fig. 2B). Because
the TiO2/AA/Co catalyst has a strong absorption at about
493 nm, the light in the wavelength range of 450–550 nm
contributions the highest light-induced conversion. This fur-
ther confirms that the light absorbed by Co complex is the
major driving force of the reaction.
The action spectra for synthesis of 2-[4-chlorophenyl]
benzimidazole has been shown in Fig. 3. The action spec-
trum is a useful tool for determining whether an observed
reaction occurs via a photoinduced process or a thermocat-
alytic process which should show one-to-one mapping
between the wavelength-dependent photocatalytic rate and
1 3

A Tandem Aerobic Photocatalytic Synthesis of Benzimidazoles by Cobalt Ascorbic Acid Complex…
Table 1 Synthesis of
benzimidazole derivatives
from benzyl alcohols and
o-phenylenediaminesa
R
NH2
NH2
H
N
R
OH
TiO2/AA/Co
+
oC
S.F, NHPI, Air,70
N
X
X
Diamine
Alcohol
X
Entry
1
Productb
Yield %c
R
H
N
H
H
90
80
65
N
H
N
2
3
H
H
4-Cl
2-Cl
Cl
N
H
N
N
Cl
H
N
4
5
6
7
H
H
H
H
4-NO2
4-OMe
4-SMe
4-Me
-
NO2
OMe
SMe
Me
N
H
N
94
37
83
N
H
N
N
H
N
N
H
N
8
9
H
H
2-Me
75
72
N
Me
H
N
4-t-Bu
N
H
N
Me
10
11
NO2
Me
4-Me
4-Me
15
79
N
N
O2
H
N
Me
N
Me
aReaction condition 1,2-phenylene diamine (1 mmol), Benzyl alcohol (1.2 mmol), catalyst (0.06 mol%),
NHPI (5 mol%) at 70 °C after 3h under air, visible light and solven free conditions
bThe products were identified by 1H NMR spectroscopy
cIsolated yield
1 3

F. Feizpour et al.
Table 2 Synthesis of
benzimidazole derivatives
from benzaldehydes and
o-phenylenediaminesa
CHO
H
N
R
NH2
NH2
R
TiO2/AA/Co
EtOH, Air,60
+
X
oC
N
X
Productb
Time Yield (%)c
(min)
Diamine
R
Aldehyde
X
Entry
1
H
N
H
H
50
30
85
96
N
H
N
2
H
4-Cl
Cl
N
H
N
3
4
H
H
2-Cl
20
95
46
N
Cl
H
N
4-NO2
NO2
120
N
H
N
5
H
2-NO2
180
50
N
N
O2
H
N
6
7
H
H
4-Me
95
96
Me
50
65
65
N
H
N
OMe
4-OMe
N
H
N
8
9
H
H
4-OH
2-OH
94
OH
N
H
N
60
60
90
95
N
HO
H
N
4-OMe
3-OH
OMe
10
11
H
H
N
OH
H
N
CHO
45
95
N
1 3

A Tandem Aerobic Photocatalytic Synthesis of Benzimidazoles by Cobalt Ascorbic Acid Complex…
Table 2 (continued)
Product b
Time Yield (%)c
(min)
Diamine
R
Aldehyde
X
Entry
12
CHO
H
N
H
95
85
N
H
N
13
14
15
16
17
NO2
Me
Cl
H
180
40
40
25
70
60
96
90
95
42
N
N
O2
H
N
H
N
H
N
Cl
Cl
Cl
4-Cl
4-Cl
4-Cl
N
Cl
H
N
Me
N
Me
Br
H
N
Br
NH2
NH2
N
N
N
aReaction condition 1,2-phenylene diamine (1 mmol), Benzaldehyde (1.2 mmol), catalyst (0.02 mol%) in
ethanol (0.8 ml) at 60 °C under air and visible light conditions
bThe products were identified by 1H NMR spectroscopy
cIsolated yield
Table 3 Screening of the
photocatalytic activity of TiO2/
AA/Co
Entry
Catalyst
Condition
Method Aa yield% Method Bb yield% (time)
(time)
1
2
3
4
5
6
7
8
9
TiO2
UV lightc
UV light
20 (3 h)
25 (3 h)
85 (2 h)
10 (3 h)
10 (3 h)
90 (3 h)
10 (3 h)
10 (3 h)
20 (3 h)
30 (30 min)
25 (30 min)
100 (15 min)
20 (30 min)
30 (30 min)
100 (30 min)
20 (30 min)
15 (30 min)
20 (30 min)
TiO2/AA
TiO2/AA/Co
TiO2
TiO2/AA
TiO2/AA/Co
TiO2
UV light
Visible lightd
Visible light
Visible light
Darkness
TiO2/AA
TiO2/AA/Co
Darkness
Darkness
aReaction condition Method A: 1,2-phenylene diamine (1 mmol), Benzyl alcohol (1.2 mmol), catalyst
(0.06 mol%), NHPI (5 mol%) at 70 °C under air and solvent free conditions
bMethod B: 1,2-phenylene diamine (1 mmol), 4-chlorobenzaldehyde (1.2 mmol), catalyst (0.02 mol%) in
ethanol (0.8 ml) at 60 °C under air
cλ=200–290 nm
dFluorescent lamp
1 3

