Combined Pd/C and montmorillonite catalysis for one-pot synthesis of benzimidazoles

Article abstract of DOI:10.1002/ejoc.201201101

A series of nineteen benzimidazoles were prepared from ortho-nitroanilines by one-pot transfer hydrogenation, condensation, and dehydrogenation enabled by the concurrent use of two heterogeneous catalysts: montmorillonite-K10 and Pd/C. This strategy was further employed to accomplish a five-step, three-component synthesis of an antifungal benzimidazoquinazoline by using a simple one-pot procedure.

Full text of DOI:10.1002/ejoc.201201101

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201101
Combined Pd/C and Montmorillonite Catalysis for One-Pot Synthesis of
Benzimidazoles
Nicholas A. Weires,[a] Jared Boster,[a] and Jakob Magolan*[a]
Keywords: Synthetic methods / Heterogeneous catalysis / Tandem concurrent catalysis / Nitrogen heterocycles / Clays
A series of nineteen benzimidazoles were prepared from
ortho-nitroanilines by one-pot transfer hydrogenation, con-
densation, and dehydrogenation enabled by the concurrent
use of two heterogeneous catalysts: montmorillonite-K10 and
Pd/C. This strategy was further employed to accomplish a
five-step, three-component synthesis of an antifungal benz-
imidazoquinazoline by using a simple one-pot procedure.
ortho-nitration of anilines and subsequent reduction of the
nitro moiety.[9] The two principal synthetic strategies for
benzimidazole formation are: (a) condensation of ortho-ary-
lenediamines with carboxylic acids and derivatives
thereof[10] and (b) dehydrogenation of benzimidazoline in-
termediates generated from condensation of ortho-arylened-
iamines with aldehydes.[11] The first of these methods entails
acidic conditions and high temperatures, while the second
typically requires stoichiometric or excess oxidants. Cata-
lytic aerobic methods have recently emerged as suitable
green alternatives for benzimidazoline oxidation.[12] Transi-
tion-metal-catalyzed aryl amination,[13] C–H functionaliza-
tion,[14] and oxidation of alcohols[15] have also been em-
ployed in the efficient preparation of benzimidazoles.
Introduction
The use of multiple catalysts in one-pot reactions to
achieve sequential transformations, termed “tandem con-
current catalysis”, is documented predominantly for homo-
geneous systems.[1] Extension of this strategy to hetero-
geneous catalysts offers noteworthy advantages. In such
cases, the inherent economic and environmental benefits of
one-pot multistep sequences are paired with facile catalyst
removal by filtration and opportunities for convenient cata-
lyst recycling.[2] Indeed, the development of multifunctional
heterogeneous catalysts enables multistep transformations
with remarkable efficiency.[3] While incorporation of com-
plex catalytic materials, such as polyfunctional mesoporous
silicas,[3d,3i] into routine use in synthetic research laborato-
ries is limited by catalyst availability, the pragmatic chemist
can now choose from an abundance of simpler hetero-
geneous options.[4] In principle, any two or more chemically
compatible heterogeneous materials are potential tandem
catalysts for one-pot multistep transformations. Herein we
aim to draw attention to this underutilized strategy by dem-
onstrating a facile preparation of benzimidazoles from
ortho-nitroanilines and aldehydes enabled by the simulta-
neous use of two heterogeneous catalysts, palladium on car-
bon and the acid-treated clay montmorillonite-K10.[5] Not
only is the isolation of two intermediates avoided but also
are products of high purity recovered by simple filtration.
Benzimidazoles demonstrate wide-ranging biological ac-
tivities including efficacy against cancers,[6] HIV,[7] and
HSV-1.[8] Benzimidazoles are most commonly prepared
from ortho-arylenediamines, which, in turn, are available by
Results and Discussion
We envisioned a three-step sequence starting from ortho-
nitroaniline (1) in which Pd/C would catalyze both the ini-
tial hydrogenation of the nitro moiety and the terminal
benzimidazoline dehydrogenation, while montmorillonite-
K10 would catalyze the condensation of diamine 2 with an
aldehyde (Scheme 1).
[a] Department of Chemistry, University of Idaho,
Moscow, ID, 83844-2343, USA
Fax: +1-208-885-6173
Scheme 1. Proposed one-pot synthesis of benzimidazoles.
E-mail: jmagolan@uidaho.edu
Homepage: http://www.uidaho.edu/sci/chem/faculty/jakobmag-
olan
The reduction of the aryl nitro moiety by transfer hydro-
genation over Pd/C is a well-documented process.[16] The
compatibility of montmorillonite-K10 and Pd/C has also
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201101.