F. Feizpour et al.
the light extinction spectrum [59, 60]. In this study, the reac-
tion rates of the synthesis of 2-[4-chlorophenyl] benzimi-
dazole using TiO2/AA/Co at 60 °C under irradiation with
different wavelengths were determined. To obtain an action
spectrum in this photocatalytic system, the AQY versus the
respective wavelengths of the reaction is plotted. AQY was
calculated with the following equation:
was easily separated by centrifugation and reused in four
consequent runs in the reaction of 1,2-phenylenediamine and
benzyl alcohol (Method A) as well as in condensation of
1,2-phenylenediamine and 4-chlorobenzaldehyde (Method
B) (Table 5). Slight reduction in the product yields was
observed after 4th run indicating the efficiency and stabil-
ity of title nanocatalyst. More evidences for this issue were
observed by comparing the FT-IR and TEM image of used
catalyst with fresh one (Fig. 4). The structure, size and mor-
phology of catalyst remained intact after recycling. Further,
[(
)
AQY (%) = Yvis − Ydark
]
×2∕(photon number entered into the reaction vessel) × 100
where Ylight and Ydark are the amounts of products formed
under light irradiation and dark conditions, respectively. As
shown in Fig. 3, a good correlation is observed between
AQY and the diffuse reflectance spectrum of the TiO2/AA/
Co nanohybrid which confirms that the reaction is taking
place photocatalytically.
3.4 Stability and Reusability of Catalyst
Finally, recycling experiments were conducted to find out
the stability and reusability of the catalysts. The catalyst
Table 4 The comparison of catalytic activity of TiO2/AA/Co with
other nanocomposites and nanooxometals
Entry
Catalyst
Method Aa,b
Method Bc
yield% (selectiv-
ity %)
yield (%)
1
2
3
TiO2/AA/Co
TiO2/AA
AA/Co
90
30
45
0
96 (100)
30 (66)
50 (80)
0
4
AA
5
6
7
8
9
10
TiO2
ZrO2
MoO3
MoO3/AA/Co
Fe2O3/AA/Co
Co (OAc)2
20
20
55
65
25
60
20 (50)
20 (50)
65 (69)
55 (90)
25 (60)
65 (76)
aReaction condition Method A: 1,2-phenylene diamine (1 mmol),
Benzyl alcohol (1.2 mmol), catalyst (0.06 mol%), NHPI (5 mol%) at
70 °C after 3 h under air, visible light and solvent free conditions
b100% selectivity
Fig. 2 Dependence of 2-[4-chlorophenyl] benzimidazole yield on the
irradiation wavelength (A), and the action spectrum of the photocata-
lytic reaction, in which the light driven conversion is plotted against
the irradiation wavelength (B)
cMethod B: 1,2-phenylene diamine (1 mmol), 4-chlorobenzaldehyde
(1.2 mmol), catalyst (0.02 mol%) in ethanol (0.8 ml) at 60 °C after
30 min under air and visible light
1 3

A Tandem Aerobic Photocatalytic Synthesis of Benzimidazoles by Cobalt Ascorbic Acid Complex…
AAS revealed a very small amount of Co (<2 ppm) in the
filtrate using a hot filtration method.
Additional advantageou of these methodologies is the
possibility of applying the scale-up conditions. When the
reaction performed using 20 mmol 1,2-phenylenediamine
and 24 mmol of benzyl alcohol, 86% of the corresponding
benzimidazole was secured at the same time and conditions
mentioned in Table 1.
4 Conclusion
In conclusion, we have developed an expedient and direct
method for the synthesis of benzimidazoles by TiO2/AA/
Co nanohybrid under visible light irradiation conditions
in good to excellent yields, starting directly from a vari-
ety of alcohols or aldehydes and 1,2-phenylenediamine
derivatives. In case of alcohols, the protocol involves
photooxidation of alcohols to aldehydes followed by
cyclocondensation with 1,2-phenylenediamine to afford
benzimidazoles in one-pot operation with good to excel-
lent yield. The methods afford environmentally friendly
reaction conditions. The process is also efficient even on
a multi-mmole scale and provides easy product isolation.
Therefore, this process has the potential to enable a more
sustainable benzimidazole synthesis. The combination of
photocatalytic and catalytic reactions presented here may
help to develop a new strategy towards the development
of photocatalysis-based organic synthesis.
Fig. 3 Photocatalytic action spectrum for synthesis of 2-[4-chloro-
phenyl] benzimidazole using TiO2/AA/Co photocatalyst. The light-
absorption spectrum (left axis) is the DR−UV/vis spectra of TiO2/
AA/Co nanohybrid (blue curve)
Table 5 Recyclability of the catalytic systems in the synthesis of
benzimidazoles
Reaction
Yield %
(run 1)
Yield %
(run 2)
Yield %
(run 3)
Yield
% (run
4)
Method Aa
Method Bb
90
96
90
96
90
95
88
94
aThe aerobic oxidative coupling of benzyl alcohol with 1,2-phenylene
diamine
bThe aerobic oxidative coupling of 4-chloro benzaldehyde with
1,2-phenylene diamine
Fig. 4 (Left) FT-IR spectra of
TiO2/AA/Co itself (a) and after
4 times reuses. (b) (Right) TEM
image of TiO2/AA/Co after
reuses
1 3

F. Feizpour et al.
18. Wang R, Lu X, Yu X, Shi L, Sun Y (2007) Acid-catalyzed
solvent-free synthesis of 2-arylbenzimidazoles under microwave
irradiation. J Mol Catal A 266:198–201
Acknowledgements Support for this work by Research Council of
University of Birjand and “Iran Science Elites Federation” is highly
appreciated. We also thank Dr. M. M. Najafpour and Dr. S. Zakavi from
IASBS, Zanjan, Iran, for their valuable comments on photochemical
reactions.
19. Hornberger KR, Adjabeng GM, Dickson HD, Davis-Ward RG
(2006) A mild, one-pot synthesis of disubstituted benzimida-
zoles from 2-nitroanilines. Tetrahedron Lett 47:5359–5361
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Compliance with Ethical Standards
Conflict of interest The authors declare no conflict of interest.
22. Tandon VK, Kumar M (2004) BF
3.Et2O promoted one-pot
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