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Pd/C–Montmorillonite-Catalyzed Synthesis of Benzimidazoles
been previously established.[17] Thus, as anticipated, the re-
duction of ortho-nitroaniline in the presence of montmoril-
lonite-K10 was readily accomplished in toluene heated to
reflux with ammonium formate (Ͼ3 equiv.) as the hydrogen
donor. Cyclohexene and ammonium acetate were also
found to be suitable hydrogen donors, but ammonium for-
mate was preferred because of the comparative ease of
handling. We were pleased to observe that inclusion of
benzaldehyde in this reaction mixture resulted in formation
of the desired 2-phenyl benzimidazole 5, albeit in modest
yield (65%) and contaminated by a small amount of 1,2-
disubstituted benzimidazole 6 (Table 1, Entry 1). A con-
siderable amount of unreacted phenylenediamine was also
recovered, which indicates that some of the benzaldehyde
had likely undergone reduction.
Scheme 2. Rationale for the formation of benzimidazole 6.
A brief screening of conditions for the reaction of phen-
ylenediamine with benzaldehyde in the presence of both
catalysts revealed that the selectivity between 5 and 6 was
remarkably temperature-dependent (Table 2). Above 60 °C,
a substantial amount of 6 was produced, while at or below
room temperature the desired benzimidazole 5 was formed
exclusively. In fact, when the reaction was performed in an
ice bath (4 °C – room temp.) formation of only traces of 6
was observed even in the presence of two equivalents of
benzaldehyde (Table 2, Entry 6).
Table 1. Initial attempted one-pot benzimidazole synthesis.
Entry Conditions
Yield[a] (5+6) Ratio (5/6)
1
2
2
PhCHO (1.0 equiv. added
at start of reaction)
Reflux 1 h, then PhCHO
(1.0 equiv.), then reflux 16 h
Reflux 1 h, then PhCHO
(2.0 equiv.), then reflux 16 h
65%
71%
85%
50:1
8:1
Table 2. Optimization of selectivity of benzimidazole 5 over 6.
1:1
[a] Yield after chromatography (5 and 6 were co-eluted).
When benzaldehyde addition was withheld until com-
plete reduction of the nitro moiety (1 h), a modest improve-
ment in yield was observed along with a drop in selectivity
for 5 over 6 (Table 1, Entry 2).
Formation of the undesired 1,2-disubstituted benzimid-
azole 6 was a predictable complication. Under a variety of
conditions, ortho-arylenediamines are documented to react
with two aldehyde equivalents to yield 1,2-disubstituted
benzimidazoles.[18] This presumably proceeds through isom-
erization of intermediate 9 to benzimidazole 6 (Scheme 2).
This aromatization has been previously proposed to pro-
ceed by a 1,3-hydride shift.[18c,18d] However, given that sup-
Entry
Conditions
Ratio (5/6)[a]
1
2
3
4
5
6
120 °C, 1 h
60 °C, 16 h
45 °C, 16 h
22 °C, 16 h
4:1
4:1
20:1
Ͼ100:1
Ͼ100:1
4 °C – room temp., 16 h
4 °C – room temp., 16 h, 2 equiv. of PhCHO 100:1
[a] Determined by 1H NMR spectroscopy of the crude reaction
mixture.
After the issue of selectivity was resolved, a suitable set
rafacial 1,3-hydride shifts are symmetry-forbidden and the of conditions was established for the one-pot, three-step re-
antarafacial pathway would be geometrically disfavored, it action. The substrate was first treated with NH4HCO2
is conceivable that an intermolecular hydride transfer may (3.3 equiv.), Pd/C (0.05 equiv. Pd), and montmorillonite-
be occurring. Addition of two molar equivalents of benzal- K10 (300 mg/mmol of substrate) in toluene heated to reflux
dehyde to our reaction did not yield exclusively 6 but rather for 1 h to complete the reduction of the nitro moiety. Next,
a 1:1 mixture of 5 and 6 (Table 1, Entry 3).
the reaction mixture was cooled, and an aldehyde (1 equiv.)
Given the equilibrium between compounds 7 and 9 un- was added before stirring for a further 16 h while allowing
der acidic conditions, exclusive formation of monosubsti- the reaction mixture to warm from 4 °C to ambient tem-
tuted benzimidazole 5 requires that dehydrogenation of perature. Notably, we found that both Pd/C and montmoril-
benzimidazoline (7 to 5) is favored over isomerization of 9 lonite-K10 are needed for high-yield benzimidazole synthe-
to 6 (Scheme 2). While a number of oxidants are known to sis, but some product formation (Ͻ20%) was also observed
mediate selective formation for 5 over 6 in such sys- in the absence of montmorillonite. Using this optimized
tems,[11,12] dehydrogenation with Pd/C has not previously procedure, we prepared 19 benzimidazoles (5, 10–27;
been reported.
Table 3).
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N. A. Weires, J. Boster, J. Magolan
SHORT COMMUNICATION
Table 3. One-pot synthesis of benzimidazoles.
sioned a one-pot sequence whereby 4,5-dimethyl-2-nitro-
aniline (29) is initially reacted with ortho-nitrobenzaldehyde
(30) to yield to benzimidazole 31, which is not isolated but
rather subjected to a second nitro-reduction and condensa-
tion with para-anisaldehyde (32, Scheme 3).
Scheme 3. One-pot, five-step synthesis of antifungal agent 34.
Gratifyingly, the one-pot five-step procedure proceeded
as planned, giving the desired heterocyclic product 34 in
60% overall yield. We found that the last stage of the reac-
tion sequence, condensation of anisaldehyde 33, required a
mild elevation of temperature (40 °C) to proceed at a rea-
sonable rate. We were cautious to avoid excessive heating at
this stage to prevent the potential further dehydrogenation
of 34 or condensation between 34 and a second equivalent
of anisaldehyde 33 followed by isomerization to the fully
aromatic species (analogous to the formation of 6,
Scheme 2). Although 1H NMR spectroscopic analysis of
the crude reaction mixture containing 33 suggested a trace
amount of dehydrogenation of this product, we were unable
to isolate any substantial byproducts of the reaction. From
a procedural standpoint, this one-pot, five-step synthesis of
33 was exceptionally facile, requiring no workup apart from
a single filtration followed by chromatographic purification
of the product.
[a] Reaction conditions: substrate (1.0 mmol), NH4CHO2
(3.3 equiv.), Mont-K10 (300 mg), Pd/C (0.05 equiv. Pd), reflux, 1 h;
then aldehyde (1 equiv.), 4 °C – room temp., 16 h. [b] Isolated yield
after chromatography. [c] Isolated as a mixture of tautomers that
equilibrate at elevated temperatures (see Supporting Information).
[d] 2-Nitrophenol was used as the substrate.
Conclusions
The procedure was found to be suitable for a variety of
electron-rich and electron-poor substituted benzaldehydes
Synthetic innovation today increasingly stresses the
10–19. These included ortho-substituted benzaldehydes 16, themes of efficiency, procedural simplicity, and minimal en-
18, and 19 as well as meta-hydroxy benzaldehyde 12. Sev- vironmental impact. In this context, multiple reactions con-
eral aliphatic aldehydes were also found to be suitable sub- ducted in one pot, employing easily removable and reusable
strates for the reaction (compounds 20–24), and alkyl sub- heterogeneous catalysts and non-polluting redox pro-
stitution on the 1,2-nitroaniline substrate was tolerated for cedures, constitute desirable tools with the potential to im-
13, 24, and 26. Three heteroaromatic aldehydes, 25, 26, and prove access to valuable compounds. We have combined
27, were also suitable substrates. When 2-nitrophenol and these themes to prepare benzimidazoles from ortho-nitro-
benzaldehyde were used as substrates, 2-phenyl benzoxazole anilines by means of a one-pot transfer hydrogenation, con-
(28) was obtained in low yield.
densation, and dehydrogenation sequence made possible by
To further highlight the synthetic value of this general the concurrent use of montmorillonite-K10 and Pd/C. In
strategy, we set out to synthesize a substituted dihydro- this manner, nineteen benzimidazoles were prepared in
benzo[4,5]imidazo[1,2-c]quinazoline 34 using a one-pot, good yields. The process was further extended to a one-pot,
three-component, five-step sequence (Scheme 3). Com- five-step, three-component synthesis of a bioactive tetracyc-
pound 34 has previously been shown to have narrow-spec- lic benzimidazoquinazoline, compound 34. With this meth-
trum antifungal activity against E. flocossum.[19] We envi- odology we hope to highlight the largely untapped potential
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Pd/C–Montmorillonite-Catalyzed Synthesis of Benzimidazoles
Supporting Information (see footnote on the first page of this arti-
of tandem concurrent heterogeneous catalysis to facilitate
multistep organic synthesis.
1
cle): General information, characterization data, and copies of H
and 13C NMR spectra for compounds 5, 10–28, and 34.
Acknowledgments
Experimental Section
General Procedure for the One-Pot, Three-Step Synthesis of Benz-
imidazoles 5 and 10–27: To a 25 mL round-bottomed flask charged
This work was made possible by the University of Idaho and by
the IDeA Network of Biomedical Research Excellence (INBRE)
program, National Institutes of Health (NIH) Grant Nos. P20
RR016454 (National Center for Research Resources) and P20
GM103408 (National Institute of General Medical Science).
N. A. W. received generous support from the Brian and Gayle Hill
Undergraduate Research Fellowship. We gratefully acknowledge
the assistance of Dr. Alex Blumenfeld with NMR spectroscopic
analysis.
with
a magnetic stirring bar were added: the nitroaniline
(1.00 mmol), ammonium formate (3.30 mmol), montmorillonite-
K10 (300 mg), and 5% Pd/C (106 mg, 0.05 mmol Pd). Solids re-
maining on the sides of the flask were then rinsed to the bottom
with toluene (10 mL) to give a bright yellow solution interspersed
with black heterogeneous particles. This reaction mixture was
heated to reflux vigorously at 120 °C for 1 h. Effervescence of CO2
was evidenced by a frothing of the reaction mixture that was distin-
guishable from reflux. This settled after about 30 min. The resulting
solution was colorless, interspersed with black heterogeneous par-
ticles. The reaction mixture was allowed to cool, and the desired
aldehyde (1.00 mmol) was added by means of a micropipette (li-
quid) or funneled weighing paper (solid). The reaction mixture was
stirred in an ice bath and warmed to ambient temperature over-
night (16 h). The reaction mixture was filtered through a pad of
Celite and rinsed with methanol under mild air pressure. The sol-
vent of the filtrate was then removed in vacuo to give the corre-
sponding benzimidazole in typically high purity. All benzimid-
azoles were further purified by liquid chromatography by using a
mixture of 1% methanol and 99% dichloromethane as the eluent.
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One-Pot, Five-Step Procedure for the Synthesis of 6-(4-Meth-
oxyphenyl)-9,10-dimethyl-5,6-dihydrobenzo[4,5]imidazo[1,2-c]quin-
azoline (34): To a 25 mL round-bottomed flask equipped with a
magnetic stirring bar were added: 4,5-dimethyl-2-nitroaniline
(166.9 mg, 1.00 mmol), ammonium formate (208.7 mg, 3.31 mmol),
montmorillonite-K10 (300 mg), and Pd/C (108.7 mg, 5%
0.05 mmol Pd). Solids remaining on the sides of the flask were then
rinsed to the bottom with toluene (10 mL). This mixture was
heated to reflux vigorously at 120 °C for 1 h. Effervescence of CO2
was observed. This reaction mixture was cooled to room tempera-
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with black heterogeneous particles. To this was added p-anisal-
dehyde (138.8 mg, 1.02 mmol) with a micropipette to yield a yellow
mixture. This reaction was heated at 40 °C in an oil bath for an-
other 16 h to yield a colorless solution of the crude benzimidazo-
quinazoline 34 interspersed with black heterogeneous particles. The
reaction mixture was filtered through a pad of Celite and rinsed
with methanol. The solvent was removed in vacuo, and the product
was purified by chromatography (mixture of 1% methanol and
99% dichloromethane as eluent). Yield 215.3 mg (60%, m.p. 111–
114 °C); yellow solid; Rf 0.70 (5%MeOH/95%DCM). 1H NMR
(500 MHz, [D6]DMSO): δ = 7.90 (dd, J = 8.0, 1.5 Hz, 1 H), 7.45–
7.39 (m, 2 H), 7.23–7.18 (m, 1 H), 7.17–7.11 (m, 2 H), 6.94–6.90
(m, 2 H), 6.89–6.85 (m, 2 H), 6.84–6.77 (m, 2 H), 3.69 (s, 3 H),
2.27 (s, 3 H), 2.21 (s, 3 H) ppm. 13C NMR (125 MHz, [D6]DMSO):
δ = 159.5, 146.1, 142.8, 142.4, 132.7, 131.3, 131.1, 130.5, 130.4,
127.1, 124.3, 118.7, 118.0, 114.7, 114.0, 112.3, 110.6, 67.3, 55.1,
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Received: August 14, 2012
Published Online: October 22, 2012
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